Homologous binary mixtures and improved hole conduction of self-assembled discotic liquid crystals

Ammar A Khan,Girish Rughoobur,Muhammad A Kamarudin,Alessandro Sepe,James A Dolan,Andrew J Flewitt,Malik M Qasim,Timothy D Wilkinson

doi:10.1016/j.orgel.2016.05.027

Ammar A Khan, Girish Rughoobur + Show 6 more

Open Access

https://doi.org/10.1016/j.orgel.2016.05.027

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Abstract

Discotic liquid crystals (DLCs) are considered promising materials for organo-electronic applications. Columnar alignment of DLCs leads to anisotropic charge transport with high charge carrier mobility. However, pure DLCs exhibit low intrinsic charge carrier density which limits bulk conductivity. This research studies the alignment and conductivity properties of small molecule triphenylene-based DLCs to develop hole transport layers for potential applications in organic semiconductor devices. Binary mixtures of homologous DLCs of the hexakis(n-alkyloxy)triphenylene series (HAT6 and HAT10) are formulated. Mesophase characteristics and columnar alignment of these mixtures are characterized using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Alignment, orientation and order of columnar packing in the mixtures is studied using X-ray diffraction (XRD) and grazing incidence wide angle X-ray scattering (GIWAXS) measurements. It is identified that binary mixture formation strongly effects the columnar alignment in solution processed films. Furthermore, to increase charge carrier density in the DLC films a strong electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is added as a p-type dopant, followed by an extensive characterization of its doping effect. POM, DSC thermal scans, UV–visible spectroscopy, photo-luminescence spectroscopy (PL) and I-V measurements are utilized to characterize and establish the improvement of hole conduction in the doped films. It is observed that F4TCNQ-doped triphenylene DLC films exhibit two-fold increase in hole conductivity, making the materials highly relevant for charge transport applications.

Highlights

Discotic liquid crystals (DLCs) typically exhibit columnar mesophase characteristics, where the mesophase exists in a given temperature range [1]
In this study we focus on both the alignment and conductivity of hexakis (n-alkyloxy)triphenylene DLC derivatives HAT6 and HAT10 in the plastic crystalline (Cr) phase [28]
Conclusion drawn by analyzing the grazing incidence wide angle X-ray scattering (GIWAXS) scattering profiles do not support the hypothesis that the dark regions seen in Fig. 3c are regions of improved homeotropic alignment as the scattering peaks corresponding to homeotropic alignment (Fig. 3i) are smaller in intensity relative to the planar alignment peaks and do not show significant enhancement when compared to the homeotropic scattering peaks of HAT6 and HAT10

Summary

Introduction

Discotic liquid crystals (DLCs) typically exhibit columnar mesophase characteristics, where the mesophase exists in a given temperature range [1]. The mesophases are typically comprised of disc-shaped molecules that form molecular columns due to phase segregation of the aromatic and aliphatic segments, in addition to p-p interactions between the poly-aromatic molecular condensed cores. Individual polyaromatic molecules exhibit significant pi-conjugation, and overlap within well-aligned molecular columns have motivated significant efforts to realize their potential as self-assembled organic semiconductors [2,4,8e11]. A.A. Khan et al / Organic Electronics 36 (2016) 35e44 principles that makes almost all types of organic semiconductors (polymeric and small molecule) possible is bond conjugation (alternating p-s bonds), that leads to the splitting of molecular energy levels, to induce band-like energy levels: conduction (lowest unoccupied molecular orbital (LUMO)) and valence (highest occupied molecular orbital (HOMO)) [18]

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