Simple Molecular-Engineering Approach for Enhancing Orientation

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Organic Electronic Devices

Simple molecular-engineering approach for enhancing orientation and out-coupling efficiency of thermally activated delayed fluorescent emitters without red-shifting emission Yi-Ting Lee, Po-Chen Tseng, Takeshi Komino, Masashi Mamada, Ruby Janet Ortiz, Man-Kit Leung, Tien-Lung Chiu, Chi-Feng Lin, Jiun-Haw Lee, Chihaya Adachi, Chao-Tsen Chen, and Chin-Ti Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16199 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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ACS Applied Materials & Interfaces

Simple Molecular-Engineering Approach for Enhancing Orientation and Out-Coupling Efficiency of Thermally Activated Delayed Fluorescent Emitters without Red-Shifting Emission Yi-Ting Lee,†,‡ Po-Chen Tseng,§ Takeshi Komino,‖ Masashi Mamada,‖ Ruby Janet Ortiz,‡ Man-kit Leung,† Tien-Lung Chiu,┴ Chi-Feng Lin,¶ Jiun-Haw Lee,*,§ Chihaya Adachi,*,‖,※ Chao-Tsen Chen,*,† and Chin-Ti Chen*,‡

†Department

‡Institute

of Chemistry, National Taiwan University, Taipei 10617, Taiwan

of Chemistry, Academia Sinica, Taipei 11529, Taiwan

§Department

of Electrical Engineering, Graduate Institute of Electronics Engineering and

Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan ‖Center

for Organic Photonics and Electronics Research (OPERA), JST ERATO Adachi

Molecular Exciton Engineering Project, and Education Center for Global Leaders in

ACS Paragon Plus Environment

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Molecular System for Devices, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ※International

Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ┴Department

¶Department

of Photonics Engineering, Yuan Ze University, Taiyuan 32003, Taiwan

of Electro-Optical Engineering, National United University, Miaoli 36003,

Taiwan KEYWORDS: thermally activated delayed fluorescence, organic light-emitting diodes, light

out-coupling factor, molecular orientation, transition dipole moment

ABSTRACT: The inclusion of a tetraphenylbenzene (4Ph) unit in thermally activated delayed fluorescence (TADF) emitters is demonstrated as a novel strategy for greatly enhancing the horizontally oriented alignment of the emitters without shifting the emission spectrum to longer wavelengths. Doping of blue-emitting 4PhOXDDMAC or greenish-blueemitting 4PhOXDPXZ into o-DiCbzBz host layers yielded much higher degrees of horizontally oriented alignment for the emitter (up to 92%) compared to when the 4Ph unit was excluded (69% and 75%, respectively). The enhanced alignment results in high outcoupling efficiencies of 24% and 35% in organic light-emitting diodes based on 4PhOXDDMAC and 4PhOXDPXZ, respectively, and boosts the external quantum ACS Paragon Plus Environment

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efficiencies to values (8.8% and 29.2%, respectively) that are higher than would be expected for randomly oriented emitters (out-coupling efficiency of 20%). These enhancements are achieved while avoiding the red-shift that often occurs using the common strategy of increasing molecular length, and, thereby, conjugation, to increase orientation.

█ INTRODUCTION

In recent years, intense efforts have been devoted toward breaking the 25% upper limit for the internal quantum efficiency (ηint) in organic light-emitting diodes (OLEDs) that emit through the fluorescence of organic materials.1 In 2012, Adachi and co-workers designed and synthesized organic materials that exhibit thermally activated delayed fluorescence (TADF) and achieved an ηint of nearly 100%, thereby launching the third generation of OLED emitters.2-5 In well-designed devices based on TADF, similar ηint values and external quantum efficiencies (ηext) of over 20% are now common.6-9 As ηint of 100% is already possible, the next step for further increasing ηext is the enhancement of the light out-coupling efficiency (ηout). One strategy for enhancing ηout is through the engineering of the external optics, such as by attaching a film containing a microlens array to the surface of the OLED,10-13 but this method increases the complexity and cost of production. Alternatively, the internal optics of the device can be modified by



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changing the thickness and refractive index of the layers and the position and the orientation of the light-emitting molecules.14-15 Experimental results have demonstrated that thin-film OLEDs with light-emitting molecules or host materials that preferentially align horizontally to the substrate often exhibit ηout, and thereby ηext, of more than 20%,16-24 which is the average value for a conventional OLED with randomly oriented molecules and no additional light out-coupling enhancements. Yokoyama and co-workers studied the relationships between emitter orientation and molecular length, end groups, and shape25-27 and found that long linearshaped molecules generally exhibit higher degrees of horizontal alignment than shorter ones. However, a longer π-conjugation always results in a bathochromic shift of the emission wavelength. Furthermore, molecular aggregation and intermolecular contact, which often reduce ηint, tend to be more serious among molecules with extended πconjugation. Therefore, a molecular engineering approach to enhance horizontal molecular alignment without shifting the emission and reducing ηint is desired. Kim and co-workers have reported that the orientation of phosphorescent dopants is strongly related to the intermolecular interaction between the host molecules and dopant emitters and that both their transition dipole moment and molecular structure (or shape) are


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key factors.28-30 So, not only the structure of dopant emitters but also the molecular interactions with host molecules can influence the orientation of dopant emitters. Here we report the inclusion of a bulky tetraphenylbezene (4Ph) unit in TADF molecules as a new molecular design approach that can increase ηout and ηext without compromising emission wavelength. Addition of the 4Ph unit allows the molecular length to be increased while the twisted nature of the sterically hinder phenyl groups limits the increase of conjugation length. We compare two new 4Ph-containing emitters, 4PhOXDDMAC and 4PhOXDPXZ, with their 4Ph-free counterparts, OXDDMAC and the previously reported OXDPXZ31 (chemical structures are shown in Figure 1). In addition to achieving a higher degree of orientation when doped in a host matrix, the 4Ph-containing emitters exhibit photoluminescence (PL) peaks wavelengths (maxPL) that are shorter than those of the corresponding emitter without 4Ph. The horizontal transition dipole ratios (Θ) measured for host films doped with 4PhOXDMAC or 4PhOXDPXZ were 92%, which is one of the highest reported Θ for TADF materials in guest-host thin films.16, 32-33



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Figure 1. Chemical structures of 4PhOXDPXZ, 4PhOXDDMAC, OXDPXZ, OXDDMAC, and

o-DiCbzBz. The synergetic conformation variation tetraphenyl (4Ph) moiety (in green color) or carbazole (Cbz) and N-phenylbenzimidazole (Bz) moieties (in orange color) is schematically illustrated.

█ EXPERIMENTAL SECTION

General Measurements and Instruments. 1H nuclear magnetic resonance (NMR) and

13C

NMR spectra were recorded on a Bruker AV-300 MHz or AV-400 MHz Fourier transform spectrometer at ambient temperatures. Chemical shifts (δ) are given in parts per million (ppm) relative to tetramethylsilane (TMS; δ = 0) as the internal reference. Elemental analyses (on a Perkin-Elmer 2400 CHN elemental analyzer) and fast atom bombardment (FAB) mass spectroscopy (MS) were performed by the Elemental Analyses Laboratory and the Mass Spectroscopic Laboratory, respectively, in-house services of the Institute of Chemistry, Academia Sinica. Melting temperatures (Tm) and glass transition temperatures ACS Paragon Plus Environment

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(Tg) of the materials were measured by differential scanning calorimetry (DSC) using Perkin-Elmer DSC-6 analyzer systems. UV-visible absorption spectra were recorded on a spectrophotometer (LAMBDA 950-PKA, PerkinElmer). The ionization potentials (or highest occupied molecular orbital (HOMO) energy levels) of the materials were determined using a low-energy photo-electron yield spectrometer (Riken-Keiki AC-2). Room-temperature fluorescence spectra were recorded on a spectrofluorometer (FluoroMax-4, Horiba Jobin-Yvon). The PL quantum efficiencies (ΦPL) were measured using an absolute ΦPL spectrometer (C11347-01 Quantaurus-QY, Hamamatsu Photonics, Japan) with an excitation wavelength of 340 nm, which corresponds to the absorption peak of

the

o-DiCbzBz

(9,9'-(2-(1-phenyl-1H-benzo[d]imidazol-2-yl)-1,3-phenylene)bis(9H-

carbazole)) host.34 The light source was a 150 W xenon light. Neat films of the TADF materials and 10-wt%-emitter-doped o-DiCbzBz films with thicknesses of 50 nm were used for the measurement. The ΦPL of the TADF emitters in toluene were also measured in air and after bubbling with nitrogen for 10 min (oxygen-free conditions). The transient PL decay of the doped thin films was recorded by using a streak camera (C4334, Hamamatsu Photonics, Japan). A nitrogen gas laser (KEN-X) with a wavelength of 337 nm was used as an excitation light source.



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Angular dependent PL spectra of 10-wt%-emitter-doped host films (15 nm thick) were measured. The thin films were fabricated by vacuum thermal deposition and encapsulated with a coverslip under nitrogen atmosphere. Each sample was attached to a half-cylinder prism with a refractive index-matching oil, and an excitation CW laser at the wavelength of 375 nm with a power less than 20 mW was irradiated onto the film. Through a 410-nm cutoff filter, a polarizer, and a collimating lens, PL intensity in a transverse magnetic mode was detected by a monochromator (PMA-11, Hamamatsu Photonics). The PL intensities were acquired in each out-of-plane angle from 0° (vertical to the substrate surface) to 90° (horizontal to the substrate surface) with a step of 1° (C14234-11, Hamamatsu Photonics). The obtained PL intensity angle-dependent patterns were analyzed using a commercial software package (Setfos 3.4, Fluxim).15,35 Grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at the beamline 13A at National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) Calculations. Electron density distributions of the HOMO and lowest unoccupied molecular orbital (LUMO) of the TADF materials were obtained using density functional


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theory (DFT) calculations.36 The molecular structures were optimized by applying the hybrid B3LYP functional and 6-31+G (d, p) basis set using the Gaussian 16 program package. Device Fabrication and Testing. OLEDs were fabricated on glass substrates with patterned indium tin-oxide (ITO) layers. The substrates were treated with O2 plasma before use. The OLEDs fabricated in the study have a pixel size of 2 mm  2 mm. The substrates were transferred into a multisource vacuum chamber for film deposition. All the thin-film depositions were conducted via thermal evaporation under a high vacuum of ~10−6 Torr. The electro-optical performance of the OLEDs was measured using a spectrometer (Minolta CS1000) driven by a multisource meter (Keithley 2400). The OLEDs were fabricated with a configuration of ITO/TAPC (50 nm)/mCP (10 nm)/10 wt% TADF emitter doped in o-DiCbzBz (30 nm)/DPPS (50 nm)/LiF (1 nm)/Al (100 nm), where ITO is the anode, TAPC (4,4'-cyclohexylidenebis[N,N-bis(4-methylphenyl)-benzenamine]) is the hole transporting layer (HTL), mCP (N,N-dicarbazolyl-3,5-benzene) is the electron blocking layer (EBL), DPPS (diphenylbis(4-(pyridin-3-yl)phenyl)silane) is the electron transporting layer (ETL), LiF (lithium fluoride) is the electron injection layer (EIL), and Al metal is the reflective cathode. The commercially available bipolar o-DiCbzBz was used as the high performance host material for the emitting layer (EML) of the TADF OLEDs studied here. A schematic energy level diagram of the device and the corresponding chemical structures are



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displayed in Figure S1. The synthetic details and structural characterization data for OXDDMAC, 4PhOXDDMAC, and 4PhOXDPXZ can be found in the Supporting Information.

█ RESULTS AND DISCUSSION

Thermal Properties and Energy Levels. Because of their bulky 4Ph groups, 4PhOXDPXZ and 4PhOXDDMAC have Tg and Tm higher than those of OXDPXZ and OXDDMAC, respectively (Figure S2 and Table S1). Moreover, DSC thermograms of 4PhOXDPXZ and 4PhOXDDMAC lack exothermic signals corresponding to crystallization temperatures (Tc), while Tc of 101–157 and 118–134 °C were clearly recorded for OXDPXZ and OXDDMAC, respectively. These results suggest that the 4Ph moiety effectively inhibits molecular interaction and, hence, crystallization in 4PhOXDPXZ and 4PhOXDDMAC. Optical energy gaps (Eg) of 3.03, 3.05, 2.68, and 2.73 eV for OXDDMAC, 4PhOXDDMAC, OXDPXZ, and 4PhOXDPXZ, respectively, were estimated from the onset wavelengths of the absorption spectra of neat films (Figures 2 and S3). Addition of 4Ph slightly increased Eg in both cases, indicating that 4Ph suppresses π-π interactions between TADF molecules in the condensed phase and does not extend the π-conjugation. The solid-state HOMO energy level (EHOMO), as determined by photoelectron yield spectroscopy of neat films (Figure S4), decreases more with the addition of 4Ph for the 10H-phenoxazine (PXZ) based emitters (−5.28 to −5.51 eV) than the for 9,9-dimethyl-9,10ACS Paragon Plus Environment

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dihydroacridine (DMAC) based emitters (−5.54 to −5.60 eV). This is likely due to the acceptor moiety more strongly interacting with PXZ than with DMAC because PXZ is a

4PhOXDDMAC

Absorption, neat film PL, toluene PL, neat film


1.0

0.5

Absorption

0.0

0.5

maxonset 406 nm (3.05 eV)

1.0

Absorption

maxonset

409 nm (3.03 eV)

0.0 300 350 400 450 500 550 600 650 Wavelength (nm)

4PhOXDPXZ

OXDPXZ Absorption, neat film PL, toluene PL, neat film


1.0

0.5

1.0

0.5

Absorption

maxonset

454 nm (2.73 eV)

Absorption

maxonset

462 nm (2.68 eV)

0.0

0.0 300 350 400 450 500 550 600 650

Normalized intensity (a.u.)

OXDDMAC

Normalized intensity (a.u.) Absorbance intensity (a.u.)

stronger donor.

Absorbance Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Wavelength (nm)

Figure 2. UV-visible absorption and PL spectra of the four TADF emitters.

Gaussian Simulations. For all four emitters, theoretical calculations predict good spatial separation of the HOMO and LUMO, which is critical for reducing the energy gap (ΔEST) between the lowest singlet (S1) and triplet (T1) and achieving TADF. Indeed, the estimated ΔEST are small enough (< 0.2 eV) for all the emitters, although the compounds with DMAC donor was calculated to have ΔEST larger than those of the PXZ series by optimizing their structures at the excited states (Table S2). This can be ascribed to both PXZ and DMAC having a near orthogonal conformation (dihedral angles of 83° and 81°, respectively) with the acceptor (Figure S5). Calculation results also show that the dihedral angle between the 1,3,4-oxadiazole ring and the acceptor side benzene ring increases from 0° to 33° with



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inclusion of the 4Ph moiety (Figure S5). This increased angle should help suppress the extension of π-conjugation length and is consistent with the increased Eg values. Emission Properties. The attachment of 4Ph to OXDDMAC or OXDPXZ results in little to no shift of λmaxPL in solution (Figure 2), further confirming that the 4Ph moiety has little effect on the π-conjugation length of the acceptor moiety. Although neat-film λmaxPL is substantially red-shifted relative to solution λmaxPL for OXDDMAC (12 nm) and OXDPXZ (24 nm), the shift is much smaller for 4PhOXDDMAC (6 nm) and 4PhOXDPXZ (1 nm). These results clearly show that the bulky 4Ph moiety effectively inhibits intermolecular π-π interactions as well as red-shifting of the PL in the solid state. As summarized in Table 1, all of the emitters exhibit higher ΦPL after nitrogen bubbling (toluene solutions) or under argon atmosphere (neat films), which is consistent with T1 states contributing to emission, such as from TADF. In either the solution or neat films, OXDPXZ and 4PhOXDPXZ exhibit a larger increase in ΦPL when changing from air to oxygen-free conditions than OXDDMAC and 4PhOXDDMAC, suggesting the PXZcontaining molecules have a larger contribution from delayed fluorescence.

In toluene

Neat film


In o-DiCbzBz host

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ΦPL PL ΦPL max TADF Emitter λ(nm) in air in N2 (%)

OXDDMAC

450

4PhOXDDMAC 445

(%)

λmaxP

λmaxP

(nm)

ΦPL ΦPL in air in Ar (%) (%)

(nm)

ΦPL ΦPL in air in Ar (%) (%)

L

L

9.1

15.7

462

20.4

26.4

455

27.7

44.0

20.7

23.5

451

23.8

26.5

453

27.8

35.9

OXDPXZ

495

12.6

31.3

519

22.5

32.1

498

58.6

86.5

4PhOXDPXZ

495

15.8

26.4

496

39.1

58.5

495

61.0

83.0

Table 1. Photoluminescence properties of OXDDMAC, 4PhOXDDMAC, OXDPXZ, and 4PhOXDPXZ in toluene, in neat films, and in o-DiCbzBz films doped with 10 wt% of the emitter.

To evaluate the ΦPL values of the emitters in the same conditions as used for OLEDs, 10wt%-emitter-doped films of o-DiCbzBz were prepared by vacuum codeposition. The host oDiCbzBz possesses high S1 and T1 energies (3.4 and 3.1 eV, respectively), which are sufficiently large to suppress back-energy transfer from the emitters to the host and to confine the T1 excitons on the emitters.34 As listed in Table 1, the ΦPL values of the oDiCbzBz films doped with OXDPXZ and 4PhOXDPXZ reach 86.5% and 83.0%, respectively. These ΦPL values are much higher than those measured in solution or neat films. Such a remarkable PL enhancement can be attributed to effective suppression of nonradiative internal conversion (because of the rigidified structure of the material in solid state) and bimolecular deactivation processes like triplet-triplet annihilation (because of dilution of the emitter in the host matrix). Similar results were also observed for OXDDMAC and 4PhOXDDMAC but with lower ΦPL. ACS Paragon Plus Environment


While inclusion of 4Ph generally enhances ΦPL, the 4Ph-containing emitters exhibited slightly lower ΦPL in oxygen-free conditions compared to the corresponding 4Ph-free emitters when doped in o-DiCbzBz films. This decline was also observed for the PXZcontaining emitters in oxygen-free solution. Nevertheless, the declines are small and may not necessarily impact performance in OLEDs, in which the environment of the layers is slightly different and more factors come into play. Prompt fluorescence with a transient lifetime (τp) of 4.1–7.8 ns and delayed fluorescence with a transient lifetime (τd) of 0.37–0.77 ms were observed for the four emitters (Figure 3). Qualitatively, τd can be seen to be longer for both 4PhOXDDMAC and OXDDMAC than 4PhOXDPXZ and OXDPXZ. Detailed quantitative analysis of the transient PL, including the intersystem crossing rate (kISC) and reverse intersystem crossing rate (kRISC), can be found in Supporting Information (Table S3).37-39 OXDDMAC OXDPXZ

105

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4PhOXDDMAC 4PhOXDPXZ Host:

4

10

N Ph

N N

N

103 o-DiCbzBz

102 101 0

1

2 3 Time (ms)

4

5

Figure 3. Transient PL decay curves for o-DiCbzBz films doped with 10 wt% of an emitter at 300 K.


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The ΔEST values of OXDDMAC, 4PhOXDDMAC, OXDPXZ, and 4PhOXDPXZ were estimated to be 0.23, 0.27, 0.07, and 0.07 eV, respectively, from the short wavelength onsets of their fluorescence (prompt) and phosphorescence (delayed) spectra at 10 K in doped films of o-DiCbzBz (Figure 4). These results are in good agreement with the calculated data by the DFT. Assuming comparable spin-orbit couplings for all of the emitters, the larger EST of the DMAC-containing emitters would account for their longer τd compared to the PXZ-containing emitters. Figure S6 shows the temperature dependence of the transient PL decay of the emitter-doped o-DiCbzBz films. The delayed fluorescence components (intensity about 0.9–1.1 ms decrease with increasing temperature, which is likely due to reduced phosphorescence.

OXDDMAC OXDPXZ 1.0 @10K

0.5

Prompt Prompt

4PhOXDDMAC 4PhOXDPXZ 1.0 @10K

Delayed Delayed



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 350

400

Delayed Delayed

0.5

425 nm

394 nm

Prompt Prompt

433 nm 443 nm

450 500 550 Wavelength (nm)

600

422 nm 386 nm

0.0 350

400

430 nm 439 nm


600

Figure 4. Fluorescence and phosphorescence spectra of doped o-DiCbzBz films measured at 10 K. Solid lines and symbol-solid lines are the fluorescence and phosphorescence spectra, respectively. Electroluminescence Performance. Figure 5 presents the performance of OLEDs (ITO/TAPC (50 nm)/mCP (10 nm)/10 wt%-emitter:o-DiCbzBz (30 nm)/DPPS (50 nm)/LiF (1 ACS Paragon Plus Environment


nm)/Al (100 nm)) employing the four emitters, and the performance parameters are

1.0 a) 0.8

102 101 100 10-1 10-2 10-3 10-4 10-5

0.6 0.4 0.2 0.0

400

450


650

700

OXDDMAC 4PhOXDDMAC OXDPXZ 4PhOXDPXZ

c) 102 101 100

b)

Current Density (mA/cm2)


0

Electroluminescence (cd/m2)

Normalized EL intensity (a.u.)

summarized in Table 2.

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2

4

Voltage (V)

104 d) 103 102 101 100

10


-1

10-2

10-1 10

-2

-1

0

1

10 10 10 Current Density (mA/cm2)

10

2

6

OXDDMAC 4PhOXDDMAC OXDPXZ 4PhOXDPXZ 8 10 12

0

2

4

6 8 Voltage (V)

10

12

Figure 5. a) EL spectra, b) current density-voltage (J-V) characteristics, c) external electroluminescence quantum efficiency-current density (EQE-J), and d) electroluminescence-voltage characteristics (L-V) characteristics of the OLEDs.


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Table 2. Device performance of the OLEDs. TADF Emitter

Von a)

λmaxEL

(V)

(nm)

OXDDMAC

4.8

472

4PhOXDDMAC

4.5

462

OXDPXZ

3.6

508

4PhOXDPXZ

3.5

500

a)Turn-on

voltage at 10 cd/m2.

b, c, d)

Lmax

ηc b)

(cd/m2) (cd/A) 8.5, 286 2.6, 10.5, 461 3.9, 62.2, 4650 52.5, 29.3 74.1, 6870 65.4, 40.1

ηp c)

ηext d)

CIEx,y e)

(lm/W) 7.6, 1.1, 9.4, 1.9, 55.8, 38.2, 14.8 66.6, 48.1, 20.8

(%) 7.2, 1.3, 8.8, 3.7, 21.7, 18.3, 10.3 29.2, 25.9, 15.9

(x, y) (0.17, 0.24) (0.15, 0.17) (0.23, 0.49) (0.21, 0.45)

Maximum value, values at 100 cd/m2, and values at

1000 cd/m2 for current efficiency (ηc), power efficiency (ηp), and external quantum efficiency (ηext).

e)

Color spaces of 1931 International Commission on Illumination

measured at ~100 cd/m2 in this study.

The OXDPXZ and 4PhOXDPXZ devices exhibited greenish-blue emission with 1931 CIEx,y of (0.23, 0.49) and (0.21, 0.45), respectively, while the OXDDMAC and 4PhOXDDMAC devices exhibited blue emission with 1931 CIEx,y of (0.17, 0.24) and (0.15, 0.17), respectively. These emission spectra are consistent with the corresponding PL spectra and also follow the trend of blue-shifting with addition of the 4Ph moiety.

17 ACS Paragon Plus Environment


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Whereas very high ηext of 21.7% and 29.2% were obtained for the OXDDPXZ and 4PhOXDPXZ OLEDs, respectively, the OXDDMAC and 4PhOXDDMAC OLEDs exhibited relatively poor ηext of 7.2% and 8.8%, respectively. We attribute their poor ηext to their low ΦPL (0.23 eV). Nonetheless, these ηext all exceed the theoretical limit for conventional fluorescent OLEDs (~5%). All of the OLEDs suffer from efficiency roll-off (Figure 5c and S7). Like most TADF OLEDs, the efficiency roll-off is attributed to the formation of excess triplet excitons at high current densities because of a slow RISC process, resulting in exciton quenching by triplet-triplet and polaron-triplet annihilation.40 However, the 4PhOXDDMAC OLED does show a slightly reduced efficiency roll-off compared to that of the OXDDMAC OLED. We attributed this to the bulky 4Ph reducing intermolecular interactions to somewhat alleviate the triplet-triplet annihilation. Orientation of Transition Dipole Moment and Out-Coupling. Maximum ηext values of 21.7% and 29.2% for the OXDDPXZ and 4PhOXDPXZ OLEDs are rather intriguing considering that the ΦPL of the emitters are only ~85%. Assuming 100% for both the 18 ACS Paragon Plus Environment

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radiative exciton fraction (ηr) and the efficiency of electron-hole recombination (γ), lower limits for ηout can be estimated to be 24% for OXDDPXZ and 35% for 4PhOXDPXZ from

ηext = γ  ηr  ΦPL  ηout. These ηout are 1.25–1.75 times those of regular OLEDs even though we used ITO-coated glass with regular refractive indices and no additional optics to enhance light out-coupling. We conjecture that these high ηout are due to preferential horizontal alignment of the molecules. To verify our conjecture, we investigated the molecular orientation (alignment) in thin films by analyzing the angular-dependent PL intensity.33,

41-42

Figure 6a shows the

normalized PL intensities as a function of the emission angle for the four emitters (10wt%-emitter-doped films of o-DiCbzBz with 15-nm thickness). Both 4PhOXDDMAC and 4PhOXDPXZ have a more horizontal orientation (Θ// of 92%) than OXDDMAC and OXDPXZ (Θ// of 69% and 75%, respectively). Surveying the literature, 92% is one of the highest reported Θ// for TADF materials.16, 32-33


1.4

1.4

a)

1.2

OXDDMAC  = 69% (sim)

1.0 0.8 0.6 0.4 0.2 0.0

b)

OXDPXZ  = 75% (sim)

0

1.0 0.8 0.6 PPT = 61% DPEPO = 65% mCBP = 67% o-DiCbzBz = 75%

0.2 0.0

0

4PhOXDPXZ  = 92% (sim)

10 20 30 40 50 60 70 80 90 Angle (degree) OXDPXZ

1.2

0.4

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4PhOXDDMAC  = 92% (sim)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1.4

c)

1.2

PPT = 78% DPEPO = 79%

4PhOXDPXZ

1.0 0.8 0.6 0.4 0.2 0.0

10 20 30 40 50 60 70 80 90 Angle (degree)

mCBP = 85% o-DiCbzBz = 92%

0

10 20 30 40 50 60 70 80 90 Angle (degree)

Figure 6. a) Angle-dependent PL intensity of the p-polarized light from emitter-doped (10 wt%) o-DiCbzBz films (15-nm thickness) at 460 and 495 nm. For the preferred horizontal transition dipole ratio Θ//, 100% and 67% correspond to fully horizontal and isotropic dipoles, respectively. b) and c) Angular dependent PL measurements of OXDPXZ and 4PhOXDPXZ doped in each host. This high order of orientation is also partially a result of the choice of host. Figure 6b, and 6c shows the normalized PL intensities as a function of the emission angle for OXDPXZ and 4PhOXDPXZ doped in 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP), 2,8bis(diphenyl-phosphoryl)dibenzo[b,d]-thiophene

(PPT),

bis[2-


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(diphenylphosphino)phenyl]ether oxide (DPEPO), and o-DiCbzBz as hosts. Although 4Ph unit increases Θ// in all four host materials that we have tested, it is the o-DiCbzBz host material that pushes Θ// of 4PhOXDPXZ or OXDPXZ to the highest limit. It seems that the o-DiCbzBz host material is special in promoting the horizontal orientation of OXDDMAC or OXDPXZ having a 4Ph unit. Examining by the GIWAXS of the neat film of four host materials, o-DiCbzBz exhibits the most distinct - stacking-like signal (corresponding to a d-spacing ~4.4 Å) in the out-of-plane direction (Figure 7). Molecular shape of o-DiCbzBz is not flat and the - stacking-like signal observed by GIWAXS is rather unusual.



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Figure 7. 2D GIWAXS pattern of four host materials, o-DiCbzBz (a), mCBP (b), DPEPO (c), and PPT (d).

In light of GIWAXS results, we suggest that the rotation of the three substituents (two Cbz units and one Bz unit) of o-DiCbzBz enables a synergetic tilt conformation (see the orange illustration in Figure 1), facilitating a tilt edge-on interaction to the substrate below and an aromatic C-H--- interaction to the molecules above. Therefore, considering such molecular interaction, we propose that molecules of o-DiCbzBz form a regular mosaic packing structure particularly along the out-of-plane direction (Figure 8). Similar synergetic rotation may happens to the 4Ph unit of 4PhOXDDMAC or 4PhOXDPXZ (see the green illustration in Figure 1). Accordingly, the moiety with transition dipole of 4PhOXDDMAC or 4PhOXDPXZ (the greenish blue bars in Figure 8) can be aligned essentially in horizontal direction in the frame work of o-DiCbzBz matrix, probably also through the aromatic C-H--- molecular interaction. Horizontal alignment with Θ// = 92% of 4PhOXDDMAC or 4PhOXDPXZ dopant is thus realized by the molecular packing frame work of o-DiCbzBz host material.


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Figure 8. A 2D schematic illustration of mosaic packing of o-DiCbzBz molecules with the indication of some aromatic C-H--- interactions. Dopant molecules embedded in the o-DiCbzBz packing frame work are also depicted.

█ CONCLUSIONS

In summary, we have demonstrated that 4Ph is a potent moiety for promoting horizontal orientation of molecules without extending the -conjugation. Moreover, structural attachment of 4Ph onto TADF molecules in fact affords no significant spectral shift in the solid state. To our best knowledge, this is the first report of a high ηext (near 30%) TADF material based on deliberate molecular engineering (addition of 4Ph in this study) to achieve a very high degree of horizontal orientation (Θ// of 92%) without red-



Page 24 of 34

shifting the emission wavelength. This strategy can potentially be applied to a wide range of TADF emitters

█ ASSOCIATED CONTENT

Supporting Information

Synthetic procedures and structural characterization data, thermal, computational, and photophysical characterizations, OLED device structure and chemical structure of corresponding materials, and efficiency roll-off (EQE vs cd/m2) of 4PhOXDPXZ, 4PhOXDDMAC, OXDPXZ, and OXDDMAC OLEDs.

The supporting information is

available free of charge on the ACS Publications website at DOI: xxxxxxx/xxxxxxxxxxx.

█ AUTHOR INFORMATION

Corresponding Authors * E-mail: [email protected] (Chin-Ti Chen). * E-mail: [email protected] (Chao-Tsen Chen). * E-mail: [email protected] (Chihaya Adachi). * E-mail: [email protected] (Jiun-Haw Lee).


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ORCID Chin-Ti Chen: 0000-0002-1493-2533 Chao-Tsen Chen: 0000-0002-7225-4873 Chihaya Adachi: 0000-0001-6117-9604 Jiun-Haw Lee: 0000-0003-3888-0595 Masashi Mamada: 0000-0003-0555-2894 Notes The authors declare no competing financial interest.

█

ACKNOWLEDGMENT We thank the Ministry of Science and Technology (MOST 103-2113-M-001-021-MY3),

iMATE Program of Academia Sinica, National Taiwan University, and JST ERATO Grant Number JPMJER1305 for financial support. Thanks to Prof. Hao-Wu Lin of National Tsing Hua University for discussions about the analysis of horizontal transition dipole ratio and Dr. W. J. Potscavage, Jr., for deep and critical editing of the manuscript.

█ REFERENCES



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Simple Molecular-Engineering Approach for Enhancing Orientation

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