Lead-Free Perovskite Solar Cells
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A VIRTUAL ISSUE ON MAKING HYBRID PEROVSKITES UNLEADED The success of achieving high-efficiency solar cells with methylammonium lead halide perovskites has enabled the field to explore new lead-free perovskite materials. The challenges of operational stability of organic lead halide perovskites and the environmental concern of the heavy metal content have led to the design of new perovskite materials (Figure 1). This virtual issue focuses on recent
gap and band energy. Cesium tin halide has also been studied and provides additional routes to enable study of the optical and PV properties of this new class of perovskites. Addition of SnF2 can further improve the performance of PV devices.10 On the basis of density functional theory (DFT) calculations, one can expect a nearly direct band gap for this material and a high absorption coefficient similar to that of CH3NH3PbI3 perovskites.11 Antimony- and Bismuth-Based Perovskites. Optical properties of Sb- and Rb-based perovskites have offered new insights into the suitability of these materials for PV applications.13−15 A layered form of Cs3Sb2I9 having an Sbdeficient derivative of the ubiquitous perovskite structure has been proposed for high-band gap PV applications.14 A high defect density in these materials manifests itself through low emission yields. Sulfur doping of the bismuth halide perovskite also offers an interesting opportunity to tune the band gap of the material.16 Additional understanding of photodynamics and the working principle of these new materials is needed before they can be introduced into practical devices. Halide Double Perovskites. Halide double perovskites are drawing significant attention because of the possibility of interplay between two metal ions to obtain a desired structure.17−22 For example, a combination of trivalent Bi3+ and monovalent Ag+ cations is necessary to occupy two Pb2+ sites of the three-dimensional perovskite architecture and form a double perovskite (Figure 2). On the basis of first-principle calculations, it was demonstrated that the halide double
Figure 1. Schematic relation between the crystal structures of Pb perovskites and lead-free perovskite derivatives. From ref 1. Copyright American Chemical Society.
developments in the area of “lead-free” metal halide perovskites. The papers compiled in this issue highlight initial success in synthesizing new classes of perovskite materials. The low power conversion efficiency and stability of these new perovskite materials still pose major hurdles with respect to finding suitable replacements to the lead-based compounds. Concerted efforts are needed to overcome these barriers, to make lead-free perovskites a contender for photovoltaic (PV) applications. The advances and scientific issues related to lead-free perovskites have been presented in recent perspective/review articles.1−4 These articles underscore the need to prioritize effective synthesis methods to develop new perovskite materials with a direct band gap with a strong oscillator strength for optical transitions. Although double halide double perovskites hold promise, their PV performance in actual sunlight operating conditions has yet to be tested. Tin-Based Perovskites. One candidate that has emerged among the pack of lead-free perovskites is the methylammonium tin halide perovskite.5−12 Both vapor-assisted solution processing5,8 and a hot injection method9 have been employed to prepare CH3NH3SnX3 (X = I, Br, Cl) perovskite films. Interestingly, excess SnX2 creates a secondary phase, which in turn aids in improving the air stability of the devices.6 Anomalous optical and optoelectronic properties of mixed halide tin perovskites have been observed upon increasing the concentration of SnCl2, as a result of the variation of the band © 2017 American Chemical Society
Figure 2. Cubic crystal structure of the halide double perovskite Cs2AgBiBr6. Silver atoms are in light gray, bismuth is in purple, cesium is in turquoise, and bromine is in brown. From ref 22. Copyright American Chemical Society. Received: March 20, 2017 Accepted: March 20, 2017 Published: April 14, 2017 904
DOI: 10.1021/acsenergylett.7b00246 ACS Energy Lett. 2017, 2, 904−905
Energy Focus
http://pubs.acs.org/journal/aelccp
Energy Focus
ACS Energy Letters
(9) Wang, A. F.; Yan, X. G.; Zhang, M.; Sun, S. B.; Yang, M.; Shen, W.; Pan, X. Q.; Wang, P.; Deng, Z. T. Controlled Synthesis of LeadFree and Stable Perovskite Derivative Cs2SnI6 Nanocrystals via a Facile Hot-Injection Process. Chem. Mater. 2016, 28, 8132−8140. (10) Gupta, S.; Bendikov, T.; Hodes, G.; Cahen, D. CsSnBr3, A LeadFree Halide Perovskite for Long-Term Solar Cell Application: Insights on SnF2 Addition. ACS Energy Lett. 2016, 1, 1028−1033. (11) Saparov, B.; et al. Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs2SnI6. Chem. Mater. 2016, 28, 2315−2322. (12) 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. (13) Hebig, J.-C.; Kühn, I.; Flohre, J.; Kirchartz, T. Optoelectronic Properties of (CH3 NH 3 ) 3 Sb 2 I 9 Thin Films for Photovoltaic Applications. ACS Energy Lett. 2016, 1, 309−314. (14) Saparov, B.; Hong, F.; Sun, J. P.; Duan, H. S.; Meng, W. W.; Cameron, S.; Hill, I. G.; Yan, Y. F.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622−5632. (15) Harikesh, P. C.; et al. Rb as an Alternative Cation for Templating Inorganic Lead-Free Perovskites for Solution Processed Photovoltaics. Chem. Mater. 2016, 28, 7496−7504. (16) Vigneshwaran, M.; et al. Facile Synthesis and Characterization of Sulfur Doped Low Bandgap Bismuth Based Perovskites by Soluble Precursor Route. Chem. Mater. 2016, 28, 6436−6440. (17) Wei, F.; et al. Synthesis and Properties of a Lead-Free Hybrid Double Perovskite: (CH3NH3)2AgBiBr6. Chem. Mater. 2017, 29, 1089−1094. (18) Sansom, H. C.; et al. AgBiI4 as a Lead-Free Solar Absorber with Potential Application in Photovoltaics. Chem. Mater. 2017, 29, 1538− 1549. (19) Volonakis, G.; Filip, M. R.; Haghighirad, A. A.; Sakai, N.; Wenger, B.; Snaith, H. J.; Giustino, F. Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals. J. Phys. Chem. Lett. 2016, 7, 1254−1259. (20) Volonakis, G.; et al. Cs2InAgCl6: A New Lead-Free Halide Double Perovskite with Direct Band Gap. J. Phys. Chem. Lett. 2017, 8, 772−778. (21) Filip, M. R.; Hillman, S.; Haghighirad, A. A.; Snaith, H. J.; Giustino, F. Band Gaps of the Lead-Free Halide Double Perovskites Cs2BiAgCl6 and Cs2BiAgBr6 from Theory and Experiment. J. Phys. Chem. Lett. 2016, 7, 2579−2585. (22) Savory, C. N.; Walsh, A.; Scanlon, D. O. Can Pb-Free Halide Double Perovskites Support High-Efficiency Solar Cells? ACS Energy Lett. 2016, 1, 949−955. (23) Ran, C.; Wu, Z.; Xi, J.; Yuan, F.; Dong, H.; Lei, T.; He, X.; Hou, X. Construction of Compact Methylammonium Bismuth Iodide Film Promoting Lead-Free Inverted Planar Heterojunction Organohalide Solar Cells with Open-Circuit Voltage over 0.8 V. J. Phys. Chem. Lett. 2017, 8, 394−400. (24) Song, T.-B.; Yokoyama, T.; Aramaki, S.; Kanatzidis, M. G. Performance Enhancement of Lead-Free Tin-Based Perovskite Solar Cells with Reducing Atmosphere-Assisted Dispersible Additive. ACS Energy Lett. 2017, 2, 897−903.
perovskites possess wide indirect band gaps with large carrier effective masses owing to a mismatch in angular momentum of the frontier atomic orbitals. The estimated theoretical efficiency of solar cell devices with double halide perovskites containing Cl and Br as the halide component is estimated spectroscopically to be less than 10%. By replacing Ag with In or Tl, it may be possible to boost the efficiency, but thermodynamic instability may pose a challenge.22 Solar Cells. There have been a few efforts to explore the PV properties of several non-lead-based perovskites.10,12,22,23 Compared to methylammonium lead halide films, all other types of perovskite films studied to date behave poorly. However, improved stability and tunable optical properties make some of these lead-free perovskites suitable candidates for solar cell and display devices. A recent effort to use excess tin iodide (SnI2) in Sn-based halide perovskite solar cells combined with a reducing atmosphere to stabilize the Sn2+ state has led to improved solar cell performances with a maximum power conversion efficiency of 4.81%.24
Prashant V. Kamat, Editor-in-Chief, ACS Energy Letters University of Notre Dame, Notre Dame, Indiana 46556, United States
Juan Bisquert, Senior Editor, Journal of Physical Chemistry Letters Universitat Jaume I, 12006 Castelló, Spain
Jillian Buriak, Editor-in-Chief, Chemistry of Materials
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University of Alberta, Edmonton, Alberta T6G 2G2, Canada
AUTHOR INFORMATION
Notes
Views expressed in this Editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.
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REFERENCES
(1) Giustino, F.; Snaith, H. J. Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 1233−1240. (2) Boix, P. P.; Agarwala, S.; Koh, T. M.; Mathews, N.; Mhaisalkar, S. G. Perovskite Solar Cells: Beyond Methylammonium Lead Iodide. J. Phys. Chem. Lett. 2015, 6, 898−907. (3) Hoye, R. L. Z.; et al. Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations. Chem. Mater. 2017, 29, 1964−1988. (4) Chakraborty, S.; Xie, W.; Mathews, N.; Sherburne, M.; Ahuja, R.; Asta, M.; Mhaisalkar, S. Rational Design - A High-Throughput Computational Screening and Experimental Validation Methodology for Lead-free and Emergent Hybrid Perovskites. ACS Energy Lett. 2017, 2, 837−845. (5) Yokoyama, T.; Song, T. B.; Cao, D. H.; Stoumpos, C. C.; Aramaki, S.; Kanatzidis, M. G. The Origin of Lower Hole Carrier Concentration in Methylammonium Tin Halide Films Grown by a Vapor-Assisted Solution Process. ACS Energy Lett. 2017, 2, 22−28. (6) Tsai, C. M.; et al. Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode. ACS Energy Lett. 2016, 1, 1086−1093. (7) Dang, Y.; Zhong, C.; Zhang, G.; Ju, D.; Wang, L.; Xia, S.; Xia, H.; Tao, X. Crystallographic Investigations into Properties of Acentric Hybrid Perovskite Single Crystals NH(CH3)3SnX3 (X = Cl, Br). Chem. Mater. 2016, 28, 6968−6974. (8) Yokoyama, T.; Cao, D. H.; Stoumpos, C. C.; Song, T.-B.; Sato, Y.; Aramaki, S.; Kanatzidis, M. G. Overcoming Short-Circuit in LeadFree CH3NH3SnI3 Perovskite Solar Cells via Kinetically Controlled Gas−Solid Reaction Film Fabrication Process. J. Phys. Chem. Lett. 2016, 7, 776−782.
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EDITOR'S NOTE This virtual issue is a compilation of selected papers published in ACS Energy Letters, Chemistry of Materials, and The Journal of Physical Chemistry Letters. The online publication of this virtual issue appears at http://pubs.acs.org/page/vi/ metalhalideperovskites.
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DOI: 10.1021/acsenergylett.7b00246 ACS Energy Lett. 2017, 2, 904−905
