Optoelectronically Optimized Colored Thin-Film Solar Cells

Photovoltaic solar cells generally have a black or blue appearance that makes them aesthetically very different from traditional rooftops that either comprise burned-clay tiles or composite-material shingles. Rooftop solar cells may become more acceptable if they are colored, e.g., red or bluish green. This objective requires that a certain part of the incoming solar spectrum be reflected. The reflected part becomes unavailable for photovoltaic generation of electricity. We implemented and optimized an optoelectronic model for Cu₂ZnSn(S_ζSe_1-ζ )₄ (CZTSSe) solar cells containing (i) a conventional 2200-nm-thick CZTSSe layer with homogeneous bandgap, or (ii) a 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 9% if 50% red photons are reflected or by 12% if 50% blue-green photons are reflected. The efficiency reduction increases to 20% if all red photons are reflected or 25% if all blue-green photons are reflected.