Analysis of the Voltage Losses in CZTSSe Solar Cells of Varying Sn Content

The performance of kesterite (Cu2ZnSn-(S,Se)(4), CZTSSe) solar cells is hindered by low open circuit voltage (V-oc). The commonly used metric for V-oc-deficit, namely, the difference between the absorber band gap and qV(oc) is not well-defined for compositionally complex absorbers like kesterite where the bandgap is hard to determine. Here, nonradiative voltage losses are analyzed by measuring the radiative limit of V-oc, using external quantum efficiency (EQE) and electroluminescence (EL) spectra, without relying on precise knowledge of the bandgap. The method is applied to a series of Cu2ZnSn(S,Se)(4) devices with Sn content variation from 27.6 to 32.9 at. % and a corresponding V-oc range from 423 to 465 mV. Surprisingly, the lowest nonradiative loss, and hence the highest external luminescence efficiency (QE(LED)), were obtained for the device with the lowest V-oc. The trend is assigned to better interface quality between absorber and CdS buffer layer at lower Sn content.