Hua Kong , Wentao Sun, , and Huanping Zhou

1Schoolof MaterialsScienceand Engineering, PekingUniversity, Beijing100871, China2Key Laboratory for the Physics and Chemistry of NanodevicesDepartment of Electronics, Peking University, Beijing 100871, China

Abstract: Perovskite solar cell has emerged as a promising candidate in flexible electronics due to its high mechanical flexibility, excellent optoelectronic properties, light weight and low cost. With the rapid development of the device structure and materials processing, the flexible perovskite solar cells (FPSCs) deliver 21.1% power conversion efficiency. This review introduces the latest developments in the efficiency and stability of FPSCs, including flexible substrates, carrier transport layers, perovskite films and electrodes. Some suggestions on how to further improve the efficiency, environmental and mechanical stability of FPSCs are provided. Specifically, we considered that to elevate the performance of FPSCs, it is crucial to substantially improve film quality of each functional layer, develop more boost encapsulation approach and explore flexible transparent electrodes with high conductivity, transmittance, low cost and expandable processability.

Key words: perovskitesolar cells; flexible electronics; thin film deposition; carrier transport

1.Introduction

The development of photovoltaics that convert sunlight into electricityisapromising strategyto meet the rapidly growing energy needs. In the past few decades, silicon-based photovoltaictechnology has madesignificant progresswithrespect to the cost and efficiency, which promoted clean energy production. Along with the advance of silicon solar cells,a variety of thin-film photovoltaics have been studied in order to further meet the application requirements of lightweight and low-cost. Especially in recent years, great interest inflexible andwearable electronic products has led to growing researchactivitieson flexibleand stretchablethin-film photovoltaic. Integrating these thin and soft solar collectors into walls, windows and portable objects will change the current way of energy production, reduce pollution, and greatly expand the usage scenarios and ways of obtaining energy.

Recently, the organic–inorganic hybrid perovskites have become the ideal candidate to develop flexible solar cells due totheir excellentoptoelectronicproperties.In particular,due to its excellent light harvesting ability,only about300 nmthickfilm issufficient toabsorbessentiallyall visiblelight above its band gap. Thus, ultra-thin and ultra-light solar cells can be prepared[1?8]. The structure and physical properties of organic–inorganic halide perovskites were first reported in 1978[9,10]. They have a cubic structure with the general formula of ABX3, where A is typically CH3NH3+(MA+) or NH2CHNH2+(FA+), BisPb2+orSn2+, andX is halogen anion(Cl–,Br–, orI–). Byadjustingthe composition of A,B andX in thecrystal,the bandgapof the metal halide perovskite can be continuously tunedinthe rangeof 1.15 to 3.06 eV.CH3NH3PbI3and CH3NH3PbI3–xClxare the most commonly studied absorbers in the early perovskite solar cells (PSCs) research. Since the CH3NH3PbI3perovskite material was first used as absorber in solar cells in 2009, the power conversion efficiency(PCE)of PSCs has rapidly increased from3.81% to a certified 25.5% within ten years development[5,7,8,11?16].Similar to rigid PSCs, flexible perovskite solar cells (FPSCs) have also made great progress with respect to device efficiency. Recently, the PCE of FPSCs has reached 21.1%[17], Fig. 1 shows thePCEimprovement of FPSCs from 2013 to2021 under variousoptimizationapproaches. However, theirpoor long-term stability and relatively lower PCE compared with rigid counterparts are the main challenges that limit its further commercialization.

Fig. 1. (Color online) ThePCE evolutionof FPSCsfrom2013 to 2021[17–26].

Fig. 2. (Color online) High-efficiency FPSC based on PEN and PET substrates. (a) Schematic diagram of the FPSC structure based on a perovskite layer doped with artemisinin. (b) J–V curves on rigid and flexible substrates with and without artemisinin doping[17]. (c) Scanning electron microscope characterization of thin film deposited on glass/fluorine-doped tin oxide (FTO) substrate[26]. (d) The room temperature sheet resistance of conductive PET/ITO, PEN/ITO, glass/ITO, and glass/FTO substrates after heat treatment at different temperatures for 30 min[29].

FPSCsare mainlycomposed of substrate (PEN,PET, etc.),bottomelectrode(FTO, ITO, etc.), electrontransport layer(ETL), perovskite active layer, hole transport layer (HTL), and the counter electrode (Au, Ag, etc.). At present, the typical FPSCs structure are sandwich structure, in which the perovskite active layer is in the middle, withelectron (or hole)blockinglayers andelectrodeson both sides.When theperovskite material is excited by light, it will produce photo-generated electron–hole pairs. Since the valence band maximum(VBM) of ETL is much lower than the VBM of perovskite materials, and their conductionbandminimum (CBM) is at the same position,ETL playsthe role of blockingholesand transporting electrons. The CBM of the HTL on the other side is much higher than the CBM of the perovskite material, and the positions of the VBM of them are the same, so the HTL playsthe role ofblocking electrons andtransportingholes.The ETL and HTLare connectedto theexternalcircuit through the electrodes on both sides, which can realize the directional transfer of charges to form current.

Table1. Performanceparameters of polymer substrate[28].

Here we analyze the factors that affect the performance of FPSCs, mainly lies in 1) the roughness of the flexible substrate affectsthe quality of the perovskitefilm, 2)the high resistanceandlowlighttransmissionoftheflexible substrate lead to thelowshort-circuitcurrent of thesolarcelldevices,3) the permeability of the flexible substrate to water and oxygen results in long-term instability, 4) the mechanical stress occurred during the device fabrication brings into the fracture of perovskite layers. Then we summarized the development offlexible substrates, carrier transportlayers,lightabsorptionlayers and electrodes to address the above concerns to improvethe performance of flexibleperovskitesolar cells.Finally, we discussed the packaging process of FPSCs to further prolong the device's lifetime.

2.Performance improvement of flexible substrates

The most obvious difference between the flexible substrate and rigid substrate is whether the resulting devices can be bent and stretched. Its bending performance is mainly determinedby thephysical properties ofthe flexiblesubstrate.Anexcellent flexible substrate is a basis for buildingahigh-performance flexibleperovskitesolar cell.Itshould have thefollowing characteristic. 1) Excellent mechanical properties, 2)Good optical properties: A suitable substrate should be transparent to the light absorption range of perovskite, particularly in the visible light region. 3) Good chemical properties: Becausea variety of chemical compounds areused in the preparation of FPSC, the substrate should have chemical stability. 4)Blocking the permeation of oxygen and water: The permeationofoxygen and wateris the main factor causing the performance degradation of perovskite solar cells. At present,thecommonlyused substrates for flexible perovskitesolar cells include polymer substrates, metal substrates, ultra-thin flexible glass substrates and other special materials, etc.

2.1.Polymer substrates

Polymersubstrates have been most widelyused inFPSC due totheir advantages of highlight transmittance,highflexibility, low cost, and good chemical stability[1,23,25,27]. The parametersof commonly used polymer substrates,suchas polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyimide (PI) and polydimethylsiloxane (PDMS), are shown inTable 1[28].Among them,PET and PEN are themost commonly used flexible substrates, which are further coated with transparent conductive indiumtinoxide (ITO)to serve as the electrode. So far, the efficiency of single-junction FPSCs using PEN and PET substrates has reached 21.0%[17,26].Yanget al.prepared an FPSC with a PCEof21.1% in 2020,and the device structure is PEN/ITO/HfOx/SnO2/perovskite:artemisinin/Spiro-OMeTAD/Au. The composition of the perovskiteabsorberis Rb0.05Cs0.05(FA0.83MA0.17)0.9Pb(I0.95Br0.05)3,doping with artemisinin molecules. Artemisinin molecules with carbonyl groupscan easilyinteractwithexposedsurface Pb2+ions through the Lewis acid-base interaction, which effectively passivates grain boundaries and surfaces (as shown inFigs.2(a) and2(b))[17]. Shiet al. preparedFPSC based on PET/ITO/SnO2/(CsPbI3)0.04[(FAPbI3)0.9(MAPbBr3)0.1]0.96/lowdimension perovskite/Spiro-OMeTAD/Au structure[26]. In this structure, the 2D perovskite is formed by in situ spin-coating 3-CBAI solution on the three-dimensional perovskite,whichcan effectively "seal" surface defects as shown in Fig. 2(c), obtain a PCE of 21.0%. However, there are some urgent issues in polymer substrate thatneedto besolved, such as its limited processing temperature tolerance, and the subsequent processingshouldbeperformedat lessthan 150°C.When elevating the processing temperatures to 150 °C, the sheet resistanceofplasticsubstrate will increaseaccompanying with substrate deformation, as shown in Fig. 2(d)[29]. Another key challenge inplastic substrates istheirpoor isolation from oxygen and moisture. This will further lead to severe device degradation,since perovskite materials are unstable in humid environments. Therefore, further packaging is required to improve the device stability.

Fig. 3.(Coloronline) (a)Schematic diagram of FPSCstructure basedon Ti foil.(b) J–V curvesof Au/Cu/HTM/CH3NH3PbI3/TiO2/Ti cellsunder 100 mW/cm2 AM 1.5Gsolar light with the same oxidizedthickness of TiO2 layer (~50 nm) basedonthe same ambience, air, withdifferentannealingtemperatures[35]. (c)FPSCcross-sectionSEMbasedon ultra-thinWillowGlasssubstrate[37].(d)Static contact angleof deionized water on PDMSlayers with differentaspect ratios. (e) Photographof aflexible perovskitemodule. (f) J–V curve of the champion flexibleperovskite modules[38].

2.2.Metal substrates

Comparedwith polymer substrates, metal foil has better thermal stability, flexibility and conductivity, which makes them promisingflexiblesubstrates for FPSCs. The metalfoilis generally used as both the substrate and the electrode, thus simplifyingthepreparationprocess. Flexible perovskitesolar cells need to have light transmission on at least one side, but theopacity of themetalfoil requires thetopelectrode on the other side to have light transmission.

Ti foils is the most commonly used metal foil substrate in FPSCs due to the compatibility of Ti foils with the subsequent growth of TiO2electron transport layer.In2015, Leeet al. used Ti foil as a flexible substrate and fabricated a flexible perovskite solar cell device with a Ti/compact TiO2/mesoporous TiO2/perovskite/Spiro-OMeTAD/Ag structurefor the first time, with an initial efficiency of 6.15%[30]. After that,they further used Ag nanowire grids and Ag embedded indiumtinoxide as the top transparent electrodeto increase the PCE of the device to 7.45% and 11.01%[31,32]. Watsonet al. reported that the Ti foil-based mesoporous FPSC using Al2O3mesoporous filmas the scaffold[33], usingex-situ prepared and laminated transparent electrodes, theyobtained a PCEas high as 10.3%. They also predicted the potential application of"self-generating" TiO2through the thermallyinduced in situ oxidationof titaniumfoil. Leeet al. proved thisconcept,and they used thermal oxidation on titanium foil to obtain TiO2, whichbecome theelectrontransport layerofFPSCs[34].By controlling the annealing temperature, the concentration of oxygen vacancies in TiO2is modulated, which significantly affectedtheelectron collectionefficiencyof thedevice. The TiO2obtainedinthe two-step annealing process of the Ti foil at 400 °C enabled the device to deliver a PCE of 14.9%. During this period, the second annealing step in an O2atmosphereisvery importantforremoving oxygen vacancies without dramatically increasing the thickness of TiO2. FPSC based on Ti foil exhibits excellent flexibility. After 1000 cycles under15 and 4 mmbendingconditions, the devicemaintains almost 100% of the initial PCE, and 77% of the initial PCEismaintainedeven after 1000 cycles under 1mmbend radius. They further found that the resistivity of the TiO2layer on the Ti foil did not change under the conditions ofR≈ 15 and4 mm, butit decreased significantlyunderthe conditions ofR≈ 1 mm. Cracks are proven to be generated in the surface-oxidized Ti foil due to bending at a low radius, and leakage current emergedon the inner surface ofthe cracks,which leads to a deterioration of device performance. In addition, TiO2nanotubes and nanowire arrays grown on Ti foil were usedto fabricate FPSC,and13.07% of PCE was obtained[35].

In addition to Ti foil, Cu foil is also used to prepare FPSCs.Ahmadiet al.prepared cuprous iodide onthecopper foil by exposing the pre-cleaned Cu foil to iodine vapor, and then useditastheholetransport layer ofFPSC. Usingzinc oxide as the electron transport layer and sprayed silver nanowires asthe top electrode, the maximum PCEof the device is 12.80%[36].

2.3.Ultra-thin flexible glass substrate

Ultra-thin flexibleglass substrates retainmostof theadvantages of rigid glass substrates, such as resisting high temperatures, blocking moisture and oxygen, and high light transmittance. Tavakoliet al.used50μmthick willow glass as a flexible substrate for the first time to prepare a flexible perovskitesolar cell with aPCEof12.06%[37]. Adding aPDMS anti-reflective nanostructure film on the front surface of the devicecan further boostthe efficiency to13.14%. At the same time, the prepared FPSC exhibits excellent stability because of the poor permeability of the flexible glass to oxygen and moisture, as shown in Fig. 3(d). Learned from this strategy, Douet al. produced an FPSC efficiency of 18.1% by using MgF2as the anti-reflection layer and replacing ITO with indium zinc oxide (IZO)onultra-thin flexible glass[39].

Fig.4. (Coloronline) (a) Flexibleperovskite devicediagram[18]. (b)J–V curve of FPSC based on ZnO prepared at lowtemperature. (c) Lightand dark J–V curves of FPSC[41].(d) J–V curveunderdifferent ALD cycles. (e)Optimized FPSC structure and itsJ–V curve. (f) Variationof VOC, JSC,FF and PCEwithbendingtimes[42].

Compared with widely used polymer substrates, the moreprominent advantage ofultra-thinflexible glass is its smooth andnon-deformable surfaceathigh temperatures.Basedonthis advantage, a large-area module (42.9 cm2)was preparedwithan efficiency of 15.86%[38],facilitatedbythe high-qualityperovskitefilmobtained bydoctorbladecoating with an NH4Cl additive. Theflexible perovskitemoduleis shown inFig.3(e). However, compared with other flexible substrates, fragility,highweight and relatively high cost maybecomethe big concerns,which also limits itswideapplication.

3.Carrier transport layer

The carrier transport layer in the perovskite solar cell plays an importantrole,whichisresponsiblefor extracting electrons and holes from the interface and transporting them tothe corresponding electrodes[40]. Therefore,the requirementsfor the carrier transport layer are band structure matched with perovskite, high light transmittance and excellent carrier mobility. For FPSCs, low-temperature processingrequirements are posed. Therefore, research on flexible perovskite solar cells mainly focuses on low-temperature preparationof carrier transport layers.

3.1.Electron transport layer

The excellent electron transport layer (ETL) should have high electron mobility and high light transmittance. At the same time, the conduction band maximum of ETL is essential to match the energy level of the perovskite, which can effectively transport electrons and block holes.

Because of its high electron mobility and low-temperature processability, ZnO was first used as an electron transport layer to prepare FPSCs[18]. But the PCE of this device is only 2.62%, due to the low-temperature (<100 °C) processed ZnO dense layer exhibits low mobility and high defect density. Liuet al. improved the preparation method of ZnO[41].They spin-coated the mixed solution of butanol/chloroform containing ZnO nanoparticle dispersion directly on ITO substrate without calcination. The PCE of corresponding FPSC was over 10% as shown in Fig. 4(b). Heoet al. increased the heat treatment temperature of the electron transport layer to 150 °C, resulting in a more compact ZnO layer.Fig. 4(c)shows that the PCE is increased to 15.6%.

Fig. 5. (Color online) (a) The first FPSC based on the TiO2 electron transport layer and (b) its J–V curve as the FPSCs performance of the electron transport layer[49]. (c) Steady-state PL spectra of glass/perovskite, FTO/anatase-TiO2/perovskite and FTO/amorphous-TiO2/perovskite film[50]. (d)FPSC cross-section scanning electron microscope with ALD deposited TiO2 dense layer and UV-irradiated mesoporous TiO2[51]. (e) Impedance diagram (Z"– Z')[57].

Although the ZnO has excellent electronic properties,there are hydroxylor residual organic acetate ligands onits surface, which leads to charge recombination at the interface andpoor qualityof perovskite deposits. Inaddition, there is proton transfer reaction between ZnO and perovskite, which leads todecompositionreactionof interface[43?48]. Therefore,Jinet al. used low-temperature atomic layer deposition (ALD)on the surface of ZnO to obtain TiO2. This ultra-thin TiO2protectivelayercan effectively passivate surfacedefectsand prevent decomposition reactions. Figs. 4(d)–4(f) show that as the number of ALD cycles (x)increases,the thickness of the TiO2layer increases, and the PCE based on a rigid substrate reaches a maximum PCE of 18.26% atx= 60. The PCE of FPSC basedonthis technologyhasincreased to17.11%[42]. In addition to ZnO electron transport materials, researchers also pay great attention tothe developmentof low-temperature treatment of TiO2electron transport layer for FPSC.

Compared with ZnO, TiO2has better environmental stability and better perovskite compatibility. However, the most efficient TiO2electron transport layer needs to be obtained through an annealing process at 450 °C. In 2013, Doampoet al. first tried to use TiOxfor p–i–n structured FPSC[49]. The TiOxprecursor solution is composed of titanium isopropoxide and isopropanol solution, and the TiOxflat film is annealed at 130 °C. The corresponding PCE of FPSC is 6.4% (as shown in Figs. 5(a) and 5(b)). So far, a variety of low-temperature processing technologies for TiO2havebeendeveloped,such as sputtering, atomic layer deposition(ALD),and vacuum evaporationdeposition.Yanget al. sputteredan amorphous TiO2film by DC magnetron at room temperature, and this film has good solar spectral transmittance[50]. The steadystate photoluminescence (PL) shows that compared with the anatase-TiO2/perovskite film, the PL intensity of the amorphous-TiO2/perovskite film is weaker, indicating that the charge transfer effectively occurs before the carrier recombination at the interface. And amorphous-TiO2improves the extraction of electrons from the absorption layer to the electron transport layer (as shown in Fig. 5(c)). Giacomoet al. used plasmaenhanced atomiclayer deposition (PEALD) to deposit adense TiO2filmona flexible PET/ITOsubstrate atlow temperature,andthenspin-coated or screen-printed to depositmesoporous TiO2[51]. Thecross-section oftheobtained deviceis shown inFig. 5(d), andthe PCE is 8.4%. The prepared photovoltaic module with anarea of 8 cm2showeda PCE of 4.3%.Inaddition, depositionof TiO2colloidalparticles and subsequent low-temperature annealing or ultraviolet treatment are also effective methods for forming the TiO2electron transport layer[52,53]. Deep charge traps at the interface between TiO2and perovskite will cause TiO2-based PSCs to exhibit severe hysteresis. In addition, due to the photocatalytic effect of TiO2,deep traps will be introduced on the surface[54?56], the passivation interface layer is necessary. Kimet al. coated PC60BM on PEALD-deposited TiO2to passivate traps and improve electron extraction, and finally achieved the highest efficiency of 17.70%, while significantly reducingthehysteresis[57]. The FPSC was analyzedby electrochemical impedancespectroscopy (EIS)in Fig. 5(e), where the first semicircle in the highfrequencyrange(> 1 kHz)can interact with thedielectric response of theperovskitelayer (Cg/R2)is related,and the second semicircle inthe low frequency range(≈ 1 Hz)can be due to thesurfacecharge or ion chargeaccumulation at the interface causing electrode polarization (Cs/R1)[58]. The capacitancedifference shownin the lowfrequencyrangecanbe seenthat TiO2/PCBM/Perovskitehasa smaller capacitance.

Comparedwith TiO2, SnO2hasadeeper conduction band and higher electron mobility. It is easy to process by solution method at low temperature, and it is more stable under illumination[59]. The investigations on SnO2as electron transport layer are mainly focusing on how to improve the conductivity and mobility and modify the conduction band to enhance the electron extraction and transmission in FPSCs. Parket al. used Li-doped SnO2as the electron transport layer, and the prepared FPSCs had a high PCE of 14.78%. Li-doping could reduce the conduction band and increase the conductivityof SnO2layers, whichwill facilitate electron extraction and transport[60].Wanget al. employedPEALD SnO2treated with watervaporas electron transportlayer toimprovethe photovoltaic performanceof FPSC. Itwasfound thatthewatervapor treatmentat 100 °C can improve the electricalconductivity andmobilityof SnO2,because annealing withwater vapor can promote the complete reactionof organic materials and the formation of pure SnO2. Based on that, PCE up to 18.36% was achieved in the devices[61]. Liuet al. reported a similar strategy to improve the quality of SnO2by hydrothermally annealing the SnO2layers. Huanget al. precisely controlled the deposition of SnO2layer by adjusting the solution concentration to form a fully covered SnO2layer. The best PCE of FPSCs based on SnO2was increased to 19.51%[24].

Zn2SnO4is a new type of ETL for high-efficiency FPSC applications. Shin has developed a new route to synthesize highly dispersedZn2SnO4by the introduction ofaZn–N–H–OH complex derived from hydrazine viasimple solution processat low temperature (90°C)[20]. Therefractiveindex of Zn2SnO4islow(about 1.37), which showsa transmission increase ofabout 20% in the entire visible light range, dueto theanti-reflection effect. The PCEofFPSC basedon Zn2SnO4withtheemployment of MAPbI3astheabsorber layer is 15.3%. On this basis, they introduced Zn2SnO4quantum dots on top of the Zn2SnO4nanoparticles layer. Among them, the relativelylarge-sized nanoparticles(>10nm) have a higher work function, which is beneficial to the electron transfer at theinterfacebetween the electron transport layer and the ITO. The Zn2SnO4quantum dots are favorable for generating largebuilt-inpotential at theZn2SnO4quantumdots/perovskite interface, and the combination of the quantum dots andnanoparticlereduces the energy barrier. Accordingly,based on Zn2SnO4, PCE of FPSC reached 16%[62].

Inaddition to all thesewell-developed inorganicETLs,Yooet al. also demonstrated a simple interfacial layer that replaces TiO2with C60with the aid of polyethyleneimine ethoxylation (PEIE)[31]. In addition to serving as an electron collecting interfacial layer, the PEIE layer also helps to C60 processingandprotect them from corrosion during the subsequent solution spin coating process. Through this low-temperature-treated double-layer ETL, FPSCwitha PCE of 13.3%was demonstrated. Interestingly, the solid ionic liquid 1-benzyl-3-methyl-imidazolium chloride wasalsousedasETL and processed at low temperature. The resulting device achieves 16.09%efficiency in FPSCwithno hysteresis, and good repeatability[21].

Thevarious electrontransportlayers described aboveare all prepared under low temperature processes. Among them,thewidely usedinorganic electrontransportlayer materials TiO2, SnO2and ZnO have been successfully developed with low temperature preparation processes. ZnO, which is the most readily developed FPSCs electron transport layer material, is limited for further application by the decomposition reaction at the interface. Tosolve this problem, interfacepassivation, e.g. using PCBM or TiO2, become effective solution[42,63].TiO2is oftenlimited by the relatively poor materials quality obtained from the low-temperature preparation process, and theefficiency of resulting devicehas only developedto 17.7%[57]. At present, modification on low-temperature preparation methods, including electron beam evaporation, magnetron sputtering, atomic layer deposition and liquid phase method, seems to be meaningful to improve the materials optoelectronic properties. SnO2, which is easily obtained by solution processingat lowtemperatures,standsoutand developsrapidly with respect to the device efficiency, and is currently the most widely used one.

3.2.Hole transport layer

Thehole transport layer(HTL) inFPSC is essentiallyto have hole extraction and hole transport capability, as well as bendingdurability.In then–i–p structure, the holetransport layer does not need to be transported because that it doesnot hinder the light absorption oftheperovskite. Spiro-OMeTAD, which is widely used in traditional n–i–p structured PSCs, can provide excellentperformance[1,17,23,24,64,65].For p–i–n-structured FPSCs, the hole transport layer requires high optical transparency. The NiOxwith superiorhole transport performance, high optical transparency, and low-temperature manufacturing processes iscurrentlythe most promising materials. By introducing low-temperature processed(130 °C) NiOxHTL, better energy alignment can be achieved,and the device efficiencybasedon the PET/ITO substrate is 13.43%[65]. Interestingly, as shown in Fig. 6(a), the deposited NiOxnanoparticles (NPs)canalso beprocessed atroom temperature without any other post treatment. By introducing a defect-free NiOxnanostructured films, the PCE of 14.53% can be achieved withnegligible hysteresis[66]. Generally, the original NiOxhaslowconductivity, and thethickness needsto be carefully optimized during the device fabrication process. Li and his colleagues developed Cu-doped NiOxNP as HTL to improve its conductivity as shown in Fig.6(b), thereby enhancing the PCE of FPSC to 17.16%[67].

Docampoet al. used PET/ITO/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/MAPbI3–xClx/PCBM/TiOx/Al device structure to obtain the first FPSC[49]. The device based on this structure has a PCE of 6.4%. The commonlyused PEDOT:PSS HTL show somedisadvantages, such as strong acidity and high water absorbability. In addition,it cannot provide perfect hole extraction ability when contact with perovskite (energy alignment ofholetransfer:PEDOT:PSS: ≈ 5.0 eV; MAPbI3: ≈ 5.4 eV), which will cause voltage loss in the device. Besides, polymer hole transport materials copolymerized by1,4-bis(4-sulfobutoxy)benzeneand thiophene group (PhNa-1T) have been successfully developed to replace PEDOT: PSS as HTL. The structure of PhNa-1Tis shown in Fig. 6(c). Because PhNa-1T can effectively reduce the potentialenergyloss at the HTL/perovskite interface and achieve better contact (as shown in Fig. 6(d)), it enhances the extraction of holes from the perovskite to the HTL and inhibits interfacialrecombination. The FPSCwith PhNa-1T HTL achieved a PCE of 14.7% with increasedVoc[68].

Fig. 6. (Color online) (a) Schematic diagram of the fabrication of nanostructured NiOx thin films[65]. (b) Cu-doped NiOx FPSC device structure[67].(c) PhNa-1T structure diagram (d) Energy band diagram using different hole transport layers[68].

Poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA)isoften usedas HTM layer, sincethefilmcan beobtained by low-temperature spin coating that is compatible with FPSC preparation. Moreover, PTAA are of high conductivity and reliabledurability. Unlike Spiro-OMeTAD,whichrequires doping to improve performance, undoped PTAA still performs well in high-efficiency PSC. For example, Qiuet al. prepared an FPSC based onthePET/ITO/TiO2/MAPbI3?xClx/PTAA/Au structure in 2015 and achieved a PCE of 13.5%[69]. In 2017, Huanget al.used PTAA and double fullerene layers (phenyl-C61-butyric acidmethyl ester (PCBM) and C60) asholetransport layer and ETL, respectively. In this architecture, the charge transfer from the perovskite to the PTAA layer is fast, and the efficiency of theprepared FPSC has reached 18.1%[70].

4.High-quality perovskite films

As thelight absorptionlayer, the quality of the perovskite film directly affects the performance of the flexible perovskitesolar cells.Comparedwiththe perovskite filmpreparedon the glass substrate, the qualityof theperovskite film on the flexible substrate is different, which is limited by the high roughness and high thermal resistance of the flexible substrate.Becausethe resistanceof themostwidely used low-cost flexible polymer substrate will rise sharply when the temperature is higher than 150 °C, the developmentof flexible perovskite solarcells mainly focuses on the preparation of perovskite thin films at low temperature.

4.1.Perovskite film preparation method

4.1.1. Spin-coating

The quality of the perovskite film on the flexible substrate is generally not satisfactory because of the poor wettability and high roughness of the surface of the flexible substrate. Choosing an appropriate preparation process to control the nucleation and crystallization process of the film is the effectivewayto obtain ahigh-quality film. As asimple and effective method, the spin coating process can be adopted to the preparation of small-area devices and are able to obtainperovskite films withawide range of thickness. In general, there are one-step and two-step spin coating methods.

In the one-step deposition method, the mixed precursor solution containing MA/FA halides and metal halides is directly depositedonto thedesired substratesthrough aonestep spin-coating procedure. The sample is thenannealed at low temperature(100–150 °C)to produce aperovskite film.The one-stepspincoating process usuallyinvolves anantisolvent. Chenget al. added chlorobenzene (CB) as an effective anti-solvent during the spin-coating of perovskite in N, Ndimethylformamide (DMF) to prepare a highly uniform perovskite layer[71]. Seok's team used toluene (TB) as an antisolvent to prepare perovskite in a mixed solvent of dimethyl sulfoxide (DMSO) and gamma-butyrolactone (GBL)[72]. Consideringthe precise control ofthe stepofadding anti-solvent,this methodismore challenging toproducelarge-areaPSCs.Ontheotherhand, duetobetter morphology and interface control, the two-step method is better than the one-step method. However, some problems in the two-step method, such as the incomplete conversion of PbI2to perovskite, severely limit its large-scale application in high-efficiency PSCs with good reproducibility. Although this method is an antisolvent-free process, there willbe edge effects,andthelarger thearea, the more seriousthe impact.

4.1.2. Scalable deposition method

Spin coating can effectively be used to produce a uniform perovskite layer on a small area, but this method is difficult to obtain a large area of uniform perovskite. In order to fabricate large-scale flexible perovskites, it is particularly important to use scalable printing technology to deposit perovskite absorbers.Among them, bladecoatinghasbecome one of the promising laboratory technologies due to its easy handlingandlesswaste of raw materials. In 2015, Jenet al.used a blade coating method to sequentially deposit poly(3,4-ethylenedioxy-thiophene): poly(4-styrenesulfonate)(PEDOT:PSS)/CH3NH3PbIXCl3–x/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)/Bis-C60, the Agwas usedas the top electrode in the device. The prepared FPSC exhibited a PCE of 7.14%[73]. Huanget al.deposited theperovskite film by gas-assisted blade coating in the ambient environment at room temperature[38]. As shown in Fig. 7(b), the air blade moving with the blade blows N2gas on the coated wet film at the same time to remove the solvent and induce crystallization. The PCE resultsare showninFigs. 7(c) and 7(d). They achieved 19.72% PCE on a small area, and at the same time obtained 15.86% PCE ona 42.9cm2large areamodule. However, their devices are made on corning willow ultrathin glass, which is limited in flexibility and cost. Wanget al. deposited the perovskite which is added with Lewis base thiourea (TU) on the PEN/ITO polymersubstrate bybladecoating[74].As shown in Fig. 7(e), they have improved the interface design and adopted adouble holetransport layer(PEDOT:PSSandpoly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)). On the one hand, itachieves more effective carrier extraction(asshown in Figs. 7(f) and 7(g)), on the other hand, it enhances the flexibility of the ITO substrate. This interface optimization design increases the device efficiency of FPSC to 19.41%.

Fig. 7. (Color online) (a)Schematic diagram of the device preparedby blade coating method[73]. (b) Blow N2 gas and precursor solutionwiththe addition of NH4Cl[38].(c) J–V curveunder anarea of 8mm2.(d) J–V curve under an area of 42.9cm2. (e)Double hole transport Energy banddiagram.(f, g)Under thelayer MAPbI3, PTAA/MAPbI3andPEDOT:PSS/MAPbI3diagramof PL and itspartial enlargement[74].

Vacuum thermal evaporation is a relatively mature technology that can depositlarge-areaand uniform thinfilms, and its relatively low temperature characteristics can also be compatible withthe preparationprocessofFPSCs[7,75?78]. Wuet al. developed a full-vacuum strategy, using a two-step flash evaporationmethod to prepare perovskite films. First, PbI2film and MAI were vapor-deposited sequentially[79]. Meanwhile,the post-annealing process wasaddedto realize the"solid-solid" reaction of the film, and the PCE of 13.15% was realized onthe 16 cm2flexibleperovskitesolar module.However, the current vacuum evaporation still has some issuessuch asdifficultyin detecting organic sources and high vacuum requirements.

In perovskite roll-to-roll deposition compatible technology, gravure printing is continuous and stable. Seoet al. obtained a FAPbI3film by gravure printing and soaked it in tertbutanol (tBuOH) to obtaina moreuniform and highlycrystalline film. The FPSC prepared entirely using the roll-to-roll process showed thebest PCE of 13.8%[80].

The combination of slot-die coating and roll-to-roll process exhibitsadvantages,including less material waste, good film thickness control, one-dimensional patterning suitable fortandem interconnected cellswithout the needfora large number of patterning processes, and high manufacturing speed[81,82]. Gaoet al. used a blow-assisted drop-casting method compatible with roll-to-roll to prepare CH3NH3PbI3films.The formulation was optimizedand appliedto the flexible substrate, so that 11.16% of PCE was obtained through slot-die coating[83].

4.2.Composition engineering

Composition engineering has been proven to be aneffective strategy to improve the quality of perovskite films. In 2017, Huanget al. proposed that the deposition conditions of theperovskite onthe rigid substrateneed to be optimized on the flexible substrate to improve the film morphology[70]. They proposed that the difference of the perovskite film quality may be causedby thedifferencein the thickness,surface roughness and thermal conductivity of the rigid glass/ITO relative tothe flexiblePET/ITO substrate. Theyadopted a method of regulating the ratio of precursors (PbI2: FAI),using a non-stoichiometric solution of 1 : 0.95. The reduced organic precursorcontent can form auniform and pinhole-free perovskite film on the PET/ITO/PTAA substrate, and has a longer carrier lifetime, a smaller trap state density, and a reduced precursorresidue.Afterimprovingthefilm morphology and optoelectronic properties of perovskites, the PCE on the flexibleITO/PET substrate is18.1%.

4.3.Additive engineering

The grainboundaries of perovskite,often as charge carrier recombination centers and ion migration channels, are currently animportant factoraffecting theefficiency and stability of PSCs[84]. The additives can effectively improve the crystalline quality of the film. Fenget al. used dimethyl sulfide (DS)as the additive to control the growth of perovskite films deposited on flexible substrates[23]. Fourier transform infrared spectroscopy shows that the DS additive can react with Pb2+and forman intermediate phase, whichdelayedthe crystallization process. The prepared perovskite films exhibit large grains and lowdefectdensities, and the PCE isimprovedto 18.4%. Wuet al. used NMP and MACl as synergistic additives to optimize FAPbI3perovskites. The introduction of NMP can form FAI·PbI2·NMPintermediate,whichdelayed the crystallization process. The employment of MACl can facilitate the phase transition of perovskites at low temperature. Based on the collaborativeimprovements, theFAPbI3basedFPSCs showed a PCE of 19.38%[64].

Menget al.incorporated a self-healing polyurethane (s-PU) with dynamic oxime-carbamate bonds in the grain boundaries of perovskite films. The s-PU could release the mechanical strain andrepair cracks at the grain boundaries,whichsignificantly improved the stretchability and deformability of perovskite films. The prepared FPSCs could recover 88% of initial PCEs after1000 cycles at20%stretching.In addition,due to the defect passivation of s-PU at the grain boundaries, the stretchableFPSCs exhibited abest efficiency of 19.15%[85].

5.Electrode

In order to absorb sufficient incident light, one of the two electrodes in the solar cell device should be transparent,while the otheris usually opaque. Foropaque electrodes,the metal foil conductive substrate has been introduced above.In addition,the evaporated thick metal film and printed carbon layerhavealso been mature and widelyused. They possess the characteristic including good electrical conductivity,easy manufacturing, andappropriate energylevels that aligned with other functional layers of the device. In the transparent electrode, it is necessary to have the characteristics of hightransparency, highconductivity, mechanicalflexibility and chemical stability. ITO, which is widely used in glass substrates, has alsobeenused in FPSCs. Apartfromthis, some new transparent electrodes, such as conductive polymers, ultra-thin metal films, Ag nanowires based electrodes, carbon nanotubes and graphene, have also been explored.

5.1.Indium tin oxide (ITO)

Inthe transparent electrode, ITO is the most commonly used material in FPSC, mainly because it is easy to obtain,and has good transmittance and conductivity. However, the high costandhighmechanical rigidityofITOstill plague the development of FPSCs. According to the published work,FPSCs basedonITO-PEN lose about 50% of the initial photoelectric conversion efficiency after a bending radius of 4 mm for 1000cycles[86]. Inorder to exploretheorigin of thedegradation during the period, they compared the resistance of two flexible devices (PEN/ITO/TiOx/perovskite/Spiro-OMeTAD and PEN/TiOx/perovskite/Spiro-OMeTAD) after bending. As shown in Fig. 8(a), on the PEN/ITO substrates, a sharp increase in resistance was observed at the beginning. Thisisbecausethe Young's modulus of the ITO and the substrate are different,which causesthe ITOlayer coated onthe flexible substrateto form large crack lines perpendicular to the bending direction(as shown in Figs. 8(b) and 8(c)), resulting in a sharp increase inthe resistance of the FPSC.

In order to overcome the poor mechanical strength of ITO,someorganicmolecules orpolymerscaffolds are usedto improve the interface contact. Inspired by the flexible structure of vertebrae, Menget al. designedan adhesive polymer interface layer (PEDOT:EVA, Poly(3,4-ethylenedioxythiophene):poly(ethylene-co-vinyl acetate)) (As shown in Fig. 8(d))[25]. The brittleITOandperovskiteare tightly bondedtogether dueto the viscosity of EVA in the PEDOT:EVA film, so the flexibility of theflexible substrate is improved. ThePEDOT:EVA ink is synthesized by miniemulsion method, which has good dispersionand stability. Finally, the FPSC preparedby meniscus-coating has a PCE of 19.87%. As shown in Figs. 8(e) and 8(f), when the bending radius is 10, 5, or 3 mm, after 7000 cycles, it can still maintain about 96%,95%, and 85% of the initial PCE, respectively.

Fig. 8. (Color online) (a) Resistance change of multilayer structure with bending cycle (ΔR/R0 (%)). (b, c) Low-magnification SEM images of PEN/ITO/TiOx/perovskite and PEN/TiOx/perovskite after 300 bending cycles, scale bar: 100 μm[86]. (d) The bionic mechanism of vertebrae and FPSCs. (e) PCE of FPSC after 500 cycles of bending at different bending radii. (f) The average PCE value of FPSC with a bending radius of 3 mm and bending 7000 cycles[25].

5.2.Conductive polymer

PEDOT:PSS composed of conductive polymerPEDOT and water-dispersible PSS:PSS is by far the most successful and most widely used conductive polymer. PSS has the dual effectsof dopingcharge andimproving water solubility[87].Therefore, the conductivity of PEDOT:PSS can be adjusted by theratioof PEDOTand PSS[88]. PEDOT:PSS filmhas excellent visible light transmittance and mechanical properties, making it very suitable for transparent conductive electrodes of flexible solar cells. For example, spin-coatedPEDOT:PSSonto a Noland Optical Adhesive 63 substrate or an ultra-thin PET substrateas atransparentconductive electrodetoobtain a device with a PCE higher than 10%[89]. The prepared ultrathinandlightweight devices haveexcellent flexibility and stretchability, and its power per unit weight is calculated as high as 23 W/g.

5.3.Ag-nanowire-based composites

Ag nanowires and gridshave attracted widespread attention as alternative materials to ITO. Ag nanowire networks andAg grids have good electrical conductivity, light transmittance and mechanical flexibility, which are used as transparent conductive electrodes in many optoelectronic devices. In FPSC, theyareusedsolely or togetherwith other materialsas top or bottom transparent conductive electrodes. For example, a bare Ag nanowire grid is used as the top transparent conductiveelectrode ofFPSC[30]. Since silvernanowires are used as bottom electrodes, their rough surfaces usually hindertheimprovement of deviceperformance, so some other transparent conductive components can be combined to prepare a compositeelectrode with a smooth surface, such as doped Fluorine-based zinc oxide[90], graphene oxide[91],andPEDOT[92]. Amongthem, thesilver grid/PEDOT:PSScomposite electrode can be simply fabricated in an embedded Ag-meshing flexible PETsubstrate byspin coating[93].Accordingly, the sheet resistance of this composite is as low as 1.2 Ω per square inch. The obtained FPSCs showed a relatively high PCE, with almost no decrease even at 2 mm bending radius.Based on the excellent flexibility of the device, even after 1500bending cycleswitha bendingradius of 5mm, PCE can remain stable, maintaining 95.4% of its initial value after 5000 cycles.For comparison, PET/ITO-baseddevices showed asignificant performance degradation after 100 bending cycles. At thesame time, FPSCalso shows relatively good storage stability. After 500 h of storage in an inert atmosphere at room temperature,PCE can maintain aninitial valueof 91.6%, which is better than a rigid control device. However, Maet al. also reportedconflictingresult aboutstabilityin another report.They found that the device based on the PH1000/Ag electrode is unstable due to the rapid reaction between Agand the perovskite precursor solution caused by the redox reaction between Ag and PH1000[94]. While, the ammonia water and PEI modified layer on PH1000 can inhibit this redox reaction and effectively improve device performance.

5.4.Carbon nanomaterials

Carbon nanomaterials havebeen usedin rigidPSCs in different forms (such as carbon black/graphite composites, carbon nanotubes(CNT)and graphene) as backelectrodes,partly because of their low cost and good electrical conductivity.Extensive researchon carbonnanotubes and graphene electrodes is mainly based on their excellent properties, including good chemicalstability, mechanicalflexibility, high electrical conductivity and visible transparency. In 2014, the laminated CNTsnetwork has been usedasboth the top electrode and the hole transport material in rigid PSC[95]. For FPSCs,Wonget al. transferred the CNT film grown by chemical vapor deposition (CVD) to the top of the perovskite film as a transparent conductive electrode and a hole collector, in which a Tifoilsubstrateisusedasbottom contact[96]. Infact,CNT networks can also be directly transferred to transparent plasticsubstrates, where theyare used asbottom transparent conductive electrodes, similar to ITO and FTO. Matsuoet al.reported thattheprimary PCEof the FPSC devicewas 5.38%, and this efficiency was further improved to 12.8% by thesamegroupin2017,partly dueto theoptimization of the device structure and the quality of the functional layer[97]. Compared with ITO-PEN control sample,CNT-basedFPSC alsohas higher mechanical strength. Carbon nanotubes are also used tofabricatetop electrodes withITO-PENas the bottom substrates, and their stability is much higher than that of control devices based on silver electrodes[98]. In addition, CNTs are also used astransparent conductive electrodes in irregular stretchable solar cells (such as fiber cells)[99].

Graphene wasfirstusedasan interfaciallayer inFPSC with an ITO-PET substrate in 2015[100]. Later, Yanet al. used graphene as the transparent conductive electrode of FPSC, in which they grow a single graphene on a copper substrate by CVD and then transfer it to a PET substrate[101]. The PCE of the resulting device is 11.5% with high flexural durability. Doping strategies are often used to improve the conductivity of graphene. Choi evaporated 2 nm MoO3on graphene to induce hole doping of graphene, and achieved 16.8% PCE in the resulting FPSC device[22]. FPSCs based on all-carbon electrodes were also realized by Guoet al. The graphene film formed on the PET substrate is used as bottom transparent conductive electrode, while the cross-laminated CNT film is used as the top counter electrode[102]. The all-carbon electrode device has excellent flexibility. Even after bending with a radius of curvature of 4 mm for 2000 times, the device retains approximately 84% of its original PCE. After 200 bends at a radius of curvature of 2.2 mm, it still remains 85% of the initial PCE. As a comparison, a device with a PEN/ITO/TiO2/PCBM/CH3NH3PbI3/Spiro-OMeTAD/gold structure loses 87%of itsinitial PCEwitha radius of curvatureof4mm, and is almost completely damaged when bent to 2.2 nm. The results show that the performance degradation of the reference device ismainly caused bythe conductivity loss in the brittle ITOlayer. Theslightdegradationof the all-carbon electrodebased device is most likely due to fractures in the crystalline perovskite layer and delamination in the device during its deformation. Inaddition,the all-carbon electrodedevice exhibits excellent stability,and it retains about90% oftheoriginal efficiency after 1000 h light or 1570 h heat treatment in the dark and air at 60 °C, respectively.

6.Encapsulation methods

While the PCE is a crucial criterion to evaluate the performance of a PSC, the lifetime is also important standard since perovskite materials are very sensitive to many stressors, e.g. moisture, oxygen, light, heat, etc.[87,103]. In this context, device encapsulation is necessary. However, some well-established encapsulationtechniques for rigid PSCs, e.g.,attaching a glass plate ontoadeviceby thermosettingepoxy, cannotbe readily used in FPSCs. Therefore, new encapsulation materials and methods are required to be developed.

Asan efficient encapsulation material, various factors needto beconsidered: light transmission,oxygentransmission rate (OTR), water vapor transmission rate (WVTR), resistance to ultraviolet (UV), chemical inertness, mechanical flexibility,and strength,etc.

The combination ofpolyurethane resin encapsulation and Cr2O3-Cr interlayer proves that the air stability of ultrathin FPSC has been improved. In particular, it was found that the use ofachromium oxide-chromium intermediatelayer can effectivelyprotect the metal top contact fromthe perovskite reaction, which is beneficial to prolong the long-term stability[104].

NanoconePDMShasanti-reflectionand waterproof functions andis attachedtothefront of the flexiblesubstrate.These nanocone PDMS packaging materials help improve optical transmittance and achieve waterproof effects[37]. The multi-layerencapsulationfilm composedofa superhydrophobic layer and a relatively hydrophilic layer significantly enhances the stability of the PSC in a very humid environment[109,110]. The superhydrophobic film containing polymethylmethacrylate (PMMA),polyurethane(PU)and SiOnanoparticles is in contactwith theenvironment, which can repel water in a humid environment and prevent water from penetrating into the fragile perovskite layer. Among them, the moderately hydrophilic PMMA layeracts asa desiccant, extracting residual water from the perovskite layer itself during the operation of the solar cell. When exposed to humidity exceeding 95%, the dual function of the coating film helps the PSCs to keep at 17.3% PCE for 180minutes[110].

7.Conclusion and outlook

Organic–inorganichalide perovskitehas become a promisingcandidatefor flexiblesolarcellsdue to itsexcellent optoelectronic performance, excellent mechanical tolerance and low-temperature solution processability. Facilitated by the endeavor on exploration offlexible substrate, charge transport layer, high-qualityperovskite filmandtransparent topelectrode, the highest PCE of flexible PSCs reached 21.1%.

For the flexible substrates,thepolymersubstrates based FPSCs show the best PCE, but bring to poor environmental stability. Metal substrates, ultra-thin flexible glass substrates and other special materials, etc.have been exploredto improve the devicestability.

For the charge transport layer in FPSCs, the main focus is developing low-temperature process approach. At present,SnO2isthe best choice for electrontransport layer in n–i–p structure due to its better stability and low-temperature processability.

For perovskite films, effortshave been devotedto the preparationofdense,uniformlycovered, andhigh-qualityfilms on flexible substrates. In terms of thin film growth, the spin coating is suitable for small-area device, but limited for large scaleproduction.Componentand additive engineering are indispensable in thin film fabrication,whichcan effectivelypassivate defects and improve crystal crystallization. Also, the band structure and mechanical property of perovskite layers can dramatically influencethe photovoltaicperformance of the resultant PSCs under mechanical strain.

For transparent conductive electrodes, ITO is the most common choice,but silver nanowiresand carbon materials also becomepromising alternatives.Benefiting fromitsbetter mechanical flexibility, FPSCs based on silver nanowires and carbon materials show better bending stability.

The further development of FPSCs is discussed in the following aspects. 1) Preparation of largearea solar cell modules. So far, the reported FPSCs with excellent PCE are all prepared by spin coating, and the effective area is small. With the increase ofdevice area, more stringentrequirements are posedonthe quality of the perovskite film, and the density and uniformity of other functional layers (carrier transport layerand perovskite layer) are also crucial. While methods such as doctor blade coating, thermal deposition, and gravure printing can be selected for large-scale production, the roll-to-roll manufacturing technology may be an effective way to developlarge-areaFPSCs. 2) The environmental instabilityis also the most challenging issue for the development of FPSCs. In addition to the device degradation under heat, light, and electricitystressors, thepolymer substrate of FPSCs accelerates the permeation of water and oxygen, which further brings to water and oxygen instability. It thus requires more robust encapsulationapproachtofabricate FPSCs with improved long term stability. 3) In addition to improve the PCE, it is also necessary to pay attention to mechanical stability of FPSCs, since the applicationscenarios ofFPSCsincludewearable devices.Exploring flexible transparent electrodeswithhigh conductivity and transmittance, low cost, and scalable processability is still an urgent need to meet the commercialization goal.