Diketopyrrolopyrrole-containing polyfluorenes: Facile method to tune emission color and improve electron affinity

Article abstract of DOI:10.1021/ma0615349

A series of novel diketopyrrolopyrrole (DPP)-containing polyfluorenes, coded as PF-DPP01-50, were synthesized through palladium-catalyzed Suzuki polycondensation with different feed ratios between fluorene dibromide and DPP dibromide. Their chemical structures and compositions were verified by ¹H NMR and elemental analysis. DSC and TGA results show that they have good to excellent thermal stabilities. Absorption and photoluminescence (PL) properties of PF-DPP01 -50, determined in both CHCl₃ solutions and thin films, exhibit regular changes with increasing of DPP contents in copolymers, with absolute PL efficiencies being in the range 13.8-26.9%. Electroluminescence (EL) properties of all the copolymers were investigated with device configurations of ITO/PEDOT/copolymer/Ba/Al and ITO/PEDOT/copolymer/Al. A very low content of DPP units (1 %) is needed to achieve full energy transfer from fluorene segments to DPP units; hence, exclusive emission of the latter occurs. EL colors of these copolymers vary from orange to red, corresponding to CIE coordinates from (0.52, 0.46) to (0.62, 0.37). The best performance was achieved by orange-emitting PF-DPP01 in device configuration of ITO/PEDOT/copolymer/Ba/Al, with maximum EQE of 0.45% and maximum brightness of 520 cd/m². Devices with configuration of ITO/PEDOT/copolymer/Al prove that DPP units can effectively improve the electron affinity of these copolymers. One of these devices (ITO/PEDOT/PF-DPP25/Al) can realize maximum EQE of 0.14% and maximum brightness of 127 cd/m². Therefore, color-tuning (red-shift) and improvement of electron affinity can be achieved at the same time through incorporation of DPP units.

Full text of DOI:10.1021/ma0615349

Macromolecules 2006, 39, 8347-8355
8347
Diketopyrrolopyrrole-Containing Polyfluorenes: Facile Method To
Tune Emission Color and Improve Electron Affinity
Derong Cao,*,†,‡ Qilin Liu,‡ Wenjing Zeng,§ Shaohu Han,§ Junbiao Peng,§ and
Shouping Liu
College of Chemistry, South China UniVersity of Technology, Guangzhou 510640, China;
LCLC, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China;
Institute of Polymer Optoelectronic Materials and DeVices, South China UniVersity of Technology,
Guangzhou 510640, China; and Singapore Eye Research Institute, Singapore National Eye Center,
168751 Singapore
ReceiVed July 7, 2006; ReVised Manuscript ReceiVed September 3, 2006
ABSTRACT: A series of novel diketopyrrolopyrrole (DPP)-containing polyfluorenes, coded as PF-DPP01-50,
were synthesized through palladium-catalyzed Suzuki polycondensation with different feed ratios between fluorene
dibromide and DPP dibromide. Their chemical structures and compositions were verified by 1H NMR and elemental
analysis. DSC and TGA results show that they have good to excellent thermal stabilities. Absorption and
photoluminescence (PL) properties of PF-DPP01-50, determined in both CHCl3 solutions and thin films, exhibit
regular changes with increasing of DPP contents in copolymers, with absolute PL efficiencies being in the range
13.8-26.9%. Electroluminescence (EL) properties of all the copolymers were investigated with device
configurations of ITO/PEDOT/copolymer/Ba/Al and ITO/PEDOT/copolymer/Al. A very low content of DPP
units (1%) is needed to achieve full energy transfer from fluorene segments to DPP units; hence, exclusive emission
of the latter occurs. EL colors of these copolymers vary from orange to red, corresponding to CIE coordinates
from (0.52, 0.46) to (0.62, 0.37). The best performance was achieved by orange-emitting PF-DPP01 in device
configuration of ITO/PEDOT/copolymer/Ba/Al, with maximum EQE of 0.45% and maximum brightness of 520
cd/m2. Devices with configuration of ITO/PEDOT/copolymer/Al prove that DPP units can effectively improve
the electron affinity of these copolymers. One of these devices (ITO/PEDOT/PF-DPP25/Al) can realize maximum
EQE of 0.14% and maximum brightness of 127 cd/m2. Therefore, color-tuning (red-shift) and improvement of
electron affinity can be achieved at the same time through incorporation of DPP units.
Introduction
white-emitting polymers are currently lack;6 (2) electron affinity
of polyfluorene should be enhanced to achieve charge carrier
balance, which is important for high efficiency and low driving
voltage of devices. Generally, there two approaches to these
issues: (1) blending and doping system or devices of multilayer
structure7 and (2) attaching functional groups to the ends or
side chains of polyfluorene or copolymerization with dedicated
comonomers. The first approach is based on the intermolecular
interactions, i.e., either energy transfer or charge transfer. To
successfully tune emission color to longer wavelength or im-
prove electron injection and transport, proper blending com-
ponents or dopants should be selected and the concentrations
usually need to be strictly controlled.8 As for multilayer devices,
the fabrication is somewhat complicated and difficult.9 Further-
more, the phase separation or interface contact problems limit
the application of this method. The second approach takes the
advantage of structural flexibility of polyfluorene and is based
on the intramolecular interactions, which is more efficient and
quicker than the intermolecular interaction and hence is greatly
favored. Many kinds of n-type structures have been utilized to
facilitate electron injection and transport,10 and various narrow-
band-gap monomers (or fluorescent dyes and pigments) have
been used for color tuning.11 However, the exploration of novel
classes is still needed for further improvements.
The initial report1 in 1990 on electroluminescence (EL) of
poly(p-phenylenevinylene) (PPV), sandwiched between indium
tin oxide (ITO) and aluminum electrodes, opened a totally novel
and highly interesting field for polymeric light-emitting diodes
(PLED). Compared with traditional inorganic semiconductors
and more recently explored organic molecular materials,
conjugated polymers combine the advantages of optoelectronic
properties of semiconductors and mechanical properties and
processability of polymers,2 which are well-suited for fabrication
of flexible large-area flat panel displays.
Enormous efforts have been put and considerable progress
has been obtained on electroluminescent conjugated polymers
ever since 1990.3 Polyfluorene derivatives are a promising
family of such polymers, as they possess high luminescence
efficiency, excellent thermal, oxidative, chemical and photo-
chemical stability, good solubility in common organic solvents,
and easy chemical modification of structures.4 Polyfluorene is
typically wide-band-gap and p-type material; hence, it emits
blue light, and the injection and transport of holes are more
efficient than electrons in PLEDs with polyfluorene as active
layer.5
Thereby, two main issues still remain or exist: (1) red and
green emission are needed for full color display, as sufficient
We are particularly interested in diketopyrrolopyrrole (DPP)
derivatives, which have pyrrolo[3,4-c]pyrrole-1,4-dione units as
underlying chromophore system12 and are commercially used
as brilliant, strongly fluorescent, so-called high performance13
for their potentials as building blocks for polyfluorene deriva-
tives to achieve red emission and high electron affinity, based
† College of Chemistry, South China University of Technology.
‡ Chinese Academy of Sciences.
§ Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology.
Singapore National Eye Center.
* Corresponding author: Fax (+86) 20 87110245; e-mail drcao@
scut.edu.cn.
10.1021/ma0615349 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/04/2006

8348 Cao et al.
Macromolecules, Vol. 39, No. 24, 2006
(ITO)-coated glass substrates were cleaned with acetone, detergent,
distilled water, and 2-propanol, subsequently in an ultrasonic bath.
After treatment with oxygen plasma, 50-60 nm of poly(3,4-
ethylenedioxythiophene) (PEDOT) doped with poly(styrenesulfonic
acid) (PSS) (Batron-P 4083, Bayer AG) was spin-coated onto the
ITO substrate followed by drying in a vacuum oven. A thin film
of PF-DPP01-50 was coated onto the anode by spin-casting inside
a drybox. The film thickness of the active layer was 70-80 nm, as
measured with an Alfa Step 500 surface profiler (Tencor). For LED
with Ba as cathodes, a thin layer of Ba (4-5 nm) and subsequently
150 nm layers of Al were vacuum-evaporated subsequently on the
top of an EL polymer layer under a vacuum of 1 × 10-4 Pa; for
LED with Al as cathodes, only 150 nm layers of Al were vacuum-
evaporated on the top of an EL polymer layer under a vacuum of
1 × 10-4 Pa.
on their red fluorescence and electron-withdrawing lactam units.
We have previously demonstrated that the different linkage of
DPP into main chains obviously affects optoelectronic properties
of DPP and fluorene alternating copolymers. Also, we have
found that DPP units endow these polymers with reversible
reduction processes and greatly lowered LUMO levels.14 Herein
we investigate the structure-property relationships of DPP-
fluorene copolymers with different DPP contents to get a full
understanding of the role of DPP units in conjugated polyfluo-
rene derivatives.
Experimental Section
1
Measurement and Characterization. H NMR spectra were
collected on a Bruker DRX 400 spectrometer in CDCl3 with
tetramethylsilane as inner reference. Number-average (Mn) and
weight-average (Mw) molecular weights were determined by a
Waters GPC 515-410 in tetrahydrofuran (THF) using a calibration
curve of polystyrene standards. Thermogravimetric analysis (TGA)
was conducted on a Pyris 1 TGA under a heating rate of 10 °C/
min and a N2 flow rate of 20 mL/min. Differential scanning
calorimetry (DSC) measurement was performed on a Diamond DSC
under N2 at a heating rate of 10 °C/min. Elemental analysis was
performed using a Vario EL III instrument. UV-vis absorption
spectra were recorded on a HP 4803 Instrument. PL and EL spectra
were recorded on an Instaspec IV CCD spectrophotometer (Oriel
Co.). The absolute PL quantum yields were determined in an
Integrating sphere IS080 (Labsphere) with 325 nm excitation of
He-Cd laser (Mells Griod) or with 405 nm excitation of solid-
state laser (Crystal Laser), as the percent of the total output photons
in all directions vs the total input photons. The current-luminance-
voltage (I-L-V) were measured using a Keithley 236 sorce
measurement unit and a calibrated silicon photodiode. The lumi-
nance was calibrated using a PR-705 SpectraScan spectrophotom-
eter (Photo research). The EL quantum efficiency (QE) was
collected by measuring the total light output in all directions in an
integrating sphere. It was determined as the percent of the total
output photons vs the total input electrons from each electrodes.
Materials. 9,9-Dihexylfluorene-2,7-bis(trimethylene boronate)
(1) and tetrakis(triphenylphosphine)palladium, Pd(PPh3)4, were
purchased from Lancaster and Aldrich, respectively, and were used
without any further purification; other chemicals were obtained from
commercially available resources. 2,5-Dioctyl-3,6-bis(4-bromophen-
yl)yrrolo[3,4-c]pyrrole-1,4-diones (2) was synthesized through two
steps of reactions as we described before.14 2-Bromo-9,9-di-n-octyl-
fluorene and 2,7-dibromo-9,9-di-n-octyl-fluorene (3) were synthe-
sized from fluorene as a starting material according to the published
literature.15
General Procedure of Polymerization.14 9,9-Dihexylfluorene-
2,7-bis(trimethylene boronate) (1), 2,5-dioctyl-3,6-bis(4-bromophen-
yl)pyrrolo[3,4-c]pyrrole-1,4-diones (2), 2,7-dibromo-9,9-di-n-octyl-
fluorene (3), K2CO3 (17 equiv), and Pd(PPh3)4 (2 mol %) were
dissolved in a mixture of THF and water (50/50, v/v) under N2.
Then the mixture was heated to 80 °C and stirred vigorously for
48 h. Additional 1 (6 mol %) was added, and the mixture was
refluxed under N2 for 6 h; subsequently, 2-bromo-9,9-di-n-
octylfluorene (6 mol %) was added, and the mixture was refluxed
for another 6 h. The reaction mixture was cooled, and the solid
was precipitated in methanol. The solid was filtered off, washed
with dilute HCl, dissolved in chloroform, and precipitated in
acetone. After being extracted with acetone in a Soxhlet apparatus
for 24 h, polymers were obtained as pale brown to dark red solid
with yields of 67-81%.
Results and Discussion
Synthesis and Characterizations. Palladium-catalyzed Su-
zuki polycodensation (SPC) was adopted for the synthesis of
copolymers to make sure that DPP units were isolated from
both sides by fluorene segments to form more regular arrange-
ments. Moreover, on the basis of the AA-BB approach of
SPC,17 monomers required are conveniently available. These
synthetic procedures used to prepare monomer and polymers
are outlined in Scheme 1. Synthetic details of monomer 2 were
described elsewhere,14 and the overall yield of the two-step-
reaction route was about 32%. The ratio of 1 to 2 and 3 was
always kept as (1):(2 + 3) ) 1:1, and comonomer feed ratios
of 2 to 3 were 0.2:99.8, 2:98, 10:90, 20:80, 30:70, 50:50, 70:
30, and 100:0, respectively. The corresponding copolymers were
named PF-DPP01, PF-DPP1, PF-DPP5, PF-DPP10, PF-DPP15,
PF-DPP25, PF-DPP35, and PF-DPP50, respectively. Their
chemical structures and compositions were verified by 1H NMR
and elemental analysis.
In an effort to optimize the conditions of SPC for this specific
system, several trials were carried out (Table 1) in the case of
PF-DPP50. The effect of inorganic bases was quite obvious as
molecular weights of polymers increased according to the
tendency of base strength: NaHCO3 < K3PO4‚3H2O e Na2-
CO3 < K2CO3. No stronger base was tried for the consideration
that the decomposition of monomer 2 may occur under such
condition and that the molecular weight achieved (item 6 in
Table 1) is quite enough for PLED application. Of course, other
factors such as solvent and time were also preliminarily
investigated, and a standard Suzuki protocol18 was also tried
(item 7 in Table 1). Other copolymers were prepared applying
the optimized condition (item 6 in Table 1) with high yields.
All the polymers were obtained as powder, with colors varying
from pale brown to orange, pink, red, and dark red as DPP
contents increased, and they are readily soluble in common
organic solvents such as CH2Cl2, CHCl3, and THF, but only
polymers with low DPP contents such as PF-DPP01 and PF-
DPP1 have good solubility in nonpolar solvents like toluene.
Molecular weights and polydispersity indices of them are listed
in Table 2. Mn of PF-DPP01-35 ranges from 7900 to10 700,
without large differences, indicating that reactivities of two
dibromides (2 and 3) are comparable. Since PF-DPP50 was
prepared with Pd(PPh3)4 freshly purchased, and other polymers
were synthesized after Pd(PPh3)4 had been stored under nitrogen
for a period of time, the much higher molecular weight of PF-
DPP50 is a direct evidence for decreased activity of Pd(PPh3)4,
which has been reported in the literature.17,19 However, all of
them possess no fewer than 10 repeating units; hence, typical
polymeric behavior can be ensured. The polydispersity index
(PDI) of PF-DPP35 is somewhat higher with the value of 4.65,
while other values are moderate lying in the region of 2.00-
The purpose of end-capping with additional 1 and 2-bromo-9,9-
di-n-octyl-fluorene, at the end of polymerization was to avoid
possible emission quenching and red-shifting of EL caused by boron
and bromides units.16
Light-Emitting Devices (LED) Fabrication and Characteriza-
tion. PF-DPP01-50 were dissolved in toluene-THF (or toluene)
and filtered through a 0.45 µm filter. Patterned indium tin oxide

Macromolecules, Vol. 39, No. 24, 2006
Diketopyrrolopyrrole-Containing Polyfluorenes 8349
Scheme 1
Table 1. Several Trials for the Optimization of Conditions of Polymerization (in the Case of PF-DPP50)
1
2
3
4
5a
6
7b
base
NaHCO3
THF/H2O
48
72
9700
K3PO4
THF
5
41
9900
3.48
K3PO4
THF
48
79
18000
2.69
Na2CO3
THF/H2O
48
68
18100
2.65
Na2CO3
THF/H2O
72
73
17500
2.38
K2CO3
THF/H2O
48
69
28700
2.00
Na2CO3
Tolc/H2O
48
70
17600
2.16
solvent
time (h)
yield (%)
Mn
PDI
2.46
a The amount of solvent was doubled. b Triethylbenzylamonium chloride was added as phase-transfer catalyst. c “Tol” stands for toluene.
3.16. The chemical structures of polymers were confirmed by
1H NMR, as shown in Figure 1. Most signals are broadened
and structureless, and only some specific peaks can be clearly
assigned; this condition is common in the case for conjugated
polymers.20 As expected, the intensity of signals corresponding
to protons of DPP moieties (protons b, g, and h), relative to
protons of fluorene moieties (protons f), increase gradually as
DPP contents in polymers increase, and only when DPP contents
are high enough (310 mol %) can these peaks be observed. The
actual compositions of present polymers are calculated from N
contents in polymers measured by elemental analysis, and related
data are summarized in Table 2. For polymers PF-DPP5-50,

8350 Cao et al.
Macromolecules, Vol. 39, No. 24, 2006
Table 2. Yields, Molecular Weights, N Contents, DPP Contents, and Thermal Properties of Polymers
polymers
yield (%)
Mn
PDI
N (%)a
DPP (%)b
Td (°C)c
Tg (°C)d
PF-DPP01
PF-DPP1
PF-DPP5
PF-DPP10
PF-DPP15
PF-DPP25
PF-DPP35
PF-DPP50
81
83
75
67
81
73
70
69
9700
10700
7900
9900
9500
10300
10300
28700
2.17
2.50
2.64
2.68
2.96
3.16
4.65
2.00
434
422
338
256
315
419
399
398
107
103
104
0.32
0.68
1.01
1.54
2.16
3.07
4.21
9.15
13.65
21.51
31.11
46.1
a Nitrogen contents in polymers found by elemental analysis. b Contents of DPP units calculated according to N%. c Based on 5% weight loss. d PF-
DPP01 exhibits a melting peak at around 175 °C.
Figure 1. 1H NMR spectra of polymers in CDCl3.
actual DPP contents are a bit lower than the feed ratios of
copolymerization. This is reasonable and rational as all the
polymers were end-capped with fluorene units (see Experimental
Section), which will inevitably add to the contents of fluorene
moieties in polymers to an extent that cannot be neglected,
especially when molecular weights are not so high for polymers
PF-DPP5-35. N contents of PF-DPP01 and PF-DPP1, with
theoretical values of 0.0078% and 0.078%, respectively, are too
low to be detected. But from their absorption and PL spectra
and verified DPP contents of PF-DPP5-50, there are adequate
reasons to believe that DPP contents of PF-DPP01 and PF-DPP1
are in good agreement with feed ratios.
Thermal properties of PF-DPP01-50 were evaluated by TGA
and DSC, and the results are shown in Table 2. TGA reveals
that onset decomposition temperatures (based on 5% weight loss,
Td) are in the range of 256-434 °C, indicative of good to
excellent thermal stability. As for DSC results, two thermally
induced phase transitions are observed for PF-DPP01: a glass
transition (Tg ) 107 °C) and a melting peak (Tm ) 175 °C).
PF-DPP1 and PF-DPP5 only exhibit glass transitions with Tg
of 103 and 104 °C, respectively; no obvious phase transitions
are found for other copolymers until 250 °C where slight
decompositions begin. This phenomenon represents an apparent
enhancement of thermal stability (with respect to thermally
induced phase transitions) and demonstrates satisfactorily how
the variation of DPP contents will affect the properties of
copolymers, as rigid, planar, and polar DPP structure will
effectively reduce segmental mobility.
Photophysical Properties. Absorption and photolumines-
cence properties of copolymers were investigated both in CHCl3
solutions and in spin-coated thin films. All characteristic data
of absorption and photoluminescence are summarized in Table
3, and spectra are shown in Figures 2-4.
Figure 2 displays UV-vis absorption spectra and photolu-
minescence spectra of copolymers in CHCl3, and related spectra
of monomer 2 are also included for comparison. For random
polymers PF-DPP01-35, the absorption spectra in CHCl3
solutions comprise two distinguished peaks, implying that the
electronic states of the two components in random copolymers
are not well mixed. Peaks centered at around 380 nm are
associated with absorption of fluorene segments,21 while those
located at around 500 nm are caused by DPP segments as their
intensity increases almost linearly with the increasing of DPP
contents in copolymers and their positions are close to the

Macromolecules, Vol. 39, No. 24, 2006
Diketopyrrolopyrrole-Containing Polyfluorenes 8351
Table 3. Photophyscial Properties of Polymers
solution
thin film
polymers
λmax (nm)
λ
pl (nm)
λ
max (nm)
λ
pl (nm)
fwhm (nm)a
η
PL (%)b
PF-DPP01
PF-DPP1
PF-DPP5
PF-DPP10
PF-DPP15
PF-DPP25
PF-DPP35
PF-DPP50
382
380
419,439,560
433, 567(603)
432, 568(609)
431, 568(608)
431, 569(608)
426, 569(610)
570(611)
384
383
421, 443, 565
421, 443, 571
417, 439, 579
417, 582
584
587
594
609
70
64
64
63
67
68
70
140
14.20
26.90
21.60
20.49
18.00
15.60
14.20
13.80
379, 501
379, 503
376, 503
365, 505
364, 506
330, 382, 512
383, 504
378, 505
376, 508
373, 509
362, 511
336, 383, 511
578(620)
b
a Fwhm of emission at longer wavelength. ηPL value of PF-DPP50 was measured under excitation of 325 nm; others were measured under excitation
of 405 nm.
Figure 3. Photoluminescent spectra of PF-DPP10 in CHCl3 solutions
with different concentrations.
being the dominant one,23 while emission at around 560 nm is
barely discernible. A distinct growth of 560 nm band to almost
the same intensity of 419-439 nm emission appears with PF-
DPP1, and the vibronic structures of emission in 419-439 nm
region are weakened. When it comes to PF-DPP5, the long-
wavelength (560 nm) emission is dominant and the short-
wavelength (420-440 nm) emission becomes completely
featureless. The relative intensity of the 419-439 nm band to
the 560 nm band decreases further when DPP contents reach
10, 15, and 25 mol %. The emission of fluorene segments
completely disappears with DPP contents equal to or higher
than 35 mol %. Given the low concentration of these solutions,
the evolution of PL spectra is considered to be mainly caused
by intrachain energy transfer,24 since narrow-band-gap DPP
units, isolated from both sides with wide-band-gap fluorene
units, can function as efficient traps for excited states.25 This
mechanism was further studied using CHCl3 solutions of PF-
DPP10 at different concentrations of 5 × 10-6-4 × 10-2 g/L.
As shown in Figure 3, even in a very dilute concentration of
5 × 10-6 g/L (low enough to neglect interchain interactions),
the PL spectrum of PF-DPP10 is dominated by emission of DPP
units. As the concentration decreases, the emission spectrum
contains more contribution from fluorene segments. This means
interchain interactions play a certain role in energy transfer from
flourene segments to DPP units, and energy transfer becomes
more incomplete with reduced concentration which leads to less
Figure 2. Absorption top and photoluminescent (bottom) spectra of
polymers in CHCl3 solutions.
absorption of monomer 2 (475 nm). The alternating copolymer
PF-DPP50 is slightly different from others for absorption peaks
in the range of 300-400 nm overlap to a continuous band
probably due to its unique alternating sequence, which can lead
to more efficient conjugation hence more well-mixed electronic
states. The gradual blue shift of short wavelength absorption
from 382 to 330 nm, together with a small red shift of long-
wavelength absorption from 501 to 512 nm, observed in these
polymers when DPP contents increase, can be attributed to
decrease of conjugation length of fluorene segments and more
efficient conjugation between DPP and fluorene as more DPP
units are inserted into fluorene units with the increasing of DPP
contents.22
interchain interactions. Through a rather wide range (5 × 10-6
-
4 × 10-2 g/L), the relative intensity of emission originated from
fluorene to emission originated from DPP units changes very
gradually. So it can be concluded that the effect of interchain
interactions is quite limited, and energy transfer or charge
transfer is largely accomplished via intrachain interactions. Also
it should be pointed out that PF-DPP10, with Mn of 9900, has
nearly 25 units, so on average there are no fewer than 2 DPP
units per chain, which can definitely facilitate very quick and
As illustrated in Figure 2 (bottom), PL spectra of copolymers
in CHCl3 solutions (4 × 10-2 g/L) also change regularly with
the variation of DPP contents. Photoluminescence of PF-DPP01,
similar to polyfluorene homopolymer, exhibits two well-resolved
peaks at 419 and 439 nm with the 0-0 transition at 419 nm

8352 Cao et al.
Macromolecules, Vol. 39, No. 24, 2006
Figure 5. Electroluminescent spectra of polymers in devices with the
configuration of ITO/PEDOT/copolymer/Ba/Al.
increases to 1 mol %, PL efficiency achieves a relatively high
value of 26.9%. Then, with further increasing of DPP contents,
PL efficiencies of copolymers fall gradually. Carbonyl groups
which facilitate intersystem crossing of singlet to triplet with
their n-π* transition,28 and possible intramolecular charge
transfer similar to those in scientific literature29 is responsible
for the loss of PL efficiency. The obvious enhancement of PL
efficiency from 14.2% of PF-DPP01 to 26.9% of PF-DPP1 is
probably due to more complete energy transfer originating from
bigger amount of DPP units, since the PL spectrum of
PF-DPP01 is dominated by blue emission while the PL spectrum
of PF-DPP1 is dominated by orange emission, and this variation
of degree to which energy transfer is accomplished will lead to
less nonradiative decay of excitons. A similar result was
obtained with silole-fluorene copolymers though no detailed
explanation was available.30
Figure 4. Absorption and photoluminescent spectra of polymers in
thin films.
efficient intrachain energy transfer or charge transfer. In
copolymers where DPP units are much fewer, maybe intrachain
interactions will be more important.
Figure 4 depicts absorption and photoluminescence spectra
of copolymers in thin films, which closely resemble spectra in
CHCl3 solutions. Some differences should be noticed: first, the
maximum emission of photoluminescence is somewhat red-
shifted for all copolymers, compared with those in CHCl3
solutions; second, the threshold of DPP contents for complete
quenching of emission from fluorene segments is lowered to
15 mol %; third, the PL spectrum of PF-DPP50 is greatly
broadened in thin films with full width at half-maximum (fwhm)
of 140 nm, in comparison with that in CHCl3 solution or with
those of PF-DPP01-35 in thin films. All these phenomena are
related to packing or aggregation of main chains in the solid
state arising from inherent rigid nature of backbones.26 The red
shift of PL spectra and the more efficient quenching of high-
energy emission attributed to this factor were also reported with
other systems.21,24,27 The reason for surprising broadening of
PL spectrum of PF-DPP50 in thin films is probably twofold:
on one hand, better conjugation and hence well-mixed electronic
states are achieved via alternating sequence as mentioned
previously; on the other hand, a more regular alternating
sequence, along with high polarity of DPP units, is favorable
for packing or aggregation of main chains.
Absolute PL efficiencies of all copolymers in thin films were
determined under excitation of 405 and 325 nm for random
copolymers PF-DPP01-35 and alternating copolymer PF-
DPP50, respectively, depending on their relative absorption
strength at 405 and 325 nm as revealed by UV-vis absorption
spectra in thin films. As listed in Table 3, a tiny incorporation
of DPP units 0.1 into polyfluorene main chain can dramatically
decrease PL efficiency to 14.8%, compared with PL efficiency
of polyfluorene homopolymer.21 However, when DPP content
Electroluminescence. The EL performances of all copoly-
mers were examined in the device configuration of ITO/PEDOT/
copolymer/Ba/Al and ITO/PEDOT/copolymer/Al, where PE-
DOT doped with poly(styrenesulfonic acid) (PSS) functions as
a buffer layer and a hole injection/transport layer.
Typical EL spectra of PLEDs constructed from copolymers
with the configuration of ITO/PEDOT/copolymer/Ba/Al are
presented in Figure 5. In contrast to PL spectra, EL emission
consists exclusively of orange or red emission peaked around
575-602 nm originating from DPP units for copolymers
containing DPP units equal to or more than 1 mol %. Even for
copolymer PF-DPP01, the emission of fluorene segments and
excimers31 or keto defects,32 responsible for the 400-500 nm
band, is much weaker than emission of DPP units at 575 nm.
This difference clearly suggests the difference in recombination
zone for photoexcitation and electric excitation. Long-wave-
length emission in PL spectra arises from energy transfer from
fluorene segments to DPP units, while the dominant long-
wavelength emission in EL spectra is dominantly produced
through charger trapping mechanism as the DPP units serve as
efficient electron traps and generated excitons are efficiently
confined at them.33 The trapping process is quick and efficient
enough to suppress radiative decay of excitons through other
channels; thus, emission from fluorene segments fully disappears
even when the DPP content is as low as 1 mol %. This
phenomenon has also been reported and discussed before.21 With
increasing DPP contents, EL peaks are gradually red-shifted
from 575 nm for PF-DPP01 to 602 nm for PF-DPP50, and this
change pattern coincides with the trend of the red shift of PL
peaks. All the EL spectra possess a fwhm of about 90 nm.

Macromolecules, Vol. 39, No. 24, 2006
Diketopyrrolopyrrole-Containing Polyfluorenes 8353
Table 4. Device Performances with the Configuration of ITO/PEDOT/Copolymer/Ba/Al
EL emission
EL fwhma
(nm)
CIE coordinate
turn-on
max EQE
(%)
max brightness
(cd/m2)
polymers
λ
max (nm)
(x,y)
voltage (V)b
PF-DPP01
PF-DPP1
PF-DPP5
PF-DPP10
PF-DPP15
PF-DPP25
PF-DPP35
PF-DPP50
575
579
580
584
587
593
596
602
98
88
91
85
84
91
88
92
(0.52, 0.46)
(0.54, 0.45)
(0.56, 0.44)
(0.57, 0.43)
(0.57, 0.42)
(0.60, 0.40)
(0.61, 0.39)
(0.62, 0.37)
9.6
13
7.5
20
10.5
10
10
0.45
0.10
0.24
0.018
0.048
0.19
0.13
0.13
520
305
570
18
50
195
124
153
8
a Full width at half-maximum. b Turn-on voltage corresponds to luminance of about 1.5 cd/cm2.
Table 5. EL Performances of Devices with the Configuration of
ITO/PEDOT/Copolymer/Al
turn-on
max EQE
(%)
max brightness
(cd/m2)
polymers
voltage (V)a
PF-DPP01
PF-DPP1
PF-DPP5
PF-DPP10
PF-DPP15
PF-DPP25
PF-DPP35
PF-DPP50
15
15
7
21
5.3
12.2
14
9.6
0.045
0.065
0.0095
0.0036
0.0015
0.14
72
159
12
3
5
127
75
18
0.10
0.021
a Turn-on voltage corresponds to luminance of about 1.5 cd/cm2.
which means they will not affect EL performances so obviously.
Also, more DPP units will largely improve electron affinity as
we have confirmed with the alternating copolymer PF-DPP50.14
All these factors contribute to the somewhat strange variation
of EL performances as a function of copolymer composition.
To investigate the expected enhancement of electron injection
arising from the lactam structures of DPP units, we fabricated
another kind of devices with high work function metal (Al) as
cathodes (ITO/PEDOT/copolymer/Al). Responding EL spectra
(not shown) are almost identical to those mentioned above. Other
parameters are summarized in Table 5. EL properties of
copolymers with medium DPP contents are still poor. We find
that turn-on voltages are only slightly increased when cathodes
have been replaced with Al. Furthermore, in comparison of
maximum EQE and maximum brightness with these devices
that possess configuration of ITO/PEDOT/copolymer/Ba/Al,
performances of devices constructed from PF-DPP01-15 with
the configuration of ITO/PEDOT/copolymer/Al are dramatically
decreased. However, for PF-DPP25 and PF-DPP35, their
performances change little. Figure 7 shows the current intensity-
voltage (J-V) and luminance-voltage (L-V) characteristics of
devices based on PF-DPP25 and PF-DPP35, with Al (labeled
as “Al” on the curves) and Ba (labeled as “Ba” on the curves)
as cathodes, respectively. From these results, we can conclude
that electron injection from an Al cathode is comparable with
that from a low work function Ba cathode, which indicates the
improvement of electron affinity by DPP units. It is worthy to
note that performances of devices using PF-DPP01 and PF-
DPP1 as active layers with Al as cathodes are comparable to
devices from polyfluorene homopolymers.36 Given the much
lower PL efficiencies, these values can also be taken as evidence
for the improved electron affinity. The reduced LUMO level
and reversible reduction process caused by DPP units, as
detected in alternating systems,14 is an important reason for the
improvement. But there may also exist Al-N interaction (the
formation of complex) on the copolymer-aluminum interface
due to the presence of electron-rich nitrogen atoms in DPP units.
This mechanism has been extensively studied, and relevant
phenomena have been observed with spectroscopy.37 This kind
of interaction can substantially reduce energy barrier for electron
Figure 6. CIE coordinates of standard red demanding by NTSC and
emission colors of polymers in devices with the configuration of ITO/
PEDOT/copolymer/Ba/Al (from bottom right to top left): a, standard
red; b-f, emission colors of polymers PF-DPP50-01.
Commission International de l’Eclairage (CIE) coordinates34 of
all these EL spectra vary from (0.52, 0.46) to (0.62, 0.37), as
summarized in Table 4. Figure 6 shows the CIE chromaticity
diagram with coordinates corresponding to the emission of all
copolymers and Standard Red demanded by National Television
System Committee (NTSC).35 The more DPP contents in
copolymers, the more saturated red can be achieved. Maximum
external quantum efficiency (EQE), maximum brightness, and
turn-on voltages of these devices are also listed in Table 4. The
best results were realized using PF-DPP01 as emissive materials,
with a maximum EQE of 0.45% and maximum brightness of
520 cd/m2. The EL performances of these copolymers changes
in a different pattern from PL efficiencies, implying a much
more complicated mechanism for electroluminescence than
photoluminescence.3c Copolymers exhibit good EL perfor-
mances with low DPP contents (PF-DPP01-5) and high DPP
contents (PF-DPP25-50), while EL performances of copoly-
mers with medium DPP contents (PF-DPP10-15) are very poor.
With respect to main chain structure, copolymers loading few
DPP units (PF-DPP01-5) resemble polyfluorene homopolymer
to a great degree, while copolymers loading much more DPP
units (PF-DPP25-50) are very similar to alternating copolymers.
The segments’ distribution of these two kinds of copolymers is
relatively regular, greatly differing from those most “random”
copolymers (PF-DPP10-15); thus, PF-DPP01-5 and PF-
DPP25-50 might be superior to PF-DPP10-15 in consideration
of conjugation that is responsible for carrier mobility, which is
critical for efficient EL.3c Moreover, PL efficiencies of these
copolymers do not change so dramatically (26.90%-13.80%),

8354 Cao et al.
Macromolecules, Vol. 39, No. 24, 2006
With all these results, it can be concluded that when
incorporating DPP derivatives into polyfluorene, a balance
among color purity, efficiency, and brightness should be
maintained by cautious selection of DPP contents. Moreover,
DPP units can enhance electron injection of copolymers from
cathodes and hence can realize efficient PLEDs with air-stable
high work function cathodes such as Al to elevate device
durability. More efforts will be paid to structure modification
of DPP units themselves to get saturated red emission with
relatively low contents so that high efficiency and brightness
can be ensured at the same time.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (20272059; 20572111)
and the Guangdong Natural Science Foundation of China
(04002263). The authors thank Dr. Candidate Chun Li in
Institute of Polymer Optoelectronic Materials and Devices,
South China University of Technology, for the fabrication and
characterization of some LED.
Figure 7. Brightness-voltage and current intensity-voltage curves
(inset) of PF-DPP25-35 with device configuration of ITO/PEDOT/
copolymer/Al and ITO/PEDOT/copolymer/Ba/Al, respectively.
injection.38 On the other hand, this kind of interaction can also
lead to disruption of conjugation along copolymer backbone,
thus resulting in significant falling of efficiency and brightness.39
Hence, the EL properties of alternating copolymer PF-DPP50,
where the most intensive and regular Al-N interactions can
occur, with Al as cathode are much inferior to PF-DPP25 and
PF-DPP35, while their EL properties in devices with the
structure of ITO/PEDOT/copolymer/Ba/Al are of the same
order. Also, electroluminescence of PF-DPP5-15, whose
conjugation may be already not good due to random segment
distribution as speculated from behaviors of ITO/PEDOT/
copolymer/Ba/Al devices, is further weakened by this interac-
tion. Similar results that can be attributed to Al-N interactions
have been reported previously in scientific literature.35a
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