A generic approach for aligning liquid crystals using solution-processed 2D materials on ITO-free surfaces

Abstract

Conducting and alignment layers are critical for industrial scale manufacture of quality liquid crystal (LC) devices. To date, the conventional approaches utilize ITO-based electrodes as the conducting layer and rubbed polyimide as the aligning layer. However, usage of ITO has issues like scarcity of indium reserves, high cost, and toxicity, whereas employing rubbed polyimide demands high-temperature baking and mechanical rubbing that causes debris generation and electrostatic charges. Although offering a new outlook, CVD-based 2D materials provided a non-traditional route for fabricating the alignment layers. Furthermore, the previous studies are restricted to ITO-based conducting substrates and also calamitic molecules. However, from the utility point of view, the uniform alignment realized over only small areas necessitates alternative approaches to fabricate interfacial layers on non-ITO-based substrates utilizing solution-processed routes and deploying non-calamitic LCs also. Here, a generalized approach is presented to address the above-mentioned issues by considering an aluminium-doped zinc oxide (AZO) conducting layer and a hexagonal boron nitride (h-BN) alignment layer deposited via the electrophoretic deposition technique. Upon fabrication of LC devices using h-BN/AZO, large-area (>3 cm²) unidirectional alignment of LC molecules is observed. The method works equally effectively for bent-core molecules also. The alignment quality is established by azimuthal angle dependence of transmission and birefringence intensity analysis. Several critical parameters were found to be comparable between h-BN/AZO and PI/ITO devices; like operating voltage (∼1 V), switching response (in ms), image retention aspect, and ionic parameters (e.g., ionic concentration on the order of 10¹⁹ m⁻³). This study underscores the potential of h-BN/AZO as a promising candidate for display technologies.