Efficiency limiting factors in Cu(In,Ga)Se2 thin film solar cells prepared by Se-free rapid thermal annealing of sputter-deposited Cu-In-Ga-Se precursors

We report an efficiency limiting factor of CuInGaSe2 (CIGS) thin film solar cells fabricated by rapid thermal annealing (RTA) of sputter-deposited Cu-In-Ga-Se films without an additional supply of Se vapor. The CIGS thin films show a single-phase chalcopyrite structure without Ga segregation, and there is no apparent Se deficiency in overall composition measured by routine compositional analysis. However, detailed investigation on properties of the CIGS films and devices reveals that the formation of Se vacancies on the CIGS film surface during Se-free RTA is a main limiting factor of the device efficiency.