Heat Soaking for Improving Rollover From S at the Back of CIGSSe Solar Cells

A simple method of low-temperature heat-soaking (HS) treatment on copper indium gallium sulfur selenide (CIGSSe) solar cells is investigated for mitigating the rollover effect. Excessive sulfur accumulation on the backside of the CIGSSe absorber leads to downward valence band bending and forms a Schottky barrier, resulting in the rollover current–voltage characteristic at room temperature. HS treatment effectively reduces current saturation in the first quadrant of the current–voltage curves and improves cell efficiency on average by more than 6%. The depth profile on exfoliated CIGSSe absorbers from the Auger electron spectroscopy analysis demonstrates the elemental redistribution, indicating that HS treatment can change the profile gradient of sulfur. The width of sulfur accumulation is significantly reduced after HS, which enables Fowler–Nordheim tunneling by Schottky barrier width shortening. This study showcases a straightforward process and easy-to-implement strategy to reduce the second reverse diode effect caused from the sulfurization process.