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 (ΔVOC) in the radiative (ΔVOCRad) and non-radiative (ΔVOCNrad) 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 ΔVOC in these devices and offer directions for future research. We find that ΔVOCRad, 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 ΔVOCNrad, 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 ΔVOCNrad.