Cyanopyrimidine-Carbazole Hybrid Host Materials for High Efficiency

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Organic Electronic Devices

Cyanopyrimidine-Carbazole Hybrid Host Materials for High Efficiency and Low Efficiency Roll-off TADF OLEDs Shu-Wei Li, Cheng-Hung Yu, Chang-Lun Ko, Tanmay Chatterjee, Wen-Yi Hung, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Materials & Interfaces

Cyanopyrimidine-Carbazole Hybrid Host Materials for High Efficiency and Low Efficiency Roll-off TADF OLEDs Shu-Wei Li,a Cheng-Hung Yu,b Chang-Lun Ko,b Tanmay Chatterjee,a Wen-Yi Hung,b* Ken-Tsung Wonga,c* a

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan

b

c

Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202,

Taiwan Corresponding Authors W.-Y. Hung ([email protected]), K.-T. Wong ([email protected]).

Keywords: TADF host, OLED, 4CzIPN, 2CzTPN, horizontal dipole ratio

1

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Abstract Two isomeric host materials (Sy and Asy) comprising carbazole (donor) and CN-substituted pyrimidine (acceptor) were synthesized, characterized, and utilized as host materials for green and blue TADF OLEDs. Both molecules have high triplet energy and small energy difference between singlet and triplet states , leading feasible TADF. The different linking topologies of carbazole and CN groups on pyrimidine core provide distinct photophysical property and molecular packing manners, which further influence the efficiency as they served as hosts in TADF OLEDs. As compared to Asy-based cases, the Sy-hosted TADF OLED device gave higher maximum external quantum efficiency (EQE) of 24.0% (vs. 22.5%) for green (4CzIPN as dopant) and 20.4% (vs.15.0%) for blue (2CzTPN as dopant) and low efficiency roll-off. The high horizontal dipole ratio (Θ~88%) for both emitters dispersed in Sy and Asy hosts accounts for the high device efficiency. A clear molecular structure-physical property-device performance relationship has been established to highlight the importance of symmetrical structure on TADF host materials design.

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Introduction Organic materials with thermally activated delayed fluorescence (TADF) are promising candidates for OLED display and lighting applications due to the small singlet−triplet energy splitting, which allows effective reverse intersystem crossing (RISC).1 One of the promising design strategies for TADF materials is to use a highly twisted structure between the donor and acceptor constituents. The twisted structure has rather small overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and thus results in small singlet−triplet energy gap (ΔEST),1-7 which is essential for fast RISC and then efficient TADF. In principle, TADF OLEDs can realize 100% internal quantum efficiency (IQE) as high as electrophosphorescent devices. However, the issue of efficiency roll-off in TADF-based OLED is still challenging. Typically, the emitting layer of TADF-based OLED is configured with an emitter as guest homogeneously dispersed into a suitable organic host to reduce the possibility of triplet-triplet annihilation (TTA) and thus enhance the device efficiency. As compared to the explosive growth of TADF-based emitters in the last five years8-10, the choice of suitable host materials is still relatively limited. The host materials for TADF-based OLEDs must satisfy a number of crucial requirements.11 First, high singlet and triplet energy provide competent energy transfer and sufficiently confine the emissive excitons on the TADF emitters. Second, the host 3



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materials are better to have suitable energy levels for charge injection and bipolar charge transporting characters for charge balance in the emitting layer.

12-15

However,

molecules exhibiting high S1 and T1 energies together with appropriate energy levels for good charge injection are hard to obtain. Especially, host materials suitable for both blue and green TADF emitters are rarely reported.11, 16 The use of host material with TADF characters is expected to result in both singlet and triplet energy transfer to the lower energy TADF emitter and further improve device performance. In addition, the up-conversion process from triplet to singlet in the host system can reduce the density of triplet exciton density of host and further render low efficiency roll-offs in TADF-based OLEDs.17-21 In this work, two new efficient TADF host materials, Sy and Asy, were synthesized and characterized. They are employed as TADF-host for blue and green TADF OLEDs. The two isomers, Sy and Asy, with different linkage topologies between cyanopyrimidine and two carbazoles can be easily synthesized in two steps by using commercially available starting materials. The two molecules showed hidden TADF features and performed well as TADF host materials for blue (2CzTPN22-23 as dopant) and green (4CzIPN7 as dopant) TADF-based devices, where the efficiency of devices using Sy as host material outperformed the Asy-hosted devices. The Sy-hosted devices exhibited maximum EQE as high as 24.3% for green device and record-high EQE of 4


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20.4% for blue device adopting 2CzTPN as emitter. From the angular distribution of the p-polarized fluorescence, we deduced the horizontal dipole ratio (Θ) is ca. 88% of those emitters that can rationalize the high device efficiencies. We believe that this work will contribute to the molecular design of TADF host materials. Particularly interesting, all devices employing Sy and Asy as host materials exhibited low efficiency roll-off and low turn-on voltage.

Results and Discussions. Carbazole, which is known for its high triplet energy (ET = 3.0 eV) and efficient hole-transporting ability, has been widely used as a donor structure in OLED materials. In

contrast,

pyrimidine

is

usually

introduced

as

an

acceptor

unit

for

electron-transporting materials due to its heteroaryl character and electron-withdrawing ability. Recently, pyrimidine was also introduced as acceptor part of high efficiency TADF emitters2, 24-25. The combination of carbazole and cyano-substituted pyrimidine should afford good opportunity to give new TADF molecules with high S1 and T1 energy levels. In addition, the unsymmetrical structural feature of pyrimidine affords extra flexibility for tuning the donor-acceptor linkage topology, resulting in isomers with different charge distributions and distinct photophysical properties for the study of structure–property relationship. 5



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We simply controlled the adding sequence of the NaH-deprotonated carbazole and 2,4,6-trichloropyrimidine to produce different isomeric ratios of intermediates, which were separated and utilized to make the targets by final cyanation. Scheme 1 depicts the synthetic pathways and corresponding molecular structures of Sy and Asy. The key step is

the

first

nucleophilic

aromatic

substitution

reaction

(SNAr),

while

2,4,6-trichloropyrimidine was rapidly added into deprotonated carbazole to give Sy-Cl as the major product (48%). Whereas, Asy-Cl appeared as the major product (43%) when deprotonated carbazole was slowly added into 2,4,6-trichloropyrimidine. More importantly, the two isomers can be easily separated due to the poor solubility of Sy-Cl. Sy-Cl and Asy-Cl were then converted to Sy and Asy, respectively, by Pd-catalyzed coupling reaction with Zn(CN)2 in good yields. All products were purified via column chromatography and further purified by sublimation before undergoing physical characterizations and device fabrications.

6


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Cl N

1. NaH DMF 2. quick addition into Cl N

N Cl

N N

N

N N

N

N

Cl

Cl

N H 1. NaH DMF

Asy-Cl (9%)

Sy- Cl (48%)

Asy-Cl (43%)

Sy- Cl (7%)

2. slow addition into Cl N Cl

N Cl

N

Zn(CN)2, Pd(PPh 3)4 Asy-Cl NMP 100

oC

48hr

N

N

NC

N

Asy (85%)

CN N Zn(CN)2, Pd(PPh 3)4 Sy- Cl

N

N N

NMP 10 0oC 48hr

Sy (72%)

Scheme 1. The synthetic route for Asy and Sy.

To study the morphological and thermal stabilities of bipolar molecules Sy and Asy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to probe decomposition temperatures (Td) and glass-transition temperature (Tg), respectively. The Td, relative to 5% weight loss for Sy and Asy are 278 and 280 °C, respectively. The Tg of Sy is 150 °C and no Tg was detected for Asy. Relevant data are 7



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summarized in Table 1. The high Td and Tg of these compounds imply that they are suitable for OLED devices by vacuum thermal deposition. The electrochemical behavior of molecules Sy and Asy were studied by cyclic voltammetry (CV), where tetra(n-butyl)ammonium hexafluorophosphate (TBAPF6) in CH2Cl2

and

tetra(n-butyl)ammonium

perchlorate

(TBAP)

in

degassed

N,N-dimethylforamide were used as supporting electrolytes for oxidation and reduction scans, respectively. Both Sy and Asy showed one irreversible oxidation and one quasi-reversible reduction assigned to the carbazole and CN-substituted pyrimidine, respectively (Figure S1). The Sy exhibits a slightly lower reduction potential (-2.83 eV) than that of Asy (-2.97 eV). This result indicates that the 2-cyanopyrimidine exhibits weaker electron-withdrawing character as compared to that of 4-CN substituted counterpart. For a more practical consideration of designing device structure, the HOMO energy levels −6.04 and −5.95 eV of Sy and Asy films were determined by photoemission spectrometer (PES). The LUMO energy levels were then calculated by adding the HOMO level and the optical energy gap (Eg), where Eg is determined from the onset of film absorption band. The data are summarized in Table 1.

Table 1. The physical properties of Sy and Asy. 8


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λ

λ

λ

[nm]

[nm]

[nm]

Sy

342

471

481

3.13

3.06

69

21.2

-2.83

3.21

-6.04

-2.83

278

150

Asy

371

518

483

3.03

2.92

114

16.6

-2.97

2.92

-5.95

-3.03

281

n.d.

a

PL

phos

ET a [eV]

△EST PLQY b Erev c [%] [eV] [meV]

ES a [eV]

abs

Eg d [eV]

HOMOe [eV]

LUMOf [eV]

Td [oC]

Tg [oC]

ES and ET were estimated from the onset of emission peak. b PLQY was measured in film. c Estimated

from CV measured in DMF.

d

Estimated from the onset of absorption peak.

e

HOMO energy level

determined by using photoelectron yield spectroscopy (AC-2). f LUMO = HOMO + Eg.

Figure 1 depicts the electronic absorption and photoluminescence (PL) spectra of Sy and Asy recorded in toluene solution. The data are summarized in Table 1. The absorption peak around 300 nm is assigned as the inherent π-π* transition of the carbazole and carbazole-pyrimidine moiety11, 26-28, and the long wavelength with lower absorptivity is attributed to the charge transfer absorption from carbazole unit to cyanopyrimidine units. Obviously, the charge transfer absorption features are different between Sy and Asy, indicating the different electronic characters governed by different substitution patterns of carbazole and cyano groups on pyrimidine core. The PL spectra of Sy and Asy in toluene solution at room temperature showed broad emission bands with PLmax centered at 471 and 518 nm, respectively. The featureless emission spectra are ascribed to the excited states with intramolecular charge transfer (ICT) character. The photophysical behaviors of Sy and Asy were also probed at 77 K. A clear vibronic feature of phosphorescent spectrum was observed for Asy, indicating the lowest triplet state may localize on the carbazole moiety, whereas the broad emission indicates that 9



the triplet state of Sy still performs charge transfer character. The singlet (ES) and triplet energy (ET) of Sy and Asy were obtained from the onset of fluorescent and phosphorescent spectra, respectively. The calculated ES/ET are 3.13/3.06 and 3.03/2.92 eV for Sy and Asy, respectively, rendering them suitable as host materials for green and blue OLEDs. In addition, the ∆EST of Sy and Asy was estimated to be 69 and 114 meV, respectively, indicating the possibility to give TADF. To confirm the TADF characteristics of Sy and Asy, neat films were fabricated and their transient photoluminescence and time-resolved emission spectra were measured. However, the transient PL decay showed very weak delayed component with τ ≈ 5 μs as shown in Figures S2 (a) and (c). The prompt ratio (φp) and delay ratio (φd) were calculated to be 99% and 1%, respectively. However, we found that delay fluorescence can still be detected after 1 μs from the time-resolved emission spectra at room temperature. The delay fluorescence spectra of Sy and Asy are slightly red-shifted as compared to their prompt fluorescence spectra. This result indicated that under photo-excitation, the emission from S1 state to ground state is very efficient (φp = 99%) and only a small amount of singlet excitons intersystem cross to triplet state and then up-converted back to singlet state. The RISC rate constant (kRISC) of Sy and Asy was estimated to be 9.1x105 s−1, which is in the typical range of TADF materials.1-5,7,29-32 The weak delay component of photoluminescent characteristics implies molecules Sy and 10


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Asy may perform hidden TADF behavior.33

Figure 1. Room temperature absorption and PL spectra of (a) Sy and (b) Asy as well as their corresponding fluorescence (Fluo.) and phosphorescence (Phos.) spectra recorded at 77 K in toluene solution.

To further understand the structure–property relationship, theoretical calculations of frontier molecular orbitals were conducted using density function theory (DFT) with 6-31G* basis sets (B3LYP/6-31G*) for geometry optimization in Figure 2. The result indicates that the LUMO is localized on the cyanopyrimidine for both Sy and Asy. However, the HOMO of Sy is equally distributed over two carbazole units, whereas the HOMO of Asy is only localized at the carbazole attaching to the C2 of pyrimidine.

11



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Obviously, the charge-transfer characteristics of Sy and Asy are different due to their distinct HOMO distributions, imparting their different emission behavior. One of the important factors that govern the ∆EST is the overlap between HOMO and LUMO. From the theoretical analysis, Sy exhibits a HOMO/LUMO overlap only at the C5 of pyrimidine, whereas Asy has evident HOMO/LUMO overlaps at the central pyrimidine unit. This result is consistent with the observed larger ∆EST of Asy as compared to that of Sy.

Figure 2. DFT(B3LYP/6-31G**)-calculated HOMO and LUMO distributions of Sy and Asy.

In Figure 3, the crystal structures of Sy and Asy were resolved by X-ray diffraction analysis

of

their

single

crystals

obtained

from

two

phase

system

from

dichloromethane/hexane solutions. The detail crystal data are summarized in Table S1. The dihedral angle between carbazole and pyrimidine of Sy is 32.1o, leading to a 12


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twisted donor-acceptor conformation that benefits to the HOMO/LUMO separation and thus small ∆EST. In contrast, the lack of ortho-ortho steric interactions between 2-carbazole and pyrimidine gives a dihedral angle of 5.3o, rendering a more planar donor-acceptor conformation and thus a larger ∆EST due to the better HOMO/LUMO overlap. Moreover, the different connection patterns of carbazole and cyano groups on the pyrimidine also affected the molecular packing motifs. Sy with symmetric molecular structure exhibits a higher packing density than asymmetric molecule Asy. The columnar packing with short intermolecular distances of 3.33 and 3.48 Å for Sy and Asy, respectively, indicate good π-π stacking that may create suitable channels for facilitating charge carrier transportation.

Figure 3. Crystal structures and packing manners of (a) Sy and (b) Asy.

The charge-carrier mobilities were characterized with Time-of-Fight (TOF) technique34. Figure 4 (a) and (b) depicts TOF transient photocurrents of Sy and Asy at room temperature under an applied electric field. Figure 4 (c) shows that the field dependence of hole mobility of Sy and Asy. The extracted data indicate that Sy and Asy 13



exhibit hole mobility in a range of 10−5 cm2/Vs order. Unfortunately, the transient photocurrent profiles are too dispersive to enable the clear determination of electron mobility.

Figure 4. Typical transient photocurrent signals for (a) Sy (1.0m thick) at E = 9.5 × 105 V/cm, (b) Asy (1.1m thick) at E = 5.8 × 105 V/cm and insets are the double logarithmic plots. (c). Hole mobilities versus E1/2 for Sy and Asy.

14


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Scheme 2. Molecular structures used in the devices and energy level alignments.

The Sy and Asy were introduced as host in a device structure configured as ITO/4% ReO3:CzSi (60 nm)/CzSi (15 nm)/EML (20 nm)/PO-T2T (50 nm)/ Liq/Al. ITO was used as transparent metal oxide anode, 4% ReO3 was doped in CzSi as hole injection layer (HIL). CzSi26 and PO-T2T35 were served as the hole- and electron-transporting materials, respectively, based on the high electron transport capability and high triplet for the suppression of exciton quenching. The emitting layer (EML) is comprised of Sy and Asy as hosts for TADF green emitter 4CzIPN7 and blue emitter 2CzTPN23. The molecular and device structures are depicted in Scheme 2. The devices were optimized with various doping concentrations (Figure S3-6 and Table S2-5). The best device performances for both green and blue TADF-based OLEDs were 15



achieved with 10wt% doping concentration. Figure 5 depicts the current density-voltage-luminance (J-V-L) and device efficiencies of green 4CzIPN- and blue 2CzTPN-doped emitting devices adopting Sy and Asy as hosts. The EL characteristics of devices are summarized in Tables 2. All devices exhibited low turn-on voltages of 2.6 V. The Sy-hosted green device with 10wt% 4CzIPN as dopant exhibits a maximum brightness (Lmax) of 221,500 cd m−2 at 13.8 V, and a maximum EQE, current efficiency (CE) and power efficiency (PE) of 24.0%, 74.4 cd A−1 and 81.3 lm W−1, respectively. More importantly, the device showed low efficiency roll-off, while the EQE remains at 22.1 % as the brightness increases to 1000 cd m−2. As compared to Sy-based device, the Asy-hosted green TADF OLEDs performed inferiorly, but still gave rather good maximum EQE, CE, and PE of 22.5%, 65 cd A−1 and 53.8 lm W−1 with limited efficiency roll-off. Similarly, the Sy-hosted blue TADF OLED outperformed the Asy-hosted counterpart as 10wt% TADF emitter 2CzTPN was introduced as dopant. The Sy-based blue device delivers a maximum brightness (Lmax) of 122,100 cd m−2 at 14.8 V with EQE, CE and PE of 20.4%, 47.7 cd A−1 and 46.8 lm W−1, respectively. Based on our knowledge, the EQE of Sy-hosted blue TADF-based OLED with 2CzTPN as an emitting dopant is the best among the reported cases23. However, both Sy- and Asy-hosted blue TADF OLEDs show relatively obvious efficiency roll-off as compared to those of green devices. The transient spectra of 10% 16


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emitter doped Sy and Asy films were then examined (Figure S7). The results indicate that the delay behaviors are well matched to the TADF character of emitters 4CzIPN and 2CzTPN. The EL efficiency comparison of represented TADF-OLEDs based on 4CzIPN and 2CzTPN emitters are listed in Table S6. The factors behind such high EQE of the device are attributed to the high emission quantum yield (PLQY) of thin-film as well as optical out-coupling properties of devices depending on the horizontal orientation of the emission dipoles. However, the PLQY of doped films is very different in green and blue cases, for example, the PLQY of Sy- and Asy-hosted 4CzIPN film is 0.76 and 0.72, respectively. Whereas, the PLQY drops in blue emitter-doped films, particularly, the Sy-hosted 2CzTPN film only gives a PLQY of 0.60 as compared to 0.50 of Asy-based film. The higher PLQY may be originated from the better overlap between the emission of host and absorption of emitter (Figure S8). This result agrees with the observed superior EQEs in green devices as compared to those of blue counterparts. Particularly, a short delay fluorescence life time (3.9 μs) was observed in 2CzTPN-doped Sy film, which is significantly shorter as compared to the case reported previously,23

and account for the high device efficiency of Sy-hosted

device. The horizontal transition dipole moment in EML can enhance the light out-coupling efficiency of devices.36 To understand the orientation of the transition 17



dipole moment of the emitters dispersed in Sy and Asy hosts, we measured and simulated the angular dependence of the p-polarised PL intensity shown in Figure 6. The horizontal dipole ratio (Θ) is ca. 88% of those emitters, thus indicating that 4CzIPN and 2CzTPN possessed a similar but rather high emission dipole orientation distribution in host matrix. The theoretical EQE of the devices were simulated using the classical dipole model37 based on the measured Θ and PLQY, which are consistent with the measured EQE values (Table S7). The devices that introduced either Sy or Asy as host materials for TADF OLEDs showed high efficiency and low efficiency roll-off, revealing that both materials are suitable as host materials for TADF emitters. In spite of emitting colors, the device using Sy as TADF host material showed better performance as compared to those of Asy-hosted ones, indicating that the advantages of symmetrical molecular structure in giving host material with smaller ∆EST for better triplet harvesting.

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Figure 5. (a) current density-voltage-luminance (J-V-L) characteristics, (b) external quantum (EQE) and power efficiencies (PE) as a function of luminance, and (c) EL spectra of green and blue devices, respectively.

19



Figure 6. Variable-angle PL intensity of p-polarized light at peak emission for (a) Sy: 4CzIPN, (b) Asy: 4CzIPN, (c) Sy: 2CzTPN, and (d) Asy: 2CzTPN (circle) compared to simulated profiles (lines) with a different ratio of horizontal dipoles (Θ).

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Table 2. The device characteristics of Sy- and Asy-hosted TADF OLEDs.

EML

Vona

Lmax 2

Imax

EQEmax CE max 2

PEmax

at 103 cd/m2

CIE

[%, V]

[x,y]

[V]

[cd/m ]

[mA/cm ]

[%]

Sy:10wt% 4CzIPN

2.6

221500 (13.8 V)

4130

24.0%

74.4

81.3

22.1%, 4.0

0.28,0.55

Asy:10wt% 4CzIPN

2.6

195000 (17.8 V)

2030

22.5%

65.0

53.8

20.4%, 5.6

0.25,0.56

Sy:10wt% 2CzTPN

2.6

122100 (14.8 V)

3780

20.4%

47.7

46.8

16.9%, 4.6

0.19,0.41

Asy:10wt% 2CzTPN

2.6

134200 (14.8 V)

3230

15.0%

37.6

42.2

12.7%, 4.4

0.21,0.44

a

[cd/A] [lm/W]

b

Turn-on voltage at which emission became detectable. b The values of driving voltage

and EQE of device at 1000 cd m–2 are depicted in parentheses.

Conclusion Two isomeric bipolar molecules, Sy and Asy, were synthesized and characterized. The different linking topologies between carbazole and cyanopyrimidine resulted in different charge transfer behaviors and HOMO/LUMO overlaps, therefore different but small ∆EST for possible TADF. However, the transient PL of Sy and Asy films only reveals weak delayed fluorescence, indicating rather hidden TADF characters upon photo-excitation. The substitution patterns of carbazole and cyano groups on pyrimidine also affect the crystal packing of Sy and Asy and lead to different hole transport characteristics. Sy and Asy were then employed as host to give highly efficient blue and 21



green TADF-based OLEDs. As compared to Asy, Sy-based device showed superior EQE as high as 24.3% and 20.4% for green and blue TADF devices with low turn-on voltage and low efficiency roll-off. Importantly, the observed high horizontal emission ratio (Θ~88%) of these emitters as dispersed in As and Asy hosts is evidently beneficial for the overall device performance. This work highlights the advantage of symmetrical structure in serving as host material for better TADF-based OLEDs owing to the smaller ∆EST for better triplet utilizations.

Supporting Information Details of the general experimental procedures, photophysical and time-of-flight mobility measurements, OLED fabrication, and 1H,

13

C NMR spectra of Sy and Asy.

This materials available free of charge via the Internet athttp://pubs.acs.org.

Acknowledgement Hung W.-Y. and Wong K.-T. thank the Ministry of Science and Technology, Taiwan (MOST 106-2112-M-019-002-MY3, 104-2113-M-002-006-MY3) for the financial support.

References 1. Goushi, K.; Yoshida, K.; Sato, K.; Adachi, C., Organic Light-emitting Diodes Employing Efficient Reverse Intersystem Crossing for Triplet-to-Singlet State Conversion. Nature Photonics 2012, 6, 253-258. 2. Pan, K.-C.; Li, S.-W.; Ho, Y.-Y.; Shiu, Y.-J.; Tsai, W.-L.; Jiao, M.; Lee, W.-K.; Wu, C.-C.; Chung, C.-L.; Chatterjee, T.; Li, Y.-S.; Wong, K.-T.; Hu, H.-C.; Chen, C.-C.; 22


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Graphic for Table of Content:

TADF Host N NC

N

CN

N

N

CN

N

N

N

CN

N

NC N

N

EQE 24.0%

EQE 20.4%

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Cyanopyrimidine-Carbazole Hybrid Host Materials for High Efficiency

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