PbI2

Letter pubs.acs.org/JPCL

Exploring the Photovoltaic Performance of All-Inorganic Ag2PbI4/ PbI2 Blends Lyubov A. Frolova,† Denis V. Anokhin,†,‡ Alexey A. Piryazev,‡ Sergey Yu. Luchkin,§ Nadezhda N. Dremova,† and Pavel A. Troshin*,§,† †

IPCP RAS, Semenov Prospect 1, Chernogolovka 142432, Russia Faculty of Fundamental Physical and Chemical Engineering, Moscow State University, Leninskie gory 1-51, Moscow 119991, Russia § Skolkovo Institute of Science and Technology, Nobel St. 3, Moscow 143026, Russian Federation ‡

S Supporting Information *

ABSTRACT: We present an all-inorganic photoactive material composed of Ag2PbI4 and PbI2, which shows unexpectedly good photovoltaic performance in planar junction solar cells delivering external quantum efficiencies of ∼60% and light power conversion efficiencies of ∼3.9%. The revealed characteristics are among the best reported to date for metal halides with nonperovskite crystal structure. Most importantly, the obtained results suggest a possibility of reaching high photovoltaic efficiencies for binary and, probably, also ternary blends of different inorganic semiconductor materials. This approach, resembling the bulk heterojunction concept guiding the development of organic photovoltaics for two decades, opens wide opportunities for rational design of novel inorganic and hybrid materials for efficient and sustainable photovoltaic technologies.

apid development of hybrid “perovskite” solar cells resulted in the demonstration of impressive power conversion efficiencies of >22% for the best small-area laboratory prototypes.1,2 However, practical implementation of the hybrid perovskite solar cells is still hampered by their severe degradation at exposure to light, elevated temperatures, and environmental factors such as oxygen and/or humidity.3−6 While some research groups work on the stabilization of the hybrid perovskite solar cells by optimization of the processing technology7−9 and compositional engineering (e.g., by insertion of larger organic or smaller inorganic cations instead of methylammonium),10,11 others aim at the development of principally new materials with improved optoelectronic and physicochemical properties.12 The most recent efforts are focused on the design of all-inorganic perovskite CsBX3 (where B = Pb or Sn, X = Cl, Br, I) materials, which promise to deliver significantly enhanced photochemical and thermal stability.13−18 In particular, two independent research groups reported by the end of 2016 CsPbI3-based planar junction solar cells with the power conversion efficiencies of >10%, thus proving that all-inorganic materials can compete successfully with the conventional hybrid lead-halide-based perovskites in terms of photovoltaic performance.17,18 Unfortunately, the black perovskite phase of CsPbI3 is thermodynamically unstable, which represents another stability issue hampering practical application of this material in photovoltaics.19 The range of thermodynamically stable all-inorganic metalhalide-based perovskites suitable for photovoltaic applications is very limited. In particular, tin(II) and germanium(II) systems

R

© 2017 American Chemical Society

are inherently unstable because of spontaneous disproportionation of the bivalent state to the zero-valence metal and the corresponding tetravalent species.20−22 At the same time, beryllium-, thallium-, cadmium-, and mercury-containing compounds should be excluded from consideration because of their high toxicity and negative impact on the environment. Therefore, there is a growing interest in nonperovskite types of materials, which can potentially deliver the desired combination of properties required for successful implementation in photovoltaic devices. While screening the literature data on such materials, we can refer only to the Ruddlesden−Popper layered perovskites (e.g., A2(MA)2Pb3I10 or A2(MA)3Pb4I13, where A is a bulky organic cation), which deliver decent power conversion efficiencies of 5−12% when the Pb−I layers in the highly crystalline films are grown in an orthogonal direction with respect to the substrate.23−25 Originally reported good photovoltaic performance of the layered wide band gap (MA)2Pb(SCN)2I2 material (Eg = 2.1−2.3 eV)26,27 was misinterpreted, most probably because of its facile decomposition to the conventional MAPbI3 perovskite phase.28,29 To the best of our knowledge, all inorganic metal-halide-based materials with the nonperovskite crystal structure reported to date delivered rather modest power conversion efficiencies (PCEs) below 1.5%.30−35 Control of the electronic dimensionality of emerging materials is one of the crucially important Received: January 27, 2017 Accepted: March 21, 2017 Published: March 21, 2017 1651

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Ag2PbI4, and cubic AgI phases in the AgI−PbI2 films (1:1 component ratio) subjected to annealing at 130 °C within 15 min and subsequent cooling to room temperature (Figure 2 and Table T1).

issues which must be addressed in order to design a successful structure.36 Here we explored a model AgI−PbI2 system, which enabled the fabrication of planar junction solar cells with the efficiency of ∼3.9%, thus featuring the potential of all-inorganic metal halide semiconductors with nonperovskite crystal lattice. Considerable attention has been paid to a variety of AgI− PbI2 compositions for quite some time because they demonstrate well-pronounced ionic conductivity37−39 and interesting semiconductor and photophysical characteristics.40,41 In this work, we initially focused on the 1:1 component ratio, which mimics a perovskite-like “AgPbI3” compound reported previously, though the chemical identity of this material has not been revealed unambiguously yet.42 Vacuum coevaporation of the binary AgI and PbI2 iodides was applied for growing the thin films following a general approach developed in our group for the CsPbI3 perovskite.18 The chemical composition of the deposited films was reconfirmed using energy-dispersive X-ray (EDX) analysis (Figure S1 in the Supporting Information). Thermal annealing of the grown films under inert atmosphere (nitrogen glovebox) was shown to affect significantly their optical characteristics. Indeed, new features start to appear at 550−635 nm at the temperature of 80 °C, thus suggesting a chemical transformation of the material. The evolution of the spectrum was complete at 100−130 °C, while increase in the temperature up to 140 °C resulted in some further irreversible changes (Figure 1). Considering the edge of the absorption band at long wavelengths, we can estimate the optical band gap of the material formed at 120−130 °C as 1.95−1.98 eV.

Figure 2. 2D (a) and 1D (b) X-ray patterns of AgI−PbI2 1:1 film after annealing at 130 °C. On the 2D pattern, indexes of Ag2PbI4, PbI2, and AgI are colored in white, blue, and red, respectively.

The one-dimensional (1D) spectrum reveals the most intense out-of-plane reflections assigned to the 0011 peak of PbI2 and 003 peak of Ag2PbI4 (Figure 2b). It is notable that the detected hexagonal Ag2PbI4 phase with the parameters a = b = 8.31 Å and c = 21.82 Å, obtained by indexing the diffraction pattern, differs from the previously reported cubic Ag2PbI4 structure stable at 130 °C (Table T2).43 There are also noticeable impurities of AgI, which cannot be eliminated completely presumably because of the thermodynamic instability of Ag2PbI4 phase at room temperature43,44 and its slow splitting to AgI and PbI2. The evolution of the AgI−PbI2 film morphology after annealing was investigated by scanning electron microscopy (SEM) and Kelvin probe force microscopy (KPFM), as shown in Figure 3. The films before annealing show poorly ordered domains of, presumably, PbI2 and AgI, with the average lateral size of ∼80−100 nm. Thermal annealing of the films at 130 °C leads to a significant evolution and overall ordering of the film nanostructure. The SEM image of the annealed film shows the presence of microscopic domains (average size of 0.5−1.0 μm) of two distinct phases. These phases probably correspond to Ag2PbI4 and PbI2, which we observed by GIWAXS. While comparing the SEM images for AgI−PbI2 blends with different component ratios (Figure S3), we were able to conclude that

Figure 1. Evolution of the optical spectra of the AgI−PbI2 (1:1 ratio) film after annealing at different temperatures.

The observed behavior was very consistent with the previously reported phase diagrams of the AgI−PbI 2 pseudoternary system.43,44 It was shown that AgI and PbI2 form solid solutions at room temperature, while annealing at 100−130 °C results in the formation of Ag2PbI4. Annealing at higher temperatures results in the formation of Ag4PbI6.43 Though the Ag2PbI4 phase was believed to exist only at 110− 130 °C,43,44 we showed that Ag2PbI4−PbI2 equimolecular blends are also stable at room temperature. Most probably, PbI2 has a stabilizing effect on the Ag2PbI4 phase. Indeed, grazingincidence wide-angle X-ray scattering (GIWAXS) analysis revealed the presence of hexagonal PbI2, orthorhombic 1652

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Figure 3. Microscopy images of the AgI−PbI2 films (1:1 component ratio) before (a, c, e) and after (b, d, f) thermal annealing.

decent photovoltaic performances can be obtained using only the initially investigated equimolar composition (Figures 4b and S5 and Table T4). This result implies that only the Ag2PbI4−PbI2 blends with balanced composition show reasonably good photovoltaic efficiencies, while the enrichment of the device active layer with either component leads to a serious deterioration of the solar cell performances. Microscopy images shown in Figure 3 are fully consistent with the presence of the separate Ag2PbI4 and PbI2 domains with the average size of 0.5−1.0 μm. Apparently, we fabricated inorganic Ag2PbI4/PbI2 bulk heterojunction solar cells similar to the well-explored organic counterparts based on, e.g., solution-processed fullerene− polymer blends. This finding might lead to the development of a new photovoltaic technology emerging from the extensive exploration of a wide variety of all-inorganic binary and, potentially, ternary semiconductor composite materials. While optimizing the photovoltaic performance of the Ag2PbI4/PbI2 system, we explored standard and inverted solar cells fabricated using TiO x (P3HT) and [60]PCBM (PEDOT:PSS) as electron transport (hole transport) layer materials, respectively (Figure 5a,b). Systematic variation of the active layer thickness and AgI/PbI2 deposition rates resulted in improved efficiencies of 3.3−3.9% (Table 1). The external quantum efficiencies of the champion devices approached 60%

darker areas correspond to the PbI2 (or PbI2-rich) phase, while lighter regions can be attributed to the Ag-containing material. The photovoltaic performance of the AgI−PbI2 (1:1 molar ratio) films annealed at different temperatures was investigated first in an inverted planar junction solar cell architecture ITO/ PEDOT:PSS/AgI−PbI2/[60]PCBM/Ag (experimental procedure is given in the Supporting Information). The obtained results are shown in Figures 4a and S4 and Table T3. It is evident from Figure 4a that the best photovoltaic performance with a decent power conversion efficiency (PCE) of ∼3% is delivered by the films annealed at 130 °C, which is fully consistent with the formation of Ag2PbI4 phase. It is notable that annealing at just slightly higher temperature of 140 °C ruins the photovoltaic performance of the material, thus suggesting that the high-temperature Ag4PbI643 phase might not be suitable for photovoltaic applications because of inferior optical or electrical (e.g., charge transport) characteristics. Considering that Ag2PbI4 seems to be the key photoactive component, as follows from the Figure 4a, we attempted to fabricate the solar cells using the stoichiometric AgI−PbI2 2:1 component ratio. Surprisingly, these films showed inferior photovoltaic performances (Figure 4b), while the GIWAXS data supported the formation of the predominant Ag2PbI4 phase with the admixtures of binary iodides AgI and PbI2 (Figure S2). Screening other AgI:PbI2 ratios evidenced that 1653

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(Figure 5c), which confirms the relatively high efficiency of the charge generation and transport in the investigated Ag2PbI4/ PbI2 blends. We can assume that even higher efficiencies are feasible via more careful control of the nanomorphology of the photoactive composite films. Deposition of the AgI/PbI2 blends from DMF solution with subsequent thermal annealing at 130 °C within 20 min also allowed us to fabricate working solar cells, though their characteristics were inferior in comparison with the parameters of the devices produced by thermal coevaporation of AgI and PbI2 (Table 1). SEM images of the solution-processed film revealed that the Ag-containing phase covers almost the whole film surface, while the PbI2-rich phase is apparently localized underneath (Figure S8). Such vertical phase segregation is known to be unfavorable for photoinduced charge generation in the bulk heterojunction systems. It was rather intriguing to observe the absence of any hysteresis in the current−voltage characteristics of the inverted perovskite solar cells (Figure 5d). Devices fabricated in a standard configuration using TiOx as electron transport layer also showed negligibly small or virtually no hysteresis depending on the scanning rate (Figure S7). We emphasize that all-inorganic perovskite solar cells based on CsPbI3 and CsPbBr3 showed considerable hysteresis effects even when they were assembled in the inverted device configuration using PEDOT:PSS and [60]PCBM as materials for hole transport and electron transport layers, respectively.18 This observation strongly suggests that the commonly observed hysteresis in J− V characteristics of perovskite solar cells is an intrinsic feature of the three-dimensional (3D) metal−halogen network. In conclusion, we have shown that planar junction solar cells based on all-inorganic photoactive Ag2PbI4/PbI2 blends can

Figure 4. Photovoltaic performance of AgI−PbI2 composite films (1:1 component ratio) as a function of the annealing temperature (a). Dependence of the solar cell efficiency on the AgI:PbI2 ratio (b).

Figure 5. General layout of the planar junction solar cells with standard (TiOx and P3HT are used) and inverted (using PEDOT:PSS and [60]PCBM) configurations (a). Dark and light-on J−V characteristics for the best standard and inverted devices (b) and EQE spectrum (c) of the champion device with inverted configuration. Current−voltage characteristics of an average device with inverted configuration measured in forward (F) and reverse (R) directions with different scan rates (d). 1654

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Table 1. Best (Average) Open-Circuit Voltages (VOC), Short Circuit Current Densities (JSC), Fill Factors (FF), and Power Conversion Efficiencies (η) of the Devices Extracted from J−V Measurements in Forward Direction with the Scan Rate of 0.1 V/sa device architecture

VOC (mV)

JSC (mA/cm2)

FF (%)

η (%)

standard ITO/TiOx/AgI+PbI2/P3HT/Au inverted ITO/PEDOT:PSS/AgI+PbI2/[60]PCBM/Ag Inverted (solution-processed) ITO/PEDOT:PSS/AgI+PbI2/[60]PCBM/Ag

950 (901 ± 49) 890 (872 ± 18) 798 (744 ± 54)

6.2 (5.9 ± 0.3) 6.2 (5.9 ± 0.3) 3.5 (3.1 ± 0.4)

56 (54 ± 2) 71 (60 ± 11) 60 (45 ± 15)

3.3 (2.9 ± 0.4) 3.9 (3.4 ± 0.5) 1.6 (1.0 ± 0.6)

Average values ± standard deviation from the mean obtained for a batch of eight devices are given in parentheses. J−V curves for the batch of eight inverted devices are shown in Figure S6.

a

deliver decent external quantum efficiencies of ∼60% and light power conversion efficiencies coming close to 4%. It should be mentioned that this particular system shows very low photostability, as do virtually all silver and lead halides, which indicates it has little promise with respect to practical applications. However, the obtained results highlight a significant potential of further exploration of different binary and ternary inorganic bulk heterojunction composite systems (far beyond Pb and Ag halides) with tailored optical and electronic characteristics in order to develop a new generation of materials for highly stable and efficient solar cells.

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(5) Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137, 1530−1538. (6) Misra, R. K.; Aharon, S.; Li, B.; Mogilyansky, D.; Visoly-Fisher, I.; Etgar, L.; Katz, E. A. Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight. J. Phys. Chem. Lett. 2015, 6, 326−330. (7) Guo, Q.; Li, C.; Qiao, W.; Ma, S.; Wang, F.; Zhang, B.; Hu, L.; Dai, S.; Tan, Z. The Growth of a CH3NH3PbI3 Thin Film Using Simplified Close Space Sublimation for Efficient and Large Dimensional Perovskite Solar Cells. Energy Environ. Sci. 2016, 9, 1486−1494. (8) Li, C.; Guo, Q.; Qiao, W.; Chen, Q.; Ma, S.; Pan, X.; Wang, F.; Yao, J.; Zhang, C.; Xiao, M.; et al. Efficient Lead Acetate Sourced Planar Heterojunction Perovskite Solar Cells with Enhanced Substrate Coverage via One-step Spin-coating. Org. Electron. 2016, 33, 194−200. (9) Li, C.; Wang, F.; Xu, J.; Yao, J.; Zhang, B.; Zhang, C.; Xiao, M.; Dai, S.; Li, Y.; Tan, Z. Efficient Perovskite/Fullerene Planar Heterojunction Solar Cells with Enhanced Charge Extraction and Suppressed Charge Recombination. Nanoscale 2015, 7, 9771−9778. (10) Zhang, Y.; Wang, J.; Xu, J.; Chen, W.; Zhu, D.; Zheng, W.; Bao, X. Efficient Inverted Planar Formamidinium Lead Iodide Perovskite Solar Cells via a Post Improved Perovskite Layer. RSC Adv. 2016, 6, 79952−79957. (11) Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; et al. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206−209. (12) Ganose, A. M.; Savory, C. N.; Scanlon, D. O. Beyond Methylammonium Lead Iodide: Prospects for the Emergent Field of ns2 Containing Solar Absorbers. Chem. Commun. 2017, 53, 20−44. (13) Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T.; et al. Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6, 1502458. (14) Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 2452−2456. (15) Chen, L.-J.; Lee, C.-R.; Chuang, Y.-J.; Wu, Z.-H.; Chen, C. Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Quantum Rods with High-Performance Solar Cell Application. J. Phys. Chem. Lett. 2016, 7, 5028−5035. (16) Ma, Q.; Huang, S.; Wen, X.; Green, M. A.; Ho-Baillie, A. W. Y. Hole Transport Layer Free Inorganic CsPbIBr2 Perovskite Solar Cell by Dual Source Thermal Evaporation. Adv. Energy Mater. 2016, 6, 1502202. (17) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum Dot-induced Phase Stabilization of α-CsPbI3 Perovskite for High-efficiency Photovoltaics. Science 2016, 354, 92−95. (18) Frolova, L. A.; Anokhin, D. V.; Piryazev, A. A.; Luchkin, S. Y.; Dremova, N. N.; Stevenson, K. J.; Troshin, P. A. Highly Efficient AllInorganic Planar Heterojunction Perovskite Solar Cells Produced by Thermal Coevaporation of CsI and PbI2. J. Phys. Chem. Lett. 2017, 8, 67−72.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00210. Experimental procedures; results of EDX analysis for the deposited AgI−PbI2 films (Figure S1); additional GIWAXS data (Figure S2 and Tables T1 and T2); photovoltaic characteristics of the solar cells based on the AgI−PbI2 films processed under different conditions (Tables T3 and T4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +7-496-522-1418. Fax: +7-496-522-3507. ORCID

Pavel A. Troshin: 0000-0001-9957-4140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed in the frame of the Next Generation Skoltech-MIT collaboration program. REFERENCES

(1) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-performance Solar Cells. Nature 2015, 517, 476−480. (2) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D.; Levi, D. H.; Ho-Baillie, A. W. Y. Solar Cell Efficiency Tables (Version 49). Prog. Photovoltaics 2017, 25, 3−13. (3) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; et al. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, 1500477. (4) 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. 1655

DOI: 10.1021/acs.jpclett.7b00210 J. Phys. Chem. Lett. 2017, 8, 1651−1656

Letter

The Journal of Physical Chemistry Letters (19) Eperon, G. E.; Paternò, G. M.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.; Cacialli, F.; Snaith, H. J. Inorganic Cesium Lead Iodide Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 19688− 19695. (20) Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; et al. Lead-free Organic−inorganic Tin Halide Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 3061−3068. (21) Kai Wang, S.; Liu, H.-G.; Toriumi, A. Kinetic Study of GeO Disproportionation into a GeO2/Ge System Using X-ray Photoelectron Spectroscopy. Appl. Phys. Lett. 2012, 101, 061907. (22) Krishnamoorthy, T.; Ding, H.; Yan, C.; Leong, W. L.; Baikie, T.; Zhang, Z.; Sherburne, M.; Li, S.; Asta, M.; Mathews, N.; et al. Leadfree Germanium Iodide Perovskite Materials for Photovoltaic Applications. J. Mater. Chem. A 2015, 3, 23829−23832. (23) Tsai, H.; Nie, W.; Blancon, J.-C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; et al. High-efficiency Two-Dimensional Ruddlesden− Popper Perovskite Solar Cells. Nature 2016, 536, 312−316. (24) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843−7850. (25) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414− 11417. (26) Jiang, Q.; Rebollar, D.; Gong, J.; Piacentino, E. L.; Zheng, C.; Xu, T. Pseudohalide-Induced Moisture Tolerance in Perovskite CH3NH3Pb(SCN)2I Thin Films. Angew. Chem., Int. Ed. 2015, 54, 7617−7620. (27) Daub, M.; Hillebrecht, H. Synthesis, Single-Crystal Structure and Characterization of (CH3NH3)2Pb(SCN)2I2. Angew. Chem., Int. Ed. 2015, 54, 11016−11017. (28) Xiao, Z.; Meng, W.; Saparov, B.; Duan, H.-S.; Wang, C.; Feng, C.; Liao, W.; Ke, W.; Zhao, D.; Wang, J.; et al. Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study. J. Phys. Chem. Lett. 2016, 7, 1213−1218. (29) Umeyama, D.; Lin, Y.; Karunadasa, H. I. Red-to-Black Piezochromism in a Compressible Pb−I−SCN Layered Perovskite. Chem. Mater. 2016, 28, 3241−3244. (30) Park, B.-W.; Philippe, B.; Zhang, X.; Rensmo, H.; Boschloo, G.; Johansson, E. M. J. Bismuth Based Hybrid Perovskites A3Bi2I9 (A: Methylammonium or Cesium) for Solar Cell Application. Adv. Mater. 2015, 27, 6806−6813. (31) Kim, Y.; Yang, Z.; Jain, A.; Voznyy, O.; Kim, G.-H.; Liu, M.; Quan, L. N.; García de Arquer, F. P.; Comin, R.; Fan, J. Z.; et al. Pure Cubic-Phase Hybrid Iodobismuthates AgBi2I7 for Thin-Film Photovoltaics. Angew. Chem., Int. Ed. 2016, 55, 9586−9590. (32) Harikesh, P. C.; Mulmudi, H. K.; Ghosh, B.; Goh, T. W.; Teng, Y. T.; Thirumal, K.; Lockrey, M.; Weber, K.; Koh, T. M.; Li, S.; et al. Rb as an Alternative Cation for Templating Inorganic Lead-Free Perovskites for Solution Processed Photovoltaics. Chem. Mater. 2016, 28, 7496−7504. (33) McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Cs2AgBiX6 (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors. Chem. Mater. 2016, 28, 1348− 1354. (34) Saparov, B.; Hong, F.; Sun, J.-P.; Duan, H.-S.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622−5632. (35) Mao, L.; Tsai, H.; Nie, W.; Ma, L.; Im, J.; Stoumpos, C. C.; Malliakas, C. D.; Hao, F.; Wasielewski, M. R.; Mohite, A. D.; et al. Role of Organic Counterion in Lead- and Tin-Based Two-Dimensional Semiconducting Iodide Perovskites and Application in Planar Solar Cells. Chem. Mater. 2016, 28, 7781−7792.

(36) Xiao, Z.; Meng, W.; Wang, J.; Mitzi, D. B.; Yan, Y. Searching for Promising New Perovskite-based Photovoltaic Absorbers: The Importance of Electronic Dimensionality. Mater. Horiz. 2017, 4, 206−216. (37) Mallick, T. K.; Ganguli, S.; Chatterjee, N. Investigation of Ionic Conduction in the Mixed AgI-PbI2 System. J. Phys. Chem. Solids 1984, 45, 661−663. (38) Nawaz, M. S. Rafiuddin. Effect on Electrical Properties of Cation Substituted Ag-Rich Binary System (AgI)2/3−(PbI2)1/3. Mater. Focus 2014, 3, 392−396. (39) Brightwell, J.; Buckley, C.; Miller, L.; Ray, B. Structural Studies and Electrical Conductivity Versus Temperature Measurements in Mixed Silver-lead Iodide Phases. Solid State Ionics 1983, 9−10, 1169− 1173. (40) Mikhailovskaya, E. V.; Sharyi, V. M. Photovoltaic Effect and Photodiffusion in the System Based on Silver and Lead Iodide. J. Sci. Appl. Photogr. Cinematogr. 1991, 36, 373−377. (41) Mikhailovskaya, E. V.; Sharyi, V. M. Electric Conductivity of Silver-lead Diiodide (Ag0.66(Pb2)0.34) in a Polarized Wagner Cell. Ukr. Fiz. Zh. (Russ. Ed.) 1990, 35, 286−290. (42) Pokhrel, S.; Dhakshinamoorthy, A.; Sundaram, R.; Nagaraja, K. S. Crystal Growth and Characterization of PbI2-AgI Binary System. Cryst. Res. Technol. 2005, 40, 752−756. (43) Brightwell, J.; Buckley, C.; Ray, B. Hot Stage X-ray Diffractometer Studies on Ag2PbI4 and Ag4PbI6. Solid State Ionics 1985, 15, 61−63. (44) Brightwell, J. W.; Buckley, C. N.; Ray, B. Electrical and Phase Behaviour of the System AgI-PbI2. Solid State Commun. 1982, 42, 715−716.

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