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.