Organic semiconductors lose energy through invisible “dark” states that standard techniques cannot see, and this project will use quantum optics to watch those states form and fade one molecule at a time. These materials power OLED displays and solar cells, but their efficiency is limited by excited-state losses—triplets, charge-transfer states, and polarons—that emit no light and are buried in the average signal from millions of molecules. Current single-molecule spectroscopy can only track bright states, missing the dark ones that matter most. Two teams, in Glasgow and Tokyo, will combine quantum light sources with single-molecule transient absorption and spectrally resolved photon antibunching to directly observe non-emissive states and their picosecond-to-nanosecond spectral evolution. They will test two material classes: a widely studied OLED polymer and a set of high-efficiency photovoltaic materials. If successful, the project will deliver fundamental understanding of how chemical structure governs optoelectronic behaviour, identifying molecular motifs that reduce losses and enabling design rules for better organic semiconductors. This is curiosity-driven fundamental science with no immediate practical application, but similar single-molecule studies have previously revealed mechanisms that later guided the development of brighter displays and more efficient solar cells.
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Organic semiconductors are important materials that find wide usage in organic light emitting diodes (OLEDs), solar cells and next generation optoelectronic devices. Their performance is governed by how excited electronic states form, evolve, interact and decay, with numerous loss channels frequently limiting their efficiency. It is a pressing question to obtain knowledge of these losses and identify materials or chemical motifs that reduce them. Measuring molecules one at a time, with single molecule spectroscopy (SMS), offers unrivalled access to fundamental knowledge, including observing emergent behaviour not possible with ensemble techniques, which suffer from simply measuring a single “average” of millions of molecules’ properties. However, SMS typically relies on recording the emission of light by molecules, and thus does not directly enable understanding of many important processes in the excited state that involve weak or dark transitions, for example triplets, charge transfer, or polaronic states. Additionally, the spectral evolution of excited state losses in SMS is typically hard to access, as this occurs on picosecond/nanosecond timescales and is not trivially measurable with single pixel single photon detectors. In this project two teams, based in Glasgow and Tokyo, will combine their knowledge and expertise to realise two new techniques to enable unrivalled new information on excited states in organic semiconductors to be obtained, including directly measuring non-emissive states and observing fast (ps/ns) spectral evolution of excited state quenching. This will be achieved by exploiting quantum optical sources and detection schemes to measure single molecule transient absorption and spectrally resolved photon antibunching. The former will give direct observation of important non-emissive excited states, enabling how they form, evolve and decay on the femtosecond to millisecond timescales to be tracked, while the latter will enable measurement of the energetic heterogeneity in excited state losses to be recorded. Two classes of materials will be examined with these techniques, one widely studied polymers used in OLEDs, while the other a progression of high efficiency photovoltaic materials. The outcomes of this project will be to contribute important new fundamental understanding of the behaviour of excited states in organic semiconductors. By measuring single molecules one at a time, unique information on how their chemical structure relates to their optoelectronic properties can be obtained that is otherwise not possible. Here chemical motifs that are responsible for advantageous properties will be identified and design rules for new materials devised.
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