17.4: A Novel Technique to Study OLED Function

We employ electrically induced absorption (EIA) spectroscopy to probe the internal operation of a series of NPB/Alq 3 (1) OLED structures. Our results complement the capacitance‐voltage measurements of Brütting et al [3] and support the proposal of a fixed negative charge population at the NPB/Alq 3 interface. We show a direct signature for hole injection under reverse bias (as predicted in [3,4]) and demonstrate that the voltage onset for hole injection, V h (equivalent to transition voltage, V o in [3,4]) is largely independent of anode material. Conversely, V h appears to depend sensitively on the cathode material (an exchange of LiF/Al for CsF/Al raises V h by +0.5V). Finally, we observe a rise in V h after a prolonged period of current stress and propose that this finding originates in a gradual neutralization of the negative interfacial charge population.