Effect of ionic additives on the optical and electronic properties of a dyesensitized TiO2 heterointerface: absorption, charge injection and aggregation

Saurabh Agrawal^a, Mariachiara Pastore^a, Filippo De Angelis^a, Enrico Roca^a^,^c, Henry Snaith^b, Tomas Leijtens^b

^aComputational Laboratory for Hybrid Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, via Elce di Sotto 8, I-06123, Perugia

^bDepartment of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, United Kingdom

^cUniversity of Perugia, Via dell' Elce di Sotto, 8, Perugia, Italy

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

Poster, Saurabh Agrawal, 411
Publication date: 1st March 2014

We present a combined experimental and theoretical (DFT/TD-DFT) study on effects of two ionic additives namely Lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) and 1- ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) on D102 dye in solution and upon its adsorption on TiO₂ metaloxide as utilized in a solid state Dye-sensitized Solar Cells (ssDSSCs). Although these additives have been reported to improve the cell performance, the exact role of them is not yet established. The experimental results show that while the addition of these ionic additives on the dye in solution has only a marginal or no spectral shifts, they contribute to a slight red-shift with the appearance of a lower-energy shoulder in the absorption spectrum upon adsorption of dye on a TiO₂ film. Theoretical results confirm that the addition of lithium as well as EMIM cations could result in week stabilization of dye/additive complexes in solution. Further computational modelling predicts appreciable spectral red-shifts as a consequence of the interaction of the dye with added ionic additives. Addition to this our results show a strong effect of Li⁺ and EMIM⁺ cations on the electronic coupling between the dye's LUMO and the TiO₂ conduction band states, reflecting on the calculated injection rates. Further, by modeling the formation and the optical response of selected dye/additive aggregate models, we find a general broadening of the absorption band, accounting for the experimentally observed lower-energy shoulder in the D102 absorption spectrum recorded on TiO₂ films where Li and EMIM cations are added.

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