High photo detectivity and responsivity under time-dependent laser-irradiation of Cu40Sb40S20 thin films for photodetector application

The emerging ternary Cu–Sb–S materials, belonging to the I–V–VI semiconductor family, with p-type conductivity are considered alternative absorber materials due to their tunable band gap energy in the 0.5–2 eV range and large absorption coefficient. Herein, we report the laser irradiation-induced optical, structural, and surface wettability changes in Cu₄₀Sb₄₀S₂₀ thin films at different lasing times. As observed from XRD data, the reduction in crystallite size upon laser irradiation increased lattice strain and dislocation density. The elemental composition of the film was verified using EDS spectra and elemental mapping. The surface structure changed with laser treatment, as probed from FESEM images, making the film more porous. The hydrophobicity of a laser-irradiated film decreased with increased surface energy, as confirmed by contact angle data. Microstructural changes were observed in Raman spectra, and defect density decreased, as revealed by photoluminescence spectra. Such a reduction in structural defects resulted in an increase in the energy gap to 1.792 eV from 1.596 eV upon 60-minute laser irradiation. Transparency is also enhanced, thus decreasing the extinction coefficient and optical density with illumination. The decrease in refractive index thus reduced optical nonlinearity in terms of third-order nonlinear susceptibility and nonlinear refractive index. Increased photosensitivity increased detectivity of the 60 min laser-illuminated film to 4.81 × 10⁷ Jones from 1.7 × 10⁶ Jones of the unirradiated film, which is beneficial for photodetection in the UV region.