High-performance composite transparent electrode composed of ultra-long silver nanowires and antimony-doped tin oxide nanoparticles

Composite transparent conductive electrodes (TCEs) consisting of silver nanowires (AgNWs) and conductive metal oxides are very promising for flexible optoelectronic devices due to their smooth surface morphology and high chemical stability. However, it is still challenging to ensure high optoelectronic performance and long-term stability in practical applications. Here, we solved these problems by coating antimony-doped tin oxide (ATO) nanoparticles dispersion on ultra-long AgNWs network using waterborne polyurethane (WPU) binder. Ultra-long nanowires occupy less wire-wire junctions and space than short nanowires, thus increasing the optoelectronic performance and flexibility of the composite TCE. WPU improves the adhesion and stability of ATO nanoparticles to the substrate and AgNWs network. The fabricated composite TCE showed a low sheet resistance of 11.9 Ω sq−1, good optical transmittance of 83% at 550 nm and a figure of merit (FOM) of 162 compared to PET/ITO electrode. It also showed excellent mechanical flexibility, adhesion to the substrate and solvent stability. Furthermore, the long-term conductivity was maintained under ambient conditions for 60 days.


Introduction
Transparent conductive electrodes (TCEs) are widely used as essential components in many optoelectronic devices such as solar cells, touch screens, light-emitting diodes, and film heaters [1,2].Currently, indium tin oxide (ITO) is the most commonly used transparent electrode material due to its low sheet resistance and high optical transmittance [3,4].However, due to the high manufacturing cost and brittleness of ITO, it is not a suitable electrode material for a flexible, low cost, large-scale fabrication [5].Hence, nanomaterials such as conducting polymers (poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate)) (PEDOT:PSS) [6], carbonbased materials (carbon nanotubes (CNTs) [7], graphene [8]), and metal nanowires (silver nanowires [9,10] and copper nanowires [11]) have been extensively investigated to replace ITO.Of them, TCE-based AgNWs are considered the most ideal choice for ITO in optoelectronic devices due to its low cost, high conductivity, high optical transparency and roll-to-roll fabrication process [12,13].
However, the practical application of AgNW electrodes to optoelectronic devices is limited by their high contact resistance, rough surface morphology and low chemical stability.In particular, many void regions between AgNWs could reduce the efficiency of electron transport and carrier collection in solar cells [14].Embedding a disordered AgNWs network with conductive metal oxides such as indium tin oxide (ITO) [15], fluorine-doped zinc oxide (FZO) [16], aluminum-doped zinc oxide (AZO) [17], and antimony-doped tin oxide (ATO) [18] is considered one of the key ways to solve the above problems.ATO is an important transparent conductive oxide with high electrical conductivity, optical transparency, stability, infrared light insulation and low cost [19,20].In the previous work, we fabricated AgNW/ATO composite TCEs by low-temperature solution coating of ATO nanoparticles dispersion on AgNWs network using polyvinyl alcohol (PVA) binder and demonstrated its applicability in flexible perovskite solar cells (PSCs) [18].
Fabrication of AgNW/ATO composite TCEs with high optoelectronic performance and long-term stability still remains an attractive target to ensure practical applications in optoelectronic devices, including PSCs.In order to achieve superior optoelectronic performance (optical transmittance and electrical conductivity), AgNWs should be free of defects to minimize the number of wire-wire junctions and possess a low junction resistance [21].Xu et al synthesized ultra-long AgNWs with a high aspect ratio of more than 1000 by a modified polyol method and fabricated transparent electrodes with a sheet resistance of 8.6 Ω sq −1 and a transmittance of 91.3% without any post-treatment [22].Silk fibroin (SF) films were spin-coated with ultra-long AgNWs of 100 ∼ 200 μm in length and sputtered with platinum to reduce the sheet resistance of the composite films to 6.9 Ω sq −1 [23].The network with ultra-long nanowires is rarer than that with short nanowires, and it can, therefore, adapt effectively to mechanical deformations, prevent crack formation and improve flexibility.Therefore, using ultralong AgNWs is an effective way to enhance the optoelectronic performance and flexibility of AgNW composite TCEs.In addition, waterborne polyurethane (WPU) emulsions have been widely applied to form films with excellent adhesion properties, high abrasion resistance, solvent stability and great flexibility with substrates [24,25].Many researchers have used WPU binder to improve the adhesion of conductive ATO nanoparticles to the substrate.For example, Dai et al prepared nanocomposite films with excellent thermal insulation properties by ultrasonically dispersing ATO nanoparticles into WPU emulsions [26].The ATO nanoparticles were mixed in a commercial WPU suspension and cast onto poly(dimethylsiloxane) (PDMS) stamps replicated from lotus leaves, resulting in a superhydrophobic and thermally insulating polymer film [27].
In this work, we fabricated a high-performance composite TCE by coating ATO nanoparticles dispersion on ultra-long AgNWs network using WPU binder.We have studied in detail the surface morphology, optoelectronic properties, flexibility, adhesion properties and solvent and long-term stability of the composite TCE.Ultra-long nanowires occupy less wire-wire junctions and less space than short ones, thus increasing the conductivity and optical transparency of the composite TCE and improving the flexibility.WPU binder improves the adhesion of conductive ATO nanoparticles to the substrate and AgNWs network and also significantly increases the stability.This composite TCE could be an excellent candidate for the development of various low cost, stable and flexible optoelectronic devices.

Synthesis of ultra-long AgNWs solution
Ultra-long AgNWs were synthesized using a typical polyol method [28].
First, 0.42 g of PVP was dissovled in a three-necked flask containing 42 ml of ethylene glycol (EG) solution.750 μl of FeCl 3 and 300 μl of KBr (5 mM for EG, respectively) were then dissolved in it, followed by addition of 0.3 g of AgNO 3 .The flask was stirred with a magnetic stirrer during all the above steps.Being stirred with the magnetic stirrer, the solution was then heated at 75 °C for 30 min under nitrogen atmosphere and reacted at 135 °C for 5 h.The resulting product was cooled to room temperature.It was then washed and centrifuged at 3000 rpm with acetone and ethanol to remove the solvent (ethylene glycol), PVP and and other impurities.The purification process was repeated several times until the supernatant became colorless.Finally, the as-obtained purified AgNWs were redispersed to the desired concentration in ethanol for the further use.

Fabrication of AgNW/ATO composite TCE
Ultra-long AgNWs dispersed in ethanol were evenly dispersed by sonication for 10 min.A 0.2 mm thick polyethylene terephthalate (PET) substrate was sonicated in ethanol for 5 min and then dried.
The fabrication process of AgNW/ATO composite TCE is shown in figure 1.First, ultra-long AgNWs solutions with different concentrations (1 mg ml −1 , 3 mg ml −1 , 5 mg ml −1 , and 10 mg ml −1 ) were dropped onto a 2 × 2 cm 2 PET substrate and spin coated at 3000 rpm for 15 s to form an AgNWs network, respectively.The AgNWs network was hot-pressed at a rate of 5 mm s −1 using 25 MPa roller at 160 °C to improve the mechanical contact of AgNWs.ATO nano-suspension (solid content 30 wt%) was prepared by dispersing ATO powder in water containing PAM dispersant (dispersant/ATO powder = 1 wt%) [29].The as-prepared ATO (4 ml) and WPU (2 ml) suspension diluted to 4 wt% were mixed being magnetically stirred for 10 min.The AgNW/ATO composite TCE was prepared by spin-coating the mixed suspension three times at 2000 rpm on the AgNWs network.Additionally, the composite TCEs were thermally annealed at 120 °C for 10 min.

Characterization
Scanning electron microscopy (SEM, Quanta 2000, FEI, Netherland) was used to characterize the morphology of the films.Atomic force microscopy (AFM, SPA400) was used to investigate the surface roughness of the films.The film thickness was measured using a surface profiler (Veeco Dektak 150).The transmission spectra of the films were measured using a UV-vis spectrophotometer (UV 2600, Shimadzu, Japan).The scanning wavelength ranged from 300 nm to 800 nm.The sheet resistance was measured using a four-probe measurement method.

Morphology and characterization
The SEM image of ultra-long AgNWs synthesized by the typical polyol method is shown in figure 2(a).
With respect to the diameter and length distribution of the nanowires, the synthesized nanowires have a high aspect ratio with an average diameter of 70 nm and an average length of 120 μm.
After coating the substrate with ultra-long AgNWs, there are many void regions and wire-wire junctions between the wires.In order to avoid the weak contact between individual nanowires, the spin-coated AgNWs network was hot-pressed using a roller.The heat-coupled rolling pressure causes the nanowires to weld and press together to flatten the surface, resulting in a good adhesion of the nanowires to the PET substrate surface.Figure 2(b) shows the SEM image of the bare AgNW film after hot-pressing.However, the surface is still rough due to randomly dispersed nanowires.This rough surface is problematic for practical fabrication of thin-film optoelectronic devices using AgNW conductive films [15].Also, when AgNW films are directly exposed to the atmosphere and used as top electrode, they have a negative effect on sheet resistance due to local oxidation or  corrosion [30].This problem could be solved by coating conductive ATO nanoparticles on the surface of the AgNWs network.Conductive ATO nanoparticles might effectively fill the void regions by fully coating the AgNWs network.With an increase in the number of coatings, ATO nanoparticles fully covered the preformed AgNWs network.As a result, the nanowires are not visible in the SEM image (figure 2(c)).The thickness of the coated ATO layer was about 280 nm.
The surface morphology of the films was imaged in detail using AFM.(figure 3(a)) The average surface roughness of the hot-pressed bare AgNW film is 84 nm.However, the average surface roughness of the AgNW/ ATO composite film is 21 nm, which is almost the same order of magnitude as the size of ATO nanoparticles (average particle size 20 nm) and has a relatively smooth surface morphology (figure 3(b)).The surface roughness is expected to be further improved by reducing the diameter of AgNWs and using small-sized ATO nanoparticles.

Optoelectronic performance of composite TCEs
The optical transmittance of AgNW/ATO composite TCEs with respect to concentration of AgNWs solution in the visible region is shown in figure 4(a).The optical transmittance of the PET/ITO electrode with sheet resistance of 9.4 Ω sq −1 is also shown as a reference.
The optical transmittance of the pure PET substrate at 550 nm wavelength is 92%.The optical transmittance of the composite TCE decreases with increasing concentration of AgNWs solution (figure 4(a)).This is because the shielding effect caused by light absorption and reflection of the AgNWs network increases with the number of nanowires.In addition, ATO nanoparticles are slightly darker compared to other conductive metal oxides [31].When the concentration of AgNWs solution is less than 5 mg ml −1 , the composite TCEs at 550 nm show a good optical transmittance (>82%).Even when the concentration of AgNWs solution is increased to 10 mg ml −1 , the composite film still shows a transmittance comparable to PET/ITO electrode (9.4 Ω sq −1 ).
The specular transmittance and diffusive transmittance of AgNW/ATO composite TCEs at 550 nm were also investigated (figure 4(b)).The specular transmittance was measured by detecting only the light from the sample parallel to the incident light, while the diffusive transmittance was measured by examining all the forward parallel light and the scattered light from the sample [10].As the concentration of AgNWs solution increases, the specular transmittance and diffuse transmittance of AgNW/ATO composite TCEs decrease.Optical haze is the difference between specular transmittance and diffusive transmittance, which plays an important role in evaluating the light scattering degree of a transparent film [3].The average Optical haze of the composite TCEs was about 3.5 ∼ 4% and that of the PET/ITO electrode was about 1%.The reason for the high Optical haze of the composite TCEs might be attributed to light scattering by AgNWs.Although this could be problematic for display applications, it is preferable for light absorption and contributes to improved conversion efficiency in optoelectronic devices such as solar cells [10].
The sheet resistance of the TCEs before and after coating of ATO nanoparticles with respect to concentration of AgNWs solution is shown in figure 4(c).After coating ATO nanoparticles, the sheet resistance of the composite TCE is significantly reduced compared to that of the bare AgNW film.For example, when the concentration of AgNWs solution is 1 mg ml −1 , the sheet resistance of the bare AgNW film decreases rapidly from 182.5 Ω sq −1 to 98.3 Ω sq −1 after coating ATO nanoparticles.It might be because the ATO nanoparticles are surrounded by a junction of nanowires and fused very efficiently to form a favorable pathway for the electrical conduction of the AgNWs network.Another possible reason might be that the horizontal electron transport of ATO nanoparticles in the confined space between AgNWs could be fully possible.Given the size of the nanowires and the number density of wires, the sheet resistance of the AgNWs network is determined by the wire-wire junction resistance.The mechanical method by hot-pressing greatly reduced the wire-wire junction resistance, but it is still not enough.The ATO nanoparticles coated on the AgNWs network are relatively small in size and could densely fill the gap of wire-wire interface.On the other hand, the C=O group of WPU and the -OH group on the surface of ATO nanoparticles form hydrogen bonds, making ATO nanoparticles close to each other.In composite films, this polymer binder acts as an insulator at high content, whereas at low content it enhances the electrical conductivity of the membranes by increasing the contact between conductive ATO nanoparticles (figure S1, Supporting Information).The conductivity in the space between the nanowires determines the carrier collection in the horizontal direction in optoelectronic devices, including solar cells.Electron transport by ATO nanoparticles in composite film is sufficient once reaching the nearest neighboring nanowires.Hence, the electron transport pathways of ATO nanoparticles in the horizontal direction are limited to the average spacing width (several μm to dozens of μm) between nanowires and ATO nanoparticle film could fully satisfy the electron transport performance in optoelectronic applications even at a sheet resistance of dozens of kΩ/sq.Moreover, as the concentration of AgNWs solution increases, the sheet resistance of bare AgNW film decreases with an increase in amount of the conductor, while the sheet resistance of composite TCEs decreases.When the concentration of AgNWs solution is 1 mg ml −1 , the sheet resistance of the composite TCE is reduced from 98.3 Ω sq −1 to 9.2 Ω sq −1 at 10 mg ml −1 , indicating that the conductivity of the composite TCE strongly depends on the number of AgNWs.
The optoelectronic properties of TCEs could be evaluated by the figure of merit (FOM).The FOM depends on the transmittance (T) and the sheet resistance (R S ) [32]: where Z 0 is the impedance of free space (377 Ω), R S is the sheet resistance, and T is the transmittance at 550 nm.
Table 1 shows the figure of merit (FOM) of PET/ITO electrode and different AgNW/ATO composite TCEs.As the concentration of AgNWs solution increases, the FOM value of the composite TCE also increases.The FOM value is 162 at 5 mg ml −1 and 132 at 10 mg ml −1 , respectively, ensuring optoelectronic properties superior to those of PET/ITO electrode (130).In addition, the FOM value was much higher than that (84) of the AgNW/ ATO composite TCE (sheet resistance: 18.0 Ω sq −1 , diameter: 50 ∼ 60 nm, length: ∼24 μm) in our previous work.The main reason might be that when ultra-long AgNWs are used for the fabrication of composite TCEs, there could be less wire-wire junctions and less space.It should be noted that optical transparency and conductivity are opposing factors.As the AgNWs concentration increases, the conductivity becomes better, whereas the optical transparency would decrease.Therefore, the balance should be chosen reasonably depending on applications.

Stability of composite TCEs
The TCE should have a high mechanical, solvent and ambient stability to ensure the conductivity stability of the device in practical applications.Figure 5 shows the test results of mechanical flexibility of AgNW/ATO composite TCE (sheet resistance 11.9 Ω sq −1 , optical transmittance 83%), PET/ITO electrode and AgNW/ATO composite TCE reported previously, respectively.During the measurements, the TCEs were characterized by measuring the change in sheet resistance after 100 bending cycles at different bending radii of 15 ∼ 3 mm [18].
The PET/ITO electrode used as a reference shows a rapid decrease in sheet resistance at a radius of less than 12 mm due to brittleness, whereas the AgNW/ATO composite TCEs maintain the initial conductivity at a bending radius of 6 mm.However, at a bending radius of 3 mm, the initial resistance was increased 1.8 times for the AgNW/ATO composite TCE used as reference, whereas 1.3 times for the AgNW/ATO composite TCE prepared in this work.It might come from the outstanding bending behavior of AgNWs in composite TCE [3].Finally, the composite TCE fabricated by ultra-long AgNWs shows excellent bending properties.Furthermore, the tape test was used to investigate the adhesion of AgNWs to PET substrate.The bare AgNWs network was easily peeled off from the substrate during the 3 M tape test, but the AgNW/ATO composite TCE was not embedded into the tape and maintained an excellent adhesion.It might be because of that WPU suspension improves the bonding of ATO nanoparticles and achieves strong contact with PET substrate.
The solvent stability of AgNW/ATO composite TCEs with respect to ATO binders is shown in table 2.  The solvent stability test was carried out by measuring the change in sheet resistance (ΔR/R 0 ) after immersion in toluene, isopropyl alcohol (IPA), DMF and DMSO at 20 °C for 30 min, respectively [18].The experimental results show that the addition of WPU binder has a much smaller change in the sheet resistance of the composite TCE in various solvents than the addition of PVA binder.Furthermore, the change in sheet resistance in DMF and DMSO was less than 3% ensuring the maintenance of the initial conductivity.This may be due to the poor stability of the polar solvent because PVA is a hydrophilic polymer with hydroxyl groups (OH) on carbon atoms.The excellent solvent stability of ATO and WPU composite layers is expected to further improve the stability of the device during the fabrication of optoelectronic devices including PSCs.
Figure 6 shows the results of conductivity stability test of AgNW/ATO composite TCE in comparison with bare AgNW film and previously reported AgNW/ATO composite TCE under ambient conditions for 60 days.
The long-term stability of the TCE in the ambient conditions was tested under sunlight irradiation at relative humidity (RH) of 50 ∼ 90% and temperature of 15 ∼ 35 °C.For a comparison, the sheet resistance changes of bare AgNW film and previously reported AgNW/ATO composite TCE were also shown.The sheet resistance of bare AgNW film was increased to more than 35 times within 30 days due to the oxidation of AgNWs, but the AgNW/ATO composite TCE almost maintained an excellent conductivity.This is because the AgNWs network was well coated by ATO nanoparticles with high stability.The sheet resistance of the composite TCE using PVA binder showed an increase of 21% of the initial value after 60 days, especially that the composite TCE using WPU binder showed no obvious change of sheet resistance.PVA undergoes photodegradation when irradiated by UV light [33].However, the WPU binder did not cause UV photodegradation, effectively preventing the corrosion of AgNWs network and thus almost maintaining the initial performance.The results confirmed that the AgNW/ATO composite TCE with WPU binder has excellent long-term stability and is promising for practical applications in optoelectronic devices.

Conclusion
In this work, we fabricated a high-performance composite TCE by coating ATO nanoparticles dispersion on ultra-long AgNWs network using WPU binder.Ultra-long nanowires occupy less wire-wire junctions and space than short nanowires, thus increasing the optoelectronic performance and flexibility of the composite TCE.WPU improves the adhesion and stability of ATO nanoparticles to the substrate and AgNWs network.The best composite TCE showed a low sheet resistance (11.9 Ω sq −1 ), good optical transmittance (83%) and FOM of 162.It also showed an excellent flexibility, adhesion to the substrate and solvent stability.Furthermore, the long-term conductivity was proved under ambient conditions for 60 days.The AgNW/ATO composite TCE with improved performance and stability would be an excellent candidate for various low cost, stable and flexible optoelectronic devices, including flexible PSCs.

Figure 1 .
Figure 1.Schematic diagram of the fabrication of AgNW/ATO composite TCE.

Figure 2 .
Figure 2. (a) SEM image of the synthesized ultra-long AgNWs, (b) SEM image of bare AgNW film after hot-pressing, (c) SEM image of AgNW/ATO composite film.

Figure 4 .
Figure 4. (a) Optical transmittance of AgNW/ATO composite TCEs with concentration of AgNWs solution in the visible region, (b) Diffusion transmittance and specular transmittance of AgNW/ATO composite TCEs at 550 nm, (c) Sheet resistance of TCEs before and after coating of ATO nanoparticles with concentration of AgNWs solution.

Figure 5 .
Figure 5. Sheet resistance of different TCEs after 100 bending cycles at different bending radii of 15 ∼ 3 mm.

Figure 6 .
Figure 6.Variations in sheet resistance of AgNW/ATO composite TCE in comparison with bare AgNW film and previously reported AgNW/ATO composite TCE under ambient conditions for 60 days.

Table 2 .
Solvent stability of AgNW/ ATO composite TCEs with respect to ATO binders.