Light soaking in thin film solar cells shows both beneficial and detrimental impact on efficiency depending upon the absorber–buffer pair. We here numerically simulated the advantageous and detrimental impact of light soaking behavior as experimentally observed in the literature by utilizing suitable defect models for the two cases, respectively. The proposed models are based on the photoconductivity and metastable photo-induced activity of absorber and buffer layers. It is observed that light soaking in absorber layer vis-a-vis photo-induced defect in absorber is beneficial for device performance. Photo-generated defect in absorber acts as hole doping which is turn enhance the built-in voltage. The photoconductivity of the buffer layer is detrimental to the device performance. The photo-generated defects in the buffer cause lowering of fill-factor, current, accelerate interfacial recombination and depreciate overall efficiency. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.