Formation of Bi - Bi Dimers in Heavily Bi -Doped Lead Halide Perovskites: Origin of Carrier Density Saturation

Heterovalent Bi doping in lead halide perovskites (LHPs) is of particular interest in recent research to improve the electronic and optoelectronic properties. In experiments, the incorporation of Bi not only converts the conductivity type of crystals from p type to n type, leading to the significant enhancement of charge-carrier concentration, but also results in the appearance of near-infrared photoluminescence (NIR PL) along with the decrease of band-edge PL intensity. However, with a high concentration of Bi doping, the carrier concentration exhibits a saturated tendency and the intensity of NIR PL decreases abnormally. The origins behind these interesting phenomena are not yet well understood. Herein, based on first-principles calculations, we demonstrate that Bi dimers form n type and intrinsic LHPs with a high concentration of Bi doping. They act as effective n-type limiting defects, and hence, cause carrier saturation in experiments. Moreover, the NIR PL arising from the BiPb defect is suppressed due to the density conversion of isolated BiPb defects to Bi dimers when Bi is heavily doped. Meanwhile, nonradiative recombination is also promoted due to the deep charge-state transition level introduced by the Bi dimer. Our work not only reveals the mechanism of highly concentrated Bi limiting the electronic and optical properties of LHPs, but also suggests the precise control of Bi doping in electronic and photovoltaic applications.