Solution-processed copper zinc tin sulfide thin films from metal xanthate precursors

The quaternary semiconductor copper zinc tin sulfide (Cu ₂ZnSnS₄, CZTS) is one of the most promising alternatives to Ga and In based semiconductors for thin film solar cells. It consists of non-toxic, cheap, and abundant elements and displays highly beneficial optical as well as electronic properties for photovoltaic applications. In this work we present a solution-based preparation method for CZTS thin films using exclusively metal xanthates as precursor materials. The introduction of branched alkyl side chains (3,3-dimethyl-2-butyl) into the metal xanthates leads to highly soluble precursors with low decomposition temperatures. In addition, these precursors already contain the sulfur needed for the formation of the metal sulfide. Therefore, no external sulfur source such as thiourea, thioacetamide, or elemental sulfur is necessary. For the preparation of CZTS thin films, solutions containing these metal xanthates were used to coat precursor layers, which were subsequently annealed at temperatures between 180 and 350 C. Depending on the temperature, nanocrystalline films with primary crystallite sizes ranging from 3 nm (180 C) up to approximately 43 nm (350 C) were obtained. A combined X-ray diffraction, Raman spectroscopy, and TEM-EDX study showed that a precursor solution with a Cu/(Zn + Sn) ratio of 0.6 has to be used to obtain CZTS films, which show high optical absorption (>2 × 10⁵ cm^-1) and an optical band gap of approximately 1.31 eV. First experiments concerning photovoltaic activity of the solution processed CZTS layers were carried out.