Factors affecting the stability of perovskite solar cells: a comprehensive review

Abstract. Accelerated depletion of fossil fuel, energy demands, and pollution force us to choose renewable and eco-friendly energy resources. Solar cells are considered as an efficient replacement for fossil fuel. In the family of solar cells, silicon-based solar cells and perovskite solar cells (PSC) have displayed significant power conversion efficiency (PCE). Perovskites have been investigated extensively over the past two decades, due to their advantageous properties, such as high absorption coefficient, efficient carrier mobility, long charge diffusion length, and direct bandgap. These features make PSC a prospective candidate to replace silicon in solar cells. By 2018, PSC achieved an encouraging PCE of 23.3%. However, low stability and toxicity have retarded the commercialization of PSC. With the aim of assisting junior researchers, we consider the latest achievements in this domain and review the field from a stability and performance perspective. We emphasize recent developments and methodologies to overcome drawbacks concerning stability and toxicity.


Introduction
The survival of the human species on Earth is a function of human health, proper climatic conditions, and their affecting factors. Due to heavy energy demands, there has been an increase in the consumption of fossil fuel, which has elevated the pollution levels leading to global warming. Thus, the development of an alternate eco-friendly power source is vital to sustain the life on Earth. The global requirement of power is estimated to grow exponentially every year. Solar cells are considered as a promising way to meet these energy crises and reduce pollution. 1 Since the last few decades, a silicon-based solar cell acts as an ideal solution. The production of siliconbased solar cells has increased by 10 times from 2005 to 2013. Silicon-based solar cells have obstacles such as cost factor, complicated production procedure, and conversion efficiency far less than the optimum value. Perovskites solar cells (PSC), which have various technical advantages, are considered as a better replacement for silicon solar cells and have a high chance for commercialization. To avail this opportunity, PSC and their associated factors have been extensively investigated by researchers.
Currently, silicon-based solar cells have efficiencies over 15%. However, they have a high energy pay-back time and CO 2 emission rate. 2 Cost of module, balance of systems, 1 and operation management, though reducing, is reaching a stagnation value far below the optimum. These drawbacks have generated demands for exploration of new material with high-performance capability. The field of conversion of solar energy to electrical energy has experienced a substantial boost due to development of perovskite-based solar cells. The reported high efficiency of solid-state-sensitized thin film (solar cell based on perovskite CH 3 NH 3 PbI as light harvester) 3 has encouraged investigation in this domain. Figure 1 shows the increment in research papers published in the last few years on perovskites. This increased research confirms that PSC has rapidly evolved as an answer to energy crises; presently (by 2018), it can achieve an efficiency of 23.3% (Table 1). 8,9,12,13,[15][16][17] Further, the National Renewable Energy Laboratory (NREL) graph shows the rapid pace of increment in efficiency achieved in the last few years. 18 Despite such aggressive progress, the contribution of solar cells, to meet the world's electricity requirement is less than 1%. 19 In solar cells, light-to-electricity conversion efficiency η is defined as a product of open circuit current density (J sc ), open circuit voltage (V oc ), and fill factor (FF). Research is going on to achieve power conversion efficiency (PCE) up to Shockley-Queisser limit, which is 33.5% for the single-junction solar cell. 20 Theoretical limits as calculated for a conventional PSC employing CH 3 NH 3 PbI 3-x Cl x absorber (E g ∼ 1.55 eV) are J scðSQ-limitÞ is 27.20 (mA∕cm 2 ), V ocðSQ-limitÞ is 1.28 V, FF ðSQ-limitÞ is 90.2, and PCE ðSQ-limitÞ is 31.4%. 21 However, actual results concerning PCE are limited to 15.4%. It has also drawbacks such as lead toxicity, low operational stability, and large-scale production issues. Recently, Christians et al. achieved operational stability of >1000 h, by tailoring the interfaces of PSC. 22 Although it is a positive sign, perovskites have to accomplish the levelized cost of electricity for residential use as 5.0 cents per kWh and achieve a stable lifespan of at least 20 years by 2030 to achieve successful commercialization. 23 Production of 1 kWh of energy from perovskites would incur a consumption of 64.77% embodied energy and 31.38% of incorporated materials. This wastage of energy and materials assures that there is enormous room for improvement. 24 Figure 2 shows the blue-dotted curve and a solid circle, which indicates the scope for performance improvement. 25 The following four conditions would enable a solar cell to maximize its power output.
• Obey Maxwell-Boltzmann statistics and exhibit a mobility of charge carrier sufficiently high reaching as high as a 10 cm 2 V −1 s −1 . 26 • Radiative band-to-band recombination. Radiative efficiency from band-to-band transition for PSC is higher than organic solar cells. 26,27 • Internal conversion efficiency reaches 100%. • Photon recycling effect occurs in the cell. [27][28][29][30][31] CH 3 NH 3 PbX 3 (where X denotes I and Br) with a band gap of 1.5 to 2.3 eV, 32 close to optimum gap, shows the best performance in PSC. The success of hybrid PSC is due to the properties of organic and inorganic components in it. Organic elements render stability for perovskites and facilitate self-assembly, whereas inorganic component produces extended meshwork by covalence and ionic interactions. This hybrid combination and their interactions allow precise crystalline structure formation. Efficient carrier mobility, (multi) ferroics, 33,34 high absorption coefficient (5.7 × 10 4 cm −1 at 600 nm), long charge diffusion length, [35][36][37][38] low nonradiative Auger recombination, 39 and direct band gap properties define good performance and efficiency of perovskite. In addition, Perovskite materials show a strong PL with a narrow full width at half maximum. 40,41 CaTiO 3 is the original perovskite mineral, which was discovered and named after Russian mineralogist Lev Perovski. In 1978, Dieter Weber was the first to show hybrid perovskites. They initially explored lead halide and later tin, bromide, and iodide. Incidentally, they did not incorporate any improvements as they considered it as another semiconductor. Perovskites started their journey as light absorbers in dye-sensitized solar cells (DSSC). O'Regan and Grätzel reported fabricating DSSC by use of a nanoscopic titanium dioxide (TiO 2 ) particle layer achieving PCE of 7.1%. 42 PCE of a different PSC structure ranges from 17% to 20.1% (9.3% for single junction, 43 20.1% for regular structure, 19% for planar heterojunction, 21.6% for mesoporous, 17% for planar heterojunction, and 18% for inverted planar heterojunction). New metal halide perovskite films were fabricated by a single step method utilizing a molar ratio of 1.05 between PbI 2 /formamidinium Iodine, providing efficiency of 20.8%. 44 These films are used in lightemitting diodes, 45,46 solution-processed photodetectors, [47][48][49] and lasers. 50 Transition metal oxide perovskites, such as LaVO 3 (mott insulators) displays an indirect band gap of 1.08 eV. The d-d transitions and p-d transitions ensure adequate absorption and utilization in solar cells. 51 Heterostructures, such as LaVO 3 grown on SrTiO 3 with a direct band gap of 1.1 eV, are also     In addition, research for fabricating hybrid PSC with various organic cations and metal halide anions is on.
New structures such as double perovskites (A 2 B I B III X 6 ), replace Pb 2þ with nontoxic trivalent/monovalent metals, are reported to yield high efficiency. 62 The family of PSC constitutes structures such as dye-sensitized cells, mesoporous structure, planar (p-i-n and n-i-p) structure, HTL and ETL free cells, flexible cells, and tandem cells. The prominent structures of solar cells are as follows.

Mesoporous Structured Solar Cell
Mesoporous structured solar cell (MSSC) is a third-generation device with low fabrication cost. TiO 2 is the most common mesoporous layer in them. Though TiO 2 nanoparticles are the most commonly used in the mesoporous layer, success reports are available while employing TiO 2 nanosheets, 63 nanorods, and nanofibers. MSSC consists of a single-phase triple-conducting oxide (TCO), blocking layer, mesoporous TiO 2 or Al 2 O 3 scaffold, perovskite absorber, hole transport layer (HTL), and a metal electrode in its construction (Fig. 8). 64 Usually, MSSC fabrication done by a two-step deposition method. The processing temperature is in the range of 150°C. However, highly efficient MSSC devices have a high sintering temperature of 500°C. 65 During high-temperature sintering, the mesoporous layer allows rapid extraction of photoinduced electrons, which shorten the electron transport length. This shortening of length makes MSSC an efficient light harvester. However, they suffer from the various disadvantage of having low open-circuit voltage (V oc ) and lower light absorbance at wavelengths greater than 700 nm. Mesoporous Al 2 O 3 , which acts as an insulator, is found to be more stable than TiO 2 , under continuous illumination. TiO 2 introduces nonstoichiometry defects, such as titanium interstitials and oxygen vacancies, which causes trap sites. 66 Encapsulation of layers can prevent moisture ingress and degradation. Etgar et al. 63

Planar Structure
Film structure has a simple construction as it does not have a mesoporous scaffold or electronic layer in it, facilitating ease in fabrication. Figure 9 shows the architecture of n-i-p and p-i-n variants with different location of the charge transport layer, which is their primary difference. 67 A simple planar junction designed between perovskite and HTM layer, without scaffolding, corresponds to a simple planar thin-film cell. The major problem in the fabrication of this structure is to avoid contact between HTM and blocking layer. Vapor deposition and dualsource vapor method excel in achieving this required feature. Docampo et al. 68 applied NiO x and VO x in planar heterojunction solar cells with CH 3 NH 3 PbI 3-x Cl x as photoactive material. However, these NiO x and VO x interlayers resulted in low photovoltaic performance due to poor coverage on the substrate by perovskite film. 68 Alternatively, UV treatment was reported to enhance surface wetting properties with increased PCE by 7% on these materials. HCl improves crystallization and coverage of PbI 2 in thin film, which subsequently improves the morphology achieving a PCE of 14.8% for perovskite thin film. 69 Planar heterojunction additionally assists in evaluating and testing of the new materials and their effect, for solar cell fabrication. SnO 2 was also experimented to replace TiO 2 as electron-selective contact. SnO 2 was  spin coated and sintered at a much lower temperature of 200°C than TiO 2 . Its fabrication included the solvent vapor annealing, which crystallizes the residual PbI 2 and adds it at the interface of SnO 2 ∕CH 3 NH 3 PbI 3 . PSC devices based on SnO 2 displayed high stability of holding the PCE of 13% till 700 h. 70

HTL/ETL Free Cells
The fundamental task of any photovoltaic device is to create charge carriers, i.e., holes and electrons on the absorption of light energy. Further separating them at respective heterojunction and making them available at their respective electrodes. This separation depends on energy offsets versus exciton binding energy. It defines the formation of two primary interfaces, perovskite/ electron transport layer (ETL) interface and perovskite/HTL interface. The architecture of a few PSCs does not have HTL or ETL layers. As these are not mandatory layers, numerous researchers have developed PSC without HTL 71,72 or ETL with good PCE. Perovskite layer has Au deposited over it while fabricating the HTL-free PSC. In this, perovskite layer conducts the duty of absorber and hole conductor by forming a heterojunction with ETL. Aharon et al. 71 have reported a two-step deposition method to fabricate highly optimized HTL-free PSC with a PCE of 10.85%, FF of 68%, and V oc of 0.84 V. Their performance is strongly dependent  on the width of the depletion layer formed by TiO 2 -CH 3 NH 3 PbI 3 . Depletion layer supports charge separation and restricts back reaction from TiO 2 to CH 3 NH 3 PbI 3 . The width of the depletion layer is evaluated using Mott Schottky analysis. 73 Figure 10 shows a schematic diagram of HTL-free PSC with their energy levels. 71 Figure 11(b) shows ETL-free PSC in-which an insulator (Al 2 O 3 ) replaces TiO 2 scaffold, 64 which proves that perovskite material could effectively transport electrons without an ETL. Production cost for HTL-less cells is high, leading to an increase in device cost. Further, the presence of the HTL layer complicates the mechanism followed for extraction.

Tandem Cells
Tandem cells consist of two formats, i.e., four-terminal and two-terminal devices. Figure 12 shows the arrangement of four-terminal tandem device, which has two-terminal devices/ cells. They are either mechanically stacked or monolithically integrated through tunneling junction. Baillie and his coworkers 74 with copper indium gallium selenide, Asadpour et al. 75 with a highly efficient a-Si:H/c-Si heterojunction bottom cell and Mailoa et al. using the c-Si bottom cell on perovskite/silicon multijunction solar cell have obtained PCE of 30%, 13.4%, and 13.7%, respectively. Due to the limitation of band gap mismatch and current matching constraints, a traditional tandem cell of Si and perovskites could only achieve PCE of 24% and 20%, respectively. Top cell absorbs high energy photons and bottom cell harness the remaining low-level energy photons. MAPbI 3 makes an excellent top layer in c-Si solar cells. Hydrogenated indium oxide, having high mobility, is used as a rear electrode to fabricate semitransparent PSCs. 76 Fabrication of two-terminal tandem solar cells with perovskite as the top layer and kesterite copper zinc tin sulfide (CZTS) as bottom cells was reported to attain a PCE of only 4.6%. This reduction is primarily due to semitransparency of the top aluminum electrode, resulting in transmission loss. 77 Thus, the highly efficient and transparent top layer is a very critical factor. Further, they should be immune to thermal instability and hygroscopicity. Numerous transparent electrodes used here are ultrathin metal such as Au, 78 graphene, 79 carbon nanotubes, 80 and Ag nanowires. 19 However, efficiency achieved was below 10%, and their contacts featured strong absorption in near-infrared (NIR). PSC that are individually efficient underperformed when used as tandem cells, due to sensitivity to a thickness of perovskite layer. Thus, bifacial tandem cell was fabricated, which utilizes scattered light back from the ground (Fig. 13) to achieve a PCE of ∼33%. 75 Further, they are immune to variations in perovskites layer thickness.

Fabrication
Fabrication is a prominent factor in PSC. Fabrication factors such as time, solvent mixtures, doping, and humidity have a marked effect on crystallization of perovskites. 81 Further processing routes and solvents selected induce intermediate phases and influence device performance with film quality. 81 Synthesis and analysis protocol for large-scale production and laboratory analysis are different. Most of the PSC that achieved high efficiency in the lab, on small devices, does not possess commercialization advantage. Large-scale production of PSC should be cost-effective, good stability, and maintain high performance. Figure 14 shows a one-step and two-step spin coating method. The perovskites precursor solution is thoroughly stirred and dropped on to the substrate and accelerated for evaporation of the solvent. 83 In the second step, crystalline perovskites are achieved, after thermal annealing the sample, by removing the residual solvent. CH 3 NH 3 PbI 3 or CH 3 NH 3 PbI 3−x Cl x when fabricated by spin coating led to the formation of scattered nanodots 3 and extremely thin layer. 64 General drawbacks such as poor quality film, small grain sizes, and dense pinholes can be controlled to a limited extent by additive engineering 84,85 to obtain required grain sizes. This process has achieved a maximum PCE of 19.19%. Another major drawback of spin coating is the formation of nonuniform thickness of the film from center to edges. Two-step coating provides full coverage of PbI 2 with cube-like structure. 82 Two-step spin-coated deposition method, as a valid mode for fabrication of PSC, was reported in 2013. It involves step-by-step spin coating of PbI 2 solution and solution of methylammonium iodide (MAI) on the substrate. Whole set up is thermally annealed at 100°C driving the interdiffusion of precursors. Single step method produces uncontrolled precipitation of perovskite, resulting in large morphological variations, which hampers the prospects for practical applications. Thus, in performance sequential deposition process, 12 PbX 2 solution is first spincoated on top of the substrate and then is dipped into a solution of MAI. The performance gets varied according to different soaking time, from 5 s to 2 h. The optimum soaking period was found to be 15 min, which resulted in enhancement of current density and voltage of the device from 10.1 mA∕cm 2 and 933 mV (5 s) to 15.1 mA∕cm 2 and 1036 mV (15 min), 86 respectively. This method can perform successfully for nanostructured devices with planar heterojunction structured PSC.
The sequential vapor deposition process is very similar to the two-step spin coating method except that this method uses vapor deposition, which yields layered multistack thin films over large areas. 17 MAI and PbI 2 were vapor deposited on a substrate as layers before thermal annealing. Yang and his coworkers used vapor-assisted solution process (VASP), also called a blended deposition method, to produce MAPbI 3 films as shown in Fig. 15. 87 A solution of PbI 2 in N; N-dimethylformamide was deposited on the substrate by spin coat method. Later, PbI 2 -coated substrate is annealed in MAI vapor at 150°C in a nitrogen atmosphere for the desired time to yield perovskite films. Though spin-coating techniques offer good thin film uniformity for lab-scale experiments, its drawbacks are concerning wastage of solution during coating, unsuitable for large-scale production, and low material transfer efficiency. However, vacuum and solution processes are two primary methods to fabricate perovskite films starting with inorganic and organic counterparts of PSC. Solution process results in the rough film surface. Vacuum process requires high energy for creation of vacuum conditions, and the flaky nature of MAI powder makes synthesis less controllable. VASP resulted in enhanced controllability of film quality. VASP method takes advantage of the characteristics of the hybrid nature of perovskite materials, particularly the low sublimation temperature of organic halides and fast reaction rate between inorganic and organic species. Full surface coverage, 100% precursor transformation, uniform grain structure, and print size up to 2 μm are highlights of perovskite film derived from this approach. It is free of pinhole defects and yields an efficiency of 15.4%. 17 Snaith and his coworkers synthesized PSC using dual-source vacuum deposition method. In this process, MAI and PbCl 2 were preheated to 120°C and 325°C, respectively, and evaporated together using separate sources, to attain superior uniformity in films. 17 Vapor-based deposition processes have few issues in comparison to solution-based deposition processes, such as an increase in manufacturing cost and capability to achieve increased efficiencies. 88,89 Analysis showed that large crystal would have a noticeable effect on PCE, leading to the development of hot-casting technique. This method guarantees large output and ultrafast production. 90 In this, precursor (iodide) is maintained at 70°C and is immediately transferred to spin coater as drops in less than 5 s. Rotating for 30 s assists in cooling and quenching it fast, results in a uniform film with large grain size. Figure 16 shows a schematic diagram of the hot casting process and displays the proportional relation between temperature and large grain formation. Figure 16(d) shows a comparative chart of grain size and processing temperature for the hot-casting method and conventional annealing method. 91 The comparison confirms that the evaporation rate defines the formation of larger crystals. In 2015, Vak's group have fabricated a fully printed planar-heterojunction PSC by slot-die coating. 92,93 Large-scale perovskites production for roll-to-roll fabrication consisted of various methods, such as slot-die coating, knife coating, spray coating, screen printing, gravure printing, flexographic printing, and inkjet printing. 94 Slot-die is a premetered coating process, in which mass flow determines the thickness of the film. Metered solution feed controls the formation of a uniform thickness of film stripes. In addition, transferring the complete solution to the substrate would lead to zero loss of solvent. This process involves two steps. The first step consists of heating the substrate at 70°C, which results in dense and glassy PbI 2 . 12, 94 The second approach replicates the quenching mechanism to avoid large crystals. It consists of high-pressure nitrogen flow allowing layers to dry faster. Spray coating and doctor blading are few methods that are advantageous when integrated with the roll-to-roll process. The genesis of spray coating is from polymer solar cell fabrication. 95 It is combined with ultrasonic spray to enable large-scale production in room temperature. PSC achieved a PCE of 13% with 0.065 cm 2 active area on glass substrates 96 by this method. Spray coating is reported to be a favorable method for fabricating tandem structural devices and mixed cation halide perovskites for achieving enhanced performance and stability. 97 It utilizes minimum solid concentration for large-scale production but has a disadvantage of excessive restrictions on boiling points of solvents used. Inkjet process is programmable production process but has limitations induced due to ink properties. 98 Doctor blading method has an advantage of slower evaporation time, cost-efficient, higher coverage area, better crystal quality, nil solution waste, and favorable for ambient temperature processing. 99 It is reported to be highly compatible with the roll-to-roll production process for optoelectronic device fabrication. 92,100,101 By applying blade coating for preparation of CH 3 NH 3 PbI 3-x Cl x perovskite films, under ambient conditions, yields large crystalline domains, high quality, and moisture/air-resistant films. Slower evaporation time leads to better quality crystals and higher coverage. For perovskites, the formation of large crystalline domains is encouraged by a relatively slow solvent drying process, on the uniformly wet film formed by solution blading. 102,103 4 Stability Stability and degradation though have different meaning; they together define the issues and parameters, which affect existence, availability, and commercialization of PSC. Stability of PSC is our concern, as they degrade significantly after a few hundred hours of usage. Even after the necessary coating, they can sustain only 80% of initial efficiency after 500 h. Performance and efficiency of PSCs decrease, as degradation increases with time. Evaluation of stability is as per standard of International Electrotechnical Commission damp heat test (at 85°C, 85% relative humidity). Stable devices can maintain its performance with less than 10% reduction of PCE after 1000 h of exposure. 104,105 Hybrid PSCs are still struggling to reach this goal. Significant factors affecting stability are sensitivity to moisture, ultraviolet exposure, thermal stress, and so forth. (Fig. 17). 106,107 In addition, a prominent hurdle for commercialization of PSC is toxicity and safety issues due to lead usage. 108 Optimum fabrication method for large-scale production of PSC is still under research. Material instability can be regulated utilizing cross-linking additives and compositional engineering. 109 The best example is adding a combination of PbðCH 3 CO 2 Þ 2 3H 2 O and PbCl 2 in precursors, 110 additionally using cation cascade, including Cs and Rb cations. 7,111 Associated layers of PSC induce instability into PSC. Monitoring of deterioration by different performance parameters, such as J sc , V oc , FF, and η, describe its stability status. Fabrication processes, light absorption, charge separation, charge carriers transport mechanisms of different layers influence performance parameters. Table 2 shows various methodologies undertaken to increase the stability factor. Unlike organic solar cells, the decay behavior of V oc in hybrid PSC, CH 3 NH 3 PbI 3 , follows three steps. These three steps are a fast partial decay within 10 to 100 μs, followed by an extremely slow decay between 1 and 100 ms, and then a final rapid decay on the 10-s timescale. 130 Initial decay is continuous until 70% and holds at that sector for a hybrid PSC, whereas an organic solar cell decays very fast, within 1 s.
Half-life is the time required to reach half of the initial absorbance in set conditions. It determines the strength of decomposition, mainly due to RH. For RH of 98%, 80%, 50%, and 20%, the half-life of MAPbI 3 -based PSC films are estimated to be 4, 34, 1000, and 10,000 h, respectively. The half-life is found to be independent of film depth confirming that moisture diffuses into the bottom of the PSC layer along the grain boundaries. 131 Methodology for evaluating stability is also an essential factor. Researchers at Aalto University have found that only a fraction of stability tests done on perovskite-based solar cells and DSSCs meet proper requirements. 132 The team analyzed 261 aging tests, conducted on solar cells, and saw significant shortcomings in the procedure of reporting test results and testing methodology. These defects emphasized the importance of standardization of testing methodology. Presently, tests are   conducted only on one cell and in dark region only. 132 Many reports state that samples that are stable in humid conditions were found to be unstable in UV light. 132 Only 15 tests conducted outdoors and three with cells connected, leading to erroneous results; checks need to be undertaken in real-world conditions in groups of several cells to bring in standardization. 132 The same team has developed a new method, which saves time, effort, and could assess deterioration to the smallest range. PSC solar films, when they get heated, they turn from black to lead iodide color, which is yellow, displaying degradation. This degradation does not happen in its bulk form, due to the catalytic activity of moisture and reduced surface area as compared to films. 132 PSC can be protected from moisture to a certain extent using PMMA [poly(methylmethacrylate)]. It also helps in nucleation and formation of smooth and fine films. 133 The next section discusses various direct and indirect factors that affect stability.

Structure
Materials and structure are also essential factors that decides the stability of PSC. The mesoporous structure formed by TiO 2 -, Al 2 O 3 -, or ZrO-coated substrates, exhibited the pore-filling problem, low stability, and degraded performance. 17 . This low value shows that MAPbðI 1-x Br x Þ 3 would constitute the entire composition of the compound and enables convenient band gap tailoring. For subservient Br fraction (x < 2.0), PCE remained unchanged with improved stability factor when tested for 20 days at 55% RH. Diminished cross-section and transformation from tetragonal to cubic stage, due to higher Br content, has led to enhanced stability. New materials such as CH 3 NH 3 SnI 3 can resolve lead toxicity problems. Whole fabrication was conducted in N 2 environment as it decolorizes if exposed to the room environment. This decolorization is primarily due to oxidation of Sn 2þ ions, which lead to the formation of oxides of Sn and residual perovskites material. Sn-based PSC shows an enhanced PCE of more than 6% and has a short diffusion length. It restricts the fabrication of devices with planar architecture. A combination of Sn with other halides, namely CH 3 NH 3 SnðI 3-x Br x Þ was developed to achieve PCE of 6%. 139 Schoonman has explained the degradation in the absence of moisture. 140 Crystal structure of 3-D perovskites incorporating corner-sharing PbI 6 octahedra is broken into 1-D double chains consisting of two adjacent octahedral (MAPbI 3 · H 2 O, monohydrate form) and subsequently into 0D isolated octahedral (MA 4 PbI 6 · 2H 2 O, dihydrate form) by intercalation of H 2 O½4. Structural degradation occurs due to ionization-simulated degradation; it happens when the hole of halide member, which the inner layers produce, migrates and reacts with negative iodine on the surface. The reports confirm that doping improves crystallization. GeO 2 doping, in planar PSC forming PEDOT∶PSS-GeO 2 composite films, displayed active sites for crystal nucleus during annealing. 141 Doping has an overwhelming influence on performance, due to various reasons. Primarily it is due to undesired band bending at ETL and HTL interfaces with perovskites, increase in Auger recombination, and traps/defects. For higher PCE, the Fermi level in perovskite should not be above (below) Fermi level of ETL (HTL) in equilibrium. 25

Materials of PSC
Perovskite materials exhibit features such as direct band gap, high optical extinction coefficients (10 4 to 10 5 cm −1 ) in the NIR-visible region, and high carrier mobility (μ ∼ 1 to 10 cm 2 V −1 s −1 ) making it a prospective member for light harvesting. A high optical extinction coefficient of perovskite makes it a good absorption of light, at low mesoporous film thickness. Tuning the band gap of perovskite materials is achieved by modifying the chemical composition of perovskite 61 or by alloying different perovskites. 142 In addition to other properties, perovskite materials also exhibit ambipolar charge transport property (can be n-or p-type), which conducts electrons and holes through them, due to small effective mass of both electrons. 36,[143][144][145] High dielectric constant leads to effective charge screening. By bandgap engineering through modification of the chemical composition of PSC, such as CH 3 NH 3 PbI 1-x Br x , the entire spectrum is utilized. 146 Enhancement in PCE happens due to reduction in binding energy. The same was confirmed by incorporating core-shell Au 2 SiO 2 in PSC and achieved PCE of 11.4%, J sc ¼ 16.91 mA∕cm 2 ). 146 On addition of PbI 2 , Sn halide perovskite exhibits photovoltaic properties. It attained a maximum photon to current conversion of 4.18% with 1060-nm-thick Sn layer, with a redshift at Sn thickness of 260 nm. 142 Variation in prominent layers and adaptation of methodology can increase stability and PCE. Addition of 5% by volume of water into MAI solution of isopropyl alcohol assists in preferential crystallization in (110) plane, achieving a large grain size of ∼600 nm. This method achieved a PCE of 12.42%. 147

Electron conducting material layer
A layer that supports electron transport and aids in avoiding hole-electron recombination is called ECM. LUMO level should be lower than the conduction band of perovskites to achieve high electron transfer, and for perfect hole blocking HOMO, should be below perovskites. Thus, normal ETM in CH 3 NH 3 PbI 3 perovskites will have LUMO of −3.9 eV and HOMO of −5.4 eV. TiO 2 is the most established and favorite ECM layer. It is highly photocatalytic in the UV regime and thus is a prime member in inducing instability due to UV exposure. 148 It provides a high surface area to perovskites loading and can exist with all nanostructures of 1-D, 2-D, and 3-D variants. Compact-TiO 2 results in lower PCE; however, ethanolamine (EA)-based solvent treatment of compact-TiO 2 would enhance the transport feature along with its defects. EA and ionic liquid molecules (ILMs) are spin coated on compact-TiO 2 modifying surface morphology of a planar perovskites. 149 Various other useful solvents that reduce defects of metal oxide and energy barrier mismatch are conjugated polyelectrolyte, 150 ILMs, 151 a self-assembled monolayer, 152,153 polar solvents, or alcohol/water-soluble-conjugated polymers. 154,155 TiO 2 is unsuitable on flexible substrates and large-scale production due to high sintering temperature. 64 ZnO, organic materials such as [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), and ICBA (indene C 60 bis adduct), as well as carbon nanoforms and polymers, are an equivalent replacement of TiO 2 with good performance data. 156,157 Nanostructured ZnO in PSC can be fabricated at a low temperature (<150°C), making it viable for photovoltaic devices. 10 It has good electron mobility along with energy alignment. However, ZnO nanostructured, quadruple the rate of degradation due to deprotonation reaction of the organic part in perovskite. 158 PCE of 17.2% and enhancement of 23.2% compared to a nonadditive solution was observed by addition of nonhalogen additives such as acetate (CH 3 COO − , Ac-) salts (NH 4 Ac, NaAc) in CH 3 NH 3 I solution. 159 Modification of ZnO layer using ILMs, such as 1-benzyl-3-methylimidazolium chloride (benmim-Cl), 151 elevates performance. Further, TiO 2 is doped with various metals to overcome drawbacks. SnO 2 was experimented to replace TiO 2 as ECM material to achieve a high PCE of 13%. 70 In few devices, the TiO 2 layer is between Al anode and fullerene C 60 for ensuring proper electronic contacts. Further research has led to the formation of valence bands of lower binding energies, with Fermi levels, within the band gap, by inserting a C 60 monolayer between perovskite and TiO 2 contacts. 160 The ability of C 60 to accept electrons results in high PCE, of 6.7%, along with enhancement of V oc and J sc . Planar devices using C 60 as ETM and NiO as HTM achieved a PCE of 9.6%. For polymer ETM, solubility is a major issue. Thus, polymer ETM deposition will affect performance and perovskites layer. In addition to C 60 , other ETM example consists of higher fullerene-like C 70 , PC 71 BM. Fullerene C 60 and its derivative have increased PCE from 7.3% to 14.9% due to reduced trap density and lower degree of symmetry. However, due to nonequivalent double bonds, it leads to regioisomeric mixtures, which are difficult to separate -TiO 2 nanocomposites, as ETM, achieved PCE of 15.6%. Graphene quantum dots, due to their confinement in 3-D modify the band gap. Thus, incorporating these between TiO 2 and perovskites achieved PCE of 10.2% with a V oc of 0.94 V and J sc of 1.71 mA∕cm 2 .

Hole conducting material layer
The primary function of HTM is to extract charges selectively from perovskite and its interface. The basic advantage of this HCM is its ability to penetrate in mesoporous layer easily. However, it induces degradation in PSC film. 161 Classification of HCM consists of small molecules, polymers, and inorganics. Small molecule HCM such as spiro-OMeTAD is a favorite for mesoporous and planar structured PSC. Electrolytes have an inbuilt problem of low ionic conductivity. HTM is less conductive than perovskite material (∼10 −3 S cm −1 ), leading to the requirement of thick HTM layer. High thickness is necessary to avoid pinholes and leakage, but in devices, it would lead to high series resistance and imbalance between series and shunt resistance. Thus, spiro-OMeTAD as solid-state HCM is introduced to nullify these limitations. However, multiplex synthetic approach and purification are the hindrances for its commercialization. Doping with lithiumbisimide (trifluoromethanesulfonyl) salt (LiTFSI), 4-tert-butylpyridine (TBP) and cobalt complex 162 assists in improving its conductivity. Due to absorption, device instability increases and oxidized form of spiro-OMeTAD starts acting as a filter in the visible region at 520 nm. 163 Spiro-OMeTAD shows an immediate initial decrease of 20% and, further, it declines steadily. New materials nicknamed EH44 are replacing spiro-OMeTAD, as it repels water and does not contain lithium. 164 PEDOT:PSS is another proven organic HTL for planar structured devices. There is energy level misalignment between PEDOT: PSS and VB (5.0 eV) of MAPbI 3 (5.46 eV), which results in lower V oc (0.80 V). A higher work function of HTL would lead to strong recombination at the grain boundary, very low PCE, and affect the V oc of the device. Doping and addition of additives were used to remove the misalignment and increase efficiency. Improved HTL doping ratio of 4:1 in volume for PEDOT∶PSS-GeO 2 has achieved PCE of 15.15%, J SC of 21.55 mA∕cm 2 , V oc of 0.96 V, and FF of 0.74 under 100 mW∕cm 2 AM 1.5 illumination. 141 MoO 2 nanoparticles/toluene solution an inorganic hole transporting material for planar CH 3 NH 3 PbI 3 PSCs is reported to exhibit stable efficiency of 14.8% under illumination of 1 Sun (100 mW∕cm 2 ). 165 Numerous reports conclude NiO as a good candidate for HTM in PSC device fabrication. It was able to achieve PCE of 16.40% with high V oc (1.04 V) and FF (0.72) with negligible current-voltage hysteresis. 166 The conjunction of mesoporous TiO 2 film and a hole conducting polymer led to the formation of PSC having 123% PCE under 1.5 Am. 158 CuSCN as HTM, along with a buffer layer of thin mesoporous alumina (Al 2 O 3 ) and an added coat of insulating polymethyl methacrylate on PSC, was utilized to achieve PCE of 13.3%. In addition, it displayed stability by retaining the efficiency of 77% after 1000 h at 85°C in air. 161 Utilization of bifunctional polymer nanocomposites as HTLs ½CH 3 NH 3 PbI ð3-xÞ Cl x for light harvesting is also recently investigated with positive results. A gold (Au) nanoparticle assists in attaining a fourfold enhancement of electrical conductivity and carrier mobility. Scattering effects of doped nanoparticles have led to a 25% improvement in PCE along with amplification of the photon absorption on the photoactive layer. 167

Blocking layer
Efficient lead halide PSCs use hole-blocking layers to help collection of photogenerated electrons and to achieve high open-circuit voltages. Deactivation of reactions at the surface of TiO 2 enhances stability, which is due to the blocking effect of this layer. Spiro-OMeTAD as HTL and TBP as additives to HTL would develop a reduction of perovskites film absorption. Introduction of montmorillonite layer between the perovskite and spiro-OMeTAD layers, as buffer, provides the solution for reduction in film absorption. 168 Experiments showed that nonencapsulated devices had higher stability than encapsulated devices because oxygen would remove the surface states and ensuring better stability. 66 Thus, oxygen is necessary for these devices. When Al 2 O 3 replaces n-type TiO 2 , as an insulating layer in MSSC, devices achieve stability of 1000 h. 169 Performance enhancement occurs when a hole-blocking layer is formed by Mg-doped TiO 2 , with a PCE of 12.28%; high FF and V oc (1.08 V). This enhancement is due to the enlargement of the gap between valence band and conduction band for better optical transmission properties. 170 Ke et al. have fabricated PSC without blocking layers, by growing perovskite directly in fluorine-doped tin oxide-coated. 171 Solution method and ultraviolet-ozone treatment can form layers. Its planar architecture consists of Au/hole-transporting material/ CH 3 NH 3 PbI 3-x Cl x /substrate. It achieved a PCE of 14% and V oc of 1.06 V, which is at par with conventional PSC. This success is due to ultraviolet-ozone treatment and involvement of Cl in synthesis. These results confirm that TiO 2 is not an ultimate interfacial material for achieving high-performance PSCs. 171

Electrode
Electrode design for large-scale production is still a challenge. The electrode should ensure good catalytic activity, electron conduction, and high proton diffusion rates. A single-phase TCO is reported to have enhanced proton and charge transport capability for perovskites. 141 Ag and Au are the favorite elements. 172,173 However, due to the cost factor, Ag is a better alternative than Au. 146 However, PSC with Ag as electrode has extremely low stability. Exposure of element to humidity (RH ∼ 50%) led to the formation of Agl, which deteriorates PCE. Agl transforms the electrode from a reflective metal to yellow color, which is highly influenced by the HTL layer. Antireflection coatings for front side of the transparent glass and plastic substrates are being explored extensively. [174][175][176][177] Nanophotonic front and rear electrodes for improved light transmittance in the solar cell is desirable for long wavelengths range operation in tandem device PSC. Successful nanopatterns are being replicated into transparent substrates for large-scale applications at low cost. 178 Long-term performance is a function of thermal stability of perovskite other layers in a photovoltaic device. Inorganic blocking layer and scaffold layer display appreciable stability compared to organic material in devices.

Hysteresis
Around 2014, scientists observed hysteretic current-voltage behavior of PSC. 179,180 Hysteresis, which is a specific property, is formed likely due to slow transient effects, leading to an over or underestimation of solar cell efficiency. J-V hysteresis property of PSC is a function of dielectric response of perovskite material, mobile ions, 181 and trap-state discharging. 179 The shape of measured current-voltage (IV) curves and efficiency are function of scan direction, light source, delay in measurement time, and voltage bias conditions before measurement. It was established by various modeling and experimentation that hysteresis is weak when surface recombination is low and diffusion length is long. 182 Improved crystallinity, better fabrication process, and improved contacts are going to continue to improve hysteresis. HTMs influence hysteresis and J-V curve. Cu-based, CZTS HTM express less hysteresis than with spiro-MeOTAD HTM. 183 J-V hysteretic behavior of CuI-based devices with an efficiency of 7.5% is better than spiro-OMeTAD devices. Faster polarization relaxation was observed using EIS and OCVD for CuI-based devices. 184 Recently, in addition to normal-type of hysteresis, an inverted hysteresis that exhibits an opposite behavior to the normal is also reported. 185,186 The inverted hysteresis is due to interfacial charge extraction barriers formed by ionic defect accumulation and space charge build-up. 187 The inverted hysteresis is noted to be a function of bias range and sweep rate. Different bias scanning conditions lead to a varying ionic movement at the interface of the ETH or HTL/perovskite, which assists in the tuning of these hysteresis. 186 Deposition of fullerene layers onto perovskites, which assists in the elimination of photocurrent hysteresis, is reported. 188 Methodology and accuracy for evaluating performance characteristics of PSCs are crucial. Hysteresis influences the evaluation of performance parameters such as I-V curves and P max . Measurement conditions such as sweep direction, start voltage, end voltage, hold time, and sweep time have an overwhelming effect. 189 Inverted devices (p-i-n structured) are reported to have less hysteresis. 182 Figure 18 shows that normalized time-dependent J sc of the regular film has degraded to a higher rate than inverted structured PSC. 190 Though moving of electron contact to the rear side is a possible explanation, detailed reasoning requires further investigation. Incorporation of a [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) interlayer and high-quality perovskite absorber fabricated by thermal evaporation-spin coating method produces a hysteresis-free planar perovskites. 76 Ions accumulate at the ion-blocking interface. The "p" and "n" doping induce ion migration at ion-blocking interface. These doping will screen the applied voltage and alter charge collection capability, resulting in hysteresis. In addition, ion migration is responsible for unusual behaviors in perovskites materials such as photo-induced phase separation, 191 photoinduced giant dielectric constant, 192 and absence of transistor behavior at room temperature. 193 In high efficient PSC cells, ion migration does not cause hysteresis. 182 Ion migration within crystal structure, 194 which is also a major contributing factor for hysteresis, has a profound effect on device stability. 195 Experimental proof and test results show that ions migrate along the grain boundaries. 196

Moisture
Moisture acts as a catalyst, and its presence tends to induce hydrolysis 107 in PSC, thereby converting PSC back to precursors, which is irreversible. Experiments to evaluate the stability of PSC against moisture were conducted in inert and other carrier gases to ascertain degradation exclusively due to moisture. 12 When exposed to moisture, perovskite structure tends to hydrolyze, undergoing irreversible degradation and decomposing back into precursors. Highly hygroscopic CH 3 NH 3 X converts to CHðNH 2 Þ 2 X salts and PbX 2 , with X = halide. 197 Heat, electric field, and ultraviolet exposure 198 accelerate this process. Figure 19 shows the mechanism of degradation: Fig. 19 Proposed decomposition pathway of CH 3 NH 3 PbI 3 in the presence of a water molecule. 197  Stability of PSC, due to moisture, requires evaluation in three modes. One is purely because of moisture, the second is under the influence of moisture along with light, and finally the third mode is a combination of moisture along with light and oxygen. Degradation of perovskites is reported to follow the mechanism of Grotthuss. 199 In this, one water molecule reacts with perovskite material and degrades to HI and CH 3 NH 2 . Further, HI reacts with O 2 to form water, to initiate more deterioration. After exposing perovskite film CH 3 NH 3 PbI 3 to 80% RH for 2.5 h, the possibility of intermediate crystalline phase such as a hydrated compound ðCH 3 NH 3 Þ 4 :PbI 6 :H 2 O exists. Grazing incidence x-ray diffraction (GIXRD) confirms the formation of intermediate phase. XRD pattern shows strong peaks of hydrate at 8.42 deg and 10.46 deg, which confirm the existence of intermediate phase. Exposure of solar cell to RH of 0% to 50% has led to very little degradation. However, under 90% RH, it showed a rapid decrease in PV performance efficiency from 12% to less than 1% in just 3 days. Third day values are J sc ¼ 3.1 mA cm −2 , V oc ¼ 0.637, and FF ¼ 0.36, η ¼ 0.7. 200 Increase in RH level beyond 55% resulted in a drastic increase in degradation rate. In addition to general characterization, a powerful method such as photothermal deflection spectroscopy evaluates the changes in absorption after exposing perovskites to moisture. 201 In Fig. 20, CH 3 NH 3 PbI 3 shows absorption edge as ∼1.57 eV, which shifts to 2.3 eV on exposure to moisture, this correspondence to PbI 2 formation, and decomposition of CH 3 NH 3 PbI 3 . Figure 21 shows PCE versus heterojunction solar cells variation based on MAPbðI 1-x Br x Þ 3 (x ¼ 0, 0.06, 0.20, 0.29) as per time, stored without encapsulation at room temperature with RH ranging from 35% to 55%. 202 It also shows variation in UV-vis spectra as per moisture content and variation in the morphology. CH 3 NH 3 PbBr 3 -based perovskites are less sensitive to moisture than PbI X -based Perovskites. However, the absorption capacity of pure CH 3 NH 3 PbI 3 is not comparable to that of CH 3 NH 3 PbBr 3 . 55,203 The first line of defense against the moisture is the top charge-transporting layer of the device. Resistance to moisture can be increased by inherently strengthening the bond between cation and metal halide. 200 Few other approaches are inclusion of thin blocking layer between perovskites and HTL, such as Al 2 O 3 , 169,204 moisture-blocking hole transporters, 205 and hydrophobic carbon electrodes. 2,116,[206][207][208][209] Incorporation of 2-D PSC with large aromatic phenyl ethylammonium (PEA) cations could achieve average PCE of 5.5% with excellent stability against moisture. After exposure of these 2-D PSCs for more than 90 h at 72 AE 2% RH, they were stable and maintained 50% PCE of initial value. 210 Incorporating polyethylenimine Fig. 20 PDS spectra of CH 3 NH 3 PbI 3 films before (initial state) and after exposure to a relative humidity of 30% to 40% for varied time period. Reprinted with permission from Ref. 201. © 2013 by the American Chemical Society cations in 2-D PSC helps in attaining better film quality and resistance to moisture degradation with PCE of 8.77%. 211 In addition, near-single-crystalline-type thin films are fabricated, which have, preferential out-of-plane alignment of crystallographic planes, good charge transport, and moisture resistance. 117 Thus, 2-D PSC has better moisture resistance capability than the 3-D PSC. 212,213 Solution-processed MAPbI 3 was immune to changes when exposed to O 2 . The decomposition on exposure to moisture or ambient air is due to H 2 O. 202 However, very low quantity H 2 O acts as an n-dopant for MAPbI 3 , affecting the energy level. Proposed decomposition of MAPbI 3 eventually releases NH 3 and HI as volatile species, leaving PbI 2 and making decomposition reaction irreversible. Reports confirm that moisture coupled with light directly led to decomposition of MAPbI 3 to PbI 2 , which was an irreversible reaction. Residuals such as PbI 2 and I 2 are formed easily under sunlight, instead of reversible hydrate in the dark, when MAPbI 3 exposed to an environment of 18 h at 60% RH. 107 Though the analysis of stability in dark is a basic test, stability research under illumination is essential for evaluation of working efficiency and degradation factor. Unlike degradation in the dark, exposure to moisture in light leads to irreversible decomposition of MAPbI 3 to PbI 2 .

Light Spectrum (UV and Visible)
Mostly laboratory experiments are conducted under controlled light source such as LED. However, actual utilization involves sunlight with UV element, which accelerates degradation Fig. 21 Effect of moisture: (a) in situ absorbance measurements of CH 3 NH 3 PbI 3 exposed to 98% relative humidity. Measurements were taken at 15 min intervals, (b) normalized absorbance measurements (taken at 410 nm) for CH 3 NH 3 PbI 3 films exposed to different relative humidity, (c) SEM images of uncoated CH 3 NH 3 PbI 3 , CH 3 NH 3 PbI 3 ∕spiro-OMeTAD, CH 3 NH 3 PbI 3 ∕PTAA, and CH 3 NH 3 PbI 3 ∕P3HT films after exposure to 98% RH for 24 h. pattern. The TiO 2 element in PSC, which is photocatalytic in the UV regime, assists degradation. 148 To further surprise, as shown in Fig. 22, degradation is higher in encapsulated devices than the unencapsulated devices for a conventional device. This phenomenon is due to electrons injected into TiO 2 are entrapped in deep-lying unoccupied sites. 148 However, encapsulation along with UV-filter can enable attaining better results. Degradation is directly proportional to efficiency and follows a pattern of reduction in J sc . A TiO 2 -free mesoporous structured solar cell is comparatively less degradable than MSSC with TiO 2 , which is confirmed by transient absorption spectroscopy. MSSC has a property of degradation being high at maximum power and rather low at open circuit condition. However, for practical applications installation of a UV filter on PSC overcomes the degradation process. Degradation is attributed to the instability problem 214 and the catalytic role of metal oxide when exposed to the UV spectrum. A general degradation mechanism for UV exposure is shown in Fig. 23. On exposure to UV light, photogenerated holes that are generated by absorption of energy react with the oxygen radicals adsorbed at surface oxygen vacancies. Molecular oxygen is absorbed from these sites, leading to a formation of unoccupied deep surface traps sites and a free electron per site. These electrons will recombine with the excess of holes in doped-hole transporter. On photoexcitation of sensitizer, electrons can follow in two ways. In the first route, they get injected into conduction band by which they get deeply trapped and in other they directly enter into the deep surface traps. These deeply trapped electrons are not mobile and recombine readily with holes on hole transporter. 148 The primary bottleneck that exists is transporting the charge. In quantum dots, polydispersity strongly affects transport. In quantum dot film, if few dots are of different sizes, then trap states manifest within the bandgap and lead to recombination. Similar is the case with 1-D carbon nanotubes. Three schemes are formulated to nullify the effects of UV. First being the reduction of trap sites or pacifying it and second is to undertake measures to avoid UV light reaching ETL (especially TiO 2 ). Finally, the third scheme is to replace TiO 2 scaffold with another material or make it TiO 2 free, which has been explored by Leijtens at el. 148 The efficiency of PSC device reduces to almost 0% after 12 h of exposure in a nonencapsulated format. However, with the introduction of Sb 2 S 3 buffer layer in PSC, efficiency increased to ∼65% of its initial value. This increase is due to the Sb 2 S 3 blocking layer, which blocks the UV-induced photocatalysis in TiO 2 , by providing a form of passivation. In addition, this extra layer assists in maintaining the crystalline structure of CH 3 NH 3 PbI 3 . 215 A methodology such as usage of the downshifting molecule such as N-(2-(6-chloros-tetrazin-3-yloxy)ethyl)-naphthalimide (NITZ) with a strategy to suppress UV from the source is also explored, for UV protection in PSC. 216 They have a specialty to get excited in the UV spectrum and to emit into the visible spectrum, allowing UV photons utility.

Thermal Stability
Variation in efficiency and increase in temperature will result in amplification of deformities. These anomalies are due to deterioration in perovskites materials and behavior of HTM. It is understood that when the heat of the device ingresses into cell materials, mechanical stress of the materials at interfaces increases, thereby decreasing the lifetime of the PSC device. Accelerated aging test results showed that PbI 2 deteriorates much faster than PbBr 2 , approximately after 60 min. High stability of Br is due to shorter and stronger bonds of Pb-Br compared to Pb-I. 217 Heating of CH 3 NH 3 PbI 3-x Cl x in N 2 environment at 90°C not only leads to the formation of 3-D perovskites structure but also results in a high rate of degradation at 100°C. 218 Hybrid perovskite assists in maintaining stability, as inorganic part of hybrid perovskites increases thermal stability and its organic part increases the stability of device. Comparison of formaldinium lead trihalide ðHCðNH 2 Þ 2 PbI 3 Þ and CH 3 NH 3 PbI 3 -based device reveals that the former is more stable as it deteriorates only after 290°C than the later, which decomposes at 230°C. 219 In PSC, deterioration rate due to temperature increases in the presence of moisture. Reports reveal that at 85°C time of decomposition is 283, 208, and 133 min when RH is 12%, 57%, and 85%, respectively. 220 Period of exposure is also an influential factor for evaluation and characterization by hard x-ray photoelectron spectroscopy instead of conventional XRD, because assessment is done on chemical content regardless of crystallinity of the sample. Mechanism of degradation is as follows: E Q -T A R G E T ; t e m p : i n t r a l i n k -; s e c 4 . 6 ; 1 1 6 ; 1 7 1 CH 3 NH 3 PbI 3 -→ PbI 2 þ CH 3 NH 2 ↑ þHI ↑ : Films are made independent of parameters such as moisture and oxygen by subjecting it to ultrahigh vacuum heating chamber, to evaluate effects purely due to temperature. 221 I/Pb and N/Pb ratios define the characterization of films for degradation. Reduction of these ratios indicates degradation of perovskites into PbI 2 , as shown in Fig. 24. Variation in temperature induces a different degree of degradation in various layers of PSC. For example, perovskites made of scaffolded ZnO are found to be more stable than TiO 2 . 222,223 To understand degradation on perovskites purely due to thermal effects, semifabricated PSC, without HTL, ETL, and the top electrode are tested. 106 These semifinished planar structures undergo exposure to inert, O 2 , and ambient conditions for 24 h. Surprisingly, degradation in the form of delamination happens in an ambient environment rather than in O 2 . GIXRD and surface photovoltage measurements assisted in characterization and revealed that at temperature above 140°C and 160°C perovskite materials decomposed into PbI 2 in vacuum. 224 Up to 150°C, an increase in the Braggs peak with enhancement of perovskite film occurs. From 150°C to 200°C , these peaks reduced due to PbI 2 formation and further heating to 330°C lead to complete decomposition into Pb. 225 Empirical studies revealed that thermal annealing conducted for production of perovskite materials induces slight PbI 2 production, causing drop in efficiency. Heat treatment of layers such as ZnO before deposition lead to an increase in efficiency and thermal stability, due to reduction in deportation of methylammonium at the interface. 223 Thermal stability is a highly potent factor that affects different layers especially the HTM. Initiation of crystallization and formation of large crystals causing morphological hole traps affecting charge transportation and effecting PCE was reported for PSC with spiro-OMeTAD as HTM. 218 This crystalline process gets aggravated by additives or doping done in the layer to increase charge due to the hygroscopic nature of dopants. 226 To achieve better stability, many new HTM materials were explored, such as PTAA and 5,10,15-trihexyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c] carbazole (KR131). 227 Introduction of bulky aromatic PEA cations was reported to produce thermally stable 2-D PSC, which maintained 65% of its initial PSC after exposure to 100°C for 70 h. 210 In addition, multidimensional 3-D/2-D perovskites with an intermediate dimension between them led to the formation of thermally stable PSC, due to reduced defect density. 228

Fabrication and Preparation
Fabrication is a vital factor for sustaining stability in PSC. It primarily constitutes two elements such as the synthesis method used and the substrate precursor. Synthesis temperature and time strongly influence the morphology, stability, and performance of PSC. In one-step deposition process, perovskite solution is dispensed on the substrate while turning at a fixed velocity. Though excess perovskite is spun off, perovskites crystallize on evaporation of the solvent.
Uncontrolled precipitation yields nonuniform large-and small-size crystals, resulting in poor performance and solar cell efficiency. Adhikari et al. have proposed the usage of an antisolvent method for better crystal formation and growth. It involves annealing conditions and defines charge transport capability, as measured by nanoscale Kelvin probe force microscopy (KPFM). 229 Annealing temperature effect on CH 3 NH 3 PbI 3 is analyzed from 80°C to 140°C as shown in Fig. 5. The peak of PbI 2 at 12.7°C, in XRD, increases proportionally as per the annealing temperature. The annealing time is optimized to achieve correct crystal size, charge transport layer such as HTL formed CZTS nanoparticles (CZTS-30). 230 Thermal annealing of CH 3 NH 3 PbBr 3 films at 90°C, for short duration such as ≤10 min, would lead to increased crystallinity, eliminating residual solvent and achieving complete conversion of the precursor to crystal, in comparison to longer annealing time. 231 Greater annealing time would initiate sublimation, so accurate monitoring of time is essential. For TiO 2 films, photonic curing technique is more effective than high-temperature annealing (∼500°C), as it creates more conductive films. 96 Photonic curing is also called pulse-thermal processing, which utilizes high-intensity plasma arc lamp. It delivers a peak sintering power up to 20;000 W∕cm 2 for milliseconds and produces rapidly annealed thin films without damaging underlying plastic substrates. PEDOT:PSS-based PSC grown at room temperature without annealing was reported to exhibit a PCE of 14.27%. 232 They have high crystallinity when subjected to 40% RH at room temperature. 232

Device Encapsulation
Air sensitivity of PSC elements, in conjunction with moisture sensitivity of perovskite absorbers, emphasizes the requirement of encapsulation of high level to attain a stable outdoor lifetime of over 25 years. 3,64,61 Without encapsulation, highly efficient devices were reported to degrade rapidly, showed 80% degradation when subjected to ambient temperature. 233 Usually, a thin glass coverslip is used to cover device, which is further sealed using a UV curable epoxy resin. 148,169,234 Ramos et al. 235 reported that single step high-temperature encapsulation (>150°C) resulted in delamination of active and metallization layers in devices. A two-step encapsulation approach minimizes the defect due to mechanical and thermal stress. The first step in a pre-encapsulation process consists of making electrical contact and later embedding it in a polymer matrix to ensure mechanical stability. The next step involves laminating the preencapsulated device using low temperature, to retain good photovoltaic characteristics of PSC. During the process and after, samples are monitored electrically (J-V curves). There are two types of configuration for encapsulation. Han et al. have compared these two types, 168 in first method, UV curable epoxy resin used to fill the area between silver contact and plain glass cover. 131,[236][237][238] The other configuration has an electrode, which has a top glass cover with a gap in between. Water absorbent, desiccant material was made to fill this gap, thus absorbing moisture that had penetrated through epoxy resin sealant. 168 For 2-D PSC films, encapsulation becomes very critical. Therefore, plastic barrier films are used. It is available in two formats, namely partial and complete. They laminate commercial encapsulate adhesive on to films at 100°C. This type of encapsulation could only retard the ingress of moisture penetration and can maintain the operational performance of PSC steady for more than 1 year. 131,236,[239][240][241] Conductive tapes made of commercial carbon assist in achieving a dual purpose such as terminal contact with perovskite and encapsulation. Encapsulated devices are leak tested with helium gas to assure moisture penetration. Investigations reveal that incorporation of impermeable barrier layers can be a long-term remedy to eliminate the ingress of moisture and oxygen.

Recombination
PSC (for MAPbI 3 ) has achieved laboratory PCE of 20.1% and an open-circuit voltage (V oc ) of 1.1 V, which is below theoretical V oc limit of 1.32 V. 242 Loss of 200 mV is due to nonradiative recombination, which manifests itself at low external quantum yield (⪅0.01%) for electroluminescence, corresponding to V oc . 242,243 KPFM results of a single step and sequentially deposited films show that increase in potential barrier suppresses back recombination between electrons in TiO 2 and holes in perovskite. 244 A blocking layer is very crucial in a conventional PSC for PbI 2 to inhibit charge-interception/recombination. Conversion of PbI 2 to MAPbI 3 occurs at solution interface of TiO 2 ∕PbI 2 interface. 86

Lead and Other Toxicity
Lead composition 245 leading to toxicity 108 and bioaccumulation in ecosystems 246 hamper the commercialization of organic-inorganic lead halide-based PSC. A PSC with CH 3 NH 3 PbI 3 as absorber material would contain about one-third lead by weight. On exposure to rainwater, perovskite will completely dissolve in water leaving behind PbI 2 . 75 Evaluations show that a PSC panel releases only 70 ppm of lead. This concentration is moderately low in comparison to the standards for low levels of contamination. 247 Though it is undesirable and not catastrophic, there is no accepted safe threshold for lead exposure.
In additional, other elements such as iodine 248,249 and methylamine 250 also have a high degree of toxicity. MAPbI 3 nanoparticles get airborne and affect the respiratory system, mix with fluids of the body, and reach brain and nerves. 251 Daniele et al. have reported the formation of PSC without lead using transition materials such as Cu. They developed ðCH 3 NH 3 Þ2CuCl x Br 4−x series and confirmed green photoluminescence due to Cu+ ions formation. Mixing of halides MA 2 CuCl x Br 4−x and chlorine with it will ensure appreciable stability, as it highly deliquescent. 252 Impedance spectroscopy was used to evaluate performance. Strong anisotropy and low absorption coefficient hamper these cells. The exact material combination decides the thickness of films and light harvesting requirement with limited diffusion length. Tin (Sn) [253][254][255][256][257] is also considered an apt replacement of Pb as it belongs to group 14 metal, with four electrons in its outer shell. However, due to the bigger size of metal in group 14, it induces instability of 2+ oxidation state. 139 Theoretical investigations reveal that I-5p orbitals of I 3 − groups influence band-edge reconstruction. This leads to new conduction-band minimum at Brillouim zone, which facilitated the formation of lead-free perovskites. 258 Lead-free perovskites films such as CH 3 NH 3 SnX 3 (X ¼ I, Br, Cl) is synthesized by vapor-assisted solution processing 253,259 and hot injection method. 255 Halide double perovskites utilize interplay between two metal ions to achieve this goal. 260,261 Replacing Pb 2þ sites with a combination of trivalent Bi 3þ and monovalent Ag þ cations has led to the formation of 3-D double perovskites. 262,263 7 Stability Comparison of 2-D and 3-D PSC A comparative study shows 2-D PSC is more stable, up to 1400 h than 3-D PSC, when tested under the light source of AM 1.51 without any UV filters. 117 They retain about 80% of their original value, whereas 3-D PSC has a dramatic loss in efficiency. Thus, 2-D perovskites showed superior stability and water resistance. 117,264,265 Quasi-2-D (n-butylammonium)2(MA)2Pb3I10 has an efficiency of 12% with a drop in performance only after 2250 h. 117 Grancini and coworkers have integrated 2-D/3-D perovskite to combine advantages of both. Their interface has a combined benefit of excellent stability, excellent charge transport of 2-D perovskites, and absorption quality of 3-D perovskites. 266 The ability to collect ions to assemble into a stable 3-D perovskite structure is predicted by the Goldschmidt tolerance factor, "t." It is a stability indicator of a perovskite crystal, which describes the compatibility of ions with other materials in PSC. An ideal Goldschmidt tolerance factor should be below or equal to 1; for guanidinium, it is 1.03. For a perfect cubic perovskite structure t ¼ 1. However, practically, one can expect ions to form a cubic 3-D structure if 0.9 ≤ t ≤ 1. However, when 0.7 ≤ t ≤ 0.9, it enables the formation of lower symmetry structures, such as tetragonal, rhombohedral, hexagonal, and orthorhombic. 267 Recent research at EPFL has reported enlarging the stability of methylammonium lead iodide perovskites, by introducing large organic cation. A promising member such as guanidinium (CH6N3+) is an example of a large organic cation group. It enhances the material's overall thermal and environmental stability for 3-D perovskites. Figure 25 shows the SEM image of 3-D PSC achieved efficiency over 19% (19.2 AE 0.4%) and sustaining its performance for 1000 h under continuous light illumination. 268 Chen et al. 269 have reported to achieve exceptional PCE for 2-D format using simple solution process and to achieve a vertical alignment of 2-D perovskite layers to electrodes. Figure 26 shows the vertical orientation of (101) planes and (202) planes parallel to the substrate. 269

Outlook
PSCs are an excellent and advantageous replacement for silicon solar cells. Positive features such as excellent absorption coefficient, ability to tune bandgap, ability to form flexible solar cells, compatibility with the solar spectrum, and good charge mobility make it a good performance solar cell. The hybrid feature of PSC, utilizing the advantages of organic and inorganic components in it, has enabled it to achieve efficiency at par with silicon solar cell and beyond. Perovskites absorber materials coupled with silicon solar cells, in a tandem cell format, could obtain a PCE above 30%. Replacing PSC with better HTM and innovative materials would enhance the PCE. Various combinations and materials are first tested in N 2 environment in the encapsulated form before further investigation and exploration. Major hindrances in its commercialization are its stability factor and toxicity. Our survey of the literature assures that continuous progress and development is going on to resolve these hindrances. In addition to the energyrelated application, perovskites have an enhanced implication in numerous other domains such as flexible optoelectronics, supercapacitors, rechargeable batteries, and other hybrid systems that couple with photovoltaic cells. The futuristic application is to paste PSC films on buildings and skyscrapers enabling them to produce power and become self-sufficient. Testing methodology  process is also wholly restructured to reduce research time on combinations, which cannot be commercialized. Recently, a new fast and low threshold photography method has been developed by Aalto University in Finland to detect even a slight disintegration in a perovskite structure. This method provides more reliable results than optical measurement devices and is less complicated with fewer labor requirements than conventional x-ray crystallography. Encapsulation methodology is also revamped to handle moisture ingress issues. As this adds to the cost of manufacturing, researchers have recently reported the fabrication of air-stable PSC in 2-D format. To date, PSC has achieved a high PCE of 23.3%. Development of innovative materials for perovskites and their combination with PSC to tailor a new increased PCE along with applications in other domains is still yet to be unlocked.