The impact of sodium on the sub-bandgap states in CZTSe and CZTS

We compare the optically active sub-bandgap states in polycrystalline Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) thin films as a function of sodium content. In all samples studied, we find that CZTSe has a lower concentration of radiative defect-derived states compared to CZTS and that the states are also shallower in CZTSe compared to CZTS. Further, we find that sodium impacts the relative ratios of two sub-bandgap peaks in the 4 K photoluminescence (PL) spectra of CZTSe (one at ∼0.85 eV and another at ∼0.92 eV). We propose that both of these sub-bandgap peaks stem from intrinsic point defects in CZTSe rather than from electronic states introduced by sodium; this is supported by a measurement on a sodium-free single-crystal of CZTSe. We also show that films with stronger emission through the shallower sub-bandgap states at 4 K display room-temperature PL closer to the bandgap energy. For a…