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Nov 21, 2017 - WOLEDs is almost identical to the summation of unit devices (9.9 V) because of good interconnecting units and the same .... ETL materia...
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Article Cite This: ACS Photonics 2018, 5, 655−662

Optical Design and Optimization of Highly Efficient Sunlight-like 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 130-701, Republic of Korea S Supporting Information *

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 a 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 a 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 WOLEDs is almost identical to the summation of unit devices (9.9 V) because of good interconnecting units and the same charge balance in the tandem WOLEDs. In addition, the experimentally measured power efficiency of the tandem device is similar to an average value of the unit devices, and most importantly the EQE is nearly equal to the summation of the unit devices with an almost matched white spectrum. KEYWORDS: optical design, color rendering index, three-stacked OLED, white OLED, tandem OLED

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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 the short distance between the light emissive layer and the reflective electrode. However, these thin OLEDs are sensitive to the particles and surface roughness and also exhibit short operational lifetimes. Similarly, these devices also indicate a main issue of color stability with respect to the applied electric field by transferring energy, especially when several emitters are doped in a single EML.20,21 In addition, after saturating emitter sites of low exciton energy, especially red emitters, the radiative exciton ratio of emitters with various energies is different compared to before saturation with respect to the driving voltage, and additionally the electron and hole recombination

hite organic light emitting diodes (WOLEDs) have been considered as the most promising technology for largesize and high-resolution active matrix displays as well as solid state lighting applications.1 In recent years, extensive research works have been reported on WOLEDs as luminaires because of their high efficiencies, and they 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, such as simple © 2017 American Chemical Society

Received: November 15, 2017 Published: November 21, 2017 655

DOI: 10.1021/acsphotonics.7b01379 ACS Photonics 2018, 5, 655−662

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ACS Photonics zone also shifts along with the mobility of charge carriers.22,23 Hence, it requires an extremely low doping concentration of low-energy dopant and fine doping technology, but this also creates an issue of process reproducibility. In the case of a multiple EML structure, a number of concerns such as energy transfer and shift in the recombination zone occurred at the interfaces of each adjacent color EML.24 In order to eliminate the above-defined issues of co-doped and multiple EML structures, a tandem OLED structure is highly recommended, where individual EMLs are vertically stacked and electrically connected in series through a charge generation unit (CGU).25 Normally, tandem OLEDs show summation of driving voltages of each unit device when there is no energy barrier at the interconnecting units. In addition, current efficiency and external quantum efficiencies (EQEs) are expected to be the sum of each unit device, without considering optical interference effects in the tandem OLEDs. Indeed, tandem OLEDs ensure higher power efficiency despite their high driving voltage because of enhanced current performances. On the other hand, tandem structures suffer from a serious optical interference issue, which is a result of 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 threestacked WOLEDs for lighting applications, and almost no reports are available on their accurate fabrication method with proper theoretical optical evaluation. Herein, a systematic and theoretical approach was used to enhance the out-coupling ratio of the emitted light and to tune the color properties of three-stacked WOLEDs by evaluating the appropriate position of EMLs and to tune the color properties in electrically connected individual electroluminescent devices because of severe optical interference by a long cavity length. To achieve a high CRI factor, a fluorescent deep blue emitter with phosphorescent red and green emitters was considered because of their high spectral intensity in the longwavelength range and low intensity in the short-wavelength range. Based on an optical and electrical calculation method, the 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%).

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 nm)/p-bPPhenB (25 nm)/pbPPhenB: 5% Li (5 nm)/Al (100 nm) We applied bis(10-hydroxybenzo[h]quinolinato)beryllium complex (Bebq 2 ), bis[2-(2-hydroxyphenyl)pyridinato]beryllium (Bepp 2 ), and 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) as phosphorescent red, phosphorescent green, and fluorescent blue host materials, respectively. Bis(4methyl-2-(3,5-dimethylphenyl)quinolinato (N,C2′)tetramethylheptadionate iridium(III) (Ir(mphmq)2(tmd)), (acetylacetonate)bis(2-phenylpyridine)iridium(III) (Ir(ppy)2(acac)), and 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) were used as fluorescent blue, phosphorescent green, and phosphorescent red emitters, respectively. In order to enhance the hole injection property from the 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, an n-doped electron transport layer (ETL) was used to improve the electron injection property from the aluminum (Al) cathode. Lithium (Li) was used as an 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 (5.8 × 10−3 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 and 2.50 eV as well as singlet energy of 3.50 and 3.36 eV, respectively, which are high enough to confine generated excitons in the EML. As shown in Figure S1, the band gaps of red and green host materials are wide enough to include the highest occupied molecular orbital (HOMO) and LUMO energy levels of red and green emitters. In the case of the red unit device, electrons are injected from the LUMO of p-bPPhenB into the LUMO of Ir(mphmq)2(tmd) and Bebq2. The injected electrons in the dopant can easily travel toward TAPC because the LUMO energy gap between the dopant and host is only 0.1 eV, which is sufficient enough to thermally activate the host. However, holes will be trapped on the dopant level if holes enter into the host; then they can hop on the HOMO level of the dopant due to the HOMO gap of 0.9 eV. The HOMO and LUMO energy gaps between host and dopant materials, Bepp2 and Ir(ppy)2(acac), are 0.4 and 0.3 eV, respectively. Herein, holes can somewhat experience trap sites due to an energy gap of 0.4 eV, while the 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 the 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, the blue unit device shows charge trapping emission, where excitons are directly generated in the emitter,

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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 position and cavity thickness management is essentially needed. Hence, we performed an extensive optical simulation of three-stacked WOLEDs 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 a 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) 656

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ACS Photonics which is overwhelmingly dominant than those of other unit devices. We designed an OLED structure with a higher emitter doping concentration of about 12% for the broad recombination zone with a minimized exciton quenching and maximized efficiency, because fluorescent exciton shows a short lifetime in the nanometer range. It is appropriate to utilize the mechanism that directly forms excitons on the dopant site due to small HOMO and LUMO gaps of the MADN and BCzVBi system. Also, a thin layer of p-bPPhenB (25 nm) rather than that of the red and green units forms the main recombination region at the HTL/EML interface. From a device physics perspective, our presumption about exciton recombination zone position could be adequate. Furthermore, a lithium-doped ETL was used to lower the electron injection barrier from the aluminum cathode (work function of 4.2 eV) toward the LUMO energy level of pbPPhenB (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 a charge balance issue due to an excessive electron density.37 Hence, the doping concentration and doping region of the 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 the material choice to the optimization and analysis is very useful to fabricate highly efficient stacked WOLED devices. Figure S2 shows the current efficiency and EQE 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 efficiencies of 42.7, 88.7, and 9.8 cd/A are obtained for the 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 the 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, PLQY of about 100% was used because our fluorescent blue unit device displayed an almost ideal EQE of 5.9%.41 In addition, we also assumed a relatively 100% charge balance factor (γ) for the optical simulation of each EML due to their ideal EQE performances with consideration of their best connection in a tandem device. We calculated the optical and electrical performances of a three-stacked white OLED with the characteristics of unit devices at a 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 the HTL side for green devices. 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 the HTL and ETL. These radiance distributions result from the weak microcavity effect by increasing the proportion of multiple-beam interference with the two-beam interference as

Figure 1. (a) Optical constants of glass substrate, anode (ITO), cathode (Al), and organic layers (TAPC and p-bPPhenB); (b) internal emission spectra of each red, green, and blue device used for the calculation of the EML distribution.

the total device thickness increases. Indeed, the radiance distribution is more sensitive to the ETL thickness than HTL due to 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 out-coupling efficiency is appeared to be at the cathode side EML location (termed as first antinode) regardless of the light emission wavelength when the thickness of the devices is identical. The number of appropriate EML positions is equal to the cavity order in the same cavity length, and indeed, a higher cavity order slightly reduces the intensity of the radiated light, as shown in Figure 2. On the basis of the radiance distribution results, we considered six possible three-stacked WOLED structures with red, green, and blue EMLs, shown in Figure 3. These structures with the calculated thickness of three-stacked WOLEDs have shown EML positions with maximum radiance (at antinodes) because EMLs in no antinode positions cause the reduction in performances, although located EMLs in these positions can tune the device spectra. For the high CRI with a warm white spectrum, structures 1 and 2 are good candidates because of their strong out-coupled positions in the long-wavelength range at the first antinode. Herein, an antinode is referred to as a first EML position from the cathode, and it has a small ETL thickness. The green EML should properly occupy its second antinode for the maximum power efficiency of the tandem devices. It is also reasonable to place a red EML instead of a green EML at the first antinode site to balance the intensity of green, because the studied green unit device has a higher spectral intensity and EQE value. Also, structure 1 just requires connecting unit with an almost unchanged thickness of transporting layers from the unit devices, which indicates charge balance in each EML of the tandem device, similar to those of unit devices. Additionally, this structure can illustrate an apparent merit in terms of performance, because the power efficiency of the OLEDs decreases by raising the driving voltage, especially when the thickness of the 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 region and reflective cathode. Hence, we fabricated threestacked WOLEDs with structure 1, which has less thickness than connected three-unit devices. To evaluate electrical characteristics (such as current efficiency (cd/A), power efficiency (lm/W), and EQE (%)) of three-stacked WOLEDs, 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 657

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Figure 2. Simulated normalized radiance of (a) red, (b) green, and (c) blue EMLs with respect to the thickness of the ETL and HTL. The following structure was considered for the calculation of the 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).

devices (Table S1). We also modified structure 1 by comprising a high refractive index material (p-bPPhenB) as ITO (50 nm)/ organic layer (OL, 82 nm)/OL as blue EML (15 nm)/pbPPhenB (30 nm)/OL (27 nm)/OL as green EML (15 nm)/ p-bPPhenB (60 nm)/OL (82 nm)/OL as red EML (15 nm)/pbPPhenB (60 nm)/Al (100 nm). The changed thickness of pbPPhenB in the modified structure 1 is approximated by considering the below equation for the optical path length.42

∑ (nd) = ∑ (n′d′)

where d represents the physical thickness of the medium. The organic layer thickness of 27, 82, and 60 nm is derived from eq 2 on the basis of a long-range spectral peak (610 nm) to attain a high CRI. The thickness at the short-range peak (450 nm) is varied by less than 5 nm. By considering this correction, spectra of the red device in the first antinode of the second order, the green device in the second antinode of the second order, and the blue device in the third antinode 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)/p-bPPhenB (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 structures, 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), CE(g,2,0), and CE(b,3,0) were calculated to be 38.0, 41.4, and 5.6 cd/A at a given constant current density of 1 mA/cm2. Our assumption on nonchanged charge balance from the excellent interconnecting interfaces without any increase in operational voltage can derive the current efficiency (CEt(θ)) of a three-stacked WOLED by using the following equation:

Figure 3. Possible three-stack structures with red, green, and blue EML positions.

cavity order hold the same charge balance. Then, we can expect the current efficiency of each EML in the designed threestacked WOLED as shown in the equation below. 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)

(2)

(1)

where CE and L denote the current efficiency and luminance at a certain current density condition. RL is the luminance ratio between different cavity spaces. The order of the cavity is represented as m and n. The factors c and θ indicate emission color and viewing angle of the devices. Herein, we just considered the change of out-coupling efficiency (or radiance distributions as shown in Figure 2) with respect to the cavity order by optical interference. The EML positions of the unit devices are in the first-order cavity space. In order to calculate L(c,1,0) values of each color, we performed optical simulations for the predicted EL spectra with the same thickness of the fabricated unit devices. However, the calculated EL spectra using the same optical constants were in discord with the measured spectra of each unit device, 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 a slightly higher refractive index (see Figure S3(b)). By incorporating measured optical constants of ETL in the calculation, we have obtained wellcorrelated EL spectra as shown in Figure S3(c). These results show different spectral tailed regions by applying EL spectra of unit devices including the optical interference effect for EMLs. With our consideration about γ of 100% in the 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

CEt(0) = CE(r,2,0) + CE(g,2,0) + CE(b,3,0)

(3)

According to eq 3, the expected CE of the 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 the 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 658

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ACS Photonics (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, a small shoulder peak at the longwavelength region and reduced spectral intensity at the shortwavelength range were obtained in the designed spectrum compared to that of the 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 a barely negligible effect on the factor “RL” in eq 1, although the calculated efficiencies from eq 3 are slightly lower than those of measured values. To evaluate the power efficiency (PE) of the three-stacked WOLED, we used the following equation:43 PE =

∫θ S(λ)|Et(θ)|2 dθ /[(Vr + Vg + Vb)I ]

mA/cm2. This indicates that a higher PE can be obtained in the three-stacked OLED, if devices with efficient HTL, ETL, and host materials are used for enhanced EQE of the unit devices. Hence, to attain excellent performances with high power efficiency, it is essential to fabricate highly efficient unit devices and to reduce energy barriers at the interconnected interfaces between unit devices. It is a very valuable method to predict the electrical and optical characteristics by designing complex stacked OLEDs. Additionally, it is also suggested that this method can evaluate whether the performances of fabricated stacked devices correlate with those of the unit devices. Three-Stacked WOLEDs. By considering the optical design calculation, a 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)/p-bPPhenB: 5% Li (5 nm)/ HATCN (7 nm)/TAPC (78 nm)/Bebq 2 : 3% Ir(mphmq)2(tmd) (15 nm)/p-bPPhenB (55 nm)/p-bPPhenB: 5% Li (5 nm)/Al (100 nm). 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 the blue and green unit devices 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 the fabricated three-stacked WOLED. The measured current density is improved over the calculated value in 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 the same as the unit OLEDs. The three-stacked WOLED exhibits CIE color coordinates of (0.467, 0.423) and a high CRI of 93. Herein, we need to reduce the driving voltage of the abovediscussed WOLED to further enhance the PE performance. Two types of three-stacked WOLEDs were fabricated with two kinds of modified structures as shown in Figure 5. The interconnecting unit of 3-RGB2 between the green and red unit devices is modified by decreasing the ETL thickness from 55 nm to 45 nm and increasing the TAPC thickness from 78 nm to 88 nm in order to intensify the concentration of electrons at the green EML, while it has almost the same cavity length as the 3-RGB1 device due to the modified thickness by 10 nm. Figure 6(a) shows a higher current density for 3-RGB2 than that of the 3-RGB1 device with the modified CGU. At the constant current density of 1 mA/cm2, the operational voltage of 9.3 V is measured with a 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 the PE and EQE (Figure 6(d)), while the emission spectrum is still maintained at the high CRI of 92.

(4)

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 the spectrum of the three-stacked WOLED as |Et(θ)|2, which is the same as the white spectrum shown in Figure S4. S(λ) and I represent the human eye sensitivity function as the wavelength and current, respectively. According to eq 4, the driving voltage of the three-stacked WOLED was expected to be 9.9 V at a 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, 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 the three-stacked WOLED with the considered refractive index of p-bPPhenB was different from that of structure 1 because the modified thickness of pbPPhenB was calculated for enhancing the long-range emission wavelength. Figure S5 shows normalized spectra with their color coordinates 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. At 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 properties. Figure 4 shows PE in accordance with the charge balance ratio and driving voltage at the presumed current density of 1

Figure 4. Calculated power efficiency versus driving voltage characteristics of an OLED with different charge balance in unit EMLs (or EQE of the unit devices). 659

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Figure 5. Fabricated device structures of 3-RGB1, 3-RGB2, and 3-RGB3.

(0.465, 0.442) and a 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 the validity of our performance prediction method by reversely calculating the performances of fabricated 3-RGB2 and 3-RGB3 devices. First, 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 the 3-RGB1 device (charge balance = 1). In order to fully correlate calculated spectra with the measured spectra, other factors need 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 balances due to the altered thickness of the HTL and ETL for the green and blue parts in the tandem devices compared to those of the original unit devices. By considering a 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 3RGB2 and 3-RGB3 were found to be 34.6 lm/W (48.3%) and 38.4 lm/W (46.9%), respectively, through eqs 1, 3, and 4. The theoretical evaluation results through our performance prediction method are correlated with the characteristics of fabricated devices as summarized in Table S4. From the reverse calculation procedure, it is worth noting that our studied performance prediction method is particularly valid to speculate the performances of complex tandem devices. Similarly, this method suggests a theoretical performance index with the design guide of optically complex OLEDs from the characteristics of simple unit devices.

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 the 3-RGB1 device, (c) power efficiency versus current density versus EQE performances, and (d) measured EL spectra of the fabricated WOLEDs.

In addition, 3-RGB3 is designed by reducing the nondoped ETL thickness of the blue unit by 10 nm to strengthen the green spectral intensity, which could enhance the luminous efficiency of structure 1. We expected to further reduce the driving voltage property by decreasing the thickness of the ETL instead of the HTL, because electron mobility is normally slower than the hole mobility. As shown in Figure 6(a), 3RGB3 shows the highest current density characteristic as compared to those of the other three-stacked WOLEDs and also demonstrates a 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 out-coupled red and green spectra despite the shift in the blue spectrum toward a 450 nm wavelength. This shift in blue spectrum is attributed to the altered blue EML position within the decreased cavity length of 10 nm. Hence, a slightly high PE of 38.6 lm/W is exhibited despite the relatively low EQE value of 49.0% (Figure 6(c)). Furthermore, 3-RGB3 shows CIE color coordinates of

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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

Table 1. Experimental and Theoretical Performances of Three-Stack WOLEDs

a

tandem

voltage [V]a

power efficiency [lm/ W]a

power efficiency [lm/ W]b

external quantum efficiency [%]a

simulationc 3-RGB1 3-RGB2 3-RGB3

9.9 10.0 10.0 9.1

30.1 33.4 36.5 38.6

24.7 28.7 29.7

42.8 49.4 51.7 49.0

CRI (CCT [K])a

CRI (CCT [K])b

91 93 92 90

92 (2683) 92 (2776) 90 (2823)

(2571) (2703) (2786) (2861)

color coordinatea (0.471, (0.467, (0.466, (0.465,

0.421) 0.423) 0.433) 0.442)

At a current density of 1 mA/cm2. bAt a current density of 5 mA/cm2. cFor 3-RGB1. 660

DOI: 10.1021/acsphotonics.7b01379 ACS Photonics 2018, 5, 655−662

Article

ACS Photonics

(10) Wang, Q.; Ho, C. L.; Zhao, Y.; Ma, D.; Wong, W. Y.; Wang, L. Reduced efficiency roll-off in highly efficient and color-stable hybrid WOLEDs: The influence of triplet transfer and charge-transport behavior on enhancing device performance. Org. Electron. 2010, 11, 238−246. (11) Chang, C. H.; Tien, K. C.; Chen, C. C.; Lin, M. S.; Cheng, H. C.; Liu, S. H.; Wu, C. C.; Hung, J. Y.; Chiu, Y. C.; Chi, Y. Efficient phosphorescent white OLEDs with high color rendering capability. Org. Electron. 2010, 11, 412−418. (12) Chen, S.; Tan, G.; Wong, W. Y.; Kwok, H. S. White Organic Light-Emitting Diodes with Evenly Separated Red, Green, and Blue Colors for Efficiency/Color-Rendition Trade-Off Optimization. Adv. Funct. Mater. 2011, 21, 3785−3793. (13) Jou, J. H.; Shen, S. M.; Lin, C. R.; Wang, Y. S.; Chou, Y. C.; Chen, S. Z.; Jou, Y. C. Efficient very-high color rendering index organic light-emitting diode. Org. Electron. 2011, 12, 865−868. (14) Zou, J.; Wu, H.; Lam, C. S.; Wang, C.; Zhu, J.; Zhong, C.; Hu, S.; Ho, C. L.; Zhou, G. J.; Wu, H.; Choy, W. C. H.; Peng, J.; Cao, Y.; Wong, W. Y. Simultaneous Optimization of Charge-Carrier Balance and Luminous Efficacy in Highly Efficient White Polymer LightEmitting Devices. Adv. Mater. 2011, 23, 2976−2980. (15) Xu, F.; Lim, J. M.; Kim, H. U.; Mi, D.; Lee, J. Y.; Joo, C. W.; Cho, N. S.; Lee, J. I.; Hwang, D. H. High color rendering white organic light-emitting diodes fabricated using a broad-bandwidth red phosphorescent emitter for lighting applications. Synth. Met. 2012, 162, 2414−2420. (16) Zhang, B.; Tan, G.; Lam, C. S.; Yao, B.; Ho, C. L.; Liu, L.; Xie, Z.; Wong, W. Y.; Ding, J.; Wang, L. High-efficiency single emissive layer white organic light-emitting diodes based on solution-processed dendritic host and new orange-emitting iridium complex. Adv. Mater. 2012, 24, 1873−1877. (17) Yu, J.; Yin, Y.; Liu, W.; Zhang, W.; Zhang, L.; Xie, W.; Zhao, H. Effect of the greenish-yellow emission on the color rendering index of white organic light-emitting devices. Org. Electron. 2014, 14, 2817− 2821. (18) Fan, C. C.; Huang, M.; Lin, W. C.; Lin, H. W.; Chi, Y.; Meng, H. F.; Chao, T. C.; Tseng, M. R. Org. Electron. 2014, 15, 517−523. (19) Wu, Z.; Ma, D. Recent advances in white organic light-emitting diodes. Mater. Sci. Eng., R 2016, 107, 1−42. (20) Zhao, Y.; Zhu, L.; Chen, J.; Ma, D. Improving color stability of blue/orange complementary white OLEDs by using single-host double-emissive layer structure: comprehensive experimental investigation into the device working mechanism. Org. Electron. 2012, 13, 1340−1348. (21) Park, Y. S.; Kang, J. W.; Kang, D. M.; Park, J. W.; Kim, Y. H.; Kwon, S. K.; Kim, J. J. Efficient, Color Stable White Organic LightEmitting Diode Based on High Energy Level Yellowish-Green Dopants. Adv. Mater. 2008, 20, 1957−1961. (22) Shen, F.; Xia, H.; Zhang, C.; Lin, D.; Liu, X.; Ma, Y. Spectral investigation for phosphorescent polymer light-emitting devices with doubly doped phosphorescent dyes. Appl. Phys. Lett. 2004, 84, 55−57. (23) Lee, J.; Lee, J. I.; Chu, H. Y. Efficient and color stable phosphorescent white organic light-emitting devices based on an ultra wide band-gap host. Synth. Met. 2009, 159, 991−994. (24) Son, Y. H.; Park, M. J.; Kim, Y. J.; Yang, J. H.; Park, J. S.; Kwon, J. H. Color stable phosphorescent white organic light-emitting diodes with double emissive layer structure. Org. Electron. 2013, 14, 1183− 1188. (25) Liao, L. S.; Klubek, K. P.; Tang, C. W. High-efficiency tandem organic light-emitting diodes. Appl. Phys. Lett. 2004, 84, 167−169. (26) Lee, T. W.; Noh, T.; Choi, B. K.; Kim, M. S.; Shin, D. W. Highefficiency stacked white organic light-emitting diodes. Appl. Phys. Lett. 2008, 92, 043301. (27) Furno, M.; Rosenow, T. C.; Gather, M. C.; Lussem, B.; Leo, K. Analysis of the external and internal quantum efficiency of multiemitter, white organic light emitting diodes. Appl. Phys. Lett. 2012, 101, 143304. (28) Adamovich, V. I.; Levermore, P. A.; Xu, X.; Dyatkin, A. B.; Elshenawy, Z.; Weaver, M. S.; Brown, J. J. High-performance

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 cavity thickness. 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 in this study for threestacked WOLEDs will be helpful for future commercial lighting products.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01379. Additional information (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: 82-2-961-0948. Fax: 82-2961-9154. ORCID

Jang Hyuk Kwon: 0000-0002-1743-1486 Notes

The authors declare no competing financial interest.

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

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DOI: 10.1021/acsphotonics.7b01379 ACS Photonics 2018, 5, 655−662