Among the most plentiful and environmentally friendly renewable energy sources, solar energy has the ability to both lessen the environmental effects of fossil fuels and supply the world’s growing electricity needs. Still, there are a number of obstacles facing modern photovoltaic (PV) technologies, including high costs, poor efficiency, inconsistent performance, and environmental concerns. Consequently, in order to circumvent these restrictions and allow for the widespread use of solar cells, new materials and technologies must be created. Given their remarkable advancement in power conversion efficiency (PCE), which has increased from 3.5 to 25.8% in just ten years, perovskite solar cells (PSCs) have emerged as a promising candidate for the next generation of PV technology ^[1][2][1,2]. PSCs are made up of a layer of perovskite materials, which are hybrid organic–inorganic compounds with the general formula ABX₃, where X is a halide anion (like iodide, bromide, or chloride), A is a monovalent cation (like methylammonium, formamidinium, or cesium), and B is a divalent metal (like lead or tin). The perovskite layer is positioned between two electrodes, usually a transparent conductive oxide (TCO) and a metal, and two charge transport layers (CTLs), typically an electron transport layer (ETL) and a hole transport layer (HTL). PSCs operate on a similar principle as conventional dye-sensitized solar cells (DSSCs) or organic solar cells (OSCs), in which light absorption by the perovskite layer produces electron–hole pairs (excitons), which are subsequently transported to the electrodes and separated by the electric field at the perovskite/CTL interfaces, producing an open-circuit voltage (V_oc) and a photocurrent. Compared to DSSCs and OSCs, PSCs have a number of advantages, including a higher absorption coefficient, a longer diffusion length, a lower rate of recombination, and a higher degree of defect tolerance. These factors raise PCE because they increase both the V_oc and the short-circuit current density (J_sc) ^[3][4][3,4].

As anticipated above, a conventional PSC device consists of five fundamental layers: the conducting substrate (typically indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO)), the HTL, the perovskite light-absorber layer, the ETL, and the metal electrode (mainly copper (Au) or silver (Ag)) [5]. When the solar cell is illuminated, the ETL/HTL extracts photogenerated electrons/holes from the perovskite absorber layer and transports them to the cathode/anode [6].

Limiting the analyses to the role of HTL in maximizing the PCE of the solar cell, the latter should exhibit highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels properly aligned with those of the other device components, namely the perovskite absorber layer and the back electrode. Taking the vacuum condition as a reference, the HOMO level of HTL should be higher in energy than the perovskite HOMO so as to facilitate charge transportation. Secondly, the LUMO of the HTM should be as high as possible with respect to the perovskite LUMO, since this energy misalignment would avoid electrons moving from the perovskite to the HTL, thus reducing the recombination exerting an electron-blocking effect. Moreover, the HOMO level of the HTL should lie just below the Fermi level of the back electrode in order to assure a fast charge collection [7].

In recent decades, two typical PSC structures have been proposed, i.e., mesoporous and planar [8]. The mesoporous structure consists of an FTO/ITO substrate, a hole blocking layer, a scaffold that can be either conductive TiO₂ or insulating Al₂O₃, a perovskite absorber, an HTL, and the top metal contact electrode. Planar PSCs are less difficult to manufacture due to the absence of high-temperature processes and are commonly classified into two types based on the location of charge transporting layers in the devices: conventional n-i-p structures and inverted p-i-n structures [8]. In conventional planar n-i-p type perovskite devices, the HTL is located between the perovskite layer and the metal electrode (Ag or Au) and the ETL layer is deposited on the bottom transport layer. However, in inverted planar p-i-n structured PSCs the depositions of HTL and ETL are inverted. In inverted p-i-n PSCs, the electrons are extracted by electron transporting materials from the perovskite layer and transported into the metal electrode [9].

Even with PSCs’ outstanding accomplishments, there are still plenty of obstacles to overcome and chances to advance the development and application of this technology. Among the principal difficulties, it is worthwhile mentioning:

(i)

Stability: PSCs’ performance and lifespan can be negatively impacted by exposure to moisture, oxygen, light, heat, and mechanical stress. The primary causes of PSC instability are the interfacial reactions between the perovskite and CTLs, the intrinsic instability of the perovskite materials, and the deterioration of the electrodes and encapsulation materials ^[10][11];

(ii)

Scalability: PSCs are primarily made using solution-based techniques like inkjet printing and spin coating, which work well for small-area devices but not for large-area modules. Therefore, the development of scalable fabrication techniques that can produce high-quality, uniform perovskite films and devices over large areas is required. Examples of these techniques include roll-to-roll processing, doctor blading, slot–die coating, and spray coating;

(iii)

Toxicity: Lead, which is a common ingredient in PSCs, is a heavy metal that is toxic and poses major health and environmental risks. Thus, it is necessary to reduce the amount of lead by alloying it with other metals or using mixed-halide perovskites, or to replace lead with less toxic or non-toxic alternatives like tin, bismuth, or antimony.

The following are some of the primary avenues for PSCs development:

(i)

Tandem cells: By varying the perovskite materials’ composition, PSCs can exhibit a band gap that is tunable. In order to harvest a wider spectrum of solar radiation and achieve higher PCE, this allows for the fabrication of tandem or multi-junction cells, where two or more PSCs with different band gaps are stacked on top of each other, or on top of a silicon or thin-film solar cell;

(ii)

Flexible and wearable devices: Using low-temperature and solution-based techniques, PSCs can be fabricated on flexible substrates like plastic or metal foils. This allows for the integration of PSCs with wearable electronics like smart watches, sensors, or displays, as well as the creation of flexible and lightweight solar modules for a variety of uses ^[11][12][13][12,13,14];

(iii)

Perovskite light-emitting diodes (LEDs) and lasers: PSCs can also function as light-emitting devices by introducing electrons and holes into the perovskite layer through an applied voltage; within this layer, electrons and holes recombine and emit light. High brightness, color purity, and tunability have been demonstrated by perovskite LEDs and lasers, which can be used for illumination, displays, or communication.

Solution- and vapor-based techniques are the main methodologies for producing perovskite absorber layers, the vapor-assisted method providing better film uniformity. Solution-processed deposition is performed using various techniques such as spin coating, doctor blading, screen printing, slot-die coating, spray coating and inkjet printing. Vapor-based techniques, instead, are further classified into two main categories, namely physical- and chemical-based ^[14][137]. Vapor deposition process produces highly crystalline and uniform nanometer-thick films compared to micrometer-thick films using solution-processed techniques. Furthermore, vapor-deposited thin films are uniform, whereas solution-processed layered thin films exhibit much larger crystal grain sizes than the field of view. The main advantages of vapor deposition processes over solution processing techniques are that multilayer films can be produced over large areas and charge collection at interfaces can be easily tuned by using vapor deposition processes. Vapor deposition is therefore one of the preferred methods for producing solar cell layers of uniform thickness ^[15][139]. The shortcoming of vapor-based techniques is the requirement of vacuum. In vapor-based techniques, vacuum is used to increase the mean free path of the vapors for producing highly uniform thin films of very high purity ^[14][137]. The main solution- and vapor-based PSC fabrication techniques are thoroughly reviewed in the following paragraphs.

2. Solution-Based Techniques

2.1. Spin Coating

Spin coating is the simplest and most cost-effective solution-processed technique for uniform deposition of perovskite layers of the PSCs. This technique is mainly exploited to produce small-area solar cells. Spin coating is divided into one-step and two-step methods. In the one-step method, a mixture of PbX₂ (X = Cl, I, Br) and AX (A = MA, FA, Cs) is dissolved in a polar aprotic solvent such as N, N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) ^[16][140]. The perovskite film is formed by spin coating the synthesized perovskite precursor solution onto the substrate ^[17][141]. In the two-step method, films are deposited by separately synthesizing two different precursors; using this method, the control of the process is easier compared to the one-step method ^[18][142]. Typically, the films are baked after spin coating to produce a well-crystallized perovskite layer, as baking results in strong adhesion and bonding between metal cations and halogen anions ^[19][143]. By adjusting spin speed, acceleration, and spin coating time, film thickness and quality can be optimized. In 1998, Liang et al. ^[20][144] first reported the preparation of PSCs using a two-step spin coating method, while the highest efficiency recorded at laboratory scale using this technique was about 22.1% ^[21][145]. The spin coating technique can be employed to fabricate inverted as well as regular PSC structures. The efficiency achieved with this technique is very high with a good level of reproducibility and morphology control in case of smaller sizes of modules, conversely this technique has the drawback of not producing a uniform film on larger areas ^[14][137]. Moreover, due to the slow processing speed and consistent material wastage, this technique is not a good solution for large-scale PSC production ^[22][146].

2.2. Doctor Blading

Doctor blading, also known as blade coating, is a cost-effective, efficient, and simple method to produce films. In this technique, a continuous and uniform wet film is formed after scraping of the precursor solution dropped on the substrate ^[23][147]. Then, the film transforms into a perovskite film after annealing ^[24][148]. Doctor blading consistently reduces the consumption of precursor solution, requiring only 10 to 20 μL for a film with an area of 2.25 cm², much less than the spin coating method (50–100 μL), thus clearly highlighting a higher utilization ratio of raw materials ^[23][147]. Therefore, it is suitable for large-scale industrial production due to its simplicity and cost savings. The thickness and quality of blade-coated films can be optimized by varying the distance between scraper and substrate, the speed and shape of scraper, the concentration of precursor solution, and the wettability of substrate, etc. ^[25][149]. Yang et al. ^[26][150] reported the first fully blade coated PSC in 2015. To improve the solubility and slow down the crystallization rate, the researchers added 1,8-diiodooctane (DIO) as an additive to the precursor solution, creating a dense and uniform perovskite film with large grains. In addition, the whole manufacturing process was carried out in ambient environment, so the impact of humidity variation on device performance was thoroughly investigated. As the humidity gradually decreased from 70%, the PCE markedly increased from 0.53 to 10.71%, which is very close to that exhibited by the device prepared by spin coating in glove box (11.32%). Moreover, flexible PSCs with efficiency of 7.14% were fabricated on PET substrate through the same method. The device also displayed a good stability, namely 80% of the initial PCE was maintained after bending for 100 times with a curvature radius of 1 cm. This work clearly demonstrated the feasibility of blade coating method to achieve high-performance PSCs. In another work, Tang et al. ^[27][151] introduced compositional engineering in (MAPbI₃)_0.6(FAPbI₃)_0.4 precursor solution to reduce the substrate temperature during blade deposition. Interestingly, the addition of a small amount of Cs⁺ (less than 5 mol%) and Br⁻ (2.5 mol%) into the perovskite precursor solution was found to enhance phase purity at a lower blade temperature of 120 °C. In addition to composition engineering in the perovskite precursor solution, 5 mol% of excess methylammonium chloride (MACl) was added into the precursor solution to improve film coverage and eliminate pinholes, resulting from a delayed crystallization. The blade coated PSCs showed a PCE of 19.5% with a J_sc of 23.1 mA/cm², a V_oc of 1.09 V, and an FF of 77%.

2.3. Screen Printing

Screen printing is a widely employed film deposition technique in which a mesh (screen) is used to transfer a paste (ink) onto a flat substrate. The printed patterns are determined by the open mesh apertures of the screen. Unprinted areas are made impermeable to the paste by a blocking stencil. The paste is placed on the unprinted areas, and a printing squeegee is moved across the surface of the screen to fill the open mesh apertures with paste. The paste that is in the mesh apertures is pumped or pressed by capillary action to the substrate in a controlled and prescribed amount (the printed wet film is proportional to the thickness of the mesh and/or stencil). As the squeegee moves toward the back of the screen, the tension of the mesh pulls the mesh up away from the substrate (called snap-off), leaving the paste on the substrate surface ^[28][152]. For screen printing, the deposition area can reach several square meters, and the material utilization can be as high as 100% for a continuous process. The production capacity can be determined by the subsequent drying and/or sintering process, not limited by the screen printing process. The reproducibility can be affected by the characteristics of the paste and the screen voltage variations. In addition to the above-mentioned advantages, screen printing is also a low-cost and low-consumption technique that can provide PSCs with low production costs ^[28][152]. A triple mesoporous carbon-based hole transport layer (HTL)-free PSC (C-PSC) stack fabricated at large scale A4 size modules was demonstrated, using commercially available screen printable pastes and using only 1.6 mL of perovskite solution per module ^[29][153]. As expected, patterning the TiO₂ blocking layer (BL) led to the improvement of both V_oc and FF due to the reduced contact resistance of the carbon/FTO connection compared to carbon/BL/FTO. A further improvement simply came from storage in the dark at ambient conditions, while no deterioration was observed even after hundreds of hours at 70% RH. Interestingly, the best module with patterned BL still yielded a PCE as high as 6.6% (6.3% stabilized) two months after production.

2.4. Slot-Die Coating

Slot-die coating is one of the most promising techniques to fabricate large-scale PSCs through roll-to-roll strategy ^[30][154]. This methodology works similarly to blade coating and the main difference is that the scraper is replaced with a coating head composed of two metal sheets, which feeds precursor solution from the narrow gap ^[31][155]. The use of slot-die printing in PSCs manufacturing can help preventing precursor contamination during the coating process. This is due to the isolation of the solution in the feeding system, which offers an advantage in terms of film quality and repeatability. Film uniformity and thickness can be finely tuned by varying the precursor solution concentration and the relative speed of coating head. Meanwhile, this technology shows a high solution utilization rate and exhibits a greater tolerance towards solution viscosity, concentration, and composition. Furthermore, as a typical non-contact method, slot-die coating can avoid the scratch between the coating head and substrate especially on rough surfaces ^[32][156]. Hwang et al. ^[33][157] first fabricated PSCs using a two-step slot-die coating method. They discovered that the drying process, which employed high-pressure nitrogen, played an important role in obtaining a uniform PbI₂ film. Meanwhile, increasing the annealing temperature facilitated the growth of perovskite grains. More specifically, a perovskite film with small round grains and poor uniformity formed at room temperature, while compact perovskite films with large grain size of about 1 μm were obtained at 70 °C. As a result, a PCE of 11.96% was achieved by slot-die coating, confirming its feasibility in PSCs production. The feasibility of manufacturing modules using slot-die coating is raising increased interest after optimization of processing parameters. Lee et al. ^[34][158] integrated wind blade on slot-die coating and a PCE of 8.3% was obtained on a module with an active surface area of 10 cm² by adjusting the additives ratio (PbAc₂ to PbCl₂). Combined with the laser scribing, Di Giacomo et al. ^[35][36][159,160] fabricated a PSC module with an area of 12.5 × 13.5 cm². The average PCE obtained over a small area (30 × 30 mm²) was 16.4%, which is comparable with the one manufactured by spin coating.

2.5. Spray Coating

Spray coating has attracted widespread attention as an effective method to prepare large-area perovskite films ^[37][161]. The precursor solution is initially atomized into small droplets, which are then sprayed onto the substrate with the help of a carrying gas, resulting in the formation of a uniform, compact, and continuous wet film, and finally crystallized into a perovskite film upon solvent evaporation by heating. A spray equipment is usually assembled with three main parts: an atomizing nozzle, an injection pumping system and a heating plate ^[23][147]. Depending on the operating mode of the nozzle, spray equipment can be classified into pneumatic, ultrasonic, and electro spraying, of which the first two types are more commonly used ^[38][39][162,163]. The quality of the perovskite film fabricated by spray coating can be influenced by a series of processing parameters, which include the distance and relative speed between nozzle and substrate, crystallization temperature, flow rate, viscosity and composition of the precursor solution, etc. However, the possibility of obtaining low-cost, high-quality perovskite films and a highly reproducible manufacturing process make spray coating a very suitable strategy for preparing large-scale PSCs ^[40][164]. Spray coating was first applied by Barrows et al. ^[41][165] for PSC fabrication in 2014. A precursor solution consisting of PbCl₂: MAI was sprayed onto a heated substrate (75 °C), then CH₃NH₃PbI_3−xCl_x was formed after annealing at a low temperature of 90 °C. In this study, it was proved that perovskite film quality can be effectively tuned by the substrate temperature, which plays a major role in the film nucleation and growth. In addition, the film thickness can be adjusted by controlling the nozzle height and relative speed. Finally, a PCE of 11.1% was achieved on a device area of 0.025 cm². This study demonstrated the feasibility of using spray coating as an effective fabrication method for PSCs. Heo et al. ^[42][166] prepared a MAPbI_3−xCl_x film by a two-step spray coating method. A low volatility solvent, γ-Butyrolactone (γ-GBL), was introduced into the perovskite precursor solution. The relationship between perovskite crystal size and the solvent inward (F_in) and outward (F_out) flux was thoroughly studied. The bottom of the perovskite film couldn’t dry completely when F_in ≫ F_out (DMF:GBL = 7:3), resulting in a rough perovskite film. While F_in ≪ F_out (DMF:GBL = 10:0) led to rapid crystallization, forming small-grained perovskite films. Only when F_in was close to or slightly higher than F_out (DMF:GBL = 8:2), precursor solution could re-infiltrate into crystallized perovskite layer and dissolve the smaller grains, thereby creating a uniform film with improved quality after the recrystallization. Consequently, a PCE of 18.3% was achieved on a 2.5 × 2.5 cm² PSC. Additionally, a small module (10 × 10 cm²) with the structure FTO/TiO₂/MAPbI_3−xCl_x/PTAA/Au was fabricated and a PCE of 15.5% was achieved on an effective area of 40 cm² after process optimization.

2.6. Inkjet Printing

Inkjet printing is a process that is similar to using a printer. More in detail, a small nozzle drips a tiny amount of solution onto a substrate at a specific speed, and the uniform droplets coalesce into a wet film by carefully adjusting the spacing. Similarly to spray coating and slot-die coating, it is a non-contact, simple and low-cost film deposition method featured by a high utilization ratio of precursor solution. Moreover, specific patterns can be printed directly without the need for a mask by precisely tuning the size and position of droplets ^[23][147]. Inkjet printing can be divided into two categories based on the solution delivery method: continuous inkjet (CIJ) and on-demand inkjet (DOD) printing ^[43][167]. DOD is a reliable and cost-effective industrial technology widely used for printing electronic and optoelectronic materials, such as metal nanoparticles, polymers, PSCs, etc. ^[44][168]. The properties of the film can be adjusted by varying the nozzle distance to substrate, velocity, droplets size and patterns. Wei et al. ^[45][169] exploited the scalable inkjet printing technology to fabricate PSCs with FTO/TiO₂/MAPbI₃/carbon configuration. A two-step deposition method was adopted, firstly spin coating the PbI₂ film and subsequently inkjet printing the MAI/carbon ink. The resultant PSCs, based on the mixed MAI/carbon ink, exhibited a markedly reduced charge recombination due to enhanced interfacial contact between MAPbI₃ and the carbon layer compared to the counterpart based on infiltrated MAI on FTO/TiO₂/PbI₂/carbon ink. The inkjet printed PSCs showed a PCE of 11.60% with a J_sc of 17.20 mA/cm², a V_oc of 0.95 V, and an FF of 71%. Li et al. ^[46][170] reported the effects of printing table temperature and MACl additive in precursor solution on crystal growth in correlation with photovoltaic performance. The inkjet printed PSCs showed a PCE of 12.3% with a J_sc of 19.55 mA/cm², a V_oc of 0.91 V, and an FF of 69%.

3. Vapor-Based Techniques

3.1. Physical Vapor Deposition (PVD)

PVD covers a wide range of film deposition techniques such as evaporation, laser ablation deposition, vacuum arc deposition and many different modes of physical sputter deposition ^[47][171]. PVD processes usually involve individual atoms or small clusters of atoms which are not normally found in the gas phase. Typically, these atoms are removed from a solid or liquid source, pass through an evacuated chamber, and impinge on a solid surface at which point the atoms stick and form a film. The ways to remove the atoms from the original source can be by heating the source or energetic particle bombardment by electrons, atoms, ions, molecules or photons. PVD differs from chemical vapor deposition (CVD) in that the main source of deposited species is solid or liquid, unlike a gas, and takes place at a vapor pressure much below the working pressure of the deposition system. In 2013, Liu et al. ^[48][172] first reported the fabrication of perovskite films (MAPbI_3−xCl_x) using PVD. More specifically, the perovskite thin films were deposited onto a spiro- OMeTAD substrate using the dual-source co-evaporation approach: the organic source used was MAI, while the inorganic source was PbCl₂. Perovskite films produced by PVD were compared to those fabricated through spin coating method, and their X-ray diffraction (XRD) analysis confirmed that both the thin films showed an orthorhombic crystal structure. Additionally, the perovskite films manufactured by the vapor deposition method were more uniform and denser compared to the spin coated ones. Finally, PSCs fabricated through PVD method exhibited a PCE of 15.4%, thereby demonstrating the feasibility of using PVD for the production of PSCs. Ono et al. ^[49][173] exploited PVD technique to fabricate large-area (5 × 5 cm²) semi-transparent perovskite films, reaching a PCE of 9.9%. It is worthwhile mentioning that the semi-transparent perovskite films with a thickness of approximately 50 nm can be easily fabricated through PVD, while it is challenging to achieve such thin perovskite layers through spin coating. The successful fabrication of large-area perovskite films represents a significant step forward in advancing the commercialization process of this technology.

3.2. Chemical Vapor Deposition (CVD)

CVD is a process in which semiconducting materials and thin-films are prepared exploiting the chemical reaction of vapor-phase precursors. Production involves evaporation and transport of reactants through gas flow into the reactor’s quartz tube. The reactions between intermediate ions occur in the reaction zone of the quartz tube. The precursor formed after the reaction is transported to the substrate and subsequently is adsorbed onto the substrate surface from thin film surface diffusion, nucleation and chemical reaction. The unreached fragments flow out of the reaction zone of the chamber to minimize the unreacted impurities ^[50][174]. In 2015, Fan and co-workers ^[51][175] reported an easy one-step CVD to deposit MAPbI₃ and MAPbI_3–xCl_x perovskites. Inorganic (PbI₂ or PbCl₂) and organic sources (i.e., MAI) were loaded in the high-temperature zone in positions determined by their sublimation temperatures. Meanwhile, the substrates were placed in the low-temperature zone. During the deposition process, a carrier gas (i.e., Ar) was constantly flowed from the source toward the substrate to facilitate the chemical reaction. MAPbI₃ and MAPbI_3–xCl_x films with large grains (>1 µm) and long carrier lifetime were deposited on substrates and, once employed in PSCs, allowed achieving a PCE in the range of 9–11%.

3.3. Hybrid Vapor Deposition

Hybrid chemical vapor deposition process has been developed to fabricate PSCs based on methylammonium lead iodide (MAPbI₃) and formamidinium lead iodide (FAPbI₃) ^[52][176]. In this method, perovskite films are formed in two stages. In the first step, a lead halide film such as PbI₂, PbCl₂ or PbBr₂ is deposited through thermal evaporation. In the second step, the as-prepared lead halide film is reacted with organic halide species under a controlled vapor atmosphere and pressure to form perovskite films inside a tube furnace. Yokoyama et al. ^[53][177] proved the advantageous role of kinetically regulated gas-solid reactions in the fabrication of CH₃NH₃SnI₃ thin films by the introduction of a low-temperature vapor assisted solution method. Compared to the one-step process, this method allowed fabricating compact films with excellent surface coverage. Furthermore, it has been demonstrated that by using this technique, the short-circuit behaviour frequently observed in conventional one-step manufacturing perovskite devices can be effectively avoided. However, there is a lack of in-depth research on this two-step vapor deposition method for inorganic CsPbBr₃ PSCs.

4. Other Techniques

4.1. Sol-Gel Method

The sol-gel method is a bottom-up synthesis technique commonly adopted to produce a wide range of materials, including inorganic membranes, monolithic glasses and ceramics, thin films, and ultra-fine powders. Today, it is even used to synthesize 1D nanomaterials ^[54][178]. The basis of the sol-gel method is to produce a homogeneous sol from the precursors and transform it into a gel. The solvent present in the gel is then removed from the gel structure and the remaining gel is dried. The properties of dried gel largely depend on the drying method. In other words, the “solvent removal method” is chosen based on the application for which the gel will be used ^[55][179]. The sol-gel method highlights many advantages over traditional processing technologies, including low reaction temperatures, precise composition control, high purity levels, and the ability to create processes for large-area applications ^[54][178]. Within this framework, in 2013 Grätzel and co-workers ^[56][180] applied for the first time a two-step sol-gel process to fabricate PSCs with a noticeable PCE above 15%. The two-step sol-gel process allowed an easy control of the surface morphology of perovskite films and the reduction of the defect content of the films. For the two-step sol-gel process, the perovskite is in direct contact with the residual PbI₂ film and not with the porous TiO₂ layer to avoid direct contact between the mesoporous TiO₂ layer and the HTL layer. Although the direct contact between the perovskite and the residual PbI₂ film increases the contact resistance, this is beneficial for PSCs when the perovskite light-absorber layer is not well fabricated.

4.2. Electrodeposition

Electrodeposition is a versatile and roll-to-roll compatible technique used for the manufacturing of PSCs ^[14][137]. Its cost-effectiveness, rapidness, and high degree of uniformity make it a desirable technique for the deposition of perovskite layers ^[57][181]. Unlike spin coating, this technique does not require heating of the substrate because heating will produce a rough film. When heat is applied, film rupture and island formation occur randomly on the surface of the substrate ^[58][182]. The thin films produced by electrodeposition are very uniform with large-area coverage and the absence of sheer forces ^[59][183]. Deposition of perovskite layer on complex shaped substrates, which is not possible with the other techniques mentioned so far, makes it a very attractive approach for large-scale production ^[57][181].

4.3. Laser Ablation

The laser ablation process is a subtractive method that allows fabricating micropatterns through the removal (ablation) of a small fraction of a substrate material under a focused pulsed laser beam. A group of materials scientists from the University of Rome Tor Vergata, in collaboration with researchers in Germany, exploited the laser ablation technique to reduce the cell-to-module losses in perovskite solar modules ^[60][184]. The scientists investigated the layer structure of planar PSCs in three patterning steps, i.e., P1, P2 and P3, and determined the width of the perovskite cells to electrically isolate the two from each other by separating the two contact layers with P1 and P3. An absorber layer was obtained by maintaining the P2 structuring between P1 and P3 to provide an electrical interconnection. To achieve a higher cell-to-module efficiency ratio, the area between P1 and P3 was kept as small as possible to avoid the challenges of preserving the integrity of the edge regions of the absorber layer during P2 patterning. By optimizing the laser design, establishing a relationship between geometrical fill factor, cell area width, and P1–P2–P3 laser parameters, a 20 × 20 cm² minipanel was fabricated showing 11.9% stabilized efficiency and 2.3 W on an active area of 192 cm², among the highest reported in literature at this size ^[60][184].