I. INTRODUCTION

Organic light emitting diode (OLED) has been regarded as the most promising display device because thin, light, and flexible display can be realized ^[1-3]. Although the internal quantum efficiency (${\eta}$IQE) of recent OLED was improved almost at 100 %, its external quantum efficiency (${\eta}$EQE) has been no more than 20 % without any light extraction structure ^[4-6]. The limitation can be resulted from the light trapping in substrate caused by the total reflection at the interface between glass and air and the wave-guiding effect caused by a much higher refractive index of organic and transparent electrode layer than substrate ^[7-9]. The surface plasmon polariton (SPP) mode is also considered as an attenuation factor of light at the interface between organic layer and cathode electrode ^[10-14]. For the enhancement of external quantum efficiency, micro-lens array and micro particle scattering layer have been proposed to overcome the total internal reflection and wave-guided light has been extracted by the modulation of photon states through photonic crystals and optical gratings ^[15-17]. Moreover, the wave-guided light has also been scattered through the inserted corrugated layer in the waveguide structure and the addition of nano particle scattering film between transparent anode electrode and substrate ^[18-20]. Recently, Ag inserted ITO/Ag/ITO layer has been used as anode electrode of OLED instead of ITO because of the high conductivity of Ag. From the inserted Ag layer and cathode metal layer, micro-cavity effect can be also expected as a result of laser effect ^[21,22]. However, the contact of Ag to dielectric layer can bring about the reduction in light extraction efficiency caused by SPP mode.

In this study, for an OLED fabricated with an ITO/Ag/ITO anode, a finite difference time domain (FDTD) simulation was carried out to investigate the effect of the inserted Ag thickness on the light extraction efficiency of OLED. 2-dimensional (2D) OLED structures were proposed for the calculation of the light extraction efficiency and various polar-plots for far-field intensities were drawn by the FDTD simulations on 3-dimensional (3D) OLED structures.

II. EXPERIMENTAL DETAILS

For the comparison of ITO/Ag/ITO multilayer and ITO layer in conductivity and transmittance, ITO and Ag was deposited on the soda-lime glass substrate using pulsed-DC magnetron sputtering and radio-frequency (RF) sputtering, respectively. Transmittance and sheet resistance of ITO/Ag/ITO and ITO was measured using an UV-vis spectrophotometer (Perkin Elmer-Lambda 35) and a 4-point probe (CMT-SR2000N, Advanced Instrument Technology), respectively. FDTD simulation was carried out using a wave-optics based Rsoft® simulator (Synopsys) solving linear Maxwell equations.

An OLED device designed for the simulation is composed of ITO/Ag/ITO anode electrode, 40 nm hole-transporting layer (HTL), 60 nm light-emitting layer (EML), 5 nm electron injection layer (EIL), and 50 nm Al cathode electrode. As the refractive index of material is dependent on the wavelength of light, the refractive indices of ITO and Ag were obtained through the spectroscopic ellipsometry of the deposited ITO and Ag layers. For the refractive indices of materials including all organics, ITO and soda-lime glass, the nk data measured using the spectroscopic ellipsometry were applied to the real (n) and complex (k) parts of all materials in the design for simulations. However, because the simulations were performed at the fixed wavelength of 520 nm, the n and k values corresponding to a wavelength of 520 nm were used in the simulations.

III. RESULTS AND DISCUSSION

Fig. 1 shows the transmittance measured for ITO single layer (47.5 nm), Ag single layer (10,5 nm), and ITO/Ag/ITO multilayer (39.2/10.7/39.2 nm) deposited on soda-lime glass substrates. In a visible spectral range above 400 nm, Ag single layer showed a lower transmittance than those of ITO and ITO/Ag/ITO. ITO/Ag/ITO showed a higher transmittance than that of the single ITO for a range from 420 to 650 nm despite its much higher ITO thickness of 78.4 nm. On the other hand, the sheet resistance of ITO/Ag/ITO and ITO was about 7.1 ${\Omega}$/${\square}$ and 172 ${\Omega}$/${\square}$, respectively. The results showed that ITO/Ag/ITO multilayer has similar transmittance to ITO despite its much-advanced conductivity. The transmittance by UV-vis spectro-photometry does not consider the attenuation by a total reflection because the transmittance is measured for a monochromic light normally incident to thin film. Therefore, the transmittance is expected to have a margin of error with much higher value compared to the light extraction ratio because a natural light generated in OLED EML layer is attenuated due to several loss effects when it is emitted through the adjacent OLED layers.

Fig. 2 shows a 2D CAD structure designed for the optimal simulation of the bottom emission OLED with ITO/Ag/ITO anode. On the ITO/Ag/ITO anode, HTL (40~nm)/ EML (60 nm)/ EIL (5 nm)/Al cathode (50 nm) structure was sequentially designed, and an LED utility was established at the midpoint of EML region for the generation of point source of light. For a development of coherent light, 3 dipole point sources were established and each light source vibrates in x, y, and z direction, respectively. The result is done by the addition of 3 simulation results calculated in 3 different vibration directions. Perfectly matched layer (PML) boundary condition was applied to absorb all the reflected lights at the boundaries. The size of simulation domain was set to 5 ${\mu}$m in x-direction and 1.42 ${\mu}$m in z-direction in which light is propagated. The wavelength of the light from a light source was set to 520 nm.

Fig. 3 shows the simulation results of the light extraction ratio depending on the Ag thickness for the OLED in Fig. 2. The thickness of top and bottom ITO in the ITO/Ag/ITO anode structure was fixed to 50 nm. The light extraction ratio decreased from 30 % to 12 % when the Ag thickness increased from 10 nm to 30 nm, as shown in the figure.

For each Ag thickness in Fig. 2, the increasing trends in the light extraction ratio depending on the simulation domain size were investigated as shown in Fig. 4. It is possible to divide a simulation domain into multiple domains. The number of divided domains is defined as ‘monitor num’ and the monitor num was set to 11 in this simulation. After the light extraction ratios were calculated in a simulation domain, the process was repeated as the monitor num was increased from 1 to 11. All the results were presented as a function of monitor num. From Fig. 4, the light extraction ratio was increased depending on the monitor num and was almost saturated above the monitor num of 7. Because the light emitted from a point source is spherically spread, the lights spread more obliquely to the ITO/Ag/ITO layer are not extracted owing to the total internal reflection as the monitor num increases. That is, the saturation of light extraction ratio above the monitor num of 7 is expected to be resulted from the wave-guide mode effect that the mode obliquely spread lights than a critical angle are confined in the OLED structure as a result of a total internal reflection. In Fig. 4, all the saturations of light extractions are investigated at the same monitor num irrespective of Ag thickness. It is possible to infer that the OLED structure with ITO/Ag/ITO anode has a constant critical angle regardless of Ag thickness.

The far-field intensity depending on the viewing angle is presented in a polar plot of Fig. 5. For all the Ag thickness of Fig. 3 and 4, similar profiles were obtained. However, the light intensity was diminished for all angles as the Ag thickness was increased. With an increasing angle relative to being vertical with respect to the OLED structure, the light intensity was abruptly decreased. The result is compatible with Fig. 4 as a result of the total internal reflection effect caused by Ag layer. Moreover, as the Ag thickness was increased, the light intensities were diminished for all viewing angles. The reduction in light intensity correspond to the result of Fig. 3.

Fig. 6 shows the x-z cross-sectional electric field distributions of the OLED structure in Fig. 2. The electric fields are developed from a dipole light source vibrating in x-direction. Almost all electromagnetic waves are almost spherically spread in case of the Ag thickness of 10 nm as shown in Fig. 6(a). Whereas Fig. 6(b) shows that the electromagnetic waves are propagating to the parallel direction along Ag/ITO interface for a Ag thickness of 20 nm. With a Ag thickness of 30 nm, it is possible to investigate more electromagnetic waves spreading horizontally at the interface of Ag layer as shown in Fig. 6(c). The SPP mode is expected to be generated at the interface between Ag electrode and dielectric layer such as ITO or OLED organic material layers. The light extraction ratio may be influenced and reduced by the SPP mode.

In the final analysis, for an OLED structure with an ITO/Ag/ITO anode, the light extraction ratio is diminished as a result of the wave-guide mode caused by the total internal reflection when the Ag thickness is increased above 10 nm for an advanced conductivity. With an Ag thickness above 20 nm, the light extraction ratio can be more reduced because of the generation of SPP mode. Surface plasmons are light waves that propagate along the surface of a conductor, usually a metal. SPs are constituted from the resonant interaction between the collective oscillation of free electrons, and the electromagnetic field of the incoming light wave. These light waves are essentially trapped on the surface, resulting in nonradiative losses ^[23,24]. Through this study, it was confirmed that the thinner the Ag electrode, the better it is in terms of light extraction of OLED. This study confirmed that the thinner the Ag electrode, the better it is in terms of light extraction in an OLED. However, in order to optimize the actual OLED device, it is necessary to optimize the Ag thickness with respect to both light extraction in the OLED, and the electrical conductance of the anode. In fact, previous studies have shown the optimal performance is obtained in the vicinity of 10-20 nm for a semi-transparent Ag electrode-based OLED, like the trend in this study ^[25,26].

Fig. 1. Transmittance measured for ITO, Ag, ITO/Ag/ITO using UV-vis spectrophotometry.

Fig. 2. A 2D CAD structure designed for the optical simulation of the bottom emission OLED with ITO/Ag/ITO anode.

Fig. 3. The simulated extraction ratio depending on the Ag thickness for the OLED in Fig. 2.

Fig. 4. The simulated extraction ratios depending on the simulation domain size-monitor num with different Ag thickness for the OLED in Fig. 2.

Fig. 5. The simulated polar plots for a far-field intensity on the viewing angle with different Ag thickness for the 3D OLED structure with ITO/Ag/ITO anode.

Fig. 6. The simulated cross-sectional electric field distributions of the OLED structure with ITO/Ag/ITO anode for a different Ag thickness of (a) 10; (b) 20; (c) 30 nm.

IV. CONCLUSIONS

When an ITO/Ag/ITO multilayer was used as an anode electrode of a bottom emission OLED instead of ITO, the light extraction efficiency of OLED depending on the inserted Ag thickness was obtained and investigated through FDTD simulation. From the simulation results, the reduction in light extraction ratio was analyzed in terms of the total internal reflection caused by the insertion of Ag layer. Moreover, the light extraction ratio was more diminished as a result of SPP mode as increasing Ag thickness. Therefore, the Ag thickness in the OLED structure with an ITO/Ag/ITO anode should be optimized for a compromise between the conductivity of anode electrode and the light extraction efficiency of OLED.