Temperature Dependence of Current Transport and Electroluminescence in Vacuum Deposited Organic Light Emitting Devices

We measure the temperature dependence of the current-voltage (I-V) and electroluminescence (EL) characteristics of organic light emitting devices based on the molecular compound; tris(8-hydroxyquinoline) aluminum ( Alq 3 ). We find that the current is limited by a large density of traps with an exponential energy distribution below the Alq 3 lowest unoccupied molecular orbital. The measured trap density suggests that each molecular site may serve as an electron trap, and that EL arises from the recombination of trapped charge. The relationship between the I-V characteristics and EL provides insight into the optimization of vacuum deposited organic light emitting devices.