Maximizing solar cell efficiency through singlet fission

A team of researchers hopes to push solar cell performance to the limit through a process that better exploits the visible light spectrum. Northwestern University researchers, in partnership with energy giant Exelon, want to increase the maximum of efficiency of solar cells from 33 percent to 45 percent. 

Their research centers around a process called singlet fission, which enables a photon of sunlight to generate two electrons, as opposed to the usual single electron. 

The increase in electrons – “two for the price of one!” as Michelle Chen, a Northwestern graduate student and lead researcher in the project, told Medill Reports Chicago – results in a corresponding boost in efficiency.

Singlet fission was first discovered in the 1960s but only recently has been recognized for its potential to enhance the efficiency of solar cells. 

Led by Michael Wasielewski, chemistry professor and director of the Institute for Sustainability and Energy at Northwestern University, the research team is developing new organic semiconductors that can carry out the singlet fission process. 

Conventional solar cells use silicon semiconductors, which are only able to exploit the red, low-energy end of the visible light spectrum.  The cells absorb a photon and generate a single electron, which is then harnessed as electricity.  The maximum possible efficiency of these cells is approximately 33 percent, though few solar cells in everyday use exceed 20 percent efficiency.

In the singlet fission process, organic semiconductors tap into the unused blue end of the visible light spectrum and increase the energy of absorbed photons.  In doing so, each higher-energy photon can be converted into two electrons, rather than one, generating more electricity and boosting the cell’s maximum efficiency to 45 percent.

The researchers are deriving the semiconductors capable of carrying out this process from commercial dye molecules found in paints.

“Singlet fission is a phenomenon that’s been known for 50 years but was not recognized until 10 years ago as being relevant to photovoltaics,” Wasielewski said. “By developing new materials that will carry out singlet fission, we hope to deliver a new solar cell technology that can be commercialized.”

According to a Northwestern news release, “the process has an array of potential applications for producing flexible solar cells that can be processed in a roll-to-roll fashion or even printed. Examples include rolled solar shades, solar coatings for buildings, and solar clothing or solar backpacks.”

The simplest application of the organic semiconductors would be to enhance the performance of conventional silicon solar cells by coating them with the materials that can undergo singlet fission, the researchers said.

“What’s important about singlet fission is that it offers us the possibility of replacing or enhancing the photo-active materials in a solar cell rather than completely redesigning or redeveloping the solar cell’s basic structure,” Wasielewski said.  “The more we learn about singlet fission, the more widely it will be applied to practical energy conversion devices.”