Fermi level pinning and effects on CuInGaSe2-based thin-film solar cells

This paper briefly summarizes the knowledge accumulated in the literature on the copper indium gallium diselenide (CIGS) material and solar cells based on CIGS. After reviewing the present use of solid-state physics principles to describe thin-film solar cells based on CIGS, a new concept is proposed with the aid of latest findings on electrical contacts to the CIGS material. It has been shown that the Fermi level pinning takes place at one of the few experimentally observed defect levels. The main levels observed to date are at 0.77, 0.84, 0.93 and 1.03 eV with a ±0.02 eV error and are situated above the top of the valence band edge. As a result, discrete values of open circuit voltages are observed, and the situation is very similar to the recent work reported on CdS/CdTe solar cells. Based on these new observations, different ways for further development of CIGS solar cells are proposed.