Letter pubs.acs.org/JPCL
Electric-Field-Induced Degradation of Methylammonium Lead Iodide Perovskite Solar Cells Soohyun Bae,† Seongtak Kim,† Sang-Won Lee,† Kyung Jin Cho,† Sungeun Park,†,‡ Seunghun Lee,†,‡ Yoonmook Kang,*,§ Hae-Seok Lee,*,† and Donghwan Kim*,† †
Department of Materials Science and Engineering, Korea University, Seongbuk-gu, Anam-dong, 136-713 Seoul, Korea Department of Solar Cells - Development and Characterization, Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany § KU•KIST Green School, Graduate School of Energy and Environment, Korea University, Seongbuk gu, Anam dong, 136-713 Seoul, Korea ‡
S Supporting Information *
ABSTRACT: Perovskite solar cells have great potential for high efficiency generation but are subject to the impact of external environmental conditions such as humidity, UV and sun light, temperature, and electric fields. The long-term stability of perovskite solar cells is an important issue for their commercialization. Various studies on the stability of perovskite solar cells are currently being performed; however, the stability related to electric fields is rarely discussed. Here the electrical stability of perovskite solar cells is studied. Ion migration is confirmed using the temperature-dependent dark current decay. Changes in the power conversion efficiency according to the amount of the external bias are measured in the dark, and a significant drop is observed only at an applied voltage greater than 0.8 V. We demonstrate that perovskite solar cells are stable under an electric field up to the operating voltage. electric field mainly because they have a low activation energy for migration.24,26 Direct evidence of this has been found by the elemental distribution after bias is applied.20−22 Second, changes in the light current−voltage (light I−V) characteristics of perovskite solar cells are reported when an electric field is applied.22,27,28 This phenomenon is explained by the changes in the internal electric field owing to ion migration or dipole orientation. Finally, when an electric field is applied to the intrinsic perovskite layer, the photovoltaic effect is observed.20−22 This is explained by self-doping effects of perovskite caused by the migration of internal ions. In summary, it can be understood from these studies that ions can migrate inside the solar cell structure. As shown in Figure 1, when solar cell is exposed to light, photovoltage is generated by separation of photogenerated carriers and by splitting of Fermi level between contacts. Then, ions inside of perovskite, depicted as balls, can be migrated by the photovoltage that has the same direction of forward bias as common p-n diodes. This indicates that the forward bias is continuously applied to the perovskite solar cells during operation conditions. Therefore, photovoltage generated by light irradiation is assumed to induce an electric field in the perovskite solar cells, and changes in their characteristics
I
nterest has been shown in perovskite solar cells based on organic−inorganic hybrid metal halide materials owing to their high performance and rapid recent increases in power conversion efficiency.1−4 Recently, an efficiency greater than 21% has been reported.5 Organometallic perovskite used for fabricating solar cells has many advantages such as direct band gap properties, high absorption coefficient,6,7 long diffusion length,6,8,9 and a tunable band gap.10 Therefore, perovskite solar cells become a spotlight as a new structure to replace silicon solar cells. Despite their promising properties, perovskite solar cells are known to be subject to significant degradation by the external environment. According to studies on stability, the characteristics of perovskite solar cells are affected and degraded by humidity,11−14 UV and sunlight,14−16 temperature,17−19 and electric fields.20−23 Owing to their operation under outdoor conditions, continuous external environmental stress may be applied to perovskite solar cells. Therefore, to enhance the stability of perovskite solar cells, the effects of the environment, degradation mechanisms, and possible solutions are widely studied. However, few studies on the degradation caused by electric fields exist. Studies on the electrical stability are also important because ion migration can occur inside perovskite under an electric field. When the voltage is applied to perovskite solar cells, three important phenomena occur as reported. First, ions in perovskite can be caused to migrate by the electric field.22−25 I− and organic cations are reported to be migrated by the © 2016 American Chemical Society
Received: May 30, 2016 Accepted: July 27, 2016 Published: July 27, 2016 3091
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The Arrhenius plot of current decay velocity k and the inverse time constant calculated from the dark I−V curve are shown in Figure 2b. The activation energy calculated from the slope of the curve is 0.19 eV, which is similar to those reported by other researchers. The activation energies from the literature are summarized in Table 1. The results of the dark current Table 1. Activation Energy for Ion Migration Reported in the Literature
Figure 1. Schematic band diagram of a perovskite solar cell during the operation.
according to the voltage are observed. In this study, the effects of ion migration and electric-field-induced degradation of perovskite solar cells are investigated, and other phenomena affected by voltage, such as interface trapping, are excluded by controlling the test environment. First, to clarify the effect of ion migration in the perovskite solar cells owing to the electric field, we measured the dark current at a given forward voltage with the biasing time. Figure 2a shows the normalized dark current curve of the perovskite solar cell at 0.6 V as a function of temperature. The current decays within a few tens of seconds and becomes saturated. Moreover, the velocity of the decay increases with temperature. In the case of crystalline silicon solar cells, which have no mobile charged ions, such current decay is not observed during electric field application. (Figure S1, Supporting Information (SI)) Therefore, the reason for the current decay of the perovskite solar cell is not the migration of carriers inside the semiconductor (e.g., electrons and holes) but rather charged ion migration or dipole orientation. The current decay has also been observed in other studies,24,25 and the main reason for the current decay is ion migration rather than relaxation of dipoles because of its moderate decay time.21 If a capacitor and a resistor are connected in parallel, then the discharge current caused by ion migration is expressed as the following equation. I = Io exp( −kt ) + Isteady
structure
Ea (eV)
origins
MaPbI3 MAPbxCly MaPbI3 MaPbI3 MaPbI3 MaPbI3 MaPbI3
0.1623 0.2329 0.3226 0.3621 0.4330 0.4531 0.5824
I− I− I− MA+ MA+/I− polarization I−
decay curve and calculated activation energy show that ions migrate under the external electric field and that migration is accelerated by temperature. If ions in the perovskite solar cell are caused to migrate by the electric field, then degradation can be caused by electrical or material problems. Thus, experiments were conducted under electric-field application to observe the effects of a forward electric field on the perovskite solar cell. The voltage was increased from a value below near the open-circuit voltage (Voc) to greater than Voc. Figure 3 shows the normalized efficiency changes for different external biases in the dark at 25 °C. Severe degradation was observed within a few hours when the applied voltage was >1 V. On the contrary, degradation was not observed after ∼200 h when the voltage was below 0.8 V. Figure S3 shows the other parameters of the solar cells as a function of the biasing time. The results show that the shortcircuit current (Jsc) of the solar cells mainly decreases after the biasing test. This indicates an interruption of the migration and collection of photogenerated carriers. The Voc of the perovskite solar cell used in this experiment was ∼1 V, and the maximum power point voltage (Vmp) was ∼0.8 V. The actual forward bias applied to the perovskite solar cell during the operation follows Vmp, nearly 0.8 V. Thus, light-induced degradation can be understood with regards to the forward bias generated by the photovoltage. Changes in the efficiency of similarly
(1)
Figure 2. (a) Dark current decay with temperature and (b) the Arrhenius plot of the time constant. Original current curve of panel a is listed in the Supporting Information. (Figure S2). 3092
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charged ions affecting the dielectric properties of the perovskite solar cell is increased, as discussed in the literature.31 Here it is considered that contribution of Li+ ions in the Spiro-OMeTAD is weak because applied voltage to the hall transport material (HTM) layer is low owing to its low resistivity, and LixOy is formed after oxidation33 of HTM. Therefore, ions attributed to migration are confined to the perovskite components. It is considered that the increase in capacitance is because of increasing ion density in the space-charge region or electrical junction owing to migration from the bulk perovskite layer to the interface of the contact layer. The space-charge region does not have mobile carriers, and the internal electric field is built at the interface. When a forward bias up to the similar amount of Voc, the built-in potential, is applied to the perovskite solar cell, the amount of migrated ions does not compensate the spacecharge regions; however, a huge amount of migrated ions may invert the initial space-charge density biased over Voc, and the net charge density in the solar cell is increased. In both cases, the space charge will be reduced by the electric field, and the photogenerated current flow will be restricted. Although capacitance did not vary significantly during the 1 V bias test, it is observed that diffusion length and collection probability were decreased, as shown in Figure S7. The integrated charge density was then calculated from the dark transient I−V curve (e.g., Figure 2) as a function of the applied bias in order to clarify the reason for degradation under a high electric field and to observe the amount of ions affected by the electric field. Figure S8 shows the dark current for different bias voltages for 5 min. The charge density is calculated by integrating the area of the unsaturated transient part of the I−V curve. The results are plotted in Figure 5. Although total integrated charged density does not directly correspond to the total migrated ion density because values are not calculated from the transient current at 0 V after biasing, it shows information regarding migrated ions at a given voltage. For reference, the reported defect density of perovskite is around 1020 cm−3.34 As shown in the graph, the integrated charge density increased with increasing voltage. This implies that migrated ion density will increase exponentially as the applied voltage is increased. Also, it is observed that a large amount of total charge is calculated at 1.2 V. Therefore, a forward bias over Voc, or the built-in potential of a perovskite solar cell, may affect the performance of solar cells by migrating a large amount of charged ions in perovskite materials. Irreversible degradation can be accompanied by the anomalous total charge density under high voltages over Voc (>1 V). As
Figure 3. Normalized efficiency changes of the perovskite solar cells for different applied voltages.
structured compounds of perovskite with the time under light soaking have been reported by Wei et al.32 After the perovskite solar cell was light-soaked under 1 sun for 90 min, an efficiency drop of >80% was observed compared with its initial value. We suggest that the power drop mainly originates from the continuous forward bias with a Voc >1 V under 1 sun and not from the light. Similar degradation results are observed (Figure S5) during light soaking without biasing, compared with Figure 3. In conclusion, it is expected that degradation induced by the electric field generated by sunlight is not severe during operation because the photogenerated voltage and current follow the maximum power point. However, a forward bias near or exceeding Voc can affect the characteristics of the perovskite solar cell. Finally, to observe light-induced degradation or light soaking effects of the perovskite solar cell, the perovskite solar cell should be connected for operation. Measurements of the capacitance−frequency characteristics were conducted when the voltage was applied to the perovskite solar cell to observe the most strongly affected electrical elements. Figure 4a,b shows the capacitance as a function of frequency after 1 and 1.2 V biasing tests, respectively. The results of 0.6 and 0.8 V biasing tests are shown in Figure S6a,b. Changes in the capacitance were not observed during the 0.6 and 0.8 V biasing tests. After the 1 V biasing test, the efficiency decreased, as shown in Figure 3, and the capacitance varied slightly as a function of the frequency. On the contrary, the capacitance at low frequencies below 1 kHz significantly increased after the 1.2 V biasing test. The increase in the capacitance at low frequencies indicates that the amount of
Figure 4. Capacitance−frequency curves (a) biased at 1 V and (b) 1.2 V. 3093
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the contrary, the space-charge region is depleted at a voltage near Voc and inverted at a voltage greater than Voc. However, the performance of the pristine perovskite solar cell can be recovered by the rearrangement of ions owing to thermodynamic equilibrium over time. To observe the degree of recovery, the biased solar cells were measured again after 1 week. (Figure S10). From the results, the light I−V result of the perovskite solar cell biased at 1 V is fully recovered, whereas the sample biased at 1.2 V did not fully recover for the same amount of time. The results also show that the electron or hole transport layer was not changed because this would be an irreversible process. Therefore, electric-field stress at a voltage greater than Voc can lead to permanent degradation by changing the space-charge region and the built-in potential inside the solar cell. In conclusion, electric-field-induced degradation of perovskite solar cells is discussed in this paper. It is confirmed that forward bias can be introduced in the solar cell during operation by the photovoltaic effect. To observe degradation induced by an electric field, biasing tests were conducted under dark conditions at different magnitudes of the electric field to exclude any effects caused by light. Ionic migration is proved indirectly from the dark current decay under the applied electric field. The activation energy calculated from the dark current decay time constant is 0.19 eV. The biasing test results show that the performance of the perovskite solar cells is severely degraded at voltages greater than the built-in potential, or Voc. Additionally, migration of ions inside the perovskite solar cell depends on the magnitude of the forward electric field from the calculated density of migrated ions and the capacitance as a function of the frequency. Increasing the bias voltage causes the electric field of the space-charge region to be affected by migrating ions. Moreover, permanent degradation occurs when a voltage greater than the built-in potential is applied to the solar cells owing to depleting and inverting the space-charge region. Nevertheless, a perovskite solar cell having Voc of ∼1 V can be demonstrated to be suitable for operation under outdoor conditions if other causes of degradation, such as humidity, temperature, and UV radiation, are excluded, because the photogenerated voltage follows the maximum power point during operation. Although degradation of the perovskite solar cell by electric field is explained by introducing
Figure 5. Integrated charge density as a function of the voltage calculated from integrating the transient parts of the dark I−V curve shown in Figure S8.
shown in Figure S9, the application of a continuous electric field at 1.2 V leads to device degradation, which will be further discussed in the next section. In summary, the amount of migrated ions is dependent on the magnitude of the applied forward bias. Thus, the total amount of migrated ions influences the characteristics of solar cells. As can be understood from the above results, degradation of the solar cell performance caused by the electric field does not occur below the maximum power point of 0.8 V. When the applied voltage is higher than Vmp, migrated ions infiltrate the space-charge region and reduce the space-charge width to degrade the solar cell performance. From the experimental results, it is observed that a large amount of ions can migrate when a voltage greater than Voc, the built-in potential, is applied to the perovskite solar cell. Migrated ions may deplete or inverse the space-charge region and act as a barrier to photogenerated carriers. Figure 6a shows schematics of the ion migration and Figure 6b shows the energy band diagram after the biasing tests with different magnitudes of the voltages. No changes in the space-charge region are expected to occur under Vmp because movements of additional ions are screened by the internal electric field generated by previously migrated ions. On
Figure 6. (a) Schematics of ion migration under forward bias and (b) energy band diagram changes with the voltage. 3094
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(10) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganic− Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (11) Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ Techniques. ACS Nano 2015, 9, 1955−1963. (12) Leguy, A. M.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; Van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J. Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, 3397−3407. (13) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; Van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584− 2590. (14) Niu, G.; Li, W.; Meng, F.; Wang, L.; Dong, H.; Qiu, Y. Study on the Stability of CH3NH3PbI3 Films and the Effect of Post-Modification by Aluminum Oxide in All-Solid-State Hybrid Solar Cells. J. Mater. Chem. A 2014, 2, 705−710. (15) Ito, S.; Tanaka, S.; Manabe, K.; Nishino, H. Effects of Surface Blocking Layer of Sb2S3 on Nanocrystalline TiO2 for CH3NH3PbI3 Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 16995−17000. (16) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized TiO2 with Meso-Superstructured Organometal Tri-Halide Perovskite Solar Cells. Nat. Commun. 2013, 4, 2885. (17) Divitini, G.; Cacovich, S.; Matteocci, F.; Cinà, L.; Di Carlo, A.; Ducati, C. In Situ Observation of Heat-Induced Degradation of Perovskite Solar Cells. Nat. Energy 2016, 1, 15012. (18) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, 1500477. (19) Han, Y.; Meyer, S.; Dkhissi, Y.; Weber, K.; Pringle, J. M.; Bach, U.; Spiccia, L.; Cheng, Y.-B. Degradation Observations of Encapsulated Planar CH3NH3PbI3 Perovskite Solar Cells at High Temperatures and Humidity. J. Mater. Chem. A 2015, 3, 8139−8147. (20) Leijtens, T.; Hoke, E. T.; Grancini, G.; Slotcavage, D. J.; Eperon, G. E.; Ball, J. M.; De Bastiani, M.; Bowring, A. R.; Martino, N.; Wojciechowski, K.; et al. Mapping Electric Field-Induced Switchable Poling and Structural Degradation in Hybrid Lead Halide Perovskite Thin Films. Adv. Energy Mater. 2015, 5, 1500962. (21) Yuan, Y.; Chae, J.; Shao, Y.; Wang, Q.; Xiao, Z.; Centrone, A.; Huang, J. Photovoltaic Switching Mechanism in Lateral Structure Hybrid Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500615. (22) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2014, 14, 193−198. (23) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8, 2118−2127. (24) Eames, C.; Frost, J. M.; Barnes, P. R.; O’regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Commun. 2015, 6, 7497. (25) Unger, E.; Hoke, E.; Bailie, C.; Nguyen, W.; Bowring, A.; Heumüller, T.; Christoforo, M.; McGehee, M. Hysteresis and Transient Behavior in Current−Voltage Measurements of HybridPerovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7, 3690− 3698. (26) Haruyama, J.; Sodeyama, K.; Han, L.; Tateyama, Y. FirstPrinciples Study of Ion Diffusion in Perovskite Solar Cell Sensitizers. J. Am. Chem. Soc. 2015, 137, 10048−10051. (27) Deng, Y.; Xiao, Z.; Huang, J. Light-Induced Self-Poling Effect on Organometal Trihalide Perovskite Solar Cells for Increased Device Efficiency and Stability. Adv. Energy Mater. 2015, 5, 1500721.
the concept of ion migration, further study is required to provide direct evidence of the degradation being caused by migrating ions.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01176. Details of the methods for the fabrication of the perovskite solar cells and their characterization and the supporting graphs and results referred to in the paper. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.K.). *E-mail:
[email protected] (H.-S.L). *E-mail:
[email protected] (D.K.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20138520011170).
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REFERENCES
(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) 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.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (3) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (4) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234−1237. (5) Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; et al. Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989−1997. (6) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (7) Yin, W.-J.; Shi, T.; Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26, 4653−4658. (8) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-Hole Diffusion Lengths > 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (9) Ponseca, C. S., Jr; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T. r.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A. Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and MicrosecondLong Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189−5192. 3095
DOI: 10.1021/acs.jpclett.6b01176 J. Phys. Chem. Lett. 2016, 7, 3091−3096
Letter
The Journal of Physical Chemistry Letters (28) Li, X.; Wang, X.; Zhang, W.; Wu, Y.; Gao, F.; Fang, J. The Effect of External Electric Field on the Performance of Perovskite Solar Cells. Org. Electron. 2015, 18, 107−112. (29) Li, C.; Tscheuschner, S.; Paulus, F.; Hopkinson, P. E.; Kießling, J.; Köhler, A.; Vaynzof, Y.; Huettner, S. Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells. Adv. Mater. 2016, 28, 2446− 2454. (30) Yang, T. Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. The Significance of Ion Conduction in a Hybrid Organic−Inorganic LeadIodide-Based Perovskite Photosensitizer. Angew. Chem. 2015, 127, 8016−8021. (31) Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G. Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1645−1652. (32) Wei, D.; Wang, T.; Ji, J.; Li, M.; Cui, P.; Li, Y.; Li, G.; Mbengue, J. M.; Song, D. Photo-Induced Degradation of Lead Halide Perovskite Solar Cells caused by the Hole Transport Layer/Metal Electrode Interface. J. Mater. Chem. A 2016, 4, 1991−1998. (33) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I.; et al. Lithium Salts as “Redox Active” p-type Dopants for Organic Semiconductors and their Impact in Solid-State Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 2572−2579. (34) Walsh, A.; Scanlon, D. O.; Chen, S.; Gong, X. G.; Wei, S. H. Self-Regulation Mechanism for Charged Point Defects in Hybrid Halide Perovskites. Angew. Chem. 2015, 127, 1811−1814.
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