Organic thin-film diodes with internal charge separation zone

We demonstrate the fabrication of new organic thin-film diodes with an internal bipolar charge separation (ICS) zone. We fabricated an organic double-layer diode with the structure of indium-tin oxide (ITO)/tris(8-quinolinolato)aluminum(III) (Alq3)/N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/Al. The stacking order of Alq3 and TPD of this diode is reversed compared with conventional organic double-layer LEDs. In the ITO/Alq3/TPD/Al device, only a small current flows in both cases when the ITO electrode is biased positive or negative, because the device has large charge injection barriers and transport resistance. When the combined zone composed of Mg-doped Alq3 and vanadium oxide layers was inserted between the Alq3/TPD interface, large current flow was observed at the positive bias on ITO electrode. The diode behaved quite similar with the conventional organic LED, ITO/TPD/Alq3/Al. The large increase of forward current can never be ascribed to the decrease of injection barriers nor charge transport resistance, because no change of device configuration was added except for the addition of the zone at the Alq3/TPD interface. This large forward current flow was ascribed to the internal bipolar charge separation within the added zone.