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Strategies for the Molecular Design of Donor-Acceptor Type Fluorescent Emitters for Efficient Deep Blue Organic Light Emitting Diodes Seung-Je Woo, Youheon Kim, Myeong-Jong Kim, Jang Yeol Baek, Soon-Ki Kwon, Yun-Hi Kim, and Jang-Joo Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04437 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018
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Chemistry of Materials
Strategies for the Molecular Design of Donor-Acceptor Type Fluorescent Emitters for Efficient Deep Blue Organic Light Emitting Diodes Seung-Je Woo,1+ Youheon Kim,2+ Myeong-Jong Kim,3 Jang Yeol Baek2, Soon-Ki Kwon,3* Yun-Hi Kim2* and Jang-Joo Kim1* 1
Department of Materials Science and Engineering, RIAM, Seoul National University, Seoul 151-742, South Korea Department of Materials Engineering and Convergence Technology and ERI , Gyeongsang National University, Jinju 660-701, South Korea 2
3
Department of Chemistry and RIGET, Gyeongsang National University, Jinju 66-701 South Korea
ABSTRACT: We describe several strategies for the molecular design of high-efficiency blue fluorescent emitters. Asymmetric donor-acceptor (D-A) and symmetric A-D-A type fluorescent emitters were designed with spiroacridine donors and diphenyltriazine acceptors. Substituting a toluene or xylene moiety for the phenyl group connecting the donor and acceptor and replacing the diphenylsilane group with a fluorene moiety resulted in a deeper blue emission without any losses in luminescence efficiency. Based on these substitutions, deep blue organic light emitting diodes (OLEDs) with Commission Internationale de L’Eclairage (CIE) coordinates of (0.149, 0.082) and an external quantum efficiency (EQE) of 7.7% were fabricated using a D-A type emitter. Symmetrizing the D-A structure to an A-D-A structure increased the proportion of horizontally oriented emission dipoles in the organic film from 70% to 90%. OLEDs incorporating symmetric A-D-A type emitters had EQEs as high as 8.5% due to increased outcoupling efficiencies and also showed deep blue emission with CIE coordinates of (0.142, 0.116). The molecular design strategies described herein can be applied to donor-acceptor type fluorescent emitters for the fabrication of efficient deep blue OLEDs.
INTRODUCTION Since the invention of organic light emitting diodes (OLEDs) by Ching Tang in 1987,1 their performance in display applications has improved significantly. Many electronics companies are now replacing liquid crystal displays (LCD) with OLED displays because of the superior display quality of the latter. Green and red phosphorescent OLEDs are particularly popular for use in displays because of their high efficiency and long operational lifetimes. Blue OLEDs typically incorporate conventional fluorescent emitters or triplet-triplet fusion (TTF) emitters, which combine good color quality with greater operational stability than blue phosphorescent OLEDs.2-7 However, the efficiency of blue conventional fluorescent or TTF emitters is much lower than that of phosphorescent emitters due to the theoretical limits of maximum internal quantum efficiency: 25% for conventional emitters and 62.5% for TTF emitters.8-22 A donor-acceptor (D-A) structure is often adopted for deep blue fluorescent OLEDs, especially for non-doped OLEDs, in order to improve charge balance and reduce driving voltage.814 The bipolar nature of D-A emitters reduces efficiency losses stemming from charge imbalances in emitting layers (EMLs). However, their extended π-conjugation makes them difficult to realize deep blue emission. Recently, D-A structures have been widely adopted in thermally activated delayed fluorescence (TADF) emitters in order to convert excitons from the triplet state to the singlet state via reverse intersystem crossing. However, attaining deep blue emission is challenging because of the bathochromic shift associated with intramolecular charge
Figure 1. (a) Molecular structures of DTPSAF, DTTSAF, DTXSAF, BDTSAF, and BDTPDDA. (b) Absorption spectra of toluene solution andphotoluminescence spectra of 10 wt% emitter:mCP host 30 nm films.
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Figure 2. Optimized S1 state geometries and Natural Transition Orbitals for S1→S0 transition of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA. Dihedral angles of donor/linker and linker/acceptor for S1 geometries of DTPSAF, DTTSAF, and DTXSAF are indicated (dihedral angles for S0 geometries are in parentheses). (ωPBE ω=0.10, PBF toluene)
transfer. Blue TADF OLEDs with external quantum efficiencies (EQEs) close to 20%23 and higher than 20% have been realized24-25, but deep blue TADF emitters that match the blue standard Commission Internationale de L’Eclairage CIEy value of the National Television System Committee (NTSC) still show EQEs lower than 10%, low maximum brightness, and large efficiency roll-offs.26-27 Future applications will require more efficient fluorescent deep blue emitters. We herein report strategies for the design of donor-acceptor type fluorescent emitters to achieve pure blue emission and high efficiency at the same time. We designed five fluorescent emitters employing acridine or azasiline as a donor and diphenyltriazine as an acceptor (Figure 1(a)). DTPSAF, DTTSAF, and DTXSAF are asymmetric D-A type emitters, and BDTPSAF and BDTPDDA are symmetric A-D-A type emitters. The para-linked donor-acceptor structure was used to control the emitting dipole moment (EDO) of emitters in the organic solid film. If both donor and acceptor moieties are connected within a molecule, then emission usually occurs via charge transfer. The S1→S0 transition dipole moment (TDM) points from the acceptor toward the donor, parallel to the molecular long axis, and the EDO follows the overall molecular orientation in the organic thin film. A high proportion of horizontal EDOs can be achieved by increasing the aspect ratio of the emitting molecule along the direction of TDM, which results in higher outcoupling efficiency.28-30 Diphenyltriazine, which is an acceptor widely used for TADF emitters31-32, was selected as the acceptor because of its planar molecular structure. By attaching diphenyltriazine to the donor moiety, the molecule’s aspect ratio can be easily tuned along the long molecular axis. Spiroacridine, which is a popular donor for green and blue TADF emitters33-34, was selected as a donor because of its rigid structure and potential for deep blue emission. The spirocarbon or silicon connects the fluorene or diphenylsilane moiety to acridine, respectively, which makes the donor moiety rigid without increasing the length of π-conjugation. The rigid structure reduces the degree of nonradiative decay through vibrational modes, leading to higher photoluminescence quantum yields (PLQY).18,35 The πconjugation length does not extend between the acridine and spirofluorene or diphenylsilane moieties because it is blocked
by an sp3-hybridized carbon or silicon atom. Due to the larger size of the silicon atom, the π-conjugation blocking effect is greater with silicon and a larger bandgap can be expected for BDTPDDA relative to that of BDTPSAF.36-38 The spirofluorene or diphenysilane moiety is orthogonal to the acridine donor plane, which facilitates deeper blue by inhibiting intermolecular interactions. Donor-acceptor connecting units are used to prevent the complete separation of HOMO and LUMO, providing sufficient oscillator strength for efficient radiative decay. The donor and acceptors are connected with para linkage of the linker moieties to separately control the dihedral angle between donor and linker and between linker and acceptor. For the D-A type emitters, phenyl, toluene, and xylene are used as linker moieties to shorten π-conjugation between the donor and acceptor for deeper blue emission.
RESULTS AND DISCUSSION DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were prepared using various organic reactions such as bromination, Suzuki coupling reactions, and N-arylation. Structures of the synthesized compounds were confirmed by 1H-NMR, 13 C-NMR, and mass spectroscopy (see Supporting Information). Thermal stabilities were measured by thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) (Figures S1–S2.) The decomposition temperatures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 428, 374, 403, 463, and 600°C, respectively. The melt transition temperatures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 344, 316, 289, 389, and 344°C. The glass transition temperatures of DTPSAF, DTTSAF, DTXSAF, BDTPSAF, and BDTPDDA were 171, 162, 156, 200, and 259°C, respectively. All compounds exhibited sufficiently high thermal stabilities. For PL measurements, high T1 (2.9 eV) N,N’-dicarbazolyl3,5-benzene (mCP) was used for the host of the emitters to confine triplet within the emitters as it could affect the delayed emission of the emitters. However, the PL measurement results showed that the emitters are conventional fluorescent materials and high T1 of the host would not be necessary. The measured photophysical properties of emitters are listed in Table 1.
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Chemistry of Materials Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed using the Schrodinger Table 1. Photophysical properties of DTPSAF,
DTTSAF,
DTXSAF,
Experimental Data
. S1 b) (eV)
λPL c) (nm)
S1 d) (eV)
FWHM e) (nm)
DTPSAF
HOMO /LUMO a) (eV) -5.51/-2.63
2.76
459
2.70
DTTSAF
-5.55/-2.56
2.80
450
2.76
DTXSAF
-5.52/-2.48
2.82
446
BDTPSAF
-5.46/-2.65
2.74
469
Material
Materials Suite’s Jaguar to understand the photophysical BDTPSAF,
and
BDTPDDA
Calculation Data PLQY f) (%)
kr g) (108 s-1)
EDO h)
S1, VA i) (eV)
S1, VE j) (eV)
T1 k) (eV)
∆EST l) (eV)
69
82
3.13
70:30
2.89
2.72
2.18
0.60
0.7388
70
100
3.23
70:30
2.91
2.74
2.29
0.53
0.5951
2.78
69
98
3.13
74:26
3.00
2.80
2.31
0.57
0.6451
2.64
62
94
3.85
90:10
2.84
2.66
2.19
0.50
1.6613
f m)
BDTPDDA -5.45/-2.53 2.76 459 2.70 60 96 3.85 90:10 2.90 2.71 2.18 0.54 1.7740 a) HOMO levels measured by cyclic voltammetry (Figures S3). LUMO levels estimated by adding the optical band gaps to the HOMO levels. b) PL peak S1 energy of toluene solution. c) PL peak of 10 wt% emitter:mCP host film. d) S1 energy of PL peak. e) Full-width half maximum of PL from 10 wt% emitter:mCP host film. f) Photoluminescence quantum yield of 10 wt% emitter:mCP host film. g) Radiative decay rate constant measured from 10 wt% emitter:mCP host film. h) Emitting dipole orientation of emitters. (horizontal:vertical) i) Calculated vertical absorption S1 energy. j) Calculated vertical emission S1 energy. k) Calculated relaxed T1 state energy. l) Calculated adiabatic(relaxed) singlet-triplet energy gap (∆EST). m) Calculated oscillator strength of S1→S0 transition. Calculation was performed using LC-ωPBE functional with modified range-separation parameter (ω=0.10).
properties of the emitters. 39,40 Ground state geometries were optimized with B3LYP functional and excited S1 state geometries were optimized with CAM-B3LYP functional. Single point excitation energies and natural transition orbitals (NTOs) were calculated using LC-ωPBE functional with modified range-separation parameter (ω=0.10), employing PBF (Poisson Boltzmann Finite-element solver) solvent model with dielectric medium (toluene, ɛ = 2.3741).41 Optimized geometries and NTOs are shown in Figure 2, S4 and calculation data are shown in Table 1, S3-4. DFT/TDDFT calculations were also performed with B3LYP functional. Optimized ground state geometries, HOMO/LUMO frontier orbitals, NTOs, and excitation energies calculated with B3LYP are shown in Figure S5-7 and Table S1. For all five emitters, the electron wave function and hole wave function of NTOs are localized at the triazine acceptor moiety and the acridine donor moiety repectively (Figure 2, S4). NTO analyses revealed that the S0- S1 absorption and S1S0 emission are mainly comprised of charge transfer (CT) transition. Strong solvatochromism of the PL in solution (Figure S8) also indicates that emission occurs from the charge transfer excited state.42,43 However, the hole wave function and electron wave function of the emitters’ NTOs are not completely separated. Instead, there is a slight overlap at the donor-acceptor connecting unit (phenyl, toluene, or xylene). This overlap generates sufficient oscillator strength between the S1 and S0 states, resulting in a very short radiative decay time of about 3 ns without delayed emission (Figure S9). The transient PL of DTXSAF doped in various hosts were also measured and all of them showed very fast radiative decays without delayed fluorescence (Figure S10). No delayed fluorescence in the molecules even with the D-A structures can be supported by the large calculated ∆EST values. The radiative decay rate constants of the symmetric A-D-A type emitters were 3.85×108 s-1, which are larger than those obtained from the asymmetric D-A emitters (Table 1). This can be explained by the larger overlap between the hole and electron wave function of the A-D-A type emitters than that of the D-A emitters due to doubled donor-acceptor connections.44 The calculated oscillator strengths of the A-D-A emitters are also larger than those of the D-A emitters. Due to their fast radiative decay and rigid
molecular structures, all of the designed emitters exhibited very high PLQYs. For example, DTTSAF attained a PLQY of unity. As expected from the vertical emission S1 energies obtained from TDDFT calculations, the emission spectra of the D-A type emitters were blue-shifted following a change in the donor-acceptor connector unit from a phenyl moiety to a toluene or xylene moiety. The dihedral angle between the acridine moiety and the connecting units, and the dihedral angle between the connecting units and the diphenyltriazine moiety, increased as the phenyl connector was changed to a toluene or xylene due to steric hindrance by the methyl groups. 45 The calculated dihedral angles for S1 and S0 geometries are indicated in Figure 2. The π-conjugation length of the LUMO was shortened by increasing the dihedral angle between the aromatic rings, resulting in higher LUMO levels and blue-shifted PL spectra.46-48 The emission spectra of the A-D-A type emitters were also blue-shifted by substituting diphenylsilane for spirofluorene. The HOMO and LUMO levels were increased by 0.01 eV and 0.12 eV, respectively, resulting in an overall bandgap increase of 0.11 eV and an increase in the peak PL energy from 2.65 eV (469 nm) for BDTPSAF to 2.70 eV (459 nm) for BDTPDDA. Thus, deeper blue emission was realized without reducing PLQY and the radiative decay rate by employing two distinct design strategies: changing the phenyl connector to a toluene or xylene, and changing the fluorene moiety to a diphenylsilane. The full-width half maximum (FWHM) of the PL emission from the A-D-A type emitters was smaller than those of the DA type emitters. The electron wavefunction of the NTO is more delocalized over donor to triazine in the A-D-A type emitters than in the D-A type emitters. Thus, the donor-linker bond of the A-D-A type emitters are more rigid in their excited state geometries than the D-A type emitters, and intramolecular vibration and rotation are suppressed. Therefore, narrow spectra are expected in the A-D-A type emitters. 49 The featured spectra of the A-D-A type molecules compared to the featureless spectra of the D-A type molecules also indicate that the vibration and rotation are suppressed in the A-D-A type molecules.
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Figure 3. Top view and side view of optimized S1 geometries and the S1→ S0 transition dipole moments of DTPSAF and BDTPSAF
Figure 4. (a) Current density-voltage-luminescence curves of OLEDs. (b) Electroluminescence intensity and CIE 1931 coordinates (inset) of the OLEDs at 500 cd/m2. (c) Current density-EQE curves of the OLEDs. (d) EQE of mCP host EML and mCP:TSPO1 host EML BDTPDDA-doped OLEDs.
The angle-dependent PL intensity of p-polarized light was measured from a film composed of 10 wt% emitter in mCP host. Measured data were fitted with an optical simulation using a classical dipole model and are shown in Figure S11. The EDOs were 70(horizontal):30(vertical), 70:30, and 74:26 for DTPSAF, DTTSAF, and DTXSAF and 90:10 for the symmetric A-D-A type emitters. BDTPDDA doped in an mCP:TSPO1 (1:1 wt%) mixed host exhibited the same molecular orientation as in a mCP host. Since the emitter doping ratio in the EML is only 10 wt%, the Tg of the emitting layer will be close to the Tg of the host molecule mCP (65°C) and the Tg of the emitters are expected to have little effect to the EDO of the emitters. 50-51 As shown in Figure 3, the S1→S0
TDM of the molecules direct toward the acridine donor from the diphenyltrazine acceptor, almost parallel to the long axis of the molecule. Hence, the orientation of the TDM follows the molecular orientation in the organic thin film. The A-D-A type emitters are oriented more horizontally than the D-A emitters probably due to the elongated molecular structures and it resulted in higher horizontal EDO ratio of the A-D-A type emitters than the D-A type emitters. Since the PLQYs of both emitter types were approximately the same, higher external quantum efficiencies (EQEs) are expected for OLEDs incorporating the A-D-A type emitters because of their higher outcoupling efficiencies.
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Chemistry of Materials We fabricated OLEDs using all five emitters as dopants. The optimized device structure was as follows: ITO (75 nm)/6 wt% ReO3:mCP (45 nm)/mCP (x nm)/10 wt% emitter:mCP (15 nm)/TSPO1(15 nm)/6 wt% Rb2CO3:TSPO1 (35 nm)/Al (100 nm). The thickness (x) of the mCP layer was 10 nm for DTPSAF/DTTSAF/DTXSAF, and 15 nm for BDTPSAF/BDTPDDA, respectively. mCP was used as the HTL and TSPO1 was used as the electron transporting layer. ReO3 and Rb2CO3 were used as charge injection dopants for the HTL and ETL, respectively. mCP was also used as the host of EML because of the large spectral overlap between the PL of mCP and the absorption of the five emitters, which facilitates efficient energy transfer from the host to the emitters. Current density-voltage-luminance (J-V-L) curves, electroluminescence spectra, and current density-EQE curves of the five devices are shown in Figures 4(a), (b), and (c), respectiveTable 2. Electroluminescent Properties of OLEDs.
ly. Electroluminescent characteristics of the OLEDs are summarized in Table 2. Replacing the phenyl moiety with toluene or xylene in the D-A type emitters lowered the CIEy value in the EL spectra as expected from the PL spectra. The EQEs of the DTTSAF- and DTXSAF-doped OLEDs (6.2 and 7.7%, respectively) were higher than that of the DTPSAFdoped device (5.7%) due to the higher PLQYs of DTTSAF (100%) and DTXSAF (98%) over that of DTPSAF (82%). The device doped with DTXSAF had the deepest blue emission with CIE coordinates of (0.149, 0.082) at 500 cd/m2, almost reaching the NTSC blue CIE coordinate (0.14, 0.08). The EQE of the DTXSAF device (7.7%) was very high relative to those of conventional fluorescent blue OLEDs with CIEy
