Photovoltaic properties of the nanoporous titanium dioxide TiO2/Poly(3-hexylthiophene) heterojunction solar cell are studied by photovoltammetry under periodic photoirradiation, which is effective in examining the photo- and dark-current potential profiles simultaneously in the same experiment. The open-circuit voltage (V-oc) of the TiO2/poly(3-hexylthiophene) cell significantly depends on the atmosphere in the measurements. Accompanying the increase of the dark current, the photovoltammogram in vacuo shows the lower V-oc than that in air. The stepwise increase of the dark current after pulse irradiation is observed in the transient current-time profiles for the TiO2/poly(3-hexylthiophene) cell in vacuo. The difference spectra of the TiO2/poly(3-hexylthiophene) cell during photoirradiation in vacuo show the increase of a broad absorption band attributed to trapped charge carriers in the near-IR region, which is the origin of the dark current increment. The photovoltaic properties of the TiO2/Poly(3-hexylthiophene) heterojunction are seriously affected by the trapped charge carriers. (c) 2005 Elsevier B.V. All rights reserved.