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.