Black Phase-Changing Cathodes for High-Contrast Organic Light

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Black Phase-changing Cathodes for Highcontrast Organic Light-Emitting Diodes Jia-Xiu Man, Shou-Jie He, Tao Zhang, Deng-Ke Wang, Nan Jiang, and Zheng-Hong Lu ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Black Phase-changing Cathodes for High-contrast Organic Light-Emitting Diodes

Jia-Xiu Man,†,‡ Shou-Jie He,†,‡ Tao Zhang,†,‡ Deng-Ke Wang,†,‡ Nan Jiang,†,‡ and Zheng-Hong Lu*,†,‡,§

†

Department of Physics, and

‡

Yunnan Key Laboratory for Micro/Nano Materials &

Technology, Yunnan University, Kunming, Yunnan 650091, China §

Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario

M5S 3E4, Canada *

To whom correspondences should be addressed. E-mail: [email protected]

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ABSTRACT: High-contrast organic light-emitting diodes (HC-OLEDs) with black phase-changing (PC) cathodes have been designed and fabricated. The black PC-cathode consists of a front thin semi-transparent metal layer, an organic PC layer and a rear thick reflective metal layer. To achieve a targeted high-contrast over the full visible spectrum, several organic molecules and their binary and ternary compounds were researched to meet optical design requirement. By combining a design in the complex refractive index and in the thickness of the PC layer, it is demonstrated that the optical reflectance of the HC-OLED can be reduced from ~ 80% to below that of the glass substrate (~ 5%) by using a ternary organic compound as the PC layer. This black PC-cathode produces extremely high optical contrast and yet it does not alter the current-voltage characteristics of the device.

Keywords: high-contrast OLED, black phase-changing cathode, ternary organic compound, complex refractive index


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Displays based on organic light-emitting diodes (OLEDs) are capturing growing attention due to their light weight, flexibility, fast response time, wide color gamut, high brightness and low power consumption.1-5 A typical OLED device consists of organic layers sandwiched between two electrodes. One of the electrodes has to be transparent or semi-transparent, while the other electrode is usually made of a highly reflective metal. The strong reflection from the metal electrode benefits the optical out-coupling efficiency of the OLEDs as the back-emitted light from the organic layer is reflected forward. However, this reflective electrode also leads to OLEDs having a very low contrast ratio (CR) under ambient lighting condition. The CR can be expressed mathematically as a ratio of the brightest and darkest elements of a display, taking into account the ambient reflected light from the device:6

CR =

௅೚೙ ାோವ ௅ೌ೘್೔೐೙೟ ௅೚೑೑ ାோವ ௅ೌ೘್೔೐೙೟

(1)

where Lon and Loff are the luminance values of “on” and “off” pixels on the display, respectively; Lambient is the ambient luminance. RD is the reflectance of the display, given by:

ܴ஽ =

ഊ భ

‫׬‬ഊ మ ௏ሺఒሻ·ோሺఒሻ·ௌሺఒሻௗఒ ഊ భ

‫׬‬ഊ మ ௏ሺఒሻ·ௌሺఒሻௗఒ

(2)

where V(λ) is the photopic function defined in 1931,7 R(λ) is the reflectance from the display, S(λ) is the optical spectrum of ambient light (CIE standard such as D65 source). As many OLED displays are targeted for outdoor use, high CR under strong external lighting (particularly under sunlight) is one of the most important parameters for displays. Therefore, enhancing CR for OLED displays is a significant issue. Equation 1 indicates that the strategies for improving CR can be improving Lon and reducing RD for OLED displays, and the latter has been intensively researched.8-10 Circular polarizer (CP) is currently the only technology for LCD and OLED displays. CP is composed of a quarter wavelength plate and a linear polarizer.11 CP, however, is very difficult to be used in flexible form, and thus display 3 ACS Paragon Plus Environment

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R&D scientists have been seeking new alternative solutions for next generations of light weight and flexible OLED displays, for instance, antireflection coatings,12 black cathode,13,14 absorbing transport layer,15 destructive interference structures,8, 16 and tandem structure.17, 18 However, these methods have trade-offs in providing a similar luminous reflectance to CP while keeping a high out-coupling efficiency. Moreover, in some of these approaches, additional layers are needed in the electrical active region causing drift on the OLED’s electrical properties. Metal-organic-metal (MOM) cathode structure is a simple black cathode technology to improve the CR of OLED displays.13, 14, 19 MOM cathode structure consists of a front thin semi-transparent metal layer, a phase-changing (PC) organic layer and a rear thick reflective metal layer. This cathode structure is referred to as black PC-cathode in the following discussion. The low reflection is realized by the cancellation (destructive interference) of the two reflected light waves, one from the front thin metal layer and another one with a π-phase shift from the rear thick metal layer. Compared with a metal-inorganic-metal cathode, this black PC-cathode structure can be easily fabricated by low-temperature evaporation of organic molecules.20, 21 Most organic molecules, however, have limited optical absorption in a few narrow spectral regions. This prevents the use of a simple organic molecule in eliminating cathode reflection from the entire visible spectrum. In this paper, we design the black PC-cathode by optimizing both the complex refractive index and the thickness of PC layer. We vary the complex refractive index n+iκ (n = 0~5, κ = 0~5) of the organic material to find the ideal optical constants which render the device lowest reflectance. With this computed ideal optical constants set as the target, we researched several organic molecules and their binary and ternary mixtures. We discovered that a SubPc: PbPc: C60 ternary mixture meets the optimal optical design requirement. Based on this ternary PC layer, we demonstrated that the reflectance of HC-OLED can be reduced to as low as that from a single face glass (~ 5%) over the full visible spectrum.


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EXPERIMENTAL SECTION All organic materials used in this work were purchased from Luminescence Technology Corporation and used in as-received form for device fabrication. All devices were fabricated in tri-chamber high-vacuum system with a base pressure of ~10-7 torr. The substrates are commercial stripes patterned ITO coated glass with a sheet resistant of 15 Ω/□. The same glass substrates without ITO were used to make the black PC-cathode structure. The substrates were sequentially cleaned in an ultrasonic bath for 5 min by detergent, acetone, and ethanol. The substrates were then treatment by UV-Ozone for 15 min. Finally the substrates were placed on the sample holders and transferred into the organic chamber. The deposition rate was 1Å/s for organics and 0.1 Å /s for LiF and MoO3 in one vacuum chamber, and finally the cathode Al was deposited in another vacuum chamber without breaking vacuum. The device active area is 2 mm2. Layer thickness and deposition rate of evaporated films were monitored using a quartz crystal microbalance. The optical constants of organics were measured by ELLIP-SR-I spectroscopic ellipsometer. The current density-voltage (J-V) characteristics were measured using HP4140B picoammeter in air ambient and at room temperature. The luminance measurements were taken using a Minolta LS-110 luminance meter. The electroluminescence spectra were measured using Ocean Optics USB-4000 spectrometer. The near-normal incidence reflectivity spectrums were taken using Hitachi 4100 spectrophotometer with a tungsten lamp source. RESULTS AND DISCUSSION Figure 1a and b illustrate the structure of a conventional OLED (Control Device) and a black PC-cathode OLED (HC-OLED Device), respectively. The Control Device consists of the following layers: Indium tin oxide (ITO) as anode, N,N’-Bis(naphthalen-1-yl)-N,N′bis(phenyl)-benzidine (NPB) as hole transport layer, Tris(8-hydroxy-quinolinato) aluminium (Alq3)

as

electron

transport

layer,


and

1

wt.%

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2,3,6,7-Tetrahydro-1,1,7,7,-tetramethyl-1H,5H,11H-10-(2-benzothiazolyl)quinolizino[9,9a,1g h] coumarin (C545T) doped in Alq3 as the emission layer, and aluminum (Al) as the cathode. MoO3 and LiF are used as hole injection and electron injection layer respectively. Compared with the Control Device, the HC-OLED use a black PC-cathode which includes a front thin Al semi-transparent layer (5 nm), an organic PC layer and a rear thick reflective Al (100 nm) layer. (a)

(b) Al (100 nm)

PC-Cathode

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PC Layer LiF (1 nm)/Al (100 nm)

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HC-OLED

Figure 1. (a) The schematic structure diagram of a Control OLED device, and (b) a high-contrast OLED device with a black PC-cathode As the PC layer has little influence on the electrical properties of OLED device,21 it is feasible to design the optical properties of black PC-cathode to enhance the CR of OLED device. Fullerene-C60, plumbum (II) phthalocyanine (PbPc) and Boron subphthalocyanine chloride (SubPc) are selected as possible candidates for the PC layer of HC-OLED. The transfer matrix method22-26 was used to calculate the reflectance of HC-OLED device (Figure S1), and the results are shown in Figure 2a-c. The color map represents the reflectance of each OLED with different thickness PC layer in 300 ~ 800 nm incident wavelength. The darker of the blue color indicates the lower of the reflectance of device. We find that there is no sweet PC layer thickness that satisfies ultra-low reflectance over the entire visible spectrum from a single component organic layer. As the refractive index of the mixed layer follows a linear combination of individual component’s refractive index,27 a broad dark blue belt nearly 6 ACS Paragon Plus Environment

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parallel to wavelength axis at 70 nm thickness is shown in Figure 2d. To understand this phenomenon, we treat the PC layer as an optical coating28,

29

in order to calculate the

reflectance of HC-OLED with a PC layer having n+iκ (n = 0~5, κ = 0~5) complex refractive index. This helps to find the ideal theoretical complex refractive index values for the PC layer (Figure S2). (b)

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Figure 2. Calculated reflectance (shown in color scale) map of HC-OLED device with various organic molecules as the PC layer: (a) C60, (b) PbPc, (c) SubPc and (d) SubPc: PbPc: C60 (the weight ratio is 1:1:1). Figure 3 shows the ideal computed complex refractive index (n and κ) of a 70 nm thick PC layer (black dots) and the experimental complex refractive indexes of C60, SubPc, PbPc, SubPc: C60 binary mixture (the weight ratio is 1:1) and SubPc: PbPc: C60 ternary mixture (the weight ratio is 1:1:1). As the n, κ values of real organic materials hardly satisfy the theory values in the whole visible spectrum, the range for n, κ values of HC-OLED reflectance less than 10% are also shown as the shaded regions. Obviously, the n, κ values of SubPc and PbPc fall almost outside the shaded regions. Although all the κ values of C60 lie in the shaded region, but its n values at λ = 420 ~ 570 nm fall outside of this region. Complex refractive index of the ternary mixture, however, fall almost in the shaded region except for a narrow 7 ACS Paragon Plus Environment

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blue region (λ = 450 ~ 470 nm) for n. The fit for Psi and delta of C60, SubPc, PbPc, and SubPc: PbPc: C60 ternary mixture (the weight ratio is 1:1:1) measured by ellipsometer are shown in Figure S3. (a)

3.5 Theory C 60

3.0

SubPc PbPc SubPc:C 60

2.5

n

SubPc:PbPc:C 60

2.0 1.5 1.0

(b)

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0.6 0.4 0.2 0.0

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Figure 3. Computed ideal theoretical (n,κ) values of a 70 nm thick PC layer which leads a zero-reflectance HC-OLED (black dot), and (n,κ) values for less than 10% reflectance HC-OLED (shaded region). The measured (n,κ) values from various organic materials: C60, SubPc, PbPc, SubPc: C60 binary mixture (the weight ratio is 1:1), and SubPc: PbPc: C60 ternary mixture (the weight ratio is 1:1:1). To verify the validity of the theoretical calculations, we fabricate a few test samples. As black PC-cathode structure is relatively simple to fabricate, we made several test samples having a structure as: Al (5 nm)/SubPc: PbPc: C60 (x nm) /Al (100 nm), x = 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, the weight ratio of SubPc, PbPc and C60 is 1:1:1. The measured reflectance spectra of the black PC-cathodes are shown in Figure 4a. Of all tested samples, the 70 nm thick SubPc: PbPc: C60 ternary mixture PC-cathode has the lowest reflectance. Figure 4c and d show computed reflectance map from the black PC-cathode stack and the whole HC-OLED stack. The computed maps show the same trend in PC layer thickness, although 8 ACS Paragon Plus Environment

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the reflectance from the PC-cathode stack is generally higher than that from the whole HC-OLED stack. Next we proceed to fabricate ITO/MoO3 (1 nm)/NPB (60 nm)/Alq3:C545T (1 wt.%, 30 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm) Control OLED structure and ITO/MoO3 (1 nm)/NPB (60 nm)/Alq3:C545T (1 wt.%, 30 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (5 nm)/SubPc: PbPc: C60 (70 nm)/Al (100 nm) HC-OLED structure, the weight ratio of SubPc, PbPc and C60 is 1:1:1. The test results are shown in Figure 4b, which shows a ~ 80% reflectance from the control OLED and a ~ 5% reflectance from the HC-OLED. Because the complex refractive index of ternary mixture falls outside of the shaded region (reflectance less than 10% region) at λ = 450 ~ 470 nm, the HC-OLED have a relatively higher reflectance in wavelengths shorter than 480 nm. (a)

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Figure 4. Near-normal incidence (5°) reflectance spectra measured from (a) black PC-cathodes with 50 nm, 60 nm, 70 nm, 80 nm, 90 nm thick PC layer and from (b) a Control OLED device, HC-OLED with a 70 nm thick PC layer and a glass substrate. Theoretical computed reflectance (shown in color scale) map of (c) black PC-cathodes and (d) HC-OLEDs. The PC layers are made of SubPc: PbPc: C60, the weight ratio is 1:1:1. In this work, a 70 nm thick SubPc: PbPc: C60 ternary mixture is used as the PC layer to 9 ACS Paragon Plus Environment

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test electrical characteristics of the HC-OLED. Figure 5 depicts the L-I-V characteristics and current efficiency vs. current density curves of Control OLED and HC-OLED. Current efficiency vs. voltage and power efficiency vs. voltage curves are depicted in Figure S4. The black PC-cathode not only eliminates the ambient light, it also eliminates the emitted light striking the cathode. So the luminance of HC-OLED at a given driving voltage is relatively lower than that of the Control OLED. As the optical output intensity of OLED is proportional to square of a light wave’s electric field amplitude (I∝E2)22, the optical output intensity is expected to be reduced by 75% if the PC cathode absorbs 100% of the light. From Figure 4a, we observe that the reflection of PC-cathode is ~15%, the current efficiency is reduced by ~60%, as expected. For the current density-voltage characteristics, however, the data show similar behavior for these two devices. This is because the thin semi-transparent metal layer functions as a floating electrode, where the electron is directly injected to the emission zone.21 Furthermore, the SubPc: PbPc: C60 is extremely conductive (see Figure S6), so there is little voltage loss, especially at low voltages. Both devices have the same EL spectrum (Figure S5). Figure 5d shows the photographic pictures of a control OLED pixel and a HC-OLED pixel operated at 1000 cd/m2 on the same substrate. The pictures clearly demonstrate the visual difference in contrast between these two OLED structures.

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Figure 5. (a) L-V characteristics, (b) J-V characteristics, (c) current efficiency vs. current 10 ACS Paragon Plus Environment

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density curves, and (d) photographic pictures of the Control OLED pixels and the HC-OLED pixels. The rectangles in (d) indicate the active pixels region in “on” and “off” states. In the above we discussed specifically the case of a 70 nm thick PC layer for producing HC-OLED. In theory, any thickness of PC layer may be used to render ultra-low reflection by finding a matching complex refractive index. Figure 6 summarizes calculated theoretical (i.e. required) complex refractive index (shown in color scale) of materials for the PC layer for zero reflection HC-OLED: (a) n(λ) values and (b) κ(λ) values for different PC layer thicknesses. As the incident light wavelength increases, the required n values increases for a given thickness of PC layer. The required κ values have two peaks, one at 400 nm and another at 590 nm. For a given incident light wavelength, the required n and κ values of the PC layer decreases as the thickness increases. So one can vary the thickness of PC layer to select different sets of optical constants for the HC-OLED, provided one can obtain this set of optical constants (n and κ values) from a new organic compound. This new compound can then be used to fabricate HC-OLED.


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Figure 6. Calculated complex refractive index color charts of the materials in the PC layer for zero reflection HC-OLED: (a) n value (shown in color scale), and (b) κ value (shown in color scale) at different PC-layer thicknesses. CONCLUSIONS In summary, we have successful designed and fabricated a black PC-cathode for HC-OLED device. The optical reflectance of HC-OLED with a 70 nm thick SubPc: PbPc: C60 ternary mixture as PC layer in visible spectrum is reduced to below that of the glass substrate (~ 5%). This high contrast device also shows excellent diode characteristics, and has the same EL spectrum as a normal OLED. Theoretically, if the complex refractive index of real materials fits the calculated ideal value, zero optical reflectance HC-OLEDs can be made. As there is not a single organic molecule possess the ideal theoretical n and κ values, blending two or more organic molecules is demonstrated as an effective method to make new optical functional compounds for high contrast OLED. 12 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The transfer matrix method for optical calculation, optical design PC-layer method, details of the ellipsometry studies of organic thin films, efficiency and EL spectra of the Control OLED and the HC-OLED AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ORCID Zheng Hong Lu: 0000-0003-2050-0822 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This research was supported by National Natural Science Foundation of China (Grant No. U1402273), Natural Science and Engineering Research Council of Canada, and the Award for Excellent Doctoral Student in Yunnan Province.


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REFERENCES (1) Ding, W.; Wang, Y.; Chen, H.; Chou, S. Y. Plasmonic Nanocavity Organic Light-Emitting Diode with Significantly Enhanced Light Extraction, Contrast, Viewing Angle, Brightness, and Low-Glare. Adv. Funct. Mater. 2014, 24, 6329-6339. (2) Qian, M.; Shi, X. B.; Ma, J.; Liang, J.; Liu, Y.; Wang, Z. K.; Liao, L.-S. A stacked Al/Ag anode for short circuit protection in ITO free top-emitting organic light-emitting diodes. RSC Adv. 2015, 5, 96478-96482. (3) Lim, B. W.; Jeon, H. S.; Suh, M. C. Top-emission organic light emitting diodes with lower viewing angle dependence. Synth. Met. 2014, 189, 57-62. (4) Zhou, L.; Ou, Q. D.; Shen, S.; Zhou, Y.; Fan, Y. Y.; Zhang, J.; Tang, J. X. Tailoring Directive Gain for High-Contrast, Wide-Viewing-Angle Organic Light-Emitting Diodes Using Speckle Image Holograpy Metasurfaces. ACS Appl. Mater. Interfaces 2016, 8, 22402-22409. (5) Cai, X.; Gerlach, C. P.; Frisbie, C. D. Current-Voltage Hysteresis and Memory Effects in Ambipolar Organic Thin Film Transistors Based on a Substituted Oligothiophene. J. Phys. Chem. C 2007, 111, 452-456. (6) Dobrowolski, J. A.; Sullivan, B. T.; Bajcar, R. C. Optical interference, contrast-enhanced electroluminescent device. Appl. Opt. 1992, 31, 5988-5996. (7) Schanda, E. J. Colorimetry: understanding the CIE system, Wiley-Interscience, John Wiley and Sons, USA, 2007. (8) Cho, H.; Chung, J.; Lee, J.; Kim, E.; Yoo, S. Dual optical role of low-index injection layers for efficient polarizer-free high contrast-ratio organic light-emitting diodes. Opt. Express 2015, 23, 10259-10265. (9) Singh, R.; Narayanan Unni, K. N.; Solanki, A.; Deepak, Improving the contrast ratio of OLED displays: An analysis of various techniques. Opt. Mater. 2012, 34, 716-723.


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(10) Cho, H.; Yoo, S. Polarizer-free, high-contrast inverted top-emitting organic light emitting diodes: effect of the electrode structure Opt.Express 2012, 20, 1816-1824. (11) Kim, S. Y.; Lee, J. H.; Lee, J. H.; Kim, J. J. High contrast flexible organic light emitting diodes under ambient light without sacrificing luminous efficiency. Org. Electron 2012, 13, 826-832. (12) Chen, S.; Xie, J.; Yang, Y.; Chen, C.; Huang, W. High-contrast top-emitting organic light-emitting diodes with a Ni/ZnS/CuPc/Ni contrast-enhancing stack and a ZnS anti-reflection layer J. Phys. D: Appl. Phys. 2010, 43, 365101-365106. (13) Kim, Y. H.; Lee, S. Y.; Song, W.; Meng, M.; Lu, Z. H.; Kim, W. Y. High contrast green OLEDs using inorganic metal multi layer. Synth. Met. 2011, 161, 2211-2214. (14) Zhou, Y. C.; Ma, L. L.; Zhou, J.; Gao, X. D.; Wu, H. R.; Ding, X. M.; Hou, X. Y. High contrast organic light-emitting devices with improved electrical characteristics. Appl. Phys. Lett. 2006, 88, 233505. (15) Xie, W. F.; Zhao, Y.; Li, C. N.; Liu, S. Y. Contrast and efficiency enhancement in organic light-emitting devices utilizing high absorption and high charge mobility organic layers. Opt. Express 2006, 14, 7954-7959. (16) Poitras, D.; Kuo, C. C.; Py, C. Design of high-contrast OLEDs with microcavity. Opt. Express 2008, 16, 8003-8015. (17) Ding, B. F.; Alameh, K. High-Contrast Tandem Organic Light-Emitting Devices Employing Semitransparent Intermediate Layers of LiF/Al/C60. J. Phys. Chem. C 2012, 116, 24690-24694. (18) Yang, C. J.; Cho, T. Y.; Lin, C. L.; Wu, C. C. Energy-recycling high-contrast organic light-emitting devices. J. Soc. Inf. Display 2008, 16, 691-694. (19) Xie, Z. Y.; Hung, L. S. High-contrast organic light-emitting diodes. Appl. Phys. Lett. 2004, 84, 1207-1209.


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For Table of Contents Use Only Black Phase-changing Cathodes for High-contrast Organic Light-Emitting Diodes

Jia-Xiu Man,†,‡ Shou-Jie He,†,‡ Tao Zhang,†,‡ Deng-Ke Wang,†,‡ Nan Jiang,†,‡ and Zheng-Hong Lu*,†,‡,§ 100

Reflectance (%)

80 Cathode Organic Layers

60

Anode Substrate PC-Cathode

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40 Metal PC Layer Metal Organic Layers

20

Anode Substrate

0

400

500

600 Wavelength (nm)

700

800


Black Phase-Changing Cathodes for High-Contrast Organic Light

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