We studied the optimization of an ultrathin CuIn1-ξGaξSe2 (CIGS) solar cell with a nonhomogeneous CIGS absorber layer and backed by a 1D metallic periodically corrugated back-reflector (PCBR) with a rectangular profile. Nonhomogeneity in the CIGS absorber layer was modeled through either a sinusoidal or a linear bandgap variation along the thickness direction. The maximum power density for the AM1.5G spectrum was determined from the spectrum of the useful solar absorptance computed using the rigorous coupled-wave approach. Ultrathin solar cells with optimized PCBR and homogenous bandgap depending on the thickness of the CIGS layer were found to deliver the best photonic absorption characteristics. The open-circuit voltage, efficiency, and fill factor were calculated for the optimal designs using values of the reverse-saturation current density, ideality factor, and the series resistance density obtained from experimental results. The overall trend is that the effect of the PCBR becomes less prominent as the thickness of the CIGS absorber layer increases. Higher efficiency and fill factor can be achieved with a solar cell containing as 400-nm-thick CIGS layer compared to the conventional solar cell with a 2200-nm-thick CIGS layer.