Effect of ITO Carrier Concentration on the Performance of Organic Light-Emitting Diodes
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Furong Zhu Institute of Materials Research & Engineering, 3 Research Link, Singapore 117206 Keran Zhang, C. H. A. Huan, and A.T.S.Wee Department of Physics, National University of Singapore Lower Kent Ridge Road, Singapore 119260 Ewald Guenther, Chua Soo Jin Institute of Materials Research & Engineering, 3 Research Link, Singapore 117206 Abstract The indium tin oxide (ITO) anodes for organic light emitting diode (OLED) were made from an oxidised target with In2O3 and SnO2 in a weight proportion of 9:1 using the RF magnetron sputtering method. The comparable ITO anodes with different carrier concentrations were prepared by varying the hydrogen partial pressure during film
deposition.
The
current-luminance-voltage
characteristics
of
the
devices
indicated that a high carrier concentration in ITO plays a role in improving OLED performance. A maximum efficiency of 3.8 cd/A was achieved when an ITO anode with a higher carrier concentration of 9×1020 cm-3 was used in a fluorene based OLED. This efficiency is about 1.5 times higher than that of an identical device made with an ITO anode having a lower carrier concentration of 5×1020 cm-3. The increase in electroluminescent efficie ncy reflects an enhanced hole-injection in the device. We consider that enhanced hole injection is due to the reduced band bending in ITO when it has a high carrier concentration.
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1. Introduction
Thin film of indium tin oxide (ITO) has attracted much interest due to its unique optoelectronic properties. ITO used in different opto-electronic devices is often optimized according to its application and the operating principle of the devices involving ITO. For example, the use of ITO as a heat mirror is based on its distinctive high reflection over the infrared light region [1]. The ITO electrodes with textured feature and suitable thickness can boost the internal absorbance and hence increase the conversion efficiency of an ITO/p-i-n/ITO/metal type thin film silicon solar cells [2-4]. An ITO contact parallel to the substrate is essential for providing a uniform electric field in liquid crystal displays. In organic light emitting diode (OLED), ITO serves both as a transparent electrode and more importantly as an anode for hole-injection. The optoelectronic properties of ITO anode and different ITO cleaning processes have shown to influence the performance of OLED [5-9]. The understanding of a good anode for OLED is of practical and technical interest.
The use of high work function materials including p-type silicon [10, 11], thin film of gold [12], and some high work function metal oxides [13] as anodes for OLEDs has been reported. Since these materials have relatively higher values in the work function, an enhanced hole-injection was observed when these materials were tested as anode in the devices. Efforts on growing a self-assembled layer on ITO surface to increase the carrier injection were also reported [14, 15]. Inserting layers of hole transport layer (HTL) to narrow the band offset between ITO anode and emissive layer is common
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