Synthesis of a polythieno[3,4-b]thiophene derivative with a low-lying HOMO level and its application in polymer solar cells

Article abstract of DOI:10.1039/c1cc12643a

A planar benzodithiophene with lower HOMO was copolymerized with the thieno[3,4-b]thiophene unit to obtain a new low band gap polymer of PBDPTT-C, which exhibited a higher open-circuit voltage (V_oc) of 0.8 V and a promising efficiency of 5.2%.

Full text of DOI:10.1039/c1cc12643a

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COMMUNICATION
Synthesis of a polythieno[3,4-b]thiophene derivative with a low-lying
HOMO level and its application in polymer solar cellsw
Lijun Huo,a Xia Guo,b Yongfang Li*b and Jianhui Hou*a
Received 5th May 2011, Accepted 16th June 2011
DOI: 10.1039/c1cc12643a
A planar benzodithiophene with lower HOMO was copolymerized
with the thieno[3,4-b]thiophene unit to obtain a new low band
gap polymer of PBDPTT-C, which exhibited a higher open-
circuit voltage (Voc) of 0.8 V and a promising efficiency of 5.2%.
In recent years, tremendous developments in polymer solar
Scheme 1 Molecular structures of PBDTTTs and PBDPTT-C.
cells (PSCs) intrigue more researchers to focus on studying
new conjugated polymer materials. The design and preparation
introduction of fluorine reduced the HOMO level of PBDTTTs
effectively, the synthesis routes of the fluorine substituted
derivatives are quite tedious and also their HOMO levels are
still not deep enough to get high Voc materials. Thus it would
be useful and also necessary to find another effective way to
lower the HOMO level of TT-based LBG polymers.
of low band gap (LBG) conjugated polymer donor materials
has been proved to be an effective approach to realize high
power conversion efficiencies (PCEs) for the PSCs.1–10 LBG
polymers exhibit broad absorption range, which extend to the
near-infrared region, correspondingly enhancing the short-circuit
current (Jsc) and PCE value. Besides the broad absorption
range, appropriately lowering the HOMO energy level of the
LBG polymers is also very important for achieving higher
open-circuit voltage (Voc) and higher PCEs, because the Voc of
PSC is closely related to the gap between the HOMO level of
the donor and LUMO level of the acceptor.11,12 Therefore, to
lower the HOMO level of the LBG polymers is one of the
effective ways to get highly efficient donor material of PSCs.
Thieno[3,4-b]thiophene (TT)-benzo[1,2-b:4,5-b0]dithiophene
(BDT)-based LBG polymers (PBDTTTs) are one kind of star
photovoltaic polymer donor materials in PSCs, and several
promising results have been reported in the past three years.4,6,7
In this series of materials, the HOMOs of PBDTTTs have
been tuned effectively by introducing electron-withdrawing
functional groups on the TT unit and hence the Voc of the
PSC based on PBDTTTs has been successfully improved from
0.6 V to 0.78 V.4 For example, the HOMO level of PBDTTTs,
as shown in Scheme 1, can be lowered from ꢀ5.01 eV (for
PBDTTT-E in Scheme 1) to ꢀ5.12 eV (for PBDTTT-C in
Scheme 1) by replacing the carboxylate group with the carbonyl
group,13 and when a fluorine was introduced, the HOMO of
PBDTTT-CF is further lowered to ꢀ5.22 eV.4 Although the
Two isomers of benzodithiophene, benzo[1,2-b:4,5-b0]-
dithiophene14(BDT) and benzo[2,1-b:3,4-b0]dithiophene8,15
(named as BDP for easier discrimination), have been proved
to be useful conjugated building blocks in photovoltaic
polymer materials. However, although the relationship
between molecular structures and photovoltaic properties of
PBDTTTs has been well investigated, the alternate copolymer
of BDP and TT is still not studied.15,16 From molecular
simulation and experimental results in some papers, the
HOMO levels of the homopolymers of BDT and BDP are
ꢀ5.16 eV and ꢀ5.70 eV, respectively.8b,14 Obviously, the latter
exhibits a lower HOMO level than that of the former, which
indicates that the new polymer containing the BDP unit can
show a lower-lying HOMO than those similar structure based
PBDTTT-C polymers. Therefore, a new conjugated polymer,
poly [(4,5-bis-(3,7-dimethyloctyl)-benzo[2,1-b:3,4-b0]dithiophene-
2,7-diyl)-alt-(2-(2-ethyl-hexanoyl)-thieno[3,4-b]thiophene-4,5-
diyl)] (PBDPTT-C), as shown in Scheme 1, was designed and
synthesized in this work.
PBDPTT-C was prepared through the synthetic route
shown in Scheme 2. In order to keep good solubility, branched
alkyl groups, 3,7-dimethyloctyl (DMO), were used as the
substituents on 4 and 5 positions of the BDP unit. And on
the TT unit, the 2-ethylhexynoyl group was used to keep a
good electron-withdrawing effect of the side group. The
dialkyl-yne, compound 2, was prepared through two steps
by lithium–halogen exchange reaction with B70% yield for
each step. Compound 4 was synthesized from compound 3
with 3 equiv. of dialkyl-yne in the presence of 5% equiv. of
palladium(II) acetate as a catalyst by an annulative coupling
a State Key Laboratory of Polymer Physics and Chemistry,
Beijing National Laboratory for Molecular Sciences Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: hjhzlz@iccas.ac.cn; Tel: +86-10-82615900
b CAS Key Laboratory of Organic Solids, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China.
E-mail: liyf@iccas.ac.cn; Tel: +86-10-62536989
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc12643a
c
8850 Chem. Commun., 2011, 47, 8850–8852
This journal is The Royal Society of Chemistry 2011

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Scheme 2 Synthetic route of the copolymer PBDPTT-C. (i) 1-bromo-
3,7-dimethyloctane, DMSO, 0 1C, then 12 h at ambient temp.;
(ii) n-butyllithium, THF, ꢀ25 1C, 20 min; hexamethylphosphoramide
and 1-bromo-3,7-dimethyloctane; (iii) Zn, HOAc, CH3OH, 0 1C;
(iv) compound 2, Pd(OAc)2, Et3N, DMF, 130 1C, 12 h; (v) NBS,
DMF, 15 h; (vi) n-butyllithium, THF, ꢀ78 1C, 30 min, then trimethyl-
tin chloride, 1 h at ambient temp.; (vii) Pd(PPh3)4, toluene, DMF,
110 1C, 18 h.
Fig. 2 Cyclic voltammogram of a PBDPTT-C film on a platinum
electrode in acetonitrile solution containing 0.1 mol Lꢀ1 Bu4NPF6 at a
scan rate of 20 mV sꢀ1
.
at a negative potential range. Based on the onset oxidation
potentials (Eox) and the onset reduction potentials (Ered) of the
polymer, HOMO and LUMO energy levels and the energy gap
of the polymer were calculated: EHOMO is ꢀ5.35 eV, ELUMO is
ꢀ3.80 eV, and the electrochemical bandgap (Eegc) of PBDPTT-C
is 1.58 eV which well matches its optical bandgap of 1.55 eV.
As listed in Table 1, for PBDTTT-CF which exhibits a
lowest HOMO level in PBDTTTs, the HOMO level value is
ꢀ5.22 eV.4 In comparison with PBDTTTs, PBDPTT-C
exhibits a deeper HOMO level, which is helpful to get higher
Voc in PSCs.
reaction in 50% isolated yields. The BDP monomer, M1, can be
prepared directly from compound 4; however, since compound 4
can be easily oxidized by the air, it would be better to make the
bromide, compound 5, for long term storage. The TT monomer,
M2, was synthesized by the reported method.13 The final
product, PBDPTT-C, was prepared through the Stille coupling
reaction, which shows a low molecular weight (Mw = 16.5 K)
and a rather big polydispersity index (PDI = 2.8). The
polymer can be readily dissolved in common organic solvents
like toluene, chlorobenzene, tetrahydrofuran or others, and also
possesses good film-forming ability. It has to be mentioned
that since M1 cannot be purified by flash column or recrystall-
ization, we tried different polymerization conditions but failed
to make a higher molecular weight polymer.
The mobility of a blend of PBDPTT-C and PC70BM was
measured by the space-charge-limited-current (SCLC) method.
A hole mobility of 2.7 ꢁ 10ꢀ4 cm2 Vꢀ1 ꢀ1 was observed. PSC
S
devices were fabricated with a structure of ITO/PEDOT:PSS/
PBDPTT-C: PC70BM/Ca/Al. Different donor/acceptor weight
ratio values were tested to optimize the photovoltaic process,
the devices results are listed in Table 2. It was found that the
best photovoltaic performance was obtained under a 1 : 2 ratio
(w/w, polymer/PC70BM) by using dichlorobenzene with 3%
(v/v) DIO (diiodooctane)18 as the solvent to make solution for
a spin-coating process of the active layers. As shown in the
AFM images (see ESIw), we found that when DIO was used as
an additive, smaller domain size can be observed in the active
layer. Fig. 3 shows the I–V curve of the PBDPTT-C-based
PSC device with champion performance. It shows that under
Fig. 1 shows the absorption spectra of PBDPTT-C solution
and film. The main absorption peak and the absorption onset
of the solution are at 619 nm and 752 nm, respectively, and the
thin film exhibits a peak at 662 nm and a onset of 800 nm. The
obvious red-shifted absorption can be attributed to the
strong aggregation of the polymer in the solid state. Based
on the absorption edge, optical band-gap (Eogpt) of PBDPTT-C is
1.55 eV, which is similar with the optical band gap of PBDTTTs.4
Electrochemical cyclic voltammetry is employed to investigate
the redox behavior.17 The cyclic voltammogram (see Fig. 2)
shows that there is a reversible p-doping/dedoping (oxidation/
re-reduction) process at a positive potential range and an
irreversible n-doping/dedoping (reduction/reoxidation) process
Table 1 Properties of several TT-based polymers
HOMO/eV
Eogpt/eV
Voc/V
Ref.
PBDTT-E
ꢀ5.01
ꢀ5.12
ꢀ5.12
ꢀ5.22
ꢀ5.35
1.60
1.59
1.63
1.58
1.55
0.62
0.70
0.74
0.76
0.82
4
4
6
4
—
PBDTTT-C
PBDTTT-EF
PBDTTT-CF
PBDPTT-C
Table 2 Photovoltaic results based on PBDPTT-C:PC70BM with
different weight ratios
D : A (weight ratio)
Voc/V
Jsc/mA cmꢀ2
FF (%)
PCE (%)
1 : 1
1 : 2
1 : 2a
1 : 3
0.79
0.78
0.82
0.80
ꢀ9.52
ꢀ10.98
ꢀ11.76
ꢀ7.17
53.7
51.1
54.1
42.2
4.04
4.38
5.21
2.42
Fig. 1 Normalized UV-Vis absorption spectra of PBDPTT-C in
a
3% DIO additive (v/v).
diluted chloroform solution and as a solid thin film.
c
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Chem. Commun., 2011, 47, 8850–8852 8851

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HOMO level at ꢀ5.35 eV, a similar narrow bandgap of
1.55 eV and a similar broad visible absorption in comparison
with PBDTTTs. The PSC based on PBDPTT-C as a donor
and PC70BM as an acceptor with a weight ratio of 1 : 2,
w/w showed a high PCE of 5.21% with a higher Voc of
0.82 V. Moreover, even higher Voc can be expected if the
fluorine-substituted thieno[3,4-b]thiophene, like that used in
PBDTTT-CF,4 was used in the copolymer. Furthermore,
since the photovoltaic properties of conjugated polymers are
susceptible to molecular structural changes, even a very little
change in their side chain can make big difference in photo-
voltaic properties. Further optimization of the molecular
structure would be necessary to fully explore the potential of
the copolymers (PBDPTTs) of BDP and TT. Therefore,
PBDPTTs should be another type of promising high perfor-
mance photovoltaic polymer donor materials for PSCs.
This work was financially supported by NSFC (No.
20874106, 20821120293, 50633050, and 20721061) and National
Science Foundation (CHE-0822573).
Fig. 3 I–V curve of the PSC device based on PBDPTT-C under
illumination of AM 1.5G, 100 mW cmꢀ2
.
the illumination of AM 1.5G, 100 mW cmꢀ2, the device
exhibited an open circuit voltage (Voc) of 0.82 V, a short
circuit current (Jsc) of 11.76 mA cmꢀ2, a fill factor (FF) of
0.54, and a power conversion efficiency (PCE) of 5.21%. As
listed in Table 1, benefitting from the deeper HOMO level
value, the Voc of the device based on PBDPTT-C is higher
than that of the PSCs based on PBDTTTs.4 Since the length of
alkyl side chains and the molecular weight influence the
photovoltaic properties of a conjugated polymer greatly, the
alkyl side chains and the molecular weight of PBDPTT-C
should be optimized further to explore its photovoltaic properties
fully. For example, different side chains, like octyl, dodecyl,
or 2-ethyl-hexyl, can be used to get an optimized molecular
structure.
Notes and references
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From the external quantum efficiency (EQE) plot of the
PSC based on PBDPTT-C/PC70BM (3% additive), as shown
in Fig. 4, it can be seen that the device has a broad response
range in the whole visible range, from 380 nm to 780 nm,
which is much similar to the PSC devices based on the
PBDTTTs. However, the maximum EQE value of PBDPTT-C-
based PSC device, which is near to 50%, is lower than the best
value of ca. 70% for the PBDTTT-based PSCs, which agrees
with the lower Jsc of the device based on PBDPTT-C in
comparison with that of the PBDTTTs-based devices.
In conclusion, a new thieno[3,4-b]thiophene-based LBG
polymer, PBDPTT-C, was synthesized for the application as
donor material in PSCs. PBDPTT-C exhibited a deeper
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Fig. 4 EQE curves of the PSCs based on PBDPTT-C.
c
8852 Chem. Commun., 2011, 47, 8850–8852
This journal is The Royal Society of Chemistry 2011