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Title: Long-lived excitations in conjugated polymer blend films and devices
Author: Ford, T. A.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2006
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This thesis discusses the formation and decay of long-lived excited states within thin films and light-emitting diodes based on blends of polyfluorene-derived conjugated polymers. A polymer blend comprises two different conjugated polymers mixed together in a complex morphology. The formation of triplet excitons is studied in the blend system comprising poly(9,90dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamene) (PFB). The formation efficiency of triplet excitons is found to be several times higher in the blend than in either of the pristine polymers. Triplet excitons are formed at the interfaces between the two different polymers and the formation rate may be reduced with an external electric field. Two distinct interfacial states are known to exist in the F8BT:PFB polymer blend, a short-lived polaron pair and a long-lived exciplex state. Efficient intersystem crossing is shown to occur in both states. The rate of intersystem crossing is calculated as (4.6 ± 1.6) x 106s-1 in the exciplex state and at least (2.2 ± 0.6) x 107s-1 in the polaron pair. Once formed, triplet excitons decay with two characteristic lifetimes, corresponding to free and trapped triplets. In a polymer LED, polaron pair states are formed from the association of injected electrons and holes. The fast rate of intersystem crossing noted in the polaron pair suggests that the single formation fraction in a polymer LED may differ from the 25% suggested by simple spin statistics. Two experimental methods are used to calculate the singlet fraction in an F8BT:PFB LED. The singlet fraction, at low applied voltage, is found to be below 20% which corresponds to a fast relaxation from the triplet polaron pair states to the triplet exciton.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available