Photoelectrochemical characterisation. The performance of the encapsulated organic photocathodes was evaluated using a Newport Oriel 67005 solar light simulator with an AM 1.5G optical filter (1 sun, 100 mW cm -2 ). Cyclic voltammetry (CV) and chronoamperometry (CA) scans were performed using Ivium CompactStat.e potentiostats for a two-compartment PEC H-cell separated by a Selemion ion exchange membrane (AGC Engineering). PEC experiments were conducted in a pH 8.5 buffer solution for HER experiments (0.1 M H3BO3 and 0.1 M K2SO4, with pH adjustment made using 10 M KOH) and a pH 7.4 buffer solution (CO2 saturated 0.5 M KHCO3) for CO2R experiments, at room temperature. The buffer solutions were purged with N2 and CO2 respectively (both with 2% CH4 as an internal standard) for at least 30 minutes before each experiment and no stirring was performed during the experiments, unless stated otherwise. The three-electrode setup comprised of an organic photocathode as working electrode, a Ag/AgCl reference electrode (in saturated NaCl, BASi MW-2030) and a platinum mesh counter electrode, with the entire cell kept gas tight by sealing the septa with Loctite superglue. CV scans were recorded between -0.1 V vs. RHE and 1.1 V vs. RHE at a scan rate of 10 mV s -1 and CA experiments were conducted at 0 V vs. RHE for the stated duration under chopped irradiation (50 min on, 10 min off). Potentials measured against Ag/AgCl were converted to the RHE scale using the equation: E (V vs. RHE) = E (V vs. Ag/AgCl) + 0.059 × pH + 0.197 V (at 298 K). Multiple consecutive CV scans were performed when needed to account for any transient charging effects. Tandem devices were investigated under similar conditions, in a one-compartment PEC cell and a two-electrode set-up. In this back-to-back configuration, the irradiation window was defined by black masking tape on the front BiVO4|TiCo photoanode, preventing illumination of the organic photocathode by unfiltered light. No masking tape was needed for characterising the individual photocathodes. A Shimadzu GC-2010 Plus chromatograph was employed to quantify the amount of H2 and CO produced with time in the headspace of the PEC cell. O2 evolution was monitored in a N2filled glovebox using a NeoFox-GT fluorometer and Fospor-R fluorescence oxygen sensor probe (Ocean Optics). Henry's law was used to determine the amount of dissolved H2, CO and O2 in the aqueous buffer solution. Product amounts obtained over the 96 h CA test of OPV|GE|CoMTPP@CNT -BiVO4 tandems were corrected based on the leakage of the 2% CH4 internal standard, following a reported analytic model. The faradaic yield (FY) was calculated using the equation: FY (%) = nZF/Q × 100%, where n is the number of moles of gaseous product produced, Z is the number of electrons needed per molecule of product (Z = 2 for both H2 and CO generation, Z = 4 for oxygen production), F is the Faraday constant (96485 C mol -1 ), and Q is the total charge passed. The total charge Q was determined by integrating the current trace over a defined period. EQE measurements were conducted using a LOT MSH-300 monochromator, a Thorlabs PM100D power meter equipped with a Thorlabs S302C thermal power sensor and an Ivium CompactStat potentiostat. The wavelength (full-width at halfmaximum of 15 nm) was increased in 25 nm steps from 300 nm to 800 nm every 30 s. EQE was calculated using the equation: EQE (%) = hcJ/(eλPλ) × 100%, where h is the Planck constant, c is the speed of light, J is the photocurrent density, e is the elementary charge, λ is the wavelength and Pλ is the wavelengthdependant light intensity flux. The solar-to-fuel efficiencies (i.e. for H2, CO and O2) were calculated using the equation: STF (%) = (J × ΔU × FY)/P × 100%, where J is the absolute value of the operating photocurrent density, FY is the faradaic yield for the solar fuel produced and P is the incident solar power. Under zero applied bias, ΔU = 1.23 and 1.34 V for H2O splitting and CO production coupled with O2 evolution, respectively. Materials characterisation. Field emission scanning electron microscope imaging and energy-dispersive X-ray (EDX) spectroscopy mapping were conducted on a TESCAN MIRA3 FEG-SEM instrument equipped with an Oxford Instruments X-maxN 80 EDS system. Absorbance spectra were recorded on an Agilent Cary 60 UV-Visible spectrophotometer equipped with a diffuse reflectance probe. CoMTPP CO2R catalyst loadings on the CNT sheets were determined by Inductively Coupled Plasma Optical Emission spectroscopy on a Thermo Scientific iCAP 7400 ICP-OES DUO spectrometer. The CoMTPP@CNT sheets were digested in 68% HNO3 and the solution diluted with MilliQ water before analysis.