Optical determination of the band gap and band tail of epitaxial Ag2ZnSnSe4 at low temperature

We report on the precise determination, in ${\mathrm{Ag}}_2\mathrm{Zn}\mathrm{Sn}{\mathrm{Se}}_4$ epitaxial layer, of both the band gap $E_{\text{g}}$ and the characteristic Urbach energy $U$ that describes the density of localized, defect states in the gap. Various origins for these defect band tail states have been considered, together with the corresponding modeling for their density of states, in order to fit the whole of the optical spectral data. The interest of the methodology developed here is to account quantitatively not only for the absorption and steady-state photoluminescence data but also for the time-resolved photoluminescence spectra. We compare the different origins of localized band tail states to select the standard textbook, Urbach tail model that corresponds to short-range band gap fluctuations. Such an approach is different from the one most often used to evaluate the energy extent of the localized states, which is the Stokes shift between the energies of the photoluminescence emission and the absorption threshold. The advantage of the present method is that no arbitrary choice of the low power excitation has to be done to select the photoluminescence emission spectrum and its peak energy. Thanks to this systematic study of both photoluminescence excitation and time-resolved photoluminescence spectra at low temperature (6 K), the values $E_{\text{g}}=1226\pm 5$ meV and $U=20\pm 3$ meV are found for this promising absorber for thin films photovoltaics.