The existence of a ground-state charge-transfer (CT) complex in a conjugated polymer:metal oxide nanoparticle bulk heterojunction photovoltaic cell is demonstrated by Fourier-transform photocurrent spectroscopy (FTPS). The CT complex between poly(3-hexylthiophene) (P3HT) and titanium dioxide ( Ti O ₂) is characterized by a weak additional photocurrent band (onset 1 eV ) in the FTPS spectra, situated below the conjugated polymer bandgap of 2 eV . The presence of CT interaction between P3HT and Ti O ₂ in relation to frontier orbital alignment is discussed, as well as the contribution of a sub-bandgap interfacial CT state to the electron transfer process in P 3 HT : Ti O ₂ solar cells.