Theoretical characterization of the charge-transport and electroluminescence properties of pi-conjugated organic materials

Conjugated organic materials are currently the object of much interest because of the intrinsic scientific challenges they present and the technological potential they offer, including light-emitting diodes, solar cells, and field-effect transistors. The charge-transport mechanisms in organic conductive materials are still subject to considerable uncertainty and vary substantially as a function of the nature of the materials. However, around room temperature, conjugated oligomers and polymers usually transport charge via a thermally activated hopping-type mechanism, which depends on the relative orientations and solid-state packing of the species involved. The key parameters in hopping transport are the reorganization energies accompanying the geometric relaxations associated with electron transfer and the effective electronic coupling matrix elements between neighboring species, dictated largely by orbital overlap. A first contribution described in this thesis has been to underline that there is another but often ignored parameter affecting charge transport that results from the polarization of the localized electronic states by intermolecular interactions. We demonstrate that this contribution can be significant and, as in the case of the electronic coupling, is also very sensitive to the details of the system environment. Application of our methodology to a number of organic semiconductors is described.In order to reach high performance in organic light-emitting diodes, efficient light emission from excited state(s) is critical. Iridium (III) organometallic complexes are of great interest because they can harvest both singlet and triplet states from electrically generated excitons as a result, theoretical efficiencies of up to 100% can be achieved, whereas fluorescent molecules can only exploit singlet excitons and thus have a maximum theoretical efficiency of 25%. Of special interest for Ir(III) complexes is the color tunability of the emission from red to green and, in particular, to blue. Tuning of emission over the entire visible spectrum could be achieved by judicious modification of the coordinated ligands however, the relation between the structure of the ligands and the emission characteristics of such complexes is still not well understood. Part of our work has thus focused on the description of the ground- and excited-state characteristics of Ir(III) complexes, to better understand the interactions between these states and help establish the relationships between the ligand structure and the photophysical properties.A careful choice of the host material for organometallic phosphors is also very important for optimizing emission properties. It is desirable that the host has a large enough bandgap for effective energy transfer to the guest, good carrier transport properties for a balanced carrier recombination in the emitting layer, and energy-level matching with electrodes for effective charge injection. Because these conditions are difficult to meet simultaneously, host materials for blue triplet emitters are relatively scarce. Part of this thesis, therefore, has described the ground- and excited-state properties of several classes of host materials including carbazoles, phosphine oxides, oxadiazoles and organosilicon compounds with the aim to understand their structure-property relationships and thus develop guidelines for the design of effective host materials for blue phosphorescence emitters.