AbstractThis paper presents structural, optical, and electronic properties of Si coated with a thin epitaxial layer of n‐type BP, which serves as protective optical window. n‐Si/n‐BP, as well as p‐Si/n‐BP heterostructures have been grown by CVD and investigated. p‐Si/n‐BP heterojunction photo‐electrodes produce large and stable photocurrents up to 15 mA/cm2 when applied in liquid junction solar cells. The origin of this photocurrent is discussed. Impedance spectroscopy is used to obtain Mott‐Schottky plots from which the flatband potential of crystalline boron phosphide is determined. The band structure of the heterojunction is shown to agree with the optical features. Finally, the distribution of the potential drop in the hetero‐structure is elucidated.