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, Mössbauer 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 fellow’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 Phys. Chem. Chem. Phys., was an excellent fit for the broader objective of this project by providing ideal test systems for the correlation of changes in structural parameters with changes in magnetic parameters detected by EPR.
During the course of the project, the research fellow also gained experience in Electrically Detected Magnetic Resonance (EDMR) spectroscopy and was trained in the preparation of thin-film samples of organic semiconductors and of organic solar cell devices for EDMR spectroscopy as well as electrical characterisation of organic electronic devices. With this additional expertise acquired during the fellowship, the researcher is now poised to further pursue research in this field and establish a career as an independent researcher.
