Fullerene-free organic solar cells achieve record conversion efficiency
The achievement is an important step to bringing organic photovoltaic cells to a higher level in the competitive thin-film photovoltaics marketplace.
Organic solar cells are a thin-film photovoltaic technology with commerical relevance because of their compatibility with flexible substrates and a tunable absorption window.
Although the power conversion efficiency of organic solar cells has increased rapidly in the last decade, further enhancements will be needed to make the production of organic photovoltaics more easily scalable into industrial production processes.
Imec’s organic solar cells with 8.4 percent power conversion efficiency were realized by introducing two innovations. Firstly, the implementation of fullerene-free acceptor materials resulted in high open-circuit voltages and useful absorption spectra in the visible. Secondly, high short-circuit currents were achieved by developing a multilayer device structure of three active semiconductor layers with complementary absorption spectra, and an efficient exciton harvesting mechanism.
Fullerenes are the dominant acceptor materials in current 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. Imec implemented two fullerene-free materials as acceptor, increasing open-circuit voltages compared to OPV cells with fullerene acceptors.
Imec’s novel fullerene-free OPV cell concept was used to process an OPV module (156cm2) with a conversion efficiency of 5.3%.
To increase the efficiency of organic solar cells, complex tandem architectures are often proposed to combine the exciton harvesting of multiple photo-active materials. The imec team now proposes a simple three-layer stack to improve the spectral responsivity range. This device architecture comprises two fullerene-free acceptors and a donor, arranged as discrete heterojunctions.
In addition to the traditional exciton dissociation at the central donor-acceptor interface, the 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 percent between 400nm and 720nm. With an open-circuit voltage close to 1 V, a remarkable power conversion efficiency of 8.4 percent is achieved. The results confirm that multilayer cascade structures are a promising alternative to conventional donor-fullerene organic solar cells.
The results were presented in Nature Communications (DOI: 10.1038/ncomms4406).
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