Novel embedded metal-mesh transparent electrodes : vacuum-free fabrication strategies and applications in flexible electronic devices

Abstract

This dissertation reports a new structure of flexible transparent electrodes featuring a metal mesh fully embedded and mechanically anchored in a flexible substrate, termed an “embedded metal-mesh transparent electrode (EMTE)”. In addition, this thesis presents cost-effective and vacuum-free fabrication strategies for this novel transparent electrode, and its applications in flexible electronic devices.The embedded nature of EMTE provides a series of advantages, including surface smoothness which is crucial for device fabrication, mechanical stability under high bending stress, strong adhesion to the substrate with excellent flexibility, and favorable resistance against moisture, oxygen, and chemicals. Our novel fabrication techniques are solution-processed and vacuum-free and therefore are potentially suitable for high-throughput, large-volume and low-cost production. In particular, these strategies enable fabrication of high-aspect-ratio (thickness to linewidth) metal meshes, substantially improving conductivity without considerably sacrificing transparency.

Our first vacuum-free fabrication strategy combines lithography, electroplating, and imprint transfer (LEIT) for making EMTEs. We utilized this technique to fabricate various prototype flexible micro-EMTEs and flexible nano-EMTEs with transmittance higher than 90% and sheet resistance below 1 ohm/sq, as well as extremely high figures of merit (ratio of electrical conductivity to optical conductivity) up to 1.5 × 104, which are among the highest reported values in recent studies. Although LEIT is a cost-effective approach for fabrication of EMTEs, yet it comprises a mandatory lithography step in making each sample which limits its suitability for high-throughput and large-volume industrial production and needs further simplifications.

Therefore, we demonstrate improved techniques based on templated electrodeposition for fabrication of EMTEs by eliminating the obligatory lithography step from the unit-production cycle of LEIT. In these templated electrodeposition and imprint transfer (TEIT) strategies, reusable templates are employed to simplify the fabrication of the EMTEs. Similarly, based on these improved techniques, prototype micro-EMTEs and nano-EMTEs are fabricated on flexible substrates, demonstrating excellent electrical and optical performances. The mechanical robustness of the reusable templates are also tested by utilizing them for repeated fabrication cycles.

As practical applications, the EMTEs are utilized in flexible bifacial dye-sensitized solar cells (DSSCs) and flexible transparent thin-film heaters (FTTHs). A novel counter electrode (CE) is developed for DSSCs, containing a micro-EMTE with catalytic platinum nanoparticles (PtNPs) in-situ electrodeposited only on the surface of the nickel mesh without considerably reducing its optical transparency. The flexible bifacial DSSCs based on this hybrid PtNP-decorated micro-EMTE demonstrate remarkable power conversion efficiencies (PCEs) both under front-side illumination and rear-side illumination. Furthermore based on EMTEs, a FTTH is fabricated and characterized. This device shows a rapid response, requires a low input power density, and operates at ultra-low voltage. These promising results reveal the enormous potential of our EMTEs in production and commercialization of low-cost and efficient flexible electronic devices.