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Abstract
We demonstrate independently and simultaneously controlled color-tunable organic light-emitting diodes (OLEDs) with vertically stacked blue, green, and red elements. The blue, green, and red elements were placed at the bottom, middle, and top positions, respectively, forming color-tunable OLEDs. The independently driven blue, green, and red elements in the color-tunable OLEDs exhibited low driving voltages of 5.3 V, 3.0 V, and 4.6 V, as well as high external quantum efficiencies of 11.1%, 10.9%, and 9.6%, respectively, at approximately 1000 cd/m2. Each element in the color-tunable OLEDs showed high-purity blue, green, and red colors with little parasitic emission owing to the delicately designed device structure resultant from optical simulations. The color-tunable OLEDs could produce any colors inside the triangle formed with blue (0.136, 0.261), green (0.246, 0.697), and red (0.614, 0.386) Commission Internationale de l'éclairage (CIE) 1931 color coordinates. In addition, the correlated color temperatures (CCTs) of white colors in the color-tunable OLED can be easily changed from the warm white to the cool white by controlling the red, green, and blue emissions simultaneously. The white colors in the color-tunable OLED have the CIE 1931 color coordinate of (0.304, 0.351), with a CCT of 6289 K and (0.504, 0.440), with a CCT of 2407K at the driving voltage of 5 V (blue), 2.8 V (green), 4.4 V (red), and 4.6 V (blue), 3 V (green), 5 V (red), respectively. Furthermore, the white color in the color-tunable OLED exhibited a high color rendering index (~88.7) due to vertically stacked three color system. Moreover, we successfully fabricated a large-sized, 14 × 12 pixel array of the color-tunable OLEDs to demonstrate lighting and display applications, respectively.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic light-emitting diodes (OLEDs) have recently been applied to a variety of devices such as a TVs, mobile phones, wearable devices, and solid-state lighting for indoor and automotive applications. This widespread use is due to their many advantages which include a wide range of colors, high contrast ratio, fast response times, low power consumption, and a high form factor with flexible properties [1–3]. There is a growing demand for high- resolution display panels, with the increasing interest in augmented reality (AR) and virtual reality (VR). Increased display pixel resolution can improve the immersion of the display and reduce eye fatigue [4]. In addition, an ultra-high-resolution display panel is also required for the development of full stereoscopic displays such as light-field displays or holograms [5].
Current display panels typically arrange blue (B), green (G), and red (R) sub-pixels laterally to realize full color. In other words, three sub-pixels are required for each pixel, and this low geometric fill-factor inherently limits the enhancement of the resolution of display panels. B, G, and R OLEDs can be vertically stacked to make color-tunable OLEDs that can produce full color with only one pixel, because OLEDs can be transparent [6,7]. This device architecture reduces the fine pattering and aligning process of B, G, and R sub-pixels and enables small pixels with a high fill factor, resulting in ultra-high-resolution displays. Moreover, this device architecture can be applied to solid-state lightings, making it possible to easily fabricate color-tunable lightings without the complex sub-pixel formation processes.
Since the first demonstration of color-tunable OLEDs by Burrows et al. in the 1990s [8], many papers on color-tunable OLEDs were reported [9–12]. Two vertically stacked OLEDs of different colors were commonly used to alter the color temperature of white light. However, white OLEDs of two colors, which are B and R or B and yellow (Y), generally have a low color rendering index (CRI) [13, 14]. Three-wavelength color-tunable OLEDs are much more advantageous than the conventional two-wavelength color-tunable OLEDs in terms of improving the CRI of the white color. For the proper application of a color-tunable OLED to the pixels of the display as well as lighting, a color-tunable OLED should have three elements, B, G, and R, and these three elements should be independently controlled to produce various colors. Some papers have reported full-color, color-tunable OLEDs using B, G, and R elements; however, their devices have high driving voltages and low efficiencies [15–17].
In this work, we fabricated color-tunable OLEDs with a vertical stack of three primary colors (B, G, and R). Phosphorescent OLEDs were utilized to B, G, and R elements for high efficiency. Through careful device structure design using optical simulations, we successfully demonstrated efficient color-tunable OLEDs with independently controlled B, G, and R elements which have little color-distortion caused by the micro-cavity effect. In addition, we fabricated a large-sized color-tunable OLED panel for lighting applications and a 14 × 12 color-tunable OLEDs pixel array for display applications.
2. Experiments
Patterned ITO glass substrates were used for device fabrication. The substrates sequentially cleaned with acetone and methanol using ultrasonic bath, followed by rinsing with deionized water. After drying the substrates, organic, inorganic materials and metals were deposited in succession using the vacuum thermal evaporation method without breaking the vacuum. When all materials were deposited, the temperature of the substrate was not controlled. During the deposition of the doping layers, the deposition rates of both the host and dopant materials were controlled with a quartz crystal oscillator. The silver (Ag) for the intermediate electrodes (IEs) was deposited under high-vacuum conditions (~7 × 10−8 Torr) using a boron nitride boat and the deposition rate of the Ag was approximately 2 Å/s. The fabricated OLEDs were transferred to an environmentally inert glove-box, where they were encapsulated using a UV-curable epoxy and a glass cap containing a desiccant.
The current density-voltage (J–V) characteristics were measured using a Keithley-238 source-measure unit, and the luminance (L) and electroluminescence (EL) spectra were examined using a spectroradiometer (Model CS-2000, Konica Minolta). The J–V–L measurements were conducted at room temperature in a dark room. The efficiencies were calculated from the luminance, current density, and EL spectrum, and were calibrated using an integrating sphere (HM series, Otsuka Electronics). To operate B, G, and R elements in the color-tunable OLEDs independently, three source-measure units (Keithley-238, Keithley-2400, Keithley-6517A) were used. To simulate the optical properties of the devices, we used all the measured refractive indices (refractive index (n) and extinction coefficient (k)) of the organic materials, as determined using an ellipsometer (M-2000D, J.A. Woollam Co.). The n and k values that were used for the Ag were found in the literature [18].
3. Results and discussion
Figure 1 illustrates the fabrication process of the color-tunable OLEDs with three primary colors. The pre-patterned ITO glass had four electrodes for the bottom, intermediate 1 and 2, and top electrode contacts. The organic layers for the bottom OLEDs were deposited onto the pre-patterned ITO glass, following which semi-transparent metal layers were deposited onto the organic layers for the IE 1. The same processes were repeated for the middle and top OLEDs. A common shadow mask was used for all the organic layer depositions, which means that fine metal masks are not required for red, green, and blue patterning. The IE 1 was used as a common electrode to the top electrode of the bottom OLED and the bottom electrode of the middle OLED, and the IE 2 was used as a common electrode to the top electrode of the middle OLED and the bottom electrode of the top OLED.
Fig. 1 Fabrication process of the color-tunable OLEDs with three primary colors. The intermediate electrode 1 is used as a common electrode to the top electrode of the bottom OLED and the bottom electrode of the middle OLED, and the intermediate electrode 2 is used as a common electrode to the top electrode of the middle OLED and the bottom electrode of the top OLED, respectively.
Figure 2(a) shows the device structure of the color-tunable OLED with the independently controlled B, G, and R OLEDs. To design the structure of the color-tunable OLEDs, we optically simulated and calculated the efficiency and color gamut by changing the thicknesses of the HTLs and ETLs of the bottom, middle, and top OLEDs elements in the devices. We also considered electrical properties such as an electron-hole charge balance when we determine the thicknesses of each functional layers of the devices because the electron-hole charge balance is one of critical factors to decide the performance of the device [19]. The B, G, and R OLEDs were at the bottom, middle, and top, respectively, in order to reduce the optical loss from their different optical bandgaps (Eg), as shown in Fig. 2(b). The characteristics of the devices can be varied, depending on the stacking order of the B, G, and R OLEDs, and the systematic study for optimization of the device by controlling the thickness of each functional layer and stacking order of B, G, and R are under way. In the case of the bottom OLED, a blue phosphorescent OLED was used, which comprised an ITO anode, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) hole-injection layer (HIL), 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC) hole-transport layer (HTL), 4,4',4”-tri(N-carbazolyl)triphenylamine (TcTa) electron-blocking layer (EBL), 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) doped with iridium(III) bis[(4,6-difluorophenyl) pyridinato-N,C2’]picolinate (FIrpic) as the blue phosphorescent emitting layer (EML), 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB) electron-transport layer (ETL), and lithium fluoride (LiF) and aluminum (Al)/silver (Ag) as the electron-injection layer (EIL) and semi-transparent cathode, respectively. This thin Ag layer could also be used as the common cathode for the middle OLED. To operate bottom, middle, and top OLED simultaneously with low operating voltages in the color-tunable OLED, the top electrode of the bottom OLED and the bottom electrode of the middle OLED should play common role of the cathode and the top electrode of the middle OLED and the bottom electrode of the top OLED should play common role of the anode. Therefore, the middle OLED should be an inverted structure for the common IE [20]. A green phosphorescent, inverted OLED was used for the middle OLED. The green device consisted of a bottom Ag common cathode, lithium (Li)-doped tris(3-(3-pyridyl)mesityl)borane (3TPYMB) EIL due to its very efficient electron injection properties [21], pristine 3TPYMB ETL, 26DCzPPy doped with tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) as the green phosphorescent EML, TcTa EBL, TAPC HTL, HAT-CN HIL, and a top Ag as a semi-transparent anode, respectively. Finally, a red phosphorescent OLED was used for the top OLED. Thin Ag which was used as the anode for middle device was also used as the bottom anode for the top OLED. HAT-CN, TAPC, and TcTa were used as the HIL, HTL, and EBL, respectively. For the red emission, 26DCzPPy doped with Bis(2-methyldibenzo[f,h]quin-oxaline) (acetylacetonate)iridium(III) (Ir(MDQ)2(acac)) was used as the EML. BmPyPB, LiF, and Al were used as the ETL, EIL, and top cathode, respectively.
Fig. 2 (a) Schematic structure of the color-tunable OLED with independently controlled B, G, R OLEDs and (b) schematic optical bandgaps, depending on colors.
Figure 3(a) shows the J-V-L characteristics of the independently controlled B, G, and R OLEDs in the color-tunable OLEDs. Each device shows typical OLED J-V characteristics regardless of the device position. The blue device in the bottom region had a driving voltage of approximately 5.3 V for 1,000 cd/m2. We also fabricated single blue OLEDs with same structure (ITO (150 nm)/HAT-CN (10 nm)/TAPC (50 nm)/TcTa (10 nm)/26DCzPPy:FIrpic (8%, 10 nm)/BmPyPB (60 nm)/LiF (1 nm)/Al (1.5 nm)/Ag (14 nm)) and compared the J-V-L characteristics as shown in Fig. 3(b). The blue devices in the single and color-tunable OLEDs showed nearly the same J-V characteristics regardless of the state of the middle OLED. This result implies that the middle OLED in the color-tunable OLED rarely affects the electrical properties of the bottom OLED. In addition, both devices had a similar luminance at the same voltage, which implies that most of the light from the EML emitted to the bottom side in the color-tunable OLED. The middle green device showed a high current density and very low driving voltage when compared with the other devices. The device had an extremely low turn-on voltage of approximately 2.5 V and a driving voltage of approximately 3.0 V for 1,000 cd/m2. This is due to efficient hole and electron injection by the HAT-CN and Li-doped 3TPYMB, respectively [21,22]. The red device in the top region of the device had a low turn-on voltage of approximately 3.4 V and driving voltage of approximately 4.6 V for 1,000 cd/m2. We fabricated single red OLEDs with similar structure (ITO (150 nm)/Ag (14 nm)/HAT-CN (10 nm)/TAPC (63 nm)/TcTa (10 nm)/26DCzPPy:MDQ2Ir(acac) (7%, 10 nm)/BmPyPB (50 nm)/LiF (1 nm)/Al (100 nm)) and compared their J-V-L characteristics as shown in Fig. 3(c). The red devices in the single and color-tunable OLEDs showed nearly the same J-V characteristics regardless of the state of the middle OLED as the blue devices. This result suggests that the middle OLED in the color-tunable OLED minimally affects the electrical properties of the top OLED. However, the single red device showed a slightly higher luminance when compared with the red device in the color-tunable OLED at the same voltage. For example, the single red device had a luminance of 2,170 cd/m2, whereas the red device in the color-tunable OLED showed 1334 cd/m2 at the same voltage of 4.8 V. This is because the red device was placed in the top position of the color-tunable OLED. The light from the red EML must pass through multiple layers of organic and thin-metal IE materials, resulting in optical loss. The B, G, and R OLEDs in the color-tunable OLED devices showed external quantum efficiencies (EQEs) of 11.1% at 5 mA/cm2 (~1,000 cd/m2), 10.9% at 1.65 mA/cm2 (~1,000 cd/m2), and 9.6% at 9 mA/cm2 (~1,000 cd/m2) as shown in Fig. 3(d). These values were slightly lower, compared with the EQEs of the single devices due to optical loss from many layers of organic and thin-metal IE. For example, the single blue and red devices have the EQEs of 12.1% (bottom) and 5.9% (top) at 5 mA/cm2 (bottom: ~1,100 cd/m2 and top: ~240 cd/m2) and 13.8% at 9 mA/cm2 (~1,500 cd/m2), respectively. Nevertheless, to the best of our knowledge, the efficiencies observed for these B, G, and R OLEDs in the color-tunable OLED device with three vertically stacked elements are the highest values obtained to date.
Fig. 3 J-V-L characteristics of (a) independently controlled B, G, R OLEDs in the color-tunable OLED, (b) B OLEDs, (c) R OLEDs in the single and the color-tunable (middle G OLED off (0 V) and on (3 V) state) device structures, (d) EQE and luminous efficacy (LE) (at ~1,000 cd/m2) of independently controlled B, G, R OLEDs in the color-tunable OLED.
Figure 4(a) shows the normalized EL spectra of the independently controlled B, G, and R devices in the color-tunable OLEDs at about 500 cd/m2. Each color clearly operated with minimal parasitic emission. The blue EL spectra had a main emission peak at 472 nm with a 496 nm shoulder peak, which is the typical emission peak for FIrpic-based devices. It also showed a very low parasitic emission peak at 578 nm due to micro-cavity effect [15]. The green and red EL spectra had main emission peaks at 533 nm and 608 nm, respectively, without any parasitic emission peaks. An optical simulation of the device was also performed using commercial software (SETFOS, Fluxim). In this simulation, we assumed that the orientation of the dipole was isotropic and it was located at a middle of each EML because 26DCzPPy is a typical bipolar host material [23]. The simulated EL spectra of the B, G, and R devices are well-matched with the measured EL spectra of B, G, and R devices, respectively as shown in Fig. 4(a). The Commission Internationale de l'éclairage (CIE) 1931 color coordinates of B, G, and R were (0.136, 0.261), (0.246, 0.697), and (0.614, 0.386), respectively. From the CIE 1931 color coordinates, we can calculate the color-gamut of the color-tunable OLED, which was approximately 86.8% based on the sRGB [24]. Figure 4(b) shows that the EL spectrum of the color-tunable OLED can be changed to be various colors by independently driving each color element. Compared with the conventional two-stack, color-tunable OLEDs, this three-stack color-tunable OLED can output more colors. For example, white OLEDs with different correlated color temperature (CCT) can be achieved as shown in Fig. 4(c). When the driving voltage of each device was B (5 V), G (2.8 V), and R (4.4 V), the CIE 1931 color coordinate was (0.304, 0.351) with a CCT of 6289 K and the LE was approximately 17.8 lm/W. The CCT of the white light can be decreased to 4281 K with CIE 1931 color coordinates of (0.375, 0.395) and LE of approximately 18.1 lm/W by reducing the driving voltage of the blue from 5 V to 4.8 V. When we calculated the maximum CRI using the blue and red spectra of the color-tunable OLED, the CRI value was approximately 57.2. However, the maximum CRI can be dramatically enhanced to 89.0 by using the blue, green, and red spectra of the color-tunable OLED for the calculation. When the driving voltage of each device was B (4.6 V), G (3 V), and R (5 V), the CRI of the color-tunable OLED was approximately 88.7 with a CCT of 2407 K, which is similar to the value of the calculated result. Figure 4(d) demonstrates successful operation of the color-tunable OLEDs with various colors. We estimated color coordinate change of B, G, and R devices in the color-tunable OLED depending on viewing angles as shown in Figs. 4(e) and 4(f). The color coordinates of B, G, and R devices in the color-tunable OLED were slightly changed due to micro-cavity effect by reflective metal electrodes [25]. To improve the color stability with viewing angles, low reflective electrodes are required and that study is under way.
Fig. 4 (a) Normalized EL spectra of independently controlled B, G, R OLEDs (Solid line: measured values, dashed line: simulated values), (b) Normalized EL spectra, (c) CIE 1931 color coordinates of three independently controlled B, G, R OLEDs under various driving voltages, (d) photograph of color-tunable OLEDs with various colors, (e) CIE x, and (f) CIE y of B, G, R OLEDs, depending on viewing angles.
We also calculated the EL spectra of B, G, and R with different thicknesses of the HTLs and ETLs as well as the IEs of bottom, middle, and top elements in the color-tunable OLED as shown in Fig. 5. The device structure in Fig. 2(a) was used for the calculation and the thicknesses of other functional layers were fixed when the thickness of each functional layer was changed. The thicknesses of the HTL and ETL of the bottom element were the main parameters that mainly affected the EL spectra of the bottom element, but they did not significantly affect the EL spectra of the middle and top elements as shown in Fig. 5(a) and (b). However, the EL spectra of B, G, and R were considerably changed depending on the thicknesses of HTL and ETL of the middle and top elements as shown in Fig. 5(c)–5(f). This result suggests that the element with metal anode and cathode is more sensitive when compared with the element with a non-metal electrode, such as ITO, due to the micro-cavity effect. In addition, the total thickness of each element is a critical factor which determines the EL spectra of each element. This is because the trend of change in the EL spectra of B, G, and R were nearly the same with the same thickness change regardless of whether it was the HTL or ETL. The thickness of the IE also affected the EL spectra of each element as shown in Fig. 5(g) and 5(h).
Fig. 5 Calculated EL spectra of B, G, R in the color-tunable OLEDs depending on the thicknesses of HTLs, ETLs, and IEs ((a) HTL1 and (b) ETL1: TAPC and BmPyPB in the bottom OLED, (c) HTL2 and (d) ETL2: TAPC and 3TPYMB/3TPYMB:Li in the middle OLED, and (e) HTL3 and (f) ETL3: TAPC and BmPyPB in the top OLED).
We fabricated large-sized, color-tunable OLEDs as shown in Fig. 6(a). The size of the glass substrate was 90 mm × 85 mm and the emission area was 54 mm × 46 mm. In spite of the size expansion, the color-tunable panel showed various colors with uniform-emission, allowing for solid-state lighting applications. Moreover, the 14 × 12 color-tunable OLED pixel arrays were demonstrated as shown in Fig. 6(b). The pixel size was 2 mm × 2mm and the R, G, and B colors were clearly observed. The dynamic operation of the color-tunable panel and pixel arrays are provided in Visualization 1 and Visualization 2 in Supplementary Materials, respectively.
Fig. 6 Photographs of (a) large-sized color-tunable OLEDs with different colors (see Visualization 1) and (b) 14 × 12 color-tunable OLEDs pixel arrays with B, G, R (see Visualization 2).
4. Conclusion
In summary, we have demonstrated color-tunable OLEDs with three vertically stacked primary colors of blue, green, and red. Each element was placed in the bottom, middle, and top position of the color-tunable OLED, respectively, and could be independently controlled. The blue, green, and red elements in the color-tunable OLEDs show a high efficiency of 11.1%, 10.9%, and 9.6% at approximately 1000 cd/m2, respectively, in spite of containing many organic functional layers and intermediate electrodes. Each color element can simultaneously operate, resulting in various potential colors such as yellow, warm white, and cool white, by controlling the driving voltage or current of each element. In addition, a large-sized and a 14 × 12 pixel array of the color-tunable OLEDs were successfully fabricated for solid-state lighting and full-color display applications. We believe that the color-tunable devices in this work can be helpful for improving the resolution of the display panels and expand the application range of OLEDs.
Funding
Korean government (MSIT)/Institute for Information and Communications Technology Promotion (IITP) (2017-0-00065); Korean government/the Electronics and Telecommunications Research Institute (ETRI) (18ZB1270).
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