Infoscience

Thesis

De l'effet de la polarisation électronique sur le transport de charge dans les semi-conducteurs moléculaires

This work aims at a better understanding of transport mechanisms which take place in organic semiconductors such as pentacene. More accurately, we believe that the electronic polarization plays a leading part, which we try to identify in the particular case of the acenes. These semiconductors are very polarisable molecules that can crystalise easily enough into ordered structures. Moreover, there is quite an important transfer integral between neighbouring molecules, which is very uncommon for such an organic. After some general remarks regarding quantum transport in chapter 1, we present the main features of this family of semiconductors (chapter 2). The question that arises then is : how the characteristic properties of the acenes can influence the ground state of the charge carrier in the cristal (chapter 3)? In order to build this ground state, we take into account the electronic polarisability and the coupling to the lattice, which dress the charge with a cloud of polarization and a cloud of phonons, and also the overlap and the thermal disorder. The time scales related to these different phenomenons play an essential part. By comparig them with the characteristic time of transfer between molecules, we classify those phenomenons into slow and fast effects. The fast ones characterize the effective mass of the charge carrier, whereas the slow ones determine the localization of the charge in a more or less disordered crystal. We will see that the slowest of these effects act in the same way as the static disorder and are able to induce electronic localization. Then comes a quantitative survey of the electronic polaron (chapter 4). We show that, on the one hand, electronic polaron leads to a renormalisation of the transfer integral whereas, on the other hand, it amplifies the low geometric disorder. We finally achieve a modelisation of the system by an Anderson hamiltonian, in which the energetic disorder –of which we study the statistic distribution– is the translation of the former geometric one. Before the quantitative study of the eigenstates of this hamiltonian, we take a glance at the different models already developped in order to understand the experimental data from organic semiconductors (chapter 5). There is one important feature which is rarely well taken into account : slow disorder. Yet, such a disorder can disturb the building of the Bloch states and thus lead to electronic localization that we study quantitatively (chapter 6). Finally, we gather both the fast and slow effects in a single picture : a model providing an expression of mobility according to the microscopic parameters (chapter 7). As a conclusion, we take a further step towards the devices. We leave the bulk out to consider a charge in the canal of a transistor, i. e. layers of pentacene on a dielectric medium, in order to take into account the interface effects (chapter 8).

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