Ultrafast Charge Transfer in Organic Nanostructures for Energy Conversion Applications

T. Roland^a, Stéphane Méry^a, Pierre-Olivier Schwartz^a, Jérémie Léonard^a, Li Liu^a, Stefan Haacke^a, Irene Burghardt^b, Mathias Polkehn^b

^aInstitut de Physique et Chimie des Matériaux de Strasbourg, CNRS UMR 7504, 67034 Strasbourg cedex

^bInstitutef. Theoretical and Physical Chemistry, Chemistry Dept., W.-Goethe-University Frankfurt,60438 Frankfurt

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2015 (NFM15)

Santiago de Compostela, Spain, 2015 September 6th - 15th

Invited Speaker, Stefan Haacke, presentation 315
Publication date: 8th June 2015

In the last ten years, many alternatives relying on organic molecules have been explored that could replace the traditional solar cells made by inorganic semiconductors. Organic cells are thin, flexible, and cheap-to-produce, but suffer from inherent limitations of organic materials, in particular their low carrier mobility and high exciton binding energies. Nevertheless, the record power conversion efficiency recently reported is as high as 10% [1] for a single junction solar cell. Solar cells using liquid crystal (LC) films of donor-acceptor (DA) molecules are a new approach, in which the ratio of DA interface-to-volume is maximized, leading to fairly good performances [2-4]. The motivation is to make the distance to the D-A interface shorter than the exciton diffusion length (typically 10 nm). We have studied LC films of bisthiophene-derivatives forming D and perylenendiimide as A, by femtosecond transient absorption (TA) spectroscopy. Due to the strong electronic coupling between D's the initial laser excitation, selectively tuned in the absorption of D, excites a coherent superposition of many D molecules (exciton) that decays within 60 fs into a charge transfer (CT) state, that localizes on a slower 0.4 ps time scale [3]. Simulating the quantum kinetics of these intermolecular excittons highlights the dominance of nearest neighbour CT states due to the specific molecular arrangement in the LCs [5]. We will present results obtained for a new type of donor family incorporated in the DA's that bear moieties with different electron-donating and -accepting character, thereby offering a handle to control the localization of HOMO and LUMO orbitals. CT lifetimes larger than 2 ns are now observed in chloroform, and with unexpectedly large effects of the solvant polarity. Simulations of the electronic structure and of intramolecular reorganisation energies find a particular interplay of two CT states of different dipole moments. Results are confronted with a description within Marcus theory. The research was funded by the ANR grant PICASSO and the ANR-DFG grant MolNanoMat.

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