Molecular evolution of host materials by regular tuning of n/p ratio for high-performance phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c8tc02432d

By linking pyridine and 1,2,4-triazole as an electron-deficient group, two “dual n-type unit” bipolar host materials, namely 2-(3,5-di(9H-carbazol-9-yl)phenyl)-5-(1H-triazol-1-yl)pyridine (PyTzDCz) and 2-(3-(9H-carbazol-9-yl)phenyl)-5-(1H-triazol-1-yl)pyridine (PyTzSCz), and one bipolar reference compound 2-(3,5-di(9H-carbazol-9-yl)phenyl)pyridine (PyDCz) were designed and synthesized for use in phosphorescent organic light-emitting diodes (PhOLEDs). From the parent 1,3-bis(9H-carbazol-9-yl)benzene (mCP) via PyDCz and PyTzDCz to PyTzSCz, the molar ratio of n-type pyridine/triazole to p-type carbazole was regularly tuned as 0:2, 1:2, 2:2, and 2:1. With a regular increase of n/p ratio from 0:2 gradually to 2:1, the LUMO levels of these hosts are regularly deepened to enhance electron injection, and the turn-on voltages of their blue and green PhOLEDs are regularly decreased. Particularly, the PyTzSCz-hosted blue PhOLED turned on at 2.6 V with high power efficiency of 48.1 lm W^-1 (corresponding to an external quantum efficiency of 26.4% and a current efficiency of 45.9 cd A^-1), which is the lowest turn-on voltage so far and one of the highest power efficiencies reported for FIrpic PhOLEDs containing a single host. The PyTzSCz-hosted green device also exhibited low turn-on voltage of 2.4 V and high external quantum efficiency of 28.0%. This study manifests the molecular evolution trend of host materials from hole-transporting via bipolar to “dual n-type unit bipolar” for performance optimization.

Full text of DOI:10.1039/c8tc02432d

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ISSN 2050-7526
PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Page 1 of 27
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DOI: 10.1039/C8TC02432D
Molecular Evolution of Host Materials by Regular Tuning N/P Ratio
for High-Performance Phosphorescence Organic Light-Emitting
Diodes
Di Liu,*a Fang Wang,b Ruijuan Yaoa
a State Key Laboratory of Fine Chemicals, College of Chemistry, Dalian University of
Technology, Dalian 116024, China. *E-mail: liudi@dlut.edu.cn
b College of Chemical Engineering, Dalian University of Technology, Dalian 116024,
China.
Abstract
By linking pyridine and 1,2,4-triazole as electron-deficient group, two “dual
n-type
unit”
bipolar
host
materials,
named
2-(3,5-Di(9H-carbazol-9-yl)-phenyl)-5-(1H-triazol-1-yl)pyridine (PyTzDCz) and
2-(3-(9H-carbazol-9-yl)-phenyl)-5-(1H-triazol-1-yl)pyridine (PyTzSCz), and one
bipolar reference compound 2-(3,5-di(9H-carbazol-9-yl)-phenyl)-pyridine (PyDCz)
were designed and synthesized for use in phosphorescence organic light-emitting
diodes (PhOLEDs). From the parent 1,3-bis(9H-carbazol-9-yl)benzene (mCP) via
PyDCz and PyTzDCz to PyTzSCz, the molar ratio of n-type pyridine/triazole to
p-type carbazole was regularly tuned as 0:2, 1:2, 2:2, and 2:1. With regular increase of
n/p ratio from 0:2 gradually to 2:1, the LUMO levels of these hosts are regularly
deepened to enhance electron injection, and the turn-on voltages of their blue and
1

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DOI: 10.1039/C8TC02432D
green PhOLEDs are regularly decreased. Particularly the PyTzSCz-hosted blue
PhOLED turned on at 2.6 V with high power efficiency of 48.1 lm W-1
(corresponding to an external quantum efficiency of 26.4% and a current efficiency of
45.9 cd A-1), which are the lowest turn-on voltage so far and one of the highest power
efficiencies reported for FIrpic PhOLEDs containing a single host. The
PyTzSCz-hosted green device also exhibited low turn-on voltage of 2.4 V and high
external quantum efficiency of 28.0%. This study manifests the molecular evolution
trend of host materials from hole-transporting via bipolar to “dual n-type unit bipolar”
for performance optimization.
Keywords: Phosphorescence OLEDs, bipolar host materials, pyridine, 1,2,4-triazole,
n/p molar ratio
Introduction
Phosphorescent Organic light-emitting diodes (PhOLEDs), which incorporate a
phosphorescent emitter can convert both singlet and triplet excitons into photons to
realize internal quantum efficiency as high as 100%, have progressed rapidly in
energy-saving flat-panel displays and eco-friendly solid-state lighting.1-4 To suppress
the severe triplet-triplet annihilation (TTA) and aggregation-caused quenching (ACQ),
phosphorescent emitters should be dispersed in a suitable host material generally.5,6 In
principle, an ideal host should have sufficient high triplet energy (ET) to prevent
reverse energy transfer and confine triplet excitons on guest emitters, appropriate
frontier molecular orbital level to match will the adjacent function layers, high
2

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morphological and thermal stability to maintain device reliability at high luDmOiI:n1a0.n10c3e9/C8TC02432D
level.7-9 Carbazole derivatives are widely used as host materials due to its high ET
(over 3.0 eV)10 and excellent hole-transporting property of cabazole, e.g.
1,3-bis(9H-carbazol-9-yl)benzene (mCP)8 is one of the most classical carbazole-based
host materials having a simple molecular structure and a triplet energy of >2.9 eV.
However high driving voltages were often observed for the devices due to its unipolar
(hole) carrier-transport property. For industrialized application of OLEDs, low driving
voltage and high power efficiency are primary requirements, thus the practical
application of mCP in high-performance OLEDs is restricted.11 In order to improve
the positive and negative charge balancing state in OLEDs, bipolar host materials
containing both hole-transporting p-type group and electron-transporting n-type group
have been invented. It has been established that the bipolar materials have better
charge carrier balancing abilities than the traditional unipolar hosts and can shift the
charge recombination zone away from the emitting layer/hole or electron transporting
layer interface and hence improve the device performance. Large amount of bipolar
host materials have been developed by using pyridine, pyrazine, pyrimidine,
imidazole, triazole, oxadiazole, diphenylphosphine oxide, cyanobenzene as n-type
unit for application in OLEDs and good performance were achieved in their
PhOLEDs.3,4,12,13 For example, 4,4′-Bis[(9H-carbazol-9-yl)-3,30-dicyano]biphenyl
(CBP-CN)5,9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN)12,
2,6-bis(3-(carbazol-9-yl)phenyl)pyridine
(26DCzPPy)4
and
bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide (BCPO)13 were developed on the
3

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basis of their unipolar parent molecules and exhibited better device performaDnOcIe: 10th.10a3n9/C8TC02432D
the parents due to improved charge balancing abilities.
In recent years, a new type of bipolar host materials, i.e. “dual n-type unit”
bipolar host materials were invented by our research team,14-16 which are
characterized by the presence of two n-type units that are directly linked to each other
and away from the p-type group. As the electron-injection and -transportation are
further improved by these two n-type units, the positive and negative charge
balancing ability of these “dual n-type unit” bipolar hosts are further improved in
comparison with the common bipolar hosts and finally lead to increased emission
efficiency and reduced efficiency roll-off.16 However, some important issues still
remain unclear, for example, whether the dual n-type units strategy has universal
applicability, whether the dual n-type units strategy match all kinds of p-type groups,
and so on. In this work, we combined pyridine and 1,2,4-triazole as a novel dual
n-type unit 5-(1H-triazol-1-yl)pyridine (PyTz), and incorporated it in unipolar host
mCP
to
construct
bipolar
host
2-(3,5-Di(9H-carbazol-9-yl)-phenyl)-5-(1H-triazol-1-yl)pyridine (PyTzDCz). Since
the electron mobility of most organic n-type groups are lower by orders of magnitude
than the hole mobility of most p-type units14, we removed one carbazole unit from
PyTzDCz to increase the molar ratio of n-type unit and designed another analogue,
named
2-(3-(9H-carbazol-9-yl)-phenyl)-5-(1H-triazol-1-yl)pyridine
(PyTzSCz).
Different from the reported 1,3,5-trissubstituted or 3,4,5-trissubstituted 1,2,4-triazole
based host materials that use the 1,2,4-triazole ring as the linking bridge for other
4

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groups or as the molecular geometrical core17,18, the 1,2,4-triazole units in PDyOTI:z1D0.1C03z9/C8TC02432D
and PyTzSCz were placed at the pyridine units by grafting it from its 1-site N atom,
which is more favorable for controlling π-conjugation length, and this strategy
endows each of them with high ET value of 2.74 eV, which are high enough to
guarantee their capability to act as hosts for RGB emitters. To evaluate the dual n-type
unit
PyTz,
a
bipolar
reference
compound
2-(3,5-di(9H-carbazol-9-yl)-phenyl)-pyridine (PyDCz) which contains only one
pyridine unit was prepared for comparison. In this way, along the order of mCP,
PyDCz, PyTzDCz and PyTzSCz, the n/p molar ratio was regularly tuned as 0:2, 1:2,
2:2, and 2:1. In addition to develop novel high-performance host materials, this work
is emphasized to systematically study the influence of n/p ratio to the optoelectronic
parameters and device performance and finally to disclose the molecular evolution
trend of host materials for PhOLEDs from the traditional unipolar host via bipolar
host to “dual n-type unit” bipolar hosts.
Results and Discussion
Synthesis and thermal properties
The chemical structures and synthetic routes of PyTzDCz and PyTzSCz are
shown in Scheme 1. Two target compounds were prepared in 71-76% yields by a
tetrakis-(triphenylphosphine)palladium(0) mediated typical Suzuki cross-coupling
between brominated n-type unit and carbazole-containing boronic acid.19 The key
intermediate 2-bromo-5-(1H-triazol-1-yl)pyridine (A1) was prepared through a CuI
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catalyzed Ullmann reaction between 2,5-dibromopyridine and 1,2,4-triazole according
to the literature procedure16, and all other chemicals were commercial available and
used without further purification. Since they have good solubility in common organic
solvents, PyTzDCz and PyTzSCz were thoroughly purified by silica gel column
chromatography and repeated recrystallization in methanol/chloroform to reach a high
purity for OLEDs application. The detailed synthetic procedures and structure
characterization are provided in Electronic Supporting Information (ESI†). The
synthesis of the reference compound PyDCz is described in the ESI†.
Scheme 1 Chemical structures and synthetic routes of PyTzDCz and PyTzSCz.
PyTzDCz and PyTzSCz exhibited good thermal stability and amorphous stability
as verified by thermogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) study, and all the pertinent dates are summarized in Table 1. PyTzDCz and
PyTzSCz showed thermal decomposition temperatures (Td) at 408.0 and 330.6 ℃
(corresponding to 5% weight loss), respectively, as depicted by the TGA thermograms
in Fig. S1 (ESI†). The glass transition temperatures (Tgs) of these two compounds
were detected by DSC traces in Fig. 1 at 132.7 ℃ for PyTzDCz and 69.4 ℃ for
6

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PyTzSCz. The remarkable higher Td and Tg of PyTzDCz than PyTzSCz should be
caused by the higher molecular weight of PyTzDCz and the special contribution from
one additional carbazole group since it is a famous chemically and morphologically
stable building block.16 Both PyTzDCz and PyTzSCz exhibit higher glass transition
temperatures than the widely used host material mCP (Tg = 60.0 ℃)8, implying that
improved device stability can be expected for these novel host materials. The melting
point of PyTzSCz was observed at 176 ℃.
PyTzDCz
PyTzSCz
69.4 ℃
132.7 ℃
40 60 80 100 120 140 160 180 200
Temperature (℃
)
Fig. 1 DSC traces (at the second heating cycle) of PyTzDCz and PyTzSCz recorded at
a heating rate of 10 ℃ min-1.
Table 1. Physical data of PyTzDCz, PyTzSCz, PyDCz and mCP.
em
max
a
a
b
b
c
e
λabs
[nm]
λ
ES
ET
Eg
HOMO/LUMO
(eV) d
Td
Tg
Compound
[nm]
(eV) (eV)
(eV)
(℃)
(℃)
PyTzDCz 297,334,345 391
3.51
3.41
2.74
2.75
2.84
3.03
3.08
2.91
-5.64/-2.56
-5.54/-2.63
-5.58/-2.43
-5.65/-2.09
408.0
330.6
132.7
69.4
PyTzSCz 296,334,340 400
PyDCz
mCP f
a
b
Absorption and fluorescence wavelengths in neat film.
Measured in
c
2-MeTHF at 77 K. The electrochemical band gap determined as the potential
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difference between oxidation onset and reduction onset multiplied by the DeOlIe: 1c0t.r10o3n9/C8TC02432D
d
e
charge (e).
Determined using electrochemical potentials.
Td is the thermal
f
decomposition temperature corresponding to a 5% weight loss.
HOMO/LUMO of mCP are cited from literature8.
ET and
Photophysical Properties
The UV-vis absorption and photoluminescence (PL) spectra of PyTzDCz and
PyTzSCz were measured in thin films on quartz substrates at room temperature (Fig.
2a), and all photophysical data are summarized in Table 1. Owing to the same
molecular skeletons of PyTzDCz and PyTzSCz, they exhibited similar absorption
spectra position and profiles, and the absorption peak at 296 nm could be assigned to
the π-π∗ transition of carbazoles and the long-wavelength peaks around 340 nm can
be attributed to n-π* transitions of the conjugated structure surrounding the carbazole
unit.20 Upon optical excitation at 296 nm, PyTzDCz and PyTzSCz have shown bluish
violet emission with peaks at 391 and 400 nm, respectively. Meanwhile, the optical
bandgaps of PyTzDCz and PyTzSCz are 3.40 eV, which were estimated from the
absorption threshold wavelengths of neat films by absorption edge technique.15 In
order to investigate their excited states and solvatochromic behavior, both absorption
and fluorescence spectra were measured in various solvents with different polarities
(n-hexane, toluene, dichloromethane (DCM), acetonitrile and ethanol). As shown in
Fig. S2 (ESI†), the absorption spectra of PyTzDCz and PyTzSCz are almost
independent of the solvent polarity, indicating that solvent polarity has a negligible
effect on the dipole moments of the ground states of these compounds.21 However,
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with gradually increasing solvent polarity, the fluorescence spectra of PyTzDCz and
PyTzSCz exhibited regular bathochromic-shift, confirming the photoinduced
intramolecular charge transfer excited states of these hosts resulted from their
donor-acceptor bipolar feature.16
1.00
0.75
0.50
0.25
0.00
(b)
1.00
0.75
0.50
0.25
0.00
1.00
0.75
0.50
0.25
0.00
PyDCz
PyTzDCz
PyTzSCz
(a)
p-PyTzDCz
p-PyTzSCz
350
400
450
500
550
600
200
250
300
350 400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 2 UV-vis absorption and PL spectra of (a) PyTzDCz and PyTzSCz in thin films
on quartz substrates at room temperature, and phosphorescence spectra of (b)
PyTzDCz, PyTzSCz and PyDCz in frozen 2-MeTHF matrix at 77 K.
Fig. 2b shows the phosphorescence spectra of PyTzDCz, PyTzSCz and the
reference PyDCz measured in a frozen 2-methyltetrahydrofuran (2-MeTHF) matrix at
77 K. Since the 1,2,4-triazole unit was grafted onto pyridine from its saturated 1-site
N atom and does not extend the π-conjugation, PyTzDCz, PyTzSCz and PyDCz have
similar phosphorescence spectra position, and the triplet energies (ETs) of PyTzDCz
and PyTzSCz determined to be 2.74 eV from the highest energy vibronic band of
phosphorescence spectra (452 nm) are close to that of PyDCz (2.84 eV). Evidently to
incorporate 1,2,4-triazole unit from its 1-site N atom onto pyridine as the second
n-type unit is an effective strategy to adjust the n/p molar ratio without at the expense
of the triplet energy. Both PyTzDCz and PyTzSCz have sufficiently high ETs to act as
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phosphor
the
hosts
for
the
typical
blue
iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic, ET=2.63
eV)22 and green phosphor tris[2-phenylpyridinato-C2,N] iridium(III) (Ir(ppy)3,
ET=2.40 eV).23 The singlet-triplet energy differences (ΔEST) were estimated from the
low-temperature photoluminescence (LT PL) spectra (Fig. S3 in ESI † ) that
simultaneously gave both singlet energies (ESs) and triplet energies (ETs) as 0.77 eV
for PyTzDCz and 0.67 eV for PyTzSCz, which are similar to those for the majority of
reported host materials (0.5-1 eV).8,11,24
Electrochemical properties and theoretical calculations
The electrochemical properties of PyTzDCz and PyTzSCz were probed by cyclic
voltammetry (CV) measurements in deoxygenated dilute DCM (for anodic scan) and
N,N-dimethylformamide (DMF) solutions (for cathodic scan) containing 0.1 M
tetra(n-butyl)ammonium hexafluorophosphate (n-Bu4NPF6) as the supporting
electrolyte at a scan rate of 100 mV s-1,14 and their cyclic voltammograms are shown
in Fig. 3. These two compounds showed reversible oxidation and irreversible
reduction waves with quite similar profiles due to their same electron-donating and
onset
-withdrawing units. The onset potential of the first oxidation wave (E ) were used to
ox
onset
ox
calculated the HOMO levels according to the equation of EHOMO = −e(E
+ 4.4)3,
while the LUMO levels were calculated from the onset potential of the reduction
onset
red
onset
red
wave (E ) using equation of ELUMO = −e(E
+ 4.4)3. The HOMO/LUMO were
determined as ca. −5.64 eV/−2.56 eV for PyTzDCz and −5.54 eV/−2.63 eV for
PyTzSCz. The relatively shallow HOMO levels indicate a small hole-injection barrier
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material
from
a
widely
used
hole
transporting
1,1-bis(4-(N,N-di(p-tolyl)-amino)phenyl)-cyclohexane (TAPC, HOMO = −5.50 eV)3
to these hosts in the emitting layer of OLEDs. Under identical measuring conditions,
the reference compound PyDCz exhibited a similar HOMO level (−5.58 eV) and a
higher LUMO level (−2.43 eV) than these two new hosts. It is clear that, with host
going from mCP via PyDCz to PyTzDCz and further to PyTzSCz, the n/p ratio is
gradually increased from 0:2 via 1:2 to 2:2 and further to 2:1, the LUMO levels are
regularly deepened in the order of -2.408 < -2.43 < -2.56 < -2.63 eV, reflecting the
stepwise improvement of electron injection from unipolar hole-transporting host, to
bipolar host and further to “dual n-type unit” bipolar hosts. Hence, it can be expected
that combining pyridine and 1,2,4-triazole as dual n-type unit should be an effective
strategy to enhance the electron injection into the emitting layer and reduce the
driving voltage of PhOLEDs. The electrochemical bandgaps (Egs) of these hosts were
onset
ox
onset
red
calculated from the potential difference between E
and E
multiplied by the
electron charge as 3.08 eV for PyTzDCz and 2.91 eV for PyTzSCz.
PyTzDCz
PyTzSCz
PyDCz
2000
1000
0
-1000
-2000
Potential (mV vs SCE)
Fig. 3 Cyclic voltammograms of PyTzDCz, PyTzSCz and PyDCz measured in dilute
DCM (anodic) and DMF (cathodic) solutions at a scan rate of 100 mV s-1.
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The geometrical and electronic properties of PyTzDCz and PyTzSCz were
calculated by density functional theory (DFT) calculations using B3LYP hybrid
functional theory with Gaussian 0925, and their optimized steric structures and
HOMO/LUMO distribution are given in Fig. 4. HOMO orbitals are mainly located on
the electron-donating carbazoles with a small contribution from the adjacent phenyl
ring, and LUMO orbitals are mainly distributed in dual n-type unit PyTz and the
adjacent phenyl. In comparison with the reference PyDCz whose LUMO is localized
on pyridine-phenyl part, the LUMOs of PyTzDCz and PyTzSCz spread onto the
triazole ring, confirming that the terminal triazole ring contributes to determine the
LUMO of these molecules in combination with pyridine-phenyl part. This provides a
solid evidence for the “dual n-type unit” strategy in the molecular design of these new
host materials. The sufficient spatial separation of HOMO and LUMO further
confirmed the bipolar charge transporting feature of these compounds. From the
optimized steric molecular structures of PyTzDCz and PyTzSCz, the dihedral angles
between carbazole and adjacent phenyl ring are greater than 50°, which helps to
guarantee the distorted molecular configuration and high triplet energy.
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Fig. 4 Geometry optimized steric molecular structures and HOMOs/LUMOs
distribution of PyTzDCz, PyTzSCz and PyDCz.
Single-carrier devices and charge-transporting properties
To further verify the function of PyTz, single-carrier devices were fabricated with
the structure of ITO/PEDOT:PSS (40 nm)/TAPC (7 nm)/host (100 nm)/TAPC (7
nm)/Al (200 nm) for hole-only devices, and ITO/TmPyPB (7 nm)/host (100
nm)/TmPyPB (7 nm)/LiF (1 nm)/Al (200 nm) for electron-only devices. The unipolar
host mCP and the reference bipolar molecule PyDCz were also studied in the same
way for comparison. The thickness of the host layer is set deliberately to be
tremendously larger than adjacent ancillary layers to reveal the intrinsic charge
transporting nature of these host materials. The chemical structure of relevant
materials and the energy level diagram of single carrier devices are illustrated in Fig.
S4 in ESI†. TAPC/Al and ITO/TmPyPB interfaces were used to prevent electron and
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hole injection due to their large barrier of 2.3 eV and 1.88 eV, respectively. As shown
in Fig. 5a and 5b, mCP devices only got hole current, but almost zero electron current,
consistent with its unipolar hole-transporting nature. All other single-carrier devices
exhibited significantly high current densities in the typical voltage range of OLEDs,
suggesting that all other three compounds possess bipolar charge-transporting features
with sufficient hole- and electron-transporting abilities.26 Obviously, the hole currents
of the hole-only devices are directly correlated with the p-group number in each host
molecule, rather than with the p/n ratior. It is reasonable that the PyTzDCz, PyDCz
based hole-only devices have similar hole current with mCP devicesince they have
same carbazole number in each molecule, which are all higher than that of PyTzSCz
device possessing only one carbazole unit. By incorporating PyTz as dual n-type unit,
PyTzDCz and PyTzSCz exhibited much higher electron currents than PyDCz, clearly
confirming the function of the second n-type unit 1,2,4-triazol. The regular increase of
electron current in the order of mCP, PyDCz, PyTzDCz and PyTzSCz should benefit
from the stepwise improvement in both electron injection and transportation caused
by the corresponding evolution of the n-type unit in these host materials. The
improved electron injection can be verified by the ascending of the current
density-voltage curves at reduced voltage with n-type unit going from pyridine to
PyTz. The improved electron transportation can get evidence from the more oblique
current density-voltage curves with n/p ratio changing from 1:2, via 2:2 to 2:1.
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300
160
140
120
100
80
(b)
PyTzDCz
PyTzSCz
PzDCz
(a)
PyTzDCz
PyTzSCz
PzDCz
250
200
150
100
50
mCP
mCP
60
40
20
0
0
0
2
4
6
8
10 12 14 16
0
2
4
6
8
10 12 14 16
Voltage (V)
Voltage (V)
Fig. 5 The current density versus voltage curves of the (a) hole-only and (b)
electron-only devices for PyTzDCz, PyTzSCz, PyDCz and mCP.
Electroluminescent devices
In order to evaluate PyTzDCz and PyTzSCz as potential host materials,
FIrpic-doped sky-blue PhOLEDs B1 and B2 were fabricated with the typical
sandwiched structure of ITO/PEDOT:PSS (40 mn)/TAPC (20 nm)/TCTA (5
nm)/host:FIrpic (4wt%, 30 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (200 nm). mCP- and
PyDCz-hosted blue device B3 and B4 were also fabricated with the same structures
for comparison. PEDOT:PSS and TAPC were used as the hole-injecting and
-transporting layers (HIL, HTL), TmPyPB as electron-transporting layer. Particularly,
a thin layer of TCTA was used as the second HTL15,27 to facilitate charge balance in
emitting layer by slowing down hole transportation due to its relatively lower hole
mobility than TAPC. The energy level diagrams of these devices are shown in Fig. S4
in ESI†. The current density-voltage-brightness (J-V-B) characteristics and efficiency
curves of these devices are shown in Fig. 6 and Fig. S5 and the electroluminescence
(EL) data are summarized in Table 2. The turn-on voltage (Von, to deliver a brightness
of 1 cd m-2) of devices B1 and B2 were lower than devices B3 and B4 by about 1.0 V,
15

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DOI: 10.1039/C8TC02432D
which should be attributed to the deeper LUMO levels caused by the dual n-type unit
PyTz and thus the improved electron injection and transportation. A regular trend can
be observed in these four blue devices: the turn-on voltage is gradually reduced from
3.6, 3.5, 2.7 to 2.6 V with the n/p ratio is increased from 0:2, 1:2, via 2:2 to 2:1 in the
corresponding hosts mCP, PyDCz, PyTzDCz and PyTzSCz, once again manifesting
the special function of the dual n-type unit strategy. As shown in Fig. 6a, the
PyTzDCz and PyTzSCz hosted devices B1 and B2 exhibited much higher current
densities and brightness than PyDCz based device B3, furthermore, the brightness
increased much rapidly than the current density in these two devices, as a result,
devices B1 and B2 achieved much higher efficiencies than B3. The PyTzDCz hosted
device B1 achieved a maximum power efficiency (ηp) of 43.9 lm W-1 (corresponding
to a maximum external efficiency (ηext) of 21.9% and a current efficiency (ηc) of 37.8
cd A-1). PyTzSCz-hosted B2 exhibited the best performance among all these blue
devices with 48.1 lm W−1 (26.4%, 45.9 cd A−1). PyDCz-hosted B3 exhibited mild
performance of 17.8% (36.1 cd A−1, 28.3 lm W−1). While device B4 with the
widely-used unipolar host mCP exhibited efficiency of 14.7% (32.5 cd A-1, 25.5 lm
W-1). It is obvious that the device efficiencies including ηc, ηp, and ηext were enhanced
constantly with the n/p ratio is increased from 0:2, 1:2, via 2:2 to 2:1 in the
corresponding hosts mCP, PyDCz, PyTzDCz and PyTzSCz. Evidently the superior
performance in PyTzDCz and PyTzSCz hosted devices should benefit from the
improved electron injection and transportation and thus the better charge balancing
abilities of these dual n-type unit bipolar hosts. As indicated by FIrpic device data
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DOI: 10.1039/C8TC02432D
within the recent 10 years in Table S1, a turn-on voltage of 2.6 V and a power
efficiency of 48.1 lm W-1 in present PyTzSCz hosted device B2 should be the lowest
turn-on voltage and one of the highest power efficiencies reported for FIrpic
PhOLEDs containing a single host so far.14,16,18,26,28-30 The turn-on voltage of 2.6 V is
even lower than the theoretical limit of FIrpic, i.e. the band gap voltage of 2.64 V that
is determined as the emitted photo energy hv (1240/469=2.64 eV) at the EL peak
wavelength (469 nm, Fig. S5) divided by the electron charge (e). The similar case was
previously observed in Ir(ppy)3 based green OLEDs.31-33 In addition to the appropriate
selection of dual n-type unit host materials in present work, it is generally believed
that the extra lower turn-on voltage than the theoretical limit is caused, at least
partially, by the thermal energy kT carried by the electrons and holes. The extra
thermal energy would make these high-energy carriers more readily diffuse over to
the side of opposite type where they recombine than those low-energy ones.31-33
100
(b)
B1
B2
B3
(a)
10000
1000
100
10
300
200
100
0
100
10
1
10
1
B1
B2
B3
0.1
1
0.1
2
3
4
5
6
7
8
9
10 11
100
1000
Brightness (cd m)
10000
-2
Voltage (V)
Fig. 6 (a) The current density-voltage-brightness characteristics and (b) the efficiency
curves for PyTzDCz, PyTzSCz and PyDCz-hosted blue PhOLEDs B1 to B3.
Table 2: Electroluminescence characteristics of the sky-blue and green PhOLEDs.a
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DOI: 10.1039/C8TC02432D
b
b
Von
Lmax
ηc
ηp,max
ηext
CIE
Device
Host
Dopant
(V)
(cd m−2)
(cd A−1)
(lm W−1)
(%)
(x, y) c
13924
(8V)
37.8, 19.5,
13.3
21.9, 11.4,
7.6
B1
B2
B3
B4
G1
G2
G3
G4
PyTzDCz
PyTzSCz
PyDCz
mCP
2.7
2.6
3.5
3.6
2.4
2.4
3.2
3.1
43.9
48.1
28.3
25.5
106.7
97.2
40.0
35.6
0.14, 0.27
20920
(8V)
45.9, 35.6,
21.2
26.4, 20.4,
12.2
0.14, 0.27
0.14,0.35
0.15, 0.33
0.27, 0.62
0.27, 0.62
0.30,0.63
0.28, 0.63
FIrpic
8483
36.1, 19.2,
14.0
17.8, 9.5,
6.9
(10V)
15627
(9V)
32.5, 27.5,
22.9
14.7, 11.3,
10.5
112860
(9V)
85.0, 79.3,
75.4
25.8, 23.7,
22.4
PyTzDCz
PyTzSCz
PyDCz
mCP
102640
(8V)
92.9, 88.6,
82.5
28.0, 26.3,
25.0
Ir(ppy)3
58330
(10V)
64276
(9V)
66.4, 63.3,
53.2
18.8, 17.9,
15.1
51.0, 48.9,
42.5
15.0, 14.4,
12.5
a
Abbreviations: Von, turn-on voltage. Lmax, maximum luminance. ηext, external
quantum efficiency. ηc, current efficiency. ηp, power efficiency. CIE (x, y),
b
Commission International de I’Eclairage coordinates. Order of measured values:
maximum, then at 1000 and 5000 cd m−2. c Measured at 6 V.
Enlightened by the excellent performance of these dual n-type unit bipolar host
materials in above sky-blue phosphorescent OLEDs, we further explored the
versatility of these hosts for green emitter Ir(ppy)3. The green devices G1 to G4 have
the same configuration as the above blue ones but with 7 wt% Ir(ppy)3 doped in these
hosts as the emitting layers. The J-V-B characteristics and efficiency curves are shown
in Fig. 7 and the EL data are summarized in Table 2. G1 and G2 displayed typical
green emission from Ir(ppy)3 (EL spectra shown in Fig. S5) with CIE coordinates of
around (0.27, 0.62), and there were no residue emission from the host or adjacent
layers. A similar trend in device performance was observed in these green devices: as
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the n/p ratio is increased from 0:2, 1:2, via 2:2 to 2:1 in the corresponding hoDstOsI:m10.C10P39,/C8TC02432D
PyDCz, PyTzDCz and PyTzSCz, the green emission efficiencies are regularly
increased, and the turn-on voltage of G1 and G2 are also lower than G3 and G4 by
about 0.7 V. Among these green devices, the PyTzSCz-hosted G2 achieved the best
performance with 97.2 lm W-1 (28.0%, 92.9 cd A-1). The G1 also demonstrated a good
device performance with 106.7 lm W-1 (25.8%, 85.0 cd A-1), and both G1 and G2
turned on at low voltage of 2.4 V, which is close to the theoretical limit of the band
gap voltage (2.37 V) of Ir(ppy)3.15 In recent years, Kido34,35 and Li14,16 reported high
efficiencies for Ir(ppy)3 PhOLEDs containing single bipolar host, such as 24% (128
lm W−1), 28% (105 lm W−1), 27.3% (96.1 lm W−1), and 28.2% (102.8 lm W−1), which
stand for the typical data for single host based green PhOLEDs so far. It is obvious
that the efficiencies 28.0% (97.2 lm W-1) of G2 and 25.8% (106.7 lm W-1) of G1 are
among the best data ever reported for Ir(ppy)3 PhOLEDs containing a single host
material. Evidently the superior performance of G1 and G2 should be attributed to the
more balanced carrier in emitting layer caused by the appropriate n/p ratio in hosts
PyTzDCz and PyTzSCz. Furthermore, the efficiencies of green devices G1 and G2
are quite stable with very low efficiency roll-offs. For example, at a practical
brightness of 1000 cd m-2, the efficiency roll-offs of G1 and G2 are 8.1% and 6.1%
respectively, and even at an extremely high brightness of 5000 cd m-2, G1 and G2 still
maintained high external quantum efficiency of 22.4% and 25.0%. Evidently the
“dual n-type unit” designing strategy and the consequent excellent charge balancing
abilities and optimized HOMO/LUMO levels and triplet energies of these bipolar
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DOI: 10.1039/C8TC02432D
hosts, especially PyTzSCz that possesses a n/p ratio of 2:1, should be the essential
factors accounting for their excellent performance in both sky-blue and green
PhOLEDs.
800
100000
10000
1000
100
(b)
G1
G2
G3
(a)
100
10
1
100
10
1
600
400
200
0
10
G1
G2
G3
1
0.1
2
3
4
5
6
7
8
9
10 11
10
100
1000
Brightness (cd m)
10000
-2
Voltage (V)
Fig. 7 (a) The J-V-B characteristics and (b) efficiency curves for PyTzDCz, PyTzSCz
and PyDCz-hosted green PhOLEDs G1 to G3.
Conclusion
Two novel “dual n-type unit” bipolar host materials PyTzDCz and PyTzSCz
have been developed by using 1,2,4-triazole-pyridine as the dual n-type unit. From the
traditional unipolar mCP to the bipolar reference compound PyDCz and further to
PyTzDCz and PyTzSCz, the n/p ratio shows a regular change from 0:2 to 1:2 further
to 2:2 to 2:1. By means of both experimental study and theoretical calculations, it has
been demonstrated that with regular increase of the n/p ratio in each host, the LUMO
level was pulled down and the electron injection barrier was reduced to certain extent,
and the positive and negative charge balancing ability of the host is stepwisely
improved. As a result, the driving voltage of the sky-blue and green PhOLED with
these host materials is gradually reduced and the emission efficiency is regularly
increased. Particularly for the host PyTzSCz with n/p ratio of 2:1, the sky-blue
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PhOLED turn-on at rather low voltage of 2.6 V with high power efficiency of 48.1 lm
W-1 (EQE of 26.4%), which are the lowest turn-on voltage so far and one of the
highest power efficiencies reported for FIrpic PhOLEDs containing a single host. The
systematic study of these simple host materials has confirmed the charge balancing
ability of the host materials, rather than its amorphous stability, dominates to
determine the overall performance of PhOLEDs, and further demonstrated that the
“dual n-type unit” bipolar hosts may be the ideal type of host materials favorable for
best PhOLEDs performance owing to the excellent charge balancing ability.
Experimental Section
General information
A 500 and 126 MHz Varian Unity Inova spectrophotometer was used to record
the 1H NMR and 13C NMR spectra, and the mass spectra were taken on
HP1100LC/MSD MS spectrometer. The fluorescence and UV-vis absorption spectra
measurements were performed on a Hitachi F-7000 spectrometer and a PerkinElmer
Lambda 750s spectrophotometer, respectively. The phosphorescence spectra were
measured on a Hitachi F-7000 spectrometer at 77 K in 2-MeTHF. Thermogravimetric
analyses (TGA) and differential scanning calorimetry (DSC) measurements were
carried out using Perkin-Elmer thermogravimeter (Model TGA7) and a Netzsch DSC
201, at a heating rate of 10 ℃ min-1 under a nitrogen atmosphere, respectively. The
cyclic voltammetry properties were studied on an electrochemical workstation
(BAS100B, USA) and using a conventional three electrode configuration at a scan
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rate of 100 mV s-1. A glass carbon working electrode, a Pt-wire counter electrDoOdI:e1,0a.10n3d9/C8TC02432D
a saturated calomel electrode (SCE) reference electrode were used. All the
measurements were made at room temperature on samples dissolved in
dichloromethane or N,N-dimethylformamide, deoxygenated with argon, and with 0.1
M [Bu4N]PF6 as the electrolyte. Density functional theory (DFT) calculations using
B3LYP functional were performed. The basis set used for the C, H, N atoms was
6-31G. There are no imaginary frequencies for both the optimized structures. All these
calculations were performed with Gaussian 09.
OLED fabrication and measurements
The pre-cleaned ITO glass substrates, with a sheet resistance of 15 Ω per square,
were treated by UV-ozone for 20 min. A 40 nm thick PEDOT:PSS film was first
deposited on the ITO glass substrate and baked at 120 ℃ for 30 min in air.
Subsequently, the substrate was transferred into a vacuum chamber to deposit the
organic layers with a base pressure less than 10-6 Torr (1 Torr = 133.32 Pa). Finally,
the device fabrication was completed through the thermal deposition of LiF (1 nm)
and then capping with Al metal (200 nm) as a cathode. The emitting area of each pixel
was determined by overlapping of two electrodes as 9 mm2. The EL spectra, CIE
coordinates and J-V-B curves of the devices were measured with a PR705 photometer
and a source-measure-unit Keithley 236, under ambient conditions at room
temperature. The forwarding viewing external quantum efficiency (ηext) was
calculated by using the current efficiency, EL spectra and human photopic sensitivity.
The details of compound synthesis and characterization are provided in
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DOI: 10.1039/C8TC02432D
Electronic Supporting Information.
Acknowledgements
We thank the National Natural Science Foundation of China (21421005 and
21374013), the Liaoning Natural Science Foundation (20170540152), the
Fundamental Research Funds for the Central Universities (DUT18ZD212) and the
Program for DUT Innovative Research Team (DUT2016TB12) for financial support
of this work.
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TOC Graphic Abstract
By gradually increasing the molar ratio of n-type to p-type groups from the unipolar
to bipolar and further to dual n-type unit bipolar host materials, the overall
performance of the phosphorescence organic light-emitting diodes is constantly and
regularly improved. The novel dual n-type unit bipolar host with n/p ratio of 2:1
exhibits a turn-on voltage of 2.6 V and an efficiency of 48.1 lm W-1, which are the
lowest turn-on voltage and one of the highest power efficiencies reported so far for
FIrpic devices containing a single host.