Multi-Functional Film Based on Aligned CNT Sheet for Liquid Crystal Display Application
Authors: Lee Gwanyong , Park Hyoung Soo , Lee Hong Gwon , Lee Junhyeok , Kim Tae Hyung , Lee Seung Hee 
Published: December 2020

Carbon nanotube (CNT), an extremely long hollow cylinder with nanometer-sized diameter, is a promising carbon material with unique thermal, electrical, and physical properties. Due to their peculiar and fascinating properties, CNTs have been studied and used in various industries since their discovery. The CNTs are potentially viable alternatives for conventional materials in electric devices considering their mechanical resilience, high electrical conductivity, and optical transparency. The alignment of CNTs into ordered structures garnered a special interest as it enhances electrical conductivity and optical transparency, and generates a new intriguing anisotropic optical absorption property. The optical properties of highly oriented CNT sheets have been investigated to apply as optical polarizer and polarized light source. Further, the flexible electrodes derived from an aligned CNTs have unveiled applications in super-capacitors, solar cell, and lithium-ion battery. Therefore, to utilize CNTs in LCD and many other flexible electronic device applications it is essential to attain highly ordered CNTs which should be static even under external stresses such as bending, pressing, and strain.

In this work, we fabricated the aligned CNT sheet films (a-CSFs) in which the grown CNTs are stretched unidirectionally on the substrate and its orientation is fixed via polymerization of a coated monomer and then measured its optical and mechanical properties, testing feasibility of its application for multi-functioning films such as polarizer, electrode, and liquid crystal (LC) alignment. The fabricated a-CSF exhibits polarization efficiency (PE) (87.23% of 10 layers of a-CSF), sheet resistance (361.7 Ω/sq), and an excellent aligning capability of LC molecules as shown in Fig. 1 (a~c). Besides, flexibility and stability test of a-CSF via repeating bending test of 1,000 cycles at bending radius of 2 mm exhibited very low variation in the change ratio of sheet resistance and transmittance less than 8% and 10%, respectively.

Utlilizing the mutifunctionalty of a-CSF, a liquid crystal display (LCD) with a twisted nematic(TN) mode was fabricated and exhibited feasibility of dynamic swicthing between bright and dark states on application of voltage off/on even without using conventional components of LCD such as transparent electrode, film-type polarizer, and polyimide-type alignment layer. We measured the voltage-dependent transmittance of both devices based on the a-CSF with 5 layers and 8 layers. The threshold voltage (Vth) and operating voltage (Vop) are defined as the voltage required to achieve 90% and 10% of transmittance relative to maximum transmittance, respectively. The measured Vth of both devices with 5 and 8 layers are 1.7 Vrms and 1.3 Vrms, respectively and the measured Vop of both devices are 7.6 Vrms and 4.3 Vrms, respectively. In the case of 8 layers, it shows slightly lower Vth and Vop, which is mainly due to the lower sheet resistance of a-CSF with 8 layers than 5 layers as shown Fig 2.

In conclusion, we fabricated the multi-functional a-CSF with properties of the electrode, polarizer, and alignment layer based on the a-CSF using the simple fabrication process. We confirmed the a-CSFs exhibit better conductivity and PE in the visible regime. Furthermore, it has the excellent grooved morphology along the drawn direction of a-CS for homogeneous alignment of LC molecules. We carried out the bending test to confirm the stability of the films from mechanical stress. It showed excellent stability after bending tests of 1,000 cycles. The performance of the fabricated TN cells using a-CSFs were considerably working good.

Figure 1