Optical Design and Optimization of Highly Efficient Sunlight-like Three

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Optical design and optimization of highly efficient sunlight liked three-stacked warm white organic light emitting diodes Mi Jin Park, Young Hoon Son, Hye In Yang, Seong Keun Kim, Raju Lampande, and Jang Hyuk Kwon ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01379 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Photonics

Optical design and optimization of highly efficient sunlight liked three-stacked warm white organic light emitting diodes Mi Jin Park, Young Hoon Son, Hye In Yang, Seong Keun Kim, Raju Lampande* and Jang Hyuk Kwon* Department of Information Display, Kyung Hee University, Dongdaemoon-gu, Seoul 130701, Republic of Korea

ABSTRACT In this paper, we report an optical structure design method with the predicted performances of

highly efficient three-stacked white organic light emitting diodes

(WOLEDs) for solid state lighting applications. The efficiency and color properties of stacked WOLEDs are strongly affected by optical interference inside the thick cavity length; therefore appropriate emissive layer (EML) position is determined by thorough theoretical optical simulations to prevent such optical effect. The theoretically evaluated, three-stacked hybrid WOLED with entirely separated phosphorescent red and green as well as fluorescent blue EML has displayed CRI and power efficiency of 91 and 33.5 lm/W, respectively. Based on our assumptions, design method and optical simulation results, the fabricated three-stacked WOLEDs showed CRI of 93, external quantum efficiency (EQE) of 49.4% and power efficiencies of 33.4 lm/W. These experimentally measured characteristics are fully correlated with the performances of optically simulated devices. It is important to note that the driving voltage (10.0 V) of optically designed WOLED is almost identical to the summation of unit devices (9.9 V) because of good interconnecting unit and same charge balance in the tandem WOLED. In addition, experimentally measured power efficiency of tandem device is similar to an average value of unit devices and most importantly EQE is nearly equal to the summation of unit devices with almost matched white spectrum. KEYWORDS: Optical design, color rendering index, stacked OLED, white OLED, tandem OLED

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White organic light emitting diodes (WOLEDs) have considered as the most promising technology for large size and high resolution active matrix display as well as solid state lighting applications.1 In recent years, extensive research works have been reported on the WOLEDs as luminaires because of its high efficiencies and it can easily represent the natural sunlight spectrum due to broad emission spectra of organic light emissive materials. Normally, color rendering index (CRI), correlated color temperature and power efficiency (lm/W) are crucial factors in lighting applications. In order to enhance these vital characteristics, it is essential to adopt appropriate device architecture with high photoluminescence quantum yield (PLQY) based light emissive and efficient host materials for the comprehensive exciton confinement as well as proper charge carrier transport/injection materials with high mobility to attain low driving voltage. The single and multiple emissive layer (EML) OLED structures have shown important advantages, like simple device configuration and time efficient process due to their total organic thickness of about 100~150 nm.2-19 Indeed, the viewing angle characteristics of such devices are less affected by relatively weak optical interference because of short distance between light emissive layer and reflective electrode. However, these thin OLEDs are sensitive to the particles and surface roughness as well as also exhibits short operational lifetime. Similarly, these devices also indicate main issue of color stability with respect to the applied electric field by transferring energy, especially when several emitters doped in the single EML.20,21 In addition, after saturating emitter sites of low exciton energy, especially red emitter, the radiative exciton ratio of emitters with various energies is different compared to before saturation with respect to the driving voltage and additionally electron and hole recombination zone also shifts along the mobility of charge carriers.22,23 Hence, it requires an 3


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extremely low doping concentration of low energy dopant and fine doping technology, but this also creates an issue of process reproducibility. In case of multiple EML structure, a number of concerns such as energy transfer and shift in recombination zone are occurred at the interfaces of each adjacent color EML.24 In order to eliminate above defined issues of co-doped and multiple EML structures, tandem stacked OLED structure is highly recommended, where, individual EMLs are vertically stacked and electrically connected in series through charge generation unit (CGU).25 Normally, tandem OLEDs show summation of driving voltages of each unit devices, when there is no energy barrier at the interconnecting units. In addition, current and external quantum efficiencies (EQE) are expected to be sum of each unit devices, without considering optical interference effect in the tandem OLEDs. Indeed, tandem OLEDs ensure higher power efficiency despite its high driving voltage because of enhanced current performances. On the other hand, tandem stacked structures suffer from the serious optical interference issue, which is obtained from the weak cavity effect due to large device thickness. To alleviate this issue, an optical design method becomes important as more layers are stacked. There are several reports available on two-stacked OLEDs such as red-green/blue or yellow/blue devices.26-31 However, very limited studies have been reported on three-stacked WOLEDs for lighting applications and almost no reports available on its accurate fabrication method with proper theoretical optical evaluation. Herein, systematic and theoretical approach was used to enhance an out-coupling ratio of the emitted light and to tune the color properties of three-stacked WOLED by evaluating appropriate position of EMLs and to tune color properties in electrically connected individual electroluminescent devices because of severe optical interference 4


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by thick cavity length. To achieve high CRI factor, a fluorescent deep blue emitter with phosphorescent red and green emitters was considered because of their high spectral intensity at the long wavelength range and low intensity at the short wavelength range. Based on optical and electrical calculation method, EML positions of red, green and blue units as well as power efficiency and CRI values are evaluated. The optically designed, optimized and fabricated three-stacked WOLED showed a power efficiency and CRI of 33.4 lm/W and 93 (EQE of 49.4%).

RESULTS AND DISCUSSION Optical Simulation. In order to enhance the out-coupling ratio of emitted light and to attain appropriate color properties in three-stack devices, proper theoretical evaluation of the emissive layer (EML) position and cavity thickness management are essentially needed. Hence, we performed the extensive optical simulation of three-stacked WOLED devices to find suitable red, green and blue EML positions and to attain appropriate color properties for lighting applications. Initially, red, green and blue unit OLEDs were fabricated to obtain electroluminescence (EL) spectra as well as recombination zone in the emissive layers for optical calculations. Here, red, green and blue unit OLEDs are fabricated with the following configurations: Red unit: ITO (50 nm)/ HATCN (7 nm)/ TAPC (75 nm)/ Bebq2: 3% Ir(mphmq)2(tmd) (15 nm)/ p-bPPhenB (55 nm)/ p-bPPhenB: 5% Li (5 nm)/ Al (100 nm), Green unit: ITO (50 nm)/ HATCN (7 nm)/ TAPC (75 nm)/ Bepp2: 3% Ir(ppy)2(acac) (15 nm)/ p-bPPhenB (55 nm)/ p-bPPhenB: 5% Li (5 nm)/ Al (100 nm), Blue unit: ITO (50 nm)/ HATCN (7 nm)/ TAPC (58 nm)/ MADN: 12% BCzVBi (15 5


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nm)/ p-bPPhenB (25 nm)/ p-bPPhenB: 5% Li (5 nm)/ Al (100 nm). We applied bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq2), bis[2-(2hydroxyphenyl)pyridinato]beryllium (Bepp2), 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) as phosphorescent red, phosphorescent green and fluorescent blue host materials,

respectively.

Bis(4-methyl-2-(3,5-dimethylphenyl)quinolinato

N,C2')tetra-methylheptadionate Iridium(III) (Ir(mphmq)2(tmd)), (acetylacetonate)bis(2phenylpyridine)iridium(III) (Ir(ppy)2(acac)), 4,4'-bis(9-ethyl-3-carbazovinylene)-1,1'biphenyl (BCzVBi) were used as fluorescent blue, phosphorescent green and phosphorescent red emitter, respectively. In order to enhance hole injection property from indium tin oxide (ITO) anode, 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) was additionally used as a hole injection layer (HIL), because its deep lying lowest unoccupied molecular orbital (LUMO) of about 6.0 eV can generate charges at the HIL/hole transport layer (HTL) interface and simultaneously improve injection of holes into HTL.32 In addition, n-doped electron transporting layer (ETL) was used to improve electron injection property from aluminum (Al) cathode. Lithium (Li) was used as a n-dopant in ETL because of its low work-function (2.5 eV).33 Herein, our new ETL material,

1,3-bis(2-phenyl-1,10-phenanthrolin-4-yl)benzene

(p-bPPhenB),

was

considered because of its high electron mobility (5.8x10-3 cm2 V-1 s-1) as well as superior

Li

doping

performances.34

Likewise,

1,1-bis[(di-4-

tolylamino)phenyl]cyclohexane (TAPC) was also applied as HTL due to its high hole mobility of 10-2 cm2 V-1 s-1. 35 It is important to note that, both TAPC and p-bPPhenB have sufficiently high triplet energy of 2.87 eV and 2.50 eV as well as singlet energy of 3.50 eV and 3.36 eV, respectively, which are high enough to confine generated excitons in the EML. 6


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As shown in Figure S1, bandgap of red and green host materials are wide enough to include highest occupied molecular orbital (HOMO) and LUMO energy level of red and green emitters. In case of red unit device, electrons are injected from the LUMO of pbPPhenB into the LUMO of Ir(mphmq)2(tmd) and Bebq2. The injected electrons in the dopant can easily travel towards TAPC because LUMO energy gap between dopant and host is only 0.1 eV, which is sufficient enough to thermally activate on the host. However, holes will be trapped on the dopant level, if holes enter into the host then it can hop on HOMO level of dopant due to HOMO gap of 0.9 eV. The HOMO and LUMO energy gap between host and dopant materials, Bepp2 and Ir(ppy)2(acac), are 0.4 eV and 0.3 eV, respectively. Herein, holes can somewhat experience trap sites due to energy gap of 0.4 eV, while LUMO energy level difference is sufficient for the electrons to pass through the EML by using thermal energy. These traps are relatively small compared to that of red EML system. Hence, this is a reasonable analysis to establish the main recombination zone at the HTL/EML interface and the HTL side in each EML for red and green devices because electrons rapidly move through their electron type host materials, Bebq2 and Bepp2, with low doping concentration. On the other hand, blue unit device shows charge trapping emission, where, excitons directly generate in the emitter, which is overwhelmingly dominant than those of other unit devices. We designed OLED structure with higher emitter doping concentration of about 12 % for the broad recombination zone with the minimized exciton quenching and maximized efficiency, because fluorescent exciton shows short lifetime in the nanometer range. It is appropriate to utilize the mechanism which directly forms excitons on the dopant site due to small HOMO and LUMO gap of MADN and BCzVBi system. Also, thin layer of p-bPPhenB (25 nm) than that of red and green units forms main recombination region at 7


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the HTL/EML interface. In device physics perspective, our presumption about exciton recombination zone position could be adequate. Furthermore, lithium doped ETL was used for lowering the electron injection barrier from aluminum cathode (work-function of 4.2 eV) towards LUMO energy level of p-bPPhenB (3.0 eV) instead of an additional electron injection layer such as thin LiF.36 It has been previously reported that the lithium dopant caused an issue of efficiency drop by scattering or absorption and charge balance issue by an excessive electron density.37 Hence, the doping concentration and doping region of Li n-dopant were decided to be minimum values of about 5% and 5 nm, respectively, for the proper electron injection ability. Our device strategy from material choice to the optimization and analysis is very useful to fabricate highly efficient stacked WOLED devices. Figure S2 shows the current efficiency and external quantum efficiency versus current density characteristics of unit devices, and their respective values are summarized in Table S1. The maximum EQEs of our red, green and blue unit devices are found to be 32.4 %, 29.6 % and 5.9 %, respectively. These results indicated that our devices are well optimized although efficiency values are quite reduced by exciton annihilation at the high current density range. Additionally, the current efficiency of 42.7 cd/A, 88.7 cd/A and 9.8 cd/A are obtained for red, green and blue devices at the given constant current density of 1 mA/cm2. In this simulation work, the horizontally aligned exciton dipole moment factors in host matrix were considered to be 82% and 77% for Ir(mphmq)2(tmd) and Ir(ppy)2(acac), respectively.38,39 Similarly, the PLQYs of 96% and 100% were assigned for red and green dopants, respectively 38,40. For the blue dopant, BCzVBi, about 100 % PLQY was used because our fluorescent blue unit device displayed an almost ideal EQE of 5.9 %41. In addition, we also assumed relatively 100% 8


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charge balance factor (γ) for the optical simulation of each EMLs due to their ideal EQE performances with consideration of their best connection in tandem device. We calculated the optical and electrical performances of three-stacked white OLED with the characteristics of unit devices at the current density of 1 mA/cm2. By considering the device structures and materials of unit OLEDs, we establish the main exciton recombination zones at the HTL/EML interface for red and blue as well as HTL side for green devices.

Figure 1. The (a) optical constants of glass substrate, anode (ITO), cathode (Al), organic layers (TAPC and p-bPPhenB), (b) internal emission spectra of each red, green and blue devices used for the calculation of EML distribution. The refractive index (n) of about 1.7~1.9 (TAPC) as well as extinction coefficient (k) of almost zero are used for TAPC and other organic layers except for p-bPPhenB in the optical simulation of OLEDs (Figure 1 (a)). The EL spectra of fabricated unit devices were incorporated as an internal emission spectrum of EMLs in the calculation of OLEDs as shown in Figure 1 (b). Figure 2 shows the calculated radiance of red, green and blue EMLs depending on the thickness of HTL and ETL. These radiance distributions are resulted from the weak micro-cavity effect by increasing the portion of multiple-beam interference with the two-beam interference as the total device thickness

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increases. Indeed, the radiance distribution is more sensitive to the ETL thickness than HTL due to such weak cavity effect as shown in Figure 2. It is important to note that the blue EML with shorter emitting wavelength shows fast periods. The highest outcoupling efficiency is appeared to be at the cathode side EML location, (termed as first anti-node) regardless of the light emission wavelength, when the thickness of devices is identical. The number of appropriate EML position is equal to the cavity order in the same cavity length, and indeed, the higher cavity order slightly reduces the intensity of radiated light as shown in Figure 2.

Figure 2. Simulated normalized radiance of (a) red, (b) green and (c) blue EML with respect to the thickness of ETL and HTL. The following structure considered for the calculation of radiance: ITO (50 nm)/organic layer as HTL (10~500 nm)/organic layer as EML (15 nm)/organic layer as ETL (10~500 nm)/LiF/Al (100 nm). Based on the radiance distribution results, we considered six possible three-stacked WOLED structures with red, green and blue EMLs are shown in Figure 3. These structures with the calculated thickness of three-stacked WOLEDs have shown EML positions with maximum radiance (at anti-nodes) because EMLs in no anti-node positions cause the reduction in performances although located EML in these positions can tune the device spectra. For the high CRI with warm white spectrum, Structure 1

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and 2 are good candidates because of its strong out-coupled position for long wavelength range at the first anti-node. Herein, an anti-node is referred to as a first EML position from the cathode, and it has small ETL thickness. The green EML should properly occupy its second anti-node for the maximum power efficiency of tandem devices. It is also reasonable to place red EML instead of green EML at the first antinode site to balance the intensity of green, because studied green unit device has higher spectral intensity and EQE value. Also, structure 1 just requires connecting unit with almost unchanged thickness of transporting layers from the unit devices, which indicates charge balance in each EMLs of tandem device, exactly similar to those of unit devices. Additionally, this structure can illustrate an apparent merit in terms of performances, because power efficiency of OLEDs decreases by raising the driving voltage, especially when the thickness of charge transporting layers is increased. In particular, the thick tandem device can reduce the viewing angle characteristics by increasing the optical path length between each light emitting regions and reflective cathode. Hence, we fabricated three-stacked WOLEDs with structure 1, which has less thickness of connected three-unit devices.

Figure 3. Possible 3-stack structures with Red, Green and Blue EML positions. 11


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To evaluate electrical characteristics (such as current efficiency (cd/A), power efficiency (lm/W) and EQE (%)) of three-stacked OLEDs we utilized the ratio of luminance (L) from the radiance distribution results (Figure 2) along with the increased cavity order for each color. The factor L can be calculated with the expected spectra, which are used in the radiance evaluation. It is assumed that the devices with different cavity order hold the same charge balance. Then, we can expect the current efficiency of each EML in designed three-stacked WOLED as shown in below equation. The parameters used in the following equation are explained in the Supporting Information. CE(c,m,θ)=L(c,m,θ)·CE(c,n,θ)/L(c,n,θ)= RL(c,θ) ·CE(c,n,θ)

(1)

Where, CE and L denote current efficiency and luminance at the certain current density condition. RL is the luminance ratio between different cavity spaces. The order of cavity is represented as m and n. The factor ‘c’ and ‘θ’ indicate emission color and viewing angle of devices. Herein, we just considered the change of out-coupling efficiency (or radiance distributions as shown Figure 2) with respect to the cavity order by optical interference. The EML positions of unit devices are in the first order cavity space. In order to calculate L(c,1,0) values of each colors, we performed optical simulation for the predicted EL spectra with same thickness of fabricated unit devices. However, the calculated EL spectra using same optical constants were in discord with the measured spectra of each unit devices as shown in Figure S3 (a). Therefore, we have modified this assumption by measuring the optical constants of p-bPPhenB, because its planar molecular structure was able to display slightly higher refractive index. (see Figure S3 (b).) By incorporating measured optical constants of ETL in the calculation, we have obtained well correlated EL spectra as shown in Figure S3 (c). These results show different spectral tailed region by applying EL spectra of unit devices including optical 12


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interference effect for EMLs. With our consideration about γ of 100% in studied system, the simulated unit devices showed EQEs of 24.2 %, 23.6 % and 6.7 % for red, green and blue, respectively. These EQE values are fully correlated with the characteristics of fabricated unit devices (Table S1). We also modified structure 1 by comprising high refractive index material (p-bPPhenB) as ITO (50 nm)/organic layer (OL, 82 nm)/OL as blue EML (15 nm)/p-bPPhenB (30 nm)/OL (27 nm)/OL as green EML (15 nm)/pbPPhenB (60 nm)/OL (82 nm)/OL as red EML (15 nm)/p-bPPhenB (60 nm)/Al (100 nm). The changed thickness of p-bPPhenB in the modified structure 1 is approximated by considering bellow equation about an optical path length.42 ∑(n·d) = ∑(n’·d’)

(2)

Where, d represents physical thickness of medium. The organic layer thickness of 27, 82 and 60 nm is derived from Equation (2) on the basis of long ranged spectral peak (610 nm) to attain high CRI. The thickness at short-ranged peak (450 nm), is varied by less than 5 nm. By considering this correction, spectra of red device in the first antinode of second order, green device in the second anti-node of second order and blue device in the third anti-node of third order for L(r,2,0), L(g,2,0) and L(b,3,0) were calculated for the following modified structure: ITO (50 nm)/organic layer (OL, 97 nm)/pbPPhenB (30 nm)/OL (42 nm)/p-bPPhenB (60 nm)/OL (85 nm)/OL as red EML (15 nm)/p-bPPhenB (60 nm)/Al (100 nm), ITO (50 nm)/OL (97 nm)/p-bPPhenB (30 nm)/OL (27 nm)/OL as green EML (15 nm)/p-bPPhenB (60 nm)/OL (100 nm)/pbPPhenB (60 nm)/Al (100 nm) and ITO (50 nm)/OL (82 nm)/organic layer as blue EML (15 nm)/p-bPPhenB (30 nm)/OL (42 nm)/p-bPPhenB (60 nm)/OL (100 nm)/p-bPPhenB (60 nm)/Al (100 nm). From these optically designed structure, the RL(c,0) values were evaluated to be 0.89, 0.47 and 0.58 for red, green and blue, respectively. Hence, CE(r,2,0), 13


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CE(g,2,0) and CE(b,3,0) were calculated to be 38.0 cd/A, 41.4 cd/A and 5.6 cd/A at the given constant current density of 1 mA/cm2. Our assumption about non-changed charge balance from the excellent interconnecting interfaces without any increase of operational voltage can derive the current efficiency (CEt(θ)) of three-stacked WOLED by using following equation: CEt(0)=CE(r,2,0)+CE(g,2,0)+CE(b,3,0)

(3)

According to equation (3), the expected CE of three-stacked WOLED is 82.5 cd/A. To confirm our calculation validity, the summed spectrum of |E(r,2,0)|2, |E(g,2,0)|2 and |E(b,3,0)|2 was compared with the spectrum of following optically designed three-stacked WOLED. The optically designed WOLED structure is as follows: ITO (50 nm)/HTL (82 nm)/Blue EML (15 nm)/ETL (30 nm)/HTL (27 nm)/Green EML (15 nm)/ETL (60 nm)/HTL (82 nm)/Red EML (15 nm)/ETL (60 nm)/Al (100 nm). Herein, |E(c,n,θ)|2 signifies an out-coupled spectrum as a function of wavelength and it is also used for the luminance calculation. The EL spectra shown in Figure S4 are well matched and clearly indicate the validity of our calculation method. However, small shoulder peak at the long wavelength region and reduced spectral intensity at the short wavelength range was obtained in designed spectrum compared to that of summed spectrum. Such changes in the EL spectrum might be ascribed to the optical interference in the thick cavity space for various wavelengths. These modified spectral regions by the optical interference have barely negligible effect on the factor ‘RL’ in Equation (1), although the calculated efficiencies from Equation (3) are slightly lower than those of measured values. To evaluate the power efficiency (PE) of three-stacked WOLED, we used following equation: 41 PE=∫θ S(λ)·|Et(θ)|2 dθ/[(Vr+Vg+Vb)·I]

(4) 14


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Where, Vr, Vg and Vb represent operational voltages for red, green and blue at the same current density condition, respectively. In this equation, we used spectrum of threestacked WOLED as |Et(θ)|2, which is same to the white spectrum as shown in Figure S4. S(λ) and I represent human eye sensitivity function as wavelength and current, respectively. According to Equation (4), the driving voltage of three-stacked WOLED expected to be 9.9 V at the constant current density of 1 mA/cm2, when unit devices are connected with energy barrierless interfaces because, prefabricated unit devices demonstrated driving voltages of 3.3 V, 3.7 V and 2.9 V for red, green and blue, respectively. The theoretical PE and EQE were found to be 30.1 lm/W and 42.8%, respectively. The simulated spectrum demonstrated a high CRI of about 91. Indeed, the simulated spectrum of three-stacked WOLED with the considered refractive index of pbPPhenB was differ from that of structure 1 because the modified thickness of pbPPhenB was calculated for enhancing long range emission wavelength. Figure S5 shows normalized spectra with their color coordinate and CCT of both designed devices. If other assumptions and considerations are included in the optical simulation, then we can design other tandem OLEDs with various spectra and efficacies. In this point, we suggest that our optical design and electrical prediction method is useful to control the spectra (or color, CCT and CRI) for the complex stacked OLEDs with materials having diverse electrical and optical property. Figure 4 shows PE in accordance with the charge balance ratio and driving voltage at the presumed current density of 1 mA/cm2. It indicates that higher PE can be obtained in three-stacked OLED, if device with efficient HTL, ETL and host materials are used for enhanced EQE of unit devices. Hence, to attain excellent performances with high power efficiency, it is essential to fabricate highly efficient unit devices and to reduce 15


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energy barriers at the interconnected interfaces between unit devices. It is very valuable method to predict the electrical and optical characteristics by designing the complex stacked OLEDs. Additionally, it also suggests that this method can evaluate whether the performances of fabricated stacked devices is correct with those of unit devices.

Figure 4. The calculated power efficiency versus driving voltage characteristics of OLED with different charge balance in unit EMLs (or EQE of unit devices)

Three-stacked WOLEDs. By considering the optical design calculation, hybrid three-stacked WOLED was fabricated and experimentally evaluated. The fabricated WOLED configuration is as follows (Figure 5): (3-RGB1):- ITO (50 nm)/HATCN (7 nm)/TAPC (75 nm)/MADN: 12 % BCzVBi (15 nm)/p-bPPhenB (25 nm)/p-bPPhenB: 5 % Li (5 nm)/HATCN (7 nm)/TAPC (20 nm)/Bepp2 : 3 % Ir(ppy)2(acac) (15 nm)/p-bPPhenB (55 nm)/pbPPhenB: 5 % Li (5 nm)/HATCN (7 nm)/TAPC (78 nm)/Bebq2: 3 % Ir(mphmq)2(tmd) (15 nm)/p-bPPhenB (55 nm)/p-bPPhenB: 5 % Li (5 nm)/Al (100 nm).

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Figure 5. The fabricated device structures of 3-RGB1, 3-RGB2 and 3-RGB3. To reduce the energy barrier at the interconnected unit devices, the CGU composed of n-doped p-bPPhenB/HATCN/TAPC is incorporated. Here, TAPC thickness in blue and green unit device is modified from 58 nm to 75 nm and from 75 nm to 20 nm to optimize the optically designed structure. Figure 6(a) shows current density and luminance characteristics of fabricated three-stacked WOLED. The measured current density is improved than the calculated value at the high voltage range, because of the decreased total HTL thickness compared to those of the sum of unit devices. The driving voltage at the current density of 1 mA/cm2 is 10.0 V and the measured spectrum is almost identical with the simulated device as shown in Figure 6(b). The fabricated three-stacked WOLED illustrates a PE of 33.4 lm/W and EQE of 49.4 %, these values are completely correlated with the calculated efficiencies. We anticipate that the converted ratio of injected charges to the exciton in each EML is almost similar to the unit OLEDs. The three-stacked WOLED exhibits CIE color coordinate of (0.467, 0.423) and high CRI of 93.

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Figure 6. (a) The relative comparison of calculated and experimental luminance versus voltage versus current density characteristics of the fabricated three-stacked WOLEDs, (b) comparison of measured and simulated EL spectrum of 3-RGB1 device, (c) power efficiency versus current density versus EQE performances and (d) measured EL spectra of the fabricated WOLEDs. Herein, we need to reduce the driving voltage of above discussed WOLED to further enhance the PE performance. Two types of three-stacked WOLED devices were fabricated with two kinds of modified structures as shown in Figure 5. The interconnecting unit of 3-RGB2 between green and red unit devices is modified by decreasing ETL thickness from 55 nm to 45 nm and increasing TAPC thickness from 78 nm to 88 nm in order to intensify the concentration of electron at the green EML, while it owns almost similar cavity length with 3-RGB1 device due to modified thickness by 10 nm. Figure 6 (a) shows higher current density for 3-RGB2 than that of 3-RGB1 device with the modified CGU. At the constant current density of 1 mA/cm2, the 18


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operational voltage of 9.3 V is measured with the PE of 36.5 lm/W. This improved performance of 3-RGB2 clearly indicates that the modified CGU is very effective to control charge density in the device. The EQE of 3-RGB2 is also enhanced from 49.4 to 51.7% as shown in Figure 6 (c). The increased intensity of the green spectral peak at around 525 nm enhances PE and EQE (Figure 6 (d)), while the emission spectrum is still maintained at the high CRI of 92.

Table 1. The experimental and theoretical performances of three stack WOLEDs Tandem

Voltage [V]a)

Power Efficiency [lm/W]a)

Power Efficiency [lm/W]b)

External Quantum Efficiency [%]a)

Simulationc)

9.9

30.1

-

3-RGB1

10.0

33.4

3-RGB2

10.0

3-RGB3

9.1

CRI (CCT [K])a)

CRI (CCT [K])b)

Color Coordinatea)

42.8

91 (2571)

-

(0.471, 0.421)

24.7

49.4

93 (2703)

92 (2683)

(0.467, 0.423)

36.5

28.7

51.7

92 (2786)

92 (2776)

(0.466, 0.433)

38.6

29.7

49.0

90 (2861)

90 (2823)

(0.465, 0.442)

a)

at the current density of 1 mA/cm2; b)at the current density of 5 mA/cm2; c)for 3-RGB1

In addition, 3-RGB3 is designed by reducing the non-doped ETL thickness of blue unit by 10 nm to strengthen green spectral intensity, which could enhance the luminous efficiency of structure 1. We expected to further reduce the driving voltage property by decreasing thickness of ETL instead of HTL, because electron mobility is normally slower than the hole mobility. As shown in Figure 6 (a), 3-RGB3 shows highest current density characteristic as compared to those of other three-stacked WOLEDs and also demonstrates relatively low operating voltage of 9.1 V at the current density of 1 mA/cm2. The luminance-voltage characteristic is also improved due to more outcoupled red and green spectra despite the shift in blue spectrum toward 450 nm 19


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wavelength. This shift in blue spectrum is attributed to the altered blue EML position within the decreased cavity length of 10 nm. Hence, slightly high PE of 38.6 lm/W is exhibited despite relatively low EQE value of 49.0 % (Figure 6 (c)). Furthermore, 3RGB3 shows CIE color coordinate of (0.465, 0.442) and CRI value of 90. Herein, all three fabricated WOLEDs demonstrated excellent electrical performances with warm white spectra, which is appropriate for lighting applications. The performances of simulated and fabricated three-stacked WOLEDs are summarized in Table 1. Here, we also verified a validity of our performance prediction method by reversely calculating the performances of fabricated 3-RGB2 and 3-RGB3 devices. Firstly, the simulated spectra of 3-RGB2 and 3-RGB3 are different from the measured spectra, as shown in Figure S9, when the same charge balance factor was assumed like 3-RGB1 device (charge balance =1). In order to fully correlate calculated spectra with the measured spectra, other factors required to be established. Therefore, we changed the charge balance factors as illustrated in Table S3, because both devices (3-RGB2 and 3RGB3) exhibited an enhanced green and reduced blue spectral intensity as shown in Figure 6 (d). Normally, it is common to assign a relatively higher charge balance ratio such as 1.1 and 1.2 because both 3-RGB2 and 3-RGB3 devices might have different charge balance due to altered thickness of HTL and ETL for green and blue parts in tandem devices compared to those of original unit devices. By considering modified charge balance ratio, we attained a nearly correlated simulated and measured spectra as shown in Figure S10. The power efficiency (and EQE) values of both 3-RGB2 and 3RGB3 were found to be 34.6 lm/W (48.3 %) and 38.4 lm/W (46.9 %), respectively, through equation (1), (3) and (4). The theoretical evaluation results through our performance prediction method were correlated with the characteristics of fabricated 20


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devices as summarized in Table S4. From reverse calculation procedure, it is worth to note that our studied performance prediction method is practically valid to speculate the performances of complex tandem devices. Similarly, this method suggests theoretical performance index with the design guide of optically complex OLEDs from the characteristics of simple unit devices.

CONCLUSION In summary, we optically designed and fabricated a highly efficient three-stacked hybrid WOLED structure for lighting applications. The fruitful optical simulation was performed to calculate the appropriate EML position in the three-stacked WOLED devices. This optical design helps to enhance the efficiency of three-stacked WOLEDs by tuning the color of each unit device, which is strongly affected by the thick cavity length. The fabricated three-stacked WOLEDs by incorporating optical design criteria demonstrated a maximum power efficiency of 33.4 lm/W and CRI of 93. We believe that the optical design approach described here for three-stacked WOLED will be helpful for the future commercial lighting products.

ACKNOWLEDGEMENTS This work was supported by the Human Resources Development program (No. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and this work was also supported by the World Class 300 Project (R&D)(S2317456) of the SMBA(Korea).

AUTHOR INFORMATION 21


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Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Tel : 82-2-961-0948 Fax : 82-2-961-9154

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