Constructing high-performance blue, yellow and red electroluminescent devices based on a class of multifunctional organic materials

Article abstract of DOI:10.1039/c3tc31271b

Two new indolo[3,2-b]carbazole derivatives (DPDT-ICZ and DNDT-ICZ) with multifunctionality were designed and synthesized. They were employed as deep-blue emitters, hole-transporting materials and hosts to fabricate organic light-emitting devices (OLEDs). The devices which used them as emitters displayed deep-blue emissions with CIE coordinates of (0.15, 0.08). They have been employed as hole-transporting and host material simultaneously to construct high performance yellow and red phosphorescent OLEDs. High power efficiencies (78.3 lm W^-1 for yellow devices and 20.4 lm W^-1 for red devices) for phosphorescent OLEDs were achieved. Importantly, these devices displayed the feature of low roll-off of efficiencies. At the luminance of 1000 cd m^-2, roll-off of current efficiencies was 1.3% for the yellow device and 14.6% for the red device.

Full text of DOI:10.1039/c3tc31271b

Journal of
Materials Chemistry C
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PAPER
Constructing high-performance blue, yellow and red
electroluminescent devices based on a class of
multifunctional organic materials†
Cite this: J. Mater. Chem. C, 2013, 1,
594
6
Shanyong Chen, Jinbei Wei, Kai Wang, Chenguang Wang, Dong Chen, Yu Liu*
and Yue Wang*
Two new indolo[3,2-b]carbazole derivatives (DPDT-ICZ and DNDT-ICZ) with multifunctionality were
designed and synthesized. They were employed as deep-blue emitters, hole-transporting materials and
hosts to fabricate organic light-emitting devices (OLEDs). The devices which used them as emitters
displayed deep-blue emissions with CIE coordinates of (0.15, 0.08). They have been employed as hole-
transporting and host material simultaneously to construct high performance yellow and red
Received 3rd July 2013
ꢀ1
ꢀ1
phosphorescent OLEDs. High power efficiencies (78.3 lm W for yellow devices and 20.4 lm W for
Accepted 9th August 2013
red devices) for phosphorescent OLEDs were achieved. Importantly, these devices displayed the feature
DOI: 10.1039/c3tc31271b
ꢀ2
of low roll-off of efficiencies. At the luminance of 1000 cd m , roll-off of current efficiencies was 1.3%
www.rsc.org/MaterialsC
for the yellow device and 14.6% for the red device.
materials with multifunctionality is still a challenge in the area
of organic electroluminescence.
Introduction
Organic light-emitting devices (OLEDs) are of current interest
In this context, we report here two indolo[3,2-b]carbazole
7–9
from both scientic and practical points of view due to their (ICZ) analogues DPDT-ICZ and DNDT-ICZ (Scheme 1). The
applications in the next generational full-color at-panel two compounds exhibit high uorescent quantum efficiencies
1
displays and solid-state lighting sources. During the last two and intense deep-blue emissions with narrow FWHM (the full
decades great progress has been made in the exploitation of width at half maximum). Meanwhile, as candidates for hole-
2
,3
OLED materials and the optimization of device structures.
transporting materials, the high HOMO energy levels (about
5.0 eV) are suitable for hole injection. Their hole mobilities are
well matched with the electron mobilities of some electron-
Many new emitters with red (R), green (G) or blue (B) emission,
hole/electron transporting materials and hosts have been
developed for full-color and white OLEDs. To realize high-
ꢀ
transporting materials leading to balanced carriers in devices,
performance full-color display or white color lighting devices, which is favorable for obtaining high efficiencies and low roll-
the R, G and B emission materials together with corresponding off of efficiencies. Importantly, the two materials have relatively
hosts, hole- and electron-transporting materials are needed. In small singlet–triplet splitting that can reduce the intrinsic
other words, 6–8 or more kinds of organic materials are energy loss from S to T to improve the power efficiencies of
1
1
4
–6
needed to construct a full-color display.
The full-color devices. They have been employed as deep-blue emitters, hole-
OLEDs were oen established based on a set of complex
materials systems and high cost organic syntheses. In this
sense, achieving high-performance R, G and B electrolumi-
nescence through a simple material system with an aim to
reduce the production cost of materials and simplify the
manufacturing process is an important issue for OLED appli-
cations. Therefore, the design and synthesis of organic
transporting materials and hosts to fabricate high-performance
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, P.R. China. E-mail: yuliu@jlu.edu.
cn; yuewang@jlu.edu.cn
†
Electronic supplementary information (ESI) available: TGA/DSC plots, OLED
1
13
device data, H NMR, C NMR, mass spectra. CCDC reference numbers 941266
and 941267. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3tc31271b
Scheme 1 Synthetic procedures and structures of compounds.
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OLEDs. A comprehensive investigation that encompasses the cheap synthetic procedures make them promising candidates
crystal structures, photophysical properties, electrochemical for practical applications.
properties, charge-transporting properties as well as the
Synthesis of DPDT-ICZ. 1 (408 mg) and DMSO (15 ml) were
detailed electroluminescent (EL) characteristics of the materials added to a 50 ml two-neck round bottom ask. A solution of 1 g
will be presented in this contribution.
sodium hydroxide in 1 ml distilled water was added to the ask
slowly. Then 736 mg of n-iodobutane was added to the mixture
immediately. The mixture turned yellow and was stirred at room
temperature for 20 h. The mixture was poured into 200 ml
distilled water and then stirred for 20 min. The resulting solid
was isolated by ltration, washed with water and dried to give a
yellow solid. The solid was then puried by column chroma-
Experimental section
General information
1
Instruments. H NMR spectra were measured on a Bruker
AVANCE 500 MHz or a Varian Mercury 300 MHz spectrometer
with tetramethylsilane as the internal standard. Mass spectra
were recorded on a GC/MS mass spectrometer. Elemental
analyses were performed on a ash EA 1112 spectrometer. UV-
Vis absorption spectra were recorded by a Shimadzu UV-2550
spectrophotometer. The emission spectra were recorded by a
Shimadzu RF-5301 PC spectrometer. The absolute uorescent
quantum yields were measured on an Edinburgh FLS920 steady
state uorimeter (excited at 365 nm). Differential scanning
calorimetric (DSC) measurements were performed on a
tography on silica gel using CH Cl /petroleum ether (1 : 8 v/v) as
2
2
1
the eluent to give an olive-coloured solid. Yield: 62%; H NMR
500 MHz, CDCl
, d): 7.65 (d, 14H; ArH), 6.52 (d, J ¼ 1.4 Hz, 4H;
ArH), 1.52 (s, 4H; –CH ), 0.94 (s, 6H; –CH
), 0.765 (t, J ¼ 1.1 Hz,
). C NMR (500 MHz, CDCl ): 13.77, 20.03, 128.16,
28.94. m/z: 520.47 [M] (calcd: 520.29). Anal. Calcd for
(
3
2
3
1
3
8
H; –CH
2
3
+
1
C H N : C 87.65, H 6.97, N 5.38; found: C 87.80, H 6.98, N 5.44.
38 36
2
Synthesis of 2. A mixture of indole (2.34 g), 1-naph-
thaldehyde (3.12 g), 0.75 ml of HI solution (45% w/w) and 60 ml
ꢁ
ꢀ1 of acetonitrile was reuxed for 16 h under a nitrogen atmo-
NETZSCH DSC204 instrument at a heating rate of 10 C min
ꢁ
ꢁ
sphere. Aer cooling to 0 C, the mixture was ltered and then
from 20 to 360 C under a nitrogen atmosphere. Thermogravi-
the solid was washed with cold acetonitrile. The solid was put
into a 100 ml two-neck round bottom ask, which contained
metric analyses (TGA) were performed on a TA Q500 thermog-
ravimeter by measuring the weight loss while heating at a rate of
ꢁ
ꢀ1
ꢁ
2.0 g iodine and 60 ml of acetonitrile. The mixture was heated at
1
0 C min from 25 to 800 C under nitrogen. Electrochemical
ꢁ
8
0 C for 10 h under a nitrogen atmosphere. Aer cooling to
measurements were performed with a BAS 100W Bioanalytical
electrochemical work station, using Pt as the working electrode,
platinum wire as the auxiliary electrode, and a porous glass wick
room temperature, the solvent was removed by rotary evapora-
tion. The solid was added to a solution of sodium thiosulfate,
then the mixture was stirred for 20 min. The product was
extracted with dichloromethane. Aer removing the solvent and
+
Ag/Ag as the pseudo-reference electrode with standardization
against ferrocene/ferrocenium. The oxidation potentials were
crystallization in diethyl ether, the slightly yellow solid 2 was
measured in CH
2 2 4 6
Cl solution containing 0.1 M of n-Bu NPF as
1
ꢀ
1
obtained. H NMR (300 MHz, CDCl
8
3
, d): 8.89 (s, 2H; –NH), 6.57–
the supporting electrolyte at a scan rate of 100 mV s
.
+
.17 (m, 22H; ArH). Yield: 70%. m/z: 508.33 [M] (calcd: 508.19).
Synthesis of DNDT-ICZ. DNDT-ICZ was synthesized and
puried by a procedure similar to that for DPDT-ICZ except that
CH Cl /petroleum ether (1 : 10 v/v) was used as the eluent
Single crystal structures. The single crystals suitable for X-ray
structural analysis were obtained by slow evaporation of the
solutions of both compounds in dichloromethane. Diffraction
data were collected on a Rigaku RAXIS-PRID diffractometer
using the u-scan mode with graphite-monochromated Mo Ka
radiation. The structure was solved with direct methods using
the SHELXTL programs and rened with full-matrix least-
2
2
1
instead of CH
2 2
Cl /petroleum ether (1 : 8 v/v). Yield: 50%. H
NMR (300 MHz, CDCl
3
, d): 8.16 (d, J ¼ 8.4 Hz, 2H; ArH), 8.05 (d,
J ¼ 8.4 Hz, 2H; ArH), 7.85–7.88 (m, 2H; ArH), 7.72–7.78 (m, 2H;
ArH), 7.61 (d, J ¼ 8.4 Hz, 2H; ArH), 7.49–7.56 (m, 2H; ArH), 7.19–
2
squares on F . Non-hydrogen atoms were rened anisotropi-
7
3
.34 (m, 6H; ArH), 6.61 (s, 2H; ArH), 6.22 (d, J ¼ 8.1 Hz, 2H; ArH),
cally. The positions of hydrogen atoms were calculated and
rened isotropically.
Preparation of materials. All commercially available reagents
were used as received unless otherwise stated. All reactions were
carried out using Schlenk techniques under a nitrogen
atmosphere.
.53 (s, 4H; –CH ), 1.15–1.39 (m, 4H; –CH ), 0.48–0.66 (m, 10H;
2
2
1
3
–
1
1
1
CH , CH ). C NMR (500 MHz, CDCl ): 13.44, 13.50, 19.79,
9.83, 108.20, 117.89, 122.32, 122.74, 125.17, 125.86, 125.88,
26.39, 126.42, 126.56, 126.67, 126.73, 128.27, 128.32, 128.59,
2 3 3
+
28.60, 133.41, 133.79, 133.82. m/z: 620.40 [M] (calcd: 620.32).
Anal. Calcd for C46
40 2
H N : C 88.99, H 6.49, N 4.51; found: C
89.28, H 6.49, N 4.59.
Synthesis
Scheme 1 outlines the synthetic procedures and structures of
Device fabrication and measurements
compounds. Compounds 1 and 2 were synthesized according to
10
the reported procedures. Then a simple alkylation of 1 and 2 Before device fabrication, the ITO glass substrates were pre-
produced compounds DPDT-ICZ and DNDT-ICZ with good cleaned carefully and treated with plasma for 7 min. Then
yields. Aer column chromatography, subsequent purication the sample was transferred to the deposition system. The devices
ꢀ
6
by vacuum sublimation produced the pure samples. Both were prepared in vacuum at a pressure of 5 ꢂ 10 Torr. The hole-
1
13
materials were fully characterized by H NMR, C NMR, mass transporting material NPB (1,4-bis(1-naphthylphenylamino)
spectra (ESI, Fig. S1–S5†) and element analysis. The simple and biphenyl), hole-blocking and electron-transporting material
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Bepp
available and thermally evaporated at a rate of 0.2 A˚ s . Aer the distance of 2.97 A, and the interaction continues on to the next
organic lm deposition, 1.5 nm of LiF and 100 nm of aluminum molecules. The two sides of the herringbone have no interac-
were thermally evaporated onto the organic surface. The thick- tions. Unlike DPDT-ICZ, the DNDT-ICZ molecules adopt a
nesses of the organic materials and the cathode layers were parallel arrangement model with two kinds of molecular rows.
controlled using a quartz crystal thickness monitor. All of the Within one molecular row, there is no intermolecular interac-
used organic materials were puried by a vacuum sublimation tion. In the other molecular row, the naphthalene units are
approach. The electrical characteristics of the devices were connected to the ICZ units by C–H/p interactions with a
Paper
2
(hydroxyphenylpyridine beryllium) were commercially of neighbour molecules by C–H/p interactions with a contact
ꢀ1
˚
˚
measured with a Keithley 2400 source meter. The EL spectra and contact distance of 2.75 A. Between the two kinds of molecular
luminance of the devices were obtained on a PR650 spectrometer. rows no contact interaction has been observed. The nonco-
All the device fabrication and device characterization steps planar molecular conformation and weak intermolecular
were carried out at room temperature under ambient interactions enable the two compounds to have high uores-
laboratory conditions. Current–voltage characteristics of single- cent quantum efficiencies in the solid state.
carrier devices were measured using the same semiconductor
parameter analyzer as for OLED devices. The single-carrier
device measurements were performed under dark and ambient
Thermal analysis
Thermal properties were investigated using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) (see
conditions.
Fig. S6† and Table 1). Both compounds have high melting
points (298 C for DPDT-ICZ and 319 C for DNDT-ICZ) and
decomposition temperatures (corresponding to 5% weight loss)
Results and discussion
Crystal structure
ꢁ
ꢁ
ꢁ
ꢁ
The yellowish crystals of DPDT-ICZ and DNDT-ICZ were (362 C for DPDT-ICZ and 398 C for DNDT-ICZ). The glass
ꢁ
obtained by slow evaporation of their dichloromethane solu- transition temperature (T
) of DNDT-ICZ is 124 C, which is
g
tions and the single crystal structures were examined (Fig. 1). In much higher than the normal hole-transporting materials NPB
0
0
crystals, the ICZ unit is almost planar with a very small torsion and CBP (4,4 -N,N -dicarbazolylbiphenyl), and should be
ꢁ
angle of 0.84 , and the side aromatic units are almost perpen- attributed to the rigid structure of DNDT-ICZ. The good thermal
ꢁ
dicular to the ICZ plane with a dihedral angle of 88.3 . In the stability is favorable for the formation of uniform thin lms
solid state, DPDT-ICZ molecules adopt a typical herringbone upon thermal evaporation and can reduce the possibility of
ꢁ
packing model with a dihedral angle of 49.2 . On each side of phase separation during operation. This characteristic may
the herringbone, benzene units are connected to the ICZ units suppress the formation of aggregation in lms, thus prolonging
11
the device operational lifetime. Therefore, DPDT-ICZ and
DNDT-ICZ have desirable properties for their application in
OLEDs.
Photophysical properties
The UV-Vis absorption and photoluminescence (PL) spectra of
DPDT-ICZ and DNDT-ICZ in dilute CH
thin lms together with the phosphorescence spectra in CH
2
Cl
2
solutions and neat
Cl
2
2
at 77 K are shown in Fig. 2, and the optical data of the two
compounds are summarized in Table 1. According to the earlier
Table 1 Physical properties of DPDT-ICZ and DNDT-ICZ
DPDT-ICZ
DNDT-ICZ
ꢁ
f
T
l
g
/T
m
/T
d
( C)
/298/362
124/319/398
286,339,421/
286,343,423
431,450/439,450
0.39/0.30
5.00/2.14
2.86/2.41
0.47
a
b
abs/nm [sol /lm ]
286,337,418/
2
86,342421
a
b
l
PL/nm [sol /lm ]
f
[sol /lm ]
442,464/433,454
0.46/0.51
4.96/2.10
2.86/2.45
0.36
a
b
F
c
HOMO/LUMO (eV)
d
e
E
g
(eV)/E
T
(eV)
DEST (eV)
a
ꢀ5
b
c
Measured in CH
2
Cl
2
solution (10 M). Measured in lm. HOMO
was measured from the onset of oxidation potentials, LUMO was
d
deduced from HOMO and E
g
.
Singlet energy measured from the
e
Fig. 1 Molecular structures with 50% thermal ellipsoids and packing structures
diluted solution at 298 K. Triplet energy measured in CH Cl at 77 K
2
2
ꢀ5
f
of (a) DPDT-ICZ; (b) DNDT-ICZ.
(10 M). Not observed.
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report, the strong absorption peaks around 286 and 337 nm are splitting DEST values are 0.36 eV for DPDT-ICZ and 0.47 eV for
ascribed to the p–p* transition, peaks around 421 nm are owing DNDT-ICZ that are hardly reached for the commonly used hosts
13
14
15
16
to the n–p* transition of the ICZ unit. Both compounds based on carbazole, silane, phosphine oxide and pyridine.
exhibited intense deep-blue emissions in solutions and lms. The small DEST can reduce the intrinsic energy loss from S to
1
They have narrow FWHM (22.2 nm for DPDT-ICZ and 33.3 nm
1
T , and this is favourable for achieving OLEDs with high power
for DNDT-ICZ in solution; 40.1 nm for DPDT-ICZ and 39 nm for efficiencies.
DNDT-ICZ in lms), which is very important for color purity of
f
deep-blue emitters. The PL quantum yields (F ) in solution are
Electrochemical properties
0
.46 for DPDT-ICZ and DNDT-ICZ, and these in lms are 0.39
The electrochemical properties were investigated by cyclic vol-
tammetry (CV) (Fig. 3). Both compounds showed reversible
oxidation behaviors suggesting the potential of possessing good
hole transporting abilities. The HOMO energy levels are deter-
mined to be 4.96 eV for DPDT-ICZ and 5.00 eV for DNDT-ICZ,
respectively. The high HOMO energy levels may be favored for
hole injection. No reduction waves were observed for the two
compounds, and the LUMO energy levels (2.1 eV for DPDT-ICZ
and 2.14 eV for DNDT-ICZ) were deduced from HOMO and Eg.
The high LUMO energy levels can effectively prevent the elec-
tron injection. The HOMO and LUMO energy levels indicate
that DPDT-ICZ and DNDT-ICZ may be good hole-transporting
materials.
for DPDT-ICZ and 0.30 for DNDT-ICZ. These indicate that they
are candidates for deep-blue emitters. For DPDT-ICZ, the PL
quantum yield in solution is lower than that in lm form.
Contrarily, for DNDT-ICZ, the PL quantum yield in solution is
higher than that in lm form. A rational explanation for these
phenomena is that in the solid state of DPDT-ICZ the chromo-
phores adopt a J-aggregation state (Fig. 1a) that can enhance the
emission efficiency, while in the solid state of DNDT-ICZ the
chromophores adopt a H-aggregation state (Fig. 1b) that can
12
induce the emission quenching.
From the absorption spectra, the energy difference between
) is estimated as 2.86 eV for DPDT-ICZ and
DNDT-ICZ. From the phosphorescence spectra, the energy
differences between S0,0 and T1,0 (E ) are estimated as 2.45 eV
for DPDT-ICZ and 2.41 eV for DNDT-ICZ, which are enough for
S
0,0 and S1,0 (E
g
T
Single-carrier devices
yellow and red phosphorescent emitters. The singlet–triplet The hole-only and electron-only devices with structures of [ITO/
DPDT-ICZ or DNDT-ICZ or NPB (70 nm)/MoO (10 nm)/Al] for
3
hole-only and [ITO/TPBi (20 nm)/DPDT-ICZ or DNDT-ICZ
(60 nm)/LiF (1.5 nm)/Al] for electron-only devices (Fig. 4) were
fabricated. Because the LUMO of MoO
3
(ꢀ2.3 eV) is much
higher than that of Al (ꢀ4.3 eV), the electron cannot be injected
into the device, so we can get the pure hole current. By using
TPBi (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) thin lm
as hole-blocking layer, we can also get the pure electron current
in electron-only devices. The mobility was estimated from the
single carrier devices based on the space charge limited current
(
SCLC) method and I–V characteristics (Fig. 4a and b) of single-
17
carrier devices. For both compounds, the hole mobilities
(
ꢀ
5
2
ꢀ1 ꢀ1
ꢀ5
2
ꢀ1
3.26 ꢂ 10 cm V
S
for DPDT-ICZ and 1.02 ꢂ 10 cm V
Fig. 2 Room-temperature UV-Vis absorption and PL spectra in CH
2
Cl
2
solution
Cl
2
ꢀ5
(
10 M) and in neat films as well as the phosphorescence spectra in CH
2
ꢀ
5
solution (10 M) at 77 K of (a) DPDT-ICZ and (b) DNDT-ICZ.
2 2
Fig. 3 Cyclic voltammograms of DPDT-ICZ and DNDT-ICZ in CH Cl .
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Electroluminescent properties
To check the electroluminescent properties of DPDT-ICZ and
DNDT-ICZ deep-blue-emitting devices with the structures of
2
[ITO/NPB (45 nm)/DPDT-ICZ or DNDT-ICZ (20 nm)/Bepp (35
nm)/LiF (1.5 nm)/Al] were fabricated. Undoped thin lms of
DPDT-ICZ (device B1) and DNDT-ICZ (device B2) were employed
as the emitting layer in devices B1 and B2, respectively. NPB was
18
used as the hole-transporting layer, Bepp (Fig. 5) served as the
2
electron-transporting layer and LiF was utilized as the electron-
injecting layer. The electroluminescent performances of devices
B1 and B2 are shown in Fig. 6, 8, S7† and Table 2. Devices B1
and B2 displayed deep-blue emissions with Commission
International de l'Eclairage (CIE) coordinates of (0.15, 0.08),
which are very close to the National Television Standards
Committee (NTSC) standard (0.15, 0.07). Meanwhile, both
devices exhibited good EL color stabilities with the variation of
driving voltages. This characteristic is important for high
performance displays and lighting sources.
Devices B1 and B2 had low turn-on voltages (2.6 V), which
can be ascribed to their easy hole injection because of the high
HOMO energy levels (ꢀ5.0 eV) of DPDT-ICZ and DNDT-ICZ. The
maximum brightness (Lmax), maximum external quantum effi-
ciencies (hext.max), current (hc.max) and power (hp.max) efficiencies
ꢀ
2
ꢀ1
of device B1 are 4117 cd m , 0.96%, 1.31 cd A and 1.58 lm
W
ꢀ
1
, respectively.
To demonstrate the possibility that the two compounds may
be used as hole-transport materials and hosts of phosphors
simultaneously in devices, the phosphorescent OLEDs were
fabricated. The device structures, materials used in devices and
their energy level diagram are summarized in Fig. 5. Phospho-
19
20
2 2
rescent (ppy) Ir(tipg) and (bt) Ir(dipba) were used as yellow
and red emitters, respectively, in the OLEDs. The current
density–voltage–luminance (J–V–L) characteristics, efficiencies
Fig. 4 Current density versus voltage characteristics of the hole-only and elec-
tron-only devices of (a) DPDT-ICZ and (b) DNDT-ICZ, and the hole-only devices of
(
c) NPB, DPDT-ICZ and DNDT-ICZ.
ꢀ
1
S
for DNDT-ICZ) are much higher than their electron mobil-
ꢀ
8
2
ꢀ1 ꢀ1
ꢀ9
ities (2.38 ꢂ 10 cm V
S
for DPDT-ICZ and 1.83 ꢂ 10
2
ꢀ1 ꢀ1
cm V
S
for DNDT-ICZ). It is worth noting that the hole
mobilities of the compounds are slightly lower than that of NPB
within the driving voltage range from 2.4 to 7.5 V (Fig. 4c), which
is the voltage range of device operation. This characteristic may
be favourable for the carrier balance in devices because the
electron mobilities of many electron-transporting materials are
much lower than the hole mobilities of hole-transporting
materials. Therefore, constructing OLEDs with high efficiencies
and low roll-off of efficiencies based on the two compounds can
be expected.
Fig. 5 The materials used in devices, their energy level diagram and device
structures.
6
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1.7% for Y2, 22% for R1, 14.6% for R2. The low roll-off of effi-
ciencies may be attributed to the well matched hole- and
electron-transporting ability between the host and electron-
19
transporting material. Although DPDT-ICZ and DNDT-ICZ are
typical hole-transporting materials with poor electron-trans-
porting ability, the bipolar carrier transporting properties of
(
ppy)
2
Ir(tipg) and (bt)
2
Ir(dipba) have been conrmed in our
20,21
previous studies.
This characteristic ensures that electrons
Fig. 6 (a) Current density–voltage–luminance (J–V–L) characteristics, current
efficiency and power efficiency versus luminance curves of devices B2, Y2, R2
based on DNDT-ICZ; (b) EL spectra and CIE 1931 coordinates of devices B2, Y2, R2
based on DNDT-ICZ; (c) PL spectra of the emitting layers of devices Y1, Y2, R1, R2.
versus luminance curves, EL spectra as well as external quantum
efficiencies (EQE) are shown in Fig. 6, 8 and S7.† These devices
have low turn-on voltages (about 2.5 V) and rapidly increased
luminance feature. To achieve high luminance at low driving
voltage and current density is very benecial for prolonging
device lifetimes. The devices display high efficiencies of 58.9 cd
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
A
1
and 73.8 lm W for Y1, 64.8 cd A and 78.3 lm W for Y2,
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
5.6 cd A and 19.6 lm W for R1, 17.8 cd A and 20.4 lm W
for R2. Their high efficiencies can be ascribed to good carrier
injection, balanced carriers, and small singlet–triplet splitting
of hosts. From tens to thousands of luminance units, the effi-
ciencies of these devices decrease slowly. At the luminance of
Fig. 7 (a) Current density–voltage–luminance (J–V–L) characteristics; (b) current
efficiency and power efficiency versus luminance curves of devices Y1f, Y2f,
ꢀ2
1000 cd m , roll-off of current efficiencies are 1.3% for Y1, R1f, R2f.
a
Table 2 Electroluminescent properties of the devices
ꢀ2
b
c
ꢀ1
b
p
ꢀ1
b
c
Dopant
Von [V]
Lmax [cd m
]
h
[cd A
]
h
[lm W
]
h
ext [%]
CIE (x,y)
B1
B2
Y1
Y1f
Y2
Y2f
R1
R1f
R2
R2f
—
—
(ppy)
(ppy)
(ppy)
(ppy)
(bt)
(bt)
(bt)
(bt)
2.6
2.6
2.4
2.4
2.5
2.5
2.5
2.5
2.5
2.5
4117
6010
1.31, 1.02, 0.56
1.36, 1.29, 0.88
58.9, 58.9, 58.1
61.2, 60.6, 57.8
64.8, 64.8, 63.7
66.2, 64.3, 63.6
15.6, 14.8, 12.1
16.3, 15.2, 14.6
17.8, 17, 15.2
1.58, 0.82, 0.28
1.64, 1.13, 0.49
73.8, 70.8, 61.5
70.0, 69.4, 63.0
78.3, 73.6, 59.9
80.5, 74.4, 58.6
19.6, 15.7, 12.3
20.1, 15.6, 10.2
20.4, 17.9, 17.1
22.1, 17.9, 16.5
0.96, 0.71, 0.40
1.35, 1.25, 0.85
18.1, 15, 14.7
17.3, 16.8, 17.3
18.1, 18.0, 17.7
18.6, 18.3, 18.0
10.3, 9.8, 8
10.3, 10.2, 9.3
11.8, 11.2, 10.1
12, 11.7, 10.6
0.15, 0.08
0.15, 0.08
0.43, 0.56
0.43, 0.56
0.43, 0.56
0.43, 0.56
0.63, 0.37
0.63, 0.37
0.63, 0.37
0.63, 0.37
2
2
2
2
Ir(tipg)
Ir(tipg)
Ir(tipg)
Ir(tipg)
Ir(dipda)
Ir(dipda)
Ir(dipda)
Ir(dipda)
49 340
47 930
52 670
48 060
21 170
31 460
29 590
37 730
2
2
2
2
18.5, 17.9, 16.7
a
c p
Abbreviations: Von: turn-on voltage; Lmax: maximum luminance; h : the maximum current efficiency; h : the maximum power efficiency; hext: the
c
b
ꢀ2
ꢀ2
maximum external quantum efficiency. In the order of maximum, then values at 100 and 1000 cd m
. Measured at 100 cd m .
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Paper
can be injected into the emitting layer efficiently. Close (50 nm)/LiF/Al] for DPDT-ICZ in Y1f and DNDT-ICZ in Y2f, [ITO/
comparison of the EL data (Table 2) revealed that the DNDT-ICZ NPB (30 nm)/DPDT-ICZ or DNDT-ICZ : 10% (bt) Ir(dipba)
2
based devices (B2, Y2 and R2) showed slightly higher perfor- (10 nm)/Bepp (60 nm)/LiF (1.5 nm)/Al] for DPDT-ICZ in R1f and
2
mance than those of DPDT-ICZ based devices (B1, Y1 and R1). It DNDT-ICZ in R2f were fabricated for comparison. The current
is worth noting that the EL data summarized in Table 2 were density–voltage–luminance (J–V–L) characteristics, efficiencies
recorded within the driving voltage range of 2.4 to 4.0 V. The versus luminance curves are shown in Fig. 7 and 8, and the
better EL performance of DNDT-ICZ may be attributed to the device performances are listed in Table 2. The performances of
slightly higher hole transport ability of DNDT-ICZ compared the four reference devices are comparable to those devices
with DPDT-ICZ within the driving voltage range of 2.4 to 4.0 V which use DPDT-ICZ or DNDT-ICZ as hole-transporting mate-
(Fig. 4). On the other hand, DNDT-ICZ possesses slightly lower rials, suggesting that the two compounds are good hole-trans-
LUMO energy level compared with DPDT-ICZ (Table 1), which porting materials.
may enhance the electron injection from the electron transport
layer to the emitting layer and improve the EL performance.
Conclusions
The electroluminescent (EL) spectra of devices are shown in
Fig. 6 and S7.† There are only peaks of guests, which means In summary, two ICZ derivatives (DPDT-ICZ and DNDT-ICZ) have
complete energy transfer from hosts to guests. We examined the been synthesized and employed as emitters, hole-transporting
PL spectra of doped lms, which were identical to those of the materials and hosts to fabricate high performance OLEDs. Deep
emitting layers (Fig. 6c). Energy had been efficiently transferred blue OLEDs and highly efficient phosphorescent OLEDs (yellow
to guests at the doping concentrations used in the devices. We and red colors) were achieved based on the two compounds. The
conclude that energy transfer plays an important role in the multifunctional characteristics enable DPDT-ICZ and DNDT-ICZ
emitting mechanism and the two compounds are good hosts for to successfully carry out dual roles (hole-transporting materials
yellow and red PhOLEDs.
and hosts of phosphors) in highly efficient OLEDs. Our study
Phosphorescent OLEDs with structures of [ITO/NPB (30 nm)/ presents a strategy for the construction of high performance
DPDT-ICZ or DNDT-ICZ : 5% (ppy)
2
Ir(tipg) (20 nm)/Bepp
2
OLEDs based on simple material systems.
Acknowledgements
This work was supported by National Basic Research Program of
China (2009CB623600 and 2013CB834805) and National
Natural Science Foundation of China (51173064 and 21221063).
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