Potential Fluctuations and Localization Effects in CZTS Single Crystals, as Revealed by Optical Spectroscopy

Abstract

Reports on Cu $$_2$$ ZnSn(S $$_x$$ Se $$_{1-x}$$ ) $$_4$$ (CZTSSe) solar cell devices all show an open-circuit voltage lower than expected, especially when compared to CuIn $$_x$$ Ga $$_{1-x}$$ (S,Se) $$_2$$ devices, which reduces their power efficiency and delays their development. A high concentration of intrinsic defects in CZTSSe, and their stabilization through neutral complex formation, which induces some local fluctuations, are at the origin of local energy shifts in the conduction and valence band edges. The implied band tail in Cu $$_2$$ ZnSnS $$_4$$ is studied in this work by combining three types of optical spectroscopy data: emission spectra compared to photoluminescence excitation spectroscopy, emission spectra as a function of excitation power, and time-resolved photoluminescence spectra. All these data converge to show that both the bandgap and the band tail of localized states just below are dependent on the degree of order/disorder in the Cu/Zn cation sublattice of the quaternary structure: in the more ordered structures, the bandgap increases by about 50 meV, and the energy range of the band tail is decreased from about 110 to 70 meV.