LAUSR

This paper describes a Si photoelectrode with an ultra‐long minority carrier diffusion length (1940 µm) passivated by an amorphous Si layer, which provides a chemically passivated surface. With this extremely long carrier diffusion length, it is possible to separate the catalyst layer (metal) with the light absorption region on different sides of the Si photoelectrode, forming a double‐side Si photoelectrode for photoelectrochemical water reduction and oxidation. The obtained photocathode exhibits a photocurrent of 39 mA cm−2 and applied bias photon‐to‐current efficiencies (ABPE) of 15.4% with stability up to 100 h. Meanwhile, 38.5 mA cm−2 photocurrent and ABPE of 5.8% with a 200 h stability are achieved when this structure is used as a photoanode. A monolithic unbiased artificial leaf is constructed, yielding an unbiased solar to hydrogen conversion efficiency of 3.7%. This chemically passivated Si photoelectrode breaks the trade‐off between carrier transport and surface passivation in conventional Si photoelectrodes.