## Introduction

Stretchable optoelectronics are widely used in smartwatches, wearable devices, stretchable and textile displays, and health care systems1,2,3,4,5. However, unlike curved displays or flexible smartphones that have only slight curvatures, stretchable devices need to maintain their efficiency under various mechanical deformations, such as folding, twisting, shrinking, and stretching6,7,8. To fabricate these stretchable devices, all components, including the substrate, organic materials, and electrodes, that constitute the device must be mechanically stretchable. The substrate should be chemically and mechanically stable. Substrate candidate groups include rubber1, polyimide9, polyurethane10, polydimethylsiloxane11, and Norland optical adhesive (NOA)6,12. Furthermore, the substrate for bottom-emitting organic light-emitting diodes (OLEDs) must be transparent and should satisfy the aforementioned conditions; thus, a highly transparent and stretchable NOA substrate is considered as the ideal material for bottom-emitting OLEDs. Moreover, organic materials are generally stretchable and can be readily applied in stretchable devices. However, OLEDs exhibit different efficiencies depending on the type of organic material. Luminescence mechanisms are categorized into fluorescence and phosphorescence mechanisms. Conventional fluorescent OLEDs exhibit an internal quantum efficiency (IQE) of up to 25%, while the IQE of phosphorescent OLEDs reaches up to 100%. However, the high cost and scarcity of the metals used in phosphorescent OLEDs continue to be problematic. To resolve these issues and improve the efficiency of OLEDs, thermally activated delayed fluorescence (TADF)-based active materials have been used for blue OLEDs13,14,15. Compared to conventional materials, TADF materials of current interest exhibit 100% IQE because all of the triplets can be used as excitons without platinum group metals16. In addition to these issues, stretchable transparent electrodes for bottom-emitting OLEDs should be considered to enhance the efficiency of stretchable OLED (SOLED) devices. In general, indium tin oxide (ITO) electrodes are used for organic/inorganic LEDs owing to their high transmittance and low sheet resistance17. However, the work function (WF = 4.6–7 eV) of ITO electrodes is lower than that of indium zinc oxide (IZO) electrodes. In addition, ITO exhibits brittle characteristics18. Hence, the application of ITO is hindered in flexible OLEDs and SOLEDs. IZO electrodes exhibit a high WF (~5.1 eV) as well as good optical and electrical properties19. Moreover, the fabrication of mechanically stretchable and optically transparent electrodes is important for obtaining high-performance stretchable electronic devices because the device performance is critically affected by the mechanical, electrical, and optical properties of the electrodes. Therefore, several groups have suggested graphene20, metal nanowires21,22, PEDOT:PSS23 and metal nanomesh24,25,26 electrodes to address these issues. However, it is difficult to prepare a high-quality multilayer electrode with graphene, and Ag nanowires have problems related to junction resistance and oxidation. Moreover, metal nanomesh electrodes are affected by the Moiré effect and low transmittance21,27. There have been some other efforts to utilize metal oxide-based mesh structures28,29 to compensate for the drawbacks of conventional metal nanomesh; however, all of them simply reported their potentials as flexible electrodes without any experimental verifications at the device level.

In this study, TADF-based blue OLEDs were fabricated on highly stretchable NOA substrate using Ti particle-incorporated IZO (Ti/IZO) mesh electrodes. A uniform array of IZO mesh structures was fabricated by simple photolithography. The sheet resistance of the IZO mesh electrode was relatively high and increased further with the open area ratio. We theoretically and experimentally optimized the mesh ratio and incorporated Ti particles in the open region to reduce the sheet resistance of the mesh structure while maintaining its high transmittance. On the other hand, a large number of photons generated from photoactive layers of the bottom-emitting OLED were observed to be wave-guided by a difference in refractive index when coupled out through the substrate. However, in the presence of Ti particles, the extracted light was scattered at an angle greater than the critical angle, thus improving the light outcoupling efficiency of OLEDs. The IZO mesh structure also led to a significant improvement in the stretchable characteristics compared with the ITO and IZO planar electrode devices. Consequently, the SOLED using a Ti/IZO mesh electrode exhibited both high external quantum efficiency (EQE) and luminance intensity regardless of the viewing angle.

## Experimental section

### Fabrication of the NOA substrate

The NOA 63 (Norland Products, Inc.) material was dropped on a 1-inch glass substrate using a pipette, spun-coated at a speed of 4000 rpm at room temperature, and then cured by UV for 5 min. After depositing the organic material onto the substrate, the four sides were cut and peeled off using tweezers.

### Fabrication of the electrodes

Planar electrodes: ITO material was deposited with a thickness of 100 nm at a 2 Å/s growth rate, an 80 W RF power and a 5 mTorr working pressure, while IZO material was deposited with a thickness of 200 nm at a 2 Å/s growth rate, an 80 W RF power, and a 2 mTorr working pressure using an RF magnetron sputtering system.

Mesh electrodes: A 2-nm-thick Ti island layer was deposited on NOA substrate using an e-beam evaporator at a growth rate of 0.2 Å/s. After a simple photolithography process (positive photoresist: GXR–601, AZ Electronic Materials), a 200-nm-thick IZO layer was deposited on a mesh-patterned Ti-island/NOA substrate using an RF magnetron sputtering system at a rate of 2 Å/s. Finally, the Ti/IZO mesh structure was formed by removing the photoresist with acetone through sonication. The IZO mesh electrode was fabricated in the same manner as the Ti/IZO mesh but without Ti deposition.

### Fabrication of TADF-based blue SOLEDs

To fabricate a TADF-based blue SOLED, organic materials (N,N‐dicarbazolyl‐3,5‐benzene (mCP) as a hole transport layer, bis[4‐(9,9‐dimethyl‐9,10‐dihydroacridine)phenyl] sulfone (DMAC-DPS) as a photoactive layer, and bis[2‐(diphenylphosphino)-phenyl] ether oxide (DPEPO) as an electron transport layer), inorganic materials (MoO3 as a hole injection layer (HIL), LiF as an electron injection layer), and Al metal as a cathode were evaporated on the Ti/IZO mesh structure using a thermal evaporator system (Daedong High Technologies Co., Ltd) at below 2 × 10−7 Torr. The LiF was deposited at a rate of 1 Å/s, and the Al electrode was deposited at a rate of 5 Å/s, while all other organic materials and MoO3 were deposited at a rate of 0.5 Å/s. All materials were deposited and patterned through shadow masks.

### Characterization

The transmittance of each electrode was measured as a function of wavelength using a UV–vis spectrophotometer (Lambda 35, Perkin Elmer). The sheet resistance was measured using a four-point probe measurement system, while the stretch characteristics of each electrode were measured using a stretching tester (COAD.121E, Ocean Science). Furthermore, the characteristics of each OLED, including the current density, voltages, luminance, EL intensity, EQE, and light output intensity, were measured according to the viewing angle using an OLED measurement system (M6100 with a Keithley 2400, McScience Inc.). The surface images and morphologies were measured using scanning electron microscopy (SEM) (SU-70, Hitachi, Ltd.) and AFM (XE-100, Park systems Corp.). The WF mapping images were measured using a Kelvin probe system (KP01, KP technology).

### FDTD simulation

Finite-difference time-domain modeling was performed using a finite-difference time-domain (FDTD) simulation (FDTD solutions, Lumerical Inc.). The refractive index of each material was obtained using an ellipsometer in the wavelength range of 380–1050 nm (F20, Filmetrics Inc.). The transmission monitor was set on the substrate. The electric and near-field distribution was then observed through the frequency-domain field and power monitor. The simulation time for the results was set to 1000 fs.

## Results and discussion

Figure 1a shows a schematic of the SOLED structure with the Ti/IZO mesh electrode. A stretchable substrate was prepared by coating the NOA material on a 1-inch × 1-inch glass (Fig. S1). When UV treatments were performed after coating, the NOA substrate exhibited a transmittance of ~99% at a wavelength of 480 nm (Fig. S2). On the substrate, a 2-nm-thick Ti island was formed through an e-beam evaporator. Thereafter, IZO mesh patterns were designed and transferred onto the photoresist for photolithography. The mesh hole diameter was 10 µm, and the spacing was designed to be 10 µm. Subsequently, 200-nm-thick IZO was deposited through a radio-frequency (RF) magnetron sputtering system, followed by a lift-off process for the formation of the Ti/IZO mesh electrode. Next, 1-nm-thick MoO3, 40-nm-thick mCP, 30-nm-thick DMAC-DPS, 50-nm-thick DPEPO, 0.8-nm-thick LiF, and 100-nm-thick Al were deposited sequentially using a thermal evaporator to fabricate the SOLED. Finally, the stretchable device was formed by peeling off the NOA substrate from the glass substrate. The NOA substrate is not a material that can be stretched up to 100% by itself when it is thermally treated. Therefore, the fabricated sample was attached to 3 M VHB tape that was prestretched, and the released state was regarded as the initial state, as indicated in Figure 1b. In other words, the sample was stretched from the wrinkled state to the original position. Figure 1c shows photographs of the SOLED before and after strain. Next, the thickness information of the fabricated SOLED was examined by cross-sectional SEM with a focused ion beam system and is depicted in Fig. 1d. Fig. 1e shows the highest occupied molecular orbital-lowest unoccupied molecular orbital energy levels of the OLED structure with the Ti/IZO electrode. Herein, the WF of the anodic IZO-based films (~5.1 eV) was higher than that of typical ITO (~4.74 eV) and was expected to be beneficial for aligning the energy level of the film with that of the HIL (5.2 eV for MoO3), facilitating hole injection efficiency.

Figure 2 shows the data related to the optimization of the mesh pattern electrode. Prior to fabrication, we investigated the optimal condition of the mesh structure in the IZO electrode (i.e., thickness, diameter and spacing of the pattern) through FDTD simulation. Fig. S3 shows a schematic for the FDTD simulation of the mesh-based OLED structure. Specifically, periodic boundary and perfectly matched layer conditions were applied to the x- and y-axes, respectively. The step of the mesh grid was 1 nm × 1 nm. The transmittance was calculated using a plane wave source, and the field intensity simulation was applied through the sum of the dipole sources in the x, y, and z directions after calculation.

First, we determined the optimal thickness of the IZO film using the calculated contour plot for the transmittance as a function of the wavelength and the thickness, as shown in Fig. 2a. The transmittance versus emission wavelength varied periodically with IZO thickness due to Fabry–Perot interference30, where the highest transmittance was observed for 100- and 200-nm thick IZO films at a wavelength of 480 nm, as indicated by the dotted line (Fig. 2a). Next, the mesh electrode structure was simulated to determine the size and the periodicity of the mesh patterns based on the 200-nm thick IZO film. Figure 2b shows the transmittance curves as a function of the diameter and spacing of the mesh structure, calculated via FDTD simulation. In this figure, the optical and electrical properties of the mesh electrode were found to vary with the diameter and spacing of the mesh structure. To theoretically investigate the correlation between the mesh diameter and the spacing, the mesh diameter was set as r, and the spacing between the two centers was set as p; hence, their ratio = r/p. The simulation results regarding this ratio between the mesh diameter and the spacing indicates that the ideal transmittance can be observed at a ratio of 0.5. As the ratio increases, the open area increases, and accordingly, the transmittance increases; however, the sheet resistance significantly increases31. These results indicated the same trend for the nanoscale mesh structure as for the microscale mesh structure (Fig. S4). To further understand the effect of the pattern size on the optical properties of the mesh electrode, the transmittance was calculated for the nanomesh structure as well as the micromesh structure. As a result, for instance, the 250 nm mesh structure showed a 93.9% transmittance at 480 nm, while the 10-μm-wide mesh structure showed a 94.5% transmittance at 480 nm for the same ratio (Fig. S5)32. Therefore, we decided to use the micromesh electrode (with 10 μm diameter and 10 μm spacing between the patterns) because of its advantages in productivity as well as uniformity (or reproducibility) that are obtained without incurring losses in optical and electrical properties in regard to the fabrication of the mesh electrode.

Based on the simulation results, we prepared 200-nm-thick IZO films deposited under different RF power (40, 80, and 120 W) and working pressure (2 and 5 mTorr) conditions and measured their transmittance (Fig. 2c) and sheet resistance, respectively, as summarized in Table 1. By comparison, a 200-nm-thick IZO film, deposited at an 80 W RF power and a 2 mTorr working pressure, was found to show the highest transmittance (91.6%) and the lowest sheet resistance (16.2 Ω/sq); therefore, the optimal IZO thickness was set at 200 nm. Regarding the deposition, all IZO films were initially deposited at a 5 mTorr working pressure and an 80 W RF power; then, the parameters, such as working pressure and RF power, were tuned up or down to find the optimal condition. Note that the crystallinity of metal-oxide films (i.e., IZO) generally decreases and the surface roughness increases as the working pressure increases; moreover, the carrier concentration decreases, and the Burstein-Moss effect, which narrows the energy band gap, was also observed. In addition, the electrical and optical properties were initially improved, but they were degraded at a certain point due to the increased scattering of ionized impurities. As the deposition power increases, the grain size increases with an energy higher than 120 W, the quality of the thin layer may deteriorate, and the adhesion to the substrate may decrease33. Based on these optimal conditions of the sputtering system, the optical properties were examined by the deposition of the materials on the substrate by the sputtering system. Figure 2d shows the measured transmittance versus the wavelength of each electrode. The obtained results were similar to the simulation results (Fig. S6). The IZO planar and mesh structures exhibited a similar shape; however, the overall transmittance increased with the wavelength, and the additional Ti structure revealed a transmittance loss at other wavelengths but no loss at the peak wavelength34.

Figure 3a shows WF mapping images of the ITO, IZO, IZO mesh and Ti/IZO mesh electrodes captured over an area of 3 × 3 mm2 using a Kelvin probe system. The WF values obtained from this equipment were calibrated by using Ag as a reference sample (~4.6 eV). The WF of the ITO electrode was measured to be 4.74 ± 0.03 eV, while the IZO and IZO mesh electrodes exhibited almost equivalent WF values of 5.09 ± 0.03 eV and 5.1 ± 0.04 eV, respectively. The WF of the Ti/IZO mesh electrode was measured to be 4.97 ± 0.08 eV, similar to that of IZO rather than that of Ti. This may be due to the use of a very thin Ti film. It is known that at least 6-nm-thick Ti films are required to illicit the WF behavior of Ti35. Therefore, the WF of the Ti/IZO mesh electrode used in this study should be similar to that of the IZO electrode. Consequently, the WF matching (or band alignment) between the Ti/IZO mesh electrode and MoO3 HIL leads to charge balance in the EML, enhancing the exciton coupling efficiency. Figure 3b shows the resistance change of each electrode with respect to the mesh ratio. Compared to the sheet resistance of ITO (15 Ω/sq) and IZO (Rplanar = 16.2 Ω/sq) planar electrodes, IZO mesh patterned electrodes exhibited higher sheet resistance (R’ = 22.3–51.2 Ω/sq) with an increase in the mesh ratio.

$$R^\prime \approx \frac{1}{{1 - ratio}}R_{planar}$$
(1)

For example, the sheet resistance of the IZO mesh electrode increases by approximately 2 times compared to that of the IZO planar electrode at ratio = 0.5, according to Eq. (1)33. The resistance changes of the fabricated IZO mesh electrode as a function of the mesh ratio were in good agreement with those of the calculated electrode. To reduce the sheet resistance of the IZO mesh electrode, we deposited approximately 2-nm-thick Ti particles below the IZO surface. As a result, the sheet resistance of the Ti/IZO mesh electrode was reduced by the electrons hopping through the Ti particles laid between the IZO mesh holes36, which might be beneficial for increasing the current injection. Interestingly, the sheet resistance of the IZO mesh electrode was reduced from 22.3 to 20.1 Ω/sq after incorporating Ti particles at a ratio of 0.3 and was steadily maintained up to a ratio of 0.5 in the range of 20.1–22.1 Ω/sq. However, it was increased significantly at a ratio of 0.6 or higher (i.e., 30.3 and 44.1 Ω/sq at a ratio of 0.6 and 0.7, respectively). Consequently, the sheet resistance of the IZO mesh decreased as the mesh ratio became small. However, considering the figure of merit (FoM) and mechanical properties, the Ti/IZO electrode with a mesh ratio of 0.5 was selected as the anode of the present TADF-based blue SOLED.

Next, we fabricated hole-only devices (HODs) with four different electrodes (ITO, IZO, IZO mesh and Ti/IZO mesh) to compare the hole-injection efficiency, as shown in Fig. 3c, d. Fig. 3c shows the energy band diagram of the HODs. The HOD structure consists of anodes (ITO, IZO, IZO mesh and Ti/IZO mesh)/MoO3 (1 nm)/mCP (40 nm)/DMAC-DPS (100 nm)/mCP (40 nm)/MoO3 (1 nm)/Al (100 nm)). In Fig. S7, the IZO planar electrode exhibits the highest current density among the four samples, which is followed by the Ti/IZO mesh, IZO mesh, and ITO electrodes. The sheet resistance of IZO is slightly higher than that of ITO; however, the WF difference between IZO and HIL is much lower than that between ITO and HIL, increasing the hole-injection efficiency. The IZO mesh electrode has advantages in WF matching (or energy level alignment) with HIL as well as in transmittance when compared to ITO, but the increased sheet resistance due to the perforated structure offsets its advantages. Furthermore, the hole mobility of the device was analyzed by the space-charge-limited current (SCLC) method. The current density (J) in the SCLC regime can be expressed by the field-dependent modified Mott–Gurney equation as follows37,38:

$$J = \frac{9}{8}\varepsilon \varepsilon _0\mu _0\frac{{E^2}}{L}{{{\mathrm{exp}}}}\left( {0.89\beta \sqrt E } \right)$$
(2)

where J denotes the current density, ε0 is the permittivity of free space, ε corresponds to the relative dielectric constant, μ0 is the zero-field mobility, E represents the electric field, L is the thickness of the active layer, and β denotes the field activation factor. The μ0 and β of the HODs with four different electrodes can be extracted from the y-intercept and the slope of the fitted curves of ln(J/E2) versus the square root of E (Fig. 3d), derived from Eq. 2 and the J-V curves in the SCLC regime of Fig. S7.

From Fig. 3d, the μ0 values of the ITO-, IZO-, IZO mesh-, and Ti/IZO mesh-based devices are estimated to be 5.7 ×10−9 cm2/V·s, 3.3 ×10−7 cm2/V·s, 1.8 ×10−8 cm2/V·s, and 1.3 × 10−7 cm2/V·s, respectively. Specifically, the μ0 of the Ti/IZO mesh-based device is much higher than that of the ITO- and IZO mesh-based devices, although it is slightly lower than that of the IZO-based device owing to the perforated structure of the mesh electrode. On the other hand, the β value of the Ti/IZO mesh-based device is observed to be 4.5 × 10−3 cm1/2/V1/2, which is smaller than that (8.1 × 10−3, 6.9 × 10−3 cm1/2/V1/2, respectively) of the ITO- and IZO mesh-based devices and slightly higher than that (3.2 × 10−3 cm1/2/V1/2) of the IZO-based device. However, the field mobility (μ(E) = μ0 exp⁡(0.89βE)) of the Ti/IZO mesh-based device is calculated to be higher than that of ITO- and IZO mesh-based devices in the SCLC regime because of its significantly higher μ0 value. This result indicates that the hole mobility of the Ti/IZO mesh-based device is higher than that of the ITO- and IZO mesh-based devices and even comparable to that of the IZO-based device. Therefore, the Ti/IZO mesh structure can overcome the issue related to the high resistance of the IZO mesh, which allows for the full utilization of the high transmittance, low sheet resistance, WF matching with the HIL (thus, high hole injection efficiency), and mechanical stretchability.

Table 2 shows the measured transmittance, sheet resistance and mechanical stability values of the present experimental set and those reported in the literature for comparison12,22,26,28,29,30,39,40,41,42. The ITO, IZO planar, IZO mesh, and Ti/IZO mesh electrodes exhibited transmittance values of 90.7%, 91.6%, 92.7%, and 92.5% at 480 nm and sheet resistances of 15, 16.2, 28.9, and 22.1 Ω/sq, respectively. In addition, the corresponding FoMs can be calculated from the transmittance (T) and sheet resistance (Rsheet) using the equation below43.

$$FoM = {{{\mathrm{T}}}}^{10}/{{{\mathrm{R}}}}_{{{{\mathrm{sheet}}}}}({{{\mathrm{{\Omega}}}}}^{ - 1})$$
(3)

The FoMs of each electrode were 0.0251, 0.0257, 0.0162, and 0.0208 Ω−1. From previous studies, the FoMs are 0.00296 Ω−1 for MoO3/Au/MoO312, 0.0163 Ω−1 for AgNW/Graphene22, 0.007 Ω−1 for Au nanomesh26, 0.0002 Ω−1 for ITO mesh28, 0.0066 Ω−1 for ITO/Ag mesh/ITO29, 0.0144 Ω−1 for Ag Fiber/IZO30, 0.0029 Ω−1 for WO3/Ag/WO343, 0.0097 Ω−1 for TiO2/Au/TiO2, 0.0073 Ω−1 for TiO2/Ag/TiO239, 0.0038 Ω−1 for CuNW40, 0.005 Ω−1 for MoCl5-Graphene41 and 0.07 Ω−1 for AgNW42. By comparing the FoM values of all electrodes mentioned above, including those from previous reports, it was confirmed that the IZO-based electrodes have advantages in transmittance and sheet resistance compared to other electrodes. In particular, the Ti/IZO mesh electrodes are considered to be the most advantageous because the island-shaped Ti particles improve the electrical properties, while the mesh structure provides excellent mechanical properties. Based on these findings, we fabricated IZO-based mesh electrodes with island-shaped Ti particles using an RF sputtering system and investigated the surface condition and uniformity of the fabricated mesh patterns via SEM analysis.

Figure 4a shows the SEM surface image of the Ti/IZO mesh patterns at a low magnification, by which the mesh patterns (with a 10 μm diameter and a 10 μm spacing) are uniformly fabricated on the IZO film. Figure 4b shows the SEM image of the Ti particles under the IZO mesh structure. By the deposition of a 2-nm-thick Ti layer at a low rate using the e-beam evaporator, an island shape was formed instead of a thin film. The size range of the resulting Ti islands was found to be from approximately a few nanometers to 200 nm. Figure 4c shows the energy-dispersive X-ray spectroscopy (EDS) mapping image of the mesh structure.

The EDS mapping images of each element are shown in Fig. S8. Owing to the IZO-material-based mesh structure, the presence of In and Zn was revealed by color mapping. In the case of oxide, oxygen was also detected on the NOA substrate, indicating that it is present in the entire area. As revealed in the aforementioned SEM image, Ti was observed as an island, but it was distributed over the entire area. The EDS spectrum of the Ti/IZO mesh electrode on the NOA63 substrate is shown in Figure 4d. EDS spectrum analysis was performed to determine the composition of the Ti particles dispersed on the surface. In particular, the measured K and L peak values of Ti were checked to confirm the quantitatively and qualitatively composed materials. As a result, each material of In, Zn, O and Ti on the surface was observed in atomic percentages of 54.5%, 5.6%, 39% and 0.9%, respectively. Figure 4e and f show atomic force microscopy (AFM) images before and after Ti deposition, respectively. The root mean square (RMS) values of the IZO layer and the deposition of the Ti layer were 0.491 nm and 0.921 nm, respectively. Interestingly, the RMS value increased by approximately twice as much for the Ti particles, and an island was also formed (Fig. 3b). With the increase in the RMS value of Ti, the reflected beam of the emitted light toward the substrate can be extracted to a great extent because the produced Ti island caused a scattering effect that reduced the waveguide mode between the substrate and electrode. Moreover, the Ti/IZO mesh structure was the most suitable electrode because the mechanical stretchable properties are also important in stretchable devices.

To verify the electrical performance of each electrode as it was stretched, the change in the sheet resistance was measured under strains ranging from 0 to 100% and stretching ranging from 0 to 1000 cycles. Figure 5a presents the graph of the sheet resistance as a function of strain. In the case of the ITO electrode, owing to its brittle characteristics, the sheet resistance increased nearly 100 times even at a strain of only 10% and further increased nearly 10,000 times at a strain of 20%. For the IZO planar electrode, the sheet resistance increased 7 times at a strain of 30%. However, for the IZO mesh electrode, the sheet resistance increased by less than 2 times, even when the strain increased by 100%. Likewise, when the Ti/IZO mesh electrode was stretched by 100%, the sheet resistance also increased by less than 2 times; however, the sheet resistance did not change significantly because the Ti was already in the island form. Figure 5b shows the change in the resistance as a function of stretching cycles from 0 to 1000. The sheet resistance of the ITO electrode was too high to be measured even with a 10% increase, while that of the IZO planar electrode increased by more than 5 times when strained by 40% during 1000 cycles. However, after 100% straining of the IZO mesh and Ti/IZO mesh electrodes during 1000 cycles, the sheet resistance increased by only 1.29 times. Based on the previous results, Figure 5c shows the SEM images for the Ti/IZO mesh electrode after 0 to 1000 stretching cycles with 100% stretching. At deposition, the surface was clean, but after 10 cycles, some wrinkles were observed on the surface. Moreover, after 1000 cycles, the number of wrinkles increased. However, the mesh shape was still maintained, and the electrical/optical properties did not change significantly.

Figure 6 displays the near- and far-field intensity distributions of the OLEDs with three different anode electrodes (IZO, IZO mesh, and Ti/IZO mesh). Figure 6a-c shows the calculated near-field intensity of each device by FDTD simulation. In the case of the IZO-based OLED (Fig. 6a), the electric field distributions of the photons toward the outside of the substrate were found to be considerably weak due to the trapping of light by the waveguide mode. In contrast, the waveguide mode can be reduced for IZO mesh-based OLEDs, as shown in Fig. 6b, because the reflection angle of the photons changes due to the corrugated structure of the IZO mesh. Moreover, owing to the scattering effect via Ti particles, the trapped light can be emitted to the outside of the substrate to increase the light extraction intensity, as shown in Fig. 6c. These results reveal that the light enhancement is mainly due to the reduced total internal reflection (TIR) at the interface. In addition, we analyzed the far-field intensity distributions of each OLED device, as shown in Fig. 6d-f. Compared to the IZO-based OLED, the angular-dependent electric field intensity was increased for IZO mesh-based OLEDs due to the wide distribution profiles (or angles) via mesh electrode patterns. To investigate the role of the metal particles, 1000 Ti particles with sizes of 5–50 nm were deposited in a random distribution, and a higher radiation intensity was observed out of the device through the mesh patterns. The light extraction efficiency was found to increase for the entire wavelength as well as for wider angles at 480 nm. These results indicate that the light that is trapped by the waveguide mode can be emitted outside the device by scattering through the Ti particles. Based on these simulation results, we conclude that light outcoupling can be enhanced further via the IZO mesh and Ti particle structure, rather than ITO or IZO planar electrodes, eventually increasing the EQE of the device.