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Ideal molecular design of blue thermally activated delayed fluorescent emitter for high efficiency, small singlet-triplet energy splitting, low efficiency roll-off and long lifetime Dong Ryun Lee, Jeong Min Choi, Chil Won Lee, and Jun Yeob Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05877 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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ACS Applied Materials & Interfaces

Ideal Molecular Design of Blue Thermally Activated Delayed Fluorescent Emitter for High Efficiency, Small Singlet-Triplet Energy Splitting, Low efficiency Roll-off and Long Lifetime

Dong Ryun Lee1, Jeong Min Choi1, Chil Won Lee2 and Jun Yeob Lee1* 1

School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea

2

Department of Polymer Science and Engineering, Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Korea E-mail: [email protected]

Abstract Highly efficient thermally activated delayed fluorescent (TADF) emitters, 5-(2-(4,6-diphenyl1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (oBFCzTrz), 5-(3-(4,6-diphenyl1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (mBFCzTrz) and 5-(4-(4,6diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2-c]carbazole (pBFCzTrz), were synthesized to study effects of ortho-, meta-, and para- linkages between donor and acceptor moieties. oBFCzTrz having ortho- linked donor and acceptor moieties showed smaller singlet-triplet energy gap, shorter excited state lifetime and higher photoluminescence quantum yield than mBFCzTrz and pBFCzTrz which are interconnected by meta- and parapositions. TADF device using oBFCzTrz as a blue emitter exhibited high external quantum efficiency over 20%, little efficiency roll-off and long device lifetime.

KEYWORDS: delayed fluorescence⋅ singlet energy⋅ ortholinkage⋅ charge transfer⋅ triplet energy

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Introduction Quantum efficiency (QE) is one of key performances of organic light-emitting diodes (OLEDs) because several device characteristics like power consumption, driving voltage and lifetime are closely related with the QE. High QE is preferred in the OLEDs for low power consumption and low driving voltage. Therefore, the development of high QE OLEDs such as phosphorescent OLEDs1-7 or thermally activated delayed fluorescent (TADF) OLEDs9-31 is a hot issue. In particular, the TADF OLEDs are promising as high efficiency blue OLEDs for practical application because the TADF emitters have only chemically stable sp2 type chemical bond instead of unstable Ir-C and Ir-N bonds of blue phosphorescent emitters possessing a phenylimidazole type ligand according to molecular calculation results.6,7,8 It is true that TADF emitters have a potential as the high efficiency blue emitters, but only a few design methods have been reported to develop the blue TADF emitters. The most popular method was to couple donor and acceptor moieties using moderate acceptors like triazine9-18, benzophenone19,20 and diphenylsulfone21-23, and strong donors like acridine16,21 and carbazolylcarbazole11,19,24,25 using a phenyl linker. The donor and acceptor moieties were properly chosen for blue emission and they were typically interconnected through paraposition of the phenyl linker in order to obtain high photoluminescence quantum yield (PLQY) although the origin of the high PLQY is not clear yet. This method was successful to report several highly efficient blue TADF emitters. Triazine-carbazolylcarbazole11, diphenylsulfone-acridine21, and benzophenone-carbazolylcarbazole19 combinations provided the blue TADF emitters. However, the diphenylsulfone and benzophenone have a potential problem of instability during operation of the device. Therefore, triazine is one of the best acceptor moieties in the blue material design. Several blue TADF emitters like 2a11 and 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) (TCzTrz)10 were examples of the highly efficient blue TADF emitters. Although the 2a and TCzTz


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showed high QE as the blue emitters, all TADF devices fabricated using the 2a and TCzTrz emitters exhibited serious efficiency drop at high luminance above 1,000 cd/m2. Several factors can explain this behavior, but the most crucial factor was the long excited state lifetime for TADF emission due to large singlet-triplet energy splitting (∆EST).22,23 This hurdle can be overcome by developing TADF emitters with the small ∆EST. Although a few blue TADF emitters offered the small ∆EST, only unstable acridine type TADF emitters were successful as the small ∆EST emitters.16,21 Therefore, a new method of designing the small ∆EST blue TADF emitters using a stable donor and a stable acceptor is needed. Herein, a new molecular design connecting benzofurocarbazole and diphenyltriazine moieties through ortho- position of the phenyl linker was proposed as a method to reduce the ∆EST of the TADF emitter and efficiency roll-off of the blue TADF devices. The ortholinkage based molecular design resulted in small ∆EST of 0.002 eV and short excited state lifetime of 5.4 µs in the 5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2c]carbazole (oBFCzTrz) emitter. As a result, a blue TADF device with a QE of 20.4% and efficiency decrease of only 2.7% at 1,000 cd/m2 was demonstrated in this work. Furthermore, the oBFCzTrz TADF emitter showed stable lifetime as the blue TADF emitter. This is the first work demonstrating high QE of 20.4%, little efficiency roll-off, and stable lifetime in the blue TADF device simultaneously.

Results and discussion Acridine is a well-known donor moiety in the design of TADF emitters because of strong donor character, but the instability of the acridine moiety destabilizes the TADF device having the acridine type TADF emitter. Moreover, too strong donor strength of the acridine moiety offered green emission rather than blue emission when it was coupled to a



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diphenyltriazine acceptor in spite of a very short excited state lifetime.16 This motivated us to come up with a blue TADF emitter design made up of a benzofurocarbazole moiety and a diphenyltriazine moiety interconnected through ortho- position of a phenyl linker. The diphenyltriazine and benzofurocarbazole moieties are chemically stable moieties enabling stable driving of the TADF devices and the orho- connection of the two moieties may strengthen CT character, decrease ∆EST, and shorten excited state lifetime by the distortion of the donor and acceptor moieties. To prove the ortho- linking design concept, three TADF emitters, oBFCzTrz, 5-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5H-benzofuro[3,2c]carbazole (mBFCzTrz), and 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5Hbenzofuro[3,2-c]carbazole (pBFCzTrz), having ortho-, meta- and para- linkages between diphenyltriazine and benzofurocarbazole, respectively, were synthesized.

Scheme 1. Synthetic procedure of oBFCzTrz, mBFCzTrz and pBFCzTrz.


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Figure 1. (a) Calculated geometrical structure and (b) molecular orbital distributions of the oBFCzTrz, mBFCzTrz and oBFCzTrz. (c) Fluorescence and phosphorescence spectra of the oBFCzTrz, mBFCzTrz and pBFCzTrz. (Fluorescence spectra are prompt emission of solid film at room temperature and phosphorescence spectra are delayed emission spectra at 77 K after delay time of 1 ms.



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Synthesis of the oBFCzTrz, mBFCzTrz and pBFCzTrz emitters was carried out by coupling the benzofurocarbazole moiety to F or Br functionalized triphenyltriazine moieties. F functionalized moiety was used for the synthesis of oBFCzTrz and pBFCzTrz, while Br functionalized moiety was utilized for the preparation of mBFCzTrz. Each F or Br functionalized intermediate was prepared by Suzuki coupling of chloro-3,5-diphenyltriazine with corresponding boronic acid intermediate. oBFCzTrz, mBFCzTrz and pBFCzTrz were produced at synthetic yields of 67, 92 and 89% after wet purification. After wet purification, all TADF emitters were sublimed in vacuum sublimator to remove residual impurities. Final purity level of oBFCzTrz, mBFCzTrz and pBFCzTrz was over 99.0 % by high performance liquid chromatography analysis. Scheme 1 describes preparation procedure of oBFCzTrz, mBFCzTrz and pBFCzTrz. As the main objective of designing the oBFCzTrz emitter was to decrease ∆EST by distorting the donor and acceptor moieties each other, the geometrical structure of oBFCzTrz, mBFCzTrz and pBFCzTrz was examined. Figure 1(a) shows calculated geometrical structure of oBFCzTrz, mBFCzTrz and pBFCzTrz by geometry optimization using Gaussian 09 software simulation based on B3LYP 6-31G* basis set. The oBFCzTrz emitter is distinguished from the mBFCzTrz and pBFCzTrz by large dihedral angle of 36.1° (diphenyltriazine-phenyl linker) and 66.4° (phenyl linker-benzofurocarbazole) induced by steric hindrance between the diphenyltriazine and benzofurocarbazole moieties. As the donor and acceptor moieties are close each other, they were distorted from the phenyl linker connecting the donor and acceptor moieties. The large dihedral angle of the oBFCzTrz emitter may decrease singlet energy by the strong CT character and increase triplet energy by short conjugation length. The dihedral angle of the oBFCzTrz, mBFCzTrz and pBFCzTrz emitters was correlated with photophysical properties by electronic molecular orbital calculation. Figure 1(b) and


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Table 1 present calculated molecular orbital distribution and calculated photophysical parameters related with TADF emission. The different linkages had little effect on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) dispersion of the TADF emitters. Typically observable extensive HOMO-LUMO separation and small HOMO-LUMO overlap at the phenyl linker were apparent in the molecular orbital diagram irrespective of the interconnecting position. However, the calculated molecular parameters were dissimilar depending on the link position of the TADF emitters. The singlet energy/triplet energy values of oBFCzTrz, mBFCzTrz and pBFCzTrz were 2.78/2.73, 2.79/2.68 and 2.89/2.64 eV, respectively. The singlet energy of the TADF emitter was in the order of oBFCzTrz < mBFCzTrz < pBFCzTrz due to the geometrical distortion, while the triplet energy was in the order of oBFCzTrz > mBFCzTrz > pBFCzTrz by decreased conjugation length. As a result, the order of ∆EST was oBFCzTrz < mBFCzTrz < pBFCzTrz. Significant decrease of ∆EST was noticeable in the oBFCzTrz by the singlet energy decreasing and triplet energy increasing effect of the ortholinkage. Therefore, the oBFCzTrz emitter has the potential to perform better than mBFCzTrz and pBFCzTrz as TADF emitters.

Table 1. Calculated photophysical properties of oBFCzTrz, mBFCzTrz and pBFCzTrz. Singlet energy (eV)

Triplet energy (eV)

ΔEST (eV)

HOMO (eV)

LUMO (eV)

Oscillator strength

oBFCzTrz

2.78

2.73

0.05

-5.19

-1.84

0.0128

mBFCzTrz

2.79

2.68

0.11

-5.24

-2.00

0.0033

pBFCzTrz

2.89

2.64

0.25

-5.29

-1.99

0.2999

The effect of the ortholinkage on the photophysical parameters observed in the molecular calculation results was experimentally confirmed by measuring fluorescence (singlet energy)



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and phosphorescence (triplet energy) of oBFCzTrz, mBFCzTrz and pBFCzTrz. Fluorescence and phosphorescence spectra of oBFCzTrz, mBFCzTrz and pBFCzTrz are shown in Figure 1(c). Onset wavelength of the fluorescence and phosphorescence spectra was used to calculate the singlet and triplet energy. The singlet energy of the oBFCzTrz emitter was lower than that of other emitters in spite of geometrical distortion possibly due to increased CT character. The triplet energy of oBFCzTrz was higher than that of pBFCzTrz because of geometrical distortion. The ∆EST of oBFCzTrz was only 0.002 eV, but that of mBFCzTrz and pBFCzTrz was 0.191 and 0.302 eV, respectively. The ortho- linking design method could decrease the ∆EST of the TADF emitter to a large extent. Singlet energy, triplet energy and ∆EST of the synthesized emitter are also listed in Table 2.

Table 2. Photophysical properties of oBFCzTrz, mBFCzTrz and pBFCzTrz. Singlet energy (eV)[a]

Triplet energy (eV)[b]

∆EST (eV)

PLQY (%) [c/d]

Excited state lifetime (µs)

oBFCzTrz

2.994

2.992

0.002

6.2/97.9

5.4

mBFCzTrz

3.204

3.013

0.191

4.7/31.1

29.6

pBFCzTrz

3.188

2.886

0.302

44.3/85.3

31.2

[a]: singlet energy was estimated from onset of fluorescence spectrum. [b]: triplet energy was estimated from onset of phosphorescence spectrum. [c]: under oxygen. [d]: under nitrogen.

As the ∆EST of the TADF emitters has great influence on the delayed fluorescence phenomenon of the TADF emitters, transient PL characterization of oBFCzTrz, mBFCzTrz and pBFCzTrz was carried out. Figure 2 represents transient PL decay of oBFCzTrz, mBFCzTrz and pBFCzTrz at room temperature. Excited state lifetime values calculated from the transient PL decay curves by assuming exponential decay were 5.4, 29.6, and 31.2 µs for oBFCzTrz, mBFCzTrz and pBFCzTrz, respectively. A short delayed fluorescence lifetime of


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5.4 µs was obtained in the oBFCzTrz emitter due to the small ∆EST of 0.002 eV. Therefore, the oBFCzTrz emitter is superior to mBFCzTrz and pBFCzTrz emitters to up-convert triplet excitons into singlet excitons. The small ∆EST also affected PL quantum yield (PLQY) of the TADF emitters. PLQY under oxygen/PLQY under nitrogen of oBFCzTrz, mBFCzTrz and pBFCzTrz was 6.2/97.9, 4.7/31.1 and 44.3/85.3%, respectively. Significant increase of the PLQY under nitrogen was detected in the oBFCzTrz emitter, suggesting efficient upconversion of triplet excitons. In the case of mBFCzTrz and pBFCzTrz, PLQY under nitrogen was relatively low although the PLQY was increased under nitrogen by activating triplet excitons for light emission. The reason for the high PLQY of oBFCzTrz is due to efficient up-conversion, while the low PLQY of mBFCzTrz is caused by small oscillator strength and poor up-conversion by large ∆EST. The excited state lifetime and PLQY of the oBFCzTrz, mBFCzTrz and pBFCzTrz emitters are summarized in Table 2.

Figure 2. Transient PL decay curves of the 10 wt.% oBFCzTrz, mBFCzTrz and pBFCzTrz doped DPEPO films.



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From the PL analysis of the TADF emitters, the oBFCzTrz emitter was found to be ideal as the blue TADF emitter because of high PLQY of 97.9% and short excited state lifetime of 5.4 µs. Based on the material characterization data, blue TADF devices were grown by doping the TADF emitters in the bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) matrix. Optimization of the device performances was performed by tuning the doping concentration of the TADF emitters. Device characteristics at optimized doping concentration are presented in Figure 3(a). Device performances at other doping concentrations are in Figure S2 (supporting information). It is well-known that DPEPO is an electron transport type host material and hole trapping dictates the device characteristics of the TADF devices. As the HOMO/LUMO of oBFCzTrz, mBFCzTrz and pBFCzTrz are -6.1/-3.4, -6.1/-3.3 and 6.1/-3.2 eV, respectively, hole trapping is the dominant process dictating the light emission of the three TADF emitters as can be predicted from the energy level diagram (Figure 3 (b)). The hole trapping mechanism optimized the device performances at high doping concentration due to poor hole injection and transport. Above 20% doping concentration, the QE was decreased due to concentration quenching effect. Optimized QE values of the oBFCzTrz, mBFCzTrz and pBFCzTrz emitters were 20.4, 13.2, and 16.7%, respectively. The QE of the blue oBFCzTrz emitter was above 20% and was higher than that of mBFCzTrz and pBFCzTrz. The QE value is also comparable to that of recently reported paper about sky blue TADF device.31 Especially, the high QE of the oBFCzTrz device was maintained even at high luminance. The QE values at 100 and 1,000 cd/m2 of the oBFCzTrz device were 20.0 and 17.7%, respectively. Considering the rapid drop of the QE according to luminance in other blue TADF emitters derived from the triazine acceptor moiety9,10, great progress of the efficiency roll-off of the blue TADF device was made in the oBFCzTrz device. The high QE of the oBFCzTrz device is due to high PLQY of the oBFCzTrz emitter and the little efficiency roll-off is caused by short delayed fluorescence lifetime. As the main factors of the


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efficiency roll-off are triplet-triplet exciton quenching and triplet-polaron quenching, short excited state lifetime of the oBFCzTrz emitter in triplet excited state lead to the weak efficiency roll-off behavior.

Figure 3. (a) Quantum efficiency-luminance curves of the oBFCzTrz, mBFCzTrz and pBFCzTrz devices at optimized doping concentration (20 wt.%). (b) Energy level diagram of the TADF OLEDs. (c) Emission spectra of the oBFCzTrz, mBFCzTrz and pBFCzTrz devices.



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Figure 3(c) represents emission spectra of the oBFCzTrz, mBFCzTrz and pBFCzTrz devices collected at a luminance of 1,000 cd/m2. Blue emission spectra were exhibited in the TADF devices and the peak wavelength was in the order of oBFCzTrz (477 nm)> mBFCzTrz (473 nm)> pBFCzTrz (465 nm). Color coordinates of the oBFCzTrz, mBFCzTrz and pBFCzTrz devices were (0.18, 0.31), (0.17, 0.25), and (0.15, 0.18), respectively.

Figure 4. Device lifetime data of the oBFCzTrz, mBFCzTrz and pBFCzTrz devices at a fixed current density of 5 mA/cm2.

Lifetime is also one of the key attributes of the oBFCzTrz emitter because it was developed for stable operation as well as high QE and little efficiency roll-off. Lifetime of the oBFCzTrz device measured at a fixed current density (5.0 mA/cm2) is shown in Figure 4 along with the lifetime of the mBFCzTrz and pBFCzTrz devices. In the practical application, lifetime up to 95% of initial luminance is important, so the lifetime up to 95% of initial luminance was compared at a current density of 5.0 mA/cm2. The oBFCzTrz device was comparable to the pBFCzTrz device and better than the mBFCzTrz device when the


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operational lifetime was considered. Considering that the well-known blue emitting bis[4(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) device lived less than 1 min at the same driving condition, the oBFCzTrz device was much better than the wellknown DMAC-DPS device. Therefore, the oBFCzTrz device could reach the performance level satisfying the general requirements of high efficiency blue emitters such as high QE, little efficiency roll-off, blue emission spectrum and stable lifetime for the first time.

Conclusions In conclusion, a novel molecular design connecting a benzofurocarbazole donor and a diphenyltriazine acceptor through ortho- position of a phenyl linker provided an oBFCzTrz emitter having small ∆EST of 0.002 eV, short delayed fluorescence lifetime of 5.4 µs, and high PLQY of 97.9%. The ideal photophysical performances of the oBFCzTrz emitter could offer high QE over 20%, little efficiency roll-off and blue emission color in addition to stable lifetime. Therefore, the ortho- linker based TADF material design can be a solution resolving the challenges of low QE, serious efficiency roll-off and short lifetime of the blue TADF devices.

Experimental

Synthesis Detailed synthesis of oBFCzTrz, mBFCzTrz and pBFCzTrz is explained in supporting information.

Device fabrication and measurement Device fabrication and measurement methods were described in our previous work[8]. The device structure was ITO/PEDOT:PSS (60.0 nm)/TAPC (20.0 nm)/mCP (10.0



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nm)/DPEPO:TADF emitter (25.0 nm)/TSPO1 (5.0 nm)/TPBi (30.0 nm)/LiF (1.5 nm)/Al (200.0 nm). In the device structure,

PEDOT:PSS, TAPC, mCP, DPEPO, TSPO1, and TPBi

represent poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate), 4,4'cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline], tris(4-carbazol-9-ylphenyl)amine, 1,3bis(N-carbazolyl)benzene, bis[2-(diphenylphosphino)phenyl]ether oxide, diphenylphosphine oxide-4-(triphenylsilyl)phenyl, and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene. Doping concentrations of oBFCzTrz, mBFCzTrz and pBFCzTrz in the DPEPO host were 5 to 30% and optimized at 20%. Device structure for lifetime test was DNTPD(60.0 nm)/BPBPA(30.0 nm)/mCBP:TADF emitter(30.0 nm)/ LG201(35.0 nm) LiF(1.5 nm)/ Al(200.0 nm). In the device structure, DNTPD, BPBPA and mCBP represent N,N’-Bis[4-[bis(3-methylphenyl)amino]phenyl]N,N’-diphenyl-[1,1’-biphenyl]-4,4’-diamine N,N,N’,N’-tetra[(1,10-biphenyl)-4-yl]-(1,10biphenyl)-4,4’-diamine and 3,3-di(9H-carbazol-9-yl)biphenyl. Doping concentrations was 10%.

Supporting Information Additional solution photoluminescence spectra and quantum efficiency-luminance curves at other doping concentrations. This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and future Planning (2013R1A2A2A01067447, 2016R1A2B3008845).


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TOC

Ideal molecular design of blue thermally activated delayed fluorescent emitter for high efficiency, small singlet-triplet energy splitting, low efficiency roll-off and long lifetime Dong Ryun Lee1, Jeong Min Choi1, Chil Won Lee2 and Jun Yeob Lee1*

A molecular design of interconnecting donor and acceptor moieties via an ortho- position of a phenyl linker was effective to reduce singlet-triplet energy splitting, decrease efficiency rolloff, shorten excited state lifetime, improve photoluminescence quantum yield and extend lifetime of blue thermally activated delayed fluorescent emitters.


Ideal Molecular Design of Blue Thermally Activated ... - ACS Publications

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