Phase-Separation-Induced Crystal Growth for Large-Grained Cu₂ZnSn(S,Se)₄ Thin Film

Large-grained Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber layers are highly desirable for high-performance CZTSSe thin film solar cells. However, solution-deposited CZTSSe thin films usually consist of a large-grained top layer and a smaller-grained bottom layer. In this work, we adopt a phase-separation strategy to promote the crystal growth of the CZTSSe thin film. By choosing ZnCl₂, SnCl₂, CuCl (instead of CuCl₂), and thiourea as the starting materials, a Cu₂S/SnS/ZnS hybrid precursor thin film can be prepared, the composition of which has been testified by X-ray diffraction, X-ray photoelectron spectroscopy, Raman, and scanning electron microscopy-energy-dispersive spectrometry characterization. Owing to the volume expansion caused by Se incorporation and the high migration abilities of Cu(I) and Sn(II) ions during selenization, large-grained and compact CZTSSe films with a thickness up to 5 μm can be obtained. The corresponding thin film solar cell devices have achieved active power conversion efficiencies above 10% (8.78% for total area), much higher than those of CuCl₂-based CZTSSe devices in our lab.