Revealing the new potential of an indandione unit for constructing efficient yellow thermally activated delayed fluorescence emitters with short emissive lifetimes

Article abstract of DOI:10.1039/C8TC01656A

Simultaneously accomplishing a high efficiency and a slow efficiency roll-off at practical luminance levels remains challenging for thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). In this study, for the first time, we reveal the new potential of an indandione unit featuring double carbonyl moieties, which is widely used in organic solar cells, as an electron-accepting core for constructing efficient TADF emitters. As a proof of concept, two TADF emitters, 5PXZ-PIDO and 5,6PXZ-PIDO, are developed by connecting an indandione (IDO) core with electron-donating phenoxazine (PXZ) units via phenylene π-bridges. Impressively, both emitters exhibit a distinct TADF nature with short DF lifetimes of ～2 μs, and display relatively high photoluminescence quantum yields (Φ_PLs) of over 70%. More importantly, a yellow OLED based on 5,6PXZ-PIDO achieves a high external quantum efficiency of 14.2% and an ultra-slow efficiency roll-off of 16.0% at a practical luminance of 1000 cd m^-2, which is outstanding among previously reported carbonyl-based TADF emitters. This finding unlocks the huge potential of indandione-based molecules as TADF emitters for high-performance OLEDs.

Full text of DOI:10.1039/C8TC01656A

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC01656A
Journal Name
ARTICLE
Revealing new potential of an indandione unit for constructing
efficient yellow thermally activated delayed fluorescence emitters
with short emissive lifetimes
Received 00th January 20xx,
Accepted 00th January 20xx
Yepeng Xiang,a,b Ze-Lin Zhu,c,d Dongjun Xie,a Shaolong Gong,*a Kailong Wu,a Guohua Xie,a Chun-
DOI: 10.1039/x0xx00000x
Sing Lee,*c,d Chuluo Yang*a,b
www.rsc.org/
Simultaneously accomplishing a high efficiency and a slow efficiency roll-off at the practical luminance levels remains
challenging for thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). In this study, for the
first time, we reveal new potential of indandione unit featuring double carbonyl moieties, which is widely used in organic
solar cells, as an electron-accepting core for constructing efficient TADF emitters. As a proof of concept, two TADF
emitters, 5PXZ-PIDO and 5,6PXZ-PIDO, are developed by connecting an indandione (IDO) core with electron-donating
phenoxazine (PXZ) units via the phenylene π-bridges. Impressively, both emitters exhibit distinct TADF nature with short
DF lifetimes of ~ 2 μs, and display relatively high photoluminescence quantum yields (ΦPLs) of over 70%. More importantly,
a yellow OLED based on 5,6PXZ-PIDO achieves a high EQE of 14.2% and an ultra-slow efficiency roll-off of 16.0% at a
practical luminance of 1000 cd m-2, which is outstanding among previously reported carbonyl-based TADF emitters. This
finding unlocks the huge potential of indandione-based molecules as TADF emitters for high-performance OLEDs.
simultaneously accomplishing a high efficiency and a slow
efficiency roll-off at a practical luminance levels remains
challenging for TADF OLEDs.19-21 New efficient multi-color
TADF emitters with reduced DF lifetimes of less than 5 μs are
therefore highly required to address this issue.7-12,17
Introduction
Due to the unique capability of effectively harvesting triplet
excitons in organic light-emitting diodes (OLEDs), thermally
activated delayed fluorescence (TADF) emitters have attracted
considerable attentions in academic and industrial
communities during the past few years. Since the pioneering
discovery of introducing TADF emitters into OLEDs by Adachi
and co-authors, much endeavor has been devoted to
extending molecular diversity of TADF emitters and enhancing
their device performance.1-6 Although blue TADF emitters have
been the most important concern, other color TADF emitters
including yellow emitters are also necessary for developing
efficient white OLEDs, hyperfluorescence OLEDs and so on. So
far, considerable progresses have been achieved in multi-color
TADF OLEDs with external quantum efficiencies (EQEs) of over
20%.7-18 However, most of TADF OLEDs generally suffer from
drastic efficiency roll-offs at a practical luminance because of
the relatively long DF lifetimes of TADF emitters. In this sense,
A common way to reduce DF lifetimes of TADF emitters is to
minimize energy gap (ΔEST) between the lowest singlet and
triplet states through increasing twisting angles between
electron donor and acceptor units, and lowering overlapping
of their frontier molecular orbitals. However, this approach
normally leads to low oscillator strength (f) of TADF emitters,
and thus sacrifices their photoluminescence quantum yields
(ΦPLs). Recently several groups developed some TADF emitters
with reduced DF lifetimes by adopting multi-dipolar transition
structure in molecular design.22-24 For example, Xu et al.
reported a multi-dipolar TADF emitter featuring phosphoine
oxide units, and the corresponding device exhibited blue
emission and reduced efficiency roll-offs at a practical
luminance.24 This may provide a feasible strategy of reducing
DF lifetimes of TADF emitters without sacrificing their ΦPLs.
Carbonyl-based moieties are widely used as electron
acceptors for constructing organic functional materials for
OLEDs and organic solar cells (OSCs).25-28 Among them, an
indandione (IDO) unit containing double carbonyl moieties has
been established to be an effective electron acceptor, in virtue
of its strong electron-withdrawing ability and multiple
modification sites. However, the introduction of IDO-based
emitters into OLEDs has not been explored. In consideration of
intrinsic double accepting structure in the IDO unit, we, herein,
a.Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of
Chemistry, Wuhan University, Wuhan, 430072, P. R. China.
*E-mail: slgong@whu.edu.cn, clyang@whu.edu.cn.
b.Shenzhen Key Laboratory of Polymer Science and Technology, College of
Materials Science and Engineering, Shenzhen University, Shenzhen 518060, P. R.
China.
c. Center of Super-Diamond and Advanced Films (COSDAF) and Department of
Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China.
d.City University of Hong Kong Shenzhen Research Institute, Shenzhen, Guangdong,
P. R. China.
* E-mail: apcslee@cityu.edu.hk.
† Electronic Supplementary Information (ESI) available
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ARTICLE
Journal Name
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-10H-phenoxazine
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DOI: 10.1039/C8TC01656A
(424 mg, 1.1 mmol), 5-bromo-2,2-dimethyl-1H-indene-1,3(2H)-
dione (PIDO-Br) (252 mg, 1 mmol), potassium carbonate (276
mg, 2 mmol) and Pd(PPh3)4 (10 mg, 0.01 mmol) was added 20
mL of degassed tetrahydrofuran and 10 mL of degassed water.
After stirring at 80 oC under an argon atmosphere for 24 h, the
mixture was cooled down to room temperature and extracted
with 3 × 40 mL of dichloromethane. The collected organic
phase was washed with brine and dried with anhydrous
Na2SO4. After removal of the solvent, the residue was purified
by column chromatography on silica gel (eluent: petroleum
/dichloromethane = 1:1, v/v) to afford the product as red
1
powder (300 mg, yield: 70%). H NMR (400 MHz, CDCl3 + TMS,
25 oC) δ [ppm]: 8.24 (d, J = 1.5 Hz, 1H), 8.15 (dd, J = 8.0, 1.6 Hz,
1H), 8.10 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 8.4 Hz, 2H), 7.51 (d, J =
8.4 Hz, 2H), 6.77-6.54 (m, 6H), 6.00 (d, J = 7.6 Hz, 2H), 1.36 (s,
6H). 13C NMR (100 MHz, CDCl3, 25 oC) δ [ppm]: 204.53, 204.06,
147.94, 143.96, 141.20, 139.97, 139.03, 134.84, 134.05,
131.80, 130.17, 124.35, 123.30, 121.85, 121.65, 115.63,
+
113.24, 50.40, 20.40. MS (HRMS): Anal. Calcd for C29H22NO3
432.15942 [M+H]+. found: 432.15927 [M+H]+.
5,6-Bis(4-(10H-phenoxazin-10-yl)phenyl)-2,2-dimethyl-1H-
indene-1,3(2H)-dione (5,6PXZ-PIDO): Prepared according to
the same procedure as 5PXZ-PIDO but using PIDO-2Cl. Yield:
1
o
73%. H NMR (400 MHz, CDCl3 + TMS, 25 C) δ [ppm]: 8.17 (s,
2H), 7.43 (d, J = 8.4 Hz, 4H), 7.31 (d, J = 8.4 Hz, 4H), 6.77-6.41
(m, 12H), 5.85 (d, J = 7.5 Hz, 4H), 1.40 (s, 6H). 13C NMR (100
MHz, CDCl3) δ [ppm]:204.12, 148.02, 143.86, 139.64, 138.96,
133.97, 132.39, 130.95, 125.29, 123.45, 121.59, 115.55,
Fig. 1 (a) The chemical structures of 5PXZ-PIDO and 5,6PXZ-PIDO. (b) The FMO, energy
levels, oscillator strength (f) of 5PXZ-PIDO and 5,6PXZ-PIDO [upon B3LYP level and 6-
31G(d) set for DFT; upon LC-ω*PBE level and 6-31G(d) set for TD-DFT]. (c) ORTEP
diagram molecular packing diagram of 5,6PXZ-PIDO with hydrogen atoms omitted.
+
112.97, 50.48, 20.44. MS (HRMS): Anal. Calcd for C47H32N2O4
688.23566 [M]+. found: 688.23566 [M]+.
developed two TADF emitters, namely, 5PXZ-PIDO and 5,6PXZ-
PIDO, by incorporating electron-donating phenoxazine (PXZ)
units into an IDO core via the phenylene π-bridges (Scheme 1).
Methyl groups are introduced into the active methylene group
of IDO unit to avoid side reactions and suppress intermolecular
π-π interaction. Remarkably, both emitters display distinct
TADF features and relatively short DF lifetimes of ~ 2 μs, and
exhibit relatively high ΦPLs of over 70%. To the best of our
knowledge, this is the first report on IDO-based compounds
with TADF feature. As a result, 5,6PXZ-PIDO successfully
achieves an intense yellow emission with a high EQE of 16.9%.
Moreover, its EQE remains as high as 14.2%, corresponding to
an ultra-slow efficiency roll-off of 16.0%, at the practical
luminance of 1000 cd m-2., which is prominent among
previously reported carbonyl-based TADF emitters.8,20,29-35
Results and discussion
Frontier molecular orbitals (FMOs) and the excited states of
both molecules were predicted by density functional theory
(DFT) and time-dependent DFT (TD-DFT) calculations. Fig. 1b
depicted the HOMO and LUMO distributions of both
molecules. Apparently, both molecules exhibited well-
separated HOMO and LUMO distributions, accompanied with
small overlaps on the phenylene bridges. Such distributions
imparted both molecules with small ΔESTs of around 0.45 eV,
indicating that RISC process based on T1
→S1 may occur in
these molecules. In comparison with 5PXZ-PIDO with a single-
dipolar structure, 5,6PXZ-PIDO possessed the higher oscillator
strength (f) originating from its double-dipolar transition
structure, suggesting that 5,6PXZ-PIDO may perform as the
better emitter.37,38
The synthetic routes for two IDO-cored compounds were
outlined in Scheme 1. Both compounds were produced by Pd-
catalyzed C-C cross-coupling reaction of 10-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-10H-phenoxazine
with the corresponding halogen precursors in high yields,
respectively. 5PXZ-PIDO and 5,6PXZ-PIDO were purified by
recrystallization and further gradient sublimation, and their
structures were verified by 1H NMR, 13C NMR and high-
Experimental section
The corresponding halogen precursors of PIDO-Br and PIDO-
2Cl were prepared according to literature precedence (Scheme
S1).36
5-(4-(10H-phenoxazin-10-yl)phenyl)-2,2-dimethyl-1H-indene-
1,3(2H)-dione (5PXZ-PIDO): To a mixture of 10-(4-(4,4,5,5-
2 | J. Name., 2012, 00, 1-3
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Scheme 1 Synthesis of 5PXZ-PIDO and 5,6PXZ-PIDO
resolution mass spectrometry (Fig. S1-S10). Furthermore, the
molecular structure of 5,6PXZ-PIDO was identified by the
single-crystal X-ray crystallographic analysis. As shown in Fig.1c,
5,6PXZ-PIDO preferred a quasi-symmetry orientation with the
large torsion angles of 89.3° and 85.8° between the PXZ unit
and the adjacent phenylene bridge, which is consistent with
the DFT results.
Both compounds exhibited relatively high decomposition
temperatures (Td, corresponding to 5% weight loss) over 400
oC. The DSC traces revealed their glass transition temperatures
(Tgs) of 134 oC for 5PXZ-PIDO and 164 oC for 5,6PXZ-PIDO,
respectively (Fig. 2a). These results clearly established the high
thermal stability of both compounds, which could benefit
device stability under long-term operation condition.15,39 The
electrochemical properties of the two compounds were
studied by cyclic voltammetry (CV) experiments. According to
the half-wave potentials of two compounds in CV curves (Fig.
2b), the HOMO/LUMO levels of 5PXZ-PIDO and 5,6PXZ-PIDO
were established to be -5.10/-2.80 eV and -5.08/-2.81 eV,
respectively (Table 1). The favorable HOMO/LUMO levels of
both compounds could benefit charge injection between the
emitting layer and the adjacent charge-transporting layers in
OLEDs.
Fig. 2 (a) TGA curves of 5PXZ-PIDO and 5,6PXZ-PIDO. (b) DSC curves of 5PXZ-PIDO and
5,6PXZ-PIDO. (c) Oxidation and reduction behaviors of 5PXZ-PIDO and 5,6PXZ-PIDO.
Fig. 3b, 5PXZ-PIDO exhibited the same FWHM (full-width at
half-maximum) of 85 nm in its Ph. and low-temperature
fluorescence spectra. The same case was observed for 5,6PXZ-
PIDO (a FWHM of ~ 87 nm) in its Ph. and low-temperature
fluorescence spectra. At room temperature, the more active
vibrations and rotations in both emitters widened their FL
spectra. According to the onsets of the corresponding
fluorescence and phosphorescence spectra, the singlet and
triplet state energy of both emitters were estimated to be
2.61/2.57 eV and 2.58/2.56 eV, respectively. As a result, the
ΔESTs of both emitters in CBP host were calculated to be 0.04
eV for 5PXZ-PIDO and 0.02 eV for 5,6PXZ-PIDO. These results
are basically consistent with the outcomes of both emitters
doped into PMMA (1.5 wt.%) films (Fig. S12), clearly revealing
that these emissive characteristics originate from the emitters.
The small ΔESTs of both compounds suggest that RISC process
As presented in Fig.
3, 5PXZ-PIDO and 5,6PXZ-PIDO
exhibited two types of absorption bands simultaneously in
toluene: strong absorption bands in the high-energy region
(280-350 nm), mainly attributed to locally excited (LE)
transitions of PXZ and/or IDO units (Fig. S11), and tailed
absorption bands in the region of 360-450 nm, assigned to
intramolecular charge transfer (ICT) transition from PXZ to IDO
units. Moreover, both emitters displayed structureless
emission with main peaks at 564 and 573 nm for 5PXZ-PIDO
and 5,6PXZ-PIDO, respectively, in toluene. Under degassed
condition, both emitters exhibited double exponential decays
containing prompt and delayed fluorescence components. The
emissive characteristics of both emitters in film state were also
investigated in 4,4’-di(9H-carbazol-9-yl)-1,1’-biphenyl (CBP)
host, which is a widely used host in OLEDs because of its
bipolar charge-transporting ability and relatively high triplet
energy. Both compounds in CBP (1.5 wt.%) host exhibited
broad emission bands with the emission peaks at 535 and 544
nm for 5PXZ-PIDO and 5,6PXZ-PIDO, respectively. As shown in
based on T1→S1 may easily occur in both emitters. To figure
out emissive nature of both emitters, transient PL spectra of
5PXZ-PIDO and 5,6PXZ-PIDO in CBP host (1.5 wt.%) were
studied. Apparently, both emitters displayed second-order
exponential decay with prompt (16.0 ns for 5PXZ-PIDO and
15.0 ns for 5,6PXZ-PIDO) and delayed (2.37 µs for 5PXZ-PIDO
and 1.98 µs for 5,6PXZ-PIDO) components in CBP host.
Compared with 5PXZ-PIDO, 5,6PXZ-PIDO with a double-dipolar
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Table 1 Thermal, electrochemical and photophysical properties of 5PXZ-PIDO and 5,6PXZ-PIDO
λaabs/λb
[nm]
HOMOc/LUMOc
[eV]
h
h
ΔESTf/ΔEST
τp
τd
kfk/kISCk [107
PL
d
e
Tg /Td [oC]
compounds
Φi [%]
Φp/Φdj [%]
kRISCk [105 s-1]
g[eV]
[ns]
[μs]
s-1]
5PXZ-PIDO
291/535
283/544
-5.10/-2.80
-5.08/-2.81
134/421
164/417
0.04/0.11
0.02/0.06
16
15
2.37
1.98
72
76
48/24
47/29
6.25/3.25
6.67/3.53
4.06
5.89
5,6PXZ-PIDO
a Measured in toluene solution (10-5 M) at room temperature. bMeasured in doped CBP films (1.5 wt.%) at room temperature. cObtained from cyclic voltammograms in
e
f
CH2Cl2 solution. dObtained from DSC measurements. Obtained from TGA measurements (Td, corresponding to 5% weight loss). Calculated from the onset of the
fluorescence (room temperature) and phosphorescence spectra (77K) of the two emitters doped in CBP films (1.5 wt.%). gCalculated from the onset of the
fluorescence (77 K) and phosphorescence spectra (77 K) of the two emitters doped in CBP films (1.5 wt.%). hThe prompt and delayed fluorescence lifetimes of emitters
doped into CBP films (1.5 wt.%) at room temperature. iThe total PLQYs of emitters doped into CBP films (1.5 wt.%) under oxygen-free condition at room temperature.
jThe prompt and delayed fluorescence PLQY under oxygen-free condition. kThe rate constants of radiative, ISC and RISC process in CBP host.
Fig. 4 The transient photoluminescence decay spectra of (a) 5PXZ-PIDO and (b) 5,6PXZ-
PIDO in toluene (10-5 M) under aerated and degassed condition. The transient photo-
luminescence decay spectra of (c) 5PXZ-PIDO and (d) 5,6PXZ-PIDO doped into CBP films
(1.5 wt.%) at various temperatures.
Fig. 3 (a) The UV-Vis absorption and fluorescence curves of 5PXZ-PIDO and 5,6PXZ-PIDO
the 5PXZ-PIDO.
Inspired by the excellent features of high ΦPLs, obvious TADF
in toluene (10-5 M), and (b) low temperature fluorescence (77 K), room temperature
fluorescence (300 K) and phosphorescence (77 K) spectra of the two compounds doped
feature and short delayed lifetimes, 5PXZ-PIDO and 5,6PXZ-
into CBP films (1.5 wt.%).
PIDO were employed as emitters in multi-layer OLEDs with an
architecture of ITO/TAPC (30 nm)/TCTA (5 nm)/CBP: TADF
emitters (1.5 wt.%, 20 nm)/TmPyPB (65 nm)/LiF (1 nm)/Al (120
nm) (Fig. 5a). Wherein, TAPC and TmPyPB acted as the hole-
transition structure possessed smaller ΔEST and shorter delay
lifetime, consistent with the previous work.24,38 Moreover, the
intensity of the delayed component for both emitters
and electron-transporting layer, respectively; TCTA was
gradually increased with increasing temperature from 100 to
inserted between TAPC and the emitting layer to facilitate hole
300 K, clearly establishing distinct TADF nature of both
injection/transport and suppress the detrimental quenching
emitters. The ΦPLs of 5PXZ-PIDO and 5,6PXZ-PIDO in CBP host
were estimated to be 72% and 76%, respectively, at room
temperature. In comparison with 5PXZ-PIDO, the slightly
enhanced ΦPL of 5,6PXZ-PIDO could be ascribed to the higher
process between the excess holes and the emissive excitons;
5PXZ-PIDO and 5,6PXZ-PIDO were doped into CBP host with an
optimal doping concentration of 1.5 wt.% (Table S4) as the
emission layer for devices A and B, respectively. Devices A and
oscillator strength (f) originating from its double-dipolar
B
displayed yellow light and favorable Commission
transition structure. To further understand the ISC and RISC
processes for both emitters in CBP host, the rate constants of
ISC and RISC process for both emitters were estimated to be
International de l’Eclairage coordinates of (0.39, 0.54) and
(0.42, 0.53), respectively. Furthermore, the EL spectra of both
devices remained the same under difference current density
3.25*107/4.06*105 s-1 and 3.53*107/5.89*105 s-1
Apparently, both emitters exhibited almost the same kISC
(
Table 1).
(Fig. S13), indicative of the balanced charge carrier injection
;
and transportation in the emission layer for both devices. Both
devices exhibited similar current density-voltage (J-V)
characteristics with the almost same turn-on voltages (c.a. 3.5
eV), which can be attributed to the same OLED architecture
and the similar HOMO/LUMO levels of both emitters. As
shown in Fig. 5d, device A based on 5PXZ-PIDO exhibited a
while, the smaller ΔEST endowed 5,6PXZ-PIDO with the larger
kRISC. Furthermore, the radiative rate constant kf of 5PXZ-PIDO
and 5,6PXZ-PIDO in CBP films were estimated to be 6.25*107 s-
1 and 6.67*107 s-1, respectively, which match well with the DFT
results that the f of the 5,6PXZ-PIDO was larger than that of
4 | J. Name., 2012, 00, 1-3
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Fig. 5 (a) The energy level diagrams and (b) chemical structures of the materials used for the devices. (c) Current density-voltage-luminance curves of the devices. Inset:
Electroluminescence spectra of devices A and B at a driving voltage of 10 V. (d) Power efficiency (PE), current efficiency (CE) and external quantum efficiency (EQE) versus
luminance curves of the devices A-B. (e) Electroluminescence spectra of devices A and B at a driving voltage of 10 V. (f) The external quantum efficiency (EQE) as a function of
current density and the fitting results according to the triplet-triplet annihilation (TTA, pink lines) mechanism.
Table 2 Electroluminescence characteristics of the devices A and B.
Maximum Efficiencya Luminance at 1000 cd m-2 Luminance at 10000 cd m-2
Device
Emitter
EQE roll-offsb
CIEc(x, y)
CE, PE, EQE
CE, PE, EQE
CE, PE, EQE
A
B
5PXZ-PIDO
5,6PXZ-PIDO
43.3, 33.1, 14.4
49.3, 38.7, 16.9
32.3, 19.9, 10.8
41.7, 26.2, 14.2
13.5, 5.6, 4.6
23.6, 10.8, 8.1
25.0, 68.1
16.0, 52.1
(0.39, 0.54)
(0.42, 0.53)
aThe maximum efficiencies of CE (cd A-1), PE (lm W-1) and EQE (%). bThe EQE roll-offs (%) at 1000 cd m-2 and 10000 cd m-2. cThe Commission Internationale de l’Eclairage
coordinates recorded at 8 V.
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maximum current efficiency (CEmax) of 43.3 cd A-1, a maximum
combination of high efficiency and ultra-slow efficiency roll-off
power efficiency (PEmax) of 33.1 lm W-1 and a maximum
external quantum efficiency (EQEmax) of 14.4%. Comparatively,
the higher ΦPL as well as larger kRISC of 5,6PXZ-PIDO endowed
device B with a better device performance of 49.3 cd A-1, 38.7
lm W-1, and 16.9% (Table 2). The charge-transporting balance
could be another important factor for different device
performance for devices A and B. To figure out this point, the
hole-only device having the structure of ITO/TAPC (25
nm)/TCTA (5 nm)/CBP: 1.5 wt.% dopant (50 nm)/TCTA (5
nm)/TAPC (25 nm)/Al (100 nm) and electron-only device
for 5,6PXZ-PIDO-based device
B is prominent among
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previously reported carbonyl-based TADF emitters (Table
S5).20,30-32,34,35,41 Generally, triplet-exciton-involved quenching
processes, such as triplet-triplet annihilation (TTA), singlet-
triplet annihilation (STA) and triplet-polaron quenching (TPQ),
are the main reasons for efficiency roll-off at high
luminance.42-45 As shown in Fig. 6, TTA simulation matched
well with the EQE-J curves for devices A and B, which suggests
that TTA mechanism mainly accounts for efficiency roll-off of
having the structure of ITO/TmPyPB (30 nm)/CBP: 1.5 wt.% both devices. The equation for the TTA fitting is expressed as
follows:19
dopant (50 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (100 nm) were
fabricated. As shown in Fig. S14, both devices exhibited
dominant hole-transporting ability and poor electron-
transporting ability. This suggests that both devices possessed
unbalanced charge-transporting ability. In this sense, the
charge-transporting balance may not be the decisive factor for
the better EL performance of 5,6PXZ-PIDO than 5PXZ-PIDO.
Remarkably, the TADF feature of both emitters imparted both
η
J0
J
(√1+8 -1)
J0
=
η0 4J
where η represents the EQE of the device, η0 refers to the
initial EQE in the absence of TTA, J stands for the current
devices with over 10% EQEs, which are significantly higher density of the device, and J0 is the “onset” current density at
than the theoretical limit of 5-10% for the conventional
fluorescent OLEDs. These results clearly establish effective
utilization of triplet excitons through TADF process. To
determine how many excitons are utilized, exciton utilization
efficiencies (ηeues) were estimated on the basis of the following
equation:40
η=η0/2. When assuming the perfect charge balance in our
devices, the J0 of device B based on 5,6PXZ-PIDO was
estimated to be 44.0 mA cm-2, which is over two times as high
as that of device A based on 5PXZ-PIDO (20.5 mA cm-2). We
also fitted the EQE-J curves of both devices with the TPQ
model. As shown in Fig. S16, the TPQ model did not fit well
with the experimental curves for both devices. Furthermore,
the TTA fitting lines deviated from the experimental EQE-J
curves at high current density. This suggests that the STA
mechanism may become dominant with the increased current
density, which is consistent with the previous reports on
excitons quenching processes in TADF OLEDs.43-46 These results
manifest that the short DF lifetime and large kRISC of 5,6PXZ-
PIDO alleviate TTA process in the EL process, and thereby
suppress efficiency roll-off. Despite of their good device
performance, the luminescence of both devices lasted only ca.
1 hour without encapsulation. This could be more associated
with instability of the CBP host and the charge-transporting
materials of TAPC and TmPyPB.47 Maybe the lifetime of our
devices would be improved by using suitable host and charge-
transporting materials in the further study.
휂푒푥푡 = 휂푒푢푒훾Ф푃퐿휂표푢푡
where γ is the charge balance factor, ηout stands for the out-
coupling efficiency (EQE), considered as a constant of 0.2-0.3
for ITO glass substrate, ΦPL refers to the intrinsic ΦPL of the
emissive layer. Given the perfect charge balance (γ = 1) and
relatively high ηout of 0.25 in the devices, ηeues of both devices
were estimated to be 80% and 88.9% for 5PXZ-PIDO and
5,6PXZ-PIDO, respectively.
To determine the possible energy loss in our devices, we
have estimated the theoretical maximum ηext for both devices
using the following equation:38
Ф푑
[ ( )]
휂푒푥푡 = 훾휂표푢푡{0.25Ф푃 + 0.75 + 0.25 1 − Ф푃 ∗
}
1 − Ф푃
Considering the perfect charge balance (γ = 1) and relatively
high ηout of 0.2-0.3 in the devices, the theoretical maximum
ηexts were estimated to be 10.5%-15.7% for the 5PXZ-PIDO-
based device and 12.0%-18.9% for the 5,6PXZ-PIDO-based
device. Consequently, the experimental maximum EQEs of
both devices (14.4% for the 5PXZ-PIDO-based device and 16.9%
for the 5,6PXZ-PIDO-based device) are very close to their
theoretical values. These results suggest that both devices
exhibited small energy loss.
Conclusion
In summary, we have designed and synthesized two TADF
emitters by an appropriate combination of an IDO core with
PXZ units via the phenylene π-bridges. Both emitters exhibited
excellent thermal stability, suitable HOMO/LUMO levels,
relatively high ΦPL and distinct TADF feature with short DF
lifetimes. As a result, the 5,6PXZ-PIDO-based device exhibited
intense yellow emission with a high EQE of 14.2% and an ultra-
slow efficiency roll-off of 16.0% at the practical luminance of
1000 cd m-2. This work demonstrates the great potential of
indandione-cored compounds as TADF emitters in OLEDs.
Inspiringly, the short DF lifetimes of both emitters endow
both devices with slow efficiency roll-off at high luminance
(
Table 2). Device A achieved a high EQE of 10.8% and a slow
EQE roll-off of 25.0% at a high luminance of 1000 cd m-2.
Comparatively, the shorter DF lifetime of 5,6PXZ-PIDO
imparted device B with an ultra-slow efficiency roll-off
character: at a practical luminance of 1000 cd m-2, the EQE
slightly dropped to 14.2% with an ultra-slow EQE roll-off value
of 16.0%; even at an extremely high luminance of 10000 cd m-2,
the EQE remained as high as 8.1%. The superior performance
Acknowledgements
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 6
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Journal of Materials Chemistry C
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ARTICLE
19 Y. P. Xiang, Y. B. Zhao, N. Xu, S. L. Gong, F. Ni, K. L. Wu, J. J.
We acknowledge financial support from the National Natural
Science Foundation of China (51573141 and 91433201), the
National Key R&D Program of China (2016YFB0401002), the
National Basic Research Program of China (2015CB655002),
Shenzhen Peacock Plan (KQTD20170330110107046) and the
Natural Science Foundation for Distinguished Young Scholars
of Hubei Province (2017CFA033).
Luo, G. H. Xie, Z. H. Lu and C. L. Yang, J. MaVteierw. ACrthicelemOn.linCe,
DOI: 10.1039/C8TC01656A
2017, 5, 12204.
20 X. Cai, D. Chen, K. Gao, L. Gan, Q. Yin, Z. Qiao, Z. Chen, X.
Jiang and S.-J. Su, Adv. Funct. Mater., 2018, 28, 1704927.
21 T.-L. Wu, M.-J. Huang, C.-C. Lin, P.-Y. Huang, T.-Y. Chou, R.-W.
Chen-Cheng, H.-W. Lin, R.-S. Liu and C.-H. Cheng, Nat.
Photon., 2018, 12, 235.
22 P. K. Samanta, D. Kim, V. Coropceanu and J.-L. Brédas, J. Am.
Chem. Soc., 2017, 139, 4042.
23 M. K. Etherington, J. Gibson, H. F. Higginbotham, T. J. Penfold
and A. P. Monkman, Nat. Commun., 2016, 7, 13680.
Conflicts of interest
There are no conflicts to declare.
24 C. Duan, J. Li, C. Han, D. Ding, H. Yang, Y. Wei and H. Xu,
Chem. Mater., 2016, 28, 5667.
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47 L. Zhang, Y.-X. Zhang, Y. Hu, X.-B. Shi, Z.-Q. Jiang, Z.-K. Wang
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Table of contents
Two indandione-based compounds are demonstrated as yellow TADF emitters with
short emissive lifetimes to simultaneously afford a high efficiency and a slow
efficiency roll-off at the practical luminance levels.