Improving the efficiency of Cu₂ZnSnS₄ solar cells by promoting the homogeneous distribution of Sn element

Homogeneous element distribution of absorber, as well as high crystallinity, is very critical for the highly efficient Cu₂ZnSnS₄ (CZTS) thin-film solar cell. However, it is more difficult to prepare high quality CZTS absorber layer by post sulfurization of sputtered metallic stack precursors. Herein, we report a simple process, i.e., the optimization for heating rate during the sulfurization, to effectively improve both the uniformity of Sn element distribution and crystallinity of CZTS absorber. CZTS films are prepared by sulfurizing sputtered metallic stack precursors under S and SnS mixed atmosphere. The effects of heating rates on phase structure, morphology, composition distribution of CZTS films and resultant solar cells performance have been comprehensively investigated. CZTS film, sulfurized with a heating rate of 15 °C/min, demonstrates the uniformity of composition distribution, especially for Sn element, besides good crystallinity. Consequently, the series resistance of CZTS solar cells is reduced and the performance of solar cells is improved remarkably. A CZTS solar cell with efficiency of 7.02% (active area efficiency 7.69%) without MgF₂ anti-reflection coating is obtained. The optimization for the heterojunction interface is expected to further improve efficiency.