Nondestructive Transfer Strategy for High-Efficiency FlexiblePerovskite Solar CellsJiali Guo,† Yue Jiang,*,† Cong Chen,† Xiayan Wu,† Xiangyu Kong,† Zhuoxi Li,† Xingsen Gao,†

Qianming Wang,§ Xubing Lu,† Guofu Zhou,‡ Yiwang Chen,∥ Jun-Ming Liu,†,⊥

Krzysztof Kempa,†,# and Jinwei Gao*,†

†Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, and Guangdong Provincial Key Laboratoryof Quantum Engineering and Quantum Materials, ‡Guangdong Provincial Key Laboratory of Optical Information Materials andTechnology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, and §School ofChemistry, South China Normal University, Guangzhou 510006, China∥Institute of Polymers and Energy Chemistry, College of Chemistry, Nanchang University, Nanchang 330031, China⊥Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China#Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, United States

*S Supporting Information

ABSTRACT: Flexible perovskite solar cells (F-PSCs) have been developingfast with the power conversion efficiency (PCE) exceeding 19%. However,aiming at the high-efficiency F-PSCs, to get a desired perovskite morphologybefore and after glass-supportive device transfer is still a challenge. Herein, wethoroughly investigated the effect of adhesive materials of substrates on theperovskite film and the solar cell performance and developed a nondestructiveF-PSC transfer method by introducing a double-side tape/protective film/epoxy binder. This nondestructive transfer strategy leads to a uniformmorphology of perovskites, even after transfer process, yielding an enhancedPCE up to 16.55%.

KEYWORDS: nondestructive transfer technology, adhesive materials, double-side tape, epoxy, flexible perovskite solar cells

■ INTRODUCTION

Perovskite solar cells (PSCs) have been widely considered asthe most promising candidates for next-generation photo-voltaics because of their high power conversion efficiency(PCE), solution processability, low-cost, etc.1−3 Particularly,the low-temperature solution fabrication process of PSCsmakes them compatible with flexible substrates and therealization of flexible PSCs (F-PSCs). In the past 6 years,the F-PSCs have experienced a rapid development with PCEimproved from 2.62 to 19.5%,4−7 showing a bright future inthe application of portable and bendable devices.In spite of the significant progress, the PCE of F-PSCs still

lags behind that of PSCs on rigid substrates. Except theintrinsic limitation of the flexible substrate, the flat contactduring the film-forming process is also considered playing afundamental role in determining the final efficiency. To date,the preparation technology of F-PSCs mainly includes spin-coating,8 spray deposition,9 roll-to-roll fabrication,10 andvacuum deposition.11 The spin-coating method is the mostcommonly used in the laboratory, and the porous sucker wasusually applied to fix flexible substrates. Yet, the surfacefluctuation of the substrate and the formed films caused by the

porous suction cup is a big hurdle. Instead, by utilizing glass asthe rigid support followed by the transfer process to fabricateF-PSCs, the efficiency as high as 12.3% was reported.12

However, few information about the materials used to attachflexible substrates onto glass and the transfer technology hasbeen reported. Although polyimide has been used as a flexiblesubstrate for high-performance organic solar cells,13 it needscomplex pretreatment, especially at high temperature. Thus, tofabricate high-efficiency F-PSCs is not as reproducible as thaton the rigid substrates. Besides, this technology is only limitedin several groups.Herein, we have thoroughly investigated four kinds of easily

accessible adhesives [oil, glue, double-side tape (DST), andepoxy] as binders at the flexible substrate/glass interface forthe preparation of F-PSCs and developed a nondestructive F-PSC transfer method by combining DST and its protective film(PF) and epoxy. With the weak adhesion between PF andepoxy, this method realizes the complete nondestructive

Received: October 10, 2019Accepted: November 20, 2019Published: November 20, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 47003−47007

© 2019 American Chemical Society 47003 DOI: 10.1021/acsami.9b18090ACS Appl. Mater. Interfaces 2019, 11, 47003−47007

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transfer of the device and retains the high quality of theperovskite layer. After device transfer, the PCE of our F-PSCsis as high as 16.55%.

■ RESULTS AND DISCUSSION

The indium tin oxide (ITO)/polyethylene naphthalate (PEN)substrates were attached on the rigid glass with oil, glue, DST,and epoxy, separately (Figure 1a). Perovskite (CH3NH2PbI3)films of F-PSCs were obtained by one-step depositiontechnique on the ITO/PEN substrate, followed by thermalannealing at 100 °C.14 The morphology of the corresponding

obtained perovskite films was examined by a scanning electronmicroscope as shown in Figure 1b−f. Because of the weakadhesive of oil, the ITO/PEN substrates were detached fromthe glass during spin-coating, causing the deterioratemorphology of the perovskite (Figure 1c). While via directporous sucker adsorption (Figure 1b), glue (Figure 1d), andDST (Figure 1e), rough surface and pinholes from theperovskite films could be observed. One of the possible reasonscomes from the rough surface of ITO/PEN substrates whenheld by a porous sucker, which shows the irregular thickness ofthe perovskite as illustrated in Figure S1a−d. For oil and glue,

Figure 1. (a) Schematic illustration of the substrate. (b−f) Top-view scanning electron microscopy (SEM) images of perovskite films obtained byporous sucker adsorption, oil, glue, DST, and epoxy adhesive. Scale bars: 5 μm. The insets show high magnification of the SEM images. Scale bars:200 nm.

Figure 2. Instant thermal image of (a) bare PEN substrates and the PEN substrates attached on the rigid glass with (b) oil, (c) glue, (d) DST, and(e) epoxy adhesive on hot stage at 100 °C. (f−j) Their corresponding thermal images after 5 min heating.

Figure 3. (a) Peeling force of PF/DST, epoxy/PF, PEN/DST, and PEN/glue. (b) Schematic illustration of the transfer process. (c-e) Top-viewSEM images of the perovskite film obtained by DST/PF/epoxy, glue, and DST after transfer (scale bars: 5 μm).

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the wavy contact mainly arose from the bubbles which wereformed in the binders and expanded during thermal annealing(Figure S2a−d). By contrast, in Figure 1f, a dense, smooth, andhighly crystalline CH3NH2PbI3 film was achieved when epoxywas utilized, suggesting the firm and tight contact at flexiblesubstrate/glass interfaces.On the other hand, the thermal annealing process was

reported to play a critical role during perovskite crystallization,thus influencing the morphology of the perovskite.15 In thispaper, we have systematically investigated the perovskitethermal annealing process with temperature distributionimages based on the perovskite/PEN/binder/glass system(Figure 2). Figure 2a,f shows that the surface temperaturedistribution of PEN is inhomogeneous when it comes incontact with a hot plate at 100 °C, which was caused by theinevitable curving of PEN. The same phenomenon alsohappened to oil or glue binder because of the weak contactand the bubble formation (Figure 2b,c). Even after 5 min, theirtemperature distribution was still uneven (Figure 2g,h). Bysharp contrast, with DST and epoxy binder, at either thebeginning or the stable state of thermal annealing, they gave auniform temperature distribution as illustrated in Figure2d,e,i,j, possibly due to their tight contact between PEN andglass.The adhesive force of glue, DST, and epoxy with PEN plays

a fundamental role in the device detachment process, whichwas conducted by the peel-off test. Because of the strongadhesion of epoxy, the attached PEN was seriously ripped afterdetachment, resulting in the absence of reasonable strippingforce−displacement curve. While for glue and DST, theiradhesion with PEN was 0.23 and 12.44 N (Figure 3a),respectively, indicating that it is difficult to separate PEN fromDST. In addition, the glue residue on PEN could hamper thelight absorption. Simultaneously, we found that the adhesionstrength between DST and PF and between PF and epoxy wasrather weak (0.09 and 0.52 N, respectively), which indicatesthat the separation of DST/PF and PF/epoxy could be easiercompared to other adhesives. Therefore, we combined DSTwith epoxy by sticking the DST on glass and leaving its PFcovered with epoxy (DST/PF/epoxy, Figure 3b). The flexibleITO/PEN was then attached on top for the subsequentfabrication of F-PSCs. On account of the wide usage of epoxyin PSC capsulation as well as its high transmittance over thevisible light spectrum in Figure S3, the influence of epoxy atthe bottom of the devices could be fully ignored. The detailedscheme of this technology is shown in the SupportingInformation (Figure S4).Further, the influence of the F-PSC transfer process on the

morphology of perovskite films was studied. Figure 3c−eshows the SEM images of the perovskite film on ITO/PENafter transfer. It is noted that with DST/PF/epoxy, aftertransfer no destruction of the perovskite film could beobserved from Figure 3c. While by sharp contrast, withadhesive of glue or DST, rough surface and pinholes in Figure3d or cracking in Figure 3e were quite obvious. Hence, theimpact of the DST/PF/epoxy system on the perovskite films isnegligible.In Figure 4a, steady-state photoluminescence (PL) was

conducted to assess the quality of the perovskite filmstransferred from different adhesive materials. The PL intensitybased on the DST/PF/epoxy binder displays a stronger PLintensity compared with those from other binders (poroussucker, oil, glue, and DST) and only slightly lower than that on

ITO/glass. Thus, DST/PF/epoxy favors to give a betterelectronic property, namely, the reduced trap density of theperovskite film; thus, DST/PF/epoxy is considered as anappropriate binder for glass-support F-PSCs.Subsequently, a series of n−i−p-type planar F-PSCs with

configuration of PEN/ITO/SnO2/MAPbI3/Spiro-OMeTAD/Ag were fabricated with the porous sucker or by binding withoil, glue, DST, and DST/PF/epoxy on glass, respectively. Toexclude the possible processing variations, the devices werefabricated in the same batch, with identical procedures andparameters. Figure 4b shows the J−V curves of the F-PSCsbefore transfer, while Figure 4c shows the J−V curves afterdevice transfer. The parameters were concluded as shown inTable S1. When the solar cells were fixed via porous sucker,oil/glass, or glue, the PCE was 11.32% [open-circuit voltage(VOC) of 1.05 V, short-circuit current density (JSC) of 20.32mA cm−2, fill factor (FF) of 52.68%], 13.31% (VOC of 1.04 V,JSC of 20.88 mA cm−2, FF of 60.74%), and 11.69% (VOC of1.01 V, JSC of 19.18 mA cm−2, FF of 60.00%), respectively. Thelow PCE mainly comes from the low FF, which was caused bythe defects in the perovskite layer as shown in Figures 1a−c,S1a. While as expected, with the DST binder, because of thedamaged perovskite film caused by the strong pull force duringtransfer, PCE was as low as 0.32% (VOC of 0.13 V, JSC of 8.91mA cm−2, FF of 26.72%). Notably, with the DST/PF/epoxybinder, the PCE was improved to as high as 16.55% with a VOCof 1.06 V, a JSC of 21.46 mA cm−2, and an FF of 72.21%.Compared with the parameters after device transfer, the initiallower PCE is due to the limited JSC caused by light absorptionby DST. To study the reproducibility of the devices, 10 PSCsbased on the porous sucker and four kinds of adhesives werefabricated and characterized after transfer. The histograms ofPCE are provided in Figure S5. The average PCEs of thesecells are 7.91, 11.01, 11.09, 5.97, and 13.8% built upon theporous sucker and four kinds of adhesive materials, which arein accordance with the PCEs obtained from the best devices.The smaller standard deviation (2.29 vs 2.17 vs 1.19 vs 3.34 vs1.69) implies the superior excellent repeatability for the devicewith DST/PF/epoxy.Figure 4d shows the Nyquist plots of the F-PSCs measured

at 1 V bias. The results of electrochemical impedance

Figure 4. (a) Steady-state PL spectra of perovskite/ITO/PEN (orglass). J−V curves of F-PSCs before (b) and after (c) transfer. (d) EISwith the inset showing the local EIS.

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spectroscopy (EIS) spectra show that F-PSCs attached viaDST/PF/epoxy present a small characteristic arc in the high-frequency range, which is comparable with PSCs based onITO/glass, illustrating a low transfer resistance (Rtr). It is alsosuggestive that a better interfacial contact was formed in ourdesigned binder.16 This result agrees well with its surfacemorphology in Figure 3, as well as the observed high FF.To further confirm our nondestructive transfer technology

(DST/PF/epoxy), the stability and flexibility of F-PSCs aftertransfer were studied. The PCE of nonencapsulated F-PSCsand PSCs on the rigid glass was monitored at the humidity ca.35%. After 2 weeks, F-PSCs maintained 79% of their initialefficiency, which is close to that on ITO/glass (78.7%, inFigure 5a). Meanwhile, regarding the flexibility, F-PSCs were

tested under different bending curvature radii over 2000 cycles.Figure 5b gives the PCE vs curvature radius plots, with theinset showing bending radius as defined by a Vernier caliper.Although the cracks of ITO could be observed at the bendingradius less than 14 mm,17 our F-PSCs still exhibit more than90% of their initial efficiency at the radius of 11.11 mm.Therefore, with DST/PF/epoxy, the F-PSCs displayed theoutstanding stability and flexibility, demonstrating that ourtransfer technology is nondestructive and fully applicable forflexible solar cells.

■ CONCLUSIONSTo conclude, we have thoroughly investigated the effect ofadhesive materials on F-PSCs with the glass-supportivefabrication process and developed a nondestructive F-PSCtransfer method with the DST/PF/epoxy binder. With ourmethod, the morphology and quality of the perovskite werewell retained, and the F-PSCs show excellent stability andflexibility. The PCE was enhanced up to 16.55%, which ismuch higher than those with other binders. Moreover, ourmethod is also adaptable for the fabrication of other flexibledevices, such as flexible transistors and supercapacitors.

■ EXPERIMENTAL SECTIONMaterials. 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9-

spirobifluorene (Spiro-OMeTAD), methylammonium iodide, leadiodide (PbI2, 99%, Yingkou You Xuan Trade Co. Ltd), bis-

(trifluoromethanesulfonyl)imide (Li-TFSI), tert-butylpyridine (t-BP,Sigma-Aldrich), and SnCl2·2H2O (Alfa Aesar) were used as received.Dimethyl formamide (DMF, 99.8%), dimethyl sulfoxide (DMSO,99.9%), chlorobenzene (CB, 99.8%), acetonitrile, ethanol, isopropa-nol, and butanol were purchased from Sigma-Aldrich without furtherpurification.

Siloxane oil (Tianjin Zhi Yuan Chemical Reagent Co. Ltd), glue(Deli group co. Ltd), DST (Shenzhen Chang Da Sheng ElectronicsCo. Ltd), epoxy resin, and curing agent of epoxy resin (Kunshan LvXun Materials Co. Ltd) were also used.

Substrate Preparation. Liquid epoxy resin was prepared bymixing epoxy resin and epoxy resin curing agent (2:1 V/V). Theglasses with specification 15 mm × 15 mm were coated with siloxaneoil, glue, epoxy, DST, and DST/PF/epoxy, which were subsequentlycovered by ITO/PEN and cured for 3 h at room temperature.

Perovskite Precursor Preparation. Perovskite precursor(CH3NH3I·PbI2) was followed from the reported literature.18 PbI2(922 mg) and 320 mg of CH3NH3I were dissolved in 1.6 mL of mixedsolvent of DMF and DMSO (7:3 V/V) and stirred at 300 rpm in aglovebox until dissolved.

PSC Fabrication. The substrates were further cleaned by UV−ozone for 10 min prior to use. The SnO2 solution was followed fromthe reported literature.19 The SnO2 electron transport layers wereprepared by spin-coating SnO2 solution on ITO/PEN at 2000 rpm for30 s, followed by thermal annealing at 120 °C for 1 h to totallyremove the solvent. Then, the MAPbI3 precursor solution was spin-coated on SnO2/ITO/PEN substrates at 500 rpm for 3 s and 4000rpm for 30 s, with quick dripping of 400 μL of CB onto the rotatingperovskite film at the beginning of 8−10 s of the second spin-coatingstep. Then, the films were annealed at 100 °C for 10 min. Twenty-fivemicroliters of Spiro-OMeTAD solution [72.3 mg of Spiro-OMeTAD,17.5 μL of Li-TFSI solution (520 mg in 1 mL of acetonitrile), 28.8 μLof t-BP, and 1 mL of CB] was spin-coated onto the perovskite layer at3000 rpm for 30 s. Finally, the Ag electrode was thermally evaporatedon top of the device under high vacuum (<5 × 10−4 Pa). The ITO/PEN with devices was peeled off from the substrate. Finally, the PF ofDST was completely separated from the ITO/PEN substrate.

Characterization. The morphology and microstructures werecharacterized by field emission SEM (ZEISS Ultra-55) and trans-mission electron microscopy (JEM-2100). Thermal images wereobtained by an infrared camera (FLUKE Ti29). The adhesionstrength test 180° peel-off test was conducted by the adhesion testingmachine (AGS-X 100N). The adhesive tape 20 mm in width wasadhered to the glass. Steady-state PL was measured by a fluorescencespectrometer (HITACHI F-5000) excited at 450 nm. The J−Vcharacteristics of the devices were measured with a Keithley 2440source under a simulated AM1.5G spectrum. With a solar simulator(Newport, 91160), the light intensity was calibrated using a standardsilicon solar cell device by the NREL. The EIS measurements wereperformed on the Zahner Zennium electrochemical workstationunder dark condition.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acsami.9b18090.

Detailed photographs of different binder systems and theJ−V curve parameters of F-PSCs (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: yuejiang@m.scnu.edu.cn (Y.J.).*E-mail: gaojinwei@m.scnu.edu.cn (J.G.).ORCIDXiangyu Kong: 0000-0003-3062-1978Xingsen Gao: 0000-0002-2725-0785Qianming Wang: 0000-0003-2795-6056

Figure 5. (a) Stability test and (b) flexibility test at different bendingdistances after 2000 bending cycles of F-PSCs with the DST/PF/epoxy binder and ITO/glass.

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Xubing Lu: 0000-0002-2552-9571Guofu Zhou: 0000-0003-1101-1947Yiwang Chen: 0000-0003-4709-7623Jun-Ming Liu: 0000-0001-8988-8429Jinwei Gao: 0000-0002-4545-1126Author ContributionsJ.G. designed and conducted this experiment. C.C., X.W., andX.K. contributed to the materials and device characterization.J.G., Y.J., and J.G. contributed to the writing. K.K., J.-M.L., andG.Z. participated in the results discussion. All authorscommented on the final paper. J.G. and Y.J. directed theproject.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank the financial support from NSFC-Guangdong JointFund (no. U1801256) and Guangdong Provincial Foundation(2016KQNCX035), National Key R&D Program of China(no. 2016YFA0201002), NSFC grant (nos. 51803064,51571094, 51431006, 51561135014, U1501244), SCNUUniversity Foundation (16KJ06), Program for Chang JiangScholars and Innovative Research Teams in Universities (no.IRT_17R40), and Guangdong Innovative Research TeamProgram (no. 2013C102). We also thank the support from theGuangdong Provincial Engineering Technology ResearchCenter for Transparent Conductive Materials, National Centerfor International Research on Green Optoelectronics (IrGO),and MOE International Laboratory for Optical InformationTechnologies and the 111 Project.

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