Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells

Transcription

1 Revealing Underlying Processes Involved in Light Soaking Effects and Hysteresis Phenomena in Perovskite Solar Cells Chen Zhao, Bingbing Chen, Xianfeng Qiao, Lin Luan, Kai Lu, and Bin Hu * Organic inorganic halide perovskites are becoming attractive candidates for developing photovoltaic solar cells. [1 5 ] Materials processing and device engineering have led to high power conversion efficiencies based on the intense light absorption, superior charge diffusion length, long charge carrier lifetimes, and low exciton binding energy. [6 12 ] It is noted that perovskite solar cells often exhibit light soaking effects and hysteresis phenomena under operating condition: a continuous light illumination can gradually change the device performance. [2,13,14 ] The experimental studies have shown that the light soaking and hysteresis phenomena are related to the electrode interfaces in planar-heterojunction perovskite solar cells. [2 ] In general, the light soaking and hysteresis effects can be attributed to the reorientation of ferroelectric organic cations, [15 17 ] lattice distortion-induced polarization, [14 ] trapping/detrapping of charge carriers, [18 ] and ion migration in perovskite solar cells. [18 ] Furthermore, optical measurements have found that bulk trapping can strongly influence the photoconductivity response and consequently generates photocurrent hysteresis phenomena. [19 21 ] In addition, the impedance and ultraviolet photoemission spectroscopy (UPS) studies indicate that energy level alignment and trap filling action can also cause light soaking behaviors. [22 28 ] However, it still demands further in-depth investigations to understand how bulk and interface parameters are internally coupled in light soaking and hysteresis phenomena to control the useful and nonuseful photovoltaic processes in perovskite solar cells. Here, we investigate the mutually connected bulk and interface parameters involved in light soaking effects and hysteresis phenomena by using capacitance voltage ( C V ), timedependent photoluminescence (PL), and frequency-dependent capacitance ( C f ) measurements based on indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS)/CH 3 NH 3 PbI 3 x Cl x /[6,6]-phenyl C61-butyric acid methyl ester (PCBM)/aluminum (Al) devices. We observe that the perovskite solar cells can exhibit reversible light soaking phenomena: the open-circuit voltage ( V OC ) and fill factor (FF) C. Zhao, B. Chen, Dr. X. Qiao, L. Luan, K. Lu, Prof. B. Hu Wuhan National Laboratory for Optoelectronics and School of Optical and Electronic Information Huazhong University of Science and Technology Wuhan , China bhu@utk.edu Prof. B. Hu Department of Materials Science and Engineering University of Tennessee Knoxville, TN 37996, USA DOI: /aenm continuously increase with light illumination while the shortcircuit current ( J SC ) experiences a quick increase and then a decrease upon light exposure. The C V measurements find that light soaking can decrease the charge accumulation at the electrode interfaces. Essentially, the light soaking-decreased charge accumulation at electrode interface can be attributed to following two possible processes. First, the photogenerated carriers can neutralize the interfacial defects at electrode interface upon light illumination. Second, the migration of ions can change the built-in electric field and then affects the charge accumulation at electrode interfaces. In particular, these two processes can largely increase the V OC by increasing interfacial potential barrier at electrode interfaces during light illumination. The time-dependent PL and frequency-dependent capacitance ( C f ) find that the bulk defects within perovskite film are mainly positively charged and can be neutralized by photogenerated electrons upon light illumination. In particular, our frequency-dependent capacitance provides the first direct evidence that light soaking can decrease bulk-electrical polarization within organo-metal halide perovskites. Especially, decreasing the bulk-electrical polarization causes a decrease on J SC in light soaking. However, neutralizing the defects at electrode interfaces and bulk perovskite film can enhance the transport of the dissociated charge carriers to respective electrodes, increasing the FF during light illumination. Clearly, our experimental studies provide an in-depth understanding on internal coupling between electrode and bulk parameters in light soaking and hysteresis phenomena in perovskite solar cells under deviceoperating condition. Figure 1 shows the light soaking effects on device performance for perovskite solar cells. On initial light exposure the device shows a lower photovoltaic performance with V OC = 0.51 V, J SC = ma cm 2, FF = 53.1%, and power conversion efficiency (PCE) = 4.97%. With continuous light exposure, device performance is significantly improved over time. After 20 min of light soaking, the enhanced V OC = 0.83 V, J SC = ma cm 2, FF = 69.5% are obtained, resulting in a PCE = 10.49%. As we know, a continuous light illumination can cause heating and charge trapping in the development of light soaking and hysteresis effects. Here, our studies indicate that the light soaking and hysteresis effects come from charge trapping rather than heating. Specifically, we observe that the surface temperature of device can be increased from 26 to 42 C when the cells are continuously exposed to light illumination. However, the device performance only slightly decreases, according to the J V characteristics (inset in Figure 2 ), when the temperature increased to 42 ºC equivalent to the temperature produced by continuous light illumination. After removing the heating and cooling the device to room temperature, we can see that the device performance is again significantly 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 6)

2 Figure 1. Light soaking effects on photovoltaic performance for ITO/ PEDOT:PSS/perovskite/PCBM/Al device under illumination. enhanced with continuous light exposure. This confirms that light soaking effects are responsible for enhancement of device performance rather than heating effects in perovskite solar cells upon light illumination. In order to determine whether the light soaking effects are reversible phenomena, we perform the test at different times, separately, on the same device. We can see in Figure 3 that the three-cycle tests performed at different times give similar light soaking effects: t 0 (initial time), t 1 (1 h), and t 2 (24 h). The reversible and irreversible effects are about 95% and 5%, respectively. This confirms that light soaking effects are reversible phenomena. The small portion of irreversible component (5%) can be attributed to the structural changes caused by light illumination. In light soaking effects we can see that the V OC and FF values increase first and then saturate with light illumination, while the J SC value experiences a quick increase and then a decrease upon light exposure. To understand the light soaking effects on device performance based on mutually connected bulk and interface parameters, the C V, time-dependent PL, and C f measurements are performed for perovskite solar cells. Figure 4 a shows the C V characteristics for perovskite solar cells at dark condition and under illumination of 30 min. Figure 2. Effects of heating and light soaking on device performance for ITO/PEDOT:PSS/perovskite/PCBM/Al device. The inset shows the heating effects on device performance. The C V measurement, usually used to reflect the interface parameters, can characterize the surface accumulation of photogenerated charge carriers and investigate the effect of surface charge accumulation on V OC. [29,30 ] Here, we discuss four detailed information from the C V characteristics. First, the device capacitance increases as the applied bias changes from negative bias toward V peak under photoexcitation. Changing the applied bias toward the V peak provides an external electric field ( E ex ) against the built-in electric field ( E bt ), leading to a decrease on effective field to drift photogenerated carriers to respective electrodes. In this case, the photogenerated charge carriers become more accumulated at electrode interfaces, showing an increased capacitance as the applied bias increase to V peak. Second, the device capacitance reaches a maximum value at V peak and then quickly decreases as the applied bias further increases beyond V peak. In this case, the charge injection and recombination can occur, leading to a reduced capacitance. Third, the initial photoexcitation generates V peak value of 0.58 V relative to the dark V peak value of 0.60 V. V peak shift between dark and photoexcitation conditions indicates that photogenerated charges are accumulated on the electrode interfaces under photoexcitation. [31,32 ] Fourth, V peak value increases from 0.58 to 0.85 V under the light illumination of 30 min, generating a V peak shift of 0.27 V (Figure 4 b). At the same time, the capacitance value continuously increases with light illumination. It is known that V peak value is determined by the condition in which the external electric field ( E ex ) completely cancels the built-in electric field ( E bt ) under photoexcitation. We should note that E bt consists of two components: (i) the dark field ( E dark ) caused mainly by the workfunction difference between two electrodes and (ii) the electric field developed by the surface accumulation, namely surface accumulation-induced field ( E ac ), as shown in Equation (1). Ebt = Edark Eac (1) It should be further noted that E ac is in the opposite direction to E dark. Increasing E ac can essentially decrease the overall E bt, decreasing V peak (Figure 4 c). Therefore, V peak value can reflect the amount of charge accumulation at electrode interfaces. The larger and smaller V peak values correspond to less and more interfacial accumulation of photogenerated charge carriers at electrode interfaces, respectively. As a result, the increase of V peak value indicates that light soaking can decrease the charge accumulation at electrode interfaces. In general, the charge accumulation at electrode interfaces can be decreased in two possible ways in light soaking effects. First, the photogenerated carriers can neutralize the surface defects at electrode interfaces and consequently reduce the density of surface defects upon light illumination. Second, the recent studies have found that the ion migration can occur in organo-metal halide perovskites toward respective electrodes driven by an electric field. [33 ] These migrated ions can consequently generate space charges. [34 ] The space charges can form local electric fields, decreasing the accumulation-induced field ( E ac ). This can essentially lead to a decrease on the charge accumulation at electrode interfaces. Furthermore, decreasing the charge accumulation can lead to an increase on E bt, according to Equation ( 1). This can cause an increase on V OC when the photocurrent is balanced by the (2 of 6) wileyonlinelibrary.com 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3 Figure 3. The reversibility of photovoltaic parameters ( V OC, J SC, FF, and PCE) upon light illumination. Three-cycle tests on the same device measured at different times: initial time (black symbols), after 1 h dark condition (red symbols), and after 24 h dark condition (blue symbols). injection current upon applying E ex, as shown in Figure 4 c. The increase of V OC due to reduction on the charge accumulation can be further discussed as follows. V OC is essentially related to the effective potential barrier for charge injection at electrode interfaces when the photocurrent is counteracting the injection current. The dark injection barrier in the metal semiconductor contact can be described by the following equation φi ni = Nt exp kt (2) B where n i and N t are surface charge density and volume density of molecular sites, respectively. φ i is the potential barrier at the interface between the active medium and the electrode. Obviously, the lower surface charge density will result in a higher potential barrier φ i. It is noted that, at V OC condition, the surface charge density contains both dark carriers (the carriers injected from electrodes) and photogenerated carriers (the carriers accumulated at electrode interfaces). As a result, decreasing the accumulation of photogenerated charge carriers during light soaking can increase the interfacial potential barrier at electrode interfaces and then lead to an increase on V OC when the photocurrent is canceled by the injection current. Nevertheless, our C V analysis indicates that the light soaking can decrease the surface charge accumulation by neutralizing the charged surface defects and consequently leads to a large increase on V OC with increasing interfacial potential barrier at electrode interfaces in perovskite solar cells. To study light soaking effects in bulk perovskites, we have measured the time-dependent PL for perovskite, perovskite/ PCBM, perovskite/spiro-meotad films on quartz substrates ( Figure 5 ). We can see that the PL intensity gradually increases with light illumination, showing light soaking effects. It is noted that the perovskites can have bulk defects. [18,35 37 ] On the other hand, a photoexcitation can generate a large number of free electrons and holes due to the low-binding energy and fast dissociation. [5,10,38,39 ] As a result, the photogenerated charge carriers can occupy the bulk defects, decreasing the density of bulk defects under continuous light illumination. As the density of bulk defects is decreased upon light illumination, more photogenerated charge carriers become available to recombine into excitons to increase the PL in the perovskite layer, as shown in Figure 5 a. Nevertheless, the increase of PL intensity indicates that the density of bulk traps can be decreased by light illumination due to the charge occupation in the perovskite film. In order to determine the type of bulk traps existed in the perovskite film, we select two different materials: electron-extracting PCBM layer and hole-extracting spiro-meotad layer, to study the light soaking effects on PL (Figure 5 b). We can see that the PL intensity is quenched with different degrees in the doublelayer perovskite/pcbm and perovskite/spiro-meotad structures (Figure 5 c,d). The PL lifetime measurements have also shown the existence of bulk defects in perovskite films. [40 ] Furthermore, the Stark spectroscopy studies indicate that the oriented dipoles are formed at the mesoporous oxide/perovskite interface due to specific interactions between the perovskite and the mesoporous oxide. [41 ] It has been also found that the PL quenching is mainly due to the oriented dipoles at the mesoporous oxide/perovskite interface. [41 ] Here, we use the PL quenching separately with electron-extracting PCBM and hole-extracting spiro-meotad based on double-layer structures to determine the type of bulk traps. In our design the 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (3 of 6)

4 Figure 4. a) C V characteristics for ITO/PEDOT:PSS/perovskite/PCBM/Al device under illumination of 30 min as compared to dark condition and. b) The change of V peak as a function of light illumination time. c) Schematic diagram to show overall electric fi eld in perovskite solar cells. The effective built-in fi eld E bt consists of the dark fi eld ( E dark ) and surface accumulation-induced fi eld ( E ac ). E ex represents an external electric fi eld. photogenerated electrons and holes become majority charge carriers, respectively, in the double-layer perovskite/spiro- MeOTAD and perovskite/pcbm structures. Therefore, we can use the PL quenching to investigate whether the photogenerated electrons or holes are responsible for occupying the bulk traps in light soaking effects. In particular, because the PL mainly comes from the bulk perovskite film, studying the PL quenching can indeed show the properties of bulk traps in the perovskite film. It can be seen in Figure 5 c that the double-layer perovskite/pcbm, where the photogenerated holes are kept in the bulk perovskite, exhibits negligible light soaking effects on PL. On contrast, the double-layer perovskite/spiro-meotad structure, where the photogenerated electrons are kept in the bulk perovskite, shows clear light soaking effects on PL. This comparison indicates that the photogenerated electrons can effectively occupy the bulk traps and consequently leads to a PL enhancement upon light illumination. As a result, we can show that the bulk traps are essentially the p-type defects in the bulk perovskite film. This is consistent with the presence of positively charged traps in both solid and mesosuperstructured perovskite films investigated by photoconductivity studies. [19 ] We should also note that both bulk and interfacial traps can be involved in photoconductivity measurements in solar cells. Here, our PL studies can eliminate the interfacial effect and provide a direct method to show positively charged bulk defects within perovskite film. We should further note that reducing the density of charged bulk traps can generate more effective channels in the perovskite film for dissociated charge carriers to transport to respective electrodes. Combining the decreased charge accumulation at electrode interfaces and the reduced density of bulk traps, we can expect that continuous light illumination can lead to an increase on the FF, as shown in Figure 3. In general, occupying the bulk defects can inevitably influence the local electrical polarizations within the perovskite film in perovskite solar cells. Here, we use the C f measurements to directly explore the light soaking effects on local polarizations in bulk perovskite film under light illumination. We can see from Figure 6 that the C f characteristics can be divided into two zones, namely zone I (< Hz) and zone II (> Hz). The C f results show two interesting phenomena upon light illumination. First, in the zone I, the capacitance increases with light exposure time at low frequencies (< Hz). We estimate the RC constant of the device to be about 600 µs. Therefore, the low-frequency zone I can be attributed to the polarizations at electrode interfaces. From our C V results shown in Figure 4 we can see that the light illumination can decrease the surface accumulation at electrode interfaces. This can lead to a reduction on E ac, consequently increasing the effective E bt, according to Equation ( 1). As a con sequence, the light illumination can increase the surface polarization in the perovskite solar cells. Second, in the zone II, the capacitance decreases at high frequencies with light illumination. This high-frequency capacitance is essentially related to the bulk polarizations in the perovskite layer. Clearly, the capacitance reduction at high frequency provides a direct evidence that the light illumination (4 of 6) wileyonlinelibrary.com 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Figure 5. The time-dependent PL measured at 780 nm for a) perovskite, c) perovskite/pcbm, and d) perovskite/spiro-meotad fi lms by using the 500 nm excitation. Each cycle was measured for 20 min under photoexcitation and subsequently placed in dark condition for 30 min. b) Schematic diagram of energy levels for double-layer perovskite/pcbm and perovskite/spiro-meotad structures. can decrease the bulk polarizations in the perovskite film. We should note that decreasing bulk polarizations can cause two possible effects: weakening the charge dissociation and increasing the charge recombination. This can consequently decrease J SC in the perovskite solar cells upon light illumination. Combining with time-dependent PL and C V results we can see that the interface and bulk parameters are coupled in light soaking effects and hysteresis phenomena. Specifically, our results indicate that light soaking can decrease the charge accumulation at electrode interfaces which can occur by Figure 6. C f characteristics measured as a function of illumination time for ITO/PEDOT:PSS/perovskite/PCBM/Al device. neutralizing the charged surface defects or migrating ions. The charge accumulation at electrode interfaces can directly change the built-in electric field and consequently affect the charge trapping in the bulk. On the other hand, the charge trapping in the bulk can indirectly change the built-in field and then modifies the charge accumulation at electrode interfaces. Clearly, the interface charge accumulation at electrode interfaces and the bulk charge trapping are internally coupled in light soaking and hysteresis phenomena in perovskite solar cells. In summary, mutually connected bulk and interface parameters involved in light soaking effects and hysteresis phenomena are studied by using C V, time-dependent PL, and C f measurements. The C V measurements find that light soaking can decrease the surface accumulation of photogenerated charge carriers at the electrode interfaces. This suggests that the photogenerated carriers can neutralize the interfacial defects and consequently increases V OC. On the other hand, the time-dependent PL studies suggest that continuous light illumination can reduce the density of charged bulk defects within the perovskite layer through charge trapping effects. Our time-dependent PL studies on the double-layer perovskite/ PCBM and perovskite/spiro-meotad structures show that photogenerated electrons can more effectively occupy the bulk defects upon continuous light illumination, leading to a reduction on the density of bulk defects. This provides a direct evidence that the bulk traps are mainly positively charged defects. In particular, light soaking can decrease the density of charged bulk defects through charge trapping. This can enhance the 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (5 of 6)

6 transport of dissociated charge carriers to respective electrodes, causing an increase on the FF. Furthermore, our C f results indicate that light soaking decreases the bulk polarizations within the perovskite film. Decreasing bulk polarizations can reduce charge dissociation and increase the charge recombination, consequently generating a decrease on J SC. Clearly, our C V, time-dependent PL and C f measurements provide new understanding on the light soaking effects and hysteresis phenomena in generating photovoltaic processes in perovskite solar cells. Experimental Section Device Fabrication : The CH 3 NH 3 I and PbCl 2 were purchased from 1-Material and Alfa Aesar, respectively. The solar cells were fabricated with the architecture of ITO/PEDOT:PSS/CH 3 NH 3 Pb 3 x Cl x /PCBM/Al. The ITO glass substrates were cleaned sequentially by using detergent, deionized water, acetone, and ethanol in ultrasonic bath, followed by drying in vacuum oven. The PEDOT:PSS fi lms with the thickness of 35 nm were spin-coated on the oxygen plasma-treated ITO substrates and then annealed at 150 C for 30 min. For the perovskite layer, the CH 3 NH 3 I and PbCl 2 (molar ratio of 3:1) were mixed in anhydrous dimethylformamide (Aldrich) with the concentration of 30 wt%. This solution was spin coated with the thickness of 200 nm onto the PEDOT:PSS layer and then annealed at 80 C for 2 h. Afterward, the 20 mg ml 1 PCBM in chlorobenzene (Aldrich) was coated with the thickness of 40 nm onto the perovskite layer. Then the aluminum (Al) electrodes were thermally deposited with the thickness of 100 nm under the vacuum of Torr to fabricate perovskite solar cells. Device Characterizations and Measurements : The I V characteristics were recorded by using Keithley 2400 source meter under illumination of AM 1.5G 100 mw cm 2 from Newport solar simulator calibrated by a silicon reference cell. The C V and C f measurements were performed by using a dielectric spectrometer (Agilent, 4294A) with alternating voltage of 50 mv. All devices were measured with encapsulation. The PL measurements were measured by a fluorescence spectrometer (Edinburgh Instruments FLS920). The samples were encapsulated before the measurements to avoid reaction with air atmosphere. Acknowledgements This research was supported by National Signifi cant Program (2014CB and 2013CB922104). The authors also acknowledge the support from photovoltaic Project (Gant No ) funded by the National Natural Science Foundation of China. Received: February 6, 2015 Revised: March 27, 2015 Published online: [1] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [2] P. Docampo, J. M. Ball, M. Darwich, G. E. Eperon, H. J. Snaith, Nat. Commun. 2013, 4, [3] J. You, Z. Hong, Y. M. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou, Y. Yang, ACS Nano 2014, 8, [4] P. Gao, M. Grätzel, M. K. Nazeeruddin, Energy Environ. Sci. 2014, 7, [5] T. C. Sum, N. Mathews, Energy Environ. Sci. 2014, 7, [6] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [7] Best Research-Cell Effi ciencies, - ciency_chart.jpg (accessed: May 2015). [8] S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 341. [9] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344. [10] C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. 2014, 26, [11] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum, Y. M. Lam, Energy Environ. Sci. 2014, 7, 399. [12] A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst, R. van de Krol, T. Moehl, M. Grätzel, J.-E. Moser, Nat. Photonics 2014, 8, 250. [13] A. T. Barrows, A. J. Pearson, C. K. Kwak, A. D. F. Dunbar, A. R. Buckley, D. G. Lidzey, Energy Environ. Sci. 2014, 7, [14] E. L. Unger, E. T. Hoke, C. D. Bailie, W. H. Nguyen, A. R. Bowring, T. Heumüller, M. G. Christoforo, M. D. McGehee, Energy Environ. Sci. 2014, 7, [15] J. Wei, Y. Zhao, H. Li, G. Li, J. Pan, D. Xu, Q. Zhao, D. Yu, J. Phys. Chem. Lett. 2014, 5, [16] H.-S. Kim, N.-G. Park, J. Phys. Chem. Lett. 2014, 5, [17] R. S. Sanchez, V. Gonzalez-Pedro, J.-W. Lee, N.-G. Park, Y. S. Kang, I. Mora-Sero, J. Bisquert, J. Phys. Chem. Lett. 2014, 5, [18] H. J. Snaith, A. Abate, J. M. Ball, G. E. Eperon, T. Leijtens, N. K. Noel, S. D. Stranks, J. T.-W. Wang, K. Wojciechowski, W. Zhang, J. Phys. Chem. Lett. 2014, 5, [19] X. Wu, M. T. Trinh, D. Niesner, H. Zhu, Z. Norman, J. S. Owen, O. Yaffe, B. J. Kudisch, X. Zhu, J. Am. Chem. Soc. 2015, 137, [20] T. Leijtens, S. D. Stranks, G. E. Eperon, R. Lindblad, E. M. J. Johansson, I. J. McPherson, H. Rensmo, J. M. Ball, M. M. Lee, H. J. Snaith, ACS Nano 2014, 8, [21] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 2014, 5, [22] L. Cabau, L. Pellejà, J. N. Clifford, C. V. Kumar, E. Palomares, J. Mater. Chem. A 2013, 1, [23] T. Kobayashi, H. Yamaguchi, T. Nakada, Prog. Photovolt: Res. Appl. 2014, 22, 115. [24] P. Tiwana, P. Docampo, M. B. Johnston, L. M. Herz, H. J. Snaith, Energy Environ. Sci. 2012, 5, [25] L. Yang, B. Xu, D. Bi, H. Tian, G. Boschloo, L. Sun, A. Hagfeldt, E. M. J. Johansson, J. Am. Chem. Soc. 2013, 135, [26] C. E. Small, S. Chen, J. Subbiah, C. M. Amb, S.-W. Tsang, T.-H. Lai, J. R. Reynolds, F. So, Nat. Photonics 2012, 6, 115. [27] J. Kim, G. Kim, Y. Choi, J. Lee, S. H. Park, K. Lee, J. Appl. Phys. 2012, 111, [28] S. Trost, K. Zilberberg, A. Behrendt, A. Polywka, P. Görrn, P. Reckers, J. Maibach, T. Mayer, T. Riedl, Adv. Energy Mater. 2013, 3, [29] H. Zang, Y. Liang, L. Yu, B. Hu, Adv. Energy Mater. 2011, 1, 923. [30] H. Zang, Z. Xu, B. Hu, J. Phys. Chem. B 2010, 114, [31] C. Zhao, X. Qiao, B. Chen, B. Hu, Org. Electron. 2013, 14, [32] H. Zang, Y.-C. Hsiao, B. Hu, Phys. Chem. Chem. Phys. 2014, 16, [33] W. Tress, N. Marinova, T. Moehl, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grätzel, Energy Environ. Sci. 2015, 8, 995. [34] Z. Xiao, Y. Yuan, Y. Shao, Q. Wang, Q. Dong, C. Bi, P. Sharma, A. Gruverman, J. Huang, Nat. Mater. 2015, 14, 193. [35] H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F.-S. Francisco, E. J. Juarez-Perez, N.-G. Park, J. Bisquert, Nat. Commun. 2013, 4, [36] W.-J. Yin, T. Shi, Y. Yan, Appl. Phys. Lett. 2014, 104, [37] I. A. Shkrob, T. W. Marin, J. Phys. Chem. Lett. 2014, 5, [38] W. Zhang, M. Saliba, S. D. Stranks, Y. Sun, X. Shi, U. Wiesner, H. J. Snaith, Nano Lett. 2013, 13, [39] V. Roiati, S. Colella, G. Lerario, L. De Marco, A. Rizzo, A. Listorti, G. Gigli, Energy Environ. Sci. 2014, 7, [40] Y. Yamada, M. Endo, A. Wakamiya, Y. Kanemitsu, J. Phys. Chem. Lett. 2015, 6, 482. [41] V. Roiati, E. Mosconi, A. Listorti, S. Colella, G. Gigli, F. De Angelis, Nano Lett. 2014, 14, (6 of 6) wileyonlinelibrary.com 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim