ZnO/n-Si heterojunction solar cells were fabricated on n-Si substrates by spin coating of the ZnO precursor solution produced by the sol-gel gel process. In order for the ZnO films to have proper electrical conductivity, the films were doped with an n-type dopant such as aluminum, and were subsequently annealed at a temperature of 450 degrees C under reducing environments. The ohmic contacts were formed between n-Si and Al for a bottom electrode by doping the rear surface of the Si substrate with phosphorous atoms. The front surface of the Si substrate was also doped with phosphorous atoms for improving the efficiency of the solar cells. The substrate doping was carried out with PSG (phosphosilicate glass) films produced by the sol-gel process. As a result, the conversion efficiencies of the fabricated solar cells ranging up to about 5.3% were obtained. These efficiencies were found to decrease rather quickly with time because of silicon oxide film grown at the ZnO/Si interface as a result of oxygen penetration through the porous structure of the ZnO films. Oxygen barrier layers should be needed in order to prevent the decrease of conversion efficiencies.