Injection and charge transport in polyfluorene polymers
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Injection and charge transport in polyfluorene polymers Dmytro Poplavskyy, Theo Kreouzis, Alasdair Campbell, Jenny Nelson, Donal Bradley Blackett Laboratory and Centre for Electronic Materials and Devices, Imperial College, Prince Consort Road, SW7 2BZ, London, United Kingdom ABSTRACT An overview of recent results concerning the injection and transport of holes in a range of conjugated fluorene polymers, provided by the Dow Chemical Company, is presented. Time-of-flight measurements in poly(9,9-dioctylfluorene) (PFO) are performed in a range of electric fields and temperatures (200-415 K). It is found that annealing at 380 K results in an irreversible increase of the hole mobility by one order of magnitude. Analysis of the TOF data within the Gaussian disorder model of Bässler and coworkers shows that this effect mainly contributes to the mobility prefactor µ0, which grows from 2.3×10-2 to 2.6×10-1 cm2/Vs after annealing, while the disorder parameters σ and Σ increase only slightly. Dark-injection transient measurements are performed in poly(9,9-dioctylfluorene-co-bisN,N’-(4-methoxyphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFMO) and poly(9,9dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) polymers for the range of electric fields and in a wide range of sample thicknesses. The lowest studied thickness (0.22 µm) for PFB is much closer to typical device thicknesses (≤0.1 µm) than the thicknesses (~1 µm) required for TOF measurements. It is shown that there are no significant differences in hole transport across the range of thicknesses from 0.22 µm to 1.1 µm indicating that for this material TOF technique can be a reliable tool to characterise materials for device operation. There is found to be an influence on stability of the metal counter-electrode used to perform dark-injection measurements. Specifically Ag and Au are found to give less stable structures than Al. INTRODUCTION Since the first report of electroluminescence in organic conjugated polymers in 1990 [1], a significant effort has been put into research and development on new conjugated polymers that can be used for high efficiency polymer light-emitting diodes (PLEDs). Such PLEDs are expected to be suited to fabrication on flexible substrates and at very low cost. Charge transport properties of the organic material are of enormous importance for the overall efficiency of the device. First, the injection and transport of electrons and holes should be balanced in order to ensure maximum quantum efficiency. Second, low driving voltages are necessary for commercially viable devices, a requirement that can be addressed via highmobility materials that are able to carry high current at low voltage. Poly(9,9-dioctylfluorene) (PFO) and its copolymers are considered to be some of the most promising materials for use in such commercial organic light-emitting diodes [2-4]. Time-offlight measurements show that PFO exhibits non-dispersive hole transport with mobilities of 4⋅10-4 cm2/Vs at room temperature [5]. Further improvem
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