Property modulation of benzodithiophene-based polymers via the incorporation of a covalently bonded novel 2,1,3-benzothiadiazole-1,2,4-oxadiazole derivative in their main chain for polymer solar cells

Article abstract of DOI:10.1039/c4tc01558d

Two new electron accepting monomers (BBOB and BOB) containing two serially connected different electron deficient units, such as 2,1,3-benzothiadiazole and 1,2,4-oxadiazole, were prepared and copolymerized with electron-rich benzodithiophene (BDT) derivative to afford polymers P(BDT-BBOB) and P(BDT-BOB), respectively. The optical band gaps of P(BDT-BBOB) and P(BDT-BOB) are calculated to be 2.32 eV and 1.99 eV, respectively, and their highest occupied molecular energy levels are determined to be -5.31 eV and -5.27 eV, respectively. Each of the newly synthesized polymers, i.e.P(BDT-BBOB) and P(BDT-BOB), is used as an electron donor, along with PC₆₁BM as an electron acceptor, in the preparation of polymer solar cells (PSCs). The PSCs made with the configuration of ITO/PEDOT:PSS/P(BDT-BBOB) or P(BDT-BOB):PC₆₁BM (1 : 2 wt%)/LiF/Al gave a maximum power conversion efficiency (PCE) of 1.76% and 2.46%, respectively, and the device performance was further improved to 3.31% and 4.21%, respectively, by simply treating the photoactive layer of PSCs with isopropyl alcohol. Overall, the opto-electrical and photovoltaic properties of the two polymers are found to be quite dependent on the configuration of the covalently bonded 2,1,3-benzothiadiazole and 1,2,4-oxadiazole units incorporated in the polymer main chain.

Full text of DOI:10.1039/c4tc01558d

Journal of
Materials Chemistry C
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PAPER
Property modulation of benzodithiophene-based
polymers via the incorporation of a covalently
bonded novel 2,1,3-benzothiadiazole-1,2,4-
oxadiazole derivative in their main chain for
polymer solar cells
Cite this: J. Mater. Chem. C, 2014, 2,
515
8
a
a
b
Rajalingam Agneeswari, Vellaiappillai Tamilavan, Myungkwan Song*
and Myung Ho Hyun*a
Two new electron accepting monomers (BBOB and BOB) containing two serially connected different
electron deficient units, such as 2,1,3-benzothiadiazole and 1,2,4-oxadiazole, were prepared and
copolymerized with electron-rich benzodithiophene (BDT) derivative to afford polymers P(BDT-BBOB)
and P(BDT-BOB), respectively. The optical band gaps of P(BDT-BBOB) and P(BDT-BOB) are calculated to
be 2.32 eV and 1.99 eV, respectively, and their highest occupied molecular energy levels are determined
to be ꢀ5.31 eV and ꢀ5.27 eV, respectively. Each of the newly synthesized polymers, i.e. P(BDT-BBOB)
and P(BDT-BOB), is used as an electron donor, along with PC61BM as an electron acceptor, in the
preparation of polymer solar cells (PSCs). The PSCs made with the configuration of ITO/PEDOT:PSS/
P(BDT-BBOB) or P(BDT-BOB):PC61BM (1 : 2 wt%)/LiF/Al gave a maximum power conversion efficiency
(
PCE) of 1.76% and 2.46%, respectively, and the device performance was further improved to 3.31% and
Received 17th July 2014
Accepted 19th August 2014
4
.21%, respectively, by simply treating the photoactive layer of PSCs with isopropyl alcohol. Overall, the
opto-electrical and photovoltaic properties of the two polymers are found to be quite dependent on the
configuration of the covalently bonded 2,1,3-benzothiadiazole and 1,2,4-oxadiazole units incorporated in
the polymer main chain.
DOI: 10.1039/c4tc01558d
www.rsc.org/MaterialsC
part of the solar spectra, and consequently, tandem PSCs have a
higher PCE than single layer PSCs. The theoretical studies
1
. Introduction
2–7
Recent progress in polymer solar cells (PSCs) demonstrates that suggest that the maximum PCE of single layer PSCs could reach
they are promising renewable energy production techniques, up to 17% and that tandem PSCs have the capability for a
14
due to their advantages, such as exible and light weight device maximum PCE of 24%.
fabrication at low cost via standard roll-to-roll (R2R) printing
It is essential to develop structurally novel polymers
1
techniques. The maximum solar to electrical energy conversion showing better opto-electrical and photovoltaic properties
efficiency (PCE) of PSCs made from the blends of an electron than the known polymers to boost the PSC performances
donating p-conjugated polymer and an electron accepting further. The literature reveals that polymers incorporating
9
fullerene derivative (such as PC BM, PC BM or ICBM) is electron decient 2,1,3-benzothiadiazole (BT), thieono[3,4-b]-
6
1
71
1
0
11
improved in the range of 9–10.5% with the use of tandem thiophene (TT), thieno[3,4-c]pyrrole-4,6-dione (TPD), and
structured PSCs, while a PCE in the range of 7–9% is obtained pyrrolo[3,4-c]pyrrole-1,4-dione (DKPP)
2–7
12,13
derivatives show
8–13
for single layer PSCs.
large band gap) is used in the photoactive layer of single layer quently high PCE in both single and tandem PSCs.
Only one type of polymer (either low or excellent absorption, and high carrier mobility, and conse-
2
–13
Among
while both large and low band gap polymers are used these, the BT analogue is considered as the most promising
8–13
PSCs,
in tandem PSCs, to utilize the entire energy in the visible-NIR acceptor unit, and polymers incorporating a BT unit gave a
maximum PCE of 8.2% and 10.6%, respectively, in single layer
6
,9
and tandem PSCs.
a
Department of Chemistry, Chemistry Institute for Functional Materials, Pusan
Dennler et al. discussed about the importance of tuning
the HOMO and LUMO energy levels of the alternating donor–
acceptor (D–A) polymers via the modication of the donor or
acceptor unit in the polymer main chain to maximize the
device efficiency of single or tandem PSCs. They also clearly
National University, Busan 690-735, Republic of Korea. E-mail: mhhyun@pusan.ac.
kr; Fax: +82-51-516-7421; Tel: +82-51-510-2245
b
Surface Technology Division, Korea Institute of Materials Science (KIMS), Changwon,
Gyeongnam 641-831, Republic of Korea. E-mail: smk1017@kims.re.kr; Fax: +82-55-
2
80-3570; Tel: +82-55-280-3686
This journal is © The Royal Society of Chemistry 2014
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Paper
demonstrated that the PCE of the PSCs is related to the band
gap and HOMO–LUMO energy levels of the donor martial.
Consequently, many studies have shown that the photovoltaic
2
. Experimental section
8
2.1 Materials and instruments
properties of the polymers incorporating BT-based acceptor The reagents were received from Aldrich or TCI chemicals and
units can be tuned via the incorporation/insertion of electron used without further purication. The common organic
1
5–28
donating or accepting units on the BT moiety.
the studies, electron donating units,
In most of solvents were distilled and handled in a moisture-free atmo-
such as benzene, sphere. The column purication of the compounds was per-
1
5–21
thiophene, thienothiophene, and furan derivatives, are cova- formed on silica gel (Merck Kieselgel 60, 70–230 mesh ASTM).
lently attached on both sides of the BT unit or electron-rich The compounds were characterized by using nuclear magnetic
2
0,21
8,28
alkoxy,
or electron-attracting uorides
are inserted on resonance (NMR) spectra performed on a Varian Mercury Plus
1
the back bone of the BT unit with the aim being to improve spectrometer (300 MHz and 75 MHz, respectively, for H and
the absorption and carrier mobility and to tune the energy
1
3
C) and high resolution mass spectra performed on a JEOL
levels of the BT-based polymers. In addition, electron JMS700 mass spectrometer. The molecular weights of the
2
2
23
accepting units, such as thiazole and benzimidazole, are polymers were determined from gel permeation chromatog-
2
4,25
also attached on both sides of the BT units or quinoxaline,
raphy (GPC) analyses by using an Agilent 1200 Innity Series
2
6
27
and thiadiazole and N-alkylpyrrolidine-2,5-dione rings are separation module using polystyrene as a standard and chlo-
fused on the BT back bone with the aim being to increase the roform as an eluent. Thermogravimetric analyses (TGA) were
ꢁ
electron attracting ability of the BT unit. Some examples of conducted with a TA instrument Q500 at a heating rate of 10 C
ꢀ
1
BT-based acceptor units are presented in Fig. 1. Recently, we min under nitrogen. The absorption spectra of the polymers
ꢁ
reported a series of 1,2,4-oxadiazole (Oxa)-based polymers for were recorded on a JASCO V-570 spectrophotometer at 25 C in
2
9
PSC applications. Polymers incorporating separate BT and chloroform or as thin lms on quartz. The cyclic voltammetry
Oxa units in the polymer main chain were found to show (CV) measurements were performed on a CH Instruments
2
9
broad absorption in the range 300–900 nm. However, poly- Electrochemical Analyzer. Electrochemical measurements were
mers incorporating covalently bonded BT-Oxa units have not performed with polymer cast platinum as a working electrode,
yet been reported. We believe that the covalent attachment of Ag/AgCl as a reference electrode, and platinum wire as a counter
the high electron transporting Oxa unit on the BT unit electrode in 0.1 M tetrabutylammonium tetrauoroborate
increases the electron deciency, as well as improves the (Bu NBF ) acetonitrile solution. The impedance responses of
4
4
electron mobility. Consequently, when the BT-Oxa unit is polymer:PC BM blends were measured over the range from 1
6
1
polymerized with electron-rich units, broad absorbing poly- Hz to 1 MHz with an oscillation amplitude of 15 mV (Bio-Logic
mers with high electron mobility are expected. In order to VMP-3). The experimental data were simulated using commer-
clearly understand the opto-electrical and photovoltaic cial Z-view soware to estimate the values for each component
properties of polymers incorporating the covalently bonded of the corresponding equivalent circuits.
BT-Oxa unit, we prepared two different electron decient
monomers, namely, BBOB (Oxa-BT-Oxa derivative) and BOB
BT-Oxa derivative). The strong electron decient monomers,
2
.2 Fabrication and characterization of PSCs
(
The device fabrication and characterization of PSCs are
summarized below. Indium tin oxide (ITO)-coated glass
substrates were cleaned stepwise by acetone, deionized water,
and isopropyl alcohol in an ultrasonic bath. Then a 50 nm poly-
such as BBOB and BOB, were copolymerized with a distannyl
0
derivative
of
4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]-
dithiophene (BDT), to give polymers P(BDT-BBOB) and
P(BDT-BBOB), respectively. In this study, we report the
synthesis, optical, electrochemical and photovoltaic proper-
ties of the two new polymers, P(BDT-BBOB) and P(BDT-
BBOB), incorporating covalently bonded BT-Oxa units in their
main chain.
(
(
ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)
Clevious P: isopropyl alcohol, 1 : 2 v/v%) buffer layer was spin-
coated on the top of the ITO substrate at 5000 rpm for 40 s and
dried at 150 C for 10 min under vacuum to remove the
residual water. The active layer of the polymer and PC61BM
ꢁ
Fig. 1 Few examples of 2,1,3-benzothiadiazole (BT)-based acceptor units.
8516 | J. Mater. Chem. C, 2014, 2, 8515–8524
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blend (1 : 1 wt%, 1 : 2 wt%, and 1 : 3 wt%) was spin-coated at carbonyldiimidazole (CDI, 0.19 g, 1.20 mmol), followed by
0 0
00 rpm from dichlorobenzene (DCB) solution on the top of the stirred for 30 min. Then, (4Z,7Z)-N4 ,N7 -dihydroxybenzo[c]-
8
ITO/PEDOT:PSS substrate. The ITO/PEDOT:PSS/polymer: [1,2,5]thiadiazole-4,7-dicarboxamidine (2) (0.13 g, 0.50 mmol)
PC BM substrate was allowed to dry for 10 min in a glove box in DMF (10 mL) was added dropwise to the stirred solution. The
6
1
ꢁ
and then subjected to pre-annealing at 80 C for 10 min. The solution was stirred overnight and then poured into water. The
thickness of the photoactive layer was found to be around 80 aqueous layer was extracted (3 ꢃ 30 mL) with chloroform
3
nm. Isopropyl alcohol (IPA) was spin-coated on the top of the (CHCl ). The combined organic layer was dried over anhydrous
2 4
ITO/PEDOT:PSS/polymer:PC61BM substrate at 3000 rpm for 40 s Na SO . The solvent was removed by rotary evaporation and the
for the devices subjected to solvent treatments, and then ꢂ0.7 resulting sticky mass was dissolved in anhydrous toluene (30
nm thick layer of LiF was deposited on the substrate. Subse- mL). The solution was reuxed for 24 h and then toluene was
quently, a 100 nm thick layer of Al was deposited through a removed by rotary evaporation. The crude product was puried
shadow mask on the top of the ITO/PEDOT:PSS/polymer: by column chromatography (silica gel, CH Cl ) to afford BBOB
2
2
ꢀ6
1
PC71BM/LiF substrate under a high vacuum (1.2 ꢃ 10 torr). as a yellow colored solid. Yield: 0.32 g (58%). H NMR (300 MHz,
The top metal electrode area, comprising the active area of the CDCl ): d (ppm) 8.74 (s, 2H), 7.73 (s, 2H), 7.32 (s, 2H), 4.10 (q,
solar cell, was found to be 0.36 cm . The J–V characteristics of 8H), 1.80–2.00 (m, 8H), 1.40–1.60 (m, 8H), 1.10–1.40 (m, 32H),
3
2
1
3
the devices were measured using a Keithley 2400 source 0.80–1.10 (m, 12H); C NMR (75 MHz, CDCl
3
): d (ppm) 175.0,
measure unit under a calibrated AM 1.5G solar simulator 165.8, 152.9, 152.3, 150.0, 130.9, 122.9, 119.3, 115.2, 112.7, 70.5,
+
(
1
Oriel® Sol3A™ Class AAA solar simulator, models 94043A) at 70.4, 32.1, 32.0, 29.6, 29.5, 29.4, 26.2, 22.9, 14.3; HRMS (EI , m/z)
ꢀ2
+
00 mW cm . The intensity of the sunlight illumination was [M ] calcd for C H Br N O S 1092.3757, found 1092.3750.
54 74 2 6 6
calibrated using a standard Si photodiode detector with a KG-5
3-Methylbenzene-1,2-diamine (4). According to the reported

lter. The IPCE measurement system (Oriel IQE-200) was procedure, the nitro group on 3-methyl-2-nitrobenzenamine
composed of a 250 W quartz–tungsten–halogen (QTH) lamp as was reduced to an amino group by using SnCl to afford
the light source, as well as a monochromator, optical chopper, compound 4. However, in this study, the nitro to amino group
2
31
lock-in amplier, and calibrated silicon photodetector.
conversion was performed by using palladium/carbon. The
modied procedure decreases the reaction time and gives a
high yield compared to the reported procedure. To a stirred
solution of 3-methyl-2-nitrobenzenamine (5.0 g, 33.0 mmol) in
2.3 Synthesis of monomers
Benzo[c][1,2,5]thiadiazole-4,7-dicarbonitrile (1). To a stirred methanol (200 mL) was added 0.75 g of palladium on carbon (10
solution of 4,7-dibromobenzo[c][1,2,5]thiadiazole (7.35 g, 25 wt% loading, matrix carbon, dry support) and ammonium
mmol) in N,N-dimethylformamide (DMF, 70 mL) was added formate (15.5 g, 165 mmol). Then, the mixture was heated to 70
ꢁ
copper cyanide (9.0 g, 100 mmol). The mixture was heated to
1
C for 3 h. The completion of the reaction was conrmed by
ꢁ
50 C for 72 h and then it was cooled to room temperature (RT). TLC, and the palladium on carbon was ltered out. The solvent
The mixture was poured into 2 N HCl (200 mL) and extracted (2 was removed and the resulting solid material was dissolved in
ꢃ 100 mL) with ethyl acetate (EA). The combined organic layer EA (100 mL). The organic solution was washed once with
was washed well with brine solution and then the organic layer aqueous 2 N sodium hydroxide solution and then the organic
2 4 2 4
was dried over anhydrous Na SO
. The solvent was removed by layer was dried over anhydrous Na SO . The solvent was
rotary evaporation and the crude product was puried by removed by rotary evaporation and the product was dried under
column chromatography (silica gel, hexane:CH Cl , 20/80) to vacuum to afford compound 4 as a brown colored solid. Yield:
2
2
1
1
afford compound 1 as a colorless solid. Yield: 2.2 g (47%). H 3.94 g (98%). H NMR (300 MHz, CDCl
3
): d (ppm) 6.60–6.70 (m,
1
3
13
NMR (300 MHz, CDCl
CDCl ): d (ppm) 152.6, 134.7, 114.1, 110.9; HRMS (EI , m/z) [M ] (ppm) 134.1, 133.7, 123.6, 122.3, 119.4, 115.3, 17.71.
calcd for C
3
): d (ppm) 8.15 (s, 2H); C NMR (75 MHz, 3H), 3.38 (s, 4H), 2.20 (s, 3H); C NMR (75 MHz, CDCl ): d
3
+
+
3
4-Methylbenzo[c][1,2,5]thiadiazole (5). The synthetic proce-
8
H
2
0
N
4
S 186.0000, found 186.0006.
0
31
(
4Z,7Z)-N4 ,N7 -Dihydroxybenzo[c][1,2,5]thiadiazole-4,7- dure for compound 5 was already reported, but, in this study,
dicarboxamidine (2). To a stirred solution of compound 1 (1.0 g, we developed a slightly modied, more efficient and facile
5
.40 mmol) in chloroform (10 mL) was added ethanol (60 mL) procedure. A stirred solution of compound 4 (3.7 g, 30.3 mmol)
ꢁ
and 50% aqueous hydroxylamine solution (10 mL). Then, the in dry CH
2
Cl
2
(150 mL) was cooled to 0 C in an ice bath for 15
whole mixture was heated to reux for 3 h. The completion of min and then triethyl amine (17 mL, 121 mmol) was slowly
the reaction was conrmed by TLC, and ethanol was removed by added. Then, thionyl chloride (4.4 mL, 60.6 mmol) in dry
ꢁ
rotary evaporation. The crude product was washed with cold CH
2
Cl
2
(50 mL) was added dropwise at 0 C. Aer the addition,
ꢁ
ethanol, diethyl ether and hexane. The solid was dried under the solution was allowed to stir for 1 h and then heated to 40 C
vacuum to afford compound 2 as a yellow colored solid. Yield: for 15 h. The reaction mixture was cooled to RT and poured into
1
1
2
.1 g (81%). H NMR (300 MHz, DMSO-d
H), 7.99 (s, 2H), 6.24 (s, 4H).
6
): d (ppm) 10.00 (s, 2 N HCl (200 mL) and the organic layer was then separated. The
organic layer was dried over anhydrous Na SO . The crude
2
4
4
,7-Bis(5-(4-bromo-2,5-bis(octyloxy)phenyl)-1,2,4-oxadiazol- product was puried by column chromatography (silica gel,
3
-yl)benzo[c][1,2,5]thiadiazole (BBOB). To a stirred solution of hexane:EA, 80/20) to afford compound 5 as a brown colored
1
3
compound 3 (0.50 g, 1.10 mmol), which was prepared by the liquid. Yield: 4.2 g (92%). H NMR (300 MHz, CDCl ): d (ppm)
reported procedure, in DMF (5 mL) was added 1,1 - 7.83 (1, 2H), 7.47 (t, 2H), 7.33 (d, 1H), 2.74 (s, 3H); C NMR
30
0
13
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1
3
(
3 3
75 MHz, CDCl ): d (ppm) 155.6, 155.2, 131.9, 129.8, 128.2, CDCl ): d (ppm) 7.96 (d, 2H), 7.91 (d, 2H); C NMR (75 MHz,
1
19.2, 18.2.
CDCl ): d (ppm) 153.3, 152.6, 136.1, 131.6, 121.5, 115.0, 105.6.
3
0
4-Bromo-7-methylbenzo[c][1,2,5]thiadiazole (6). Compound
(Z)-7-Bromo-N -hydroxybenzo[c][1,2,5]thiadiazole-4-carbox-
31
6
5
was prepared via the similar reported procedure. Compound amidine (10). To a stirred solution of compound 9 (0.7 g, 2.9
(3.2 g, 21.3 mmol) and 48% HBr (80 mL) were stirred at RT. To mmol) in ethanol (60 mL) was added 50% aqueous hydroxyl-
2
this solution was added Br (1.20 mL, 22 mmol) in 48% HBr (80 amine solution (10 mL). The mixture was heated to reux for 3
mL) dropwise. Then, the mixture was heated to reux for 16 h. h. The completion of the reaction was conrmed by TLC, and
The solution was cooled to RT and the mixture was poured into ethanol was removed by rotary evaporation. The crude product
saturated sodium bisulte solution. The mixture was stirred for was washed with diethyl ether and hexane. The solid was dried
3
0 min and then extracted (2 ꢃ 100 mL) with CH Cl . The under vacuum to afford compound 10 as a yellow colored solid.
2
2
1
combined organic layer was dried over anhydrous Na SO , Yield: 0.65 g (82%). H NMR (300 MHz, CD OD): d (ppm) 7.97 (d,
2
4
3

ltered, and then the solvent was evaporated by rotary evapo- 2H), 7.86 (d, 2H), 3.25 (s, 1H).
ration. The crude product was puried by column chromatog-
4-Bromo-7-(5-(4-bromo-2,5-bis(octyloxy)phenyl)-1,2,4-oxa-
raphy (silica, hexane:EA, 90/10) to afford compound 6 as a diazol-3-yl)benzo[c][1,2,5]thiadiazole (BOB). Under an argon
1
colorless solid. Yield: 4.5 g (92%). H NMR (300 MHz, CDCl
(
3
): d atmosphere, to the solution of compound 3 (0.64 g, 1.4 mmol)
1
3
0
ppm) 7.72 (d, 1H), 7.22 (d, 1H), 2.70 (s, 3H); C NMR (75 MHz, in DMF (10 mL) was added 1,1 -carbonyldiimidazole (CDI, 0.23
CDCl
3
): d (ppm) 155.4, 153.4, 132.3, 131.5, 129.1, 111.4, 17.9.
g, 1.4 mmol). The whole mixture was stirred for 30 min. Then,
4
-Bromo-7-(bromomethyl)benzo[c][1,2,5]thiadiazole (7). compound 10 (0.38 g, 1.4 mmol) in DMF (10 mL) was added
31
Compound 7 was prepared via the similar reported procedure.
dropwise to the stirred solution. The solution was stirred over-
Compound 6 (4.0 g, 17.4 mmol), N-bromosuccinimide (NBS, 3.1 night and then poured into water. The aqueous layer was
g, 17.5 mmol) and benzoyl peroxide (10 mg) were dissolved in extracted (3 ꢃ 30 mL) with chloroform (CHCl ). The combined
CCl (140 mL). The solution was heated to reux and 33% HBr organic layer was dried over anhydrous Na SO . The solvent was
3
4
2
4
in acetic acid (1 mL) was added. Aer 3 h, the reaction mixture removed by rotary evaporation and the resulting sticky mass
was poured into water (100 mL). The organic layer was sepa- was dissolved in anhydrous toluene (30 mL). The solution was
rated and dried over anhydrous Na SO , ltered, and then the reuxed for 24 h and then the toluene was removed by rotary
2 4
solvent was evaporated by rotary evaporation. The crude evaporation. The crude product was puried by column chro-
product was puried by column chromatography (silica, hex- matography (silica, hexane:EA, 80/20) to afford BOB as a yellow
1
ane:EA, 90/10) to afford compound 7 as a colorless solid. Yield: colored solid. Yield: 0.53 g (55%). H NMR (300 MHz, CDCl ): d
3
1
4
7
1
.5 g (92%). H NMR (300 MHz, CDCl
.53 (d, 1H), 4.94 (s, 2H); C NMR (75 MHz, CDCl
55.4, 153.4, 132.3, 131.5, 128.8, 114.5, 27.9.
3
): d (ppm) 7.81 (d, 1H), (ppm) 8.46 (d, 1H), 8.00 (d, 1H), 7.69 (s, 1H), 7.30 (s, 1H), 4.08 (q,
1
3
3
): d (ppm) 4H), 1.80–2.00 (m, 4H), 1.40–1.60 (m, 4H), 1.20–1.40 (m, 16H),
1
3
0.90–1.00 (m, 6H); C NMR (75 MHz, CDCl
(8). 165.7, 154.1, 152.9, 151.3, 150.0, 132.0, 131.7, 120.1, 119.3,
Compound 8 was prepared via the slightly modied proce- 119.2, 118.2, 115.2, 112.7, 70.5, 70.4, 32.1, 32.0, 29.5, 29.4, 26.2,
3
): d (ppm) 174.8,
7
-Bromobenzo[c][1,2,5]thiadiazole-4-carbaldehyde
32
+
+
dure. Compound 7 (2.8 g, 9.1 mmol) and sodium periodate 22.9,14.3; HRMS (EI , m/z) [M ] calcd for C H Br N O S
3
0
38
2 4 3
(
NaIO ) (0.42 g, 2.3 mmol) were added in a round bottom ask 692.1031, found 692.1036.
4
0
and then DMF (30 mL) was added. The reaction mixture was
heated at 150 C for 3 h. Again, NaIO
Poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]dithiophene-
(0.42 g, 2.3 mmol) was alt-4,7-bis(5-(2,5-bis(octyloxy)phenyl)-1,2,4-oxadiazol-3-yl)benzo-
ꢁ
4
ꢁ
added to the solution with continuous stirring for 2 h at 150 C. [c][1,2,5]thiadiazole) (P(BDT-BBOB)). A solution of the respective
The reaction mixture was cooled and poured into water (100 monomers, such as BDT (0.23 g, 0.3 mmol) and BBOB (0.33 g, 0.3
mL) and then extracted with EA (2 ꢃ 50 mL). The combined mmol) in chlorobenzene (30 mL), was degassed well with argon
2 4 2 3 3
organic layer was dried over anhydrous Na SO , ltered, and for 45 min. Then, Pd dba (0.14 g) and P(o-tol) (0.30 g) were
then the solvent was evaporated by rotary evaporation. The added. The stirred solution was heated to reux under an argon
crude product was puried by column chromatography (silica, atmosphere for 48 h. Then, the solution was dropwise added to
hexane:EA, 90/10) to afford compound 8 as a yellow colored the vigorously stirred methanol (200 mL). The precipitate was
1
solid. Yield: 1.2 g (54%). H NMR (300 MHz, CDCl
1
3
): d (ppm) recovered by ltration, and then extracted with methanol for 24
1
3
0.73 (s, 1H), 8.11 (d, 1H), 8.06 (d, 1H); C NMR (75 MHz, h and acetone for 24 h in a Soxhlet apparatus to afford pure
CDCl ): d (ppm) 188.6, 154.5, 152.6, 132.4, 132.0, 127.1, 122.2. polymer P(BDT-BBOB) as a light brown colored solid. Yield: 0.37
-Bromobenzo[c][1,2,5]thiadiazole-4-carbonitrile (9). g (90%). GPC (Chloroform): M
Compound 9 was prepared via a previously reported proce- ꢃ 10 g mol ; PDI ¼ 1.45. Elemental analysis calcd for
3
4
ꢀ1
7
n
¼ 1.85 ꢃ 10 g mol ; M
w
¼ 1.28
4
ꢀ1
3 n
33
dure. A stirred solution of compound 8 (1.0 g, 4.1 mmol) and (C H N O S ) : C, 69.53; H, 8.17; N, 6.08; S, 6.96. Found: C,
8
0
112 6 8
hydroxylamine hydrochloride (0.57 g, 8.2 mmol) in DMSO (30 70.30; H, 7.86; N, 5.70; S, 7.01.
ꢁ
0
mL) was heated at 100 C for 2 h. Then, the reaction mixture was
Poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]dithiophene-
cooled to RT and poured into water (100 mL). The mixture was alt-4-(5-(2,5-bis(octyloxy)phenyl)-1,2,4-oxadiazol-3-yl)benzo[c]-
stirred for 30 min and then allowed to settle down. The [1,2,5]thiadiazole) (P(BDT-BOB)). For the preparation of P(BDT-
precipitates were ltrated off and washed well with MeOH and BOB), the synthetic procedure similar to that for the prepara-
hexane. The solid was dried under vacuum to afford compound tion of (P(BDT-BBOB)) was used. In this reaction, monomers
1
9
as a colorless solid. Yield: 0.82 g (84%). H NMR (300 MHz, BDT (0.23 g, 0.3 mmol) and BOB (0.21 g, 0.3 mmol) were used
8518 | J. Mater. Chem. C, 2014, 2, 8515–8524
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33
2
for the polymerization. Polymer (P(BDT-BOB)) was obtained as a by treating with hydroxyl amine hydrochloride (NH OH$HCl).
dark brown colored solid. Yield: 0.26 g (90%). GPC (chloro- Compound 10 and monomer BOB were prepared via the
4
ꢀ1
4
ꢀ1
form): M ¼ 2.13 ꢃ 10 g mol ; M ¼ 1.30 ꢃ 10 g mol ; PDI ¼ synthetic procedure used for the synthesis of compound 2 and
n
w
1
13
1
7
.63. Elemental analysis calcd for (C H N O S ) : C, 68.53; H, BBOB, respectively. The H and C NMR spectra of BBOB and
56 76 4 5 3 n
.81; N, 5.71; S, 9.80. Found: C, 66.95; H, 7.93; N, 4.74; S, 9.16. BOB are presented in Fig. 2.
The Stille polymerization between the most promising elec-
tron-rich BDT and the new electron-accepting BBOB and BOB
afforded polymers P(BDT-BBOB) and P(BDT-BOB), respectively.
Both the polymers P(BDT-BBOB) and P(BDT-BOB) show good
solubility in chloroform, chlorobenzene, and dichlorobenzene,
3
. Results and discussion
3.1 Synthesis and structural characterization
The synthetic route for the monomers (BBOB and BOB) and and their weight average molecular weights (M ) and poly-
w
polymers (P(BDT-BBOB) and P(BDT-BOB)) are outlined in dispersities (PDI) determined by GPC analysis with chloroform
4
4
Scheme 1 and Scheme 2, respectively. Benzo[c][1,2,5]thiadia- as an eluent were 1.85 ꢃ 10 , 2.13 ꢃ 10 and 1.45 and 1.63,
zole-4,7-dicarbonitrile (1) was prepared by treating 4,7-dibro- respectively. The 5% weight loss temperatures of P(BDT-BBOB)
ꢁ
ꢁ
mobenzo[c][1,2,5]thiadiazole with copper cyanide. Compound 1 and P(BDT-BOB) were found to be 407 C and 415 C, respec-
was converted into compound 2 by treating with 50% aqueous tively, from thermogravimetric analysis (TGA). The good
hydroxyl amine solution. On the other hand, compound 3 was solubility and high thermal stability of P(BDT-BBOB) and
30
prepared by using the reported procedure. The acid group on P(BDT-BOB) allow them to be utilized in PSCs. The molecular
compound 3 was activated by treating with coupling agent (CDI) weights and 5% weight loss temperatures of P(BDT-BBOB) and
and then reaction with compound 2, followed by cyclization, to P(BDT-BOB) are summarized in Table 1.
afford the monomer BBOB containing two Oxa units and one BT
unit. Another monomer, namely, BOB, was prepared from 3-
methyl-2-nitrobenzenamine. In this route, the nitro group on 3-
3.2 Optical properties
methyl-2-nitrobenzenamine was reduced to the amino group The absorption spectra of the polymers P(BDT-BBOB) and
with the use of Pd/C, to give compound 4. Compound 4 was P(BDT-BOB) measured in chloroform and as a thin lm on glass
treated with thionyl chloride and TEA in dichloromethane to are presented in Fig. 3. Polymer P(BDT-BBOB) displays an
afford compound 5. The bromination of compound 5 with absorption band in the range of 300–520 nm, with two
bromine afforded compound 6, and then compound 6 was maximum absorption peaks at 399 and 454 nm in solution and
31
32
treated with NBS to afford compound 7. The NaIO
4
oxidation
416 and 455 nm as a lm. On the other hand, polymer P(BDT-
of compound 7 gave compound 8, and the formyl group of BOB) showed a broad absorption band in between 300 nm and
compound 8 was converted to the cyano group of compound 9 600 nm, with maximum absorption peaks at 431 and 509 nm,
Scheme 1 Synthetic route to monomers BBOB and BOB.
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Scheme 2 Synthetic route to polymers P(BDT-BBOB) and P(BDT-BOB).
Fig. 3 Absorption spectra of polymers P(BDT-BBOB) and P(BDT-BOB)
in chloroform and as a thin film.
both in solution and as a lm. The absorption bands of P(BDT-
BBOB) are expected to originate from the p–p transition, while
those of P(BDT-BOB) might originate from the combined elec-
tronic transitions, such as by p–p transitions and donor–
acceptor internal charge transfer (ICT) transitions between BDT
and BT units. The presence of electron accepting Oxa units on
both sides of the BT unit of P(BDT-BBOB) might restrict the ICT,
but the covalent bond between the strong electron donating
1
13
Fig. 2 The H and C NMR spectra of BBOB and BOB.
Table 1 Polymerization results and thermal, optical, and electrochemical properties of P(BDT-BBOB) and P(BDT-BOB)
a
w
ꢀ1
a
b
ꢁ
c
d
e
f
f
g
Polymer
M
(g mol
)
PDI TGA ( C)
lmax in solution (nm)
lmax as lm (nm)
E
g,opt (eV) HOMO (eV) LUMO (eV)
Eg,elect (eV)
4
P(BDT-BBOB) 1.85 ꢃ 10
1.45 407
1.63 415
399, 454
431, 509
416, 455
431, 509
2.32
1.99
ꢀ5.31
ꢀ5.27
ꢀ2.90
ꢀ3.10
2.41
2.17
4
P(BDT-BOB) 2.13 ꢃ 10
a
b
Weight average molecular weight (M
weight loss temperature measured by TGA under N
the glass substrate. Band gap estimated from the onset wavelength of the optical absorption in thin lm. The HOMO and LUMO levels were
estimated from cyclic voltammetry analysis. The electrochemical band gap values estimated from the HOMO and LUMO values.
w
) and polydispersities (PDI) of the polymers were determined by GPC using polystyrene standards. 5%
c
d
2
.
Measurements in chloroform solution. Measurements in thin lm were performed on
e
f
g
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BDT and electron accepting BT units of P(BDT-BOB) might LUMO energy levels of polymers P(BDT-BBOB) and P(BDT-BOB)
allow the ICT. Earlier reports conrmed that the incorporation are included in Table 1.
of sterically hindered electron donor groups, such as 3-
octylthiophene or alkoxy benzene, or an electron accepting
3
.4 Photovoltaic properties
The PSCs were fabricated with the device structure of ITO/
PEDOT:PSS/P(BDT-BBOB) or P(BDT-BOB):PC61BM/LiF/Al.
benzimidazole derivative on both sides of the BT restrict the ICT
from the donor to the acceptor units when they are polymerized
15,17,23,34
with electron-rich units.
These results suggest that the
Among the devices prepared with three different donor:acceptor
ratios of 1 : 1 wt%, 1 : 2 wt%, and 1 : 3 wt% for each polymer
P(BDT-BBOB) or P(BDT-BOB) as a donor and PC61BM as an
acceptor, the highest power conversion efficiency (PCE) was
obtained for the devices made from 1 : 2 wt% blends. The
current density–voltage (J–V) curves of the PSCs were measured
incorporation of Oxa units on both sides of the BT unit also
diminish the ICT between the D–A units. The absorption onset
wavelengths of P(BDT-BBOB) and P(BDT-BOB) were calculated
to be 534 nm and 622 nm, respectively, from which the optical
band gaps (Eg,opt) were determined to be 2.32 eV and 1.99 eV,
respectively. The optical properties of polymers P(BDT-BBOB)
and P(BDT-BOB) are summarized in Table 1.
ꢀ2
under the illumination of AM 1.5 G (100 mW cm ), and their
corresponding J–V curves are shown in Fig. 5. The characteristic
parameters of the J–V curves of PSCs, such as open circuit
voltage (V ), short-circuit current density (J ), ll factor (FF),
3.3 Electrochemical properties
oc
sc
and power conversion efficiency (PCE) are presented in Table 2.
The device made from the blends of P(BDT-BBOB):PC61BM
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of P(BDT-
BBOB) and P(BDT-BOB) were estimated from cyclic voltammetry
(
1 : 2 wt%) and (BDT-BOB):PC61BM (1 : 2 wt%) gave an overall
ꢀ
2
PCE of 1.76% (Voc of 0.69 V, a Jsc of 7.10 mA cm , and a FF of
3
(CV) analysis, and the CV spectra of polymers are displayed in
ꢀ2
6%) and 2.46% (Voc of 0.71 V, a Jsc of 8.21 mA cm , and a FF of
Fig. 4. The HOMO and LUMO energy levels of P(BDT-BBOB) and
P(BDT-BOB) were calculated from the onset oxidation (Eox) and
onset reduction (Ered) potential values, by using the following
43%), respectively. Recently post-solvent treatment on the
photoactive layer of the PSCs was reported to improve the
35–37
photoactive performances of the PSCs signicantly.
We also
equations, EHOMO ¼ [ꢀ(E
ꢀ E
) ꢀ
ox,onset vs. Ag/AgCl
ferrocene vs. Ag/AgCl
tried post-solvent treatment with various solvents, such as
acetone, methanol, ethanol, and isopropyl alcohol (IPA), and
the best result was obtained with IPA treatment. The PCE of the
PSCs prepared from the blends of P(BDT-BBOB):PC61BM (1 : 2
4
.8] eV and ELUMO ¼ [ꢀ(Ered,onset vs. Ag/AgCl ꢀ E
) ꢀ
ferrocene vs. Ag/AgCl
4.8] eV. The Eferrocene vs. Ag/AgCl is calculated from the oxidation
potential of ferrocene standard to be 0.55 V. The Eox,onset vs. Ag/AgCl
and Ered,onset vs. Ag/AgCl values of P(BDT-BBOB) and P(BDT-BOB)
were determined to be 1.06 V, 1.02 V and 1.35 V, 1.15 V, respec-
tively. The HOMO/LUMO energy levels of P(BDT-BBOB) and
P(BDT-BOB) were calculated to be ꢀ5.31 eV/ꢀ2.90 eV and
ꢀ
5.27 eV/ꢀ3.10 eV, respectively. The electrochemical band gap
values were 2.41 eV and 2.17 eV, respectively. The electrochemical
band gap (Eg,elect) values are found to be slightly higher than that
of the optical band gap values (Eg,opt). The deeper HOMO energy
levels of P(BDT-BBOB) and P(BDT-BOB), and most importantly,
the signicant energy difference between the LUMO levels of
newly synthesized polymers and PC61BM ensure that the polymers
are suitable electron donors for PSCs application. The HOMO and
Fig. 4 Cyclic voltammograms of polymers P(BDT-BBOB) and P(BDT-
BOB) films cast on a platinum working electrode in 0.1 M TBATFB/ Fig. 5 J–V characteristics of PSCs prepared with the device structure
ꢀ1
+
acetonitrile at 100 mV s , potential vs. Fe/Fe .
of ITO/PEDOT:PSS/P(BDT-BBOB) or P(BDT-BOB):PC61BM/LiF/Al.
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Table 2 PSCs performance of polymers P(BDT-BBOB) and P(BDT-BOB)
a
ꢀ2 b
c
d
2 e
ꢀ2 f
Polymer:PC61BM ratio
Voc (V)
Jsc (mA cm
)
FF (%)
PCE (%)
R
s
(U cm )
Integrated Jsc (mA cm )
P(BDT-BBOB):PC61BM (1 : 1 wt%)
P(BDT-BBOB):PC61BM (1 : 2 wt%)
P(BDT-BBOB):PC61BM (1 : 3 wt%)
0.67 ꢄ 0.02
0.69 ꢄ 0.02
0.69 ꢄ 0.01
4.09 ꢄ 0.16
7.10 ꢄ 0.18
3.96 ꢄ 0.01
9.59 ꢄ 0.28
4.44 ꢄ 0.26
8.21 ꢄ 0.09
5.07 ꢄ 0.05
25.96 ꢄ 0.01 0.72 ꢄ 0.10 36
35.75 ꢄ 0.35 1.76 ꢄ 0.10 15
41.77 ꢄ 0.01 1.15 ꢄ 0.02 20
49.43 ꢄ 0.01 3.31 ꢄ 0.08 10
33.33 ꢄ 0.02 1.04 ꢄ 0.03 31
42.60 ꢄ 0.03 2.46 ꢄ 0.16 12
45.43 ꢄ 0.02 1.62 ꢄ 0.05 14
3.96 ꢄ 0.12
7.03 ꢄ 0.10
3.77 ꢄ 0.10
9.57 ꢄ 0.16
4.31 ꢄ 0.17
8.10 ꢄ 0.02
5.00 ꢄ 0.10
11.07 ꢄ 0.06
P(BDT-BBOB):PC61BM (1 : 2 wt%) + IPA 0.70 ꢄ 0.01
P(BDT-BOB):PC61BM (1 : 1 wt%)
P(BDT-BOB):PC61BM (1 : 2 wt%)
P(BDT-BOB):PC61BM (1 : 3 wt%)
P(BDT-BOB):PC61BM (1 : 2 wt%) + IPA
0.70 ꢄ 0.01
0.71 ꢄ 0.03
0.70 ꢄ 0.01
0.70 ꢄ 0.01 11.23 ꢄ 0.16
53.82 ꢄ 0.01 4.21 ꢄ 0.10
6
a
b
c
d
Open-circuit voltage. Short-circuit current density. Fill factor. Power conversion efficiency (averages of 5 PSC devices are reported for each
e
f
blend). Series resistance. Estimated from IPCE curves.
wt%) and (BDT-BOB):PC61BM (1 : 2 wt%) was further improved
ꢀ2
to 3.31% (Voc of 0.70 V, a Jsc of 9.59 mA cm , and a FF of 49%)
ꢀ
2
and 4.21% (Voc of 0.70 V, a Jsc of 11.23 mA cm , and a FF of
4%), respectively, with IPA treatment on the active layer of
PSCs.
The comparison of the device performances of P(BDT-
5
BBOB) and P(BDT-BOB) suggests that the difference in the
PCE mainly originates from their different Jsc and FF values.
Usually, the Jsc and FF of the PSCs are affected by the
Fig. 6 (a) Schematic diagram of the hole-only device. (b) Current
density–voltage (J–V) characteristics of the hole-only devices.
absorption ability, series resistance (R ), charge-transport
s
3
6
properties, charge extraction, and charge recombination.
Polymer P(BDT-BOB) displayed a broad and red-shied
absorption band compared to polymer P(BDT-BBOB), and
consequently, the PSCs made from P(BDT-BOB) gave higher
strongly supports the higher FF and Jsc obtained for the P(BDT-
BOB)-based PSCs.
J
sc values than the PSCs made from P(BDT-BBOB). The lower
In order to investigate the effect of solvent treatment on the
interface resistance of the PSCs, the electrochemical impedance
spectra (EIS) of the PSCs were measured in the dark. Fig. 7
shows the Nyquist plots of the ac impedance of the PSCs based
on P(BDT-BBOB) and P(BDT-BOB) without and with solvent
treatment at 0 V with a frequency range from 1 Hz to 1 MHz. The
ac impedance consists of a single depressed semicircle for each
PSC, which can be used to calculate the internal resistance of
the thin lm for the PSC. For example, the equivalent resistance
of the PSC made from P(BDT-BBOB) with solvent treatment is
about 2086 U, which is much smaller than the value of about
FF value obtained for P(BDT-BBOB) indicates that the pres-
ence of the additional Oxa unit on the polymer main chain
might reduce their hole mobility, because oxadiazoles have a
tendency to facilitate electron transport more effectively than
3
8–40
the hole transport,
values compared with those of P(BDT-BOB). The higher series
resistance (R ) values (measured from the dark J–V curves) of
and consequently, it offered lower FF
s
the PSCs made from P(BDT-BBOB):PC61BM blends compared
to those of the PSCs made from P(BDT-BOB):PC61BM
(
1 : 2 wt%) agree with our arguments. In both cases, the series
resistances of the PSCs were dramatically decreased aer IPA
treatment, as shown in Table 2, and consequently, higher Jsc
and FF were obtained with IPA treatment.
3886 U for the PSC without solvent treatment (see Fig. 6a). This
indicates that the solvent treatment layer can improve the
contact between the photoactive layer and the top Al electrode
and, consequently, decreases the resistance of the whole device.
It is worth noting that the resistance of P(BDT-BOB)-based PSCs
To make clearer our argument, we estimated the hole
mobility of each polymer and of the polymer:PC61BM blends by
the space charge limited current (SCLC) method. The device
structure used in this study and the corresponding SCLC
characteristics for the hole-only devices made with the polymer
or polymer:PC BM blend as active layers are presented in
6
1
Fig. 6. The device made with P(BDT-BBOB) and the P(BDT-
BBOB):PC BM blend displayed relatively lower hole mobility, m
6
1
ꢀ
5
2
ꢀ1 ꢀ1
ꢀ6
2
ꢀ1 ꢀ1
ꢂ 6.11 ꢃ 10 cm
V
s
and 9.61 ꢃ 10 cm
V
s ,
compared with that of the corresponding device made with the
ꢀ44
P(BDT-BOB) and P(BDT-BOB):PC61BM blend (m ꢂ 3.31 ꢃ 10
2
ꢀ1 ꢀ1
ꢀ5
2
ꢀ1 ꢀ1
cm V
s
and 4.27 ꢃ 10 cm V
s , respectively). The
higher hole mobility obtained for the P(BDT-BOB)-based device Fig.
7
Electrochemical impedance spectra of PSCs made from
P(BDT-BBOB):PC61BM (1 : 2 wt%) and P(BDT-BOB):PC61BM (1 : 2 wt%)
blends with/without IPA treatment.
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Fig. 9 AFM images of the blend films of P(BDT-BBOB):PC61BM (1 : 2
wt%) and P(BDT-BOB):PC61BM (1 : 2 wt%) without (images a and c,
respectively) and with solvent treatment (images b and d, respectively).
treatment were recorded, as shown in Fig. 9. The rms roughness
of the lms made from P(BDT-BBOB):PC61BM (1 : 2 wt%) and
P(BDT-BOB):PC61BM (1 : 2 wt%) increased from 0.98 nm to
1
.3 nm, and from 0.38 nm to 0.58 nm, respectively, aer
applying the solvent treatment. The rough surface increases the
Fig. 8 IPCE spectra of the PSCs made from P(BDT-BBOB):PC61BM contact area between the photoactive layer and the Al cathode,
(
1 : 2 wt%) and P(BDT-BOB):PC61BM (1 : 2 wt%) blends with/without
which may benet a more efficient charge collection at the
interface. Moreover, the increased surface roughness may also
IPA treatment.
41
increase the internal reection and light collection, increasing
42
the PSCs' efficiency.
are lower than that of P(BDT-BBOB)-based PSCs, which also
supports our earlier arguments regarding the higher FF values
obtained for P(BDT-BOB)-based PSCs.
The IPCE spectra of the PSCs made from P(BDT-
BBOB):PC61BM (1 : 2 wt%) and P(BDT-BOB):PC61BM (1 : 2 wt%)
blends with and without IPA treatment are displayed in Fig. 8.
The IPCE responses were found to cover the region of 300–580
nm and 300–650 nm, respectively, for the PSCs made from
P(BDT-BBOB) and P(BDT-BOB). The maximum IPCE values at
4
. Conclusions
Two new electron accepting monomers (BBOB and BOB) con-
taining two serially connected different electron decient units,
such as 2,1,3-benzothiadiazole (BT) and 1,2,4-oxadiazole (Oxa),
were prepared and copolymerized with an electron-rich dis-
0
tannyl derivative of 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b ]-
420 nm appeared to be 53% and 58%, respectively, for the PSCs
dithiophene (BDT) to afford polymers P(BDT-BBOB) and P(BDT-
BOB), respectively. Polymer P(BDT-BBOB) incorporating two
Oxa units on both sides of the BT unit was found to show a blue-
shied absorption with a slightly lower HOMO level compared
to those of polymer P(BDT-BOB) incorporating one Oxa unit
attached to the BT unit. The maximum PCEs obtained for the
PSCs made with the conguration of ITO/PEDOT:PSS/P(BDT-
BBOB) or P(BDT-BOB):PC BM (1 : 2 wt%)/IPA/LiF/Al were
made from P(BDT-BBOB) without and with IPA treatment. On the
other hand, the PSCs made from P(BDT-BOB) without and with
IPA treatment showed the IPCE maximum values of 60% and
70%, respectively, at 420 nm. In both cases, the IPCE responses of
the PSCs signicantly increased aer IPA treatment. These
results strongly agree with the higher Jsc values obtained for the
PSCs made with IPA treatment and also support the higher Jsc
6
1
values obtained for the PSCs made from P(BDT-BOB), compared
3.31% and 4.21%, respectively, measured using an AM 1.5 G
with those obtained for the PSCs made from P(BDT-BBOB). The
integrated Jsc value of the PSCs determined from the IPCE spectra
and the Jsc values obtained from the J–V curve of the PSCs are
found to be within a 5% error range, and the integrated Jsc values
are summarized in Table 2 for clear comparison.
ꢀ2
solar simulator at 100 mW cm light illumination. The pres-
ence of the additional Oxa unit on the polymer main chain was
found to decrease the FF and Jsc values signicantly and
consequently, gave a lower PCE. This study concludes that the
ratio or conguration of two covalently bonded different elec-
tron acceptor units present in the polymer main notably alters
the optical, electrical, and photovoltaic properties of the poly-
mers. We hope that replacement of the Oxa unit in polymer
3.5 Active layer surface morphology
The tapping mode atomic force microscopy (AFM) images of the P(BDT-BOB) with other electron accepting units, such as thia-
lms made from P(BDT-BBOB):PC61BM (1 : 2 wt%) and diazole, tetrazine, or thiazole, might enhance the PCE further,
P(BDT-BOB):PC61BM (1 : 2 wt%) without and with IPA and currently progress is underway in this regard.

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2
0 L. Fan, R. Cui, X. Guo, D. Qian, B. Qiu, J. Yuan, Y. Li,
W. Huang, J. Yang, W. Liu, X. Xu, L. Li and Y. Zou, J.
Mater. Chem. C, 2014, 2, 5651–5659.
Acknowledgements
This research was supported by the National Research Foun-
dation of Korea (NRF-2013R1A2A2A04014576). This study was 21 X. Wang, S. Chen, Y. Sun, M. Zhang, Y. Li, X. Li and H. Wang,
also supported by the research fund (PKC 3670) of Korea
Institute of Materials Science.
Polym. Chem., 2011, 2, 2872–2887.
22 Y. Cao, T. Lei, J. Yuan, J.-Y. Wang and J. Pei, Polym. Chem.,
2013, 4, 5228–5236.
2
3 V. Tamilavan, M. Song, R. Agneeswari, S. Kim and
M. H. Hyun, Bull. Korean Chem. Soc., 2014, 35, 1098–1104.
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5
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