Copyright © 2018 John Wiley & Sons, Ltd. Potassium-treated Cu (In,Ga)(S,Se)2 (CIGSSe)-based solar cell with power conversion efficiency (η) of 19.4% is obtained using high transparent Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al layers to minimize optical loss at short wavelength (~520 nm) and to control total conduction band minimum alignment. To further enhance η, the post treatment named HLS + LS process, including heat-light soaking (HLS) at 110°C under AM 1.5G illumination followed by light soaking (LS) under AM 1.5G illumination, is conducted successively on the as-fabricated solar cell. It is revealed that HLS in the HLS + LS process mainly yields the increase in open-circuit voltage. On the other hand, LS in the HLS + LS process primarily leads to the increase in fill factor, attributable to the decrease in sheet resistance of Zn0.88Mg0.12O:Al. The HLS + LS process consequently gives rise to not only the enhancement of carrier concentration but also the decrease in the recombination rate at the buffer/absorber interface through passivating the recombination centers. As a result, 21.2%-efficient CIGSSe solar cell with the Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al layers is attained after the HLS + LS process, which is an effective process to enhance photovoltaic performances.