Synthesis and photovoltaic properties of thieno[3,4-c]pyrrole-4,6-dione- based donor-acceptor Copolymers

Article abstract of DOI:10.1002/pola.26164

Three donor-acceptor (D-A) 1,3-di(thien-2-yl)thieno [3,4-c]pyrrole-4,6- dione-based copolymers, poly{9,9-dioctylfluorene-2,7-diyl-alt-1,3-bis(4- hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, poly{N-(1-octylnonyl) carbazole-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4, 6-dione}, and poly {4,8-bis(2-ethylhexyloxyl) benzo[1,2-b:3,4-b']dithiophene- alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c] pyrrole-4,6-dione} were synthesized by Suzuki or Stille coupling reaction. By changing the donor segment, the bandgaps and energy levels of these copolymers could be finely tuned. Cyclic voltammetric study shows that the highest occupied molecular orbital (HOMO) energy levels of the three copolymers are deep-lying, which implies that these copolymers have good stability in the air and the relatively low HOMO energy level assures a higher open-circuit potential when they are used in photovoltaic cells. Bulk-heterojunction photovoltaic cells were fabricated with these polymers as the donors and PC₇₁BM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.22, 0.74, and 3.11% with large open-circuit potential of 1.01, 0.99, and 0.90 V under one sun of AM 1.5 solar simulator illumination (100 mW/cm²).

Full text of DOI:10.1002/pola.26164

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Synthesis and Photovoltaic Properties of Thieno[3,4-c]pyrrole-4,6-dione-
Based Donor–Acceptor Copolymers
Shanpeng Wen, Weidong Cheng, Pengfei Li, Shiyu Yao, Bin Xu, Hui Li, Yajun Gao,
Zilong Wang, Wenjing Tian
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, People’s Republic of China
Correspondence to: W. Tian (E-mail: wjtian@jlu.edu.cn)
Received 20 March 2012; accepted 1 May 2012; published online
DOI: 10.1002/pola.26164
ABSTRACT: Three donor–acceptor (D–A) 1,3-di(thien-2-yl)thieno
[3,4-c]pyrrole-4,6-dione-based copolymers, poly{9,9-dioctylfluor-
ene-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-
4,6-dione}, poly{N-(1-octylnonyl)carbazole-2,7-diyl-alt-1,3-bis(4-
hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}, and poly
{4,8-bis(2-ethylhexyloxyl) benzo[1,2-b:3,4-b0]dithiophene-alt-1,3-
bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c] pyrrole-4,6-dione} were
synthesized by Suzuki or Stille coupling reaction. By changing the
donor segment, the bandgaps and energy levels of these copoly-
mers could be finely tuned. Cyclic voltammetric study shows that
the highest occupied molecular orbital (HOMO) energy levels of the
three copolymers are deep-lying, which implies that these copoly-
mers have good stability in the air and the relatively low HOMO
energy level assures a higher open-circuit potential when they are
used in photovoltaic cells. Bulk-heterojunction photovoltaic cells
were fabricated with these polymers as the donors and PC71BM as
the acceptor. The cells based on the three copolymers exhibited
power conversion efficiencies of 0.22, 0.74, and 3.11% with large
open-circuit potential of 1.01, 0.99, and 0.90 V under one sun of AM
1.5 solar simulator illumination (100 mW/cm2).
2012 Wiley
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KEYWORDS:
copolymerization; electrochemistry; high per-
formance polymers
INTRODUCTION Polymer solar cells (PSCs) have obtained
considerable research interests because of their prominent
merits, such as low cost, easy processing, light weight,
mechanically flexible, and suitability for large-area fabrica-
tion.1–6 So far, the most efficient device structure of PSCs
was based on the concept of bulk heterojunction (BHJ),
which consists of a blend of conjugated polymers and fuller-
ene derivative as electron donors and acceptors, respec-
tively.7–10 During the past decade, BHJ solar cells based on
regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phe-
nyl-C61-butyric acid methyl ester blend have been widely
investigated and power conversion efficiencies (PCEs) up to
4–5% have been reported.11–14 However, further improve-
ment of P3HT-based photovoltaic devices is difficult due to
P3HT’s intrinsic absorption limit. To enlarge the absorption,
some low-bandgap polymers have been synthesized success-
fully and applied to PSCs as donors. Although these low-
bandgap polymers exhibited high short-circuit current (Jsc),
the device performance suffers owing to the low open-circuit
voltage (Voc) of 0.5–0.7 V. According to the BHJ solar cell
model and experimental results, the Voc is related to the
energy difference between the highest occupied molecular
orbital (HOMO) level of the donor and the lowest unoccu-
pied molecular orbital (LUMO) level of the acceptor.15–17
Therefore, in order for the realization of a higher PCE, an
ideal conjugated polymer is required to have both lower
bandgap and deeper HOMO level.
The donor–acceptor (D–A) strategy has demonstrated itself to
be one efficient approach, in which conjugated electron-rich
(donor) and electron-deficient (acceptor) units are alternatively
copolymerized to manipulate optical bandgap and optimize the
HOMO and LUMO energy levels via intramolecular charge trans-
fer (ICT).18 In the recent years, several efficient D–A copolymers
have exhibited promising potentialities for PSCs. For instance,
electron-deficient units derived from 4,7-dithien-2-yl-2,1,3-ben-
zothiadiazole,19 thiazolo[5,4-d]thiazole,20,21 3,6-diaryl-2,5-dihy-
dropyrrolo[3,4-c]pyrrole-1,4-dione,22,23 and thieno[3,4-b]thio-
phene-2-carboxylate24,25 units, when these acceptor units are
copolymerized with various donor units, such as fluorene,26
carbazole,27
dithienosilole,28
4H-cyclopenta[2,1-b;3,4-b0]
dithiophene,29 and benzo[1,2-b:3,4-b0]dithiophene (BDT),30–33
the corresponding BHJ devices have shown attractive PCE of up
to 7% after process optimization.34,35
Among various acceptor units, thieno[3,4-c]pyrrole-4,6-dione
(TPD) has attracted much interests in the design of new con-
jugated copolymers for the application in photovoltaic devi-
ces.36–41 This TPD unit has a compact planar structure which
Additional Supporting Information may be found in the online version of this article.
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is favorable for the electron delocalization along the poly-
meric backbone. Moreover, the strong electron withdrawing
effect from TPD unit could adjust both HOMO and LUMO
energy levels lower within the resulting copolymer. As we
know that the lower HOMO level is desirable for the stability
and Voc in PSCs. Very recently, Leclerc and coworkers synthe-
sized a series of D–A copolymers based on 1,3-di(thien-2-
yl)thieno[3,4-c]pyrrole-4,6-dione (DTTPD) unit for polymer
solar cell (PSC) and the devices demonstrates a relatively
high PCE of 3.9%.42 DTTPD unit, being a derivative of TPD,
owns some advantages comparing with TPD. First, for
DTTPD unit, two thiophene units are added in both ends to
enlarge the planar structure, which would greatly promote
facial p–p stacking of the molecules, thus benefit red shift of
the absorption spectra. Second, the two electron-rich, flank-
ing thienyl units would help improve the hole mobility, as
thiophene-based polymers (such as P3HT) have shown very
high hole mobility.43 All of these advantages make DTTPD to
be another potential acceptor unit for the design of photo-
voltaic donor polymers.
cal measurements of these derivatives were performed with
Bioanalytical Systems BAS 10 B/W electrochemical
workstation.
a
Photovoltaic Device Fabrication and Characterization
The solar cells were fabricated with a device structure
ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/Polymer:PC71BM Blend/LiF/Al. The ITO glass
substrates were precleaned by detergent, acetone and boil-
ing in H2O2. Highly conducting (PEDOT:PSS, Baytron P,
Al4083) was spin-casted (3000 rpm) at a thickness of ꢁ40
nm from aqueous solution (after passing through a 0.45-lm
ꢀ
filter). The substrate was annealed at 120 C for 15 min on
hot plate. The active layer contained a blend of copolymers
as electron donor and PC71BM as electron acceptor, which
was prepared by weight ratio (1:2, w/w) in chlorobenzene
(7 mg/mL) for copolymers. The active layers were obtained
by spin coating the blend solutions at 1000 rpm for 50 s
and the thickness of films were ꢁ90 nm, as measured with
the Ambios Technology XP-2. Subsequently, LiF (0.6 nm) and
Al (100 nm) electrodes were deposited via thermal evapora-
tion in vacuum (5 ꢂ 10ꢃ4 Pa) in thickness of approximately.
The active area was about 5 mm2. Current–voltage (J–V)
characteristics were recorded using Keithley 2400 Source
Meter in the dark and under 100 mW/cm2 simulated AM
1.5 G irradiation (Sciencetech SS-0.5K Solar Simulator). All
the measurements were performed under ambient atmos-
phere at room temperature.
In this article, we designed and synthesized three new D–A
conjugated copolymers consisting of alkylated DTTPD
acceptor unit coupled to different electron-donating units:
poly{9,9-dioctylfluorene-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-
5-octylthieno[3,4-c]pyrrole-4,6-dione} (PF-DTTPD), poly{N-(1-
octylnonyl)carbazole-2,7-diyl-alt-1,3-bis(4-hexylthien-2-yl)-5-
octylthieno[3,4-c]pyrrole-4,6-dione} (PC-DTTPD), and poly
{4,8-bis(2-ethylhexyloxyl) benzo[1,2-b:3,4-b0]dithiophene-alt-
1,3-bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c] pyrrole-4,6-dione}
(PBDT-DTTPD). The hexyl side chains added on the thiophene
parts of DTTPD unit are used to improve solubility and inter-
chain packing properties of the copolymers. And in fact, these
three conjugated copolymers do show good solubility in com-
mon organic solvents, such as chloroform, toluene, and chloro-
benzene, and so forth. The photovoltaic performance of the PSC
devices based on the three copolymers, PF-DTTPD, PC-DTTPD,
and PBDT-DTTPD with a cell structure of ITO/PEDOT:PSS/
copolymer:PC71BM/LiF/Al exhibit PCEs of 0.22, 0.74, and
3.11% under one sun of AM 1.5 solar simulator illumination
(100 mW/cm2). It should be noted that high Vocs of 1.01, 0.99,
and 0.90 V have been achieved from these cells due to the
deep-lying HOMO energy levels of the copolymers.
Materials
All starting materials were purchased from either Acros or
Aldrich Chemical and used without further purification,
unless otherwise noted. In synthetic preparations, diethyl
ether, and THF were dried by distillation from sodium/ben-
zophenone under nitrogen. Similarly, N, N-dimethyl formam-
ide (DMF) and dichloromethane were distilled from CaH2
under nitrogen. The following compounds were synthesized
according to procedures in the literature: 4-(hexyl-2-thienyl)-
stannane (4),44 2,7-bis(40,40,50,50-tetramathyl-10,30,20-dioxaboro-
lan-20-yl)-9,9-dioctylfluorene (monomer 6), 2,7-bis(40,40,50,50-
tetramethyl-10,30,20-dioxaborolan-20-yl)-N-9-octylnonylcarbazole
(monomer 7),45 and 2,6-bis(trimethyltin)-4,8-diethylhexyloxyl-
benzo[1,2-b:3,4-b0]dithiophene (monomer 8).16
Synthesis of Monomer
EXPERIMENTAL
5-Octylthieno[3,4-c]pyrrole-4,6-dione (2)37
Measurements and Characterization
A solution of thiophene-3,4-dicarboxylic acid (5.00 g, 29.0
mmol) in acetic anhydride (150 mL) was stirred at 140 ꢀC
overnight. The solvent was removed and the crude product
was used for the next step without any purification. The
brown solid (assuming 29.0 mmol) was dissolved in toluene
(300 mL) then n-octylamine (5.70 g, 43.9 mmol, 7.3 mL)
was added. The reaction mixture was refluxed for 24 h. The
reaction mixture was cooled down and the solvent was
removed under reduced pressure. The resulting solid was
dissolved into 250 mL of thionyl chloride. The mixture was
mixture was refluxed for 4 h then cooled. The solvent was
removed under reduced pressure and the crude product was
purified by column chromatography using dichloromethane/
1H NMR spectra were recorded on Bruker AVANCE 300-MHz
spectrometer with chloroform-d as solvent and tetramethyl-
silane (TMS) as internal standard. The elemental analysis
was carried out with a Thermoquest CHNS-Ovelemental ana-
lyzer. The gel permeation chromatographic (GPC) analysis
was carried out with a Waters 410 instrument with tetrahy-
drofuran as the eluent (flowrate: 1 mL/min, at 35 ꢀC) and
polystyrene as the standard. The thermogravimetric analysis
(TGA) was performed on a Perkin Elmer Pyris 1 analyzer
under nitrogen atmosphere (100 mL/min) at a heating rate
of 10 ꢀC/min. UV–visible absorption spectra were measured
using a Shimadzu UV–3600 spectrophotometer. Electrochemi-
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petroleum ether (3:2) to afford 4.12 g of the product as a
white solid (yield: 54%).
1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.63 (s, 2H), 3.64
(t, J ¼ 7.5, 2H), 2.58 (t, J ¼ 7.5, 4H), 1.70–1.55 (m, 6H),
1.42–1.23 (m, 22H), 0.92–0.80 (m, 9H). 13C NMR (75 MHz,
1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.80 (s, 2H), 3.61
(t, J ¼ 7.5, 2H), 1.69–1.59 (m, 2H), 1.35–1.24 (m, 10H), 0.87
(t, J ¼ 7.2, 3H). Anal. Calcd for C14H19NO2S: C, 63.36; H,
7.22. Found: C, 63.09; H, 7.48.
CDCl3, TMS):
d (ppm) 162.34, 143.76, 135.37, 131.85,
130.22, 128.29, 113.49, 38.66, 31.77, 31.56, 29.59, 29.52,
29.15, 28.92, 28.45, 26.97, 22.60, 14.06. Anal. Calcd for
C34H45Br2NO2S3:C, 54.04; H, 6.00. Found: C, 53.86; H, 6.24.
1,3-Dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (3)37
5-octylthieno[3,4-c]pyrrole-4,6-dione (3.12 g, 11.7 mmol)
was dissolved in a mixture of sulfuric acid (16.0 mL) and tri-
fluoroacetic acid (52 mL). The solution was kept in the dark.
N-Bromosuccinimide (NBS, 6.27 g, 35.2 mmol) was added in
four portions, and the reaction mixture was stirred at room
temperature overnight. The brown-red solution was poured
into water and extracted with chloroform. The organic phase
were combined and dried over anhydrous magnesium sul-
fate. After the removal of the solvent, the crude product was
by column chromatography using dichloromethane/ petro-
leum ether (1:1) as an eluent to obtain 3 (3.73 g, yield:
75%).
Synthesis of Polymer
Poly{9,9-dioctylfluorene-2,7-diyl-alt-1,3-di(4-hexylthien-2-
yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}
Monomer 6 (122 mg, 0.191 mmol) and monomer DTTPD
(144 mg, 0.191 mmol) and dry toluene (3 mL) and a 2 M
aqueous solution of Na2CO3 (2 mL) were added to a 25-mL-
double-neck round-bottom flask. The reaction container was
purged with nitrogen for 30 min to remove oxygen, and then
Pd(PPh3)4 (6.6 mg, 3%) was added. The reaction was stirred
for 72 h at 95 ꢀC under nitrogen. The reactant was cooled
down to room temperature and was slowly poured into
methanol (200 mL) and then filtered through a Soxhlet thim-
ble; subsequently, the filter residue was subjected to Soxhlet
extraction with methanol, hexane, and chloroform. The frac-
tion from chloroform was concentrated under reduced pres-
sure and precipitated into methanol, collected by filtration.
The final product was dried under vacuum overnight to
afford PF-DTTPD as a red solid (145 mg, yield: 77%).
1H NMR (300 MHz, CDCl3, TMS): d (ppm) 3.59 (t, J ¼ 7.5,
2H), 1.68–1.57 (m, 2H), 1.35–1.24 (m, 10H), 0.87 (t, J ¼ 7.2,
3H). Anal. Calcd for C14H17Br2NO2S: C, 39.74; H, 4.05. Found:
C, 39.52; H, 4.31.
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 8.10 (s, 2H), 7.98
(br, 2H), 7.78 (d, J ¼ 8.0, 2H), 7.49 (d, J ¼ 8.0, 2H), 7.45 (br,
2H), 3.70 (br, 2H), 2.74 (t, J ¼ 6.0, 4H), 2.03 (br, 4H), 1.76–
1.64 (m, 6H), 1.41–1.26 (m, 22H), 1.24–1.05 (m. 22H), 0.88
(t, J ¼ 6.0, 9H), 0.82 (t, J ¼ 6.0, 6H), 0.72 (br, 4H). Anal.
Calcd for C63H85NO2S3:C, 76.85; H, 8.70. Found: C, 76.62; H,
8.98.
1,3-Bis(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-
dione (5)
To a solution of compound 3 (0.50 g, 1.18 mmol) and 4-
(hexyl-2-thienyl)stannane (4) (1.19 g, 2.60 mmol) in dry
THF (40 mL), PdCl2(PPh3)2 (49.76 mg, 3 mol %) was added.
The solution was refluxed for 24 h, then cooled and poured
into water. The mixture was extracted three times with chlo-
roform. The organic phase was combined and dried over an-
hydrous magnesium sulfate. After the removal of the solvent,
the crude product was purified by column chromatography
using dichloromethane/petroleum ether (1:1) as an eluent to
afford 0.56 g of product 5 as a yellow solid (yield: 80%).
Poly{N-(1-octylnonyl)carbazole-2,7-diyl-alt-1,3-bis
(4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione}
PC-DTTPD was prepared using the procedure for the synthe-
sis of PF-DTTPD, but monomer 7 (131 mg, 0.198 mmol) and
monomer DTTPD (150 mg, 0.198 mmol) were used as start-
ing materials. Yield: 140 mg, 71%.
1H NMR (300 MHz, CDCl3, TMS): d (ppm) 7.87 (s, 2H), 7.02
(s, 2H), 3.65 (t, J ¼ 7.5, 2H), 2.63 (t, J ¼ 7.5, 4H), 1.72–1.56
(m, 6H), 1.42–1.25 (m, 22H), 0.95–0.83 (m, 9H). Anal. Calcd
for C34H47NO2S3:C, 68.30; H, 7.92. Found: C, 68.12; H, 8.15.
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 8.05 (s, 2H), 7.69
(br, 2H), 7.51 (s, 2H), 7.49 (t, J ¼ 8.5, 4H), 4.60 (s, 1H), 3.71
(br, 2H), 2.81 (br, 4H), 2.32 (br, 2H), 1.98 (br, 2H), 1.73 (br,
6H), 1.44–1.04 (m, 46H), 0.91–0.85 (m, 9H), 0.81 (t, J ¼ 7.0,
6H). Anal. Calcd for C63H86N2O2S3:C, 75.70; H, 8.67. Found:
C, 75.38; H, 8.92.
13-Bis(5-bromo-4-hexylthien-2-yl)-5-octylthieno[3,4-c]pyr-
role-4,6-dione
Compound 5 (0.599 g, 1.00 mmol) and NBS (0.374 g, 2.10
mmol) were dissolved into a mixture of acetic acid and chlo-
roform in a two-neck round flask under argon protection,
and then the solution was protected from light and stirred at
room temperature. After 24 h, the reaction mixture was
poured into water and extracted three times with chloro-
form. The organic phase were combined and dried over an-
hydrous magnesium sulfate. After the removal of the solvent,
the crude product was purified by column chromatography
using dichloromethane/petroleum ether (1:1) as an eluent to
afford 0.59 g of the product DTTPD as a bright yellow solid
(yield: 78%).
Poly{4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:3,4-
b0]dithiophene-alt-1,3-bis(4-hexylthien-2-yl)-5-octylth-
ieno[3,4-c]pyrrole-4,6-dione}
Monomer 8 (147 mg, 0.191 mmol) and monomer DTTPD
(145 mg, 0.191 mmol), and dry toluene (5 mL) and DMF
(0.5 mL) were added to a 25-mL-double-neck round-bottom
flask. The reaction container was purged with nitrogen for
30 min to remove oxygen, and then Pd(PPh3)4 (6.6 mg, 3%)
was added. The reaction was stirred and refluxed for 24 h
under nitrogen. The reactant was cooled down to room tem-
perature and was poured into methanol (200 mL) and then
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SCHEME 1 Synthesis of monomer DTTPD.
RESULTS AND DISCUSSION
filtered through a Soxhlet thimble; subsequently, the filter
residue was subjected to Soxhlet extraction with methanol,
hexane, and chloroform. The fraction from chloroform was
concentrated under reduced pressure and precipitated into
methanol, collected by filtration. The final product was dried
under vacuum overnight to afford PBDT-TTz as a dark solid
(168 mg, yield: 85%).
Synthesis and Characterization
The synthetic routes to the monomer and polymers are out-
lined in Schemes 1 and 2. Monomer DTTPD was synthesized
in a multistep synthesis. Starting from commercially available
thiophene 3,4-dicarboxylic acid, in three steps, 1,3-dibromo-
5-octylthieno[3, 4-c]pyrrole-4, 6-dione (3) was obtained. By
Stille coupling of compound 3 with 4-(Hexyl-2-thienyl)stan-
nane (4), compound 5 was obtained in good yield. The
monomer DTTPD was prepared by the bromination of 5
with NBS in a mixture of chloroform and acetic acid at
1H NMR (500 MHz, CDCl3, TMS): d (ppm) 8.10 (br, 4H), 4.20
(br, 4H), 3.72 (br, 2H), 2.75 (br, 4H), 2.10–1.25 (m, 42H),
0.12–0.78 (m, 21H). Anal. Calcd for C60H81NO4S3:C, 69.25; H,
7.85. Found: C, 68.95; H, 8.13.
SCHEME 2 Synthesis of copolymers.
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TABLE 1 Polymerization Results and Thermal Properties of
Copolymers
Yield
(%)
Mn
(104)
Mw
(104)
TGA
Polymers
PDI
(Td)
PF-DTTPD
77
71
85
1.36
1.17
3.92
2.62
2.14
1.92
1.83
2.95
412
379
334
PC-DTTPD
PBDT-DTTPD
11.55
ambient temperature with a good yield of 78%. In 1H NMR
spectra of DTTPD, as shown in Supporting Information Fig-
ure S1, thiophene proton peaked at about 7.02 ppm, disap-
peared after the bromination, which confirmed the success
of the bromination. The purity of the monomers and the in-
termediate compounds were confirmed by 1H NMR and ele-
mental analysis. The alternative copolymers of PF-DTTPD
and PC-DTTPD were synthesized by Suzuki coupling reac-
tion, and PBDT-DTTPD was synthesized by Stille coupling
reaction. In the 1H NMR spectrum of PF-DTTPD (Supporting
Information Fig. S1), the characteristic peaks at 8.12–7.95,
7.82–7.75, and 7.53–7.44 ppm can be assigned to the reso-
nance of protons on the fluorene ring and the thiophene
ring. The peak due to protons in ACH2A linked to the N
atom is at 3.70 ppm; the peaks at 2.74–0.72 ppm correspond
to the protons of the long alkyl chain.
FIGURE 1 TGA curves of copolymers at the heating rate of 10
ꢀC/min under nitrogen atmosphere.
respectively. The absorption peak around 336 nm could be
originated from the p–p* absorption of conjugated polymer
backbone, while the strong absorption peak at about 450 is
All these copolymers are soluble in common organic solvents
such as chloroform, toluene, and chlorobenzene. The good
solubility of the synthesized copolymers provides the solu-
tion-processed thin films for electronic and optoelectronic
applications. The molecular weights and polydispersities of
the resulting copolymers were determined by GPC analysis
with a polystyrene standard calibration. The weight-average
molecular weights (Mw) of PF-DTTPD, PC-DTTPD, and PBDT-
DTTPD were 2.6 ꢂ 104, 2.1 ꢂ 104, and 11.5 ꢂ 104 with pol-
ydispersity indices (PDIs, Mw/Mn) of 1.92, 1.83, and 2.95,
respectively. Table 1 summarizes the polymerization results
including molecular weights, polydispersity index (PDI), and
thermal stability of the copolymers.
Thermal Properties
Thermal stability of the copolymers was investigated with
TGA, as shown in Figure 1. The TGA analysis reveals that the
onset temperatures with 5% weight-loss (Td) of PF-DTTPD,
PC-DTTPD, and PBDT-DTTPD are 412, 379, and 334 ꢀC,
respectively. The high thermal stability of the resulting
copolymers prevents the deformation of the copolymer mor-
phology and the degradation of the polymeric active layer
under applied electric fields. The Td of the copolymers is
also summarized in Table 1.
Optical Properties
The optical absorption spectra of PF-DTTPD, PC-DTTPD, and
PBDT-DTTPD in dilute (10ꢃ5 M) chloroform solution and
thin films are shown in Figure 2, and the main optical prop-
erties of the copolymers are listed in Table 2. In solution,
PF-DTTPD and PC-DTTPD show similar absorption spectra
with the two absorption peaks around 336 and 450 nm,
FIGURE 2 UV–vis absorption spectra of PF-DTTPD, PC-DTTPD,
and PBDT-DTTPD in (a) chloroform solution and (b) in thin
films.
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TABLE 2 Optical and Electrochemical Properties of Copolymers
abs;sol
max
abs;film
max
onset
onset
red
Polymers
k
(nm)
k
(nm)
Egopt (eV)
E
ox
(V)
HOMO (eV)
E
(V)
LUMO (eV)
Egec (eV)
PF-DTTPD
336/450
336/450
512
340/456
341/460
517
2.30
2.28
1.87
0.92
0.86
0.70
ꢃ5.66
ꢃ5.60
ꢃ5.44
ꢃ1.08
ꢃ1.15
ꢃ1.13
ꢃ3.66
ꢃ3.59
ꢃ3.61
2.00
2.01
1.83
PC-DTTPD
PBDT-DTTPD
attributed to the ICT transition between the donor unit and
DTTPD acceptor unit.46 In contrast, the solution absorption
spectrum of PBDT-DTTPD, with an absorption maximum
(kambasx) at 512 nm, is red-shifted compared to those of PF-
DTTPD and PC-DTTPD, which can be explained by much
stronger ICT effect in PBDT-DTTPD than that in PF-DTTPDT
and PC-DTTPD.
onset
red
LUMOðeVÞ ¼ ꢃeðE
þ 4:74Þ
onset
onset
EgecðeVÞ ¼ eðE
ꢃ E
Þ
ox
red
onset
ox
onset
red
where E
and E
are the measured onset potentials
relative to Ag/Agþ.
The results of the electrochemical measurements and calcu-
lated energy levels of the copolymers are listed in Table 2.
The estimated HOMO and LUMO energy levels of PF-DTTPD
are ꢃ5.66 and ꢃ3.64 eV, respectively. The LUMO energy lev-
els of PC-DTTPD and PBDT-DTTPD are at ꢃ3.59 and ꢃ3.61
eV, respectively, which are similar to that of PF-DTTPD. It
indicates that the different donor units have almost no effect
on the LUMO energy level of copolymers. The HOMO energy
levels of the copolymers PF-DTTPD, PC-DTTPD, and PBDT-
DTTPD are in the range of ꢃ5.66 to ꢃ5.44 eV, which is
clearly affected by the varied electron-donating ability of the
donor units due to the modulation of ICT inside the copoly-
mers. For PBDT-DTTPD, the HOMO energy level shift upward
by 0.2 eV compared with those of the other two polymers,
which results in the decrease of the bandgap. The electro-
chemical bandgap of PBDT-DTTPD is similar with the optical
bandgap estimated from the UV–vis absorption onset. For
PF-DTTPD and PC-DTTPD, the electrochemical bandgaps are
2.02 and 2.01 eV, which are slightly lower than the optical
bandgaps.
Figure 2(b) shows the optical absorption spectra of thin
films of the copolymers. The thin film absorption spectra are
generally similar in shape to those in dilute solution. The
maximum absorption peak of copolymers in thin film red
shifts 5, 10, and 5 nm compared that in solution, respec-
tively, presumably indicating the formation of a p-stacked
structure in the solid state.47,48 From the low energetic edge
of the absorption spectrum of the individual copolymer, the
bandgaps of PF-DTTPD and PC-DTTPD were estimated to be
abs
2.30 eV (k
¼ 540 nm), and smaller bandgap of 1.87 eV
max
abs
max
was obtained for PBDT-DTTPD (k
¼ 661 nm), respec-
tively. These results suggest that by changing the donor units
with different electron-donating ability, the absorption spec-
tra can be tuned, which is very useful for the design of the
photovoltaic materials.
Electrochemical Properties
Electrochemical cyclic voltammetry has been widely
employed to investigate the redox behavior of the polymer
and to estimate its HOMO and LUMO energy levels. Cyclic
voltammetry of the copolymers in films was performed in
acetonitrile with 0.1 M tetrabutylammonium hexafluorophos-
phate (TBAPF6) as supporting electrolyte at scan rates of 50
mV/s. Platinum wire electrodes were used as both counter
and working electrodes, and silver/silver ion (Ag in 0.1 M
AgNO3 solution, from Bioanalytical Systems) was used as a
reference electrode. Ferrocene/ferrocenium (Fc/ Fcþ) was
used as the internal standard.
The cyclic voltammogram of the copolymers are shown in Fig-
ure 3. On the anodic sweep, all copolymers showed an oxida-
tion with an onset potentials of 0.92 V (vs. Ag/Agþ) for PF-
DTTPD, 0.86 V for PC-DTTPD, and 0.70 V for PDBT-DTTPD,
respectively. In contrast, the cathodic sweep showed an onset
reduction potentials of ꢃ1.10 V (vs. Ag/Agþ) for PF-DTTPD,
ꢃ1.15 V for PC-DTTPD, and ꢃ1.13 V for PBDT-DTTPD.
From the onset oxidation potentials (Eooxnset) and the onset
onset
red
reduction potentials (E
) of the copolymers, HOMO and
LUMO energy levels as well as the energy gaps were calcu-
lated according to the following equations:44,49
FIGURE 3 Cyclic voltammograms of copolymer thin films on
Pt wires in 0.1 M TBAPF6 in acetonitrile. The scan rates used
were 50 mV/s.
onset
HOMOðeVÞ ¼ ꢃeðE
þ 4:74Þ
ox
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TABLE 3 Photovoltaic Properties of the Copolymer
Photovoltaic Cells
Donor
Blend
Jsc
(mA/cm2)
Copolymers
Ratio D:A
Voc (V)
FF
PCE
PF-DTTPD
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
1.03
1.01
1.00
1.01
0.99
0.99
0.97
0.97
0.91
0.90
0.90
0.89
0.38
0.62
0.56
0.42
0.98
1.83
1.80
1.15
3.52
6.11
5.33
3.18
0.30
0.35
0.34
0.35
0.37
0.41
0.40
0.43
0.45
0.56
0.52
0.54
0.15
0.22
0.19
0.15
0.36
0.74
0.70
0.48
1.44
3.11
2.40
1.53
PC-DTTPD
PBDT-DTTPD
FIGURE 4 DFT-calculated LUMO and HOMO of the geometry
optimized structures of analogous monomers of PF-DTTPD,
PC-DTTPD, and PBDT-DTTPD.
To better understand the oxidative and reductive properties
of these polymers, the electronic properties of their analo-
gous monomers were calculated by density functional theory
(DFT) at the DFT B3LYP/6-31G* level.50 The HOMO and
LUMO wave functions of the analogous monomers are shown
in Figure 4. To simplify the calculation, only one repeating
unit of each polymer was subject to the calculation, with
alkyl chains replaced by CH3 groups. The electron density
distributions of LUMO levels for resulting polymers are
nearly identical and mainly localized on the DTTPD-based
acceptor unit. Thus, the change of donor units has almost no
effect on the LUMO levels. However, the electron density of
HOMO is distributed over the whole conjugated backbone
(both the acceptor and donor units), which indicates the do-
nor unit significantly affects the HOMO level of these poly-
mers. The results from the calculation are in agreement with
the experimental results estimated from the cyclic
voltammograms.
tor (FF) of 0.35, giving a PCE of 0.22%. Another two cells
based on PC-DTTPD:PC71BM and PBDT-DTTPD:PC71BM
showed Voc of 0.99 and 0.90 V, significantly increasing Jsc to
1.83 and 6.11 mA/cm2, FF to 0.41 and 0.56, and PCEs to
0.74 and 3.11%, respectively. It is worth noting that these
devices show quite high Voc (up to 1.01 V in the PF-DTTPD
device). The Voc of the cells based on PF-DTTPD and PC-
DTTPD showed similar values above 0.99 V, while the cell
based on PBDT-DTTPD showed a lower Voc of 0.90 V. This
difference of Voc could result from the upward shift the
HOMO energy levels of PBDT-DTTPD because the Voc is
mainly determined by the difference between of the LUMO
of acceptor (PCBM) and the HOMO of donor (the conjugated
polymer).
As for the Jsc, the device based on PBDT-DTTPD:PC71BM
exhibits the highest Jsc among the three polymers due to the
absorbance of PBDT-DTTPD matches the solar radiation
Photovoltaic Properties
To investigate the photovoltaic properties of the copolymers,
the bulk-heterojunction (BHJ) photovoltaic cells with a struc-
ture of ITO/PEDOT-PSS/copolymer:PC71BM/LiF/AI were fab-
ricated. The active layer was a blend of copolymers with
PC71BM spin-coated from DCB solution. The blending ratio
between copolymers and PC71BM in the active layer has
obviously effect on the photovoltaic performance, so BHJ
photovoltaic cells with varied weight ratios (copoly-
mer:PC71BM from 1:1 to 1:4) in the active layer were investi-
gated (the results are listed in Table 3). The optimized per-
formances were achieved with weight ratio of copolymers:
PC71BM at 1:2 (w/w). Solar cells were tested under AM 1.5G
illumination of 100 mW/cm2 and the active area of the devi-
ces was 5 mm2. Current density–voltage (J–V) curves are
shown in Figure 5 and data on BHJ solar cells are summar-
ized in Table 3.
The typical density–voltage curves of copolymer-based devi-
ces are shown in Figure 5. The based on PF-DTTPD:PC71BM
(1:2 w/w) showed an open-circuit voltage (Voc) of 1.01 V, a
short circuit current density (Jsc) of 0.62 mA/cm2, a fill fac-
FIGURE 5 J–V curves of the copolymer photovoltaic cells
based on PF-DTTPD, PC-DTTPD, and PBDT-DTTPD under the
illumination of AM 1.5, 100 mW/cm2.
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FIGURE 6 AFM images showing the morphology of the blend films spin-coated from DCB for copolymers: (a) height image of PF-
DTTPD/PC71BM (w/w, 1:2); (b) height image of PC-DTTPD/PC71BM (w/w, 1:2); (c) height image of PBDT-DTTPD/ PC71BM (w/w, 1:2).
CONCLUSIONS
better than that of PF-DTTPD and PC-DTTPD. Furthermore,
the amount of the absorbed light depends not only on the
We have successfully synthesized three donor–acceptor (D–
edge of absorption wavelength but also on the optical den-
A) copolymers containing DTTPD acceptor unit and different
sity. Comparing the absorption of PF-DTTPD, PC-DTTPD, and
donor units including fluorene, 2,7-carbazole, and BDT, by
PBDT-DTTPD in Figure 2(b), PBDT-DTTPD have higher
the Pd-catalyzed Suzuki or Stille coupling reaction. The
absorption coefficient than PF-DTTPD and PC-DTTPD, and
copolymers exhibit moderate to high molecular weights and
PF-DTTPD and PC-DTTPD have similar absorption coeffi-
are readily soluble in common organic solvents. The electro-
cients in film. That is one of the reasons why the device
chemical properties indicated that these copolymers had rel-
based on PBDT-DTTPD: PC71BM has the higher Jsc than the
devices based on PF-DTTPD: PC71BM and PC-DTTPD:
PC71BM.
ative low HOMO energy level and high air-stability, and the
low HOMO energy level assured a relative high open-circuit
potential when they were used in PSCs. The photovoltaic de-
vice based on a PBDT-DTTPD:PC71BM BHJ gives a higher
PCE of 3.11%. It indicates that PBDT-DTTPD is a promising
polymer donor material for application in PSCs.
In BHJ solar cells, the surface morphology of the active layer
is very important to their photovoltaic performance. Figure 6
shows the surface morphology of the active layers of PF-
DTTPD:PC71BM,
PC-DTTPD:PC71BM,
and
PBDT-
ACKNOWLEDGMENTS
DTTPD:PC71BM in atom force microscopy (AFM) images. As
is clearly evidenced by AFM, the PF-DTTPD:PC71BM blend
film shows a most coarse surface with the root-mean-square
(rms) of 3.05 nm and a ‘‘sphere’’ shape with a diameter over
150 nm. The percolation pathways were poorly formed,
which limits the transport of the mobile charge carriers to
the respective electrodes and leads to poor short circuit cur-
rent.51 This finding is fully consistent with the rather low
short-circuit current densities obtained for the PF-
DTTPD:PC71BM cell (0.62 mA/cm2). Compared with the PF-
DTTPD:PC71BM blend film, PC-DTTPD:PC71BM blend film
shows small roughness with a rms of 1.94 nm and no signifi-
cant aggregation, suggesting that the PC-DTTPD is good com-
patible with PC71BM molecules. So, the Jsc of PC-
DTTPD:PC71BM (1:2 w/w)-based device increases to 1.83
mA/cm2, which is nearly three times higher than that of PF-
DTTPD:PC71BM based device. In the case of PBDT-
DTTPD:PC71BM blend film, clear phase separation and a
small rms are showed in Figure 6(c), which is good for exci-
ton transportation and carrier collection efficiency and
results in a reduction of recombination loses and an increase
in Jsc. Consequently, the photovoltaic performance of PBDT-
DTTPD:P71CBM cell is higher than those of PF-
DTTPD:PC71BM cell and PC-DTTPD:PC71BM cell.
This work was supported by the State Key Development Pro-
gram for Basic Research of China (Grant No. 2009CB623605),
the National Natural Science Foundation of China (Grant No.
20874035), the 111 Project (Grant No. B06009), and the Pro-
ject of Jilin Province (20080305).
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