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Abstract
We report a corrugated structure to effectively extract the surface plasmon polaritons (SPP) and waveguiding modes in organic light-emitting diodes (OLEDs). This structure is formed by nano-imprint of blazed gratings. To study the optimum extraction condition in terms of grating pitches, we compare the light extraction efficiency of corrugated OLEDs with three kinds of pitches, showing a 42.00% external quantum efficiency (EQE) enhancement ratio with this internal structure. Due to the transfer of SPP and waveguiding modes into substrate mode, the EQE enhancement ratio can be further pushed to 103.02% by attaching a macrolens. The simulation verifies the experimental results and shows the extraction mechanism of the corrugated structure towards transverse electric (TE) and transverse magnetic (TM) waves. We foresee that this method is able to enhance the optical efficiency of devices for both mass-production OLED lighting and display in a cost-effective way.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic light-emitting diodes (OLEDs) have become a promising candidate for the applications of lighting and display due to their flexibility, low-power consumption and excellent color quality [1,2]. However, the low outcoupling efficiency strongly limits the overall efficiency of OLEDs because only a small fraction of light can penetrate the multilayer structure due to the large refractive index difference between air (n = 1.0), glass (n ≈1.5), and organic layers (n ≈1.7 – 2.0) [3]. Although the internal quantum efficiency has already reached close to 100% using phosphorescent emitters [4], the external efficiency is typically about 35% [5] since the emitted photons are trapped in the substrate and internal modes [6,7]. Depending on where the optical modes occur, the internal modes can be categorized into waveguiding modes that mostly concentrate in the ITO/ organic layers and surface plasmon polaritons (SPPs) that exist as surface waves along the metal-organic interface. Both of them accounts for close to 50% of the total light output in an OLED [8].
To overcome these obstacles, many methods have been suggested to increase the light extraction efficiency. The substrate mode can be extracted effectively using macrolenses, microlens arrays, patterned high-index fillers or light scattering layers [9–18]. The waveguiding modes can be extracted by nanoparticles, photonic crystals, low-index grids, and high-index substrates [19–23]. Although these processes contribute to the enhancement of light extraction efficiency, most of them accompany several disadvantages such as high cost, the use of complicated fabrication techniques, degradation in the electrical characteristics, spoiling of the color purity or distortion of the angular spectral emission [24,25]. To minimize the efficiency loss due to the SPPs, one can either get rid of the metal electrode or use corrugated structures. Especially, corrugated OLEDs fabricated on buckling structures with a semi-random periodicity [26–34] have shown to give significant enhancements in efficiency across the entire visible spectrum. In addition, the reflective subelectrode grid [35] is able to recover the SPPs by transforming them into visible light in top-emitting OLEDs. Although the techniques mentioned above are effective to extract either the internal modes, they still require complicated processes to achieve the outcoupling.
Here, we show that a poly(methyl methacrylate) (PMMA) layer with a blazed grating pattern is effective for the extraction of both SPP and waveguiding modes. The elements on the blazed grating endow the vertical contribution for SPPs, giving rise to extracting the SPPs into the propagating air mode by corrugating the OLED structure. Both FDTD simulation and experimental results show that a 42.00% enhancement is obtained for a commonly used green OLED. In addition, a macrolens implemented on the corrugated OLED is able to further extract the substrate mode, pushing the enhancement ratio to 103.02%. This 2X outcoupling efficiency is produced by a simple blazed grating stamp process and the macrolens attachment. We envision that this design can be achieved in a cost-effective way for mass-production, and the high optical efficiency is promising for both OLED lighting and display.
2. Design principle
2.1 Theory
SPPs are electromagnetic surface waves traveling at the interface between a dielectric medium and a metal with an exponentially decay on intensity into both surrounding media. The dispersion of SPPs on the organic layers-metal interface is governed by:
where |k0| = ω / c denotes the vacuum wave vector with the speed of light c; εm is the relative permittivity of the metallic layer; ωp is the plasma frequency; neff is the effective refractive index of the “waveguiding layers” (organic layers and the IZO layer); di and εi are the corresponding thickness and dielectric constant of each layer in the waveguiding layers.It is obvious that |kSPP| is larger than the in-plane wave vector kx = k0sinθ with θ denoting the angle of propagation with respect to the substrate normal (as shown in Fig. 1(a)). Therefore, for normal planar interfaces, it is not possible for the near-field SPPs to couple with the far-field propagating light, resulting in inevitable heat and loss.
Fig. 1 Schematic show of scattering properties of an OLED with (a) planar interfaces and (b) corrugated interfaces. The arrows in red denote the propagating directions of SPPs and the green arrows indicate the direct emitting air mode.
Typically, the trapped SPPs can be extracted in air by a Bragg grating with a subwavelength periodic interface, which satisfies momentum conservation in SPPs plane:
where k’SPP and kSPP describes the scattered and original wave vector of SPPs, respectively; |kg| = 2π/ Λ denotes the wave vector generated by the grating with period of Λ, and m is an integer defines the increasing or reducing orders of the scattering. Thus, the energy and momentum conservation allows the scattered SPPs to be partially coupled into the air mode by tuning the period of the grating and the order of the scattering. Similarly for the waveguiding mode, the scattered and original wave vectors k’wg and kwg are described by:Moreover, the coupled SPPs can be further enhanced by forming the periodic grating into a corrugated shape. These corrugated interfaces change the propagating direction of SPPs that reduce the component of kSPP||, increasing the SPPs outcoupling (as shown in Fig. 1(b)).
3. Experiment
3.1 Corrugated substrate formation
The polydimethylsiloxane (PDMS) liquid is mixed with the hardener in the ratio of 10:1 in a low-pressure container. In order to remove the bubble in the PDMS film, we repeat pumping under 1 × 10−1 torr and venting ten times to expel all the air inside. Then the PDMS is spin-coated on the commercial blazed grating mother mold under the vacuum chamber at 60°C for 1 min to transfer the reverse nanostructure from the grating onto the PDMS film, as shown in Fig. 2(a). After 4 hours solidifying at room temperature, the corrugated PDMS film is de-molded as the nano-imprint mold. Finally, the poly(methl methacrylate) (PMMA) liquid is dropped on the glass substrate and nano-imprinted by the PDMS mold. The corrugated substrate can then be obtained followed by ultraviolet (UV) curing process (100 W for 500 s).
3.2 OLED formation
The organic and metallic cathode materials of our OLED are evaporated layer by layer onto the substrate. The anode composed of Indium-Zinc-Oxide (IZO) is sputtered under room temperature. Thickness of IZO is chosen to be 125 nm according to the electrical test shown in Tab. 1. The transmission of the IZO keeps over 80% at the wavelength ranging from 400 nm to 800 nm. A hole transporting layer (HTL) is made of N,N’-Di(naphthalen-1-yl)-N,N’-diphenyl-benzidine (NPB) with the thickness of 60 nm. For the emissive layer, the tris-(8-hydroxyquinoline)aluminum (Alq3) is used with the thickness of 10 nm. The electron transporting layer (ETL) is made of 4,7-Diphenyl-1,10-phenanthroline (BPhen) with thickness of 40 nm, and the top metallic contact is composed of lithium fluoride/aluminum (LiF/ Al) with thickness of 0.9 nm and 120 nm, respectively. Figure 2(b) shows the schematic diagram for the corrugated OLED.
Table 1. Sheet resistance of IZO at difference thickness
To find out the optimum grating parameters of the corrugated structure, we fabricate three corrugated OLEDs with different grating pitches. We choose the reflective diffraction gratings from Edmund Optics (#43-772, #43-776, and #43-228 with the pitches corresponding to 833, 417, and 277 nm, respectively) as the mother mold. Figure 3(a) shows the scanning electronic microscope (SEM) image of the corrugated OLED with the pitch of 417 nm in top-view. Figure 3(b) shows its cross section image at a 52° tilted angle. The physical dimensions for the corrugated OLEDs with different pitches are shown in Table 2.
Table 2. Physical dimensions for corrugated OLEDs with different pitches
Maintaining electrical stability when introducing the corrugation is a key requirement to achieve stable, long-term device operation and high efficiency. Therefore, the current-voltage (J-V), luminance-voltage (L-V), and power efficiency-voltage (P-V) characteristics of the OLEDs are tested and shown in Fig. 4. Even though the textured metal may partially increase the parasitic absorption in metallic electrodes, it is shown in Fig. 4(a) that the current density increases with decreasing grating pitches under the same driving voltage. In the forward bias, the driving voltage at 20 mA/cm2 decreases from 6.37 V for a planar OLED to 4.97 V for corrugated OLED with the smallest grating pitch. As the number of carriers injected into an OLED is proportional to its total surface area, the increase of current density is ascribed to larger total surface area of the metallic electrode with smaller grating pitch. For the one-dimensional blazed grating, the surface area per mm2 is defined by:
where A is the surface area, L = 1 mm is the length of the OLED; x, y, height and pitch are given in Table 2. In addition, it has been reported that larger current in the corrugated device also results from a stronger electric field because of the partially reduced organic layer thickness in the intermediate region between the peak and valley of the patterned gratings [26,27].4. Results and discussion
4.1 Light extraction of corrugated OLEDs with different pitches
After the preparation of samples, we then perform a series of optical measurements for them. Firstly, the electroluminescence (EL) test is performed by a spectroradiometer (CS-1000, Konica Minolta). It is found that the emission is enhanced at certain viewing angles for the corrugated OLEDs compared to the planar one, which is caused by the light extraction of waveguiding or SPP modes. Figure 5(a) - 5(f) illustrate the enhancement from −80° to 80°, respectively. Therefore, the enhancement ratio for spectra at different viewing angles can be also calculated, as shown in Fig. 6(d) – 6(f). The angular radiation pattern for each type of OLED is illustrated in Fig. 7 [36]. Since the corrugated structures are asymmetric, we analyze the peak intensity enhancement ratio as well as the enhancement ratio for normal emission at 550 nm. Table 3 lists the corresponding results. Among all the three cases, 94.50% maximum enhancement is achieved at 550 nm measured at + 30 o for the OLED with the grating pitch of 417 nm.
Fig. 5 EL spectra for planar and corrugated OLEDs with (a) (d) 833 nm, (b) (e) 417 nm and (c) (f) 277 nm pitches at the viewing angles from −80° to 80°. The black lines denote the EL spectra for planar OLED and red lines for corrugated OLED.
Fig. 6 The enhancement ratio from planar and corrugated OLEDs with (a) (d) 833 nm, (b) (e) 417 nm and (c) (f) 277 nm pitches at the viewing angles from −80° to 80°. The black lines denote the EL spectra for planar OLED and red lines for corrugated OLED.
Table 3. Peak enhancement ratio of OLEDs with different nanostructure pitches.
4.2 External quantum efficiency of the corrugated OLEDs
Secondly, the external quantum efficiency (EQE) of the corrugated OLEDs with different grating pitches is obtained by an integrating sphere. Table 4 shows the measuring results. As the internal gratings extract a portion of the waveguiding and SPP modes to the substrate mode, a macrolens is also attached to further extract the substrate mode into air. The macrolens is made of quartz to match the refractive index of the glass substrate, and its diameter is 1 cm in order to collect all the emitting light from our 2 mm × 2 mm devices. For the case without the macrolens, the maximum EQE enhancement ratio is 42.00% for the corrugated OLED with the 417 nm pitch. Attachment of the macrolens further improves the EQE and its maximum enhancement ratio is 103.02%, which is the most efficient one among the three cases. It is worth noting that although the OLED with the 277 nm pitch exhibits highest EQE without the macrolens (47.62%), the percentage of substrate mode extracted by the macrolens is the lowest (26.88%). It is attributed that this grating redirects most of the light directly into the air mode rather than into the substrate mode. Figure 8 shows the intensity spectra of different OLEDs with and without the macrolens. Figure 9 shows the comparison of actual performance of OLEDs with and without the macrolens.
Table 4. EQE measured by integrating sphere.
Table 5 also shows the EQE of the OLEDs measured by a spectroradiometer. The OLEDs are mounted on a rotational stage; the EL characteristics from different viewing angles can be measured by changing the angle of the rotational stage. Besides, the transverse-electric (TE) and transverse-magnetic (TM) emission from different viewing angles can be separately measured by putting a polarizer in front of the spectroradiometer. As shown in Table 5, the EQE improves for both TE and TM emission; however, it is more efficient for enhancing TM than TE emission, indicating that the corrugated structures is polarization dependent. The integrating sphere and the spectroradiometer are complementary methods to characterize the EQE: through the integrating sphere, substrate mode extracted by a macrolens can be completely collected and hence the maximum attainable light extraction from waveguiding and SPP modes can be obtained; while the TE/ TM characteristics and the viewing-angle dependent spectra can be separately measured by the spectroradiometer.
Table 5. EQE performance measured by the spectroradiometer.
5. Discussion
To understanding the light extraction mechanisms of the light extraction by corrugated structures, we build up a corrugated OLED FDTD simulation (Lumerical, Inc.) with the structure shown in Fig. 1(b) to confirm the experimental results. The central wavelength of the electroluminescent Alq3 is 550 nm. We show the FDTD simulation with the pitch of 833, 417, and 277 nm, respectively, in Fig. 10.
Fig. 10 Dispersion relation of (a)-(d) planar and (e)-(p) corrugated OLED with the pitch of 833, 417, and 277 nm. (a), (e), (i), and (m) are simulation results of TE emission. (b), (f), (j), and (n) are experimental results of TE emission. (c), (g), (k), and (o) are simulation results of TM emission. (d), (h), (l), and (p) are experimental results of TM emission. The yellow solid lines denote the enhancement of waveguiding modes and the red dash lines show the enhancement of SPPs according to the calculation from Eq. (1) – (5).
By performing the calculations of Eq. (1) – (5) with ωp = 1.8 × 1016 rad/s at 550 nm, we then can depict the enhancing wave vectors of the waveguiding (yellow solid lines) and SPP modes (red dash lines) in the dispersion plot (ω-kx plot) in Fig. 10. The simulated TE and TM emission intensities of the corrugated OLED are shown in Fig. 10(e), 10(i), 10(m), and 10(g), 10(k), 10(o), and the corresponding intensities of a planar OLED are illustrated in Fig. 10(a) and 10(c). It is obviously seen that the emission intensity in TE mode is mainly enhanced by the extraction of waveguiding mode, while the TM intensity is enhanced by the extraction of both waveguiding and SPP modes. The enhancing peaks locate right at the position in coincidence with the theoretical calculation, which verify that our approach is able to effectively extract both modes. It is also shown that the experimental results are consistent with the simulation results (as illustrated in Fig. 10(b), 10(d), 10(f), 10(h), 10(j), 10(l), 10(n), and 10(p)). It should be noted that the simulation could not simulate the actual dispersion due to the usage of finite dipole models; however, the enhancement introduced by the corrugated structures is irrelevant to the quantity of dipoles, making the simulated enhancement valid in this case. The experimental EQE enhancement ratio (w/o macrolens) for extracting waveguiding mode in air is 19.3% in TE and 12.4% in TM mode, while it is 15.9% for SPP mode.
6. Conclusion
In summary, we show that a blazed grating on a PMMA layer is effective for light extraction of both waveguiding and SPP modes. The pitches on the blazed grating endow the vertical attribute for SPPs, giving rise to extracting the SPP mode into the propagating air mode by corrugating the OLED structure. Both Optical simulation and experimental results show that a 42.00% enhancement is obtained for a commonly used green OLED. In addition, a macrolens is implemented on the corrugated OLED, which is able to further extract the substrate mode, pushing the enhancement ratio to 103.02%. This 2X outcoupling efficiency is produced by a simple blazed grating stamp process and the macrolens attachment. We foresee that this method is able to enhance the optical efficiency of devices for both mass-production OLED lighting and display in a cost-effective way.
Funding
National Natural Science Foundation of China (11704421), Guangdong Natural Science Foundation (2016A03031333), Guangzhou Science and Technology Project (201805010004), Fundamental Research Funds for the Central Universities, 2017 (171gpy20).
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