Substance and shadow of formamidinium lead triiodide based solar cells

The current decade has witnessed a surge of progress in the investigation of methyl ammonium lead iodide (MAPbI 3 ) perovskites for solar cells fabrication due to their intriguing electro-optical properties, despite the material’s intrinsic degradation that has restricted its commercialisation. As a promising alternative, its’ formamidinium ananlogue, FAPbI 3 based solar cells are currently being actively pursued having demonstrated a certified efficiency of 24.4%, while, the room-temperature conversion to a non-perovskite  -phase impedes its further commercialisation and strategies have been adopted to overcome the phase instability. In-depth and real time understanding of microstructural relationships with optoelectronic properties and its underlying mechanisms through operando in-situ spectroscopic techniques is paramount. Thus the designing and the development of new process, data driven methodology, charecterization and evaluation protocols for perovskite absorber layer and the fabricated devices are judicious research direction. Here, in this perspective, we shed light on the compositional, surface engineering and crystallization kinetics manipulations for FAPbI 3 followed by a proposition for unified testing protocols in order to uplift the devices from the lab to the


Introduction
In an era, where greenhouse gases emission is showing steep rise and carbon neutral technology are sought after [1] , photovoltaic (PV) deployment is gaining momentum as a green and a clean source of energy. PV technology is suited to fulfil the growing energy demand by being cost effective and has demonstrated grid parity in parts of the world. To cater to the planet's demand and large-scale deployment, thin film PV technology other than Si are being investigated. [2] Thin film PV devices equipped with added functionality, such as the prospect of being lightweight and cost competitive make them strong contenders for future portable and utility scale applications. In recent years, the investigation of hybrid inorganic-organic halide perovskite (Fig. 1a) for solar cell fabrication has gained significant attention owing to its potential for high solar-toelectric efficiencies at lab scale, low-precursor cost of materials and ability for solution processability. [3,4] The reported power conversion efficiency (PCE), is on par with the mature PV technology and has reached beyond 25%, which is also suited for self-powered internet of things based devices [5] or utility scale application. Among the materials for emerging thin film PV, perovskites supersede other technology in terms of performance (Fig. 1b). Perovskites are known for over a century, however, their application for solar cells was exploited a decade ago and it turned out to be a game changer in PV research. The extremely high absorption coefficient of the semiconducting perovskite and its intriguing opto-electrical properties, enable the perovskites to be efficient light harvesters and arguably, less than a micrometre (300-500 nm) thick perovskite layer can harvest the same amount of light as effectively as a thick silicon (200-300 µm) wafer does. Typically, a perovskite solar cell (PSC) is a thin film device consisting of a transparent conducting oxide as the charge collector, perovskite as the light harvester and charge selective layers as efficient charge extractors (Figure 1c). Both mesoscopic and planar (n-i-p) architectures are being fabricated (Figure 1c), while inverted planar, (p-i-n) structures have also been reported, where moderate heat treatment for hole selective layer is not an issue. Upon light irradiation, electron-hole pair are created in the absorber layer, which dissociate and transport to their respective charge selective layers, i.e., the electrons travel to electron transport layer (ETL), while the positive charge (holes) travel to the hole transporting layer (HTL); thus collected charge carriers at FTO and metal electrode generates photocurrent at the outer circuit. The structural and optoelectronic properties of halide perovskite is well studied and we direct the readers to the reviews. [6,7] It is imperative to confine the structural and optoelectronic credentials of halide perovskites within a range for its implementation in PSCs. Importantly, the structural stability of ABX3 perovskite is determined by the Goldschmidt tolerance factor (t), and is represented by the formula, = ( + ))/(√2( + ) where , and represent the ionic radii of A, B and X respectively. Arguably, the value of 0.9 ≤ t ≤ 1 is the ideal tolerance factor range to form the cubic perovskite structure. It can be deduced from Figure 1d, any deviation from this range will lead to structural distortion and unfavourable phase transition. [8,9]  Moreover, the intrinsic as well extrinsic degradation pathways prevalent in PSCs have plagued its commercial endeavour. Plausibly, in the case of the widely exploited MAPbI3 (methyl ammonium lead iodide), the volatile nature of the organic cation, halide segregation and crystal size expansion are undesirable processes that accelerate device degradation. [10] Ions have the natural tendency to move, an undesirable process that promotes various type of recombination losses in the fabricated PSCs. [11] Studies reviled that such degradations, induced further defects, which limit the long-term stability of the device. [10][11][12][13] Baikie and co-workers introduced a perovskite with broader absorption and improved thermodynamic stability. [14] The replacement of MA cation by the larger Formamidinium (FA) cation maintained the ABX3 symmetry with a large t value which eventually reduced the band-gap to ~1.47 eV closer to the optimum value of ~1.4 eV. In a parallel work, Snaith et al., reported a champion PCE of 14.2% and suggested FAPbI3 as the future alternative for degradation prone MAPbI3. [15] However, the X-ray and neutron powder diffraction studies of FAPbI3 reported the existence of two polymorphs, a black photoactiveperovskite material with a cubic symmetry (-phase) and a yellow non-perovskite (-phase) hexagonal photo-inactive counterpart (Figure 2a,b). In contrast to MAPbI3, a phase transition from to -phase takes place upon moisture exposure at low temperatures. Though it lacks the detailed understanding of factors affecting the degradation dynamics of FAPbI3, the thermal decomposition studies on powder FAPbI3 revealed the formation of 1,3,5-triazine (s-triazine), hydrocyanic acid, and ammonia at 95 o C [16] as depicted in scheme 1. Recent studies suggests that the intrinsic lattice strain and extrinsic induced strain such as thermal expansion coefficient is a mismatch between the perovskite and the substrate. Disparity in local lattice at the perovskite interface and crystal phase in doped perovskites have deleterious effects on the optoelectronic properties of perovskite caused by the formation of defects and dislocations in grains and grain boundaries. [17][18][19][20][21] The produced defects not only accelerate the photo-, thermal-and moisture induced chemical degradation but also adversely affect the charge carrier mobility to destabilize the PSCs. [19][20][21][22] In addition to the unfavourable phase transition and thermal degradation, smart strain engineering has also emerged as a challenge to stabilize the FAPbI3, where low defect density and superior charge transportation can be achieved.
To an extent, such adverse processes can be controlled, by extrinsic doping of perovskites and these interfacial doping agents provide avenues for improvement of PSC performance. [23] The use of such dopants or additional layer or new crystallization routes protect or curtail moisture-, thermaland photo-damage, induce better charge carrier dynamics and improve the intrinsic structural stability which can synergically boost the PV performances. Studies dealing with compositional engineering, interfacial modification, charge carrier dynamics and controlled crystallization of metal halide perovskites have been reported [24][25][26][27] . In this perspective, we have shed lights on the role of compositional engineering, interfacial modifications, controlled crystallization, and deciphering device kinetics on structural stability, charge carrier dynamics and defect engineering for fabricating high quality FAPbI3 layers and PSCs thereof. Further, we have extended the discussion to the need of unified testing protocols to foster the commercialisation of PSCs.

Compositional and dimensional engineering
Compositional engineering of 'A', 'X' and 'B' sites in ABX3 is an effective pathway to improve the physical and structural without any phase segregation at room temperature, where, Bein et al. attributed the stabilisation of FAxMA1-xPbI3 to the higher dipole moment magnitude of MA which creates stronger hydrogen bond interaction with the inorganic cage and optimised the MA content to <20% of FA [34] . Though the MA content improved the phase stability and efficiency of FAPbI3, the chemical instability of MA cation due to its photo-and thermal sensitivity hindered its employment in PSCs. Akin to MAPbI3, formation of volatile methylamine through the reversible acid-base decomposition induced the deprotonation of MA cations which is reported in MA doped FAPbI3 [35,36] . The 1 H NMR studies on precursor solutions (Figure 2c-f) demonstrated that only a small amount of volatile methylamine leaves the system and the remainder immediately condensates with formamidinium iodide (FAI) to form the unwanted Nmethyl FAI and N,N-dimethyl FAI. We can deduce ( Figure 2f) the gradual increase in the N-methyl FAI upon aging of precursor solution [36] . By capitalizing the reported knowledge on the compositional engineering of MAPbI3, Cs, K or Rb amalgamated FAPbI3 along with the Br or Cl doping delivered improved device performances at the expense of higher bandgap though the device long-term stability under harsh testing conditions remains unclear. The enhanced photo-and moisture-stabilities demonstrated through smaller Cs amalgamation into FAPbI3 opened a new pathway whereby they demonstrated the contraction of cubo-octahedral volume ( Figure 3a) and a reduction of trap density (Figure 3b) by an order of magnitude, subsequently increasing the open circuit voltage (VOC) and the fill factor (FF), leading to PCE and stability hike. [37] The inclusion of Cs motivated researchers to expand the cationic dopant pool with Rb and K, and although it improves the device performance marginally, only Cs displayed structural incorporation. [38] Various methodologies were adopted for Cs amalgamation and >20% of PCE was reported. [39] The understanding of mechanisms and dynamics with Cs inclusion is of significant interest. The microscopic understanding of structural energy landscape revealed that higher entropy contribution from an organic cation reduces the Gibbs free energy thereby assisting in circumventing phase transitions. FA cation in FAPbI3 was reported to have an isotropic orientation with a large entropy at higher temperatures and upon cooling to room temperature it adopts a strong preferential orientation with a lower entropy. [40] The entropy enhancement and reduction of annealing temperature upon Cs inclusion which was optimized over a range of 3 -20% to balance the bandgap widening and the increase of density of states (DOS) below the valance band maxima (VBM). [41] In addition to the phase stability, different imaging or mapping techniques such as Synchrotron-based nanoscale X-ray fluorescence (n-XRF) [42] , photoluminescence (PL), hyperspectral imaging [43] showed that inorganic Cs doping homogenizes the halide distribution between Cl, I and Br and reduces the heterogeneity in the charge carrier dynamics [42,43] which eventually highlighted the ability of the triple-cationmixed halide perovskites to function as capable light harvesters. The addition of dopants as additives in the precursor solution, a non-uniform distribution at perovskite surface and bulk has been reported in thin films which significantly affects the structural, optoelectronic and device properties. [44,45] The dopant impact at the grain and grain boundaries was studied on device performance, understanding of dopant distribution and the effects at FAPbI3 surface and bulk is paramount research for further optimisation. As discussed in the previous section, the intrinsic lattice strain is detrimental to the successful working of PSCs and it stems from twisting and tilting of the PbI6 octahedra and lattice expansion or contraction. The A or X site doping stabilizes the α-FAPbI3 phase at the expense of lattice expansion or contraction which eventually generates the residual strain and influences the 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins optoelectronic properties. Although strain engineering was initially reported for low dimensional and MAPbI3 perovskites, Kim et al., reported a strain relaxation strategy of using smaller and larger cations together as dopants to reduce tensile strain and comprehensive strain in the FAPbI3 lattice. [20] The grazingincidence wide-angle X-ray scattering (GIWAXS) patterns of FAPbI3 modified with equal mole % of smaller (Cs) cation and larger (methylene diammonium) cation showed a reduction of lattice strain and trap density to deliver an unparalleled device with a PCE of 24.4% through prolonged carrier lifetime and reduced Urbach energy. Additionally, the non-encapsulated devices showed superior thermal stability of retaining >80% of the initial PCE after 1300 h of annealing at 85 o C. [20] In addition to the single site doping, co-doping strategies have been introduced to relax the residual strain. The anisotropic strain on the (111) lattice plane of FAPbI3 is reported as the driving force for the phase transition and a co-doping strategy where both the A and X sites get doped with comparatively smaller ions (MA + , Br -) which reduced the strain along (111) plane through lattice shrinkage (Figure 3c) [46] . The micro-strain ( Figure 3d) calculated from the Williamson−Hall plot where MABr alloying reduced the strain at the (111) lattice plane of α-FAPbI3 where they found MA contributes more towards the phase stabilization. However, the current limitation to obtain a defectfree strained FAPbI3 or defect rich FAPbI3 without strain restricts the individual study of strain and defects to their relationships.  The usage of long organic spacer cations will help to address parts of the existing stability concerns in PSCs. The low dimensional/layered perovskites (in scientific literature also referred as 2D or quasi 2D) derived from large organic spacers with high intrinsic and extrinsic structural as well as thermal stability will enhance the physico-chemical properties, whereas, its poor charge-transport properties owing to the large quantum well and narrow light absorption ability impede its development. Moreover, the layered perovskite surface is known to induce the hydrophobicity and the contact angle measurements with water droplet has been used to probe the same. Layered FAPbI3 perovskite showed higher hydrophobicity than the pristine FAPbI3 surface [48] (Figure 4a and 4b).
Apparently, the employment of alternate layer of layered (2D) and conventional (3D) i.e., 2D/3D hybrid perovskites was reported to align the interface in order to maximize charge collection and transportation. [24,25,49,50] The approach of having perpendicular orientation of the layered perovskites atop of 3D (conventional) perovskite to allow uni-directional charge transport and collection was adopted. This bilayer approach was optimized by structural and interfacial packaging control to maximize light harvesting and induced stability. [51] Researchers focussed on the tuning of opto-electrical and structural properties of layered, bilayer, and layered passivation by manipulating the elemental composition, organic spacer, layer thickness and crystallization routes of MA based system and demonstrated reduced exciton binding energy, improved charge carrier dissociation and conductivity. FA cations with a low band gap were also employed in layered PSCs and a PCE of >19% was achieved. [52] Liu et al. showed the superior absorption ability of FA over MA that led to high photocurrent and PCE along with superior stability of the layered perovskite over the FAPbI3 under harsh moisture and thermal testing conditions. In another report, Chen et al., compared the MA and FA based layered PSCs and the (PDA)(FA)3Pb4I13 demonstrated superior stability. Here, both the perovskite were annealed at 150 o C under N2 and the digital images were recorded ( Figure 4c). (PDA)(MA)3Pb4I13 films turned to brown within 1 hour and gradually changed to non-perovskite (yellow) phase after 3 hours while the FA counterpart showed exceptional stability until 12 hours of continuous thermal stress. Further, the corresponding PSCs were tested for thermal stability at 85 o C under N2 atmosphere in dark (Figure 4d) [53] . The knowledge capitalized from MA system, it is vital to focus on the rational selection of organic spacer and crystallization routes to optimize the FA systems. To this end, the employment of an organic spacer with an electron-donating or an electron-with drawing group [54] or hetero-atoms can improve the charge transport. Another approach was to introduce functional donor groups on the cations in order to fine-tune the electrical properties of the layered perovskite. Such organic cations can actively participate in the energy band structure Please do not adjust margins Please do not adjust margins of the layered perovskite, in the charge-transfer processes, and subsequently contribute to the optimization of device performance. The incorporation of cations tailored for π-π stacking is imperative for improving charge-transport properties in low-dimensional perovskites. [55,56] Cations can be tailored to stabilize lead iodide based perovskites through stronger hydrogen bonding coupled with π-π stacking interactions. However, most of the reports on low-dimensional PSCs are restricted to MA cations while the FA cations have rarely been studied and much efforts are needed to elucidate the charge carrier dynamics and thereby boost the PCE for future commercialisation.

Controlled crystallization
Defects at the grain boundaries and surfaces of polycrystalline perovskite layers are sensitive to moisture and ion migration, these promote intrinsic as well as extrinsic instability of PSCs under operando conditions. Any methodology to decrease the super-saturation of perovskite solution by increasing the precursor-solvent interaction will limit the nucleation rate and thereby control the defects at grain boundaries. The use of oxygen-, sulphur-and nitrogen-donor Lewis bases/acids with high dipole moment is a well explored methodology to increase the molecular interactions and the nucleation energy barrier to slow down the crystallization. [57][58][59] The choice of non-volatile additives induces uniform layer formation by residing at grain boundaries and passivating charged point defects. In the recent years, ionic liquids (ILs) have been employed as non-volatile additives due to high hydrophobicity, high surface tension properties, low-toxicity and excellent electrochemical properties and among them, the imidazolium based ILs are preferred owing to their high ionic conductivity. [60][61][62][63][64][65] Enhancement in the PCE of FAPbI3 from 17.1 to 20.06% with a 1-hexyl-3-methylimidazolium iodide (HMII) additive was reported. HMII addition improved the carrier life time from 229 ns to 1382 ns along with an increase in the PL emission suggesting the reduced non-radiative recombination and authors attributed this to the passivation of vacancies through ionic interactions of HMII. [64] Improved crystallinity and phase purity were observed from morphological and structural analysis. The low trap density of the modified device calculated from space charge limit current (SCLC) measurements supported the crystallinity improvements and reduced nonradiative recombination which resulted in higher Voc and photocurrent. Moreover, HMII enhanced the stability and the device retained 85% of its initial PCE after 250 hours of maximum power point tracking (MPPT) under ISOS-L-1I protocol. A polymerized ionic liquid (PIL) in a PSC achieved a PCE of 21.4% and excellent long term operational stability of maintaining 92% of its initial PCE after 1200 hours under 1 sun illumination at 70-75 o C. [66] The synthesized PIL (Figure 5a) containing multi-anchoring sites forms high quality films with large grains. The authors reported that the PIL species were immobilized at the grain boundaries and passivate the undercoordinated Pb ion defects which eventually resulted in excellent stability under harsh conditions. Though the role of the alkyl chain on the imidazole ring has been well studied, the understandings of the effects of the anionic part and the interactions with FAPbI3 require further investigation. Spin coating method followed by an anti-solvent dripping and high temperature (150 o C) annealing is the way to fabricate high quality FAPbI3 layers. The higher thermal expansion coefficient of FAPbI3 over the substrate and the temperature gradient between the bottom and top surface of the FAPbI3 layer upon annealing will induce large residual strain. [67,68] The faster 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins annealing or expansion at the bottom surface and faster cooling from the top surface of FAPbI3 due to temperature gradient leads to tensile strain on the surface and comprehensive strain in the bulk (Figure 5c). [69] Moreover, the thermal coefficient gradient between the perovskite and the substrate induces an additional tensile strain to the bulk eventually leads to a gradient residual strain in the perovskite layer to affect the optoelectronic properties. A protocol to minimize the gradient residual tensile strain of the perovskite layer by a flipping method, where the spun coated perovskite was annealed from the perovskite surface instead of substrate surface, has been introduced. [69] The residual strain distribution was calculated from grazing incident X-ray diffraction (GIXRD) measurement as (Figure 5d and 5f) for pristine and flipped thin films respectively where a smaller strain gradient was visible for the film fabricated through flipping method. The detailed analysis of strain distribution across the film thickness showed homogeneous lattice parameter for the flipped film and a vertical gradient lattice structure for the pristine film as depicted in Figure 5g and 5e respectively. To this end, it is crucial to design effective strategies to reduce the annealing temperature of FAPbI3 film and to introduce new substrate materials with thermal coefficients similar to that of FAPbI3 which could synergically lower the temperature and strain gradients, throughout the perovskite layer.

Interfacial engineering
Optimization in the performance of PSCs can be achieved through interfacial engineering, which allows the tuning of interfacial processes. Since charge recombination plays an important role in controlling the magnitudes of the FF and VOC, ongoing research efforts should concentrate on the search for suitable interlayers between perovskites and selective contacts. So far, the reported FF and VOC are quite close to the theoretical limit values, however, improvement in the JSC value can be a new direction. The ligand-chemistry-dependent nature of the interfacial agents at the perovskite/hole transport layer junction plays a key role in the charge transport/extraction. The passivation of FAPbI3 surface reduces non-radiative losses and suppress ion migration in PSCs, [70][71][72] and can be made through the application of surface passivation agent atop of the perovskite layers. Pure FAPbI3 composition has not been very effective in improving the JSC value. Interfacial engineering of the perovskite absorber assists in forming high quality polycrystalline nature, possessing monolithic grains with negligible defects in the bulk compared with the surface defects. Thus, passivation of the interface can contribute towards the PCE and stable PSCs. The passivation of the FAPbI3 surface with various molecules, reported so far, includes the Lewis acid-base adduct approach to produce a high-quality perovskite layer. [73] It is known that the non-bonding pair of electrons in the Lewis base are shown to coordinated with the under/coordinated Pb 2+ or I vacancies to form a Lewis adduct. The use of phenylalkylamines to passivate the FAPbI3 surface that improves the moisture resistance, while simultaneously enhancing the electronic properties was reported. [74] The champion PSC passivated by benzylamine exhibited a PCE of 19.2 % and VOC of 1.12V ( Figure 6). The device also retained stability up to >2800 h air exposure. Surface passivation by sulphur containing thiophenes and nitrogen containing pyridine compounds are widely exploited due to their ability to form coordination bonds with under-coordinated Pb ions in perovskites. Improvements were also registered by using interfacial layers of chromium, MoOX and PCBM, which supress metal migration. [75,76] Recent efforts allowed us to replace the well-exploited TiO2 with ZnOx and SnOx as electron transport materials, to tap the benefits of their higher electron mobility and favourable band alignment with the perovskite layer [77] . Preliminary results of utilizing surface modifications of TiO2 layer with organic and inorganic passivation will allow us to direct the research to develop new electron selective contacts. [77,78] For hole transporting materials (HTMs), various competitive materials were designed and integrated in PSCs to achieve high performances. The classical Spiro-OMeTAD, performs well in terms of efficiency, however the long-term performance of the device is compromised by the usage of the ionic dopant and additives such as lithium salt (LiTFSI), t-butyl pyridine (t-BP) and/or cobalt complex. The dopant used in the HTM can migrate and the intercalation of Li + in the perovskite layer accelerates the device degradation. [80] Organic semiconductors based on small molecules or polymers, organo-metallic compounds, and inorganic p-type semiconductors, are being investigated to substitute Spiro-OMeTAD. Small molecules offer the choice of molecularly engineering the building blocks to tune semiconducting properties. HTMs based on cores such as triazatruxene, thiophene, carbocyclic moieties have been explored and exhibited high performance in PSCs. [86][87][88] The pyridine based HTM acts as a Lewis base and offers synergistic effect for it to passivate the defects on the perovskite surface along with hole transportation. [81] Please do not adjust margins Please do not adjust margins

Advanced perovskite processing methods
Though the conventional precursor solution synthesized through the dissolution of high-grade precursors (PbI2, FAI etc.) in organic toxic solvents have been widely employed to acquire superior device performances, the reproducibility and costcompetitiveness were highly compromised. To overcome such challenges, application of pre-synthesized halide perovskite powder as precursor is emerging recently which scores with its higher reproducibility and adaptability of both wet-and dryprocessibility. [82][83][84] Similar to the field of ceramics and metals [85] ; various techniques such as mechanochemical, precipitation, and sonochemical processes can be utilised for the powder synthesis. -FAPbI3 powder was synthesized through a roomtemperature precipitation technique and the PSCs gave higher reproducibility and low trap density along with the improved efficiency was shown as compared to the conventional route. [82,83] Followed by this significantly we reported, Cs amalgamated -Cs0.1FA0.9PbI3 powder from low-grade PbI2.
Remarkably the fabricated PSs thereof, where the double cation based absorber layer was annealed at low temperature (80 o C), showed reduced hysteresis and on par performance. [84] We propose the powder precursor route as one of the future directions in FAPbI3 based PSCs. Much efforts are needed to find the extend of incorporating dopants and passivating materials into the powder. The proposed processing techniques has both advantages and disadvantages. Mechanochemical route, which is solvent free will produce highly stable perovskites at the expense of high cost of equipment and contamination possibilities from its abrasions. While, the recrystallization possibilities in precipitation method will reduce the cost of powder synthesis through the use of low-grade precursors along with control morphological homogeneity, but, the requirement of large volume of solvents and stoichiometric balancing will be challenging. However, the powder precursor will open the wide possibilities of utilization through wetfabrication techniques such as dispersion and dissolution along with the dry-fabrication techniques such as physical vapour deposition and aerosol deposition methods. Additionally, the monocrystalline perovskite-based PSCs can be an attractive direction to address the environmental instability. Recent reports revealed exceptional long charge carrier diffusion lengths (>10 μm), low trap density (10 9 − 10 10 cm -3 ), high charge mobility and low band gap for MAPbI3 single crystal perovskites as compared to their polycrystalline counterparts. [86,87] The device performance showed an unparalleled enhancement from 5.49% in 2016 to 21.09% in 2019. [88,89] The current focus needs to turn from the MAPbI3 based single crystal perovskite to the promising FAPbI3 perovskite. Most importantly, large area PSCs are a pre-requisite for their commercialisation. Besides, the conventional laboratory techniques such as spin coating and chemical vapour deposition, new cost-effective, user-friendly, and reproducible techniques need to be employed to fabricate high quality large area FAPbI3 thin films to step-up from the laboratory setup. There are reports in the literature dealing with large area thin films fabricated through blade coating, slot-die coating, inject printing and spray coating [90][91][92] . All these techniques are solution based which will retain the cost-effectiveness of device fabrication. Among the above four methods, blade coating is the least inexpensive way to fabricate large area thin films along with an additional advantage of monocrystalline structure [93] . In slot-die coating, a die is designed to have a thin channel to spread the perovskite ink to a flexible substrate. [94] Inject printing is a material efficient and contactless technique where a pressure pulse generated by a piezoelectric transducer in the continuous fluid supply controls the ejection of ink droplets to the moving substrate. [95] Moreover, spray coating has also been used for the fabrication of perovskite layers and this can be adapted as a future tool for fabrication of large area PSCs due to its wide atomisation possibilities. [96] However, most of the large area PSCs fabricated till now are focussed on MA rich perovskites and attention on FA rich perovskites will accelerate enhancement in device performances and outdoor applications.

Device kinetics and understanding
A synergistic approach that can ensure the fabrication of a stable yet efficient perovskite layer can be achieved by screening new perovskite materials and their designing, to validate relationships between microstructure, phase purity, dimensionality, charge transport and device stability in a holistic manner. In this context, the development of structural characterization techniques having superior spatial resolution will enable us to visualize perovskite degradation processes and pursue in-situ under operando conditions. The induced structural changes and their influence should be analysed with electro-optical techniques. This will unravel the real time monitoring of charge dynamics at the nanometre scale under operational conditions. Such information will establish the corelationship between the nanostructure and the physical properties, essential for advancing the design of new materials and interfaces. Research groups are studying techniques such as in-situ grazing-incidence small-angle neutron scattering (GISANS) to gain information about the water uptake [97] , grazing-incidence wide-angle X-ray scattering (GIWAXS) to track moisture-and thermal-induced structural changes [98][99][100][101] , timeof-flight secondary-ion mass spectrometry (ToF-SIMS) to measure the ion distribution and migration during the decomposition [102][103][104] , in-situ high resolution X-ray photoelectron spectroscopy (HR-XPS) to analyse thermal degradation induced compositional changes at the surface [104] , in-situ point-resolved valence electron energy loss spectroscopy (VEELS) to understand the chemistry of the perovskite during thermal degradation [105] and in-situ high resolution transmission electron microscopy (HR-TEM) [106] coupled with their fast Fourier transforms (FFTs) [107] , selected area electron diffraction (SAED) [108] and energy dispersive X-ray analysis (EDX) [109] to visualize the microstructural variations during thermal-, moisture-and electric stress-induced degradation of different perovskites and complete devices. In this direction, FAPbI3 requires a more detailed study in order to grab the real information on composition-microstructure-property-  [110] Nucleophilic amino group in FA o generated through the deprotonation of FAI undergoes additionelimination reactions to form s-triazine with removal of NH3. The authors noted the addition of boronic derivatives with vacant boron orbital can inhibit the deprotonation step and there by the FAI degradation through its strong the interaction with lone pair of electrons in I -. This observation highlights the need of in-depth and real time understanding of underlying reactions and mechanisms taking place throughout the process, i.e, the quality of the precursor materials to evaluation of fabricated PSCs.

Prerequisite of unified protocols
Reliability of PSCs is a predominant research direction for the uniform reporting of testing conditions and for this, standardized stress testing conditions will play a crucial role in substantiating the research. Environmental stress conditions to assess the long-term durability in the temperature range from -20 − 85 o C including external factors such as irradiance and humidity is important. In the absence of a unified protocol and degree of condition used to investigate the devices under stress conditions, it is unreasonable to draw a rational comparison for stability improvements among scientific labs reporting either innovative chemistry or new process protocols. Degradation induced by light irradiance/soaking at elevated temperatures testing should be performed firstly, and secondly, the reporting of steady-state PCEs, which is significantly different from the maximum power point, obtained from J-V curves due to the presence of the hysteresis behaviour in imprecise devices through J-V scanning. In this context, accelerated environmental stress test and performance characterization methods to probe the stability, adoption of international standards protocol that is designed for silicon or thin film based photovoltaics (IEC 61215) [111] needs to put in action for module or mini module type devices testing. For lab scale devices, the initial testing should be reported exceeding 1000 hours under light soaking conditions at an elevated temperature for any investigation focused on stability enhancement. Addressing the stability challenges is paramount for the transfer of PSC technology from the lab to fab, by validating a standard testing protocol for long-term stability. For this, the nature of the organic component and the ionic characteristics, must be investigated in depth, in order to achieve long-term durability, by monitoring operational stability conducted at the maximum power point tracking under continuous light illumination. Further, the substitution of environmentally unfriendly and toxic lead is a daunting but an essential task. Although lead can be recycled or reused to minimize its pollution or employed for other applications, the presence of ppm amounts of lead in our body has shown neurotoxicity.

Future adaptability
The following measures will allow to further push the performance of FAPbI3 based PSCs and its adaptability. i. Engineering of FAPbI3 to extend light absorption, crystals optimization and processing conditions to control the formation of defect-free, segregation-free, strain-free perovskite layers. Large quantity presynthesized FAPbI3 powder will provide easy processability and reproducibility.
ii. Designing innovative charge selective contacts, which are cost effective and easy to synthesize (inorganic as well as organic) to allow the tailoring of interfaces for enhanced charge extraction, and stability improvement.
iii. Elucidating basic understanding of the underlying process and kinetics through real-time in-situ characterisations to allow materials' optimization and interface tailoring. iv. New processing techniques for scaling up modules and development in a phased manner (30 × 30 cm² − 100 × 100 cm²), which represent uniform, pin-hole free layers and precise control over thickness. v. Efficiency in excess of 18-20% at the module level, and increasing the aperture area to maximize device efficiency and FF. vi. Optimized processing techniques (slot die, blade coating, chemical bath deposition and printing) to achieve reproducibility through the wet chemistry route for the deposition of different charge selective layers. vii. Accelerated ageing test to validate long-term (20+ years) performance in accordance with the IEC-61215 test conditions is required.

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Please do not adjust margins viii. Outdoor testing and characteristics correlation with accelerated ageing tests for lifetime prediction. ix. For solar value chain, life-cycle assessment, energy-payback time evaluation and negative environmental impact valuation are required. The development of emerging PV technology that can improve the competitiveness and dispatchability in the energy mix, will allow energy transition to take place at a faster rate.

Conflicts of interest
There are no conflicts to declare.