The effects of selenium content on Cu(InGa)Se2 thin film solar cells by sputtering from quaternary target with Se-free post annealing

Two kinds of Cu(InGa)Se2(CIGS) absorbers and the corresponding solar cells have been fabricated by sputtering from Se-poor and Se-rich quaternary CIGS targets, respectively. The CIGS thin films fabricated by Se-poor target are mainly composed of chalcopyrite CIGS phase together with small amount of Cu2-xSe compound, while the CIGS thin films fabricated by Se-rich target show single chalcopyrite CIGS phase. The electrical properties of Se-poor films are deteriorated severely due to the formation of Cu2-xSe compared with that of Se-rich film. The highest device efficiency of 12.5% has been achieved in the work with absorber sputtering from Se-rich target with Se-free post annealing. It was found that the key limiting factor to success by this method is sufficient selenium in CIGS absorbers.