The incident photon to current efficiency (IPCE) for an ATM-WSe 2 photocathode was measured in the photon energy range from 1.2 eV to 3.7 eV, where its behavior is similar to the absorption spectrum of the ATM-WSe 2 photocathode (Fig. ). The peak at 1.6 eV corresponds to the excitonic absorption (A exciton) of WSe 2 , characteristic for TMDCs, which was also observed in the SPV measurement (Fig. ). The APCE reaches a maximum of 34 % at a photon energy of 2.7 eV. The decrease of the APCE spectrum for E ph > 2.7 eV points to recombination losses at the photocathode/electrolyte interface, since the theoretical APCE value reaches 80 % (see the absorption spectrum). The absorption of the light at energies lower than 1.5 eV is due to trap states in the bandgap, which were also observed in the SPV spectra of ATM-WSe 2 film (Fig. ).