Scientists were pretty excited when they discovered you could convert light energy directly into electricity by capturing photons in semiconductors, exciting them into “excitons” (bound electron with negative charge and hole with positive), and capturing the resultant current through electrodes. Now a group of four chemists from the University of California, Riverside, has worked out a way for one photon to generate a pair of excited states rather than just one.
It’s called “singlet fission,” and by using it, we should be able to boost solar cell efficiency by as much as 30%, providing “Third Generation” solar power. The Journal of Physical Chemistry Letters published the research results in an Editor’s Choice perspective article last month.
Christopher Bardeen, the chemistry professor whose lab led the research, explains what sent him along this line of inquiry:
Our research got its launch about ten years ago when we started thinking about solar energy and what new types of photophysics this might require. Global warming concerns and energy security have made solar energy conversion an important subject from society’s point of view. More efficient solar cells would lead to wider use of this clean energy source.
“If a triplet exciton has half the energy of a singlet, then it is possible for one singlet exciton, generated by one photon, to split into two triplet excitons,” Dr. Bardeen explains. “Thus, you could have a 200% yield of excitons—and hopefully, electrons—per absorbed photon.”
Here’s the Bardeen lab’s diagram of how singlet fission works to spontaneously split into two triplets, effectively dodging the efficiency barrier of the Shockley-Queisser limit.
“The exact mechanism is unknown, but it does happen quickly—at the sub-nanosecond timescale—and with high efficiency,” Bardeen says. His lab’s work has shown that it is very sensitive to molecular alignment and position.
Bardeen cites recent work at MIT that has already demonstrated an organic photovoltaic cell with more than 100% external quantum efficiency based on this effect. Bardeen believes we can use this effect to raise the efficiency of inorganic semiconductors.
Next steps: finding new materials that exhibit singlet fission, figuring out how to turn the triplet excitons into photocurrent efficiently, and determine how the spin properties of the electrons affect exciton dynamics.