Synthesis of Cu₂CoSnS₄ chalcogenide thin films by spray pyrolysis and efficient Au/p-CCTS/n-SnO₂ diode for cells solar applications

Quaternary chalcogenide Cu₂CoSnS₄ (CCTS) thin films were successfully prepared by spray pyrolysis on glass substrates in varying the precursor concentration and the thiourea volume. The effects of the precursor concentration and the thiourea volume on the formation of the CCTS phase were systematically investigated by X-ray diffraction, Raman spectroscopy, Energy-Dispersive X-ray Spectroscopy. The electrical resistivity at room temperature has been measured by the four probes method. It is shown that the crystallization of the stannite CCTS phase depends on both the precursor concentration and thiourea volume. The minimum volume of thiourea required to crystallize the CCTS phase depends on the precursor concentration, i.e. the CCTS crystallization needs higher volume of thiourea at low precursor concentration. For higher precursor concentration, the CCTS is poorly crystallized with the presence of secondary phases such as CuS₂, CoS₂, and CuCo₂S₄, while the crystallization domain is relatively large. The CCTS well crystallizes only in a narrow range of precursor concentration and thiourea volume. The relatively poor crystallization combined with the presence of secondary phases are harmful for the optical and electrical features of the CCTS film. Besides, optical transmission measurements of the optimized film showed an optical gap of 1.7 eV (~ 730 nm) and an absorption coefficient included between 10⁴ and 10⁵ cm⁻¹ in the visible solar range. Furthermore, the optimized stannite CCTS films are compact exhibiting a relatively rough and granular surface due to the formation of copper sulfide nodules. The electrical resistivity of the optimized CCTS film is about 0.4 10^–2 Ω cm. These results suggest that the deposited films fit the requirements of absorbing layer in solar cells. The diode composed of Au/Cu₂CoSnS₄/SnO₂ films was investigated with current–voltage (I–V) measurements performed under dark and bright conditions. The series resistance, saturation current and ideality factor of typical diode is estimated to be 689 Ω, 1.15 10^–5 A and 3, respectively.