Light out-coupling of organic light-emitting diodes (OLEDs) with silver (Ag)-nanomesh electrodes was examined. Experimental results for the OLEDs with nine different nanomesh dimensions were compared with simulation results to elucidate the dimensional effect of the nanomesh on the light out-coupling behavior of the devices. The Ag-nanomesh electrodes did not only increase the transparency of the Ag electrode due to periodic nanoholes but also enhanced light extraction from surface plasmon polaritons and substrate/waveguide modes in the devices. The simulation results show similar trends with the experimental results for the optical transmittance, emission spectrum, and efficiency enhancement. With a nanomesh dimension of 480 nm period and 50% fill factor, the OLEDs with the Ag-nanomesh electrode showed 1.66 times higher efficiency than those with a planar Ag electrode did. Using our validated simulation, we construct an external quantum efficiency map in full ranges of the period and fill factor of the Ag-nanomesh electrode to find out the optimum nanomesh dimension.
© 2016 Optical Society of America
In recent years, intensive research and development have been conducted on organic light-emitting diodes (OLEDs) for the realization of high efficiency displays and solid-state lightings. However, a low light out-coupling efficiency limits the overall efficiency of OLEDs because only a small fraction (typically 20%) of light can be coupled out of the multilayer structure due to a large difference in refractive index between air, glass, and organic layers . The rest of light is lost due to the confinement in glass substrate or organic layers as well as the excitation to surface plasmon polaritons (SPP) which is a guided electromagnetic surface waves traveling along the interface between the organic dielectric material and metal layer [2–4]. In order to extract light in the organic layers, i.e. waveguide modes, several techniques was proposed such as the utilization of micro- or nanostructures or insertion of low refractive-index material [5–9]. As a nanostructure to extract light in the waveguide mode, a silver (Ag)-nanomesh has been utilized as a transparent electrode for OLEDs [9–11]. Unlike other alternative transparent electrodes such as Ag-nanowire networks or metal grids, the periodic subwavelength nanostructures of the Ag-nanomesh provides a surface plasmonic effect to increase or decrease the light transmission at certain wavelength ranges. When nanometer-thin OLEDs on the nanostructured anode with nanohole arrays, corrugated organic and cathode metal layers can be formed due to the perforated anode nanostructure. The corrugated metal cathode can alter the SPP wave vector on a planar cathode with the corrugation wave vector to result in the coupling out the SPP modes bound to the metal cathode. The light extraction of trapped SPP modes from OLEDs has been reported in the literature [12–14].
In this study, we fabricated Ag-nanomesh structures as the transparent anode of OLEDs by a colloidal lithography using polystyrene (PS) nanoparticles. The dimension of the Ag-nanomesh electrode is characterized by the period of the nanohole array and fill factor (FF) which is defined by the ratio of the total area of nanoholes to the total electrode area. The dimensions were controlled by the initial diameter of the PS nanospheres and final diameters after partial etching of the nanospheres. Blue OLEDs were fabricated on the Ag-nanomesh electrodes with different dimensions. The efficiencies and emission spectra of the OLEDs were measured experimentally and compared with simulation results to elucidate the dimensional effect of the nanomesh on the light out-coupling behavior of the devices. Using the simulation results, a full external efficiency map of OLEDs was presented in full ranges of the dimensions of the Ag-nanomesh structure to search an optimal dimension producing the highest device efficiency. Even though the colloidal lithography using PS nanoparticles is useful for proving the light out-coupling of OLEDs, it is difficult to apply directly to mass production. For the mass production of Ag-nanomesh electrode for real applications, other fabrication methods such as nanoimprint or deep UV lithography should be implemented.
2. Fabrication of Ag-nanomesh electrode and OLEDs
With a glass substrate of 2.5 × 2.5cm2 size, the surface was cleaned with acetone and methanol in a sonicated bath for 10 min each and blow-dried with nitrogen. The substrate was additionally treated with the piranha solution for 30 min to have the surface hydrophilic. In order to prepare periodic Ag-nanomesh structures on the substrate surface, we utilized a colloidal lithography using PS nanospheres with three different diameters of 210, 400, and 480 nm. A schematic diagram of the preparation method is shown in Fig. 1. At first, a PS colloidal solution was spin-coated onto a 15 × 15 cm2 size glass substrate using the PS size-dependent spin speeds of 500 rpm for 200 nm PS, 1,000 rpm for 400 nm PS, and 2,000 rpm for 480 nm PS. A surfactant (2% dodecyl sodium sulfate solution, Sigma-Aldrich) was dropped on the nanosphere floating solution to make a close-packed monolayer of the PS. The packed PS monolayer was lifted up with a pre-cleaned 2.5 × 2.5 cm2 glass substrate and dried at 60°C. The PS nanospheres were etched in an oxygen plasma to control the size. A 20 nm-thick Ag layer was then evaporated at the speed of 1 Å/s in vacuum (base pressure: 1.2 × 10−6 torr). Finally, The PS nanospheres were removed by an adhesive tape (3M) to complete periodic Ag-nanomesh structures. In Fig. 1(h), the period (P) depends on the initial size of the PS nanospheres while the diameter (D) can be controlled by the etch duration.
Figures 2(a) and 2(b) show the scanning electron microscopy (SEM) images before and after the plasma etch of the close-packed PS nanosphere monolayer, respectively. After the Ag deposition and removal of the PS nanosphere, the periodic Ag-nanomesh electrode was obtained as shown in Fig. 2(c). The removal of the PS nanosphere can be done by ultra-sonification in a solvent  or using an adhesive tape. In this study, we removed the nanosphere with the adhesive tape because the ultra-sonification could cause the Ag-surface damage. Figures 2(d)-2(f) represent the tiled images and atomic force microscopy image of the fabricated Ag-nanomesh electrode. The angle between the Ag-nanomesh and glass substrate at the surface was around 135°.
As shown in Fig. 3(a), the nanohole diameter decreases at the average rate of 50 nm/min regardless of the initial nanosphere size as the etch time increases. The decrease in the nanohole diameter results in the increase of FF, which is defined as the ratio of nanohole array area to total area. For this study, we selected three different FF of 20, 50, and 65% for the three different nanosphere sizes. As an example, Fig. 3(c) shows the average sheet resistance of the nanohole array with 210 nm period as a function of FF. As the FF decreases, it converges to the sheet resistance of a planar Ag film of 1.89 Ω/square. In Fig. 3(d), the optical transmittance of the nanohole array electrodes was around 70% at 550 nm wavelength while that of the planar Ag electrode was 33%. The higher transmittance is due to the nanohole perforation of the planar Ag electrode. The transmittance spectra of the Ag-nanomesh are not monotonic unlike that of the planar Ag electrode because of the transmittance enhancement effect due to surface plasmon polaritons (SPPs) on metal nanomeshes [15–17].
We fabricated bottom-emission OLEDs on the perforated Ag-nanomesh anode by depositing organic layers and aluminum (Al) cathode as shown in Fig. 4. The period of the nanomesh anode were varied to 480, 400, and 210 nm with the different fill factor (FF) of 20, 50, and 65%. The characteristics of the OLEDs were compared with the reference device fabricated on the planar Ag anode. As a hole injection and transport materials, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) and N,N’-di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) were utilized, respectively. The 4,4’,4”-tris(carbazol-9-yl)-triphenylamine (TCTA) was the electron blocking material and LG201 (LG Chem. Ltd) doped with 50% 8-hydroxy-quinolato lithium (Liq) was the electron transport materials. As the blue-emitting materials, 2-methyl-9,10-di(2-naphthyl) anthracene (MADN) and 1,6-bis(N-phenyl-p-CN-phenylamino)-pyrenes (Pyrene-CN) were utilized as the host and dopant. The structure of OLED was Ag-nanomesh anode (20 nm)/HAT-CN (30 nm)/NPB (80 nm)/TCTA (20 nm)/MADN:Pyrene-CN (5%, 20nm)/LG201:Liq (50%, 38.5 nm)/Liq (1.5 nm)/Al (100 nm).
Figures 5(a), 5(c), and 5(e) show the characteristics of OLEDs with different Ag nanomesh periods (P 480 nm, P 400 nm, P 210 nm) at the fixed FF of 50%. On the other hand, Figs. 5(b), 5(d), and 5(f) explain the effect of FF on the OLED characteristics by changing FF (65%, 50%, 20%) with the fixed period of 480 nm. The current density-voltage-luminance curves for OLEDs with different Ag-nanomesh structures were compared with that with the planar Ag electrode (the reference OLED) in Figs. 5(a) and 5(b). The curves were measured at the direction normal to the emitting surface. The leakage current density of OLEDs with the Ag nanomesh electrodes was lower than that of the planar Ag electrode. It means that the operational stability of the OLEDs with the Ag-nanomesh electrodes is similar to that of the referece OLED. In experiments, the OLEDs with the Ag-nanomesh electrodes were working stably in a number of repeated measurements. In all cases, the higher luminance was observed for the OLEDs with Ag-nanomesh electrodes. As shown in Figs. 5(c) and 5(d), the power efficiency of OLEDs with Ag-nanomesh electrodes was found higher than that with the planar Ag electrode except the case of P 210 nm. The higher power efficiency is attributed to the enhanced light extraction due to the periodic nanomesh structure. Figures 5(e) and 5(f) show the angular dependence of the external quantum efficiency (EQE) of OLEDs. This measurement was done at the constant current density which corresponds to the luminance of 1,000 cd/m2 of the reference OLED at the normal direction to the surface. The reference OLED shows the strong angular dependence of emission (or EQE) due to the characteristic microcavity effect. On the other hand, the OLEDs with the Ag-nanomesh electrodes exhibit weaker angular dependence of EQE due to the reduced microcavity effect. Using the total EQE calculated by integrating the angular EQE with angle, the EQE enhancement factor of the OLEDs with the Ag-nanomesh electrodes was compared to that of the reference OLED. At the fixed FF of 50%, the enhancement factor was 1.66, 1.59, and 1.11 for P 480 nm, P 400 nm, and P 210 nm, respectively. The factor increases with the Ag-nanomesh period. Varing the FF with the fixed period of P 480 nm, the factor was 1.53, 1.66, and 1.52 for the FF of 65%, 50%, and 20%, respectively. Among the nine different Ag-nanomesh structures tested in this study, the highest EQE was obtained for the OLED with 480 nm period and 50% FF.
3. Simulation model verification and validation by comparion with experimental results
Commercially available finite difference time domain (FDTD) software (FDTD solutions, Lumerical) was utilized to simulate optical characteristics of OLEDs fabricated in this study. The simulation was done in 3 dimension to enhance the accuracy. The simulated area and height was 8 × 8 μm2 in x-y plane and 1 μm (Al cathode 100 nm, organics 195 nm, Ag anode 20 nm, and thick glass substrate with a perfectly matched layer) in z direction, respectively. Three dipole sources were placed per a nanohole at (0,0), (P,0), and (0,P) coordinates in x-y plane, where P is the nanohole period. In z direction, the dipole source was placed in the middle of the emission layer. A dipole has a spectrum at the center wavelength of 460 nm with a full-width-half-maximum of 40 nm.
Using the 3D optical simulation, the effect of Ag-nanomesh electrode was investigated on the energy flux density, emission peaks, and angular distribution of OLEDs by comparing them with those of the reference OLED. The calculated energy flux density was shown in Fig. 6 for the reference OLED and OLED with an Ag-nanomesh electrode. For the reference OLED, a large amount of energy is radiated to far field from x- and y-oriented horizontal dipoles while most of energy from z-oriented dipoles is confined as the SPP at the cathode. On the contrary, more light is extracted from all directions of dipoles in OLED with Ag-nanomesh electrode.
Calculated emission spectra were compared with experimentally obtained emission spectra for the reference OLED and OLED with an Ag-nanomesh electrode. Because the reference OLED is a conventional microcavity OLED with a planar Ag anode, the emission spectra show a strong angular dependence due to the microcavity effect as shown in Fig. 7(a). The simulation [Fig. 7(b)] predicts the experimental emission spectra well. For an OLED with a Ag-nanomesh electrode (P 480 nm, FF 65%), the measured and simulated emission spectra were shown in Figs. 7(c) and 7(d). The emission spectra were broadened and the angular dependence were weakened when compared with those in Figs. 7(a) and 7(b). It is because the microcavity effect was reduced due to the nanohole perforation of the Ag anode. The calculated spectra of the OLED with the Ag-nanomesh electrode explain the spectrum broadening and reduction of the angular dependence, but the relative intensity and shape of the spectra exhibit considerable errors when compared with the measured spectra. The error could be caused by the fact that the periodicity of experimentally prepared Ag nanomesh was not perfect because of the polycrystal grains in the Ag layer.
In order to investigate the cause of the angular broadening of the emission spectra, we calculated the far-field intensity distributions depending on the emission wavelength and angle. Figure 8(a) represents the light outcoupling and confinement in a planar microcavity OLED. There are light losses due to two SPP modes between metal (Al cathode and Ag anode) and organic layers, and the substrate mode between glass and air. Figure 8(b) shows light loss in an OLED on Ag-nanomesh electrode. Due to the corrugated cathode, the light scattering of SPP may occur to reduce the SPP light loss. At the same time, the Ag-nanomesh reduces the SPP propagation and the light transmission through the nanoholes in the Ag-nanomesh electrode reduces the microcavity effect to weaken the angular dependence of emission. Figures 8(c)-8(l) show far-field intensity distribution of light emission from OLEDs. Deeper color represents stronger light intensity. The simulation result of the reference planar cavity shown in Fig. 8(c) shows strong light intensity at near vertical direction to the emission surface and the intensity decreases continuously with increasing angle. On the contrary, Figs. 8(d)-8(l) for the OLEDs with the Ag-nanomesh electrodes show the dispersed and descrete intensity distribution in enlarged regions of the angle and wavelength. This results coincide with the experimental data of Figs. 5(e) and 5(f), i.e, the reduced angular dependence of the emission for the OLEDs with Ag-nanomesh electrodes. The simulation results of Figs. 8(d)-8(l) show dispersive light-intensity features unlike Fig. 8(c), which were originated from the scattering of SPP due to the different periods of the corrugation. The black solid and dashed lines in Figs. 8(d)-8(l) represent a few characteristic SPP and substrate-mode diffraction patterns, respectively. Other lines seem to be originated from the waveguide-mode diffraction. Since the features depend on the corrugation periodicity, each of Figs. 8(d)-8(f), Figs. 8(g)-8(i), and Figs. 8(j)-8(l) shows their characteristic diffraction patterns. The light-intensity pattern from the OLEDs with the same nanomesh period shows the reduced intensity dispersion and thus approaches to that of the planar device as the FF decreases. As the nanohole size decreases with the same FF [Figs. 8(f), 8(i), and 8(l)], the light-intensity pattern approaches to that of the planar device since the areas of the Ag anode increases so that the SPP loss increases and becomes similar to the planar microcavity device. On the other hand, the light loss from the substrate mode increases as the size of nanoholes increases due to the increased interface area between organic and glass. Therefore, it is expected that there is an optimum FF corresponding to each nanomesh peroid.
For the OLED with P 480 nm and FF 65% as an example, the light-dispersion patterns were assigned to the SPP or substrate mode. Using the Bragg grating equation, we calculated the angle extracted from the substrate mode.
The diffraction angle extracted from the SPP mode was calculated with the following equation where m is the diffraction order.
For the calculation, εAg and εAl were −8 and −25.5 at 460 nm wavelength and norg was 1.95. The calculated diffraction order m was shown in Fig. 9 and Table 1. The dispersion patterns with the maximum 3rd order was included in the light zone and a few among them was confined in the glass substrate due to the total internal reflection (TIR) at angles higher than the critical angle. Based on the calculation, we assigned the diffraction peaks in Fig. 9(b) to the dispersion relations for the SPP and substrate modes. Other unassigned peaks at 0° and ± 17° seem to be originated from the waveguide-mode diffraction. Using the Bragg equation with the refractive index of the organic material, θair is obtained at ± 2°, ± 16°, ± 57°, and ± 68° for the 460 nm wavelength emission. These diffractions should exist in the far-field intensity pattern in Fig. 9(a) even though they were unassigned. The Ag-nanomesh electrode is periodic in two directions (x direction in which the period P is defined; and y direction perpendicular to the x direction). Therefore, the diffraction order should be a combination of order numbers in the two directions. However, the calculation shows that the major periodicity is in the x direction and the y direction has little effect on the SPP diffraction patterns.
Finally, we calculated the EQE enhancement factors for the light extraction with the Ag-nanomesh electrodes and compared them with the experimental values. The enhanement factor was defined as the ratio of the EQE of the corrugated devices to that of the planar device. The experimental EQE was obtained by the integration of the angularly measured EQEs. The EQE calculation was done using the following equation.
Here, x, y, and z are the parallel plane to the axis and + and – means the directions. The calculated and measured enhancement factors are shown in Fig. 10 for the OLEDs with different Ag-nanomesh electrodes. Even though the absolute values of the calculated enhancement factor were found lower than the measured values, the trends of the values were the same when varying FF and P at a fixed P and FF, respectively. Among the nine different structures that we have tested, the best structure was P 480 nm at FF 50% and showed 166% increase in EQE due to the enhanced light extraction.
4. Simulated EQE map for OLEDs with Ag-nanomesh electrodes
We showed that our simulated performance of OLEDs with nine different geometries of Ag-nanomesh electrodes agrees well with the experimentally measured performance. As shown in Fig. 10(b), the EQE enhancement increases as the period P increases. To further find out the optimum P value, we simulated the performance of the corrugated OLEDs with Ag-nanomesh electrodes at additional P values of 600, 800, and 1,000 nm. A simulated EQE map is shown in Fig. 11 for the period ranging from 210 nm to 1,000 nm and the fill factor ranging from 20% to 65%. As shown in the figure, the highest EQE was obtained in the P region from 420 nm to 650 nm or in the FF region from 35% to 65%. The highest EQE region covers the widest range in FF when P is 600 nm. The maximum EQE obtained in this experiment is located in the highest EQE region in the figure. The simulated EQE map for the blue emission may not be applicable to other color emissions such as green and red for full color display applications. Even though it is possible to obtain the highest EQE condition of P and FF for green and red emissions, it is extremely difficult to form different Ag-nanomesh structures for separate pixels of blue, green, and red for the full color display applications. Instead, it is more reasonable to utilize the optimum Ag-nanomesh electrode for blue OLEDs and change colors from blue to green or red using color changing phosphors or dyes. In this respect, we think that the optimization of Ag-nanomesh electrodes for the blue emission is meaningful for the display or lighting applications.
Light out-coupling of OLEDs with Ag-nanomesh electrodes was studied. The OLEDs with nine different nanomesh dimensions were fabricated and their performance was compared with simulation results to elucidate the dimensional effect of the nanomesh on the light out-coupling behavior of the devices. We proved that our simulation results show similar trends with the experimental results for the optical transmittance, emission spectrum, and EQE enhancement. The maximum EQE enhancement ratio of 1.66 was obtained for OLEDs with the Ag-nanomesh dimension of P 480 nm and FF 50%. Using the proven simulation, we constructed an EQE map in full ranges of the period and fill factor of the Ag-nanomesh electrode and showed that the Ag-nanomesh of P 480 nm and FF 50% was the optimum dimension for the corrugated OLED.
Ministry of Science, ICT and Future Planning (MSIP) under the Information Technology Research Center (ITRC) support program (IITP-2016-H8501-16-1009) supervised by the Institute for Information & communications Technology Promotion (IITP), South Korea.
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