Large exciton binding energy in atomically thin Cs3Bi2I9−xClx halide perovskite

Abstract

Two-dimensional lead-free perovskite Cs${}_3$Bi${}_2$I${}_9$ has emerged as a promising optoelectronic material due to its unique composition and quantum confinement effects. Herein, we present a comprehensive first-principles computational analysis of the many-body optoelectronic properties arising from the isoelectronic substitution of Cl into the I site in atomically thin Cs${}_3$Bi${}_2$I${}_{9-x}$Cl${}_x$. By solving the Bethe–Salpeter equation, we reveal the distinct optical properties of these doped and pristine monolayers, characterized by strongly bound excitons within an ultralow dielectric screening environment. Specifically, we demonstrate that the isoelectronic substitution of Cl in Cs${}_3$Bi${}_2$I${}_9$ enables significant tunability of quasiparticle states, leading to an unprecedented increase in the exciton binding energy from 1.32 eV to 2.66 eV, and a corresponding increase in the optical gap from the visible to the ultraviolet spectrum. The dominance of these strongly bound excitons in the ground-state properties highlights the substantial potential of these materials for high-efficiency optoelectronic applications, including light-emitting diodes, laser diodes, solar-blind photodetectors, and reconfigurable optical systems.