  A coupled optoelectronic model was implemented along with the differential
evolution algorithm to assess the efficacy of grading the bandgap of the CZTSSe
layer for enhancing the power conversion efficiency of thin-film CZTSSe solar
cells. Both linearly and sinusoidally graded bandgaps were examined, with the
molybdenum backreflector in the solar cell being either planar or periodically
corrugated. Whereas an optimally graded bandgap can dramatically enhance the
efficiency, the effect of periodically corrugating the backreflector is modest
at best. An efficiency of 21.74% is predicted with sinusoidal grading of a
870-nm-thick CZTSSe layer, in comparison to 12.6% efficiency achieved
experimentally with a 2200-nm-thick homogeneous CZTSSe layer. High
electron-hole-pair generation rates in the narrow-bandgap regions and a high
open-circuit voltage due to a wider bandgap close to the front and rear faces
of the CZTSSe layer are responsible for the high enhancement of efficiency.

Applied Physics