Tuning the electronic dimensionality and bandgap in Cs2AgBiX6 (X = Br, Cl) for photovoltaic applications: a DFT-1/2 study of cation disorder

Nontoxic, stable, and experimentally realized lead-free halide double perovskites, Cs2AgBiX6 (X = Br, Cl), attracted much attention for solar cell applications. However, their reduced electronic dimensionality and indirect (wide) bandgap, limiting solar energy absorption efficiency, are not mostly suitable. To address such issues, we employ the computationally efficient DFT-1/2 + SOC method to study the electronic structure of cation-ordered and cation-disordered materials comparatively. Our study explores the impact of cation disorder in tuning the electronic dimensionality, demonstrating how the disorder effect reduces bandgaps, increases solar energy absorption, enhances band dispersion, and decreases carrier effective masses for better photovoltaic performance. We observe an evolution of the electronic dimensionality in the disordered systems, influencing the carrier effective masses and absorption properties. Fractional (and non-integer) electronic dimensionality appears to be an essential concept in understanding the optoelectronic properties. The direct bandgap, high absorption in the desired energy range, and mostly lower effective masses of the disordered systems make them suitable for solar cell applications.