Imec achieves OPV cell efficiency of 8.4%

Micro- and nanoelectronics research centre Imec has achieved 8.4% conversion efficiency in fullerene-free organic solar cells. The achievement is said to be an important step to bring organic photovoltaic cells to a higher level in the competitive thin-film photovoltaics marketplace.

Imec achieved the efficiency 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 (156 cm²) 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 quantum efficiency above 75% between 400 and 720 nm. With an open-circuit voltage close to 1 V, a remarkable power conversion efficiency of 8.4% is achieved. These results confirm that multilayer cascade structures are a promising alternative to conventional donor-fullerene organic solar cells.