25 Years of Light-Emitting Electrochemical Cells: A Flexible and Stretchable Perspective.

Light-emitting electrochemical cells (LECs) are simple electroluminescent devices comprising an emissive material containing mobile ions sandwiched between two electrodes. The operating mechanism of the LEC involves both ionic and electronic transport, distinguishing it from its more well-known cousin, the organic light-emitting diode (OLED). While OLEDs have become a leading player in commercial displays, LECs have flourished in academic research due to the simple device architecture and unique features of its operating mechanism, inviting exploration of new materials and fabrication strategies. These explorations have brought LECs to an exciting frontier in advanced optoelectronics: flexible and stretchable light-emitting devices. Flexible and stretchable LECs are discussed herein, presenting the LEC system as a robust and fault-tolerant development platform. The engineering of emissive composites is highlighted to control mechanical properties, and how the tolerance of LECs to electrode work function and roughness has enabled the incorporation of new electrode materials to achieve flexibility and stretchability. As part of this story, the solution processability of LECs has led to exciting demonstrations of flexible and printed LECs. An outlook is provided for LECs that builds on these strengths, potentially leading to flexible, stretchable, low-cost devices such as illuminated tags, smart packaging, flexible signage, and wearable illumination.

Patterned, Flexible, and Stretchable Silver Nanowire/Polymer Composite Films as Transparent Conductive Electrodes.

The emergence of flexible and stretchable optoelectronics has motivated the development of new transparent conductive electrodes (TCEs) to replace conventional brittle indium tin oxide. For modern optoelectronics, these new TCEs should possess six key characteristics: low cost, solution-based processing; high transparency; high electrical conductivity; a smooth surface; mechanical flexibility or stretchability; and scalable, low-cost patterning methods. Among many materials currently being studied, silver nanowires (AgNWs) are one of the most promising, with studies demonstrating AgNW films and composites that exhibit each of the key requirements. However, AgNW-based TCEs reported to date typically fulfill two or three requirements at the same time, and rare are examples of TCEs that fulfill all six requirements simultaneously. Here, we present a straightforward method to fabricate AgNW/polymer composite films that meet all six requirements simultaneously. Our fabrication process embeds a AgNW network patterned using a solution-based wetting-dewetting protocol into a flexible or stretchable polymer, which is then adhered to an elastomeric poly(dimethylsiloxane) substrate. The resulting patterned AgNW/polymer films exhibit ∼85% transmittance with an average sheet resistance of ∼15 Ω/sq, a smooth surface (a root-mean-square surface roughness value of ∼22 nm), and also withstand up to 71% bending strain or 70% stretching strain. We demonstrate the use of these new TCEs in flexible and stretchable alternating current electroluminescent devices that emit light to 20% bending strain and 60% stretching strain.

Developing the Surface Chemistry of Transparent Butyl Rubber for Impermeable Stretchable Electronics.

Transparent butyl rubber is a new elastomer that has the potential to revolutionize stretchable electronics due to its intrinsically low gas permeability. Encapsulating organic electronic materials and devices with transparent butyl rubber protects them from problematic degradation due to oxygen and moisture, preventing premature device failure and enabling the fabrication of stretchable organic electronic devices with practical lifetimes. Here, we report a methodology to alter the surface chemistry of transparent butyl rubber to advance this material from acting as a simple device encapsulant to functioning as a substrate primed for direct device fabrication on its surface. We demonstrate a combination of plasma and chemical treatment to deposit a hydrophilic silicate layer on the transparent butyl rubber surface to create a new layered composite that combines Si-OH surface chemistry with the favorable gas-barrier properties of bulk transparent butyl rubber. We demonstrate that these surface Si-OH groups react with organosilanes to form self-assembled monolayers necessary for the deposition of electronic materials, and furthermore demonstrate the fabrication of stretchable gold wires using nanotransfer printing of gold films onto transparent butyl rubber modified with a thiol-terminated self-assembled monolayer. The surface modification of transparent butyl rubber establishes this material as an important new elastomer for stretchable electronics and opens the way to robust, stretchable devices.

A Self-Assembled, Low-Cost, Microstructured Layer for Extremely Stretchable Gold Films.

We demonstrate a simple, low-cost, and green approach to deposit a microstructured coating on the silicone elastomer polydimethylsiloxane (PDMS) that can be coated with gold to produce highly stretchable and conductive films. The microstructured coating is fabricated using an aqueous emulsion of poly(vinyl acetate) (PVAc): common, commercially available white glue. The aqueous glue emulsion self-assembles on the PDMS surface to generate clustered PVAc globules, which can be conformally coated with gold. The microstructured surface provides numerous defect sites that localize strain when the structure is stretched, resulting in the initiation of numerous microcracks. As the structure is further elongated, the microcracks interact with one another, preventing long-range crack propagation and thus preserving the conduction pathway. The resistance of PDMS/glue/gold structures remains remarkably low (23 times the initial resistance) up to 65% elongation, making these structure useful as stretchable interconnects. Decreasing the concentration of the PVAc aqueous emulsion reduces the density of defect sites of the microstructure, which increases the change in resistance of the gold films with stretching. In this way, we can tune the resistance changes of the PDMS/glue/gold structures and increase their sensitivity to strain. We demonstrate the use of these structures as wearable, soft strain sensors.

Silver nanowire/optical adhesive coatings as transparent electrodes for flexible electronics.

We present new flexible, transparent, and conductive coatings composed of an annealed silver nanowire network embedded in a polyurethane optical adhesive. These coatings can be applied to rigid glass substrates as well as to flexible polyethylene terephthalate (PET) plastic and elastomeric polydimethylsiloxane (PDMS) substrates to produce highly flexible transparent conductive electrodes. The coatings are as conductive and transparent as indium tin oxide (ITO) films on glass, but they remain conductive at high bending strains and are more durable to marring and scratching than ITO. Coatings on PDMS withstand up to 76% tensile strain and 250 bending cycles of 15% strain with a negligible increase in electrical resistance. Since the silver nanowire network is embedded at the surface of the optical adhesive, these coatings also provide a smooth surface (root mean squared surface roughness<10 nm), making them suitable as transparent conducting electrodes in flexible light-emitting electrochemical cells. These devices continue to emit light even while being bent to radii as low as 1.5 mm and perform as well as unstrained devices after 20 bending cycles of 25% tensile strain.

Stretchable light-emitting electrochemical cells using an elastomeric emissive material.

Dispersing an ionic transition metal complex into an elastomeric matrix enables the fabrication of intrinsically stretchable light-emitting devices that possess large emission areas (∼175 mm(2)) and tolerate linear strains up to 27% and repetitive cycles of 15% strain. This work demonstrates the suitability of this approach to new applications in conformable lighting that require uniform, diffuse light emission over large areas.