Decreasing Exciton Binding Energy in Two-Dimensional Halide Perovskites by Lead Vacancies

Two-dimensional (2D) perovskites, such as (BA)(2)(MA)(n-1)PbnX3n+1 (X = Cl, Br, or I), have shown great promise in optoelectronic applications because of their stability, tunability, and unique electronic properties. However, their photovoltaic applications are hindered by high exciton binding energies in phases when n is small. On the basis of first-principles calculations, we predict that the exciton binding energy in the atomically thin 2D perovskites (n = 1 and 2) can be decreased significantly by neutral lead vacancies so that free charge carriers can be readily generated at room temperature. Moreover, the photogenerated electrons and holes are delocalized and spatially separated in the presence of lead vacancies; hence, radiative charge recombination can be suppressed. The lead vacancies are predicted to be formed in the 2D perovskites in an I-rich synthetic environment. Some intriguing experimental observations may be related to the presence of lead vacancies in the 2D perovskites.