High Bending Durability of Efficient Flexible Perovskite Solar Cells

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High Bending Durability of Efficient Flexible Perovskite Solar Cells Using Metal Oxide Electron Transport Layer Fengjiu Yang,† Jiewei Liu,‡ Hong En Lim,† Yasuhisa Ishikura,‡ Keisuke Shinokita,† Yuhei Miyauchi,† Atsushi Wakamiya,*,‡ Yasujiro Murata,‡ and Kazunari Matsuda*,† †

Institute of Advanced Energy and ‡Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

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S Supporting Information *

ABSTRACT: With a rapid progress in photovoltaic performance over the past several years, organic−inorganic perovskite solar cells (PSCs) have been regarded as a promising candidate for next-generation photovoltaic devices such as lightweight and flexible photovoltaic equipment and portable systems. However, the photovoltaic performance and its durability during the mechanical bending test of flexible PSCs (fPSCs) are insufficient for the realistic application. This inadequacy stems from a lack of a superior electron transport material (ETM) for fPSCs. Here, we describe the application of SnO2 as an ETM for fPSCs (SnO2-fPSCs), achieving outstanding photovoltaic performance and excellent mechanical bending durability. We demonstrate the high power conversion efficiency (PCE) of 17.1 and 16.2% with negligible hysteresis and the high stabilized power output of 17.0 and 15.9% of normal-architecture SnO2-fPSCs with a photoactive area of 0.1 and 1.0 cm2, respectively. Moreover, the SnO2-fPSCs exhibit an excellent bending durability that retains 76.5% of the initial PCE even after 2000 times harsh bending cycle with a small bending radius of 4.0 mm on 0.1 cm2 active area.

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approximately 17.0%.26 Although the photovoltaic performances of devices are enhancing with the enormous endeavors using metal oxide ETMs, the serious and large shortcoming of J−V curve hysteresis still exists.15,23 Moreover, the mechanical bending durability of the fPSCs in normal architectures is not enough. The further acceleration of the development in normal-type fPSCs with high photovoltaic performance and its mechanical durability, is strongly required for making fPSCs more competitive in the industrial application. It is necessary to develop an ETM with high carrier mobility and durability on the flexible substrate. Moreover, a large photoactive area of fPSCs has also become a crucial factor enhancing the core competence of fPSCs. The high carrier mobility and conductivity of inorganic metal oxide material of the SnO2 have triggered enormous attention as an excellent ETM because of its successful application in organic solar cells, dye-sensitized solar cells,27,28 standard PSCs with rigid substrate,29−34 and fPSCs.35−42 The high transport mobility and the well-matched energy level of SnO2 in the band alignment for electron extraction from the perovskite have been verified, leading to the enhancement of the PCE of PSCs and elimination of hysteresis.29 Moreover, the fabrication temperature of SnO2 film is relatively low (∼150 °C), which enables its application as an effective ETM for fPSCs.29,30,32,33 However, the performance of fPSCs using

INTRODUCTION Hybrid organic−inorganic perovskite solar cells (PSCs) have attracted enormous attention in recent years, with their highest power conversion efficiency (PCE) reaching 22.7%.1−4 One of the attractive research topic in this area is the low-temperature solution-processed flexible PSCs (fPSCs), which are recognized as a promising candidate for next-generation photovoltaic devices in lightweight and wearable photovoltaic equipment, portable systems, and bendable displays.5−13 However, the photovoltaic performance of fPSCs is still far behind that of the conventional PSCs that are fabricated on rigid indium tin oxide (ITO) or fluorine-doped tin oxide substrates. So far, the fPSCs can be realized either with the normal (n−i−p) 5,14,15 or inverted (p−i−n) architectures.6,9,12,13,16−18 In the inverted-type fPSCs, the PCE of 18.1%12 and superior mechanical bending durability8,9 have been achieved. However, the complicated fabrication process and extremely small photoactive area reported for inverted architecture’s devices have become a main limitation for their industrial application and competition.9 Moreover, the hydrophilic property of the most common hole transport material, poly(3,4-ethylenedioxythiophene)polystyrene sulfonate, seriously weakens the stability of the PCE in ambient condition for inverted-type fPSCs. On the other hand, the development of normal-architecture fPSCs has also progressed. In normal-architecture fPSCs, ZnO,15,19−22 TiO2,5,7,23,24 ZnSnO4,14 and fullerene derivatives25,26 are typically employed as electron transport materials (ETM), where the highest PCE reported to date is © XXXX American Chemical Society

Received: May 25, 2018 Revised: July 3, 2018

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DOI: 10.1021/acs.jpcc.8b05008 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Cross-sectional SEM image of SnO2-based planar PSCs (SnO2-PSCs). The inset shows a schematic of the architecture of SnO2-PSCs. (b) Surface SEM image of the perovskite layer on an ITO substrate. (c) Current density and voltage (J−V) curves of TiO2- and SnO2-PSCs under forward (0 → 1.2 V) and reverse (1.2 → 0 V) scans. (d) Time evolution of an SPO PCE under maximum power point (MPP) tracking of TiO2and SnO2-PSCs under forward scanning conditions. (e) External quantum efficiency (EQE) and integrated short-circuit current density of TiO2and SnO2-PSCs. (f) Histograms of PCEs in TiO2- and SnO2-PSCs (red: 35 SnO2-PSCs; black: 26 TiO2-PSCs).

rigid ITO substrate using the same procedure as that for SnO2; the resultant films were annealed at 150 °C for 30 min. Fabrication Process of PSCs. The rigid ITO substrates (25 × 25 mm2) were treated with O3/UV light before spin coating of the perovskite precursor solution. The perovskite precursor solution with excess Pb44,45 contained FAI (1 M), PbI2 (1.1 M), MABr (0.2 M), and PbBr2 (0.2 M) in anhydrous dimethylformamide/dimethylsulfoxide (DMSO) 4:1 (v/v), and a predissolved 1.5 M solution of CsI in DMSO was added to the precursor solution. The mixed precursor solution was dissolved at 40 °C for 40 min so that all of the chemicals were fully dissolved. The dissolved perovskite precursor solution was spin-coated onto ITO/SnO2 and TiO2 rigid substrates via two steps at 1000 and 6000 rpm for 10 and 20 s, respectively. Six seconds from the end of the spin-coating program, 400 μL of toluene as an antisolvent was quickly dropped onto the substrate within 2 s. The color of substrate immediately changed from transparent to dark-orange after dropping of the antisolvent. The substrate was annealed at 150 °C for 10 min on a hotplate to obtain a large crystal grain size. The process of spin coating and annealing on the flexible PEN/ITO was the same as that on the rigid substrates. The hole transport layer (HTL) solution was prepared according to the method described in a previous study.2 All the procedures involving rigid and flexible PSCs were conducted in a N2-filled glovebox. Characterization. All characterization and evaluation procedures were conducted under ambient conditions using methods similar to those described in our previous study.2 The mechanical bending durability tests were conducted using home-made systems. The continuous light soaking measurement was conducted by the system (Bunkoukeiki,BIR-50P1) in ambient condition at 30 min intervals.

an SnO2 ETM is far behind the expectation even though it achieved efficiency of over 18.0% by expensive process of the plasma enhanced atomic layer deposition,36 especially the nonnegligible hysteresis, poor bending durability, and extremely small active area,35−40 which seriously limits the application of fPSCs. In this study, we demonstrated a successful utilization of a facile low-temperature solution-processed SnO2 as the ETM for normal-type fPSCs (SnO2-fPSCs) with a Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 perovskite photoactive layer. We achieved an outstanding PCE of 17.1% without hysteresis and a stabilized power output (SPO) of 17.0% for normal structured SnO2-fPSCs with a 0.1 cm2 photoactive area. The SnO2-fPSCs can sustain an excellent bending durability of 76.5% in normalized PCE even after a harsh bending test of 2000 times at the fixed bending radius of 4.0 mm. Moreover, we also obtained a high PCE value of 16.2% and an SPO of 15.9% in the SnO2-fPSCs with a large photoactive area of 1.0 cm2 by the facile low-temperature solution-processed SnO2. The outstanding photovoltaic performance and bending durability of the fPSCs using the SnO2 nanoparticle film as the ETM demonstrate a promising and facile technique for the future industrialization of fPSCs.

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EXPERIMENTAL SECTION Rigid and Flexible Polyethylene Naphthalate (PEN)/ ITO Film Fabrication. Details of the materials used in this study are described in the Supporting Information. The ITO substrates were etched using the same procedures to our previous work.2 For the flexible PEN/ITO, the PEN/ITO substrates were etched by zinc powder and 6 M HCl aqueous solution. The etched films were washed with distilled water, and the water was subsequently removed by wiping the water using bibulous papers. The flexible PEN/ITO films were then pasted onto the glass substrates for the subsequent experiments. A 5 wt % suspension of SnO2 nanoparticles was directly spin-coated onto rigid and flexible PEN/ITO substrates, spun at 3000 rpm for 30 s, and then annealed at 150 °C for 30 min. The TiO2 crystal was synthesized using a nonhydrolytic sol− gel approach; details of synthesis can be found in previously reported work.43 The TiO2 crystals were spin-coated onto the

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RESULTS AND DISCUSSION We first investigated the application of SnO2 electron transport layer (ETL) on the rigid ITO substrate for PSCs (SnO2PSCs), based on perovskite photoactive layer with the composition of Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3. The detailed fabrication procedures for the PSCs are described in the Experimental Section. The inset of Figure 1a depicts the B

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photoresponse of both devices ranges from 300 to 790 nm, whereas the maximum value of EQE of 90.5% in SnO2-PSCs is much higher than that of 84.2% in TiO2-PSCs. Moreover, the integrated photocurrent density of 21.1 mA/cm2 calculated from the IPCE spectrum of the SnO2-PSCs is also much higher than that of 19.1 mA/cm2 in the TiO2-PSCs, which are well consistent with the values of JSC in J−V curves in Figure 1c. We have also tested a large number of devices (35 SnO2and 26 TiO2-PSCs) to confirm the reproducibility of devices’ performance. Figure 1f presents the histograms of PCEs in SnO2- and TiO2-PSCs. The SnO2-PSCs exhibit an average PCE of 18.1 ± 0.2% with a narrow distribution ranging from 17.5 to 18.6%. These values are much higher than that of 9.1 ± 0.8% and ranging from 7.4 to 11.6% in TiO2-PSCs. The parameters of JSC, VOC, and FF values of SnO2-PSCs are also significantly higher than those of TiO2-PSCs, as shown in Figure S5. These excellent results of PCE, stabilized efficiency, and reproducibility of photovoltaic performance for the SnO2-PSCs indicate the great potential of applying SnO2 in efficient flexible photovoltaic devices. The uppermost advantage of low annealing treatment temperature of SnO2 film enables us to apply it as an efficient ETL for the flexible PSCs. Figure 2a

schematic of SnO2-PSCs, where the thickness of SnO2, perovskite, 2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)9,9'-spirobifluorene(spiro-OMeTAD), and Au electrode is about 25, 500, 250, and 80 nm, respectively. Figure 1b shows the surface scanning electron microscopy (SEM) image of perovskite film deposited on the SnO2 ETL surface. A uniform and compact photoactive layer of perovskite fully covers on the surface SnO2 film without any observable voids or cracks with an average crystals grain size about 700 nm and the largest one reaches over 1.0 μm. This crystal grain size is substantially larger than the previously reported value for Csperovskite44,46 and the perovskite film deposited on the TiO2 ETL, as shown in Figure S1d (see Supporting Information). This suggests that we can achieve an excellent photovoltaic performance of PSCs on the SnO2 substrate with high VOC and suppressed hysteresis, where the large grain size indicates a reduction of the trap states and charge recombination loss.9 The configuration of TiO2-based planar PSCs (TiO2-PSCs) is depicted in the inset of Figure S1d. Moreover, we used atomic force microscopy (AFM) to check the surface morphology of the corresponding perovskite layers (Figure S2, see Supporting Information) and summarized the surface roughness in Table S1. The results show smaller surface roughness of perovskite layer on the SnO2 with a mean value of 62.0 nm because of the difference of grain boundary morphology, comparing to that of 78.0 nm for perovskite layer on TiO2, which indicates that the SnO2 film can form more flat perovskite layer with long carrier diffusion length. Figure 1c depicts the photovoltaic performance of PSCs. The current density−voltage (J−V) curves were obtained from different scanning directions: forward scans from the shortcircuit current density (JSC) to the open-circuit voltage (VOC) and vice versa in the reverse scanning directions with the scanning rate of 50 mV/s. The championed SnO2-PSC in our work shows a JSC of 21.2 mA/cm2, a VOC of 1.17 V, a fill factor (FF) of 75.2%, and a PCE of 18.6% under forward scanning conditions. Noted that the hysteresis of J−V curves in SnO2PSCs is negligibly small. We also show the J−V curves of TiO2PSCs in Figure 1c for comparison. The SnO2-PSCs exhibit much higher photovoltaic performances and much smaller hysteresis in comparison to those in TiO2-PSCs devices, which are also supported by the impedance spectroscopy in Figure S3. These results indicate that the electron mobility and electron extraction effectiveness of SnO2 are higher than those of TiO2. This is further confirmed by the photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy of perovskite films deposited on either SnO2 or TiO2 (in Figure S4a,b, see Supporting Information), where PL quenching is faster for SnO2/perovskite sample comparing with TiO2/perovskite sample. We also measured the SPO to confirm the reliability of the PCE value extracted from J−V curve. Figure 1d depicts the time evolutions of the SPO of SnO2- and TiO2-based PSCs under forward MPP voltage of 0.95 and 0.80 V, respectively. The SPO of 18.2% in the SnO2-PSCs is much higher than that of 13.3% in the TiO2-PSCs. These values well agree with those obtained from the J−V curves in Figure 1c. The inset of Figure 1d shows the long time evolution of SPO in SnO2-PSCs. The SnO2-PSCs can sustain relatively high stabilized efficiency without obvious decrease even after 800 s under continued light soaking condition. Figure 1e presents typical incident-photon to currentefficiency (IPCE) spectra of SnO2- and TiO2-PSCs. The

Figure 2. (a) Schematic of SnO2-based flexible planar PSCs (SnO2fPSCs). (b) Surface SEM image of the perovskite layer on a flexible PEN/ITO substrate. (c) Cross-sectional SEM image of an SnO2fPSCs. (d) AFM images of the perovskite layer on the flexible PEN/ ITO/SnO2 substrate.

presents the schematic of SnO2-PSCs fabricated with a flexible PEN/ITO substrate (SnO2-fPSCs). The SnO2 thin film was fabricated by directly spin-coating SnO2 nanoparticles on the PEN/ITO substrate and annealing the film at 150 °C for 30 min. The detailed fabrication procedures for the SnO2-fPSCs are similar with the rigid ITO substrate and are described in the Experimental Section. Figure 2b shows the surface SEM image for SnO2 film as the ETL of perovskite photoactive layer on flexible substrate. The surface morphology and grain size of the perovskite film on the PEN/ITO/substrate are almost the same as those on the rigid ITO substrate in Figure 1a. Some white plates are observed in the grain boundaries because of the excess of PbI2, which is consistent with that observed on the rigid ITO substrate and previous reported results.29,44 Figure 2c depicts the cross-sectional SEM image of SnO2C

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detailed photovoltaic parameters of JSC, VOC, and FF of the SnO2-fPSCs in Figure S5 (see Supporting Information). The mechanical bending durability is one of the most important evaluation indices for specific applications of fPSCs such as wearable and portable electronics. Figure 4a depicts the

fPSCs. The thickness of each layer is almost the same as SnO2PSC. Figure 2d shows the surface morphology of the perovskite photoactive layer on flexible substrate measured by AFM and the value of surface roughness is summarized on Table S1. The perovskite photoactive layer exhibits a little higher value of 71.0 nm surface roughness comparing with perovskite layer on the rigid substrate. Figure 3a shows the J−V curves of the championed SnO2fPSC with high photovoltaic performance in our work. The JSC

Figure 4. (a) Normalized PCE of SnO2-fPSCs measured during 400 bending cycles with various radii: 13.5, 9, and 4 mm. The average values of PCEs with error bars are also shown. The inset shows a photograph of SnO2-fPSCs bent under R9 condition. (b) Normalized PCE as a function of the number of bending cycles until 2000 bending cycles under R9 and R4 bending conditions. (c) Evolution of the J−V curves of SnO2-fPSCs during 2000 cycles under the R9 conditions. (d) Series resistance (RS) as a function of the number of bending cycles under the R9 condition.

Figure 3. (a) J−V curves of SnO2-fPSCs under forward (0 → 1.2 V) and reverse (1.2 → 0 V) scans. (b) Time evolution of an SPO PCE under MPP tracking of SnO2-fPSCs under forward scanning conditions. Inset shows a photograph of SnO2-fPSCs with a 0.1 cm2 photoactive area. (c) EQE and integrated short-circuit current density of SnO2-fPSCs. The transmittance spectra of flexible PEN/ITO and flexible PEN/ITO/SnO2 films are also depicted in the figure. (d) Histograms of the PCEs of 28 evaluated SnO2-fPSCs.

bending durability of a normalized PCE with various bending curvature radii (R) of 13.5 mm (R13.5), 9.0 mm (R9), and 4.0 mm (R4) after 400 bending cycles. A photograph of the bending test is shown in the inset of Figure 4a. The bending durability of the normalized PCE exhibits a slight decrease with the decrease of R and the magnitude of decrease becomes larger. However, the bending durability of normalized PCE sustains over 80.0% of its initial value even for the smallest bending radius (R4). As shown in Figure S6a−c (see Supporting Information), after 400 bending cycles, the normalized photovoltaic parameters of JSC, VOC, and FF retain over 98.1, 94.6, and 86.2% of their initial values, respectively. Furthermore, we investigated the photovoltaic performance of SnO2-fPSCs under R9 and R4 with bending cycles up to 2000 times and depicts the results in Figures 4b and S6d−f. The normalized PCEs at R9 and R4 exhibit almost constant values of about 80.0% from 1000 to 2000 times bending cycle. In the R4 condition, the SnO2-fPSCs can maintain the normalized photovoltaic performance parameters of 76.5, 97.8, 94.1, and 83.2% of initial PCE, JSC, VOC, and FF, respectively. These outstanding results of the bending durability of SnO2-fPSCs are comparable to that of poly(bis(4-phenyl)(2,4,6trimethylphenyl)amine) HTL-based fPSCs12 and much better than those in normal-architecture devices with inorganic ETM5,19−24 and inverted-architecture fPSCs,12,13,16,18 especially superior than the SnO2-fPSCs fabricated by the other approaches.35−37,39 The changes of J−V curves for SnO2-fPSCs during 2000 times bending cycle at R9 are shown in Figure 4c. We observed

of 20.1 mA/cm2, VOC of 1.16 V, and FF of 72.8% resulted in an excellent PCE value of 17.1% under forward scanning condition, and the JSC of 20.0 mA/cm2, VOC of 1.16 V, and FF of 73.1% resulted in a PCE of 17.0% under reverse scanning condition. These PCE values of 17.1% (17.0%) under reverse (forward) condition of the SnO2-fPSCs are higher than the reported values of fPSCs with a normal architecture5,7,8,11,14,15,19−26,35,38−41 and inverted structure.6,9,16−18 Moreover, the SnO2-fPSCs achieve a superior SPO of ∼17.0% and sustain superior SPO in more than 1500 s under continued measurement, as shown in Figure 3b. This value of SPO is well consistent with the PCE obtained from J− V curves. Figure 3c shows the transmission spectra of flexible PEN/ ITO and flexible PEN/ITO/SnO2 substrates. The transmission spectrum of the flexible PEN/ITO/SnO2 is almost identical to that of the flexible PEN/ITO, suggesting that the SnO2 film is highly transparent at wavelengths longer than 400 nm. Figure 3c also depicts the EQE spectrum of SnO2-fPSCs. The photoresponse of EQE ranges from 300 to 790 nm, which is well consistent with that of the SnO2-PSCs on the rigid ITO substrate. The maximum value of EQE reaches approximately 87.0% with an integrated current density of 19.6 mA/cm2. A large number of SnO2-fPSCs (28 devices) have been tested, and the histograms of photovoltaic performance are presented in Figures 3d and S5. The histogram of PCE shows an average value 16.2 ± 0.4% with a narrow distribution. We show the D

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Figure 5. (a−c) Surface morphology of flexible PEN/ITO, flexible PEN/ITO/SnO2, and flexible PEN/ITO/SnO2/perovskite, respectively. (d−f) Surface morphology of flexible PEN/ITO, flexible PEN/ITO/SnO2, and flexible PEN/ITO/SnO2/perovskite after 400 bending cycles under the R9 condition.

by the lines is much slow in comparison with that in PEN/ ITO. Moreover, the SnO2-fPSC exhibits a high value of VOC (=1.18 V), as presented on the Figure S9a, which is the highest VOC for flexible PSCs5−26,34−41 and also comparable to that of 1.20 V for devices fabricated on the rigid substrate in Figure S5. The SnO2-fPSC with highest VOC also achieves high PCE and SPO of 15.9 and 15.7%, respectively, as is shown in the Figure S9. We present the storage stabilities of SnO2-fPSCs in ambient condition (humidity ≈ 33.0%) in comparison with that of SnO2-PSCs and TiO2-PSCs in Figure S10. The SnO2fPSCs exhibit an excellent stability that maintains >95.0% of its initial PCE for over 35 days. This value is comparable with SnO2-PSCs on rigid substrate and is much higher than that of 75.0% TiO2-PSCs under the same condition. We also confirmed that the higher long time continued light soaking stability in SnO2-PSCs is much better than that in TiO2-PSCs, as shown in Figure S11, indicating that the TiO2 is quite unstable under the light including UV component and seriously affect the light soaking stability.48 Furthermore, this outstanding stability is comparable to the highest reported stability according to the degradation rate in Table S5. The enlargement of photoactive area and photovoltaic stability of flexible PSCs is also important for their industrial applications. Even though the great achievement was obtained on fPSCs recently, the active area of them were extremely small and less than ∼0.1 cm2. Such a small active area of fPSCs is far behind the requirements of application. The solutionprocessed and low-temperature-processed SnO2 used in this study is suitable for application to the large area fPSCs. Figure 6a shows the J−V curves of SnO2-fPSCs with a large photoactive area of 1.0 cm2. The photograph of the large photoactive area SnO2-fPSCs is also shown in Figure 6b. The JSC of 19.9 mA/cm2, VOC of 1.17 V, and FF of 69.4% resulted in an excellent PCE value of 16.2% under forward scanning condition, and the JSC of 19.8 mA/cm2, VOC of 1.17 V, and FF of 69.4% resulted in a PCE of 16.1% under reverse scanning condition. The PCE value of 16.2% (16.1%) under forward (reverse) scanning conditions with negligible hysteresis in SnO2-fPSCs with a 1.0 cm2 photoactive area is the highest value on the large active area fPSCs as our knowledge. Figure 6b shows the time evolution of SPO PCE in the SnO2-fPSCs with a large photoactive area of 1.0 cm2. The SPO reaches 15.9%, which is consistent with the PCE from J−V curves. The

a decrease of photovoltaic performances, which mainly stems from the reductions of the FF and VOC. The detailed parameters are summarized at Table S4 (see Supporting Information). The series resistance (RS) evaluated from the J− V curves is plotted as a function of bending cycles in Figure 4d. The RS shows a gradually increase within 200 bending cycles and reaches a relative stable value after 900 bending cycles. As the RS value does not increase so much with the increase of the bending cycles, it demonstrates the reason of the moderate decrease in photovoltaic performance on the harsh bending testing. The surface morphology of each component in SnO2-fPSCs was observed by SEM images to investigate the reason of degradation of photovoltaic performance during the bending test. Figures 5a−f and S7 depict the surface SEM images of flexible PEN/ITO, flexible PEN/ITO/SnO2, flexible PEN/ ITO/SnO2/perovskite, flexible PEN/ITO/SnO2/perovskite/ spiro-OMeTAD, and flexible PEN/ITO/SnO2/perovskite/ spiro-OMeTAD/gold (Au) at R9 before and after 400 consecutive bending cycles. The surface morphology of SnO2 , perovskite, and spiro-OMeTAD layer show no observable changes comparing to their morphology in the SEM images collected before the bending test. By contrast, the PEN/ITO substrate shows some cracks in the Figure 5a because of the brittles of ITO, consistent with the previously reported results.5,9,21,47 Moreover, the linear cracks are also observed in Au electrode after 400 bending cycles, which is a similar phenomenon as what happened on the previous observation.9 The deteriorated properties of ITO and Au electrodes lead to the increase of the RS value, which further results in the decrease of the photovoltaic performance of SnO2-fPSCs. Importantly, the surface morphology of SnO2 film in Figure 5b does not show any changes after bending, indicating that the SnO2 film can sustain the excellent charge extraction effectiveness and mobility during the hard bending process. Notably, the high resistance to the mechanical bending of ETL should be mainly attributed to the thin SnO2 film composed of nanoparticles, where the stress would be absorbed between the grain boundaries of SnO2 nanoparticles in the SnO2-fPSCs. This has been investigated by the relative resistance deteriorated during the mechanical bending tests of PEN/ITO and PEN/ITO/SnO2 film in Figure S8. The increase of relative resistance in PEN/ITO/SnO2 as indicated E

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.W.). *E-mail: [email protected] (K.M.). ORCID

Fengjiu Yang: 0000-0002-5205-2465 Yuhei Miyauchi: 0000-0002-0945-0265 Atsushi Wakamiya: 0000-0003-1430-0947 Yasujiro Murata: 0000-0003-0287-0299 Kazunari Matsuda: 0000-0002-3990-8484

Figure 6. (a) J−V curves of SnO2-fPSCs with 1.0 cm2 active area. (b) Time evolution of an SPO PCE under MPP tracking of the SnO2fPSCs under forward scanning conditions. The inset shows the photograph of SnO2-fPSCs with 1.0 cm2 active area.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI grant numbers JP24681031, JP16H00911, JP15K13337, JP15H05408, JP23340085, JP25610074, and JP16H06331, the Canon Foundation, the Asahi glass foundation, Exploratory Research for Advanced Technology (JPMJER1302) and the Center of Innovation Program (COI) from the Japan Science and Technology Agency (JST), New Energy and Industrial Technology Development Organization (NEDO), and research program of the Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University. F.Y. was financially supported by China Scholarship Council (CSC). The purified PbI2 (L0279) sample and the flexible PEN/ITO substrates were kindly gifted by Tokyo Chemical Industry Co., Ltd. (TCI) and Oike & Co., Ltd., respectively.

SPO maintains a relatively high value even after 1000 s continued light soaking conditions. The high photovoltaic performance and stabilized photovoltaic efficiency under long time light soaking are more difficult in comparison with those in the small area fPSCs because interface condition becomes more complicated with the increase of contact area. The demonstration of fPSCs with large photoactive area and high stabilized efficiency strongly indicates that SnO2 has superior properties as the ETM for fPSCs and could enhance the competitive competence of SnO2-fPSCs toward the industrial application.

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CONCLUSIONS In summary, we demonstrated fPSCs employing the SnO2 nanoparticles as the ETL achieving both high photovoltaic performance and outstanding mechanical bending durability. A high PCE (SPO) of 17.1% (17.0%) with negligible hysteresis is demonstrated in the SnO2-fPSCs with a 0.1 cm2 photoactive area. The SnO2-fPSCs exhibit the highest bending durability of normalized PCE (76.5%) even after 2000 times bending test with a small bending radius of 4.0 mm on metal-oxide ETMbased fPSCs. The superior resistance of the mechanical bending arises from the excellent property of ETL composed of SnO2 nanoparticles. Moreover, we also achieved a high PCE (SPO) value of 16.2% (15.9%) in the SnO2-fPSCs with a large photoactive area of 1.0 cm2, which could enhance its competitiveness in flexible photovoltaic devices and shed light on further practical applications in the large active area fabrication techniques such as roll-to-roll, doctor blade, printing, and so on. Our finding that an SnO2 nanoparticle film functions as an excellent ETM for fPSCs with excellent mechanical properties and large active area device represents an important milestone for the future industrial application of fPSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05008. Experimental description of PL, absorption, impedance spectroscopy, and continuous light soaking condition; additional SEM images; AFM topographical images; impedance resistance; steady-state PL and TRPL spectra; device statistical distribution; bending stability; highest VOC current density−voltage curves; and stability of PSCs data (PDF) F

DOI: 10.1021/acs.jpcc.8b05008 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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