Quaternary Sputtered Cu(In,Ga)Se2 Absorbers for Photovoltaics: A Review

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se 2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H 2 Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed