Towards highly efficient thin-film solar cells with a graded-bandgap CZTSSe layer

Abstract

A coupled optoelectronic model was implemented along with the differential evolution algorithm to assess the efficacy of grading the bandgap of the Cu 2 ZnSn(S ξ Se 1−ξ ) 4 (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-nmthick 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.

Optoelectronically optimized colored thin-film CZTSSe solar cells

“…Finally, we considered the sinusoidally nonhomogeneous bandgap described by Eq (3). Values of J sc ,V oc , η, F F and % reduction in η are shown in Table 1.…”

“…Accordingly, either the red part (620-700 nm wavelength) or the bluish-green part (400-550 nm wavelength) of the incoming solar spectrum must be substantially reflected so that it becomes mostly unavailable for the photovoltaic generation of electricity. Therefore, we used an optoelectronic model of CZTSSe solar cells 3 to optimize them for maximum power conversion efficiency while reflecting either red photons or blue-green photons.…”

“…The nonhomogeneous bandgap in the CZTSSe layer was taken to be either sinusoidally or linearly graded. 3 Linearly graded bandgaps of two types were considered: (a) forward graded and (b) backward graded. The linear backward bandgap grading was modeled as…”

Double-absorber thin-film solar cell with 34% efficiency

“…Similarly, it has been experimentally demonstrated that J sc can be improved without reducing V oc by grading the bandgap energy of the absorber layer in CZTSSe solar cells [7]. Detailed optoelectronic modeling indicates that the proper grading of the bandgap energy of the absorber layer can enhance both V oc and J sc in thin-film solar cells [10,11]; efficiencies as high as 27.70% and 21.74% have been predicted for CIGS solar cells [10] and CZTSSe solar cells [11], respectively.…”

“…Thereafter, the electron-hole-pair generation rate G(z) was determined in the ZnO, CdS, CIGS, and CZTSSe layers of the double-absorber solar cell [10], assuming normal illumination by unpolarized polychromatic light endowed with the AM1.5G solar spectrum [20]. The frequencydependent relative permittivity of every material in the double-absorber solar cell is available elsewhere [10,11].…”