nphoton.2013.242

Elastomeric polymer light-emitting devicesand displaysJiajie Liang, Lu Li, Xiaofan Niu, Zhibin Yu and Qibing Pei*

The emergence of devices that combine elasticity with electronic or optoelectronic properties offers exciting newopportunities for applications, but brings signicant materials challenges. Here, we report the fabrication of an elastomericpolymer light-emitting device (EPLED) using a simple, all-solution-based process. The EPLED features a pair of transparentcomposite electrodes comprising a thin percolation network of silver nanowires inlaid in the surface layer. The resultingEPLED, which exhibits rubbery elasticity at room temperature, is collapsible, and can emit light when exposed to strains aslarge as 120%. It can also survive repeated continuous stretching cycles, and small stretching is shown to signicantlyenhance its light-emitting efciency. The fabrication process is scalable and was readily adapted for the demonstration ofa simple passive matrix monochrome display featuring a 53 5 pixel array.

Stretchable electronics has been perceived as an alternative tech-nology for the realization of the next generation of electronicapplications. Stretchable displays and solid-state lighting

systems would enable expandable and foldable screens for smart-phones, wearable or fashionable electronic clothing, rollable or col-lapsible wallpaper-like lamps and biocompatible light sources for invivo or epidermal medical devices15. Combining elastic intercon-nects with discrete rigid inorganic light-emitting diodes (LEDs) ororganic light-emitting diodes (OLEDs) has been used in the manu-facture of stretchable displays610. The rigid and brittle LEDs areembedded in or bonded onto the surface of soft rubbery polymers.The resulting displays and lighting systems show high stretchabilityand efciency. An alternative approach to achieving stretchable dis-plays is based on a different kind of mechanics, whereby intrinsicallystretchable OLEDs are fabricated in which all the constituentmaterials are elastic1,5. We recently reported an intrinsically stretch-able OLED comprising a pair of transparent carbon nanotubes(CNTs)polymer composite electrodes sandwiching an electrolumi-nescent polymer blend layer5. The composite electrodes exhibitedthe property of shape memory, and the devices could be stretchedrepeatedly at 70 8C for several cycles. A light-emitting deviceusing an elastic electroluminescent blend, an ultrathin goldcoating on polydimethylsiloxane substrate, and galliumindiumeutectic alloy liquid metal as the opposite electrode has also beenreported1. However, the limited stretchability1 and conductivity5

of electrodes, low electroluminescent performance, or complicatedprocessing methods still constitute signicant obstacles to the fabri-cation of a stretchable display based on these light-emittingdevice architectures.

Here, we report an elastomeric polymer light-emitting device(EPLED) comprising an electroluminescent polymer layer sand-wiched between a pair of new transparent elastomeric compositeelectrodes. The composite electrode is based on a thin silver nano-wire (AgNW) network inlaid in the surface layer of a rubbery poly(urethane acrylate) (PUA) matrix. It has high visual transparency,good surface electrical conductivity, high stretchability and highsurface smoothness, all features essential to the fabrication of theEPLED. The electroluminescent polymer layer is formulated to beable to form a light-emitting PIN junction in situ for efcient

injections of both electrons and holes from the AgNW network.The EPLED is semitransparent, and emits from both surfaceswith a high light-emitting efciency. Moreover, the EPLED is col-lapsible at room temperature. It can survive stretching to amaximum linear strain of up to 120%, and can be stretched at30% strain repeatedly for 1,000 stretchingreleasing cycles.Stretching can signicantly enhance the light-emitting efciency.A fully stretchable EPLED array of 5 5 pixels has been fabricatedfor the rst time to demonstrate the applicability of the EPLEDarchitecture for stretchable OLED displays.

Fabrication and characterization of composite electrodesA schematic representation of the manufacturing process for therubbery AgNWPUA composite electrodes is presented inSupplementary Scheme S1. According to a model for a one-dimen-sional random network11, the surface nanowire percolation densityis inversely proportional to the length of the AgNWs. TheCOMSOL numerical simulation also shows that a percolationnetwork made from longer nanowires has better compliancy thanone made from shorter nanowires12. In this work, AgNWs with alength-to-diameter aspect ratio of 500 were used as the conduc-tive material to form a percolation network with high electricalconductivity and mechanical compliancy1219. The matrixpolymer PUA of the composite electrode is a copolymer of a sili-conized urethane acrylate oligomer (UA) and an ethoxylatedbisphenol A dimethacrylate (EBA). UA and EBA were chosen forthe high transparency and excellent stretchability of the homopoly-mer of UA and the good bonding force between the homopolymerof EBA and AgNWs20. Various weight ratios of UA:EBA werestudied, and a ratio of 5:1 was found to give the optimal overall per-formance in terms of optical transmittance, stretchability andbonding force with AgNWs (Supplementary Fig. S1). The trans-mittance of the resulting PUA lms (150 mm thick) is greaterthan 92% in the wavelength range 5501,100 nm (Fig. 1a). Theelongation at break for the PUA matrix is greater than 140%,and Youngs modulus is 38 MPa (Supplementary Fig. S1). Scotchadhesive tape was applied to the conductive surface of theAgNWPUA composite electrode and peeled off to test theadhesion between the AgNWs and PUA. After 100 such tests, it

Department of Materials Science and Engineering, Henry Samueli School of Engineering and Applied Science, University of California, Los Angeles,California 90095, USA. *e-mail: [email protected]

ARTICLESPUBLISHED ONLINE: 22 SEPTEMBER 2013 | DOI: 10.1038/NPHOTON.2013.242

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2013 Macmillan Publishers Limited. All rights reserved.

was found that the sheet resistance remained unchanged, indicatinggood bonding force between the AgNWs and PUA. In addition, theAgNWPUA composites show high mechanical exibility

(Fig. 1b). A composite sample with an area of 10 7 cm2 couldbe easily wrapped around a 7-mm-diameter steel rod withoutany damage.

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PUA composite lms with specied sheet resistance (thickness, 150mm). b, Optical photograph of a 15Vsq21 AgNWPUA composite partially rolled on arod. c,d, SEM micrographs of 15Vsq21 AgNW coating on a glass substrate (c) and the conductive surface of a 15Vsq21 AgNWPUA composite electrode

(d). e, Sheet resistance for AgNWPUA composite electrodes with increasing strain (sample area, 7 10 mm2; stretch speed, 1 mm s21). f, Transientresistance measured during 1,500 cycles of stretchingrelaxing between 0% and 30% strains for a 15Vsq21 AgNWPUA composite electrode (sample size,

5 5 mm2; stretch speed, 1 mm s21). g,h, SEM images of a 15Vsq21 AgNWPUA composite electrode under 30% elongation (arrow indicates stretchingdirection) (h) and after 100 cycles of stretchingrelaxing between strains of 0% and 30% (g).

ARTICLES NATURE PHOTONICS DOI: 10.1038/NPHOTON.2013.242

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Figure 1c presents a top-view scanning electron microscopy(SEM) image of a AgNW percolation coating on a glass substratewith 15V sq21 sheet resistance. The AgNWs have an average diam-eter of 2535 nm and length in the range 1020 mm. The aspectratio of 500 rivals, or even surpasses, certain CNTs modied for pro-cessability21. Figure 1d presents a top-view SEM image of the con-ductive surface of a 15V sq21 AgNWPUA composite electrode.The nanowires are inlaid into the surface layer of the composite.The transmittance of the AgNWPUA composite electrodes withvarious sheet resistances are depicted in Fig. 1a andSupplementary Table S1. The transmittances at 550 nm for the10V sq21, 15V sq21 and 25V sq21 AgNWPUA compositeelectrodes are 80%, 83% and 84% respectively, comparable withthose of indium tin oxide (ITO)/glass and better than commercialITO/polyethylene terephthalate (PET) electrodes. The 15V sq21

AgNWPUA composite electrode displays a transmittance higherthan 81% in the range 5001,000 nm. Transmittance in the deepblue region (400450 nm) is low, which may be due to the localizedsurface plasmon resonance of the AgNWs22.

To test the stretchability of the transparent AgNWPUA compo-site electrodes, samples were subjected to repeated stretch and relax-ation cycles at a stretch speed of 1 mm s21. The sheet resistancechanges of the 10V sq21 and 15V sq21 AgNWPUA compositeelectrodes with increasing strain up to 100% are shown in Fig. 1e.The 15V sq21 sample shows a steady increase in resistance withincreasing strain up to 80%, after which the sheet resistanceincreases steeply, but still remains below 1 kV sq21 at 100%strain. For the 10V sq21 sample, the sheet resistance exhibits asteady increase to 235V sq21 at 100% strain.

The resistance evolution of AgNWPUA composite samplesduring 1,500 cycles of continuous stretchingrelaxing with 30%peak strain is shown in Fig. 1f. With an initial resistance of15 V, the peak resistance for the 15V sq21 AgNWPUA sampleincreases from 39 V to 56 V after the rst 100 cycles with 30%peak strain, and then rises at a slower pace, to 111 V after the sub-sequent 1,400 cycles. The baseline resistance at 0% strain alsoincreases at a similar pace, to 34 V after 100 cycles and then to65 V in the subsequent 1,400 cycles. After being kept in therelaxed state for 30 min, the resistance can still be restored to alow resistance of 45 V (that is, 45V sq21). This partial recoveryindicates that the gradual increase in resistance during thestretch and relaxation cycles is partially caused by the viscoelasti-city of the PUA matrix5,18. Supplementary Fig. S2 shows that thePUA matrix has a loss factor of 0.26 at 1 Hz, indicative ofsome degree of viscoelastic behaviour in the PUA matrix. The per-manent increase in sheet resistance after continuous cyclic loadingis probably caused by partial loss of interconnection among nano-wires and sliding of the nanowires in the AgNW networkembedded in the PUA matrix.

SEM imaging was carried out to further examine the AgNWnetwork. Figure 1g presents an image of a 15V sq21 AgNWPUA composite electrode at 30% elongation. A general alignmentof the AgNWs along the stretching direction is observed. After100 cycles of stretching and relaxing between 0% and 30% strain,the dense AgNW network appears intact, and the AgNWs retaintheir high aspect ratio (Fig. 1h). Composite electrodes fabricatedby in situ substrate formation and transfer generally have asmooth conductive surface20. The elastomeric composite electrodesretain this property. After 100 stretchingreleasing cycles at strainsbetween 0% and 30%, the conductive surface remains smooth, asshown in Fig. 1h, with no cracks, voids or buckling patterns obser-vable on the surface. The surface can remain smooth even afterrelaxation from 80% strain (Supplementary Fig. S3). Having asmooth surface is critically important for the use of transparent elec-trodes in thin-lm light-emitting devices1,5,23. The preservation ofsurface smoothness after large-strain elongation is a manifestation

of the strong interfacial bonding between the PUA matrix and theAgNWs. An example of a composite electrode lacking such stronginterfacial bonding is shown in Supplementary Fig. S4 (the matrixpolymer is poly(tert-butylacrylate))18.

Fabrication and investigation of an elastomeric PLECBased on this rubbery and transparent AgNWPUA compositeelectrode, a polymer light-emitting electrochemical cell (PLEC)was fabricated by an all-solution processing procedure(Supplementary Scheme S1). The PLEC device architecture wasselected for the EPLED instead of a conventional OLED becauseof the simplicity of the PLEC device structure, the lack of require-ment for specic electrode workfunctions for charge injection,and the straightforward fabrication process, which is compatiblewith conventional polymer processing techniques13,2426. AAgNWPUA composite electrode with 15V sq21 sheet resistancewas rst spin-coated with a thin layer of poly(3,4-ethylenedioxythio-phene):poly(styrenesulphonate) (PEDOT:PSS), to be used as theanode. The thin PEDOT layer protects the PUA matrix fromsolvent attack in the subsequent coating of the electroluminescentpolymer layer. The electroluminescent polymer layer consists of ablend of a yellow light-emitting polymer (SuperYellow), ethoxylatedtrimethylolpropanetriacrylate (ETPTA), polyethylene oxide (PEO)and lithium triuoromethane sulphonate (LiTf). SuperYellow wasselected for its very high molecular weight, which is benecial forlarge-strain stretchability, and its reported high electroluminescentperformance2730. ETPTA was chosen for its capability to (1)conduct ions and (2) to polymerize to form a highly crosslinkedpolymer network that ceases to conduct ions. This property isimportant for the formation of a stable PIN junction5,29,30. PEO,an ionic conductor widely used for solid electrolytes, was addedto enhance the stretchability of the crosslinked ETPTA network.LiTf is a widely used salt in solid electrolytes. In the PLEC, LiTfprovides the ionic dopants for the doped polymers in the for-mation of a PIN junction. The weight ratio of these ingredientswas initially selected based on the previously reported fabricationof PLEC29,30 and then optimized for the present work. The electro-luminescent layer was deposited by spin-coating a solutioncontaining these ingredients co-dissolved in tetrahydrofuran(THF). The resulting two-layer lm was laminated with a second15V sq21 AgNWPUA composite electrode (as cathode) tocomplete the device fabrication.

The PLEC was rst driven at 9 mA cm22 for 600 min to evaluatethe timeframe for establishing the PIN junction and the device life-time. As shown in Supplementary Fig. S5, the brightness of thedevice gradually rises in the rst 10 min to a peak value of211 cd m22 due to the gradual formation of a PIN junction in theelectroluminescent polymer layer5,29,30. The time frame for establish-ing the PIN junction is mostly determined by the speed of ionicmigration in the emissive layer. The emission intensity then gradu-ally decreases to 106 cd m22 over the following 10 h. The lightemission turn-on response from the PIN junction in the activelayer of a PLEC immediately after initial charging was also investi-gated in a pulse voltage operation, as shown in SupplementaryFig. S6. The pre-charged PLEC has a rapid turn-on, similar toconventional OLEDs.

Systematic device characterization was carried out after thePLEC was initially charged under a constant 9 mA cm22 for10 min. The current densityluminancedriving voltage charac-teristic curves and current efciencyluminance characteristiccurves of a typical pre-conditioned PLEC device are presentedin Fig. 2a,b. Light emission in this device turns on at 6.8 V andreaches a peak brightness of 2,200 cd m22 (measured from theanode side) at 21 V. The current efciency increases rapidlywith brightness, and reaches 5.7 cd A21 at the high end of thebrightness range. This efciency is much higher than in

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2013.242 ARTICLES



previously reported stretchable light-emitting devices5. The PLECdevice (active area of 21 mm2) can be driven to 10 cd m22,120 cd m22 and 320 cd m22 at 9 V, 14 V and 16 V, respectively.

These voltages are comparable to those for typical polymerOLEDs. Supplementary Table S2 also shows that the PLEC deviceexhibits fairly uniform light emission, even at very low brightness

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Figure 2 | Device characterization of a stretchable PLEC. a, Current densityluminancedriving voltage characteristics of an elastomeric PLEC device.

b, Current efciencyluminance characteristics of the device. Insets: photographs of the PLEC (original emission area, 3.0 7.0 mm2) unbiased, biased at12 V, and deformed to show light emission from both surfaces. c, Current density and luminance characteristics of a PLEC device at 12 V with increasing

strain. d, Current efciency characteristics of the device with strain. e, Photographs of a PLEC (original emission area, 5.0 4.5 mm2) biased at 14 V atspecied strains. f, Images of a PLEC (original emission area of 3.0 7.0 mm2, biased at 12 V) wrapped around the edge of 400-mm-thick cardboard. Allmeasurements were carried out at room temperature.




around the emission threshold. It is known that electrochemicaldoping took place on either side of the light-emitting polymerwhen the turn-on voltage was applied29,30. The doped polymer withimproved electrical conductivity would also promote uniformcharge transportation, thereby resulting in uniform light emissionover the entire active area29,30. More than ten devices were tested,with performance (brightness and efciency) uctuating in a fairlynarrow range of+10%, indicating good reproducibility for the PLEC.

The PLEC is composed mostly of transparent components, andthe PLEC is therefore semitransparent (Fig. 2b, inset). Light pro-duced in the electroluminescent polymer layer is emitted in alldirections27. The photograph of the bent device shown in theinset to Fig. 2b shows the emission of light from both surfaces ofthe thin-lm device. Because the transmittance of the PEDOT:PSSlayer on the anode is quite high (Supplementary Fig. S7), measure-ments of the emission intensity from both sides of the devices showalmost identical brightness and efciency (Supplementary Fig. S8).Accordingly, the actual maximum external current efciency of thePLEC at a brightness of 2,200 cd m22 should account for emissionsfrom both sides, and adds to 11.4 cd A21. The calculated externalquantum efciency is 4.0%, which is comparable to that for a state-of-the-art PLEC based on SuperYellow and fabricated using anITO/glass substrate with an evaporated aluminium cathode27,28,30.

The stretchability of the PLEC was investigated at room tempera-ture in a glovebox. The device can be uniaxially stretched to 120%strain (along the length direction) before failing to emit light(Supplementary Movie S1). Figure 2c,d shows the current density,luminescence and current efciency characteristics of the PLECsbiased at 12 V and stretched from 0 to 120% strains. The decreasein current density with strain can be accounted for by the increasein sheet resistance of the composite electrodes with strain. Thebrightness of the device initially increases from 0% to 20% strain,and then decreases with higher strains (Fig. 2c). The oppositechange of current density and brightness at small strains indicatesa rapid rise in electroluminescent efciency. The current efciencyshows a 200% increase, from 1.0 cd A21 before stretching, to3.0 cd A21 at 40% strain. It levels off at up to 80% strain and thenbegins to decrease, but still retains a fairly high value of 2.1 cd A21

at 120% strain (Fig. 2d), which is still 100% higher than its originalvalue. The increase in efciency is an indication of themore balancedinjection of electrons and holes. To investigate this, electron-onlyand hole-only devices were fabricated (Supplementary Fig. S9 andcorresponding description in Supplementary section Chargecarrier transporting characteristic of the emissive layer underdifferent strain) and their currentvoltage characteristics weremeasured at various strains. Supplementary Fig. S9a shows that thecurrent density of hole-dominated devices decreases with increasingstrain from 0% to 100% strain. The electron-dominated device showsan enhanced current injection from 0% to 20% strain, and thecurrent begins to decrease at higher strains (SupplementaryFig. S9b). The initial efciency increase of the PLEC device withstrain may therefore be a result of a more balanced injection ofelectrons and holes.

Figure 2e presents photographs of a PLEC (initial brightness of130 cd m22) at various elongations (biased at 14 V). The stretcheddevice displays uniform (Supplementary Fig. S10) and bright emis-sion across the entire luminous area, even when the device isstretched up to 120% strain (Supplementary Movie S1). The picturesare saturated because of the high brightness. To observe the uni-formity of the light emission, the PLEC with an original luminancebelow 20 cd m22 was imaged while being stretched to variousstrains. Supplementary Fig. S11 shows that the emission is fairlyuniform over the entire emissive area, even at 60% strain whenthe emission intensity has diminished to 0.5 cd cm22.

The PLEC is bendable and can be collapsible. Figure 2f demon-strates a PLEC device (biased at 12 V) emitting brightly and

uniformly even when being wrapped around the edge of 400-mm-thick cardboard. Bending or folding causes no mechanical orelectrical damage to the device because of the high exibility andconductivity of the AgNWPUA composite electrodes. The PLECis also subjected to repeated cycles of stretch and relaxation.Supplementary Movie S2 shows a PLEC device with fairlyuniform and bright light emission during the stretching cycles atroom temperature. Supplementary Figs S12S15 (and correspond-ing description in Supplementary section Investigation of PLECsubjecting to continuous cycles of stretchingrelaxing) demonstratethat the PLEC can exhibit fairly stable current efciency and bright-ness, even after 1,000 continuous stretchrelaxation cycles withstrains between 0% and 30%, indicating the fairly high elasticity ofthe PLEC at small strains at room temperature. When the strain is40% or larger, the electroluminescent performance of the devicesdeteriorates rapidly (Supplementary Fig. S13). This is attributed toirreversible changes at large strains in the emissive and PEDOTlayers (Supplementary Figs S14 and S15).

The mechanical properties of the PLEC were also investigated. Asshown in Supplementary Fig. S16a, the Youngs modulus for thePLEC stack is 38 MPa, and the PLEC device can be stretched upto 125% strain when it fractures. The stressstrain response of thedevices is almost the same as that of the neat PUA polymermatrix (Supplementary Fig. S1b), which is not surprising as PUAcomprises over 99% of the total material in the devices. Moreover,the measured mechanical loss factor of 0.26 (SupplementaryFig. S16b) is much smaller than the values of 0.42 and 0.64 foracrylic interpenetrating elastomer and VHB acrylic copolymer elas-tomers, respectively, two dielectric elastomers that have been widelyinvestigated for electrically induced actuation strains greater than at100% (refs 31,32). The PLEC reported here can indeed be con-sidered to be elastomeric.

The results for PLEC using 10V sq21 and 25V sq21 AgNWPUAcomposite electrodes are presented in Supplementary Figs S17, S18and S19. The PLEC based on 15V sq21 AgNWPUA compositeelectrodes exhibits the best overall performance.

The aforementioned device fabrication and testing were allcarried out in a glovebox protected with dry nitrogen. To take thedevices out of the box and test in air, a thermally crosslinked poly-urethane (TCPU) was selected to seal the PLEC33,34. SupplementaryFig. S20 and Supplementary Movie S3 show an encapsulated PLECdevice being twisted and stretched repeatedly in air while beingdriven at 12 V. Furthermore, taking full advantage of the elastomericPLEC and the high conductivity of the rubbery composite electrode,monolithic arrays of the EPLED consisting of 5 5 pixels were fab-ricated by the same technique, except that the anode and cathodewere patterned into row and columns, respectively (Fig. 3a).AgNWs were spray-coated onto a release substrate through ashadow mask to deposit parallel strips of AgNW coating, whichwas then transferred to PUA matrix to form a patterned compositeelectrode. Figure 3b shows that high-space-resolution AgNW pat-terns with 100 mm line width and 80 mm separation can beachieved. A well-dened edge can be clearly seen in the SEMimage for the patterned composite electrode (Fig. 3c). AfterPEDOT:PSS and an electroluminescent layer were successivelycoated onto the patterned composite electrode (anode), anotherpatterned composite electrode (cathode) was stacked at an angleof 908, then hot pressed. The resulting display was sealed usingTCPU. The cross-sectional areas of the orthogonal AgNW stripsdene the pixels of the display. Figure 3d shows an optical imageof an encapsulated fully stretchable display consisting of a 5 5array of pixels. The column and row electrodes are 1.0 mm inwidth with a spacing of 0.2 mm. The display is transparent (thelogo can be clearly seen through it). The insets of Fig. 3d show adisplay driven with selected pixels. Figure 3e,f depicts a displaybeing folded and stretched, respectively.




Summary and outlookWe have shown that a high-performance, elastomeric PLEC can befabricated using a relatively simple, all-solution-based process. Thecomposite electrodes fabricated by in situ polymerization and trans-fer combine the necessary properties of high optical transmittance,surface electrical conductivity, surface smoothness and the rubberyelasticity of the matrix polymer. For efcient charge injection,particularly the injection of electrons into the conduction band, aelectroluminescent polymer semiconductor layer is formulatedthat is capable of forming a PIN junction in situ. The resulting

PLEC exhibits rubbery elasticity at room temperature, can emitlight at strains as large as 120%, and shows signicantly improvedefciency in the stretched state. The fabrication process is quitefacile and scalable, and is readily adapted for the demonstrationof a simple passive matrix display. The display retains therubbery elasticity of the individual PLEC pixels. This is an impor-tant step towards producing fully stretchable electronics. Finally, weanticipate that, with the future development of elastomeric thin-lm transistors, rubbery sealing materials and stretchable electrolu-minescent polymers, fully stretchable active-matrix OLED displays

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Figure 3 | Demonstration of encapsulated fully stretchable display. a, Schematic (left) and top-view (right) illustrations of an encapsulated fully stretchable

EPLED display comprising 5 5 pixels. b, Optical image of the surface of a patterned AgNWPUA composite electrode with 100mm line width and 80mmlineline spacing. c, SEM image of the surface of a patterned AgNWPUA composite electrode (lower light grey area comprises AgNWs). d, Photograph of a

stretchable display. Insets: display driven with selected pixels (pixel size, 1 mm 1 mm). e,f, Demonstrations of EPLED displays being folded (e) and stretched(f) (pixel size, 1 mm 1 mm). All displays are operated in air and at room temperature.




for the high-resolution display of information will be achieved inthe near future.

MethodsMaterials. AgNWs were synthesized with an average diameter in the range2535 nm and average length between 10 and 20 mm. UA (CN990), EBA (SR540)and ETPTA (SR9035) were all supplied by Sartomer. DMPA, LiTf (99.995% purity),PEO (Tg267 8C; Mv 100,000, where Mv is the average molecular weightdetermined by viscosity) and anhydrous THF were obtained from Sigma-Aldrich.The soluble SuperYellow (phenyl substituted poly(1,4-phenylene vinylene)) wasobtained from Merck (catalogue no. PDY-132). High-conductivity PEDOT:PSS wasobtained from H.C. Starck (Clevios VP PH1000). A thermal crosslinkable urethaneliquid rubber compound (Clear Flex 50, from Smooth-On USA, mixed at a weightratio of 1:2 parts A:B) was used as the elastic encapsulation material, TCPU.

Preparation of AgNWPUA composite electrode. A dispersion of AgNWs inisopropanol (concentration of 2 mg ml21) was coated on glass substrates using aMeyer rod (RD Specialist) or airbrush (Paasche), as shown in SupplementaryScheme S1. The resulting transparent conductive coating on the glass substrates wasthen coated with a precursor solution consisting of 100 weight parts UA, 20 partsEBA and 1 part DMPA. The coatings were cured on a Dymax ultraviolet curingconveyor equipped with a 2.5 W cm22 Fusion 300S type H ultraviolet curing bulb,at a speed of 0.9 feet per minute for one pass, and then peeled off as a free-standingcomposite electrode.

Fabrication of stretchable PLEC. AgNWPUA composite electrodes were cleanedby sequential 30 min treatments with detergent followed by deionized water in anultrasonic bath. PEDOT:PSS was then spin-coated on the composite electrode at4,500 r.p.m. for 60 s, followed by vacuum evaporation for 24 h to remove residualwater. A AgNWPUA composite electrode coated with PEDOT:PSS was used as theanode. A solution of SuperYellow, ETPTA, PEO and LiTf in THF (weight ratio20:2:2:1) with 7 mg ml21 of Super Yellow was spin-coated onto the anode at3,000 r.p.m. for 60 s. The lms were then dried at room temperature under vacuumfor 1 h before use. The electroluminescent polymer layer was 200 nm thick, asmeasured by a Dektak prolometer. A second AgNWPUA composite electrode (ascathode) was faced down, stacked onto the emissive polymer layer, and the stack washeated to 90 8C to enhance adhesion between the layers. The stack was then fedthrough a hot-press set-up at 150 8C. To encapsulate the stretchable PLEC, thedevice was laminated between a pair of pre-crosslinked Clear Flex 50 layers and leftto fully crosslink overnight at room temperature. All stacking and laminationoperations were carried out in a glovebox, with oxygen and moisture levels bothbelow 0.5 ppm.

Preparation of fully stretchable display. A AgNW dispersion (concentration of2 mg ml21) was spray-coated onto a release substrate through a shadow mask toform parallel strips of AgNW coating. The coating was transferred to the PUAmatrix in a manner similar to that for the AgNWPUA composite. PEDOT:PSS wasspin-coated onto the patterned AgNWPUA composite electrode, which was thenused as the anode. A solution of SuperYellow, ETPTA, PEO and LiTf in THF (weightratio 20:2:2:1; 7 mg ml21 SuperYellow) was spin-coated onto the anode at3,000 r.p.m. for 60 s, followed by vacuum drying for 1 h. A second patternedAgNWPUA composite electrode (as cathode) was stacked, face down onto theelectroluminescent polymer layer with the patterned AgNW strips perpendicular tothose of the anode. The stack was heated to 90 8C to enhance adhesion between thelayers, then fed through a hot-press set-up at 150 8C.

Characterization. The stretching and relaxing tests were performed on a motorizedlinear stage with a built-in controller (Zaber Technologies). A Keithley 2000 digitalmultimeter was used to monitor resistance changes. Strain and resistance data wererecorded via a custom-made LabView code. All measurements were carried out atroom temperature. Transmittance spectra were recorded by a Shimadzu UV-1700spectrophotometer. SEM was performed on a JEOL JSM-6701F scanningelectron microscope.

The currentvoltagelight intensity curves for the stretchable PLECs in theiroriginal state were measured with a Keithley 2400 source meter and a calibratedsilicon photodetector by sweeping the applied voltage from 0 to 21 V in 100 mVincremental steps. The currentvoltagelight intensity curves for the stretchablePLECs under the stretched state were measured with a Keithley 2400 source meterand a Photoresearch PR-655 (measurement spot size, ,1 mm). All PLECmeasurements were carried out at room temperature in the glovebox unlessspecied otherwise.

Received 21 February 2013; accepted 14 August 2013;published online 22 September 2013

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AcknowledgementsThis work was supported by the National Science Foundation (ECCS-1028412) and the AirForce Ofce of Scientic Research (FA9550-12-1-0074). The authors thank Zhi Ren andKwing Tong for experimental assistance.

Author contributionsJ.L. and Q.P. conceived and designed the research. X.N. carried out the mechanicalmeasurements. J.L., L.L., X.N., Z.Y. and Q.P. participated in materials preparation, device

fabrication and data interpretation. J.L. and Q.P. wrote the paper. Q.P. supervisedthe project.

Additional informationSupplementary information is available in the online version of the paper. Reprints andpermissions information is available online at www.nature.com/reprints. Correspondence andrequests for materials should be addressed to Q.P.

Competing nancial interestsThe authors declare no competing nancial interests.




Elastomeric polymer light-emitting devices and displaysFabrication and characterization of composite electrodesFabrication and investigation of an elastomeric PLECSummary and outlookMethodsMaterialsPreparation of AgNWPUA composite electrodeFabrication of stretchable PLECPreparation of fully stretchable displayCharacterization

Figure 1 Visual transparency, stretchability and SEM characterization of composite electrodes.Figure 2 Device characterization of a stretchable PLEC.Figure 3 Demonstration of encapsulated fully stretchable display.ReferencesAcknowledgementsAuthor contributionsAdditional informationCompeting financial interests

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