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 performance, stability, and electronic properties of organic light-emitting diodes (OLEDs). We fabricate and operate the devices in a controlled ultrahigh-vacuum (LTHV; pressure below 10(-9) mbar) environment. The UHV system allows complete control of the exposure to impurity gases, and the measurement of current-voltage characteristics, electroluminescence, and impedance spectroscopy before and after exposure to oxygen or air. These measurements showed that both pure oxygen and air increase the threshold voltage for light emission. Exposure to air leads to a higher efficiency and to a degradation of the devices with the appearance of black spots. This is not the case for oxygen. Additional impedance spectroscopy measurements also confirm that exposure to air has a large influence on the stability of the electrode interfaces, with the appearance of an additional capacitance in the equivalent circuit for a device with an Mg cathode exposed to air, a fact that could 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.