Degradation mechanism of Cu(In,Ga)Se2 solar cells induced by exposure to air

The degradation mechanism of unencapsulated Cu(In,Ga)Se-2 (CIGS) solar cells upon exposure to air has been investigated. Exposure to air at room temperature slightly reduces the conversion efficiency of CIGS solar cells. However, this conversion efficiency decreases significantly under damp heat testing at 85 degrees C and a relative humidity of 85% for 15 h. The shunt resistance and conversion efficiency are completely recovered after removing the side edges of the CIGS solar cells by mechanical scribing. This result suggests that low-resistive layers are formed on the sidewalls of the solar cells during damp heat testing. In addition, alkaline solution etching has been confirmed to be an effective way of removing the low-resistive layers. The low-resistive layers on the sidewalls are identified to be molybdenum oxides and sodium molybdate by Auger electron spectroscopy. After etching the oxides on the sidewalls, the saturation current density and ideality factor are confirmed to be improved. (C) 2016 The Japan Society of Applied Physics.