Effects of residual copper selenide on CuInGaSe2 solar cells

► The conductive Cu2-xSe becomes the shunt paths, especially for a high Cu content. ► The residual Cu2-xSe on CIGS destroys the pn junction behavior. ► The Cu-rich CIGS cells treated by KCN cannot show acceptable efficiency. ► Exhausting the residual Cu2-xSe is necessary for high-efficiency CIGS solar cells.