The perovskite PV devices were next interfaced to Pt or Cu92In8 catalysts for proton or CO2 reduction using graphite epoxy (GE) paste, which acted as a conductive encapsulant (see Methods). The resulting photocathodes (abbreviated PVK|GE|Pt or PVK|GE|Cu92In8) displayed early onset potentials of 1.0 and 0.8 V vs. RHE, respectively, resulting in a good overlap with CV curves of BiVO4 photoanodes (Fig. , Supplementary Fig. ). Photocurrents under 1 sun irradiation remained similar when light was filtered through BiVO4, which confirms the limiting effect of sheet resistance on perovskite photocathodes. Further controls indicated that only negligible photocurrents below -5 µA cm -2 are observed in the absence of a catalyst, i.e., GE has no intrinsic catalytic activity (Supplementary Fig. ). The perovskite photocathodes were attached to BiVO4 photoanodes in a back-to-back, tandem configuration (Fig. , Supplementary Fig. ). The PEC performance of these 10 cm 2 artificial leaves was tested in a 2-electrode, onecompartment setup, using a custom-built Perspex (acrylic glass) reactor with an inner volume of 884 cm 3 (Fig. , Supplementary Fig. ).