The influence of oxygen and air on the characteristics of organic light-emitting devices studied by in vacuo measurements

Organic light-emitting devices (OLEDs) based on end-capped sexithiophene or aluminium-quinolate (Alq3) with aluminium and ITO contacts were prepared by vapour deposition in high-vacuum (10−6 mbar). Current–voltage (I–V), capacitance–voltage (C–V) and impedance measurements were performed in situ without breaking the vacuum. The OLEDs were then deliberately exposed to oxygen or air. For both materials, the I–V curves of pristine OLEDs are non-rectifying and show significant anomalies at about ±5 V. The capacities are found to be voltage- and frequency-independent. Whereas doping with dry oxygen has only small influence on the electrical properties, doping with air yields a rectifying I–V behaviour without anomalies, but at significantly reduced currents. This is attributed to a decrease in carrier mobility. The capacitance of the Alq3-OLEDs remains voltage- and frequency-independent, whereas for thiophene-based OLEDs, a “Schottky-like” capacitance with a net carrier concentration of about Neff≈1017 cm3 is observed.