Fully Solution-Based Fabrication of Flexible Light-Emitting Device at

Jul 9, 2013 - Here, we report the fabrication of fully solution processed polymer light-emitting electrochemical cells (PLECs) as an alternative emiss...
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Fully Solution-Based Fabrication of Flexible Light-Emitting Device at Ambient Conditions Jiajie Liang,† Lu Li,† Xiaofan Niu, Zhibin Yu, and Qibing Pei* Department of Materials Science and Engineering, Henry Samueli School of Engineering and Applied Science, University of California, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Organic light-emitting devices (OLEDs) have long been perceived as a low-temperature, low-cost technology that can be fabricated through solution-based processes. However, how to achieve all-solutionprocessed OLEDs has been a challenge. Here, we report the fabrication of fully solution processed polymer light-emitting electrochemical cells (PLECs) as an alternative emissive device by spin-coating, rod-coating, and/or blade-coating at ambient conditions. The all-solution-based device, which can be fabricated under ambient air and in large-size, exhibits good flexibility and uniform light emission. The entire fabrication process of the PLEC, include the formation of electrodes, emissive polymer layer, and substrate, are carried out by solution processing under ambient conditions. Moreover, it is notable that employing a solution-processed and transparent SWCNT/AgNW bilayer composite electrode as anode, this fully solution processed PLEC shows even higher device performance than the conventional control devices fabricated on indium tin oxide anode coated on glass and using evaporated aluminum as cathode.

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balanced injections of electrons and holes regardless of the work-function of the electrodes. The ability to use high workfunction, air-stable materials for the cathode9 is an important step to rid of physical vapor deposition of reactive metals in high vacuum. Thus, solution-based materials such as industrial silver paste or PEDOT-PSS ink10 can be used as the cathode. Moreover, a relatively thick emissive polymer layer may be employed in the PLECs. Conventional industrial coating techniques such as rod coating or slot-die coating may be used to deposit large areas at extremely high throughput rate.13 The recent progresses in improving the device performance and lifetime has made the PLECs an attractive technology for product development.14−17 These developments have paved the way toward all-solution processing of large-area PLECs.13,18−20 For solution-processed transparent electrodes, SWCNT, graphene, and conducting polymers have been extentively investigated.5,10,21,22 Matyba et al. realized a solution-processed PLEC that used chemically derived graphene as anode and commercial PEDOT:PSS “ink” as cathode.10 This PLEC exhibited limited device performance probably due to the high sheet resistance of the graphene anode (5k Ω/sq). Metallic nanowires, such as AgNW and copper nanowires, can form conductive coatings with figure-of-merit conductivitytransmittance performance comparable to ITO,23,24 and may be

rganic light-emitting diodes (OLEDs) have emerged as an important technology for flat panel displays and solid state lighting.1−3 While the efficiency of OLEDs can surpass that of fluorescent tubes, some of the originally perceived major advantages of the OLEDs, such as solution-based roll-to-roll process for low cost and flexible form factors have not been fully realized in commercial products.4,5 Zeng et al. reported solution-processed polymer OLEDs (PLEDs) by printing both the emissive layer and cathode on indium tin oxide (ITO)/glass anode.6 However, several key challenges have yet to be overcome for all-solution processing: (1) the limited selection of transparent anode. ITO coating requires high vacuum and high temperature. While several alternative technologies, including silver nanowires (AgNWs) and single-walled carbon nabotubes (SWCNT) coatings and ITO or polyester, have been studied,7 none can match the performance of ITO/glass. (2) The cathode generally uses low-work-function metals such as aluminum (Al) deposited by physical vapor deposition in high vacuum. (3) For efficient injections of charge carriers, the emissive organic layer has to be thinner than 100 nm, and multiple layers are preferred, which make all-solution processing rather difficult.5 Polymer light-emitting electrochemical cells (PLECs) have been investigated to potentially address the charge injection issues.8,9 The devices employ a single layer of conjugated polymer sandwiched between two contact electrodes. The polymer layer comprises a blend of a light-emitting conjugated polymer and an ionic-conducting electrolyte, and can form in situ a light-emitting p-i-n junction when an external voltage is applied.9−12 This junction formation allows the efficient and © 2013 American Chemical Society

Received: June 5, 2013 Revised: July 3, 2013 Published: July 9, 2013 16632

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Figure 1. (a) Schematic illustration of the all-solution fabrication process of an SWCNT/AgNW bilayer polymer composite electrode and PLEC device. (b) A bilayer SWCNT/AgNW-polymer composite electrode, 10 × 10 cm2 in area, bent to a 1.5 cm convex. (c) A doctor-blade coater depositing a luminescence polymer coating on the composite electrodes, (d) A finished all-solution processed PLEC device with OC1C10 coating (5 × 5 cm2) and printing Ag paste cathode (active area ∼4 × 4 cm2) coated using the doctor-blade coater.

The entire fabrication process of the PLEC, include the formation of electrodes, emissive polymer layer, and substrate, are carried out by solution processing under ambient conditions. Moreover, this fully solution processed PLEC is provided with good flexibility and uniform light emission, and the electroluminescent performance is even higher than typical control devices on ITO/glass anode and using evaporated Al cathode. The fully solution processing of a PLEC device is illustrated in Figure 1a. The process starts with coating a layer of SWCNT on glass substrate from dispersion in isopropanol (IPA) and deionized water, followed by a layer of AgNWs from dispersion in IPA. Both coatings were done using a Meyer rod,30 with high uniformity and scalability as shown in Figure S1. The SWCNT layer before the deposition of AgNWs had a sheet resistance of 10 000 Ω/sq. Its scanning electron microscopy (SEM) image displayed in Figure S2 reveals a dense network of SWCNTs. After the deposition of AgNWs, the bilayer coatings on glass is annealed to form a highly conductive AgNW network, with a total measured sheet resistance controlled in the range of 10 to 30 Ω/sq. Subsequently, a precursor solution containing an urethane acrylate resin (UA), a difunctional methacrylate monomer (DMA), tert-butyl acrylate (tBA), and a photo initiator were cast on top of the SWCNT/AgNW bilayer and cured under UV light to complete the fabrication of the bilayer

a suitable choice for the fabrication of all-solution processed PLECs. Solution-processed AgNW-polymer composite electrodes appear particularly attractive thanks to the simple process and low surface roughness.25−27 However, one shortcoming of using AgNWs is the low surface coverage of AgNWs: the nanowires exposed on the surface to form electrical contact with light emitting polymer coated on top of the electrode cover only a small fraction of the surface area.25 This low surface coverage could cause nonuniform charge injection across the electrode area and leave certain areas of the devices nonactive. A conducting polymer layer of PEDOT:PSS on the surface of AgNW electrodes or mixing AgNW with carbon nanotubes can increase the conductive surface coverage and thus enhance the hole injection.20,28,29 However, the absorption of water by the PEDOT:PSS layer would corrode the silver nanowires and thus lower the performance of the light-emitting devices.30 Herein, we report a technique to fabricate a fully solutionbased and large-size PLEC based on a SWCNT/AgNW bilayerpolymer composite electrode anode and printable Ag paste cathode using spin-coating, rod-coating, and/or blade-coating at ambient conditions. The solution-processed bilayer composite electrode anode, which comprises a top layer of dense SWCNT network and a bottom layer of highly conductive AgNW network, can highly improve the performance of PLEC. 16633

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Figure 2. SEM micrographs of (a) a SWCNT/AgNW bilayer -polymer composite electrode with 30 Ω/sq sheet resistance and (b) an AgNWpolymer composite electrode with 30 Ω/sq sheet resistance. (c) Transmittance spectra of the copolymer matrix and specified composite electrodes based on this copolymer (the SWCNT layer in all bilayers is controlled to have a sheet resistance of 10 000 Ω/sq on its own). All transmittance data are relative to air. (d) Sheet resistance of a 15 Ω/sq bilayer composite electrode (10 × 10 cm2) after specified cycles of bending to a 1.0 cm convex.

composite electrode.25−27 The weight ratios of the comonomers were adjusted to balance the mechanical strength, flexibility, transmittance, and solvent resistance of the composite electrode. The stress−strain response of the copolymer with UA:DMA:tBA weight ratio of 5:5:1 is shown in Figure S3. Solvent resistance of composite electrodes based on this copolymer is presented in Figure S4. This copolymer, called poly(urethane acrylate) for convenience, was selected as the flexible substrate for all-solution processed device fabrication as further described below (Figure 1b). A luminescent polymer layer comprising a red light-emitting polymer poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylenevinylene] (OC1C10), admixed a cross-linkable ethoxylated trimethylolpropanetriacrylate (ETPTA) as an ionic conductor (IC) and lithium trifluoromethane sulfonate (LiTf) as an ionic source, and was then coated onto the conductive surface of the composite electrode by spin or doctor-blade coating (Figure 1c). OC1C10, whose chemical structure is displayed in Figure S5, is a red light-emitting copolymer with good solubility in tetrahydrofuran (THF). An OC1C10/THF solution was first drop-casted on a bilayer composite electrode, and spread uniformly by pulling the doctor-blade over the solution. The wet coating was then spinned on a spin-coater to remove the residual THF solvent. The thickness of the OC1C10 layer was controlled via the gap between the doctor-blade and the substrate surface as well as the spin-coating rate. Figure 1d shows a 5 × 5 cm2 uniform coating of OC1C10 on a composite electrode. Finally, a layer of printing Ag paste was coated by doctor-blade coating to complete the all-solution fabrication of a PLEC device (Figure 1(d)). It is notable that the entire fabrication process of the fully solution-based PLEC was carried under ambient air. The SWCNT/AgNW bilayer-polymer composite electrode has a conductive surface covered by the dense SWCNT network. The surface microstructure of a composite electrode

imaged by SEM is shown Figure 2a, which reveals a dense, highly entangled network of SWCNTs. The underlying AgNW percolation network is also clearly seen due to the much higher conductivity of AgNW than that of SWCNT. The AgNW percolation network resembles that formed on bare glass substrate. Compared to the sparse network of AgNW on the surface of AgNW-polymer composite electrode (Figure 2b),25−27 the surface of the bilayer composite electrode is covered with a dense SWCNT network, which should facilitate uniform charge distribution across the entire electrode surface. The transmittance spectra for the 10 000 Ω/sq SWCNT coating and various SWCNT/AgNW bilayer coatings on glass substrate are shown in Figure S6. The transmittance of the 10 000 Ω/sq SWCNT coating at 550 nm is around 89% (inclusive of glass substrate). Despite such a high transmittance, SEM image (Figure S2) shows a dense packing of SWCNT network in stark contrast with that of an AgNW coating with 30 Ω/sq sheet resistance and 85% transmittance at 550 nm. The sheet resistance of SWCNT coatings could be lowered to 1.5 kΩ/sq in thicker coatings. However, the transmittance would diminish to only 76% at 550 nm (Figure S7). To lower to less than 100 Ω/sq, the transmittance would be less than 60% at 550 nm, which is undesirable as a transparent electrode for PLECs. Thus, the 10 000 Ω/sq SWCNT coatings were selected to prepare the SWCNT/AgNW bilayers in this work. Figure 2c and Table 1 further depict the figure-of-merit transmittance-sheet resistance for the SWCNT/AgNW bilayerpolymer composite electrodes. Note that the SWCNT/AgNW bilayer-polymer composite electrodes retain the same surface conductance as the neat coatings of SWCNT/AgNW bilayers on glass, indicating the complete transfer of SWCNT/AgNW bilayers into the copolymer matrix. In the composites, most of the SWCNT/AgNW bilay networks are inlaid in the polymer surface, except for a small fraction of SWCNTs that are exposed on the surface. The poly(urethane acrylate) copolymer matrix 16634

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electrode. Repeated Scotch tape peeling test also did not affect the sheet resistance of the composite electrodes. For a comparison purpose, 15 Ω/sq AgNW-polymer composite electrode with 83% transmittance at 550 nm was also prepared according to our previous reported method (Figure S8).25−27 To evaluate these solution-processed SWCNT/AgNW bilayer composite electrodes as the PLEC anode, we first prepared the sandwich-structure PLEC devices based on bilayer composite electrode anode and conventional evaporated Al cathode, following the typical procedure used to fabricate PLECs on ITO/glass anode.17 A electroluminescent (EL) polymer layer, comprising a yellow light-emitting polymer SuperYellow, a cross-linkable ETPTA as an ionic conductor and LiTf as the ionic source, was spin-coated onto the conductive surface of the composite electrodes as the anode. PLECs employing a highly cross-linkable ionic conductor can form a stable p-i-n junction within the emissive layer upon voltage bias, and thus enhance the charge injections and electroluminescent efficiency of the PLECs.17,32 A thin layer of Al was evaporated on top of the EL polymer layer as the top contact (cathode). Control devices employing ITO/glass as anode and evaporated Al as cathode were also fabricated for comparative study. All the PLEC devices were initially charged under a constant voltage (8 V) for 10 min to allow the formation of a stable p-i-n junction within the EL polymer layer. 17,32 The EL property of the devices was then characterized. Five devices were tested for each type of devices, and similar results can be found in other repeated devices as shown in Figures S9 and S10. Figure 3a,b respectively shows the typical current density− driving voltage characteristic curves (J−V) and luminance− driving voltage characteristic curves (L−V) of PLEC devices processed on conventional ITO/glass, a AgNW−polymer composite electrode, and a bilayer composite electrode. It can be seen that in the voltage scan from 0 to 10 V, the PLEC

Table 1. Transmittance and Sheet Resistance Data for Composite Electrodes electrodes copolymer matrix SWCNT-polymer composite electrode SWCNT/AgNW bilayer composite electrode 1 SWCNT/AgNW bilayer composite electrode 2 SWCNT/AgNW bilayer composite electrode 3

transmittance @ 450 nm (%)

transmittance @ 550 nm (%)

transmittance @ 850 nm (%)

sheet resistance (Ω/sq)

92 90

92 90

93 90

10000

82

83

81

30

79

81

79

15

77

78

75

10

has a transmittance better than 92% in the wavelength range between 450 and 1100 nm. The SWCNT/AgNW bilayerpolymer composite electrodes with 10 Ω/sq, 15 Ω/sq and 30 Ω/sq sheet resistance have a transmittance of 78%, 81% and 83% at 550 nm respectively. In comparison, a 10 Ω/sq ITO/ glass electrode has a transmittance of 86% at 550 nm. Unlike ITO/glass, the bilayer composite electrodes are highly flexible. A 10 × 10 cm2 sheet could be repeatedly bent to concave and convex with 1.0 cm radius (Figure 1b). The sheet resistance did not change much with bending. After 2000 bending cycles, the sheet resistance only increased from 15 Ω/ sq to 16 Ω/sq as shown in Figure 2d. This high flexibility of the composite electrode coincides with a strong bonding between the SWCNT/AgNW bilayer film and the copolymer matrix: any delamination or microstructural change would have caused (1) the loss of AgNW interconnections and (2) the sliding of AgNWs, and thus can significant increase contact resistance of AgNW network and the surface resistance of composite

Figure 3. Current density-driving voltage characteristics (a), luminance-driving voltage characteristics (b) and current efficiency-brightness characteristics (c) of yellow PLEC devices on ITO/glass, 15 Ω/sq AgNW-polymer composite electrode, and 15 Ω/sq SWCNT/AgNW bilayer composite electrode. The voltage scanning rate is at 0.1 V/s. (d) Stress test for a yellow PLEC device on bilayer composite electrode at 1.5 mA/cm2 constant current density. 16635

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Figure 4. Current density−driving voltage characteristics (a), luminance−driving voltage characteristics (b) and current efficiency−brightness characteristics (c) of red PLEC devices on ITO/glass and 15 Ω/sq bilayer composite electrode. The voltage scanning rate is at 0.1 V/s.

composite can reach a maximum efficiency of 7800 cd/m2, significantly lower than the 14 600 cd/m2 maximum brightness of the PLEC on the bilayer composite. The stability of the yellow PLEC devices based on the bilayer composite electrode was also evaluated under 1.5 mA/cm2 constant current, as shown in Figure 3d. The brightness of the device initially increases with time, peaks at 147 cd/m2 in 60 min and then turn to gradual descending to 75 cd/m2 in 16 h. Red PLEC devices were also fabricated on the bilayer composite electrodes utilizing similar sandwich device structure of composite electrode/EL layer/Al, following the typical procedure used for device fabrication on ITO/glass. The red EL polymer layer comprising a light-emitting polymer OC1C10, admixed with ETPTA as an ionic conductor and LiTf as ionic source. The current density, emission density, and current efficiency characteristics of the red PLECs on bilayer composite electrode and control device measured at a scanning voltage at 0.1 V/s are displayed in Figure 4a−c. The red PLEC on bilayer composite electrode turns on at 3.0 V (the same as the control device), and reaches a maximum current efficiency of 2.76 cd/A at 2040 cd/m2. The device on bilayer composite electrode also shows higher current density, higher brightness, and higher current efficiency at high brightness levels than the control device on ITO/glass. These results indicate that the solution-processed bilayer composite electrode is an excellent choice of transparent anode in the application of PLECs. Subsequently, to investigate the performance of PLEC devices using Ag paste as cathode, we further fabricated red PLEC devices using conventional ITO/glass as anode, but with the printing Ag paste (coated with doctor blade) as cathode. The active area of the devices for measurement is controlled in the range of 0.15 − 0.18 cm2. The J−L−V curve and η−L curve of the Ag paste device measured at a scanning voltage of 0.1 V/ s are presented in Figure 5a,b. This Ag paste device (ITO/ OC1C10:IC:LiTt/Ag paste) has similar turn-on voltage at 3.0 V, a slightly lowered maximum brightness (1360 cd/m2), and maximum current efficiency (1.76 cd/A) than the control

devices exhibit characteristic turn-on of LEC. The turn-on voltage of the devices is ∼2.2 V, regardless of the anode/ substrate, and comparable to controls on ITO/glass (Figure 4b). The turn-on voltage corresponds to the band gap of the SuperYellow polymer. However, the PLEC device on AgNWpolymer composite electrode has lower current density and emission intensity than the control device at the same bias above 3.5 V. On the other hand, the device on the bilayer composite electrode resembles the control device on ITO/glass except for at high driving voltage. The device on the bilayer composite electrode shows higher emission intensity than that of control device when bias is higher than 7.5 V. The large difference between the AgNW−polymer composite electrode and the bilayer composite electrode can be explained by the difference surface coverage as shown in Figure 2. Note that on the surface of the AgNW−polymer composite, there are areas as large as 10 μm2 that do not have any AgNWs. This dimension is significantly larger than the thickness of the EL polymer layer (200 nm). Charge distribution into the uncovered areas should be difficult, and these areas are probably unlit or under-lit compared to the areas covered by AgNWs. Also note that doping in the EL layer can propagate horizontally to a certain degree. Therefore, the actually unlit area should be smaller than the areas uncovered by AgNWs. The current efficiency−brightness (η−L) responses of the PLEC devices are depicted in Figure 3c. It can be seen that the PLEC devices on the composite electrodes and ITO/glass exhibit comparable current efficiency at low brightness up to 1000 cd/m2. At high brightness, the current efficiency for control device on ITO/glass diminishes rather rapidly with brightness. The efficiency of the PLECs on bilayer composite electrode remains stable with brightness up to 13 000 cd/m2. The higher efficiency of the PLEC on the composite electrodes than on ITO/glass may be attributed, at least in part, by surface plasmon scattering of the emitted light by AgNWs, and consequently, the out-coupling efficiency is increased.27,33−35 Of the two composite electrodes, PLEC on the AgNW 16636

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Figure 5. (a) Current density−luminance−driving voltage characteristics and (b) current efficiency−brightness characteristics of a red PLEC device using ITO/glass as anode and Ag paste as cathode. Current density−luminance−driving voltage characteristics (c) and current efficiency−brightness characteristics (d) of an all-solution processed red PLEC device; inset in panel d shows a photograph of a red PLEC driven at 8 V; lighting area ∼0.18 cm2. Current density−luminance−driving voltage characteristics (e) and current efficiency−brightness characteristics (f) of the all-solution processed red PLEC before and after 10 cycles of bending-recovery.

concave and convex curvatures with 5.0 mm radius for 10 cycles. Compared to the freshly prepared device, the device after 10 cycles of bending still shows the same turn-on voltage at 3.0 V. Slight degradation of current efficiency was observed: the maximum current efficiency decreased to 2.73 cd/A as a result of the slight increase in current density and reduction in light intensity. The maximum bending curvature of ITO electrode was limited to a 7 mm radius,36 the all solutionprocessed PLEC devices show better flexibility than the conventional light-emitting devices using ITO as anode and evaporated Al as cathode. In summary, we have demonstrated an all-solution processing technique to fabricate high-performance PLEC devices. The solution-processed anode (including substrate) comprising SWCNT/AgNW bilayer networks embedded in a surface layer of the polymer composite exhibits high figure-ofmerit-conductivity-transmittance performance and mechanical flexibility. The dense SWCNT network embedded on the outer surface can distribute charges uniformly across the electrode surface area, whereas the sparsely distributed AgNW networks buried underneath the SWCNT network contribute to low sheet resistance. The bilayer composite electrode can be used to fabricate high-performance polymer PLEC devices without the use of a conducting polymer layer to enhance hole injection. Both the emissive layer and the cathode can be

device using ITO/glass as anode and evaporated Al as cathode (Figure 4). These results indicate that printing Ag paste is a suitable material for solution-processed cathode for PLECs. Thus, taking full advantage of the solution-processed and high-performance bilayer composite electrode anode and Ag paste cathode, all-solution processed red PLECs were fabricated, wherein all the component materials (including anode, cathode, the EL polymer, and the substrate) were solution-processed (from a solution, dispersion, or paste). The characteristic J−L−V and η−L responses of the all-solution processed red PLEC using bilayer composite electrode (anode) and Ag paste (cathode) are depicted in Figure 5c,d. The device has a turn-on voltage of 3.0 V, and exhibits a maximum current efficiency of 3.02 cd/A at maximum brightness of 2080 cd/m2. The efficiency represents a 53% improvement over the control device (Figure 5). It is also higher than the device using bilayer composite electrode as anode and evaporated Al as cathode (Figure 5). The inset in Figure 5d displays an optical photograph of an all-solution processed device at 8 V bias, showing fairly uniform and bright light emission over the entire active area (∼0.18 cm2). More than 10 devices were made and tested, and the performance (brightness and efficiency) fluctuation remained in a fairly narrow range of ±10%. The flexibility of this all solution-processed red PLEC is also measured as shown in Figure 5e,f. The devices were bent to 16637

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mg/mL were spin-coated onto the composite electrodes at 2000 rpm for 60 s. The films were then dried at room temperature in vacuum for 1 h before use. The film thickness was approximate 200 nm, as measured by a Dektak profilometer. Then, a layer of Al (100 nm) was deposited in a vacuum thermal evaporator through a shadow mask at a pressure of 10−6 Torr as cathode according to the earlier reported works.18 Fabrication of All-Solution Processed PLECs. A solution of OC1C10, ETPTAm and LiTf in anhydrous THF (weight ratio 20: 5: 1) with the concentration of polymer about 6 mg/ mL was coated onto the composite electrodes by either spin coating at 2000 rpm for 60 s or doctor-blade coating (the distance between the conductive surface and blade was controlled at 100 μm) on precleaned bilayer composite electrode. The films were then dried at room temperature in vacuum for 1 h before use. A predetermined amount of Ag paste was then dropped cast onto one end of the light-emissive polymer layer. The doctor-blade was drawn down to spread the Ag paste to form a uniform and smooth cathode layer. The resulting coatings were dried at room temperature in vacuum for additional 1 h to completely remove the solvent of the Ag paste. To define the active area, we would cut the irregular part to make sure the active area is a regular shape. Device Characterization. All the electrical measurements were carried out under nitrogen in the glovebox with oxygen and moisture levels below 0.5 ppm. More than 5 devices were tested for each type of PLECs. The current−voltage−light intensity curves were measured with a Keithley 2400 source meter and a calibrated silicon photodetector by sweeping the applied voltage from 0 to 20 V at 100 mV increments per step. The transmittance spectra were recorded utilizing a Shimadzu UV-1700 spectrophotometer.

coated from solutions, by doctor-blade and rod coating to complete the all-solution process. The all-solution-processed PLECs exhibit even higher performance than control devices using conventional ITO/glass anode or evaporated Al as cathode. While investigation is underway to optimize the allsolution processing technique and further improve device performance, the approach presented here may open up a new avenue toward the manufacturing of low-cost, high-performance polymer light-emitting devices and displays.

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METHODS Raw Materials. AgNWs were synthesized with average diameter of these AgNWs was 25−35 nm, and average length was 10−20 μm. The SWCNTs, which contain 1.0−3.0 atomic% carboxylic acids, were purchased from Carbon Solutions, Inc. Urethane acrylate (UA), ethoxylated bisphenol A dimethacrylate (DMA) and ethoxylated (15) trimethylolpropanetriacrylate (ETPTA) were all supplied by Sartomer. 2,2-dimethoxy-2phenylacetophenone (photo initiator), lithium trifluoromethanesulfonate (LiTf) (99.995% purity), tert-butyl acrylate (tBA) and anhydrous inhibitor-free tetrahydrofuran (THF) were obtained from Sigma-Aldrich. The conducting Ag paste was supplied by Add-Vision, Inc. The soluble phenyl substituted poly(1,4-phenylene vinylene) (SuperYellow, catalogue No. PDY-132), and the red light-emitting polymer poly[2methoxy-5-(3,7-dimethyloctyloxy)-p-phenylenevinylene] (OC1C10) for the PLECs were obtained from Merck. Preparation of Solution-Processed Composite Electrode. The SWCNT film was first prepared using Meyer Rod coating. In a typical procedure, 10 mg functional SWCNTs were dispersed in 10 mL distilled water (DI-water) and isopropanol alcohol (IPA) mixture solution (volume ratio: DIwater/IPA = 1/4) with the aid of sonication. The dispersion was subjected to centrifuge (at 8000 rpm for 20 min) to remove undispersed SWCNT agglomerates and other large impurities. The resulting solution was then coated employing the Meyer rod (RD Specialist, Inc.) on glass release substrates. To make the AgNW transparent conducting films, AgNWs were first dispersed in the methanol and IPA mixture solution with a concentration of around 2 mg/mL, followed by coating on glass release substrates utilizing Meyer rod.24 For the fabrication of SWCNT/AgNW bilayer transparent conducting films, a layer of SWCNT and a layer of AgNW were coated successively on glass substrates using the Meyer rod. Subsequently, the above transparent conducting films on glass were overcoated with a precursor solution that contained 45 weight parts of UA, 45 parts of DMA, 9 parts of tBA, and 1 part of DMPA, followed by polymerizing in situ through photoinitiated free-radical polymerization. Finally, the resulting composite electrodes were peeled off the release substrate, thus the AgNWs or SWCNTs that were in contact with the glass surface were exposed on the conductive surface of the composite electrodes, as shown in Figure 1. Fabrication of PLECs. The as-prepared composite electrodes, which were used as anode, were cleaned in an ultrasonic bath with detergent, DI-water, acetone and IPA for 5 min, respectively. The 10 Ω/sq ITO coated glass substrates were cleaned successively with detergent, DI-water, acetone and IPA for 30 min. To coat the light-emissive polymer layer, a solution of SuperYellow, ETPTA, and LiTf in THF (weight ratio 20:4:1) with concentration of polymer about 7 mg/mL or a solution of OC1C10, ETPTA and LiTf in THF (weight ratio 20:5:1) with the concentration of the emissive polymer about 6

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ASSOCIATED CONTENT

S Supporting Information *

Optical photographs of SWCNT/AgNW bilayer coatings on glass, SEM micrographs SWCNT coating on glass substrate, mechanical property and solvent-resistance characterization of composite electrodes, the chemical structure of the lightemitting polymer OC1C10, transmittance spectra of SWCNT films on glass substrates with 10 000 Ω/sq sheet resistance and various SWCNT/AgNW bilayer coating on glass substrates, transmittance spectra of the 1500 Ω/sq SWCNT-poly(urethane acrylate) composite electrode, transparent spectra of AgNW-PUA composite electrodes, and stress test of yellow PLEC. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

Jiajie Liang and Lu Li contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work reported here was supported by National Science Foundation (ECCS-1028412) and the Air Force Office of Scientific Research (FA9550-12-1-0074). We also acknowledge the use of SEM facilities at Nano and Pico Characterization 16638

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The Journal of Physical Chemistry C

Article

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Laboratory at the California NanoSystems Institute, University of California, Los Angeles.

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dx.doi.org/10.1021/jp405569q | J. Phys. Chem. C 2013, 117, 16632−16639