Method for developing high-efficiency solar cells using carbon nanotubes

Method for developing high-efficiency solar cells using carbon nanotubes

Cornell University researchers have created the basic elements of highly efficient solar cells, which hold great promise for converting sunlight into electricity in a more efficient way, using carbon nanotubes instead of traditional silicon technology.

Researchers have built, tested and demonstrated a simple solar cell called a photodiode, which is formed from a single carbon nanotube. The research was supported by the Cornell NanoSystems Center and the Cornell Nanoscale Science and Technology Laboratory, as well as the National Science Foundation and Advanced Materials, Structures, Devices and Microelectronics Research. The study, led by Paul McEuen, Cornell Goldwin Smith Professor of Physics, and Jiwoong Park, assistant professor of chemistry and biochemistry, describes how the device can convert light energy into electricity very efficiently by increasing the number of charge carriers. This approach demonstrates the importance of the next generation of high-efficiency solar cells.

"We're not just studying new materials, we're actually putting them to use to create real solar cell devices," said Nathan Gabor, a graduate student in McEuen's lab and first author of the paper.

The researchers used single-walled carbon nanotubes, which are rolled up from graphene sheets, to make the solar cells. Comparable in size to a DNA molecule, the nanotubes conduct electricity between two electrical connections and are close to two electrodes that carry a positive and negative charge, respectively. The research using single-walled nanotubes was inspired in part by previous work on diodes, which are simple forms of transistors that only allow current to move in one direction. The Cornell team wanted to see what results they could get using a similar approach, this time shining light on the nanotubes.

Using lasers of different colors to illuminate the nanotubes, the researchers found that the current generated by higher energy photons has an amplifying effect in the process of converting light energy into electrical energy. Further research found that the narrow, cylindrical carbon nanotubes allow electrons to squeeze through the nanotubes smoothly one by one. After passing through the nanotubes, the electrons are excited and generate new electrons to continue moving. They said that nanotubes may be an ideal photovoltaic cell because by using the excess light energy of light, it can enable electrons to recreate more electrons.

This doesn't look like current solar cells, where extra energy is often lost as heat and the cells require constant external cooling.

Although they have made the device, it is still a big challenge for engineers to make it cheap and reliable in mass production, Gabor said. "All we observe is its physical behavior."

In a photodiode made of carbon nanotube material, when sunlight shines on the device, electrons (blue) and holes (red) release excess energy, creating more electron-hole pairs more efficiently.