Efficient Quantum-Dot Light-Emitting Diodes Employing Thermally

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Efficient Quantum-Dot Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence Emitter as Exciton Harvesters Ying Nan Zhang, Yu Sheng Liu, Min Ming Yan, You Wei, Qi Lun Zhang, and Yong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16579 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Applied Materials & Interfaces

Efficient Quantum-Dot Light-Emitting Diodes Employing

Thermally

Activated

Delayed

Fluorescence Emitter as Exciton Harvesters Ying-Nan Zhang, Yu-Sheng Liu, Min-Ming Yan, You Wei, Qi-Lun Zhang, and Yong Zhang* Laboratory of Nanophotonic Functional Materials and Devices, Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou, 510631, China

ABSTRACT: The utilization of triplet excitons plays a key role in obtaining highly efficient quantum-dot light-emitting diodes (QD-LEDs). However, to date, only phosphorescent materials have been implemented to harvest the triplet excitons in QD-LEDs. In this work, we introduced a thermally activated delayed fluorescence (TADF) emitter, 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN), doped into poly(N-vinylcarbazole) (PVK) as exciton harvester in red QD-LEDs by solution-processed. As a result, electrons leaking to PVK layer will be trapped by 2CzPN to form long lifetime TADF excitons in 2CzPN:PVK layer and then these harvesting exciton energy can be effectively transferred to the adjacent QDs by Förster resonance energy transfer process. The fabricated red CdSe/CdS core/shell QD-LEDs show a maximum luminescence efficiency of 17.33 cd/A and longer lifetime. Our results demonstrate that the TADF sensitizer would be a promising candidate to develop highly efficient QD-LEDs.

KEYWORDS: quantum-dot light-emitting diodes, thermally activated delayed fluorescence, exciton harvester, förster resonance energy transfer, solution-processed

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INTRODUCTION Colloidal quantum-dots (QDs) as emerging optoelectronic materials have been widely used in biomedical,1,

2

light-emitting diodes (LEDs),3,

4

light detection,5 and solar cells.6,

7

QD light-

emitting diodes (QD-LEDs) have attracted many of the interests due to tunable colors, saturated color emission, high luminescence efficiency, and simple fabrication process.3,

8-11

Therefore,

QD-LEDs have potential application in next-generation display technologies. Nowadays, the widely

used

QD-LED

structure

is

ITO/poly(3,4-ethylenedioxythioxythiophene):

poly(styrenesulfonate) (PEDOT:PSS)/hole-transporting layer (HTL)/QDs/electron-transporting layer (ETL)/cathode, where the QD emissive layer is usually fabricated by spin-coating.12 Metal oxides like ZnO and TiO2 are commonly used as ETL because of its high electron mobility and similar lowest unoccupied molecular orbit (LUMO) level with QDs.11, 13, 14 On the other hand, holes is quite difficult to inject into the QD emissive layer because most QDs have a much lower valence band than highest occupied molecular orbit (HOMO) level of HTL.15 Therefore, electrons will more easily inject into the QD emissive layer than holes. As a result, excess electrons occur in the QD emissive layer and lead to QDs charged and electron leaking to HTL without recombination, which can greatly deteriorate the device performance of QD-LEDs.16 Many approaches have been proposed to optimize the electron and hole-injection balance and device performance in QD-LEDs, Solution-processed NiO with rather low HOMO levels have been utilized as HTL in QD-LEDs with high luminance and air stability.17 Ho et al. showed polymer and small molecule mixture as HTL can enhanced luminescence efficiency of QDLEDs.18 Dai et al. reported an insulating PMMA layer inserting between the QD emissive layer and ZnO ETL can effectively tune charge balance in QD-LEDs by solution-processed and exhibited excellent device performance and reproducibility.16 Recently, we found that Mg-doped


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ZnO can elevate the conduction-band minimum (CBM) and reduce the electron mobility of ZnO, resulting in enhancing the performance of blue QD-LEDs due to a balanced charge-injection.19 In addition to increasing hole injection or reducing election injection, energy transfer has often been used as enhancing the performance of QD-LEDs. In this process, excitons are generated on organic charge transporting layer surrounding the QD emissive layer and transfer their energy into the adjacent QDs by Förster resonant energy transfer (FRET). Phosphorescent materials have much longer exciton lifetimes,20, 21 and thus may act as more effective donors transferring their energy to QD acceptors.22 Zhang et al. showed that a 2.5-fold increase in the quantum efficiency of

green

CdSe/ZnS

QD-LEDs

by

employing

bis(4,6-difluorophenylpyridinato-N,C2)

picolinatoiridium (FIrpic) as efficient exciton harvesters to capture holes overinjected from the QDs.23 Later, Multugun et al. demonstrated a QD-LED with enhanced color purity by incorporating an intermediate layer between the QD emissive layer and the Ir(ppy)3 sensitizer.24 Siboni et al. reported very efficiency and bright green QD-LEDs by the combined use of a FIrpic sensitizer and thermal annealing and found that the efficiency of energy transfer from the sensitizer to the QDs is strongly dependent on the QD size.25 Recently, Liu et al. demonstrated improving the performance of inverted QD-LEDs by doping FIrpic into HTL to trap leaked electrons from the QD emissive layer to form the excitons and the harvested energy was then transferred to the adjacent QDs by the FRET process.26 Although phosphorescent materials as exciton harvester are known to be an effective route to promoting the performance of QD-LEDs, phosphorescent emitters suffer from expensive and nonrenewable heavy metals which would restrict the practical application in the future. In contrast, metal-free thermally activated delayed fluorescence (TADF)27, 28 is a novel strategy to utilize the triplet excitons via the reverse intersystem crossing (RISC) and is regarded as promising materials for next-generation organic light-emitting diodes (OLEDs) owing to their 3 Environment ACS Paragon Plus


potential of low cost and high performance.29-31 In the TADF process, electrically generated triplet excitons can be efficiently converted into singlet excitons through RISC when the energy gap between the lowest singlet excited state (S1) and the lowest triplet excited state (T1) (∆EST) is sufficiently small.32 Recently, these novel triplet-harvesting pathways were proposed for realizing high performance fluorescence OLEDs by TADF molecular as an exciton harvester for conventional fluorescent emitters.33, 34 In these host-guest systems, Triplet excitons generated on TADF molecules are upconverted to the S1 via RISC and then the S1 excitons are transferred to the conventional fluorescent dopants via FRET processes. Nakanotani et al. reported a series of fluorescent OLEDs with external quantum efficiencies (EQE) of 13.4-18.0% by utilizing efficient energy transfer from TADF molecules to the conventional fluorescent emitters.34 Similarly, Zhang et al. used TADF materials with ∆EST of 0.06-0.11 eV as hosts of conventional fluorescent emitters and realized a high EQE of 11.7 %.33 However, no studies have been reported about the utilization of TADF molecular as an exciton harvester for the development of highly efficient QD-LEDs. In this work, we used 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN) doped into poly(Nvinylcarbazole) (PVK) as exciton harvester by solution-processed. For a 2CzPN:PVK HTL, the 2CzPN molecules can effectively capture and utilize the leakage electrons from the QD emissive layer to form both singlet and triplet excitons. Triplet excitons created on the 2CzPN molecules will be upconverted to the singlet excited state of the 2CzPN molecule via RISC due to its low ∆EST and the singlet excitons of 2CzPN are subsequently transferred to the adjacent QDs through FRET processes. At the same time, the electron accumulation in PVK/QDs interface can be alleviated because of the effective capture of electron, thus reducing the QDs charging and enhancing the luminescent efficiency of QD-LEDs. As a result, we successfully developed highly efficient red QD-LEDs based on TADF sensitizer. For a 20 nm 2CzPN(20%):PVK HTL, red QD4 Environment ACS Paragon Plus

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LED exhibited a 2.08-fold enhancement in luminescence efficiency as well as a longer operational lifetime. RESULTS AND DISCUSSION

Figure 1. (a) Device structure of QD-LEDs and (b) energy level alignment of the used materials.

Figure 1 shows the device structure of red QD-LEDs and the energy level diagrams of the used materials in this work. The HOMO and LUMO, conduction and valence band levels of the materials are taken from the literature.18,

35

During the all-solution-processed QD-LEDs, the

solvent selection of QDs plays an important effect on the device performance. In this work, QDLEDs with structures of glass/ITO/PEDOT:PSS/PVK or 2CzPN:PVK/QDs/ZnO/Al were fabricated by solution-processed. The exciton harvester layer of 2CzPN:PVK were fabricated by spin-coating from chlorobenzene and then the QD emissive layer were generally spin-coated from octane to form a smooth and compact QD film.36 Solvent resistance of the exciton harvester layer was investigated by monitoring the UV-Vis absorbance spectra of the 2CzPN:PVK blend layer before and after rinsing with octane. As shown in Figure 2, the absorbance spectrum of the 2CzPN:PVK blend film was unchanged after washing with octane. This indicates that the 2CzPN:PVK film has a sufficient solvent resistance during spin-coating from the QD octane dispersion. A 40 nm thick ZnO layer was spin-coated on the top of QDs and used as an ETL, and Al was used a cathode.



Figure 2. Absorbance spectra of 2CzPN:PVK blend film before and after washing with octane.

Figure 3a shows luminescence efficiency and brightness properties of red QD-LEDs based on the different 2CzPN-doped concentrations at 20 nm thickness. It can be seen that the luminescence efficiency of red QD-LEDs increased and then decreased as the 2CzPN-doped ratios increased. The corresponding device performances are summarized in Table 1. The maximum luminescence efficiency (LE) and external quantum efficiency (EQE) of QD-LEDs increased from 8.32 cd/A and 5.94 % for pure PVK to 17.33 cd/A and 12.37 % for 20% 2CzPNdoped PVK. Figure 3b plots the electroluminescent (EL) spectra of the corresponding devices at 30 mA/cm2. It is important to note that the EL spectral line shape of red QD-LEDs remains almost unvaried for the different 2CzPN-doped ratios, suggesting that the increase in luminescence efficiency cannot come from any luminescence of 2CzPN. The increase in luminescence efficiency should be attributed to the increase of red QD emitting, indicating it may be as a result of FRET from 2CzPN to red QDs. The current density and brightness versus voltage (J-V-L) characteristics of the corresponding QD-LEDs are presented in Figure 3c. At low driving voltages, 2CzPN-doped PVK can reduce the current of red QD-LEDs. But, for higher driving voltages, the current density and brightness are elevated greatly. For instance, at a driving


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voltage of 8 V, the current density and brightness are 620 mA/cm2 and 19258 cd/m2 for red QDLED from 2CzPN(20%):PVK while that of the control devices are only 383 mA/cm2 and 6983 cd/m2, respectively. For our QD-LEDs, the interface between QDs and PVK has a relatively high electron and holes accumulation. This is due to the relatively deep conduction band level of QDs, which lead to electrons injecting them from the adjacent ZnO ETL relatively easy.16 On the contrary, it is much more difficult for holes to inject from PVK into QDs due to the large hole injection barrier at the QDs/PVK interface. We note that 2CzPN has a lower LUMO level (3.0 eV)35 than that of PVK (2.2 eV)18 and they have a similar HOMO energy level (5.8 eV). Therefore, electrons will be more easily trapped by 2CzPN, thus reducing current for 2CzPN doped PVK devices at low driving voltage. With the driving voltage increasing, the trapped electrons recombine with holes and thus enhance the recombination current. On the other hand, the trapped electrons pair with holes and then form TADF excitons on 2CzPN molecules. 2CzPN is a purely aromatic TADF compound with a ∆EST of 0.09 eV.35 Therefore, triplet excitons created on 2CzPN molecules are upconverted to the singlet state of 2CzPN molecules via RISC and then these singlet exicton energy is transferred to the adjacent QDs via the FRET process, resulting in enhancing the intensity of the QD emission.

Figure 3. (a) Luminescence efficiency vs brightness, (b) normalized EL spectra at 30 mA/cm2, and (c) current density-voltage-brightness of red QD-LEDs at the various 2CzPN concentrations.



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Table 1. Device properties of red QD-LEDs based on the various 2CzPN concentrations 2CzPN-doped ratios 0% 10 % 20 % 30 %

Von (V) 3.3 3.5 3.1 3.0

EQEmax (%) 5.94 8.10 12.37 8.25

LEmax (cd/A) 8.32 11.35 17.33 11.58

PEmax (lm/W) 5.50 7.75 13.77 8.34

The FRET process is dependent on the distance between donor molecules and acceptors. Figure 4a shows the luminescence efficiency and current density characteristics of red QD-LEDs with 20% 2CzPN doped PVK as HTL at the different thicknesses. The device performance of red QD-LEDs is strongly dependent on the thickness of the 2CzPN:PVK layer. The maximum luminescence efficiency of red QD-LEDs increased from 13.71 cd/A for 10 nm 2CzPN:PVK to 17.33 cd/A for 20 nm 2CzPN:PVK and then reduced to 13.42 cd/A when the thickness of 2CzPN:PVK further increased to 40 nm. The EL spectra of the corresponding red QD-LEDs at 30 mA/cm2 are shown in Figure 4a. It can be found that there has weak emission from 2CzPN with the increasing of the 2CzPN:PVK thickness. This is because of incomplete FRET from 2CzPN to the adjacent QDs at the thicker 2CzPN:PVK layer. However, thin 2CzPN:PVK layer easily make PEDOT:PSS quench the 2CzPN excitons. The 2CzPN:PVK layer acts as HTL and exciton harvesters. Therefore, for the 2CzPN:PVK layer, the optimal doped concentration and thickness is around 20 % and 20 nm, respectively, which guarantees efficient FRET processes and remains highest efficiency.


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Figure 4. (a) Luminescence efficiency and current density, and (b) normalized EL spectra at 30 mA/cm2 characteristics of the 20 % 2CzPN-doped PVK as HTL at the different thicknesses.

To obtain insights about the role of the FRET mechanism in the efficiency improvement, photophysical measurements on thin films of QDs, PVK/QDs, 2CzPN(20%):PVK, and 2CzPN(20%):PVK/QDs were performed. The UV-Vis absorbance spectrum of QD film and the PL spectra of QDs and 2CzPN(20%):PVK films are superimposed in Figure 5a. The large area of spectra overlap implies that an efficient FRET process maybe occur from 2CzPN to the adjacent QDs. Figure 5b shows PL emission spectra of PVK/QDs and 2CzPN(20%):PVK/QDs bilayer films under the excitation of 340 nm laser. It can be observed that for the PVK/QDs bilayer film, the emission from QDs is dominating with weak emission from PVK due to incomplete FRET from PVK to QDs. However, when 20% 2CzPN was doped into PVK, the emission intensity of QDs has further enhanced to about 5-fold. There has no emission from PVK and partial emission from 2CzPN is observed (as shown insert of Figure 5b). The PL change suggests the FRET process from 2CzPN to the adjacent QDs. We note that simple reabsorption of 2CzPN luminescence by the QD film does not explain the observed PL intensity because the 20 nm thick QD film has very weak absorbance during the 2CzPN PL spectrum. Data from time-resolved PL measurements are plotted in Figure 5c and 5d. As can be seen from Figure 5c that the initial fluorescence lifetime and the delay fluorescence lifetime of 2CzPN from a single layer of 2CzPN:PVK film reduce from 29 ns and 297 ns to 18 ns and 194 ns for a bilayer of 2CzPN:PVK/QDs film, respectively. QDs adjacent to 2CzPN:PVK can accelerate the 2CzPN PL decay rate, suggesting that energy transfer from 2CzPN molecules to the adjacent QDs indeed occurs. At the same time, it can be observed from Figure 5d that the QD lifetime increases from 29 ns for PVK/QDs to 40 ns for 2CzPN:PVK/QDs. The PL decay rate of the QDs becomes 9 Environment ACS Paragon Plus


slower when 2CzPN molecules are doped into PVK to be substituted for the PVK, further indicating that this delayed QD PL is because of FRET from 2CzPN excitons to the adjacent QDs.

Figure 5. (a) Absorbance spectrum of QD film and PL spectra of 2CzPN(20%):PVK and QD films, (b) PL spectra of PVK/QDs and 2CzPN(20wt%):PVK/QDs films, Time-resolved photoluminescence decay of (c) 2CzPN in 2CzPN(20wt%):PVK and 2CzPN(20wt%):PVK/QDs films, and (d) QDs in PVK/QDs and 2CzPN(20wt%):PVK/QDs films. The thicknesses of the used films are the same as those in red QD-LEDs

Transient EL measurements were carried out for red QD-LEDs based on PVK and 2CzPN(20%):PVK as HTLs under electric pulse excitation (voltage: 10V, pulse width: 200 ns) and plotted in Figure 6. The EL decay lifetime of QD-LEDs increase from 84.1 ns for PVK to 102.0 ns for 2CzPN(20%):PVK. The device based on 2CzPN(20%):PVK exhibited a much longer EL decay lifetime to further account for effective utilization of both singlet and triplet


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excitons of 2CzPN via RISC between T1 and S1 and then all singlet excitons were transferred to the adjacent QDs by FRET processes.

Figure 6. Transient EL decay profiles of QD-LEDs based PVK and 20% 2CzPN doped PVK as HTLs (λem=628 nm) excited by a pulse voltage (amplitude: 10V, pulse with: 200 ns, repetition rate: 200 KHz).

Figure 7. Brightness and driving voltage versus operation time characteristics of QD-LEDs using PVK and 2CzPN(20%):PVK as HTLs.

The utilization of exciton harvester of 2CzPN doped into PVK HTL provides not only a significant luminescence efficiency improvement but also an enhancement of device operational



stability under electrical driving. For example, the normalized brightness of red QD-LEDs as a function of operation time at a constant current density of 10 mA/cm2 is plotted in Figure 7. A rapid decrease in luminance was observed in the QD-LED with PVK as HTL while the QD-LED with 2CzPN-doped PVK as HTL showed improved luminance decay characteristics. Because of efficient leakage electrons captured and FRET process, the electron accumulation in the PVK/QD interface is significantly relieved, resulting in enhancing the stability of the device.17 QD-LED with 2CzPN(20%):PVK exhibited a longer half-lifetime of 9.61 h while that of the reference device is only 0.85 h. The device improvement can be attributed to the accumulated electrons in the QD/HTL interface alleviating due to 2CzPN capturing leakage electrons from QDs to PVK and then forming excitons transferred to the adjacent QDs versus the FRET process. The driving voltage of the device based on 2CzPN(20%):PVK as HTL is also rather stable (as shown in Figure 7), displaying a rise of less than 0.9 V after 10 h of operation. However, the driving voltage of the control device showed a quick rise of 1.5 V after 3 h of operation, indicating that 2CzPN-doped PVK as HTL provides the improvement device performance. We note that the morphology of 2CzPN:PVK blend film is nearly similar to that of the pure PVK film, indicating that the device stability is not influenced by a morphology change caused by 2CzPN-doping as shown in Figure 8.

Figure 8. AFM surface morphology of PVK (a) and 2CzPN(20%):PVK (b) films.


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CONCLUSION In summary, utilizing the long delay fluorescence lifetime of TADF material, highly efficiency red QD-LEDs have been realized based on 2CzPN-doped PVK as HTL and exciton harvesters by solution-processed. 2CzPN could capture leakage electrons from the QD emissive layer to form TADF excitons. The corresponding triplet excitons generated on the 2CzPN molecules are upconverted to the S1 via RISC and then the S1 exciton energy is transferred to the adjacent QDs via the FRET process. Red QD-LED with 20 nm 2CzPN(20%):PVK as HTL showed a maximum luminescence efficiency of 17.33 cd/A and longer operational lifetime relative to the control QDLED because of relieving QD charged. Our work demonstrates the TADF sensitizer would be a promising candidate to enhance the efficiency and reliability of QD-LEDs.

AUTHOR INFORMATION Corresponding Author *Tel: +86-20-85215603-807. Fax: +86-20-85211435. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (No. 61377065 and 61574064), the Science and Technology Planning Project of Guangdong Province (No. 2013CB040402009, 2015B010132009), and the Science and Technology Project of Guangzhou City (No. 2014J4100056). EXPERIMENTAL PROCEDURE



In this work, QD-LEDs with structures of glass/ITO/PEDOT:PSS/2CzPN:PVK/QDs/ZnO/Al were prepared and measured. Before the device fabrication, the ITO glass substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopropyl alcohol sequentially for 10 min. PEDOT:PSS aqueous solution by filtering with a 0.45µm filter was spincoated onto the precleaned and O2-plasma-treated ITO substrates at 2000 rpm for 60 s and then heated on the hotplate at 150 °C for 20 min to remove residual water. The PEDOT:PSS-coated ITO substrates were then moved into a nitrogen-filled glove box to deposit the subsequent multilayer films. 2CzPN and PVK solutions dissolved separately in chlorobenzene were then mixed in the different ratios to give the appropriate weight ratios. The blend solutions of 2CzPN and PVK were deposited on the top of PEDOT:PSS by spin-coating at 2000 rpm for 60 s as HTLs and exciton harvesters. The 2CzPN:PVK blend layers were then baked at 100 °C for 30 min. Red QDs (in octane, 10 mg/mL) and ZnO nanocrystals (in methanol, 30 mg/mL) were sequentially spin-coated on the top of 2CzPN:PVK blend film at 2000 rpm for 60 s as the emissive layer and ETL, respective. The QD and ZnO layers were heated at 100 °C for 20 min and 100 °C for 30 min, respectively, before the spin-coating next layer. Finally, the samples were transferred into the thermal evaporation system and Al cathodes (100 nm) were evaporated on the top of ZnO under a vacuum of 4×10-4 Pa. PEDOT:PSS, 2CzPN, and PVK were purchased from Heraeus, Xi'an Polymer Light Technology Corp., and Sigma Aldrich, respectively. Red CdSe/CdS/ZnS QDs (core/shell type) was purchased from Guangdong Poly Optoelectronics Co. Ltd. All materials were used as received. ZnO nanoparticles were prepared by low-temperature solution-precipitation method reported by Kwak et al.3 The testing of device properties was carried out in the nitrogen-filled glove box at room temperature. A Keithley 2400 voltage and current source meter and a Konica Minolta LS-150 14 Environment ACS Paragon Plus

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luminance meter were used for current-voltage and light output measurements, respectively. The electroluminescence (EL) spectra of the corresponding QD-LEDs were collected by an Ocean Optics fiber optical spectrometer (Maya 2000 Pro). UV-visible absorption spectra of 2CzPN:PVK, and QDs films were recorded on a HP8453A spectrophotometer. The photoluminescence (PL) spectra of 2CzPN:PVK, QDs, PVK/QDs and 2CzPN:PVK/QD films were measured by Horiba FluoroMax-4 Spectrofluorometer. Transient PL measurements were based on a lifetime spectrometer (FL920, Edinburgh Instrument) at room temperature and the samples were excited by a 340 nm pulsed diode laser. A function wavelength generator (RIGOL DG4162) was used to provide pulse voltages for the transient EL tests. The AFM images of the films were carried out with NT-MDT

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Efficient Quantum-Dot Light-Emitting Diodes Employing Thermally

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