In order to obtain further insight into the PEC operation of our photoanodes, the wavelength-resolved incident photon-to-current conversion efficiency (IPCE) was measured at +0.74 V vs. RHE under illumination from the backside (BS, i.e. from the substrate side) and frontside (FS, i.e. from the electrolyte side) (Fig. ). The photoaction spectra show an onset at ca. 510 nm, which is in line with the electronic absorption properties of the photoanodes (compare with Fig. ). The IPCE values are, independently of the illumination side, the highest for the Mo-BiVO4/CoPOM photoanode, followed by Mo-BiVO4 and pure BiVO4 over the whole wavelength range, which is in line with the photocurrent performance under polychromatic light at the same bias, as shown in Fig. † in ESI (see also Fig. ). From the mechanistic point of view, it is often worth looking at the difference in IPCE values under the FS and BS illumination for a given photoelectrode. The most notable difference is in the behavior of the Mo-free pristine BiVO4 photoanode as compared to Mo-BiVO4 and Mo-BiVO4/CoPOM photoanodes. While the latter two photoanodes show a very small difference in IPCE values recorded under the FS and BS illumination, the Mo-free pristine BiVO4 photoanode exhibits much larger (by the factor of ~10) IPCE values under the BS illumination than under the FS illumination (for a detailed view see ESI, Fig. ). This is exactly what one expects for a porous photoanode in which the photocurrent is mainly limited by the transport of electrons through the porous network, i.e. by the electronic conductivity of BiVO4. Under the FS illumination, most of the photogenerated electron-hole pairs are generated close to the film/electrolyte interface, and while the holes can readily react with water within the pores of the electrode, the electrons have to travel through the porous electrode to the underlying FTO back contact. Since under the BS illumination the electron-hole pairs are generated much closer to the FTO/film interface, the photogenerated electrons have a much shorter pathway to reach the FTO, resulting in significantly higher IPCE values under the BS illumination. Interestingly, the pristine BiVO4 photoanode even exhibits a wavelengthdependent switching (at ~460 nm) of the sign of the photocurrent from anodic to cathodic under the BS illumination and from cathodic to anodic under the FS illumination. This behavior can be again rationalized by the dominant effect of the poor electron transport properties of pristine BiVO4 on the photocurrent response, which, in combination with the different penetration depth of short (below 460 nm) and long (above 460 nm) wavelengths of light and with the generally higher intensity of the visible light as compared to UV light in our setup, can even results in the reversal of the photocurrent sign. In this context, we note that while the potential-dependent photocurrent switching has been reported for BiVO4 photoelectrodes, the fact that at a specific bias potential (in our case a rather moderate bias potential of +0.74 V vs. RHE) the photocurrent switching at BiVO4 can be controlled even by the light wavelength and by the illumination side has not been reported before, and would deserve further investigation given the importance of such phenomena for the development of photoelectrochemical optoelectronic switches. In contrast to the behavior of the pristine BiVO4 photoanode that is dominated by its low electronic conductivity, the very small difference in IPCE values under the FS and BS illumination observed for both Mo-doped samples Mo-BiVO4 and Mo-BiVO4/CoPOM (Fig. ) nicely illustrate that the beneficial effect of Mo-doping consists mainly in improving the electronic conductivity of Mo-BiVO4 by increasing the electron concentration, as discussed in detail above.