Photo-patternable and transparent films using cellulose nanofibers for stretchable origami electronics

Abstract

Substantial progress in flexible or stretchable electronics over the past decade has extensively impacted various technologies such as wearable devices, displays and automotive electronics for smart cars. An important challenge is the reliability of these deformable devices under thermal stress. Different coefficients of thermal expansion (CTE) between plastic substrates and the device components, which include multiple inorganic layers of metals or ceramics, induce thermal stress in the devices during fabrication processes or long-term operations with repetitions of thermal cyclic loading–unloading, leading to device failure and reliability degradation. Here, we report an unconventional approach to form photo-patternable, transparent cellulose nanofiber (CNF) hybrid films as flexible and stretchable substrates to improve device reliability using simultaneous electrospinning and spraying. The electrospun polymeric backbones and sprayed CNF fillers enable the resulting hybrid structure to be photolithographically patternable as a negative photoresist and thermally and mechanically stable, presenting outstanding optical transparency and low CTE. We also formed stretchable origami substrates using the CNF hybrid that are composed of rigid support fixtures and elastomeric joints, exploiting the photo-patternability. A demonstration of transparent organic light-emitting diodes and touchscreen panels on the hybrid film suggests its potential for use in next-generation electronics. A substrate containing transparent nanofibres makes it easier for flexible electronic devices to combat the effects of thermal stress. Plastic substrates respond differently to hot and cold conditions than inorganic circuits do, and this mismatch often leads to premature failure of stretchable devices. Jang-Ung Park and co-workers in South Korea developed an alternative substrate by electrospinning a three-dimensional web of epoxy fibres while simultaneously spraying cellulose-based nanofibres into the polymer's pores. The nanofibre—epoxy hybrid substrate had a significantly lower thermal coefficient of expansion and an improved mechanical stability, and it was compatible with photolithographic patterning. The team demonstrated the potential of this approach with three prototypes: a flexible touch-screen panel, a transparent organic light-emitting diode, and ‘origami’-type substrates that can be reversibly folded and formed into arbitrary shapes. Photo-patternable and transparent cellulose hybrid films as flexible and stretchable substrates exhibit outstanding mechanical, optical and thermal properties. Exploiting the photo-patternability of the hybrid film, reversibly foldable and stretchable forms of the substrate can be produced by integrating patterns of this hybrid film with elastomeric joints. Flexible touchscreen panels and transparent organic light-emitting diodes using hybrid films are demonstrated.