Band Structure Engineering of Cs2AgBiBr6 Perovskite through Order-Disordered Transition: A First-Principle Study.

Cs2AgBiBr6 was proposed as one of the inorganic, stable, and nontoxic replacements of the methylammonium lead halides (CH3NH3PbI3, which is currently considered as one of the most promising light-harvesting material for solar cells). However, the wide indirect band gap of Cs2AgBiBr6 suggests that its application in photovoltaics is limited. Using the first-principle calculation, we show that by controlling the ordering parameter at the mixed sublattice, the band gap of Cs2AgBiBr6 can vary continuously from a wide indirect band gap of 1.93 eV for the fully ordered double-perovskite structure to a small pseudodirect band gap of 0.44 eV for the fully random alloy. Therefore, one can achieve better light absorption simply by controlling the growth temperature and thus the ordering parameters and band gaps. We also show that controlled doping in Cs2AgBiBr6 can change the energy difference between ordered and disordered Cs2AgBiBr6, thus providing further control of the ordering parameters and the band gaps. Our study, therefore, provides a novel approach to carry out band structure engineering in the mixed perovskites for optoelectronic applications.