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Combining these two approaches appears to yield better solar cell
materials than either one alone does, according to Jin Zhang,
professor of chemistry at the University of California, Santa Cruz.
Zhang led a team 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 performed
better than predicted - as if the functioning
of the whole material was greater than the sum of its two individual
components.
"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," said Zhang.
"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 group's findings were reported in the Journal of Physical
Chemistry in a paper posted online on January 4, 2008.
Lead author of the paper was Tzarara Lopez-Luke, a graduate student
visiting in Zheng's lab who is now at the Instituto de Investigaciones
Metalurgicas, UMSNH, Morelia, Mexico.
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.
"We think what's happening is that it's easier for the charge to hop
around in the material," he explained. "That can only happen if you
have both the quantum dot sensitizing and the nitrogen doping at the
same time."
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 (see below). 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."
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.
"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."
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