Ultra-high vacuum reveals interface dependent and impurity-gas dependent charge-injection in organic light-emitting diodes

摘要

We present a complete characterization of Organic Light Emitting Diode (OLED) structures performed in an ultrapure 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, conrollable 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".