The organometal halide perovskite solar cells (PSCs) have attracted attention and achieved efficiencies compared with traditional solar cells. There are several ways to develop perovskite solar cells like effect of moisture, degradation, and understanding the reason for instability of perovskite. In this chapter, we are specified how to make coating and film fabrication are affected by the existing methods. Improvement in the photovoltaic performance of PSCs can be achieved by enhanced processing technique. These techniques include the spin-coating PbI2 solution controlling the substrate temperature and crystal quality of the morphology for perovskite films. There is no doubt that film coating indicates that the crystallization and morphology of perovskite films affect the absorption intensity and obviously influence the short-circuit current density. This study points out an enhancement of the stability of perovskite films and solar cells by reducing residual strains in perovskite films.
- perovskite solar cells
- working mechanism
- photovoltaic parameters
- low cost
Metal halide perovskite solar cells (PSCs) have emerged as a kind of encouraging alternative to existing photovoltaic technologies with both solution processability and superior photovoltaic performances. Fundamental studies on perovskite materials , device designs [2, 3], fabrication processes [4, 5, 6, 7, 8, 9, 10], and materials engineering [11, 12, 13, 14, 15, 16] have boosted the rapid development of PSCs. Consequently, a certified power conversion efficiency (PCE) of 22.1% has been obtained after the past several years of vigorous work. However, despite the overwhelming achievements in terms of performance of PSCs, the long-term stability and current–voltage hysteresis still remain critical .
Perovskite solar cells, the most promising new technology in the academia and industry, have promised a highly competitive alternative to silicon solar cells and other commercial alternatives. Perovskite solar cells are high-performance photovoltaic devices which have the potential to enter in the market in the near future. Low processing costs and highly abundant raw materials may permit a short-energy payback time and low overall CO emissions. After an impressive increase in PCE from ~10% in 2012 to ~22.1% in early 2016, experts expect to discover further improvements in efficiency in the next several years . Perovskite solar cell research is still in its infancy considering that the first work was only published in 2009 . The commercialization of perovskite solar cell needs to address several fundamental issues in the near future: for example, control the growth of thin film and deposition, make scale and numerous process, achieve high stability and long lifetime, and low toxicity. To be competitive, cost will be a concern for manufacturing companies. Although the raw materials for making perovskite solar cells are inexpensive and abundant, recent analyses of cost-performance and commercialization requirements are not entirely positive . To make perovskite solar cells competitive, several goals are needed to be achieved. For example, the levelized cost of electricity (LCOE) for residential use is 9.0 cents per kWh by 2020 and is expected to decline to 5.0 cents/kWh by 2030 . This is a huge challenge for perovskite technology at present.
In this review, we summarize recently developed perovskite film deposition techniques and evaluate their suitability for industrial production of perovskite solar cells and modules. Our discussion of stability and device lifetime focuses mainly on the measurement standards and issues relative to commercialization. Thereafter, we present techniques that used to fabricate the perovskite solar cells such as one-step spin-coating and two-step deposition techniques, solvent-solvent extraction, vapor-assisted solution processes, dual-source vacuum deposition, hybrid deposition, hybrid chemical vapor deposition, sequential vapor deposition, and flash evaporation. The control of the morphology for perovskite thin films has been observed by many efforts.
2. The common architecture of PSC
Generally, PSC has three main types of device architectures: (i) mesoscopic structures using mesoporous semiconducting materials as electron-transporting layers (ETLs), i.e., TiO2 , WO3 , SrTiO3 , ZnO , Zn2SnO4 , and SnO2 ); (ii) meso-superstructures employing mesoporous insulators such as Al2O3  and ZrO2  as scaffolds, while perovskite itself acts as ETL; and (iii) planar structures implementing ultrathin compact layer materials for both hole-blocking and electron-conducting purposes. Among these types, the planar PSC was motivated by the requests of more simple process and lower cost for future applications . Figure 1 reveals the evolution of device configuration as sequence from mesostructure to planar heterojunction. The categories related to the types of device architecture were discussed here in summary.
2.1. The mesoporous scaffold
The best performance for PSCs have reported and reached a certified PCE of 22.1%, have depend on high temperature processing (450–550°C) and the mesoporous structure used TiO2 as ETLs . Moreover, in another work, a 16.2% efficiency was obtained from the combination of uniform and dense MAPbI3 and MAPbBr3 bilayer architecture consisting of perovskite-infiltrated mesoporous TiO2 electrodes . Similarly, the same group proceeded the FAPbI3-MAPbBr3 system with an architecture as follows: FTO/blocking TiO2 (70 nm)/mesoporous TiO2 and perovskite composite layer (200 nm)/perovskite upper layer (300 nm)/PTAA (50 nm)/Au (100 nm) with a PCE of 18.4% at maximum power point condition [31, 32].
A simple binder-free colloid Al2O3 nanoparticle meso-superstructured scaffold with annealing temperature of 150°C deposited over compact TiO2 layer delivered PCEs of up to 12.3%. Furthermore, it has been revealed that a solid thin film of the perovskite absorber was formed on top of the scaffold thin porous Al2O3 films. This supported charge separation and transport of both carrier species with an internal quantum efficiency approaching 100% . Mahmood et al. reported two-dimensional (2D) nanosheets with enhanced absorber infiltration as compared with 1D nanostructures as revealed in Figure 2. The author utilized WO3 n-type semiconductor as a nanostructured porous ETL to obtain highly efficient perovskite solar cells with PCE of 11.2% .
2.2. The planar heterojunction
Planar PSCs have been demonstrated by Tan et al. with smooth and pinhole-free TiO2-Cl as the ETL. The film also exhibited negligible parasitic absorption loss over the entire visible to near-infrared spectrum. Solar cells fabricated on TiO2-Cl exhibit considerably better performance than those on TiO2 for all PV parameters. Correspondingly, TiO2-Cl resulted in a higher average PCE (19.8%) than the Cl-free TiO2 (15.8%) [35, 36]. The best-performing small-area CsMAFA solar cell (0.049 cm2) exhibited a high laboratory PCE of 21.4% without hysteresis in J-V sweeps. Similarly, large-area (1.1 cm2) cells fabricated on TiO2-Cl showed a PCE value >20% with negligible hysteresis. Zhou et al. manipulated carrier behavior with planar heterojunction perovskite solar cells. Yttrium-doped TiO2 (Y-TiO2) was fabricated at <150°C annealing temperature as the ETL to enhance electron extraction and transport over reduced work function ITO treated with polyethylenimine ethoxylated (PEIE) solution. These treatments produced a PCE of 19.3% [37, 38]. The use of CdSe nanoparticles (solution processed at 150°C) has been investigated to replace the widely used TiO2 as the ETLs for the conventional planar heterojunction PSCs. Devices with CdSe nanoparticle ETLs were performed well, with the PCE of 11.7% . A planar device with high PCE was applicable through Hagfeldt et al. and Jiang et al., using SnO2 [24, 40, 41, 42].
By modifying the surface of a planar structure of the TiO2 compact layer with C60-SAM molecules (Wojciechowski et al.), a PCE of 15.7% has been obtained (Figure 3) . Pablo Docampo et al. have demonstrated 10% PCEs for inverted planar PSCs with bilayer of PC60BM and compact TiOx as ETL . Further, hysteresis-less inverted planar hybrid solar cells with 18.1% PCE has been fabricated by Heo et al. Better PCE and stability were attributed to the electron extraction from MAPbI3 into PCBM, the increased EQE value by the better charge injection/separation efficiency, and the improved FF by the increased diffusion coefficient (Dn) and charge carrier lifetime (τ n). In addition, the air and humid stability was improved by the corrosive additive-free device architecture and hydrophobicity of the PCBM top layer .
3. Film formation of ETL
There is no doubt that the crystallinity, thickness, material film morphology, and purity have impact on the efficiency of solar cell performance. The film formation has been relied on deposition techniques such as one-step spin-coating [7, 46, 47, 48, 49, 50] and two-step deposition techniques [51, 52], solvent-solvent extraction , vapor-assisted solution processes [54, 55, 56, 57], dual-source vacuum deposition [58, 59, 60, 61], hybrid deposition [62, 63], hybrid chemical vapor deposition [62, 64, 65, 66], sequential vapor deposition [67, 68], and flash evaporation . The control of the morphology for perovskite thin films has been observed by many efforts. These efforts include the optimization of the annealing time, temperature [70, 71], selection of the underlayer material and thickness [3, 72, 73, 74, 75], and the use of alternative deposition methods such as two-step deposition and vacuum sublimation [4, 76, 77]. Herein, some of these methods have been discussed.
3.1. Vacuum thermal co-evaporation
In this method, the authors used vacuum thermal co-evaporation of organic halide and metal halide to resulting perovskite thin films with homogeneous morphology and improved thin-film coverage. These results achieved a high performance of 12–15% PCE [5, 78]. Despite the promising results, however, to date, only limited reports have utilized this vacuum sublimation technique to fabricate perovskite layers [5, 78]. The main reason could be due to the small molecular weight of organic halide and make the monitoring without control of the CH3NH3I deposition rate using quartz microbalance sensors [5, 78]. In another way, Zhu et al. reported how to develop this technique deposition to fabricate pinhole-free cesium-substituted perovskite films and enhance the surface coverage as shown in Figure 4. The same method used to promise tunable bandgap reduced trap-state density and longer carrier lifetime, with efficiency 20.13%, which is the highest fabrication for planar perovskite solar cells .
3.2. Layer-by-layer sequential vacuum sublimation
Chen et al. reported a novel method of perovskite thin-film deposition via a layer-by-layer sequential vacuum sublimation. This method has been easier than the previous technique. The very uniform perovskite thin films can achieve high coverage via incorporating the thin films of perovskite with a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole-transporting layer (HTL) and thermally evaporated C60/bathophenanthroline (Bphen) electron-transporting layers (ETLs). The cells here attain efficiencies as high as 15.4% because the devices were free of high-temperature-prepared metal oxide layers .
3.3. Vapor deposition by dual source
The large-scale production in optoelectronic applications has been achieved by vapor deposition techniques because this technique is widely used in semiconductor industry. The feasibility of organometal halide perovskite materials via vapor deposition techniques has advantages like the possibility to fabricate films with high purity; these techniques are more proper to prepare multilayered structures of thin films, and suitable optimization of perovskite films also can be deposited by vapor deposition [5, 81]. Liu et al.  reported preparation of CH3NH3PbI3−xClx by dual-source vapor deposition technique in the presence of PbCl2 and CH3NH3I. This method leads to high-efficiency photovoltaic devices of 15.4%. Malinkiewicz et al. used the same technique to deposited CH3NH3PbI3 in the presence of PbI2 and CH3NH3I and give uniform film formation with root-mean-square roughness of 5 nm measured by AFM . In addition, the films showed uniform grainy structures with an average grain size of 150 nm . Schematic illustration of the dual-source vacuum deposition process is shown in Figure 5 .
3.4. Spin coating
Spin coating is widely used to fabricate a small area from thin films in lab scale. Spin coating used a small amount of solution, which was then dropped on the substrate as shown in Figure 6a. Then, the substrate has been covered by a layer of solution and spun to accelerate evaporation of the solvent . This technique controls the thickness of the film by the concentration of the solution and speed . In general, with regular spin coating, a one-step process with PbI2/MAI or PbCl2/MAI with gamma-butyrolactone (GBL), dimethylformamide (DMF), or dimethyl sulfoxide (DMSO) as solvents prompts poor film quality [71, 84]. Despite the fact that all preparing conditions have been considered, spin-coated perovskite film quality is regularly poor, with a high thickness of pinholes and little grain sizes. These pinholes cause shunt that debase the efficiency. With added substances designing the crystallization of perovskite could be finely tuned, and perovskite films with altogether enhanced quality can be set up for superior power conversion [85, 86]. For instance, by utilizing lead acetic acid as lead source, the crystallization of perovskite is significantly speedier. The free pinhole perovskite films were shown by a basic one-step coating process .
Concerning the two-step process, made to create good morphology perovskite films, convolutes control over the change rate of PbI2 to perovskite . In the recent date, it has been demonstrated that the order of PbI2-DMSO-MAI essentially improves the film morphology and quality utilizing a propelled hostile to dissolved designing strategy [6, 88, 89]. Furthermore, this hot-throwing procedure could be exchanged to a considerably less difficult plunge-covering process for large-area film deposition. There is no doubt that the high performance and scale of large area for PSCs are closely related to perovskite film quality. A 1-cm2 PSC was fabricated for the first time with a modified interface layer and certified efficiency of 15.6% . Then, enhancing the gradient of heterojunction structure for charge separation/transport, its performance increased to 18.21% . Moreover, a vacuum, flash-assisted process has been produced by a solar cell with a 1-cm2 area . This technique showed a maximum efficiency of 20.5% and a certified efficiency of 19.6% .
3.5. Spray coating
Spray coating has been broadly utilized to deposit perovskite films and compact TiO2 films and is perfect with large-scale, high-throughput manufacture (Figure 6b). The first thinking about how to get perovskite films via spray coating came from polymer solar cell fabrication. An ultrasonic spray-coating technique has been discovered in ambient conditions. In this method a system from DMF or DMSO and perovskite materials was investigated with deposition parameter to achieve higher coverage of perovskite films. The parameters which are related to spray coating require to form high-coverage perovskite film such as drying time, substrate temperature, solvent volatility, and post-annealing conditions. With PCE of 11% and an active area about 0.025 cm2, the potential of spray coating in fabricating perovskite solar cells has been indicated . A similar work was performed in TiO2 and achieved PCE of 13% with 0.065 cm2 active area on a glass/ITO substrate. In low-temperature PET/ITO substrate in the presence of TiO2, an efficiency of 8.1% was attained on a flexible device, which is comparable with roll-to-roll processing . This spray-coating process is suitable for various perovskite precursor solutions; for example, spray-coating deposition of large bandgap CsPbIBr2 thin films has a potential for tandem structure devices . Other mixed cations and halide perovskites, FA1-xCsxPbI3 mixed cation films, were prepared with a spray-assisted solution process . Solar cell devices based on this mixed cation film showed enhanced stability and performance compared with those based on the other. Efficiency increased from 11.3 to 14.2% for the mixed cations.
3.6. Screen printing
Screen printing is a technique used to fabricate PSCs that could be easily fabricated with a printing process. The layer-by-layer printing process starts with screen printing of TiO2, followed by printing of ZrO2 and carbon electrodes. Then, perovskite solution is dropped onto the porous carbon electrode so that it infiltrates into the mesoporous TiO2 and ZrO2. Herein, ZrO2 functions as a porous insulating layer to prevent direct contact between the carbon electrode and the TiO2/FTO substrate. Although it is easy to fabricate solar cells with this printing technique, infiltration of perovskite precursor solution remains a challenge and is the main reason for lower efficiency compared to those of devices fabricated by other means. The most intriguing properties of this carbon-coated, printed, mesoscopic device are the high stability and outstanding outdoor performance. A certified PCE of 12.8% and stable performance over >1000 h in ambient air under full sunlight has been recorded for a device with an active area of 0.28 cm2 [26, 99]. Figure 7 shows noncontact inkjet printing offering rapid and digital deposition combined with excellent control over the layer formation for printed perovskite solar cells. Mathies et al.  reported that inkjet printing is used to deposit triple cation perovskite layers with 10% cesium in a mixed formamidinium/methylammonium lead iodide/bromide composite for solar cells with high temperature and moisture stability. A reliable process control over a wide range of perovskite layer thickness from 175 to 780 nm and corresponding grain sizes is achieved by adjusting the drop spacing of the inkjet printer cartridge. A continuous power output at constant voltage, resulting in a power conversion efficiency of 12.9%, is demonstrated, representing a major improvement from previously reported inkjet-printed methylammonium lead triiodide perovskite solar cells . Compared with solution processes, dry deposition processes may be more environmentally friendly, as they do not require toxic solvents (DMSO, GBL, DMF, chloroform, chlorobenzene, isopropanol, toluene, diethyl ether, etc.), and they are compatible with high-quality, large-area perovskite film deposition, such as vacuum deposition and chemical vapor deposition.
4. Summary and future outlook
The effectivity of photon capture in PSCs has resulted in tangible action and contributed to scientific community . These achievements have an economic impact for future endeavor. Therefore, the innovation in PSC field required a large amount of effort and attention to be reliable and highly efficient at converting sunlight to electricity. Furthermore, the improvement of device engineering methods is urgent. In particular, investigation of photophysical mechanism of the materials also plays an important role. The continuous investigation on current density and voltage characteristics of PSCs would provide a good understanding point for the semiconducting behavior . The improvement in PSC efficiency relies on deposition techniques and material composition . The solution processed in PSCs is more important . We are noticed that the one-step spin coating is broadly the used method because of its simplicity and low cost. The films synthesized by this method have a poor morphology and incomplete coverage, for instance, in the case of planar architecture [84, 85, 105]. On the other hand, in the two-step coating, a layer of lead halide was deposited by spin coating then followed by immerging in organic salt solution and the perovskite films formed by a chemical reaction [51, 52]. The high reaction rates of perovskite materials are important to optimize the coating conditions with sufficient reproducibility. To record high efficiencies by solution processing, it is revealed that the reaction kinetics are required to control and maintain consistent device to minimize batch-to-batch variations. The different vapor-based methods to deposit perovskite films are also discussed, which in many cases show properties different from their counterparts prepared by solution-based methods.
Conflict of interest
The authors declare no conflict of interest.