Study of structural properties in CZTSSe absorber thin films fabricated by pulsed laser deposition

Abstract

Kesterite (CZTSSe) based thin-film solar cells have attracted intensive attention due to their earth-abundant and cost-effective nature AND non-toxic composition. They have a high absorption coefficient in the visible region and a direct band gap. However, the CZTSSe compound is a complex quaternary material, and obtaining a single phase with no secondary phases is known to be complicated, which can have a significant impact on the performance of the device. In this work, the purpose is to investigate the effect of thermal annealing treatment temperature and the variation in the sulfur-to-selenium ratio, on the morphological, structural, and optical properties of Cu2ZnSn(S, Se)4 (CZTSSe) thin films. In this context, firstly, CZTSSe thin films were grown using the Pulsed Laser Deposition method. Post-deposition annealing treatments were applied at 425°C and 450°C to enhance the homogeneity and crystallinity of the deposited thin films. Subsequently, structural and optical characterisations of the annealed films were carried out, including GIXRD, Raman spectroscopy, UV-Vis-NIR spectroscopy and EDS. The results show that films annealed at 425 °C generally exhibit larger cristallite sizes and narrower XRD peaks, indicating improved crystallinity compared to those annealed at 450 °C. Intermediate compositions with sulfur content between 50% and 75% demonstrate optimal structural properties. Raman analysis confirms the formation of a kesterite phase with the presence of secondary phases, particularly in samples with balanced S/Se ratios. Optical measurements reveal strong absorption in the visible range, with absorption coefficients exceeding 104 cm-1. Band gap values increase progressively with sulfur content, ranging from approximately 0.9 eV (CZTSe) to 1.5 eV (CZTS), aligning with theoretical expectations. Compositions with x = 0.75–0.95 show favourable band gaps and high absorption, highlighting them as promising candidates for photovoltaic absorber layers.