Narrow band gap D-A copolymer of indacenodithiophene and diketopyrrolopyrrole with deep HOMO level: Synthesis and application in field-effect transistors and polymer solar cells

Article abstract of DOI:10.1002/pola.25042

A novel fused ladder alternating D-A copolymer, PIDT-DPP, with alkyl substituted indacenodithiophene (IDT) as donor unit and diketopyrrolopyrrole (DPP) as acceptor unit, was designed and synthesized by Pd-catalyzed Stille-coupling method. The copolymer sh

Full text of DOI:10.1002/pola.25042

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Narrow Band Gap D–A Copolymer of Indacenodithiophene and
Diketopyrrolopyrrole with Deep HOMO Level: Synthesis and Application
in Field-Effect Transistors and Polymer Solar Cells
Xiaochen Wang,1 Hao Luo,2 Yeping Sun,1,2 Maojie Zhang,2 Xiaoyu Li,1 Gui Yu,2 Yunqi Liu,2
Yongfang Li,2 Haiqiao Wang1
1Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, College of Materials Science and Engineering,
Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China
2Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, People’s Republic of China
Correspondence to: H. Wang (E-mail: wanghaiqiao@mail.buct.edu.cn), X. Li (E-mail: lixy@mail.buct.edu.cn), Y. Li (E-mail: liyf@
iccas.ac.cn), or G. Yu (E-mail: yug@iccas.ac.cn)
Received 24 August 2011; accepted 3 October 2011; published online 31 October 2011
DOI: 10.1002/pola.25042
ABSTRACT: A novel fused ladder alternating D–A copolymer,
PIDT–DPP, with alkyl substituted indacenodithiophene (IDT) as
donor unit and diketopyrrolopyrrole (DPP) as acceptor unit, was
designed and synthesized by Pd-catalyzed Stille-coupling
method. The copolymer showed good solubility and film-form-
ing ability combining with good thermal stability. PIDT–DPP
exhibited a broad absorption band from 350 to 900 nm with an
absorption peak centered at 735 nm. The optical band gap deter-
mined from the onset of absorption of the polymer film was 1.37
eV. The highest occupied molecular orbital level of the polymer
is as deep as ꢀ5.32 eV. The solution-processed organic field-
effect transistor (OFETs) was fabricated with bottom gate/top
contact geometry. The highest FET hole mobility of PIDT–DPP
reached 0.065 cm2 Vꢀ1 sꢀ1 with an on/off ratio of 4.6 ꢁ 105. This
mobility is one of the highest values for narrow band gap conju-
gated polymers. The power conversion efficiency of the polymer
solar cell based on the polymer as donor was 1.76% with a high
open circuit voltage of 0.88 V. To the best of our knowledge, this
is the first report on the photovoltaic properties of alkyl substi-
C
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tuted IDT-based polymers. 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 50: 371–377, 2012
KEYWORDS: conjugated polymers; diketopyrrolopyrrole; indaceno-
dithiophene; organic field-effect transistors; polymer solar cells
INTRODUCTION During the last decade, conjugated polymer
materials have been the focus of both scientific research and
industry, because of their excellent electronic and optoelec-
tronic properties. Up to now, conjugated polymers have been
successfully used in many optoelectronic devices, such as or-
ganic light emitting diodes, organic field-effect transistors
(OFETs), polymer solar cells (PSCs), and so on.1–10 Recently,
indacenodithiophene (IDT) containing polymers have
attracted much attention because the fused ring unit in IDT
can enhance both the coplanarity of the molecular backbones
and the p–p stacking in the solid state of polymers and con-
sequently qualify these kinds of polymers as particularly
promising candidates for applications in PSCs and OFETs.
The copolymers based on tetraalkyl substituted IDT unit
polymers are much lower (10ꢀ9–10ꢀ3 cm2 Vꢀ1 ꢀ1) than
s
that of alkyl substituted IDT-based polymers. This is because
the phenyl rings in the IDT unit could induce steric hin-
drance to interrupt coplanarity of polymer backbones and
intermolecular stacking between polymer chains. From this
point of view, tetraalkyl substituted IDT-based copolymers
would facilitate high performance of PSCs and OFETs.
Despite a large number of reports on copolymers based on
tetraphenyl substituted IDT, there have been very few studies
on the syntheses of the alkyl substituted IDT copolymers and
the applications in the PSCs and OFETs. Here, we designed
and synthesized a novel fused ladder alternating D–A copoly-
mer, PIDT–DPP, with alkyl substituted IDT as donor unit and
diketopyrrolopyrrole (DPP) as acceptor unit. The structure of
PIDT–DPP is shown in Scheme 1. In PIDT–DPP, the IDT unit
was used to enhance both the coplanarity of the molecular
backbone and the p–p stacking in the solid state of the copol-
ymer, whereas the electron deficient nature of DPP unit was
showed hole mobilities as high as 1 cm2 Vꢀ1 sꢀ1 11
Some of
.
the tetraphenyl substituted IDT-based polymers were also
investigated in PSCs. Although the PSCs based on tetraphenyl
substituted IDT polymers exhibited power conversion effi-
ciency (PCE) of 2.0–6.41%,12–15 the hole mobility of these
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2011 Wiley Periodicals, Inc.
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model UV-3150. Absorption spectra measurements of the
polymer solutions were performed in chloroform (analytical
reagent) at 25 ꢂC. Absorption spectra measurements of the
polymer films were performed on the quartz plates with the
polymer films spin-coated from theꢂ polymer solutions in
chloroform (analytical reagent) at 25 C. The electrochemical
cyclic voltammetry was conducted on a Zahner IM6e electro-
chemical workstation with a Pt plate, Pt wire, and Ag/Agþ
electrode as the working electrode, counter electrode, and
reference electrode, respectively, in a 0.1 mol/L tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) acetonitrile so-
lution. Polymer thin films were formed by drop casting 1.0
mm3 of polymer solutions in THF (analytical reagent, 1 mg/
mL) on the working electrode and then drying in air.
SCHEME 1 Molecular structure of the PIDT–DPP.
Fabrication of Field-Effect Transistor Devices
used to construct donor–acceptor structure with narrow
band gap, which would facilitate high performance of PSCs or
OFETs.16–19 In addition, the well-defined alternating D–A
structure of polymer combined with the linear alkyl side
chains is expected to optimize the molecular packing in the
solid state. In this article, OFETs and PSCs based on the poly-
mer were fabricated and characterized. The highest FET hole
mobility of PIDT–DPP reached 0.065 cm2 Vꢀ1 sꢀ1 with an
on/off ratio of 4.6 ꢁ 105. The PCE of the PSCs was 1.76%,
under the illumination of AM 1.5 and 100 mW/cm2, with a
broad photoresponse spectrum covering from 300 to 900
nm. To the best of our knowledge, this is the first report on
the photovoltaic properties of alkyl substituted IDT-based
polymer.
Thin-film OFETs were fabricated on highly doped silicon sub-
strates with thermally grown 300-nm-thick silicon oxide
(SiO2) insulating layer, where the substrate served as a com-
mon gate electrode. Before polymer semiconductor deposition,
the substrates were treated with the silylating agent octyltri-
chlorosilane (OTS). Thin semiconductor films were then de-
posited by spin-coating the polymer solutions in CHCl3 on the
substrates. The film thickness was measured by an XP-2 sur-
face profilometer (Ambios Technology). The samples were
ꢂ
then dried and annealed at 80–100 C under nitrogen. Source
and drain gold electrodes of transistor were vacuum deposited
on the polymer layer to form top-contact geometry. Then, the
Au electrode was deposited on the polymer layer by vacuum
evaporation under 7 ꢁ 10ꢀ4 Pa. The electrical characterization
of the transistor devices was performed using a Keithley 4200
semiconductor parameter analyzer.
EXPERIMENTAL
Fabrication of Photovoltaic Devices
Materials
PSCs were fabricated with indium tin oxide (ITO) glass as a
positive electrode, Ca/Al as a negative electrode, and the blend
film of the polymer/PCBM between them as a photosensitive
layer. The ITO glass was precleaned and modified by a thin
layer of poly(3,4-ethylene dioxythiophene):poly(styrene sulfo-
nate) (PEDOT:PSS), which was spin-cast from a PEDOT:PSS
aqueous solution (Clevious P VP AI 4083 H. C. Stark, Germany)
on the ITO substrate and the thickness of the PEDOT:PSS layer
was about 60 nm. The photosensitive layer was prepared by
spin-coating a blend solution of the polymer and PC70BM with
a weight ratio of 1:1 in o-dichlorobenzene at 1500 rpm on the
ITO/PEDOT:PSS electrode. Then, the Ca/Al cathode was de-
posited on the polymer layer by vacuum evaporation under 3
ꢁ 10ꢀ4 Pa. The thickness of the photosensitive layer was
about 60 nm, measured on an Ambios Tech XP-2 profilometer.
The effective area of one cell was about 4 mm2. The current–
voltage (I vs. V) measurement of the devices was conducted
on a computer controlled Keithley 236 source measure unit. A
xenon lamp with an AM 1.5 filter was used as the white-light
source and the optical power at the sample was 100 mW/cm2.
3,6-Bis(5-bromothien-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]
pyrrole-1,4(2H,5H)-dione, (DPP), was synthesized as reported
in the literature.20 Tetrahydrofuran (THF) was dried over
Na/benzophenone ketyl and freshly distilled before use. Other
reagents and solvents were commercial grade and used as
received without further purification. All reactions were per-
formed under nitrogen atmosphere.
Measurements and Characterization
The molecular weight of the polymer was measured by gel
permeation chromatography (GPC) method. The GPC meas-
urements were performed on Waters 515-2410 with poly-
styrenes as reference standard and THF as an eluent. All
new compounds were characterized by nuclear magnetic res-
onance (NMR) spectra. The NMRs were recorded on a
Bruker AV 600 spectrometer in CDCl3 or DMSO at room tem-
perature. Chemical shifts of 1H NMR were reported in ppm.
Splitting patterns were designated as s (singlet), t (triplet), d
(doublet), m (multiplet), and br (broaden). Elemental analy-
ses were performed on a Flash EA 1112 analyzer or Elemen-
tar vario EL III. Thermal gravimetric analysis (TGA, Netzsch
TG209C) and the differential scanning calorimetry (DSC, TA-
Q100) measurements were performed under a nitrogen
atmosphere at a heating rate of 10 ꢂC/min. UV–Vis absorp-
tion spectra were recorded on a Shimadzu spectrometer
Synthesis of the Monomers
2,5-Dithien-2-yl-terephthalic Acid Diethyl Ester (1)
To a mixture of diethyl 2,5-dibromoterephthalate (16.7 g, 44
mmol), 2-thiophenylboric acid (12.8 g, 100 mmol) in 300 mL
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THF, NaHCO3 (21 g, 250 mmol) and 100 mL H2O were
added. The solution was flushed with nitrogen for 10 min
and then Pd(PPh3)4 (0.6 g, 0.5 mmol) was added. After
another flushing with nitrogen for 20 min, the reactant was
heated to reflux for 48 h. After cooled to room temperature,
the reaction mixture was poured into saturated NH4Cl solu-
tion. The product was extracted with ethyl acetate (3 ꢁ 150
mL). The extracts were combined and washed with water
and brine then dried over Na2SO4. After filtration, the solvent
was removed under reduced pressure. The residue was puri-
fied by column chromatography on silica, eluting with petro-
leum ether/ethyl acetate (from 20:0 to 20:1), to give 2,5-
dithien-2-ylterephthalic acid diethyl ester as white crystal
4,9-Dihydro-4,4,9,9-tetrahexadecyl-s-indaceno
[1,2-b:5,6-b0]-dithiophene (6)
KOH (6.72 g, 0.12 mol) and KI (0.5 g, 3 mmol) were added
to a suspension of compound 5 (3.19 g, 12 mmol) in anhy-
drous DMSO (100 mL). The reaction mixture was stirred for
1 h, followed by the dropwise addition of 1-bromododecane
(18.5 g, 74 mmol). Then, the mixture was heated at 80 ꢂC
over night. CHCl3 (200 mL) was added when the mixture
ꢂ
cooled to about 50 C. The mixture was stirred for another 1
h and then poured into cold water. The CHCl3 phase was
separated and concentrated. The residue was purified by col-
umn chromatography on silica, eluting with petroleum ether,
1
to give a light yellow crystal (9 g, 80%). H NMR (600 MHz,
1
(15 g, 88%). H NMR (600 MHz, CDCl3, ppm): d 7.82 (s, 2H,
ArAH), 7.41 (dd, 2H, ArAH), 7.08–7.10 (m, 4H, ArAH), 4.22
(q, 4H, CH2), 1.16(t, 6H, CH3).
CDCl3, ppm) d 7.27 (s, 2H, ArAH), 7.25 (d, J ¼ 4.8 Hz, 2H,
ArAH), 6.96 (d, 2H, ArAH), 1.97–1.96 (m, 4H, CH2), 1.85–
1.84 (m, 4H, CH2), 1.28 (m, 8H, CH2), 1.25–1.13 (m, 48H,
CH2 ), 1.09(m, 24H, CH2), 0.87 (m, 12H, CH3).
2,5-Dithien-2-yl-terephthalic acid (2)
2,7-Bis(trimethyltin)-4,9-dihydro-4,4,9,9-
To a solution of compound 1 (13.5 g, 35 mmol) in ethanol
(500 mL), a solution of sodium hydroxide (16 g NaOH in
120 mL water) was added. This mixture was heated at reflux
over night. After cooled to room temperature, the solvent
was removed under reduced pressure. Then, the residue was
added to concentrated hydrochloric acid. The precipitate was
collected by filtration and washed with water then dried in
vacuo to afford product as off-white solid (9.1 g, 79%). 1H
NMR (600 MHz, DMSO-d6, ppm): d 13.43 (s, 2H, COOH), 7.69
(s, 2H, ArAH), 7.67 (dd, 2H, ArAH), 7.26 (m, 2H, ArAH),
7.15 (m, 2H, ArAH).
tetradodecyl-s-indaceno[1,2-b:5,6-b0]-dithiophene
Compound 6 (2.83 g, 3 mmol) was dissolved in dry THF
(100 mL). The solution was cooled to ꢀ78 ꢂC and butyl-
lithium (2.5 M, 3 mL, 7.5 mmol) was added dropwise over
10 min. The reaction mixture was stirred at this temperature
for 1 h. Trimethyltin chloride (1 M in hexanes, 9.0 mL, 9.0
mmol) was added dropwise. The reaction mixture was
allowed to warm to room temperature and stirred overnight.
Water was added, and the reaction mixture was extracted
with diethyl ether. The organic layer was washed with water
and dried over magnesium sulfate. Evaporation of the sol-
vent afforded the bis(trimethyltin) monomer as a light
4,9-Dihydro-s-indaceno[1,2-b:5,6-b0]-dithiophene-
4,9-dione (4)
1
brownish viscous oil (3.5 g, 92%). H NMR (400 MHz, CDCl3,
To a suspension of compound 2 (6.6 g, 20 mmol) in anhy-
drous DCM (200 mL, containing 1 mL DMF), a solution of ox-
alyl chloride (10.2 g, 80 mmol) in 100 mL DCM was added
dropwise at room temperature. The mixture was stirred
overnight. The solvent was removed under reduced pressure
to afford crude acid dichloride as a yellow solid. The residue
was redissolved in anhydrous DCM (150 mL) and then
added to a suspension of anhydrous AlCl3 (13 g) in DCM
(300 mL) at 0 ꢂC. The resultant mixture was allowed to
warm to room temperature and stirred overnight and then
poured into cold 2 M hydrochloric acid. The precipitate was
collected by filtration and washed with diluted hydrochloric
acid, water, and acetone and then dried in vacuo to afford a
deep blue solid (5.17 g, 86%). MS(m/z): 294 (Mþ, 100%),
281, 266, 207, 193; IR (KBr, thin film, cmꢀ1) 1706 (C¼¼O).
4,9-Dihydro-s-indaceno[1,2-b:5,6-b0]-dithiophene (5)
A mixture of compound 4 (7.35 g, 25 mmol), hydrazine
monohydrate (25 g, 0.5 mol) and KOH (28 g, 0.5 mol) in
diethylene glycol (250 mL) was heated at 180 ꢂC for 24 h
and then poured into cold 2 M hydrochloric acid. The precip-
itate was collected by filtration and washed with water and
acetone, and dried in vacuo to give 4,9-dihydro-s-inda-
ceno[1,2-b:5,6-b0]-dithiophene as a brown solid (5.2 g, 78%).
1H NMR (600 MHz, DMSO-d6, ppm): d 7.71 (s, 2H, ArAH),
7.54 (d, J ¼ 4.8 Hz, 2H, ArAH), 7.20 (d, J ¼ 4.8 Hz, 2H,
ArAH), 3.78 (s, 4H, CH2).
ppm) d 7.17 (s, 2H, ArAH), 6.97 (s, 2H, ArAH), 1.91–1.95
(m, 4H, CH2), 1.81–1.84 (m, 4H, CH2), 1.00–1.53 (m, 88H,
CH2), 0.84–0.87 (m, 30H, CH2 and CH3).
Synthesis of the Polymer
IDT monomer (259 mg, 0.2 mmol) and DPP monomer (136
mg, 0.2 mmol) were put into a 25-mL two-neck flask and
then 6 mL of chlorobenzene was added. The mixture was
stirred and purged with argon for 10 min and then
Pd2(dba)3 (10 mg, 0.01 mmol) and (o-tol)3P (25 mg, 0.08
mmol) were added. After being purged for 15 min, the mix-
ture was heated at 140 ꢂC for 72 h. After cooled to room
temperature, the reaction mixture was added dropwise to
200 mL acidic methanol (HCl þ CH3OH) and then collected
by filtration and washed with methanol. The black solid
was filtered into a Soxhlet funnel and extracted by metha-
nol, ether, and chloroform successively. The polymer recov-
ered from chloroform was purified by preparative GPC.
Then, the product was dried under vacuum for 1 day to
recover the target polymer PIDT–DPP as a black solid.
(Yield 76%, Mn ¼ 20.8 kDa, Mw ¼ 50.0 kDa, PDI ¼ 2.4.) 1H
NMR (600 MHz, CDCl3, ppm) 8.97 (m, 2H), 7.45 (br, 2H),
7.36 (br, 2H), 7.22 (m, 2H), 4.10 (br, 4H), 2.00 (br, 4H),
1.91 (br, 4H), 1.54 (br, 8H), 1.43 (br, 8H), 1.33–1.13 (br,
50H), 0.96–0.92 (br, 32H), 0.84 (br, 24H). Anal. Calcd for
(C94H142N2O2S4)n: C, 77.24; H, 9.72; N, 1.92. Found: C,
77.31; H, 9.73; N, 2.00.
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SCHEME 2 Synthetic routes of the monomers. (i) 2-Thiophenylboric acid, Pd(PPh3)4, THF, NaHCO3, H2O, reflux; (ii) NaOH, H2O,
reflux and then concentrated hydrochloric acid; (iii) oxalyl chloride, DCM, DMF, room temperature; (iv) AlCl3, DCM, 0 ꢂC to room
temperature; (v) hydrazine monohydrate, KOH, diethylene glycol, 180 ꢂC; (vi) bromododecane, KOH, KI, DMSO, 80 ꢂC; and (vii) n-
BuLi, THF, ꢀ78 ꢂC to room temperature and then trimethyltin chloride, ꢀ78 ꢂC to room temperature.
RESULTS AND DISCUSSION
benzene, even in cold chloroform. It can be readily processed
to form smooth and pinhole-free films on spin-coating.
Synthesis of Monomers and Polymer
The synthetic routes of monomers and corresponding polymer
are outlined in Schemes 2 and 3, respectively. The synthesis of
the polymer was performed using palladium-catalyzed Stille-
coupling21 between monomer IDT and DPP. All starting mate-
rials, reagents, and solvents were carefully purified, and all
procedures were performed under an air-free environment.
The structure of the polymer was confirmed by 1H NMR spec-
troscopy and elemental analysis. PIDT–DPP has good solubility
in common organic solvents such as THF, toluene, and chloro-
Thermal Analysis
The thermal properties of the polymer were determined by
TGA and DSC under nitrogen. PIDT–DPP has good thermal
stability with onset decomposition temperatures with 5%
weight loss at 415 ꢂC, as shown in Figure 1. When investi-
gating the thermal behavior of the polymer using DSC, we
observed no clear thermal transitions in the temperature
ꢂ
range from 0 to 300 C.
SCHEME 3 Synthetic route of the copolymer. (i)Pd2(dba)3, P(o-Tol)3, PhCl, 140 ꢂC, 72 h.
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FIGURE 1 TGA plot of the polymer with a heating rate of 10 ꢂC/
FIGURE 3 Cyclic voltammogram of the polymer film on Pt
electrode in 0.1 mol/L Bu4NPF6, CH3CN solution with a scan
rate of 100 mV/s.
min under an inert atmosphere.
Optical Properties
the lowest unoccupied molecular orbital (LUMO) energy lev-
els can be readily estimated, which correspond to ionization
potential and electron affinity, respectively.2
Figure 2 shows the absorption spectra of the polymer in
chloroform solution and solid film on a quartz plate. In chlo-
roform solution, the polymer exhibited a broad absorption
from 350 to 850 nm. In solid film, the absorption spectrum
was even expanded to 900 nm with an enhanced shoulder at
675 nm. The absorption peak of PIDT–DPP in solution and
solid film are both centered at 735 nm. The optical band gap
(Eogpt) determined from the onset of absorption of PIDT–DPP
film was 1.37 eV. The small value of Eg of the copolymer is
due to intramolecular charge transfer22–27 between the IDT
moieties and DPP unit. A small value of Eg should improve
light harvesting and, therefore, could enhance the photocur-
rent of the PSCs.
Cyclic voltammogram of the polymer film is shown in Figure
3. The onset oxidation potential (Eox) and onset reduction
potential (Ered) of PIDT–DPP are 0.61 V versus Ag/Agþ and
ꢀ0.98 V versus Ag/Agþ, respectively. The HOMO and LUMO
energy levels of the polymer are calculated from the onset
oxidation potential and the onset reduction potential accord-
ing to the equations: HOMO ¼ ꢀe (Eox þ 4.71) (eV) and
Electrochemical Properties
Cyclic voltammetry has been used and considered as an effec-
tive tool in investigating electrochemical properties of conju-
gated oligomers and polymers.28 From the onset oxidation
and reduction potentials in the cyclic voltammogram, energy
levels of the highest occupied molecular orbital (HOMO) and
FIGURE 4 (a) Output and (b) transfer characteristics of top-
contact OFET using PIDT–DPP as the active layer (annealed at
80 ꢂC).
FIGURE 2 Normalized absorption spectra of the PIDT–DPP in
chloroform solution and solid film on a quartz plate.
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TABLE 1 FET Properties of Devices with the PIDT–DPP Films
Spin-Coated on OTS-Modified SiO2/Si Substrates
Threshold
l (cm2 Vꢀ1 sꢀ1
)
I
on/Ioff
Voltage (V)
Without annealing
Annealed at 80 ꢂC
Annealed at 100 ꢂC 0.023
0.042
0.065
9.8 ꢁ 105 ꢀ1
4.6 ꢁ 105
2 ꢁ 105
3
0
LUMO ¼ ꢀ e (Ered þ 4.71) (eV).29,30 The calculated HOMO
and LUMO energy levels of the polymer are ꢀ5.32 eV and
ꢀ3.73 eV, respectively. It is obvious that the optical and elec-
trochemical bandgaps of polymer are not similar (1.37 vs.
1.59 eV). The deviation could be attributed to the fact that
the values obtained for the frontier-orbital energy levels can-
not be taken as absolute due to strongly aggregate tendency
of the polymer in solid state; therefore, the choice of solvents
and concentration of a spin-coated film can greatly affect the
morphology of polymer and as a result can affect UV–vis
measurements. Compared to the HOMO of P3HT (ꢀ4.76
eV30), that of PIDT–DPP is about 0.5 eV lower, which means
a higher Voc could be expected, because Voc is linearly corre-
lated with the difference of the HOMO of the donor and the
LUMO of the acceptor.24,31
FIGURE 6 The EQE spectrum of the device based on PIDT–
DPP/PC71BM (1:1 w/w).
for 2.5 min led to improved hole mobility of up to 0.065 cm2
Vꢀ1 sꢀ1 with an on/off ratio of 4.6 ꢁ 105 and a threshold
voltage of 3 V.
Photovoltaic Properties of the Polymer
Another motivation of the design and synthesis of the poly-
mer PIDT–DPP is to look for a novel alternating D–A copoly-
mer based on fused ladder alkyl substituted IDT for the
application in PSCs. We fabricate the PSCs with the structure
of ITO/PEDOT: PSS/polymer:PC71BM (1:1 w/w)/Ca/Al,
where the polymer (PIDT–DPP) was used as the electron do-
nor and PC71BM was used as the electron acceptor. Figure 5
shows the I versus V curve of the PSCs under the illumina-
tion of AM 1.5 and 100 mW/cm2. It showed a PCE of 1.76%
with an open circuit voltage (Voc) of 0.88 V, a short circuit
current density (Jsc) of 3.66 mA/cm2, and a fill factor of
0.54. The high value of Voc of the device based on PIDT–DPP
is benefited from the low HOMO energy level of the polymer.
The external quantum efficiency (EQE) spectrum of the de-
vice are shown in Figure 6. The device exhibited photores-
ponse covering from 300 to 800 nm with an EQE value of
more than 10% in the region between 340 and 715 nm. The
Jsc calculated from the EQE curves are 3.63 mA/cm2 for the
device, showing a mismatched factor less than 1% compared
with those values measured under AM 1.5G illumination.
The relatively lower EQE value of the PSC based on PIDT–
DPP could be due to low LUMO energy level (ꢀ3.73 eV) of
the polymer, which is only 0.18 eV higher than that (ꢀ3.91
eV)32,33 of PC71BM. If we can move the LUMO level of the
polymer up-shifted and then combine the broad absorption
spectrum, high charge carrier mobility, and deep HOMO, the
polymer could become a promising high efficiency donor ma-
terial in PSCs.
Field-Effect Transistor Properties of the Polymer
Figure 4 shows the typical output and transfer curves of the
OFET device with PIDT–DPP as an active layer. The device
performances of the polymer are summarized in Table 1.
The output behavior closely followed the metal-oxide semi-
conductor FET gradual-channel model with very good satura-
tion [Fig. 4(a)]. The transfer characteristics of PIDT–DPP-
based devices showed a low drain current at zero gate volt-
age [Fig. 4(b)]. Without annealing, the OFET showed a hole
mobility of 0.042 cm2 Vꢀ1 sꢀ1 in the saturation regime, to-
gether with an on/off ratio of 9.8 ꢁ 105 when measured
ꢂ
under an ambient conditions. Annealing the devices at 80 C
CONCLUSIONS
We designed and synthesized a novel D–A copolymer, PIDT–
DPP, based on alkyl substituted IDT as a new donor unit and
DPP as acceptor unit. The polymer possesses good thermal
stability (Td > 415 C) and exhibits a broad absorption spec-
trum from 350 to 900 nm with a narrow band gap of
FIGURE 5 Current density–potential characteristic of PSC
based on PIDT–DPP:PC71BM (1:1 w/w) under illumination of
AM 1.5G, 100 mW/cm2.
ꢂ
376
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15 Chen, Y. C.; Yu, C. Y.; Fan, Y. L.; Hung, L. I.; Chen, C. P.;
Ting, C. Chem. Commun. 2010, 46, 6503–6505.
1.37eV. The solution-processed OFET with PIDT–DPP as the
active layer showed a hole mobility of 0.042 cm2 Vꢀ1 sꢀ1 to-
gether with an on/off ratio of 9.8 ꢁ 105. Annealing the devi-
16 Bronstein, H.; Chen, Z. Y.; Ashraf, R. S.; Zhang, W. M.; Du,
J. P.; Durrant, J. R.; Tuladhar, P. S.; Song, K.; Watkins, S. E.;
Wienk, Y. G. M. M.; Janssen, R. A. J.; Anthopoulos, T.; Sirring-
haus, H.; Heeney, M.; McCulloch, I. J. Am. Chem. Soc. 2011,
133, 3272–3275.
ꢂ
ces at 80 C greatly increased the hole mobility up to 0.065
cm2 Vꢀ1 sꢀ1 with an on/off ratio of 4.6 ꢁ 105. The PSC fab-
ricated from the blend of PIDT–DPP and PC71BM exhibited
PCEs of 1.76% with a high Voc of 0.88 V, which is benefitted
from the deep HOMO levels (ꢀ5.32 eV) of the polymer.
These results indicate that this kind D–A copolymer based
on the alkyl substituted IDT may become a promising active
material for field-effect transistor, PSCs and other organic
electronic devices.
17 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv.
Mater. 2008, 20, 2556–2560.
18 Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk,
M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. J. Am.
Chem. Soc. 2009, 131, 16616–16617.
19 Ashraf, R. S.; Chen, Z.; Leem, D. S.; Bronstein, H.; Zhang,
W.; Schroeder, B.; Geerts, Y.; Smith, J.; Watkins, S.; Anthopou-
los, T. D.; Sirringhaus, H.; de Mello, J. C.; Heeney, M.; McCul-
loch, I. Chem. Mater. 2011, 23, 768–770.
This work was supported by NSFC (Nos. 20874106 and
21021091), The Ministry of Science and Technology of China
and Chinese Academy of Sciences.
20 Woo, C. H.; Beaujuge, P. M.; Holcombe, T. W.; Lee, O. P.;
Frechet, J. M. J. J. Am. Chem. Soc. 2010, 132, 15547–15549.
21 Bao, Z. N.; Chan, W. K.; Yu. L. P. J. Am. Chem. Soc. 1995,
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