Growth of a void-free Cu2SnS3 thin film using a Cu/SnS2 precursor through an intermediate-temperature pre-annealing and sulfurization process

Ternary earth-abundant Cu2SnS3 (CTS) absorbers were synthesized from a Cu/SnS2 stacked precursor by direct annealing in a S atmosphere and by pre-annealing at lower temperature followed by sulfurization. The existing S within the chosen Cu/SnS2 precursor allows avoiding the interface voids commonly generated from a metal precursor. We found that direct annealing of the S-containing precursor at 570 °C in a S atmosphere also generated voids mostly in the middle of the film because a CuS layer is formed on the precursor surface resulting in the discharge of excess S from the SnS2 layer. To eliminate the voids in the CTS film, we developed a two-step annealing process that consists of pre-annealing at 400 °C in N2 and sulfurization at 570 °C in a S atmosphere. The developed process yields a void-free CTS film with a smooth surface and tightly-connected grains. The phase evolution in the CTS films was analyzed by X-ray and Raman spectroscopy, and reaction pathways to form a dense Cu2SnS3 film from the Cu/SnS2 precursor are revealed. Our study demonstrated that appropriate design of annealing could grow a large-grain and dense CTS absorber required for a cost-effective thin film solar cell. Photoluminescence analysis confirmed that the CTS film grown by the two-step annealing process exhibited fewer deep-level defects compared to the film grown by direct annealing in a S atmosphere. The conversion efficiency of the solar cell based on the developed absorber is higher than that of a device using a CTS absorber synthesized by direct sulfurization. However, low values of the open-circuit voltage and fill factor indicate that fine control of the CTS composition is necessary to improve the device performance.