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A Renewed Interest A new ORNL process seeks to improve solar cells. Research priorities are indeed cyclical. In the 1980s, Oak Ridge National Laboratory was a leader in the development of solar-cell fabrication techniques. The work's focus was laser processing, using light from a pulsed high-power laser instead of high-temperature heat to create solar cells. For a brief time in 1986 under Richard Wood's leadership, ORNL's 19.5% single-crystal silicon solar cell fabricated with laser processing held the world record for efficiency in converting sunlight's energy into electrical energy.
After 1987, the programmatic direction of the Department of Energy, and subsequently ORNL, changed. The Laboratory abandoned research on photovoltaics, or PV. Twenty years later, the challenges of the changing energy sector are putting ORNL back into the PV business. DOE's Solar America Initiative recognizes that materials researchers at ORNL and other national labs could help industry raise solar-cell efficiencies and lower the costs of manufacturing PV materials and devices. As a result, ORNL researchers are encouraged to request funding for solar projects. Through a project sponsored by ORNL's Laboratory Directed Research and Development program, researchers led by Ron Ott demonstrated that pulsed thermal processing using the Laboratory's plasma arc lamp could increase the collection efficiency of thin-film amorphous silicon solar cells by introducing a nanocrystalline microstructure within the amorphous material. As a result of this successful LDRD-funded collaboration with a leading manufacturer of thin-film amorphous silicon solar cells, the ORNL researchers received funding from the Defense Advanced Research Projects Agency. Their mission for DARPA is to help develop methods to increase solar-cell efficiencies on polymer substrates, reduce fabrication costs and improve productivity. This type of thin-film solar cell is fabricated using amorphous silicon-germanium alloys consisting of three layers. Amorphous, or noncrystalline, materials are considerably cheaper to manufacture but are less efficient than standard crystalline silicon solar cells. The top layer is pure amorphous silicon, while the underlying layers consist of different silicon-germanium alloys. Each layer absorbs a different portion of the solar wavelength range. The amorphous silicon-germanium layers are deposited on a metal substrate along with a transparent conducting layer that forms the topside electrical contact, as well as an anti-reflective coating that reduces the amount of light reflected from the cell. The entire device is placed under a protective cover layer. All the cells are then connected to make a PV module. The Oak Ridge team believes the solar cells' efficiency might be improved by changing the amorphous silicon to nanocrystalline silicon. To explore this idea in the DARPA project, Ott will "flash" amorphous silicon thin-film specimens with the plasma arc lamp. "We have shown that pulsed thermal processing will initiate solid-phase crystallization, which will introduce a nanocrystalline structure with fewer defects and higher efficiencies," says Ott, ORNL's program manager for solar energy technologies. "Our process will produce a purer structure faster than what an in-situ, deposition process can provide." ORNL seeks funding from DOE to work with Georgia Institute of Technology on improving the university's process for increasing the efficiency of multicrystalline silicon solar cells. Georgia Tech researchers add to multicrystalline ribbon—grown silicon cells a hydrogen—containing silicon nitride antireflective layer. The researchers have tried using currently available rapid thermal annealing to drive the hydrogen from the antireflective layer into the multicrystalline silicon to "passivate," or reduce, defects and increase electron travel distances. "They found their process cannot heat silicon nitride to high enough temperatures fast enough to force the hydrogen into the multicrystalline silicon," Ott says. "Pulsed thermal processing has higher heating rates and higher processing temperatures that should take the material to the next level in efficiency." The findings are encouraging enough to warrant further funding. To characterize these new PV materials, ORNL has invested in the Center for Advanced Thin-film Solar Cells, managed by Jay Jellison. The center will house several instruments to measure solar-cell efficiency, spectral response, thin-film thickness and other parameters used to evaluate the recordbreaking potential of PV materials.—Carolyn Krause Contact: Ronald D. Ott
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