If you take a close look at a leaf from a tree and you’ll notice the veins that run through it. The structure these veins take are what’s called a quasi-fractal hierarchical networks. Fractals are geometric shapes in which each part has the same statistical character of the whole. Fractal science is used to model everything from snowflakes and the veins of leaves to crystal growth.
Now an international team of researchers led by Helmholtz-Zentrum Berlin have mimicked leaves’ quasi-fractal structure and used it to create a network of nanowires for solar cells and touch screen displays.
Indium tin oxide (ITO) has been the go-to material for transparent conductors in displays and solar cells. While the costs associated with ITO have been one of the main knocks against it, it’s been difficult for the various nanomaterials proposed as alternatives to replace it. Nanomaterials—including silver nanowires, carbon nanotubes and graphene—have not only been handicapped by their own relative high costs, but their performance has been somewhat lacking as well.
With this new method of distribution, nanowires are able to surpass the performance of traditional ITO layers. The reason for this becomes a little clearer when you go back and look at the leaf.
The distribution of veins in the leaf is determined in part by the amount of shade and sunlight the leaf receives. With ITO, the material is spread out in one continuous, uniform film. However, the way the sunlight strikes a solar cell or the way a finger presses on a touch-screen display are not uniform. This reduces the ITO layer’s efficiency.
In research described in the journal Nature Communications, the international research team used a quasi-fractal hierarchical network to optimize the distribution of the nanowires on a solar cell according to three conditions: provide maximum surface coverage, achieve a uniform current density, and have a minimal overall resistance.
“On the basis of our studies, we were able to develop an economical transparent metal electrode," Michael Giersig, a professor at Helmholtz-Zentrum Berlin and who led the research, said in a press release. “We obtain this by integrating two silver networks. One silver network is applied with a broad mesh spacing between the micron-diameter main conductors that serve as the ‘highway’ for electrons transporting electrical current over macroscopic distances.”
Next to this broad highway for the electrons, the researchers added randomly distributed nanowire networks that serve as local conductors to cover the surface between the large mesh elements.
“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,” according to Giersig.
Solar cells with the leaf-vein network had an efficiency of 5.91 percent in experiments. Those with a standard ITO had 5.37 percent.