Charge Carrier Dynamics of Multiple-Cation Mixed-Halide Perovskite Thin Films

We investigate the influence of the stoichiometric ratio between formamidinium iodide and methylammonium bromide in mixed lead halide perovskite photovoltaic devices (CH3NH3)1–x (CH­(NH2)2)xPbBr3(1–x)I3x, 0.6 ≤ x ≤ 1.0, on the material and solar cell characteristics, respectively. The control over ratio x is achieved by changing the perovskite precursor stoichiometry. Absorption and external quantum efficiency measurements reveal that the exciton binding energy and the Urbach energy have a local minimum for a ratio of 0.83, which has been reported to cause the lowest lattice strain and also gives the highest power conversion efficiency. These findings are confirmed by photoluminescence measurements, absorption measurements, and solar cell characteristics. Our results predict that nonradiative recombination still plays a major role in this perovskite variation, although the stoichiometric composition can be optimized for less lattice strain to achieve lower energetic disorder and exciton binding energies for a higher solar cell performance.