Direct vs. indirect mechanisms for electron injection in DSSC: Catechol and alizarin

Catechol and alizarin have become model sensitizers for Dye Sensitized Solar Cells in recent years due to their capability to rapidly inject photoexcited electrons into the semiconductor conduction band. Because of their different geometries and electronic structures both dyes present important differences as sensitizers and operate through different mechanisms for electronic injection into semiconductor conduction band. employing alizarin are classified as type I DSSC. In these systems electron injection occurs through an indirect mechanism. First, there is a photoexcitation of the dye to an excited state, followed by the electron injection from this to the semiconductor conduction band. In contrast, solar cells employing catechol as sensitizer are representative examples of type II DSSCs, in which the electrons are injected in the semiconductor conduction band by a direct one-step mechanism from the dye ground state. s work, the electronic structure and the optical response of both dyes, free and bound to TiO2 (one of the most common semiconductors used in DSSC) have been analysed and compared using time-dependent density functional theory (TD-DFT) performing calculations both in real time and frequency domains. Results illustrate those characteristic aspects of each sensitizer to follow different electron injection mechanisms.