Laser Patterning Raises Soft Electronics' Operation Possibilities

Using a laser ablation patterning technique, researchers from the Korea Institute of Industrial Technology and Pohang University of Science and Technology (POSTECH) fabricated deformable microsupercapacitors (MSCs) for storing energy in soft electronic devices. The work addresses the need for energy storage systems in emerging stretchable devices for health monitoring and other applications.

MSCs offer customizable form factors, reliable performance, efficient use of space, and easy integration with electronic components, making them strong contenders for filling this need. However, the solid metals that are typically used to collect the current in MSCs, like gold, have limited stretchability, restricting deformation potential.

To fabricate an MSC capable of bending and stretching without breaking or losing functionality, the researchers selected a liquid metal, eutectic gallium-indium alloy (EGaIn), as the current collector. EGaIn has high conductivity and, due to its liquid nature, is easily deformable. However, fabricating high-density interdigitated patterns with EGaIn to ensure high energy storage performance is a challenge.

The highly deformable microsupercapacitor (MSC) is based on a liquid metal current collector. The researchers used laser ablation patterning to achieve a high level of capacitance and flexibility in the MSC. Courtesy of the Korea Institute of Industrial Technology.

To shape the EGaln, the researchers used graphene as the material for the electrode and polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene copolymer (SEBS) as the material for a flexible substrate. They fabricated EGaIn film with a uniform thickness on the SEBS substrate using a brushing method. They coated graphene onto the EGaIn film and fabricated an interdigitated graphene-EGaIn electrode using laser ablation.

The researchers optimized the laser ablation process to ensure that the complete ablation of both the graphene and the EGaIn occurred without damaging the SEBS. During laser irradiation, the graphene and EGaIn film absorbed the laser light at a wavelength of 355 nm, while the SEBS material did not absorb any light. Thus, the team achieved selective ablation of the graphene-EGaIn electrode without losing the flexibility of the SEBS substrate.

The energy storage performance of MSCs depends on the areal density of the interdigitated electrodes. Ideally, the gap between neighboring interdigitated electrodes should be minimized for as long as the electrodes maintain mechanical and electrical stability under deformation.

By carefully controlling the gap between neighboring interdigitated electrodes and the mass loading of graphene, the researchers achieved a high areal capacitance of 1336 µF cm^-2 with reliable rate performance. The researchers were able to stretch and shrink the MSC for 1000 cycles without impairing energy storage performance.

To demonstrate the feasibility of using the MSC as a deformable power source, the researchers fabricated a soft electronic system comprising serial and parallel connected MSC arrays integrated with LEDs. Because of the liquid characteristics of the EGaIn current collector and the flexibility of the SEBS substrate, the MSC demonstrated a strong energy storage performance, and the soft electronic system demonstrated stable operation under various mechanical deformations, including folding, stretching, wrinkling, and twisting.

The intense energy of the laser used to pattern EGaIn enabled the liquid metal to be used effectively as a current collector for the MSC. “Laser technology allows for precise work while also speeding up the process,” researcher Chanwoo Yang said.

The laser ablation technology could be used to pattern various electrode materials, including carbon materials, metal oxides, and Mxene, for deformable, high-performance energy storage systems.

As micro- and optoelectronics move beyond foldable and rollable devices and into the era of stretchable devices for electronic textiles, healthcare, and other applications, it is essential to develop small, elastic energy storage devices, like the deformable MSC created using laser ablation technology. “This research will be useful in various industrial fields and contribute to the development and commercialization of elastic energy storage devices,” Yang said.

The research was published in npj Flexible Electronics (www.doi.org/10.1038/s41528-024-00306-2).