The biggest issue facing the commercialization of perovskite solar cells is a lack of stability. These problems arise from the chemical interactions within the perovskite structure. The interactions mostly consist of weak ionic bonds, e.g., the Pb-I bond has a bonding energy of 142 kJmol⁻¹ [115]. Other secondary interactions are hydrogen bonding and van der Waals forces, giving the material its soft nature [116]. Perovskite solar cells are inherently sensitive to heat, light, moisture, electric fields and oxygen that can all degrade the cell [117]. Particular sensitivity to water and moisture was highlighted by Niu et al., concluding that iodide perovskites have a negative standard Gibbs free energy with regards to moisture degradation [118]. These stability issues must be addressed in order for perovskite cells to have a long operational life and be commercialized. Further research must be conducted to determine the exact degradation mechanisms, allowing appropriate stabilisation and encapsulation approaches to be developed. Figure 7 summarizes various strategies that can be adopted to improve the device stability along with overall device performance.

Composition engineering is an effective approach for the stability and performance enhancement of the PSCs [119]. In this strategy, the anion and/or cation of the perovskite material with crystal structure ABX₃ is substituted either partially or totally with a desirable material improving the optoelectronic property of the device. In the perovskite material MAPbI₃, the conduction band minima are directed by the p-orbital of Pb. The valance band maxima are directed by s and p orbital of Pb and p-orbital of I. The A-site of the perovskite structure does not contribute to the band edge but contributes to the deep energy level of the material. Studies suggest that on tuning A-site the optoelectronic properties of the material can be easily tuned [120]. These modifications also improve other factors, such as thermal stability. The substitution of FA in place of MA at A-site, leads to a stronger bond formation with PbX₆ octahedra due to the presence of a larger number of H-bonds. This not only modifies the optoelectronic property of the material but also enhances the thermal stability [121]. The substitution of an inorganic cation such as Cs in place of MA has proved to form stronger bonds as compared to H-bonds. Similarly, mixed halide perovskites have exhibited better stability and efficiency [122]. The partial substitution of X-site I by other halides is another strategy of composition engineering. The substitution of I by Br and Cl increases bandgap and reduces the device efficiency. Therefore, limited substitution has to be done such that efficiency is not much reduced and better stability is attained [123]. Apart from the basic components, the addition of certain materials as an additive has the capability to enhance the performance of the PSC. Several studies have been conducted and certain materials are proven beneficial for the PSCs. Additives can be classified into two categories: (i) additives with precursor of one-step method; (ii) additives with organic solvent at two-step solution method, and additives with antisolvents. This is a broad research field and several reviews based on it can be easily obtained from the literature [124]. The material used along with the quality of the charge transport layer (CTL) affects the performance of the PSCs. The material for the electron transport layer (ETL) must be selected such that the conduction band edge is either equal to or lower than the conduction band of perovskite material, such that the photogenerated electrons from the perovskite layer can be easily received. The valance band of the ETL must lie below the perovskite layer, such that the generated holes can be blocked [125,126]. Similarly, the material for the hole transport layer (HTL) must be selected such that the valance band maxima of the material must be in the proper position to receive the photogenerated holes and the conduction band must be at a high position to stop the photogenerated electrons. In order to attain higher efficiency, proper band alignment between the perovskite layer and CTLs must be done. Energy band tuning is the sole field of the study conducted in the PSCs to attain the best possible performance [127]. The proper energy band tuning ensures minimal interfacial recombination [128]. Apart from proper band alignment of the CTL other properties such as thermal stability, chemical reactivity, etc. impact the performance of the cell to a great extent. Defect passivation is another important strategy to overcome the issue of stability. Certain defect passivation techniques such as interlayer engineering, perovskite film formation conditions, film post-treatment, grain boundary passivation and more are being used by the researchers. The intrinsic stability of the perovskite film is attributed to the crystallographic and molecular structure of the material. The lattice imperfections within the material are dependent on stoichiometry, annealing time, annealing temperature, film deposition method, deposition time, and more [129]. The existence of the defects is determined by the defect formation energy of the material. All the above-stated conditions decide the defect formation energy of the material. Apart from the synthesis conditions, external factors such as moisture, heat, temperature, oxygen, etc. lead to the formation of the new defects during the operation of the PSCs, which in turn has a degrading impact [130]. This rapid degradation of the PSCs has hampered the commercialization of the PSCs despite attaining good performance.

Long-term device stability is a vital parameter to determine the commercialization of PSCs. The device stability is evaluated in terms of device lifetime tested under 1-sun illumination at electric load. However, PSCs have not attained stability comparable to Si PVs because of their van der Waals interaction causing ion immigration, photo-degradation and phase segregation. Further, the poor intrinsic stability and soft ion lattice due to weak H-bond lead to poor stability.

The various commercialization companies are Oxford PV GmbH (Brandenburg, Germany), Swift solar (Sancarlos, CA, USA), Solaronix (Aubonne, Switzerland), Saule Technology (Wroclaw, Poland), Microquanta Semiconductor (Hangzhou, China) etc. [131]. Oxford PV developed the world’s first full-size 100MW production line [132]. Many commercial companies in China and other countries are working on industrialization of PSCs such as GCL perovskites, Microquanta and a few more. Table 3 summarizes the details of the large-scale PSC modules. One potential issue for perovskite solar cells is the scalability needed for commercialization. The spin-coating method used for the majority of laboratory-scale tests is not effective for producing large-scale uniform coatings. This is because of the lack of consistency in film thickness over a large area, large material waste and lack of compatibility with the roll-to-roll processing that has a high throughput [133,134]. Industrial-scale techniques such as screen printing and slot-die coating have been identified as the most promising solutions to this [135]. They have already been used successfully to fabricate modules over 100 cm².

For more than half a century, silicon PV technology dominates the largest PV market. Studies suggest that in order to obtain the highest efficiency from tandem cells, a wide bandgap perovskite material of bandgap of order 1.7 eV with a thickness of the order of 1 µm must be used. The synthesis of such perovskite material is a hot topic of research among the researchers of the PV community. If a perovskite material of bandgap 1.7 eV is obtained, then extracted voltage will reach about 1.3 V rendering the overall voltage of the device to be 2.0 V. In order to extract similar currents from both the cells, the thickness of the perovskite material must be of order 1 µm with a bandgap of about 1.7 eV. However, preparing high-quality microns-thick perovskite material is still a challenge. Another way to enhance the efficiency of the solar cells is to modify the tunnel junction material [145]. The widely used transparent conductive oxide (TCO) is indium tin oxide (ITO). ITOs are not considered ideal due to their improper transmissivity. The ITOs exhibit parasitic absorption at a range of 800 nm [146]. An ideal tunnelling junction must have high conductance and high transmittance in order to minimize recombination loss. An appropriate refractive index and thickness are also important to minimize the anisotropic conductance, and internal reflection and avoid lateral breakdown. It can be said that industrialization of tandem cells is completely dependent upon the development of perovskite materials.

Currently, state-of-the-art perovskite solar cells still require the use of lead (Pb²⁺) as the B-cation site. Lead is a toxic element and its use could present problems if released into the environment, eventually working its way into the human food chain [147]. Therefore, a large amount of research has been conducted into alternative lead-free perovskite materials. Perovskite solar cells based on different elements such as antimony, copper, germanium, bismuth and others have all been tested [148,149,150,151,152]. The strongest candidate appears to be tin, having both a similar ionic radius and electronic configuration. This allows direct replacement of the lead ion in the B-site without a significant phase change. Tin-based perovskite cells have a PCE of around 10–12%, which is significantly lower than lead-containing perovskites [153,154]. It is also important to make sure that environmental burden-shifting is not taking place, as studies have found the oxidation of Sn²⁺ to Sn⁴⁺ can lead to the formation of toxic by-product hydroiodic acid [155]. Overall, Ju et al. state that only once the degradation and toxicity mechanisms of current perovskites are understood will lead-free, stable perovskites be fabricated [156]. Single-junction perovskite solar cells are not the only technology that has seen a large jump in PCE over the previous decade. Tandem solar cells involving perovskite have been developed and are not constrained to the single-junction Shockley–Quessier limit. The efficiency limit for a tandem solar cell is 47%, much higher than the 31% for single-junction [157,158]. This is possible as Tandem solar cells better utilize small wavelength radiation from the spectrum. The tandem cell will have a top layer with a large bandgap material; this will absorb the short wavelength part of the spectrum. The longer wavelength radiation will pass through the top layer and be absorbed by the smaller band-gap bottom layer [159]. Perovskites can be combined with a variety of materials to create two-terminal tandem cells. Perovskite-Silicon has shown good performance reaching a PCE of 29.15%, outperforming the highest achieving monocrystalline silicon cell [160]. Perovskite–Perovskite cells have reached an impressive PCE of 24.8% [161]. Another successful combination of materials is perovskite with second-generation material CIGS, achieving 24.2% [162]. Overall, the high theoretical efficiency of tandem perovskite solar cells is predicted to allow the price of PV to continue to fall over the coming decades [163].

Two-dimensional (2D) Perovskites have also been explored as light absorbers for solar cells due to their wide structural diversity and superior stability compared to conventional 3D perovskites [164]. 2D perovskites are produced by using larger ammonium or diammonium cations. These cations are too large and divide the perovskite structure into 2D layers. The layered structures are placed into several categories, with Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) being by far the most common [165,166,167,168]. RP structures are stacked so that they have two offset layers per unit cell having pairs of monovalent interlayer spacer cations. DJ structures can be stacked directly on top of each other and only require one divalent interlayer spacer per formula unit [169]. A successful example of a RP 2D-perovskite (PEA)₂(MA)₂[Pb₃I₁₀] was synthesized and showed excellent resistance to moisture and allowed high-quality films to be produced after just 1-step [170,171]. However, this material only achieved a PCE of 4.73% [172]. Further improvement of the efficiency was made by improving the charge transport mechanism, reaching a PCE of 12.52% [173]. Although, 2D-perovskites show excellent stability they still lack the efficiency of 3D-perovskite cells. For this reason, they are not suitable for single-junction cells but may be an excellent option for use as the high band-gap absorber in a tandem cell [174]. The applications of perovskite material in PV involve usage in electric vehicles and building integrated technology because of its flexibility and tunable bandgap [175,176]. The increment in stability and performance enables the researchers to work not only on performance enhancement but to seek new applications as well. The proper surface engineering and interface engineering improved the efficiency and stability to a great extent. The performance improvement occurred due to a reduction in defect density within the material leading to a decrement in non-radiative recombination. The performance of PSCs is further improved by the surface passivation using hydrophobic molecules. Liang et al. used organic hydrophobic molecules (Benzylamine) with a side chain to enhance the performance of Formamidinium lead iodide (FAPbI3) films. This modification not only enhanced the voltage from 1 to 1.12 V but the stability improved from three days to four months [177,178]. It can be said that depositing high-quality perovskite film with minimum defects can enhance the performance of the PSCs.

Building integration of perovskite-based PV is probably one of the promising approaches. Building energy currently consumes significant energy which must be reduced by employing an energy-positive building envelope [179,180,181,182,183]. For decades, PV technology is placed in a building in terms of roof integrated or wall integrated technology. These are mainly known as building attached or applied PV. Building integrated type PV application semitransparent PV is a precondition [184,185]. However, previously most investigation was devoted based on first-generation silicon [186,187,188] or second-generation thin film-based PV [189,190,191]. However, the booming Perovskite industry gives high hope for the building industry [192,193]. Perovskite which can have variable [194] or static [195] transparency is the most suitable for building window or façade applications. The recent trend is to include a smart switchable or adaptive window in a building [196,197,198,199,200,201,202,203]. However, these most effective windows can only reduce energy consumption [204,205,206,207,208]. In the future, switchable perovskite can be a dominant player in the building industry which can have the potential to generate benign electricity and also tune the transparency concomitantly.