Charge-transport simulations in organic semiconductors
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In this thesis we have extended the methods for microscopic charge-transport simulations for organic semiconductors. In these materials the weak intermolecular interactions lead to spatially localized charge carriers, and the charge transport occurs as an activated hopping process between
diabatic states. In addition to weak electronic couplings between these states, different electrostatic environments in the organic material lead to a broadening of the density of states for the charge energies which limits carrier mobilities.\r\nThe contributions to the method development include\r\n(i) the derivation of a bimolecular charge-transfer rate,\r\n(ii) the efficient evaluation of intermolecular (outer-sphere) reorganization energies,\r\n(iii) the investigation of effects of conformational disorder on intramolecular reorganization energies or internal site energies\r\nand (iv) the inclusion of self-consistent polarization interactions for calculation of charge energies.These methods were applied to study charge transport in amorphous phases of small molecules used in the emission layer of organic light emitting diodes (OLED).\r\nWhen bulky substituents are attached to an aromatic core in order to adjust energy levels or prevent crystallization, a small amount of delocalization of the
frontier orbital to the substituents can increase electronic couplings between neighboring molecules. This leads to improved charge-transfer rates and, hence, larger charge-mobility. We therefore suggest using the mesomeric effect (as opposed to the inductive effect) when attaching substituents to aromatic cores, which is necessary for example in deep blue OLEDs, where the energy levels of a host molecule have to be adjusted to those of the emitter.\r\nFurthermore, the energy landscape for charges in an amorphous phase cannot be predicted by mesoscopic models because they approximate the realistic morphology by a lattice and represent molecular charge distributions in a multipole expansion. The microscopic approach shows that a polarization-induced stabilization of a molecule in its charged and neutral states can lead to large shifts, broadening, and traps in the distribution of charge energies. These results are especially important for multi-component systems (the emission layer of an OLED or the donor-
acceptor interface of an organic solar cell), if the change in polarizability upon charging (or excitation in case of energy transport) is different for the components. Thus, the polarizability change upon charging or excitation should be added to the set of molecular parameters essential for understanding charge and energy transport in organic semiconductors.\r\nWe also studied charge transport in self-assembled systems, where intermolecular packing motives induced by side chains can increase electronic couplings between molecules. This leads to larger charge mobility, which is essential to improve devices such as organic field effect transistors, where low carrier mobilities limit the switching frequency.\r\nHowever, it is not sufficient to match the average local molecular order induced by the side\r\nchains (such as the pitch angle between consecutive molecules in a discotic mesophase) with maxima of the electronic couplings.\r\nIt is also important to make the corresponding distributions as
narrow as possible compared to the window determined by the closest minima of the\r\nelectronic couplings. This is especially important in one-dimensional systems, where charge transport is limited by the smallest electronic couplings.\r\nThe immediate implication for compound design is that the side chains should assist the self-assembling\r\nprocess not only via soft entropic interactions, but also via stronger specific interactions, such as hydrogen bonding.\r\n\r\n\r\n\r\n