Synthesis and applications of 2,7-carbazole-based conjugated main-chain copolymers containing electron deficient bithiazole units for organic solar cells

Article abstract of DOI:10.1002/pola.24356

A series of low-band-gap (LBG) donor-accepor conjugated main-chain copolymers (P1-P4) containing planar 2,7-carbazole as electron donors and bithiazole units (4,4′-dihexyl-2,2′-bithiazole and 4,4′-dihexyl-5,5′-di(thiophen-2-yl)-2,2′-bithiazole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with optical band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. Moreover, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (V_oc) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C ₆₁-butyric acid methyl ester (PC₆₁BM) or (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC₇₁BM (1:1.5 w/w) showed the best power conversion efficiency value of 1.01% with a short circuit current density (J_sc) of 4.83 mA/cm², a fill factor (FF) of 35%, and V_oc = 0.60 V under 100 mW/cm² of AM 1.5 white-light illumination.

Full text of DOI:10.1002/pola.24356

Synthesis and Applications of 2,7-Carbazole-Based Conjugated
Main-Chain Copolymers Containing Electron Deficient Bithiazole
Units for Organic Solar Cells
1
1
1
2
2,3
1
DHANANJAYA PATRA, DURYODHAN SAHU, HARIHARA PADHY, DHANANJAY KEKUDA, CHIH-WEI CHU, HONG-CHEU LIN
1
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
2
Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
3
Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Received 11 July 2010; accepted 21 August 2010
DOI: 10.1002/pola.24356
Published online 5 October 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A series of low-band-gap (LBG) donor–accepor con-
jugated main-chain copolymers (P1–P4) containing planar 2,7-
provided good air stability and high open circuit voltages (Voc
in the PV applications. The BHJ PV devices were fabricated
using polymers P1–P4 as electron donors and (6,6)-phenyl-C61
)
0
carbazole as electron donors and bithiazole units (4,4 -dihexyl-
-
0
0
0
0
2
,2 -bithiazole and 4,4 -dihexyl-5,5 -di(thiophen-2-yl)-2,2 -bithia-
butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric
acid methyl ester (PC71BM) as electron acceptors in different
weight ratios. The PV device bearing an active layer of polymer
blend P4:PC71BM (1:1.5 w/w) showed the best power conver-
zole) as electron acceptors were synthesized and studied for
the applications in bulk heterojunction (BHJ) solar cells. The
effects of electron deficient bithiazole units on the thermal,
optical, electrochemical, and photovoltaic (PV) properties of
these LBG copolymers were investigated. Absorption spectra
revealed that polymers P1–P4 exhibited broad absorption
bands in UV and visible regions from 300 to 600 nm with opti-
cal band gaps in the range of 1.93–1.99 eV, which overlapped
with the major region of the solar emission spectrum. More-
over, carbazole-based polymers P1–P4 showed low values of
the highest occupied molecular orbital (HOMO) levels, which
sion efficiency value of 1.01% with a short circuit current den-
2
sity (Jsc) of 4.83 mA/cm , a fill factor (FF) of 35%, and Voc
¼
2
0.60 V under 100 mW/cm of AM 1.5 white-light illumination.
VC 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem
48: 5479–5489, 2010
KEYWORDS: bithiazole units; bulk heterojunction; copolymers;
donor–acceptor
INTRODUCTION In the 21st century, to reduce carbon emis-
sions and green house effects, solar energy is one of the
V) and large band gaps, which limit their net light harvesting
capabilities. Hence, the utilization of newly developed low-
band-gap (LBG) conjugated polymers likely to be the promis-
ing alternatives of P3HT for PSCs. Recently, PCE values up to
‘‘green’’ and ‘‘sustainable energy’’ sources to create better
environment. Recently, organic semiconducting materials,
1
2
including p-conjugated polymers and small molecules, have
been used in various optical and electronic devices because
of their unique advantages, such as light weight, low-cost
production, and large area device fabrication by solution pro-
6
.0–7.7% were obtained by using LBG conjugated polymers
4
in the BHJ solar cells as electron donors. Nevertheless, these
PCEs are not sufficient for commercialization of PSCs. There-
fore, promising efforts are required to develop new donor–
acceptor (D–A) polymer structures with higher molecular
crystallinity which can result in better p–p stacking,
extended absorption, higher mobility, and balanced charge
3
cess. The highly efficient organic solar cell devices belong to
the bulk heterojunction (BHJ) solar cells, in which p-conju-
gated polymers are used as electron donors and the fuller-
ene derivatives, such as [6,6]-phenyl-C -butyric acid methyl
6
1
1(e)
transport to get higher PCE values in PSCs.
ester (PC BM) or [6,6]-phenyl-C -butyric acid methyl ester
6
1
71
5
–14
Later, there were several reports on D–A PSCs,
which
(
PC BM), as electron acceptors. After an extensive investiga-
71
harvest maximum solar spectrum ranging from visible to
near infrared absorptions which appealed high short circuit
current density (Jsc) values. It has been verified that Voc is
directly proportional to the difference between the highest
occupied molecular orbital (HOMO) levels of donor poly-
mers and the lowest unoccupied molecular orbital (LUMO)
tion on polymer solar cells (PSCs), the BHJ devices based on
polymer blends (with various weight ratios and thicknesses)
of poly(3-hexylthiophene) (P3HT) and PC BM were taken as
standard devices. However, the enhancements of power con-
version efficiency (PCE) values in these devices are quite dif-
6
1
ficult because of low open circuit voltage (V ) values (ꢀ0.6
oc
Correspondence to: H.-C. Lin (E-mail: linhc@mail.nctu.edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 5479–5489 (2010) V
2010 Wiley Periodicals, Inc.
C
COPOLYMERS CONTAINING BITHIAZOLE UNITS, PATRA ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
levels of acceptor PCBM derivatives (i.e., PC BM and
conducting thiophene (thiophene or bithiophene) as elec-
tron-donating segments and bithiazole derivatives as elec-
tron-accepting segments. The copolymers were synthesized
by Pd(0)-catalyzed Stille coupling polymerization with 1:1
(molar ratio) donor–acceptor ratio. The resulting polymers
P1–P4 exhibited broad absorption bands located in the UV–
visible regions from 300 to 600 nm with optical band gaps
of 1.99–1.93 eV. From the preliminary investigation, the pho-
tovoltaic (PV) performance of the PSC device containing P4
(as an electron donor) blended with PC71BM (as an acceptor)
6
1
1
(c,d),3
PC71BM).
In BHJ solar cells, where PCBM is used as an
acceptor, the ideal band gap (in order to achieve a high Voc
value) of donor polymer should be in the range of 1.2–1.9
eV which corresponds to a HOMO energy level between ꢁ5.8
and ꢁ5.2 eV and a LUMO energy level between ꢁ4.0 and
7
,1(c,d)
ꢁ
3.8 eV.
Furthermore, to facilitate efficient electron
transfer from donor to acceptor, the minimum energy differ-
ence between LUMO levels of electron donor and acceptor
should be ca. 0.3 eV.
8
,10(b,c)
Consequently, to obtain the
showed the best PCE value of 1.01% with a V ¼ 0.60 V, a
oc
desired molecular energy levels of the conjugated LBG poly-
mers, electron-donating groups or electron-withdrawing
groups can be substituted alternatively in the polymer back-
bones either to raise the HOMO energy level or to reduce
ꢁ
2
Jsc ¼ 4.83 mAcm , and a fill factor (FF) of 35.0% measured
2
under 100 mW/cm of AM 1.5 white-light illumination.
1
(c),3
EXPERIMENTAL
the LUMO energy level.
Materials
Conjugated polymers having D–A architectures have been
extensively studied by using fused heterocyclic electron rich
segments, such as carbazole, dibenzosilole, cyclopentadi-
All chemicals and solvents were reagent grades and pur-
chased from Aldrich, ACROS, Fluka, TCI, TEDIA, and Lancas-
ter Chemical Co. Toluene and tetrahydrofuran (THF) were
distilled from sodium-benzophenone under nitrogen before
use. Unless otherwise specified, the other solvents were
degassed by nitrogen 1 h before use. All the other chemicals
were used as received.
1
0
11
1
2
13
14
15
thiophene,
dithienopyrrole,
dithienosilole,
fluorene,
1
6
and phenothiazine as an electron donating building block
for PSCs as well as organic field effect transistors. Owing to
the easy modulations of physical properties, it has been pro-
ven that 2,7-carbazole derivatives are one of the excellent
1
0(b)
potential donor candidates for BHJ solar cells.
Using 2,7-
Measurements and Characterization
carbazole-based alternating copolymer (PCDTBT) as an elec-
1
13
H and C NMR spectra were recorded on a Varian Unity
00 MHz spectrometer using CDCl3 solvent and chemical
1
0(a)
tron donor, Leclerc et al. achieved a PCE value of 3.6%.
3
Hence, with improving absorption characteristics and
charge-carrier mobilities, Heeger et al. reported an ever high
PSC device containing PCDTBT with a PCE value of 6.1% and
shifts were reported as d values (ppm) relative to an internal
tetramethylsilane standard. Elemental analyses were per-
formed on a HERAEUS CHN-OS RAPID elemental analyzer.
Thermogravimetric analyses (TGA) were conducted with a
4
(c)
an internal quantum efficiency approaching ca. 100%.
ꢂ
TA Instruments Q500 at a heating rate of 10 C/min under
The five-membered heterocyclic electron deficient moiety,
nitrogen. Gel permeation chromatography (GPC) analyses
were conducted on a Waters 1515 separation module using
polystyrene as a standard and THF as an eluent. UV–Visible
that is, thiazole, induces larger p–p stacking and higher
1
7,18
coplanarity
in D–A polymers so as to have a stronger
tendency to self-assemble into stacked solid structures,
which not only minimize steric hindrances but also provide
extended conjugation lengths. Introduction of thiazole units
with electron-withdrawing imine nitrogen (AC ¼¼ N) generally
enhances the electron-accepting (n-doping) properties of the
D–A polymers. Moreover, thiazole-based polymers exhibit
high oxidative stabilities which favor the polymers to lower
its HOMO energy level and thus to increase their open circuit
(
UV–vis) absorption spectra were recorded in dilute THF sol-
ꢁ
6
utions (10 M) on a HP G1103A spectrophotometer. Thin
films for UV–vis measurements were spin-coated on a glass
substrate from THF solutions with a concentration of 5 mg/
mL. Cyclic voltammetry (CV) measurements were performed
using a BAS 100 electrochemical analyzer with a standard
three-electrode electrochemical cell in a 0.1M tetrabutylam-
2
0
monium hexafluorophosphate (TBAPF ) solution (in acetoni-
6
voltages. Though Shim et al. firstly reported that the poly-
mer containing bithiazole and fluorine units achieved a low
trile) at room temperature with a scanning rate of 100
mV/s. During the CV measurements, the solutions were
purged with nitrogen for 30 s. In each case, a carbon work-
ing electrode coated with a thin layer of copolymers, a plati-
num wire as the counter electrode, and a silver wire as the
quasi-reference electrode were used, and Ag/AgCl (3 M KCl)
electrode was served as a reference electrode for all poten-
tials quoted herein. The redox couple of ferrocene/ferroce-
2
0(b)
PCE value of 0.52%,
we reached a much higher PCE
value of 3.04% using a copolymer containing bithiazole and
2
1(a)
cyclopentadithiophene units recently.
As a consequence,
the copolymers containing the planar electron-withdrawing
bithiazole units as acceptors and 2,7-carbazole units as
donors to produce D–A polymers will be very interesting
LBG polymers for the applications of PSCs. In addition, Li
et al. have newly reported one D–A copolymer containing
þ
nium ion (Fc/Fc ) was used as an external standard. The
corresponding HOMO and LUMO levels were calculated using
Eox/onset and Ered/onset for experiments in solid films of poly-
mers, which were performed by drop-casting films with the
similar thickness from THF solutions (ca. 5 mg/mL). The LUMO
levels of PC BM or PC BM employed were in accordance
2,7-carbazole and bithiazole moieties as electron-donor and
electron-acceptor segments, respectively, but only possessed
1
0(e)
a maximum PCE value of 0.30%.
In this article, we synthesized and characterized a series of
copolymers consisting of a planar 2,7-carbazole moiety with
61
71
with the literature data. The onset potentials were determined
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ARTICLE
from the intersections of two tangents drawn at the rising
currents and background currents of the CV measurements.
2,7-Dibromocarbazole (2)
Mixture of compound 2 (20 g, 56.02 mmol) and triphenyl-
phosphine (36.73 g, 140.05 mmol) were dissolved in 220
mL of DCB and the reaction mixture was refluxed for 12 h.
The excess DCB was removed by high vacuum distillation
and the residue was purified by column chromatography
Device Fabrication and PV Measurements of PSCs
The polymer PV cells in this study were composed of an
active layer of blended polymers (P1–P4:PCBM) in solid
films, which were sandwiched between a transparent indium
tin oxide (ITO) anode and a metal cathode. Before the device
(
Silica gel) using a mixture of hexane:ethyl acetate (7:3) to
1
get a white solid (12.90 g, 70.87%). H NMR (300 MHz,
CDCl
2
fabrication, ITO-coated glass substrates (1.5 ꢃ 1.5 cm ) were
3
), d (ppm): 8.08 (br, 1H), 7.87 (d, J ¼ 8.7 Hz, 2H), 7.56
ultrasonically cleaned in detergent, deionized water, acetone,
and isopropyl alcohol sequentially. After routine solvent
cleaning, the substrates were treated with UV ozone for
(
d, J ¼ 1.5 Hz, 2H), 7.35 (dd, J ¼ 1.8 Hz, J ¼ 8.4 Hz, 2H).
1-Hexylheptanol (3)
1
5 min. Then, a modified ITO surface was obtained by spin-
In a 500 mL flame-dried two-neck round bottom flask, ethyl
formate (10 mL,123.78 mmol) was dissolved in 100 mL of
coating a layer of poly(ethylene dioxythiophene):polystyrene-
ꢂ
sulfonate (PEDOT:PSS) (ꢀ30 nm). After baking at 130 C for
ꢂ
anhydrous THF and cooled to ꢁ78 C under N atmosphere.
2
1
h, the substrates were transferred to a nitrogen-filled glove
A freshly prepared hexylmagnesium bromide, which was
obtained by adding 1-bromohexane (48.90 mL, 346.59
mmol) to a suspension of magnesium turning (10.40 g,
box. The PSC devices were fabricated by spin-coating solu-
tions of blended polymers (P1–P4):PCBM (with various
weight ratios of a copolymer and one of PCBMs, that is,
PC61BM or PC71BM) onto the PEDOT:PSS modified substrates
at 1500 rpm for 60 s (ca. 80 nm), and placed in a covered
glass Petri dish. Initially, the blended polymer solutions were
433.24 mmol) in dry THF (150 mL), was added dropwise
into the previous solution, and then the reaction mixture
was stirred overnight at room temperature. The reaction was
quenched by the addition of MeOH, and then followed by
prepared by dissolving both copolymers and PC BM (with a
6
1
adding saturated aqueous NH Cl. The crude compound was
4
1
:1 weight ratio) initially and then with various weight ratios
extracted three times with ethyl acetate. The combined or-
ganic fractions were washed with brine, dried over MgSO4,
and concentrated by rotary evaporation. After vacuum distil-
for the optimum copolymer with PC BM in 1,2 dichloroben-
zene (DCB) (20 mg/mL), followed by continuous stirring for
7
1
ꢂ
12 h at 50 C. In the slow-growth approach, blended poly-
lation, the final compound was isolated as a white solid
1
mers in solid films were kept in the liquid phase after spin-
coating by using the solvent with a high boiling point.
Finally, a calcium layer (30 nm) and a subsequent aluminum
layer (100 nm) were thermally evaporated through a shadow
(
21.87 g, 87.94%). H NMR (CDCl , 300 MHz): d (ppm): 3.58
3
(
m, 1H), 1.46–1.25 (m, 21H), 0.87 (t, J ¼ 6.4 Hz, 6H).
Tridecan-7-yl 4-Methylbenzenesulfonate (4)
In a 250 mL flame-dried two neck round bottom flask, 1-
ꢁ
6
mask at a pressure below 6 ꢃ 10 Torr. All PSC devices
hexylheptanol (10.0 g, 49.91 mmol), Et N (17.40 mL, 124.77
were prepared and measured under ambient conditions,
3
2
mmol), and Me
3
NꢄHC1 (4.77 g, 49.91 mmol) were mixed in
where the active area of the devices was 0.12 cm . The solar
cell testing was done inside a glove box under simulated AM
1
ꢂ
4
0 mL of CH Cl and then cooled to 0–5 C. A solution of p-
2
2
2
toluenesulfonyl chloride (11.90 g, 62.38 mmol) in CH Cl (39
.5G irradiation (100 mW/cm ) using a Xenon lamp-based
2
2
mL) was added dropwise over 90 min and kept the reaction
at room temperature. After 2 h, water was added and the
solar simulator (Thermal Oriel 1000W). The external quan-
tum efficiency (EQE) action spectrum was obtained at short-
circuit condition. The light source was a 450 W Xe lamp
crude compound was extracted with CH Cl . The organic
2
2
fraction was washed with water and brine, and dried over
(
Oriel Instrument, model 6266) equipped with a water-based
MgSO , and concentrated by rotary evaporation. Subse-
IR filter (Oriel Instrument, model 6123NS). The light output
from the monochromator (Oriel Instrument, model 74100)
was focused onto the PV cell under test.
4
quently, the crude product was purified by column chroma-
tography (Silica gel, hexane/ethylacetate 9:1) to yield a vis-
1
cous colorless liquid (15.15 g, 85.60%). H NMR (300 MHz,
Synthesis of Monomers and Polymers
CDCl ): d (ppm): 7.79 (d, J ¼ 8.2 Hz, 2H);7.32 (d, J ¼ 8.1 Hz,
3
0
0
4
4
,4 -Dibromo-2-nitrobiphenyl (1)
2
1
H); 4.53 (m, J ¼ 6.0 Hz, 1H); 2.43 (s, 3H); 1.52 (m, 4H);
.22 (m, 20H); 0.88 (t, J ¼ 6.9 Hz, 6H).
,4 -Dibromobiphenyl (20 g, 64 mmol) in 300 mL of glacial
ꢂ
acetic acid was heated (100 C) to dissolve completely. Then,
90 mL of fuming nitric acid was added dropwise for a period
2,7-Dibromo-9-(Tridecan-7-yl)-9H-Carbazole (5)
of 30 min. The resulting mixture was further stirred vigo-
rously for 1 h at 100 C to get a reddish brown precipitate.
The reaction mixture was cooled to room temperature and
poured into ice cold water. The precipitate was filtered and
washed with excess of water, then the obtained product was
2,7-Dibromocarbazole (4.0 g, 12.30 mmol) and potassium hy-
droxide powder (3.45 g, 61.50 mmol) were dissolved in 50
mL of dimethylsulfoxide (DMSO) at 60 C. Then, a solution
of tridecan-7-yl 4-methylbenzenesulfonate (6.25 g, 18.45
mmol) with 30 mL of DMSO was added dropwise through a
dropping funnel over 1.5–2 h and stirred overnight. The
reaction mixture was cooled to room temperature and
poured into 500 mL of water. The crude compound was
extracted with ethylacetate and washed with brine. The com-
ꢂ
ꢂ
further purified by recrystalization from ethanol to get a yel-
1
low solid (20.30 g, 88.72%). H NMR (300 MHz, CDCl
3
), d
(
ppm): 8.03 (d, J ¼ 3.0 Hz, 1H), 7.75 (dd, J ¼ 9.0 Hz, J ¼ 3.0
Hz, 1H), 7.55 (d, J ¼ 9.0 Hz, 2H), 7.30 (d, J ¼ 9.0 Hz, 1H),
.15 (d, J ¼ 6.0 Hz, 2H).
7
bined organic layer was dried over MgSO and concentrated
4
COPOLYMERS CONTAINING BITHIAZOLE UNITS, PATRA ET AL.
5481

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Synthetic schemes of compound 5 and polymers P1–P4.
in rotary evaporator. The crude compound was purified by
column chromatography (silica gel) using hexane as an elu-
ent to give a white solid (4.62 g, 74.03%). H NMR (300
26.98; 22.76; 14.23; EIMS (m/z): Anal. Calcd for C H Br N:
25 33 2
C, 59.18; H, 6.56; N, 2.76. Found: C, 59.58; H, 6.12; N, 2.77.
MS (FAB): m/z [Mþ] 505.0; calcd m/z [Mþ] 505.10.
1
MHz, CDCl ): d (ppm): 7.90 (br, 2H); 7.70 (s, 1H); 7.54 (s,
3
1
1
H); 7.33 (d, J ¼ 6.0 Hz, 2H); 4.42 (m, 1H); 2.19 (m, 2H);
General Synthetic Procedures of Polymers P1-P4
The synthetic route of copolymers is shown in Scheme 1.
Into a 25 mL two-necked flask, 2,7-dibromo-9-(tridecan-7-
1
3
.91 (m, 2H); 1.15 (m, 16H); 0.83 (t, J ¼ 6.3 Hz, 6H).
C
NMR (75 MHz, CDCl ): d (ppm): 130.61; 130.15; 122.58;
3
0
121.71; 121.48; 114.75; 112.39; 57.22; 33.76; 31.79; 29.25;
yl)-9H-carbazole, 2,5-bis(tributylstannyl)thiophene (or 5,5 -
5482
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ARTICLE
0
0
0
bis(tributylstannyl)-2,2 -bithiophene), and 5,5 -dibromo-4,4 -
TABLE 1 Molecular Weights and Thermal Properties of
Polymers P1–P4
0
0
dihexyl-2,2 -bithiazole (or 5,5 -bis(5-bromothiophen-2-yl)-
,4 -dihexyl-2,2 -bithiazole were added. The mixture was
deoxygenated with nitrogen for 30 min, after which dry tolu-
ene (15 mL) and Pd(PPh ) (1 mol %), was transferred into
0
0
4
a
b
PDI
Yield
(%)
T
d
ꢂ
a
w
a
Polymer
M
M
n
(M
w
/M
n
)
( C)
3
4
the mixture in a dry environment. The reaction mixture was
stirred at 110 C for 3 days, and then an excess amount of
P1
P2
P3
P4
a
41,900
25,100
8,900
25,900
18,500
7,400
1.62
1.36
1.20
1.19
72
69
66
64
452
423
418
361
ꢂ
2-bromothiophene was added to end-cap the trimethyl-
stannyl groups for 4 h. The reaction mixture was cooled to
ꢂ
8,600
7,200
4
0 C and added slowly into a vigorously stirred mixture of
methanol/acetone (3:1). The polymers were collected by fil-
tration and reprecipitation from methanol. The crude poly-
mers were further purified by washing with acetone and EA
for 2 days in a Soxhlet apparatus to remove oligomers and
catalytic residues.
Molecular weights (Mn and M ) and polydispersity index (PDI) values
w
were measured by GPC, using THF as an eluent, polystyrene as a stand-
ard. M
ular weight.
n w
, number average molecular weight; M , weight average molec-
b
ꢂ
Temperature ( C) at 5% weight loss measured by TGA at a heating
ꢂ
rate of 10 C/min under nitrogen.
RESULTS AND DISCUSSION
P1
Following the general polymerization procedure, compound
Syntheses and Characterization
5
(0.5 equiv), M1 (0.5 equiv), and compound 6 (1.0 equiv)
The synthetic routes of 2,7-carbazole-based donor monomer
(compound 5) and polymers P1–P4 are outlined in Scheme
were used in this polymerization to acquire a red powder.
Yield: 72%. GPC: M : 41,900; polydispersity index (PDI):
1
1. Compound 5 was adequately characterized by H NMR,
w
1
13
1.62; H NMR (300 MHz, CDCl ): d (ppm) 8.10 (broad),
C NMR, MS spectroscopies, and elemental analyses. The
3
7
.79–7.39 (broad), 7.12 (s), 4.62 (broad), 2.84 (broad), 1.83
broad), 1.75–0.80 (broad), 0.77–0.61(broad). Anal. Calcd for
: C, 72.21; H, 7.72; N, 4.95. Found: C, 71.81; H,
thiophene- and bithiophene-based donor monomers (com-
(
(
pounds 6 and 7, respectively) were prepared according to
2
2
C
51
H
65
N
3
S
4
)
n
the methods described elsewhere. In addition, the syn-
thetic procedures of bithiazole-based acceptor monomers
7.59; N, 4.87.
2
1(a)
M1 and M2 were also reported earlier by our group.
In
this study, polymers P1–P4 consisting of 2,7-carbazole and
thiophene (or 2,2 -bithiophene) as electron-donating moeities
P2
Following the general polymerization procedure, compound
(0.5 equiv), M2 (0.5 equiv), and compound 7 (1.0 equiv)
0
and bithiazole as electron-accepting moieties were synthe-
sized by Pd(0)-catalyzed Stille coupling polymerization in
5
were used in this polymerization to acquire a deep red pow-
ꢂ
toluene at 110 C with a feed-in molar ratio of m:n ¼ 1:1.
1
der. Yield: 69%. GPC: M : 25,100; PDI: 1.36; H NMR (300
w
MHz, CDCl ): d (ppm) 8.10 (broad), 7.89–7.22 (broad), 7.12
All these copolymers are readily soluble in common organic
solvents such as chloroform, THF, and chlorobenzene at
room temperature and completely soluble in high boiling
point solvents (e.g., chlorobenzene) at high temperature. The
molecular weights of polymers P1–P4 determined by GPC
against polystyrene standards in THF are summarized in Ta-
ble 1. These results show that considerable molecular
weights with high yields (64–72% after Soxhlet extractions)
were obtained in these copolymers, where the average mo-
3
(
(
broad), 4.62 (broad), 2.97 (broad), 1.83 (broad), 1.75–0.80
broad), 0.77–0.61(broad). Anal. Calcd for (C H N S ) : C,
5
9 69 3 6 n
69.98; H, 6.87; N, 4.15. Found: C, 69.18; H, 6.79; N, 4.25.
P3
Following the general polymerization procedure, compound
5
(0.5 equiv), M1 (0.5 equiv), and compound 6 (1.0 equiv)
were used in this polymerization to acquire a black powder.
lecular weights (M ) were in the range of 41,900–8600 with
w
1
Yield: 66%. GPC: M : 8,880; PDI: 1.20; H NMR (300 MHz,
w
PDI (PDI ¼ M
w n
/M ) values of 1.62–1.19. The thermal stabil-
CDCl
3
): d (ppm) 8.10 (broad), 7.79–7.19 (broad), 7.15
broad), 4.62 (broad), 2.99 (broad), 1.85 (broad), 1.75–0.80
broad), 0.77–0.61(broad). Anal. Calcd for (C H N S ) : C,
ities of conjugated polymers play an important role for opto-
electronic applications. As shown in Figure 1, the thermal
stabilities of polymers P1–P4 were investigated by TGA, and
their corresponding results are summarized in Table 1. All
polymers showed good thermal stabilities and exhibited Td
values (temperatures at 5% weight loss by a heating rate of
(
(
5
9 69 3 6 n
69.98; H, 6.87; N, 4.15. Found: C, 69.30; H, 6.98; N, 4.22.
P4
ꢂ
ꢂ
Following the general polymerization procedure, compound
10 C/min under nitrogen) between 361 and 452 C, where
the value was reduced as the molecular weight
decreased.
5
(0.5 equiv), M2 (0.5 equiv), and compound 7 (1.0 equiv)
T
d
2
3
were used in this polymerization to acquire a black powder.
1
Yield: 64%. GPC: M : 8600; PDI: 1.19; H NMR (300 MHz,
w
CDCl
(
(
3
): d (ppm) 8.10 (broad), 7.79–7.39 (broad), 7.11
broad), 4.59 (broad), 2.97 (broad), 1.81 (broad), 1.75–0.80
broad), 0.77–0.61(broad). Anal. Calcd for (C H N S ) : C,
Optical Properties
The photophysical features of the copolymers were investi-
gated by UV–vis absorption spectroscopy in dilute THF solu-
tions and spin-coated films on glass substrates, which are
6
7 73 3 8 n
68.38; H, 6.25; N, 3.57. Found: C, 67.82; H, 6.38; N, 3.64.
COPOLYMERS CONTAINING BITHIAZOLE UNITS, PATRA ET AL.
5483

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 TGA measurements of polymers P1–P4 with a heat-
ꢂ
ing rate of 10 C/min.
presented in Figures 2(a) and 2(b), respectively. The normal-
ized absorption spectra of polymers P1–P4 and their optical
data, including the absorption wavelengths (k
) and the
max,abs
optical band gaps (E go pt), and absorpation coefficients (amax),
are summarized in Table 2. All polymers (P1–P4) shows rel-
atively high absorption coefficients (amax, calculated from
4
ꢁ1
Beer’s law) with the range of 4.2–5.7 and 2.0–4.2 ꢃ 10 M
ꢁ
1
cm in dilute solutions and solid films, respectively, which
assures the copolymers to harvest enough photons. The
absorption wavelengths (kmax,abs) of polymers P1, P2, P3,
and P4 in dilute solutions were located at 455, 459, 464,
and 466 nm, respectively, which can be attributed to p–p*
transition of the conjugated copolymer backbones and the
p–p interaction between the electron donor (carbazole) and
acceptor (bithiazole) units. It is obvious that, by tuning the
numbers of thiophene units in the polymer-conjugated heter-
ocyclic main-chains, the absorption spectra of carbazole-
based copolymers will be effectively influenced (in both solu-
tions and solid films). In contrast to solutions (see Table 2),
the absorption wavelengths (kmax,abs) of polymers P1–P4 in
solid films were found red-shifted to the range of 463–503
FIGURE 2 Normalized absorption spectra of P1–P4 in dilute
chloroform solutions.
nm obviously. These red-shifted wavelengths in solid films
are ascribed to the interchain associations and p–p stackings
of these copolymers as well as the highly rigid and planar
TABLE 2 Optical and Electrochemical Properties of Polymers P1–P4
a
b
4
ꢁ1
ꢁ1
g
Solution
Solid Film
a
max (ꢃ10
M
cm
)
Energy Levels
Band Gaps
ox
red
E
ec
Eg
opt
k
max,abs
k
max,abs
k
onset,abs
Solid
E
E
g
onset
onset
d
e
f
f
Polymer
(nm)
(nm)
(nm)
Solution
Film
(V)/HOMO (eV)
(V)/LUMO (eV)
(eV)
(eV)
c
P1
P2
P3
P4
a
(314) 455
314, 463
464
623
632
626
642
4.2
4.5
5.2
5.7
2.0
2.1
3.4
4.2
1.07/ꢁ5.42
1.05/ꢁ5.40
1.03/ꢁ5.38
0.99/ꢁ5.34
ꢁ0.75/ꢁ3.60
ꢁ0.76/ꢁ3.59
ꢁ0.77/ꢁ3.58
ꢁ0.80/ꢁ3.55
1.82
1.81
1.80
1.79
1.99
1.96
1.98
1.93
c
(364) 459
464
466
490
290, 504
f
In THF dilute solution.
E
HOMO/ELUMO
¼
[ꢁ(Eonset
ꢁ
E
onset(FC/FCþvs.Ag/Agþ)
)
ꢁ
4.8] eV where
b
c
Spin coated from THF solution on glass surface.
Shoulder peak.
4.8 eV is the energy level of ferrocene below the vacuum level and
E
onset(FC/FCþvs.Ag/Agþ) ¼ 0.45 eV.
d
e
g
ec
g
Absorption coefficient determined at kmax in THF.
Absorption coefficient of the solid film at kmax
Band gaps, electrochemical band gap E ¼ Eox/Onset – Ered/Onset and
opt
.
optical band gap Eg ¼ 1240/kedge
.
5484
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
The HOMO and LUMO energy levels were estimated by the
oxidation and reduction potentials from the reference energy
level of ferrocene (4.8 eV below the vacuum level) according
2
1(a),26
to the following equation
: EHOMO/ELUMO ¼ [ꢁ(E
–
–
onset
E
) ꢁ 4.8] eV and band gap ¼ E
onset(FC/FCþ vs. Ag/Agþ)
onset/ox
Eonset/red (where 4.8 eV is the energy level of ferrocene
below the vacuum level and E
¼ 0.45
onset(FC/FCþ vs. Ag/Agþ)
eV). It can be seen that polymers P1–P4 possess quasi-
reversible p-doping/dedoping (oxidation/rereduction) proc-
esses at positive potentials and reversible n-doping/dedop-
ing (reduction/reoxidation) processes at negative potentials.
The onset oxidation and reduction potentials of polymers
P1–P4 were in the ranges of 1.07–0.99 V and (ꢁ0.75)–
(
ꢁ0.82) V, respectively, from which the estimated HOMO and
LUMO levels were found in the range of (ꢁ5.34)–(ꢁ5.42) eV
and (ꢁ3.55)–(ꢁ3.60) eV, respectively. The lower HOMO
energy levels of the polymers were desirable for high open
circuit voltages of PSCs, as the polymers were taken as do-
FIGURE 3 Cyclic voltammograms of P1–P4 in solid films at a
scan rate of 100 mV/s.
8
nor materials. The noticeably higher oxidation potentials of
P1–P4 can be explained by that the resulting conjugated
2
4,6(b)
segments of polymer backbones.
Moreover, the red
copolymers were more electron deficient because of the
1
8(c),19(b)
shifts of absorption wavelengths from solutions to solid films
are 8, 5, 26, 38 nm for polymers P1–P4, respectively. Hence,
the larger numbers of thiophene units in P3 and P4 with
longer conjugation lengths induced stronger p–p stackings in
solid films, and thus to have larger red-shifted absorption
nitrogen atoms in their planar p-conjugated systems.
On the other hand, the LUMO energy level of the electron
donor (polymer) has to be positioned above the LUMO
energy level of the electron acceptor (PCBM) at least 0.3 eV,
so the exciton binding energy of polymer could be overcome
and result in efficient electron transfer from donor to
2
5
wavelengths.
3
As shown in Table 2, the optical band gaps (Eopt) of poly-
mers P1–P4 in solid films were found in the range of 1.93–
acceptor. The high reduction potentials of polymers P1–P4
g
represent high electron affinities to make these copolymers
suitable donors to inject and transport electrons to PCBM
1
.99 eV, which were determined from the cutoffs of the
3
(a),20(b)
acceptor in PSC devices.
The differences between the
absorption wavelengths. The optical band gaps of the copoly-
mers were reduced from P1 to P2 and from P3 to P4 owing
to the enhancement of electron donating capabilities, be-
cause more thiophene units and longer conjugation lengths
ec
band gap values directly measured by CV (Eg between 1.79
and 1.82 eV) and the optical band gap values obtained from
UV–vis spectra (Eg between 1.93 and 1.99 eV) lied within
opt
6
(b),23
an acceptable range of errors.
were introduced in the polymer backbones.
These
results imply that the light harvesting capabilities along with
optical band gaps can be tuned by electron D–A segments in
the polymer backbone, which is one of the efficient ways to
design the organic PV materials.
PV Properties
To investigate the potential applications of copolymers in
PSC devices, BHJ solar cells were fabricated by using poly-
mers P1–P4 as electron donors and fullerene [6,6]-phenyl-
Electrochemical Properties
C61-butyric acid methyl ester (PC61BM) as an electron
To investigate the redox behavior of the random copolymers
and determine their electronic states (i.e., HOMO/LUMO lev-
els), the electrochemical properties of polymers P1–P4 were
investigated by CV. The oxidation and reduction cyclic vol-
tammograms of the copolymers are shown in the Figure 3.
The electrochemical properties, such as onset potentials of
oxidation and reduction, that is, the estimated positions of
the upper edges of the valence band (HOMO) and the lower
edges of the conduction band (LUMO), respectively, and elec-
trochemical band gaps are summarized in Table 2. The CV
measurements were carried out in a 0.1M TBAPF6 solution
acceptor with a device configuration of ITO/PEDOT:PSS(30
nm)/(P1–P4):PCBM(1:1 w/w)(ꢀ80 nm)/Ca(30 nm)/Al(100
nm). This weight ratio of polymer blends with PCBM (P1–
P4:PCBM ¼ 1:1 w/w) was found to have the optimum PCE
value. Figure 4 shows the J-V curves of all PSCs containing
2
P1–P4 under the condition of AM 1.5 at 100 mW/cm , and
the open circuit voltage (Voc), short circuit current density
(J ), FF, and PCE values of the devices are summarized in
sc
Table 3. To have the great performance in PSC devices, DCB
was chosen as the solvent to obtain the blended polymer
active layers with good film qualities. The obtained PCE val-
ues of polymers P1–P4 were in the range of 0.36–0.57%,
where P3 and P4 possessed the highest PCE value (0.57%).
However, the similar alternating copolymer reported by Li
et al., which comprised of a planar carbazole unit as an elec-
tron donor and a bithiazole unit as an electron acceptor
(
in acetonitrile) at a scan rate of 100 mV/s under nitrogen.
A carbon electrode, which was coated with the polymer film
by dip coating, was used as a working electrode and Ag/
AgCl was served as a reference electrode, and it was cali-
1
=2
brated by ferrocene (E
¼ 0.45 mV versus Ag/AgCl).
ferrocene
COPOLYMERS CONTAINING BITHIAZOLE UNITS, PATRA ET AL.
5485

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 4 J–V characteristics of ITO/PEDOT:PSS/P1-P4:PC61BM
2
(
1:1 w/w)/Ca/Al under illumination of AM 1.5 at 100 mW/cm .
sandwiched between thiophene units, only achieved a lower
1
0(e)
PCE value of 0.30%.
Although both PSC devices containing P3 and P4 possessed
the highest PCE value (PCE ¼ 0.57%), P4 generated a higher
J
value, a higher absorption coefficient, and an efficient red
sc
shift in UV–vis spectrum compared with those of P3. There-
fore, the PSC device containing P4 was chosen to be opti-
mized in further PV studies. To acquire the advantage of a
2
7
higher absorption coefficient of PC BM than PC BM, the
7
1
61
BHJ PSC devices with different weight ratios of P4 (as an
electron donor) and PC BM (as an electron acceptor) were
7
1
fabricated, and their J–V characteristics and PV properties
are illustrated in Figure 5(a) and Table 4, respectively. The
optimum PCE value of 1.01% was obtained in the PSC device
having a weight ratio of P4:PC BM ¼ 1:1.5 (with V ¼ 0.60
7
1
oc
FIGURE 5 (a) J–V characteristics of ITO/PEDOT:PSS/P4:PC71BM/
2
V, J ¼ 4.83 mA/cm , and FF ¼ 35%). Using a lower weight
2
sc
Ca/Al under illumination of AM 1.5 at 100 mW/cm . (b) EQE
ratio of PCBM in blended polymer P4:PC BM (1:1 w/w) led
7
1
curves of PSC devices based on polymer blends P4/PC71BM in
various weight ratios.
to a reduction in the Jsc value, which could be attributed to
the inefficient charge separation and electron transporting
2
8
properties, resulting in the lower PCE value. However,
loading larger weight ratios of PCBM in blended copolymers
a
TABLE 4 Photovoltaic Properties of Bulk-Heterojunction
TABLE 3 Photovoltaic Properties of Polymer Solar
PSC Devices Containing Different Weight Ratios of Blended
Polymers P4:PC71BM and Blend Film Roughness by
AFM Measurements
Cell (PSC) Devices with a Configuration of
a
ITO/PEDOT:PSS/P1–P4:PC61BM(1:1 w/w)/Ca/Al
b
Active Layer
V
oc
J
sc
FF
PCE
(%)
Weight Ratios
2
(
Polymer:PC61BM¼1:1)
(V)
(mA/cm )
(%)
of Blended
V
oc
J
sc
FF
R
rms
PCE
(%)
2
b
P4:PC71BM
(V)
(mA/cm )
(%)
(nm)
P1
P2
P3
P4
a
0.53
0.56
0.62
0.58
1.93
2.31
2.52
2.87
2
35
36
37
34
0.36
0.46
0.57
0.57
1:1
1:1.5
1:2
a
0.60
0.60
0.58
3.30
4.83
3.42
39
35
28
2
0.20
0.17
0.22
0.77
1.01
0.55
Measured under AM 1.5 irradiation, 100 mW/cm .
Measured under AM 1.5 irradiation, 100 mW/cm .
b
b
Active layer of blended polymers with the weight ratio of P1–
P4:PC61BM¼1:1.
Rrms: root-mean-square values of roughnesses measured from AFM
images.
5486
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
devices, the EQEs for polymer P4 blended with PC BM in
7
1
various weight ratios were further investigated in Figure
(b), where the PSC devices exhibited a very broad response
5
range covering from 400 to 700 nm with the maximum EQE
values of 27%, 34%, and 22% for P4:PC BM ¼ 1:1, 1:1.5,
7
1
and 1:2 (w/w), respectively. Therefore, the photocurrent
generation in the PSC device with P4:PC BM ¼ 1:1.5 (w/w)
7
1
is higher and leading to the highest PCE value because of
more light harvest in the visible region.
Surface morphology of the active layer is also the key param-
2
9
eter for device performance in PSC devices. The AFM topo-
graphic images of the polymer blends of P4:PC BM in vari-
7
1
ous weight ratios (1:1, 1:1.5, and 1:2) are presented at
Figure 6(a–c) and their root-mean-square values of rough-
ness (Rrms) are presented in Table 4. It is clearly seen that
all the phase images possessed almost similar coarse surfa-
ces, which were attributed to the domains of highly stacked
2
1(a)
polymer chains in P4.
The most coarse surface of Rrms
¼
0
.22 nm in P4:PC BM ¼ 1:2 (w/w) indicated a large scale
7
1
phase separation, which might decrease the diffusional
escape probability for mobile charge carriers and thus to
3
0,16(d)
increase charge recombination.
However, the decrease
of PC71BM content in P4:PC71BM ¼ 1:1 (w/w) reduced the
Rrms value to 0.20 nm, which led to a similar J value with
sc
that of P4:PC BM ¼ 1:2 (w/w). Compared with the other
7
1
blending ratios of P4:PC BM, a smoothest surface with R
rms
7
1
¼
0.17 nm was obtained in P4:PC BM ¼ 1:1.5 (w/w),
7
1
which enhanced the J value and yielded the highest effi-
sc
3
0
ciency (PCE ¼ 1.01%) in PSCs.
CONCLUSIONS
In conclusion, a series of conjugated main-chain copolymers
consisting of 2,7-carbazole electron-donating unit and bithia-
zole electron-accepting unit were synthesized by Pd(0)-cata-
lyzed Stille coupling polymerization. Carbazole-based poly-
mers exhibited broad absorption bands located in the UV
and visible regions from 300 to 600 nm with optical band
gaps of 1.93–1.99 eV. The HOMO and LUMO energy levels of
the polymers can be finely tuned via the molecular engineer-
ing of donor/acceptor moieties and conjugated linkers inside
the copolymers, which possessed relatively lower HOMO lev-
els for PSC applications. The BHJ PV devices using polymers
P1–P4 as electron donors and PC BM as electron acceptors
6
1
were fabricated, and the optimization of PSC devices with
P4:PC71BM in different weight ratios were investigated.
Finally, the PV device bearing an active layer of polymer
blend P4:PC BM (1:1.5 w/w) showed the best PCE value of
7
1
FIGURE 6 AFM images of blended polymer P4:PC71BM spin
1
.01%, with a short circuit current density (J ) of 4.83 mA/
sc
coated from DCB in the ratios of (a) 1:1 (w/w), (b) 1:1.5 (w/w),
2
2
2
cm , a FF of 35%, and V ¼ 0.60 V under 100 mW/cm of
oc
and (c) 1:2 (w/w) with a size of 1 ꢃ 1 lm .
AM 1.5 white-light illumination. AFM images reveled that
there were a better mixing between polymers and PC71BM
to generate a less scale phase separation. Although the PCE
values of all PSC devices were not sufficiently high, the tuna-
ble optoelectronic properties could be achieved by the struc-
tural modifications of electron donor and acceptor units.
P4:PC BM (1:2 w/w) also reduced the J and PCE values,
7
1
sc
which could be probably attributed to the increased aggrega-
tion of PCBM so as to influence the separation of charges.
Hence, both Jsc and PCE values decreased with larger PCBM
molar ratios of 1:2 (w/w) because of the reasons described
The authors thank to the National Center for High-performance
Computing for computer time and facilities. The financial
1
5(e)
here.
To investigate the different efficiencies of the PSC
COPOLYMERS CONTAINING BITHIAZOLE UNITS, PATRA ET AL.
5487

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
supports of this project provided by the National Science Coun-
cil of Taiwan (ROC) through NSC 97-2113-M-009-006-MY2,
National Chiao Tung University through 97W807, and Energy
and Environmental Laboratories (charged by Dr. Chang-Chung
Yang) in Industrial Technology Research Institute (ITRI) are
acknowledged.
Leclerc, M. J Am Chem Soc 2008, 130, 732–742; (c) Zou, Y.;
Gendron, D.; A ı¨ ch, R. B.; Najari, A.; Tao, Y.; Leclerc, M. Macro-
molecules 2009, 42, 2891–2894; (d) Qin, R. P.; Li, W. W.; Li, C.
H.; Du, C.; Veit, C.; Schleiermacher, H. F.; Andersson, M.; Bo, Z.
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Zhang, Z.; Min, J.; Zhang, J.; Zhao, G.; Zhan, X.; Li, Y. Macro-
molecules 2010, 43, 5706–5712.
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