M. Lang, C. Zimmermann, C. Krämmer, T. Renz, C. Huber, H. Kalt, und M. Hetterich

Phys. Rev. B 95, 155202 (2017)

The power conversion efficiency of Cu₂ZnSn(S,Se)₄ kesterite thin-film solar cells is mainly limited by a low open-circuit voltage V_OC. In the literature, this low V_OC has been amongst others attributed to electrostatic potential fluctuations leading to fluctuating band edges. This assignment was mainly based on the observation of a shift of the photoluminescence (PL) maximum to higher photon energies as a function of excitation power, which was interpreted in terms of a screening of the electrostatic potential fluctuations. However, in this paper, we show evidence that the observed shift of the PL maximum is dominantly caused by state filling rather than screening. Our assignment is based on the investigation of the full PL line shape (not only of the PL maximum) and the PL yield as a function of excitation power and temperature. Further support of our interpretation is given by the observation of additional band-tail emission showing up as a second high-energy PL band at the highest excitation powers. The spectral position of this additional band coincides with the absorption-tail deduced from photoluminescence excitation spectroscopy (PLE).