Injection and charge transport in polyfluorene polymers

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-bis-N,N'-(4-methoxyphenyl)-bis-N,N'-phenyl-1,4-pheny lenediamine) (PFMO) and poly(9,9-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenyle nediamine) (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 A1.