Large enhanced conversion efficiency of perovskite solar cells by CsBr doping

Perovskite solar cells single-doped with Br⁻ or Cs⁺ ions have been proved to be an effective approach to improve their efficiency and stability. In our work, we took advantage of co-doping with Br⁻ and Cs⁺. At our studied doping levels from CH₃NH₃I:PbI₂:CsBr = 1:1:0 (x = 0) to 0.85:1:0.15 (x = 0.15), CsBr doping does not introduce any detectable impurity, and the crystal grains grow larger with increasing CsBr doping level. Furthermore, when the CsBr doping level is less than x = 0.1, it can progressively enhance the optical absorption of the perovskite film, although the absorption begins to decrease when the doping level rises above x = 0.1. X-ray photoelectron spectroscopy measurements show that Br⁻ has successfully replaced I⁻ and bonds with Pb²⁺ after CsBr doping. At the optimized doping level of x = 0.1, the incorporation of CsBr in the reaction system can improve the morphology of perovskite films and greatly enhance the efficiency from 9.8% for undoped sample to 13.6%, better than single Br⁻ or Cs⁺ doping. Our result shows that CsBr doping is an effective method to enhance the efficiency of perovskite solar cells.