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ITE Trans. on MTA Vol. 3, No. 2, pp. 121-126 (2015)

1. Introduction

Flexible displays have attracted a great deal of

attention because they have excellent characteristics,

such as flexibility, light-weightiness, ultra-thinness, and

resistance to impact. They are expected to be used for

not only high-definition mobile TVs but also large-area

sheet-type TVs to produce realistic visual images such as

"8K Super-Hi-Vision", namely, the ultra-high-definition

TV in next-generation broadcasting systems1).

To create large-screen and high-definition flexible

displays, high-speed driving, low-power-consumption,

and a technique for fabricating devices on flexible films

are required. Organic light-emitting diodes (OLEDs) are

expected to be used as the light-emitting devices for such

displays because of their self-emission property, thin-

film structure, and high efficiency2)3). The luminance of

OLED displays is determined by the amount of current

passing through the OLEDs in them, which can be

provided through an active-matrix backplane. Therefore,

active-matrix circuits with thin-film transistors (TFTs)

that can output high current density are needed to drive

large-screen and high-definition OLED displays4)-6).

Oxide-TFTs are suitable for the driving devices mounted

on the backplane of such displays because the carrier

mobility of oxide semiconductors is higher than that of

amorphous-silicon ones and because oxide TFTs have

better uniformity than low-temperature polycrystalline-

silicon TFTs. Moreover, oxide TFTs can be formed on a

typical plastic film at low processing temperature. These

advantages make oxide TFTs a strong candidate for the

backplanes of flexible OLED displays.

Besides, shorter lifetime of flexible OLED displays

using plastic substrate is often significant problem for

practical demonstration. The OLED device sandwiched

with typical plastic films are generally degraded by

exposure to ambient moisture, and dark spots form in

the luminous area because of the poor barrier properties

of the plastic film7)8). It is therefore necessary to protect

the devices with high barrier films on which inorganic

gas barrier layers are fabricated. However, it is difficult

for the single barrier layer to sufficiently prevent

moisture because the layer might have pinholes or

cracks as critical defects. Therefore, multilayer or hybrid

barrier structure often utilized for the flexible OLED

displays, however it requires complicated process and

higher process cost.

On the other hand, recently, air-reactive electrode-free

OLEDs with an inverted device structure have been

proposed as "inverted" OLEDs (iOLEDs)9)10). In iOLED,

electrons are injected from the bottom cathode to the

Abstract An 8-inch oxide-TFT-driven flexible display using inverted organic light-emitting diodes (iOLEDs)

with an inverted device structure was demonstrated. We employed iOLEDs with an air-stable electron injection

layer and longer lifetime. An oxide-TFT backplane having good electrical performances (mobility ~7 cm2/Vs,

on/off ratio >107) was also fabricated on a plastic substrate at low temperature (i.e., below 160 ˚C). The

fabricated flexible iOLED display showed clear and stable color moving images and produced uniform RGB

emissions from each pixel, even when it was bent.

Keywords: flexible display, oxide TFT, inverted OLED.

Received September 19, 2014; Revised December 2, 2014; AcceptedDecember 9, 2014† NHK Science & Technology Research Laboratories

(Tokyo, Japan)

†† Nippon Shokubai Co., Ltd.(Osaka, Japan)

A Flexible Display Driven by Oxide-Thin-Film Transistorsand Using Inverted Organic Light-Emitting Diodes

Genichi Motomura† (member), Yoshiki Nakajima† (member), Tatsuya Takei† (member),

Toshimitsu Tsuzuki† (member), Hirohiko Fukagawa† (member), Mitsuru Nakata† (member),

Hiroshi Tsuji† (member), Takahisa Shimizu† (member), Katsuyuki Morii†† (member),

Munehiro Hasegawa†† (member), Yoshihide Fujisaki† (member) and Toshihiro Yamamoto† (member)

Copyright © 2015 by ITE Transactions on Media Technology and Applications (MTA)

emitting layer by means of air-stable injection layers. A

flexible display using iOLEDs on a plastic film can

potentially suppress the growth of dark spots and

achieve longer lifetime. In this work, we developed an 8-

in. oxide-TFT-driven flexible display using iOLEDs as

light-emitting devices. The oxide-TFT backplane was

directly fabricated on plastic film, and red, green, and

blue (RGB) iOLEDs were formed on the backplane.

2. Inverted OLED (iOLED)

The structures of a conventional OLED and an iOLED

are shown in Figure 1. These structures consist of an

anode, a hole-injection layer (HIL), a hole-transport

layer (HTL), an emitting layer (EML), an electron-

transport layer (ETL), an electron-injection layer (EIL)

and a cathode. In the case of a conventional OLED, an

air-reactive cathode or EIL is easily oxidized by moisture

and oxygen. On the other hand, an iOLED has a bottom

electron-injection cathode and a top hole-injection anode.

An iOLED can thus be fabricated without use of air-

reactive layers, such as alkali metals. The stability of an

iOLED employing an air-stable EIL with a metal-oxide

layer and a suitable organic layer was reported11) .Dark-

spot formation was clearly observed in the conventional

OLED encapsulated with a film after 15 days, whereas it

was not observed in the iOLED after 250 days.

Furthermore, the iOLED exhibited similar luminance-

voltage characteristics, external quantum efficiency of

15%, and operational stability to those of a conventional

OLED fabricated for comparison.

Moreover, iOLEDs are more convenient to integrate

with pixel circuit based on n-type TFTs. A schematic of

the pixel circuit is shown in Figure 2. Each pixel consists

of two oxide TFTs, a storage capacitor, and an OLED.

The switching TFT (Sw-TFT) was used to select pixels,

and the driving TFT (Dr-TFT) was used to apply current

to the OLED. The current through the OLED is

generally related to gate-source voltage (Vgs) of the Dr-

TFT. Slight changes in Vgs will affect the current of the

OLED significantly. In the case of a conventional OLED,

Vgs of the Dr-TFT is dependent on the driving voltage of

the OLED. Since the voltage of an OLED typically

increases with driving stress, Vgs of the Dr-TFT

decreases; thus, the current and luminance of the OLED

decreases during driving12). On the contrary, in the case

of an iOLED, Vgs of the Dr-TFT does not change after

stress, as can be seen from the driving scheme.

Therefore, the driving current of an iOLED can be kept

constant without the "image-sticking" phenomenon

occurring13).

3. Fabrication of flexible display

The specifications of the fabricated flexible display

using iOLEDs are listed in Table 1. The display is eight

inches across diagonally and has 640 (RGB) × 480 pixels

(video graphic array, VGA). Its resolution is 100 pixels

per inch (ppi). The TFTs have a bottom-gate and top-

contact configuration, and the iOLEDs have a bottom-

emission structure.

ITE Trans. on MTA Vol. 3, No. 2 (2015)

122

Fig.1 Multilayered structures of a conventional OLED and an

iOLED.

Fig.2 Pixel circuit diagrams of an OLED display using a

conventional OLED and an iOLED.

Table 1 Specifications of the fabricated flexible display using

iOLEDs.

A schematic of the process flow for fabricating the

flexible display is given in Figure 3. A polyethylene

naphthalate (PEN) film with a thickness of 100 µm was

used as the transparent flexible substrate. The

transmittance in the visible range is about 87% and the

heat resistance is below 180 ˚C. First, the PEN film was

laminated onto glass with an adhesive layer that was

used as the carrier substrate. Second, a foundation layer

consisting of SiOx and olefin type polymer14) was formed

on the PEN film. The SiOx layers and the polymer layers

take a role as a gas barrier and planarization,

respectively.

Gate electrodes of stacked metals were deposited and

patterned with photolithography. Then, a SiOx gate

insulator with a thickness of about 400 nm was

deposited by DC pulse sputtering at room temperature.

Pixel electrodes made of indium-tin-oxide (ITO) and a

semiconductor, In-Ga-Zn-O (IGZO), were deposited by

sputtering and patterned with photolithography on the

gate insulator. Next, via holes of a gate insulator for the

electrodes of the TFTs were formed with

photolithography and dry etching using CF4 plasma.

After that, source and drain electrodes of stacked metals

were formed using a lift-off process. Then, the fabricated

TFT backplane was baked at 160 ˚C in ambient air.

After the TFTs were fabricated, a polymer passivation

layer with a thickness of about 3.5 µm was formed on

the backplane15). This layer could be photo-patterned

and was also used as a bank structure for the OLEDs.

After patterning, it was baked below 160 ˚C. An optical

micrograph of the fabricated pixels after formation of the

passivation is shown in Figure 4.

Next, organic layers for OLEDs were formed on the

pixel electrodes in the bank structure. First, an organic

insertion layer as an EIL was deposited by spin-coating.

Next, EMLs were deposited through shadow masks that

were aligned in a vacuum. The phosphorescent

materials used in the red and green emitting layers were

a red guest material, tris[1-phenylisoquinolinato-

C2,N]iridium(III) [Ir(piq)3], and a green guest material,

tris(2-phenylpyridine)iridium(III) [Ir(ppy)3]. A

fluorescent guest material, (E)-1,2-bis(4-(1-phenyl-1H-

phenanthro[9,10-d]imidazol-2-yl)phenyl)ethane

(PPIE)16), was used in the blue emitting layer. The

concentration of the dopants in each EML was 5wt%.

After that, a HTL, a HIL, and common anode of

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Paper » A Flexible Display Driven by Oxide-Thin-Film Transistors and Using Inverted Organic Light-Emitting Diodes

Fig.3 Schematic of process for fabricating a flexible display.

Fig.4 Optical micrograph of a fabricated pixel on the TFT

backplane.

aluminum were formed.

Finally, the fabricated panel was peeled off from the

glass substrate, and encapsulation films (with water

vapor transmission rate of ~10-4 g/m2/day) were

laminated onto both sides of the panel. A peeling of the

fabricated flexible display is shown in Figure 5. The

fabricated display was then connected to a driving

system with flexible printed circuits.

4. Performance of flexible display

Typical transfer characteristics of the fabricated Dr-

TFT on the backplane are shown in Figure 6. The off-

current was low enough to turn off the light emissions

from the OLED pixels. The TFT exhibited a current

on/off ratio of over 107 at a gate-voltage swing of about

10 V. The field-effect mobility was estimated from the

transfer characteristic to be about 7 cm2/Vs. These

values directly affected the brightness, response speed,

and contrast ratio of the image quality on the display. A

turn-on voltage is negatively shifted, but the

characteristic will be improved by optimizing

photolithographic conditions such as cleaning the

surface of some layers. These results indicate that the

fabricated oxide-TFTs successfully operated in the 8-

inch OLED display formed on the plastic film.

A photograph of the flexible display using iOLEDs in

operation is shown in Figure 7. The display was driven

at a typical frame rate of 60 Hz. As can be seen in Figure

8(a), uniform RGB emissions were observed from each

iOLED pixel. The colors of the RGB emissions on a CIE

chromaticity diagram are shown in Figure 8(b).

Although the characteristics of the blue iOLED under

development were inferior to those of a conventional

OLED, the red and green emissions showed good color

saturation in comparison to those with HDTV. The total

thickness of the fabricated panel is about 0.3 mm, which

makes the display mechanically flexible. A stable

moving image was observed even when the display was

bent. These results demonstrate that a flexible display

employing iOLEDs can be fabricated on a plastic film

and indicate the possibility of realizing a practicable

flexible display with a barrier film that would be

insufficient for a conventional OLED.

ITE Trans. on MTA Vol. 3, No. 2 (2015)

124

Fig.5 Peeling of the fabricated flexible display.

Fig.6 Typical transfer characteristics of the fabricated TFT.

Fig.7 Photograph of the flexible display using iOLEDs in

operation.

Fig.8 RGB emissions from each pixel of the fabricated display.

5. Conclusion

An 8-inch oxide-TFT-driven flexible display using

iOLEDs was fabricated and evaluated. Such a flexible

display employing iOLEDs can be fabricated on a plastic

film at low temperature (i.e., below 160˚C). IGZO and

SiOx film deposited by sputtering at room temperature

and polymer insulators formed at low temperature were

used as a passivation layer, which acted as a bank

structure of OLEDs, on the backplane. Good enough

TFT performance to drive the OLED display (with VGA

pixels) on the backplane was achieved. Stable and clear

moving images could be observed on the fabricated

display using iOLEDs, even when it was bent. These

results demonstrate that a flexible iOLED display can

be fabricated on a plastic film and that iOLEDs are

promising devices for large-area flexible displays

encapsulated with a barrier film.

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Toshimitsu Tsuzuki received the BE, ME,and PhD degrees from Osaka University, Osaka,Japan, in 1994, 1996 and 1999, respectively. In 1999,he joined Toyota Motor Corporation, Japan. In 2002, hejoined NHK and had been researching on flexibleorganic light-emitting diode displays until 2008 at theScience and Technology Research Laboratories, Tokyo.He worked at NHK Matsuyama and NHK KochiBroadcasting stations from 2008 to 2010 and from 2010to 2013, respectively. Since 2013, he has beenresearching on flexible displays at the Science andTechnology Research Laboratories, Tokyo.

Tatsuya Takei graduated from ChiyodaInstitute of Technology and Art, Tokyo, in 1991. In1991, he joined the Science and Technology ResearchLaboratories of NHK, Tokyo, and has been engaged indevelopment of plasma display panels, field emissiondisplays, and flexible displays.

Yoshiki Nakajima received the BE, ME, andPhD degrees in electrical and electronic engineeringfrom Tokyo University of Agriculture and Technology,Tokyo, Japan, in 2000, 2002, and 2004, respectively. In2004, he joined Japan Broadcasting Corporation(NHK), Tokyo, and worked at the BroadcastEngineering Department. Since 2005, he has been atthe Science and Technology Research Laboratories ofNHK. He is currently working on development of thefabrication process and driving technologies of aflexible active-matrix organic light-emitting diodedisplay driven by organic and oxide thin-filmtransistors (TFTs).

Genichi Motomura received his BE and MEdegrees from Keio University, Kanagawa, Japan, in2004 and 2006, respectively. In 2006, he joined NHK.Until 2008, he worked at NHK Matsuyamabroadcasting station, Ehime. Since then, he has beenworking at the Science and Technology ResearchLaboratories of NHK, Tokyo, and has been researchingflexible OLED displays.

ITE Trans. on MTA Vol. 3, No. 2 (2015)

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Mitsuru Nakata received the BE, ME, andPhD degrees from Tokyo Institute of Technology,Tokyo, Japan. In 2001, he joined the Central ResearchLaboratories, NEC Corporation, Kawasaki, Japan,where he was engaged in the research anddevelopment of polycrystalline silicon and oxidesemiconductor TFTs. Since 2010, he has been with theScience and Technology Research Laboratories, NHK,Tokyo, Japan, where he has been engaged in researchon oxide semiconductor TFTs and flexible displays.

Hiroshi Tsuji received the BE, ME, and PhDdegrees in electronic engineering from OsakaUniversity, Osaka, Japan, in 2001, 2003, and 2005,respectively. From 2005 to 2007, he was a ResearchFellow of the Japan Society for the Promotion ofScience. He was engaged in process modeling of metal-oxide-semiconductor largescale-integration fabrication.From 2007 to 2011, he was a Researcher with theDivision of Electrical, Electronic, and InformationEngineering, Osaka University, Osaka, Japan, wherehe was engaged in compact modeling of poly-Si TFTs.Since 2011, he has been with the Science andTechnology Research Laboratories, NHK, Tokyo, wherehe has been engaged in research on oxidesemiconductor TFTs.

Toshihiro Yamamoto received the MEdegree from Osaka University, Osaka, Japan, in 1984.In 1984, he joined NHK. Until 1987, he worked atNHK Kumamoto Broadcasting Station, Kumamoto.Since then, he has been working at the Science andTechnology Research Laboratories of NHK, Tokyo, andhas been engaged in research of plasma display panels,field emission displays, and flexible displays.

Yoshihide Fujisaki received his MS degreein electronics and communication from WasedaUniversity, Tokyo, Japan, in 1998 and PhD degree in2010 from the Department of Electronic Chemistry,Graduate School of Science and Engineering, TokyoInstitute of Technology, Tokyo, Japan. In 1998, hejoined NHK Tokyo. Since 2001, he has been at theScience and Technology Research Laboratories ofNHK. He is currently working on the processdevelopment and characterization of organic transistorand flexible displays.

Munehiro Hasegawa received the BEdegrees from Himeji Institute of technology, Hyogo,Japan, in 2002. He received the ME and PhD degreesfrom Kyoto University, Kyoto, Japan, in 2004 and2010, respectively. In 2007, he joined Nippon Shokubaiand he has been researching design and synthesis ofOLED materials.

Katsuyuki Morii received his MS, and PhDdegrees in Japan advanced institute sciencetechnology, Ishikawa, Japan, in 1995, and 1998,respectively. In 1999, he has joined the EPSON CO.,Ltd., Nagano, and he has been researched the OLEDdisplay fabricated by an inkjet printing. He has startedto study a hybrid organic-inorganic LED with air-stability in 2004. In 2009, he joined the NIPPONSHOKUBAI CO.,LTD., Osaka, and he has beenresearching an air-stable OLED.

Takahisa Shimizu received his BE, ME, andPhD degrees in Tokyo Institute of Technology, Tokyo,Japan, in 1995, 1997, and 2000, respectively. In 2010,he joined the Science and Technical ResearchLaboratories of NHK, Tokyo, and he has beenresearching a printing process of OLED.

Hirohiko Fukagawa received the BE, ME,and PhD degrees from Chiba University, Chiba, Japan,in 2003, 2004, and 2007, respectively. In 2007, hejoined the Science and Technology ResearchLaboratories of NHK, Tokyo, and he has beenresearching on organic light-emitting diode (OLED)suitable for flexible display.