Electronic Properties of {111} Twin Boundaries in a Mixed-Ion Lead Halide Perovskite Solar Absorber

We present first-principles theoretical predictions of the electronic properties of {111} twin boundaries in pure formamidinium lead iodide (FAPI) as well as a mixed-ion lead halide perovskite containing formamidinium, Cs, I, and Br. We find that the {111} twin boundary is extremely stable in pure FAPI but introduces no electron or hole trapping states and presents relatively small barriers (<100 meV) to transport of electrons and holes, suggesting that they are relatively benign for solar cell performance. However, in the mixed-ion perovskite, twin boundaries serve as a nucleation site for formation of an I- and Cs-rich secondary phase. The reduced band gap in this segregated phase leads to hole trapping and is likely to enhance electron hole recombination and lead to reduced open-circuit voltage in solar cell devices. These results highlight the role of twin defects as nucleation sites for secondary phases, which can be extremely detrimental to solar cell performance.