Rapid Recombination by Cadmium Vacancies in CdTe.

CdTe is currently the largest thin-film photovoltaic technology. Non-radiative electron-hole recombination reduces the solar conversion efficiency from an ideal value of 32% to a current champion performance of 22%. The cadmium vacancy (V) is a prominent acceptor species in -type CdTe; however, debate continues regarding its structural and electronic behavior. Using defect techniques, we calculate a negative-U double-acceptor level for V, while reproducing the V hole-polaron, reconciling theoretical predictions with experimental observations. We find the cadmium vacancy facilitates rapid charge-carrier recombination, reducing maximum power-conversion efficiency by over 5% for untreated CdTe-a consequence of tellurium dimerization, metastable structural arrangements, and anharmonic potential energy surfaces for carrier capture.