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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SpinSolar (Characterisation method for spin-dependent processes in solar energy technology)

Teaser

Summary

Innovations in the field of solar energy technology have the potential to replace fossil fuels with energy from renewable resources and thereby help reach sustainable development goals. However, progress now crucially relies on an improved fundamental understanding of the processes involved in energy conversion on the molecular level. Significant research effort is currently devoted to the investigation of organic molecules and polymers as the active component in solar cells, with significant advantages compared to traditional silicon-based solar cells in terms of tunability through chemical modification, flexibility and reduced manufacturing costs. Similar organic materials also hold promise for applications in organic light emitting diodes and organic thin-film transistors. Investigation of how molecular structure affects the electronic properties of these materials is at the basis of the development of new design rules resulting in improved high-efficiency devices.

Since the function of organic photovoltaic and optoelectronic devices is based on the generation and transport of charge carriers, typically characterised by the presence of unpaired electrons, Electron Spin Resonance (ESR), or Electron Paramagnetic Resonance (EPR), spectroscopy provides the ideal characterisation tool for these systems. The main objective of the project is to demonstrate how EPR spectroscopy can contribute to the characterisation of materials for photovoltaics and optoelectronics and how the resulting in-depth understanding of structural and electronic properties, as well as of their interdependence, can lead to new insights enabling progress in the development of new materials.

Work performed

Early on during the fellowship, an opportunity for a collaboration on doping of organic semiconductors with two of the leading groups in this field presented itself. Organic semiconductors are widely explored for application in photovoltaics, light-emitting diodes and field-effect transistors, but in order to reach their full potential, precise and reliable control over their conductivity is required. Molecular doping is a promising approach to achieve this, however the mechanistic details of the doping process and their dependence on the structural and electronic properties of the dopant molecule and the host material are at present only poorly understood. Efficient doping relies on electron transfer between dopant and host followed by charge separation, essentially analogously to charge carrier generation by photoexcitation in organic bulk-heterojunction solar cells, and therefore leads to the formation of paramagnetic species. The potential of EPR to provide information complementary to other traditionally used techniques and to thereby lead to fundamental insights into the doping mechanism is far from being fully exploited and this ideal opportunity to promote the use of this technique prompted a shift of focus towards the use of EPR to investigate the molecular doping of organic semiconductors.

In one collaboration, doping of the prototypical hole conductor material in organic photovoltaics, poly(3-hexylthiopene) (P3HT), with a novel highly reactive two-coordinate boron cation was compared to a previously proposed boron-based dopant. In addition to characterising the properties of doped host material and demonstrating a significantly higher doping efficiency for the new dopant, evidence from EPR led to the proposal of the formation of bipolarons at high concentrations of the borinium ion dopant. In a second collaboration, the doping process for two forms of P3HT characterised by different extents of long-range order was investigated in solution. Correlation of EPR and UV-vis-NIR measurements revealed that for ordered P3HT, doping proceeds via integer charge transfer for all investigated dopants, while for the more disordered form charge-transfer complex formation seems to dominate. The more extensive spin delocalization determined by ENDOR in the former material suggests that the ability of the charge to delocalize on the host molecule influences the type of doping mechanism. Thus, the local morphology in the polymer chain appears to be a more crucial parameter in determining the doping mechanism than the shape or size of the dopant molecule. The results of both studies are expected to be published in high-impact journals in the near future. An additional, currently still ongoing, investigation of the host-dopant pair P3HT-F4TCNQ by multifrequency continuous wave and pulse EPR is shedding new light on the interactions between the paramagnetic species generated during the doping process and has the potential to provide a more nuanced picture of the doping process beyond the current distinction between the two extremes of integer charge transfer and charge transfer complex formation.

As set out in the proposal, inorganic Fe-N-C catalysts for proton exchange fuel cells were also investigated in a collaborative combined EPR, MÃ¶ssbauer and nuclear inelastic scattering study published in Angewandte Chemie. Identification of iron centres in different molecular environments highlighted the importance of advanced characterisation methods capable of distinguishing catalytically active centres from spectator species for the development of efficient and clean catalytic systems.

Furthermore, thanks to the extensive collaboration network of the host group, there was an opportunity to apply the fellowâ€™s previous research experience to an investigation of the influence of the twist angle in a series of helically-locked anthracene-based twisted acenes on the properties of their photoexcited triplet state. This study, published in Ph

Final results

The project has contributed to substantially improve the understanding of molecular doping of organic semiconductors with implications for the fields of photovoltaics and optoelectronics. Knowledge gained on the molecular parameters affecting efficiency will guide the design of new materials and fabrication methods leading to advancements in the development of electronic devices based on organic materials. In the longer term, this has potential socio-economic impacts through the establishment of new technologies for clean and renewable energy.

On a general note, this project has contributed to promoting EPR as a technique for the characterisation of organic materials for photovoltaics and optoelectronics through the establishment of several collaborations with groups devoted to material and device design. Insights gained with this technique on structure-property relationships have the potential to influence rational design of new functional materials.