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GERMAN researchers are using silicon micro-funnels inspired by mammalian eyes to improve the efficiency of solar cells.
In a classic case of bio-mimicry, the researchers from the Institute for Nanoarchitectures for Energy Conversion at the Helmholtz-Zentrum Berlin (HZB) and the Max Planck Institute for the Science of Light (MPL) were inspired by the fovea centralis, which sits in the middle of the macula of the retina.
The fovea consists of slender, funnel-like ocular cones that are closely packed together. This structure means this small region provides the greatest acuity, since each cone is connected to a nerve cell.
The researchers designed an inorganic counterpart based on these cones, for use in solar cells. They used conventional semiconductor processes to etch micron-sized vertical funnels shoulder to shoulder in a silicon substrate.
They then used mathematical models and experiments to test how the funnel arrays collect incident light and conduct it to the active layer of a silicon solar cell. They found that the arrangement of funnels increases photo absorption by about 65 percent in a thin-film solar cell, compared with a silicon film of the same thickness, representing a considerable increase in solar cell efficiency.
While the researchers already knew that arrangements of very thin vertical cylinders, in the form of silicon nanowires, can absorb light well. However, they were surprised at the improvements that came with changing the shape of the cylinders into funnels.
Manufacturing the light funnels requires no special effort and can be done with conventional semiconductor processes such as reactive ion etching or wet chemical etching.
The researchers also found the reason for the improved light trapping capabilities: optical modes in nanowires mutually interfere with each other. A field of closely arrayed nanowires therefore takes in light less efficiently than an identical number of single nanowires could. With light funnels, neighbouring light funnels mutually strengthen one another’s absorption.
The researchers will now work on further improving thin-film solar cells based on silicon, with a few of building funnels into robust cell designs that can be economically realised over large surfaces.