Parity-Forbidden Transition as an Origin of the Large Optical Bandgap of Cs$_2$AgInCl$_6$ Lead-Free Halide Double Perovskite

Research paper by Weiwei Meng, Xiaoming Wang, Zewen Xiao, Jianbo Wang, David Mitzi, Yanfa Yan

Indexed on: 12 Feb '17Published on: 12 Feb '17Published in: arXiv - Physics - Materials Science


Lead-free Cs$_2$B$^{1+}$B$^{3+}$X$_6$ (B = metal, X = Cl, Br, I) metal halide double perovskites have attracted extensive attention because of their nontoxicity and relative air-stability as compared to their organic-inorganic lead halide perovskite counterparts. Recently, Cs$_2$AgInCl$_6$ has been reported as a new lead-free halide double perovskite with a direct bandgap, which is a preferred property for thin-film solar cell applications. However, while the synthesized Cs$_2$AgInCl$_6$ powders exhibit white coloration with an experimentally measured optical bandgap of 3.3 eV, the photoluminescence emission energy is only 2.0 eV. Here, we unveil the origin of this mystery by density-functional theory calculations. We show that the large observed optical bandgap is caused by parity-forbidden transitions, which are due to the centrosymmetry of double perovskite and the lowest conduction band being derived from the unoccupied In 5s orbitals. The large difference between the fundamental (independent of allowed/forbidden transition) and the optical (derived from allowed transition) bandgaps renders this double perovskite unsuitable for efficient thin-film photovoltaic applications. Similar parity-forbidden transitions are also found in some other Cs$_2$B$^{1+}$B$^{3+}$X$_6$ double perovskites, e.g., when B$^{1+}$ = group IA or IB, B$^{3+}$ = Ga or In. Our results suggest that the centrosymmetry-induced parity-forbidden transitions must be taken into consideration when one designs new double perovskites for optoelectronic application.