Effects of residual copper selenide on CuInGaSe sub(2) solar cells

Large-grain, copper-poor CuInGaSe sub(2) (CIGS) films are favored in the fabrication of highly efficient solar cells. However, the degradation of cell performance caused by residual copper selenide (Cu sub(2-x)Se) remains a problem. This work studies the formation and behavior of excess Cu sub(x)Se and further compares the cell performance of typical copper-poor with that of copper-rich solar cells. Since excess Cu sub(2-x)Se cannot be exhausted during the growth, it fully surrounds the polycrystalline CIGS grains. Excess Cu sub(2-x)Se in the CIGS film produces serious shunt paths and causes the pn junction to be of poor quality. A short circuit in copper-rich CIGS solar cells is attributable to the conductive Cu sub(2-x)Se. The best way to ensure high-efficiency of the cells is to exhaust Cu sub(2-x)Se during growth. Otherwise, a dense, chemically treated CIGS film is required to prevent the negative effects of excess Cu sub(2-x)Se.