Investigation of TiO₂ surface modification with [6,6]-phenyl-C₆₁-butyric acid for titania/polymer hybrid solar cells

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.