Dual-functional conjugated polymers based on trifluoren-2-yl-amine for RGB organic light-emitting diodes

Article abstract of DOI:10.1039/c1jm13533c

A series of blue-light-emitting conjugated polymers that possess high-lying HOMO energy levels were synthesized by introducing trifluoren-2-yl-amine (TFA) as a building block. The emission color could be effectively tuned in the region of deep-blue and light-blue by introducing various substituents onto the TFA as the pendants. Deep-blue light emission was achieved for the device based on P4FNCz as an emitting layer without using any hole-transport layer (HTL), giving a maximum current efficiency (CE) of 2.44 cd A^-1, corresponding to an external quantum efficiency of 3.00%, with Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.12). Thanks to the improved hole injection and transport ability, red- and green-light-emitting devices based on P4FNCz as an HTL were also successfully fabricated, giving a maximum CE of 7 and 25 cd A^-1 with CIE coordinates of (0.64, 0.34) and (0.29, 0.64), respectively, which are comparable to those of the devices based on a general HTL of 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB). The CIE coordinates of the RGB devices are very close to those of the National Television System Committee (NTSC) and High Definition Television (HDTV) standard colors. The RGB devices achieved by tuning the carrier recombination zone with different electron-transport layers will facilitate the development of the polymer-based full-color flat-panel displays in an alternative process.

Full text of DOI:10.1039/c1jm13533c

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PAPER
Dual-functional conjugated polymers based on trifluoren-2-yl-amine for RGB
organic light-emitting diodes†
*
Hua Ye, Baofeng Zhao, Ming Liu, Xiao Zhou, Yanhu Li, Dongyun Li, Shi-Jian Su, Wei Yang and Yong Cao
Received 25th July 2011, Accepted 30th August 2011
DOI: 10.1039/c1jm13533c
A series of blue-light-emitting conjugated polymers that possess high-lying HOMO energy levels were
synthesized by introducing trifluoren-2-yl-amine (TFA) as a building block. The emission color could
be effectively tuned in the region of deep-blue and light-blue by introducing various substituents onto
the TFA as the pendants. Deep-blue light emission was achieved for the device based on P4FNCz as an
emitting layer without using any hole-transport layer (HTL), giving a maximum current efficiency (CE)
of 2.44 cd Aꢀ1, corresponding to an external quantum efficiency of 3.00%, with Commission
Internationale de L’Eclairage (CIE) coordinates of (0.16, 0.12). Thanks to the improved hole injection
and transport ability, red- and green-light-emitting devices based on P4FNCz as an HTL were also
successfully fabricated, giving a maximum CE of 7 and 25 cd Aꢀ1 with CIE coordinates of (0.64, 0.34)
and (0.29, 0.64), respectively, which are comparable to those of the devices based on a general HTL of
4,40-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB). The CIE coordinates of the RGB devices are
very close to those of the National Television System Committee (NTSC) and High Definition
Television (HDTV) standard colors. The RGB devices achieved by tuning the carrier recombination
zone with different electron-transport layers will facilitate the development of the polymer-based full-
color flat-panel displays in an alternative process.
the large energy band-gaps of the fluorene-based blue-light
Introduction
emitters, it is also difficult to inject electrons from the cathode in
Organic light-emitting diodes (OLEDs) have great potential
blue OLEDs. To enhance the electron injection and transport,
applications in full-color flat-panel displays, back-lighting sour-
the introduction of electron-withdrawing components into the
ces for liquid-crystal displays, and next-generation solid-state
polymer has been proven to be effective.5 To achieve a reduced
lighting sources.1–3 To realize commercialization of OLEDs in
hole-injection barrier from the anode to the emitting layer
such applications, the development of high-efficiency and stable
(EML), poly(N-vinylcarbazole) (PVK) is generally utilized as
RGB emitters, especially for a blue one, remains a huge
a hole-transport layer (HTL) inserted between the anode and the
EML.5–7 Liu et al. realized highly efficient blue OLEDs with
challenge.
Fluorene-based polymers and oligomers have been recognized
Commission Internationale de L’Eclairage (CIE) coordinates of
as promising candidates for blue emitters due to their high
(0.16, 0.19) by using PVK as an HTL.5 With the same device
photoluminescence (PL) efficiency, good chemical and thermal
configuration, Wang et al. reported deep-blue OLEDs based on
stability, film-forming ability, and high electroluminescence (EL)
efficiency.4 However, the energy barrier for hole injection from
a fluorescent p-conjugated dendrimer G0 with CIE coordinates
of (0.155, 0.086).7 Although different solvents are utilized for
the indium-tin oxide (ITO) anode to the fluorene-based polymers
spin-coating the HTL and EML, respectively, a mix of the two
are quite high because of their low-lying highest occupied
layers at the interface is unavoidable since PVK is soluble in the
molecular orbital (HOMO) energy levels. In addition, because of
general organic solvents used for the EML. Moreover, the
operating voltage of the devices with PVK as an HTL is higher
State Key Laboratory of Luminescent Materials and Devices (South China
University of Technology) and Institute of Polymer Optoelectronic
Materials and Devices, South China University of Technology,
Guangzhou, 510640, China. E-mail: mssjsu@scut.edu.cn
than that of the devices without PVK, and this is a great disad-
vantage to achieving a high power conversion efficiency, a well-
known factor that is crucially important to power consumption.
Another effective route is to modify their chemical structures
in order to bring the HOMO energy levels closer to the work
function of ITO,8 and thus to enhance the hole injection into the
EML. Organic compounds containing triphenylamine (TPA)
have been widely used as an HTL in organic electroluminescent
† Electronic supplementary information (ESI) available: Detailed
synthetic procedures, elemental analysis, and 1H NMR spectroscopic
characterizations of the polymers, UV-vis absorption and PL spectra of
the polymers in dilute toluene solutions, cyclic voltammograms of the
polymers, electroluminescent characteristics of the single-layer devices.
See DOI: 10.1039/c1jm13533c
17454 | J. Mater. Chem., 2011, 21, 17454–17461
This journal is ª The Royal Society of Chemistry 2011

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devices,9 and most organic compounds containing TPA units
have high-lying HOMO energy levels (ca. ꢀ5.3 eV), which is very
close to the work function of ITO and thus allows efficient hole
injection.9c,10 To overcome the barrier for the hole injection of
polyfluorenes (PFs), an effective approach is to modify their
chemical structures through incorporating electron-donating
moieties, such as TPA11a and carbazole,11b into the polymer main
corresponding polymers are outlined in Scheme 1 and Scheme 2,
respectively. The four polymers of P4FN, P4FNCN, P4FNCz,
P4FNtBuCz were synthesized by the palladium(0)-catalyzed
Suzuki cross-coupling polycondensation of an equimolar
mixture of two monomers of TFA derivatives (11–14) and 16. In
comparison, considering the relatively weak catalytic activity of
Pd(PPh3)4, Pd(PPh3)4 was replaced by Pd(OAc)2 and tricyclo-
hexyl phosphine (PCy3) for the preparation of P4FNOXDPh.
The chemical structures of the polymers were verified by
1H NMR and elemental analysis.
€
chain. Mullen et al. reported a PF derivative by introducing
bulky TPA groups at the 9-position of fluorene as side chains.12a
End-capping the polymer chain with triarylamines12b or blocks
bearing triarylamine pendants12c is another alternative approach
to introduce hole-transport units into the emitting polymers. The
resultant polymers have the charge injection units distributed
only at the ends of the chain,12b where they are unable to affect
the aggregation tendencies. Although a few electroluminescent
TPA-based polymers have been developed, very strong and even
only excimer emission was observed for the devices utilizing them
as the emitters.12c,d It is still a challenge to develop triarylamine-
based polymers with improved hole injection and transport
property and no aggregation/excimer emission.
The developed TFA-based polymers are soluble in conven-
tional organic solvents, like toluene, chloroform, and tetrahy-
drofuran (THF). Molecular weights of the polymers were
determined by gel permeation chromatography (GPC) utilizing
THF as a solvent and polystyrene as a standard. As shown in
Table 1, the number-average molecular weights (Mn) of the
polymers are from 17.5 to 27.8 kDa with a polydispersity index
(PDI, weight-average molecular weight (Mw)/Mn) between 1.67
and 3.86. The C, H and N contents for the five polymers are close
to those calculated from the feed compositions.
Considering that the TPA derivatives usually exhibit
morphological instabilities due to the non-rigid TPA molecular
configuration,13 in this article, fluorene was chosen to replace
benzene of TPA to give trifluoren-2-yl-amine (TFA) because of
its ambipolar carrier transport property,14 high PL efficiency,
and amorphous film-forming ability.4 A series of blue-light-
emitting polymers based on a building block of TFA were
developed and thoroughly characterized. To the best of our
knowledge, this is the first report on conjugated polymers based
on a building block of TFA. As expected, similar to the TPA-
based polymers, HOMO energy levels of the TFA-based poly-
mers are higher-lying than those of the PFs to give a reduced hole
injection barrier. Due to the improved hole injection and trans-
port ability, efficient blue-light-emitting devices were achieved by
using the polymers as an EML without an HTL of PVK. In
addition, their emission colors can be effectively tuned in the
region of deep-blue and light-blue by introducing various
substituents onto the TFA as the pendants. More importantly,
red- and green-light-emitting devices were also successfully
fabricated by using P4FNCz as an HTL, which are comparable
to those based on a general HTL of 4,40-bis(N-(1-naphthyl)-N-
phenylamino)biphenyl (NPB). The RGB devices based on
a single polymer achieved by the utilization of different electron-
transport layers (ETL) for tuning the carrier recombination zone
will facilitate the development of the polymer-based full-color
flat-panel displays in an alternative process.
Thermal properties
The thermal properties of the five polymers were determined by
differential scan calorimetry (DSC) and thermogravimetric
analysis (TGA) under a nitrogen atmosphere. As shown in Table
1, all the polymers exhibit good thermal stability with 5% weight-
loss temperatures (Td) around 420 ꢁC due to the introduction of
the bulky TFA building blocks into the main chain, which impart
the chains with rigidity and non-coplanarity. With the change of
the substituents at the C-7 position of the pendant fluorene of
TFA, relatively high glass transition temperatures (Tg) ranging
from 60 to 90 ꢁC were achieved. For P4FNCz and P4FNtBuCz,
which possess the bulky pendant of carbazole, a Tg up to 90 ꢁC
ꢁ
was achieved, which is 30 C higher than that of P4FN without
such a bulky pendant. Owing to the longer pendant of phenyl
oxadiazole, the Tg of P4FNOXDPh increases to 78 ꢁC compared
with P4FN. Even for P4FNCN with the smallest substituent
ꢁ
group of cyano, its Tg is 12 C higher than that of P4FN. The
increment of Tg in comparison with P4FN can be attributed to
the different substituents at the pendant fluorene of TFA that
induce the change in rigidity of polymer chain, polarity of the
side chain, and thus the stronger molecular interaction.15a
Optical properties
In toluene solution, the polymers exhibit one distinct absorption
band at about 400 nm, which corresponds to the p–p* transition
of the conjugated polymer backbone (Fig. S1, see Supporting
Information†).16 As a reference of P4FN, its absorption band
peaks are at 404 nm. In comparison, the absorption bands for
P4FNCz and P4FNtBuCz are slightly narrower than that of
P4FN combined with a broad absorption band in the range of
300ꢂ350 nm, which can be attributed to the pendant carbazole.
By introducing electron-withdrawing cyano and phenyl oxadia-
zole as the pendants, bathochromic shifts of 4 and 10 nm were
observed for P4FNCN and P4FNOXDPh, respectively, due to
the intramolecular charge-transfer (ICT) interaction between
the electron-withdrawing cyano and oxadiazole and the
Results and discussion
Synthesis and chemical characterization
Take a consideration that the TFA unit is an electron donor.
Various substituents, such as electron acceptors of cyano and
oxadiazole groups and bipolar transporters of carbazole and
tert-butyl substituted carbazole were introduced onto the TFA as
the pendants to tune the HOMO and the lowest unoccupied
molecular orbital (LUMO) energy levels of the polymers and
thus to achieve a well-balanced carrier in the EML. The struc-
tures and synthetic routes of the TFA derivatives and the
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Scheme 1 Synthetic routes of the trifluoren-2-yl amine (TFA) derivatives. Reagents and conditions: i: n-C8H17Br, toluene, 50% NaOH, Bu4NBr, 60 ꢁC;
ii: hydrazine monohydrate, 5% Pd/C, EtOH, reflux; iii: iron, CHCl3, Br2, 0 ꢁC; iv: CuCN, DMF, reflux; v: carbazole, Cu, K2CO3, DMSO, 150 ꢁC; vi: 3,6-
di-tert-butyl-9H-carbazole, CuI, K2CO3, 18-crown-6, DMPU, 180 ꢁC; vii: 2,7-dibromo-9,9-dioctylfluorene, Pd2(dba)3, DPPF, t-BuONa, toluene,
110 ꢁC; viii: 1) NaN3, NH4Cl, anhydrous DMF, 100 ꢁC; 2) benzenecarbonyl chloride, anhydrous pyridine, reflux.
Table 1 Molecular weights and thermal properties of the polymers
electron-donating TFA, giving reduced energy band-gaps. For
the films, besides the absorption bands at ꢂ400 nm, another
intense band is observed at ꢂ220 nm (Fig. 1(a)), which can be
attributed to the n–p* transitions of fluorene. These absorption
bands are absent for the solutions since they are overlapped by
the absorption of toluene. Two specific bands of 313 and 332 nm
were observed for P4FNOXDPh, and they can be attributed to
the p–p* transitions of the phenyl oxadiazole pendant. For
P4FNCz and P4FNtBuCz, two absorption bands were observed
Polymer
Mn/kDa
Mw/kDa
PDI
Td/ꢁC
Tg/ꢁC
P4FN
P4FNCN
P4FNCz
P4FNtBuCz
P4FNOXDPh
23.2
17.5
26.3
20.9
27.8
59.0
32.2
43.8
37.5
107
2.55
1.84
1.67
1.80
3.86
415
421
416
423
422
60
72
89
90
78
Scheme 2 Synthetic route of the polymers based on trifluoren-2-yl amine (TFA). Reagents and conditions: Pd(PPh3)4 or Pd(OAc)2/PCy3, toluene,
Et4NOH, deionized water, 90ꢂ100 ꢁC.
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ferrocene/ferrocenium (Fc/Fc+) measured to the SCE reference
electrode under the same experimental conditions is ꢀ0.462 V.
Note that the redox potential of Fc/Fc+ has an absolute energy
level of ꢀ4.8 eV to vacuum,18b,c and the redox potential of Fc/Fc+
should be 0.4 V vs. SCE. As thus, the potentials were calibrated
to the Fc/Fc+ external standard by positively shifting in
0.862 V.19 The HOMO energy levels are calculated according to
the following equations:18a HOMO ¼ ꢀe (Eox + 4.4) (eV), where
Eox is the onset oxidation potential. The LUMO energy levels of
the polymers are estimated from the HOMO energy levels and
the optical energy band-gaps. The energy band-gaps of the
polymers are estimated from the absorption edges of the UV-vis
spectra of the thin films (Table 2). As anticipated, the HOMO
energy levels of all the TFA-based polymers are about ꢀ5.4 eV,
which is nearly 0.5 eV higher-lying compared to the PFs (HOMO
energy level is as low as ꢀ5.9 eV),20 thus rendering efficient hole-
injection from the anode. With introducing two tert-butyl groups
at the 3-, and 6-positions of the carbazole component in
P4FNCz, slightly reduced HOMO and LUMO energy levels
were achieved for P4FNtBuCz. As is well-known, the tert-butyl
group involves little electron affinity, and they do not participate
in the p conjugation of the TFA main chain or the carbazole
component. Additionally, according to the absorption edges of
the solid films, the energy band-gap of P4FNCz is estimated to be
2.80 eV, which is equal to that of P4FNtBuCz. Although p–p
stacking of the pendant carbazoles in P4FNtBuCz might be
reduced due to the bulky tert-butyl group, the lowest energy
absorption band according to the polymer backbone may be
hardly influenced to give similar energy band-gaps. The
decreased HOMO and LUMO energy levels of P4FNtBuCz can
be attributed to the tert-butyl groups that might break the p–p
stacking of carbazoles and fluorenes. Taking P4FN as a refer-
ence, slightly higher-lying HOMO and LUMO energy levels were
achieved for P4FNCz. It can be attributed to the introduction of
the electron-donor of carbazole at the pendant. In contrast,
slightly reduced HOMO and LUMO energy levels were achieved
for P4FNCN and P4FNOXDPh due to the introduction of the
electron-acceptors of cyano and oxadiazole at the pendant.
Fig. 1 Normalized UV-vis absorption (a) and PL (b) spectra of the
polymers in thin films.
at 295 and 300 nm, respectively, which can be attributed to the
p–p* transitions of the pendant carbazole.
Similar bathochromic shifts were also observed for their PL
spectra recorded in both toluene solution and thin film. For the
toluene solutions of P4FN, P4FNCz, and P4FNtBuCz, their PL
spectra peak at 442 nm with a shoulder at 470 nm (Fig. S2, see
Supporting Information†). In comparison, PL spectrum of
P4FNCN solution shifts to the longer wavelength and peaks at
446 nm, which can be attributed to the ICT interaction formed
between the electron-donor of TFA and the electron-acceptor of
cyano. The PL band of P4FNOXDPh solution shifts further to
the longer wavelength, which can also be explained by the strong
electron affinity of the oxadiazole component and the ICT
interaction between the electron-donating segments of TFA and
the electron-acceptor of the oxadiazole component. Moreover,
the oxadiazole unit does not interrupt the p conjugation of TFA
to the benzene unit,17 and P4FNOXDPh exhibits a red-shifted
PL spectrum due to the elongated p conjugation in the side chain
in comparison to the other polymers. As compared with the PL
spectra of the polymers in toluene solution, the PL spectra of the
polymers in thin films are slightly red-shifted (Fig. 1(b)), which
can probably be ascribed to the aggregation and dielectric envi-
ronment of the solid film.15
Electroluminescence
To evaluate their electroluminescence properties, double-layer
devices with a configuration of ITO/PEDOT:PSS (40 nm)/EML
(70ꢂ80 nm)/TPBI (30 nm)/CsF (1.5 nm)/Al were fabricated,
Table 2 Energy band-gaps and electrochemical properties of the
polymers
Energy
band-gapa/eV Eoxb/V HOMOc/eV LUMOd/eV
Polymer
P4FN
2.79
2.77
2.80
2.80
0.98
1.04
0.95
1.04
1.01
ꢀ5.38
ꢀ5.44
ꢀ5.35
ꢀ5.44
ꢀ5.41
ꢀ2.59
ꢀ2.67
ꢀ2.55
ꢀ2.64
ꢀ2.69
P4FNCN
P4FNCz
P4FNtBuCz
Electrochemical properties
The electrochemical properties of the polymers were investigated
by cyclic voltammetry (CV) with a platinum working electrode
and a platinum wire counter electrode at a scan rate of 50 mV sꢀ1
against a saturated calomel electrode (SCE) reference electrode
with an argon saturated anhydrous solution of 0.1 mol Lꢀ1
P4FNOXDPh 2.72
a
Estimated from the absorption edges of the polymer films. b Eox is the
onset oxidation potential that is calibrated to the Fc/Fc+ external
c
standard. Calculated according to HOMO
Calculated from the HOMO energy level and the energy band-gap.
¼
ꢀe (Eox
+ 4.4).
d
Bu4NPF6 in acetonitrile. The redox potential (E1/2
) of
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where the polymers were used as an EML and TPBI (2,20,200-
(1,3,5-benzenetriyl)-tris-(1-phenyl-1H-benzimidazole)) is used as
an electron-transport and hole-block layer to enhance the elec-
tron-injection/transport into the EML and to confine carrier
recombination in the EML.
but show much higher Von values of 4.8ꢂ5.8 V.5 Note that no
such HTL is used for the current devices. The reduced driving
voltage can be attributed to the introduction of TFA as
a building block of the polymers that gives a higher-lying HOMO
energy level and thus facilitates hole injection and transport from
the anode.
As shown in Fig. 2(a), the device based on P4FNCz exhibits
the highest current density. It can be attributed to its relatively
high-lying HOMO energy level that facilitates hole-injection
from the anode. The current density of the device based on
P4FNtBuCz is lower than that of the device based on P4FNCz.
Taking into consideration that the only structural difference
between these two polymers is that P4FNtBuCz possesses two
tert-butyl groups at the 3- and 6-positions of the pendant
carbazole component, the bulky tert-butyl groups may inhibit
the p–p stacking of carbazoles and fluorenes and thus induce
a reduced carrier transport property. A similar phenomenon was
also found for the single-layer devices without using TPBI
(Fig. S3, see Supporting Information†). As shown in Table 3,
turn-on voltages (Von) for electroluminescence (a luminance of
1 cd mꢀ2 was detected) of the double-layer devices are between
2.9 and 3.5 V, and further lower Von values of 2.8ꢂ3.3 V were
achieved for the single-layer devices. In comparison, some of the
previously reported blue OLEDs generally use PVK as an HTL
The current efficiency (CE) versus current density character-
istics of the double-layer devices are shown as Fig. 2(b). The
device based on P4FN exhibits a maximum CE of 1.08 cd Aꢀ1
,
which is the lowest one among all the double-layer devices. The
relatively low efficiency can be attributed to the low carrier
balance achieved in the device since P4FN is comprised of only
the electron-donor of TFA. Due to this reason, the single-layer
device based on P4FN exhibits a further lower maximum CE of
0.15 cd Aꢀ1, since there is no TPBI that acts as a hole-block layer
to give a further reduced carrier balance. Both the EL spectra of
the single-layer device and the double-layer device show peaks at
445 nm with a subsidiary peak at 471 nm, which is consistent with
its PL spectrum in film, indicating that both EL and PL emissions
mostly arise from the same excited state or the same type of
excitons.21 However, different from the double-layer device,
a long tail was found in the EL spectrum at wavelengths longer
than 500 nm for the single-layer device (Fig. S5, see Supporting
Information†), which is absent in the PL spectrum. Note that the
LUMO and HOMO energy levels of TPBI are ꢀ2.7 and ꢀ6.2 eV,
respectively.22 For the double-layer devices, hole extraction from
the EML to the cathode can be effectively impeded by such
a deep HOMO energy level, thus enhancing the recombination
probability of holes and electrons to form emissive excitons in the
EML.23 In contrast, it is anticipated that the carrier recombina-
tion zone in the single-layer device should be close to the cathode
because of the relatively high-lying HOMO energy level and hole
transport capacity of TFA. As thus, it is speculated that the long
tail found for the P4FN-based single-layer device should be
according to the excimer formed at the interface of P4FN and the
cathode. But compared with the previously reported TPA-based
polymer that shows only excimer emission,12d the excimer emis-
sion is significantly suppressed by solely introducing fluorene
instead of benzene as building blocks.
By introducing carbazole at the pendant fluorene of P4FN to
give P4FNCz, significantly improved efficiency was achieved to
give a maximum CE of 2.44 cd Aꢀ1, corresponding to an external
quantum efficiency (EQE) of 3.00% (Fig. 2(b)). Compared with
the best value previously reported for the polymer-based pure
blue OLEDs by using a single-layer device structure (CEmax
¼
4.57 cd Aꢀ1, EQEmax ¼ 4.54%),24 the efficiency is slightly lower,
but the current device exhibits a ꢂ1 V lower turn-on voltage.
Since carbazole is a well-known bipolar transporter, the intro-
duction of carbazole as the pendant may facilitate both the hole
and electron transport in the EML to give an improved current
density and reduced driving voltage. In addition, the carrier
balance was also improved to give an improved CE. A similar
phenomenon was also found for the corresponding single-layer
device. Aside from the improved efficiency, the improved elec-
tron transport also induces the carrier recombination zone apart
from the interface of the EML and cathode for the single-layer
device, restraining the formation of the excimer at the interface of
the EML and cathode to give an improved color purity with CIE
coordinates of (0.19, 0.20). A slightly lower CE was obtained for
Fig. 2 (a) Current density and luminance versus voltage and (b) current
efficiency versus current density characteristics of the double-layer devices
with the configuration of ITO/PEDOT:PSS (40 nm)/EML (70ꢂ80 nm)/
TPBI (30 nm)/CsF (1.5 nm)/Al. EML: P4FN (,), P4FNCN (>),
P4FNCz (B), P4FNtBuCz (O), and P4FNOXDPh (P).
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Table 3 Summary of the electroluminescent performance of the double-layer devices with the configuration of ITO/PEDOT:PSS (40 nm)/EML (70ꢂ80
nm)/TPBI (30 nm)/CsF (1.5 nm)/Al
EML
Vona/V
Lmax/cd mꢀ2
CEmax/cd Aꢀ1
EQEmax (%)
CIE (x, y)
P4FN
P4FNCN
P4FNCz
P4FNtBuCz
P4FNOXDPh
3.3
3.5
2.9
3.2
2.9
1462
2583
2092
1653
4824
1.08
2.10
2.44
1.70
4.10
1.15
1.76
3.00
1.53
2.80
(0.16, 0.12)
(0.19, 0.27)
(0.16, 0.12)
(0.18, 0.19)
(0.18, 0.30)
a
Turn-on voltage for electroluminescence (a luminance of 1 cd mꢀ2 was detected).
the double-layer device based on P4FNtBuCz in contrast to the
device based on P4FNCz, and the decreased efficiency can be
attributed to the reduced carrier transport property and thus the
carrier balance.
the pendants (Table 3). In addition, CIE coordinates of the deep-
blue devices are close to the National Television System
Committee (NTSC) and High Definition Television (HDTV)
standard blue with CIE coordinates of (0.16, 0.07) and (0.15,
0.06), respectively.
Compared with the device based on P4FN, improved effi-
ciency was also achieved for the device based on P4FNCN,
giving a maximum CE of 2.10 cd Aꢀ1, corresponding to an EQE
of 1.76%. The improved efficiency can be attributed to the
introduction of the electron-acceptor of cyano that gives the
polymer a lower-lying LUMO energy level and thus a lower
electron injection barrier. As thus, an improved carrier balance
was achieved to give an improved efficiency. Further higher
efficiency of 4.1 cd Aꢀ1 was achieved for the device based on
P4FNOXDPh, and it can be attributed to its relatively lower-
lying LUMO energy level (ꢀ2.69 eV) that is very close to that of
TPBI (ꢀ2.70 eV) to give a reduced electron injection barrier from
TPBI.22 The EL spectra of the double-layer devices based on the
P4FNCN and P4FNOXDPh have peaks at 471 and 474 nm
(Fig. 3), respectively, with smaller full-width at half-maximum
(FWHM) values compared with the EL spectra of the corre-
sponding single-layer devices. Similar to the PL spectra, bath-
ochromic shifts were also observed in the EL spectra compared
with the device based on P4FN, and it can be attributed to the
strong electron affinity of the electron-acceptors of cyano and
oxadiazole that induces an ICT interaction with the electron-
donor of TFA. Owing to the ICT interaction, the emission colors
could be effectively tuned in the region of deep-blue with CIE
coordinates of (0.16, 0.12) and light-blue with CIE coordinates of
(0.18, 0.30) by introducing various substituents onto the TFA as
Considering that the devices based on P4FNCz exhibit the
lowest driving voltage and the highest EQE among these poly-
mers in both single-layer and double-layer configurations, it is
anticipated that P4FNCz should possess a good hole injection
and transport ability to be used as an HTL for OLEDs, although
few fluorene-based polymers have been reported as an emitting
material as well as a hole-transport material to date. In addition,
different from the previously mentioned color-tuning method-
ology of molecular design, it might also be possible to tune the
emission color of the device by device engineering, the tuning of
the carrier recombination zone with the polymers as an HTL. To
validate this idea, the following two devices with configurations
of ITO/PEDOT:PSS (40 nm)/P4FNCz (50 nm)/Alq3:DCJTB (3
wt%) (30 nm)/Alq3 (20 nm)/LiF (1 nm)/Al and ITO/PEDOT:PSS
(40 nm)/P4FNCz (50 nm)/Alq3:C545T (1 wt%) (30 nm)/Alq3 (20
nm)/LiF (1 nm)/Al were fabricated, where P4FNCz is expected to
be utilized as an HTL. DCJTB is 4-(dicyanomethylene)-2-tert-
butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran,
a well-known red-light-emitting dye, and C545T is 10-(2-benzo-
thiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)
benzopyropyrano(6,7-8-I,j) quinolizin-11-one,
a well-known
green-light-emitting dye. In these configurations, tris(8-hydroxy-
quinoline)aluminium (Alq3) is used as the host for the dyes as
well as the ETL. Note that the HOMO energy level of Alq3 is
ꢀ5.70 eV, which is only 0.35 eV lower-lying than that of
P4FNCz. Different from the previously used TPBI, it is expected
that holes can be easily injected into the co-deposited layer of
Alq3 and dye from P4FNCz, and the carrier recombination in
the co-deposited layer will excite the dye to give its emission. For
comparison, a well-known hole-transport material of (4,40-bis(N-
(1-naphthyl)-N-phenylamino)-biphenyl) (NPB) was also used as
an HTL instead of P4FNCz to give two devices as the references.
As shown in Fig. 4(a), for the devices based on DCJTB, the
current density of the device based on P4FNCz is almost
consistent with that of the device based on NPB. For the devices
based on C545T, although the current density of the device based
on P4FNCz is somewhat lower than that of the device based on
NPB at voltages higher than 3 V, the same threshold voltage
(Vth) of 2.2 V for current was achieved for both the devices. It
indicates that holes can also be effectively injected from the
anode when P4FNCz is used as an HTL instead of NPB. As
shown in Fig. 4(b), for the devices based on C545T, a maximum
Fig. 3 EL spectra of the double-layer devices based on the polymers as
an EML in the configuration of ITO/PEDOT:PSS (40 nm)/EML (70ꢂ80
nm)/TPBI (30 nm)/CsF (1.5 nm)/Al.
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Fig. 5 EL spectra of the devices with the configurations of ITO/PEDOT:
PSS (40 nm)/P4FNCz (B) or NPB (,) (50 nm)/Alq3:DCJTB (3 wt%)
(30 nm)/Alq3 (20 nm)/LiF (1 nm)/Al and ITO/PEDOT:PSS (40 nm)/
P4FNCz (>) or NPB (O) (50 nm)/Alq3:C545T (1 wt%) (30 nm)/Alq3
(20 nm)/LiF (1 nm)/Al.
Conclusions
In summary, a series of blue-light-emitting polymers based on
a building block of TFA were synthesized via the palladium-
catalyzed Suzuki coupling polycondensation. Their emission
colors could be effectively regulated in the region of deep-blue
and light-blue by solely changing the substituents at the pendant
fluorene of TFA. Efficient hole injection from the anode to the
polymers was achieved even without any HTL due to their high-
lying HOMO energy level achieved by the introduction of the
electron-donor of TFA. Efficient deep-blue light emission was
observed for the device based on P4FNCz as an EML that shows
a maximum CE of 2.44 cd Aꢀ1, corresponding to an EQE of
3.00%, with CIE coordinates of (0.16, 0.12). Thanks to the
improved hole injection and transport, red- and green-light-
emitting devices were also successfully fabricated by using
P4FNCz as an HTL, giving maximum CE values of 7 and 25 cd
Aꢀ1, respectively, which are comparable to the ones based on
NPB as an HTL. Moreover, the CIE coordinates of the RGB
devices are very close to those of the NTSC and HDTV standard
colors, indicating that color-tuning achieved by the molecular
design as well as the tuning of the carrier recombination zone will
facilitate the development of the polymer-based full-color flat-
panel in an alternative process.
Fig. 4 Current density (closed symbols) and luminance (open symbols)
versus voltage (a) and current efficiency (CE) versus current density (b)
characteristics of devices with the configurations of ITO/PEDOT:PSS
(40 nm)/P4FNCz (B) or NPB (,) (50 nm)/Alq3:DCJTB (3 wt%) (30
nm)/Alq3 (20 nm)/LiF (1 nm)/Al and ITO/PEDOT:PSS (40 nm)/P4FNCz
(>) or NPB (O) (50 nm)/Alq3:C545T (1 wt%) (30 nm)/Alq3 (20 nm)/LiF
(1 nm)/Al.
CE of 25 cd Aꢀ1 was achieved for the device based on P4FNCz,
which is comparable to that of the device based on NPB and is
very close to the best value of the C545T-based devices previ-
ously reported (29.8 cd Aꢀ1).25 For the devices based on DCJTB,
a maximum CE of 7 cd Aꢀ1 was achieved for the device based on
P4FNCz, which is lower than that of the device based on NPB,
and it can be attributed to the reduced carrier balance in the
EML.
Experimental
EL spectra of the devices based on P4FNCz are quite similar to
those of the devices based on NPB, and no emission from
P4FNCz or Alq3 was detected (Fig. 5). It further indicates effi-
cient hole injection from P4FNCz to the EML for carrier
recombination and effective energy transfer from Alq3 to the
dyes. In addition, CIE coordinates of the devices based on C545T
and DCJTB are (0.29, 0.64) and (0.64, 0.34), respectively, which
are very close to the NTSC standard green (0.31, 0.60) and red
(0.63, 0.34) and HDTV standard green (0.30, 0.60) and red (0.64,
0.33). It proves that color-tuning can also be achieved by using
P4FNCz as an HTL through the tuning of the carrier recombi-
nation zone by utilizing an appropriate ETL. The current find-
ings will enable us to develop the polymer-based full-color flat-
panel in an alternative process.
Measurement and characterization
1H NMR and 13C NMR spectra were recorded on a Bruker AV-
300 (300 MHz) in deuterated chloroform (CDCl3) with tetra-
methylsilane as a reference. Mn and Mw were determined by
a Waters GPC 2410 in THF using a calibration curve with
standard polystyrene as a reference. Elemental analysis was
performed on a Vario EL elemental analysis instrument (Ele-
mentar Co.). DSC measurements were performed on a Netzsch
DSC 204 under N2 flow at a heating rate of 10 ꢁC minꢀ1 and
a cooling rate of 20 ꢁC minꢀ1. TGA measurements were per-
formed on a Netzsch TG 209 under N2 flow at a heating rate of
10 C minꢀ1. UV-vis absorption spectra were recorded on a HP
ꢁ
8453 spectrophotometer. PL spectra were measured using
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Acknowledgements
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The authors greatly appreciate the financial support from the
National Natural Science Foundation of China (Project No.
51073057), the Ministry of Science and Technology (Project No.
2009CB930604 and 2011AA03A110), and the Fundamental
Research Funds for the Central Universities (2011ZZ0002).
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