Long-term electrically stable silver nanowire composite transparent electrode under high current density

Silver nanowire (AgNW) network has been employed into many electronic devices as transparent electrode. However, the poor electrical stability under current shock has been seriously holding its practical application, and we still lack a long-term electrically stable AgNW system to study the underlying fundamentals of electrical failure. In this work, the electrical stability performance and failure mechanism of chitosan–ascorbic acid (Chi-AsA)/AgNW composite under current stress were thoroughly studied. The composite electrode maintained stable above 24,000 h under high current density of 100 mA cm−1. The main failure in AgNW composite is found to be a wave break perpendicular to the current instead of traditional uniform degradation across the entire AgNW networks. More interestingly, the AgNWs in failed composite electrode maintained their original smooth morphology excepting the crack region, while the AgNWs in pristine networks degraded to nanoparticles or became disconnected everywhere. The patterned AgNW composite in microscale exhibits long lifetime in resisting current stress as well. The effect of over-coating location, electrical stress, temperature, and over-coating materials on the electrical stability were studied. The over-coating layer of Chi-AsA is proven to suppress the silver atoms from electromigration, reduce the concentrated Joule heating at junctions, and inhibit the corrosion. The Chi-AsA/AgNW composite enables electrically stable transparent conductor for long-serving optoelectronics, and the mechanism investigation deepens the comprehension of preparing electrically stable AgNW system.

ABSTRACT Silver nanowire (AgNW) network has been employed into many electronic devices as transparent electrode. However, the poor electrical stability under current shock has been seriously holding its practical application, and we still lack a long-term electrically stable AgNW system to study the underlying fundamentals of electrical failure. In this work, the electrical stability performance and failure mechanism of chitosan-ascorbic acid (Chi-AsA)/AgNW composite under current stress were thoroughly studied. The composite electrode maintained stable above 24,000 h under high current density of 100 mA cm -1 . The main failure in AgNW composite is found to be a wave break perpendicular to the current instead of traditional uniform degradation across the entire AgNW networks. More interestingly, the AgNWs in failed composite electrode maintained their original smooth morphology excepting the crack region, while the AgNWs in pristine networks degraded to nanoparticles or became disconnected everywhere. The patterned AgNW composite in microscale exhibits long lifetime in resisting current stress as well. The effect of over-coating location, electrical stress, temperature, and over-coating materials on the electrical stability were studied. The over-coating layer of Chi-AsA is proven to suppress the silver atoms from electromigration, reduce the concentrated Joule heating at junctions, and inhibit the corrosion. The Chi-AsA/AgNW composite enables electrically stable transparent conductor for long-serving optoelectronics, and the mechanism investigation deepens the comprehension of preparing electrically stable AgNW system.

Introduction
Transparent electrode constitutes crucial component of various optoelectronic devices. The main transparent conductive materials have been metal oxides for decades, but alternative flexible film electrodes have attracted intense attention because of emerging flexible optoelectronics. Particularly, silver nanowire (AgNW) networks can be fabricated with easy solution process, and exhibit comparable or even better opto-electronic performance compared with traditional indium tin oxide (ITO), showing advantages to other candidates [1]. It has been efficiently integrated into various devices, such as solar cells [2], organic light-emitting diodes (OLEDs) [3], film heaters [4], touch screens [5], sensors [6], and energy electronics [7,8].
Despite the attractive application in lab scale, the AgNW transparent electrode has not been widely considered in industry because of the instability, especially the poor electrical stability. For example, the lifetime of AgNW film with sheet resistance of 12 X sq -1 was only 18 days at 17 mA cm -2 [9], and sharply reduced to several hours under higher current density of 125 mA cm -2 [10]. However, the optoelectronics require service life of at least years under working state, which means the current is inevitable to flow through the transparent electrode. For example, the film heaters request current density of 10-30 mA cm -2 to achieve saturation temperature above 100°C, and * 100 mA cm -2 to higher than 200°C [4,[11][12][13]. The short-circuit current density in OPV can reach 20 mA cm -2 [14][15][16][17][18] or above; and 10-50 mA cm -2 is required for OLEDs to obtain the maximum efficiency, which exceeds 100 mA cm -2 at the upper test bias voltage [10,19,20]. It is obviously that the poor electrical stability of AgNWs seriously cut down the lifetime of optoelectronic devices.
Unlike the continuous flat film, e.g., the traditional metal oxide, AgNW transparent electrode is composed of randomly interconnected silver nanowires with high aspect ratio of * 1000, which brings additionally new challenging problems. First, every single silver nanowire bears much higher current density compared with that loaded to the whole film. For example, a current density of 150 mA cm -2 on the AgNW network film turns out to an extremely high current density of 5 9 10 6 A cm -2 on individual nanowire [10]. Second, the Joule heating caused by the current in cross-linked junctions between nanowires is quite strong due to the high contact resistance, which tends to melt the nanowires to droplets in junctions followed with capillary or Rayleigh instability [9,10,21,22]. Third, the deposition of AgNW networks in nanoscale is not homogeneous, and may cause the nonuniform current flow, resulting in local hotspots and local failures [22,23]. Forth, the surface of pristine AgNW is prone to be corroded with sulfides, oxygen, moisture, or other chemicals in ambient environment [24][25][26]. Last, the synergistic effect of electrical stress, Joule heating, and corrosion may speed up the failure of AgNW networks. Therefore, it has been a great challenge to improve electrical stability of AgNW networks.
Silver nanowire composites with passivation layer on the top or with mixed inert materials have been explored to slow down the nanowire degradation under current flow. Generally, there are two main strategies. One is the introduction of solution processed carbon nanomaterials or conductive polymer. For example, graphene oxide (GO), reduced graphene oxide (rGO) obtained from chemical reaction, carbon nanotube (CNT), single wall carbon nanotube (SWCNT), or neutral poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), was deposited on the top of AgNW networks [27][28][29][30][31][32][33]; SWCNT was mixed with AgNW solution to prepare composite electrodes [34,35]; GO was inserted into the multilayer AgNW networks [36]. The composites demonstrated better stability as compared to pristine AgNW networks under current stress. The best result comes from the rGO/AgNW composite film with a lifetime of 12 days at 20 mA cm -1 [29]. However, the GO or CNT formed a network with gap rather than a dense passivation layer on the top, and impurities and inorganic ions in GO and CNT raw materials are inevitable and accelerate the failure somehow. Another strategy is the physical deposition of a fully covered layer of dense inorganic oxides on the top of AgNWs. For example, zinc oxide (ZnO) or titanium dioxide (TiO 2 ) was deposited on individual nanowires through atomic layer deposition process [10,[37][38][39][40]. A 5 nm-thick TiO 2 layer helped increase the sustained voltage up to 13 V from 8 V [40], and 4.5 nm of ZnO was reported to improve the lifetime of AgNW network from 9.5 to 12 h at 125 mA cm -2 [10]. The imported metal oxide is purer and denser than the previous carbon solution, but the similar deposition process and brittle nature with traditional ITO transparent electrode make it less advantage to low cost and flexible optoelectronics. Irrespective of the strategy, the best reported lifetime of AgNW composite only extends to hours at high current density and less than half a month at low current flow (Table 1). It is still far away from the requirement of long-term operational devices. Furthermore, we still hardly know the failure mechanism of AgNW composite, and lack of good sample with long-term electrical stability to enable the failure mechanism study. Here we identified a highly electrical stable AgNW composite with biopolymer whose conductivity remained good after more than 24,000 h under high current density of 100 mA cm -1 . We thoroughly investigated the failure mechanism of the AgNW composite transparent electrode under different conditions via various characterizations. Wave break perpendicular to current flow direction is found to be the main failure in AgNW composite electrode instead of local partial failure or uniform degradation across the film. The patterned AgNW composite in microscale exhibits similar long lifetime in resisting current stress as the bulk composite film. The effect of electrical stress, environment surrounding the electrode, chemical corrosion, and over-coating were studied.

Materials
AgNW in isopropanol (IPA) dispersion was purchased from Zhejiang Kechuang Advanced Materials Company. It is * 30 nm in diameter and * 15 lm in length. Chitosan was obtained from Energy Chemical with deacetylation of 94.6% and 100-200 mPas of molecular weight. PEDOT:PSS was provided by Cool Chemistry Technology (Beijing) Co., Ltd. Polydimethylsiloxane (PDMS, Sylgard 184) was purchased from Dow Corning.

Fabrication of Chi-AsA/AgNW composite film
The scheme of the fabrication process is presented in Fig. S1. Chi-AsA solution with chitosan concentration of 1 wt% was prepared as reported previously [14]. Diluted the AgNW dispersion with IPA to 0.2 wt% and shook it to disperse AgNWs uniformly. Firstly coated the AgNW solution on clean glass or PI substrates by rod coating method. AgNW films with different deposition densities were prepared by controlling the number of coating times. Then the prepared Chi-AsA solution was spin-coated on the AgNW film at 3000 rpm for 60 s, followed with drying at 110°C for 10 min. The thickness of the Chi-AsA layer on the network void was measured to be about 55 nm as reported previously [14], and the Chi-AsA on the top of AgNWs is expected to be much thinner than that. The Chi-AsA/AgNW composite film or pristine AgNW film was swabbed into rectangle strip of 0.8 cm in width by wet cotton swab. Finally, silver paste was cured at the two ends of the strip with 0.8 cm in separation distance. The final effective area was 0.8 cm 9 0.8 cm.

Characterization of the samples
Scanning electron microscopy (SEM) images were obtained using Zeiss ultra 55 field-emission scanning electron microscope with SE2 detector. Transmittance was measured over the wavelength range of 300-900 nm on a T6 New Century UV spectrophotometer, and the data at 550 nm are used to describe the transmittance of the film. X-ray photoelectron spectroscopy (XPS) was obtained with an Axis Ultra DLD electron spectrometer from Kratos using Al target. The thermal images were recorded by FLIR T640 IR thermometer.

Stability test
As shown in Fig. 1, the samples were connected in series in the circuit. When a sample failed, it was removed and the circuit was reconnected. The constant current was applied using a Tektronix PWS4602 current generator, and the data of voltage, current, and temperature were collected and recorded by an Agilent 34970A acquisition unit. The resistance was calculated from the ratio of voltage to current. Resistances of the failed samples were measured with a VICTOR VC9808 digital multimeter, which showed that all the resistances were beyond 2000 X. It is noted that the current density of 100 mA cm -1 in this work is defined by dividing the current value by sample width. If change the definition to dividing by area, the current density of 100 mA cm -1 will shift to 125 mA cm -2 , and 200 mA cm -1 to 250 mA cm -2 accordingly. Both methods have been used in the literature to define current density [9,10,29].

Results and discussion
As shown in Fig. 1a, we studied the electrical stability of pristine AgNW network film and chitosan-ascorbic acid (Chi-AsA)/AgNW composite film. The IR thermal images show the broken pristine AgNW film while the composite film is still in good state. To test multiple samples simultaneously, we built up an electrical measurement system as shown in Fig. 1b. The samples were connected in series through a screw type wire connector with current resource, and in parallel with voltage measuring system. In this way, we can keep the same current flow into every sample, and measure the individual voltage from every sample. We fixed the initial resistance of the films around 13 X. Constant current mode was applied to maintain the current density at 100 mA cm -1 and 200 mA cm -1 , respectively. The lifetime is defined until the moment when the sample lost conductivity under current stress [9,29]. As shown in Fig. 1c, the pristine AgNW film demonstrated almost linearly increase in resistance from 13 to 32 X under 100 mA cm -1 and then spiked up at 729 h, suggesting the disconnection among AgNW networks at this point. In contrast, Chi-AsA/AgNW composite film maintained stable conductivity for more than 24,000 h, and its resistance only slightly changed from 13 to 18 X. The electrical stability is significantly improved after introduction of Chi-AsA to AgNW networks. We further increased the current density to 200 mA cm -1 . The composite film showed resistance change below 30% during a long duration of 846 h, and maintained in good condition until 889 h, while the pristine one quickly exceeded 30% in resistance only after 34.5 h and completely lost its conductivity in 82 h (Fig. 1d). Although lifetime drops predictably with the current density, the lifetime of composite at 200 mA cm -1 is still longer than that of the pristine film at 100 mA cm -1 . Since patterning technology is essential for electronic devices, we further tested the patterned films in a width of 350 lm at the current density of 200 mA cm -1 . It is found the pattern has similar long lifetime in resisting current stress as the bulk composite film as shown in Fig. 1e. As illustrated in Fig. 1f and Table 1, the previously reported composite AgNW films mostly operated less than hundreds of hours at the current density below 100 mA cm -2 . Our composite worked normally for more than 24,000 h under 100 mA cm -1 (i.e. 125 mA cm -2 ), exhibiting much better stability under current loading. As most devices operate below 100 mA cm -1 , the composite film with improved electrical stability to * 3 years at 100 mA cm -1 could enable long-term stable devices to meet the commercial longevity requirement. The mechanism behind the improvement of electrical stability is highly interesting. First, we used dual-probe ohmmeter to check the conductivity across the failed film after longevity current loading test. As shown in Fig. 2a, one probe of the ohmmeter was secured at silver paste connection electrode coated on one side of the composite film, and another probe was moved freely to contact with any points. Afterwards, change the fixed probe to another silver paste contact electrode and repeat the check. We found that actually most regions across the failed film still had good conductivities, and the resistances only slightly shifted to * 20-40 X. However, there was always one site with resistance larger than 2000 X when the probe was moved up and down, perpendicular to the current flow direction. It suggests a disconnection line perpendicular to the current flow. We then used IR thermal camera to track the failure process, and found most samples kept uniform thermal distribution in the early stage, and then local hot spots showed up and grew to a wave line in the latter stage (Fig. 2b). It demonstrates that the silver nanowire networks undergo a slight degradation until the first obvious defeat boosting the full degradation. Once the weak regions in AgNWs networks degraded, it will accelerate degradation to extend the damaged area appearing as hot spots. The current in the broken area redistributes to the surroundings vertical to current flow. The increased current density and corresponding higher temperature accelerate the degradation of surrounding area, and in turn, affect further region. Thus, the crack extends vertically to the current flow in this cascade process. When the remaining AgNWs along the wave break are insufficient to withstand the crowded current, rapid electrical failure occurs in the AgNW film, consistent with the phenomenon in Fig. 1c, d. The optical and SEM images further confirmed the failure mode. As shown in Fig. 2c, f, and i, an obvious wave break formed in Chi-AsA/AgNW composite network and pristine AgNW network. In the enlarged SEM images, the composite electrode shows smooth silver nanowire without obvious damage in the region aside the wave break (Fig. 2d), and serious silver migration inside the break (Fig. 2e). It suggests that the degradation is restricted inside the wave break area. Such distinct degradation at different locations has been seldom reported in long-term electrical failure of AgNW composite electrode. While as shown in the enlarged SEM images of pristine AgNWs, serious degradation happened inside the waved crack and across the whole film. Disconnections in the cross-linked junctions and hillocks on surface of nanowires were observed both at 200 and 100 mA cm -1 aging (Fig. 2g-k). We alsofound that inside the waved crack of AgNWs composite, the silver migration was constrained along the original nanowire deposition path, and limit nanoparticles formed among network voids. While the pristine AgNWs showed free movement of silver migration to all the voids between nanowires. The results suggest that the over-coating layer is able to efficiently prevent silver migration under current. Besides, several local failures which are similar with the reported hotspots were also found in the AgNW composites (Fig. S2), especially when there is an external contaminant nanoparticle on the surface.
The position of wave break in pristine samples depends on the competition of Joule heating and electromigration as we reported before [44]. The wave break locates in the middle region of the film when rapid thermal fusing dominates the short-term failure. It moves towards anode when electromigration and electrochemical effects play a role in the long-term failure [44]. We sum up the locations of the wave break of failed samples with initial resistance of 10-15 X at 200 mA cm -1 in Fig. S3. For pristine samples, the wave break mainly occurs near the anode. In contrast, the composite samples show variable failure locations because of the effective inhibition of electromigration by Chi-AsA coating. Owing to the suppression of electromigration near the electrode in composite samples, the influence from accumulated high temperature in the center area, local inhomogeneity of the network, and impurities in the film, might be prominent, resulting in various break locations.
We studied the function of Chi-AsA coating via changing the covering position as shown in Fig. 3a. b Fig. 1  First, the Chi-AsA was only coated on the central area of AgNW networks and exposed the area near connection electrodes (Sample I). The lifetime at the current density of 200 mA cm -1 was drastically reduced from 889 to 116 h when compared with fully covered composite film (Fig. 3b). The wave break appeared at the exposed AgNWs area where was next to the covered area in the side towards positive connection electrode. It showed obvious migration of silver to form spheres (Fig. 3c). Second, the Chi-AsA coating was extended to the whole surface of AgNW networks, but the part beneath connection electrodes was still the bare AgNWs (Sample II). The sample exhibited improved lifetime to 561 h, and the wave break located at the vicinity of the positive connection electrode (Fig. 3e). It is similar with Sample I, in which the wave break both happened near the boundary of Chi-AsA coating. Third, only around 30% of the AgNW network in the central area was exposed, and all the other areas including the part beneath connection electrodes was coated with Chi-AsA (Sample III). It showed a lifetime of 382 h, which is much higher than the first sample, but lower than the second one. The wave break line appeared at the border between the covered and the exposed networks with serious silver migration (Fig. 3g). Pristine AgNWs inside exposed area also exhibited obvious degradation with disconnected nanoparticles on the surface of AgNWs and gaps among silver nanowire networks, and some big clusters of silver agglomeration at the nanowire junctions (Fig. 3h). For all the three types of samples, the area covered with Chi-AsA remained intact silver nanowire network, good contact junctions, and smooth surface after the electrical failure of the samples (Fig. 3d, f  and i). It demonstrates the effectiveness of Chi-AsA protection on electrical stability. The main failure always occurred at the boundary region of Chi-AsA coating no matter where it is. When the location of Chi-AsA covering layer moves, the wave break line follows instead of only appearing near the connection electrode. Inside the failure area of wave break, rut voids along the deposition path of silver nanowire left after silver migration, which is similar with the morphology of fully covered composite samples. Since the Chi-AsA polymer is coated on the surface and fills the voids of AgNW network, silver atoms are strongly suppressed from moving along the AgNWs or migrating to the surrounding area under current stress. Thus, the electrical and thermal stability of Chi-AsA/AgNW film is significantly enhanced. In addition to electromigration, the degradation caused by concentrated Joule heating at the junctions also contributes to the electrical failure [22,36]. Since Chi-AsA layer helps form a tighter contact and smaller contact resistance between AgNWs [14], the risk of contact junctions being molten with Joule heating is reduced compared with pristine AgNW networks. Besides, it is learned from sample II that even all AgNWs are covered with Chi-AsA, the lifetime under current stress is still lower than that of the fully covered one in which the part beneath connection electrode is also protected with Chi-AsA. It indicates that the protection of AgNWs beneath conductive connection points is also important. It requires the coating material should be thin enough to transfer electron between AgNW networks and connection electrodes, but thick enough to provide sufficient protection on the AgNWs from current stress. Some traditional insulating polymers such as polydimethylsiloxane or epoxy can protect AgNW networks as overcoatings, but generally make the composite surface insulate from the top connection electrode. In contrast, Chi-AsA layer with tens of nanometers can deposit on AgNWs beneath connection points with unchanged conductivity and good protection.
According to electrical failure theory of bulk metal film, electromigration highly depends on the current density and temperature. The current density on single AgNW is related to both applied current value and network density. We fabricated pristine and composite samples with various AgNWs deposition densities by changing the coating times. The higher deposition density represents lower transmittance. As shown in Fig. S4, the transmittance at 550 nm decreases linearly by 1.1% on average for each b Fig. 2 a Schematic illustration of the fragment formed in Chi-AsA/AgNW film and the conductivity inspection by dual-probe ohmmeter. b Infrared thermal images of the degradation process under 200 mA cm -1 at 45%, 95%, 97% and 100% of the lifetime, respectively. c-k Optical microscope and SEM images of the failed samples: Chi-AsA/AgNW composite film at 200 mA cm -1 (c-e), pristine AgNW film at 200 mA cm -1 (f-h), and pristine AgNW film at 100 mA cm -1 (i-k). SEM images demonstrate the preserved conductive area (d, g, j) and the crack area (e, h, k). The scale bar in c, f, i is 60 lm, and in d, e, g, h, j, k is 300 nm additional coating time. We investigated the electrical stability of the composite films with 2-7 coating times and corresponding transmittance of 96.8-91.6%. As shown in Fig. 4a, b, the lifetime of the composite samples increases obviously with AgNWs deposition density. For example, composite film with 96.1% of transmittance only subsisted for 223 h at current density of 200 mA cm -1 , which expanded to 1582 h for the one with 92.5% of transmittance. Pristine samples also exhibit the similar relationship between lifetime and transmittance, but the lifetime is below 10% of the composite sample with identical deposition density. Higher coating deposition density means lower current density on every single AgNW, resulting in slower electromigration, less Joule heating, and hence better electrical stability. In addition, denser AgNW networks benefit in more uniform distribution of current, and thus reduce the occurrence of hotspots [21,23,29,45]. The current density along single AgNW is calculated via dividing the current by the quantity of silver nanowires at the cross-sectional area perpendicular to the current flow (Fig. S5). The calculated current density on single AgNW is inversely proportional to the coating times (Fig. S6). As shown in Fig. 4c, it demonstrates a power function relationship with a power index of -3.7 between lifetime and current density on single AgNW. It illustrates that the lifetime could exceed 1000 h if the current density of single AgNW is less than 8.4 9 10 5 A cm -2 , which is approximately corresponding to the films with 94% of transmittance at 200 mA cm -1 , or 97% of transmittance at 100 mA cm -1 . Therefore, as long as the transmittance is among the application requirement, increasing the AgNWs deposition density would significantly improve the lifetime under current stress. We then studied the performance of composite and pristine AgNWs under elevated current densities, and compared their tolerance threshold to current shock. The AgNW networks were fabricated on polyimide (PI) instead of glass in this experiment because it can withstand higher current density before the substrate break-down by the elevated temperature induced thermal stress. Meanwhile, the silver nanowire network on PI substrate exhibits higher temperature induced by current stress because of the slower heat dissipation of PI substrate compared to glass. It suggests the electrical stability of silver nanowire on PI is more challenging than that on glass. The pristine and composite films with initial resistance of 14 X were tested under current density starting from 50 mA cm -1 with increased step size of 25 mA cm -1 per 10 min. As shown in Fig. 4d, the resistance of AgNWs network increased with current density due to Joule heating. Before 175 mA cm -1 , the composite and pristine films exhibited similar resistance change with current density. Afterwards the composite film showed slower resistance change, suggesting better resistance to high current shock. Finally the pristine film failed at the current density of 250 mA cm -1 when the substrate PI was still kept intact. In contrast, the Chi-AsA/AgNW composite film performed well until the test was stopped by the rupture of substrate because of high current density at 350 mA cm -1 . It illustrates that the Chi-AsA/AgNW composite film itself could withstand even higher current shock if providing a stronger substrate. The longterm current aging and short-term current shocking both reveal that the composite AgNW film is more stable at high current density.
The influence from temperature on the electrical stability of composite films was further investigated shown in Fig. 5. First we studied whether Chi-AsA coating would block the dissipation of current induced thermal heating of AgNW network. Fig. 5a showed temperature increased as current applied to film due to Joule heating, and reached a steady stage in several minutes. We define the temperature increase as the difference between the room temperature (T 0 ) and the steady temperature. As shown in Fig. 5b, the temperature increase changes linearly with the resistance. Both the composite and pristine samples meet the same linear fit at 100 mA cm -1 and 200 mA cm -1 , respectively. It suggests that the Chi-AsA coating has no adverse effect on the heat dissipation of AgNW network. Tens of nanometers thickness of the coating layer is negligible relative to b Fig. 3 a Schematic of AgNW networks with partial Chi-AsA coating area and b the corresponding resistance vs time curves. c-i SEM images corresponding to the sites in a: the pristine region near positive connection electrode (c) and the Chi-AsA covered region in the middle (d) of Sample I; the boundary region between Chi-AsA coating and positive connection electrode (e) and the Chi-AsA covered region in the middle (f) of Sample II; the boundary region between Chi-AsA coating and middle pristine network (g), the middle pristine region (h) and the Chi-AsA covered region (i) of Sample III. The bar is 400 nm the substrate and shows minimal obstacle to surface heat dissipation. Besides, Chi-AsA enables stronger adhesion between AgNWs and substrate [14], which could enhance the heat dissipation through substrate. Both internal Joule heating and external thermal environment contribute to the temperature of AgNW films, and thus influence their electrical stability. Chi-AsA/AgNW composite samples of 13 X were applied with current from a DC power and thermal heating from a hot plate simultaneously. The sample temperature was controlled by the current density and/or hot plate heating. The composite film at 200 mA cm -1 without external heating cause a temperature increase of * 50°C due to internal Joule heating, showing lifetime up to 889 h. With external heating from hotplate to maintain at 60°C, the lifetime of composite film was shortened to 450 h, and further to 136 h at 200 mA cm -1 /85°C (Fig. 5c). Most area of the network at 200 mA cm -1 /85°C showed degradation with breakage and aggregations ( Fig. 5c-I), and that at 200 mA cm -1 /60°C showed some fractures (Fig. 5c-II). It is expected that high temperature not only enhances thermal melting, but also provides energy for silver atoms to migrate and accelerates the corrosion, causing poor electrical stability of the film. Interestingly, although temperature shows significant effect on electrical stability, the lifetime of the sample at same temperature is heavily dependent on current stress. We use hotplate heating to keep two samples at different current density at the same measured temperature. As shown in Fig. 5c, the composite film at 100 mA cm -1 /85°C showed a lifetime of 866 h with slight degradation in most area (Fig. 5c-III). It is about 6.4 times that at 200 mA cm -1 /85°C. Meanwhile, the composite film at 85°C purely from hotplate heating without any current stress only showed 5 X increase in resistance after more than 2000 h of aging without obvious fragmentation (Fig. 5c-IV). It means that even at the same macroscopic temperature the electrical stability still depends on the applied current density. The effect of current stress is not only generating Joule heating, but also causing electromigration due to momentum exchange and electrochemical action. In addition, the AgNW network only accounts for a small deposition areal percentage of the entire film surface area. Thus, the temperature of individual AgNW, particularly at the contact junctions, is expected to be higher than the apparent macroscopic temperature measured from the substrate. Even at the same macroscopic temperature controlled by current stress and external heat simultaneously, higher current density results in more Joule heating on individual AgNW and thus reduced its lifetime.
The importance of the current density and temperature to the electrical stability is further confirmed by the comparison of different samples at same temperature. The composite films of 28.8 X/ 100 mA cm -1 and 8.5 X/200 mA cm -1 both reached a apparent temperature of 43°C. However, the 8.5 X/200 mA cm -1 film showed a lifetime much longer than that of the 28.8 X/100 mA cm -1 film as shown in Fig. 5d. To achieve the same macroscopic temperature, individual AgNW in sparser network (28.8 X) should produce more Joule heat, which means higher current density in single AgNW and thus shorter lifetime. Therefore, the current density and temperature inside single AgNW are more important parameters in evaluating the long-term electrical stability than the apparent macroscopic temperature measured from the film.
In addition to electromigration and Joule heating, the effect of corrosion on the electrical stability cannot be ignored as well. Ambient substances such as sulfides, oxygen and moisture have been reported to cause corrosion of AgNWs, and sulfidation has been proven to be the dominant effect [24][25][26]. The corrosion is expected to function synergistically with electromigration and Joule heating, and even probably brings about complex electrochemical effect. Therefore, it will accelerate the electrical failure. XPS was taken from the failed pristine and composite films in Fig. 1c, d. As shown in Fig. 6, the ratio of silver to sulfur content was 82: 18 in the pristine AgNW film that sustained for 729 h under 100 mA cm -1 . The ratio drops to 96: 4 for the pristine AgNW film under 200 mA cm -1 for 82 h. It suggests that corrosion by sulfur took place under current stress and developed over time. The corrosion of silver decreases the nanowire diameter and thus degrades the network conductivity. It causes elevated temperature and current density in AgNWs, which plays a role in long-term failure process under current. Coating is an effective way to inhibit the corrosion. Remarkably, even though the Chi-AsA/ AgNW composite film underwent longer time and higher temperature test at 200 mA cm -1 than the pristine film at 100 mA cm -1 , XPS showed no obvious signal of sulfur, demonstrating good resistance to corrosion by ambient chemicals.
AgNW networks are usually integrated with other materials in optoelectronic devices. Some materials are harmful to the stability of AgNW network. AgNW film is often compounded with PEDOT:PSS when used in OLEDs and solar cells as electrode. We imported PEDOT:PSS onto the top of AgNW film and Chi-AsA/AgNW film, both with initial resistance of 10 X, and then tested the electrical stability at 200 mA cm -1 . As shown in Fig. 7, the lifetime of PEDOT:PSS/AgNW film decreased to merely 109 h as compared to 156 h of pristine AgNW film. The result is consistent with the previously reported work that the acidic property and water-absorbing tendency of commercial PEDOT:PSS lead to declining stability of AgNW network [41]. In contrast, the PEDOT:PSS/Chi-AsA/AgNW film subsisted for 751 h. The results illustrate that the antipermeation property of Chi-AsA coating not only benefits in resisting ambient corrosion, but also contributes to suppressing the destructive effect of composite materials on AgNW network. AgNW network is often packaged for protection such as in film heaters. Here we used PDMS as a protective cover on the top of the sample. The PDMS/AgNW sample exhibited improved lifetime of 792 h at 200 mA cm -1 . Meanwhile, the lifetime was significantly elevated to 4193 h for the PDMS/Chi-AsA/AgNW composite film. It demonstrates that no matter what kind of materials are composited with, the Chi-AsA/AgNW composite films always show superior advantages in electrical stability in contrast to pristine AgNW films.

Conclusion
The Chi-AsA/AgNW composite transparent conductive film exhibited excellent long-term electrical stability under current stress. A 13 X composite film operated functionally at the current density of 100 mA cm -1 for more than 24,000 h without significant damage, while the pristine AgNW film failed within 729 h. A failure analysis reveals a wave break perpendicular to the current flow direction in the composite film instead of uniform degradation across the entire networks. Chi-AsA is able to prevent Ag atoms from electromigration, reduce the Joule heating at the contact junctions, and inhibit the corrosion of AgNWs, resulting in robust AgNW networks resisting current shock. The electrical stability is highly related to the temperature and current density along individual silver nanowire, and the lifetime increases remarkably with lower environment temperature and higher film conductivity. Even in harsher environment or with sparser AgNW densities, the Chi-AsA/AgNW composite film exhibites advantages in electrical stability compared to the pristine AgNW network, which demonstrates a good potential in practical application.