***************************************************************** Experimental ***************************************************************** Photoanodes devices fabrication - FTO glass TEC 7 (Sigma-Aldrich) was etched with zinc powder and HCl (2 M). It was then cleaned in 2vol% Hellmanex solution in water, followed by de-ionised (DI) water, acetone (VWR), 2-propanol (IPA) (VWR) and ethanol (VWR) before being treated with UV-Ozone cleaning (in ProCleaner PLUS for 20 min). A compact TiO2 layer was deposited by spray pyrolysis, using a hand held atomiser to spray 20mM solution of titanium diisopropoxide bis(acetylacetonate) (75wt%, Sigma-aldrich) in EtOH onto the substrates held at 500 °C. Substrates were then sintered at this temperature for 10 minutes. PbBr2 solution (1 M in DMF) was spin coated on top of the substrates held at 70 °C at 2500 rpm for 30 s and each cell was further sintered at 70 °C for 30 min. The cell was then immersed in the CsBr solution (17 mg ml/mg in methanol) kept at 50 °C in a vertical staining jar for 30 min before being annealed at 150 °C for 30 min. A carbon paste (black carbon+graphite, Gwent Electronic Materials) was doctor bladed as top contact on top of the CsPbBr3 film. Devices were further post-annealed at 400 °C for 30 min in air for the mesoporous carbon (m-c) layer formation. A graphite thermal sheet (RS, Panasonic,1600Wm-1K-1, 180 x 115 mm, 0.025 mm, self-adhesive on one side) was stuck by hand onto the mesoporous carbon top layer. The device was finally sealed (except an active area of 0.25 cm-2) with commercial silicon and epoxy resin left to harden out at room temperature overnight. C-PSC fabrication - The iridium precursor used to synthesise the water oxidation catalyst (WOC) was [Cp*Ir(pyalc)Cl]. 48mg (0.1 mmol) were added to 20 ml of DI water in an open beaker and the clear orange solution was stirred vigorously for 5 min. Under constant stirring, 540 mg (2.5 mmol) of NaIO4 were added to the beaker and the dark blue solution was allowed to stir overnight at room temperature. A piece of GS was floated on the unstirred, blue Ir-WOC solution overnight to bind the catalyst. It was then rinsed with DI water and left to dry at room temperature. ***************************************************************** Characterisation ***************************************************************** JV curves - Solar cell JV curves were measured using Keithley 2601A potentiostat, under simulated sunlight AM 1.5 G. (100mWcm-2) with a solar simulator Class AAA with a HMI Lamp (200W/70V). A WPVS reference cell (Fraunhofer ISE) in accordance with international standard requirements of IEC 60904-2 was used to calibrate the light. The cell was held at 1,5 V under illumination for 5 s before scanning in reverse. The PV performance was not confirmed from independent certification laboratories. The voltage was swept from 1.5 to 0 V and back to 1.5V at 100 mV/s. The folder 'JVcurves' contains the data from the JV scans. The data are in .csv files. Column 1: Voltage [V], Column 2: Current density [mAcm-2]. There are two subfolders: - 'statistic' contains files of JV scans of cells with structure FTO|TiO2|CsPbBr3|m-c used to plot the Box Plots available in the SI of the publication for statistical analysis of reproducibility. The files use the naming convention [SampleName].csv - 'waterStability' contains files of JV scans of cells with structure FTO|TiO2|CsPbBr3|m-c|GS. The files use the naming convention [DeviceArchitecture]_[MeasurementDetails].csv XRD - Powder x-ray diffraction (XRD) patterns were collected using a Bruker Advance D8 X-ray diffractometer with a Cu K source. Measurements were taken from 2? values in the range 5-80°. The folder 'XRD' contains XRD patters of CsPbBr3 films before and after immersion in water for 1 second. The data are in .xlsx files. Column 1: 2? [deg], Column 2: intensity [counts]. UV-Vis - Thin film UV-vis optical transmission and reectance measurements were performed on a Perkin-Elmer Lambda 750S UV/Vis spectrometer, from 900 to 300 nm. Absorption was calculated as ?=log((1-R)/T). The folder 'UV-Vis' contains the reflection (R) and transmission (T) spectrum of CsPbBr3 collected before and after wtaer immersion for 1 second. The files use the naming convention [MeasurementDetails]_[method].xlsx. Column 1: Wavelength [nm], Column 2: Spectrum [%]. SEM - The cross-sectional microscopy (SEM) morphology was studied using a JEOL SEM6480LV scanning electron microscope (SEM) (20 kV acceleration voltage and a magnification of 5000). SEM images of FTO|TiO2|CsPbBr3|m-c are grouped in the folder 'SEM'. Images are in .tif files, and specifications of images are in the text file. Word file contains the EDX chemical characterisation of the cross section. 'FESEM' folder contains the image and specifications of the high resolution cross section image measured with a Field Emission Scanning Electron Microscope (FESEM). IPCE - The incident photon to current efficiency (IPCE) measurements were performed from 300 to 900nm with a 300W Xe source (Lot-OD) from the back side. The light source was passing through a monochromator (MSH-300F LOT QuantumDesign) without the AM 1.5 G filter. The intensity of the monochromatic light was measured by a SEL033/U photodetector (International Light Technologies). IPCE data are grouped in the folder 'IPCE' which contains 2 subfolders: -'RawData' contains raw data measurements of the photocurrent generated overtime by changing the wavelength of the lighting source (measurements set to have an increase of 10 nm every 5 seconds from when the light is on). The data are in .csv files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl]. -'ExtraFiles' contains the calibration file of the light source and the calculated IPCE considering the lamp intensity. The excel file shows data and graphs of the integrated photocurrent density of FTO|TiO2|CsPbBr3|m-c (sheet 2 - sample IP543) and FTO|TiO2|CsPbBr3|m-c|GS (sheet 1 - sample IP683) and the calculated IPCE (sheet 3 - plotIPCE). -'Revision' folder contains IPCE curve shown in the main manuscript (Fig 3c. 'IPCE_TiO2-CsPbBr3-mc-GS.xlsx' contains the calculated IPCE considering the lamp intensity. The excel file shows data and graphs of the integrated photocurrent density and the calculated IPCE. Each column of the excel file is labeled. 'Raw_data.csv' contains raw data measurement of the photocurrent generated overtime by changing the wavelength of the lighting source (measurements set to have an increase of 10 nm every 5 seconds from when the light is on). The data are in .csv files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl]. PEC - The photoelectrochemical (PEC) performances of the devices were investigated in aqueous solutions 0.1M KNO3, with pH adjusted by using H2SO4 or KOH. The measurements were carried out in a three electrode system with Pt as a counter electrode and Ag/AgCl as a reference electrode. The measured potentials versus Ag/AgCl were then converted to the RHE scale using the Nernst equation ERHE = EAg=EAgCl + 0.059pH + 0.197. The PEC performance, such as Linear sweep voltammetry (LSV) and chronoamperometry, was measured with a Compactstat IVIUM potentiostat under simulated solar illumination (AM 1.5 G filtered 100mWcm-2) with 300W Xe source (Lot-OD) from the back side. The intensity of the light at the working electrode position was measured by certified and calibrated SEL033/U photodetector (International Light Technologies). LSV curves were measured at a scanning rate of 20mV/s and chronoamperometry experiments were conducted at 1.23VRHE. The folder 'PEC' five subfolders: - 'Eoc' contains data measurements of the open circuit potential (OCP). The files use the naming convention [SampleArchitecture]_[Eoc]_[pH electrolyte].csv. The data are in .csv files with Column 1: Time [sec], Column 2: Current [A], Column 3: OCP [V versus Ag/AgCl]. - 'Von' contains an excel file with LSV curves for photoanodes with (WOC) and without catalyst (GS) measured at different pHs (4, 7 and 12.9). Column 1: Applied Voltage [V versus RHE], Column 2: Normalised Current [normalised to the maximum value]. O2 evolution - The O2 evolution was probed using a compact fiber-optic oxygen meter (FireStingO2) using a robust oxygen probe (XB7-546-206) in the three electrode system previously purged with N2 in a gas-tight quartz cell. TDIP temperature sensor was used to give automatic compensation for minor uctuation in the PEC cell temperature. The faradaic efficiency was calculated by dividing the experimental O2 produced and the theoretical O2 calculated from chronoamperometric traces, assuming that O2 formation liberates 4 electrons. The amount of oxygen dissolved in the liquid was calculated according to Henry's law (KH=769:23 atmM-1) and added to the amount detected in the headspace. Data files are grouped in the folder 'O2_production'. PEC data collected during the O2 evolution detection are in the .csv format file, with Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl]. .txt file is the number of O2 moles detected by the sensor. CV - cyclic voltammetry (CV) scans of GS and GS|WOC were performed using an AUTOLAB potentiostat in H2SO4 solution (pH 3.5) with 0.1M KNO3 at a scan rate of 50mV/s (software GPES). Ag/AgCl and Pt were used as reference and counter electrodes, respectively. The folder 'CV scans' contains the cyclic voltammogram scans obtained for GS electrodes, functionalised GS (GS|WOC) and TiO2|CsPbBr3|mc|GS|WOC photoanodes. - 'CV_GS_and)_GS-WOC.xlsx' contains CV scans of GS electrodes and GS|WOC electrodes measured at different pH (each pH indicated in the sheet name). - 'CV_TiO2-CsPbBr3-mc-GS-WOC.xlsx' contains CV scans of TiO2|CsPbBr3|mc|GS|WOC photoanodes at different pH. EDX - EDX characterisation was done on GS|WOC electrodes to chemically detect the presence of the catalyst on the surface. The folder 'EDX' contains the images of the elemental composition spectra in .JPEG (the word file contains the image with higher resolution). The Chrome HTML document contains the top-view SEM image highlighting the interest area of the surface and the elemental composition of each interest area. The files use the naming convention [Magnification]_[SiteOfInterest]_[spectrumNumber]. Tensile strength - Ultimate stress and strain was measured using a 50kN Instron Instrument and samples prepared according to the standard for tensile testing of polymer thin films. Sample was 75 mm long and approximately 6 mm wide along the working region of the tensile test sample. Stress calculated according to Stress=Force/area. Data are collected in the folder 'stress' in a .xlsx format file. The folder 'Stability' contains chronoamperometric traces recorded at 1.23 V versus RHE at different pH for different devices. The files use the naming convention Stability_[pH electrolyte]_[SampleArchitecture].csv. - 'Stability_pH2.5_TiO2-CsPbBr3-mc-GS-WOC.csv' contains the raw data of TiO2|CsPbBr3|m-c|GS|WOC at pH 2.5. The data are in .csv files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl], Column 4: Current density [mAcm-2]. - 'Stability_pH7_TiO2-CsPbBr3-mc-GS70.csv' contains the raw data of TiO2|CsPbBr3|m-c|GS70 at pH 7 (showed in the manuscript). The data are in .csv files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl]. - 'Stability_TiO2-CsPbBr3-mc-GS.csv' contains the raw data of TiO2|CsPbBr3|m-c|GS pH 2.5 (Sheet 1), pH 7 (Sheet 2) and at pH 12.5 (Sheet 3). The data are in .csv files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl], Column 4: Current density [mAcm-2]. The folder 'two-electrode measurement' contains chronoamperometric traces recorded in a two electrode measurement with working electrode and platinum as counetr electrode and no bias applied between the 2 electrodes. - '2-electrodeMeasurement-pH2.5.xlsx' contains the raw data of TiO2|CsPbBr3|m-c|GS and TiO2|CsPbBr3|m-c|GS|WOC at pH 2.5. The data are in .xlsx files. Column 1: Time [sec], Column 2: Current [A], Column 3: Applied Voltage [V versus Ag/AgCl], Column 4: Current density [mAcm-2]. - '2-electrodeMeasurement-long-O2production.xlsx' contains the raw data of TiO2|CsPbBr3|m-c|GS|WOC at pH 2.5 (showed in the ESI) and the calculated theoretical O2 produced with the generated current (and their plots), assuming that oxidation of water to O2 requires abstraction of 4 electrons. 'Hg_porosimetry.xlsx' contains the Hg porosimetry data of GS and mc shown in the SI. 'SoakingTimeGS_TiO2-CsPbBr3-mc-GS_pH7.xlsx' contains the chronoamperometric traces of TiO2|CsPbBr3|m-c|GS recorded three consecutive times with interval time of 1 hour between each measurement (Fig shown in SI).