To construct a bias-free tandem device, OPV|IO-TiO2|enzyme photocathodes were coupled to BiVO4 photoanodes in a back-to-back configuration to achieve overall unassisted H2O splitting and CO2 conversion (Fig. ) . The complementary light absorption of BiVO4 (< 500 nm) and PCE10:EH-IDTBR (< 800 nm) supported good utilisation of the solar spectrum . Amorphous TiCoOx (TiCo) was spin-coated onto BiVO4, serving as a water oxidation catalyst . To further improve electrocatalytic current densities, H2ase and FDH loadings were increased to 500 pmol, while keeping the CA loading constant at 100 pmol (electrochemical optimisation in Figs. S25,26 and Table ).