  CdTe is a key 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_Cd) is a
prominent acceptor species in p-type CdTe; however, debate continues regarding
its structural and electronic behavior. Using ab initio defect techniques, we
calculate a negative-U double-acceptor level for V_Cd, while reproducing the
V_Cd^-1 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.
