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New stretchable electrode enables high-performance flexible electronics
(Nanowerk Spotlight) Flexible electronics have emerged as a transformative technology, promising to revolutionize how we interact with devices and integrate them into our daily lives. The ability to bend, stretch, and conform to irregular surfaces opens up possibilities for wearable health monitors, foldable displays, and energy-harvesting systems that seamlessly blend with the human body or adapt to dynamic environments. However, creating electronic components that can withstand repeated deformation while maintaining consistent performance has posed significant challenges.
A critical bottleneck in the development of flexible and stretchable electronics has been the electrode - the conductive layer that carries electrical signals. Traditional electrode materials like indium tin oxide are brittle and crack under strain, rendering devices non-functional. Researchers have explored various approaches to create stretchable electrodes, including patterning metal films into serpentine shapes or embedding conductive nanoparticles in elastic polymers. Yet these methods often involve complex fabrication processes or trade-offs between stretchability and electrical conductivity.
Recent advances in nanomaterials have reinvigorated the pursuit of high-performance stretchable electrodes. Silver nanowires, in particular, have garnered attention for their excellent conductivity and mechanical flexibility. When properly integrated with elastomeric substrates, silver nanowire networks can maintain electrical pathways even when stretched. However, challenges remain in achieving robust adhesion between the nanowires and substrate, as well as protecting the nanowires from environmental degradation.
Against this backdrop, a team of researchers has developed a novel and straightforward method for fabricating stretchable electrodes with silver nanowires embedded in a thermoplastic elastomer. Their approach, detailed in a paper published in Advanced Functional Materials ("Intrinsically Stretchable Organic Solar Cells and Sensors Enabled by Extensible Composite Electrodes"), offers a promising path toward scalable production of highly stretchable and durable electrodes for next-generation flexible electronics.
stretchable electrode
a) Schematic diagram of the preparation method of the new stretchable electrode (i.e., Strem-AT) in this work. b) The application of Strem-AT in stretchable strain sensor. c) The application of Strem-AT in intrinsically stretchable organic photovoltaics. d) The relative resistance change rate (ΔR/R0) with the cyclic stretching of strain sensor at 100% strain for 100 time. e) Efficiency retention statistics of intrinsically stretchable organic solar cells in cyclic stretching under different strains. f) Statistics of the strain at PCE80% of intrinsically stretchable organic solar cells. (Image: Reproduced with permission by Wiley-VCH Verlag) (click on image to enlarge)
The researchers' method involves a spray-transfer technique where a solution of silver nanowires is sprayed onto a heated glass substrate, followed by pouring a liquid thermoplastic polyurethane (TPU) solution over the nanowire layer. The silver nanowires are shallowly embedded within the TPU matrix, which is crucial for enhancing both the mechanical stability and electrical performance of the electrodes. As the TPU cures, it partially envelops the silver nanowires, creating a composite structure where the conductive network is shallowly embedded within the elastomer surface.
This embedding strategy offers several key advantages, including improved electrical stability by protecting the nanowires from mechanical damage and environmental degradation, which are common issues when nanowires are simply deposited on top of an elastic substrate.
By integrating the nanowires into the TPU matrix, the electrode exhibits excellent mechanical stability under repeated deformation. When stretched, the elastic polymer absorbs much of the strain, reducing stress on the conductive nanowire network. This allows the electrode to maintain consistent electrical performance even when subjected to 100% strain over hundreds of cycles. The embedding also protects the silver nanowires from environmental factors like oxidation that can degrade conductivity over time.
Importantly, the fabrication process is relatively simple and amenable to large-scale production. Unlike some previous approaches that rely on complex lithography or transfer printing steps, this spray-coating and pouring method can be readily scaled up to create large-area flexible electrodes. The researchers also optimized the solvent composition and curing conditions to achieve uniform films without defects.
To demonstrate the versatility of their stretchable electrode, dubbed "Strem-AT," the team incorporated it into two key applications: a wearable strain sensor and a flexible organic solar cell. As a strain sensor attached to various body joints, the electrode could accurately detect and quantify complex motions. Its high stretchability allowed it to conform to skin and maintain stable electrical signals even during large deformations like bending an elbow or knee.
Perhaps even more impressively, the researchers used the Strem-AT as the bottom electrode in a fully stretchable organic solar cell. This intrinsically stretchable photovoltaic device achieved a power conversion efficiency exceeding 12.5% - among the highest reported for flexible organic solar cells. Crucially, it retained over 80% of its initial efficiency when stretched to 51% strain. Even after 1000 cycles of 50% strain, the solar cell maintained 76% of its original performance. This combination of high efficiency and mechanical durability represents a significant step forward for flexible energy harvesting devices.
The enhanced stability stems from the embedded electrode structure, which provides a smooth surface for depositing subsequent device layers while protecting the conductive network. This allows the entire solar cell stack to deform cohesively without delamination or cracking that would severely degrade performance.
While promising, some limitations and areas for further improvement remain. The researchers noted that incorporating silver nanowires slightly reduced the elasticity of the TPU substrate compared to the pure polymer. Additionally, maintaining consistent conductivity after thousands of deformation cycles remains a challenge, indicating the need for further optimization of nanowire concentration and embedding depth.
There may be room to optimize the nanowire concentration and embedding depth to balance conductivity and stretchability. Additionally, while more durable than surface-deposited nanowires, the embedded network still experienced some damage at very high strains or after thousands of deformation cycles.
Nevertheless, this work represents an important advance in stretchable electronics, providing a straightforward yet effective strategy for fabricating high-performance flexible electrodes. The simplicity of the process, combined with the impressive mechanical and electrical properties achieved, makes it a promising candidate for scaling up to industrial production. This method addresses efficiency issues found in other techniques, such as spin-coating, making it more suitable for large-scale applications in flexible electronics.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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