Bipolar Blue Host Emitter with Unity Quantum Yield Allows Full-Exciton

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Organic Electronic Devices

Bipolar Blue Host Emitter with Unity Quantum Yield Allows Full-Exciton Radiation in Single-EmissiveLayer Hybrid White Organic Light-Emitting Diodes Chen Cao, Wen-Cheng Chen, Jia-Xiong Chen, Lei Yang, Xue-Zhi Wang, Hu Yang, Bin Huang, Ze-Lin Zhu, Qing-Xiao Tong, and Chun-Sing Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01105 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Bipolar Blue Host Emitter with Unity Quantum Yield Allows FullExciton Radiation in Single-Emissive-Layer Hybrid White Organic Light-Emitting Diodes Chen Cao,1,2 Wen-Cheng Chen,2,* Jia-Xiong Chen, 2 Lei Yang,1 Xue-Zhi Wang,1 Hu Yang,1 Bin Huang,2,3 Ze-Lin Zhu,2 Qing-Xiao Tong,1,* and Chun-Sing Lee2,* 1Department

of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, 243 University Road, Shantou, Guangdong, 515063, P.R. China E-mail: [email protected] 2Center of Super-Diamond and Advanced Films (COSDAF) and Department of Chemistry, City University of Hong Kong, Hong Kong S.A.R & City University of Hong Kong Shenzhen Research Institute, Shenzhen, Guangdong, P.R. China. E-mail: [email protected]; [email protected] 3College of Life Sciences and Chemistry, Jiangsu Key Laboratory of Biofunctional Molecule, Institute of New Materials for Vehicles, Jiangsu Second Normal University, Nanjing, 210013, P.R. China. KEYWORDS: bipolar blue fluorophore, unity quantum yield, electron trap engineering, hybrid white OLED, full-exciton radiation, ABSTRACT Phosphorescence/fluorescence hybrid white organic light-emitting diodes (OLEDs) are highly appealing for solid-state lighting. One major challenge is how to fully utilize the electrically generated excitons for light output. Herein, an efficient strategy to realize full exciton radiation is successfully revealed by judicious molecular design and suitable device engineering. A blue host emitter TP-PPI is designed and synthesized, exhibiting a near 100% photoluminescence quantum yield and a high triplet energy level enabling high-performance blue fluorescence and sensitization of yellow phosphorescent dopant. Full exciton radiation in hybrid white OLEDs is demonstrated with single emitting-layer formed by doping a yellow phosphor (PO-01) into TP-PPI. Near 100% exciton utilization and state-of-the-art EQE of 27.5% are achieved with the high efficiency blue-emitting host and an electron-trap engineered device architecture.

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INTRODUCTION The demonstration of high power efficiency (PE), homogenous large-area emission, good color rendering, environmental protection has brought white organic lightingemitting diodes (OLEDs) to the lighting market, which have drawn a great deal of attention in the scientific and industrial communities in recent years.1,2 Several approaches to produce white light with OLEDs have been proposed.3–5 Hybrid white OLEDs (WOLEDs) that integrate blue fluorescence and low-energy phosphorescence emitters have received increasing attention because they can fully harness all the excitons for radiation.6–8 Furthermore, hybrid WOLED can avoid the use of unstable blue phosphors, rendering it to be a promising candidate for next generation of lighting sources. To realize high-performance hybrid WOLEDs, singlet and triplet should be isolated and then respectively captured by fluorescence and phosphorescence emitters. Successful tactics to achieve full-exciton radiation have been demonstrated in hybrid WOLEDs with multi-9–11 and single-emissive layer12–14 (EML) configurations. Ma et al. proposed a mixed-host concept for blue-emitting layer in multi-EML hybrid WOLED.10 In this device structure, exciton quenching between fluorescence and phosphorescence emitters was limited by virtue of bipolar transporting blue EML, and thus the interlayer between fluorescence and phosphorescence region can be avoided. Recently, Fung and co-workers developed power-eﬃcient single-EML hybrid WOLEDs by employing a blue exciplex.15 Because of the bimolecular exciplex with barrier-free carrier injection ability, superior PE up to 105 lm W-1 and extremely low turn-on voltage (Von, 2.5 V, at 1 cd m-2) were achieved. To date, most high-performance hybrid WOLEDs require employment of multiple-EML and/or multicomponent hostguest systems. For low-cost large-scale application, to realize full exciton radiation in hybrid WOLEDs simple structure are still a major research task. Consider that a single-EML system contains only two components, for example a blue host emitter and a yellow phosphorescent dopant, in a hybrid WOLED. If the blue host emitter has a high triplet energy (ET) over that of the yellow phosphor but possesses 2


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a low photoluminescence (PL) quantum yield (PLQY), there is an energy loss pathway via S1 to S0 deactivation in blue host emitter (Scheme 1a), leading to an exciton utilization efficiency lower than 100%. This situation often happens in strong intramolecular charge transfer (ICT) emitters.16 On the other hand, a highly emissive blue host emitter with a low ET may suffer from downhill energy leakage, leading to incomplete energy transfer from blue host to yellow guest (Scheme 1b), and eventually the triplet excitons are unintendedly quenched by the nonradiative T1 of blue fluorophor. This situation often happens in highly emissive polycyclic aromatic hydrocarbon emitters, such as anthracene derivatives.17 100% internal quantum efficiency (IQE), i.e. full exciton radiation is most typically achieved by using blue host emitter that exhibits superior PLQY and high enough ET, simultaneously (Scheme 1c). However, such blue host emitters are still rare in literature.18 While hybrid WOLEDs using single-EML with only two components (1 dopant + 1 host) have been reported, the highest forwardviewing EQE reported so far is only around 20% without optical out-coupling enhancement.19 It is thus highly desirable to explore ways to further improve efficiency of such simple-structured hybrid WOLEDs.

Scheme 1. Schematic diagrams for exciton management in single-EML hybrid WOLED based on a) blue fluorophors with low PLQY and high ET; b) blue fluorophors with high PLQY and low ET; c) blue fluorophors with high PLQY and ET, and the yellow dopants harvest triplet exciton via energy transfer from blue fluorophors. Superscripts “H” and “G” denote blue host emitter and yellow phosphorescent guest, respectively. Grey dotted arrows indicate the unpreferred processes, while the black dotted arrows are the potential energy loss pathways.

In this study, we employ a novel strategy to realize high performance in simple 3



single-EML hybrid WOLEDs by an appropriate molecular design and device engineering. A bipolar blue fluorophor, named TP-PPI, comprising [1,2,4]triazolo[1,5a]pyridine20 (TP) electron acceptor coupled with phenanthro[9,10-d]imidazole21–23 (PI) donor, is designed and characterized (Figure 1a). TP-PPI demonstrates high PLQY approaching unity, bipolar charge transporting properties and high ET (~2.3 eV), showing ideal blue host emitter characteristics. TP-PPI can emit highly efficient blue electroluminescence (EL) in a nondoped device with a Von of 2.7 V and an external quantum efficiency (EQE) of 5.85% at 1000 cd m-2. Using TP-PPI as the host for a common

yellow

phosphor,

iridium(III)

bis(4-phenylthieno[3,2-c]pyridinato-

N,C2’)acetylacetonate (PO-01), a high-performance yellow phosphorescence OLED is also achieved. We then employ an electron-capture regulating strategy by using a deepLUMO (lowest unoccupied molecular orbital) electron transport layer (ETL) tris(2,4,6trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB) that can facilitate direct exciton capture on the yellow dopant and thus improve the exciton harvesting efficiency, which is more efficient than the energy transfer counterpart.24 The optimized WOLED exhibits excellent EL performance with a low Von of 2.6, forward-viewing maximum EQE and PE of 27.5% and 94.8 lm W-1, respectively. To the best of our knowledge, this is the first report on hybrid WOLEDs using a simple two-component (1 host + 1 dopant) EML showing forward-viewing EQE (PE) over 25% (90 lm W-1), which are also comparable to those of the state-of-the-art counterparts using much more complicated multi-component systems.15,19,25–29

4


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Figure 1. a) Chemical structure of TP-PPI. b) Frontier molecular orbital distribution. c) PL spectrum of neat thin film, fluorescence and phosphorescence spectra measured in 2-Me-THF at 77 K. d) Molecular stacking arrangement in single crystal.

RESULTS AND DISCUSSION Molecular Design and Characterization The new blue fluorescent host reported here has a D-π-A structure with an electronwithdrawing TP segment incorporated to an electron-donating PI via a biphenyl linker to achieve appropriate ICT30 (Figure 1a). TP moiety has been used as a mild electron acceptor in host materials for phosphorescence OLEDs.20 It is believed that the nitrogen-rich heterocyclic TP moiety would enhance intermolecular interaction to improve electrical properties of the material. On the other hand, the use of PI group is critical in the molecular design to boost fluorescence and device efficiency because of its rigid and planar skeleton.31 It has been found that the two distinct nitrogen atoms with electron-rich (N1) and electron-deficient (N3) endow the PI group with bipolar characteristics.23 Upon linking with a suitable electron acceptor, it would act as electron-donating moiety to realized ICT in excited state.32 The ICT feature is also 5



predicted by molecular calculation: highest occupied molecular orbital (HOMO) is dominated by the PI motif, while the LUMO tends to distribute on the TP moiety and extend to the biphenyl linker, as shown in Figure 2b. The moderate D-A pair and the πlinker can provide suitable electron cloud overlap in frontier molecular orbital, and thus result in a large oscillator strength (f = 1.025). This special electronic feature is conducive for obtaining bipolar charge transport and high PLQY. ICT feature of TPPPI is also confirmed by its solvent-dependent PL spectra (Figure S1). TP-PPI shows fine and structural PL bands in nonpolar hexane but broad gaussian PL profile in polar acetonitrile (ACN), indicating a sign of ICT in excited state. But the ICT extent is moderate, only 47-nm PL redshift is observed from hexane (395 nm) to ACN (442 nm). A PL spectrum of its thin film prepared on a fused silica substrate by thermal evaporation is displayed in Figure 1c, showing a deep-blue emission peaking at 455 nm. The thin film exhibits fast PL decay with two fluorescence lifetimes of 1.65 and 4.81 ns (Figure S2), which are attributed to local emissive and ICT decay, respectively. Note that the absolute PLQY is high up to 97.9%, and the corresponding radiative rate is beyond 108 s-1. The superior fluorescence efficiency of TP-PPI is also verified in the doped films with different concentrations (Figure S3). Such high PLQY of TP-PPI is very important for realizing high EL performance. TP-PPI possesses singlet energy level (ES) and ET of 3.08 and 2.35 eV respectively, estimated by the fluorescence and phosphorescence spectra in 4-methyltetrahydrofuran glass matrix at 77 K, which are high enough to act as the host for yellow or orange phosphors. TP-PPI is further characterized via single-crystal X-ray diffraction (XRD). The crystalline phase of TP-PPI belongs to the monoclinic system with P 21 space group (See Table S1). The molecules grow along [010] in single crystal, and pack in a herringbone arrangement (Figure 1d). As shown in Figure S4a, the TP and the PI moieties lie in the same plane, with 9.4° and 38.4° torsions to the biphenyl linker, respectively. The molecules adopt staggered face-to-face stacking hold by π interactions with some short contacts (Figure S4b), which could facilitate charge transport. On the other hand, the card-packed structure has an interplanar separation of 3.87 Å, which is slightly longer than typical π-π stacking (3.3-3.7 Å).33 This can help 6


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to preserve the pure blue emission of TP-PPI in solid state.34 TP-PPI possesses high decomposition (5% weight loss) and glass transition temperatures of 434 and 212 °C (Figure S5), respectively, which are high enough for thermal evaporation and against Joule heat during device operation. The HOMO level of TP-PPI is measured to be -5.37 eV by cyclic voltammetry (Figure S6), while its LUMO level is -2.36 eV estimated by subtracting of optical energy gap (3.01 eV, as determined from the absorption onset of thin film, Figure S7) from the HOMO energy level.

Monochrome OLEDs To study TP-PPI’s EL characteristic as a blue fluorescent emitter, a nondoped OLED (denoted as device B) is fabricated with a configuration of ITO/TAPC (30 nm)/TCTA (20 nm)/mCP (10 nm)/EML (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). Indium tin oxide (ITO) and Al are the anode and the cathode, respectively; 4,4’cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine]

(TAPC)

is

the

hole

injection layer; 4,4',4"-tris(carbazol-9-yl)triphenylamine (TCTA) and 1,3-bis(carbazol9-yl)benzene (mCP) act as hole-transporting and exciton-blocking layers, respectively; 1,3,5-tris(1-phenyl-1H-benzimidazol- 2-yl)benzene (TPBI) and LiF serve as the EML and electron injection layers, respectively. The current density-voltage-luminance (JV-L) characteristic is shown in Figure 2a and key device data are listed in Table 1. Device B has a low Von of 2.7 V and high PEmax of 5.66 lm W-1 (Figure 2b) by virtue of TP-PPI’s bipolar electronic properties. Bipolar charge transport is confirmed from the J-V characteristics of single-carrier devices (Figure S8). Device B emanates deep blue EL peaking at 440 nm with Commission Internationale de l’Éclairage (CIE) coordinates of (0.15, 0.11) (Figure 2a inset). The EQE of device B reaches 6.58% at maximum and slightly rolls off to 5.85% at a luminance of 1000 cd m-2, revealing one of the best performances in deep blue nondoped OLEDs.35–42 Performances of TP-PPI as a host for a typical yellow phosphorescence dopant, PO-01, is then investigated. Regarding to the energy level, TP-PPI can host PO-01 which has an ET of 2.2 eV, eliminating the probability of energy loss via nonradiative 7



T1 state of TP-PPI.43 Significant overlap between PO-01’s absorption and TP-PPI’s PL spectra suggests possible efficient energy transfer from TP-PPI to PO-01 (Figure S9). The device structure is ITO/TAPC (30 nm)/TCTA (5 nm)/mCP (5 nm)/TP-PPI: 3 wt% PO-01 (20 nm)/TPBI (50 nm)/LiF (1 nm)/Al (100 nm), denoted as device Y, and its key performance parameters are summarized in Table 1. J-V-L characteristic and efficiency curves against luminance are shown in Figure 2c and 2d, respectively. Device Y demonstrates yellow-orange EL emission peaking at 568 nm and impressive maximum CE and EQE of 80.6 cd A-1 and 26.1%, respectively. Its Von is as low as 2.6 V, resulting in a superior PEmax of 82.4 lm W-1. Performances of device B and device Y confirm that TP-PPI is a high performance bipolar blue fluorescent emitter and a suitable host for yellow phosphorescence dopant. These attributes suggest that TP-PPI can potentially be an ideal blue host emitter for realizing full exciton radiation in hybrid WOLEDs.

Figure 2. a) J-V-L characteristics (insert: the EL spectrum and a photograph of the operating blue device at 1000 cd m-2) and b) EQE-L-PE curves of the nondoped blue 8


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OLED based on TP-PPI. c) J-V-L characteristics (insert: the EL spectrum and a photograph of the operating yellow device at 1000 cd m-2) and d) EQE-L-PE curves of the PO-01 based yellow OLED using TP-PPI as host.

Table 1. EL performances of the fabricated OLEDs. Device

Votagea (V)

CIE coordinatesb

CEc (cd A-1)

PEc (lm W-1)

EQEc (%)

B

2.7, 3.8, 5.2

(0.15, 0.11)

6.30, 5.61

5.66, 3.42

6.58, 5.85

Y

2.6, 3.2, 5.0

(0.51, 0.48)

80.6, 78.1

82.4, 66.1

26.1, 25.3

W1

3.2, 5.0, 7.3

(0.45, 0.47)

79.1, 52.3

70.5, 33.5

25.5, 17.2

W2

2.6, 2.8, 3.3

(0.45, 0.46)

84.0, 75.4

94.8, 72.9

27.5, 24.1

Voltage at 1, 100, 1000 cd m , respectively. Measured at 1000 cd m . Performances recorded at maximum and 1000 cd m-2,

a

-2

b

-2 c

respectively.

White OLEDs As mentioned in the introduction, it is highly desirable to development high performance WOLED with simple device architecture. To achieve this, device engineering is also crucial. Full radiative exciton is only achieved in the situation that blue emitter harvests less than 25% exciton for light output.7 Considering that diffusion lengths of triplet excitons are much larger than those of singlets and host-to-guest energy transfer is generally efficient, the dopant concentration should be strictly controlled within a low level (< 1 wt%).12,44 Accordingly, TP-PPI based hybrid WOLEDs with the configuration of ITO/TAPC (30 nm)/TCTA (5 nm)/mCP (5 nm)/TPPPI: 0.4 wt% PO-01 (20 nm)/ETL (50 nm)/LiF (1 nm)/Al (100 nm) were fabricated as device W1 and W2, corresponding to the those using TPBI and 3TPYMB as ETL, respectively. Performances of W1 and W2 are shown in Figure 3 and Table 1. As shown in Figure 3a, W1 show a Von of 3.2 V. Device W1 exhibits decent efficiencies with forward-viewing EQE and CE up to 25.5% and 70.5 lm W-1 (Figure 3b). In comparison, W2 exhibits higher efficiencies after replacing the ETL to 3TPYMB. As shown in Figure 3b, W2 displays impressive EL performances, with forward-viewing EQEmax and PEmax of 27.5% and 94.8 lm W-1, respectively. Even at a high brightness of 1000 cd m-2, its EQE just slightly rolls off to 24.1%. These performances are corresponding to near 100% exciton utilization efficiency (assuming the out-coupling efficiency as 0.2 9



to 0.3).45,46 Both OLEDs emit warm-color white light, with CIE coordinates of (0.45, 0.47) and (0.45, 0.46) at 1000 cd m-2 for W1 and W2, respectively (Figure 3c). The EL spectra of the hybrid WOLEDs have more blue light components upon increasing driving voltages, demonstrating color indices of (0.40, 0.41) and (0.41, 0.42) at 10000 cd m-2 for W1 and W2, respectively. These spectral shifts should be ascribed to triplettriplet annihilation of the yellow phosphorescence at high exciton concentrations.7 The Performances of W2 are comparable to state-of-the-art hybrid WOLEDs without employing out-coupling enhancement. To the best of our knowledge, this performance is the highest among reported hybrid WOLEDs with single bicomponent EMLs.19,47–50

10


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Figure 3. a) J-V-L characteristics, b) EQE-L-PE curves and c) EL spectra (inset: a photography of operating W2 at 10000 cd m-2) of the W1 and W2.

Mechanism of Direct Electron Capture on Dopant Because TP-PPI is the main component (> 99 wt%) in the EML and it transports hole 11



better than electron (Figure S8), we reasonably consider that the recombination zone is near the EML/ETL interface. It can be seen from Figure 4a that 3TPYMB has a deeper LUMO (-3.3 eV), it is more easily for electrons to be injected directly into PO-01 instead of TP-PPI. On the other hand, for the device using TPBI as ETL, the electrons can be injected into both the TP-PPI matrix or the PO-01 dopant. Given that amount of PO-01 is small in the EML (

Bipolar Blue Host Emitter with Unity Quantum Yield Allows Full-Exciton

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