Performance improvement of Cu2ZnSn(S,Se)4 thin-film solar cells by optimizing the selenization temperature

In this work, Cu2ZnSnS4 (CZTS) precursor films were deposited using a water-based solution approach. Subsequently, selenization was performed at different temperatures in the range of 480–610 °C to prepare Cu2ZnSn(S,Se)4 (CZTSSe) absorber-layer films. The effects of the selenization temperature on the crystallinity, structure, morphology, and photoelectric properties of CZTSSe thin films, as well as the performance of solar cells constructed using these films, were systematically studied. The absorber-layer films selenized at different temperatures all formed pure-phase CZTSSe and had basically the same film thickness. It was found that application of an optimal selenization temperature can enhance the crystallinity, crystal grain size, and mobility and reduce the resistivity of CZTSSe films. Selenization at 550 °C resulted in the largest grain size (∼μm), the highest crystallinity, the highest mobility (4.29 cm2 V−1 s−1), the lowest resistivity (3.13 × 102 Ω cm), the thinner fine-grained layer, a bandgap value of 1.21 eV, and a Cu-poor, Zn-rich elemental composition [Cu/(Zn + Sn) = 0.85 and Zn/Sn = 1.16]. The power-conversion efficiency was improved from 3.04% in a CZTSSe cell device with an absorber layer selenized at 480 °C to 4.69% in a film selenized at 550 °C. This was mainly due to the improvement of the crystallinity, crystal grain growth, and reduction of the fine-grained layer of the CZTSSe film. These results show that optimizing the selenization temperature is essential for enhancing the performance and the ultimate device efficiency of CZTSSe absorber layers prepared using a water-based solution approach.