Influence of the process vacuum on the device performance of organic light-emitting diodes

We demonstrate that by growing organic light-emitting diodes (OLEDs) under ultra high vacuum (UHV) conditions at a base pressure of 10−9 mbar with the organic molecular beam deposition (OMBD) technique, the device performance can be significantly improved. Our devices consist of a CuPc (copper phthalocyanine) hole injection layer, a 4,4′-bis(3-methylphenylphenylamino)-biphenyl (TAD) hole transport layer and an aluminum-tris-(8-hydroxychinoline) (Alq3) emitter layer. The operating voltage is reduced from 7.6 to 5.4 V compared to a device realized under high vacuum (HV) conditions at 10−6 mbar. Beside this we also found a decrease of quantum efficiency and an increased formation of black spots in our OLEDs at higher base pressures of the fabrication process.