A photoelectrochemical cell with an n-Si/RbAg4I5 interface showed a high open-circuit potential (V∞ = 0.69 V) and a high incident photon-to-current conversion efficiency (> 70%) in a wide range of wavelengths from visible to near-infrared light (500- 900 nm). An electronic structure model at the interface was proposed from electrochemical data involving electrochemical impedance spectra to elucidate the origin of the high V-oc. The flatband potential was determined to be -0.09 V obtained from a Mott-Schottky plot, which led to band-edge potentials of -0.33 and 0.79 V for the conduction band and the valence band, respectively. The redox potential of the I-/I-2 couple (0.65 V), which donates electrons to photogenerated holes in the valence band, was higher but close to the band-edge potential of the valence band, resulting in the high Voc. © 2005 The Electrochemical Society. All rights reserved.