Single-Walled Carbon Nanotube Composites as Hole Injection Layer for Organic Light Emitting Diode Applications
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Single-walled carbon nanotube composites as hole injection layer for organic light emitting diode applications C. C. Oey,1 A. B. Djuriši ,2 C. Y. Kwong,1 C. H. Cheung,2 W. K. Chan,3 and P. C. Chui1 1 Dept. of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong 2 Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong 3 Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong ABSTRACT In this work, blends of dispersed short (~500 nm) single-walled carbon nanotubes (SWCNTs) with poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) were investigated as hole injection layers in OLEDs consisting of N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transporting and tris-(8-hydroxyquinoline) aluminum (Alq3) as electron transporting and emitting layer. The devices were characterized by electroluminescence and current-voltage measurements. By comparing the performance of devices fabricated using different surfactants (polyethyleneimine (PEI) and Gum Arabic (GA)) in dispersing SWCNTs and those prepared without surfactants, it was found that the use of appropriate surfactants can improve the OLEDs performance. Improved efficiency was obtained for optimized SWCNTs concentration compared to the devices with pure PEDOT:PSS, although maximum luminance is lower. The PEDOT:PSS:SWCNT nanocomposite layers are characterized and the reasons for the improved OLED performance are discussed. INTRODUCTION As a potential candidate for the next generation flat panel color displays, organic light-emitting diodes (OLEDs) have gained much attention in both academic and commercial fields. OLEDs are thin film structures, in which organic materials are sandwiched between two electrodes. The carriers (holes injected from anode and electrons injected from cathode) move towards the oppositely charged electrodes and some of them can recombine radiatively to produce light [1]. In recent years, it has suggested that the dispersion of nanomaterials into the charge-carrier medium can effectively increase the luminance efficiency of devices. One promising candidate as the nanocomposite component is the single-walled carbon nanotubes (SWCNTs) [2-4]. Therefore, we investigated blends of dispersed short (~500 nm) SWCNTs with poly(3,4-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) coated on top of ITO as hole injection layers in OLEDs consisting of N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transporting and tris-(8-hydroxyquinoline) aluminum (Alq3) as electron transporting and emitting layer in this
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work. We also compared the use of different surfactants (polyethyleneimine (PEI) and Gum Arabic (GA)) in dispersing SWCNTs with those prepared without surfactants on device performance. Since PEDOT:PSS is the layer in contact with ITO, it is expected that presence of SWCNTs can affect both injection of holes from ITO and the hole transport within the PEDOT:PSS based composite. The influence of
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