Electron spin as a fingerprint for charge generation and transport in doped organic semiconductors

In this contribution, we use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80-280K) and doping concentrations (0-5wt%). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two paramagnetic species distinguished by two different g-tensors, which are assigned to a positive polaron on the host and a radical anion on the dopant based on DFT calculations. Combined with modelling, the inspection of the EPR spectra shows that anions on the dopants couple in an antiferromagnetic manner at high doping concentrations and that polarons on the host move with an activation energy much smaller than that inferred from electrical conductivity measurements. We rationalize this difference in terms of the disorder-free, intra-grain motion of the polarons probed by EPR, compared to disorder-limited, inter-grain transport probed via electrical measurements.