Na role during sulfurization of NaF/Cu/SnS2 stacked precursor for formation of Cu2SnS3 thin film as absorber of solar cell

Cu2SnS3 (CTS) is one of the intriguing absorbers owing to the earth-abandant and nontoxic element. Reaction paths for formations of CTS films through sulfurization of NaF/Cu/SnS2 (with NaF) and Cu/SnS2 (without NaF) stacked precursors are examined. It is revealed that Cu2SnS3 in the resulting film with NaF starts to be formed at the substrate temperature from 400 °C, while Cu2SnS3 in the film without NaF begins to be formed at the substrate temperature over 500 °C. The crystalline grains of resulting CTS films are markedly enlarged by Na addition, where Na is accumulated in the large grain area. The Na has a significant impact on the formation of CTS crystalline grains at lower substrate temperature, thus inducing the enlargement of the crystalline grains. In addition, the monoclinic CTS structure in the absorbers of the CTS solar cells can be formed at the lower temperature with the help of NaF. In addition, Cu2Sn3S7 phase and/or NaxCuSnS3 (x = 0.5-1.0) are only observed in CTS films with NaF.