Polymer and cathode emission studies of polymer-based light-emitting diodes under strong electrical pulse excitation

We present spectroscopic and time-resolved studies of polymer-based light-emitting diodes under strong pulse excitations. The devices were fabricated using soluble poly(p-phenylene vinylene) (PPV) derivatives. Above some threshold voltage, new emission lines were observed in addition to the electroluminescence (EL) emission associated with the polymer. These new emission lines are attributed to excitation of the cathode material by collisions with holes at electric fields above 3.2 MV/cm. Time-resolved studies of the cathode emission and the polymer EL show similar time delays between the EL emissions and the onset of the voltage pulse. This supports the idea that the polymer EL emission at high fields and the cathode emission are governed by hole transport. Current-voltage characteristics were studied up to 6 MV/cm. The data can be described by the space charge limited current model for electric fields up to 2.7 MV/cm but show deviation from this model at higher electric fields. EL quantum efficiency decreases for fields above 2.5 MV/cm. This decrease is attributed to exciton dissociation at high fields. Based on the results in this article we suggest that hybrid EL devices containing layers of polymers and inorganic nanoparticles operating under high field excitation may have some advantages for future electrically pumped lasers. (C) 2000 American Institute of Physics. [S0021-8979(00)05215-4].