Atomic-scale Observation of Multi-conformational Binding and Energy Level Alignment of Ruthenium-based Photosensitizers on TiO2 Anatase

Christian Dette^a, Gordon Rinke^a, Soon Jung Jung^a, Stephan Rauschenbach^a, Jan Cechal^a, Christopher Kley^a, Sebastian Stepanow^a, Klaus Kern^a^,^d, Christopher Patrick^b, Feliciano Giustino^b, Markus Baur^c, Michael Dürr^c

^aMax Planck Institute for Solid State Research, Stuttgart, Heisenbergstraße, 1, Stuttgart, Germany

^bDepartment of Materials, University of Oxford, Parks Road, Oxford OX1 3PH

^cInstitute of Applied Physics, Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany

^dLaboratory of Physics of Complex Matter (LPMC), Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Ecublens, Switzerland, 2014 May 11th - 14th

Organizers: Michael Graetzel and Mohammad Nazeeruddin

Oral, Christopher Kley, presentation 209
Publication date: 1st March 2014

Dye-sensitized solar cells (DSSCs) constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dye’s energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO₂, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence.

Here we report on our latest results on the observation of multi-conformational adsorption and energy level alignment of single N3 dyes (cis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)-ruthenium(II)) on TiO₂ anatase (101). In situ electrospray ionization deposition combined with scanning tunnelling microscopy and spectroscopy studies in ultra high vacuum at low temperature provide direct access to the dye-substrate interface at the atomic level.The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection.

Most importantly our work suggests that optimization strategies based solely on the electrochemical properties of the dye should be replaced by a more comprehensive approach where the focus is on the engineering of the chromophore−semiconductor interface at the atomic scale. This work shows unambiguously that in order to design more efficient photosensitizers it is crucial to consider kinetic aspects in the dye-substrate coupling, possible strain in the molecular chromophore, and the atomic-scale structure of the dye−semiconductor interface in such a way to achieve optimal photoelectron injection from all adsorbed photosensitizers.

First atomic-scale insights into photosensitizer-TiO2 anatase interfaces
Kley, C. S.; Dette, C.; Rinke, G.; Patrick, C. E.; Cechal, J.; Jung, S. J.; Baur, M.; Dürr, M.; Rauschenbach, S.; Giustino, F.; Stepanow, S.; Kern, K. Atomic-scale observation of multi-conformational binding and energy level alignment of ruthenium-based photosensitizers on TiO2 anatase. Nano Letters 2014, doi: 10.1021/nl403717d

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