Charge Generation Mechanism of Metal Oxide Interconnection in Tandem Organic Light Emitting Diodes

Abstract

The mechanism of charge generation in metal oxide-based charge generation layers (CGLs) in tandem organic light emitting diodes (OLEDs) was studied via in situ synchrotron radiation photoelectron spectroscopy (SRPES) and in situ ultraviolet photoemission spectroscopy (UPS). The energy band structure and interface dipole energy of a CGL architecture comprising Ca doped tris(8-hydroxyquinoline) aluminum (Alq3), 4,4′-bis[N-(1-naphtyl)-N-phenylamino]biphenyl (α-NPD), and various kinds of metal oxides are studied. The charge generation property is contributed to the amount of work function and interface dipole energy of metal oxide CGLs. The hole injection barrier at the metal oxide/α-NPD interface decreased as a function of the work function of the metal oxide. However, contrary to common belief, the large interface dipole resulted in a small hole injection barrier and low operation voltage of the device. Using data on interface energetics measured by in situ SRPES and UPS, it is shown that the work function of the metal oxide is a key factor in determining the charge generation process. The low work function (<4.50 eV) of metal oxides such as Sb2O3 and CoO showed a large hole injection barrier (>1.0 eV). Meanwhile, due to the high work function of AgO (5.40 eV), the hole injection barrier at the AgO/α-NPD interface could be reduced to 0.36 eV. Thus, the tandem OLEDs with AgO showed the lowest turn-on voltage (15 V) and highest current efficiency (41 cd/A) out of all the OLEDs studied in this work.