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Ning Sun, Qi Wang, Yongbiao Zhao, Yonghua Chen, Dezhi Yang, Fangchao Zhao, Jiangshan Chen,* and Dongge Ma* White organic light-emitting devices (WOLEDs) can be used as solid-state lighting sources, backlights for liquid crystal displays (LCDs), and full color OLEDs.[1] Recently, hybrid WOLEDs have drawn considerable attention due to their unique advantages of combining the blue fluorophores excellent stability and the high efficiency of long wavelength phosphors.[2,3] In general, hybrid WOLEDs can be constructed using two architectures, including multi-emissive-layers (multi-EML),[2] or a single-emissive-layer (single-EML),[3] with different color emitting dopants. The concentrations of the phosphors in the single-EML hybrid WOELDs need to be precisely controlled in order to realize the desired color since a small variation in the dopant concentration would result in a pronounced change in the energy transfer between the emitting dopants. Besides, the notorious spectra variations associated with different applied voltages represent another drawback.[3] Compared to the single-EML counterpart, the multi-EML structure allows flexible manipulation of each EML as well as precise control of the exciton distributions or charges in different EMLs. Despite the complexity of device fabrication, better EL performance can be achieved. Thus, the multi-EML structure provides a reliable strategy to fabricate hybrid WOLEDs. Research work relying on such multi-EML structures has been reported by a number of groups.[2] Unfortunately, their Dr. N. Sun, Dr. D. Z. Yang, Dr. F. C. Zhao, Prof. J. S. Chen, Prof. D. G. Ma State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry University of Chinese Academy of Sciences Changchun, 130022, People’s Republic of China E-mail: [email protected]; [email protected] Prof. Q. Wang Department of Chemistry Faculty of Science Xi’an Jiao Tong University Xi’an, 710049, People’s Republic of China Dr. Y. B. Zhao Luminous! Center of Excellence for Semiconductor Lighting and Displays School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore Dr. Y. H. Chen Center of Advanced Science and Engineering for Carbon (Case4Carbon) Department of Macromolecular Science and Engineering Case School of Engineering Case Western Reserve University 10900, Euclid Avenue, Cleveland, Ohio, 44106, USA
DOI: 10.1002/adma.201304779
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High-Performance Hybrid White Organic Light-Emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions
exhibited external quantum efficiencies (EQEs) at a practical brightness of 1000 cd m−2 are much lower than the theoretically maximum EQE (EQEmax) of 20%, and the corresponding power efficiencies are not yet high enough for practical applications. One common structural element for these devices is an interlayer between the fluorophore and phosphor to prevent mutual exciton transfer and quenching processes.[2] It seems that this interlayer has become indispensable for multi-EML hybrid WOLEDs. Nevertheless, the use of the interlayer has several disadvantages that limit the device quantum efficiency and power efficiency. Firstly, the voltage drop across the interlayer is not negligible, leading to lower power efficiency (PE). Moreover, the addition of an interlayer brings additional interfaces which inevitably increase the possibility of exciplex formation which, to some extent, impairs the quantum efficiency of hybrid OLEDs.[2a] Finally, the additional fabrication step for an interlayer is also unfavorable for commercial applications. Thus, if mutual quenching in a non-interlayer structure can be controlled well, the efficiency would be further improved. In this study, we propose a novel architecture for mutil-EML hybrid WOLEDs that does not follow the conventional principle with an interlayer between the fluorophore and phosphors. The core of our concept is to introduce mixed-hosts with bipolar transport to construct the fluorescent blue emission layer (EML). The importance of constructing the hybrid WOLEDs was the selection of guest emitters and its host. Here, we used N,N′-di-1naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1′″-quaterphenyl]-4,4′″diamine (4P-NPD) as the fluorescent blue emitter due to its high fluorescence quantum yield, that is, 92%.[4] The commonly used bis(2-phenylpyridine)iridium acetylacetonate (Ir(ppy)2(acac)) and [iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)) were selected as the phosphorescent green and red dopants, respectively. As shown in Figure 1, these three primary-color emitters were arranged with a R–G–B sequence from the anode to the cathode. In order to reduce the structural heterogeneity and facilitate charge transport between the two adjacent emitting layers, 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) served as a common host for all lumophores. In addition, an electron transport material 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPb) was introduced in the blue EML to combine with TCTA as mixed hosts for 4P-NPD. A pure TmPyPb layer was also used as the electron transport layer (ETL), and a 4% cesium carbonate (Cs2CO3) doped TmPyPb acted as the n-ETL to achieve a low driving voltage. Finally, a Cs2CO3/Al was used as a bilayer cathode. Meanwhile, PEDOT:PSS provided an
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intermediate energy level to facilitate hole injection from the indium tin oxide (ITO) anode to the hole transport layer (HTL), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB). (see Figure 1) As shown in Figure 2, the optimized device exhibits very impressive EL performance with a turn-on voltage of 3.1 V. The maximum EQE, current efficiency (CE), and PE of the device are 19.0%, 45.2 cd A−1, and 41.7 lm W−1, respectively. Furthermore, at the practical brightness of 1000 cd m−2, they remain as high as 17.0%, 40.5 cd A−1, and 34.3 lm W−1, exhibiting less pronounced efficiency roll-off. The critical current density jc, where EQE declines by half from its peak,[5] is about 105 mA cm−2, which is comparable to or even higher than some of the high-performance single- or multi-EML WOLEDs demonstrated before.[2a,6] All the results were achieved in the forward direction without the use of any outcoupling enhancement techniques. Unlike common hybrid WOLEDs with their brightness-dependent color shifts, the EL spectrum of our device turns out to be rather stable within the investigated brightness range, indicating balanced exciton generation. Furthermore, the device can also give off white light with a high CRI of 82. To our knowledge, this is one of the best results obtained considering both efficiency and color-rendition issues. The motivation for conceiving this non-interlayer strategy is to enhance device efficiency. However, the critical problem in the hybrid WOLEDs without the interlayer is the mutual quenching between fluorescent blue and phosphorescent green emitters when they directly contact each other. On the one hand, the relatively lower triplet energy of 4P-NPD (2.3 eV) compared to that of Ir(ppy)2(acac) (2.4 eV) leads to a possible Dexter energy transfer from the triplet state of Ir(ppy)2(acac) to the lower lying non-radiative triplet state of 4P-NPD,[4,7] resulting in energy loss and thus a reduction in device efficiency. On the other hand, the singlet excitons created in the blue EML can either undergo a direct radiative decay to produce blue emission or be transferred via Förster transfer to
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Figure 1. Energy level scheme for materials used in the hybrid WOLED, and exciton (S0, S1, and T1) energy diagram of the emitter layers. The gray filled rectangle represents the main exciton generation zone. R, G, B, and Tm represents Ir(MDQ)2(acac), Ir(ppy)2(acac), 4P-NPD, and TmPyPb, respectively. Solid lines and dashed lines correspond to HOMO and LUMO energy levels, respectively; circles and diamonds refer to the exciton (S0, S1, and T1) energies, respectively.
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the TCTA: Ir(ppy)2(acac) layer to enhance the green emission. Although this Förster transfer is not a loss mechanism for light generation, it would lead to the insufficient blue emission which is required for balanced white light if this process is dominant. The negative influence of Dexter type transfer that favors the formation of the non-luminescent triplet excited state of 4P-NPD can be minimized by reducing the concentration of 4P-NPD. But the presence of Ir(ppy)2(acac) perturbing the utilization of the singlet excitons for blue emission have a negative effect on the white light attained when the concentration of 4P-NPD is low. In order to resolve the issues mentioned above, we employed TCTA and TmPyPb as mixed-hosts for 4P-NPD allowing for bipolar charge transporting ability in the blue EML, which is the key feature of this novel structure. To prove the positive effect of the bipolar transport property in the blue EML, we fabricated two other devices (W1 and W2) for comparison. Apart from the variation of the blue EML components, all other parameters of both devices were kept the same as the optimized structure. The blue EMLs for W1 and W2 are TCTA:2% 4P-NPD (7 nm) and TmPyPb:2% 4P-NPD (7 nm), respectively (see Figure 3). According to the transport properties
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can be achieved at a low concentration of 4P-NPD. Assuming equal density in the cubic closed packing and 1 nm molecule size for both host and guest molecule, one can calculate the average distance between 4P-NPD molecules to be 3.7 nm at a 2 mol% doping level,[2a] which lies beyond the Förster transfer radius (3 nm),[9] resulting in an energetically unfavorable singlet excited state transition between guest molecules. The singlet excitons are inherently localized on 4P-NPD molecules and thus spatially confined from each other. Instead of diffusing to the interface and then quenched by Ir(ppy)2(acac), the majority of singlet excitons generated in the blue EML are mainly used for the blue emission. Assuming an optical outcoupling efficiency of roughly 20%, the EQEmax of 19% corresponds to an almost unity internal quantum efficiency, which indicates that the number of triplet excitons lost to nonradiative decay on 4P-NPD is negligible. On the one hand, the observed delayed green emission from device S1 (in contrast to that of device S2) proves the existence of endothermic energy transfer from the triplet state of 4P-NPD to that of Ir(ppy)2(acac) (Supporting Information, Figure S1). Thereby, the energy loss on the nonradiative triplet state of 4P-NPD can partially survive via the endothermic energy transfer, which would improve the efficiency of the device. On the other hand, due to the low doping concentration of 4P-NPD, the generated triplet excitons in the mixed-hosts cannot be efficiently transferred to the fluorophore by Dexter mechanism which typically happens within a short interaction distance (≈1 nm).[10] Hence, it can be assumed that the majority of triplet excitons generated in the blue EML reside in the mixed-hosts. Considering the fact that the triplet energy level of TmPyPb (2.78 eV) is lower than that of TCTA (2.83 eV),[11,12] the triplet excitons can diffuse into nearby green emitters by successive Dexter transfer between adjacent TmPyPb molecules. In order to prove this hypothesis, device A and device B are discussed (see Figure 4). A vivid
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of TCTA and TmPyPb, the main exciton generation zone of the optimized device with the mixed-hosts should be located across the whole blue EML where the majority of injected holes and electrons meet with each other. Conversely, W1 and W2 would have a narrow exciton generation zone since there is an abrupt interface between the blue EML and adjacent layer. As shown in Figure 3, the strong blue emission in W1 suggests that the main recombination zone is concentrated near the blue EML and ETL interface, which is far away from the Ir(ppy)2(acac)-containing layer. Meanwhile, the extremely low efficiency at high current density inhibits the feasibility of this configuration. Alternatively, the almost negligible blue emission for W2 is mainly because the recombination zone is located at the interface of blue EML and green EML and the singlet excitons in the blue EML are largely quenched by Ir(ppy)2(acac), which results in a poor white emission. Furthermore, it can be seen that both spectra for W1 and W2 also show more brightness-dependent color shifts than that of the optimized device. This can be attributed to the quenching effect of space charge accumulation and high density of triplet excitons in the narrow exciton generation zone.[3a,4,5,8] From the EL spectrum just described (see Figure 2), it is clear that the sufficient blue emission required for white light
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www.MaterialsViews.com Table 1. Summary of EL Performance (Forward-Viewing) of the Device S3 EQEa) [%] 16.5, 15.7
CEa) [cd A−1] 37.2, 33.8
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green emission from Ir(ppy)2(acac) in device B and a barely green emission in device A provides a strong experimental evidence for the above assumption. Consequently, the effective TmPyPb triplet excitons sequentially harvested by the green and red emitters form a cascade energy transfer mechanism. Except for this emission mechanism, the direct exciton formation following charge trapping on guest sites also contributes to the green and red emission, owing to the lower LUMO levels of Ir(MDQ)2(acac) (2.8 eV) and higher HOMO levels of Ir(ppy)2(acac) (5.6 eV) than those of the host material (2.3 eV and 5.7 eV).[3b,13,14] The recombination zone extending into the red and green EMLs not only decreases the density of triplet excitons in the main exciton generation zone, but also reduces the charge carrier accumulation in that region, suppressing both triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) which have been reported to be responsible for efficiency roll-off at high brightness.[15] Additionally, the bipolar transport property in the blue EML that helps broaden the exciton formation region is another reason for the low efficiency roll-off.[16] To demonstrate the universality of this novel design concept, we replace 4P-NPD with another fluorescent blue emitter 4,4′-bis[2-{4-( N , N -diphenylamino)phenyl}vinyl]biphenyl (DPAVBi) with an extremly low triplet energy level (
Ning Sun, Qi Wang, Yongbiao Zhao, Yonghua Chen, Dezhi Yang, Fangchao Zhao, Jiangshan Chen,* and Dongge Ma* White organic light-emitting devices (WOLEDs) can be used as solid-state lighting sources, backlights for liquid crystal displays (LCDs), and full color OLEDs.[1] Recently, hybrid WOLEDs have drawn considerable attention due to their unique advantages of combining the blue fluorophores excellent stability and the high efficiency of long wavelength phosphors.[2,3] In general, hybrid WOLEDs can be constructed using two architectures, including multi-emissive-layers (multi-EML),[2] or a single-emissive-layer (single-EML),[3] with different color emitting dopants. The concentrations of the phosphors in the single-EML hybrid WOELDs need to be precisely controlled in order to realize the desired color since a small variation in the dopant concentration would result in a pronounced change in the energy transfer between the emitting dopants. Besides, the notorious spectra variations associated with different applied voltages represent another drawback.[3] Compared to the single-EML counterpart, the multi-EML structure allows flexible manipulation of each EML as well as precise control of the exciton distributions or charges in different EMLs. Despite the complexity of device fabrication, better EL performance can be achieved. Thus, the multi-EML structure provides a reliable strategy to fabricate hybrid WOLEDs. Research work relying on such multi-EML structures has been reported by a number of groups.[2] Unfortunately, their Dr. N. Sun, Dr. D. Z. Yang, Dr. F. C. Zhao, Prof. J. S. Chen, Prof. D. G. Ma State Key Laboratory of Polymer Physics and Chemistry Changchun Institute of Applied Chemistry University of Chinese Academy of Sciences Changchun, 130022, People’s Republic of China E-mail: [email protected]; [email protected] Prof. Q. Wang Department of Chemistry Faculty of Science Xi’an Jiao Tong University Xi’an, 710049, People’s Republic of China Dr. Y. B. Zhao Luminous! Center of Excellence for Semiconductor Lighting and Displays School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore Dr. Y. H. Chen Center of Advanced Science and Engineering for Carbon (Case4Carbon) Department of Macromolecular Science and Engineering Case School of Engineering Case Western Reserve University 10900, Euclid Avenue, Cleveland, Ohio, 44106, USA
DOI: 10.1002/adma.201304779
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High-Performance Hybrid White Organic Light-Emitting Devices without Interlayer between Fluorescent and Phosphorescent Emissive Regions
exhibited external quantum efficiencies (EQEs) at a practical brightness of 1000 cd m−2 are much lower than the theoretically maximum EQE (EQEmax) of 20%, and the corresponding power efficiencies are not yet high enough for practical applications. One common structural element for these devices is an interlayer between the fluorophore and phosphor to prevent mutual exciton transfer and quenching processes.[2] It seems that this interlayer has become indispensable for multi-EML hybrid WOLEDs. Nevertheless, the use of the interlayer has several disadvantages that limit the device quantum efficiency and power efficiency. Firstly, the voltage drop across the interlayer is not negligible, leading to lower power efficiency (PE). Moreover, the addition of an interlayer brings additional interfaces which inevitably increase the possibility of exciplex formation which, to some extent, impairs the quantum efficiency of hybrid OLEDs.[2a] Finally, the additional fabrication step for an interlayer is also unfavorable for commercial applications. Thus, if mutual quenching in a non-interlayer structure can be controlled well, the efficiency would be further improved. In this study, we propose a novel architecture for mutil-EML hybrid WOLEDs that does not follow the conventional principle with an interlayer between the fluorophore and phosphors. The core of our concept is to introduce mixed-hosts with bipolar transport to construct the fluorescent blue emission layer (EML). The importance of constructing the hybrid WOLEDs was the selection of guest emitters and its host. Here, we used N,N′-di-1naphthalenyl-N,N′-diphenyl-[1,1′:4′,1″:4″,1′″-quaterphenyl]-4,4′″diamine (4P-NPD) as the fluorescent blue emitter due to its high fluorescence quantum yield, that is, 92%.[4] The commonly used bis(2-phenylpyridine)iridium acetylacetonate (Ir(ppy)2(acac)) and [iridium(III)bis(2-methyldibenzo-[f,h]quinoxaline)(acetylacetonate) (Ir(MDQ)2(acac)) were selected as the phosphorescent green and red dopants, respectively. As shown in Figure 1, these three primary-color emitters were arranged with a R–G–B sequence from the anode to the cathode. In order to reduce the structural heterogeneity and facilitate charge transport between the two adjacent emitting layers, 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) served as a common host for all lumophores. In addition, an electron transport material 1,3,5-tri(m-pyrid-3-ylphenyl)benzene (TmPyPb) was introduced in the blue EML to combine with TCTA as mixed hosts for 4P-NPD. A pure TmPyPb layer was also used as the electron transport layer (ETL), and a 4% cesium carbonate (Cs2CO3) doped TmPyPb acted as the n-ETL to achieve a low driving voltage. Finally, a Cs2CO3/Al was used as a bilayer cathode. Meanwhile, PEDOT:PSS provided an
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intermediate energy level to facilitate hole injection from the indium tin oxide (ITO) anode to the hole transport layer (HTL), N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB). (see Figure 1) As shown in Figure 2, the optimized device exhibits very impressive EL performance with a turn-on voltage of 3.1 V. The maximum EQE, current efficiency (CE), and PE of the device are 19.0%, 45.2 cd A−1, and 41.7 lm W−1, respectively. Furthermore, at the practical brightness of 1000 cd m−2, they remain as high as 17.0%, 40.5 cd A−1, and 34.3 lm W−1, exhibiting less pronounced efficiency roll-off. The critical current density jc, where EQE declines by half from its peak,[5] is about 105 mA cm−2, which is comparable to or even higher than some of the high-performance single- or multi-EML WOLEDs demonstrated before.[2a,6] All the results were achieved in the forward direction without the use of any outcoupling enhancement techniques. Unlike common hybrid WOLEDs with their brightness-dependent color shifts, the EL spectrum of our device turns out to be rather stable within the investigated brightness range, indicating balanced exciton generation. Furthermore, the device can also give off white light with a high CRI of 82. To our knowledge, this is one of the best results obtained considering both efficiency and color-rendition issues. The motivation for conceiving this non-interlayer strategy is to enhance device efficiency. However, the critical problem in the hybrid WOLEDs without the interlayer is the mutual quenching between fluorescent blue and phosphorescent green emitters when they directly contact each other. On the one hand, the relatively lower triplet energy of 4P-NPD (2.3 eV) compared to that of Ir(ppy)2(acac) (2.4 eV) leads to a possible Dexter energy transfer from the triplet state of Ir(ppy)2(acac) to the lower lying non-radiative triplet state of 4P-NPD,[4,7] resulting in energy loss and thus a reduction in device efficiency. On the other hand, the singlet excitons created in the blue EML can either undergo a direct radiative decay to produce blue emission or be transferred via Förster transfer to
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the TCTA: Ir(ppy)2(acac) layer to enhance the green emission. Although this Förster transfer is not a loss mechanism for light generation, it would lead to the insufficient blue emission which is required for balanced white light if this process is dominant. The negative influence of Dexter type transfer that favors the formation of the non-luminescent triplet excited state of 4P-NPD can be minimized by reducing the concentration of 4P-NPD. But the presence of Ir(ppy)2(acac) perturbing the utilization of the singlet excitons for blue emission have a negative effect on the white light attained when the concentration of 4P-NPD is low. In order to resolve the issues mentioned above, we employed TCTA and TmPyPb as mixed-hosts for 4P-NPD allowing for bipolar charge transporting ability in the blue EML, which is the key feature of this novel structure. To prove the positive effect of the bipolar transport property in the blue EML, we fabricated two other devices (W1 and W2) for comparison. Apart from the variation of the blue EML components, all other parameters of both devices were kept the same as the optimized structure. The blue EMLs for W1 and W2 are TCTA:2% 4P-NPD (7 nm) and TmPyPb:2% 4P-NPD (7 nm), respectively (see Figure 3). According to the transport properties
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Figure 3. External quantum efficiency and power efficiency versus luminance characteristics of W1 and W2. The inset shows the corresponding normalized EL spectra.
can be achieved at a low concentration of 4P-NPD. Assuming equal density in the cubic closed packing and 1 nm molecule size for both host and guest molecule, one can calculate the average distance between 4P-NPD molecules to be 3.7 nm at a 2 mol% doping level,[2a] which lies beyond the Förster transfer radius (3 nm),[9] resulting in an energetically unfavorable singlet excited state transition between guest molecules. The singlet excitons are inherently localized on 4P-NPD molecules and thus spatially confined from each other. Instead of diffusing to the interface and then quenched by Ir(ppy)2(acac), the majority of singlet excitons generated in the blue EML are mainly used for the blue emission. Assuming an optical outcoupling efficiency of roughly 20%, the EQEmax of 19% corresponds to an almost unity internal quantum efficiency, which indicates that the number of triplet excitons lost to nonradiative decay on 4P-NPD is negligible. On the one hand, the observed delayed green emission from device S1 (in contrast to that of device S2) proves the existence of endothermic energy transfer from the triplet state of 4P-NPD to that of Ir(ppy)2(acac) (Supporting Information, Figure S1). Thereby, the energy loss on the nonradiative triplet state of 4P-NPD can partially survive via the endothermic energy transfer, which would improve the efficiency of the device. On the other hand, due to the low doping concentration of 4P-NPD, the generated triplet excitons in the mixed-hosts cannot be efficiently transferred to the fluorophore by Dexter mechanism which typically happens within a short interaction distance (≈1 nm).[10] Hence, it can be assumed that the majority of triplet excitons generated in the blue EML reside in the mixed-hosts. Considering the fact that the triplet energy level of TmPyPb (2.78 eV) is lower than that of TCTA (2.83 eV),[11,12] the triplet excitons can diffuse into nearby green emitters by successive Dexter transfer between adjacent TmPyPb molecules. In order to prove this hypothesis, device A and device B are discussed (see Figure 4). A vivid
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device A Normalized intensity (a.u.)
of TCTA and TmPyPb, the main exciton generation zone of the optimized device with the mixed-hosts should be located across the whole blue EML where the majority of injected holes and electrons meet with each other. Conversely, W1 and W2 would have a narrow exciton generation zone since there is an abrupt interface between the blue EML and adjacent layer. As shown in Figure 3, the strong blue emission in W1 suggests that the main recombination zone is concentrated near the blue EML and ETL interface, which is far away from the Ir(ppy)2(acac)-containing layer. Meanwhile, the extremely low efficiency at high current density inhibits the feasibility of this configuration. Alternatively, the almost negligible blue emission for W2 is mainly because the recombination zone is located at the interface of blue EML and green EML and the singlet excitons in the blue EML are largely quenched by Ir(ppy)2(acac), which results in a poor white emission. Furthermore, it can be seen that both spectra for W1 and W2 also show more brightness-dependent color shifts than that of the optimized device. This can be attributed to the quenching effect of space charge accumulation and high density of triplet excitons in the narrow exciton generation zone.[3a,4,5,8] From the EL spectrum just described (see Figure 2), it is clear that the sufficient blue emission required for white light
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Wavelength (nm) Figure 4. The normalized EL spectra for both devices, device A with structure of ITO/PEDOT/NPB/TCTA (10 nm)/Ir(ppy)2(acac) (0.06 nm)/TCTA (10 nm)/TCTA:TmPyPb:4P-NPD = 73%:25%:2% (7 nm)/TmPyPb (20 nm)/nEML/Cs2CO3/Al and device B with structure of ITO/PEDOT/NPB/TCTA (20 nm)/TCTA:TmPyPb:4P-NPD = 73%:25%:2% (7 nm)/TmPyPb (10 nm)/ Ir(ppy)2(acac) (0.06 nm)/TmPyPb (10 nm)/n-EML/Cs2CO3/Al
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www.MaterialsViews.com Table 1. Summary of EL Performance (Forward-Viewing) of the Device S3 EQEa) [%] 16.5, 15.7
CEa) [cd A−1] 37.2, 33.8
PEa) [lm W−1] 40.3, 27.0
a)Order
CIE [x, y]b) (0.43, 0.43)
CRIb) 87
of measured value: maximum, then values at 1000 cd 1000 cd m−2.
J0 [mA cm−2] 161
m−2; b)Measured
at
green emission from Ir(ppy)2(acac) in device B and a barely green emission in device A provides a strong experimental evidence for the above assumption. Consequently, the effective TmPyPb triplet excitons sequentially harvested by the green and red emitters form a cascade energy transfer mechanism. Except for this emission mechanism, the direct exciton formation following charge trapping on guest sites also contributes to the green and red emission, owing to the lower LUMO levels of Ir(MDQ)2(acac) (2.8 eV) and higher HOMO levels of Ir(ppy)2(acac) (5.6 eV) than those of the host material (2.3 eV and 5.7 eV).[3b,13,14] The recombination zone extending into the red and green EMLs not only decreases the density of triplet excitons in the main exciton generation zone, but also reduces the charge carrier accumulation in that region, suppressing both triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) which have been reported to be responsible for efficiency roll-off at high brightness.[15] Additionally, the bipolar transport property in the blue EML that helps broaden the exciton formation region is another reason for the low efficiency roll-off.[16] To demonstrate the universality of this novel design concept, we replace 4P-NPD with another fluorescent blue emitter 4,4′-bis[2-{4-( N , N -diphenylamino)phenyl}vinyl]biphenyl (DPAVBi) with an extremly low triplet energy level (
