Highly Transparent, Colorless Optical Film with Outstanding Mechanical Strength and Folding Reliability Using Mismatched Charge-Transfer Complex Intensification

The development of flexible displays and electronics is hindered by the disadvantages of polymeric window films, such as low mechanical strength, poor optical properties, and lack of folding and rolling reliability. To overcome these critical limitations, a novel colorless polyimide window film is prepared in this study using the concept of charge-transfer complex (CTC) intensification. The resulting window film has a tensile modulus of 8.4 GPa, total transmittance of ≈90%, and yellow index below 3, which is the best recorded balance between mechanical strength and optical properties for a highly flexible optical film. Unlike commercially available optical-grade engineering plastic films and glass substrates, the prepared window film has both pencil hardness grade over 2H and folding reliability over 200 000 folding/unfolding cycles. These remarkable properties are attributed to the unique supramolecular structure with multiple hydrogen bonding and salt complexation interactions, which exhibits CTC intensification. The CTC intensification mechanism is also proposed in this study.