New all-fused ring spiro blue host and dopant system: Application in sky-blue fluorescent organic light-emitting materials

Article abstract of DOI:10.1002/bkcs.10543

Highly efficient sky-blue organic light emitting devices (OLEDs) were fabricated using fused-ring spiro host materials 1,1'-di(2-naphthyl)spiro[benzo[ij]tetraphene-7,9'-fluorene] (H1) and 1-[1-(4-biphenyl)]-10-phenylspiro[benzo[ij]tetraphene-7,9'-fluorene

Full text of DOI:10.1002/bkcs.10543

Article
DOI: 10.1002/bkcs.10543
BULLETIN OF THE
J.-R. Cha et al.
KOREAN CHEMICAL SOCIETY
New All-Fused Ring Spiro Blue Host and Dopant System: Application
in Sky-Blue Fluorescent Organic Light-Emitting Materials
Jae-Ryung Cha,† Chil-Won Lee,‡ and Myoung-Seon Gong†,
*
†DepartmentofNanobiomedicalScienceandBK21PLUSNBMGlobalResearchCenter, DankookUniversity,
Chungnam 330-714, Republic of Korea. *E-mail: msgong@dankook.ac.kr
‡Department of Polymer Science and Engineering, Dankook University 126, Yongin 448-701,
Republic of Korea
Received March 12, 2015, Accepted July 21, 2015, Published online October 4, 2015
Highly efficient sky-blue organic light emitting devices (OLEDs) were fabricated using fused-ring spiro host
materials 1,10-di(2-naphthyl)spiro[benzo[ij]tetraphene-7,90-fluorene] (H1) and 1-[1-(4-biphenyl)]-10-phe-
nylspiro[benzo[ij]tetraphene-7,90-fluorene] (H2). H1 and H2 were doped with a fluorescent dopant based
on 5,9-di(di-p-tollylaminospiro[benzo[c]fluorene-7,90-fluorene] (D3). Typical blue fluorescent OLEDs were
fabricated with indium tin oxide (ITO)/N,N0-diphenyl-N,N0-bis[4-(phenyl-m-tolyl-amino)phenyl]-biphenyl-
4,40-diamine (DNTPD)/N,N,N0,N0-tetra(1-biphenyl)-biphenyl-4,40-diamine (TBB)/SBTF hosts: D3/9,10-di
(naphthalene-2-yl)anthracen-2-yl-(4,1-phenylene)(1-phenyl-1H-benzo[d]imidazole) (LG201)/LiF/Al config-
urationandweresubsequentlyinvestigated. TheH1:5%D3device exhibitedanelectroluminescentemission at
472 nm, with efficiency 6.91 cd/A, external quantum efficiency 4.56% at 107.84 mA/cm2, and a sky-blue color
at Commission Internationale de l0Eclairage (CIE) (x,y) coordinates of (0.150, 0.217) at 7447 cd/m2.
Keywords: Fluorescence organic light emitting devices, Spirobenzo[c]fluorene-7,90-fluorene, Spirobenzo[ij]
tetraphene-7,90-fluorene, Blue dopant, Blue host
Introduction
In our previous study, we synthesized and characterized
spiro[benzo[c]fluorene-7,90-fluorene] (SBFF) derivatives
with extended conjugated structures for use as blue host and
dopant materials.22–26 These conjugated spiro molecules
based on SBFF exhibited good thermal stability and unique
optical and electrochemical properties that are desirable for
use in new emitting materials for OLEDs technologies. In
addition, the uniquestructure of highly conjugatedspiro mole-
cules allows their optical and electrochemical properties to be
delicately tuned over a wide range through appropriate chem-
ical modification to their aromatic moieties.27,28 Our study
was extended to include a wider variety of fused ring
spiro molecules, and spiro[benzo[ij]anthracene-7,90-fluorene]
(SBAF)28–31 and spiro[benzo[ij]tetraphene-7,90-fluorene]
(SBTF)32–36 units were introduced in blue OLEDs to improve
their morphologic stability to achieve a highly efficient blue
light emission.
Over the last decade, highly efficient blue organic light emit-
ting diodes (OLEDs) have attracted a considerable amount of
attention due to their potential use in full-color flat-panel dis-
plays or in lighting sources.1–3 However, blue-emitting mate-
rials with high efficiency and pure, saturate color are difficult
to design because of their wide bandgap. A number of anthra-
cene, distyrylarylene, pyrene, fluorene, and silole derivatives
have been presented in the literature as blue host materials,3–7
and among these, anthracene derivatives have played
an important role in the development and application of host
and dopant materials for OLED applications.8–11 However,
the number of optimized blue dopant materials with
energetic matching is still limited, except for monostyryl-
amine, distyrylamine, and iminodibenzyl-substituted
distyrylarylenes.12–15
Compounds with a spiro-sp3 carbon center are promising
candidates for use in blue organic light-emitting materials
because of their high energy gap and low highest occupied
molecular orbital (HOMO) levels.12 Spirobifluorene deriva-
tives have received much attention because they not only pre-
vent close packing and intermolecular interaction but also
increase the molecular rigidity.16,17 As a result, spirobifluor-
ene linkages have been introduced into the structure of the
small molecules, leading to a reduction inthe tendency tocrys-
tallize, improvement in the solubility, and an increase in the
glass transition temperature.18–21
Here, we report on the development of a sky-blue
fluorescent dopant material 5,9-di(di(p-tolyl))aminospiro
[benzo[c]fluorene-7,90-fluorene] (D3), which was doped into
morphologically stable fused ring host materials 1,10-
dinaphthylspiro[benzo[ij] tetraphene-7,90-fluorene] (H1) and
1-[1-(4-biphenyl)]-10-phenylspiro[benzo[ij]tetraphene-7,90-
fluorene] (H2). The H1 and H2 blue host materials
employed an SBTF core structure to improve the light-
emitting efficiency because of its extended conjugated struc-
ture, while the thermal and morphological stability were
improved by implementing a twisted spiro structure. The
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electroluminescence (EL) of devices using H1 and H2 host
materials and D3 dopant was investigated, and a high lumi-
nance efficiency of 6.91 cd/A and an external quantum effi-
ciency (EQE) of 4.56% were demonstrated in the blue
fluorescence of the sky-blue OLEDs.
8.02–8.00 (d, J = 8.45 Hz, 1H, Ar-CH-benzene), 7.69–7.67
(d, J = 7.60 Hz, 2H, Ar-CH-fluorene), 7.60–7.58 (t, J =
8.00 Hz, 1H, Ar-CH-naphthalene), 7.39–7.37 (t, J = 7.57
Hz, 1H, Ar-CH- naphthalene), 7.28–7.26 (t, J = 7.90 Hz,
2H, Ar-CH-fluorene), 7.12–7.10 (s, 2H, Ar-CH-benzene),
7.12–7.10 (t, 4H, Ar-CH-benzene), 7.10–7.08 (d, 2H, Ar-
CH-fluorene), 7.04–7.02 (d, 2H, Ar-CH-benzene),
6.96–6.94 (d, 1H, Ar-CH-benzene), 6.90–6.88 (t, 4H, Ar-
CH-benzene), 6.88–6.86 (d, 2H, Ar-CH-naphthalene),
6.80–6.78 (d, 2H, Ar-CH-benzene), 6.74–6.72 (d, 2H, Ar-
CH-benzene), 6.72–6.68 (d, 2H, Ar-CH-benzene),
2.08–2.09 (s, 12H, Ar-CH3). 13C NMR (CDCl3) δ 151.5,
148.1, 147.8, 147.5, 147.4, 146.8, 142.1, 139.0, 138.8,
136.9, 135.4, 131.9, 130.8, 129.2, 129.0, 128.8, 127.9,
126.9, 126.1, 125.7, 124.4, 124.1, 123.8, 123.6, 123.3,
123.0, 122.5, 122.1, 121.4, 121.1, 120.4, 119.2, 118.6,
77.4, 77.2, 76.9, 66.4, 21.4, 21.3. Anal. Calcd. for
C57H44N2 (Mw, 756.97): C, 90.44; H, 5.86; N, 3.70.
Found: C, 90.38; H, 5.88; N, 3.67. MS (FAB) m/z 757.97
[(M + 1)+]. UV–vis (THF): λmax (Absorption) = 415 nm, λmax
(Emission) = 474 nm.
Experimental
Materials and Measurements. Palladium(II) acetate,
sodium tert-butoxide, tri-t-butylphosphine, and di-p-
tolylamine(AldrichChemical. Co., St. Louis, MO, USA)were
used as received. Tetrahydrofuran and toluene were
distilled over sodium and calcium hydride. 5,9-Dibromo-
SBFF was prepared as previously reported.15,19 1,10-Di(2-
naphthyl)spiro[benzo[ij]tetraphene-7,90-fluorene] (H1) and
1-[1-(4-biphenyl)]-10-phenylspiro[benzo[ij]tetraphene-7,90-
fluorene] (H2) were prepared by the methods reported
previously.32–34 9,10-Di(2-naphthyl)anthracene (β-ADN)
was used as commercial host material. Photoluminescence
(PL) spectra were recorded on a fluorescence spectrophotom-
eter (Jasco FP-6500; Tokyo, Japan) and UV–vis spectra were
obtained by a UV–vis spectrophotometer (Shimadzu, UV-
1601PC, Tokyo, Japan). Energy levels were measured with
a low-energy photoelectron spectrometer (AC-2; Riken-
Keiki, Union City, CA, USA). FT-IR spectra were obtained
with a Thermo Fisher Nicolet 850 spectrophotometer
(Waltham, MA, USA), and elemental analysis were per-
formed using a CE Instrument EA1110 (Hindley Green,
Wigan, UK) apparatus. Differential scanning calorimetry
(DSC) measurements were performed on a Shimadzu DSC-
60 instrument under nitrogen. Thermogravimetric analysis
(TGA) measurements were performed on a Shimadzu TGA-
50 thermogravimetric analyzer. High-resolution mass spectra
were recorded usingan HP 6890 (Brea, CA, USA) and Agilent
Technologies 5975C MSD in the fast atom bombardment
(FAB) mode (Palo Alto, CA, USA).
OLED Fabrication. A basic device configuration of
indium tin oxide (ITO) (150 nm)/N,N0-di(1-naphthyl)-N,N0-
bis[(4-diphenylamino)
phenyl]-biphenyl-4,40-diamine
(DNTPB, 60 nm)/N,N,N0,N0-tetra(1-biphenyl)-biphenyl-4,40-
diamine (TBB, 30 nm)/SBTF hosts: D3 (30 nm, 5%)/
9,10-di(naphthalene-2-yl)anthracen-2-yl-(4,1-phenylene)(1-
phenyl-1H-benzo[d] imidazole) (LG201, 20 nm)/LiF (1 nm)/
Al (200 nm) was used for device fabrication. The organic
layers were deposited sequentially onto the substrate at a rate
of 1.0 Å/s by thermal evaporation from heated alumina cruci-
bles. The concentration of the dopant materials was varied in
steps of 5%. The devices were encapsulated with a glass lid
and a CaO getter after cathode deposition. Current density–
voltage luminance and EL characteristics of the blue fluores-
cent OLEDs were measured with a Keithley 2400 source
measurement unit (Cleveland, OH, USA) and a CS 1000
spectroradiometer (Minota GmbH, Arensburg, Germany).
Preparation of 5,9-di(di-p-tolyl)aminospiro[benzo[c]
Fluorene-7,9’-fluorene] (D3).
5,9-Dibromo-SFBF
(1) (7.06 g, 13.47 mmol), di-p-tolylamine (5.31 g, 26.94
mmol) and palladium acetate (0.30 g, 1.35 mmol) were dis-
solved in anhydrous toluene (70 mL) under nitrogen atmos-
phere. To the reaction mixture was added a solution of tri-t-
butylphosphine (0.55 g, 2.69 mmol) and sodium t-butoxide
(3.24 g, 33.68 mmol) in toluene (70 mL) dropwise slowly.
The reaction mixture was stirred for 3 days at 100 ꢀC. Follow-
ing the removal of the solvent in vacuo, an ammonia solution
(70 mL) was added, and the mixture was left to stand for 2 h.
Dichloromethane (150 mL) and water (100 mL) were added,
and the organic phase was separated. The organic layer was
dried over anhydrous MgSO4 and evaporated in vacuo to give
the crude product, which was purified by column chromatog-
raphy bydichloromethane/hexane (v/v, 1/3). The final product
was obtained as a yellowish green powder.
Results and Discussion
Synthesis and Characterization. Highly conjugated fused-
ring spiro host materials H1 and H2 were prepared by multi-
step synthetic routes, as previously reported (Figure 1).32–34
5,9-Dibromo-SBFF was prepared by selective bromination
of 5-bromo-SBFF in carbon tetrachloride solvent.15,19 The
dopant material D3 was prepared by amination reactions of
5,9-dibromo-SBFF with di(p-tolyl)amine in the presence of
Yield 74%. 1H NMR (500 MHz, CDCl3) δ (ppm)
8.78–8.76 (d, J = 8.47 Hz, 1H, Ar-CH-naphthalene),
8.22–8.20 (d, J = 8.53 Hz, 1H, Ar-CH-naphthalene),
H1
H2
Figure 1. Chemical structure of SBTF host materials.
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Fused Ring Spiro Blue Host and Dopant Materials
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a: naphthalene-1-boronic acid/tetrakis(triphenylphosphine)
palladium(0)/K2CO3/THF; b: NaOH/HCl/Ethanol; c:
CH3SO3H/30oC/24 h; d: n-BuLi/-78oC/3/HCl/CH3COOH; e:
Br2/CCl4; f: di(p-tolyl)amine/palladium acetate
Figure 2. Preparation of SBFF dopant material.
a palladium catalyst as shown in Figure 2. The synthetic yield
of D3 was 74%. Chemical structures and compositions of the
resulting SBFF dopant were characterized by 1H-NMR, 13C-
NMR, FT-IR spectroscopy, elemental analysis, and gas
chromatography-mass spectrometry. The chiral carbon peak
was observed at 60.0 ppm in the 13C-NMR spectra. Results
of elemental analysis and mass spectroscopy also supported
formation of D3 and matched well with the calculated data.
Optical Properties and Energy Levels. Figure 3(a) presents
the UV–vis absorption and photoluminescence (PL) spectra
of D3 in a dilute solution (THF) and as a neat film on a quartz
plate. The absorption bands at ~300 nm for D3 originate from
the diarylamine-centered n–π* transition. The absorption
maximum of D3 in dilute solution, centered at 415 nm, is sim-
ilar to that provided by its thin film (421 nm), implying that no
significant intermolecular interactions occur in the ground
state. The PL maximum of D3 was at 474 nm in solution,
whereas in the solid-state PL maximum was observed at
479 nm. The nonplanar structure of D3 facilitates its use as
dopant in sky-blue OLEDs owing to the efficient inhibition
of π–π* stacking in the film state.
The UV–vis absorption and PL spectra of the H1 and H2
host materials are summarized in Figure 3(b) and (c), and
Table 1. H1 and H2 showed absorption peaks at 387 and
368 nm, respectively. PL maxima of H1 and H2 were at
440 and 441 nm in solution, respectively, whereas the solid-
state PL maxima were observed at 445 nm for H1 and 447
nm for H2. These solid-state PL spectra were slightly red-
shifted compared to those of other nonspiro molecules.37
Thermal Properties. The molecular configuration of D3
endowed it with high thermal stability, as indicated by a high
decomposition temperature (Td = 412 ꢀC, corresponding to
5% weight loss during TGA) and a rather high glass transition
temperature (Tg = 136 ꢀC, determined using DSC). No
melting points were observed during the second heating,
even though the sample was given enough time to cool
in air. Once the sample had become an amorphous solid, it
did notrevert toa crystalline state. Asa result, D3forms homo-
geneous and stable amorphous films upon thermal evapora-
tion. The Td values of H1 and H2 were 428 and 439 ꢀC,
respectively, whereas their Tg values were 179 and 168 ꢀC,
respectively. Tg of H1 was increased by 11 ꢀC compared to
that of H2 because of the 2-naphthyl moiety at both 5- and
9-position.
Theoretical Calculations and Energy Levels. To gain more
insight intotheelectronic structure ofD3, densityfunction the-
ory (DFT) calculations were performed at Beck’s three-
parameter Lee–Yang–Parr (B3LYP) functional with 6-
31G* level for the geometry optimization. As shown in
Figure 4, the HOMO of D3 is mainly populated over both
the di(p-tolyl)amine and SBAF with considerable contribu-
tion, whereas the lowest unoccupied molecular orbital
(LUMO) has a sizable contribution from the SBAF core.
The di-(p-tolyl)amine at the 5-position of D3 was twisted with
respect to the naphthalene plane, with a dihedral angle of 52ꢀ,
whereas the di-(p-tolyl)amine at the 9-position of D3 was
twisted with respect to the phenylene plane with a dihedral
angle of 38ꢀ. Using low-energy photoelectron spectroscopy,
we estimated the energy level of HOMO to be 5.29 eV. The
crossing point of the background and the yield line is the pho-
toemission threshold energy, also called the work function or
the ionization potential, asshown inFigure 5.38 By subtracting
the optical energy gap (Eg = 3.12 eV) from the HOMO energy
level, we calculated the energy level of LUMO to be 2.53 eV.
The HOMO of H1 and H2 was also calculated from the pho-
toemission threshold energy curve. The HOMO and LUMO
energy levels were 5.82 and 2.82 eV for H1 and 5.75 and
2.73 eV for H2, respectively. Figure 6 shows the energy levels
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(a)
474 479
(b)
440
415 421
D3
445
H1
387
Solution Abs
Solid Abs
Solution PL
Solid PL
Solution Abs
Solution PL
Solid PL
(c)
442
H2
389
Solution Abs
Solution PL
Solid PL
448
Figure 3. UV–vis and PL spectra of (a) D3, (b) H1, and (c) H2.
Table 1. UV absorption, PL, energy levels, and thermal properties of two SBTF hosts and SBFF dopant.
Sample properties
H1
H2
D3
Puritya
HPLC
DSC
(%)
99.9
99.9
99.26
b
Thermal analysis
Tg (ꢀC)
179
428
387
440
43
445
54
3.00
5.82
2.82
168
439
389
441
63
447
67
3.02
5.75
2.73
136
412
415
474
47
479
58
2.70
5.29
2.53
c
Td (ꢀC)
Optical analysis
UV (THF)
PL (THF)
Maximum (nm)
Maximum (nm)
FWHMd (nm)
Maximum (nm)
FWHMd (nm)
(eV)
PL (Solid)
e
Bg
Electrical analysis
AC-2f
HOMO (eV)
LUMO (eV)
a The purity of the samples were finally determined by high-performance liquid chromatography (HPLC) using the prepared samples above after train
sublimation.
b Glass transition temperature.
c Decomposition temperature.
d Full width at half-maximum.
e Bandgap.
f photoelectron spectroscopy (60 nm film).
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-1.6
-1.8
-2.0
-2.2
-2.4
-2.6
-2.8
-3.0
-3.2
-3.4
-3.6
-3.8
-4.0
-4.2
-4.4
-4.6
-4.8
-5.0
-5.2
-5.4
-5.6
-5.8
-6.0
-6.2
-6.4
HIL
2.0
Unit:eV
HTL
2.48
EML
EIL
2.8
2.53
2.82
3.6
LiF/Al
3.8
ITO
4.8
5.2
5.29
5.82
5.45
6.01
Figure 6. Energy diagram of the blue fluorescence device using
SBTF host doped with SBFF dopant.
Figure 4. HOMO and LUMO electronic density distributions of
SBFF dopant material D3, calculated at the DFT/B3LYP/6-31G*
level for optimization and time-dependent DFT (TDDFT) using
Gaussian 03 (Gaussian, Inc., Wallingford, CT, USA).
468 nm
468 nm
472 nm
H1
H2
1.0
0.8
0.6
0.4
0.2
0.0
ꢀ-ADN
400
450
500
550
600
Wavelength (nm)
Figure 7. Electroluminescence spectra of H1, H2, and β-ADN
devices doped with D3.
Commission Internationale de l0Eclairage (CIE) color coordi-
natesofthedevices basedonH1, H2, andβ-ADN were(0.150,
217), (0.136, 0.202), and (0.139, 0.208), respectively. This
apparent resistance tocolorchangeundervarious drivecurrent
densities suggests that the charge carriers for recombination
are well balanced in this blue emitter and both excitation
mechanism of charge trapping and Förster energy transfer
may be prevalent in the D3 doped devices.
OLED Device Properties. The device structure used was
ITO (150 nm)/DNTPB (HIL, 60 nm)/TBB (HTL, 30 nm)/
SBTF hosts: D3 (30 nm, 5%)/LG201 (ETL, 20 nm)/LiF (1
nm)/Al (200 nm), where the emitting layer was composed
of the fluorescent dopant D3 and hosts H1, H2, and
β-ADN. The current density–voltage–luminance characteris-
tics of the devices with three hosts doped with 5% D3 dopant
are shown in Figure 8. All devices show the typical character-
istics of OLEDs, and the turn-on voltages of devices H1, H2,
Figure 5. Low-energy photoelectron spectra (from AC-2) of the
SBFF dopant D3.
of devices using the H1 and H2 hosts and D3 as dopant
materials.
ELProperties. Figure 7showsthenormalized ELspectra ofa
device composed of H1, H2, and β-ADN doped with 5% D3 at
7 V. All the devices showed an intense sky-blue emission in
the EL spectra at 472 nm for H1, 468 nm for H2, and 468
nm for β-ADN. The devices H1 showed a 4 nm longer wave-
length in the EL spectra than H2.
Looking at the EL spectra for all three hosts, we can con-
clude that thereis almost exclusivelyemission from thedopant
with a neat single peak (λmax = 474 nm) in THF, suggesting
that the energy transfer from the host to D3 dopant was quite
efficientattheoptimumdopantconcentrationemployedinthis
experiment. Sky-blue emission was observed, and the
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and β-ADN were 3.6, 3.6, and 4.0 V, respectively. It is found
that the luminance is higher for the device H1 than for the
device H2 at the same driving voltage. The maximum bright-
ness of the devices H1 and H2 was 7447 and 5798 cd/m2,
which is an improvement over the device β-ADN with 812
cd/m2 due to the different hole–electron recombination prob-
ability in the emitting layer for each blue OLED. In addition,
β-ADN showed much lower current density characteristics
than the two SBTF hosts H1 and H2.
Devices H1 and H2 have the small energy gap of 3.00 and
3.02 eV (β-ADN = 3.03 eV) between HTL and ETL, which
makes it easier for electron injection from the ETL to the emit-
ting layer, thereby able to reach the high luminance efficiency
(Figure 9). We believe that the more balanced carriers for
recombination in the H1 and H2 devices may be another rea-
son for the enhanced luminance efficiency in addition to the
more effective Förster energy transfer.39
Figure 10(a) presents the quantum efficiency–luminance
and power efficiency–luminance curves of the sky-blue
OLEDs. The quantum efficiency was also dependent on the
host materials, and the device using β-ADN doped with D3
showed a lower efficiency than the devices H1 and H2. It is
noteworthy that we obtained an almost upper EQE of 5.20%
at 5.1 V in the device H2 based on the fluorescent dopant
D3. The device H2 shows higher EQE than the device H1
(5.10%) although the device H2 shows lower luminance effi-
ciency than the device H1 at a fixed voltage.
The power efficiency of the sky-blue OLEDs is also shown
in Figure 10(b). In spite of the low quantum efficiency of the
device H1, the power efficiency of the device H1 was higher
than that of the device H2. The power efficiency at a low cur-
rent density of 48.57 cd/m2 was 6.6 lm/W, which is much
higher than that of β-ADN (3.0 lm/W) (Table 2).
Thedependenceofthechromaticityofthedevice onthecur-
rent density was measured to evaluate its stability (Figure 11).
10
H1
H1
H2
ꢀ–ADN
10000
9
200
H2
8
ꢀ-ADN
7
6
5
4
3
2
1
0
1000
100
100
10
0
1
0
2
4
6
8
10
Voltage (V)
0
2
4
6
8
10
12
Voltage (V)
Figure 8. Current density–voltage–luminescence characteristics of
the devices using H1, H3, and β-ADN host materials doped with
5% D3.
Figure 9. Luminance efficiency–voltage characteristics of the device
using H1, H2, and β-ADN host materials doped with 5% D3.
7
8
(a)
(b)
7
6
5
4
3
2
1
0
6
5
4
3
2
1
1
10
100
1000
10000
100000
1
10
100
1000
10000
Luminance (cd/m2)
Luminance (cd/m2)
Figure 10. (a) Quantum efficiency–luminance and (b) power efficiency–luminance curves of the sky-blue light-emitting diodes.
Bull. Korean Chem. Soc. 2015, Vol. 36, 2669–2676
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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BULLETIN OF THE
Article
Fused Ring Spiro Blue Host and Dopant Materials
KOREAN CHEMICAL SOCIETY
Table 2. Electroluminescence properties of the devices obtained from SBTF hosts and SBFF dopant materials.
H1
H2
D3
β-ADN
468
Material properties
EL at 7 V
Host dopanta
max (nm)
λ
472
50
468
50
FWHM (nm)
J (mA/cm2)
Lb (cd/m2)
LEc (cd/A)
LE (cd/A)d
EQEb (%)
EQEe (%)d
PEf (%)
51
107.84
7447
6.91
7.78 (5.1 V)
4.56
85.70
5758
6.72
7.65 (5.1 V)
4.65
5.20 (5.1 V)
3.06
5.72 (3.6 V)
0.136
19.50
812
4.16
4.29 (5.4 V)
2.82
2.86 (5.7 V)
1.90
2.72 (4.8 V)
0.139
5.04 (5.1 V)
3.14
6.10(3.6 V)
PEf (%)d
CIE-x
CIE-y
0.150
0.217
0.202
0.208
a 5% concentration.
b Luminance.
c Luminance efficiency.
d Values at a highest peak.
e External quantum efficiency.
f Power efficency.
0.150
0.145
0.140
0.25
0.20
0.15
EL efficiency. Thus, the HOMO level of D3 was suitable
for hole-trapping and hole transportation as a dopant, and
the SBTF hosts were moderate for balancing holes and elec-
trons in the emitting layer. It is therefore evident that the over-
lap between the hypsochromic-shifted emission peak of H1
and H2 and the absorption peak of D3 is better than that of
commercial β-ADN, which is essential for efficient Förster
energy transfer.
D3 Dopant
H1
H2
ꢀ-ADN
Conclusion
0.135
D3 Dopant
H1
H2
ꢀ-ADN
In conclusion, we designed and prepared a new SBFF deriv-
ative D3, which is a thermally stable and highly fluorescent
blue dopant material because of its rigid and spiro molecular
structure. The fused ring host materials H1 and H2 doped with
D3 devices show blue emissions with the maximum peak at
472 nm and have the EL spectra identical to the PL spectrum
of D3. The ITO (150 nm)/DNTPB (60 nm)/TBB (30 nm)/H1
hosts: D3 (30 nm, 5%)/LG201 (20 nm)/LiF (1 nm)/Al (200
nm) device displayed sky-blue emission, a maximum EQE
of 4.56%, a maximum luminance efficiency of 6.91 cd/A at
the current density of 107.84 mA/cm2, and maximum lumi-
nance of 7447 cd/m2.
0.130
1
10
100
Current density (mA/cm2)
Figure 11. Stability of the chromaticity depicted by CIE coordination
for the devices using H1 and H2 hosts doped with 5% D3 dopant.
The CIE chromatic variation of H1 and H2 devices is negligi-
ble (x = 0.001, y = 0.01) with the current density increasing
from 5 to 100 mA/cm2. When β-ADN and 5% D3 dopant were
used, the color stability was even worse: its chromatic varia-
tionwas(x = 0.002, y = 0.012). According tothe colorstability
of SBTF-based devices, we can confirm that the energy levels
of SBTF hosts and SBFF dopant match well with neighboring
layers, and the charge carrier recombination at the emitting
layer is efficient at all current densities.38 Device H2 shows
a near-flat color variation versus current density and is resist-
ant to color shift with increasing current density as well. Based
on these results, we attribute this apparent resistance to color
shift to the inserted spiro linking group, which effectively pre-
vents the dyes from aggregation at high concentration.38
The sky-blue dopant D3 had a large capacity to catch the
holes, and thus minimize the loss of holes resulting in good
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Fused Ring Spiro Blue Host and Dopant Materials
KOREAN CHEMICAL SOCIETY
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Bull. Korean Chem. Soc. 2015, Vol. 36, 2669–2676
© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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