Development of polycrystalline CuIn(x)Ga(1-x)Se2 thin-film solar cells with band gap of 1.3 to 1.5 eV

Achievements in the development of CuIn(x)Ga(1-x)Se2 (CIGS) thin-film solar cells with band gap of 1.3 to 1.5 eV, in order to obtain a better match to the solar spectrum, are reported. The solar cells have an efficiency of 10 percent over ZnO/CdS/CIGS thin-film solar cells. Our achievements are as follows: 13.5 percent efficiency with 44.5 percent Ga (band gap 1.32 eV) and 10.7 percent efficiency with 81.5 percent Ga (band gap 1.57 eV, graded Ga structure). These results clearly demonstrate: (1) high efficiency of over 13 percent 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 In(Cu) antisite donor defects be replaced with Ga(Cu) antisite donor defects, thereby increasing the fractional Ga content in the CIGS thin films. The development of a ZnSe buffer layer, which has a better lattice match to CIGS thin films than CdS, is also reported. (Author)