The role of traps in polymer-based light-emitting devices

We describe ‘injectionless’ electroluminescence (EL) mechanism in organic light-emitting diodes (LED) based on ‘thick’ films of poly-(phenylene vinylene) (PPV) and poly-[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) sandwiched between ITO and Al electrodes. We provide evidence that this mechanism is associated with detrapping of charge carriers from ‘shallow’ traps. A pump-probe experiment allows an estimate of the trapping–detrapping time scale. This time scale suggests the location of trap levels at 0.15 eV above the HOMO band. We also demonstrate spatially-resolved EL enhancement in thin film devices that can be achieved by simultaneous illumination and biasing the device. We attribute the EL enhancement to charging of traps located near Al/polymer interface. Slow temporal decay of the EL enhancement corresponds to charge release process from ‘deep’ (0.8 eV) traps.