Role of buffer layers in CIS-based solar cells

This paper describes investigations of the effects of buffer layers on CIGSS-based solar cells. Two general types of cells have been investigated, namely, direct ZnO devices and cells with either a CdS or i-ZnO buffer layer. The CIGSS absorber in all cases is Siemens Solar material. The CdS buffer layers were grown by CBD at Siemens Solar, and the i-ZnO buffer layers were grown by MOCVD at WSU. Typical efficiencies for the three types of cells were; 7 to 9 % for direct ZnO/CIGSS structures; 11 to 14 % for cells with CdS or i-ZnO buffer layers. Characterization studies included current-voltage analysis to identify current loss mechanisms, and SIMS depth concentration profiles. These studies indicate that a key purpose of the buffer layer is to provide a barrier to diffusion of impurities into the absorber.