Stretchable active-matrix organic light-emitting

Stretchable active-matrix organic light-emitting

(Parte 1 de 2)

Stretchable active-matrix organic light-emitting diode display using printable elastic conductors

Stretchability will significantly expand the applications scope of electronics, particularly for large-area electronic displays, sensors and actuators. Unlike for conventional devices, stretchable electronics can cover arbitrary surfaces and movable parts. However, a large hurdle is the manufacture of large-area highly stretchable electrical wirings with high conductivity. Here, we describe the manufacture of printable elastic conductors comprising single-walled carbon nanotubes (SWNTs) uniformly dispersed in a fluorinated rubber. Using an ionic liquid and jet-milling, we produce long and fine SWNT bundles that can form well-developed conducting networks in the rubber. Conductivity of more than 100Scm−1 and stretchability of more than 100% are obtained. Making full use of this extraordinary conductivity, we constructed a rubber-like stretchable active-matrix display comprising integrated printed elastic conductors, organic transistors and organic light-emitting diodes. The display could be stretched by 30–50% and spread over a hemisphere without any mechanical or electrical damage.

Recently, new technologies have been emerging in stretchable electronics1–8, motivating intensive efforts to grant various kinds of surface some form of ‘intelligence’9–17. The largest obstacle, however, has been the development of stretchable or elastic electrical wiring that is both highly conductive and highly stretchable. Various types of high-conductivity stretchable material have been developed, such as wavy thin metals1–7, metalcoated net-films15, graphene films18 and single-walled carbon nanotubes19,20 (SWNTs)/fluorinated copolymer composite8. These stretchable materials exhibit excellent conductivity and mechanical stretchability by exploiting structures such as waves and nets; the manufacturing process used is vacuum evaporation, photolithographic patterning or mechanical punching and/or cutting. However, because these manufacturing processes are not scalable, it is difficult to apply to large-area electronics. One solution would be the use of direct printing technologies using printable inks that have high conductivity and stretchability, but none has been reported thus far. Our group has reported an elastic conductor comprising SWNT–fluorinated copolymer composite8; however, the material was incompatible with printing owing to its low viscosity and tendency for reaggregation before drying and, furthermore, it required an extra rubber coating and mechanical process to improve the elasticity.

We have developed a printable elastic conductor comprising

SWNTs uniformly dispersed in a highly elastic fluorinated copolymer rubber by using an imidazolium ion-based ionic liquid21–23 and a jet-milling process. Taking advantage of a process that can uniformly disperse finer bundles of SWNTs in a rubber matrix without shortening the length of the nanotubes, the

1Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, 2R&D Center, Dai Nippon Printing Co., Ltd, 250-1 Wakashiba, Kashiwa-shi, Chiba-ken 277-0871, Japan, 3Functional Soft Matter Engineering Laboratory, Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, 4Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, 5Nanospace Project, Exploratory Research for Advanced Technology–Solution Oriented Research for Science and Technology, Japan Science and Technology Agency, National Museum of Emerging Science and Innovation, 2-41 Aomi, Koto-ku, Tokyo 135-0064, Japan, 6Research Center for Advanced Carbon Materials, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8565, Japan, 7Institute for Nano Quantum Information Electronics, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153-8505, Japan, 8Department of Electrical and Electronic Enginering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. *e-mail:someya@e.t.u-tokyo.ac.jp.

SWNT–rubber composite gel becomes increasingly viscous, and consequently this material can be finely patterned using direct printing technologies. Furthermore, a printed elastic conductor requires no extra coating and mechanical process, and can stretch by 118% and has an extraordinarily high conductivity of 102Scm−1. To the best of our knowledge, this is the highest value reported for a stretchable, printable conductor so far. This printable elastic conductor can be used for stretchable wires and contacts in electrical integrated circuits. We have integrated such printed elastic conductors with organic transistors24–27 and organic light-emitting diodes28 (LEDs) to realize a truly rubberlike active-matrix organic LED display. A 16 × 16 grid of transistors was used for driving the display and the effective size of the active matrix was 10 × 10cm2. The display could be stretched by 30–50% and spread over a hemisphere without any mechanical or electrical damage. Furthermore, it remained functional even when folded in two or crumpled up, indicating excellent durability.

Stretchable, conductive inks

A schematic representation of the manufacturing process is shown in Supplementary Fig. S1. As a chemically stable and highly conductive dopant, we used super-growth SWNTs of high purity and with a high aspect ratio (>9.98% in purity, >1mm in length and 3nm in diameter)20. Typically, a mixture of super-growth SWNTs (30mg), an ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide, BMITFSI, 60mg) and 4-methly-2-pentanone (20ml) was stirred at 25◦C with a magnetic stirrer (>700r.p.m.) for 16h. The resulting swollen

NATUREMATERIALSDOI:10.1038/NMAT2459 ARTICLES

Figure 1 | Printable elastic conductors. a, Printed elastic conductors on a PDMS sheet. Printed elastic conductors patterned by screen printing can be stretched by 100% without electrical or mechanical damage. The insets show SWNTs dispersed in paste and a micrograph of printed elastic conductors with a linewidth of 100µm. The picture was taken without stretching. b, An SEM image of the surface of printed elastic conductors. SWNTs were uniformly dispersed in a fluorinated copolymer matrix (Daikin, Daiel-G912), which is a very soft rubber, using a room-temperature ionic liquid (BMITFSI) and jet-milling. c, A magnified SEM image of the elastic conductor. Finer or exfoliated bundles of SWNTs were uniformly dispersed in the rubber, and formed well-developed conducting networks.

gel was processed on a high-pressure jet-milling homogenizer (60MPa; Nano-jet pal, JN10, Jokoh), giving a black paste-like substance, referred to as bucky gel. To the gel were successively added 4-methly-2-pentanone (80ml) and a fluorinated copolymer, vinylidenefluoride–tetrafluoroethylene–hexafluoropropylene (50–1,500mg; Daiel-G912, Daikin; referred to as G912 henceforth), and the mixture was stirred at 25◦C for 16h, poured onto a glass platebydrop-castingandairdriedfor6htoaffordaSWNT–rubber composite gel (referred to as SWNT paste henceforth; Fig. 1a). The paste viscosity was approximately 10Pas. Furthermore, when the SWNT paste was air dried for more than 12h, an elastic conductor was formed.

The SWNT paste was patterned on rubber sheets made of a silicone elastomer (polydimethylsiloxane, PDMS; Sylgard 184, Dow-Corning) using screen printing through shadow masks. The patterned SWNT pastes were air dried for more than 6h to afford fine SWNT-based elastic conductors with a linewidth of 100µm(Fig. 1a).Theresolutioniscurrentlylimitedbytheadhesion between the SWNT paste and the PDMS substrate. The printed elastic conductors can be stretched by approximately 100% without mechanical damage or delamination from the PDMS matrix. Note that the stretchability can be enhanced when printed elastic conductors are sandwiched between rubber substrates because encapsulation prohibits the elastic conductors from delamination. More detailed information on the printability can be found in Supplementary Information.

Figure 1b shows an image of the surface of printed elastic conductors taken using a scanning electron microscope (SEM, S4300; Hitachi). The smooth surface confirms that the

C o nductivit y ( S cm

Stretchability (%)

Str e tchabilit y (%) C o nductivit y ( S cm

SWNT (wt%)

Conductivity Stretchability

Sample 1 (SWNT 15.8 wt%)

Sample 2 (SWNT 5.1 wt%)

Sample 3 (SWNT 1.4 wt%)

Figure 2 | Electrical and mechanical characteristics of printed elastic conductors. a, Conductivity of printed elastic conductors as a function of stretchability. Typical elastic conductors with 15.8, 5.1 and 1.4wt% SWNTs are shown. Conductivity increases with SWNT content, whereas stretchability decreases. Furthermore, conductivity does not change with stretching. b, Stretchability and conductivity as a function of SWNT content.

bundles of SWNTs were made much finer through the process using ionic liquid and jet-milling. In fact, the SWNTs were significantly untangled or exfoliated after the jet-milling process (see Supplementary Fig. S2). We clearly demonstrated that the process including an ionic liquid and jet-milling significantly untangled or exfoliated the bundle of SWNTs, and formed well-developed nanotube networks in a rubber, as shown in Fig. 1c.

The electrical and mechanical properties of the printed elastic conductors under tensile stress were investigated using a precision mechanical stretching system (Autograph AG-X; Shimazu). Figure 2a shows a plot of the conductivity as a function of stretchability as the SWNT content was changed from 1.4 to 15.8wt%. A SWNT content of 15.8wt% produced an extraordinarily high conductivity of 102Scm−1 and stretchability of 29%. In contrast, the conductors with 1.4wt% SWNTs exhibited an extraordinarily high stretchability of 118% and conductivity of 9.7Scm−1. Importantly, the elastic conductors did not show significant changes in conductivityoranysignofmechanicaldamagewhenfullystretched. For comparison, a conventional conducting rubber containing carbon particles was also tested. Although the stretchability of the conventional conducting rubber exceeded 150%, the conductivity was as low as 0.1 S cm−1, which is insufficient for electronic circuit applications. In fact, low conductivity of electrical wirings may result in a low-speed response, large voltage depression and large power consumption.

There are three important factors determining the conductivity and stretchability of the printable elastic conductors. The first is the mixing ratio of SWNTs, the ionic liquid (BMITFSI) and the fluorinated copolymer matrix (G912). The SWNT content was changed from 1.4 to 15.8wt% (Fig. 2b) while the BMITFSI content

ARTICLES NATUREMATERIALSDOI:10.1038/NMAT2459 was maintained at twice the SWNT content. When the SWNT content was greater than 6wt%, the conductivity was higher than 50Scm−1, and the stretchability was less than 40%. In contrast, at less than 6wt% of SWNTs, the stretchability was higher than 50%, and the conductivity was less than 40Scm−1. Importantly, we found that the elastic conductors have very smooth surfaces and high film quality over the wide range of SWNT contents. This is due to the compatibility of the constituent materials, as described later. Furthermore, Fig. 2b clearly indicates that the important parameters—conductivityandstretchability—canbesystematically controlled and tailored to the demands of electronic applications, although there exists a trade-off between the two parameters. One method to achieve a better balance between conductivity and stretchability would be the use of mechanical punching to form a net-shaped structure, although this manufacturing process is more complex.

The second factor is the method used to exfoliate the bundles of

SWNTs and make them finer so that they are uniformly dispersed in the elastic fluorinated copolymer matrix. Typical dispersing techniques such as ultrasonication, grinder-milling and ball-milling can make the bundles finer, but at the same time, they make the SWNTs shorter, thus reducing the conductivity. We used a jet-milling high-pressure homogenizer that breaks materials into fragments with a high-pressure jet. It is particularly worth noting that this process enables us to uniformly disperse finer bundles of SWNTs into a rubber matrix without shortening the length of the nanotubes during the process. The resulting longer and finer bundles of SWNTs can form well-developed conducting networks in rubbers, as shown in Fig. 1c, thus leading to higher conductivity and stretchability, simultaneously. The formation of SWNT networks in rubbers also realizes printability of this material because the SWNT paste becomes increasingly viscous (more than 10 Pa s),whichisverysuitableforhigh-definitionscreenprinting.

Besides the mixing ratio and dispersing process, the compatibility of the materials—SWNTs, ionic liquid and copolymers—is also very essential. We carried out material compatibility tests to determine the combination of elastic polymers and ionic liquid that would be compatible with each other (see Supplementary Fig. S3). We examined three different fluorinated copolymers: G912 and vinylidene fluoride/hexafluoropropylene copolymers with composition ratios of 0.78:0.2 (Daiel-G801, Mw = 150,0, Daikin; referred to as G801 henceforth) and 0.8:0.12 (KYNAR-FLEX2801, Mw = 30,0, Arkema; referred to as Kyner henceforth). G912 is very soft and elastic, but this often results in separation of the G912 and ionic liquid phases. We used BMITFSI, which is compatible with several fluorinated copolymers. In fact, when G912 and BMITFSI were used, the resulting SWNT rubber films were very smooth, flat and uniform (Fig. 1b and Supplementary Fig. S2c), and hence exhibited high conductivity and stretchability. In contrast, when Kyner (which is a hard resin) was used, the resulting films exhibited a conductivity of only 21Scm−1 and were very fragile and not stretchable. When G801 (which is also a soft rubber) was used, the conductors exhibited a stretchability of 87% and conductivity of 3Scm−1 with 1.8wt% SWNTs, and a stretchability of 24% and conductivity of 22Scm−1 at 15.7wt% SWNTs. The results of the compatibility tests convinced us that combination of G912 and BMITFSI is the best from the points of view of both the electrical and mechanical performance in the process with jet-milling.

We also investigated the durability under stretching cycles.

Although durability depends on stretchability, the elastic conductors with 1.4wt% SWNTs, 2.8wt% BMITFSI and 95.8wt% G912 did not show changes in conductivity after 1,0 cycles of stretching by 70%.

Carbon-nanotube-based conducting materials or carbonnanotube/conducting-polymer composites have also been

C urr ent densit y (mA cm

Luminanc e ( cd m

Voltage (V)

¬40 V

¬30 V ¬20 V

¬10 V

¬5 V a b

Data-line Bias-voltage-line

Scanning-line VData

VScan VBias

IOLED Trselector

Iselector Trdriver

Ground-line

I selector

VScan (V)

IOLED VBias = ¬30 VIselector

Figure 3 | Stretchable display cells comprising an organic LED and a 2T1C driving cell. a, Circuit diagram of one stretchable display cell comprising an organic LED and a 2T1C driving cell. Iselector and IOLED represent currents passing through the selector transistor Trselector and through the organic LED, respectively. Display cells are interconnected with each other through printed elastic conductors that work as scanning-, data-, bias-voltage- and ground-lines. VScan, VData and VBias represent voltages applied to scanning-, data- and bias-voltage-lines, respectively. The capacitance of the circuit (C) is 215pF. The resistor symbols represent printed elastic conductors. b, Current density and luminance of a typical stand-alone organic LED as a function of bias voltage. Experiments were carried out in ambient air.

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