Intramolecular Band Alignment and Spin-Orbit Coupling in Two-Dimensional Halide Perovskites

In two-dimensional (2D) halide perovskites, four distinct types of intramolecular band alignment (I_a, I_b, II_a, and II_b) can be formed between the organic and inorganic components. Molecular design to achieve desirable band alignments is of crucial importance to the applications of 2D perovskites and their heterostructures. In this work, by means of first-principles calculations, we have developed molecular design strategies that lead to the discovery of 2D halide perovskites with favorable band alignments toward light-emitting and photovoltaic applications. The same design strategies can be extended to vertical and lateral heterostructures of 2D perovskites with selective light emissions from the organic and/or inorganic layer of constituent 2D perovskites. For each intramolecular band alignment, the charge density and binding energy of the lowest energy exciton are examined. The effect of spin-orbit coupling (SOC) on the band structures is assessed. While SOC significantly lowers the band gaps in type-I_a and type-II_a alignments, it has a negligible effect in type-I_b and type-II_b alignments.