Orientation-Engineered Semitransparent Ta₃N₅ Photoanode for Unbiased Solar Water Splitting

Semitransparent tantalum nitride (Ta₃N₅) photoanodes are promising front absorbers for tandem solar water splitting, but their development is hindered by the incompatibility of high-temperature nitridation with conventional transparent conductive substrates and by the intrinsically anisotropic carrier transport of Ta₃N₅. Here, we report an orientation-engineered semitransparent Ta₃N₅ photoanode enabled by two synergistic strategies. A homologous W-incorporated Ta₃N₅ semitransparent conductive layer is deposited on quartz glass, providing electronic compatibility with the Ta₃N₅ absorber and favorable transmittance in the >600 nm range. In parallel, Pt-assisted lattice matching guides the preferential growth of Ta₃N₅ with a high-mobility (023) orientation. Theoretical and experimental analyses confirm that this (023)-oriented architecture promotes charge separation and transport. The optimized photoanode delivers 9.7 mA cm^-2 at 1.23 V versus the reversible hydrogen electrode and a maximum applied-bias photon-to-current efficiency of 3.47%. When integrated with a specially designed narrow-bandgap perovskite solar cell, the tandem device achieves unbiased water splitting with a solar-to-hydrogen efficiency of 7.2%, representing one of the highest values reported for two-photon tandem PEC water-splitting systems. This work establishes a general strategy for coupling transparent conduction with crystallographic orientation control to advance efficient unassisted solar fuel production.