photon energies below about 1.3. eV, i.e. in the band gap of the WSe 2 absorber, the SPV signal shows a plateau, presumably caused by defects in the band gap. The maximum SPV value appears at a photon energy of about 1.6 eV, which correlates with the excitonic A transition in WSe 2 caused by a direct transition of electrons from the split valence band to the conduction band. This is also in accordance with the optical absorption spectrum, published recently by us. In the sub-bandgap region (E ph < 1. The optical properties of a single, thick (690 nm) ATM layer were determined by transmittance and reflectance measurements (Supplementary Fig. ). From the Tauc plot of the absorption coefficient, a bandgap of 1.9 eV was derived (Supplementary Fig. ). This means, ammonium thiomolybdate itself is a semiconductor and can absorb solar light and contribute to the water splitting process. However, due to the very thin (10 nm or below according to XPS probe depth)