Impurity photovoltaic and split spectrum for efficiency gain in Cu2ZnSnS4 solar cells

Abstract

Cu2ZnSnS4 (CZTS) solar cells are potential terawatt scale photovoltaic materials due to optimal bandgap and cost-effective synthesis process. However, it shows low efficiency. We have simulated two avenues of efficiency enhancement in Cu2ZnSnS4 solar cells, viz., impurity photovoltaic, and Split-spectrum strategy. An impurity energy level 0.4 eV above valence band with high electron/hole capture cross-section ratio shows an optimum sub-bandgap absorption of wavelengths up to 1127 nm leading to a short-circuit current (JSC), an observation of beyond Shockley-Queisser (SQ) limit. Secondly, the bandgap tunability of Cu2ZnSnS4 is utilized to design a point focusing spectral splitting system, where we numerically showed an enhancement in efficiency. The spectrum is split into three regions, which are focused on bandgap-matched solar cells to absorb a wider spectrum, and reduced thermalization, transmission losses. A conservative calculation using point-focused spectral splitting shows an efficiency up to 19.4 %, a significant improvement compared to the best-reported efficiency of 12.6 % in the literature.