High-performance non-doped blue OLEDs based on 1,2,4-triazole-phenanthroimidazole derivatives with negligible efficiency roll-off

Article abstract of DOI:10.1039/d1tc01079d

Triazole and phenanthroimidazole moieties are commonly used to construct efficient blue-emitting materials as acceptors and donors, respectively. Benefitting from the D-A coupling of the two moieties, two novel bipolar blue emitters,TAZ-PPIand4NTAZ-PPI, w

Full text of DOI:10.1039/d1tc01079d

View Article Online
View Journal
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: W. Cao, A.
Abdurahman, P. Zheng, M. Zhang and F. Li, J. Mater. Chem. C, 2021, DOI: 10.1039/D1TC01079D.
Volume
Number
6
5
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
February 2018
Pages 911-1266
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2050-7526
PAPER
Nosang V. Myung, Yong-Ho Choa et al.
A
noble gas sensor platform: linear dense assemblies of
a multi-layered
single-walled carbon nanotubes (LACNTs) in
ceramic/metal electrode system (MLES)
rsc.li/materials-c

Please do not adjust margins
Journal of Materials Chemistry C
Page 1 of 7
View Article Online
DOI: 10.1039/D1TC01079D
ARTICLE
High-Performance non-doped blue OLEDs based on 1,2,4-triazole-
phenanthroimidazole derivatives with negligible efficiency roll-off
Wenjuan Cao, Alim Abdurahman, Ping Zheng, Ming Zhang*, Feng Li
Received 00th January 20xx,
Accepted 00th January 20xx
Triazole and phenanthroimidazole moieties are commonly used to construct efficient blue-emitting materials as acceptors
and donors, respectively. Profiting from the D-A coupling of the two moieties, two novel bipolar blue emitters, TAZ-PPI and
4NTAZ-PPI were synthesized. They showed excellent thermal stability and high photoluminescence quantum yield (PLQY).
Non-doped blue organic light-emitting diodes (OLEDs) based on 4NTAZ-PPI and TAZ-PPI present maximum external quantum
efficiencies of 7.3% and 5.9%, respectively, and exhibit negligible efficiency roll-offs. The CIE coordinates are (0.149, 0.131)
and (0.149, 0.130) at the luminance of 1000 cd m-2 , respectively. Because the combination of electron donors and acceptors
is a favorable method to balance the carrier injection and transport, 4NTAZ-PPI and TAZ-PPI exhibit excellent holes and
electrons transporting capabilities.
DOI: 10.1039/x0xx00000x
result in inferior carrier injection and transport, low fluorescent
efficiency.6,24 To address these issues, molecules based on
donor (D)-acceptor (A) types are usually applied to improve
charge balance.25,26 However, considerable numbers of them
would produce an intramolecular charge transfer (ICT) effect,
resulting in a red shift of emission. The selection of appropriate
electron-donating and -withdrawing moieties can play a vital
role in determining the color.27,28
1. Introduction
Organic light-emitting diodes (OLEDs) have great potential for
display and lighting application due to their light-weight,
flexibility and fast respons.1-6 In recent years，a lot of effort has
been put into developing highly efficient blue luminescent
material, because efficient blue emitters not only act as energy
donor to transfer energy to red and green dopants in full-color
display but also significantly reduce device power
consumption.7,8 Up to now, a large amount of efficient blue
organic materials based on phosphorescence and thermally
activated delayed fluorescence (TADF) have been designed and
synthesized.9-14 Generally, both phosphorescent and TADF
materials are doped into host materials, which would
complicate the structure of devices and increase the
manufacturing costs.15-18 In addition, the performance of most
devices is highly dependent on doping concentrations and
matrix materials, and the efficiency roll-off at high brightness is
mainly ascribed to the accumulation of triplet excitons.19-21
Triplet–triplet annihilation (TTA) materials can make use of
triplet excitons and boosts the theoretical limit of the internal
quantum efficiency (IQE) from 25% to 62.5% through fusing two
triplets to a singlet.22,23 In rare cases, TTA materials are
accessible to non-doped OLEDs with negligible efficiency roll-
off. Therefore, it is still a significant challenge to develop high-
performance blue emitters suitable for the non-doped devices.
Blue emission implies a wider band gap and limited molecular
conjugate size for emitters, and these properties generally
Phenanthroimidazole (PI) derivatives are commonly used to
construct efficient blue-emitting materials. Typically, the PI unit
has bipolar character, good thermal stability, and high
photoluminescence quantum yield.29-34 Triazole (TAZ)
derivatives have been employed as building blocks for efficient
electron-transporting and hole-blocking materials due to the
electron-deficient properties.35-37 For the past few years，it has
been reported that 1,2,4-triazole derivatives were used as the
acceptors to construct highly efficient pure blue or near-
ultraviolet electroluminescence emitters.38-41 We have reported
a blue OLED based on a TAZ derivative with a maximum EQE of
6.8% and CIE coordinates of (0.158, 0.043) and the extra singlets
are attributed to the triplet-polaron interaction (TPI) induced
upconversion from triplet to singlet.38 In this work, triazole
derivatives as the acceptors and phenanthroimidazole
derivatives as the donors were firstly exploited to construct two
highly efficient donor-acceptor (D-A) type blue emitters, TAZ-
PPI and 4NTAZ-PPI. Besides, the ICT effect is weakened by the
introduction of the benzene ring between phenanthroimidazole
and triazole units. The EL performance can be greatly improved
by changing the structure design of triazole derivatives linked
with phenyl or naphthalene. Notably, the PLQY of 4NTAZ-PPI in
the pristine film is 60%, and that of TAZ-PPI is 67%. Non-doped
organic devices based on 4NTAZ-PPI and TAZ-PPI exhibited EQEs
of 7.3% and 5.9%, and CIE coordinates of (0.149, 0. 131) and
(0.149, 0.130) at a luminance of 1000cd m-2, respectively. More
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry, Jilin University, Changchun 130012, P. R. China
E-mail: zhming@jlu.edu.cn
† Electronic Supplementary Information (ESI) available: TGA, DSC, CV curves,
transient decay curves, additional electroluminescence data of the OLED devices,
NMR and other characterization. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 2 of 7
ARTICLE
Journal Name
importantly, these devices demonstrated negligible efficiency
roll-offs.
2. Experimental Section
View Article Online
DOI: 10.1039/D1TC01079D
Scheme 1 Synthetic routes and chemical structures of TAZ-PPI and 4NTAZ-PPI.
Synthesis
biphenyl]
of
2-(4'-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-[1,1'-
3. Results and discussion
-4-yl)-1-phenyl-1H-phenanthro[9,10-d] imidazole (TAZ-PPI)
TAZBr (0.55 g, 1.46 mmol), PPIB (0.8 g, 1.61 mmol), potassium
carbonate (1.8 g, 13 mmol), tetrahydrofuran (30 mL), deionized
water (16 mL) and Pd(PPh3)4 (0.084 g, 0.073 mmol) were placed in a
100 ml round-bottom flask. The mixture was stirred at 70 °C for 36 h
under an argon atmosphere. After the reaction is over, extract with
dichloromethane for several times, dry organic solution with
anhydrous MgSO4. The residue was purified via column
chromatography by using dichloromethane/ethyl acetate (3:1, v/v)
mixture as eluent. A white power was obtained. (0.58 g, Yield: 60%).
1H NMR (500 MHz, CDCl3) δ 8.80 (d, J = 8.3 Hz, 1H), 8.74 (d, J = 7.9 Hz,
1H), 7.87 – 7.74 (m, 1H), 7.74 – 7.62 (m, 6H), 7.51 (m 15H), 7.43 –
7.35 (m, 1H), 7.35 – 7.31 (m, 3H), 7.22 (d, J = 7.2 Hz, 3H). 13C NMR
(151 MHz, CDCl3) δ 154.92, 154.35 ， 141.16 ， 135.39, 130.34,
130.01, 129.67, 129.61, 129.13, 128.82, 128.38, 127.91, 126.93,
126.82, 126.41, 124.19, 123.09, 120.91. MS(m/z) calculated for
C47H31N5 ： 665.26; Found [M]+ 665.24. Elem. Anal. Calcd (%) for
C47H31N5: C, 84.79, H, 4.69, N, 10.52. Found: C,84.74; H, 4.65; N, 10.50
Synthesis and characterization of compounds
The synthetic scheme and molecular structures of TAZ-PPI and
4NTAZ-PPI are depicted in Scheme 1. The synthesis of PPIBr and
PPIB were according to previous reports.30,42 The last target
molecules were synthesized by the Suzuki cross-coupling
reaction with good yields. Those chemical structures were
confirmed by mass spectrometry and NMR. The synthetic
details can be seen in the supporting information and
experimental section.
Thermal stability and electrochemical properties
Synthesis of 2-(4'-(4-(naphthalen-1-yl)-5-phenyl-4H-1,2,4-triazol-3-
yl)-[1,1'-biphenyl]-4-yl)-1-phenyl-1H-phenanthro[9,10-d]
imidazole(4NTAZ-PPI)
The synthesis process of 4NTAZ-PPI was similar to the detailed
method of TAZ-PPI. The residue was purified via column
chromatography by using dichloromethane/ethyl acetate (3:1,
v/v) as eluent to give 4NTAZ -PPI as a yellowish white solid (0.57
1
g, Yield: 55%). H NMR (500 MHz, CDCl3) δ 8.79 (d, J = 8.3 Hz,
1H), 8.72 (d, J = 8.3 Hz, 1H), 8.05 (d, J = 8.3 Hz, 1H), 7.98 (d, J =
8.3 Hz, 1H), 7.78 (t, J = 7.5 Hz, 1H), 7.73 – 7.61 (m, 6H), 7.60 - Figure 1. TGA curves of 4NTAZ-PPI and TAZ-PPI.
7.52 (m, 5H), 7.44 (m, 11H), 7.33-7.22(m, 3H), 7.17 (m, 3H) 13
C
NMR (151 MHz, CDCl3) δ 155.72, 155.19, 141.07, 134.21, The thermal stability of TAZ-PPI and 4NTAZ-PPI were
131.88, 131.80, 130.58, 130.26, 129.86, 129.59, 129.07, 128.61, characterized by thermal gravimetric analysis (TGA) (Figure 1)
128.37, 128.31, 128.10, 127.36, 126.84, 126.70, 126.36, 125.41, and differential scanning calorimetry (DSC) under a nitrogen
124.16, 123.07, 122.04, 120.89. MS (m/z) calculated for atmosphere (Figure S1). The two compounds exhibited
C51H33N5:715.27; Found [M]+ 715.27.Elem. Anal. Calcd (%) for extremely high decomposition temperature (Td, corresponding
C51H33N5: C, 85.57, H, 4.65, N, 9.78. Found: C, 85.58; H, 4.59; N, to 5% weight loss) of 491oC due to the bulky and rigid skeleton
9.76.
of two molecules. As shown in Figure S1, no distinct glass
transition temperatures (Tg) were observed for the two
compounds, indicating that the thermal morphology is stable in
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 7
Journal of Materials Chemistry C
Please do not adjust margins
Journal Name
ARTICLE
the solid film. From those thermal analysis data, both the emission peak only has a red-shifted of 14 nm Vtioew4A5rt0iclenOmnlinine
compounds display outstanding thermal stability, which is pristine films, indicating that ICT effect is weDaOkeI:n1e0d.10to39s/oDm1TeC0e1x0te79nDd
conducive to get devices with remarkable performance.
due to the rigidly twisted molecular conformation. The solvation
effect was tested to further investigate the excited state properties
of the two molecules. The PL spectra of 4NTAZ-PPI shift 24 nm from
420 nm in low polar solvent n-hexane to 444 nm in high-polarity
acetonitrile. TAZ-PPI exhibits 19 nm bathochromic variations from
421 nm in n-hexane to 440 nm in acetonitrile. The shift of emission
peak in PL spectra of the two compounds is not remarkable and
vibration fine structure exists in low polarity solution, indicating
mainly localized excited states (LE) characteristic of the lowest
excited state S1 for the two compounds. In addition, the dipole
Furthermore, electrochemical properties of TAZ-PPI and
4NTAZ-PPI were measured by cyclic voltammetry (CV) and the
results are shown in Figure S2. The oxidation potentials of TAZ-
PPI and 4NTAZ-PPI are determined to be 1.17 and 1.18 V, which
correspond to the HOMO levels of -5.97 and -5.98 eV,
respectively. The reduction potentials of TAZ-PPI and 4NTAZ-PPI
are -2.08 and -2.17 V，which correspond to the LUMO levels of
-2.72 and -2.63 eV, respectively.
Photophysical properties
휇
moments ( 푒) of the excited states of 4NTAZ-PPI and TAZ-PPI are
To further understand the basic photophysical properties of TAZ-
PPI and 4NTAZ-PPI, the normalized UV–vis absorption and
photoluminescence (PL) spectra in dichloromethane (DCM) and
pristine films are displayed in Figure 2. TAZ-PPI shows almost the
same absorption and PL spectra as those of 4NTAZ-PPI. The
absorption band centred at 288 nm can be ascribed to the π–π*
transition of triazole moiety40 and the absorption at 347 nm and 368
nm belong to the π−π* transition of PPI group.31 The optical band gap
of TAZ-PPI and 4NTAZ-PPI were estimated to be 2.92 eV based on the
onset of absorption. The PL spectra of DCM solution show an
emission maximum at 436 nm for the two compounds. Moreover,
estimated by the Lippert-Mataga model.For the two compounds, the
휈 ― 휈 )
(
plot of the Stokes shift
versus the solvent polarity function
푓
a
푓
휇
was drawn. The result shows 푒 is 10.2 D and 8.9 D for 4NTAZ-PPI
and TAZ-PPI, respectively. These values are little larger than that of
conventional LE emitters such as anthracene (approximately 4.0-6.0
휇
=
푒
D) while much smaller than the typical CT molecule DMABN with
23 D,43 indicating that the excited states of both compounds are
mainly LE state (Figure S3). Further, the dipole moment of 4NTAZ-PPI
is a bit greater than TAZ-PPI, which indicates the excited state of
4NTAZ-PPI retains a slight CT component.
Figure 2. (a) Normalized UV-PL spectra of 4NTAZ-PPI and TAZ-PPI in DCM solutions. (b) 4NTAZ-PPI and TAZ-PPI in pristine film. (c) PL spectra of 4NTAZ-PPI and (d) TAZ-
PPI measured in various solvents with increased solvent polarity at room temperature.
Importantly, the PLQYs of both compounds were 73% in THF and 2.91 eV for 4NTAZ-PPI and TAZ-PPI from the fluorescence
solution. 4NTAZ-PPI and TAZ-PPI exhibited high PLQYs of 60% and spectra, respectively. According to the first vibrational
67% in the film, respectively. The transient PL decay spectra of phosphorescent peak, the T1 of both compounds were obtained to
4NTAZ-PPI and TAZ-PPI in THF solution were measured (Figure S4). be 2.27 eV. Therefore, the energy bandgap (ΔEST) between S1 and T1
Both compounds show short fluorescent lifetimes of approximately were estimated to be 0.61 and 0.64 eV for 4NTAZ-PPI and TAZ-PPI,
1.5 ns in the THF solvent, and there is no delayed fluorescence. Thus, respectively, which is not conducive to the effective reverse
the radiative transition rate (Kr) was estimated to be near 5.0 × 108 intersystem crossing (RISC) from T1 to S1 states. Fluorescence and
s-1 for both of them. The S1 (ES1) and T1 (ET1) energy levels were phosphorescence (at 77 K) spectra of 4NTAZ-PPI and TAZ-PPI pristine
estimated by recording fluorescence and phosphorescence spectra films were also measured, as shown in Figure S7. Similar results as
at 77 K in THF solution (Figure S6). The ES1 was determined to be 2.88
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 4 of 7
ARTICLE
Journal Name
the THF solvent (at 77 K) are acquired. Detailed photophysical
properties are listed in Table 1.
View Article Online
DOI: 10.1039/D1TC01079D
Table 1 Photophysical data and energy levels of 4NTAZ-PPI and TAZ-PPI.
λabs
(nm)a
λPL
(nm)b
Eg
(eV)c
S1
(eV)d
T1
(eV)d
HOMO
(eV)e
LUMO
(eV)e
PLQY
(%)b
τ
κr
8(s-1)f
κnr
8(s-1) f
Compound
(ns)a
10
10
4NTAZ-PPI
TAZ-PPI
284,338,363
284,338,363
450
450
2.92
2.92
2.88
2.91
2.27
2.27
-5.98
-5.97
-2.63
-2.72
60
67
1.5
1.5
5.00
5.03
1.85
1.86
aMeasured in dilute DCM solution at room temperature. bMeasured in a thin film. cOptical band-gap calculated from the absorption spectra onset in THF solution. dEstimated from
fluorescent and phosphorescent spectra at 77 K in THF solution. eMeasured by CV measurement. fCalculated from the equation: κr = φ/τ; τ=1/(κr +κnr), meased in THF solution.
Density functional theory (DFT) calculations
LUMO of 4NTAZ-PPI dominantly concentrates on the naphthyl ring.
It can be seen that the electron-cloud spatial distributions between
HOMO and LUMO for naphthyl-based 4NTAZ-PPI is more separated
than that of phenyl-based TAZ-PPI. In addition, the nearly full spatial
separation between HOMO and LUMO for 4NTAZ-PPI is favorable to
the effective transport of carriers, while the orbitals overlap for TAZ-
PPI is propitious for the radiative transition.27,28 The calculated
results of the energy levels of first-ten singlet and triplet excited
states are summarized (Figure S8). The S1 of both compounds were
obtained to be 3.3 eV and The T1 was calculated to be 2.15 and 2.57
eV for 4NTAZ-PPI and TAZ-PPI by DFT calculation, respectively.
To better understand the relationship between the structure and
properties of the two compounds. We carried out density functional
theory calculations by using the B3LYP/6-31G (d, p) method of the
Gaussian 09 program package. Optimized molecular geometries can
be seen in Figure 3, TAZ-PPI and 4NTAZ-PPI adopted highly twisted
biphenyl units to regulate the molecular conjugation, which is in
favour of preventing the inter-molecular π-π stacking. The highest
occupied molecular orbitals (HOMO) are mainly located on the donor
unit (PI) and the biphenyl bridge for the two molecules. The lowest
unoccupied molecular orbital (LUMO) of TAZ-PPI is predominantly
delocalized on the triazole and adjacent phenyl ring, whereas the
Figure 3. Optimized geometries, the HOMO and LUMO orbitals distributions of (a) TAZ-PPI (b) 4NTAZ-PPI.
Electroluminescence properties
transport properties of 4NTAZ-PPI are slightly better than that of
TAZ-PPI.
The non-doped OLEDs were fabricated with the optimized device
The balanced carrier injection and transport are very important to
obtain excellent device efficiencies. Figure S9 shows the current
density versus voltage characteristics of the hole-only (HOD) and
electron-only devices (EOD).30 The carrier mobility can be calculated
from the J1/2-V curves of single-carrier devices according to the space
charge limited current (SCLC) model (Figure S10, Supporting
Information). The electron and hole mobilities were calculated to be
1.6×10-5 cm2 V-1s-1 and 1.1×10-6 cm2 V-1s-1 for 4NTAZ-PPI, 1.4×10-5cm2
V-1s-1 and 1.0×10-6 cm2 V-1s-1 for TAZ-PPI. The results suggest TAZ-PPI
and 4NTAZ-PPI have good holes and electrons transporting
capabilities. Additionally, both compounds transport electrons
better than holes, which is attributed to the good electron
transporting property of triazole groups. Further, the carrier
structure of ITO/MoO3(3 nm)/TCTA(40 nm) /TAZ-PPI or 4NTAZ-PPI
(20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm), where ITO (indium tin
oxide) is the anode, MoO3 is the hole injecting layer, TCTA (tris(4-
carbazoyl-9-ylphenyl)amine) acts as the hole transporting layer
(HTL), 4NTAZ-PPI and TAZ-PPI are the emitting layers and TPBI (1,3,5-
tris-(N-phenylbenzimidazol-2-yl)benzene) plays the role of electron
transporting layer. The device structure along with the HOMO/LUMO
levels of organic materials are shown in Figure 4 (a).
The non-doped device based on 4NTAZ-PPI exhibits blue emission
with a maximum EQE of 7.3 % and CIE coordinates of (0.149, 0.131).
The EL spectrum has an emission peak at 460 nm and the maximum
power efficiency (PE) and current efficiency (CE) reach 6.3 lm W-1 and
8.0 cd A-1, respectively. Compared with 4NTAZ-PPI, the maximum
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 7
Journal of Materials Chemistry C
Please do not adjust margins
Journal Name
ARTICLE
EQE of the TAZ-PPI based device is 5.9 % and maximum PE, CE up to which is not conducive to the effective reverse intersystem crossing
View Article Online
6.1 lm W-1 and 6.7 cd A-1, respectively. The EL spectrum shows blue (RISC) from T1 to S1 states. The energy of two triplet excitons lies
DOI: 10.1039/D1TC01079D
emission with a peak at 460 nm and the corresponding CIE slightly above that of the singlet exciton, (E(S1) ≲2E(T1)), which
coordinates of (0.149, 0.130). The EL spectra almost remain facilitates TTA mechanism.46-48 However, for TAZ-PPI and 4NTAZ-PPI
unchanged as driving voltage increases from 5 to 8.5 V for all devices compounds, the energy of two triplet excitons is much higher than
(Figure S11), which demonstrated the excellent spectra stability of that of the singlet exciton according to the experimental and
the EL process. Interestingly, although 4NTAZ-PPI has lower PLQY in theoretical results (Figure S7 and Figure S8). In addition, the EL
film ，the EQE of the 4NTAZ-PPI-based device is a little higher than intensities of the devices linearly increase as the current density
that of the TAZ-PPI-based device. The slightly superior charge increases (Figure S12), together with no delayed fluorescence in the
mobility of 4NTAZ-PPI is considered to be an influencing factor. transient PL decay measured at 77 K (Figure S5). These results
4NTAZ-PPI and TAZ-PPI based devices show very small efficiency roll- suggest TTA are not dominant mechanism. Time-dependent DFT
off and remain EQEs of 7.0 % and 5.5% at the luminance of 1000 cd calculations were performed and the results indicate that there is no
퐸푄퐸푚푎푥 ― 퐸푄퐸1000
large energy gap (over 0.5 eV) between T1 and Tn for the compounds
-2
η
=
m , respectively. Further, according to
,
푟표푙푙 ― 표푓푓
퐸푄퐸푚푎푥
which is the prerequisite of HLCT (Figure S8).45 The solvatochromic
effect (Figure S3) also indicates HLCT has the undominant
contribution to the emission of the devices. In view of the above
results, it can be inferred that TADF, TTA and HLCT are not the main
mechanism of converting triplet to singlet excitons in this work. As
previously reported, some blue OLEDs with much higher singlet
ratios than 25% and without observable delayed fluorescence may
have the TPI process38,39. The TPI process does not need the spin-flip.
Moreover, TPI can occur circularly, that is TPI can continuously
convert triplet to singlet, which may induce much higher singlet ratio
in OLEDs. TPI can also happen at high current density. Therefore, one
advantage of the TPI mechanism is that the efficiency roll-off of the
OLEDs are not remarkable. So, according to the negligible efficiency
roll-offs, we conjectured that the extra singlets here were attributed
to the TPI mechanism. The deeper study about the TPI mechanism of
the triazole cored D-A type emitters needs to be further performed.
η
)
of non-doped 4NTAZ-PPI and TAZ-
the efficiency roll-offs (
푟표푙푙 ― 표푓푓
PPI devices at 1000 cdm-2 were 4.1% and 6.8%, respectively.44 The
performance of devices based on 4NTAZ-PPI is comparable to those
of the reported high-efficiency blue devices.
The electrically excited singlet exciton ratios were obtained in the
range of 60-40% and 44-30% for 4NTAZ-PPI and TAZ-PPI,
respectively, which greatly exceeded the theoretical limitation of
25% from the spin statistics for fluorescent OLEDs assuming the out-
coupling efficiency is 20-30%.17 This means that there exist extra
ways to create singlet excitons. Presently, the proposed different
ways to create singlet excitons converted from triplets include: TADF,
TTA, hybridized local and charge transfer (HLCT),45 and etc. A small
energy gap between singlet and triplet (ΔEST), generally smaller than
0.30 eV, is the primary feature of TADF materials. In our case, the
energy bandgap (ΔEST) between S1 and T1 were estimated to be 0.61
and 0.51 eV for 4NTAZ-PPI and TAZ-PPI pristine films, respectively,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 6 of 7
ARTICLE
Journal Name
Figure 4. (a) The device structure of 4NTAZ-PPI and TAZ-PPI. (b) The characteristic curves of current density-voltage-luminescence (J-V-L). (c) The curves of CE and PE versus luminance.
View Article Online
DOI: 10.1039/D1TC01079D
(d) The EL spectra of non-doped OLEDs based TAZ-PPI and 4NTAZ-PPI, the inset shows EL spectra of the OLEDs.
Table 2 Device performances of the devices based on 4NTAZ-PPI and TAZ-PPI.
Von
(V) a
3.0
3.0
2.7
3.5
3.6
3.4
2.8
3.1
3.2
3.0
Lmax
(cd m-2)
20210
14750
14600
5162
-
58679
11006
4970
3113
6228
CE
(cd A-1) b
8.0
PE
(lm W-1)c
6.3
6.1
4.49
EQE
(%) d
7.3
λEL
(nm)
460
460
CIE
(x, y)
Emitter
Ref
4NTAZ-PPI
TAZ-PPI
IP-PPI
pCzphAnBzt
m-PO-ABN
CzAnTAZ
4NDTPA-TAZ
TPATZ
(0.149,0.131)
(0.149,0.130)
(0.153,0.09)
(0.15, 0.12)
(0.148, 0.099)
(0.16, 0.25)
(0.158,0.038)
(0.155, 0.047)
(0.17, 0.07)
(0.158, 0.124)
This Work
This Work
[6]
[23]
[24]
[27]
[39]
[40]
[41]
6.7
5.9
4.49
7.73
5.2
13.0
—
2.41
2.85
4.98
4.86
6.75
5.9
7.96
6.3
5.92
6.57
4.63
5.51
456
448
468
426
430
408
452
11.7
—
2.20
—
PCZTZ
PPI-2BI
4.82
[49]
aTurn-on voltage at 1 cd m-2. bMaximum current efficiency. cMaximum power efficiency. dMaximum EQE.
7
8
9
Z. Wu, Y. Liu, L. Yu, C. Zhao, D. Yang, X. Qiao, J. Chen, C. Yang,
H. Kleemann, K. Leo and D. Ma, Nat Commun, 2019, 10, 2380.
Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and S.
R. Forrest, Nature, 2006, 440, 908-912.
P. L. Dos Santos, D. Chen, P. Rajamalli, T. Matulaitis, D. B.
Cordes, A. M. Z. Slawin, D. Jacquemin, E. Zysman-Colman and I.
D. W. Samuel, ACS Appl. Mater. Interfaces, 2019, 11, 45171-
45179.
4. Conclusions
In summary, we successfully designed and synthesized two novel
highly efficient D-A type blue emitters, 4NTAZ-PPI and TAZ-PPI. They
exhibit excellent thermal stability with the Td of 491°C and PLQYs in
the pristine film exceeding 60%. Besides, all non-doped devices show
negligible efficiency roll-off at high current densities. Non-doped
electroluminescent devices employing 4NTAZ-PPI as emitters has a
maximum external quantum efficiency of 7.3% and CIE coordinates
of (0.149, 0.131) at luminance of 1000cd m-2. The results are
comparable to the reported non-doped blue fluorescent OLEDs. The
strategy proposed here to connect triazole derivatives as the
acceptors and phenanthroimidazole derivatives as the donors has
shown the great potential to obtain high-performance organic blue
emitters.
10 Q. Wei, N. Fei, A. Islam, T. Lei, L. Hong, R. Peng, X. Fan, L. Chen,
P. Gao and Z. Ge, Adv. Opt. Mater., 2018, 6. 1800512.
11 S. Gan, W. Luo, B. He, L. Chen, H. Nie, R. Hu, A. Qin, Z. Zhao and
B. Z. Tang, J. Mater. Chem. C, 2016, 4, 3705-3708.
12 S. Wu, M. Aonuma, Q. Zhang, S. Huang, T. Nakagawa, K.
Kuwabara and C. Adachi, J. Mater. Chem. C, 2014, 2, 421-424.
13 C.J. Chiang, A. Kimyonok, M. K. Etherington, G. C. Griffiths, V.
Jankus, F. Turksoy and A. P. Monkman, Adv. Funct. Mater.,
2013, 23, 739-746.
14 T.-T. Bui, F. Goubard, M. Ibrahim-Ouali, D. Gigmes and F.
Dumur, Appl. Sci., 2018, 8,494.
15 X. Cai and S.J. Su, Adv. Funct. Mater., 2018, 28, 1802558.
16 J. Lee, H.F. Chen, T. Batagoda, C. Coburn, P. I. Djurovich, M. E.
ThompsonandS. R. Forrest, Nat. Mater., 2015, 15, 92-98.
17 H. Uoyama, K. Goushi, K. Shizu, H. Nomura and C. Adachi,
Nature, 2012, 492, 234-238.
Conflicts of interest
The authors declare that they have no conflict of interest.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (grant nos. 21875083, 51925303 and 91833304),
the program “JLUSTIRT” (grant no. 2019TD-33) and the China
Postdoctoral Science Foundation (No. 2020TQ0117).
18 C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo,
M. E. Thompson and S. R. Forrest, Appl.Phys. Let., 2001, 79,
2082-2084.
19 M. Y. Lai, C. H. Chen, W. S. Huang, J. T. Lin, T. H. Ke, L. Y. Chen,
M. H. Tsai and C. C. Wu, Angew.Chem., 2008, 47, 581-585.
20 H. Liu, J. Zeng, J. Guo, H. Nie, Z. Zhao and B. Z. Tang,
Angew.Chem., 2018, 57, 9290-9294.
21 J. Zhao, B. Liu, Z. Wang, Q. Tong, X. Du, C. Zheng, H. Lin, S. Tao
and X. Zhang, ACS Appl. Mater. Interfaces, 2018, 10, 9629-
9637.
22 P. Y. Chou, H. H. Chou, Y. H. Chen, T. H. Su, C. Y. Liao, H. W. Lin,
W. C. Lin, H. Y. Yen, I. C. Chen and C. H. Cheng, ChemComm,
2014, 50, 6869-6871.
23 Y. Wang, W. Liu, S. Ye, Q. Zhang, Y. Duan, R. Guo and L. Wang,
J. Mater. Chem. C, 2020, 8, 9678-9687.
References
1
2
C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987,51, 913.
H. H. Chou, Y. H. Chen, H. P. Hsu, W. H. Chang, Y. H. Chen and
C. H. Cheng, Adv. Mater., 2012, 24, 5867-5871.
M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963-4976.
Z. L. Zhu, S. F. Ni, W. C. Chen, M. Chen, J. J. Zhu, Y. Yuan, Q. X.
Tong, F. L. Wong and C. S. Lee, J. Mater. Chem. C, 2018, 6, 3584-
3592.
3
4
5
6
J. H. Lee, C. H. Chen, P. H. Lee, H. Y. Lin, M. k. Leung, T. L. Chiu
and C. F. Lin, J. Mater. Chem. C, 2019, 7, 5874-5888.
X. H. Zheng, J. W. Zhao, X. Chen, R. Cai, G. X. Yang, J. J. Zhu, S.
S. Tang, Z. H. Lin, S. L. Tao and Q. X. Tong, Chem. Eur. J., 2020,
26, 8588-8596.
24 S. Kang, J.-S. Huh, J.J. Kim and J. Park, J. Mater. Chem. C, 2020,
8, 11168-11176.
25 X. Tang, Q. Bai, Q. Peng, Y. Gao, J. Li, Y. Liu, L. Yao, P. Lu, B. Yang
and Y. Ma, Chemistry of Materials, 2015, 27, 7050-7057.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 7
Journal of Materials Chemistry C
Please do not adjust margins
Journal Name
ARTICLE
26 Z. Li, C. Li, Y. Xu, N. Xie, X. Jiao and Y. Wang, J Phys Chem Lett,
2019, 10, 842-847.
View Article Online
DOI: 10.1039/D1TC01079D
27 Y. Liu, H. Liu, Q. Bai, C. Du, A. Shang, D. Jiang, X. Tang and P. Lu,
Appl. Mater. Interfaces, 2020, 12, 16715-16725.
28 F. Liu, X. Man, H. Liu, J. Min, S. Zhao, W. Min, L. Gao, H. Jin and
P. Lu, J. Mater. Chem. C, 2019, 7, 14881-14888.
29 Z. Wang, X. Li, K. Xue, H. Li, X. Zhang, Y. Liu, Z. Yu, P. Lu and P.
Chen, J. Mater. Chem. C, 2016, 4, 1886-1894.
30 Y. Xu, X. Liang, X. Zhou, P. Yuan, J. Zhou, C. Wang, B. Li, D. Hu,
X. Qiao, X. Jiang, L. Liu, S. J. Su, D. Ma and Y. Ma, Adv. Mater.,
2019, 31, e1807388.
31 X. Tang, Q. Bai, T. Shan, J. Li, Y. Gao, F. Liu, H. Liu, Q. Peng, B.
Yang, F. Li and P. Lu, Advanced Funct. Mater., 2018, 28,
1705813.
32 Y. Xu, X. Liang, Y. Liang, X. Guo, M. Hanif, J. Zhou, X. Zhou, C.
Wang, J. Yao, R. Zhao, D. Hu, X. Qiao, D. Ma and Y. Ma, Appl.
Mater. Interfaces., 2019, 11, 31139-31146.
33 J. Tagare and S. Vaidyanathan, J. Mater. Chem. C, 2018, 6,
10138-10173.
34 C. He, H. Guo, Q. Peng, S. Dong and F. Li, J. Mater. Chem. C,
2015, 3, 9942-9947.
35 S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin and D. Ma,
Adv. Mater., 2011, 23, 4956-4959.
36 J. Zhuang, W. Su, W. Wu, W. Li, Q. Shen and M. Zhou,
Tetrahedron, 2013, 69, 9038-9044.
37 Y. Tao, Q. Wang, L. Ao, C. Zhong, C. Yang, J. Qin and D. Ma, J.
Mater. Chem. C, 2009, 114, 601-609.
38 A. Obolda, Q. Peng, C. He, T. Zhang, J. Ren, H. Ma, Z. Shuai and
F. Li, Adv. Mater., 2016, 28, 4740-4746.
39 A. Abdurahman, A. Obolda, Q. Peng and F. Li, Dyes and
Pigments, 2018, 153, 10-17.
40 J. Shi, Q. Ding, L. Xu, X. Lv, Z. Liu, Q. Sun, Y. Pan, S. Xue and W.
Yang, J. Mater. Chem. C, 2018, 6, 11063-11070.
41 S. Xue, X. Qiu, S. Ying, Y. Lu, Y. Pan, Q. Sun, C. Gu and W. Yang,
Adv. Opt. Mater., 2017, 5, 1700747.
42 W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B. Yang and
Y. Ma, Adv. Funct. Mater., 2012, 22, 2797-2803.
43 Z. R. Grabowski, K. Rotkiewicz and W. Rettig, Chem. Rev., 2003,
103, 3899.
44 X. Lv, L. Xu, W. Cui, Y. Yu, H. Zhou, M. Cang, Q. Sun, Y. Pan, Y.
Xu, D. Hu, S. Xue and W. Yang, ACS Appl. Mater. Interfaces.,
2021, 13, 970-980.
45 L. Yao, S. Zhang, R. Wang, W. Li, F. Shen, B. Yang and Y. Ma,
Angew. Chem. Int. Ed., 2014, 53, 2119-2123.
46 D. Di, L. Yang, J. M. Richter, L. Meraldi, R. M. Altamimi, A. Y.
Alyamani, D. Credgington, K. P. Musselman, J. L. MacManus-
Driscoll and R. H. Friend, Adv. Mater., 2017, 29,1605987
47 J. Zhou, Q. Liu, W. Feng, Y. Sun and F. Li, Chem Rev, 2015, 115,
395-465
48 T. A. Lin, C. F. Perkinson and M. A. Baldo, Adv. Mater., 2020, 32,
e1908175.
49 B. Liu, Z. L. Zhu, J. W. Zhao, D. He, Z. Y. Wang, C. Y. Luo, Q. X.
Tong, C. S. Lee and S. L. Tao, Chem. Eur. J., 2018, 24, 15566-
15571.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins