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Title: Electronic structure of interfacial states formed at polymeric semiconductor heterojunctions
Author: Huang, Ya-shih
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2008
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This thesis focuses on the nature and role of interfacial states formed at the heterojunctions between polymer donor and polymer acceptor systems. Two classes of organic semiconductors are investigated: (1) conjugated polymers, 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, and (2) helical supra-molecular assemblies of copolymers consisting of the Pt-porphyrin and perylene chromophores. First, a molecular model is developed to describe such interfacial excited electronic excitations. It is found that for stable ground-state geometries, the excited state has a strong charge-transfer character. Furthermore, when partly covalent, modelled radiative lifetimes (~10-7s) and off-chain axis polarisation (30°) matched 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. Second, the interchain and intrachain interactions are disentangled by systematically varying the interchain distance and extent of planarisation – two primary parameters that change with applied pressure. It is shown that the photophysical properties of these interfacial excited states are determined by the tradeoff between the electronic couplings and the energy mismatch between the charge-transfer and locally excited excitations upon reducing the interchain distance. In addition, increasing planarisation yields a decreased energy mismatch and a larger mixing from the locally excited excitations. Finally, a well-defined, close-packing multi-chromophoric system is investigated using a variety of optical spectroscopy techniques. The photophysical processes taking place in the system are found to strongly depend on which component of the copolymer is excited.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available