We have investigated modification of TiO2 surfaces with [6,6]-phenyl-C61-butyric acid (PCBA) used for fabrication of TiO 2/poly(3-hexylthiophene-2,5-diyl) (P3HT) hybrid solar cells. The surface modification process was monitored using in-situ infrared absorption spectroscopy in the multiple-internal reflection geometry (MIR-IRAS). IR data showed that longer exposure of TiO2 surfaces to an organic solution of PCBA leads to undesirable formation of a physisorbed PCBA overlayer that cannot be removed by rinsing the surface in pure solvent. We found that ultrasonic cleaning of the TiO2 surface removed most of the physisorbed PCBA molecules. Modification of TiO2 surfaces with PCBA molecules drastically increased the short circuit current of TiO 2/P3HT-based hybrid solar cells, which is ascribed to improved charge separation efficiency at the TiO2/P3HT interface. The physisorbed PCBA molecules decreased the open circuit voltage and the fill factor. We demonstrated that the power conversion efficiency is improved by ultrasonic cleaning following PCBA deposition.