Investigation of the light soaking behaviors in two-step sputter and selenization Cu(In,Ga)(Se,S)2 solar cells with different sulfur ratios

Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells with different S/(S+Se) (SSSe) ratios were fabricated using a sequential two-step sputtering and selenization/sulfurization process. The origins of the light soaking behavior in the CIGSSe absorber layer were studied based on the light soaking time (or dark storage time) dependence of the open circuit voltage (Voc) and built in potential (Vbi) as well as the temperature dependence of the current-voltage curve. In CIGSSe devices with a SSSe ratio >0.35, Voc and Vbi increase with increasing light soaking time and decrease with increasing dark storage time. In contrast, the Voc and Vbi of the CIGSSe devices with a SSSe ratio of approximately 0.25 showed slightly different behaviors, an increase with increasing light soaking time but little change with increasing dark storage time. The reasons for the different behavior in the CIGSSe devices with different SSSe ratios were attributed mainly to changes in the interface state by the SSSe ratio at the CIGSSe/buffer interface. The results suggest that the control of the SSSe ratio in the CIGSSe absorber fabricated by the proposed two-step process is an important key to determining the light soaking characteristics of the solar cell devices.