The importance of band tail recombination on current collection and open-circuit voltage in CZTSSe solar cells

Abstract

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells typically exhibit high short-circuit current density (Jsc), but have reduced cell efficiencies relative to other thin film technologies due to a deficit in the open-circuit voltage (Voc), which prevent these devices from becoming commercially competitive. Recent research has attributed the low Voc in CZTSSe devices to small scale disorder that creates band tail states within the absorber band gap, but the physical processes responsible for this Voc reduction have not been elucidated. In this paper, we show that carrier recombination through non-mobile band tail states has a strong voltage dependence and is a significant performance-limiting factor, and including these effects in simulation allows us to simultaneously explain the Voc deficit, reduced fill factor, and voltage-dependent quantum efficiency with a self-consistent set of material parameters. Comparisons of numerical simulations to measured data show that reasonable values for the band tail parameters (characteristic energy, capture rate) can account for the observed low Voc, high Jsc, and voltage dependent collection efficiency. These results provide additional evidence that the presence of band tail states accounts for the low efficiencies of CZTSSe solar cells and further demonstrates that recombination through non-mobile band tail states is the dominant efficiency limiting mechanism.