High-mobility low-bandgap conjugated copolymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units for thin film transistor and photovoltaic applications

Article abstract of DOI:10.1039/c1jm11564b

Two new semiconducting polymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units were synthesized via Stille coupling polymerization. The polymers, PIDTPyT and PIDTDTPyT, exhibited main absorption bands in the range of 550-800 nm while their absorption maxima were located at around 700 nm in films. With two additional thiophene spacers, PIDTDTPyT showed a broader absorption band but a 20 nm blue-shifted maximum peak compared to that of PIDTPyT. Both of the polymers possess low bandgaps (～1.6 eV) and deep energy levels for both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Organic field-effect transistors (OFETs) device measurements indicate that PIDTPyT and PIDTDTPyT have high hole carrier mobilities of 0.066 and 0.045 cm² V^-1 s ^-1, respectively, with the on/off ratio on the order of 10 ⁶. Bulk heterojunction photovoltaic devices consisting of the copolymers and PC₇₁BM gave power conversion efficiencies (PCE) as high as 3.91% with broadband photo-response in the range of 300-800 nm. The relationships between the photovoltaic performance and film morphology, energy levels, hole mobilities are discussed.

Full text of DOI:10.1039/c1jm11564b

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PAPER
High-mobility low-bandgap conjugated copolymers based on
indacenodithiophene and thiadiazolo[3,4-c]pyridine units for thin film
transistor and photovoltaic applications
Ying Sun,ab Shang-Chieh Chien,ac Hin-Lap Yip,a Yong Zhang,a Kung-Shih Chen,a David F. Zeigler,d
c
Fang-Chung Chen, Baoping Lin and Alex K.-Y. Jen
b
a
*
*
Received 12th April 2011, Accepted 30th June 2011
DOI: 10.1039/c1jm11564b
Two new semiconducting polymers based on indacenodithiophene and thiadiazolo[3,4-c]pyridine units
were synthesized via Stille coupling polymerization. The polymers, PIDTPyT and PIDTDTPyT,
exhibited main absorption bands in the range of 550–800 nm while their absorption maxima were
located at around 700 nm in films. With two additional thiophene spacers, PIDTDTPyT showed
a broader absorption band but a 20 nm blue-shifted maximum peak compared to that of PIDTPyT.
Both of the polymers possess low bandgaps (ꢀ1.6 eV) and deep energy levels for both the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Organic
field-effect transistors (OFETs) device measurements indicate that PIDTPyT and PIDTDTPyT have
high hole carrier mobilities of 0.066 and 0.045 cm2 Vꢁ1 sꢁ1, respectively, with the on/off ratio on the
order of 106. Bulk heterojunction photovoltaic devices consisting of the copolymers and PC71BM gave
power conversion efficiencies (PCE) as high as 3.91% with broadband photo-response in the range of
300–800 nm. The relationships between the photovoltaic performance and film morphology, energy
levels, hole mobilities are discussed.
film morphology and high hole mobility.2,17–19 Up to now,
1. Introduction
significant progress has been made for BHJ polymer solar cells
with a PCE as high as 7–8%.7,8
Polymer solar cells (PSCs) are considered as one of the most
promising alternative to conventional solar cells due to their
unique advantages of low cost, light weight, flexibility and
processability in large-area substrates.1–4 So far, the bulk heter-
ojunction (BHJ) structure, which comprises a blend of an elec-
tron-rich polymer as donor and an electron-deficient fullerene
derivative as acceptor, is the most efficient architecture for
PSCs.5–8 In the past decade, a remarkable amount of efforts to
improve the power conversion efficiency (PCE) of PSCs has been
devoted to the development of novel conjugated donor poly-
mers.9–16 It has been demonstrated that the ideal p-type polymer
in the BHJ layer should simultaneously possess a strong
absorption overlap with the solar spectrum, suitable HOMO–
LUMO energy levels to facilitate efficient exciton dissociation in
the BHJ while maintaining high open-circuit voltage (Voc), good
Recently, indacenodithiophene (IDT) has emerged as a prom-
ising donor unit for constructing donor–acceptor (D–A) alter-
nating conjugated copolymers for PSCs.20–25 The structure is
characterized as two thiophene rings rigidified together with
a central phenyl ring, which can provide strong intermolecular
interactions for ordered packing to improve the charge carrier
mobility.21 Previously, a series of random copolymers based on
indacenodithiophene, benzothiadiazole (BT) and thiophene units
were synthesized.24 These polymers showed broad absorption
bands, reasonable field-effect hole mobilities (10ꢁ4 to 10ꢁ3 cm2
Vꢁ1 sꢁ1) and promising OPV performance. Recently, Ting et al.23
and Chen et al.22 have reported that an alternating copolymer
based on IDT and BT units exhibits a high hole mobility with the
PCE over 6%, which is higher than the analogous random
copolymers. The PCE of IDT-based polymers can be potentially
improved by further deepening the HOMO level to increase the
Voc and narrowing the bandgap to enhance light absorption.
However, the trade-off between the bandgap and the HOMO–
LUMO relationships of the polymer donor and fullerene
acceptor makes it difficult to simultaneously optimize the energy
levels and bandgap. So further development of new IDT based
polymers is very essential to achieve more understanding of the
structure–performance relationships.
aDepartment of Materials Science and Engineering, University of
Washington, Seattle, Washington, 98195, USA. E-mail: ajen@
u.washington.edu; Fax: +1 206 543 3100; Tel: +1 206 543 2626
bSchool of Chemistry and Chemical Engineering, Southeast University,
Jiangning District, Nanjing, Jiangsu Province, P.R. China 211189.
E-mail: lbp@seu.edu.cn; Fax: +86-25-52090616; Tel: +86-25-52090616
cDepartment of Photonics & Display Institute, National Chiao Tung
University, Hsinchu, 30010, Taiwan
dDepartment of Chemistry, University of Washington, P.O. Box 351700,
Seattle, Washington, 98195, USA
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Thiadiazolo[3,4-c]pyridine (PyT), which is expected to be
a stronger electron acceptor compared to 2,1,3-benzothiadiazole
due to the more electron-deficient property of the pyridine, has
been less explored in D–A copolymers. Blouin et al.26 incorpo-
rated thienyl-flanked PyT with the carbazole unit to yield a low
molecular weight copolymer showing limited mobility and effi-
ciency. They demonstrated that the asymmetric structure of the
PyT unit may diminish the structural organization of the poly-
mer and consequently decreased the device performance.
Recently, a series of polymers copolymerized with various donor
moieties and an alkylated thienyl-flanked PyT unit were reported
by Zhou et al.27 Encouragingly, these polymers showed good
solubility, reduced bandgaps, deeper HOMO–LUMO levels and
better performance than their corresponding 1,4-dithien-2-yl-
2,1,3-benzothiadiazole (DTBT) based polymers. In spite of the
achieved high efficiency based on PyT units, the relationships
between the structure, deep energy levels, charge mobility, and
device performance still need to be further explored.
with 0.1 M of tetrabutylammonium hexafluoro-phosphate using
a scan rate of 100 mV sꢁ1. ITO, Ag/AgCl and Pt mesh were used
as working electrode, reference electrode and counter electrode,
respectively. The differential scanning calorimetry (DSC) was
performed using a DSC2010 (TA instruments) under a heating
rate of 10 ꢂC minꢁ1 and a nitrogen flow of 50 mL minꢁ1. The
AFM images under tapping mode were taken from the actual
devices fabricated for photovoltaic measurement on a Veeco
multimode AFM with a Nanoscope III controller.
Synthesis of monomers
Synthesis of compound 1. To a solution of 1-bromo-4-hex-
ylbenzene (3.0 g, 12.5 mmol) in THF (25 mL) was added drop-
ꢂ
wise n-BuLi (5.5 mL, 2.5 M in hexane, 13.75 mmol) at ꢁ78 C.
The resulting solution was stirred for 1 h and then quenched with
a solution of diethyl 1,4-bis (thiophen-2-yl)-2,5-benzene-dicar-
boxylate (1.0 g, 2.59 mmol) in THF (15 mL). And the reaction
was kept at ꢁ78 ꢂC for 1 h, slowly warmed to room temperature
and stirred overnight. Water was added to the solution and the
mixture was extracted with ethyl acetate. The combined organic
layers were dried with Na2SO4 and then solvent was removed by
rotary evaporation. The resulting white solids were added to
acetic acid (30 mL). After the addition of concentrated H2SO4
(1 mL), the reaction was allowed to reflux for 5 h and then
quenched with water (150 mL). The mixture was extracted with
CH2Cl2 and the combined organic layer was washed with water
for 3 times and dried over Na2SO4. Solvent was removed under
reduced pressure and the residue was purified by silica gel
chromatography (eluent: hexane/dichloromethane ¼ 10/1) to
afford a white solid (1.3 g 67%). 1H NMR (CDCl3, ppm): 7.44 (s,
2H), 7.26 (d, J ¼ 4.89 Hz, 2H), 7.15 (d, J ¼ 8.28 Hz, 8H), 7.08 (d,
J ¼ 8.31 Hz, 8H), 7.02 (d, J ¼ 4.89 Hz, 2H), 2.57 (t, 8H), 1.58 (m,
4H), 1.31 (m, 28H), 0.88 (t, 12H). 13C NMR (CDCl3, ppm):
156.05, 153.60, 142.27, 141.57, 141.47, 135.30, 128.48, 128.10,
127.59, 123.33, 117.69, 62.85, 35.78, 31.94, 31.57, 29.37, 22.82,
14.32. HRMS (ESI): (M+, C64H74S2), calcd: 906.5232; found:
906.5201.
To understand these relationships, two new D–A copolymers
(PIDTPyT and PIDTDTPyT) were synthesized based on PyT
and thienyl-flanked PyT acceptor moieties copolymerized with
the IDT donor monomer. After introducing the more electron-
deficient PyT unit, both PIDTPyT and PIDTDTPyT showed
reduced bandgaps (1.60 eV for PIDTPyT and 1.62 eV for
PIDTDTPyT) and deep HOMO energy levels compared to their
BT conterparts.22 OFET measurements showed that the poly-
mers had excellent charge transporting properties with hole
mobilities as high as 0.066 cm2 Vꢁ1 sꢁ1 and an on/off ratio of 106.
BHJ solar cells based on the blend of the polymers and PC71BM
showed the PCEs up to 3.41% and 3.91% for PIDTPyT and
PIDTDTPyT, respectively. The main limitation on device
performance was the fill factor (FF) though both devices
exhibited superior broadband photo-response spectra in the
range of 300–800 nm. The small energy offset between the
LUMO levels of the polymer and PC71BM as well as the inferior
vertical mobility may be the limitation factors for the efficiency.
2. Experimental section
Materials
Synthesis of compound 2. n-BuLi (1.1 mL, 2.5 M in hexane) was
added dropwise to a solution of compound 1 (1 g, 1.1 mmol) in
THF (20 mL) at ꢁ78 ꢂC. The mixture was kept at ꢁ78 ꢂC for 30
min and then warmed to room temperature for another 30 min.
After cooling to ꢁ78 ꢂC again, trimethyltin chloride (2.5 mL,
1 M in hexane) was added. The reaction was stirred overnight at
room temperature and then quenched with water, extracted with
hexane, and dried over Na2SO4. After removal of the solvent,
ethanol was added to the mixture and the precipitate was
All chemicals were purchased from Aldrich and TCI without
further purification. Diethyl 1,4-bis(thiophen-2-yl)-2,5-benzene-
dicarboxylate was prepared as reported.28 Toluene was distilled
from sodium and benzophenone ketyl under nitrogen prior to
use.
Measurement and characterization
1
UV-Vis spectra were recorded using a Perkin-Elmer Lambda-9
1
collected as a white solid (1.1 g, 82%). H NMR (CDCl3, ppm):
spectrophotometer. The H NMR and 13C NMR spectra were
7.40 (s, 2H), 7.15 (d, J ¼ 8.04 Hz, 8H), 7.07 (d, J ¼ 8.28 Hz, 8H),
7.03 (s, 2H), 2.56 (t, 8H), 1.58 (m, 4H), 1.31 (m, 28H), 0.89 (t,
12H), 0.35 (s, 18H). 13C NMR (CDCl3, ppm): 157.72, 153.77,
147.55, 142.66, 141.35, 141.15, 134.93, 130.81, 128.39, 128.20,
117.96, 62.33, 35.79, 31.95, 31.55, 29.39, 22.82, 14.32, ꢁ7.82.
HRMS (ESI): (M+, C70H90S2Sn2), calcd: 1234.4528; found:
1234.4480.
collected on a Bruker AV 300 or 500 spectrometer operating at
300 or 125 MHz in deuterated chloroform solution with TMS as
reference. ESI-MS spectra were recorded on a Bruker APEX Qe
47e Fourier transform (ion cyclotron resonance) mass spec-
trometer. The molecular weight was measured by a Waters 1515
gel permeation chromatograph (GPC) with a refractive index
detector at room temperature (THF as the eluent). Cyclic vol-
tammetries of polymer films were conducted on a BAS CV-50 W
voltammetric system with a three-electrode cell in acetonitrile
Synthesis of compound 3. To a solution of 3,4-diaminopyridine
(3.93 g, 36 mmol) in 50 mL 48% aqueous hydrobromic acid was
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added slowly 6 mL bromine and the mixture subsequently was
allowed to reflux overnight. The mixture was cooled down to
room temperature and then filtered. The resultant precipitation
was washed sequentially with aq. Na2CO3, aq. Na2S2O3 and
water. The crude product was further refluxed in a 10% solution
of Na2CO3 for 1 h and then obtained by filtration. Purification by
silica gel chromatography (eluent: dichloromethane/ethyl
flask under N2 protection. Toluene (4 mL) and DMF (0.4 mL)
were added and the mixture was degassed following the quick
addition of Pd2(dba)3 (4 mg) and P(o-tol)3 (10 mg). The mixture
was heated at reflux for 2 days, cooled to RT and then poured
into methanol. The precipitate was collected by filtration and
washed by Soxhlet extraction with acetone and hexane for 12 h
respectively. Then the resulting solid was dissolved in chloroform
and the solution was poured into hexane to afford the polymer
1
acetate ¼ 10/1) gave 3 (3.5 g, 37%) as a yellow solid. H NMR
1
(CDCl3, ppm): 7.89 (s, 1H), 4.49 (s, 2H), 3.68 (s, 2H).
PIDTPYT (85 mg, 74%). H NMR (500 MHz, CDCl3, d) 8.80
(br, 1H), 8.63 (br, 1H), 8.10 (br, 1H), 7.57 (dd, 2H), 7.12 (m,
16H), 2.58 (br, 8H), 1.57 (m, 4H), 1.29 (m, 28H), 0.88 (m, 30H).
Elemental analysis: calcd C, 79.57; H, 7.16; N, 4.03; found C,
78.81; H, 6.81; N, 3.73%. Mn ¼ 115 kDa, Mw ¼ 267 kDa,
PDI ¼ 2.32.
Synthesis of compound 4. To a stirred, cooled solution of 3,4-
diamino-2,5-dibromopyridine (1 g, 3.78 mmol) in pyridine (12 mL)
at 0 ꢂC was added dropwise SOCl2 (0.4 mL). Stirringwas continued
for an additional 2 h and then the mixture was poured into water.
The crude product was collected by filtration and washed with
water. Further purification by silica column chromatography
(eluent: hexane/dichloromethane ¼ 4/1) afforded 4 as a pale yellow
solid (0.62 g, 56%). 1H NMR (CDCl3, ppm): 8.57 (s, 1H). 13C NMR
(CDCl3, ppm): 155.20, 150.22, 145.23, 136.65, 111.68. HRMS
(ESI): (M+, C5HN3SBr2) calcd: 292.8258; found: 292.8341.
Synthesis of polymer PIDTPYT. Compounds 2 (123.4 mg,
0.1 mmol) and 6 (45.7 mg, 0.1 mmol) were charged in a 25 mL flask
under N2 protection. Toluene (5 mL) and DMF (0.5 mL) were
added and the mixture was degassed following the quick addition
of Pd2(dba)3 (4 mg) and P(o-tol)3 (10 mg). The mixture was heated
at reflux for 2 days, cooled to RT and then poured into methanol.
The precipitate was collected by filtration and washed by Soxhlet
extraction with acetone and hexane for 12 hours respectively. Then
the resulting solid was dissolved in chloroform and the solution
was poured into hexane to afford the polymer PIDTDTPYT
(67 mg, 56%). 1H NMR (500 MHz, CDCl3, d) 8.79 (br, 1H), 7.72
(m, 4H), 7.55 (m, 4H), 7.23 (dd, 8H), 7.14 (dd, 8H), 2.60 (br, 8H),
1.70 (m, 4H), 1.37 (m, 28H), 0.92 (m, 30H). Elemental analysis:
calcd C, 76.70; H, 6.52; N, 3.48; found C, 75.45; H, 6.16; N, 3.44%.
Mn ¼ 32 kDa, Mw ¼ 87 kDa, PDI ¼ 2.73.
Synthesis of compound 5. A mixture of compound 3 (0.292 g,
1 mmol) and 2-(tributylstannyl)thiophene (0.970 g, 2.6 mmol) in
THF (15 mL) was degassed several times over 1 h. Pd2(dba)3
(37 mg) and P(o-tol)3 (73 mg) were added and the resultant
mixture was heated at reflux for 15 h and then allowed to cool.
Water was added to the mixture and the aqueous phase was
extracted with dichloromethane for three times. The combined
organic extracts were dried over Na2SO4, filtered and the solvent
was removed under vacuum. The crude was further purified with
silica column chromatography (eluent: hexane/dichlorome-
thane ¼ 2/1) to give compound 5 as a red solid (0.241 g, 80%). 1H
NMR (CDCl3, ppm): 8.88 (s, 1H), 8.74 (dd, J1 ¼ 3.81 Hz, J2 ¼
1.12 Hz, 1H), 8.14 (dd, J1 ¼ 3.66 Hz, J2 ¼ 1.1 Hz, 1H), 7.64 (dd,
J1 ¼ 4.80 Hz, J2 ¼ 1.1 Hz, 1H), 7.52 (dd, J1 ¼ 5.10 Hz, J2 ¼ 1.1
Hz, 1H), 7.30 (dd, J1 ¼ 5.01 Hz, J2 ¼ 3.8Hz, 1H), 7.27 (dd, J1 ¼
4.80 Hz, J2 ¼ 3.7 Hz, 1H). 13C NMR (CDCl3, ppm): 154.94,
148.03, 146.48, 141.79, 140.80, 136.55, 131.89, 130.54, 128.89,
128.15, 127.87, 127.25, 120.50. HRMS (ESI): (M+, C13H7N3S3)
calcd: 300.9802; found: 300.9875.
Fabrication of the OFET device. The Field-effect transistors
were fabricated with a bottom-gate, top-contact configuration.
The heavily n-doped silicon substrates with a 300 nm thick
thermally grown SiO2 dielectric (from Montco Silicon Technol-
ogies, Inc.) were first cleaned by sonicating in acetone and iso-
propanol and then exposed to air plasma. The cleaned substrates
were then treated with hexamethyldisilazane (HMDS) through
ꢂ
vapor phase deposition in a vacuum oven (200 mTorr, 100 C,
3 h). Subsequently, the semiconductor polymer films were spin-
coated in a glove box from their 10 mg mLꢁ1 chloroform solu-
tions which were stirred overnight and filtered with a 0.2 mm
PTFE filter. Interdigitated source and drain electrodes (L ¼ 1000
mm, W ¼ 12 mm) were deposited by evaporating a 50 nm thick
gold film and defined with a shadow mask. The transfer and
output characteristics were measured in a glove box using an
Agilent 4155B semiconductor parameter analyzer. The satura-
tion field-effect mobility (m) was calculated from the following
equation:
Synthesis of compound 6. To a solution of compound 4 (0.2 g,
0.664 mmol) in 5 mL chloroform was added the n-bromosucci-
nimide (0.26 g, 1.462 mmol) and the mixture was stirred in the
dark for 24 h. The reaction was quenched with water and the
water layer was extracted with chloroform. The combined
organic layer was dried over Na2SO4 and the solvent was
removed under vacuum. Purification by the silica column chro-
matography (eluent: hexane/dichloromethane ¼ 2/1) gave the
compound 6 as a dark red solid (0.27 g, 86%). 1H NMR (CDCl3,
ppm): 8.73 (s, 1H), 8.43 (d, J ¼ 4.05 Hz, 1H), 7.83 (d, J ¼ 3.99
Hz, 1H), 7.22 (d, J ¼ 4.08 Hz, 1H), 7.18 (d, J ¼ 3.99 Hz, 1H).
HRMS (ESI): (M+, C13H5Br2N3S3) calcd: 456.8012; found:
456.8095.
Ids ¼ m(W/2L)Ci(Vgs ꢁ Vth)2
where W and L are the channel width and length, respectively. Ci
is the capacitance of the insulating SiO2 layer per unit area, Vgs
and Vth are the gate voltage and the threshold voltage, respec-
tively. The threshold voltage (Vth) was obtained as the x-axis
intercept of the linear section of the plot of (Ids)1/2 vs. Vgs. The
subthreshold swing was estimated by taking the inverse of the
slope of Ids vs. Vgs in the region of exponential current increase.
Synthesis of polymers
Synthesis of polymer PIDTPYT. Compounds 2 (123.4 mg,
0.1 mmol) and 4 (29.3 mg, 0.1 mmol) were charged in a 25 mL
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Fabrication of the photovoltaic and hole only device. To fabri-
cate conventional configuration solar cells, ITO-coated glass
substrates (15 U sqꢁ1.) were first cleaned with detergent, de-
ionized water, acetone, and isopropyl alcohol. Subsequently,
a PEDOT:PSS (Baytronꢀ PVP AI 4083, filtered at 0.45 mm)
layer (ꢀ45 nm) was spin-coated onto the cleaned ITO-coated
glass substrates at 5000 rpm and then annealed at 120 ꢂC for 30
min under ambient conditions. After that, the substrates were
loaded into a nitrogen-filled glove-box. Following that, the active
layer was spin-coated onto the PEDOT:PSS layer from a homo-
geneous blending solution of polymer:PC71BM. The solution
was prepared by dissolving the polymer and PC71BM with
a certain blending weight ratio in o-dichlorobenzene (o-DCB)
overnight and filtered with a 0.2 mm PTFE filter. Finally, the
substrates were transferred into the evaporator with shadow
masks to define the active area of the devices (10.08 mm2) and
pumped under high vacuum (<2 ꢃ 10ꢁ7 Torr). Then calcium
(30 nm) and aluminium (100 nm) were thermally evaporated
onto the active layer sequentially. The un-encapsulated solar cells
were measured in glove box conditions using a Keithley 2400
SMU source measurement unit and an Oriel Xenon lamp
(450 W) with an AM1.5 filter as the solar simulator. A reference
silicon solar cell with a KG5 filter, which has been previously
standardized by the National Renewable Energy Laboratory,
was used to calibrate the light intensity to 100 mW cmꢁ2. To
fabricate the hole only device, the same procedure as that used
for the photovoltaic device was followed except that MoO3 was
used to replace the calcium.
undesired product with the bromine atom at the 4-position
replaced by chlorine atom, pyridine was employed to dissolve the
amine compound and absorb the hydrochloric acid. The amount
of thionyl chloride was controlled to be around 1.4–1.5 eq. and
the reaction was carried out at low temperature. Under these
conditions, the pure dibromo functionalized monomer 4 was
obtained with a reasonable yield (56%). The purity of the
monomer was confirmed by GC-MS and NMR. The Stille
coupling reaction between 4 and tributyl (2-thienyl) stannane
yielded compound 5 and subsequent bromination by n-bromo-
succinimide gave the target monomer 6, which was carefully
purified using silica column chromatography.
Scheme 2 depicted the synthetic methods for the polymers. It is
worth noting that the polymer structure is regiorandom due to
the asymmetric PyT unit. Copolymers PIDTPyT and
PIDTDTPyT were then synthesized by Stille cross-coupling
reaction of the dibromo monomers (4 and 6) with the bis-stannyl
compound 2, using tris(dibenzylidene-acetone)dipalladium(0)
(Pd2dba3) as catalyst and tri(o-tolyl) phosphine (P(o-tol)3) as the
corresponding ligand in toluene/DMF solution. The polymeri-
zation was taken at 110 ꢂC under nitrogen atmosphere for 48 h.
The resulting polymers were collected by directly precipitating
the solution in methanol followed by filtration. After Soxhlet
extraction with acetone and hexane, the final polymers were
further purified by dissolving in CHCl3 and then precipitating in
hexane. The two polymers have good solubilities in chlorinated
solvents such as chloroform, chlorobenzene and dichloroben-
zene. The molecular weights of the two polymers were measured
by GPC with polystyrene as standard and THF as eluent. The
number-average molecular weight (Mn) of PIDTPyT and
PIDTDTPyT is 111.0 kDa and 37.7 kDa, with polydispersity
indices (PDI) of 2. 32 and 2.73 (listed in Table 1) respectively.
The thermal properties were investigated with differential scan-
ning calorimetry (DSC) but no thermal transition signal was
observed in the range from 20 to 350 ꢂC for both of the polymers.
3. Results and discussion
Synthesis part
General synthetic routes of the monomers are described in
Scheme 1. The synthesis of compound 1 involved the double
addition of the 1-hexylbenzene lithium to the di-ester compound
to give the corresponding alcohol, which subsequently under-
went an acid-mediated intramolecular cyclization reaction.28
Lithiation of the compound 1 followed by quenching with tri-
methyltin chloride afforded the functionalized IDT based
monomer 2 in a good yield (82%). Compound 3 was synthesized
according to previously reported procedures.26 The conversion of
the compound 3 to the target monomer 4 was conducted through
the ring closure reaction with thionyl chloride. To avoid the
Optical and electrochemical properties
The UV-vis absorption spectra of the two polymers in CHCl3
solutions and films are shown in Fig. 1 and the summarized data
are listed in Table 1. Two obvious absorption bands can be
observed for the two polymers both in films and solutions. The
shorter wavelength absorbance can be assigned to the delocalized
p–p* transition in the polymer chains while the absorption band
Scheme
2
Synthetic routes for polymers (regiorandom polymer
Scheme 1 Synthetic routes for monomers.
structure).
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Table 1 Physical, optical and electrochemical properties of polymers
UV-vis absorption
Cyclic voltammetry
HOMO/
eV
In CHCl3
In film
Mn/kg
molꢁ1
lmax
/
lonset
nm
/
lmax
nm
/
lonset
nm
/
Bandgap/ Mobility/ Threshold
LOMO/eV eV
cm2 Vꢁ1 sꢁ1 voltage
On/off
ratio
a
Polymer
PDIa nm
PIDTPyT
111.0
2.32 707
2.73 650
765
745
707
680
775
765
ꢁ5.40
ꢁ5.32
ꢁ3.80b/–
ꢁ3.70b/–
3.38c
1.60d/1.89c 0.066e/
0.062f
ꢁ18.9e/–14.9f
ꢁ14.5e/–12.4f
106
106
3.51c
PIDTDTPyT 37.7
1.62d/1.94c 0.037e/
0.045f
a
Determined from GPC against the polystyrene standard using THF as eluent. b Calculated from the optical bandgap and HOMO level. c Measured
from the cyclic voltammetry. Optical bandgap. As cast. f Annealing at 110 ꢂC.
d
e
introduced to the backbone, the concentration of the acceptor
units decreases and consequently the accessible ICT decreases to
the point where most energy transitions are localized on the
donor segments.33 Therefore, by introducing one thiophene
spacer between the donor and the acceptor, the ICT character-
istic may be diminished in PIDTDTPyT. Another possible
reason may be the less rigid and planar polymer backbone of
PIDTDTPyT due to the twist of the polymer chain introduced by
the thiophene units, causing reduced conjugation.34
Fig.
1 UV-vis absorption spectra of polymers PIDTPyT and
PIDTDTPyT in chloroform solution (a) and in film (b).
Evaluated from the onset of the UV-vis absorption, the bandg-
aps of the polymers PIDTPyT and PIDTDTPyT were determined
to be 1.60 and 1.62 eV, respectively. Compared to the polymer
copolymerized with IDT and BT units,22 PIDTPyT exhibited
a bathochromic shift (ꢀ60 nm) in the absorption maximum and
a reduced optical bandgap (ꢀ0.14 eV), which also indicated that the
stronger electron-accepting ability of PyT induces more efficient
ICT. These low bandgap polymers were expected to give more light
absorption and broader photo-response.
in the longer wavelength has been attributed to intramolecular
charge transfer (ICT) from the electron-rich IDT units to the
electron-deficient PyT functionalities.2
The absorption maxima in chloroform solution were observed
at 707 and 650 nm for PIDTPyT and PIDTDTPyT, respectively.
In the solid state, PIDTPyT showed an absorption spectrum
similar to that obtained in the solution with only slightly broader
absorption range and a red-shifted (ꢀ7 nm) absorption onset.
Considering the high molecular weight and the rigid backbone of
the polymer, the absence of red-shift in the absorption maxima
may originate from the aggregation of the polymer chains
formed in solution.29 The strong internal charge transfer between
IDT and PyT would facilitate the polymer backbone to adopt
a more planar structure, thereby enhancing the stacking and
aggregation of polymer even in solution. The absorption maxima
and the onset of PIDTDTPyT were red-shifted from the solution
to the solid state with a pronounced shoulder at approximately
680 nm. This indicates a higher structural organization and an
ordered packing which may be related to stronger aggregation
induced by intermolecular interactions in the solid state.30
According to many results published in the literature,16,31,32 the
bandgap of the polymer decreased when extra thiophene units
were introduced between the donor and the acceptor due to the
more extended and delocalized p-electron system. It is interesting
that PIDTDTPyT exhibits a broader absorption range but with
blue-shifted absorption maxima and the onset both in solution
and in film compared to PIDTPyT. The increased conjugation
length can be indicated by the red-shifted short-wavelength
absorption band of polymer PIDTDTPyT toward PIDTPyT.
However, the magnitude of the red shift in the longer-wavelength
absorption band induced by increasing the monomer conjuga-
tion length is less pronounced. When more donor units are
Cyclic voltammetry (CV) was employed to evaluate the
HOMO and LUMO levels of polymers which were calculated
using the following equation:
HOMO ¼ ꢁ[Eox + 4.80] eV
LOMO ¼ ꢁ[Ered + 4.80] eV
where Eox and Ered are the onset of the oxidation and reduction
potentials, respectively. The CV curves of polymers are shown in
Fig. 2 and the detailed results are listed in Table 1. The HOMO
levels of two polymers were found to be ꢁ5.40 and ꢁ5.32 eV for
PIDTPyT and PIDTDTPyT, respectively. The deep HOMO
levels may contribute to improve oxidative stability and higher
open-circuit voltage (Voc).35
The LUMO levels measured by CV were determined to be
ꢁ3.51 and ꢁ3.38 eV for PIDTPyT and PIDTDTPyT, respec-
tively. However, calculated from the HOMO level and the optical
bandgap of the polymer films (lonset of the absorption spectra),
the LUMO energy levels were estimated to be ꢁ3.80 and
ꢁ3.70 eV for PIDTPyT and PIDTDTPyT, respectively. The
close energetic proximity of the polymer and the LUMO level of
PC61BM and PC71BM may diminish the efficient charge sepa-
ration and lead to increased charge recombination.36
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Charge transport properties
The charge transport properties of the active materials in BHJ
play an important role in the OPV performance. A good carrier
mobility ensures efficient exciton dissociation, charge transfer
and reduces the charge recombination.37 Top contact organic
field-effect transistor (OFET) devices were fabricated to evaluate
the hole mobilities of the polymers. The OFET devices without
annealing exhibited the typical p-channel characteristics. Fig. 4
showed the output curves at different gate voltage (Vgs) and
transfer characteristics in the saturation regime under constant
source–drain voltage (Vds ¼ ꢁ100 V) for PIDTPyT and
PIDTDTPyT. We also investigated the annealing effect on the
OFET characteristics and the detailed data including mobilities,
on/off ratios, and threshold voltages obtained from the OFETs
based on these two polymers before and after annealing are listed
in Table 1.
Fig. 2 Cyclic voltammograms of PIDTPyT and PIDTDTPyT (working
electrode: ITO; reference electrode: Ag/AgCl; counter electrode: Pt
mesh).
The hole-mobilities of PIDTPyT and PIDTDTPyT without
annealing were determined to be 0.066 and 0.037 cm2 Vꢁ1 sꢁ1,
respectively, with the on/off ratio at the order of 106. After
annealing at 110 ꢂC, the mobility of PIDTPyT remained essentially
constant while PIDTDTPyT showed an increased mobility of
0.045 cm2 Vꢁ1 sꢁ1. The high charge mobility indicates that there are
strong intermolecular interactions induced by the aforementioned
p–p stacking and donor–acceptor interactions. The fused ring
aromatic structure of the IDT unit may generate better molecular
organization and provide highly efficient pathways for charge
carrier transport. PyT may help promote the donor–acceptor
interaction and decrease the intramolecular stacking distance.36
The negligible annealing effect on the mobility for PIDTPyT also
indicated this strong intermolecular interaction of polymer chains.
The lower mobility of PIDTDTPyT may be ascribed to weaker
intermolecular interactions due to the additional thiophene
To further understand the geometric and electrical properties,
density functional theory (DFT) theoretical calculations were
performed at the B3LYP/6-31G (d) level by modeling the trimer
compounds. Particularly, the hexyl-phenyl side chains were
replaced with ethyl–phenyl groups for computational simplicity
considering that side-chain substituents have minimal effects on
the oxidative and reductive properties of the polymers. The
optimized molecular geometries and HOMO and LUMO wave
functions are depicted in Fig. 3. For both the polymers, the
HOMO wavefunctions are delocalized over the whole conjugated
polymer backbone, which indicates that the HOMO levels are
determined by both donor and acceptor units. However, the
LUMO wave functions for PIDTPyT are primarily localized on
PyT units, while they extend onto the thiophene units for
PIDTDTPyT. This implies that adding the extra thiophene units
has impacts on both the HOMO and LUMO levels. These results
from the calculations are in agreement with the experimental
values from CV tests. As can be observed, the optimized geom-
etries of the polymers revealed highly planar molecular struc-
tures, which can provide more evidence for the planar structure
and good packing as previously discussed.
Fig. 3 Molecular geometries and HOMO, LUMO wavefunctions of the
Fig. 4 Output (a and c) and transfer (b and d) characteristics of
trimer model of PIDTPyT and PIDTDTPyT.
PIDTPyT (a and b) and PIDTDTPyT (c and d).
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spacers. And the interaction can be enhanced upon annealing,
therefore promoting polymer chains form more ordered organi-
zation in film to result in higher mobility.
Photovoltaic properties
The photovoltaic properties of PIDTPyT and PIDTDTPyT were
investigated with the conventional device structure of ITO/
PEDOT:PSS(40 nm)/polymer:PC71BM/Ca(30 nm)/Al(100 nm),
where the device area is 10.08 mm2. PC71BM, which features
a wider absorption region compared to PC61BM, was chosen as
the n-type acceptor in order to efficiently utilize the solar light.
The BHJ active layer was prepared by spin-coating dichloro-
benzene solution of the polymer:PC71BM on a PEDOT:PSS
layer. All the devices were annealed in 150 ꢂC for 15 min before
the electrode deposition. The variation effects of the blending
ratio, which seriously influence the charge balance characteristics
and consequently determine the device performance, were
investigated in detail. The summarized electrical parameters of
these OPV devices are listed in Table 2. Higher current density
(Jsc), FF and the resulting PCE were obtained under the ratio of
1 : 3, which may be due to the optimized electron–hole charge
balance and reduced charge recombination.
Fig. 5 Current–voltage characteristics (a) and EQE wavelength depen-
dencies (b) of photovoltaic devices based on PIDTPyT and PIDTDTPyT
with the blending ratio as 1:3 under illumination with 100 mW cmꢁ2 (AM
1.5G).
Optimal phase segregation and formation of a bicontinuous
interpenetrating network between the polymer donor and PCBM
acceptor are critical to achieve high device performance and
largely influence the charge separation and exciton dis-
socation.38–40 Tapping-mode atomic force microscopy (AFM)
was used to investigate the morphology of thin films of the BHJ
blends of the polymer and PC71BM. As shown in Fig. 6, the BHJ
films showed very smooth surfaces with the rms roughness
around ꢀ0.5 nm and no obvious large-scale phase separation
was observed. This indicated that both polymers have good film-
forming abilities and compatibilities with PC71BM.
Photovoltaic devices based on PIDTPyT:PC71BM (1 : 3) with
the optimized thickness of 68 nm showed a PCE value up to
3.41% with a Jsc of 10.03 mA cmꢁ2, a Voc of 0.82 V, and a FF of
42%. For PIDTDTPyT-based devices, a highest resulting PCE of
3.91% with the optimized thickness of 80 nm was achieved when
the FF improved to 50%, and the Jsc and Voc decreased slightly to
9.89 mA cmꢁ2 and 0.79 V compared to PIDTPyT. Representative
J–V curves of these two devices measured under illumination
(100 mA cmꢁ2, AM 1.5G) are plotted in Fig. 5(a) based on the
optimized conditions. Notably, the active layer thickness of the
devices is only around 60–80 nm corresponding to the photo-
current of ca. 10 mA cmꢁ2, indicating that the two polymers
blended with PC71BM featured good absorption properties.
Moreover, Fig. 5(b) displays the external quantum efficiency
(EQE) and UV-vis spectra of the two polymers and PC71BM.
The EQE spectra were tightly correlated with the UV-vis
absorption of the bulk heterojunctions. Compared to
PIDTDTPyT, the EQE curve for PIDTPyT exhibited weaker
photoresponse between 350 and 610 nm but increased EQE
between 610 and 800 nm, which can be explained by the more
red-shifted absorption of PIDTPyT. To conclude, both devices
based on these two low-bandgap materials blended with PC71BM
exhibited superior broadband photo-response spectra covering
300–800 nm. The limitation to the better performance was the
poor fill factor which has been found to be related to the film
morphology, charge transfer and the recombination process.1,3,18
Additionally, the difference in the LUMO energy level
between the acceptor and donor is one another of the critical
factors determining the exciton dissociation rate and charge
recombination.18,41 From prior discussion on device perfor-
mance, the FF is the main limitation on device performance. We
speculate that a certain degree of electron–hole recombination
would occur in these blending systems, thereby lowering the FF.
From the energy level point of view, the LUMO levels of
PIDTPyT and PIDTDTPyT are respectively ꢁ3.80 eV and
ꢁ3.70 eV, which may be too close to the LUMO level of PC71BM
(ꢁ4.3 eV). Thus, the exciton may not efficiently dissociate into
Fig. 6 AFM images (1 mm ꢃ 1 mm) for the films of polymer:PC71BM
(1 : 3): (a) PIDTPyT and (b) PIDTDTPyT as cast from o-DCB solutions.
Table 2 Summarized performances of devices containing PIDTPyT and PIDTDTPyT with different polymer:PC71BM blending ratios
Polymer
Blend ratio
Voc/v
Jsc/mA cmꢁ2
FF (%)
PCE (%)
PIDTPyT
1 : 2
1 : 3
1 : 4
1 : 2
1 : 3
1 : 4
0.80
0.82
0.76
0.77
0.79
0.62
8.86
10.03
9.35
8.88
9.89
3.65
39
42
40
44
50
45
2.73
3.41
2.87
3.03
3.91
1.03
PIDTDTPyT
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method were determined as three orders of magnitude lower than
the FET mobilities. The photovoltaic properties of the polymers
were investigated and the highest achieved PCEs for PIDTPyT
and PIDTDTPyT were 3.41% and 3.91%, respectively. The deep
LUMO level of the polymers combined with the low vertical
mobilities may not allow efficient charge dissociation and sepa-
ration, which may give rise to the limitation on photovoltaic
performance.
Acknowledgements
This work is supported by the National Science Foundation’s
NSF-STC program under Grant No. DMR-0120967, the
AFOSR (FA9550-09-1-0426), the Office of Naval Research
(N00014-11-1-0300), and the World Class University (WCU)
program through the National Research Foundation of Korea
under the Ministry of Education, Science and Technology (R31-
21410035). A.K.-Y.J. thanks the Boeing-Johnson Foundation
for financial support. Y. Sun thanks the State-Sponsored
Scholarship for Graduate Students from China Scholarship
Council. S. C. Chien thanks the National Science Council of
Taiwan (NSC98-2917-I-009-112) for supporting the Graduate
Students Study Abroad Program.
Fig. 7 Current density and voltage (J–V) curves of the hole-only devices
containing polymer and PC71BM with the ratio of 1 : 3. (V ¼ Vapplied
Vbi, Vbi ¼ 0.1 V).
ꢁ
separated free charges and the charge recombination rate may
potentially increase. The higher FF obtained from PIDTDTPyT
based devices might be correlated with its higher LUMO level
compared to the PIDTPyT-based device.
Although the results based on a field-effect transistor indicated
that both PIDTPyT and PIDTDTPyT feature superior lateral
hole carrier-transport abilities, the vertical hole carrier-transport
behaviours of the polymers blended with the fullerene play an
essential role in the photovoltaic devices performance.42 Herein,
hole-only devices based on ITO/PEDOT:PSS/BHJ/MoO3/Al
were fabricated to measure the vertical hole mobility. MoO3 was
used as an interfacial layer to suppress electron injection from Al
so that the hole-only current was measured.43 Fig. 7 shows the
current density and voltage curve (J–V) of the hole-only devices
containing polymer and PC71BM with the blending ratio of 1 : 3.
The space charge limited current (SCLC) model was employed to
investigate the vertical hole mobilities using the following equa-
tion, J ¼ 9303rmV2/8L3, where J is the current density (mA cmꢁ2),
303r is the permittivity of the polymer, m is the carrier mobility,
and L is the active layer thickness. The mobilities were extracted
by modelling the dark current in the SCLC region. The calcu-
lated vertical hole-transport mobilities of PIDTPyT and
PIDTDTPyT are 6.34 ꢃ 10ꢁ5 and 7.97 ꢃ 10ꢁ5 (cm2 Vꢁ1 sꢁ1),
respectively, which are lower than the FET mobilities by three
orders of magnitude. This discrepancy indicates that the vertical
hole-transporting ability was greatly inferior to the lateral
mobility, which might cause charge recombination issues,
thereby leading to lower FF and thinner optimized device
thickness.41
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