Meter-scale transparent conductive circuits based on silver nanowire (AgNW) networks are fabricated for transparent light-emitting diode (LED) screens on both rigid and flexible substrates. A 25-cm long AgNW transparent conductive strip is fabricated with a strip resistivity of 9.95 Ω/cm. A high uniformity is achieved in terms of film optical transmission (up to 84.5% in average) and sheet resistance (as low as 4.7 Ω/sq in average), superior to ITO. A transparent LED screen based on a 1.2-m ultralong AgNW circuit is demonstrated with LEDs emitting bright red, green and blue lights under different biases. The AgNW strip on a polyethylene terephthalate substrate shows mechanical flexibility and stable performance in bending tests. Based on this, a flexible transparent LED screen is proposed and presented. It works well when dynamically bent to a radius as small as ∼15 mm. Therefore, the AgNW transparent conductive circuits are very promising as a replacement to ITO circuits for such smart screens, to be integrated into modern glass architectures and display videos in various public places.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Nearly 70% of modern buildings have large glass areas. Providing information and entertainment on glass has attracted much attention worldwide. Static information can be incorporated through acidic treatment, engraving, or thin-film coating of glass. Incorporating electrochromic materials or liquid crystals enables transparent or translucent dynamic information but is hard to achieve in multiple colors [1,2]. In recent years, transparent light-emitting diode (LED) screens have emerged and quickly become an effective alternative to traditional passive or switchable functional glass. A number of LEDs connected with transparent conductive circuits constitute a transparent screen. By programming the LEDs to work together, an image or video can be created. These screens have the same general function with conventional liquid crystal displays (LCDs) or LED panels. The inherent transparency enables many applications that were not possible with conventional LCD or LED screens. They can be installed on glass walls for video display, audience interaction, etc. Indoor windows incorporated with transparent LED screens can be informative without blocking out light or the view on both sides.
Transparent LED screens require highly transparent and conductive circuits to connect LEDs. For meter-scale large-format screens in glass walls, ultralong transparent conductive circuits are required. Fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) transparent conductive circuits are typically employed in commercial products [3,4]. Screen-printed metallic wires have also been proposed to connect LEDs for transparent screens , which are too thick and clearly visible even 10 m away. FTO is cheaper than ITO because indium is expensive, but has the same shortcomings, including the requirement of high-cost meter-scale vacuum deposition, the difficulty of etching them into circuits, and the brittleness which exclude flexible applications [6–9]. This electrode design constraint is primarily responsible for the lack of flexible transparent display [3,4].
In order to simplify the fabrication process, reduce the cost and extend the application of current transparent LED screens, ITO or FTO circuits must be replaced. Alternatives have been proposed, such as conducting polymers [10,11], carbon nanotubes [12,13], graphene [14–16], and metallic networks [17,18]. Conducting polymers are unstable and weakly conductive ; semiconducting carbon nanotubes have high junction resistances ; graphene has low conductivity induced by rather low carrier density . Metallic networks, especially silver nanowires (AgNWs) [17,18], are the most promising alternative to ITO or FTO, because of their excellent optical transmittance, electrical conductance, and mechanical flexibility. Some metallic networks require complicated deposition and patterning [19–24], but AgNWs can be easily synthesized in solution and distributed over a large area via various methods, e.g., spin-coating [25–31], spray deposition [32,33], Meyer rod coating [34,35], etc. With these methods, transparent conductive films have been easily fabricated [25–35]. However, fabrication of transparent circuits is challenging and has been rarely reported, which limits progress towards transparent LED screens. Most AgNW circuits reported previously only had transparent substrates and the conductive circuits were opaque [36–38]. Transparent AgNW circuits, where both substrates and circuits were transparent, were developed . Based on microfluidic technology, AgNWs were spin-coated into microfluidic channels on polydimethylsiloxane (PDMS), forming chip-scale complex conductive patterns . However, limitations of the mold and the size of the spin-coater prevent application of this technique to transparent LED screens.
In this work, with a spray coating method using sacrificial masks, 25-cm long AgNW transparent conductive strips are achieved. A transparent LED screen is demonstrated with a 1.2-m ultralong AgNW transparent conductive circuit. These substrates show superior mechanical flexibility over those based on ITO. Flexible transparent LED screens are presented in this work. The whole fabrication process is straightforward. AgNW transparent conductive circuits can reduce the cost and expand the applications of the transparent LED screens to flexible and large-angle curved areas.
2. Experimental section
AgNWs (average diameter: ∼60 nm; average length: ∼16 µm) suspended in ethanol (5 mg/mL) were purchased from Blue Nano. Pieces of glass (7.5 cm × 2.5 cm coverslip for electrical and optical characterizations, 30 cm × 30 cm toughened glass for demonstrating transparent screens) and polyethylene terephthalate (PET) films (0.25 mm thick) served as rigid and flexible substrates, respectively. Poly-L-lysine (SIGMA-ALDRICH) dissolved in deionized water (0.1 mg/mL) was used to improve the affinity of the AgNWs for the substrates.
2.2 Treatment of glass or PET substrates
A piece of glass or PET film was sonicated in acetone, methanol and isopropanol in sequence, each for 5 min. Then it was immersed in poly-L-lysine solution for 5 min. An ultrathin layer of poly-L-lysine was coated on the surface to promote uniform distribution of AgNWs.
2.3 Fabrication of AgNW transparent conductive strips
Figs. 1(a)–1(d) schematically demonstrate the fabrication procedure of AgNW transparent conductive strips. Before treatment, the substrate was covered with a sacrificial polyvinyl chloride (PVC) mask, which contains several straight lines (2 mm wide) to organize the AgNWs (Fig. 1(a)). Subsequently, the PVC covered substrate was treated with poly-L-lysine solution for 5 min and the bare substrate surface along the lines became sticky (Fig. 1(b)). Then, AgNWs in ethanol (5 mg/mL) were sprayed with different doses on the top (Fig. 1(c)). Since the spray nozzle was kept 10 cm above the substrate, the sprayed AgNWs were well within a very small area with diameter of about 1.5 cm. To make a long conductive strip, the spray nozzle was moved along the pattern step by step. The distance between the adjacent spray steps was varied to tune the uniformity of the AgNW distribution in the strip. After it was dry, the PVC mask was peeled off. Only the AgNWs on the sticky areas were left, forming AgNW transparent conductive strips on the substrate (Fig. 1(d)). In order to measure the optical transmission and sheet resistance, as well as to analyze the uniformity of AgNW distribution, AgNW transparent conductive films were fabricated according to the above procedure without the employment of the sacrificial PVC mask. For comparison, ITO transparent conductive strips and films were also fabricated by sputtering deposition (Shenyang Tengao Machinery Manufacturing Co., Ltd., JSS-450-1) on either glass coverslip or PET substrates. The thickness was 160 nm.
2.4 Preparation of AgNW transparent conductive circuits for demonstration of transparent led screens
As shown in Fig. 1(e), the PVC mask design for the LED-connected circuits was more complex than that used for transparent conductive strips. Here SMD (surface mounted device) LEDs (Shenzhen Best LED Optoelectronic Co., Ltd) were applied. Each LED had three pairs of pins. Therefore, three parallel transparent conductive strips had to be designed in order to apply different voltages on the LED to generate different colors (2 V for red, 3 V for green, and 3 V for blue). The three parallel strips (separated by 2 mm for isolation) were truncated into two parts with a 5 mm intermediate spacing for the LED connection. In order to guarantee connection with the LED pins, when they approached LED, they became a little bit narrower and less separated, but still isolated from each other. After surface treatment and AgNWs deposition, the PVC mask was removed and three AgNW transparent conductive strips remained. Simply connecting such AgNW transparent conductive strips one by one could make a long circuit for accommodating more LEDs (which were separated by about 1.0 cm, according to commercial products fabricated based on ITO or screen-printed Ag thick wires). In the following, a number of SMD LEDs were stuck to the reserved positions in the transparent conductive circuits with Ag conductive adhesive. All the pins of the LEDs were electrically connected to the AgNW transparent conductive strips within the circuits correspondingly (Fig. 1(f)). By changing the PVC mask patterns, various LED-connected transparent conductive circuits were obtained, which formed simple transparent LED screens for demonstration. Different voltages were applied manually to the circuits from one end to the other to make the LEDs emitting different colors. Once the LEDs were lit, the circuit or the screen (either rigid or flexible) would have various colorful appearances. When they were turned off, the screen became very transparent.
The morphology of the AgNW transparent conductive strips was characterized with a field emission scanning electron microscope (SEM; Carl Zeiss Ultra 55). In order to avoid charging, an ultrathin film of gold was sputtered onto the samples. The resistances of AgNW transparent conductive strips with different probe separating lengths were measured by a multimeter (Victor VC890). Assuming constant contact resistances between the probes and the AgNW networks, the resistivity per unit length of each conductive strip was calculated through fitting the curve of resistance versus length. Since light transmission and sheet resistance were difficult to characterize for lines, we fabricated transparent conductive films with the same spray coating method as described above and measured these properties to represent those of the transparent conductive strips. The light transmission of the film was measured through our home-built integrating sphere based spectrometer and normalized to that of a pure substrate (glass or PET). The sheet resistance of the film was measured with our home-built four-probe measurement system. The external quantum efficiency (EQE) of the LED was characterized with an integrating sphere based EQE measurement system. As schematically shown in Fig. 5(a), the effect of the AgNW transparent conductive circuit on the electroluminescence (EL) performance were investigated with a spectrometer (Ocean Optics) collecting the emission through a multimode fiber. For bending tests, a PET film deposited with a AgNW or ITO transparent conductive strip was bent to different curvatures or to a fixed minimal radius of curvature of 9 mm for many different cycles via two aluminum supports (one fixed, the other moving; the photo was shown in the inset of Fig. 8(b)). The line resistance after each bending cycle was recorded by a multimeter through the conductive supports. This apparatus was also employed for the dynamic demonstration of flexible transparent LED screens on PET.
3. Results and discussion
3.1 Ultralong and uniform AgNW-based transparent conductive strips
Five AgNW transparent conductive strips with different spray doses were fabricated on a piece of toughened glass (see Section 2.3). As shown in Fig. 2(a), each strip was 2 mm wide and 25 cm long, which could be made arbitrarily long. As the spray dose decreased, the university logo under the glass became increasingly clear, otherwise, the line became increasingly translucent. For SEM inspection, a small sample was fabricated on a piece of glass coverslip with the same method. The SEM images in Figs. 2(b)–2(f) showed that AgNWs were randomly distributed, forming networks along the strip. As observed in the zoomed-in image of the case with spray dose of 30 µL (Fig. 2(c1)), the AgNWs were well connected with each other. The density of the AgNWs increased with increasing spray dose from 20 µL to 60 µL. Due to light scattering and absorption, denser AgNW networks led to less light transmission as demonstrated in Fig. 2(a). The strip edge was very smooth with no wires extending out (not shown).
For each AgNW transparent conductive strip and typical ITO, the relationship between the measured resistance (R) and length were plotted in Fig. 3. The AgNW strips were fabricated with different spray doses but the same spray step distance of 1.3 cm and AgNW concentration of 5 mg/mL. In order to show the process repeatability of AgNW strips, three strips were fabricated with the spray dose of 30 µL. At each length, the average R values were plotted and indicated by red up-triangles. The rather small error bars (vertical black sticks) showed very good repeatability of both fabrication and measurement. Therefore, for other cases with different spray doses, only one strip was fabricated and characterized. As shown in Fig. 3, for each case, R rose almost linearly with the length, which was fitted with a straight line. The slope of the fitting line represented a constant resistivity per unit length (Fig. 3). The intersection of the fitting line with the vertical ordinate axis approached zero, meaning negligible contact resistances between the two probes and the AgNW networks or ITO. Due to the sparsely distributed AgNWs in the strip fabricated with the spray dose of 20 µL, its resistivity (29.59 Ω/cm) was very large. Probably because of the contact instability, the measured resistance varied largely from the fitting line. As the spray dose increased, more and more AgNWs became interconnected. Therefore, the resistances along the transparent conductive strips became more stable and matched with the fitting lines. The resistivity became lower than 10 Ω/cm with a spray dose of 30 µL and then was reduced to 4.88 Ω/cm with a spray dose of 50 µL. In this case, the AgNWs became dense enough to allow smooth current flow (Fig. 2(e)). Further increasing the spray dose or the AgNW density would not introduce an apparent reduction of the resistivity. The resistivity of the ITO strip was much larger than those of all the AgNW strips, indicating superior performance of the AgNWs. Overall, 30 µL seemed to be an optimal spray dose for fabrication of AgNW strips, which permitted a rather small strip resistivity of 9.95 Ω/cm (more than 18 times smaller than that of the ITO strip; Fig. 3) and a rather clear appearance of the background (Fig. 2(a)).
In order to quantitatively analyze the optical performance of AgNW transparent conductive strips, five AgNW transparent conductive films were fabricated and characterized on small pieces of glass coverslip with the same spray dose of 30 µL but different spray step distances. The films were assumed to have the same optical performance as their strip counterparts. To analyze the uniformity, for each sample, two transmission spectra were recorded at the central position of a spray area and at the intersection position of adjacent spray areas, respectively (Appendix Fig. 10). Their averaged spectrum (Tave) and spectrally averaged absolute difference (ΔT) were plotted in Figs. 4(a) and 4(b), respectively. For each sample, sheet resistances at five random positions were also measured. Their averaged value (Rsh_ave) and mean square deviation (ΔRsh) were plotted in Fig. 4(c) as an overall sheet resistance and a non-uniformity value. The transmission spectrum and the sheet resistance value for the ITO film were also plotted in Figs. 4(a) and 4(c), respectively.
Figure 4(a) demonstrated quite flat transmission spectra of all AgNW network films over the considered visible and near-infrared wavelength range. The spectrum rose with increasing spray step distance, indicating increasingly thinner AgNW networks distributed on the glass substrate. When the distance was 1.3 cm, the transmission spectrum was well above 80% (average transmittance up to 84.5%) and was also higher than that for the ITO film, particularly in the visible regime (average transmittance of only 67.8%). In this case, the difference in light transmission between the center and the intersection of spray areas was the smallest among the five samples (Fig. 4(b)), meaning the best uniformity of AgNW networks along the spray steps. Appendix Fig. 9 demonstrated that when the distance was smaller, the central spray area had a larger light transmission than areas on which AgNWs were double sprayed and the density was higher. When the distance became larger, less AgNWs were sprayed on the intersection area. The transmission spectrum rose to a higher level than that of the AgNW network at the central spray area. Both too small and too large spray step distances were undesirable to a uniform AgNW distribution. In terms of sheet resistance as shown in Fig. 4(c), Rsh_ave stayed around 4.7 Ω/sq when the spray step distance was no more than 1.3 cm. Further increasing this distance made Rsh_ave rise substantially due to sparsely distributed AgNW networks. Like ΔT shown in Fig. 4(b), ΔRsh behaved similarly. It was also the smallest among all cases when the distance was 1.3 cm. This further indicated that 1.3 cm was an optimal spray step distance, which allowed uniform AgNW distribution along the spray steps from both the optical and electrical points of view.
3.2 Transparent LED screens based on AgNW transparent conductive circuits on toughened glass
In order to characterize the commercial LED and investigate the effect of the AgNW transparent conductive circuit on the LED luminescent performance, we fabricated on a piece of toughened glass a very simple AgNW transparent conductive circuit (spray dose: 30 µL; spray step distance: 1.3 cm) with only one SMD LED connected in the middle, as schematically shown in Fig. 5(a). The intrinsic EQE of the LED was measured to be 90.7%, 90.8%, and 85.8% for red, green and blue emissions. The EL spectra of the LED were measured when different biases were applied to the circuits 0-5 cm (in step of 1 cm) away from the pins (Fig. 5(a)). As shown in Fig. 5(b), the LED emitted the brightest red (main peak: 621.9 nm, FWHM (full width at half maximum): 14.1 nm), green (main peak: 521.8 nm; FWHM: 26.6 nm) and blue (main peak: 470.9 nm; FWHM: 24.7 nm) lights with 2 V, 3 V and 3 V biases directly applied to the corresponding LED pins. When the probes were separated away from the LED, under the same biases, the main emission peaks remain almost unchanged, but the relative emission intensities dropped gradually. This indicated the increased resistivity of the long transparent circuits (Fig. 3). Therefore, 1 cm separation was chosen for LEDs connected in the AgNW transparent conductive circuits. This separation is also a standard distance for commercial transparent LED screens based on FTO or ITO transparent conductive circuits [3,4].
With the same spray parameters, a transparent LED screen was experimentally demonstrated based on a 1.2-m long transparent conductive circuit with 80 SMD LEDs connected on a piece of toughened glass of 30 cm × 30 cm (see Section 2.4). It is the longest AgNW transparent conductive circuit we are aware of. The photos were shown in Fig. 6. In Fig. 6(a), the AgNW transparent conductive circuits were nearly invisible and the background green trees could be clearly seen; while in Fig. 6(b), under room light illumination, the circuits appeared due to the shadows of LEDs. A local zoomed-in picture in Fig. 6(b1) showed that for each LED, its three pairs of pins were well connected by three faint parallel lines based on AgNW networks. Since the transmittance of the AgNW network was sufficiently high (Fig. 4(a)), it was not necessary to reduce the width of the strip to further increase the overall transparency of the screen. The AgNW strips are ∼2 mm wide and separated by ∼2 mm. Adjacent LEDs were separated by about 1 cm. As mentioned above, for a LED to emit red, green and blue lights, voltages of 2 V, 3 V and 3 V had to be applied, respectively. In order to make all 80 LEDs work, at least 120 V and 220 V were required. In this work, we divided the long circuit into several segments and applied a 30 V bias (maximum available) to each segment of the LEDs to make them emit. As shown in Fig. 6(c), the circuit was divided into 5 segments to allow the 30 V bias to make the LEDs in each segment emit red light. For emission of green or blue light, a larger bias to each LED was necessary. Since the necessary biases were equal for these two cases, the same 8 segments were divided, as shown in Figs. 6(d) and 6(e). In Figs. 6(c-e), LEDs in different segments appeared to produce different emission intensities. It was likely induced by the silver/black background, room light illumination, as well as the angle at which the pictures were taken. LEDs in all segments were able to emit bright lights in red, green and blue, confirming the conductivity of the AgNW transparent conductive circuits and their good contacts with LEDs. When the LEDs were lit up, the circuits became invisible.
Such transparent LED screens could be varied into different patterns by changing the transparent conductive circuits. For example, we fabricated a transparent LED screen with three transparent conductive circuits forming “Z”, “J”, “U” letters, respectively. When biases were applied, they could appear different colors (Fig. 7). As the area of glass walls increased in modern architecture, taking full advantage of them for advertising or beautifying the urban environment seemed to be increasingly important. Such large-area transparent LED screens could also be used as curtain walls for large-scale performances or events. Some small-area discrete glass screens could be used in bus stops, railway stations, airports or other public places for dynamic advertising or announcement.
3.3 Flexible transparent LED screens based on AgNW transparent conductive circuits on polyethylene terephthalate
In some circumstances, rigid transparent LED screens could not be applied and flexible screens that could be bent were highly desirable. Considering that the screen flexibility was mainly determined by the AgNW transparent conductive circuits, we first investigated the flexibility of a pure AgNW strip without LED connection on a PET substrate. For comparison, a pure ITO strip was also fabricated and tested on a PET substrate. The AgNW or ITO strip was fixed by two aluminum supports, one of which was moving and the other of which was immovable (inset of Fig. 8(b)). The measured resistance (R) normalized to its initial value (R0) was recorded as a function of bending radii and bending cycles (see Experimental Section). Figure 8(a) showed that as the bending radius decreased, R of the ITO strip rose dramatically while R of the AgNW strip remained nearly constant even when the bending radius was as small as 2 mm. When the bending radius was fixed to 9 mm, R of the AgNW strip remained nearly constant while R of the ITO strip increased quickly, over 500 bending cycles, as shown in Fig. 8(b). Both bending tests clearly indicated the superiority of our AgNW strip over its ITO counterpart in terms of mechanical flexibility.
The flexibility of the transparent LED screen based on flexible AgNW transparent conductive circuits was characterized. Figure 9(a) showed that when the PET substrate was attached to a 65 mm diameter bottle, the AgNW transparent conductive circuits were still conducting and the LEDs in the screen were still able to emit bright red, green or blue lights. Even when the screen was being dynamically bent into a radius as small as about 15 mm (Fig. 9(b)), the LEDs could still work (see Visualization 1). This indicated very good flexibility of these smart screens, which could be attached to any surface of various shape or inserted into two pieces of glass of any shape. The good dynamic flexibility also enables installation in dynamic architecture such as doors.
All the transparent LED screens demonstrated in this work (either on rigid glass or flexible PET substrates) are prototypes. Simple treatments are necessary before practical applications. For example, the AgNW transparent conductive circuits can be reasonably designed for easily programing the connected LEDs and displaying videos. Since silver can oxidize in air (Appendix Fig. 11), the circuits can be protected by e.g., a thin transparent PDMS coating [36,37], to avoid any chemical reactions of AgNWs exposed to the environment and meanwhile to enhance their adhesion to the substrates. These screens will also be integrated with sandwiched glass or transparent plastic films for further protection and easy maintenance.
In summary, we have successfully fabricated ultralong and uniform transparent conductive circuits based on AgNW networks for transparent LED screens on both rigid and flexible substrates. The 25-cm long AgNW transparent conductive strip fabricated on toughened glass with the spray dose of 30 µL had a strip resistivity of 10.19 Ω/cm. The strip performances were further characterized through its film counterpart fabricated with the same spray coating parameters. The AgNW network film showed a high uniformity in terms of optical transmission (with an average value up to 84.5% in the wavelength range from 400 to 800 nm) and sheet resistance (with an average value as low as 4.7 Ω/sq), both of which were superior to ITO. A transparent LED screen based on a 1.2-m ultralong AgNW transparent conductive circuit has been designed and fabricated on toughened glass. Under different biases, the 80 LEDs emitted bright red, green and blue lights. The AgNW transparent conductive strip on a flexible PET substrate demonstrated very good mechanical flexibility, whose resistance kept stable even when the bending radius was as small as 2 mm and when it was bent 500 cycles at a bending radius of 9 mm. A flexible transparent LED screen was proposed based on the AgNW transparent conductive circuit, which functioned when the circuit was attached to a 65-mm diameter bottle or when it was dynamically bent to a radius as small as ∼15 mm. Therefore, such LED screens are very promising to be integrated in modern glass architectures, e.g., shopping centers, bus stops, railway stations, airports or other public places, and the mapped LEDs in the screens could be programmed to display videos for dynamic advertising, announcement, beautifying the urban environment, or as display walls for large-scale performances or events.
National Natural Science Foundation of China (91833303, 61775195); National Key Research and Development Program of China (2017YFA0205700); Fundamental Research Funds for the Central Universities (2019FZA5002).
The authors would thank Dr. Julian Evans for his improvement of the manuscript.
The authors declare no conflicts of interest.
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