Synthesis and characterization of new indeno[1,2-b]indole-co- benzothiadiazole-based π-conjugated ladder type polymers for bulk heterojunction polymer solar cells

Article abstract of DOI:10.1016/j.polymer.2013.07.004

We have developed a new ladder type, tetrafused, π-conjugated building block with indeno[1,2-b]indole (II) and extended the π-conjugation of the indeno[1,2-b]indole ring by incorporating a thiophene π-spacer in the 2nd and 8th positions to attain dithioph

Full text of DOI:10.1016/j.polymer.2013.07.004

Polymer 54 (2013) 4883e4893
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of new indeno[1,2-b]indole-
co-benzothiadiazole-based
p-conjugated ladder type polymers
for bulk heterojunction polymer solar cells
Pachagounder Sakthivel a, Ho Sung Song a, Nallan Chakravarthi a, Jae Wook Lee b,
,
Yeong-Soon Gal c, Sungu Hwang d, Sung-Ho Jin a
*
a Department of Chemistry Education, Graduate Department of Frontier Materials Chemistry, and Institute for Plastic Information and Energy Materials,
Pusan National University, Busan 609-735, Republic of Korea
b Department of Chemistry, Dong-A University, Busan 604-714, Republic of Korea
c Polymer Chemistry Lab, College of General Education, Kyungil University, Hayang 712-701, Republic of Korea
d Department of Applied Nanoscience, Pusan National University, Miryang 627-706, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed a new ladder type, tetrafused, p-conjugated building block with indeno[1,2-b]indole
Received 23 April 2013
Received in revised form
22 June 2013
Accepted 2 July 2013
Available online 9 July 2013
(II) and extended the
p-conjugation of the indeno[1,2-b]indole ring by incorporating a thiophene p-
spacer in the 2nd and 8th positions to attain dithiophene-substituted indeno[1,2-b]indole (IIDT). The two
newly synthesized donor monomers II and IIDT were polymerized with 2,1,3-benzothiadiazole (BT)
acceptor to obtain poly[4-(5-ethyl-10,10-dioctyl-5,10-dihydroindeno[1,2-b]indol-2-yl)benzo[c][1,2,5]
thiadiazole] (PIIBT) and poly[4-(5-(5-ethyl-10,10-dioctyl-8-(thiophen-2-yl)-5,10-dihydroindeno[1,2-b]
indol-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PIIDTBT). The photophysical studies of PIIBT and
PIIDTBT reveal the bandgaps 1.99 and 1.75 eV, respectively. Both the polymers showing a deep-lying
highest occupied molecular orbital (HOMO) energy levels wꢀ5.37 eV. The hole mobilities of PIIBT
(4.03 ꢁ 10ꢀ6 cm2/V s) and PIIDTBT (1.94 ꢁ 10ꢀ4 cm2/V s) were studied by the space charge limited
Keywords:
Indeno[1,2-b]indole
Ladder type
Bulk heterojunction polymer solar cells
current (SCLC) method. Owing to the incorporation of the thiophene moiety as a
p-spacer, PIIDTBT
displayed extended -electron conjugation and improved intermolecular interaction in the polymer
p
pep
backbone. The bulk heterojunction polymer solar cells (BHJ PSCs) fabricated using PIIBT:PC71BM (1:3,
1.5 wt%) exhibited a power conversion efficiency (PCE) of 1.23%, which was enhanced to 3.72% for the BHJ
PSCs based on PIIDTBT:PC71BM (1:3, 1.5 wt%) blend. The high hole mobility of PIIDTBT is in good
agreement with its high short-circuit current density (Jsc) and fill factor (FF). This is the first report of an
enhanced PCE using indeno[1,2-b]indole derivative as a donor material in BHJ PSCs.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
photovoltaic properties when blend with (6,6)-phenyl-C61-butyric
acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl
The bulk heterojunctionpolymer solar cells (BHJ PSCs) consisting
of an electron donating -conjugated polymer and electron
accepting fullerene derivatives have an enormous industrial and
ester (PC71BM) acceptor than the bench mark structure, poly(3-
hexylthiophene) (P3HT)/PCBM system [3,4]. By implementing this
approach, a power conversion efficiency (PCE) up to 9% has been
reported [5e12].
p
scientific applications [1,2]. The most powerful approach for de-
signing
p
-conjugated polymers is arranging electron-rich (donor)
-conjugated
In order to deepen the highest occupied molecular orbital
(HOMO)energy level in donor/acceptorcopolymeric systems, which
causes to improve the open-circuit voltage (Voc), several re-
and electron-deficient (acceptor) segments along the
polymer backbone. This kind of donor polymers have superior
p
search groups have intensively concentrated on p-conjugated fused
ring systems of fluorenes, carbazoles, cyclopentadithiophenes, and
dibenzothiophenes [1,2,6,10,13] as donor moieties. Among the re-
ported acceptor moieties so far, 2,1,3-benzothiadiazole (BT) and
* Corresponding author. Tel.: þ82 51 510 2727; fax: þ82 51 581 2348.
E-mail addresses: jlee@donga.ac.kr (J.W. Lee), shjin@pusan.ac.kr (S.-H. Jin).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.004

4884
P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
its derivatives have gained intensive attention because of strong
electron-accepting ability and synthetic ease. More importantly,
when this BT unit was introduced in between two thiophene
rings, it impose the planarity to the backbone of the corre-
sponding polymer due to hydrogen bonding between the N in BT
and 3H in thiophene [2,13e16]. If the donor/acceptor units in the
copolymeric systems is planar in nature resulted to increase the
indeno[1,2-b]indole are linked via vinyl units. The trans position of
the vinyl carbons is cyclized to the ortho position of the phenyl
group via sp3 carbon and nitrogen bridge. The vinylenic double
bond encourages
p-electron delocalization in the indeno[1,2-b]
indole ring. The alkyl chain (octyl) is introduced on the sp3 carbon
of the indene ring to enhance the solubility which may lead to
improve device performance. We extend the
p-conjugation of the
p
e
p
stacking between the backbone, the hole mobility, short-
backbone by using a thiophene -spacer attached in the 2nd and
p
circuit current density (Jsc) and fill factor (FF) will also be
improved [6,10]. Collectively, upon introduction of these fused
rings systems as donor and BT based moieties as acceptors re-
8th positions. The synthesized derivative of indeno[1,2-b]indole
donor is polymerized with BT acceptor by Suzuki polymerization
to generate poly[4-(5-ethyl-10,10-dioctyl-5,10-dihydroindeno[1,2-
b]indol-2-yl)benzo[c] [1,2,5]thiadiazole] (PIIBT) and poly[4-(5-(5-
ethyl-10,10-dioctyl-8-(thiophen-2-yl)-5,10-dihydroindeno[1,2-b]
indol-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PIIDTBT).
PIIDTBT exhibits a deep-lying HOMO of ꢀ5.37 eV and an LBG of
1.75 eV, with a maximum PCE of 3.72%.
sults to increase the planarity of corresponding
polymer.
p-conjugated
Carbazole and fluorene are considered as the best example for
producing tricyclic fused ring systems that was used in plenty of
attractive applications such as polymer light-emitting diodes and
PSCs [2,17e20]. Noticeably, the fluorene and carbazole-based poly-
mers BisDMOePFDTBTand PCDTBT deliver deep-lying HOMO energy
levels, high Voc, high hole mobility, and achieved PCEs of 4.5% and
6.1%, respectively [21,22]. Currently, many research groups have
2. Experimental section
2.1. Materials
focused on constructing more fused
p-conjugated ring systems
(ladder type donor) to tune the electronic energy levels of se-
miconducting polymers. Suh et al. reported indenoindene-based
tetracycle fused ring systems containing polymers of BT and dime-
thylbenzimidazole (MBI) and the BT-based polymers showed a deep-
lying HOMO energy level of ꢀ5.60 eV [23,24]. Indolocarbazole-based
ladder type polymers possess higher red shift (w100 nm) in film
state, which is attributed to the planarization of the aryl rings,
implying high charge mobility, and achieved Jsc of 9.17 mA/cm2 [25].
All the reagents and chemicals were purchased from Sigmae
Aldrich Chemical Co. Ltd and TCI, and were used without further
purification unless otherwise noted. Moisture sensitive reactions
were conducted in the presence of nitrogen atmosphere. Equipment
like glassware, syringes, and magnetic bars for polymerization were
carefully dried at 100 ꢂC for 20 h before use. Tetrahydrofuran (THF)
was distilled over sodium and benzophenone. PC71BM was pur-
chased from SES Research Co. Ltd.
In p-conjugated ladder type, heptacyclic fused ring systems based on
fluorene, replacementof bulkyaryl groupsbyflexiblealiphaticgroups
showed better properties like improved hole mobility, enhanced
absorption intensity, reduced optical bandgap, a deep-lying HOMO
energy level of ꢀ5.30 eV, and high PCE of 7% [26,27].
2.2. Measurements
1H and 13C NMR spectra were recorded on a Varian Mercury Plus
600 MHz spectrometer in CDCl3 using tetramethylsilane as an in-
ternal reference. The chemical shifts were accounted in ppm
related to the singlet of CDCl3 at 7.26 and 77 ppm and CD3COCD3 at
2.09 and 30.60 ppm for 1H and 13C NMR, respectively. The UVe
visible spectra were recorded with JASCO V-570 spectrophotome-
ters at room temperature. The absorption spectra were measured
by 10ꢀ5 mol/L chloroform (CF) solution and polymer thin films spin
coated on glass substrates. The molecular weight and polydis-
persity index of the polymers were determined by gel permeation
Yongfang Li et al. reported a series of p-conjugated polymers of
indacenodithiophene (IDT) donor moiety with different acceptor
units. They described that bis(thiophene-2-yl)-BT (DTBT) acceptor
containing PIDTeDTBT showed a deep-lying HOMO energy level
of ꢀ5.24 eV, a high hole mobilityof 2.24 ꢁ10ꢀ3 cm2/V s, and a superior
Jsc of 13.27 mA/cm2 [28]. Cheng et al. reported ladder type, carbazole-
dicyclopentathiophene (CDCT-C8)-based polymers PCDCTBT-C8 and
PCDCTDTBT-C8BT containing BT and DTBT as acceptors, respectively.
The BT and DTBT facilitate intramolecular charge transfer (ICT) be-
tween donor and acceptor units and the resulting polymers produced
low bandgap (LBG), lower-lying lowest unoccupied molecular orbital
(LUMO) energy levels, and good photovoltaic properties [29]. In
addition, a variety of fluorene- and carbazole-based ladder type
polymers having BTas an acceptor were reported with high PCE [30e
35]. Gordon and coworkers deeply investigated the electronic struc-
chromatography (GPC) using PL gel 5 mm MLXED-C column on an
Agilent 1100 series liquid chromatography system with THF as an
eluent and calibrated with polystyrene standards. Thermal analysis
was carried out on a Mettler Toledo thermogravimetric analysis/
simultaneous differential thermal analysis (TGA/SDTA) 851e, dif-
ferential scanning calorimetry (DSC) 822e analyzer under N2 at-
mosphere at a heating rate of 10 ꢂC/min. The cyclic voltammetry
(CV) studies were carried out with a CHI 600C potentiostat (CH
Instruments) at a scan rate of 100 mV/s in 0.1 M solution of tetra-
butylammonium tetrafluoroborate in o-dichlorobenzene (ODCB).
A platinum wire was used as the counter electrode and an Ag/
AgCl electrode as the reference electrode. The HOMO energy levels
tures and photophysical properties of the thiophene p-spacer incor-
porated between the electron-rich carbazole donor unit and the
electron-deficient BT acceptor unit by resonance Raman, transient
absorption spectroscopy, and density functional theory (DFT) calcu-
lations. They noticed that the thiophene
the extended -conjugation but also the planarity of the polymer
backbone [36].
p-spacer not only increases
p
of the
HOMO ¼ -eV (E
p-conjugated polymers were calculated using the equation
onset
ꢀ E1/2 ferrocene þ 4.8) [31]. Where the E1/2
¼
ox
Though fluorene- and carbazole-based polymers have been
deeply investigated so far in the literature, the synergetic effect of
both carbazole- and fluorene-containing polymers has not yet
been explored in BHJ PSCs. Therefore, the present study examines
the effect of the new building block indeno[1,2-b]indole as a
donor moiety, when polymerized with efficient acceptor systems
like BT.
0.46 V was calculated from the oxidation and reduction potential of
ferrocene.
2.3. Synthesis of monomers and polymers
2.3.1. Synthesis of 5,10-dihydroindeno[1,2-b]indole (1)
5,10-Dihydroindeno[1,2-b]indole was synthesized according to
a modified version of the reported procedure [37]. Indanone
(16.3 g, 123 mmol) was dissolved in 80 mL of absolute ethanol,
In this work, we report indeno[1,2-b]indole derivatives, which
possess a fluorene and carbazole skeleton. Two phenyl units of

P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
4885
after which phenyl hydrazine (12.1 mL, 123 mmol) and a few drops
of acetic acid were added. The reaction mixture was refluxed for
15 min and then cooled to room temperature. The intermediate
phenylhydrazone derivative was formed as light yellow needles,
was filtered out via vacuum filtration and then used without
further purification. The above solid was dissolved in 250 mL of
conc. HCl and then refluxed at 100 ꢂC for 10 min. After the solution
was cooled to room temperature and poured into ice water, the
product precipitated out as a brown color solid. The crude solid
was recrystallized in acetone to yield product as a light brown
solid (17.17 g, 68%). mp 260e261 ꢂC. 1H NMR (600 MHz, CDCl3):
DMSO (10 mL) and added to the reaction mixture, which was then
stirred at 80 ꢂC for 1 h and 1-bromooctane (2.95 mL, 15.3 mmol)
was added dropwise. The mixture was further stirred at 90 ꢂC for
1 h under N2 atmosphere. The reaction mixture was poured into
crushed ice and extracted with EA (2 ꢁ 300 mL). The combined
organic layers were washed with 2% aq. NaHCO3 solution (300 mL),
brine (300 mL), and dried over anhydrous MgSO4. The solvent was
removed under reduced pressure and the residue was purified by
column chromatography on silica gel (hexane:MC, 8:2 v/v) to
afford white solid 4 (2.6 g, 83%). mp 87e89 ꢂC. 1H NMR (600 MHz,
CDCl3):
d (ppm) 7.46e7.47 (d, 1H), 7.44 (d, 2H), 7.35e7.36 (d, 1H),
d
(ppm) 8.28 (br, 1H, sp3 NH), 7.62e7.63 (d, 1H), 7.52e7.53 (d, 1H),
7.24e7.29 (m, 2H), 4.36e4.44 (q, 2H), 2.59e2.64 (t, 2H), 2.01e2.07
(t, 2H), 1.45e1.47 (t, 3H), 1.13e1.17 (m, 4H), 1.02e1.11 (m, 16H),
0.77e0.79 (t, 6H), 0.64e0.66 (m, 2H), 0.47e0.49 (m, 2H). 13C NMR
7.44e7.45 (d, 1H), 7.41e7.42 (d, 1H), 7.30e7.33 (t, 1H), 7.20e7.22 (t,
1H), 7.14e7.19 (m, 2H), 3.72 (s, 2H, sp3 CH2). 13C NMR (600 MHz,
CDCl3):
d
(ppm) 148.04, 143.54, 140.87, 135.23, 126.76, 125.71,
(600 MHz, CDCl3): d (ppm) 156.70, 144.72, 142.28, 142.18, 133.12,
125.00, 124.96, 121.94, 119.19, 117.51, 112.27, 30.57.
126.33, 126.08, 122.91, 117.89, 114.17, 113.69, 113.63, 113.58, 112.08,
111.95, 52.78, 39.96, 39.62, 39.46, 31.99, 29.40, 29.33, 23.99, 22.80,
15.64, 14.27.
2.3.2. Synthesis of 5-ethyl-5,10-dihydroindeno[1,2-b]indole (2)
5,10-Dihydroindeno[1,2-b]indole (30.5 g, 148.6 mmol) was dis-
solved in 200 mL of THF and stirred at 0 ꢂC for 10 min, after which
NaH (7.13 g, 297.20 mmol) was added. The green mixture was
stirred at 0 ꢂC for 2 h. 1-Bromoethane (19.43 g, 178.32 mmol) was
dissolved in 50 mL of THF and slowly added to the reaction mixture.
This mixture was refluxed at 80 ꢂC for 24 h under N2 atmosphere,
and then poured into crushed ice in order to separate the organic
phase. The aqueous layer was extracted with ethyl acetate (EA)
(2 ꢁ 300 mL). The combined organic layers were washed with 2%
aq. NaHCO3 solution (300 mL) and brine (300 mL). The collected
organic layer was dried over anhydrous MgSO4 and the solvent was
removed under reduced pressure. The residue was then purified by
column chromatography on silica gel (hexane:methylene chloride
(MC), 8:2 v/v) to give white solid 2 (28 g, 81%). mp 133e135 ꢂC. 1H
2.3.5. Synthesis of 5-ethyl-10,10-dioctyl-2,8-di(thiophen-2-yl)-
5,10-dihydroindeno[1,2-b]indole (5)
2,8-Dibromo-5-ethyl-10,10-dioctyl-5,10-dihydroindeno[1,2-b]
indole (4.5 g, 7.31 mmol) was dissolved in toluene (30 mL) and
stirred at room temperature for 10 min under N2 atmosphere.
Pd(PPh3)4 (4 mol%, 340 mg) was added to the reaction mixture
which was then purged with N2 atmosphere for 10 min. Tribu-
tyl(thiophen-2-yl)stannane (7.64 g, 20.47 mmol) was slowly added
and the resulting mixture was then stirred at 90 ꢂC overnight
under N2 atmosphere. The reaction mixture was poured into
crushed ice and extracted with EA (3 ꢁ 300 mL). The combined
organic layers were washed with 2% aq. NaHCO3 solution
(300 mL) and brine (300 mL), and dried over anhydrous MgSO4.
The solvent was removed under reduced pressure and the residue
was purified by column chromatography (hexane:MC, 8:2 v/v) to
furnish semi solid 5 (3.16 g, 69%). 1H NMR (600 MHz, CD3COCD3):
NMR (600 MHz, CDCl3):
d (ppm) 7.67e7.69 (d, 1H), 7.61e7.62 (d,
1H), 7.57e7.58 (d, 1H), 7.42e7.43 (d, 1H), 7.37e7.40 (t, 1H), 7.24e
7.27 (m, 2H), 7.18e7.20 (t, 1H), 4.47e4.51 (q, 2H), 3.74 (s, 2H), 1.52e
1.54 (t, 3H). 13C NMR (600 MHz, CDCl3):
d
(ppm) 148.41, 144.05,
d (ppm) 7.88e7.89 (d, 1H), 7.67e7.68 (d, 1H), 7.59e7.60 (dd, 1H),
140.94, 135.51, 126.77, 125.80, 124.75, 124.39, 121.31, 119.68, 119.31,
117.79, 117.77, 109.88, 39.53, 30.34, 16.03.
7.51e7.52 (dd, 1H), 7.39e7.40 (d, 1H), 7.33e7.34 (m, 2H), 7.30e7.32
(t, 1H), 7.22e7.23 (d, 1H), 7.21e7.22 (t, 1H), 7.17e7.18 (d, 1H), 7.11e
7.13 (t, 1H), 4.68e4.71 (q, 2H), 1.71e1.73 (m, 2H), 1.56e1.58 (m,
2H), 1.50e1.53 (t, 3H), 0.95e1.12 (m, 20H), 0.77e0.79 (t, 6H), 0.63e
0.66 (m, 2H), 0.35e0.39 (m, 2H). 13C NMR (600 MHz, CDCl3):
2.3.3. Synthesis of 2,8-dibromo-5-ethyl-5,10-dihydroindeno[1,2-b]
indole (3)
5-Ethyl-5,10-dihydroindeno[1,2-b]indole (15 g, 64.29 mmol)
was dissolved in 100 mL of CF and stirred at 0 ꢂC for 10 min, after
which bromine (6.65 mL, 129.87 mmol) dissolved in CF (50 mL)
was added dropwise. The mixture was stirred at 0 ꢂC for 1 h and
then stirred further at room temperature overnight. The reaction
mixture was quenched with Na2CO3 solution. The aqueous layer
was extracted with MC (2 ꢁ 300 mL). The combined organic
layers were washed with 2 N HCl (300 mL), NaHCO3 (300 mL),
and brine solution (300 mL). The organic layer was dried over
anhydrous MgSO4 and the solvent was removed under reduced
pressure. The residue was then purified by column chromatog-
raphy on silica gel (hexane:MC, 8:2 v/v) to furnish white solid 3
(15.50 g, 61%). mp 146e148 ꢂC. 1H NMR (600 MHz, CDCl3):
d (ppm) 157.16, 145.73, 144.99, 143.62, 141.61, 134.66, 127.83,
127.62, 127.01, 126.86, 126.43, 125.66, 125.07, 123.25, 122.96,
122.66, 122.00, 117.63, 107.20, 52.86, 39.65, 38.97, 32.01, 29.56,
29.51, 24.09, 22.79, 15.81, 14.28.
2.3.6. Synthesis of 2,8-bis(5-bromothiophen-2-yl)-5-ethyl-10,10-
dioctyl-5,10-dihydroindeno-[1,2-b]indole (6)
To a stirred solution of 5-ethyl-10,10-dioctyl-2,8-di(thiophen-2-
yl)-5,10-dihydroindeno[1,2-b]indole (3.2 g, 5.14 mmol) in 10 mL of
dimethylformamide (DMF), N-bromosuccinimide (NBS) (2.74 g,
15.43 mmol) in DMF (5 mL) was added dropwise at 0 ꢂC. The green
color solution was stirred at room temperature under N2 atmo-
sphere overnight. The reaction mixture was poured into ice water
and extracted twice with MC (200 mL). The combined organic
layers were washed with water, 2 N HCl solution, and brine, after
which the organic extract was dried over anhydrous MgSO4. The
solvent was removed by rotary evaporator and the crude liquid was
purified by column chromatography on silica gel (hexane:MC, 9:1
v/v) to obtained greenish yellow semi solid 6 (1.64 g, 41%). 1H NMR
d
(ppm) 7.51e7.53 (t, 2H), 7.38 (d, 2H), 7.34e7.37 (t, 1H), 7.24e
7.27 (t, 1H), 4.27e4.31 (q, 2H), 3.79 (s, 2H), 1.43e1.45 (t, 3H). 13C
NMR (600 MHz, CDCl3): (ppm) 148.42, 144.06, 141.21, 141.17,
d
134.22, 129.06, 126.90, 123.76, 120.81, 118.93, 114.22, 113.84,
111.98, 111.89, 39.99, 31.28, 15.87.
2.3.4. Synthesis of 2,8-dibromo-5-ethyl-10,10-dioctyl-5,10-
dihydroindeno[1,2-b]indole (4)
2,8-Dibromo-5-ethyl-5,10-dihydroindeno[1,2-b]indole (2 g,
5.1 mmol) was dissolved in DMSO (20 mL) and heated at 80 ꢂC for
10 min. Sodium tert-butoxide (1.47 g, 15.3 mmol) was dissolved in
(600 MHz, CD3COCD3): d (ppm) 7.77 (s, 1H), 7.71e7.72 (d, 1H),
7.68e7.69 (d, 1H), 7.32e7.36 (m, 2H), 7.26e7.27 (d, 1H), 7.24e7.26
(t, 1H), 7.19e7.20 (d, 2H), 7.10e7.11 (d, 1H), 7.08e7.09 (d, 1H), 4.68e
4.71 (q, 2H), 1.65e1.76 (m, 4H), 1.49e1.51 (t, 2H), 1.10e1.14 (m, 4H),
0.92e1.09 (m, 16H), 0.76e0.79 (t, 6H), 0.63e0.65 (m, 2H), 0.30e

4886
P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
0.34 (m, 2H). 13C NMR (600 MHz, CD3COCD3):
d
(ppm) 157.80,
dried at room temperature for 1 h. Lithium fluoride (LiF) (0.5 nm)
and aluminum (Al) cathodes (100 nm) were deposited on top of
the active layer under vacuum less than 5.0 ꢁ 10ꢀ6 torr to yield an
active area of 9 mm2 per pixel. The evaporation thickness was
controlled by a quartz crystal sensor. The film thickness was
146.87, 143.76, 142.95, 142.18, 141.97, 135.15, 134.93, 128.75, 127.44,
126.54, 126.46, 126.32, 124.32, 124.21, 123.31, 119.12, 111.57, 111.50,
106.82, 53.53, 40.24, 39.47, 32.52, 30.54, 30.13, 29.98, 24.79, 23.31,
16.06, 14.39.
measured with an a-Step IQ surface profiler (KLA Tencor, San Jose,
2.3.7. Synthesis of poly[4-(5-ethyl-10,10-dioctyl-5,10-
CA). The performance of the BHJ PSCs were measured using cali-
brated air mass (AM) 1.5G solar simulator (OrielÒ Sol3AÔ Class AAA
solar simulator, models 94043A) with a light intensity of 100 mW/
cm2 adjusted using a standard PV reference cell (2 cm ꢁ 2 cm
monocrystalline silicon solar cell, calibrated at NREL, Colorado, USA)
and a computer controlled Keithley 236 source measure unit. The
incident photon to current conversion efficiency spectrum was
measured using OrielÒ IQE-200Ô. All device fabrication procedures
and measurements were carried out in air at room temperature.
dihydroindeno[1,2-b]indol-2-yl)benzo[c] [1,2,5]-thiadiazole] (PIIBT)
Suzuki polymerization condition was used for the synthesis of
novel donoreacceptor type polymers [15,38,39]. A 100 mL three
neck flask was charged with 2,8-dibromo-5-ethyl-10, 10-dioctyl-
5,10-dihydroindeno[1,2-b]indole (4) (615 mg, 1 mmol), 4,7-bis(4,4,
5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]thiadiazole
(388 mg, 1 mmol), K2CO3 (4 N, 10 mL), and Pd(PPh3)4 (4 mol%,
60 mg) in dry THF (25 mL) and the resultant mixture was degassed.
The polymerization mixture was refluxed at 80 ꢂC for 36 h under N2
atmosphere. The solvent was concentrated to 5 mL and the polymer
was precipitated in methanol, then collected by filtration and
washed with water followed by methanol. The polymer was further
purified via Soxhlet extraction with methanol, acetone, hexane, and
CF. The CF fraction was evaporated and reprecipitated in methanol.
The polymer was filtered and washed with methanol and acetone,
and dried in vacuo to give PIIBT as a red solid (300 mg, 51%). 1H
2.5. Hole mobility studies
The hole mobilities of the polymers were measured by space
charge limited current (SCLC) method using the device structure
ITO/PEDOT:PSS/PIIBT (or) PIIDTBT:PC71BM/Al. For hole mobility
measurements, SCLC is described by the equation [40].
2
3
NMR (600 MHz, CDCl3):
d
(ppm) 8.09e8.13 (m, 2H), 8.00e8.05 (m,
3 m
V =8L
J ¼ 9
3
r
0
4H), 7.92e7.97 (m, 2H), 4.36e4.62 (br, 2H), 2.77e2.94 (br, 2H),
2.02e2.26 (br, 2H),1.53e1.63 (br, 5H), 1.10e1.14 (m, 18H), 0.70e0.88
where
3 0 is the permittivity of free space,
3 r the dielectric constant
(m,10H). 13C NMR (600 MHz, CDCl3):
d (ppm) 157.43,154.59,145.04,
of the polymer,
m
the hole mobility, V the voltage drop across the
device, and L the polymer thickness. V ¼ Vap ꢀ Vbi ꢀ Vr, Vap is the
applied voltage to the device, Vr the voltage drop due to contact
resistance and series resistance across the electrodes, and Vbi the
built-in voltage due to the difference in work function of the two
electrodes. The resistance of the device as measured using a blank
142.31, 141.95, 141.68, 131.27, 129.91, 128.32, 128.09, 125.20, 125.01,
123.73, 121.22, 119.69, 117.68, 113.62, 113.55, 111.21, 111.17, 110.25,
110.01, 108.91, 53.12, 40.09, 39.83, 39.67, 39.07, 30.19, 29.92, 29.58,
29.53, 22.91, 22.84, 22.82, 16.16, 15.92, 15.76, 14.40, 14.30. Anal.
Calcd for C39H49N3S: C 79.40, H 8.03, N 7.11. Found: C 78.79, H 7.84,
N 6.81.
configuration ITO/PEDOT/Al was about 10e20
deduced from the best fit of the J0.5 versus Vap plot at voltages
above 2.5 V was about 1.5 V. The dielectric constant r was assumed
U. The Vbi as
3
2.3.8. Synthesis of PIIDTBT
to be 3 in our analysis, which is a typical value for conjugated
polymers.
PIIDTBT was synthesized using 2,8-bis(5-bromothiophen-2-yl)-
5-ethyl-10,10-dioctyl-5,10-dihydroindeno[1,2-b]indole (6) and 4,7-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]
thiadiazole as monomers with a similar method to that of PIIBT
3. Results and discussion
(170 mg, 54%). 1H NMR (600 MHz, CDCl3):
d (ppm) 7.95e8.10 (br,
2H), 7.78e7.81 (br, 2H), 7.35e7.65 (br, 4H), 6.85e7.15 (br, 4H), 4.75e
4.89 (br, 2H), 2.01e2.03 (m, 3H), 1.65e1.78 (br, 4H), 1.35e1.51 (br,
4H), 0.93e1.19 (br, 20H), 0.69e0.88 (m, 6H). 13C NMR (600 MHz,
3.1. Synthesis and characterization
Two novel ladder type polymers based on indeno[1,2-b]
indole-co-BT were synthesized via Suzuki polymerization method
and the synthetic route outlined in Scheme 1. The tetracycle fused
CDCl3):
d (ppm) 157.99, 157.52, 155.79, 152.59, 144.99, 143.54,
141.78, 137.99, 134.08, 131.41, 126.59, 125.48, 124.63, 124.21, 123.38,
123.33, 121.54, 119.69, 118.03, 106.68, 53.22, 39.07, 37.60, 32.02,
30.19, 29.81, 29.58, 25.42, 22.82, 15.77, 15.58, 15.54, 14.44, 14.40,
14.33, 14.30. Anal. Calcd for C47H53N3S3: C 74.65, H 6.79, N 5.55.
Found: C 74.02, H 6.91, N 5.12.
p-conjugated indenoindole [37,41,42] was synthesized via Fischer
Indole method. Indanone and phenyl hydrazine were dissolved in
absolute ethanol and refluxed at 80 ꢂC to form the intermediate
phenylhydrazone, which was cyclized in the presence of conc.
HCl. Then ethyl chain was introduced on the sp3 nitrogen of the
indole ring by 1-bromoethane and NaH. The purpose of intro-
ducing a short ethyl chain is to reduce the steric hindrance and
maintain the solubility and device processability. The obtained N-
alkylated product was brominated in the presence of bromine in
CF. The solubilizing group was introduced on the sp3 carbon of
the indene ring by 1-bromooctane and sodium tert-butoxide to
obtain freely soluble 2,8-dibromo-5-ethyl-10,10-dioctyl-5,10-
dihydroindeno[1,2-b]indole (II), which is used as a monomer for
2.4. Fabrication of BHJ PSCs
The BHJ PSCs were fabricated using the following configuration:
ITO/PEDOT:PSS/PIIBT (or) PIIDTBT:PC71BM/LiF/Al. Each glass sub-
strate was coated with a transparent ITO electrode (110 nm thick,
10e15
U/sq). The ITO-coated glass substrates were ultrasonically
cleaned with detergent, distilled water, acetone, and isopropyl
alcohol. The layer of 40 nm thick PEDOT:PSS (Clevios PH1000) was
spin-coated onto the pre-cleaned and UVeozone treated ITO sub-
strate followed by annealing in air at 150 ꢂC for 30 min. The
PIIBT:PC71BM and PIIDTBT:PC71BM blend was prepared in chlo-
robenzene (CB), ODCB, mixed solvents using CF and ODCB (1:1 v/v)
at a 1:1, 1:2, and 1:3 weight ratio with a total blend concentration
the synthesis of PIIBT. The thiophene
p-spacer was introduced in
the 2nd and 8th positions of the indeno[1,2-b]indole ring by
palladium-catalyzed Suzuki coupling reaction. The bithiophene-
substituted indeno[1,2-b]indole was easily brominated at the 5th
position of the thiophene ring by NBS bromination condition to
obtain 2,8-bis(5-bromothiophen-2-yl)-5-ethyl-10,10-dioctyl-5,10-
dihydroindeno[1,2-b]indole (IIDT) and it was used as a monomer
of 15 mg/mL. The blending solution was filtered with 0.45
mm PTFE
(hydrophobic) syringe filter. The active layer was spin-coated and

P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
4887
H
N
EtOH
NaH
THF
N
S
NH2
+
+
Br
N
H
Conc. HCl (12N)
Reflux 30 min
O
Reflux
2
1
24 h
N
N
1-Bromooctane
Br2/CHCl3
Sn(C4H9)3
+
Br
Br
Sodium tert-butoxide
DMSO, 90 oC, 1 h
RT, overnight
Br
Br
C8H17
C8H17
3
II
N
N
O
Toluene
S
S
DMF
+
N
Br
Pd(PPh3)4
Reflux, overnight
Br
C8H17
C8H17
C8H17
5
C8H17
0 oC-RT, overnight
S
S
O
Br
NBS
IIDT
S
S
N
N
N
N
N
N
Pd(PPh3)4
O
O
B
B
Br
+
Br
O
THF, 4N-K2CO3
80 oC, 36 h
O
C8H17
C8H17
n
C8H17
II
C8H17
BT
PIIBT
S
S
N
N
N
N
N
Pd(PPh3)4
N
O
O
S
B
B
+
Br
THF, 4N-K2CO3
80 oC, 36 h
S
O
O
C8H17
C8H17
S
C8H17
S
C8H17
Br
n
BT
IIBT
PIIDTBT
Scheme 1. Synthetic route of the PIIBT and PIIDTBT.
for the synthesis of PIIDTBT. The Suzuki polymerization condition
was applied for the synthesis of novel donor-acceptor type PIIBT
and PIIDTBT and the polymerization results are summarized in
Table 1. The acquired intermediates, monomers, and polymers
(PIIBT and PIIDTBT) were confirmed by 1H and 13C NMR,
elemental analysis, GPC, DSC, TGA, CV, UVevisible, and photo-
luminescence (PL) spectra. 1H and 13C NMR spectra of the in-
termediates, monomers, and polymers are shown in Figs. S1eS16.
PIIBT and PIIDTBT were completely soluble in MC, THF, CF, CB,
ODCB, and toluene.
temperature is 403 and 436 ꢂC for PIIBT and PIIDTBT, respectively
(Fig. S17), indicates high thermal stability of both the polymers. The
DSC analysis of PIIBT and PIIDTBT did not show any crystallization
peaks between 25 and 350 ꢂC. In addition, PIIBT and PIIDTBT
showed a glass transition (Tg) at 121 and 196 ꢂC, respectively
(Fig. S18). The DSC result revealed a higher Tg for the thiophene p-
spacer-incorporated PIIDTBT than that of PIIBT. These TGA and DSC
results provide strong evidence that PIIBT and PIIDTBT will not be
distorted easily under high temperature annealing and will retain
stable durability following device fabrication.
3.3. Photophysical properties
3.2. Thermal properties
The photophysical properties of PIIBT and PIIDTBT were ex-
amined by UVevisible absorption and PL spectrophotometry. The
absorption and emission spectra of PIIBT and PIIDTBT are shown
in Figs. 1 and 2 and these results are summarized in Table 2. Both
PIIBT and PIIDTBT showed two absorption bands, in accordance
with other donoreacceptor polymers [24,43]. An absorption band
in the shorter wavelength region was found between 350 and
The thermal stability of the newly synthesized PIIBT and
PIIDTBT were examined by TGA and DSC and their results are listed
in Table 1. According to TGA, the onset of decomposition
Table 1
Polymerization results and thermal properties of PIIBT and PIIDTBT.
450 nm, which was attributed to the
pep* transition in p-con-
Polymer
Mn (kDa)
Mw (kDa)
PDI
Tg (ꢂC)
Td (ꢂC)a
jugated polymer backbone. Another broad band was observed in
the longer wavelength region between 450 and 720 nm, which is
indicative of ICT between the electron-rich tetracyclic backbone
segment and the electron-deficient BT segment. The absorption
PIIBT
PIIDTBT
10.2
13.6
23.2
35.5
2.28
2.61
121
196
403
436
a
Decomposition temperature at 5% weight loss.

4888
P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
Table 2
Photophysical and electrochemical properties of PIIBT and PIIDTBT.
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
PIIBT (s)
PIIBT (f)
Polymer Solution
Film
Eogpt
(eV)b (eV)
HOMO LUMO
PIIDTBT (s)
PIIDTBT (f)
(eV)
UV lmax
PL lmax UV lmax
PL lmax
(nm)
(nm)a
(nm)a
(nm)
PIIBT
367, 506 622
368, 521 635
419, 609 706
1.99
1.75
ꢀ5.37
ꢀ5.37
ꢀ3.38
ꢀ3.62
PIIDTBT 416, 584 688
a
Measurements performed in CF.
Estimated from the onset of the absorption in thin films (Eg ¼ 1240/(lonset eV)).
b
Overall, these photophysical results suggest that both
gated polymers exhibit superior characteristics in terms of
bandgap.
p-conju-
400
500
600
700
800
3.4. Electrochemical properties
Wavelength (nm)
Fig. 1. Normalized UVevisible spectra of PIIBT and PIIDTBT in CF solution and thin
films.
The electrochemical properties of PIIBT and PIIDTBT thin films
were measured by CV. The obtained CV spectra are shown in Fig. 3
and their results are summarized in Table 2. The onset oxidation
potential of PIIBT and PIIDTBT was observed at 1.03 V. The HOMO
energy levels of both polymers were calculated to be ꢀ5.37 eV from
their onset oxidation potential observed in CV. The Voc of a
photovoltaic device is related to the energy difference between the
HOMO of the donor and the LUMO of the acceptor [33]. The HOMO
energy level of PIIBT and PIIDTBT is located in an ideal range and
comparable with carbazoleecyclopentadithiopheneeco-BT (or)
DTBT-based polymers [29]. The LUMO energy levels of the poly-
mers were calculated from the optical bandgap and the HOMO
energy level obtained from CV. The calculated LUMO energy levels
of ꢀ3.38 and ꢀ3.62 eV for PIIBT and PIIDTBT, respectively were
higher than that of PC71BM (ꢀ4.0 eV), which ensured energetically
favorable electron transfer from donor to acceptor in the photo-
voltaic device [44,45]. The energy band diagrams of the materials
used in this study, shown in Fig. 4 suggest that PIIBT and PIIDTBT
are suitable donor materials for BHJ PSCs.
maxima of PIIBT appeared at 367 and 506 nm, compared to 416
and 584 nm for whereas PIIDTBT, in CF solvent. This reveals a
bathochromic shift for the thiophene
p-spacer-incorporated
PIIDTBT. The maximum emission peak of PIIDTBT was 688 nm,
which was more red-shifted compared to PIIBT (622 nm), which
provides further evidence of extended
p-conjugation in PIIDTBT.
The measured molar absorbance coefficients (
3
) of PIIBT and
PIIDTBT were 2.68
ꢁ
104 Mꢀ1 cmꢀ1 at 506 nm and
5.82 ꢁ 104 Mꢀ1 cmꢀ1 at 584 nm, respectively, which demon-
strated that thiophene
p-spacer-incorporated PIIDTBT had
greater light absorbing ability than PIIBT with no thiophene. The
absorption peaks at longer wavelength in the thin film of PIIBT
and PIIDTBT were red shifted by about 15 and 25 nm, respec-
tively, as compared with those in the solution state. The relative
red shift infers the presence of intermolecular interactions and
aggregation in solid state. These results are more favorable for
improving charge mobility in thin film. The optical bandgaps of
the PIIBT and PIIDTBT thin films were estimated from the ab-
sorption onsets at 622 and 710 nm, respectively [26,30]. This
result implies a bandgap for PIIBT and PIIDTBT of 1.99 and
1.75 eV, respectively. The reduction in the PIIDTBT bandgap of
0.24 eV compared to PIIBT was attributed to the result of effective
3.5. Hole mobility of PIIBT and PIIDTBT
Hole mobility is an important parameter for
p-conjugated
polymers used in BHJ PSCs. The hole mobilities of PIIBT and
ICT between the extended
p-conjugated donor and BT acceptor.
PIIBT (s)
1.0
PIIBT (f)
PIIDTBT (s)
PIIDTBT (f)
0.8
0.6
0.4
0.2
0.0
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 3. Cyclic voltammograms of PIIBT and PIIDTBT in films state at the scanning rate
Fig. 2. Normalized PL spectra of PIIBT and PIIDTBT in CF solution and thin films.
of 100 mV/s.

P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
4889
10
PIIBT
PIIDTBT w/o DIO
PIIDTBT with DIO
8
6
4
2
Fig. 4. Energy diagram of HOMO and LUMO energy levels for the polymers and
PC71BM.
0
0.0
0.2
0.4
Voltage (V)
0.6
0.8
PIIDTBT were measured using SCLC with the configuration of ITO/
PEDOT:PSS/PIIBT (or) PIIDTBT/Al. From Fig. 5 the hole mobilities
for PIIBT and PIIDTBT of 4.03 ꢁ 10ꢀ6 cm2/V s and 1.94 ꢁ 10ꢀ4 cm2/
V s, respectively were extracted, which are comparable to that of
fused donor polymers and P3HT systems [26,29,35,46]. The higher
Fig. 6. JeV characteristics of BHJ PSCs based on polymer:PC71CBM blend films as active
layer using CB solvent.
hole mobility of thiophene
compared to PIIBT was attributed to the incorporation of the
thiophene unit, which increases the -conjugation length, gives
p-spacer-incorporated PIIDTBT
enhancement in the photovoltaic parameters with increasing
PC71BM ratio. For PIIBT:PC71BM based devices with the ratio of
1:3, Jsc was increased by 93% (from 2.02 mA/cm2 to 3.90 mA/cm2)
and FF by 14% (from 26.10% to 29.76%) compared to that of the 1:1
weight ratio. The higher concentration of PC71BM leads to aggre-
gation with the polymer in the active layer and enhances the
exciton dissociation at the donoreacceptor interface [47,48]. Sub-
sequently, we fixed the weight ratio of PIIBT:PC71BM (1:3) in CB
solvent and increased the active layer thickness to 120 and
140 nm. The JeV curves of these two PIIBT:PC71BM-based devices
are shown in Fig. S19 and the corresponding device results are
summarized in Table S1. The 120-nm-thick active layer device
showed improved photovoltaic performance with a Jsc of 5.10 mA/
cm2, Voc of 0.75 V, FF of 32.31% and PCE of 1.23%, as shown in Fig. 6.
The corresponding photovoltaic performance is shown in Table 3
(see also Fig. S19 and Table S1). The external quantum efficiency
(EQE) curves of the PIIBT:PC71BM-based devices are shown in
Fig. S20.
The device optimization of PIIDTBT-based BHJ PSCs comprised
fine tuning of solvents, PIIDTBT:PC71BM weight ratio, and solvent
additive. The PIIDTBT-based BHJ PSCs using a mixed solvent
composed of CF:ODCB (1:1 v/v), with different blending ratios (1:1,
1:2, and 1:3) of PIIDTBT and PC71BM and a 100-nm-thick active
layer were considered for device study and further characteriza-
tion. The JeV curve of these samples are shown in Fig. S21 and
their photovoltaic performances are summarized in Table S2. The
1:3 weight ratio PIIDTBT:PC71BM blend showed improved Jsc
(from 5.29 mA/cm2 to 8.81 mA/cm2) and FF (from 35.40% to
47.64%) compared to that of the 1:1 weight ratio sample. We
studied the BHJ PSC performance with other solvents and found CB
p
better intermolecular
packing, enables better
p
e
p
interaction [6,36], improves close
pep* stacking, and thus improves charge
mobility. The polymers possessing superior
pep* stacking absorb
maximum sunlight and consequently give higher Jsc in the BHJ PSCs
[38,40]. The PIIDTBT mobility data indicate that the thiophene
spacer-incorporated polymer will have higher Jsc and FF.
p-
3.6. Photovoltaic properties
The BHJ PSCs were fabricated using PIIBT and PIIDTBT as an
electron donor and PC71BM as an electron acceptor with the device
configuration of ITO/PEDOT:PSS/PIIBT (or) PIIDTBT:PC71BM/LiF/Al.
The PIIBT-based BHJ PSCs were characterized with diverse donors
and acceptor weight ratios of 1:1, 1:2, and 1:3 in CB solvent and an
active layer thickness of 100 nm. The JeV curves of devices are
shown in Fig. S19 and the photovoltaic performances are sum-
marized in Table S1. The PCEs of BHJ PSCs at three different weight
ratios were 0.40%, 0.57%, and 0.90%, respectively, indicating an
PIIBT
PIIDTBT
0.5
0.4
0.3
0.2
0.1
0.0
Table 3
PSC performances of the Polymer:PC71BM device.
Polymer
Weight ratio Jsc
(1.5 wt%)
Integrated Jsc Voc (V) FF (%) PCE (%)
(mA/cm2) (mA/cm2)
PIIBTa
1:3
5.10
9.46
9.21
5.08
8.91
8.92
0.75
0.77
0.78
32.31 1.23
47.91 3.51
51.36 3.72
PIIDTBTb 1:3
PIIDTBTc 1:3
0.5
1.0
1.5
2.0
2.5
V
-V -V (V)
r
ap bi
a
Active layer thickness of 120 nm.
Without DIO additive.
With DIO additive, active layer thickness of 100 nm.
b
c
Fig. 5. JeV characteristics of PIIBT and PIIDTBT for the measurement of hole mobility
by the SCLC method.

4890
P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
to be superior with effective PCE of 3.51%, Jsc of 9.46 mA/cm2, Voc of
0.77 V, and FF of 47.91%, as shown in Fig. 6 and Table 3 (see also
Fig. S21 and Table S2). The EQE curves of the PIIDTBT:PC71BM-
based devices are shown in Fig. S22.
As compared to PIIBT, the PIIDTBT-based device showed
better BHJ PSC performance with PCE of 3.51%, compared to 1.23%
for PIIBT. The much higher Jsc and FF values for PIIDTBT com-
pared to the PIIBT-based devices was attributed to the broad
absorption area, LBG, better planarity, and high hole mobility of
PIIDTBT. The addition of a solvent additive to the PIIDTBT:
PC71BM blend further enhances the photovoltaic performance.
1,8-Diiodooctane (DIO) has been demonstrated as a remarkable
and universal additive for improving the morphology of polymer
films, and hence the effective charge dissociation and charge
transport [49,50]. Fig. 6 shows the JeV characteristics of PIIDTBT:
PC71BM (1:3) using processing additive (CB, 4% DIO). The corre-
sponding BHJ PSCs results are summarized in Table 3. The addi-
tion of DIO improved the PCE to 3.72% with a higher Voc of 0.78 V
and FF of 51.36%.
Atomic force microscopy (AFM) was used to investigate the
surface morphology of PIIBT:PC71BM and PIIDTBT:PC71BM (both
without and with DIO) blend films, as shown in Fig. 7. The root
mean square (rms) roughness of PIIBT:PC71BM and
PIIDTBT:PC71BM (without and with DIO) blend films were 0.41,
0.30, and 0.26 nm, respectively. All the blend films showed a
smooth surface, which suggested that the donor polymers are
highly compatible with PC71BM. Normally, nanoscale phase sepa-
ration should be recognized to allow efficient charge separation,
which improves charge transport property and FF of the BHJ PSCs
[28,51e53]. The PIIDTBT:PC71BM (without DIO) blend film showed
a smoother surface with a rms of 0.30 nm, and it showed the
maximum PCE of 3.51%. Furthermore, the surface morphology was
further improved (rms 0.30 nme0.26 nm) by adding 4% DIO to
PIIDTBT:PC71BM, and PCE was slightly increased to 3.72%.
The EQE studies were conducted to analyze the accuracy of the
optimized devices based on PIIBT:PC71BM and PIIDTBT:PC71BM
(without and with DIO), as shown in Fig. 8. The EQE curves of the
optimized PIIBT and PIIDTBT donor-based device covered a broad
wavelength from 400 to 725 nm and showed a maximum EQE of
38.6% at 482 nm, 56.5% at 489 nm, and 57.7% at 489 nm for the
BHJ PSCs based on PIIBT, PIIDTBT (without DIO), and PIIDTBT
(with DIO), respectively. The lower EQE of PIIBT compared to
60
PIIBT
PIIDTBT w/o DIO
50
40
30
20
10
0
PIIDTBT withDIO
300
400
500
600
700
Wavelength (nm)
Fig. 7. AFM phase images of (a) PIIBT (120 nm), (b) PIIDTBT (without DIO), and (C)
PIIDTBT (with DIO). The scanned area is 1000 nm ꢁ 1000 nm.
Fig. 8. EQEs curve of polymer:PC71CBM blend films as active layer.

P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
4891
Fig. 9. Optimized structures of the four backbone units and their torsional energy curves in repeating units of PIIBT and PIIDTBT: a) II-BT linkage, b) T-II linkage, c) II-T linkage, and
d) T-BT linkage.
PIIDTBT was attributed to charge recombination [54,55]. The Jsc
values calculated from the integration of the EQE spectra agreed
with those obtained from the JeV measurements, as shown in
Table 3.
was considered in the computation. Fig. 9 shows the optimized
structures of the four backbone units and their torsional energy
curves in repeating units of PIIBT and PIIDTBT. In the II-BT, T-II,
and II-T linkages, two rings are twisted from each other in the
minimum energy structures around 30 and 150ꢂ with barrier
heights around 3 kcal/mol. The T-BT linkage exhibited a coplanar
structure with an energy barrier of around 4.5 kcal/mol. Based on
these torsional profiles, we obtained the most stable structure
with one repeating unit of PIIDTBT. We choose 24 conformers
with various initial torsion angles in T-II, II-T, and T-BT linkages,
and minimized the structures. Fig. 10 shows the most stable
3.7. DFT calculations
DFT calculations were performed to obtain the electronic
structures and optical properties of the polymers. Calculations
were carried out at the B3LYP/6-31G** level using Jaguar quan-
tum chemistry software [56]. One repeating unit of each polymer

4892
P. Sakthivel et al. / Polymer 54 (2013) 4883e4893
Fig. 10. Optimized structures, HOMOs and LUMOs of one repeating unit of PIIBT and PIIDTBT: (a) PIIBT and (b) PIIDTBT.
energy structures, HOMOs, and LUMOs of one repeating unit of
Acknowledgments
PIIBT and PIIDTBT. The almost planar (176ꢂ) torsion angle of the
T-BT linkage in PIIDTBT contributes to the
polymer backbone, which is reflected in the photovoltaic per-
formance of the BHJ PSCs based on the two polymers.
p
e
p
stacking of the
This work was supported by grant fund from the National
Research Foundation of Korea (NRF) of the Ministry of Education,
Science and Technology (MEST) of Korea (No. 2011-0028320).
4. Conclusions
Appendix A. Supplementary data
A new class of
p-conjugated, tetracycle-based, ladder type
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.07.004.
building block is introduced for application to BHJ PSCs. PIIBT and
PIIDTBT were synthesized via Suzuki polymerization conditions. A
thiophene
p-spacer unit was incorporated in PIIDTBT for increased
References
p-conjugation and planarity. The PIIDTBT-based polymer showed
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p-electron delo-
calization. Moreover, PIIDTBT exhibited a deep-lying HOMO energy
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These results demonstrated the suitability of the new thiophene
spacer-incorporated -conjugated indeno[1,2-b]indole as a build-
ing block for fabricating high performance -conjugated polymers.
p-
p
p
Now, we are presently focusing on improving the photovoltaic
performance of indeno[1,2-b]indole as a basic backbone by struc-
tural modification with different acceptor combinations.

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