Effects of Co-Addition of Sodium Chloride and Copper(II) Bromide to Mixed-Cation Mixed-Halide Perovskite Photovoltaic Devices

The effects of the co-addition of NaCl and CuBr2 to CH3NH3PbI3–xClx-based photovoltaic devices are investigated. The lattice constant of CH3NH3Pb(Cu)I3–xClx(Br) is slightly increased by the addition of NaCl, and the X-ray diffraction peaks corresponding to the tetragonal symmetry disappears after several weeks, which indicates a structural transition from the tetragonal to the cubic system by the co-addition of NaCl and CuBr2. A microstructural model, where Na occupies the CH3NH3 vacancy site, is proposed. The Jahn–Teller effect of the Cu d electrons leads to the formation of a cubic system at elevated temperatures, and the conversion efficiency is maintained for 10 weeks for the device with a co-addition of CuBr2 and NaCl.