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Bright Inkjet Printed Macromolecular Organic Light emitting Diodes on Flexible Substrates

Published online by Cambridge University Press:  31 January 2011

Madhusudan Singh ,
Hyun Sik Chae ,
Jesse D. Froehlich ,
Takashi Kondou ,
Sheng Li ,
Amane Mochizuki  and
Ghassan Jabbour
Madhusudan Singh
Affiliation:
singh.madhusudan@gmail.com, Arizona State University, School of Mechanical, Aerospace, Chemical and Materials Science Engineering, 85284, Arizona, United States
Hyun Sik Chae
Affiliation:
hyunsik_chae@gg.nitto.co.jp, Nitto Denko Technical Corporation, Oceanside, California, United States
Jesse D. Froehlich
Affiliation:
jesse_froehlich@gg.nitto.co.jp, Nitto Denko Technical Corporation, Oceanside, California, United States
Takashi Kondou
Affiliation:
takashi_kondou@gg.nitto.co.jp, Nitto Denko Technical Corporation, Oceanside, California, United States
Sheng Li
Affiliation:
sheng_li@gg.nitto.co.jp, Nitto Denko Technical Corporation, Oceanside, California, United States
Amane Mochizuki
Affiliation:
amane_mochizuki@gg.nitto.co.jp, Nitto Denko Technical Corporation, Oceanside, California, United States
Ghassan Jabbour
Affiliation:
Jabbour@asu.edu, Arizona State University, School of Mechanical, Aerospace, Chemical and Materials Science Engineering, Tempe, Arizona, United States
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Abstract

We present results of printing solution-processable organic light emitting diodes (OLEDs) based on electrophosphorescent Ir(III) stellate polyhedral oligomeric silsesquioxane (POSS) macromolecules. The macromolecules are doped into a polymer-based ink containing a hole transporting polymer, poly(9-vinylcarbazole) (PVK) and an electron transporting material, 2-4-biphenylyl-5-4-tertbutyl-phenyl-1,3,4-oxadiazole (PBD), and the resulting ink is printed on a layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) spin-coated on indium tin oxide (ITO). An exciton-blocking layer consisting of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is thermally evaporated onto the printed ink layers, followed by a LiF/Al cathode. While the photoluminescence (PL) spectrum of printed ink on glass indicates a significant contribution from the PVK:PBD exciplex, electroluminescence measurements (EL) suggest a much smaller effect of these states, which implies significant charge trapping at dopant molecular sites. Our latest experiments with devices printed on PEDOT:PSS/ITO/glass indicate that the devices exhibit high luminances (∼ 13,000 cd/m2 at 70 mA/cm2) and a fairly consistent quantum efficiency (∼ 2.5%) across a wide range of luminances. The corresponding figures for spin-coated devices are expectedly higher, with efficiencies ∼ 4.5% at similar levels of brightness. We use white light interferometry as a non-contact technique to quantify surface roughness and thickness of printed layers. Our current results thus indicate that inkjet printing of macromolecular phosphor doped polymer inks is suitable for fabrication of OLEDs with high brightness, in spite of the low glass transition temperature of some of the species. Our initial work with these devices on flexible ITO-coated plastic substrates shows devices with moderately high luminance values (∼ 2,500 cd/m2 at 35 mA/cm2). Our more recent devices show higher brightnesses (∼ 9, 600 cd/m2). We are working on the development of inkjet printing materials and processes for pure macromolecular OLEDs that are not performance and reliability limited by the presence of the PVK:PBD matrix with low glass transition temperatures. This requires the development of macromolecules that subsume all the functions of the host matrix and have high enough Tg to be usable in high concentrations needed. In conjunction with the inkjet printing technology demonstrated in this work, this has the potential to yield low-cost manufacturing of these devices.

Keywords

luminescencemacromolecular structureink-jet printing
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1197: Symposium D – Organic Materials for Printable Thin-Film Electronic Devices , 2009 , 1197-D04-23
Copyright
Copyright © Materials Research Society 2010

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