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Highly Bendable Flexible Perovskite Solar Cells on a Nanoscale Surface Oxide Layer of Titanium Metal Plates Gill Sang Han, Seongha Lee, Matthew Duff, Fen Qin, and Jung-Kun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16499 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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ACS Applied Materials & Interfaces

Highly Bendable Flexible Perovskite Solar Cells on a Nanoscale Surface Oxide Layer of Titanium Metal Plates Gill Sang Han, ‡ Seongha Lee, ‡ Matthew Lawrence Duff, Fen Qin, Jung-Kun Lee*

Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States. *E-mail: [email protected]

‡These authors contributed equally to this work

ABSTRACT: We report highly bendable and efficient perovskite solar cells (PSCs) that use thermally oxidized layer of Ti metal plate as an electron transport layer (ETL). The power conversion efficiency (PCE) of flexible PSCs reaches 14.9 % with short-circuit current density (Jsc) of 17.9 mA/cm2, open circuit voltage (Voc) of 1.09, and fill factor (ff) of 0.74. Moreover, the Ti metal based PSCs exhibit a superior fatigue resistance over ITO/PEC substrate. Flexible PSCs maintain 100 % of their initial PCE even after PSCs are bent 1000 times at the bending radius of 4 mm. This excellent performance of flexible PSCs is due to high crystalline quality and low oxygen vacancy concentration of TiO2 layer. The concentration of oxygen vacancies in the oxidized Ti metal surface controls the electric function of TiO2 as ETL of PSCs. A decrease in the oxygen vacancy concentration of the TiO2 layer is critical to improving the electron collection efficiency of the ETL. Our results suggest that Ti metal based PSCs possess excellent mechanical properties which can be applied to the renewable energy source for flexible electronics. Keyword: flexible perovskite solar cell, titanium metal plate, oxygen vacancies, thermal annealing, surface oxidation

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INTRODUCTION A general device structure of PSCs with very high PCE is similar to n-i-p type semiconductor solar cells.1-4 The perovskite layer is coated on mesoporous or planar TiO2 layer on the rigid transparent conducting oxide (TCO) substrate such as ITO/glass or FTO/glass. Recent studies show that highly crystalline TiO2 is necessary as an electron transport layer (ETL) of high efficiency PSCs.5, 6 Therefore, it is necessary to anneal TiO2 layer at high temperature (ca. 500 oC) before the perovskite layer is coated. However, this process cannot be applied to the flexible polymer substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) that are suitable for flexible electronics. To address this problem, the fabrication of n-type materials at low temperature (under 150 oC) has been explored.7-9 For example, B. J. Kim et al. deposited a low-temperature processed TiOx compact layer on ITO/PEN substrate through atomic layer deposition (ALD) at 80 oC, achieving high PCE of 12.2%.10 D. Yang et al. reported that amorphous TiO2 grown by magnetron sputtering at room temperature increases the PCE of PSCs on ITO/PEN to 15.07%.11 In recent years, flexible PCSs of an inverted structure (p-i-n) were designed. The flexible inverted PCSs are composed of NiOx, PhNa-1T, or PEDOT: PSS as p-type hole transport layer (HTL) on ITO/polymer substrates.12-14 The PCE of flexible inverted PSCs reached 14.7% which is not as good as the best performing PSCs on rigid substrates (> 22%).13 Recent results indicate that there is a limitation in increasing the PCE of flexible PSCs without using hightemperature annealing of TiO2 layer. In addition, the mechanical strength of PSCs on ITO/polymer substrate is weak due to low fatigue resistance of thick ITO layer (ca. 200 nm). Since Poisson’s ratio of ITO is smaller than that of CH3NH3PbI3, ITO is less bendable than CH3NH3PbI3.15 Consequently, as PSCs are bent repeatedly, a crack is formed in the ITO layer and propagates through the perovskite layer, resulting in the degradation of PSCs. There are several alternatives to ITO/plastic substrates such as graphene coated polymers, surface treated metal plates and metal meshs.16-18 Among these alterative materials, a metal plate is an attractive candidate, due to the capability of high temperature annealing, low manufacturing cost, and excellent mechanical properties.19 In fact, there are several studies on PSCs of titanium (Ti) metal substrate and transparent top electrode.20-22 However, the maximum PCE of PSCs built on the surface oxidized TiO2 /Ti substrates was only 13.1 % which is much smaller than PCE of other flexible PSCs.19 Furthermore, there has been no 2


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systematic research on the structure and property of TiO2 layer that is produced by oxidizing Ti plates. Here, we report highly bendable PSCs with high PCE which use thermally oxidized layer of Ti metal plates as the ETL of PSCs. The role of surface oxidized Ti metal in PSCs was systematically investigated as a function of thermal annealing conditions. The concentration of oxygen vacancies in the oxidized TiO2 layer on Ti metal surface is found to control the electric function of TiO2 as ETL of PSCs. A decrease in the oxygen vacancy concentration of TiO2 is key to improving its electron collection efficiency. The highest PCE of flexible PSC on Ti plate is 14.9 % with a short-circuit current density (Jsc) of 17.9 mA/cm2, open circuit voltage (Voc) of 1.09, and fill factor (ff) of 0.74 under AM 1.5 illumination. This highest value of ff and Voc is due to high crystallinity and low oxygen vacancy concentration of the TiO2 layer. Moreover, the Ti metal based PSCs exhibit superior fatigue resistance over ITO/PEC substrates. PCE of flexible PSCs does not decrease even after PSCs are bent 1000 times at the bending radius (R) of 4 mm. This suggests that Ti metal based PSCs possess excellent mechanical properties which can be applied to the renewable energy source of flexible electronics. RESULTS AND DISCUSSION Figure 1a and Table 1 presents a structure of flexible PSCs cell based on titanium (Ti) foil (thickness ~25µm). Briefly, the TiO2 layer was formed by thermal oxidation of Ti foil in air. The defect concentration of TiO2 layer was controlled by changing annealing temperature. After thermal annealing, CH3NH3PbI3 and hole transport material (HTM) were coated sequentially onto the TiO2/Ti substrate. On top of HTM, a transparent electrode was formed by coating a very thin metal film (Au ~ 7 nm/ Cu ~ 1nm).23 This structure was designed for highly bendable PSCs with high quality TiO2 film on Ti metal substrate. One of the important experimental variables which can influence the quality and thickness of the oxide layer is annealing temperature. Hence, we systematically investigated the thermal growth rate of TiO2 layer as function of temperature in the range of 400 oC to 700 oC for 1 hour. The results of the ellipsometer measurement show that the thickness of TiO2 grown at 400 oC, 500 oC, 600 oC, and 700 oC in air for 1 hr was 2 µm, respectively. Figure S1 (Supporting information 1) shows that experimental results are in good agreement with theoretically predicted thicknesses which were calculated using the oxygen diffusion 3



coefficient at various temperatures.24 This difference in the thickness of TiO2 on Ti is confirmed in optical micrographs which presents a change in the color of the oxide layer due to optical interference.25 These surface oxidized Ti foils were used as flexible substrates of PSCs. To find the optimal thickness of the TiO2 layer for PSCs, we fabricated PSCs using Ti foils that were annealed at 400 oC for different oxidation times. Since Ti metal plate at the bottom of PSCs was not transparent, the photovoltaic measurement of PSCs was conducted in a top illumination mode. Current density - voltage (J-V) curve of Ti foil based PSCs is compared as a function of the thickness, as shown in Figure S2. When the thickness of TiO2 on Ti is ~50 nm, PSC exhibits the best PCE of ~8.56%. A thinner TiO2 layer on Ti decreases the shunt resistance, which lowered ff and Voc. As a result, the efficiency of PSCs was only ~4.2%. On the other hand, a thicker TiO2 significantly decreases Jsc, which is attributed to an increase in the series resistance. Therefore, the thickness of the oxidized TiO2 layer in the following experiments was fixed at 50 nm. Figure 1b shows J-V curves of PSCs on Ti foils that were annealed at different temperature but all of them had ~50 nm thick TiO2 layer. By changing thermal annealing time at different temperature, we changed only the crystalline quality of TiO2 layer with the film thickness fixed at 50 nm. To grow 50 nm thick TiO2 layer, Ti foil was annealed at 400 oC for 72 hours, 500 oC for 50 minutes, 600 oC for 8 minutes or 700 oC for 50 seconds. PSCs on 400 oC oxidized Ti foil shows the average PCE of 8.56%. Jsc, Voc, and ff of this PSC are 16.2 mA/cm2, 0.91 V, and 0.58, respectively. (Figure S3) The relatively small Jsc of the best PSC is due to the loss of incident light at the top transparent electrode and the HTL (Figure S4). Transmittance is also simulated using the thickness of each layer which was measured by SEM and ellipsometry (Figure S4). Transmittance of the thin metal electrode ranges from 61 to 86 % for visible light. In addition, since HTL (spiro-OMeTAD) is coated on the top of the perovskite layer, the HTL captures solar light of the wavelength smaller than ~420 nm.26 An increase in the annealing temperature decreases PCE and shunt resistance of PSCs, thought the thickness of TiO2 layer is same for all samples. To understand the effect of the oxidation temperature of TiO2 on the PV performance of PSCs, the microstructure, phase transformation, surface roughness, and electrical properties of TiO2 layer on Ti were analyzed. Figure S5 shows optical profiler and SEM images of oxidized Ti foil surface. The TiO2 layers which were oxidized at different temperature exhibit the very 4


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similar morphology and the surface roughness of all oxide layers is ~150 nm. In addition, no TiO2 layer has microcraks which can be formed by the volume expansion of the oxidation process. Therefore, the effect of the annealing temperature on the PV performance of PSCs cannot be explained by a change in the microstructure of the TiO2 layer. To examine the effect of the annealing temperature on the electric properties, the electrical resistance of the TiO2 layer was also measured (Figure 2a). An electrode configuration of TiO2 layer for I-V curve measurement is illustrated in inset Figure 2a. Interestingly, the annealing temperature has a huge impact on the resistance and I-V of TiO2 layer. TiO2 layer grown at 400 oC exhibits non-linear I-V curve which was reported in previous studies of highly crystalline TiO2 films for resistive random-access memory (ReRAM) applications.27 TiO2 layer grown at or higher than 500 oC exhibits linear I-V curve and higher temperature annealing significantly decreases the electric resistance of TiO2 layer. This suggests that the effect of the oxidation temperature on the PV performance of PSCs is to change the electric properties of TiO2 layer (Figure 1b). In addition, the phase of TiO2 changes as a function of temperature and the rutile phase is more stable than the anatase phase at high temperature. However, a change in the crystal structure of this study cannot explain an increase in the electric conductivity of TiO2 film which are annealed at different temperature. Figure S6 shows XRD patterns of 100 nm thick TiO2 film on Ti plate as a function of annealing temperature. The rutile phase starts to show up at 500 oC and becomes dominant at 700 oC. The intensity of the anatase peak is the strongest after long-time annealing at 400 oC. In addition, XPS spectra in Figure 2c shows the narrowest Ti peak. This indicates that the crystallinity of the anatase phase is the best for 400 oC treated TiO2 film. TiO2 is an inherently n-type semiconductor. The conductivity of TiO2 is often controlled by the concentration of defects such as oxygen vacancies and aliovalent impurities (e.g. Mg2+, Nb5+, Ta5+, Al3+ or Y3+).28-33 Given that all TiO2 films were prepared from the same type of Ti foil, the impurity defect concentrations must be same for all samples. Therefore, the change in the electric properties of the TiO2 layer can be attributed to a dependence of the equilibrium oxygen vacancy concentration on temperature. The equilibrium oxygen vacancy concentration exponentially increases as temperature increases.34 Since the oxygen vacancy works as a shallow donor in TiO2, more oxygen vacancies at higher temperature mean that an increase in the annealing temperature raises the concentration of oxygen vacancy and free 5



electrons.35 A remaining question is how an increase in the concentrations of oxygen vacancies and electrons deteriorate PV performance of PSCs. There are several studies showing that donor impurities such as Nb increases the electron concentration and improves the charge collection efficiency of PSCs.[28] Based on recent studies, we speculate that the oxygen vacancy is responsible for the decreased PV performance of TiO2 film grown at high temperature. To understand the role of the oxygen vacancy, J-V curves in Figure 1b are fitted using a stand diode equation of solar cells. Figure 2b shows the shunt resistance of the PSCs as function of the oxidation temperature. An increase in the annealing temperature decreases the shunt resistance. We postulate that the change in the shunt resistance is related to an inhomogeneous agglomeration of oxygen vacancy in TiO2 film by the electric field which is applied during J-V measurement.

Hwang et al. have experimentally proven the alignment

of the oxygen vacancies in TiO2 film and its effect on the electric property by combining TEM analysis and electric characterization.36 The formation of the oxygen vacancy filament under electric field provides a low resistance path for electron conduction and decreases overall resistance of TiO2 film. In this study, though the oxidized layer thickness of Ti metal foils is same for all samples, higher annealing temperature increase the oxygen vacancy concentration and promotes the agglomeration of the oxygen vacancies in TiO2 layer under electric field. The 1-dimensional agglomeration of the oxygen vacancy is illustrated in an inset of Figure 2b.37,38 Once the filament of the oxygen vacancies is formed, there is a shortcut for the electron transport. As the oxygen vacancy concentration increases, the kinetic energy barrier for the formation of the electron path decreases. Consequently, when high temperature annealed TiO2 layer is used, the leakage current through TiO2 layer increases and the shunt resistance of PSCs decreases. To examine the relative oxygen vacancy concentration, XPS analysis was performed for TiO2 layers. Figure 2c shows Ti 2p spectra. Between two peaks, a peak at the binding energy of ~459.2 eV is assigned to Ti4+.39 The other peak of lower binding energy (457.5 eV) is due to a presence of Ti3+ which is the titaniumdefect site associated with oxygen vacancies or other donor impurities.39 The XPS analysis clearly indicates that an increase in the thermal oxidation temperature also increases the amount of oxygen vacancy which easily agglomerates to form the inhomogeneous path for the electron transport. To further understand the effect of the oxygen vacancy concentration on the electric 6


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properties, TiO2 layer which was thermally grown at 400 oC for 72h in air was additionally treated in oxygen atmosphere at 400 oC. Since the second annealing was conducted for a short time in comparison to the first annealing, the second annealing has little impact on the oxidation of Ti foil. During the second annealing in pure oxygen atmosphere, oxygen atoms preferentially diffuse into TiO2 layer through a vacancy-assisted diffusion. As a result of oxygen inward diffusion, oxygen vacancies can be removed from TiO2 layer. A hypothesis on the decrease in the oxygen vacancy is verified by measuring the resistivity of two-time thermal treated Ti substrate. As shown in Figure S7, the TiO2 layer exhibits larger resistance and more non-linear J-V curve after the TiO2 layer was retreated in oxygen for 1 hr. Moreover, the second treatment almost removed the Ti3+ peak at 457.5 eV in Ti 2p spectra. Figure 3a shows J-V curve of the PSC built on the two-time treated Ti foil under solar light radiation. After the oxygen vacancy concentration is reduced in the TiO2 layer on Ti foil, the PSC exhibits the best performance. Jsc, Voc, ff, and a PCE are 17.9 mA/cm2, 1.09 V, 0.74, and 14.9 %, respectively. The ETL property of other TiO2 layers grown at higher temperature in air is also significantly improved when Ti foils are treated in oxygen at 400 oC for 1 hr, as shown in Figure S8 and S9. We characterized PSCs after the second annealing at 400 oC was done for 15, 30, 60, and 120 min. Results are shown Figure S10. PCE was significantly increased even at annealing for 15 min and a change in PCE is saturated at 60 min annealing. This suggests that the defect concentration near the perovskite - TiO2 interface rather than a total defect concentration of TiO2 controls the charge transport and recombination behaviors. A small hysteresis in Figure 3a also indicates that the high crystalline TiO2 with less defects can help to suppress the hysteresis on the perovskite solar cells. In our study, the PCE of forward and reverse sweep direction exhibit the 14.7% and 13.2% and a hysteresis index is about 0.1. This level of the hysteresis is consistent with recent reports.40-42 Additionally, Figure S11 shows the dark current of Ti based PSCs. As the annealing temperature increase, dark current starts to increase at low bias. This indicates that oxygen vacancies act as trap states and cause charge recombination at the TiO2/perovskite interface. After these oxygen vacancies are passivated, PSCs recover diode characteristics in a dark condition. J-V curve of the passivated sample (retreated in oxygen after 400 oC annealing in air) in Figure S11 shows the highest onset bias and the smallest current density at 1.2V, which confirms the effect of the passivation on the device performance. Moreover, the 7



oxygen vacancy passivated surface on TiO2 suppressed the hysteresis behaviour on PV performance. Results in this study demonstrate that the oxygen vacancy plays a critical role in lowering the performance of PSCs and the passivation of defects in the oxidized layer of Ti foil helps to recover the shut resistance of PSCs. Figure 3b show IPCE spectra of the best PSC with a negligible oxygen vacancy concentration in the TiO2 layer. The integrated of current is ~16.46 mA/cm2 which is in a good agreement with Jsc of J-V curve. IPCE of Ti based PSCs drops to 0% at ~420 nm which is longer than the cut-off wavelength of conventional glass based PSCs (~300 nm). This is because solar light is incident on the top electrode and passes through the HTL before light hits the perovskite layer. The absorption of high energy photons by HTM, in turn, limits the theoretical efficiency of PSCs. This explains relatively smaller PCE of PSCs on the oxidized Ti foil. It is also noteworthy that the removal of UV component from solar light may improve the light stability of PSCs, since UV is a major source of light induced aging of halide perovskite. One of the most attractive features of Ti foil based PSCs is that the solar cell is very flexible. We characterized the mechanical strength of Ti foil based PSCs by repeatedly bending PSCs at different radii of curvature (R). Previously, the flexible perovskite solar cells have been studied using polymer substrates such ITO/PEN substrates. It was found that cracking of ITO transparent electrode was a source of degradation of photovoltaic performance. Even at R ~4 mm, PCE of PSCs on ITO/PEN decreases by approximately 50 % after the bending was repeated 1000 times.[10] Figure 4a shows a change in normalized PCE while PSCs are bent 1000 times. Surprisingly, the Ti foil base PSCs maintain the initial PCE of ~100% at R ~15 and 4 mm while 1000 cycles of the bending experiment is conducted. This is due to high ductility of Ti foil in comparison to transparent metal oxide such as ITO. Figure S12 shows the SEM image on the surface of TiO2/Ti substrate after 1000 cycle bending with R ~4 mm. Large cracks were not developed on the surface till then bending was repeated 1000 times. Even at an extreme bending condition (R ~1 mm), Ti foil based PSCs keep the original PCE until 100 cycles of the bending test. Even after 1000 times bending at the condition of R = 1mm, PCE of PSCs is as high as 77% of the initial value. To find out the origin of the fatigue of foil based PSCs, we also measured the change of I-V curve of surface oxidized Ti foils as a function of bending time in a dark condition. For the 8


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electric characterization, the electrode structure of Au/TiO2/Ti in the inset of Figure 2a was used. The resistivity of TiO2 layers on Ti foil did not change at conditions of R ~ 15 and 4 mm. On the other hand, the resistivity of TiO2 significantly decreases at the condition of R ~ 1 mm. After the bending test repeated for 1000 cycles at R ~1 mm, J-V curve exhibits a linear relation. The microstructure of Au/TiO2/Ti was further investigated using SEM at a plan-view geometry. Figure 4c shows that the crack produced and the direction of cracks is perpendicular to the bending direction, after the Ti foil was bent 1000 times at R ~1 mm. Cracks the surface oxidized Ti foil can cause the degradation of PSCs by generating leakage current through the internal surface of cracks. As a result, TiO2 layer does not work as ETL and the device exhibits ohmic behavior rather than diode behavior, as shown in Figure 4b. CONCLUSION We have demonstrated PSCs on thermally oxidized Ti foil. PCE of PSCs is as high as 14.9 %, though the top illumination through HTM decreases the theoretical efficiency of PSCs by ~8 %). The concentration of oxygen vacancy in the TiO2 layer is a very important parameter to control the electronic function of TiO2 as ETL of PSC. Oxygen vacancy agglomeration forms a low-resistance bypass of electron transport and decreases the shunt resistance of PSCs. Moreover, the Ti metal based PSCs exhibit an excellent fatigue resistance during mechanical bending tests. Even at R = 4 mm, the PSCs do not show any decrease in PCE till the bending test is repeated 1000 times. This fatigue resistance of the flexible PSCs on surface oxidized Ti metal is higher than that of flexible PSCs on other substrates such as ITO/PET substrates. Our results suggest that Ti metal based PSCs possess excellent mechanical properties and can be applied flexible electronics. MATERIALS AND METHODS Preparing of the Ti samples: TiO2/Ti substrates were prepared by thermal oxidation process of Ti foils. Ti foils (Alfa Aesar, 99.7 wt% purity, 0.025 mm thickness) were cut in the form of 20 x 20 mm sheets. Ti sheets were cleaned by sequential sonication in acetone, deionized (DI) water, and ethanol for 30 minutes, respectively. After washing, Ti foils were annealed at different temperatures (400 oC, 500 oC, 600 oC, 700 oC) to form TiO2 layer at the top of the Ti foils. For second heat-treatment, the oxidized TiO2 layer was thermally treated in oxygen ambience for 1 hour at 400 oC. 9



Fabrication of the PSCs: Perovskite solar cells were fabricated with a structure of Au (7 nm)/Cu (1 nm)/HTM/CH3NH3PbI3/TiO2/Ti. Perovskite layer, the CH3NH3PbI3 solution was synthesized by dissolving lead iodide (PbI2), methyl ammonium iodide (MAI, CH3NH3I) and dimethyl sulfoxide (DMSO) with a molar ratio of 1:1:1 in N,N-dimethylformamide (DMF).[4] This synthesized perovskite layer was spin-coated on the top of TiO2/Ti at 4500rpm for 25 sec while slowly dropping 5ml of ethyl ether. The spin-coated samples were annealed on a hot plate at 65 oC, and then at 100 oC for 1 min and 5 min, respectively. And then, the HTL solution was deposited by spin-coating from 36 mg of the 2,2′,7,7′-tetrakis[N,N-di(4methoxyphenyl)amino]-9,9′-spirobi-fluorene (spiro-OMeTAD) in 500 µl of chlorobenzene with added 14.4 µl of 4-tert-butyl pyridine and 8.8 µl of lithium -bis(trifluoromethanesulfonyl)-imide (Li-TFSI) solution. Li-TFSI solution was prepared by dissolving 180 mg of Li-TFSI in 250 µl acetonitrile for 15 min. Finally, each 1 nm-thick Cu and 7 nm-thick Au were deposited by e-beam evaporation on the top of HTL as a transparent electrode. Characterization: The photovoltaic performances of the device were conducted by using electrochemical workstation (CHI660, CHI Instrument) system equipped with a solar simulator (Oriel Sol 3A class AAA, Newport) as light source with a light intensity 1 sun calibrated to a reference Si solar cell (PVM 95). The effective area of the cell under illumination was defined to be 0.14 cm2 using a non-reflective metal mask. The evaluated external quantum efficiency (EQE) and integrated current were measured by using the incident photo-to-current conversion efficiency (IPCE) spectra from 300 nm to 800 nm in air under short-circuit conditions using a monochromatic light grating system (PV Measurements. The thickness of layers was measured by ellipsometry equipment (UVISEL Spectroscopic Phase Modulated ellipsometer, Horiba Jobin Yvon) under incident angles of 70° for photon energies between 1.5 and 6 eV with a 50 meV increment. The surface roughness was measured by using a 3D profilometer (Contour GT, Bruker). X-ray photoelectron spectroscopy (XPS) was carried out in ESCALAB 250Xi XPS system to detect oxygen vacancy. UV/vis transmittance spectra of the films were recorded with PerkinElmer Lambda35 UV/vis spectrometer equipped with an integrating sphere. The surface morphology of TiO2 was examined using a scanning electron microscopy (FESEM, XL-30F, Philips FEI). ASSOCIATED CONTENT 10


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx, The diffusion coefficient and diffusion length of oxygen at various temperature, J-V curves of the flexible Ti based PSCs depending on different thickness, Histogram of the PCE of oxidized Ti foil based PSCs at different temperature, The transmittance of the Cu (1nm)/ Au (7nm) film on the glass and simulated transmittance of spiro-OMeTAD (200nm) /Cu (1nm)/ Au, Surface roughness of oxidized TiO2 layer, The resistance and XPS spectra of oxidized TiO2 layer after second annealing, Representative J-V characteristics of the Ti based PSCs after second annealing. Acknowledgement This work was supported from the Global Frontier R&D Program on Center for Multiscale Energy System, Korea (2012M3A6A7054855) and National Science Foundation (Grant No. CMMI-1333182, EPMD-1408025).

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Reference (1) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Grätzel, M.; Park, N. -G. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. (3) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (4) Ahn, N.; Son, D. Y.; Jang, I. H.; Kang, S. M.; Choi, M.; Park, N. G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 86968699. (5) Son, D.-Y.; Lee, J.-W.; Choi, Y. J.; Jang, I.-H.; Lee, S.; Yoo, P. J.; Shin, H.; Ahn, N.; Choi, M.; Kim, D.; Park, N. -G. Self-Formed Grain Boundary Healing Layer for Highly Efficient CH3NH3PbI3 Perovskite Solar Cells. Nat. Energy 2016, 1, 16081. (6) Bi, D.; Yi, C.; Luo, J.; Décoppet, J.-D.; Zhang, F.; Zakeeruddin, S. M.; Li, X.; Hagfeldt, A.; Grätzel, M. Polymer-Templated Nucleation and Crystal Growth of Perovskite Films for Solar Cells with Efficiency Greater Than 21%. Nat. Energy 2016, 1, 16142. (7) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient Organometal Trihalide Perovskite Planar-Heterojunction Solar Cells on Flexible Polymer Substrates. Nat. Commun. 2013, 4, 2761. (8) Liu, D.; Kelly, T. L. Perovskite Solar Cells with a Planar Heterojunction Structure Prepared using Room-Temperature Solution Processing Techniques. Nat. Photonics. 2013, 8, 133-138. (9) Qiu, W.; Paetzold, U. W.; Gehlhaar, R.; Smirnov, V.; Boyen, H.-G.; Tait, J. G.; Conings, B.; Zhang, W.; Nielsen, C. B.; McCulloch, I.; Froyen, L.; Heremans, P.; Cheyns, D. An Electron Beam Evaporated TiO2 Layer for High Efficiency Planar Perovskite Solar Cells on Flexible Polyethylene Terephthalate Substrates. J. Mater. Chem. A 2015, 3, 2282412


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Figure 1. (a) Device architecture of the highly flexible cells based on metal substrates tested in this study. (b) J-V curves of Au/Cu/HTM/CH3NH3PbI3 /TiO2/Ti cells under 100 mW cm-2 AM1.5G solar light with the same oxidized thickness of TiO2 layer, ~50 nm, based on the same ambience, air, with different annealing temperature.

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Figure 2. (a) The corresponding resistances of oxidized TiO2 layer with the same thickness depending on different temperatures in air and the measured device structure of the Au/TiO2/Ti. (b) Changes of representative shunt resistance determined from J-V curves of Au/Cu/HTM/CH3NH3PbI3/TiO2/Ti flexible cells under different oxidation temperature in air and diffusion time calculated at the same condition. The series and shunt resistance was derived in Figure 1 by using a stand diode equation as follow; -dV/dJ at Voc and -dV/dJ at Jsc. (c) XPS spectra of the TiO2 layer according to different oxidization temperature in air.

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Page 19 of 22

20

2

Current density (mA/cm )

(a)

15

5

Jsc= 17.9 mA/cm Voc= 1.09 V ff = 0.74 PCE = 14.7%

0 0.0

0.2

10

0.4

2

0.6

0.8

1.0

1.2

Voltage (V)

(b) 100

400

500

600

700

IPCE(%)

80

800 20 15

60 10 40 5

20 0

500

600

700

0 800

2

400

Integrated current (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm)

Figure 3. (a) J-V characteristics measured under 100 mW cm-2 AM1.5G illumination for the highest-performing of Au/Cu/HTM/CH3NH3PbI3/TiO2/Ti device after a two-step annealing process (first annealing: 400 oC for 72 hours in air, second annealing: 400 oC for 1 hour in oxygen). (b) IPCE spectrum with the highest-performing device. The integrated product of the IPCE spectrum with the AM1.5G photon flux is also shown (blue line) under the same condition.

19



Figure 4. (a) Changes in normalized PCEs of flexible perovskite solar cells based on Ti foil during repeated bending cycles, 10, 100, and 1000 cycles, at different bending radius of 15, 4, and 1mm. The inset gives a real photograph of the bending tests. (b) The electrical resistance measured from I-V curve of the Au/TiO2/Ti device before and after 10, 100, and 1000 bending cycles at different bending radius. (c) Plan-view SEM image of Au/TiO2/Ti: before bending test (c-1), after bending test at 1mm bending radius (c-2, c-3).

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Table 1. Device properties of the PCSs fabricated on Ti metal plates which were oxidized at different temperatures in air. Temperature (oC)

Jsc (mA/cm2)

Voc (V)

ff

Efficiency (%)

Rsh (Ω cm2)

400 oC

16.2

0.91

0.58

8.56

1696

500 oC

12.6

0.70

0.40

3.53

233

600 oC

9.64

0.56

0.30

1.63

140

700 oC

6.46

0.32

0.26

0.54

56

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Table of Contents (TOC)

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