Authors: Lili Meng | Min Zhang | Huanhuan Deng | Bojie Xu | Hongqin Wang | Yunjun Wang | Lei Jiang | Huan Liu
Source: CCS Chemistry (article, 2020-09-30)
Field: Materials Science — Materials Chemistry

Direct-Writing Large-Area Cross-Aligned Ag Nanowires Network: Toward High-Performance Transparent Quantum Dot Light-Emitting Diodes Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2021Direct-Writing Large-Area Cross-Aligned Ag Nanowires Network: Toward High-Performance Transparent Quantum Dot Light-Emitting Diodes Lili Meng, Min Zhang, Huanhuan Deng, Bojie Xu, Hongqin Wang, Yunjun Wang, Lei Jiang and Huan Liu Lili Meng Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 , Min Zhang Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 , Huanhuan Deng Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 , Bojie Xu Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 , Hongqin Wang Suzhou Xingshuo Nanotech Company Limited (Mesolight), Suzhou 215213 , Yunjun Wang Suzhou Xingshuo Nanotech Company Limited (Mesolight), Suzhou 215213 , Lei Jiang Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 and Huan Liu *Corresponding author: E-mail Address: [email protected] Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191 https://doi.org/10.31635/ccschem.020.202000402 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Flexible transparent electrodes (FTEs) made of silver nanowires (AgNWs) have been widely used in wearable and foldable electronics devices. For obtaining FTEs with both high transparency and low resistance, the AgNWs network should be highly cross-aligned with a low density. Various solution processes have been developed, but most suffer from poor control of the distribution of the AgNWs. Here, we developed a facile direct-writing solution process guided by a conical fiber array (CFA) for preparing large-area highly cross-aligned AgNW networks with low density. The CFA enables both fine-tuning the triphase contact line during the film formation and the soft conformal contact with the substrate to facilitate multiple liquid transfers without deteriorating the underlayer. Consequently, a high-performance FTE was prepared, showing a transmittance of 93.8%, a sheet resistance of 21.4 Ω sq−1, and good stability against bending. With this process, a large-area transparent quantum dot light-emitting diode (T-QLED) device was fabricated, which gives an impressive external quantum efficiency of 15.47% (indium-tin oxide [ITO] side: 7.17%; FTE side: 8.3%). Moreover, we also demonstrated a large-area flexible T-QLED. We envision that these results will shed new light on easy fabrication of high-performance FTEs. Download figure Download PowerPoint Introduction Flexible transparent electrodes (FTEs) have been widely used in various wearable and foldable electronic devices including organic photovoltaic devices and light-emitting diodes and displays.1–8 Diverse conductive nanomaterials, such as graphene, carbon nanotubes, silver nanowires (AgNWs), and metal nanoparticles, have been used for making FTEs.9–14 Among them, AgNWs serve as the most frequently used material due to high transmittance, low sheet resistance, and good flexibility.15,16 For an FTE with both high transparency and low resistance, the AgNWs network require a low density but high crossing. Thus, various AgNW networks have been prepared for making FTEs.17,18 However, AgNWs are distributed randomly in most cases, which normally leads to a high percolation threshold and sometimes even electric short circuits.19 Such a decrease in performance is attributable to the inhomogeneous distribution of AgNWs and the highly resistive AgNW–AgNW junctions. Furthermore, the crossing angle of the AgNWs junction affects the conductivity, and a large crossing angle is desirable according to the percolation theory.20 Therefore, preparing a vertically cross-aligned AgNWs (CA-AgNWs) network with homogeneous distribution is apparently crucial for balancing a good trade-off between the electrical conductivity and the optical transmittance. To date, various solution-coating techniques have been developed for fabricating AgNW-based FTEs,21 including Langmuir–Blodgett,22 rod coating,20,23 blade coating,24 capillary printing,25 spin coating,26,27 inkjet printing,18 and so on. However, these techniques can hardly control the wetting/dewetting of the liquid film on the substrate especially at micrometer scale, which leads to the random distribution of AgNWs with undesirable aggregations. What is more, the uniformity of the as-deposited AgNW films decreases drastically as the film area increases. To address these issues, finely controlling the transfer of AgNWs solution onto the substrate, as well as their stacking pattern at micrometer scale, is required. Current strategies for making CA-AgNW networks22,23,25 normally involve complicated processes of making templates to assist the orientation of AgNWs. Very recently, we demonstrated that the conical fibers can control the receding of the triphase contact line during the film formation, by which AgNWs can be aligned.28 It suggests that the conical fibers can control the orientation of AgNWs. Herein, we demonstrated a facile directional solution coating approach guided by the conical fiber array (CFA), which enables direct writing of CA-AgNW networks in a large area of 5 × 5 cm2. Thus, an FTE with an optical transmittance of 93.8% and a sheet resistance of 21.4 Ω sq−1 was fabricated. It is proposed that the CFA enables fine-tuning the receding of the triphase contact line to generate parallel directional stresses to align AgNWs. Moreover, the CFA is capable of soft conformal contacting with the substrate, which facilitates multiple liquid transfers onto the targeted area without deteriorating the underlayer film. The as-prepared FTE shows a good electric stability against bending with no obvious failure in conductivity (ΔR/R ≈ 10%) at a bending angle of 60° for more than 1000 bending cycles, which we attribute to the nonanchored nanowire junctions. Using the FTE and indium-tin oxide (ITO) as electrodes, a transparent quantum dot light-emitting diode (T-QLED) device was fabricated, showing a rather high external quantum efficiency (EQE) of 15.47% (ITO side: 7.17%; FTE side: 8.3%). Furthermore, the FTE is also applicable for making both a flexible T-QLED and touch screen. We envision that the results will assist the easy fabrication of large-area high-performance FTEs. Experimental Methods Materials AgNWs solution was purchased from Suzhou Cold Stones Technology Co., Ltd (Suzhou, China). Poly(ethylene terephthalate) (PET) (200 μm in thickness) was purchased from Colleagues Hardware Co., Ltd. (Shanghai, China). Sodium hydroxide (NaOH, Mw: 40), glucose (C6H12O6, Mw: 180.16), and ammonia solution (NH3·H2O, 25%, Mw: 17.03) were purchased from Beijing Lanyi Chemical Products Co., Ltd. (Beijing, China). Sliver nitrate (AgNO3, 99.8%, Mw: 169.87) was purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). Preparation of CA-AgNWs The PET substrates were cleaned by ultrasonication in deionized water, acetone, and isopropanol for 15 min in sequence, and then dried under N2 stream. AgNWs with the length of 30 ± 5 μm and the diameter of 40 ± 10 nm dispersed in isopropyl alcohol with a concentration of 3 mg mL−1 were used. The unidirectionally oriented AgNWs array was initially transferred onto the substrate, following by a second brushing process in the perpendicular direction on top of the prealigned AgNWs array. Electroless-welding process for the CA-AgNWs film The electroless welding was performed according to the literature procedure. The silver-ammonia solution was first prepared by added 1 mL sodium hydroxide solution (1%) and 6 mL aqueous ammonia solution (0.2%) dropwise into 10 mL silver nitrate solution (0.2%). Then, the AgNWs film was dipped into a mixed solution of silver-ammonia and glucose (0.5%) with a 1∶1 ratio for 3 min. Subsequently, the film was rinsed three times with ultrapure water and vacuum dried at 60 °C for 1 h.17 Characterization Scanning electron microscopy (SEM) images were obtained by a Hitachi S-4800 microscope (Hitachi Limited, Tokyo, Japan). The sheet resistances were obtained using a four-point probe resistivity measurement system (Probes Tech Co., Ltd., Guangzhou, China). The optical properties were measured using a Shimadzu UV-2600 UV–vis spectrophotometer (Shimadzu, Kyoto, Japan) in the wavelength range of 400–800 nm. The bending test of the CA-AgNW films was measured using a linear bending machine and an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co., LTD., Shanghai, China). Preparation of the T-QLEDs The poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) solution was spin coated onto an ITO glass, and the coated ITO was transferred into a glovebox followed by the deposition of poly(9,9-dioctylfluorene-co-N-[4-(3-methy-lpropyl)]-diphenylamine) (TFB). The red quantum dot (QD) films were spin coated on top of TFB layer in air. Then, the multilayer samples were loaded into a high vacuum chamber for spin coating ZnO as an electron transport layer. Finally, the as-prepared CA-AgNWs were used as a transparent counterelectrode. The voltage–current density/luminance, current density–current efficiency/EQE, and electroluminescence (EL) spectral performance of T-QLED devices were characterized by Spectrascan PR-655 (Photoresearch, California, USA). Preparation of the flexible T-QLED The as-prepared CA-AgNWs FTEs were used for both the bottom and the top electrodes. Other processes were the same as that of making T-QLED described earlier. Measurement on the EQE The EQE performances of QLEDs were characterized using a Spectrascan PR-655. All measurements were conducted under ambient conditions, and shadow masks were used to prevent error due to optical and electrical edge effects during measurements. The current–voltage–luminance characteristics were measured with a source measure unit (Keithley 236). The as-prepared QLEDs were nailed to a background plate. Then, the beam spot was focused and collimated with a set of optical lenses under an applied output voltage. The EQE spectra were recorded from 380 to 780 nm in 4 nm steps. During the data acquisition, the voltage was performed from 3 to 7 V in 0.2 V steps. Fabrication of a transparent touch screen The transparent touch screen was operated by connecting a four-wire resistive controller (STP-RAP45U2U-S; Shenhuiheng Electronic Technology Co., Ltd., Shenzhen, China) to a laptop computer. Using the CA-AgNWs FTE as top electrode, a resistive touch sensor without any dot spacers was fabricated. The top and bottom electrodes were separated by an air gap (thickness of ∼200 μm) developed by attaching PET around the substrate edges. Results and Discussion As shown schematically in Figure 1a, the CFA-guided direct-writing process entails two sequential steps of aligning nanowires in two mutually perpendicular directions. Guided by the CFA, the AgNWs solution was initially transferred onto a flexible PET substrate along the X direction (Figure 1a1), followed by a secondary writing and brushing process in the Y direction on the top of the prealigned AgNW layer (Figure 1a2). By such a direct-writing process, a vertical CA-AgNWs network was prepared on the PET substrate (Figure 1a3), which is conductive and transparent and can be used as the FTE. For comparison, the as-prepared CA-AgNWs were treated with an electroless-welding process (EWCA-AgNWs) by dipping the CA-AgNW into a mixed solution of silver-ammonia and glucose resulting in a reduced contact resistance between AgNWs (Figure 1a4).17 The representative SEM images of the CA-AgNWs show a homogeneous AgNWs network containing numerous vertically crossed junctions on the whole substrate (Figure 1b). Such uniform distribution suggests the quality of the underlayer AgNW film was maintained during the secondary aligning of the AgNWs in the vertical direction. Statistics on the cross angle of the CA-AgNWs network in an area of 50 × 50 μm2 shows that a cross angle between 80° and 100° accounts for 65.75%, which further verified the quasi-vertical crossing angle of the as-prepared CA-AgNWs network (Figure 1c). The single junction of AgNWs with the crossing angle of ca. 90° is shown in Figure 1d. The EWCA-AgNWs show essentially identical surface morphology (Figure 1e) in large scale, but with clearly welded junctions (Figure 1f and Supporting Information Figure S1). Therefore, the CFA-guided directing writing strategy is capable of aligning AgNWs multiple times on the same targeted substrate without deteriorating the quality of the underlayer film. Figure 1 | (a) Schematic illustration of the direct-writing process guided by the CFA along X and Y direction in sequence, followed by an optional process of electroless welding. (b) Large-area SEM image of the as-prepared CA-AgNWs on the PET substrate. (c) Statistics on the cross angle of the CA-AgNWs network. (d) The enlarged SEM image of a single vertical-crossing junction. (e) Large-area SEM image of the EWCA-AgNWs, and (f) the enlarged SEM image of a single-welded crossing junction. CFA, conical fiber array; SEM, scanning electron microscopy; CA-AgNW, cross-aligned silver nanowire; PET, poly(ethylene terephthalate). Download figure Download PowerPoint Using CA-AgNWs, a large-area high-performance FTE was developed on a flexible PET substrate. The optical picture shows good transparency and homogeneous distribution in an active area of 5 × 5 cm2 (Figure 2a). With increasing the concentrations of AgNW solutions from 1 and 2 to 3 mg mL−1, the sheet resistance of the as-prepared FTEs decreased from 355 and 121 to 21.4 Ω sq−1, respectively (Figure 2b and Supporting Information Figure S2), accompanied by a slight decrease of the optical transparency at the wavelength of 550 nm from 95.8% and 94.7% to 93.8% (Figure 2c). Further increasing the concentration of AgNW solution to 3.5 mg mL−1 led to only minimal improvement in the conductivity (19.8 Ω sq−1), but a big decrease in the transparency (90.3%) (Figure 2c and Supporting Information Figure S2). To obtain high conductivity and high transparency, the AgNW solution with a concentration of 3 mg mL−1 and a shearing speed of 800 μm s−1 was used, which enabled better alignment, uniform coverage, and high crossing of AgNWs on the substrate (Figure 2c and Supporting Information Figure S3). For comparison, both the conductivity and the transparency of the EWCA-AgNWs FTE were also characterized. The sheet resistance after being electroless welded decreased from 21.4 to 13 Ω sq−1 (Figure 2b), accompanied by an impaired optical transparency of 90.7% (Figure 2c). The spatial electric uniformity of the CA-AgNWs FTE was also characterized by measuring the sheet resistance values in 25 local regions in a 5 × 5 cm2 area (Figures 2d and 2e). The CA-AgNWs FTE exhibits a uniform distribution of the sheet resistance with a small standard deviation of 2.3. Taken together, the as-prepared CA-AgNWs FTE exhibits rather good performances of high conductivity and transparency with a large-area uniformity. Furthermore, by summarizing the sheet resistance versus optical transmission at 550 nm for AgNWs-based FTEs fabricated by other solution processes,6,20–26,29–32 the as-prepared CA-AgNWs FTE shows a good balance between the electrical conductivity and the optical transmittance (Figure 2f). The area of our CA-AgNWs FTEs is also comparable with other reported AgNWs-based FTEs ( Supporting Information Figure S4).8,30,33–35 Figure 2 | (a) An optical picture of the as-prepared CA-AgNWs FTE with an area of 5 × 5 cm2. (b) The sheet resistance, and (c) the optical transmittance over the visible spectrum of the FTEs based on CA-AgNWs and EWCA-AgNWs at different concentrations. (d) Schematic illustration showing 5 × 5 pixels of the CA-AgNWs FTE, and (e) the corresponding mapping images (5 × 5 pixels) of the resistance distribution. (f) The summary of the sheet resistance versus optical transmission at 550 nm for the AgNWs-based FTEs reported. (g) The resistance at different bending angle for the CA-AgNWs FTE (red line), the EWCA-AgNWs FTE (blue line), and the commercial ITO/PET (black line). The inset depicts a higher bending angle. (h) The resistance changes under bending/unbending motion at bending angle of 60° for the CA-AgNWs (red line), the EWCA-AgNWs (blue line), and the commercial ITO/PET (black line), and (i) depiction of high stability during 1000 cycles. CA-AgNW, cross-aligned silver nanowire; FTE, flexible transparent electrode; ITO, indium-tin oxide; PET, poly(ethylene terephthalate). Download figure Download PowerPoint Notably, the FTE based on the CA-AgNWs shows better bending stability than that based on EWCA-AgNWs. To explore the effect of the electroless welding on the bending stability of the AgNW films, the bending tests for CA-AgNWs and EWCA-AgNWs FTEs were conducted, and the results are summarized in Figures 2g–2i. When the bending angle was lower than 130°, the sheet resistance for both FTEs remained constant (Figure 2g). The resistance shows a clear positive linear increase with surpassing a bending angle of 130° for both FTEs. Particularly, the slope for the EWCA-AgNWs FTE (Figure 2g, blue line) is much bigger than that of CA-AgNWs FTE (Figure 2g, red line), which directly supports the better stability of the CA-AgNWs FTE. For comparison, we measured the sheet resistance of a commercial ITO/PET film. The sheet resistance was dramatically amplified with an increasing compression ratio because of the brittle nature of ITO (Figure 2g, black line). Considering its application in flexible devices, the bending stability was also investigated. As shown in Figure 2h, the electrical response of the CA-AgNWs FTE exhibited high stability during 1000 bending and unbending cycles under a bending angle of 60°. The resistance variation (ΔR/R) was measured as ca. 0.1 and ca. 0.4 after 1000 bending cycles for the CA-AgNWs FTE and the EWCA-AgNWs FTE, respectively, indicating a better mechanical stability of the CA-AgNWs FTE under bending stress. For the commercial ITO/PET, the ΔR/R reaches a rather large value of ca. 162 after 1000 bending cycles (Figure 2i and Supporting Information Figure S5). Here, the soft conformal contact between the CFA and the substrate is essential for realizing multiple liquid transfer while maintaining the quality of the underlayer film. As we previously suggested,28 the CFA enabled steady holding the solution is imparted by the Laplace pressure difference FL along each individual conical fiber. As a result, the liquid can be transferred onto the substrate in a controllable manner (Figure 3a), without any undesirable leakage. Guided by the CFA, the liquid/gas/solid triphase contact line was shaped into multiple small meniscus curves, by which numerous liquid surface tensions can be aligned to facilitate orientation of the AgNWs in certain directions (Figure 3a). Particularly, during writing the second layer, the bending of the conical fiber tends to vary depending on the local position, which is attributable to the fibrous flexible nature. As schematically shown in Figure 3b, the underlayer aligned AgNWs can be viewed as a prepatterned substrate with uniform roughness, on which the fibers are capable of conformal contacting by varying the bending.36 Consequently, the secondary aligning of AgNWs in a vertical direction was guaranteed without destroying the underlayer AgNWs. Figure 3 | (a and b) Schematic illustration for the direct writing of the second layer AgNWs. (c) Cracking of the CA-AgNWs film after bending at an angle of 180° due to the movable crossing junction. (d) Cracking occurs in the EWCA-AgNWs films after bending because the crossing junction is anchored and easily broken under external stress. CA-AgNW, cross-aligned silver nanowire. Download figure Download PowerPoint To investigate the reason for the improved bending stability of the CA-AgNWs over that of the EWCA-AgNWs, SEM observation of cracking was conducted after bending at a 180° angle. As shown, there are little cracks on the CA-AgNWs network after bending (Figures 3c1 and 3c2), which is attributable to the movable crossing point. The AgNWs in the nonanchored crossing junctions tend to rotate and move as resistance to the external bending imparted by the substrate (Figure 3c3). However, the EWCA-AgNWs were liable to break due to the anchored crossing junctions (Figures 3d1 and 3d2). When compressing the EWCA-AgNWs, no spare space was available for the AgNWs movement because of the confining anchored junctions, thereby leading to undesired cracks under the external stress (Figure 3d3). Taking advantage of both the high conductivity and the high transparency of the CA-AgNWs FTE, a high-performance T-QLED was constructed with a device structure of ITO/PEDOT:PSS/TFB/QDs/ZnO/CA-AgNWs FTE (Figure 4a). The optical picture for the device operated at 4 V directly confirmed its transparency because the letters were clearly visible the film (Figure As shown in Figures and the current density and with a of over at 6 V (ITO FTE Here, the of the FTE is higher than that of the ITO which is attributable to the in light of the AgNW The further increase of the and an decrease the voltage V ( Supporting Information Figure The as-prepared T-QLED gives a current efficiency of (ITO side: FTE side: and rather high on both of ITO and FTE (Figures and spectrum shows at nm with a at of nm (Figures and The and the remained constant increasing the the high stability of the for the T-QLED. Taken together, the as-prepared CA-AgNWs FTE can be used for a high-performance Furthermore, we demonstrated a flexible transparent by using the CA-AgNWs FTEs as both the top and bottom electrodes (Figure As shown in Figure a uniform was obtained for under bending Figure 4 | The performance of the as-prepared T-QLED (a) Schematic of the T-QLED using the CA-AgNWs FTE as the counterelectrode. (b) The optical image of the T-QLED device under showing good The current density and the characteristics for (c) the FTE and (f) the ITO of the The current efficiency and the EQE characteristics for (d) the FTE and (g) the ITO of the spectral for (e) the FTE and (h) the ITO of the T-QLED with increasing voltage. (i) Schematic illustration of the device The optical image of the device under bending transparent quantum dot light-emitting FTE, flexible transparent electrode; ITO, indium-tin oxide; CA-AgNW, cross-aligned silver nanowire; external quantum flexible transparent Download figure Download PowerPoint We also demonstrated that the CA-AgNWs FTE is applicable for making a transparent touch screen. As in Supporting Information Figure and the CA-AgNWs FTE was used as the top transparent electrode, and was with a touch screen controller (a four-wire touch By the two resistive that were from each at the touch and resistance the resulting electrical in both the X and Y directions was to the touch screen The touch screen various such as and As shown in Supporting Information Figure movement on the touch screen that the CA-AgNWs FTE is applicable for various flexible devices. facile CFA-guided solution process was developed for direct-writing large-area CA-AgNWs, both the high crossing and low density of the AgNWs can be The as-prepared CA-AgNWs FTE shows a high optical transmittance and a low sheet resistance Ω We proposed that the multiple parallel directional surface by CFA-guided solution shearing on the substrate, aligning AgNWs multiple times in desirable directions. Here, the soft conformal contact between the CFA and the substrate to multiple liquid transfers while maintaining quality of the underlayer film. Notably, the CA-AgNWs FTE a good stability against imparted by the the nonanchored crossed junctions. the as-prepared FTE can be used to a large-area high-performance T-QLED which shows rather high EQE on both of ITO and FTE The as-prepared CA-AgNWs FTE is also applicable for making both an and touch screen. The new in fabricating CA-AgNWs networks applied in wearable and foldable electric devices. 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