Polymer-graphene composite with variable modulus for flexible display applications

Abstract

Flexible displays consist of multilayered structure, and the individual layeres have to be able to withstand significant bending strain. It is important, therefore, to engineer the multilayer structure such that minimal strain is applied to the device by controlling the neutral plane

close the layer of interest is to the neutral plane, the samller imposed the bending strain. One method in changing the position of the neutral plane is to choose materials with different modulus or by varying the thicknesses of the layers. In this study, we explore the usage of polymer-graphene composite with different concentrations of graphene flakes to systematically vary the modulus in attempt to control the modulus to the desirable values. In addition, graphene is known to have low permeation level to gases or water molecules that has the potential to enhance the barrier properties of encapsulation in organic light emitting diodes (OLEDs). Therefore, the effect of graphene flake contents in the poly(methyl methacrylate) (PMMA) on the gas transition rate to $O_2$ and water molecules was also investigated for PMMA/graphene composites. Different contents of graphene flakes from 0 wt% to 1.0 wt% were mixed to PMMA solution, which is then spin-coated on the polycarbonate (PC) substrates. Nanoindentation for the PMMA/graphene composites revealed that increasing the graphene flake contents in PMMA led to the enhancement of mechanical properties of PMMA/graphene composites. The hardness and Young’s modulus of PMMA with 1.0 wt% graphene flake was 0.21 GPa and 7.6 GPa, respectively, which is ~2.0 times larger than those of PMMA without graphene. The increased Young’s modulus resulted in the shift of neutral plane toward organic emissive layers by $~1.2 \mum$, which in turn, reduced the applied strain to the organic layer, thereby enhancing the mechanical reliability of OLEDs. Furthermore, gas permeability tests to $O_2$ and water molecules indicated that the gas transition rate of PMMA/graphene composites decreased as the contents of graphene in PMMA increased. Randomly mixed graphene in the PMMA efficiently block the $O_2$ and water molecules to transit through PMMA/graphene composites, thereby leading to the enhanced barrier property.