Competition of Iodide/Bromide Ions in the Formation of Methylammonium Lead Halide in Different Solvents

The influence of the solvent on the halide exchange reaction of methylammonium lead tri-iodide (MAPbI3) by bromide ions in solution by forming bromide related plumbates like PBr3– or PbBr42– was investigated by photoluminescence (PL) spectroscopy. Dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL) and N,N-dimethylformamide (DMF) were used as coordinating solvents. It was found that iodide is slowly replaced by bromide ions using DMSO and DMF. In the case of DMSO, we calculated an exchange of about 15 mol % of the initial MAPbI3 concentration within 100 h using equimolar amounts of KBr. Surprisingly, using GBL as solvent leads to very small exchange of PbI3– ions to PbBr3–/PbBr42– ions even after 7 days. Furthermore, the PL investigation reveals that if PbI42– is formed iodide is no longer replaced by bromide ions. Both effects are sustained by DFT calculations that support electronic shielding of the formed complexes against exchange with bromide anions.