Stoichiometry Control Reveals Compensating Donor-Type Defects Limiting Minority-Carrier Lifetime in Cu2O

The minority-carrier lifetime of Cu2O has been limited to a few nanoseconds or less, constraining its photovoltaic and photoelectrochemical performance, yet the origin of this rapid recombination remains incompletely understood. We tune the Cu/O stoichiometry of large-grained polycrystalline Cu2O plates across its phase stability region by postannealing under controlled oxygen partial pressures (PO2) between the Cu-rich limit (near Cu precipitation) and the Cu-poor limit (near CuO formation). The minority-carrier lifetime increases by more than an order of magnitude, from 0.7 ns at the Cu-rich limit to 28 ns at the Cu-poor limit. The corresponding diffusion length reaches the micrometer scale, comparable to absorber thicknesses in Cu2O devices. This enhancement occurs despite the higher hole concentration at the Cu-poor limit, where a shorter lifetime would conventionally be expected. Time-resolved photoluminescence, subgap photocurrent spectroscopy, and first-principles calculations link the short lifetime under Cu-rich conditions to donor-type Cu interstitials, which act as compensating defects and are accompanied by stronger subgap absorption and increased band-edge disorder. In contrast, both are suppressed toward the Cu-poor limit, consistent with the observed lifetime trend. These results show that the recombination behavior cannot be explained by a model based on abundant acceptor-type copper vacancies. Suppressing compensating donor-type defects through stoichiometry control therefore offers a route to extending the minority-carrier lifetime in Cu2O and may also be relevant to other compensated semiconductors.