A poly(3-methylthiophene) (P3MeT) film was electrochemically deposited on TiO2 layers prepared by firing, and the junction characteristics of the P3MeT/TiO2 cells were investigated and compared with those of the P3MeT/n-Si and P3MeT/n-GaAs cells. A high-quality junction was obtained by electrochemical deposition on the TiO2, as opposed to the P3MeT/n-Si cell which requires specific post-treatments for the modification of its electrical properties. More important, the rectifying behavior of the P3MeT/TiO2 cell spontaneously improved during its storage in the air over 40 days, in marked contrast to the deterioration behaviors of the P3MeT/n-Si and P3MeT/n-GaAs cells when exposed to air. These experimental findings were well explained by assuming the existence of chemically active species on the TiO2 surface and their chemical bondings to the P3MeT. This assumption was experimentally supported by surface work function measurements of the TiO2.