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.