Density Functional Theory Treatment of Site-Disordered Silver Iodo-Bismuthate Semiconductors*

Silver iodo-bismuthates, a class of derivative metal-halide perovskite materials, are promising candidates for applications including indoor photovoltaics, radiation detectors, and flexible electronics.  Their large degree of site-disorder has prevented first principles calculations of all but the simplest material structures.  In this study, we use a cluster expansion description of the total energy and a simulated annealing minimization routine to determine low-energy structures of the NaVO₂-like Ag-Bi-I materials.  It is found that 92 clusters are capable of describing greater than 10¹⁷ possible cell configurations to an average accuracy of 11 meV/atom.  With the low-energy cells, we employ density functional theory to calculate optoelectronic properties such as the effective masses and absorption spectra.  It is found that fundamental properties, such as the experimental band gaps, only agree with experimental values for the lowest-energy structures, indicating a need for the proposed efficient ground-state search method.