Journal Pre-proof Advances in stable and flexible perovskite solar cells Qamar Wali, Faiza Jan Iftikhar, Naveen Kumar Elumalai, Yaseen Iqbal, Sidra Yousaf, Shahid Iqbal, Rajan Jose PII: S1567-1739(20)30061-4 DOI: https://doi.org/10.1016/j.cap.2020.03.007 Reference: CAP 5173 To appear in: Current Applied Physics Received Date: 7 August 2019 Revised Date: 3 February 2020 Accepted Date: 9 March 2020 Please cite this article as: Q. Wali, F.J. Iftikhar, N.K. Elumalai, Y. Iqbal, S. Yousaf, S. Iqbal, R. Jose, Advances in stable and flexible perovskite solar cells, Current Applied Physics, https://doi.org/10.1016/ j.cap.2020.03.007. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. 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Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. on behalf of Korean Physical Society. 1 2 3 4 5 6 7 8 9 10 11 Advances in stable and flexible perovskite solar cells Qamar Walia*, Faiza Jan Iftikhara, Naveen Kumar Elumalaib, Yaseen Iqbalc Sidra Yousafc, Shahid Iqbala, and Rajan Josed a School of Applied Sciences & Humanities, National University of Technology, Islamabad, 42000, Pakistan, Faculty of Engineering and Science, Curtin University, Sarawak Malaysia, 98009 Miri, Sarawak, Malaysia, c Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan, dNanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang 26300, Malaysia b * Correspondence: qamar@nutech.edu.pk 12 Abstract 13 Roll-to-roll (R2R) production is an innovative approach and is fast becoming a very popular 14 industrial method for high throughput and mass production of solar cells. Replacement of costly 15 indium tin oxide (ITO), which conventionally has served as the transparent electrode would be a 16 great approach for roll to roll production of flexible cost effective solar cells. Indium tin oxide 17 (ITO) and fluorine-doped tin oxide (FTO) are brittle and ultimately limit the device flexibility. 18 Perovskite solar cells (PSCs) have been the centre of photovoltaic research community during 19 the recent years owing to its exceptional performance and economical prices. The best reported 20 PSCs fabricated by employing mesoporous TiO2 layers require elevated temperatures in the 21 range of 400-500°C which limits its applications to solely glass substrates. In such a scenario 22 developing flexible PSCs technology can be considered a suitable and exciting arena from the 23 application point of view, them being flexible, lightweight, portable, and easy to integrate over 24 both small, large and curved surfaces. 25 Keywords: Renewable energy; lightweight device; portable device; flexible perovskite solar 26 cell; fibre shaped solar cells; 1 27 1. Introduction 28 Solar-to-electrical energy conversion lies at the forefront of human advancements for an 29 energy-secured world with a clean environment. Consequently, improving the performance and 30 lowering the cost of the current solar cell technologies as well as searching for new solar energy 31 conversion protocols have become a dynamic area for clean and sustainable energy research. 32 Achievement of these objectives needs optimization of solar cell materials and configuration so 33 that, (i) the photoactive materials have a high photon absorption cross-section, (ii) photo- 34 generated electrons are long-lived so that a high-volume fraction of these electrons can be 35 transported within the device over an appreciable length scale (of the order of few hundred 36 micrometres), and (iii) high hole mobility so that the electron loss in the photoactive material can 37 be quickly compensated. The presence of these characteristics in the first and second generation 38 solar cells enabled their commercial availability; yet the cost of electricity which has indeed, 39 significantly reduced over the past decade is still pricier than that from the conventional fossil 40 fuels and hence is an area where scope of ample research is evident. 41 It is imperative for adopting adaptable technological applications that solar cells are highly 42 efficient, stable and economical supported by transparent flexible electrodes and 43 favourable operating conditions. Moreover, the first three conditions were successfully 44 realized by the first two generation solar cells however the third generation solar cell 45 offered additional benefits such as being flexible, transparent and solution processable. 46 However, soon the interest waned due to failure of earlier third generation solar cells to 47 accomplish efficiency, stability and cost effectiveness. Nonetheless, the advent of hybrid 48 organic-inorganic PSCs has witnessed a revival of confidence on the third-generation solar 49 cells in terms of boosting the efficiency and stability with economical cost. The fascinating 2 50 features that have captured attention of scientists towards PSC are their extraordinary 51 charge mobilities due to their efficient light absorbing property that is responsible for 52 creation of charge carriers [1-5]. 53 Many organometallic hybrid perovskite compounds have been tested in PSCs and show 54 dramatic performance at laboratory scale: from a photo-conversion efficiency (η) of merely 55 ~3.8% (the first architecture of PSCs reported in 2009) to a record η = ~25.2[6]%, exceeding 56 the η of the present photovoltaic (PV) cells based on polycrystalline silicon, consuming ~1000 57 times less light harvesting material than silicon [7]. Additionally, perovskite devices are 58 fabricated employing relatively simpler low-cost solution processing methods without the need 59 for vacuum steps or elevated temperatures. PSCs have remained at par with the efficient copper 60 indium gallium di-selenide (CIGS) solar cells with an η =23.4%[6] since their inception [8]. 61 Substrates coated with metal oxides such as TiO2, ZnO, SnO2 and other similar materials play a 62 vital role in PSCs because they not only support the perovskite molecules, but also accept and 63 transport photoinduced-electrons[9, 10]. The substrates must possess the following features; (i) 64 high specific surface area for anchoring large amount of perovskite molecule, (ii) a suitable 65 morphology to support electrons’ diffusion and additionally scatter light and (iii) a favourable 66 conduction band alignment (The conduction band of the metal oxide must be lower than the dye 67 lowest unoccupied molecular orbitals in order to favour easy electrons injections) with respect to 68 the lowest unoccupied molecular orbitals of the perovskite molecule[9-13]. Various substrates 69 have been employed so far in PSCs owing to their unique characteristic. For instance, the 70 commonly used flexible substrates such as poly(ethylene terephthalate) (PET) could not endure 71 high processing temperature significantly higher than >200 °C. Hence, most of the PSCs 72 fabricated are being reported on either glass or PET substrates however a new avenue in this 3 73 direction has been to use paper as a substrate because of it being easily available, flexible and 74 disposable along with being cost effective. Hence the first ever paper based PSC device with 75 architecture: 76 reported with a power efficiency of 2.7% steering future research towards sustainable solutions 77 of using such a substrates[14]. Paper/Au/SnO2/mesoTiO2/CH3NH3PbI3/Spiro-OMeTAD/MoOx/Au/MoOx is 78 For scalable roll to roll (R2R) production of truly flexible, cost effective solar cells, 79 alternatives are needed to replace indium tin oxide (ITO), which conventionally serves as the 80 transparent electrode. Flexible, thin film and lightweighted PSCs based on plastic substrates 81 can be utilized in niche applications for instance, electronic textiles, portable electronic 82 chargers, large scale industrial roofing and electronic textile industry[15]. Employing an 83 extremely thin (~1 µm) light absorber of organometal halide layer in PSCs delineates itself as 84 the most promising candidate for flexible thin film solar cells. The solution processable and 85 low temperature fabrication of perovskite substrate layers in PSCs presents itself as a great 86 prospect to realize its importance as flexible devices. The high performance of PSCs arises 87 mostly from a photoactive perovskite comprised of organic-inorganic hybrid lead halide 88 (ABX3) as shown in Figure 1. In this ABX3-type structure, A is an organic monovalent cation 89 (methyl ammonium (MA+, CH3NH3+)), formamidinium (FA+, CH(NH2)2+ [16-19], B is a metal 90 cation such as lead (Pb2+), germanium (Ge2+) or tin (Sn2+) [20-23] and X is a halogen anion such 91 as iodine (I−), chlorine (Cl−), bromine (Br−), hexafluoroborate (BF6-) or tetrafluoroborate (BF4-) 92 [24-28]. Similarly, modifying the chemistry of perovskites via suitable substitutions or exploring 93 different configurations as well as tailoring various interfacial layers and microstructures have 94 notable effects on the performance and stability of the PSCs devices [29-37]. 4 95 96 97 98 Figure 1: (a) Crystal structure of organic-inorganic perovskite, where central big atom is the methyl ammonium while the Pb atom is surrounded by six halogen atoms and (b) A cation coordinated by X ions in12-fold cuboctahedral symmetry. 99 100 Perovskite acts as a light absorber as well as provides a transport medium for 101 photoinduced charge species (holes and electrons). The concept of PSCs originated from the dye- 102 sensitized solar cells (DSSCs) where a thick (10 − 20 µm) mesoporous-TiO2 (m-TiO2) scaffold 103 was required to anchor sufficient dye and enable complete light absorption [38]. During 2009, 104 Miyasaka et al., [39] employed CH3NH3PbI3 and CH3NH3PbBr3 as sensitizers in a solar cell 105 device with architecture similar to DSSCs which resulted in η ≈3.81% and a significant high 106 photovoltage (VOC = 0.96 V). In a similar device architecture, Park and co-workers [40] 107 optimized the m-TiO2 layer’s thickness and treated it with Pb(NO2)2 prior to the deposition 108 which yielded η ≈ 6.51%. However, these devices were unstable due to rapid dissolution of the 109 perovskite layer by the liquid electrolyte. 110 The issue of rapid and abrupt degradation of perovskite layer in PSCs by liquid 111 electrolyte was partially solved with the inclusion of a novel hole transport layer (HTL) named 112 as 2,2',7,7'-tetrakis (N,N-p-dimethoxy-phenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) into 5 113 the device. The newly developed PSC employed a HTL Spiro-OMeTAD accompanied with 114 halides in the form of CH3NH3PbI3 and mixed halides CH3NH3PbI3-xClx as a light absorber 115 coated with a very thin ≈0.6 µm m-TiO2 scaffold layer yielding η ≈9.7%. The replacement of 116 liquid electrolyte by spiro-OMeTAD significantly improved the device stability under ambient 117 conditions for about 500 h. Unfortunately, these devices were unstable for long term use [18]. 118 Despite a number of attempts to eliminate m-TiO2, this class of PSCs has shown the highest 119 η >20% so far in a device structure similar to DSSCs (with a significant reduction in the 120 thickness of m-TiO2 layer from 10 µm to 300 nm). 121 Device designs for PSC have been further explored by changing the orientation and/or tailoring 122 the material’s thickness in relation to TiO2 with meso-porous framework (m-TiO2) with η > 21% 123 [41], regular planar (η ≈ 20.44%) [42], inverted planar (η ≈19.9%) [43], meso- superstructure 124 TiO2 (η ≈15.9%) [44], flexible devices (η ≈18%) [45-48], HTL free (η ≈16.0%) [49], electron 125 transport layer free (ETL) (η ≈14.14%) [50], and fibre-shaped PSC architecture (η ≈5.3%) [14, 126 51]. Figure 2 shows how efficiencies have improved over the same period for both systems and 127 it is clear that in recent years, the efficiencies have seen a tremendous surge in flexible PSC 128 devices versus the normal PSC devices. 6 129 130 131 Figure 2: Comparison of progress made in efficiency in normal and flexible perovskite solar cells. Scopus (14.10.2019). 132 133 As illustrated in Figure 3(a-b), the number of published articles on PSC (according to Scopus) 134 has exponentially increased since 2013 and a similar trend can be seen with flexible PSC devices 135 since 2012 with 2018 heralding the emergence of new, implementable and rational innovations 136 in PSCs. 137 7 138 8 139 140 141 Figure 3: Progress have been made (a) PSCs and (b) flexible PSCs. Data was taken from Scopus (14th October, 2019) 142 This review will briefly elaborate upon PSCs and perovskite crystal structures highlighting the 143 progress that has been made in flexible PSCs. In addition, stability and large scale production has 144 also been deliberated on. 145 146 2. Progress made in flexible PSCs 147 Flexible and light-weight solar cells have considerable significance in terms of supplying 148 power to wearable and portable devices as well as reducing the cost of solar panels. R2R 149 manufacturing of PSCs has proven itself as a fast and economical method which is ideal for mass 9 150 production. PSCs with different architectures have been explored and investigated in detail since 151 their inception. These include, mesoporous, planar (regular and inverted), HTL-free and ETL- 152 free PSCs constructs. Among these, the inverted class of PSCs possessing a p-i-n structure is an 153 excellent choice for flexible devices than the regular planar devices owing to its lower hysteresis 154 and fabrication at a low temperature through an in-situ solution process assisted by organic 155 vapours. In high performing planar PSCs, the TiO2 layer is usually deposited by spray pyrolysis 156 or spin coating which requires very high sintering temperatures >450 oC, [44, 52] in order to 157 obtain highly dense structure and crystallinity. This high sintering temperature is not compatible 158 with most of the polymeric substrates and therefore complicates the fabrication process. 159 Most PSCs are made on ITO or FTO transparent conductive electrodes which have 160 several disadvantages. The scarcity and costliness of Florine or Indium leads to an increase in the 161 overall cost of PSCs production [53, 54]. Additionally, a 50% decrease in power conversion 162 efficiency η at low bending cycles is reported when ITO is used as the transparent conducting 163 oxide attributed to mechanical deformity produced in ITO. On the other hand, a very high 164 temperature is required for indium-free substrate such as FTO which is unsuitable for plastic 165 substrates. Properties such as mechanical flexibility, high optical transmittance, spatially 166 homogenous and low sheet resistance are a prerequisite to employ flexible substrates for PSCs 167 [55, 56], yet very few materials exhibit all these features of the substrate, such as metal film 168 typically possesses low transmittance when it is thick while thinner films lead to poor 169 conductivity. 170 (styrenesulfonate) (HC-PEDOT:PSS) electrode could be fabricated with a low sheet resistance, 171 and high mechanical flexibility [57-59]. Similarly, Highly conductive poly(3,4ethylenedioxythiophene): poly 10 172 In order to investigate ITO-free transparent conducting electrode TCE, Yoon et al., [47] 173 employed graphene as a TCE on top of a thin film of polyethylene naphthalate (PEN) in flexible 174 PSCs which was reported to exhibit η ~16.8% without showing any hysteresis. At the same time, 175 the inferior conductivity of the deposited graphene layer on PEN was significantly improved by 176 introducing a molybdenum trioxide (MoO3) layer. A schematic of the flexible PSCs based on 177 graphene as an anode is depicted in Figure 4a, with device architecture represented as 178 PEN/graphene-MoO3/PEDOT:PSS/MAPbI3/C60/BCP/LiF/Al where the HTL, ETL and hole 179 blocking layers are represented as PEDOT:PSS, C60 and BCP (bathocuproine) respectively. The 180 annealing temperature for all these layers was kept below 110 oC during fabrication process as 181 the glass transition temperature of PEN is about 120 oC. The as fabricated flexible devices 182 comprised of PEN substrate with Graphene doped MoO3 electrode Gr-Mo/PEN and ITO based 183 electrode ITO/PEN were compared with each other and their J-V characteristics as concluded are 184 shown in Figure 4b-c. The figure shows similar JSC curves for both the devices; however, the 185 device based on Gr-Mo/PEN resulted in a lower fill factor FF ~0.78 than that of ITO/PEN (0.83) 186 owing to the relatively higher sheet resistance of the former. 187 These devices were scanned in both the reverse and forward directions and gave a 188 significantly high η= ~17.3 for ITO/PEN based device and η= ~16.8% for the Gr-Mo/PEN based 189 device with nominal hysteresis. Bending stability was also checked for the ITO/PEN and Gr- 190 Mo/PEN devices for its practical applications. The devices were bent with radius (R = ∞, 6, 4, 191 and 2 mm) for 1000 bending cycles. Figure 4d shows the relationship of normalized η of the 192 ITO/PEN and Gr-Mo/PEN based devices. The latter has maintained 90% of its original 193 performance when bended at R = 6 mm, 4 mm, and 2 mm while the performance of the 194 ITO/PEN based device deteriorated to 60% (for R= 6 mm) and 25% (for R= 4 mm) from its 11 195 original values. Figure 4e shows the relationship between η and bending cycles for both the 196 devices particularly at R = 4 mm. One can see a similar trend for both the devices at the 197 beginning of the bending upto 250 cycles; however, there was a sharp drop in η for the ITO/PEN 198 device when bending increased. In stark contrast, the Gr-Mo/PEN based device showed a stable 199 performance over bending for 1000 cycles at R = 4 mm. Such an outstanding performance of the 200 Gr-Mo/PEN based PSCs against the mechanical distortion paves the way for these devices to be 201 realized as potential candidates for new foldable PV applications. 202 ITO has been the photoanode material of choice because it offers high conductivity and 203 transparency on both flexible plastic and rigid glass substrates. [60] However it has its own 204 limitation of being expensive and brittle and hence affects flexibility of the device [61, 62], yet it 205 is reported to improve bending by using with configuration as ITO/PET and ITO/PEN. [63] 206 Besides Kaltenbrunner et al., have employed PEDOT: PSS in place of ITO as promising 207 candidate in flexible PSCs with a thickness of only 3 µm exhibiting an η value of 12.5%. [64] 208 Metal foil has also been use as a flexible valid robust substrate for PSC fabrication that has high 209 electrical conductance and thermal endurance with low rate of water transmission as compared to 210 plastic substrates. Thus Ti foils have been selected for flexible SCs because of their excellent 211 resistance to corrosion as well excellent ratio of strength to weight. However the opaque nature 212 of metals needs to be countered by using a light transparent top counter electrode. [60] It also 213 poses difficulty in fabricating solar modules connected in series unless cell is divided. Hence a 214 physical separation is required to avoid short circuiting which is in contrast to the glass 215 substrates where laser etching helps to prevent short circuiting. Cu and stainless steel foils apart 216 from Ti foil have been used as substrates [65, 66] in flexible PSCs while fibrous shaped metal 217 photoanode has also been investigated and found to be flexible enough to be integrated with 12 218 wearable electronic gadgets. [67] While currently flexible PSCs on metal substrates are restricted 219 to development of single cells where the metal foil is used as carrier and bottom electrode only. 220 Additionally, the roughness needs to be dealt with by pretreating the metal foil with electro- 221 polishing. [68] 222 Most of the metal based PSCs report using a mesoporous TiO2 layer with an n-i-p architecture 223 and hence titanium foil is the most exhaustively employed substrate. [68-70] A most efficient 224 PSC obtained by anodizing a TiOx layer to nanotube TiO2 used as the ETL with η of 8.3% is 225 reported. [67] In another study, spray pyrolysis was used to obtain a compact TiO2 layer along 226 with perovskite layer and demonstrated an η value of 11% where the top electrode was 227 comprised of Ag/ITO layer that resulted in improved performance. [68] However metal film may 228 confer poor conductivity to the flexible PSC which can be overcome by using a mesh of metal 229 together with conducting polymer as a hybrid counter electrode. Al2O3 scaffold was reported 230 with lamination of a transparent electrode composed of Ni mesh impregnated onto a 231 polyethylene phthalate (PET) foil covered with PEDOT: PSS with an adhesive that ensured good 232 electrical conductivity. The η value was demonstrated to be 10.3%. [71] Ag mesh deposited 233 through spray pyrolysis and Cu foil employed as substrate was reported by [65]. Ag nanowires as 234 metal fibers with highly efficient flexible PSCs have been reported. Ag nanowires can be an 235 effective alternative to transparent conducting oxides where a top electrode of Ag nanowires was 236 used in flexible PSCs in the form of fibers.[72] Nevertheless the fabrication procedure as 237 employed was different from the ones used for glass or flexible substrates. Electro deposition can 238 be an effective method to deposit Pb salt e.g., by a two-step synthesis of PSCs which leads to an 239 increase in performance. The result portrays versatility in fabrication process adopted by a 240 variety of substrates for designing high performance PSCs. 13 241 Flexible PSCs printed on a low priced metal foil is reported. R2R high purity PSC fabrication 242 offers an alternative route to a high heat treatment process in the halide perovskite processing. 243 Doojin Vak working on development of protocol and instrument for R2R fabrication of flexible 244 PSCs on plastic substrates has also discussed the heat treatment issue and have used a slot-die 245 coating method instead of spin coating one to avoid rapid evaporation of solvent. Hence 246 extensive work in this domain will lead to advancement of the film fabrication process on a 247 suitable substrate for its large scale production.[73] Due to high conductivity and flexibility, 248 carbon nanotubes (CNT) and graphene have also been used as photoanodes. Hence CNT 249 deposited on PET substrate was reported to show an efficiency of 5.38%. [74] Graphene 250 modified with MoO3 to reduce its hydrophobicity has also been introduced on PET substrate in 251 flexible PSCs with a higher efficiency of 16.8% than CNT modified PET. [47] It showed even 252 higher flexibility and transmittance than ITO films. 253 254 255 Figure 4: (a) Basic architecture of graphene-based flexible perovskite solar cell where the inset image is the complete photograph of the device, (b&c) J-V curves of the prepared flexible devices based on Gr-Mo/PEN and 14 256 257 258 259 260 ITO/PEN with respect to both scan directions, (d) normalized efficiency after 1000 bending cycles with various bending radii: flat, 6, 4, and 2 mm, and (e) normalized efficiency vs bending cycles with a fixed radius 4 mm of the respective devices (reprinted from ref.[47]) Copyright 2017 RSC. 261 nanoparticles synthesized at temperatures <100 oC for developing a flexible PSC [75]. The 262 inclusion of n-type Zn2SnO4 layer significantly enhanced the transmittance performance of 263 flexible polyethylenenaphthalate/indium-doped tin oxide (PEN/ITO) up to 75 – 90% over the 264 entire wavelength’s range of light as compared to PEN/ITO as a bare flexible electrode. Recently, a high η >15% has been reported in a study utilizing metal oxide Zn2SnO4 265 In another similar study on flexible devices, a very condensed amorphous TiO2 (am- 266 TiO2) film was prepared by magnetron sputtering process at room temperature which was 267 employed as ETL in flexible PSCs [76]. The synthesized am-TiO2 dense layer was highly 268 transparent across the solar spectrum and yielded an impressive η >15% with reasonable values 269 for key photovoltaic PV parameters as shown in Table 1. The J-V curve of the various flexible 270 devices are shown in Figure 5 with inset showing the bent devices. The performance of the 271 flexible device showed no degradation even after mechanical bending at 65º for 100 times, thus, 272 assuring mechanical stability (Figure 5a). Similarly, highly bendable flexible PSCs based on 273 amorphous am-TiO2 with a bending diameter of ≈1mm has shown significant η= ≈12.2% based 274 on PEN/ITO substrate [62] with the corresponding J-V curve shown in Figure 5b. A thick 275 amorphous TiOx (≈20 nm) layer was deposited by plasma enhanced atomic layer deposition at a 276 very low temperature (80 oC). The stability and durability of the fabricated devices were 277 investigated through bending tests. Three different radii of curvature were chosen i.e. 400 mm, 278 10 mm, and 4mm (R400, R10, and R4, respectively). The devices’ performance with R400 and R10 279 maintained the quality without any significant reduction in η during 1000 bending cycles 15 280 whereas for the device with R4, η was maintained at 90% after 25 cycles, however at the end of 281 1000 cycles, the device lost 50% of the initial η. 282 Zhang et al [77] fabricated a novel flexible HTL-free PSC with notable performance of 283 η ≈9.7% and 12.5% with flexible PET/ITO and glass/ITO substrate using Fast deposition 284 crystallization (FDC) technique for perovskite deposition as shown in Figure 5c. The low 285 performance regarding η of PET/ITO based PSC may be attributed to high resistance of ITO on 286 PET. The HTL-free device was tested for stability in comparison to the most commonly 287 employed PEDOS: PSS HTL under (30 ± 10%) humid environment at room temperature. The 288 device with PEDOT: PSS as HTL lost 99% of the original η in just 50 min while the same 289 reduction of the HTL-free device occurred in 300 min. Chen et al., [78] employed a layer by 290 layer approach for mixed perovskite CH3NH3PbI3-xClx deposition on ITO-coated PET flexible 291 substrate. The J-V curve of the flexible device based on CH3NH3PbI3-xClx shown in Figure 5d 292 resulted in η= ~12.25%. The flexible device bent repeatedly at 60o for 1000 times without 293 showing any drastic change in its performance. 16 294 295 296 297 298 Figure 5: J-V curve and bent photographs of the best flexible PSCs (a)PET/ITO/am-TiO2/ CH3NH3PbI3-xClx /spiroOMeTAD/Au [76] (b) PET/ITO/MAPbI3/PC61BM/Al [62], (c)PEN/ITO/am-TiO2/ CH3NH3PbI3-xClx/spiroOMeTAD/Ag [77] (d) PEN/ITO/PEDOT:PSS/CH3NH3PbI3-xClx/PCBM/Ca/Al [78]. Reproduced from ref. 77, 62, 78 and 79. 299 300 Jin et al., [79] employed ZnO as ETL in flexible PSCs and compared its performance 301 with a TiO2-based device. Figure 6(a-b) illustrate a schematic energy band diagram of ZnO and 302 TiO2 with respect to MAPbI3 perovskite. Upon light illumination, the photoinduced charge 303 carriers are transported from MAPbI3 layer into ZnO or TiO2 as ETL and Polytriarylamine 304 (PTAA) as HTL, respectively. It has been reported that TiO2 as ETL in planar PSCs possesses 305 hysteresis with respect to the scan direction owing to its lower electron conductivity (≈6×10–5 306 mS.cm–1) than the hole conductivity of PTAA (≈3.4×10–2 mS.cm–1) and is responsible for 17 307 accumulation of electron at the TiO2/perovskite interface [80]. On the other hand, ZnO as ETL 308 could minimize the hysteresis due to its high electron conductivity. Moreover, ZnO conducting 309 layer can be formed at room temperature by sol gel method so that it is more feasible for flexible 310 PSCs. The SEM cross sectional micrograph of the ZnO (~40 nm) as ETL based flexible PSCs is 311 shown in Figure 6c. 312 The J-V characteristic curves of ZnO and TiO2 ETL based devices with respect to 313 direction under 200 ms of delay time (scan rate = 10 mV.200 ms-1) are shown in Figure 6(d-e). 314 It is evident from the figure that the device based on ZnO ETL showed less hysteresis with 315 respect to the scan direction as well as delivered improved VOC in comparison to its TiO2 ETL 316 counterpart. The maximum η= ~17.7% (in reverse direction) and 17.5% (in forward direction) 317 were achieved for ZnO ETL based device in comparison to 17.3% and 16.8% for TiO2 ETL- 318 based device, respectively. The other PV parameters are listed in Table 1. The external quantum 319 efficiency (EQE) of both the devices were almost the same and the calculated JSC using EQE 320 (20.0 mA.cm−2 for TiO2 and 20.1 mA.cm−2 for ZnO) were in close agreement with the directly 321 measured J-V data. Moreover, similar values for EQE curves were expected because the UV-Vis 322 absorbance of FTO/TiO2/MAPbI3 and FTO/ZnO/MAPbI3 films showed similar absorbance over 323 the same spectral range of light. 324 The reproducibility of the devices based on ZnO and TiO2 ETLs is shown in histograms 325 (Figure 6f). The figure shows deviation in average value of η for each of the 40 devices where 326 the ZnO based device has slightly higher value of η (15.96 ± 1.07%) than its TiO2 counterpart 327 (15.20 ± 1.23%). The better performance of ZnO ETL based device was further investigated by 328 studying the I-V characteristics of the electrodes: ITO/TiO2/Au and ITO/ZnO/Au in sandwich 329 cells using the relation I = σοAd-1V, where A is the active area (0.16 cm2) and d is film thickness 18 330 (50 nm) of the sample. The calculated conductivity of the ZnO was 0.0031 mS.cm-1 which was 331 about 50-folds higher than that of TiO2 (0.00006 mS.cm-1). With ZnO ETL employed on 332 PEN/ITO, the planar flexible device yielded η= ~15.4% in the forward direction while 15.6% in 333 the reverse direction as shown in Figure 6g. The other PV parameters are listed in Table 1. 334 Regarding the mechanical durability, the flexible device based on ZnO ETL showed no visible 335 change in η when bent into curvature of different radii. 336 337 338 339 340 341 342 343 Figure 6: Schematic band energy illustration of (a) TiO2 and (b) ZnO ETL with (c) SEM cross sectional image of representative devices, (d)J-V of TiO2 and (e) ZnO ETL based devices with respect to scan direction, (d), (f) efficiency deviation of 40 sample of TiO2 and ZnO, (g) J-V curve of the flexible device based on ZnO ETL along with bended flexible cell photographs shown in inset (reproduced with permission from Ref. [79]). Copyright of RSC. 19 344 Yang et al., [81] employed solid-state ionic-liquids (ss-IL) as ETL in flexible PSCs with a device 345 architecture shown in Figure 7a along with energy band diagram. The ss-IL is known for its 346 unique characteristics (such as high electrical conductivity, high carriers’ mobility, better thermal 347 stability) substantiating its superior performance in batteries, supercapacitors, actuators, and 348 organic solar cells [82, 83]. Figure 7b shows the SEM cross sectional micrograph of the full 349 device while the upper-right inset displays the photograph of the flexible device where the 350 thickness of the ss-IL was only about 10 nm. The flexible device based on ss-IL ETL exhibited 351 no hysteresis with respect to scan direction (Figure 7c). 352 Owing to the outstanding properties of ss-IL (such as wide bandgap, anti-reflection, high 353 electron mobility, and suitable work function WF), the fabricated device exhibited no hysteresis. 354 The device without ss-IL showed inferior performance due to maximal recombination of charge 355 carries in comparison to devices based on ss-IL as reported earlier. The insertion of a thin ss-IL 356 layer between ITO and MAPbI3 significantly enhanced the performance of the cell and the η lept 357 from 8.19% (without ss-IL) to an impressive η ≈16.09% (with ss-IL) which has been reported as 358 the highest value achieved so far for such devices. The Incident photon to current conversion 359 efficiency (IPCE) of ss-IL based device was notably higher than the device without ss-IL 360 (Figure 7d). Additionally the integrated JSC deduced from IPCE was in close agreement with the 361 J-V profile. The fabricated device was bent at curvatures of 12, 7, and 5 mm and flexed for 300 362 cycles which showed no notable change in η value (Figure 7e). In comparison to ITO/MAPbI3 363 electrode, the Photoluminescence (PL) intensity of ITO/ss-IL/MAPbI3 significantly quenched 364 showing excellent charge extraction capability due to its high electron mobility and reasonable 365 WF. 366 20 367 368 369 370 371 372 373 Figure 7:(a) Schematic architecture along with energy band diagram of flexible PSC, (b) SEM colourful cross section and the upper-right inset shows the photograph of the flexible bended PSCs, (c) J-V profile of flexible PSC with ss-IL with respect to scan direction and without, (d) IPCE and integrated JSC, and (d) J-V profile of bending at curvature radius of 12, 7, and 5 mm and flexed for 300 cycles (reproduced with permission from Ref.[81]). Copyright of John Willy & Sons. 374 375 Li et al., [84] fabricated flexible PSCs based on p-i-n architecture with significantly high 376 value of η ≈14%. The J-V curve (Figure 8a) showed a small hysteresis with respect to the scan 377 directions. For mechanical flexibility and durability the inverted flexible device was bent into 378 five curvatures of radii r = ∞, 7, 5, 3.5, and 2 mm in one bending cycle. The resulting η showed a 379 stable trend for all the radii and therefore validated the superior flexibility of PET/Ag- 380 mesh/PH1000 electrode. Yin et al., [85] employed solution derived NiOx as HTL in a flexible 381 device which yielded a decent η= ~13.43%. Figure 8b shows the J-V curve and its comparison 21 382 with the device based on ITO glass. However, η (≈12.63%) slightly dropped for the reverse scan 383 direction (VOC →JSC). 384 Jung et al., [86] employed a solvent engineering approach for perovskite film deposition 385 at a very low annealing temperature of 70oC and fabricated inverted planar flexible PSCs which 386 delivered η ≈9.43%. The J-V curve with the inset showing the photograph for the flexible device 387 as seen in Figure 8c exhibits a high FF ≈75%. After bending 20 times, the device still exhibited 388 η ≈8.42%. 389 Yoon et al., [87] reported fabrication of highly efficient, hysteresis free and stable 390 flexible PSCs with η ≈16%. The J-V profile (Figure 8d) exhibited no hysteresis with respect to 391 the scan direction. Its mechanical stability was investigated by bending test where 1000 392 consecutive bending cycles at different radii of curvatures (10 mm and 5 mm) were employed for 393 its evaluation. The results revealed that bending showed almost no loss of η for 10 mm curvature 394 while a 20% decrease in η was observed after 1000 bending cycles. Xi et al., [88] reported η 395 ≈13.03% in a flexible device employing α-FAPbI3 (formamidinium lead iodide) perovskite as a 396 light absorber. Figure 8e depicts the representative J-V profile along with photographs of bent 397 flexible device in the inset. As can be seen from the figure, negligible dependence was observed 398 with respect to the scan direction i.e., from FB-SC (VOC→JSC) and from SC-FB (JSC→VOC). In 399 addition, the J-V profile exhibited reduction of 0.18%, 0.61% and 0.81% in η accompanied at 400 bending angles of 15°,30°and 45° after 1000 times, respectively, showing excellent working 401 ductility. Hu et al., [89] employed FAPbI3 as light absorber in flexible PSCs and reported an η of 402 ≈12.70%. The FAPbI3 film was directly grown onto an ITO with a simplified device 403 architecture. Its J-V curve is shown in Figure 8f. The bending test was performed in order to 22 404 examine the robustness of the device which showed a slight effect when the device was bent 20 405 times. 406 407 408 409 410 411 412 413 414 Figure 8: J-V characteristic with inset photographs of the best flexible PSC (a) p-i-n based architecture PET/Agmesh/PH1000/PEDOT:PSS(35 nm)/MAPbI3(≈280 nm)PCBM(≈60 nm)/Al(100 nm) hysteresis free η≈14%[84], (b) ITO/NiOx/MAPbI3/PCBM/Ag device[85](c)PET/PEDOT:PSS/MAPbI3(300 nm) /PC61BM (50 nm) /Bis-C60(10 nm)/Ag(150 nm)[86](d)PEN/ITO/C60/Perov/Spiro/Au, J-V curve with inset shows a digital camera image of the flexible device [87], (e)ITO/PEDOT:PSS/α-FAPbI3/PCBM/BCP/Ag[88](f) ITO(PET)/Cl-FAPbI3(350 nm)/spiroOMeTAD(150 nm)/Au(100 nm) η ≈12.70% [89]. 415 performance of flexible PSCs with a p-i-n structure as compared to glass/ITO is lower largely 416 due to its surface roughness leading to significant leakage of current. Hence to improve η values 417 for flexible PSCs, PEDOT: PSS was deposited onto ITO surface with NiO film immobilized 418 over the bare ITO along with 1,8-diiodooctane (DIO) and H2O added as dual additives to the 419 precursor solution. This improved the uniformity and grain size with a high flexibility and high 420 value of 18% for η. The η value of flexible PSCs has indeed reached from 6.4% to 18% as reported by [90]. The 23 421 Solar cells with PET/ITO architecture and integrated with a wide variety of metal oxides as 422 flexible 423 perovskite/mAl2O3/N2 plasma SnO2/ITO/PET have displayed an outstanding η of 18.1% with 90 424 % performance from its initial value after 1000 bending cycles. [91] It is shown that the UV 425 radiation with low media frequency N2 and N2O plasma emission results in good quality film of 426 SnO2 at low enough temperatures and the film retains same properties as of thermally annealed 427 SnO2 as a compact layer. Flexible solar cells have long been pursued due to extensive advantages 428 as compared to rigid substrates. However oxide layer deposition as a charge extraction layer is 429 shown to be deposited by an energy intensive process through traditional thermal treatment. 430 Co doped NiOx NPs for inverted planar PSCs as HTL have been employed at different 431 concentrations of Co and have demonstrated a η of 14.5% as compared to pristine NiO-based 432 HTL processed at low temperature that shows limited conductivity while high temperature Li 433 Mg doped NiOx based HTL reporting a η of 16.2% is disadvantaged with respect to the high 434 pyrolysis process used for its fabrication.[92] Therefore alternatives in the form of Co-NiOx 435 nanostructured HTL is sought. Whereas Fe-doped NiOx based planar PSC improved η by 436 17.57% compared to pure NiOx at 15.41 %, the flexible PSC have shown a η of 14.42% for the 437 doped HTL as compared to 13.37% for NiOx /perovskite delineating promising candidature for 438 HTL for constructing flexible PSCs in future. [93] Sr-NiOx film as HTL in comparison to 439 pristine NiOx at different concentrations of Sr exhibits η at 20.07% retaining 60% of original 440 η for 100 days.[94] This is attributed to reduction of carrier recombination, matching of energy 441 levels with perovskite layers and improvement of conductivity by doping NiOx with Sr. Cs 442 doped NiO2 have shown a remarkable η of 19.35% used as HTL at a relatively low annealing 443 temperature for inverted planar PSCs. [95] A 2-step process comprising of a hydrothermal substrates have demonstrated interesting results. Thus Au/spiro-OMeTAD/ 24 444 process followed by an in situ hydrolysis and oxidation of SnCl2 for synthesis of SnO2 NPs as 445 ETL for flexible PSC (ITO/PEN) is reported in [96]. SnO2 NPs is reported to have improved the 446 crystallinity of the PSC layer with an η of 15.21%. Zn2SO4 as ETL was developed as a nanoink 447 on perovskite based CH3NH3Pb(I0.9Br0.1)3 achieving a η of 16.5% for low temperature flexible 448 PSCs with regular architecture by improving the transport ability of photogenerated electrons in 449 the energy levels with inverted flexible PSCs. [97] Yoon et al., reported an η of 16.8% with high 450 efficiency and flexibility constructed from graphene as the anode deposited by CVD having the 451 architecture 452 photoanode counterpart deposited with graphene and 2nm MoO3 to overcome graphene 453 hydrophobicity showed an η of 17.3% which is the highest so far and is expected to improve 454 further. [47] 455 Exposure of spin coated metal oxide salts (SnO2) deposited on FTO to deep UV (DUV) 456 irradiation as a low temperature alternative to conventional annealing step was used as an ETL 457 with MAPbI3-xClx as the absorber resulting in 16.2% of η while photonically annealed 458 counterpart resulted in 15% of η at low substrate temperatures for planar PSCs. The device 459 parameters are lower than the thermally annealed SnO2. However using thin film of metal oxide 460 at low enough temperatures seems a plausible solution to high temperature based flexible 461 polymer PSCs. [98] 462 Yang et al [48] employed Ethylene diamine tetraacetic acid (EDTA) complexed tin oxide (SnO2) 463 as electron-transport layer 464 transport layer is better matched with perovskite that resulted in higher open circuit voltage. A η 465 of 18.28% has been achieved using EDTA –SnO2 complex as ETL in F-PSCs. Feng et al[46] 466 employed a novel dimethyl sulfide (DS) additive and effectively improved the performance of 467 the F-PSCs. Such an effective additive technique helps tremendously to control the perovskite as PEN/Graphene-MO/PEDOT:PSS/CH3NH3I3/C60/BCP/LiF/Al. Its ITO in PSCs. The fermi level of EDTA complexed SnO2 electron- 25 468 morphology i.e., the grain size and crystallinity improves and the trap state density in the 469 resultant perovskite film is reduced on flexible substrates for F-PSCs. The schematic diagram of 470 the device is shown in Figure 9 a-b. The η of 18.40% has been achieved for flexible devices 471 with good mechanical tolerance, the highest reported so far for the F-PSCs. In addition, the 472 environmental stability of the F-PSCs significantly enhances by 1.72 times compared to the 473 device without the additive, likely due to the large grain size that suppresses perovskite 474 degradation at grain boundaries (Figure 9 c-e). 475 476 26 477 478 479 Figure 9: (a -b) schematic illustration of the F-PSC structure, cross-sectional SEM image of a MAPbI3– DS absorber, (c-d) J–V curves of F-PSCs of pure and DS additive MAPbI3 and (e) The η of flexible device based on MAPbI3–DS at different bending curvature radii after 5000 flexing cycles. 480 481 482 483 Cost analysis 484 to low cost, high efficiency and stability together with additional features such as flexibility and 485 easy tandem with other technologies. [5] Si solar cells have proven to exhibit long lifetime and 486 higher efficiency than PSCs (20-25%) however are rigid, hefty and expensive though the price 487 has seen a fall since their inception. New generation organic photovoltaics OPV and DSSC 488 suffer from low efficiencies and short lifetimes yet exhibit interesting and remarkable 489 functionalities including flexibility and transparency. At the same time, the PSCs have shown to 490 demonstrate similar features concurrently with a comparable η of 21% with Si and an easiness to 491 be printed on flexible substrates. It is estimated that the cost of flexible substances such as 492 PET/PEN is much lower (∼30%) as compared to when glass conducting surfaces are used. [99] 493 The cost breakdown for PSCs includes fabrication method chosen and its architecture with 494 relation to substrate and other components such as sealants and electrical connections,labor, 495 overhead cost and capital investment for its large scale production. [60] Flexible PSCs have the 496 characteristics of OPVs with the fabrication method also mimicking the OPV method of 497 fabrication and hence similar in investment. The cost effectiveness for flexible device can be 498 calculated as in [100] while prices of materials used for fabrication of flexible PSC can be taken 499 from the local market. It is however worth mentioning here that commercialization of PSCs 500 requires installation of facilities to fabricate it at a large scale which is still not available. Unlike 501 organic SCs, PSCs can be constructed in different architectures with a number of technologies. To allow PSCs technology to overwhelm the market trend, it needs to be considered with regards 27 502 Among the materials used for preparing PSCs, substrates are considered to leap the fabrication 503 costs up. Hence the cost will increase to ∼40% for ITO/glass ∼36% for PET/PEN and ∼5% for 504 metallic substrates and this area largely affects the economics of its fabrication. R2R processing 505 at low temperature for flexible substrate such as PET/PEN holds great promise to reduce labor 506 cost for its fabrication as compared to glass based counterparts. The cost economics for 507 fabrication of solar cells depends on its different applications and hence cost needs to be tailored 508 according to it. 509 The quest to attain par excellence in domotics by investing on light harvesting technology using 510 ambient or poor indoor light conditions has been the topic of much discourse now [101, 102]. 511 Harvesting energy provides smart solutions to make our wireless sensing and buildings efficient 512 in term of energy use. Hence intelligent sensors that are self-powered, low maintenance, are 513 flexible, ensure easy instalment for managing building systems without the need to change the 514 older infrastructure have attracted a lot of attention. It is also noteworthy to mention that due to 515 low cost of materials used, cost of fabrication and the efficiency of the device, photovoltaics 516 based on perovskites are the chosen candidates for harvesting light in the contemporary era as 517 compared to the more efficient organic and inorganic solar cells. Planar PSC utilizing composite 518 layer of SnO2/MgO as ETL is shown to demonstrate efficiencies of 25% at 200 lux and 26.9% at 519 400 lux as compared to a SnO2 counterpart PSCs. The MgO overlayer is shown to exhibit better 520 stability and can better resist recombination of charges at the interface leading to better 521 efficiencies[103]. Milder indoor condition in comparison to harsh outdoor conditions calls for a 522 more focus on stabilities for modulating the devices to be used outdoor for commercial outlets. 523 Additionally, small area PSCs using inverted ITO/PEDOT:PSS/oragno-perovskite/ETL/1,3,5- 524 tri(m-pyrid-3-yl-phenyl)benzene/Ag have demonstrated an η of 27.4% without an hysteresis 28 525 effect under fluorescent illumination at 100 lux while large area device has shown an η of 20.4% 526 with very long lasting stability[104]. In the study, [6,6]-phenyl-C 61 -butyric acid methyl ester, 527 C60 and C70 were used as different ETLs. However, flexible in comparison to rigid devices are 528 lightweight, economical, thin and adapt effectively to the substrate which are important 529 properties for integrating them seamlessly in indoor environments. Flexible photovoltaics 530 especially amorphous silicon, DSSC, organic voltaic cell and PSCs have been the candidates of 531 choice to accomplish the aim to develop buildings that are smarter especially if ambient light 532 could be used[104-107]. These harvester technologies have a huge potential to replace older 533 batteries and/or extend shelf life of rechargeable batteries for electronics. This is bound to 534 provide a growing platform for developing sustainable solutions through the internet of 535 things[108]. Flexible planar and mesoscopic PSC for harvesting artificial light are reported for 536 device 537 xClx/spiro-MeOTAD/Au 538 compared with planar PSC based solar cells fabricated[109]. It was found that the mesoporous 539 flexible PSCs exceeded the performed of DSSC and flexible amorphous Si based solar cell under 540 illumination of LED with η = 10.8% for 200 lux and 12.1% for 400 lux. Yet, there is scope of 541 improvement to reach the best values of rigid substrates standing at η= 24% with LED light 542 illumination of 400 lux. It was established that planar devices show poor efficiencies with 543 mesoscopic cells using atomic layer deposition of c-TiO2 layer and device architecture as 544 PET/ITO/c-TiO2/CH3NH3PbI3–xClx/spiro-MeOTAD/Au. Furthermore, mesoscopic devices based 545 on a sol gel deposited TiO2 compact layer also exhibited poor performance efficiencies at 546 standard testing conditions in indoor conditions. Similarly Flexible PSCs were fabricated using 547 solution processible SnO2 and m-TiO2 as the ETL and it was found out that the cell with device configuration PET/ITO/atomic layer deposited-c-TiO2/meso-TiO2/CH3NH3PbI3– by illuminating it with artificial white LED lamp at 200 and 400 lux 29 548 configuration PET/ITO/SnO2/m-TiO2/CH3NH3PbI3/Spiro-MeOTAD/Au delivered an efficiency 549 of 14.8% under light source of AM1.5G which was better than the PSCs using only SnO2 as the 550 ETL. The cell were illuminated with 200 and 400 lux artificial indoor light and exhibited an 551 η=12.8% and η= 13.3% which is the highest so far achieved in comparison to PSC with SnO2 as 552 ETL highlighting its importance and feasibility for commercial applications. This was scaled up 553 to be used as a highly efficient PSC module that was fabricated with an active area of 12 cm2 554 with laser scribing for the first time ever and showed η of 8.8% illuminated under 1 sun[110] 555 which was better than the efficiency reported in [111]. While, previously a MAPbI3 based PSC 556 module on a glass substrate with an active area of 10 cm2 showed an η of 13-14% [112]. Hence 557 the results on flexible PSC for module fabrication demonstrate its potential to be used to further 558 boost the efficiency of flexible perovskite by using its intrinsic qualities of being flexible and 559 lightweight. 560 561 562 563 Table 1: Available data in the literature for flexible and fibre PSCs, device architecture, PV parameters JSC (mA.cm2 ), VOC (V), FF (%) and η(%) along with perovskite deposition method Regular device architecture/PSCs Deposition Method JSC 2 (mA/cm ) VOC FF (V) (%) η Ref (%) PEN/ITO/ZnO/ MAPbI3/PTAA/Au (RS) One step SC 18.7 1.10 76 15.6 [79] PEN/ITO/ZnO/ MAPbI3/PTAA/Au (FS) One step SC 18.7 1.10 75 15.4 ----- ITO/Zn2SnO4/MAPbI3/PTAA/Au Two step SC 21.6 1.05 67 15.3 [75] PET/Ag//PH1000/PEDOT:PSS/MAPbI3/PCBM/Al (RS) Two step SC 19.5 0.91 80 14.2 [84] PET/Ag//PH1000/PEDOT:PSS/MAPbI3/PCBM/Al (FS) Two step SC 19.3 0.90 79 13.7 ----- ITO/Li-SnO2/perovskite/Spiro/Au One step SC 20.57 1.06 67.27 14.78 [113] PEN/ITO/PEDOT:PSS/Perovskite/PC60BM/Ag One step SC PEN/ITO/PhNa-1T/Perovskite/PC60BM/Ag 14.4 0.88 66.2 8.4 [114] 18.4 1.03 77.4 14.7 [114] ITO/PEDOT/Hybrid Perv/PCBM/BCP/Ag (RS) Two step SC 18.93 0.994 69.2 13.03 [88] ITO/PEDOT/Hybrid Perv/PCBM/BCP/Ag (FS) Two step SC 18.89 0.994 69.3 13.02 [88] PEN/ITO /NiOx/MAPbI3/PCBM/Ag (FS) One step SC 18.74 1.04 68.9 13.43 [85] PEN/ITO /NiOx/MAPbI3/PCBM/Ag (RS) -------------- 19.37 1.03 63.3 12.63 ---- PET/ITO/TiO2/MAPbIx-3Clx/PTTA/Au One step SC 21.3 0.91 69 13.5 [115] PET/ITO/am-TiO2/ pervoskite/Au Thermal Eva2 20.90 1.03 72 15.07 [76] 30 PEN/ITO/TiOx/MAPbI3-xClx/spiro-OMeTAD/Ag One step SC 21.4 0.95 60 12.2 [62] PDMS/ITO/ZnO/Perovskite/Spiro/Au w/o nanocone Two step SC 17.7 0.97 70 12.06 [116] PDMS/ITO/ZnO/Perovskite/Spiro/Au with nanocone Two step SC 19.3 980 69 13.14 ---- IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Au (RS) One step SC 18.2 1049 69 13.2 [117] IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Au (FS) One step SC 17.7 1031 56 10.2 ---- PET/ITO/TiO2/MAPbI3-xClx/Spiro-OMeTAD/Ag Ultra- spray 20.6 1.03 61.5 13.0 [118] IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Ag(RS) One step SC 16.8 1020 71 12.2 [119] IZO/PET/TiO2/MAPbI3/Spiro-OMeTAD/Ag(FS) One step SC 17.0 1010 63 10.8 ----- Epoxy/ITO/ZnO/MAPbI3/Spiro-OMeTAD/Au Two step SC 19.2 0.87 67.6 11.29 [120] 17.0 983 61 10.3 [69] PET/PEDOT:PSS/Spiro/MAPbI3/Al2O3/bl/Ti Ti/TiO2 NTs/MAPbI3/Spiro-OMeTAD/CNTs Two step SC 14.36 0.99 68 8.31 [67] PET/ITO/TiO2 ALD/TiO2-MAPbI3-xCl/Spiro/Au One step SC 12.6 0.828 71 7.4 [121] 16.5 0.996 55 9.1 [68] ITO/Spiro-OMeTAD/TiO2-MAPbI3/bl/Ti 564 2 565 3. PET/ITO/ MAPbI3/PC61BM/Al FDC 14.8 0.96 68.1 9.7 [77] PET/AZO/Ag/AZO/PEDOT:PSS/MAPbI3/PCBM/Au One step SC 14.3 1.04 47 7.0 [122] PET/PEDOT:PSS/MAPbI3/PCBM/PFN-P1/Ag One step SC 12.73 0.94 66.24 7.92 [123] AgNWs/Spiro-OMeTAD/MAPbI3/Ti One step SC 16.48 0.918 49.0 7.45 [124] PEN/ITO/C60/Perovskite/Spiro/Au SC(R) 23.21 1.02 67.33 16.00 [87] PEN/ITO/C60/Perovskite/Spiro/Au SC(F) 23.45 1.03 63.80 15.45 [87] PEN/ITO/ZnO/Perovskite/Spiro/Au Two step SC 21.92 0.90 62.67 12.34 [125] Thermal evaporation system Fibre shaped PSCs (FPSCs) 566 Fibre-shaped solar cells (FPSCs) are linear in shape and can harvest solar energy from 3D 567 space. Due to their symmetrical structure and adjustable aspect ratio, FPSCs can be potential 568 flexible power sources [126]. The importance of flexible fibre-shaped devices had been realized 569 for DSSC and organic solar cells by considering their η ≈8% reported in [127-130]. Hu et al., 570 [51] fabricated PSCs into a fibre-shaped device and achieved η ≈5.35%. The device architecture 571 comprised of Ti/c-TiO2/m-TiO2/perovskite/Spiro-OMeTAD/Au is shown in Figure 10a. The c- 572 TiO2 layer was coated in situ on a titanium (Ti) wire through electric heating followed by coating 573 with mesoporous-TiO2 (m-TiO2) via a dipping process. Each of these layers in flexible PSCs can 574 be seen in SEM cross sectional view (Figure 10b). The typical device micrograph is shown in 575 Figure 10c where a 25 µm thick Au layer was wrapped around as a lead electrode. The 31 576 perovskite and HTL layers were obtained via a dipping solution process while Au electrode of 577 nanoparticle size dimensions was produced by magnetron sputtering as a semi-transparent 578 electrode. Figure 10(d-e) presents the J-V characteristics of the best device with respect to scan 579 direction. After optimization of each layer, the best performing fibre-shaped device yielded 580 η ≈5.35% with JSC ≈12.32 mA.cm-2, VOC ≈0.714 V, and FF ≈60.9%. 581 582 583 584 585 586 Figure 10: (a) Schematic of the fibre-shaped PSC structure. (b) SEM cross-sectional micrograph, (c) Image of typical device, (d) J-V of the best fibre PSCs, and (e) J-V curve with respect for scan direction (reproduced with permission from Ref.[51]). Copyright of Royal Society of Chemistry. 587 Deng et al., [131] reported FPSCs with η= ~4.81 – 5.22% with schematics of its 588 preparation shown in Figure 11a. Similarly, another report by Qui et al., [132] reported 589 fabrication of FPSCs using cathodic deposition solution process for perovskite layers with high 590 coverage and uniformity on curved surfaces of Ti wires. The modified Ti wire accompanied by 591 the transparent aligned CNT sheet was used as the two electrodes to produce a coaxial perovskite 592 solar cell fibre, A significantly high η of 7.1% was achieved with VOC ~ 0.85: the highest value 32 593 so far for a fibre-shaped device. Figure 11b shows a schematic of the fabrication and structure of 594 the FPSC device where TiO2 nanotube array engaged as an ETL was grown by an anodization 595 process on the Ti nanowires acting as a substrate. 596 Li et al., [133] presented CNT fibre-supported double-twisted PSCs (Figure 11c) 597 prepared using a homemade heat-assisted coating for setting up a dense perovskite layer. Lee et 598 al., [134] fabricated a fibre-shaped device with η= ~3.85% using silver nanowires as back 599 contact electrode instead of Au, and the schematic of the device is shown in Figure 11d. He et 600 al., [67] employed ZnO array as ETL in fibre-shaped devices, the step by step fabrication process 601 is depicted in Figure 11e. Similarly, Qui et al., [66] reported coaxial fibre-shaped PSCs for the 602 first time with η= ~3.3%. For the fabrication of a typical coaxial fibre-shaped device, a stainless 603 steel fibre with a compact blocking layer was used as an anode followed by deposition of 604 MAPbI3 as shown in Figure 11f. The spiro-OMeTAD layer as HTL was then deposited and 605 finally a CNT sheet was wound as a cathode. The collected data of PV parameters for flexible 606 and FPSCs are enlisted in Table 2. 33 607 608 609 610 611 612 613 614 615 Figure 11: (a) Schematic illustration of FPSC, (a) Fabrication steps and final device structure [131], (b) Schematic illustration to the fabrication and structure of the perovskite solar cell fiber (PSCF) [132], (c) Structure (PMMA layer not shown) of each layer in the double-twisted fibrous perovskite solar cell [133], (d) Schematic showing structure of a fiber-shaped PSC using silver nanowires as the top electrode [134], (d) Schematic illustration showing the fabrication process (e) Fiber-shaped PSC based on the aligned ZnO nano-obelisks like ETL[67], and (f) Co-axial fiber-shaped PSCs [66]. 616 617 Table 2: Available data in the literature for flexible and fibre PSCs, device architecture, PV parameters JSC (mA.cm2 ), VOC (V), FF (%) and η(%) along with perovskite deposition method Fibre shaped perovskite solar cells PEN/ITO/c-TiO2/Perovskite/CNT Deposition Method Dip-coating JSC (mA/cm2) 15.9 VOC (V) 0.91 FF (%) 65.6 η (%) 9.49 [135] Ti wire/c-TiO2/TiO2 NTs/perovskite/CNT sheet Deposition 14.5 0.85 56 7.1 [132] Ti wire/c&m-TiO2/MAPbI3-xClx/Au Dip-coating 12.32 0.714 60.9 5.35 [51] Stainless steel/c&m-TiO2/ MAPbI3/Spior/CNT sheet dip-coating 10.2 66.4 48.7 3.3 [66] Ti-polished /TiO2- MAPbI3/spiro-OMeTAD/Ag Nws Dipping proc 11.87 73.2 37 3.21 [134] Ti-dimple/TiO2-MAPbI3/spiro-OMeTAD/Ag Nws Dipping proc 6.97 56.5 29 1.14 [134] Regular device architecture/PSCs Ref 618 34 619 PSCs have offered robust application for flexible and wearable electronics however high 620 temperature requirement for mesoscopic TiO2 limits its application as flexible polymers 621 substrates.[136] Efficient PSCs based on mesoporous TiO2, are however reported by adopting 622 low temperature process on a glass substrate that have their own limitations; one being rigid and 623 the other being their inability to twist in 3 D space as they appear in a planar structure making 624 then unsuitable to be integrated for wearable devices, thus demonstrating a lack in softness and 625 curvature.[62, 137-139] While at the same time flexible planar PSC can be made thin but still are 626 hefty and massive and hence cannot be incorporated into wearable electronics’ technology. 627 Hence the need to develop 3D light weight flexible PSC for its application in wearable electronic 628 devices with a highly efficient and low cost fabrication process is necessary. Unlike the 629 conventional DSSC and quantum dots (QD) based SCs, the PSCs layer is thinner and has high 630 surface area to fabricate flexible devices with high η and light absorbing efficiency. Electronic 631 textile (e-textile) have unique features of being light weight, flexible and wearable just like 632 textiles in addition to being warm and snug and hence have evolved in e-apparels leading to its 633 significant commercialization and so smart watches, bands and glasses have seen a tremendous 634 boost. [140] 635 A blend film such as poly(3-hexylthiophene) P3HT: PCBM as an alternative to dye molecules 636 used as the light harvesting layer for fabricating flexible fiber shaped PSC, is reported 637 demonstrating η = 1.23 % which was able to tolerate the strain upto 30%.[141] Further 638 improvement was carried out by using active layer of MAPbI3-xClx PSC layer to improve η. A 639 double twisted PSC warped over pristine CNT fiber with mesoporous and n-TiO2, was 640 sequentially prepared by solution process. [142] P3HT/SWCNT (single walled carbon nanotubes) 641 was used as the HTL on Ag nanowire to increase the contact area. η value of 3.03% with a 35 642 stability of 96 Hr was reported. A mild solution low temperature process to synthesize tunable 643 sized obelisk-like ZnO NPs arrays aligned perpendicularly on the substrate to substitute for TiO2 644 layer is reported for fabrication of highly efficient, light weighted and flexible PSC fiber and 645 fabrics acting as the ETL. [67] The voids in ETL scaffold comprising nano-obelisk ZnO array 646 was infiltrated with perovskite layer and was deposited over with HTL while the CNT was used 647 as back contact electrode. The size of CH3NH3PbI3 was compatible with ZnO scaffold which 648 otherwise may had resulted in poor performance and collapsing of scaffold by a high charge 649 recombination of charges with a small surface area. The ZnO scaffold provided an η=3.8 % 650 higher than PSC based on ZnO nanorods on polymer substrate with planar architecture and 651 offered 3D deformation of fiber shaped PSC with η maintained at 84% with a twisting angle of 652 30∘ and surviving 100 cycles of bending without being damaged. Additionally, cathodic 653 deposition method followed for perovskite layer to form a coaxial perovskite SC fiber with 654 η = 7.1 % was reported. [143] More recently η = 9.49% has been realized by using instead of 655 curved surface that limits fabrication of a dense perovskite layer, a 1D flat substrate to increase 656 the coverage and formation of micrometer sized crystal as the perovskite layer. [144] 657 The breakthrough in η for planar PSCs have led researchers to apply perovskite material as fiber 658 shaped PSCs, however limited by the difficulty to fabricate film on it and also to obtain high 659 purity and quality perovskite film on fiber surface, is a feat in itself. Hence, vapor assisted 660 deposition method for its preparation by applying novel changes to its fabrication process has 661 been employed and has demonstrated a η = 10.79% which is the highest so far reported. 662 However still certain issues including operational and environmental stability and washability 663 needs to be looked into.[145] Most PSCs have been fabricated employing glass or polyethylene 664 terephthalate (PET) substrates with an indium tin oxide (ITO). Recently a report shows for the 36 665 first time that paper has employed as substrate in F-PSCs which deliver η of 2.7%[14]. The 666 architecture of the device is paper/Au/ SnO2/ meso-TiO2/CH3NH3PbI3/Spiro- OMeTAD/MoOx/ 667 Au/MoOx. 668 environmentally friendly recyclable substrate made from abundantly available cellulose 669 materials such as plants. Gao et al [145] used transparent nanocellulose paper (NCP) coated with 670 acrylic resin as a substrate in F-PSCs which yielded η 4.25%. 671 4. As we know that paper is a lightweight, flexible, inexpensive, ubiquitous, and Stability and durability of Flexible PSCs 672 Stability is one of the crucial aspects when considering the commercial prospects of 673 perovskite solar cells. Having said that, the PSCs are highly susceptible to degradation under the 674 influence of moisture, oxygen, temperature and ultra-violet UV radiation that serves as a 675 detriment for outdoor deployment.[29, 146-148] Though improvement and enhancement of 676 performance has been shown in numerous studies involving flexible PSCs, only few reports 677 discuss the stability of flexible PSCs.[149, 150] In this scenario, material selection is an important 678 criterion for ensuring the durability of flexible PSCs. Integration of hydrophobic materials in the 679 device structure or in the fabrication process significantly contributes to mitigation of moisture 680 degradation. 681 Numerous ways to mitigate the degradation has been explored so far. Recently, Hou et 682 al., developed a flexible PSC based on double hole transport layers of PEDOT:PSS/NiO 683 deposited on the surface of PET/ITO.[90] The resultant devices exhibited high η of 18%. The as- 684 fabricated devices also employed dual additives with 2 wt% DIO and 2 wt% H2O added to the 685 CH3NH3PbI3−xClx solution. The devices also exhibited high stability maintaining over 80% 686 efficiency of η even after 60 days. The H2O/DIO dual additives used in this work contributed to 37 687 the improvement of the interfacial energy of the perovskite film during deposition onto the 688 PEDOT:PSS surface. This resulted in a smooth perovskite film with less impurities, contributing 689 towards high stability and enhanced mechanical durability. The η decreased only by 8.3% 690 compared with the initial efficiency even after 1000 cycles.[90] Najafi et al., demonstrated 691 stability enhancement in flexible PSCs incorporating solution‐derived NiOx and ZnO 692 nanoparticle extraction layers.[151] The triple cation devices with ZnO interlayer retained >95% 693 of the initial performance after its storage for 210 days in the dark under nitrogen atmosphere 694 without encapsulation. The cells also exhibited high stability under continuous light soaking in 695 N2 atmosphere retaining more than 85% of the initial η after 1000 hr .[151] 696 Counter electrode and its material properties play a significant role in determining the 697 rate of degradation and the prospect of the devices’ stability in the long run. Metal electrodes 698 undergo oxidation in the presence of oxygen and result in the formation of halide compounds at 699 the interfaces. For example, in the case of gold (Au) electrode, the redox couple of the iodide 700 system from perovskite reacts readily with the Au resulting in the formation of AuI2- and 701 AuI3.[152] Lee et al., demonstrated the application of Silver nanowire (AgNW)‐based transparent 702 electrodes prepared via an all‐solution‐process as bottom electrodes for flexible PSCs.[153] In 703 their work, the authors deposited a layer of pinhole-free amorphous aluminium doped ZnO (a- 704 AZO) to enhance the chemical stability of the AgNW network. The a‐AZO/AgNW/AZO 705 composite electrode exhibits a transmittance of 88.6% at 550 nm and a sheet resistance of 11.86 706 Ω sq−1 comparable to that of commercial FTO. The PSCs fabricated using this composite 707 electrode a‐AZO/AgNW/AZO exhibited high stability and mechanical integrity retaining 94% of 708 its initial η even after 400 bending cycles with a bending radius of 12.5 mm.[153] Chu et al., 709 developed a self-healing electrode based on liquid metal microcapsules and polymer composites 38 710 (LMC/polymer) for application in flexible PSCs as shown in Figure 12.[154] The devices 711 demonstrated perfect recovery of the photovoltaic parameters even after cutting the metal 712 contact, exhibiting a η retention of about 99% relative to the initial value (η = 15.07%). Such 713 self-repairing contact could overcome the mechanical durability issues arising out of bending 714 cycles and its negative influence on the device performance could be repressed. The self-healing 715 property of the contact was attributed to the filling of the damaged sites with out-flowing liquid 716 metal from the ruptured capsules. [154] 717 718 719 720 721 722 Figure 12. a) Schematic of a solar‐powered smart watch embedded with self‐healing conductors. The interconnects are passivated with an electrically self‐healing film. b) Schematic illustrating the healing mechanism of a metal conductor by a passivation film made of an LMC/polymer composite. Liquid metal 39 723 724 flows out from the ruptured microcapsules and is transported to the damaged site. [154] Copyright Wiley Publications 2018. 725 726 Ryu et al., synthesized nanocrytsalline Ti-based metal–organic framework (nTi-MOF) 727 particles of about 6 nm in diameter.[155] The nanoparticle layer served as an intermediate layer 728 between the perovskite and the substrate, thereby protecting it from chemical etching as well as 729 facilitating electron transport. The flexible PSCs thus fabricated exhibited η of about 17% and 730 demonstrated high mechanical durability even after 700 bending cycles retaining η at 731 ~15.5%.[155] Ligand capped ultrafine SnO2 quantum-dots (QDs) have also demonstrated to 732 serve as a multifunctional interlayer improving the charge transport across the interface as well 733 as enhancing the stability of the flexible PSCs.[156] The device exhibited an η value of 17.7% 734 and retained 92% of the initial η even after 1000 bending cycles for a bending radius of 18 735 mm.[156] It was also reported that the use of Cr/Cr2O3 interlayer as buffer layer between the 736 perovskite and counter electrode reduces the chemical etching of the metal caused by the iodide 737 component disintegrated from the perovskite.[152] Bu et al., demonstrated the efficacy of 738 SnO2 nanocrystals (NCs) as ETLs for flexible PSCs in small devices and modules.[157] Power 739 conversion efficiency (η) of 16.5% was achieved using 120 oC sintered and UVO treated SnO2 740 ETLs in small flexible devices. In flexible modules of size 5 x 5 cm2, η of 12.4% was achieved 741 which retained 80% of its initial performance even after 1000 bending cycles with a bending 742 radius of 25 mm.[157] Zhou et al., reported the synergetic effect of anatase TiO2 NPs and 743 pristine C60 (TiO2/C60) interlayer in enhancing the stability and mechanical durability of flexible 744 PSCs.[158] The TiO2/C60 based flexible devices exhibited η= ~16% while retaining 80% of its 745 initial value even after 1500 cycles. In addition, the devices also demonstrated longer device 40 746 lifetime under light soaking conditions retaining 100% of its original η even after 100 minutes of 747 continuous illumination. The reason behind such an enhancement is attributed to superior charge 748 extraction at the TiO2/C60 interface, reduction of interfacial trap states by fullerene acting as a 749 passivation layer and immobilization of iodine ions (I-). It is also shown to reduce the formation 750 of PbI2 originating from the reaction between the iodine ions of the perovskite and oxygen 751 vacancies of TiO2, preventing thus material degradation. [158] 752 Similarly, Carbon-based counter electrodes exhibit non-oxidative and hydrophobic 753 properties which makes it non-reactive with the perovskite components, and a suitable candidate 754 for developing highly stable devices.[159] Luo et al., reported an all-carbon-electrode based 755 flexible PSCs employing graphene as transparent anode and carbon nanotubes (CNT) as 756 cathode.[159] The devices exhibited maximum η of 11.9% incorporating Spiro-OMeTAD as 757 HTL in conjunction with CNT. The carbon-based flexible PSCs retained 86% of their initial 758 η even after 2000 bending cycles at a curvature radius of 4mm which is exceptionally high 759 compared to conventional flexible PSCs fabricated with transparent ITO/PEN electrode and 760 metal based counter electrode. The all-carbon based electrodes also demonstrated enhanced 761 device lifetime of the flexible PSCs retaining over 90% of the initial η after 1000 hr of 762 continuous illumination in humid air as shown in Figure 13. The authors attributed the enhanced 763 device stability to the hydrophobic surface of CNTs and also to the thickness of CNT-Spiro- 764 OMeTAD layer. The combined effect together lowers the moisture ingression and also improves 765 thermal stress stability of the flexible PSCs.[159] 766 41 767 768 769 770 771 772 773 Figure 13. a) η of the flexible devices upon increasing bending curvature radius after 200 bending cycles. b) Bending durability of the ITO/PEN‐based and all‐carbon‐electrode‐based flexible solar cells as a function of bending cycles. c) Efficiency stability of the standard and all‐carbon‐electrode‐based flexible PSCs as a function of soaking time in different conditions c) in ambient atmosphere under AM 1.5G illumination in air without UV filter, and d) in ambient atmosphere with constant heating temperature of 60 °C. [159] Copyright Wiley Publications 2018. 774 775 Encapsulation is one of the prominent ways to improve the device lifetime which could facilitate 776 the successful commercialization of flexible PSCs in future. Weerasinghe et. al., demonstrated the 777 encapsulation effects on flexible PSCs.[117] In their work, flexible PSCs based on the 778 configuration (PET/ITO/TiO2/MAPbI3/SM-Au) was studied in three categories namely, non- 779 encapsulated, partially-encapsulated and completely encapsulated PSCs. The plastic encapsulant 780 employed is about 240 µm in thickness with a water vapour transmission rate (WVTR) of 10−3 g 781 m−2day−1 and 89% transparency in the visible light spectrum. The device was sealed with the 42 782 encapsulant on the metal electrode side using acrylic adhesive. The moisture content in the 783 devices are removed by drying in vacuum for 12hr before employing for the stability tests. The η 784 of the fully encapsulated devices retained over 80% of the initial efficiency even after 500 hr 785 while the partially encapsulated devices exhibited significant drop in performance after 400 hr. 786 Non-encapsulated devices deteriorated rapidly even before 100 hr. Investigation of the moisture 787 ingression pathways was carried out by employing metallic calcium (Ca) sensor. It was found 788 that the moisture enters the partially encapsulated devices through the sides or edges while the 789 moisture ingression happens in the fully encapsulated devices through the copper wire contacts 790 provided for electrical connections in the modules.[117] 791 Recent advances in encapsulation technology applied for rigid PSCs could also be 792 extended for flexible devices with appropriate modifications. Cheacharoen et al., reported a 793 comparative study of PSCs packaged between glass and two commonly used encapsulants with 794 different elastic moduli (i.e., Surlyn and EVA).[160] It was found that the devices encapsulated 795 with Surlyn, which has a stiffer ionomer, degraded rapidly with temperature cycling and 796 delaminated. On the other hand, PSCs encapsulated with softer ethylene vinyl acetate (EVA) 797 endured temperature cycling and retained over 90% of their initial performance even after 200 798 temperature cycles. The results indicate that the encapsulant with a low elastic modulus is much 799 suitable for device packaging to ensure mechanical stability and long operating lifetime under 800 high thermal stresses.[160] . Park et al[159] uses Noland Optical Adhesive 63, a shape 801 recoverable polymer as a substrate in PSC. The employed stretchable substrate was quite 802 impressive and prevented mechanical damage of the perovskite layer. Before bending at a 803 radius of 1 mm, an efficiency (η η) ~ 10.75% and after bending at a similar radius, η ~10.4% 43 804 have been reported. In addition, the shape recoverable device exhibits η= ~6.07% even after 805 crumpling. 806 Hu et al., [88] employed single piezoelectric ZnO microwire as an ETL on a flexible substrate in 807 PSCs. The incorporation of piezo-phototronics microwire can significantly enhance the 808 performance of ZnO/PSCs with η= ~0.0216%. The improved performance is due to the fact that 809 at the vicinity of ZnO/perovskite interface the strain-induced piezoelectric polarization charges 810 can modulate the transport and separation processes of photo-generated carriers within the 811 photovoltaic device. It has been reported that there is a profound effect of the c-axis of the ZnO 812 NW when pointed towards perovskite. The VOC and JSC of the device improved by 25.42% and 813 629.47%, respectively resultantly the η enhanced by 1280% (from 0.0216% to 0.298%) under a 814 compressive strain of 0.8%. 815 816 Progress in flexible module 817 Organic SCs had witnessed a boom in production due to being cheap, having a low energy 818 budget, adopting a solution process and being flexible SCs. This originated in utilizing 819 economical techniques such as spin coating and thermal evaporation for their fabrication and 820 constitute a small area cell[161]. With progress of time, the focus shifted to improving 821 η efficiencies compatible to the supply of global energy. However, the recent focus has reverted 822 back to high throughput production of SCs. The methods used previously were not economical 823 for R2R processing and hence advancements in large area SCs was limited due to these factors 824 where η for large area cell stands at < 3.5%. [162, 163] The focus has been on large area 825 production with emphasis on R2R processing to attain high throughput production of SCs. [164] 44 826 PSC have amassed a lot of interest due to their ease of processing as well as quest for improving 827 low cost technology by favourable combination of highly efficient small lab devices with 828 η above 20% [165, 166] and η (11-16%) for small area modules.[112, 167, 168] 829 Flexible PSCs have theoretically a low cost of fabrication than their counterparts made up of 830 glass substrates, however exhibit low η values for laboratory scale PSCs. [169] Yet they are 831 better than the existing DSSCs and organic photovoltaics and are fast emerging as the state-of-art 832 flexible PSC technology. [170, 171] In future the gap between the glass and plastic PSCs has to 833 be closed. 834 There is ample scope of developing large area modules. [60] Flexible PSCs have been reported 835 as small area devices while large area modules have also been actively followed for their 836 characteristics such as being flexible, light weight and thin leading to upscaling of small area 837 devices to large area modules which is supported by rather a few reports. The following must be 838 considered for upscaling the facility: 839 1. Coating techniques applied to large area PSC modules 840 2. Patterning 841 3. Optimization of the cell 842 4. Interconnection and integration of design of cell [60] 843 Giacomo et al., first showed large area flexible PSCs module by using porous ITO/PET precisely 844 etched through laser patterning over a compact TiO2 layer deposited through atomic layer 845 deposition and a mesoporous n-i-p type TiO2 layer by UV screen printing. [111]The PSC module 846 requires as other thin film technologies, optimization of the cell architecture through laser 847 scribing which has resulted in η=3.1% on a plastic substrate of area 5.6 x 5.6 cm2.[172] Small 45 848 device fabrication is closely mirrored. At the same time, Hwang et al., have reported slot die 849 coating and introduced R2R processing for flexible PSC module fabrication for large area 850 manufacturing, however the module has very low efficiency. [173] Hu et al., have manufactured 851 a PSC module with 6 small scale devices with a η= 12.32% and an active area of 1.01 cm2. 852 However, still the performance of large PSC modules posits a great deal of challenge that needs 853 to be overcome to successfully implement it for large scale production. This is an active area of 854 research which is yet to be fully understood which is far inferior to same design as ITO and FTO 855 based PSC but rigid substrate with an η= 18.84%. [174] Blade coating technique as well as R2R 856 with inverted structure with η < 5% has also been investigated for exploring large area 857 perovskite solar cells.[175, 176] Currently the efficiency for flexible PSCs with single junctions 858 for large area module stands at 11.7% [177] while world’s highest performing PSC large area 859 module is reported in [178] employing 2 step process by adopting bar coating methodology to 860 fabricate the same PSC module.. This shows that small devices are highly efficient and robust 861 but do not give satisfying results when it comes to large modules. PSCs with rigid substrates 862 have been developed into large area module and have displayed superb performance and 863 efficiency with relation to their flexible counterparts. [179, 180] Additionally tandem devices of 864 PSC with Si sub cell or CIGS have demonstrated η =20% [181, 182]. 865 866 5. Conclusion and outlook 867 There has been a soaring interest in perovskites solar cells (PSCs) due to their unique 868 features especially such as wearabilty because of being light weight, portable and deployment for 869 near space and very light weight space applications. PSCs with different architectures have been 46 870 explored and investigated in detail which include, mesoporous, planar (regular and inverted), 871 HTL-free and ETL-free PSCs constructs. Among these, the inverted class of PSCs possessing a 872 p-i-n structure is an excellent choice for flexible devices than the regular planar devices owing to 873 its lower hysteresis and fabrication at a low temperature through an in-situ solution processable 874 route which presents itself as a great prospect to realize its importance for its 875 commercialization. Similarly, modifying the chemistry of perovskites via suitable substitutions 876 or exploring different configurations as well as tailoring various interfacial layers and 877 microstructures have notable effects on the performance and stability of the PSCs devices and 878 have shown to have a positive impact on its efficiencies. Various device designs for PSC by 879 changing the orientation or tailoring the material’s thickness which includes TiO2 with different 880 architectures have been employed. It is very crucial to carefully select materials for PSC from the 881 substrate to the top counter electrode that can guarantee good resistance to deformation of 882 various kinds. Research has been directed in the recent years to investigating ITO free TCE by 883 employing graphene on top of polymer film to overcome challenges associated with using ITO 884 and FTO based transparent conductive electrode. Other studies have shown that ZnO as ETL 885 could be used for flexible PSCs in comparison to its TiO2 ETL counterpart. The ss-IL has also 886 been known for its unique characteristics and used as ETL exhibiting no hysteresis with respect 887 to scan direction. Fibre-shaped solar cells due to their symmetrical structure and adjustable 888 aspect ratio, can be the potential flexible power sources. The importance of flexible fibre-shaped 889 devices had been realized for DSSC and organic solar cells with η ≈8%. FPSCs with η= ~4.81 – 890 5.22% have also been reported. 891 Counter electrode and its material properties play a significant role in determining the 892 rate of degradation and the prospect of the devices’ stability in the long run. Thus protecting 47 893 bottom electrode for flexibles PSCs by employing nanoparticles or ligand capped ultrafine SnO2 894 quantum-dots from chemical etching as well as facilitating the electron transport is incumbent. 895 Similarly, Carbon-based counter electrodes exhibit non-oxidative and hydrophobic properties 896 which makes them non-reactive with the perovskite components, and a suitable candidate for 897 developing highly stable devices. Encapsulation is one of the prominent ways to improve the 898 device lifetime which could facilitate successful commercialization of flexible PSCs in future. 899 Recent advances in encapsulation technology applied for rigid PSCs could also be extended for 900 flexible devices with appropriate modifications. The results indicate that the encapsulant with a 901 low elastic modulus is much suitable for device packaging to ensure mechanical stability and 902 long operating lifetime under high thermal stresses. In the future novel, implementable and 903 rational innovations in PSCs by using R2R are projected. 904 It goes without saying that so far the conductive substrates such as ETL and HTL incur the most 905 cost for fabrication of the flexible PSCs. Hence striving to search for new materials as HTLs, 906 ETL as well perovskites and their combinations seems a plausible solution to advance in the 907 realm of renewable energy. Substrate selection also poses challenges as well, which needs to be 908 investigated especially with relation to flexible substrates. The substrate stability comes to fore 909 when especially polymer substrates are employed which allows ingression of moisture and O2. 910 Additionally lead based perovskite offer promising efficiencies yet is rapidly degraded in 911 presence of moisture and UV light.[181, 183] Lead plays a crucial role in PSC with highest 912 efficiency so far. Lead is however toxic yet the environmental impact seems minimal, [184] 913 however, research is ongoing to substitute lead with suitable nontoxic metals. It is argued that 914 non-lead based perovskites have demonstrated significantly low value of efficiencies as 915 compared to their lead based counterparts and hence challenges associated with lead replacement 48 916 and sustaining a high efficiency are still unresolved. It is noted that the η value for flexible PSCs 917 are far lower as compared to its rigid substrate counterpart due to a high transmittance value 918 which stands at 22% efficiency. While it is also true that studies on optimisation of devices in 919 terms of efficiencies has been more a subject of research rather the stability and degradation 920 studies of PSCs. Hence focus should be on arenas which will fetch the best results in terms of 921 effectively harvesting the solar energy to our best use. 922 Temperature consideration is an important parameter to ponder upon especially for low 923 temperature solution processing of conducing layers that will direct the research towards 924 achieving high performance PSCs on flexible substrates. This along with proper selection of 925 substrate will lead to large scale production of flexible PSCs by employing different solution 926 processing methods that include but not limited to slot dies coating, spray coating, etc., and 3D 927 printing in future. Integrating these flexible PSCs into wearable ingenious gadgetry with n-i-p or 928 inverted p-i-n configuration thus requires an adequate understanding, discovery and selection of 929 materials, techniques for depositing different layers, novel engineering processing under ambient 930 conditions, device architecture, cost analysis and an emphasis on durability and stability. This 931 may lead to replacing the existing materials with further research without compromising the 932 efficiency of the device.[149] This is important if large scale production of PSCs has to be 933 realized that promises outstanding resistance to bending and hence stability. It is however 934 contended that the road leading to manufacturing large scale PSCs devices is paved with 935 challenges that need to be overcome in order to attain high efficiency ,stability and low cost for 936 flexible PSCs modules by adopting large area techniques applicable to all layers just as has been 937 demonstrated for rigid substrate based PSCs module.[180, 185, 186] Thus results achieved for 938 lab scales PSC devices have not shown satisfying results when up-scaled to large modules. 49 939 Despite the achievements with small devices, large modules do not yet show satisfying results. In 940 addition, due to the absence of a large-scale manufacturing process, it is currently difficult to 941 provide a detailed cost analysis for flexible PSCs. 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Cost of module and small area F-PSCs are compared. Declaration The authors Qamar Walia*, Faiza Jan Iftikhara, Naveen Kumar Elumalaib, Yaseen Iqbalc Sidra Yousafc, Shahid Iqbala, and Rajan Josed declared no conflict of interest. a School of Applied Sciences and Humanities, National University of Technology, Islamabad, 44000, Pakistan, Faculty of Engineering and Science, Curtin University, Sarawak Malaysia, 98009 Miri, Sarawak, Malaysia, c Materials Research Laboratory, Department of Physics, University of Peshawar, 25120, Pakistan, dNanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang 26300, Malaysia b