Current Conduction and Electroluminescence Mechanisms in Molecular Organic Light Emitting Devices.

Abstract

Electroluminescence (EL) from vacuum deposited organic molecular heterojunction (HJ) devices can potentially yield red, green and blue light at levels of brightnesses and efficiencies adequate for viable flat panel displays1. However, to date, there has been little detailed analysis of the origin of EL and the current conduction mechanisms in these novel devices. The spatial distribution of EL in HJ devices using tris-(8-hydroxyquinoline) aluminum (Alq3) as the light emitting layer has been measured and shown to occur within a few hundred Ångstroms of the organic HJ.2 This was assigned to excitons created at the organic HJ subsequently diffusing into the bulk organic layer before radiatively recombining. Charge injection mechanisms in such structures, however, were not elucidated. In polymeric OLEDs, tunneling3 into the conduction bands of the organic material has been invoked to explain the observed current-voltage (I-V) and EL characteristics. However, given the low carrier mobilities characteristic of many organic materials and the difficulties in measuring and interpreting band offset data, the validity of simple band structure and tunneling models remains unclear.