Ambipolar transport in the smectic E phase of 2-propyl-5''-hexynylterthiophene derivative over a wide temperature range.

5-Hexyl-5''-hexynyl-2,2':5',2''-terthiophene exhibits the smectic E phase below 200 degrees C and does not crystallize when it is cooled to -100 degrees C. Between 200 and -100 degrees C, non-dispersive transport is observed for holes and electrons with time-of-flight spectroscopy. Over the entire temperature range, the electron mobility is approximately twice as high as that of the hole. The hole and electron transport characteristics in the smectic phase below 0 degrees C are explained by the Gaussian disorder model, which was proposed for amorphous organic semiconductors. The disorder parameters, sigma and Sigma, are almost the same for holes and electrons. However, the pre-exponential parameter mu(0) for the electron is twice as large as that for the hole, which can be attributed to the difference in the extension of the LUMO of the molecules. The energetic disorder sigma is primarily determined by the disorder in the orientation of the permanent dipoles of liquid crystal molecules.