Band gap dispersion of Pb-free double perovskites induced by metal cation ordering

Metal halide perovskites (MHP) attract broad interests in the fields of next-generation photovoltaic material developments. Since the Miyasaka group first implemented methylammonium (MA, CH3NH3+) lead tri-iodides (MAPbI3) as a... [ view full abstract ]

Metal halide perovskites (MHP) attract broad interests in the fields of next-generation photovoltaic material developments. Since the Miyasaka group first implemented methylammonium (MA, CH₃NH₃⁺) lead tri-iodides (MAPbI₃) as a sensitizer in dye-sensitized solar cells, there have been dramatic increases in the power conversion efficiency (PCE) of perovskite-based solar cells, of which the certified PCE has already exceeded 22%. However, stability and toxicity are main concerns for MHP solar cell commercialization. Recently, there have been much effort to develop Pb-fee perovskites by expanding the concept of common perovskite lattice family. Double perovskite is one of the promising candidates for Pb-free MHPs which can be made by replacing two divalent Pb²⁺ ions with one monovalent M⁺ and one trivalent M³⁺ ions (A₂M⁺M³⁺X₆).

Although there have been many theoretical and experimental studies on the development of dual perovskites, the importance of the sequence of metal cations has not been examined. This study presents the electronic structure changes induced by metal cation ordering of double perovskites through density functional theory calculation. The band gaps of double perovskites are significantly influenced by metal cation ordering. Hybrid density functional calculation including spin orbit coupling predicts that the band gaps can be modulated by even 2 eV for a unit cell configuration of Cs₂BiAgCl₆. This study reveals the importance of the cation alignment control and suggests the physical origin of the band gap dispersion of double perovskites.