Belgian research institute, Interuniversity Microelectronics Centre (IMEC), has achieved a record-breaking 8.4% efficiency for its new organic solar cell made without fullerenes, marking yet another step towards making organic solar cells more commercially relevant.
Organic solar cells, like all photovoltaic devices, require a material that can donate and accept electrons (called donor and acceptor materials). Fullerenes, a general class of carbon-based materials that include carbon nanotubes and spherical buckyballs, are typically used in organic solar cell designs due to their good stability as an acceptor material and the low resistance they offer to the movement of charge within the cell.
However, fullerenes also limit the maximum voltage (open-circuit voltage) extractable from the cell. Since organic solar cells already have lower efficiencies than inorganic thin-film solar cells, researchers have been trying to find ways to improve the voltage output of organic cells by using an alternative to fullerenes. However, finding a suitable alternative has been challenging.
Researchers at IMEC have not just managed to find an alternative but have also broken the record efficiency for fullerene-free organic solar cells using their new material. The resulting cell voltage (close to 1V) is even higher than that of silicon solar cells, which have open-circuit voltages in the range of 0.6-0.7 V.
IMEC’s latest publication in Nature Communications also sheds light on a second key innovation that led to the record efficiency: a multi-layer material stack in which each stack absorbs a slightly different spectrum of the sun’s light, resulting in a higher current output. This is similar to the way tandem junctions improve the current and efficiency of inorganic solar cells.
Organic solar cells are a promising PV technology as they can be easily made on flexible substrates and have tunable optical properties. Although power conversion efficiencies of organic cells have seen rapid growth in recent years, the lack of industrially applicable manufacturing processes has held them back from commercial adoption. IMEC is hoping its new fullerene-free cell will help solve that problem and one day make organic PV cells competitive in the thin-film PV marketplace.
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