A promising passivation method for the AlGaAs window layer of GaAs-based compound solar cells using photoelectrochemical (PEC) oxidation process was investigated. The advantage of this passivation method was attributed to the reduction of interface states between the window layer and oxide layer, and the decrease in original contaminants on the window layer surface by self-oxidation. The conversion efficiency of the solar cell could be improved by more than 3.68% due to the PEC treatment. The obtained results demonstrate that the PEC treatment is an effective low temperature technique for surface passivation, which enhances the performance of GaAs-based compound solar cells. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3431542] All rights reserved.