Host-free Yellow-Green Organic Light-Emitting Diodes with External

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Host-free Yellow-Green Organic Light-Emitting Diodes with External Quantum Efficiency over 20% based on a Compound Exhibiting Thermally Activated Delayed Fluorescence Xiaoqing Zhang, Matthew W. Cooper, Yadong Zhang, Canek FuentesHernandez, Stephen Barlow, Seth R. Marder, and Bernard Kippelen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18798 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Host-free Yellow-Green Organic Light-Emitting Diodes with External Quantum Efficiency over 20% based on a Compound Exhibiting Thermally Activated Delayed Fluorescence Xiaoqing Zhang 1, Matthew W. Cooper 2, Yadong Zhang 2, Canek Fuentes-Hernandez 1, Stephen Barlow 2, Seth R. Marder 2, and Bernard Kippelen 1,* 1

Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer

Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA 2

Center for Organic Photonics and Electronics (COPE), School of Chemistry and Biochemistry,

Georgia Institute of Technology, Atlanta, Georgia 30332, USA KEYWORDS : organic light-emitting diode, thermally activated delayed fluorescence, guesthost systems, host-free emissive layers, concentration quenching.

ABSTRACT: Thermally activated delayed fluorescent (TADF) materials are advantageous as emitters in organic light-emitting diodes (OLEDs) due to their ability to utilize all excited states formed by charge recombination for light emission, potentially leading to 100% internal quantum efficiency. As in conventional fluorescent or phosphorescent OLEDs, TADF emitters

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are commonly doped at a relatively low concentration in a host matrix. However, increasing evidence suggest that balanced ambipolar transport properties and small aggregation-induced fluorescence quenching allows TADF emitters to be used alone in so-called host-free OLEDs. Here, we report on host-free OLEDs in which the EMLs consist solely of a yellow-greenemitting

TADF

compound,

5,5'-(2,3,5,6-tetra(carbazol-9-yl)-1,4-phenylene)bis(2-(4-(tert-

butyl)phenyl)-1,3,4-oxadiazole), TCZPBOX. Devices with this host-free EML yield a maximum external quantum efficiency (EQE) of 21%, current efficacy (CE) of 73 cd/A, and power efficacy (PE) of 79 lm/W at a luminance of 10 cd/m2. At a high luminance of 10,000 cd/m2, a high EQE of 13% is maintained. A maximum luminance of 120,000 cd/m2 is reached at an applied voltage of 9.8 V. When TCZPBOX was doped in the host of 2,6-di(carbazol-9-yl)-pyridine (PYD2) at 40 wt. %, the device yielded a maximum EQE of 28%, CE of 94 cd/A, and PE of 100 lm/W at 10 cd/m2.

INTRODUCTION Achieving high external quantum efficiency (EQE) in organic light-emitting diodes (OLEDs) is crucial for their use in displays and in solid-state lighting. However, charge recombination results in ca. 75% of triplet excitons, which are non-emissive in emissive layers utilizing conventional fluorescent emitters. Utilizing phosphorescent emitters incorporating heavy metals such as iridium or platinum allows these triplets to contribute to light emission through phosphorescence due to intersystem crossing enabled by strong spin-orbit coupling to achieve internal quantum efficiency (IQE) values of nearly 100%.1-4 An alternative to using phosphorescent emitters is based on using emitters displaying delayed fluorescence, in particular E-type delayed fluorescence, also called thermally activated delayed fluorescence (TADF), a process by which long-lived non-emissive triplets are thermally excited to emissive singlet


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states. In this approach, the use of heavy metals is not required; all-organic compounds can be designed to exhibit efficient TADF. One important parameter in the design of TADF emitters is the singlet/triplet energy separation, ΔEST, which approximates the minimum barrier that must be overcome to convert triplet excitons to singlet excitons through reverse intersystem crossing (RISC) at room temperature. Fast RISC requires not only a small energy barrier, but also high spin-orbit coupling which can be realized through careful design of the nature of the relevant excited states.5 In recent years, emitters with ΔEST approaching 0 eV have been synthesized and shown to yield highly efficient TADF-based OLEDs with EQE up to 30% in blue-, 6 green-,7 yellow- and red-emitting devices.8,9 This level of performance is comparable to that achieved in devices with phosphorescent compounds. The common approach used to realize emissive layer (EML) having emitters displaying TADF, is to dope them in a host matrix. In recent years, an increasing number of studies have found that the efficiency of OLEDs containing TADF emitters is optimum at emitter concentrations that are much larger than those typically used in OLEDs having fluorescent and phosphorescent emitters.10 At large emitter concentrations, the distinction between host and guest roles is blurred, as electron and holes can be transported and radiatively recombine in the emitter domains. Hence, it is apparent that the donor-acceptor (D-A) building blocks in TADF emitters can also lead to materials displaying balanced ambipolar transport. Furthermore, a few recent reports suggest that TADF emitters can be used as a single material in the EML and yet yield devices with high efficiency. For instance, an EQE value of 19.5% was achieved at 100 cd/m2 in a host-free blue-emitting device,11 and a maximum EQE of 20% was achieved at 20 cd/m2 in a green-emitting device.12 It is worth noting that some emitters showing aggregationinduced delayed fluorescence were used as a single material in the EML of a device and showed


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high EQE and small EQE roll-offs.13-15 For example, the most efficient device reported was a yellow-green-emitting device using a carbazole- and phenothiazine-substituted ketone, CP-BPPXZ, as the EML, which achieved a maximum EQE of 18.4%, a maximum current efficacy (CE) of 59.1 cd/A, and a maximum power efficacy (PE) of 65.7 lm/W, at a luminance level of 10 cd/m2; and retained an EQE value of 18.2% at 1,000 cd/m2.13 The TADF emitters used in host-free devices generally show high photoluminescence quantum yields (PLQY) in the solid state, indicating limited aggregation-induced fluorescence quenching. As suggested by recent studies, 16, 17 the high PLQY of these emitters in the solid state can be ascribed to their highly twisted conformations in space. It was demonstrated that the exciton-quenching rate of TADF emitters with D-A structures are dominated by their intermolecular distance (described by the Dexter energy transfer model), which means the spatial geometric structures can inhibit strong intermolecular interactions of emitters in condensed solid phases, and as a result, can reduce concentration fluorescence-quenching and preserve a high PLQY even at high emitter concentrations. If in addition, TADF emitters display balanced ambipolar transport properties, OLEDs with a high performance can be achieved in EMLs having large TADF emitter concentration and even in host-free devices.10 In this work, we report a new compound comprised of 9-carbazolyl donors paired with a pphenylene bis-1,3,4-oxadiazole acceptor moiety, TCZPBOX (see Figure 1, for the synthetic scheme see Supporting Information), which exhibits efficient TADF and can be used as host-free EMLs or doped in the host of 2,6-di(carbazol-9-yl)pyridine (PYD2)18 as EMLs in high-efficiency yellow-green emitting OLEDs.

RESULTS AND DISCUSSION


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Compounds containing the 1,3,4-oxadiazole moiety have been used extensively as electrontransport materials and as emitters in OLED devices due to their high electron mobility, good thermal stability, and high PLQYs.19,20 Similarly, carbazoles have also found extensive use in OLEDs, displaying excellent hole-transport properties as well as being a common electron donor in D-A compounds. 21, 22 Furthermore, ambipolar host materials have been effectively obtained previously by combining these functionalities.23, 24 The compound TCZPBOX combines these common organic-electronic functionalities into a symmetric, D-A compound in which high steric crowding around the bridging phenyl group ensures twisting amongst the constituents and a nonplanar geometry that may minimize the detrimental intermolecular interactions that lead to concentration quenching in the solid state. Calculations using density functional theory (DFT) performed on the compound found two distinct dihedral angles (φ) between carbazolyl donor substituent and the bridging phenyl groups of φ1 = 72° and φ2 = 58°. Furthermore, as is typical for TADF emitters, the highest occupied molecular orbital (HOMO), which in this case is well confined to an ortho-substituted pair of carbazolyl donor groups, and the lowest unoccupied molecular orbital (LUMO), here spread across the bridging phenyl group and onto both oxadiazole moieties, are well-separated spatially. The molecule exhibited a sufficiently reversible reduction in solution to determine a half-wave potential for reduction and was slightly easier to reduce than a typical diphenyloxadiazole, with E1/2 of -1.97 V vs. ferrocenium/ ferrocene, which is consistent with the LUMO spreading out over both oxadiazole groups and the bridging phenylene group as seen in Figure 1. The oxidation was nonreversible, as is typical for carbazoles, and the oxidation potential was determined via the onsets to be 1.09 V. Good thermal stability was confirmed by thermogravimetric analysis (Figure S2), where a


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decomposition temperature (Td) of 450 °C was determined. These data are summarized in Table 1.

Figure 1. (a) Chemical structure of TCZPBOX. DFT calculated geometry and HOMO (b) and LUMO (c) orbitals of TCZPBOX. The absorption and photoluminescence (PL) were characterized in toluene at room temperature (r.t.) as shown in Figure 2a. The PL has an emission peak of 569 nm and shows a yellow-green color. When collected in acetonitrile, the emission maximum is moderately redshifted to 597 nm, consistent with the assignment of the lowest singlet excited state to a chargetransfer (CT) excitation (Figure S5). The relatively small red shift in this particular fluorophore is likely due to a quadrupolar CT character to the excited state (due to the presence of the symmetrical molecular architecture), rather than the dipolar CT found in many other examples. To estimate the value of ΔEST, the prompt PL and delayed PL (500 μs delay after excitation) of TCZPBOX were collected at 77 K in a toluene matrix. As shown in Figure 2b, the spectrum of the delayed PL, which is often attributed to phosphorescence from the triplet state, shows negligible change compared with that of the prompt PL, which is attributed to fluorescence from the singlet state. With these assumptions, the ΔEST would be estimated from either the highenergy onset of the two spectra or the high energy vibronic peaks to be 0.02 eV; however, for molecules such as this, with near-superimposable prompt and delayed low-temperature PL, the low-temperature delayed PL possibly represents TADF, in which case ΔEST cannot be quantified


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in this way, although it must clearly be very small for RISC to be occurring at an appreciable rate at this temperature.

Figure 2. (a) Normalized absorbance and PL intensity of TCZPBOX characterized in toluene at room temperature (r.t.); (b) prompt and delayed PL (500 μs) spectrum characterized in toluene at 77 K. TADF in the compound was confirmed through transient photoluminescence measurements, where a biexponential decay was observed in a nitrogen-sparged toluene solution, but was fully quenched upon equilibrating with air (Figure S4). The temperature dependence of the RISC process was reflected in the increased lifetime of the delayed component upon decreasing the temperature of the solution from 308 K (τDF = 6.27 μs) down to 248 K (9.49 μs). The RISC rate constant (kRISC) in TCZPBOX was found to be ca. 7.2 × 105 s-1, which is quite fast. 25 Solutionstate photophysical data in toluene are collected in Table 1. Table 1. Physical data for TCZPBOX in solution. Photophysical characterization in dilute (ca. 5 × 105 M) toluene solution at room temperature. λabs /nm

λPL /nm

τPF /nsa

φPFa

krad /106 s-1

τDF /μsb

φDFb

kRISC /105 s-1

Eox /Vc

Ered /Vc

Td /°C d

290, 318, 330, 396

569

12.7

0.07

5.5

6.79

0.27

7.2

1.09

-1.97

450

aRecorded

in air-equilibrated solution. bRecorded in nitrogen-sparged solution. cOxidation potential (Eox) and reduction potential (Ered), collected in dichloromethane, values are vs. FeCp2+/FeCp2 internal standard; see SI for details. dThermal decomposition temperature at 95% weight loss; see SI for details.


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In order to study the potential of TCZPBOX as emitters in OLED devices, PL and PLQY of the solid-state TCZPBOX were characterized in thin films. The emitter material was thermally co-evaporated with the host material (PYD2) at doping concentrations of 5 wt. %, 40 wt. %, and 100 wt. %. The PL spectra of the three films were measured at room temperature as shown in Figure 3a. Compared with PL spectrum measured in toluene, the PL spectra of all three solid films were blue-shifted. In the 5 wt. % doped film, the PL spectrum was centered at 507 nm, and a second peak at 373 nm was observed, which is attributed to the fluorescent emission of PYD2 in the solid film, indicating an incomplete energy transfer from host to the emitter molecules in the sparsely doped (5 wt. %) film. When the concentration of the emitter in the film increases from 5 wt. % to 40 wt. % and 100 wt. %, the emission peak is red-shifted from 507 nm to 527 nm and 546 nm, respectively, suggesting an increasing emitter aggregation in the solid film and/or an increased stabilization of the PL transition by the local environment provided by TCZPBOX versus that of PYD2. Meanwhile, the emission peak attributable to PYD2 disappears in the PL spectrum of the 40 wt. % doped film, indicating a more efficient host to emitter energy transfer in heavily doped films. The PLQY of these films all showed high values as shown in Table 2. Specifically, the 100 wt. % neat film showed a high PLQY of 71%, implying the potential of TCZPBOX to be used as an efficient emitter in host-free devices.


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Figure 3. (a) Normalized PL spectrum, and (b) transient PL decay of variously doped solid thin films, measured under room temperature with an excitation wavelength of 300 nm. Furthermore, the PL spectra and the transient PL of these variously doped films were also investigated. In contrast to some previously reported TADF emitters, 14 the apparent ΔEST of TCZPBOX, estimated using 77 K prompt and delayed PL (Figure 4), in the solid film did not change significantly with an increase of its concentration in the host material (Table 2); the values measured in the solid state are comparable to those measured in dilute toluene solutions (0.02 eV), indicating an efficient RISC within the emitter that is independent of the intermolecular aggregation. This is also confirmed by the transient PL decay of the three solid films. As shown in Figure 3b, there is no significant difference observed in the fluorescence decays, which can be best fitted by a tri-exponential decay with τ1 and τ2 decay constants shown in Table 2 (the shortest lifetime corresponds to the prompt fluorescence but cannot be resolved in the same experiment and thus is not reported here). The short term, τ1, is ca. 4 μs and corresponds to ca. 50% of the total amplitude of the decay, and it is assumed to mainly contribute to the efficient RISC in the emitters.


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Figure 4. Prompt and delayed PL (500 µs) of variously doped solid thin films characterized at 77 K. Table 2. PLQY, ΔEST, and lifetime of delayed PL of variously doped solid thin films. Solid film

Emitter concentration (wt. %)

PLQY (%)

∆𝑬𝑺𝑻 (eV)

PYD2:TCZPBOX

5

93

PYD2:TCZPBOX

40

TCZPBOX

100

Delayed PL (μs) τ1 (weighted amplitude, %)

τ2 (weighted amplitude, %)

0.01

4 (50)

33 (15)

89

0.03

4 (55)

30 (16)

71

0.03

4 (42)

26 (16)

Given the PLQY and TADF properties of TCZPBOX shown in heavily doped (40 wt. %) and neat (100 wt. %) solid films, we fabricated two types of devices using these EMLs with different TCZPBOX concentration. First, devices with host-free neat films of TCZPBOX as EMLs were fabricated with the following structure: ITO/MoO3(15 nm)/Poly-TriCZ(70 nm)/


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TCZPBOX(25 nm)/TPBi(60 nm)/LiF(1 nm)/Al(50 nm)/Ag (100 nm). In the other type of devices, TCZPBOX was heavily doped into PYD2 as an EML in the device with the structure of ITO/MoO3(15 nm)/Poly-TriCZ(70 nm)/PYD2:TCZPBOX(40 wt. %, 25 nm)/TPBi(60 nm)/LiF(1 nm)/Al(50 nm)/Ag(100 nm). The device geometry and chemical structures of organic materials used are shown in Figure 5. TPBi and Poly-TriCZ were used as electron- and hole- transporting layers, respectively; LiF and MoO3 are used as electron- and hole- injection layers, respectively (for device fabrication details see the Supporting Information).

Figure 5. Device geometry and chemical structures of host (PYD2), electron-transporting material (TPBi), and hole-transporting material (Poly-TriCZ). The performances of both host-free and doped devices are shown in Figure 6. The J (current density)-V (voltage) curves of the two devices showed good diode behaviors, and the two L (luminance)-V curves both showed a low turn-on voltage (defined as the voltage used to achieve luminance of 10 cd/m2) of 2.9 eV (Figure 6a). The luminance of the two devices achieved similar values at different voltage bias and both reach values above 100,000 cd/m2 at ca. 10V. Note that these values are among the highest values reported in OLEDs with host-free or doped EMLs. In the EL spectrum (Figure 6b), the emission peak of devices with host-free EMLs is red-shifted compared with that of devices with doped EMLs, with the color of emission changing from


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green (CIE (0.34, 0.58)) to yellow-green (CIE (0.40, 0.56)). The EL spectrum of both devices exhibit more structured profile compared with their corresponding PL spectra measured in solid films at room temperature as shown in Figure 3a; a secondary-maximum peak wavelength of ca. 580 nm was observed. However, for each device, the maximum emission peak wavelength of the EL and of the corresponding PL are the same; and the full width half maximum (FWHM) of the EL and PL showed comparable value of ca. 90 nm. Structured EL profiles have been seen in several other TADF emitters. 10

Figure 6. (a) J-V and L-V characteristics; (b) EL spectrum; (c) EQE, CE and PE performance of host-free and doped (40 wt. %) devices.


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Figure 6c shows the efficiency performance (EQE, CE, and PE) of two types of devices as a function of luminance. In the host-free device, a maximum EQE of 21 %, with a maximum CE of 73 cd/A and PE of 79 lm/W, was achieved at 10 cd/m2. The EQE shows a small efficiency roll-off and retains a value of 18% at 1,000 cd/m2 and 13% at 10,000 cd /m2. When using PYD2 as a host in the EML, considering the effects of doping concentration on the efficiency performance of TADF-based OLED as we previously reported,

10

the doping concentration of

TCZPBOX in PYD2 was varied between 30 wt. % to 50 wt. %. The highest efficiency performance was found for a 40 wt. % doped EML. In the doped device, a maximum EQE of 28%, a CE of 94 cd/A and a PE of 100 lm/W was achieved at 10 cd/m2. Detailed device performance parameters are summarized in Table 3. Table 3. Average performance with standard deviation of host-free device and doped devices (over 6 devices) Device name

Emissive layer

Host-free

TCZPBOX

Doped

PYD2:TCZPBOX (40 wt. %)

Maximum luminance (cd/m2)

Von (V) @10 cd/m2

EQE (%) @ 10/100/1,000/10,000 cd/m2

CE (cd/A) @ 1,000 cd/m2

PE (lm/W) @ 1,000 cd/m2

120000 ± 11000 2.9 ± 0.1

20.9 ± 1.1/19.8 ± 0.3/ 18.0 ± 0.1/13.4 ± 0.2

61.6 ± 0.7

45.4 ± 2.1

100000 ± 7000 2.9 ± 0.1

27.9 ± 1.1/26.6 ± 0.3/ 24.3 ± 0.2/17.5 ± 0.2

84.4 ± 0.8

65.2 ± 1.2

CONCLUSION A yellow-green emitting organic small molecule, TCZPBOX, was demonstrated to be an efficient TADF emitter in OLEDs. It exhibits a high PLQY value of 71% in neat films. Hence it can be used efficiently as a TADF emitter doped into a host or in neat films. OLEDs with a hostfree EML yield a maximum EQE of 21%, a CE of 73 cd/A, and a PE of 79 lm/W at 10 cd/m2. A high luminance of 120,000 cd/m2 was achieved in the host-free OLEDs at a voltage of 9.8 V.


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When the EML consists of TCZPBOX doped in a PYD2 host at an optimized concentration of 40 wt. %, devices achieve a maximum EQE of 28%, a CE of 94 cd/A, and a PE of 100 lm/W at 10 cd/m2. These high performance levels suggest that TADF materials used as host-free EMLs in OLEDs can lead to good charge balance, suggesting effective bipolar transport properties resulting from the donor and acceptor moieties present in the TCZPBOX molecular structure. We believe that this work represents a significant step towards realizing the potential of OLEDs with host-free TADF emitters, which we believe is an attractive route to further simplify the device architecture of OLEDs to be used in display and lighting applications. ASSOCIATED CONTENT Supporting Information. Synthesis of TCZPBOX, thermogravimetric analysis (TGA), cyclic voltammetry (CV), transient photoluminescence, solvatochromic data, and details of OLED fabrication and measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ACKNOWLEDGMENT The authors wish to thank Timothy C. Parker for providing the DFT data. This material is based in part upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Solid-State Lighting Program Award Number DEEE0008205. The work is sponsored in part by the Department of the Defense, Defense Threat


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Reduction Agency under Award Number HDTRA1-18-1-0033. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. This work is also supported by the Samsung GRO program.

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Host-free Yellow-Green Organic Light-Emitting Diodes with External

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