Although 12&percent; to 14&percent; cells have been fabricated with highly resistive ZnO buffer layers grown by MOCVD, an improved understanding of the required processing to achieve high efficiency is still required. For example, it is found that it is beneficial to “age” i-ZnO/CIS cell structures in air for several weeks before completing the cell with a TCO and collector grid. SIMS depth concentration profiles have been acquired for i-ZnO/CIS film structures grown on polycrystalline CIS and also for epitaxial CIS grown on GaAs. These profiles clearly establish that Zn and oxygen diffuse along grain boundaries during ZnO growth, and probably in such a manner that the Zn concentration exceeds that of oxygen. It is proposed that the beneficial effect of air exposure in the aging process is due to additional oxygen diffusing along grain surfaces and combining with excess zinc such that recombination centers are passivated, and values of FF and Voc are increased. ©1999 American Institute of Physics.