Cells for Solar

By Lee Anna Sherman

The diatom — an ancient form of single-celled algae — may hold the key to a new generation of cheap, clean solar technology.

Teeming in Earth’s oceans for nearly 200 million years, diatoms have hard silica shells intricately patterned with tiny pinholes and latticework. These nano- and micro-scale pores are super-efficient harvesters of sunlight. Now, a team of researchers at Oregon State and Portland State universities has discovered a way to exploit the diatoms’ solar-energy prowess through “bio-mineralization.” Organisms use this process to produce hard compounds such as silicates (in shells) and carbonates (in bones), which they incorporate into their structures.

In the marine environment, where they drift in large colonies, diatoms build shells from silicon dioxide. In the lab, the microorganisms grow in photo-bioreactors, illuminated vessels for cultivating algae. Lead researcher Gregory Rorrer has coaxed them to use titanium in addition to their usual minerals. As a result, their shells contain titanium dioxide, the active semiconductor in a new type of solar cell scientists call “dye-sensitized.”
Such solar cells are made of thin films of titanium oxide nano-crystals that have been infused with light-absorbing dyes. They imitate the way algae and other plants convert sunlight into energy.

When incorporated into these films, the lab-altered diatom shells trap light at a remarkable rate. That’s because photons (particles of light) “bounce around more inside the pores of the diatom shell” than they do when striking the titanium-oxide nano-crystals alone, explains Rorrer, a professor in the OSU School of Chemical, Biological, and Environmental Engineering.

In laboratory studies, the ability of the titanium oxide-coated diatom shells to trap light tripled the conversion of light to electricity relative to titanium dioxide nano-crystals alone. Therefore, there is significant potential to enhance the efficiency of existing dye-sensitized solar cells for commercial prototypes.

Viewed under electron microscopes, diatoms reveal an astonishing array of geometric forms. Of the roughly 100,000 species, some have radial symmetry, resembling antique coins, buttons, thimbles or Swedish rosettes. Others are axial, their shapes suggesting peapods, cigars, peanut shells, hair combs, zippers and other familiar objects. Each species is uniquely patterned. As his model diatom, Rorrer used a species called Pinnularia, chosen for its nanoscale “rectangular lattice” whose “concentric array of fine features” lines the base of each pore (see photo).

Today, most solar cells are made from highly refined silicon by a complicated and expensive process. In contrast, diatoms can be easily grown on a mass scale at room temperature, needing only light, air, water and nutrients to perform chemical conversions. So far, dye-sensitized solar cells, while easier to manufacture than silicon solar cells, have lagged behind at only about a quarter of the efficiency. But the promise of semiconductor-enriched diatoms could change that.

“There is tremendous interest in bio-inspired approaches for synthesis of semiconductor and metal oxide nano-materials,” Rorrer and his team of fellow researchers noted in the American Chemical Society publication ASC Nano in 2008. This is the first reported study, the team says, of using a living organism to fabricate titanium dioxide semiconductor nano-structures that could be scaled up for potential commercial use.

The research is funded by a $1.3 million National Science Foundation grant to Rorrer, OSU chemical engineering professor Chih-hung (Alex) Chang and Jun Jiao, a physics professor who directs PSU’s Electron Microscopy and Microanalysis Facility. All are affiliated with the Oregon Nanoscience and Microtechnologies Institute (ONAMI).

See a video about Greg Rorrer’s research produced by for the Discover Channel program Brink.

For more about Rorrer’s diatom research, see this OSU news release:

Ancient Diatoms Lead to New Technology for Solar Energy, April 8, 2009

To support energy research at OSU, contact the Oregon State University Foundation.