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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 (UHV; 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 Alq3/Mg system.

We present electroluminescence and electrical measurements on organic light emitting diodes (OLEDs) which were produced and operated inside a contamination-free ultra high vacuum (UHV) system, at a purity level corresponding to a pressure of 10−9 mbar. We characterized the devices with a simple magnesium (Mg) cathode, where Mg atoms diffuse into the 8-hydroxyquinoline aluminum (Alq3) layer during evaporation, and devices where a 1-nm thin silver (Ag) layer acts as a diffusion barrier between Mg and Alq3. All OLEDs could be operated in UHV during 1 day at a brightness level of 1000 cd/m2, without the appearance of any “black spots”. When exposed to air however, a significant degradation of the device performance was observed. The lifetime in air was a few seconds for the device with the simple Mg cathode, and a few hours for the device with the Ag layer, indicating the importance of Mg diffusion when the device is operated in air.

We show that the contacts formed by magnesium on tris (8-hydroxyquinoline) aluminum (Alq3) are intrinsically ohmic when they are fabricated and operated in ultrahigh vacuum. Under the same conditions, the injected current shows a steep increase approximately proportional to the seventh power of the applied voltage that we assign to trap filling. Only a subsequent contact with oxygen leads to an injection-limited behavior, where the observed steep current increase is caused by potential barriers at the contacts. In addition, we observe that electron injection in oxidized structures can be very well described by Fowler–Nordheim tunneling in the case when electrons are injected from the magnesium (Mg) contact evaporated onto Alq3.

We show that tris (8-hydroxyquinoline) aluminum (Alq3) thin films produced and characterized under ultrahigh vacuum conditions present a well-defined squared-law dependence of the injected current on the applied voltage at applied electric fields of the order of 0.25–1 MV/cm. From this, one derives an electric-field-independent electron mobility of the order of 10−7 cm2/(V s), with a variation between different samples of about one order of magnitude. Observations of current–voltage characteristics with clear indications of trap-filling and space-charge-limited conduction at high fields in Alq3 excludes the existence of traps with an exponential distribution of trap energies, as is commonly assumed in amorphous materials.

We present a complete characterization of Organic Light Emitting Diode (OLED) structures performed in an ultra-pure Ultra High Vacuum (UHV) environment and under controlled influence of oxygen and atmospheric gases. We fabricated and characterized standard NPB/Alq 3 devices with an Indium Tin Oxide (ITO) anode and a magnesium cathode in an UHV system. With this system we are able to study the injection properties of very clean, controllable interfaces in the absence of any impurity gas. We found that the threshold voltage for OLED operation always increased after exposure to any atmospheric gas, an indication of deteriorated injection properties. However, the luminescence efficiency can become higher after exposure to impurity gases. Without contact to air the OLED do not degrade with appearance of so called 'black spots.' To investigate the intrinsic stability of the OLEDs in ultra-high vacuum we performed a realtime observation on the surface of a 35 nm thin magnesium cathode with Ultraviolet Photoelectron Spectroscopy (UPS). We found that even with a 35 nm thin magnesium-cathode, the underlying organic layer never appeared at the surface also after hours of operation in the ultra-pure conditions. The only sign of deterioration at the cathode is a slow oxidation of the magnesium surface. Thus, OLEDs with semitransparent cathodes are stable if the are operated under ultra-high vacuum conditions.