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

Abstract

Polymerizable liquid crystalline semiconductors, referred to as reactive mesogens ͑RMs͒, consist of -conjugated cores with reactive end groups decoupled by an aliphatic spacer. These can be polymerized within the mesophase, maintaining the self-assembled morphology and charge transport characteristics. The polymerized films can then be used in organic electronic applications such as charge transport layers in organic light emitting diodes and field effect transistors. We present a systematic study of the effect of reactive end groups on charge transport in calamitic liquid crystals ͑RMs͒ using the time-of-flight technique. Several different compounds were synthesized with a variation in both the liquid crystal ͑LC͒ 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 reduction 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. Polymerized reactive mesogens do maintain long-range transport, with comparable mobilities to those of the phase in which they were polymerized over a broad temperature range, including room temperature. The hole and electron mobilities found in polymerized systems are explored using the Holstein small polaron model in the nonadiabatic limit, yielding the relevant polaron binding energies and bandwidths, and using the Bässler Gaussian disorder model, yielding the relevant energetic disorder parameters.

Photocrosslinkable π-conjugated cruciform molecules for electronic/optoelectronic applications

“…Polymer networks formed from reactive mesogens provide a unique and advantageous combination of properties compared to other approaches: they are monodisperse after standard purification procedures; they form insoluble, intractable polymer films by spin-coating and subsequent polymerization; their films are photopatternable and some exhibit higher photoluminescence (PL) efficiency and improved current-voltage characteristics in prototype OLEDs than the monomers themselves before crosslinking [2,7], and the charge-carrier mobility can also exhibit a low field dependence [9]. However, the carrier mobility of the linear thiophene-based liquid crystalline molecule was observed to decrease after the photocrosslinking reaction due to disruption of the preferred anisotropic molecular packing (figure 1) [10][11][12].…”

“…Photopolymerization is preferred to thermal polymerization because of the pixellation capability and also because high temperatures can reduce the order parameter of uniformly oriented reactive mesogens and lead to photodegradation. A very limited number of reactive mesogens with acrylate, methacrylate, oxetane or nonconjugated diene polymerizable end-groups have been used so far as charge-transporting semiconductor material in OFET and light-emitting polymer networks in multilayer OLEDs [2][3][4][5][6][7][8][9][10][11][12]. The polymerizable end-groups should preferentially be polymerized by a radical mechanism to avoid the presence of ionic initiator and reaction products within the resultant crosslinked polymer network.…”

Functional Liquid-Crystalline Polymers for Ionic and Electronic Conduction

“…Kreouzis et al studied the carrier transport properties of photopolymerizable phenylnaphthalene, diphenylbithiophene, and quaterthiophene derivatives having an oxetane moiety or 1,4-pentadien-3-yloxy in their alkyl side chain (Fig. 21) [107,108].…”

“…These results suggest that molecular ordering was not perturbed during the polymerization process in the smectic phases. Photopolymerization in the isotropic phase produces thin films that do not exhibit excellent carrier transport properties [108].…”

“…The carrier transport characteristics were analyzed using Bässler's disorder formalism and Holstein's small polaron theory; however, the temperature range in which the carrier mobility was measured precisely was not so wide that the transport mechanism could be determined clearly [108].…”