Stacked thin film organic solar cell catches up with silicon

Stacked thin film organic solar cell catches up with silicon

Technology News |
By Nick Flaherty

The team at the University of Michigan used a carbon-based system that combines stacked layers to absorb both visible and infrared light. The bottom layer absorbs light from the visible spectrum starting at 350nm while the top layer absorbs near-infrared light up to 950nm, but still allowing a flexible, low cost thin film manufacturing process.

At 15 percent efficiency and given a 20-year lifetime, researchers estimate organic solar cells could produce electricity at a cost of less than 7 cents per kilowatt-hour. In comparison, the average cost of electricity in the US was 10.5 cents per kilowatt-hour in 2017, according to the US Energy Information Administration. The team believes it can reach 18 percent efficiency with a high volume manufacturing process.

“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, professor of engineering at Michigan who led the work, published in Nature Energy.

“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 a new study published in Nature Energy. “By themselves, the cells achieve 10- to 11-percent efficiency,” he said. “When we stack them together, we increase light absorption and efficiency improves to 15 percent with an antireflection coating.”

Stacking the cells required a key development, interconnecting layers that prevent damage to the first cell, and still 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,” said Che.

The bottom 930nm layer is a fullerene-based subcell grown by vacuum thermal evaporation, while the top layer is a solution-processed non-fullerene-acceptor-based infrared absorbing subcell. The hydrophilic–hydrophobic interface within the charge-recombination zone that connects the two subcells leads to a fabrication yield over 95% for more than 130 devices, and with areas up to 1 cm2, essential for scaling up fabrication to an industrial level. The team is working on boosting both the conversion efficiency and the fabrication yield.  

“We can improve the light absorption to increase electric current, and minimize the energy loss to increase voltage,” said Che. “Based on calculations, an 18-percent efficiency is expected in the near future for this type of multijunction device.”

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