Shallow defects in Cu₂ZnSnS₄

Cu₂SnZnS₄ is a promising candidate for thin film solar cells with absorbers made of non-toxic and abundant elements. So far very little effort has been put into understanding the basic material properties. We investigate vapour phase grown Cu₂SnZnS₄ crystals by temperature and intensity dependent photoluminescence measurements. We observe for the first time narrow photoluminescence peaks, which allow us to determine defect levels and to propose a defect recombination model for Cu₂SnZnS₄. Assuming an exciton binding energy of 10 meV, we find an energy gap of 1.519 eV at 10K. From the observed DA transitions we derive the energies of two shallow acceptor states 10 +/- 5 and 30 +/- 5 meV above the valence band and one shallow donor state 5 +/- 3 meV below the conduction band.