Stimuli directed alignment of self-organized one-dimensional semiconducting columnar liquid crystal nanostructures for organic electronics

Abstract

Electronic devices are ubiquitous and inorganic semiconductors generally serve as their active components. Recently, organic semiconductors, i.e., conjugated polymers, oligomers and small molecules with delocalized π-electron clouds, have been employed in electronic and optoelectronic devices and their performances are being constantly improved. In this context, the columnar phases of discotic liquid crystals have been investigated as a new generation of organic semiconductors in organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs), organic photovoltaic (OPV) solar cells, etc. These self-organizing and soft materials possess the necessary charge carrier mobility and anisotropic conduction characteristics for employment in semiconductor devices, however little control over the desired molecular orientation in device architectures has been a major roadblock in the exploration of the full potential of these fascinating functional materials. Since spontaneous alignment capability of these intriguing materials is poor, various methods and techniques have been developed to direct their molecular orientation with suitable configuration using different stimuli to demonstrate their optimal performance in device structures. Accordingly, a variety of physical and chemical methods have been used to obtain highly ordered columns with desired orientation on single substrates as open supported films and in-between two substrates as confined flat films. Moreover, their controlled organizations in micro and nano grooves, trenches and pores have also been demonstrated. This article deals with the different methods involving various stimuli used for the large area alignment control of discotic columnar phases both parallel (uniaxial planar) and perpendicular (homeotropic) to the substrates. Various strategies utilizing one or more stimulus to control the alignment have been described. The applications of achieved alignment control over the supramolecular columnar nanostructures in the fabrication optoelectronic devices have been highlighted. The article concludes with a brief perspective on the challenges and opportunities in this area of research and development involving these intriguing self-healing materials.