Novel NIR-absorbing conjugated polymers for efficient polymer solar cells: Effect of alkyl chain length on device performance

Article abstract of DOI:10.1039/b818885h

Three low bandgap conjugated polymers, i.e., PDTPBT-C8, PDTPBT-C6 and PDTPBT-C5, which consist of alternating N-alkyl dithieno[3,2-b:2′, 3′-d]pyrrole and 2,1,3-benzothiadiazole units and carry 1-octylnonyl, 1-hexylheptyl and 1-pentylhexyl as side chains,

Full text of DOI:10.1039/b818885h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel NIR-absorbing conjugated polymers for efficient polymer solar cells:
effect of alkyl chain length on device performance
a
a
a
Wei Yue,ab Yun Zhao,ab Shuyan Shao,ab Hongkun Tian, Zhiyuan Xie, Yanhou Geng and Fosong Wanga
*
*
Received 24th October 2008, Accepted 5th January 2009
First published as an Advance Article on the web 12th February 2009
DOI: 10.1039/b818885h
Three low bandgap conjugated polymers, i.e., PDTPBT-C8, PDTPBT-C6 and PDTPBT-C5, which
consist of alternating N-alkyl dithieno[3,2-b:20,30-d]pyrrole and 2,1,3-benzothiadiazole units and carry
1-octylnonyl, 1-hexylheptyl and 1-pentylhexyl as side chains, respectively, were synthesized. These
polymers show strong absorption in the wavelength range of 600–900 nm with enhanced absorption
coefficient as the length of alkyl chain decreases. The film morphology of the polymers and 1-(3-
methoxycarbonyl)propyl-1-phenyl-[6,6]-C-61 (PCBM) blends is also dependent on the alkyl chain
length. As the length decreases, the film becomes more uniform and the domian size decreases from
400–900 nm for PDTPBT-C8 to ꢀ50 nm for PDTPBT-C5. Bulk heterojunction photovoltaic solar cells
(PSCs) were fabricated based on the blend of the polymers and PCBM with a weight ratio of 1:3. The
device performance is dramatically improved as the length of the side chain decreases, due to enhanced
film absorption coefficient and improved film morphology. With the polymer PDTPBT-C5, which
carries the shortest alkyl chain, power conversion efficiency (PCE) up to 2.80% has been achieved. This
result indicates that optimizing the structure of the solublizing alkyl chain is also crucial for the design
and synthesis of high performance PSC polymeric materials.
and poly(phenylene vinylene)s (PPVs),7,8 exhibit optical bandg-
Introduction
aps larger than or around 2.0 eV, and therefore can only harvest
Bulk-heterojunction polymer photovoltaic solar cells (PSCs)
visible light. This mismatch of the absorption to the solar spec-
have been emerging as one of the attractive renewable energy
trum significantly limits the device performance of PSCs.
sources for the possibility of chemically manipulating the mate-
Therefore, development of NIR-absorbing or low-bandgap
rial properties of the polymers combined with easy and cheap
polymers is very important.
processing techniques.1 In the last few years, continuous devel-
To construct polymers consisting of alternating electron-rich
opment of new polymers together with optimizing the
and electron-deficient aromatic units is the most successful
morphology of the active layer and device structure has led to
strategy to lower the optical bandgap of the conjugated poly-
dramatic improvement of the device performance. Single-layer
mers.9 Various electron-deficient units, e.g., electron-deficient
and tandem PSCs with power conversion efficiencies (PCEs)
heterocycles,10,11 perylene diimides,12 and cyanovinylenes,13 and
beyond 5%2 and up to 6.5%,3 respectively, have been reported.
several electron-rich units, e.g., thiophene, pyrrole, fluorene and
Moreover, lifetimes of the devices have been significantly
carbazole, have been used to construct low bandgap polymers.
improved, and an appropriate industrial processing technique
However, of those with NIR absorption,10d,e,i,14 only a few exhibit
has also been developed.4
PCE beyond 1%.10e,14e–f
In typical bulk-heterojunction PSCs, conjugated polymers and
Dithieno[3,2-b:20,30-d]pyrrole is an easily made aromatic unit,
fullerene derivatives (e.g, 1-(3-methoxycarbonyl)propyl-1-
which has been used to synthesize PTh-like soluble conducting
phenyl-[6,6]-C-61 (PCBM)) function as the light-absorbing/
homo-polymers15 and copolymers for PSCs16 and thin film
electron-donating and electron-accepting materials, respec-
transistors (TFTs).17 Herein, we report the synthesis of donor
tively.5 Therefore, the energy gap and film absorption coefficient
(D)–acceptor (A) type low bandgap polymers consisting of
alternating dithieno[3,2-b:20,30-d]pyrrole and 2,1,3-benzothia-
of conjugated polymers are two of the most important parame-
ters to be optimized for efficient harvesting of the solar energy,
diazole units for bulk-heterojunction PSCs, and the length of the
and hence realizing high performance PSCs. The solar spectrum
alkyl side-chain was optimized in order to improve device
covers a very broad wavelength from ultraviolet to near-infrared
performance.
(NIR) regions. Theoretically, the ideal bandgap of conjugated
polymers for PSCs should be around 1.5 eV.6 However, most
Experimental
conjugated polymers reported so far, e.g., polythiophenes (PThs)
Materials
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P. R. China. E-mail: yhgeng@ciac.jl.cn
bGraduate School of Chinese Academy of Sciences, Beijing, 100049, P. R.
China
Tetrahydrofuran (THF), toluene and diethyl ether were distilled
over sodium/benzophenone. Absolute ethanol was obtained by
refluxing with magnesium ethoxide and then distilled. Other
reagents were used as received without further purification.
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3,30-Dibromo-2,20-bithiophene,18
2,6-di(trimethyltin)-N-(1-
Hz, 2 H), 7.69 (dd, J ¼ 3.0 Hz, J ¼ 5.4 Hz, 2 H), 4.17 (m, 1 H),
2.04 (m, 2 H), 1.67 (m, 2 H), 1.17–1.27 (m, 12 H), 0.80–0.84 (m, 6
H). 13C NMR (CDCl3, 75 MHz) d (ppm) 169.1, 134.1, 132.3,
123.4, 52.63, 32.79, 31.80, 26.66, 22.82, 14.28. MALDI-TOF:
m/z: 302.3 [M + H]+ (Calcd: 301.2).
octylnonyl)dithieno[3,2-b:20,30-d]pyrrole15 and 4,7-dibromo-
2,1,3-benzothiadiazole11a were prepared according to literature
procedures.
1-Hexylheptanol (2b)
1-Hexylheptylamine (4b)
A solution of ethyl formate (10.1 g, 136 mmol) in dry diethyl
ether (25 mL) was added dropwise to a fresh solution of hexyl-
magnesium bromide prepared from 1-bromohexane (49.5 g,
300 mmol) and magnesium (8.02 g, 330 mmol) in dry diethyl
ether (150 mL) at such a rate that gentle reflux was maintained.
After addition, stirring was continued for 10 min, and water
(12 mL) was added at such a rate that rapid refluxing occurred.
This was followed by addition of a mixture of concentrated
sulfuric acid (11 mL) in water (80 mL). The solid was filtered off
and washed thoroughly with diethyl ether. The combined
organic layers were washed with saturated Na2CO3 solution (2 ꢁ
100 mL) and brine (2 ꢁ 100 mL) and dried over MgSO4. After
removal of the solvent, the residue was purified by recrystalli-
zation from acetonitrile to afford 2b as a white solid in a yield of
Into a solution of 3b (20.0 g, 60.7 mmol) in absolute ethanol (115
mL) was added hydrazine monohydrate (3.30 mL, 66.8 mmol)
dropwise. The mixture was heated to reflux for 2 h followed by
addition of aqueous HCl (5 M, 13 mL). The mixture was refluxed
for an additional 15 min and cooled to room temperature. The
precipitate was filtered off and washed with water. Excess NaOH
solution (2 M, 80 mL) was added into the filtrate, and the mixture
was extracted with diethyl ether (3 ꢁ 100 mL). The combined
organic layers were washed with brine (2 ꢁ 100 mL) and dried
over MgSO4. After the solvent was removed, the residue was
purified by vacuum distillation to afford 4b as a colorless oil in
a yield of 54.5% (6.60 g). 1H NMR (CDCl3, 300 MHz): d (ppm)
2.65 (m, 1 H), 1.24–1.37 (m, 22 H), 0.84–0.89 (m, 6 H). 13C NMR
(CDCl3, 75 MHz) d (ppm) 51.08, 38.06, 31.92, 25.72, 22.51,
13.84. MALDI-TOF: m/z: 200.2 [M + H]+ (Calcd: 199.2).
1
94.3% (25.8 g). H NMR (CDCl3, 300 MHz): d (ppm) 3.57 (m,
1 H), 1.27–1.46 (m, 21 H), 0.87 (t, J ¼ 6.4 Hz, 6 H). 13C NMR
(CDCl3, 75 MHz) d (ppm) 72.32, 37.87, 32.22, 29.76, 25.99,
22.98, 14.39. The purity was 99.5% according to GC-MS. MS: m/
z: 199.2 [M ꢂ H]+ (Calcd: 200.2).
1-Pentylhexylamine (4c)
Compound 4c was synthesized following the procedure for
preparation of 4b as a colorless oil in a yield of 40.4% (2.30 g). 1H
NMR (CDCl3, 300 MHz): d (ppm) 2.65 (m, 1 H), 1.14–1.38 (m,
18 H), 0.85–0.89 (m, 6 H). 13C NMR (CDCl3, 75 MHz) d (ppm)
51.55, 38.53, 32.21, 29.83, 26.48, 22.9614.36. MALDI-TOF: m/z:
172.2 [M + H]+ (Calcd: 171.2).
1-Pentylhexanol (2c)
Compound 2c was synthesized following the procedure for
preparation of 2b as a white solid in a yield of 59.9% (14.7 g). 1H
NMR (CDCl3, 300 MHz): d (ppm) 3.58 (m, 1 H), 1.27–1.52 (m, 17
H), 0.88 (t, J ¼ 6.6 Hz, 6 H). 13C NMR (CDCl3, 75 MHz) d (ppm)
72.30, 37.81, 32.30, 25.70, 23.00, 14.35. The purity was 100%
according to GC-MS. MS: m/z: 171.2 [M ꢂ H]+ (Calcd: 172.2).
N-(1-Hexylheptyl)dithieno[3,2-b:20,30-d]pyrrole (6b)
Into a mixture of 5 (0.60 g, 1.85 mmol), sodium t-butoxide (0.43
g, 4.44 mmol), tris(dibenzylideneacetone)dipalladium (Pd2dba3,
0.042 g, 0.046 mmol) and 2,20-bis(diphenylphosphino)-1,10-
binaphthyl (BINAP, 0.115 g, 0.185 mmol) were added the solu-
tion of 4b (0.41 g, 2.04 mmol) in toluene (4 mL). The resultant
mixture was stirred at 110 ꢃC for 24 h under an argon atmo-
sphere. After the mixture was cooled to room temperature, water
(30 mL) was added, and the aqueous layer was extracted with
diethyl ether (3 ꢁ 50 mL). The combined organic layers were
washed with brine (2 ꢁ 100 mL) and dried over MgSO4. After the
solvent had been removed, the residue was purified by column
chromatography on silica gel with petroleum ether as eluent to
N-(1-Hexylheptyl)phthalimide (3b)
Into a solution of triphenylphosphine (28.8 g, 110 mmol),
phthalimide (16.3 g, 110 mmol), and 2b (22.0 g, 110 mmol) in dry
diethyl ether (100 mL) was added a solution of diisopropyl
azodicarboxylate (DIAD, 23.0 mL, 110 mmol) in dry diethyl
ether (40 mL) dropwise. The resultant mixture was stirred
overnight at room temperature, and the precipitate was filtered
off and washed thoroughly with diethyl ether. After removal of
the solvent, the residue was purified by column chromatography
on silica gel with petroleum ether/dichloromethane (2/1) as the
eluent to afford 3b as a yellow oil in a yield of 93.1% (33.7 g). 1H
NMR (CDCl3, 300 MHz): d (ppm) 7.80 (dd, J ¼ 3.0 Hz, J ¼ 5.7
Hz, 2 H), 7.69 (dd, J ¼ 3.3 Hz, J ¼ 5.7 Hz, 2 H), 4.17 (m, 1 H),
2.05 (m, 2 H), 1.68 (m, 2 H), 1.20–1.26 (m, 16 H), 0.80–0.86 (m, 6
H). 13C NMR (CDCl3, 75 MHz) d (ppm) 168.9, 134.0, 132.3,
123.3, 53.72, 52.52, 32.79, 31.97, 29.24, 26.92, 22.82, 14.23.
MALDI-TOF: m/z: 330.4 [M + H]+ (Calcd: 329.2).
1
afford 6b as a white solid in a yield of 77.6% (0.52 g). H NMR
(CDCl3, 300 MHz): d (ppm) 7.09 (d, J ¼ 5.4 Hz, 2 H), 7.01 (d, J ¼
5.4 Hz, 2 H), 4.20 (m, 1 H), 2.01 (m, 2 H), 1.82 (m, 2 H), 1.03–
1.25 (m, 16 H), 0.78–0.82 (m, 6 H). 13C NMR (CDCl3, 75 MHz)
d (ppm) 144.5, 122.7, 115.2, 112.2, 60.19, 35.56, 31.96, 29.36,
26.94, 22.93, 14.38. Anal. Calcd for C21H31NS2 (%): C, 69.75; H,
8.64; N, 3.87. Found (%): C, 69.71; H, 8.59; N, 3.87. MALDI-
TOF: m/z: 361.2 [M]+ (Calcd: 361.2).
N-(1-Pentylhexyl)phthalimide (3c)
N-(1-Pentylhexyl)dithieno[3,2-b:20,30-d]pyrrole (6c)
Compound 3c was synthesized following the procedure for
preparation of 3b as a yellow oil in a yield of 89.9% (20.4 g). 1H
NMR (CDCl3, 300 MHz): d (ppm) 7.81 (dd, J ¼ 3.0 Hz, J ¼ 5.4
Compound 6c was synthesized following the procedure for
preparation of 6b as a white solid in a yield of 86.8% (2.17 g). 1H
2200 | J. Mater. Chem., 2009, 19, 2199–2206
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NMR (CDCl3, 300 MHz): d (ppm) 7.09 (d, J ¼ 5.4 Hz, 2 H), 7.01
(d, J ¼ 5.4 Hz, 2 H), 4.21 (m, 1 H), 2.01 (m, 2 H), 1.81 (m, 2 H),
1.04–1.19 (m, 12 H), 0.75–0.80 (m, 6 H). 13C NMR (CDCl3, 75
MHz) d (ppm) 144.5, 122.8, 115.2, 112.2, 60.20, 35.53, 31.91,
26.65, 22.80, 14.36. Anal. Calcd for C19H27NS2 (%): C, 68.42; H,
8.16; N, 4.20. Found (%): C, 68.45; H, 8.02; N, 4.25. MALDI-
TOF: m/z: 333.2 [M]+ (Calcd: 333.2).
7.96 (s, br, 2 H), 4.17 (s, br, 1 H), 1.81–2.61 (m, 4 H), 0.35–1.72
(m, 30 H). Anal. Calcd for C31H41N3S3 (%): C, 67.71; H, 7.15; N,
7.64. Found (%): C, 67.58; H, 7.13; N, 7.99.
Poly[2,6-(N-(1-hexylheptyl)dithieno[3,2-b:20,30-d]pyrrole)-alt-
4,7-(2,1,3-benzothiadiazole)] (PDTPBT-C6)
PDTPBT-C6 was synthesized following the procedure for
preparation of PDTPBT-C8 in a yield of 81.6% (310 mg). GPC:
Mn 15600; PDI 1.36. 1H NMR (CDCl3, 300 MHz): d (ppm) 8.89
(s, br, 2 H), 8.00 (s, br, 2 H), 4.16 (s, br, 1 H), 1.77–2.47 (m, 4 H),
0.24–1.71 (m, 22 H). Anal. Calcd for C27H33N3S3 (%): C, 65.41;
H, 6.71; N, 8.48. Found (%): C, 65.65; H, 6.48; N, 8.26.
2,6-Di(trimethyltin)-N-(1-hexylheptyl)dithieno[3,2-b:20,30-d]-
pyrrole (7b)
Into a solution of 6b (0.800 g, 2.21 mol) in THF (25 mL) was
added n-BuLi (1.90 mL, 4.65 mmol, 2.5 M in hexane) at ꢂ78 ꢃC.
The mixture was maintained at this temperature for 1 h, warmed
to room temperature for another 15 min, and then re-cooled to
ꢂ78 ꢃC. Trimethyltin chloride (0.970 g, 4.86 mmol) was added at
once. The mixture was stirred overnight at room temperature
and poured into water for extraction with diethyl ether (3 ꢁ 50
mL). The combined organic layers were washed with brine (2 ꢁ
100 mL) and dried over MgSO4. After the solvent had been
removed under reduced pressure, the residue was washed with
ethanol to afford 7b in a yield of 42.8% (0.65 g). 1H NMR
(CDCl3, 300 MHz): d (ppm) 6.98 (s, 2 H), 4.21 (m, 1 H), 2.01 (m,
2 H), 1.82 (m, 2 H), 1.09–1.27 (m, 16 H), 0.78–0.83 (m, 6 H), 0.39
(s, 18 H). 13C NMR (CDCl3, 75 MHz) d (ppm) 147.2, 135.1,
120.4, 118.9, 59.62, 35.24, 31.68, 29.05, 26.62, 22.65, 14.14,
ꢂ8.01. Anal. Calcd for C27H47NS2Sn2 (%): C, 47.19; H, 6.89; N,
2.04. Found (%): C, 47.42; H, 7.11; N, 2.19. MALDI-TOF: m/z:
687.0 [M]+ (Calcd: 687.1).
Poly[2,6-(N-(1-pentylhexyl)dithieno[3,2-b:20,30-d]pyrrole)-alt-
4,7-(2,1,3-benzothiadiazole)] (PDTPBT-C5)
PDTPBT-C5 was synthesized following the procedure for
preparation of PDTPBT-C8 in a yield of 88.0% (370 mg). GPC:
Mn 17900; PDI 1.63. 1H NMR (CDCl3, 300 MHz): d (ppm) 8.86
(s, br, 2 H), 8.06 (s, br, 2 H), 4.17 (s, br, 1 H), 1.82–2.42 (m, 4 H),
0.28–1.71 (m, 18 H). Anal. Calcd for C25H29N3S3 (%): C, 64.20;
H, 6.25; N, 8.98. Found (%): C, 64.14; H, 6.05; N, 9.25.
Measurements
1H NMR and 13C NMR spectra were recorded on a Bruker AV
300 spectrometer at 300 MHz. Elemental analysis was performed
on a VarioEL elemental analysis system. Gas chromatography-
mass spectrometry (GC-MS) measurements were carried out on
an Agilent 5975/6890N GC-MS instrument equipped with an
Agilent HP-5 column (30 m) with 1,4-bishexyloxy-benzene as the
internal reference. Matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectra were recorded on
a Bruker/AutoflexIII Smartbean MALDI Mass Spectrometer
with dithranol as the matrix. GPC measurements were conducted
on a Waters 510 system equipped with a Waters 410 Differential
refractometer using polystyrene as standard and THF as the
eluent. Differential scanning calorimetry (DSC) was run on a TA
Instruments Q100 DSC under a flow of N2 at heating/cooling
2,6-Di(trimethyltin)-N-(1-pentylhexyl)dithieno[3,2-b:20,30-d]-
pyrrole (7c)
Compound 7c was synthesized following the procedure for
preparation of 7b in a yield of 77.4% (2.30 g). 1H NMR (CDCl3,
300 MHz): d (ppm) 6.98 (s, 2 H), 4.21 (m, 1 H), 2.01 (m, 2 H),
1.82 (m, 2 H), 1.09–1.20 (m, 12 H), 0.78–0.81 (m, 6 H), 0.39 (s, 18
H). 13C NMR (CDCl3, 75 MHz) d (ppm) 149.4, 137.3, 124.4,
121.1, 61.88, 37.37, 33.78, 28.51, 24.67, 16.26, ꢂ5.82. Anal. Calcd
for C25H43NS2Sn2 (%): C, 45.55; H, 6.58; N, 2.12. Found (%): C,
45.69; H, 6.66; N, 2.03. MALDI-TOF: m/z: 659.0 [M]+ (Calcd:
659.1).
rates of 10 C$minꢂ1. UV-vis absorption spectra were recorded
ꢃ
on a Shimadzu UV-3600 UV-vis-NIR spectrometer. All films for
absorption measurements were spin-coated at 1500 rpm for 1
min with the solution concentration of 5 mg$mLꢂ1. Cyclic vol-
tammetry (CV) was performed on a CHI660a electrochemical
analyzer with a three-electrode cell in a solution of 0.1 mol $ Lꢂ1
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in
acetonitrile at a scan rate of 100 mV$sꢂ1. A glassy carbon elec-
trode with a diameter of 10 mm, a Pt wire, and a saturated
calomel electrode were used as the working electrode, the counter
electrode, and the reference electrode, respectively. The potential
was calibrated against the ferrocene/ferrocenium couple. The
highest occupied molecular orbital (HOMO) and the lowest
Poly[2,6-(N-(1-octylnonyl)dithieno[3,2-b:20,30-d]pyrrole)-alt-4,7-
(2,1,3-benzothiadiazole)] (PDTPBT-C8)
A mixture of 7a (0.520 g, 0.700 mmol), 4,7-dibromo-2,1,3-ben-
zothiadiazole (0.202 g, 0.690 mmol), tris(dibenzylideneacetone)-
dipalladium (Pd2dba3, 1.26 ꢁ 10ꢂ2 g, 1.38 ꢁ 10ꢂ2 mmol) and
triphenylphosphine (2.90 ꢁ 10ꢂ2 g, 0.110 mmol) were degassed
with argon, and then toluene (30 mL) was added. The mixture
was further purged with argon for 10 min and heated to 120 ꢃC
for 48 h. 4-Bromotoluene (24.0 mg, 0.140 mmol) was added and
the reaction was continued for another 12 h. The polymer was
precipitated in methanol. The crude polymer was collected by
filtration, and then extracted on a Soxhlet’s extractor with
acetone, hexane and chloroform in succession. The final product
was obtained by precipitating the chloroform solution in meth-
anol as a blue solid in a yield of 81.6% (310 mg). GPC: Mn 14900;
PDI 1.37. 1H NMR (CDCl3, 300 MHz): d (ppm) 8.89 (s, br, 2 H),
unoccupied molecular orbital (LUMO) energy levels were esti-
onset
mated by the equations: HOMO ¼ ꢂ(4.80 + E ), and LUMO
ox
¼ ꢂ(4.80 + E ).19 Atomic force microscopy (AFM) measure-
onset
red
ments were performed in tapping mode on a SPA400HV
instrument with an SPI 3800 controller (Seiko Instruments).
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Photovoltaic device fabrication and characterization
The PSCs were fabricated with the device structure of ITO/
poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS)/polymer:PCBM/LiF/Al. The ITO glass was pre-
cleaned and modified by a 30 nm PEDOT:PSS (Baytron P4083)
layer which was spin-cast from a PEDOT:PSS aqueous solution
on the ITO substrate, and was dried subsequently at 120 ꢃC for
30 min. The active layer of the blend of polymer and PCBM was
prepared by spin-coating the chlorobenzene solution of the
polymers and PCBM (1:3, w/w) with the polymer concentration
of 6 mg/mL on the ITO/PEDOT:PSS electrode. The thickness of
the active layer was controlled to ca. 80–140 nm by adjusting the
rotating speed of the spin-coating. Then the LiF/Al cathode was
deposited at a vacuum level of 4 ꢁ 10ꢂ4 Pa. The thicknesses of the
LiF and Al layers are 1 and 100 nm, respectively. The effective
area of the unit cell is 12 mm2. The current–voltage (I–V)
measurement of the devices was conducted on a computer-
controlled Keithley 236 Source Measure Unit. A xenon lamp
(500 W) was used as the white light source, and the optical power
at the sample was 100 mW/cm2. The external quantum efficiency
(EQE) was measured using a Model SR830 DSP Lock-in
Amplifier coupled with a SBP500 monochromator, a DSC102
data acquisition system and a Model SR540 chopper controller.
The light intensity at each wavelength was calibrated with
a standard single-crystal Si photodiode.
Scheme 1 Synthesis of PDTPBT-Cn (n ¼ 5, 6, 8). (i) (1) Diethyl ether; (2)
H3O+. (ii) PPh3, DIAD, Et2O, room temperature (rt). (iii) (1) Hydrazine
monohydrate, absolute ethanol, reflux; (2) 5.0 M HCl, reflux. (iv) Pd2dba3,
BINAP, NaOtBu, toluene, 110 ꢃC. (v) (1) n-BuLi, THF, ꢂ78 ꢃC to rt; (2)
Me3SnCl, ꢂ78 ꢃC to rt. (vi) Pd2dba3, PPh3, toluene, 120 ꢃC.
Table 1 Molecular weights and thermal properties of PDTPBT-Cn (n ¼
5, 6, 8)
a
a
b
Td (ꢃC)
Polymer
Mw
Mn
PDIa
PDTPBT-C8
PDTPBT-C6
PDTPBT-C5
20300
21700
29200
14900
15600
17900
1.37
1.36
1.63
345
344
361
Results and discussion
a
Synthesis and characterization
Determined by GPC with polystyrene as standard and THF as eluent.
Decomposition temperature (1% weight loss) determined by TGA in
b
The structures and synthetic route to the polymers are shown in
Fig. 1 and Scheme 1, respectively. Compound 7a was prepared
according to the literature procedure.15 Compounds 7b and 7c
were synthesized in an identical approach. First, alcohols 2b,c
were prepared from ethyl formate 1 by reaction with 2 equiv of
Grignard reagent. Next, N-substituted phthalimides 3b,c were
prepared using a Mitsunobu reaction from 2b,c, and were finally
converted to 4b,c by hydrazinolysis. Dithienopyrroles (DTP)
6b,c was synthesized via Buchwald–Hartwig reaction of 3,30-
dibromo-2,20-bithiophene (5)18 and 4b,c in yields of 78% and
87%, respectively. Compounds 6b,c were converted to 7b,c by
deprotonation with n-butyllithium followed by addition of tri-
methyltin chloride in yields of 43% and 77%, respectively.
Subsequently, polymers PDTPBT-C8, PDTPBT-C6 and
PDTPBT-C5 were synthesized by Stille coupling10h between
7a,b,c and 4,7-dibromo-2,1,3-benzothiadiazole with Pd2dba3 as
catalyst in yields of 82, 82 and 88%, respectively. All of the three
polymers are soluble in common organic solvents, such as THF,
chloroform and chlorobenzene. The molecular weights and the
polydispersity indices (PDIs) measured by GPC with THF as
eluent and polystyrene as the standard are listed in Table 1.
N2.
Thermal properties
The thermal stability of PDTPBT-Cn was characterized by
thermogravimetric analysis (TGA). As shown in Fig. 2 and Table
1, the three polymers are thermally stable with 1% weight-loss
temperatures of 345, 344 and 361 ꢃC, respectively. No phase
transitions below the decomposition temperatures were observed
from DSC measurements.
Photophysical properties
Absorption spectra of the polymers were recorded in both
solution (CHCl3) and film, and related data are collected in Table
2. In solution, the polymers exhibit two distinct vibronic
absorption peaks (Fig. 3a) with wavelength identical to poly-
[cyclopenta[2,1-b:3,4-b0]dithiophene-alt-4,7-(2,1,3-benzothiadia-
zole)] (PCPDTBT),10e a state-of-the-art NIR-absorbing polymer
for PSCs. This indicates that PDTPBT-Cn have a rigid chain-
conformation. The film absorption spectra are 7–13 nm red-
shifted in comparison to those in solution with the maxima of
776, 777 and 771 nm for PDTPBT-C8, PDTPBT-C6 and
PDTPBT-C5, respectively (Table 2). The optical bandgaps of
PDTPBT-Cn in the solid state were estimated to be around 1.42
eV (Table 2), which is close to the ideal bandgap of the conju-
gated polymers for PSCs.6a Compared with homo-polymers of
Fig. 1 The chemical structures of PDTPBT-Cn (n ¼ 5, 6, 8).
2202 | J. Mater. Chem., 2009, 19, 2199–2206
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Fig. 2 TGA plots of PDTPBT-Cn with a heating rate of 10 ꢃC/min
under nitrogen atmosphere.
N-substituted DTPs,15 the film absorption spectra of the current
polymers exhibit a red-shift of up to 230 nm. As shown in Fig. 3b,
all of the three polymers show strong absorption between 600
and 900 nm, which are in the maximum flux region of the solar
emission spectrum.20 It is known that, in addition to the band-
width of the absorption, high absorption coefficient of conju-
gated polymer is very important for efficiently harvesting
photons and thereby getting large short circuit current in PSCs.21
As shown in Fig. 3b, the film absorption coefficients of the
polymers increase as the length of alkyl chains decreases, and the
films of PDTPBT-C5 and PDTPBT-C6 can absorb 60 and 35%
more light than PDTPBT-C8 at the same thickness.
Fig. 3 UV-vis absorption spectra of PDTPBT-Cn in CHCl3 solution (a,
1 ꢁ 10ꢂ5 M of the repeating unit) and film (b).
Electrochemical properties
The electrochemical characteristics of the polymers in thin film
were studied by cyclic voltammetry with Bu4NPF6 (0.1 M in
acetonitrile) as the electrolyte at a scan rate of 100 mV$sꢂ1. As
shown in Fig. 4, all of the three polymers exhibited one reversible
reduction process and one broad oxidation process. The oxida-
onset
tion onsets (E ) of PDTPBT-Cn have minor differences, which
ox
are 0.09, 0.06 and 0.01 V vs. Fc/Fc+ (Table 2), respectively, while
onset
the reduction onsets (E ) are same (ꢂ1.72 V). The HOMO and
red
LUMO energy levels and electrochemical bandgaps (Eegc) of the
onset
polymers calculated from E
ox
onset
are listed in Table 2.
red
and E
The three polymers have the same LUMO levels at ꢂ3.08 eV, and
their HOMO levels have minor difference in the range of ꢂ4.81
to ꢂ4.89 eV. For comparison, the HOMO and LUMO levels of
regioregular poly(3-hexylthiophene) (P3HT) were also measured
under the same conditions, which are ꢂ4.86 eV and ꢂ2.46 eV,
Fig. 4 Cyclic voltammograms of PDTPBT-Cn films on a glassy carbon
electrode measured in 0.1 mol/L Bu4NPF6 solution in acetonitrile.
Table 2 Photophysical and electrochemical properties of PDTPBT-Cn (n ¼ 5, 6, 8)
abs
max
l
(nm)
solutiona
film
E
(V vs Fc/Fc+)/HOMO (eV)
E
(V vs Fc/Fc+)/LUMO (eV)
onset
red
Eegc (eV)b
Eogpt (eV)c
onset
ox
polymer
PDTPBT-C8
PDTPBT-C6
PDTPBT-C5
765
764
764
776
777
771
0.09/ꢂ4.89
0.06/ꢂ4.86
0.01/ꢂ4.81
ꢂ1.72/ꢂ3.08
ꢂ1.73/ꢂ3.07
ꢂ1.72/ꢂ3.08
1.72
1.79
1.73
1.41
1.42
1.43
a
b
c
1 ꢁ 10ꢂ5 M of the repeating unit in anhydrous chloroform. Eegc ¼ E
ꢂ E
eV. The optical bandgap was calculated according to the equation Egopt
onset
ox
onset
red
¼ 1240/ledge, where ledge is the onset value of the absorption spectrum in the long wavelength range.
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are noticeably higher than PDTPBT-C8. At 429 nm, PDTPBT-
C5 exhibits an EQE of 39%, and the EQE is still as high as 27% at
800 nm. This is among the best for the polymers with absorption
edge beyond 800 nm.10e,14e
The photovoltaic properties of the devices with different pol-
ymer:PCBM (1:3) thicknesses are listed in Table 3, and Fig. 6
shows the illuminated I–V curves of the PSCs based on
PDTPBT-C8, PDTPBT-C6 and PDTPBT-C5 with a film
thickness of 90 nm. The best performance was obtained at the
thickness of 90 nm for PDTPBT-C8 and PDTPBT-C5, and 100
nm for PDTPBT-C6. The best device of PDTPBT-C8 shows an
open circuit voltage (Voc) of 0.43 V, a short circuit current (Jsc) of
6.19 mA/cm2, and a fill factor (FF) of 0.46, leading to a PCE of
1.23%. The best device of PDTPBT-C6 shows a Voc of 0.43 V,
a Jsc of 11.1 mA/cm2, and a FF of 0.43, leading to a PCE of
2.06%. The polymer PDTPBT-C5 exhibits the highest PCE of
2.80%, ascribed to the largest Voc of 0.54 V, although Jsc (11.9
mA/cm2) and FF (0.44) are similar to those of PDTPBT-C6.
It has been reported that film morphology could significantly
affect the performance of PSCs.23 Therefore, the film
morphology of PDTPBT-Cn:PCBM (1:3) was studied in order
to understand the reason for the improved device performance as
the length of the alkyl chain decreases. As shown in Fig. 7,
tapping-mode AFM studies show that the films for all three
polymers have a low root mean square (RMS) roughness, which
are 0.59, 0.80 and 0.35 nm for PDTPBT-C8, PDTPBT-C6 and
PDTPBT-C5, respectively. However, the films have very
different morphologies. The film of PDTPBT-C8:PCBM
exhibits phase separation into large islands with a domain size of
Fig. 5 (a) UV-vis absorption spectra of PDTPBT-Cn:PCBM (1:3) blend
films and (b) EQE spectra of the polymer photovoltaic cells with
a PDTPBT-Cn:PCBM (1:3) film thickness of 90 nm.
respectively. The electrochemical bandgaps are 0.3–0.4 eV larger
than optical bandgaps, which is also observed in other polymers
measured by a similar method.22
Photovoltaic properties
The PSCs with the device structure of ITO/PEDOT:PSS/poly-
mer:PCBM(1:3, w/w)/LiF/Al were fabricated, and the film
thickness were optimized. Depicted in Fig. 5 are the typical
absorption and EQE spectra. The shapes of the EQE spectra are
identical to the absorption spectra (Fig. 5a), indicating that all
the absorption of the polymers contributed to the photovoltaic
conversion. As shown in Fig. 5b, the EQE values of PDTPBT-C6
and PDTPBT-C5 in the wavelength range from 350 to 800 nm
Fig. 6 I–V curves of the polymer solar cells based on PDTPBT-Cn with
a polymer:PCBM (1:3) thickness of 90 nm.
Table 3 Photovoltaic performance of polymers PDTPBT-Cn measured under 100 mW/cm2 white light
polymer
thickness (nm)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
PDTPBT-C8
80
90
100
90
100
140
80
90
100
0.40
0.43
0.40
0.42
0.43
0.38
0.46
0.54
0.55
5.63
6.19
6.66
10.8
11.1
8.17
8.86
11.9
12.3
0.50
0.46
0.45
0.41
0.43
0.33
0.47
0.44
0.41
1.11
1.23
1.20
1.88
2.06
1.02
1.91
2.80
2.76
PDTPBT-C6
PDTPBT-C5
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Fig. 7 AFM topography (a–c) and phase contrast (d–f) images of PDTPBT-Cn:PCBM (1:3) films: PDTPBT-C8 (a, d), PDTPBT-C6 (b, e), PDTPBT-
C5 (c, f). The film thickness is about 90 nm.
400–900 nm. The film of PDTPBT-C6:PCBM also shows phase
separate into islands, but the domains are more regular and the
size is smaller (100–150 nm). PDTPBT-C5:PCBM exhibits the
best film morphology, and the AFM images indicate a much
more uniform mixing of PDTPBT-C5 and PCBM with a domain
size of ꢀ50 nm, as shown in Fig. 7c,f. This alkyl chain length
induced difference of film morphology and accordingly variation
of Voc were also observed in other polymer systems.24 Mean-
while, hole only device measurements of PDTPBT-C8,
PDTPBT-C6 and PDTPBT-C5 indicate that the length of the
alkyl chain has a negligible effect on mobility. Consequently, the
improved PSC performance with the decrease of the length of
alkyl chain can tentatively be attributed to the improved film
morphology and the more efficient harvest of light. It is worth
noting that the device performance of PDTPBT-C5 is better than
those of most of the NIR-absorbing polymers and comparable to
that of the state-of-the-art NIR-absorbing polymer PCPDTBT
with similar device structure and fabrication conditions,10e,i but
its synthesis is straightforward and much easier than PCPDTBT.
We expect further improvement of the device performance
through optimization of polymer structure, film morphology and
device configuration.2d
heterojunction polymer solar cells with the polymers as donor
and PCBM as acceptor (polymer:PCBM ¼ 1:3, w/w) were
fabricated. The device performance was improved dramatically
with a decrease of the alkyl chain length. Polymer PDTPBT-C5
exhibits the best device performance with a power conversion
efficiency of 2.80%. The enhanced performance from PDTPBT-
C8 to PDTPBT-C6 and then to PDTPBT-C5 can tentatively be
ascribed to the increased absorption coefficients and the
improved film morphology as the length of alkyl chain decreases.
Easy synthesis and structural modification of the dithienopyrrole
unit together with possible device performance improvement
through optimization of the film morphology and device
configuration make this type of polymer promising donor
materials for high performance polymer solar cells.
Acknowledgements
This work is supported by NSFC (Nos. 20423003, 20521415 and
50703041), Science Fund for Creative Research Groups of NSFC
(20621401), and Chinese Academy of Sciences (KJCX2-YW-
M11).
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