New donor-acceptor benzotrithiophene-containing conjugated polymers for solar cells

Full text of DOI:10.1134/S0012500814020037

ISSN 0012ꢀ5008, Doklady Chemistry, 2014, Vol. 454, Part 2, pp. 25–31. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © M.L. Keshtov, S.A. Kuklin, V.S. Kochurov, I.O. Konstantinov, M.M. Krayushkin, N.A. Radychev, A.R. Khokhlov, 2014, published in Doklady Akademii
Nauk, 2014, Vol. 454, No. 4, pp. 417–423.
CHEMISTRY
New Donor–Acceptor
BenzotrithiopheneꢀContaining
Conjugated Polymers for Solar Cells
M. L. Keshtova, S. A. Kuklina, V. S. Kochurovb, I. O. Konstantinovc, M. M. Krayushkinc,
N. A. Radychevd, and Academician A. R. Khokhlova
Received October 23, 2013
DOI: 10.1134/S0012500814020037
absorption spectrum and narrows the band gap. In
addition to the combination of existing donor and
acceptor fragments, it is also important for design of
new donor–acceptor copolymers to develop new
donor and acceptor structures for improving the
understanding of the physical processes and the effiꢀ
ciency of solar cells.
In recent years, much effort has focused on develꢀ
oping organic solar cells [1–3]. Their electronic propꢀ
erties have still not achieved those of inorganic semiꢀ
conductors; however, this disadvantage is compenꢀ
sated by the manufacturability, low cost, and
possibility of producing flexible and light largeꢀarea
devices by solution spreading, centrifuging, and jet
printing. To use organic solar cells as an alternative to
inorganic solar cells, it is necessary to improve the
electronic properties of organic semiconductors. In
the last years, electron–donor polymeric materials
used in combination with fullerene as an acceptor in
bulk heterojunction organic photovoltaic devices have
been widely developed.
In this paper, we report a new electron–donor benꢀ
zotrithiophene (BTT) structure, which is not only a
promising donor in donor–acceptor copolymers but
also can be used as a holeꢀtransporting copolymer in
organic field transistors. The large BTT structure is
planar and leads to an effective intermolecular packꢀ
age in the solid state. BTTꢀbased polymers have a
highly ordered crystal structure and exhibit high
mobility of charge carriers. However, structures of this
type have been little studied [7–9].
Steady progress in energy conversion efficiency (an
efficiency of more than 8%) is observed [4–6]. The
donor–acceptor approach is most commonly used for
producing suitable donor materials with a wide
absorption band of the solar spectrum and the correꢀ
sponding energy levels to ensure an efficient transfer of
an electron to the acceptor in donor–acceptor copolꢀ
ymers. Intramolecular charge transfer extends the
In this work, we have designed a new BTTꢀcontainꢀ
ing monomer as a promising “building block” and
used it in synthesis of four new BTTꢀcontaining narꢀ
rowꢀgap donor–acceptor conjugated polymers as an
electroactive material for solar cells.
a Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, ul. Vavilova 28, Moscow,
119991 Russia
RESULTS AND DISCUSSION
To obtain new narrowꢀgap donor–acceptor copolꢀ
ymers, we synthesized a new monomer—5,8ꢀbis(triꢀ
b Moscow State University, Moscow, 119991 Russia
c Zelinskii Institute of Organic Chemistry, Russian Academy
of Sciences, Leninskii pr. 47, Moscow, 119991 Russia
d Carl von Ossietzky University of Oldenburg,
26129 Oldenburg, Germany
methylstannyl)ꢀ2ꢀdodecylbenzo[1,2ꢀ
'']trithiophene ( )—as the donor in D–A copolyꢀ
mers according to Scheme 1.
b:3,4ꢀb':5,6ꢀ
d
8
25

26
KESHTOV et al.
Br
S
Br
S
O
Br
C11H23
AlCl3
B(OH)2
NꢀBTT
DMF
S
Cl
O
Pd(PPh3)4, DMF
Br
O
Br
S
O
S
C11H23
S
S
H C
C1213H2131
S
H C
C H
S
23 11
11 23
1
2
3
4
Br
FeCl3
C12H25
S
O
C11H23
O
C11H23
C12H25
8
2
S
7
S
1
S
N2H4
Pd(PPH3)4
nꢀBuOH
nꢀBuLi
S
S
ClSn(CH3)3
S
S
6
1 S
S
S
S
5
Sn
Sn
2
Br
Br
4
3
8
7
6
5
Scheme 1.
Target monomer
of lauroyl chloride with 2,3ꢀdibromothiophene
give ketone , which was then treated with a twoꢀmole
8 was synthesized by the reaction nonequivalent aromatic hydrogen atoms. In the highꢀ
1
to
field region 2.99–0.50 ppm, the spectrum shows mulꢀ
tiplets due to 43 aliphatic hydrogen atoms (Fig. 1a).
The aliphaticꢀtoꢀaromatic ratio of integrated intensiꢀ
ties is consistent with the suggested structure. The 13C
NMR spectrum shows 12 signals in the range 120–
150 ppm due to 12 different aromatic carbon atoms,
and the upfield region of the spectrum shows also
12 signals assigned to 12 different aliphatic carbon
atoms. The spectrum also shows a singlet at –8.12 ppm
characteristic of the carbon atoms of the trimethylꢀ
stannyl group (Fig. 1b).
2
excess of thiopheneꢀ3ꢀboronic acid under Suzuki–
Miyaura conditions to form 5ꢀdodecanoylꢀ2,3ꢀbis(2ꢀ
thienyl)thiophene
then cyclized in the presence of FeCl3 to form 2,5ꢀ
dibromoꢀ8ꢀdodecanoylbenzo[1,2ꢀ :3,4ꢀ ':5,6ꢀ
'']trithiophene ( ). Compound was debrominaꢀ
ted to 2ꢀdodecanoylbenzo[1,2ꢀ :3,4ꢀ ':5,6ꢀ
'']trithiophene ( ), and the latter was reduced by
hydrazine hydrate N2H4 to 2ꢀdodecylbenzo[1,2ꢀ :3,4ꢀ
':5,6ꢀ '']trithiophene ( ) in high yield (74%). Finally,
compound was selectively lithiated at two free
positions in the presence of ꢀbutyllithium, and then
trimethylstannyl groups were introduced to give target
product
3. The latter was brominated and
b
b
d
5
5
b
b
d
6
b
b
d
7
7
α
ꢀ
As electronꢀacceptor monomers, we synthesized
n
5,8ꢀdibromoꢀ2ꢀheptadecylbisthieno[3,2ꢀ
][1,3]benzothiazole (14) (Scheme 2), 5,8ꢀdibromoꢀ
2ꢀ(4ꢀ((2ꢀethylhexyl)oxy)phenyl)ꢀ1ꢀoctylꢀ1
dithieno[3',2':3,4;2'',3'':5,6]benzo[1,2ꢀ ]imidazole
15) [10], 4,9ꢀbis(5ꢀbromothiophenꢀ2ꢀyl)ꢀ6,7ꢀdi(2ꢀ
ethylhexyl)ꢀ1,2,5]thiadiazolo[3,4ꢀ ]quinoxaline (16
[11], and 2,5ꢀdibromoꢀ8ꢀpentadecylꢀ10Hꢀ
bisthieno[2',3':6,7:3'',2'':4,5]indeno[1,2ꢀ ][1,3]thiazꢀ
singlets in the range 7.87–7.31 ppm attributed to three ole (17) [12].
e:2',3'ꢀ
g
8.
H
ꢀ
d
The compositions and structures of all intermediꢀ
ate compounds 1–7, as well as of monomer , were
confirmed by elemental analysis data, IR spectrosꢀ
(
8
g
)
1
1
copy, and H and 13C NMR. In particular, the H
d
NMR spectrum of compound show three lowꢀfield
8
DOKLADY CHEMISTRY Vol. 454
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2014

NEW DONOR–ACCEPTOR
27
Cl–
S
S
+
Br
N
Br
S
O
S
O
S
S
Br
Br2/AlCl3
PPh3/CCl4
OH
Cl
HO
S
Br
Br
O
O
9
10
11
12
6
7
S
Br
S
5 Br
Br
8
S
S
Br
4
S
H
C
C
H
N
35 17
17 35
FeCl3/CH3NO2
1S
N3
2C17H35
14
N
S
C17H35
13
Scheme 2.
suggested structure. The 13C NMR spectrum of comꢀ
pound 14 shows signals due to eleven different aroꢀ
matic carbon atoms in the range 113–175 ppm and
signals due to 17 different aliphatic carbon atoms in
the range 14–35 ppm.
The compositions and structures of all intermediꢀ
ate compounds 9–13 and target product 14 were conꢀ
firmed by elemental analysis data, IR spectroscopy,
and H and C NMR. In particular, the aromatic
region of the 1H NMR spectrum of target monomer 14
shows two singlets due to two nonequivalent protons of
the thiophene moiety; the spectrum also shows a tripꢀ
let at 3.17 ppm typical of the –CH2 group adjacent to
the thiazole moiety and a signal at 0.86 ppm due to the
1
13
The reaction of monomer 8 and dibromides 14–17
as, respectively, the donor and acceptors under the
Stille reaction conditions led to four new narrowꢀgap
terminal –CH3 group. The aliphaticꢀtoꢀaromatic conjugated polymers with the strict alternation of the
ratio of integrated intensities is consistent with the donor and acceptor units according to Scheme 3.
Table 1. Molecularꢀweight and thermal characteristics of polymers P1
–P4
Polymer
Mn
×
10–4
Mw
×
10–4
Mw
/
Mn
Tg
,
°
C
T5%, °C*
290
ꢀꢀꢀꢀꢀꢀ
P1
1.55
1.31
1.20
1.44
3.25
2.78
2.74
3.15
2.10
2.12
2.28
2.19
180
253
231
197
301
320
ꢀꢀꢀꢀꢀꢀ
P2
P3
P4
329
303
ꢀꢀꢀꢀꢀꢀ
310
298
ꢀꢀꢀꢀꢀꢀ
308
* In air, above the line, and under argon, below the line.
DOKLADY CHEMISTRY Vol. 454 Part 2
2014

28
KESHTOV et al.
C12H25
S
C12H25
S
S
S
S
S
Pd(PPh3)4
Sn
Sn + Br–Ar–Br
Ar
n
8
P1–P4
ꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
⎛
⎜
⎜
⎜
⎜
⎜
⎜
⎞
⎟
⎟
⎟
⎟
⎟
⎟
O
H C
C1351H3115
N
C17H35
N
⎜ꢀꢀ
ꢀꢀꢀ
Ar equal:
ꢀꢀ⎟
⎟
ꢀꢀꢀ
⎟
S
C8H17
⎜
;
;
;
N
N
S
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
N
N
S
S
S
S
S
S
S
S
N
N
S
Scheme 3.
Polycondensation was carried out by reacting the suggested structures of macromolecules. These
equimolar amounts of monomer and aromatic results, in combination with elemental analysis data,
dibromides 14 17 in a toluene–DMF solvent under demonstrate that the reaction was successful and
8
–
argon in the presence of tetrakis(triphenylphosꢀ occurred completely.
phine)palladium as a catalyst. To remove the metal
The molecularꢀweight distribution of the polymers
complex catalyst residues and lowꢀmolecularꢀweight
impurities, the resulting polymers were reprecipitated
twice from the solution into methanol and successively
extracted with methanol, hexane, acetone, and chloꢀ
roform in a Soxhlet apparatus. The chloroform fracꢀ
tion was evaporated to a minimum and precipitated
into methanol. The precipitate was filtered off and
vacuumꢀdried for one day. The yield of the polymers
was 65 and 79%, respectively.
was determined by gel permeation chromatography
(GPC) in THF with polystyrene as a reference. The
numberꢀaverage molecular weight (Mn) and dispersity
Mw/Mn) of polymers P1–P4 vary within 12 000–
15 500 and 2.10–2.28, respectively. The polymers are
soluble in organic solvents, such as DMF, Nꢀmethꢀ
(
ylpyrrolidone, chloroform, THF, and oꢀdichlorobenꢀ
zene, owing to side alkyl substituents, which decrease
the interaction of copolymer molecules and increase
their solubility.
The compositions and structures of copolymers
P1–P4 were confirmed by elemental analysis data and
1H NMR. In particular, the 1H NMR spectra of polyꢀ
mers P1 and P2 show multiplet signals corresponding
to aromatic protons in the δH ranges 7.65–7.09 and
7.72–7.27 ppm, respectively. In the δH range 4.22–
4.10 and 4.40–4.15 ppm, respectively, there are
broad signals corresponding to the protons of СН2
groups directly bonded to the oxygen atoms of the 2ꢀ
(4ꢀ((2ꢀethylhexyl)oxy)phenyl)ꢀ1ꢀoctylꢀ1 Hꢀ
The thermal properties of the copolymers were
studied by differential scanning calorimetry (DSC)
and thermogravimetry (TGA). The glass transition
temperature of polymers P1
within 180–253 (Table 1). All polymers are rather
thermally stable. The 5% weight loss temperature
Т5%) of the polymers in air and under argon is within
290–320 and 301–329 , respectively. These results
–P4 evaluated by DSC is
°С
(
°С
show that the polymers exhibit thermal stability suffiꢀ
cient for use in photovoltaic and other optoelectronic
devices.
dithieno[3',2':3,4:2'',3'':5,6]benzo[1,2ꢀd]imidazole
moiety (О–CH2–), and signals in the δH ranges 3.15–
2.60 and 2.08–0.80 ppm, respectively, are due to the
other protons of the alkyl substituent of the polymer
The optical properties of polymers P1
Vis spectroscopy (Table 2). The correꢀ
P4 is consistent with sponding absorption spectra of the polymers in chloꢀ
–P4 were
unit. The aliphaticꢀtoꢀaromatic ratio of integrated studied by UV
–
intensities for all polymers P1
–
DOKLADY CHEMISTRY Vol. 454
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2014

NEW DONOR–ACCEPTOR
29
p−v
o
n
(a)
l
m
j
k
h
i
a; b
f−n
f
g
c
d
e
c
S
S
S
p
t
8.0
c
7.8
7.6
7.4
7.2
Sn
v
Sn
s
a
b
r
u
o
a; b
d
e
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
, ppm
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
δ
26
27
24
25
22
23
(b)
20
21
18
19
16
22; 20
19
17
2
3
S
5
11
4
15
16
23; 24
6
21
18
S
S
17
25
29
13
31
9
1110
2
8
5
8
13
Sn
35
Sn
12
9
33
33
34
4
32
12
15; 6
32.0 31.5 31.0 30.5 30.0 29.5 29.0
16−25
29; 31; 32; 33; 34; 35
27
26
137
135
133
131
129
15; 6
11
3
10
5; 13; 9
2; 8
4
12
150 140 130 120 110 100
90
80
70
, ppm
60
50
40
30
20
10
0
−
10
δ
1
13
Fig. 1. (a) H and (b) C NMR spectra of monomer
8.
DOKLADY CHEMISTRY Vol. 454
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2014

30
KESHTOV et al.
1.0
0.8
0.6
0.4
0.2
0
4
1
1
2
3
3
2
4
400
600
800
1000
1200
−
2.5
−
2.0
−
1.5
−1.0
−
0.5
0
0.5 1.0 1.5
V, V
Wavelength, nm
Fig. 2. Absorption spectra of polymers (
1
)
P1, (2)
P2,
Fig. 3. Cyclic voltammograms of polymers (1) P1, (2) P2,
(3)
P3, and ( P4 in a chloroform solution.
4
)
(
3)
P3, and (4) P4.
roform are shown in Fig. 2. As follows from this figure,
polymer P2 has two absorption peaks in solution and
in films, which are usually specific features of D–A
0
0.4
0.8
3
copolymers. The absorption maxima of polymers P1
,
P3, and P4 in solution and in films vary within 413–
422 and 421–425 nm, respectively. For all polymers
P1–P4, a weak bathochromic shift is observed in
going from the solution to the film (Table 2) owing to
4
−
−
2
1
the increase in the conjugation length and planarity of
the polymer chain caused by the interchain
stacking effect in solid films. The optical band gap
widths of polymers P1 P4 determined from the onset
π π
–
–
−
0.2
0
0.2
V, V
0.4
0.6
of the absorption spectra of polymer films vary within
the range 1.1–2.3 eV and are rather close to the values
obtained electrochemically (Table 2).
Fig. 4. Current–voltage curves of solar cells based on polyꢀ
mers P1 P4 and PC BM.
–
The electrochemical properties of the polymers
were studied by cyclic voltammetry (Table 2). Typical
voltammograms are shown in Fig. 3. The cyclic voltaꢀ
mmograms exhibit one reduction peak at negative
potentials and one oxidation peak at positive potenꢀ
tials. The cathodic reduction peak is presumably due
to the formation of thiophene radical anions, and the
anodic oxidation peak corresponds to the thiophene
60
(
Eooxns ) and the reduction onset potential (Eroends ) of the
polymers, the energies of the highest occupied molecꢀ
ular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) and the band gap width of
radical cations. From the oxidation onset potential the polymers were calculated by the equations
Table 2. Optical and electrochemical properties of polymers P1–P4
ons
ox
ons
red
Eegc
Eogpt
λmax, nm
HOMO
LUMO
E
E
Polymer
in solution
in film
V
eV
P1
P2
P3
P4
420
443; 800
413
423
445; 812
421
0.6
0.2
0.2
0.3
–1.4
–1.2
–2.2
–2.1
–5.4
–5.0
–5.0
–5.1
–3.4
–3.6
–2.6
–2.7
2.0
1.4
2.4
2.4
2.3
1.1
2.3
2.3
422
425
DOKLADY CHEMISTRY Vol. 454
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2014

NEW DONOR–ACCEPTOR
Table 3. Photoelectric measurements of solar cells based on
31
REFERENCES
polymers P1–P4
1. Facchetti, A., Chem. Mater., 2011, vol. 23, no. 3, p. 733.
FF
Efficiency
Jsc
,
2. Kroon, R., Lenes, M., Hummelen, J.C., Blom, P.W.M.,
Polymer
Vi, V
mA/cm2
%
and Boer, B., Polym. Rev., 2008, vol. 48, no. 3, p. 531.
3. Chochos, C.L. and Choulis, S.A., Prog. Polym. Sci.
2011, vol. 36, no. 10, p. 1326.
,
P1
P2
P3
P4
0.93
0.69
0.13
0.27
0.52
0.45
0.33
0.57
28.4
32.2
21.0
23.4
0.14
0.10
0.01
0.04
4. Venkataraman, D., Yurt, S., Venkataraman, B.H., and
Gavvalapalli, N., J. Phys. Chem. Lett., 2010, vol. 1,
p. 947.
5. Arias, A.C., Mackenzie J.D., McCulloch, I., Rivnay, J.,
and Salleo, A., Chem. Rev., 2010, vol. 110, p. 3.
HOMO = –е( ,
Eooxns + 4.8) eV
6. Piliego, C., Holcombe, T.W., Douglas, J.D., Woo, C.H.,
Beaujuge, P.M., and Frechet, J.M.J., J. Am. Chem.
Soc., 2010, vol. 132, p. 7595.
LUMO = –
е
(
Eroends + 4.8) eV
,
Eegc
=
е(
E
– E ) eV.
ons ons
ox
red
7. Schroeder, B.C., Nielsen, C.B., Kim, Y.J., Smith, J.,
Huang, Z., Durrant, J., Watkins, S.E., Song, K.,
Anthopoulos, T.D., and McCulloch, I., Chem. Mater.,
2011, vol. 23, p. 4025.
It follows from Table 2 that the band gap width of
polymers P1 P4 varies within 1.4–2.4 eV. From the
electronic standpoint, these data are evidence that
polymers P1 P4 can act as efficient donors in bulk
–
–
8. Nielsen, C.B., Fraser, J.M., Schroeder, B.C., Du, J.,
White, A.J.P., Zhang, W., and McCulloch, I., Org.
Lett., 2011, vol. 13, no. 9, p. 2414.
heterojunction solar cells.
To study the photovoltaic properties of copolymers
P1–P4, polymeric bulk heterojunction solar cells
9. Taerum, T., Lukoyanova, O., Wylie, R.G., and
Perepichka, D.F., Org. Lett., 2009, vol. 11, no. 15,
p. 3230.
ITO/PEDOTꢀPSS/copolymer:PC60BM/CaAl with a
copolymer : PC60BM ratio of 1 : 1 in the active layer
were developed. The current–voltage characteristics
10. Keshtov, M.L., Marochkin, D.V., Kochurov, V.S.,
Komarov, P.V., Parashchuk, D.Yu., Trukhanov, V.A.,
and Khokhlov, A.R., Vysokomol. Soedin., Ser. B., 2014,
vol. 56, no. 1, p. 1.
of photovoltaic cells based on polymer P1
exposed to simulated AM1.5 sunlight (100 mW/cm2)
are presented in Table 3 and Fig. 4. ꢀDichorobenzene
was used for preparing the active layer. The idle voltage
Vi), shortꢀcircuit current (Jsc), fill factor (FF), and
–P4 : PC60BM
o
11. Keshtov, M.L., Toppare, L., Marochkin, D.V.,
Kochurov, V.S., Parashchuk, D.Yu., Trukhanov, V.A.,
and Khokhlov, A.R., Vysokomol. Soedin., Ser. B., 2013,
vol. 55, no. 6, p. 723.
(
solar cell efficiency vary within 0.33–0.57 V, 0.13–
0.93 mA/cm2, 21.0–32.2%, and 0.01–0.14%, respecꢀ
tively.
12. Shirinian, V.Z., Lonshakov, D.V., Kachala, V.V., Zavarꢀ
zin, I.V., Shimkin, A.A., Lvov, A.G., and Krayushꢀ
kin, M.M., Org. Chem., 2012, vol. 77, p. 8112.
ACKNOWLEDGMENTS
This work was supported by the Ministry of Educaꢀ
tion and Science (agreement no. 8848).
Translated by G. Kirakosyan
DOKLADY CHEMISTRY Vol. 454
Part 2
2014