**5. Property enhancement by doping**

Doping, in general is an effective approach to enhance the properties of semiconductor materials by intentionally introducing impurity or heteroatom into the target lattices. The doping with anions and cations of different charge is also known to modify their micro structural, electrical, and magnetic properties. Considering the fact that halide materials possess interesting properties, researchers recognized that introduction of other materials into the halide perovskite could potentially lead to relevant discoveries and applications, with this aim, over the past few years, different kind of dopant materials are introduced. Among them most relevant ones are discussed here [43].

### **5.1 Metal ion**

Due to the high abundance in the earth's crust, varying oxidation state and mostly nontoxic nature, doping by transition metals are the best choice to enhance the properties of perovskite materials. There are two methods to doping by metal ions in perovskite materials, alloying to partially replace the metal frame and inserting a small amount of transition metal in the lattices. Among these two insertions of small amount of transition metal ion into the perovskite is the preferred one, cause the energy transfer and charge carrier transfer processes between the dopant and perovskite crystals [44].

So far a large number of reports were being available on CsPbX3 perovskite nanocrystals doped with divalent transition metals like Mn2+, Co2+, Cu2+ and Zn2+. These metal ions play a role in eliminating the defects and distortions of perovskite crystals and exhibit dual color emission and efficient charge transfer. Among various methods available incorporating metal ion into perovskite material through hot injection is the feasible method, moreover, transition metals are economical and eco-friendly compared to other toxic materials and shows excellent properties without destroying the crystal structure [45]. Among these transition metals, Mn2+ need special attention due to its excellent properties. Mn-doping in nanocrystals increased the exciton luminescence and attribute the efficient energy transfer between exciton and host. Thus, for Mn doped CsPbX3 achieved up to 60% luminescence quantum yield [16]. MAPbBr3 the one of the most studied organic−inorganic perovskites show structural instabilities, which can be improved by doping with Zn metal ions. The Zn doped MAPbBr3 reveals excellent optoelectronic properties and environmental stability due to increased lattice strain which leads to the improved interaction between bonds [46]. Cs2SbAgCl6 and its Cu2+ doped perovskite shows well-ordered double perovskite cubic structure with excellent conductivity. The researchers proposed that Cu2+ doping creates cation defect, which leads to increased conductivity of double perovskite. In conclusion, the antimony−silver based double perovskite doped with copper ion exhibit desirable properties comparison to the bare perovskite in greater bandgap tunability and stability and has impact on their morphological, optical, electronic behaviors [47].

In addition to the transition metals, alkali and alkaline earth metals, other group metals are also used as dopant in perovskite materials. The possibility of doping with alkali metal ion like Rb was explored and the result shows that Rb ion doping suppress the forming of impurity phases, and also increase the lifetime of perovskite materials [43]. In search of other metal doping, it has been found that Na+ , in bulk, Cs2SbInCl6 bring an increase in PL emission by three order magnitude compared to pure double halide material with an optimum Na content of 40%. This is where explained as the result of improved crystal quality and increased rate of radiative recombination [17]. There is also a reported result of increased hole concentration and mobility by Na doping and promoted electron injection in devices by Li doping in solar cell application [43]. Alkaline earth metals, Ca, Sr., and Ba, are proposed to be able to enhance the properties of perovskite with doping and has remarkable impact on their morphological, optical, and electronic behaviors. Besides alkali and alkaline earth metals, metals like Al3+ can provide tremendous morphological control to improve the properties of halide perovskite. The incorporation of indium (In3+) has also been reported to influence morphology by facilitating preferential growth of grains in several orientations [48]. In addition to that, Bi-doped bulk crystals underwent a significant band gap narrowing and shows improved stability and excellent optoelectronic and magnetic properties.

#### **5.2 Lanthanides**

To date, several successful doping of inorganic or hybrid perovskite by metal ions have been reported. However, the emissions for transition metal ions are broad band and confined to specific wavelength region limiting their application to limited energy structure. Hence lanthanide ions or called rare earth elements would be the most suitable candidate for energy and optical applications, because they possess rich and unique optical properties and emissions are in wide range with sharp line from UV to infrared region. In addition to that moving from Ce to Lu, a gradual decrease in ionic radii of lanthanides provide varying electrical, magnetic and chemical features provide the opportunities to study the changes of doping. Various lanthanide doped halide perovskite are studied and successful doping of Ce3+,Sm3+, Eu3+, Tb3+, Dy3+, Er3+ and Yb3+ into the CsPbCl3 perovskite through hot injection methods are reported. Lanthanide doped double halide perovskite of the type Cs2AgInCl6 are also reported. For the lanthanide doped perovskite nanocrystals stable and tunable multi-color emission from visible to NIR regions are obtained [23]. However, few RE metals have been reported so far make it an attractive field. Eu3+ is reported to stabilize CsPbI3 thin films.

Various materials like main group metals, transition metals and rare earth metals have been successfully doped into perovskite nanocrystals, single crystal, and polycrystalline film, giving rise to enhanced properties of perovskite materials. The various properties obtained through doping technology including improved stability, improved quality of thin films with reduced defect and enlarged grain size found application in optoelectronic devices including LEDs and solar cells. It is evident that dopant engineering has emerged as one of the excellent tools to enhance the properties of perovskite. Despite the fact that doping improves the properties our understanding about the mechanism related to doped halide perovskite is still limited. And number of scientific issues in terms of synthesis, doping methods, and structure and property relation are remaining unsolved. To fully exploit the possibilities of doping, there are certain questions that are needed to be answered, such as the role of metal ions in crystallization, their doping capability, the true position of metal ions, and how these differ with altered perovskite compositions.

Although 3D halide perovskite has been the prime target of doping, lower dimensional perovskite including 2D halide perovskite could be the next frontier for doping study, since they provide a rich space to study the interaction of electrically-, optically-, or magnetically-active do pants with quantum confinement effects [43].

### **5.3 Small molecules**

Generally, in semiconductor devices, small molecules are used as dopant to enhance the properties of materials. The mechanism behind these doping is surface and interface charge transfer pathway. The small molecules like HATCN (Hexaazatriphenylenehexacarbonitrile), F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane) are used as dopant in low concentration in 2D perovskite materials. Also, molecules like cobaltocene and zethrene was used in inorganic perovskite materials and shows strong charge transfer from dopant to perovskite [44]. Even though doping with small molecule provide enhancement in property, formation of composite with small molecule is considered as more effective way to improve the properties of perovskite materials. The development of low cost, largest surface to volume ratio, low resistivity and high sensitivity along with ecofriendly perovskite-graphene based composite attain widespread attention in electrochemistry, sensing and optoelectrical applications. For example, CsPbBr3 QDs/GO composite, that is cesium perovskite quantum dots and graphene oxide composite were used as photo catalyst for artificial CO2 reduction. Quantum dot photo catalyst found wide attention, because of their large surface area and charge transfer mechanism. Moreover, the quantum confinement effect causes the shift in band position provides sufficient energy for photochemical reaction [49]. A perovskite and dual additive composite are reported in recent years, the PEO (polyethylene oxide) and TPBi (2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) blended perovskite act as the barrier for the excitonic dissociation at interface, which contribute to the increase in PL intensity. The possible reason for the change in PL is the smaller grain and higher surface coverage provided by the dual additive perovskite material, which can be possibly used as LEDs with high luminance and current efficiency [50].

## **5.4 Polymer**

Perovskite-polymer composite received great attention in recent years due to their combination of properties from polymers and perovskite. The technique involves the formation of perovskite and polymer matrix in one pot reaction, to avoid the complexity of separate preparation. Compared with other technique like coating and ligand cross linking, forming composite with polymers is relatively easy to handle. In addition to the stability, polymers provide other advantages like convenient device fabrication mechanical performance, and enhanced luminescent properties. Recently poly (methyl methacrylate) (PMMA) and perovskite composite based solar cell provide a power conversion efficiency of 22.1%. Along with this, a lot of composites with excellent stability is reported. Even though excellent applications are reported, some serious issues are also found with polymers blending with perovskite. The blending of perovskite with polymers may result into serious aggregation. The large polarity difference between polymers and perovskite material results the aggregation of perovskite material, which reduce the efficiency of perovskite material. Another issue is the stability, the preparation and storage of perovskite material required high attention, to optimize the property of composite material. The time-consuming blending

#### **Figure 4.**

*Illustration of one-pot strategy to prepare perovskite-polymer composites (CsPbBr3-polymer or CH3NH3PbBr3 polymer). a) Formation of perovskite crystals in bulk monomers. The photo taken under room light is for the emissive bulk styrene after adding precursors. b) the UV- or thermal- polymerized perovskite-polymer composites. Representative disks (under room and UV light) are shown in photos reprinted with permission from [51]. Copyright 2018 American Chemical Society.*

process, especially with high molecular weight polymer, result the decomposition of perovskite by moisture. The organic solvent used for dissolve polymers should be anhydrous to avoid the aggregation, and these solvents are not environment friendly and of high cost. Also needed to consider the fact that physical properties of polymers are different, hence it is necessary to know the characters of polymers before using as a composite with perovskite material. Commercially available polymers like PMMA, polystyrene (PS), polyethylenimine (PEI), polyvinylpyrrolidone (PVP), and poly (butyl methacrylate) (PBMA) are used with CSPbBr3 perovskite material to form composite. The one pot synthesis of perovskite-polymer composite is illustrated in **Figure 4** [51]. The PMMA and perovskite material have difference in polarity make them less efficient, hence butyl methylacrylate with long alkyl chain of similar polarities was selected to prepare CsPbBr3-PBMA shows deep green color than those of PMMA, indicating more stability. The organic halide perovskite-polymer composite of MAPbBr3-PMMA could also synthesize via injection of precursor solution into the bulk MMA. The PL intensity of both CsPbBr3-PMMA, CsPbBr3-PBMA could remain high as 70% and 78% for month indicating stability. A white LED device was prepared based on the green emissive composite with phosphor of red emission. Also, the diverse selection of monomer provides controlled mechanical properties and flexibility to the composite also enables the preparation of device in large area. Recently, CsPbBr3-PMMA was successfully prepared without using organic solvent provides a new direction to the perovskite-polymer composite synthesis and will broaden the use of polymer in perovskite science [51].
