Controllable Molecular Doping and Charge Transport in Solution-Processed Polymer Semiconducting Layers

Here, controlled p-type doping of poly(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) deposited from solution using tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) as a dopant is presented. By using a co-solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current-voltage characteristics of MEH-PPV-based hole-only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of I F4-TCNQ dopant per 600 repeat units of MEH-PPV leads to a free carrier density of 4 x 10(22)m(-3). Neglecting the density-dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4-TCNQ doped MEH-PPV and a silver electrode.