Nondispersive hole transport in an electroluminescent polyfluorene

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, ${\mu }_p{\text{=4×10}}^{\text{−4}}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V\hspace{0.17em}s}$ at a field of ${\text{5×10}}^5\hspace{0.17em}\mathrm{V/cm}.$ The field dependence of the mobility is weak: ${\mu }_p{\text{=3×10}}^{\text{−4}}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V\hspace{0.17em}s}$ at ${\text{4×10}}^4\hspace{0.17em}\mathrm{V/cm}$ and increases only modestly to ${\mu }_p{\text{=4.2×10}}^{\text{−4}}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V\hspace{0.17em}s}$ at ${\text{8×10}}^5\hspace{0.17em}\mathrm{V/cm}.$ 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.