WO3/CuWO4 Ratio Controls Open-Circuit Photovoltage and Photocurrent in Type II Heterojunction Solar Fuel Photoelectrodes

Abstract

WO₃/CuWO₄ photoelectrodes for the oxygen evolution reaction benefit from a type II heterojunction for charge separation. However, the impact of the WO₃/CuWO₄ ratio on the photocurrent and the photovoltage is not clear. To probe the effect of composition, Cu_xW_1–xO_y thin films with variable W:Cu ratios were prepared on FTO by reactive magnetron cosputtering of W and Cu, followed by air annealing at 500 °C. EDS, XRD, Rietveld refinement, and Raman spectroscopy confirm the presence of crystalline WO₃ and CuWO₄ in the W-rich films and increasing amounts of amorphous copper oxides in the Cu-rich films. Band gaps were determined by optical absorption spectroscopy, surface photovoltage spectroscopy (SPS), and photoaction spectra. Optical band gaps are found to decrease from 2.7 to 1.2 eV with increasing copper oxide content. SPS reveals n-type semiconductor photoanode behavior for WO₃/CuWO₄ samples and p-type photocathode behavior for CuO_x-rich films. Photoelectrochemical experiments confirm stable water oxidation with Faraday efficiency near unity for all W-rich films and photocurrents that are increasing with CuWO₄ content. Optimal performance is seen for WO₃/CuWO₄ mixed phases containing 47–75 mass% CuWO₄. These compositions maximize charge separation at the type II heterojunction interface between the two materials. Additionally, according to incident photon-to-current efficiency (IPCE) data, WO₃ improves photon conversion below 350 nm, while CuWO₄ improves conversion at 450–525 nm. Overall, this work shows for the first time how the WO₃/CuWO₄ ratio controls the photovoltage and the photocurrent in type II heterojunction solar fuel photoelectrodes and how copper oxides in the copper-rich films severely degrade the performance. These results are useful in the context of bulk-heterojunction electrodes for the conversion of solar energy into fuels.