Adhesive calcium alginate networks enable robust silver nanowire electrodes for flexible electronics

In the field of flexible electronics, devices such as photodetectors, organic field-effect transistors, and organic solar cells demonstrate vast potential for application in smart devices, wearable technologies, and portable electronics due to their unique flexibility, lightweight design, excellent adaptability to curved surfaces, and cost-effective roll-to-roll production processes. However, the absence of high-performance flexible transparent electrodes often results in flexible devices that struggle to match the performance of their rigid counterparts. Silver nanowires (AgNWs), with their remarkable conductivity, transparency, flexibility, and ease of solution processing, have emerged as ideal candidates for transparent conductive electrodes in flexible optoelectronic devices. Nevertheless, the inherent limitations of AgNWs, including high junction resistance, weak adhesion to substrates, high surface roughness, tendency to agglomerate, and insufficient stability, significantly hinder their ability to enhance device performance. To overcome these challenges, researchers have explored various strategies, including: (a) employing chemical or physical welding techniques to reduce sheet resistance; (b) coating AgNWs with additional layers (such as conductive metal oxides, two-dimensional materials, or conductive polymers) to improve surface roughness, enhance interfacial adhesion, and boost stability. However, while these strategies enhance the performance of AgNWs electrodes, they often introduce new issues, such as increased manufacturing complexity, higher costs, and potential compatibility issues with the original material properties. Additionally, some methods may adversely affect the transparency or conductivity of the electrodes, thereby impacting the overall device performance. Therefore, the current research focus lies in discovering an innovative solution that can effectively address the inherent defects of AgNWs while avoiding the introduction of new performance bottlenecks or manufacturing challenges, thereby driving the further development of flexible electronics technology.

The research team led by Professor Weiwei Li at Beijing University of Chemical Technology has innovatively proposed an efficient and direct preparation strategy aimed at creating flexible transparent electrodes with AgNWs of exceptional performance. At the heart of this strategy lies the initial treatment of the AgNWs suspension with sodium alginate (SA), followed by a refined post-treatment of the AgNWs film using an aqueous solution of calcium chloride (CaCl₂). SA, as a biomacromolecule, harnesses its abundant carboxyl (-COO^-) and hydroxyl (-OH) functional groups on its molecular chain to play multiple pivotal roles in this process. Firstly, SA can tightly adsorb onto the surface of AgNWs, either through electrostatic attraction with Ag⁺ ions or by forming stable hydrogen bonds with residual polyvinylpyrrolidone (PVP) molecules on the AgNWs surface, thereby achieving strong binding among AgNWs. This mechanism not only effectively prevents the aggregation of AgNWs but also significantly enhances the viscosity of the AgNWs suspension, ensuring uniform distribution, high coverage, and extremely low surface roughness of AgNWs in the film. Secondly, as a polymeric binder, SA forms stable hydrogen bonds with the polyethylene terephthalate (PET) substrate, vastly improving the interfacial adhesion between AgNWs and the PET substrate, thereby safeguarding the robust structure of the flexible transparent electrode. Through subsequent CaCl₂ treatment, SA undergoes a crosslinking reaction with Ca²⁺ ions, transforming the originally linear SA molecules into a three-dimensional network of calcium alginate (CA). This transformation not only markedly enhances the solvent resistance and mechanical stability of the flexible transparent electrode but also further consolidates the overall structure of the electrode. Meanwhile, with the participation of dissolved oxygen in the aqueous solution, Cl⁻ ions facilitate the redox reaction at the contact points of AgNWs, achieving chemical sintering that effectively reduces contact resistance and further optimizes the surface roughness of the electrode. Ultimately, the AgNWs@CA/PET flexible transparent electrode prepared using this strategy exhibits remarkable comprehensive performance: a root mean square (RMS) roughness as low as 8.4 nm, ensuring an extremely smooth electrode surface; a sheet resistance (R_sh) of only 8.3 Ω/cm², demonstrating exceptional electrical conductivity; and a transmittance (T₅₅₀) of up to 91.2% at a wavelength of 550 nm, preserving outstanding optical transparency. These outstanding properties lay a solid foundation for the further development and application of flexible electronic devices.

By adopting a treatment strategy of SA and CaCl₂, the team not only significantly optimized the functional characteristics of AgNWs flexible transparent conductive electrodes, but also maintained their cost effectiveness, making them an attractive material choice for large-scale industrial production. This innovative approach not only provides a strong technical support for the development of high-performance flexible electronic devices, but also opens up new possibilities for the future development of wearable devices, smart packaging, environmental monitoring, and foldable displays.

This research successfully fabricated high-performance silver nanowire FTE through a series of well-designed processing steps, including the pretreatment of AgNWs suspension with SA and the subsequent fine post-treatment of AgNWs film with CaCl2 aqueous solution. As a biomacromolecule, SA, with its abundant carboxylate and hydroxyl groups on its molecular chain, played a unique role in enhancing the performance of AgNWs FTE. Through this series of surface treatment and chemical treatment processes, not only were the electrical and optical properties of the electrodes significantly enhanced, but also their stability and mechanical durability under complex environmental conditions were greatly improved. This study not only provides a simple and efficient method for the preparation of high-performance flexible transparent electrodes, but also opens up new avenues for the performance improvement and durability enhancement of flexible electronic devices, especially flexible organic solar cells. This achievement has laid a solid foundation for the widespread application of flexible electronic devices in practical scenarios, vigorously promoting the rapid development and popularization of flexible electronics technology.