The wavelength-resolved incident photon-to-current conversion efficiency (IPCE) was measured at E = 0.80 V vs. RHE both under irradiation from the back-side (BS, substrate side) and the front-side (FS, electrolyte side) (Figure ). The photoconversion efficiencies show a photoconversion onset at ca. 530 nm, which is in line with the optical absorption edge of all samples (Figure , Supporting Information). Under the BS illumination, which is typically beneficial for mesoporous photoanodes such as ours since the performance-limiting process is the transport of electrons through the porous network into the underlying FTO substrate, the IPCE values of the both doped and surface-modified Composite 2 photoanode are the highest in the whole wavelength range, as shown in Figure . Notably, at the doped Composite 1 photoanode the IPCE values under the FS illumination are nearly the same as under the BS illumination (Figure ). This implies that the CoPOMdoping has a notable enhancing effect on the electron transport properties of BiVO4, and the electron transport no longer limits the photocurrents, even if the electrons have to be transferred across longer pathways to the FTO support under the FS illumination. On the other hand, in the case of the Composite 2, the expectedly higher IPCE values under the BS illumination as compared to the FS illumination are observed, which is most probably due to a more efficient hole extraction by the co-catalyst, rendering the electron transport through the electrode limiting again. To gain further insights into the factors limiting the performance of our photoanodes, the charge separation efficiency (𝜂 𝑠𝑒𝑝 ) and the hole transfer efficiency (𝜂 𝑡𝑟 ) were calculated by employing the methodology developed by Dotan et al. and Hamann et al. (see Experimental Section). According to the electronic absorption properties shown in Figure , Supporting Information, the 𝐽 𝑚𝑎𝑥 was calculated as 8.12 mA cm -2 (BiVO4), 7.96 mA cm -2 (Composite 1) and 8.09 mA cm -2 (Composite 2). In the presence of Na2SO3 as an effective hole scavenger, the photocurrent densities reach 3.22 mA cm -2 (BiVO4), 6.32 mA cm -2 (Composite 1) and 6.55 mA cm -2 (Composite 2) at 1.23 V vs. RHE in Figure , Supporting Information. As a result (Figure ), the 𝜂 𝑠𝑒𝑝 for Composite 1 and Composite 2 photoanodes show much higher values than for pristine BiVO4 in the potential ranging from +0.35 to 1.23 V vs. RHE. The 𝜂 𝑠𝑒𝑝 of Composite 1 photoanode reaches 76.5% at 1.23 V vs. RHE, which is two times higher compared to the 38.3% of the non-modified photoanode. This indicates that CoPOM-doping effectively improves the conductivity and facilitates the charge transport through the photoanode. Interestingly, at very low bias potentials (< 0.35 V vs. RHE) the detrimental effects of doping are apparent as the 𝜂 𝑠𝑒𝑝 decreases as compared to pristine BiVO4. In our previous study on Mo-doped BiVO4, we discussed in detail these detrimental effects of doping as related to the higher concentration of electron polaronic states due to the presence of the dopant. The electrons trapped in these polaronic states limit the maximum achievable photovoltage and enhance the charge recombination unless a sufficiently high bias is applied. Indeed, the negative effect of the CoPOM-doping is clearly corroborated by the reduced quasi-Fermi level of electrons after the photodoping, as apparent from reduced open-circuit photopotentials at Composite 1 and Composite 2 as compared to pristine BiVO4 (see Figure , Supporting Information). Furthermore, the hole transfer efficiency 𝜂 𝑡𝑟 (Figure ) is significantly enhanced over the whole potential range and shifted to cathodic potentials for both modified photoanodes. Specifically, the 𝜂 𝑡𝑟 increased from 12.7% for BiVO4 to 47.8% for Composite 1 and 59.2% for Composite 2 photoanodes at 1.23 V vs. RHE. Notably, the beneficial effects of the CoPOM co-catalyst become particularly significant at very low bias potentials (< 0.45 V vs. RHE), where it can partially compensate the detrimental effects of the electron polaronic states from doping.