Cu(In/sub 1-x/,Ga/sub x/)Se/sub 2/ thin film solar cells using transparent conducting oxide back contacts for bifacial and tandem solar cells

Cu(In/sub 1-x/,Ga/sub x/)Se/sub 2/ (CIGS)-based thin film solar cells have been fabricated using transparent conducting oxide (TCO) back contacts allowing light to pass through the entire solar cell structure. The cell performance of CIGS devices using tin oxide (SnO/sub 2/) and indium tin oxide (ITO) back contacts were almost the same as that of a conventional CIGS solar cell fabricated using a Mo back contact. In contrast, fairly low efficiency was achieved for a device fabricated using a ZnO:Al back contact. The best cell performance was obtained using a substrate temperatures of 500-520/spl deg/C. However, the cell performance was observed to deteriorate above 550/spl deg/C. This is attributed to increased resistivity of the TCO's due to the outdiffusion of fluorine dopant atoms from SnO/sub 2/ and the formation of a Ga/sub 2/O/sub 3/ interfacial layer on the ITO layer. We propose a low-cost bifacial CIGS thin film solar cell for the first time as one of the key applications of this type of CIGS device; preliminary results of cell performance are presented. A four-terminal tandem CIGS solar cell is also briefly discussed.