MEASURING INTERNAL ELECTRIC-FIELDS IN ORGANIC LIGHT-EMITTING-DIODES USING ELECTROABSORPTION SPECTROSCOPY

A widely applicable electroabsorption technique to measure internal el ectric fields in organic light-emitting diodes is presented. The techn ique exploits the change in the a.c. electroabsorption response in the presence of a d.c. electric field. The electroabsorption signal is mo dulated at the fundamental frequency of the a.c. test signal, in addit ion to the usual modulation at the second harmonic frequency, when a d .c. bias is present. In metal/organic film/metal devices employing dif ferent metal contacts there is a built-in electric field in the organi c film caused by the difference in work function between the two conta cts. The electroabsorption response at the fundamental frequency of th e applied a.c, bias is measured as a function of an external d.c. bias . The electroabsorption signal is nulled when the applied d.c. bias ca ncels the built-in electric field established by the different metals. We apply this technique to measure changes in metal-polymer Schottky barrier heights as a function of the contact metal. In metal/multiple organic films/metal structures the electroabsorption signals from the constituent organic films are identified spectroscopically and measure d at both the fundamental and second harmonic frequency of the a.c. te st signal. The amplitudes of the electroabsorption responses are then used to determine the a.c. and d.c. electric fields present in the org anic layers. We apply this technique to determine the d.c. electric fi eld distribution within a multi-layer organic light-emitting diode. Th ese results highlight the general applicability of electroabsorption m ethods to probe internal electric fields in organic light-emitting dio des (C) 1997 by John Wiley & Sons, Ltd.