Role of ionic species in modifying properties of organic LEDs

Abstract

Ionic species in emissive polymer: The performance of polymer LEDs having the configuration Al/MEH-PPV/ITO (where MEH-PPV equals poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene- vinylene) have been improved by light iodine p-doping of the emissive MEH-PPV polymer layer to presumably produce (MEH- PPV) x +y (I 3 ) xy - whereby the turn-on voltage is reduced from approximately 10 V to approximately 5 V and the external quantum efficiency is increased by an order of magnitude. It differs from non-doped MEH-PPV LEDs in that light emission is observed in both forward and reverse bias modes. The presence of (MEH-PPV) x +y ions at the cathode facilitate electron injection due to (partial) compensation of the injected charge while I 3 - ions at the anode analogously facilitate injection of holes. Ionic species in non-emissive polymer: LEDs having the configuration Al/MEH-PPV/ITO, at a constant applied potential of approximately 11 V, reached maximum intensity of light- emission after approximately 0.07 minute. Those having the configuration Al/MEH-PPV/EB/ITO reached maximum intensity (at approximately 11 V) after approximately 4.2 minutes (where 'EB' equals emeraldine base, the non-doped form of polyaniline), while those having the configuration Al/MEH- PPV/EB.HCSA/ITO reached maximum intensity (at approximately 11 V) after approximately 7.2 minutes (where 'EB.HCSA' is EB completely doped by camphor sulfonic acid). In each case, decay in intensity began to occur after the maximum was reached. The behavior of both systems are consistent with a non-electrochemical model wherein the new phenomena are controlled by an electric field induced diffusion of positive and negative ions towards the appropriate electrodes.