Supramolecular self-assembly and physical-gel formation in disc-like liquid crystals: a scalable predictive model for gelation and an application in photovoltaics.

Sehrish Iqbal,Ammar A Khan

doi:10.1039/c8ra09278h

Abstract

The application of triphenylene-based discotic liquid crystal derivatives as physical gelators is investigated. In particular, we focus on 2,3,6,7,10,11-hexakis-pentyloxytriphenylene (HAT5) and the longer alkyl chain homologue (HAT6). The driving mechanisms behind and parameter space of non-covalent physical gel formation is studied. A Hansen solubility parameter (HSP) approach is used to predict physical gelation of these disc-like liquid crystalline molecules in a variety of common organic and inorganic solvents important to electrochemical devices. Our results show that HSP analysis is very useful for the prediction of gel formation. The results are transferrable and can form the basis for future investigations into liquid crystalline physical gels. Furthermore, we use acetonitrile as a solvent and apply the gels as electrolytes in dye sensitized solar cells. It is observed that using a binary mixture of gelators results in average photovoltaic power conversion efficiencies as high as 7.21%, compared to a 5.9% reference electrolyte. This is attributed to a reduction in electron recombination at the n-type interface and provides further insight about hybrid gelators. Coupled with an increase in device stability, the results are promising for gel-based dye sensitized solar cells as well as potentially other electrolytic devices such as batteries and supercapacitors.

Highlights

Self-assembled physical gelators have attracted signi cant attention due to their applications as structural as well as functional materials in optoelectronic devices.[1,2,3] The gels consist of a solvent matrix that is restrained within a network of elongated bres and can be used in optoelectronic devices to give favourable physical properties such as stability, mechanical support, catalysis and self-assembly.[4]
It has been shown that triphenylene discotic liquid crystals (DLCs) can be used in organic semiconducting applications owing to high charge carrier mobility supported by the columnar morphology
Optical absorption data (Fig. 1c) con rms that the materials have high optical bandgaps >3.5 eV, so room temperature charge densities are low. It is the ordered morphology and high charge carrier mobility that makes DLCs promising for device applications

Summary

Introduction

Self-assembled physical gelators have attracted signi cant attention due to their applications as structural as well as functional materials in optoelectronic devices.[1,2,3] The gels consist of a solvent matrix that is restrained within a network of elongated bres and can be used in optoelectronic devices to give favourable physical properties such as stability, mechanical support, catalysis and self-assembly.[4]. Preliminary device lifetime measurements were performed on un-encapsulated DSSCs. In previous studies, it was demonstrated that a HAT6 gel electrolyte leads to signi cant increases in device stability.[10] Here, we compare a 50 : 50 HAT5/6 gel electrolyte with a reference liquid electrolyte, and the results are demonstrated in Fig. 3e It is observed that device lifetime is de nitely longer in the gel electrolyte, as expected.

Results

Conclusion