Light Extraction Enhancement in Flexible Organic Light-Emitting

Sep 14, 2018 - School of Mechanical and ICT Convergence Engineering, Sun Moon University, Asan-si , Chungcheongnam-do 31460 , Republic of Korea...
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Light Extraction Enhancement in Flexible Organic Light-Emitting Diodes by a Light-Scattering Layer of Dewetted Ag Nanoparticles at Low Temperatures Junhee Choi,† Seonju Kim,†,‡ Cheol Hwee Park,† Jin Ho Kwack,†,‡ Chan Hyuk Park,† Ha Hwang,† Hyeong-Seop Im,§ Young Wook Park,*,∥ and Byeong-Kwon Ju*,†

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Display and Nanosystem Laboratory, Department of Electrical Engineering, and §Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea ‡ Samsung Display Co., Samsung Street 181, Tangjeong-Myeon, Asan-si, Chungcheongnam-do 31454, Republic of Korea ∥ School of Mechanical and ICT Convergence Engineering, Sun Moon University, Asan-si, Chungcheongnam-do 31460, Republic of Korea S Supporting Information *

ABSTRACT: We demonstrated light extraction improvement by applying a scattering layer of Ag nanoparticles physically synthesized through a low-temperature annealing process to flexible organic light-emitting diodes (OLEDs). In general, increasing the size of Ag nanoparticles is preferred to increase light scattering, but a high-temperature annealing process (∼400 °C) is required to produce them. However, flexible substrates generally cannot withstand high-temperature processes. In this study, we formed Ag nanoparticles at a low temperature of ∼200 °C by inserting a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate buffer layer, thus promoting Ag dewetting. As a result, the scattering layer of enlarged Ag nanoparticles formed at low temperatures increased the external quantum efficiency by 24% in a flexible OLED compared to a reference device. KEYWORDS: organic light-emitting diodes (OLEDs), plasmonics, metal nanoparticles, light extraction, light scattering

1. INTRODUCTION

most of these light extraction techniques have been reported on rigid glass substrates. There are some reports on light extraction techniques applied to flexible substrates such as photonic crystal structures,17 phase separation of a polymer,18 and substrate surface modulation.19,20 However, these light extraction techniques for flexible OLEDs have focused on external light extraction which adds a light-extraction layer on the outer plastic substrate (i.e. the opposite side of OLED deposition). There are only a few reports on internal light extraction, which inserts a light-extraction layer between the substrate and the anode (i.e. OLED deposition side) because the structure of the OLED device can be affected by this layer, and more elaborate techniques are required for application in flexible substrates. Nanostructured metal/dielectric composite electrodes exhibit angle-independent light out-coupling while reducing the microcavity effect and the surface plasmonic loss.21 Industrial-grade polyethylene naphthalate (PEN) substrates with builtin scattering particles can enhance the extraction efficiency of flexible OLEDs.22 The air-gaps embedded in a flexible substrate act as light scattering centers, enhancing the light

Organic light-emitting diodes (OLEDs) have been intensively studied for decades because of their considerable potential in solid-state lighting sources and future flat panel displays such as flexible, wearable, and stretchable displays. It is well known that the internal quantum efficiency can theoretically be 100% with the development of phosphorescent materials and thermally activated delayed fluorescence (TADF) materials.1−4 However, the light out-coupling efficiency of conventional OLEDs is limited to only about 20% owing to light losses such as the waveguide mode, the substrate mode, electrode absorption, and surface plasmons at the metal−organic interface. Therefore, an effective light extraction technique is still required. Many studies have been carried out on light extraction techniques such as the low-index grid structure,5,6 random scattering layer,7−9 buckled polymer structure,10−12 capping layer,13 and hybrid mode, which combines the effects of microcavity and the tamm plasmon−polariton mode.14 It has recently been shown that manipulating the refractive index of an organic material by depositing it at an oblique angle improves its light extraction efficiency.15 Microporous polymer films improved the light extraction efficiency by reducing the amount of light trapped in the substrate mode.16 However, © XXXX American Chemical Society

Received: April 30, 2018 Accepted: September 4, 2018

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DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces extraction efficiency of flexible OLEDs.23 High-refractive-index polymer substrates with spontaneously developed surface nanostructures and scattering properties enhance light outcoupling efficiency in flexible OLEDs.24 Corrugated polycarbonate substrates successfully reduce the photon losses of flexible OLEDs.25 However, there have been no reports on Ag nanoparticles used as light scattering particles in flexible OLEDs. A lightscattering layer is generally produced by coating with a polymer solution containing dispersed dielectric particles,7,8 but this process is not suitable for mass production because of uncontrollable discrete nanoparticle agglomeration. On the other hand, physically synthesized metal nanoparticles have the advantage of a controllable nanoparticle size and surface coverage, with the control of the deposition thickness and the heat treatment temperature without the agglomeration of discrete nanoparticles. Also, Ag nanoparticles have lower light absorption than other metal nanoparticles, and have strong light scattering cross-section.26,27 It has already been reported that light scattering by Ag particles improves the performance of various devices such as solar cells,28−30 sensors,31−33 and infrared photo-detectors34,35 because of their strong light scattering. In general, increasing the diameter of metal nanoparticles can result in most of the light being scattered by metal nanoparticles rather than being absorbed.26,27 However, in most studies, a high-temperature heat treatment process, which the flexible substrates cannot withstand, is performed to increase the diameter of the metal nanoparticles.35−37 In this study, we demonstrated the enhanced light extraction efficiency of flexible OLEDs by introducing a light-scattering layer of Ag nanoparticles. The Ag nanoparticles were physically synthesized by thermal evaporation and post-annealing to increase the size of the Ag nanoparticles. By introducing a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) buffer layer that promotes the dewetting of Ag nanoparticles, the size of the Ag nanoparticles can be further increased at a temperature of ∼200 °C without any problems from the flexible substrate. The light-extraction effect of Ag nanoparticles was demonstrated through a finite-difference time-domain (FDTD) simulation. As a result, the external quantum efficiency (EQE) of a flexible OLED was improved by 24% using a light-scattering layer of Ag nanoparticles.

(naphthalen-1-yl)-N,N′bis (phenyl)benzidine (NPB)/thick tris(8hydroxy-quinolinato) aluminum (Alq3) were deposited as hole injection (5 nm), hole transport (60 nm), and emission layers (80 nm), respectively. The TADF white OLED consisted of 3 nm thick molybdenum trioxide (MoO3)/20 nm thick 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA)/15 nm thick bis[4-(9,9-dimethyl-9,10dihydroacridine)phenyl]sulfone (DMAC-DPS): 0.6 wt % rubrene/ 40 nm thick 4,7-diphenyl-1,10-phenanthroline (Bphen). The hole injection, hole transport, and electron transport layers are MoO3, TCTA, and Bphen, respectively. DMAC-DPS was used as the TADF host, and rubrene was the orange fluorescent dopant. The cathode was produced by depositing 0.7 nm thick lithium fluoride (LiF) and 100 nm thick Al. 2.3. Characterization. The surface morphologies of the Ag nanoparticles were observed using a field emission scanning electron microscope (S-4800, Hitachi High-Technologies Co., Ltd.). The diameter and surface coverage of the Ag nanoparticles were measured using an Image-Pro Plus 4.5. The transmittances were measured by a UV−visible spectrometer (Cary 5000, Agilent Technologies, Inc.). Mechanical bending was carried out by using a bending tester (Z-Tec. Co., Inc.). The electroluminescence (EL) and J−V characteristics of the OLEDs were measured using a high-voltage-source measurement unit (model 237, Keithley Instruments, Inc.) and a spectroradiometer (PR-670 SpectraScan, Photo Research, Inc.). 2.4. Optical Simulation. Commercial FDTD software (FDTD Solutions, Lumercial, Inc.) was used for simulation. The normalized scattering cross-section was calculated by using a total-field scatteredfield source, and the optical constants (n, k) of the organic materials, SU-8 and PEDOT:PSS, were measured by a F20 thin-film analyzer (Filmetrics, Inc.). To investigate the E-field distribution of the OLEDs, vertical and horizontal dipoles were applied and the Ag nanoparticles were arranged. The FDTD size was x = 14 μm and y = 4 μm. The mesh size was applied at x = 8 nm and y = 15 nm. The radius of the Ag nanoparticles was 75 nm. The nanoparticles had a period of 2.2 μm, considering the surface coverage.

3. RESULTS AND DISCUSSION 3.1. Optical Properties of Ag Nanoparticles. Figure 1 shows the normalized scattering cross-section of spherical Ag

2. EXPERIMENTAL SECTION 2.1. Preparation of Ag Nanoparticles. PEN films were cleaned by UV−ozone treatment for 15 min. The as-received PEDOT:PSS (Heraeus Clevios P) was spin-coated at 4000 rpm on the prepared PEN substrates. The neutral PEDOT:PSS can prevent the corrosion of Ag nanoparticles that can be caused by the as-received PEDOT:PSS, which is an acidic solution.38−41 The PEDOT:PSS film was annealed on a 150 °C hotplate for 10 min. Ag thin films were deposited on the PEDOT:PSS layer by using a thermal evaporator (Digital Optics Vacuum Co., Ltd.). The Ag deposition rate was 0.2 Å/ s. Ag thin films were annealed on a 200 °C hotplate for 10 min to obtain large-sized Ag particles. The Ag nanoparticles were covered with a 2.4 μm layer of SU-8 (Negative photoresist, MicroChem) for surface planarization. 2.2. Fabrication of Flexible OLEDs. A flexible electrode of indium zinc oxide (IZO) was deposited on the prepared Ag-scattering layer by sputtering. The organic layers and the cathode were deposited by using a thermal evaporator in a high vacuum chamber below 10−6 Torr. Fluorescent green OLEDs and TADF white OLEDs were fabricated. To fabricate the fluorescent green OLED, hexaazatriphenylenehexacarbonitrile (HATCN)/N,N′-bis-

Figure 1. Scattering efficiency of spherical Ag nanoparticles at various diameters. (Calculated by FDTD optical simulation).

nanoparticles inside an SU-8 medium at various nanoparticle diameters. It was calculated by the FDTD optical simulation, and the simulation schematic is shown in Figure S1. As the diameter of the Ag nanoparticles increases, the scattering spectra of Ag nanoparticles are generally red-shifted, and broadened. When the diameters of the Ag nanoparticles are 25 and 50 nm, they exhibit only dipole resonance at approximately 400 nm. On the other hand, when the diameter B

DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Fabrication process of the light-scattering layer using Ag nanoparticles.

Figure 3. SEM images of the Ag nanoparticles on different substrates. The annealed Ag nanoparticles on the PEN substrate for the nominal thicknesses of (a) 2, (b) 3, and (c) 4 nm. The annealed Ag nanoparticles on the PEDOT:PSS buffer layer for the nominal thicknesses of (d) 2, (e) 3, and (f) 4 nm.

Figure 4. (a) Total transmittance and (b) diffuse transmittance of the Ag nanoparticle-based scattering layers depending on nominal thicknesses with and without the buffer layer.

exceeds 100 nm, quadrupole resonance is observed at a wavelength of approximately 400 nm. Ag nanoparticles with diameters of 100, 150, and 200 nm show dipole resonances at approximately 500, 550, and 600 nm, respectively.20,30 The resonance peak is controlled by adjusting the diameter of Ag nanoparticles. This implies that the diameter of Ag particles is an important factor to induce strong light-scattering resonance. Therefore, to maximize light scattering, the optimum diameter of Ag nanoparticles should be selected considering the target wavelengths such as ∼450 nm (blue), ∼520 nm (green), or ∼650 nm (red). 3.2. Light-Scattering Layer of Ag Nanoparticles. Figure 2 shows the fabrication process of the light-scattering

layer composed of Ag nanoparticles. This layer of Ag nanoparticles is fabricated by annealing an Ag thin film. In general, larger-sized Ag nanoparticles are more effective for inducing strong light scattering than small-sized nanoparticles.26,27 By inserting a PEDOT:PSS buffer layer between the flexible substrate and the silver thin film, it is possible to further increase the size of Ag nanoparticles in a lowtemperature annealing process.42 This is because the buffer layer promotes dewetting of the Ag nanoparticles. Ag thin films with nominal thicknesses of 2, 3, and 4 nm are deposited on the PEDOT:PSS layer, followed by annealing on a 200 °C hotplate. After the annealing process, SU-8 polymer solution is spin-coated on Ag nanoparticles as a surface planarization layer C

DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. EL characteristics of the reference and the OLEDs with a scattering layer on the PEN substrate, (a) J−V and V−L characteristics, (b) EQE as a function of current density.

diameter of Ag nanoparticles increases when the Ag nominal thickness increases. The scattering layer with a nominal thickness of 3 nm has the highest diffuse transmittance, which means that the scattering layer with 3 nm nominal thickness has the optimum diameter of Ag nanoparticles to induce strong light scattering. The optimum nanoparticle diameter is smaller than the simulation results because the actually prepared Ag nanoparticles are elongated ovals rather than spheres, shifting the scattering resonance peak to the red region.29,30,42 To observe the mechanical durability of the scattering layer, the transmittances were measured at various bending cycles up to the value of 2000 (shown in Figure S4). The transmittances of the scattering layer did not change depending on the number of bending cycles. Thus, it has been demonstrated that the scattering layer with Ag nanoparticles exhibited mechanical stability through the bending test. To further investigate the scattering properties of the Agscattering layers, Rayleigh scattering and dark-field images were obtained and are shown in Figure S5. The Rayleigh scattering spectrums are similar to those of the diffuse transmittances, and the dark field images become brighter as the Ag thickness increases. There is a difference between the Rayleigh scattering and the diffusion transmittance. Rayleigh scattering increases with Ag thickness, but the diffuse transmittance of the scattering layer with a thickness of 3 nm is the highest because of the absorption of the scattering layer with a thickness of 4 nm. 3.3. OLED Fabrication. To demonstrate the performance of the scattering layer in OLEDs, we fabricated a reference device without a scattering layer and an OLED device with a scattering layer. We fabricated green fluorescent OLEDs with a device structure of IZO/HATCN/NPB/Alq3/LiF/Al. The reference device has no scattering layer including Ag nanoparticles, SU-8, and buffer layers. All devices were measured at various viewing angles (10°, 20°, 30°, 40°, 50°, 60°, and 70°) to observe the EL efficiency at different viewing angles. Figure 5a presents J−V−L characteristics of the fabricated OLEDs at various nominal deposited thicknesses. All OLEDs show identical current−voltage (J−V) characteristics. This implies that the scattering layer of Ag nanoparticles does not affect electrical properties of the OLEDs. Figure 5b presents the EQE of the OLEDs. The OLEDs with nominal thicknesses of 2, 3, and 4 nm show EQEs of 1.88, 2.19, and 1.68%, respectively, and the reference device EQE is 1.77% at 1000 cd/m2. The EQE values are summarized in Table 1. The OLED with a nominal thickness of 3 nm has the highest

which prevents electrical instability. Figure 3 shows the scanning electron microscopy images of the Ag nanoparticles after the annealing process at 200 °C with and without the buffer layer at nominal deposited thicknesses of 2, 3, and 4 nm. Figure 3a−c shows the Ag nanoparticles on the bare PEN substrate at nominal thicknesses of 2, 3, and 4 nm. Although, the diameter of Ag nanoparticles is slightly increased when increasing the nominal thickness, Ag nanoparticles remain small at each thickness. On the other hand, the diameter of Ag nanoparticles is significantly enlarged by the introduction of the PEDOT:PSS buffer layer, as shown in Figure 3d−f. Further, the surface coverage of Ag nanoparticles is considerably decreased. As the nominal thickness increases, the diameter of Ag nanoparticles on the buffer layer is gradually increased, and the surface coverage is also increased. The surface energy of PEDOT:PSS is about 70 mJ/m2, which is larger than that of PEN (about 40 mJ/m2).43−45 Therefore, Ag dewetting may have been promoted by the outgassing of SO2/SO3 in PEDOT:PSS rather than by the change in the surface energy, similar to the previous study in which Ag dewetting was altered by the sublimation of the Sb sacrificial layer.46−48 Figure S2 shows the average diameters of the Ag nanoparticles in all cases. To investigate the optical properties of the scattering layer, total and diffuse transmittances were measured using a UV−vis spectrometer. Figure 4a exhibits the total transmittance of the scattering layers with various nominal thicknesses. The total transmittance decreased with an increase in the Ag nominal thickness. The scattering layers without the buffer layer present a lower total transmittance than those with the buffer layer at all nominal thicknesses in a broad spectral range. In the case of 3 nm nominal thickness (at a wavelength of 550 nm), the total transmittance of the scattering layer without the buffer layer is approximately 40%, whereas the scattering layer with the buffer layer shows a total transmittance of approximately 70%. This is because the insertion of the buffer layer increases the diameter of the Ag nanoparticles and decreases their surface coverage, which reduces the absorption. Figure S3 shows the reflectance and absorbance of the scattering layers as functions of wavelength. Figure 4b shows the diffuse transmittance of the scattering layers with and without the buffer layer at various nominal thicknesses. For the scattering layer without the buffer layer, the diffuse transmittance is very low at all nominal thicknesses. On the other hand, in the case of the scattering layer with the buffer layer, an apparent diffuse transmittance peak exists near a wavelength of 500 nm. This peak is redshifted with increasing Ag nominal thickness because the D

DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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layer is more effective at a tilted angle. The angular distribution of the OLED with the Ag-scattering layer is related to the direction of the light scattered by the Ag nanoparticles. The direction of the light scattered by the nanoparticles is determined by the shape and size of the nanoparticles and the surrounding medium.50−52 The layer of Ag nanoparticles is expected to scatter more in the oblique direction than in the front direction. To further investigate the improvement in the light extraction of OLEDs with Ag nanoparticles, the E-field profile of the OLEDs with periodic Ag nanoparticles was calculated through a FDTD optical simulation. The simulated E-field intensity profile of OLEDs with and without Ag nanoparticles is presented in Figure 7. Compared to an OLED without Ag

Table 1. Summary of EL Characteristics of the Fabricated OLEDs EQE (%) 2

devices

(at 1000 cd/m )

(at 4000 cd/m2)

reference 2 nm 3 nm 4 nm

1.77 1.88 2.19 1.68

1.64 1.70 2.08 1.48

efficiency, and it confirms the light-scattering effects of Ag nanoparticles observed in the previous results of diffuse transmittance. Compared to the reference device, the enhancement ratio of the OLED with a nominal thickness of 3 nm is approximately 24%. Ag nanoparticles typically exhibit strong absorption and scattering because of localized surface plasmon resonance. However, our Ag nanoparticle-scattering layer reduced parasitic absorption and increased scattering through the PEDOT:PSS buffer layer (shown in Figures 4, S3, and S5). Therefore, Ag nanoparticles with strong scattering and reduced absorption can be used as dielectric scattering particles in OLEDs because they can increase the light extraction efficiency in OLEDs by reducing waveguide losses. However, the OLED with a nominal thickness of 4 nm shows a decrease in the EQE compared with that of the reference; this is because the increased surface coverage of the Ag nanoparticles results in parasitic absorption losses. Strong absorption and scattering of Ag nanoparticles have been reported to change their emission wavelength,49 but our Ag scattering layer did not reveal much change in the wavelength because it reduced absorption and optimized scattering to green wavelengths. The CIE color space of our green OLED is shown in Figure S6. We also investigated the light-extracting efficiency of the white OLED with Ag nanoparticles (shown in Figure S7). The EQE improved by 43% in the white OLED because of the scattering of light by the Ag nanoparticles over a broad range of wavelengths. Figure 6 shows the angular distribution of the normalized radiative intensity of OLEDs. The reference OLED demonstrates almost an Lambertian light distribution. On the other hand, the OLEDs with a light-scattering layer show a nonLambertian distribution, having their strongest intensity at 20°−30°, meaning that the light extraction of the scattering

Figure 7. Simulated E-field distribution of OLEDs (a) without and (b) with Ag nanoparticles (light source: vertical dipole).

nanoparticles, the E-field of an OLED with Ag nanoparticles is obviously enhanced in air but reduced in the PEN substrate and the organic layer. Although the Ag nanoparticles are periodic, the results reveal a possibility that the light trapped in the OLED can escape through the scattering layer. The simulated E-field by the horizontal dipole is illustrated in Figure S8, and it presents similar results. As a result, the scattering layer of Ag nanoparticles is effective for light extraction from flexible OLEDs.

4. CONCLUSIONS We demonstrated the fabrication of a scattering layer consisting of Ag nanoparticles for flexible OLEDs. The scattering layer of Ag nanoparticles was easily fabricated by low-temperature annealing after Ag deposition. By inserting a PEDOT:PSS buffer layer, the scattering layer can be fabricated at a low temperature of ∼200 °C on a flexible substrate. We analyzed the optimal size of Ag nanoparticles using a UV−vis spectrometer, and the optimal-sized Ag nanoparticles improved the light extraction efficiency in flexible OLEDs by strong light scattering. We also demonstrated that the trapped light can be extracted from waveguide modes via an FDTD optical simulation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07026. Simulation scheme for scattering cross-section (Figure S1), average diameter of Ag nanoparticles (Figure S2), reflectance and absorbance of the scattering layer

Figure 6. Angular distribution of normalized radiative intensity with (red line) and without (black line) scattering layers with an optimal diameter of 3 nm Ag nominal thickness. Dotted line represents an ideal Lambertian emitter. E

DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(Figure S3), transmittance at various bending cycles (Figure S4), dark-field images and Rayleigh scattering (Figure S5), CIE 1931 color space of the green OLED (Figure S6), EL characteristics of the white OLED (Figure S7), simulated E-field distribution induced by the horizontal dipole (Figure S8) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.W.P.). *E-mail: [email protected] (B.-K.J.). ORCID

Young Wook Park: 0000-0003-3652-591X Byeong-Kwon Ju: 0000-0002-5117-2887 Author Contributions

J.C. and S.K. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (no. 2016R1A2B4014073) and the Ministry of Education (no. NRF-2017R1D1A1B03036520), and the Industry Technology R&D program (10048317, Development of red and blue OLEDs with EQE over 20% using delayed fluorescent materials) funded by MOTIE/KEIT.

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DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.8b07026 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX