Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application

*Rajan Kumar Singh, Neha Jain, Sudipta Som, Somrita Dutta, Jai Singh and Ranveer Kumar*

## **Abstract**

Most recently, organic-inorganic semiconductor light harvester materials, have arisen as a new class of functional element and attracts the research community due to its outstanding optoelectronic properties. Organic-inorganic perovskites are solution process that is easy for the fabrication of devices at low temperature. Additionally, up to date, perovskite quantum dots have emerged as the most efficient light harvester for LEDs and display applications, with high color purity, color tunability, and photoluminescence quantum yield up to 100%. However, the presence of lead in organic-inorganic perovskites and the stability issue of perovskite materials are the significant challenges for the research community. To date, some lead substitute materials have been tried to enhance the film morphology and production of the less toxic light harvester. In this chapter, we focus on the lead substitution on B sight with homovalent cations like Sn2+, Mn2+, Cd2+, and Zn2+ cations. These lead substitutions not only reduce the toxicity of perovskite material while these dopants also enhance the optical and performance of LEDs. We also included the LEDs application of lead substituted perovskite quantum dots (PQDs) that may be useful for the environmental friendly and highly performing perovskite quantum dot LEDs (PQ-LEDs) shortly.

**Keywords:** hybrid perovskite, quantum dots, lead-free, light emitting diodes

## **1. Introduction**

Organic-inorganic perovskite light harvesting materials have attracted passionate interest in past few years and raised as one of the most promising solar cells with power conversion efficiency up to 23% as well as excellent performance in LED display devices with high color gamut and color purity [1]. For the photovoltaic (PV) application, perovskite materials were first used in 2009 and for QDs LED applications, perovskite material was used firstly in 2014 [2]. Perovskite-based devices are solution processable that makes it cost-effective and helps in large scale production. Presently, such kind of perovskite solar cells is now comparable to the commercially available solar cells including, silicon, CdTe, CuInGaSe, and GaAs [3]. Perovskite quantum dots (PQDs) also are the most research centered topic in the field of nano-materials and colloidal QDs due to its excellent optical behavior as explained earlier [4].

After long research and development, semiconductor QDs (CdSe, CdZnSe, InP, etc.) and OLEDs have emerged as the leading technologies in the field of display devices [3]. OLEDs have lit pixels, less energy consuming properties, superior contrast, and wider viewing angles. However, synthesis and fabrication process of OLEDs are very complicated that raise the price of devices. On the other hand, PQDs possess tunable, narrow band emission and high PLQY that significantly improve the performance in both EL and PL devices as explained earlier. Existing CdSe and InP QDs have taken decades to be suitable as the robust candidates for displays. Samsung QLED TVs have been in the forefront serving the premium market along with Chinese brands, TCL and Hisense. Samsung Display is building a pilot production facility for QD OLED, which will start production in 2019. Most research is centered on PQDs due to their high photoluminescence (PLQY), tunable band gaps, and narrow emission wavelength. With quantum confinement effects, the emission of PQDs can be controlled with size and ingredients. Hence, PQDs are useful in photovoltaic cells, laser applications, light-emitting diodes (LEDs), and bio-imaging. Promising color purity and emission tunability of QDs make them promising contenders for next-generation displays as a light converter (white-light LEDs) and active-mode QD-LEDs (QLEDs) [3, 5]. Additionally, PQDs work faster than other QDs due to the absence of a deep state, where the electron-hole pair does not immediately revert to its ground state. In this context, PQDs have become promising candidates for the next-generation of w-LEDs for lighting and other display applications [6–9]. After the discovery of PQDs, more than 7000–8000 peer-reviewed literatures have been published. The fast rate of scientific research publication reveals the interest of the research community in the PQDs topic. Besides excellent performances of PQD technology, some challenges restrict the lead-containing perovskite materials for commercialization. Lead halide perovskite both bulk as well as nanoparticles/QDs are soft and very sensitive with humidity, moisture, light, air, temperature, etc. [10]. Therefore, the most apparent work necessity is to develop the new synthesis routes and new approaches of their more stabilization concerning ambient conditions, water, light, and temperature. Bromide-based perovskite has excellent performance, but the major shortcoming is the stability and presence of toxic element Pb. In this regards, a more in-depth understanding of the mechanism on the radiative and nonradiative recombination is necessarily required [11, 12].

Thus it is an open question that does the organic-inorganic perovskite-based device industry fabrication comes soon? The answer may be controversial because research community will say yes while some will say no because of the toxicity of Pb and long term stability of the materials. According to the U. S. EPA, the maximum amount of Pb2+ in air and water should not be more than 0.1 μg L<sup>−</sup><sup>1</sup> and 15 μg L<sup>−</sup><sup>1</sup> respectively [13]. Hence mass production of pure Pb-based perovskite may pollute the Earth due to its toxicity, long degradation lifetime and easy solubility in water. Therefore, we need to develop a new approach to explore environmental friendly Pb free or limited amount of Pb-based perovskite materials for optoelectronic applications. Thus these two main drawbacks of perovskite material restrict it in commercialization. Hence, the demand for nontoxic element-based perovskite materials continues to grow in the past few years [14]. Among these nontoxic alternatives, divalent tin cation has been considered as a right candidate in replacing Pb2+, and the application of tin-halide perovskites in optoelectronic devices has also been investigated. However, Sn2+ undergoes facile oxidation to its tetravalent state (Sn4+), creating a high defect density in the perovskite lattice. These defects would generate trap states in the middle of the band gap, leading to rapid non-radiative relaxation of the exciton. The highest PLQY value of tin-based perovskite nanomaterials can only reach 6.4%, which is still far inferior to the lead-based perovskite

**119**

MA<sup>+</sup>

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

nanomaterials (~84%) [15]. Potential applications of Sn-based perovskite material in optoelectronic applications, significant efforts have been made recently to improve the photoluminescence properties of tin perovskite-based nanomaterials. Besides Sn, in place of Pb2+, Zn2+, Mn2+, Ge2+, Cu2+, and Bi3+ already have been investigated for less lead of lead-free perovskite materials for versatile applications. However, in the case of organic-inorganic halide PQDs, there is very less work reported and some research work based on Pb2+ substitution in only inorganic CsPbX3 perovskite systems are going on, and this will be the main focus for the

Perovskites structure derives from the crystal structure of titanium calcium oxide (CaTiO3). This was the first discovered perovskite material in the year 1839 by German mineralogist Gustav Rose and structure of perovskite material was studied by Russian mineralogist Lev A. Perovski [16]. After this discovery, a huge amount of perovskite materials have been found from natural resources and as well as synthesized in the lab also. So perovskite materials can be categorized mainly into two categories, one is based on oxides (oxygen anion) and another based on halides (halogen anions). On the basis of this categorization, perovskite materials adopt ABO3 and ABX3 chemical formula, where A and B are the cations and X is halogens such as I, Br, Cl, or mixed halides. ABO3 structured perovskite is generally used for thermoelectric, superconductive and ferromagnetic applications which were discovered in the 18th century. On the other hand, halide-based perovskite material; CsPbX3 was first reported in 1958, and CH3NH3PbX3-based perovskite was seen in 1978. Thus ABX3 perovskite further can be categorized into two subparts, pure inorganic halide perovskite, and organic-inorganic halide perovskite. Mitzi et al. have first time reported the excitonic property of halide perovskites for light-emitting diodes (LEDs) and thin film transistors in the year 1990 [17]. Nowadays, most extensively studied materials are ABX3 structured perovskite semiconductor due to its excellent properties. In ABX3 chemical formula, A,

, Rb<sup>+</sup>

+

Sn2+, Ge2+, Ca2+, Sr2+, Ba2+ and X is anion (I<sup>−</sup>, Br<sup>−</sup> and Cl<sup>−</sup>). The properties of perovskite material can be easily tuned by changing the A, B or X site in ABX3 structure [18]. Generally, perovskite materials have cubic structure consists of close-packed AX3 sub-lattice with divalent B-site cations within the six-fold coordinated cavities. B-X bonding of perovskite governs the electronic behavior of perovskite semiconductors while A cation has no direct role for the electronic properties. But the size of the A cation may cause the distortions in symmetry of the material. Besides the cubic phase, perovskite has tetragonal and orthorhombic phase also. For example, MAPbI3 has a tetragonal structure while MAPbBr3 and MAPbCl3 have a cubic structure at room temperature. The symmetry of MAPbI3 perovskite also depends on the temperature and symmetry increases with increase in temperature such as, at a lower temperature it has octahedral symmetry, and at room temperature to (162.2–327 K) has tetragonal white at a higher temperature above 178.8 K shows cubic phase. FAPbI3 perovskite has higher symmetry than MA-based perovskites hence has been most widely investigated [19]. FAPbI3 perovskite has nearly cubic structure at room temperature and inert atmosphere. At open atmosphere, FA shows its yellow phase that is non-perovskite phase of this material, and it is not useful for optoelectronic devices because of the high band gap.

, Cs<sup>+</sup>

, EA), methylammonium (CH3NH3

+2), FA), B-site stands for divalent cation Pb2+,

or monovalent organic

+ ,

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

**2. Structure of halide perovskite**

belongs to monovalent inorganic cation such as K<sup>+</sup>

cation like ethylammonium (CH3CH2NH3

), formamidinium ((CH(NH2)

present chapter.

nanomaterials (~84%) [15]. Potential applications of Sn-based perovskite material in optoelectronic applications, significant efforts have been made recently to improve the photoluminescence properties of tin perovskite-based nanomaterials. Besides Sn, in place of Pb2+, Zn2+, Mn2+, Ge2+, Cu2+, and Bi3+ already have been investigated for less lead of lead-free perovskite materials for versatile applications. However, in the case of organic-inorganic halide PQDs, there is very less work reported and some research work based on Pb2+ substitution in only inorganic CsPbX3 perovskite systems are going on, and this will be the main focus for the present chapter.

## **2. Structure of halide perovskite**

*Perovskite Materials, Devices and Integration*

is necessarily required [11, 12].

After long research and development, semiconductor QDs (CdSe, CdZnSe, InP, etc.) and OLEDs have emerged as the leading technologies in the field of display devices [3]. OLEDs have lit pixels, less energy consuming properties, superior contrast, and wider viewing angles. However, synthesis and fabrication process of OLEDs are very complicated that raise the price of devices. On the other hand, PQDs possess tunable, narrow band emission and high PLQY that significantly improve the performance in both EL and PL devices as explained earlier. Existing CdSe and InP QDs have taken decades to be suitable as the robust candidates for displays. Samsung QLED TVs have been in the forefront serving the premium market along with Chinese brands, TCL and Hisense. Samsung Display is building a pilot production facility for QD OLED, which will start production in 2019. Most research is centered on PQDs due to their high photoluminescence (PLQY), tunable band gaps, and narrow emission wavelength. With quantum confinement effects, the emission of PQDs can be controlled with size and ingredients. Hence, PQDs are useful in photovoltaic cells, laser applications, light-emitting diodes (LEDs), and bio-imaging. Promising color purity and emission tunability of QDs make them promising contenders for next-generation displays as a light converter (white-light LEDs) and active-mode QD-LEDs (QLEDs) [3, 5]. Additionally, PQDs work faster than other QDs due to the absence of a deep state, where the electron-hole pair does not immediately revert to its ground state. In this context, PQDs have become promising candidates for the next-generation of w-LEDs for lighting and other display applications [6–9]. After the discovery of PQDs, more than 7000–8000 peer-reviewed literatures have been published. The fast rate of scientific research publication reveals the interest of the research community in the PQDs topic. Besides excellent performances of PQD technology, some challenges restrict the lead-containing perovskite materials for commercialization. Lead halide perovskite both bulk as well as nanoparticles/QDs are soft and very sensitive with humidity, moisture, light, air, temperature, etc. [10]. Therefore, the most apparent work necessity is to develop the new synthesis routes and new approaches of their more stabilization concerning ambient conditions, water, light, and temperature. Bromide-based perovskite has excellent performance, but the major shortcoming is the stability and presence of toxic element Pb. In this regards, a more in-depth understanding of the mechanism on the radiative and nonradiative recombination

Thus it is an open question that does the organic-inorganic perovskite-based device industry fabrication comes soon? The answer may be controversial because research community will say yes while some will say no because of the toxicity of Pb and long term stability of the materials. According to the U. S. EPA, the maximum

respectively [13]. Hence mass production of pure Pb-based perovskite may pollute the Earth due to its toxicity, long degradation lifetime and easy solubility in water. Therefore, we need to develop a new approach to explore environmental friendly Pb free or limited amount of Pb-based perovskite materials for optoelectronic applications. Thus these two main drawbacks of perovskite material restrict it in commercialization. Hence, the demand for nontoxic element-based perovskite materials continues to grow in the past few years [14]. Among these nontoxic alternatives, divalent tin cation has been considered as a right candidate in replacing Pb2+, and the application of tin-halide perovskites in optoelectronic devices has also been investigated. However, Sn2+ undergoes facile oxidation to its tetravalent state (Sn4+), creating a high defect density in the perovskite lattice. These defects would generate trap states in the middle of the band gap, leading to rapid non-radiative relaxation of the exciton. The highest PLQY value of tin-based perovskite nanomaterials can only reach 6.4%, which is still far inferior to the lead-based perovskite

and 15 μg L<sup>−</sup><sup>1</sup>

amount of Pb2+ in air and water should not be more than 0.1 μg L<sup>−</sup><sup>1</sup>

**118**

Perovskites structure derives from the crystal structure of titanium calcium oxide (CaTiO3). This was the first discovered perovskite material in the year 1839 by German mineralogist Gustav Rose and structure of perovskite material was studied by Russian mineralogist Lev A. Perovski [16]. After this discovery, a huge amount of perovskite materials have been found from natural resources and as well as synthesized in the lab also. So perovskite materials can be categorized mainly into two categories, one is based on oxides (oxygen anion) and another based on halides (halogen anions). On the basis of this categorization, perovskite materials adopt ABO3 and ABX3 chemical formula, where A and B are the cations and X is halogens such as I, Br, Cl, or mixed halides. ABO3 structured perovskite is generally used for thermoelectric, superconductive and ferromagnetic applications which were discovered in the 18th century. On the other hand, halide-based perovskite material; CsPbX3 was first reported in 1958, and CH3NH3PbX3-based perovskite was seen in 1978. Thus ABX3 perovskite further can be categorized into two subparts, pure inorganic halide perovskite, and organic-inorganic halide perovskite. Mitzi et al. have first time reported the excitonic property of halide perovskites for light-emitting diodes (LEDs) and thin film transistors in the year 1990 [17]. Nowadays, most extensively studied materials are ABX3 structured perovskite semiconductor due to its excellent properties. In ABX3 chemical formula, A, belongs to monovalent inorganic cation such as K<sup>+</sup> , Rb<sup>+</sup> , Cs<sup>+</sup> or monovalent organic cation like ethylammonium (CH3CH2NH3 + , EA), methylammonium (CH3NH3 + , MA<sup>+</sup> ), formamidinium ((CH(NH2) +2), FA), B-site stands for divalent cation Pb2+, Sn2+, Ge2+, Ca2+, Sr2+, Ba2+ and X is anion (I<sup>−</sup>, Br<sup>−</sup> and Cl<sup>−</sup>). The properties of perovskite material can be easily tuned by changing the A, B or X site in ABX3 structure [18]. Generally, perovskite materials have cubic structure consists of close-packed AX3 sub-lattice with divalent B-site cations within the six-fold coordinated cavities. B-X bonding of perovskite governs the electronic behavior of perovskite semiconductors while A cation has no direct role for the electronic properties. But the size of the A cation may cause the distortions in symmetry of the material. Besides the cubic phase, perovskite has tetragonal and orthorhombic phase also. For example, MAPbI3 has a tetragonal structure while MAPbBr3 and MAPbCl3 have a cubic structure at room temperature. The symmetry of MAPbI3 perovskite also depends on the temperature and symmetry increases with increase in temperature such as, at a lower temperature it has octahedral symmetry, and at room temperature to (162.2–327 K) has tetragonal white at a higher temperature above 178.8 K shows cubic phase. FAPbI3 perovskite has higher symmetry than MA-based perovskites hence has been most widely investigated [19]. FAPbI3 perovskite has nearly cubic structure at room temperature and inert atmosphere. At open atmosphere, FA shows its yellow phase that is non-perovskite phase of this material, and it is not useful for optoelectronic devices because of the high band gap.

This is the only drawback with FA material otherwise FA-based perovskite would exceed those of MA [20].

Every ABX3 structure cannot be a perovskite phase. There is some fixed criteria for the existence of perovskite structure: (1) charge neutrality: that means the charges of cation and anion should be equal. For example, for ABX3 structure, A and B cations have 1+, and 2+ charges (total = 3+) and X anion has 3− charges. So both cation and anion have equal costs, i.e., N(A) + N(B) = 3 N(X), here N belongs to the valence of the A, B and X ions; (2) the Goldschmidt tolerance factor, t and (3) octahedral factor μ. t and μ are the most potent factor that decides the existence and structure of the perovskite material. Tolerance factor t and octahedral factor μ are defined as the following equation:

$$\mathbf{t} = \frac{(rA - r\mathbf{x})}{\sqrt{2}} \text{ and } \boldsymbol{\mu} = \frac{rB}{r\bar{X}} \tag{1}$$

where rA, rB, and rX are the ionic radii of A, B, and X, respectively. For the perovskite structure, the value of t and μ must be lies between given limit; 0.89 < t < 1.11 and 0.442 < μ < 0.895 [21]. Thus from the above formula, it is clear that the size of the A, B and X ions play an essential role for the perovskite materials. These days the Goldschmidt tolerance factor concept is beneficial for the finding the lead-free perovskite materials based on the ionic radii of implicated ions. Beside B-site replacement, there are too many possibilities of the other ion positions such as A as well as X-site. On the basis of this concept, Kieslich and co-workers have theoretically found out more than 600 hypothetical perovskites that have not been reported yet [22]. Such kind of study can help develop less lead of lead-free perovskite materials on the practical level for commercialization.

## **3. Optical properties of halide perovskite**

Organic-inorganic perovskite materials demonstrate a strong optical absorption and bandgap tuning, long diffusion length, high charge carrier mobility, ambipolar charge transport and high tolerance of defects. Strong light absorbing properties of the material ideally suited for solar cell devices. On the other hand, band tuning properties and a wide range of light emission in the visible range with narrow emission bandwidth (FWHM) suitable for the light emitting devices. Organicinorganic perovskite materials have sharp optical band edge that indicates the direct bandgap and minimal disordered materials [23]. The bandgap of perovskite material can be controlled by their chemical compositions and crystallite size. For example, CH3NH3PbI3 (red emission), CH3NH3PbBr3 (green emission) and CH3NH3PbCl3 (blue emission) hybrid perovskites have band gap of 1.55, 2.3 and 3.1 eV, respectively [24].

The band gap energy levels of hybrid perovskite are determined by an antibonding hybrid state between the cation B-s and anion X-p orbitals. This state is related to the valance band maxima and conduction band minimum. Thus due to their unique ABX3 structure and compositional flexibility, optical properties can be tuned by varying elements at each site. The change in I/Br ratio in CH3NH3PbX3 perovskite can modify the bandgap from 1.55 to 2.3 eV, i.e., PL emission can be tuned red, orange and yellow. This band gap tuning mechanism is due to the hybridization of p and s orbitals such as Br-4p orbital is overlapped with the I-5p orbital and the Pb-6 s orbital. Similarly, with varying the concentration of Cl/Br, we can tune bandgap from 2.3 to 3.1 eV for hybrid perovskites [25]. Another approach is tuning of the band gap in perovskite structure is substitution of (CH3NH3) + site

**121**

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

ing the A cation, the bond length with B and X site is also changed that is responsible for the band gap tuning. CsPbI3, CH3NH3PbI3 and FAPbI3 has band gap of 1.73, 1.55 and 1.48 eV, respectively [26]. So from these values, we can see that the band gap of the material decreases with increasing the cation size. Besides A and X site, the band gap energy can be tuned with changing the Pb2+ (B) from other divalent

Depending on the particle size, dimensions and morphology hybrid perovskite materials show different optical energy gap than their bulk counterparts due to their quantum confinement effect. For traditional nano-materials, quantum confinement effect has been widely studied. In recent times, the development of size-controlled perovskite QDs has enabled exhaustive research and developments of quantum confinement effect in QDs. Size of perovskite QDs can be controlled or tuned by ligand OLA. Friend et al. have studied on the size-dependent photon emission from CH3NH3PbBr3 PQDs, where the particle size and the PL emission peak could be tuned by varying the concentration of the perovskite precursors [27]. Quantum confinement effects have also been reported by Kovalenko group in all inorganic CsPbBr3 PQDs. They found that, with a decrease in the diameter of particle size from 11.8 to 3.8 nm, the PL emission peak gradually blue-shifted from 512 to 460 nm [28]. The thickness of the QDs also responsible for the band gap tuning. Thus trough band gap tuning, multi-colored LEDs and white LED can be formed by simple and easy way.

Perovskite quantum dots are considered as the most capable aspirant to the next generation of optoelectronic devices and solar cells technology. But the presence of toxicity due to Pb restricts the commercialization. Hence, currently, most of the research is going on less lead or Pb-free perovskites. The nontoxic and environmentally free perovskite can be prepared by substituting Pb with nontoxic elements that can be achieved through two approaches, i.e., homovalent and heterovalent substitution. Homovalent elements for lead substitution with an isovalent cation such as Sn2+, Mn2+, Cd2+, Zn2+, etc. while heterovalent are aliovalent cations like Bi3+, Sb3+.

For the Pb-free perovskite materials, many homovalent elements with +2 stable oxidation states can be used. For example, the group- 14 elements Sn2+ and Ge2+, i.e., the same group of Pb2+ can be the best choice for lead substitution. Beside this, transition metals as Cd2+, Mn2+, Fe2+, Cu2+ and Zn2+; alkaline-earth metals such as Ba2+, Sr2+ and Ca2+; rare-earth elements like Eu2+, Yb2+ can be considered for the Pb-free perovskites [29]. Theoretically, these Pb-substitutes are perfect for perovskite structure according to the tolerance factor calculations. But some elements like Ba, Sr, and Ca have a large band gap, so these are not suitable for semiconducting materials. However, working with Cu and Zn in ambient condition will be difficult. In recent years, Sn2+ and Mn2+ are the promising candidates for

The most suitable Pb-substitute is Sn2+ for lead-free perovskite because of the around same ionic radii, binding energy and the same electronic configuration of

In this work, we will only discuss homovalent cation substitution of PQDs.

or mixing of both Cs/other alkyl-ammonium cations. With chang-

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

cations like Sn2+, Mn2+, Cd2+, etc. [26].

**4.1 Homovalent substitution**

**4. One step towards lead-free perovskite materials**

homovalent substitution in perovskite quantum dots.

**4.2 Sn-based perovskite quantum dots**

(A) by Cs+

, Rb+

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

(A) by Cs+ , Rb+ or mixing of both Cs/other alkyl-ammonium cations. With changing the A cation, the bond length with B and X site is also changed that is responsible for the band gap tuning. CsPbI3, CH3NH3PbI3 and FAPbI3 has band gap of 1.73, 1.55 and 1.48 eV, respectively [26]. So from these values, we can see that the band gap of the material decreases with increasing the cation size. Besides A and X site, the band gap energy can be tuned with changing the Pb2+ (B) from other divalent cations like Sn2+, Mn2+, Cd2+, etc. [26].

Depending on the particle size, dimensions and morphology hybrid perovskite materials show different optical energy gap than their bulk counterparts due to their quantum confinement effect. For traditional nano-materials, quantum confinement effect has been widely studied. In recent times, the development of size-controlled perovskite QDs has enabled exhaustive research and developments of quantum confinement effect in QDs. Size of perovskite QDs can be controlled or tuned by ligand OLA. Friend et al. have studied on the size-dependent photon emission from CH3NH3PbBr3 PQDs, where the particle size and the PL emission peak could be tuned by varying the concentration of the perovskite precursors [27]. Quantum confinement effects have also been reported by Kovalenko group in all inorganic CsPbBr3 PQDs. They found that, with a decrease in the diameter of particle size from 11.8 to 3.8 nm, the PL emission peak gradually blue-shifted from 512 to 460 nm [28]. The thickness of the QDs also responsible for the band gap tuning. Thus trough band gap tuning, multi-colored LEDs and white LED can be formed by simple and easy way.

## **4. One step towards lead-free perovskite materials**

Perovskite quantum dots are considered as the most capable aspirant to the next generation of optoelectronic devices and solar cells technology. But the presence of toxicity due to Pb restricts the commercialization. Hence, currently, most of the research is going on less lead or Pb-free perovskites. The nontoxic and environmentally free perovskite can be prepared by substituting Pb with nontoxic elements that can be achieved through two approaches, i.e., homovalent and heterovalent substitution. Homovalent elements for lead substitution with an isovalent cation such as Sn2+, Mn2+, Cd2+, Zn2+, etc. while heterovalent are aliovalent cations like Bi3+, Sb3+. In this work, we will only discuss homovalent cation substitution of PQDs.

## **4.1 Homovalent substitution**

For the Pb-free perovskite materials, many homovalent elements with +2 stable oxidation states can be used. For example, the group- 14 elements Sn2+ and Ge2+, i.e., the same group of Pb2+ can be the best choice for lead substitution. Beside this, transition metals as Cd2+, Mn2+, Fe2+, Cu2+ and Zn2+; alkaline-earth metals such as Ba2+, Sr2+ and Ca2+; rare-earth elements like Eu2+, Yb2+ can be considered for the Pb-free perovskites [29]. Theoretically, these Pb-substitutes are perfect for perovskite structure according to the tolerance factor calculations. But some elements like Ba, Sr, and Ca have a large band gap, so these are not suitable for semiconducting materials. However, working with Cu and Zn in ambient condition will be difficult. In recent years, Sn2+ and Mn2+ are the promising candidates for homovalent substitution in perovskite quantum dots.

#### **4.2 Sn-based perovskite quantum dots**

The most suitable Pb-substitute is Sn2+ for lead-free perovskite because of the around same ionic radii, binding energy and the same electronic configuration of

*Perovskite Materials, Devices and Integration*

defined as the following equation:

t = (*rA* <sup>−</sup> *rx*) \_\_\_\_\_\_\_\_\_\_\_\_\_

**3. Optical properties of halide perovskite**

√ \_\_ 2 (*rB* − *Rx*)

perovskite materials on the practical level for commercialization.

exceed those of MA [20].

This is the only drawback with FA material otherwise FA-based perovskite would

for the existence of perovskite structure: (1) charge neutrality: that means the charges of cation and anion should be equal. For example, for ABX3 structure, A and B cations have 1+, and 2+ charges (total = 3+) and X anion has 3− charges. So both cation and anion have equal costs, i.e., N(A) + N(B) = 3 N(X), here N belongs to the valence of the A, B and X ions; (2) the Goldschmidt tolerance factor, t and (3) octahedral factor μ. t and μ are the most potent factor that decides the existence and structure of the perovskite material. Tolerance factor t and octahedral factor μ are

where rA, rB, and rX are the ionic radii of A, B, and X, respectively. For the perovskite structure, the value of t and μ must be lies between given limit; 0.89 < t < 1.11 and 0.442 < μ < 0.895 [21]. Thus from the above formula, it is clear that the size of the A, B and X ions play an essential role for the perovskite materials. These days the Goldschmidt tolerance factor concept is beneficial for the finding the lead-free perovskite materials based on the ionic radii of implicated ions. Beside B-site replacement, there are too many possibilities of the other ion positions such as A as well as X-site. On the basis of this concept, Kieslich and co-workers have theoretically found out more than 600 hypothetical perovskites that have not been reported yet [22]. Such kind of study can help develop less lead of lead-free

Organic-inorganic perovskite materials demonstrate a strong optical absorption and bandgap tuning, long diffusion length, high charge carrier mobility, ambipolar charge transport and high tolerance of defects. Strong light absorbing properties of the material ideally suited for solar cell devices. On the other hand, band tuning properties and a wide range of light emission in the visible range with narrow emission bandwidth (FWHM) suitable for the light emitting devices. Organicinorganic perovskite materials have sharp optical band edge that indicates the direct bandgap and minimal disordered materials [23]. The bandgap of perovskite material can be controlled by their chemical compositions and crystallite size. For example, CH3NH3PbI3 (red emission), CH3NH3PbBr3 (green emission) and CH3NH3PbCl3 (blue emission) hybrid perovskites have band gap of 1.55, 2.3 and

The band gap energy levels of hybrid perovskite are determined by an antibonding hybrid state between the cation B-s and anion X-p orbitals. This state is related to the valance band maxima and conduction band minimum. Thus due to their unique ABX3 structure and compositional flexibility, optical properties can be tuned by varying elements at each site. The change in I/Br ratio in CH3NH3PbX3 perovskite can modify the bandgap from 1.55 to 2.3 eV, i.e., PL emission can be tuned red, orange and yellow. This band gap tuning mechanism is due to the hybridization of p and s orbitals such as Br-4p orbital is overlapped with the I-5p orbital and the Pb-6 s orbital. Similarly, with varying the concentration of Cl/Br, we can tune bandgap from 2.3 to 3.1 eV for hybrid perovskites [25]. Another approach is tuning of the band gap in perovskite structure is substitution of (CH3NH3)

Every ABX3 structure cannot be a perovskite phase. There is some fixed criteria

and μ = \_\_\_\_ *rB*

*rX* (1)

+ site

**120**

3.1 eV, respectively [24].

s 2 valence [29]. MASnI3 is the most studied lead-free perovskite material. Sn-based perovskites are direct bandgap semiconductors, like Pb-based perovskites. This means, the valence band maximum and conduction band minimum recline at the same position in k-space (reciprocal space). Similar to the APbX3 structure, the structural, optical and electrical properties of ASnX3 perovskite is also affected by the size of A-site cation and X-site anion. MASnI3 perovskite also has a tetragonal structure, but the band gap is near unity that is lower than those of the MAPbI3 [30]. Due to the quantum confinement effect, PQDs shows different behavior from their bulk counterpart.

In case of the pure inorganic perovskite CsPbI3, band gap of PQDs is 1.85–1.97 eV. Such kind of higher bandgap highly encouraged to find some lower Pb-containing, low band gap, and more stable perovskite material. CsSnI3 pure inorganic can be the satisfied the all requirements for the good PQDs because of its lower binding energy and broad absorption from visible to near infrared region. Due to this different behavior, such kind of perovskite can exhibit higher light harvesting efficiency than other perovskite materials [31]. However, the stability of the Sn2+-based perovskite material is the biggest issue due to higher sensitivity to air, moisture and sometimes with nonpolar organic solvents also [32]. In recent years, many significant approaches have been tried for the stability of the Sn2+ perovskites. For example, SnBr2, SnCl2, and SnF2 had used as a dopant into the CsSnI3 perovskite for the stability of the materials. In all, depends, SnF2 and SnCl2 were found more suitable for the air resistive property. However, stability of the Sn-based material was not satisfactory due to the degradation of the sample within only 3hs [33–35].

In the year 2017, Liu et al. reported, the alloyed CsSn1−xPbxI3 PQDs, which was more phase stable than its parent CsPbI3 and CsSnI3 PQDs for months in ambient air [36]. They have prepared CsSn1−xPbxI3 PQDs via the standard Schlenk line method under a nitrogen atmosphere. In this method, SnI2/PbI2 mixture was rapidly dissolved in trioctylphoshine into the cesium olate at 120–170°C and purified by antisolvent methyl acetate (MeOAc) washing process. The 140°C reaction temperature was found the best for homogeneous and proper morphology of CsSn1−xPbxI3 PQDs. With the incorporation of Sn, red shifting occurred that can be observed in absorption and PL spectra as shown in **Figure 1a**.

Additionally, the increase in Sn concentration drastically enhanced the light absorption of CsSn1−xPbxI3 PQDs; however, PLQY was found up to 3% only. On the other hand, CsPbI3-based PQDs had up to 100% efficiency to date. Low PLQY with Sn-based PQDs indicates the increase in intrinsic defects that were allied with Sn vacancies. Such kind of vacancies creates deep level defects which act as nonradiative recombination centers and hence this reduces the efficiency of PQDs. **Figure 1b** also suggested the average lifetime of CsSn1−xPbxI3 PQDs dramatically decreases with increasing the concentration of Sn. This behavior of Sn containing materials also confirms the occurrence of a large density of quenching defects that support the lower PLQY of the PQDs. Liu also reported that CsSn0.60Pb0.40I3 PQDs was best for performance and stability because of the less- Pb content and widest range of light absorption among the alloyed PQDs. Phase, as well as chemical Stability of CsSn0.60Pb0.40I3 PQDs, is confirmed via XRD and XPS analysis (see **Figure 1c** and **d**). XRD **Figure 3b** suggested that there is no degradation was absorbed up to 5 months that is the first time reported such long time stability with Sn-based material.

Furthermore, there was not found Sn4+ oxidation stage in the XPS analysis of the sample. In **Figure 1d** XPS graph showed 2 spectra at 486.2 and 494.5 eV that is associated with Sn2+ 3d5/2 and 3d3/2 state, while deficiency of spectra at 486.9 indicates the absence of Sn4+ in perovskite compound. Thus this less lead-containing and air-stable Sn-based PQDs can be the best choice for solar cell devices but for highly

**123**

**Figure 1.**

and anion exchange between Br<sup>−</sup><sup>1</sup>

*(as prepared) and those after storage for 150 days.*

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

efficient LED there will be a need in the improvement of its PLQY. Jellicoe and co-workers had also tried for the Sn-based perovskite, CsSnX3 (X = Cl, Br, I) for the lead-free perovskite PQDs but due to the instability of Sn it cannot be used for practical applications [35]. To overcome the stability problem, Wang group had replaced Sn2+ with Sn4+ and developed stable CsSnI6 PQDs with cubic structure. However it's PLQY was just 0.48% that is also not sufficient for optoelectronic applications [36]. From the several studied we absorbed that, CsSnI3-based perovskites have low PLQY but good hole conducting properties. Hence it is mostly used for solar cell applications. For sufficient PLQY and more stable Sn-based material was still needed for display devices. For this purpose, CsSnBr3-based perovskite was found

*(a) Optical absorption spectra of different Sn doped PQDs and the inset fig shows their corresponding normalized steady photoluminescence spectra, (b) time-resolved photoluminescence decay for various stoichiometries of Sn, (c) X-ray diffraction patterns and (d) XPS spectra for the CsSn0.60Pb0.40I3 PQDs* 

Yang group have reported the fast and less Pb or Pb- free perovskite nanocrystals

, the PLQY of CsSnI3 and CsPbBr3 NCs was

by introducing the Sn in Pb site and I at Br site [37]. They have used one pot hot injection method for preparation of SnI2 doped CsPbBr3 perovskite nanocrystals. After cation and anion exchange between the CsPbBr3 and SnI2, the lead-free CsSnI3 NCs were obtained. These Pb-free NCs maintained the high PLQY as well as morphology in an inert atmosphere. **Figure 2a** indicated that, with increasing the amount of SnI2 into CsPbBr3 colloidal solution, dramatically red shifting observed in PL emission and absorption spectra. There was one fascinating result was reported regarding the stability of Sn2+. XRD result predicted that Sn2+ converted into Sn4+ and formed the small amount of Cs2SnI6 phase that indicated the cation as well as anion exchange in the sample. After cation exchange between Pb2+ and Sn2+

and I<sup>−</sup><sup>1</sup>

more suitable candidate than iodine-based perovskites.

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

**Figure 1.**

*Perovskite Materials, Devices and Integration*

absorption and PL spectra as shown in **Figure 1a**.

their bulk counterpart.

 valence [29]. MASnI3 is the most studied lead-free perovskite material. Sn-based perovskites are direct bandgap semiconductors, like Pb-based perovskites. This means, the valence band maximum and conduction band minimum recline at the same position in k-space (reciprocal space). Similar to the APbX3 structure, the structural, optical and electrical properties of ASnX3 perovskite is also affected by the size of A-site cation and X-site anion. MASnI3 perovskite also has a tetragonal structure, but the band gap is near unity that is lower than those of the MAPbI3 [30]. Due to the quantum confinement effect, PQDs shows different behavior from

In case of the pure inorganic perovskite CsPbI3, band gap of PQDs is 1.85–1.97 eV. Such kind of higher bandgap highly encouraged to find some lower Pb-containing, low band gap, and more stable perovskite material. CsSnI3 pure inorganic can be the satisfied the all requirements for the good PQDs because of its lower binding energy and broad absorption from visible to near infrared region. Due to this different behavior, such kind of perovskite can exhibit higher light harvesting efficiency than other perovskite materials [31]. However, the stability of the Sn2+-based perovskite material is the biggest issue due to higher sensitivity to air, moisture and sometimes with nonpolar organic solvents also [32]. In recent years, many significant approaches have been tried for the stability of the Sn2+ perovskites. For example, SnBr2, SnCl2, and SnF2 had used as a dopant into the CsSnI3 perovskite for the stability of the materials. In all, depends, SnF2 and SnCl2 were found more suitable for the air resistive property. However, stability of the Sn-based material was not satisfactory due to the degradation of the sample within only 3hs [33–35]. In the year 2017, Liu et al. reported, the alloyed CsSn1−xPbxI3 PQDs, which was more phase stable than its parent CsPbI3 and CsSnI3 PQDs for months in ambient air [36]. They have prepared CsSn1−xPbxI3 PQDs via the standard Schlenk line method under a nitrogen atmosphere. In this method, SnI2/PbI2 mixture was rapidly dissolved in trioctylphoshine into the cesium olate at 120–170°C and purified by antisolvent methyl acetate (MeOAc) washing process. The 140°C reaction temperature was found the best for homogeneous and proper morphology of CsSn1−xPbxI3 PQDs. With the incorporation of Sn, red shifting occurred that can be observed in

Additionally, the increase in Sn concentration drastically enhanced the light absorption of CsSn1−xPbxI3 PQDs; however, PLQY was found up to 3% only. On the other hand, CsPbI3-based PQDs had up to 100% efficiency to date. Low PLQY with Sn-based PQDs indicates the increase in intrinsic defects that were allied with Sn vacancies. Such kind of vacancies creates deep level defects which act as nonradiative recombination centers and hence this reduces the efficiency of PQDs. **Figure 1b** also suggested the average lifetime of CsSn1−xPbxI3 PQDs dramatically decreases with increasing the concentration of Sn. This behavior of Sn containing materials also confirms the occurrence of a large density of quenching defects that support the lower PLQY of the PQDs. Liu also reported that CsSn0.60Pb0.40I3 PQDs was best for performance and stability because of the less- Pb content and widest range of light absorption among the alloyed PQDs. Phase, as well as chemical Stability of CsSn0.60Pb0.40I3 PQDs, is confirmed via XRD and XPS analysis (see **Figure 1c** and **d**). XRD **Figure 3b** suggested that there is no degradation was absorbed up to 5 months that is the first time reported such long time stability with

Furthermore, there was not found Sn4+ oxidation stage in the XPS analysis of the sample. In **Figure 1d** XPS graph showed 2 spectra at 486.2 and 494.5 eV that is associated with Sn2+ 3d5/2 and 3d3/2 state, while deficiency of spectra at 486.9 indicates the absence of Sn4+ in perovskite compound. Thus this less lead-containing and air-stable Sn-based PQDs can be the best choice for solar cell devices but for highly

s 2

**122**

Sn-based material.

*(a) Optical absorption spectra of different Sn doped PQDs and the inset fig shows their corresponding normalized steady photoluminescence spectra, (b) time-resolved photoluminescence decay for various stoichiometries of Sn, (c) X-ray diffraction patterns and (d) XPS spectra for the CsSn0.60Pb0.40I3 PQDs (as prepared) and those after storage for 150 days.*

efficient LED there will be a need in the improvement of its PLQY. Jellicoe and co-workers had also tried for the Sn-based perovskite, CsSnX3 (X = Cl, Br, I) for the lead-free perovskite PQDs but due to the instability of Sn it cannot be used for practical applications [35]. To overcome the stability problem, Wang group had replaced Sn2+ with Sn4+ and developed stable CsSnI6 PQDs with cubic structure. However it's PLQY was just 0.48% that is also not sufficient for optoelectronic applications [36]. From the several studied we absorbed that, CsSnI3-based perovskites have low PLQY but good hole conducting properties. Hence it is mostly used for solar cell applications. For sufficient PLQY and more stable Sn-based material was still needed for display devices. For this purpose, CsSnBr3-based perovskite was found more suitable candidate than iodine-based perovskites.

Yang group have reported the fast and less Pb or Pb- free perovskite nanocrystals by introducing the Sn in Pb site and I at Br site [37]. They have used one pot hot injection method for preparation of SnI2 doped CsPbBr3 perovskite nanocrystals. After cation and anion exchange between the CsPbBr3 and SnI2, the lead-free CsSnI3 NCs were obtained. These Pb-free NCs maintained the high PLQY as well as morphology in an inert atmosphere. **Figure 2a** indicated that, with increasing the amount of SnI2 into CsPbBr3 colloidal solution, dramatically red shifting observed in PL emission and absorption spectra. There was one fascinating result was reported regarding the stability of Sn2+. XRD result predicted that Sn2+ converted into Sn4+ and formed the small amount of Cs2SnI6 phase that indicated the cation as well as anion exchange in the sample. After cation exchange between Pb2+ and Sn2+ and anion exchange between Br<sup>−</sup><sup>1</sup> and I<sup>−</sup><sup>1</sup> , the PLQY of CsSnI3 and CsPbBr3 NCs was

found 59.1 and 73.47% respectively. From this result, the author concluded that the ion exchange did not cause the structural and surface defects. **Figure 4b** shows the schematic representation of the cation and anion exchange in CsPbxSn1−x(BryI1−y)3 NCs. Halide anions have high migration and the sufficient vacancy diffusion ability that is why after incorporation of SnI2 into CsPbI3, halide anions reacts very fast [38]. On the other hand, cation takes more time to reaction with parent material due to the obligation of high activation energy. Moreover, in most of the cases, cation substitution is obsessed with the concentration of halide vacancies [39]. Thus with this cation and anion approach creates a possibility of higher PLQY of Pb-free perovskite NCs.

The 100% Pb replacement with any other divalent cation is impossible till date because of the lower conductivity of substitute element like Sn. In the case of the Sn lower ionic conductivity of PQDs causes many surface defects that make material unstable. Some of the Pb-free perovskite with Sn4+ support that instability and toxicity problem can be resolve by partial substitution of Pb2+ with Sn4+. In this regards, Liu et al. have reported the less lead-containing CsPb1−xSnxBr3 PQDs with PLQY up to 83% [40]. They have performed cation exchange via replacing Pb2+ cation with Sn4+ cation using hot injection method. Partial replacement with Sn4+ in CsPb1−xSnxBr3 exhibited the enhancement in PL emission and after some time increasing the more concentration of Sn4+ ion, decreases the PL performance due to the increasing the impurity phase of Cs2SnBr6. In this work, they found CsPb0.67Sn0.33Br3 was the best composition, and PQDs of material, displayed the high external quantum efficiency and current efficiencies. **Figure 3a** Shows the variation of PL spectra of CsPb1–xSnxBr3 PQDs and digital picture in UV irradiation (365 nm). This PL spectrum show the highest PL intensity for x = 33% while increasing the concentration of Sn; PL intensity starts to decrease due to impurity and more surface defects. **Figure 3b** shows the absorption and PL emission spectra of samples that indicated the absence of blue shift that means there is no big effect on band gap after Sn4+ incorporation, but exciton recombination can affect the

#### **Figure 2.**

*(a) UV-Visible and PL spectra of different Sn doped CsPbBr3 NCs (A1–6); the colored image of PQDs under 365 nm UV radiation and (b) schematic diagram of the ion exchange process in SnI2 doped CsPbBr3 NCs.*

**125**

**Figure 4.**

**Figure 3.**

*x = 0.67.*

*and (e) band positions of CsPbX3 and Mn2+ d-state.*

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

lifetime and lower PLQY with a higher concentration of Sn4+. A higher concentration of Sn4+ also affected the size; morphology and structure of the QDs (see **Figure 3c–g**). After x = 0.33 Sn4+ amount in CsPbBr3, the impure phase can be seen in the XRD spectrum and irregular shape and agglomerated particles in TEM due to

Mn2+ is a transition metal cation, and doping of Mn2+ in II-VI semiconductors has been widely investigated due to its excellent optical, electrical and magnetic properties. All ready many studied have been done on Mn2+ doped ZnS, CdSe, CdS, and ZnSe inorganic QDs in which Mn2+ incorporation can enhance the long-lifetime and interaction between host d-electron of Mn dopants [41]. In compare to the traditional II-VI group semiconductors, the soaring tolerance in perovskite can be the better candidate for promoting the exciton energy transfer to Mn d-d emission

The band gap of Mn2+ doped in perovskite materials depends upon the presence of halide anions in a host as well as a dopant. For example, change from Cl<sup>−</sup> to Br<sup>−</sup> to I<sup>−</sup>, the PL emission of perovskite is tuned from blue region to red. In the year 2018, Zhao et al. have reported the Mn2+ doped CsPbCl3 NCs and after that intensive exploration are carried on this work [42]. After incorporation of Mn2+

*(a–d) Digital images of CsPbCl3, CsPbBr3, CsPbI3 and Mn: CsPbCl3 nanocrystals under 365 nm UV radiation* 

*(a) Variation in PL intensity of different Sn4+ doped CsPbBr3 PQDs. Inset of samples under 365 nm UV light (b) UV-Visible absorbance and PL spectra of CsPb1−xSnxBr3 (c) XRD pattern of CsPb1−xSnxBr3 PQDs, impurity peaks are shown by stare sign. (d–g) TEM images with (d) x = 0, (e) x = 0.33, (f) x = 0.50 and (g)* 

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

Cs2SnBr6 byproduct.

to Mn d-state.

**4.3 Mn2+-based perovskite QDs**

## *Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

lifetime and lower PLQY with a higher concentration of Sn4+. A higher concentration of Sn4+ also affected the size; morphology and structure of the QDs (see **Figure 3c–g**). After x = 0.33 Sn4+ amount in CsPbBr3, the impure phase can be seen in the XRD spectrum and irregular shape and agglomerated particles in TEM due to Cs2SnBr6 byproduct.

## **4.3 Mn2+-based perovskite QDs**

*Perovskite Materials, Devices and Integration*

perovskite NCs.

found 59.1 and 73.47% respectively. From this result, the author concluded that the ion exchange did not cause the structural and surface defects. **Figure 4b** shows the schematic representation of the cation and anion exchange in CsPbxSn1−x(BryI1−y)3 NCs. Halide anions have high migration and the sufficient vacancy diffusion ability that is why after incorporation of SnI2 into CsPbI3, halide anions reacts very fast [38]. On the other hand, cation takes more time to reaction with parent material due to the obligation of high activation energy. Moreover, in most of the cases, cation substitution is obsessed with the concentration of halide vacancies [39]. Thus with this cation and anion approach creates a possibility of higher PLQY of Pb-free

The 100% Pb replacement with any other divalent cation is impossible till date because of the lower conductivity of substitute element like Sn. In the case of the Sn lower ionic conductivity of PQDs causes many surface defects that make material unstable. Some of the Pb-free perovskite with Sn4+ support that instability and toxicity problem can be resolve by partial substitution of Pb2+ with Sn4+. In this regards, Liu et al. have reported the less lead-containing CsPb1−xSnxBr3 PQDs with PLQY up to 83% [40]. They have performed cation exchange via replacing Pb2+ cation with Sn4+ cation using hot injection method. Partial replacement with Sn4+ in CsPb1−xSnxBr3 exhibited the enhancement in PL emission and after some time increasing the more concentration of Sn4+ ion, decreases the PL performance due to the increasing the impurity phase of Cs2SnBr6. In this work, they found CsPb0.67Sn0.33Br3 was the best composition, and PQDs of material, displayed the high external quantum efficiency and current efficiencies. **Figure 3a** Shows the variation of PL spectra of CsPb1–xSnxBr3 PQDs and digital picture in UV irradiation (365 nm). This PL spectrum show the highest PL intensity for x = 33% while increasing the concentration of Sn; PL intensity starts to decrease due to impurity and more surface defects. **Figure 3b** shows the absorption and PL emission spectra of samples that indicated the absence of blue shift that means there is no big effect on band gap after Sn4+ incorporation, but exciton recombination can affect the

*(a) UV-Visible and PL spectra of different Sn doped CsPbBr3 NCs (A1–6); the colored image of PQDs under 365 nm UV radiation and (b) schematic diagram of the ion exchange process in SnI2 doped CsPbBr3 NCs.*

**124**

**Figure 2.**

Mn2+ is a transition metal cation, and doping of Mn2+ in II-VI semiconductors has been widely investigated due to its excellent optical, electrical and magnetic properties. All ready many studied have been done on Mn2+ doped ZnS, CdSe, CdS, and ZnSe inorganic QDs in which Mn2+ incorporation can enhance the long-lifetime and interaction between host d-electron of Mn dopants [41]. In compare to the traditional II-VI group semiconductors, the soaring tolerance in perovskite can be the better candidate for promoting the exciton energy transfer to Mn d-d emission to Mn d-state.

The band gap of Mn2+ doped in perovskite materials depends upon the presence of halide anions in a host as well as a dopant. For example, change from Cl<sup>−</sup> to Br<sup>−</sup> to I<sup>−</sup>, the PL emission of perovskite is tuned from blue region to red. In the year 2018, Zhao et al. have reported the Mn2+ doped CsPbCl3 NCs and after that intensive exploration are carried on this work [42]. After incorporation of Mn2+

#### **Figure 3.**

*(a) Variation in PL intensity of different Sn4+ doped CsPbBr3 PQDs. Inset of samples under 365 nm UV light (b) UV-Visible absorbance and PL spectra of CsPb1−xSnxBr3 (c) XRD pattern of CsPb1−xSnxBr3 PQDs, impurity peaks are shown by stare sign. (d–g) TEM images with (d) x = 0, (e) x = 0.33, (f) x = 0.50 and (g) x = 0.67.*

#### **Figure 4.**

*(a–d) Digital images of CsPbCl3, CsPbBr3, CsPbI3 and Mn: CsPbCl3 nanocrystals under 365 nm UV radiation and (e) band positions of CsPbX3 and Mn2+ d-state.*

cation in place of Pn2+ site, the XRD spectra shied towards higher angle due to the smaller radii of Mn2+ while PL emission shifted towards higher wavelength because of the d-d transition. Hence undoped CsPbCl3 NCs shows blue emission, but after doping of Mn2+, it tuned into orange emission as shown in **Figure 4a–c**. It was reported that Cl-based Mn dopant and perovskite host is the best rather than other manganese (II) salts [43]. Via hot injection method, it is tough to doping of Mn2+ into CsPbBr3 and CsPbI3 perovskite structure but through the anion exchange process Mn doping is possible. Such kind of possibility depends upon the bond strength and dissociation energy between Mn-X and Pb-X bond. **Figure 4d** shows the band gap energy diagram for Mn-doped different perovskite host [44]. As we know Cl-based perovskite NCs has very low PLQY (<5%) in compared to Br- and I-based perovskites. Besides reduction in toxicity, Mn2+ doping helps to enhance the PLQY and lifetime of the perovskite NCs. In this regards, Liu and coworkers fabricated CsPbxMn1−xCl3 PQDs via hot injection method [45]. Mn2+ incorporation in CsPbCl3 not only increases the PLQY of host material from 5 to 54% but also created an additional intense PL emission peak at 580 nm that is stand for bright orange color (see **Figure 5a**). With increasing the Mn2+ concentration secondary, PL peak gradually shifted from 569 to 587 nm. The highest PLQY (54%) was found for the 46% Mn-doped CsPbCl3 perovskite. The various Mn2+ compositions with tunable emission are shown in schematic **Figure 5b**. CsPbxMn1–xCl3 PQDs has two emission peaks with two different lifetime values such as 13.9 ns at 390 nm and 1.6 ms at 580 nm as shown in **Figure 5a** and **b**. The long decay time for second emission peak near 580 nm is assigned to emission from d-d transition energy transfer mechanism for Mn2+ doped semiconductors. Hence Mn2+ doping also increases

#### **Figure 5.**

*(a) UV-Visible and PL spectra of CsPb0.54Mn0.46Cl3 PQDs (b) schematic illustration of Mn incorporation in CsPbCl3 PQDs by different molar ratio and reaction temperature, (c and d) PL lifetime of CsPbCl3 and CsPb0.73Mn0.27Cl3 PQDs.*

**127**

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

the overall lifetime of PQDs that may be an improvement in the stability of PQDs also. A similar study was carried by Yuan et al. in which they have obtained PLQY of up to 60% for 14.6% Mn2+ doped CsPbCl3 [46]. With the help of ligand assisted room temperature method (LART), same work was reported by Zhu group with maximum 41.6% PLQY for 28% Mn2+ incorporation in place of Pb2+ [47]. Through LART, Mn2+:CsPbCl3 can be prepared in concise time duration and room temperature that reduce the cost of experiment, easy, simple method and high production

In some cases, Mn2+ is beneficial for stability of CsPbX3 PQDs. Zou et al. have found that Mn2+ incorporation in place of Pb2+ not only reduced toxicity while also enhance the thermal as well as air stability of the perovskite samples [48]. Similar to previous work, Zou group also have worked with CsPbCl3: Mn2+ PQDs and studied different structural and optical enhanced properties. Besides this work, they have also provided experimental data for CsPbBr3: Mn and CsPbI3: Mn PQDs to prove the absence of energy transfer in Br and I containing perovskites. **Figure 6(a–c)** show the comparison of PL spectra for CsPbCl3: Mn, CsPbBr3: Mn and CsPbI3:Mn PQDs with various Mn2+ concentrations. In this case, PL emission intensity first increases with increasing the Mn2+ concentration and then decreases with further increasing doping concentration which suggests that the Mn2+ ions have a noteworthy impact on the optical properties of PQDs. Presence of additional peak in CsPbCl3: Mn, is the proof of energy transformation, on the other hand, there is no peak shift in Br and I containing PQDs samples due to the mismatch between their optical bang gap

and in Br and I increases up to 20% Mn2+ doping and after that dramatically decreases with increasing the Mn2+ concentration (see **Figure 6d–f**) while lifetime of CsPbBr3: Mn2+ continuously decreases as shown in **Figure 6g**. Mn2+ doping in CsPbBr3 and CsPbI3 PQDs also enhance air stability as demonstrated in **Figure 6h** and **i**. **Figure 6h** indicates, the degradation of pure CsPbBr3 PQDs after 30 days while Mn2+ doped PQDs is stable up to 120 days while 50%Mn: CsPbI3 PQDs was found stable up to 4 days. As we know CsPbI3 PQDs are unstable in the air, so it is challenging of use for optoelectronic devices. Enhanced stability of Mn: CsPbI3 materials may open the new door for stable red PQDs. The same methodology was reported by Manna group by theoretical and experimental approach also [49]. They reported that unstable CsPbI3 perovskite could be stabilized by incorporation the 10% of Mn2+ and there were no major changes in structural and optical properties.

Cd and Zn2+ Cation exchange in traditional NCs have been extensively studied, but for perovskite material, there are few reports are present till date. A detailed study of cation exchange has not yet been explained, but halide exchange has been explained well. Halide exchange is easier than cation exchange because of low activation energy and diffusion of anion vacancies in the perovskite materials. Stam et al. have experimentally proved that cation exchange in perovskite takes long time [50]. They reported the cation exchange such as Sn2+, Cd2+, and Zn2+ in place of Pb2+ in CsPbBr3 perovskite host. 10% cation exchange in perovskite system results in the reduction in toxicity due to lead and also maintained the good PLQY as well as high color purity. But with these, cation exchange, the blue shift was observed in PL emission due to smaller ionic radii of Cd and Zn than Pb (as shown in **Figure 7k**). On the other hand, there is very less shift was obtained in Sn2+ due to similar ionic radii as Pb. The Cd2+ doping in CsPbBr3 PQDs was resulting in PL emission variation between 452 and 512 nm while Zn showed between 462 and 512 nm. Furthermore, over 60% of PLQYs was obtained for cation doped PQDs and good

T1 transition of Mn2+ [48]. PLQYs of the Cl-based increases

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

of products.

absorption and <sup>4</sup>

T1 → <sup>6</sup>

**4.4 Cd2+ and Zn2+ doped perovskite quantum dots**

the overall lifetime of PQDs that may be an improvement in the stability of PQDs also. A similar study was carried by Yuan et al. in which they have obtained PLQY of up to 60% for 14.6% Mn2+ doped CsPbCl3 [46]. With the help of ligand assisted room temperature method (LART), same work was reported by Zhu group with maximum 41.6% PLQY for 28% Mn2+ incorporation in place of Pb2+ [47]. Through LART, Mn2+:CsPbCl3 can be prepared in concise time duration and room temperature that reduce the cost of experiment, easy, simple method and high production of products.

In some cases, Mn2+ is beneficial for stability of CsPbX3 PQDs. Zou et al. have found that Mn2+ incorporation in place of Pb2+ not only reduced toxicity while also enhance the thermal as well as air stability of the perovskite samples [48]. Similar to previous work, Zou group also have worked with CsPbCl3: Mn2+ PQDs and studied different structural and optical enhanced properties. Besides this work, they have also provided experimental data for CsPbBr3: Mn and CsPbI3: Mn PQDs to prove the absence of energy transfer in Br and I containing perovskites. **Figure 6(a–c)** show the comparison of PL spectra for CsPbCl3: Mn, CsPbBr3: Mn and CsPbI3:Mn PQDs with various Mn2+ concentrations. In this case, PL emission intensity first increases with increasing the Mn2+ concentration and then decreases with further increasing doping concentration which suggests that the Mn2+ ions have a noteworthy impact on the optical properties of PQDs. Presence of additional peak in CsPbCl3: Mn, is the proof of energy transformation, on the other hand, there is no peak shift in Br and I containing PQDs samples due to the mismatch between their optical bang gap absorption and <sup>4</sup> T1 → <sup>6</sup> T1 transition of Mn2+ [48]. PLQYs of the Cl-based increases and in Br and I increases up to 20% Mn2+ doping and after that dramatically decreases with increasing the Mn2+ concentration (see **Figure 6d–f**) while lifetime of CsPbBr3: Mn2+ continuously decreases as shown in **Figure 6g**. Mn2+ doping in CsPbBr3 and CsPbI3 PQDs also enhance air stability as demonstrated in **Figure 6h** and **i**. **Figure 6h** indicates, the degradation of pure CsPbBr3 PQDs after 30 days while Mn2+ doped PQDs is stable up to 120 days while 50%Mn: CsPbI3 PQDs was found stable up to 4 days. As we know CsPbI3 PQDs are unstable in the air, so it is challenging of use for optoelectronic devices. Enhanced stability of Mn: CsPbI3 materials may open the new door for stable red PQDs. The same methodology was reported by Manna group by theoretical and experimental approach also [49]. They reported that unstable CsPbI3 perovskite could be stabilized by incorporation the 10% of Mn2+ and there were no major changes in structural and optical properties.

## **4.4 Cd2+ and Zn2+ doped perovskite quantum dots**

Cd and Zn2+ Cation exchange in traditional NCs have been extensively studied, but for perovskite material, there are few reports are present till date. A detailed study of cation exchange has not yet been explained, but halide exchange has been explained well. Halide exchange is easier than cation exchange because of low activation energy and diffusion of anion vacancies in the perovskite materials. Stam et al. have experimentally proved that cation exchange in perovskite takes long time [50]. They reported the cation exchange such as Sn2+, Cd2+, and Zn2+ in place of Pb2+ in CsPbBr3 perovskite host. 10% cation exchange in perovskite system results in the reduction in toxicity due to lead and also maintained the good PLQY as well as high color purity. But with these, cation exchange, the blue shift was observed in PL emission due to smaller ionic radii of Cd and Zn than Pb (as shown in **Figure 7k**). On the other hand, there is very less shift was obtained in Sn2+ due to similar ionic radii as Pb. The Cd2+ doping in CsPbBr3 PQDs was resulting in PL emission variation between 452 and 512 nm while Zn showed between 462 and 512 nm. Furthermore, over 60% of PLQYs was obtained for cation doped PQDs and good

*Perovskite Materials, Devices and Integration*

cation in place of Pn2+ site, the XRD spectra shied towards higher angle due to the smaller radii of Mn2+ while PL emission shifted towards higher wavelength because of the d-d transition. Hence undoped CsPbCl3 NCs shows blue emission, but after doping of Mn2+, it tuned into orange emission as shown in **Figure 4a–c**. It was reported that Cl-based Mn dopant and perovskite host is the best rather than other manganese (II) salts [43]. Via hot injection method, it is tough to doping of Mn2+ into CsPbBr3 and CsPbI3 perovskite structure but through the anion exchange process Mn doping is possible. Such kind of possibility depends upon the bond strength and dissociation energy between Mn-X and Pb-X bond. **Figure 4d** shows the band gap energy diagram for Mn-doped different perovskite host [44]. As we know Cl-based perovskite NCs has very low PLQY (<5%) in compared to Br- and I-based perovskites. Besides reduction in toxicity, Mn2+ doping helps to enhance the PLQY and lifetime of the perovskite NCs. In this regards, Liu and coworkers fabricated CsPbxMn1−xCl3 PQDs via hot injection method [45]. Mn2+ incorporation in CsPbCl3 not only increases the PLQY of host material from 5 to 54% but also created an additional intense PL emission peak at 580 nm that is stand for bright orange color (see **Figure 5a**). With increasing the Mn2+ concentration secondary, PL peak gradually shifted from 569 to 587 nm. The highest PLQY (54%) was found for the 46% Mn-doped CsPbCl3 perovskite. The various Mn2+ compositions with tunable emission are shown in schematic **Figure 5b**. CsPbxMn1–xCl3 PQDs has two emission peaks with two different lifetime values such as 13.9 ns at 390 nm and 1.6 ms at 580 nm as shown in **Figure 5a** and **b**. The long decay time for second emission peak near 580 nm is assigned to emission from d-d transition energy transfer mechanism for Mn2+ doped semiconductors. Hence Mn2+ doping also increases

*(a) UV-Visible and PL spectra of CsPb0.54Mn0.46Cl3 PQDs (b) schematic illustration of Mn incorporation in CsPbCl3 PQDs by different molar ratio and reaction temperature, (c and d) PL lifetime of CsPbCl3 and* 

**126**

**Figure 5.**

*CsPb0.73Mn0.27Cl3 PQDs.*

#### **Figure 6.**

*Comparison of photoluminescence (PL) emission spectra for (a) CsPbCl3:Mn, (b) CsPbBr3:Mn and (c) CsPbI3:Mn QDs doped with different Mn2+ contents (d) absolute PL quantum yields (QYs) for CsPbCl3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 80 mol% (e) absolute PL QYs for CsPbBr3:Mn and CsPbI3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 60 Mol% (f and g) PL decay curves (left) and lifetimes (right) for excitonic luminescence of CsPbBr3:Mn QDs upon excitation by a 397-nm pulsed laser (h) PL emission photographs for CsPbBr3:Mn QDs coated on the surface of a glass slide with different Mn2+ contents from 0 to 6.2 mol% taken under UV irradiation at indicated time periods and (i) red PL emission photographs for CsPbI3:Mn QDs with different nominal Mn2+ doping concentrations of 0, 20, 40, 50, 60, 80, 100 mol% from left to right respectively, taken at daylight or under UV irradiation at indicated time period.*

stability in ambient conditions. Cation exchange in PQDs reduced the PL lifetime of the CsPbBr3 PQDs. **Figure 7a–j** TEM and energy dispersive X-ray spectroscopy mapping of Cd2+ and Zn2+ doped CsPbBr3 PQDs. EDS mapping helped to identify the presence of different doped elements in PQDs. **Figure 7a**–**e** shows the Cd and **Figure 7f**-jshows the Zn doped CsPbBr3 PQDs. These mapping analyses also predicted the uniform distribution and lower concentration of dopant into the host material. Zn2+ incorporation enhanced the stability of CsPbI3 black phase in the air also due to improvement in lattice contraction and alloy form of perovskite.

As it is well reported that lower PLQYs of Cl-based perovskite materials due to the large band gap. There are many have published on Mn2+ doped CsPbCl3 PQDs, but with doping of, we get orange color so in this case to obtain pure blue color is impossible. On the other hand, other heterojunction-based PQDs such as CH3NH3Bi2X9, Cs3Bi2Br9 and Cs3Sb2Br9 have been developed for blue emission [50]. But still, these blue color emitted PQDs have 46–52% and very unstable that is not suitable for blue LEDs fabrication. To overcome this issue, Mondal et al. reported the effect of Cd incorporation in CsPbCl3 PQDs by hot injection method to enhance the stability and PLQYs of blue color PQDs [51]. For CdCl2 treatment, CsPbCl3 colloidal solution with CHCl3 was mixed with CdCl2 solution (CdCl2 solution was prepared in ethanol) and sonicated for 2–3 min. The obtained colloidal solution was

**129**

**Figure 8.**

*addition of CdCl2 (right).*

**Figure 7.**

*CdBr2 (orange lines) and ZnBr2 (blue lines).*

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

*Energy dispersive X-ray spectroscopy mapping of CsPb1−xCdxBr3 and CsPb1−xZnxBr3 nanocrystals. (a) HAADF-STEM image of CsPb1−xCdxBr3 NCs and corresponding maps of (b) Cs, (c) Pb, (d) Br, and (e) Cd, demonstrating the presence of Cd in the perovskite NCs. The inset in panel shows a photograph of a colloidal suspension of the NCs under UV illumination. (f) HAADF-STEM image of CsPb1−xZnxBr3 NCs and the corresponding maps of (g) Cs, (h) Pb, (i) Br, and (j) Zn, indicating the presence of Zn in the perovskite NCs. The inset in panel f shows a photograph of a colloidal suspension of the NCs under UV illumination (k) parent CsPbBr3 NCs (green lines) and product NCs obtained after reaction with different concentrations of* 

purified by centrifugation with methyl acetate treatments. The CdCl2 treatment of CsPbCl2 dramatically enhanced the from 3–96% PLQYs and narrow PL emission

*UV-Visible absorption and PL spectra (a) of the CsPbCl3 NCs before and after the CdCl2 treatment. PL spectra showing dramatic enhancement of PL upon treatment (b) the inset shows the untreated (i) and treated samples (ii) under a UV lamp. (c) PL decay dynamics (λex = 375 nm, λPL = 406 nm) of CsPbCl3 NCs before and after the treatment with CdCl2. PL spectra (d) and PL dynamics (e) of CsPbBr3 NCs before and after the CdBr2 treatment. (f) Change in PL on addition of CdCl2, starting from treated CsPbBr3 NCs (left) to increased* 

spectra that also indicates the higher color purity of the treated sample.

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

#### **Figure 7.**

*Perovskite Materials, Devices and Integration*

stability in ambient conditions. Cation exchange in PQDs reduced the PL lifetime of the CsPbBr3 PQDs. **Figure 7a–j** TEM and energy dispersive X-ray spectroscopy mapping of Cd2+ and Zn2+ doped CsPbBr3 PQDs. EDS mapping helped to identify the presence of different doped elements in PQDs. **Figure 7a**–**e** shows the Cd and **Figure 7f**-jshows the Zn doped CsPbBr3 PQDs. These mapping analyses also predicted the uniform distribution and lower concentration of dopant into the host material. Zn2+ incorporation enhanced the stability of CsPbI3 black phase in the air

*Comparison of photoluminescence (PL) emission spectra for (a) CsPbCl3:Mn, (b) CsPbBr3:Mn and (c) CsPbI3:Mn QDs doped with different Mn2+ contents (d) absolute PL quantum yields (QYs) for CsPbCl3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 80 mol% (e) absolute PL QYs for CsPbBr3:Mn and CsPbI3:Mn QDs doped with different nominal Mn2+ contents ranging from 0 to 60 Mol% (f and g) PL decay curves (left) and lifetimes (right) for excitonic luminescence of CsPbBr3:Mn QDs upon excitation by a 397-nm pulsed laser (h) PL emission photographs for CsPbBr3:Mn QDs coated on the surface of a glass slide with different Mn2+ contents from 0 to 6.2 mol% taken under UV irradiation at indicated time periods and (i) red PL emission photographs for CsPbI3:Mn QDs with different nominal Mn2+ doping concentrations of 0, 20, 40, 50, 60, 80, 100 mol% from left to right respectively, taken at daylight or under UV* 

also due to improvement in lattice contraction and alloy form of perovskite. As it is well reported that lower PLQYs of Cl-based perovskite materials due to the large band gap. There are many have published on Mn2+ doped CsPbCl3 PQDs, but with doping of, we get orange color so in this case to obtain pure blue color is impossible. On the other hand, other heterojunction-based PQDs such as CH3NH3Bi2X9, Cs3Bi2Br9 and Cs3Sb2Br9 have been developed for blue emission [50]. But still, these blue color emitted PQDs have 46–52% and very unstable that is not suitable for blue LEDs fabrication. To overcome this issue, Mondal et al. reported the effect of Cd incorporation in CsPbCl3 PQDs by hot injection method to enhance the stability and PLQYs of blue color PQDs [51]. For CdCl2 treatment, CsPbCl3 colloidal solution with CHCl3 was mixed with CdCl2 solution (CdCl2 solution was prepared in ethanol) and sonicated for 2–3 min. The obtained colloidal solution was

**128**

**Figure 6.**

*irradiation at indicated time period.*

*Energy dispersive X-ray spectroscopy mapping of CsPb1−xCdxBr3 and CsPb1−xZnxBr3 nanocrystals. (a) HAADF-STEM image of CsPb1−xCdxBr3 NCs and corresponding maps of (b) Cs, (c) Pb, (d) Br, and (e) Cd, demonstrating the presence of Cd in the perovskite NCs. The inset in panel shows a photograph of a colloidal suspension of the NCs under UV illumination. (f) HAADF-STEM image of CsPb1−xZnxBr3 NCs and the corresponding maps of (g) Cs, (h) Pb, (i) Br, and (j) Zn, indicating the presence of Zn in the perovskite NCs. The inset in panel f shows a photograph of a colloidal suspension of the NCs under UV illumination (k) parent CsPbBr3 NCs (green lines) and product NCs obtained after reaction with different concentrations of CdBr2 (orange lines) and ZnBr2 (blue lines).*

#### **Figure 8.**

*UV-Visible absorption and PL spectra (a) of the CsPbCl3 NCs before and after the CdCl2 treatment. PL spectra showing dramatic enhancement of PL upon treatment (b) the inset shows the untreated (i) and treated samples (ii) under a UV lamp. (c) PL decay dynamics (λex = 375 nm, λPL = 406 nm) of CsPbCl3 NCs before and after the treatment with CdCl2. PL spectra (d) and PL dynamics (e) of CsPbBr3 NCs before and after the CdBr2 treatment. (f) Change in PL on addition of CdCl2, starting from treated CsPbBr3 NCs (left) to increased addition of CdCl2 (right).*

purified by centrifugation with methyl acetate treatments. The CdCl2 treatment of CsPbCl2 dramatically enhanced the from 3–96% PLQYs and narrow PL emission spectra that also indicates the higher color purity of the treated sample.

There are the best thing is that there was no PL or absorbance spectrum shifting observed with CdCl2 treated samples as shown in **Figure 8a**. From **Figure 8b** also indicates the huge improvement in PL emission and bright blue emission under UV light radiation compared to untreated CsPbCl3 PQDs. Generally, with doping of Cd2+ or other lower atomic radii elements than Pb2+, has PL emission shift towards blue region due to lower ionic radii. But in this work, the authors did not get any shifting. CdCl2 incorporation not only enhances the PLQYs while it also improved the lifetime and stability of CsPbCl3 PQDs by the four times of the untreated PQDs (**Figure 8c**). A similar strategy was applied with CsPbBr3 PQDs and improvement in PLQY and lifetime also absorbed in green PQDs also (**Figure 8d–f**). Thus this work suggests the Pb2+ cation exchange with Cd2+ not only provide high PLQYs while long term stability without disturbing any peak shifting. So such kind of blue and green PQDs can be beneficial for good quality of blue, green or white LEDs and a backlight system. Thus Cd and Zn doped PQDs may be useful for different optoelectronic applications similar to pure PQDs due to its low toxicity and excellent properties.

## **5. Applications of lead substituted PQDs for LEDs**

Lead halide-based perovskite QDs, as a promising light-harvesting material for light absorbing and converting light energy, have attracted research community as well as industrialists due to unique properties of PQDs for solid-state lighting (SSL) and flexible color tuning thin film display application. In recent years, organic LED (OLED) and quantum dot LED (QDLED) join the competition of display market and mean to wrest the dominance from LCD. OLED has the advantage of self-luminous, large area fabrication, fast response time, high contrast and application on flexible substrates [2–4]. Due to such type of benefits, many manufacturers have invested in the development of OLEDs, and now various products based on OLEDs have been commercially available. However, there are some limitations with OLEDs like wide PL emission wavelength, instability of organic molecules, etc. Due to this issue, it is difficult to achieve high color purity and a high-quality full-color display. So it is still required to develop a new variety of display technology with high color purity and PLQY to meet the higher demand of consumers. To overcome the less color purity problem, quantum dots can be used because of its excellent luminescence and color characteristics as we have already discussed in the previous section. There is a chance to achieve the high-resolution color contrast and a better full-color display device in near future moreover in contrast to another semiconductor (inorganic) employed in solid state lighting device perovskite material have also benefited regarding the synthesis and manifesting process. PQDs can also fabricate at low temp, solution processed approach to precisely control the size, shape, and purity of perovskite material which makes PQDs ideal candidates for the display device. In addition, due to the colloidal solution characteristics, PQDs is cheaper to use, easier to process and easier to fabricate in large area, large scale production thus PQDs are often touted as disruptive material that could completely replace traditional inorganic phosphor LEDs or OLEDs, QLEDs can be mainly divided into types one is photoluminescence QD LEDs (based on photoexcited), and another is electroluminescence (EL) QDLEDs (electron- excited QDs) [3]. PL QDLEDs is the most commonly used a type of LEDs. QDs LEDs usually have applications in the high-quality display, high resolution, and high contrast display device as well as in lighting application. For commercialization, much less lead-containing perovskite QDLEDs have been investigated in recent years. Lead substituted perovskites also used for PL LEDs and electroluminescence LEDs.

**131**

**Figure 9.**

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

Zou et al. fabricated the EL-based LEDs using CsPbBr3: Mn perovskite QDs [48]. Active LEDs was fabricated by spin coating of PEDOT: PSS on ITO glass substrate as a hole transporting layer, Mn-doped CsPbCl3 as the active layer and TPBi, LiF/Al were used as electrodes by thermal evaporation technique. The schematic illustration of the device is shown in **Figure 9a**. This active led produced electroluminescence with green color (512–515 nm) for CsPbBr3 and Mn-doped perovskites with high color purity and narrow emission spectrum FWHM of 20 nm (see **Figure 9b**). There were no significant shifting observed in EL as well as in current density-voltage properties (**Figure 9c**) of Mn-doped perovskites. Mn doping enhanced the luminescence of LEDs by 1.3 times of the pure PLEDs and EQE and CE also improved from 0.81% and 3.71 cd/A for the CsPbBr3 PLED to 1.49% and 6.40 cd/A for the 3.8 mol% Mn-doped CsPbBr3 PLED device. Similarly, Liu group also fabricated lead substituted active PLEDs with doping of 33 mol% Sn4+ ion consisting of the same device structure as Zou reported [40]. They reported that the presence of Sn4+ helped in the easy injection of charge carriers that is responsible for small turn off voltage as well as large current density. CsPb0.67Sn0.33Br3 PLEDs showed 4.13% EQE,

of CE, 6.76 lm W<sup>−</sup><sup>1</sup>

age. Authors have also claimed that such kind of high performance is the best results found in Sn-based PLEDs. So this lead substituted Sn-based PQDs LED can be suitable for different active LEDs as well for backlight-based display devices. On the other hand, CsPbCl3: Mn PQDs has also been fabricated in PLEDs with 2.2 lm W<sup>−</sup><sup>1</sup> of luminous intensity (**Figure 9d–f**) and good stability after continuous applying 3.5 V of voltage for 200 h [45]. 27 mol% Mn: CsPbCl3 was mixed with curable resin and coated on 365 nm commercial UV-LED chip for the PL PLED device. Due to d-d transition, Mn doped CsPbCl3 PLED gives bright orange emission with 54% PLQY in ambient conditions. In most of the optoelectronic devices, photo and thermal

*(a) Schematic illustration of a typical multilayer-structured PLED device by using pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs are used as green light emitters. (b) Comparison of normalized EL spectra at an applied voltage of 6 V and their corresponding PL emission spectra for CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs when dispersed in cyclohexane solution. The inset shows a photograph of the EL of a representative PLED device. Current density (c) and luminance characteristics for three types of PLEDs based on the pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%), and CsPbBr3: Mn (3.8 mol%) QDs (d) fluorescent image (e), PL emission spectrum (f) CIE chromaticity coordinate of the* 

*LED from CsPbxMn1−xCl3 QDs. Inset: Optical image of the LED.*

and 3.6 V turn-on volt-

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

luminescence, 11.63 cd A<sup>−</sup><sup>1</sup>

12500 cd m<sup>−</sup><sup>2</sup>

## *Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

Zou et al. fabricated the EL-based LEDs using CsPbBr3: Mn perovskite QDs [48]. Active LEDs was fabricated by spin coating of PEDOT: PSS on ITO glass substrate as a hole transporting layer, Mn-doped CsPbCl3 as the active layer and TPBi, LiF/Al were used as electrodes by thermal evaporation technique. The schematic illustration of the device is shown in **Figure 9a**. This active led produced electroluminescence with green color (512–515 nm) for CsPbBr3 and Mn-doped perovskites with high color purity and narrow emission spectrum FWHM of 20 nm (see **Figure 9b**). There were no significant shifting observed in EL as well as in current density-voltage properties (**Figure 9c**) of Mn-doped perovskites. Mn doping enhanced the luminescence of LEDs by 1.3 times of the pure PLEDs and EQE and CE also improved from 0.81% and 3.71 cd/A for the CsPbBr3 PLED to 1.49% and 6.40 cd/A for the 3.8 mol% Mn-doped CsPbBr3 PLED device. Similarly, Liu group also fabricated lead substituted active PLEDs with doping of 33 mol% Sn4+ ion consisting of the same device structure as Zou reported [40]. They reported that the presence of Sn4+ helped in the easy injection of charge carriers that is responsible for small turn off voltage as well as large current density. CsPb0.67Sn0.33Br3 PLEDs showed 4.13% EQE, 12500 cd m<sup>−</sup><sup>2</sup> luminescence, 11.63 cd A<sup>−</sup><sup>1</sup> of CE, 6.76 lm W<sup>−</sup><sup>1</sup> and 3.6 V turn-on voltage. Authors have also claimed that such kind of high performance is the best results found in Sn-based PLEDs. So this lead substituted Sn-based PQDs LED can be suitable for different active LEDs as well for backlight-based display devices. On the other hand, CsPbCl3: Mn PQDs has also been fabricated in PLEDs with 2.2 lm W<sup>−</sup><sup>1</sup> of luminous intensity (**Figure 9d–f**) and good stability after continuous applying 3.5 V of voltage for 200 h [45]. 27 mol% Mn: CsPbCl3 was mixed with curable resin and coated on 365 nm commercial UV-LED chip for the PL PLED device. Due to d-d transition, Mn doped CsPbCl3 PLED gives bright orange emission with 54% PLQY in ambient conditions. In most of the optoelectronic devices, photo and thermal

#### **Figure 9.**

*Perovskite Materials, Devices and Integration*

**5. Applications of lead substituted PQDs for LEDs**

lent properties.

There are the best thing is that there was no PL or absorbance spectrum shifting observed with CdCl2 treated samples as shown in **Figure 8a**. From **Figure 8b** also indicates the huge improvement in PL emission and bright blue emission under UV light radiation compared to untreated CsPbCl3 PQDs. Generally, with doping of Cd2+ or other lower atomic radii elements than Pb2+, has PL emission shift towards blue region due to lower ionic radii. But in this work, the authors did not get any shifting. CdCl2 incorporation not only enhances the PLQYs while it also improved the lifetime and stability of CsPbCl3 PQDs by the four times of the untreated PQDs (**Figure 8c**). A similar strategy was applied with CsPbBr3 PQDs and improvement in PLQY and lifetime also absorbed in green PQDs also (**Figure 8d–f**). Thus this work suggests the Pb2+ cation exchange with Cd2+ not only provide high PLQYs while long term stability without disturbing any peak shifting. So such kind of blue and green PQDs can be beneficial for good quality of blue, green or white LEDs and a backlight system. Thus Cd and Zn doped PQDs may be useful for different optoelectronic applications similar to pure PQDs due to its low toxicity and excel-

Lead halide-based perovskite QDs, as a promising light-harvesting material for light absorbing and converting light energy, have attracted research community as well as industrialists due to unique properties of PQDs for solid-state lighting (SSL) and flexible color tuning thin film display application. In recent years, organic LED (OLED) and quantum dot LED (QDLED) join the competition of display market and mean to wrest the dominance from LCD. OLED has the advantage of self-luminous, large area fabrication, fast response time, high contrast and application on flexible substrates [2–4]. Due to such type of benefits, many manufacturers have invested in the development of OLEDs, and now various products based on OLEDs have been commercially available. However, there are some limitations with OLEDs like wide PL emission wavelength, instability of organic molecules, etc. Due to this issue, it is difficult to achieve high color purity and a high-quality full-color display. So it is still required to develop a new variety of display technology with high color purity and PLQY to meet the higher demand of consumers. To overcome the less color purity problem, quantum dots can be used because of its excellent luminescence and color characteristics as we have already discussed in the previous section. There is a chance to achieve the high-resolution color contrast and a better full-color display device in near future moreover in contrast to another semiconductor (inorganic) employed in solid state lighting device perovskite material have also benefited regarding the synthesis and manifesting process. PQDs can also fabricate at low temp, solution processed approach to precisely control the size, shape, and purity of perovskite material which makes PQDs ideal candidates for the display device. In addition, due to the colloidal solution characteristics, PQDs is cheaper to use, easier to process and easier to fabricate in large area, large scale production thus PQDs are often touted as disruptive material that could completely replace traditional inorganic phosphor LEDs or OLEDs, QLEDs can be mainly divided into types one is photoluminescence QD LEDs (based on photoexcited), and another is electroluminescence (EL) QDLEDs (electron- excited QDs) [3]. PL QDLEDs is the most commonly used a type of LEDs. QDs LEDs usually have applications in the high-quality display, high resolution, and high contrast display device as well as in lighting application. For commercialization, much less lead-containing perovskite QDLEDs have been investigated in recent years. Lead substituted perovskites also used for PL LEDs and electroluminescence LEDs.

**130**

*(a) Schematic illustration of a typical multilayer-structured PLED device by using pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs are used as green light emitters. (b) Comparison of normalized EL spectra at an applied voltage of 6 V and their corresponding PL emission spectra for CsPbBr3 and CsPbBr3: Mn (2.6 mol%) and CsPbBr3: Mn (3.8 mol%) QDs when dispersed in cyclohexane solution. The inset shows a photograph of the EL of a representative PLED device. Current density (c) and luminance characteristics for three types of PLEDs based on the pure CsPbBr3 and CsPbBr3: Mn (2.6 mol%), and CsPbBr3: Mn (3.8 mol%) QDs (d) fluorescent image (e), PL emission spectrum (f) CIE chromaticity coordinate of the LED from CsPbxMn1−xCl3 QDs. Inset: Optical image of the LED.*

stability are the crucial issues. So for improving such kind of stability SiO2 coating and KCl/polystyrene or other inorganic materials has been used for encapsulation of perovskite LEDs [42].

## **6. Summary**

Lead halide-based perovskite material opens a new opportunity in optoelectric since 2012, because of their excellent optoelectronic properties and high power conversion efficiency within a brief time. The big challenge for the perovskite QDs is the presence of toxicity due to lead. Already many groups have worked on leadless or lead-free perovskite materials. 100% replacement of Pb will be complicated but with replacement up to 60–70%, Pb with non-toxic cation may help to the commercialization of less lead-containing perovskite materials. Shortly, less lead or lead-free PQDs have the potential for next-generation optoelectronic devices like backlights, QD TV, flexible and wearable devices, etc. However, to achieve high PLQY, good performances, and stability of lead-free or less lead containing PQDs also be challenging for the research community. Many homovalent lead substitutions like Sn, Mn, Cd, and Zn have been successfully done in place of Pb in different halide containing perovskite nanomaterials. Such kind of replacement of lead not only reduces the toxicity of perovskite while it also improves the performances and stability of the perovskite light-harvesting material. In the halide-based perovskites, dopant engineering is a compelling strategy to tuning the optical, structural, and electrical properties and day by day new depends arrive that make the PQDs more suitable for lead-free perovskite devices. Mn and Cd incorporation in Cl-based perovskite NCs was increased the PLQYs but for Br- and I-based PNCs it is still challenging to get high PLQYs than pure PQDs. So, we need to find a more suitable strategy for less lead-containing high PLQYs perovskite material.

## **Author details**

Rajan Kumar Singh1 \*, Neha Jain1 , Sudipta Som<sup>2</sup> , Somrita Dutta3 , Jai Singh1 and Ranveer Kumar1


\*Address all correspondence to: rajanphysicssgo@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

Journal of Materials Chemistry C.

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[14] Lyu M, Yun J-H, Chen P, Hao M, Wang L. Addressing toxicity of lead: Progress and applications of low-toxic metal halide perovskites and their derivatives. Advanced Energy Materials.

[15] Babayigit A et al. Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism danio rerio. Scientific Reports. 2016;**6**:18721

[17] Singh RK et al. Exploring the impact of the Pb2+ substitution by Cd2+ on the structural and morphological properties of CH3NH3PbI3 perovskite. Applied Nanoscience. 2019:1-10. https://doi. org/10.1007/s13204-019-01021-5

[16] Mitzi DB. N4H9Cu7S4: A hydrazinium-based salt with a layered Cu7S4-framework. Inorganic Chemistry. 2007;**46**(3):926-931

Science. 2016;**354**:203-206

2019;**1808843**:1-25

2017;**2**:897-903

2017;**7**:1602512

[12] Wang R et al. A review of perovskites solar cell stability. Advanced Functional Materials.

Kanatzidis MG. Performance enhancement of lead-free tin-based perovskite solar cells with reducing atmosphere-assisted dispersible additive. ACS Energy Letters.

2018;**6**:2189-2209

2019;**48**:310-350

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

[1] Noh JH, Im SH, Heo JH, Mandal TN, Seok SI. Chemical management for colorful, efficient, and stable inorganicorganic hybrid nanostructured solar cells. Nano Letters. 2013;**13**:1764-1769

[2] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society.

[3] Guerrero A et al. How the chargeneutrality level of interface states controls energy level alignment in cathode contacts of organic bulkheterojunction solar cells. ACS Nano.

[4] Yang X et al. Author correction: Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nature Communications. 2018;**9**:1169

[5] Schmidt LC et al. Nontemplate synthesis of CH3NH3PbBr3 perovskite nanoparticles. Journal of the American Chemical Society. 2014;**136**:850-853

[6] Kovalenko MV, Protesescu L, Bodnarchuk MI. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals.

[7] Gonzalez-Carrero S, Galian RE, Pérez-Prieto J. Maximizing the emissive properties of CH3NH3PbBr3 perovskite nanoparticles. Journal of Materials Chemistry A. 2015;**3**:9187-9193

[8] Moyen E et al. Ligand removal and photo-activation of CsPbBr3 quantum dots for enhanced optoelectronic devices. Nanoscale. 2018;**10**:8591-8599

[9] Hong K, Van Le Q, Kim SY, Jang HW. Low-dimensional halide perovskites: Review and issues.

Science. 2017;**358**:745-750

2009;**131**:6050-6051

**References**

2012;**6**:3453-3460

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

## **References**

*Perovskite Materials, Devices and Integration*

of perovskite LEDs [42].

**6. Summary**

**Author details**

Ranveer Kumar1

Rajan Kumar Singh1

stability are the crucial issues. So for improving such kind of stability SiO2 coating and KCl/polystyrene or other inorganic materials has been used for encapsulation

Lead halide-based perovskite material opens a new opportunity in optoelectric since 2012, because of their excellent optoelectronic properties and high power conversion efficiency within a brief time. The big challenge for the perovskite QDs is the presence of toxicity due to lead. Already many groups have worked on leadless or lead-free perovskite materials. 100% replacement of Pb will be complicated but with replacement up to 60–70%, Pb with non-toxic cation may help to the commercialization of less lead-containing perovskite materials. Shortly, less lead or lead-free PQDs have the potential for next-generation optoelectronic devices like backlights, QD TV, flexible and wearable devices, etc. However, to achieve high PLQY, good performances, and stability of lead-free or less lead containing PQDs also be challenging for the research community. Many homovalent lead substitutions like Sn, Mn, Cd, and Zn have been successfully done in place of Pb in different halide containing perovskite nanomaterials. Such kind of replacement of lead not only reduces the toxicity of perovskite while it also improves the performances and stability of the perovskite light-harvesting material. In the halide-based perovskites, dopant engineering is a compelling strategy to tuning the optical, structural, and electrical properties and day by day new depends arrive that make the PQDs more suitable for lead-free perovskite devices. Mn and Cd incorporation in Cl-based perovskite NCs was increased the PLQYs but for Br- and I-based PNCs it is still challenging to get high PLQYs than pure PQDs. So, we need to find a more suitable

strategy for less lead-containing high PLQYs perovskite material.

, Sudipta Som<sup>2</sup>

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

, Somrita Dutta3

, Jai Singh1

and

\*, Neha Jain1

3 National Chiao Tung University, Hsinchu, Taiwan

provided the original work is properly cited.

1 Dr. Harisingh Gour Central University, Sagar, MP, India

\*Address all correspondence to: rajanphysicssgo@gmail.com

2 National Taiwan University, Taipei, Taiwan (Republic of China)

**132**

[1] Noh JH, Im SH, Heo JH, Mandal TN, Seok SI. Chemical management for colorful, efficient, and stable inorganicorganic hybrid nanostructured solar cells. Nano Letters. 2013;**13**:1764-1769

[2] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society. 2009;**131**:6050-6051

[3] Guerrero A et al. How the chargeneutrality level of interface states controls energy level alignment in cathode contacts of organic bulkheterojunction solar cells. ACS Nano. 2012;**6**:3453-3460

[4] Yang X et al. Author correction: Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nature Communications. 2018;**9**:1169

[5] Schmidt LC et al. Nontemplate synthesis of CH3NH3PbBr3 perovskite nanoparticles. Journal of the American Chemical Society. 2014;**136**:850-853

[6] Kovalenko MV, Protesescu L, Bodnarchuk MI. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science. 2017;**358**:745-750

[7] Gonzalez-Carrero S, Galian RE, Pérez-Prieto J. Maximizing the emissive properties of CH3NH3PbBr3 perovskite nanoparticles. Journal of Materials Chemistry A. 2015;**3**:9187-9193

[8] Moyen E et al. Ligand removal and photo-activation of CsPbBr3 quantum dots for enhanced optoelectronic devices. Nanoscale. 2018;**10**:8591-8599

[9] Hong K, Van Le Q, Kim SY, Jang HW. Low-dimensional halide perovskites: Review and issues.

Journal of Materials Chemistry C. 2018;**6**:2189-2209

[10] Wei Y, Cheng Z, Lin J. An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs. Chemical Society Reviews. 2019;**48**:310-350

[11] Bella F et al. Improving efficiency and stability of perovskite solar cells with photocurable fluoropolymers. Science. 2016;**354**:203-206

[12] Wang R et al. A review of perovskites solar cell stability. Advanced Functional Materials. 2019;**1808843**:1-25

[13] Song T-B, Yokoyama T, Aramaki S, Kanatzidis MG. Performance enhancement of lead-free tin-based perovskite solar cells with reducing atmosphere-assisted dispersible additive. ACS Energy Letters. 2017;**2**:897-903

[14] Lyu M, Yun J-H, Chen P, Hao M, Wang L. Addressing toxicity of lead: Progress and applications of low-toxic metal halide perovskites and their derivatives. Advanced Energy Materials. 2017;**7**:1602512

[15] Babayigit A et al. Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism danio rerio. Scientific Reports. 2016;**6**:18721

[16] Mitzi DB. N4H9Cu7S4: A hydrazinium-based salt with a layered Cu7S4-framework. Inorganic Chemistry. 2007;**46**(3):926-931

[17] Singh RK et al. Exploring the impact of the Pb2+ substitution by Cd2+ on the structural and morphological properties of CH3NH3PbI3 perovskite. Applied Nanoscience. 2019:1-10. https://doi. org/10.1007/s13204-019-01021-5

[18] Kim HS et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Scientific Reports. 2012;**2**:591

[19] Singh RK et al. Novel synthesis process of methyl ammonium bromide and effect of particle size on structural, optical and thermodynamic behavior of CH3NH3PbBr3 organometallic perovskite light harvester. Journal of Alloys and Compounds. 2018;**743**:728-736

[20] Chen Q et al. Under the spotlight: The organic–inorganic hybrid halide perovskite for optoelectronic applications. Nano Today. 2015;**10**:355-396

[21] Peña MA, Fierro JLG. Chemical structures and performance of perovskite oxides. Chemical Reviews. 2001;**101**:1981-2017

[22] Zhang F et al. Colloidal synthesis of air-stable CH3NH3PbI3 quantum dots by gaining chemical insight into the solvent effects. Chemistry of Materials. 2017;**29**:3793-3799

[23] Zhang S et al. Efficient red perovskite light-emitting diodes based on solution-processed multiple quantum wells. Advanced Materials. 2017;**29**:1606600

[24] Singh RK et al. Solution processed hybrid organic-inorganic CH3NH3PbI3 perovskite material and optical properties. Materials Today: Proceedings. 2017;**4**:12661-12665

[25] Price MB et al. Hot-carrier cooling and photoinduced refractive index changes in organic–inorganic lead halide perovskites. Nature Communications. 2015;**6**:8420

[26] Sheng R et al. Photoluminescence characterisations of a dynamic aging process of organic–inorganic

CH3NH3PbBr3 perovskite. Nanoscale. 2016;**8**:1926-1931

[27] Tan Z-K et al. Bright light-emitting diodes based on organometal halide perovskite. Nature Nanotechnology. 2014;**9**:687

[28] Shamsi J, Urban AS, Imran M, De Trizio L, Manna L. Metal halide perovskite nanocrystals: Synthesis, post-synthesis modifications, and their optical properties. Chemical Reviews. 2019;**119**:3296-3348

[29] Hoefler SF, Trimmel G, Rath T. Progress on lead-free metal halide perovskites for photovoltaic applications: A review. Monatshefte für Chemie—Chemical Monthly. 2017;**148**:795-826

[30] Konstantakou M, Stergiopoulos T. A critical review on tin halide perovskite solar cells. Journal of Materials Chemistry A. 2017;**5**:11518-11549

[31] Shum K et al. Synthesis and characterization of CsSnI3 thin films. Applied Physics Letters. 2010;**96**:221903

[32] Chung I et al. CsSnI3: Semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material. High hole mobility and phasetransitions. Journal of the American Chemical Society. 2012;**134**:8579-8587

[33] Hou S, Guo Y, Tang Y, Quan Q. Synthesis and stabilization of colloidal perovskite nanocrystals by multidentate polymer micelles. ACS Applied Materials & Interfaces. 2017;**9**:18417-18422

[34] Fang G et al. Reverse synthesis of CsPbxMn1−x(Cl/Br)3 perovskite quantum dots from CsMnCl3 precursors through cation exchange. Journal of Materials Chemistry C. 2018;**6**:5908-5915

**135**

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application*

[44] Liu F et al. Highly luminescent phase-stable CsPbI3 perovskite quantum dots achieving near 100% absolute photoluminescence quantum yield. ACS Nano. 2017;**11**:10373-10383

[45] Liu W et al. Mn2+-doped lead halide perovskite nanocrystals with dual-color emission controlled by halide content. Journal of the American Chemical Society. 2016;**138**:14954-14961

[46] Yuan X et al. Photoluminescence temperature dependence, dynamics, and quantum efficiencies in Mn2+-doped CsPbCl3 perovskite nanocrystals with varied dopant concentration. Chemistry

of Materials. 2017;**29**:8003-8011

[47] Zhu J et al. Room-temperature synthesis of Mn-doped cesium lead halide quantum dots with high Mn substitution ratio. Journal of Physical Chemistry Letters. 2017;**8**:4167-4171

[48] Zou S et al. Stabilizing cesium lead halide perovskite lattice through Mn(II) substitution for air-stable light-emitting

Chemical Society. 2017;**139**:11443-11450

[49] Akkerman QA, Meggiolaro D, Dang Z, De Angelis F, Manna L. Fluorescent

nanocrystals with high structural and optical stability. ACS Energy Letters.

diodes. Journal of the American

alloy CsPbxMn1–xI3 perovskite

[50] van der Stam W et al. Highly emissive divalent-ion-doped colloidal CsPb1–xMxBr3 perovskite nanocrystals through cation exchange. Journal of the American Chemical Society.

[51] Mondal N, De A, Samanta A.

photoluminescence efficiency for blueviolet-emitting perovskite nanocrystals. ACS Energy Letters. 2019;**4**:32-39

2017;**2**:2183-2186

2017;**139**:4087-4097

Achieving near-unity

*DOI: http://dx.doi.org/10.5772/intechopen.86836*

[35] Marshall KP, Walker M, Walton RI, Hatton RA. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics. Nature

[36] Liu F et al. Colloidal synthesis of air-stable alloyed CsSn1–xPbxI3 perovskite nanocrystals for use in solar cells. Journal of the American Chemical Society. 2017;**139**:16708-16719

[37] Wang A et al. Controlled synthesis of lead-free and stable perovskite derivative Cs2SnI6 nanocrystals via a facile hot-injection process. Chemistry of Materials. 2016;**28**:8132-8140

[38] Eames C et al. Ionic transport in hybrid lead iodide perovskite solar cells. Nature Communications. 2015;**6**:7497

[40] Akkerman QA et al. Tuning the optical properties of cesium lead halide perovskite nanocrystals by anion exchange reactions. Journal of the American Chemical Society.

[39] Wang H-C et al. Highperformance CsPb1−xSnxBr3 perovskite quantum dots for light-emitting diodes. Angewandte Chemie, International Edition.

2017;**56**:13650-13654

2015;**137**:10276-10281

2013;**25**:1305-1317

2018;**10**:19435-19442

Letters. 2017;**2**:1014-1021

[41] Buonsanti R, Milliron DJ. Chemistry of doped colloidal

nanocrystals. Chemistry of Materials.

[42] Zhao J. Enhanced luminescence and energy transfer in Mn2+ dopedB-site doped CsPbCl3-xBrx perovskite nanocrystals. Nanoscale.

[43] Guria AK, Dutta SK, Das Adhikari S, Pradhan N. Doping Mn2+ in lead halide perovskite nanocrystals: Successes and challenges. ACS Energy

Energy. 2016;**1**:16178

*Lead-Free Hybrid Perovskite Light-Harvesting Material for QD-LED Application DOI: http://dx.doi.org/10.5772/intechopen.86836*

[35] Marshall KP, Walker M, Walton RI, Hatton RA. Enhanced stability and efficiency in hole-transport-layer-free CsSnI3 perovskite photovoltaics. Nature Energy. 2016;**1**:16178

*Perovskite Materials, Devices and Integration*

[18] Kim HS et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Scientific

CH3NH3PbBr3 perovskite. Nanoscale.

[27] Tan Z-K et al. Bright light-emitting diodes based on organometal halide perovskite. Nature Nanotechnology.

[28] Shamsi J, Urban AS, Imran M, De Trizio L, Manna L. Metal halide perovskite nanocrystals: Synthesis, post-synthesis modifications, and their optical properties. Chemical Reviews.

[29] Hoefler SF, Trimmel G, Rath T. Progress on lead-free metal halide perovskites for photovoltaic

applications: A review. Monatshefte für Chemie—Chemical Monthly.

solar cells. Journal of Materials Chemistry A. 2017;**5**:11518-11549

[31] Shum K et al. Synthesis and characterization of CsSnI3 thin films. Applied Physics Letters.

[32] Chung I et al. CsSnI3:

[30] Konstantakou M, Stergiopoulos T. A critical review on tin halide perovskite

Semiconductor or metal? High electrical conductivity and strong near-infrared photoluminescence from a single material. High hole mobility and phasetransitions. Journal of the American Chemical Society. 2012;**134**:8579-8587

[33] Hou S, Guo Y, Tang Y, Quan Q. Synthesis and stabilization of colloidal perovskite nanocrystals by multidentate polymer micelles. ACS Applied Materials & Interfaces.

[34] Fang G et al. Reverse synthesis of CsPbxMn1−x(Cl/Br)3 perovskite quantum dots from CsMnCl3 precursors through cation exchange. Journal of Materials Chemistry C.

2016;**8**:1926-1931

2019;**119**:3296-3348

2017;**148**:795-826

2010;**96**:221903

2017;**9**:18417-18422

2018;**6**:5908-5915

2014;**9**:687

[19] Singh RK et al. Novel synthesis process of methyl ammonium bromide and effect of particle size on structural, optical and thermodynamic behavior of CH3NH3PbBr3 organometallic perovskite light harvester. Journal of Alloys and Compounds. 2018;**743**:728-736

[20] Chen Q et al. Under the spotlight:

[21] Peña MA, Fierro JLG. Chemical structures and performance of perovskite oxides. Chemical Reviews.

[22] Zhang F et al. Colloidal synthesis of air-stable CH3NH3PbI3 quantum dots by gaining chemical insight into the solvent effects. Chemistry of Materials.

The organic–inorganic hybrid halide perovskite for optoelectronic

applications. Nano Today.

2015;**10**:355-396

2001;**101**:1981-2017

2017;**29**:3793-3799

2017;**29**:1606600

[23] Zhang S et al. Efficient red perovskite light-emitting diodes based on solution-processed multiple quantum wells. Advanced Materials.

[24] Singh RK et al. Solution

processed hybrid organic-inorganic CH3NH3PbI3 perovskite material and optical properties. Materials Today: Proceedings. 2017;**4**:12661-12665

[25] Price MB et al. Hot-carrier cooling and photoinduced refractive index changes in organic–inorganic lead halide perovskites. Nature Communications.

[26] Sheng R et al. Photoluminescence characterisations of a dynamic aging process of organic–inorganic

Reports. 2012;**2**:591

**134**

2015;**6**:8420

[36] Liu F et al. Colloidal synthesis of air-stable alloyed CsSn1–xPbxI3 perovskite nanocrystals for use in solar cells. Journal of the American Chemical Society. 2017;**139**:16708-16719

[37] Wang A et al. Controlled synthesis of lead-free and stable perovskite derivative Cs2SnI6 nanocrystals via a facile hot-injection process. Chemistry of Materials. 2016;**28**:8132-8140

[38] Eames C et al. Ionic transport in hybrid lead iodide perovskite solar cells. Nature Communications. 2015;**6**:7497

[39] Wang H-C et al. Highperformance CsPb1−xSnxBr3 perovskite quantum dots for light-emitting diodes. Angewandte Chemie, International Edition. 2017;**56**:13650-13654

[40] Akkerman QA et al. Tuning the optical properties of cesium lead halide perovskite nanocrystals by anion exchange reactions. Journal of the American Chemical Society. 2015;**137**:10276-10281

[41] Buonsanti R, Milliron DJ. Chemistry of doped colloidal nanocrystals. Chemistry of Materials. 2013;**25**:1305-1317

[42] Zhao J. Enhanced luminescence and energy transfer in Mn2+ dopedB-site doped CsPbCl3-xBrx perovskite nanocrystals. Nanoscale. 2018;**10**:19435-19442

[43] Guria AK, Dutta SK, Das Adhikari S, Pradhan N. Doping Mn2+ in lead halide perovskite nanocrystals: Successes and challenges. ACS Energy Letters. 2017;**2**:1014-1021

[44] Liu F et al. Highly luminescent phase-stable CsPbI3 perovskite quantum dots achieving near 100% absolute photoluminescence quantum yield. ACS Nano. 2017;**11**:10373-10383

[45] Liu W et al. Mn2+-doped lead halide perovskite nanocrystals with dual-color emission controlled by halide content. Journal of the American Chemical Society. 2016;**138**:14954-14961

[46] Yuan X et al. Photoluminescence temperature dependence, dynamics, and quantum efficiencies in Mn2+-doped CsPbCl3 perovskite nanocrystals with varied dopant concentration. Chemistry of Materials. 2017;**29**:8003-8011

[47] Zhu J et al. Room-temperature synthesis of Mn-doped cesium lead halide quantum dots with high Mn substitution ratio. Journal of Physical Chemistry Letters. 2017;**8**:4167-4171

[48] Zou S et al. Stabilizing cesium lead halide perovskite lattice through Mn(II) substitution for air-stable light-emitting diodes. Journal of the American Chemical Society. 2017;**139**:11443-11450

[49] Akkerman QA, Meggiolaro D, Dang Z, De Angelis F, Manna L. Fluorescent alloy CsPbxMn1–xI3 perovskite nanocrystals with high structural and optical stability. ACS Energy Letters. 2017;**2**:2183-2186

[50] van der Stam W et al. Highly emissive divalent-ion-doped colloidal CsPb1–xMxBr3 perovskite nanocrystals through cation exchange. Journal of the American Chemical Society. 2017;**139**:4087-4097

[51] Mondal N, De A, Samanta A. Achieving near-unity photoluminescence efficiency for blueviolet-emitting perovskite nanocrystals. ACS Energy Letters. 2019;**4**:32-39

**137**

years [1].

**Chapter 8**

**Abstract**

memristors

**1. Introduction**

*Jiaqi Zhang and Wubo Li*

Perovskite Materials for Resistive

Resistive random access memory (RRAM) utilizes the resistive switching behavior to store information. Compared to charge-based memory devices, the merits of RRAM devices include multi-bit capability, smaller cell size, and energy per bit (~fJ/bit). In this chapter, we review different perovskite material-based resistive random access memories (RRAMs). We first introduce the history of RRAM development and operational mechanism of conduction, followed by a review of two types of materials with perovskite crystal structure. One is conventional perovskite oxides (PCMO, a-LCMO, etc.), and the other is perovskite halides (organic-inorganic hybrid perovskites and inorganic perovskites) that have recently emerged as novel materials in optoelectronic fields. Our goal is to give a comprehensive review of perovskite-based RRAM materials that can be used for neuromorphic computing

Random Access Memories

and to help further ongoing development in the field.

**Keywords:** memory devices, RRAM, perovskite oxides, perovskite halides,

Resistive random access memory (RRAM) devices use the resistance switching (RS) behavior to save information, which has attracted much attention due to its outstanding performance compared with traditional semiconductor electronic devices. In comparison to charge-based memory cells, RRAM device possesses various advantages, such as multi-bit capability, simpler device structure (electrode/ active layer/electrode), as well as lower energy consumption (~fJ/bit) [1]. Another advantage of RRAM is its good compatibility to the conventional CMOS, which allows it to be integrated into current integrated circuit (IC) technology [2].

RRAM has various potential applications. First, RRAM is considered as the most promising candidate as the next-generation memory device because it acts excellently as both main memory and working memory. As main memory, RRAM is nonvolatile with high memory capacity. As working memory, the operation voltage and power of RRAM are very low, and the write/erase rate is very high. Apart from the memory application, RRAM is also utilized in low-energyconsumption computing as nonvolatile logic circuit [3, 4] and in neuromorphic computing as a synaptic cell, with the latter being a research hotspot in recent

A typical RRAM device is a metal/insulator/metal (MIM) stack, in which an insulating active layer is sandwiched between the top and the bottom metal electrodes, as shown in **Figure 1**. The device resistance can be tuned by applying an

## **Chapter 8**

## Perovskite Materials for Resistive Random Access Memories

*Jiaqi Zhang and Wubo Li*

## **Abstract**

Resistive random access memory (RRAM) utilizes the resistive switching behavior to store information. Compared to charge-based memory devices, the merits of RRAM devices include multi-bit capability, smaller cell size, and energy per bit (~fJ/bit). In this chapter, we review different perovskite material-based resistive random access memories (RRAMs). We first introduce the history of RRAM development and operational mechanism of conduction, followed by a review of two types of materials with perovskite crystal structure. One is conventional perovskite oxides (PCMO, a-LCMO, etc.), and the other is perovskite halides (organic-inorganic hybrid perovskites and inorganic perovskites) that have recently emerged as novel materials in optoelectronic fields. Our goal is to give a comprehensive review of perovskite-based RRAM materials that can be used for neuromorphic computing and to help further ongoing development in the field.

**Keywords:** memory devices, RRAM, perovskite oxides, perovskite halides, memristors

## **1. Introduction**

Resistive random access memory (RRAM) devices use the resistance switching (RS) behavior to save information, which has attracted much attention due to its outstanding performance compared with traditional semiconductor electronic devices. In comparison to charge-based memory cells, RRAM device possesses various advantages, such as multi-bit capability, simpler device structure (electrode/ active layer/electrode), as well as lower energy consumption (~fJ/bit) [1]. Another advantage of RRAM is its good compatibility to the conventional CMOS, which allows it to be integrated into current integrated circuit (IC) technology [2].

RRAM has various potential applications. First, RRAM is considered as the most promising candidate as the next-generation memory device because it acts excellently as both main memory and working memory. As main memory, RRAM is nonvolatile with high memory capacity. As working memory, the operation voltage and power of RRAM are very low, and the write/erase rate is very high. Apart from the memory application, RRAM is also utilized in low-energyconsumption computing as nonvolatile logic circuit [3, 4] and in neuromorphic computing as a synaptic cell, with the latter being a research hotspot in recent years [1].

A typical RRAM device is a metal/insulator/metal (MIM) stack, in which an insulating active layer is sandwiched between the top and the bottom metal electrodes, as shown in **Figure 1**. The device resistance can be tuned by applying an

**Figure 1.**

*(a) Schematic diagram of an RRAM device; (b) cross-sectional view of a RRAM device with conductive filament mechanism. Reproduced with permission [2]. Copyright 2016, Elsevier.*

electric bias across two electrodes, forming high resistance state (HRS) and low resistance state (LRS). So the nonvolatile memory phenomenon in RRAM device is realized by electrically modulating the RS between HRS and LRS.

The transformation from HRS to LRS is normally named the Set process, and the transformation from LRS to HRS is named the Reset process. There are three kinds of RS switching behaviors. One is unipolar switching, in which the Set and Reset processes can happen at the same polarity of the external bias. Second is bipolar switching, in which RS switching occurs at different polarities of the applied bias. The third is nonpolar resistive memory, in which both RS switching from HRS to LRS and RS switching from LRS to HRS can be achieved without altering the voltage polarity (unipolar), while they can also be achieved by altering the polarity (bipolar).

The schematic I-V characteristics of unipolar switching and bipolar switching are illustrated in **Figure 2**, respectively. In addition to the Set and Reset processes, a Forming process typically exists for many initially prepared RRAM devices, in which an applied forming voltage (Vform) drives the formation of CFs with the compliance current limitation. The forming process is normally accomplished before the RRAM device enables to work, and the Vform is usually larger than the setting voltage (Vset).

#### **Figure 2.**

*Typical current voltage (I-V) characteristics of memory devices with (a) unipolar switching and (b) bipolar switching. Reproduced with permission [1]. Copyright 2018, Springer.*

**139**

*Perovskite Materials for Resistive Random Access Memories*

As aforementioned, a basic RRAM device usually consists of two electrodes and an active layer. The top and bottom electrodes can use various materials, including elementary substantial metals (Ag, Cu, Al, Au, Pt, W, etc.) [5], metallic alloys (Pt-Al, Cu-Ti, etc.) [6], and oxides (ITO, SrRuO3, Nb:SrTiO3, etc.) [7–9]. Based on the functions in RS conversion, the electrode materials can be divided into two types. One is active electrodes (Cu, Ag, etc.), which contribute to the RS conversion by the migration and/or redox reaction of the electrode ions around the electrode/active layer junction. The other is inert electrodes (Pt, Au, etc.), which do not directly

As for the active layer, various materials have been utilized in memory devices, such as amorphous metal oxides, polymers, hybrid composites, perovskite oxides, and perovskite halides [10, 11]. The material choice of RRAM active layer has significant influence on the device performance. In this chapter, we will mainly introduce oxides and halides with perovskite structures. Perovskite oxides are a conventional active material family for memories. In addition to high-endurance, chemically stable, and high-speed operation, the strong electron correlation induces many unique properties for perovskite oxides, which makes it remain as one of the most promising materials for RRAM yet. Compared with conventional RRAMs, perovskite halides can make flexible devices with low-cost fabrication, compositional flexibility, and excellent optoelectronic properties, enabling the perovskite halides as a promising next-generation memory material family. Next, we will introduce the memory devices with perovskite oxides and halides separately.

In 2002, perovskite oxide Pr0.7Ca0.3MnO3 was firstly utilized in a 64-bit RRAM array by a 500-nm complementary metal oxide semiconductor (CMOS) process [1]. Nearly two decades passed, transition metal perovskite oxides are still one of the best materials for RRAM active layers. Perovskite oxides possess stable crystal structure with high defect tolerance and structure flexibility, which enables the accommodation of nonstoichiometric ions. The nonstoichiometric ions contribute to the local ionic migration and thermochemical reaction therefore allowing for the RS conversion. In addition, the strongly correlated electrons in perovskite oxides provide various electronic phases and bring out multifunctionality, e.g., colossal magnetoresistance/electroresistance, ferroelectricity, multiferroics, superconductivity, etc. [12]. Also, the competing behavior among different electronic phases brings out the metal-insulator transition (MIT) phenomenon, which allows a significant change of resistance with a tiny electric stimulus [12, 13]. In the following, based on different resistance switching mechanisms, we will introduce RRAM with transition

metal perovskite oxides via filament mode and uniform mode, respectively.

Conductive filament (CF) is the most common mechanism to explain the resistance switching of memristor devices, in which the formation and the breakage of filaments are in Set and Reset process, respectively. Many works have proven the conductive filament in RRAM devices. For instance, a co-doped BaTiO3-based device is forming as LRS, and the top electrode is divided into two portions (TE-I and TE-II). Then people measured the resistance between the bottom electrode and TE-I or TE-II. Significantly different resistances are found for two parts, which indicates inhomogeneous conductivity inside the whole memory device [14]. Besides, in an YMnO3-based memory device, people found that the switching I-V curves of

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

participate in the RS conversion.

**2. Perovskite oxide memory devices**

**2.1 Conductive filament mechanism**

*Perovskite Materials, Devices and Integration*

the polarity (bipolar).

**Figure 1.**

electric bias across two electrodes, forming high resistance state (HRS) and low resistance state (LRS). So the nonvolatile memory phenomenon in RRAM device is

*(a) Schematic diagram of an RRAM device; (b) cross-sectional view of a RRAM device with conductive* 

The transformation from HRS to LRS is normally named the Set process, and the transformation from LRS to HRS is named the Reset process. There are three kinds of RS switching behaviors. One is unipolar switching, in which the Set and Reset processes can happen at the same polarity of the external bias. Second is bipolar switching, in which RS switching occurs at different polarities of the applied bias. The third is nonpolar resistive memory, in which both RS switching from HRS to LRS and RS switching from LRS to HRS can be achieved without altering the voltage polarity (unipolar), while they can also be achieved by altering

The schematic I-V characteristics of unipolar switching and bipolar switching are illustrated in **Figure 2**, respectively. In addition to the Set and Reset processes, a Forming process typically exists for many initially prepared RRAM devices, in which an applied forming voltage (Vform) drives the formation of CFs with the compliance current limitation. The forming process is normally accomplished before the RRAM device enables to work, and the Vform is usually larger than the setting voltage (Vset).

*Typical current voltage (I-V) characteristics of memory devices with (a) unipolar switching and (b) bipolar* 

*switching. Reproduced with permission [1]. Copyright 2018, Springer.*

realized by electrically modulating the RS between HRS and LRS.

*filament mechanism. Reproduced with permission [2]. Copyright 2016, Elsevier.*

**138**

**Figure 2.**

As aforementioned, a basic RRAM device usually consists of two electrodes and an active layer. The top and bottom electrodes can use various materials, including elementary substantial metals (Ag, Cu, Al, Au, Pt, W, etc.) [5], metallic alloys (Pt-Al, Cu-Ti, etc.) [6], and oxides (ITO, SrRuO3, Nb:SrTiO3, etc.) [7–9]. Based on the functions in RS conversion, the electrode materials can be divided into two types. One is active electrodes (Cu, Ag, etc.), which contribute to the RS conversion by the migration and/or redox reaction of the electrode ions around the electrode/active layer junction. The other is inert electrodes (Pt, Au, etc.), which do not directly participate in the RS conversion.

As for the active layer, various materials have been utilized in memory devices, such as amorphous metal oxides, polymers, hybrid composites, perovskite oxides, and perovskite halides [10, 11]. The material choice of RRAM active layer has significant influence on the device performance. In this chapter, we will mainly introduce oxides and halides with perovskite structures. Perovskite oxides are a conventional active material family for memories. In addition to high-endurance, chemically stable, and high-speed operation, the strong electron correlation induces many unique properties for perovskite oxides, which makes it remain as one of the most promising materials for RRAM yet. Compared with conventional RRAMs, perovskite halides can make flexible devices with low-cost fabrication, compositional flexibility, and excellent optoelectronic properties, enabling the perovskite halides as a promising next-generation memory material family. Next, we will introduce the memory devices with perovskite oxides and halides separately.

## **2. Perovskite oxide memory devices**

In 2002, perovskite oxide Pr0.7Ca0.3MnO3 was firstly utilized in a 64-bit RRAM array by a 500-nm complementary metal oxide semiconductor (CMOS) process [1]. Nearly two decades passed, transition metal perovskite oxides are still one of the best materials for RRAM active layers. Perovskite oxides possess stable crystal structure with high defect tolerance and structure flexibility, which enables the accommodation of nonstoichiometric ions. The nonstoichiometric ions contribute to the local ionic migration and thermochemical reaction therefore allowing for the RS conversion. In addition, the strongly correlated electrons in perovskite oxides provide various electronic phases and bring out multifunctionality, e.g., colossal magnetoresistance/electroresistance, ferroelectricity, multiferroics, superconductivity, etc. [12]. Also, the competing behavior among different electronic phases brings out the metal-insulator transition (MIT) phenomenon, which allows a significant change of resistance with a tiny electric stimulus [12, 13]. In the following, based on different resistance switching mechanisms, we will introduce RRAM with transition metal perovskite oxides via filament mode and uniform mode, respectively.

## **2.1 Conductive filament mechanism**

Conductive filament (CF) is the most common mechanism to explain the resistance switching of memristor devices, in which the formation and the breakage of filaments are in Set and Reset process, respectively. Many works have proven the conductive filament in RRAM devices. For instance, a co-doped BaTiO3-based device is forming as LRS, and the top electrode is divided into two portions (TE-I and TE-II). Then people measured the resistance between the bottom electrode and TE-I or TE-II. Significantly different resistances are found for two parts, which indicates inhomogeneous conductivity inside the whole memory device [14]. Besides, in an YMnO3-based memory device, people found that the switching I-V curves of

devices with various electrode areas are not remarkably different and that the lateral distribution of filaments is not uniform [15]. Real-time observation of conductive filament was also conducted by TEM in oxide-based memory devices, which shows direct evidence of conductive filament mechanism [16, 17].

Next, we will give a brief introduction on the microscopic mechanisms of filaments, mainly including two types: ion migration and metal-insulator transition.

### *2.1.1 Ion migration*

For the conductive filaments caused by ion migration, the presence of CFs is very random owing to the random distribution of ions and defects in the active layer. Therefore, the formation and rupture of the CFs are strongly dependent on the initial distribution of ions and defects. For bipolar RRAM devices, the microscopic mechanism of ion migration can be classified into three types.

First, the filament is formed by the local redox reaction of metallic cations from the active metal electrode. For example, in an Ag/a-LSMO (amorphous Sr-doped LaMnO3)/Pt device, Ag is demonstrated as the main component of the CFs [18]. When a positive bias is applied to the Ag electrode, some Ag atoms around the Ag/a-LSMO interface will be oxidized into Ag cations, and these Ag cations will move toward the opposite cathode under the external electrical field and are eventually reduced back to Ag atoms around the cathode/a-LSMO interface, therefore forming the CFs between two electrodes. Now if we apply a negative bias to the Ag electrode, the Ag atoms around the cathode will be re-oxidized into Ag+ and move back toward the Ag electrode, thus leading to the breakage of the filaments. A typical RRAM device of this mode is an insulating active layer sandwiched between an active metallic electrode and an inert electrode [17].

A second mechanism of ion migration is that the positive-charged defects drift under the external voltage. The charged defects, such as oxygen vacancies and excess cations, can tune the Fermi level and correspondingly change the electrical conductivity in the local area. For instance, in a TiO2-based memory device, the filaments are formed by the oxygen-deficient Ti4O7 phase under positive voltage [19]. When applying a negative voltage, the reverse redox occurs with the backward electric field and the parasitic Joule heating and consequently leads to the rupture of CFs [20].

Redox reactions both exist in the aforementioned two microscopic mechanisms. However, bipolar RS phenomenon also can be caused by the ion migration without redox. Pt/NSTO (Nb-doped SrTiO3) device is taken as an example. Under electrical field, the movement of oxygen vacancies can change the Schottky barrier height and the depletion width of the Pt/NSTO junction at some local areas of the interface, resulting in the change of the electrical conductivity alongside the Pt/NSTO interface [21, 22].

For unipolar RS behavior with ion migration, the rupture of the CFs is different from that in the bipolar counterparts. In bipolar devices, the filament rupture is caused by the retraction of the initially moved ions or by the change of interfacial junction barriers. However, in unipolar memory devices, the filament rupture is driven by the Joule heat-assisted thermochemical reaction. For example, in a Au/ YMn1−δO3/Pt unipolar memory device, after the filament is formed under forward bias, a reverse bias with a similar value cannot supply sufficient energy to retract the initially migrated ions and activate the local reverse redox [15]. Instead, the electrical current can provide enough Joule heat in local areas of the filament; sometimes the local temperature can be increased by several hundreds of Kelvin [15, 23], thus assuring the local reverse redox and the corresponding rupture of CFs. As for the HRS to LRS transformation, accompanied by the ion migration, the electron

**141**

*Perovskite Materials for Resistive Random Access Memories*

hopping barriers and the related trapping states which exist in HRS are removed by the further increase of the applying voltage [15, 17, 24]. The same mechanism has also been proven in the Au/co-doped BaTiO3/Pt unipolar memory device [14].

Metal-insulator transition (MIT) effect has been found in many perovskite oxides, in which the electronic charges are injected into the insulating material to induct the current with an external bias [25–27]. Pr1−xCaxMnO3 (PCMO), now one of the most developed memory materials for neuromorphic computing, is taken as an example, in which resistive switching behavior was first discovered in late 1990s [25]. The electron injection distorts the superlattice structure and the mixed valence band in the strongly electronic correlated PCBM system, which acts as an ion doping process. The rebuilding of the electronic phase separation state can also contribute to the MIT, induced by the external electrical stimulus and parasitical Joule heating, which exhibits CFs-based unipolar RS phenomenon [28]. In addition, filament-type RS behavior may also derive from the Mott transition, which has been

demonstrated in many transition metal perovskite oxides [26, 29, 30].

In CF-based memory devices, the conductive filaments are formed under electric stimuli in local areas. The I-V characteristics are not proportional to the electrode area due to the random distribution of the filaments. Apart from the CF-based RRAMs, uniform resistance switching mechanism has already been demonstrated, in which the device resistance variation is spatially uniform. Thus the variations of HRS and LRS are both proportional to the electrode area. Uniform RS behavior mainly includes two types, one is the carrier trapping/detrapping, and the other is

Carrier trapping/detrapping and the migration of charged defects can tune the Schottky barrier at the metal-insulator interface thus modulating the device resistance. This modulation could occur in local regions near the interface (i.e., filament mode) or occur laterally uniform near the interface (i.e., uniform mode), which is strongly dependent on the interfacial electrical and morphological uniformity. A smooth interface with uniform distribution of charges and defects may bring out uniform RS. Otherwise, filaments may tend to form with nonuniform interfaces. Researchers found that the uniform migration of charged defects (e.g., oxygen vacancies) is too slow thus leading to very slow RS [31]. Nevertheless, the charge trapping/ detrapping at the junction can be very fast, enabling uniform RS with fast response,

which has been confirmed in the Au/Nb-doped SrTiO3 heterojunction [14].

electric field, the resistance state is obviously changed.

For the uniform RS by ferroelectric polarization, ferroelectric tunnel junctions (FTJs) are utilized for the RRAM devices, including a ferroelectric tunnel barrier sandwiched by two electrodes. Many perovskite oxide materials have been utilized in the FTJ-based memory devices, such as Pt/BaTiO3/SrRuO3 [8], Pt/BiFeO3/SrRuO3 [32], Co/BaTiO3/La0.7Sr0.3MnO3 [33], etc. The polarization at the ferroelectric/metal junction has a significant influence on the junction barrier profile and modulates the electron tunneling. Thus when the polarization is varied with the external

Although uniform RS behavior exhibits many advantages, currently the practical application of uniform-type memory devices is still restricted by some intrinsic demerits. The key issue is that the uniform-type device performance is closely dependent on the quality of the films and the junction [6]. For the carrier trapping-/detrapping-based devices, the LRS and the HRS often show considerable relaxation, which deteriorates

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

*2.1.2 Metal-insulator transition*

**2.2 Uniform resistance switching**

the ferroelectric polarization.

hopping barriers and the related trapping states which exist in HRS are removed by the further increase of the applying voltage [15, 17, 24]. The same mechanism has also been proven in the Au/co-doped BaTiO3/Pt unipolar memory device [14].

## *2.1.2 Metal-insulator transition*

*Perovskite Materials, Devices and Integration*

*2.1.1 Ion migration*

direct evidence of conductive filament mechanism [16, 17].

devices with various electrode areas are not remarkably different and that the lateral distribution of filaments is not uniform [15]. Real-time observation of conductive filament was also conducted by TEM in oxide-based memory devices, which shows

Next, we will give a brief introduction on the microscopic mechanisms of filaments, mainly including two types: ion migration and metal-insulator transition.

For the conductive filaments caused by ion migration, the presence of CFs is very random owing to the random distribution of ions and defects in the active layer. Therefore, the formation and rupture of the CFs are strongly dependent on the initial distribution of ions and defects. For bipolar RRAM devices, the micro-

First, the filament is formed by the local redox reaction of metallic cations from the active metal electrode. For example, in an Ag/a-LSMO (amorphous Sr-doped LaMnO3)/Pt device, Ag is demonstrated as the main component of the CFs [18]. When a positive bias is applied to the Ag electrode, some Ag atoms around the Ag/a-LSMO interface will be oxidized into Ag cations, and these Ag cations will move toward the opposite cathode under the external electrical field and are eventually reduced back to Ag atoms around the cathode/a-LSMO interface, therefore forming the CFs between two electrodes. Now if we apply a negative bias to the Ag

and move

scopic mechanism of ion migration can be classified into three types.

electrode, the Ag atoms around the cathode will be re-oxidized into Ag+

active metallic electrode and an inert electrode [17].

back toward the Ag electrode, thus leading to the breakage of the filaments. A typical RRAM device of this mode is an insulating active layer sandwiched between an

A second mechanism of ion migration is that the positive-charged defects drift under the external voltage. The charged defects, such as oxygen vacancies and excess cations, can tune the Fermi level and correspondingly change the electrical conductivity in the local area. For instance, in a TiO2-based memory device, the filaments are formed by the oxygen-deficient Ti4O7 phase under positive voltage [19]. When applying a negative voltage, the reverse redox occurs with the backward electric field and the parasitic Joule heating and consequently leads to the rupture

Redox reactions both exist in the aforementioned two microscopic mechanisms. However, bipolar RS phenomenon also can be caused by the ion migration without redox. Pt/NSTO (Nb-doped SrTiO3) device is taken as an example. Under electrical field, the movement of oxygen vacancies can change the Schottky barrier height and the depletion width of the Pt/NSTO junction at some local areas of the interface, resulting in the change of the electrical conductivity alongside the Pt/NSTO

For unipolar RS behavior with ion migration, the rupture of the CFs is different from that in the bipolar counterparts. In bipolar devices, the filament rupture is caused by the retraction of the initially moved ions or by the change of interfacial junction barriers. However, in unipolar memory devices, the filament rupture is driven by the Joule heat-assisted thermochemical reaction. For example, in a Au/ YMn1−δO3/Pt unipolar memory device, after the filament is formed under forward bias, a reverse bias with a similar value cannot supply sufficient energy to retract the initially migrated ions and activate the local reverse redox [15]. Instead, the electrical current can provide enough Joule heat in local areas of the filament; sometimes the local temperature can be increased by several hundreds of Kelvin [15, 23], thus assuring the local reverse redox and the corresponding rupture of CFs. As for the HRS to LRS transformation, accompanied by the ion migration, the electron

**140**

of CFs [20].

interface [21, 22].

Metal-insulator transition (MIT) effect has been found in many perovskite oxides, in which the electronic charges are injected into the insulating material to induct the current with an external bias [25–27]. Pr1−xCaxMnO3 (PCMO), now one of the most developed memory materials for neuromorphic computing, is taken as an example, in which resistive switching behavior was first discovered in late 1990s [25]. The electron injection distorts the superlattice structure and the mixed valence band in the strongly electronic correlated PCBM system, which acts as an ion doping process. The rebuilding of the electronic phase separation state can also contribute to the MIT, induced by the external electrical stimulus and parasitical Joule heating, which exhibits CFs-based unipolar RS phenomenon [28]. In addition, filament-type RS behavior may also derive from the Mott transition, which has been demonstrated in many transition metal perovskite oxides [26, 29, 30].

## **2.2 Uniform resistance switching**

In CF-based memory devices, the conductive filaments are formed under electric stimuli in local areas. The I-V characteristics are not proportional to the electrode area due to the random distribution of the filaments. Apart from the CF-based RRAMs, uniform resistance switching mechanism has already been demonstrated, in which the device resistance variation is spatially uniform. Thus the variations of HRS and LRS are both proportional to the electrode area. Uniform RS behavior mainly includes two types, one is the carrier trapping/detrapping, and the other is the ferroelectric polarization.

Carrier trapping/detrapping and the migration of charged defects can tune the Schottky barrier at the metal-insulator interface thus modulating the device resistance. This modulation could occur in local regions near the interface (i.e., filament mode) or occur laterally uniform near the interface (i.e., uniform mode), which is strongly dependent on the interfacial electrical and morphological uniformity. A smooth interface with uniform distribution of charges and defects may bring out uniform RS. Otherwise, filaments may tend to form with nonuniform interfaces. Researchers found that the uniform migration of charged defects (e.g., oxygen vacancies) is too slow thus leading to very slow RS [31]. Nevertheless, the charge trapping/ detrapping at the junction can be very fast, enabling uniform RS with fast response, which has been confirmed in the Au/Nb-doped SrTiO3 heterojunction [14].

For the uniform RS by ferroelectric polarization, ferroelectric tunnel junctions (FTJs) are utilized for the RRAM devices, including a ferroelectric tunnel barrier sandwiched by two electrodes. Many perovskite oxide materials have been utilized in the FTJ-based memory devices, such as Pt/BaTiO3/SrRuO3 [8], Pt/BiFeO3/SrRuO3 [32], Co/BaTiO3/La0.7Sr0.3MnO3 [33], etc. The polarization at the ferroelectric/metal junction has a significant influence on the junction barrier profile and modulates the electron tunneling. Thus when the polarization is varied with the external electric field, the resistance state is obviously changed.

Although uniform RS behavior exhibits many advantages, currently the practical application of uniform-type memory devices is still restricted by some intrinsic demerits. The key issue is that the uniform-type device performance is closely dependent on the quality of the films and the junction [6]. For the carrier trapping-/detrapping-based devices, the LRS and the HRS often show considerable relaxation, which deteriorates

the device performance [9]. An effective interfacial modification is commonly required to solve this problem. For the FTJ-based devices, the tunneling current tuned by the polarization is normally remarkably small, which hinders its actual application. In addition, the ferroelectric layer is usually ultrathin, and how to maintain the ultrathin film uniformity in a large scale is another technical issue.

## **3. Perovskite halide memory devices**

In recent years, halide perovskites (HPs) have become a star material due to its excellent optical and charge transport properties. The rapid advance in powerconversion efficiency (PCE) of perovskite solar cells has exceeded by 20% [34], and the simple and solution-based preparation enables low-cost production. HPs have excellent electron migration ability and good optical absorption. With the development of HPs, the hysteresis in the current voltage curve was observed and described [35]. It is found that the hysteresis has a strong dependence on the voltage scanning rate and transient response. Ion migration is thought to be a possible origin of the slow response [36]. This discovery paves the way for HPs' applications in other electronic devices, for example, resistive switching memory (memristors) [37–39], field-effect transistors [40–42], and artificial synapse devices [43, 44]. Owing to its unique features and manufacturing advantages, rapid progress has been made, and HPs are considered as a promising candidate for the next generation of electronic devices [45, 46].

## **3.1 Tunable bandgap**

The perovskite material is an ABX3 compound with a 3D framework (**Figure 3a**), where A is a monovalent cation. A-site can be an inorganic or organic cation, for example, methylammonium (MA<sup>+</sup> , CH3NH3 + ), formamidinium (FA<sup>+</sup> , HC(NH2) 2+), or Cs+ ; B is a divalent cation, and X is an anion. B is typically Pb (also Sn) and X is a halide such as Cl, Br, or I. Based on the composition flexibility of

#### **Figure 3.**

*(a) Crystal structure of HPs [47]. (b) A schematic diagram of device structure halide perovskite memory [48]. (c) FETs based on CsPbBr3 and fabrication method [47]. (d) Biological synapse compared to artificial synapses [49]. (a, c) Reproduced with permission [47]. Copyright 2015, American Chemical Society. (b) Reproduced with permission [48]. Copyright 2018, American Chemical Society. (d) Reproduced with permission [49]. Copyright 2019, American Chemical Society.*

**143**

*Perovskite Materials for Resistive Random Access Memories*

HPs, the bandgap can be tuned by replacing elements at each position. In addition, the bandgap can be tuned by controlling the crystalline structure of HPs and the

It is a feasible method to change the length of the bonds between A and B/C sites. In theory, the crystal lattice of the perovskite ABX3 is expandable, and the gap of the forbidden band is narrow. For example, the material obtained by replacing

an arylamine cation is generally a two-dimensional layered structure. The length of alkylammonium cations at position A was reported in 1990 by Calabrese et al., and the synthesized HPs demonstrated a two-dimensional layered structure [51]. With the increase of the length of cation at site A, the maximum absorption peak is red-shifted from 390 to 450 nm [51]. The modulation of the perovskite bandgap by the substitution of A-site has been demonstrated by the density functional theory (DFT), showing an obvious change, i.e., FA (1.5 eV), MA (1.55 eV), and Cs (1.73 eV) [52, 53]. The angle of B-X-B bond in perovskite structure plays an important role in regulating the bandgap of perovskite materials. Therefore, the change of different metal ions (B) to regulate the structure and properties of perovskite materials is also of great concern. By substituting Pb by Sn at B site, MASnI3 (1.3 eV) exhibits a smaller bandgap than MAPbI3 (1.55 eV) [53]. For the X site, when I ions in MASnI3 are doped with Br in different proportions, the bandgap of the materials can be modulated between 1.3 and 2.15 eV, and the corresponding absorptions are

Another important way to adjust the bandgap is to control the quantum confinement in the nanoscale. Compared with 0D quantum dots or 1D nanowires, 2D geometry provides a natural way to accurately control the thickness of quantum wells for perovskite halides, resulting in a confinement effect. Huang et al. found that a two-dimensional MAPbBr3 perovskite layer could be regulated by the concentration of oleic acid and the balance between surfactant and precursor in two phases [55]. Two-dimensional MAPbBr3 nano-sheets with different layers show different

Many perovskite photovoltaic cells have exhibited I-V hysteresis behavior, as shown in **Figure 4** [36, 56, 57]. Ion migration is thought to be an origin of the photocurrent hysteresis. Low formation energy of ionic defects combined with low activation energy for ion migration enables easy and fast ion migration in perovskite halide materials. Although raising potential stability is an issue in HP solar cells, the ion migration combined with the excellent optical and electrical properties of the material also provides an opportunity for new devices such as optically controlled

HPs are good ionic conductors with fast ion migration ability. There are many factors affecting ion migration, such as component ions, defects, cation rotation, etc. We first briefly introduce the defect ions in perovskite crystals. HPs possess various intrinsic point defects, such as vacancies, interstitial defects, and antisite defects. Shao et al. found that ion migration at grain or grain boundary of MAPbI3 perovskite membrane is different [58]. Ion migration in perovskite membranes can be regulated by the introduction of other foreign substances into grain boundaries, such as large fullerene derivatives (PC60BM) or small chloride ions [59, 60]. The modulation of ion migration is desirable for the development of highperformance perovskite-based optically adjustable resistors and synaptic devices [60]. Perovskite has been proven as an excellent ion conductor. Because of the ion motion, the semiconductor material can be changed from p-doped to n-doped.

representative absorption spectra and photoluminescence spectra [55].

in the MAPbI3 with an ethylamine, a propylamine, a long-chain alkyl, or

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

between 950 and 650 nm [54].

memory and switched diodes.

**3.2 Ion migration**

grain size [50].

the MA+

*Perovskite Materials for Resistive Random Access Memories DOI: http://dx.doi.org/10.5772/intechopen.86849*

*Perovskite Materials, Devices and Integration*

**3. Perovskite halide memory devices**

cation, for example, methylammonium (MA<sup>+</sup>

film uniformity in a large scale is another technical issue.

the device performance [9]. An effective interfacial modification is commonly required to solve this problem. For the FTJ-based devices, the tunneling current tuned by the polarization is normally remarkably small, which hinders its actual application. In addition, the ferroelectric layer is usually ultrathin, and how to maintain the ultrathin

In recent years, halide perovskites (HPs) have become a star material due to its excellent optical and charge transport properties. The rapid advance in powerconversion efficiency (PCE) of perovskite solar cells has exceeded by 20% [34], and the simple and solution-based preparation enables low-cost production. HPs have excellent electron migration ability and good optical absorption. With the development of HPs, the hysteresis in the current voltage curve was observed and described [35]. It is found that the hysteresis has a strong dependence on the voltage scanning rate and transient response. Ion migration is thought to be a possible origin of the slow response [36]. This discovery paves the way for HPs' applications in other electronic devices, for example, resistive switching memory (memristors) [37–39], field-effect transistors [40–42], and artificial synapse devices [43, 44]. Owing to its unique features and manufacturing advantages, rapid progress has been made, and HPs are considered as a promising candidate for the next generation of electronic

The perovskite material is an ABX3 compound with a 3D framework (**Figure 3a**), where A is a monovalent cation. A-site can be an inorganic or organic

Sn) and X is a halide such as Cl, Br, or I. Based on the composition flexibility of

, CH3NH3

; B is a divalent cation, and X is an anion. B is typically Pb (also

+

), formamidinium (FA<sup>+</sup>

,

**142**

**Figure 3.**

devices [45, 46].

HC(NH2)

**3.1 Tunable bandgap**

2+), or Cs+

*Copyright 2019, American Chemical Society.*

*(a) Crystal structure of HPs [47]. (b) A schematic diagram of device structure halide perovskite memory [48]. (c) FETs based on CsPbBr3 and fabrication method [47]. (d) Biological synapse compared to artificial synapses [49]. (a, c) Reproduced with permission [47]. Copyright 2015, American Chemical Society. (b) Reproduced with permission [48]. Copyright 2018, American Chemical Society. (d) Reproduced with permission [49].* 

HPs, the bandgap can be tuned by replacing elements at each position. In addition, the bandgap can be tuned by controlling the crystalline structure of HPs and the grain size [50].

It is a feasible method to change the length of the bonds between A and B/C sites. In theory, the crystal lattice of the perovskite ABX3 is expandable, and the gap of the forbidden band is narrow. For example, the material obtained by replacing the MA+ in the MAPbI3 with an ethylamine, a propylamine, a long-chain alkyl, or an arylamine cation is generally a two-dimensional layered structure. The length of alkylammonium cations at position A was reported in 1990 by Calabrese et al., and the synthesized HPs demonstrated a two-dimensional layered structure [51]. With the increase of the length of cation at site A, the maximum absorption peak is red-shifted from 390 to 450 nm [51]. The modulation of the perovskite bandgap by the substitution of A-site has been demonstrated by the density functional theory (DFT), showing an obvious change, i.e., FA (1.5 eV), MA (1.55 eV), and Cs (1.73 eV) [52, 53]. The angle of B-X-B bond in perovskite structure plays an important role in regulating the bandgap of perovskite materials. Therefore, the change of different metal ions (B) to regulate the structure and properties of perovskite materials is also of great concern. By substituting Pb by Sn at B site, MASnI3 (1.3 eV) exhibits a smaller bandgap than MAPbI3 (1.55 eV) [53]. For the X site, when I ions in MASnI3 are doped with Br in different proportions, the bandgap of the materials can be modulated between 1.3 and 2.15 eV, and the corresponding absorptions are between 950 and 650 nm [54].

Another important way to adjust the bandgap is to control the quantum confinement in the nanoscale. Compared with 0D quantum dots or 1D nanowires, 2D geometry provides a natural way to accurately control the thickness of quantum wells for perovskite halides, resulting in a confinement effect. Huang et al. found that a two-dimensional MAPbBr3 perovskite layer could be regulated by the concentration of oleic acid and the balance between surfactant and precursor in two phases [55]. Two-dimensional MAPbBr3 nano-sheets with different layers show different representative absorption spectra and photoluminescence spectra [55].

### **3.2 Ion migration**

Many perovskite photovoltaic cells have exhibited I-V hysteresis behavior, as shown in **Figure 4** [36, 56, 57]. Ion migration is thought to be an origin of the photocurrent hysteresis. Low formation energy of ionic defects combined with low activation energy for ion migration enables easy and fast ion migration in perovskite halide materials. Although raising potential stability is an issue in HP solar cells, the ion migration combined with the excellent optical and electrical properties of the material also provides an opportunity for new devices such as optically controlled memory and switched diodes.

HPs are good ionic conductors with fast ion migration ability. There are many factors affecting ion migration, such as component ions, defects, cation rotation, etc. We first briefly introduce the defect ions in perovskite crystals. HPs possess various intrinsic point defects, such as vacancies, interstitial defects, and antisite defects. Shao et al. found that ion migration at grain or grain boundary of MAPbI3 perovskite membrane is different [58]. Ion migration in perovskite membranes can be regulated by the introduction of other foreign substances into grain boundaries, such as large fullerene derivatives (PC60BM) or small chloride ions [59, 60]. The modulation of ion migration is desirable for the development of highperformance perovskite-based optically adjustable resistors and synaptic devices [60]. Perovskite has been proven as an excellent ion conductor. Because of the ion motion, the semiconductor material can be changed from p-doped to n-doped.

#### **Figure 4.**

*A typical I-V hysteresis behavior of perovskite solar cells, with forward bias to short circuit sweep (FB-SC) and short circuit to forward bias sweep (SC-FB) [36]. Reproduced with permission [36]. Copyright 2014, American Chemical Society.*

By applying external bias, the device structure can be changed from p-i-n structure to n-i-p structure, thus gradually changing the resistance of the device. This memory characteristic of perovskite materials can simulate the signal processing, learning, and memory functions of the nervous system [61]. Perovskite memristors can reduce the energy consumption required for the primary signal transmission of artificial synaptic devices to femto-Joule/(100 nm)<sup>2</sup> which is similar to the ultralow energy consumption required for primary signal transmission in biological synapses. Due to the excellent optical and electrical properties of perovskite materials, some biological functions read by optical signals have also been discovered [62].

### **3.3 Flexibility**

Flexible devices have enormous potentials for applications in emerging areas such as wearable electronics, portable chargers, remote power supplies, automobiles, and aircrafts. The fabrication of the substrate is very important for the flexibility of the final device, and the flexible device based on the polymer substrate is usually needed, resulting in general processing, and manufacturing only in lowtemperature environments cannot withstand high-temperature processes. But HP materials do not require high temperatures and can be processed at low temperatures, and HPs provide mechanical flexibility. These make HPs a great advantage in flexible device applications (**Figure 5**). For typical HPs, MAPbBr3, they have weak interactions between organic elements. This combination is relatively weak, so the shear between perovskite surfaces is easy to occur, which explains why this perovskite can provide elasticity under mechanical deformation. The annealing temperature of HPs is generally only one hundred degrees. Therefore, high flexible polymer substrates can be used in HP-based flexible devices because of the low processing temperature. Many repeated bending tests of HP solar cells and storage devices have been reported. These studies show that the materials have a good mechanical flexibility.

**145**

memory devices.

**Figure 5.**

**3.4 Thin film preparation methods**

great importance in the fabrication process.

*Perovskite Materials for Resistive Random Access Memories*

*permission [37]. Copyright 2016, American Chemical Society.*

HP-based flexible resistive switch storage device has been fabricated on a plastic substrate. After more than 100 times of bending radius of 1.5 cm, the storage device still has electrical performance (**Figure 5b**) [37]. The first fiber-shaped perovskite memristor was developed in 2016 [64]. In particular, fiber morphology is expected to promote the application of perovskite materials in wearable memory and computing device. Therefore, thanks to the good mechanical and electrical reliability, HP-based devices are very promising for the next-generation flexible

*(a) Photograph of a flexible RRAM device with the Al/CsPbBr3/PEDOT:PSS/ITO structure [63]. (b) Memory device based on flexible substrate of the Au/perovskite/ITO structure and I-V characteristics [37]. (a) Reproduced with permission [63]. Copyright 2017, American Chemical Society. (b) Reproduced with* 

In the field of perovskite solar cells, it is necessary to improve the photoelectric

In the early preparation of the solar cell, the perovskite precursor solution is usually spin-coated on the hydrophilic TiO2 layer, and due to the hydrophilicity, the perovskite is easily deposited on the TiO2 [67, 68]. Resistive switch memory and logic device is a kind of device with an insulating layer sandwiched between two metal layers. Thus when we use a solution method to prepare HP layers for the memristor devices, the perovskite precursor solution needs to be deposited on the hydrophobic metal layer. However, it is difficult to use the solution method to deposit a HP film on a hydrophobic metal surface. Because of the hydrophobicity of metal electrodes, for example, a simple spin coating method of MAPbI3 precursor solution may produce island growth on the metal surface. One-step spin coating therefore is not suitable for the fabrication of memory and logic device structures without interfacial modification. In order to solve this problem, the surface of metal electrode is usually treated with ultraviolet ozone (UVO) or O2 plasma to change the hydrophobicity of metal electrode. However, it is still not easy for HP thin films to get a good uniformity, both in one-step spin coating and two-step spin coating. This mainly originates from the difference between the general perovskite layer annealing temperature and the solvent boiling point temperature. For example, the annealing temperature of MAPBI3 is generally 100–150°C, while for the solvent perovskite, such as γ-butyrolactone and N,N dimethylformamide, possessing high boiling points of 204 and 153°C, respectively [44]. Thus the nucleation during the

conversion efficiency of perovskite solar cells with good thin film preparation technology [65, 66]. Generally good perovskite thin films have smooth surface and large grain size with relatively few defects. In order to achieve high-performance devices, controlling the uniformity, thickness, and grain size of the HP layer is of

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

*Perovskite Materials for Resistive Random Access Memories DOI: http://dx.doi.org/10.5772/intechopen.86849*

**Figure 5.**

*Perovskite Materials, Devices and Integration*

By applying external bias, the device structure can be changed from p-i-n structure to n-i-p structure, thus gradually changing the resistance of the device. This memory characteristic of perovskite materials can simulate the signal processing, learning, and memory functions of the nervous system [61]. Perovskite memristors can reduce the energy consumption required for the primary signal

*A typical I-V hysteresis behavior of perovskite solar cells, with forward bias to short circuit sweep (FB-SC) and short circuit to forward bias sweep (SC-FB) [36]. Reproduced with permission [36]. Copyright 2014, American* 

similar to the ultralow energy consumption required for primary signal transmission in biological synapses. Due to the excellent optical and electrical properties of perovskite materials, some biological functions read by optical signals have also

Flexible devices have enormous potentials for applications in emerging areas such as wearable electronics, portable chargers, remote power supplies, automobiles, and aircrafts. The fabrication of the substrate is very important for the flexibility of the final device, and the flexible device based on the polymer substrate is usually needed, resulting in general processing, and manufacturing only in lowtemperature environments cannot withstand high-temperature processes. But HP materials do not require high temperatures and can be processed at low temperatures, and HPs provide mechanical flexibility. These make HPs a great advantage in flexible device applications (**Figure 5**). For typical HPs, MAPbBr3, they have weak interactions between organic elements. This combination is relatively weak, so the shear between perovskite surfaces is easy to occur, which explains why this perovskite can provide elasticity under mechanical deformation. The annealing temperature of HPs is generally only one hundred degrees. Therefore, high flexible polymer substrates can be used in HP-based flexible devices because of the low processing temperature. Many repeated bending tests of HP solar cells and storage devices have been reported. These studies show that the materials have a good

which is

transmission of artificial synaptic devices to femto-Joule/(100 nm)<sup>2</sup>

**144**

mechanical flexibility.

been discovered [62].

**3.3 Flexibility**

**Figure 4.**

*Chemical Society.*

*(a) Photograph of a flexible RRAM device with the Al/CsPbBr3/PEDOT:PSS/ITO structure [63]. (b) Memory device based on flexible substrate of the Au/perovskite/ITO structure and I-V characteristics [37]. (a) Reproduced with permission [63]. Copyright 2017, American Chemical Society. (b) Reproduced with permission [37]. Copyright 2016, American Chemical Society.*

HP-based flexible resistive switch storage device has been fabricated on a plastic substrate. After more than 100 times of bending radius of 1.5 cm, the storage device still has electrical performance (**Figure 5b**) [37]. The first fiber-shaped perovskite memristor was developed in 2016 [64]. In particular, fiber morphology is expected to promote the application of perovskite materials in wearable memory and computing device. Therefore, thanks to the good mechanical and electrical reliability, HP-based devices are very promising for the next-generation flexible memory devices.

## **3.4 Thin film preparation methods**

In the field of perovskite solar cells, it is necessary to improve the photoelectric conversion efficiency of perovskite solar cells with good thin film preparation technology [65, 66]. Generally good perovskite thin films have smooth surface and large grain size with relatively few defects. In order to achieve high-performance devices, controlling the uniformity, thickness, and grain size of the HP layer is of great importance in the fabrication process.

In the early preparation of the solar cell, the perovskite precursor solution is usually spin-coated on the hydrophilic TiO2 layer, and due to the hydrophilicity, the perovskite is easily deposited on the TiO2 [67, 68]. Resistive switch memory and logic device is a kind of device with an insulating layer sandwiched between two metal layers. Thus when we use a solution method to prepare HP layers for the memristor devices, the perovskite precursor solution needs to be deposited on the hydrophobic metal layer. However, it is difficult to use the solution method to deposit a HP film on a hydrophobic metal surface. Because of the hydrophobicity of metal electrodes, for example, a simple spin coating method of MAPbI3 precursor solution may produce island growth on the metal surface. One-step spin coating therefore is not suitable for the fabrication of memory and logic device structures without interfacial modification. In order to solve this problem, the surface of metal electrode is usually treated with ultraviolet ozone (UVO) or O2 plasma to change the hydrophobicity of metal electrode. However, it is still not easy for HP thin films to get a good uniformity, both in one-step spin coating and two-step spin coating. This mainly originates from the difference between the general perovskite layer annealing temperature and the solvent boiling point temperature. For example, the annealing temperature of MAPBI3 is generally 100–150°C, while for the solvent perovskite, such as γ-butyrolactone and N,N dimethylformamide, possessing high boiling points of 204 and 153°C, respectively [44]. Thus the nucleation during the

substrate annealing could be very slow, which tends to achieve poor film morphology, such as cracks or even pores. Anti-solvent engineering can be applied in spin coating process to eliminate this issue. In anti-solvent engineering, toluene, chloroform, and other substances are often used as anti-solvents. Because they are insoluble to perovskite, when anti-solvent is added, the anti-solvent begins to diffuse and permeate into HPs solution. It is helpful for rapid nucleation. Anti-solvent engineering has been successfully used in the fabrication of HP-based flexible resistive switch memory [37]. However, the use of anti-solvent engineering will also bring some problems. With the addition of anti-solvent, it gradually begins to diffuse and permeate in HP solution. However, it is not possible for the anti-solvent to diffuse and penetrate uniformly throughout the perovskite film, which may result in a large distribution of the perovskite crystal size throughout the film. In order to prepare more uniform membranes, it is usually necessary to add additives such as alkane dimercaptan to control the crystallization kinetics of perovskite [69].

## **3.5 HPs for resistive switching memories**

With the advent of the information age and the rapid development of the Internet, the information that needs to be stored has been explosively increased, and the traditional storage equipment is more and more difficult to meet the demand. As a new-generation storage device, the memristor has great potential in the field of storage. In terms of storage performance, excellent memory devices need to have the advantages of fast working time, long service life, low power consumption, and low cost.

For memristor applications, many materials have been used, from organic materials and binary metal oxides to perovskite halide. Among them, metal oxide-based resistive switch devices have been extensively studied and applied in many fields. However, the technology has many demerits, such as high-power consumption and complicated fabrication process, which is not suitable for fabrication of flexible/ wearable devices. As discussed above, perovskite halides are an ideal alternative to fabricate flexible devices [46].

For example, the change in the resistance switching for the MAPbI3 memristor is a filament-type mechanism with the direct reaction of the charge carriers with the defects [39]. For RRAMs, fast charge transfer can reduce energy consumption. In HPs, the carrier transport capacity can be enhanced with appropriate concentrations of defects. For instance, doping MAPbI3 with Br reduces the "SET" voltage, thereby reducing the power consumption of the device. This is because the activation energy of ion migration with Br vacancies is smaller than that with I. Thus, the HRS to LRS switching energy is reduced, and the switching response is accelerated.

At mentioned above, anti-solvent engineering has been utilized in the preparation of HP thin Films. The MAPbI3 thin films treated with toluene as anti-solvent exhibit extremely low electric field about 3.25 × 103 V/cm and high switch-specific resistance switching behavior [70].

As a low-cost material, HPs have a great potential for the development of wearable and portable devices. Yan et al. developed the first fiber-shaped perovskite memristor [64]. In particular, fiber morphology is expected to promote the application of perovskite materials in wearable memory and computing device.

As the volume of information increases, devices that can store more data in the same size are the trend in the future. So, it is important to develop the memory device into a device with high storage density. Hwang et al. fabricated MAPbI3 layer for nano-RRAM devices on 250 nm perforated silicon wafers by vapor deposition [71]. The device has the characteristics of bipolar resistance switch, low operating voltage, high switching speed (200 ns), high durability, and high data retention

**147**

**Figure 6.**

*Chemical Society.*

*Perovskite Materials for Resistive Random Access Memories*

area device fabrication for high-density memory devices [71].

HP-based RRAMs are compared, as shown in **Table 1**.

s). In addition, the continuous vapor deposition technology is extended

to MAPbI3 memristor with a cross-point array structure. This method enables large

All-inorganic perovskite halides, such as CsPbBr3, have also demonstrated as working flexible nonvolatile memories, with a filament-type RS mechanism (**Figure 6a**) [63]. CsPbBr3 quantum dots are also developed for the memory cells [73, 74]. Besides, due to lead in HPs being a component that pollutes the environment and is harmful to humans, it is also necessary to develop lead-free devices. Han et al. successfully fabricated RRAMs based on lead-free inorganic cesium iodide (CsSnI3) perovskite material, as shown in **Figure 6b** [72]. Some typical

In addition to the traditional perovskite halides with ABX3 structure, other new types of materials have also been utilized in memory devices. 2D perovskite is another promising candidate for RRAM. The conductivity of HPs is low, but it has a good carrier transport ability. At present, most of the HP RRAMs are based on 3D MAPbX3 and some 2D Ruddlesden-Popper (RP) phase perovskite. 2D perovskite material has high Schottky barrier, 2D anisotropic structure, and electrothermal activation energy characteristics. Compared with 3D perovskite devices, the off current of 2D perovskite devices can be greatly reduced. Tian et al. reported the utilization of single-crystalline 2D (PEA)2PbBr4 and graphene for RRAM [76]. The two sides of 2D HPs are entrapped by graphene and Au, respectively. Due to the low conductivity of 2D HPs caused by multilayer organic ligands, there is no leakage current channel in perovskite grain boundaries of 2D HPs. The off current is limited to 1 pA. It is proven that the switching behavior has good reproducibility by switching devices at 10 pA program current circulate 100 times. Cheng et al. fabricated into Al/2D (CH3NH3)2PbI2(SCN)2 perovskite film/indium-tin oxide [78]. The RRAM shows ternary switching. The three states have a conductivity

, with long retention over 10,000 s. A transparent 2D perovskite

(C4H9NH3)2PbBr4 has also been developed for compliance-free multilevel RS devices [79]. Ultrathin bismuth halide Cs3Bi2I9 is also used as an electronic memory

For most exploited devices, the data only transiently converts the optical signal into a circuit under illumination, which requires the use of additional converters to further store the output signal and record the occurrence of optical stimuli. HPs have a very strong optical absorption ability, low exciton binding energy, and long life carrier transmission time, so HPs can display a short signal under illumination, which can be used in light-stimulated devices. Chen et al. first introduced the concept of floating gate flash memory and successfully fabricated HP floating gate photomemory with a multilevel memory behavior [81]. Wang et al. first introduced a photonic RRAM based on CsPbBr3 quantum dots. The CsPbBr3 quantum dot layer

*(a) Schematic drawing of the CsPbBr3-based flexible resistive switching memory [63]. (b) Schematic diagram of the Ag or Au/PMMA/CsSnI3/Pt/SiO2/Si vertical stack structure [72]. (a) Reproduced with permission [63]. Copyright 2017, American Chemical Society. (b) Reproduced with permission [72]. Copyright 2019, American* 

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

time (>105

ratio of 1:103

:107

device with a typical bipolar RS behavior [80].

*Perovskite Materials, Devices and Integration*

**3.5 HPs for resistive switching memories**

consumption, and low cost.

fabricate flexible devices [46].

exhibit extremely low electric field about 3.25 × 103

resistance switching behavior [70].

substrate annealing could be very slow, which tends to achieve poor film morphology, such as cracks or even pores. Anti-solvent engineering can be applied in spin coating process to eliminate this issue. In anti-solvent engineering, toluene, chloroform, and other substances are often used as anti-solvents. Because they are insoluble to perovskite, when anti-solvent is added, the anti-solvent begins to diffuse and permeate into HPs solution. It is helpful for rapid nucleation. Anti-solvent engineering has been successfully used in the fabrication of HP-based flexible resistive switch memory [37]. However, the use of anti-solvent engineering will also bring some problems. With the addition of anti-solvent, it gradually begins to diffuse and permeate in HP solution. However, it is not possible for the anti-solvent to diffuse and penetrate uniformly throughout the perovskite film, which may result in a large distribution of the perovskite crystal size throughout the film. In order to prepare more uniform membranes, it is usually necessary to add additives such as alkane dimercaptan to control the crystallization kinetics of perovskite [69].

With the advent of the information age and the rapid development of the Internet, the information that needs to be stored has been explosively increased, and the traditional storage equipment is more and more difficult to meet the demand. As a new-generation storage device, the memristor has great potential in the field of storage. In terms of storage performance, excellent memory devices need to have the advantages of fast working time, long service life, low power

For memristor applications, many materials have been used, from organic materials and binary metal oxides to perovskite halide. Among them, metal oxide-based resistive switch devices have been extensively studied and applied in many fields. However, the technology has many demerits, such as high-power consumption and complicated fabrication process, which is not suitable for fabrication of flexible/ wearable devices. As discussed above, perovskite halides are an ideal alternative to

For example, the change in the resistance switching for the MAPbI3 memristor is a filament-type mechanism with the direct reaction of the charge carriers with the defects [39]. For RRAMs, fast charge transfer can reduce energy consumption. In HPs, the carrier transport capacity can be enhanced with appropriate concentrations of defects. For instance, doping MAPbI3 with Br reduces the "SET" voltage, thereby reducing the power consumption of the device. This is because the activation energy of ion migration with Br vacancies is smaller than that with I. Thus, the HRS to LRS switching energy is reduced, and the switching response is accelerated. At mentioned above, anti-solvent engineering has been utilized in the preparation of HP thin Films. The MAPbI3 thin films treated with toluene as anti-solvent

As a low-cost material, HPs have a great potential for the development of wearable and portable devices. Yan et al. developed the first fiber-shaped perovskite memristor [64]. In particular, fiber morphology is expected to promote the applica-

As the volume of information increases, devices that can store more data in the same size are the trend in the future. So, it is important to develop the memory device into a device with high storage density. Hwang et al. fabricated MAPbI3 layer for nano-RRAM devices on 250 nm perforated silicon wafers by vapor deposition [71]. The device has the characteristics of bipolar resistance switch, low operating voltage, high switching speed (200 ns), high durability, and high data retention

tion of perovskite materials in wearable memory and computing device.

V/cm and high switch-specific

**146**

time (>105 s). In addition, the continuous vapor deposition technology is extended to MAPbI3 memristor with a cross-point array structure. This method enables large area device fabrication for high-density memory devices [71].

All-inorganic perovskite halides, such as CsPbBr3, have also demonstrated as working flexible nonvolatile memories, with a filament-type RS mechanism (**Figure 6a**) [63]. CsPbBr3 quantum dots are also developed for the memory cells [73, 74]. Besides, due to lead in HPs being a component that pollutes the environment and is harmful to humans, it is also necessary to develop lead-free devices. Han et al. successfully fabricated RRAMs based on lead-free inorganic cesium iodide (CsSnI3) perovskite material, as shown in **Figure 6b** [72]. Some typical HP-based RRAMs are compared, as shown in **Table 1**.

In addition to the traditional perovskite halides with ABX3 structure, other new types of materials have also been utilized in memory devices. 2D perovskite is another promising candidate for RRAM. The conductivity of HPs is low, but it has a good carrier transport ability. At present, most of the HP RRAMs are based on 3D MAPbX3 and some 2D Ruddlesden-Popper (RP) phase perovskite. 2D perovskite material has high Schottky barrier, 2D anisotropic structure, and electrothermal activation energy characteristics. Compared with 3D perovskite devices, the off current of 2D perovskite devices can be greatly reduced. Tian et al. reported the utilization of single-crystalline 2D (PEA)2PbBr4 and graphene for RRAM [76]. The two sides of 2D HPs are entrapped by graphene and Au, respectively. Due to the low conductivity of 2D HPs caused by multilayer organic ligands, there is no leakage current channel in perovskite grain boundaries of 2D HPs. The off current is limited to 1 pA. It is proven that the switching behavior has good reproducibility by switching devices at 10 pA program current circulate 100 times. Cheng et al. fabricated into Al/2D (CH3NH3)2PbI2(SCN)2 perovskite film/indium-tin oxide [78]. The RRAM shows ternary switching. The three states have a conductivity ratio of 1:103 :107 , with long retention over 10,000 s. A transparent 2D perovskite (C4H9NH3)2PbBr4 has also been developed for compliance-free multilevel RS devices [79]. Ultrathin bismuth halide Cs3Bi2I9 is also used as an electronic memory device with a typical bipolar RS behavior [80].

For most exploited devices, the data only transiently converts the optical signal into a circuit under illumination, which requires the use of additional converters to further store the output signal and record the occurrence of optical stimuli. HPs have a very strong optical absorption ability, low exciton binding energy, and long life carrier transmission time, so HPs can display a short signal under illumination, which can be used in light-stimulated devices. Chen et al. first introduced the concept of floating gate flash memory and successfully fabricated HP floating gate photomemory with a multilevel memory behavior [81]. Wang et al. first introduced a photonic RRAM based on CsPbBr3 quantum dots. The CsPbBr3 quantum dot layer

#### **Figure 6.**

*(a) Schematic drawing of the CsPbBr3-based flexible resistive switching memory [63]. (b) Schematic diagram of the Ag or Au/PMMA/CsSnI3/Pt/SiO2/Si vertical stack structure [72]. (a) Reproduced with permission [63]. Copyright 2017, American Chemical Society. (b) Reproduced with permission [72]. Copyright 2019, American Chemical Society.*


#### **Table 1.**

*Summary of hybrid perovskite RRAMs in this review.*

is sandwiched by two PMMA layers. Silver is selected as the top electrode by thermal evaporation [82]. In the absence of light, the device displays a bipolar resistive switch memory. By inputting the light field and electric field signals, the current will be used as the output signal to realize the switching logic operation.

The traditional von Neumann architecture requires a large amount of data transmission directly in CPU and memory (memory wall). This leads to increased power consumption. In order to solve this problem, Tian et al. proposed a fully distributed architecture based on optical synapse. The optical synapse based on layered 2D (PEA)2PbI4 perovskite structure was prepared [83]. This 2D perovskite-type optical synapse is similar to the biologic optical synapse with light-induced excitation/inhibition. Based on the unique optical gate control effect, the ultrahigh light response rate can reach 730 a/w. Lead-free 2D perovskite was also utilized for the first time in the study of flexible optical synaptic devices [84]. A flexible optical synapse based on 2D perovskite (PEA)2SnI4 can mimic the short-term plasticity of biological synapses.

In addition to the single device operation, we should also pay attention to crossarray arrangement of RRAMs. A large number of RRAMs can be connected to each other in micro-space to form a cross-array structure. This architecture combines the memory advantage of the RRAMs and the massively parallel processing of the cross array. Cross arrays exhibit the characteristics of large-scale parallel processing, distributed information storage, self-organization, self-adaptation, etc. RRAM cross array provides a more convenient storage structure for binary images and a new storage scheme for gray-scale images. Hwang et al. prepared homogeneous perovskite thin films by sequential evaporation deposition and then prepared 16 × 16 cross-point array of RRAM [71]. The I-V characteristics of the memory cells show a variation among different points, while the setting voltages remain similar, and the on/off ratios are large for all devices. The memory characteristics prove the feasibility of HPs in the application of high-density cross-point memory. Kang et al. fabricated perovskite RRAM devices with high yield in 8 × 8 cross-bar arrays using solution-treated perovskite films [77]. Among the 64 memory cells, 55 cells are functional. These results are of great significance for the practical perovskite storage equipment with low cost and high density through a simple solution.

## **4. Conclusions**

In this chapter, we have outlined an overview of the application of perovskite oxides and perovskite halides in memory devices. In the new era, artificial

**149**

*Perovskite Materials for Resistive Random Access Memories*

these promising materials for the next-generation electronics.

The authors declare that they have no conflict of interest.

**Acknowledgements**

University (grant: 2019-24).

**Conflict of interest**

**Author details**

Jiaqi Zhang\* and Wubo Li

Engineering, Jilin University, Changchun, China

provided the original work is properly cited.

\*Address all correspondence to: zhangjiaqi@jlu.edu.cn

intelligence and IoTs are dramatically developing. Correspondingly, memory cells are getting more and more important, especially in low-power information storage and in neuromorphic computing. Although already developing for around two decades, perovskite oxides are still one of the most promising materials for RRAM owing to its high-endurance, chemically stable, and high-speed operation. However, more efforts are expected for perovskite oxide-based memories. Technologically, improving the endurance of the RS is still required for better actual application. Fundamentally, the basic operational mechanism of perovskite oxide RRAM device needs further investigation, especially considering the strong electron correlation system. For the perovskite halide, as a rising star, it has exhibited a great potential in the application of memristors. Flexible devices, low-cost fabrication, compositional flexibility, and excellent optoelectronic properties enable the perovskite halides to obtain potential application into wearable memory devices and artificial synapse. However, the film quality of HPs should be further improved, because the memory device performance is significantly dependent on the film uniformity. In addition, the intrinsic stability issue needs to be addressed by intended doping and interfacial passivation. Overall, further investigation is required to fulfill the expectation on

JZ acknowledges the National Science Foundation of Jilin Province (grant: 20190201208JC), the Science and Technology Foundation of Department of Education, Jilin Province (grant: JJKH20190136KJ), and the Open Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin

Key Laboratory of Automobile Materials of MOE, College of Materials Science and

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*DOI: http://dx.doi.org/10.5772/intechopen.86849*

*Perovskite Materials for Resistive Random Access Memories DOI: http://dx.doi.org/10.5772/intechopen.86849*

intelligence and IoTs are dramatically developing. Correspondingly, memory cells are getting more and more important, especially in low-power information storage and in neuromorphic computing. Although already developing for around two decades, perovskite oxides are still one of the most promising materials for RRAM owing to its high-endurance, chemically stable, and high-speed operation. However, more efforts are expected for perovskite oxide-based memories. Technologically, improving the endurance of the RS is still required for better actual application. Fundamentally, the basic operational mechanism of perovskite oxide RRAM device needs further investigation, especially considering the strong electron correlation system. For the perovskite halide, as a rising star, it has exhibited a great potential in the application of memristors. Flexible devices, low-cost fabrication, compositional flexibility, and excellent optoelectronic properties enable the perovskite halides to obtain potential application into wearable memory devices and artificial synapse. However, the film quality of HPs should be further improved, because the memory device performance is significantly dependent on the film uniformity. In addition, the intrinsic stability issue needs to be addressed by intended doping and interfacial passivation. Overall, further investigation is required to fulfill the expectation on these promising materials for the next-generation electronics.

## **Acknowledgements**

*Perovskite Materials, Devices and Integration*

**Table 1.**

**Device structure Set voltage** 

*Summary of hybrid perovskite RRAMs in this review.*

is sandwiched by two PMMA layers. Silver is selected as the top electrode by thermal evaporation [82]. In the absence of light, the device displays a bipolar resistive switch memory. By inputting the light field and electric field signals, the current

The traditional von Neumann architecture requires a large amount of data transmission directly in CPU and memory (memory wall). This leads to increased power consumption. In order to solve this problem, Tian et al. proposed a fully distributed architecture based on optical synapse. The optical synapse based on layered 2D (PEA)2PbI4 perovskite structure was prepared [83]. This 2D perovskite-type optical synapse is similar to the biologic optical synapse with light-induced excitation/inhibition. Based on the unique optical gate control effect, the ultrahigh light response rate can reach 730 a/w. Lead-free 2D perovskite was also utilized for the first time in the study of flexible optical synaptic devices [84]. A flexible optical synapse based on 2D perovskite (PEA)2SnI4 can mimic the short-term plasticity of biological synapses. In addition to the single device operation, we should also pay attention to crossarray arrangement of RRAMs. A large number of RRAMs can be connected to each other in micro-space to form a cross-array structure. This architecture combines the memory advantage of the RRAMs and the massively parallel processing of the cross array. Cross arrays exhibit the characteristics of large-scale parallel processing, distributed information storage, self-organization, self-adaptation, etc. RRAM cross array provides a more convenient storage structure for binary images and a new storage scheme for gray-scale images. Hwang et al. prepared homogeneous perovskite thin films by sequential evaporation deposition and then prepared 16 × 16 cross-point array of RRAM [71]. The I-V characteristics of the memory cells show a variation among different points, while the setting voltages remain similar, and the on/off ratios are large for all devices. The memory characteristics prove the feasibility of HPs in the application of high-density cross-point memory. Kang et al. fabricated perovskite RRAM devices with high yield in 8 × 8 cross-bar arrays using solution-treated perovskite films [77]. Among the 64 memory cells, 55 cells are functional. These results are of great significance for the practical perovskite storage equipment with low cost and high density through a simple solution.

In this chapter, we have outlined an overview of the application of perovskite

oxides and perovskite halides in memory devices. In the new era, artificial

will be used as the output signal to realize the switching logic operation.

**[V]**

**On/off ratio**

Au/MAPbI3−xClx/FTO 0.8 10 1 × 104 102 [38] Ag/MAPbI3/Pt 0.13 106 1 × 104 400 [70] Ni/ZnO/CsPbBr3/FTO −0.95 105 1 × 104 — [75] Al/CsPbBr3/PEDOT:PSS/ITO/PET −0.6 102 — 50 [63] Ag/PMMA/CsPbI3/Pt 0.18 106 — 300 [62] Graphene/PEA2PbBr4/Au 2.8 10 1 × 103 100 [76] Ag/PMMA/CsSnI3/Pt/SiO2/Si 0.13 103 7 × 103 600 [72] Au/MAPbI3/Au 0.96 108 1 × 104 1000 [77]

**Retention [s]**

**Endurance [cycles]**

**Ref.**

**148**

**4. Conclusions**

JZ acknowledges the National Science Foundation of Jilin Province (grant: 20190201208JC), the Science and Technology Foundation of Department of Education, Jilin Province (grant: JJKH20190136KJ), and the Open Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (grant: 2019-24).

## **Conflict of interest**

The authors declare that they have no conflict of interest.

## **Author details**

Jiaqi Zhang\* and Wubo Li Key Laboratory of Automobile Materials of MOE, College of Materials Science and Engineering, Jilin University, Changchun, China

\*Address all correspondence to: zhangjiaqi@jlu.edu.cn

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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**Chapter 9**

*Sajad Ahmad Dar*

**Abstract**

Osmium Containing Double

Spintronic Applications

been predicted in the temperature range of 0–1000 K.

mechanical behavior, thermodynamics

**1. Introduction**

**157**

**Keywords:** Ba2XOsO6 (X = Mg, Zn, Cd), spintronics, ferromagnetic, elastic,

The need of advanced materials with novel properties for industrial and technological use has strained the material community to have a deep and appropriate understanding of the periodic table elements, along with their combinations. Therefore, materials community consequently observes the vital changes in innovative designing of novel materials. A tremendous increase in simulation power, along with algorithmic improvements in quantum theory allows one to have

Perovskite Ba2XOsO6 (X = Mg,

Zn, Cd): Important Candidates for

In this chapter, Osmium-based double perovskites Ba2XOsO6(X = Mg, Zn, Cd) have been investigated for their magnetic structure, electronic, elastic, mechanical and thermodynamic belongings. These materials have been recently reported experimentally for their magnetic structure. Here, we report the first successful ab initio calculations on the physical properties of these materials. The structural optimization for these Ba2XOsO6(X = Mg, Zn, Cd) double perovskite compounds has been finalized within density functional theory via full potential linearized augmented plane wave (FP-LAPW) method. The structural investigation exposes the ferromagnetic phase stability of these compounds. The spin-polarized electronic and magnetic properties were calculated within generalized gradient approximation (GGA), Hubbard approximation (GGA+U) and modified Becke-Johnson approximation (mBJ). The electronic profile establishes the half-metallic nature for all the three compounds. The total spin magnetic moment was found to be an integer value of 2 μb. The elastic constants have been calculated and used to predict mechanical stuffs like Shear modulus (G), Poisson ratio (*v*) and anisotropic factor. The calculated B/G and Cauchy pressure (C12-C44) both characterize these materials as brittle. The thermodynamic parameters like heat capacity and Debye temperature have

Half-Metallic Ferromagnetic and

## **Chapter 9**
