Silver Nanowire Electrodes: Conductivity Improvement Without Post-treatment and Application in Capacitive Pressure Sensors
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Transparent electrode based on silver nanowires (AgNWs) emerges as an outstanding alternative of indium tin oxide film especially for flexible electronics. However, the conductivity of AgNWs transparent electrode is still dramatically limited by the contact resistance between nanowires at high transmittance. Polyvinylpyrrolidone (PVP) layer adsorbed on the nanowire surface acts as an electrically insulating barrier at wire–wire junctions, and some devastating post-treatment methods are proposed to reduce or eliminate PVP layer, which usually limit the application of the substrates susceptible to heat or pressure and burden the fabrication with high-cost, time-consuming, or inefficient processes. In this work, a simple and rapid pre-treatment washing method was proposed to reduce the thickness of PVP layer from 13.19 to 0.96 nm and improve the contact between wires. AgNW electrodes with sheet resistances of 15.6 and 204 Ω sq−1 have been achieved at transmittances of 90 and 97.5 %, respectively. This method avoided any post-treatments and popularized the application of high-performance AgNW transparent electrode on more substrates. The improved AgNWs were successfully employed in a capacitive pressure sensor with high transparency, sensitivity, and reproducibility.
KeywordsSilver nanowire Pre-treatment Transparent electrode Pressure sensor
Transparent electrodes are regarded as essential components in optoelectronic applications such as solar cells, touch screens, organic light-emitting diodes, and sensor devices [1, 2, 3, 4], and indium tin oxide (ITO) thin films are the most widely used material for such applications. However, there are several drawbacks to use ITO thin film, such as the inherent brittleness, the expensive deposition process, and also the emerging indium scarcity. Several alternatives have been investigated, including carbon nanotubes (CNTs), graphene, conductive polymer, and metal nanowires [5, 6, 7]. Among these, transparent electrode based on silver nanowire (AgNW) networks is being studied intensively and attracting commercial interest owing to their great potential for flexible, cost-efficient, and large-scale fabrication [8, 9, 10, 11].
Although bulk silver exhibits very low electrical resistivity, the conductivity of AgNW networks is limited, especially at high transmittance, by the contact resistance between wires due to the residual of polyvinylpyrrolidone (PVP) layer, which is usually employed as the capping agent to control nanostructure size and disperse nanowires during AgNW synthesis [12, 13]. Several methods have been developed to enhance the contact between nanowires, such as high-temperature (above 200 °C) or long-duration thermal annealing [14, 15], rinsing and pressure treatment , and photonic sintering . Other techniques such as nanosoldering, microwire enhancement, and galvanic displacement aim to remove the PVP layer or enlarge the contact area at wire–wire junctions [3, 18, 19]. However, all of these are the post-treatment methods after AgNWs are deposited on the substrate or the surface of device. Therefore, not only do these methods complicate the fabrication process, they also inevitably influence the performance of the heat-sensitive, pressure-sensitive, or chemical-sensitive substrates.
Recently, a new strategy was proposed using long nanowires to reduce the number of wire–wire junctions in conductive paths, thereby leading to low sheet resistance [20, 21]. However, the PVP-induced resistance problem still has not been solved. The residual PVP layer adsorbing on the surface of AgNW still acts as an electrically insulating barrier at the wire–wire junctions and undermines the conductivity of the electrode. As the saying goes, “sharpen the knife before cutting the wood,” i.e., high-performance AgNW ink is prerequisite before the fabrication of AgNW transparent electrode. Hence, decreasing the thickness of PVP layer beforehand will be a suitable, simple, and effective method for conductivity improvement. Unfortunately, many efforts are still focused on various post-treatments of AgNW electrodes as mentioned above. So far, few reports have paid attention to the important process to improve AgNW ink before electrode fabrication. AgNW paste washed by water has been reported to joint copper at room temperature without pressure , but this close-packed AgNW layer with thickness over 20 μm was quite different from transparent electrode. The actual effect of filtration washing and sonication dispersing process could improve the conductivity of AgNW films ; however, it is too time-consuming to produce nanoscale ink by filtration in practical applications. Moreover, the washing effect on long AgNW has not been discussed. In this paper, a simple and rapid washing method is proposed to tailor the thickness of PVP layer on the surface of very long AgNWs, and the nanowire ink for high-performance transparent electrode regardless of substrate properties was achieved accordingly. Solvents and washing parameters were carefully selected and adjusted to meet the dispersion and preservation requirements of the nanowires. Finally, as-washed AgNWs were used in capacitive pressure sensor, which showed high transparency, sensitivity, and reproducibility.
2.1 Preparation of AgNWs
AgNWs for the preparation of transparent electrodes were synthesized using a one-step polyol method starting with two solutions. For the first, 1.0 g silver nitrate was dissolved in 40 mL ethylene glycol at room temperature, while for the second, 0.8 g PVP (Mw = 360,000) was gradually dissolved in 50 mL ethylene glycol at 60 °C while stirring at 300 rpm. After complete dissolution, the two solutions were mixed and 13.6 g of FeCl3 solution (600 μmol L−1 in ethylene glycol) was added to the mixture at room temperature and stirred at 300 rpm for 3 min. The mixture was then heated at 110 °C without stirring for a 12-h redox reaction. Finally, the solution was mixed with acetone at a volume ratio of 1:4 to precipitate AgNWs for the subsequent steps.
2.2 Washing Method
Thickness tailoring of PVP nanolayer was performed by first mixing AgNW precipitate with ethanol at a mass ratio of 1:15 and stirring at 150 rpm for 15 min at room temperature, followed by centrifugation of the suspension at 3,000 rpm for 3 min. The supernatant was carefully decanted, and the residual precipitate was dispersed in ethanol and prepared for further washing. This procedure constitutes one cycle of ethanol washing (E1) and was applied twice, thrice, or four times to obtain three more AgNW ink-labeled E2, E3, and E4. Then, E4-AgNWs were mixed with deionized (DI) water at either 25 or 90 °C, with stirring at 150 rpm for 15 min, to obtain W25-AgNWs and W90-AgNWs, respectively. Meanwhile, E4-AgNWs were also mixed with dimethylformide (DMF), at either 25 or 140 °C, with stirring at 150 rpm for 15 min to form D25-AgNWs and D140-AgNWs, respectively. The ink in DI water and DMF were then centrifuged at 3,000 rpm for 3 min to obtain AgNW precipitate. Since the wetting properties of these three solvents on PET were different, the AgNWs were finally dispersed in ethanol for coating. All these AgNW inks were fixed at 1.2 wt % concentration. All the reagents mentioned above were purchased from Wako Pure Chemical Industries, Ltd.
2.3 Fabrication of AgNW Transparent Electrodes
PET films with a thickness of 100 μm were employed as substrates for AgNW electrodes. The PET substrates were cleaned in ethanol with ultrasonic treatment and then dried in air. The well-dispersed AgNW inks were drop-coated on the PET substrates for various transmittances at 550 nm wavelength. The dropped ink spread on the surface until uniformly coating the substrate, and the specimens were dried in air for 2–3 min until the solvent is evaporated. AgNW ink was also coated on glass beaker, PET bottle, tissue paper, and bacterial cellulose to verify its adaptability on various substrates. Coating method for glass beaker and PET bottle was dip-coating, and for tissue paper and bacterial cellulose was also drop-coating.
2.4 Fabrication of Pressure Sensors
AgNW transparent electrodes on PET substrates were employed in capacitive pressure sensors. Pure PVP was dissolved in ethanol at 5 wt % concentration. The PVP solution was spin-coated on the as-prepared AgNW electrodes at room temperature. After drying in air for about 30 s, two pieces of sandwiched PET/AgNWs/PVP structures were assembled with the two PVP layers in contact. Two pieces of conductive tapes were pasted on AgNW electrodes for the capacitance measurement.
2.5 Measurements and Characterization
The morphology of the PVP layer adsorbed on the AgNWs was investigated by optical microscope (VHX-600, Keyence), scanning electron microscope (SEM, SU8020, Hitachi High-Technologies), and transmission electron microscope (TEM, JEM-2100, JEOL). The sheet resistance of 30 mm × 20 mm AgNW electrodes was measured using a four-point probe meter (Loresta GP T610, Mitsubishi Chemical Analytech). The transmittance investigated here was the transmittance of parallel light and does not include the transmittance of diffused light. The parallel transmittance (Tp) of the AgNW electrode for wavelengths in the range 300–900 nm was measured using a UV–visible/near-infrared spectrophotometer (V670, JASCO). The testing window size of the spectrophotometer is 12 mm × 6 mm, and three different places of one sample were tested and the average value was used as the transmittance for each sample. Thermogravimetry analysis (TGA) was carried out on a thermal analyzer (TG–DTA 2000SA, Netzsch Japan). The electrode haze was measured using a D65 illumination haze meter with a strong visible light source (HZ-V3, Suga Test Instruments). For the capacitive sensor, changes in capacitance were measured using a digital multimeter (34410A, Agilent Technologies).
3 Results and Discussion
The sheet resistances of electrodes employing these four kinds of AgNWs were also compared with the ones employing E4-AgNWs (Fig. 4b). A similar pattern emerged that high-transmittance electrodes were more sensitive to changes in the thickness of the PVP nanolayer: for transmittance below 95 % at the wavelength of 550 nm, the sheet resistance was similar to that of ethanol-treated samples, while for electrodes with higher transmittance, a clear reduction in the sheet resistance was obtained. For example, the sheet resistance of electrodes using W90-AgNWs dramatically dropped from 2.1 × 106 to 204 Ω sq−1 at the transmittance of 97.5 %. To the best of our knowledge, this result is the lowest value ever reported without any post-treatment and far exceeds that of ITO.
It should be noticed that the agglomeration of nanowires gradually emerged and finally precipitated when washing time in DI water or DMF was prolonged, corresponding to the excessive removal of PVP. The agglomeration degraded the dispersion of nanowires and thus the electrode performance at high transmittance. Therefore, it is important to maintain well-distributed nanowire percolation networks while reducing the contact resistance of individual wire–wire junction. In other words, the washing parameters should be carefully controlled to obtain PVP layer with a thickness optimal for peak electrical performance. The washing parameters included washing temperature, washing times, stirring speed, and the solvent type. Simple, time-saving or cost-effective washing pre-treatment could be easily achieved with different washing parameters to improve the electrical performance for various applications of AgNW transparent electrodes.
The conductivity of AgNW transparent electrode has been dramatically improved at high optical transmittance by a simple and rapid washing method without any post-treatment. In this washing pre-treatment process, increasing the number of washing cycles leads to the gradual reduction in the thickness of PVP layer and the corresponding decrease in the sheet resistance especially at high transmittance. Washing temperature and solvent type are also important factors in the pre-treatment process. Therefore, AgNW electrodes with sheet resistances of 15.6 and 204 Ω sq−1 at transmittances of 90 and 97.5 %, respectively, were produced without any post-treatment at room temperature. A capacitive pressure sensor based on the pre-treated AgNWs that performs with high sensitivity, reproducibility, and transparency, is demonstrated. The AgNW ink after washing pre-treatment also avoids the substrate limitation induced by the usual post-treatment and greatly expands the application of AgNW electrode on various substrates.
This work was partly supported by Showa Denko Co. Ltd, Grant-in-Aid for Scientific Research (Kaken S, 24226017) and COI Stream Project. J. Wang acknowledges the financial support from China Scholarship Council for his PhD research in Osaka University.
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