University of Michigan researchers have made a major stride in creating efficient organic solar cells, one step closer to the commercialization of these solar cells. The new method creates a more flexible and inexpensive solar cell that has 15 percent efficiency. This efficiency is in the range of solar panels that are currently on the market.
"Organic photovoltaics can potentially cut way down on the total solar energy system cost, making solar a truly ubiquitous clean energy source," said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering and Paul G. Goebel Professor of Engineering, who led the work.
The new solar cells have a 20 year lifetime and could produce electricity at 7 cents per kilowatt per hour. The average cost of electricity in the U.S. is 10.5 cents per kilowatt-hour in 2017 according to the U.S. Energy Information Administration.
Organic solar cells have major advantages over in-organic solar cells because they have carbon in the construction. Silicon-based inorganic solar panels are expensive, thick, rigid and require fixed installation points. The carbon-based organic solar cells are inexpensive to create and can be made into rolls that are thin so they can bend and curve around structures or in clothing. The organic solar cells can also be made in any color so they can blend in with the environment.
While there are clearly major advantages to organic solar cells, they are not widely used yet because they have not had an efficiency that competed with current energy sources.
"For the last couple of years, efficiency for organic photovoltaics was stuck around 11 to 12 percent," said Xiaozhou Che, a doctoral candidate in the Applied Physics Program and first author of the study.
In order to break this efficiency barrier, the team created a system that combines specialized layers that absorb visible and infrared light. One layer is capable of absorbing light from the visible spectrum starting at 3560 nanometers in wavelength and the other layer can absorb near-infrared light up to 950 nanometers in wavelength.
"By themselves, the cells achieve 10- to 11-percent efficiency," Che said. "When we stack them together, we increase light absorption and efficiency improves to 15 percent with an antireflection coating."
This cell stacking is a huge breakthrough for organic solar cells. Along with these layers, the team developed interconnecting layers that prevent damage to the first cell while making them open enough to allow light and electrical charges to pass through.
"That's considered a difficult process because there's a chance the liquid used in processing the top cell will dissolve the layers already deposited underneath," Che said.
The organic solar cells have proved to have a high fabrication yield at over 95 percent, creating almost all the devices without short circuits. This is a major step towards production for the industrial level.
But this breakthrough is not stopping the researchers from focusing on further development.
"We can improve the light absorption to increase electric current, and minimize the energy loss to increase voltage," Che said. "Based on calculations, an 18-percent efficiency is expected in the near future for this type of multijunction device."
The paper on the breakthrough was published in Nature Energy.