Encyclopedia
Please note this is a comparison between Version 1 by Melvin M. Mashingaidze and Version 2 by Peter Tang.
Solar energy is one of the most encouraging, abundant, green, and renewable sources for decreasing or even replacing traditional energy in the future. The energy provided by the sun in one hour is sufficient to supply the Earth’s needs for an entire year. The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance.
- perovskite solar cells
- power conversion efficiency
- large-area perovskite films
- preparation and fabrication methods
- scalable
- quality and stability
- building-integrated photovoltaics
1. Introduction
Fossil fuels must be replaced by clean and renewable energy in order to reduce environmental pollution and meet an increase in global energy demands [1][2][3][4][1,2,3,4]. Solar energy is one of the most encouraging, abundant, green, and renewable sources for decreasing or even replacing traditional energy in the future. The energy provided by the sun in one hour is sufficient to supply the Earth’s needs for an entire year [5].
Various innovative photovoltaic technologies have been developed to capture and convert solar energy into electricity, but affordable solar cells with high power conversion efficiency are needed. One of the frontrunner technologies uses inorganic and organic hybrid compounds (CH3NH3-PbX3, X = I, Br, Cl) with the crystal structure of perovskite. Photovoltaic solar cells utilizing such light absorbers are called perovskite solar cells (PSCs). Some of the PSCs’ favorable attributes compared to conventional solar cells such as the silicon-based rigid modules include ease of fabrication [6], diversity in device architectures [7], small energy band gaps, high carrier mobility [8], panchromatic sunlight absorption, superior carrier diffusion length, and long carrier lifetimes [7][9][7,9]. Unlike silicon, which only absorbs light near the red end of the visible light spectrum, perovskites can be tuned to absorb different wavelengths (https://www.technologyreview.com/2021/06/29/1027451/perovskite-solar-panels-hype-commercial-debut, accessed on 8 February 2022). However, PSCs tend to display poor long-term stability due to the natural instability of the active perovskite layer. The lifespan of PSCs is severely reduced by exposure to water, heat, oxygen, and light (https://resources.system-analysis.cadence.com/blog/msa2021-the-pros-and-cons-of-halide-perovskite-solar-panels, accessed on 8 February 2022). The power conversion efficiency of PSCs is generally lower than for comparable conventional silicon solar cells and drops with increasing size, which is a huge disadvantage for commercialization efforts. PSC power conversion efficiency is also dependent on the quality of the perovskite layer, which in turn is dependent on the synthesis method deployed. This leads to wide variability in reported PSC performance, further compounding the PSCs’ commercialization challenge.
The reader is referred to the works of [10][11][12][13][10,11,12,13] for the structures (Figure 1) and characteristics of PSCs, the classifications of different materials used to enhance PSC performance, and the encapsulation materials and methods related to PSCs. A thorough analysis of PSCs’ chemical stability is provided by [6][8][10][13][6,8,10,13], and the reader is advised to consider these works. The working principle of PSC architecture is as follows: the perovskite layer absorbs the incident light, and produces electrons and holes that are extracted and carried by electron transport materials (ETMs) and hole transport materials (HTMs), respectively, which are finally collected by electrodes, forming PSCs [11].
Figure 1 shows the archetypal PSC structures, including the mesoporous structure in Figure 1a, the planar heterojunction structure in Figure 1b, and the inverted planar heterojunction structure in Figure 1c. The PSCs with standard configurations have a transparent, conductive, oxide (TCO)/blocking layer ETL/perovskite absorber layer/HTL material/gold (Au). The perovskite absorption layer absorbs light and generates charges while light is incident on the PSC. The photon energy creates pairs that diffuse and separate through electrons’ and holes’ selective contacts, as shown in Figure 1d [14]. Once electrons and holes are present at the cathode and anode, an external load can be powered by connecting a circuit to the cell.
Figure 1. Structural configurations of perovskite solar cells: (a) mesoporous structure, (b) planar heterojunction structure, (c) inverted planar heterojunction structure, and (d) schematic diagram of electron and hole transportation [15].
The power conversion efficiency (PCE) of PSCs has reportedly risen from 3.8% to more than 25% over the past few years, surpassing established thin-film solar cells, such as CuInGaSeCdTe [16]. However, most high PCEs of PSCs have been reported for cells of small areas, from 0.04 to 0.2 cm2 [17], and also for those manufactured by spin coating methods of areas ~0.1 cm2 [18][19][20][18,19,20]. Few researchers have attempted to manufacture large-area solar cells. For PSCs to be evaluated for practical commercial applications, large-area cells (minimum active area > 1 cm2) must be fabricated cost-effectively and achieve PCEs comparable to small-area cells.
Perovskite materials and preparation methods for large-area modules are essential for scalable deposition [21]. According to Kim et al., the two main challenges are currently preventing the synthesis of larger efficient PSCs and limiting the maximum device area to a few cm2. These challenges include a severe decrease in perovskite film quality and uniformity for sizes greater than 1 cm2 and the nearly linear rise in series resistance with cell area [22].
Wang et al. cited the drop in PSC efficiency accompanying the device’s enlargement as a stumbling block to PSCs’ commercial application and attribute the inefficiencies to each functional layer’s imperfections in coverage, uniformity, and flatness, which arise from the solution processing method. Wang et al. questioned the high PCEs reported by some researchers for small-area PSCs since measurement errors normally increase as the active cell area decreases [12]. According to these authors, the second challenge for the commercialization of PSCs is poor stability, which worsens with the increase in cell size. The third challenge reported by Wang et al. is the cost of PSCs with PCEs being >20% higher, since they have to be manufactured from costly materials, such as Spiro-OMeTAD, Au, using costly vacuum deposition technology.
Despite the fact that spin coating has been extensively used to fabricate a dense and uniform perovskite film for PSC modules, film uniformity declines significantly as cell size grows, leading to very poor PCEs and limiting the development of large-area PSCs [22]. Most recent research has focused on fabrication methods for large-area PSCs [10][22][23][24][25][26][10,22,23,24,25,26]. Apart from spin coating, other PSC fabrication methods include spray pyrolysis, dip coating [23][27][28][23,27,28], two-step interdiffusion [29][30][29,30], chemical vapor deposition, ink-jet printing, atomic layer deposition, and blade coating deposition [31][32][33][34][31,32,33,34]. These methods will be discussed in detail, with supportive diagrams provided where necessary.
Figure 2 from the NREL website shows the best solar cell efficiencies reported so far [35]. Certified PCEs as high as 25.5% were attained from single-junction PSCs with active areas of 0.0937 cm2. However, the present average PCE for PSCs larger than 10 cm2 (minimodules) is 18.04% (19.276 cm2) [36], which falls below the PCE of a 79 cm2 silicon PV cell, which is 26.7%. Figure 2 shows nearly 50 articles published during the past years, including research on the PCEs of mesoporous and planar (usual and inverted) structural large-area PSC minimodules, of which the typical active area is from 10 to 100 cm2; the five cases with sizes larger than 100 cm2 are labeled.
Figure 2.
2. Large-Area PSC Preparation Methods and Fabrication Technologies
There are three critical factors for successful PSC commercialization: performance, cost, and stability. Improvements in device architecture and fabrication processes that have improved device performance have also resulted in PSC commercialization becoming an unstoppable trend. Commercialization of PSCs is now gradually being realized with the establishment of pilot production lines for perovskite photovoltaic devices possessing good performance and manufactured cost effectively using established techniques. However, device stability has continued to be an issue for researchers and industries.
Thin metal–organic perovskite films have traditionally been fabricated through solution-processed spin-coating, which permits fast iteration, optimization, and research development but is not a scalable technique for fabricating photovoltaic cells. The critical requirement for a scalable production method is low cost, which consists of capital expenditures for the necessary equipment as well as operational expenditures, including energy usage, material costs, the cost of post-treating production waste, and quality control [27][37][38][27,37,38]. Since the cost of contemporary PSC materials is negligible due to their ready availability, the costs of PSC manufacturing are dominated by capital expenditures and lowered by higher and faster throughputs [39]. Due to the different substrate materials, it is essential to distinguish between rigid and flexible PSCs when studying their synthesis methods. The large-area solar cell preparation methods shown in Table 1 have been successfully used to manufacture flexible and rigid PSCs [16][40][41][16,40,41]. Perovskite thin-film fabrication methods are divided into solution processing and vapor deposition methods [7][40][7,40]. Table 1 shows the different methods in each of these clusters. In contrast, Figure 3 shows the graphic timeline on thin-film PSC fabrication methods for inorganic and hybrid halide perovskites developed by [42] shows that for the five years, most researchers have mainly focused on solution-based then vacuum base method.
Figure 3. Timeline for application of vacuum deposition methods vs. solution processing methods in the fabrication of inorganic and mixed halide perovskites. The symbology legend defines fabrication methods used; reported National Renewable Energy Laboratory (NREL) record efficiencies, modules, and multi-cation/multi-halide compositions [42].
Table 1. Large-area perovskite thin-film fabrication methods used in the current work [15][17][23][40][41][43].
|
Solution Processing Method |
Vapor Deposition Method |
|---|---|
|
Spray coating |
Vacuum thermal evaporation |
|
Ink-jet printing |
Co-evaporation |
|
Spin coating |
Sequential evaporation |
|
Slot-die coating |
Flash evaporation |
|
Blade coating/Knife-over edge coating |
Vapor assisted solution process |
|
Vacuum flash-assisted solution process |
Chemical vapor deposition |
Some conclusions can be drawn from the two cluster methods in Table 1 and related works of literature:
- In recent years, vacuum thermal evaporation has lost its position as the fabrication method of choice.
- Spray coating and blade coating have also seen a reduction in use by PSC researchers.
- PSC researchers have increasingly adopted ink-jet printing and slot-die coating.
- PSC researchers are working on overcoming the technological impediments to the synthesis and commercialization of large-area PSCs.
- Not all perovskite thin-film fabrication technologies have been used to create large-area solar cells despite some methods such as thermal evaporation having significant scalability potential.
- In recent publications, the slot-die coating has demonstrated the highest PCE for large-area PSCs, implying that it has a high potential for ushering in PSC commercialization.
Progress in the fabrication of large-area PSCs indicates that PSC commercialization is now a reality. As a matter of fact, Saule Technologies, a start-up from Poland, launched the world’s first industrial production line of solar panels based on groundbreaking perovskite technology in May 2021. The company manufactures perovskite solar modules on thin, flexible substrates, in a variety of different colors, using a novel, room-temperature ink-jet printing procedure invented by one of the company co-founders, Olga Malinkiewicz. Malinkiewicz developed this fabrication technique in 2013 while still a Ph.D. student at the University of Valencia in Spain and this work was published in Nature Photonics in 2013 [44].
2.1. Spin Coating
Spin coating is a batch method in which a liquid film is spread by centrifugal force onto a rotating substrate [45]. The method has been extensively used to manufacture small PSCs of about 0.1 cm2 and large-area devices of 1 cm2. This method is categorized into one-step and two-step processes. Perovskite devices fabricated through spin coating have reached PCEs of over 9.4% [46]. Spin coating has potential for the production of moderately large-area PSCs if evaporation of the solvent can be closely regulated [2][40][2,40], and this has been demonstrated by the authors of [47], who prepared a large-area perovskite film of 57 cm2. The two-step sequential processing method provides better performance than the one-step method for perovskite deposition. In addition, film quality can be enhanced through controlled crystal growth and the post-annealing time [32][48][32,48].2.2. Spray Coating Methods: Spray Printing, Spray Deposition, Spray Pyrolysis, and Ultrasonic Spray
Spray coating is a low-temperature and low-ink-concentration coating method that is suitable for large-area technology and it is a widely used deposition method in the industry [16]. It is an easily scalable method for fabricating large-area thin perovskite films. Perovskite film obtained through this method displays high uniformity over large areas. Spray coating is accomplished through a series of four distinct activities: production of the ink droplets, placement of the droplets on the substrate, amalgamation of the droplets into a wet film, and film drying. Of all the scalable methods, spray coating is the most diverse. It presently encompasses two methods of deposition: one-step and two-step. One-step film deposition solutions comprise aprotic solvents [16][49][50][51][16,58,59,60]. Two-step methods use metal salts deposited by either spray coating or spin coating in an aprotic solvent lacking an acidic proton and hydroxyl and amine groups [40][52][40,61]. Spray coating is the fastest process by which a subjected fluid can be automatically driven to exhibit capillary waves and obtain a scalable substrate [21]. It is based on the atomization of a fluid and the depositing of the atomized fluid droplets onto a suitable surface. Atomization can be generated through various methods: high flow gas, ultrasonic stimulation, or cavitation of the ink itself [51][53][60,62]. The aforementioned atomization methods are generally scalable. Ultrasonic spray coaters are the latest technology for efficiently preparing various functional thin films for photovoltaic cells [54][55][65,66]. Liquid thin-film coating technology has been developed for different applications. Organic salts are deposited on the substrate by spray coating or immersion in an alcohol solution, using isopropanol as the solvent [56][57][58][59][67,68,69,70]. The results vary depending on the method of perovskite solution deposition employed. The single-step deposition method has been used in compositions encompassing MAI, lead iodide, and iodide chloride varieties [56][60][67,71]. The solvents used were DMF, DMSO, gamma-butyrolactone (GBL), DMF-DMSO, and GBL-DMSO; some studies used processing additives such as hydrogen iodide (HI) [60][71]. The highest reported PCE for perovskite films made through this method was 18% for small-area samples and 15.5% for the maximum device area of 40 cm2 [61][64]. Robert et al. reported the device area with the highest efficiency and good morphology without using an ultrasonic spray coater [52][61]. They used single-cation (MAI) mixed halide for perovskite preparation by spray coating on a preheated substrate, and this showed excellent crystallization morphology [49][51][58,60]. Ultrasonic atomization plays a crucial role in nucleation, metal polyhalide complex formation, solution optimization processes, and temperature control. Kim et al. developed devices using the antisolvent spray coating method, which allows high-quality perovskite film to be deposited over a large area. An FTO/glass/bl-TiO2/m-TiO2/perovskite/spiro-OMeTAD/gold 16 cm2 device on the cellular level, as opposed to the module level, was created via the modified solution process in combination with a metal lattice. This cell device was found to be 12.1% efficient and overcame low PCE and poor film quality challenges associated with large-area PSCs [22]. Tait et al. used concurrently pumped ultrasonic spray coating for precise and fast optimization of the precursor ratios of PbCl2, Pb(CH3CO2)2·3H2O, PbBr2, MABr, and MAI to attain pinhole-free perovskite films with high crystallinity, and they achieved a PCE of 11.7% for 3.8 cm2 modules [16][50][16,59]. Ye et al. manufactured high-efficiency large-area PSCs using NiO-based HTLs synthesized through a spray pyrolysis method. PSCs with active areas of 1 cm2 exhibited notable mean PCEs of 17.6% [62][72], 18.21% [63][73], and 19.19% [64][74]. A larger PSC with an active area of 5 cm2 attained a mean PCE of 15.5% [65][75]. The PSC based on the mesoscopic TiO2/Al2O3/NiO/carbon framework showed a PCE of up to 15.03% [66][76]. Using a recent facile spray deposition method for CuI film, PSCs exhibited a mean PCE of 17.6% and excellent device stability [67][77]. The method frequently used in large-area C-TiO2 for electron transport material fabrication is spray pyrolysis deposition [34][67][68][69][70][71][72][34,77,78,79,80,81,82]. Compared to the traditional spin coating method, the pyrolysis spray deposition method produces a denser TiO2 film, lessening material loss and deposition time, which results in desirable low-cost production. Nanomaterial spray coating has been widely investigated as a means for developing semi-transparent devices due to its simple process. For example, using transparent electrodes of spray-coated silver nanowires, carbon nanotubes, and the respective composites, small 0.25 cm2 entirely solution-processed, semi-transparent PSCs with more than 10% PCEs were successfully fabricated [73][74][75][76][77][83,84,85,86,87].2.3. Slot-Die Coating Method
Slot-die coating is a process in which ink is metered through a microfluidic metal die machine. The die is machine structured with a thin channel to spread ink over a moving substrate surface [45].2.4. Blade-Coating Method
Blade coating is a method in which a blade moves across a surface or vice-versa in the case of roll-to-roll coating [45][78][45,99]; it is also known as doctor-blading and knife-over-edge coating. The blade spreads pre-dispensed ink and forms it into a thin liquid film. The film is then dried, creating a solid thin film. This is the most used synthesis technique for fabricating large-area perovskite films. It has been used in several PSC studies to synthesize high-performance cells with areas of over 10 cm2. Various studies have demonstrated that the perovskite film quality can be enhanced by controlling the processing temperature [79][80][100,101]. Recently, additives have been utilized to realize dense perovskite films with smaller pinholes and homogeneous crystal morphology [81][82][102,103]. Blade coating has been commonly used as a single-step deposition method for perovskite films [33][34][78][83][84][85][86][87][33,34,99,104,105,106,107,108] and recently for producing perovskite PVs with a 20% [83][84][88][104,105,109] scalable solution method. Razza et al. used this method to fabricate a PSC module with an active area of 10.1 cm2 and recorded an average PCE of 10.4% [34], while previously, these authors reported a PCE of 4.3% for a cell area of 100 cm2 [34]. Gao et al. reported a method for fabricating ultra-long nanowire arrays and highly oriented CH3-NH3PbI3 thin films in ambient environments. This method integrated large-scale roll-to-roll micro gravure printing and doctor blading to fabricate perovskite nanowires of 15 mm in length [89][110].2.5. Ink-Jet Printing Method
This is a non-contact printing method with direct control of ink deposition, which greatly reduces material utilization and waste. Quintilla et al. reported the fabrication and optimization of multipass inkjet-printed PSCs [90][118]. The perovskite film’s thickness and grain size were carefully controlled during multipass ink-jet printing with MAPbI3 ink, producing PSCs with a high average PCE of 11.3% [90][118].
Ink-jet printing is a method through which a microfluidic cavity is subjected to pressure change, thereby causing the solution to jet out of a microfluidic nozzle. This pressure change can be created through various methods, including thermal and structural sources acting on the microfluidic nozzle. Most ink-jet printing methods demonstrate piezoelectricity of the ink-jet printhead based on a micro-electro-mechanical system, which provides controllable microfluidic jetting through a silicon-etched nozzle. Ink-jet printing, such as ink-jet printers, uses numerous jetting nozzles in a single mobile print head to control the planar thin film thickness and improve the reliability and speed [91][119].
2.6. Vacuum Flash-Assisted Solution Method
The vacuum flash-assisted solution (VAS) method allows fast and well-controlled removal of the solvent. In so doing, it promotes rapid crystallization of the perovskite precursor phase [92][128].
2.7. Chemical Vapor Deposition Method
Chemical vapor deposition (CVD) is an established, low-cost, and highly efficient technology for fabricating various semiconductor materials from gases. Compared to the PSC device fabrication process using the spin coating method, CVD methods produce significantly higher device performance. The CVD variants include dual-source evaporation [93][134], vapor–solid reaction [94][135], and the vapor-assisted method, [14]. Liu et al. fabricated high-quality and uniform perovskite films using the dual-source co-evaporation method [93][134]. This method is reliant on high temperatures and high vacuum conditions. Alternatively, the vapor–solid reaction method can be used as a substitute, which deposits the perovskite film at a low temperature [94][135]. Chen et al. used this method on a 64 cm2 PSC device and obtained a mean PCE of 6.0% over an active area of 1.5 cm2 [94][135].2.8. Sequential Evaporation Method
This method involves the separate vapor deposition of multiple film layers on top of each other and the subsequent conversion of these multiple film layers through diffusion and recrystallization. First, the metal halide layer is deposited on top of the conductive glass and then on the organic halides. The sequential deposition demonstration of MAPbI perovskite films showed a significantly lower small-scale PCE of 5.4% for a device without a hole-transporting layer [95][140]. However, this method might not be ideal for optimum commercial scaling as the throughput and alternating evaporation might slow material utilization. The highest efficiency achieved for sequentially evaporated PSCs was 17.6% for small-area devices; this was accomplished by optimizing the system’s pressures through the evaporation steps, which also had a binding effect on the morphology [96][141].
2.9. Co-Evaporation Method
This method is the most applicable vacuum-based process for several applications. The perovskite films are prepared inside a high-vacuum chamber with a pressure of 10−5–10−6 mbar, where the precursor solutions are loaded in separate crucibles and heated to their corresponding sublimation temperatures [97][132]. Co-vapor-deposited PSC films are smooth and homogeneous, with modules achieving high PCEs of 16.5% [98][130]. Additionally, the method enables the preparation of multilayer films and is completely compatible with conventional semiconductor manufacturing methods.2.10. Flash Evaporation Method
In this process, the perovskite materials are positioned on a metallic heater and transferred to a vacuum. A large high-current voltage is transmitted through the metal heater, which activates all the perovskite substance, causing it to swiftly vaporize and condense onto a substrate to form a thin perovskite film. Longo et al. established that the flash evaporation method could be applied for the deposition perovskite materials [98][130].
Two-dimensional (2D) and three-dimensional (3D) layered metal halide perovskites doped with aliphatic or aromatic ammonium cations have been synthesized through this method. The hybrid film is synthesized at temperatures that are sufficiently high for the inorganic material to volatilize without deteriorating the organic part [40][98][99][40,130,145]. Homogeneous and smooth polycrystalline MAPbI3 thin films of the required thickness can be achieved by carefully adjusting the process variables. The final layer’s thickness and homogeneity are attained if the metal heater is coated with the deposited material. Longo et al. obtained a PCE of 12.2% for a PSC fabricated through this technique [98][130].
2.11. Vacuum Thermal Evaporation Method
With this method, materials are sublimated by heating them under high vacuum conditions (pressures ≤ 10−6 Pa) and allowing the resulting vapor to condense onto a cooler substrate [40]. The evaporated particles are extended under high vacuum conditions. The sublimated particles move away from the heated source in a deposition cone to reach the substrate. The film’s deposition uniformity depends on the distance from the evaporation source to the substrate. The reduced material production resulting from parasitic condensation on the vacuum chamber walls is a trade-off [40]. Large-scale manufacturing processes use linear deposition sources with a substrate that moves orthogonally through the elongated evaporation cone.Cimaroli et al. used the vacuum thermal evaporation method to produce perovskite films for the comparison of low-temperature processed SnO2 at 185 °C with high-temperature processed SnO2 at 500 °C [100][146].
