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Large grain and copper-poor CuInGaSe2 (CIGS) film is preferred to fabricate high efficiency solar cells. However, the degradation caused by excess copper selenide (Cu2-xSe) is still a problem. This study investigates the formation and behavior of excess CuxSe and further compares the cell performances between typical copper-poor and copper-rich solar cells. As the excess Cu2-xSe can not be exhaust during the growth, the excess Cu2-xSe fully surrounded by the polycrystalline CIGS grain. The CIGS film with excess Cu2-xSe would result in serious shunt paths and poor PN junction. A short circuit in the copper-rich CIGS solar cells can be attributed to the conductive Cu2-xSe. For high efficiency cells, the best way is to exhaust Cu2-xSe during the growth. Or otherwise a dense CIGS film with chemical treatments is necessary to avoid the negative effects of the excess Cu2-xSe.