Investigating The Influence of Multivalent Transition Metal Dopants on Copper Antimony Sulphide Thin Films for Enhanced Photovoltaic and Electrochemical Properties

Undoped non-stoichiometric semiconducting copper antimony sulphide (CAS) thin films were deposited at 500, 550 and 600 °C onto glass substrates by aerosol-assisted chemical vapour deposition (AACVD) at different flow rates and copper concentrations using metal diethyldithiocarbamate precursors. For thin films deposited at an optimal flow rate, data from powder X-ray diffraction (p-XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) suggest a correlation of composition of the non-stoichiometric sulphur-deficient tetrahedrite phase (cubic structure) microcrystalline CAS (Cu₁₂Sb₄S₁₃) thin films with particle sizes ranging from 0.1 to 4 µm. For undoped thin films, visible optical absorption and cyclic voltammograms (CV) show a bandgap of ~2.1 eV is likely associated with compositional variations involving intrinsic lattice defects, including shallow electronic states such as copper interstitials (Cu_i^●/●●) and vacancies of sulphur (V_s^●●) as deep-lying donors, and copper-antimony anti-sites (Cu_Sb^'/'') and antimony vacancies (V_Sb^''') as deep-lying acceptors. Additionally, upper first row transition metal divalent (M²⁺) ions such as Mn²⁺, Co²⁺, Ni²⁺ and Zn²⁺ were incorporated into tetrahedrite phase CAS (<i>t</i>CAS) thin films at optimal temperature, 550 °C and flow rate, 150 sccm. M²⁺ ions incorporated (M<i>_x</i>-<i>t</i>CAS) exhibit tunable composition-driven electronic structure for small M²⁺ content influencing narrower bandgaps between 1.7 and 1.9 eV. The Raman data suggest that the phase purity is affected by small fractions of the famatinite (tetragonal) phase. Additionally, <i>t</i>CAS and M<i>_x</i>-<i>t</i>CAS thin films were deposited on conducting substrates (ITO/FTO) and were found to be efficient as a novel catalyst for the hydrogen and oxygen evolution reactions in water splitting. Overall, the thin films display broad emission of fast dual radiative recombination, and an additional recombination pathway exhibited for M<i>_x</i>-<i>t</i>CAS associated with divalent ion-related point defects. These results show the utility of AACVD in tuning compositional and optical and electrochemical properties of undoped <i>t</i>CAS and M<i>_x</i>-<i>t</i>CAS thin films for possible applications in photovoltaics and electrochemical conversion.