Influence of Structural Organization On Optical and Transport Properties in Organic Materials
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Figure 1: Illustration of the molecular arrangement in the bc layer of crystalline 6T (left) and BDT (right); the full chemical structure of the two compounds is also given.
The choice of these two compounds is motivated by their very different crystal packings (see Figure 1), which are expected to lead to significant variations in transport and optical properties. The supermolecular approach also allows us to define different strategies to enhance the bulk mobility of charge carriers in FETs and the internal electroluminescence quantum yield in LEDs. THEORETICAL METHODOLOGY A large number of molecular aggregates are extracted from the crystalline structure of BDT and 6T in order to selectively assess the influence of various parameters on the calculated properties. We calculate the one-electron structure of the clusters by means of the semiempirical Hartree-Fock Intermediate Neglect of Differential Overlap (INDO) Hamiltonian, as parameterized by Zerner and co-workers [6]. We then characterize their excited states by coupling the INDO method to a Single Configuration scheme. All the single excitations between π-molecular orbitals are systematically involved in the active space in order to ensure size consistency, thus mixing explicitly intrachain and charge-transfer one-electron excitations. This represents a major improvement with respect to the traditional excitonic theories where the charge-transfer contributions are usually neglected. RESULTS AND DISCUSSION We first estimate the amplitude of the splitting of the HOMO and LUMO levels in dimers formed by two adjacent 6T molecules aligned along the nearest-neighbor stacking directions in the crystal. The results show that the electronic splittings (equal to twice the value of the corresponding interchain transfer integrals in a tight-binding model) are significant only along the c crystal axis and the d axis connecting the inequivalent molecules within the bc layers. Since theories based on polaron transport (giving a coherent band motion at low temperatures and a hopping regime at high temperatures) relate the bulk mobility of the electrons and holes in a given direction to the square of corresponding interchain transfer integrals [7], we are led to the conclusion that the charge transport in the 6T crystal has a two-dimensional character and mainly takes place within the bc layers; this is in agreement with experimental data [8]. Figures 2 displays the evolution of the bandwidths formed by the interacting HOMO and LUMO levels as a function of the number of 6T molecules aligned along the c and d axes, respectively. 0.30
LUMO
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Figure 2: Evolution of the INDO-calculated bandwidths formed by the HOMO (open symbols) and LUMO (filled symbols) levels of 6T molecules along the c (solid lines) and d (dashed lines) directions as a function of the number of interacting units.
The HOMO [LUMO] splittings have similar amplitudes in the two directions and s
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