In then‐GaAs/Se=–Se=x–OH−/C liquid junction solar cell, modification of the semiconductor surface by incorporation of ruthenium increases both the fill factor and the open‐circuit voltage and improves the reproducibility of performance. The power conversion efficiency of the modified cell is 12% under ∼100 mW/cm2sunlight. Surface metal atoms or ions are shown to alter GaAs cell behavior widely; Ru represents a case for which the effect on cell performance is both positive and persisting.