Double Perovskite Cs₂BBiX₆ (B = Ag, Cu; X = Br, Cl)/TiO₂ Heterojunction: An Efficient Pb-Free Perovskite Interface for Charge Extraction

Motivated from the recent success in synthesizing bismuth-based double perovskites (J. Am. Chem. Soc. 2016, 138, 2138-2141), we perform a comprehensive study of interfacial properties of bismuth-based double perovskites Cs₂AgBiX₆ (X = Br, Cl) and TiO₂ interfaces. The bismuth-based double perovskites possess desirable electronic and optical properties as excellent light absorber and thus may serve as lead-free alternatives to the organic-inorganic perovskites. On the basis of density functional theory computation, we systematically study the Cs₂BBiX₆ (B = Ag, Cu; X = Br, Cl)/TiO₂ interfaces and analyze the trend of charge transfer across the interfaces. We find that the Cs₄X₄ (X = Br and Cl)/TiO₂-mediated interfaces are prospective interfaces for charge extraction and separation due largely to the withdrawn trap states for the TiO₂ part when in contact with the Cs₄X₄ termination. Moreover, the ionic interaction and charge redistribution across the specific interfaces can lead to the appropriate band alignment, reduced band gap for the rock-salt double perovskite part, and smooth gradient distribution for the locally projected density of states along the normal direction to the interfaces, further facilitating the charge transfer. Overall, we predict that bismuth-based double perovskites Cs₂AgBiX₆ (X = Br, Cl) and TiO₂ interfaces are highly efficient for charge extraction, suggesting high potential for interfacial engineering optoelectronic devices.