Effect of the number of thiophene rings in polymers with 2,1,3-benzooxadiazole core on the photovoltaic properties

Article abstract of DOI:10.1016/j.orgel.2013.07.009

A series of polymers, poly{5,6-bis(decyloxy)-4-(thiophen-2-yl)benzo[c][1,2, 5]oxadiazole} (1T-BO20), poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)benzo[c] [1,2,5]oxadiazole} (2T -BO20), poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)- 7-(thiophen-2-yl)b

Full text of DOI:10.1016/j.orgel.2013.07.009

Organic Electronics 14 (2013) 2673–2681
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Effect of the number of thiophene rings in polymers
with 2,1,3-benzooxadiazole core on the photovoltaic properties
⇑
Thu Trang Do, Ye Eun Ha, Joo Hyun Kim
Department of Polymer Engineering, Pukyong National University, Yongdang-Dong, Nam-Gu, Busan 608-739, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of polymers, poly{5,6-bis(decyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]oxadiazole}
(1T-BO20), poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)benzo[c][1,2,5]oxadiazole} (2T
-BO20), poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)-7-(thiophen-2-yl)benzo[c][1,2,5]
oxadiazole} (3T-BO20) containing 2,1,3-benzooxadiazole derivative and different
thiophene rings are synthesized. Effect of the number of thiophene rings on the optical,
electrochemical and photovoltaic properties of the polymers is investigated. The maximum
absorption wavelength and the optical band gap of the polymers are almost the same, indi-
cating the polymers exhibit similar intramolecular charge transfer effect. The HOMO levels
are in the order of 1T-BO20 (ꢀ5.60 eV) < 2T-BO20 (ꢀ5.45 eV) < 3T-BO20 (ꢀ5.36 eV), reveal-
ing that the HOMO level of the polymers are dependent of number of thiophene ring in the
back bone. Under the illumination of AM 1.5G, 100 mW/cm2, the power conversion effi-
ciency (PCE) of PSCs based on these polymers increases in the order of 1T-BO20 (1.66%),
2T-BO20 (1.71%) and 3T-BO20 (1.92%). Besides, we find that the efficiency of PSCs showed
very different responses by the addition of DIO as a processing additive. The devices based
on 1T-BO20 and 2T-BO20 with DIO exhibit an enhancement of PCE from 1.66% to 3.65% and
from 1.71% to 2.40%, respectively, whereas PCE of the device based on 3T-BO20 with DIO
decreased from 1.92% to 1.76%.
Received 17 April 2013
Received in revised form 7 July 2013
Accepted 11 July 2013
Available online 26 July 2013
Keywords:
Polymer solar cells
Thiophene
2,1,3-Benzooxadiazole
1,8-Diiodooctane
Morphology
Power conversion efficiency
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
and only in the wavelength range between 350 nm and
650 nm. The limitation in the absorption is detrimental
Polymer solar cells (PSCs), especially bulk heterojunc-
tion (BHJ) PSCs comprising -conjugated polymers and ful-
to the short circuit current (Jsc) of the resulting photovol-
taic cells as well as the high-lying HOMO energy level of
P3HT limits the open circuit voltage (Voc) of its solar cell
[7,8]. In recent years, the development of novel materials
has contributed to significant improvements in device per-
formance. Among various types of polymers, the conju-
gated polymers based on the alternating electron-rich
and electron-deficient units along the polymer backbone
have been shown to be able to achieve high power conver-
sion efficiencies [9]. The state of internal charge transfer
(ICT) transition from donor to acceptor inside the copoly-
mer absorbs photons with long wavelengths which could
extend the absorption and decrease the band gap of the
copolymer [10].
p
lerene derivatives have attracted much interest in recent
years due to their solution processability and flexibility,
which potentially allow them to be manufactured using
low-cost, high-throughput processes such as roll-to-roll
printing and inkjet printing [1–3]. In the past years,
poly(3-hexylthiophene) (P3HT) was used as the most com-
mon donor material because of its high carrier mobility
[4,5]. However, the relatively large band gap of P3HT
(Eg = 1.9 eV) limits the harvest of photons from the solar
spectrum. It is calculated that P3HT is only capable of
absorbing about 46% of the available solar photons [6]
Moreover, the morphology of the polymer:fullerene
blend is a critical factor which affects the solar cell
⇑
Corresponding author. Tel.: +82 51 629 6452.
E-mail address: jkim@pknu.ac.kr (J.H. Kim).
1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.07.009

2674
T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
performances [11,12]. Various methods have been devel-
oped to optimize the morphology such as thermal anneal-
ing [13–16], solvent annealing [17–19] and the use of
co-solvents as the processing additives [20–22]. Consider-
able improvement of the PCE of PSCs by tuning the
morphology of the active layers with various additives has
been demonstrated and become a more attractive method
due to its simplicity without an additional fabrication steps
and production compatibility compared with thermal treat-
ment [12,21,23,24]. They influence the size of the fullerene
domains and enhance the crystallinity of the self-organized
polymers by improving the solubility of the fullerenes and
slightly elongating the drying time of the active layer [25].
Recently, 2,1,3-benzooxadiazole (BO) derivatives have
been paid much attention as strong electron-deficient moi-
eties used for PSC [26–29]. A BO possesses a high electron
transporting ability due to two electron withdrawing
imine (C = N) groups [30]. Besides, it also exhibits capabil-
ity to adopt the quinoid structure in the polymer, resulting
in a low band gap and coplanar polymer [27,29]. In com-
number of thiophene rings and DIO as the processing addi-
tive will be studied.
2. Experimental section
2.1. Materials
Methylene chloride (MC) was distilled over CaH2. All
other chemicals were purchased from Sigma–Aldrich Co.,
Tokyo Chemical Industry (TCI) or Alfa Aesar (A Johnson
Matthey Company) and used as received unless otherwise
described.
2.2. Synthesis of monomers and polymers
2.2.1. Synthesis of 1,2-bis-decyloxy-benzene (2)
Sodium hydroxide (NaOH, 3.56 g, 89.1 mmol) was dis-
solved in methanol (100 mL) at 0 °C for 30 min and then
catechol (4.46 g, 40.5 mmol) was added slowly. White
solution turned to deep green and finally dark. At this time
1-bromodecane (19.7 g, 89.1 mmol) was added into the
mixture dropwise and then the mixture was refluxed under
the nitrogen atmosphere. After 8 h, reaction mixture was
cooled down to room temperature and then the solvent
was removed under the reduced pressure. A portion of
250 mL of water was added into a reaction mixture and
then the organic layer was extracted using a portion of
250 mL of ethyl acetate (EA). The organic layer was dried
over anhydrous magnesium sulfate (MgSO4) and the sol-
vent was removed using a rotary evaporator. The combined
organic layer was triturated from MC and methanol to gain
the off-white solid (11.7 g, 73.8%). mp: 40.3 °C. 1H NMR
(400 MHz, CDCl3, ppm): d 6.87 (s, 4H), 3.99–3.95 (t,
J = 7.0 Hz, 4H), 1.83–1.76 (m, 4H), 1.48–1.41 (m, 4H),
1.33–1.26 (m, 24H), 0.88–0.85 (t, J = 7.0 Hz, 6H). 13C NMR
(100 MHz, CDCl3, ppm): d 150.29, 122.34, 115.46, 65.81,
32.89, 30.24, 30.51, 30.44, 30.19, 29.65, 26.09, 23.67, 15.05.
parison with
a typical electron-withdrawing moiety
2,1,3-benzothiadiazole (BT), a BO has a low-lying oxidation
potential which can decrease both the HOMO and LUMO
energy levels [31]. The devices based on the polymers hav-
ing BO moiety exhibit pretty high open circuit potentials
[32]. Furthermore, it has been recently reported that the
PCE of device based on copolymer PBDTBO containing ben-
zo[1,2-b:4,5-b0]dithiophene and BO derivatives reached
5.7% [27]. Therefore polymers based on BO derivatives
are potential photoactive material candidates for PSCs.
In this research, we synthesize two new
p-conjugated
alternating copolymers, poly{4-(2,20-bithiophen-5-yl)-5,6-
bis(decyloxy)benzo[c][1,2,5]oxadiazole} (2T-BO20), and
poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)-7-(thio-
phen-2-yl)benzo[c][1,2,5]oxadiazole} (3T-BO20), based on
BO. The absorption, electrochemical and photovoltaic
properties of 2T-BO20 and 3T-BO20 were explored and
compared with that of poly{5,6-bis(decyloxy)-4-(thio-
phen-2-yl)benzo[c][1,2,5]oxadiazole} (1T-BO20) which
was reported previously [28]. These polymers have similar
structure but different number of thiophene rings in their
repeat unit, one thiophene ring for 1T-BO20, two thio-
phene rings for 2T-BO20 and three thiophene rings for
3T-BO20. Thiophene was used because it has been shown
to have relatively strong interchain interaction and high
charge carrier mobility in BHJ type solar cells [10,33]. Fur-
thermore, it has been reported that the optical and physi-
cal properties of copolymers can be controlled with
suitable modification of thiophene conjugated length
[34,35]. Therefore, we expected that an extension of thio-
phene ring can improve planarity and increase effective
conjugation of polymer main chain which is translated into
broader absorption and better photovoltaic performances
[36]. In addition, these polymers are designed with two
decyloxy chains on the 5- and 6-positions of BO unit to im-
prove solubility of polymers in common organic solvents
and make it more feasible to synthesize the polymers with
high molecular weight. Herein, we will discuss the synthe-
sis, characterization, and optical properties of the polymers
as well as the photovoltaic properties of the devices based
on these polymers. On the other hand, the influence of the
2.2.2. Synthesis of 1,2-dinitro-4,5-bis-decyloxy-benzene (3)
Compound 2 (9.57 g, 24.5 mmol) was dissolved in MC
(150 mL) and acetic acid (AcOH, 150 mL). A portion of
28.5 mL of nitric acid (HNO3, 65.0%) was added dropwise
into a reaction mixture at 10 °C. The reaction was allowed
to warm to room temperature and stirred for 1 h. The mix-
ture was again cooled to 10 °C and 65.5 mL of 95.0% HNO3
was added dropwise into a reaction mixture. The mixture
was allowed to warm to room temperature and the mixture
was stirred for 24 h. After completion of the reaction, the
reaction mixture was poured into ice-water and the MC
layer separated. The water phase was extracted with MC.
The combined organic layer was washed with water, aque-
ous solution of NaHCO3, brine and dried over anhydrous
MgSO4 and then removed using a rotary evaporator. The
crude product was recrystallized from methanol. The yel-
low solid product yield was 10.4 g (88.3%). mp: 83.5 °C.
1H NMR (400 MHz, CDCl3, ppm): d 7.27 (s, 2H), 4.09–4.06
(t, J = 6.5 Hz, 4H), 1.88–1.81 (m, 4H), 1.49–1.42 (m, 4H),
1.34–1.25 (m, 24H), 0.87–0.84 (t, J = 6.5 Hz, 6H). 13C NMR
(100 MHz, CDCl3, ppm): d 152.81, 137.48, 108.92, 71.20,
32.87, 30.55, 30.51, 30.30, 30.21, 29.69, 26.80, 23.66, 15.07.

T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
2675
2.2.3. Synthesis of 5,6-bis-decyloxy-benzo[c][1,2,5]oxadiazole
(4)
A mixture of compound 3 (9.85 g, 20.5 mmol), sodium
azide (NaN3, 6.69 g, 103 mmol), and tetrabutyl ammonium
bromide (n-Bu4NBr, 1.32 g, 4.10 mmol) was heated under
reflux in toluene (100 mL) for 12 h under N2 atmosphere.
1.37–1.26 (m, 24H), 0.89–0.85 (t, J = 6.6 Hz, 6H). 13C NMR
(100 MHz, CDCl3, ppm): d 152.69, 147.74, 133.90, 131.85,
129.00, 128.14, 114.05, 75.50, 32.92, 31.28, 30.62, 30.58,
30.56, 30.35, 26.88, 23.69, 15.11.
2.2.6. Synthesis of 4,7-bis(5-bromothien-2-yl)-5,6-bis-
decyloxy-benzo[c][1,2,5] oxadiazole (7)
A
portion of 6.45 g of triphenylphosphine (PPh3,
24.6 mmol) was added in small portions into a reaction
mixture. After completion of addition of PPh3, the mixture
was heated under reflux for an additional 24 h. The reac-
tion mixture was cooled to room temperature and the or-
ganic solvent was removed under the reduced pressure.
The crude product was purified by silica gel column eluting
with MC:hexane (1:10 v/v). The yellow solid yield was
5.25 g (59.2%). mp: 110.7 °C. 1H NMR (400 MHz, CDCl3,
ppm): d 6.78 (s, 2H), 4.06–4.02 (t, J = 6.6 Hz, 4H), 1.90–
1.83 (m, 4H), 1.51–1.44 (m, 4H), 1.35–1.25 (m, 24H),
0.87–0.84 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3,
ppm): d 156.19, 147.82, 91.66, 70.35, 32.89, 30.57, 30.53,
30.32, 30.28, 29.58, 26.96, 23.66, 15.08.
A
portion of N-bromosuccinimide (NBS, 2.81 g,
15.8 mmol) was added portion wise into a solution of com-
pound 6 (3.76 g, 6.30 mmol) in 40.0 mL of N,N-dimethyl-
formamide (DMF) in ice bath. After addition of NBS, the
reaction mixture was stirred in the dark for 24 h and the
temperature being gradually raised to room temperature.
A portion of aqueous sodium bisulfite (NaHSO3, 5 wt.%)
was added into the reaction mixture and stirred for
20 min to remove excess of NBS. The mixture was then ex-
tracted with 100 mL of EA. The organic layer was combined
and dried over anhydrous MgSO4. The organic solvent was
removed under the reduced pressure. The residue solid
was purified by flash chromatography on silica gel column
eluting with MC:hexane (1:10 v/v). The reddish orange so-
lid product yield was 4.21 g (88.6%). mp: 76.7 °C. 1H NMR
(400 MHz, CDCl3, ppm): d 8.22–8.21 (d, J = 4.0 Hz, 2H),
7.15–7.14 (d, J = 4.4 Hz, 2H), 4.15–4.11 (t, J = 7.4 Hz, 4H),
2.0–1.93 (m, 4H), 1.47–1.41 (m, 4H), 1.36–1.27 (m, 24H),
0.89–0.85 (t, J = 6.6 Hz, 6H). 13C NMR (100 MHz, CDCl3,
ppm): d 152.27, 147.23, 135.31, 132.21, 131.09, 117.39,
113.71, 75.83, 32.99, 31.24, 30.09, 30.65, 30.55, 30.43,
26.89, 23.77, 15.19.
2.2.4. Synthesis of 4,7-dibromo-5,6-bis-decyloxy-benzo[c]
[1,2,5]oxadiazole (5)
A portion of 14.5 g of bromine (90.6 mmol) was added
into a solution of compound 4 (5.41 g, 12.5 mmol) in MC/
acetic acid (200 mL/100 mL) at room temperature. The
resulting mixture was stirred in the dark for ca. 3 days at
room temperature. A portion of 200 mL of aqueous NaOH
solution (10.0 g in 200 mL) was slowly added into the reac-
tion mixture. The aqueous phase was extracted with
100 mL of MC 3 times then the combined organic layer
was dried over anhydrous MgSO4 and concentrated under
reduced pressure to afford a crude solid. The crude product
was purified through column chromatography (SiO2, hex-
ane:MC (9:1 v/v). The white solid product yield was
5.62 g (76.2%). mp: 52.4 °C. 1H NMR (400 MHz, CDCl3,
ppm): d 4.13–4.10 (t, J = 6.6 Hz, 4H), 1.87–1.80 (m, 4H),
1.52–1.45 (m, 4H), 1.34–1.25 (m, 24H), 0.87–0.84 (t,
2.2.7. General procedure for Stille polymerization
2.2.7.1. Poly{5,6-bis(decyloxy)-4-(thiophen-2-yl)benzo[c][1,
2,5]oxadiazole}
0.25 mmol), 2,5-bis(trimethylstannyl)thiophene (102 mg,
0.25 mmol), Pd2dba3 (11.4 mg, 12.5 mol) and tri-(o-
tolyl)phosphine (30.4 mg, 100 mol) were mixed in dry
degassed toluene (6.0 mL). The reaction mixture was
heated to reflux for 48 h under nitrogen. After cooling to
room temperature, the mixture was poured into 200 mL
methanol and the polymer was allowed to precipitate.
The polymer was filtered and purified by Soxhlet extrac-
tion using methanol, hexane and chloroform. The chloro-
form fraction was concentrated by rotary evaporator and
then poured into methanol to precipitate again. The poly-
mer was filtered and dried in vacuum to give 1T-BO20
(89.3 mg, 69.6%) as black solid. 1H NMR (400 MHz, CDCl3,
ppm): d 8.67–8.24 (br, 2H), 4.43–4.21 (br, 4H), 2.26–2.18
(br, 4H), 2.13–1.97 (br, 4H), 1.61–1.40 (br, 4H), 1.31–1.23
(br, 20H), 0.86–0.84 (br, 6H).
(1T-BO20). Compound
5
(148 mg,
l
l
J = 6.2 Hz, 6H). 13C NMR (100 MHz, CDCl3, ppm):
d
156.66, 148.44, 100.55, 76.39, 32.88, 31.18, 30.56, 30.55,
30.36, 30.30, 26.88, 23.66, 15.09.
2.2.5. Synthesis of 5,6-bis-decyloxy-4,7-di(thien-2-yl)benzo[c]
[1,2,5]oxadiazole (6)
Compound 5 (5.61 g, 9.50 mmol), 2-tributylstannylthi-
ophene (8.84 g, 23.7 mmol), [1,10-bis(diphenylphosphino)
ferrocene] dichloropalladium (II), complex with dichloro-
methane (Pd(dppf)Cl2ꢁCH2Cl2, 0.39 g, 0.48 mmol) were dis-
solved in dry toluene (50.0 mL) and then the reaction
mixture was heated under reflux for overnight under N2
atmosphere. After the reaction mixture was cooled to room
temperature, water and EA were added. The aqueous phase
was extracted with 100 mL of EA 3 times and combined or-
ganic layer was dried over anhydrous MgSO4. The solvent
was removed under the vacuum and the residue was puri-
fied by column chromatography (SiO2, hexane:chloroform,
10:1 (v/v)) afforded a yellow solid (3.81 g, 67.2%). mp:
84.2 °C. 1H NMR (400 MHz, CDCl3, ppm): d 8.45–8.44 (dd,
J1 = 1.1 Hz, J2 = 3.7 Hz, 2H), 7.49–7.41 (dd, J1 = 1.1 Hz,
J2 = 5.2 Hz, 2H), 7.21–7.19 (t, 4.6 Hz, 2H), 4.15–4.11 (t,
J = 11.0 Hz, 4H), 2.02–1.94 (m, 4H), 1.48–1.41 (m, 4H),
2.2.7.2. Poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)benzo
[c][1,2,5]oxadiazole} (2T-BO20). 2T-BO20 was synthesized
by the Stille coupling reaction between compound 5
(148 mg, 0.25 mmol) and 5,50-bis(trimethylstannyl)-2,20-
bithiophene (123 mg, 0.25 mmol). The same reaction con-
dition and procedure were used as in the polymerization
of 1T-BO20. The yield of the polymer was 63.1 mg
(42.4%) as black solid. 1H NMR (400 MHz, CDCl3, ppm): d
8.46–7.94 (br, 4H), 4.25–4.06 (br, 4H), 2.21–2.01 (br, 4H),
1.57–1.46 (br, 4H), 1.44–1.17 (br, 24H), 0.91–0.82 (br, 6H).

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T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
2.2.7.3. Poly{4-(2,20-bithiophen-5-yl)-5,6-bis(decyloxy)-7-(thi
ophen-2-yl)benzo[c][1,2,5]oxadiazole} (3T-BO20). 3T-BO20
was synthesized by the Stille coupling reaction between
compound 7 (189 mg, 0.25 mmol) and 2,5-bis(trimethyl-
stannyl)thiophene (103 mg, 0.25 mmol). The same reaction
condition and procedure were used as in the polymeriza-
tion of 1T-BO20. The yield of the polymer was 82.6 mg
(48.8%). 1H NMR (400 MHz, CDCl3, ppm): d 8.52–7.82 (br,
4H), 7.02–6.53 (br, 2H), 4.43–3.92 (br, 4H), 2.37–2.14 (br,
4H), 2.07–1.91 (br, 4H), 1.59–1.17 (br, 24H), 0.98–0.79
(br, 6H).
and followed by baking at 150 °C for 10 min under the
ambient condition. The active layer was spin-cast from
the blend solution of polymer/PCBM (5 mg of polymer
and 10 mg of PCBM were dissolved in 1 mL chloroform
(CF) with or without 1,8-diiodooctane (DIO) (1 vol%). Prior
to spin coating, the active solution was filtered through a
0.45 lm membrane filter. Then, the aluminum cathode
deposited with a thickness of 110 nm through a shadow
mask with a device area of 0.13 cm2 at 2 ꢂ 10ꢀ6 Torr.
3. Results and discussion
2.3. Measurements
3.1. Synthesis and characterization
Synthesized compounds were characterized by 1H NMR
and 13C NMR spectrum, which were obtained with a JEOL
JNM ECP-400 spectrometer. Melting point of compounds
was measured by MPA100 OptiMelt Automated Melting
Point System with heating rate of 10 °C/min. The gel per-
meation chromatography (GPC) measurements were con-
ducted by GPC system equipped with a Varian 212-LC
pump, a Rheodyne 6-port sample injection valve, a Waters
Temperature Control Module, a Waters 410 differential RI
detector and two Waters Styragel HR4E columns using
polystyrene as standard and toluene as eluent. The ther-
mogravimetric analysis (TGA) was carried out under the
N2 atmosphere at a heating rate of 10 °C/min with a Per-
kin–Elmer TGA 7 thermal analyzer. The UV–Vis spectrum
was recorded using a JASCO V-530 UV–Vis Spectrophotom-
eter. The cyclic voltammetry (CV) was performed by an Ivi-
um B14406 with a three electrode cell in a 0.1 M n-
tetrabutylammonium hexafluorophosphate (n-Bu4NPF6)
acetonitrile solution at a scan rate of 100 mV/s. A Pt coil
was used as the counter electrode and dip-polymer on a
Pt wire was used as the working and a Ag/Ag+ electrode
was used as the reference electrode. Atomic Force Micro-
scope (AFM) images were taken on a Digital Instruments
(USA), Multi Mode™ SPM. The AFM images were obtained
by the tapping mode and a scan rate of 2 Hz. The J–V mea-
surements under the 1.0 sun (100 mW/cm2) condition
from a 150 W Xe lamp with an AM 1.5G filter were per-
formed using a KEITHLEY Model 2400 source-measure
unit. A calibrated Si reference cell with a KG5 filter certified
by National Institute of Advanced Industrial Science and
Technology was used to confirm 1.0 sun condition. The
thickness of films was measured by an Alpha-Step IQ sur-
face profiler (KLA-Tencor Co.).
Scheme 1 shows the general synthetic route for mono-
mers and polymers. Compound 2 was prepared by alkyl-
ation of catechol with 1-bromodecane in methanol and
sodium hydroxide at 80 °C. Compound 3, 4, 5, 6 and 7 were
synthesized by the reported procedures [26]. The polymers
(1T-BO20, 2T-BO20 and 3T-BO20) were synthesized by the
typical Stille coupling reaction. The molecular weight of
the polymers was determined by the gel permeation chro-
matography (GPC) relative to polystyrene standards by
using a toluene eluent. The number average molecular
weight (Mn) of 1T-BO20, 2T-BO20 and 3T-BO20 was
68500, 11500, and 36600 g/mol, respectively; with poly-
dispersity index (PDI) of 1.73, 7.61, and 1.66, respectively.
The thermal stability of the polymers was determined by
the thermogravimetric analysis (TGA). As shown in Fig. 1,
1T-BO20, 2T-BO20 and 3T-BO20 were thermally stable up
to 308, 279, and 309 °C. A temperature of 5%-weight loss
in TGA thermogram of 1T-BO20, 2T-BO20 and 3T-BO20
were 308, 279, and 309 °C, respectively. Among the poly-
mers, 2T-BO20 exhibit lower thermal stability. This is pre-
sumably due to its lower molecular weight than that of the
others. The thermal stability of the resulting polymers pre-
vents degradation of the active layer during fabrication of
PSCs [37,38].
3.2. Optical and electrochemical properties
Fig. 2 shows the UV–Vis absorption spectra of 1T-BO20,
2T-BO20 and 3T-BO20 films. The optical properties are
summarized in Table 1. All of the absorption spectra were
exhibited two broad absorption bands near 310–460 nm
and 460–740 nm. The former corresponds to localized
p–
p
transition of the conjugated polymer backbone and the
2.4. Fabrication of PSCs
latter corresponds to intramolecular charge transfer (ICT)
between the electron rich thiophene ring(s) and electron
deficient BO unit. The absorption spectrum of 1T-BO20
resembles that of PTTTBO [28] whose structure is similar
to 1T-BO20. The maximum absorption peaks at longer
wavelength region of 1T-BO20 film (kmax = 608, 662 nm)
show red-shift of 6, 11 nm compared to those of 3T-BO20
(kmax = 602, 653 nm). Whereas the maximum absorption
peaks at longer wavelength region of 2T-BO20 (kmax = 611,
661 nm) are similar to those of 1T-BO20. This is because of
the similar chemical structural feature of the three poly-
mers resulting in similar ICT effect. The maximum absorp-
tion peak at shorter wavelength region of 2T-BO20
PSCs were fabricated in the configuration of the common
sandwich structure: ITO/PEDOT:PSS/active layer (polymer:
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM))/Al. The
ITO-coated glass substrates were cleaned with deionized
water, acetone, methanol, 2-propanol in an ultrasonic bath.
After the substrates were dried, they were treated with UV/
O3 for 120 s prior to use. A layer of poly(3,4-ethylenedioxy-
thiophene):poly(styrenesulfonate) (PEDOT:PSS, Celvios P)
with 2-propanol (IPA) (PEDOT:PSS/IPA = 1/2 by volume)
was deposited onto the ITO substrate (sheet resistance =
15
X/sq) with a thickness of ca. 40 nm by spin-coating

T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
2677
Scheme 1. Synthesis route of monomers and polymers.
Fig. 1. TGA thermograms of polymers.
Fig. 2. UV–Vis absorption spectra of polymers.
(383 nm) and 3T-BO20 (408 nm) are red-shifted of 44 and
69 nm, than 1T-BO20 (339 nm). In addition, the absorption
range at the longer wavelength region of the polymers be-
comes broader as increases the number of thiophene rings.
This means that the effective conjugation length increases
with the increasing number of the thiophene rings in a re-
peat unit, which leads red-shift of the absorption spectrum
of the polymer and thereby increases photon harvesting
[39]. Interestingly, there is one other example with oppo-
site effect: after adding the thiophene rings into polymer
backbones lead to a blue shift in the absorption spectrum
[40]. This might be the result of decrease in the degree of
ICT between donor and acceptor. The optical band gap
(Eg,opt) of 1T-BO20, 2T-BO20 and 3T-BO20 estimated from
the absorption edges in the UV–Vis spectra were 1.73 eV.
As expected, the low Eg,opt of BO-based alternating copoly-
mers, which is lower than that of P3HT (ꢃ2.0 eV) and is
originated from the strong
tion bands [41,42].
p–p stacks and the ICT absorp-
To understand the energy levels of the polymers, and
expect the parameters of the resulting solar cells, it is nec-
essary to determine the HOMO and LUMO level of the
polymers. Thus, the electrochemical behaviors of the poly-
mers were investigated by CV measurements (See Fig. 3)
were carried out in anhydrous acetonitrile solution con-
taining 0.1 M Bu4NPF6 as the supporting electrolyte. The
reference electrode (Ag/Ag+) was calibrated by the

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T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
Table 1
Optical absorption and electrochemical properties of polymers.
Polymers
kmax (nm)
Eg,opt (eV)a
HOMO (eV)b
LUMO (eV)c
Eg,CV (eV)d
1T-BO20
2T-BO20
3T-BO20
608, 662
611, 661
602, 653
1.73
1.73
1.73
ꢀ5.60
ꢀ5.45
ꢀ5.36
ꢀ3.87
ꢀ3.72
ꢀ3.63
1.93
1.94
1.82
a
The optical band gap was obtained from the equation Eg,opt = 1240/kedge, where kedge is the onset value of absorption spectrum in long wavelength
direction.
b
Calculated from the oxidation onset potential.
Calculated from the HOMO and optical band gap energy (LUMO = HOMO + Eg,opt).
The electrochemical band gap was calculated according the equation: Eg,CV = oxidation onset potential – reduction onset potential.
c
d
ferrocene/ferrocenium (Fc/Fc+) redox couple (4.8 eV below
the vacuum level). From the values of the onset of oxida-
tion potentials of the polymers, the HOMO levels of the
polymers are estimated. The estimated HOMO and LUMO
energy levels of the polymers were compiled in Table 1.
The HOMO energy levels of the polymers were in the order
of 1T-BO20 (ꢀ5.60 eV) < 2T-BO20 (ꢀ5.45 eV) < 3T-BO20
(ꢀ5.36 eV). Thiophene is very strong electron donor and
has a high-lying ionization potential so that the HOMO en-
ergy level depends on the number of thiophene rings in the
polymer. These data agree with previously published re-
ports demonstrating that adding more thiophene rings re-
sulted in a higher HOMO energy level [42,43]. Besides, the
relatively low HOMO levels of these polymers promise
higher the Voc values of the photovoltaic cells compared
to that of P3HT:PCBM based device. The electrochemical
band gap (Eg,CV) of 1T-BO20, 2T-BO20 and 3T-BO20 were
estimated from the difference between onset potential of
reduction and oxidation, which are 1.93, 1.94 and
1.82 eV, respectively, which are quite different from the
Eg,CV of the polymers. This discrepancy might have been
caused by the presence of an energy barrier at the interface
between the polymer film and the electrode surface [44–
46]. Similar phenomena have been reported for other poly-
mer systems [47]. It was found that, although the HOMO
and LUMO energy levels of these polymers are quite differ-
ent, they have the same optical band gap. This possibly due
to the addition of thiophene ring(s) concurrently pulled the
HOMO and LUMO energy levels up. Beside, as discussed
above, three polymers have almost the same features in
the long wavelength region.
3.3. Photovoltaic properties
The photovoltaic performances of polymers were inves-
tigated in PSCs having the sandwich structure of ITO/PED-
OT:PSS/polymer:PCBM/Al. Fig. 4 shows the typical J–V
curves of polymer:PCBM devices in the dark and under
the illumination of AM 1.5G, 100 mW/cm2 without DIO.
The photovoltaic parameters of the devices are summarized
in Table 2. The devices prepared from PCBM blended with
1T-BO20, 2T-BO20 or 3T-BO20 exhibited Voc of 0.82, 0.82,
and 0.73 V, respectively, which are related with the HOMO
energy level of the polymers [48] and the difference
between the HOMO energy level of the polymer and the
LUMO of PCBM. The device with 1T-BO20 exhibited a
higher Voc than that of the device based on 3T-BO20 which
benefits from its a lower-lying HOMO energy level. Surpris-
ingly, the devices based on 2T-BO20 and 1T-BO20 have the
same Voc value, even though 2T-BO20 has higher-lying
HOMO energy level. This might be ascribed to the differ-
ence in Voc of devices is not only affected by their HOMO
levels but also other factors such as morphology and carrier
recombination rate [49]. The Jsc values of the devices with
the 1T-BO20, 2T-BO20 and 3T-BO20 films were 4.74, 3.39
and 4.98 mA/cm2, respectively. When 1T-BO20 based
device compared to the device based on 3T-BO20, with
increasing of thiophene rings in the polymer backbone,
Fig. 4. J–V curves of PSCs based on 1T-BO20, 2T-BO20 and 3T-BO20 in the
dark condition (inset) and under the illumination of AM 1.5G, 100 mW/
cm2.
Fig. 3. Cyclic voltammograms of polymers.

T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
2679
Table 2
Photovoltaic parameters of PSCs based on polymers:PCBM blend films.
Active layer polymer/PCBM
Thickness (nm)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rs
(X
cm2)
Rp (kX
cm2)
1T-BO20
1T-BO20a
2T-BO20
2T-BO20a
3T-BO20
3T-BO20a
150
115
125
145
120
125
4.74
8.37
3.39
5.55
4.98
4.30
0.82
0.85
0.82
0.82
0.73
0.79
42.7
48.0
61.5
52.8
52.8
51.9
1.66
3.65
1.71
2.40
1.92
1.76
10.5
3.29
7.03
2.36
9.06
12.6
10.7
57.9
6.64
29.7
11.2
24.6
a
DIO was added for additive.
the Jsc slightly increased. However, in case of 2T-BO20, with
increasing the thiophene ring in the polymer compared to
that of 1T-BO20, the Jsc decreased; even though absorption
spectrum of 2T-BO20 is red-shifted compared to that of 1T-
BO20. This is probably the result of lower molecular weight
of 2T-BO20 compared to that of 1T-BO20. It has been re-
ported that the low Jsc of solar cells fabricated using a low
molecular weight polymer originates mainly from the re-
duced charge carriers mobility in the donor phase of the
heterojunction [50,51]. The FF values of device based on
1T-BO20, 2T-BO20 and 3T-BO20 were 42.7%, 61.5% and
52.8%, respectively. We can recognize that 2T-BO20
showed the highest FF. It was possibly due to the good con-
tact between layers of device which possessed the lowest
series resistance (Rs) value compared to those of 1T-BO20
and 3T-BO20. It has been reported that FF of the devices
is more sensitive to the polymer–metal interface morphol-
ogy unlike other parameters [52]. Therefore, the low molec-
ular weight of 2T-BO20 not seemed to be much effect on FF.
The PCE of 1T-BO20, 2T-BO20 and 3T-BO20 based devices
were 1.66%, 1.71% and 1.92%, respectively. This result indi-
cates that modification polymer structures by introducing
the optimal morphology in the blend without DIO [55].
As for the device with DIO, the Voc of the device based on
1T-BO20, 2T-BO20, and 3T-BO20 were 0.85, 0.82, 0.79 V,
which are strongly related with the HOMO energy level
of the polymers.
To investigate the effects of the additives on the surface
morphology, we obtained the images by a atomic force
microscopy (AFM). Fig. 6 shows the surface topography
of BHJ films as directly measured from the same solar cell
devices before and after the incorporation of DIO referred
to in Figs. 4 and 5, respectively. As shown in Fig. 6 without
DIO, the root-mean-square roughness (r.m.s.) of the 1T-
BO20/PCBM, 2T-BO20/PCBM and 3T-BO20/PCBM films
were 7.48, 3.98 and 5.66 nm, respectively. However, when
DIO was used, the r.m.s. values of the blend films increased
to 10.72, 5.89 and 11.54 nm for the films of 1T-BO20/
PCBM, 2T-BO20/PCBM and 3T-BO20/PCBM, respectively.
The morphology of 1T-BO20/PCBM film processed with
DIO (Fig. 6b) showed larger scale phase separation and
higher surface roughness than that in the BHJ film made
without processing additive. Because of the more opti-
mized morphology, this polymer showed significant
enhancement of Jsc and FF, which lead to an increase in
PCE [55]. In case of 2T-BO20/PCBM blend film with DIO,
a significant reduction in phase separation and small do-
main size is observed in Fig. 6d. The morphology of this
blend film was more uniform than that of 2T-BO20/PCBM
blend film without DIO. Besides, there is no large phase
separation, showing good miscibility between 2T-BO20
and PCBM [57]. This leads to a large interface for exciton
thiophene rings into polymer back bone to increase p-con-
jugated length and improve planarity could lead to improve
performances of solar cell devices. The similar results have
been reported for other polymer systems [39,42,53]. Inter-
estingly, adding thiophene rings into polymer based on
2,1,3-benzothiadiazole [54] whose structures are similar
to our research showed the opposite results with ours.
The absorption wavelength was blue shifted and perfor-
mances of device decreased.
In order to further optimized photovoltaic performance,
we added 1% (by volume) DIO as additive to the solution of
the blend to examine its effect on the performance of the
devices. We found that after adding DIO, the devices based
on 1T-BO20 and 2T-BO20 exhibited significantly increased
PCEs, which are 3.65% and 2.40%, respectively. This result
revealed that DIO impacted on the morphology of active
layer and it benefits for the exciton dissociation and carrier
collection efficiency, thus leading to an increase in the
short-circuit current density as well as the device effi-
ciency [55]. Interestingly, the device of 3T-BO20 showed
a very different response to the addition of DIO compared
to 1T-BO20 and 2T-BO20. This polymer gave better perfor-
mances without adding DIO. The Jsc and PCE of the device
based on 3T-BO20 decreased to 4.30 mA/cm2 and 1.76%,
respectively. The low Jsc obtained for this polymer possibly
due to the morphology not being fully optimized through
the addition of DIO [56] or 3T-BO20 has already reached
Fig. 5. J–V curves of PSCs based on 1T-BO20, 2T-BO20 and 3T-BO20 with
1% DIO in the dark condition (inset) and under the illumination of AM
1.5G, 100 mW/cm2.

2680
T.T. Do et al. / Organic Electronics 14 (2013) 2673–2681
e
c
a
d
b
f
Fig. 6. AFM height images of polymer:PCBM blend films: (a) 1T-BO20 without DIO; (b) 1T-BO20 with 1% DIO; (c) 2T-BO20 without DIO; (d) 2T-BO20 with
1% DIO; (e) 3T-BO20 without DIO and (f) 3T-BO20 with 1% DIO (size: 5 m).
m ꢂ 5
l
l
dissociation and bicontinuous interpenetrating networks
for free charge transportation, which is well consistent
with the improved Jsc in devices [58]. In contrast, the blend
film of 3T-BO20/PCBM with DIO (Fig. 6f) exhibited rough
morphology and much larger domain size with a r.m.s. of
11.54 nm indicating large phase separation took place.
The severe phase separation in active layer reduces the
area for charge separation and increases the recombination
of excitons, leading to low photocurrent l [2].
The PCE, Voc, Jsc and FF of the device based on 3T-BO20
were 1.92%, 0.73 V, 4.98 mA/cm2 and 52.8%, respectively.
This study also revealed the different effect trend in film
morphology and performances of devices based on these
polymer:PCBM blends when using DIO as a processing
additive. Both of polymers 1T-BO20 and 2T-BO20 based
devices exhibited an enhancement of PCE from 1.66% to
3.65% and from 1.71% to 2.40%, respectively; whereas 3T-
BO20 gave the best performance without DIO. The PCE of
device based on 3T-BO20 reduced from 1.92% to 1.76% by
the addition of DIO. This result implies that number of thi-
ophene rings in polymer back bones may reduce the posi-
tive effect of additives for optimum control of the
nanomorphology.
4. Conclusions
We have demonstrated that the modification in chemi-
cal structure of conjugated polymers by adding thiophene
rings into polymer back bone could lead to change in opti-
cal, electrochemical, and photovoltaic properties. The max-
imum absorption wavelength and the band gap of the
polymers were almost the same because the polymers
exhibited similar ICT effect. However, there are some re-
ports showed the opposite results to ours. Adding thio-
phene rings into polymer back bones could lead to blue
shifted absorption [40,54]. Therefore, it is necessary to
investigate more examples to study these phenomena. Thi-
ophene is very strong electron donor and has a high-lying
ionization potential so that the HOMO energy levels of the
polymers depend on the number of thiophene rings in the
polymer. The polymer (3T-BO20) containing three thio-
phene rings in a repeat unit showed the best photovoltaic
performances compared to those of 1T-BO20 and 2T-BO20.
Acknowledgments
This work was financially supported by Converging Re-
search Center Program through the Ministry of Education,
Science and Technology (2012K001279) and Basic Science
Research Program through the National Research Founda-
tion of Korea (NRF) funded by the Ministry of Education,
Science and Technology (2013020225).
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