**1. Introduction**

Organic-inorganic perovskites materials have emerged as a promising material for high-efficiency nanostructured devices such as light-emitting diodes (LEDs), detectors, field-effect transistors, and photovoltaic devices etc. [1, 2]. Organicinorganic perovskites have attracted extensive attention due to their promising optical and electronic properties, excellent crystallinity, adjustable bandgap, long charge diffusion length, electroluminescence, and conductivity [3, 4]. As the most fascinating new-generation photovoltaic materials, organic–inorganic perovskite due to their facile synthesis, low temperature deposition, and capability to make flexible devices has been considered as a vigorous component of the efficient, low-cost, lightweight and flexible Perovskite solar cells [3, 4]. Perovskite solar cells (PSCs) have rapidly become the leading edge of third generation 3G photovoltaic technologies [5, 6]. PSCs based on the organic inorganic perovskite materials have fascinated great consideration, with their power conversion efficiencies (PCEs) reaching 25.2% certified [7, 8]. Over the past several months, it has been observed

a surprising revolution and rapid progress in the field of emerging photovoltaic, with the understanding of highly efficient solar cells based on organic inorganic perovskite materials. This perovskite technology is now well-matched with the 1G and 2G technologies and is thus probably be embraced by the conventional photovoltaic community and industry [9, 10].

#### **1.1 Organic inorganic perovskite materials in solar cells**

The advent of organic inorganic perovskite based solar cells has resulted in rapid growth in photovoltaic history. Organic inorganic perovskite materials have recently, fascinated greater attention due to its outstanding light-harvesting features [7].

Organic-inorganic perovskite absorbers have appeared in the field of DSSCs since 2009. The first perovskite-sensitized DSSCs were developed by Kojima et al. [11] which obtained PCE of 3.13% using liquid electrolytes. However, continuous irradiation produced a photocurrent decay in an open cell when exposed to air. Later, the electron transporting layer (Titania) surface and perovskite processing were optimized, and in 2011, Im et al. [12] developed first stable PSC, using CH3NH3PbI3-based iodide liquid electrolyte offered a PCE of 6.5%. However, the perovskite nanocrystals dissolved in the liquid iodide electrolyte solution, and the cell degraded within 10 minutes. To avoid the problem of perovskite dissolution in an electrolytic solution, the liquid electrolyte was replaced by a solid in 2012, and a PCE of 9% was achieved showing good stability up to 500 h without significant losses [13, 14]. Afterward, Al2O3 an insulating network was used to substitute conducting nano porous TiO2. By using mixed MAPbI3-XClX as the sensitizer, an enhanced open-circuit voltage (VOC) and PCE (10.9%) was achieved [15]. In 2013, a successive deposition method for the perovskite layer within the porous metal oxide film was developed. The fabrication technique for solid-state mesoscopic solar cells greatly improved the reproducibility of cell performance and produced a high PCE of 15%. Many PSC devices are now attaining PCE > 20% since 2015 and 25% in 2019 [4, 16]. National Renewable Energy Laboratory (NREL), on 3rd August 2019 declared a new world record PCE of 25.2% for PSCs. This PCE value is improved up to ∼28% for perovskite-silicon tandem structures [4].

#### **1.2 Device architecture of perovskite solar cells**

PSC consists of a perovskite absorbing material sandwiched between electron transporting layer (ETL) and hole-transporting layer (HTL) along with the transparent conducting oxide substrate (FTO) and a top electrode such as gold, silver [17]. In PSCs, the effective charge separation and the light harvesting efficiency are significantly affected by the properties like particle size, porosity, surface area, surface morphology, band gap, thickness of semiconductor materials, and the nature of organometal halide perovskites [18].

The primary function of an **ETL** is to extract a photo generated electron from perovskite and then transfer to electrodes. The basic criterion for an ideal ETL is high optical transmittance, excellent electron mobility, high conductivity, and an appropriate work function. ETL also performs as hole-blocking layer (HBL) [19]. The configuration and the choice of ETL are essential factors to understand the electronic mechanisms in PSCs, which control processes such as carrier separation, extraction, transport, and the recombination. Hence, the configuration of device structure is critical to alter different materials for ETL, electrode contacts, and the barrier layer of insight these processes and mechanisms [18]. TiO2, which has a wide band gap, has been extensively studied as an efficient electron transport material

**73**

**Figure 1.**

*Representative architecture of PSC.*

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material*

inorganic HTMs for PSCs are NiO, CuSCN, CuI, CsSnI3 etc. [21, 25].

A PSC includes an organic-inorganic perovskite material as the light-harvesting active layer. Amongst the component's PSCs, perovskite materials perform a key role. Perovskite is comprised of earth abundant and inexpensive materials. It is processed at lower temperature rather via the printing techniques [26]. The organic−inorganic perovskites can exhibit appropriately good ambipolar charge transport and the primary functions of photovoltaic operation comprising light absorption, generation of charges, and transport of both electrons and holes. They perform both as efficient light absorbers and charge carriers [21]. The commonly used perovskites are Methylammonium lead triiodide (CH3NH3PbI3) and formamidinium lead triiodide (CH3(NH2)2PbI3) [27]. Moreover, The PSC architecture is

(ETM). Moreover, ZnO and other *n*-type semiconductors such as SnO2, Nb2O5 and BaSnO3 are frequently used as ETMs and are used in flexible perovskite solar

The HTL lies in the heart i.e. in between the metal electrode and perovskite of device. It plays a central-role in the PSC and extracts holes from the perovskite and transfer them to top-electrode. It avoids the direct contact of perovskite and top electrode [22]. For efficient hole transport, the highest occupied molecular orbit (HOMO) must match the valence band (VB) of perovskite materials. According to the chemical composition, HTMs in PSCs can be divided into two types: organic and inorganic HTMs. Spiro-OMeTAD is the most used organic HTM, which displays good penetration in perovskite and is an appropriate match with the VB energy of perovskite, though its hole mobility is not as superior as that of other organic HTMs. [21, 23]. Hence, in order to improve the hole mobility, polymers are doped with *p*-type (i.e., cobalt or Lithium salts) or some additives (i.e., bis(trifluoromethane) sulfonimide lithium, LiTFSI, and 4-tert-butyl pyridine, TBP) [21, 24]. Other organic materials reported as HTMs in PSCs are PTAA, PEDOT:PSS, P3HT etc. [25]. Inorganic *p*-type semiconductor materials, due to their advantages such as high hole mobility, wide band gap, and easy solvent treatment process as compared with organic HTMs exhibit the possibility to replace organic HTMs. The reported

*DOI: http://dx.doi.org/10.5772/intechopen.94104*

cells [20, 21].

represented in **Figure 1**.

1.mesoscopic

2.planar

The two main device architectures of PSC are

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material DOI: http://dx.doi.org/10.5772/intechopen.94104*

(ETM). Moreover, ZnO and other *n*-type semiconductors such as SnO2, Nb2O5 and BaSnO3 are frequently used as ETMs and are used in flexible perovskite solar cells [20, 21].

The HTL lies in the heart i.e. in between the metal electrode and perovskite of device. It plays a central-role in the PSC and extracts holes from the perovskite and transfer them to top-electrode. It avoids the direct contact of perovskite and top electrode [22]. For efficient hole transport, the highest occupied molecular orbit (HOMO) must match the valence band (VB) of perovskite materials. According to the chemical composition, HTMs in PSCs can be divided into two types: organic and inorganic HTMs. Spiro-OMeTAD is the most used organic HTM, which displays good penetration in perovskite and is an appropriate match with the VB energy of perovskite, though its hole mobility is not as superior as that of other organic HTMs. [21, 23]. Hence, in order to improve the hole mobility, polymers are doped with *p*-type (i.e., cobalt or Lithium salts) or some additives (i.e., bis(trifluoromethane) sulfonimide lithium, LiTFSI, and 4-tert-butyl pyridine, TBP) [21, 24]. Other organic materials reported as HTMs in PSCs are PTAA, PEDOT:PSS, P3HT etc. [25]. Inorganic *p*-type semiconductor materials, due to their advantages such as high hole mobility, wide band gap, and easy solvent treatment process as compared with organic HTMs exhibit the possibility to replace organic HTMs. The reported inorganic HTMs for PSCs are NiO, CuSCN, CuI, CsSnI3 etc. [21, 25].

A PSC includes an organic-inorganic perovskite material as the light-harvesting active layer. Amongst the component's PSCs, perovskite materials perform a key role. Perovskite is comprised of earth abundant and inexpensive materials. It is processed at lower temperature rather via the printing techniques [26]. The organic−inorganic perovskites can exhibit appropriately good ambipolar charge transport and the primary functions of photovoltaic operation comprising light absorption, generation of charges, and transport of both electrons and holes. They perform both as efficient light absorbers and charge carriers [21]. The commonly used perovskites are Methylammonium lead triiodide (CH3NH3PbI3) and formamidinium lead triiodide (CH3(NH2)2PbI3) [27]. Moreover, The PSC architecture is represented in **Figure 1**.

The two main device architectures of PSC are


*Perovskite and Piezoelectric Materials*

features [7].

photovoltaic community and industry [9, 10].

**1.1 Organic inorganic perovskite materials in solar cells**

to ∼28% for perovskite-silicon tandem structures [4].

**1.2 Device architecture of perovskite solar cells**

nature of organometal halide perovskites [18].

a surprising revolution and rapid progress in the field of emerging photovoltaic, with the understanding of highly efficient solar cells based on organic inorganic perovskite materials. This perovskite technology is now well-matched with the 1G and 2G technologies and is thus probably be embraced by the conventional

The advent of organic inorganic perovskite based solar cells has resulted in rapid growth in photovoltaic history. Organic inorganic perovskite materials have recently, fascinated greater attention due to its outstanding light-harvesting

Organic-inorganic perovskite absorbers have appeared in the field of DSSCs since 2009. The first perovskite-sensitized DSSCs were developed by Kojima et al. [11] which obtained PCE of 3.13% using liquid electrolytes. However, continuous irradiation produced a photocurrent decay in an open cell when exposed to air. Later, the electron transporting layer (Titania) surface and perovskite processing were optimized, and in 2011, Im et al. [12] developed first stable PSC, using CH3NH3PbI3-based iodide liquid electrolyte offered a PCE of 6.5%. However, the perovskite nanocrystals dissolved in the liquid iodide electrolyte solution, and the cell degraded within 10 minutes. To avoid the problem of perovskite dissolution in an electrolytic solution, the liquid electrolyte was replaced by a solid in 2012, and a PCE of 9% was achieved showing good stability up to 500 h without significant losses [13, 14]. Afterward, Al2O3 an insulating network was used to substitute conducting nano porous TiO2. By using mixed MAPbI3-XClX as the sensitizer, an enhanced open-circuit voltage (VOC) and PCE (10.9%) was achieved [15]. In 2013, a successive deposition method for the perovskite layer within the porous metal oxide film was developed. The fabrication technique for solid-state mesoscopic solar cells greatly improved the reproducibility of cell performance and produced a high PCE of 15%. Many PSC devices are now attaining PCE > 20% since 2015 and 25% in 2019 [4, 16]. National Renewable Energy Laboratory (NREL), on 3rd August 2019 declared a new world record PCE of 25.2% for PSCs. This PCE value is improved up

PSC consists of a perovskite absorbing material sandwiched between electron transporting layer (ETL) and hole-transporting layer (HTL) along with the transparent conducting oxide substrate (FTO) and a top electrode such as gold, silver [17]. In PSCs, the effective charge separation and the light harvesting efficiency are significantly affected by the properties like particle size, porosity, surface area, surface morphology, band gap, thickness of semiconductor materials, and the

The primary function of an **ETL** is to extract a photo generated electron from perovskite and then transfer to electrodes. The basic criterion for an ideal ETL is high optical transmittance, excellent electron mobility, high conductivity, and an appropriate work function. ETL also performs as hole-blocking layer (HBL) [19]. The configuration and the choice of ETL are essential factors to understand the electronic mechanisms in PSCs, which control processes such as carrier separation, extraction, transport, and the recombination. Hence, the configuration of device structure is critical to alter different materials for ETL, electrode contacts, and the barrier layer of insight these processes and mechanisms [18]. TiO2, which has a wide band gap, has been extensively studied as an efficient electron transport material

**72**

**Figure 1.** *Representative architecture of PSC.*

The conventional PSC consists of mesoscopic n-i-p structure and is the novel architecture of PSC devices which consists of an FTO, an electron transport layer (ETL), a mesoporous oxide layer such as TiO2, or SnO2, perovskite (light absorbing) layer, a hole transport layer (HTL), and an electrode layer. The mesoporous TiO2 layer played a significant role in the electron transfer process and as a scaffold providing mechanical support of the perovskite crystal. The use of mesoporous materials in PSC permit the perovskite material to adhere to the mesoporous metal oxide framework to increase the light-receiving area of the photosensitive material and results in improving the efficiency of the device. The mesoporous layer was usually less than 300 nm. The presently mesoporous structure of PSCs is one of the most common structures with a power conversion efficiency (PCE) greater than 20% [28]. The mesoscopic structure due to the fabrication ease and outstanding best efficiencies is the most extensively adopted in research labs. However, high temperature (˃450°C) sintering is required for mesoporous layer-based devices, which prevents the use of plastic substrates [29–31]. To overcome this problem, the planar perovskite solar cell was developed that showed comparable performance for mesoporous perovskite solar cell. Planar heterojunction PSCs have been reported by several researchers in which only compact layers of ETM and HTM is used without a mesoporous layer at a temperature lower than 200°C [19, 21, 32]. Hence the planar structure turns out to be very attractive for basic research purposes. The mesoscopic and planar structures of PSC are represented in **Figure 2**.

#### **1.3 Structure of perovskite materials**

Perovskites materials are designated by the formula ABX3, where A and B are cations of different sizes (A being larger than B) and X is an anion [7]. The crystal structure of perovskites is depicted in **Figure 3** and it has a cubic crystal structure with three-dimensional (3D) framework sharing BX6 octahedron with the A ion placed at the octahedral interstices [33, 34]. In organic-inorganic materials, the A is organic cations generally methylammonium, ethylammonium and formamidinium and B is usually metal ions of group IV such as Pb2+, Sn2+ and Ge2+ whereas the X are VII group anions I− , Cl− and Br− [2, 7, 34].

The crystallographic stability and probable structure of perovskite can be inferred by studying a "tolerance factor" t and an "octahedral factor" μ. A "tolerance factor" is defined as the "ratio of the A-X distance to the B-X distance in an idealized solid-sphere model" and is represented by the formula:

$$\mathbf{t} = \frac{\left(\mathbf{R\_A} + \mathbf{R\_X}\right)}{\left[\sqrt{2\left(\mathbf{R\_B} + \mathbf{R\_X}\right)}\right]} \tag{1}$$

**75**

**Figure 3.**

**Figure 2.**

*Structure of perovskite.*

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material*

(ascribed to the oxidation of Sn to SnI4 in the iodide perovskite). The anion X is a halogen, generally iodine (RX = 0.220 nm) is used, however Br and Cl are also used (RX = 0.196 nm and 0.181 nm) [35, 36]. The commonly used organic inorganic

MAPbX3 perovskite show multiple phases as a function of composition and temperature. These different phases have markedly different optical and electrical properties as well as stability. MAPbI3 displayed α-phase, δ-phase, and γ-phases with transition temperatures of 400 K, 333 K, and 180 K, respectively. Generally, the δ-phase MAPbI3 is used as absorber in solar cell due to its thermodynamically stable nature at room temperature and its increased conductivity and absorption coefficient (>26 mm\_1) in contrast to the α-phase. Though, a phase transition from

The deposition technique of organic-inorganic perovskites films is quite an important issue for perovskite studies, because the possible use of perovskite materials depends on the availability of simple and perfect thin film deposition method. As concerns the preparation methods of organometallic halide perovskite CH3NH3PbX3 thin films, solution-based procedures have been proposed to manufacture thin films.

perovskite material is methylammonium lead triiodide (CH3NH3PbI3).

δ-phase to α-phase may occur under continuous 1 sun illumination [15].

**2. Synthesis of inorganic–organic solar cells materials**

*DOI: http://dx.doi.org/10.5772/intechopen.94104*

*Representative scheme of a mesoporous (right) and planar PSC (left).*

*X*

where RA, RB and RX are the ionic radii of the corresponding ions. An "octahedral factor" is defined as "the ratio *<sup>B</sup> R R* ".

For halide (X = F, Cl, Br, I) perovskites, generally 0.81 < t < 1.11 and 0.44 < μ < 0.90 [35]. If t value lies in the narrow range 0.89–1.0, the structure is cubic, but, if it is lower, symmetric tetragonal or orthorhombic structures is expected [2]. Regardless of these limitations, conversions between these structures are common on heating, at the high-temperature cubic phase is generally obtained.

For the organic–inorganic perovskites, organic cation A usually methylammonium (CH3NH3 + ) with RA = 0.18 nm, ethylammonium (CH3CH2NH3 + ) (RA = 0.23 nm) and formamidinium (NH2CHNH2 + ) (RA = 0.19–0.22 nm) are used. The cation B is commonly Pb (RB = 0.119 nm); however, Sn (RB = 0.110 nm) forms similar compounds with more ideal bandgap but exhibits lower stability

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material DOI: http://dx.doi.org/10.5772/intechopen.94104*

#### **Figure 2.**

*Perovskite and Piezoelectric Materials*

The conventional PSC consists of mesoscopic n-i-p structure and is the novel architecture of PSC devices which consists of an FTO, an electron transport layer (ETL), a mesoporous oxide layer such as TiO2, or SnO2, perovskite (light absorbing) layer, a hole transport layer (HTL), and an electrode layer. The mesoporous TiO2 layer played a significant role in the electron transfer process and as a scaffold providing mechanical support of the perovskite crystal. The use of mesoporous materials in PSC permit the perovskite material to adhere to the mesoporous metal oxide framework to increase the light-receiving area of the photosensitive material and results in improving the efficiency of the device. The mesoporous layer was usually less than 300 nm. The presently mesoporous structure of PSCs is one of the most common structures with a power conversion efficiency (PCE) greater than 20% [28]. The mesoscopic structure due to the fabrication ease and outstanding best efficiencies is the most extensively adopted in research labs. However, high temperature (˃450°C) sintering is required for mesoporous layer-based devices, which prevents the use of plastic substrates [29–31]. To overcome this problem, the planar perovskite solar cell was developed that showed comparable performance for mesoporous perovskite solar cell. Planar heterojunction PSCs have been reported by several researchers in which only compact layers of ETM and HTM is used without a mesoporous layer at a temperature lower than 200°C [19, 21, 32]. Hence the planar structure turns out to be very attractive for basic research purposes. The mesoscopic

Perovskites materials are designated by the formula ABX3, where A and B are cations of different sizes (A being larger than B) and X is an anion [7]. The crystal structure of perovskites is depicted in **Figure 3** and it has a cubic crystal structure with three-dimensional (3D) framework sharing BX6 octahedron with the A ion placed at the octahedral interstices [33, 34]. In organic-inorganic materials, the A is organic cations generally methylammonium, ethylammonium and formamidinium and B is usually metal ions of group IV such as Pb2+, Sn2+ and Ge2+ whereas the X are

and planar structures of PSC are represented in **Figure 2**.

**1.3 Structure of perovskite materials**

, Cl−

and Br−

An "octahedral factor" is defined as "the ratio *<sup>B</sup>*

[2, 7, 34].

idealized solid-sphere model" and is represented by the formula:

t

where RA, RB and RX are the ionic radii of the corresponding ions.

For halide (X = F, Cl, Br, I) perovskites, generally 0.81 < t < 1.11 and 0.44 < μ < 0.90 [35]. If t value lies in the narrow range 0.89–1.0, the structure is cubic, but, if it is lower, symmetric tetragonal or orthorhombic structures is expected [2]. Regardless of these limitations, conversions between these structures are common on heating, at the high-temperature cubic phase is generally obtained. For the organic–inorganic perovskites, organic cation A usually methylam-

The crystallographic stability and probable structure of perovskite can be inferred by studying a "tolerance factor" t and an "octahedral factor" μ. A "tolerance factor" is defined as the "ratio of the A-X distance to the B-X distance in an

> ( ) ( ) A X B X

(1)

+ )

) (RA = 0.19–0.22 nm) are

R R

2R R <sup>+</sup> <sup>=</sup> <sup>+</sup>

) with RA = 0.18 nm, ethylammonium (CH3CH2NH3

used. The cation B is commonly Pb (RB = 0.119 nm); however, Sn (RB = 0.110 nm) forms similar compounds with more ideal bandgap but exhibits lower stability

*X R R* ".

+

VII group anions I−

monium (CH3NH3

+

(RA = 0.23 nm) and formamidinium (NH2CHNH2

**74**

*Representative scheme of a mesoporous (right) and planar PSC (left).*

**Figure 3.** *Structure of perovskite.*

(ascribed to the oxidation of Sn to SnI4 in the iodide perovskite). The anion X is a halogen, generally iodine (RX = 0.220 nm) is used, however Br and Cl are also used (RX = 0.196 nm and 0.181 nm) [35, 36]. The commonly used organic inorganic perovskite material is methylammonium lead triiodide (CH3NH3PbI3).

MAPbX3 perovskite show multiple phases as a function of composition and temperature. These different phases have markedly different optical and electrical properties as well as stability. MAPbI3 displayed α-phase, δ-phase, and γ-phases with transition temperatures of 400 K, 333 K, and 180 K, respectively. Generally, the δ-phase MAPbI3 is used as absorber in solar cell due to its thermodynamically stable nature at room temperature and its increased conductivity and absorption coefficient (>26 mm\_1) in contrast to the α-phase. Though, a phase transition from δ-phase to α-phase may occur under continuous 1 sun illumination [15].

## **2. Synthesis of inorganic–organic solar cells materials**

The deposition technique of organic-inorganic perovskites films is quite an important issue for perovskite studies, because the possible use of perovskite materials depends on the availability of simple and perfect thin film deposition method. As concerns the preparation methods of organometallic halide perovskite CH3NH3PbX3 thin films, solution-based procedures have been proposed to manufacture thin films. However, deposition of organic-inorganic perovskite materials is often challenging due to different physical and chemical properties of the organic and inorganic parts of perovskite materials [15]. Despite of this, several significant methods are used for thin film deposition of organic-inorganic hybrid perovskites. Various methods used for perovskite deposition are solution-processed (one-step and two-step) deposition, evaporation method, and vapor assisted solution process (VASP) are the typically adopted methods for film deposition [15, 37, 38].


### **2.1 One-step precursor solution deposition (spin-coating technique)**

One-step processing (spin-coating) is a suitable technique extensively applied for uniform thin film deposition and is based on the co-deposition of both the inorganic and organic components either through solution processing or thermal evaporation. In solution processing, a mixture of both MX2 (M ¼ Pb, Sn; X ¼ Cl-, Br-, I-) and AX (A ¼ methylammonium MA); formamidinium, FA) is dissolved in an organic solvent and deposited through the spin coating to form a film (**Figure 4**), followed by annealing to produce the perovskite layer [15]. The post deposition annealing of the films at low temperature (T < 250°C) is sometimes used to increase phase purity and crystallinity [6]. Spin-coating allows deposition of hybrid perovskites on various substrates, containing glass, quartz, plastic, and silicon. Selection of suitable parameters such as substrate, spin speed and the substrate temperature are essential for this technique and can be selected accordingly. The wetting properties of the solution on the chosen substrate can be improved by pre-treating the substrate with a suitable adhesion agent. The spin-coating technique does not involve cumbersome equipment and it gives high-quality films in quite short time at room temperature. It is considered as a distinct case of solution crystal growth, which results in the formation of highly oriented perovskites layer on a substrate. In order to obtain a layer with the desired thickness, optimization of various parameters such as concentration of perovskites solution, and spin-coating parameters (spin speed, acceleration and spin duration) can be carried out. Generally, 2D homogeneous perovskites films with a thickness ranging 10 nm to 100 nm can be obtained by carefully choosing the parameters. The selection of solvent is also important by considering the solubility for both the organic ammonium and the inorganic lead halide. The usually used solvents for spin coating technique are Dimethylformamide (DMF) or Dimethyl sulfoxide (DMSO) [39]. These spin-coated perovskites films are very reproducible, and this technique is suitable for all PSC structures (mesoporous vs. planar) [39].

**77**

planar PSCs.

*Organic Inorganic Perovskites: A Low-Cost-Efficient Photovoltaic Material*

Mitzi [40] first time reported the two-step dipping technique in 1998, and later

M. Era et al. [43] first used thermal evaporation method. They used the dual source vapor deposition by using ammonium iodide RNH3I and lead iodide PbI2, organic and inorganic source were co-evaporated and deposited on quartz. The pressure of evaporation chamber was about 10−6 Torr. By using this method, it is possible to precisely control the smoothness and thickness of the films. However, it is often hard to balance the organic and inorganic rates, which is important in attaining the correct composition of the resultant perovskite films. Furthermore, Mitzi et al. [40] developed another method, by using a single evaporation source to deposit perovskites thin films called single source thermal ablation (SSTA) technique. This consists of a vacuum chamber, with an electrical feed-through to a thin tantalum sheet heater. A suspension of insoluble powders in a drying solvent is placed on the heater. Under a suitable vacuum, the temperature goes to approximately 1000°C in 1–2 second, the whole starting charge ablates from the heater. After ablation, the organic and inorganic parts reassemble on the substrates to yield films of the chosen product. Liu et al. [44] in 2013, improved this technique as a dual-source vapor deposition method for pinhole-free MAPbI1−xClx perovskite films with a thickness of hundreds of nanometers for

Later on, the chemical vapor deposition (CVD) method was reported by Leyden

et al. [45], which precisely control the crystallization process. Vapor deposition methods are appropriate for multi-layered thin-film and a variety of substrates, though needs high vacuum [39]. However, this method has drawbacks of yield and therefore is not very effectively employed at industrial scale [46]. Though great achievements have been attained, researchers still meet some challenges, involving reproducibility and grain boundaries of perovskite films which are considered as a defect region initiates carrier recombination and accelerates device degradation. Hence, efforts to increasing grain size and reducing grain boundary of films are

by Burschka et al. [41] in 2013. In a two-step dip-coating deposition process, a metal halide PbI2 layer is first deposited by vacuum evaporation or spin-coated on a substrate. Then this coated film is altered into the perovskite by dipping into an organic MAI solution as it is shown in **Figure 4**. This method offered PCE of 15% and certified 14.14% [39]. Suitable selection of solvent is important for the dipping process. The solvent is selected such that can dissolve organic salt but cannot metal halide and the final organic-inorganic perovskite, toluene/2-propanol mixture is an appropriate solvent for the organic salt. The organic cations in solution intercalate into and react with metal halide on the substrate and form a crystalline film [6]. The dipping times are quite short: several seconds to minutes, depending on the system. This method is a suitable method for a variety of inorganic and organics, even if they have an incompatibility in solubility. This process effectively reduces the chemical reaction between the perovskite and the underlying ETL. The development of successive deposition methods has offered a variety of ETL options, though allowing for perovskite films to be prepared successfully at room temperature [15]. In addition, Chen et al. [42] developed a vapor assisted solution processing (VASP) method that used the reaction between MAI vapor and pre-deposited PbI2 to form the completed perovskite film. The resulting MAPbI3

*DOI: http://dx.doi.org/10.5772/intechopen.94104*

**2.2 Two-step dip-coating**

exhibits excellent film quality.

**2.3 Thermal evaporation technique**

critical for stable and highly efficient PSCs.

**Figure 4.** *Schematic of the spin-coating process [4].*
