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Flexible InP based quantum dot light-emitting diodes using Ag nanowire-colorless polyimide composite electrode
TIME:2017-6-15 8:35:00

ABSTRACT
A smooth, flexible, and transparent electrode was fabricated by embedding a percolated network of Ag nanowires (AgNWs) at a surface of colorless polyimide utilizing an inverted layer film-processing approach. The electrode with buried AgNWs showed excellent physical characteristics: a smooth surface roughness (less than 1 nm in root-mean-square roughness), high flexibility (resisted cyclic bend testing with a curvature radius of 500 ¦Ìm for 1000 iterations), high transparency (higher than 84% at a wavelength of 550 nm), and good conductivity (12.38 ¦¸/sq for sheet resistance). One demerit that remained for this electrode was a limited surface coverage of conductive pathways for electric carrier transference, originated from the fully embedded structural configuration. Here, the authors could successfully deal with this issue by an Ar plasma treatment in order to partially excavate the embedded AgNWs without a significant increase in surface roughness. Green quantum-dot light-emitting diodes (QLEDs) using InP based quantum dots were fabricated using these composites as a bottom electrode. Hole-injection was poor for an electrode without the plasma treatment since the AgNWs were fully buried beneath the composite's surface. As a result of the plasma treatment, however, the authors could obtain much improved electroluminescence properties of the flexible QLEDs due to the enlarged conductive pathways. The fabricated flexible devices showed durable performance even under a bending with a curvature radius of 5 mm.

I. INTRODUCTION

Colloidal quantum dots (CQDs) have been highly spotlighted as new materials for the next generation display due to their unique properties such as a narrow full width at half maximum (FWHM), color tunable luminescence, and cost-effective solution processability.1,2 Since the first practical application of electroluminescence (EL) of CQD in 1994, the performance of quantum-dot light-emitting didoes (QLEDs) has been dramatically increased by many research groups over the last two decades.3 Nowadays, flexible displays have been emerged as a new trend in consumer electronics, and therefore, new materials for flexible devices and various fabrication techniques for stable flexibility have been investigated intensively. The research for performance enhanced QLEDs is now branching out into its flexible application, as well.4¨C6 Since most EL devices using CQDs have been focused into the charge balance in the emission layer to improve the current efficiency, only very limited results have been reported so far dealing with the flexible QLEDs using Cd-free CQDs.

In general, QLEDs have been fabricated by the sequential deposition of thin organic or inorganic layers and a metallic layer on a patterned indium tin oxide (ITO) anode. Since ITO is not a suitable material for flexible electronics due to its brittleness, many new materials such as carbon-based materials,7,8 conductive polymers,9,10 and metal-based electrodes11¨C13 have been vigorously researched for a possible candidate for substituting the ITO. Among them, a percolated network of silver nanowires (AgNWs) has been considered one of the most promising materials with their high flexibility, conductivity, and transparency.14¨C16 However, the well-known poor surface roughness originated from its networked structure percolated in low density hinders active application in optoelectronic devices. Meanwhile, AgNW-polymer composites showed a solid and smooth conductive surface by burying the AgNW film at the surface of the polymer matrix. However, since most AgNWs are buried inside the polymer matrix, only a very small amount of AgNWs is exposed at the surface, resulting in a limited conductive pathway. Consequently, the charge injection into the light emitting layer of such a QLED is poor. To address this matter, employing an additional conducting layer on the AgNW layer was reported.16 However, the use of an additional material can increase the fabrication cost, decrease transmittance, and even induce damage to the AgNWs due to the acidity of the additive material. Therefore, it is necessary to develop a smooth electrode with large conductive surface coverage, without employing any additional conductive materials, for application in the highly efficient QLED devices.

Here, we used a colorless polyimide (cPI) to make an embedded-AgNW composite electrode and hence a smooth and flexible transparent anode for use in InP based QLEDs. The cPI employed in this study was highly transparent and colorless, while exhibiting superior mechanical properties. To fabricate our composite electrodes for QLEDs without additional planarization materials, we introduced a sacrificial layer for an improved peeling-off procedure. To enlarge the surface coverage of the buried AgNWs, a simple Ar plasma etching was performed without severe deterioration of the composite surface's smoothness. Finally, highly efficient and flexible green InP based QLEDs were successfully fabricated on the AgNW-composite electrodes.


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