Carrier traps and effect of oxygen in MEH-PPV polymer diodes

We investigated carrier transport and capture in poly[2-methoxy-5-(2'-ethyl-hexyloxy)1,4-phenylene vinylene] (MEH-PPV) Schottky diodes by thermally stimulated currents and current-voltage characteristics. Two main electron traps were found after the white light excitation. Their effective activation energies were in the ranges 0.207 - 0.355 eV and 0.75 - 0.91 eV, and the total charges were up to (2-8)x 10(14) cm(-3) and similar to (1-2)x 10(16) cm(-3) respectively. We report effect of oxygen on these traps that was never observed before. The activation energy and level filling could be increased significantly by exposing the sample to the air. The deeper trap is likely located nearby surface (probably in the contact region), meanwhile the shallower trap is most probably distributed over the sample depth. In contrast none of the traps could be recharged by applied voltage. Instead the injected carriers created a long-living sample polarization. The non-exponential depolarization lasted for several thousands seconds and was not thermally activated even above the glass transition temperature. These facts make it necessary to include into analysis other possible physico-chemical mechanisms, e.g., reversible chemical reactions or chain structure reorganization induced by electric field.