Why are efficient and affordable solar cells so highly coveted? Volume. The amount of solar energy lighting up Earth's land mass every year is nearly 3,000 times the total amount of annual human energy use. But to compete with energy from fossil fuels, photovoltaic devices must convert sunlight to electricity with a certain measure of efficiency. For polymer-based organic photovoltaic cells, which are far less expensive to manufacture than silicon-based solar cells, scientists have long believed that the key to high efficiencies rests in the purity of the polymer/organic cell's two domains – acceptor and donor. Now, however, an alternate and possibly easier route forward has been shown.
Working at Berkeley Lab's Advanced Light Source (ALS), a premier source of X-ray and ultraviolet light beams for research, an international team of scientists found that for highly efficient polymer/organic photovoltaic cells, size matters.
"We've shown that impure domains if made sufficiently small can also lead to improved performances in polymer-based organic photovoltaic cells," says Harald Ade, a physicist at North Carolina State University, who led this research. "There seems to be a happy medium, a sweet-spot of sorts, between purity and domain size that should be much easier to achieve than ultra-high purity."
Ade, a longtime user of the ALS, is the corresponding author of a paper describing this work in Advanced Energy Materials titled "Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71 BM Solar Cells." Co-authors are Brian Collins, Zhe Li, John Tumbleston, Eliot Gann and Christopher McNeill.
Solar cell conversion efficiency in polymer/organic photovoltaic cells hinges on excitons – electron/hole pairs energized by sunlight – getting to the interfaces of the donor and acceptor domains quickly so as to minimize energy lost as heat. Conventional wisdom held that the greater the purity of the domains, the fewer the impedances and the faster the exciton journey.
Ade and his co-authors became the first to simultaneously measure the domain size, composition and crystallinity of an organic solar cell. This feat was made possible by ALS beamlines 18.104.22.168, a Resonant Soft X-ray Scattering (R-SoXS) facility; 7.3.3, a Small- and Wide-Angle X-Ray Scattering (SAXS/WAXS/) end-station; and 5.3.2, an end-station for Scanning Transmission X-Ray Microscopy (STXM).
Says Collins, the first author on the Advanced Energy Materials paper, "The combination of these three ALS beamlines enabled us to obtain comprehensive pictures of polymer-based organic photovoltaic film morphology from the nano- to the meso-scales. Until now, this information has been unattainable."
The international team used the trifecta of ALS beams to study the polymer/fullerence blend PTB7:PC71BM in thin films made from chlorobenzene solution with and without the addition (three-percent by volume) of the solvent diiodooctane. The films were composed of droplet-like dispersions in which the dominant acceptor domain size without the additive was about 177 nanometers. The addition of the solvent shrank the acceptor domain size down to about 34 nanometers while preserving the film's composition and crystallinity. This resulted in an efficiency gain of 42-percent.
"In showing for the first time just how pure and how large the acceptor domains in organic solar devices actually are, as well as what the interface with the donor domain looks like, we've demonstrated that the impact of solvents and additives on device performance can be dramatic and can be systematically studied," Ade says. "In the future, our technique should help advance the rational design of polymer-based organic photovoltaic films."
This research was primarily supported by the DOE Office of Science, which also supports the ALS.
DOE/Lawrence Berkeley National Laboratory: http://www.lbl.gov
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
A skin-eating fungus that infiltrated Europe through the global wildlife trade is threatening to inflict massive losses on the continent's native salamanders including extinction of whole species and could do the same in North America, scientists say.
A gray wolf was recently photographed on the north rim of the Grand Canyon in Arizona in what would be the first wolf sighting in the national park since the last one was killed there in the 1940s, conservation groups said on Thursday.
Two years after Sandy, researchers in New Jersey want to bring more powerful predictions to the people
In order to save the Amazon, it's not enough for deforestation to stop; areas that have been denuded also need recuperation. A Brazilian research scientist has released a report with the World Wildlife Fund that suggested actions to curb the effect of humans on the world's largest rainforest.
Smithsonian photographer Laurie Penland details the exhausting, but rewarding, work of scraping invasive species off the hull of a boat
Politicians who ignore message cannot in future say they take science seriously, open letter says
In the wake of Hurricane Sandy two years ago, shocking photos showed the huge extent of the destruction caused by the storm. Only days after the storm struck, before and after satellite images from Google revealed the widespread damage to coastal areas of New York and New Jersey. Last year, before and after photos showed progress but still a lot of work to be done.
Australian scientists say they have successfully tested a vaccine aimed at protecting wild koalas from chlamydia.
A startup that might have a record-breaking solar cell is in danger of going out of business.The power unit is a rectangular slab about the size of a movie theater screen. It’s mounted on a thick steel post, and equipped with a tracking mechanism that continuously points it at the sun. The slab is made of over 100,000 small lenses and an equal number of even smaller solar cells, each the size of the tip of a ballpoint pen. This contraption is part of one of the most efficient solar power devices ever made.
There are several captive breeding programs for pandas around the world; but one facility in China hopes to release the endangered animals back into the wild