CHARGE-TRANSPORT IN ORGANIC LIGHT-EMITTING-DIODES - POLARONS OR HOLES

Electroluminescence in organic light-emitting diodes (LEDs) based on poly(p-phenylenevinylene) (PPV) or related materials is usually attributed to the radiative decay of polar excitons. A consistent description also requires the understanding of the carrier transport through the active layer. We analyse d.c. conductivity measurements on PPV and show that the conventional semiconductor description can hardly be applied. A model for the transport is proposed, which is based on assuming the existence of a bipolaron ground state and hopping transport of thermally activated polarons. The resulting conductivity mechanism is an admixture of thermal activation and variable range hopping.