Hole mobilities (μ) in rubrene single crystals (space group Cmca) along the crystallographic c-axis have been investigated as a function of temperature and applied electric field by the time-of-fight method. Measurements demonstrate an inverse power law dependence on temperature, namely, μ=μ0T-n with n = 1.8, from room temperature down to 180 K. At 296 K, the average value of μ was found to be 0.29 cm2/Vs increasing to an average value of 0.70 cm2/Vs at 180 K. Below 180 K a decrease in mobility is observed with further cooling. Overall, these results confirm the anisotropic nature of transport in rubrene crystals as well as the generality of the inverse power law temperature dependence that is observed for field effect mobility measurements in the a-b crystal plane.
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