Charge transport, injection and optical properties of fluorene based copolymers

Abstract

In this work we investigate the charge transport, injection and optical properties of several series of fluorene based copolymers. These properties are key areas for polymer semiconductor research studies that are intended to address the challenges of achieving electrically pumped polymer lasing. In general, high charge carrier mobility and luminescence efficiency have been found to be mutually exclusive, however, optimising both together, as well as demonstrating high optical gain will be a significant step to offset losses associated with the mechanisms for lasing. A further requirement is that it is necessary to achieve efficient charge injection in the particular materials that are found to have both of these properties optimised. We concentrate our investigation on fluorene-based copolymers, which have been amongst the earliest classes of materials of interest for polymer lasers (including studies in optically pumped polymer lasers), and since it has previously been shown that high hole mobility and luminescence efficiency may be achieved in fluorene based copolymers with simple alterations to side chain configurations. However, ohmic hole injection into poly(9,9-dioctylfluorene) (PFO) has been difficult to achieve because of its deep-lying highest occupied molecular orbital (HOMO) level of ~5.8eV below the vacuum level. Therefore, in our first experimental study, we revisit the charge transport properties of PFO based on recent developments in the use of transition metal oxides as hole injecting electrodes. With the ability to achieve ohmic injection in PFO, we are able to use Dark Injection Space Charge Limited Current (DI-SCLC) and field effect transistor (FET) techniques, both of which require ohmic injecton for charge transport measurements, and make comparisons with measurements from Time-of-Flight (ToF) photocurrent experiments. Together, these techniques span several orders of magnitude in charge carrier density, which provides an additional perspective on charge transport and the presence of charge traps. The subsequent chapters introduce three series of fluorene-based copolymers. The first series of blue emitting polymers has a resemblance to familiar fluorene based polymers with hexyloxyphenyl substituents instead of linear alkyl side chains and the series contains copolymers with a phenoxazine (BPPX) unit derived from more familiar triarylamine structures. Optical measurements show these polymers are free of morphologies such as the beta-phase and we obtain a hole mobility of ~10-3 cm2/Vs, photoluminescence quantum efficiency (PLQE) of 65.7% and optical gain of 41cm-1 for poly(9,9-di(4-hexyloxyphenyl)fluorene). Transient DI-SCLC measurements are used to show differences in charge trapping behaviour in the series. The final chapter of experimental results is based on novel poly(indenofluorene-fluorene) and poly(indenopyrazine-fluorene) copolymers. In the poly(indenofluorene-fluorene) series we consider the role of increased backbone rigidity compared to polyfluorene and the variation of side chain length and structure on charge transport and optical properties. Copolymers in this series yield a time-of-flight hole mobility of ~10-2 cm2/Vs, PLQE of 61.5% and optical gain of 43cm-1. Finally, we show that optical gain is possible in the indenopyrazine-fluorene series, which has been designed with the aim of achieving balanced electron and hole mobility.