Large excitonic effects in 2D Cs₃Bi₂I_9-xCl_x perovskites

Two-dimensional Cs₃Bi₂I₉ (2D CBI) belongs to a family of soft nanomaterials and has been previously shown to host large electrophotonic responses and optically addressable valley degrees of freedom governed by flat bands, large carrier effective masses, and quantum confinement [Phys. Rev. Mater. 7, 2, 024002 (2023)]. Using advanced ab initio methods, we show the evolution of the excitonic features of 2D CBI with isoelectronic substitution of iodine with chlorine: Cs₃Bi₂I_9-xCl_x (0 ≤ x ≤ 9). Our calculations reveal a significant increase in the excitonic behavior with exciton binding energy E_b as high as 3.0 ± 0.2 eV. Such E_b is significantly larger than those reported in any 2D materials. Our analysis shows notable deviation from established binding energy scaling laws for generic 2D materials.