University of Michigan researchers have found a way to coax electrons to travel much further than was previously thought possible in the materials often used for organic solar cells and other organic semiconductors.
An organic solar cell testing inside the Randall Lab. University of Michigan researchers have found a way to coax electrons to travel much further than was previously thought possible in the materials often used for organic solar cells and other organic semiconductors. Image credit: Robert Coelius, Michigan Engineering
Unlike the inorganic solar cells widely used today, organics can be made of inexpensive, flexible carbon-based materials like plastic. Manufacturers could churn out rolls of them in a variety of colors and configurations, to be laminated unobtrusively into almost any surface.
* Organic solar cells can be printed on a roll to roll basis on the cost and scale of newspapers
Roll to roll printing of solar cells could be 100 to 1000 times cheaper than current solar panels
The New York Times is printed in a 515,000-square-foot building in Queens. Every weeknight, the plant will print more than 300,000 copies—double that on the weekend. Up to 2231 of one-ton paper rolls are used. The presses can print up to 80,000 papers per hour.
The New York Times has sent out editions that have weighed 12 lbs. and presented 1,612 pages. More typically the Sunday editions are 4 to 5 pounds and about 500-600 pages. Broadsheets are (600mmX750mm or 2 feet by 2.5 feet) 5 square feet per page. So a Sunday New York Times is about 3000 square feet or 278 square meters. The NY Times Sunday Edition costs $5. In 2015, solar panels were costing about $10 per square foot.
Roll to roll solar cells could become 100 to 1000 times cheaper than existing solar panels. This is a rough calculation of the future potential. The eventual roll to roll solar cells would have multiple layers of printing.
The Conductance breakthrough
Organics’ notoriously poor conductivity, however, has slowed research. Forrest believes this discovery could change the game. The findings are detailed in a study published Jan. 17 in Nature.
The team showed that a thin layer of fullerene molecules—the curious round carbon molecules also called Buckyballs—can enable electrons to travel up to several centimeters from the point where they’re knocked loose by a photon. That’s a dramatic increase; in today’s organic cells, electrons can travel only a few hundred nanometers or less.
The ability to make electrons move more freely in organic semiconductors could have far-reaching implications. For example, the surface of today’s organic solar cells must be covered with a conductive electrode that collects electrons at the point where they’re initially generated. But freely moving electrons can be collected far away from their point of origination. This could enable manufacturers to shrink the conductive electrode into an invisible grid, paving the way for transparent cells that could be used on windows and other surfaces.
Electrons, moving from one atom to another, make up the electric current in a solar cell or electronic component. Materials like silicon, used in today’s inorganic solar cells and other semiconductors, have tightly bound atomic networks that make it easy for electrons to travel through the material.
Burlingame says that the initial discovery of the phenomenon came as something of an accident as the team was experimenting with organic solar cell architecture in hopes of boosting efficiency. Using a common technique called vacuum thermal evaporation, they layered in a thin film of C60 fullerenes—each made of 60 carbon atoms—on top of an organic cell’s power-producing layer, where the photons from sunlight knock electrons loose from their associated molecules. On top of the fullerenes, they put another layer to prevent the electrons from escaping.
They discovered something they’d never seen before in an organic—electrons were skittering unfettered through the material, even outside the power-generating area of the cell. Through months of experimentation, they determined that the fullerene layer formed what’s known as an energy well—a low-energy area that prevents the negatively charged electrons from recombining with the positive charges left behind in the power-producing layer.
“You can imagine an energy well as sort of a canyon—electrons fall into it and can’t get back out,” said Caleb Cobourn, a graduate researcher in the U-M Department of Physics and an author on the study. “So they continue to move freely in the fullerene layer instead of recombining in the power-producing layer, as they normally would. It’s like a massive antenna that can collect an electron charge from anywhere in the device.”
Forrest cautions that widespread use of the discovery in applications like solar cells is theoretical at this point. But, he is excited by the discovery’s larger implications for understanding and exploiting the properties of organic semiconductors.
“I believe that ubiquitous solar power is the key to powering our constantly warming and increasingly crowded planet, and that means putting solar cells on everyday objects like building facades and windows,” Forrest said. “Technology like this could help us produce power in a way that’s inexpensive and nearly invisible.”