Synthesis and correlation between structure and photovoltaic performance of two-dimensional BDT-TPD polymers

Article abstract of DOI:10.1016/j.orgel.2016.05.007

Four BDT-TPD polymers (PA-PD) were synthesized by modifying the alkylthienyl chains on BDT, placing spacer group between BDT and TPD, and installing extended conjugated side chains on the BDT of the polymer to investigate the correlation between structure and photovoltaic performance for these polymers. The molecular weight of PA-PD polymers ranged from the highest (Mn = 80 kDa for PA) to the lowest (Mn = 7.9 kDa for PD), and their decomposition temperatures at 5% weight loss were in the range 401-435 °C. PA, PB, and PC showed similar UV-vis absorption spectra; however, PD showed much broader absorption spectrum in the entire UV-vis region, because of the extended conjugated side chains. The HOMO levels of the polymers were -5.72, -5.63, -5.48, and -5.61 eV for PA, PB, PC, and PD, respectively, indicating very low-lying HOMO energy levels. The bandgaps of these polymers were calculated and found to be in the range 1.85-1.88 eV. The theoretical calculations clearly show that the torsional angles between the alkylthienyl group and BDT unit of the simplified dimer correlated to the π-orbital delocalization, suggesting that the HOMO π-electrons of vertically aligned conjugated side chains do not delocalize well in the polymers such as PA, PB, and PC bearing high torsional angles. The optimized weight ratios of the polymer to PC₆₁BM were determined to be 1:1, 1:1.5, and 1:1 for PA, PC, and PD, respectively, and the average PCEs of the devices were 5.36%, 4.62%, and 2.74% for PA, PC, and PD, respectively, after optimization with 1,8-diiodooctane (DIO). A relatively small amount of DIO as an additive was necessary to reach the optimal PCEs of the devices, and the device incorporating PC needed only 0.5% DIO to obtain the best PCE. The AFM study reveals that the blend films after adding DIO showed much smooth morphologies, and the blend film of PA exhibited more crystalline property, as shown by the XRD analysis.

Full text of DOI:10.1016/j.orgel.2016.05.007

Organic Electronics 35 (2016) 101e111
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Synthesis and correlation between structure and photovoltaic
performance of two-dimensional BDT-TPD polymers
,
,
Nam Jeong Hong a 1, Hanok Park b 1, Myoung-Jin Baek b, Soo-Hyoung Lee b, Jo Jeong a,
Jun Hui Park a **, Kyukwan Zong a
,
, *
a Department of Chemistry Education, Div. of Science Education, Chonbuk National University, 567 Baekje-ro, Jeonju 561-756, Republic of Korea
b Department of Chemical Engineering, Chonbuk National University, 567 Baekje-ro, Jeonju 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Four BDT-TPD polymers (PAePD) were synthesized by modifying the alkylthienyl chains on BDT, placing
spacer group between BDT and TPD, and installing extended conjugated side chains on the BDT of the
polymer to investigate the correlation between structure and photovoltaic performance for these
polymers. The molecular weight of PAePD polymers ranged from the highest (Mn ¼ 80 kDa for PA) to
the lowest (Mn ¼ 7.9 kDa for PD), and their decomposition temperatures at 5% weight loss were in the
range 401e435 ꢀC. PA, PB, and PC showed similar UVevis absorption spectra; however, PD showed much
broader absorption spectrum in the entire UVevis region, because of the extended conjugated side
chains. The HOMO levels of the polymers were ꢁ5.72, ꢁ5.63, ꢁ5.48, and ꢁ5.61 eV for PA, PB, PC, and PD,
respectively, indicating very low-lying HOMO energy levels. The bandgaps of these polymers were
calculated and found to be in the range 1.85e1.88 eV. The theoretical calculations clearly show that the
Received 21 March 2016
Received in revised form
4 May 2016
Accepted 5 May 2016
Available online 20 May 2016
Keywords:
Polymer solar cells
BDT-TPD polymers
Two-dimensional structure
PCE
torsional angles between the alkylthienyl group and BDT unit of the simplified dimer correlated to the
p-
orbital delocalization, suggesting that the HOMO -electrons of vertically aligned conjugated side chains
p
do not delocalize well in the polymers such as PA, PB, and PC bearing high torsional angles. The opti-
mized weight ratios of the polymer to PC61BM were determined to be 1:1, 1:1.5, and 1:1 for PA, PC, and
PD, respectively, and the average PCEs of the devices were 5.36%, 4.62%, and 2.74% for PA, PC, and PD,
respectively, after optimization with 1,8-diiodooctane (DIO). A relatively small amount of DIO as an
additive was necessary to reach the optimal PCEs of the devices, and the device incorporating PC needed
only 0.5% DIO to obtain the best PCE. The AFM study reveals that the blend films after adding DIO showed
much smooth morphologies, and the blend film of PA exhibited more crystalline property, as shown by
the XRD analysis.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
excellent control of the electronic bandgap, resulting in tunable
light absorption for organic photovoltaic. Conjugated polymers
Polymer solar cells (PSCs) have recently received significant
attention as renewable energy sources owing to their potential in
fabricating large-area, flexible, and cost effective solar panels by roll
to roll printing process [1e4]. Donoreacceptor conjugated co-
polymers have been widely used in high-performance organic
electronic devices. When the polymers are prepared by careful
selection of the donor and the acceptor units, they may offer
incorporating alternating donor and acceptor units provide rela-
tively low bandgaps, probably because of the introduction of
intermolecular charge transfer complexes, and these low bandgap
polymers are considered as promising candidates for highly effi-
cient photovoltaic benefited with a broad range of absorption up to
near IR region [5e7]. To design high efficiency photovoltaic poly-
mers, understanding the correlation between the molecular
structures and photovoltaic properties of the conjugated polymers
is of critical importance. Recently, several methods were proposed
to improve the photovoltaic properties of conjugated polymers by
* Corresponding author.
** Corresponding author.
using
provided valuable guidelines for the design of conjugated molec-
ular structure [8e10]. Recently,
benzo[1,2-b:4,5-b0]
p-spacers or conjugated side chains, and these studies have
E-mail addresses: echem@jbnu.ac.kr (J.H. Park), kzong@jbnu.ac.kr (K. Zong).
These authors are equally contributed to this work.
1
http://dx.doi.org/10.1016/j.orgel.2016.05.007
1566-1199/© 2016 Elsevier B.V. All rights reserved.

102
N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
dithiopheneethieno[3,4-c]pyrrolo-4,6-dione (BDT-TPD) polymers
applied for PSCs have received significant attention owing to their
extraordinary high open circuit voltage (Voc) values, which are
crucial for achieving high power conversion efficiency (PCE) values
of PSCs. Several research groups reported the PCE, Voc, short circuit
current density (Jsc), and fill factor (FF) values of 4.2e6.6%,
0.85e0.91 V, 9.1e11.5 mA cmꢁ2, 0.51e0.68, respectively, for the
PSCs based on PBDT-TPD [11e14]. Replacing the O-alkoxy groups by
the alkylthienyl groups on BDT unit significantly improved the
thermal stabilities and Voc (up to 1 V) of PSCs [15e17]. More
recently, replacing the alkylthienyl groups with more extended
conjugated side chains on the BDT units of the polymers signifi-
cantly enhanced PCEs not only of the BDT-TPD polymers [18], but
also of other polymers by significantly improving the optical ab-
sorption in the range 300e500 nm [19,20]. Therefore, investigating
the correlation between structure and photovoltaic performance of
BDT-TPD polymers is of great interest to us to find the room to
further improve PCEs of BDT-TPD polymers by structural engi-
neering and then designing new polymers based on BDT-TPD
structure with high PCE. In this study as illustrated in Fig. 1, we
designed and synthesized four BDT-TPD polymers based on alkyl
group engineering on the BDTT unit, spacer engineering between
the donor and acceptor units of the polymer backbone, and side
chain engineering on the BDT unit. The correlation between the
structural and photovoltaic property of these polymers were
comprehensively studied, and the results provide thoughtful
insight for improving the performance of PSCs and designing new
polymers based on BDT-TPD for developing better performing PSCs.
ambient temperature and stirred for 2 h. The reaction mixture was
quenched with saturated NH4Cl and extracted with ether
(3 ꢂ 30 mL). The combined organic extract was washed with water,
brine, and dried over anhydrous MgSO4. After filtering off MgSO4,
the supernatant was evaporated by rotary evaporator and further
concentrated under high vacuum. The crude product was used in
the next step without further purification. To a solution of 2-(2-
ethylhexyl)-5-iodothiophene (3.2 g, 10 mmol) in diisopropyl-
amine (50 mL) was added CuI (95 mg, 0.5 mmol) and Pd(PPh3)2Cl2
(350 mg, 0.5 mmol) at room temperature. The reaction mixture was
stirred for 20 h at room temperature and refluxed for 2 h. The re-
action mixture was allowed to warm to ambient temperature,
quenched with 1 M HCl, and extracted thrice with ether. The
combined organic extract were washed with water, brine, and dried
over anhydrous MgSO4. After filtering MgSO4, the filtrate was
concentrated by rotary evaporation, and the residue was dissolved
in 2 M methanolic NaOH solution to remove TMS and stirred for
10 min. The reaction mixture was quenched with water and
extracted with ether (3 ꢂ 30 mL). The product was isolated by
column chromatography on silica gel using hexane as the eluent.
Pale yellow oil; (60%); 1H NMR (400 MHz, CDCl3, d): 0.87e0.89 (m,
6H), 11.33 (m, 8H), 1.55 (t, J ¼ 5.8, 1H), 2.73 (dd, J ¼ 6.8 Hz, 2H), 3.27
(s, 1H) 6.60 (d, J ¼ 3.9 Hz, 1H) 7.08 (d, J ¼ 3.4 Hz 1H); 13C NMR
(100 MHz, CDCl3,
41.38, 77.46, 80.39, 119.38, 124.94, 132.99, 147.15. Anal. Calcd. for
14H20S: C, 76.30; H, 9.15; S, 14.55. Found: C, 76.28; H, 9.16; S, 14.50.
d): 10.77, 14.07, 22.94, 25.46, 28.79, 32.28, 34.05,
C
2.2. General procedure for preparation of compound 2
2. Experimental
A solution of 2-(2-ethylhexyl)thiophene (11.78 g, 60 mmol) in
dry THF was slowly treated with n-BuLi (24 mL, 60 mmol, 2.5 M in
hexane) at ꢁ78 ꢀC. The reaction mixture was allowed to warm to
50 ꢀC and then stirred for 2 h. To the solution was added 4,8-
dihydrobenzodithiophen-4,8-dione (3.3 g, 15 mmol) at 50 ꢀC and
further stirred for 2 h. The reaction was cooled down to room
temperature, followed by the addition of SnCl2$H2O (27 g, 0.12 mol)
dissolved in 10% HCl (48 mL) and further stirred for 2 h at room
temperature. Ice water was poured into the reaction and extracted
with CH2Cl2 (3 ꢂ 30 mL). The combined organic extract was washed
2.1. 5-(2-ethylhexyl)-2-ethynylthiophene (D)
A solution of 2-ethylhexylthiophene (25.24 g, 0.1 mol) in dry
THF (50 mL) was cooled to ꢁ78 ꢀC under argon, and n-BuLi (62 mL,
0.11 mol, 2.5 M in hexane) was slowly added to the solution. The
reaction mixture was warmed to 50 ꢀC and stirred for 2 h. Then, the
solution was again cooled to ꢁ78 ꢀC, followed by adding a solution
of I2 in 50 mL dry THF. The reaction mixture was allowed to warm to
R1
R2
S
N
O
O
n
Spacer
Engineering
R1
S
S
R2
Alkyl Chain
S
S
S
N
Engineering
O
O
n
(Branched)
S
S
PC (PBDTTbo-DTTPDo)
S
S
R1 = 2-butyloctyl
R2 = octyl
R1
R1
PA (PBDTTeh-TPDo)
S
R
R
N
R
1 = 2-ethylhexyl
R1
R2 = octyl
S
S
O
O
R2
N
S
O
O
n
S
S
S
n
S
Side Chain
Engineering
S
R1
S
S
R2
PBDTT-TPD
(Linear)
PD (PBDTTEeh-TPDo)
N
O
R
O
n
R1 = 2-ethylhexyl
R2 = octyl
S
S
S
S
R1
PB (PBDTTn-TPDo)
S
R1 = nonyl
R2 = octyl
R1
Fig. 1. Schematic illustration for polymer structure engineering carried out in this study.

N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
103
with water, followed by brine, and then dried over anhydrous
2.8. 2,6-Bis(trimethyltin)-4,8-di(5-(2-ethylhexyl)thiophen-2-yl)-
MgSO4. After filtering off MgSO4, the solution was concentrated
under reduced pressure, and the residue was isolated by column
chromatography on silica gel.
benzo[1,2-b:4,5-b0]dithiophene (3A)
Yellow crystal; 66% yield. Known compound and also see the
reference [23].
Anal. Calc. for C40H58S4Sn2: C, 53.11; H, 6.46; S, 14.18; Sn, 26.25;
Found: C,53.49; H, 6.48, S, 14.58; Sn, 25.45.
2.3. 4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]
dithiophene (2A)
Pale yellow oil; 76% yield; 1H NMR (400 MHz, CDCl3,
d): 7.64 (d,
2.9. 2,6-Bis(trimethyltin)-4,8-di(5-(2-nonyl)thiophen-2-yl)-benzo
[1,2-b:4,5-b0]dithiophene (10B)
2H), 7.45 (d, 2H), 7.29 (d, 2H) 6.89 (d, 2H), 2.86 (d, 4H), 1.75 (m, 2H),
1.53e1.18 (m, 16H), 0.92e0.83 (m, 12H). 13C NMR (300 MHz, CDCl3,
ppm) d: 145.84,139.11,137.34,136.62,127.83,127.58,125.50,124.20,
123.56, 41.60, 34.38, 32.61, 29.05, 25.84, 23.17, 14.32, 11.06. Anal.
Calcd. for C34H42S4: C, 70.53; H, 7.31; S, 22.15. Found: C, 70.50; H,
7.25; S, 22.11.
Yellow crystals; 64.3%; 1H NMR (400 MHz, CDCl3,
d): 0.39(t,
J ¼ 28.2 Hz 9H), 0.88 (t, J ¼ 6.83 Hz, 3H), 1.27e1.45 (m, 12H),
1.75e1.83 (m, 2H), 2.92 (t, J ¼ 7.8 Hz, 2H), 6.92 (d, J ¼ 3.4 Hz, 1H)
7.31(d, J ¼ 3.4 Hz 1H) 7.69 (t, J ¼ 15.1 Hz, 1H); 13C NMR (100 MHz,
CDCl3,
d): 14.12, 22.69, 29.32, 29.43, 29.57, 30.28, 31.57, 31.90,
122.39, 124.15, 127.61, 131.01, 131.15, 137.31, 137.74, 142.22, 143.27,
146.78. Anal. Calc. for C42H62S4Sn2: C, 54.19; H, 6.70; S; 13.75, S,
25.46, found: C, 54.29; H,6.67, S, 14.36; Sn, 24.68.
2.4. 4,8-Bis(5-(2-nonyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]
dithiophene (2B)
Yellow powder; 70.6% yield; 1H NMR (400 MHz, CDCl3,
d): 0.82
(t, J ¼ 6.6 Hz, 3H), 1.22e1.37 (m, 14H), 2.84 (t, J ¼ 7.41 Hz, 2H), 6.83
2.10. 2,6-Bis(trimethyltin)-4,8-di(5-(2-butyloctyl)thiophen-2-yl)-
benzo[1,2-b:4,5-b0]dithiophene (10C)
(d, J ¼ 3.9 Hz, 1H), 7.22 (d, J ¼ 3.9 Hz, 1H), 7.38 (d, J ¼ 5.8 Hz, 1H)
7.57(d, J ¼ 5.8 Hz 1H); 13C NMR (100 MHz, CDCl3,
d): 14.12, 22.69,
29.24, 29.30, 29.38, 29.53, 30.26, 31.62, 31.89,123.42,124.07,124.25,
127.42, 127.76, 136.52, 136.97, 138.99, 147.16; HRMS (FAB) m/z Calc.
for C36H46S4: 606.2482, found: 606.2476.
Light brown oil (the product was purified by passing thorough
activated charcoal pad, because trimethyltin group was too labile to
survive against chromatography on silica gel or alumina). 70.6%
yield; 1H NMR (400 MHz, CDCl3, d): 0.39 (s, 9H), 0.88e0.91 (m, 6H),
1.29e1.37 (m, 16H), 2.04 (s, 1H), 2.86 (d, J ¼ 6.34 Hz, 2H), 6.89 (d,
J ¼ 3.41 Hz, 1H) 7.31 (d, J ¼ 3.41 Hz 1H) 7.68 (s, 1H); 13C NMR
2.5. 4,8-Bis(5-(2-butyloctyl)thiophen-2-yl)benzo[1,2-b:4,5-b0]
dithiophene (2C)
(100 MHz, CDCl3, d): 14.13, 14.17, 22.68, 23.03, 26.54, 26.69, 28.93,
Light brown oil; 84% yield; 1H NMR (400 MHz, CDCl3,
d):
29.68, 31.90, 33.05, 33.47, 34.71, 40.01, 60.37, 122.40, 125.28, 126.39,
127.51, 131.12, 137.29, 137.96, 142.20, 143.27, 145.33; FAB-HR m/z
Calc. for C48H74S4Sn2: 1018.2717, found: 1018.2724.
0.84e0.97 (m, 6H), 1.29e1.34 (m, 16H), 1.72 (m, 1H), 2.84 (d,
J ¼ 6.8 Hz, 2H), 6.87 (d, J ¼ 3.41 Hz, 1H), 7.28 (d, J ¼ 3.41 Hz, 1H)
7.44(d, J ¼ 5.8 Hz 1H) 7.63 (d, J ¼ 5.8 Hz 1H); 13C NMR (100 MHz,
CDCl3, d): 14.13, 22.69, 23.01, 26.65, 28.91, 29.66, 31.58, 31.89, 33.07,
2.11. 2,6-Bis(trimethyltin)-4,8-di(5-(2-ethylhexyl)thiophene-2-
ylethynyl)benzo[1,2-b:4,5-b0]dithiophene (10D)
33.40, 34.69, 40.02, 123.39, 124.09, 125.36, 127.42, 127.68, 136.52,
137.20, 139.01, 145.72; HRMS (FAB) m/z Calc. for
690.3421, found: 690.3425.
C42H58S4:
Yellow powder; 47% yield; 1H NMR (400 MHz, CDCl3, d): 0.47 (t,
J ¼ 0.9 Hz, 9H), 0.89e0.91 (m, 6H), 1.31e1.39 (m, 8H), 1.54 (s, 1H),
2.78 (d, J ¼ 6.8 Hz, 2H), 6.73 (d, J ¼ 3.9 Hz, 1H) 7.27 (d, J ¼ 3.4 Hz,1H)
7.68 (t, J ¼ 14.1 Hz, 1H); 13C NMR (100 MHz, CDCl3, d): 10.83, 14.12,
22.99, 25.54, 28.86, 32.38, 34.30, 41.52, 89.38, 92.26, 109.94, 120.49,
125.48, 130.87, 132.51, 138.83, 143.49, 144.36, 147.90. Anal. Calcd. for
2.6. 4,8-di(5-(2-ethylhexyl)thiophene-2-ylethynyl)benzo[1,2-b:4,5-
b0]dithiophene (2D)
Light brown oil; 60% yield; 1H NMR (400 MHz, CDCl3,
d):
C
44H58S4Sn2: C, 55.48; H, 6.14; S, 13.46; Sn, 24.92; Found: C, 55.38;
0.86e0.97 (m, 6H),1.26e1.42 (m, 8H),1.61 (t, J ¼ 5.8 Hz,1H), 2.78 (d,
J ¼ 6.3 Hz, 2H), 6.72 (d, J ¼ 3.4 Hz, 1H) 7.25 (d, J ¼ 1.7 Hz, 1H) 7.56(d,
J ¼ 5.3 Hz 1H) 7.66 (d, J ¼ 5.3 Hz, 1H); 13C NMR (100 MHz, CDCl3, d):
10.83, 14.12, 22.98, 25.53, 28.86, 32.37, 34.28, 41.51, 88.76, 92.98,
111.80, 120.13, 123.20, 125.55, 128.06, 132.65, 137.97, 139.95, 148.20;
HRMS (FAB) m/z Calc. for C38H42S4: 626.2169, found: 626.2171.
H, 6.12; S, 15.24; Sn, 23.26.
2.12. Typical procedure of the preparation of PBDTTeh-TPDo
BDTTeh (400 mg, 0.442 mmol), TPDoct-diBr2 (187 mg,
0.442 mmol), Pd2(dba)3 (6.07 mg, 1.5 mol%), and p(o-tolyl)3 (4 mg,
3 mol%) were added to a 50 mL schlenk tube and evacuated and
backfilled with argon four times. Toluene (10 mL, degassed by 4
freeze/pump/thaw cycles) was then added, and the reaction
mixture was heated to 110 ꢀC and stirred for 3 days. The reaction
mixture was then cooled to 50 ꢀC, and the large spatula tip of
diethylammoniumdiethyldithiocarbamate and 10 mL toluene was
added and stirred for 1 h. The mixture was then pipetted into
300 mL methanol, precipitating the polymer, and this was filtered
into a cellulose thimble and purified by soxhlet extraction with
methanol (1 day), acetone (1 day), hexane (1 day), dichloromethane
(1 day), and chloroform (3 h). Chloroform soluble fraction was
added to 300 mL methanol, and the solid was collected by passing
2.7. General procedure for preparation of compound 3
To a solution of 4,8-bis(5-(2-ethylhexyl)thiophene-2yl)benzo
[1,2-b:4,5-b0]dithiophene (2A, 3.4 g, 5.9 mmol) in dry THF was
added n-BuLi (5.9 mL, 14.75 mmol, 2.5 M in hexane) at ꢁ78 ꢀC. The
reaction was stirred for 1 h at ꢁ78 ꢀC and for 0.5 h at room tem-
perature. Then, the reaction was cooled to ꢁ78 ꢀC and treated with
a solution of trimethyltinchloride (3.53 g, 17.7 mmol) via a syringe.
Water (50 mL) was poured into the reaction and extracted with
ethyl acetate (3 ꢂ 30 mL). The combined organic extract was
washed with water, followed by brine and dried over anhydrous
MgSO4. After concentrating under reduced pressure, the residue
was recrystallized from ethyl acetate/methanol.
through a 20 mm pore size nylon filter paper.

104
N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
2.13. PA (PBDTTeh-TPDo)
3.2. Optical properties
Dark brown solid (lustrous when filmed); 278 mg (75% yield);
GPC: Mn ¼ 80,455, Mw ¼ 151,310, PDI ¼ 1.88. Anal. Calc. for
The optical properties of the polymers, PAePD were investi-
gated by UVevis absorption spectroscopy in dilute chloroform so-
lutions and as thin films. The detailed absorption data are listed in
Table 2. Fig. 3 shows absorption spectra of PA in solution and as thin
film. Although the lmax in solution is consistent with the previous
report, lmax as thin film is 7 nm more red-shifted than that reported
previously [16]. The absorption spectra of PB installed with linear
alkyl chains are shown in Fig. 3. No significant bathochromic shift
was observed on going from solution to the solid state, suggesting
that strong aggregation of this polymer in solution is promoted by
more orderly layered polymers with linear alkyl chain. Another
distinct characteristic of the spectrum from PB compared to that of
PA is that a shoulder peak next to the absorption maximum be-
comes much pronounced in solution and is also consistent with the
findings reported previously [16]. The structural variation in the
polymer by placing a spacer between donor and acceptor units (PC)
dramatically changed the absorption spectra, whereas the shoulder
of PA and PB became the absorption maximum in PC. Similarly as
shown in PB, no significant bathochromic shift was observed be-
tween in solution and as thin film, suggesting that a high degree of
macromolecular ordering is favored even in dilute solution. The
UVevis spectrum of PD exhibited two lmax at 367 and 563 nm and a
shoulder peak at 625 nm. The absorption maximum at the shorter
wavelength (367 nm) may attribute to the BDT unit bearing con-
jugated thienylethynyl side chain and is approximately consistent
with the previous report [18]. Two peaks at 563 and 625 nm may
attribute to the intramolecular charge transfer (ICT) between the
acceptor (TPD) and the donor (BDT) units [24].
C
50H63NO2S5: C, 69.0; H, 7.30; N, 1.61; S, 18.42, found: C 68.33; H,
7.19; N, 1.65; S, 20.73.
2.14. PB (PBDTTn-TPDo)
Dark brown solid (lustrous and porous when filmed); 285 mg
(77% yield). GPC: Mn ¼ 52,243, Mw ¼ 84,915, PDI ¼ 1.627. Anal. Calc.
for C50H61NO2S5: C, 69.16; H, 7.08 N, 1.61; S,18.46, found: C, 68.7; H,
6.98; N, 1.71; S, 17.78.
2.15. PC (PBDTTbo-TPDTo)
Dark brown solid (lustrous when filmed); 190 mg (55% yield).
GPC: Mn ¼ 24,915 Mw ¼ 54,715, PDI ¼ 2.196. Anal. Calcd. for
C
66H83NO2S7: C,69.12; H 7.2; N, 1.22; S, 19.57, found: C, 69.12; H,
7.21; N, 1.3; S 18.88.
2.16. PD (PBDTTEeh-TPDo)
A dark brown solid, 185 mg (45% yield). GPC: Mn ¼ 7557
Mw ¼ 15,747 PDI ¼ 2.084, Anal. Calcd. for C54H63NO2S5: C, 70.62; H,
6.91; N, 1.53; S, 17.46, found: C, 69.89; H,6.54; N, 1.50; S, 19.50.
3. Results and discussion
3.3. Electrochemical properties
3.1. Synthesis and characterization of the polymers
Cyclic voltammetry (CV) was used to investigate the electro-
chemical properties of the polymers. The HOMO of the polymers
was measured by anodic sweep to þ1.5 V vs. Fc/Fcþ. The LUMO
energy levels of the polymers were calculated based on the corre-
lation between the optical band gap from the absorption spectra
(film) and the HOMO level (ELUMO ¼ EHOMO þ Eg). It should be noted
that the obtained ELUMO is an approximated value because the
optical bandgap used as Eg in our calculation neglects the exciton
binding energy [25]. The CV curves and the energy levels of the
polymers PAePD are shown in Fig. 4. As shown in Fig. 4a, the CV
plots of p-doping process are provided, and they were measured
using a Pt disk electrode in the presence of 100 mM TBAPF6 in
acetonitrile at a scan rate of 100 mV/s. The onset oxidation poten-
The synthetic routes for monomers and polymers are shown in
Scheme 1. The detailed experimental procedures for the monomers
and polymers are given in the Experimental Section. BDT mono-
mers, 3AeD, were synthesized according to reported methods
[16,21e23]. Especially, for the synthesis of 3D as a new side chain, D
was synthesized from A through two steps, iodation at the 5-
position of 2-(2-ethylhexyl)thiophene and ethynylation of A-1
under the Sonogashira reaction condition. These four polymers
were prepared by employing the Stille coupling reaction with
tris(dibenzylideneaceton)dipalldium (Pd2dba3) and tri(o-tilyl)
phosphine (P(o-tol)3) as catalysts [16]. All the polymers showed
good solubility in chloroform, chlorobenzene, and 1,2-
dichlorobenzene at ambient temperature except PB. Gel perme-
ation chromatography (GPC) using monodispersed polystyrene as
the standard and chloroform as the eluent was used to measure the
number-average molecular weights (Mn) and polydispersity index
(PDI) of the polymers (Table 1). Mn of the polymers are as follows:
PA (PBDTTeh-TPDo), 80.4 KDa (PDI ¼ 1.88); PB (PBDTTn-TPDo),
52.2 KDa (PDI ¼ 1.63); PC (PBDTTbo-TPDTo), 24.9 KDa (PDI ¼ 2.19);
and PD (PBDTTEeh-TPDo), 7.6 KDa (PDI ¼ 2.08). Notably, PA
exhibited noticeably high molecular weight with a reasonable PDI
(1.88), which is considered to be highly promising for the device
stability.
tials (Фox) of PA, PB, PC, and PD were 0.92, 0.83, 0.68, and 0.81 eV,
respectively. The HOMO energy levels of the polymer solutions
were calculated by assuming the HOMO energy for the Fc/Fcþ
standard of 4.8 eV with respect to the zero vacuum level. The
relation is expressed as follows: EHOMO ¼ ꢁ(Eox þ 4.8) eV, where Eox
is the onset oxidation potential relative to the ferrocene external
standard [17]. According the equation, the HOMO levels of the
polymers were calculated to ꢁ5.72, ꢁ5.63, ꢁ5.48, and ꢁ5.61 eV for
PA, PB, PC, and PD, respectively, and are included in the energy
level diagram in Fig. 4b. The trend of low-lying HOMO energy levels
related to the BDT-TPD polymers well matched with the previous
works [16]. Interestingly, the HOMO energy level of ꢁ5.72 eV for PA
turned out even lower than that (ꢁ5.61 eV) reported previously,
suggesting that the difference of ꢁ0.11 eV of HOMO energy level is
considered to be further beneficial for achieving higher Voc in PSCs.
To the best of our knowledge, the HOMO energy level of ꢁ5.72 eV is
the lowest one among the BDT-TPD polymers. Notably, the addition
of a spacer between the BDT and TPD unit as exemplified by PC
increased the HOMO level significantly (ꢁ5.72 eV / ꢁ5.48 eV),
The thermal stability of the polymers was evaluated by ther-
mogravimetric analysis (TGA), and the plot is shown in Fig. 2. The
thermal decomposition temperature (5% weight loss) of PA, PB, PC,
and PD were 435, 434, 401, and 413 ꢀC, respectively. The thermal
stabilities of PA and PB having high molecular weight appeared to
be enhanced in comparison with those of PC and PD. This result
indicates that the polymers are thermally stable enough for
photovoltaic applications.

N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
105
O
R
R
1) A~D
S
S
S
n-BuLi, THF
1) n-BuLi, THF
Me3Sn
SnMe3
2) SnCl2 2H2O
S
S
2) Me3SnCl
S
O
R
R
2A~D
1
3A~D
C4H9
C4H9
C6H13
C2H5
C2H5
C4H9
C9H19
S
R
S
S
S
(A)
(B)
(C)
(D)
TMS
Pd(PPh3)2Cl2, CuI
1) n-BuLi, THF
2) I2, THF
I
S
S
S
diisopropylamime
D
A
A-I
C8H17
N
C8H17
N
O
O
O
O
Pd2dba3, P(o-tol)3
+
3A~D
PA~D
OR
S
toluene, 110oC
3 days
S
Br
Br
Br
Br
S
S
4
5
C2H5
C4H9
C2H5
C6H13
C4H9
C4H9
S
C9H19
S
C8H17
C8H17
N
C8H17
C8H17
N
S
S
N
N
O
O
O
O
O
O
O
O
S
S
S
S
S
S
n
n
n
S
S
n
S
S
S
S
S
S
PBDTTbo-DTTPDo
PBDTTEeh-TPDo
PBDTTeh-TPDo
S
PBDTTn-TPDo
S
S
(PA)
(PC)
(PD)
(PB)
C9H19
S
C4H9
C4H9
C2H5
C6H13
C4H9
C2H5
Scheme 1. Synthetic routes for the proposed monomers and polymers and the structures of the synthesized polymers.
Table 1
Polymerization results and thermal properties.
110
100
90
80
70
60
50
40
30
Copolymers
Yields (%)
Mn (KDa)a
Mw (kDa)a
PDI
Td (ꢀC)b
PA
PB
PC
PD
75
77
55
45
80.4
52.2
24.9
7.6
151.3
84.9
54.7
15.7
1.88
1.63
2.19
2.08
435
434
401
413
PA
PB
PC
PD
a
From gel permeation chromatography.
From thermogravimetric analysis.
b
whereas the bandgap almost remained unchanged, supporting that
decreasing the electron-deficiency of the polymer by inserting a
donor spacer can increase the HOMO energy level of the polymer.
More interestingly, PD bearing extended conjugated side chains
was found to have low-lying HOMO energy level (ꢁ5.61 eV), which
is considered as a valuable insight for designing 2-D polymers for
highly efficient PSCs.
0
100
200
300
400
500
600
700
800
Temperature(oC)
Fig. 2. TGA plots of PA, PB, PC, and PD.

106
N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
Table 2
Optical and electrochemical properties of the polymers.
Polymers
UVevis
Cyclic voltammetry
Solutiona
Film
b
d
f
ELUMOc (eV)
lmax (nm)
lonset (nm)
Eg (eV)
ELUMOc (eV)
EHOMO (eV)
ox
e
lmax (nm)
lonset (nm)
Eg (eV)
E
(V)
onset
PA
PB
PC
PD
609
628
562
367
654
660
659
664
1.90
1.88
1.88
1.87
ꢁ3.83
ꢁ3.75
ꢁ3.59
ꢁ3.74
621
626
559
371
670
659
659
662
1.85
1.88
1.88
1.87
ꢁ3.87
ꢁ3.74
ꢁ3.59
ꢁ3.74
0.92
0.83
0.68
0.81
ꢁ5.72
ꢁ5.63
ꢁ5.48
ꢁ5.61
a
In CHCl3.
Solution optical bandgap.
b
c
d
e
f
Calculated from optical bandgap and electrochemically obtained HOMO energy.
Thin film optical bandgap.
Potentials (vs Fc/Fcþ).
Solution electrochemical bandgap.
Fig. 3. Absorption spectra of PA, PB, PC, and PD in chloroform solution and as thin films.
3.4. Theoretical calculations
partially delocalized on the side chains. However, the HOMO sur-
faces of PD are well delocalized along both its backbone and side
Theoretical calculations were performed to predict the energies
and distribution of the frontier molecular orbitals by density
functional theory (DFT) using the Gaussian 09 program at the
B3LYP/6-31Gþ(d) level in the gas phase [26]. The optimized mo-
lecular geometries were confirmed to be the minimum-energy
conformations by computing vibrational frequencies at the same
level. To reduce the time and cost of calculation, long alkyl chains
were replaced by methyl group, and the polymers were simplified
to dimer. The molecular geometries of the dimers for optimization
process were referred from those reported in the literature [16,27]
and optimization was carried out starting from the planar structure
of the dimer only. The optimized molecular geometries and frontier
molecular orbitals are shown in Fig. 5, indicating that the HOMO
surfaces of PA and PB are well delocalized along its backbone and
chains. One of the reasons that p-orbitals of the side chains on PA
and PB are partially delocalized can be found that their torsional
angles to the polymer backbone are as much twisted as 47.20ꢀ and
52.75ꢀ for PA and PC, respectively, indicating that the
p-orbitals of
the backbone and the side chains may not overlap effectively for the
delocalization, because of highly twisted torsional angles. This
phenomenon is also found in the previous studies [27,28] and may
attribute to the poor absorption in the shorter wavelength of the
UVevis spectra of these BDTT polymers even if they have two-
dimensional structures. However, the torsional angle to the poly-
mer backbone for PD was calculated as 0.76ꢀ, indicating that the
side chain is coplaner with its polymer backbone. As shown in Fig. 3
(PD), the electrons in the side chains and BDT unit are well delo-
calized, and this may attribute to significantly enhanced absorption

N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
107
the solvent. The current densityevoltage (JeV) curves of the de-
vices with photovoltaic parameters such as Jsc, Voc, FF, and PCE, for
the optimization processes based on the polymer:PC61BM ratios
were collected and are provided in Figs. S1eS12. The best devices
from the three polymers, PA, PC, and PD, fabricated from chloro-
form without any additives showed the PCE values of 2.91%, 2.58%,
and 1.71%, respectively (see Figs. S1, S5, and S9). 1,8-Diiodooctane
(DIO) has been proven as an efficient and universal additive to
improve the morphology of polymers [29,30]. The introduction of
an optimal amount of DIO into the polymer:PC61BM blends
significantly enhanced the device performance. The JeV charac-
teristics of some representative PSCs based on PA, PB, and PD with
different amounts of DIO additive measured under simulated AM
1.5 G illumination (100 mW cmꢁ2) are presented in Fig. 6a, and the
devices’ photovoltaic parameters, Jsc, Voc, FF, and PCE are listed in
Table 3. The optimum amounts of additive to the blend solutions
were determined as 1.0%, 0.5%, and 1.0% for PA:PC61BM, PC:PC61BM,
and PD:PC61BM respectively. For the JeV curves of the device
incorporating PC:PC61BM as shown in Fig. 6a, the maximal device
performance was achieved upon only 0.5% addition of DIO to the
polymer blends and did not significantly change by adding more
additive. This result suggests that PC tends to easily transform into
its optimal geometric microstructure even with a small amount of
additive, because the torsional energy between the donor and
acceptor units is significantly lowered by placing the spacer to form
the polymer backbone to be more planar for effective intermolec-
ular
pep interactions. After the optimization, the PCEs of the de-
vices significantly improved to 5.36%, 4.62%, and 2.74% for PA, PC,
and PD, respectively. Notably, the addition of DIO to the poly-
mer:PC61BM blends significantly enhanced both the FF and Jsc, but
only slightly improved the Voc. The device fabricated from PA
showed the optimum PCE of 5.36% benefited by high Voc (>1 V) and
is comparable to the optimum PCE (6.17%, PBDT-T8-TPD: PC61BM)
reported previously [16]. A high Voc observed in this device is well
in accordance with that reported previously and can be expected
from the low-lying HOMO energy level of PA (ꢁ5.72 eV). As shown
in Fig. 6b and Table 3, the device fabricated from PC showed the
best of PCE (4.62%), which is lower than that from PA, mainly
because of the decreased Voc (0.87 V). Concerning the role of spacer
in the polymer, Leclerc and others reported that the copolymers
with unsubstituted thiophene spacers tend to have a low PCE value
(less than 1%), because of the bad morphology of the active layer
[31]. However, we found that the morphology of the polymer PC
with unsubstituted thiophene spacers can be improved with the
choice of proper alkyl chain on the BDT unit, and the addition of
proper additive (see the AFM results) dramatically improved the
PCE up to 4.62% from the PC based device. Moreover, less amount of
additive (0.5% of DIO) can be used with the polymer with spacers
and is desirable for long term-stability of the device. This phe-
nomenon indicates that the introduction of spacer group may
eliminate the torsional angle strain of the backbone by moving
apart the neighboring alkyl chains, helping the polymer backbone
Fig. 4. (a) Cyclic voltammograms of the drop casted polymers (PAePD) on Pt disk
electrode in the presence of 100 mM TBAPF6 in Acetonitrile at a scan rate of 100 mV/s,
the potential is referenced to the Fc/Fc, (b) HOMO and LUMO energy level diagram of
the polymers and PC61BM.
in the shorter wavelength of the UVevis spectra of PD. In addition,
the torsional angles between BDT and TPD units of the polymers are
12.37 and 1.14ꢀ for PA and PD, respectively, whereas that of be-
tween BDT and thiophene units of the polymers is 10.49ꢀ for PC.
These data indicate that the polymer backbones are not much
deviated from the planar conformation, and the
p-orbitals of the
backbone can be well delocalized by effective overlapping.
3.5. Photovoltaic properties
PSC devices were fabricated to evaluate and compare the
photovoltaic properties of three differently engineered polymers by
utilizing a conventional BHJ device structure of ITO/PEDOT:PSS/
polymer:PC61BM/LiF/Al, where PC61BM was used as the electron
acceptor. The device fabrication process was optimized by fine
adjustment of solvents, polymer:fullerene ratios, and additives. For
PSC device fabrication, various solvents such as chloroform, chlo-
robenzene, o-dichlorobenzene were screened, and chloroform was
chosen as the processing solvent owing to its good solubility for
polymers and wide variety of uses. However, PB was excluded in
device fabrication, because the best solvent for its fabrication could
not be obtained in spite of the scans of various solvents. Then,
different polymer:fullerene (D/A) weight ratios were scanned, and
the optimized weight ratios of the polymer and PC61BM were
determined to be 1:1,1.5, and 1:1 for PA, PC, and PD, respectively, at
a polymer concentration of 10 mg/mL (1 wt%) using chloroform as
to be more planar for more efficient polymer pep stacking. Finally,
we turned our attention to PD polymer equipped with an extended
conjugated side chain on the BDT unit. The device fabricated from
PD/PC61BM (1:1) blend with 1% DIO showed the optimum PCE of
2.74% with a Voc, Jsc, and FF of 0.98 V, 6.34 mA/cm2, and 44.03%,
respectively. The PCE of 2.74% appears to be much lower than those
obtained from PA and PC, mainly because of much decreased Jsc and
FF. This low PCE can be attributed from the deteriorated polymer
morphology, probably because of the low solubility and low mo-
lecular weight of PD.
External quantum efficiency (EQE) spectra of the devices by
varying the amount of DIO were measured to evaluate the photo-
responses of the polymers and further confirm the accuracy of the

108
N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
Fig. 5. Optimized molecular structures and frontier molecular orbitals of the dimers for PA, PC, and PD.
Fig. 6. (a) Current densityevoltage (JeV) curves and (b) EQE curves of the PSCs based on polymer:PC61BM blend without DIO (closed symbols) and with DIO (open symbols).
relatively high photo-to-current conversion efficiency over the
wavelength range 340e650 nm, with monochromatic EQE values
in the range ~40e60%. The device from the optimized PC:PC61BM
blend with 0.5% DIO showed a similar EQE curve as that from the PA
blend. As shown in Fig. 7c, the device from the optimized
PD:PC61BM blend with 1% DIO showed relatively low photo-
conversion efficiency over the entire wavelength range in com-
parison with those from PA and PC, with the EQE values in the
range ~38e45%. Although the device incorporating PD showed low
EQE values, likely attributed to its lower Jsc value, the EQE values
were more evenly distributed over the entire wavelength range and
can be understood from its absorption spectrum.
Table 3
Photovoltaic properties of PSCs based on polymer:PC61BM blend without DIO and
with DIO.
Blend
DIO
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PA:PC61BM (1:1)
Without
With 1%
Without
With 0.5%
Without
With 1%
1.01
1.01
0.88
0.87
0.95
0.98
6.08
8.74
6.50
8.85
4.93
6.34
47.43
60.48
45.07
60.29
36.68
44.03
2.91
5.36
2.58
4.62
1.71
2.74
PC:PC61BM (1:1.5)
PD:PC61BM (1:1)
photovoltaic measurements. Fig. 6b shows the EQE curves of the
PSCs based on PA:PC61BM (1:1), PC:PC61BM (1:1.5), and PD:PC61BM
(1:1) by varying the amount of DIO. The EQE curves of the devices
exhibited broad spectral responses ranging from 300 to 700 nm,
strongly correlating with their absorption spectra (Fig. 3). The de-
vice from the optimized PA:PC61BM blend with 1% DIO showed a
3.6. Morphology study by AFM
Atomic force microscopy (AFM) was used to further characterize
the surface morphology and phase separation of the poly-
mer:PC61BM blend films to investigate the relationship between

N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
109
Fig. 7. Tapping-mode AFM images: topography images of the blend films of PA: PC61BM (a), PC: PC61BM (c), and PD: PC61BM (e) without DIO (left) and the blend films of PA:
PC61BM (b, 1.0%), PC: PC61BM (d, 0.5%), and PD: PC61BM (f, 1.0%) with DIO (right).
the morphology and the photovoltaic characteristics of the devices.
DIO as an additive is believed to allow a slower crystallization
process during spin-coating, thus improving morphology through
enhanced intermolecular ordering and well-developed phase sep-
aration. Fig. 7 shows the AFM images of the surface morphologies of
the each solar cell device without and with DIO. As shown in
Fig. 7(a, c, and e), the root-mean-square roughness (rms) of the
blend films of PA:PC61BM, PC:PC61BM, and PD:PC61BM without DIO
were 2.0, 2.7, 1.2 nm, respectively. In Fig. 7(a), the nanofibril
structure was observed in PA:PC61BM film indicating a crystallinity
of PA polymer with (100) and (010) peaks as shown in Fig. 8. The
nanofibril structure with a high crystallinity is help to achieve high
performance by improving charge transport in organic solar cells
[34]. As shown in the images in Fig. 7c, although the blend film of
PC:PC61BM shows regularly aggregated domains (~250 nm) like
tulip buttons, which is distinct morphological features compared to
the other two films, the Jsc of PC:PC61BM device has a higher value
than those of other devices, because of the high absorption prop-
erty of PC polymer in the range 350e600 nm. PD:PC61BM film
showed smaller granular domain size (~100 nm) than that of
PC:PC61BM film, however, this device shows a lower Jsc and FF than
those of other polymers due to the amorphous nature (Fig. 6) and
low molecular weight (Table 1) of PD polymer, which is mainly
affected in charge transport and light absorption [35,36]. When an
additive was added into the blends of polymer:PC61BM, the surface
roughness decreased to 1.8 and 2.3 for PA:PC61BM and PC:PC61BM,
respectively, as evidenced by significantly improved PCEs of the
devices fabricated from those blends. Unusually, the surface
roughness of PD:PC61BM blend with DIO increased from 1.2 to 2.7
despite of some improvement in the photovoltaic performance. To
our surprise, the morphology of the blend of PA:PC61BM incorpo-
rated with DIO exhibited better crystalline properties, as shown in
the blend film of PA:PC61BM with DIO (Fig. 7b). Recently, Cho and
coworkers reported that BHJ-PSCs composed of highly crystalline
polymer nanowires (PNWs) yielded an outstanding improvement
in the PCE compared to the PCE of a device prepared without
nanowires [32,33]. They also mentioned that only a few polymers
formed crystalline polymer nanowires (PNWs) in the processing of
the blend film. Therefore, we believe that this polymer would be a
promising candidate as a polymer to be processed as crystalline
polymer nanowires (PNWs). In PC:PC61BM and PD:PC61BM films,
the additive contributed to forming distinct nanoscale phase sep-
aration by removing the grain boundary, thus increasing the Jsc and
PD
PC
PA
5
10
15
20
25
2 Theta(Degree)
Fig. 8. XRD patterns of PA, PC, and PD films.

110
N.J. Hong et al. / Organic Electronics 35 (2016) 101e111
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3.7. XRD analysis
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q
¼ 4.05 and 4.09ꢀ for PA and PC polymers,
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extended conjugated side chains to investigate the correlation be-
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mers. All the polymers showed high thermal stabilities, broad
absorption spectra, and noticeably enhanced absorption in the
entire wavelength for PD, because of the extended conjugated side
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of PA and PC are quite twisted against the polymer backbone,
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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