The effect of ozone and O2 plasma treatment of ITO on the charge-carrier injection in ITO/N, N'-bis-(1-napthyl)-N, N'-
diphenyl-1,1'biphenyl-4,4'-diamine (NPB)/tris(8-quinolinolato)-aluminum (Alq3)/Al organic heterojunction devices
have been studied through the analysis of current-voltage characteristics. From the experiments, it is demonstrated that
the average electric field inside Alq3 layer is larger than the average field in the NPB layer. The investigation
demonstrated that the hole injection into NPB from anode is Fowler-Nordheim (FN) tunneling and the electron injection
into Alq3 from cathode is Richardson-Schottky (RS) thermonic emission.
A new structure of organic light-emitting diode (OLED) was fabricated by inserting a thin molybdenum trioxide (MoO3)
layer into hole transport layer (HTL) N,N'-diphenyl-N,N'-bis(1-napthyl-phenyl)-1,1'-biphenyl-4,4'-diamine (NPB). The
device structure is ITO/NPB(10 nm)/MoO3(3 nm)/NPB (30 nm)/tris-(8-hydroxyquinoline) aluminum (Alq3) (60
nm)/LiF(0.5 nm)/Al. The control device is set without MoO3 interlayer. The driving voltage at 100 cd/m2 is only 4.87 V
corresponding to the device with MoO3 interlayer, which is much lower than the control device of 6.40 V. The novel
device also shows higher power efficiency compared to the control device. The improvement of device performance is
attributed to the Charge Transfer complex (CT complex), generated at the NPB/MoO3/NPB interfaces, contributing here
as hole transfer enhancer. Our finding additionally demonstrates the practical applicability of MoO3 as a buffer layer in
OLEDs.
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