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Two Host-Dopant Emitting Systems Realizing Four-Color Emission: A Simple and Effective Strategy for Highly Efficient WarmWhite OLED with High Color-Rendering Index at High Luminance Xuming Zhuang, Hao Zhang, Kaiqi Ye, Yu Liu, and Yue Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03374 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 25, 2016

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ACS Applied Materials & Interfaces

Two Host-Dopant Emitting Systems Realizing FourColor Emission: A Simple and Effective Strategy for Highly Efficient Warm-White OLED with High Color-Rendering Index at High Luminance Xuming Zhuang, Hao Zhang, Kaiqi Ye, Yu Liu* and Yue Wang State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun, 130012, P. R. China KEYWORDS. white organic light-emitting diode (WOLED), four-color, warm-white, high CRI, phosphorescent material

ABSTRACT. A four-color warm-white OLED employing simple adjacent two-EML structure as blue host-orange dopant/green host-red dopant has been fabricated, which exhibited stable high EL performance: the EQE of 23.3% and PE of 63.2 lm W–1 at an illumination-relevant luminance of 1000 cd m–2 with a high CRI of 92 and maintained the high levels of 21.6% and 48.8 lm W−1 with the CRI range of 93 at the extremely high luminance of 5000 cd m−2. To our knowledge, this should be the best result so far for a WOLED with CRI >90, simultaneously exhibiting very high efficiencies based on high-luminance level for the solid-state lighting.

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Currently, organic light-emitting diodes (OLEDs) with high electricity-light conversion efficiency have great potential for energy saving solid-state lighting (SSL), and many research efforts have been made for high-efficiency white OLEDs (WOLEDs) by employing all the electrically generated excitons, thereby achieving nearly 100% internal quantum eifficiency.1-4 In addition, a high color rendering index (CRI≥80) is another important parameter for white illumination source.5,6 In particular, very-high CRI is essential for lighting applications in house, museums, art galleries and other commercial places, and great developments in efficient WOLEDs with the CRI values of >90 have been achieved recently.7-9 In this regard, the use of three or more fluorescent and/or phosphorescent emitters has been a prerequisite for realizing a broad electroluminescent (EL) spectrum. Unfortunately, in addition to the high CRI achieved at a cost of low device efficiency accompanied by significant efficiency roll-off (90), simultaneously exhibiting very high efficiencies based on high-luminance level for the solid-state lighting.



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Figure 1. a) Molecular structures of Bepp2, FPPCA, BZQPG and BTIPG. b) UV-Vis absorption of BZQPG and BTIPG in neat thin film and PL spectra of neat films of Bepp2, FPPCA, BZQPG and BTIPG, and the films of BZQPG and BTIPG doped in Bepp2 and FPPCA respectively with 5 wt% doping concentration. The chemical structures of Bepp2, FPPCA, BZQPG and BTIPG and the absorption and photoluminescence (PL) spectra in neat and doped films are shown in Figures 1a and 1b. The spectral overlap between the emission peaks of Bepp2/FPPCA and the MLCT absorption band of BZQPG/BTIPG, together with their triplet energy alignment: 2.6, 2.4, 2.1 and 2.0 eV for T1 of Bepp2, FPPCA, BZQPG and BTIPG those estimated from the 0−0 band of their phosphorescence spectra respectively previous reports,13-15 suggested effective energy transfer from Bepp2 to


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BZQPG, and from FPPCA to BTIPG. This has been demonstrated in the PL spectra of Bepp2:BZQPG and FPPCA:BTIPG thin films with the concentration of 5 wt%, which show the emission only from BZQPG and BTIPG at λmax of ∼570 and 620 nm with the quantum yields of 0.68±0.03 and 0.55±0.02, respectively. To obtain high efficiency and high CRI WOLED with the optimal EL spectral curve, four emitting components have been arranged elaborately in two EMLs (B-O EML and G-R EML) according to their respective charge-transporting and luminous characters, and the energy diagram of the optimized WOLED (Device I: ITO/NPB (35 nm)/Bepp2:BZQPG (2.0 wt%, 5 nm)/FPPCA:BTIPG (0.8 wt%, 15 nm)/TPBi (30 nm)/LiF (0.5 nm)/Al) as shown in Figure 2a. Here, NPB and TPBi are the hole-transporting layer (HTL) and the electron-transporting layer (ETL), respectively. Theoretically, both singlet and triplet excitons are created with a ratio of 1:3 in either of the EMLs.22 In the G-R layer based on BTIPG doped in FPPCA, all the generated excitons can produce either green emission through direct radiative decay on the host FPPCA molecule, or red emission from the dopant BTIPG molecules through host-guest energy transfer. In the B-O EML consisting of Bepp2 doped BZQPG, the Bepp2 molecules not only generate blue singlet emission, but also sensitize the emission of BZQPG in this EML, leading to the increase in both blue and orange emission intensity. As we known, Bepp2 is an effective electrontransporting/hole-blocking material due to its high electron mobility of 1.3 × 10−3 cm2 V−1 s−1.12,21 Normally, this could prevent the recombination zone confined in this Bepp2-EML from the excitons generating in another EML, leading to higher blue and/or orange intensity in the whole EL spectrum that suffered from the low CRI. However, a rather thin Bepp2-based B-O EML (5 nm) adopted here would effectively induce more recombination taken place in G-R



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EML with the thicker thickness of 15 nm, which dramatically decreased the relative EL intensity from the B-O EML.

Figure 2. a) Proposed energy diagram of the materials used in OLEDs. b) EL spectra and the corresponding CRI values of Device I at different luminances (L). Inset: a photograph of Device I illuminating at 5000 cd m−2 with a CRI of 93. Figure 2b showed the white EL spectra and the corresponding CRIs of Device I at different luminance levels. These spectral curves covered all visible wavelengths from 400 to 750 nm due to the simultaneous fluorescent blue from Bepp2 (~450 nm), and phosphorescent green, orange 6 Environment ACS Paragon Plus

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and red from FPPCA (~510 nm) and BZQPG (~570 nm) and BTIPG (~620 nm) respectively. Clearly, they are primarily due to the red emitter BTIPG, with the moderate intensities of green FPPCA and orange BZQPG as well as the weakest blue Bepp2. This strongly supports the hypothesis above that the recombination zone mainly occurred in the G-R EML, where the more EL-efficient all-phosphor host-guest emitting system14 would facilitate the desirable CRI values as well as the very high EL efficiency. Furthermore, it can be seen that the relative contribution of the green and red emission slightly increased compared to the blue and orange emission as the driving voltages (equivalent to the luminance) gradually increased. This is due to the inherent hole- and exciton-blocking character of Bepp2-based EML, although its thickness has been controlled at a very low level as thin as 5 nm, which still had the ability to confine the certain holes within this layer. Thus, the increased driving voltage can actuate more holes flowing into the G-R EML through the B-O EML, leading to the more recombination of holes and electrons in G-R EML, where both the green and red emission increased gradually. Figure 3a show the efficiency-luminance (PE/EQE-L) characteristics of Device I, and the detailed data are summarized in Table 1. Device I exhibit rather excellent EL performance with turn-on voltage at 2.5 V, and the driving voltages for the luminances of 1000 and 5000 cd m−2 are 3.7 and 4.8 V respectively. The corresponding CIEx,y coordinates changed only marginally from (0.431, 0.449) at 1000 cd m−2 to (0.433, 0.458) at 5000 cd m−2, with the correlated color temperature (CCT) of 3230 K and 3290 K, corresponding to the warm-white color emission at these illumination-relevant luminance levels. At a luminance of 1000 cd m−2 this WOLED with the CRI of 92 has the EQE and PE values as high as 22.5% and 63.2 lm W−1, respectively. These efficiencies slightly rolled off to 21.6% and 48.8 lm W−1 at a critical luminance level of 5000 cd m−2 for solid-state lighting, based on a further enhanced CRI value of 93. As we expected, these



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EQEs and PEs represented the highest reported to date among WOLEDs exhibiting the decent CRI values (>90) in the scientific literature.7-9

Figure 3. a) Power efficiency (PE)-luminance (L)-external quantum efficiency (EQE) curves of Devices I, II, III and IV. b) EL spectra and the corresponding CIE coordinates of Devices I, II, III and IV at the luminance of 5000 cd m−2


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In addition, three exploratory devices with the same configuration as Device I were fabricated by using the single doping films of Bepp2:BZQPG (2.0 wt%, 20 nm) and FPPCA:BTIPG (0.8 wt%, 20 nm) and the double doping films of Bepp2:BZQPG (2.0 wt%, 10 nm)/FPPCA:BTIPG (0.8 wt%, 10 nm) as the EML for Device II, III and IV respectively (see Figure S1 in ESI file). The PE/EQE-L curves of Device II, III and IV and the EL spectral comparison of Device I-IV at the luminance of 5000 cd m−2 are shown in Figures 3a and 3b respectively. Clearly, the EMLs in Device II and III adopted the same doping concentration as the B-O and G-R layers respectively in Device I, thus they could generally reflect the EL processes occurred in Device I. In the EL spectra of Devices II and III, the emission peaks originating from both the host and dopant molecules are observed, due to suppression of energy transfer of singlet/triplet excitons at such low doping concentrations as 2.0 or 0.8 wt%. Since Bepp2 is an effective electrontransporting/hole-blocking material,12 the recombination zone in this Bepp2-based EML of Device II should be confined in the interface of NPB/Bepp2, which led to the decrease of carrier utilization and increase of the triplet-triplet annihilation (TTA) process evidently in the narrow exciton generation zone, especially under high current density. Indeed, the relatively low EL efficiency and obvious efficiency roll-off has been observed for this device. This double-color fluorescent/phosphorescent hybrid device just showed the moderate EL efficiency with the EQEs and PEs of 17.1, 14.0% and 31.8, 20.6 lm W−1 at 1000 and 5000 cd m−2 respectively, as shown in Figures 3a and Table 1. For Device III, The bipolar charge transport ability of the host FPPCA will effectively eliminate the accumulated carriers,14 resulting in balanced and sufficient charge fluxes within this EML, where the low-content of BTIPG (0.8 wt%) could not have the substantial effect on the inherent charge transport property of FPPCA. Thus, the resulting exciton recombination region broadened as wide as the entire FPPCA-based EML, together with the



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better similarity in their molecular structure and lots of optoelectric properties between two phosphors compared with the conventional doping cases based on fluorescent host and phosphorescent dopant, are indeed the advantages of the well-matched FPPCA:BTIPG combination, where all the electrically generated excitons could be utilized, leading to very high EL efficiencies at high luminance levels: 23.6% and 23.3% for EQE and 65.5 lm W−1 and 51.1 lm W−1 for PE at 1000 and 5000 cd m−2 respectively, also rendering this all-phosphorescent hostguest system remarkably efficient in nature. Table 1. Summary of EL performance for Devices I, II, III and IV. Device Va [V] ηpb [lm W−1] ηextb [%]

CIEx,y b/CRIb

I

2.5, 3.7, 4.8

63.2, 48.8

22.5, 21.6

(0.431, 0.449)/92, (0.433, 0.458)/93

II

2.7, 4.5, 5.6

31.8, 20.6

17.1, 14.0

(0.416, 0.422)/--, (0.404, 0.413)/--

III

2.3, 3.6, 4.6

65.5, 51.1

23.6, 23.3

(0.518, 0.447)/--, (0.493, 0.467)/--

IV

2.6, 4.0, 5.5

52.7, 37.8

20.5, 19.1

(0.348, 0.360)/76, (0.375, 0.382)/79

a

Applied voltage recorded at 1, 1000 and 5000 cd m−2, respectively. bValues at 1000 and 5000 cd m−2, respectively. Besides, it is apparent that as the relative thickness of B-O layer increased, Device IV showed different-quality white emission from Device I, where the EL intensities of blue (~450 nm) and orange (~570 nm) emission being assigned to Bepp2 and BZQPG respectively, were significantly enhanced, becoming the dominant emission peaks as shown in the Figures 3b. This should be because the host Bepp2 is a typical hole-blocking material, which could block a majority of holes at this B-O layer in Device IV due to its thicker thickness (10 nm) than that (5 nm) in Device I. While electrons can easily enter this EML and consequently recombine with the confined holes, resulting in that the charge carriers mainly recombined in the less EL-efficient B-O layer based on fluorescent/phosphorescent hybrid host-dopant emitting system,9 causing the lower EL efficiency and CRI of Device IV than those of Device I. Compared with Device I, the variation


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of the relative spectral intensity for four emission peaks in Device IV, corresponded to a considerable decease in the EL efficiencies: 20.5% and 19.1% for EQE, 52.7 lm W−1 and 37.8 lm W−1 for PE, and its rather low CRI values of 76 and 79 at 1000 and 5000 cd m−2 respectively (see Figures 3a and Table 1), indicated that it can not provide adequate quality for illumination, although it possessed rather pure-white CIEx,y coordinates. As we can see, the obviously difference in the EL spectra, colorimetric parameters as well as the efficiencies between Device I and IV, together with the systemic comparison for the EL characters of all the four devices above, illustrated that the color and EL performance of each white device (I and IV) can be easily and effectively tuned by adjusting the relatively thickness of the B-O and G-R EMLs, according to their respective emitting and charge-transporting characteristics. Actually, it is more importantly, the desirable EL performance of Device I demonstrated that the ROGB four-color white EL emission can be realized by utilizing a simple and novel two-layer emitting system, where four emitters had been employed as two host and two dopant materials, and radiated the light respectively based on their own emission color, through either the direct charge carrier recombination on the host emitters (Bepp2 and FPPCA), or the energy transfer to the dopant dye (BZQPG and BTIPG). This strategy avoided introducing any additional host molecule into the emitting system and the corresponding problems about the host choice and matching. Furthermore, this free recombination zone consisting of double EMLs without an interlayer in Device I, effectively reduced the undesirable charge accumulation as well as the resulting triplet-polaron and/or polaron-polaron quenching processes in the device.23 Thus, it is well understood that the surplus excitons could easily diffuse into the adjacent EML and sensitize the emitters with a lower energy, allowing all the excitons to decay radiatively. Of course, as mentioned above, the careful manipulation for the excitons generated in their specific



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location in both EMLs as we expected, is the essential factor for maximizing our WOLED overall quantum efficiency and simultaneously achieving the high-quality white light for lighting. Actually, this is also the optimal result obtained through the more detailed optimizing processes (shown in ESI file) for the emitting systems based on these four complexes. In summary, using two host-dopant emitting systems based on blue, green, orange and red dyes achieving a ROBG four-color white EL emission, which showed almost excellent performances including its efficiency and color characters, is indeed a novel and effective approach to realize the adequate white OLED for illumination sources. It is more important that, such simple adjacent two-EML structure as blue host-orange dopant/green host-red dopant adopted here, without any interlayer and specialized host introduced additionally, could effectively reduce the total number of constituent components and layers within the whole system, which enabled not only the simplified device fabrication process, but also the favorable charge-transporting and energy transfer processes, leading to very high EL efficiency through reducing exciton/charge accumulation as much as possible. These desirable EL performances of this WOLED are commonly believed to be the prerequisites for its use in future solid-state lighting.

ASSOCIATED CONTENT Supporting Information. Figure S1. Configurations for Devices I, II, III and IV; Figure S2. Current density-voltage-luminance (J-V-L) of Devices I, II, III and IV;


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Figure S3. PL spectra and PLQY values of the doping films of BZQPG doped in Bepp2 (2.0 wt%) and BTIPG doped in FPPCA (0.8 wt%); Figure S4. Configurations for two reference Devices V and VI; Figure S5. EL spectra and the corresponding CIE coordinates and CRI values of two reference Device V and VI at the luminance of 5000 cd m−2; Figure S6. Power efficiency (PE)-luminance (L)-external quantum efficiency (EQE) curves of two reference Devices V and VI; This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program, 2013CB834805), Natural Science Foundation of China (91333201, 21221063, 51373062, 51473028), the key scientific and technological project of Jilin province (20150204011GX).



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