Tunable motional narrowing of polaron charge carrier hyperfine field distributions in ethylene glycol doped poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

The conductivity of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) thin films can be enhanced by adding ethylene glycol (EG) to its aqueous solution. We have investigated the effect of this on charge carrier (polaron) spin states by measuring multifrequency continuous wave (cw) and pulsed (p) electrically detected magnetic resonance (EDMR) on PEDOT:PSS based bipolar injection devices with varying EG content [1]. At T=300K, all devices show nearly Ohmic behavior whose conductivity increases with EG concentration. At T=5K, all devices reveal diode-like behavior. Using EDMR, we have been able to spectroscopically separate the hyperfine (HF) interaction from spin-orbit interaction induced g-factor distributions. We observe a reduction of the HF fields with increasing EG content, yet, at the same time, an increase of the spin coherence times T₂, in spite of the increased charge carrier mobility. This behavior is consistent with motional narrowing of the observed resonance lines and it shows that EG doping can be used to tune the effective local hyperfine fields. [1] G. Joshi et al., Appl. Phys. Lett. 109, 103303 (2016).