Effect of High-temperature (>320 °C) Thermal Annealing on Electrochemically Deposited Cu₂ZnSnS₄ (CZTS) Thin Films for Photovoltaic and Optoelectronic Applications

Abstract

This study investigated the effect of high-temperature thermal annealing on the structural, optical, morphological, and electrical properties of electrochemically deposited Cu₂ZnSnS₄ (CZTS) thin films for photovoltaic and optoelectronic applications. CZTS thin films were deposited on fluorine-doped tin oxide (FTO) substrates using an electrochemical deposition technique and subsequently annealed at 340, 360, 380, and 400 °C. The deposited films were characterised using UV–visible spectrophotometry, Hall-effect measurements, scanning electron microscopy (SEM), and X-ray diffraction (XRD) to determine the influence of annealing temperature on their performance. Optical characterisation revealed direct band gap energies of 2.19, 2.29, 2.33, and 2.27 eV for films annealed at 340, 360, 380, and 400 °C, respectively, indicating that annealing temperature significantly influenced the electronic structure of the films. UV–visible analysis further showed that optical absorbance increased progressively with annealing temperature, reaching its highest value at 380 °C, while transmittance decreased correspondingly, demonstrating enhanced photon absorption suitable for solar energy conversion. SEM micrographs revealed gradual grain enlargement, improved grain connectivity, reduced void density, and enhanced surface compactness with increasing annealing temperature, with the 380 °C sample exhibiting the most homogeneous morphology. XRD analysis confirmed progressive improvement in crystallinity through sharper and more intense diffraction peaks as annealing temperature increased, indicating enhanced crystal growth and structural ordering. Hall-effect measurements showed improved electrical transport properties after annealing, with the films annealed at 360–400 °C exhibiting superior charge-carrying characteristics compared with the 340 °C sample due to reduced defect density and improved grain connectivity. Thermal annealing considerably enhanced the structural, optical, morphological, and electrical quality of electrochemically deposited CZTS thin films, with 380 °C providing the optimum combination of crystallinity, optical absorption, and surface morphology, making the films promising absorber materials for high-efficiency thin-film solar cells, photodetectors, optical sensors, and other optoelectronic devices.