More light into solar cells? Tiny funnels can help

Silicon is widely used for solar cells and other devices. But if silicon as a bulk material does not have big secrets any more, nanostructured silicon remains to be explored.  “With nanoarchitectures, we can change optical and electronical properties sometimes dramatically, it is like designing a material with new properties out of the same elemental building blocks”, physicist Sebastian Schmitt explains. Especially light absorption, one of the most relevant properties for solar cells, can be greatly enhanced by tiny structures. The young scientist and his colleague Gil Shalev, both working in the team of Silke Christiansen, created an array of densely packed, tiny silicon funnels, which, according to simulations, absorbs 65 percent more light than a silicon-layer of comparable thickness and which reminds of a biological structure in the mammalian eye.

It was in fact only in hindsight that they were reminded of a biological counterpart: densely packed, slender, funnel-like ocular cones are sitting also in a small region of the fovea centralis, in the middle of the retina. It is in this small region that we see an image with greatest acuity, because each cone there is connected to a nerve cell. Of course, the inorganic silicon structure cannot be directly compared with the organic structure in the eye. But it does, as its biological paragon, an amazing job in absorbing light of different frequencies.

The scanning electron microscopies (SEM) show how regularly the funnels etched in a silicon substrate are arranged (left: the line segment = 5 microns; right: 1 micron). The funnels measure about 800 nanometres in diameter above and run down to about a hundred nanometres at the tip. Credit: S. Schmitt / MPL

The scanning electron microscopies (SEM) show how regularly the funnels etched in a silicon substrate are arranged (left: the line segment = 5 microns; right: 1 micron). The funnels measure about 800 nanometres in diameter above and run down to about a hundred nanometres at the tip. Credit: S. Schmitt / MPL

Small change – large effect!

“Nearly for one year, I was working every day with Gil on this project,” Sebastian recalls. Both physicists were surprised at just how large the effect of this new architecture was, however. It was known from previous studies that arrangements of very thin vertical cylinders (a “carpet” of silicon nanowires) absorb light well. But even tiny deviations in the shape of the cylinders right down to the shape of a funnel increase absorption further. In comparison to the carpet of nanowires, the funnel fields clearly perform better.

And now, Gil and Sebastian can even tell us why!  Within a computer simulation they modeled the propagation of light in different silicon nanostructures, either nanowires some hundreds of nanometers thick or funnels, whose thickness varied from 1000 to only 100 nanometers. “Our modelling shows that 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. Just the opposite occurs with the light funnels: closely packed light funnels mutually strengthen one another’s absorption”, explains Sebastian.

The simulation shows how the concentration of light (red = high concentration, yellow= low concentration) rises in the funnels with declining diameter of the lower end of the funnel. Credit: G. Shalev, S. Schmitt/MPL

The simulation shows how the concentration of light (red = high concentration, yellow= low concentration) rises in the funnels with declining diameter of the lower end of the funnel. Credit: G. Shalev, S. Schmitt/MPL

Processing requires nothing special

Yet manufacturing such talented light funnels requires no special effort and is feasible with conventional semiconductor processes. The silicon wafers were structured by so called nanosphere lithography and reactive ion etching, which are quite conventional processes in semiconductor technology. First, they deposited a monolayer of polystyrene nanospheres of 1 μm in diameter on the silicon substrate. Then they used reactive ion-etching-techniques to remove material below the nanospheres. Finally, the polystyrene nanospheres were removed in an ultrasonic bath. Thus, a dense array of micron-sized vertical funnels remained, shoulder-to-shoulder in a silicon substrate. The simulations indicate that compared with a silicon film of the same thickness, a layer of the light funnels increases the absorption of sunlight about 65%.

A look into the future

“Following these interesting initial results, we are pressing ahead in various directions”, says Silke Christiansen, head of the team. She now plans to develop robust cell designs with arrays of microfunnels that can be economically realised over large surfaces. To this purpose, they can count on experts of PVcomB, the photovoltaics competence Center in Berlin at Helmholtz-Zentrum Berlin, who have lots of experience in upscaling tiny lab structures to the 30×30 centimeter devices required by the industry. Sebastian Schmitt wants also to explore funnel structures for other photonic applications and sensor components, though. “If we can use a structure for converting light into electric energy, this structure could in principle also convert electric energy into light, such as in an LED or lasers” Schmitt explains.

Notes

Prof. Dr. Silke Christiansen heads the Institute for Nanoarchitectures for Energy Conversion at the Helmholtz-Zentrum Berlin (HZB) and leads as well a team at Max-Planck-Institut for the Science of Light (MPL), where Sebastian Schmitt does his PhD-project and Gil Shalev has been a Postdoc.

The results have been published in Scientific Reports.


Antonia RötgerAntonia Rötger is a science writer working at Helmholtz-Zentrum Berlin in the communication department. She has a PhD in physics and left the lab in order to report what other scientists are doing and why. During the last 20 years she was writing about nearly everything else but physics, now she is back to the roots and enjoys meeting physicists trying to understand materials which may contribute to a more sustainable

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