Optical Analysis of Power Distribution in Top-Emitting Organic Light

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Optical analysis of power distribution in top-emitting organic light emitting diodes integrated with nano-lens array using finite difference time domain Kyung-Hoon Han, Young-Sam Park, Doo-Hee Cho, Yoonjay Han, Jonghee Lee, Byoung-Gon Yu, Nam Sung Cho, Jeong-Ik Lee, and Jang-Joo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02631 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Materials & Interfaces

Optical analysis of power distribution in topemitting organic light emitting diodes integrated with nano-lens array using finite difference time domain Kyung-Hoon Han,

†,§

Young-Sam Park,*, ‡ , § Doo-Hee Cho,

‡

Yoonjay Han,

†

Jonghee Lee,

⊥

Byounggon Yu, ‡ Nam Sung Cho, ‡ Jeong-Ik Lee‡ and Jang-Joo Kim*, †

† Department

of Materials Science and Engineering, Seoul National University, Seoul, 08826,

Republic of Korea ‡Flexible

Information Device Research Section, Electronics and Telecommunications Research

Institute (ETRI), Daejeon, 34129, Republic of Korea ⊥ Department

of Creative Convergence Engineering, Hanbat National University, Daejeon,

34158, Republic of Korea §Co-first

authors

ACS Paragon Plus Environment

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KEYWORDS: nano lens, organic light emitting diode (OLED), top-emitting blue OLED, light extraction, finite difference time domain (FDTD)


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ABSTRACT: Recently, we have addressed that a formation mechanism of a nano-lens array (NLA) fabricated by using a maskless vacuum deposition is explained as the increase in surface tension of organic molecules induced by their crystallization. Here, as another research using finite difference time domain (FDTD) simulations, not electric field intensities but transmitted energies of electromagnetic waves inside and outside top-emitting blue organic light emitting diodes (TOLEDs), without and with NLAs, are obtained, to easily grasp the effect of NLA formation on the light extraction of TOLEDs. Interestingly, the calculations show that NLA acts as an efficient light extraction structure. With NLA, larger transmitted energies in the direction from emitting layer (EML) to air are observed, indicating that NLAs send more light to air otherwise trapped in the devices by reducing the losses by waveguide and absorption. This is more significant for higher refractive index of NLA. Simulation and measurement results are consistent. A successful increase in both light extraction efficiency and color stability of blue TOLEDs, rarely reported before, is accomplished by introducing the highly process compatible NLA technology using the one-step dry process. Blue TOLEDs integrated with a N,N′-Di(1-

naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) NLA with a refractive index of 1.8 show a 1.55-times-higher light extraction efficiency, compared to those without it. In addition, viewing angle characteristics are enhanced and image blurring is reduced, indicating that the manufacturer-adaptable technology satisfies the requirements of highly efficient and color stable top emission displays.


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INTRODUCTION Organic light emitting diodes (OLEDs) are classified as top- or bottom-emitting devices, depending on their light emission direction. Generally, top-emitting OLEDs (TOLEDs) better fit into active matrix OLEDs (AMOLEDs), because TOLEDs are structurally unaffected by the number of thin film transistors (TFTs) integrated on the substrate.1 Additionally, TOLEDs do not show significant image blurring issues, as they emit light through a thin transparent top electrode rather than a thick glass substrate.2 Comparing red, green and blue subpixels in full-color OLED displays, it has been shown that external quantum efficiency (EQE) of fluorescent blue emissive units is low than those of phosphorescent red and green elements.3 The low EQE leads to a low luminance as well as a challenge in the control of color gamut. Moreover, blue emitters are the most unstable,3,4 causing burn-in on the display.5 Therefore, highly efficient and color stable blue TOLEDs that consume low operating power are essential for the successful replacement of a liquid crystal display (LCD) technology. As replacements for the typical fluorescent blue OLEDs, many researchers have adopted phosphorescent materials, and have focused on their design and synthesis to obtain high EQE.3,4,6-19 Nevertheless, blue phosphorescent emitters had short lifetimes that are not suitable for real applications.3 A thermally activated delayed fluorescent (TADF) device can be another alternative that provides a different emitting layer (EML) material. However, TADF elements show lifetime and efficiency roll-off.20-24 A 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) light outcoupling layer25 and a phase adjustment layer26 have been considered a film of the blue TOLEDs. Chen et al.25 proposed that,


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the use of BCP layer led to a suppressed multiple beam interference, an improved blue emission and

a

better

chromaticity.

Meng

et

al.26

asserted

that

a

60-nm-thick

Tris(8-

hydroxyquinolinato)aluminum (Alq3) employed as the phase adjustment layer showed a high current efficiency of 3.4 cd/A, and a better international commission on illumination (CIE) of (0.13, 0.15). A micro-lens array (MLA) with a lens diameter of several hundred micrometers is widely used as a light extraction technology to improve the efficiency of OLEDs.2,27-29 However, the lens fabrication using a dry method is a two-step fabrication, as it requires to perform an additional process such as masking and molding.2 Furthermore, MLAs can cause image quality degradation of AMOLED displays which have a pixel size of same or less than 50 um. Even though it is very important for the development of blue TOLEDs, manufacturer-adaptable information about efficiency and color stability has been little posted. We have recently developed a nano-lens array (NLA, lens size: 99.9%) were loaded into a vaporizer. Next, the vaporizer was heated to produce NPB vapors. Then, they moved to the specimens by means of an inert gas. The current density-voltage-luminance (J-V-L) was measured using a Keithley 2400 programmable source meter. By using the programmable source meter, a rotation stage, and an Ocean Optics S2000 fiber optic spectrometer, the angular distribution characteristics of the electroluminescence (EL) intensity were measured.


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RESULTS AND DISCUSSION Manufacturer-adaptable NLA technology Visible light has a wavelength of several hundred nanometers, hence NLA with a diameter of several hundred nanometers is required to achieve the desired properties of blue TOLEDs. The recent study has reported that, even though NPB melting temperature is constant at 280 oC regardless of the pressure, the boiling temperatures decrease linearly with the decrease in the pressure from 102 Torr to 10-2 Torr.32 This indicates that lower pressure conditions (down to 10-2 Torr) in the vaporizer provide higher NPB vapor pressures. Figures 1a (10-1 Torr, ~100 nm) and 1b (5 x 10-2 Torr, ~150 nm) show that larger hemispherical nano-lenses are obtained by decreasing the vaporizer pressure. By optimizing the OVPD process parameters, NLA with a size of 200-600 nm are obtained on the IZO layer (Figs. 1c and 1d), and integrated on the device (Fig. 1e). The areas of nano-lenses and IZO are in the ratio of 7:10. During the process, due to the spontaneous nature of the organic droplet formation, neither a mask nor an additional treatment is required, thus greatly simplifying the entire process.


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Significantly increased transmitted energies of electromagnetic waves in the direction to air by NLA integration The previous researches using FDTD simulations have schematically illustrated the distributions of electric fields of OLEDs having a scattering layer.33-35 Here, as another investigation, timely integrated Poynting vectors inside and outside blue TOLEDs, without and with NLAs, in the direction from EML to air are described to quantitatively understand which optical modes are dominantly suppressed to improve light extraction efficiency (Fig. 2 and Table 1). The distribution and the diameter (ie, 200 nm) of the nano-lenses in the calculations follow those in the scanning electron microscope (SEM) experiments (Figs. 1c and 1d). The refractive indices of NLAs are ranged from 1.7 to 2.2 to monitor their effects on the transmitted energies of the electromagnetic waves. The dotted line at the center of EML (ie, x1+65 in Fig. 2a) is a position where the total of 100 dipoles are randomly distributed. The light direction is assumed to be positive from EML to air, negative from EML to Al. The differences in the transmitted energies at the dotted line indicate Purcell factors of the dipole arrays in a microcavity structure, and exceed one. In organic, NLA and air regions where energy absorptions do not occur, respectively, the transmitted energies are constant, respectively, because the integrated energy values with times are constant, even though electromagnetic waves are reflected or penetrated at a given time. In contrary, in Al and IZO regions, the transmitted energies are decreased due to energy absorptions. Thus, when electromagnetic waves arrive at the Organic-Al interface, their energies begin to decrease mainly due to surface plasmon polariton (SPP) loss, and are zero inside Al. Similarly, when light propagates by waveguide mode, its energy is decreased in the IZO. Unlike corrugated devices, the differences in the transmitted energies at the dotted line are almost same (ie, ~1.45) regardless of NLA introduction, because the distance between EML and


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NLA is large. Compared to device without NLA, those with NLAs have larger transmitted energies in the direction from EML to air, indicating that NLAs send more light to air otherwise trapped in the device. The tendency is more significant for higher refractive indices of NLAs. This suggests that light extraction efficiency can be further increased by using a higher refractive index material for NLA, corresponding to the results shown in Fig. 2d. NPB with a refractive index of 1.8 is used for the NLA fabrication in the experiments, hence it is predicted that the device efficiency is increased approximately 1.42 times. The total amount of energy absorbed in the Al and the IZO electrodes (ie, ‫ܣ‬஺௟,ூ௓ை ሺ‫ݎ‬Ԧ, ‫ݐ‬௦ ሻ) is calculated by using the following equation.36

ଶ

‫ܣ‬஺௟,ூ௓ை ሺ‫ݎ‬Ԧ, ‫ݐ‬௦ ሻ = ߱ × Im[ߝ஺௟,ூ௓ை ] × ห‫ܧ‬஺௟,ூ௓ை ሺ‫ݎ‬Ԧ, ‫ݐ‬௦ ሻห

(1)

where, ߱ is frequency, Imൣߝ஺௟,ூ௓ை ൧ is an imaginary part of the dielectric constant of the Al and ଶ

the IZO, and ห‫ܧ‬஺௟,ூ௓ை ሺ‫ݎ‬Ԧ, ‫ݐ‬௦ ሻห is the electric field intensity at specific position and time, respectively. Optical mode fractions of elements without and with NLAs are summarized in Table 1. By introducing NLA and by increasing its refractive index, light extraction efficiencies increase up to 1.8 times, by mainly reducing waveguide and absorption losses. This indicates that NLA efficiently sends light out to the air, as an optically excellent scattering structure. It has been reported that scattering layers based on 240 nm size nanoparticles fabricated by using solution processes enhance the efficiency and improve the color stability of bottom-emitting white OLEDs.34,37 The nanoparticle process is not compatible with the processes in the current OLED industry, because it uses a wet method. The nanoparticles are internal scattering layers, as they are located between a substrate and a bottom electrode. NLA in this study is an external


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scattering layer because it is formed on the top electrode. To experimentally demonstrate the effectiveness of NLA on blue OLEDs, devices are fabricated and measured.


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Simultaneous increase in light extraction efficiency and color stability of blue TOLEDs by NLA introduction Figure 3a shows the J-V-L characteristics of blue TOLEDs without and with NLAs. The two J-V curves are nearly overlapped, and voltage at a current density of 10-2 mA/cm2 is equal (5.5 V) in both cases. This indicates that OVPD process does not damage the device at all. The L-V curves display that the device with NLA has a higher luminance at the same voltage compared to that without NLA. Figure 3b illustrates that, at 1 mA/cm2, current efficiencies of TOLEDs without and with NLAs are 4.6 and 7.3 cd/A, respectively. Their power efficiencies are 1.7 and 2.6 lm/W, respectively. This reveals that NLA integration increases light extraction efficiency by approximately 1.55 times, which agrees with the simulation results. In developing blue TOLEDs with high color uniformity, a viewing angle property and an image blurring are key measurement parameters. Figures 4 illustrates angular spectrums of TEOLEDs without and with NLAs. The first main EL intensity peaks (at a wavelength of 462 nm) do not change between 0 and 60o regardless of NLA introduction, However, with NLA, the second main peaks (at a wavelength of 493 nm) are scarcely changed between 0 and 30o and vary less between 0 and 60o. This agrees well with the results of color coordinate change measurements, ∆u’v’ between viewing angles 0 and 60o (without NLA: 0.020; with NLA: 0.015). As the peak values of the second main picks of a NLA device increase from 40o, it can be seen that NPB NLA does not completely alleviate microcavity effects in blue TOLEDs. NLA device shows larger slope change with a distance from OLED center (Fig. 5), indicating that NLA reduces image blurring.


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CONCLUSION Power simulations are carried out to theoretically evaluate the effectiveness of curved shape NLAs with a lens diameter of several hundred nanometers in blue TOLEDs. The calculations indicate that NLA acts as an excellent exterior light extraction system, which efficiently sends light out to the air. This agrees well with experimental data showing that NLA integration increases light extraction efficiency by 1.55 times. Interestingly, NLA introduction improves the angular color stability in a wide viewing angle ranging from 0 to 60o, and provides less image blurring, indicating that NLA increases color stability. Therefore, we believe that this manufacturer-acceptable technology contributes to the OLED industry in pursuing optically efficient blue TOLEDs.


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Corresponding Author *E-mail: [email protected] (J. -J. K.) *E-mail: [email protected] (Y. -S. P.)

Author Contributions K. -H. Han and Y. -S. Park contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors greatly appreciate the use permit of the organic vapor phase deposition equipment from Dr. S. Ahn (ETRI). This work was partially supported by the Industrial Strategic Technology Development Program (10048317) through the Korea Evaluation Institute of Industrial Technology (KEIT). This work was also partially supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korea government [18ZB1240, Technology development of OLED light source and panel for Photostimulation device]. This work was also partially supported by Institute for Information & communications


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Technology Promotion (IITP) grant funded by the Korea government (MSIT) (2018-0-00202, Development of Core Technologies for Transparent Flexible Display Integrated Biometric Recognition Device).

Supporting Information Brief statement indicating that NLA technology is available in the blue TOLED employing the thin metal top electrode supplied as Supporting Information.


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Figure 1 by Han and Park et al.


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Figure 1. Continued.


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FIGURE CAPTIONS

Figure 1. SEM images of NLAs on glass/IZO substrates: (a) planar, ~100 nm, (b) planar, ~150 nm, (c) planar and (d) cross sectional, 200-600 nm. (e) The integration of NLA with a lens size of 200-600 nm on IZO top electrode of blue TOLED.

Figure 2. (a) Profiles of transmitted energies of electromagnetic waves inside and outside blue TOLEDs, without and with NLAs with a radius of 100 nm. Device structures (b) without, and (c) with NLAs used in the simulations. The arrows show positive directions in the X axis in (a). (d) Enhancement ratios of light extraction efficiencies of blue TOLEDs with refractive indices of NLAs.

Figure 3. (a) The J-V-L, and (b) current and power efficiencies of blue TOLEDs without and with NLAs.

Figure 4. Angular spectrums of blue TOLEDs between 0 and 60o (a) without and (b) with NLAs.

Figure 5. Image blurring data of blue TOLEDs without and with NLAs.


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Table 1. Optical mode fractions of blue TOLEDs without and with NLAs. n is a refractive index of NLA.

Extracted Structure Fraction

w/o w/; n=1.7 w/; n=1.8 w/; n=1.9 w/; n=2.0 w/; n=2.1 w/; n=2.2

0.144 0.192 0.205 0.220 0.231 0.248 0.259

Enhancement ratio 1.33 1.42 1.53 1.60 1.72 1.80

Waveguide loss

Absorption/SPP

Fraction

Fraction

0.346 0.316 0.305 0.302 0.301 0.296 0.288

0.510 0.492 0.490 0.478 0.468 0.456 0.453


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Table of Content Graphic


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Optical Analysis of Power Distribution in Top-Emitting Organic Light

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