"Swiss Cheese" Solar Cells Use Less Silicon

A new design for thin-film solar cells that requires significantly less silicon — and may boost their efficiency — is the result of an industry and academia collaboration between Oerlikon Solar in Switzerland and the Institute of Physics' photovoltaics group at the Academy of Sciences of the Czech Republic.

One long-term option for low-cost, high-yield industrial production of solar panels from abundant raw materials can be found in amorphous silicon solar cells and microcrystalline silicon tandem cells (micromorph), providing an energy payback within a year.

A drawback to these cells, however, is that the stable panel efficiency is less than the efficiency of currently dominant crystalline wafer-based silicon, said Milan Vanecek, who heads the photovoltaics group at the Institute of Physics in Prague.

This SEM micrograph shows the nanostructured ZnO layer, Swiss cheese design for micromorph solar cells. (Image: Milan Vanecek, Institute of Physics, Prague)

"To make amorphous and microcrystalline silicon cells more stable, they're required to be very thin because of tight spacing between electrical contacts, and the resulting optical absorption isn't sufficient," Vanecek said. "They're basically planar devices. Amorphous silicon has a thickness of 200 to 300 nanometers, while microcrystalline silicon is thicker than one micrometer."

The team's new design focuses on optically thick cells that are strongly absorbing, while the distance between the electrodes remains very tight.

"Our new 3-D design of solar cells relies on the mature, robust absorber deposition technology of plasma-enhanced chemical vapor deposition, which is a technology already used for amorphous silicon-based electronics produced for liquid crystal displays. We just added a new nanostructured substrate for the deposition of the solar cell," Vanecek said.

This nanostructured substrate consists of an array of zinc oxide (ZnO) nanocolumns or, alternatively, from a "Swiss cheese" honeycomb array of microholes or nanoholes etched into the transparent conductive oxide layer.

"This latter approach proved successful for solar cell deposition," Vanecek said. "The potential of these efficiencies is estimated within the range of present multicrystalline wafer solar cells, which dominate solar cell industrial production. And the significantly lower cost of micromorph panels, with the same panel efficiency as multicrystalline silicon panels (12 to 16 percent), could boost its industrial-scale production."

The next step is a further optimization to continue improving efficiency.

For more information, visit: www.aip.org