Capturing the effect of long low-temperature anneals on the sub-bandgap defect structure of CZTSSe

In this study, Cu2ZnSn(S,Se)4 (CZTSSe) thin films are subjected to long, low-temperature annealing treatments which have been suggested to bring the material through an a€order/disordera€ transition. The samples are then characterized by intensity-dependence photoluminescence measurements at low temperature. We observe that annealing the films at 150°C for 1 day causes a shift in the sub-band gap (Eg) states towards higher photon energies. One week of annealing appears to result in a similar electronic structure as 1 day of annealing, and therefore the measurements performed after 1 day roughly represents the equilibrium (kinetically-limited) defect structure for this temperature. Importantly, all samples measured in this study display strong recombination through deep states up to ∼330 meV below the band gap. Therefore, while some improvements are observed to occur after long low-temperature annealing, we find that this approach does not fully remedy the band tailing states found to limit the Voc in CZTSSe thin film photovoltaics.