Solution-processed, undoped, deep-blue organic light-emitting diodes based on starburst oligofluorenes with a planar triphenylamine core

Article abstract of DOI:10.1002/chem.201200062

A series of starburst oligomers (T1-T3) that contained a fully diarylmethene-bridged triphenylamine core and oligofluorene arms were designed and synthesized through Suzuki cross-coupling reactions. Their thermal, photophysical, and electrochemical properties were also investigated. These materials showed high glass transition, in the range of 123-129 °C, and good film-forming abilities. They displayed deep-blue emission both in solution and as thin films. Solution-processed devices based on these oligomers exhibited highly efficient deep-blue electroluminescence and the device performances were significantly enhanced with the extension of the oligofluorene arms. The double-layered device that contained T3 as an emitter showed a maximum current efficiency of 3.83 cd A^-1 and a maximum external quantum efficiency of 4.19 % with CIE coordinates of (0.16, 0.09), which are among the highest values for undoped deep-blue OLEDs that are based on solution-processable starburst oligomers. Copyright

Full text of DOI:10.1002/chem.201200062

FULL PAPER
DOI: 10.1002/chem.201200062
Solution-Processed Undoped Deep-Blue Organic Light-Emitting Diodes
Based on Starburst Oligofluorenes with a Planar Triphenylamine Core
Cui Liu,[a] Yanhu Li,[b] Yingyue Zhang,[a] Chuluo Yang,*[a] Hongbin Wu,*[b]
Jingui Qin,[a] and Yong Cao[b]
Abstract: A series of starburst oligo-
mers (T1–T3) that contained a fully di-
played deep-blue emission both in so-
lution and as thin films. Solution-pro-
cessed devices based on these oligo-
mers exhibited highly efficient deep-
blue electroluminescence and the
device performances were significantly
enhanced with the extension of the oli-
gofluorene arms. The double-layered
device that contained T3 as an emitter
showed a maximum current efficiency
of 3.83 cdAÀ1 and a maximum external
quantum efficiency of 4.19% with CIE
coordinates of (0.16, 0.09), which are
among the highest values for undoped
deep-blue OLEDs that are based on
solution-processable starburst oligo-
mers.
arylmethene-bridged
triphenylamine
core and oligofluorene arms were de-
signed and synthesized through Suzuki
cross-coupling reactions. Their thermal,
photophysical, and electrochemical
properties were also investigated.
These materials showed high glass tran-
sition, in the range of 123–1298C, and
good film-forming abilities. They dis-
Keywords: conjugation · cross-cou-
pling · fluorescence · OLEDs · olig-
omers
Introduction
but can also be utilized to generate light of other colors by
energy cascade to a lower-energy fluorescent or phosphores-
cent dopant.[13–15]
Since the pioneering work by Tang et al. in 1987,[1] organic
light-emitting diodes (OLEDs) have attracted considerable
scientific and industrial attention because of their applica-
tions in flat-panel displays and in solid-state lighting.[2–12]
Full-color displays require primary RGB emission of rela-
tively equal stability, efficiency, and color purity. However,
the performance of blue-light-emitting devices is often infe-
rior to that of their green and red counterparts for the in-
trinsic wide band-gap of blue-light-emitting materials, which
makes it hard to inject charge into an emitting layer. Thus,
the development of blue OLEDs—in particular deep-blue
OLEDs—with high electroluminescence (EL) efficiency is
a pressing concern to realize commercial full-color displays.
The deep-blue color is defined as having blue EL emission
with a Commission Internationale de l’Eclairage (CIE) y co-
ordinate value of <0.15. Such emitters can not only effec-
tively reduce the power consumption of a full-color OLED
Solution-processable OLEDs are critical for realizing low-
cost and large-area displays.[16] However, a few solution-
processable deep-blue OLEDs have been reported.[17–24] Re-
cently, monodisperse conjugated star-shaped oligomers that
can be deposited by solution processing have been of great
interest for use in OLEDs.[25–35] By combining the advantag-
es of polymers and small molecules, starburst compounds
are characterized by a well-defined molecular structure and
superior chemical purity as well as good solution processa-
bility. Furthermore, their optoelectronic and thermal proper-
ties can be independently optimized by the judicious choice
of different cores and branches. In OLED applications, oli-
gofluorene is favorably used as the arm because of the asso-
ciated high quantum yield, good optical stability, and film-
forming ability.[26–35] The core usually plays a major role in
the molecular shape, the emission color, and the thermal
and morphological properties of the starburst molecules.
Different centers, such as benzene,[34] triazatruxene,[26–27]
pyrene,[29] porphyrin,[31] truxene,[32] 4,4’,4’’-tris(carbazol-9-yl)-
triphenylamine (TCTA),[28] and spirofluorene,[33] have been
used to construct starburst molecules with interesting prop-
erties. For example, Lai and co-workers reported a highly ef-
ficient deep-blue OLED with hc,max =2.07 cdAÀ1 and CIE
coordinates of (0.15, 0.09) by utilizing triazatruxene as
a core and six oligofluorene arms as the branches.[27] Liu
and co-workers synthesized a series of deep-blue-emitting
starburst oligomers that contained a TCTA core and six oli-
[a] C. Liu, Y. Zhang, Prof. C. Yang, Prof. J. Qin
Department of Chemistry
Hubei Key Lab on Organic and
Polymeric Optoelectronic Materials
Wuhan University, Wuhan, 430072 (P.R. China)
Fax : (+86)27-68756757
E-mail: clyang@whu.edu.cn
[b] Y. Li, Prof. H. Wu, Prof. Y. Cao
Institute of Polymer Optoelectronic Materials and
Devices, State Key Laboratory of Luminescent Materials
and Devices, South China University of Technology
Guangzhou 510640 (P.R. China)
gofluorene arms, and obtained
a device with hc,max =
E-mail: hbwu@scut.edu.cn
0.47 cdAÀ1 and CIE coordinates of (0.16, 0.07).[28] By using
a pyrene core and four oligofluorene arms, Liu and co-work-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200062.
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ers obtained an sky-blue-emitting OLED with hc,max
=
ized by H and 13C NMR spectroscopy, MS (MALDI-TOF),
1.75 cdAÀ1 and a maximum luminance of 2716 cdmÀ2 with
and elemental analysis.
CIE coordinates of (0.20, 0.32).[29]
We have recently reported a fully diarylmethene-bridged
triphenylamine (FATPA), which had an almost planar tri-
phenylamine (TPA) skeleton and exhibited excellent ther-
mal and morphological stability.[36–37] Herein, we designed
and synthesized a series of starburst oligofluorenes that
were based on the FATPA core. Their thermal, photophysi-
cal, and electrochemical properties, as well as the character-
istics of devices that used these starbursts as a blue emitter
were investigated. All of the compounds showed deep-blue
emission with almost unit fluorescence quantum yields in so-
lution. Solution-processed devices based on these oligomers
exhibited efficient deep-blue electroluminescence. Further-
more, the device performances were significantly enhanced
with the extension of the oligofluorene arms. The double-
layered device with T3 as the emitter showed a maximum
current efficiency of 3.83 cdAÀ1 and a maximum external
quantum efficiency of 4.19% with CIE coordinates of (0.16,
0.09). To the best of our knowledge, this device performance
is among the highest for deep-blue fluorescent OLEDs that
are based on solution-processable conjugated starbursts.
Thermal properties: The good thermal stability of these
compounds was indicated by their high decomposition tem-
peratures (Td, which corresponded to 5% weight loss),
which were in the range of 409–4358C, as determined by
TGA (Figure 1). Their glass-transition temperatures (Tg),
Figure 1. TGA thermograms of oligomers T1–T3 at a heating rate of
108CminÀ1; inset: DSC traces of oligomers T1–T3 at a heating rate of
108C minÀ1
.
Results and Discussion
which were determined by DSC, increased with increasing
arm length (T1: 1238C to T3: 1298C; Figure 1, inset). These
values were significantly higher than those for correspond-
ing star-shaped oligofluorenes that were based on a benzene
core[34] and poly(9,9-dihexylfluorene) (Tg =1038C).[38] This
result demonstrated that the construction of a bulky star-
shaped architecture with a rigid FATPA core could effec-
tively suppress the crystallizing tendency and improve the
morphological stability.
Synthesis and characterization: Oligofluorene boronic acids
of different chain lengths (F1–F3) were prepared according
to a literature procedure.[32–33] FATPA-Br was synthesized
according to our previous procedure.[36–37] Starburst oligo-
fluorenes T1–T3 were prepared through Suzuki cross-cou-
pling reactions of FATPA-Br and the oligofluorene boronic
acids (Scheme 1). All of the new compounds were character-
Scheme 1. Synthesis of oligomers T1–T3; reagents and conditions: a) [PdACTHNUTRGNE(UNG PPh3)4], 2m Na2CO3, THF, 858C, 48 h.
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Deep-Blue OLEDs Based on Starburst Oligofluorenes
FULL PAPER
Figure 2. AFM topographic images of solid thin films of oligofluorenes T1–T3 (thickness: 50 nm, size: 5 mmꢁ5 mm).
We also recorded atomic force microscopy (AFM) images
to investigate the morphological stability of these materials.
Figure 2 shows the AFM images of oligomers T1–T3 on an
ITO substrate. The thin film was prepared by spin-coating
and annealing under a N2 atmosphere at 1008C for 10 min.
The annealed films had a fairly smooth surface morphology
with root-mean-square (rms) roughness of 0.361, 0.365, and
0.417 nm for oligomers T1, T2, and T3, respectively. The ex-
cellent thermal and morphological stability of these oligo-
mers enabled the preparation of homogeneous and stable
amorphous thin films through solution processing.
Photophysical properties: Figure 3 shows the absorption and
fluorescence spectra of oligomers T1–T3 both as a solution
in toluene and as solid-state films. The photophysical data of
the compounds are summarized in Table 1. The absorption
À
band from 378–390 nm was due to the p p* transition of
the FATPA core, whilst the absorption peaks at around 296,
À
335, and 353 nm were attributed to the p p* transition of
the oligofluorenes. The absorption bands showed a red-shift
on increasing the length of the oligofluorene arms, owing to
the extended conjugation. The emission maxima of these
Figure 3. UV/Vis absorption and PL spectra of oligomers T1–T3 as a) a
solution in toluene (5ꢁ10À6 m) and b) a thin film.
compounds were in the range of 427–441 nm in solution and
431–450 nm in the solid-state films. The absorption and
emission spectra of the films were similar to those in solu-
tion and only exhibited a small red-shift in the emission
maxima, thereby indicating the absence of intermolecular
interactions in the thin films. This result could be attributed
to the bulky star-shaped structures that effectively restrained
intermolecular aggregation. The emission bands showed
a significant red-shift from oligomers T2 to T1, but only
a slight red-shift from oligomers T3 to T2, thus suggesting
that the emission was rapidly saturated with the extended
arms. The fluorescence quantum yields of the starbursts
were measured in a dilute solution in toluene by using 9,10-
diphenyl-anthracene (DPA; F=0.90) as a reference. These
deep-blue emitters showed almost unit quantum efficiencies,
of 0.97, 0.98, and 1.00, respectively.
Electrochemical properties: The electrochemical behavior
of compounds T1–T3 was investigated by CV (Figure 4). All
of these compounds showed two well-defined stepwise-oxi-
dation waves: The first reversible oxidation process at
around 0.4 V (versus Fc+/Fc) could be attributed to the oxi-
dation of the bridged triphenylamine core whilst the second
wave could be attributed to the oxidation of the oligofluor-
ene arms. As shown in Figure 4, an increase in the conjugat-
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C. Yang, H. Wu et al.
Table 1. Photophysical and thermal data of oligomers T1–T3.
0.27% for the T1-based device to 0.82% for the
T3-based device (see the Supporting Information,
Figure S1 and Table S1). To optimize the device
structure, double-layer devices were fabricated with
the configuration of ITO/PEDOT:PSS (40 nm)/T1–
T3 (50 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)-
benzene (TPBI) (40 nm)/CsF (1.5 nm)/Al (120 nm)
(emitter: T1, device A; T2, device B; T3, device C),
in which TPBI was inserted between the EML and
CsF layers as an electron-transporting layer. As
a result, the EL performances of the bilayer devices
showed significant improvement (Table 2). This
result could be attributed to the lower LUMO
energy (2.7 eV) and the much-lower HOMO
energy level (6.2 eV) of TPBI than those of oligo-
[a]
[b]
[c]
[c]
[d]
[d]
[g]
Compound Tg
[8C]
Td
labs
lem,max
labs
lem,max
HOMO[e]
/
FPL
G
E
[nm]
427
[nm]
[nm]
431
LUMO[f] [eV]
T1
T2
T3
123
125
129
409 296,
378
435 335,
389
432 353,
389
296,
384
339,
397
359,
394
5.15/2.22
0.97
AHCTUNGTRENNUNG
439
442
445
450
5.14/2.29
5.15/2.32
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] Obtained from DSC measurements; [b] obtained from TGA measurements;
[c] measured in toluene; [d] measured in a thin film; [e] determined from the onset of
the oxidation potentials; [f] deduced from the HOMO and Eg values; [g] fluorescence
quantum yields in toluene were measured with 9,10-diphenyl-anthracence (F=0.90)
as a standard, and the fluorescence quantum yields in solid-state films were measured
on a quartz plate with an integrating sphere (in parenthesis).
mers T1–T3, which could facilitate electron transport and ef-
ficiently block the holes and excitons.
Table 2. Electroluminescence characteristics of the devices.[a]
Device Von
Lmax [cdmÀ2], hc,max
hp,max
[lm WÀ1
hmax
CIE
ACHTUNGTRENNUNG
[x, y][b]
[V]
V [V]
G
]
G
]
[%]
A
B
C
4.4
3.8
3.4
365, 9.0
602, 7.8
732, 6.6
1.63
1.96
3.83
0.98
1.23
3.16
1.82 0.19, 0.11
2.30 0.16, 0.11
4.19 0.16, 0.09
[a] Abbreviations: Von
: turn-on voltage; Lmax: maximum luminance;
hc,max: maximum current efficiency; hp.max: maximum power efficiency;
hext,max: maximum external quantum efficiency; CIE [x, y]: Commission
Figure 4. CVs of oligomers T1–T3 in CH2Cl2.
International de l’Eclairage coordinates. [b] Measured at 10 mAmÀ2
.
ed length from T1 to T3 led to a shift in the peaks of the
second wave from 1.02 to 0.86 V (versus Fc+/Fc). Moreover,
the ratio of the current value of the second wave to the first
wave was significantly enhanced owing to the increase in
number of fluorene units from oligomers T1 to T3. The
HOMO energy levels, estimated from the E1/2 (half-wave
potential) values of the oxidation, were around 5.15 eV with
regard to the energy level of ferrocene (4.8 eV below
vacuum), which matched well with that of poly(3,4-ethylene-
Oligomers T1–T3 showed deep-blue emission in their EL
spectra with CIE coordinates of (0.19, 0.11) for device A,
(0.16, 0.11) for device B, and (0.16, 0.09) for device C
(Figure 5). The EL spectra of T2- and T3-based devices are
similar to their PL spectrum, thus indicating the efficient
suppression of the excimer or exciplex during the EL pro-
cess. However, the EL spectrum of the T1-based device
shows a residue residual emission band at 520–620 nm,
which may have been due to the formation of an exciplex at
the interface between TPBI and the emitting layer.
dioxythiopene):poly(styrenesulfonate)
(PEDOT:PSS)
(5.1 eV) and should result in an efficient hole-injection from
PEDOT:PSS to the emissive layer. The band gaps (Eg) of
oligomers T1–T3, on the basis of the red edge of the longest
absorption wavelength for the solid-film sample, were esti-
mated to be 2.93–2.83 eV, and decreased with increasing the
length of the oligofluorene arms. The LUMOs were in the
range of 2.22–2.32, which were deduced from the HOMO
and Eg values (Table 1).
Deep-blue electroluminescence: The good solubility, deep-
blue emission, and good film-forming ability enabled the
starburst oligofluorenes to be used as the solution-processa-
ble deep-blue emitters in electroluminescent devices. Initial-
ly, single-layer devices were fabricated with the structure of
indium tin oxide (ITO)/PEDOT:PSS (40 nm)/T1–T3
(50 nm)/CsF (1.5 nm)/Al (120 nm). PEDOT:PSS and CsF
acted as hole- and electron-injecting layers, respectively. The
maximum external quantum efficiency increased from
Figure 5. Normalized EL spectra of devices A–C. Inset: CIE coordinates.
Figure 6 shows the current-density–voltage–brightness (J–
V–L) characteristics and plots of efficiency versus current
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Deep-Blue OLEDs Based on Starburst Oligofluorenes
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The EL performance was enhanced with the lengthening
of the oligofluorene arms from T1 to T3: the turn-on voltag-
es decreased from 4.4 to 3.4 V, whilst the maximum efficien-
cies (current efficiency, power efficiency, and external quan-
tum efficiency) increased from 1.63 to 3.83 cdAÀ1, from 0.98
to 3.16 lmWÀ1, and from 1.82 to 4.19%, respectively. This
improvement could be elucidated from the following obser-
vations: 1) The carrier-transport ability of the emitters
would be gradually enhanced from T1 to T3 because of the
extended conjugation. 2) As shown in Figure 7, the LUMO
Figure 7. Energy level diagrams for devices A–C.
level of T3 (2.32 eV) was 0.03 eV lower than that of T2, and
0.10 eV lower than T1. Thus, the electron injection barriers
between TPBI (LUMO: 2.7 eV) and the EML decreased
from T1 to T3; therefore, more-balanced charge flux could
be anticipated in T3. 3) T3 showed the highest photolumi-
nescence quantum yield in the solid-state film among the
three compounds.
Figure 6. a) Current-density–voltage–brightness (J–V–L) characteristics
for devices A–C; b) current efficiency c) EQE and power efficiency
versus current density curves for devices A–C.
Conclusion
density for the three devices. All of the devices had a low
turn-on voltage (below 5 V), which should be attributed to
the high-lying HOMO levels of these oligomers, which
matched well with the HOMO level of the adjacent PE-
DOT:PSS and thus facilitated the hole injection of the
device.
We have developed a series of starburst oligofluorenes (T1–
T3) that were based on a rigid planar triphenylamine core.
All of the starbursts showed excellent thermal stability, pro-
nounced PL efficiency, and good solution-processability.
Double-layer devices that used the oligofluorenes as the
emission layer were fabricated by using a spin-coating
method. All three devices showed high EL performance and
the T3-based device achieved a maximum current efficiency
of 3.83 cdAÀ1 and maximum external quantum efficiency of
4.19% with CIE coordinates of (0.16, 0.09). This strategy
will be valuable for the rational design and development of
efficient blue EL oligomers and other related materials.
The T1-based device showed a turn-on voltage of 4.4 V,
a maximum current efficiency (hc,max) of 1.63 cdAÀ1, a maxi-
mum power efficiency (hp,max) of 0.98 lmWÀ1, and a maxi-
mum external quantum efficiency (hext,max) of 1.82%. The
performance of the T2-based device was significantly im-
proved, with a turn-on voltage of 3.8 V, hc,max =1.96 cdAÀ1,
hp,max =1.23 lmWÀ1, and hext,max =2.30%. The best perfor-
mance was achieved by T3, with a low turn-on voltage of
3.4 V, hc,max =3.83 cdAÀ1, hp,max =3.16 lmWÀ1, and hext,max
=
Experimental Section
4.19%. The device performances are summarized in Table 2.
To the best of our knowledge, the performance of the T3-
based device is comparable with the other solution-pro-
cessed deep-blue fluorescent OLEDs,[20–24] and among the
highest reported for undoped deep-blue OLEDs that are
based on solution-processable starburst oligofluorenes.[26–30]
General information: 1H and 13C NMR spectra were measured on
a MECUYR-VX300 spectrometer. Elemental analysis was performed on
a Vario EL III microanalyzer. MS was measured on a ZAB 3F-HF mass
spectrophotometer. MALDI-TOF (matrix-assisted laser-desorption/ioni-
zation time-of-flight) MS was performed on Bruker BIFLEX III TOF
mass spectrometer. UV/Vis absorption spectra were recorded on a Shi-
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C. Yang, H. Wu et al.
madzu UV-2500 recording spectrophotometer. Photoluminescence spec-
tra were recorded on a Hitachi F-4500 fluorescence spectrophotometer.
The PL quantum yields of the solid-state films were measured according
to an absolute method by using an Edinburgh Instruments (FLS920) inte-
grating sphere that was excited with a Xe lamp. Differential scanning cal-
orimetry (DSC) was performed on a NETZSCH DSC 200 PC unit from
RT to 3008C at a heating rate of 108C minÀ1 under an argon atmosphere.
The glass-transition temperature was determined from the second heating
scan. Thermogravimetric analysis (TGA) was performed on a NETZSCH
STA 449C instrument. The thermal stability of the samples under a nitro-
gen atmosphere was determined by measuring their weight loss whilst
heating from 25–8008C at a rate of 158C minÀ1. Cyclic voltammetry was
carried out in nitrogen-purged THF (reduction scan) and CH2Cl2 (oxida-
tion scan), with a CHI voltammetric analyzer. Tetrabutylammonium hex-
afluorophosphate (TBAPF6, 0.1m) was used as the supporting electrolyte.
The conventional three-electrode configuration consisted of a platinum
working electrode, a platinum wire auxiliary electrode, and a Ag wire
pseudo-reference electrode with ferrocenium/ferrocene (Fc+/Fc) as the
d=151.63, 151.28, 143.29, 141.12, 140.21, 139.55, 135.67, 135.36, 134.96,
130.64, 129.74, 128.52, 127.11, 126.47, 125.28, 123.18, 121.14, 119.98, 56.07,
55.21, 40.67, 31.79, 30.09, 24.05, 22.94, 21.34, 14.36 ppm; MS (MALDI-
TOF): m/z: 1819.7; elemental analysis calcd (%) for C138H147N: C 90.97,
H 8.55, N 0.99; found: C 91.09, H 8.14, N 0.77.
Synthesis of T2: Oligomer T2 was synthesized according to a similar pro-
1
cedure as T1. Yield: 71%; H NMR (300 MHz, CDCl3): d=7.79–7.59 (m,
33H), 7.37–7.25 (m, 12H), 6.94 (d, J=6.3 Hz 12H), 6.84 (d, J=7.2 Hz,
12H), 2.35 (s, 18H), 2.15–1.80 (m, 24H), 1.18–0.96 (m, 72H), 0.85–
0.65 ppm (m, 60H); 13C NMR (75 MHz, CDCl3): d=151.62, 151.32,
150.87, 140.69, 140.20, 135.23, 130.22, 128.08, 126.71, 125.92, 122.78,
121.32, 119.76, 55.03, 54.92, 40.27, 31.33, 29.60, 23.67, 22.46, 20.90,
13.91 ppm; MS (MALDI-TOF): m/z: 2816.6; elemental analysis calcd
(%) for C213H243N: C 90.89, H 8.72, N 0.44; found: C 90.81, H 8.69,
N 0.50.
Synthesis of T3: Oligomer T3 was synthesized according to a similar pro-
1
cedure as T1. Yield: 73%; H NMR (300 MHz, CDCl3): d=7.83–7.60 (m,
33H), 7.40–7.17 (m, 30H), 6.94 (d, J=8.1 Hz 12H), 6.85 (d, J=8.1 Hz,
12H), 2.36 (s, 18H), 2.15–1.82 (m, 36H), 1.18–1.02 (m, 108H), 0.85–
0.67 ppm (m, 90H); 13C NMR (75 MHz, CDCl3): d=151.70, 142.86,
140.46, 139.93, 135.29, 130.26, 129.38, 128.14, 126.94, 126.07, 122.84,
121.42, 119.86, 55.09, 40.32, 31.37, 29.61, 23.73, 22.49, 20.93, 13.94 ppm;
MS (MALDI-TOF): m/z: 3814.5; elemental analysis calcd (%) for
C288H339N: C 90.57, H 9.08, N 0.19; found: C 90.68, H 8.96, N 0.37.
internal standard. CVs were obtained at a scan rate of 100 mVsÀ1
.
Formal potentials are calculated as the average of the cyclic voltammetric
anodic and cathodic peaks. The onset potential was determined from the
half-wave oxidation potential.
Device fabrication and measurement: Patterned ITO-coated glass with
a sheet resistance of 15–20 WsquareÀ1 were cleaned by a surfactant scrub
followed by a wet-cleaning process inside an ultrasonic bath, beginning
with deionized water, and followed by acetone and isopropanol. After
oxygen plasma cleaning for 4 min, 40 nm of PEDOT:PSS (Bayer Baytron
P 4083), which was used as a hole-injection layer at the anode interface,
was spin-coated on the ITO substrate and then dried in a vacuum oven
at 808C overnight. The emissive layer (EML) was coated onto the anode
by spin-coating from a solution of chlorobenzene, and then annealed at
1008C for 10 min to remove the solvent residue. The thickness of the
EML was about 50 nm. Finally, an electron-transporting layer of TPBI
(30 nm) and a cathode that was composed of CsF (1.5 nm) and an Al
(120 nm) layer were evaporated with a shadow mask at a base pressure
of 3ꢁ10À4 Pa. The thickness of the evaporated cathode was monitored by
a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon).
The overlapping area between the cathode and the anode defined a pixel
size of 19 mm2. Except for the deposition of the PEDOT layers, all of the
fabrication processes were carried out inside a controlled atmosphere of
a nitrogen dry-box (Vacuum Atmosphere Co.) that contained less than
10 ppm oxygen and moisture. The current-density–luminance–voltage
characteristic was measured by using a Keithley 236 source measurement
unit and a calibrated silicon photodiode. The forward-viewing luminance
was calibrated by a spectrophotometer (SpectraScan PR-705, Photo Re-
search) and the forward-viewing LE was calculated accordingly. Herein,
the luminance and LE values were for the forward-viewing direction
only. The external quantum efficiency of EL was collected by measuring
the total light output in all directions in an integrating sphere (IS-080,
Labsphere). The EL spectra were collected on a PR-705 photometer.
Acknowledgements
C.Y. thanks the National Science Fund for Distinguished Young Scholars
of China (No. 51125013), the National Basic Research Program of China
(973 Program, 2009CB623602), the National Natural Science Foundation
of China (No. 90922020), and the Fundamental Research Funds for the
Central Universities of China; H.W. thanks the National Natural Science
Foundation of China (No. 61177022).
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Deep-Blue OLEDs Based on Starburst Oligofluorenes
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Received: January 6, 2012
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
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www.chemeurj.org
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C. Yang, H. Wu et al.
Sheꢀs a star: Three starburst oligomers
that have a rigid planar triphenylamine
core and oligofluorene arms were syn-
thesized (see figure). The undoped
deep-blue OLED with T3 as the emit-
ter showed a maximum current effi-
ciency of 3.83 cdAÀ1 and a maximum
external quantum efficiency of 4.19%
with CIE coordinates of (0.16, 0.09),
which is among the highest values for
undoped deep-blue OLEDs that are
based on solution-processable conju-
gated starbursts.
OLEDs
C. Liu, Y. Li, Y. Zhang, C. Yang,*
H. Wu,* J. Qin, Y. Cao ...... &&&& — &&&&
Solution-Processed Undoped Deep-
Blue Organic Light-Emitting Diodes
Based on Starburst Oligofluorenes
with a Planar Triphenylamine Core
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!