Crystalline low band-gap alternating indolocarbazole and benzothiadiazole-cored oligothiophene copolymer for organic solar cell applications

Article abstract of DOI:10.1039/b811031j

A low band-gap alternating copolymer of indolocarbazole and benzothiadiazole-cored oligothiophene demonstrated balanced crystallinity and solubility; a solar cell combining this polymer with PC₆₁BM in a preliminary test demonstrated power conversion efficiencies of 3.6%. The Royal Society of Chemistry.

Full text of DOI:10.1039/b811031j

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Crystalline low band-gap alternating indolocarbazole and
benzothiadiazole-cored oligothiophene copolymer for organic
solar cell applicationswz
Jianping Lu,*a Fushun Liang,yb Nicolas Drolet,a Jianfu Ding,*b Ye Tao*a and
Raluca Movileanua
Received (in Cambridge, UK) 28th June 2008, Accepted 12th August 2008
First published as an Advance Article on the web 15th September 2008
DOI: 10.1039/b811031j
A low band-gap alternating copolymer of indolocarbazole and
benzothiadiazole-cored oligothiophene demonstrated balanced
crystallinity and solubility; a solar cell combining this polymer
with PC61BM in a preliminary test demonstrated power conver-
sion efficiencies of 3.6%.
Meanwhile, a sufficient amount of flexible substituent has to
be introduced to the polymer as side chains to improve the
solubility for solution processing. As a side effect, these side
chains may dilute the content of electroactive components and
lower the polymer crystallinity, resulting in lower charge
mobility. We report herein an innovative way to balance the
solubility and crystallinity of conjugated polymers by ration-
ally designing the structures and patterns of the substitu-
tent, so that high charge mobility of the polymers can be
maintained.
Low cost, high efficiency and long term stability are the key
factors for commercializing photovoltaic (PV) technology.
Organic PV is one of the most promising candidates due to the
low material cost and compatibility with the high throughput wet
processes.1 Although significant progresses have been made in
organic solar cells based on heterojunction networks of p- and
n-type materials in the past 10 years, the cell power conversion
efficiency (PCE) of 5–6% on laboratory scale is still too low.2 To
further enhance the PCE, it is of paramount importance to
develop new p-type organic/polymer semiconductors with high
charge mobility and narrow energy band gaps.1i
The synthesis of the benzothiadiazole-cored oligothiophenes
involved a repetitive bromination and Stille coupling proce-
dure starting from commercially available 2,1,3-benzothiadia-
zole (Scheme 1). Each cycle extended the resulting oligomers
by adding one more thiophene unit on each side of the
benzothiadiazole core. It is worth pointing out that the
bromination condition was different for each cycle in order
to get high bromination yield at the desired position. For
example, 4,7-dibromo-2,1,3-benzothiadiazole (1) was syn-
thesized using liquid Br2 as the brominating agent in a refluxing
The concept of internal electron donor–acceptor (D–A)
interaction has been extensively exploited to prepare low
band-gap organic semiconductors.3 Using this strategy, new
low band-gap materials have been developed to better cover
the solar spectrum, especially in the 1.4–1.9 eV region.4 In
addition, it is generally accepted that the formation of strong
intermolecular p–p stacking is necessary to achieve high
charge mobility in organic semi-conductors. Rigid coplanar
fused aromatic rings efficiently enhance this interaction and
hence improve charge mobilities.5,6 Therefore, copolymers
consisting of an electron-accepting benzothiadiazole-cored
oligothiophene and an electron-donating indolo[3,2-b]carba-
zole unit has been designed. We expect this polymer would
have narrow band-gaps and high charge mobility owing to the
large fused heterocyclic aromatic ring of indolocarbazole and
the D–A structure in the polymer backbone.
a Institute for Chemical Process and Environmental Technology
(ICPET), National Research Council of Canada (NRC), 1200
Montreal Road, Ottawa, Canada ON K1A 0R6.
E-mail: Jianfu.Ding@nrc-cnrc.gc.ca; Ye.Tao@nrc-cnrc.gc.ca
b Institute for Microstructural Sciences (IMS), National Research
Council of Canada (NRC), 1200 Montreal Road, Ottawa, Canada
ON K1A 0R6. E-mail: Jianping.Lu@nrc-cnrc.gc.ca
w NRCC publication Number: 49170.
z Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b811031j
y Current address: Department of Chemistry, Northeast Normal
University, 5268 Renmin Street, Changchun 130024, P. R. China
Scheme 1 Synthetic route to P(InCzTh2BTD).
Chem. Commun., 2008, 5315–5317 | 5315
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47% HBr solution,7 while 4,7-bis(4-octyl-2-thienyl)-2,1,3-benz-
othiadiazole (3) was brominated with N-bromosuccinimide
(NBS) at room temperature in the presence of silica gel as a
catalyst. The bromination of 4,7-bis(3,40-dioctyl-2,2’-bithio-
phen-5-yl)-2,1,3-benzothiadiazole (5) should be done under
even milder condition, at ꢁ10 1C to avoid the bromination at
the other positions. 3,9-Dibromoindolo[3,2-b]carbazole was
synthesized based on literature procedures,8 and then alkylated
with 2-ethylhexyl bromide, 2-butyloctyl bromide or 2-hexyldecyl
bromide. The resulting alkylated dibromoindolocarbazoles
were converted to the corresponding diboronates via lithiation
with n-butyllithium at ꢁ78 1C, followed by the treatment with
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Low
band-gap conjugated polymers were synthesized by Suzuki
coupling reaction between indolocarbazole diboronates and
benzothiadiazole-cored oligothiophene dibromides in reflux-
ing toluene using Pd(PPh3)4 as a catalyst under Ar. It was
found that when the alkyl group attached at the N atoms of
indolocarbazole was 2-ethylhexyl, the solubility of the result-
ing polymer in organic solvents was very poor. The solubility
was improved when the size of alkyl chain was increased from
C8 (2-ethylhexyl) to C12 (2-butyloctyl), leading to an enhanced
solubility in hot dichlorobenzene. However, being soluble only
in hot solvents is not convenient for device fabrication. With
the aim of further enhancing the solubility, a branched C16
chain (2-hexyldecyl) was selected and the obtained polymers,
P(InCzTh2BTD) were easily soluble in chlorinated hydrocar-
bons such as chlorobenzene, dichlorobenzene and chloroform
even at room temperature. The polymer is also well soluble in
the polymerization solution, possesses a high molecular weight
(Mn = 19.5 kDa) with a narrow distribution (PDI = 2.08),
and was obtained in high yield (above 80%). The purity was
confirmed by elemental analysis.
the benzothiadiazole-cored oligothiophene unit. Compared
with the solution absorption, the film sample prepared by
spin-coating a dichlorobenzene solution using a slow solvent
evaporation process,2c showed pronounced peak broadening
and a significant red shift of B100 nm of the absorption edge.
More importantly, this feature remained in the polymer/
PC61BM blending film used in the PV device (see ESIz). This
phenomenon is attributed to the planarization of aryl rings
and the presence of interchain interactions in the solid state.9
More interestingly, in contrast to the strong solution fluores-
cence, the thin films only exhibited very weak fluorescence
(not shown). This suggests the formation of strong interchain
interaction in the solid state, leading to fluorescence
self-quenching.10
The thermal properties of P(InCzTh2BTD) were analyzed
by DSC and TGA, showing high thermal stability with onset
decomposition temperatures 4410 1C under nitrogen. Fig. 2
shows that the polymer has a strong tendency to crystallize, as
evidenced by the appearance of a very sharp crystallization
peak at 209 1C, and the lack of an apparent glass transition.
The heats of melting and crystallization are both about
17 J gꢁ1, similar to regioregular P3HT.11 The physical proper-
ties of the polymer are summarized in Table 1.
Cyclic voltammetry (CV) measurements of this polymer
were carried out under argon in a three-electrode cell using
0.1 M Bu4NPF6 in anhydrous CH3CN as the supporting
electrolyte. The polymers were coated on the platinum-work-
ing electrode. The CV curves were recorded referenced to an
Ag quasi-reference electrode, which was calibrated using a
ferrocene–ferrocenium (Fc/Fc+) redox couple (4.8 eV below
the vacuum level) as an external standard. The E1/2 of the Fc/
Fc+ redox couple was found to be 0.40 V vs. the Ag quasi-
reference electrode. The CV scans of the polymer film resulted
in highly reproducible cathodic reduction and anodic oxida-
tion processes (see ESIz), indicating a potentially high stability
of the material for both hole and electron injection. As shown
in Fig. 3, an additional reduction peak was observed for
P(InCzTh2BTD), probably owing to the electron-withdrawing
benzothiadiazole units in the polymer backbone. The forma-
tion of this peak narrowed the energy gap, in comparison with
P3HT.2 Meanwhile, its HOMO energy level remains low at
B5.17 eV, which is important for maintaining the large open-
circuit voltage (Voc) and high air stability.
The absorption spectra of P(InCzTh2BTD) measured in
chloroform are characterized with a strong, broad and struc-
tureless absorption peak at 538 nm (labsmax), corresponding to
the intramolecular charge-transfer (ICT) transition, together
with a strong absorption band at shorter wavelength (B395 nm)
owing to higher energy transitions such as a p–p* transition.
Fig. 1 shows that the polymer emits deep red fluorescence in
CHCl3 solution with an emission peak at 691 nm, regardless of
excitation at 400 or 535 nm. This suggests that there is a very
efficient energy transfer from the indolocarbazole unit to
Fig. 1 Absorption spectra of P(InCzTh2BTD) in chloroform and in a
thin film of 100 nm in thickness; its fluorescence spectrum in chloro-
form is also included.
Fig. 2 DSC curves of P(InCzTh2BTD) at a heating–cooling rate of
10 1C minꢁ1 under nitrogen.
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Table 1 Physical properties of P(InCzTh2BTD)
PCE
=
3.6%, Voc
=
0.69 V, short circuit current
(Jsc) = 9.17 mA cmꢁ2, and fill factor (FF) = 0.57 have been
obtained in some preliminary devices. This result confirms our
previous finding that high crystallinity of the photoactive
material is important to achieve high device performance.12
In future work, we will focus on device optimization to further
improve device efficiency and stability.
labsmax/nm (10ꢁ4emax/Mꢁ1 cmꢁ1 a
)
538 (4.39)
Eox1/2; Ered1/2/Vb
HOMO, LUMO/eVc
EgO,d EgE e/eV
Tmf, Tdecompg/1C
a
1.13; ꢁ1.29, ꢁ1.75
ꢁ5.17, ꢁ3.15
1.89, 2.02
258, 424
b
Measured in CHCl3. Half wave potential referred to Ag quasi-
c
d
reference electrode. Calculated from onset potentials. Optical en-
ergy gap estimated from the absorption edge of polymer solutions in
e
f
Notes and references
CHCl3. Energy gap = HOMO ꢁ LUMO. Determined by DSC
from 2nd heating after cooling with a rate of 10 1C minꢁ1 under
N2. 1 wt% Loss temperature, determined by TGA with a heating
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Fig. 3 Cyclic voltammograms of P(InCzTh2BTD) and P3HT thin films
on Pt electrode in 0.1 M Bu4NPF6 acetonitrile solution at 50 mV minꢁ1
.
The potential of this polymer to be used as a hole-transport-
ing light-absorbing component in photovoltaic cells were ex-
plored. Bulk heterojunction PV cells with a device structure of
ITO|PEDOT-PSS|P(InCzTh2BTD) : PC61BM (1 : 2 w/w)|LiF
(1 nm)|Al (100 nm) were fabricated using a slow solvent
evaporation process.2c Fig. 4 shows the current–voltage char-
acteristics of the fabricated PV cells in the dark and under the
AM 1.5 simulated solar illumination at irradiation intensity of
100 mW cmꢁ2. These PV cells gave relatively larger Voc than
P3HT based devices (0.60 V), which are attributed to the high
oxidation potential of P(InCzTh2BTD). These devices also
exhibited low series resistance (B10.3 O cm2), implying higher
charge mobility, being related to the higher crystallization
tendency of the polymer as shown in Fig. 2. As a result,
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Fig. 4 Current density–voltage characteristics of the P(InCzTh2BTD)
based devices under AM 1.5 simulated solar illumination of 100 mW cmꢁ2
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Chem. Commun., 2008, 5315–5317 | 5317