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

The photovoltaic performance of CuZnSn(S,Se) is limited by open-circuit voltage losses (ΔV) in the radiative (ΔV) and non-radiative (ΔV) 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 in these devices and offer directions for future research. We find that ΔV, arising from bandgap fluctuations and Urbach tails, has been significantly suppressed, with values comparable to those of commercial Cu(In,Ga)(S,Se) solar cells. However, the recombination parameter J, which is more directly related to ΔV, shows only modest improvement and must be reduced by four to six orders of magnitude to compete with Cu(In,Ga)(S,Se). To approach the theoretical efficiency limit, future work should focus on more directly addressing deep defects and ΔV.