for 12 hours to deposit the hydrogen evolution reaction (HER) catalyst. Finally, the films are annealed at 120°C to improve catalyst adhesion without altering molecular integrity. This was confirmed by Raman measurements which showed identical peaks before and after annealing. The [Mo3S4] 4+ illustrated in Figure is synthesized from the reaction of ammonium tetrathiomolybdate with sodium borohydride and HCl in air. Crystals suitable for single-crystal X-ray diffraction (XRD) were obtained after anion exchange with p-toluenesulfonic acid. The [Mo3S13] 2-shown in Figure was synthesized according to procedures developed by Streb and co-workers, starting from ammonium heptamolybdate. A reaction with elemental sulphur and ammonium sulphide over four days yields the [Mo3S13] 2-as dark red crystals suitable for X-ray crystallography. Mimicking the MoS2 catalytically active edge sites, molybdenum sulfide clusters have a maximum dimension of approximately 0.7 nm with a high ratio of active sites to non-active ones (e.g., the basal plane in MoS2); therefore, more active species and thus catalytically active sites can be packed on the photoabsorber surface, increasing reactivity with a thinly deposited catalyst layer ranging between 5-30 nm, as observed from the profilometer measurements reported in Table . This difference is attributed to the different overpotentials of the two catalysts (see also Figure ). This leads to a shift of the JV curve of the [Mo3S4] 4+ to more negative potentials, while the [Mo3S13] 2-catalyst device exhibits a photocurrent of approximately 2.5 mA cm -2 higher at -0.2 V vs RHE compared to the [Mo3S4] 4+ catalyst device indicated by their J-V curves. Figure illustrates that the [Mo3S13] 2-catalyst has a higher incident photon to current conversion efficiency (IPCE) between 400-850 nm at a similar applied potential, indicating less surface recombination and more efficient charge collection compared to [Mo3S4] 4+ , which suffers from slower charge transfer due to higher overpotential. Both catalysts have similar IPCE in the 850-1100 nm range, as differences in extinction coefficients of Sb2Se3 affect photon absorption and charge carrier generation. In the 400-850 nm range, high charge density near the semiconductor-electrolyte interface enhances [Mo3S13] 2-performance. In contrast, the 850-1100 nm range sees charge generation in the semiconductor bulk, resulting in similar IPCE for both. The higher overpotential for [Mo3S4] 4+ shown in Figure underscores the [Mo3S13] 2-catalyst's efficiency under conditions of high charge density at the electrolyte interface. The integrated currents in Figure are slightly higher than the values observed in the CV measurements above. The slight discrepancy in values can be attributed to the light intensity dependence of the Sb2Se3 devices, as shown in previous studies. Furthermore, the activity of both catalysts was investigated across various pH levels. As shown in Figure , both catalysts exhibit a similar trend, performing best at pH 0, although they remain effective at more basic pH levels as well. This study highlights the flexibility and adaptability of these catalysts in diverse pH environments; however, stability remains limited on FTO.