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Cs<sub>2</sub>InAgCl<sub>6</sub>: A New Lead-Free Halide Double Perovskite with Direct Band Gap

George VolonakisDepartment of Materials, University of Oxford, Parks Road OX1 3PH, Oxford, United KingdomAmir A. HaghighiradDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomRebecca L. MilotDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomWeng Hong SioDepartment of Materials, University of Oxford, Parks Road OX1 3PH, Oxford, United KingdomMarina R. FilipDepartment of Materials, University of Oxford, Parks Road OX1 3PH, Oxford, United KingdomBernard WengerDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomMichael B. JohnstonDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomLaura M. HerzDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomHenry J. SnaithDepartment of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United KingdomFeliciano GiustinoDepartment of Materials, University of Oxford, Parks Road OX1 3PH, Oxford, United Kingdom

2017en

ABI

Annotatsiya

A2BB′X6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B3+ and B+ cations are In3+ and Ag+, respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3̅m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidt’s rules, and we find that it should also be possible to form Cs2InAg(Cl1–xBrx)6 for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

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DOI: 10.1021/acs.jpclett.6b02682

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