Elsevier

Carbon

Volume 91, September 2015, Pages 304-310
Carbon

A 46-inch diagonal carbon nanotube field emission backlight for liquid crystal display

https://doi.org/10.1016/j.carbon.2015.04.093Get rights and content

Abstract

A large area carbon nanotube field emission backlight that is built with a new cathode structure is reported. The approach involves unique gate insulator formation by glass etching, highly populated multi wall nanotube tips, and gate electrode assembly by anodic bonding. Impressive lamping performances are noted through dynamic control of the built in fine 4320 local dimming blocks as small as 1 cm2. The liquid crystal display lit by the backlight demonstrates superior luminance uniformity 97%, and a native contrast ratio 400,000:1, whilst showing less than 10% decay in emission current of the sealed panel during continuous operation over 8000 h.

Introduction

Exploiting carbon nanotube (CNT) field emitters in cathodoluminescent devices has widely been demonstrated [1], [2], [3], [4], [5], [6], [7], such as field emission display, backlight unit (BLU) for liquid crystal display (LCD), and household lamp. Among the numerous applications of CNTs, these luminescent devices can be the most promising because the commercial market is huge, and the impact is supposed to be very direct to the consumers to experience nanotechnology. CNTs provide the most ideal geometry for field emission, known as great aspect ratio over 1000. In addition, they possess superior chemical stability thanks to strong covalent bonds, leading to more robust and resistant to electromigration under strong electric field than traditional metallic structures. Though CNT emitters have shown their promising properties including extraction of high currents (a few mA per emitter) under such an ideal condition as ultra high vacuum (UHV, <10−8 Torr), the details of time-dependent emission behaviors in the sealed large area devices are strongly affected by emitter distribution and evolution of the internal vacuum condition. Under an extreme condition (10−10 Torr), CNTs can emit electrons stably at even high temperature heated by Joule heating, up to 1600 K. This high temperature stability is sustainable by self-surface cleaning through desorption process [8]. However, such a condition, meaning monolayer formation time ∼104 s at 10−10 Torr, is hardly attainable in the normal consumer electronic devices, where getter materials are used for in-situ pumping. Practically, low 10−5–10−6 Torr of vacuum is readily available in the getter installed devices. Moreover, sustaining vacuum condition becomes harder in large-area flat panels due to its limited conductance. In an ensemble of CNT arrays that appears in usual cathode structures, the field enhancement factor (β) of the emitters depends on the spacing between neighboring emitters (d) due to the field screening effect. Nilsson et al. [9] showed that the overall β can be maximized at d  2l, where l is emitter length. Assuming all of the prepared emitters having equal height l = 1 μm, the attainable emitter density with minimum field screening is estimated less than 2.5 × 107 cm2. However, in the sealed devices under moderate vacuum condition with a range of tip height distribution, preparation of the highly populated emitter tips (⩾109 cm−2) at a little sacrifice of β is desirable to ensure the lifetime stability of the emitters via minimizing average current load (average Joule heating) on the field emitting CNTs.

Meanwhile, a “local dimming” technology is implemented in the current LCDs lit by an array of light emitting diode (LED) arranged at the edge of the frame or at the direct backplane of the panel. The local dimming can dim the desired area of the screen, while keeping the bright parts of the screen bright to improve contrast ratio, in which however, only a broad areal local dimming is available. The edge-lit LEDs supply dimming units as large as a 100 cm2 through horizontal or vertical direction of the panel, while even in the direct back-lit LEDs, individual control of the equipped LEDs are not technically probable, but they do zone dimming in an area as large as a few tens of cm2. Therefore, if a zone is lit during the rest zones are not lit, resulting in a bloom (halo) as that part of the image becomes unnaturally brighter than its neighboring zones (known as blooming artefact). By contrast, LCD lit by CNTBLU suggested a possible fine local dimming block as small as 1 cm2 by cost-effect design with a simple matrix operation, providing high contrast and fast response time [10].

What is missing till now in the CNTBLU is a cost-effective architecture in the cathode expendable to large substrates with good reliability in the emission current and brightness uniformity. Here, we detail the fabrication and optimization of our 46 inch diagonal CNTBLU based on a new cost-effective cathode frame and its reliability for over 8000 h.

Section snippets

Experimental

The fabrication flow of our cathode structure and unique features of the CNTBLU are illustrated in Fig. 1, starting with forming groove lines (60 μm in depth and 765 μm in pitch) through spray etching on a soda-lime glass (1.8 mm t, and 1088 × 653 mm2 in dimension) patterned with thick printable photo-resist. In the groove lines, a series of screen printing and firing was followed successively to build cathode lines by stacking triple layers of barrier (glass paste, Bi2O3–B2O3–SiO2)/cathode electrode

Results and discussion

Highly crystallized multi-wall CNT (MWCNT) produced by arc discharge process (JFE Engineering Corp., Japan) was selected for the formulation of emitter paste (see Supporting Information), having ten to twelve graphitic walls (Fig. 3a). A very low integrated D (disordered, 1351 cm−1) to G (graphitic, 1588 cm−1) intensity ratio (ID/IG) ∼0.33 in the Raman scattering data (Fig. 3b) was observed from the raw material named MW3, which is unusually lower value than the other comparative MWCNTs (MW1 and

Conclusions

A 46-inch diagonal CNT field emission BLU that is built with a new cathode structure is reported. The approach involves unique gate insulator formation by glass etching, highly populated MWCNT tips, and gate electrode assembly by anodic bonding. Impressive lamping performances are noted through dynamic control of the built in fine 4320 local dimming blocks as small as 1 cm2. The LCD-TV lit by the backlight demonstrates superior luminance uniformity 97%, and a native contrast ratio 400,000:1,

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