Understanding efficiency losses from radiative and nonradiative recombination in Cu₂ZnSn(S,Se)₄ solar cells

The photovoltaic performance of Cu₂ZnSn(S,Se)₄ is limited by open-circuit voltage losses (ΔV_OC) in the radiative (ΔV_OC^Rad) and non-radiative (ΔV_OC^Nrad) limits, due to sub-bandgap absorption and deep defects, respectively. Recently, several devices with power conversion efficiencies approaching 15% have been reported, prompting renewed interest in the possibility that the key performance-limiting factors have been addressed. In this work, we analyze the sources of ΔV_OC in these devices and offer directions for future research. We find that ΔV_OC^Rad, arising from bandgap fluctuations and Urbach tails, has been significantly suppressed, with values comparable to those of commercial Cu(In,Ga)(S,Se)₂ solar cells. However, the recombination parameter J₀, which is more directly related to ΔV_OC^Nrad, shows only modest improvement and must be reduced by four to six orders of magnitude to compete with Cu(In,Ga)(S,Se)₂. To approach the theoretical efficiency limit, future work should focus on more directly addressing deep defects and ΔV_OC^Nrad.