Our most promising visions of a clean-powered future typically position cheap, abundant solar energy as an important cornerstone. And solar panels would be a lot cheaper and abundant if they were made out of plastic instead of silicon—that way, they could be 'printed' in a flexible material and easily applied to walls, windows, you name it. Researchers just made an important discovery about the nature of plastic solar, which might speed the arrival of thin, bendable energy strips.
"In a nutshell, we can print solar panels just as we print newspapers, which is a large scale and cheap way to put a photovoltaic technology in the hands of everyone on the planet," Francoise Provencher, the lead author of the new study from the University of Montreal, told me.
That vision, of cheap, plastic solar panels rolling like the early edition off clean-power printing presses isn't a new one: Konarka, a Massachussetts-based company, began mass-producing polymer-based panels in 2008. But major hurdles have prevented them from taking off: plastic, or organic solar panels, which are made of thin, flexible polymers instead of rigid silicon, aren't nearly as efficient. Yet.
Still, researchers are pushing onward. Engineers at Georgia Tech built a new, more efficient kind of plastic solar cell in 2012. Last year, scientists at Northwestern designed a polymer that was nearly 9 percent efficient in suboptimal conditions.
Now, researchers at the University of Montreal have made an important breakthrough in understanding how light beams excite the chemicals in plastic solar panels, which will allow solar power printers to improve efficiency even further.
"Our findings are of key importance for a fundamental mechanistic understanding, with molecular detail, of all solar conversion systems—we have made great progress towards reaching a 'holy grail' that has been actively sought for several decades," Provencher said in a statement upon the study's release.
Three laser beams are needed to record the excited vibrational modes of PCDTBT with the method called femtosecond stimulated Raman spectroscopy. Image: University of Montreal
"Our study is a big breakthrough as we are the first to observe how the cloud of electrons on the semiconductor polymer moves while the electric charges are generated," Provencher wrote me in an email. "I was personally surprised by the results because it challenges the way we usually view charge separation.
"The classic scenario happens in 4 steps: (1) the donor molecule is first excited by the light, (2) then it transfers an electron to a nearby acceptor molecule to form an electron-hole pair (a hole is a positive charge that is created by the absence of the electron on the polymer), (3) then the electron-hole pair separates into free charges and (4) they create electricity as they move away," she said. "The big surprise is that our experiment shows that it happens all at once in the system that we studied."
Which is all a little wonky—but the takeaway is that solar researchers used to worry that if that electron-hole pair didn't separate right, it would put a hard limit on the efficiency of plastic solar. But it does. Which means the sky's the limit for plastic solar. Okay, not quite the sky—the efficiency is still expected to be less than the silicon stuff, but perhaps not by enough to matter, when costs and ease of use come into play.
"Current estimations for the thermodynamic limit of the efficiency of organic solar cells are around 22 to 27 percent," Provencher said. "For comparison, silicon solar cells have a thermodynamic efficiency limit of 33 percent, which is not that much higher. The goal of the community is to reach 20 percent, but a major breakthrough in understanding is required to come up with the right materials design rules, and our paper is an important step towards this."
Remember, plastic solar is thin—it can be produced at about the width of a human hair—and cheap. What it lacks in efficiency it can make up in cost. The giant solar plants of the future—the vast desert arrays and industrial solar fields—will still be silicon. But plastic solar is primed to slip into more of our daily lives.
Provencher believes that plastic panels "will find their own niche thanks to their unique properties: flexible, light, printable, cheap (e.g. photovoltaic tinted windows, portable solar panels for off-grid expeditions, third world country applications)" and will become a major force on the market.
One of the perverse realities about Kontarka's unfortunate decline was that the demand was steadily growing: The company had more orders in 2012, the year it shuttered, than ever before. And wearable, printable tech is officially Silicon Valley's next big gadget play. The dream of printing out solar panels en masse, in other words, is very much alive.
"This means we might be able to push solar cells efficiency a few percents more, just what we need for a solar energy revolution," she said.