Structure-simplified and highly efficient deep blue organic light-emitting diodes with reduced efficiency roll-off at extremely high luminance

Article abstract of DOI:10.1039/c6cc08501f

Based on a series of new fluorescent emitters, deep blue non-doped multilayer OLEDs with EQEs exceeding 5.10% and single layer devices excluding any charge carrier transporting materials with an EQE of 4.22% were obtained at an extremely high luminance of 10 000 cd m^?2.

Full text of DOI:10.1039/c6cc08501f

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Structure-Simplified and Highly Efficient Deep Blue Organic Light-
Emitting Diodes with Reduced Efficiency Roll-Off at Extremely
High Luminance†
Received 00th January 20xx,
Accepted 00th January 20xx
Xiang-Long Li††, Ming Liu††, Yunchuan Li, Xinyi Cai, Dongcheng Chen, Kunkun Liu, Yong Cao and
Shi-Jian Su*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Based on a series of new fluorescent emitters, deep blue non- emitters were designed and synthesized for non-doped
doped multilayer OLEDs with EQE exceeding 5.10% and single electroluminescent (EL) devices. For the multilayer non-doped
layer devices excluding any charge carrier transporting materials devices, all of the deep blue fluorophores exhibited excellent
with an EQE of 4.22% were achieved at extremely high luminance EQEs exceeding 5.10% at 10000 cd m-2. As for the simplified
of 10000 cd m-2.
OLEDs excluding carrier transporting layers, a peak EQE of
5.12% was achieved with Commission Internationale de
L'Eclairage (CIE) coordinates of (0.146, 0.106), and the EQE can
be maintained at 4.22% at 10000 cd m-2. Representive works
were shown in Table S1 for a clear comparison (some data
were not shown directly in the references and they were
fetched from the exhibited figures reasonably), which can
show the excellence of our current work.
In the past decades of years, numerous efforts have been
devoted to developing deep blue emitters, and impressive
external quantum efficiency (EQE) has been achieved
particularly in development of noble metal containing
phosphorescent materials1 and thermal activated delayed
fluorescence (TADF) materials.2 However,
a significant
drawback of previously demonstrated deep blue OLEDs is that
they are subjected to a pronounced EQE roll-off at practical
display and illumination relevant high luminance.3 Recently,
outstanding works have been reported on high performance
deep blue phosphorescent and TADF emitters, but the limited
maximum luminance (< 10000 cd m-2) confined their
applications.4 Moreover, deep blue phosphorescent and TADF
devices require appropriate hosts and other functional
materials with enough high triplet energy level for exciton
confinement, and device structures turn out to be
complicated, which is undesirable for realization of simple
device fabrication process and low cost commercial
production. Among conventional fluorescent emitters,
materials based on triplet-triplet annihilationm (TTA)
mechanism can be potential for the capability of efficient
NH2
H
N
N
N
N
N
Br
N
O
O
N
B
1-1
NI-1-PhBr
NI-1-DPhTPA
Br
CHO
NH
EtOEtOH
Pd(PPh3
)4, Na2CO3
N
N
NH2
N
N
N
THF, toluene
Br
O
2-1
NI-2-PhBr
NI-2-DPhTPA
N
N
N
N
B
Br
Br
N
O
N
O
O
O
O
O
3-1
3-2
3-3
4-3
B
B
Br
I
B B
O
O
O
Pd(dppf)2Cl2 AcOK
dioxane
Pd(PPh3
)4, Na2CO3
Pd(dppf)2Cl2 AcOK
dioxane
N
N
N
O
THF, toluene
Br
Br
B
N
N
N
O
4-1
4-2
N
N
N
O
B
O
N
N
3-4
4-4
BIDPhTPA
Br
N
N
N
N
O
B
O
N
Pd(PPh3
)4, Na2CO3
N
THF, toluene
PhIDPhTPA
triplet exciton fussion process, yielding theoretically maximum Scheme 1. Molecular structures and synthetic routes of the developed
deep blue fluorescent emitters NI-1-DPhTPA, NI-2-DPhTPA, BIDPhTPA,
and PhIDPhTPA.
internal quantum efficiency (IQE) of 62.5%, and can realize
high efficiency at high luminance. Nevertheless, the undesired
hosts hunting and complicated device doping process are also
indispensable.5
A general electron-rich group triphenylamine (TPA) was
utilized to construct all of these fluorophors, and three para
interconnected benzene rings between TPA and electron-poor
imidazole derivative groups play a role of link unit. Molecular
structures of these newly designed and synthesized emitters,
NI-1-DPhTPA, NI-2-DPhTPA, BIDPhTPA, and PhIDPhTPA, and
their synthetic routes are shown in Scheme 1. All of these
molecules were thoroughly characterized by 1H, 13C NMR
spectrometry, and mass spectrometry. Density functional
theory (DFT) calculations were performed to evaluate their
In this communication, a group of deep blue fluorescent
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frontier molecular orbitals and energy band gaps (Figure S1 their HOMO levels and Egopts to be -2.35, -2.35, -2.28, and -2.28
and Table S2). Although different imidazole derivatives were eV, respectively. Finally, all the molecules reported herein
adopted, the calculated lowest unoccupied molecular orbitals show high photoluminescence quantum yields (PLQYs) of
(LUMOs) are quite similar, and also are the highest occupied almost 100% in DCM solutions at a uniform concentration of
molecular orbitals (HOMOs) because of the same electron-rich 1×10-5 M, while for NI-1-DPhTPA, the PLQY in solid film
TPA unit. On the basis of the theoretical calculation, the declines sharply to 53%, showing severe quenching effect. DFT
synthesized compounds were also predicted to exhibit quite optimized molecular configurations indicate easier molecular
approximate energy band gap (Eg) and singlet excited energy stacking may occur in NI-1-DPhTPA solid film (Figure S6), which
(S1) in comparison with each other (Table S2).
would result in quenching and thus a much lower PLQY. In
These compounds show quite good thermal stability for contrast, high PLQYs of 81-85% were observed for the other
application in EL devices, as indicated by high glass transition molecules in solid films.
temperatures (Tg) of 139.5, 136.6, 119.6, and 148.5 oC
In order to probe their EL properties, non-doped multilayer
recorded by differential scanning calorimetry (DSC)
OLEDs were initially investigated in a configuration of indium
thermograms and decomposition temperatures (Td,
tin oxide (ITO)/ NPB ( 40 nm)/ TCTA (5 nm)/ emitter (20 nm)/
corresponding to 5% weight loss) of 500.3, 448.5, 440.4, and
TPBi (40 nm)/ LiF (1 nm)/ Al. In these devices, N,N'-
o
502.6 C obtained from thermogravimetric analyses (TGA) for
bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) and
NI-1-DPhTPA, NI-2-DPhTPA, BIDPhTPA, and PhIDPhTPA,
4,4',4"-tris(carbazol-9-yl)triphenylamine (TCTA) serve as
a
respectively (Figure S2).
hole-transporting and an electron-blocking layer, respectively,
and 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
Cyclic voltammograms (CV) were taken to estimate their
HOMO levels. Each of them displayed reversible oxidated
wave, which could be attributed to the oxidation of the TPA
unit (Figure S3). HOMO levels of these compounds were
calculated from the onset oxidation potential (Eox vs.
ferrocene/ferrocenium) and determined to be almost the
same (-5.17 ~ -5.18 eV), due to the same electron rich unit.
Their representative UV-vis absorption and PL spectra were
studied in various solvents, as shown in Figure S4, and some
key data are summarized in Table S3. Firstly, insignificant
changes were found in their absorption spectra with the
increase of solvent polarity, indicating negligible polarity
change at the ground state in different solvents. Secondly, all
the compounds show deep blue emission at 418-420 nm with
obvious vibronic structure features in low-polarity toluene
solutions, while in higher polarity solutions, their PL spectra
were broadened and gradually bathochromic-shifted without
vibronic structure features. When the solvent polarity was
increased gradually from low-polarity toluene to high-polarity
dichloromethane (DCM), the compounds exhibited large red-
shift of 43-47 nm. The large solvatochromic shift, which is
commonly consistent with the large dipole moment of the
charge transfer (CT) state, indicates that the low-lying excited
benzimidazole) (TPBi) serves as an electron-transporting and
hole-blocking layer, and the emitter is NI-2-DPhTPA (marked as
M1), BIDPhTPA (marked as M2) or PhIDPhTPA (marked as M3),
NI-1-DPhTPA is precluded here due to its relatively poor PLQY
(its EL performance can be seen in Figure S7 and Table S4).
Current density-voltage-luminance (J-V-L), EQE and power
efficiency (PE) versus luminance characteristics and other key
parameters of the multilayer devices are shown in Figure 1 and
Table 1
.
14
12
10
8
a)
8
6
4
2
b)
4
1000
10
NI-2-DPhTPA
BIDPhTPA
PhIDPhTPA
800
600
400
200
3
10
2
6
10
4
1
10
2
0
0
2
10
40
10
0
1
2
3
100
10
10
10
-2
3
4
5
6
7
8
Voltage(V)
Luminance(cdm )
8
1
c)
d)
7
6
5
4
3
2
1
0.8
0.6
0.4
0.2
0
state, S1, of the compounds possesses
a
CT-state
0
0
characteristic, and tiny stronger CT-state hallmarks of the
current four molecules when compared with their anolagues
with less conjugated benzene rings,6 which means the
emission color of these molecules should be preserved.
400 450 500 550 600 650 700 750
2000
4000
6000
8000
10000
-2
Wavelength(nm)
Luminance(cdm )
Figure 1. a) Current density and luminance versus driving voltage (J-V-
L), b) current efficiency and power efficiency versus luminance (CE-L-
PE), c) external quantum efficiency versus luminance (EQE-L)
characteristics, and d) electroluminescence (EL) spectra at 10 mA cm-2
for the non-doped multilayer OLEDs in a structure of ITO/ NPB (40
nm)/ TCTA (5 nm)/ emitter (20 nm)/ TPBi (40 nm)/ LiF (1 nm)/ Al,
Comparing the absorption spectra in solid films (Figure S5
)
with those in toluene solutions, an obviously broader
intramolecular charge transfer (ICT) band and slight red-shift
of the absorption peak can be discovered for all of these
compounds. These phenomena imply that the emitters
reported herein in solid film states form compact stacking
where emitter is NI-2-DPhTPA (●, marked as M1), BIDPhTPA (
▲,
marked as M2), or PhIDPhTPA (◆, marked as M3).
The EQEs are 5.15, 5.10, and 5.26% at an extremely high
structure. The optical energy band gaps (Egopt) of NI-1-DPhTPA, luminance of 10000 cd m-2 (corresponding to current
NI-2-DPhTPA
BIDPhTPA, and PhIDPhTPA were determined to efficiencies (CE) of 5.46, 5.61, and 5.67 cd A−1) for the NI-2-
,
be 2.83, 2.83, 2.89, and 2.89 eV, respectively, according to the DPhTPA, BIDPhTPA, and PhIDPhTPA based devices,
onset of absorptions in solid films. Their photophysical data respectively. Such high values at high luminance for non-doped
are shown in Table S3. Their LUMO levels were estimated from deep blue OLEDs have never been reported before to our best
2 | J. Name., 2012, 00, 1-3
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knowledge. Especially, another excellent character of the cannot surport TTA again.8 Considering the excellent
fabricated devices is their maximum power efficiency (PE) of comprehensive device performance, a miraculous balance of
7.04, 7.62, and 7.79 lm W-1, which still can be kept at 5.44, low driving voltages, highly efficiency, deep blue emission, and
5.96, and 6.01 lm W-1 at 1000 cd m-2, and 3.21, 3.39, and 3.43 extremely low efficiency roll-off was realized simultaneously,
lm W-1 at 10000 cd m-2 for the emitters NI-2-DPhTPA, making the current non-doped blue OLEDs very shining herein.
BIDPhTPA, and PhIDPhTPA, respectively. The high PE values for
400
350
300
250
200
150
100
50
400
350
300
250
200
150
100
50
a)
b)
deep blue OLEDs benifit from the extremely low driving
voltages of these devices (Figure 1, Table 1). As can been seen
in Table 1, a turn-on voltage as low as 2.6 V and driving
voltages as low as 5.3 ± 0.1 V at 10000 cd m-2 have been
observed.
NI-2-DPhTPA
BIDPhTPA
PhIDPhTPA
Besides the impressive efficiencies at high luminance, the
peak EQEs of the non-doped multilayer OLEDs can also reach
up to 5.92, 6.18, and 6.43% for the NI-2-DPhTPA, BIDPhTPA,
and PhIDPhTPA based devices, respectively. The wonderful
peak values can compare favourably with the best result of the
0
0
3
4
5
6
7
8
9
10
4
6
8
10
12
Voltage(V)
Voltage(V)
Figure 2. Current density-voltage (J-V) characteristics of the carrier-only
devices in configurations of a) ITO/ NPB (40 nm)/ TCTA (5 nm)/ emitter (20
nm)/ NPB (40 nm)/ Al for hole-only devices (hollow), and ITO/ TPBi (40
nm)/ emitter (20 nm)/ TPBi (40 nm)/ LiF (1 nm)/ Al for electron-only devices
(solid), and b) ITO/ HATCN (5 nm)/ emitter (80 nm)/ TAPC (10 nm)/ Al for
hole-only devices (hollow), and ITO/ TmPyPB (10 nm)/ emitter (80 nm)/ LiF
(1 nm)/ Al for electron-only devices (solid), where emitter is NI-2-
7
non-doped blue fluorescent devices published recently.6a,
Along with the excellent maximum values, the high efficiencies
at 10000 cd m-2 indicate that all the multilayer devices exhibit
extremely low efficiency roll-off uniformly, and this could be
attributed to excellent carrier balance in virtue of the felicitous
molecular design. Single-carrier devices with multilayer
structure were fabricated to understand the tiny efficiency
roll-off of the multilayer devices. As can be seen from their
current density-voltage (J-V) characteristics in Figure 2a, the
hole current and the electron current of each emitter is pretty
similar, which reveals that holes and electrons injected into
the emission layers are very balanced. To gain further insights
into the observed high peak efficiency, lifetimes of the
compounds in DCM solutions with a uniform concentration of
DPhTPA(●○), BIDPhTPA (▲△), or PhIDPhTPA (◆◇).
Multilayer device configuration is commonly adopted for
highly efficient OLEDs because it is relatively convenient to
tune the carrier transporting and/or to control the exciton
distribution by choosing appropriate adjacent materials.
Nevertheless, for commercially available OLED applications, it
is always highly desirable to simplify the OLED structures and
fabrication process greatly further. As has been demonstrated
before,6a a coordination effect of the exposed nitrogen atom
of a compound and LiF would reduce the energy difference
between the interface of the compound and the cathode,
giving a much better electron injection than these without
1×10-5 M were measured firstly. All the solutions present very exposed nitrogen atoms. Then NI-2-DPhTPA and BIDPhTPA
with exposed nitrogen atoms were selected to fabricate
simplified OLEDs in a configuration of ITO/ HATCN (5 nm)/
emitter (80 nm)/ LiF (1 nm)/ Al, where emitter is NI-2-DPhTPA
(marked as S1), or BIDPhTPA (marked as S2). As can be seen in
Table 1 and Figure 3, both emitters give excellent device
performance. In contrast, NI-1-DPhTPA and PhDPhTPA exhibit
very poor performance (Figure S10 and Table S4). Low turn-on
voltages and high EQEs have been obtained for devices S1 and
S2, exceeding the landmark reports with similar device
structures published before.6a, 9 For the device based on the
1H-naphtho[1,2-d]imidazole molecule NI-2-DPhTPA, a record-
short lifetimes (1.32 ~ 1.79 ns, Figure S8a), making it clear that
these emitters are totally different from the TADF materials in
the inherent device operating mechanism. Short lifetimes
(1.67 ~ 3.10 ns, Figure S8b) of neat solid films can prove the
conclusion further. Additionally, the EL characters of these
devices indicate that the high efficiencies cannot be attributed
to TTA effect either. Firstly, the EQE versus luminance (EQE-L)
curves show
a slight decrease in efficiency for higher
luminance (Figure 1c). Another evidence to oppose TTA effect,
which is a second-order process proportional to the square of
the triplet exciton concentration, is that the dependence of breaking EQE of 4.22% have been achieved with deep blue CIE
coordinates of (0.146, 0.104) at 10000 cd m-2 and the
luminance on current density as shown in Figure S9 is almost a
maximum EQE is 5.12%. The EQE values indicates extremely
low EQE roll-off and outstanding performance of the current
simplified device. The maximum EQE value of the structure-
linear curve at low injection current density. In addition, the
luminance increased slower than the current density, which
Table 1. Summary of the electroluminescent performance of the fabricated non-doped deep blue OLEDs.
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unprecedented EQE of 4.22% was achieved at 10000 cd m-2 for
NI-2-DPhTPA, which can even rival the reported multilayer
non-doped deep blue OLEDs. Without utilizing any charge
carrier transporting materials, this fabricated deep-blue
fluorescent OLEDs with brand new record performance and
extraordinary simple device structure can be very promising
for future cheap and large scale OLED application at high
luminance.
simplified device apparently reach the EQE limit of traditional
fluorescent OLEDs for the first time (5.0 ~ 7.5%, assuming the
light out-coupling factor ηop = 0.2 ~ 0.3) and the result is
comparable with most multilayer non-doped blue devices,10
demonstrating a bright prospect of the simplified OLEDs. For
the BIDPhTPA based device, unfortunately, the EQE value
drops sharply compared with the device based on NI-2-
DPhTPA and remains only 1.11% at 10000 cd m-2. These data
indicate that the injected holes and electrons should be much
more imbalanced in the devices based on BIDPhTPA than in
the devices based on NI-2-DPhTPA.
The authors greatly appreciate the financial support from
the National Key Research and Development Plan
(2016YFB0401004), 973 Project (2015CB655003), the National
Natural Science Foundation of China (51625301, 51573059 and
91233116), and Guangdong Provincial Department of Science
and Technology (2016B090906003).
b)
10
a)
5
104
1000
NI-2-DPhTPA
BIDPhTPA
8
6
4
2
4
3
2
1
103
102
101
100
800
600
400
200
Notes and references
2
3
4
5
6
7
8
9
2103
4 103
6 103
8103
1 104
-2
Voltage(V)
Luminance (cdm )
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1.2
1
6
5
4
3
2
1
c)
d)
0.8
0.6
0.4
0.2
0
0
2103
4103
Luminance(cdm )
6103
8103
1104
400 450 500 550 600 650 700 750
-2
Wavelength(nm)
Figure 3. a) Current density and luminance versus driving voltage (J-V-L), b)
current efficiency and power efficiency versus luminance (CE-L-PE), c)
external quantum efficiency versus luminance (EQE-L) characteristics, and
d) electroluminescence (EL) spectra at a 10 mA cm-2 of the simplified non-
doped OLEDs in a structure of ITO/ HATCN (5 nm)/ emitter (80 nm)/ LiF (1
nm)/ Al, where emitter is NI-2-DPhTPA (
marked as S2).
●, marked as S1), or BIDPhTPA (▲,
Single-carrier devices with single layer structure were also
fabricated and characterized (Figure 2b). It turns out that both
holes and electrons could be easily injected into the emitters,
and the hole currents perform very similar while the situations
for the electron injection from the LiF/Al cathode and
transport in each material appeared to be quite different, even
though their LUMO levels are pretty alike. The big difference
should be ascribed to the coordination effect between the
exposed nitrogen atom of BIDPhTPA and LiF, which should be
stronger than that of NI-2-DPhTPA for its smaller spacial
hindrance (Scheme 1, Figure S1, and Figure S6), reducing the
energy difference between the interface of BIDPhTPA and the
cathode, resulting in better electron injection, leading to
unbalanced carriers and thus rapid efficiency roll-off. In
addition to the excellent efficiency performance, bluer
emission can also be seen from the CIE coordinates (Table 1)
and EL spectra (Figure S11). This may be ascribed to the
microcavity effect, which generally lead to narrower EL
spectra.11
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In summary, a series of deep blue emitters were designed
and synthesized for the fabrication of non-doped OLEDs. All
the multilayer devices exhibited excellent performances with
remarkable EQEs exceeding 5.10% at 10000 cd m-2 and
extremely reduced efficiency roll-off, which is the best result
reported by far. For the simplified single layer devices
excluding any charge carrier transporting materials, an
4 | J. Name., 2012, 00, 1-3
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