Photovoltaic cells made from organic compounds, rather than silicon wafers, are set to transform the solar energy industry. Because organic solar cells are made using solution processes, they can be spread onto flexible substrates, like films or fabrics, in the same manner as inks or paints. These properties open the way for intriguing new applications, such as light-harvesting clothes and window coatings — but only if scientists can find organic materials that combine high solar conversion efficiency with favorable behavior in solutions.
Prashant Sonar and co-workers from the A*STAR Institute of Materials Research and Engineering in Singapore have now synthesized a class of molecules, based on substances called diketopyrrolopyrroles (DPPs), that may make organic solar cells cheaper and easier to produce1. These new compounds are set to replace the use of electron-transporting materials based on fullerenes — spherical hydrocarbons shaped like mini soccer balls — in organic photovoltaic devices. While fullerenes are excellent electron acceptors, their specialized structures make it difficult to fabricate them reliably and inexpensively.
The search for an efficient, solution-friendly material led Sonar and his team to DPP, a molecule containing four pentagonal aromatic rings and sulfur, nitrogen and oxygen functional groups. This compound has an electron-attracting, conjugated framework that can be easily modified to improve solvent compatibility. “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,” says Sonar.
Converting photons into electricity using organic molecules requires the fabrication of devices known as bulk heterojunction (BHJ) cells. In these systems, electron-donating materials are blended with acceptors so that light-generated charges can be dissociated and transported to their respective electrodes, creating an electric circuit. To improve the energy levels of DPP for BHJ devices, Sonar and his co-workers modified its structure by adding fluorine-based acceptor units on either side of the central rings. In just a few synthetic steps, this produced small molecules with enhanced optical capabilities that are suitable for solution processing.
Fabricating the new DPP derivative molecules into working BHJ cells revealed activity over a range of light frequencies and power conversion efficiencies of up to 1%, the highest ever reported for a DPP-based electron-accepting organic semiconductor. “These materials can be prepared on large scales with high yields, and there are several possibilities for structural modifications to improve conversion efficiencies,” says Sonar, who believes it is a great start to transforming the solar industry.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering