K. J. Donovan

Demonstration of an ultra-high mobility organic polymer

Abstract Experimental evidence is given for a high mobility (μ ∼ 20 m2 s−1 V−1) and a low field-saturated drift velocity (v d ∼ 2·2 × 103 m s−1) for carrier motion along a polymer chain. Photoconduction experiments in time domains from 10−9 to 102 seconds and fields from 102 V m−1 to 106 V m−1 are described. The polymer, the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol, is a single crystal in which the conjugated semiconducting carbon chains are all parallel to each other. Carriers travel a distance approaching 1 mm along a chain before trapping at a defect. It is possible to cut the chains and detect the resulting reduction of range of carrier motion.

Temperature-independent hole mobility in discotic liquid crystals

Experimental measurements are presented of the hole mobilities of four conjugated discotic systems, forming columnar liquid crystals, as a function of temperature. The measurements cover the crystalline/glassy phase and mesophase of these materials. It is a remarkable fact that the mobility is almost independent of temperature in the range 30 °C–170 °C. Various explanations of a weak temperature dependence exist and these are explored. They include the small polaron of Holstein in the nonadiabatic limit and the effect of the dynamic disorder present in the system.

Enhanced electronic transport properties in complementary binary discotic liquid crystal systems

Abstract The electronic transport properties of the molecular stacks in discotic liquid crystals (DLCs), have generated much recent interest. In particular, among a certain class of these DLCs, the triphenylenes, some derivatives exhibit high hole mobilities as demonstrated by the time of flight transit signals obtained in transient photoconduction experiments. The stacks have also been shown to exhibit one-dimensional transport behaviour. If the DLCs are to find application as new electronic materials it is desirable to improve their electronic transport properties. In this paper we demonstrate that the hole mobility and range are greatly increased as a result of the ordering imposed in complementary binary mixtures formed by addition of a large core discogen to the triphenylene molecules.

Transient Photoconductivity and Dark Conductivity in Discotic Liquid Crystals

A comparative study of transient photoconductivity and dark conductivity in the discotic liquid crystal hexahexyloxytriphenylene (HAT6) is reported. In both experiments the transport is dispersive, and the carrier mobilities are essentially identical demonstrating that the presence of the counterions in the chemically doped conductors does not significantly perturb the mobility.

Photocarrier creation in one dimension

Abstract Experimental results, and theoretical discussion, are presented on photocarrier creation in large single crystals of a conjugated polymer: the bis(p-toluene sulphonate) ester of 2,4-hexadiyne-1,6-diol. The quantum efficiency for carrier creation as a function of photon energy is found absolutely. The probability φ of carrier pairs avoiding geminate recombination is characteristic of a one-dimensional system, and well described by a theory due originally to Onsager. φ is linear in field at low fields, going over to Poole–Frenkel–Schottky variation at high fields. φ is exponentially dependent on temperature, with an activation energy depending on field, and on the mechanism of carrier creation.

Observation of Charge Motion along Molecular Wires Laid Down as Langmuir-Blodgett Multilayers

Abstract Fast photocurrents have been observed in Langmuir-Blodgett multilayers of polymerised 10, 12 penta cosa diynoic acid (PDA 12–8). They are linear in electric field and vary sublinearly with light intensity.

2,3,7,8,12,13-Hexakis[2-(2-methoxyethoxy)ethoxy]tricycloquinazoline: a discogen which allows enhanced levels of n-doping

A synthesis has been developed for 2,3,7,8,12,13-hexakis[2-(2-methoxyethoxy)ethoxy]-tricycloquinazoline (TCQ6EO2M) in which the ethylenoxy side chains are introduced before elaboration of the heterocylic core. This discogen gives a hexagonal columnar phase (Colh) between 77 and 233°C. n-Doping using potassium metal is facilitated firstly by the electron poor/π-deficient nature of the core and secondly by the polyethylenoxy side chains which complex the potassium K+ counter-ions. The conductivity of the Colh phase of TCQ6EO2M doped with 10 mol % potassium (σ ∥ = 1.1 x 10-3 S m-1) is substantially higher than that previously reported for 2,3,7,8,12,13-hexa(hexylthio)tricycloquinazoline doped with 6 mol % potassium (σ ∥ = 2.9 x 10-5 S m-1). Photoconductivity studies of TCQ6EO2M using a time of flight sample configuration show transient photocurrents for both holes and electrons. From these an upper limit on the mobility for the electrons is estimated as ~10-4 cm2 V-1 s-1 at 150°C which is of the same magnitude as that for hole mobilities in other columnar discotic liquid crystals.

Electron motion perpendicular to Langmuir-Blodgett multilayers of conjugated macrocyclic compounds: The organic quantum well

Abstract The principle of a three-dimensional molecular memory is outlined. Initial experiments aimed at understanding the transfer of electrons between the layers of a Langmuir-Blodgett multilayer are described.

Anomalous electric field dependence of carrier drift in one-dimensional systems. Example: organic PDA-TS

Some years ago Donovan and Wilson (1981) concluded that carrier motion on a single chain of the quasi-one-dimensional polymer crystal PDA-TS exhibits highly unusual features. From the electric field dependence of the peak photocurrent and carrier generation rate they concluded that carriers had an ultra-high mobility and a low-field saturated drift velocity. The authors report new measurements in the t- alpha time-dependent domain and at the same time propose a transport model to explain the data. They show that both the t- alpha decay law and the sublinear field scaling of the effective drift velocity can be associated with one and the same phenomenon: one-dimensional carrier drift in the presence of strongly scattering defects.

Determination of the parallel and perpendicular intermolecular tunnelling rates in two Langmuir-Blodgett quantum well systems

Abstract The first experimental determination of the anisotropy of the electron transfer rate in two Langmuir-Blodgett insulator multilayer systems is presented. A key feature of these two multilayer systems is the presence of the planar conjugated phthalocyanine ring. The first system is composed of tetra-tertiary-butyl phthalocyanine monosulphonic acid. In it the ring planes are approximately normal to the layer plane. The second system is composed of the two-ring phthalocyanine HOSiPcOSiPcOSI( n C 6 H 13 ) 3 . In it the rings lie in the layer plane. The anisotropy of the systems is determined from a model of the recombination rate, at high density, of photoelectrons and photoholes on the same layer. The ratio of the intralayer to interlayer transfer rates between adjacent rings is found to be 1500 and 570 respectively. Coupled with a previous determination of the interlayer transfer time of 1.6 and 0.30 ns, the intralayer transfer times are then determined as 1.07 and 0.56 ps respectively. The in-plane mobilities can then be determined as 0.34 and 1.2 cm 2 s −1 V −1 respectively. The results justify regarding these systems as organic multiple-quantum wells.