How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites

Varying the halide ratio (e.g., Br–:I–) is a convenient approach to tune the bandgap of organic lead halide perovskites. The complexation between Pb2+ and halide ions is the primary step in dictating the overall composition, and optical properties of the annealed perovskite structure. The complexation between Pb2+ and Br– is nearly 7 times greater than the complexation between Pb2+ and I–, thus making Br– a dominant binding species in mixed halide systems. Emission and transient absorption measurements show a strong dependence of excited state behavior on the composition of halide ions employed in the precursor solution. When excess halide (X = Br– and I–) are present in the precursor solution (0.3 M PbX2 and 0.9 M CH3NH3X), the exclusive binding of Pb2+ with Br– results in the formation of CH3NH3PbBr3 perovskites as opposed to mixed halide perovskite.