Using an Interlayer to Toughen Flexible Colorless Polyimide-Based Cover Windows

Colorless polyimide (PI)-based flexible cover windows are a critical component of flexible electronics to protect devices from unwanted chemical and mechanical damage. The integration of flexible colorless PI-based windows into electronics applications is limited by the embrittlement of some colorless PI films when they are coated with hard coats. Here, we investigate the embrittlement mechanism of hard-coated colorless PI films and the role of interlayers in toughening the colorless PI-based cover windows for flexible electronics applications. A fracture mechanics approach combined with finite element analysis (FEA) models is employed to compute fracture strain, epsilon(c), for different crack cases in the bilayer (hard coated colorless PI) and trilayer (with an additional interlayer) cover windows. For the model inputs and validation, the material properties of the cover windows are characterized. We show that the embrittlement is attributed to the fracture behavior of the cover windows, and placing a ductile interlayer increases the epsilon(c) of colorless PI films. Using the fracture analysis as a design guide, we fabricate a trilayer cover window with an acrylic thermoset interlayer and demonstrate an improvement of the epsilon(c) of the colorless PI cover window by similar to 42%. We believe our analysis provides insights into design guides for mechanically robust cover windows using colorless PI films and flexible HCs for emerging flexible electronics.