In silico investigation of catechol-based sensitizers for type II Dye Sensitized Solar Cells (DSSCs)

A series of catechol-based sensitizers, Cat-I to Cat-XV, for a possible application in type II DSSCs is presented. The electronic structures and excited state properties of free and bound to (TiO2)9 dyes have been analysed and compared by means of Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) methods. In particular, the effect of introducing electron-donating and electron-withdrawing substituents, as well as the effect of introducing an ethylene spacer as π-bridge between the substituents and the catechol unit to increase the conjugation has been investigated. Moreover, key aspects that could strongly influence the direct (type II) electron injection mechanism are analyzed and discussed. The results of the calculations suggest that the introduction of the π-bridge contributes to enhance the dye-to-TiO2 charge transfer (DTCT) efficiency regardless of the presence of electron-donating or electron-withdrawing substituents.