Development of new buffer layers for Cu(In,Ga)Se2 solar cells

Recent progress in the field of Cu(In,Ga)Se2 (CIGS) thin film solar cell technology is briefly reviewed. New wide-bandgap In x (OOH,S) y and ZnS x (OH) y O z buffers for CIGS solar cells have been developed. Advances have been made in the film deposition by the growth process optimization that allows the control of film properties at the micro- and nanolevels. To improve the CIGS cell junction characteristics, we have provided the integration of the developed Cd-free films with a very thin CdS film. Transmittances of the developed buffers were greatly increased compared to the standard CdS. In x (OOH,S) y buffer has been applied to low-bandgap CIGS devices which have shown poor photovoltaic properties. The experimental results obtained suggest that low efficiency can be explained by unfavorable conduction band alignment at the In x (OOH,S) y /CIGS heterojunction. The application of a wide-gap Cu(In,Ga)(Se,S)2 absorber for device fabrication yields the conversion efficiency of 12.55 %. As a result, the In x (OOH,S) y buffer is promising for wide-bandgap Cu(In,Ga)(Se,S)2 solar cells, however, its exploration for low-bandgap CIGS devices will not allow a high conversion efficiency. The role played by interdiffusion at the double-buffer/CIGS heterojunction and its impact on the electronic structure and device performance has also been discussed.