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-e mitting diodes under strong pulse excitations. The devices were fabricated using soluble poly(p-phenylene vinylene) (PPV) derivatives. Above some thre shold voltage, new emission lines were observed in addition to the electrol uminescence (EL) emission associated with the polymer. These new emission l ines are attributed to excitation of the cathode material by collisions wit h holes at electric fields above 3.2 MV/cm. Time-resolved studies of the ca thode emission and the polymer EL show similar time delays between the EL e missions and the onset of the voltage pulse. This supports the idea that th e 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 high er electric fields. EL quantum efficiency decreases for fields above 2.5 MV /cm. This decrease is attributed to exciton dissociation at high fields. Ba sed on the results in this article we suggest that hybrid EL devices contai ning layers of polymers and inorganic nanoparticles operating under high fi eld excitation may have some advantages for future electrically pumped lase rs. (C) 2000 American Institute of Physics. [S0021-8979(00)05215-4].