Fabrication of Cu2MSnS4 (M = Co2+, Ni2+) nanocrystal thin films and their application in photodetectors

Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) can be regarded as Cu2ZnSnS4 compounds with the Zn2+ replaced by Co2+ and Ni2+, and are alternative absorber materials in thin film solar cells. Here, we fabricated high-quality CCTS and CNTS nanocrystal thin films by employing the ion substitution strategy. We systematically studied the crystal structures, surface morphologies and compositions of the as-prepared and selenized thin films via X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The selenized CCTS and CNTS films showed an obvious phase separation process for the oxidizing reaction of Co2+ and Ni2+ with high-temperature selenium steam, and then formed Cu2Sn(S,Se)3, Co(S,Se)2 and Ni(S,Se)2 compounds. We successfully fabricated photodetector devices using CCTS and CNTS nanocrystal films, and achieved a strong and reversible photoresponse performance.