Time-of-flight study of electrical charge mobilities in liquid-crystallinezinc octakis(beta-octoxyethyl) porphyrin films

Time-of-flight measurements performed on micron-thick films of liquid-cryst alline zinc octakis(beta -octoxyethyl) porphyrin indicated that charge carr iers possess significantly high drift mobilities, attaining approximately 0 .01 cm(2) V(-1)s(-1) and 0.008 cm(2) V(-1)s(-1) for holes and electrons, re spectively, at room temperature. Upon heating the samples from 300 to 420 K , causing the porphyrin to go from the solid-crystalline to the discotic me sophase, the mobilities did not decrease drastically, and remained at value s slightly larger than half those observed at room temperature. Charge tran sport in this material conformed to the Scher-Montroll model, which attribu tes a distribution of hopping times to the propagation of the initially for med charged carrier packet. Analysis of the "universal" plots prescribed by this model yielded a dispersion factor of 0.5 for both charge carriers.