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 (ZrO2) 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. ZrO2 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 × 106 Hz), resulting in significantly enhanced foldability and sensitivity. Meanwhile, because of the nano size of CNCs and ZrO2, 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.
Keywords: cellulose nanocrystals; colorless polyimide; cover window; flexible OLED displays; zirconia.