Time-of-flight measurements have been used to study carrier transport in films of poly(9,9-dioctylfluorene). We find nondispersive hole transport with a room-temperature mobility, μp=4×10−4 cm2/V s at a field of 5×105 V/cm. The field dependence of the mobility is weak: μp=3×10−4 cm2/V s at 4×104 V/cm and increases only modestly to μp=4.2×10−4 cm2/V s at 8×105 V/cm. Both the relatively high mobility and weak field dependence point to a high degree of chemical regularity and purity that makes polyfluorene attractive for use as an electroluminescent polymer and for other device applications. We have not been able to measure clean electron current transients, suggesting highly dispersive transport with possible deep trapping, as found in many other conjugated polymers.

Any material can show w (3) effects, as there is no symmetry selection rule excluding this effect for special systems. Even isotropic systems such as solutions or amorphous polymers can have strong third-order effects in certain cases. Therefore, it is important for our conclusions to provide evidence that it is, in fact, the crystal that is responsible for the effects measured and not the silicone glue or the glass slides used for its preparation. To this end we checked the lateral variations of the SH signal. The crystal is projected onto the entrance slit of the monochromator so that simply moving the sample enables spatial resolution of the signal. This set-up allowed a resolution of ca. 30 mm. Careful examination of the variations of the EFISH signal clearly showed that the crystal is the only source of the SH light. Nevertheless, another effect from the silicone glue was observed: After a change of the static voltage, a waiting time of 30 s was inserted before measuring the SHG. Within this time, shifts of the signal can be observed. In addition, a slight dependence of the signal on the history of applied voltages was recognized. In Figure 3, the measured points are connected according to the sequence of their measurement. This effect can be minimized when the amount of silicone glue between the crystal and the electrodes is minimized, and the effect is thus ascribed to the silicone glue, which influences the actual electric field at the crystal.

Homogeneous alignment of poly(9,9′-dioctylfluorene) films on rubbed polyimide results in a more than one order of magnitude increase in time-of-flight hole mobility normal to the alignment direction. We find μ=8.5±1×10−3 cm2/V s at an electric field of E=104 V/cm. Hole transport is found to be nondispersive, indicating a low degree of energetic disorder.

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