A comprehensive study of the effect of reactive end groups on the charge carrier transport within polymerized and nonpolymerized liquid crystals

Abstract

Polymerizable liq. cryst. semiconductors, referred to as reactive mesogens (RMs), consist of p-conjugated cores with reactive end groups decoupled by an aliph. spacer. These can be polymd. within the mesophase, maintaining the self-assembled morphol. and charge transport characteristics. The polymd. films can then be used in org. electronic applications such as charge transport layers in org. light emitting diodes and field effect transistors. We present a systematic study of the effect of reactive end groups on charge transport in calamitic liq. crystals (RMs) using the time-of-flight technique. Several different compds. were synthesized with a variation in both the liq. crystal (LC) mesogenic core group and the functional end groups. The reactive end groups in most cases affect the mesophase charge transport compared to the nonreactive LC mesophase transport. This manifests itself as a redn. in mobility, varying from a factor of 4 in the best case to as large as two orders of magnitude. In the best systems studied, however, the reactive end group effect on the transport, compared to the nonreactive mesophase transport, is negligible. Polymd. reactive mesogens do maintain long-range transport, with comparable mobilities to those of the phase in which they were polymd. over a broad temp. range, including room temp. The hole and electron mobilities found in polymd. systems are explored using the Holstein small polaron model in the nonadiabatic limit, yielding the relevant polaron binding energies and bandwidths, and using the Bassler Gaussian disorder model, yielding the relevant energetic disorder parameters.

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