Balancing Charge Injection and Transport in Organic Light-emitting Diodes with a Transparent Conductive Tungsten Oxide Layer

Abstract

High-brightness green phosphorescent hybrid inorganic-organic light-emitting diodes (HyLEDs) and inverted HyLEDs (IHyLEDs) have been demonstrated. The devices comprised a transparent and conductive WO 3 layer deposited by thermal evaporation, which improved both hole injection and transport, and led to more balanced charge injection and significant performance enhancement. At 20 mA/cm 2 , the HyLEDs had a low operation voltage of 6.1 V, 0.8 V lower than that of OLEDs with an organic hole transport layer. With an optimized layer structure, the HyLEDs reached 10 4 cd/m 2 brightness at 7.3 V. At this brightness level, the current efficiency was 55.2 cd/A, 57% higher than that of the OLEDs. In the IHyLEDs, facile hole injection and transport through WO 3 was balanced by electron injection from the indium-tinoxide (ITO) cathode overcoated with nanometer-thick Ca, leading to a low turn-on voltage of ~6 V. Brightness of 8133 cd/m 2 was reached at 20 mA/cm 2 and the corresponding current efficiency was 40 cd/A. The hybrid devices also exhibited markedly improved stability under constantcurrent stressing due to the robust WO 3 hole transport layer.