Functional Layers of Inverted Flexible Perovskite Solar Cells and Effective Technologies for Device Commercialization

Perovskite solar cells (PSCs) are now one of the most promising solar cells due to advantages such as high‐power conversion efficiency (PCE), low cost, and ease of fabrication. Among PSCs, flexible perovskite solar cells (FPSCs) are lightweight and bendable, making them easier to transport and install than rigid PSCs, and hence can be used in wearable and portable electronics, space energy systems, multifunctional integrated buildings, unmanned systems, and other applications. Compared with regular n–i–p FPSCs, inverted p–i–n FPSCs possess advantages of reliable operational stability, reduced hysteresis effect, low‐temperature fabrication process, and strong potential for tandem devices. At present, the PCEs of single‐junction and tandem‐inverted FPSCs have reached 21.76% and 24.7%, respectively, indicating promising commercial applications. In this review, first, the developments of device functional layers including flexible substrates, flexible conductive electrodes, charge transport layers, and perovskite active layers in inverted FPSCs are elucidated and discussed thoroughly. Then, the technologies for accelerating commercialization of inverted FPSCs are summarized in detail. Finally, perspectives on the development and commercialization of inverted FPSCs are provided.

to the addition of corrosive and hygroscopic additives including 4-tert-butylpyridine (4-TBP) and bis (trifluoromethane) sulfonimide lithium salt (LiTFSI), [71,72] which is bad for the long-term stability of flexible devices and hinders their commercialization. [73,74] While for inverted FPSCs, neither electron transport layers (ETLs) nor hole transport layers (HTLs) require dopants, which is helpful for the long-term stability of inverted FPSCs. Furthermore, regular FPSCs suffer from serious hysteresis due to the ion migration and low electron extraction, while in inverted FPSCs, the most commonly used electron transport material-1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)-C 61 (PCBM) can not only passivate interface defects and suppress ion migration but also improve electron extraction at the interface, greatly suppressing the hysteresis effect through the interfacial function mediated by PCBM. [48,49] Additionally, for most temperature-sensitive flexible substrates with relatively poor resistance to heat, low-temperature annealing process is beneficial to obtain high-quality films. Fortunately, to yield high-quality films, the required annealing temperature for HTLs in inverted FPSCs is relatively lower than that of ETLs in regular FPSCs, which is consistent with the low-temperature processing technology of flexible substrates. [41][42][43][44][45][73][74][75][76] More importantly, compared with their regular counterparts, inverted FPSCs are more suitable for the preparation of tandem solar cells to improve PCEs of FPSCs to accelerate commercialization. [47,[77][78][79] Therefore, inverted FPSCs with excellent optoelectronic properties have triggered intense research interests from scientists especially in recent years. The efficiency of single-junction inverted FPSCs has been improved from 6.4% in 2013 to 21.76% in 2022 (Figure 1), and the PCE of tandem inverted FPSCs has even reached 24.7%, indicating their promising commercialization prospects. [39,77,78,[80][81][82][83][84][85][86] Although a number of reviews about FPSCs have been published so far, most of them focused on regular FPSCs rather than inverted ones. Thus, a specifical and comprehensive review of inverted FPSCs is required covering not only their research developments in recent years but also the facing issues and challenges. In this review paper, as shown in Figure 2, recent advances of inverted FPSCs based on different functional layers are first presented, including flexible substrates, flexible bottom electrodes, HTLs, perovskite active layers, ETLs, and flexible top electrodes, to study their impacts on device performance. Then, the technologies for accelerating device commercialization are reviewed, primarily focusing on large-area inverted FPSCs, scalable deposition techniques, tandem-inverted FPSCs, integrated inverted FPSCs, semitransparent inverted FPSCs, and device encapsulation technology, all of which are key issues for future commercialization. Finally, the challenges and opportunities to promote the commercialization of inverted FPSCs are discussed.

Flexible Substrates
Inverted FPSCs are designed based on a p-i-n architecture composing of a flexible substrate with a bottom transparent electrode on it, a p-type HTL, a perovskite active layer, a n-type ETL, and a top electrode. [44] The essential difference between rigid PSCs and FPSCs is substrate, where a flexible substrate is one of the key components for flexible devices. The surface morphology of flexible substrates is usually worse than that of rigid substrates, making it quite difficult to obtain high-quality perovskite films. [43] Therefore, high-quality flexible substrates are conducive to obtain high-performance FPSCs. For inverted FPSCs, a good substrate affects not only the following depositing layers and final efficiency but also the environmental and mechanical stability of the devices, which is consistent with regular FPSCs.  [39,77,78,80,82,85,86]  Generally, an ideal flexible substrate should consider the factors as following: robust mechanical and sufficient flexible properties to sustain the bending and stretchable process, high optical transmittance to absorb efficient incident photons, high conductivity to reduce series resistance, high chemical stability to resist chemical solvents, outstanding thermal stability to sustain the thermal annealing process, low surface roughness to grow high-quality subsequent layers, high hermeticity to block water vapor and oxygen transmission, and environmentally friendly to avoid environmental pollution. [42,43,87] However, no material could satisfy all the advantageous requirements in practical applications. Therefore, it is necessary to comprehensively consider the influences of flexible substrates on the performance of FPSCs. Possible flexible substrates that can be used for inverted FPSCs include polymer substrates, metal substrates, and other novel flexible substrate materials, as summarized in Table 1.

Polymer Substrates
So far, for inverted FPSCs, polymer materials are the most commonly used flexible substrates, which include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyimide (PI), etc. [44] There are many advantages such as low cost, excellent flexibility, high optical transparency, strong chemical stability, lightweight, and R2R processability for polymer substrates. [87] Among those polymer substrates, PET and PEN are the most  Cellulose paper/Carbon/Perovskite/C 60 /BCP/Cu/Au Cellulose paper 9.05 2018 [104] NCP commonly used ones although several drawbacks, such as low heat resisting property with a low T g (glass-transition temperature) of about 78 and 120°C for PET and PEN, respectively, and an increased sheet resistance at high temperature still remain. [88] Therefore, PET and PEN would deform when the processing temperature is too high due to their relatively poor heat resistance, which limits the thermal annealing temperature of all manufacturing processes. As early as 2013, Snaith et al. fabricated inverted FPSCs on indium tin oxide (ITO)-coated PET substrates using a low-temperature solution-processed organometal trihalide perovskite absorber CH 3 NH 3 PbI 3Àx Cl x and obtained a PCE of 6.5%, opening up a new era of inverted FPSCs. [80] Recently, the PCEs of inverted FPSCs on PET substrates have been promoted to 21.76% and 24.7% for singlejunction and tandem-inverted FPSCs, [78,86] showing strong development prospects.
As for PEN substrates, FPSCs with a PCE of 16.6% were reported by Najafi and co-workers, who employed metal oxide nanoparticles (NiO x and ZnO) as charge extraction layers (HTL and ETL). [89] Recently, researchers employed PEN as flexible substrates to fabricate-inverted FPSCs and achieved an efficiency of 20.50% with excellent bending resistance by doping a functional polymeric adhesive (AD-23) to improve the film quality and self-healing ability. [90] Notably, the PCE value of PEN-based inverted FPSCs has already reached 21.08% in 2022. [84,[90][91][92] Compared with PET and PEN, PI shows relatively better heat resisting property with a high T g of over 200°C, but a poor optical transparency due to its light brown color, restricting the achievement of high-performance light-absorbing devices. [93] Therefore, it is important to prepare colorless and transparent PI films for fabricating high-performance FPSCs. Jeon et al. synthesized a single-walled carbon nanotubes (SWNTs)-PI composite film as a foldable transparent conductor in inverted FPSCs (Figure 3a), which exhibited smooth morphology and high transparency. [94] The further doping of MoO x in SWNT-PI resulted in films with thickness of 7 μm, and hence the achievement of foldable inverted FPSCs with a PCE of 15.2%. [94] Figure 3. Polymer substrates of inverted FPSCs. a) Schematic illustration of the fabrication process of SWNT-PI ultrathin conductors. Reproduced with permission. [94] Copyright 2021, Wiley-VCH. b) Schematic illustration of textile-based perovskite solar cells (left), SEM image of the polyurethane-coated textile substrate (middle), and cross-section SEM images of PH1000/CNT-coated textile substrate (right). Reproduced with permission. [95] Copyright 2018, Elsevier. c) Schematics of the fabrication process for ultrathin parylene-based FPSCs. Reproduced with permission. [96] Copyright 2022, Springer.
However, it should be pointed out that the price of transparent PI is expensive, limiting its wide applications. Besides, other polymer substrates could also be explored in inverted FPSCs. For example, Lee and co-workers introduced a thin layer of polyurethane (PU) onto polyester substrates to improve the processability and successfully achieved FPSCs with a PCE of 5.72% and good environmental stability (Figure 3b). [95] Recently, a flipover transferring (FOT) process was utilized to fabricate ultrathin inverted FPSCs on a 3 μm-thick parylene-C substrate by chemical vapor deposition (CVD), which exhibited a PCE of 20.0% with an outstanding power-per-weight of 30.3 W g À1 (Figure 3c). [96] Stretchable polydimethylsiloxane (PDMS) was also a possible substrate material in FPSCs, as reported by Daoud et al., who achieved inverted FPSCs with a PCE of 15.44% and high mechanical stability. [97] Although inverted FPSCs based on polymer substrates delivered high performance, the heat resisting property of these substrates is relatively poor due to their generally low T g , making the temperature control extremely important. [40] Fortunately, thanks to the relatively low-temperature processing property of most HTLs, [43] the required temperature of high-performance inverted FPSCs is relatively low, making it possible to construct inverted FPSCs with polymer substrates. However, it should be noted that the polymer substrates with higher water vapor and oxygen transmission rates than other substrate materials such as metal substrates [98] suffer issues of long-term stability. Hence, subsequent process optimizations are still required to improve stabilities, such as encapsulation technology, which will be discussed in the following subsections.

Metal Substrates
Metal substrates are considered as promising flexible substrates with advantages such as high flexibility, strong heat resisting property, excellent barrier property, and good electrical conductivity. [99,100] The smooth surface of metal substrates is found helpful for obtaining high-quality perovskite active layers with suppressed charge recombination. Possible metal substrates include Ti foils, Cu foils, and stainless steel foils. [101] Cu foils are light weight and cheap conductive substrate, [102] which has been chosen as the flexible substrate to construct inverted FPSCs with a structure of Cu/CuI/MAPbI 3 /ZnO/silver nanowires (Ag NWs). [102] In this study, Cu foil not only worked as copper atom source to grow the following cuprous iodide layer but also benefited the collection of extracted holes from HTL, resulting in inverted FPSCs with an optimized efficiency of 12.80%. [102] Although metal substrates display many advantages, the light opacity causes a loss of optical absorption, which ultimately results in low PCEs of inverted FPSCs. To solve such issues, it is necessary to explore high transparent and conductive top electrodes. However, metal substrates possess relatively high heat resisting property. Thus, metal substrates are suitable for fabrication of flexible devices with special requirements, such as thermal stable devices.

Biomass Substrates
In addition to traditional polymer and metal substrates, many novel biomass substrates have also been developed to fabricate inverted FPSCs owing to their excellent properties. [43] Biomass substrates possess many merits, such as paper substrates with high thermal stability (over 200°C), excellent flexibility, strong foldability, and low production cost, and show a great potential in fabrication of flexible devices. Specifically, cellulose paper with advantages of flexible, light weight, low cost, and accessibility is a suitable flexible substrate. [103] Yu and co-workers fabricated HTLfree inverted FPSCs on cellulose paper to realize a PCE of 9.05%, suggesting broad application prospects in wearable and portable electronics. [104,105] Besides, nanocellulose paper (NCP) is a suitable flexible substrate owing to their abundant source, excellent thermal and chemical stability, high mechanical strength, high transparency, and low surface roughness (Figure 4a). [106] Inverted FPSCs with NCP as the substrate were already reported to show a PCE of 4.24% (Figure 4b,c), while having high power per weight and excellent stability, paving a new direction for novel inverted FPSCs. [107] Using bamboo cellulose nanofibril (b-CNF) flexible substrates, Zou et al. reported lightweight inverted FPSCs with a PCE of 11.68% by integrating transparent conductive oxide (Figure 4e-g). [108] The same group also use nature polylactic acid (PLA) obtained from plants of potato, corn, or wheat as substrates to assemble into composite electrodes ( Figure 4h) and reported biomass-based inverted FPSCs with an efficiency of 11.44% (Figure 4i-k) and remarkable mechanical durability. [109] With advantages of outstanding flexibility and malleability, silk fibroin has also been widely used in the field of photoelectronic. [110] Meanwhile, their properties could be modified owing to its carbonyl and amino groups. Malleable and pliable silkderived electrodes (SDEs) were made by researchers to fabricate deformable inverted FPSCs driven by moisture, showing a PCE of 10.40% with excellent malleability and pliability, indicating the great potential for SDEs. [111] Although biomass substrates hold many advantages for preparing flexible devices, the efficiency of resulted inverted FPSCs still needs to be improved. Therefore, it is urgent to design and synthesize biomass substrates with high transparency, low surface roughness, high electrical conductivity, and low cost for the fabrication of high-performance inverted FPSCs. Biomass substrates can also be applied to other electronic devices, which is helpful for the development of new green flexible electronic products.

Other Substrates
Besides the rapid developed polymer, metal, and biomass substrates, other functional substrates for inverted FPSCs have also made impressive progress especially in recent years. For example, Noland Optical Adhesive 63 (NOA 63) was exploited as flexible substrates to prepare ultraflexible-shaped recoverable inverted FPSCs without transparent and conducting oxides, resulting in FPSCs with a PCE of over 10% (Figure 5a-c) and high bending durability. [112] Photoresists (SU-8) were also utilized as the flexible substrate to fabricate inverted FPSCs, which showed an efficiency of 9.05%. [113] Alternatively, with properties of strong thermal tolerance, high transmittance, impermeability to water and oxygen, and high chemical stability to resist chemical solvents, flexible glass is considered as suitable flexible substrates for FPSCs. [42] Inverted FPSCs with a PCE of 19.72% were reported by Huang et al., using willow glass (WG) assisted by additive engineering (Figure 5d-f ). [114] However, flexible glass substrates suffer from disadvantages of high weight, fragility, and low specific power, limiting their wide applications in FPSCs.
Other possible flexible substrates also include mica, which showed outstanding properties including strong thermal tolerance, outstanding mechanical flexibility, and excellent impermeability to water and oxygen. Li et al. developed high performance inverted FPSCs on mica substrates, which exhibited a high PCE of 18.0% (Figure 5g-i), and held amazing mechanical durability, thermal and environmental stability. [115]

Flexible Bottom Electrodes
It is critical to explore transparent conductive electrodes with high conductivity for the development of high-performance inverted FPSCs. An ideal bottom electrode in inverted FPSCs should hold advantages of high transparency, good mechanical flexibility, high conductivity, low-temperature processability, good chemical stability, good endurance, and low sheet resistance. Up to now, transparent conductive oxides, metal-based materials, carbon-based materials, and highly conductive poly(3,4-ethylenedioxythiophene): polystyrene-sulfonate (PEDOT:PSS, PH1000) have been widely used in inverted FPSCs, acting as flexible bottom electrodes. A summary of flexible bottom electrodes used in inverted FPSCs is illustrated in Table 2.

Transparent Conductive Oxide
Transparent conductive oxides (TCOs) are the most commonly used bottom electrodes in inverted FPSCs, which include ITO, [90,116] aluminum-doped zinc oxide (AZO), [81] indium zinc oxide (IZO), [85] and so on. As the first reported bottom electrode for PSCs, ITO is the most frequently used TCO material owing to its simple low-temperature fabrication process, excellent chemical stability, and outstanding photovoltaic properties. [90,116] Besides, it could be deposited on both PET and PEN substrates, enabling the further applications in flexible electronics. Actually, up to now, almost all recorded efficiency in inverted FPSCs are based on ITO. Recently, the PCE of ITO-based single-junction and tandem inverted FPSCs have reached 21.76% and 24.7%, respectively. [78,86] Although ITO-based inverted FPSCs have obtain outstanding efficiencies, the intrinsic brittleness of ITO may cause the crack of films and thus affecting the mechanical stability, which limits the future commercialization of FPSCs. [49] In addition, ITO possesses relatively high cost, and the scarcity of indium resources hinders large-scale manufacturing of flexible devices. All above a-c) Reproduced with permission. [107] Copyright 2019, Nature Publishing Group. d) A photograph of the extremely lightweight b-CNF electrode on bamboo leaves without deformation. e,f ) Device structure and a cross-section SEM image of b-CNF-based inverted FPSCs. g) J-V curve of inverted FPSCs prepared on c-CNF. d-g) Reproduced with permission. [108] Copyright 2019, Wiley-VCH. h) Photograph of the PLA/Ag NWs/PH1000 (PLAAP) substrate. i-k) Device structure, a cross-section SEM image, and J-V curve of inverted FPSCs prepared on PLAAP. h-k) Reproduced with permission. [109] Copyright 2020, Wiley-VCH. shortcomings limit the future commercial applications. Therefore, it is urgent to explore novel flexible transparent electrodes for the construction of high-performance FPSCs. For example, an AZO/Ag/AZO-coated PET foil was exploited to fabricate inverted FPSCs with an improved mechanical stability. [81] Recently, a TCO of Zr-, Ti-, and Ga-doped indium oxide (ITGZO) film was also developed to prepare inverted FPSCs with high performance. [96] Moreover, in order to further reduce the material cost and improve the price advantage of inverted FPSCs, it is urgent to develop novel TCO-free electrodes to replace the traditional TCO electrodes (ITO, etc.).

Metal Nanowires
Currently, it is a great challenge to develop highly flexible transparent electrodes with high transparency and low sheet resistance. The advantages of high transparency, mechanical flexibility, and high conductivity have made solution-processable metal nanowires be widely used to replace ITO in inverted FPSCs. As a promising bottom electrode, Ag NWs can be prepared by hard-template and soft-template processes. [117] However, Ag NWs still face several critical issues including high surface roughness, poor thermal and chemical stability, weak adhesion to flexible substrates, and serious current leakage between perovskite layer and Ag NWs electrode. [118] Many methods have been conducted to resolve the above problems. Bae et al. manufactured a robust and high-performance all-in-one platform containing c-ITO/metal NW composite electrodes on a glass fabric-reinforced plastic (GFRHybrimer) (Figure 6a) to fabricate highly efficient and stable inverted FPSCs (Figure 6b). [119] The device exhibited a PCE of 14.15% (Figure 6c), while possessing both outstanding mechanical durability and excellent thermal/chemical stability. [119] During the fabrication of inverted FPSCs, Ag NWs might be damaged by the halogen ions diffused from the perovskite active Figure 5. Other novel substrates for inverted FPSCs. a-c) Device structure, a cross-section SEM image. and J-V curves of NOA 63-based inverted FPSCs. a-c) Reproduced with permission. [112] Copyright 2015, Wiley-VCH. d) Device structure of inverted FPSCs based on willow glass. e) A photograph of a flexible perovskite module. f ) J-V curves of inverted FPSCs prepared on willow glass. d-f ) Reproduced with permission. [114] Copyright 2019, Wiley-VCH. g,h) Device structure and a photograph of mica-based inverted FPSCs. i) J-V curves of inverted FPSCs based on mica and glass. g-i) Reproduced with permission. [115] Copyright 2019, Elsevier. layer to form a nonconducting silver halide phase, which is detrimental to the conductivity of Ag NWs. [120] Besides, the random Ag NW networks lead to low transmittance and high surface roughness due to NW aggregation, which limits the application of Ag NWs in high-performance FPSCs. The orthogonal Ag NWs display highly oriented NW surface morphology with excellent surface coverage to prevent the formation of silver halide, which is conducive to improving the conductivity. For instance, Ko and co-workers demonstrated orthogonal Ag NWs, which displayed smooth surface morphology and excellent photovoltaic properties. The flexible devices based on orthogonal Ag NWs obtained a PCE of 15.18%, benefiting from the suppressed formation of nonconducting silver halides. [120] This demonstrated that Ag NWs are promising alternatives to ITO, which may play an increasingly important role in the field of FPSCs.

Metal Grids
Metal grid electrodes were developed to work as bottom electrodes due to the high mechanical flexibility, excellent conductivity, and high transparency. [42][43][44] However, a cap layer need to be used to solve the problem of chemical instability and high surface roughness. Li et al. fabricated inverted FPSCs based on Ag-mesh electrodes embedded PET substrates. [121] To further improve device performance, researchers utilized highly conductive PEDOT:PSS (PH1000) coated on Ag-mesh electrodes to improve the flexibility and conductivity (Figure 6d,e). The composite PET/ Ag-mesh/PH1000 electrodes showed a higher transmittance and a lower sheet resistance. The flexible devices based on TCO-free electrodes displayed a efficiency of 14.0% with high durability and superior bending stability (Figure 6f ). [121] Zheng and coworkers used a hybrid of protruded Cu grids on PET and PEDOT:PSS (PH1000) as the bottom transparent electrode to construct inverted FPSCs and obtained a PCE of 13.58% with outstanding mechanical flexibility and improved long-term storage stability. [122] Additionally, an ultraflexible embedded PET/nickel (Ni)-mesh electrode was developed through co-planar and the film inversion process, as reported by Wang and co-workers (Figure 6g,h). [123] The inverted FPSCs based on PET/Ni-mesh electrodes displayed a record efficiency of 17.3% (Figure 6i) with greatly enhanced mechanical stability and environmental stability.

Ultrathin Metal Films
Metal films are one of the most promising bottom electrodes in inverted FPSCs due to their excellent ductility and conductivity. Moreover, metal films are beneficial for the growth of high-quality  perovskite films due to the highly smooth surface. It must be pointed out that the conductivity is related to the continuity of the metal film, while the continuity of metal film is related to the nucleation and the growth kinetics on the flexible substrates. [124] Generally, the optical transparency and electrical conductivity can be balanced by adjusting the thickness of metal films (%10 nm). Ultrathin metal films with both high conductivity and excellent transmittance could be used as bottom electrodes of inverted FPSCs. [124] Although ultrathin metal films (%10 nm) possess high optical transparency, metal films fabricated by thermal evaporation process follow the Volmer-Weber growth mode to grow films with rough surface topography. [125] By employing nucleation-inducing seed layers of molybdenum trioxide (MoO 3 ) and gold (Au) to regulate the surface energy and optimize the growth conditions of metal films, researchers obtained a continuous ultrathin Au electrode and MoO 3 /Au-based flexible device with increased flexibility. [125]

Carbon Nanotubes
SWNTs are the most widely used CNTs due to their wide source, abundant carbon composition, outstanding flexibility, and direct roll-to-roll processability. As early as 2015, Matsuo et al. utilized HNO 3 -doped SWNTs as the bottom electrode to make TCO-free planar heterojunction inverted FPSCs, which reached a PCE of 6.32%. [127] Zhang and co-workers fabricated TCO-free inverted  [119] Copyright 2016, Nature Publishing Group. d) An image of a large-area flexible PET substrate with embedded Ag-meshs (FEAMs) and detail parameters. e,f ) Device architecture, cross-section SEM images J-V curves of inverted FPSCs fabricated on FEAMs. d-f ) Reproduced with permission. [121] Copyright 2016, Nature Publishing Group. g) Optical image of the large-area PET/Ni-mesh substrate. h,i) Device structure and J-V curves of inverted FPSCs based on Ni-mesh:PH1000. g-i) Reproduced with permission. [123] Copyright 2020, Wiley-VCH.
FPSCs on flexible substrates with small-bundle SWNTs as bottom electrodes by a simple dry transfer process (Figure 7a, b). [128] The device displayed a high efficiency of 18.1% with negligible hysteresis (Figure 7c), and outstanding environmental stability and mechanical robustness. Simultaneously, the use of SWNTs as electrodes could greatly reduce the cost of bottom electrodes and simplify the fabrication procedure.
Although SWNTs were proven to be promising alternatives to traditional TCO electrodes in a series of photovoltaic and optoelectronic fields, it should be pointed out that the contact resistance between SWNTs is relatively high, which might affect the conductivity. Hence, doping engineering is needed to improve the photovoltaic properties of SWNTs. Additionally, the inherent hydrophobicity of SWNTs is not conducive to the preparation of uniform and high coverage films. Therefore, corresponding measures should be taken to solve the above problems.

Graphene
Graphene (GR) with high electrical conductivity, excellent transparency, and smooth surface has also been exploited as alternative bottom electrodes to TCO electrodes. The high transmittance of GR films can promote the absorption and utilization of light to improve the performance of inverted FPSCs. [129] Despite the highly theoretical carrier mobility of GR, the practical conductivity is not satisfactory owing to its poor film morphology. Therefore, Im and co-workers doped GR with bis(trifluoromethanesulfonyl)-amide[((CF 3 SO 2 ) 2 NH)] (TFSA) to prepare p-type transparent bottom electrodes for inverted FPSCs and achieved devices with an outstanding bending stability (Figure 7d,e) and a PCE of 18.3% (Figure 7f ). [83] Park et al. reported a new flexible hybrid bottom electrode containing a Cu grid-embedded polyimide film and a GR capping layer (GCEP) (Figure 7g,h). [130] The resulted devices based on this flexible hybrid electrode Figure 7. Carbon-based bottom electrodes of inverted FPSCs. a-c) Device structure, a cross-section SEM image, and J-V curves of SWCNT-based inverted FPSCs. a-c) Reproduced with permission. [128] Copyright 2021, Wiley-VCH. d-f ) Device structure, energy band diagram, and J-V curves of TFSA-doped GR-based inverted FPSCs. d-f ) Reproduced with permission. [83] Copyright 2018, The Royal Society of Chemistry. g,h) Cross-section SEM image and energy band diagram of GCEP-based FPSCs. i) J-V curves of corresponding inverted PSCs. g-i) Reproduced with permission. [130] Copyright 2020, American Chemical Society. exhibited an efficiency of 16.4% (Figure 7i) with improved mechanical and chemical stability.

Highly Conductive PEDOT:PSS (PH1000)
As a conductive polymer, PEDOT:PSS can be used not only as an efficient hole transport material but also as an alternative to TCO for bottom electrodes owing to its high transmittance, excellent flexibility, and solution processability. [126,[131][132][133][134][135] Moreover, PEDOT:PSS possesses a tunable work function, which enables good energy-level alignment. It is found that cracks in TCO electrodes after repeated bending cycles can lead to photovoltaic devices with low performance, thus Kelly et al. employed highly conductive poly(3,4-ethyl-enedioxythiophene):poly(styrene sulfonate) (HC-PEDOT) as bottom electrodes to fabricate inverted FPSCs and achieved devices with a PCE of 7.6% and high flexibility. [131] Ouyang and co-workers used methane sulfonic acid (MSA)-treated PEDOT:PSS (PH1000) as conductive bottom electrodes to construct inverted FPSCs with a PCE of 8.6% and superior mechanical flexibility (Figure 8a-c). [132] Brunetti and co-workers reported inverted FPSCs using highly conductive ethylene glycol-modified PEDOT:PSS (PH1000) as bottom electrodes prepared by spray coating. [133] Choi et al. used PEDOT: PSS (PH1000) as bottom electrodes for inverted FPSCs and obtained a PCE of 17.03% (%0.09 cm 2 ). By employing hybrid electrodes composing of a PEDOT:PSS (PH1000) and a Au metal mesh grid, they also achieved an efficiency of 13.6% for devices with a larger area of 1.2 cm 2 (Figure 8d-f ). [134] Although great progresses have been made in the efficiency of FPSCs, mechanical stability is still an important issue for flexible devices considering their practical applications such as mobile energy systems and aerospace systems. Hence, Que and coworkers reported an in-depth analysis of mechanical stability for inverted FPSCs based on PEDOT:PSS (PH1000) bottom electrodes (Figure 8g-i). [135] Compared with their counterparts of  [134] Copyright 2019, The Royal Society of Chemistry. g,h) Cross-section SEM image and J-V curve of PEDOT:PSS (PH1000)-based FPSCs. i) Normalized PCEs of FPSC devices as a function of bending cycles with a radius of 10 mm. g-i) Reproduced with permission. [135] Copyright 2022, American Chemical Society.
ITO-based inverted FPSCs, flexible devices based on PEDOT: PSS (PH1000) delivered improved flexibility. Although PEDOT:PSS (PH1000) has been widely utilized as a transparent electrode for the fabrication of high-performance FPSCs, its acidic properties are not conducive to the long-term stability of flexible devices.

Hole Transport Layers
As one of the most key functional layers in PSCs, HTL can extract photogenerated holes from light absorbing layers and transport them to the anodes. [43] HTLs in inverted FPSCs require much lower processing temperature to obtain high-quality films compared to ETLs in regular FPSCs, which is beneficial for most temperature-sensitive flexible substrates. The HTL needs to meet the following requirements: good band alignment with absorber layer, excellent hole mobility, high optical transmittance, compatibility with adjacent functional layers, and solution processability. [73] Furthermore, considering the high water transmittance of flexible substrates, HTL with high moisture resistance is helpful for the long-term stability of the device. A summary of HTLs used in inverted FPSCs is illustrated in Table 3.

PEDOT:PSS (Al4083)
As the first inverted HTL, conductive polymer of PEDOT:PSS has been widely used in inverted FPSCs owing to its tunable conductivity, high transmittance, and low-temperature processability. Low-temperature annealing process of PEDOT:PSS is beneficial for most temperature-sensitive flexible substrates. Back in 2014, Yang et al. chose PEDOT:PSS as the HTL to fabricate inverted FPSCs through a low-temperature and solution processable technique and attained an efficiency of 9.2%. [136] Subsequently, Jen and co-workers adopted a blade-coating technique to construct inverted FPSCs with PEDOT:PSS as the HTL. [137] Chen and co-workers used PEDOT: PSS as the HTL and employed methylammonium acetate as an ink additive to induce a more favorable crystal growth for high-quality films. [138] Simultaneously, 4-chlorobenzenesulfonic acid (Cl-BSA) was introduced to passivate perovskite deep defects. As a result, the flexible device (1.01 cm 2 ) achieved a PCE of 18.12% with improved stability. [138] Nevertheless, the inherent shortcomings of PEDOT:PSS limit its further development in the field of inverted FPSCs. First, the solution of PEDOT:PSS is acidic, which can corrode the electrode and react with the perovskite layer, thus degrading the device performance. Second, PEDOT:PSS possesses strong hygroscopicity, which is not conducive to the long-term stability of inverted FPSCs. Third, PEDOT:PSS displays a relatively low work function, which limits the open-circuit voltage (V OC ) value of inverted FPSCs. Therefore, it is urgent to explore more ideal HTLs.

Poly[bis(4-Phenyl)(2,4,6-Trimethylphenyl)amine
Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) has been widely used in inverted devices due to its excellent properties. [76] First, PTAA can be prepared by a low-temperature solution process, which greatly reduces the possible thermal damage to most polymer substrates. Second, PTAA has higher humidity stability than PEDOT:PSS, beneficial for improving the long-term stability of devices. Third, PTAA has negligible chemical reactions with perovskite layers owing to its relatively high chemical stability, which is beneficial to achieve long-term stability of devices. Fourth, PTAA possesses a higher work function than PEDOT: PSS, which closely matches the band structure of most perovskite layers such as CH 3 NH 3 PbI 3 , thus could reduce the energy loss and facilitate high open-circuit voltage (V OC ) and fill factor (FF). Last, PTAA displays high optical transmittance and strong mechanical tolerance, making it suitable for inverted FPSCs.
A double HTLs composed of PEDOT:PSS/PTAA was reported by researchers to form a cascading energy funnel between bottom electrodes and perovskite layers, facilitating the improvement of carrier extraction and transportation ( Figure 9a). [139] The flexible devices based on such double HTLs demonstrated a high PCE of 19.41% (0.09 cm 2 ) and 16.61% (1 cm 2 ) (Figure 9b,c) with excellent mechanical stability and outstanding environmental stability. In order to optimize the fabrication process, a thermal evaporation process was adopted by researchers to deposit HTLs of PTAA with a low thickness (2-10 nm), which had a good match with perovskite layers for band alignment and quenching effects. [140] The inverted FPSCs based on this preparation process achieved a PCE of 17.27% with negligible hysteresis and excellent stability. Actually, the highest efficiency of inverted FPSCs reported so far was achieved using PTAA as the HTL. [90] Although PTAA is one of the most promising hole transport materials for inverted FPSCs, there are still many issues that need to be addressed before practical applications. In particular, conductivity, substrate coverage, substrate wettability, and cost are factors that require significant attention from researchers in the future. To further improve the performance of PTAAbased inverted FPSCs, efforts can be made from the following aspects. In order to obtain high-quality perovskite films, the top and bottom of PTAA can be optimized, such as the optimization of bottom electrodes or the optimization of PTAA using other hole transport materials. Furthermore, novel deposition processes need to be explored for the fabrication of PTAA layers, such as vapor deposition and spray deposition. Additionally, more attention should be paid to the coverage and uniformity of perovskite layer deposited on PTAA. In order to reduce the material cost of PTAA, it is necessary to explore and develop simple synthetic routes, cheap reagents, and simple purification procedures. Moreover, novel HTLs should be prepared by reasonable organic synthesis design for replacement of expensive PTAA.

Other Organic Hole Transport Layers
Besides PEDOT:PSS and PTAA, other novel organic HTLs have also been developed for producing high-performance FPSCs. Poly(3-hexylthiophene) (P3HT) is a promising p-type organic hole transport material for PSCs owing to its excellent photovoltaic properties, low cost, and facile fabrication. [40] More importantly, high charge transfer and outstanding environmental stability make P3HT a suitable hole transport material for inverted FPSCs. [40] In order to improve environmental stability of inverted FPSCs, Yan et al. used P3HT as the HTL in inverted flexible devices. [141] Specially, P3HT can be coated uniformly on surface of GR electrodes to form a more homogeneous film than PEDOT:PSS. Additionally, P3HT displays more matched band alignment with CH 3 NH 3 PbI 3 compared with PEDOT: PSS to improve V OC of devices. [141] Therefore, P3HT is a potential material for the preparation of low-cost and high-performance inverted FPSCs. Nevertheless, since the poor contact between P3HT and perovskites impedes efficient hole transfer, PCEs of P3HT-based inverted FPSCs are still low, and interface engineering may be an effective strategy to solve it.
is also a promising hole transport material in inverted FPSCs owing to its properties of excellent hole mobility (1.0 Â 10 À4 cm 2 V À1 s À1 ), low annealing temperature (<120°C), superior hydrophobicity, and high chemical and thermal stabilities. Poly-TPD has been applied to fabricate single-junction and tandem inverted FPSCs and have obtained outstanding photovoltaic performance. [77,81,142] Furthermore, Poly-TPD could be also deposited by slot-die process to obtain a uniform film. [143] Rizzo and co-workers focused on slot-die deposition, which provides a facile and scalable solution processing for several materials. They employed Poly-TPD as HTL and fabricated inverted FPSCs through the strategy of a polymer starch template-assisted perovskite growth. [143] However, Poly-TPD is acidic and may be eroded by solvent, which can be solved by the PFN passivation layer. Besides, the high cost of Poly-TPD limits its potential for large-scale application, so more efforts should be paid to reduce its cost or replace it by other novel HTLs.
In the case of utilizing PEDOT:PSS as HTL for FPSCs, the work function (WF) of PEDOT:PSS results in potential energy loss at the HTL/perovskite (CH 3 NH 3 PbI 3 ) interface, reducing the built-in potential energy of PSCs. To solve this problem, a selforganized HEL (SOHEL) with a high WF was developed to modify the energy-level alignment. [144] SOHEL is composed of a PEDOT: PSS and a perfluorinated ionomer (PFI), where PFI enriched on the film surface due to its self-organization to increase the surface WF. Hence, the use SOHEL successfully increased the built-in potential, photocurrent, and performance of PSCs. [144] Rational design of HTLs can effectively improve the charge extraction from perovskite layers to HTLs, while suppressing the charge recombination in perovskite layers and at HTL/perovskite interfaces. Son and co-workers displayed a novel polymer-based HTL of 1,4-bis(4-sulfonato-butoxy)benzene and thiophene moieties (PhNa-1 T) to fabricate inverted FPSCs with excellent environmental and mechanical stability. [145] Jin and co-workers  [139] Copyright 2020, Wiley-VCH. d) Molecular structure of CzPAF-TPA. e,f ) Energy band diagram and J-V curves of corresponding FPSCs. d-f ) Reproduced with permission. [146] Copyright 2017, Elsevier. g) Molecular structures of BTF5 and BTF6. h,i) Energy band diagram and J-V curves of corresponding FPSCs. g-i) Reproduced with permission. [147] Copyright 2021, Elsevier. rationally designed and synthesized a HTL material named N-(4-(9 H-carbazol-9-yl)phenyl)-7-(4-(bis(4-methoxyphenyl)amino) phenyl)-N-(7-(4-(bis(4-methoxyphenyl)amino)phenyl)-9,9-dioctyl-9H-fluoren-2-yl)-9,9-dioctyl-9H-fluoren-2 amine (CzPAF-TPA) to construct inverted FPSCs, and obtained devices with a PCE of 12.47% (Figure 9d-f ) and an improved long-term stability. [146] Jen and co-workers synthesized two novel D-A-D-type HTLs (BTF5 and BTF6) for inverted FPSCs (Figure 9g,h) and achieved a PCE of 18.06% for those based on BTF6, (Figure 9i) [147] suggesting that efficient and stable inverted FPSCs can be fabricated by reasonable design of HTLs. Novel self-assembly monolayers (SAMs) were also designed as HTLs for the efficient transfer of charge. Many SAMs like MeO-2PACz and 2PACz were systematically investigated by researchers to fabricate single junction and tandem inverted FPSCs, [79,148] indicating that SAMs are also promising materials to obtain high-performance flexible devices.

NiO x
The stability of organic HTLs is usually poor, seriously affecting long-term stability of resulting inverted FPCSs. Compared with organic HTLs, inorganic HTLs show a better stability and hence drawing attentions on the preparation of high-quality inorganic HTLs at low temperatures. [42] NiO x is the most commonly used p-type inorganic HTLs in inverted FPSCs owing to its outstanding chemical stability, low processing temperature, and excellent transmittance. [149][150][151][152][153][154][155][156][157][158][159] Despite the excellent performance of NiO x in inverted FPSCs, inorganic NiO x exhibits problems of poor electrical conductivity and bad band arrangement, hindering the development of NiO x -based inverted FPSCs. Hence, efforts should be focused on the improvement of electrical conductivity and band arrangement of NiO x . Que and co-workers employed a solution-derived NiO x film as the HTL to construct inverted FPSCs, which reached a PCE of 13.43% owing to the efficient extracting and blocking ability of holes and electrons for NiO x , respectively. [149] Choy et al. reported a surface-nanostructured and flawless NiO x film by a room-temperature, solution-processable technique. [150] The inverted FPSCs based on NiO x possess excellent stability and reproducibility. However, the inherent brittleness of inorganic NiO x limits its applications in inverted FPSCs. To solve this problem, Chen et al. developed a polydopamine (PDA) modified NiO x (NiO x :PDA) layer to optimize the interfacial contact and balance the charge mobility. [153] The flexible device based on NiO x :PDA exhibited an ideal PCE of 18.35% with an outstanding flexural endurance. It has been reported that nanostructured NiO x array structures can effectively reduce reflection losses, inhibit recombination dynamics, and release stress and strain during mechanical bending to improve the mechanical stability of flexible devices. [160,161] While it is a big challenge to prepare NiO x nanopillar arrays (NiO x NaPAs), Huang and co-workers obtained NiO x NaPAs by a vapor deposition (Figure 10a), which enhanced light harvesting and promoted hole transport and collection. [155] The inverted FPSCs obtained an efficiency of 17.23% (Figure 10b,c) with high mechanic stability. In order to solve the poor interface contact caused by the easy agglomeration of NiO x nanoparticles (NPs), researcher prepared high-quality NiO x NPs by a polymer network micro-precipitation method to fabricate inverted FPSCs and obtained an efficiency of 19.17% (1.01 cm 2 ) (Figure 10d-f ). [158] In addition to the reported problems for NiO x in inverted FPSCs, light-induced degradation at the NiO x -perovskite heterojunction is still an issue to be solved, utilizing functional interface layers may be an effective strategy to solve it.
According to previous reports, NiO x possesses obvious advantages of thermal and photostability over organic hole transport materials. Therefore, NiO x has been widely used in inverted FPSCs due to its excellent properties. [160,161] However, the low efficiency of NiOx-based flexible devices is still an obstacle to the commercialization of inverted FPSCs based on NiOx. Two main factors limiting the performance of NiO x -based FPSCS are low conductivity and mismatched energy level of NiO x . These problems can be solved through the following strategies. NiO x can be modified by metal ion dopants and organic molecular dopants to reduce defect density, improve carrier mobility, and improve device stability. Moreover, functional organic molecules, alkaline chlorides, and ozone can be utilized for surface modification of NiO x to improve uniformity of NiOx films, passivate interfacial defects, and inhibit interfacial carrier recombination.

Other Inorganic Hole Transport Layers
Another potential p-type HTL material is copper-based ones, including CuI, CuO, Cu 2 O, and Cu 2 CrO, which possess merits of high conductivity, wide band gaps, low cost, and easy synthesis. Inverted FPSCs based on CuI and Cu 2 CrO have already been reported to show high performance. [102,162] Nevertheless, the high post-heating temperature of CuI limits its applications on polymer substrates. Although Cu 2 CrO could be fabricated at a low temperature, the performance of Cu 2 CrO-based flexible devices needs to be further improved. Therefore, it is necessary to develop new inorganic hole transport materials for fabrication of high-performance inverted FPSCs at low temperature.
Considering that high-temperature treatments and acidic solvents restrict applications of most organic and inorganic HTLs in the field of inverted FPSCs, Joh et al. adopted fluorinated reduced graphene oxide (MFGO) as the HTL to fabricate inverted FPSCs. [163] The functionalized MFGO has a higher WF and a stronger hydrophobicity, enabling the resulted MFGO-based inverted FPSCs to obtain a PCE of 14.7%. Currently, GO quantum dots (GQDs) have been widely applied to PSCs owing to their excellent optoelectronic properties. So GQDs were employed as the HTL to construct inverted FPSCs and obtained FPSCs with a PCE of 15.38% and outstanding stability (Figure 10g-i). [164] Fabrication of all-vacuum-processed device devices is challenging because all layers must be deposited in a vacuum mode, especially for most solution-processed HTLs in inverted PSCs, copper (II) phthalocyanine (CuPC) was used as the HTL to make all-vacuum-processed inverted FPSCs to achieve excellent performance. [165]

Hole Transport Layer-Free Inverted Flexible Perovskite Solar Cells
Theoretically, PSCs can be fabricated without HTLs due to ambipolarity and long carrier diffusion length of perovskites. Based on this, perovskite layer can be used not only as a light absorber layer but also as HTL. [166] This type of HTL-free inverted FPSCs can simplify the fabrication process and improve device stability. Besides, HTL-free inverted FPSCs possess better compatibility with flexible substrates due to no heating treatment before depositing the perovskite layers. For example, Chen et al. fabricated a HTL-free inverted FPSC to get an efficiency of 9.7%. [167] Compared with those using PEDOT:PSS as HTLs, the stability of HTL-free inverted FPSCs has been significantly improved. More importantly, the HTL-free inverted FPSCs saved time and cost, thus offering potential for the commercialization. However, the research progress on HTL-free inverted FPSCs is slow, the optimization of perovskite layers and counter electrodes may be the focus of future studies.

Perovskite Active Layers
The preparation of high-quality perovskite films with excellent mechanical durability, strong flexibility, and outstanding environmental stability is the key to obtain high-performance FPSCs. This is mainly attributed to the undesired ion migration and trap-induced nonradiative recombination in the perovskite layer, both of which limit the photovoltaic performance of inverted FPSCs. [41][42][43][44]49] In this section, we discuss different techniques for fabricating high-quality perovskite films in inverted FPSCs. A summary of the optimizations of perovskite active layers for inverted FPSCs is illustrated in Table 4.

Perovskite Composition Engineering
Perovskites can be divided into organic-inorganic hybrid perovskites and all-inorganic perovskites according to the difference of component materials. At present, organic-inorganic hybrid perovskites are the most studied solar cell materials in the field of FPSCs. Among the optimizations of perovskites, composition engineering is a very useful strategy to obtain high-quality  [158] Copyright 2021, Wiley-VCH. g,h) Energy band diagram and J-V curves of inverted FPSCs fabricated on GQDs. i) Normalized PCE as a function of bending cycles at a fixed bending radius of 4 mm. g-i) Reproduced with permission. [164] Copyright 2019, American Chemical Society.
perovskite films in inverted organic-inorganic hybrid FPSCs, [80,82,89,[168][169][170][171][172] i.e., by optimizing the desired ratios of different precursors. Compared with traditional methylammonium (MA)-based perovskites, formamidinium (FA)-based perovskites have narrower band gaps and better thermal stability, making them one of the most promising perovskite materials. For example, Huang et al. adopted FA cations to expand the absorption spectrum of the perovskite films and boost short-circuit current ( J SC ). [82] By mixing methylammonium bromide (MABr) with FAI precursor solutions to form mixed cation perovskite films, they changed the tolerance coefficient and obtained improved thermal, humidity, and phase stability. [82] The flexible devices based on this method exhibited a record PCE of 18.1% with excellent stability.
The key to the fabrication of high-performance FPSCs is to form uniform, pinhole-free, and defect-less perovskite films on flexible substrates. Mai and co-workers fabricated high-performance FA-alloyed inverted FPSCs through manipulating film morphology and controlling defects by a solvent and dimensional engineering (Figure 11a). [169] Researchers first fabricated dense and uniform FA-alloyed high-quality perovskite films by solvent engineering and then suppressed trap state defects by building two-dimensional (2D)/three-dimensional (3D) heterostructures. The FA-alloyed device delivered high PCEs of 20.16% (0.09 cm 2 ) and 16.86% (1 cm 2 ) (Figure 11b,c) with outstanding mechanical durability. [169] The toxicity of lead-based perovskites is also a major obstacle to the commercialization of FPSCs. As an alternative, tin-based perovskites exhibit potential applications in inverted FPSCs due to their low toxicity. Chen et al. used graphite phase-C 3 N 4 (g-C 3 N 4 ) as the crystal template to grow high-quality tin-based perovskite thin films with low defect density. [171] The tin-based inverted FPSCs achieved a PCE of 8.56% with improved hydrophobicity and oxidation resistance (Figure 11d-f ). Padture and co-workers stabilized Sn 2þ oxidation in FASnI 3 by alloying with Ge 2þ , and obtained a PCE of 10.43% with outstanding operational stability and mechanical reliability (Figure 11g-i). [172] For future optimizations of organic-inorganic hybrid perovskites through composition engineering, 2D perovskites with excellent stability need to be considered. In recent years, 2D Zeolitic imidazolate framework-67; b) 2,2 0 ,7,7 0 -tetra (N,N-di-tolyl) amino-9,9-spiro-bifluorene. perovskites with unique multiquantum well structure (QW) express excellent phase and environmental stability compared with 3D perovskites due to their high formation energy and inhibition of ion migration. Hence, 2D flexible inverted FPSCs should be developed to improve long-term stability of devices. The fabrication of low-dimensional inverted FPSCs is beneficial to improve device stability and accelerate the commercialization of flexible devices. Although many breakthroughs have been made in inverted flexible devices based on organic-inorganic hybrid perovskites, there are still several issues to be solved before commercialization, such as thermal stability and UV stability. The main reason for poor thermal stability of organic-inorganic hybrid FPSCs is that the perovskite layers contain organic components that are easily decomposed by heating. [19] Therefore, it is an effective strategy to improve thermal stability of flexible devices through developing all-inorganic FPSCs. [173] In particular, composition engineering plays an important role in the field of inverted all-inorganic FPSCs. All-inorganic cesium lead halide (CsPbX 3 ) perovskites are promising due to their superior thermal stability.
To improve the thermal stability of inverted FPSCs, Bian and coworkers reported a solvent-induced method for the preparation of high-quality CsPbI 2 Br perovskite films at room temperature, resulting in FPSCs with a PCE of 7.3% and excellent thermal stability. [174] Although CsPbX 3 possesses advantages of high thermal stability and easy fabrication of tandem devices, it delivers some problems such as phase instability and narrow absorption range, which hinder the commercialization of flexible devices.

Additive Engineering
Defects may cause undesirable energy losses in nonradiative recombination, thus limiting the performance of FPSCs. More importantly, the defects are the main reason for the operational instability of FPSCs, which hinders their commercialization. Therefore, it is necessary to develop strategies to reduce formation of defects as well as to passivate defects. Additives help passivating defects in perovskites and improving the mechanical stability of inverted FPSCs. [84,90,97,[175][176][177][178] Generally, introducing cations of different sizes into perovskites causes microstrain of  [171] Copyright 2021, Wiley-VCH. g,h) Device structure and energy band diagram of FASn 0.9 Ge 0.1 I 3 inverted FPSCs. i) J-V curves of corresponding inverted FPSCs. g-i) Reproduced with permission. [172] Copyright 2022, American Chemical Society. perovskite films, which affects the optoelectronic properties of inverted FPSCs. Yan et al. reported a method of alleviating microstrain in perovskite films by additive engineering. [175] As an additive, KBF 4 can not only improve the crystallinity of perovskite films and reduce microstrain but also passivate defects and improve carrier lifetime. The flexible device reached a notable efficiency of 21.02% with excellent mechanical stability.
Recently, organic ammonium salts have been widely reported to passivate defects in PSCs and suppress nonradiative charge recombination. [179,180] Chen et al. employed alkyldiammonium ligands to passivate defects on perovskite surface and suppress nonradiative recombination. [176] As a result, the inverted FPSCs delivered an impressive PCE of 20.9% with an improved mechanical stability owing to the release of residual stress. The brittleness of perovskite film limits the development of inverted FPSCs, Song et al. exploited the polyurethane elastomers with disulfide bonds (PUDS) to fabricate self-healing inverted FPSCs. [177] PUDS possessed excellent tensile and bending properties, which is composed of hard and soft phases. When the temperature exceeds the stimulating response temperature (T S = 60°C) of disulfide bonds in the hard phase, disulfide bonds will break and then form free radicals. [177] As the temperature was cooled, disulfide bonds between free radicals were formed to complete the self-healing process. As a result, the PUDS-mediated device reached a remarkable PCE of 17.19% with outstanding flexibility and self-healing property. [177] R2R processibility is believed to accelerate the commercial promotion of PSCs. However, the high overall coordination of efficiency, stability, and flexibility during R2R processes limits the performance of flexible devices. To further improve the compatibility of R2R technology and flexible devices, a cross-linkable multifunctional ionogel (IG) was reported by Dong and co-workers to modify perovskite polycrystalline films (Figure 12a-c). [86] IG composed of ionic liquids and polymer networks improved the elongation, toughness, and rapid room-temperature self-healing ability of perovskite films, thereby regulating the mechanical properties and passivating defects in perovskite devices. As a result, the inverted FPSCs utilizing the R2R-compatible coating process achieved a record efficiency of 21.76% (Figure 12d). [86] Furthermore, this strategy significantly improved the operational stability, mechanical stability, and humidity stability of the inverted FPSCs.
Additives have been widely used in the preparation of efficient, stable, and hysteresis-free inverted FPSCs. In order to further improve the performance of FPSCs, multifunctional additives should be developed to adjust the morphology of perovskite films, stabilize perovskite phases, regulate energy levels, passivate defects, eliminate hysteresis, and improve device stability. At the same time, a better understanding of the mechanism of additives is needed, which will help to further elucidate their effects on efficiency and stability.

Interface Engineering
Notably, interface engineering has been identified as an effective strategy with remarkable achievements in improving the efficiency and stability of inverted FPSCs. [92,[181][182][183][184] Interface engineering can not only establish appropriate energy-level arrangement to facilitate charge transport but also passivate interface defects. Therefore, a lot of work have been done on interface engineering for inverted FPSCs. For example, a self-assembled monolayer (C3-SAM) was employed by researchers at the interface between HTLs and perovskite films, resulting in perovskite films with high crystallinity and high coverage, and inverted FPSCs obtained a PCE of 5.1% and improved stability. [182] Li and co-workers introduced branched polyethyleneimine hydroiodide (PEI·HI) at the interface between HTLs and perovskite layers to form 2D perovskite in situ. [183] Here, the 2D perovskite layer was used to control the morphology and grain growth of perovskite films. The inverted FPSCs exhibited a high PCE of 13.8% with an excellent stability.
The poor interfacial contacts and pinholes caused by the coffee-ring effect can increase defects within perovskite films, thereby impairing performance of inverted FPSCs. [185] Chen et al. utilized a biomimetic interface layer (Bio-IL) to suppress the coffee-ring effect of perovskite films (Figure 12e), which was beneficial to obtain perovskite films with large grain size, low defect density, and good interfacial contact. [92] The fabricated inverted FPSCs displayed impressive PCEs of 21.08% (0.1 cm 2 ) and 18.16% (1.01 cm 2 ) (Figure 12f,g), accompanying with an outstanding environmental and mechanical stability.
Currently, interface engineering plays an important role in achieving efficient and stable FPSCs. Although great progress has been made in interface engineering for optimizing perovskite layers of inverted FPSCs, significant challenges still need to be overcome. The focus of researchers should be paid on developing novel multifunctional interface materials, which can passivate defects, promote carrier extraction and improve device stability. Simultaneously, the physical and chemical properties of interfacial materials can be regulated by dopants. More importantly, interface materials with enhanced chemical and thermodynamic stability are helpful for achieving long-term stability of flexible devices.

Other Optimizations of Perovskite Layers
In addition to the above methods, other processes have also been developed for the construction of highly efficient and stable inverted FPSCs. Antisolvent engineering is an effective strategy to obtain high-quality perovskite films, which can accelerate the growth of crystals and reduce the generation of defects. An optimized antisolvent engineering was employed to prepare uniform perovskite films with high crystallinity by a simple spin-coating process. [186] The prepared films have the advantages of large grain sizes, high crystallinity, and low dependence on annealing temperature. [186] Environmental instability limits the commercialization of inverted FPSCs, so researchers utilized a layer-bylayer process to fabricate perovskite films with desirable thickness, homogeneous morphology, and oriented crystalline domains. [187] The inverted FPSCs based on this approach delivered an efficiency of 12.25% with superior stability.
Thermal process limits the high-throughput R2R fabrication of inverted FPSCs since HTLs also require high-temperature annealing for fabrication. To solve this, Hsu et al. employed a photonic curing process to replace traditional thermal annealing in fabrication of HTLs and perovskite active layers in inverted FPSCs. [188] Besides, vapor deposition could be a method to construct highly efficient and stable PSCs, as reported by Bruno et al., who explored co-evaporation process for the fabrication of perovskite films. They found graded Fermi levels along the thickness direction with compositional gradients, which was beneficial for obtaining a favorable energy alignment for inverted FPSCs (Figure 12h). [189] As a result, the co-evaporated inverted FPSCs achieved a notable PCE of 19.34% (Figure 12i,j) with an improved long-term stability.

Electron Transport Layers
In inverted FPSCs, ETL is deposited directly on the perovskite active layer, thus playing a crucial role in the overall performance.
There are several aspects to consider for an ideal ETL. First, ETL S should have high electron mobility and hole blocking ability and be able to block the erosion of oxygen and moisture. Second, ETLs are required to have a good band matching with perovskites. Third, the solvent used for ETL deposition should not damage the perovskite active layer, while the prepared ETLs should fully cover the perovskites. Fourth, potential reactions between ETLs and perovskites should be avoided, while nonradiative recombination caused by the interfacial defects should be reduced. At present, fullerenes and their derivatives (i.e., C 60 ,  [86] Copyright 2022, Wiley-VCH. e) Device structure of inverted FPSC with Bio-IL interface. f ) J-V curves of inverted FPSCs with and without Bio-IL interface. g) J-V curves of a large-area (1.01 cm 2 ) inverted FPSCs. e-g) Reproduced with permission. [92] Copyright 2022, Wiley-VCH. h,i) Energy band diagram and J-V curves of inverted FPSCs based on a vapor deposition process. J) PCE distributions from both forward and reverse scans. h-j) Reproduced with permission. [189] Copyright 2021, Wiley-VCH. Table 5.

PCBM) are the most widely used ETLs. A summary of ETLs used in inverted FPSCs is illustrated in
To achieve efficient and stable inverter FPSCs, it is necessary to improve the quality of ETLs and to optimize adjacent buffer layers. Li and co-workers modified ETLs by doping 3-aminopropyltriethoxysilane (APTS) molecules in PCBM solutions to obtain a smooth and dense PCBM film, beneficial for promoting electron collections between ETLs and perovskite layers. [190] Simultaneously, conformal tin oxide (SnO x ) layer was employed as the robust buffer layer. The inverted FPSCs delivered a PCE of 18.62% with high environmental stability.
In addition to the above ETLs, researchers have also explored other suitable materials for the preparation of high-performance inverted FPSCs. [39,191,192] Kaltenbrunner utilized vapor-deposited N,N 0 -dimethyl-3,4,9,10-tetracarboxylic perylene diimide (PTCDI) as the ETL to fabricate homogeneous films over large areas. PTCDI favored an efficient hole blocking and an excellent environmental stability owing to their deep highest occupied molecular orbital (HOMO) level. [39] Meanwhile, they also introduced chromium oxide-chromium interlayer between ETLs and top metal electrodes to avoid the reaction with the perovskites (Figure 13a,b). The resulted inverted FPSCs expressed a PCE of 12.0% (Figure 13c) with superior power-per-weight, high mechanical flexibility, and excellent operational stability. [39] Park et al. developed a novel naphthalene diimide (NDI)-based polymer named P(NDI2DT-TTCN) as the ETL to replace PCBM in inverted FPSCs (Figure 13d). [191] P(NDI2DT-TTCN) improved not only the electron extraction capability but also the environmental stability, resulting in FPSCs with a PCE of 17.0% and outstanding mechanical stability (Figure 13e,f ). [191] Wojciechowski and co-workers reported the use of a solutionprocessable fullerene derivative, [6,6]-phenyl-C61 butyric acid n-hexyl ester (PCBC6) (Figure 13g), as the ETL to improve charge extraction and reduce nonradiative recombination. [192] The inverted FPSCs based on such ETLs obtained PCEs of 20.01% (0.09 cm 2 ) and 17.04% (1.08 cm 2 ) (Figure 13h,i).
In order to obtain high-performance inverted FPSCs, the optimization of ETLs is also crucial. An ideal electron transport material should possess the following advantages: excellent electron mobility, outstanding optical transmittance, and matched energy levels with adjacent functional layers. In addition to the above properties, the following factors should also be considered for ideal electron transport materials. For the sake of conductivity and stability, novel inorganic electron transport materials should be developed and ETLs layers could be fabricated by a vapor deposition. Further, PCBM is prone to molecular aggregation during dissolution process, which affects the performance of inverted FSPCs. Additive engineering may be an effective strategy to solve molecular aggregation. To facilitate more efficient electron extraction from the perovskite layer, an interfacial between ETL and perovskite layer is demanded. Flexibility and mechanical stability also need to be considered for an ideal electron transport material. Furthermore, ETL-free inverted FPSCs can be developed to improve stability and reduce cost.

Flexible Top Electrodes
The conductivity, work function, stability, and fabrication process conditions of top electrodes play key roles in the efficiency and long-term stability of inverted FPSCs. [74] For inverted FPSCs, top electrodes are usually deposited on ETLs or buffer layers by vacuum thermal evaporation. A summary of flexible top electrodes used in inverted FPSCs is illustrated in Table 6.
Currently, top electrodes for inverted devices are mainly metal or carbon materials. [125,[193][194][195] Krebs et al. used printing process to scale up inverted FPSCs under ambient conditions. [193] The inverted FPSCs with Ag as top printed electrodes achieved a PCE of 4.9%. To avoid the potential corrode of top electrodes by perovskites under continuous bending, exposure to air, thermal environment, or light, resulting in a dramatic drop in optoelectronic properties, Fu and co-workers employed Bi 2 Te 3 as the top electrodes by a simple thermal evaporation process to construct inverted FPSCs (Figure 14a). [194] The flexible devices achieved a high PCE of 18.16% with excellent mechanical flexibility and outstanding long-term operational stability (Figure 14b,c). Wojciechowski et al. fabricated efficient, stable, and flexible large-area inverted FPSCs utilizing carbon top electrodes (Figure 14d). [195] An ultrathin buffer layer between the ETL and carbon electrodes was introduced by them to improve the electronic contact. The inverted FPSCs with a large area of 1 cm 2 reached a PCE of 15.18% with excellent operational and thermal stability (Figure 14e,f ). [195] In inverted devices, the main function of the top electrode is to collect electrons from the electron transport layer. Hence, top electrode and electron transport layer should have Fermi levels that match as closely as possible. There are also other properties to be considered, such as high light transmittance, strong corrosion resistance, high environmental stability, and low cost. Especially for flexible devices, flexibility and mechanical stability also need to be considered. PET/ITO/PEDOT:PSS/Perovskite/P(NDI2DT-TTCN)/BCP/Au P(NDI2DT-TTCN) 17.0 2018 [191] PET/IZO/PTAA/Perovskite/PCBC6/BCP/Ag PCBC6 20.01 2020 [192] a) 3-aminopropyltriethoxysilane (APTS), N,N 0 -dimethyl-3,4,9,10-tetracarboxylic perylene diimide (PTCDI), Naphthalene diimide (NDI)-based polymer named P(NDI2DT-TTCN), [6,6] -phenyl-C61 butyric acid n-hexyl ester (PCBC6). Here, Section 2-7 have reviewed recent developments and challenges of different functional layers from bottom to top in inverted FPSCs, including flexible substrates, flexible bottom electrodes, HTLs, perovskite active layers, ETLs, and flexible top electrodes. Systematic optimization of each functional layer is conducive to obtaining high-performance inverted FPSCs. Nevertheless, the ultimate goal of FPSCs is commercialization. Therefore, in addition to optimizing functional layers, efforts  [39] Copyright 2015, Nature Publishing Group. d) Molecular structure of P (NDI2DT-TTCN). e,f ) J-V curves and mechanical stability of inverted FPSCs based on different ETLs. d-f ) Reproduced with permission. [191] Copyright 2018, Wiley-VCH. g) Molecular structure of PCBC6. h,i) A cross-section SEM image and J-V curves of PCBC6based inverted FPSCs. g-i) Reproduced with permission. [192] Copyright 2020, Wiley-VCH. should also be paid to develop technologies to accelerate the commercialization process of FPSCs. Next, we also focused on effective technologies for promoting device commercialization.

Effective Technologies for Commercialization
As a promising technology, the commercialization of FPSCs has always been one of the most concerned topics for researchers. Inverted FPSCs have advantages including high stability, and negligible hysteresis, etc., beneficial for commercialization in the future. However, to accelerate commercialization of inverted FPSCs, several issues need to be addressed: 1) development of uniform large-area coating technologies, 2) development of tandem or integrated device fabrication technologies, and 3) encapsulation technologies to ensure a long-term stability and reduce the lead leakage. In this section, we will discuss different techniques to facilitate the commercialization of inverted FPSCs.

Large-Area Inverted Flexible Perovskite Solar Cells
Inverted FPSCs are more promising for commercial applications than rigid ones due to the mass production of devices. The development of large-area inverted FPSCs and modules fabricated by R2R methods is significant for accelerating the commercialization of inverted FPSCs. At present, the fabrication of large-area FPSCs with both high efficiency and stability is still in its infancy. [37,78,[195][196][197][198][199][200][201][202][203][204][205][206] A summary of large-area inverted FPSCs is illustrated in Table 7.
The rupture of perovskite films and the loss of transmittance of substrates are the main factors affecting the fabrication of high-performance inverted FPSCs. Song et al. introduced a nanocellular scaffold to relieve mechanical stress during device bending to improve film quality. [196] The inverted FPSCs were constructed in modules to obtain a PCE of 12.32% (1.01 cm 2 ) with an impressive mechanical stability, which could be used as a wearable solar power source. [196] Besides, the natural brittleness of perovskite crystals limits their mechanical robustness for the fabrication of inverted FPSCs. To avoid this, researchers reported high-quality perovskite films with an elastic "brick-andmortar" structure through a biomimetic crystallization process to resolve the "cask effect." [197] As a result, they achieved inverted FPSCs (1 cm 2 ) with an efficiency of 15.01% and an outstanding mechanical robustness, and a large-area flexible wearable power source (56.02 cm 2 ) with a relatively high PCE of 7.91%. [197] A flexible device utilizing a fluorosurfactant doped network PEDOT: PSS as the bottom electrode was reported to fabricate highperformance large-area inverted FPSCs, which showed a PCE of 10.9% (25 cm 2 ) with excellent mechanical and long-time stabilities (Figure 15a-c). [198] The translation of FPSCs from small-area devices to large-area modules results in a huge performance loss, to solve this problem, a PEDOT:EVA (EVA, poly(ethylene-co-vinyl acetate)) interface layer was introduced between the flexible bottom electrodes and perovskite layers to construct large-area inverted FPSCs (Figure 15d). [37] It was found that this biomimetic interface layer both controlled the crystallization and acted as a binder, which facilitated the inverted FPSCs to achieve efficiencies of 19.87% and 15.17% for 1.01 and 36 cm 2 areas (Figure 15e,f ), respectively, with enhanced flexibility and high mechanical stability. Moreover, they fabricated a wearable solar-power source utilizing a module (36 cm 2 ). [37] Im and co-workers employed lithium bis(trifluoromethane)sulfonimide (LiTFSI) doped GO as the bottom electrode to construct inverted FPSCs. Thanks to the  [194] Copyright 2019, American Chemical Society. d) Device structure of inverted FPSCs based on carbon electrodes. e) J-V curves of inverted FPSCs with different electrodes. f ) Evolution of normalized photovoltaic parameters of inverted FPSCs aged inside the glovebox at 85°C. d-f ) Reproduced with permission. [195] Copyright 2020, American Chemical Society. adjusted work function and work sheet resistance, the flexible devices produced a PCE of 19.01% (1 cm 2 ) with high bending and long-term photostability. [199] The low stability and toxicity caused by poor polymer encapsulation also limit the application of large-area inverted FPSCs, Chen and co-workers introduced diphosphatidyl-glycerol (Di-g) (Figure 15g) as the self-shield interface on perovskite layers to suppress lead leakage and block water, resulting in FPSCs with a PCE of 20.29% and 15.01% at areas of 1.01 and 21.82 cm 2 , respectively (Figure 15h,i), with outstanding mechanical and environmental stability. [202] Carlo et al. exploited a blade-coating process to deposit PTAA and perovskite layers in MA-free inverted FPSCs, and the resulted devices showed a PCE of 10.51% at an active area of 15.7 cm 2 with an outstanding light stability (Figure 15j-l). [200] With advantages of suitable bandgap, high transmittance, and good hole mobility, V x Ni 1Àx O was used to replace NiO x as HTLs inverted FPSCs, as reported by Seo and co-workers, achieving an efficiency of 18.75% (1.2 cm 2 ). [205] Although a lot of breakthroughs have been accomplished for large-area FPSCs, there are still several issues to be solved. Due to the rough surface of flexible substrates, it is difficult to deposit high-quality perovskite films, leading to an increase of electrode series resistance and film defect density, and hence a lower efficiency for large-area inverted FPSCs. Therefore, it is necessary to develop new fabrication processes of perovskite active layers to improve the efficiency of large-area FPSCs. In addition, new technologies need to be explored to connect subcells in series to produce FPSCs modules. Simultaneously, the stability of largearea FPSCs should also be paid enough attention by researchers.

Scalable Deposition Techniques
To date, many great achievements have been made on large-area inverted FPSCs as discussed earlier. However, most reported  [37] Copyright 2020, Nature Publishing Group. g) Molecular structure of Di-g. h) Device structure of inverted FPSCs with Di-g at interface. i) J-V curves of inverted FPSCs with and without Di-g. g-i) Reproduced with permission. [202] Copyright 2021, Wiley-VCH. j) JÀV curves and 5 min stabilized efficiency (maximum power point (MPP) tracking) of the best (15.7 cm 2 ). k) Structure of FPSCs (top) and perovskite deposition techniques (down). l) Light beaminduced current (LBIC) map of the fabricated modules. j-l) Reproduced with permission. [200] Copyright 2021, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com large-area inverted FPSCs were fabricated by spin coating strategy. Although useful for laboratory-scale devices, the spincoating strategy is obviously incompatible with continuous production. In addition, there are also problems of large-area uniformity and material losses in spin-coating process. Therefore, the fabrication technology urgently needs a scalable process with effective quality control in order to reduce costs and prepare inverted FPSCs with long-term stability and environmental protection. The R2R process can continuously produce photovoltaic devices in an efficient and low-cost manner by combining various technologies to accelerate the commercialization of FPSCs. Scalable disposition techniques compatible with the R2R process can be divided into coating or printing techniques, such as blade coating, slot-die coating, spray coating, gravure printing, screen printing, and inkjet printing. A summary of scalable deposition techniques is illustrated in Table 8.

Coating Techniques
The coating techniques can prepare a continuous wet film by moving the coating head of ink along the web direction with no direct contact between the coating head and the web. Coating techniques have been widely used to fabricate FPSCs owing to their high processing speed and accurate control of the thickness of required layer. [86,200] The quality of the final film fabricated by coating techniques depends on the properties of substrate and precursor, as well as the manufacturing process parameters. [207] According to different coating heads, coating techniques can be divided into blade coating, slot-die coating, and spray coating, etc. Among coating techniques, blade (including knife, shearing, and meniscus) coating refers to the method of sliding the preprepared ink into a wet film on the web with a moving blade. [86,204,206] A pictorial description of the process can be seen in Figure 16a. [208] Blade coating has been widely used in the field of inverted FPSCs. In inverted FPSCs with NiO x as the HTL, many defects might exist on the film surface due to the poor printability of NiO x and the enrichment of Ni 3þ compounds on the surface. [204] Chen and co-workers utilized an in situ double-sided passivation strategy for NiO x by simply soaking them in hydrogen iodic acid (HI) during R2R process. The flexible devices fabricated by meniscus-coating technique delivered PCEs of 19.04% (1 cm 2 ) and 16.15% (15 cm 2 ) (Figure 16b). [204] In order to achieve uniform crystallization and stress release of perovskite films in inverted FPSCs, Yang et al. introduced a liquid crystal 4((6(acryloyloxy)hexyl)oxy)benzoic acid (6OBA) with liquid fluidity and crystal orientation into the precursor to fabricate high-quality perovskite films. [206] The inverted flexible devices fabricated by blade-coating technique exhibited PCEs of 19.87% (1 cm 2 ) and 14.74% (25 cm 2 ).
The slot die coating is similar to the blade coating, but it possesses more advantages in film formation, uniformity and ink management, which can realize a continuous, R2R compatible deposition of a variety of ink. [207] The film thickness is related to ink concentration, volume, moving speed, and substrate width. In order to produce high-quality films in air through the R2Rcoating process, Gao et al. prepared perovskite films in air utilizing slot die coating method by combining blowing-assisted drop-casting (BADC) with additive engineering (NH 4 Cl) (Figure 16c). [209] The device fabricated on flexible substrate achieved a PCE of 11.16%. Kim and co-workers used a hot slot die coating technique to produce high-performance inverted FPSCs through the R2R process in air. They utilized the developed 3D printer-based slot die coater to optimize the deposition parameters, and the prepared flexible device exhibited a PCE of 11.7% (Figure 16d). [210] Antisolvent engineering is one of the most effective strategies for preparing high-quality perovskite films, Lee and co-workers employed the antisolventassisted R2R slot die coating process to prepare uniform and pinhole-free perovskite films (Figure 16e). [211] Compared with other coating techniques, spray coating relies on the formation of ink droplets. [207] A pictorial description of spray coating can be seen in Figure 16f. [208] However, spray coating has low requirements for ink, so it is convenient to employ inks with various rheological and viscosity. Spray coating has been used to fabricate inverted FPSCs due to its low cost, large volume, and high manufacturing speed. Brunetti and co-workers successfully fabricated TCO-free inverted FPSCs by spray coating PEDOT: PSS (PH1000) modified by ethylene glycol (EG) with high boiling point. [133] The flexible device exhibited a PCE of 4.9% and excellent mechanical robustness. In addition, Ag NWs have also been deposited by spray coating process to prepare inverted FPSCs. [102] 8.

Printing Techniques
Printing techniques refer to the processes of transferring the ink prefilled in the engraving or cavity of a specific pattern to the required web through direct contact, except for ink-jet printing. [207] Printing techniques can design and fabricate modules with various shapes and sizes. Additionally, the printing techniques usually require less ink than the coating techniques, and the properties of ink are extremely important for printing quality. [207] Printing techniques can be divided into screen printing, gravure printing, and inkjet printing, etc. The screen printing is applicable to the printing of highly conductive films and displays a high printing speed. Screen printing includes flat bed screen and rotary screen printing. The flat bed screen printing utilizes a squeegee to move at the top of the flat bed screen, so that the ink can pass through the mesh mouth to obtain the desired pattern. [212] Rotary screen printing uses a squeegee to move relative to the rotary screen, so that the ink passes through the opening of the mesh to obtain the desired pattern. [212] Krebs and co-workers studied two fully scalable and R2R compatible preparation processes for the production of FPSCs. The researchers first introduced screen printing electrodes (Figure 16g), and then manufactured inverted FPSCs on a roll coater by utilizing a slot-die coating on a flexible PET substrate. [193] The one-step fabricated flexible device exhibited a PCE of 4.9%.
The gravure printing is based on the gravure roller to transfer the ink from the groove to the pressed substrate through its engraving cavity. A pictorial description of gravure printing can be seen in Figure 16h. [213] Gravure printing is a highthroughput process with high-resolution patterns, but the microcavities in the carved patterns make the printed layer a rough surface. [207] In addition, the factors such as ink rheology and the pressure of gravure cylinder on the substrate have a great impact on the printing quality, so it is necessary to increase the systematic optimization of the ink surface tension. [213] Gravure printing can be easily transferred to R2R process to fabricate FPSCs.
Inkjet printing is a noncontact deposition method without any mask. A pictorial description of inkjet printing can be seen in Figure 16i. [208] Since the nozzle can be moved arbitrarily, inkjet printing exhibits the advantages of accurate spatial positioning and high pattern resolution. [207] Compared with other templatebased, inkjet printing can coat large areas at a slower rate, resulting in relatively low yields. In addition, restrictions on low viscosity inks that can form droplets may be prohibitive for certain layers of inkjet printing, while high viscosity inks can cause nozzle clogging failures. [213] Meng and co-workers successfully fabricated large-area MAPbI 3 -based inverted FPSCs based on full inkjet printing, including bottom electrode (Ag NW S ), HTL (PEDOT:PSS), perovskite layer, ETL (PCBM), and top electrode (Ag NW S ). [214] The flexible device with large area of 120 cm 2 exhibited a high PCE of 16.78%.
Although considerable efforts have been made to achieve R2R production of inverted FPSCs by scalable deposition techniques, there are still several issues to be resolved. The long-term stability and mechanical durability of inverted FPSCs fabricated by R2R process have not been effectively evaluated. Therefore, one key issue that needs to be carefully studied is the influence of various factors on long term and mechanical durability. In addition, more flexible and cost-effective transparent conducting electrodes need to be developed to replace traditional ITO or IZO. In order to obtain ideal device performance and production efficiency, parameters such as boiling point, viscosity, toxicity of perovskite solvent, and environmental compatibility of materials must also be considered. Moreover, the coating or printing temperature, environmental humidity, and other process factors that affect the quality of perovskite film need to be optimized. Also, the roll speeds of R2R process should be ideally 10 m min À1 or more to ensure productivity.

Tandem Inverted Flexible Perovskite Solar Cells
Compared with single-junction inverted FPSCs, tandem solar cells are preferred for commercialization due to their high efficiency. [42] A higher efficiency can be achieved by stacking perovskites with traditional photovoltaic materials (silicon or CIGS), or stacking two perovskites with complementary band gaps to expand the light absorption spectrum. [44,47] Among tandem solar cells, multijunction all-PSCs combine low thermalization losses in multijunction structures with the favorable properties of perovskites, and thus possess the advantages of compatibility with flexible substrates, low cost, and mass fabrication. For all-perovskite tandem solar cells, Pb-Sn mixed perovskites with a narrow bandgap are usually required, which limits the utilization of metal oxide charge transport materials due to the induced oxidation of Sn 2þ . Therefore, inverted structures utilizing organic HTLs such as PEDOT:PSS are preferred for all-perovskite tandem solar cells. A summary of tandem inverted FPSCs is illustrated in Table 9.
As one of the simplest double-junction tandem solar cells, twoterminal architecture (2T) exhibits a higher potential efficiency limit due to the reduction of two transparent contacts and potentially one substrate by integrating all elements of the two subcells more closely together. [215,216] However, the conductive composite layer of ITO used in 2T tandems might lead to a shunting of the lateral connections produced in each subcell; [217,218] meanwhile, the perovskite phase separation in the wide bandgap (WBG) subcell might cause voltage loss, resulting in lower voltages in 2T tandem solar cells. [219] To solve these problems, Moore et al. employed a functional ultrathin polymer of poly(ethylenimine) ethoxylated (PEIE) as the nucleation layer to prepare a conformal, low-conductivity conductive composite film by atomic layer deposition (ALD) to reduce shunting and solvent degradation on the existing perovskite layer (Figure 17a). [77] Next, researchers tuned the WBG of the perovskite active layer to achieve high and stable voltage by adjusting the A-site cation composition using dimethylammonium (DMA). Through above efforts, 2T architecture inverted tandem FPSCs reached a remarkable PCE of 21.3% (Figure 17b-d) with an excellent stability. [77] In order to suppress interfacial defects and further improve the efficiency of inverted tandem FPSCs, Tan and co-workers constructed a molecularly bridged interface by anchoring a mixture of two hole-selective molecules on a low-temperature-treated NiO nanocrystalline film (Figure 17e), which improved the energy-level distribution, inhibited the interfacial recombination, and facilitated the hole extraction capability. [78] As a result, the inverted tandem FPSCs exhibited a super high efficiency of 24.7% (certified 24.4%) and 23.5% for areas of 0.049 and 1.05 cm 2 , respectively, with an outstanding bending durability (Figure 17f-h), which was the highest efficiency of flexible photovoltaics reported so far. [78] Recently, Fu and co-workers exploited self-assembled monolayers as hole-selective contacts to effectively inhibit interfacial recombination and produced high-quality perovskites with a WBG of 1.77 eV (Figure 17i). [79] Then, the researchers treated the WBG cells with 2-thiophene ethyl ammonium chloride (TEACl) to inhibit the bulk and interfacial recombination, which further increased the V OC of the WBG cells to 1.29 V (Figure 17i). www.advancedsciencenews.com www.small-structures.com The first proof-of-concept four-terminal all-perovskite inverted tandem FPSCs also achieved an efficiency of 22.6% and those with 2T architecture delivered a high PCE of 23.8% with a superior V OC of 2.1 V (Figure 17j-l). [79] However, it should be pointed out that the low conductivity and low-temperature fabrication process of flexible substrates greatly limit the fabrication conditions of high-performance inverted tandem FPSCs. At present, many breakthroughs have been achieved in tandem inverted FPSCs. However, there are still several obstacles to the rapid development of tandem inverted FPSCs, including how to fabricate high-performance large-area tandem devices and how to improve the long-term stability of devices. In tandem inverted FPSCs, scalable techniques including inkjet printing, slot die coating, doctor blading, spray coating, and low-temperature vapor deposition should be integrated in fabrication process. In addition, advanced encapsulation technology is the key to improve the long-term stability of tandem devices.

Integrated Inverted Flexible Perovskite Solar Cells
In inverted tandem FPSCs, the two subcells are generally connected as a whole by a recombination layer, but the appropriate energy-level alignment and high light transmittance need to be considered comprehensively, which pose a great challenge for the manufacture of tandem cells to achieve an optimal performance. Recently, the novel integrated perovskite/bulk heterojunction (BHJ) organic solar cells (OSCs) have been developed, which has attracted much attention due to its excellent performance without any recombination layer. [220,221] Perovskites with a WBG absorb high energy photons while allowing low energy photons to pass through and being absorbed by organic BHJ layers with a low bandgap to improve utilization of photon. This facilitates the increase of photocurrent without sacrificing V OC of perovskite devices. [222] A summary of integrated inverted FPSCs is illustrated in Table 10.
Lee and co-workers fabricated integrated solar cells by combining MAPbI 3 perovskite and near-infrared (NIR) absorbent organic BHJ. [223] The BHJ was optimized with an n-type polymer (N2200) as an electron transport enhancer in combination with diphenyl ether (DPE) as a solvent processing additive, resulting in a well-distributed bicocontinuous network and an increased electron mobility. The resulted integrated inverted FPSCs showed a significantly increased J SC (17.61 to 20.04 mA cm À2 ), while maintaining high FF (77%) and V OC (1.06 V) to obtain a remarkable efficiency of 12.98% (Figure 18a-c). [223] Jen and Figure 17. Tandem inverted FPSCs. a-d) Cross-sectional SEM, J-V curves, normalized EQE spectra, and photograph of all-perovskite 2T tandem device. a-d) Reproduced with permission. [77] Copyright 2019, Elsevier. e,f ) Device structure and normalized EQE spectra of inverted tandem FPSCs. g,h) J-V curves of inverted tandem FPSCs (0.049 and 1.06 cm 2 ). e-h) Reproduced with permission. [78] Copyright 2022, Nature Publishing Group. i) Schematics of the 4T all-perovskite flexible solar cells. j) J-V curves for 4T all-perovskite flexible solar cells. k) A FIB-SEM image for 2 T all-perovskite flexible solar cells. l) J-V curve for the best-performing 2 T all-perovskite flexible solar cells. i-l) Reproduced with permission. [79] Copyright 2022, Wiley-VCH.
www.advancedsciencenews.com www.small-structures.com co-workers used a ternary blend of PM6:CH1007:PCBM as the BHJ to extend the photo-response beyond 950 nm to fabricate integrated perovskite/BHJ solar cells (Figure 18d). [85] Here, CH1007 acted as an NIR photon collector to further redshift the spectrum to over 950 nm, and the donor polymer of PM6 promoted exciton dissociation of BHJ and improved stability of devices. As a result, the integrated inverted FPSCs achieved a champion PCE of 21.73% (Figure 18e,f ) with superior long-term device stability and high mechanical-fatigue endurance, which were also employed as a power source for wearable sensors. [85] At present, it is still a challenge to extend the NIR spectral response and increase the UV utilization. Song et al. extended the NIR response to 1100 nm utilizing a mixture of CoTIC-4 F:PC 61 BM:PTB7-TH ternary organic BHJ and Au nanotriangles on PSCs (Figure 18g), which also favored charge transport, defect passivation, and moisture blocking. [224] Next, CsPbCl 3 :Yb 3þ , Ce 3þ , Cr 3þ nanophosphors with an excellent down-conversion ability were prepared and then selfassembled with polymethyl methacrylate (PMMA) to be used below Nanotubes. Figure 18. Integrated inverted FPSCs. a) EQE spectra of the optimized control PSC and integrated solar cells. b,c) J-V curve and EQE spectrum of a flexible integrated solar cell. a-c) Reproduced with permission. [223] Copyright 2016, Wiley-VCH. d-f ) Device structure, J-V curve, and EQE spectra of flexible hybrid PSCs. d-f ) Reproduced with permission. [85] Copyright 2021, Wiley-VCH. g) Device structure of the hybrid device. h) J-V curves of flexible devices after flexing at a curvature radius of 0, 14, 12, and 6 mm for 1000 cycles. i) The PCEs of flexible devices at a bending curvature radius of 4 mm for 3000 cycles. g-i) Reproduced with permission. [224] Copyright 2022, Wiley-VCH.
www.advancedsciencenews.com www.small-structures.com the perovskite layer to improve the utilization of ultraviolet light. Through above efforts, the integrated inverted FPSCs with an improved NIR and UV spectral response delivered a high PCE of 20.71% with a J SC of 23.98 mA cm À2 (Figure 18h) and exhibited an excellent long-time stability (Figure 18i). [224] Integrated devices have great potential to further improve efficiency of inverted FPSCs by combining the advantages of PSCs and near-infrared BHJ OSCs. [220] However, there are still many challenges to be solved for integrated inverted FPSCs. Until now, only a few suitable NIR BHJ materials have been used to fabricate integrated devices, which limits the development of integrated inverted FPSCs. Therefore, it is necessary to develop novel low bandgap NIR donor or acceptor materials with matched energy level and high mobility. Additionally, the effect of hydrophobic BHJ materials on device stability should be explored.

Semitransparent Inverted Flexible Perovskite Solar Cells
Compared with opaque FPSCs, semitransparent FPSCs are capable of combining some special applications of light transmission and solar energy collection. Semitransparent FPSCs are ideal candidates for multifunctional integrated buildings and vehicles owing to their excellent visible transparency. [225] Moreover, semitransparent FPSCs are suitable for the preparation of the top cell in four-terminal tandem flexible devices due to their translucency and adjustable band gaps. [226] Therefore, semitransparent FPSCs have broad application prospects in multifunctional integrated devices and tandem flexible devices and are worthy of further study. The realization of semitransparent FPSCs requires that all functional layers such as electrodes, perovskites, and carrier transport materials have certain light transmittance. For semitransparent inverted FPSCs, the current researches have mainly focused on the flexible substrates or electrodes to prove the excellent photovoltaic performance of the devices. [125,[226][227][228] A summary of integrated inverted FPSCs is illustrated in Table 11.
Although the PCE of semitransparent inverted FPSCs has been greatly improved by optimizing perovskite layers, it is still a challenge to provide electrodes with high transparency and conductivity for semitransparent inverted FPSCs. Sun and co-workers used Au as bottom electrode and dielectric/metal/ dielectric (DMD) as top electrode to fabricate efficient ITO-free semitransparent inverted FPSCs. [227] DMD consists of bottom Next, they employed MoO 3 /Au as bottom electrode and b-MoO 3 /Au/Ag/t-MoO 3 /Alq 3 as top electrode to fabricate semitransparent inverted FPSCs (Figure 19a-c). [125] The flexible devices exhibited a PCE of of 6.96% and an AVT of 18.16% under the wavelength of 380-790 nm.
The key to fabricate semitransparent inverted FPSCs is to develop the transparent electrode as the top layer without damaging the perovskite layers. Park et al. adopted a dry stamping transfer technology to directly deposit PEDOT:PSS (PH1000) electrodes on PET/ITO/PEDOT:PSS (Al4083)/perovskites/ PCBM and fabricated semitransparent inverted FPSCs (Figure 19d,e). [228] The flexible device showed a PCE of 13.6% (1 cm 2 ) and excellent mechanical bending stability. In order to further improve the performance and stability of semitransparent inverted FPSCs, Kim and co-workers constructed efficient and stable semitransparent inverted FPSCs utilizing polyimide integrated GR electrode by a lamination assisted double-sided cation exchange strategy (Figure 19f-h). [226] The semitransparent inverted FPSCs exhibited a PCE of 15.1% and excellent long-term stability and mechanical durability.
Although the performance of semitransparent inverted FPSCs has made great progress, their efficiencies are still much lower than that of opaque inverted FPSCs. Performance optimization of semitransparent inverted FPSCs focuses on PCE and AVT, which affect each other in reverse, making their design more challenging than traditional opaque inverted FPSCs. According to the meric of light utilization efficiency (LUE), a proper balance between PCE and AVT is the key to achieving high-performance semitransparent inverted FPSCs. Thus, both photon propagation management and carrier dynamics regulation are crucial for improving the performance of semitransparent inverted FPSCs.
In addition, more attention should also be paid to color regulation considering the practical application of semitransparent FPSCs. Color rendering index (CRI) and CIELAB color coordinates (a*, b*) are two key parameters for color regulation of semitransparent FPSCs. [225] Therefore, the above metrics should be comprehensively evaluated before the practical application of semitransparent inverted FPSCs.

Device Encapsulation Technology
Although the inverted FPSCs have made significant improvements in efficiency, the lead leakage and inherent instability of perovskites have severely hampered the commercialization of these devices. In this case, an encapsulation technology is a feasible and common strategy to extend the service lifetime and reduce toxicity from lead leakage of inverted FPSCs. Rigid PSCs are usually encapsulated with glass sheets and epoxy resins, while FPSCs require lightweight, flexible encapsulated materials. This results in a great challenge to employ suitable materials for effective encapsulation of FPSCs. [39,[229][230][231][232] Kaltenbrunner et al. utilized PU resin to effectively encapsulate the device to improve the long-term stability. [39] Hayase and coworkers presented a novel encapsulation technique for sealing inverted FPSCs inside a cylindrical glass tube (R = 8 mm) (Figure 20a). Surprisingly, the encapsulated flexible device retained 90% of the initial PCE after 6000 h under ambient environment, reflecting the important role of the encapsulation process in improving the long-term stability of flexible device. [229] The long-term stability of flexible devices requires appropriate encapsulation to prevent degradation caused by environmental factors. Wojciechowski and coworkers reported a holistic encapsulation protocol for large-area inverted FPSCs, where a barrier foil was combined with appropriately selected adhesive materials, effective edge sealants, and an additional protective buffer layer (Figure 20b,c). [231] The reliability of the devices was assessed by three different aging tests of damp heat, thermal cycling, and humidity freeze, which complied with IEC norms for established photovoltaic technologies (IEC 61215, IEC 61646). Next, researchers demonstrated the universality of the applied protocol by studying the stability results of different PSCs structures (p-i-n and n-i-p device configurations with metal and carbon back contact electrodes) (Figure 20d), thus providing a compellent feasibility assessment of the long-term reliability in outdoor environments. [231] In addition to the inherent instability of perovskites in water, oxygen, and UV light, lead leakage is a serious threat to the ecosystem and human health, which is also an obstacle to the commercialization of PSCs. In order to solve the lead leakage problem, Zhu and co-workers developed a simple and economical encapsulation process utilizing a mixture of cation exchange resin (CER) and UV resin as encapsulation agent on the metal surfaces of both rigid and flexible PSCs, which effectively captured the leaking lead ions (Figure 20e). [232] As a result, more than 90% of the degraded Pb 2þ can be captured by the encapsulation agent employing the interaction between CER and Pb 2þ under simulated severe weather conditions (Figure 20f,g), providing an effective strategy to solve the problem of lead leakage in flexible devices and facilitate the commercialization. [232] Figure 19. Semitransparent inverted FPSCs. a) Structure of the inverted FPSCs. b) Absorbance and transmittance spectra of perovskite films. a) Photographs of semitransparent inverted FPSCs. a-c) Reproduced with permission. [125] Copyright 2017, Optical Society of America d-f ) Device structure, J-V curve, and EQE spectra of flexible hybrid PSCs. d) Schematic illustration of the dry stamping transfer process of the PEDOT:PSS top electrode layer for semitransparent FPSCs. e) Photographs of semitransparent inverted FPSCs. d,e) Reproduced with permission. [228] Copyright 2020, American Chemical Society. f ) Device structure of inverted FPSCs. g) Cross-sectional SEM image of inverted FPSCs. h) Digital image of semitransparent inverted FPSCs. f-h) Reproduced with permission. [226] Copyright 2022, American Chemical Society. Despite many achievements have been achieved for FPSCs, long-term stability is one of the most critical challenges for their commercialization. The internal instability of perovskite devices and their poor resistance to various harsh environments are two main factors that restrict the long-term stability of flexible devices. Hence, a deep understanding of the intrinsic and extrinsic instability mechanisms is needed for inverted FPSCs. In addition, the ability of FPSCs to resist harsh natural environments, including rain, hail, sandstorm, and sun exposure, should be fully considered. Next, the selection of multifunctional encapsulation materials and the optimization of encapsulation process will be the focus in the future.
The toxicity of lead is another big issue restricting the commercialization of FPSCs. Unfortunately, the utilization of physical or chemical encapsulation is insufficient to address the issue. For physical encapsulation, it is urgent to explore novel selfrepairing polymers as encapsulated materials to suppress lead leakage from damaged FPSCs. For chemical encapsulation, some impact-resistant, self-healable, and light-weight lead capture materials should be fully developed and validation completed to solve the issue of lead leakage caused by special natural environment. Furthermore, lead capture materials that can be used as functional layer components should also be developed to reduce lead leakage from inverted FPSCs. Although the use of physical or chemical encapsulation alone is insufficient for preventing lead leakage, it is a good method to combine physical and chemical encapsulation to suppress lead leakage. More importantly, there must be a set of recycling procedures after the end of the operational lifetime of encapsulated FPSCs, to avoid lead waste and environmental pollution.

Conclusion and Outlook
At present, commercial flexible solar cells have some limitations in practical applications, such as flexible amorphous silicon solar cells holding low efficiency and poor stability, flexible gallium arsenide solar cells possessing high cost, and flexible copper indium gallium selenium solar cells displaying low efficiency and high cost. PSCs have attracted great interests because of their high efficiency and easy processing over large areas. Compared with other flexible solar cells, FPSCs have the advantages of high efficiency, low cost, and simple fabrication process, etc. More importantly, FPSCs possess excellent flexibility, high powerper-weight ratio, relatively low manufacture cost and mass production by R2R processing techniques, which can be used for the preparation of light, wearable and portable electronic devices, showing great application potential. Among FPSCs, inverted Reproduced with permission. [229] Copyright 2019, The Royal Society of Chemistry. b) Pictures of an encapsulated FPSC employing a silicone-based edge sealant. c) Schematic illustration of the perovskite solar cell encapsulation stack. d) Time evolution of normalized PCE of the encapsulated FPSCs. b-d) Reproduced with permission. [231] Copyright 2022, Wiley-VCH. e) Molecule structure of the cation-exchange resin before and after reacting with Pb 2þ . f ) Pb leaching comparison of a flexible perovskite solar module with the UVR and UVR-C as an encapsulant. g) Pb leaching comparison of a perovskite thin film on glass with UVR and UVR-C. e-g) Reproduced with permission. [232] Copyright 2022, Elsevier. FPSCs possess the advantages of negligible hysteresis, reliable operational stability, and low-temperature preparation process. In this review, we have summarized the developments and fundamental challenges in the bottom-up functional layers of inverted FPSCs and technologies to accelerate device commercialization. Significant progresses have been made so far in the development of inverted FPSCs focusing on the chemical properties, photovoltaic properties, fabrication technologies of functional layer materials, and working mechanisms of devices, as well as the deep exploration of technologies for accelerating commercialization. Up to now, under the premise of flexibility and stability, the PCE of single-junction and tandem inverted FPSCs have reached 21.76% and 24.7%, [78,86] respectively, with many breakthroughs being achieved, suggesting strong prospects for commercial applications. However, despite such achievements, there are still many issues that need to be addressed before implementing in practical commercial applications, including the improvement of PCE, stability, and flexibility, as well as the reduction cost and toxicity. For inverted FPSCs, PCE is a key parameter to be considered. However, the PCEs of single-inverted FPSCs are lower than that of the regular and rigid ones. In order to obtain inverted FPSCs with enhanced performance, the functional layers including substrates, bottom electrodes, HTLs, perovskite active layers, ETLs, and top electrodes should be systematically optimized. For substrates, PET and PEN are the most commonly used substrates, while they need to be processed at low temperatures due to the relatively poor heat resisting property. Hence, the factors such as light transmittance, mechanical durability, thermal and chemical resistance, and water and oxygen resistance of the used substrates should be considered comprehensively. For flexible bottom electrodes, ITO was the most used electrode material in high-efficiency inverted FPSCs, but the rigid characteristics of ITO would lead to cracks in the film. Therefore, more efforts should be focused on the exploration of novel high-performance ITO-free electrodes. For HTLs, NiO x deserves further attention as a stable and well-performing candidate, but more efforts should be taken to optimize it to improve performance. For perovskite layers, full coverage without pinholes, uniform morphology, highly oriented crystal structure, and suitable bandgap are important factors to be considered for achieving FPSCs with high efficiencies. Fullerenes and their derivatives (i.e., C 60 , PCBM) are the most widely used ETLs, but the price is high, so it is necessary to exploit cheap and efficient ETLs. As for top electrodes, the conductivity, work function, stability, and fabrication process conditions are all factors to be considered comprehensively. Recently, intensive studies have been conducted on tandem or integrated devices, and PCEs of 24.7% and 21.73% were achieved for all-perovskite inverted tandem and integrated FPSCs, [78,85] respectively, showing a strong application prospect. Flexible tandem devices such as flexible PSC/CIGS, flexible PSC/PSC, or flexible PSC/OPV and flexible integrated devices might draw increasing research interests in future studies. In addition, the efficiency of large-area FPSCs module also needs to be improved by optimizing fabrication process.
The stability is another key issue for inverted FPSCs, which determines whether they can be commercialized in the future. For mechanical stability, the electrodes used in inverted FPSCs are the main reasons for their low mechanical stability.
As discussed, some studies have demonstrated that the mechanical stability of flexible devices can be improved by utilizing TCOfree electrodes, but the efficiencies of these inverted FPSCs are still quite low. The advantages of different electrodes can be combined by fabricating composite electrodes to obtain devices with high efficiency and mechanical stability. Due to the excellent mechanical and environmental stability, GO and carbon materials are expected to replace traditional metal electrodes, where more attention should be paid in the future. However, large-area fabrication could be a limitation of GO in flexible devices. Furthermore, the cohesion energy (G c ) is another important factor for mechanical stability of inverted FPSCs. [233] Generally, PSCs possess a lower G c compared with other solar technologies.
Here, more efforts should be paid to enhance G c through extrinsic reinforcement and stress distribution. In addition, continuous bending will cause the accumulation of stress between the brittle perovskite and the adjacent functional layers, resulting in delamination between different layers and the formation of cracks. [234] Therefore, mechanical stability would be improved by utilizing multifunctional materials at the interface to form chemical bonds between the two layers. For environmental stability, high water transmittance of polymer substrates and ion diffusion in perovskites lead to poor environmental stability of flexible devices. 2D perovskites have been proven to offer superior ambient stability, and 2D inverted FPSCs could be prepared to inhibit ion migration and improve environmental stability. As a stable and well-performing HTL, NiO x was found also capable of improving device stability. Another possible method to solve such issues is the combination of high-quality encapsulation processes with the utilization of carbon electrodes to block the penetration of water. Flexible devices should be systematically encapsulated, including internal surface and interface encapsulation, internal grain boundary encapsulation, external device encapsulation, etc. Additionally, further research directions also include the stability of large-area FPSC modules.
Cost is another important factor for future commercialization of FPSCs. Low-cost manufacture of large-area inverted FPSCs and modules should be one of the next research priorities. For functional layer materials, inexpensive materials should be applied to replace expensive ITO as bottom electrodes; novel HTLs should be prepared by reasonable organic synthesis design for replacement of expensive PTAA and poly-TPD; new electron transport materials should be explored to replace the expensive materials of fullerenes and their derivatives (i.e., C 60 , PCBM) to reduce the cost of materials. HTL-free or ETL-free inverted FPSCs can also reduce manufacture cost by simplifying manufacturing processes. Moreover, the vacuum deposition process is beneficial to simplify the fabrication process, reduce the manufacture cost, and obtain high-quality films. Nevertheless, vacuum deposition is usually suitable for small-area devices, which is not suitable for large-scale production. R2R process has been considered as a feasible method to achieve mass production and commercialization of flexible devices. However, with the increase of perovskite film sizes, the morphology might get worse with increased defects within the film, resulting in a relatively low efficiency. Therefore, scalable techniques such as slot die coating, doctor blading, inkjet printing, and spray coating should be integrated in R2R process to fabricate one or more high-quality function layers and further reduce costs. Furthermore, it is necessary to develop large-area fabrication techniques for all kinds of functional layers in inverted FPSCs.
Flexibility is an important attribute of flexible devices in practical applications, so it is urgent to develop novel flexible substrates, transparent electrodes, and charge transport materials to meet the flexible requirements. Especially for flexible electrodes, the mechanical flexibility can be improved by reducing the thickness of electrodes. Additionally, the bending tolerance of inverted FPSCs should be studied in depth. The toxicity of Pb-based perovskites is also a major barrier to the application of portable and wearable electronics. More stable lead-free or lead-less inverted FPSCs can be developed by replacing Pb with Sn, Bi, and other nontoxic metal elements. For the sake of environmental protection and health, more efficient encapsulation techniques need to be further explored to prevent lead leakage.
Although inverted FPSCs still face many challenges toward commercialization, they have disruptive potential in photovoltaic applications owing to the combination of high efficiency and excellent flexibility and lightweight. At the current stage, tandem inverted FPSCs, integrated inverted FPSCs, semitransparent inverted FPSCs, and large-area inverted FPSCs will be the focus of studies. In short, in order to obtain highly efficient and stable inverted FPSCs, it is necessary to optimize all aspects of the functional layers to overcome the challenges. Given the current breakthroughs in inverted FPSCs, we believe that inverted FPSCs will play a major role in future flexible devices market and pave the way for new applications in wearable and portable electronics, mobile energy systems, unmanned systems, space energy systems, aerospace systems, and multifunctional-integrated buildings.