CdTe Photovoltaics: Successes and Challenges

CdTe has fundamental advantages for photovoltaics that include its near-optimal band gap for the solar spectrum and the ability to keep its defect level small in thin-film applications. At the same time panels with CdTe as an absorber can be manufactured on a large scale at low cost and demonstrated long-term stability. These advantages have translated into major commercial success in the U.S. and to some extent in China as well. Recent areas of progress have been the effective use of a CdSeTe alloy layer, enhancement of the transparent front contact, higher voltage with an organic PTAA rear layer, high luminescence efficiency of the cell structure, development of semi-transparent panels, and progress towards bifacial performance. However, several significant challenges remain, and the combination of these is limiting the voltage of the cells. Most fundamental is the large valence-band energy of CdTe, which makes it difficult to contact the typical p-type material. In addition, doping to a desired hole concentration while maintaining the low defect level has been difficult, and the passivation of interfaces and grain boundaries in a completed cell has been only partially successful. Each of these issues has been showing improvement, but the overall challenge is to find the right combination of materials and processes that can be collectively optimized. Since this challenge may be too large for an individual laboratory, researchers at several locations have formed a consortium to combine facilities and experience to jointly increase the overall progress of the CdTe technology.