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Nondestructive Transfer Strategy for High-E ﬃ ciency Flexible Perovskite Solar Cells

Jiali 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 Laboratory of Quantum Engineering and Quantum Materials,^‡Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, and^§School of Chemistry, 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 developing fast with the power conversion efficiency (PCE) exceeding 19%. However, aiming at the high-efficiency F-PSCs, to get a desired perovskite morphology before and after glass-supportive device transfer is still a challenge. Herein, we thoroughly investigated the effect of adhesive materials of substrates on the perovskitefilm and the solar cell performance and developed a nondestructive F-PSC transfer method by introducing a double-side tape/protective film/

epoxy binder. This nondestructive transfer strategy leads to a uniform morphology of perovskites, even after transfer process, yielding an enhanced PCE up to 16.55%.

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

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INTRODUCTION

Perovskite solar cells (PSCs) have been widely considered as the most promising candidates for next-generation photovoltaics because of their high power conversion efficiency (PCE), solution processability, low-cost, etc.¹⁻³ Particularly, the low-temperature solution fabrication process of PSCs makes them compatible with flexible substrates and the realization of flexible PSCs (F-PSCs). In the past 6 years, the F-PSCs have experienced a rapid development with PCE improved from 2.62 to 19.5%,⁴⁻⁷ showing a bright future in the 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 the intrinsic limitation of the flexible substrate, the flat contact during the film-forming process is also considered playing a fundamental role in determining thefinal efficiency. To date, the preparation technology of F-PSCs mainly includes spin- coating,⁸ spray deposition,⁹ roll-to-roll fabrication,¹⁰ and vacuum deposition.¹¹ The spin-coating method is the most commonly used in the laboratory, and the porous sucker was usually applied to fix flexible substrates. Yet, the surface fluctuation of the substrate and the formedfilms caused by the

porous suction cup is a big hurdle. Instead, by utilizing glass as the rigid support followed by the transfer process to fabricate F-PSCs, the efficiency as high as 12.3% was reported.¹² However, few information about the materials used to attach flexible substrates onto glass and the transfer technology has been reported. Although polyimide has been used as aflexible substrate for high-performance organic solar cells,¹³ it needs complex pretreatment, especially at high temperature. Thus, to fabricate high-efficiency F-PSCs is not as reproducible as that on the rigid substrates. Besides, this technology is only limited in several groups.

Herein, we have thoroughly investigated four kinds of easily accessible adhesives [oil, glue, double-side tape (DST), and epoxy] as binders at the ﬂexible substrate/glass interface for the preparation of F-PSCs and developed a nondestructive F- PSC transfer method by combining DST and its protectiveﬁlm (PF) and epoxy. With the weak adhesion between PF and epoxy, this method realizes the complete nondestructive

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

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transfer of the device and retains the high quality of the perovskite layer. After device transfer, the PCE of our F-PSCs is as high as 16.55%.

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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 deposition technique on the ITO/PEN substrate, followed by thermal annealing at 100°C.¹⁴The morphology of the corresponding

obtained perovskitefilms was examined by a scanning electron microscope as shown in Figure 1b−f. Because of the weak adhesive of oil, the ITO/PEN substrates were detached from the glass during spin-coating, causing the deteriorate morphology of the perovskite (Figure 1c). While via direct porous sucker adsorption (Figure 1b), glue (Figure 1d), and DST (Figure 1e), rough surface and pinholes from the perovskitefilms could be observed. One of the possible reasons comes from the rough surface of ITO/PEN substrates when held by a porous sucker, which shows the irregular thickness of the perovskite as illustrated inFigure S1a−d. For oil and glue, Figure 1.(a) Schematic illustration of the substrate. (b−f) Top-view scanning electron microscopy (SEM) images of perovskitefilms obtained by porous sucker adsorption, oil, glue, DST, and epoxy adhesive. Scale bars: 5μm. 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-view SEM images of the perovskiteﬁlm obtained by DST/PF/epoxy, glue, and DST after transfer (scale bars: 5μm).

ACS Applied Materials & Interfaces

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the wavy contact mainly arose from the bubbles which were formed in the binders and expanded during thermal annealing (Figure S2a−d). By contrast, inFigure 1f, a dense, smooth, and highly crystalline CH₃NH₂PbI₃film was achieved when epoxy was utilized, suggesting the firm and tight contact atflexible substrate/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 this paper, we have systematically investigated the perovskite thermal annealing process with temperature distribution images based on the perovskite/PEN/binder/glass system (Figure 2). Figure 2a,f shows that the surface temperature distribution of PEN is inhomogeneous when it comes in contact with a hot plate at 100°C, which was caused by the inevitable curving of PEN. The same phenomenon also happened to oil or glue binder because of the weak contact and the bubble formation (Figure 2b,c). Even after 5 min, their temperature distribution was still uneven (Figure 2g,h). By sharp contrast, with DST and epoxy binder, at either the beginning or the stable state of thermal annealing, they gave a uniform temperature distribution as illustrated in Figure 2d,e,i,j, possibly due to their tight contact between PEN and glass.

The adhesive force of glue, DST, and epoxy with PEN plays a fundamental role in the device detachment process, which was conducted by the peel-off test. Because of the strong adhesion of epoxy, the attached PEN was seriously ripped after detachment, resulting in the absence of reasonable stripping force−displacement curve. While for glue and DST, their adhesion with PEN was 0.23 and 12.44 N (Figure 3a), respectively, indicating that it is difficult to separate PEN from DST. In addition, the glue residue on PEN could hamper the light absorption. Simultaneously, we found that the adhesion strength between DST and PF and between PF and epoxy was rather weak (0.09 and 0.52 N, respectively), which indicates that the separation of DST/PF and PF/epoxy could be easier compared to other adhesives. Therefore, we combined DST with epoxy by sticking the DST on glass and leaving its PF covered with epoxy (DST/PF/epoxy,Figure 3b). Theflexible ITO/PEN was then attached on top for the subsequent fabrication of F-PSCs. On account of the wide usage of epoxy in PSC capsulation as well as its high transmittance over the visible light spectrum inFigure S3, the influence of epoxy at the bottom of the devices could be fully ignored. The detailed scheme of this technology is shown in the Supporting Information(Figure S4).

Further, the influence of the F-PSC transfer process on the morphology of perovskite films was studied. Figure 3c−e shows the SEM images of the perovskite film on ITO/PEN after transfer. It is noted that with DST/PF/epoxy, after transfer no destruction of the perovskite film could be observed from Figure 3c. While by sharp contrast, with adhesive of glue or DST, rough surface and pinholes inFigure 3d or cracking in Figure 3e were quite obvious. Hence, the impact of the DST/PF/epoxy system on the perovskitefilms is negligible.

In Figure 4a, steady-state photoluminescence (PL) was conducted to assess the quality of the perovskite ﬁlms transferred from diﬀerent adhesive materials. The PL intensity based on the DST/PF/epoxy binder displays a stronger PL intensity compared with those from other binders (porous sucker, oil, glue, and DST) and only slightly lower than that on

ITO/glass. Thus, DST/PF/epoxy favors to give a better electronic property, namely, the reduced trap density of the perovskite ﬁlm; thus, DST/PF/epoxy is considered as an appropriate 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 with oil, glue, DST, and DST/PF/epoxy on glass, respectively. To exclude the possible processing variations, the devices were fabricated in the same batch, with identical procedures and parameters. Figure 4b shows the J−V curves of the F-PSCs before transfer, while Figure 4c shows the J−V curves after device transfer. The parameters were concluded as shown in Table S1. When the solar cells werefixed via porous sucker, oil/glass, or glue, the PCE was 11.32% [open-circuit voltage (V_OC) of 1.05 V, short-circuit current density (J_SC) of 20.32 mA cm⁻²,fill factor (FF) of 52.68%], 13.31% (V_OCof 1.04 V, J_SC of 20.88 mA cm⁻², FF of 60.74%), and 11.69% (V_OC of 1.01 V,J_SCof 19.18 mA cm⁻², FF of 60.00%), respectively. The low PCE mainly comes from the low FF, which was caused by the defects in the perovskite layer as shown inFigures 1a−c, S1a. While as expected, with the DST binder, because of the damaged perovskitefilm caused by the strong pull force during transfer, PCE was as low as 0.32% (V_OCof 0.13 V,J_SCof 8.91 mA cm⁻², FF of 26.72%). Notably, with the DST/PF/epoxy binder, the PCE was improved to as high as 16.55% with aV_OC of 1.06 V, a J_SC of 21.46 mA cm⁻², and an FF of 72.21%.

Compared with the parameters after device transfer, the initial lower PCE is due to the limitedJ_SCcaused by light absorption by DST. To study the reproducibility of the devices, 10 PSCs based on the porous sucker and four kinds of adhesives were fabricated and characterized after transfer. The histograms of PCE are provided in Figure S5. The average PCEs of these cells are 7.91, 11.01, 11.09, 5.97, and 13.8% built upon the porous sucker and four kinds of adhesive materials, which are in accordance with the PCEs obtained from the best devices.

The smaller standard deviation (2.29 vs 2.17 vs 1.19 vs 3.34 vs 1.69) implies the superior excellent repeatability for the device with 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 (or glass).J−Vcurves of F-PSCs before (b) and after (c) transfer. (d) EIS with the inset showing the local EIS.

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spectroscopy (EIS) spectra show that F-PSCs attached via DST/PF/epoxy present a small characteristic arc in the high- frequency range, which is comparable with PSCs based on ITO/glass, illustrating a low transfer resistance (R_tr). It is also suggestive that a better interfacial contact was formed in our designed binder.¹⁶ This result agrees well with its surface morphology inFigure 3, as well as the observed high FF.

To further conﬁrm our nondestructive transfer technology (DST/PF/epoxy), the stability andﬂexibility of F-PSCs after transfer were studied. The PCE of nonencapsulated F-PSCs and PSCs on the rigid glass was monitored at the humidity ca.

35%. After 2 weeks, F-PSCs maintained 79% of their initial eﬃciency, which is close to that on ITO/glass (78.7%, in Figure 5a). Meanwhile, regarding theﬂexibility, F-PSCs were

tested under diﬀerent bending curvature radii over 2000 cycles.

Figure 5b gives the PCE vs curvature radius plots, with the inset showing bending radius as deﬁned by a Vernier caliper.

Although the cracks of ITO could be observed at the bending radius less than 14 mm,¹⁷our F-PSCs still exhibit more than 90% of their initial eﬃciency at the radius of 11.11 mm.

Therefore, with DST/PF/epoxy, the F-PSCs displayed the outstanding stability and ﬂexibility, demonstrating that our transfer technology is nondestructive and fully applicable for ﬂexible solar cells.

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CONCLUSIONS

To conclude, we have thoroughly investigated the effect of adhesive materials on F-PSCs with the glass-supportive fabrication process and developed a nondestructive F-PSC transfer method with the DST/PF/epoxy binder. With our method, the morphology and quality of the perovskite were well retained, and the F-PSCs show excellent stability and flexibility. The PCE was enhanced up to 16.55%, which is much higher than those with other binders. Moreover, our method is also adaptable for the fabrication of otherflexible devices, such asflexible transistors and supercapacitors.

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EXPERIMENTAL SECTION

Materials. 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9- spirobiﬂuorene (Spiro-OMeTAD), methylammonium iodide, lead iodide (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 further puriﬁcation.

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

Substrate Preparation. Liquid epoxy resin was prepared by mixing epoxy resin and epoxy resin curing agent (2:1 V/V). The glasses with speciﬁcation 15 mm×15 mm were coated with siloxane oil, glue, epoxy, DST, and DST/PF/epoxy, which were subsequently covered 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 mixed solvent of DMF and DMSO (7:3 V/V) and stirred at 300 rpm in a glovebox until dissolved.

PSC Fabrication.The substrates were further cleaned by UV−

ozone for 10 min prior to use. The SnO2solution was followed from the reported literature.¹⁹ The SnO2 electron transport layers were prepared by spin-coating SnO₂solution on ITO/PEN at 2000 rpm for 30 s, followed by thermal annealing at 120 °C for 1 h to totally remove the solvent. Then, the MAPbI₃precursor solution was spin- coated on SnO₂/ITO/PEN substrates at 500 rpm for 3 s and 4000 rpm for 30 s, with quick dripping of 400μL of CB onto the rotating perovskitefilm at the beginning of 8−10 s of the second spin-coating step. Then, thefilms were annealed at 100°C for 10 min. Twenty-five microliters of Spiro-OMeTAD solution [72.3 mg of Spiro-OMeTAD, 17.5μL of Li-TFSI solution (520 mg in 1 mL of acetonitrile), 28.8μL oft-BP, and 1 mL of CB] was spin-coated onto the perovskite layer at 3000 rpm for 30 s. Finally, the Ag electrode was thermally evaporated on top of the device under high vacuum (<5×10⁻⁴Pa). The ITO/

PEN with devices was peeled oﬀfrom the substrate. Finally, the PF of DST was completely separated from the ITO/PEN substrate.

Characterization. The morphology and microstructures were characterized by field emission SEM (ZEISS Ultra-55) and trans- mission electron microscopy (JEM-2100). Thermal images were obtained by an infrared camera (FLUKE Ti29). The adhesion strength test 180°peel-offtest was conducted by the adhesion testing machine (AGS-X 100N). The adhesive tape 20 mm in width was adhered to the glass. Steady-state PL was measured by afluorescence spectrometer (HITACHI F-5000) excited at 450 nm. The J−V characteristics of the devices were measured with a Keithley 2440 source under a simulated AM1.5G spectrum. With a solar simulator (Newport, 91160), the light intensity was calibrated using a standard silicon solar cell device by the NREL. The EIS measurements were performed on the Zahner Zennium electrochemical workstation under dark condition.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.9b18090.

Detailed photographs of diﬀerent binder systems and the J−Vcurve parameters of F-PSCs (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail:[email protected](Y.J.).

*E-mail:[email protected](J.G.).

ORCID

Xiangyu Kong:0000-0003-3062-1978

Xingsen Gao:0000-0002-2725-0785

Qianming Wang:0000-0003-2795-6056 Figure 5.(a) Stability test and (b)ﬂexibility test at diﬀerent bending

distances after 2000 bending cycles of F-PSCs with the DST/PF/

epoxy binder and ITO/glass.

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Xubing Lu: 0000-0002-2552-9571

Guofu Zhou:0000-0003-1101-1947

Yiwang Chen:0000-0003-4709-7623

Jun-Ming Liu:0000-0001-8988-8429

Jinwei Gao:0000-0002-4545-1126 Author Contributions

J.G. designed and conducted this experiment. C.C., X.W., and X.K. contributed to the materials and device characterization.

J.G., Y.J., and J.G. contributed to the writing. K.K., J.-M.L., and G.Z. participated in the results discussion. All authors commented on the ﬁnal paper. J.G. and Y.J. directed the project.

Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

We thank theﬁnancial support from NSFC-Guangdong Joint Fund (no. U1801256) and Guangdong Provincial Foundation (2016KQNCX035), National Key R&D Program of China (no. 2016YFA0201002), NSFC grant (nos. 51803064, 51571094, 51431006, 51561135014, U1501244), SCNU University Foundation (16KJ06), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (no.

IRT_17R40), and Guangdong Innovative Research Team Program (no. 2013C102). We also thank the support from the Guangdong Provincial Engineering Technology Research Center for Transparent Conductive Materials, National Center for International Research on Green Optoelectronics (IrGO), and MOE International Laboratory for Optical Information Technologies and the 111 Project.

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(1) Kozlik, M.; Forker, R.; Fritz, T. Principal Processes of Organic-^REFERENCES Inorganic Hybrid Solar Cells Using the Example of ZnPc with ZnO.

Phys. Status Solidi A2015,212, 554−560.

(2) Parks, J. W.; Schmidt, H. Flexible Optofluidic Waveguide Platform with Multi-Dimensional Reconfigurability.Sci. Rep.2016,6, 33008.

(3) Coyle, D. J.; Blaydes, H. A.; Northey, R. S.; Pickett, J. E.;

Nagarkar, K. R.; Zhao, R.-A.; Gardner, J. O. Life Prediction for CIGS Solar Modules Part 2: Degradation Kinetics, Accelerated Testing, and Encapsulant Effects.Prog. Photovolt. Res. Appl.2013,21, 173−186.

(4) Huang, K.; Peng, Y.; Gao, Y.; Shi, J.; Li, H.; Mo, X.; Huang, H.

High-Performance Flexible Perovskite Solar Cells via Precise Control of Electron Transport Layer.Adv. Energy Mater.2019,9, 1901419.

(5) Teoh, H. F.; Dzung, P.; Lim, W. Q.; Chua, J. H.; Lee, K. K.; Hu, Z.; Tan, H.; Tok, E. S.; Sow, C. H. Microlandscaping on a Graphene Oxide Film via Localized Decoration of Ag Nanoparticles.Nanoscale 2014,6, 3143−3149.

(6) Wang, C.; Guan, L.; Zhao, D.; Yu, Y.; Grice, C. R.; Song, Z.;

Awni, R. A.; Chen, J.; Wang, J.; Zhao, X.; Yan, Y. Water Vapor Treatment of Low-Temperature Deposited SnO2 Electron Selective Layers for Efficient Flexible Perovskite Solar Cells.ACS Energy Lett.

2017,2, 2118−2124.

(7) Di Giacomo, F.; Fakharuddin, A.; Jose, R.; Brown, T. M.

Progress, Challenges and Perspectives in Flexible Perovskite Solar Cells.Energy Environ. Sci.2016,9, 3007−3035.

(8) Cao, B.; Yang, L.; Jiang, S.; Lin, H.; Wang, N.; Li, X. Flexible Quintuple Cation Perovskite Solar Cells with High Efficiency. J.

Mater. Chem. A2019,7, 4960−4970.

(9) Das, S.; Yang, B.; Gu, G.; Joshi, P. C.; Ivanov, I. N.; Rouleau, C.

M.; Aytug, T.; Geohegan, D. B.; Xiao, K. High-Performance Flexible Perovskite Solar Cells by Using a Combination of Ultrasonic Spray- Coating and Low Thermal Budget Photonic Curing.ACS Photonics 2015,2, 680−686.

(10) Schmidt, T. M.; Larsen-Olsen, T. T.; Carlé, J. E.; Angmo, D.;

Krebs, F. C. Upscaling of Perovskite Solar Cells: Fully Ambient Roll Processing of Flexible Perovskite Solar Cells with Printed Back Electrodes.Adv. Energy Mater.2015,5, 1500569.

(11) Chen, C.-W.; Kang, H.-W.; Hsiao, S.-Y.; Yang, P.-F.; Chiang, K.-M.; Lin, H.-W. Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition.Adv. Mater.2014,26, 6647−6652.

(12) Dkhissi, Y.; Huang, F.; Rubanov, S.; Xiao, M.; Bach, U.; Spiccia, L.; Caruso, R. A.; Cheng, Y. B. Low Temperature Processing of Flexible Planar Perovskite Solar Cells with Efficiency over 10%. J.

Power Sources2015,278, 325−331.

(13) Xu, X.; Fukuda, K.; Karki, A.; Park, S.; Kimura, H.; Jinno, H.;

Watanabe, N.; Yamamoto, S.; Shimomura, S.; Kitazawa, D.; Yokota, T.; Umezu, S.; Nguyen, T.-Q.; Someya, T. Thermally Stable, Highly Efficient, Ultraflexible Organic Photovoltaics. Proc. Natl. Acad. Sci.

U.S.A.2018,115, 4589−4594.

(14) Song, T.-B.; Chen, Q.; Zhou, H.; Jiang, C.; Wang, H.-H.; Yang, Y.; Liu, Y.; You, J.; Yang, Y. Perovskite Solar Cells: Film Formation and Properties.J. Mater. Chem. A2015,3, 9032−9050.

(15) Dualeh, A.; Tétreault, N.; Moehl, T.; Gao, P.; Nazeeruddin, M.

K.; Grätzel, M. Effect of Annealing Temperature on Film Morphology of Organic − Inorganic Hybrid Pervoskite Solid-State Solar Cells.

Adv. Funct. Mater.2014,24, 3250−3258.

(16) Pellet, N.; Gao, P.; Gregori, G.; Yang, T.-Y.; Nazeeruddin, M.

K.; Maier, J.; Grätzel, M. Mixed-Organic-Cation Perovskite Photo- voltaics for Enhanced Solar-Light Harvesting.Angew. Chem., Int. Ed.

2014,53, 3151−3157.

(17) Zardetto, V.; Brown, T. M.; Reale, A.; Di Carlo, A. Substrates for Flexible Electronics: A Practical Investigation on the Electrical, Film Flexibility , Optical , Temperature , and Solvent Resistance Properties.J. Polym. Sci., Part B: Polym. Phys.2011,49, 638−648.

(18) Chen, H.; Liu, D.; Wang, Y.; Wang, C.; Zhang, T.; Zhang, P.;

Sarvari, H.; Chen, Z.; Li, S. Enhanced Performance of Planar Perovskite Solar Cells Using Low-Temperature Solution-Processed Al-Doped SnO 2 as Electron Transport Layers.Nanoscale Res. Lett.

2017,12, 238.

(19) Chen, C.; Jiang, Y.; Guo, J.; Wu, X.; Zhang, W.; Wu, S.; Gao, X.; Hu, X.; Wang, Q.; Zhou, G.; Chen, Y.; Liu, J. M.; Kempa, K.; Gao, J. Solvent-Assisted Low-Temperature Crystallization of SnO 2 Electron-Transfer Layer for High-Efficiency Planar Perovskite Solar Cells.Adv. Funct. Mater.2019,29, 1900557.