XPS, UV–Vis, FTIR, and EXAFS Studies to Investigate the Binding Mechanism of N719 Dye onto Oxalic Acid Treated TiO2 and Its Implication on Photovoltaic Properties

The anchoring mechanism of N719 dye molecules on oxalic acid treated TiO 2 (OA-TiO 2 ) electrodes has been investigated using extended X-ray absorption fine structure (EXAFS) measurements, Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). The FTIR spectroscopy of OA-TiO 2 electrodes revealed that the oxalic acid dissociates at the TiO 2 surface and binds through bidentate chelating and/or bidentate bridging. Analyses of EXAFS, FTIR, UV–vis, and XPS measurements of N719 dye loaded onto OA-TiO 2 revealed that the binding of N719 molecules takes place via interaction between the Ru atom of the dye and O – of bidentate bridged oxalate ions at the TiO 2 surface. This mechanism is quite different from the binding of N719 onto untreated TiO 2 (WO-TiO 2 ) surface, where −COOH and SCN groups of the dye directly bind to the TiO 2 surface. The analyses of UV–vis data show that the amount of N719 dye loading onto OA-TiO 2 surface is much higher than that onto the native TiO 2 surface. In addition, the incident photon-to-current conversion efficiency (IPCE) measurements show that the presence of oxalate ions between the dye and TiO 2 surface favors efficient electron transfer and therefore improvement in device efficiency. The dye-sensitized solar cells fabricated using N719 dye sensitized onto OA-TiO 2 showed an efficiency of ∼4.6%, which is significantly higher than that based on a WO-TiO 2 electrode (∼3.2%).