– MolNanoMat
A combined theoretical and experimental investigation of the energy conversion mechanisms in organic liquid crystal materials
New experimental and computational tools are being developed that will help designing novel photo-sensitive molecular nano-structures. The present project focuses on the primary processes responsible for the conversion of photons into free charge carriers.
A combined theoretical and experimental investigation of the energy conversion mechanisms in organic liquid crystal materials
Organic solar cells face a have to overcome a fundamental hurdle: the generation of free electrons and holes from strongly bound excitons. This charge separation is possible only at the interface of two materials with different electron affinities, between a donor D and an acceptor A. Within this context, a novel class of materials is presently emerging based on the self-organization of organic oligomers forming liquid crystals (LC).<br />We have studied the energy and charge transfer processes between D and A after selective photoexcitation of D, and occurring within the same oligomer isolated in solution or in between neighboring oligomers, when closely packed in the LC. For the first generation of oligomers bisthiophene – perylenediimide (PDI)), our results obtained by ultrafast laser spectroscopy showed interesting physical phenomena such as the generation of charge transfer states with spatial coherence, but also a very fast and efficient charge recombination that cuts short on a sufficient photo-current generation. The theoretical modeling based in calculations of the electronic structure and the quantum carrier dynamics confirmed that the spatial coherence accelerates the formation of separated charges, but indicated also a strong Coulomb binding of these states as the reason for the unwanted charge recombination.
The experimental methods used for the investigation of molecular photo-energy conversion are «pump-probe« transient absorption and fluorescence spectroscopy with a resolution of 10-13 s (100 fs). These time scales are the ones, on which excitons move, dissociate and possibly recombine. Our techniques yield time-resolved differential spectra over a broad range (300-1000 nm), so as to allow identifying the photo-chemically generated species by their absorption spectra. The comparison with the spectra of the D?+ et A?- radicals, obtained in separate spectrometry experiments in electrochemical cells, leads then to the identification of the (D?+/A?-) charge transfer states.
The molecular electronic states as well as their orbitals and the relevant vibrational modes were calculated using time-dependent density functional theory (TD-DFT), with the carefully chosen functionals CAM-B3LYP and wB97XD, tested against accurate but more time-consuming reference
methods (e.g. CC2). The validation of the functionals is crucial for the accuracy of the charge transfer states. The computation of the charge formation dynamics is based on solving the system Hamiltonian
including several electronic and vibrational states, and utilizing the ML-LCTDH (Multi-Layer Multiconfiguration Time-Dependent Hartree) method. The calculations, without any adjustable parameter, reproduce the main experimental findings with excellent agreement. They validate in particular the role of vibrational coherence for the ultrafast and highly efficient formation of charge transfer states.
In order to reduce charge recombination, a second generation of oligomers (thiophene-fluorenespacer-PDI) were designed and synthesized with a variable chemical composition of the donor block. Ultrafast spectroscopy then identified the optimal chemical composition, and an increase of the charge transfer state by more than a factor of 50 was achieved in single oligomers in solution. The calculation of the electronic orbitals and their energy then allowed understanding the observed effects.
In summary, the combined experimental and theoretical studies provide a detailed understanding of the molecular mechanism that are at work for the generation of free charges in these new materials. They open new opportunities for the design of materials for organic solar cells.
The development of new design strategies for organic solar cella presently witnesses a tremendous soaring. Against this background, the aim of the project is to demonstrate the usefulness of new analytical and theoretical tools. These should allow for a rational design of novel molecular nanomaterials with improved characteristics in terms of speed and quantum efficiency of charge carrier production.
1. «Sub-100 fs charge transfer in a Novel Donor-Acceptor-Donor Triad Organized in a Smectic Film«, T. Roland, J. Léonard, G. Hernandez Ramirez, O. Yurchenko, S. Ludwigs, S. Méry, S. Haacke, PCCP, 14, 273–279 (2012).
2. «Sub-100fs charge transfer within a new donor-acceptor-donor triad self-organized in liquid-crystal phase«, T. Roland, J. Léonard, G. Hernandez Ramirez, S. Méry, S. Ludwigs, O. Yurchenko, and S. Haacke, CNRS-EWHA winter school, Séoul, South Korea, 6-10 fev. 2012 (poster)
3. «Ultrafast spectroscopy of donor-acceptor-donor triads self-organized in liquid-crystal phase«,T. Roland, J. Léonard, G. Hernandez Ramirez, P-O. Schwartz, S. Méry, S. Ludwigs, O. Yurchenko, S. Haacke, EMRS 2012 Spring Meeting, Strasbourg, 15-17 mai 2012 (poster)
4. «Sub-100fs charge transfer within a new donor-acceptor-donor triad self organized in liquid-crystal phase«, T. Roland, J. Léonard, G. Hernandez Ramirez, P-O. Schwartz, S. Méry, S. Ludwigs, O. Yurchenko, S. Haacke, IUPAC Symposium on Photochemistry 24 , Coimbra, Portugal,15-20 Juillet 2012 (poster)
5. « Theoretical investigation of energy and charge transfer in organic donor-acceptor systems with applications in photovoltaics », J. Wenzel, thèse en master, Université de Francfort (soumis mars 2012).
6. « Investigation of elementary energy and charge transfer processes in a novel donor-acceptor-donor triad for organic solar cells », M. Polkehn, thèse en master, Université de Francfort (soumis septembre 2012).
7. « Theoretical investigation of energy and charge transfer in novel donor-acceptor-donor triad systems », J. Wenzel, I. Burghardt, S. Haacke, A. Dreuw, 48th Symposium on Theoretical Chemistry (STC 2012), Karlsruhe, September 23-27, 2012 (poster).
8. « Quantum dynamics of ultrafast energy and charge transport in extended systems », I. Burghardt, 244th American Chemical Society National Meeting, Philadelphia, August 19-23, 2012 (oral presentation).
Project coordination
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
Help of the ANR 199,975 euros
Beginning and duration of the scientific project:
- 0 Months
