From the Lab: Nanotechnology

Nanowire Solar Cells
Building photovoltaics out of nanowires

Results: In a step toward cheaper and more efficient solar cells, researchers from the University of California, Berkeley, have made solar cells out of billions of nanowires, each wire about 60 nanometers in diameter and 20 micrometers in length. The nanowires, made of zinc oxide and coated in a light-absorbing dye, conducted electrons from one end of the cell to the other about 100 times more efficiently than other nanoparticle-based solar cells currently under development. The solar cells’ overall light-conversion efficiency, however, was a relatively poor 1.5 percent.

Why It Matters: Silicon-based solar cells are expensive to make. Replacing the silicon with nanomaterials promises to lower costs. But the sunlight conversion efficiency of nano solar cells is typically low, mainly because electrons have to find their way to the external circuit by hopping between nanoparticles within the cell. Some electrons get lost along the way, leading to low light conversion efficiency. By replacing the nanoparticles with long single-crystal nanowires that run between the cell’s electrodes, the researchers were able to get the electrons moving through the solar cell more efficiently. This is an important advance that could ultimately lead to more-efficient nano solar cells.

Methods: The researchers, led by chemist Peidong Yang, made nanowire arrays by coating a conductive glass surface with zinc oxide “dots” three to four nanometers in diameter. The dots served as seeds for the subsequent growth of the wires. Yang’s team then immersed the glass in a solution of zinc oxide for 2.5 hours. A polymer in the solution controlled the rate and direction of the wires’ growth, ensuring that they remained perpendicular to the surface of the glass. The researchers dipped the array in a dye solution, placed the array between two electrodes, and filled the internal space with a liquid electrolyte. They then shone light with the same spectrum as sunlight onto the cells and measured the electrical output.

Next Step: Although the cells’ electron transport was better, their overall light conversion efficiency was low compared to that of some nanoparticle-based solar cells (which have achieved efficiencies of up to 10 percent). Zinc oxide harvests electrons from the dye less efficiently than does titanium dioxide – a material more commonly used in nano solar cells. The researchers are now making their nanowires out of titanium dioxide, a more challenging manufacturing process. The nanowires also have a smaller surface area than a network of nanoparticles, so they carry less light-absorbing dye. The researchers are consequently shrinking their nanowires to 10 nanometers in diameter so that they can fit more nanowires onto their arrays and increase the total surface area. Yang predicts that with thinner and more numerous titanium wires, his team will be able to achieve a conversion efficiency of 10 percent or more, which could make these nano solar cells a viable source of energy.

Source: Law, M., et al. 2005. Nanowire dye-sensitized solar cells. Nature Materials 4:455-459.

Making Microcapsules
Tiny chemical carriers form themselves

Results: Chemical engineers have developed a simple “mix and shake” technique for producing microcapsules – tiny shells that can hold substances such as drugs and medical imaging dyes. The technique, developed by a team at Rice University led by Michael Wong, resulted in microcapsules measuring less than a micrometer across.

Why It Matters: With microcapsules, researchers can more precisely control where, when, and in what quantities a substance is delivered and released. One current production method, which relies on meticulously depositing a coating onto a core that is then dissolved away, has produced stable microcapsules a few micrometers across. But this method is expensive to use because it requires carefully controlled conditions, such as very low pressures and high temperatures, and harsh chemicals. The Rice method works at room temperature and atmospheric pressure, and uses water as a solvent. It’s also simpler than other methods and potentially cheaper to use on a large scale.

Methods: The Rice recipe for microcapsules begins with a mixture of water and the chemical to be contained in the shells, such as a small-molecule drug. The researchers then add a salt and an organic polymer that, when mixed, form water-permeable globules. Next, the researchers pour in silicon dioxide nanoparticles about 100 times smaller than the globules. The particles stick into the walls of the globules, forming capsules that trap the chemical and water mixture inside. By adjusting the mixing intensity and the quantities and types of salts, polymers, and nanoparticles used, the researchers varied the thickness of the capsule walls and the size of the capsules, changing the timing and rate of the release of the contents.

Next Step: This method produces only grams of the shells at a time, and they are of nonuniform size. The team is now working on ways to produce the capsules by mixing the components in the form of individual streams of liquid. This would enable the continuous production of more consistently sized capsules.

Source: Rana, R., et al. 2005. Nanoparticle self-assembly of hierarchically ordered microcapsule structures. Advanced Materials 17:1145-1150.