Carbazole/sulfone hybrid D-π-A-structured bipolar fluorophores for high-efficiency blue-violet electroluminescence

Article abstract of DOI:10.1021/cm400945h

Based on a D-π-A structural strategy incorporating carbazole as a mild electron-donor and sulfone as an electron-acceptor with a π-conjugation- breaking feature, two novel blue-violet emitting materials (CzS1 and CzS2) were successfully designed and synthesized. The two compounds exhibit high-efficiency fluorescent emissions of intramolecular charge-transfer transition type, with impressively high quantum yields in both solution and film states. CIE _y below 0.06 and excellent current/power efficiencies up to 1.89 cd A^-1/1.58 lm W^-1 were achieved with their corresponding nondoped devices. These performances currently represent the best results for OLEDs with CIE_y < 0.06. Moreover, single-carrier devices were also fabricated to demonstrate the bipolar characteristics as well as to understand the different electroluminescence performance of the two fluorophores.

Full text of DOI:10.1021/cm400945h

Article
pubs.acs.org/cm
Carbazole/Sulfone Hybrid D‑π-A-Structured Bipolar Fluorophores for
High-Efficiency Blue-Violet Electroluminescence
Jun Ye,†,‡,∥ Zhan Chen,†,∥ Man-Keung Fung,‡ Caijun Zheng,† Xuemei Ou,† Xiaohong Zhang,*,†
Yi Yuan,‡,§ and Chun-Sing Lee*,‡
†Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China
‡Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Sciences, City University of
Hong Kong, Hong Kong SAR, P.R. China
§Department of Chemistry, Shantou University, Guangdong 515063, P.R. China
S
* Supporting Information
ABSTRACT: Based on a D-π-A structural strategy incorporating carbazole as a mild
electron-donor and sulfone as an electron-acceptor with a π-conjugation-breaking
feature, two novel blue-violet emitting materials (CzS1 and CzS2) were successfully
designed and synthesized. The two compounds exhibit high-efficiency fluorescent
emissions of intramolecular charge-transfer transition type, with impressively high
quantum yields in both solution and film states. CIEy below 0.06 and excellent current/
power efficiencies up to 1.89 cd A−1/1.58 lm W−1 were achieved with their
corresponding nondoped devices. These performances currently represent the best results for OLEDs with CIEy < 0.06.
Moreover, single-carrier devices were also fabricated to demonstrate the bipolar characteristics as well as to understand the
different electroluminescence performance of the two fluorophores.
KEYWORDS: blue-violet electroluminescence, carbazole, sulfone, bipolar fluorophor
they not only show high fluorescent quantum yields owing to
the effective radiative decay of their excited intramolecular
charge-transfer (ICT) state but also possess impressive bipolar
charge-transporting properties for their constituting hole- and
electron-transporting moieties.10 As a result, the use of D-π-A
type fluorophores can effectively simplify the device config-
urations10a,d,e,g and tremendously enhance the electrolumines-
cence (EL) efficiencies of OLEDs.10 However, except for the
enlarged π conjugation, the ICT trend in such molecules
indeed could also lead to remarkable fluorescence red shifts.10c
Basing on a central A group of phosphine oxide that can break
the local π conjugation, Liu et al. recently developed a series of
efficient deep-blue emitters which show electroluminescence
CIE coordinates around (0.15, 0.10).10j Whereas room for
further blue-shifting the emission still remains because the
strong electron-donating N-phenylnaphthalen-1-amine group
(with relatively low oxidation potentials)10j and long π bridges
between the D and A units (3 to 4 benzene rings) were used in
these emitters. Once replaced by other less electron-rich donors
and shorter π bridges, respectively, to limit the ICT trend and
the molecular conjugation, the light emission can be further
adjusted to shorter wavelength. Moss et al. systematically
studied the photophysics of a series of electron-accepting
dibenzothiophene S,S-dioxide (DBTO)-based fluorophores
INTRODUCTION
■
Blue-violet/violet light-emitting materials are of great signifi-
cance for their unique applications in flat-panel displays1 and
high-density information storage.2 Especially in full-color
displays, the short-wavelength emission can not only serve as
an excitation source for emission over the whole visible range
and white light emission, but also is a key component for
effectively reducing the power consumption of the display.3
Therefore, the development of high-efficiency blue-violet to
violet emitting materials and devices are important for the
rapidly developing organic light-emitting device (OLED)
technology.
However, so far, the reports of efficient OLEDs with their
color coordinates in the Commission Internationale de
L’Eclairage (CIE) chromaticity diagram beyond the standard
blue (CIE coordinates of 0.15, 0.06), are still rare.4 Many
efficient emitters, such as anthracene,5 fluorene,6 styrylarylene7
and pyrene derivatives,8 and so forth, are based on molecular
structures of large π conjugation which can give highly emissive
π−π* transition states upon excitation. However, for realizing
blue-violet or violet emission the π conjugation of organic
molecules must be strictly confined. This will then either cause
co-instantaneous decrease of fluorescent quantum yield and
charge-transporting ability,9 or to some extent increase the
synthetic difficulty when large molecular weight and/or bulky
structure are usually required for good thermal and
morphological stability. Very recently, donor-π-acceptor (D-π-
A) type fluorophores have attracted much attention because
Received: March 22, 2013
Revised: June 9, 2013
© XXXX American Chemical Society
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Scheme 1. Synthesis of CzS1 and CzS2
which are modified with different kinds of D units.11 Among
them, the carbazole/DBTO-connected fluorophores exhibit
blue to violet (450 to 408 nm) fluorescence in toluene solution.
Their fluorescent quantum yields are yet either moderate
(0.58) or even low (0.25). It is also noticeable that substantial
conjugation exists across the DBTO unit, still leaving the
chance for further blue-shifting the emission. Thereby, the key
point for designing high-efficiency blue-violet to violet
electroluminescence materials is to suitably control the π-
conjugation enlargement and the ICT trend inside the
molecules.
EXPERIMENTAL SECTION
■
General Procedures. H and 13C NMR spectra were measured
with a Varian Gemini-400 Varian spectrometer. Mass spectra were
recorded on a Finnigan 4021C GC-MS spectrometer. Differential
scanning calorimetry (DSC) was performed using a Perkin-Elmer
Pyris DSC 6 instrument operated at a heating rate of 10 °C min−1 in a
nitrogen atmosphere. The glass transition temperature (Tg) was
determined from the second heating scan. Thermogravimetric analysis
(TGA) was undertaken using a TA SDT Q600 instrument. The
thermal stability of the samples under a nitrogen atmosphere was
determined by measuring their weight losses while heating at a rate of
20 °C min−1. The absorption and emission spectra were recorded on a
Hitachi U-3010 UV−vis spectrophotometer and a Hitachi F-4500
fluorescence spectrophotometer, respectively. The film photolumines-
cence quantum yield was measured by the integrating sphere method,
and the emission decay profile was measured with an Edinburgh F900
fluorescence spectrophotometer. Cyclic voltammetry was performed
with a CHI600A analyzer with the scan rate of 100 mV s−1 at room
temperature. The electrolytic cell was a conventional three-electrode
cell in which a glassy carbon working electrode, a Pt wise auxiliary
electrode, and an aqueous saturated calomel electrode (SCE) as the
reference electrode were employed. Tetra-n-butylammoniumhexa-
fluorophosphate (TBAPF6, 0.10 M) was used as the supporting
electrolyte and CH2Cl2 as the solvent, respectively. The ferrocene/
ferrocenium couple was used as the internal standard.
1
Herein, we utilized a carbazole/sulfone hybrid design
strategy to obtain two novel efficient blue-violet fluorophores,
di(4-(4-(carbazol-9-yl)phenyl)phenyl)sulfone (CzS1) and di(4-
(9-phenylcarbazol-3-yl)phenyl)sulfone (CzS2) (Scheme 1).
The widely used hole-transporting moiety, carbazole, was
selected as the electron-donor (D) for its relatively mild
electron-donating ability as compared to other arylamines.10c
These D-π-A type molecules could thus have relatively weak
ICT trends, which is beneficial for generating short-wavelength
emissions. Besides, the sulfone group was chosen as the
electron-acceptor (A) for its excellent electron injection/
transporting properties.10a,12 More importantly, the sulfone,
as linked to two freely rotatable benzene rings in this work, can
also serve as a π-conjugation breaker because of its tetrahedral
electronic conformation which could effectively confine the π-
conjugation of the molecules. Finally, these compounds were
designed to be bulky to enhance their morphological and
thermal stability by using a symmetrical D-π-A-π-D structure.
Bipolar charge-transporting characteristics of CzS1 and CzS2
were demonstrated with single-carrier type devices. Nondoped
OLEDs based on the two novel fluorophores show blue-violet
light emissions with CIEx,y of (0.157, 0.055) and (0.157, 0.044),
respectively. The CzS1-based device exhibited excellent EL
performance with high external quantum efficiency (EQE) of
4.2%, current efficiency (CE) of 1.89 cd A−1, and power
efficiency (PE) of 1.58 lm W−1. Performances of the devices are
among the best of OLEDs with CIEy below 0.06. In particular,
to the best of our knowledge, the CE and PE obtained here
have reached the highest values of reported blue-violet/violet
OLEDs with CIEy below 0.06.
Synthesis. All solvents and materials were used as received from
commercial suppliers without further purification. Synthetic routes of
the two compounds are outlined in Scheme 1.
Di(4-bromophenyl)sulfide. Diphenylsulfide (2.34 mL, 20 mmol)
was dissolved in a mixed solution of CH2Cl2 and H2O (1:1, 150 mL).
Then 30% hydrogen peroxide (0.86 mL, 36 mmol) and bromine (4.1
mL, 80 mmol) were added, and the reaction solution was continuously
stirred for 6 h. The resulting mixture was extracted with CH2Cl2 with
surplus bromine removed by saturated Na2SO3 solution. After
removing the CH2Cl2 solvent, the residue was further purified by
silica gel column chromatography (petroleum ether) to give 5.88 g
(86%) of a white solid. 1H NMR (400 MHz, CDCl3) δ [ppm]: 7.45−
7.40 (m, 4H), 7.20−7.16 (m, 4H). EI MS (m/z): 343.86.
Di(4-bromophenyl)sulfone. Di(4-bromophenyl)sulfide (3.44 g, 10
mmol), KMnO4 (5.0 g, 31.6 mmol), and MnSO4 monohydrater (5.0 g,
36.2 mmol) were stirred continuously in CH2Cl2 (75 mL) for 10h.
After that, the reaction mixture was filtered, and the residue was
washed with additional CH2Cl2. The crude product was obtained by
evaporation of the collected CH2Cl2 and was further purified via silica
gel column chromatography (CH2Cl2/petroleum ether (1:1.5)) to
1
afford 2.3 g (61%) of a white solid. H NMR (400 MHz, CDCl3) δ
[ppm]: 7.80−7.76 (m, 4H), 7.67−7.63 (m, 4H). EI MS (m/z):
373.85.
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Di(4-(4-(carbazol-9-yl)phenyl)phenyl)sulfone (CzS1). Toluene (6
mL), ethanol (2 mL), and 2 M aqueous Na2CO3 (4 mL) were added
to a mixture of di(4-bromophenyl)sulfone (374 mg, 1 mmol), 4-
(carbazol-9-yl)phenylboronic acid (718 mg, 2.5 mmol), and Pd-
(PPh3)4 (117 mg, 0.1 mmol). With stirring, the suspension was heated
at 90 °C for 24 h under a nitrogen atmosphere. When cooled to room
temperature, the mixture was extracted with CH2Cl2 and dried over
Na2SO4. After the solvent had been removed, the residue was purified
by column chromatography on silica gel (CH2Cl2/petroleum ether
weight loss) of 498 °C (CzS1) and 486 °C (CzS2), respectively
(Figure 1). High glass transition temperatures (Tg) above 140
1
(2:1)) to give a white solid (532 mg, 76%). H NMR (400 MHz,
CDCl3) δ [ppm]: 8.15 (t, J = 7.3 Hz, 8H), 7.86−7.80 (m, 8H), 7.71−
7.68 (m, 4H), 7.49−7.40 (m, 8H), 7.33−7.29 (m, 4H). 13C NMR is
unavailable owing to the poor solubility of the solid. EI MS (m/z):
700.20. Elemental analysis calcd (%) for C48H32N2O2S: C 82.26, H
4.60, N 4.00, S 4.58; found: C 81.93, H 4.62, N 3.95, S 4.40.
Di(4-(9-phenylcarbazol-3-yl)phenyl)sulfone (CzS2). The synthetic
procedure was similar to that of CzS1, with 4-(carbozol-9-yl)-
phenylboronic acid replaced by (9-phenylcarbazol-3-yl)boronic acid,
producing a white solid (80%). 1H NMR (400 MHz, CDCl3) δ
[ppm]: 8.34 (s, 2H), 8.17 (d, J = 7.7 Hz, 2H), 8.08 (d, J = 8.3 Hz, 4H),
7.84 (d, J = 8.3 Hz, 4H), 7.62−7.55 (m, 10H), 7.50−7.39 (m, 8H),
7.32−7.29 (m, 2H). 13C NMR (100 MHz, CDCl3) δ [ppm]: 147.00,
141.59, 141.13, 139.70, 137.47, 131.31, 130.11, 128.32, 128.02, 127.88,
127.17, 126.62, 125.52, 124.15, 123.34, 120.51, 119.35, 110.45, 110.21.
EI MS (m/z): 700.20. Elemental analysis calcd (%) for C48H32N2O2S:
C 82.26, H 4.60, N 4.00, S 4.58; found: C 82.06, H 4.41, N 4.07, S
4.33.
Quantum Calculation. Theoretical calculation of the compounds
was carried out using the Gaussian-03 program. Density functional
theory (DFT) B3LPY/6-31G(d) was used to determine and optimize
the structures. Optimized structures were used for the calculation. No
negative mode was found, indicating that the systems indeed
correspond to a local energy minimum.
OLED Fabrication and Measurement. ITO coated glass with a
sheet resistance of 30 Ω square−1 was used as the substrate. Before
device fabrication, the ITO glass substrates were cleaned with
isopropyl alcohol and deionized water, dried in an oven at 120 °C,
treated with UV-ozone, and finally transferred to a vacuum deposition
system with a base pressure better than 1 × 10−6 Torr for organic and
metal deposition. The devices were fabricated by evaporating organic
layers with an evaporation rate of 1−2 Å s−1. The cathode was
completed through thermal deposition of LiF at a deposition rate of
0.1 Å s−1, and then capped with Al metal through thermal evaporation
at a rate of 10 Å s−1. The overlap between ITO and Al electrodes was
3.3 × 3.3 mm2 as the active emissive area of devices. EL luminescence,
spectra, and CIE color coordinates were measured with a Spectrascan
PR650 photometer, and the current−voltage characteristics were
measured with a computer-controlled Keithley 2400 SourceMeter
under ambient atmosphere.
Figure 1. TGA curves (inset: DSC curves) of CzS1 (a) and CzS2 (b).
°C were also obtained via differential scanning calorimetry
(DSC) (Figure 1). Such high Tgs should be attributed to those
bulky D-π-A-π-D structures and would benefit the stability of
film morphology.
Optical Properties. Photophysical properties of CzS1 and
CzS2 were analyzed using ultraviolet−visible (UV−vis) and
photoluminescence (PL) spectrometers. The data are shown in
Figure 2, 3 and Table 1. In the absorption spectra of 30 nm
thick films, the two compounds show similar absorptions where
the bands around 345 and 300 nm can be respectively
RESULTS AND DISCUSSION
■
Synthesis. CzS1 and CzS2 were similarly prepared through
three synthetic steps (Scheme 1) of good yields. Diphenylsul-
fide was first subjected to the substitution of Br2.13 The Br2
could be rapidly reproduced by H2O2 in the two-phase system
of CH2Cl2/H2O, which significantly promotes the reaction.
The resulting di(4-bromophenyl)sulfide subsequently went
through a MnSO4-assisted oxidization by KMnO4,14 giving
the next intermediate di(4-bromophenyl)sulfone. Finally,
palladium(0)-catalyzed Suzuki cross-coupling reaction between
4-(carbazol-9-yl)phenylboronic acid/(9-phenylcarbazol-3-yl)-
boronic acid and di(4-bromophenyl)sulfone resulted in the
target compounds, CzS1/CzS2. These new compounds were
characterized with mass spectrometry, H NMR, 13C NMR
1
spectroscopy, and elemental analysis.
Thermal Properties. Their good thermal stability was
demonstrated by thermogravimetric analysis (TGA), showing
high decomposition temperatures Td (corresponding to 5%
Figure 2. Normalized absorption (lines) and emission (symbols)
spectra of CzS1 and CzS2.
C
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the new compounds give violet fluorescence with maximum
wavelengths below 400 nm in the same solution. The higher
energy fluorescence of our compounds should be primarily
ascribed to the reasonable molecular design using conjugation-
limited diphenylsulfone block instead of the planar DBTO unit.
The ICT characteristics of the two emitters are shown via their
gradually red-shifted and broadened fluorescent emissions upon
increasing the polarity of solvent (Figure 3). Large increase of
the dipole moment from the ground state to the excited state
was also calculated from the Lippert−Mataga calculation, 37 D
for CzS1 and 24 D for CzS2 (see Supporting Information),15
further reflecting the typical photoinduced ICT process and the
effective electronic communication between the donor and
acceptor units. In addition, fluorescent quantum yields of CzS1
and CzS2 were estimated to be approaching unity in toluene, as
referenced to quinine sulfate (Φf = 0.56 in 1 N H2SO4).16 Such
high quantum yields (four times over that of compound 11,
0.25) are owing to the suitably modified D-π-A molecular
structures which could provide highly emissive ICT excited
states upon excitation. Even in neat films, these yields still
remain impressively high as measured by the integrating sphere
method under air condition, 0.98 for CzS1 and 0.79 for CzS2,
suggesting that the fluorescence quenching routes in the film
have been effectively suppressed, probably owing to the bulky
D-π-A-π-D structures.
Adachi and co-workers have recently made a milestone
advance in the organic electroluminescence research field by
introducing purely organic molecules that can give efficient
thermally activated delayed fluorescence (TADF) through up-
conversion of triplet to singlet states, and therefore lead to
potentially 100% internal singlet yields.10h,17 Among their
reported TADF materials, compound 3, namely, di(4-(3,6-di-
tert-butylcarbazol-9-yl)phenyl)sulfone, shows pure-blue electro-
luminescence with extremely high EQE of 9.9%.10h Considering
that the two compounds in this work have similar molecular
structures with compound 3, we also explore their potential of
TADF emission. Supporting Information, Figure S2 shows their
room-temperature transient PL decay characteristics in an
oxygen-free toluene solution. The samples were freshly
nitrogen-purged to avoid any possible triplet quenching by
oxygen. It is interesting to find that, in contrast to the case of
compound 3 in the report of Adachi et al., both of our new
compounds clearly exhibit first-order exponential decays which
completely end within a short time range of 20 ns. Indeed, no
delayed components were observed in both cases, indicating
the absence of DATF emission from CzS1 and CzS2. We may
mainly attribute this to the overwhelming emission transition
rate over the nonemission transition rate of their photoinduced
excited states. As mentioned above, CzS1 and CzS2 achieved
near-unity fluorescent quantum yields even in the air-
equilibrated toluene solution, and therefore one can deduce
that the intersystem crossing (ISC) process, one non-emission
Figure 3. PL spectra of CzS1 (a) and CzS2 (b) in solvents of different
polarities.
attributed to the ICT transition and the carbazole-centered
n−π* transition. These absorption bands slightly blue-shift in
toluene solution while another distinct shoulder band at 332
nm was found for CzS1. This may arise from the π−π*
transition in the donor units. From the film onset absorptions,
both of the band gaps (Eg) of CzS1 and CzS2 are determined
to be 3.16 eV. In the film PL spectra, CzS1 exhibits a narrow
emission around 428 nm with a fwhm (full width at half-
maximum) of 56 nm, while CzS2 shows a relatively blue-shifted
emission at 416 nm with a fwhm of 51 nm. Therefore, the film
fluorescent emissions of both compounds are located in the
short-wavelength spectral region of the whole visible light
spectrum. To illustrate the reasonable molecular design in this
work, we may also compare their solution PL characteristics
with compound 11, namely, 2,8-di(N-(2-ethylhexyl)-carbazol-
3-yl) dibenzothiophene-S,S-dioxide, which has the highest-
energy fluorescence of 408 nm in toluene among its carbazole/
DBTO analogues reported by Moss et al.11 In our case, both of
Table 1. Physical Properties of These Novel Fluorophores
a
c
c
d
e
f
g
λf
(nm)
λabs
λf
Eg
τ
ΔEST
Tg/Td
(°C)
Eox
HOMO
(eV)
LUMO
(eV)
a
b
ac
,
λabs, (nm) (log ε)
(nm)
(nm)
(eV)
(ns)
Φf
(eV)
0.62
(V)
CzS1 293(4.51), 332(4.42),
341(4.47)
399
293, 346
428
3.16
2.55
∼1 (0.98)
157/498
0.81
−5.61
−2.45
CzS2 300(4,81), 332(4.73)
381, 392 304, 347
416
3.16
1.51
∼1 (0.79)
0.65
143/486
0.82
−5.62
−2.46
a
b
c
d
Measured in an air-equilibrated toluene solution. ε is the extinction coefficient (in M−1 cm−1). Measured in 30 nm-thick film state. Energy band
e
f
gap as determined from the film absorption edge. Lifetime of the fluorescence as determined in an oxygen-free toluene solution. Estimated from the
low-temperature fluorescence and phosphorescence maxima. The onset potential of oxidation as referenced to ferrocene (Fc).
g
D
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route which is supposed to create a triplet state, is significantly
suppressed. On the other hand, as delayed fluorescence is
established by the premise of converting the triplet back to a
singlet state, the energy distance (ΔEST) between the lowest
singlet state and the lowest triplet state should be taken into
account; generally, the smaller ΔEST the more efficient the up-
conversion. Estimated from the low-temperature fluorescence
and phosphorescence maxima, we obtained the ΔEST of 0.62
and 0.65 eV respectively for CzS1 and CzS2 (see Supporting
Information, Figure S3). As a result, these compounds show
much larger ΔEST than the reported 0.32 eV for compound 3,
which should dramatically raise the difficulty of up-converting
triplet to singlet state to give delayed fluorescence.
units respectively. This separation should effectively maintain
the individual electronic properties of these functional groups
and benefit the transport of both holes and electrons.19 In
addition, it can also be observed that there are still enough
HOMO−LUMO overlaps. These are required to guarantee
effective electronic communication between the donor and the
acceptor, and thus highly emissive ICT excited states which
have already been demonstrated by the high fluorescent
quantum yields.20
Electroluminescence. To investigate the EL performance
of CzS1 and CzS2, two nondoped fluorescent OLEDs were
fabricated with a configuration of ITO/NPB (30 nm)/mCP
(10 nm)/ CzS1 or CzS2 (30 nm)/TPBI (30 nm)/LiF (1.5
nm)/Al (100 nm). Figure 6 shows the EL spectra of the devices
Electrochemical Properties. Cyclic voltammetry (CV)
was conducted to measure the HOMO levels of CzS1 and
CzS2. From the onset of the oxidation curves (Figure 4), the
Figure 4. Cyclic voltammograms of CzS1 and CzS2 in CH2Cl2.
highest occupied molecular orbital (HOMO) levels were
estimated to be −5.61 and −5.62 eV for CzS1 and CzS2,
respectively, by comparing with ferrocene (Fc) (HOMO =
−(Eox + 4.8) eV).18 Their LUMO levels were further estimated
by summing the HOMO levels and the band gaps determined
from their absorption edges. These orbital energy levels are
listed in Table 1. The similarity of molecular orbital energy
levels should be associated with the fact that these molecules
incorporate identical functional groups and similar D-π-A
structures. To gain insight into the electronic structures of these
compounds, a density functional theory (DFT) calculation was
performed at the B3LYP/6-31G (d) level. Figure 5 reveals an
obvious separation of the HOMO and the LUMO which are
dominantly located on the carbazole and the diphenylsulfone
Figure 6. EL spectra of the CzS1 (a) and the CzS2 (b) based devices
at different voltages.
Figure 5. Simulated electronic distributions of CzS1 and CzS2’s HOMOs and LUMOs.
E
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at different driving voltages. The EL spectra are almost identical
to the corresponding PL spectra of their neat films and show
little change under different driving voltages. These spectral
characteristics imply a good confinement of excitons in the
emission layers, which are attributed to the wide band gaps of
the adjacent mCP (3.5 eV) and TPBI (3.4 eV) (Figure 7).21
Figure 7. Energy level diagram of the devices.
CIE coordinates of (0.157, 0.055) and (0.157, 0.044) were
respectively observed for the CzS1 and the CzS2 devices at
brightness of 100 cd m−2. According to the CIE chromaticity
diagram, CzS1 and CzS2 show intrinsic blue-violet electro-
luminescence in these nondoped devices. Performances of
these devices are further shown in Figure 8 and Table 2. From
the current density−voltage−luminescence (J-V-L) plots, it can
be observed that CzS2 device achieved an extremely low turn-
on voltage of 2.8 V, as well as much higher current densities
under the same voltage as compared to the CzS1 device. These
might be owing to the better charge-transporting ability of
CzS2. However, the CzS2-based device exhibits relatively lower
efficiencies with maximum EQE of 2.7%, CE of 0.82 cd A−1,
and PE of 0.84 lm W−1, while the device based on CzS1
presents impressive EL performance with high maximum EQE,
CE, and PE of 4.2%, 1.89 cd A−1 and 1.58 lm W−1, respectively.
The CE and PE in this work currently have represented the
best results for OLEDs with CIEy < 0.06 (Table 2).4 Even at
the practical luminescence of 1000 cd m−2, the EQE and CE of
the CzS1 device still remain high as 3.2% and 1.45 cd A−1,
indicative of a relatively low efficiency roll-off.
Figure 8. (a) Current density−voltage−luminance characteristics, (b)
current efficiency−luminance−power efficiency plots, and (c) external
quantum efficiency−luminance plots of the CzS1 and the CzS2 based
blue-violet light-emitting devices.
Single-Carrier Devices. To understand the different EL
performance as well as to demonstrate the bipolar properties of
these fluorophores, hole-only and electron-only devices of CzS1
and CzS2 were fabricated. The configuration of the hole-only
device is ITO/NPB (30 nm)/mCP (10 nm)/ CzS1 or CzS2
(30 nm)/MoO3 (10 nm)/Al (100 nm), and the electron-only
device have the structure of ITO/TPBI (20 nm)/ CzS1 or
CzS2 (30 nm)/TPBI (30 nm)/LiF (1.5 nm)/Al (100 nm).
MoO3 in the first configuration (LUMO = −2.3 eV) and TPBI
(20 nm) in the second configuration (HOMO = −6.2 eV) were
respectively utilized to block the electron-injection from Al
(−4.3 eV, corresponding to a large energy barrier of 2.0 eV)
and the hole-injection from ITO (−4.8 eV, corresponding to a
large energy barrier of 1.4 eV).22 Moreover, the other parts for
charge injection/transport were kept unchanged from these
blue-violet light-emitting devices. As shown in Figure 9, all the
four single-carrier devices show significant currents, indicating
the bipolar charge-transporting abilities of both CzS1 and CzS2.
Additionally, it is also found that the hole and electron currents
of the CzS2 devices are both much higher, however, less
balanced as compared to those of the CzS1 devices. These
could explain why the light-emitting device based on CzS2
achieved relatively lower turn-on voltage and higher current
density/luminescence while the CzS1-based device achieved
higher EL efficiencies. It is thus reflected that charge balance is
an important factor for the device efficiencies.23
CONCLUSIONS
■
In summary, two highly efficient blue-violet fluorophores, CzS1
and CzS2, have been developed via a D-π-A structure design
F
dx.doi.org/10.1021/cm400945h | Chem. Mater. XXXX, XXX, XXX−XXX

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Table 2. Key Characteristics Summary of the CzS1 and the CzS2-Based Devices and Other High Performance Blue-Violet/
Violet Light-Emitting Devices with CIEy ≤ 0.06
a
b
c
d
emitters
Von [V]
CE [cd A−1
]
PE [lm W−1
]
EQE [%]
λf, max [nm]
CIE
ref
e
e
CzS1
3.5
1.89/1.88/1.45
0.82/0.82/0.73
1.53/ -/ -
1.1/ -/ -
1.58/1.10/0.57
0.84/0.59/0.33
-/ -/ -
4.21/4.20/3.19
2.7/2.69/2.2
5.3/ -/ -
426
417
(0.157,0.055)
(0.157,0.044)
(0.158, 0.041)
(0.160, 0.044)
(0.16, 0.05)
(0.16, 0.05)
(0.165, 0.050)
(0.166, 0.056)
(0.16, 0.06)
(0.16, 0.05)
(0.16, 0.05)
(0.15, 0.06)
this work
this work
CzS2
2.8
e
TDAF1
TDAF2
CPhBzIm
Cz-2pbb
M1
2.5
4a
e
2.5
-/ -/ -
4.1/ -/ -
4a
e
2.5
1.6/ -/ -
1.07/ -/ -
-/ -/ -
3/ -/ 2.4
4.1/ -/ -
426
410
420
428
428
425
425
432
393
4d
f
2.5−3
-/ -/ -
4e
e
0.65/ -/ -
1.53/ -/ -
0.39/ -/ -
0.9/ -/ -
0.48/ -/ -
0.86/ -/ -
-/ -/ -
1.94/ -/ -
3.02/ -/ -
-/ -/ -
4g
e
M2
4g
e
TPA(3)-F
TCPC-4
TCPC-6
Purine 1
Purine 2
4f
e
-/ -/ -
2.47/ -/ -
3.72/ -/ -
3.1/ -/ -
4h
e
1.35/ -/ -
-/ -/ -
-/ -/ -
4h
f
2.9
-/ -/ -
4i
f
3.2
-/ -/ -
-/ -/ -
1.6/ -/ -
4j
a
b
c
Current efficiency. Power efficiency. External quantum efficiency respectively corresponding to the maximum value, the value at brightness of 100
d
e
f
cd m−2 and 1000 cd m−2. The maximum electroluminescence wavelength. Nondoped. Doped devices.
Author Contributions
∥J.Y. and Z.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful to the National Natural Science
Foundation of China (Grants 50825304,51033007, 51103169,
51128301) and Beijing Natural Science Foundation (Grant
2111002), P. R. China. Prof. C. S. Lee would like to
acknowledge support from the Research Grants Council of
the Hong Kong (Project No. T23-713/11).
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ASSOCIATED CONTENT
* Supporting Information
■
S
Lippert−Mataga calculation, molecular orbital quantum calcu-
lation, emission decay in toluene, and low-temperature
luminescence and phosphorescence. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E- mail: xhzhang@mail.ipc.ac.cn (X.Z.), apcslee@cityu.edu.hk
(C.-S.L.).
■
G
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