Electrochemical characterization of blue-emitting polyfluorene LEP
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Electrochemical characterization of blue-emitting polyfluorene LEP Ilaria Grizzi, Clare Foden, Simon Goddard, Carl Towns Cambridge Display Technology Greenwich House, Madingley Rise, Madingley Road, Cambridge CB3 0TX, UK Introduction Conjugated polymers based on polyfluorene have been used for a number of optoelectronic applications from LEDs [1] to organic transistors [2] and photovoltaic cells [3]. The electrochemical properties of these polymers are central to all these applications [4]. In an operating LED, electrons are injected into the polymer layer from the cathode and holes from the anode. The injection of electrons is into the LUMO level, and the injection of holes is into the HOMO level. The ease with which injection of charge occurs is strongly dependent on the energy mismatch between the HOMO or LUMO and the work function of the injecting electrode, as well as the applied voltage. As direct LUMO level measurement is difficult the value of the electron affinity (EA) is usually inferred (often incorrectly) from the experimental HOMO level energy and the value of the optical band gap obtained from the absorption spectrum. Carefully controlled experimental conditions have enabled us to detect the polymer reduction by cyclic voltammetry, thus enabling a direct measurement of the polymer LUMO level. Direct measurement of the polymer ionization potential and electron affinity, as well as higher energy transitions, can give a unique insight into how the charges are injected on a polymer, particularly when combined with the results from theoretical models. This study focuses on the electrochemical properties of blue emitting polymers containing N,N’-bis(4butylphenyl)-N,N’-diphenyl phenylenediamine (PFB) and 9,9-dioctylfluorene (F8). The effect of varying the proportions of the two moieties in a polymer on the electrochemical properties was investigated by studying the respective homopolymers (PFB-PFB and pF8: the AB copolymer (F8-PFB) and a random copolymer of 10%PFB and 90%F8 (Copolym1). As a comparison the AB copolymer of F8 with another triarylamine 4-sec-butylphenyl diphenyl amine (F8-TFB) was also studied. Experimental The polymers were synthesized by Suzuki polymerization and purified according to the published protocol [5]. When the AB copolymers were prepared the PFB and TFB were always introduced as a dibromide monomer. 9,9-dioctylfluorene (F8) were introduced as a diethylene glycol boronic esters and, for concentrations of F8 above 50%, (as in Copolym1), as 2,7 dibromide monomers. To produce the homopolymer PFB-PFB, dibromide PFB was used in addition to the pinacolic diboronic ester. The cyclic voltammograms reported here were recorded with a computer controlled EG&G potentiostat/galvanostat at a constant scan rate of 1000 mV/s. A three electrode configuration undivided cell was used. The working electrode was glassy carbon (3mm diameter), with a Pt wire auxiliary electrode and a non-aqueous reference electrode containing 0.01M AgNO3, 0.1M tetrabutylammoniumperchlorate (TBAP) in acetonitrile, a
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