Research Promises Cheap Solar Power

A technician inspecting thin-film solar material/

Photo: Energy Conversion Devices Ovonics / AP Photo

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Combining two cutting-edge nanotech breakthroughs could drive down the cost of solar power and make it even more efficient, according to Jim Zhang, an author of new research in the Journal of Physical Chemistry.

Thin-film solar technology, which uses nano-scale metal oxides "doped" with other elements, like nitrogen, to increase the conversion of sunlight to electricity, has received a lot of press lately. Another promising technique uses nanosize crystals called quantum dots that sensitize metal oxide films to visible light.

Zhang's team combined the two techniques: a thin film doped with nitrogen and sensitized with quantum dots. The results exceeded expectations: There seemed to be unexplained gains in efficiency that could not be accounted for by just adding the benefits of the two techniques together.

"We have discovered a new strategy that could be very useful for enhancing the photo response and conversion efficiency of solar cells based on nanomaterials," Zhang, a professor at the University of California-Santa Cruz, said in a prepared statement. "We initially thought that the best we might do is get results as good as the sum of the two, and maybe if we didn't make this right, we'd get something worse. But surprisingly, these materials were much better."

Here's how UC-Santa Cruz described the research:

Zhang's team characterized the new nanocomposite material using a broad range of tools, including atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and photoelectrochemistry techniques. They prepared films with thicknesses between 150 and 1100 nanometers, with titanium dioxide particles that had an average size of 100 nanometers. They doped the titanium dioxide lattice with nitrogen atoms. To this thin film, they chemically linked quantum dots made of cadmium selenide for sensitization.

The resulting hybrid material offered a combination of advantages. Nitrogen doping allowed the material to absorb a broad range of light energy, including energy from the visible region of the electromagnetic spectrum. The quantum dots also enhanced visible light absorption and boosted the photocurrent and power conversion of the material.

When compared with materials that were just doped with nitrogen or just embedded with cadmium selenide quantum dots, the nanocomposite showed higher performance, as measured by the "incident photon to current conversion efficiency" (IPCE), the team reported. The nanocomposite's IPCE was as much as three times greater than the sum of the IPCEs for the two other materials, Zhang said.

The nanocomposite material could be used not only to enhance solar cells, but also to serve as part of other energy technologies. One of Zhang's long-term goals is to marry a highly efficient solar cell with a state-of-the-art photoelectrochemical cell. Such a device could, in theory, use energy generated from sunlight to split water and produce hydrogen fuel The nanocomposite material could also potentially be useful in devices for converting carbon dioxide into hydrocarbon fuels, such as methane.

In essence, the team has been trying to manipulate materials so that when sunlight strikes them, the free electrons generated can easily move from one energy level to another--or jump across the different materials – and be efficiently converted to electricity.

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