Charge transfer across the SEI. In order to investigate the lateral homogeneity of the charge transfer sites on the ATM surface, Pt islands were photochemically deposited onto the ATM-WSe 2 film for 540 sec at open circuit potential in 0.1 M HCl solution under AM 1.5 illumination (see Fig. and section methods). The SEM image and the EDX mapping pictures display the homogeneous distribution of Pt nanoislands on the whole ATM surface, which proves the homogeneous charge transfer over the WSe 2 vdW planes to the ATM surface, where the electrons can react with the Pt ions from the solution (Supplementary Fig. ). In the Pt photodeposition process, electrons, excited in the WSe 2 film, are transported to the ATM surface passing the ATM/WSe 2 heterojunction, and reducing the Pt ions on the ATM surface, leading to the precipitation of Pt nanoislands with diameters of 20 to 50 nm. The homogeneous distribution of Pt nanoislands shows clearly that the charge transfer from the absorber film takes place over the whole area of the van der Waals planes of the WSe 2 crystallites. This is strongly different from the photodeposition of platinum on bare WSe 2 films, which leads to the decoration of grain boundaries/edge steps and local defects, see Fig. . Solar hydrogen evolution. The hydrogen evolution at the ATM/WSe 2 photoelectrode was quantitatively determined with a two-pipette set-up, allowing both the measurement of the evolved H 2 at the cathode as well as the O 2 generated at the anode. The Faradaic efficiency of 82 % a b