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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:
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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.
ba
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800 1,000
Figure 1 | Visual transparency, stretchability and SEM
characterization of composite electrodes. a, Transmittance spectra
of a neat PUA lm and AgNW
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).
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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
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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
ba
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0% strain 40% strain 60% strain
80% strain 100% strain 120% strain
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100 120
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.
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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.
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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
50 m
80 m
Stretch
b
d e
f
c
Packaging material
Packaging material Patterned composite electrode
Patt
erne
d co
mpo
site
ele
ctro
de
One pixel
Patterned anode
PEDOT:PSS
Emissive layer
Patternedcathode
a
Stretch
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.
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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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NATURE PHOTONICS DOI: 10.1038/NPHOTON.2013.242 ARTICLES
NATURE PHOTONICS | VOL 7 | OCTOBER 2013 |
www.nature.com/naturephotonics 823
2013 Macmillan Publishers Limited. All rights reserved.
-
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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.
ARTICLES NATURE PHOTONICS DOI: 10.1038/NPHOTON.2013.242
NATURE PHOTONICS | VOL 7 | OCTOBER 2013 |
www.nature.com/naturephotonics824
2013 Macmillan Publishers Limited. All rights reserved.
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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