An effective method for enhancing the light outcoupling efficiency from top-emitting organic light-emitting diodes (TEOLEDs) with a nano-sized stochastic texture surface (NSTS) is suggested. The broadly distributed pitch and the randomly sized of islands in the NSTS enable the photons that are otherwise trapped to be emitted over the broad emission wavelength range. The NSTS-embedded TEOLEDs have wide angular-dependent emission characteristics and an enhanced external quantum efficiency (EQE). Theoretical and full-wave optical calculations were performed to understand the mechanisms of the efficiency enhancement. Optimized TEOLEDs achieved a 32% EQE enhancement compared with the reference devices without the NSTS.
© 2014 Optical Society of America
Organic light-emitting diodes (OLEDs) have been developed as an important candidate for flat panel display and lighting applications [1–3] since they have various advantages over other displays like liquid crystal display (LCD) such as the fast response time, self-emission and low energy consumption . The top-emitting OLEDs (TEOLEDs) emit light through the top electrode and can provide a larger aperture ratio of the display than that of the bottom-emitting OLEDs (BEOLEDs) because of the planarized backplane that incorporates the driving circuit . The total external quantum efficiency (EQE), which is the product of the internal quantum efficiency (IQE) and the outcoupling efficiency (ηout), is regarded as one of the critical device parameters because it directly describes the amount of emitted photons per consumed electrical energy. An IQE of nearly 100% could be achieved using well-designed phosphorescent organic materials [6–8]. However, the layered structure of OLEDs causes a low outcoupling efficiency since generated photons become trapped in waveguided modes and are wasted in the excitation of surface plasmon polaritons (SPPs) . To overcome these obstacles, many methods have been suggested to increase the outcoupling efficiency. For example, corrugated structures [10–12] or the use of a diffuser at air/substrate interfaces  can be inserted to extract the trapped photons. To reduce the waveguided modes, nanostructures, nanoparticles and grids can be integrated into the electrode [14–19]. Although these provisions could provide enhancement the outcoupling efficiency, most of them accompany several disadvantages such as the high cost, use of complicated fabrication techniques, changes in the electrical characteristics, spoiling of the color purity or distortion of the angular spectral emission characteristics. All of the desired device performance specifications, such as efficiencies, color purity and angular spectral emission characteristics, complicate the design of the TEOLEDs since these parameters are difficult to satisfy simultaneously in a single device architecture . First-order resonance microcavity OLEDs have been widely used for efficiency enhancement in previous studies [5–11]. Microcavity OLEDs have short cavity lengths that can cause low color purity or a low production yield . This may result in production of high cost OLED televisions [21–23].
We propose a method that can be used to improve the light extraction in TEOLEDs. In addition, the angular emission characteristics were closer to Lambertian emission characteristics than that of the reference device without spoiling the color purity. The proposed TEOLEDs used a nano-sized stochastic texture surface (NSTS) fabricated by thermally annealing the Ag thin film. The NSTS is positioned between the substrate and anode because it is structurally continuous and can support multi-stacked OLEDs. We performed theoretical and full-wave electromagnetic calculations to understand the underlying mechanisms and explain the experimental results. Most importantly, the suggested method is suitable for practical mass production because of the simple processing method, low cost, applicability to large-area fabrication and the ease with which it can be integrated into the TEOLED platform.
2. Experimental methods
2.1 NSTS formation
Figure 1 shows the process used to form the NSTS. First, an Ag film of 8 nm was deposited on the glass substrate using a radio frequency (RF) magnetron sputtering system. If the surface energy of the Ag was higher than that of the substrate, Ag dewetting was initiated. Any structural irregularity on the surface resulted in an irregular dewetting pattern. In this study, dewetting was facilitated via a process of thermal annealing on the surface of the Ag thin film. To form the stochastically distributed Ag nanostructure, the Ag thin film was heated using the rapid thermal annealing (RTA) system at a temperature of 250 °C for 2 min at atmospheric pressure. This process formed stochastic nano-sized Ag islands as shown in the second step of Fig. 1. The nano-sized islands form the NSTS which was formed over the entire Ag thin film deposition area on the glass.
2.2 OLED fabrication
A green phosphorescent OLED was fabricated using thermal evaporation on glass substrates that were pre-cleaned in acetone, methanol and deionized water. The base pressure was 4 × 10−4 Pa. Reference devices were fabricated using the following configuration: Al anode (120 nm)/di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):MoO3 (25%, 150 nm)/TAPC (20 nm)/8 wt % tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) doped with 4,4'-bis(carbazol-9-yl)biphenyl (CBP, 30 nm)/ 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB, 40 nm)/LiF (0.5 nm)/Al (1 nm)/Ag cathode (15 nm)/TAPC (45 nm). The 150 nm thick MoO3-doped TAPC hole injection layer (HIL) was used to achieve the second-order resonance condition without a significant voltage drop. The CBP was used as an emissive layer (EML). For the electron transport layer (ETL), we used a layer of TmPyPb with a thickness of 40 nm. A 0.5 nm thick layer of LiF was used as a hole blocking layer to confine the excitons within the EML. The TAPC layer above the Ag cathode was used for the capping layer. Figures 2(a) and 2(b) show the schematic illustrations of the proposed OLED device architecture and a three-dimensional atomic force microscope (AFM) image of the surface of the NSTS on the glass substrate, respectively. The nano-sized Ag islands had stochastic properties in both spatial distribution and size. The measured diameter, height and pitch of the structures using the AFM were distributed around 100–200 nm, 30–50 nm and 200–600 nm, respectively.
The devices emitted a green color, with a peak wavelength of 520 nm. The OLEDs were characterized by recording the current‒voltage‒luminance (J‒V‒L) characteristics. The current‒voltage characteristics of the OLEDs were recorded using a programmable voltage source (Keithley, 236). At the same time, the emission spectra were recorded using a spectroradiometer (Minolta, CS-1000) and the device luminance was measured using a calibrated Si photodiode (Hamamastu S5227-1010BQ). The angular emission characteristics were collected using an optical fiber, a monochrometer combined with a photomultiplier tube as the detector and a rotation stage. The overall active area of the devices was 2 × 2 mm. All of the measurements were performed in air at room temperature.
Instead of using the first-order resonance condition, we chose to use the second-order resonance condition as the reference because of the high color purity and high production yield. Improvement of the color purity and the production yield can be easily achieved by increasing the microcavity length [20, 21]. A low OLED production yield is a critical issue in industry and is closely connected to the high cost of large-scale OLED televisions [21–23].
3. Results and discussion
Figure 3(a) shows the current density‒voltage‒luminance (J‒V‒L) characteristics of the OLEDs as a function of the driving voltage. These values were measured in the forward direction. Measurements were collected using devices with both the planar and the NSTS-based structure. The current density of the devices with the NSTS and the planar devices are shown in the red with square and black with circle curves, respectively. The turn-on voltages (~3 V) of the NSTS-based OLEDs were lower than those of the reference OLEDs. The textured OLED structure is an effective method to reduce the operating voltage [15,16]. The electric field was stronger in the NSTS-based devices since the organic layer thickness was reduced around the interface region of the multi-layered stacks between the peaks and valleys of the NSTS. The stronger electric field causes a larger current and a lower operating voltage. The NSTS-based OLEDs have a higher current density at the same bias voltage as the reference OLEDs. The NSTS-based devices have about 1.15-fold enhancement of the luminance compared with the reference devices at forward direction during operation.
The angular dependence of the electroluminescence (EL) intensity from the devices with the NSTS and the reference devices showed interesting characteristics that NSTS-based OLEDs had emission characteristics that were closer to being Lambertian than the reference OLEDs. The broad distribution of the grating pitch and the randomly sized islands in the NSTS enabled outcoupling enhancement over all oblique angles. One of the most effective methods to enhance the outcoupling is by widening the angular dependence of the light intensity . Planar top-emitting OLEDs have a forward directed angular dependence of the light intensity compared with the proposed structure because of strong microcavity effects . Previous studies that used periodic corrugated structures to enhance the device efficiency inevitably suffered from distorted angular emission characteristics [10–12]. In Fig. 3(b), we show the EQE versus luminance of the devices. An integrating hemisphere method was used for the EQE measurements. Independent measurements verified the angular distribution of the EL and the luminance. Our proposed structures exhibited an EQE enhancement of 32% over the reference devices at a current density of 10 mA/cm2.
Our structure displayed a widely distributed angular dependence of the EL intensity. Figures 3(c) and 3(d) show the normalized EL spectra as a function of the angle for observation angles of 0°, 20°, 40° and 60°. The applied current density was fixed at 10 mA/cm2 during measuring the angle dependent EL spectra. Compared with Fig. 3(c), the peak light intensities of Fig. 3(d) decreased slowly as the observation angle increased. Also, the wide distribution of the grating pitch and the randomly sized islands in the NSTS allowed the emission with broad wavelength to be outcoupled without any distortion of the angular emission characteristics.
Generally, only about 20% of the generated photons can be emitted into the air in the conventional planar OLEDs because 80% are trapped as waveguided modes in the organic layers and the SPP modes associated with the metallic electrode/organic interface. To understand the enhancement of the extraction of the trapped modes with NSTS, we used the Bragg diffraction relation with a sub-wavelength structure. According to Bragg diffraction, the in-plane wave vector components after diffraction can be expressed using the following equationEq. (1) as followsEqs. (1) and (2), we can predict the relationship between the outcoupling angle and the grating pitch. In Fig. 4(a), the characteristics of the grating pitch in the NSTS were obtained using a Fast Fourier transform (FFT) of the power spectra and using the AFM images shown in Fig. 2(b). The power spectra of the NSTS show that the pitch range was from 200 to 600 nm and the peak intensity of the grating pitch was 270 nm. In Fig. 4(b), the outcoupling angle of the emission is plotted as a function of the grating pitch using an order of m = 1 and m = 2. The angle of 0° is the normal and 90° is in the direction parallel to the OLED layer. In Fig. 4(c), the relationship between the emission wavelength and the grating pitch is plotted. The grating pitch of the NSTS has a peak at 270 nm as depicted by vertical dotted line. This corresponds to an emission wavelength of 430 nm, which is far away from the main emission peak of 520 nm. However, the broadly distributed power spectra in Fig. 4(a) cover the entire emission wavelength range of the first order Bragg diffraction mode and the part of the range of the second-order Bragg diffraction mode. This means that the change in the angular-dependent emission characteristics can be attributed to contributions from both Bragg diffractions of first and second order modes, with most of the contribution from the first order mode. From Figs. 4(a), 4(b) and 4(c), the diffraction angle of the main emission is expected to be in a specific direction because the peak intensity of the grating pitch of 270 nm gives an outcoupling angle of 18°. However, the emitted light from the devices covers all angles because there is a broad distribution of the grating pitch and the island sizes in the NSTS, which means that the grating vector cover a wide direction.
To analyze the underlying operating mechanism and emitted power of the devices, we used full-wave electromagnetic radiation simulations. The simulations were performed using the finite element method (FEM) since it is capable of handling a complex geometry [25, 26]. The simulations are the space discretization of the wave equations that are derived from Maxwell’s equations. We can solve the governing equation for the electric and magnetic fields using the following expressionsFig. 4(d), the outcoupling efficiency and enhancement factor vary as the pitch is increased from 0 nm (planar device) to 550 nm while fixing a height of 40 nm for all pitches. The outcoupling efficiency at planar device shows 22%. The enhancement factor is defined as the ratio of the outcoupling efficiency in the nanostructure embedded device to that in the planar device. In simulation study, grating pitch was only discussed as a variable and remained parameters were fixed for all pitches. Although diameter and height of the NSTS could also be influenced to the outcoupling efficiency, pitch contributes saliently among 3 parameters. When the grating pitch of 270 nm was applied in the simulation, the enhancement factor was 1.41, while the enhancement factor was 1.32 in the experiment. This deviation may be resulted from neglecting the Ohmic losses in the driving circuit and loss through SPP excitation. The outcoupling efficiency has a peak value at a grating pitch of 350 nm, which is similar to the value of the grating pitch of 325 nm that produces an emission wavelength of 520 nm using first order diffraction as shown in Fig. 4(c).
The de-trapping of the trapped photons can be also observed by the electric field distribution in the planar and corrugated structures, as shown in Figs. 5(a)and 5(b). The dipole point source is located in the EML. In these simulation studies, the dimensions of grating pitch and height were 400 and 40 nm, respectively. In Fig. 5(b), a distorted, enhanced electric field distribution would be observed when a periodic nanostructure surface was applied to the simulation especially around the cathode/capping layer of the device and the air interface.
The introduction of NSTS in TEOLED was proved to help the extraction of trapped photons as well as widen the angular emission characteristics. An optimized TEOLED gave a 32% EQE enhancement compared with that of the conventional reference device. The underlying mechanism was explained theoretically as well as numerically calculating the electromagnetic fields of a radiating dipole positioned in the EML. The proposed architecture can be easily integrated into the TEOLED device with large sizes because of the easiness of implementation.
This work was supported by the Samsung Display Company. We were also supported by the Industrial Strategic Technology Development Program (no. 10042412) funded by the Ministry of Trade, Industry and Energy of Korea.
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