f e a t u r e :
h a r v e s t i n g
Al
new solar technologies,” says Alivisatos, “because it is at that scale that photons of light must be harnessed to produce a flow of electrons.” Alivisatos’s latest solar cell is based on years of experience building nanocrystals of different sizes and shapes. In earlier research he developed a technique for growing semiconductor nanocrystals in elongated rods. Prior to that work, nanocrystals had always been grown as simple spheres. Using rod-shaped nanocrystals rather than spheres provides a directed path for electron transport to help improve solar cell performance. The newest solar cell is created by adding semiconductor nanorods of cadmium telluride to a solvent and coating the solution onto a transparent conducting substrate of indium tin oxide. After the first layer has been heated to drive off the solvent, a second layer of cadmium selenide is deposited, followed by a reflecting and conducting backing of aluminum. When excited by light, electrons migrate along the length of the nanorods to the conducting layers, where the electrons are gathered. “By varying the size of the nanoparticles, we can tune the responsiveness of the solar cells to different frequencies of light,” says Alivisatos. “Our goal is to tune the size of the nanorods to capture as much energy as possible from the Sun’s spectrum.” Although Alivisatos and colleagues have made tremendous progress, the efficiency of the new cell is low at about 3 percent, compared to the 15–20 percent efficiency of the best commercially available solar panels. Neil Fromer, a postdoctoral researcher in the Alivisatos lab and a co-author of their paper in Science magazine that describes the new solar cell, helps put this in perspective: “In many ways, conventional solar cells are based on computer chip processing technology, which has kept the cost too high for most consumers,” says Fromer. “The challenge has been to keep the relatively high efficiencies but drive the cost down. We are approaching the problem from the other end. We are making solar cells that are cheap to mass produce but relatively less efficient, and we are working to drive the efficiency up.” Another problem to overcome is the scarcity and toxicity of some of the raw materials used in alternative solar cells. “High efficiencies have been attained with cells made from a material called CIGS — copper indium gallium diselenide,” says Fromer. “One problem is that indium is rare, and there is not nearly enough of it to meet the growing demand for solar panels.” One surprise is that the new solar cell is very robust. “Some alternative solar cells are sensitive to photo-oxidation,” says Alivisatos. “Exposure to harsh sunlight degrades them. But aging seems to improve the performance of our new cell.” “Our success with nanorods and solar cells highlights one advantage of working at the nanoscale,” say Alivisatos. “You can change the properties of materials not just by changing their chemical composition, but also by changing their size. This gives you an
l i g h t
CdSe/CdTe Bilayer
Glass
ITO
CdSe
CdTe
Deposition
Spin
Dry
Deposition
Spin
Dry
How to make a nanotech solar cell Using spin deposition, layers of cadmium telluride and cadmium selenide nanoparticles are sandwiched between layers of aluminum and the transparent conductor indium tin oxide (ITO). A layer of glass is added to protect the cell. When exposed to light, current flows between the aluminum and ITO conductors. Solar cells like these have the potential to be inexpensive and durable.
Add Electrical Contacts
added dimension to your toolbox when it comes to creating new materials and devices. “I see nanoscience not as a specific science, but as a scientific movement, a way of disciplines working together,” concludes Alivisatos. “It’s been exciting to apply these new techniques to an important problem, and I hope our research will help lead to the day when stable and low-cost solar power panels cover rooftops everywhere.”
1nm = 10-9m
How big is a nanometer? One meter is 39.37 inches, about 10 percent longer than a yard. A nanometer is one-billionth of a meter. A human hair is 100,000 nanometers thick. E. coli bacteria are about 250 nanometers wide and 1000 – 2000 nanometers long. The wavelength of visible light ranges from 400 to 700 nanometers. A typical capsid of the tobacco mosaic virus measures 18 by 300 nanometers. The nanorods in solar cells devised by the Alivisatos group are 5 by 30 nanometers. It takes 20 hydrogen atoms, but only 3 uranium atoms, to span one nanometer. A nanometer is equal to 10 angstrom units. Fall 2006 Catalyst
13