January 9, 2008

New Nanostructured Thin Film Shows Promise For Efficient Solar Energy Conversion

Many researchers and start-up companies are relying on new designs that exploit nanostructures (materials produced on a scale of a billionth of a meter) in a race to make solar cells cheaper and more efficient. Using nanotechnology, researchers can test and control how a material generates, captures, transports, and stores free electrons.

There are two unique and hopefully promising methods using nanotechnology for engineering solar cell materials. One uses thin films of metal oxide nanoparticles, such as titanium dioxide, doped with other elements, such as nitrogen. The other method employs quantum dots (nano-size crystals) that strongly absorb visible light. These minute semiconductors inject electrons into a metal oxide film to increase solar energy conversion. Both methods extend the visible light absorption of metal oxide materials.

According to professor of chemistry Jin Zhang of the University of California, Santa Cruz, combining both methods appears to yield better solar cell materials than either one does alone. Zhang led a field of researchers from California, Mexico, and China that created a thin film doped with nitrogen and sensitized with quantum dots. When tested, the new nanocomposite material worked better than predicted.

Zhang stated that "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." He went on to say, "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."

The Journal of Physical Chemistry reported the group's findings in a paper posted online on January 4th. Lead author of the paper was Tzarara Lopez-Luke, a graduate student visiting in Zhang's lab who is now at the Instituto de Investigaciones Metalurgicas, UMSNH, Morelia, Mexico.

The group characterized the new nanocomposite material using an extensive range of tools, including atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and photoelectrochemistry techniques.

They readied 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 to materials that were either just doped with nitrogen or just embedded with quantum dots, the nanocomposite showed higher performance, as measured by the "incident photon to current conversion efficiency" (IPCE), the field of researchers reported. The nanocomposite's IPCE was as much as three times greater than the sum of the IPCEs for the two other materials.

Zhang went on to explain that "We think what's happening is that it's easier for the charge to hop around in the material. That can only happen if you have both the quantum dot sensitizing and the nitrogen doping at the same time."

One of Zhang's long-term goals is to unite 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. The new strategy for engineering solar cell materials offers a promising path for Zhang's lab to explore for years to come.

"I'm very excited because this work is preliminary and there's a lot of optimizing we can do now," Zhang noted. "We have three materials--or three parameters--that we can play with to make the energy levels just right."

The team of researchers has been trying to manipulate materials so that when sunlight hits them, the free electrons generated can easily move from one energy level to another, or jump across materials, and be converted into electricity efficiently.

"What we're doing is essentially 'band-gap engineering.' We're manipulating the energy levels of the nanocomposite material so the electrons can work more efficiently for electricity generation," Zhang said. "If our model is correct, we're making a good case for this kind of strategy."