To improve the efficiency of thermally activated delayed fluorescence OLEDs by controlling the horizontal orientation through optimizing stereoscopic and linear structures of indolocarbazole isomers

Article abstract of DOI:10.1039/c8tc01419a

Molecular structures play a critical role in forming the horizontal orientation of the emission transition dipole of light-emitting TADF materials which can improve the light outcoupling efficiency of OLEDs. Here, two TADF emitters of IndCzpTr-1 and IndCzpTr-2 based indolocarbazole derivatives with stereoscopic and linear structures were designed and synthesized. With a smaller ΔE_ST, better thermal stability and preferential horizontal orientation with an orientation order parameter S of -0.264, a green TADF OLED device based on IndCzpTr-2 realized a maximum EQE of 30.0% and maximum current efficiency (CE) of 82.6 cd A^-1 without using any optical outcoupling technology; meanwhile, a sky-blue TADF OLED with a maximum EQE of 14.5% and a maximum CE of 28.1 cd A^-1 was obtained for IndCzpTr-1 with a homologous device structure.

Full text of DOI:10.1039/c8tc01419a

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Journal of
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Sun, Q. Zhang, Z. Huang, R. Guo, H. Gu, S. Liu and L. Wang, J. Mater. Chem. C, 2018, DOI:
1
0.1039/C8TC01419A.
Volume
4
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1
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January 2016 Pages 1–224
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ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

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To improve efficiency of thermally activated delayed fluorescence
OLEDs by controlling the horizontal orientation through
optimizing stereoscopic and linear structures of indolocarbazole
isomers
a
a
a
a
a
a
Songpo Xiang, Xialei Lv, Shuaiqiang Sun, Qing Zhang, Zhi Huang, Runda Guo, Honggang
b
b
a
Gu, Shiyuan Liu, Lei Wang*
a
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and
Technology, Wuhan, 430074, P. R. China
b
State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University
of Science and Technology, Wuhan, 430074, P.R. China.
*Email: wanglei@mail.hust.edu.cn
Abstract: The molecular structures play a critical role in forming horizontal
orientation of the emission transition dipole of light-emitting TADF materials which
can improve light outcoupling efficiency of OLEDs. Here, two TADF emitters of
IndCzpTr-1 and IndCzpTr-2 based indolocarbazole derivatives with the stereoscopic
and linear structures were designed and synthesized. With the smaller ΔEST, better
thermal stabilities and preferential horizontal orientation with the orientation order
parameter S of -0.264, the green TADF OLED device based IndCzpTr-2 realized the
-1
maximum EQE of 30.0%, maximum current efficiency (CE) of 82.6 cd A without
using any optical outcoupling technology, meanwhile the sky-blue TADF OLED with
-1
maximum EQE of 14.5% and maximum CE of 28.1 cd A was obtained for IndCzpTr-
1
with homologous device structure.

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DOI: 10.1039/C8TC01419A
Introduction
Organic light-emitting diodes (OLEDs) have been flourishing for application in
displays and illuminations, recently.1-6 The traditional fluorescent emitters show an
upper limit internal quantum efficiency(IQE) of 25% due to the fact that only one-
quarter of excitons are singlets.7, 8 Soon afterwards, the phosphorescent emitters
achieving IQE of 100% have been developed for which can harvest both singlet and
triplet excitons.9, 10 Given the high cost and unclear toxicities of metal complexes of
phosphorescent OLEDs, a strategy of the pure organic thermally activated delayed
fluorescence (TADF) with 100% theoretical IQE has been proposed by Adachi and co-
workers.11-13 Although the TADF emitters with high external quantum efficiency(EQE)
have been reported, there is key bottleneck of theoretic optical outcoupling to realize
higher efficiency.14-16 For achieving superior performance, an efficient strategy is to
ameliorate the molecule horizontal orientation. It was possible to achieve EQE of 46%
without any external outcoupling technology by optimizing the emitter molecule
1
7
horizontal orientation in phosphorescent OLEDs. Simultaneously, several molecules
with the almost maximum EQE in TADF emitters have been reported according to the
strategy.18-26
TADF emitters can convert the triplet excitons to singlet excitons through reverse
intersystem crossing (RISC) with a small energy gap (ΔEST) between the lowest singlet
excited state (S
1
) and lowest triplet excited state (T ), while the highest occupied
1
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TADF
molecules generally require be separated. Earlier the donor and acceptor were used in

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2
7
DOI: 10.1039/C8TC01419A
TADF molecule with twisted structures to enhance steric hindrance. However, the
stereoscopic structure is not beneficial to generate horizontal orientation. On account
of that the vertical emitting dipoles in OLEDs generally contribute little to external
emission, the radiation pattern of horizontal emitting dipoles is more suitable for optical
outcoupling due to the optimal distribution of luminous intensity and suppression of
surface plasmon loss.28-31 Consequently, another designed solution is to stretch distance
between planar donor and acceptor moieties, in addition to form more planar and linear
molecular structure to improve dipole out-coupling.22, 27 The donor and acceptor
moieties in TADF molecules both should possess planar structures basing on the
designed strategy. In numerous donor moieties, the most of indolocarbazole moieties
shows the large planar rigid conjugation structures, and series of indolocarbazole-based
TADF materials with superior properties have been reported.32-36 Obviously, the
indolocarbazole moieties are potential to constitute planar and linear TADF molecules
with superior horizontal orientation by linkage with planar electron acceptor moieties.
According to the designed strategy, two isomeric TADF emitters, 11-(4-(4,6-
diphenyl-1,3,5-triazin-2-yl)phenyl)-12-phenyl-11,12-dihydroindolo[2,3-a]carbazole
(IndCzpTr-1) and 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-11-phenyl-5,11-
dihydroindolo[3,2-b]carbazole (IndCzpTr-2) based on indolocarbazole donors with
different spatial configurations were designed and investigated. The triphenyltriazine
moieties were selected as acceptor due to its planar structure, good chemical stability
and strong electron-accepting power. With stereoscopic and linear structures,
IndCzpTr-1 and IndCzpTr-2 endow the different behaviors in photophysical and

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DOI: 10.1039/C8TC01419A
electroluminescence properties. Both of materials exhibit excellent thermal stabilities,
small ΔEST, high photoluminescence quantum yield (PLQY) and short decay lifetime.
Particularly, the IndCzpTr-2 shows better horizontal orientation with orientation order
parameter (S) of -0.264 from the results of variable angle spectroscopic ellipsometry
measurement due to linear structure. The green TADF OLED device based IndCzpTr-
2
realized a high maximum EQE of 30.0% and a maximum current efficiency (CE) of
-1
8
2.6 cd A without using any optical outcoupling technology. Meanwhile the sky-blue
-1
TADF OLED with a maximum EQE of 14.5% and a maximum CE of 28.1 cd A were
obtained for IndCzpTr-1 with homologous device structure. The results show that
TADF OLEDs with higher efficiency could be realized by tuning the molecular
structure to improve horizontal orientation.
Results and discussion
Synthesis and Characterization
Scheme 1. Synthetic routes of IndCzpTr-1 and IndCzpTr-2: (a) K
2
CO
3
, Pd(PPh
3
4
) , toluene,
o
o
ethanol, H
2
O, 100 C, 12 h. (b) PPh
3
, 1,2-dichlorobenzene, 180 C, 24 h. (c) Pd(OAc) ,
2
o
P(tBu) .HBF
3
4
, NaOtBu, toluene, 110 C, 5 h.

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The two target indolocarbazole-based compounds were synthesized by Buchwald-
Hartwig coupling reaction and the synthesis route was presented in Scheme 1. Besides,
the two important intermediate products were obtained from a pot reaction
simultaneously as two isomers. Both of the final compounds were purified by vacuum
sublimation and the chemical structures were confirmed by nuclear magnetic resonance
(NMR) spectroscopy, mass spectroscopy and elemental analysis. The detail synthesis
procedures and characterization data are showed in the Experimental Section. Both of
the materials exhibit high thermal stabilities, which were estimated via
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (as
seen in Fig. S1). The IndCzpTr-1 and IndCzpTr-2 exhibit high decomposition
o
o
temperatures (T
d
) of 426 C and 448 C, respectively. Additionally, IndCzpTr-1 shows
o
a high T
g
of 159 C, while there was no obvious T
g
observed for IndCzpTr-2 from 50
o
o
C to 250 C. The excellent thermal properties of compounds are beneficial to high-
quality film formation and device fabrication through vacuum evaporation.
Table 1. Thermal, Electrochemical, and Photophysical Properties of IndCzpTr-1 and IndCzpTr-2.
a
b
c
d
e/T
f
ΔESTg
f
f
HOMO/ LUMOi
j
λ
PL sol /film
Φ
PL
E
g
S
1
1
τ
p
τ
d
g/ d
T T
Compound
(nm)
(%)
(eV)
(eV)
(eV)
(ns)
(μs)
(eV)
(℃)
IndCzpTr-1
480/492
490/510
75.2
71.9
2.95
2.92
2.94/2.81
2.77/2.66
0.13
0.11
11.09
8.83
25.48
34.31
-5.47/-2.91
-5.29/-2.95
159/426
no/445
IndCzpTr-2
a
Measured in dilute toluene solution at room temperature. b Measured neat films at room temperature. c Absolute
d
PLQY of these emitters doped in mCBP films with integrating sphere at room temperature. Obtained from the onset
e
of the absorption spectra in toluene solvent. Obtained from the onset of fluorescence spectra in 2-MeTHF at 77K.
f
g
Obtained from the onset of phosphorescence spectra in 2-MeTHF at 77K. Energy split between S
1
1
and T estimated
from the low temperature fluorescence and phosphorescence spectra. f τ
(the prompt lifetime) and τ (the delayed
p
d
i
lifetime) were obtained from transient PL decay of doped films. HOMO and LUMO energy levels estimated from
the redox potential in cyclic voltammetry. j
d g
T obtained from TGA measurements and T obtained from DSC
measurements.

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DOI: 10.1039/C8TC01419A
Theoretical Calculations
Fig. 1. a) The optimized geometries at the ground state, the HOMO and LUMO distribution of b)
IndCzpTr-1 and c) IndCzpTr-2, and the calculated energy levels of compounds.
The molecular configuration and the frontier molecular orbital distribution of
IndCzpTr-1 and IndCzpTr-2 were analyzed by the density functional theory (DFT)
calculations using Gaussian 09 software at the B3LYP/6-31G(d) level. The optimal
structures and the distribution of HOMO and LUMO are showed in Fig. 1. For two
isomeric materials, the different configurations are exhibited. The IndCzpTr-1
o
possesses the stereoscopic structure with a twist angle of 71 between IndCz-1 and
triphenyltriazine moieties, while a nearly planar structure is found for IndCzpTr-2 with
o
a much small twist angle of 40 . For both molecules, the HOMOs are mainly dispersed
over the indolocarbazole units, while the LUMOs are fully localized on the
triphenyltriazine moiety and the bridging phenyl. The orbital overlap in the bridge
phenyl could obviously improve the PLQY of the two compounds as seen in following
measurement. In addition, the excited state was calculated using dependent density
functional theory (TD-DFT) at the B3LYP/6-31G(d) level to assess the single and triplet
excited energies, S
1
and T energies gaps (ΔEST) and transition dipole moment. As
1

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shown in Fig. S2, the HOMO value of IndCz-2 is shallower than IndCz-1, which
3
3
indicated that IndCz-2 possesses stronger electron-donating power. Thus, IndCzpTr-
exhibits smaller E of 3.02 eV, S of 2.64 eV and T of 2.45 eV from calculation,
compared with E of 3.24 eV, S of 2.81 eV and T of 2.63 eV of IndCzpTr-1. Since
2
g
1
1
g
1
1
the similar HOMOs and LUMOs distribution, the similar calculated ΔEST values of
IndCzpTr-1 and IndCzpTr-2 are 0.17 eV and 0.19 eV, which potentially enable
effective reverse inter-system crossing (RISC) process from T
1
to S .
1
Electrochemical Properties
IndCzpTr-1
IndCzpTr-2
-2.4
-1.8
0.0
0.6
1.2
Voltage (V)
Fig. 2. Cyclic voltammograms of IndCzpTr-1 and IndCzpTr-2.
The electrochemical properties of IndCzpTr-1 and IndCzpTr-2 were measured
by cyclic voltammetry using traditional three-electrode system. As seen from the Fig.
2, the reduction peaks are similar because of the same acceptor of two compounds.
Notably, the oxidation potential of IndCzpTr-2 is lower than that of IndCzpTr-1,
which demonstrates the stronger electron donating property of the donor in IndCzpTr-
2. The HOMO and LUMO energy levels were estimated to be -5.47/-2.91 eV for

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IndCzpTr-1 and -5.29/-2.95 eV for IndCzpTr-2 with regard to ferrocene. The
experiment results are also in accordance with the trend of calculation consequences.
Photophysical Properties
1
0
0
.0
.5
.0
IndCzpTr-1 1.0
IndCzpTr-2
1
.0
IndCzpTr-1 1.0
IndCzpTr-2
(a)
(b)
0.5
0.5
0.0
0.5
0.0
0.0
3
00
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3. Absorbance and photoluminescence spectra of IndCzpTr-1 and IndCzpTr-2 in a) toluene
solvent and b) 20 nm neat films. Emission spectra were excited at 360 nm.
The absorption and photoluminescence (PL) spectra of two compounds were
measured in dilute toluene solvents and films, as shown in Fig. 3. Two compounds
exhibit absorption bands at the range from 300 nm to 350 nm in solvent and film which
can be assigned to the indolocarbazole moieties. The ICT absorption bands from the
indolocarbazole and triphenyltriazine groups are showed at the around 400 nm.
Apparently, the IndCzpTr-1 shows a slightly blue-shifted absorption because of the
more stereoscopic structure leading to reduced conjugation degree. The energy gaps
(E ) of IndCzpTr-1 and IndCzpTr-2 which were obtained from the onset of the
g
absorption spectra in toluene solvent are 2.95 eV and 2.92 eV. The peaks in the PL
spectra of the IndCzpTr-1 and IndCzpTr-2 are found at 480 nm and 490 nm in toluene,
4
90 nm and 510 nm in film, respectively. The IndCzpTr-2 exhibits a larger redshift
PL, which can be ascribed to the stronger electron-donating ability of IndCz-2 and the
stronger intermolecular reaction. Both of the compounds exhibit similar absorption and

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PL spectrum shapes in toluene solvent and films, yet the red-shift between toluene and
film indicative of a small increase in electron delocalization in the bulk solid state.37
From the PL of different doped concentrations of IndCzpTr-1 and IndCzpTr-2 in
mCBP films (Fig. S3), no emission is observed from mCBP host, while the
photogenerated excitons on hosts smoothly transfer energy to two dopant materials.
And with increasing doped concentrations of these films, IndCzpTr-2 shows the larger
red-shifted PL than that of IndCzpTr-1, because of the planar structure of IndCzpTr-
2
inducing stronger intermolecular reaction. The PL of two materials in different
solvents were tested, and the emissions red-shifted from blue to orange as the polarity
of solvent increased, which exhibiting the obvious ICT-state characteristic (Fig. S4).
The low temperature fluorescence and phosphorescence spectra (Fig. S5) in 2-
methyltetrahydrofuran (2-MeTHF) solutions were measured to estimate the S
1
energy,
energy levels of IndCzpTr-1 and IndCzpTr-2 are
.94 eV and 2.77 eV, which were obtained from the onset of the low temperature
fluorescence. Meanwhile T energy levels are 2.81 eV and 2.66 eV from the onset of
the low phosphorescence spectra. As the result, the ΔEST values were evaluated to be
.13 eV for IndCzpTr-1 and 0.11 eV for IndCzpTr-2, exhibiting high potential to be
TADF materials.
3
8
T
1
energy and ΔEST values. The S
1
2
1
0

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4
3
2
1
0
4
10
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DOI: 10.1039/C8TC01419A
1
1
1
1
1
0
0
0
0
0
(a)
3
(b)
3
1
1
0
0
10
10
3
10
2
2
2
10
0
25
50
75
100
0
25
50
75
100
Time (ns)
Time (ns)
1
0
10
10
0
100
200
300
400
0
100
200
Time (us)
300
400
Time (us)
Fig. 4. Transient PL characteristics for delay component of a) IndCzpTr-1 and b) IndCzpTr-2 in
doped films at 300 K under vacuum. Inset: prompt component of fluorescence decay curve.
In order to confirm the TADF characteristic of IndCzpTr-1 and IndCzpTr-2, the
transient PLof doped films were analyzed at room temperature under vacuum. The Fig.
4
exhibits that IndCzpTr-1 possesses a prompt emission decay curve from S
1
to S
0
with a lifetime of 11.09 ns and a delayed emission component with a lifetime of 25.48
μs, and 8.83 ns of prompt compinent, 34.31 μs of delay component for IndCzpTr-2.
The delayed emission should be derived from the thermal up-conversion of energy from
T
1
to S
1
, then radiates from S
1
to S . Two materials exhibit similar short delayed
0
lifetimes in the doped films, suggesting the efficient RISC process. The temperature
dependent transient PL decay of doped films from 77 K to 300 K was also measured to
further confirm the TADF property (Fig. S6). The ratio of the delayed emission
component gradually increases with the temperature rising, due to the increased T
1
to
up-conversion. Both of materials show high PLQY values of 75.2 % for IndCzpTr-
and 71.9 % for IndCzpTr-2 in doped films, which were measured using an
S
1
1
integrating sphere under air atmosphere. Using the PLQY and decay curves, the rate
constants were calculated and summarized in Table S1 according the reported
method.39

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Molecular orientation
0
.4
0.6
2
1
1
1
1
.0 (a)
S=-0.100
(b)
S=-0.264
2
1
1
1
.1
.8
.5
.2
n
n
n
n
o
e
o
e
0.3
.8
.6
.4
.2
0.4
0.2
0
0
.2
.0
k
o
e
0.1
k
k
o
e
k
0.0
450
600
750
900
450
600
750
900
Wavelength (nm)
Wavelength (nm)
Fig. 5. Ordinary (in-plane) (solid lines) and extraordinary (out-of-plane) (dash lines) refractive
indices (n , n ) (red) and extinction coefficients (k , k ) (blue) of neat (non-doped) films of a)
IndCzpTr-1 and b) IndCzpTr-2.
o
e
o
e
Linear structure molecule is potential to realize horizontal orientation, which can
increase optical out-coupling efficiency of OLED by the optimal distribution of
luminous intensity and suppression of surface plasmon loss. As shown in Fig. 5, the
orientation order parameter (S) was estimated by variable angle spectroscopic
ellipsometry for neat film of IndCzpTr-1 and IndCzpTr-2. By the equation S=(k
e
-
k
o
)/(k +2k ), where k and k are the ordinary and extraordinary extinction coefficients
e
o
o
e
2
9
at the peak of the band attributed to the transition dipole moment. The orientation
order parameter S of two materials are -0.1 and -0.264 for IndCzpTr-1 and IndCzpTr-
2
neat films, respectively. IndCzpTr-2 exhibits better anisotropy, for which the S is
closer to the theoretical value S=-0.5 with the perfectly horizontal orientation. And
according to value S, the orientation factor (θ) was calculated as the 0.70 for IndCzpTr-
2
2
1
and 0.77 for IndCzpTr-2. The results are derived from the different structures of
two materials and the better thermal stability of IndCzpTr-2. The preferential
horizontal orientation would be formed with significant difference between the
4
0
deposition temperature (Tdeposition) and T of the film surface. In the optimized excited
g

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state structures of IndCzpTr-1 and IndCzpTr-2, the transition dipole moments are
both along the molecular long axis (Fig. S7), however the CT state transition of the
longer liner structure of IndCzpTr-2 is more along the molecular long axis. The
calculated results indicate the transition dipole moments of IndCzpTr-2 directed more
along the long molecular axis would enhance the horizontal orientation, matching well
with the results of measurement. In addition, the planar structure of IndCzpTr-2 leads
to the strong preferred orientation parallel to the substrate.
Electroluminescent Devices
1
0
0
.0
.5
.0
(a)
(b)
IndCzpTr-1
IndCzpTr-2
4
3
2
1
0
10
10
10
10
10
IndCzpTr-1
IndCzpTr-2
2
1
1
00
50
00
5
0
0
400
500
600
700
4
8
12
16
Wavelength (nm)
Voltage (V)
4
0
1
IndCzpTr-1
IndCzpTr-2
IndCzpTr-1
IndCzpTr-2
(d)
(c)
100
3
2
1
0
1
0
10
1
0
1
0
1
1
10
1
0
0
.1
1
0.1
10
0
1
2
3
4
0
1
10
2
3
4
10
0
10
10
2
10
10
10
10
2
Luminance (cd/m )
Lumiance (cd/m )
2
Fig. 6. (a) EL spectra at 100 cd/m . (b) current density–voltage–luminance (J–V–L) characteristics.
c) EQE–luminance characteristics. (d) CE–luminance characteristics and PE–luminance
characteristics.
(

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Fig. 7. Energy-level diagram and chemical structures of the materials used in the devices.
Table 2. EL performance of representative TADF OLEDs.
a
b
Maximum Efficiency
Efficiency at 100 cd m-2
e
V
on
L
max
λ
EL
Dopants
CIE(x,y)
-
2
(
V)
(cd m )
(nm)
c
c
-1
c
-1
d
d
-1
d
-1
EQE (%)
CE (cd A )
PE (lm W )
EQE (%) CE (cd A ) PE (lm W )
IndCzpTr-1
IndCzpTr-2
3.8
4.0
4452
7876
14.5
30.0
28.1
82.6
23.2
61.8
11.0
15.3
20.5
41.9
11.5
23.1
(0.17,0.27)
(0.23,0.50)
472
496
a
b
c
The maximum luminance. Operating voltages for onset. The maximum efficiencies of EQE (%), CE
-1
-1
d
-1
-1
-2 e
(
cd A ) and PE (lm W ). The efficiencies of EQE (%), CE (cd A ) and PE (lm W ) at 100 cd m .
EL peak wavelength.
With the superior properties of IndCzpTr-1 and IndCzpTr-2, multiple
electroluminescent devices were fabricated. The EL properties of these devices are
showed in Fig. 6 and summarized in Table 2, and the energy level diagram and
chemical structures of materials used in the devises is shown in Fig. 7. The doped
devices were fabricated using the structures of ITO/MoO
mCP (10 nm) /EML (20 nm) /DPEPO (5 nm) /TmPyPB (25 nm) /LiF (1 nm) /Al (100
nm), in which ITO (indium tin oxide) was used as the anode, MoO was used as hole
3
(10 nm) /TAPC (70 nm)
/
3
injection layer, TAPC (4,4'-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline)) was used
as the hole transporting layer, mCP (N,N-dicarbazoyl-3,5-benzene) was used as an
exciton blocking layer, IndCzpTr-1 and IndCzpTr-2 doped in mCBP (3,3-di(9H-
carbazol-9-yl)) host were used as the emission layer(EML), DPEPO (bis{2-
[
di(phenyl)phosphino]phenyl}ether oxide) was used as exciton blocking layer,

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TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) was used as the electron transporting
layer, LiF was used as electron injecting layer and Al was used as cathode. The doping
concentration of emitters was fixed to be 10 wt% for IndCzpTr-1 and 20 wt% for
IndCzpTr-2 in the EML after optimization of the control device efficiency. As showed
in Fig. 6a, both EL spectra of sky-blue and green EL demonstrate efficient energy
transfer from host to dopant observing for device of IndCzpTr-1 and IndCzpTr-2,
respectively. As the results, the device of IndCzpTr-2 gives maximum external
-1
quantum efficiency (EQE) of 30.0 % and current efficiency (CE) of 82.6 cd A without
using any optical outcoupling technology which exceed the theoretical value of 25%
-1
and a maximum EQE of 14.5 % and maximum CE of 28.1 cd A are obtained for
IndCzpTr-1 with homologous device structure. The high efficiency can be attributed
to the high PLQY, small ΔEST and short delayed lifetime of two materials. Obviously,
device of IndCzpTr-2 exhibits higher external efficiency comparing with that
IndCzpTr-1. One cause of the results is that the smaller ΔEST of IndCzpTr-2 is
favorable for utilizing triplet exciton by the fast up-conversion of T
1
to S . The major
1
reason is the preferred horizontal dipole orientation of IndCzpTr-2 with liner structure
substantially improving out-coupling efficiency. Meanwhile dopants are apt to improve
the horizontally orientation in a host matrix of with random orientation like mCBP.[6b,
6
c]
Furthermore, the materials with excellently horizontal dipole orientation trend to
possess higher carrier mobility due to the increasing of π-π overlap between adjacent
molecules.[6b, 18] The devices of IndCzpTr-2 is supposed to possess more balanced
carrier if the electrons or holes are trapped and transported by dopants, which would

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obviously enhance the efficiency of devices at low brightness. The devices with
different concentration of IndCzpTr-1, IndCzpTr-2 in mCBP host were fabricated
(Fig. S8 and Fig. S9), respectively. With the increasing doping concentration, both
emitters exhibit slightly red-shifted EL spectra, which could be ascribed to the stronger
interaction between emitter and host under high doping concentration. The detail data
of devices was summarized in Table S2. Moreover, the theoretical maximum EQE as
the orientation factor (θ) by optical simulation was calculated using software setfos. As
shown in Fig. S10, the theoretical maximum EQE for device of IndCzpTr-1 is
1
3
4.65527% at θ=0.70, and the theoretical maximum EQE for device of IndCzpTr-2 is
0.01294% at θ=0.77. The results are matching with the results of experiment.
As shown in Fig. S11 and Fig. S12, the hole-only devices and electron-only
devices were fabricated with the structure of ITO/MoO
3
(10 nm)/TAPC (70 nm)/mCP
(10 nm)/Al and ITO/LiF (1
(10 nm)/EML (20 nm)/mCP (10 nm)/ TAPC (70 nm)/MoO
3
nm)/TmPyPB (25 nm)/DPEPO (5 nm)/EML (20 nm)/DPEPO (5 nm)/TmPyPB (25
nm)/LiF (1 nm)/Al (100 nm). The IndCzpTr-1 and IndCzpTr-2 doped in mCBP host
with different doping concentration (5 wt%, 10 wt% and 20 wt%) were used as the
EML. The results are well-matched with the energy level analysis of device structures.
The shallow LUMO cause electrons to be almost trapped into and transported by
dopants. However, due to too large energy difference of HOMO between dopants and
mCP, the holes will be preferentially injected into host and transported via host then
41
combined with the electrons trapped into by dopants. As discussed above, the trap-
assisted mechanism in the OLED devices of IndCzpTr-2 would realize more balanced

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carrier transportation at low brightness to obtain high EQE. Moreover, the efficiency
roll-off for both OLED devices at higher brightness maybe derive from the unbalanced
charge carriers and the direct combination of electron and hole on dopants leading to
the more intense triplet concentration.
Conclusion
In summary, two TADF emitters of IndCzpTr-1 and IndCzpTr-2 based on
indolocarbazole derivatives were designed and synthesized. The isomers IndCz-1 and
IndCz-2 donor moieties were obtained simultaneously from a simple reaction, when
combined with triphenyltriazine moieties, the stereoscopic and linear structures were
realized. Both of materials possess obvious TADF features with small ΔEST and
moderate delayed lifetimes. Moreover, the IndCzpTr-2 exhibits smaller ΔEST, better
thermal stabilities and preferential horizontal orientation with the orientation order
parameter S of -0.264, the green TADF OLED device based on IndCzpTr-2 realized
the a maximum EQE of 30.0%, a maximum current efficiency (CE) of 82.6 cd A-1
without using any optical outcoupling technology. Meanwhile, the sky-blue TADF
-1
OLED with maximum EQE of 14.5 % and maximum CE of 28.1 cd A was obtained
for IndCzpTr-1 with homologous device structure. The results show that TADF
OLEDs with higher EQE could be realized by tuning the molecular structure to improve
horizontal orientation.
Experimental section
General Methods: The most of experimental conditions and equipment was
according to the our reported procedures.42 1H NMR and 13C NMR spectra were

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obtained through a Bruker-AF301 AT 400 MHz spectrometer. Mass spectrometry were
carried out on a Bruker-Daltonics microflex LT/SH mass spectrometer. Elemental
analyses of carbon, hydrogen, and nitrogen were performed on an Elementar (Vario
Micro cube) analyzer. TGA was undertaken using a PerkinElmer Instruments (Pyris1
-1
TGA) under the nitrogen environment at a heating rate of 10 °C min from 30 to 600 °C.
The temperature at 5% weight loss was used as the decomposition temperature (T ).
DSC was conducted on a PE Instruments DSC 2920 unit at a heating rate of 20 °C min
d
-1
from 50 to 250 °C under a nitrogen atmosphere. The glass transition temperature (T )
g
is determined from the second heating scan. Cyclic voltammetry (CV) was carried out
on a computer-controlled EG&G Potentiostat/Galvanostat model 283 at room
temperature with a conventional three electrode cell, which consisted of a Pt carbon
working electrode of 2 mm in diameter, a platinum wire counter electrode, and an
Ag/AgNO3 (0.1 M) reference electrode. The supporting electrolyte was 0.1 M
tetra(nbutyl)ammonium hexauorophosphate (nBu
4
NPF
6
) in CH
2
Cl . The onset
2
potential was determined from the intersection of two tangents drawn at the rising and
background current of the cyclic voltammogram. All solutions were purged with a
nitrogen stream for 10 min before measurement.
UV-Vis absorption spectra were recorded on a Shimadzu UV-VIS-NIR
Spectrophotometer (UV-3600). Absolute PLQYs were obtained using a Quantaurus-
QY measurement system (C11347-11, Hamamatsu Photonics). Fluorescence
measurements and photoluminescence decay characteristics were carried out using
Edinburgh instruments (FLS920 spectrometers). The low temperature fluorescence and

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phosphorescent spectra were measured in 2-MeTHF by a spectrofluorimeter (F-4600,
Hitachi Inc.) at 77 K cooled by liquid nitrogen.
Molecular orientation in neat films was investigated using an ellipsometry
technique. The 100 nm thick films for these measurements were prepared by thermal
evaporation on top of precleaned bare silicon substrates. The optical constants of the
films were then measured using a variable angle spectroscopic Mueller matrix
ellipsometer43, 44 (ME-L, Wuhan Eoptics Technology Co., Ltd) at several incident
o
o
o
angles varying from 60 to 70 by steps of 5 . The ellipsometry data were then analyzed
using an analytical software (Eometrics, Wuhan Eoptics Technology Co., Ltd) to
determine the anisotropic extinction coefficients and refractive indices of the films.
Synthesis
Synthesis of 2-(2-nitrophenyl)-9-phenyl-9H-carbazole: A mixture of (9-
phenyl-9H-carbazol-2-yl)boronic acid (10 g, 34.84 mmol), 1-bromo-2-nitrobenzene
(
7.74 g, 38.32 mmol), 50 mL of K
mL) was added into a three-neck round bottom flask. After bubbling through nitrogen
for 15 min, added Pd(PPh (0.4 g, 0.34 mmol) into the solution and heated to 100 ˚C
stirring for 12 h under a nitrogen atmosphere. The solution was cooled to room
temperature, extracted with dichloromethane, and dried over anhydrous MgSO . After
2
CO
3
(2 mol/L), toluene (100 mL) and ethanol (50
)
3 4
4
removal of the solvent under reduced pressure, the final residue was purified by column
1
chromatography on silica gel to afford a yellow product (11.0 g, yield 87%). H NMR
(
400 MHz, CDCl ) δ [ppm]:8.10-8.13 (t, J = 7.6 Hz, 2H), 7.77-7.79 (d, J = 8.0 Hz, 1H),
3

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7
1
.52-7.57 (m, 5H), 7.37-7.46 (m, 5H), 7.31 (s, 1H), 7.23-7.29 (m, 1H), 7.17-7.20 (m,
H). MS (APCI): calcd for C24
H
16
N
2
O
: 364.1212; found: 364.7592 [M+H]+.
2
Synthesis of 11-phenyl-11,12-dihydroindolo[2,3-a]carbazole and 5-phenyl-
,11-dihydroindolo[3,2-b]carbazole: (2-nitrophenyl)-9-phenyl-9H-carbazole (5 g,
4.75 mmol) and triphenylphosphine (9.67 g, 36.87 mmol) were added into a three-
5
1
neck round bottom flask. Then the mixture was dried in vacuum and the flask was filled
with nitrogen gas, and 150 mL of 1,2-dichlorobenzene was added to dissolve the
o
mixture and heated to 180 C for 24 hours in a nitrogen atmosphere. After removing
the solvent with vacuum distillation, the crude product was purified using column
chromatography and two isomers 11-phenyl-11,12-dihydroindolo[2,3-a]carbazole and
5
-phenyl-5,11-dihydroindolo[3,2-b]carbazole were afforded (total yield 63%). 11-
1
phenyl-11,12-dihydroindolo[2,3-a]carbazole: H NMR (400 MHz, DMSO) δ [ppm]:
9
.78 (s, 1H); 8.26-8.28 (d, J = 7.6 Hz, 1H), 8.15-8.17 (d, J = 8.0 Hz, 1H), 8.05-8.09 (t,
J = 8.8 Hz, 2H), 7.73-7.77 (t, J = 8.0 Hz, 2H), 7.63-7.67 (t, J = 8.0 Hz, 3H), 7.54-7.56
(d, J = 8.0 Hz, 1H), 7.29-7.41 (m, 4H), 7.16-7.20 (t, J = 7.6 Hz, 1H). MS (APCI): calcd
+
for C24
H
16
N
2
: 332.1313; found: 332.7628. [M+H] . 5-phenyl-5,11-dihydroindolo[3,2-
1
b]carbazole: H NMR (400 MHz, DMSO) δ [ppm]: 11.19 (s, 1H), 8.33-8.35 (d, J = 7.6
Hz, 1H), 8.25 (s, 1H), 8.14-8.16 (d, J = 8.0 Hz, 1H), 8.05 (s, 1H), 7.72-7.75 (m, 4H),
7
.54-7.57 (t, J = 5.6 Hz, 1H), 7.35-7.48 (m, 4H), 7.24-7.28 (t, J = 7.2 Hz, 1H), 7.07-
.11 (t, J = 7.2 Hz, 1H). MS (APCI): calcd for C24 : 332.1313; found: 332.6931.
Synthesis of 11-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12-phenyl-11,12-
dihydroindolo[2,3-a]carbazole (IndCzpTr-1): A mixture of 11-phenyl-11,12-
7
H
16
N
2

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dihydroindolo[2,3-a]carbazole (2 g, 6 mmol), 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-
triazine (2.32 g, 6 mmol), Pd(OAc) (0.027 g, 0.12 mmol), tri-tert-butylphosphine tetra-
2
fluoroborate (0.104 g, 0.36 mmol) and NaOtBu (1.44 g, 15 mmol) was added into a 100
mL round bottom flask with nitrogen gas blowing. Dry toluene (50 mL) was added to
dissolve the mixture and refluxed for 5 hours in nitrogen atmosphere. Upon cooling,
removed the toluene and extracted with dichloromethane, then removed the solvent
under vacuum to give the crude product. The crude product was purified with column
chromatography, then recrystallized with dichloromethane/hexane to afford light
1
yellow powder (1.9 g, yield: 50 %). H NMR (400 MHz, CDCl
3
) δ [ppm]: 8.72-8.74 (d,
J = 6.4 Hz, 4H), 8.43-8.45 (d, J = 8.4 Hz, 2H), 8.05-8.14 (m, 4H), 7.50-7.58 (m, 6H),
7
7
1
1
.16-7.33 (m, 6H), 7.05-7.09 (t, J = 7.6 Hz, 1H), 6.90-6.98 (m, 4H), 6.75-6.77 (d, J =
1
3
.6 Hz, 2H). C NMR (100 MHz, CDCl ) δ [ppm]: 171.72, 171.12, 143.99, 143.49,
3
42.94, 139.98, 136.26, 133.80, 132.62, 129.68, 129.02, 128.89, 128.72, 128.18,
27.80, 126.59, 126.37, 126.31, 125.77, 125.70, 125.36, 125.13, 121.37, 121.11, 120.03,
119.92, 114.15, 113.71, 111.01, 110.90. MS (APCI): calcd for C45
H
29
5
N : 639.2423;
+
found: 639.2841 [M+H] ; Elemental analysis (%): calcd for C45
.57; N, 10.95; found: C 84.77, H 4.44, N 10.79.
Synthesis of 5-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-11-phenyl-5,11-
H
29
N
5
: C, 84.48; H,
4
dihydroindolo[3,2-b]carbazole (IndCzpTr-2): The synthesis process was referred to
1
the detailed method of IndCzpTr-1 (yellow powder, yield: 73%). H NMR (400 MHz,
CDCl
3
) δ [ppm]: 9.09-9.11 (d, J = 5.6 Hz, 2H), 8.84-8.86 (d, J = 5.6 Hz, 4H), 8.28 (s,
1
H), 8.13-8.17 (t, J = 7.6 Hz, 2H), 8.09 (s, 1H), 7.95-7.97 (d, J = 8.4 Hz, 2H), 7.59-

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7
.70 (m, 12H), 7.52-7.54 (t, J = 4.0 Hz, 1H), 7.42-7.46 (m, 3H), 7.25-7.30 (m, 1H). C
NMR (100 MHz, CDCl
3
) δ [ppm]: 171.84, 171.04, 142.33, 142.12, 141.53, 138.28,
37.35, 136.50, 136.20, 134.70, 132.66, 130.79, 130.05, 129.05, 128.73, 127.38,
27.32, 126.78, 126.23, 126.14, 124.00, 123.76, 123.51, 123.44, 120.41, 120.31, 119.93,
19.48, 109.71, 109.56, 100.13, 100.01. MS (APCI): calcd for C45 : 639.2423;
: C, 84.48; H,
1
1
1
H
29
N
5
+
found: 639.2658 [M+H] ; Elemental analysis (%): calcd for C45
.57; N, 10.95; found: C 84.74, H 4.47, N 10.79.
Computational details: The geometrical and electronic properties were computed
H
29
N
5
4
using the Gaussian 09 program package. The ground-state geometry was optimized
using density functional theory (DFT) with B3LYP hybrid functional at the basis set
level of 6-31G(d). The singlet and triplet states were calculated by time-dependent
density functional theory (TD-DFT) with the B3LYP functional at the same basis set
level. Molecular orbitals were visualized using Gauss view 5.0 program.
Device fabrication and measurement: Indium tin oxide (ITO) with a sheet
-1
resistance of 15 Ω square was used as the substrate. Prior to use, the ITO glass
substrates were pre-cleaned carefully and treated by oxygen plasma for 5 min before
device fabrication. Then the sample was transferred to the deposition system. All
-1
organic layers and MoO
3
were deposited at a rate of 1 Å s , and subsequently LiF was
-1
-1
deposited at 0.2 Å s and then capped with Al (ca. 4 Å s ) through a shadow mask in
-6
the vacuum of 2×10 Torr. For all of the OLEDs, the emitting areas were determined
2
by the overlap of two electrodes as 0.09 cm . The EL spectra, CIE coordinates and J-V-
L curves of the devices were measured with a PHOTO RESEARCH SpectraScan PR

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6
55 photometer and a KEITHLEY2400 SourceMeter constant current source. The EQE
values were calculated according to previously reported methods. All measurements
were carried out at room temperature under ambient conditions.
Acknowledgements
This research work was supported by the NSFC/China (51573065, 51727809), China Postdoctoral
Science Foundation(2017M620321), the science and technology support program of Hubei
Province (2015BAA075) and the Analytical and Testing Centre at Huazhong University of Science
and Technology.

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Page 25 of 25
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01419A
The linear and stereoscopic structures were realized in two TADF isomers. With
the smaller ꢀEST, better thermal stabilities and preferential horizontal orientation, the
device based IndCzpTr-2 realized EQEmax of 30.0%, the blueꢁgreen TADF OLED
with EQEmax of 14.5% was obtained for IndCzpTr-1 with homologous device
structure.