Strain-resilient electrical functionality in thin-film metal electrodes using two-dimensional interlayers

Abstract

Flexible electrodes that allow electrical conductance to be maintained during mechanical deformation are required for the development of wearable electronics. However, flexible electrodes based on metal thin-films on elastomeric substrates can suffer from complete and unexpected electrical disconnection after the onset of mechanical fracture across the metal. Here we show that the strain-resilient electrical performance of thin-film metal electrodes under multimodal deformation can be enhanced by using a two-dimensional (2D) interlayer. Insertion of atomically-thin interlayers - graphene, molybdenum disulfide, or hexagonal boron nitride - induce continuous in-plane crack deflection in thin-film metal electrodes. This leads to unique electrical characteristics (termed electrical ductility) in which electrical resistance gradually increases with strain, creating extended regions of stable resistance. Our 2D-interlayer electrodes can maintain a low electrical resistance beyond a strain in which conventional metal electrodes would completely disconnect. We use the approach to create a flexible electroluminescent light emitting device with an augmented strain-resilient electrical functionality and an early-damage diagnosis capability.

Fig. 1.. Flexible metal electrode achieved by insertion of an interlayer of 2D materials.

a, Schematic illustration of a flexible metal electrode with an interlayer of atomically-thin 2D material. b, Schematic illustrations (top view) of different crack propagation modes on a metal-2D interlayer electrode (left), and on a bare thin metal film electrode (right). Inset SEM images show dominant fracture modes of deflected/multiple cracking (left) compared to straight/debonding cracking (right). c, Conceptual plots of change in resistances (R) as a function of applied strain on bare metal electrodes (black) and on metal-2D interlayer electrodes (red).

Fig. 2.. Fracture behaviours of thin film metal electrodes with 2D interlayers.

a, Crack progression at various bending strains (ε_b) in bare Au electrode (top) and Au/1LG electrode (bottom). A white arrow indicates Au film debonding from the PDMS substrate. b, Different fracture behaviours observed in bare Cu (I), Cu/1LG (II), and Cu/2LG (III) areas partitioned in one Cu-based electrode. c, Crack width on bare Au and Au/1LG electrodes as a function of bending stain. d, Fracture domain size on bare Au and Au/1LG electrodes as a function of bending stain.

Fig. 3.. Strain-resilient electrical performance with 2D interlayers.

a, Electrically ductile behaviours of multilayer-graphene integrated electrodes in response to bending deformation. Scale bar, 10 mm. b, Dependence of electrical failure strains under bending on the number of 2D-interlayers. c, Electrically ductile behaviours of multilayer-graphene integrated electrodes in response to twist deformation. Scale bar, 10 mm. d, Fatigue test of the Au/2LG electrode upon repeated bending strain (ε_b ~11%) up to 10,000 cycles.

Fig. 4.. Flexible light emitting device integrated with an electromechanically robust metal-2D interconnector.

a, Functionality of a flexible light emitting device integrated with an Au/2LG based-interconnector under bending deformation modes of tension (left) and compression (right). b, Device functionality under a twisting deformation mode. Scale bars, 2 cm. c, Normalized luminous power of flexible light emitting devices integrated with a conventional thin film metal and a metal-2D interconnector as a function of bending strain. d, Device failure strains with the conventional thin film metal interconnector and the metal-2D interconnector.