Novel photovoltaic polymers constructed from alternative donor and acceptor units having one mother structure

Article abstract of DOI:10.1016/j.polymer.2013.02.036

This work presents a new strategy for the preparation of donor-acceptor conjugated copolymers, in which both donor and acceptor units derive from one mother structure. Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (BQDT) is an electron-deficient unit. From BQDT, benzo[1,2-b:4,5-b′]dithiophene (BDT) and benzo[1,2-b:4,5-b′]dithiophene-4,8-dione dioxime (BXDT) can be easily derived and serve as electron-donating and another electron-accepting unit, respectively. Based on these three units, poly(BDT-alt-BQDT) (PBQ-C24, with 2-decyltetradecyl side chains) and poly(BDT-alt-BXDT) (PBX-C8, with 2-ethylhexyl side chains) have been designed and synthesized. Their thermal, optical and electrochemical properties have been investigated. PBQ-C24 has a broad absorption spectrum in the range of 300-700 nm, while PBX-C8 can only harvest the light from 300 to 600 nm. However, the bulk heterojunction cell based on the blend of PBX-C8 and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) displays a better performance with a power conversion efficiency of 0.51%.

Full text of DOI:10.1016/j.polymer.2013.02.036

Polymer 54 (2013) 2278e2284
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Novel photovoltaic polymers constructed from alternative donor and acceptor
units having one mother structure
*
*
Long Liang, Jin-Tu Wang, Chong-Yu Mei , Wei-Shi Li
Laboratory of Organic Functional Materials and Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
This work presents a new strategy for the preparation of donoreacceptor conjugated copolymers, in
which both donor and acceptor units derive from one mother structure. Benzo[1,2-b:4,5-b0]dithiophene-
4,8-dione (BQDT) is an electron-deficient unit. From BQDT, benzo[1,2-b:4,5-b0]dithiophene (BDT) and
benzo[1,2-b:4,5-b0]dithiophene-4,8-dione dioxime (BXDT) can be easily derived and serve as electron-
donating and another electron-accepting unit, respectively. Based on these three units, poly(BDT-alt-
BQDT) (PBQ-C24, with 2-decyltetradecyl side chains) and poly(BDT-alt-BXDT) (PBX-C8, with
2-ethylhexyl side chains) have been designed and synthesized. Their thermal, optical and electro-
chemical properties have been investigated. PBQ-C24 has a broad absorption spectrum in the range of
300e700 nm, while PBX-C8 can only harvest the light from 300 to 600 nm. However, the bulk heter-
ojunction cell based on the blend of PBX-C8 and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)
displays a better performance with a power conversion efficiency of 0.51%.
Received 3 December 2012
Received in revised form
5 February 2013
Accepted 20 February 2013
Available online 27 February 2013
Keywords:
Organic solar cells
Donoreacceptor copolymers
Benzo[1,2-b:4,5-b0]dithiophene
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
are weak acquisition capability when light wavelength is longer
than 650 nm and a little unmatched highest occupied molecular
As energy crisis and environment pollution go worse and worse,
solar light, as a permanent green natural energy resource, has
become an increasing significance for human sustainable devel-
opment. Among a variety of light-to-electric conversion technolo-
gies, organic solar cells (OSCs) have attracted huge attention in
terms of their ease of processing, low cost, flexibility, and light
weight [1]. After the pioneering work of bilayer [2] and bulk het-
erojunction [3] device configurations, the active layer of OSCs is
required to be composed of both p- and n-type organic semi-
conductors. In basic light-to-electric conversion mechanism, p-type
materials play the roles of electron donors in photo-induced
charge-separation process and conduct the separated positive
charge carriers (holes) to anode. While, n-type materials behave as
electron acceptors and transport the separated negative charge
carriers (electrons) to cathode. Since it was first reported in 2000
[4], the bulk heterojunction composed of poly(3-hexylthiophene)
(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)
represents as a typical system having a top power conversion ef-
ficiency (PCE) of 4e5% [5]. The material innovation afterward for a
long time is motivated to overcome the shortages of P3HT, which
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels relative to PC61BM. Along this line, a variety of donore
acceptor (DeA) alternative conjugated copolymers have been
designed and investigated [6], some of which have achieved PCE
over 6% [7]. The incorporation of the alternative electron-deficient
units (acceptor) in donor polymers can lower the band gap and the
molecular energy level, which enables more efficient light ab-
sorption in the far visible and near infrared region and provides
more matched HOMO and LUMO to fullerene acceptors.
Among the reported DeA copolymers, benzo[1,2-b:4,5-b0]
dithiophene (BDT) has often been used as an electron-donating
unit. The planar and rigid fused aromatic ring of BDT would be
helpful to strengthen interactions among polymer chains, which is
favourable for charge-transportation. In 2002, BDT was first applied
for synthesis of conjugated polymers, including its homopolymers
and copolymers [8]. Later, Ong et al. demonstrated that the copol-
ymer of BDT and thiophene is a good field-effect transistor with
hole mobility of w0.4 cm2 Vꢀ1 sꢀ1 [9]. In 2008, Hou, Yang and
coworkers firstly used BDT to prepare DeA copolymers with ben-
zothiadiazole, benzoselenadiazole, and thienopyrazine for photo-
voltaic application with PCE in the range of 0.1e0.9% [10]. The
outstanding work was reported by Yu et al., who copolymerized
BDT with thieno[3,4-b]thiophene, affording a family of promising
photovoltaic materials (PTB) [11]. After structural optimization, this
* Corresponding authors. Tel./fax: þ86 21 5492 5381.
E-mail addresses: meichy@sioc.ac.cn (C.-Y. Mei), liws@mail.sioc.ac.cn (W.-S. Li).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.02.036

L. Liang et al. / Polymer 54 (2013) 2278e2284
2279
family frequently renewed the PCE record [12] and preserved the
champion efficiency (9.2%) to date [13]. Recently, BDT was com-
bined with thienopyrrolodione [14], 4,7-bis(2-thienyl)-2,1,3-
benzothiadiazole [15], naphthobisthiadiazole [16], or fluorinated
benzothiadiazole [17], producing a number of high performance
photovoltaic copolymers with PCE of 6e7%. We notice that the
precursor of BDT is benzo[1,2-b:4,5-b0]dithiophene-4,8-dione, a
benzoquinone-fused dithiophene (BQDT, Scheme 1), which itself is
an electron-deficient unit. From BQDT, another electron-deficient
unit, benzoquinone dioxime-fused dithiophene (BXDT) can be
easily derived (Scheme 1). We are curious about the DeA alterna-
tive copolymers, poly(BDT-alt-BQDT) (PBQ) and poly(BDT-alt-
BXDT) (PBX), based on these units that derive from the same
mother structure. The cost for producing such DeA alternative
copolymers would be lower than those with D and A having
different mother structures. Herein, we report their synthesis and
photovoltaic properties.
a three-electrode cell with a glassy carbon as a working electrode, a
platinum wire as a counter electrode, and Ag/AgNO3 as a reference
electrode. The samples were first casted on the glassy carbon elec-
trode to form a film, and then measured in CH3CN in the presence of
0.1 M Bu4NPF6 with a scan rate of 50 mV minꢀ1. Thermogravimetric
analysis (TGA) was carried out by a TGA Q500 instrument under N2
with a temperature rate of 10 ꢁC minꢀ1. Atomic force microscopy
(AFM) was performed on a Veeco Instrument Nanoscope IIIa Multi-
mode apparatus by tapping mode with a silicon tip.
2.2. Device fabrication and characterization
The solar cell devices were fabricated with a structure of ITO/
PEDOT:PSS/active layer/Ca/Al. A layer of PEDOT:PSS (Heraeus Cle-
vios P VP. Al 4083) with a thickness around 30 nm was spin-coated
on top of well cleaned ITO glass at 4000 rpm and baked at 150 ꢁC for
20 min. After transfer into a N2-filled glove box, the active layer was
spin-coated from a chlorobenzene solution of polymer and PC61BM
(Lumitec LT-8905) at 3000 rpm. For PBX-based cells, the plates were
annealed at 100 ꢁC for 25 min, while the PBQ-based cells were
annealed at 120 ꢁC for 10 min. Finally, a 10 nm-thick Ca layer and a
100 nm-thick Al layer were subsequently thermally deposited on
the top of the active layer under a high vacuum (10ꢀ5 mbar) through
a shadow mask. The active cell area is 7 mm2. Layer thickness was
measured on a Veeco Dektak 150 profilometer. Current densitye
voltage (JeV) curves were recorded on a Keithley 2420 source me-
ter. Photocurrent was acquired upon irradiation using an AAA solar
simulator (Oriel 94043A, 450 W) with AM 1.5G filter. The intensity
was adjusted to be 100 mW cmꢀ2 under the calibration with an
NREL-certified standard silicon cell (Orial reference cell 91150).
External quantum efficiency (EQE) was detected with a 75 W Xe
lamp, Oriel monochromator 74125, optical chopper, lock-in ampli-
fier and an NREL-calibrated crystalline silicon cell.
2. Experimental
2.1. Measurements and characterization
1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 on a
Varian Mercury spectrometer operating at 300 and 75 MHz,
respectively, using tetramethyl silane as an internal reference. Elec-
tron ionization (EI) mass spectra were tested on an Agilent 5973N
Mass Spectrometer by an electron impact ionization procedure
(70 eV). Matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectroscopy was carried out on a Shimadzu
Biotech Axima Performance Mass Spectrometer using dithranol or
a-cyano-4-hydroxycinnamic acid (CHCA) as a matrix. Gel perme-
ation chromatography (GPC) was carried out on a Waters 1515 HPLC
instrument equipped with a Waters 2489 UV detector, using tetra-
hydrofuran (THF) as eluent. The molecular weight and its poly-
dispersity index (PDI) were calculated based on polystyrene
standards. UVevis absorption spectroscopy was performed on a
Hitachi U-3310 spectrophotometer. Fluorescence spectroscopy was
recorded in a Hitachi F4600 fluorophotometer. Cyclic voltammetric
(CV) measurements were performed on a CHI 660C instrument using
2.3. Materials
Unless indicated, all commercial reagents were used as received.
Tetrahydrofuran (THF), ether, and toluene were refluxed over a
mixture of sodium and benzophenone, while chlorobenzene was
dried over CaH2 under argon, and freshly distilled prior to use.
BQDT [10], N,N-dimethylthiophene-3-carboxamide [18], and
monomers 3a [19] and 3b [20] were synthesized according to
literature procedures.
O
R
O
S
S
S
S
O
O
n
R
O
O
2.3.1. 2,5-Dibromo-thiophene-3-carboxylic acid dimethylamide (1)
N,N-dimethylthiophene-3-carboxamide (16.3 g, 105.0 mmol)
and N-bromosuccinimide (NBS, 41.1 g, 231.1 mmol) were added
into 150 mL DMF. The solution was shielded from light and stirred
for 2 h at room temperature. Afterward, the reaction mixture was
poured into 600 mL water, and extracted with ethyl acetate. The
organic phase was washed with water, dehydrated with MgSO4,
filtered, concentrated under reduced pressure. The residue was
subjected to silica gel column chromatography using hexane/ethyl
acetate (6:1, v/v) as eluent, affording compound 1 (28.0 g) as yellow
S
PB
Q
S
a
B
DT
Q
R
R
O
O
S
b
S
R
O
N
N
BDT
S
oil with a yield of 85.2%. 1H NMR (300 MHz, CDCl3,
3.10 (s, 3H), 2.99 (s, 3H).
d): 6.92 (s, 2H),
R
O
S
R
N
O
O
2.3.2. 2,6-Dibromo-benzo[1,2-b:4,5-b0]dithiophene-4,8-dione (2)
BuLi (15.3 mmol, 2.5 M in hexane) was added dropwise into an
Et2O solution of compound 1 (4.9 g, 15.4 mmol, 40 mL) under dry-
ice/acetone bath. After slowly warming to room temperature, the
mixture was added with saturated NH4Cl aqueous solution (30 mL).
Then, the precipitate was collected and subjected to silica gel col-
umn chromatography using CHCl3 as eluent, affording compound 2
(1.3 g) as an orange solid with a yield of 43.6%. 1H NMR (300 MHz,
S
S
R
O
BXDT
S
S
n
R
N
R
O
PBX
Scheme 1. Molecular structures of BDT, BQDT, BXDT, and their alternative copolymers,
PBQ and PBX. Conditions: a) Zn, NaOH, ReBr, H2O, reflux; b) i. NH2OH∙HCl, pyridine,
reflux; ii. CH3ONa, CH3OH, reflux; iii. ReBr, DMSO.

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L. Liang et al. / Polymer 54 (2013) 2278e2284
CDCl3,
d
): 7.60 (s, 2H); LRMS (EI) m/z: 378 (Mþ). HRMS (EI, m/z):
was added with Na2CO3 aqueous solution, and then extracted with
CHCl3. The organic phase was dried over anhydrous MgSO4, filtered,
and concentrated. The residue was subjected to silica gel column
chromatography using hexane as eluent, affording compound 7
(135.0 mg) as a pale yellow solid with a yield of 85.0%. 1H NMR
[M þ H]þ calcd. for C10H2Br2O2S2, 375.8763; found, 375.7864.
2.3.3. Benzo[1,2-b:4,5-b0]dithiophene-4,8-dione dioxime (4)
BQDT (1.1 g, 5.0 mmol) and hydroxylamine hydrochloride (5.2 g,
75.0 mmol) were added into 15 mL pyridine. The mixture was
heated to reflux under N2 overnight. After the solvent was removed
under vacuum, the residue was poured into 1.0 M hydrochloric
solution. The precipitate was separated by filtration and washed
with water, giving pure compound 4 (1.2 g) as a yellow solid with a
(300 MHz, CDCl3,
d
): 7.60 (s, 2H), 4.32 (d, J ¼ 5.7 Hz, 4H), 1.80 (m,
2H), 1.20e1.60 (m, 16H), 0.95 (m, 12H). 13C NMR (75 MHz, CDCl3,
d
):
140.7, 135.4, 127.1, 125.0, 119.1, 79.1, 39.3, 30.2, 28.9, 23.6, 23.1, 14.0,
10.9. HRMS (MALDI, m/z): [M þ H]þ calcd. for C26H36N2O2S2Br2,
631.0658; found, 631.0639.
yield of 97.0%. 1H NMR (300 MHz, DMSO-d6,
d): 12.70 (s, 2H), 7.86
(d, J ¼ 6.0 Hz, 2H), 7.59 (d, J ¼ 6.0 Hz, 2H). MS (EI) m/z: 250 (Mþ).
2.3.6. Poly(4,8-bis((2-decyltetradecyl)oxy)-benzo[1,2-b:4,5-b0]
dithiophene-alt-benzo[1,2-b:4,5-b0] dithiophene-4,8-dione)
(PBQ-C24)
2.3.4. Benzo[1,2-b:4,5-b0]dithiophene-4,8-dione O,O-di(2-
ethylhexyl) oxime (6)
Monomer 2 (75.6 mg, 0.20 mmol), monomer 3b (244.2 mg,
0.20 mmol), and Pd(PPh3)4 (2.3 mg, 0.002 mmol) were added into
15 mL pre-degassed dry toluene. The resulting mixture was
degassed again via freezeepumpethaw cycle three times, and
heated to reflux for 60 h under Ar. Then the reaction mixture was
poured into CH3OH (150 mL) to precipitate the product. After
filtration, the crude product was washed with CH3OH, dried in
vacuum, and subjected to Soxhlet extraction with a sequence of
CH3OH, acetone, hexane and CHCl3. Finally, the CHCl3 extraction
was evaporated to dryness, affording PBQ-C24 (183.0 mg) as dark
solid with a yield of 82.4%. Mw: 29,370, PDI: 1.90.
Compound 4 (750 mg, 3.0 mmol) and CH3ONa (1.6 g, 30.0 mmol)
were added into 30 mL CH3OH. The mixture was heated to reflux
overnight. Aftercooling toroom temperature, thesolutionwas poured
into 180 mL diethyl ether, affording precipitation. After filtration, the
crude product of compound 5 was obtained as a pale yellow solid, and
was used for the next reaction without further purification.
The above crude compound 5 was then dissolved in 15 mL DMSO
and added with 1-bromo-2-ethylhexane (24 mL,135.0 mmol). After
stirring overnight, the reaction mixture was poured into water
(500 mL) and extracted with ethyl acetate (500 mL). The organic
phase was dried over anhydrous MgSO4, filtered, and concentrated.
Then, the residue was subjected to silica gel column chromatog-
raphy using CH2Cl2/hexane (1:10, v/v) as eluent, affording com-
pound 6 (900.0 mg) as yellowoil. The yield for the two steps is 63.2%.
2.3.7. Poly(4,8-bis((2-ethylhexyl)oxy)-benzo[1,2-b:4,5-b0]
dithiophene-alt-benzo[1,2-b:4,5-b0] dithiophene-4,8-dione O,O-
di(2-ethylhexyl) oxime) (PBX-C8)
1H NMR (300 MHz, CDCl3,
d): 7.67 (d, J ¼ 5.1 Hz, 2H), 7.50
Monomer 3a (280.8 mg, 0.36 mmol), monomer 7 (230.0 mg,
0.36 mmol), Pd2(dba)3 (6.7 mg, 0.007 mmol), and P(o-tol)3 (8.9 mg,
0.029 mmol) were added into a 20 mL pre-degassed chloroben-
zene. The resulting mixture was degassed again via freezeepumpe
thaw cycle three times, and heated to reflux for 60 h under Ar. Then
the reaction mixture was poured into CH3OH (200 mL) to precipi-
tate the product. After filtration, the crude product was washed
with CH3OH, dried in vacuum, and subjected to Soxhlet extraction
with a sequence of CH3OH, acetone, hexane and CHCl3. Finally, the
(d, J ¼ 5.1 Hz, 2H), 4.36 (d, J ¼ 5.4 Hz, 4H), 1.81 (m, 2H), 1.20e1.60
(m, 16H), 0.95 (m, 12H). MS (EI) m/z: 347 (Mþ).
2.3.5. 2,6-Dibromo-benzo[1,2-b:4,5-b0]dithiophene-4,8-dione O,O-
di(2-ethylhexyl) oxime (7)
Compound 6 (118.5 mg, 0.25 mmol) was dissolved in 10 mL
CHCl3. Then the solution was added with Br2 (160.0 mg, 1 mmol).
After stirring at room temperature overnight, the reaction mixture
Scheme 2. Synthesis of PBQ and PBX.

L. Liang et al. / Polymer 54 (2013) 2278e2284
2281
Table 1
Molecular weight, optical, and electrochemical properties of PBQ-C24 and PBX-C8.
Mw (g molꢀ1
)
PDIa
lmax-solution (nm)
lmax-film (nm)
lonset (nm)
Eg,opt (eV)
ELUMO (eV)
EHOMO (eV)
Eg,ecc (eV)
a
b
Polymer
PBQ-C24
PBX-C8
29,370
9400
1.90
1.35
444, 557
408, 457, 481
431, 538
412, 460, 488
720
600
1.72
2.07
ꢀ3.73
ꢀ3.55
ꢀ5.53
ꢀ5.35
1.80
1.80
a
Determined by GPC with polystyrene as a reference.
b
c
Eg,opt ¼ 1240/lonset
.
Eg,ec ¼ ELUMO ꢀ EHOMO
.
CHCl3 extraction was evaporated to dryness, affording PBX-C8
(300.0 mg) as a red solid with a yield of 75.4%. Mw: 9400, PDI: 1.35.
appears at 444 nm, assignable to pep* transition of its conjugated
backbone [21]. The other band has a peak around 557 nm but with
relatively weak absorbance, possibly originating from the intra-
molecular charge transfer (ICT) between BDT and BQDT units [22].
While, PBX-C8 in CHCl3 solution shows intense absorption only in
the range of 300e550 nm with three peaks at 408, 457, and 481 nm
3. Results and discussion
3.1. Synthesis and characterization
(Fig. 2). In film state, the pep* transition peak of PBQ-C24 appears
at 431 nm, which is 13 nm blue-shifted from that in solution. In
general, the blue shift for the absorption bands is caused by the
formation of H-aggregation of chromophores. In sharp contrast, the
absorption peaks of PBX-C8 show up at 412, 460, and 488 nm,
about 5 nm red-shifted from those in solution, suggesting the for-
mation of J-aggregation of the polymer chains. Such spectral shifts
in the absorption bands indicate the occurrence of interchain in-
teractions in film state, which would be good for charge-
transportation among polymer chains. From the onset points of
the absorption spectra in film state (lonset, 720 nm for PBQ-C24,
600 nm for PBX-C8), the optical energy band gap (Eg,opt) of PBQ-
C24 and PBX-C8 can be calculated to be 1.72 and 2.07 eV, respec-
tively (Table 1).
The syntheses of monomers and polymers are shown in Scheme
2. Monomer 2 is a key compound for the preparation of PBQ. But, it
is hard to produce from direct bromination of BQDT with NBS or Br2
[10]. Here, it was synthesized by the dimerization of compound 1 in
the presence of BuLi, a protocol similar to that for BQDT. Upon
Stille-coupling polymerization of monomers 2 and 3, two PBQ
polymers were prepared. PBQ-C24, having 2-decyltetradecyl as
side chains, can be readily dissolved in CHCl3 and chlorobenzene.
However, PBQ-C8 with 2-ethylhexyl side chains has very limited
solubility, which prevents it from further purification and charac-
terization. On the other hand, BQDT was treated with hydroxyl-
amine hydrochloride, affording compound 4 in a yield of 97.0%.
After reacting with CH3ONa and 1-bromo-3-ethylhexane, com-
pound 4 was converted into compound 6 with a yield of 63.2% for
two steps. Thus, monomer 7 was synthesized by the bromination of
6 with Br2. Finally, polymer PBX-C8 was successfully prepared via
Stille-coupling polymerization of monomers 7 and 3a in dry chlo-
robenzene. PBX-C8 has a good solubility in THF, CHCl3, chloro-
benzene, etc. The weight-average molecular weight (Mw) and its
polydispersity index (PDI) were determined to be 29,370 and 1.90
for PBQ-C24, while 9400 and 1.35 for PBX-C8 (Table 1). Thermog-
ravimetric analysis (TGA) showed that both PBQ-C24 and PBX-C8
have good thermal stability (Fig. 1). Their 5%-weight-loss temper-
atures (Td) are 321.8 and 306.7 ꢁC, respectively.
3.3. Electronic properties
The electrochemical properties of the two polymers were
investigated by cyclic voltammetry (CV) (Fig. 3). The polymer films
were first drop-casted from their CHCl3 solution onto a glass carbon
working electrode. Then, the electrochemical current was
measured at room temperature in acetonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) with a plat-
inum wire counter electrode and an Ag/AgNO3 reference electrode
at a scan rate of 50 mV sꢀ1. As shown in Fig. 3, PBQ-C24 shows an
onset oxidation potential (Eox) at 0.80 V and an onset reduction
potential (Ered) at ꢀ1.0 V, whereas those for PBX-C8 are 0.62
and ꢀ1.18 V, respectively. The redox potential of ferrocene/ferro-
cenium was measured to be 0.07 V in the same conditions, giving
the energy level of Ag/AgNO3 electrode to be 4.73 eV below vacuum
3.2. Optical properties
In CHCl3 solution, PBQ-C24 exhibits broad absorption spectrum
in the range of 300e700 nm with two bands (Fig. 2). One band
PBQ-C24
100
PBX-C8
80
60
40
100
200
300
400
500
600
Temperature (ºC)
Fig. 2. UVevis absorption spectra of PBQ-C24 and PBX-C8 in chloroform solution and
film state.
Fig. 1. TGA curves of PBQ-C24 and PBX-C8.

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L. Liang et al. / Polymer 54 (2013) 2278e2284
performance. The best cell exhibited an open-circuit voltage (VOC
)
of 0.73 V, a short-circuit current (JSC) of 1.84 mA cmꢀ2, and a fill
factor (FF) of 38.1%, giving a PCE of 0.51% (Fig. 4a, red cure with solid
circles). On the other side, the optimized conditions for fabrication
of PBQ-C24-based cell are PBQ-C24:PC61BM ¼ 1:4 (w/w) in the
presence of 1% (v/v) DIO in chlorobenzene. However, even with the
best cell, PBQ-C24 showed a much inferior photovoltaic perfor-
mance (VOC: 0.59 V, JSC: 0.21 mA cmꢀ2, FF: 36.6%, PCE: 0.045%) than
PBX-C8. The difference comes from an extremely small JSC, which is
one order lower than that of the PBX-C8-based cell. In the EQE
measurement (Fig. 4b), the PBX-C8-based cell showed photocur-
rent in the range of 360e600 nm, which is consistent with its ab-
sorption spectrum (Fig. 2). Based on EQE spectrum, JSC of PBX-C8
cell was calculated to be 1.78 mA cmꢀ2, 3% error from the JeV
measured value. In contrast, the EQE spectrum of the PBQ-C24
cell just has weak photo-response in the region of 300e500 nm, far
different from its absorption spectrum. It could be deduced that
PBX-C8
PBQ-C24
-1.0
0.0
Potential (V)
1.0
2.0
-2.0
only pep* transition band of the conjugated backbone of PBQ-C24
may contribute to photocurrent, while ICT band has less help. This
may be one of the reasons for the low JSC of PBQ-C24 cell.
Fig. 3. CV curves of PBQ-C24 and PBX-C8.
The huge difference in the EQE spectra of PBQ-C24 and PBX-C8
cells motivated us to investigate the efficiency of photo-induced
charge-separation between polymer and PC61BM. In CHCl3 solu-
tion, PBX-C8 fluoresces with an intense band centered at 568 nm
(Fig. 5a). Upon portion-wise adding PC61BM, the intensity of the
fluorescence gradually decreased with a SterneVolmer quenching
level (The standard energy level of ferrocene/ferrocenium is 4.8 eV)
[23,24]. Therefore, the HOMO and LUMO energy levels of the
polymers can be calculated according to the following equations:
EHOMO ¼ ꢀeðEox þ 4:73ÞðeVÞ
ELUMO ¼ ꢀeðEred þ 4:73ÞðeVÞ
Based on the above measured redox potential data, the HOMO
and LUMO energy levels of PBQ-C24 are ꢀ5.53 and ꢀ3.73 eV, while
those of PBX-C8 are ꢀ5.35 and ꢀ3.55 eV, respectively (Table 1). The
fact of lower HOMO and LUMO energy levels for PBQ-C24 than
those for PBX-C8, indicates that BQDT unit has a stronger electron-
withdrawing effect than BXDT unit.
constant (KSV) of 2.7 ꢂ 103
M
ꢀ1. In film state, PBX-C8 shows an
intense fluorescence band around 550 nm (Fig. 5b). This fluores-
cence was almost quenched when PBX-C8 was blended with
PC61BM at a weight ratio of 1/1, and completely disappeared when
PC61BM/PBX-C8 weight ratio was increased to 2/1. These observa-
tions clearly suggest the occurrence of an efficient photo-induced
charge-separation between PBX-C8 and PC61BM. In sharp
contrast, the fluorescence of PBQ-C24 is very weak in both solution
and film state (Fig. 5c and d) and did not quench upon addition of
PC61BM, suggesting inefficient photo-induced charge-separation
between PBQ-C24 and PC61BM. It was reported that the minimum
offset of LUMO energy level for an efficient charge-separation is
about 0.3 eV [25]. However, the LUMO energy of PBQ-C24
is ꢀ3.73 eV, only 0.17 eV higher than that of PC61BM (ꢀ3.9 eV
measured under the same conditions). Such energy level difference
may be not enough to promote exciton dissociation into free charge
carriers in the blend film of PBQ-C24 and PC61BM.
3.4. Polymer photovoltaic properties
The photovoltaic properties of PBQ-C24 and PBX-C8 were
investigated with device configuration of ITO/PEDOT:PSS/poly-
mer:PC61BM/Ca/Al. The cell active layer was spin-coated on the top
of PEDOT:PSS layer from a chlorobenzene solution containing the
checked polymer and PC61BM. For the fabrication of PBX-C8-based
devices, the optimized polymer to PC61BM weight ratio was 1:3.
The addition of 1,8-diiodooctane (DIO) did not improve the cell
The morphology of the photovoltaic active films was studied by
atomic force microscopy (AFM). As shown in Fig. 6a, a plenty
(a)
(b)30
2
1
0
20
10
0
-1
-2
-0.2
0.0
0.2
0.4
0.6
0.8
300
400
500
600
700
800
Wavelength (nm)
V (V)
Fig. 4. (a) JeV curves and (b) EQE spectra of polymer solar cells based on PBQ-C24:PC61BM (1:4) (blue curve with open squares) and PBX-C8:PC61BM (1:3) (red cure with solid
circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

L. Liang et al. / Polymer 54 (2013) 2278e2284
2283
Fig. 5. (a) The fluorescence spectra of PBX-C8 (2.0 ꢂ 10ꢀ6 M) in CHCl3 in the presence of different concentrations of PC61BM upon excitation at 483 nm at room temperature. (b) The
film fluorescence spectra of PBX-C8 and its blends with PC61BM by a weight ratio of 1:1 and 1:2 upon excitation at 493 nm at room temperature. (c) The fluorescence spectra of
PBQ-C24 (1.0 ꢂ 10ꢀ6 M) in CHCl3 in the presence of different concentrations of PC61BM upon excitation at 431 nm at room temperature. (d) The film fluorescence spectra of PBQ-
C24 and its blend with PC61BM by a weight ratio of 1:1 upon excitation at 580 nm at room temperature.
Fig. 6. AFM images of blend films of (a) PBQ-C24/PC61BM (1:4, w/w) spin-coated from chlorobenzene solution containing DIO (1%, v/v), (b) PBX-C8/PC61BM (1:3, w/w) spin-coated
from chlorobenzene solution, on the top of PEDOT:PSS/ITO/glass.
number of dark areas with estimated average size of w200 nm
were clearly observed in the spin-coated film of PBQ-C24/PC61BM
(1:4, w/w) from their chlorobenzene solution containing 1% (v/v)
DIO. The present morphology is obviously far from ideal re-
quirements for light-to-electric conversion [25], and would be one
of the reasons for its poor photovoltaic output. In a contrast, the
film of PBX-C8/PC61BM (1:3, w/w) looked more smooth (Fig. 6b).
4. Conclusions
Two conjugated polymers, PBQ-C24 and PBX-C8, having alter-
native donor and acceptor units both derived from a same mother
structure, BQDT, have been synthesized and investigated. Both
polymers show desired thermal stability and good solubility.
Although PBQ-C24 has a broader absorption spectrum than PBX-
C8, the bulk heterojunction devices based on the blend of the
former polymer with PC61BM show less photovoltaic response
than those based on the latter polymer. Inefficient photo-induced
electron-transfer between PBQ-C24 and PC61BM and the bad film
morphology would be the causes for their poor device
The root-mean-square roughness obtained in 10 ꢂ 10
m
m2 for this
film is as small as 0.99 nm, whereas that for the former PBQ-C24/
PC61BM film is 3.03 nm. These observations suggest that PBX-C8/
PC61BM has a better morphology than PBQ-C24/PC61BM, coinciding
with their photovoltaic performance.

2284
L. Liang et al. / Polymer 54 (2013) 2278e2284
(b) Chang CY, Cheng YJ, Hung SH, Wu JS, Kao WS, Lee CH, et al. Adv Mater
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[8] Shiraishi K, Yamamoto T. Synth Met 2002;130:139e47.
[9] Pan H, Li Y, Wu Y, Liu P, Ong BS, Zhu S, et al. J Am Chem Soc 2007;129:4112e3.
[10] Hou J, Park M-H, Zhang S, Yao Y, Chen L-M, Li J-H, et al. Macromolecules
2008;41:6012e8.
performance. Although the present two polymers do not show
satisfied photovoltaic properties, the so preparation strategy for De
A copolymers would reduce the material-producing cost, and open
a new avenue for material innovation.
[11] (a) Liang Y, Wu Y, Feng D, Tsai S-T, Son H-J, Li G, et al. J Am Chem Soc
2009;131:56e7;
Acknowledgements
(b) Huo L, Hou J. Polym Chem 2011;2:2453e61.
[12] (a) Liang Y, Feng D, Wu Y, Tsai S-T, Li G, Ray C, et al. J Am Chem Soc 2009;131:
7792e9;
This work is supported by NFSC (Nos. 20974119, 90922019, and
21074147) and Chinese Academy of Sciences.
(b) Liang Y, Xu Z, Xia J, Tsai S-T, Wu Y, Li G, et al. Adv Mater 2010;22:E135e8;
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2011;23:4636e43;
(d) Huo L, Zhang S, Guo X, Xu F, Li Y, Hou J. Angew Chem Int Ed 2011;50:
9697e702.
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