Exploiting novel electron-deficient moiety 2,5-diazarcarbazole to functionally construct DPA-containing electron transporting materials for highly efficient sky-blue fluorescent OLEDs

Article abstract of DOI:10.1016/j.dyepig.2020.108935

A span-new electron-deficient moiety 2,5-diazarcarbazole (25NCz) with high T₁ = 2.77 eV was firstly designed and synthesized, which had enormous potential to construct organic electronic materials in photoelectric field. Herein, 25NCz as large rigid-type periphery functionalized group, was introduced to develop novel electron transporting materials (ETMs) by grafting large rigid-type π-conjugated core anthracene. Two novel ETMs p-S25NCzDPA and p-D25NCzDPA were designed and developed. Due to the dual characteristics of rigidity and electron-accepting of the 25NCz, as expected, these two ETMs possessed good thermal stability and high electron mobility. When triplet-triplet fusion (TTF) sky-blue fluorescent OLEDs based on these two novel ETMs were manufactured, they exhibited superior performances. In particular, the maximum external quantum efficiency (EQE) reached 7.45% and the EQE kept 7.45% at the luminance of 50,000 cd m^?2 for p-S25NCzDPA based device. This work rationally demonstrated that the significance of newly developed electron-deficient moiety 25NCz in designing ETMs for high-performance OLEDs.

Full text of DOI:10.1016/j.dyepig.2020.108935

Dyes and Pigments 185 (2021) 108935
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Exploiting novel electron-deficient moiety 2,5-diazarcarbazole to
functionally construct DPA-containing electron transporting materials for
highly efficient sky-blue fluorescent OLEDs
**
*
Shaofeng Ye, Runda Guo , Yaxiong Wang, Yalei Duan, Lei Wang
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430074, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A span-new electron-deficient moiety 2,5-diazarcarbazole (25NCz) with high T1 = 2.77 eV was firstly designed
and synthesized, which had enormous potential to construct organic electronic materials in photoelectric field.
Herein, 25NCz as large rigid-type periphery functionalized group, was introduced to develop novel electron
2
,5-Diazarcarbazole
Electron transporting materials
Sky-blue fluorescent organic light-emitting di-
odes
transporting materials (ETMs) by grafting large rigid-type -conjugated core anthracene. Two novel ETMs p-
π
S25NCzDPA and p-D25NCzDPA were designed and developed. Due to the dual characteristics of rigidity and
electron-accepting of the 25NCz, as expected, these two ETMs possessed good thermal stability and high electron
mobility. When triplet-triplet fusion (TTF) sky-blue fluorescent OLEDs based on these two novel ETMs were
manufactured, they exhibited superior performances. In particular, the maximum external quantum efficiency
Extremely low efficiency roll-off
ꢀ 2
(
EQE) reached 7.45% and the EQE kept 7.45% at the luminance of 50,000 cd m for p-S25NCzDPA based
device. This work rationally demonstrated that the significance of newly developed electron-deficient moiety
5NCz in designing ETMs for high-performance OLEDs.
2
1
. Introduction
Organic light-emitting diodes (OLEDs) have been prevalent in full-
peripheral electron transporting groups is of great importance. Due to
the electron-withdrawing properties of the naphthyridine moiety and its
high thermal stability, a series of naphthyridine derivatives were
designed and developed by Kido et al. [22], their electron transporting
and injecting properties were investigated by comparing with that of the
conventional ETM in phosphorescent OLEDs. The performance of the
naphthyridine derivative DNPF based device was more outstanding than
the conventional device. Benzimidazole (BI) was used as the core
substituted with diphenylphosphine oxide (PO) groups to form
dual-encapsulated charge-exciton separation-type ETMs by Xu and
co-workers [23]. Especially, mmPBIDPO showed a relatively high elec-
tron mobility and effective core encapsulation. The extremely simple
blue thermally activated delayed fluorescence (TADF) devices based on
mmPBIDPO obtained the state-of-the-art performance. Duan and col-
leagues reported a battery of anthracene-based ETMs [24–27] equipped
with pyridine and BI, the synergistic effect between peripheral N-con-
taining heterocycles and anthracene resulted in acquisition of good in-
jection and high electron mobility materials. Our research group
reported a series of electron-deficient moiety 1,5-diazarcarbazole func-
tionally modified anthracene-based ETMs [28]. The TTF fluorescent
color high-end flexible displays and lighting equipment since the pio-
neering work of OLEDs by Tang et al. [1–10]. All the time, the devel-
opment and utilization of new organic electronic materials was the
original driving force of OLED technology. At present, some researchers
are still working to solve the technical bottleneck of blue OLEDs by
developing novel blue materials constructed by new unit, or functional
materials matching with blue devices [7,9,11–16]. Usually, functional
materials, namely hole transporting materials (HTMs), electron trans-
porting materials (ETMs) and host materials are developed to improve
device performance. However, hole mobilities of HTMs are generally
one or two orders of magnitude higher than the electron mobilities of
ETMs [17–19]. As known, one of the keys to achieving
high-performance devices is to obtain extremely balanced charge car-
riers distribution in the emission zone. Reasonably enhancing electron
mobility property of ETMs has become particularly imperative.
For the exploitation of high-performance ETMs, in addition to core
group development [20,21], the modification and functionalization of
*
Corresponding author.
* Corresponding author.
E-mail addresses: runda_guo@hust.edu.cn (R. Guo), wanglei@mail.hust.edu.cn (L. Wang).
*
https://doi.org/10.1016/j.dyepig.2020.108935
Received 7 September 2020; Received in revised form 5 October 2020; Accepted 12 October 2020
Available online 14 October 2020
0
143-7208/© 2020 Published by Elsevier Ltd.

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
OLEDs based them achieved better performance compared to the de-
vices using the common ETMs. When 1,5-diazarcarbazole and PO moi-
ety functionally modified 9,10-diphenylanthracene (DPA) at the same
time, these ETMs possessed deep-blue emission and high PLQY besides
above mentioned, which could construct the high-performance ho-
mogenous OLEDs [29]. In view of above, the introduction and functional
modification of peripheral electron-deficient or electron-withdrawing
groups have great influence on improving the performance of ETMs.
In this work, the newly-developed electron deficient moiety 2,5-dia-
3 4 2 3
mmol), Pd(PPh ) (0.36 g, 0.31 mmol), toluene (120 ml), 2.0 M K CO
aqueous solution (60 ml), ethanol (60 ml) were added to a 500 mL round
◦
bottom flask, the mixture was stirred at 110 C with a nitrogen atmo-
sphere for 12 h. After cooling to room temperature, removing liquid
phase by rotary evaporator. The remaining solid residues were extracted
with dichloromethane for three times, the organic phase dried over
4
anhydrous MgSO . The organic solid residues were obtained by rotary
evaporator, and were purified by column silica gel chromatography
using petroleum ether/ethyl acetate (2:1, V/V) as eluent to obtain the
yellow product (8.60 g, 67%). Next, the yellow product (5.00 g, 24.88
mmol), triphenylphosphine (19.55 g, 74.63 mmol) and o-dichloroben-
zene (150 ml) were added to a 250 mL round bottom flask, the mixture
zacarbazole (25NCz) with triplet energy level (T
developed as an electron-accepting moiety to construct ETMs. Mean-
while, DPA with large rigid-type -conjugated core served as an electron
1
) of 2.77 eV, was firstly
π
◦
transporting channel. In order to make full use of the synergistic effect
was stirred at 110 C with a nitrogen atmosphere for 24 h. After the
between 25NCz and anthracene group in terms of injection and trans-
reaction was completed, removing organic phase by vacuum distillation.
The remaining organic solid residues were purified by column silica gel
chromatography using petroleum ether/ethyl acetate (2:1, V/V) as
′
porting, two novel ETMs, namely 9-(4-(9-([1,1 -biphenyl]-4-yl)anthra-
cen-10-yl)phen-yl)-9H-2,5-diazarcarbazole (p-S25NCzDPA) and 9,10-
bis(4-(9H-2,5-diazarcarbazole-9-yl)phenyl)anthracene (p-D25NCzDPA)
were designed and developed. Researches showed that these two ETMs
indeed possessed good thermal stability and high electron mobility.
When the TTF sky-blue fluorescent OLEDs based on ETMs p-
S25NCzDPA and p-D25NCzDPA were manufactured, the excellent de-
vice performance with maximum luminance (Lmax) of 57,930 and
1
eluent to obtain the pale yellow compound 1 (2.4 g, 57%). H NMR
(600 MHz, CDCl
3
) δ 10.15 (s, 1H), 9.01 (s, 1H), 8.68–8.67 (dd, J = 4.5,
1.1 Hz, 1H), 8.57–8.56 (d, J = 5.3 Hz, 1H), 8.28–8.27 (d, J = 5.2 Hz,
1H), 7.90–7.88 (d, J = 8.3 Hz, 1H), 7.49–7.47 (dd, J = 8.3, 4.6 Hz, 1H).
1
9-(4-bromophenyl)-9H-2,5-diazarcarbazole (compound 2):
H
NMR (400 MHz, CDCl
3
) δ [ppm]: 8.92 (s, 1H), 8.72–8.70 (d, J = 4.6 Hz,
ꢀ
2
5
5,290 cd m , maximum current efficiency (CEmax) of 13.96 and 13.06
1H), 8.64–8.62 (d, J = 5.2 Hz, 1H), 8.29–7.28 (d, J = 6.1 Hz, 1H),
ꢀ 1
cd A , maximum external quantum efficiency (EQEmax) of 7.45% and
.51% were obtained, respectively. More impressively, these devices
achieved extremely low efficiency roll-offs of 0% and 0.9% at the
7.80–7.76 (m, 3H), 7.49–7.46 (m, 3H). MALDI-TOF MS (m/z): calcd for
+
6
C
16
H10BrN
3
, 324.18; found 324.45, [M ].
9-(4-(10-bromoanthracen-9-yl)phenyl)-9H-2,5-diazarcarbazole
(compound 3): 1H NMR (400 MHz, CDCl
) δ [ppm]: 9.19 (s, 1H),
.79–8.77 (d, J = 4.5 Hz, 1H), 8.70–8.66 (m, 3H), 8.39–8.38 (d, J = 5.8
Hz, 1H), 8.08–8.05 (d, J = 8.4 Hz, 1H), 7.83–7.81 (d, J = 8.3 Hz, 2H),
ꢀ 2
luminance of 50,000 cd m , respectively.
3
8
2
. Experimental section
7
.74–7.70 (dd, J = 16.3, 8.5 Hz, 4H), 7.65–7.63 (m, 2H), 7.58–7.56 (dd,
2
.1. Synthesis of compounds
J = 8.4, 4.5 Hz, 1H), 7.51–7.47 (m, 2H). MALDI-TOF MS (m/z): calcd for
+
C
30
H18BrN
3
, 499.07; found 499.24 [M ].
Part of the synthetic method of the intermediate compound 1–4 and
9-(4-(9-([1,1′-biphenyl]-4-yl)anthracen-10-yl)phenyl)-9H-2,5-
diazarcarbazole (p-S25NCzDPA): Compound 3 (2.0 g, 4.0 mmol), 4-
biphenylboronic acid (1.3 g, 4.8 mmol), Pd(PPh (0.05 g, 0.04
mmol), toluene (40 ml), 2.0 M K CO aqueous solution (20 ml) and
final-products referred to our previous work [30,31].
2
,5-diazarcarbazole (compound 1, 25NCz): 2-chloro-3-nitropyri-
3 4
)
dine (10.00 g, 63.29 mmol), 4-Pyridinylboronic acid (9.34 g, 75.95
2
3
Scheme 1. Synthetic routes and molecular structures of p-S25NCzDPA and p-D25NCzDPA. Reagents and conditions: (a) Pd(PPh
3
)
4
, Toluene, 2.0 M K
2
CO
3
aqueous
◦
◦
◦
solution, EtOH, N
2
, 110 C, 12 h. (b) Triphenylphosphine, o-dichlorobenzene, N
2
◦
, 180 C, 24 h. (c) CuI, K
2
CO
3
, 18-crown-6, DMF, N
2
, 160 C, 24 h. (d) NBS, CHCl
3
,
◦
◦
◦
N
2
, 70 C, 6 h. (e) n-BuLi, THF, N
2
, -78 C, 2 h; KI, NaH
2
PO
3
, HOAc, N
2
, 110 C, 2 h. (f) CuI, K
2
CO
3
, 18-crown-6, DMPU, N
2
, 180 C, 24 h.
2

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
1
Fig. 1. H NMR spectra of 25NCz.
ethanol (20 ml) were added to a 250 mL round bottom flask, the mixture
5H), 7.47–7.38 (m, 5H). 13C NMR (100 MHz, CDCl
) δ [ppm]: 142.97,
3
◦
was stirred at 110 C with a nitrogen atmosphere for 12 h. After cooling
140.97, 140.80, 140.67, 140.46, 139.47, 137.82, 137.55, 137.22,
135.43, 135.41, 135.28, 134.01, 133.35, 131.73, 130.00, 129.93,
128.93, 128.21, 127.19, 126.44, 125.59, 125.26, 122.65, 118.28,
to room temperature, removing liquid phase by rotary evaporator, then
solid residues were washed with water, the aqueous phase was removed
by suction filtration. The remaining solid residues were preliminarily
purified by silica gel column chromatography using petroleum ether/
114.72. MALDI-TOF MS (m/z): calcd for C42
H
27
N
3
, 573.22; found,
+
573.43, [M ]; Anal. calcd. (%) For C42
Found: C 87.94, H 4.75, N 7.30.
27 3
H N : C 87.93, H 4.74, N 7.32.
ethyl acetate (2:1, V/V) as eluent to afford a yellow target product p-
1
S25NCzDPA (1.7 g, 75%). H NMR (400 MHz, CDCl
3
) δ [ppm]: 9.21 (s,
9,10-bis(4-(9H-2,5-diazarcarbazole-9-yl)phenyl)anthracene (p-
D25NCzDPA): Compound 1 (1.5 g, 9.1 mmol), compound 4 (2.0 g, 4.1
1
8
7
H), 8.78–8.76 (dd, J = 4.5, 1.3 Hz, 1H), 8.69–8.68 (d, J = 5.2 Hz, 1H),
.37–8.36 (dd, J = 5.3, 1.0 Hz, 1H), 8.10–8.07 (dd, J = 8.4, 1.3 Hz, 1H),
.86–7.81 (dd, J = 12.2, 7.3 Hz, 7H), 7.79–7.77 (m, 5H), 7.58–7.51 (m,
mmol), CuI (0.16 g, 0.82 mmol), K
2 3
CO (2.8 g, 20.5 mmol), 18-crown-6
(0.2 g, 0.8 mmol) and 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-
Fig. 2. Molecular designing, optimized geometries and the HOMO/LUMO energy levels of 25NCz, p-S25NCzDPA and p-D25NCzDPA.
3

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
Table 1
Photophysical properties of p-S25NCzDPA and p-D25NCzDPA.
a
a
b
c
d
e
d/f
◦
g d
T /T ( C)
g
ETMs
λ
abs (nm) solution
λ
em (nm) solution /film
E
g
(eV)
HOMO/LUMO (eV)
HOMO/LUMO (eV)
E
T
(eV)
p-S25NCzDPA
p-D25NCzDPA
297/357/376/396
298/357/376/396
424/443
421/460
3.00
3.02
ꢀ 5.30/-1.83
ꢀ 5.50/-2.00
ꢀ 5.63/-2.63
ꢀ 5.67/-2.65
1.73/1.77
1.73/1.79
139/404
ND/491
a
Abs and PL peaks measured in CH
2 2
Cl .
b
c
d
e
f
PL peaks measured in film.
Calculated from the absorption onset in CH
2 2
Cl .
Calculated from DFT and TD-DFT.
HOMO was measured by CV measurements, LUMO = HOMO + E
g
.
Estimated by the Phos spectra.
g
Measured by DSC/TGA. ND stands for not detected.
pyrimidinone (DMPU, 6 ml) were added to a 50 mL round bottom flask,
dependent density functional theory, TD-DFT simulations) could pro-
vide a reasonable guidance to predict the physical properties of the
monomer 25NCz, p-S25NCzDPA and p-D25NCzDPA. Their molecular
geometric structures and electronic properties were analyzed clearly by
employing Gaussian 09 software with the B3LYP/6-31G(d) basis set. The
corresponding distributions and energy levels of the highest occupied
(HOMO) and the lowest unoccupied molecular orbital (LUMO) were
showed in Fig. 2, the calculated LUMO of the monomer 25NCz was
deeper than carbazole, carboline and 1,5-diazarcarbazole [30], which
could be attributed to the further strengthened electron-withdrawing
ability of 25NCz. The optimized structures of these two ETMs exhibi-
ted highly twisted geometric configurations, which contributed to good
thermal stability [26]. Meanwhile, both HOMOs and LUMOs of these
ETMs were almost distributed on the anthracene core, implying that
anthracene as the electronically active core was mainly responsible for
the fast transporting of electrons. The calculated HOMO/LUMO of these
two ETMs were ꢀ 5.30/-1.83 eV and ꢀ 5.50/-2.00 eV, respectively
(Fig. 2, Table 1). The electron-withdrawing moiety 25NCz with two
electron-deficient C=N double bonds as a functionally modified pe-
ripheral group contributed to the charge transporting and injection. The
◦
the mixture was stirred at 180 C with a nitrogen atmosphere for 24 h.
After the mixture cooled to room temperature, washed by using water,
the solid phase was collected by filtration. The solid residues were
preliminarily purified by silica gel column chromatography using pe-
troleum ether/ethyl acetate (1:1, V/V) as eluent to afford a yellow target
1
product p-D25NCzDPA (1.8 g, 65%). H NMR (400 MHz, CDCl
3
) δ 9.24
(
s, 2H), 8.81–8.79 (d, J = 5.8 Hz, 2H), 8.72–8.71 (d, J = 5.2 Hz, 2H),
8
.40–8.39 (d, J = 6.2 Hz, 2H), 8.12–8.10 (d, J = 9.7 Hz, 2H), 7.90–7.75
(
m, 12H), 7.61–7.58 (dd, J = 8.4, 4.6 Hz, 2H), 7.53–7.51 (dd, J = 6.8,
3
1
.2 Hz, 4H). 13C NMR (100 MHz, CDCl
3
) δ [ppm]: 143.05, 141.22,
37.52, 133.76, 130.98, 127.59, 125.43. MALDI-TOF MS (m/z): calcd
+
for C46
H
28
N
6
, 664.24; found, 664.86, [M ]; Anal. calcd. (%) For
C
46
H
28
6
N : C 83.11, H 4.25, N 12.64. Found: C 83.14, H 4.24, N 12.62.
3
. Results and discussion
3
.1. Synthesis and characterization of compounds
The synthetic routes and molecule structures of these novel ETMs
were depicted in Scheme 1. Firstly, it was worth mentioning that the
calculated T
TD-DFT simulation were 3.15 eV and ~1.73 eV, respectively, the latter
rooted in the large -conjugation core anthracene [27,29].
1
state energy of monomer 25NCz and these ETMs based on
′
cyclization of 3-nitro-2,4 -bipyridine just supposedly produced one
structure, that was 2,5-diazacarbazole (25NCz), due to the symmetry
caused by N atom, no matter the rotation of the C–C bond. Next, the
π
1
structure of new intermediate 25NCz was verified by H Nuclear Mag-
3.3. Thermal stability of compounds
1
netic Resonance ( H NMR) Spectra. The chemical environment and
coupling constant of the proton hydrogen were considered and analyzed
Thermal stability of p-S25NCzDPA and p-D25NCzDPA were per-
formed by thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements, as shown in Fig. 3. The decomposition
comprehensively, the position label in the molecular structure of 25NCz
1
corresponds to the position in the H NMR spectrum as shown in Fig. 1.
These novel materials have been characterized by 1H NMR, C NMR,
Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spec-
trometry (MALDI-TOF MS), and elemental analyses. The detailed mea-
surement instruments, device fabrication and measurements, the time-
of-flight (TOF) transient photocurrent technique and calculation de-
tails referenced the previous reports [29].
13
d
temperatures (T
) of p-S25NCzDPA and p-D25NCzDPA were 404 and
◦
491 C at weight loss of 5%, respectively. The glass transition temper-
◦
g
atures (T ) of p-S25NCzDPA was 139 C, however, the T
g
of p-
g
and T values indicated that
D25NCzDPA was not detected. The high T
d
these two ETMs possessed good thermal and morphological stabilities,
which could be attributed to their appropriate molecular conformations
with rigid anthracene and 25NCz moiety, should be favorable for vac-
uum thermal evaporation and hopefully realizing excellent device sta-
bility upon use in OLEDs.
3
.2. Geometric and electronic structure simulation
Theoretical simulation (density functional theory, DFT and time-
Table 2
Device performances of Devices A-C.
a
2
b
c
d
e
f
Devices
Voltage (V)
L
max/Voltage (cd/m )/(V)
CE (cd/A)
PE (lm/W)
EQE (%)
CIE (x,y)
A
B
C
3.0
2.8
3.1
57,930/8.6
55,290/7.8
38,630/9.0
13.96/13.02/13.96
13.06/12.97/12.83
13.58/13.56/–
9.12/5.68/5.22
10.80/6.36/5.30
9.74/6.23/–
7.45/6.76/7.45
6.51/6.36/6.45
6.92/6.90/–
(0.17, 0.28)
(0.18, 0.30)
(0.16, 0.29)
a
ꢀ 2
Turn-on Voltage. (L = 1 cd m ).
b
c
d
e
f
Voltage at the Lmax
.
ꢀ 2
ꢀ 2
Order of CEs: CEmax, 10,000 and 50,000 cd m
Order of PEs: PEmax, 10,000 and 50,000 cd m
.
.
ꢀ 2
Order of EQEs: EQEmax, 10,000 and 50,000 cd m
.
ꢀ 2
CIE coordinates at 20 mA cm
.
4

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
Fig. 3. (a) TGA and (b) DSC curves of p-S25NCzDPA and p-D25NCzDPA.
3
.4. Photophysical properties
eV. The PL emission peaks of these ETMs in solution were observed at
421 nm, which originated from anthracene core. And the films
~
The ultraviolet–visible absorption (Abs), photoluminescence (PL)
exhibited a bathochromic shift of the emission peak of 19 nm for p-
S25NCzDPA and 39 nm for p-D25NCzDPA, respectively, which could
and phosphorescence (Phos) spectra of monomer 25NCz, DPA, p-
S25NCzDPA and p-D25NCzDPA in dilute solutions and pure thin film
were shown in Fig. 4(a-c). Compared with the Abs spectra monomer
be attributed to the enhanced intermolecular
π
- interaction. In addi-
π
tion, there were new peak of ~550 nm in the emission spectra of the
films based on these two ETMs, which was attributed to the excimer
formation due to the interactions of peripheral 25NCzs. The Phos
2
5NCz and DPA in dilute solutions (Fig. 4a), these two ETMs showed
similar characteristic absorption peaks, these three absorption peaks at
57, 376, 396 nm were derived from the * transition of the anthra-
cenes (Fig. 4b, Table 1). Another absorption peak at ~297 nm was
derived from the * transition of 25NCz. The energy gaps (E ) of these
3
π
-
π
emission peaks of these ETMs were ~700 nm (Fig. 4c), the T
two ETMs was estimated to be ~1.77 eV, which was consistent with the
of the reported anthracene derivatives [25,28].
1
of these
π-
π
g
T
1
two ETMs were ~3.0 eV according to the onset of absorption, which
were greatly affected by absorption of anthracene core. The low-
temperature Phos spectra were measured in 2-methyl-tetrahydrofuran
at 77 K. Fig. 4a showed Phos emission of monomer 25NCz with the
3
.5. Electrochemical properties
The cyclic voltammetry (CV) measurement was used to better un-
1
emission peak of 448 nm, which could be used to estimate the T of 2.77
derstand the electron structure (HOMO/LUMO) of these ETMs. As
Fig. 4. (a) Abs, PL and Phos spectra of monomer 25NCz and DPA in dilute solutions. (b) Abs and PL spectra of these ETMs in dilute solutions and in neat film at
room-temperature, respectively. (c) Phos spectra of these ETMs at 77 K.
5

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
Fig. 7. Electron mobilities of p-S25NCzDPA and p-D25NCzDPA under the
Fig. 5. CV curves of p-S25NCzDPA and p-D25NCzDPA. (Inset: oxidation curve
different electric field.
of ferrocene).
showed difference in low electric field, which could be attributed to the
possibility that TOF transient for electrons of p-D25NCzDPA was highly
dispersive. The high electron transporting mobilities of these ETMs were
propitious to realize balanced charge carriers in the emission zone and
reduce operation voltages of devices.
shown in Fig. 5, p-S25NCzDPA and p-D25NCzDPA exhibit an oxidative
onset potential of 0.83 eV and 0.87 eV compared to ferrocene. The
HOMO energy level was calculated on the basis of the equation HOMO
=
-(Eonset,OX + (-4.8) eV, while the LUMO energy level was calculated
from the HOMO level and the E + HOMO). The HOMO/
(LUMO = E
LUMO of p-S25NCzDPA and p-D25NCzDPA were ꢀ 5.63/-2.63 eV and
5.67/-2.65 eV, respectively, which was in agreement with the ten-
g
g
ꢀ
3.7. Electroluminescent properties
dency of theoretical calculations. Results showed these 25NCz-con-
taining ETMs could appropriately regulate the deeper LUMO energy
level to improve electron injection. The relevant parameters were listed
in Table 1.
In view of high electron mobility and good thermal stability of these
ETMs, the sky-blue fluorescent OLEDs based these ETMs were fabricated
with the structure: ITO/Hexaazatriphenylenehexacabonitrile (HATCN,
1
0 nm)/di-(4-(N,N-ditolyl-amino) phenyl)cyclohexan (TAPC, 40 nm)/N,
N,N-tris(4-(9-carbazolyl)phenyl)amine (TCTA, 5 nm)/2-methyl-9,10-di
2-naphthyl)anthracene (MADN): 3% BUBD-1 (20 nm)/ETMs (30 nm)/
3
.6. Electron-transporting properties
(
The intrinsic electron transporting characteristics of p-S25NCzDPA
LiQ (1 nm)/Al (70 nm), (ETMs: p-S25NCzDPA for Device A; p-
D25NCzDPA for Device B; TPBi for Device C). The device configurations
and the molecule structures were showed in Fig. 8. Here, HATCN, TAPC,
TCTA, MADN, and BUBD-1 were employed as the hole injecting material
(HIM), the hole transporting material (HTM), the electron blocking
material (EBM)/buffer layer, the triplet-triplet fusion host material
(TTF-host) and the sky blue fluorescent guest dopant (FD), respectively.
The Current density-Voltage-Luminance (J-V-L) characteristics,
Current efficiency-Luminance-Power efficiency (CE-L-PE), EQE-L char-
acteristics, and Normalized EL spectra of the devices A-C were described
in Fig. 9. The key performance data were summarized in Table 2 As
shown in Fig. 9d, all devices uniformly showed the sky-blue emission of
BUBD-1 with the Commission International de L’Eclairage (CIE) co-
ordinates of (0.16, 0.30) [34]. Compared to Device C, the Devices A and
and p-D25NCzDPA were evaluated by means of the time-of-flight (TOF)
technique with the device structure of ITO/ETMs (2–3 m)/Al (100 nm)
32]. Their typical TOF transients under an applied field and the elec-
μ
[
tron mobility fitting were showed in Figs. 6 and 7. The electron mobil-
ities followed the Poole-Frenkel relationship and the slopes were all
negative, suggesting their evaporated film could exhibit the excellent
thermal stability, which contributed to prolonging lifetime of devices
[
25]. Impressively, the electron mobilities of p-S25NCzDPA and
ꢀ
2
3
2
ꢀ 1 ꢀ 1
p-D25NCzDPA were 1.26 × 10 cm V
s
at electric field of 3.40 ×
5
ꢀ 1
ꢀ 4
ꢀ 1 ꢀ 1
5
1
0 V cm and 6.04 × 10 cm V
s
at electric field of 5.68 × 10 V
ꢀ
1
cm , respectively, which is an order of magnitude higher than that of
the widely used ETM 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene
(
TPBi) [33]. Obviously, the electron mobilities of these novel ETMs
Fig. 6. Typical TOF transients of electrons for (a) p-S25NCzDPA and (b) p-D25NCzDPA under an applied field.
6

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
Fig. 8. (a) Device structure and energy level values for each material; (b) Chemical structures of the employed materials.
Fig. 9. (a) J-V-L curves; (b) CE-L-PE curves; (c) EQE-L curves; and (d) Normalized EL spectra. (EL properties of Devices A-C based on p-S25NCzDPA, p-D25NCzDPA
and TPBi, respectively).
Power efficiencies (PEmaxs) of 9.12 and 10.80 lm W 1, and EQEmaxs of
7.45% and 6.51%, respectively. It seemed that the p-S25NCzDPA-based
Device A was more efficient than the p-D25NCzDPA-based Device B
because of the higher electron mobility and the relatively better charge
ꢀ
B had relatively low turn-on voltages of 3.0 V and 2.8 V, which were
attributed to the high electron mobilities and good injection of these
novel ETMs; and achieved more prominent device performance with
ꢀ 2
ꢀ 1
L
maxs of 57930 and 55290 cd m , CEmaxs of 13.96 and 13.06 cd A ,
7

S. Ye et al.
Dyes and Pigments 185 (2021) 108935
carrier balance in device A. Uniformly, these outstanding device per-
formances mainly originated from higher electron mobility, which
facilitated the balance of carriers in the emission zone. Perhaps because
of that, more impressively, Device A and B displayed almost no effi-
ciency roll-off at extremely high luminance (the efficiency roll-off of 0%
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Shaofeng Ye: Conceptualization, Methodology, Investigation, Soft-
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Writing - review & editing. Yalei Duan: Software, Validation. Lei
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