Integrating Cu₂O with Upconversion Materials: Comprehensive Strategy for Effective Solar Water Splitting

Photoelectrochemical (PEC) water splitting is a promising solution for harnessing solar radiation toward a hydrogen-based future. This technology combines sunlight capture and water electrolysis processes, resulting in the production of hydrogen and oxygen that can efficiently recombine in fuel cells. Copper oxide semiconductors, specifically materials based on cuprous oxide (Cu₂O), have attracted considerable attention among a range of economically viable options due to their abundant elemental availability and scalable synthesis methods. Nevertheless, the limited efficiency of utilizing solar energy remains a hindrance in photocatalysis, primarily due to the narrow absorption range typically exhibited by photocatalysts. There were attempts to increase the efficiency of light absorption by improving the crystal quality or controlling the surface morphology. Yet, most of them absorb and utilize only UV and visible light. Taking into consideration that UV accounts for 9.3% (λ &lt; 400 nm) and visible light is 54.1% (400 &lt; λ &lt; 800 nm), the rest 36.6%, which accounts for IR light (λ &gt; 800 nm), results in significant losses of solar energy. The problem of harvesting the lost photons can be solved by implementing photon upconversion (UC) materials into a water-splitting device. Triplet-triplet annihilation-based upconversion (TTA-UC) is especially suitable for solar water splitting due to the advantage of efficient conversion at low photon intensity (excitation power).<br/>This work highlights the potential application of TTA-based UC in solar-assisted water splitting and illustrates the significance of photonic designs in addition to nanointerface engineering to improve the light-harnessing property of photoactive material. To the best of our knowledge, our research group is the first to discover the integration of photoactive materials for solar water splitting with an upconversion device based on the TTA mechanism. Here, we have developed a device where a photocatalyst based on Cu₂O film absorbs light within the UV-visible range, while the remaining infrared (IR) light is absorbed by the upconverter. Subsequently, the upconverter emits high-energy photons, which in turn are utilized by our photocatalyst. This strategy allows us to dramatically improve the light-harnessing properties of our photoelectrode by irradiating the photocatalyst from dual perspectives (front and back-side illumination). It has been demonstrated that Cu₂O coupled with an upconverter (UC) outperforms bare Cu₂O by 56% in terms of produced photocurrent density.