Putting All of the Sunshine to Work in Solar Cells

Hank Hogan

Despite extensive research and development, solar cells still fall woefully short of perfection. Commercially available units convert only about 15 percent of the light that falls on them into electricity. Now a team from the University of Notre Dame in Indiana, led by professor Prashant V. Kamat, has deposited quantum dots of various sizes onto TiO₂ nanotubes. The resulting device generated photocurrent with conversion efficiency conservatively estimated by the group to be at least 1 percent. Nonetheless, the technology could form the basis for new solar cells, according to Kamat.

This is an artist’s rendering of a “rainbow” solar cell. Quantum dots of various sizes absorb blue, green and red light at different depths. Shown in the inset is a rendering of the quantum dots attached to nanoscale tubes of TiO₂. Reprinted with permission of the Journal of the American Chemical Society.

“We are now working towards the construction of a rainbow solar cell that will utilize different-size quantum dots organized in an orderly fashion. Such a cell can capture different regions of sunlight through control of particle size, thus maximizing the utilization of low- and high-energy photons.”

The group has been engaged for a number of years in utilizing semiconductor nanostructures for conversion of light energy. Made out of complementary semiconductors such as cadmium and selenium, quantum dots have attracted interest for use in this application because their optical properties depend upon their size. A device constructed with the smallest quantum dots on top and with the biggest on the bottom would absorb blue, green and red at different depths, potentially leading to highly efficient conversion of light.

The fact that photons generate electron-hole pairs when they strike quantum dots is a problem. The electrons must be driven quickly toward the electrode for harvesting, but capturing them before they recombine with the holes is difficult because the small size of the quantum dots is a problem. However, anchoring the dots within a TiO₂ film helps achieve a good charge separation.

The researchers synthesized CdSe quantum dots ranging in size from 2.3 to 3.7 nm. They attached the particles to nanoscale TiO₂ particles and tubes, with the latter measuring 80 to 90 nm in diameter and approximately 8 μm in length.

They recorded the absorption and emission spectra of the quantum dots using a spectrophotometer from Shimadzu Scientific Instruments of Columbia, Md., determining emission lifetimes using a single-photon-counting system from Horiba Jobin Yvon of Edison, N.J. They found that the response of the quantum dots to light was not affected by the attachment process.

They then evaluated the construct’s photoelectrochemistry using an Oriel xenon arc lamp from Newport Corp. of Stratford, Conn., and a monochromator from Bausch and Lomb. They measured the response to various excitation wavelengths.

Kamat noted that the group did not focus on optimizing the device for efficiency and that the solar cell did not fully utilize light in the red or infrared regions of the spectrum. Because of that, he characterized the 1 percent result as encouraging. “We should be able to improve the power-conversion efficiency significantly.”

He added that the goal is to attain efficiency close to or greater than that of conventional solar cells. Achieving that could involve using other semiconductor quantum dots, such as ones made of PbSe.

Journal of the American Chemical Society, March 26, 2008, pp. 4007-4015.