Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers for Bulk Heterojunction Polymer Solar Cell

Article abstract of DOI:10.1080/15421406.2014.932658

A series of donor-acceptor low band gap polymers (DINIBT-C8, DINIBT-C10 and DINIBT-EH) containing 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donating unit and benzothiadiazole (BT) as an electron accepting unit with various kinds of alkyl chain

Full text of DOI:10.1080/15421406.2014.932658

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Synthesis and Characterization of
Indolo[3,2-b]indole Based Copolymers
for Bulk Heterojunction Polymer Solar
Cell
Sung-Ho Jina, Hyunyoung Yooa, Chikyu Leea, Chinnusamy Saravanana,
Yeong-Soon Galb & Jae Wook Leec
a Department of Chemistry Education, Graduate Department of
Chemical Materials and Institute for Plastic Information and Energy
Materials, Pusan National University, Busan, Republic of Korea
b Polymer Chemistry Lab., Kyungil University, Hayang, Republic of
Korea
Click for updates
c Department of Chemistry, Dong-A University, Busan, Republic of
Korea
Published online: 17 Nov 2014.
To cite this article: Sung-Ho Jin, Hyunyoung Yoo, Chikyu Lee, Chinnusamy Saravanan, Yeong-Soon Gal
& Jae Wook Lee (2014) Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers for
Bulk Heterojunction Polymer Solar Cell, Molecular Crystals and Liquid Crystals, 597:1, 135-145, DOI:
10.1080/15421406.2014.932658
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Mol. Cryst. Liq. Cryst., Vol. 597: pp. 135–145, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.932658
Synthesis and Characterization
of Indolo[3,2-b]indole Based Copolymers
for Bulk Heterojunction Polymer Solar Cell
SUNG-HO JIN,1,* HYUNYOUNG YOO,1 CHIKYU LEE,1
CHINNUSAMY SARAVANAN,1 YEONG-SOON GAL,2
AND JAE WOOK LEE3,*
1Department of Chemistry Education, Graduate Department of Chemical
Materials and Institute for Plastic Information and Energy Materials,
Pusan National University, Busan, Republic of Korea
2Polymer Chemistry Lab., Kyungil University, Hayang, Republic of Korea
3Department of Chemistry, Dong-A University, Busan, Republic of Korea
A series of donor-acceptor low band gap polymers (DINIBT-C8, DINIBT-C10 and
DINIBT-EH) containing 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donat-
ing unit and benzothiadiazole (BT) as an electron accepting unit with various kinds
of alkyl chains (octyl; C8, decyl; C10 and ethyl hexyl; EH) on the donor unit have
been designed, synthesized, and characterized for bulk heterojunction polymer solar
cell applications. Due to the presence of long alkyl chains on the donor unit, all the
polymers showing good solubility in common organic solvents. The frontier orbital
energy levels of polymers were calculated using cyclic voltametry analysis. The influ-
ence of alkyl chains on the photo-physical properties of polymers were carried out.
Among the three polymers, the DINIBT-C8 shows higher absorption coverage and
good energy level matching with acceptor PC71BM than that of the other two poly-
mers. Thus, the bulk-heterojunction polymer solar cells (BHJ PSCs) were fabricated
using DINIBT-C8:PC71BM as an active layer with the configuration of ITO/PEDOT/
DINIBT-C8:PC71BM/LiF/Al. The DINIBT-C8 shows the maximum PCE of 0.16%.
Keywords Bulk heterojunction polymer solar cell; 5,10-dihydroindolo[3,2-b]indole
(DINI); low-band gap polymers; benzothiadiazole
Introduction
The donor/acceptor low band gap polymers for bulk heterojunction (BHJ) polymer solar
cells (PSCs) have been studied intensively in the recent years because of their potential
utility as low-cost, large-area, flexible, light-weight roll to roll printing devices [1, 2].
∗Address correspondence to Sung-Ho Jin, Department of Chemistry Education, Graduate De-
partment of Chemical Materials, and Institute for Plastic Information and Energy Materials, Pusan
National University, Busan 609-735, Korea. E-mail: shjin@pusan.ac.kr and Jae Wook Lee, Depart-
ment of Chemistry, Dong-A University, Busan 604-714, Korea. E-mail: jlee@donga.ac.kr.
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
135

136
S.-H. Jin et al.
The BHJ, which is formed in the active layer of PSCs by the binary blending of p-type π-
conjugated low band gap polymers and n-type fullerene derivatives, have offered a potential
platform for dissociation of exciton into free carriers. Generally, the excitons generated in
polymeric material moves towards the D/A interfaces and dissociate into free carriers with
the aid of the energetic driving force originating from the differences in the electronic levels
of the donor (D)/acceptor (A) materials [3, 4]. Thus, the nature of BHJ highly depended on
the energy levels, miscibility, planarity of p-type donor polymers/n-type acceptor fullerene
derivatives. To this end, the benchmark BHJ PSCs based on poly (3-hexylthiophene) (P3HT)
and acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) shows the average power
conversion efficiencies (PCEs) of around 4–5% under AM 1.5 conditions [5, 6]. In order to
get further improvement in the efficiency of PSCs, many kinds of donor/acceptor low band
gap copolymers have been designed, synthesized and utilized for PSCs [7]. Particularly,
the carbazole based copolymers has become the next benchmark structure among the most
efficient, stable and low-cost material for BHJ PSCs [8].
The size and density of solubilizing alkyl chains on the polymer backbone plays an
important role in the determination of the morphology of the active layer, which also one
of the detrimental factor on the PCE of PSCs [9]. In order to facilitate the formation
of nanoscale phase separation in the active layer of PSCs, the systematic analysis of
the position and nature of alkyl chains of the particular polymer is very important. Very
recently, Owczarczyk et al. have estabilzied donor−acceptor copolymers containing 5,10-
dihydroindolo[3,2-b]indole (DINI) as electron donor unit and diketopyrrolopyrrole and
thienopyrroledione as the electron accepting units, which are the derivative of carbazole,
for BHJ PSCs [10]. Here, they explored the size and shape of the pendant alkyl substituents
attached to the DINI unit on the optical and electronic properties of the copolymers. In this
work, we have synthesized a series of copolymers containing the DINI as an electron donor
unit with various kinds alkyl chains on the nitrogen atoms and the benzothiadiazole (BT)
as electron accepting unit for BHJ PSC applications as shown in Scheme 1.
Experimental
Materials
All the chemicals and reagents purchased from Sigma-Aldirich were used without any fur-
ther purification. The 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester) and tetra-
n-buytylammonium tetrafluroborate (TBABF4) were also purchased from Sigma-Aldirich
and used as received. The 5,10-dihydroindolo[3,2-b]indole (3) was prepared according to
the reported procedures [10].
Synthesis of 5,10-dioctyl-5,10-dihydroindolo[3,2-b]-indole (4)
To a stirred of a solution of 1.24 g (6.02 mmol) of compound 3 in 55 mL of THF at
room temperature under nitrogen was added 576 mg (18.06 mmol) of NaH in 40 mL of
THF. After 10 min at room temperature, 4.65 g (24.08 mmol) of n-bromooctane was added
to the reaction mixture. The resultant mixture was refluxed for overnight, treated with
60 mL of water, extracted using 3 × 30 mL of methylene chloride, washed with water and
brine and dried over anhydrous MgSO4. The solvents was removed under reduced pressure
and purified by flash silica gel column chromatography (hexane) to give 1.17 g (45%) of
compound 4 as a white solid.

Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers
137
O2N
I
O
O2N
NO2
KMnO4
Pd(OAc)2/(Ph)3P/CuI
(Et)3N/CH3CN/H2O
HC CH
O
ClCH2CH2Cl
NO2
(2)
(1)
NO2
HCl/AcOH
SnCl2 2H2O
R
N
R
N
H
N
Br
NBS
RBr/NaH
THF
Br
DMF
N
R
N
N
R
(3)
H
(4): R = octyl-,
(5): R = decyl-,
(6): R = 2-ethylhexyl
(7): R = octyl-,
(8): R = decyl-,
(9): R = 2-ethylhexyl
S
R
N
R
N
N
N
S
N
O
O
O
*
Br
Pd(PPh3)4/K2CO3
Toluene/H2O
N
B
B
Br
O
N
N
*
R
R
DINIBT-C8: R = octyl-,
DINIBT-C10: R = decyl-,
(7): R = octyl-,
(8): R = decyl-,
DINIBT-EH: R = 2-ethylhexyl
(9): R = 2-ethylhexyl
Scheme 1. Synthesis of DINIBT-C8, DINIBT-C10 and DINIBT-EH.
1H-NMR (CDCl3, ppm): 0.84 (t, 6H), 1.41 (m, 20H), 1.92 (m, 4H), 4.61 (t, 4H), 7.15
(t, 2H), 7.29 (t, 2H), 7.60 (d, 2H),7.92 (d, 2H); 13C-NMR (CDCl3, ppm): 14.27, 22.92,
27.25, 29.30, 29.73, 30.31, 32.01, 45.61, 109.86, 114.65, 116.78, 118.07, 121.62, 126.01,
141.73.
Synthesis of 5,10-didecyl-5,10-dihydroindolo[3,2-b]-indole (5)
The compound 5 was synthesized as white colored solid with a quantitative yield using the
similar procedure for compound 4.
1H-NMR (CDCl3, ppm): 0.84 (t, 6H), 1.41 (m, 28H), 1.92 (m, 4H), 4.61 (t, 4H), 7.15 (t,
2H), 7.29 (t, 2H), 7.60 (d, 2H),7.92 (d, 2H); 13C-NMR (CDCl3, ppm): 14.39, 22.94, 27.43,
29.54, 29.70, 29.76, 29.81, 30.57, 32.13, 45.56, 109.96, 114.76, 116.88, 118.18, 121.71,
126.08, 141.74.
Synthesis of 5,10-di-2-ethylhexyl-5,10-dihydroindolo[3,2-b]-indole (6)
The compound 6 was also prepared as white colored solid with a quantitative yield by
adopting the similar procedure for compound 4.
1H-NMR (CDCl3, ppm): 0.84 (t, 12H), 1.41 (m,14H), 1.92 (m, 4H), 4.61 (t, 4H), 7.15
(t, 2H), 7.29 (t, 2H), 7.60 (d, 2H),7.92 (d, 2H); 13C-NMR (CDCl3, ppm): 11.04, 14.25,
23.24, 24.50, 28.94, 31.09, 39.48, 47.33, 109.75, 114.64, 116.76, 118.05, 121.60, 125.97,
141.62.
Synthesis of 2,7-Dibromo-5,10-dioctyl-5,10-dihydroindolo[3,2-b]-indole (7)
To a stirred solution of 1.17 g (2.72 mmol) of compound 4 in dimethylformamide (100 mL)
at 0◦C under nitrogen was slowly added 0.97 g (5.44 mmol) of N-bromosuccinimide in

138
S.-H. Jin et al.
dimethylformamide (50 mL). After stirring for 3 hrs at room temperature, the reaction
mixture was poured into water (100 mL) and the resulting white precipitate was filtered
and dried. The crude product was purified by flash silica gel chromatography (hexane) to
give 510 mg (32%) of compound 7 as a colorless solid. The compounds 8 and 9 were also
prepared by applying this procedure.
1H-NMR (CDCl3, ppm): 0.85 (t, 6H), 1.23 (m, 12H), 1.32 (m, 4H), 1.38 (m, 4H), 1.93
(m, 4H), 4.43 (t, 4H), 7.27 (dd, 2H), 7.63 (s, 2H), 7.69 (d, 2H); 13C-NMR (CDCl3, ppm):
14.26, 22.93, 27.25, 29.30, 29.74, 30.32, 32.02, 45.74, 112.96, 113.37, 115.68, 118.82,
121.64, 126.02, 141.37.
Synthesis of 2,7-Dibromo-5,10-didecyl-5,10-dihydroindolo[3,2-b]-indole (8)
1H-NMR (CDCl3, ppm): 0.84 (t, 6H,), 1.30 (m, 28H), 1.90 (m, 4H), 4.61 (t, 4H), 7.25 (dd,
2H), 7.58 (s, 2H), 7.64 (d, 2H); 13C-NMR (CDCl3, ppm): 14.38, 22.93, 27.43, 29.52, 29.71,
29.77, 29.82, 30.55, 32.14, 45.55, 112.97, 113.38, 115.66, 118.83, 121.68, 126.01, 141.35.
Synthesis of 2,7-Dibromo-5,10-di-2-ethylhexyl-5,10-dihydroindolo[3,2-b]-indole (9)
1H-NMR (CDCl3, ppm): 0.84 (t, 12H), 1.30 (m, 14H), 1.90 (m, 4H), 4.61 (t, 4H), 7.25
(dd, 2H), 7.58 (s, 2H), 7.64 (d, 2H); 13C-NMR (CDCl3, ppm): 11.04, 14.25, 23.24, 24.50,
28.94, 31.09, 39.48, 47.33, 112.93, 113.34, 115.63, 118.80, 121.65, 126.03, 141.35.
Synthesis of DINIBT-C8
Under Nitrogen atmosphere, the compound 7 (176.5 mg, 0.3 mmol), 2,1,3-
benzothiadiazole-4,7-bis(boronic acid pinacol ester) (116.4 mg, 0.3 mmol), tetrakis(triph
enylphosphine)palladium(0) (17.3 mg, 0.015 mmol) were added into 25 ml round flask.
Freshly distilled toluene (1 mL) was added and the mixture was stirred at 120◦C for
5 minutes. Then, 2 M K2CO3 (1 mL) was injected slowly into the reaction and stirred
for 2 days. To complete the end-capping reaction, bromobenzene and phenylboronic acid
was added into the reaction mixture with an appropriate time gap. After completion of
reaction, the mixture allowed to cool room temperature and poured into methanol. A solid
was obtained by filtration. The polymer was further purified by stirring it in methanol and
acetone for 24 hours and passed through celite to remove oligomers and catalyst residues.
After precipitating the resulting product in methanol, it was dried under reduced pressure.
The polymer DINIBT-C8 was obtained as dark brown colored solid.
1H-NMR (CDCl3, ppm): 0.84 (s, 6H,), 1.30 (m, 20H), 1.91 (br, 4H), 4.63 (br, 4H),
7.27–8.23 (br, 8H); 13C-NMR (CDCl3, ppm): 14.37, 22.96, 27.23, 29.32, 29.62, 30.51,
32.12, 45.53, 111.15, 114.31, 117.56, 119.43, 127.21, 128.11, 131.32, 132.43, 141.10,
154.41; Anal Calcd for C36H42N4S : C, 76.55; H, 7.85; N, 9.92. Found: C, 76.05; H, 7.82;
N, 9.55.
Synthesis of DINIBT-C10
The polymer DINIBT-C10 was also prepared by applying procedure similar for DINIBT-
C8.
1H-NMR (CDCl3, ppm): 0.84 (s, 6H,), 1.30 (m, 28H), 1.90 (br, 4H), 4.61 (br, 4H),
7.27–8.23 (br, 8H); 13C-NMR (CDCl3, ppm): 14.38, 22.93, 27.43, 29.52, 29.71, 29.77,
29.82, 30.55, 32.14, 45.55, 111.17, 114.35, 117.66, 119.63, 127.36, 128.31, 131.24, 132.40,

Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers
139
141.15, 154.42. Anal Calcd for C36H42N4S: C, 77.37; H, 8.44; N, 9.02. Found: C, 76.92;
H, 8.26; N, 8.92.
Synthesis of DINIBT-EH
The polymer DINIBT-EH was also synthesized by adopting similar procedure for DINIBT-
C8.
1H-NMR (CDCl3, ppm): 0.84 (s, 12H,), 1.30 (m, 14H), 1.91 (br, 4H), 4.63 (br, 4H),
7.27–8.23 (br, 8H); 13C-NMR (CDCl3, ppm): 11.02, 14.15, 23.18, 24.45, 28.89, 31.12,
39.47, 47.23, 111.16, 114.33, 117.63, 119.61, 127.34, 128.27, 131.18, 132.24, 141.11,
154.40. Anal Calcd for C36H42N4S: C, 76.55; H, 7.85; N, 9.92. Found: C, 76.25; H, 7.75;
N, 9.03.
Fabrication of Organic Photovoltaic Cells
The glass substrates were coated with a transparent ITO electrode (110 nm thick, 10∼15
ꢀ / sq sheet resistance), which was ultrasonically cleaned with detergent, distilled water,
acetone, and isopropyl alcohol (IPA). Then, the layer of 40 nm thick PEDOT:PSS (baytron
P) was spun-coated onto the pre-cleaned and UV-ozone treated ITO substrates and the film
baked in air at 150◦C for 30 minutes. The solution of active layer, which was prepared by
dissolving appropriate ratios of PIIBT-C8/PCBM in chloroform/o-dichlorobenzene (1:1,
v/v) co-solvent system, was filtered with 0.45 μm PTFE (hydrophobic) syringe filter
and spin coated on the top of the PEDOT:PSS with the layer thickness of 80∼90 nm,
dried at room temperature for 30–40 minute. The Lithium fluoride (LiF) (0.5 nm) and
Aluminum (Al) cathode (120 nm) were deposited on the top of the active layer under
vacuum less than 5.0 × 10−6 torr, which yields a 9 mm2 of active area per each pixel. The
film thickness was measured with a α-Step IQ surface profiler (KLA Tencor, San Jose, CA).
The fabricated BHJ PSCs performance was measured by using photocurrent density-voltage
(J-V) characteristics under light illumination. To characterize the OPV cells, measurement
was carried out using a Solar Simulator (Orel 300 W simulator, models 81150) under
a simulated solar light with ARC Lamp power supply (AM 1.5G, 100 mW/cm2) and a
standard source measure unit (Keithley 236). All processes and measurements mentioned
above were carried out in the open air at room temperature.
Results and Discussion
Scheme 1 shows the synthetic routes for the monomers and polymers. In the first step,
iodo-2-nitrobenzene was treated with acetylene gas in the presence of palladium diacetate,
triphenylphosphine, and cuprous iodide in acetonitrile and water to generate the compound
1, which was oxidized in the presence of KMnO4 in water and methylene chloride to
gives the compound 2. It was reduced using stannous chloride dihydrate in the mixture of
acetic acid and hydrochloric acid to give compound 3 [10]. The compound 3 was alkylated
using NaH in THF with n-bromooctane, n-bromodecane and 2-ethylhexylbromide afford
compound 4, 5 and 6 with quantitative yields, respectively. The compounds 4, 5 and
6 were brominated using NBS in DMF yields monomers 7, 8, and 9 with good yield,
respectively. These monomers, 7, 8 and 9, were polymerized with another monomer, 2,1,3-
Benzothiadiazole-4,7-bis(boronic acid pinacol ester), by Suzuki polymerization to get the
corresponding polymers DINIBT-C8, DINIBT-C10 and DINIBT-EH, respectively. After
polymerization, all the polymers were precipitated into methanol and purified by Soxhlet

140
S.-H. Jin et al.
Table 1. Physical properties of DINIBT-C8, DINIBT-C10 and DINIBT-EH
Td
Mna Mwa PDIa (◦C)
λabs max
(nm)
λem HOMOd LUMOe Egf
b
c
c
Polymer
(nm)
(eV)
(eV)
(eV)
DINIBT-C8 7530 14100 1.88 347 385, 552 649
DINIBT-C10 9400 14600 1.56 333 370, 533 671
DINIBT-EH 9020 17200 1.91 401 371, 501 660
−5.01
−5.03
−5.02
−3.13 1.88
−3.11 1.92
−3.10 1.92
aMn, Mw and PDI of the polymers were determined by GPC using polystyrene standards; bTd was
determined from TGA thermograms; cmeasured in the thin film onto the quartz substrate; dmeasured
in acetonirtile with 0.1M TBABF4 (film onto a counter electrode) HOMO (eV) = −4.8 - (Eonset
fdetermined from UV–visible absorption.
-
E
1/2(Ferrocene)); destimated from HOMO levels and the optical band gaps: LUMO = HOMO + Eg;
extraction in acetone and hexane to remove catalytic impurities and low molecular weight
oligomers. Under these processes, the high pure and narrow dispersed polymers were
obtained. Details of molecular weights and polydispersity index (PDI) values of all the
polymers are shown in Table 1. All the polymers have good solubility in common solvents
such as chloroform, toluene, tetrahydrofuran, etc. Molecular structures of the monomers
and polymers were confirmed by 1H-NMR and 13C-NMR spectroscopy.
The thermal properties of all the polymers were investigated by TGA and DSC thermal
analysis and the corresponding TGA and DSC thermograms of all the polymers are shown
in Fig. 1. From the TGA thermogram, the onset decomposition temperature of DINIBT-C8,
DINIBT-C10 and DINIBT-EH were found to be 347◦C, 333◦C and 401◦C, respectively.
Among the three polymers, the DINIBT-C10 has less thermal stability than the other two
polymers due to the presence of longer alkyl chain. Particularly, DINIBT-EH shows higher
11 0
DINIBT-C8
DINIBT-C10
10 0
DINIBT-EH
90
80
70
60
50
40
100
200
300
400
600
700
800
o500
Temperature ( C )
Figure 1. TGA thermograms of DINIBT-C8, DINIBT-C10 and DINIBT-EH.

Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers
141
1.0
a
DINIBT-C8
DINIBT-C10
DINIBT-EH
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
900
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
b
DINIBT-C8
DINIBT-C10
DINIBT-EH
550
600
650
700
750
800
850
900
Wavelength(nm)
Figure 2. a) UV-visible absorption and b) fluorescence spectra of DINIBT-C8, DINIBT-C10 and
DINIBT-EH in thin film.
thermal stability due to the presence of branched alkyl chain that is suitable to form a stable
solid state. The glass transition temperature of all the polymers is invisible.
In order to understand the absorption coverage of all the polymers, the UV-visible
absorption spectra of thin films of all the polymers were measured and corresponding
absorption spectra are shown in Fig. 2a. The thin films were prepared by spun coating
of corresponding chloroform solution onto a quartz slide. All the polymers show two
absorption bands; the bands in the high energy regions corresponding to the π-π∗ transition
of aromatic moieties and the bands at lower energy regions are corresponding to the intra

142
S.-H. Jin et al.
molecular charge transfer (ICT) between DINI and BT entities. The DINIBT-C8, DINIBT-
C10 and DINIBT-EH show the absorption maxima at 385 nm & 552 nm, 370 nm & 533 nm
and 371 nm & 501 nm, respectively. Among the three polymers, the DINIBT-C8 shows
broad and red shifted ICT band than the DINIBT-C10 and DINIBT-EH. It suggests that
the orientation of octyl chains in the solid state is more favorable for the efficient ICT
interactions between DINI and BT units. Particularly, the higher energy bands do not show
any significant changes with respect to the nature of aliphatic chains. The emission spectra
of thin films of all these polymers are shown in Fig. 2b. When increasing the length of
aliphatic chain from DINIBT-C8 to DINIBT-C10, the emission maxima was shifted from
649 nm to 671 nm and the DINIBT-EH shows an emission maximum at 660 nm.
To investigate the information about the charge injection and devices performance,
it is very important to calculate the frontier orbital (highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)) energy levels of all the
polymers. The HOMO of the all the polymers were measured from the cyclic voltametry
(CV) and the optical band gap from optical absorption spectra. The CV experiments of all
the polymers were carried out with TBABF4 (0.1 M in acetonitrile) as electrolyte at a scan
rate of 50−100 mV/s. Here, each polymer film as working electrode, platinum wire as
the counter electrode and Ag/AgCl as reference electrode was used to measure the cyclic
voltammograms. The potentials were calibrated with ferrocene/ferrocenium redox couple
(Fc/Fc+). The cyclic voltammograms of all the polymers are shown in Fig. 3 and the
electrochemical values are extracted in Table 1. By assuming the energy level of Fc/Fc+
to be −4.8 eV below the vaccum level, the HOMO level of polymer was determined by
EHOMO = −4.8–(Eox,onset –E1/2 (ferrocence)) eV. The LUMO levels were measured by
ELUMO = EHOMO + Eg eV. Accordingly, the HOMO/LUMO energy levels of DINIBT-C8,
DINIBT-C10 and DINIBT-EH were found to be −5.01/−3.13 eV, −5.03/−3.11 eV and
2.0x10-5
DINIBT-C8
DINIBT-C10
1.5x10-5
DINIBT-EH
1.0x10-5
5.0x10-6
0.0
-5.0x10-6
-1.0x10-5
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V vs Ag/AgCl)
Figure 3. Cyclic voltammograms of DINIBT-C8, DINIBT-C10 and DINIBT-EH thin films in
0.1 M TBABF4 solution in acetonitrile at a scan rate of 100 mV/s.

Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers
143
Figure 4. Energy level diagram of DINIBT-C8, DINIBT-C10,− DINIBT-EH and PC71BM.
−5.02/−3.10 eV and the corresponding band gaps are 1.88 eV, 1.92 eV and 1.92 eV, respec-
tively. These energy levels polymers are well matched with the energy levels of PC71BM
acceptor, which is more favorable for the dissociation of exciton at the interface between
the donor and acceptor. The corresponding energy level diagram was shown in Fig. 4.
Due to the favorable optical properties and well matched energy levels with PC71BM,
The BHJ PSCs were fabricated using the polymer DINIBT-C8 as an electron donor and
PC71BM as an electron acceptor. The devices were fabricated with the configuration of ITO
(110 nm)/ PEDOT:PSS (40 nm)/DINIBT-C8:PCBM (80 nm)/LiF (0.5 nm)/Al (120 nm)
and the corresponding current density- voltage curves (J-V) under the illumination of AM
1.5G (100 mW/cm2) are shown in Fig. 5. The device parameters are extracted in Table 2.
As shown in Table 2, upon increasing the ratio of PC71BM content in the active layer of
the devices, the Jsc is increased from 0.58 mA/cm2 to 0.76 mA/cm2 results to increase the
1.2
DINIBT-C8 : PC BM (1 : 0.5)
71
DINIBT-C8 : PC BM ( 1 : 1 )
1.0
0.8
0.6
0.4
0.2
0.0
71
DINIBT-C8 : PC BM ( 1 : 1.5 )
71
DINIBT-C8 : PC BM ( 1 : 2 )
71
0.0
0.2
0.4
0.6
Bia s[v]
Figure 5. The current density-voltage (J-V) characteristics of devices based on DINIBT-C8:PC71BM
with different ratios of PC71BM.

144
S.-H. Jin et al.
Table 2. Photovoltaic performance of devices based on DINIBT-C8:PC71BM with various
ratios of PC71BM under AM 1.5G illumination (100 mW/cm2)
Active layer
Voca (V)
Jscb (mA/cm2)
FFc (%)
PCEd (%)
DINIBT-C8:PC71BM (1:0.5)
DINIBT-C8:PC71BM (1:1)
DINIBT-C8:PC71BM (1:1.5)
DINIBT-C8:PC71BM (1:2)
0.52
0.53
0.55
0.55
0.58
0.76
0.48
0.41
29.05
27.95
26.78
26.49
0.088
0.112
0.070
0.061
aJsc: short-circuit current density; bVoc: open-circuit voltage; cFF: fill factor; dPCE: power conver-
sion efficiency.
Table 3. Photovoltaic performance of devices based on DINIBT-C8:PC71BM with various
annealing temperatures under AM 1.5G illumination (100 mW/cm2)
Annealing
Active layer
temp. (◦C)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
DINIBT-C8:PC71BM
DINIBT-C8:PC71BM
DINIBT-C8:PC71BM
DINIBT-C8:PC71BM
—
80
100
120
0.55
0.56
0.55
0.54
0.70
0.95
0.94
1.01
27.92
29.72
28.86
29.96
0.11
0.15
0.14
0.16
aJsc: short-circuit current density; bVoc: open-circuit voltage; cFF: fill factor; dPCE: power conver-
sion efficiency.
1.2
no annealing
annealing at 80OC
1.0
annealing at 100OC
annealing at 120OC
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
Vol tag e [v]
Figure 6. The current density-voltage (J-V) curves of DINIBT-C8:PC71BM based devices with
various annealing temperatures for 30 min.

Synthesis and Characterization of Indolo[3,2-b]indole Based Copolymers
145
PCE from 0.09% to 0.11%. Further increasing the ratio of PCBM, the efficiency going to
decrease. With all the ratios of PC71BM, there are no significant changes in the Voc, due
to the fact that Voc is depending on the HOMO of the donor polymer and LUMO of the
acceptor. Thus, the optimized ratio of the DINIBT-C8:PC71BM is 1:1. In order to increase
the efficiency of the devices, the annealing temperature of the devices were optimized.
Upon increasing the annealing temperature from 80◦C to 120◦C, the efficiency is increased
from 0.11% to 0.16% and corresponding J-V curves and device characteristics are shown
in Fig. 6 and Table 3, respectively.
Conclusions
In summery, we have designed and synthesized a series of low band gap copolymers,
containing DINI donor unit and BT acceptor unit for BHJ PSCs applications. All the
polymers showing better solubility in common organic solvents and exhibit narrow band
gap less than 2.0 eV. The absorption spectra of all the polymers covers from 300 to 700 nm
region. Among the absorption spectra of thin films of all polymers, the DINIBT-C8 shows
well defined ICT absorption band, which confirms that the octyl chain on the nitrogen atom
of DINI offers a more favorable environment for ICT interactions between the donor and
acceptor units. Thus, we fabricated BHJ PSCs using the DINIBT-C8:PC71BM blend as
the active layer and observed the optimized PCE of 0.11%. Upon annealing the optimized
devices with various temperatures, it shows the maximum PCE of 0.16% with annealing
temperature at 120◦C for 30 mins. In order to further improvement in the efficiency of these
polymers, the another derivatives of these moieties are currently underway.
Acknowledgments
This work was supported for two years by Pusan National University Grant.
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