Synergistic enhancement of Cs and Br doping in formamidinium lead halide perovskites for high performance optoelectronics

Abstract

Among the various developed perovskite materials, HC(NH₂)₂PbI₃ (FAPbI₃) is considered one of the best photovoltaic materials due to its high thermal stability and smaller band gap for light harvesting over a broader spectrum. However, the relatively large size of HC(NH₂)₂⁺ (FA) makes HC(NH₂)₂PbI₃ prone to a phase transition from its black phase to the yellow phase at room temperature. Reduction of the effective ion size by doping with smaller ions is an effective way to stabilize black-phase FAPbI₃. However, the effect of dopants on the optoelectronic properties of doped formamidinium lead halide perovskites has rarely been investigated. In this work, we prepared Cs-, Br- and Cs–Br co-doped FAPbI₃ single-crystal materials with doping concentrations of 5%, 10% and 15%. The impacts of dopants on the phase stability, trap state density, carrier lifetime and carrier mobility are thoroughly investigated. It is found that Cs doping is not effective at stabilizing the black phase, while Br is a good stabilizer at concentrations exceeding 6.7%. Furthermore, the Cs and Br co-doped samples could be synergistically enhanced for high performance optoelectronics and show not only better stability but also much improved carrier lifetimes of 1.23 μs and carrier mobilities of 427.3 cm² V⁻¹ s⁻¹, which resulted in a longer carrier diffusion length of approximately 30 μm as well as a photodetector with a high responsivity of 0.64 A/W.