Structural Dynamics in Organic Semiconductors

Speaker:

Henrik Till Lemke Dipl. Phys., Centre for Molecular Movies, Niels Bohr Institute

Co-Author(s):

Tine Ejdrup, Centre for Molecular Movies, University of Copenhagen, Denmark. Dag Werner Breiby, Norwegian University of Science and Technology, Trondheim, Norway. Peter Hammershøj, Centre for Molecular Movies, University of Copenhagen, Denmark. Yanhong Tong, Centre for Molecular Movies, University of Copenhagen, Denmark. Niels Harrit, Centre for Molecular Movies, University of Copenhagen, Denmark. Robert Feidenhans'l, Centre for Molecular Movies, University of Copenhagen, Denmark. Martin Meedom Nielsen, Centre for Molecular Movies, University of Copenhagen, Denmark. Peter Hammershøj, University of Copenhagen Niels Harrit, University of Copenhagen Robert Feidenhans’l Martin Meedom Nielsen, University of Copenhagen

Figure A

The structural environment in organic semiconductors during and after an electronic excitation changes the conditions for the charge carriers. The knowledge of transient structures is therefore essential for a complete understanding of processes like charge separation and charge transport. These processes are exploited in functional materials for large scale technical applications and are therefore of a high technological and scientific interest. We investigated thin, polycrystalline films of perylene (C20H12, Figure A) by time resolved X-ray diffraction. The 200-800 nm thick vapor deposited films were investigated by the optical pump – X-ray probe method: The transient structures after excitation by an ultra short optical laser pulse were probed by short X-ray pulses obtained from a synchrotron source. By this method structural information at 100 ps time resolution was obtained. The transient diffraction signal shows an expansion of the crystal lattice which decays on a time scale of 1-2 µs. This can be explained by the heat exchange to the substrate. The expansion is overlaid with oscillations of the crystal lattice in the MHz to GHz regime (Example: Azimuthal peak shift of the -221 reflection for several film thicknesses d, Figure B). They can be explained by standing modes of coherent vibrations of the lattice planes in direction of the crystal structures a-axis (arrows in Figure A). These standing waves are defined by the elastic constants of the crystal material and the boundaries of the thin film which are the contact to the underlying substrate and the free film surface (Figure C). In this continuum mechanical model only odd harmonics of vibration frequencies are allowed which are related to the speed of sound by Vsound = (2n-1) d fn. Here n represents an interger and fn the frequency of the nth mode. The results from films of different thickness show clearly the first two modes and satisfy the relation between the thin film parameters (Figure B). Both processes, expansion and vibrations, are induced by the instant dissipation (~50 ps) of vibrational energy during the decay from an initial electronically excited state to a trapped exciton state, the so called excimer state. In this state two monomers are associated into a short lived (~100ns) excited dimer varying in inter monomer distance. Our data are the first experimental insight into the structure of the excimer in perylene. They show that the geometrical change of the dimerization is smaller than previously predicted by theoretical calculations (Cohen et al., Chem. Phys. 27, 211; 1978). In this work we obtained structural information about the mechanical and thermal coupling between a thin organic film and the substrate which is of high interest for functional devices like low cost organic solar cells or field effect transistors.