Reorganization Energies in the Transports of Holes and Electrons in Organic Amines in Organic Electroluminescence Studied by Density Functional Theory

Abstract

To enable the design of efficient organic electroluminescence (OLED) devices with desirable charge carrier transport properties, the mobilities of hole and electron in a series of compounds were studied computationally based on the Marcus electron transfer theory. MO calculations were performed, using the DFT B3LYP/6-31G* method in the Gaussian 98 program suite, on the following compounds: biphenyl (Bp), 4,4‘-biphenyldiamine (BA), triphenylamine (TPA), tri-p-tolylamine (TTA), 4-biphenylphenyl-m-tolylamine (BPTA), 4,4‘-bis(phenyl-m-tolylamino)biphenyl (TPD), naphthalene (Np), 1-naphthyldiphenylamine (NDPA), 1-biphenylnaphthylphenylamine (BNPA), and 4,4‘-bis(1-naphthylphenylamino)biphenyl (NPB). The geometries of these compounds in their neutral, cationic, and anionic states were optimized. The optimized geometries were then used to calculate the ionization potential, electron affinity, and reorganization energies. For compounds containing a biphenyl moiety (Bp, BA, BPTA, TPD, BNPA, and NPB), the inter-ring distance and torsional angle followed the trend neutral ≥ cationic ≥ anionic, except NPB in which these two parameters in anionic state were larger than the corresponding parameters in the cationic state because of a small contribution from the biphenyl moiety to its LUMO. Also, the ionization potentials follow the order Bp > BPTA ≈ BNPA > BA > NPB ≈ TPD. The electron affinities were calculated to range from −1.54 to −0.05 eV for all compounds except NPB which has a positive electron affinity 0.24 eV due to the dominant contribution of two naphthyl groups to LUMO. For most compounds, the reorganization energy λ+ for the hole transport is larger than λ- for the electron transport except NPB and BApy (constrained nitrogen pyramidal geometry). These exceptions were rationalized by the special structures for their anionic states. According to the magnitudes of λ+, compounds can be divided into two groups: λ+ ≥ 0.28 eV (BApl (constrained planar nitrogen geometry) ≈ Bp > TPD ≈ NPB) for compounds containing biphenyl group with or without two amino groups and λ+ ≤ 0.2 eV (TPA ≈ TTA <BPTA < BNPA ≈ NDPA) for compounds with single triarylamine group. According to the magnitudes of λ-, compounds can be divided into three groups: λ- ≥ 0.50 eV (TPD > Bp > BPTA) for compounds with a dominating biphenyl group in their LUMO, λ- ≤ 0.32 eV (NDPA > BNPA > Np > NPB) for compounds with a dominating naphthyl group in their LUMO, and the other compounds (TPA and TTA). From these results, λ+ is determined mainly by the moiety which contributes predominantly to its HOMO, whereas λ- is determined mainly by the moiety which contributes predominantly to its LUMO. Therefore, by controlling the major contributors to the HOMO and LUMO, and by incorporating substituents to fine-tune the energy levels of these frontier orbitals (HOMO and LUMO), a systematic design of materials for OLED with desirable charge carrier transport properties should be feasible.