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OLEDs based on ITO as anode, TPD/Alq3 as organic PN junction, and Al as cathode, represent an important standard model. Since TPD and Alq3 are thin layers (< 100 nm), even at low bias voltage, V (~1 V), the electric field is relatively high (~108 V/m). During the very first bias voltage application to ITO/TPD/Alq3/Al OLEDs, some unexpected currents add to the injection of electrons and holes. These “primary processes” induce irreversible structural alterations in OLEDs. This conclusion is based on the fact that the forward bias current-voltage characteristic curves, I-V, systematically show a first scan that is very different from all subsequent ones. Below 4 V, the first scan shows two current “bumps” (near 1 V and 2.5 V). The current density is ~1 mA/cm2. For all subsequent scans, the bumps disappear, and the current density decreases to ~10 μA/cm2. Above 3 V, the I-V curves show current “spikes”, up to ~100 μA/cm2. Since no light is emitted when the bumps and spikes occur, these current surges are not due to electron-hole recombination. We suggest that four processes occur nearly simultaneously: (1) ionic transport due to ions uniformly distributed inside the organic layers, (2) ionic transport similar to the propagation of a Haynes-Shockley current pulse, (3) electrochemical cathode oxidation via the Cabrera-Mott mechanism, (4) sporadic rotations of organic molecules, similarly to liquid crystals and responsible for current spikes. Processes (1) and (2) are both responsible for the first bump, while (3) and (4) are both linked to the second bump. The theoretical model developed for the I-V characteristic curves agrees well with measurements. These irreversible primary processes could be partly responsible for OLEDs aging.
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