Electroluminescence transient of polymer light emitting diodes: field dependence of overshoot and time delay

Transient electroluminescence is widely used to obtain the effective mobility of carriers in single layer polymer light emitting diodes (PLEDs). We monitor the shape of the pulse mode electroluminescence (EL) of PLED with two different active layers viz. a new polymer derived from Bispyrrole (AVPV) and poly [2-methoxy-5-(2´-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). We show that the occurrence of EL overshoot at turn-on and the transient shape is crucially dependent on the field and the cathode used. We argue that the apparent mobility depends on the injection efficiency of the cathode probably due to the variation in the degree of electron participation in the evolution of electroluminescence. The shape of the EL transient is dependent on the material and electrode combination due to the competition between dispersive transport and relaxation of carriers within the density of states. An anomalous dependence of mobility on field is observed when calcium is used as the electron injecting contact. Our results indicate a possible role of hot carrier dynamics in defining effective time delay in EL onset. In addition, field independence of time delay in the high electric field regime has been attributed to the tunneling of hot carriers directly to the transport states of the active layer instead of transport via the intermediate interfacial and bulk trap states within the energy gap of the conjugated system.