Effect of Alkali Treatment on Photovoltaic Performances of Cu(In,Ga)(S,Se)2 Solar Cells and Their Absorber Quality Analyzed by Urbach Energy and Carrier Recombination Rates

Cd-free Cu(In,Ga)(S,Se)(2) (CIGSSe) solar cells are fabricated by all dry process, where CIGSSe absorbers with and without alkali (K and Cs) treatments are used. Urbach energy (E-U) is estimated in long-wavelength edge of external quantum efficiency. It is revealed that reduced E-U through K and Cs treatments yields decreased open-circuit voltage deficit. Carrier recombination rates across devices are reduced through K and Cs treatments, corresponding to increased carrier lifetimes and decreased E-U. Conversion efficiency of Cd-free CIGSSe solar cells with a structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:Al is enhanced from about 17.8% (none alkali) to 18.5% (with K) and to 20.3% (with Cs).