A photosynthetic semiconductor electrochemical cell which spontaneously fixes CO2 into an organic molecule such as benzyl chloride to form phenyl acetic acid was constructed. The photosynthetic cell supplies electric power to the outer circuit as well as converting the organic compound to higher quality molecules under illumination. The competitive reaction between photoelectrochemical CO2 fixation and charge recombination on the photocathode is analyzed using the information provided by the variation of the short-circuit photocurrent with irradiation intensity and benzyl chloride concentration.