Additive effects of alkali metals on Cu-modified CH₃NH₃PbI_3- Cl photovoltaic devices

We investigated the addition of alkali metal elements (namely Na⁺, K⁺, Rb⁺, and Cs⁺) to Cu-modified CH₃NH₃PbI_3- Cl photovoltaic devices and their effects on the photovoltaic properties and electronic structure. The open-circuit voltage was increased by CuBr₂ addition to the CH₃NH₃PbI_3- Cl precursor solution. The series resistance was decreased by simultaneous addition of CuBr₂ and RbI, which increased the external quantum efficiencies in the range of 300-500 nm, and the short-circuit current density. The energy gap of the perovskite crystal increased through CuBr₂ addition, which we also confirmed by first-principles calculations. Charge carrier generation was observed in the range of 300-500 nm as an increase of the external quantum efficiency, owing to the partial density of states contributed by alkali metal elements. Calculations suggested that the Gibbs energies were decreased by incorporation of alkali metal elements into the perovskite crystals. The conversion efficiency was maintained for 7 weeks for devices with added CuBr₂ and RbI.