Ammar A. Khan

Graphene and chiral nematic liquid crystals: a focus on lasing

This work presents the interaction of self-assembled liquid crystalline (LC) unidimensional photonic structures on the surface of polycrystalline graphene. Further, this surface effect is studied through different substrate geometries in the test devices. Primarily, these devices are characterised through polarizing optical microscopy (POM) and their laser emission features in the dye-doped chiral systems. Then the conductive nature of graphene is utilized to apply external electric fields to the photonic medium and its effect is envisaged. These graphene-based devices demonstrate a unique result in polarizing optical micrographs and electro-optic responses which indicates the presence of multidirectional domain formations. Additionally, the LC band-edge lasing from graphene cells is found to be anisotropic and depends on the directionality of the optical pump. This work attempts to lay the foundation for the implementation of a new class of defused chiral nematic liquid crystal based devices e.g. optical filters, notch filters, colour reflectors, and light shutters and may add toward the knowledge base necessary in the substitution of Indium Tin Oxide (ITO) with graphene in traditional LC based display devices.

Plasmonic nanohole electrodes for active color tunable liquid crystal transmissive pixels

Plasmonic pixels have been shown to offer numerous advantages over pigment-based color filters used in modern commercial liquid crystal (LC) displays. However, wideband dynamic tunability across the visible spectrum remains challenging. We experimentally demonstrate transmissive electrically tunable LC-nanohole pixels operating across the visible spectrum with unpolarized input light. An ultrathin Al nanohole electrode is designed to exhibit a polarized spectral response based on surface plasmon resonances. An output analyzer in combination with a nematic LC layer enables pixel color to be electronically controlled through an applied voltage across the device, where LC reorientation leads to tunable mixing of the relative contributions from the plasmonic color input. The nanostructured Al layer, acting as a combined electrode, polarizer, and functional color filter, is highly promising for electro-optic display applications.

Composite liquid crystal-polymer electrolytes in dye-sensitised solar cells: effects of mesophase alkyl chain length

ABSTRACT The doping of polymer electrolytes (PEs) with liquid crystal (LC) materials has been shown to improve the performance of dye-sensitised solar cells (DSSCs). This is achieved by promoting ionic conduction and increasing optical path length through multiple-light scattering within the photovoltaic devices. In LCs, it is well known that the length of the alkyl chain plays an important role since the LC morphology and mesophase stabilisation depend strongly on the alkyl group. In this work, liquid crystal-polymer composite electrolytes (LC-PEs) are prepared using nematic LCs with different alkyl chain lengths. The morphology of the LC-PEs is investigated and correlated with their electrical properties. Subsequently, DSSCs are prepared using the LC-PEs as a direct example of its application. It is shown that increasing the alkyl chain length of the LCs reduces the efficiency of the solar devices. The longer alkyl chains are speculated to intertwine, thus trapping the mobile ions and reducing the bulk ionic conductivity. For the same reason, longer alkyl chain LCs are thought to be unable to passivate the TiO2 surface through the adsorption of cyanobiphenyl groups and hence the higher probability of back recombination reaction between the electrons in TiO2 and PE. Graphical Abstract

Induced alignment of a reactive mesogen-based polymer electrolyte for dye-sensitised solar cells

Liquid crystalline materials are interesting organic molecules possessing anisotropic behaviour. The materials undergo self-assembly forming highly ordered structures, which from the opto-electronic applications point of view, have a promising future. By controlling the functionality or the mesophase of the liquid crystal materials, it is possible to develop specific device architectures. So far, controlling the morphologies of organic materials for electronic applications has proven to be difficult. Here, we prepared a liquid crystal-based polymer template using a polymer alignment layer and electric field. The resultant morphology is closely related to the fabrication technique which can be further modified to suit particular device applications. The mesophase characteristics and morphologies of these materials are characterised using polarising optical microscopy, atomic force microscopy and scanning electron microscopy. Next, we utilised these polymer electrolytes in dye-sensitised solar cells as a potential application. Device performance such as open-circuit voltage, short-circuit current, fill-factor and power conversion efficiencies also showed strong dependence on the structure of the polymer scaffold. Hierarchical polymer electrolyte structures were prepared using a reactive mesogen assisted by Smectic A liquid crystals. The morphology of these hierarchical structures was controlled by the use of alignment layers on the substrate or by applying electric fields. The highest power conversion efficiency achieved was 5.02% in cells with electric field induced alignment, as compared to 4.57% for the polyimide aligned sample. This can be attributed to the higher porosity in the case of the electric field aligned sample whereas for the polyimide aligned sample, despite having more ordered pores, the width between the pores is comparatively smaller.

Effect of dielectric permittivity on the performance of polymer dispersed liquid crystal (PDLC) electrolyte dye-sensitized solar cells (DSSCs)

Dye-sensitized solar cells (DSSCs) are a type of organic solar cell often cited for their high efficiency and easy fabrication. Recent studies have shown that modification of the standard liquid electrolyte DSSC architecture by the changing one of the components or the addition of additives often results in the improvement in one of the photovoltaic parameters and hence the overall efficiency. Here we explore a dielectric liquid crystal material which is a known insulator but possesses a high degree of order and optical anisotropy. In the presence of an applied electric field, the equilibrium of positive and negative charges are displaced in opposite directions. In this work, different mixtures with different dielectric anisotropies ranging from negative, zero and positive are formulated. These mixtures are then used to prepare polymer dispersed liquid crystal (PDLC) electrolytes and subsequently DSSC devices based on these PDLC electrolytes are fabricated. The morphology of the PDLC is observed through polarizing optical microscopy (POM) and the electrical/photovoltaic characterizations are performed through current density-voltage (J-V) measurements and electrochemical impedance spectroscopy.

Research data supporting "Homologous binary mixtures and improved hole conduction of self-assembled discotic liquid crystals"

Raw data, corresponding to each figure in the paper has been included. This includes data of all the various characterizations that were performed during the research. Excel files summarizing each figure have been included.