Highly efficient thermally activated delayed fluorescence emitters based on novel Indolo[2,3-b]acridine electron-donor

Article abstract of DOI:10.1016/j.orgel.2018.03.024

Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have attracted much interest in recent years for their great potential in full exciton utilization. However, the lack of electron-donor candidates limi

Full text of DOI:10.1016/j.orgel.2018.03.024

OrganicElectronics57(2018)327–334
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Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Highly efficient thermally activated delayed fluorescence emitters based on
novel Indolo[2,3-b]acridine electron-donor
Jia-Xiong Chena, Wen-Wen Taoa, Kai Wanga,∗∗∗, Cai-Jun Zhengb,∗∗, Wei Liua, Xing Lia,
Xue-Mei Oua, Xiao-Hong Zhanga,∗
a
Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou,
Jiangsu 215123, PR China
b
School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu, Sichuan 610054, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Organic light emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have
attracted much interest in recent years for their great potential in full exciton utilization. However, the lack of
electron-donor candidates limits further development of TADF emitters. In this study, a novel electron-donor segment
13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine (IDAC) is newly developed to construct TADF
emitters. Due to the suitable steric hindrance and large planar rigid structures, IDAC derivatives could easily obtain
small energy differences (ΔESTs) between the lowest singlet and triplet excited states for effective TADF character-
istics as well as high photoluminance quantum yields. Two novel TADF emitters, 6-(13,13-dimethyl-5-phenyl-5,13-
dihydro-11H-indolo[2,3-b] acridin-11-yl)-3-methyl-1H-isochromen-1-one (IDAC-MCO) and 11-(4-(4,6-diphenyl-
1,3,5-triazin-2-yl)phenyl)-13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine (IDAC-Trz), are accord-
ingly designed and synthesized, and both exhibit evident TADF characteristics with small ΔESTs of 0.09 and 0.02 eV
and high photoluminance quantum yields of 81% and 90%, respectively. The OLEDs using IDAC-MCO and IDAC-Trz
as emitters respectively achieve high external quantum efficiencies of 18.0% and 20.5% with remarkable low effi-
ciency roll-offs. These results demonstrate the capacity of IDAC segment as an ideal electron-donor to construct
efficient TADF emitters.
Organic light emitting diode
Thermally activated delayed fluorescence
Indolo[2,3-b]acridine
Low efficiency roll-off
Electron-donor segment
1. Introduction
mechanism brought a new possibility to realize full exciton utilization
with pure organic emitters and has become the most attractive ap-
Organic light emitting diodes (OLEDs) are considered as the next
generation technology for display and lighting applications due to their
unique advantages of full-color emission, high efficiency, high bright-
ness and flexibility [1–6]. In these devices, electrons and holes are re-
spectively injected in cathodes and anodes, and then recombine at the
emitting layers to form singlet and triplet excitons with a ratio of 1:3,
according to spin statistics [7–9]. Generally, for conventional fluor-
escent OLEDs, only singlet excitons can be used for photon generation,
resulting in a limited maximum internal quantum efficiency (IQE) of
25% [9–11]. While for phosphorescent OLEDs, both singlet and triplet
excitons can potentially be used due to the strong spin-orbit coupling
effect of the noble metal ions, thus realizing a theoretically 100% IQE
[12–14]. However, the high cost and rarity of these noble metals are
undesirable for practical productions [15–17].
proach for future OLED applications [18–20]. TADF emitters generally
have extremely small energy differences (ΔESTs) between the lowest
excited singlet (S1) and triplet (T1) states, and the originating triplet
excitons can be up-converted into radiative singlets via an efficient
reverse intersystem crossing (RISC) process [21–24]. To realize this
point, it is essential to reduce the overlaps between the highest occu-
pied molecular orbitals (HOMOs) and lowest unoccupied molecular
orbitals (LUMOs) [25–27]. Besides,
a high photoluminance (PL)
quantum efficiency (ΦPLQY) is also beneficial to realize efficient TADF
process [28]. Current TADF emitters are generally constructed based on
electron donor (D) - electron acceptor (A) frameworks since their
HOMO and LUMO could respectively localize on D and A segments,
beneficial to realize a small ΔEST [29]. Meanwhile, a moderate overlap
between HOMO and LUMO is essential to realize the efficient ICT
process and thus high ΦPLQY [30]. Thus, for these emitters, the
Recently, thermally activated delayed fluorescence (TADF)
∗
Corresponding author.
Corresponding author.
Corresponding author.
∗∗
∗∗∗
E-mail addresses: wkai@suda.edu.cn (K. Wang), zhengcaijun@uestc.edu.cn (C.-J. Zheng), xiaohong_zhang@suda.edu.cn (X.-H. Zhang).
https://doi.org/10.1016/j.orgel.2018.03.024
Received 15 February 2018; Received in revised form 11 March 2018; Accepted 12 March 2018
Availableonline13March2018
1566-1199/©2018ElsevierB.V.Allrightsreserved.

J.-X. Chen et al.
OrganicElectronics57(2018)327–334
development of constituent D and A segments is one of the most im-
portant issues to dominate their properties [15]. In last few years, great
effort has been attempted to develop new segments for TADF emitters
[2,31]. However, most of these studies are focused on the evolution of A
segments [31,32]. Correspondingly, current D candidates are generally
limited in a few conventional segments such as carbazole [33], tri-
phenylamine [34], acridine [35], phenoxazine [36] and phenothiazine
[37], while they play important roles in deciding the key properties of
the TADF emitters. In particular, with the steric hindrance enforced by
D groups gradually increased from weak restricted triphenylamine, to
carbazole with moderate restrictions and highly restricted acridine,
phenoxazine, the overlaps between HOMOs and LUMO show a regular
decline, resulting in a more efficient RISC process [33–36]. While on
the other hand, the decreasing overlaps are harmful to fluorescence
process and thus undesired to obtain high ΦPLQYs [28]. Therefore,
current D groups hardly benefit RISC and fluorescence processes si-
multaneously, such lack of D candidates greatly limits further devel-
opment of effective TADF emitters.
DMAC-Ph (2 g, 7 mmol) is added into a round-bottom flask (250 mL)
within 100 mL N,N-dimethylformamide (DMF), then 1-bromopyrroli-
dine-2,5-dione (NBS, 1.23 g, 7 mmol) in 50 mL DMF is slow added to
the solution at 0 °C. Upon completion, the mixture is added into ice-cold
and filtration. The crude product is purified by column chromatography
on silica gel to give the desired product as a white solid (2 g, 78.33%):
1H NMR (600 MHz, CDCl3) δ 7.67–7.60 (m, 2H), 7.56–7.49 (m, 2H),
7.44 (ddd, J = 7.6, 3.1, 1.6 Hz, 1H), 7.31 (ddd, J = 8.2, 3.0, 1.4 Hz,
2H), 7.04 (ddd, J = 8.8, 3.4, 2.0 Hz, 1H), 7.01–6.88 (m, 2H), 6.26 (dd,
J = 8.1, 1.4 Hz, 1H), 6.14 (dd, J = 8.8, 1.4 Hz, 1H), 1.67 (d, J = 3.0 Hz,
6H); MS (EI) m/z: [M]+ calcd for C21H18BrN 363.06, found 363.26.
N-(2-chlorophenyl)-9,9-dimethyl-10-phenyl-9,10-dihydroacridin-2-
amine (CPA-AC-Ph). CPA-AC-Ph is synthesized by the similar proce-
dure as for DMAC-Ph with Br-AC-Ph (1.8 g, 5 mmol) and 2-chloroani-
line instead of Br-Ph and DMAC. Yield: (1.27 g, 62%): 1H NMR
(600 MHz, CDCl3) δ 7.88–7.28 (m, 9H), 7.08 (d, J = 84.8 Hz, 6H), 6.12
(s, 2H), 1.61 (s, 6H). MS (EI) m/z: [M]+ calcd for C27H23ClN2 410.15,
found 410.31.
In this work, we developed a novel D segment 13,13-dimethyl-5-
phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine (IDAC) as the new
candidate to construct TADF emitters. On one hand, IDAC can provide a
suitable steric hindrance when connecting A segments to separate
HOMOs and LUMOs, which is essential to realize TADF characteristic.
On the other hand, the steric hindrance also encourages suitable
overlaps between HOMO and LUMO, which are highly desired for in-
tramolecular charge transfer (ICT) process [30]. Moreover, due to the
large planar rigid structures of IDAC, IDAC-based emitters could exhibit
large delocalization of HOMOs in structures, further benefiting to ob-
tain small ΔEST and thus result in effective TADF characteristic [25].
Based on IDAC, two novel TADF emitters 6-(13,13-dimethyl-5-phenyl-
5,13-dihydro-11H-indolo[2,3-b]acridin-11-yl)-3-methyl-1H-iso-
chromen-1-one (IDAC-MCO) and 11-(4-(4,6-diphenyl-1,3,5-triazin-2-
yl)phenyl)-13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]
acridine (IDAC-Trz) were successfully obtained with two different A
segments, 3-methyl-1H-isochromen-1-one (MCO) and 2,4,6-triphenyl-
1,3,5-triazine (Trz). They both have ideal twisted angles about 50°
between D and A segments and well-separated frontier orbitals, thus
leading simultaneously high ΦPLQY values and small ΔESTs of 81% and
0.09 eV for IDAC-MCO and 90% and 0.02 eV for IDAC-Trz, respectively.
The OLEDs based on IDAC-MCO and IDAC-Trz respectively realized
maximum external quantum efficiencies (EQEs) up to 18% and 20.5%
with remarkable low efficiency roll-offs. These results prove IDAC
segment as an ideal D candidate to construct efficient TADF emitters.
13,13-dimethyl-5-phenyl-11,13-dihydro-5H-indolo[2,3-b]acridine
(IDAC). CPA-AC-Ph (0.82 g, 2 mmol), 1,3-bis(2,6-di-i-propylphenyl)
imidazolium chloride (129 mg, 0.3 mmol), palladium acetate (22.4 mg,
0.1 mmol), potassium carbonate (0.55 g, 4 mmol) and 40 mL of N,N-
dimethylacetamide is added into a round-bottom flask (100 mL) under
N2. Then, the mixture is stirred at 130 °C for 24 h. After completion of
the reaction, water and DCM are added to the cooled mixture. The
organic layer is separated and dried over Na2SO4, then concentrated in
vacuo. The residue solid is purified by column chromatography to give
the product as white solid (0.47 g, 61.1%): 1H NMR (600 MHz, CDCl3).
δ 10.91 (s, 1H), 7.74 (t, J = 7.8 Hz, 2H), 7.61 (dd, J = 7.5, 5.1 Hz, 2H),
7.57 (s, 1H), 7.49 (dd, J = 7.7, 1.2 Hz, 1H), 7.44 (d, J = 7.2 Hz, 2H),
7.37 (d, J = 8.1 Hz, 1H), 7.24 (s, 1H), 6.97–6.93 (m, 2H), 6.87 (s, 1H),
6.80 (s, 1H), 6.21 (d, J = 8.1 Hz, 1H), 1.67 (s, 6H). 13C NMR (151 MHz,
CDCl3) δ 142.54, 141.56, 140.24, 137.13, 135.30, 133.95, 131.35,
131.00, 129.51, 128.34, 126.63, 125.97, 124.63, 123.96, 122.43,
120.42, 118.51, 114.00, 109.84, 109.34, 105.43, 104.82, 37.02, 30.39.
MS (EI) m/z: [M]+ calcd for C28H25N2 389.20, found 389.33.
6-(13,13-dimethyl-5-phenyl-5,13-dihydro-11H-indolo[2,3-b]ac-
ridin-11-yl)-3-methyl-1H-isochromen-1-one (IDAC-MCO) and 11-(4-
(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-13,13-dimethyl-5-phenyl-
11,13-dihydro-5H-indolo[2,3-b]acridine (IDAC-Trz) are synthesized
using the similar procedure as for CPA-AC-Ph. With that, Br-AC-Ph and
2-chloroaniline are replace by Br-MCO or Br-Trz and IDAC, respec-
tively.
(IDAC-MCO) yield: (0.18 g, 67%): 1H NMR (600 MHz, CDCl3). δ
8.49 (d, J = 8.4 Hz, 1H), 7.79 (d, J = 7.8 Hz, 1H), 7.73–7.68 (m, 3H),
7.63–7.55 (m, 3H), 7.49–7.43 (m, 4H), 7.33 (dd, J = 11.2, 4.0 Hz, 1H),
7.16 (t, J = 7.4 Hz, 1H), 7.02–6.91 (m, 3H), 6.38–6.31 (m, 2H), 2.34 (s,
3H), 1.71 (s, 6H). 13C NMR (151 MHz, CDCl3) δ 162.29, 155.80,
144.28, 141.57, 140.34, 139.52, 137.04, 135.43, 131.73, 131.39,
131.24, 130.97, 129.54, 128.31, 126.40, 125.89, 125.23, 124.63,
123.95, 122.41, 121.21, 120.46–119.99 (m), 117.64, 113.97, 109.53,
105.67, 104.76, 103.31, 37.00, 30.40, 19.79. MALDI-TOF MS (mass m/
z): 532.33 [M]+. Calcd for C37H28N2O2 532.22.
2. Experimental section
All the organic materials were purchased from various sources and
used as received.
2.1. Synthesis
9,9-dimethyl-10-phenyl-9,10-dihydroacridine (DMAC-Ph). Bromo
benzene (Br-Ph, 1.88 g, 12 mmol), 9,9-dimethyl-9,10-dihydroacridine
(2.1 g, 10 mmol), cesium carbonate (6.6 g, 20 mmol), tri-tert-butylpho-
sphane (300 mg, 1.5 mmol) and palladium (II) acetate (112 mg,
0.5 mmol) are dissolved in 200 mL of toluene under N2. Then, the
mixture is stirred to reflux overnight. After completion of the reaction,
dichloromethane (DCM) and water are added to the cooled mixture.
The organic layer is separated and dried over MgSO4, then concentrated
in vacuo. The residue solid is purified by column chromatography to
give the product as white solid (2.29 g, 80%): 1H NMR (600 MHz,
CDCl3) δ 7.62 (t, J = 7.8 Hz, 2H), 7.54–7.47 (m, 1H), 7.45 (dd, J = 7.6,
1.7 Hz, 2H), 7.36–7.30 (m, 2H), 6.96 (ddd, J = 8.4, 7.1, 1.6 Hz, 2H),
6.91 (td, J = 7.4, 1.4 Hz, 2H), 6.25 (dd, J = 8.1, 1.3 Hz, 2H), 1.69 (s,
6H). MS (EI) m/z: [M]+ calcd for C21H19N 285.15, found 285.28.
2-bromo-9,9-dimethyl-10-phenyl-9,10-dihydroacridine (Br-AC-Ph).
(IDAC-Trz) yield: (0.21 g, 62%): 1H NMR (600 MHz, CDCl3). δ 9.05
(d, J = 8.4 Hz, 2H), 8.86–8.82 (m, 4H), 7.83 (dd, J = 7.9, 4.2 Hz, 3H),
7.72 (dd, J = 15.4, 7.7 Hz, 3H), 7.67–7.58 (m, 7H), 7.51 (dd, J = 23.3,
7.9 Hz, 4H), 7.36 (d, J = 7.6 Hz, 1H), 7.17 (t, J = 7.4 Hz, 1H),
7.04–6.98 (m, 2H), 6.94 (d, J = 7.4 Hz, 1H), 6.38 (d, J = 8.2 Hz, 1H),
1.74 (s, 6H). 13C NMR (151 MHz, CDCl3) δ 171.77, 170.96, 142.12,
141.70 (d), 140.74, 136.72, 136.15, 135.88, 134.36, 132.63, 131.46,
131.19, 130.96, 130.67, 129.69, 129.00, 128.69, 128.25, 126.32 (d),
125.76, 124.69, 123.70, 122.16, 120.19 (d), 119.87, 113.94, 109.73,
105.87, 104.74, 37.05, 30.50. MALDI-TOF MS (mass m/z): 681.35
[M]+. Calcd for C48H35N5 681.29.
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OrganicElectronics57(2018)327–334
Scheme 1. Synthetic routes of IDAC, IDAC-MCO and IDAC-Trz.
2.2. Characterization
spectra, and EL luminance were recorded using a Spectrascan PR655
photometer, and the current density-voltage-luminance characteristics
of the devices were measured using a computer-controlled Keithley
2400 Source Meter. EQE was calculated from the luminance, EL spec-
trum and current density, assuming a Lambertian distribution.
Nuclear magnetic resonance (NMR) spectra were recorded in CDCl3
with tetramethylsilane as the internal standard for 1H NMR and 13C
NMR. Mass spectra data were achieved via Trace-ISQ gas chromato-
graphy-mass spectrometer for intermediate products and Bruker auto-
flex MALDI-TOF mass spectrometer for target products. Thermal
gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
measurement were using TAQ 500 thermogravimeter and DSC1 dif-
ferential scanning calorimeter. Absorption spectra were recorded on a
Hitachi UV–Vis spectrophotometer U-3900 and PL spectra were ob-
tained by using a Hitachi fluorescence spectrometer F-4600, respec-
tively. Transient photoluminance decays and photoluminance quantum
yield were measured with Edinburgh Instruments FLS980 spectrometer.
Cyclic voltammetry was carried out using CHI660E electrochemical
analyzer with a Pt disk as the working electrode, a silver chloride
electrode as the reference electrode at a scan rate of 50 mV s−1 with
0.1 M tetrabutylammonium perchlorate as the supporting electrolyte
and dichloromethane (DCM) as solvent.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis routes of the IDAC-based compounds are shown in
Scheme 1, and the detailed synthetic procedures are described in the
Experimental Section. IDAC is synthesized in four steps by starting with
the 9,9-dimethylacridine (DMAC). Then, Buchwald-Hartwig cross-cou-
pling reaction is taken between IDAC and the selected A segments to get
target compounds. All the compounds are further confirmed via mass
spectrometry technology and nuclear magnetic resonance spectroscopy.
3.2. Thermal analysis
2.3. Device fabrication
To examine the thermal stability of the IDAC-based compounds,
TGA and DSC were carried out and the results are shown in Fig. 1. It is
found that two compounds exhibit excellent thermal stability with high
decomposition temperatures (Td, defined at the temperature that the
materials show a 5% weight loss) around 400 °C. Moreover, high glass
transition temperatures (Tg) of 128 °C and 151 °C can also be obtained
for IDAC-MCO and IDAC-Trz, respectively. These results reveal two
compounds both have high thermal stability and suitable for vapor
depositions.
Indium tin oxide (ITO)-coated glass substrates with a sheet re-
sistance of 15 Ω per square were cleaned with acetone and deionized
water, then drying in an oven at 100 °C, and subjected to UV-ozone
treatment, and finally transferred to a vacuum deposition system at a
base pressure of about 4 × 10−4 Pa. All organic layers were deposited
onto the ITO substrates at an evaporation rate of 1–2 Å s−1. The
cathode layer was completed via the thermal deposition of LiF at a rate
of 0.1 Å s−1 and Al at 10 Å s−1, respectively. CIE color coordinates,
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OrganicElectronics57(2018)327–334
investigate the frontier orbital energy levels of two compounds, and the
results are shown in Fig. 3a and listed in Table 1. From the onset of the
oxidation curves respect to that of ferrocene, the HOMO energy levels
of IDAC-MCO and IDAC-Trz are estimated to be similar and shallow
values of −5.20 and −5.17 eV, respectively, indicating the strong
electron-donating ability of IDAC segments. While their LUMO energy
levels are respectively calculated to be −2.83 and −2.90 eV by esti-
mating from the onset of the reduction curves [38]. These results are
well consistent with the different electron-withdrawing abilities of A
segments.
3.5. Photophysical properties
The photophysical properties of two IDAC-based compounds were
then investigated. Fig. 3b shows the room-temperature UV–visible
(UV–vis) absorption spectra of these molecules in toluene solution. Both
the compounds exhibit broad and weak bands at the absorption cut-off
regions, which can be attributed to the ICT transitions from the IDAC
segments to the A segments. 10 wt% IDAC-MCO doped bis[2-(diphe-
nylphosphino)phenyl]ether oxide (DPEPO) and 9 wt% IDAC-Trz doped
4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) films are then prepared to
further investigate their characteristics in host-guest conditions. As
shown in Fig. 3b, both the PL spectra exhibit broad and structureless
emission, indicating their ICT characteristics. Specifically, with ac-
ceptor strength increased, the emission peaks shifted from 498 nm for
IDAC-MCO to 533 nm for IDAC-Trz, indicating well adjustability of
IDAC derivatives. Phosphorescent spectra of the IDAC derivatives
doped films at 77 K are shown in Fig. 3c, broad and structureless
emissions of phosphorescence spectra are observed for both IDAC-MCO
and IDAC-Trz, indicating their T1 states are also characterized as charge
transfer characters (3CT). Thus, the T1 energy levels of IDAC-MCO and
IDAC-Trz are estimated to be 2.71 and 2.61 eV, respectively. And from
Fig. 1. TGA and DSC (inset) curves for IDAC-MCO and IDAC-Trz recorded at a
heating rate of 10 °C min−1 under N2.
3.3. Theoretical calculations
To gain an insight of characteristic of two IDAC derivatives, density
functional theory (DFT) calculations were carried out on their ground
states at the B3LYP/6-31G (d) level. As shown in Fig. 2, for both IDAC-
MCO and IDAC-Trz, the HOMOs are mainly distributed over the IDAC
segment; whereas the LUMOs are mainly located on their corre-
sponding A segments. Obviously, due to the steric hindrance between
IDAC and A segments, IDAC-MCO and IDAC-Trz have twisted angles of
49.5° and 51.2°, respectively, enough to ensure their small overlaps
between HOMOs and LUMOs and thus effective TADF characteristic. On
the other hand, these angles also encourage slight conjugations between
IDAC and A segments, which are required for effective ICT transitions.
Moreover, due to the extended planar conjugations of IDAC segments,
both compounds show large delocalization of HOMOs, which is also
beneficial to realize small ΔESTs.
the onsets of the fluorescence and phosphorescence spectra, the ΔEST
s
are estimated to be 0.09 and 0.02 eV for both compounds, which are
small enough to activate triplet excitons into singlets via RISC process.
To further confirm their TADF properties, the transient PL decays of
these two compound doped films are measured. As list in Table 1 and
shown in Fig. 4a, in the range of 200 ns, IDAC-MCO and IDAC-Trz both
show prompt decays with nearly identical lifetimes of 13 and 12 ns,
respectively, which could be attributed to the direct fluorescence pro-
cess from S1 to ground states (S0). While in the order of microsecond,
3.4. Electrochemical properties
Cyclic voltammetry (CV) measurement was also carried out to
Fig. 2. Molecular structure, optimized geometry and HOMO and LUMO distribution of IDAC-MCO and IDAC-Trz.
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OrganicElectronics57(2018)327–334
Fig. 3. (a) Cyclic voltammograms of the IDAC-based compounds. (b) The room temperature normalized UV–vis absorption spectra in toluene and fluorescence
spectra of IDAC-based compounds doped film at 77 K; (c) Normalized phosphorescence spectra at 77 K of IDAC-based emitters doped film.
double-component emission decay profiles are obtained for two com-
pounds. Specifically, at room temperature, the lifetimes of delay com-
ponent are measured to be 22 μs for IDAC-MCO and 13 μs for IDAC-Trz,
respectively, which should be ascribed to the TADF processes. More-
over, with temperature gradually increased from 200 to 300 K, the
delayed lifetimes showed regular declines, which further confirm their
TADF characteristics as present in Fig. 4b and c. As listed in Table 1, in
air condition, although most triplet excitons are avoidably quenched by
oxygen, both IDAC-MCO and IDAC-Trz exhibit high ΦPLQY value of 44%
and 48%, respectively. Moreover, in oxygen-free condition, the ΦPLQY
of these IDAC derivatives doped films are measured to be 81% for
IDAC-MCO and 90% for IDAC-Trz, respectively, suggesting their high
utilizations for both singlet and triplet excitons. Thus, both emitters are
expected to have well performance during electroluminescent excita-
tions.
3.6. Electroluminescence properties
To investigate the electroluminescence (EL) properties of both the
compounds, the common multilayer device architectures are fabricated
Table 1
Physical properties of IDAC-MCO and IDAC-Trz.
HOMOe (eV)
LUMOf (eV)
ΔEST (eV)
Td/Tg (°C)
τPF (ns)
τTADF (μs)
a
b
g
λabs (nm)
λem (nm)
ΦPLQY (%)
air freec
aird
IDAC-MCO
IDAC-Trz
378
400
498
533
81
90
44
48
−5.20
−5.17
−2.83
−2.90
0.09
0.02
395/128
410/151
13
12
22
13
a
Measured in toluene at room temperature.
b
c
d
e
f
Measured in thin film at 77 K.
Measured in doped film in oxygen-free conditions.
Measured in doped film in air conditions.
Determined from the starting position of the oxidation potential in 10−3 M DCM solution by cyclic voltammetry.
Determined from the starting position of the reduction potential in 10−3 M DCM solution by cyclic voltammetry.
Estimated from the onset of fluorescence and phosphorescence spectrum.
g
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Fig. 4. (a) Transient PL decay curves of prompt emission at room temperature; and delayed emission of IDAC-MCO (b) and IDAC-Trz (c) doped film at different
temperatures.
with the optimized structures of ITO/TAPC (35 nm)/TCTA (10 nm)/
mCP (5 nm)/10 wt% IDAC-MCO: DPEPO (20 nm)/DPEPO (5 nm)/
TmPyPb (45 nm)/LiF (1 nm)/Al and ITO/TAPC (35 nm)/TCTA
(10 nm)/9 wt% IDAC-Trz: CBP (20 nm)/TmPyPb (45 nm)/LiF (1 nm)/
Al, respectively. In both device architectures, ITO (indium tin oxide)
and LiF/Al are used as the anode and the cathode, respectively; TAPC
(1,1-Bis[4-[N,N-di(p-tolyl)amino]phenyl]-cyclohexane) and TmPyPb
(1,3,5-tri[(3-pyridyl)phen-3-yl]benzene) are the hole-transporting layer
and the electron-transporting layer, respectively; TCTA (4,4′,4″-tris
(carbazol-9-yi)triphenylamine) and mCP (1,3-di(9H-carbazol-9-yl)ben-
zene) are the exciton-blocking layers. Specifically, CBP was chosen as
the host for IDAC-Trz due to its suitable T1 energy level and good
charge carrier balance [39]. While in IDAC-MCO-based devices, DPEPO
was chosen as host as well as exciton-blocking layer due to its high T1
energy level [2] enough to sensitize IDAC-MCO.
and a maximum EQE of 18.0%. While for the IDAC-Trz-based device,
due to its smaller ΔEST and larger ΦPLQY, it exhibits even better per-
formance with the maximum CE, PE and EQE of 62.3 cd A−1, 59.3 lm
W−1 and 20.5%, respectively. Noticeably, both devices exhibit small
EQE roll-off at high brightness, with EQEs remaining 10.3% for IDAC-
MCO and 15.8% for IDAC-Trz at a high luminance of 1000 cd m−2
,
respectively, suggesting the great potential of OLEDs based on IDAC
derivatives in practical utilizations.
4. Conclusion
In conclusion, we have developed a novel segment IDAC as an ideal
D candidate for the design of TADF emitters. Two novel TADF emitters
IDAC-MCO and IDAC-Trz were successfully developed with small ΔEST
s
The EL characteristics of the OLEDs are presented in Fig. 5 and the
key EL performance parameters are summarized in Table 2. Both de-
vices using IDAC-based molecules as emitters show low turn-on vol-
tages at about 3 V and exhibit EL spectra located at the green to yellow
regions with the peaks at 516 and 540 nm and the CIE coordinates of
(0.28, 0.48) and (0.36, 0.55) for IDAC-MCO and IDAC-Trz, respectively,
which are close to corresponding PL spectra of their doped films. The
device based on IDAC-MCO achieves a maximum current efficiency
(CE) of 51.7 cd A−1, a maximum power efficiency (PE) of 54.1 lm W−1
of 0.09 and 0.02 eV as well as high ΦPLQY of 81% and 90%, respectively.
The OLEDs using IDAC-MCO and IDAC-Trz as emitters realized the
maximum EQEs up to 18.0% and 20.5%, respectively. Moreover, both
devices show relatively low efficiency roll-offs with remaining high
EQEs of 10.3% and 15.8% at a luminance of 1000 cd m−2, respectively.
These results prove IDAC segment as an ideal candidate to construct
efficient TADF emitters. Moreover, such IDAC-based TADF emitters
may provide a bright prospect for highly efficient OLEDs in practical
application.
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OrganicElectronics57(2018)327–334
Fig. 5. (a) Current density-voltage-luminance characteristics; (b) the EQE-luminance characteristics and (c) the EL spectra of the devices based on IDAC-based
emitters.
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