Guanqi Tang and Feng Yan

Department of Applied Physics, TheHong Kong Polytechnic University, HongKong999077, China

Abstract: Flexible perovskite solar cells (FPSCs) are supposed to play an important role in the commercialization of perovskite solar cells due to their unique properties, such as high efficiency, thin thickness and being compatible with roll to roll (R2R) process for mass production. At present, deformable and lightweight FPSCs have been successfully prepared and applied as power supply by integrating with different wearable and portable electronics, which opens a niche market for photovoltaics. In this mini review, we will introduce the recent progress of FPSCs from the aspect of small-area flexible devices, R2R processed devices with large scale and emerging flexible cells with deformability and stretchability. Finally, conclusion and outlook are provided.

Key words: flexible perovskite solar cell; roll to roll process; stretchable and deformablesolar cell; low temperatureprocessing

1.Introduction

Perovskite solar cells (PSCs) have attracted extensive attentions as the most promising candidate for the new generation photovoltaic technologies. The power conversion efficiency (PCE) has developed rapidly from 3.8% to a certified 25.5% within about ten years due to the superior optoelectronic properties of perovskites, such as high light absorption coefficient, long charge diffusion length, desirable/tunable bandgap and high defect tolerance[1−11]. Besides, relative good mechanical flexibility of perovskite materials enables the realization of flexible perovskite solar cells (FPSCs) on various flexible substrates[12]. FPSCs are supposed to be a breakthrough for photovoltaics with high commercial value due to the compatibility with roll to roll (R2R) mass production,which could significantly decrease the production cost and increase the productivity[13,14]. Furthermore, FPSCs with intriguing properties, such as lightweight, conformality, deformability and stretchability couldfulfillvariousinteresting applicationson smart integrated buildings,wearable andportableelectronicsandunmannedsystems[15−17]. Thus, the development ofFPSCs playsacritical role inthepracticalapplications of PSCs.

Sincethe firstreportofFPSC withanefficiency of 2.62%in 2013by Kumaret al.intensiveefforts on investigating flexible substrates, transparent electrodes, charge transport materials,perovskitefilms andinterfacial layers have been madeto achievethe current recordefficiencyover 21%with small device area(<1 cm2)[18−20]. The mechanicalflexibility has also been improved significantly[21]. These methods specifically developed for FPSCs and the obtainedresults have provided valuableinsight ondeviceand materialdesign, facilitating the fabricationof large-scale FPSCs with highperformance.Inthe meantime,the fabricationof large-scale FPSCsor flexible modules byR2R or R2Rcompatible processes hasdevelopedrapidly thanks to many innovative coating methods andcompositional engineering of perovskite precursors recently developed, making it possible to prepare high-quality perovskite filmswith large scale[22,23]. So far,fully R2R processed FPSCs withsmall area(except for top metal electrodes) demonstrated a record efficiency of 13.8% by Kimet al., which is much lower than the record for a normal FPSC[24]. Therefore,there is a considerable room for improvement in the R2Rprocess FPSCs.Forpractical applications,deformable and stretchable FPSCs with light weight have been realized and applied as a power supply for different devices, such as rechargeable batteries, sensors and aircrafts[25,26].

In this mini review, we willfocusonthe recentprogress ofFPSCs including efficiency improvementof small-scale FPSCs, R2R processed large-area devices and emerging flexible photovoltaic technologies. We will summarize the criteria/requirements for different component of a FPSC, such as flexible substrate, transparent electrode, charge transportlayer and perovskite film. The innovative techniques on coating methods and perovskite precursors for R2R process are discussed. The emerging photovoltaics based on deformable and stretchable FPSCs aredemonstrated.Finally, we willconclude and address the remainingchallengesand potentialfuture directions of this research field.

2.Device components of high-performance FPSCs.

A FPSC contains several essential components, including a substrate, two electrodes with at least one being transparent, a perovskite layer and two chargetransportlayers (CTLs)forelectrons or holes respectively. Themajorthicknessof the whole device comes from the substrate since the perovskite layer and electrodes are always very thin (< 1μm) and thus the mechanical flexibility of the device is mainly influenced bythe substrate. On theotherhand, flexiblesubstrate plays a key role on the photovoltaic performance of the flexible device because the following layers deposited on it could be influenced by its chemical and mechanical properties. Therefore, the selectionof flexible substrateisimportant for the wholefabrication process. Unlike traditionalglasssubstrates,flexible substrates usually have high roughness and low-temperatureprocessingrequirement. Hence, it isnecessaryto preciselyoptimizethe depositionconditions of electrodes,CTLs and perovskite layers to ensure both high efficiency and mechanical robustnessof resultant flexible devices.

2.1.Flexible substrates

Adesirableflexible substrate for FPSCs should have high optical transmittance, high thermal tolerance, low roughness,high resistance to chemical solvents, robust mechanical flexibility andgood oxygenand water barrier properties. However,it is hard to integrate all these properties in one kind of substrate. Currently, there are mainly three types of flexible substrates applied in FPSCs, suchas plastic polymersubstrates,metalfoils, andglass substrates.

Polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films arethemostusedflexible polymer substrates for FPSCs due to their high optical transmittance, mechanical robustness, good chemical resistance and R2R processability. Thecurrent FPSCs withrecordefficiency are prepared on these polymer substrates[19]. Although thesepolymer substrates could enable FPSCs with high efficiency and mechanicalflexibility,some drawbackscannot be ignored.First,the thermal resistancesof these plasticsubstrates arerelatively poor. The glass phase transition temperatures for PET and PEN are78 and 120°C, respectively[27]. Highertemperature could deform the substratesand degrade the efficiencyand flexibility of devices. Therefore, the whole fabrication processes and working conditions must be conducted at a low temperature. Second,thepolymersubstrates have high water and oxygen transmission rates, which seriously degrade the long-term stability of FPSCs[28]. Encapsulation technologiesor newmaterialsare desired to solve thesedrawbacks[29,30].

Metal foils have better thermal resistance, lower water and oxygen transmissionrate and highermechanical durability. They can be used as substrates, electrodes and even CTLs in one cell simultaneously, which significantly simplifies the fabricationprocess. TiandCufoilsarethe most used materials for flexible substrate[31−34]. Since the metal foils are opaque, transparent top electrodes are needed. Leeet al. introduced Ti metalsubstrate toprepare FPSC witha structure of Ti/TiO2/mesoporous TiO2/perovskite/Spiro-MeTAD/Ag and a PCE of over 6% for the first time[35]. The large resistance of ultrathinAg film asthetop electrodeseriouslysuppressedthe performance. Heoet al. reported a Ti based FPSC with a record efficiency over 15% by employing graphene as top transparent electrode[36].For theFPCs based onmetal foils,the exploration of top electrode with high optical transmittance and low resistance is the critical issue.

A glasssubstrate with thicknesslower than severalhundred micrometers could become mechanically flexible[37]. The flexible glass substrate owns the similar properties with rigid glass counterparts. Tavakoliet al.firstemployed willow glasses with thickness of 50μm as flexible substrates to fabricate FPSCs with a champion efficiency of 12.06%[38]. The flexibledevicecould maintainoriginal performance after200 bending cycles with a radius of 4 cm. Daiet al. used blade-coating method to fabricated flexible perovskite module on flexible glass substrate with a high PCE of 15.86% and an area of 42.9cm2. Thesmall-area(8mm2)devicecould achieve a best efficiency of 19.72%[39]. Although flexible glass substrates have several advantages, the fragility nature and high cost prevent thepracticalapplications.

2.2.Transparent electrode

ITO is the most usedtransparent electrode due to high optical transmittance andlowresistance. While thehigh production cost is a drawback for the mass production due to the presence of noble metal. Therefore, other transparent conductive oxides electrodes have been explored as alternatives to prepare FPSCs, such as aluminum-doped zinc oxide (AZO)[37].These conductive oxide electrodes usually are fragile, which limits their applications in extreme conditions, such as deformable and stretchable devices.

Solution-processable metal nanowire/mesh are promisingmaterials for transparent electrodes dueto highconductivity, transparency and flexibility[40]. Moreover, the solution processability for metal nanowires couldenableR2R mass production. Ag/Cu/Ni nanowires/mesh have been widely used in FPSCs as electrodes[41−44]. Recently, Liet al. reported a FPSC withanefficiency of 17.3% based on nickelmesh as electrode with low sheet resistance and high transmittance[45](Fig.1(a)). The resultantflexible devicecould retain76% of the initial efficiency after 2000 bending cycles. For these metal nanowire/mesh applied in FPSCs, surface modification is usually needed due to the high roughness and chemical reaction with perovskite materials. ITO, AZO, graphene oxide(GO), Graphene and PEDOT:PSS have been coated on surface to solve these issues[48−50].

Fig. 1. (Color online) (a) Schematic illustration of the PET/Ni-mesh substrate, Ni-mesh:PH1000 hybrid electrode, and perovskite device; the top right image shows the SEM image of Ni-mesh. The right is the optical image of PET/Ni-mesh substrate. Reprinted with permission from Ref. [45].Copyright 2020, John Wiley & Sons, Inc. (b) Schematic diagram of a flexible PSC with the structure of PET/graphene/P3HT/perovskite/phenyl-C70-butyric acid methylester (PCBM)/Ag. (c) The sheet resistances of one and two layers of CVD graphene transferred by using poly(methyl methacrylate) (PMMA) or P3HT. Reprinted with permission from Ref. [46]. Copyright 2016, Elsevier Ltd. (d) The sheet resistances of SWCNT films with different optical transmittance values before and after HNO3 treatment. (e) Dry transfer procedure of a SWCNT film and transferred SWCNT on glass and PEN substrates. Reprinted with permission from Ref. [47]. Copyright 2021, John Wiley & Sons, Inc.

Carbon-based materials, such as graphene and carbon nanotubes, are investigated as transparent electrodes for FPSCs due totheir high transmittance and conductivity[51−54].Graphene was first used by Liuet al. as electrode to fabricate FPSCs with achampionefficiencyof 11.5%[46](Figs. 1(b)and 1(c)). Jeonet al. have compared the performance of FPSCs based on graphene and single-walled carbon nanotubes(SWNTs)[55]. The better morphologyandtransparency of graphene enabled higher efficiency. While the SWNTs based FPSCsexhibitedbetter mechanicalflexibility duetothe randomly oriented SWNTs. Recently, Zhanget al. used SWNTs as electrode by simple drytransfer technologytofabricate a FPSC with a high efficiency of 18.1%[47](Figs. 1(d) and 1(e)).

PEDOT:PSS is a widely used conductive polymer material inflexible and stretchable electronics[56]. As-cast PEDOT:PSS films could have high conductivity up to about 4000 S/cm by various doping and coating strategies[57]. Therefore, it is a promising alternative for ITO electrodes in solar cells. Huet al.used high conductive PEDOT:PSS toprepare FPSCswith a best efficiency up to 19%[58]. Although the high efficiency can be achieved based on PEDOT:PSS electrode, its acidic property could corrode theperovskite materials and seriously degradethe long-termstability of devices.

2.3.Charge transport layers

Inaworking PSC, a charge transport layerwould extract charge carriers from perovskite absorber and transport to corresponding electrode. Therefore, the band structure, film morphology andmobility are key factors fortheselection andpreparation of CTLs for high performance and stable PSCs[59].Due to the temperature limitation of flexible substrates, low temperature process is anadditionalrequirement for CTLs.

In FPSCs with n–i–p structure, low temperature processed ZnO, SnO2, TiO2are usually used inorganic electron transport layers(ETLs)[61−65]. Among thevarious preparation methods, deposition of ETLs with pre-synthesized nanoparticles (NPs)or nanocrystals with high crystallinity is a successful and R2R compatible strategy to prepare high-quality CTLs on flexible substrates at low temperature[66,67]. Besides the inorganic materials, other low temperature processed materials, such as C60, metal-organic framework, and ionic liquid,are alsoexplored asETLsforFPSCs[68−70]. PEDOT:PSS,poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and Ni-Oxare the most used hole transport layer (HTL) materials for FPSCs with p–i–n structure. PEDOT:PSS andPTAA could be processed at low temperature[71,72]. For NiOx, pre-synthesized nanoparticles/nanocrystals can besuccessfully deposited on flexible substrates[73]. Due to the deformation of flexible substrates during fabrication process, the deposited CTLs usuallyhave holes anddefects. Furthermodificationsareusually needed[74,75]. Chunget al. presented a new porous planar ETL based onSnO2NPsand Zn2SnO4NPs asa compactlayer and porous layer to successfully fabricate a FPSC with a PCE of 20.7%[60](Fig. 2). This porous planar structure may be an efficient wayto overcome thedrawbacks of high roughness and deformation of flexible substrates.

2.4.High-quality perovskite layer

The processing approaches and compositions of perovskite films on flexible substrates are different from counterparts on rigidonesdue tothe different properties of substrates. The preparation methods at low temperature and compatible with R2R mass production are highly desirable for practical applications.

Laser annealing isan efficient method crystalizing perovskite films at room temperature at a fast rate. Jeonet al.first used a laser with wavelength of 1064 nm to anneal the perovskite film viaphoto-thermal heating induced by the light absorption of ITO and PEDOT:PSS layers[76](Fig. 3(a)).The prepared FPSC demonstrated an efficiency of 8.0%. Youet al. used alaserwithvisible light to directlyinduce photothermal heatingin perovskitefilms,which leadto largegrain size and low defect density[79]. Vacuum deposition methods have also been used to prepare perovskite films on flexible substrates. Recently, Fenget al.reportedFA based rigid PSCs with a record efficiency of 21.32% prepared by a vacuum deposition approach assisted by 60 °C thermal annealing[77](Figs. 3(b)and 3(c)).Forvacuumdeposition,high costand energy consumption are main drawbacks.

Fig. 2. (Color online) (a) TEM image of planar layer and porous planar layer coated on ITO/glass substrate and EDS mapping. (b) J–V curves of planar (green) and porous planar (orange) flexible unit cell (active area: 0.094 cm2) measured in the lab and that of Newport certification data (purple). The device based on porous planar layer exhibits a lab PCE of 20.75% and a certified PCE of 19.9%. The planar device demonstrates a PCE of 17.5%. (c) Photograph of flexible module based on porous planar CTL with an area of 400 cm2. (d) J–V curves of best-performing flexible sub-module at an aperture area of 100, 225 and 400 cm2. Reprinted with permission from Ref. [60]. Copyright 2020, Royal Society of Chemistry.

Fig. 3. (Color online) (a) Schematic description of perovskite film formation process by laser annealing method. Reprinted with permission from Ref. [76]. Copyright 2016, American Chemical Society. (b) Schematic illustration of multisource vacuum deposition with an in-vacuum annealing process for large-area perovskite films. Photographs of FA-based perovskite films deposited on (c) glass and (d) PET substrates. Reprinted with permission from Ref. [77]. Copyright 2021, Royal Society of Chemistry. (e) Cross-section SEM images of an FPSC with a structure of PEN/ITO/SnO2/3D/2D perovskite/ Spiro-OMeTAD/Ag and J–V curves of 3D and 2D/3D FPSCs. Reprinted with permission from Ref. [20]. Copyright 2021, John Wiley & Sons, Inc. (f) Schematic illustrate of the interaction between s-GO with perovskite. (g) Schematic diagram of the enhanced water resistance with flexural endurance due to cementation and passivation of grain boundaries. Reprinted with permission from Ref.[78]. Copyright 2020, Elsevier Ltd.

Fig. 4. (Color online) (a) Diagram showing R2R processing for the fabrication of FPSCs. (b) Photograph of fully R2R processed PSCs. (c) The J–V curves of fully R2R gravure-printed PSCs. Reprinted with permission from Ref. [24]. Copyright 2020, Nature Publishing Group.

Composition tailoring for perovskite precursor is necessary since thedirecttransferofperovskitedeposition from rigid to flexiblesubstrates usually leads to poor morphology and low performance[80]. Dimensional composition engineering might be apromisingmethod for high performance FPSCs[20,81](Fig. 3(e)). Besides, various additives have been introduced into perovskite films to manipulate the crystallization process and passivate the defects. Recently, Yanget al.used artemisinin to passivate the perovskite grains to boost the efficiency of FPSCs to a record value of 21.1%[19].Moreover, special additives havebeen introducedinto grain boundaries of perovskite films toimprovethe mechanical flexibility offlexible devices[82,83]. Huet al.introduced sulfonated graphene oxideto construct acementitiousgrainboundary in perovskite films[78](Figs. 3(f) and 3(g)). The flexible devices demonstrated better photovoltaic performance and mechanical flexibility due to the tough grain boundaries.

3.R2R fabrication

Low cost andhighefficiency aretwo advantages promising the commercialization of PSCs in the near future. R2R fabrication might be a breakthrough to realize commercialization due to the cost-effective high throughout mass production. In order to realize R2R production of FPSCs, uniform and large-area deposition of sequential layers should be achieved. The R2R depositionof CTLs have already been establishedin other photovoltaic technologies,such as organicsolar cells[84]. Thus, the investigationof FPSCsby R2R process mainly focusesonthe deposition of high quality perovskite films. It is key to depositing uniform precursor wet films with following complete conversion to perovskite phase[22]. Several R2R compatible coating technologies, such as blade coating, slot-die coating, spray coating and gravure-printing, have been applied[39,85−88]. In addition, the perovskite precursor compositionneeds to be tailored toobtain stableperovskite intermediate phase during the deposition process which is critical for the final qualityof perovskite films. Therefore,solvent tuningand additiveshave beeninvestigated.

Galaganet al. used dimethyl sulfoxide (DMSO) and 2-butoxyethanol as solvents for perovskite precursor to prepare FPSCs with a best PCE of 13.5%byR2R slot-diecoating method[86].It wasfound thattheaddition of2-butoxyethanol could reduce the surface tension ofprecursorleadingtobetterwettability and formation ofuniformlayer andaccelerate the perovskite crystallization processresulting intheformation oflarge crystals.Kimet al. employeda hot slot-diecoating method toprepare perovskitefilms by heating the substrate to 130°Cduring thedeposition process[89]. Poly(ethylene oxide)PEO was introducedinto perovskite films to significantly improvethe stability. The flexible devices exhibited a best PCE of 11.7%.

Daiet al. reported that theadditionofammonium chloride(NH4Cl) intothe perovskite precursorsolutioncould form high quality perovskitefilmwithgoodcontactwith substratesbyretardingthecrystallization process[39]. The bladecoated FPSCs with areas of 8 and 42.9 cm2could reach efficiencies of 19.72% and 15.86%, respectively. Wanget al. used thiourea as an additive to modulate the crystal growth of perovskite films by blade-coating deposition. An efficiency of 19.41% and a fill factor of 81% have been achieved[90].

Kimet al. employed tert-butyl alcohol (tBuOH) as an ecofriendly anti-solvent to prepare highly crystalline and uniform FA based perovskite film by gravure printing method on flexible substrates[24](Fig. 4). It was found that the antisolvent could extract the retardation mediator and excess processing solvent without dissolving the perovskite precursors from the wet film. The flexible devices based on gravure printed FAPbI3showed a best PCE of 19.1%. The fully R2R gravure printed FPSCs except for metal electrode exhibited a record efficiency of 13.8%.

Fig. 5. (Color online) (a) Photographs of silk derived electrodes using natural silkworm cocoons as raw materials, which show high transmittance and various deformation including wave, spiral, bowknot, flower and paper crane. Reprinted with permission from Ref. [91]. Copyright 2020,John Wiley & Sons, Inc. (b) Schematic illustration of the stretching process of the kirigami structure at left) initial state and right) stretching state.(c) Voltage response of kirigami-based PSCs in a stretching cycle. Reprinted with permission from Ref. [92]. Copyright 2020, American Chemical Society. (d) Digital photographs demonstrating left) the integrating process of the self-powered smart bracelet and right) a subject wearing the self-powered smart bracelet on wrist during outdoor running and indoor biking. Reprinted with permission from Ref. [17]. Copyright 2021, American Chemical Society. (e) A photo of a solar powered wearable sensor. Reprinted with permission from Ref. [25]. Copyright 2019, Elsevier Ltd.

Although significant improvements have been obtained on photovoltaic performance of R2R processed FPSCs, the current efficiency could not reach the commercialization level.More efforts are needed to study the deposition of high quality perovskite film by R2R compatible approaches. Higher efficiency (~20%) can been expected.

4.Emerging FPSCs

Due to the easy fabrication of perovskite films, different kinds of FPSCs have been successfully preparedonvarious flexiblesubstrates extending the applicability and facilitating opening an emerging photovoltaic market. The deformable andlightweight FPSCs couldbe integrated with different functional devices to construct self-powdered smart systems.

The plastic substrates such as PET and PEN would cause environmentalpollution dueto the long decompositionperiods[93]. This would hinder the wide use of FPSCs. Biodegradableand biocompatiblematerialscouldbeafeasiblestrategy to solve this issue. Zhuet al. synthesized a biocompatible flexile cellulose nanofibril (CNF) substrate derived from the bamboos[94]. The correspondingflexible devices demonstrated a champion efficiency of 11.68% with a good bending stability. Gaoet al. used a biodegradablenanocellulosepaper as a flexible substrate to fabricate FPSCs[95]. Silk has also been used to prepare biocompatible flexible substrates for FPSCs[91](Fig.5(a)).

The foldable and stretchable FPSCs are highly desirable for wearable electronicsand vehicle-and buildingintegrated photovoltaics. Kaltenbruneret al. fabricated FPSCs on PET substrates with a thickness of 1.4μm[16]. This lightweight device could obtaina record power-per-weightashighas23W/g,which is much higher than other established photovoltaic technologies.Liet al.used acellophane substratewithkirigami design to prepare FPSCs with a high stretchability (strain up to 200%), twistability (angle up to 450˚) and bendability (radius down to 0.5 mm)[92](Figs. 5(b) and 5(c)).

The application of flexible power supply by FPSCs has alsobeendemonstrated.Zhaoet al. reported a safe, flexible and self-powderedwristbandsystembyintegrating highperformance zinc-ion batterieswithFPSCs[17](Fig.5(d)). Liet al.demonstratedsuccessful fabrication of awearable strain sensors self-powdered by a lithium-ion capacitor recharged by a FPSC[25](Fig. 5(e)). These works highly extend and enrich the application of FPSCs.

5.Conclusion and outlook

In summary,FPSCs are supposedtobe apromising breakthrough for the next generation photovoltaic technology with a high possibility for commercialization. In addition, the successful fabrication of lightweight anddeformable FPSCs applied in various self-powdered systems could open an emerging and pioneering market. Therefore, the development of FPSCs will playacritical rolein thecommercializationpathway of PSCs. Interestingly, the fabrication techniques of FPSCs have been developed very fast and the efficiency over 21%hasbeenachieved. The integration with R2Rprocesshas been reported by several groups recently with promising performance achieved in large-area devices. In this review, we summarize therecent development of FPSCs,focusing on the key issues of flexible devices, R2R process and emerging applications of FPSCs, which provides a guideline for the future development of this field.

Tofurther improve the device performance,weshould carefully check and optimize each component of the devices.In the efficiencyevolutionofFPSCs, many efforts have been made in optimizing flexible substrates, transparentelectrodes, CTLs andperovskite absorbers forhigh performance FPSCs.For the threetypes of flexible substrates,including plastic polymer substrates, metal foils and flexible glasses,the polymer substrate-based devices demonstrate high efficiency and easy fabrication. Moreover, it is convenient to functionalize the polymer substrates (e.g. coating hydrophobic or antireflection layers), which will definitely enhance the performance of the FPSCs.

For transparent electrodes,ITO-baseddevices exhibit the highestefficiencywhile relatively poor mechanicalflexibility.PEDOT:PSScould be a better electrodefor stretchable and deformabledevices. However,perovskites may degrade on the acidic PEDOT:PSS surfaces, leading to poor stability of the devices, so surface modification on PEDOT:PSS is needed,such as coating a suitable CTL on the surface.

The low-temperature processed materials with pre-synthesized nanoparticles/nanocrystals structure should be a useful strategy to prepare efficient CTLs for FPSCs. In addition,the CTLs with porous structure could successfully overcome the drawback of high roughness of flexible substrates. More effortsareneeded tosuppressthenon-radiative recombinationand facilitate the chargecarriertransfer atthe CTL/perovskite interfaces. Similarly,low-temperature crystallization technologies of perovskite films are needed. In view of mass production, laser annealing may be a promising method to crystalize the perovskite films at room temperature with fast rate. The fabrication of large-scale FPSCs by R2R procession along with laser annealing is expected to be a feasible strategy.

The developmentof R2R fabricationof FPSCs is still at early stage and the record efficiency forfullyR2R processed devices is only 13.8%. Thereis a considerable room for further improvements. The key isto deposit uniformandpinhole-free wet perovskite precursor filmsbyR2R compatible methods. Novel additives and precursor compositions need to be further probed. The deformable and stretchable FPSCs have been demonstrated as power supplies by integrating with sensors, batteries, and aircrafts. Potential future applications could be power sources for industrial monitoring and tactical security applications.

Inthefuturestudies onFPSCs, our focus should beon but not be limited to the following aspects:(1)Enhancement of long-termstability. Theplasticpolymer substrates have limited water and oxygenbarrierabilities. Additional encapsulations on topand bottom ofdevicesarenecessary.(2)Largearea R2R fabrication. Although the record efficiency for FPSCs is over 21%, the corresponding device area is relatively small.Besides, the spin-coating method is not compatible with mass production. More efforts should be focused on the deposition of perovskite film with large scale by R2R compatible coating methods. (3) Extending the applicability of FPSCs. Due to high efficiency and mechanical flexibility, FPSCs have great potential tobeintegratedwith different systems as power supply. Thesuccessfulapplication inthe field of wearable/flexible electronics maybe apreferential commercializationoption. Overall, the rapid advancements along with the practical challenges in FPSCs suggest a bright future for this active field.