The Other Crucial Factor Dominates the Holes Injection Efficiency of Ultra-thin Anode Buffer Layer on Organic Light Emitting Devices

Sheng-Yuan Chu

, Yu-Cheng Chen

, Hsin-Hsuan Huang

and Po-Ching Kao

*Department of Electrical Engineering, National Cheng Kung University, Tainan 70101, Taiwan

**Department of Applied Physics, National Chiayi University, Chiayi 60004, Taiwan

In this study, we have investigated surface energy of ITO substrates modified by ultra-thin

buffer layers plays a crucial factor as important as energy barrier. With larger surface

energy, organic light emitting devices could get superior performance even there exist

larger barrier between ultra-thin buffer layer modified ITO and organic layer

So far, various buffer layers have been adopted to improve the OLEDs efficiency. However, each

attempted buffer material reported explained by forming the dipole at interface and then energy

bending to decrease the injection barrier. Here we adopting two buffer layer materials elucidate

the surface energy plays a crucial role for holes injection efficiency as important as energy barrier..

Introduction

Abstract

Experiments, results and discussions

ITO

Buffer layer( NaF,CuF

₂

)

NPB

Alq3

LiF/Al

Sessile drop

Measurement:

Contact angle

ITO

Bufferr layer( NaF,CuF

₂

)

AC2 measurement:

Work function

a)

b)

c)

d)

Fig a) and b) reveals the device performances i.e. luminance, current density and turn on voltage all are superior than standard device when buffer layers were treated by UV-ozone and get worse with pristine buffer layers. Figure c) shows the schematic relationship between photoelectron emission and incident photon energy for buffer layers deposited on an ITO substrate, pristine and after UV-ozone treatment, as measured by a photoelectron spectrometer (AC-2). It reveals ITO modified by CuF₂ layer either UV-ozone treated or pristine shows no barrier between electrode and organic but NaF layer does. Interestingly, the results of energy diagram are drastically different with the device performances. There must exit another crucial factor dominates the hole injection efficiency between buffer layer/ITO anode and organic layer. And Fig d) shows that the results of calculated surface energy correspond with the device performances.

Increasing the surface energy and the work function simultaneously causes

superior performance of the OLEDs. Raising work function only without surface

energy at the same time, even there is no barrier between ITO and the organic layer

it still leads to inferior device performance. The results show the surface energy

indeed is another crucial factor that dominates the performance of OLEDs. It is a

crucial factor should be considered when choosing the buffer layer materials.

Conclusion