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Researchers from the Agency for Science, Technology and Research (A*STAR) in Singapore have synthesized new acceptors that could help reduce the cost of organic solar cells and make them easier to produce.

Organic solar cells are made using solution processes, and thus they can be spread onto flexible substrates such as films or fabrics. However, scientists are still seeking organic materials that offer both high solar conversion efficiency and favorable behavior in solutions.

Prashant Sonar and co-workers from A*STARs Institute of Materials Research and Engineering sought an efficient and solution-friendly material that could replace electron-transporting materials based on fullerenes in organic photovoltaic devices. Fullerenes are excellent electron acceptors, but they have specialized structures that make it difficult to fabricate them reliably and inexpensively.

The researchers started with diketopyrrolopyrroles (DPPs), which contain four pentagonal aromatic rings and sulfur, nitrogen and oxygen functional groups. The electron-attracting, conjugated framework of this class of molecules can be easily modified to improve solvent compatibility.

The researchers modified the structure of DPPs by adding fluorine-based acceptor units on either side of the central rings. In only a few steps, they produced small molecules with enhanced optical capabilities that were suitable for solution processing. These compounds had optical bandgaps from 1.81 to 1.94 eV and intense absorption bands covering a wide range from 300 to 700 nm.

Because these materials have strong absorbance in the ultraviolet, visible and near-infrared light regions, as well as straightforward synthesis, suitable energy levels, solvent-dependent morphologies and promising charge-transport properties, they can, in principle, achieve high efficiencies in organic photovoltaics, Sonar said.

The researchers tested the DPP-derived molecules by incorporating them as the acceptors in bulk heterojunction solar cells and were able to achieve power conversion efficiencies of up to 1%. These materials can be prepared on large scales with high yields, and there are several possibilities for structural modifications to improve conversion efficiencies, Sonar said.

Research Paper:

Solution processable low bandgap diketopyrrolopyrrole (DPP) based derivatives: novel acceptors for organic solar cells. Journal of Materials Chemistry 20, 36263636, DOI: 10.1039/B924404B.

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