Structural Dynamics in Organic Semiconductors – Niels Bohr Institute - University of Copenhagen

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Structural Dynamics in Organic Semiconductors

Organic semiconductors are fascinating materials of high scientific interest and with a wealth of potential technological applications as energy efficient, low-cost electronic devices, in e.g. displays, solar cells and logic circuits. The electronic function of such devices-typically built from thin layers-is strongly correlated to their structure. While the relation between their static structure and device performance has been established for a decade, the dynamical structural response of the materials to energy and charge injection, and thus the coupling between charge carriers and lattice vibrations, has-until now-been largely unknown.
In the present thesis, the dynamic structural changes in 100-1000 nm thin crystalline films of different organic semiconducting materials upon excitation by light were investigated by time-resolved grazing incidence X-ray diffraction. For the first time it was directly observed and modeled how the excitation by an ultra-short light pulse created coherent acoustic waves in the thin films. The waves are confined to the organic semiconducting material through reflection at the interfaces of the thin film. The lifetime of the acoustic waves showed to be determined by the mechanical properties of the neighboring media. Besides the fundamental interest in accurately describing the structural implications of phonon propagation in organic micro crystals, the observations may lead to an improved understanding of organic semiconducting devices.