Field-assisted ionization of molecular doping in conjugated polymers

The field- and temperature dependence of the charge transport in solution-processed poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) doped with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) is investigated. Conventional charge transport models based on a density- and field-dependent mobility in combination with a constant background hole density from the dopant do not explain the strongly enhanced hole current at high electric fields. Addition of field-assisted ionization of F4-TCNQ with a field-enhancement factor ∼exp(γ√E), which increases with decreasing temperature and doping concentration, consistently describes the charge transport in F4-TCNQ doped MEH-PPV.