On the other hand, the slow process with its characteristic time constant tslow is shorter for the sulfurized film. This slow process (>15 μs) is likely due to de-trapping of carriers from shallow defect states back into the bands followed by band to band recombination. With reduced and/or shallower defect/trap states, the trapping and de-trapping process is faster for the sulfur-treated samples, leading to a faster repopulation of the bands by thermalization from the traps and increasing the probability of band to band recombination. recombination. The ΦΣμ was compared for the different wavelengths using front vs back illumination. The shorter wavelengths (350-800 nm) exhibited significantly lower peak ΦΣμ signal when illuminated from the back, which is probably due to a high recombination velocity at the back interface (quartz/Sb2Se3) because of a high defect density. With decreasing energy of the excitation wavelength (950 and 1100 nm), the penetration depth increases and the photogenerated charge carriers have a similar and even distribution for both front and back illumination, which is evident by the similar peak ΦΣμ signal (Figure After sulfur treatment, these non-radiative recombination pathways are reduced, leading to a higher PL intensity. The PL spectra show a bandedge emission and a broad defect emission at lower energy. Investigation on the band-edge PL peak at 1.25 eV indicates that the transition is likely a free to bound state emission at 1.25 eV (as evidenced by no peak energy shift and the < 1 power law exponent of the PL intensity vs incident power plot (Figure )). The defect emission band extends over a broad energy range from 0.75 to 1.15 eV for Sb2Se3. The defect emission in the PL spectra at energies close to 0.8 eV were reduced due to sulfur treatment suggesting a removal of a deep defect/trap state (Figure ). The increased radiative recombination and decreased defect emission peak in sulfur-treated films coupled with a longer lifetime of the fast process and reduced bulk recombination observed from TRMC strongly suggest a passivation of deep defects in Sb2Se3. Recent computational studies have shown that SbSe antisite defects would occupy states at 0.4 eV above the valence band of Sb2Se3 and the suppresion of defect emission at energies close to 0.8 eV could imply the passivation of SbSe antisite defects upon sulfur treatment of Sb2Se3 films. Low-frequency Raman spectroscopy (LFRS) can be used to probe the acoustic vibrational modes (AVM) of materials. Long-range order induces AVM enhancement thus revealing otherwise hidden LFRS modes. AVMs of Sb2Se3 have been extensively studied using DFT-based theoretical models. Figure represents the LFRS spectrum of the Sb2Se3 samples before and after sulfur treatment. Experimental LFRS studies have optically resolved 15 of these theoretically predicted modes. The theoretical and experimental data agree within 5% deviation from theory. The AVM modes reported in the literature (theoretically as well as experimentally) are compared with the modes we observed, in Table , and a graphical summary is presented in Figure .