On the energy level alignment at titanium-dye interfaces: Implications for electron injection and light harvesting

Laurent Lasser^a

^aUniversité de Mons, Place du Parc, 20, Mons, Belgium

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Laurent Lasser, 050
Publication date: 5th February 2015

We have applied Density Functional Theory to describe the changes in the electronic structure of oligothiophene derivatives equipped with carboxylic acid anchoring groups upon chemical grafting on TiO₂ clusters. The adsorption promotes a partial pinning effect for the dye lowest unoccupied molecular orbital, i.e., a LUMO energy level alignment with respect to the TiO₂ conduction band edge (CBE) irrespective of the oligothiophene length, which we ascribe to a strong hybridization between the dye discrete (LUMO) level and the crystal conduction band (CB). This is borne out by the fact that no pinning is observed, when decoupling the conjugated segment from the metal oxide substrate, e.g. by introducing a phenylene (PTn-Ph) or vinylene phenylene (PTn-Vi-Ph) moiety in a meta configuration or when there is a large energy mismatch between the dye frontier levels and the oxide conduction band, as is the case for oligothiophene-S,S-dioxides (with deep LUMO levels below the oxide CBE). Implications for electron injection into the titania and the optical absorption properties of the oxide-dye hybrids are discussed.

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