Low band gap copolymers consisting of porphyrins, thiophenes, and 2,1,3-benzothiadiazole moieties for bulk heterojunction solar cells

Article abstract of DOI:10.1002/pola.24700

Two novel low band gap conjugated copolymers containing porphyrins, thiophenes, and 2,1,3-benzothiadiazole (BTZ) moieties were synthesized and applied in bulk heterojunction solar cells. The thermal, optical, electrochemical, and photovoltaic properties o

Full text of DOI:10.1002/pola.24700

Low Band Gap Copolymers Consisting of Porphyrins, Thiophenes, and
2,1,3-Benzothiadiazole Moieties for Bulk Heterojunction Solar Cells
WEIPING ZHOU,1 PING SHEN,1,2 BIN ZHAO,1,2 PENG JIANG,1 LIJUN DENG,1 SONGTING TAN1,2
1College of Chemistry and Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province,
Xiangtan University, Xiangtan 411105, People’s Republic of China
2Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Xiangtan University, Xiangtan,
Hunan 411105, People’s Republic of China
Received 21 January 2011; accepted 1 April 2011
DOI: 10.1002/pola.24700
Published online 27 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Two novel low band gap conjugated copolymers
containing porphyrins, thiophenes, and 2,1,3-benzothiadiazole
(BTZ) moieties were synthesized and applied in bulk hetero-
junction solar cells. The thermal, optical, electrochemical, and
photovoltaic properties of the two copolymers were examined
to investigate the effect of the introduction of BTZ moiety in
the backbone of the porphyrin polymers. The copolymers
exhibited good thermal stability and film-forming ability. The
absorption spectra indicated that the BTZ moiety has signifi-
cant influence on the UV–visible region spectra of the copoly-
mers: with increasing the molar amount of BTZ moieties in
conjugated main chain, the absorption in the range of 450–
700 nm is largely broadened and red-shifted compared to the
similar polymers without BTZ moiety, and the optical band
gaps of copolymers were narrowed to ꢀ1.50 eV. The photolu-
minescence spectra showed that there is effective charge trans-
fer in the whole conjugated main chain. Cyclic voltammetry
displayed that the band gaps were reduced effectively by the
introduction of the BTZ moieties. The bulk heterojunction solar
cells were fabricated based on the blend of the copolymers
and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) in a 1:2
weight ratio. The maximum power conversion efficiency of
0.91% was obtained by using P2 as the electron donor under
2
C
V
the illumination of AM 1.5, 100 mW/cm . 2011 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 49: 2685–2692, 2011
KEYWORDS: 2,1,3-benzothiadiazole; bulk-heterojunction solar
cells; low band gap; porphyrin; thiophenes
INTRODUCTION In the last few decades, solar cells based on
conjugated polymeric materials have recently received con-
siderable attention due to their excellent electrical and opti-
cal properties comparable with inorganic semiconductors,
but also because they can provide devices with flexibility,
cost-effectiveness, light weight, and easy processability.1,2 To
achieve high power conversion efficiency (PCE), bulk hetero-
junction (BHJ) devices containing polymer donors and fuller-
ene-derived acceptors have been used and represent the
most efficient polymer solar cells (PSCs) structure.3–5 A PCE
of 7.4% has been achieved from BHJ devices of PTB7/
PC71BM, which is the first polymer solar cell with the PCE
over 7% so far.6
gated system more planar to increases the delocalization of
the backbone and then the reduction of the band gap.19
Another successful and flexible strategy to design low band
gap polymers involves the alternation of electron-rich unit
(donor, e.g., bithiophene, benzodithiophene) and electron-
deficient unit (acceptor, e.g., 2,1,3-benzothiadiazole, quinoxa-
line, dithiopheneylbenzothiadiazole) along the polymer
chain.20–22 The first breakthrough on the way to developing
highly efficient low band gap donor-acceptor (D-A) polymers
is PCPDTBT investigated by Heeger and coworkers.20
Since 54.3% of the sunlight energy is distributed in the visi-
ble region from 380 to 800 nm, an ideal active layer for a
polymer solar cell should have a broad and strong absorp-
tion spectrum in this range.23 Porphyrins possess a strong
absorption band at around 425 nm, and weak Q-bands at
around 600 nm. And investigators have been aiming to red-
shift the Soret-band and broaden the Q-bands of the spectra
of the porphyrins. Bo et al. have investigated a series of solu-
ble conjugated alternating porphyrin-dithienothiophene co-
polymers (IIa and IIb) and an achieved power conversion
efficiency (PCE) of 0.3% under AM 1.5, 100 mW/cm2 for the
Conventional wisdom dictates that suitable HOMO and
LUMO energy levels, broad and strong absorption, good film-
forming property, and high hole mobility in polymer blend
are important prerequisites for the conjugated polymers.7,8
To broaden light absorption range, many low band gap con-
jugated polymers have been exploited in the past decade.9–18
There are various means to obtain the low band gap conju-
gated polymers. One effective method is to make the conju-
Correspondence to: S. Tan (E-mail: tanst2008@163.com)
C
V
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 2685–2692 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
PSC using IIb:PCBM (1:3, w/w) as active layer.24 In our pre-
vious articles, two star-shaped polymers with the structure
of porphyrin as core and four polythiophene derivatives as
arms were synthesized and characterized, to obtain a power
conversion efficiency of 0.61%.25 In addition, we introduced
thiophene derivertives into the meso-positions to synthesis
two novel soluble p-conjugated porphyrin polymers P-PTT
and P-POT to obtain the best PCE of 0.32%.26 To further
broaden the absorption spectra and enhance the light-
harvest ability of porphyrin polymers, in this article, we
design and synthesis two copolymers consisting of porphyr-
ins, thiophenes, and 2,1,3-benzothiadiazole (BTZ) moieties in
the backbone to obtain low band gap copolymers. With the
introduction of BTZ moieties, the energy gap between
HOMO and LUMO energy levels was narrowed and the elec-
tronic spectra were red-shifted and broadened compared
with previous porphyrin polymers (P-PTT and P-POT) with-
out BTZ moieties. Moreover, we introduced octyloxy and
hexyl as side chains on polymer backbones to improve the
solubility of polymers.
(Bayer AG) on a precleaned indium tin oxide (ITO)/glass
substrate, and was dried at 150 C for 30 min. After cooling
ꢁ
to room temperature, the solutions containing both copoly-
mer and PC61BM (in chloro-benzene at a concentration of
24 mg/mL) were spin coated onto the ITO/PEDOT:PSS elec-
trode, and then dried at 150 ꢁC for 10 min in a nitrogen-
filled glove-box. And the thickness of the active layers of
liF0.7 nm. Finally, the aluminium (100 nm) electrode was
evaporated onto the top of copolymer film at 6 ꢂ 10ꢃ4 Pa.
The effective area of one cell was 9 mm2. Current density–
voltage (J–V) characteristics were measured by a computer
controlled Keithley 2602 source measurement unit in dark
and under AM 1.5 illumination conditions, 100 mW/cm2. All
these measurements were carried out under ambient atmos-
phere at room temperature.
Monomer Synthesis
The synthetic routes of the monomers and copolymers are
shown in Scheme 1. The detailed synthetic pathways are as
follows.
EXPERIMENTAL
2-(Tri-n-butylstannyl)-4-Hexyl thiophene (1)
Under argon atmosphere, 3-hexylthiophene (3.36 g, 20
mmol) and 50 mL freshly distilled dry THF were added in a
Materials and Characterization
All the chemicals were purchased from Alfa Aesar and
Shanghai Medical Company (China), THF and toluene were
distilled from sodium-benzophenone prior to use. All other
reagents were used as received. The 1H NMR and 13C NMR
spectra were measured with Bruker AVANCE 400 spectrome-
ter. Element analyses were measured by an Elementar Vario
EL III element analyzer.FT-IR spectra were obtained on a
Perkin–Elmer spectrometer, using KBr pellets. UV–visible
100 mL three-necked flask. n-Butyllithium (8.8 mL, 2.5 M in
ꢁ
hexane, 22 mmol) was added dropwise at ꢃ78 C, and the
ꢁ
solution was stirred for 1 h at ꢃ78 C, then tributyltin chlo-
ride (6 mL, 22 mmol) was added. The mixture was allowed
up to room temperature slowly and stirred for another 24 h.
Finally, the mixture was poured into 100 mL of cooled water,
and extracted by hexane. The organic layer was dried over
anhydrous MgSO4. Removal of the solvent by rotary evapora-
tion, a yellow–brown liquid 1 (8.25 g, 90%) was obtained.
The crude product was used in next step without further
purified.
absorption spectra were measured on
a Perkin–Elmer
Lambda 25 UV/vis/NIR spectrometer. The photolumines-
cence emission spectra (EL) were recorded with Perkin–
Elmer LS-50 luminescence spectrometer. The average molec-
ular weight and polydispersity index (PDI) of the copolymers
were determined using Waters1515 gel permeation chroma-
tography (GPC) analysis with THF as eluent and polystyrene
as standard. Thermogravimetric analysis (TGA) measure-
ments were conducted with a Netzsch TG 209 analyzer
1H NMR (CDCl3, 400 MHz, d/ppm): 7.18 (s, 1 H, thienyl-H),
6.96 (s, 1 H, thienyl-H), 2.64 (t, 2 H, ACH2), 1.62–1.54 (m, 8
H, ACH2), 1.36–1.30 (m, 12 H, ACH2), 1.11–1.06 (m, 6 H,
ACH2), 0.93–0.87 (m, 12 H, ACH3).
ꢁ
under nitrogen at a scan rate of 20 C/min. Differential scan-
ning calorimetry (DSC) analysis was made on a TA DSCQ10
instrument at a scan rate of 20 ꢁC/min. Electrochemical
measurements were made with EG&G Princeton Applied
Research Model 273 Electrochemical Workstation. Redox
potentials of the polymers were measured by cyclic voltam-
metry (CV) in acetonitrile containing 0.1 M tetrabutylammo-
nium hexafluorophosphate as a supporting electrolyte, An Pt
disk working electrode coated with polymer films, saturated
calomel electrode (SCE) reference electrode, and Pt wire
counter electrode were employed.
2,5-Bis-(trimethylstannyl)thiophene (2)
Under argon atmosphere, thiophene (1.05 g, 12.5 mmol) and
50 mL freshly distilled dry THF were added in a 100 mL
three-necked flask. n-Butyllithium (12.5 mL, 2.5 M in hexane,
ꢁ
62.5 mmol) was added dropwise at ꢃ78 C, and the solution
was stirred for 1 h at ꢃ78 ꢁC, then trimethyltin chloride
(18.7 mL, 75 mmol) was added. The mixture was allowed up
to room temperature slowly and stirred for another 24 h.
Finally, the mixture was poured into 100 mL of cooled water,
and extracted by hexane. The organic layer was dried over
anhydrous MgSO4. Removal of the solvent by rotary evapora-
tion, then recrystallized by isopropyl alcohol to obtain a
white needle solid (1 g, 17.2%).
Fabrication and Characterization of PSCs
The PSCs were fabricated in the traditional sandwich struc-
ture (denoted as ITO/PEDOT: PSS/copolymer: PC61BM/Al).
The fabrication process was as follows: a layer about 30 nm
of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS) was spin-coated from an aqueous solution
1H NMR (CDCl3, 400 MHz, d/ppm): 7.40 (s, 2 H, thienyl-H),
0.39 (m, 18 H, ACH3). 13C NMR (CDCl3, 100 MHz, d/ppm):
143.0, 136.0, 1.01.
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SCHEME 1 Synthetic route of the monomers and copolymers.
4,7-Bis-(4-hexylthiophen-2-yl)benzo[1,2,5]thiadiazole (3)
4,7-Dibromo-2,1,3-benzothiadiazole (1 g, 3.4 mmol) and 2-
(tri-n-butylstannyl)-4-hexyl thiophene (3.1 g, 6.8 mmol),
Pd(PPh3)4 (0.084 g, 0.075 mmol), were dissolved in toluene
(50 mL) and the mixture was refluxed for 48 h. After evapo-
rating the solvent under reduced pressure, H2O (50 mL) and
chloroform (50 mL) were added. The organic layer was sepa-
rated and dried over anhydrous MgSO4. The solvent was
removed under reduced pressure. The pure product 3 was
obtained as orange solid by column chromatography on silica
gel (CH2Cl2/hexane ¼ 1/8).
1H NMR (CDCl3, 400 MHz, d/ppm): 7.79–7.77 (d, 4 H, aryl-
H), 2.68–2.64 (t, 4 H, ACH2), 1.71–1.65 (t, 4 H), 1.42–1.36
(m, 12 H), 0.94–0.92 (t, 6 H, ACH3). 13C NMR (CDCl3, 100
MHz, d/ppm): 152.2, 143.1, 138.5, 128.1, 125.2, 124.7, 111.6,
31.6, 29.7, 29.6, 28.9, 22.6, 14.1.
5,15-Bis(4-brophenyl)-10,20-bis(4-(octyloxy)
phenyl)porphyrin (5)
The monomer 5, 5,15-bis(4-brophenyl)-10,20-bis(4-(octyloxy)-
phenyl) porphyrin, was synthesized according to the methods
reported in the literature26 with some modification.
1H NMR (CDCl3, 400 MHz, d/ppm): 8.90 (d, 4H, pyrrolic-H),
8.81 (d, 4H, pyrrolic-H), 8.10 (t, 8 H, Ar-H), 7.90 (d, 4H, Ar-
H), 7.29 (d, 4H, Ar-H), 4.27 (t, 4H, OCH2), 2.00 (t, 4H, CH2),
1.54 (m, 20H, CH2), 0.95 (t, 6H, CH3), ꢃ2.80 (s, 2H, N-H). 13C
NMR (CDCl3):125.1, 119.9, 155.6, 160.8, 132.2, 137.7, 136.5,
103.1, 124.2, 127.4, 114.7, 156.7, 138.8, 128.5, 131.6, 122.2,
68.9, 29.6, 25.8, 28.9, 29.4, 31.8, 22.7, 14.4. MALDI-TOF MS
(C60H60Br2N4O2) m/z: calcd for 1026.3; found 1026.2.
Yield: 1.08 g, 68%. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.99
(s, 2 H, thienyl-H), 7.84 (s, 2 H, phenyl-H), 7.06 (s, 2 H,
thienyl-H), 2.71 (t, 4 H, ACH2), 1.72–1.60 (m, 8 H, ACH2),
1.36–1.22 (m, 8 H, ACH2), 0.94 (t, 6 H, ACH3). MALDI-TOF
MS (C26H32N2S3) m/z: calcd for 468.1; found 468.1.
4,7-Bis-(5-bromo-4-hexylthiophen-2-yl)benzo[1,2,5]
thiadiazole (4)
5,15-Bis(4-brophenyl)-10,20-bis(4-(octyloxy)phenyl)
porphyrin Zinc (6)
Compound 3 (1.00 g, 2.13 mmol) and N-bromosuccinimide
(NBS) (0.84 g, 4.72 mmol) were resolved in mixed solvents
(THF/HAc V/V ¼ 1:1) (60 mL) in the dark. The resulting
solution was stirred at room temperature under argon over-
night. The organic phase and the water phase were sepa-
rated from mixed solution. The organic phase was washed
with brine and dried with sodium sulfate, filtered and the
solvent evaporated under reduced pressure and further
The solution of 5 (0.45 g, 0.44 mmol) in chloroform
(200 mL) was added a solution of Zn(OAc)2 (0.2 g, 1 mmol)
in methanol (20 mL). The reaction mixture was refluxed for
1 h. After cooling to room temperature, the mixture was
washed with water. The organic layer was dried over anhy-
drous MgSO4 and concentrated. A purple–red solid of com-
pound 6 was obtained (1.69 g, 97%).
dried under high vacuum.
A bright orange solid was
obtained by column chromatography on silica gel with hex-
ane (1.02 g, 77%).
1H NMR (CDCl3, 400 MHz, d/ppm): 9.02–9.01 (d, 4 H,
pyrrolic-H), 8.93–8.92 (d, 4 H, pyrrolic-H), 8.12–8.09 (t, 8 H,
LOW BAND GAP COPOLYMERS, ZHOU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Polymerization Results and Thermal Properties of the
(trimethylstannyl)thiophene and 4,7-bis-(5-bromo-4-hexylth-
iophen-2-yl)benzo[1,2,5]thiadiazole by a palladium-catalyzed
Stille coupling reaction. The obtained copolymers P1 and P2
were completely soluble in common organic solvents, such
as THF and chloroform. The structures of copolymers were
characterized by 1H NMR spectroscopy and FT-IR. In the 1H
NMR spectra of copolymers the signals of the pyrrole-H of
the porphyrin unit dominated in the region of 9.01–8.88
ppm, the alkyl chains appeared at 4.26–0.91 ppm, and other
aromatic proton peaks located between 7.42 and 8.34 ppm.
In the 1H NMR spectrum of P2, the resonance integral ratio
of signals of alkyl-H versus AOCH2 is bigger than that of P1,
indicating that there are more monomer 4 incorporated in
P2. In the FT-IR spectra of P1 and P2, the strong stretching
vibration of the CAC of porphyrin and CAH of aromatic ring
are observed around 997 and 2921 cmꢃ1, respectively. Poly-
merization via Stille coupling yielded black–blue copolymers
with average molecular weight (Mw) of 11.2 and 3.2 kg/mol
and polydispersity index (PDI) of 1.8 and 1.5 for P1 and P2,
respectively (Table 1). According to the theoretical x/y ratio
of 1:2 for P1, the elemental analysis shows the theoretical
calculation (such as C/S ¼ 5.12) and experimental result (C/
S ¼ 5.06) is almost consistent. For P2, the theoretical calcu-
lation (x/y ratio of 1:4, C/S ¼ 4.05) is also approximate with
the experimental result (C/S ¼ 4.21).
Porphyrin Copolymers
Copolymer
Mn (ꢂ103)
Mw (ꢂ103)
PDI
Td (ꢁC)
Tg (ꢁC)
P1
P2
5.2
2.1
11
1.8
1.5
310
455
66
39
3.2
Ar-H), 7.91–7.89 (t, 4 H, Ar-H), 7.30–7.28 (d, 4 H, Ar-H),
4.28–4.25 (t, 4 H, AOCH2), 2.02–1.98 (m, 4 H, ACH2), 1.68–
1.61 (m, 4 H, ACH2), 1.39–1.28 (m, 16 H, ACH2), 0.98–0.95
(t, 6 H, ACH3). 13C NMR (CDCl3, 100 MHz, d/ppm): 158.9,
150.8, 150.7, 149.8, 141.8, 135.7, 135.4, 134.8, 132.4, 131.7,
129.7, 122.2, 112.6, 68.4, 31.9, 29.5, 29.3, 26.2, 22.7, 14.1.
MALDI-TOF MS (C60H58Br2N4O2Zn) m/z: calcd for 1088.2;
found 1088.1.
Synthesis of the Copolymer (P1)
To a 50 mL three-necked flask were added monomer 2
(139.8 mg, 0.3 mmol), 4 (0.2 mmol, 125.6 mg), 6 (108.8 mg,
0.1 mmol), and freshly distilled toluene (10 mL). The mix-
ture was purged with nitrogen for 30 min. Pd(PPh3)4
ꢁ
(100 mg, 0.08 mmol) was added. After stirring at 110 C for
3 days, the mixture was cooled to room temperature, the sol-
vent was removed by rotary evaporation, and the residue
was poured into 50 mL CH3OH. The precipitate was filtered
and washed with methanol and then the solid was purified
successively by Soxhlet extraction in methanol, hexane, and
chloroform for 24 h. The fraction extracted by chloroform
was evaporated under reduced pressure, then precipitated in
methanol, filtered, and finally dried over 24 h to obtain a
dark purple solid.
Thermal Properties
The thermal properties of the copolymers were investigated
by performing the differential scanning calorimetry (DSC)
and thermogravimetric analysis (TGA) measurement, and the
corresponding data were summarized in Table 1. As shown
in Table 1, the glass transition temperature (Tg) of P1 and
P2 was observed to be 66 ꢁC and 39 ꢁC, respectively. The
TGA curves (Fig. 2) of two copolymers showed that 5%
weight loss temperatures (Td) are at 310 ꢁC and 455 ꢁC,
respectively. The Td values of both polymers are higher than
Yield: 34.1%. 1H NMR (CDCl3, 400 MHz, d/ppm): 9.11–8.95
(br, pyrrolic-H), 8.35–7.42 (br, Ar-H), 4.37–4.17 (t, AOCH2),
2.91–0.87 (br, alkyl-H). FT-IR (KBr, vmax/cmꢃ1): 2923, 2850,
1604, 1504, 1464, 1338, 1244, 1171, 997, 795. Mw ¼ 11 kg/
mol, PDI ¼ 1.8. According to the theoretical x/y ratio of 1:2,
the ELEM. ANAL. for P1 Calc.: C: 70.37, H: 5.91, N: 5.34, S:
13.73,. C/S ¼ 5.12. Found: C: 69.04, H: 6.10, N: 4.60, S:
13.63, C/S ¼ 5.06.
ꢁ
300 C, indicating that two copolymers own the better ther-
mal stability. The good thermal stability of the two copoly-
mers is adequate for their applications in PSCs and other
optoelectronic devices.27
Photophysical Properties
Synthesis of the Copolymer (P2)
The photophysical properties of the copolymers were investi-
gated by UV–visible and fluorescence spectroscopy in diluted
CHCl3 solution and thin films on quartz plates. For compari-
son, we also measured the UV–visible spectra of reference
The synthetic procedure for P2 was similar to that for P1
while the molar ratio of monomer 2: monomer 4: monomer
6 is 5:4:1.
Yield: 30.3%. 1H NMR (CDCl3, 400 MHz, d/ppm): 9.05–8.88
(br, pyrrolic-H), 8.34–7.10 (br, Ar-H), 4.26 (t, AOCH2), 2.96–
0.91 (br, alkyl-H). FT-IR (KBr, vmax/cmꢃ1): 2926, 2855, 1629,
1486, 1440, 1340, 1248, 1173, 1171, 995, 793. Mw ¼ 3.2
kg/mol, PDI ¼ 1.5. According to the theoretical x/y ratio of
1:4, the ELEM. ANAL. for P2 Calc.: C: 68.76, H: 5.88, N: 5.26, S:
16.98, C/S ¼ 4.05. Found: C: 68.62, H: 5.68, N: 4.75, S:
16.28, C/S ¼ 4.21.
RESULTS AND DISCUSSION
Syntheses and Chemical Characterization
Following the synthetic routes of copolymers in Scheme 1,
monomer 6 was reacted with carefully purified 2,5-bis
FIGURE 1 The chemical structure of copolymers.
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ARTICLE
FIGURE 3 UV–Vis spectra of copolymers and reference PZn in
FIGURE 2 TGA plots of the copolymers with a heating rate of
20 ꢁC/min. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
CHCl3 solutions and in thin films.
broadened in the red region with increasing the content of
BTZ moieties in the main chain of the porphyrin polymers.
26
PZn and monomer 3 in diluted CHCl3. PZn showed a sharp
Soret-band at 420 nm and weak Q-bands at 549 and 589
nm. And the monomer 3 exhibited two strong absorption
peaks at 313 and 460 nm. After copolymerization, monomer
3 was successfully incorporated in the polymer backbone,
thus, broad and strong absorption spectra of copolymers
from 300 to 800 nm were obtained as expected. It is worth-
while to note that the copolymers not only hold the strong
Soret-band but also possess much more broadened and
stronger absorption at the locations of the Q-bands. The UV–
visible spectra of copolymers in thin films are shown in
Figure 3. Compared with those in solution, the spectra of co-
polymer films were improved significantly in intensity as
well as breadth. As shown in Figure 3, the Soret-bands of P1
and P2 are respectively located at 439 nm and 442 nm,
which are broadened and red-shifted by 14 and 18 nm com-
pared with those in solution. On the other hand, the Q-bands
of copolymer films located at 566 and 587 nm, respectively,
are 14 and 39 nm red-shifted than those in solution appa-
rently. The improvement of spectra in films can be attributed
to the partial formation of J-aggregates (head-to-tail arrange-
ment) through p–p stacking in the solid phase.28 Those
results showed that the introduction of BTZ moiety into
main chains of the porphyrin polymers can markedly
improve absorption in the red region. Moreover, it can be
seen that Q-bands of P2 red-shifted than that of P1 in films.
Thus, we can conclude that the absorption spectra would be
The photoluminescence (PL) spectra of copolymers and PZn
were measured in the chloroform and thin films spin-coated
on quartz plates (shown in Table 2 and Fig. 4). PZn showed
two emission peaks at 600 nm and 650 nm. P1 and P2
exhibited the maximum emission peaks (kem) at 692 nm and
697 nm, respectively, and obtain red-shifted spectra in com-
parison with that of PZn, which conforms to the fact that
played out in UV–visible spectra. We also measured the PL
spectra of the two copolymers in thin films, and the maxi-
mum emission peaks (kem) of P1 and P2 are 783 and 772
nm, respectively. The large red-shift of kem observed from so-
lution to solid thin film of porphyrin copolymers is similar
to that observed from the UV–visible absorption spectra.
Electrochemical Properties
The electrochemical behaviors of the copolymers were inves-
tigated by cyclic voltammetry (CV) for determining the high-
est occupied molecular orbital (HOMO) and the lowest unoc-
cupied molecular orbital (LUMO) energy levels of the
conjugated copolymers. Figure 5 showed the cyclic voltam-
mograms of copolymer films on Pt electrodes in acetonitrile
containing 0.1 M tetrabutylammonium hexafluorophosphate
as a supporting electrolyte and saturated calomel electrode
(SCE) was used as reference electrode. P2 shows an irrevers-
ible oxidation peak and a quasi-reversible reduction peak.
However, P1 shows two irreversible oxidation peaks and a
TABLE 2 UV–Vis, PL Spectral Data, and Electrochemical Properties of the Porphyrin Copolymers
Solution kmax
Film kmax
(nm)b
(nm)a
HOMOc (eV)
LUMOc (eV)
Egopt(eV)
ec
Eg (eV)
Copolymer
UV
PL
UV
PL
P1
P2
a
425
425
692
697
439
442
783
771
ꢃ5.12
ꢃ5.21
c
ꢃ3.50
ꢃ3.45
1.62
1.76
1.51
1.50
Measured in chloroform solution.
Energy levels calculated from the cyclic voltammogram.
b
Polymer cast from chloroform solution.
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FIGURE 5 Cyclic voltammograms of copolymers in CH3CN/0.1
M Bu4NPF6 at 50 mV/s. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 4 PL spectra of copolymers in CHCl3 solution and in
films. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
quasi-reversible reduction peak. Onset potentials are values
obtained from the intersection of the two tangents drawn at
the rising current and the baseline changing current of the
CV curves. P1 and P2 have onset oxidation potentials at 0.72
and 0.81 eV, and onset reduction potentials at ꢃ0.90 and
ꢃ0.95 eV, respectively. The potential of Hg/Hg2Cl2 reference
electrode was internally calibrated by using the ferrocene/
ferrocenium redox couple, which has a known reduction
potential of ꢃ4.8 eV. The HOMO and LUMO energy levels of
copolymers were calculated from the onset oxidation poten-
tials (Eox) and onset reductive potentials (Ered), respectively,
according to the following equations:
HOMO ¼ ꢃeðEox þ 4:40Þ ðeVÞ;
LUMO ¼ ꢃeðEred þ 4:40Þ ðeVÞ;
ce
Eg ¼ eðEox ꢃ EredÞ ðeVÞ
Therefore, the energy levels of HOMO/LUMO for P1 and P2
are estimated to be ꢃ5.12/ꢃ3.50 eV and ꢃ5.21/ꢃ3.45 eV,
and the calculated HOMO–LUMO energy gap are 1.62 and
1.76 eV, respectively. The location of HOMO and LUMO
energy levels of the donor polymers are very important for
application in efficient photovoltaic cell. The HOMO energy
level of an ideal donor polymer should be lower than the air
FIGURE 6 Band diagram for the donor polymers P1, P2, and the acceptor PC61BM. Dashed lines indicate the thresholds for air sta-
bility (ꢃ5.2 eV) and effective charge transfer to PCBM (ꢃ3.8 eV).
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ARTICLE
PCE (%)
TABLE 3 Photovoltaic Performances of the Porphyrin Copolymers
Copolymers
Copolymer/PC61BM (w/w)
Voc (V)
Jsc (mA/cm2)
FF
P1
1:1
1:2
1:3
1:1
1:2
1:3
0.53
0.55
0.54
0.57
0.62
0.57
3.38
3.13
2.80
4.10
5.03
3.99
0.26
0.26
0.24
0.36
0.29
0.36
0.46
0.45
0.36
0.84
0.91
0.83
P2
oxidation threshold (ca. ꢃ5.2 eV) to ensure the good air
stability (i.e., resistant to oxidation).29 Furthermore, the rela-
tively low HOMO of the polymers may lead to a high open-
circuit potential (Voc) value for the photovoltaic cell.30 The
HOMO energy level of P2 is 0.09 eV lower than P1, indicat-
ing that P2 would own higher Voc value. On the other hand,
a donor polymer intended for use with a soluble fullerene
acceptor (e.g., PC61BM) should have an LUMO offset of ꢀ0.3–
0.4 eV relative to PC61BM (ꢃ4.2 eV) for the effective charge
transfer. A complete picture of the band structure of P1, P2,
and the acceptor PC61BM is presented in Figure 6. The first
dashed line indicates the threshold for air stability (ꢃ5.2
eV), and the second dashed line represents the threshold
value for an effective charge transfer from the polymers to
PC61BM (ꢃ3.8 eV). As seen from Figure 6, both the HOMO
and LUMO of the polymers are close to the ideal range. With
the introduction of BTZ moieties, the LUMO energy levels of
P1 and P2 are lower than that of P-PTT and P-POT.26 At the
same time, the HOMO energy levels of P1 and P2 are also
lower than that of P-PTT and P-POT, which promise higher
Voc values.
P-POT (1.70 mA/cm2). This result indicates that the intro-
duction of the electron deficient BTZ moieties in the back-
bone of the porphyrin copolymers can achieve higher Jsc due
to the red-shifted and broadened UV–visible spectra in the
red region. The short-circuit current for P2 is higher com-
pared to P1, which implies that the increase molar ratio of
BTZ moiety in the polymer improved the photovoltaic per-
formances. In general, a higher Voc could result from the
lower HOMO energy levels of the studied copolymers
because Voc is related to the energy differences between the
HOMO of the donor (conjugated polymer) and the LUMO of
the acceptor.16 In Table 3, P2 possesses the higher Voc than
P1, which conforms to the result of the discussion in the
electrochemical properties. Furthermore, the Voc values of P1
and P2 are both higher than P-PTT (0.46 eV) and P-POT
(0.46 eV), which are also due to the lower HOMO energy lev-
els of the former. In summary, the photophysical and electro-
chemical properties of these porphyrin copolymers, P1 and
P2, possess red-shifted and broadened UV–visible spectra
and lower HOMO energy levels, which, respectively, caused
the higher Jsc and Voc values than P-PTT and P-POT and
resulted in the higher power conversion efficiencies.
Photovoltaic Properties
The bulk heterojunction solar cells of ITO/PEDOT:PSS/
copolymer:PC61BM/LiF/Al device structures with different
weight ratios of P1 or P2 to PC61BM were fabricated to
investigated the photovoltaic properties. We have measured
the photovoltaic properties of the devices in copoly-
mers:PC61BM weight ratio (1:1, 1:2, and 1:3). The photovol-
taic parameters including the short-circuit current density
(Jsc), the open-circuit voltage (Voc), the fill factor (FF), and
power conversion efficiency (PCE), are summarized in Table
3. For P1, the copolymers:PC61BM weight ratio doesn’t affect
the performance very much. However, it dose have effect on
P2. When the copolymers:PC61BM weight ratio is 1:2, the
PSCs devices exhibited the best performances. Under stand-
ard global AM 1.5 solar conditions and with copoly-
mers:PC61BM weight ratio of 1:2, the photovoltaic cells
based on copolymer P2 exhibits the better photovoltaic per-
formances (Jsc ¼ 5.03 mA/cm2, Voc ¼ 0.62 V, FF ¼ 0.29, and
PCE ¼ 0.91%) than P1 based devices (Jsc ¼ 3.13 mA/cm2,
Voc ¼ 0.55 V, FF ¼ 0.28, and PCE ¼ 0.45%) . Figure 7 shows
the J–V curves of the devices for P1 and P2 with the copoly-
mers:PC61BM weight ratio of 1:2. Compared with previous
copolymers P-PTT and P-POT11 the Jsc values of P1 and
P2 is much higher than those of P-PTT (2.03 mA/cm2) and
FIGURE 7 J–V curves of the photovoltaic cells based on P1 and
P2 (copolymers:PC61BM, 1:2, w/w) under the illumination of
AM 1.5, 100 mW/cm2. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
LOW BAND GAP COPOLYMERS, ZHOU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CONCLUSIONS
11 Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. Angew
Chem Int Ed 2010, 49, 1500–1503.
In summary, two novel low band gap conjugated copolymers
P1 and P2 containing porphyrins, thiophenes, and BTZ
moieties, have been successfully synthesized via the Stille
coupling reaction. The research results showed that the intro-
duction of BTZ moiety in the conjugated main chain of the por-
phyrin copolymers can broaden the absorption spectra in the
range of 450–750 nm. With the introduction of BTZ moieties,
the LUMO energy levels of the copolymers were lowered to
ꢃ3.50 and ꢃ3.45 eV and the HOMO energy levels were low-
ered to ꢃ5.12 and ꢃ5.21 eV, resulting energy gaps lowered to
1.62 and 1.76 eV, respectively. The lowered energy gaps prom-
ised the red-shifted and broadened spectra which lead to
higher Jsc, and the lower located HOMO energy levels result the
higher Voc. Thus, the synthesized porphyrin copolymers in this
article could reach a promising PCE of 0.91% which is a rela-
tively high PCE value in porphyrin polymers applied in PSCs.
12 Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.;
Zhang, Y.; Luo, J.; Jen, A. K.-Y. J Am Chem Soc 2009, 131,
13886–13887.
13 Zhang, Y.; Hau, S. K.; Yip, H.-L.; Sun, Y.; Acton, O.; Jen, A.
K.-Y. Chem Mater 2010, 22, 2696–2698.
14 Zou, Y.; Najari, A.; Berrouard, P.; Beaupre´, S.; Aich, B. R.;
Tao, Y.; Leclerc, M. J Am Chem Soc 2010, 132, 5330–5331.
15 Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon,
J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat Photon
2009, 3, 297–302.
16 Qin, R.; Li, W.; Li, C.; Du, C.; Veit, C.; Schleiermacher, H.;
Andersson, M.; Bo, Z.; Liu, Z.; Inganas, O.; Wuerfel, U.; Zhang,
¨
F. J Am Chem Soc 2009, 131, 14612–14613.
17 Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G.; Wienk,
M. M.; Turbiez, M.; Leeuw, D.; Janssen, R. A. J. J Am Chem
Soc 2009, 131, 16616–16617.
This work was supported by the National Nature Science
Foundation of China (Nos. 50973092, 51003089, 21004050)
and Specialized Research Fund for the Doctoral Program
of Higher Education of China (20094301120005,
20104301110003).
18 Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.;
Jen, A. K.-Y. Chem Mater 2008, 20, 5734–5736.
19 Mazumdar, S.; Dallakyan, S.; Chandross, M. Synth Met
2003, 139, 769–772.
20 Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.;
Dante, M.; Heeger, A. J. Science 2007, 317, 222–225.
REFERENCES AND NOTES
21 Zoombelt, A. P.; Fonrodona, M.; Turbiez, M. G. R.; Wienk,
1 Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature
1990, 347, 539–541.
M. M.; Janssen, R. J. J Mater Chem 2009, 19, 5336–5342.
22 Thompson, B. C.; Madrigal, L. G.; Mauricio, R. P.; Kang, T.-S.;
Schanze, K. S.; Reynolds, J. R. J Polym Sci Part A: Polym Chem
2005, 43, 1417–1431.
2 Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri,
N.; Heeger, A. J. Nature 1992, 357, 477–479.
23 Hou, J. H.; Chen, H. Y.; Zhang, S. Q.; Li, G.; Yang, Y. J Am
3 Thompson, B. C.; Fre´chet, J. M. J. Angew Chem Int Ed 2008,
Chem Soc 2008, 130, 16144–16145.
47, 58–77.
24 Huang, X. B.; Zhu, C. L.; Zhang, S. M.; Li, W. W.; Guo, Y. L.;
Zhan, X. W.; Liu, Y. Q.; Bo, Z. S. Macromolecules 2008, 41,
6895–6902.
4 Chen, J. W.; Cao, Y. Acc Chem Res 2009, 42, 1709–1718.
5 Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Wal-
¨
dauf, C.; Heeger, A. J.; Brabec, C. J. Adv Mater 2006, 18,
789–794.
25 Liu, Y. J.; Guo, X.; Xiang, N.; Zhao, B.; Huang, H.; Li, H.;
Shen, P.; Tan, S. J Mater Chem 2010, 20, 1140–1146.
6 Liang, Y. Y.; Xu, Z.; Xia, J. B.; Tsai, S. T.; Wu, Y.; Li, G.; Ray,
26 Xiang, N.; Liu, Y. J.; Zhou, W. P.; Huang, H.; Guo, X.; Tan,
C.; Yu, L. P. Adv Mater 2010, 22, 1–4.
Z.; Zhao, B.; Shen, P.; Tan, S. Eur Polym J 2010, 46, 1084–1092.
7 Bre´das, J.-L.; Norton, J.; Cornil, J.; Coropceanu, V. Acc Chem
27 Hou, J. H.; Chen, H.-Y.; Zhang, S. Q.; Yang, Y. J Phys Chem
Res 2009, 42, 1691–1699.
C 2009, 113, 21202–21207.
8 Inganas, O.; Zhang, F.; Tvingstedt, K.; Andersson, L. M.; Hell-
28 Fox, M. A.; Grant, J. V.; Bard, A. J Chem Mater 1998, 10, 1771.
¨
strom, S.; Andersson, M. R. Adv Mater 2010, 22, E100–E116.
¨
29 Thompson, B. C.; Kim, Y. G.; Reynolds, J. R. Macromole-
9 Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.;
cules 2005, 38, 5359–5362.
DeBoer, B. Polym Rev 2008, 48, 531–582.
30 Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.;
Rispens, M. T.; Sanchez, L.; Hummelen, J. C.; Fromherz, T.
Thin Solid Films 2002, 368, 403–404.
10 Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem Rev 2009, 109,
5868–5923.
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