  Despite the rapid rise in the performance of a variety of perovskite
optoelectronic devices with vertical charge transport, the effects of ion
migration remain a common and longstanding Achilles heel limiting the long-term
operational stability of lead halide perovskite devices. However, there is
still limited understanding of the impact of tin (Sn) substitution on the ion
dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent
current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to
show that short circuit current losses at lower scan rates, which can be traced
to the presence of mobile ions, are present in both kinds of perovskites. To
understand the kinetics of ion migration, we carry out scan-rate-dependent
hysteresis analyses and temperature-dependent impedance spectroscopy
measurements, which demonstrate suppressed ion migration in Pb-Sn devices
compared to their Pb-only analogues. By linking these experimental observations
to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key
role played by Sn vacancies in increasing the iodide ion migration barrier due
to local structural distortions. These results highlight the beneficial effect
of Sn substitution in mitigating undesirable ion migration in halide
perovskites, with potential implications for future device development.
