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Topic Name: Researchers can experiment important properties for the conversion of sunlight into electricity Using nanotechnology

Category: Nanocharacterization

Research persons: Jin Zhang

Location: University of California, Santa Cruz, United States

Details

In the race to make solar cells cheaper and more efficient, many researchers and start-up companies are betting on new designs that exploit nanostructures--materials engineered on the scale of a billionth of a meter. Using nanotechnology, researchers can experiment with and control how a material generates, captures, transports, and stores free electrons--properties that are important for the conversion of sunlight into electricity.

Two nanotech methods for engineering solar cell materials have shown particular promise. One uses thin films of metal oxide nanoparticles, such as titanium dioxide, doped with other elements, such as nitrogen. Another strategy employs quantum dots--nanosize crystals--that strongly absorb visible light. These tiny semiconductors inject electrons into a metal oxide film, or "sensitize" it, to increase solar energy conversion. Both doping and quantum dot sensitization extend the visible light absorption of the metal oxide materials.

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. 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. 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."

Note for Solar Cell
A solar cell or photovoltaic cell is a device that converts light energy into electrical energy by the photovoltaic effect. Photovoltaics is the field of technology and research related to the application of solar cells as solar energy. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight, while the term photovoltaic cell is used when the source is unspecified.
A solar cell fulfills only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity (simply put, carrying electrons off through a metal contact into a wire or other circuit). Solar cells have many applications. Individual cells are used for powering small devices such as electronic calculators. Assemblies of cells are used to make solar modules, which may in turn be linked in photovoltaic arrays. 

Note for Quantum Dot
A quantum dot is made from a semiconductor nanostructure that confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), the presence of an interface between different semiconductor materials (e.g. in core-shell nanocrystal systems), the presence of the semiconductor surface (e.g. semiconductor nanocrystal), or a combination of these. A quantum dot has a discrete quantized energy spectrum. The corresponding wave functions are spatially localized within the quantum dot, but extend over many periods of the crystal lattice. A quantum dot contains a small finite number (of the order of 1–100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.

About Atomic Force Microscope
The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction limit. The precursor to the AFM, the scanning tunneling microscope, was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s, a development that earned them the Nobel Prize for Physics in 1986. Binnig, Quate and Gerber invented the first AFM in 1986. The AFM is one of the foremost tools for imaging, measuring and manipulating matter at the nanoscale. The term 'microscope' in the name is actually a misnomer because it implies looking, while in fact the information is gathered by "feeling" the surface with a mechanical probe.

About Atomic Force Microscope
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through it. An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen, a fluorescent screen in most TEMs, plus a monitor, or on a layer of photographic film, or to be detected by a sensor such as a CCD camera. The first practical transmission electron microscope was built by Albert Prebus and James Hillier at the University of Toronto in 1938 using concepts developed earlier by Max Knoll and Ernst Ruska.

Sources of funding for this research included the U.S. Department of Energy, the National Science Foundation of China, and the University of California Institute for Mexico and the United States (UC-MEXUS).

Research collaborators included Abraham Wolcott, Li-ping Xu and Shaowei Chen at UCSC; Zhenhai Wen and Jinghong Li at Tsinghua University in Beijing, China; and Elder De La Rosa of the Centro de Investigaciones en Optica, A.C., in Leon, Guanajuato, Mexico.