Deep-blue and white organic light-emitting diodes based on novel fluorene-cored derivatives with naphthylanthracene endcaps

Article abstract of DOI:10.1039/c1jm11438g

Novel fluorene based deep-blue-emitting molecules with naphthylanthracene endcaps, namely 2,7-di(10-naphthylanthracene-9-yl)-9,9-dioctylfluorene (NAF1) and 7,7′-di(10-naphthylanthracene-9-yl)-9,9,9′,9′-tetraoctyl- 2,2′-bifluorene (NAF2), are synthesized by a Suzuki cross-coupling reaction. These materials exhibit excellent thermal and amorphous stabilities, and high fluorescence quantum yield of over 70%. Organic light-emitting devices (OLEDs) using NAF1 or NAF2 as non-doped emitter exhibit bright deep blue electroluminescence with CIE coordinates of (0.15, 0.13) for NAF1, (0.16, 0.13) for NAF2. A maximum power efficiency of 2.2 lm W^-1 (4.04 cd A ^-1, 4.04%) is achieved for NAF1, which is among the highest values ever reported for deep-blue fluorescent OLEDs. A further improved coordinates of (0.15, 0.09) with efficiencies of 3.56 cd A^-1 and 2.10 lm W ^-1 are achieved for NAF1 upon tuning device thickness, which are also among the best data for non-doped deep blue fluorescent OLEDs with a CIE coordinate of y < 0.1. NAF1 serves as excellent host emitter when doped with an orange fluorophore (4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7- tetramethyljulolidyl-9-enyl)-4H-pyran, DCJTB). Upon careful tuning the doping level, the two-emitting-component (NAF1:DCJTB) OLED realizes efficient white light emission with a power efficiency of 3.01 lm W^-1 (7.66 cd A ^-1), a brightness of 12090 cd m^-2, and a standard white light coordinates of (0.33, 0.33). This performance is among the best results ever reported for two-emitting-component white OLEDs based on fluorescent materials.

Full text of DOI:10.1039/c1jm11438g

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PAPER
Deep-blue and white organic light-emitting diodes based on novel
fluorene-cored derivatives with naphthylanthracene endcaps
a
b
b
a
Ting Zhang, Di Liu, Qian Wang, Renjie Wang, Huicai Ren and Jiuyan Li
b
a
*
Received 6th April 2011, Accepted 12th June 2011
DOI: 10.1039/c1jm11438g
Novel fluorene based deep-blue-emitting molecules with naphthylanthracene endcaps, namely 2,7-di
(10-naphthylanthracene-9-yl)-9,9-dioctylfluorene (NAF1) and 7,70-di(10-naphthylanthracene-9-yl)-
9,9,90,90-tetraoctyl-2,20-bifluorene (NAF2), are synthesized by a Suzuki cross-coupling reaction. These
materials exhibit excellent thermal and amorphous stabilities, and high fluorescence quantum yield of
over 70%. Organic light-emitting devices (OLEDs) using NAF1 or NAF2 as non-doped emitter exhibit
bright deep blue electroluminescence with CIE coordinates of (0.15, 0.13) for NAF1, (0.16, 0.13) for
NAF2. A maximum power efficiency of 2.2 lm Wꢀ1 (4.04 cd Aꢀ1, 4.04%) is achieved for NAF1, which is
among the highest values ever reported for deep-blue fluorescent OLEDs. A further improved
coordinates of (0.15, 0.09) with efficiencies of 3.56 cd Aꢀ1 and 2.10 lm Wꢀ1 are achieved for NAF1 upon
tuning device thickness, which are also among the best data for non-doped deep blue fluorescent
OLEDs with a CIE coordinate of y < 0.1. NAF1 serves as excellent host emitter when doped with an
orange fluorophore (4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-
pyran, DCJTB). Upon careful tuning the doping level, the two-emitting-component (NAF1 : DCJTB)
OLED realizes efficient white light emission with a power efficiency of 3.01 lm Wꢀ1 (7.66 cd Aꢀ1),
a brightness of 12090 cd mꢀ2, and a standard white light coordinates of (0.33, 0.33). This performance is
among the best results ever reported for two-emitting-component white OLEDs based on fluorescent
materials.
energy transfer to a blue dopant. However, the large-band-gap
Introduction
host materials are relatively rare and always keep as one of the
most challenging targets of OLED research. Fortunately, more
and more blue fluorescent host emitters have been developed to
fabricate non-doped deep blue OLEDs with CIE coordinate
value of y < 0.15 or even < 0.1.6,7
Organic light-emitting diodes (OLEDs) have attracted more and
more research attention because of their promising applications
in full-color, large-area flat-panel displays and solid-state
lighting.1,2 Compared with green-emitting materials, blue-emit-
ting materials still have some problems such as imbalance of
carrier transporting, low efficiency and especially poor color
purity.3–5 In recent years, great efforts have been taken to tune
the color of blue electroluminescence (EL) into the deep blue
region with a Commisssion Internationale de L’Eclairage (CIE)
coordinate value of y < 0.1. Charge-transporting emitters and
their non-doped OLEDs are apparently advantageous in terms
of ease of device fabrication, low cost especially in mass
production, and absence of phase separation or spectra shift with
driving voltage. Among the three primary color emitting mate-
rials, blue host emitters are especially more significant than other
two color ones, since otherwise a purple-light-emitting host
material having larger band gap would be necessary for efficient
Due to the relatively large band gaps, the blue host emitting
materials are additionally valuable to serve as the matrix emit-
ters for low-energy dopants to fabricate other single-color and
even white OLEDs.8–11 White OLEDs are of special interests in
both research and industry fields due to their practical appli-
cation as new-generation lighting source and backlight of flat-
panel displays.12,13 In general, white OLEDs can be obtained by
incorporating either three primary colors (red, green, and blue)
or two complementary color ones (blue and yellow or orange) in
a multi- or a single-layer structure. Due to the simplicity of
device structure and fabricating process and the excellent device
performance, the two-emitting-component strategy has
ascended to be one important method for fabrication of
WOLEDs.14–16 In this type of white OLEDs, the proper doping
level and thus the extent of energy transfer from the blue host to
the orange dopant are the key points to control the color purity
of white light. In addition, the luminescent efficiencies of both
the blue host and the orange dopant are essential to determine
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, 2 Linggong Road, Dalian, 116024,
China. E-mail: jiuyanli@dlut.edu.cn; Fax: +86 411 84986233
bSchool of Chemistry, Dalian University of Technology, 2 Linggong Road,
Dalian, 116024, China
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the overall efficiency of white light. Therefore, excellent blue
host emitters are sought for white OLEDs application. Recently
a few blue fluorophores were successfully used to fabricate white
OLEDs. For example, Shu et al. reported that an anthracene
derivative end-capped with tetraphenylethylene groups, namely
TPVAn, exhibited an efficiency of 5.3 cd Aꢀ1 with CIE coor-
dinates of (0.14, 0.12) in its non-doped blue OLED, and the
two-component white OLED containing TPVAn as blue host
and an orange fluorescent dopant realized a high efficiency of
4.9% with CIE coordinates of (0.33, 0.39). Both these blue and
white OLEDs performances are among the best data for fluo-
rescent materials.17
In this paper, we report the synthesis and luminescent prop-
erties of a group of novel fluorene-based deep blue emitters with
naphthylanthracene endcaps, namely NAF1 and NAF2 (Scheme
1). Anthracene18 and fluorene19 are important building blocks to
construct blue luminescent dyes due to their merits such as high
fluorescent quantum yield, wide band gap and ease to be modi-
fied. However, neither of them exhibits good color purity nor
stable thermal stability in organic light-emitting devices since
both of them are easy to recrystallize and have low glass tran-
sition temperature. This situation didn’t change until the
appearance of oligomer light-emitting materials such as oligo-
fluorenes.20 Herein, we select the oligofluorene as the core and
Scheme 1 Synthetic routes for NAF1 and NAF2.
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9-naphthylanthracene as the end caps at the C2 and C7 positions
of fluorene to construct the first examples of fluorene/anthracene/
naphthalene hybrids. Our results indicate the molecules designed
in this way have non-planar structures, which are thus favorable
to effectively inhibit self-aggregation and facilitates the forma-
tion of stable amorphous films.21,22 Furthermore, these molecules
are well soluble due to the non-planar conformation and the long
alkyl substituents at the C9 position of fluorene moieties. The
blue OLEDs containing NAF1 or NAF2 as a non-doped emitter
show high efficiency of 4.02% (3.56 cd Aꢀ1 and 2.10 lm Wꢀ1) with
CIE coordinates of (0.15, 0.09). More importantly, these blue
emitters serve as an efficient host for orange fluorescent dopant,
and the two-emitting-component white OLEDs fabricated with
NAF1 host and an traditional orange dopant exhibited an effi-
ciency of 7.66 cd Aꢀ1 with pure white color of (0.33, 0.33).
the measurements were carried out at room temperature under
ambient conditions.
Compounds synthesis
The intermediates 1, 2, and 3,23–25 and 4, 5, 6, 7, and 8,26 were
synthesized according to the literature methods.
7,70-Dibromo-9,9,90,90-tetraoctyl-2,20-bifluorene (9). Bromine
(0.26 mL, 0.8 mmol) in CCl4 (3 mL) was slowly added with
ꢁ
stirring at 0 C to a CCl4 (15 mL) solution of the compound 8
(1 g, 1.28 mmol) containing a piece of iodine. The solution was
stirred at 0 ꢁC for 2 h in the dark, and 20 mL of aqueous
NaHCO3 (15%) was added. Vigorous stirring was applied until
the red color disappeared. The organic layer was separated,
washed with water and dried over anhydrous MgSO4. The
solvent was removed under vacuum and the residue was recrys-
tallized from ethanol to give 9 as a white solid (1 g, 83% yield).
Mp: 74.5–76.8 ꢁC; 1H NMR (400 MHz, CDCl3, d): 7.74 (d, J ¼
7.6 Hz, 2H; ArH), 7.63–7.57 (m, 6H; ArH), 7.48 (d, J ¼ 8 Hz, 4H;
ArH), 2.05–1.95 (m, 8H; CH2), 1.21–1.08 (m, 40H; CH2), 0.81 (t,
J ¼ 7.2 Hz, 12H; CH3), 0.69 (m, 8H; CH2).
Experimental
General information
Chemicals, reagents and solvents from commercial sources are of
analytical or spectroscopy grade and used as received without
further purification. 1H and 13C NMR spectra were recorded on
a Varian INOVA spectrometer (400 MHz). Mass spectra were
recorded on a GC-Tof MS (Micromass, UK) mass spectrometer
for TOF-MS-EI and a MALDI micro MX (Waters, USA) for
MALDI-TOF-MS. Elemental analyses were carried out on
a Carlo–Eriba 1106 elemental analyzer. Thermogravimetric
analyses (TGA) and differential scanning calorimetry (DSC)
measurements were performed on a Perkin-Elmer thermog-
ravimeter (ꢁModel TGA7) and a Perkin-Elmer DSC 7 at a heating
rate of 10 C minꢀ1 under a nitrogen atmosphere, respectively.
The fluorescence and UV-vis absorption spectra measurements
were performed on a Perkin-Elmer LS55 fluorescence spec-
trometer and a Perkin-Elmer Lambda 35 UV-Visible spec-
traphotometer, respectively. The phosphorescence quantum
yields were determined against quinine sulfate as the standard
(F ¼ 0.55 in water). Electrochemical measurements were made
by using a conventional three-electrode configuration and an
electrochemical workstation (BAS 100B, USA) at a scan rate of
100 mV sꢀ1. A glassy carbon working electrode, a Pt-wire counter
electrode, and a saturated calomel electrode (SCE) as reference
electrode were used. All measurements were made at room
temperature on samples dissolved in dichloromethane, with
0.1 M tetra-n-butylammonium hexafluorophosphate (Bu4NPF6)
as the electrolyte, ferrocene as the internal standard.
2,7-Di(10-naphthylanthracene-9-yl)-9,9-dioctylfluorene (NAF1).
A mixture of 3 (460 mg, 1.32 mmol), 5 (302 mg, 0.55 mmol) and Pd
(PPh3)4 (63mg, 0.055 mmol) in toluene (15 mL) and 2 M aqueous
Na2CO3 solution (2.8 mL, 5.5 mmol) was degassed by pump. The
solution was heated at 80 ꢁC for 18 h under argon. After the
reaction mixture was cooled to room temperature, dichloro-
methane and water were added. The organic layer was separated
and washed with diluted HCl and brine, then dried over anhy-
drous MgSO4. The solvent was removed under vacuum and the
residue was purified by column chromatography over silica gel
with petroleum ether/CH2Cl2 (7 : 1) as the eluent to give NAF1 as
1
a light yellow solid (284 mg, 52% yield). H NMR (400 MHz,
CDCl3, d): 8.12–8.00 (m, 8H; ArH), 7.96 (d, J ¼ 8 Hz, 2H; ArH),
7.89 (d, J ¼ 8 Hz, 4H; ArH), 7.77 (d, J ¼ 8 Hz, 4H; ArH), 7.67–
7.56 (m, 10H; ArH), 7.40–7.33 (m, 8H; ArH), 2.09–2.05 (m, 4H;
CH2), 1.25–1.16 (m, 20H; CH2), 0.99 (m, 4H; CH2); 0.82 (t, J ¼
7.2 Hz, 6H; CH3); 13C NMR (400 MHz, CDCl3, d): 151.23, 140.46,
137.90, 136.91, 136.65, 133.46, 132.80, 130.28, 129.62, 128.14,
127.10, 126.47, 125.16, 119.83, 55.52, 40.52, 31.85, 30.11, 24.27,
22.62, 14.12; MALDI-TOF-MS (m/z): [M+] calcd for C77H70,
994.5477; Found, 994.5214. Anal. calcd. for C77H70: C, 92.91; H,
7.09. Found: C, 92.64; H, 6.91.
7,70-Di(10-naphthylanthracene-9-yl)-9,9,90,90-tetraoctyl-2,20-
bifluorene (NAF2). NAF2 was prepared according to the method
used for NAF1 by using 3 (230 mg, 0.66 mmol), 9 (206 mg,
0.22 mmol) and Pd(PPh3)4 (26 mg, 0.022 mmol) in toluene
(15 mL) and 2 M aqueous Na2CO3 solution (1.1 mL, 2.2 mmol).
The crude product was purified by column chromatography over
silica gel with petroleum ether/CH2Cl2 (6 : 1) as the eluent to give
NAF2 as a light yellow solid (122 mg, 40% yield). 1H NMR (400
MHz, CDCl3, d): 8.11 (d, J ¼ 8 Hz, 2H; ArH), 8.04–7.99 (m, 6H;
ArH), 7.95 (d, J ¼ 8 Hz, 4H; ArH), 7.87 (d, J ¼ 8 Hz, 4H; ArH),
7.80–7.76 (m, 8H; ArH), 7.64–7.60 (m, 6H; ArH), 7.54–7.51 (m,
4H; ArH), 7.35–7.34 (m, 8H; ArH), 2.14–2.08 (m, 8H; CH2),
Device fabrication
The pre-cleaned ITO glass substrates (30 U ,ꢀ1) were treated by
UV-Ozone for 20 min. All the organic layers were deposited by
vacuum evaporation in a vacuum chamber with a base pressure
less than 10ꢀ6 torr. The cathode was completed through thermal
deposition of LiF (1 nm) and then capping with Al metal
(100 nm). The emitting area of each pixel is determined by
overlapping of the two electrodes as 9 mm2. The EL spectra, CIE
coordinates, and current–voltage-luminance relationships of
devices were measured with computer-controlled Spectrascan
PR 705 photometer and a Keithley 236 source-measure-unit. All
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ꢁ
1.25–1.16 (m, 40H; CH2), 0.92–0.88 (m, 8H; CH2); 0.82 (t, J ¼ 7.2
Hz, 12H; CH3); 13C NMR (400 MHz, CDCl3, d): 152.01, 151.22,
140.45, 140.02, 137.69, 136.87, 136.66, 133.47, 132.80, 130.28,
129.62, 128.14, 127.11, 126.46, 125.15, 121.44, 120.10, 119.76,
55.48, 40.53, 31.95, 30.11, 24.13, 22.71, 14.10; MALDI-TOF-MS
(m/z): [M+] calcd for C106H110, 1382.8608; Found, 1383.5051.
Anal. calcd. for C106H110: C, 91.99; H, 8.01. Found: C, 91.57; H,
7.69.
stability with decomposition temperatures (Td) up to 430 C in
N2. The morphological stability of these molecules was moni-
tored by the DSC measurements. As shown by the thermograms
of NAF1 in Fig. 1, only one endothermic peak due to melting was
observed at 334 ꢁC when a crystalline powder obtained from
organic solvent is heated for the first run. The isotropic liquid
would be rapidly self-cooled into the glassy state. In the second
heating run, a glass transition was detected at 110 ꢁC (Tg). Upon
further heating beyond Tg, an exothermal peak was observed at
ꢁ
177 C, which is ascribed to the crystallization. The melting is
Results and discussion
repeated when the temperature is increased to 334 ꢁC. A glass
ꢁ
transition temperature (Tg) of 110 C indicates a relatively high
Synthesis
amorphous stability of the NAF1 solid films. We attribute the
amorphous stability to the non-plꢁanar molecular structure of this
molecule.27 NAF2 melted at 269 C and no glass transition was
observed under the present experimental condition. Based on the
thermal and amorphous stability of these compounds, homoge-
neous and stable amorphous thin films of these compounds could
be achieved by vacuum deposition, which matches the basic
requirement for host emitters used in OLEDs.
The synthetic routes for the fluorene-cored derivatives with
naphthylanthracene endcaps (NAF1, NAF2) are shown in
Scheme 1. They were prepared by C–C coupling reaction of the
boronic acid of the naphthylanthracene endcaps (3) and the bis-
brominated oligo-fluorene core (5 or 9). First, the important
intermediates for the synthesis of these target molecules,
including 9-(2-naphthyl)anthracene-10-boronic acid (3),23–25 2,7-
dibromo-9,9-dioctylfluorene (5) and 9,9,90,90-tetraoctyl-2,20-
bifluorene (8),26 were synthesized according to literature
methods, as described in Scheme 1. The 7,70-dibromo-9,9,90,90-
tetraoctyl-2,20-bifluorene (9) was then obtained at a high yield of
83% by bromination of 8 in CCl4 at low temperature of zero
degree Celsius. It is found that use of the other solvents or
increasing reaction temperature e.g. to room temperature or
higher often resulted in tris-brominated byproducts. Finally the
target compounds NAF1 and NAF2 were prepared at a moderate
yield of 52% and 40% by Suzuki coupling reactions of the
boronic acid 3 with the bis-brominated oligo-fluorene 5 and 9,
respectively. NAF1 and NAF2 have good solubility in common
organic solvents so that they can be easily purified by column
chromatography and recrystallization to high purity for spec-
troscopy characterization and OLEDs application. The chemical
Photophysical properties
The photophysical properties of NAF1 and NAF2 were investi-
gated by means of electronic absorption and steady state pho-
toluminescence (PL) spectra for both dilute solutions in
dichloromethane and the solid films on quartz plates. The data
are summarized in Table 1. Fig. 2 shows the absorption and PL
spectra for NAF1 as an example. The absorption spectrum of
NAF1 in dilute solution exhibits the characteristic vibrational
structures of NAF1 and NAF2 were confirmed by H and 13C
1
NMR spectroscopy, matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectrometry, and elemental
analysis. To the best of our knowledge, they are the first examples
of fluorene/anthracene/naphthalene hybrids.
Thermal properties
The thermal properties of NAF1 and NAF2 were investigated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) at a scanning rate of 10 ꢁC minꢀ1 and the data
are listed in Table 1. NAF1 and NAF2 exhibit high thermal
Fig. 1 DSC trace for NAF1. Tg: glass transition temperature; Tc: crys-
tallization temperature; Tm: melting point.
Table 1 Summary of physical parameters of NAF1 and NAF2
abs
max
em
max
l
(nm)
l
(nm)
Compound CH2Cl2 Film
CH2Cl2 Film F (%)a Tg (ꢁC)b Tc (ꢁC) Tm (ꢁC) Td (ꢁC) HOMO (eV)c Eg (eV)d LUMO (eV)
NAF1
NAF2
399,379 403,383 436
360 361
398,378 404,383 440
358,341 361,344
444
448
70
73
110
n.o.
177
133
334
269
438
430
ꢀ5.52
ꢀ5.53
2.90
2.87
ꢀ2.62
ꢀ2.66
a
Measured in CH2Cl2 solution with quinine sulfate as the standard (F ¼ 0.55 in water). b n.o. ¼ not observed. c Calculated with reference to ferrocene
(4.8 eV). d The energy band gap, calculated by the absorption edge technique.
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Fig.
2 Absorption and fluorescence spectra of NAF1 in dilute
dichloromethane solution and in solid film.
Fig. 3 Cyclic voltammograms of NAF1 and NAF2 measured in CH2Cl2
at a scan rate of 100 mV sꢀ1
.
patterns of the isolated anthracene group with three bands at
399, 379 and 360 nm.28 A red shift of 4 nm was detected in the
absorption spectrum of NAF1 thin film in comparison with that
of the solution. Upon photoexcitation at the absorption
maximum, the NAF1 solution exhibits deep-blue fluorescence
with emission peak at 436 nm. Similar to the absorption, a red
shift of 8 nm was detected in the PL spectra of thin film. The
absorption spectra of NAF2 have similar features with those of
NAF1, except with one additional band at 341 nm which should
be ascribed to the p–p* transition of oligo-fluorene core. NAF2
exhibits blue PL with emission peak at 440 nm in solution and
448 nm in thin film. The bathochromic effect in both absorption
and PL spectra are frequently observed due to intermolecular
interaction in solid state. However, the small red shift of only
4–8 nm in both absorption and PL spectra of present NAF1 and
NAF2 reflects that the non-coplanar conformations of NAF1 and
NAF2 molecules effectively inhibit unwanted intermolecular
interaction and consequently guarantee the deep-blue emission
color of these dyes in the solid state. The fluorescence quantum
yield of NAF1 and NAF2 in dilute dichloromethane solution
were determined as high as 70% and 73% when using quinine
sulfate (F ¼ 0.55 in water) as the standard.29 The high fluores-
cence quantum yield along with the agreeable emission color
indicate that they may be used as efficient blue-light-emitting
materials in OLEDs.
Electroluminescence of OLEDs
To investigate the electroluminescence (EL) properties of NAF1
and NAF2, blue-emitting devices with a typical three-layer
structure of ITO/NPB (40 nm)/NAF1 (device I) or NAF2 (device
II) (15 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm) were fabri-
cated. In these devices, NPB acts as the hole-transporting layer
(HTL) and TPBI as the electron-transporting and hole-blocking
layer (ETL), respectively. A 15 nm thick NAF1 or NAF2 film is
deposited as the non-doped emitting layer (EML). Both device I
and II exhibit deep-blue EL with emission peak at 448 nm and
CIE coordinates of (0.15, 0.13) for NAF1, and 450 nm and CIE
coordinates of (0.16, 0.13) for NAF2, as shown by the EL spectra
in Fig. 4. The EL spectra and the CIE coordinates of these two
OLEDs are observed to insensitive to the driving voltage, indi-
cating a stable blue emission. The EL spectra show a small red
shift of 2–4 nm compared with the PL of films. This is
a frequently observed phenomena caused by the influence of the
electrical field on the excited states in OLEDs.31 As shown by the
voltage-current density-luminance (V–J–L) characteristics and
the efficiency curves in Fig. 5, the NAF1 based device I exhibited
a maximum brightness of 6528 cd mꢀ2 at 8.5 V, and a maximum
luminance efficiency (hL) of 4.04 cd Aꢀ1 at 6 V, corresponding to
a peak power efficiency (hp) of 2.2 lm Wꢀ1 and an external
quantum efficiency (hext) of 4.04%. The detailed performance of
both device I and II are summarized in Table 2. Apparently, the
Electrochemical properties
Cyclic voltammetry (CV) measurements were performed using
a conventional three-electrode cell set-up with 0.1 M tetra-n-
butylammonium hexafluorophosphate (Bu4NPF6) as a support-
ing electrolyte in CH2Cl2, ferrocene as the internal standard, to
investigate the electrochemical behaviors of these materials. The
cyclovoltamograms are shown in Fig. 3. NAF1 and NAF2 show
a reversible one-electron oxidation process with the onset
potential of 0.72 and 0.73 V relative to ferrocene, respectively.
The HOMO energy levels were estimated from the onset poten-
tials to be 5.52 and 5.53 eV for NAF1 and NAF2 respectively,
+ 4.8 eV).30
onset
ox
according to the equation of HOMO (eV) ¼ ꢀ(E
The energy band gaps were determined by the absorption edge
technique.30 The LUMO levels were calculated by subtracting the
gap from the energy of the HOMO. The detailed electrochemical
and electronic data of these two molecules are listed in Table 1.
Fig. 4 EL spectra of blue devices I–IV.
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Fig. 6 Energy level diagrams for the OLEDs.
For this purpose, we fabricate a reference device III with struc-
ture of ITO/NPB (55 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm),
in which NPB acts as both hole transporting and emitting layer.
The thickness of NPB layer in device III is controlled the same as
the total of both HTL and EML in device I and II, to avoid any
possible performance difference caused by different film thick-
ness. The EL spectrum of device III is located at 438 nm, at least
10 nm shorter than those of device I and II. Furthermore, the
luminance and efficiencies of the reference device III are quite
lower than those of device I and II, as shown by the data in Table
2. All these results combine to confirm that EL in device I and II
are indeed emitted by the NAF1 and NAF2 emitting layer, rather
than from adjacent layers or interfaces.
Fig. 5 (a) Voltage-current density-luminance (V–J–L) characteristics
and (b) efficiencies curves for blue devices I, II and IV.
performances of NAF1 based device I are among the best results
ever reported for non-doped deep-blue OLEDs with CIE coor-
dinate of y # 0.15. It should be noted that the overall perfor-
mance of NAF2 based device II are inferior to those of NAF1
based device I, although the fluorescent quantum yield of NAF2
is slightly higher. According to the energy levels diagrams for
these two OLEDs in Fig. 6, both the hole injection barrier in
NPB/EML interface (0.22 and 0.23 eV for NAF1 and NAF2,
respectively) and the electron injection barrier in TPBI/EML
(0.08 and 0.04 eV for NAF1 and NAF2, respectively) are both
small enough to guarantee efficient hole and electron injection
into the emitting layer in both devices. However, the current
density at a given voltage in device II is much lower than that of
device I (Fig. 5a). This probably results from the possible lower
charge transporting mobility of NAF2 film than NAF1.32
In order to improve the performance of the blue emission,
especially to obtain lower value of CIE coordinate y to approach
the standard blue color, the thickness of NPB layer in device I
was reduced to 30 nm, to obtain the device IV. Device IV
exhibited lower turn-on voltage than device I due to the reduc-
tion in total thickness.33 Furthermore, a narrowed EL spectrum
(Fig. 4) and better CIE coordinates of (0.15, 0.09) were obtained.
This CIE coordinates are quite close to the National Television
System Committee (NTSC) standard blue of (0.14, 0.08).
Although the emission efficiencies are slightly decreased as the
expense in comparison with device I, a maximum luminance
efficiency hL of 3.56 cd Aꢀ1, a maximum external quantum effi-
ciency hext of 4.02% with an excellent CIE coordinates of (0.15,
0.09) for device IV (Fig. 5b and Table 2), are still among the best
results ever reported for non-doped deep-blue OLEDs with
a CIE coordinate of y < 0.1.
Since the hole transporting material NPB used in these devices
is also a typical blue emitting material, it is necessary to verify the
true origin of the deep-blue electroluminescence in these devices.
Table 2 Performance of devices I–VI
Device
Vturn-on [V]a
Lmax (cd mꢀ2) (b)
hL(max) (cd Aꢀ1) (b)
hp(max) (lm Wꢀ1) (b)
hext(max) (%)c
lmax (nm)
CIE (x, y)
I
II
III
IV
V
4.1
5
3.5
3.8
6
6528 (8.5 V)
4541 (9 V)
2308 (7.5 V)
5339 (7.5 V)
20890 (15 V)
12090 (12.5 V)
4.04 (6 V)
2.50 (7 V)
0.92 (5.5 V)
3.56 (5.5 V)
11.90 (13 V)
7.66 (10.5 V)
2.20 (5.5 V)
1.15 (6.5 V)
0.54 (5 V)
2.10 (5 V)
4.27 (6 V)
3.01 (6.5 V)
4.04
2.50
n.c.
4.02
4.17 (13 V)
3.24 (10.5 V)
448
450
438
448
564
448, 556
0.15,0.13
0.16,0.13
0.15,0.07
0.15,0.09
0.45,0.45
0.33,0.33
VI
5
a
b
c
Recorded at 1 cd mꢀ2
.
Values in the bracket are the voltages at which the data are obtained. n.c ¼ not calculated.
12974 | J. Mater. Chem., 2011, 21, 12969–12976
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View Article Online
DCJTB combines with energy transfer to be responsible for the
efficient EL of DCJTB in the doped OLEDs. When the doping
concentration is reduced to 0.2%, device VI shows a balanced
emission from both host and dopant as indicated by the EL
spectra in Fig. 7. This two-emitting-component device exhibited
a bright white light with CIE coordinates of (0.34, 0.35) at 7 V.
With increasing the voltage, the color coordinates slightly shifted
to (0.33, 0.33) at 12.5 V, which is just the standard white light of
the National Television System Committee. The voltage-current
density-luminance characteristics and the efficiency curves of the
white OLED VI are illustrated in Fig. 8 and the data are listed in
Table 2. The white OLED VI exhibited a maximum luminance
(Lmax) of 12 090 cdmꢀ2 at 12.5 V. The maximum luminance
efficiency reached to hL ¼ 7.66 cd Aꢀ1, corresponding to a peak
power efficiency of 3.01 lm Wꢀ1. Although the efficiencies of our
present white OLED are only close to the best data reported by
Shu17 for the two-emitting-component white OLEDs, the color
coordinates of our device is superior to theirs over the whole
detected voltage range. Most excitingly, the standard white light
with (0.33, 0.33) is achieved for our NAF1 based device at a high
Fig. 7 EL spectra of orange device V (at 7 V) and white device VI (at
various applied voltages).
The good performance of NAF1 in the non-doped blue
OLEDs prompts us to explore the possibility to use it as host
emitter in host : dopant type of devices and finally to fabricate
white OLEDs. For this purpose, a traditional yellow fluo-
rophore, 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetrame-
thyljulolidyl-9-enyl)-4H-pyran (DCJTB), was selected as the
yellow dopant to fabricate OLEDs with the following configu-
ration: ITO/NPB (30 nm)/NAF1:DCJTB (1 : x) (30 nm)/TPBI
(30 nm)/LiF (1 nm)/Al (200 nm) (x ¼ 2%: device V, x ¼ 0.2%:
device VI). When the doping concentration is 2%, device V shows
yellow emission (Fig. 7) from DCJTB with CIE coordinates of
(0.45, 0.45), implying that efficient forward energy transfer from
NAF1 host to DCJTB can occur. This can be confirmed by the
large spectral overlap between the absorption of DCJTB (spec-
trum not shown) and the fluorescence of NAF1. In addition, the
HOMO and LUMO of DCJTB is located in between those of
NAF1 (shown in Fig. 6), implying that direct charge trapping by
brightness over 10 000 cd mꢀ2
.
Conclusion
In summary, novel fluorene-cored deep-blue-emitting materials
with naphthylanthracene endcaps, NAF1 and NAF2, have been
synthesized and investigated. A luminance efficiency of 3.56 cd
Aꢀ1 (2.10 lm Wꢀ1 and 4.02%) and an almost standard blue
coordinates (0.15, 0.09) were obtained in the non-doped OLEDs
with NAF1 as emitter, which is among the best performance ever
reported for deep-blue OLEDs with a low CIE coordinate y <
0.1. Furthermore, NAF1 is capable of acting as host emitter in
host : dopant emitting systems. The white OLEDs using NAF1
as blue host and a traditional yellow fluorophore as dopant
realized a high luminance of 12 090 cd mꢀ2 and an efficiency of
7.66 cd Aꢀ1 with a standard white light coordinates of (0.33,
0.33), which is also among the best results ever reported for two-
emitting-component white OLEDs based on fluorescent
materials.
Acknowledgements
We thank the National Natural Science Foundation of China
(U0634003, 20704002, 20923006, and 21072026), the Ministry of
Education for the New Century Excellent Talents in University
(Grant NCET-08-0074), the NKBRSF (2009CB220009), and the
Fundamental Research Funds for the Central Universities
(DUT10LK16) for financial support of this work.
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