Bi(2)S(3)‐Cu(3)BiS(3) Mixed Phase Interlayer for High‐Performance Cu(3)BiS(3)‐Photocathode for 2.33% Unassisted Solar Water Splitting Efficiency

To realize practical solar hydrogen production, a low‐cost photocathode with high photocurrent density and onset potential should be developed. Herein, an efficient and stable overall photoelectrochemical tandem cell is developed with a Cu(3)BiS(3)‐based photocathode. By exploiting the crystallographic similarities between Bi(2)S(3) and Cu(3)BiS(3), a one‐step solution process with two sulfur sources is used to prepare the Bi(2)S(3)–Cu(3)BiS(3) blended interlayer. The elongated Bi(2)S(3)‐Cu(3)BiS(3) mixed‐phase 1D nanorods atop a planar Cu(3)BiS(3) film enable a high photocurrent density of 7.8 mA cm(−2) at 0 V versus the reversible hydrogen electrode, with an onset potential of 0.9 V(RHE). The increased performance over the single‐phase Cu(3)BiS(3) thin‐film photocathode is attributed to the enhanced light scattering and charge collection through the unique 1D nanostructure, improved electrical conductivity, and better band alignment with the n‐type CdS layer. A solar‐to‐hydrogen efficiency of 2.33% is achieved under unassisted conditions with a state‐of‐the‐art Mo:BiVO(4) photoanode, with excellent stability exceeding 21 h.