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 (Alq(3)) 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 o xygen 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 a re found to be voltage- and frequency-independent. Whereas doping with dry oxygen has only small influence on the electrical properties, doping with a ir yields a rectifying I-V behaviour without anomalies, but at significantl y reduced currents. This is attributed to a decrease in carrier mobility. T he capacitance of the Alq(3)-OLEDs remains voltage- and frequency-independe nt, whereas for thiophene-based OLEDs, a "Schottky-like" capacitance with a net carrier concentration of about N-eff approximate to 10(17) cm(3) is ob served. (C) 2000 Elsevier Science S.A. All rights reserved.