Highly Transparent and Colorless Nanocellulose/Polyimide Substrates with Enhanced Thermal and Mechanical Properties for Flexible OLED Displays

Flexible organic light-emitting diode (OLED) displays have attracted worldwide attention and colorless polyimides (CPIs) are their key substrate materials. However, desirable CPIs are difficult to obtain since the thermal and mechanical properties are sacrificed during CPI production through modification of colored polyimide. Here, a cellulose nanocrystal (CNC)/CPI hybrid substrate with high optical, mechanical, and thermal properties is introduced. Due to the outstanding mechanical and thermal properties of CNCs as well as their strong interfacial interaction with CPI matrix, the sacrificed properties are made up and hybrid substrate is demonstrated strikingly improved thermal properties and mechanical properties with thermal decomposition temperature of 555 degrees C, upper operating temperature of 320 degrees C, glass transition temperature of 289 degrees C, coefficient of thermal expansion of 31.62 ppm K-1, tensile strength of 128 MPa, elastic modulus of 3.72 GPa, and folding capacity of 160 000 times. Particularly, the substrate keeps an excellent transmittance of 86% at 600 nm and it is colorless. The OLED devices built on the hybrid substrates show outstanding performance, which is superior to that of OLED@CPI, and comparable to that of OLED@glass. It is expected that this work will open new avenues for fabricating high-performance and low-cost flexible OLED devices.