Carbon based semiconducting molecular materials now support a wide range of practical technologies, particularly as organic LEDs, OLEDs, used in smartphone and TV displays. The electronic processes that govern their semiconducting properties are strongly controlled by their low dielectric screening, so that excited states, excitons, are often spatially localised and generally show strong magnetic exchange interactions. The exchange interaction presents a challenge for the engineering of efficient OLEDs. Only 25% of electron-hole capture events in the OLED produce emissive spin singlet excitons, and 75% capture events form spin triplet excitons that are not emissive. A number of engineering approaches have been developed to overcome this challenge, including the use of organo-metallic emitters that can show efficient phosphorescence.

We have been working with spin-radical molecules that show high luminescence yield within the spin doublet manifold, and can be designed so that this ‘bright’ doublet exciton lies lower in energy than ‘dark’ higher spin states. These enable efficient OLED operation in the red and near-IR, and can be engineered to show high luminescence yield.

When coupled together, either intermolecularly or intramolecularly these spin radical systems show properties of Mott-Hubbard spin systems, where the lowest energy electronic excitation is a charge transfer between antiferromagnetically arranged neighbouring radical sites at the cost of the Hubbard U. This process is radiatively allowed and enables optical write and read of spin. We are exploring how these excited states can be used to assist charge photogeneration in the absence of a donor-acceptor heterojunction, and to engineer spin-optical interfaces that allow easy magnetic field control of luminescence.

About the speaker

Richard Friend is at the Department of Physics at the University of Cambridge. His research encompasses the physics, materials science and engineering of semiconductor devices made with carbon-based semiconductors, particularly polymers. His research advances have shown that carbon-based semiconductors have significant applications in LEDs, solar cells, lasers, and electronics. He explores novel schemes that seek to improve the performance of LEDs and solar cells, using carbon-based semiconductors. His current projects include materials with unpaired electron spins that show novel couplings of spin with luminescence.

Professor Friend is a Fellow of the Royal Society and of the Royal Academy of Engineering, and a Foreign Member of the US National Academy of Engineering. He has received many international awards for his research, including Laureate of the Millennium Prize for Technology (2010) the Harvey Prize (2011) of the Israel Institute of Technology, the von Hippel Award of the Materials Research Society (2015) and the Isaac Newton Prize of the Institute of Physics (2024). He was knighted for “Services to Physics” in the Queen’s Birthday Honours List, 2003.