Hole injection and transport in a fluorene-containing copolymer
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B6.3.1
Hole injection and transport in a fluorene-containing copolymer Rizwan U. A. Khan,† Theo Kreouzis, Dmytro Poplavskyy, and Donal D. C. Bradley Blackett Laboratory, Imperial College London, London SW7 2BW, U.K.
ABSTRACT We have studied the electrical properties of a fluorene-containing copolymer which is currently being developed for state-of-the-art blue polymer LEDs. This copolymer is made up of three functional groups which are nominally the hole-conducting, electron conducting and emissive regions. Using a combination of current/voltage, time-of-flight and dark injection transient versus temperature measurements, the injection and transport properties of the material have been investigated. Hole injection from polystyrene sulphonate doped polyethylenedioxythiophene (PEDOT:PSS) into the polymer is found to be consistent with an ohmic contact. Hole transport within the fluorene copolymer is found to possess a mobility that is two orders of magnitude lower than that for previously studied polymers containing the copolymer constituents. Using the equations for trap-free space-charge limited current, predicted J/V characteristics have been obtained from the mobility values derived using the time-of-flight technique. We discuss both the reduced hole mobility of the copolymer, and the discrepancies between the measured and predicted J/V characteristics, in terms of variations in both the trap and transport site densities and their energetic and spatial distributions. INTRODUCTION Conjugated polymers are highly promising electronic materials, primarily for use in the next generation of displays based on light-emitting diodes [1], but also for future photovoltaic and transistor-based applications [2]. A key property of a polymer-based electronic material is its ability to transport both holes and electrons in sufficient and equal quantities. The efficiency of a polymer-based light-emitting diode (PLED), for example, is determined by both the rate of creation of excitons and also the position of the zone in which they are generated [3]. The former is governed by the number of charge carriers which recombine to form excitons, which in turn is governed by the transport properties of the material and also the energy barriers to charge injection. The latter is governed by the mobility difference between the holes and electrons. Most commonly the electron mobility is strongly reduced by traps [4, 5], which leads to recombination close to the metal cathode and hence a dissipative loss of recombination energy via dipole-image dipole coupling. One approach is to incorporate functional moieties that provide electron or hole injection and transport within a single copolymer. Using such a material one may hope to balance and/or improve hole and electron transport, and also reduce the barriers to injection by modifying the ionization potential and electron affinity of the polymer layer. We report here on studies of a statistical copolymer known as SCB3 which contains three such moieties, namely 9,9-dioctylfluorene as the nomin
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