Carbon-based radicals at the frontier of solar cell technology

Researchers from Aalto University, University of Cambridge, and their international collaborators have discovered that carbon-based organic radicals can function as highly efficient semiconductors—paving the way for lightweight, flexible, and energy-efficient solar cells.

The study shows that these radicals can be used to create a new class of semiconductors that convert all absorbed light into electrical charges. Organic radicals – carbon-based molecules with an unpaired electron – can act as single-material semiconductors without the need for precisely engineered combination of two materials that conventional organic solar cells require to generate and transport charges.

It is the lone electron that makes radicals exceptionally efficient charge carriers. When a thin film of radicals is exposed to light, electrical charges are generated and separated from each other with ideal, 100% efficiency – meaning that all absorbed light can be transformed into electrical charges.

Silicon, which dominates today’s electronics market, is an excellent semiconductor but comes with clear drawbacks: it is heavy, rigid, and brittle. Traditional silicon-based solar panels can weigh up to 20 kilograms. Silicon is also opaque, which limits its integration as a visible or design element in, for example, windows, wearable electronics, or biological environment.

The results open a completely new pathway for thin-film technology and lightweight solar cells. Such cells can be manufactured as flexible, coloured films suitable for integration into windows or wearable devices.

'Energy efficiency of the devices will be improved by generating electrical charges without energy losses using radical-based semiconductors,' says Academy Research Fellow Petri Murto, who moved to Aalto University from University of Cambridge and established his own research group at the Department of Chemistry and Materials Science in 2024.

Organic semiconductors can be produced as ultra-thin layers only about 100 nanometres thick, meaning that an entire solar cell can weigh just a few hundred grams depending on its surface area. The materials can be bent, shaped, and coloured, and the manufacturing process for organic thin films is easily scalable. Radical-based materials offer a promising new approach to enhancing the energy efficiency of solar cell technology.

'This research builds on our earlier collaboration with University of Cambridge, where we developed materials for LED applications and observed that certain radicals behave in an unexpected way when in close contact with each other,' Murto explains. 'We found that radical electrons can move from one molecule to another rather than staying bound as previously believed. Our latest study utilizes this phenomenon.'

The synthetic organic molecules were developed in collaboration between Aalto University’s School of Chemical Engineering and University of Cambridge. These materials enabled experiments and practical demonstration of radicals’ unique charge transfer properties.

The findings were recently published in the prestigious journal Nature Materials.