Research strategies toward improving thin-film CdTe photovoltaic devices beyond 20% conversion efficiency

Recent studies of thin-film CdS/CdTe photovoltaic (PV) devices have suggested that a significantly higher device performance will not be achieved unless recombination in the CdTe is reduced. Although some control of CdTe recombination has been achieved historically through the careful incorporation of oxygen, chlorine, and copper, we believe a more promising avenue to higher device performance is by controlling the defects in the as-deposited CdTe. This is supported by theoretical studies that suggest much of the improvement associated with oxygen, chlorine, and copper is due to the interaction of these species with intrinsic defects related to cadmium and tellurium vacancies, interstitials, and anti-sites in the bulk as well as within the grain boundary regions. Several research projects at NREL are currently focused on altering CdTe deposition and post-deposition processes to allow for enhanced control of the as-deposited intrinsic defects. This paper discusses initial results in which process changes expected to alter the as-deposited defects are also observed to affect junction evolution and device functionality.