Thin-film solar cells are cheap and easy to manufacture but require improvements as their efficiencies are low compared to that of the commercially dominant crystalline-silicon solar cells. An optoelectronic model is formulated and implemented along with the differential evolution algorithm to assess the efficacy of grading the bandgap of the CIGS, CZTSSe, and AlGaAs photon-absorbing layer for optimizing the power-conversion efficiency of thin-film CIGS, CZTSSe, and AlGaAs solar cells, respectively, in the two-terminal single-junction format. Each thin-film solar cell is modeled as a photonic...