Electrical properties of polymeric light-emitting diodes

In this work, the electrical properties of polymeric light-emitting diodes (LEDs) based on poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, were studied by dc current–voltage (I–V) curves and impedance/admittance spectroscopy, at different temperatures. ITO/polymer/metal (Al and Au) structures were used to study the effect of the barrier height on the charge injection at the polymer/metal interfaces. For ITO/MEH-PPV/Al devices, strong temperature influence on the dc conductivity was observed in the forward direction whereas, in the reverse direction, the conductivity was almost temperature independent. On the other hand, ITO/MEH-PPV/Au devices presented temperature dependent conductivities for both polarities. This indicates that, for low barrier height at the interfaces (<0.4 eV), the carrier injection is a thermally activated process while, for higher barrier heights, the injection becomes dominated by a tunneling-type process. Application of a dc bias voltage superimposed to the alternating voltage in the impedance measurements also causes a diminution in the activation energy of the conductivity, resembling Schottky barrier lowering at the interfaces or Poole–Frenkel type effects in the bulk.