Temperature dependence and anisotropy of charge carrier mobilities in durene

Very high charge-carrier drift mobilities have been measured in durene (1,2,4,5 tetramethyl benzene) single crystals by the transient-photoconductivity technique. In the crystal ($a, b$) plane both hole and electron mobilities are isotropic, 5 ${\mathrm{cm}}^2$/V sec and 8 ${\mathrm{cm}}^2$/V sec, respectively, at room temperature, and follow a $T^{-2.5\mathrm{}}$ temperature dependence. In the $c^{\prime }$ direction only hole mobilities could be measured, which were 0.15 ${\mathrm{cm}}^2$/V sec at room temperature and followed a $T^{-2.8\mathrm{}}$ temperature dependence. These results indicate that charge-carrier transport in durene can be treated within the frame of the band model. The charge-carrier generation process for the excitation wavelength (nitrogen laser $\lambda =3371\mathrm{}$ Å) was also studied. It is shown that the process involves a direct two-photon absorption to produce singlet excitons which then dissociate at surface states giving charge carriers. A lower limit of ∼${10}^{-48\mathrm{}}$ ${\mathrm{cm}}^4$ sec/(photon molecule) was estimated for the molecular singlet-singlet excitation-rate constant.