  Power-conversion efficiency is a critical factor for the wider adoption of
solar-cell modules. Thin-film solar cells are cheap and easy to manufacture,
but their efficiencies are low compared to crystalline-silicon solar cells and
need to be improved. A thin-film solar cell with two absorber layers (instead
of only one), with bandgap energy graded in both, can capture solar photons in
a wider spectral range. With a 300-nm-thick CIGS~absorber layer and an
870-nm-thick CZTSSe~absorber layer, an efficiency of $34.45\%$ is predicted by
a detailed optoelectronic model, provided that the grading of bandgap energy is
optimal in both absorber layers.
