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Characterisation method for spin-dependent processes in solar energy technology

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Probing spins to identify new material design rules for organic photovoltaics

Photovoltaics based on organic electronics shows great promise in the field of renewable energy. However, a detailed understanding of energy conversion processes at the nanoscale is needed for the rational design of new materials and improvement of this technology.

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Organic electronic devices are based on organic small molecules or polymers that display useful electronic properties, like conductivity. Low costs and easy processability make them competitive in the energy market, while their flexibility and ease of tunability, including different colours and degrees of transparency, enable them to be easily integrated into building design. The EU-funded SpinSolar used electron spin resonance (ESR) spectroscopy for the detailed characterisation of spin-dependent processes in organic molecules and materials for photovoltaics and optoelectronics. This research was undertaken with the support of the Marie Skłodowska-Curie programme. “Detecting spins in new functional materials and characterising their environment and interactions can improve the understanding of how molecular properties determine function in potential future technological applications,” says project coordinator Jan Behrends.

Increased understanding of structure-property relationships

Researchers used ESR to gain new insights into the nature and dynamics of paramagnetic molecules in new organic functional materials. ESR is unique among the characterisation techniques currently employed to investigate these materials as it is selective for molecules carrying unpaired electrons and provides direct information on their environment on the nanoscale. Team members studied the correlation between the molecular structures of organic molecules and materials and their electronic properties. They investigated a series of twisted acenes, organic molecules based on fused benzene rings that act as active materials in a series of devices for organic electronics. Results showed how the distribution of the unpaired electrons in the photoexcited triplet state of the molecule changed as it is twisted further out of planarity and how this affected its properties. “Even small changes in the degree of twisting are revealed as clear changes in the measured ESR spectra, demonstrating the high sensitivity of this technique,” explains Marie Skłodowska-Curie fellow Claudia Tait.

New insights into relevant material properties

One major challenge in organic electronics is to control the conductivity of polymers through molecular doping. SpinSolar therefore investigated doping of P3HT, an organic polymer that can be rendered conductive by oxidation and is considered for use in solar cells, field-effect transistors and chemical sensors. “In a combined optical and ESR spectroscopic study, we compared a series of different dopants and different forms of the polymer in an attempt to identify the relevant molecular properties for efficient doping,” notes Behrends. The ability of ESR to quantify and characterise the paramagnetic species produced by doping showed that the doping mechanism is determined primarily by the ability of the polymer to delocalise the acquired charge, rather than the size and shape of the dopant molecule. They also found that different dopants can lead to significantly different ESR spectral signatures of the polymer-dopant system and that advanced pulse ESR techniques at multiple frequency bands are required for an accurate characterisation. SpinSolar successfully demonstrated the potential of the ESR technique, in particular advanced pulse methods, to address a wide range of questions. Tait and Behrends conclude: “The knowledge on correlations between molecular structure and electronic properties obtained with this technique has the potential to influence the design of new functional materials and new material combinations for photovoltaics and other organic electronic devices.”


SpinSolar, organic electronics, electron spin resonance (ESR), photovoltaics, molecular doping

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