A better understanding of the reaction pathways in the formation of Cu2ZnSn(Sx,Se1−x)4 thin films

•The phase formation of CZT(S,Se) absorber is studied at the early stage of annealing.•A strong chemical affinity between Sn and Se is observed between 350 °C and 450 °C.•This affinity triggers the Se diffusion into layers at low temperatures.•CZTSe phase tends to form at the surface while CZTS phase segregates at the bottom.•CZT(S,Se) solid solution results from a progressive intermixing between CZTS and CZTSe.

This work carries out a study on the reaction pathways during the selenization of Cu-Zn-Sn-S sulfide precursors deposited on Mo/glass. The effect of the annealing temperature on the structural and compositional properties of the materials was studied between 250 and 600 °C combining glow discharge optical emission spectroscopy (GD-OES), x-ray diffraction, and Raman spectroscopy measurements. Through a deep chemical-physical characterization of the front, bulk and back regions of the CZTSSe layers, we show that both diffusion phenomena and the chemical affinity between some elements govern the reaction pathway. The experimental observations support a formation mechanism of CZTSSe that proceeds through two different paths. At the surface, the progressive incorporation of Se triggers the diffusion of Sn to the surface, which results to a fast formation of tin selenides compared to copper selenides. In addition, the predominance of Se-rich CZTSSe is observed at the top part region.

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