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Enhanced efficiency of thermally activated delayed fluorescence emitters by suitable substitution on isonicotinonitrile
Dyes and Pigments 170 (2019) 107633
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Enhanced efficiency of thermally activated delayed fluorescence emitters by suitable substitution on isonicotinonitrile
T
Amjad Islama, Zhiheng Wangb, Shaomin Jic, Khurram Usmand, Syed Comail Abbasa, Jianguo Lia, Lihui Chena, Mudassir Iqbale, Shi-Jian Sub,∗∗, Xinhua Ouyanga,∗ a
College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, PR China State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China c School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510640, PR China d International Academy of Optoelectronics at Zhaoqing, South China Normal University, PR China e Department of Chemistry, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad, 44000, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Isonicotinonitrile Emitter Thermally activated delayed fluorescence Organic light emitting diodes
Two novel thermally activated delayed fluorescence (TADF) emitters, 3,5-bis(4-(9H-carbazol-9-yl)phenyl) Isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]-4-yl)amino)phenyl)isonicotinonitrile (2BTPAINN), have been developed and their photophysical, electrochemical and electroluminescent properties have also been studied. Both materials possess high thermal stabilities and high photoluminescence quantum yields (PLQYs). Importantly, organic light emitting device (OLED) with 2BTPA-INN as emitter showed outstanding performance with a low driven voltage (VON) of 2.9 V, a high external quantum efficiency (EQE) of 26.1%, power efficiency (PE) of 93.7 lm/W and current efficiency (CE) of 83.7 cd/A, which is among the excellent performances for TADF OLEDs. The enhanced efficiency can be ascribed to the high PLQY. These results provide an optimum strategy to design efficient materials for TADF OLED devices.
1. Introduction Thermally activated delayed fluorescence (TADF) compounds have earned an immense attraction in the research area of organic lightemitting devices (OLEDs) over the last one decade owing to their capabilities to harvest both singlet and triplet excitons via reverse intersystem crossing (RISC) to realize theoretical 100% internal quantum efficiency [1–3]. So far, high-performance electroluminescent devices containing various TADF-emitters have been demonstrated and external quantum efficiency (EQE) up to 30% has been reported, which matches well with that of phosphorescent organic light-emitting diodes (PhOLEDs). For these efficient TADF emitters, a small energy gap (ΔEST) between the singlet energy states (ES) and triplet energy states (ET) is vital to achieve a RISC. However, the molecular design strategies of TADF emitters are still not fully clear with highly efficient RISC due to the restrictions of molecular systems. In this regard, it is highly desirable to develop novel TADF materials with different structures and substitutions to elucidate the relationship of structures and properties. To realize an optimum ΔEST, a minimum overlap of highest
∗
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of donor (D) and acceptor (A) moieties is required. Since the pioneer work of Adachi in the studies of TADF emitters, many advances have been reported for the materials synthesis and device fabrications [4,5]. To date, different kinds of acceptor moieties such as; sulphones [5,6], benzonitrile [3,7,8], ketones [9], 2,4,6-triphenylpyrimidine [10–16] and boron containing compounds [17–19] have been used to prepare TADF emitters. Among these structures, most of them were focused on the molecular conformation of D-D-A and D-A-D types to replace the traditional structure of D-A type with the aim of reduced ΔEST [1,11,18,20–26]. Generally, these D-D-A and D-A-D molecules were prepared with complicated synthetic procedure, which limits their application in low-cost and large-scale fabrication. Additionally, the small overlap between HOMO and LUMO often results in a forbidden S1–S0 transition, which decreases the radiative efficiency [22,27]. In view of this, the design and synthesis of novel TADF materials with suitable overlap of HOMO and LUMO are crucial to obtain high radiative efficiency. To get the suitable overlap of HOMO and LUMO, a feasible way is to
Corresponding author. Corresponding author. E-mail address: [email protected] (X. Ouyang).
∗∗
https://doi.org/10.1016/j.dyepig.2019.107633 Received 25 April 2019; Received in revised form 22 May 2019; Accepted 6 June 2019 Available online 07 June 2019 0143-7208/ © 2019 Published by Elsevier Ltd.
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OLEDs so far.
prepare these molecules with reasonable D and A blocks. Pyridine and its derivatives have been shown as potential candidates for acceptor unit owing to their high triplet energy levels (ET) compared to other reported molecules [11,28]. Moreover, their electron-withdrawing abilities can be improved significantly by incorporating different moieties like cyano (CN) group. Meanwhile, the nitrogen atoms from CN and pyridine units also play a key role to modify the extent of intramolecular charge transfer (ICT), inducing the TADF feature to these materials. However, a very few emitters based on pyridine-carbonitriles for efficient TADF process have been explored so far [29,30]. Therefore, the TADF compounds with pyridine-carbonitrile core are considered to have the opportunity to realize high efficiency TADF devices. Recently, isonicotinonitrile (INN) or 4-cyanopyridine, one of the typical pyridinecarbonitrile compounds, has been demonstrated as effective TADF material. For example, Zheng and colleagues prepared an efficient blue TADF material (CPC) containing isonicotinonitrile [30]. OLED device fabricated using CPC as emitter observed an EQE of 21% with current efficiency (CE) of 47.7 cd/A and power efficiency (PE) 42.8 lm/W. In another attempt to enhance the EQE of OLED devices, Kido and coworkers synthesized isonicotinonitrile based blue and green materials for TADF [31]. The TADF devices were fabricated using those materials showed high EQE of 22% [31]. On the other hand, electron-donor units with effective hole transporting properties are extensively employed to prepare electroluminescent materials in the past decades, especially for the TADF materials. Carbazole and triphenylamines (TPA) have been demonstrated as excellent electron-donor moieties to construct highly efficient TADF compounds, such as 2,3,5,6-tetrakis (carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) [3], and 2-(4'-(diphenylamino)-[1,1′-biphenyl]-4yl)quinoxaline-6,7-dicarbonitrile (TPA-QCN) [32]. The development of the materials containing hole-transporting units with high glass transition temperature (Tg), excellent film forming ability and good solubility in common solvents is urgently required to fabricate high performance TADF OLED devices. Furthermore, these functional pyridines with carbazole and triphenylamine moieties will create a twisted conformation, resulting in a high photoluminescence quantum yield (PLQY) [33]. Herein, two novel INN derivatives, 3,5-bis(4-(9H-carbazol-9-yl) phenyl)isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]4-yl)amino)phenyl)-isonicotinonitrile (2BTPA-INN) with meta-substitutions of 9-phenyl-9H-carbazole (CzP) and N-([1,1′-biphenyl]-4-yl)N-phenyl-[1,1′-biphenyl]-4-amine (BTPA) have been designed and synthesized and the molecular structures are shown in Scheme 1. Their electrochemical, thermal, photophysical and electroluminescent properties were studied systematically with high thermal stabilities, high PLQYs and relatively small ΔEST. Notably, by using 2BTPA-INN as emitter the TADF device showed high performance with a low driven voltage (VON) of 2.9 V, a high EQE of 26.1%, and a high PE of 93.7 lm/ W and CE of 83.7 cd/A, which is among the best performances for TADF
2. Results and discussions 2.1. Synthesis Both TADF emitters 3,5-bis(4-(9H-carbazol-9-yl)phenyl) Isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]-4-yl) amino)phenyl)isonicotinonitrile (2BTPA-INN), were prepared in a single step through suzuki coupling aromatic reaction (as can be seen in Scheme 1) in a high yield of over 70%. After purification through column chromatography, the compounds were further purified using sublimation process and characterized by 1H NMR, 13C NMR (Figs. S1–S2) and mass spectrometry (MALDI-TOF) (Fig. S3). One of the aims to design 2CzP-INN and 2BTPA-INN was to construct efficient TADF devices by considering the benefit of high thermal and molecular stability of 2CzP-INN and 2BTPA-INN. Isonicotinonitrile is a stable acceptor unit owing to its expanded conjugation over the whole units of isonicotinonitrile. Carbazole and triphenylamine are also stable units with their aromatic characters. Like other TADF compounds, 2CzP-INN and 2BTPA-INN are also economical as compared to the phosphorescent materials. 1,2,3,5-tetrakis (carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [2] is a very famous TADF emitter, consisting of phthalonitrile and carbazole moieties. High EQE of 30% [8] and high molecular stability [34] have already been realized using 4CzIPN as emitter. Owing to the strong electron-withdrawing ability, cyano group is utilized to construct efficient TADF emitters. 2.2. Electrochemical and thermal properties Electrochemical efficiencies of 2CzP-INN and 2BTPA-INN were determined through cyclic voltammetry. With the help of onset potential (of initial oxidation connected to ferrocene) the HOMO energy value was obtained, (Fig. S4). HOMO energy level is obtained through the equation of: EHOMO = (Eonset)OX + 4.84. The HOMO energy levels of 2CzP-INN and 2BTPA-INN molecules are −5.39 eV and −5.24 eV, subsequently. For better hole injection, the charge injection barrier should be tuned close to the HOMO level (−5.3 eV) of hole transport layer, 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC), facilitating the hole injection. The values of HOMO levels of both materials are near to that of TPAC. Furthermore, the HOMO level of 2BTPA-INN is higher by 0.15 eV compared to 2CzP-INN because of increased conjugation length afforded by the TPA group. The values of energy band gaps (Eg) of these materials are around 3.06 and 2.77 eV, acquired by using absorption spectrum, respectively. The difference between the HOMO and Eg gives out the values of the LUMO energy levels (−2.33 and −2.47 eV) of 2CzP-INN and 2BTPA-INN (Table 1). Decomposition temperature (Td) and glass transition temperature (Tg) of 2CzP-INN and 2BTPA-INN were measured through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in nitrogen environment. The schematic is shown in Fig. S5 and Table 1 represent the thermal data. The TGA indicates a high thermal stability of these materials with temperature range above 400 °C. For 2CzP-INN and BTPA-INN, a 5% (Td) loss of weight has been detected at 482 °C and 480 °C, subsequently. Both emitters were found to be thermally stable beyond 450 °C. The sublimation temperature of these materials is at least 150 °C lower than these values. No Tg was observed for 2CzP-INN, while 2BTPA-INN demonstrated a high Tg ∼145.45 °C. The twisted shape of the material is thought to be the major reason for higher values of Tg, that increases the stability of the molecules at high temperatures [35]. 2.3. Theoretical calculations An investigation of molecular orbital distribution is essential to explain the optical nature of 2CzP-INN and 2BTPA-INN emitters and is displayed in Fig. 1. Theoretical calculations were conducted by
Scheme 1. Synthetic mechanism of 2CzP-INN and 2BTPA-INN. 2
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Table 1 Photophysical performance of 2CzP-INN and 2BTPA-INN. Compound
Abs.a (nm)
PLa (nm)
Tdb, Tgb, (oC)
c,d
2CzP-INN 2BTPA-INN
341 338
435 480
482/N·O 480/145.5
−5.39/-5.48 −5.24/-5.01
a b c d e f g
HOMO (eV)
c,d
LUMO (eV)
−2.33/-2.21 −2.57/-1.88
e
Eg (eV)
3.06 2.67
f
ΔEST
0.20 0.13
g
ΔEST
0.42 0.33
Measured in toulene solution at room temperature. Glass transition temperature (Tg) determined by DSC measurement, and decomposition temperature with 5% weight loss (Td) obtained from TGA analysis. HOMO measured from the onset of oxidation curves from CV and LUMO calculated from the difference between HOMO and Eg. Theoretically calculated values from DFT using gaussian software. Optical bandgap (Eg) calculated from the absorption edge of UV–Vis spectra. Theoretically calculated values from DFT using gaussian software from the difference between singlet and triplet energy. Calculated experimentally from the difference between singlet and triplet energy measured from fluorescence and phosphorescence spectra.
Fig. 1. Molecular structure and HOMO-LUMO level distribution of 2CzP-INN and 2BTPA-INN.
applying density functional theory (DFT) at the B3LYP/6-31G(d)/level using Gaussian 09 W to analyze the geometric conformation, the HOMO and LUMO energy levels, ES, ET, and ΔEST of 2CzP-INN and 2BTPA-INN (Fig. 1, and Table 1). According to the DFT calculations, ES of 2CzP-INN and 2BTPA-INN molecules were found to be at 2.8591 and 2.699 eV and ET were found to be at 2.7327 and 2.499 eV, respectively. A small energy difference (ΔEST) of 0.1264 and 0.200 eV was obtained, which indicated blue and green TADF emission. The electron density in Fig. 1 reveals that the HOMO level of 2CzP-INN was totally distributed on carbazole and phenyl units, whereas LUMO level was completely located on isonicotinonitrile unit. A very weak overlapping is observed in 2CzP-INN molecule. In the case of 2BTPA-INN molecule, the HOMO level was localized on the donor moiety (triphenylamine), and the LUMO level was populated on the acceptor moiety (isonicotinonitrile). As anticipated, both, HOMO and LUMO levels were completely disintegrated with each other and a very weak overlapping was found between them. The distribution between HOMO and LUMO of 2CzP-INN and 2BTPA-INN is very much suitable for TADF emission. Both, 2CzPINN and 2BTPA-INN molecules show a little twisted configuration and maintain the π-conjugation length in control. The π-conjugation is important for the delocalization of HOMO and LUMO orbitals in the charge transfer molecules, because the rate of radiative decay depends on this delocalization of HOMO and LUMO [1].
Fig. 2. (a) UV–Vis absorption and PL spectra of 2CzP-INN and 2BTPA-INN in toluene solution (10 −5 M); (b) Low temperature PL spectra (77K) of 2CzP-INN and 2BTPA-INN.
solution of toluene. Fig. 2 presents the UV/Vis absorption spectrum of the 2CzP-INN and 2BTPA-INN molecule measured at room temperature. Maximum absorption for both materials lies in the range of 300–400 nm. 2CzP-INN exhibited a maximum absorption peak at 341 nm, whereas maximum absorption of 2BTPA-INN was obtained at almost 338 nm. These peaks can be ascribed to the independent absorption of D-A units. The other absorption peaks of 2CzP-INN and
2.4. Photophysical properties ICT is an outstanding characteristic of TADF molecules, which can be examined through Ultraviolet/Visible (UV) absorption spectroscopy. DFT calculations in Fig. 1 are associated with the optical parameters of these TADF materials such as ES, ET, and PLQY. Photophysical properties of 2CzP-INN and 2BTPA-INN molecule were measured in a dilute 3
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2BTPA-INN at 360 nm and 400 nm, subsequently, can be assigned to the intra-molecular charge-transfer (ICT) from the electron donating carbazole and triphenylamine group to the electron accepting isonicotinonitrile group. Similar trend in absorption spectra has already been realized in the previous reports [31,36]. Photoluminescence spectra of these derivatives are also presented in Fig. 2. 2CzP-INN exhibited deep blue emission with the maximum peak located at almost 435 nm, whereas maximum emission of 2BTPA-INN was appeared at 480 nm at room temperature. The emission of 2BTPA-INN was more red-shifted as compared to 2CzP-INN owing to the extension of conjugation through the addition of phenyl ring on the TPA moiety [2,37]. We also investigated the low temperature (77K) emission of these materials. This trend of PL emission peak matched well with those values which were projected from theoretical calculations. The ΔEST values were measured through the emission onsets as 0.42 eV for 2CzPINN and 0.33 eV for 2BTPA-INN at 77 K (Fig. 2). The 2CzP-INN and 2BTPA-INN compounds also demonstrated high PLQYs of 0.775 and 0.897, respectively, in (degassed) toluene solution. The PLQY values of 10 wt% DPEPO films of 2CzP-INN and 2BTPA-INN were found to be 0.690 and 0.890, subsequently. The high PLQYs are an indication of the involvement of triplet excitons in the PL emission probably due to TADF process. The emission lifetime of 2CzP-INN and 2BTPA-INN in toluene consists of a prompt (14–20 ns), however, their delayed component are too weak to be resolved as it coincides with the instrument response function (Fig. 3a). Thus, the delayed fluorescence in the studied compounds in toluene is smaller than we can reliably detect. For the TADF behavior of devices, they were investigated by measuring transient PL decay curves of 10 wt% 2CzP-INN and 2BTPA–INN–doped DPEPO films showed a double-exponential decay (Fig. 3b and c). A fast prompt decay (15 ns) and several very long delayed emission components are found. The dominant delayed components were estimated to be 3.6 ms (77%, amplitude in multi-exponential fit to data) and 46.2 ms (14%) for compound 2CzP-INN, 1.9 ms (74%) and 28.7 ms (19%) for 2BTPA-INN, respectively. The spectroscopic evidence signifies that these compounds are indeed TADF emitters. The summary of thermal and photophysical properties is given in Table 1. 2.5. Electroluminescence performance To examine the potential of 2CzP-INN and 2BTPA-INN as emitter, OLED devices were constructed the device with the structure of ITO (95 nm)/TAPC (30 nm)/mCP (10 nm)/EML (25 nm)/DPEPO (10 nm)/ TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), where 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) is served as hole transport material, 3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB) was employed as electron transport layer as well as hole blocking layer and LiF was acted as electron injection material. N, N ′ -dicarbazolyl-3,5-benzene (mCP) was utilized as electron and exciton blocking layer to confine triplet excitons. The molecular structures of different materials employed in the device fabrication process are displayed in Fig. S6 mCP [38,39] is a well-known electron blocking material and has demonstrated excellent results in many high performance TADF devices. mCP possesses a high ET1 value (2.9 eV) which contributes to confine the excitons within the EML and minimize the efficiency roll-off [40]. Bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO) was served as the host material. A combination of mCP and DPEPO was used which is beneficial for TADF dopant. [41] A doping ratio of 10 wt% of 2CzP-INN and 2BTPA-INN emitters was used. Basically, the idea of using mCP and DPEPO simultaneously, was to confine the holes and electrons within the emissive layer (EML), resulting in a balanced charge transportation. As the ET of both, mCP and DPEPO were higher than 2CzP-INN and 2BTPA-INN emitters, triplet exciton quenching of emitter was reduced and consequently, OLED device performance was improved. 2CzP-INN based device showed deep-blue electroluminescence (EL) with a maximum peak at 440 nm with CIE of (0.15, 0.09) (Fig. 4). 2CzP-INN device achieved an EQE of 8.07% with
Fig. 3. The photo-luminance transient decay curves of 2CzP-INN and 2BTPAINN doped in DPEPO films (10 wt %) at 77 K and 298K.
CE of 6.66 cd/A and PE of 5.23 lm/W (Fig. 4). VON was observed to be 4.1 V for 2CzP-INN device. The 2BTPA-INN based device showed electroluminescence peak at 512 nm with CIE coordinates of (0.24, 0.55) (Fig. 4). EL spectrum of 2BTPA-INN was red-shifted compared to 4
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Fig. 4. (a) Current density and luminance versus driving voltage (J-V-L); (b) current efficiency versus luminance; (c) power efficiency versus luminance; (d) external quantum efficiency (EQE) versus luminance characteristics and (e) EL spectra of the devices based on 2CzP-INN and 2BTPA-INN at 1 mA cm−2 doped with DPEPO (B1 & B2).
2CzP-INN spectrum due to the presence of extra phenyl ring on the TPA unit. The EL spectrum is generated from the emissive layer and no emission was observed from other materials. EQE versus luminance curves of the 2BTPA-INN based devices are presented in Fig. 4. The maximum EQE of the 2BTPA-INN device was realized to be 26.1%. But CE and PE of the 2BTPA-INN device were significantly high (83.7 cd A−1 and 93.7 lm/W) (Fig. 4), subsequently. More interestingly, the VON was dropped from 4.1 V to 2.9 V. This value is almost five times higher than the spin statistic limit (5% theoretical) of EQE for typical fluorescent emitters and is a demonstration of efficient triplet excitons harvesting for EL emission through TADF process. The low efficiency of 2CzP-INN based device was attributed to low PLQY of 2CzP-INN. This efficiency of 2BTPA-INN device was better than many other TADF devices in terms of VON and EQE. These are not only among the best results achieved for green TADF OLEDs but also among other colors OLED devices till date [41–49]. The hole injection barrier from TAPC to the EML is very less (0.1–0.2 eV) which facilitated better hole injection. On the other hand, the electron injection barrier from TmPyPB to EML was also less (0.3 and 0.03 eV) which improved the electron injection and charge balance. Owing to its wide Eg (4.20 eV) and high ET (3.00 eV), DPEPO prevents reverse transfer of triplet excitons from dopant to host. Additionally, VON of 2BTPA-INN device is significantly reduced to 2.9 from 4.1 V. High PE of 93.7 lm/W and CE of 83.77 cd/A reflects the improved charge injection and transportation in 2BTPA-INN device owing to the better charge trapping by 2BTPA-INN dopant, and it can be emulated by the reduced VON with increased doping ratio. However, in both devices, considerable efficiency roll-off was observed. This is very common with the TADF emitters having long lifetime [2]. Generally, two types of exciton formation mechanisms exist in OLED devices. In first mechanism, energy is transferred from host to dopant, whereas in the other case, charges are directly recombined in the dopant layer [50–55]. In both devices, as the HOMO levels of 2CzP-INN and 2BTPAINN dopants are shallower compared to DPEPO hosts, exciton formation in 2CzP-INN and 2BTPA-INN would preferably take place through direct recombination of charges. In this case, the enhanced efficiencies of 2BTPA-INN based device should be primarily attributed to the effectively suppressed triplet excitons quenching. The high efficiency of 2BTPA-INN device is attributed to the high PLQY and efficient process of RISC culminated from relatively small ΔEST of 2BTPA-INN with an additional phenyl ring in TPA moiety, confirming that highly efficient TADF materials can be prepared through appropriate incorporation of substituents. The electroluminescence performances are displayed in Table 2. In the development of stable materials for OLEDs, planar geometrical conformation and stable groups are considered as the most significant factors [38]. From our results, we show that in addition to these parameters, substituent effect also plays an effective role to realize high efficiency of the device. 2CzP-INN and 2BTPA-INN have shown excellent stability which is beneficial for stable operation of blue and green OLED devices. Additionally, 2BTPA-INN exhibited a high Tg above 145.5 °C as displayed in Fig. S5. 2BTPA-INN also showed a high Td 480 °C, which indicates its high thermal stability (Fig. S5) (see Table 3). In addition to the emitters, host materials also have a significant impact on the device efficiency of OLED devices. For instance, a host with high ET (DPEPO) enhanced the EQEs of both the devices (2CzPINN and 2BTPA-INN) (Fig. 3). Apart from this, a suitable host can be useful for reducing the efficiency roll-off the emitters. Therefore, it is considered that the performances of the devices containing these TADF 5
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Table 2 Electroluminescence performance of 2CzP-INN and 2BTPA-INN devices. Device
VON (V)
Max. CE (cd/A)
Max. PE (lm/W)
Max. EQE (%)
CE/PE/EQE (V/cd A−1/lm W−1/%) at 100 cd m−2
2CzP-INN 2BTPA-INN
4.1 2.9
6.66 83.7
5.23 93.7
8.07 26.1
2.51/1.17/3.33 29.47/21.04/9.49
*VON is obtained at 1 cd m−2. Table 3 Comparison of this work with best reported work. Compound
VON
CE
PE
EQE
Ref.
2BTPA-INN 4CzCNPhPy DDCzIPN CCDD TXO-PhCz DPETPO 5CzCN Da-CNBQx AcDPA-2TP DDCzTrz
2.9 4.8 3.5 3.3 4.7 2.8 4.2 2.8 2.6 4.0
83.7 22.4 – 72.1 76.0 39.7 – 28.9 69.0 31.3
93.7 14.4 38.3 61.5 70.0 44.4 43.4 32.4 83.4 26.2
26.1 13.7 18.9 22.7 21.5 23 19.7 20.0 21.2 18.9
This Work 40 44 45 46 47 48 49 52 43
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emitters can be enhanced with appropriate selection of suitable host materials. 3. Conclusions In summary, two isonicotinonitrile based novel and efficient TADF emitters, 2CzP-INN and 2BTPA-INN were developed. Both materials exhibited ΔEST values of 0.42 and 0.33 eV, respectively. The green emitter 2BTPA-INN based device demonstrated a very high EQE of 26.1% with the CIE coordinates of (0.24, 0.55). A high CE of 83.7 cd/A with a high PE of 93.7 lm/W was achieved. Additionally, the device also operated at a very low VON of 2.9 V. This is among the excellent performances in green TADF OLED devices obtained so far. These results signify the bright potential of isonicotinonitrile based molecules as a TADF emitter. The enhancement in the device performance can be attributed to the appropriate tuning of the substituent moieties. Lastly, this design of materials will be useful to synthesize high performance materials for TADF-OLED devices. Acknowledgements This work was financially supported from the National Natural Science Foundation of China (21674123, 31700520), National Natural Science Foundation of Fujian Province (2018J01592), Project of “100 People Planning in Fujian Province”, New Century Excellent Talents in Fujian Province University (KLa17009A), International cooperation project of Fujian Agriculture and Forestry University (KXGH17003), and the Distinguished Young Scholars of Fujian Agriculture and Forestry University (No. xjq201729). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107633. Conflicts of interest The authors declare no conflicts of interest. References [1] Hirata S, Sakai Y, Masui K, Tanaka H, Lee SY, Nomura H, Nakamura N, Yasumatsu M, Nakanotani H, Zhang Q, Shizu K, Miyazaki H, Adachi C. Nat Mater 2014;14(3):330–6.
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Enhanced efficiency of thermally activated delayed fluorescence emitters by suitable substitution on isonicotinonitrile
T
Amjad Islama, Zhiheng Wangb, Shaomin Jic, Khurram Usmand, Syed Comail Abbasa, Jianguo Lia, Lihui Chena, Mudassir Iqbale, Shi-Jian Sub,∗∗, Xinhua Ouyanga,∗ a
College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, PR China State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China c School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, 510640, PR China d International Academy of Optoelectronics at Zhaoqing, South China Normal University, PR China e Department of Chemistry, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad, 44000, Pakistan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Isonicotinonitrile Emitter Thermally activated delayed fluorescence Organic light emitting diodes
Two novel thermally activated delayed fluorescence (TADF) emitters, 3,5-bis(4-(9H-carbazol-9-yl)phenyl) Isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]-4-yl)amino)phenyl)isonicotinonitrile (2BTPAINN), have been developed and their photophysical, electrochemical and electroluminescent properties have also been studied. Both materials possess high thermal stabilities and high photoluminescence quantum yields (PLQYs). Importantly, organic light emitting device (OLED) with 2BTPA-INN as emitter showed outstanding performance with a low driven voltage (VON) of 2.9 V, a high external quantum efficiency (EQE) of 26.1%, power efficiency (PE) of 93.7 lm/W and current efficiency (CE) of 83.7 cd/A, which is among the excellent performances for TADF OLEDs. The enhanced efficiency can be ascribed to the high PLQY. These results provide an optimum strategy to design efficient materials for TADF OLED devices.
1. Introduction Thermally activated delayed fluorescence (TADF) compounds have earned an immense attraction in the research area of organic lightemitting devices (OLEDs) over the last one decade owing to their capabilities to harvest both singlet and triplet excitons via reverse intersystem crossing (RISC) to realize theoretical 100% internal quantum efficiency [1–3]. So far, high-performance electroluminescent devices containing various TADF-emitters have been demonstrated and external quantum efficiency (EQE) up to 30% has been reported, which matches well with that of phosphorescent organic light-emitting diodes (PhOLEDs). For these efficient TADF emitters, a small energy gap (ΔEST) between the singlet energy states (ES) and triplet energy states (ET) is vital to achieve a RISC. However, the molecular design strategies of TADF emitters are still not fully clear with highly efficient RISC due to the restrictions of molecular systems. In this regard, it is highly desirable to develop novel TADF materials with different structures and substitutions to elucidate the relationship of structures and properties. To realize an optimum ΔEST, a minimum overlap of highest
∗
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of donor (D) and acceptor (A) moieties is required. Since the pioneer work of Adachi in the studies of TADF emitters, many advances have been reported for the materials synthesis and device fabrications [4,5]. To date, different kinds of acceptor moieties such as; sulphones [5,6], benzonitrile [3,7,8], ketones [9], 2,4,6-triphenylpyrimidine [10–16] and boron containing compounds [17–19] have been used to prepare TADF emitters. Among these structures, most of them were focused on the molecular conformation of D-D-A and D-A-D types to replace the traditional structure of D-A type with the aim of reduced ΔEST [1,11,18,20–26]. Generally, these D-D-A and D-A-D molecules were prepared with complicated synthetic procedure, which limits their application in low-cost and large-scale fabrication. Additionally, the small overlap between HOMO and LUMO often results in a forbidden S1–S0 transition, which decreases the radiative efficiency [22,27]. In view of this, the design and synthesis of novel TADF materials with suitable overlap of HOMO and LUMO are crucial to obtain high radiative efficiency. To get the suitable overlap of HOMO and LUMO, a feasible way is to
Corresponding author. Corresponding author. E-mail address: [email protected] (X. Ouyang).
∗∗
https://doi.org/10.1016/j.dyepig.2019.107633 Received 25 April 2019; Received in revised form 22 May 2019; Accepted 6 June 2019 Available online 07 June 2019 0143-7208/ © 2019 Published by Elsevier Ltd.
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OLEDs so far.
prepare these molecules with reasonable D and A blocks. Pyridine and its derivatives have been shown as potential candidates for acceptor unit owing to their high triplet energy levels (ET) compared to other reported molecules [11,28]. Moreover, their electron-withdrawing abilities can be improved significantly by incorporating different moieties like cyano (CN) group. Meanwhile, the nitrogen atoms from CN and pyridine units also play a key role to modify the extent of intramolecular charge transfer (ICT), inducing the TADF feature to these materials. However, a very few emitters based on pyridine-carbonitriles for efficient TADF process have been explored so far [29,30]. Therefore, the TADF compounds with pyridine-carbonitrile core are considered to have the opportunity to realize high efficiency TADF devices. Recently, isonicotinonitrile (INN) or 4-cyanopyridine, one of the typical pyridinecarbonitrile compounds, has been demonstrated as effective TADF material. For example, Zheng and colleagues prepared an efficient blue TADF material (CPC) containing isonicotinonitrile [30]. OLED device fabricated using CPC as emitter observed an EQE of 21% with current efficiency (CE) of 47.7 cd/A and power efficiency (PE) 42.8 lm/W. In another attempt to enhance the EQE of OLED devices, Kido and coworkers synthesized isonicotinonitrile based blue and green materials for TADF [31]. The TADF devices were fabricated using those materials showed high EQE of 22% [31]. On the other hand, electron-donor units with effective hole transporting properties are extensively employed to prepare electroluminescent materials in the past decades, especially for the TADF materials. Carbazole and triphenylamines (TPA) have been demonstrated as excellent electron-donor moieties to construct highly efficient TADF compounds, such as 2,3,5,6-tetrakis (carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) [3], and 2-(4'-(diphenylamino)-[1,1′-biphenyl]-4yl)quinoxaline-6,7-dicarbonitrile (TPA-QCN) [32]. The development of the materials containing hole-transporting units with high glass transition temperature (Tg), excellent film forming ability and good solubility in common solvents is urgently required to fabricate high performance TADF OLED devices. Furthermore, these functional pyridines with carbazole and triphenylamine moieties will create a twisted conformation, resulting in a high photoluminescence quantum yield (PLQY) [33]. Herein, two novel INN derivatives, 3,5-bis(4-(9H-carbazol-9-yl) phenyl)isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]4-yl)amino)phenyl)-isonicotinonitrile (2BTPA-INN) with meta-substitutions of 9-phenyl-9H-carbazole (CzP) and N-([1,1′-biphenyl]-4-yl)N-phenyl-[1,1′-biphenyl]-4-amine (BTPA) have been designed and synthesized and the molecular structures are shown in Scheme 1. Their electrochemical, thermal, photophysical and electroluminescent properties were studied systematically with high thermal stabilities, high PLQYs and relatively small ΔEST. Notably, by using 2BTPA-INN as emitter the TADF device showed high performance with a low driven voltage (VON) of 2.9 V, a high EQE of 26.1%, and a high PE of 93.7 lm/ W and CE of 83.7 cd/A, which is among the best performances for TADF
2. Results and discussions 2.1. Synthesis Both TADF emitters 3,5-bis(4-(9H-carbazol-9-yl)phenyl) Isonicotinonitrile (2CzP-INN) and 3,5-bis(4-(di ([1,1′-biphenyl]-4-yl) amino)phenyl)isonicotinonitrile (2BTPA-INN), were prepared in a single step through suzuki coupling aromatic reaction (as can be seen in Scheme 1) in a high yield of over 70%. After purification through column chromatography, the compounds were further purified using sublimation process and characterized by 1H NMR, 13C NMR (Figs. S1–S2) and mass spectrometry (MALDI-TOF) (Fig. S3). One of the aims to design 2CzP-INN and 2BTPA-INN was to construct efficient TADF devices by considering the benefit of high thermal and molecular stability of 2CzP-INN and 2BTPA-INN. Isonicotinonitrile is a stable acceptor unit owing to its expanded conjugation over the whole units of isonicotinonitrile. Carbazole and triphenylamine are also stable units with their aromatic characters. Like other TADF compounds, 2CzP-INN and 2BTPA-INN are also economical as compared to the phosphorescent materials. 1,2,3,5-tetrakis (carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) [2] is a very famous TADF emitter, consisting of phthalonitrile and carbazole moieties. High EQE of 30% [8] and high molecular stability [34] have already been realized using 4CzIPN as emitter. Owing to the strong electron-withdrawing ability, cyano group is utilized to construct efficient TADF emitters. 2.2. Electrochemical and thermal properties Electrochemical efficiencies of 2CzP-INN and 2BTPA-INN were determined through cyclic voltammetry. With the help of onset potential (of initial oxidation connected to ferrocene) the HOMO energy value was obtained, (Fig. S4). HOMO energy level is obtained through the equation of: EHOMO = (Eonset)OX + 4.84. The HOMO energy levels of 2CzP-INN and 2BTPA-INN molecules are −5.39 eV and −5.24 eV, subsequently. For better hole injection, the charge injection barrier should be tuned close to the HOMO level (−5.3 eV) of hole transport layer, 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC), facilitating the hole injection. The values of HOMO levels of both materials are near to that of TPAC. Furthermore, the HOMO level of 2BTPA-INN is higher by 0.15 eV compared to 2CzP-INN because of increased conjugation length afforded by the TPA group. The values of energy band gaps (Eg) of these materials are around 3.06 and 2.77 eV, acquired by using absorption spectrum, respectively. The difference between the HOMO and Eg gives out the values of the LUMO energy levels (−2.33 and −2.47 eV) of 2CzP-INN and 2BTPA-INN (Table 1). Decomposition temperature (Td) and glass transition temperature (Tg) of 2CzP-INN and 2BTPA-INN were measured through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in nitrogen environment. The schematic is shown in Fig. S5 and Table 1 represent the thermal data. The TGA indicates a high thermal stability of these materials with temperature range above 400 °C. For 2CzP-INN and BTPA-INN, a 5% (Td) loss of weight has been detected at 482 °C and 480 °C, subsequently. Both emitters were found to be thermally stable beyond 450 °C. The sublimation temperature of these materials is at least 150 °C lower than these values. No Tg was observed for 2CzP-INN, while 2BTPA-INN demonstrated a high Tg ∼145.45 °C. The twisted shape of the material is thought to be the major reason for higher values of Tg, that increases the stability of the molecules at high temperatures [35]. 2.3. Theoretical calculations An investigation of molecular orbital distribution is essential to explain the optical nature of 2CzP-INN and 2BTPA-INN emitters and is displayed in Fig. 1. Theoretical calculations were conducted by
Scheme 1. Synthetic mechanism of 2CzP-INN and 2BTPA-INN. 2
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Table 1 Photophysical performance of 2CzP-INN and 2BTPA-INN. Compound
Abs.a (nm)
PLa (nm)
Tdb, Tgb, (oC)
c,d
2CzP-INN 2BTPA-INN
341 338
435 480
482/N·O 480/145.5
−5.39/-5.48 −5.24/-5.01
a b c d e f g
HOMO (eV)
c,d
LUMO (eV)
−2.33/-2.21 −2.57/-1.88
e
Eg (eV)
3.06 2.67
f
ΔEST
0.20 0.13
g
ΔEST
0.42 0.33
Measured in toulene solution at room temperature. Glass transition temperature (Tg) determined by DSC measurement, and decomposition temperature with 5% weight loss (Td) obtained from TGA analysis. HOMO measured from the onset of oxidation curves from CV and LUMO calculated from the difference between HOMO and Eg. Theoretically calculated values from DFT using gaussian software. Optical bandgap (Eg) calculated from the absorption edge of UV–Vis spectra. Theoretically calculated values from DFT using gaussian software from the difference between singlet and triplet energy. Calculated experimentally from the difference between singlet and triplet energy measured from fluorescence and phosphorescence spectra.
Fig. 1. Molecular structure and HOMO-LUMO level distribution of 2CzP-INN and 2BTPA-INN.
applying density functional theory (DFT) at the B3LYP/6-31G(d)/level using Gaussian 09 W to analyze the geometric conformation, the HOMO and LUMO energy levels, ES, ET, and ΔEST of 2CzP-INN and 2BTPA-INN (Fig. 1, and Table 1). According to the DFT calculations, ES of 2CzP-INN and 2BTPA-INN molecules were found to be at 2.8591 and 2.699 eV and ET were found to be at 2.7327 and 2.499 eV, respectively. A small energy difference (ΔEST) of 0.1264 and 0.200 eV was obtained, which indicated blue and green TADF emission. The electron density in Fig. 1 reveals that the HOMO level of 2CzP-INN was totally distributed on carbazole and phenyl units, whereas LUMO level was completely located on isonicotinonitrile unit. A very weak overlapping is observed in 2CzP-INN molecule. In the case of 2BTPA-INN molecule, the HOMO level was localized on the donor moiety (triphenylamine), and the LUMO level was populated on the acceptor moiety (isonicotinonitrile). As anticipated, both, HOMO and LUMO levels were completely disintegrated with each other and a very weak overlapping was found between them. The distribution between HOMO and LUMO of 2CzP-INN and 2BTPA-INN is very much suitable for TADF emission. Both, 2CzPINN and 2BTPA-INN molecules show a little twisted configuration and maintain the π-conjugation length in control. The π-conjugation is important for the delocalization of HOMO and LUMO orbitals in the charge transfer molecules, because the rate of radiative decay depends on this delocalization of HOMO and LUMO [1].
Fig. 2. (a) UV–Vis absorption and PL spectra of 2CzP-INN and 2BTPA-INN in toluene solution (10 −5 M); (b) Low temperature PL spectra (77K) of 2CzP-INN and 2BTPA-INN.
solution of toluene. Fig. 2 presents the UV/Vis absorption spectrum of the 2CzP-INN and 2BTPA-INN molecule measured at room temperature. Maximum absorption for both materials lies in the range of 300–400 nm. 2CzP-INN exhibited a maximum absorption peak at 341 nm, whereas maximum absorption of 2BTPA-INN was obtained at almost 338 nm. These peaks can be ascribed to the independent absorption of D-A units. The other absorption peaks of 2CzP-INN and
2.4. Photophysical properties ICT is an outstanding characteristic of TADF molecules, which can be examined through Ultraviolet/Visible (UV) absorption spectroscopy. DFT calculations in Fig. 1 are associated with the optical parameters of these TADF materials such as ES, ET, and PLQY. Photophysical properties of 2CzP-INN and 2BTPA-INN molecule were measured in a dilute 3
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2BTPA-INN at 360 nm and 400 nm, subsequently, can be assigned to the intra-molecular charge-transfer (ICT) from the electron donating carbazole and triphenylamine group to the electron accepting isonicotinonitrile group. Similar trend in absorption spectra has already been realized in the previous reports [31,36]. Photoluminescence spectra of these derivatives are also presented in Fig. 2. 2CzP-INN exhibited deep blue emission with the maximum peak located at almost 435 nm, whereas maximum emission of 2BTPA-INN was appeared at 480 nm at room temperature. The emission of 2BTPA-INN was more red-shifted as compared to 2CzP-INN owing to the extension of conjugation through the addition of phenyl ring on the TPA moiety [2,37]. We also investigated the low temperature (77K) emission of these materials. This trend of PL emission peak matched well with those values which were projected from theoretical calculations. The ΔEST values were measured through the emission onsets as 0.42 eV for 2CzPINN and 0.33 eV for 2BTPA-INN at 77 K (Fig. 2). The 2CzP-INN and 2BTPA-INN compounds also demonstrated high PLQYs of 0.775 and 0.897, respectively, in (degassed) toluene solution. The PLQY values of 10 wt% DPEPO films of 2CzP-INN and 2BTPA-INN were found to be 0.690 and 0.890, subsequently. The high PLQYs are an indication of the involvement of triplet excitons in the PL emission probably due to TADF process. The emission lifetime of 2CzP-INN and 2BTPA-INN in toluene consists of a prompt (14–20 ns), however, their delayed component are too weak to be resolved as it coincides with the instrument response function (Fig. 3a). Thus, the delayed fluorescence in the studied compounds in toluene is smaller than we can reliably detect. For the TADF behavior of devices, they were investigated by measuring transient PL decay curves of 10 wt% 2CzP-INN and 2BTPA–INN–doped DPEPO films showed a double-exponential decay (Fig. 3b and c). A fast prompt decay (15 ns) and several very long delayed emission components are found. The dominant delayed components were estimated to be 3.6 ms (77%, amplitude in multi-exponential fit to data) and 46.2 ms (14%) for compound 2CzP-INN, 1.9 ms (74%) and 28.7 ms (19%) for 2BTPA-INN, respectively. The spectroscopic evidence signifies that these compounds are indeed TADF emitters. The summary of thermal and photophysical properties is given in Table 1. 2.5. Electroluminescence performance To examine the potential of 2CzP-INN and 2BTPA-INN as emitter, OLED devices were constructed the device with the structure of ITO (95 nm)/TAPC (30 nm)/mCP (10 nm)/EML (25 nm)/DPEPO (10 nm)/ TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm), where 1,1-bis [(di-4-tolylamino)phenyl]cyclohexane (TAPC) is served as hole transport material, 3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3′,1″-terphenyl]-3,3″-diyl)dipyridine (TmPyPB) was employed as electron transport layer as well as hole blocking layer and LiF was acted as electron injection material. N, N ′ -dicarbazolyl-3,5-benzene (mCP) was utilized as electron and exciton blocking layer to confine triplet excitons. The molecular structures of different materials employed in the device fabrication process are displayed in Fig. S6 mCP [38,39] is a well-known electron blocking material and has demonstrated excellent results in many high performance TADF devices. mCP possesses a high ET1 value (2.9 eV) which contributes to confine the excitons within the EML and minimize the efficiency roll-off [40]. Bis [2-(diphenylphosphino)phenyl]ether oxide (DPEPO) was served as the host material. A combination of mCP and DPEPO was used which is beneficial for TADF dopant. [41] A doping ratio of 10 wt% of 2CzP-INN and 2BTPA-INN emitters was used. Basically, the idea of using mCP and DPEPO simultaneously, was to confine the holes and electrons within the emissive layer (EML), resulting in a balanced charge transportation. As the ET of both, mCP and DPEPO were higher than 2CzP-INN and 2BTPA-INN emitters, triplet exciton quenching of emitter was reduced and consequently, OLED device performance was improved. 2CzP-INN based device showed deep-blue electroluminescence (EL) with a maximum peak at 440 nm with CIE of (0.15, 0.09) (Fig. 4). 2CzP-INN device achieved an EQE of 8.07% with
Fig. 3. The photo-luminance transient decay curves of 2CzP-INN and 2BTPAINN doped in DPEPO films (10 wt %) at 77 K and 298K.
CE of 6.66 cd/A and PE of 5.23 lm/W (Fig. 4). VON was observed to be 4.1 V for 2CzP-INN device. The 2BTPA-INN based device showed electroluminescence peak at 512 nm with CIE coordinates of (0.24, 0.55) (Fig. 4). EL spectrum of 2BTPA-INN was red-shifted compared to 4
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Fig. 4. (a) Current density and luminance versus driving voltage (J-V-L); (b) current efficiency versus luminance; (c) power efficiency versus luminance; (d) external quantum efficiency (EQE) versus luminance characteristics and (e) EL spectra of the devices based on 2CzP-INN and 2BTPA-INN at 1 mA cm−2 doped with DPEPO (B1 & B2).
2CzP-INN spectrum due to the presence of extra phenyl ring on the TPA unit. The EL spectrum is generated from the emissive layer and no emission was observed from other materials. EQE versus luminance curves of the 2BTPA-INN based devices are presented in Fig. 4. The maximum EQE of the 2BTPA-INN device was realized to be 26.1%. But CE and PE of the 2BTPA-INN device were significantly high (83.7 cd A−1 and 93.7 lm/W) (Fig. 4), subsequently. More interestingly, the VON was dropped from 4.1 V to 2.9 V. This value is almost five times higher than the spin statistic limit (5% theoretical) of EQE for typical fluorescent emitters and is a demonstration of efficient triplet excitons harvesting for EL emission through TADF process. The low efficiency of 2CzP-INN based device was attributed to low PLQY of 2CzP-INN. This efficiency of 2BTPA-INN device was better than many other TADF devices in terms of VON and EQE. These are not only among the best results achieved for green TADF OLEDs but also among other colors OLED devices till date [41–49]. The hole injection barrier from TAPC to the EML is very less (0.1–0.2 eV) which facilitated better hole injection. On the other hand, the electron injection barrier from TmPyPB to EML was also less (0.3 and 0.03 eV) which improved the electron injection and charge balance. Owing to its wide Eg (4.20 eV) and high ET (3.00 eV), DPEPO prevents reverse transfer of triplet excitons from dopant to host. Additionally, VON of 2BTPA-INN device is significantly reduced to 2.9 from 4.1 V. High PE of 93.7 lm/W and CE of 83.77 cd/A reflects the improved charge injection and transportation in 2BTPA-INN device owing to the better charge trapping by 2BTPA-INN dopant, and it can be emulated by the reduced VON with increased doping ratio. However, in both devices, considerable efficiency roll-off was observed. This is very common with the TADF emitters having long lifetime [2]. Generally, two types of exciton formation mechanisms exist in OLED devices. In first mechanism, energy is transferred from host to dopant, whereas in the other case, charges are directly recombined in the dopant layer [50–55]. In both devices, as the HOMO levels of 2CzP-INN and 2BTPAINN dopants are shallower compared to DPEPO hosts, exciton formation in 2CzP-INN and 2BTPA-INN would preferably take place through direct recombination of charges. In this case, the enhanced efficiencies of 2BTPA-INN based device should be primarily attributed to the effectively suppressed triplet excitons quenching. The high efficiency of 2BTPA-INN device is attributed to the high PLQY and efficient process of RISC culminated from relatively small ΔEST of 2BTPA-INN with an additional phenyl ring in TPA moiety, confirming that highly efficient TADF materials can be prepared through appropriate incorporation of substituents. The electroluminescence performances are displayed in Table 2. In the development of stable materials for OLEDs, planar geometrical conformation and stable groups are considered as the most significant factors [38]. From our results, we show that in addition to these parameters, substituent effect also plays an effective role to realize high efficiency of the device. 2CzP-INN and 2BTPA-INN have shown excellent stability which is beneficial for stable operation of blue and green OLED devices. Additionally, 2BTPA-INN exhibited a high Tg above 145.5 °C as displayed in Fig. S5. 2BTPA-INN also showed a high Td 480 °C, which indicates its high thermal stability (Fig. S5) (see Table 3). In addition to the emitters, host materials also have a significant impact on the device efficiency of OLED devices. For instance, a host with high ET (DPEPO) enhanced the EQEs of both the devices (2CzPINN and 2BTPA-INN) (Fig. 3). Apart from this, a suitable host can be useful for reducing the efficiency roll-off the emitters. Therefore, it is considered that the performances of the devices containing these TADF 5
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Table 2 Electroluminescence performance of 2CzP-INN and 2BTPA-INN devices. Device
VON (V)
Max. CE (cd/A)
Max. PE (lm/W)
Max. EQE (%)
CE/PE/EQE (V/cd A−1/lm W−1/%) at 100 cd m−2
2CzP-INN 2BTPA-INN
4.1 2.9
6.66 83.7
5.23 93.7
8.07 26.1
2.51/1.17/3.33 29.47/21.04/9.49
*VON is obtained at 1 cd m−2. Table 3 Comparison of this work with best reported work. Compound
VON
CE
PE
EQE
Ref.
2BTPA-INN 4CzCNPhPy DDCzIPN CCDD TXO-PhCz DPETPO 5CzCN Da-CNBQx AcDPA-2TP DDCzTrz
2.9 4.8 3.5 3.3 4.7 2.8 4.2 2.8 2.6 4.0
83.7 22.4 – 72.1 76.0 39.7 – 28.9 69.0 31.3
93.7 14.4 38.3 61.5 70.0 44.4 43.4 32.4 83.4 26.2
26.1 13.7 18.9 22.7 21.5 23 19.7 20.0 21.2 18.9
This Work 40 44 45 46 47 48 49 52 43
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emitters can be enhanced with appropriate selection of suitable host materials. 3. Conclusions In summary, two isonicotinonitrile based novel and efficient TADF emitters, 2CzP-INN and 2BTPA-INN were developed. Both materials exhibited ΔEST values of 0.42 and 0.33 eV, respectively. The green emitter 2BTPA-INN based device demonstrated a very high EQE of 26.1% with the CIE coordinates of (0.24, 0.55). A high CE of 83.7 cd/A with a high PE of 93.7 lm/W was achieved. Additionally, the device also operated at a very low VON of 2.9 V. This is among the excellent performances in green TADF OLED devices obtained so far. These results signify the bright potential of isonicotinonitrile based molecules as a TADF emitter. The enhancement in the device performance can be attributed to the appropriate tuning of the substituent moieties. Lastly, this design of materials will be useful to synthesize high performance materials for TADF-OLED devices. Acknowledgements This work was financially supported from the National Natural Science Foundation of China (21674123, 31700520), National Natural Science Foundation of Fujian Province (2018J01592), Project of “100 People Planning in Fujian Province”, New Century Excellent Talents in Fujian Province University (KLa17009A), International cooperation project of Fujian Agriculture and Forestry University (KXGH17003), and the Distinguished Young Scholars of Fujian Agriculture and Forestry University (No. xjq201729). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107633. Conflicts of interest The authors declare no conflicts of interest. References [1] Hirata S, Sakai Y, Masui K, Tanaka H, Lee SY, Nomura H, Nakamura N, Yasumatsu M, Nakanotani H, Zhang Q, Shizu K, Miyazaki H, Adachi C. Nat Mater 2014;14(3):330–6.
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