halide perovskite*

Using first-principles many-body theory, we present a comprehensive computational analysis of the unique electronic structure and optoelectronic properties of atomically thin Cs₃Bi₂I₉, a novel two-dimensional (2D) quantum material. We demonstrate that the low-energy physics is governed by flat electronic bands, characterized by a large bandgap and a strongly bound excitonic ground state in an ultraweak screening environment. Through defect engineering, achieved via isoelectronic substitution of Cl on the I site, we show the tunability of excitonic features, with exciton binding energies ranging from ~1.3 eV to as high as 3.0 eV.