Interfacial trapping in an aged discotic liquid crystal semiconductor

Abstract

This study reports on time-of-flight (TOF) hole mobility measurements in aged 2,3,6,7,10,11-Hexakis(pentyloxy)triphenylene columnar liquid crystals. In contrast to the original samples reported in 2006, homeotropically aligned samples yielded TOF transients with an extended non-exponential rise. The experimental data were fit to a simple model that accurately reproduces the TOF transients assuming delayed charge release from traps near the optically excited electrode. While interfacial trapping appears only in the aged materials, the bulk mobility is similar to the pristine material. The model addresses dispersive transport in quasi-one-dimensional materials, determines the charge carrier mobility in systems with interfacial traps, and provides a method for characterizing the traps.

Charge Mobility in Discotic Liquid Crystals

“…Given the requirements listed above, the sample thickness is usually between 1 and 20–30 μm, so that TOF probes macroscopic 3-D drift mobility in the time scale of 10 −7 –10 −4 s, at low charge densities around 10 13 –10 15 cm −3 . This method has been used extensively to measure charge mobility both in smectic [ 25 , 127 ] and in columnar [ 26 , 125 , 128 , 129 ] mesophases.…”

Self-assembled discotics as molecular semiconductors

“…Here, the homeotropic alignment of the columnar mesophase is favourable as the method relies on a 1D semiconducting pathway perpendicular to the substrate. 37–40…”

“…Here, the homeotropic alignment of the columnar mesophase is favourable as the method relies on a 1D semiconducting pathway perpendicular to the substrate. [37][38][39][40] For the space-charge-limited current (SCLC) measurements, Ohmic contacts are employed in a sandwich-cell geometry. The material is filled between two electrodes having work functions compatible with its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for favourable charge injection.…”