Effect of post-annealing on RF-sputtered CZTS absorber layer for solar cell application

Abstract

Thin-film solar cells (TFSCs) are important components for solar energy harvesting. CZTS (Cu₂ZnSnS₄) is composed of earth-abundant environmentally friendly elements with a commendable absorption coefficient and optical energy bandgap. However, poor crystallinity, homogeneity, and low efficiency are common issues associated with CZTS. We address these issues by growing CZTS layers using RF sputtering at a deposition temperature of 300 °C. Rapid Thermal Processing (RTP) was performed for post-deposition treatment at 400 and 500 °C. X-ray diffraction analysis revealed the formation of a tetragonal structure in CZTS. Signature peaks were revealed in the structural analysis. Secondary phases were detected in samples annealed at 500 °C. Upon increasing the RTP temperature, an enhancement in crystallinity was observed. Raman spectroscopy confirms the presence of the secondary phase. The Raman shift associated with the CZTS signature peaks was observed when grown at 300 °C and annealed at 400 and 500 °C. Scanning electron microscopy was used to analyze the surface morphology. The SEM images show that upon increasing the RTP temperature, the grain size increases, and the surface becomes more compact. Cu, Zn, Sn, and S are all present in the deposited thin layers. Variations in the elemental ratios and grading were observed as the deposition and processing temperatures changed.