Aligned silver nanowire-based transparent electrodes for engineering polarisation-selective optoelectronics

We herein report on a remarkably simple, fast, and economic way of fabricating homogeneous and well oriented silver nanowires (AgNWs) that exhibit strong in-plane electrical and optical anisotropies. Using a small quantity of AgNW suspension, the horizontal-dip (H-dip) coating method was applied, in which highly oriented AgNWs were deposited unidirectionally along the direction of coating over centimetre-scale lengths very rapidly. In applying the H-dip-coating method, we adjusted the shear strain rate of the capillary flow in the Landau-Levich meniscus of the AgNW suspension, which induced a high degree of uniaxial orientational ordering (0.37–0.43) of the AgNWs, comparable with the ordering seen in archetypal nematic liquid crystal (LC) materials. These AgNWs could be used to fabricate not only transparent electrodes, but also LC-alignment electrodes for LC devices and/or polarising electrodes for organic photovoltaic devices, having the potential to revolutionise the architectures of a number of polarisation-selective opto-electronic devices for use in printed/organic electronics.

Scientific RepoRts | 6:19485 | DOI: 10.1038/srep19485 Experimental To fabricate AgNW films on solid substrates we used AgNWs suspended in isopropyl alcohol (IPA) (0.5-2.0 wt%) (NTC-01, Nanopyxis Co., LTD.), with an average diameter and length of about 25 nm and 32 μ m, respectively. The AgNWs were synthesised using a modified polyol process 41 . After 400 mL of propylene glycol (PG) as both solvent and reducing agent was heated to 126 °C, a capping agent of polyvinyl pyrrolidone (12 g) as a dispersant was added and dissolved with vigorous stirring until clear, and then halide compounds of KBr (0.2 g) and AgCl (1.0 g) were simultaneously injected into the hot PG media as catalysts. After 90 min, AgNO 3 (4.6 g) as a metal precursor was dissolved in PG (100 ml) to obtain an AgNO 3 solution, which was then added to the reaction mixture. After about 30 min AgNWs started to form, and the reaction then continued for about 1 hour to allow complete formation of the AgNWs. The reaction mixture was then cooled to room temperature and the AgNWs were purified by precipitate extraction and resuspension in ethanol, the product then being dispersed and suspended in IPA at an appropriate concentration at room temperature. The AgNW suspension can be considered as quasi-stable over 3 hours, and within this time the dispersion and mass fraction of the AgNWs showed no obvious changes.
As shown in Fig. 1a, the AgNW suspension was coated on glass substrates using the H-dip-coating method [36][37][38][39][40] . A standard cleaning procedure was adopted in which the glass substrates were first cleaned ultrasonically in baths of detergent and alcohol for 30 min, then rinsed several times using deionised water, and were then dried using nitrogen followed by ultra-violet (UV) ozone treatment. On the glass substrate, a thin AgNW film was solution-coated using an H-dip coater; the apparatus used for this had a maximum work space of 10 cm × 10 cm. A small volume of AgNW suspension (~5 μ l) per unit of coating area (1 cm × 1 cm) was fed into the gap of the cylindrical H-dip-coating head using a syringe pump (NE-1000, New Era Pump Systems Inc.). The height of the gap h 0 was adjusted vertically using micrometer positioners mounted at the end of the coating head, and the carrying speed U was controlled by a computer-controlled translation stage (SGSP26-200, Sigma Koki Co., Ltd). After a concave meniscus of coating suspension had formed on the substrate, the substrate was transported horizontally such that the meniscus formed by the coating head caused the suspension spread evenly on the transporting substrate while maintaining the shape of the meniscus of the suspension. The AgNWs coated on the glass substrates were then annealed at 120 °C in a dry air environment for 5 s. For comparison, a reference layer of AgNWs was also prepared using spin-coating on the bare glass substrates and then baked at 120 °C for 5 s to extract the residual solvent.
The microscopic morphology of the AgNWs was observed by field emission scanning electron microscopy (SEM, Model JSM-6700F, JEOL Co.). The surface roughness and topographic properties of the AgNW surfaces were characterised using atomic force microscopy (AFM, Nanosurf easyscan2 Nanosurf AG Switzerland Inc.). The macroscopic arrangement of AgNW arrays was observed using optical microscopy (Model BA300Pol., Motic Co.). The optical properties of the AgNWs were investigated using a UV-visible spectroscopy system (8453, Agilent). The sheet resistance of the AgNW film as prepared was also tested using a four-point probe method with a sheet resistivity meter (SRM-232-2000, range 0 to 2000 Ω /square, Guardian Manufacturing).
To fabricate the LC cells, we used a nematic LC mixture (ZLI-2293, Merck), which had a nematic phase below the clearing point of 85 °C. At room temperature the extraordinary (n e ) and ordinary (n o ) refractive indices of ZLI-2293 are 1.6313 and 1.4990 (λ = 589.3 nm), respectively. An empty LC cell was fabricated by sandwiching two glass substrates together; in this case their inner surfaces were coated only with the transparent conductive AgNW films (ca. 40-60 nm thick), i.e., without any further ITO electrode or alignment layer. As a result the LC molecules interacted directly with the AgNWs. The cell gap was maintained using glass spacers 4.9 μ m thick. The H-dip-coating directions (x-directions) of the two AgNW layers on the substrates were set to be parallel or perpendicular to each other. By capillary-filling the empty cells with ZLI-2293 in its isotropic phase (~90 °C), we obtained nematic LC cells after cooling to room temperature using a micro-furnace (FP90 and 82, Mettler Toledo). The LC cell was placed between a polariser at the input end and an analyser at the output end, forming a LC device; illuminating light was normally incident on the LC device. For normally white (NW) mode operation, the polarisation axis of the polariser was set to be parallel to the LC director of the twisted nematic LC (TN-LC) cell at the input end, and the passing axis of the analyser was set perpendicular to the passing axis of the polariser. For normally black (NB) mode operation, the passing axis of the analyser was set parallel to the passing axis of the polariser. The optical characteristics of each component of the LC cells were investigated using polarised microscopy with the UV-visible spectroscopy system.
Organic PV (OPV) cells on glass substrates were fabricated using an inverted structure configuration 37 . An ITO layer (80 nm, 30 Ω /square) on a glass substrate used as the transparent cathode was ultrasonically cleaned using a sequence of detergent, deionised water, acetone, and IPA. The ITO cathode was modified by a ZnO electron-collective layer (ECL, thickness: ~70 nm), prepared by the sol-gel process using a ZnO precursor. The details of the preparation of the ZnO precursor are similar to those described in earlier experiments 37 . Next, a thin cesium carbonate (Cs 2 CO 3 ) electron-selective buffer layer (thickness: ca. 10 nm) was also spin-coated on top of the ZnO ECL using a solution of Cs 2 CO 3 (0.2 wt%) 37 . A blended solution of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT, 0.456 wt%, 1-material Chemscitech, Inc.) and [6,6]-phenyl C 71 butyric acid methyl ester (PCBM 70 , 1.824 wt%, Nanostructured Carbon, Inc.) 42 in a solvent of monochlorobenzene was then spin-coated on the ZnO/Cs 2 CO 3 layers. The PCDTBT:PCBM 70 bulk-heterojunction (BHJ) PV layer was about 85 nm thick. After spin coating, the PV layer was annealed at a temperature of 65 °C for 1 hour. In order to form a hole-collection layer (HCL) on the PV layer, a ca. 20-nm-thick molybdenum oxide (MoO 3 , Aldrich) layer was prepared using thermal deposition at a rate of 0.05 nm/s under a base pressure below 2.7 × 10 −4 Pa. Finally, to fabricate a transparent AgNW anode, the prepared AgNW suspension was deposited onto MoO 3 HCL using the H-dip-coating method (ca. 200 nm thick). The active area of the fabricated device was 3 × 3 mm 2 . For comparative purposes, we also fabricated two reference OPV cells using a vacuum-evaporated Ag anode (thickness: 100 nm) on the MoO 3 HCL (ref. 1) and using a spin-coated AgNW anode (thickness: ca. 200 nm) on the MoO 3 HCL (ref. 2). Apart from the differences in the Ag anodes described, the reference OPV cells were fabricated using exactly the same method as that used for the sample OPV cell with the H-dip-coated AgNW anode.
PV performance was measured using a source meter (2400, Keithley) and calibrated using a reference cell (BS-520, Bunkoh-keiki) under an illumination of 100 mW/cm 2 produced by an AM 1.5G light source (96000 Solar Simulator, Newport). The reported values were averaged from several (at least 10) individual cells. The external quantum efficiency (EQE) spectra were obtained using a measurement system (Oriel ® IQE-200 ™ EQE/ IQE, Newport). Figure 1a shows a meniscus of an AgNW suspension during the H-dip-coating process. With a Landau-Levich meniscus, the film thickness (h) can be described in terms of the capillary number (C a = μU/σ) of the solution, by the associated drag-out problem; h = k· C a 2/3 ·R d , for C a ≪ 1, where μ, σ, U, R d , and k represent the viscosity, surface tension, coating speed, radius of the meniscus, and constant of proportionality, respectively [36][37][38][39][40] . We first investigated the film thickness of the H-dip-coated AgNWs (Fig. 1b). It is clear that for a given h 0 , the thickness of the AgNWs showed a continuous increase with increasing U. These results were in good agreement with the associated drag-out problem. Figure 1c shows typical examples of homogeneous and transparent H-dip-coated AgNWs (thickness ~ 60 nm) formed on a 10 × 10 cm 2 glass substrate. The figure also shows a SEM image of H-dip-coated AgNWs. Interestingly, nearly all the AgNWs are aligned unidirectionally along the direction of coating (x-direction) with few deviations, in contrast with randomly assembled AgNWs formed by spin-coating ( Supplementary Fig. S1a). The surface morphology results obtained by AFM show that the values of the rms surface roughness for 40, 60, and 200 nm-thick H-dip-coated AgNWs were 12.8, 18.4, and 26.0 nm, respectively, which were almost identical at different positions on the investigated layers, while the rms surface roughness observed for the 40-nm-thick spin-coated AgNWs was ~22.1 nm. We then investigated the optical microscopic properties of the AgNWs under a polariser (the inset in the right panel of Fig. 1c): the observed texture is bright-state when the x-direction of the AgNWs is perpendicular to the transmission axis (P) of the polariser, but becomes dark-state when the x-direction is parallel to the P axis, indicating the clear dichroic properties of the AgNWs. These results confirm the two in-plane principal axes of the H-dip-coated AgNWs, where the x-direction is the light-absorbing axis and the y-direction is the polarising axis.

Results and Discussion
In contrast with previous techniques such as the spreading evaporation method 30 , the orientation of the H-dip-coated AgNWs is mainly homogeneous over the coated area. During the H-dip-coating process, the capillary fluid in the Landau-Levich meniscus near the substrate induces a shear stress 43 , particulary where the suspension passes under the coating barrier. The shear strain rate (γ ) can be estimated by 43 γ = U fluid /δ = ~50/s, where U fluid is the capillary flow velocity near the bottom surface of the barrier, which is similar to U (~2 cm/s), and δ is the fluid depth at the edge (~h 0 ). For μ = ~6.6 cp, the corresponding shear stress is τ = γ μ = ~0.33 Pa, which may be sufficient to achieve for the AgNW alignment. The Peclet number (Pe) for the AgNW suspension can also be estimated by 44 Pe = L 3 γ μ/kT ~ 2.1 × 10 6 ≫ 1, where L is the AgNW length (~30 μ m) and kT is the thermal energy, implying that the motion of the AgNWs during H-dip-coating is dominated by hydrodynamic behaviour rather than by diffusion. Subsequently, the ordered AgNWs in the suspension gradually sink on to solid substrate, and finally a unidirectional alignment of AgNWs remains on the substrate after the solvent has evaporated. In addition, large deviations in a few AgNWs with respect to the coating direction can be attributed to insufficient rotation of the AgNWs adjacent to the substrate.
We then investigated the electrical anisotropy of the AgNWs by measuring the sheet resistances R s . Figure 2a shows that R s for the AgNWs decreases with increasing U. For a given U, R s measured along the x-direction is lower than that along the y-direction. While this R s anisotropy is obvious for film thicknesses in the range < ~170 nm (U = 1.5 cm/s), it is only slight for film thicknesses > ~170 nm, which may be due to increasing inter-nanowire connectivity for thick AgNWs. In contrast, the optical anisotropy remains clear even for thick AgNWs. Figure 2b   thin (60 nm) and thick (200 nm) AgNWs: in the long wavelength region (λ > 500 nm), the transmittance observed at θ = 0° (T θ= 0°) is much lower than that observed at θ = 90° (T θ= 90°) . For short wavelengths (λ < 500 nm), T θ= 90° is higher than T θ= 0° for both AgNWs. To gain a better understanding of the optical anisotropy we observed the polarised absorption of the 200 nm-thick AgNW film. The AgNW film has a significant anisotropy of absorption (Fig. 2c), apart from near an isosbestic-like point at ~473 nm: in the short-wavelength region (e.g., λ = 350 nm), the polarised absorption for incident light polarised along the y-direction (A θ= 90°) is stronger than it is for incident light polarised along the x-direction (A θ= 0°) , while A θ= 90° is weaker than A θ= 0° in the long-wavelength region, e.g., λ = 1030 nm. Here, the absorption peak at 350 nm is mainly attributable to the transverse SP resonances (SPRs) of the AgNWs, while the strong signals in the long-wavelength region over 1000 nm (peak at around 3100 nm, not shown) may be attributable to the longitudinal SPRs of the AgNWs 6,45,46 . Such optical anisotropy is characterised by the dichroic ratio DR, defined as 46 DR = A θ= 0°/ A θ= 90°. The measured dichroic ratios are DR ⊥ = 0.58 for the transverse (⊥) SPRs at λ = 350 nm and DR // = 3.18 at λ = 1030 nm (for the longitudinal SPRs). From these DRs, we estimated the macroscopic orientational order parameter S of the AgNWs, defined as the statistical average (< > ) of the second Legendre polynomial, i.e., S = < P 2 (cos φ)> = 1/2 (3 < cos 2 φ > − 1) for an angle φ between the long-wire axis of a single AgNW and the preferred direction 22,46 ; S can be determined using the relationships: S //, ⊥ = (DR //, ⊥ − 1)/(DR //, ⊥ + 2) and S // = − 2 S ⊥ , where S // and S ⊥ are the major order parameters with respect to the x-and y(z)-direction, respectively, for uniaxial symmetry 46 . Values of S estimated for AgNWs are shown in Fig. 2d, in which the degree of nanowire ordering shows obvious high values of S // = ~0.38 for the observed range of U. Note that the S // values (~0.38) estimated from DR ⊥ at λ = 350 nm can be confirmed by those (S // = ~0.43) estimated from DR // at λ = 1030 nm, resembling a nematic ordering (S = ~0.3-0.9) of archetypal liquid crystal (LCs) materials 46 . This S // value of the H-dip-coated AgNWs is much higher than that (~0.0) of spin-coated AgNWs ( Supplementary Fig. S1b), and is one of the highest values reported for oriented AgNWs 22 .
To address the need for large-scope applications in polarisation-selective opto-electronic devices, the H-dip-coating method can be used to apply the oriented AgNWs directly to a target substrate without any additional steps such as the transfer method 27 . As an example showing the use of H-dip-coated AgNWs, we investigated LC devices with oriented AgNWs as the transparent electrode in place of a conventional ITO electrode. Figure 3a shows the polarising microscopic textures of three nematic LC cells containing the AgNW electrodes.
When the x-directions of the two H-dip-coated AgNW electrodes on glass substrates are in parallel, the nematic LC molecules are homogeneously (or planar) aligned on the oriented AgNWs in the x-direction due to the free energies of the anisotropic surface ( Supplementary Fig. S2), even in the absence of any other alignment layer or treatment (upper panel in Fig. 3a). Interestingly, when the x-directions of the two H-dip-coated AgNW electrodes are perpendicular, the nematic LC molecules between the oriented AgNW electrodes form a 90° twist of the LC director, i.e., a TN structure, resulting in a 90° rotation of the polarisation of the light after it passes through the cell, as in waveguide mode 47 (middle panel in Fig. 3a). In contrast, when the two spin-coated AgNWs on glass substrates were used as electrodes, the nematic LC molecules were inhomogeneously aligned on the randomly assembled AgNWs (lower panel in Fig. 3a). Hence, when H-dip-coated AgNW electrodes are used in LC cells, various LC modes are possible, depending on the combination of the oriented AgNW electrodes, which act not only as a transparent conductive electrode but also as an LC-alignment layer (Fig. 3b).
Next, we measured the electro-optic characteristics of the TN-LC cell with the AgNWs as a function of the voltage applied (V app ) under the NW mode 47 (Fig. 3c). It can be seen that for the voltage-off state (or V app < threshold voltage, V th , ~ 0.70 V), the transmittance is high (the absolute transmittance, T 0 ~ 25-30%), representing the bright state (Fig. 3c). In contrast, for V app > saturation voltage (V sa ) ~ 1.45 V, the effective retardation value decreases due to the field-induced re-orientation of LCs between the AgNW electrodes, and the amount of transmitted light decreases. Thus, the contrast ratio (CR) of the intensity of bright to dark increases as V app increases, giving maximum CR values of ca. 78.3, 91.7, and 175.9 for R, G, and B light, respectively. Moreover, the rising (field on) and falling (field off) times of the TN-LC cell were found to be approximately 3.4 and 33.9 ms, respectively. Such a low V th and high CR, and such fast switching times, make the AgNW TN-LC cell suitable for a variety of video-rate display applications. In addition, in the NB mode the operation of the TN-LC cell is inversely related to V app (Supplementary Fig. S3). These results show the considerable performance advantages of the LC-alignment electrodes of AgNWs in the LC devices.
Another use of the H-dip-coated AgNWs is a transparent polarising electrode in an opto-electonic device, e.g., an OPV cell. Here, we also investigated (semi-)transparent OPV cells having oriented AgNWs, whose structure and energy diagram are shown in Fig. 4a and Supplementary Fig. S4a, respectively. In view of the transparency of the OPV cells, either the glass substrate side (glass illumination) or the AgNW anode side (AgNW illumination) can be subject to light irradiation. The oriented AgNW anode may also serve as a polarisation-selective optical window for incident light. The PV layer in the OPV cells may be either isotropic or anisotropic (e.g., rubbed 48 ), but isotropic BHJ PV layers of low-band-gap PCDTBT:PCBM 70 were used in our case to investigate the effect of the oriented AgNW anode on device performance. Note that the PCDTBT:PCBM 70 layer is too soft to endure conventional rubbing for generating anisotropy, in contrast to a conventional OPV layer 48 . Figure 4a also shows the polarised absorption of the PCDTBT:PCBM 70 OPV cell with the H-dip-coated AgNW anode (sample AgNW-OPV cell). The sample AgNW-OPV cell exhibited two strong broad absorption bands with peaks at 398 and 576 nm caused by the PCDTBT, extending to an absorption onset at 720 nm, together with an absorption at ca. 450 nm caused by the PCBM 70 . In contrast to the isotropic absorptions of the PCDTBT:PCBM 70 PV layer (Fig. 4a) and the spin-coated AgNW electrode ( Supplementary Fig. S1b), the sample AgNW-OPV cell with the H-dip-coated AgNW anode shows clear absorption anisotropy due to the oriented AgNW anode, which selectively absorbs and transmits (or reflects, Supplementary Fig. S4b) incident lights polarised parallel and perpendicular to the x-direction, respectively.
We then investigated the current density-voltage (J-V) characteristics of the OPV cells. In the dark, the reference PV cell with an evaporated Ag anode clearly revealed good diodic behaviour with high rectification ratios of 10 4 at 1.5 V, while the sample AgNW-OPV cell exhibited similar but slightly different current flows (Fig. 4b), which may have been due to differences in the internal resistance and variations in the interfacial potential barrier between the PV layers and the anodes. The PV characteristics of the sample and reference OPV cells were investigated using unpolarised and polarised illumination, and their performances are summarised in Supplementary  Tables S1 and S2 The polarisation-dependent J-V characteristics of the OPV cells were then investigated ( Fig. 4b and Supplementary Table S2). Under glass illumination, the sample AgNW-OPV cell gave a clear anisotropic PV performance: for polarised incident light along the x-direction, the sample cell with the 85 nm-thick PV layer exhibited a PCE of about 2.49%, which is much higher than results (ca. 1.94%) for light polarised along the y-direction. This may have been caused by the polarised reflection of the oriented AgNW anode ( Supplementary  Fig. S4b). Thus it is clear that the sample AgNW-OPV cell showed a considerable degree of anisotropy in terms of the PV effects (PCE // /PCE ⊥ = 1.28). Interestingly, under AgNW illumination, the sample AgNW-OPV cell showed a reverse anisotropy of PV performance (PCE // /PCE ⊥ = 0.90), which may be attributed to the polarised absorption of the oriented AgNW anodes (Fig. 4a). In contrast, ref. 2 with the spin-coated AgNW anode showed clear isotropic PV effects under both illuminations, i.e., PCE // /PCE ⊥ ≈ 1.0 (Supplementary Table S2). It is also noteworthy that the anisotropic J SC behaviours of the sample cells are consistent with their EQE spectra (Fig. 4c): the polarised reflection spectra of the oriented AgNW anode (Supplementary Fig. S4b) are responsible for the EQE spectral shapes under glass illumination (Fig. 4c), while the absorption spectra of the OPV layer (Fig. 4a) together with the polarised transmission spectra of the oriented AgNW anode (Fig. 2b) are responsible for the EQE spectral shapes under Ag illumination. The foregoing results clearly show that the sample AgNW-OPV cell with the H-dip-coated AgNW anode acted as a transparent polarising OPV cell, providing clear evidence of anisotropic PV activity even for isotropic PV layers. We infer that further improvements to the PV anisotropy might be expected by incorporating anisotropic PV layers 48 in place of an isotropic PV layer in the sample AgNW-OPV cell, and a more detailed investigation of this will be presented elsewhere.
Finally, as an example to show the combined effects of our findings, using the H-dip-coated AgNWs we fabricated and tested a power-generating transmission-type TN-LC device by imbedding a polarising OPV cell, i.e., a Solar-LCD with a schematic structure, as shown in Fig. 5a. Figure 5b shows an operating Solar-LCD, displaying bright and dark states in the NW mode, and simultaneously generating electricity from the backlight illumination. This example also clearly shows that the H-dip-coated AgNW electrode can serve not only as an LC-alignment electrode but also as a polarising, hole-collecting electrode in a Solar-LCD.
Considering all these results, it is clear that oriented AgNWs produced by H-dip-coating can act as a multi-functional anisotropic optical and electrical component in various polarising opto-electronic devices.

Conclusions
In summary, we investigated homogeneous and uniaxial ordering of H-dip-coated AgNWs for large-area polarising printed/organic electronics. Self-assembled and well oriented AgNWs obtained via the simple H-dip-coating process were fabricated with a significantly high order parameter of ~0.38-0.43 due to capillary flow in the meniscus of suspension. We showed that oriented AgNWs can be used in LC devices as a transparent LC-aligning electrode, exhibiting good LC device performance. Moreover, oriented AgNWs can also be used in inverted OPV cells as a transparent polarising anode, exhibiting high PV anisotropy. These results for well oriented AgNWs fabricated by H-dip-coating provide an encouraging basis for the design and fabrication of large-area, flexible, low-cost, high-performance, and polarisation-dependent applications in advanced printed/organic optoelectronics.