Highly Foldable, Super-Sensitive, and Transparent Nanocellulose/Ceramic/Polymer Cover Windows for Flexible OLED Displays

Abstract

Polymer cover windows are important components of flexible OLED displays but they easily generate wrinkles because of their weak folding resistance. Increasing the polymer thickness can improve the folding resistance but it decreases the touch sensitivity. Thus, fabricating highly foldable and supersensitive polymer cover windows is still challenging. Here, by incorporating cellulose nanocrystals (CNCs) and zirconia (ZrO 2 ) into colorless polyimide (CPI), we developed a highly foldable and supersensitive hybrid cover window. Inspired by the theory of elasticity, we added rigid CNCs into CPI to improve the elastic modulus and hence the foldability. ZrO 2 was introduced to improve dielectric properties, which leads to improved touch sensitivity. After these modifications, the elastic modulus of the cover windows was increased from 1432 to 2221 MPa, whereas its dielectric constant was increased from 2.95 to 3.46 (@1 × 10 6 Hz), resulting in significantly enhanced foldability and sensitivity. Meanwhile, because of the nano size of CNCs and ZrO 2 , the hybrid cover windows exhibit excellent optical properties with the transmittance of ∼88.1%@550 nm and haze of 2.39%. With improved and balanced mechanical, dielectric, and optical properties, these hybrid cover windows overcome current cover windows' defects and could be widely used in next-generation flexible displays.

Nanocellulosics in Transient Technology

“…Flexible displays and optoelectronics such as hand-held foldable devices (for example, mobile phones) have gained considerable research and application possibilities in recent years due to their capacity for excellent deformation, flexibility in operational conditions, emission of self-illumination, high contrast ratio, lightweight, and considerable viewing angles, among others. ,, In this regard, organic light emitting diodes (OLEDs) are fabricated from greener materials such as cellulosics instead of conventional plastics (for example, polycarbonates), and ultrathin glasses have become the growing trends toward sustainable development among other benefits. ,, For example, a highly transparent and multifunctional membrane (referred to as HTBC), shown in Figure , was fabricated from bacterial cellulose incorporated with organic–inorganic sol composed of boehmite (Boe) nanoparticles and epoxy modified siloxane (GTPS), and subsequently coated with silicone oxide (SiO 2 ) and indium tin oxide (ITO) (HTBC/SiO 2 /ITO) and employed in the fabrication of biocompatible OLEDs . It was shown that more than 80% visible light transmission was achieved with a resistivity of 2.7 × 10 –4 Ω cm, a carrier concentration of −1.48 × 1021 cm –3 , and mobility of 15.2 cm 2 V –1 s –1 ; these values compare to those obtained from conventional glass-based substrates doped with ITO .…”

“…Elsewhere, challenges such as the wrinkling of conventional polymer cover windows used in foldable OLED displays, which is attributable to the weak folding resistance of these materials during operational uses, are well-known. , Although increasing the thickness of the used material (for example, colorless polyimide, CPI) for OLED polymer cover windows can improve the resistance to wrinkling that would trade-off the touch sensitivity of the OLED touchscreen panels . Hence, to overcome this challenge, inspiration is taken from the theory of elasticity, making it possible to manufacture nanocellulose/ceramic/polymer cover windows with high bendability, exceptional sensitivity, and impressive transparency for OLED screens .…”

“…Elsewhere, challenges such as the wrinkling of conventional polymer cover windows used in foldable OLED displays, which is attributable to the weak folding resistance of these materials during operational uses, are well-known. , Although increasing the thickness of the used material (for example, colorless polyimide, CPI) for OLED polymer cover windows can improve the resistance to wrinkling that would trade-off the touch sensitivity of the OLED touchscreen panels . Hence, to overcome this challenge, inspiration is taken from the theory of elasticity, making it possible to manufacture nanocellulose/ceramic/polymer cover windows with high bendability, exceptional sensitivity, and impressive transparency for OLED screens . The contributory properties of CNCs (such as good mechanical strength and transparency) and that of zirconia (ZrO 2 ), such as excellent chemical and thermal constancy, biocompatibility, optical, and electrical sensitivity, were momentous.…”

“…Through hydrolysis of the oxalic acid and subsequent mechanical treatment the CNC suspension were obtained, while the homogeneous ZrO 2 particles were synthesized via alkali hydrolysis and hydrothermal treatments, respectively. However, the polycondensation of 2,2′-bis (trifluoromethyl)-4,4′-diaminophenyl ether (6FODA) and 4,4′-(hexafluoroisopropene) diphthalic anhydride (6FDA) at low temperature resulted in the poly­(amic acid, PAA) . The ZrO 2 particles remarkably enhanced the touch sensitivity of the display, and the incorporation of the nanoparticles (CNCs and ZrO 2 ) appreciated the elastic modulus of the composite from 1432 to 2221 MPa, more than a 30% improvement.…”

“…The ZrO 2 particles remarkably enhanced the touch sensitivity of the display, and the incorporation of the nanoparticles (CNCs and ZrO 2 ) appreciated the elastic modulus of the composite from 1432 to 2221 MPa, more than a 30% improvement. The dielectric constant of the nanocomposite appreciated to 3.36 from 2.95 (@ 1 × 106 Hz), more than 10% improvement; thereby, demonstrating a significant enhancement in the material foldability and sensitivity . Another work has exemplified the benefits of nanocellulosic materials over plastics in OLEDs; although plastics can be used for fabricating OLEDs with good flexibility and transparency, and are suitable for the roll-to-roll (RTR) industrial process, they, however, demonstrate dimensional distortions during the high-temperature manufacturing process as a result of their high coefficient of thermal expansion (CTE); thereby, resulting in the difficulty in the laying down and precise alignment of the OLED functional materials, which subsequently affects the product output and optimization .…”