Photoinduced electron injection from an organic dye having a pyridyl anchor to Lewis acid site of TiO2 surface

Abstract

An organic dye with a pyridyl group as an anchor unit (NI4) adsorbs exclusively at the Lewis acid sites of TiO₂ surface, although its adsorbability is very weak compared with that of a similar dye with a carboxyl group anchoring to Brønsted acid sites (NI2). In the present study, photovoltaic features of two similar dyes, NI2 and NI4, are thoroughly investigated by changing dye loadings on TiO₂ surface and the intensity of the illuminated light. A remarkable finding is that photocurrents and photovoltages observed under intense illumination are larger for NI4 than for NI2, suggesting that the adsorbability of dye onto a TiO₂ surface is not necessarily a principal factor leading to a high electron injection probability from dye to TiO₂. Plausible reasons for the difference in photovoltaic features between NI2- and NI4-DSSCs are discussed on the basis of electrochemical impedance spectroscopy and transient photovoltage measurements.