Abstract

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.

Highlights

  • The most recent doping model consists of three steps, as schematically shown in Fig. 1 for the case of efficient p-type doping.[6,17] The first step involves either host-dopant frontier molecular orbitals hybridization or ground-state integer-charge transfer (ICT) from donor to acceptor molecules

  • Comparing the g-values obtained from simulation with literature values, we attribute species 1 as the radical anion localized on F6-TCNNQ and species 2 as the positive polaron on ZnPc.[40,41,42]

  • The anisotropy of the g-value could be disclosed by using higher-field electron paramagnetic resonance (EPR) spectroscopy (e.g. W-band), which is outside the scope of our work and not relevant for our conclusions

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Summary

Introduction

The most recent doping model consists of three steps, as schematically shown in Fig. 1 for the case of efficient p-type doping.[6,17] The first step involves either host-dopant frontier molecular orbitals hybridization or ground-state integer-charge transfer (ICT) from donor to acceptor molecules. We associate this larger activation energy to a dominant contribution arising from CT state dissociation, while both bound and free polarons contribute to EPR

Sample preparation
EPR measurements
Photothermal deflection spectroscopy
DFT calculations
Electrical measurements
Grazing-incidence wide-angle X-ray scattering
Results and discussion
Two spin-bearing species generated by doping
Antiferromagnetic coupling mechanism
Thermal activation energies for polaron transport
Conclusions
Full Text
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