The research field of organic electronics experiences tremendous developments since the discovery of conducting polymers upon chemical doping and the developments of applications where organic materials replace the traditionally used inorganic semiconductors. Devices such as light-emitting diodes (OLEDs), solar cells, and field-effect transistors (OFETs) based on organic ð-conjugated materials as active materials represent the key applications of the domain. In OLEDs, charge carriers (holes and electrons) are injected from the electrodes into the organic semiconductor and emit light when they meet. Solar cells have an opposite working principle compared to OLEDs: light is absorbed and dissociated in charge carriers that migrate to the electrodes to give rise to an electric current. OFET plays the role of current modulator in electronic circuits by tuning the current flowing in its channel. The gain of better device performances (better conversion efficiency for OLEDs and solar cells or high ON/OFF ratios for OFETs) requires a better understanding at a molecular scale of the charge transport properties that are quantified at the experimental level by the charge carrier mobility ì.
Since organic conjugated materials are typically disordered, the charge carriers are mostly localized over a single molecule and charge transfers between molecules occur via a hopping mechanism. In our Ph.D. thesis, we have characterized the charge transport properties at the molecular scale by calculating the parameters entering into the Marcus hopping rate by means of semi-empirical Hartree-Fock methods and Density Functional Theory (DFT) calculations. On that basis, we have propagated a single charge carrier in molecular assemblies by means of a Dynamic Monte-Carlo procedure that we have developed in order to estimate mobility values as the ratio of the total distance travelled by the charge divided by the product of the total time needed to travel that distance and the norm of the electric field. The systems under study were model one-dimensional array of pentacene molecules, single molecular crystals and structures simulated by Molecular Dynamics (liquid crystalline phthalocyanine derivatives) and by Molecular Mechanics (grain boundaries in pentacene layers). The principle results shows anisotropic behaviour and electric field dependence for the charge carrier mobility, the impact of energetic as well as the positional disorder on the charge migration were investigated and we emphasize the importance to describe both disorders at a molecular scale in order to get a reliable picture for the charge transport properties calculations.
| Identifer | oai:union.ndltd.org:BICfB/oai:umh.ac.be:ETDUMH:UMHetd-02112009-170533 |
| Date | 25 September 2008 |
| Creators | Olivier, Yoann |
| Contributors | Jérôme Cornil, Didier Villers, Zhigang Shuai, Jean-Luc Brédas, Toussaint Robert, Sven Stafström, David Beljonne |
| Publisher | Universite de Mons Hainaut |
| Source Sets | Bibliothèque interuniversitaire de la Communauté française de Belgique |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://theses.umh.ac.be/ETD-db/collection/available/UMHetd-02112009-170533/ |
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