The optimal networks for photovoltaic“The research clarifies that increasing the order number, that is the number of sub-networks at smaller scales and not the details of the branching, is key for improving performance of these transparent electrodes”Krzysztof Kempa
When it comes to research into nanotechnology applications for energy materials, there is always much to learn. However, a new study by a team of international scientists has used nature to inspire them to develop electrodes with quasi-fractal nano-features similar to the veins on leaves. They showed how grids of metallic mesh with fractal-like nano-features, structures similar to the networks of leaf veins, could expand upon other metallic networks in terms of utility, optimizing the performance of electrodes for a range of applications.
The networks combine a minimal amount of surface coverage with ultra-low total resistance, while at the same time maintaining uniform current density and improving on the performance of standard indium tin oxide layers. When tested on artificially constructed electrode networks with different topologies, the researchers, whose work was published in Nature Communications [Han et al. Nat. Commun. (2016) DOI: 10.1038/ncomms12825], found that non-periodic hierarchical organization showed less resistance and also significant optical transmittance compared to periodic organization. This led to increased output power for photovoltaic components.
The team produced their economical transparent metal electrode by integrating two silver networks, one applied with a broad mesh spacing between the micron-diameter main conductors, acting as a path for electrons transporting electrical current over macroscopic distances, the other extra nano-wire networks randomly distributed to act as local conductors covering the surface between the large mesh elements.
As team leader Michael Giersig said, “These smaller networks act as regional roadways beside the highways to randomize the directions and strengths of the local currents, and also create refraction effects to improve transparency above that of classical shadow-limited performance”. He claimed that solar cells based on such electrodes demonstrate exceptional high efficiencies, while the work also pioneers the use of fractal plasmonics for improving the networks, as it permits light to flow around the wires of the network, increasing transparency.
As Krzysztof Kempa points out, “The research clarifies that increasing the order number, that is the number of sub-networks at smaller scales and not the details of the branching, is key for improving performance of these transparent electrodes”.
The study helps to pave the way for the design of ultra-efficient, high coverage, multi-order, transparent electrodes for displays, solar cells and smart windows, and especially high-power LED light sources, as they are prone to thermal losses because of insufficient electrode conductivity. In addition, such electrodes would be able to implement all the effects discovered in the team’s work to date, such as the plasmonic enhancement of network efficiency.