The photoelectrochemical reaction was investigated at a type of photosensitizing interface between a solid electrolyte and a semiconducting electrode. RbAg4I5 thin film was used as a solid electrolyte and n-Si single-crystal plate as a photoanode. This cell showed an incident photon flux to photocurrent conversion efficiency higher than 70% in a wide range of wavelengths from visible to near-infrared lights (500-900 nm). (C) 2003 The Electrochemical Society.