Optoelectronically optimized colored thin-film CZTSSe solar cells

Rooftop solar cells may become more acceptable if they are colored, e.g., red or bluish green, which requires that a certain part of the incoming solar spectrum be reflected. We implemented and optimized an optoelectronic model for Cu2ZnSn(SξSe1-ξ)4 (CZTSSe) solar cells containing (i) a conventional 2200-nm-thick CZTSSe layer with homogeneous bandgap, or (ii) an ultrathin CZTSSe layer with optoelectronically optimized sinusoidally nonhomogeneous bandgap, or (iii) a CZTSSe layer with optoelectronically optimized linearly nonhomogeneous bandgap. Either complete or partial rejection of either red or bluish green photons was incorporated in the model. Calculations show that on average, the efficiency of a typical solar cell will be reduced by about 9% if 50% red photons are reflected or by about 13% if 50% blue-green photons are reflected. The efficiency reduction increases to about 18% if all red photons are reflected or about 26% if all blue-green photons are reflected.