High‐Performance Organic Light‐Emitting Diodes Using ITO Anodes Grown on Plastic by Room‐ Temperature Ion‐Assisted Deposition

Abstract

Organic light-emitting diodes (OLEDs) fabricated on flexible plastic substrates are the focus of increasing attention due to their broad potential applications in portable devices such as cellular phones, personal digital assistants (PDAs), and laptops, etc., which require light weight and mechanical durability. [1,2] Heeger and co-workers first reported flexible OLEDs, fabricated from a conducting polymer electrode deposited on poly(ethylene terephthalate) (PET).[3] It was generally thought at the time that mechanical flexibility could only be achieved with a polymeric electrode. However, Forrest and co-workers subsequently demonstrated flexible, vacuum-deposited, small-molecule OLEDs fabricated on indium tin oxide (ITO)-coated PET and having the structure PET/ITO/ TPD/Alq/MgAg/AgÐanalogous to those of conventional glass-based devices, and capable of repeated flexing.[4] Subsequently, small-molecule OLEDs have been fabricated on several kinds of plastic or plastic/inorganic hybrid substrates, pre-coated with a transparent conductive oxide (TCO) such as ITO, by conventional pulsed-laser deposition or sputtering. [2,5,6] TCO growth on plastic remains a significant challenge for the fabrication of truly efficient flexible OLEDs, due to the poor thermal and mechanical properties of typical polymeric substrates. This is illustrated in ITO film growth on glass, where relatively high deposition and/or post-annealing temperatures (>200 C) are typically required to achieve reasonable electrical conductivity, optical transparency, and longterm stability. Conventionally, ITO film growth on plastic has been achieved by low-temperature deposition techniques such as sputtering. However, such films are typically amorphous, leading to poor conductivity, transparency, and adhesion properties, and underscoring the need for an improved growth technique. In contrast to simple sputtering, ion-assisted deposition (IAD) is uniquely suited for producing smooth, adherent, and microstructurally dense thin oxide films at remarkably low temperatures.[7] IAD employs two ion beams to effect simultaneous film deposition, oxidation, and crystallization, resulting in smooth, dense, coherent films at low temperatures. In addition, the assisting ion bombardment generates fresh surfaces during the pre-and in-situ cleaning/activation process, creating strong interfacial adhesion and removal of voids that can trap loosely bound/physisorbed O 2 , which may degrade OLED performance. These attractions raise the interesting question of whether IAD could be effectively employed in low-temperature ITO depositions for OLEDs, especially on plastics, because ITO's physical properties, such as work function, conductivity, morphology, and surface composition, etc., which significantly influence OLED performance, are strongly dependent on the specific deposition process and post-treatment. [8] To date, there have been no reports of OLED fabrication with IAD-deposited ITO. [9] We report here the growth and characteristics of high-quality ITO thin films on bo...

Flexible Inorganic/Organic Hybrid Thin‐Film Transistors Using All‐Transparent Component Materials

“…The ITO gate electrode was deposited using the same IAD growth system at room temperature. The ITO target (In 2 O 3 :SnO 2 = 9:1) was purchased from Sputtering Materials, Inc., and the ITO growth process details are reported elsewhere [33]. A top-contact electrode structure was employed in TFT device fabrication.…”

Indium Tin Oxide Thin Films Grown on Polyethersulphone (PES) Substrates by Pulsed-Laser Deposition for Use in Organic Light-Emitting Diodes

“…ITO thin films were used as the anode in OLEDs and the device performance was measured. Electro-luminescence is slightly higher than that of an ITO/PET substrate [7], and the external quantum efficiency is similar to that of commercial ITO films [11]- [13]. This indicates that a low temperature grown ITO film by PLD is a good candidate for use as an anode in high performance OLEDs.…”

Efficient Flexible Phosphorescent Polymer Light‐Emitting Diodes Based on Silver Nanowire‐Polymer Composite Electrode

“…The commercial ITO/polyethylene terephthalate (PET) has a typical fi gure-of-merit sheet resistance of 40-100 ohm/sq., much higher than that of ITO/glass. [6][7][8][9] The diminished conductivity is a result of reduced thermal treatment of the ITO after sputtering. In addition, ITO is intrinsically brittle.…”