“This achievement is an important step to bring organic photovoltaic cells to a higher level in the thin-film photovoltaics marketplace,” said the lab.
Fullerenes (C60, buckyballs) are the dominant acceptor materials in current organic photovoltaic (OPV) cells, due to their ability to accept stable electrons and their high electron mobility.
“However, the small absorption overlap with the solar spectrum limits the photocurrent generation in fullerene acceptors, and their deep energy level for electron conduction limits the open-circuit voltage,” said IMEC.
Three mechanisms have been used to increase efficiency:
Fullerene-free acceptors to raise open-circuit voltage.
Three active semiconductor layers with different absorption spectra to broaden the wavelengths harvested.
An efficient exciton harvesting mechanism.
The cell has a three-layer stack with two fullerene-free acceptors and a donor, arranged as discrete heterojunctions.
In addition to the traditional exciton dissociation at the central donor-acceptor interface, excitons generated in the outer acceptor layer are first relayed by energy transfer to the central acceptor, and subsequently dissociated at the donor interface.
“This results in a quantum efficiency above 75% between 400nm and 720nm. With an open-circuit voltage close to 1V, power conversion efficiency of 8.4% is achieved,” said IMEC. “These results confirm that multilayer cascade structures are a promising alternative to conventional donor-fullerene organic solar cells.”
Results were published in Nature Communications (DOI: 10.1038/ncomms4406).
Funding from the European Community’s Seventh Framework Programme contributed to the research.
Photo – an 156cm2 organic photovoltaic cell with 5.3% conversion efficiency.