Highly conductive silver nanowire transparent electrode by selective welding for organic light emitting diode

Organic Electronics 60 (2018) 9–15

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Highly conductive silver nanowire transparent electrode by selective welding for organic light emitting diode

T

Lu Lian, Xin Xi, Dan Dong, Gufeng He∗ National Engineering Lab for TFT-LCD Materials and Technologies, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Silver nanowire Selective welding Hydrophilicity difference OLED

A highly smooth, conductive and air-stable flexible transparent silver nanowire (AgNW) composite film is developed by solution process without high temperature treatment. The sheet resistance of the AgNW/polyvinyl butyral (PVB) film decreases significantly after the film is immersed into the silver-ammonia and glucose mixed solution for only a few seconds. The solution selectively wets the AgNWs rather than the PVB substrate, because the AgNWs is hydrophilic while the PVB is hydrophobic. The silver-ammonia on the surface of the AgNWs is reduced into silver, which solders the nanowire assembles together. This selective welding of the AgNWs greatly increases the electrical conductivity of the composite film without obviously attenuating its transmittance. Specifically, the sheet resistance of this AgNW composite film is 10.1 Ω/sq with a transmittance of 87% at the wavelength of 550 nm, and it also shows excellent stability. The film can be easily patterned and employing this composite film as an anode, the organic light emitting diode exhibits comparable performance to the ITO device. This simple and effective solution process method shows strong potential to be applied in industrial mass production.

1. Introduction The transparent electrodes are indispensable components in optoelectronic devices, such as liquid crystal displays, touch screens, solar cells, and organic light emitting diodes (OLEDs). People are expecting more and more bendable, stretchable, and twistable devices, which particularly require high reliability, high conductivity and flexible transparent electrodes. The most widely used material for transparent electrode in the market is indium tin oxide (ITO) on account of its low sheet resistance (below 20 Ω/sq) and high transmittance (90%) [1,2]. However, the brittle ITO is hard to satisfy the requirement of transparent electrodes for flexible devices [3–5]. In addition, high-temperature manufacture procedure [6–8], indium scarcity [9,10], and high cost [3,4], restrict its application as an ideal candidate for the future. These limitations prompt researches for alternatives to ITO, which can satisfy the requirements of high conductivity, high transmittance, flexibility, reasonable cost, and scalable for large-scale mass production. Substitutes of ITO are intensively investigated, such as carbon-based materials [11–14], conducting polymers [15–17], and metallic nanowires [9,18]. Among them, films based on silver nanowires (AgNWs) are particularly promising because of their excellent electrical conductivity and transmittance [19–21]. However, the AgNW films still

∗

Corresponding author. E-mail address: [email protected] (G. He).

https://doi.org/10.1016/j.orgel.2018.05.028 Received 24 April 2018; Received in revised form 17 May 2018; Accepted 20 May 2018 Available online 21 May 2018 1566-1199/ © 2018 Published by Elsevier B.V.

have several drawbacks which restrict their applications in practical cases. For example, the contact resistance between the nanowires is relatively high [22,23], resulting in high sheet resistance of the whole film. High temperature annealing (about 200 °C) is a conventional method to reduce the junction resistance [24,25], but it is unsuitable for plastic substrates sensitive to the temperature. What's more, owing to the inherent thickness of the nanowires, the AgNW films generally suffer from high surface roughness [26,27], making them unsuitable for thin film devices like OLED. Several effective approaches are reported to solve these issues, including high force mechanical pressing, nanoscale joule heating, nanoplasmonic and electrochemical welding [28–31]. However, the processes of these methods are generally complex, and the cost is high. Thus, a low cost, easy processing method which is suitable for large-scale industrial production is urgently demanded. Here, we develop a simple and effective solution process method to fabricate AgNW flexible transparent electrode. The AgNW networks are embedded in the polyvinyl butyral (PVB) substrate through a peel-off procedure. When the AgNW/PVB film is immersed into the silver-ammonia and glucose mixture, the solution selectively wets the hydrophilic AgNW networks rather than the hydrophobic PVB surface. The remaining silver-ammonia surrounding AgNWs is reduced into silver, which solders the adjacent nanowires and reduces the contact

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Fig. 1. (a) Schematic procedure of the AgNW flexible transparent electrode fabrication. The hydrophilicity properties of (b) PVB and (c) AgNW/PVB films.

AgNWs on the top. Then the AgNW/PVB films were immersed into the silver-ammonia aqueous solution, which was prepared by mixing 20 ml AgNO3 aqueous solution (0.01 mM), 2 ml sodium hydroxide aqueous solution (NaOH, 2.5 mM), and 5 ml aqueous ammonia (NH4OH, 0.6 mM). Then 1 ml glucose aqueous solution (C6H12O6, 0.5 mM) was dropped. And after 5–90 s, the films were rinsed with deionized water and blew dry with nitrogen. 2.2. OLED device fabrication 60 nm PEDOT: PSS (Clevios AI4083, H. C. Starck) was spin-coated on the AgNW or ITO electrode as hole-injection layer and annealed at 120 °C for 15 min. Then 1 nm molybdenum trioxide (MoO3, second hole-injection layer), 80 nm N,N-dicarbazolyl-3,5-benzene (mCP, hole transporting layer), 0.2 nm iridium (III)bis (2-(4-trifluoromethylphenyl) pyridine) tetraphenylimidodiphosphinate (Ir (tfmppy)2 (tpip), emitting layer), 60 nm 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene (TPBi, electron transporting layer), 1 nm lithium fluoride (LiF, electron injection layer), and 80 nm aluminum (Al, cathode) were deposited in a high-vacuum chamber (5 × 10−6 Torr) [33–35]. The light emitting area was defined by the overlap of the anode and Al cathode.

Fig. 2. The transmittance of PVB, pristine AgNWs and AgNW/PVB film.

resistances between nanowires significantly. This selective welding AgNW composite film shows a low sheet resistance of 10.1 Ω/sq with a transmittance of 87%. Moreover, covered with the soft and hydrophobic PVB, this AgNW/PVB composite film shows great flexibility and air-stability. Using PVB as mask, the AgNWs can be easily patterned into target shape, and employing this AgNW/PVB composite film as the anode, the OLED device shows comparable performance to the device based on ITO anode. This easily operated method has the potential to produce large-scale transparent electrode in commercial production.

2.3. Characterization The morphology of the film was characterized by a field emission scanning electron microscope (FE-SEM, Sirion 200). The surface roughness was investigated by an atomic force microscope (AFM, Nanonavi E-Sweep). The optical transmittance of the film was recorded by a spectrophotometer (MAPADA UV-3100PC). The measurement of the sheet resistance was taken by a four-point probe system. The thickness of the film was evaluated by KLA-Tencor Alpha Step D-120 Stylus Profiler. A computer-controlled measurement system with Keithley 2400 source meter and Topcon BM-7A luminance colorimeter was used to characterize the performance of OLED device.

2. Experimental section 2.1. Electrode fabrication The AgNWs were synthesized according to the method reported in our previous work [22,32], and measured about 90 nm in diameter and 20 μm in length. The quartz glass was cleaned under ultrasonic bath with deionized water, acetone and isopropyl alcohol sequentially. AgNWs were spin-coated on the quartz glass at 1000 rpm for 3 times. PVB solution with a concentration of 10 wt% in ethanol was coated directly onto the AgNWs by a Meyer bar (RDS #10 3/8″). After being dried at room temperature, the PVB film was easily peeled off with

3. Results and discussion The AgNW composite film fabrication procedure is shown in Fig. 1a. The PVB ethanol solution is coated on the AgNW assembles on the quartz glass surface. After being dried, the AgNW/PVB film is easily peeled off from the glass, and the AgNWs are partly embedded in the 10

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Fig. 3. (a) The sheet resistance and transmittance (at 550 nm) of AgNW/PVB film versus immersing time in silver-ammonia solution after adding glucose. The insets show the color change of the solution after immersing. (b) SEM and (c) AFM images of the AgNW composite films after immersing in the mixture solution of silverammonia and glucose for different time. (d) The figure of merit of the AgNW/PVB films after immersing in the silver-ammonia and glucose mixture solution for different time. (e) SEM images of the AgNW/PVB film before immersing in the silver-ammonia and glucose solution. The circled parts show the nanowires extended away from the PVB surface. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

In contrast, the AgNWs is relatively hydrophilic due to the amide and hydroxyl residuals attaching on the surface during the preparation process [36,37]. In the hydrophilicity test, the water-drop covering on the surface of the AgNW/PVB film shows a lower water contact angle of 70.5° (Fig. 1c). Due to the different hydrophilicity of AgNWs and PVB, the silver-ammonia solution mainly wets the AgNWs surface rather than the PVB substrate, and then the nanowires junctions are tightly welded as the solution wrapping around AgNWs is reduced into silver according to the reaction shown in chemical equation (1).

PVB substrate. Due to the loose contact between the nanowires, the electrical conductivity of this AgNW/PVB film is very poor. However, after dipping in the solution of silver-ammonia and glucose for several seconds, its electrical conductivity increases significantly. In particular, the sheet resistance of the AgNW/PVB film fabricated from 3 mg/ml AgNW solution drops greatly from 7.06 KΩ/sq to 11.2 Ω/sq after the film is immersed into the mixed solution for only 5 s, and junction welding is obviously observed between adjacent nanowires. As shown in Fig. 1b, the water contact angle of the PVB substrate is 104.2°, demonstrating that the PVB is water-repellent. Thus, when the AgNW/ PVB film is immersed in the silver-ammonia and glucose solution, the solution hardly wets the surface of the PVB substrate. The clean PVB surface maintains the good optical transmittance of the composite film.

C6H12O6 + 2[Ag(NH3)2]OH + 3NH3 + H2O + 2Ag Interestingly, 11

compared

to

the

= pristine

C5H11O5COONH4 (1) AgNW

film,

the

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Fig. 4. (a) SEM images of the AgNW composite films with different AgNW density. (b) The sheet resistance of the AgNW/PVB films with different AgNW density before and after immersing in the silver-ammonia and glucose mixed solution, and the transmittance (550 nm) of the AgNW composite film. The immersing time is 10 s. (c) The Figure of merit of the AgNW composite film with different AgNW density.

Fig. 5. (a) The sheet resistance changes of AgNW/PVB composite film and commercial ITO film with increasing bending angle. (b) The sheet resistance changes of AgNW/PVB composite film and commercial ITO film with increasing numbers of bending cycles, and the inset shows that the selective welding AgNW/PVB composite film is flexible and transparent. (c) The sheet resistance of the AgNW/PVB composite film after tape test.

immersing and reaches the maximum value of 2.4 × 10−2 at 10 s, which is chosen as the optimal immersing time. After 10 s’ dipping, the sheet resistance of the AgNW composite film is 10.1 Ω/sq with 86.6% transmittance at 550 nm. The morphology of the film surface is investigated by SEM and AFM (Fig. 3b and c), the root mean square (RMS) of AgNW/PVB film before silver-ammonia treatment is 4.2 nm, and the junctions between nanowires stack loosely. During the peel-off procedure, AgNWs are easily extended away from the PVB surface, as shown in the circled parts in Fig. 3e, resulting in high surface roughness. Meanwhile, without welding at the intersections, the electrical conductivity of the film is poor. After the film is immersed in the solution, meniscus shaped capillary bridges form on the junctions of nanowires, yielding capillary force to attach the separate nanowires onto the PVB surface [42]. Thus the film surface become smoother (RMS 3.6 nm after 5 s), and the contact resistance between the nanowires decreases simultaneously. Furthermore, small particles are observed on the film surface after 10 s’ immersing, owing to the quick reaction of silver-ammonia and glucose mixed solution at room temperature. When the glucose is added, the solution soon turns from colorless transparent to orange, and becomes turbid after 10 s (the insets in Fig. 3a), attributing to the growth of silver particles reduced from silver ions. With the reaction time

transmittance of the film after coating 2 μm PVB layer increases from 87% to 89% at 550 nm, attributing to the decreased external scattering of the film, which is determined by the film uniformity and the gaps between the nanostructures. As shown in Fig. 2, the PVB is almost completely transparent and has little variation on the transmittance over the wavelength. When the PVB is coated on the surface of the AgNWs film, it fills up the gaps between the nanowires and increases the uniformity of the film. The scattering at air surface is reduced and thus the transmittance of the AgNW/PVB film is higher than the pristine AgNWs film [38–40]. As shown in Fig. 3a, the sheet resistance and transmittance of the AgNW/PVB film decrease with the immersing time in the silver-ammonia and glucose solution increasing. The initial sheet resistance of the film is 7.06 KΩ/sq (a transmittance of 89.2% at 550 nm), and drops around three orders of magnitude after the film is immersed in the silver-ammonia and glucose mixed solution. As the immersing time increasing from 5 to 90 s, the sheet resistance decreases from 11.2 to 6.6 Ω/sq, and its transmittance drops as well, from 87.1% to 72.3%. Figure of merit [22,41] is used to describe the optical and electrical performance of the films under different immersing time, shown in Fig. 3d. Before dipping in the silver-ammonia solution, the figure of merit of the film is 4.5 × 10−5, and it increases significantly after 12

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Fig. 6. The sheet resistance of the AgNW/PVB composite film and conventional annealed-AgNW on glass (180 °C, 10 min) after immersing in (a) water, (b) 1 wt% HCl and (c) 1 wt% ammonia water. (d) The sheet resistance changes of AgNW/PVB composite film and annealed-AgNW in ambient condition. SEM images of (e) annealed-AgNW and (f) AgNW/PVB composite film after 30 days in air.

Fig. 7. (a) Schematic procedure of the film patterning. The photos and SEM images of (b) the AgNWs film with PVB mask after 3 wt% HCl solution etching for 2 h, and (c) AgNWs transferred to PVB substrate.

is observed in the SEM images (Fig. 4a) with the concentration of the AgNW solution raising from 1 mg/ml to 5 mg/ml. The sheet resistance and transmittance variation of the AgNW/PVB films with various AgNW densities before and after silver-ammonia treatment is investigated, as shown in Fig. 4b. The sheet resistance of the composite film decreases significantly with the AgNW density increasing. In particular, after immersing for 10 s, the sheet resistance of the film drops two orders of magnitude from 7.06 KΩ/sq to 10.1 Ω/sq when the concentration of AgNWs solution increases to 3 mg/ml, due to the wellwelded consecutive AgNW networks of the film. Although the increasing AgNW density reduces the sheet resistance of the film effectively, the transmittance of the film also decreases from 92.0% to 78.2% with the concentration of AgNW solution increasing from 1 mg/ml to

increasing, the number of the silver particles in the solution increases and the sizes become larger. Although these particles attaching to the film surface can benefit the electrical conductivity, they make negative effect on the transmittance and surface roughness of the film. The RMS of the film increases to 23.8 nm after 90 s’ immersing, and silver particles protrude from the surface higher than 60 nm, which will easily cause shorting and shunting in an OLED device. Thus, in order to ensure the smooth surface of the electrode for OLED, the AgNW/PVB film immersing is limited at 10 s. AgNW density also plays a key role in achieving high electrical conductivity and transmittance of the film. A series of films with various AgNW density are fabricated by spin-coating with different concentrations of AgNW solution. A monotonic increasing of AgNW density 13

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Fig. 8. (a) The OLED structure, (b) Current density-voltage, (c) Luminance-voltage, (d) Current efficiency-luminance of the OLEDs using AgNW composite anode and reference ITO anode.

5 mg/ml. The figure of merit (Fig. 4c) reaches the maximum value at 3 mg/ml, which is selected as the optimum. In order to measure the flexibility of the AgNW/PVB composite film, a bending test with a bending radius of 3 mm is conducted. As shown in Fig. 5a, the sheet resistance of the AgNW/PVB composite film is constant with the bending angle increasing, and almost no change even when the bending angle is 180°. Commercial ITO on PEN substrate is used as a reference. And because of the brittle nature, its sheet resistant increases from 12 Ω/sq to 2.56 KΩ/sq with a ratio of 212.3 when the bending angle is 180° with a radius of 3 mm. Besides, the AgNW/PVB composite film also can stand repeated bending, which includes a concave and convex with a radius of 6 mm in a typical bending cycle. The sheet resistance of the AgNW/PVB composite film only increases from 10.5 Ω/sq to 11.9 Ω/sq after 10,000 bending cycles (Fig. 5b), while the ITO goes to poor contact only after 500 times bending. These experiments demonstrate that this AgNW/PVB composite film exhibits excellent flexibility. Moreover, tape test is used to investigate the adhesion of the AgNWs to the substrate. It is known that the adhesion of the pristine AgNWs to the substrate is poor, and the nanowires are easily torn off by the tape. In contrast, the AgNW/PVB composite film is robust to sustain repetitive taping. As shown in Fig. 5c, the sheet resistance of the film almost no change after 100 times tape test, since the AgNWs are stick tightly to the PVB substrate. The stability of the electrode to resist chemical damage is crucial for their application in the practical devices, which can be characterized by measuring the sheet resistance of the AgNW/PVB composite film. Since annealing at 180 °C for 10 min is a conventional method to solder wire junctions and consequently increase the electrical conductivity of the film, it is used as a reference (annealed-AgNW). As shown in Fig. 6, after 10 h in water, the sheet resistance of annealed-AgNW increases 14.6 times from 9.8 Ω/sq to 153.3 Ω/sq, while that of the AgNW/PVB composite film is constant at about 10.5 Ω/sq. Immersed in acid and alkaline solution, the AgNWs on the annealed sample are all corroded and the films are non-conducting after 3 h. However, the PVB film

around the AgNWs protects the nanowires efficiently and slows down the corrosion speed. The sheet resistance of the AgNW/PVB composite film is 112.3 Ω/sq after immersing in 1 wt% hydrochloric acid (HCl) for 10 h and 289.4 Ω/sq in 1 wt% ammonia water for 8 h separately. After the annealed sample is exposed in ambient condition for 30 days, its sheet resistance raises significantly from 9.8 Ω/sq to 868.2 Ω/sq with a ratio of 87.59 and an oxide layer forms around the nanowire (Fig. 6e), which has negative effects on the electrical conductivity of the film. In contrast, the sheet resistance change ratio of the AgNW/PVB composite film is only 6.63 after a month, and no obvious change is observed on the surface of the film (Fig. 6f). The hydrophobic PVB wrapping around AgNWs effectively protects them from oxidation and corrosion. Since AgNWs are easily etched by HCl and PVB can efficiently protect the AgNWs from corrosion, a scalable method is proposed by using PVB as protection layer for patterning, as shown in Fig. 7a. 10 wt % PVB ethanol solution is coated on the AgNWs film with a mask on it. The AgNWs required for electrodes are covered by PVB. After being dried at room temperature, the sample is dipped into 3 wt% HCl solution. The AgNWs without PVB protection layer is etched gradually by the HCl solution. After 2 h, the AgNWs on the uncovered part completely disappears, whereas the covered part is intact. The boundary of the two parts can be clearly seen, as shown in Fig. 7b. After rinsing and drying, a new PVB layer is deposited on the sample, and the patterned AgNWs embedded in PVB substrate is easily torn off from the quartz glass as a freestanding electrode. An OLED using the AgNW/PVB composite film as the anode has been fabricated, and its structure is illustrated in Fig. 8a. For comparison, a device with the same structure based on commercial ITO anode is used as a reference. Though the sheet resistance of the AgNW/PVB composite film is similar to ITO, the OLED based on AgNW/PVB composite film shows slightly lower current density than ITO device (Fig. 8b). At 10 V, the current density of the AgNW device is 121.4 mA/ cm2, while that of ITO is 137.6 mA/cm2. The work function of silver is 4.2 eV, which is lower than ITO (4.8 eV). The lower work function 14

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makes it more difficult to inject holes from the AgNW/PVB composite anode into the organic layer, resulting in lower current densities. The turn-on voltages of both devices are 3.5 V, and the OLED with the AgNW/PVB composite anode exhibits comparable luminance with the ITO device, shown in Fig. 8c. At the voltage of 10 V, the luminance of AgNW device is 14330 cd/m2, while that of ITO device is 15830 cd/m2. At a luminance of 1000 cd/m2, the current efficiency of the OLED with AgNW/PVB composite anode is 27.5 cd/A, which is comparable to that of ITO device (27.9 cd/A).

[15] [16]

[17]

[18]

4. Conclusion [19]

We have developed a simple and scalable method to fabricate highly smooth, conductive, air stable and flexible transparent AgNW/PVB composite film through solution process. When the composite film is dipped in the silver-ammonia and glucose mixed solution, the solution selectively wet the AgNWs network, due to the different hydrophilicity of the AgNWs and PVB substrate. This selective welding of the AgNWs greatly increases the electrical conductivity of the composite film without obviously attenuating its transmittance. The AgNW/PVB composite film owns a relative low sheet resistance of 10.1 Ω/sq with a transmittance of 87%. The hydrophobic PVB around AgNWs form a protective layer, which protects the AgNWs against bending damage, corrosion and oxidation. The PVB can also act as a protection layer for scalable patterning. OLED device employing this AgNW/PVB composite film as anode shows comparable performance to ITO device. This technique can be useful for the development of large scale solution processed ITO-free transparent electrode for flexible electronic devices.

[20]

[21]

[22]

[23]

[24]

[25]

[26]

Acknowledgement [27]

This work was supported by the National Natural Science Foundation of China (61675127).

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