Backwards Physics Experiment Could Help Solar Panels of the Future Stop Wasting Energy From the Sun

Rows of rooftop solar panels operated by the SolarSpace energy retail program at a factory in Singapore on February 29, 2016. The future of solar technology will drastically increase the efficiency of harvesting sunlight energy, and a study published on January 22 in Nature Chemistry may bring us one step closer. Reuters/Edgar Su

A perplexing and hotly debated problem with solar panels may have just been solved. If so, harvesting energy from the sun without wasting any energy could be within reach.

Inefficiencies that plague solar panels are partly holding back our use of them. But these issues have been hard to eradicate in part because they are hard to understand.

Solar panels absorb sunlight and produce energy. Often during this process, a molecule called an excimer forms. These molecules are made of two others bound together, and they decay within 10 billionths of a second. Some scientists argue that excimers that form en route during the conversion of sunlight to energy are detrimental to the process. But not all experts agree—the discussion has become so contentious that one scientist called it "a bit of a cold war" among physicists.

That scientist is Tim Schmidt, who, together with his colleagues at Australian Research Council Centre of Excellence in Exciton Science, University of Adelaide, University of Kentucky and University of Sydney, may have solved the argument.

Schmidt and his team found that the formation of excimers "traps" potential energy during a process called singlet exciton fission. That in turn affects a crucial component in the overall energy harvesting process: the exciton.

Many proposed solar energy devices work by creating two excitons from one. An exciton is created when photons of light hit a negatively charged electron held inside the device. That meeting elevates the energy level of the electron, creating a vacancy of sorts that carries a positive charge. The negatively charged electron is attracted to that positively charged vacancy, so the two stick together. That electron-hole combination is called an exciton, which can split apart—producing electricity. But, Schmidt told Newsweek, "to get the full value out of higher energy excitons, we want them to split into two lower energy excitons." This process (singlet exciton fission) is impeded by excimer formation, which causes energy from the original exciton to be "trapped in this 'energy pit.'"

The excimers that are forming are getting in the way of that exciton split. Rather than acting as an intermediary doorway to energy production, the excimers are taking the excitons and "killing them off," Schmidt told Newsweek.

The researchers demonstrated that when molecular systems absorb sunlight, moving immediately to the split-state phase helps to increase the energy efficiency. Some scientists believed that excimer formation was part of the process, but the new study confirms that it is not. Rather, it is a drain that leaks valuable energy.

The value of the finding, published Monday in Nature Chemistry, is that it could help make solar energy devices more efficient. Currently solar cells use silicon to help absorb sunlight. But silicon has limitations; namely it can absorb only around 25 percent of light from the solar spectrum. Green light in particular is the best to absorb for energy production, but silicon can absorb only half of its energy.

The findings by Schmidt and colleagues could revolutionize a way to make silicon more efficient at harvesting sunlight for electricity. The researchers envision coating the silicon with a yet-to-be-developed material that could make the process more efficient.

Fun fact: The scientists discovered that excimer formation was a trap by doing the experiment backwards. "Physics doesn't have an arrow of time," explained Schmidt, who researches molecular photonics (essentially the interaction of light and matter) at the University of New South Wales, Sydney.

The researchers were inspired by an idea known as the principle of microscopic reversibility, which says that a molecular process in reverse should be a mirror image of the process going forward. One should not use more energy than the other.

Capture 1
A visualization of what occurs in the process of energy absorption. Some energy is being lost in a "trap" of excimers. Tim Schmidt/ARC Centre for Excellence in Exciton Science

But when the team conducted their experiment backwards and forwards, the processes didn't match. The only explanation that made sense when comparing both sets of data was that the formation of excimers were acting as a trap.

The improved understanding of the solar energy process that this study advances could increase solar cell efficiency beyond 30 percent. The future—especially one with minimal wasted sunlight energy—looks bright.