Improved bulk-heterojunction polymer solar cell performance through optimization of the linker groupin donor-acceptor conjugated polymer

Article abstract of DOI:10.1016/j.polymer.2012.02.007

Two donor-acceptor conjugated polymers PCTBTC8 and PCTTBTC12 have been prepared from Suzuki coupling reactions between the 2,7-carbazole and alkoxy substituted 2,1,3-benzothiadiazole units with two different linker groups thiophene and thieno[3,2-b] thiophene, respectively. The polymer PCTTBTC12 with the thieno thiophene linker group possessed a red-shifted UV-vis absorption spectrum and similar HOMO level in comparison with PCTBTC8. BHJ polymer solar cells were fabricated to investigate the photovoltaic properties of the two polymers and the polymer PCTTBTC12 showed superior device performance than PCTBTC8 with a significantly improved current density. In addition, the hole only devices tests results indicated that the hole mobility of the polymer was increased by replacing the thiophene group with fused thieno thiophene as the linker. The relationships between the device performance with the light absorption, energy levels, BHJ film morphology and hole mobility are discussed in details. And the improved photovoltaic property of the polymer PCTTBTC12 probably can be contributed to the better light absorption spectrum and enhanced hole transporting ability related to the more electron rich nature and planar structure of the thieno[3,2-b] thiophene group.

Full text of DOI:10.1016/j.polymer.2012.02.007

Polymer 53 (2012) 1535e1542
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Improved bulk-heterojunction polymer solar cell performance through
optimization of the linker groupin donoreacceptor conjugated polymer
*
Ying Sun, Baoping Lin , Hong Yang, Xiaohui Gong
School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two donoreacceptor conjugated polymers PCTBTC8 and PCTTBTC12 have been prepared from Suzuki
coupling reactions between the 2,7-carbazole and alkoxy substituted 2,1,3-benzothiadiazole units with
two different linker groups thiophene and thieno[3,2-b] thiophene, respectively. The polymer
PCTTBTC12 with the thieno thiophene linker group possessed a red-shifted UV-vis absorption spectrum
and similar HOMO level in comparison with PCTBTC8. BHJ polymer solar cells were fabricated to
investigate the photovoltaic properties of the two polymers and the polymer PCTTBTC12 showed
superior device performance than PCTBTC8 with a significantly improved current density. In addition,
the hole only devices tests results indicated that the hole mobility of the polymer was increased by
replacing the thiophene group with fused thieno thiophene as the linker. The relationships between the
device performance with the light absorption, energy levels, BHJ film morphology and hole mobility are
discussed in details. And the improved photovoltaic property of the polymer PCTTBTC12 probably can be
contributed to the better light absorption spectrum and enhanced hole transporting ability related to the
more electron rich nature and planar structure of the thieno[3,2-b] thiophene group.
Received 10 November 2011
Received in revised form
14 January 2012
Accepted 5 February 2012
Available online 9 February 2012
Keywords:
Linker group
Donoreacceptor polymer
Polymer solar cell
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
backbone. Possessing finely tailored energy levels and bandgaps,
several donoreacceptor (DeA) copolymers have exhibited very
In the past few years, polymer solar cell (PSC) has been
attracting more and more interests of researchers all over the world
due to their low-cost, flexibility, light-weight properties and
increasing potentials to replace the conventional silicon-based
solar cell. A major boost in PSC efficiency has been achieved
through use of the bulk-heterojunction (BHJ) device architecture
consisting of an electron-donating conjugated polymer and an
electron-accepting fullerene derivative [1e5]. The BHJ PSCs based
on poly (3-hexylthiophene) (P3HT) and [6,6] e phenyl-C61-butyric
acid methyl ester ([60] PCBM) have been intensively studied and
power conversion efficiencies (PCE) up to 4e5% have been reported
[6,7]. However, the narrow absorption spectra and high HOMO
level of P3HT impose significant limitations on the further
enhancement of PCE. Within the past decade, considerable
researches have been directed toward the development of low
bandgap polymers for improved photon harvesting abilities and
electronic properties. One of the feasible strategies to obtain low
band gap polymer is to combine electron-donating (donor) and
electron-accepting (acceptor) units together in the polymer
encouraging photovoltaic properties with the PCE exceeding 7%
when blended with fullerene derivatives [1e5].
2,1,3-benzothiadiazole (BT) is the most widely used acceptor
unit for constructing DeA copolymers which have shown prom-
ising photovoltaic properties [8e10]. A thiophene ring has usually
been inserted as a linker group between the donor unit and
acceptor unit to reduce steric hindrance and result in planar poly-
mer structure [7]. Evidently, this linker group also plays an
important role in the optical, electronic and photovoltaic properties
of the resulting polymers [11]. Although significant research efforts
have been devoted to new donor development and side chain
modifications for structural optimization of the BT based polymers,
only few researches have been done on the linker group
improvement [3,12e16]. So it is very critical to develop the new
linker group attached to BT moiety and to explore the effects on the
polymer properties.
In this paper, we presented our efforts to improve the photo-
voltaic property of the polymer by introducing thieno[3,2-b] thio-
phene as the linker group and synthesizing a new polymer
PCTTBTC12 based on 2,7-carbazole donor and alkoxy substituted
2,1,3-benzothiadiazole acceptor. As compared to the polymer
PCTBTC8 based on thiophene as linker group, the new polymer
* Corresponding author. Tel./fax: þ86 25 52090616.
showed
a red-shifted absorption spectrum both in solution
E-mail addresses: ysunpaper@gmail.com (Y. Sun), lbp@seu.edu.cn (B. Lin).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.02.007

1536
Y. Sun et al. / Polymer 53 (2012) 1535e1542
(w20 nm) and in film (w30 nm). Cyclic voltammetry measure-
ments of the two polymers indicated that polymer PCTTBTC12
possessed the similar HOMO level and deeper LUMO level
compared to the polymer PCTBTC8. BHJ polymer solar cell devices
were fabricated based on the synthesized two polymers as donor
and PC71BM as acceptor. As expected, the incorporation of the fused
thieno thiophene ring as the linker group between the donor and
acceptor units in the polymer chain lead to the significantly
improved current density and hence device efficiency. In addition,
the hole only device also indicated that the polymer PCTTBTC12
possessed enhanced hole mobility than PCTBTC8. The relationships
between the photovoltaic properties with the UVevis absorption,
energy levels, film morphology and hole mobility are discussed in
details.
temperature, the reaction mixture was poured into water (100 mL).
Organic layer was separated and water phase was extracted with
dichloromethane for three times. The combined organic layer was
washed with brine (30 mL) and dried over anhydrous magnesium
sulfate. The residue was chromatographically purified on silica gel
column eluting with CH2Cl2:hexane (1:12, v:v) to afford the
compound 1 as a yellow oil (968 mg, 87%).1HNMR (CDCl3, ppm):
d
¼ 8.47 (dd, J ¼ 3.7 Hz, 2H), 7.51 (dd, J ¼ 6.3 Hz, 2H), 7.27e7.22 (m,
2H), 4.11 (t, J ¼ 7.1 Hz, 4H), 1.97e1.87 (m, 4H), 1.67e1.20 (m, 20H),
0.98e0.87 (m, 6H). 13C NMR (CDCl3):
d
¼ 152.32, 151.38, 134.57,
131.02, 127.68, 127.14, 118.03, 74.75, 32.44, 30.85, 30.22, 30.16,
30.08, 29.89, 28.76, 27.25, 26.47, 23.21, 17.69, 14.63, 14.09.
2.3.2. 4, 7-bis (5-bromothiophen-2-yl)-5, 6-bis (octyloxy) benzo[c]
[1,2,5] thiadiazole (compound 2)
2. Experimental part
NBS (653 mg, 3.67 mmol) was added in one portion to a solution
of compound 1 (1 g, 1.80 mmol) in DMF (15 mL). The mixture was
stirred at room temperature for 20 h in the dark. After that, solvent
was removed under reduced pressure and the residue was chro-
matographically purified on silica gel column eluting with
CH2Cl2:hexane (1:12, v: v) to afford3as an orange solid (1.15 g,
2.1. Materials
4,7-Dibromo-5,6-bis (octyloxy) benzo-2,1,3-thiadiazole, 4,7-
Dibromo-5,6-bis
(dodecyloxy)-benzo-2,1,3-thiadiazole,4,7-bis
(thieno[3,2-b]thiophen-2-yl)-2,1,3-benzothiadiazole, 2-(tributyl-
stannyl) thieno[3,2-b]thiophene and 2,7-Bis (4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-N-9-heptadecanylcarbazole (compound
5) were prepared according to literature procedures [17e19]. All
other chemical reagents were purchased from Aldrich or TCI and
used without further purification. Toluene and tetrahydrofuran
(THF) were purified by distillation from sodium under nitrogen
atmosphere using benzophenone as an indicator. N, N-dime-
thylformamide (DMF) was distilled under vacuum after dried over
CaH2.
90%).1H NMR (CDCl3):
2H), 4.13 (t, J ¼ 7.1 Hz, 4H), 2.18e1.97 (m, 4H), 1.52e1.20 (m, 20H),
0.88 (m, 6H). 13C NMR (CDCl3):
d
¼ 8.37 (d, J ¼ 4.1 Hz, 2H), 7.18 (d, J ¼ 4.1 Hz,
d
¼ 151.75, 150.67, 136.07, 131.41,
130.04, 117.30, 115.88, 74.91, 32.30, 30.77, 29.97, 29.77, 26.41, 23.17,
14.61.
2.3.3. 5, 6-bis (dodecyloxy)-4, 7-di (thieno[3,2-b] thiophen-2-yl)
benzo-[c] [1,2,5]-thiadiazole (compound 3)
4,7-Dibromo-5,6-bis(dodecyloxy)-benzo-2,1,3-thiadiazole
(1.33 g, 2 mmol) and 2-(tributylstannyl) thieno[3,2-b] thiophene
(2.15 g, 5 mmol) were dissolved in dry THF and then the mixture
was added the catalyst of tris(dibenzylideneacetone) dipalladium
(0) (55 mg) and tri(o-tolyl) phosphine (137 mg). Then the reaction
was refluxed for 20 h under N2. After cooling to room temperature,
the mixture solution was poured into water (100 mL) and organic
layer was separated. Water phase was extracted with dichloro-
methane for three times. The combined organic layer was washed
with brine (30 mL) and dried over anhydrous magnesium sulfate.
The residue was chromatographically purified on silica gel column
eluting with CH2Cl2:hexane (1:8, v:v) to afford the product
2.2. Measurement and characterization
1H and 13C NMR spectra were recorded on Bruker AV 300 or 500
spectrometers operating at 300 or 125 MHz in CDCl3. ESI-MS spectra
tests were performed using a Bruker APEX Qe 47e fourier transform
(ion cyclotron resonance). Gel permeation chromatography (GPC)
measurements were carried out on a Waters 1515 chromatograph
with a refractive index detector in room temperature with THF as
eluent. UVevis spectra were obtained using a PerkineElmer
Lambda-9 spectrophotometer. The differential scanning calorim-
etry (DSC) tests was testeꢁd1 by DSC2010 (TA instruments) with
compound 3 as red solid (702 mg, 45%).1H NMR (CDCl3):
d
¼ 8.82 (s,
2H), 7.48 (d, J ¼ 5.2 Hz, 2H), 7.35 (d, J ¼ 5.2 Hz, 2H), 4.18 (t, J ¼ 7.1 Hz,
a heating rate of 10 ꢀC min and a nitrogen flow of 50 mL minꢁ1
.
4H), 2.02e1.94 (m, 4H), 1.52e1.10 (m, 36H), 0.91 (m, 6H). 13C NMR
Thermogravimetric analysis (TGA) was performed on
a
Per-
(CDCl3):
d
¼ 153.21, 151.64, 138.79, 137.69, 135.79, 122.67, 121.23,
kineElmer TGA-7.Cyclic voltammetries (CV) measurement were
conducted on a BAS CV-50W voltammetric system with a three-
electrode cell in acetonitrile with 0.1 M of tetrabutylammonium
hexafluorophosphate using a scan rate of 100 mV sꢁ1 with ITO, Ag/
AgCl and Pt mesh as the working electrode, reference electrode and
counter electrode, respectively. The photoluminescence (PL) spectra
of the samples were measured on a FluoroLog 3-TCSPC spectro-
photometer. AFM images were obtained on a Veeco multimode AFM
with a Nanoscope III controller under tapping mode.
120.21, 116.89, 75.11, 32.72, 30.89, 30.11, 29.91, 29.72, 26.36, 23.21,
14.78.
2.3.4. 4, 7-bis (5-bromo-thieno [3, 2-b] thiophen-2-yl)-5, 6-bis
(dodecyloxy) benzo[c] [1,2,5]thiadiazole (compound 4)
The compound 3 (702 mg, 0.90 mmol) was dissolved in DMF
(10 mL) and then NBS (384 mg, 2.16 mmol) was added to the
solution in one portion. The mixture solution was stirred at room
temperature for 20 h in the dark. Solvent was removed under
reduced pressure and the residue was chromatographically purified
on silica gel column eluting with CH2Cl2:hexane (1:6, v:v) to afford
the compound 4as a red solid (753 mg, 89%).1H NMR (CDCl3):
2.3. Synthesis of monomers and copolymers
2.3.1. 5, 6-bis (octyloxy)-4, 7-di (thiophen-2-yl) benzo-[c] [1,2,5]
thiadiazole (compound 1)
d
¼ 8.76 (s, 2H), 7.33 (s, 2H), 4.15 (t, J ¼ 7.1 Hz, 4H), 1.99e1.91 (m,
4H), 1.52e1.20 (m, 36H), 0.89 (m, 6H).13C NMR (CDCl3):
d
¼ 152.01,
A mixture of 4, 7-Dibromo-5,6-bis (octyloxy) benzo-2,1,3-
thiadiazole (1.10 g, 2 mmol) and tributyl (thiophen-2-yl) stannane
(1.87 g, 5 mmol) were dissolved in dry THF followed by the addition
of tris(dibenzylideneacetone) dipalladium (0) (55 mg) and tri(o-
tolyl) phosphine (137 mg) under N2. Then the resulting mixture
was refluxed for 20 h under N2. After being cooled to room
150.79, 140.42, 139.70, 135.82, 122.67, 122.40, 117.93, 114.89, 74.96,
32.16, 30.57, 29.90, 29.79, 29.61, 26.19, 22.93, 14.35.
2.3.5. PCTBTC8
Compound 5 (264 mg, 0.4 mmol) and compound 2 (284 mg,
0.4 mmol) were dissolved in a mixture of toluene (6 mL) and

Y. Sun et al. / Polymer 53 (2012) 1535e1542
1537
aqueous 2 M K2CO3 solution (2 mL) in a 25 mL two-neck flask. A
catalyst of Pd(PPh3)4 (6 mg) and one drop of aliquat 336 were
added to the mixture and the resulting solution was carefully
degassed and then refluxed for 48 h under a nitrogen atmosphere.
At the end of polymerization, the polymer was end-capped with
phenyl boronic acid (40 mg) and bromobenzene (2 mL). The reac-
tion solution was poured into methanol (200 mL) and the precip-
itated polymer was obtained by filtration and purified by Soxhlet
extraction and short flash column with CHCl3 as eluent. The
concentrated polymer solution in CHCl3 was re-precipitated into
methanol, the resulting polymer was collected by filtration and
(10.08 mm2). Device testing was carried out in glove box with
a Keithley 2400 SMU source measurement unit and an Oriel Xenon
lamp (450 W) using an AM1.5 filter as the solar simulator. The
device results have been calibrated by a standard silicon solar cell
with a KG5 filter.
2.5. Hole only device fabrication
The hole only devices were fabricated employing the same
procedure with the photovoltaic devices except that the MoO3 was
deposited in replacement of calcium.
dried under vacuum (268 mg, 72.3%). 1H NMR (CDCl3):
d
¼ 8.70 (br,
2H), 8.19 (br, 2H), 7.99 (s, 1H), 7.77 (s, 1H), 7.66 (br, 4H), 4.73 (br,1H),
4.28 (br, 4H), 2.52 (br, 2H), 2.16 (br, 6H), 1.55e1.10 (br, 44H), 0.98 (s,
6H), 0.82 (m, 6H).
3. Results and discussion
3.1. Design and synthesis of monomers and polymers
2.3.6. PCTTBTC12
To be solution processed, the targeted polymer should have
reasonable solubility in common solvents and introduction of alkyl
or alkoxy substituents in the polymer side chain can improve the
solubility of the polymer. To reduce the undesired steric hindrance,
we introduced the long alkoxy chain in the benzodithiazole unit to
ensure the planarity of the resulting polymer chain. Due to the
higher planarity, thieno[3,2-b]thiophene is expected to facilitate
the molecular organization and consequently increase the charge
mobility. Moreover, the more electron rich nature of the fused ring
To a 25 mL two-neck flask containing compound 5 (264 mg,
0.4 mmol), and compound 4 (374 mg, 0.4 mmol) in a mixture of
toluene (6 mL) and aqueous 2 M K2CO3 solution (2 mL) was added
a catalyst of Pd(PPh3)4 (6 mg) and one drop of aliquat 336. The
resulting solution was carefully degassed and then refluxed for 48 h
under a nitrogen atmosphere. The polymer was end-capped by
adding phenyl boronic acid (40 mg) and bromobenzene (2 mL) at
the end of polymerization. The reaction solution was poured into
methanol (200 mL) and the precipitated polymer was recovered by
filtration and purified by Soxhlet extraction and then flash short
column with CHCl3 as eluent. The concentrated polymer solution
was re-precipitated into methanol and the resulting polymer was
collected by filtration and dried under vacuum (355 mg, 75.1%). 1H
probably can enhance the intermolecular
pep interactions and
improve the absorption ability of the polymer. So thieno[3,2-b]
thiophene was designed to be the new linker group between the
2,7-carbazole donor and alkoxy substituted benzodithiazole
acceptor to investigate the linker group effect on the polymer
properties.
NMR (CDCl3):
d
¼ 8.92 (s, 2H), 8.56 (s, 2H), 8.01 (br, 1H), 7.79 (br,
1H), 7.75 (br, 2H), 7.71 (br, 2H), 4.73 (br, 1H), 4.29(br, 4H), 2.50 (br,
The synthetic routes of the monomer are outlined in the
Scheme 1. The alkoxy substituted benzodithiazole was prepared
according to the reported procedure [17] and the Stille coupling
reaction of it with2-(tri-n-butylstannyl)-thiophene in the presence
of Pd(PPh3)4 as catalyst gave rise to the compound 1 with a high
yield of 87%. And then the compound 2 was prepared from the
bromination reaction of the compound 1. The tri-n-butylstannyl
substituted thieno[3,2-b]thiophene was prepared following the
routes in the published papers [20]. Then Stille coupling reaction of
it with alkoxy substituted benzodithiazole provided the new
compound 3 which was subsequently brominated with NBS to yield
the targeted monomer compound 4. The synthesis procedure of the
polymers was described in Scheme 2. Pd(0)-catalyzed Suzuki
copolymerization between the 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-N-9-heptadecanylcarbazole with compound 2
and 4 in the toluene gave the targeted polymer PCTBTC8 and
PCTTBTC12,respectively. The resulting polymers were obtained by
precipitating the mixture solution from methanol and purified by
2H), 2.12 (br, 6H), 1.52e1.10 (br, 60H), 0.98 (s, 6H), 0.80 (m, 6H).
2.4. Photovoltaic device fabrication
To fabricate the BHJ PSC, PEDOT:PSS layer (w45 nm) was spin-
coated from the its water solution (BaytronÒ PVP AI 4083, filtered
at 0.45
mm) onto the ITO-coated glass substrates which were
cleaned with detergent, de-ionized water, acetone, and isopropyl
alcohol in advance. Then the substrate was baked on a hotplate at
150 ꢀC for 10 min. The BHJ film was spin-coated in a nitrogen filled
glove box from a blending chlorobenzene (CB):diiodooctane (DIO)
solution of polymer with PC71BM which was stirred overnight and
filtered with a 0.2 mm PTFE filter. All the devices were annealed in
the glove box under 150 ꢀC for 10 min. Subsequently, calcium
(30 nm) and aluminum (100 nm) were thermally evaporated onto
the active layer sequentially under high vacuum (<2 ꢂ 10ꢁ7 Torr)
with a shadow mask to define the active area of the devices
Scheme 1. Synthetic routes for the monomers.

1538
Y. Sun et al. / Polymer 53 (2012) 1535e1542
Scheme 2. Synthetic routes for the polymers.
Soxhlet extraction with acetone and hexane, subsequently. Further
dissolving in CHCl3 and subsequent precipitating from methanol
provided the final polymers.
electron-donating carbazole moiety to electron-deficient benzodi-
thiazole unit while the delocalized transition in the conju-
pep
gated polymer chain may result in the other absorption band in the
shorter wavelength [23]. Polymer PCTTBTC12 exhibited a red-
shifted absorption spectrum in solution compared to PCTBTC8
with the absorption maximum red shifting from 522 nm to 542 nm.
Such an improvement in the absorption ability of the polymer can
be explained that the stronger electron-donating ability and higher
planarity of thieno[3,2-b] thiophene increased the effective
conjugation length of the polymer and extended the delocalized p-
electron system, leading to enhanced intramolecular charge
transfer [24].
Both of the two polymers possess good solubility in common
organic solvents such as chloroform, chlorobenzene and dichloro-
benzene. The molecular weights of the two polymers were deter-
mined by gel permeation chromatography with polystyrene as
standard and THF as eluent. The number-average molecular weight
(Mn) of PCTBTC8 and PCTTBTC12 is 22.9 kDa and 16.2 kDa, with
a polydispersity index (PDI) of 2. 54 and 3.00, respectively (listed in
Table 1).
The absorption spectrum of the two polymers in film were
similar in shape with those in solution while the absorption bands
were red-shifted about 20e30 nm for both of PCTBTC8 and
PCTTBTC12. These solution-to-solid-state-induced red-shifts have
been observed in many published literatures and can be assigned to
be the higher structural organization and more ordered packing
induced by the stronger intermolecular interaction in the solid
state [25].
Calculated from the film absorption onset, the bandgap of the
polymer was determined as 1.93 eV and 1.84 eV for PCTBTC8 and
PCTTBTC12, respectively. As expected, the incorporation of thieno
[3,2-b] thiophene as the linker group decreased the bandgap and
improved the light absorption ability of the polymer.
The electrochemical properties of the polymers were evaluated
by cyclic voltammetry (CV) tests and the energy levels of polymers
were calculated from the equations as following:
3.2. Thermal properties
Thermal stability of the polymer is very important for the
fabrication process of the photovoltaic device. Fig. 1 illustrated the
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) thermograms of the two polymers. The TGA tests
showed that the decomposition temperatures at of 5% weight loss
for PCTBTC8 and PCTTBTC12 are 355 and 388 ꢀC, respectively. And
as shown in the DSC curves, the heating circle thermogram of the
polymer PCTBTC8 revealed a vaguely observable glass transition at
w70e120 ꢀC while no pronounced crystallization peak was
observed in the temperature range from 30 to 250 ꢀC. For polymer
PCTTBTC12, neither glass transition nor melting peak appeared in
the heating or cooling circle, probably indicated the amorphous
nature of the polymer. The good thermal property of the photo-
active polymer can provide the superior stability of the BHJ
morphology and prevent the device performance degradation
under high temperature [21,22].
HOMO ¼ ꢁ½Eox þ 4:80ꢃeV LOMO ¼ ꢁ½Ered þ 4:80ꢃeV
in which Eox and Ered are the onset of oxidation and reduction
potential, respectively. Fig. 3 illustrated the cyclic voltammograms
of the two polymers films on Pt electrode with 0.1 mol/L tetrabu-
tylammonium tetrafluoroborate (Bu4NPF6) as the electrolyte in
acetonitrile with a scan rate of 100 mV/s using Ag/Agþ as the
reference electrode and the detailed energy level values are listed
in Table 1.
3.3. Optical and electrochemical properties
The optical properties of the polymers were investigated both in
solution and in film. Normalized UVevis absorption spectrum were
presented in Fig. 2 while detailed data were included in Table 1.
Both of the polymers showed two absorption bands in the wave-
length range of 300e700 nm. The longer absorbance may
arise from the intramolecular charge transfer (ICT) from
In bulk-heterojunction OPVs, the maximum achievable Voc is
dependent on the difference between the HOMO level of the donor
Table 1
Molecular weights, optical and electrochemical properties of the polymers.
Polymer
Mn [kg/mol]
Mw [kg/mol]
PDI
UVevis absorption
Cyclic voltammetry
In CHCl3
In film
lmax [nm]
lonset [nm]
lmax[nm]
lonset [nm]
Bandgap [eV]
HOMO [eV]
LUMO [eV]
PCTBTC8
PCTTBTC12
22.9
16.2
58.2
48.6
2.54
3.00
522
542
605
640
543
572
643
673
1.93
1.84
ꢁ5.46
ꢁ5.47
ꢁ3.30
ꢁ3.39

Y. Sun et al. / Polymer 53 (2012) 1535e1542
1539
Fig. 2. UVevis absorption spectra of the polymers in chloroform (a) and in solid
state (b).
Fig. 1. (a) TGA thermograms of the polymers (heating rate: 20 K/min). (b) DSC ther-
mograms of the polymers with a (heating rate: 10 K/min).
photovoltaic devices employing the weight ratio of 1:4 and mixture
solvent of CB/DIO.
As shown in the Table 2, higher Voc, FF and PCE were achieved in
the PSC devices based on 1:4 ratio for both of the polymers. Due to
the amorphous nature of the polymers, it is probably difficult for
the fullerene molecular exclude from the polymer, which may
polymer and the LUMO level of the acceptor [26]. The HOMO levels
of polymer PCTBTC8 and PCTTBTC12 were determined to be
similar as ꢁ5.46 and ꢁ5.47 eV, respectively. So maintaining this
deep HOMO level is critical for the BHJ photovoltaic device to
achieve high Voc. [27]. In additional, the LUMO energy level differ-
ences between the donor polymer and the acceptor have been
found to have significant effect on the exciton dissociation and
charge recombination [28,29]. As calculated from CV results, the
two polymers PCTBTC8 and PCTTBTC12 showed the LUMO levels
of ꢁ3.30 eV and ꢁ3.39 eV, respectively. So this large difference
between the LUMO levels may allow for the efficient charge sepa-
ration and decreased charge recombination rate [21,30,31].
3.4. Photovoltaic properties
To evaluate the photovoltaic properties of the polymers, poly-
mer solar cells were fabricated with the conventional device
structure of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. The detailed
device fabrication process was described in the experimental part.
To increase the light absorption ability of the active layer, PC71BM
was employed as the acceptor in the BHJ due to its wider absorption
range compared to PC61BM. The blending ratio of the donor and
acceptor in the BHJ layer was tuned from 1:2, 1:3 to 1:4 to obtain
the well balanced charge carrier transporting and enhanced device
efficiency. The electrical parameters based on the optimized
devices with different weight ratio were summarized in Table 2.
And Fig. 4(a) illustrated the device characteristics of the optimized
Fig. 3. Cyclic voltammogram of the two polymers with a scan rate of 100 mV/s.

1540
Y. Sun et al. / Polymer 53 (2012) 1535e1542
Table 2
around 67 nm. Combined with the relatively large bandgap
(1.93 eV) of the polymer PCTBTC8, the thin active layer film may
have the limited light absorption ability to generate exitons, which
may lead to the smaller current density. For PCTTBTC12:PC71BM
(1:4) based photovoltaic device, a significantly increased current
density (9.44 mA/cm2) was achieved with the optimized thickness
around 115 nm, although the Voc (0.83 V) and FF (0.43) were
decreased compared to PCTBTC8. Furthermore, the external
quantum efficiency (EQE) wavelength dependencies of optimized
photovoltaic devices under illumination with 100 mW/cm2 (AM
1.5G) were measured and the EQE spectra were plotted in Fig. 4(b).
Compared to PCTBTC8, PCTTBTC12 based device showed wider and
stronger photo response in the absorption range of 350e700 nm,
which was consistent with the red-shifted absorption spectrum
of PCTTBTC12 and higher current density of the device based on
PCTTBTC12:PC71BM (1:4).
To investigate the charge transfer properties of the blends,
photoluminescence (PL) spectra of the two polymers in the solid
state and the corresponding blending films with the ratio of 1:4
were tested and shown in Fig. 5. All the films are prepared and
measured under the same conditions to make direct comparisons.
For excitation at the corresponding maximum absorption wave-
length, both of the two polymers PCTBTC8 and PCTTBTC12
exhibited emission band in the 600e800 nm with the maximum at
around 670 nm, while the PL of the polymers were thoroughly
quenched in the blending films of the polymer and PC71BM. This PL
quenching phenomena indicated the efficient charge transfer from
the polymers to PC71BM and favorable exciton dissociation in the
films.
Photovoltaic parameters of the BHJ polymer solar cell based on the polymers and
PC71BM with different blending ratio and solvent.
Polymer
PCTBTC8
Blend Solvent (V/V)
Ratio
PCE (%) Voc (V) Jsc
(mA/cm2)
FF (%)
1:2
1:3
1:4
CB/DIO (30/1) 1.83
CB/DIO (30/1) 1.99
CB 1.97
0.79
0.82
0.84
0.89
0.78
0.80
0.81
0.83
5.91
6.22
4.54
5.99
5.77
8.84
6.25
9.44
0.394
0.391
0.52
0.51
0.40
0.37
0.39
0.43
CB/DIO (30/1) 2.72
CB/DIO (20/1) 1.80
CB/DIO (20/1) 2.64
PCTTBTC12 1:2
1:3
1:4
CB
1.97
CB/DIO (20/1) 3.40
require more PC71BM to form the continuous and conducting
network within BHJ film [32]. In addition, it has been demonstrated
in many papers that the blend composition can influence the
intermediate charge transfer state between the exciton and free
charges and consequently determine the charge separation
[33e35]. In our case, optimized charge transporting and reduced
charge recombination may be achieved in the devices based on 1:4
ratio for the two polymers.
By using the high boiling point 1,8-diiodooctane (DIO) as the
additive in the processing solvent, we have optimized the film
morphology and improved the photovoltaic performance. Opti-
mized PSC device employing PCTBTC8:PC71BM (1:4) as the active
layer showed an efficiency of 2.72 with a Voc of 0.89 V, a Jsc of
5.99 mA/cm2, and a FF of 0.51. The optimized thickness is only
Since the device performance is greatly dependant on the BHJ
film morphology, well defined film microstructure should be ach-
ieved by optimizing the process conditions [36]. Employing the
CB:DIO as the mixing solvent, we have obtained smooth and
homogeneous films for both of the polymer blended with PC71BM.
Tapping-mode atomic force microscopy (AFM) images of the BHJ
films were illustrated in Fig. 6. As shown in the pictures, there was
no obvious large-scale phase separation observed in the BHJ films
from the phase images and the rms roughness from the height
images was around 1 nm.
To explore the effect of the linker group on the hole mobility of
the polymer, hole only devices were fabricated with the device
structure of ITO/PEDOT:PSS/Polymer:PC71BM/MoO3/Al. Fig.
7
showed measured JeV characteristics of the device and the hole
Fig. 4. Respective JeV curves (a) and EQE curves (b) of optimized photovoltaic devices
Fig. 5. Photoluminescence (PL) spectra of the polymers and blending films of polymer
based on polymers and PC71BM as active layer.
and PC71BM with the ratio of 1:4.

Y. Sun et al. / Polymer 53 (2012) 1535e1542
1541
Fig. 6. Tapping mode AFM height images (a, c) and phase images (b, d) of the optimized BHJ active layer based on PC71BM with polymer PCTBTC8 (a) and PCTTBTC12 (b).
transporting characteristic was modeled by the space charge
limited current (SCLC) method using a field dependent mobility.
The hole mobility of the polymer was extracted by modeling the
dark current in SCLC region and calculated from the equation of
better molecular organization, which result in more efficient
pathways for charge transporting [37].
4. Conclusion
J ¼ 9
3
0
3
rmV2/8L3, where J is the current density,
3
is the vacuum
0
permittivity,
3
r
is the relative permittivity of the polymer, m is the
In summary, two donoreacceptor polymers PCTBTC8 and
PCTTBTC12 with different linker group between 2,7-carbazole
donor and alkoxy substituted benzothiadiazole acceptor units have
been synthesized through Suzuki coupling reaction. Thermal,
optical and electrochemical properties of the polymers were
investigated and discussed in details. The polymer PCTTBTC12
possessing the thieno thiophene linker group exhibited a red-
shifted absorption spectrum and reduced bandgap compared to
the polymer PCTBTC8 with the thiophene linker. BHJ PSC devices
were fabricated and the results indicated that the fused thiophene
linker inserted in donor unit and acceptor unit can lead to better
photovoltaic property than thiophene linker which probably can be
contributed to the improved light absorption ability and hole
mobility related to the more electron rich nature and planar
structure of the thieno thiophene ring. In the context of this work,
the study of the linker group effect can be used to obtain more
understanding on the structural-properties relationship and help
optimize the donoreacceptor conjugated polymer.
hole mobility, and L is the thickness of active layer. The calculated
hole mobility was 1.92 E-6 and 1.23 E-4 cm2 Vꢁ1 sꢁ1 for PCTBTC8
and PCTTBTC12, respectively. The higher mobility of the
PCTTBTC12 can be related to its more planar polymer structure and
Acknowledgment
Ying Sun thanks the State-Sponsored Scholarship for Graduate
Students from China Scholarship Council. Baoping Lin is grateful to
the National Natural Science Foundation of China (no.61178057).
Fig. 7. Current density and voltage (JeV) curves of the hole only devices.

1542
Y. Sun et al. / Polymer 53 (2012) 1535e1542
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