Effect of Anchoring Groups on Electron Transfer at Porphyrins-TiO₂ Interfaces in Dye-Sensitized Solar Cell Application

Porphyrin has excellent optical properties as required in solar cell applications, for example, high molar extinction coefficient and high quantum efficiency (≈80%). Excellent optical properties make porphyrin a good candidate as a photosensitizer, that is, in dye-sensitized solar cells (DSSC). Nevertheless, porphyrins exhibit poor charge transfer resulting in poor cell performance. Poor charge transfer in porphyrins is due to the aggregation, low porphyrin adsorption, and weakly bound porphyrins on the TiO₂ surface. Anchoring group attached to the porphyrin ring may play an important role to increase both charge transfer and porphyrin adsorption. This study adopts a strategy to increase the electron transfer from porphyrins to TiO₂ by applying different anchoring groups. This strategy aims both to reduce the aggregation and to enhance the binding between porphyrin and TiO₂ surface. The presence of methyl and carboxylic acid as the anchoring groups in the porphyrin system can increase the electronic coupling between the porphyrin and the TiO₂ surface while also hindering the aggregation. Thus, the presence of anchoring groups improves the electron injection efficiency and cell performance. The type and position of the anchoring groups strongly contribute to the electron injection, thus is discussed in detail.