Efficiency Improvement in a Powder‐Based Flexible Electroluminescence Device using Ag Nanothin‐Film‐Coated Transparent Electrodes

Powder‐based electroluminescent (EL) devices have attracted attention as a technology for realizing flexible curved lighting or displays. However, low efficiency is a challenge to overcome practical applications. Herein, the characteristics of EL devices are analyzed according to the properties of an indium‐tin‐oxide (ITO) electrode, and EL devices containing a Ag nanothin‐film for efficiency enhancement are reported. As the emission area of the as‐prepared EL device decreases, the power loss per unit area increases, resulting in a low device efficiency. To decrease the power loss of the device, a several‐nano meter‐thick Ag thin film is formed over a polyethylene terephthalate film to obtain 88% transmittance and 10 Ω sq−1 sheet resistance. An EL paste is printed thereon, followed by upper electrode film stacking to fabricate an EL device with a sandwich structure. Compared to an ITO film‐based device with 40 Ω sq−1 sheet resistance, the as‐prepared EL film device consumes less power and exhibits ≈12% higher device efficiency. The possibility of powder‐based EL film display by playing pattern images on 9 × 9 pixel arrays formed on Ag nanothin‐film electrodes is confirmed. It is believed that the present results promote practical research on future devices, including stretchable and bendable EL displays.

Powder-based electroluminescent (EL) devices have attracted attention as a technology for realizing flexible curved lighting or displays. However, low efficiency is a challenge to overcome practical applications. Herein, the characteristics of EL devices are analyzed according to the properties of an indium-tinoxide (ITO) electrode, and EL devices containing a Ag nanothin-film for efficiency enhancement are reported. As the emission area of the as-prepared EL device decreases, the power loss per unit area increases, resulting in a low device efficiency. To decrease the power loss of the device, a several-nano meter-thick Ag thin film is formed over a polyethylene terephthalate film to obtain 88% transmittance and 10 Ω sq À1 sheet resistance. An EL paste is printed thereon, followed by upper electrode film stacking to fabricate an EL device with a sandwich structure. Compared to an ITO film-based device with 40 Ω sq À1 sheet resistance, the as-prepared EL film device consumes less power and exhibits %12% higher device efficiency. The possibility of powder-based EL film display by playing pattern images on 9 Â 9 pixel arrays formed on Ag nanothin-film electrodes is confirmed. It is believed that the present results promote practical research on future devices, including stretchable and bendable EL displays.
To address these, the following approaches can be mainly adopted: 1) quantum-efficiency improvement, 2) optical structure optimization, and 3) electronic circuit optimization. [14][15][16][17] First, the quantum efficiency of EL devices can be increased by increasing the recombination rate of the electron-hole pairs generated in a phosphor. [18][19][20] In contrast, the efficiency decreases when electrons and holes are captured by vacancies or surface defects in the phosphor before recombination. According to the research results so far, such surface defect formation can be significantly controlled by coating the surface with a high-bandgap material, such as SiO 2 , and thus the efficiency can be improved. [21,22] The dielectric properties of the device components also significantly influence the luminance of the EL device. [23,24] The luminance can be improved by increasing the dielectric constant of binder materials and by inserting a dielectric layer between the phosphor layer and electrode. In theory, EL luminance is proportional to the dielectric constant of the device. [25] Structural optimization, another effective method, can reduce light loss due to internal scattering and, consequently, increase the device luminance efficiency. In a previous study, we proposed a novel structure that adapted retroreflective electrodes at the bottom layer, enhancing brightness up to %1000 cd m À2 . [26] However, further improvements in terms of device efficiency are required for the practical application of the device because devices implementable in products may cause performance degradation owing to the limitations imposed on the materials and device size. In particular, electrodes significantly affect the power loss and device performance degradation. The current injected into the device is limited according to the electrode area, and the voltage drop varies depending on the resistance, which affects the efficiency of the device. In order to overcome these challenges, an approach from the material point of view is required, and Ag-nanothin-film-based transparent electrodes can be considered as a candidate material with a performance comparable to that of indium-tin-oxide (ITO) electrodes. [25] However, transparent Ag thin films are difficult to deposit on a flexible substrate because Ag has a higher surface energy than base films such as polyethylene terephthalate (PET), it grows in an island mode when forming a thin film. To achieve high transmittance, a film with a thickness of several nanometers must be formed through layer-by-layer growth. [27] According to recent study, the formation of a high-surface-energy acrylic seed layer over the organic film substrate would induce layer-by-layer growth of a Ag thin film. Consequently, a flexible transparent conducting film with a low resistance of %10 Ω sq À1 and a high transmittance of ≥85% can be fabricated. [28,29] In this study, we examined the effects of an ITO electrode on EL device efficiency. Then, by incorporating a low-resistance alternative electrode of a transparent Ag nanothin film with 10 Ω sq À1 sheet resistance, we increased the efficiency of the EL device by %12%. Such an Ag thin film electrode has a very important meaning in terms of technology that can implement very low resistance to reduce signal delay of display pixels and increase device efficiency. Through the driving of individual pixels patterned on the Ag nanothin film electrode, we displayed pattern images and moving pictures to test their feasibility.

Efficiency Variation with Emission Area
First, to evaluate the change in efficiency according to the electrode size, we prepared an EL film device with an area of 1600 mm 2 using an ITO-coated PET film substrate having 100 Ω sq À1 sheet resistance. The device was then cut into sizes of 200, 400, and 800 mm 2 , and the consequent changes in emission characteristics were investigated. The devices were operated under a high-efficiency condition of 100 V@600 Hz to precisely verify the characteristic change according to the emission area of the EL device. The average luminance of the samples was 26.5 cd m À2 , as shown in Figure 1. The luminance per unit area of the EL device did not fluctuate significantly with the device size, and the deviation was within 10% of the average. However, the current density in the device changed significantly with the emission area. As shown in Figure 1a, the current density decreased with increasing device area.
In the 200 mm 2 device, the current density was 2.2 A m À2 ; when the device area was increased to 400, 800, and 1600 mm 2 , the current density decreased to 1.9, 1.8, and www.advancedsciencenews.com www.adpr-journal.com 1.4 A m À2 , respectively. The emission area variations may alter the amount of charge injected into the electrode, resulting in current density variation. We consider this to be directly related to the reduction in power consumption caused by the electrical wiring and electrodes, attributed to the increase in the light emission area. Therefore, the device efficiency increased relatively from %42%, as shown in Figure 1b.

Efficiency Variation with Electrode Resistance
The device efficiency can also be affected by the resistance of the EL device, which is attributed to its components-the transparent electrode and metal wire. Because the resistance of the transparent electrode is higher than that of the metal wire, the resistance of the device is mainly determined by the transparent electrode. To gain further insights, we fabricated 50 Â 50 mm 2sized EL devices with different resistances, using ITO films with 40, 100, and 300 Ω sq À1 sheet resistances. The efficiencies were then compared. Figure 2 shows the results of the experiment performed under high-brightness operating conditions, which are applicable for practical use. As the sheet resistance increased from 40 to 300, the efficiency decreased relatively by 23%. This result indicates that the EL device efficiency significantly depends on the resistance of the electrode. Thus, the EL device efficiency can be enhanced by decreasing the resistance of the electrode. However, manufacturing a flexible transparent conducting film with a low sheet resistance using ITO is challenging because a high concentration of indium (In) metal, a rare earth metal, is required to achieve low sheet resistance. Furthermore, the heat treatment must be conducted at high temperatures, which cannot be withstood by the organic substrate. [25] Therefore, to solve this problem, we prepared EL film devices using a conducting PET film, over which a transparent Ag electrode was formed by sputtering. Figure 3a shows the cross section of an EL film device sandwiched in a single light-emitting layer. Because of the adoption of a Ag-nanothin-film-based PET film as the substrate, the light-emitting characteristics are unaffected even if the device is bent or distorted (Figure 3b). A single-layered light-emitting device in which a transparent electrode is applied to the upper and lower electrodes exhibits double-sided light emission. This feature facilitates the visualization of the display screen from both sides, leading to potential variety in product applications. In addition, the luminescent and nonluminescent images in Figure S1a,b, Supporting Information, indicate that light entering from the rear surface of the single-emission-layer device may pass through the interparticle gaps.

Optoelectronic Properties of EL Device with Ag Electrodes
This implies that transparency can be imparted to the device, enabling a transparent display based on the degree of particle filling. In this study, the device was fabricated with 70% filling density to avoid transparency; the characteristics were evaluated. As shown in Figure 3c, the luminescence intensity increased with the operating frequency and driving voltage. This is because a voltage increase affects the number of electron-hole pairs generated in the phosphor, and a frequency change affects the electron-hole pair generation and recombination process. As shown in Figure 3d, unlike the luminescence intensity, the efficiency, which is the amount of power used for electroluminescence with respect to the applied power, first increased and then decreased rapidly with increasing voltage. The intensity depends on the number of generated electron-hole pairs and the recombination rate; moreover, it significantly depends on current decrease in the light-emitting layer, attributed to factors such as Joule heat. The generation of an electron-hole pair in the emission layer is proportional to the number of electrons that break the ionic bond and get excited, which increases with voltage.
The current, luminance, and voltage characteristics of the EL device are presented in Figure 4a; it better depicts the factors influencing the efficiency. In the low-voltage region, the device exhibits linear I-V and luminance-V relationships, implying the suitability of the proposed mechanism. In the high-voltage region (>150 V), the current increased exponentially. This is attributed to the increase in the number of electrons accumulated in the dielectric layer due to voltage increase, leading to an increase in the leakage current directly flowing from the cathode. Therefore, as the voltage increased, the leakage current increased exponentially but the luminance increased linearly,  thereby decreasing the efficiency. Figure 4b shows the change in efficiency according to frequency. As the frequency increases, the efficiency decreases, which is mainly due to the decrease in the dielectric constant of the composite constituting the device (see inset of Figure 4b) as the frequency increases. [30]

Efficiency Improvement
The performance of the devices based on ITO and transparent electrodes was compared, as shown in Figure 5. As explained earlier, the power was linearly dependent on the driving voltage, whereas the efficiency reached a maximum at 70 V and decreased with the driving voltage therefrom regardless of the electrode. Upon introducing the transparent Ag electrode, the efficiency of the device improved drastically under the applied driving conditions. The efficiency plot indicates that the device based on the transparent Ag electrodes consumed lower power to achieve the same luminance than that based on the ITO electrode; thus, it showed %12.8% higher efficiency. This can be interpreted as  www.advancedsciencenews.com www.adpr-journal.com an efficiency improvement due to reduction in the power loss, ascribed to the device components such as transparent electrodes; the remaining power directly contributes to electroluminescence when low-resistance Ag is employed as the electrode. And an important point in display applications, this electrode can be formed as a signal line with a narrow width.

Colorimetry
When the emitted light passes through the electrodes, it induces a wavelength shift. This can be visualized as a change in the color of emission, which is instrumental in the application of transparent Ag electrode-based EL devices to displays and lighting. To this end, we investigated and compared the color shifts in the samples using Ag and ITO electrodes according to the frequency and driving voltage. As shown in Figure 6a, the emitted light exhibited main peaks at 452 and 498 nm, corresponding to ZnS:Cu and ZnS:Mn emission centers, respectively, and the intensity of 452 nm peak further increased with the frequency. This is a general characteristic of ZnS-based powder EL and is not attributed to the incorporation of the Ag electrode. [31] In the device fabricated with an ITO electrode with 40 Ω sq À1 sheet resistance, the color of the emission shifted to deep blue with increasing frequency and to pale blue with increasing voltage (Figure 6b,c, respectively). Additionally, as shown in Figure 6d, same color characteristics were observed at 1 and 5 kHz for the samples prepared with ITO electrodes with 300 and 100 Ω sq À1 sheet resistances. Because the color shift is based on the change in the refractive index, no color shift can occur for the electrodes of the same material. Furthermore, almost no color change is observed in the device using the transparent Ag electrode. This could be due to the  www.advancedsciencenews.com www.adpr-journal.com application of the nanothin film, which cannot induce any appreciable change in the refractive index of the electrode.

Display Evaluation
The responsiveness of pattern pixels is an important factor for display applications. In the EL film device with a relatively large gap between electrodes, individual pixel driving has not been reported so far. We fabricated passive-type displays with 9 Â 9 pixel arrays to determine if individual pixels are driven in a multipixel structure, thereby to evaluate their application scope in displays and lighting. Figure 7a shows an image in the bent state of the display fabricated with 50 Â 50 mm 2 area, and Figure 7b shows the full-surface emission image. In addition, the "DGIST" letter pattern ( Figure 7c) and moving picture (Movie S1, S2, Supporting Information) were displayed upon operating individual pixels. Although crosstalk was partially observed between pixels owing to the implementation of a common electrode in the device structure, this can be overcome through further improvements in the driving scheme. These results well substantiate that powderbased EL film devices can be developed for both flexible lighting and self-luminous flexible display technologies.

Conclusions
The efficiency of powder-based EL devices increased with the light-emitting area owing to the decrease in the power loss per unit area. The resistivity of the electrode is another factor influencing the efficiency, motivated by which a hightransparency low-resistance electrode based on Ag was applied. Consequently, the efficiency of the device was improved by more than 12% in relation to that of the device employing the ITO electrode. Furthermore, we demonstrated the effective driving of a pixel array composed of EL devices by display fabrication.

Experimental Section
EL Paste Synthesis and Device Fabrication: Powder-based AC-driven EL devices can be manufactured by sequentially printing a phosphor layer and a dielectric layer on an electrode. The incorporation of the dielectric layer into the phosphor layer can result in a simpler single-layer structure. [26] In this study, we prepared a single-layered EL-device film by printing a UVcurable EL paste on conducting film substrates. The EL paste was prepared by mixing ZnS (Cu, Mn) particles (30 μm, Osram Silvania) and BaTiO 3 particles (1 μm, Sigma-Aldrich) in a 9:1 weight ratio, followed by blending with a photoacrylic monomer in a 7:3 weight ratio. Then, 4 wt% photoinitiator was added to induce photocuring of the monomer. Commercially available ITO-coated PET films with 40, 100, and 300 Ω sq À1 sheet resistances were used as substrates to investigate the effects of sheet resistance on the device performance. Additionally, a device with a Agcoated PET film (MSWAY Co. Ltd.) with 10 Ω sq À1 sheet resistance and 88% transparency was prepared and its performance was compared to that with a ITO-coated film.
For device fabrication, the substrate was first cleaned in an ultrasonic bath with acetone and alcohol for 10 min each and then dried. As shown in Figure S2a, Supporting Information, a photocurable EL paste was prepared by blending the phosphor powder with a UV-curable binder. The paste was then screen printed onto the Ag coating film to a thickness of %30 μm ( Figure S2b, Supporting Information). Subsequently, the upper plate film was laminated thereon and irradiated with UV light ( Figure S2c, Supporting Information), thereby completing the EL device fabrication. To fabricate the EL display, the EL paste was cured without laminating the upper plate film. The pixel array for individual driving was deposited by directly forming Ag electrodes on the cured EL paste via thermal evaporation using a shadow mask. Patterns with a size of 4 Â 4 mm 2 were arranged at 1 mm intervals on the mask.
Device Characteristics: Cross-sectional and emission images of the EL device were captured in reflection and transmission modes, respectively, through optical microscopy (EV-100, Nikon). The luminance and color of the EL film devices were measured investigated using a homemade device consisting of a function generator (DG-4202, RIGOL), a voltage amplifier (HA-405, Pintek), a source meter (Model 2400, Keithly), and a spectroradiometer (PR-655, Photo Research). The experiments were conducted in the frequency range of 1-5 kHz and the driving voltage range of 0-192 V rms .
Display Driving: The application of the driving signal to each pixel is crucial to this implementation. Accordingly, we formed a pixel array by thermally depositing 4 Â 4 mm 2 -sized Ag electrodes on the light-emitting Figure 7. Photography of the EL film display with a 9 Â 9 pixel array. a) Bending image of the EL film display (50 Â 50 mm 2 ), b) emission image, c) "DGIST" letter pattern expressed by the EL film display.
www.advancedsciencenews.com www.adpr-journal.com layer at 1 mm intervals to form a 9 Â 9 array. The driving signal was configured to be supplied to individual pixels from the controller through the bottom-pin contacts. "DGIST" letter pattern in Figure 7c was displayed by taking a picture one character by one character using a 9 Â 9 pixel array.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.