Impurity-gas-dependent charge injection properties at the electrode-organic interface in organic light-emitting diodes

We determine how impurity gases, such as oxygen and water, influence the pe rformance, stability, and electronic properties of organic light-emitting d iodes (OLEDs). We fabricate and operate the devices in a controlled ultrahi gh-vacuum (LTHV; pressure below 10(-9) mbar) environment. The UHV system al lows complete control of the exposure to impurity gases, and the measuremen t of current-voltage characteristics, electroluminescence, and impedance sp ectroscopy before and after exposure to oxygen or air. These measurements s howed that both pure oxygen and air increase the threshold voltage for ligh t emission. Exposure to air leads to a higher efficiency and to a degradati on of the devices with the appearance of black spots. This is not the case for oxygen. Additional impedance spectroscopy measurements also confirm tha t exposure to air has a large influence on the stability of the electrode i nterfaces, with the appearance of an additional capacitance in the equivale nt circuit for a device with an Mg cathode exposed to air, a fact that coul d be connected to the effect of atmospheric gases such as oxygen and water on the Alq(3)/Mg system. (C) 2001 Elsevier Science B.V. All rights reserved .