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2006-04-04 - Colloque/Présentation - communication orale - Anglais - 24 page(s)

Olivier Yoann , "Charge Transport in Conjugated Materials : From the Molecular to the Macroscopic Vision." in SPIE Photonics Europe, Strasbourg, France, 2006

  • Codes CREF : Physico-chimie générale (DI1320), Chimie quantique (DI1321)
  • Unités de recherche UMONS : Chimie des matériaux nouveaux (S817)
  • Instituts UMONS : Institut de Recherche en Science et Ingénierie des Matériaux (Matériaux)

Abstract(s) :

(Anglais) Since the discovery of high electrical conductivity in organic conjugated polymers, a growing interest has been dedicated to conjugated materials for applications such as field-effect transistors (FETs), light-emitting diodes (LEDs) and solar cells. Organic FETs are made of three metallic contacts (gate, drain, and source); their operation consists in modulating the current flowing from the source to the drain across the organic layer by means of a voltage applied at the gate. Organic solar cells and LEDs are built by sandwiching an organic layer between two electrodes and aim at converting a light into charges and at creating light upon charge injection, respectively. In order to optimize the performance of these devices, it is important to maximize the mobility of the charge carriers in the organic layer to allow for a high ON/OFF ratio and a fast switching time in FETs, an efficient charge recombination in LEDs and charge collection in solar cells. In the early nineties, Bässler and co-workers have developed Monte-Carlo methods to simulate charge transport within a hopping regime in a model cubic lattice. This approach is based on effective parameters such as the cubic lattice constant (which is the average distance between adjacent sites) and the average hopping frequency derived from the theory of Miller-Abrahams that does not take into account exact chemical structures. In the recent years, we have developed a quantum-chemical approach allowing for a better understanding of the hopping processes at the molecular scale [1,2]. In this work, the hopping rate is expressed within the semi-classical Marcus theory that implies the passage through a transition state going from the reactants (R) to the products (P). Transfer integral (t) and the reorganization energy (λ) parameters that appear in the Marcus formula were estimated from quantum-chemical calculations. In this contribution, we will present our recent advances in modelling charge transport in a hopping regime. On the one hand, we have now extended the previous work by adopting the Marcus-Levich-Jortner (M-L-J) formalism that describes possible tunnelling effects across the barrier by treating an effective vibrational mode at the quantum-mechanical level. A driving force G° has also been introduced to depict the influence of the applied electric field [3]. On the other hand, we have developed a Monte-Carlo approach to estimate charge mobilities on the basis of rates obtained via the M-L-J formalism for each pair of interacting molecules. This approach is based on a random choice of the charge propagation direction for each hop and on acceptation conditions for the transfer rates to induce the charge migration. We have thus in hand an unique tool to link the transfer rates at the molecular level to charge mobilities at the macroscopic level. For sake of illustration, this approach is applied here to a model system made of a one-dimensional stack of pentacene molecules. We will describe among others the way the charge mobility is affected by the amplitude of the electric field, by molecular disorder, and by the presence of energy traps [4]. Preliminary results obtained for actual crystalline structures with a two-dimensional charge transport will also be presented.