Discotic Liquid Crystals for Self-organizing Photovoltaics

Abstract

Development of photovoltaic solar cells using organic materials as active components is an emerging area of contemporary research endeavor toward sustainable energy harvesting. Among the organic optoelectronic materials, disc-shaped polycyclic aromatic compounds capable of exhibiting columnar liquid crystal (LC) properties are unique. Liquid crystalline materials simultaneously display the anisotropic properties of crystalline solids and flow properties of isotropic liquids. The order and dynamics of the molecules of liquid crystalline materials make them stimuli responsive smart soft materials. LCs are ubiquitous in our daily life as the active switching materials in flat panel display such as the screens of laptops, computers, cameras and mobile phones. Interestingly, the nematic LC phase of disc-like molecules has been used to fabricate optical compensation films which enable enlarging the viewing angle of LC display devices. However the columnar LC phase of disc-like compounds has been found to possess remarkable semiconducting properties. Like other LCs, they show promising characteristics such as controllable alignment and self-healing of structural defects which render them appealing candidates for use in organic semiconducting devices. In this chapter, we present the state-of-the-art of organic photovoltaics employing discotic LCs as active components as well as facilitating additives in the active layer of photovoltaic devices. Various discotic cores such as phthalocyanine, porphyrin, hexabenzocoronene, triphenylene, perylene diimide and others with different electronic properties have been used in organic photovoltaics. The discotic LCs act as electron and/or hole transporting materials in the active layer, which have been tested both in bilayer and bulk heterojunction device configurations. Though this research enterprise is in early stage, with adequate attention these fascinating self-organizing materials could contribute toward the sustainable energy harvesting from solar radiation in future.

Charge Mobility in Discotic Liquid Crystals

“…The first observation of the photovoltaic effect in DLC was reported by Gregg et al [ 218 ], who observed the photovoltaic properties of a phthalocyanine derivative in symmetric cells. A review of the results obtained using DLC in the field of OSC can be found in reference [ 219 ], while the newest results will be presented here. Bajpai and coworkers studied how the presence of a layer of the 2,3,6,7,10,11-hexabutyloxytriphenylene mesogen can influence the performance of the classic bulk-heterojunction OSC formed by poly (3-hexylthiophene) and [ 6 , 6 ]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) [ 220 , 221 ].…”

Self-assembled discotics as molecular semiconductors

“…In such a scenario, organic electronic materials have an upper hand over their inorganic counterparts in terms of reduced synthetic costs as well as reduced toxicity. With organic light-emitting diodes (OLEDs) already flourishing in the market, 1,2 and organic photovoltaics (OPVs), sensors and organic field-effect transistors (OFETs) gradually emerging as the next generation of technology, the need for lightweight and flexible devices for the global populace is becoming evident with each passing day. The effective operations of all these devices rely on the charge transporting abilities of organic semiconductors (OSCs) as the key functional parameter.…”

Spectroscopic and Spectroelectrochemical Studies of Hexapentyloxytriphenylene—A Model Discotic Molecule

“…Functional groups, such as azobenzene [ 10 ] or other conjugated moieties [ 11 , 12 ], can be introduced for tailoring of the surface or bulk properties of triphenylene-based materials. Besides the extensive studies on the photophysical [ 13 ] and charge transport properties [ 14 ] of H n Ts, which are crucial for their implementation in organic light-emitting diodes (OLEDs) [ 15 ], organic field-effect transistors (OFETs) [ 16 ], or organic photovoltaics (OPVs) [ 17 ], several aspects connected with molecular order and thin film engineering have been investigated. Due to the liquid-like properties of DLCs, a large amount of studies are concentrated on the self-assembly of H n Ts in thin films [ 18 ], under geometrical confinement [ 19 ], or the growth of nanostructures of different dimensionality [ 20 ].…”

“…Since the fabrication of organic active layers for optoelectronic devices preferably involves solution processing, e.g., spin coating [ 16 , 17 , 18 ], knowledge about the behavior of molecules in solution is highly desirable. The aggregation has a great influence on the electronic properties of solutions of polycyclic aromatic molecules, since functionalities often arise due to electronic coupling between π -conjugated systems [ 24 ].…”