Fast pulsed electroluminescence from polymer light-emitting diodes

Transient electroluminescence (EL) from polymer light emitting diodes is investigated by measurements of the response to short voltage pulses. The carrier mobility is derived from the delay time between the electrical pulse and the onset of EL, muapproximate to3x10(-4) cm(2)/V s. Bilayer devices with a polyethylene-dioxythiophene (PEDOT), hole injection layer are also studied. The delay time between the electrical pulse and the onset of EL is independent of the thickness of the injection layer, implying that the conducting PEDOT functions as a part of the electrode. When a dc forward bias is applied to the device, the delay time decreases, probably as a result of the shift of the emission zone towards the anode. The EL turn-on depends on the amplitude of the voltage pulse. The data are modeled by an equivalent circuit with a fixed capacitance connected in parallel with a nonlinear resistance. The solution of the differential equation depends on the exact form of the device's I-V curve. Two analytical solutions are provided, and an analysis based on space-charge-limited current is presented. By applying a dc forward bias in advance to precharge the space-charge capacitance, the turn-on response time is reduced to 12 ns. The EL decay consists of two components with time constants of 15 ns and 1 mus. The decay does not depend on either the amplitude of the voltage pulse or the prebias. (C) 2002 American Institute of Physics.