Electrical impedance measurements of polymer light-emitting diodes

We report electrical impedance measurements of polymer light-emitting diodes employing the soluble, conjugated polymer poly2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene (MEH-PPV) as the light-emitting layer. The diode structures were metal-polymer-metal structures utilizing thin gold as the transparent, positive contact, and calcium as the negative contact. The devices were fabricated using undoped, polymer active layers ∼40 nm thick. The polymer light-emitting diodes are accurately modeled as a resistor and capacitor in parallel for frequencies from 100 Hz to 1 MHz and for bias conditions from reverse bias to forward current densities of 0.1 A/cm2. The diode capacitance as a function of bias voltage is qualitatively different from conventional Schottky or p-n junction diodes; in reverse bias, the polymer layer is fully depleted and the capacitance is independent of bias; at small forward bias, traps are charged near the metallic contacts and the capacitance increases; under large forward bias, with significant electron and hole injection, the traps are neutralized and the capacitance decreases. From the magnitude of the initial increase in capacitance with forward bias the trap density is estimated to be only a few times 1016 cm−3.