Facile growth of Cu₂ZnSn(SSe)₄ thin films with controlled phase and microstructure evolution from green ethanol-based molecular solutions

This study presents a cost competitive approach to fabricate phase-pure kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films through an environmental-friendly, ethanol-based solution process, and subsequent dip-coating deposition. The research highlights the significance of controlled heat treatment strategies, utilizing elemental sulfur and selenium flakes, in tailoring the microstructural and opto-electronic properties of the precursor Cu₂ZnSnS₄ thin films. It is demonstrated that the selenization atmosphere, especially the amount of selenium and the duration of heat treatment, plays a critical role in determining the grain structure and phase purity. Optimal conditions involving selenization at 575 °C for 15 min with 1.5 g of Se yielded single-phase CZTSSe thin films, confirmed by the XRD and Raman analysis. Further, the Se/(S + Se) ratio of the phase-pure CZTSSe thin film was found to be 0.68, estimated from EDS analysis. These films exhibited a compact, large-grained microstructure with a thickness of 1.2 µm, marking a significant advancement in the development of CZTSSe films for efficient photovoltaic applications. The optical bandgap (E_g) of the phase-pure CZTSSe thin film was found to be 1.06 eV, and very high white light sensitivity at a bias potential of 6 V, highly suitable for photovoltaic. The resistivity (ρ), carrier concentration (n), and Hall mobility (µ) have estimated from Hall measurement. The results indicated that the opto-electronic characteristics of the grown film are promising for solar cell application.