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This paper demonstrates 2-stack and 3-stack white organic light-emitting diodes (WOLEDs) with fluorescent blue and phosphorescent yellow emissive units. The 2-stack and 3-stack WOLED comprises blue-yellow and blue-blue-yellow (blue-yellow-blue) combinations. The position of the yellow emitter and possible cavity lengths in different stack architectures are theoretically and experimentally investigated to reach Commission Internationale de L’Eclairage (CIE) coordinates of near (0.333/0.333). Here, a maximum external quantum efficiency (EQE) of 23.6% and current efficiency of 62.2 cd/A at 1000 cd/m2 as well as suitable CIE color coordinates of (0.335/0.313) for the blue-blue-yellow 3-stack hybrid WOLED structure is reported. In addition, the blue-yellow-blue 3-stack architecture exhibits an improved angular dependence compared to the blue-blue-yellow structure at a decreased EQE of 19.1%.
© 2016 Optical Society of America
1. Introduction
White organic light-emitting diodes (WOLEDs) are essential components for the fabrication of large size and high resolution AMOLED displays [1, 2]. Especially, the WOLED technique with red, green and blue (RGB) color filter has gained a lot of attention for large size AMOLED display applications. Moreover, WOLED with RGB color filter is advantageous compared to the RGB subpixel patterning technology due to less complicated manufacturing steps since the alignment of fine metal masks is not necessary and material mixing caused by mask sagging does not occur [3, 4]. Additionally, this approach exhibits low optical losses compared to the simple side-by-side RGB patterning. Therefore, the development of WOLEDs exhibiting CIE color coordinates of (0.333/0.333) qualifying as cool white light sources with high efficiency and stable angular dependency is essentially required for WRGB AMOLED displays [5–8].
Normally, a white OLED structure comprises multilayers with two or more emitting layers in single stacks or in multi-stack architectures. Among those structures, a multi-stack OLED design provides high efficiency with excellent device stability, and most importantly good color stability [3, 9–11]. Indeed, blue emitting materials play an important role in the generation of cool white light. In fact, multi-stack white OLED architectures with phosphorescence blue and yellow emitters have the potential to exhibit high external quantum efficiencies (EQE) through harvesting all triplet excitons for the conversion into photons [3, 9]. However, a highly efficient and stable deep blue phosphorescent emitter is not available yet. Even though the internal quantum efficiency (IQE) of blue fluorescent materials is low but their lifetime is considered to be acceptable for industrial manufacturing of WOLED devices [12].
Recently, 2-stack WOLEDs with fluorescent blue and phosphorescent orange/yellow have attracted a lot of attention due to their high efficiency values [3]. But in those cases, the phosphorescence emission intensity originating from its high IQE is too dominant to generate cool white color coordinates. Hence, there is a need to decrease the yellow peak to achieve perfect white CIE color coordinates. Several techniques are available to influence the yellow emission peak and intensity for example by controlling the electron transport layer (ETL) and emissive layer (EML) thickness, doping concentration and change of host material for charge trapping emission [13, 14]. In addition, the cavity length of each emitting unit in the multi-stack structure allows the exact color modification of the overall emission [15]. With the help of optical simulations, cavity trends can be derived that speed up the experimental procedure to achieve the desired CIE color coordinates. Hence, we focus on developing efficient cool white light emitting 2-stack and 3-stack OLEDs by combining fluorescent blue and phosphorescent yellow emission layers using charge generation layers (CGLs) with proper adjustment of cavity length of each emission unit. At first, the 2-stack blue-yellow (B-Y) (Y is close to cathode) device was theoretically and experimentally optimized. This device does not exhibit high efficiency characteristics due to the decreased yellow emission intensity. Moreover, balanced blue and yellow emission intensities without the need to decrease the yellow emission intensity are possible by using 3-stack architectures (blue-blue-yellow (B-B-Y); blue-yellow-blue (B-Y-B); yellow-blue-blue (Y-B-B)).
Here, hybrid multi-stack architectures are demonstrated that contain fluorescent blue and phosphorescent yellow emitting layers to realize a white emission which is close to perfect cool white CIE coordinates of (0.333/333). Based on the optical simulations, electron and hole transport layer (ETL and HTL) variations have been performed to adjust the cavity length in order to achieve the appropriate intensity ratio of blue and yellow. Apart from stack architectures which locate the yellow emitter close to the cathode (B-Y, B-B-Y) another 3-stack design (B-Y-B) were fabricated which improved the angular dependence of the white emission from 40° to 60° (degrees). In detail an EQE of 16.1% for the 2-stack device (B-Y) and 23.6% for the 3-stack (B-B-Y) device was achieved while the B-Y-B device exhibited 19.1%.
2. Experimental
2.1 Device fabrication and characterization
For the fabrication of blue emitting units, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) was chosen as blue host material and 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'-biphenyl (BCzVBi) as fluorescent blue dopant [16]. The yellow emitting unit was realized with the host material bis[2-(2-hydroxyphenyl)pyridine] beryllium (Bepp2) while iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2́)acetylacetonate (Ir(tptpy)2(acac)) was employed as phosphorescent yellow dopant [17, 18]. Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as hole transport and electron transport material, respectively [19]. In order to connect all OLED units in series within one stack architecture, dipyrazino[2,3-f:2̒,3̒-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) in combination with Lithium (Li) doped Bphen was used as charge generation unit [19]. Moreover, aluminum (Al) and lithium fluoride (LiF) were used as cathode material and electron injection layer (EIL), respectively. Patterned indium tin oxide (ITO) on glass substrates were used for the preparation of single devices and multi-stack architectures. After sequential cleaning with isopropyl alcohol, acetone and deionized water followed by 10 minute UVO treatment, all devices were fabricated under vacuum conditions (∼10−7 Torr) using a thermal evaporation system. The sequential deposition of the organic layers was performed according to the single, 2-stack and 3-stack architecture as shown in Fig. 1. After the deposition of organic layers and the metal top contact, all devices were encapsulated using glass cap technique in inert atmosphere. Optical measurements including angular dependence were performed using integrated sphere and Konica Minolta CS-2000. In case of the angular dependence measurements, the samples were moved to a 20°, 40°, and 60° positions towards the camera.
Fig. 1 Energy level diagram of a) blue single, b) yellow single, and c) 2-stack-light emitting devices as well as chemical structure of related organic materials. The blue device consist of MADN host and BCzVB blue fluorescent dopant, whereas yellow EML consists of Bepp2 and Ir(tptpy)2(acac) as host and yellow phosphorescent dopant, respectively.
2.2 Theoretical optical simulation
The theoretical evaluation of the 2-stack and 3-stack architecture was carried out using commercially available semiconducting emissive thin film optics simulator (SETFOS 4.1). This model is used for the design and development of optoelectronic thin film devices (OLED and OSC). The n and k values of the used organic materials were considered to be 1.8 including zero extinction coefficients in the visible wavelength range. Additionally, photoluminescence spectrum of BCzVBi and Ir(tptpy)2(acac) were used for the simulation of all the devices.
3. Result and discussion
3.1 Fluorescent blue and phosphorescent yellow OLED
Before preparing tandem and triple OLED devices, reference devices were fabricated consisting of single fluorescent blue and phosphorescent yellow single electroluminescent (EL) unit. Figure 1(a), 1(b) shows the fabricated device structure of single yellow and blue emission OLED device. The fabricated single device structure comprises a 7 nm HATCN layer to improve the charge transfer from ITO to the highest occupied molecular orbital (HOMO) of TAPC (HTL), whereas, a thin LiF was incorporated between Bphen and metal cathode for easy electron injection. In addition, the low lowest unoccupied molecular orbital (LUMO) of Bphen and high LUMO of TAPC served as an electron transport and electron blocking layers and confine the excitons in the emissive layer as well as prevent from non-radiative recombination of charge carriers within the charge transport layers. To achieve the highest intensity, the EML of each device is located at the first ETL and HTL optimum position.
Figure 2 shows that the position of yellow unit is sensitive to its ETL thickness. Based on the photoluminescence spectrum of BCzVBi and Ir(tptpy)2(acac), single device simulations have been carried out. Here, blue and yellow single devices are simulated with varying ETL and HTL thickness. As a result, the intensity distribution is achieved as displayed in Fig. 2. Table 1 presented the single device structure for highest emission intensity where the maximum EQE of blue device was found to be 5.2% (at 500 cd/m2) at a driving voltage of 3.6 V, while the yellow device exhibits almost 22.9% (at 3.8 V) at 1000 cd/m2. To attain highest emission intensity HTL and ETL thickness was chosen according to the simulated intensity distribution of both blue and yellow single device. By comparing the ETL (30 nm for blue and 60 nm for yellow) and HTL (45 nm for blue and 75 nm for yellow) thicknesses with simulated structures it can be stated that the experimental found layer thicknesses for high intense device emission match the simulated cavity lengths. Moreover, the experimental EQE values of the blue and yellow single devices were used for the calculation of the multi-stack devices EQE.
Fig. 2 Simulated emission intensity of (a) yellow and (b) blue single devices depending on the distance from EML to cathode (ETL) and anode (HTL).
Table 1. Electroluminescent characteristics of single blue and yellow OLEDs
3.2 Optical simulation of 2-stack and 3-stack OLED
By using a singlet-triplet harvesting host-dopant structure as yellow emitting unit, the IQE will in theory reach the maximum of 100% [20]. Compared to that, the IQE of blue fluorescent emission will only be a fraction of that reaching 25% [21]. Therefore, the yellow emission will be more dominant compared to the blue emission. In order to equalize the blue and yellow emission, two approaches were taken into account. First, the suppression of yellow intensity by choosing the right cavity length, and second the addition of an additional blue unit. In simulation and later in experiment for B-Y and B-B-Y architectures the first step consisted of finding blue cavities with highest intensities while keeping yellow at the lowest intensity. Subsequently the yellow intensity was stepwise increased by raising its ETL (close to cathode) thickness while keeping the blue cavities constant. Therefore, by following the simulated trends concerning ETL and HTL thickness variations the highest EQE for the desired color coordinates could be achieved.
In order to design efficient WOLED structures, optical simulations of 2-stack and 3-stack devices were performed. The used organic layers in this study are considered to exhibit no absorption in the visible wavelength. Initially, all possible 2-stack and 3-stack architectures are analyzed that exhibit CIE (0.333/0.333) while maintaining reasonable EQE. The simulated 2-stack (B-Y) and 3-stack (B-B-Y and B-Y-B) structures are as follows:
Blue-Yellow (B-Y): ITO (50 nm)/HTL-1 (52 nm)/Blue EML (20 nm)/ETL-1 (30 nm)/CGL 7 (nm)/HTL-2 (90 nm)/Yellow EML (15 nm)/ETL-2 (15 nm)/Al (100 nm)
Blue-Blue-Yellow (B-B-Y): ITO (50 nm)/HTL-1 (52 nm)/Blue EML (20 nm)/ETL-1 (30 nm)/CGL 7 (nm)/HTL-2 (70 nm)/Blue EML (20 nm)/ETL-2 (30 nm)/CGL 7 (nm)/HTL-3 (80 nm)/Yellow EML (15 nm)/ETL-3 (30 nm)/Al (100 nm)
Blue-Yellow-Blue (B-Y-B): ITO (50 nm)/HTL-1 (52 nm)/Blue EML (20 nm)/ETL-1 (30 nm)/CGL 7 (nm)/HTL-2 (50 nm)/Yellow EML (15 nm)/ETL-2 (40 nm)/CGL 7 (nm)/HTL-3 (90 nm)/Blue EML (20 nm)/ETL-3 (30 nm)/Al (100 nm)
Herein, HTL-1, HTL-2, HTL-3, ETL-1, ETL-2, and ETL-3 represents the hole and electron transport layers of bottom-, center- and top-light emission units of 3-stack devices (B-B-Y and B-Y-B). Similarly, HTL-1, HTL-2, ETL-1, and ETL-2 indicate the charge transport layers of blue and yellow emission units in 2-stack device.
The charge generation unit structure is explained in detail in the further discussion of this section. An essential aspect is the location of yellow emitting unit in the 2-stack or 3-stack architecture. By simulating different positions of the yellow emitting unit for 2-stack and 3-stack structures, it is possible to give a statement about the most promising structure for an experimental realization. All presented structures are bottom emission architectures with ITO as an anode and aluminum as highly reflective cathode material. The main objective of simulating each structure was to check whether the investigated structure can achieve the desired color coordinates and high efficiency. The EQE and color coordinates of both simulated 2-stack and 3-stack (B-B-Y) was calculated to be 13.1%, 23.0% and (0.343, 0.330), (0.343, 0.331), respectively. The simulated data of those architectures, which exhibit the best performance at the desired color coordinates, was compared with the experimental results of 2-stack and 3-stack devices. It can be seen that in case the yellow unit is located closely to the cathode both structures 2-stack and 3-stack show color coordinates close to CIE (0.333/0.333). Additionally another 3-stack structure was simulated which positions the yellow unit in the center between two blue units. In this configuration (B-Y-B) the EQE decreases to 21.2% but the wanted color coordinates (0.316, 0.334) was achieved. In fact, the B-Y-B structure led to improved viewing angle properties, which will be discussed later. Moreover, Y-B-B structure was not considered for simulation and fabrication because of the very low radiation intensity of the yellow unit at thicker ETL condition (Fig. 2).
3.3 2-stack (blue-yellow) white OLED
After the fabrication of reference devices the design of a 2-stack structure was considered. Since the simulation of stack architectures demonstrated that the position of yellow unit close to the cathode (B-Y, B-B-Y) leads to the desired CIE coordinates close to (0.333/0.333) at highest EQE, the design of B-Y and B-B-Y multi stack devices was considered. In order to make a high performance multi-stack structure, an electrically connecting CGL is necessary [22,23]. Required properties for an efficient CGL are transparency in the range of visible wavelength and the creation of an ohmic contact with low resistance between two EL units. Figure 3 shows the CGL stack design that was used in the fabrication of 2-stack and 3-stack white OLED architectures. Here, electrons are injected from HATCN into Li doped Bphen where they can easily travel to the LUMO energy level of non-doped Bphen. Since the low LUMO of HATCN aligns well with the HOMO of TAPC, holes can drift towards the cathode side of the device. In order to create a 2-stack and 3-stack device with a maximum of EQE, the emission of the blue unit(s) needs to be as high as possible to balance the intensity ratio of the blue and yellow emission. This stems from the fact that the yellow emission is more dominant due to its phosphorescent character. In order to determine the optimum condition for blue cavity lengths in a 2-stack architecture the simulated ETL and HTL thicknesses from Fig. 2 are considered for further experiments. Highest blue intensities are expected to be achieved for an HTL thickness of around 50 nm and a range of 150 nm to 170 nm for the ETL thickness. Likewise, Fig. 2 shows that the position of the yellow unit is sensitive to its ETL thickness. Therefore, in experiment a thickness variation of the yellow unit HTL (TAPC) was conducted with 70 nm, 80 nm, and 90 nm while the ETL thickness was kept at 20 nm. Indeed, the emission intensity of yellow unit decreases for thinner ETL because of the coupling between free charges at the surface of the aluminum cathode and the emitted electromagnetic waves. The fabricated 2-stack configuration was: ITO (50 nm)/HATCN (7 nm)/TAPC (45 nm)/MADN: 8 wt % BCzVBi (20 nm)/Bphen (25 nm)/Bphen: 5 wt% Li (5 nm)/HATCN (7 nm)/TAPC (90 nm)/Bepp2: 6 wt% Ir(tptpy)2(acac) (15 nm)/Bphen (20 nm)/LiF (1.5 nm)/Al (100 nm). The fabricated 2-stack device architecture and chemical structures of the used organic materials are shown in Fig. 1(c). The highest EQE and deepest color coordinates were found to be 16.1% and (0.343/0.330) for a TAPC thickness of 90 nm which results in a blue ETL thickness of 162 nm. This EQE value is lower than the summation of individual emissive unit because of the suppressed emission of the yellow emitter. The detail values of the current and power efficiency, CIE color coordinates, driving voltages and EQE for the 70 nm, 80 nm, and 90 nm HTL and 20 nm, 30 nm ETL thicknesses are listed in Table 2. Indeed, due to the sensitivity of yellow emission to the distance from cathode, low ETL thickness was considered for yellow emission to equalize the blue and yellow spectral region in order to reach perfect white color coordinates irrespective of its high emission at 60 nm. At this point, it can be stated that a 2-stack structure was found that meets the requirement to show CIE coordinates close to (0.333/0.333).
Table 2. Electrical characteristics of 2-stack and 3-stack devices with variation in ETL and HTL thickness
3.4 3-stack (blue-blue-yellow) white OLED
In order to improve the EQE and current efficiency, a 3-stack structure was considered that locates the yellow unit close to the cathode. Based on the previous results of the 2-stack, the ETL thickness of the second blue unit (center position) is kept constant at 162 nm while the distance of the yellow unit remains 20 nm. In order to adjust the HTL thickness of the blue unit in the center position a Bphen thickness variation of 50 nm, 60 nm and 70 nm was performed. Simultaneously this modifies the ETL thickness of the blue unit which is located to the anode. Figure 2 shows that the simulated results of emission intensity stays almost constant for a blue unit with an HTL thickness of around 50 nm and an ETL thickness of 270 nm to 300 nm.
Consequently, a Bphen thickness variation at the HTL side of the centered blue unit will not influence the intensity of the first blue unit (close to anode) but the intensity of the second blue unit (center position). The fabricated 3-stack structure was: ITO (50 nm)/HATCN (7 nm)/TAPC (45 nm)/MADN: 8 wt % BCzVBi (20 nm)/Bphen (25 nm)/Bphen: 5 wt% Li (5 nm)/HATCN (7 nm)/TAPC (70 nm)/MADN: 8 wt % BCzVBi (20 nm)/Bphen (25 nm)/Bphen: 5 wt% Li (5 nm)/HATCN (7 nm)/TAPC (80 nm)/Bepp2: 6 wt% Ir(tptpy)2(acac) (15 nm)/Bphen (30 nm)/LiF (1.5 nm)/Al (100 nm). A 3-stack structure of 70 nm thick Bphen as HTL of the centered blue unit exhibited the highest EQE of about 19% and CIE coordinates of (0.309/0.278). As already mentioned, the yellow unit is sensitive to its ETL thickness. By increasing the ETL thickness (Bphen) of the yellow unit and at the same time decreasing the HTL (TAPC) thickness the blue cavity lengths are kept constant while the yellow emission is increased (Table 2). Finally, 3-stack architecture was fabricated with CIE coordinates of (0.335/0.313) and an EQE of 23.6%, current efficiency of 62.2 cd/A at a driving voltage of 9.9 V which was measured at a luminance 1000 cd/m2 (Table 2). The experimental EQE and color co-ordinates show negligible difference with simulated results. The driving voltage of the 3-stack device (9.9 V) is less than the sum of the reference single devices where the blue single device had a driving voltage of 3.6 V at 500 cd/m2 and the yellow device had 3.8 V at 1000 cd/m2. Adding the voltages of two times the blue and one time the yellow single device results in a total driving voltage of 11 V. This confirms the lossless performance of the CGL architecture since the operating voltage of the 3-stack device at the same current density is significantly lower than the expected driving voltage (11 V). A similar tendency of the CGL performance was also observed in the 2-stack device. The current density versus voltage, EQE and power efficiency versus luminance plot of single blue, yellow, 2-stack (B-Y) and 3-stack (B-B-Y and B-Y-B) OLED devices are shown in Fig. 4. As shown in Fig. 4(b), 2-stack and 3-stack WOLEDs shows very small decrease in EQE and power efficiency upto 4000 cd/m2 brightness. This result confirmed that all the fabricated WOLEDs are useful for real display applications. Figure 5(a) illustrates the electroluminescence (EL) spectrum of fabricated 2-stack and 3-stack (B-B-Y) devices and Fig. 5(b) shows the equated EL spectrum of B-B-Y 3-stack WOLED structure with transmission spectrum of color filters. Due to the broad band color passing nature of color filters, substantial overlap in the blue-green and green-red wavelength is observed in the transmission spectra of red, green and blue colors filters. The EL spectrum of B-B-Y structure with color filters demonstrated a D65 color coordinates of (0.312, 0.315), which also represents the white light with correlated color temperature of 6504 K and the peak wavelength of 451, 562 and 594 nm for blue, green and yellowish, respectively.
Fig. 4 a) Current density versus voltage (J-V), b) power efficiency and EQE versus luminance plots of the blue (B), yellow (Y), 2-stack (B-Y), and 3-stack (B-B-Y and B-Y-B) OLED devices.
Fig. 5 Electroluminescent (EL) spectrum of a) all fabricated devices at the same current intensity b) white OLED (B-B-Y structure) with color filters and transmission spectrum of color filters.
3.5 3-stack (blue-yellow-blue) white OLED
Based on optical simulations, the B-Y-B 3-stack architecture was considered to achieve improved angular dependence characteristics and ideal white color coordinates. Apart from the position of the blue emitting unit that is close to the anode and already known from the B-B-Y architecture, the position of the remaining two emitters needs to be adjusted. The fabricated 3-stack architecture was: ITO (50 nm)/HATCN (7 nm)/TAPC (45 nm)/MADN: 8 wt % BCzVBi (20 nm)/Bphen (25 nm)/Bphen: 5 wt% Li (5 nm)/HATCN (7 nm)/TAPC (50 nm)/Bepp2: 6 wt% Ir(tptpy)2(acac) (15 nm)/Bphen (45 nm)/Bphen: 5 wt% Li (5 nm)/HATCN (7 nm)/TAPC (80 nm)/MADN: 8 wt % BCzVBi (20 nm)/Bphen (30 nm)/LiF (1.5 nm)/Al (100 nm). In this structure to balance the blue and yellow emission intensity, HTL-2 and ETL-2 thickness variations are performed, whereas HTL-2 (TAPC) and ETL-2 (Bphen) are the charge transport layers of the center yellow emission unit.
According to the cavity simulation in Fig. 2 30 nm ETL length of the anode sided blue emitter was considered to put the emission in an area of high intensity. Finally, TAPC (HTL 2) and Bphen (ETL 2) thickness of 50 and 45 nm was found for a 3-stack architecture that exhibits CIE color coordinates of (0.331/0.338), an EQE of 19.1% and a current efficiency 65.8 cd/A at a driving voltage 10.3 V (Table 2). These values are slightly lower than the simulated results due to unbalance carrier transport performances with change in HTL and ETL thickness. The 3-stack WOLED with B-Y-B arrangement illustrates a higher current efficiency but slightly inferior EQE compared to the B-B-Y structure due to its (B-Y-B) narrower emission pattern. Likewise, the 3-stack (B-Y-B) WOLED exhibits a slightly higher driving voltage of 10.3 V as compared to B-B-Y architecture (9.9 V) because of the thick Bphen layer on the ETL side of the centered yellow emitter. The EL spectrums of all the fabricated structures are presented in Fig. 5(a).
3.6 Angular dependence characteristics
The angular dependence is a crucial aspect for the application of WOLEDs in displays [24,25]. Figure 6 shows simulated and experimentally measured color shift (Δu’v’) depending on the viewing angle of 2-stack and 3-stack WOLEDs. Considering Δu′v′ in Fig. 6(b) it can be stated that the B-Y-B structure shows a slight improvement in angular dependence from 40° to 60° (degrees) compared to B-B-Y. It has been previously reported that the white color variation with viewing angle should be close to 0.02, such color change cannot be easily detected by human eyes [12]. Herein, B-Y-B structure shows a Δu′v′ close to 0.02 compared to B-B-Y (~0.03). The expected improvement of angular dependence at higher angles was confirmed (Fig. 6) due to a balanced emission of blue and yellow. Additionally, when the yellow emission unit is at center position yellow light out-coupling is enhanced at front viewing angle by micro-cavity effect. However, when yellow is close to the cathode/anode position, such effect get suppressed due to unsatisfactory cavity lengths. Therefore, due to the enhanced light intensity at the front viewing angle (0° degree) of the B-Y-B structure, viewing angle property is improved. This result also confirms the suitability of B-Y-B WOLED structure for display application over B-B-Y. Additionally, the simulated viewing angle characteristics show valid agreement with experimental trends.
Fig. 6 a)Simulated, b)experimental angular dependence characteristics of the 2-stack and 3-stack WOLEDs.
In order to investigate the emission pattern of both 3-stack architectures and to extract a trend for upcoming experiments the emission for each emitting unit was simulated depending on their cavity lengths in the stack. Figure 7 displays each emission pattern for blue and yellow emitting units for the B-B-Y and B-Y-B architecture. In case of the B-B-Y structure it can be seen that the blue emission is narrow at higher angles compared to the yellow emission which is close to the Lambertian emission pattern. Also, from the previous experiment which is displayed in Fig. 6(a) Lambertian distribution of the yellow emission is assumed since an increased yellow emission (thicker ETL) at constant blue cavity lengths leads to a constant change of delta u'v' for two different color coordinates. In contrast to B-B-Y the B-Y-B structure shows a reduced yellow intensity starting from 25 degrees while the blue intensity of the 2nd unit (close to cathode) increases and moves towards the Lambertian pattern (Fig. 7). Therefore, a change of the yellow position to the center location might lead to improved angular dependence since the reduced yellow intensity might be compensated with the slightly increased blue emission. Even though the white emission pattern moves away from the Lambertian pattern an improved ratio of blue and yellow intensity at higher angles is expected (Fig. 7). Consequently as shown in Fig. 7, the B-Y-B 3-stack architecture was considered as a promising structure to improve the angular dependence of the white emission.
Fig. 7 Emission pattern simulation of individual emitting units within a) the B-B-Y structure and b) the B-Y-B structure. Additionally the Lambertian emission pattern was inserted in both figures.
4. Conclusion
In conclusion, multi-stack architectures containing fluorescent blue and phosphorescent yellow emitting layers were fabricated. All presented structures show cool white emission which almost matched the perfect white CIE coordinates of (0.333/0.333). The highest quantum efficiency of 23.6% was achieved by the B-B-Y architecture while the B-Y structure realized 16.1%. Moreover, real position of yellow and blue emissive unit in the 3-stack WOLED architecture for cool white or required display color coordinate and high quantum efficiency was investigated. Even though the latter shows less efficiency it also exhibits a less complicated and time intense fabrication procedure which would be advantageous for large scale AMOLED display manufacturing. By locating the yellow emitter in the center position (B-Y-B) an improved angular dependence from 40° to 60° (degree) could be achieved at an EQE of 19.1%.
Funding
Human Resources Development program (no. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry & Energy and IT R&D program of MOTIE/KEIT [More than 60” transparent flexible display with UD resolution, transparency 40% for transparent flexible display in large area] as well as Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No.10063289, 'Development of High Temperature Negative tone Photosensitive Black Resin and fabrication process for Pol-less AMOLED Devices.
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