Heterojunctions between organic semiconductors are central to the operation of light-emitting and photovoltaic diodes, providing respectively for electron-hole capture and separation. However, relatively little is known about the character of electronic excitations stable at the heterojunction. We have developed molecular models to study such interfacial excited electronic excitations that form at the heterojunction between model polymer donor and polymer acceptor systems: poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with F8BT. We find that for stable ground-state geometries the excited state has a strong charge-transfer character. Furthermore, when partly covalent, modelled radiative lifetimes (approximately 10(-7) s) and off-chain axis polarization (30 degrees) match observed 'exciplex' emission. Additionally for the PFB:F8BT blend, geometries with fully ionic character are also found, thus accounting for the low electroluminescence efficiency of this system.
A series of Ir(III)-based heteroleptic complexes with phenylpyridine (ppy) and 2-(5-phenyl-4H-[1,2,4]triazol-3-yl)-pyridine (ptpy) derivatives as coordinating ligands has been characterized by a number of experimental and theoretical techniques. Density functional theory (DFT) calculations were able to reproduce and rationalize the experimental redox and excited-states properties of the Ir complexes under study. The introduction of fluorine and trifluoromethyl substituents is found not only to modulate the emission energy but also often to change the ordering of the lowest excited triplet states and hence their localization. The lowest triplet states are best characterized as local excitations of one of the chromophoric ligands (ppy or ptpy). The admixture of metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) character is small and strongly depends on the nature of the excited state; their role is, however, primordial in defining the radiative decay rate of the complexes. The extent of charge-transfer contributions depends on the energy gaps between the relevant molecular orbitals, which can be modified by the substitution pattern.
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