Development of Polycrystalline CuInxGa1-xSe2 Thin-Film Solar Cells with Band Gap of 1.3 to 1.5 eV

Achievements in the development of CuInx Ga1-x Se2 (CIGS) thin-film solar cells with band gap of 1.3 to 1.5 eV to obtain a better match to the solar spectrum are reported with efficiency over 10% on ZnO/CdS/CIGS thin film solar cells. Our achievements are as follows: 13.5% efficiency with 44.5% Ga (band gap 1.32 eV) and 10.7% efficiency with 81.5% Ga (band gap 1.57 eV, graded Ga structure). These results clearly demonstrate (1) high efficiency over 13% is attainable with CIGS thin films in the band-gap range of 1.3 to 1.5 eV and (2) the optimization of both buffer and window layers is also one of the key issues in improving the efficiency. The defect chemistry of a wide range of Cu–III–VI2 thin-film absorbers from CIS to CGS is discussed based upon the analyses of PL spectra, PL peak positions and the film composition. It is proposed for CIGS thin films that InCu antisite donor defects be replaced with GaCu antisite donor defects, increasing the fractional Ga content in the CIGS thin films. The development on a ZnSe buffer layer which has a better lattice match to CIGS thin films than CdS is also reported.