Synthesis and characterization of triple-azacrown ethers containing fluorene-cored derivatives: Application as electron injection layer for significantly enhanced performance of PLEDs

Article abstract of DOI:10.1039/c3tc31301h

To enhance electroluminescence of polymer light-emitting diodes (PLEDs) using an environmentally stable aluminum cathode, we designed a novel water/alcohol-soluble electron injection material, FTC, composed of a fluorene core and triple azacrown ether terminals. FTC significantly enhances the emission performance of PLEDs [ITO/PEDOT:PSS/EML/EIL/Al] when used as the electron injection layer (EIL), especially in the presence of metal carbonates and metal acetates. The metal carbonate-doped devices showed the best performance due to their higher dissociation rate than metal acetates. In particular, the device using K₂CO₃ doped-FTC as the electron injection layer (EIL) exhibited significantly enhanced performance compared to the device without an EIL. For the device based on PF-Green-B as the emitting layer, the performance was significantly enhanced to 17:460 cd m ^-2, 21.58 cd A^-1, and 12.42 lm W^-1, respectively, from 1220 cd m^-2, 0.72 cd A^-1, and 0.27 lm W^-1 for the non-FTC device. Using HY-PPV as the emitting layer, the device performance was also significantly enhanced to 10:990 cd m^-2, 6.93 cd A^-1, and 5.27 lm W^-1, respectively, from 680 cd m^-2, 0.07 cd A^-1, and 0.03 lm W^-1 for the non-EIL device. The results indicate that FTC with metal cations is an excellent electron injection candidate for the performance enhancement of PLEDs with a high work function Al cathode.

Full text of DOI:10.1039/c3tc31301h

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Materials Chemistry C
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Synthesis and characterization of triple-azacrown ethers
containing fluorene-cored derivatives: application as
electron injection layer for significantly enhanced
performance of PLEDs†
Cite this: DOI: 10.1039/c3tc31301h
*
Chia-Shing Wu, Huai-An Lu, Ying-Ju Lin and Yun Chen
To enhance electroluminescence of polymer light-emitting diodes (PLEDs) using an environmentally stable
aluminum cathode, we designed a novel water/alcohol-soluble electron injection material, FTC, composed
of a fluorene core and triple azacrown ether terminals. FTC significantly enhances the emission
performance of PLEDs [ITO/PEDOT:PSS/EML/EIL/Al] when used as the electron injection layer (EIL),
especially in the presence of metal carbonates and metal acetates. The metal carbonate-doped devices
showed the best performance due to their higher dissociation rate than metal acetates. In particular,
the device using K2CO3 doped-FTC as the electron injection layer (EIL) exhibited significantly enhanced
performance compared to the device without an EIL. For the device based on PF-Green-B as the
emitting layer, the performance was significantly enhanced to 17 460 cd mꢀ2, 21.58 cd Aꢀ1, and
12.42 lm Wꢀ1, respectively, from 1220 cd mꢀ2, 0.72 cd Aꢀ1, and 0.27 lm Wꢀ1 for the non-FTC
device. Using HY-PPV as the emitting layer, the device performance was also significantly enhanced to
10 990 cd mꢀ2, 6.93 cd Aꢀ1, and 5.27 lm Wꢀ1, respectively, from 680 cd mꢀ2, 0.07 cd Aꢀ1, and 0.03 lm
Wꢀ1 for the non-EIL device. The results indicate that FTC with metal cations is an excellent electron
injection candidate for the performance enhancement of PLEDs with a high work function Al cathode.
Received 8th July 2013
Accepted 25th August 2013
DOI: 10.1039/c3tc31301h
www.rsc.org/MaterialsC
electron injection is dependent on the energy barrier height
between the emitting layer and the cathode. Therefore, low
Introduction
For more than two decades, polymer light-emitting diodes
(PLEDs)1–3 have attracted continued attention because of their
low cost, easy processability, and the possibility of easily fabri-
cating exible, large-area displays. PLEDs are carrier injection
devices, which basically require balanced hole and electron
injection from the anode and the cathode respectively, and their
fast transport and recombination in the emissive layer.4,5 To
achieve high electroluminescence efficiency, it is necessary that
both electrons and holes are efficiently injected.5,6 Therefore,
substantial improvements in the device efficiency have been
obtained upon modication of the anode,7–9 by the introduction
of low-work function cathodes,10 and by using hole and electron
injecting/transporting layers.11–14 Efficient injection of electrons
from the cathode to the emissive conjugated polymers plays an
important role in improving the device efficiency and stability.
For most existing conjugated polymers, electron injection is
more difficult than hole injection.11,15 It is well-known that
work-function metals such as Ba and Ca are widely utilized as
cathode materials to facilitate efficient electron injection.
However, these metals are not stable in air, and they may
sometimes react with and diffuse into organic materials,
leading to the deterioration of the device.
For development of PLEDs industrialization, the cathode
must be stable to environmental conditions, and the use of
environmentally stable high-work-function metals such as Al,
Cu, Ag, and Au as the cathode has attracted extensive attention
recently. Use of a water- or alcohol-soluble electron injection
layer (EIL) based on a conjugated polymer graed with amino,
ammonium salt, or diethanolamino groups has been demon-
strated to allow the use of a high-work-function metal as the
cathode.16–23 This is because interfacial dipole or space charge is
formed between the emitting layer (EML) and the cathode that
reduces the electron injection barrier. In addition to the above
hydrophilic groups, crown ether groups may be expected to
serve the same purpose. Crown ethers are a special class of
ether, able to form stable complexes with alkali, alkaline-earth,
and transition-metal ions.24,25 Chen et al. rst reported the use
of water/methanol-soluble polyuorene graed with 18-crown-6
chelating to K+ as the electron-injection layer (EIL) for deep-
blue-emission PLEDs, allowing the use of environmentally
Department of Chemical Engineering, National Cheng Kung University, Tainan,
Taiwan. E-mail: yunchen@mail.ncku.edu.tw
† Electronic supplementary information (ESI) available: Synthesis and
characterization of monomers, the 1H NMR spectra, and thermal, optical,
electrochemical and optoelectronic properties of FTC, and the proposed
chemical structure of doped-FTC. See DOI: 10.1039/c3tc31301h
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stable Al as the cathode.26 The device exhibited the highest that FTC with metal carbonates and metal acetates was an
performance reported to date for a deep-blue-emission PLED excellent electron injection layer for high performance PLEDs
based on a conjugated polymer as the emitting layer, with a with a high work function Al cathode.
brightness of 54 800 cd mꢀ2 and an external quantum efficiency
of 5.42%. An azacrown ether is a crown ether that has nitrogen
donor atoms as well as oxygen donor atoms to coordinate to the
Experimental section
Materials
metal iron. The nitrogen atoms in an azacrown ether should
play an important role in coordination with metal cations. This
means that it can be a more effective EIL than a crown ether
when applied in PLEDs.
All reagents and solvents, unless otherwise specied, were
obtained from Aldrich, Acros, Showa, and J. T. Backer
and were used as received. Poly(3,4)-ethylendioxythiophene–
polystyrenesulfonate (PEDOT:PSS) was obtained from Bayer,
green-emitting polyuorene derivative (PF-Green-B) was
obtained from Dow Chemical Company (Lumationꢀ), and
yellow-emitting material HY-PPV was obtained from Merck
(Super yellow, Livilux PDY-132).
Recently, most water/alcohol-soluble electron injection
materials that have been developed and demonstrated are
mainly based on polymers.16–23,26 Nonetheless, a few studies
focused on small molecules have been reported. In particular,
small-molecular-weight materials have received interest
due to their easy purication by recrystallization or column
chromatographic techniques, their monodispersity with
well-dened chemical structures, and their good synthetic
reproducibility. In addition, among the typical uorescent
materials, uorene-based derivatives show good thermal and
chemical stabilities, exhibit wide energy band gaps, and have
high photoluminescence (PL) efficiencies (60–80%) in solution
and solid state.27–29 Moreover, vinyl groups have also been
introduced between uorene-cored and triple-azacrown ethers,
to extend the conjugation of monouorene to achieve more
suitable LUMO and HOMO energy levels. To the best of our
knowledge, little attention has been paid to the effects of
different metal cations and different anionic groups on electron
injection layers.
In this study, we propose the use of a water/alcohol-soluble
small-molecule uorene derivative, FTC, doped with M2CO3
(M ¼ Na, K, Cs) and CH3COOM (M ¼ Na, K) as the EIL in PLEDs
with PF-Green-B (Dow Chemical LUMATION30,31) and HY-PPV
(PDY-132, Merck32,33) as the emitting layer (EML). Moreover, to
the best of our knowledge this is the rst investigation into the
doping effect of different counter anions (CO3ꢀ and CH3COOꢀ)
in the electron injection layer. The solubility in highly polar
solvents (e.g., water or alcohol) provided by the azacrown can
prevent dissolution of the EML having a thin EIL atop it, and the
Measurements
All new synthesized compounds were identied by 1H NMR,
1
13C NMR, and elemental analysis (EA). The H and 13C NMR
spectra were recorded on a Bruker AMX-400 MHz FT-NMR,
with the chemical shis reported in ppm using tetrame-
thylsilane (TMS) as an internal standard. The elemental
analysis was carried out on a Heraeus CHN-Rapid elemental
analyzer. Absorption and photoluminescence (PL) spectra
were measured on a Jasco V-550 spectrophotometer and a
Hitachi F-4500 spectrouorometer, respectively. Cyclic vol-
tammograms were measured with a voltammetric apparatus
(model CV-50W from BAS) equipped with a three-electrode
cell. The cell was made up of FTC-coated glassy carbon as the
working electrode, an Ag/AgCl electrode as the reference
electrode, and a platinum wire as the auxiliary electrode. The
electrodes were immersed in acetonitrile containing 0.1 M (n-
Bu)4NClO4 as the electrolyte. The energy levels were calculated
using the ferrocene (FOC) value of ꢀ4.8 eV with respect to the
vacuum level, which is dened as zero.
Synthesis of electron injection material FTC (Scheme 1)
interaction of M+ (M ¼ Na, K, Cs) with the azacrown allows the A mixture of 4-(monoaza-15-crown-5)benzaldehyde (compound 4:
M+ ion to act similarly to a metallic state in reducing the elec- 3.36 g, 10.4 mmol) and 2,4,7-tri[methylene(triphenylphosphonium
tron injection barrier from a stable metal cathode (e.g., Al, Ag or bromide)]-9,9-dihexyluorene (compound 6: 1.6 g, 1.1 mmol)
Au) and facilitating electron injection. When FTC binding M+ in dried ethanol (20 ml) was added dropwise to sodium eth-
(M ¼ Na, K, Cs) was inserted between the emitting layer and the oxide (3.4 g, 10.4 mmol) and stirred at room temperature
Al cathode as an EIL, the device efficiency was signicantly overnight under a nitrogen atmosphere. The mixture was
enhanced. Particularly, the device using FTC doped with K2CO3 extracted with chloroform and the combined organic layer was
as the electron injection layer showed the highest performance. dried (MgSO4) and concentrated under reduced pressure. The
For the devices based on PF-Green-B, the maximum luminance, crude product was puried by column chromatography
maximum current efficiency and maximum luminous power (eluent: acetone : n-hexane ¼ 1 : 1, EA) (0.51 g, 34.5%). 1H
efficiency were 17 460 cd mꢀ2, 21.58 cd Aꢀ1, and 12.42 lm Wꢀ1
,
NMR (400 MHz, CD2Cl2, TMS, 25 ꢁC): d 7.88–7.86 (d, 1H,
respectively, which were signicantly superior to those without Ar–H), 7.70–7.66 (d, 1H, Ar–H), 7.6 (s, 1H, Ar–H), 7.54–7.42
the electron injection layer (1220 cd mꢀ2, 0.72 cd Aꢀ1, and (m, 8H, Ar–H), 7.38 (s, 1H, Ar–H), 7.20–6.98 (m, 5H, Ar–H),
0.27 lm Wꢀ1). For the devices based on HY-PPV, the maximum 6.76–6.68 (m, 6H, Ar–H), 3.80–3.53 (m, 60H, crown ether),
luminance, maximum current efficiency and maximum lumi- 2.06–2.02 (t, 4H, –CH2–), 1.16–1.03 (m, 12H, –CH2–), 0.79–
nous power efficiency were 10 990 cd mꢀ2, 6.93 cd Aꢀ1, and 0.75 (m, 6H, –CH3–), 0.67–0.65 (m, 4H, –CH2–). Anal. calc. for
5.27 lm Wꢀ1, respectively, which were also much higher than
C79H109N3O12: C, 73.40; H, 8.50; N, 3.25. Found: C, 73.42; H,
those without the electron injection layer. The results indicate 8.72; N, 3.11.
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The chemical shis at 2.06–0.65 ppm are attributed to the
protons on the long alkyl chains of FTC (Fig. 1). The protons
have been further assigned using COSY spectra (Fig. S1 in
ESI†). The chemical shis of the protons belonging to the
azacrown ether moieties are located at 3.80–3.53 ppm, in
which the chemical shis at 3.80–3.71 ppm are assigned to the
protons symmetrically adjacent to the nitrogen atom.
The chemical shis at 3.65–3.53 ppm are attributable to the
protons attached to the carbons of ether groups (C–O–C). The
complex and multiple chemical shis around 7.88–6.68 ppm
are due to the signals of the protons belonging to the aromatic
and vinylene parts of FTC. Due to the complexity of the
spectrum, the assignment of individual protons has been
assisted by two dimensional COSY and NOESY spectra (Fig. S2
and S3 in ESI†).
Homonuclear correlation spectroscopy (COSY) is used to
identify spins which are coupled to each other. COSY spectra
give information about pairs of protons that are J-coupled. This
usually indicates that the protons are on adjacent carbons, e.g.,
3 bonds away. NOESY (Nuclear Overhauser Effect Spectroscopy)
spectra indicate proton–proton through space interactions via
the NOE, which gives information about pairs of protons that
Fabrication of polymer light-emitting diodes
Multilayer polymer light-emitting diodes (PLEDs), with a
structure of ITO/PEDOT:PSS/EML/EIL/Al, were fabricated by
wet processes for the investigation of optoelectronic charac-
teristics. A glass substrate coated with an ITO conductive layer
was used as the anode, poly(3,4-ethylenedioxythiophene)–
polystyrenesulfonate (PEDOT:PSS, Bayer) was used as the hole
injection layer, the green polyuorene derivative (PF-Green-B,
Dow Chemical) or Livilux PDY-132 (HY-PPV, Merck) was used
as the light-emitting layer, FTC (or FTC in the presence of
metal cations) was used as the electron injection layer, and Al
was used as the metal cathode. The ITO-coated glasses were
washed successively in an ultrasonic bath of neutral cleaner/
de-ionized water mixture, de-ionized water, acetone and
2-propanol, followed by treatment in a UV-ozone chamber. A
thick hole injection layer of PEDOT:PSS was spin-coated on
top of the cleaned ITO glass and annealed at 423 K for 900 s in
a dust-free atmosphere. For the devices based on PF-Green-B,
the light-emitting lm was prepared by spin-coating the PF-
Green-B solution (10 mg per 1 ml toluene) on top of the
PEDOT:PSS layer at 2500 rpm and annealing at 65 ꢁC for
25 min to remove the residual solvent. For the devices based
on HY-PPV, the emitting layer (EML) was deposited by spin-
coating the HY-PPV solution (6 mg per 1 ml toluene) on top of
˚
are close in space (<5 A apart). The chemical shis at 2.06–0.65
ppm are attributed to the protons on the long chain of FTC by
1H NMR. As demonstrated in the COSY spectrum (Fig. S1 in
ESI†), there is an off-diagonal peak between 2.06 and 2.02 ppm
and 0.67–0.65 ppm. The signal at 2.06–2.02 ppm is known to be
due to the protons at position k, which indicates that the signal
at 0.67–0.65 ppm is due to the protons at position l. Also, there
is an off-diagonal peak between 0.67 and 0.65 ppm (position l)
and 1.16–1.03 ppm, so the peak at 1.16–1.03 ppm is assigned to
position m. From another off-diagonal peak between 1.16–1.03
ppm (position m) and 0.79–0.75 ppm, the peak at 0.79–0.75 ppm
can be assigned as position n. The protons at positions k, l, m
and n can be further conrmed by the integration of the 1H
NMR spectrum.
For the aromatic part (Fig. S2 in ESI†), there is an off-diag-
onal peak between 7.88 and 7.86 ppm and 7.46–7.42 ppm; the
former has been assigned as position a, hence the latter one can
be attributed to position b. Another off-diagonal peak is found
between 7.70 and 7.66 ppm and 7.20–6.98 ppm; the former
doublet has been assigned as position c, so the latter chemical
shi is due to the doublet of position d. According to the fact
that linkage with a nitrogen atom usually results in electron rich
protons and the integration of the chemical shis at 6.76–6.68
ꢁ
the PEDOT:PSS layer at 6000 rpm and annealing at 65 C for
25 min to remove the residual solvent. The electron injection
layer was cast on top of the EML by spin-coating (3000 rpm) of
FTC and M2CO3 in a mixed solvent of de-ionized water and
2-propanol (v/v ¼ 1 : 5). The ratio of FTC : M+ was xed at 1 : 9
(azacrown : M+ ¼ 1 : 3), and the concentration was 1 mg FTC
per ml. Finally, the aluminum (100 nm) was deposited as the
cathode by thermal evaporation at about 1 ꢂ 10ꢀ6 Torr.
The current–luminance–voltage (I–L–V) characteristics of the
devices were recorded using a combination of a Keithley
power source (model 2400) and an Ocean Optics usb2000
uorescence spectrophotometer. Current efficiency and
luminous power efficiency were calculated from the I–L–V
characteristics. The fabrication of the devices was done in
ambient conditions, with the following performance tests
conducted in a glove-box lled with nitrogen.
Results and discussion
Characterization of electron injection material FTC
The electron injection material FTC was synthesized from ppm, these peaks are attributed to position e of the benzene
monoaza-15-crown-5 derivative (4) and 2,4,7-tri[methylene- rings. Hence, the off-diagonal peak between 6.76 and 6.68 ppm
(triphenylphosphonium bromide)]-9,9-dihexyluorene (6) by the and 7.54–7.42 ppm is assigned to the signals of positions e and
Wittig reaction (Scheme 1). The chemical structure of FTC has f. This is further veried by observing the corresponding off-
been well characterized by 1H NMR spectra and elemental anal- diagonal peak in the NOESY spectra.
ysis, whereas its optical properties were investigated by absorp-
As shown in Fig. S3 (ESI†), an off-diagonal peak is observed
tion spectroscopy (UV-vis) and photoluminescence spectroscopy between 7.70 and 7.66 ppm and 7.60 ppm, demonstrating the
(PL). Cyclic voltammetry (CV) was used to measure the onset through space interaction between positions c and g. The off-
reduction and oxidation potentials of the FTC lm, which in turn diagonal peak between 7.60 ppm and 7.54–7.50 ppm is due to
were used to estimate the LUMO and HOMO levels respectively. positions g and h of uorene. As discussed above, by detailed
The thermal transitional properties of FTC were studied by analysis of the 1H NMR, COSY and NOESY spectra, the protons
differential scanning calorimetry (DSC).
of FTC have been assigned completely.
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Scheme 1 Synthetic procedure for FTC.
uorene plane effectively prevent close packing between the
molecules. In addition, the thermal decomposition temperature at
5% weight loss was quite high (394 ꢁC), indicating that FTC is
thermally stable enough to be applied in PLEDs.
Thermal properties
Glass transition temperatures (Tg) and thermal decomposition
temperatures (Td) (at 5 wt% loss) of FTC were evaluated using
differential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) respectively, with the representative data summa-
rized in Table 1. The glass transition temperature (Tg) and melting
Optical properties
point (Tm) of FTC were observed at 49 ꢁC and 64 ꢁC [Fig. S4 (ESI†)], Fig. 2 illustrates the absorption and photoluminescence (PL)
respectively, but no obvious crystallization temperature (Tc) was spectra of FTC in solution (CHCl3) and as a lm spin-coated
detected between 30 C and 150 C. The asymmetric structure of from CHCl3 solution, with the characteristic optical data
ꢁ
ꢁ
FTC and the twist of the 4-azacrown ether moiety relative to the summarized in Table 1. The absorption spectra of FTC in
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Fig. 1 1H NMR spectrum of compound FTC.
solution and lm state peak at ca. 416 nm and 424 nm,
respectively. The absorption could be attributed to the n–p* and
p–p* electronic transitions of the crown ether and the conju-
gated uorenyl vinyl phenyl moiety. The PL spectra of FTC in
solution and solid state are located at ca. 470 nm and ca. 535
nm, respectively. The solid state PL maximum of FTC is greatly
red-shied (65 nm) relative to the solution state, probably due
to excimer formation via intra- or inter-chain interactions.
Electrochemical properties
Cyclic voltammetry (CV) was employed to investigate the elec-
trochemical properties of FTC. The cyclic voltammograms are
shown in Fig. S5 in the ESI,†,with the representative electro-
chemical data summarized in Table 1. HOMO and LUMO
energy levels were estimated using the following equations:
EHOMO (eV) ¼ ꢀ(Eox,FOC + 4.8) and ELUMO (eV) ¼ ꢀ(Ered,FOC + 4.8),
where Eox,FOC and Ered,FOC are the onset oxidation and onset
reduction potentials, respectively, relative to the ferrocene/fer-
rocenium couple whose energy level is already known (ꢀ4.8 eV).
The HOMO and LUMO energy levels of FTC were estimated to
Fig. 2 Absorption spectra and photoluminescence spectra of FTC in the film
state and CHCl3 solution.
be ꢀ5.88 eV and ꢀ2.88 eV, respectively, and its electrochemical
band gap (Eegl) was 3.0 eV.
On the other hand, the optical band gap (Eogpt) was estimated
to 2.55 eV from the onset absorption wavelength of 486 nm
Table 1 Thermal, optical and electrochemical properties of FTC
c
Material
Tga/Tmb/Td (ꢁC)
UV-vis lmax/PL lmax (nm)
Eonset(ox) vs. FOCf (V)
Eonset(red) vs. FOCf (V)
ꢀ1.92
EHOMO/ELUMOg (eV)
Eeglh/Eogpti (eV)
FTC
49/64/394
416d (424)e/470d (535)e
1.08
ꢀ5.88/ꢀ2.88
3.00/2.55
a
b
c
Glass transition temperature determined by DSC measurement. Melting point determined by DSC measurement. The decomposition
d
e
temperature at 5 wt% loss was measured by TGA at a heating rate of 10 ꢁC minꢀ1 under nitrogen. In chloroform (1 ꢂ 10ꢀ5 M). Film state.
f
g
h
i
EFOC ¼ 0.49 V vs. Ag/AgCl. EHOMO ¼ ꢀ(Eonset(ox),FOC + 4.8) eV; ELUMO ¼ ꢀ(Eonset(red),FOC + 4.8) eV. Eg ¼ |LUMO ꢀ HOMO|. Eg ¼ 1240/
lonset(abs. spectrum) (nm).
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obtained from the absorption spectrum of FTC in the lm state or CH3COOM (M ¼ Na, K) as an EIL in a PLED with PF-Green-B
(Fig. 2). The energy band gaps obtained by the optical method or HY-PPV as the EML. The optimal conditions for processing
and cyclic voltammetry were 2.55 eV and 3 eV, respectively. The the EIL materials for use in PLEDs was to use water and alcohol
electrochemically-estimated band gap (Eegl), calculated using (v/v ¼ 1 : 5) as the solvent for spin-coating. The ratio of FTC : M+
Eegl ¼ |LUMO ꢀ HOMO|, is slightly different from the optical was xed at 1 : 9 (azacrown : M+ ¼ 1 : 3), and the concentration
band gap (Eogpt) obtained from the onset absorption. The elec- was 1 mg FTC per ml. Current density–luminance–voltage
trochemical band gap (Eegl) is the energy difference between the ( J–L–V) and luminous efficiency–power efficiency–current
LUMO and HOMO levels, which are in turn estimated from the density (LE–PE–J) characteristics of the devices are shown in
onset oxidation and reduction potentials, respectively. Fig. 4 and 5 and Fig. S6 and S7 (ESI†), with the corresponding
However, oxidation and reduction may start from different parts optoelectronic data summarized in Tables 2 and 3. The elec-
of a molecule which is composed of electron-donating and troluminescence performance was signicantly enhanced by
electron-withdrawing groups. Therefore, the discrepancy inserting the electron injection layer between the emitting layer
between the band gaps determined electrochemically (Eegl) and (PF-Green-B or HY-PPV) and the Al cathode.
optically (Eogpt) is probably due to their different estimating
Maximum luminance and current efficiency of the devices
bases. Due to the fact that FTC is comprised of a conjugated were greatly increased from 1220 cd mꢀ2 and 0.72 cd Aꢀ1 to
uorene core and three crown ethers, oxidation may start from 2920 cd mꢀ2 and 5.56 cd Aꢀ1, respectively, by inserting an
the electron-donating crown ether parts, and reduction may electron injection layer (EIL: neat FTC) between the emitting
start from the conjugated core of FTC.
layer (PF-Green-B) and the Al cathode [Fig. 4(a) and 5(a)]. Similar
trends in the enhancement of device performance were also
observed for the devices using HY-PPV as the emitting layer and
neat FTC as the EIL [Fig. 4(b) and 5(b)]. The performance
enhancement is attributable to the hole blocking and electron
injection effect of FTC, which raises the charges recombination
ratio. The lower HOMO level of FTC (ꢀ5.88 eV) than that of the
emitting layer PF-Green-B (ꢀ4.52 eV) or HY-PPV (ꢀ5.0 eV)
Electroluminescence enhancement of PLEDs by insertion of
FTC
According to the energy band diagrams depicted in Fig. 3, the
energy barrier between aluminum (Al) and PF-Green-B (2.16 eV)
is larger than that between PEDOT:PSS and PF-Green-B
(0.48 eV). Moreover, holes are usually more readily transported
than electrons in conjugated materials, leading to a reduced
recombination ratio of holes and electrons. Similar non-
matching energy barriers for electrons and holes are also
apparent for the HY-PPV based device (Fig. 3). However, the
charge injection/transport imbalance in PF-Green-B or HY-PPV
can be alleviated by inserting the electron injection and hole
blocking layer of FTC, in which both LUMO and HOMO energy
levels are lower than those of PF-Green-B or HY-PPV. The lower
LUMO and HOMO levels give rise to enhanced electron injec-
tion/transport and hole blocking, respectively, in PF-Green-B or
HY-PPV based devices, resulting in more balanced charge
recombination.
Multi-layer polymer light-emitting diodes (PLEDs) with a
conguration of ITO/PEDOT:PSS/EML (PF-Green-B or HY-PPV)/
EIL/Al (80 nm) were fabricated to investigate their electrolumi-
nescence characteristics. In this work, we propose the use of
water/alcohol-soluble FTC blended with M2CO3 (M ¼ Na, K, Cs)
Fig. 4 Luminance versus voltage and current density versus voltage charac-
teristics of PLEDs. Device structure: ITO/PEDOT:PSS/(a) PF-Green-B or (b) HY-
PPV/EIL/Al.
Fig. 3 The energy band diagrams of PF-Green-B and HY-PPV, with those of FTC,
PEDOT:PSS, and the electrodes.
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The device performances using PF-Green-B or HY-PPV as the
emitting layer were signicantly enhanced in the presence of
metal carbonates M2CO3 (M ¼ Na, K, Cs) (Tables 2 and 3). The
devices using K2CO3-doped FTC as the EIL showed the best
performance, i.e., the maximum current efficiency and power
efficiency of the device based on PF-Green-B (or HY-PPV) were
further promoted to 21.58 cd Aꢀ1 and 12.42 lm Wꢀ1 (or 6.93 cd
Aꢀ1 and 5.27 lm Wꢀ1), respectively (Fig. S6 and S7 in the ESI†).
Moreover, these efficiencies are superior to those obtained with
a conventional LiF/Al cathode (14.25 cd Aꢀ1 and 10.11 lm Wꢀ1
for PF-Green-B or 5.70 cd Aꢀ1 and 2.38 lm Wꢀ1 for HY-PPV)
(Fig. S8 and S9 in the ESI†, Table 3). In addition, the turn-on
voltages of the devices based on PF-Green-B or HY-PPV were
decreased to 3.7 V or 2.5 V, which are much lower than the
values of 5.7 V or 5.5 V obtained for the devices without FTC as
the EIL, respectively (Tables 2 and 3). The electroluminescence
enhancement is attributed to the effective promotion of elec-
tron injection/transport by the metal carbonate-doped FTC as
the EIL, which leads to a higher charge recombination ratio.
To investigate the effects of the different counter anions
(CO3 and CH3COOꢀ) on the electron injection ability, devices
ꢀ
based on PF-Green-B or HY-PPV using metal acetate [CH3COOM
(M ¼ Na, K)]-doped FTC as the EIL were also fabricated and
their electroluminescence characteristics were investigated
(Fig. 4 and 5). The device performances were also effectively
enhanced in the presence of metal acetates CH3COOM (M ¼ Na,
K) (Tables 2 and 3). The devices using CH3COOK-doped FTC as
the EIL exhibit better performance than those using
CH3COONa-doped FTC as the EIL. However, the metal
carbonate-doped devices (M2CO3, M ¼ Na, K) showed the best
performance due to their higher dissociation rate than metal
acetates (CH3COOM, M ¼ Na, K). For instance, potassium
carbonate dissociates more readily than potassium acetate,
resulting in more abundant azacrown-chelated K+. The chelated
K+ can further reduce the electron injection barrier through the
formation of an interfacial dipole and the establishment of an
intermediate step for electron injection.26 Therefore, the
potassium carbonate-doped FTC as the EIL effectively promoted
electron injection/transport to signicantly enhance the device
performance.
Fig. 5 Current efficiency versus current density characteristics of PLEDs. Device
structure: ITO/PEDOT:PSS/(a) PF-Green-B or (b) HY-PPV/EIL/Al.
results in the accumulation of holes at the interface between the
emitting and FTC layers. Moreover, the lower LUMO level of
FTC (ꢀ2.88 eV) than that of the emitting layer PF-Green-B
(ꢀ2.14 eV) or HY-PPV (ꢀ2.8 eV) promotes electron injection
(Fig. 3). Therefore, holes will readily combine with electrons
passing through the interface to increase the recombination
ratio.
To clarify the hole blocking effect of the EIL (neat FTC
and doped FTC), hole-only devices [ITO/PEDOT:PSS/EML
Table 2 Optoelectronic properties of the light-emitting diodes based on PF-Green-Ba
Lmax (cd mꢀ2
)
CEmaxd (cd Aꢀ1
)
LPEmaxe (lm Wꢀ1
)
CIE (x, y) f
b
c
EIL
Von (V)
None
LiF
FTC
FTC + Na2CO3
FTC + K2CO3
FTC + Cs2CO3
FTC + CH3COONa
FTC + CH3COOK
5.7
3.6
5.7
3.7
3.7
4.0
3.0
3.5
1220
14 390
2920
13 770
17 460
17 380
22 580
15 720
0.72
14.25
5.56
15.77
21.58
16.26
14.90
15.23
0.27
10.11
2.87
9.23
12.42
9.96
(0.35, 0.62)
(0.35, 0.62)
(0.36, 0.60)
(0.36, 0.61)
(0.36, 0.61)
(0.35, 0.62)
(0.38, 0.58)
(0.38, 0.59)
11.55
9.37
a
c
[ITO/PEDOT:PSS/PF-Green-B (ꢃ100 nm)/EIL/Al (80 nm)]. b Turn-on voltage at 10 cd mꢀ2
.
Maximum luminance. d Maximum current efficiency.
e
Maximum luminous power efficiency. f The 1931 CIE coordinate at maximum luminance.
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Table 3 Optoelectronic properties of the light-emitting diodes based on HY-PPVa
Lmax (cd mꢀ2
)
CEmaxd (cd Aꢀ1
)
LPEmaxe (lm Wꢀ1
)
CIEf (x, y)
b
c
EIL
Von (V)
None
LiF
FTC
FTC + Na2CO3
FTC + K2CO3
FTC + Cs2CO3
FTC + CH3COONa
FTC + CH3COOK
5.5
3.5
3.1
3.1
2.5
2.8
3.3
3.0
680
16 240
3170
0.07
5.70
1.18
2.88
6.93
4.61
2.34
3.09
0.03
2.38
0.72
1.34
5.27
3.02
1.12
1.49
(0.36, 0.61)
(0.41, 0.57)
(0.40, 0.58)
(0.39, 0.59)
(0.41, 0.58)
(0.40, 0.58)
(0.39, 0.59)
(0.40, 0.58)
8220
10 990
13 340
7150
9980
a
b
c
d
[ITO/PEDOT:PSS/HY-PPV (ꢃ80 nm)/EIL/Al (80 nm)]. Turn-on voltage at 10 cd mꢀ2
.
Maximum luminance. Maximum current efficiency.
e
Maximum luminous power efficiency. f The 1931 CIE coordinate at Maximum luminance.
(PF-Green-B or HY-PPV)/EIL/Au (100 nm)] were fabricated to current densities are still lower than that of the device without
investigate their current density versus bias characteristics. As an EIL. Therefore, more balanced charge injection and trans-
shown in Fig. 6, the curve shis horizontally to higher bias aer port was achieved by inserting the metal carbonate- or metal
the insertion of FTC as the EIL, indicating the diminished acetate-doped FTC layer, which promoted electron injection
current density under the same bias. However, the curve shis and adjusts hole blocking simultaneously.
slightly to a lower bias in the presence of metal carbonates or
To demonstrate the electron injection ability of the EIL (neat
metal acetates, indicating that the hole blocking effect is FTC and doped FTC), electron-only devices [glass/PEDOT:PSS/
diminished by the metal salts. However, the voltage-induced Ag (100 nm)/EML (PF-Green-B or HY-PPV)/EIL/A1 (80 nm)]34,35
Fig. 6 Current density versus voltage characteristics of hole-only PLEDs. Device
Fig. 7 Current density versus voltage characteristics of electron-only PLEDs.
structure: ITO/PEDOT:PSS/(a) PF-Green-B or (b) HY-PPV/EIL/Au.
Device structure: ITO/PEDOT:PSS/Ag/(a) PF-Green-B or (b) HY-PPV/EIL/Al.
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were fabricated to investigate their current density versus bias
characteristics (Fig. 7). The current density (under the same
bias) is increased by the EILs, especially in the device using
K2CO3-doped FTC as the EIL, which shows the highest current
density. This is attributed to the enhancement in electron
injection/transport by the EILs. Moreover, for the devices with
K2CO3-doped FTC as the EIL, the current densities of the hole-
only and electron-only devices are the closest, compared to
those with neat FTC or without EIL (Fig. 8). This suggests that
the K2CO3-doped FTC is the most effective EIL to balance charge
injection and transport.
Photovoltaic (PV) measurements were conducted to
demonstrate that the addition of metal carbonates and metal
acetates plays a crucial role in promoting electron injection in
the PLEDs. When the anode is kept the same (ITO/PEDOT:PSS),
the open-circuit voltage (Voc) is primarily determined by the
effective work-function of the cathode, which reects the elec-
tron-injection ability of the EIL materials.11,36 The Voc values
were 0.36 V and 0.96 V for the devices based on PF-Green-B and
HY-PPV without EIL, respectively, which increased to 1.28 V and
1.16 V when neat FTC was inserted as the EIL (Fig. 9). They were
further raised to 1.74 V and 1.62 V when K2CO3-doped FTC was
Fig. 9 Photovoltaic measurements of PLEDs using FTC and doped-FTC as the
electron injection layer. Device structure: ITO/PEDOT:PSS/PF-Green-B or HY-PPV/
EIL/Al.
used as the EIL. A higher Voc means that the built-in potential
(the difference in work function between the anode and the
cathode) across the anode/EML/cathode junction has been
increased. Moreover, the highest Voc values (1.74 V and 1.62 V)
correspond to the best device performances for the devices
using K2CO3-doped FTC. Therefore, a drop in the work function
of the EIL/Al cathode (or a generation of interfacial dipoles at
the EIL/metal junction) has been realized by inserting the EIL. A
lower work function of the cathode usually facilitates electron
injection. Accordingly, the signicant performance enhance-
ment in the devices using doped FTC as an EIL should mainly
be attributed to the enhanced electron injection ability. Current
results indicate that the uorene-cored FTC with azacrown
ether groups is a promising stable cathode modier for
promoting electron injection and device performance.
Conclusion
We have presented a novel water/alcohol-soluble uorene-cored
FTC with triple-azacrown ether groups as an electron injection
layer (EIL) for the fabrication of multilayer PLEDs by a spin-
coating process. The FTC is a highly efficient EIL, especially
Fig. 8 Current density versus voltage characteristics of electron-only and hole-
only PLEDs.
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aer the addition of metal carbonates M2CO3 (M ¼ Na, K, Cs) 11 H. Wu, F. Huang, Y. Mo, W. Yang, D. Wang, J. Peng and
and metal acetates CH3COOM (M ¼ Na, K). We also investi-
Y. Cao, Adv. Mater., 2004, 16, 1826.
gated the effect of different counter anions (CO3 and 12 T.-W. Lee, H.-C. Lee and O. O. Park, Appl. Phys. Lett., 2002,
ꢀ
CH3COOꢀ) on the electron injection ability of the devices
based on PF-Green-B or HY-PPV. The metal carbonate-doped 13 T. F. Guo, F. S. Yang, Z. J. Tsai, T. C. Wen, S. N. Hsieh and
devices showed the best performance due to their higher Y. S. Fu, Appl. Phys. Lett., 2005, 87, 13504.
dissociation rate than metal acetates. The devices based on 14 X. Y. Deng, W. M. Lau, K. Y. Wong, K. H. Low, H. F. Chow
PF-Green-B or HY-PPV as the emission layer with K2CO3-doped and Y. Cao, Appl. Phys. Lett., 2004, 84, 3522.
FTC as the EIL (ITO/PEDOT:PSS/PF-Green-B or HY-PPV/EIL/Al) 15 S. H. Jin, M. Y. Kim, J. Y. Kim, K. Lee and Y. S. Gal, J. Am.
revealed the best device performance. For the devices based Chem. Soc., 2004, 126, 2474.
on PF-Green-B (and HY-PPV), their maximum current effi- 16 F. Huang, H. B. Wu, D. L. Wang, W. Yang and Y. Cao, Chem.
ciency and maximum luminous power efficiency were 21.58 cd Mater., 2004, 16, 708.
Aꢀ1 (6.93 cd Aꢀ1), and 12.42 lm Wꢀ1 (5.27 lm Wꢀ1), respec- 17 F. Huang, Y. H. Niu, Y. Zhang, J. W. Ka, M. S. Liu and
tively, which were superior to those of the device without an A. K. Y. Jen, Adv. Mater., 2007, 19, 2010.
electron injection layer. In addition, the turn-on voltages were 18 F. Huang, Y. Zhang, M. S. Liu and A. K. Y. Jen, Adv. Funct.
also signicantly reduced (from 5.7 V to 3.0 V for PF-Green-B, Mater., 2009, 19, 2457.
and from 5.5 V to 2.5 V for HY-PPV). The electron- and hole- 19 S. H. Oh, D. Vak, S. I. Na, T. W. Lee and D. Y. Kim, Adv.
only devices and their open circuit voltages (Voc) were inves- Mater., 2008, 20, 1624.
tigated to clarify their charge injection/transport abilities. The 20 R. Yang, H. Wu, Y. Cao and G. C. Bazan, J. Am. Chem. Soc.,
performance enhancement has been attributed to hole 2006, 128, 14422.
blocking and promoted electron injection/transport by the 21 C. V. Hoven, A. Garcia, G. C. Bazan and T. Q. Nguyen, Adv.
inserted EIL. The present results demonstrate that FTC doped Mater., 2008, 20, 3793.
with metal carbonates and metal acetates is an effective elec- 22 X. Xu, B. Han, J. Chen, J. Peng, H. Wu and Y. Cao,
81, 214.
tron injection/transport material, applicable for use in opto-
electronic devices.
Macromolecules, 2011, 44, 4204.
23 F. Huang, P. I. Shih, C. F. Shu, Y. Chi and A. K.-Y. Jen, Adv.
Mater., 2009, 21, 361.
24 C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 2495.
25 C. J. Pedersen, Angew. Chem., Int. Ed. Engl., 1988, 27, 1021.
Acknowledgements
The authors gratefully acknowledge nancial support from 26 H. H. Lu, Y. S. Ma, N. J. Yang, G. H. Lin, Y. C. Wu and
NSC 101-2221-E-006-075-MY3 and NSC 101-2221-E-006-042-
MY3.
S. A. Chen, J. Am. Chem. Soc., 2011, 133, 9634.
27 (a) M. Kreyenschmidt, G. Klaerner, T. Fuhrer, J. Ashenhurst,
S. Karg, W. D. Chen, V. Y. Lee, J. C. Scoot and R. D. Miller,
Macromolecules, 1998, 31, 1099; (b) A. W. Grice,
D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu
and E. P. Woo, Appl. Phys. Lett., 1998, 73, 629.
Notes and references
1 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackey, R. H. Friend, P. L. Burns and A. B. Holmes, 28 (a) U. Scherf and E. J. W. List, Adv. Mater., 2002, 14, 477; (b)
Nature, 1990, 347, 539.
S. Becker, C. Ego, A. C. Grimsdale, E. J. W. List, D. Marsitzky,
¨
2 J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard,
J. Miller, R. Moon, D. B. Roitman and A. Stocking, Science,
1996, 273, 884.
3 R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
D. D. C. Bradley, D. A. D. Santos, J. L. Bredas, M. Loglund
and W. R. Salaneck, Nature, 1999, 397, 121.
4 Organic Light-Emitting Devices: Synthesis, Properties and
Applications, ed. K. Mullen and U. Scherf, Wiley-VCH,
Weinheim, 2006.
5 Y. Z. Lee, X. Chen, S. A. Chen, P. K. Wei and W. S. Fann, J. Am.
Chem. Soc., 2001, 123, 2296.
6 F. Garten, A. Hilberer, F. Cacialli, E. Esselink, Y. V. Dam,
B. Schlatmann, R. H. Friend, T. M. Klapwijk and
G. Hadziioannou, Adv. Mater., 1997, 9, 127.
7 J. S. Kim, R. H. Friend and F. Cacialli, Appl. Phys. Lett., 1999,
74, 3084.
8 Q. Huang, G. Evmenenko, P. Dutta, P. Lee, N. R. Armstrong
and T. J. Marks, J. Am. Chem. Soc., 2005, 127, 10227.
9 S. Kato, J. Am. Chem. Soc., 2005, 127, 11538.
10 D. Braun and A. J. Heeger, Appl. Phys. Lett., 1991, 58, 1982.
A. Pogantsch, S. Setayesh, G. Leising and K. Mullen, Synth.
Met., 2001, 125, 73; (c) A. Babel and S. A. Jenekhe,
Macromolecules, 2003, 36, 7759; (d) N. S. Cho,
D.-H. Hwang, B.-J. Jung, E. Lim, J. Lee and H.-K. Shim,
Macromolecules, 2004, 37, 5265.
29 (a) C.-S. Wu and Y. Chen, Macromolecules, 2009, 42, 3729; (b)
C.-S. Wu and Y. Chen, J. Mater. Chem., 2010, 20, 7700; (c)
C.-S. Wu and Y. Chen, J. Polym. Sci., Part A: Polym. Chem.,
2010, 48, 5727; (d) C.-S. Wu, S.-L. Lee and Y. Chen, J.
Polym. Sci., Part A: Polym. Chem., 2011, 49, 3099; (e)
C.-S. Wu and Y. Chen, J. Polym. Sci., Part A: Polym. Chem.,
2011, 49, 3928; (f) C.-S. Wu, Y.-J. Yang, S.-W. Fang and
Y. Chen, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 3875;
(g) C.-S. Wu, Y.-T. Lee and Y. Chen, Polym. Chem., 2012, 3,
2776; (h) C.-S. Wu, C.-T. Liu and Y. Chen, Polym. Chem.,
2012, 3, 3308; (i) C.-S. Wu, J.-W. Wu and Y. Chen, J. Mater.
Chem., 2012, 22, 23877; (j) C.-S. Wu, C.-P. Chen and
Y. Chen, J. Polym. Sci., Part A: Polym. Chem., 2013, 51, 3975;
(k) C.-S. Wu, S.-W. Fang and Y. Chen, Phys. Chem. Chem.
Phys., 2013, 15, 15121.
J. Mater. Chem. C
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Paper
Journal of Materials Chemistry C
30 T. Bernius, M. Inbasekaran, J. O'Brien and W. Wu, Adv. 33 L. Scholer, K. Seibel, K. Panczyk and M. Bohm, Microelectron.
Mater., 2000, 12, 1737. Eng., 2009, 86, 1502.
31 M. W. Lin, C. T. Wen, Y. J. Hsu and T. F. Guo, J. Mater. Chem., 34 K. Manabe, W. Hu, M. Matsumura and H. Naito, J. Appl.
2011, 21, 18840. Phys., 2003, 94, 2024.
32 T. H. Lee, J. C. A. Huang, T. F. Guo, T. C. Wen, Y. S. Huang, 35 R. Steyrleuthner, S. Bange and D. Neher, J. Appl. Phys., 2009,
C. C. Tsou, C. T. Chung, Y. C. Lin and Y. J. Hsu, Adv. Funct.
Mater., 2008, 18, 3036.
105, 064509.
36 L. S. Yu and S. A. Chen, Adv. Mater., 2004, 16, 744.
This journal is ª The Royal Society of Chemistry 2013
J. Mater. Chem. C