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The enhancement and control of the electrical conductivity of organic semiconductors is fundamental for their use in optoelectronic applications and can be achieved by molecular doping, which introduces additional charge carriers through electron transfer between a dopant molecule and the organic semiconductor. Here, we use Electron Paramagnetic Resonance (EPR) spectroscopy to characterise the unpaired spins associated with the charges generated by molecular doping of the prototypical organic semiconductor poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and tris(pentafluorophenyl)borane (BCF). The EPR results reveal the P3HT radical cation as the only paramagnetic species in BCF-doped P3HT films and show evidence for increased mobility of the detected spins at high doping concentrations as well as formation of antiferromagnetically coupled spin pairs leading to decreased spin concentrations at low temperatures. The EPR signature for F4TCNQ-doped P3HT is found to be determined by spin exchange between P3HT radical cations and F4TCNQ radical anions. Results from continuous-wave and pulse EPR measurements suggest the presence of the unpaired spin on P3HT in a multitude of environments, ranging from free P3HT radical cations with similar properties to those observed in BCF-doped P3HT, to pairs of dipolar and exchange-coupled spins on P3HT and the dopant anion. Characterisation of the proton hyperfine interactions by ENDOR allowed quantification of the extent of spin delocalisation and revealed reduced delocalisation in the F4TCNQ-doped P3HT films.

Doping the electron transport polymer poly [N,N amp; 8242; bis 2 octyldodecyl naphthalene 1,4,5,8 bis dicarboximide 2,6 diyl] alt 5,5 amp; 8242; 2,2 amp; 8242; bithiophene [P NDI2OD T2 ] with the bulky, strongly reducing metallocene 1,2,3,4,1 amp; 8242;,2 amp; 8242;,3 amp; 8242;,4 amp; 8242; octaphenylrhodocene OPR leads to an increased bulk conductivity and a decreased contact resistance. While the former arises from low level n doping of the intrinsic polymer and increased carrier mobility due to trap filling, the latter arises from a pronounced accumulation of dopant molecules at an indium tin oxide ITO substrate. Electron transfer from OPR to ITO leads to a work function reduction, which pins the Fermi level at the P NDI2OD T2 conduction band and thus minimizes the electron injection barrier and the contact resistance. The results demonstrate that disentangling the effects of electrode modification by the dopant and bulk doping is essential to comprehensively understand doped organic semiconductors

Twisting of the acene backbone out of planarity in twisted acenes leads to a variation in their optical and electronic properties. The effect of increasing twist angles on the properties of the photoexcited triplet states of a series of anthracene-based helically tethered twisted acenes is investigated here by Electron Paramagnetic Resonance (EPR) spectroscopy. Increasing signal intensities with increasing twist angles indicate increased intersystem crossing efficiencies for the twisted molecules compared to the untethered reference compound. Variations in the electron spin polarisation observed in the transient EPR spectra, in particular for the compound with the shortest tether, imply changes in the sublevel population kinetics depending on molecular geometry. Changes in the zero-field splitting parameters and in the proton hyperfine couplings for compounds with short tethers and therefore higher twist angles point towards a slight redistribution of the spin density compared to the parent compound. The experimental results can be explained by considering both an increase in twist angle and a related decrease in the dihedral angle between the phenyl side groups and the acene core. The observation of a clear excitation-wavelength dependence suggests preferential excitation of different molecular conformations, with conformers characterised by higher twist angles selected at higher wavelengths.

Fe-N-C catalysts are very promising materials for fuel cells and metal-air batteries. This work gives fundamental insights into the structural composition of an Fe-N-C catalyst and highlights the importance of an in-depth characterization. By nuclear- and electron-resonance techniques, we are able to show that even after mild pyrolysis and acid leaching, the catalyst contains considerable fractions of α-iron and, surprisingly, iron oxide. Our work makes it questionable to what extent FeN4 sites can be present in Fe-N-C catalysts prepared by pyrolysis at 900 °C and above. The simulation of the iron partial density of phonon states enables the identification of three FeN4 species in our catalyst, one of them comprising a sixfold coordination with end-on bonded oxygen as one of the axial ligands.