Solar design triumphs through thick and thin

Marie Freebody, Contributing Editor, marie.freebody@photonics.com

The quest for high power-conversion efficiency in most thin-film solar cells has often been hindered by the “thick and thin” challenge, in which a cell must be thick enough to collect a sufficient amount of light, yet thin enough to extract current. Now a novel solar cell devised by physicists at Boston College in Chestnut Hill and inspired by the coaxial cable resolves this dilemma by being both optically thick and electrically thin.

This is an electron microscope image of a nanocoax solar cell array comprising 2-μm-tall metal nanopillars conformally coated with 100 nm of amorphous silicon and 70 nm of indium tin oxide. The nanocoaxes in the array are spaced 900 nm apart (see scale bar). Images courtesy of Boston College.

“The challenge has to do with the desire to make a highly optically absorbing cell that is simultaneously highly efficient at extracting electric charge as current,” said one of the creators, Michael Naughton, a professor of physics at the college. “In simple terms, a thick cell achieves the former, while a thin cell, the latter. Our nanocoax design achieves both, which has intrinsic advantages in terms of cost, so long as good efficiency can be achieved.”

The so-called nanocoax solar cell boasts greater efficiency than any previously designed nanotech thin-film (noncrystalline) solar cell. As reported in the July 2010 issue of Physica Status Solidi, the nanocoax cells, made with amorphous silicon, yield power-conversion efficiency in excess of 8 percent, and similar cells yielded efficiencies of more than 9.6 percent.

A nanocoax solar cell array is shown at different magnifications. On the left is a completed cell array, while the middle image illustrates how a focused-ion beam is used to mill out a section of the array, exposing the constituent components. On the right is an expanded section that has been milled by a focused ion beam.

Naughton also reports that, since publication, his team – in collaboration with Solasta Inc. of Newton, Mass., and the Neuchatel Institute of Microengineering at the EPFL in Neuchatel, Switzerland – has achieved even greater initial efficiency of more than 10.5 percent, as certified by the National Renewable Energy Laboratory.

Many solar cells are based on crystalline semiconductors due to their superior energy-conversion efficiency compared with noncrystalline thin-film cells. The problem is that the dominant material (crystalline silicon) must be relatively thick to collect light. On the other hand, noncrystalline materials such as amorphous silicon are strongly absorbing but less efficient electrical conductors.

To overcome the “thick and thin” barrier, Naughton and his colleagues started by separating the optics from the electronics by forming an array of vertically oriented nanocoaxes. In each nanoscale coaxial wire in the array, an inner and outer metal surround a dielectric medium, just as in a conventional coaxial wire. The difference here is that the dielectric is a photovoltaic material such as silicon. “In such a wire, light collection is governed by the height (length) of the nanocoax, while charge (electron and hole) extraction is governed by the thinness of the photovoltaic in the coax annuli, which can be quite thin and still absorb light,” he said.

The result is a geometry that separates the “photo” from the “voltaic” and enables high energy-conversion efficiency using thinner, less expensive thin-film materials.

“The nanocoax configuration achieves efficiency more than 50 percent higher than a conventionally prepared ‘planar’ thin-film silicon cell,” Naughton said.

Solar cells of this type (amorphous silicon) face a major problem known as the Staebler-Wronski effect, in which light-induced degradation damages the conversion efficiency of the cell. The team says the ultrathin nature of the nanocoax cell reduces this destructive effect because the nanocoax architecture spatially distributes incident light in such a way that the local intensity per unit volume is less than in a planar cell.

The nanocoax concepts and the nanocoax solar cell array were invented by professors Krzysztof Kempa, Michael Naughton and Zhifeng Ren, and by Drs. Yang Wang and Jakub Rybczynski, all in the department of physics at Boston College.