Pyrazine-fused isoindigo: A new building block for polymer solar cells with high open circuit voltage

Article abstract of DOI:10.1039/c7cc01973d

Pyrazine-fused isoindigo (PzIIG) was designed and synthesized as a novel electron acceptor to construct two D-A conjugated polymers, PzIIG-BDT2TC8 and PzIIG-BTT2TC10. Both the polymers were successfully applied in polymer solar cells, and the PzIIG-BDT2TC8 based solar cell device exhibited a PCE of 5.26% with a high V_oc over 1.0 V.

Full text of DOI:10.1039/c7cc01973d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Pyrazine-Fused Isoindigo: A New Building Block for Polymer Solar
Cells with High Open Circuit Voltage
Jiu-Long Li, Yun-Fei Chai, Wei Vanessa Wang, Zi-Fa Shi, Zhu-Guo Xu, Hao-Li Zhanga,b*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pyrazine-fused isoindigo (PzIIG) was designed and synthesized as a achieve better planarity.10 Several TIIG based conjugated
novel electron-acceptor to construct two D-A conjugated copolymer exhibited enhanced mobility in field effect
polymers, PzIIG-BDT2TC8 and PzIIG-BTT2TC10. Both polymers transistors,8, 11 suggesting the important role of planarity for
were successfully applied in polymer solar cells, and the PzIIG- charge transport in conjugated copolymers.
BDT2TC8 based solar cell device exhibited a PCE of 5.26 % with a
high Voc over 1.0 V.
In this communication, we report a novel pyrazine-fused
isoindigo (named as PzIIG) unit, which possesses more planar
structure and stronger electron-withdrawing characteristic
compared with IIG. The PzIIG unit contains one pyrazine ring
inserted in the middle of the IIG. The pyrazine unit plays two
roles. First, pyrazine ring could form intramolecular hydrogen
bonding with the nearby phenyl unit to improve the molecular
planarity and rigidity, providing longer effective conjugation
length compared with IIG. Second, the pyrazine ring acts as a
strong electron deficient unit to improve the electron-
withdrawing ability. Given the more electron deficient nature
of pyrazine than thiophene, the PzIIG shall be a stronger
electron acceptor unit than TIIG. To demonstrate capability of
PzIIG as an efficient electron-withdrawing unit, two D-A
Donor-acceptor (D−A) conjugated polymers have
attracted great interest in recent years due to their
applications in organic field effect transistors (OFETs)1 and
polymer solar cells (PSCs).2 Organic optoelectronic devices
based on D-A conjugated polymers possess some attracting
characteristics, such as low cost fabrication, light-weight and
flexibility.3 Recently, new electron-deficient units have been
developed for the construction of D-A copolymers, such as the
4,5
double B
←
N bridged bipyridyl (BNBP
)
and several lactam
based electron-acceptors, including diketopyrrolopyrrole
7
(
DPP)6 and isoindigo (IIG
)
.
IIG is identified as one of the most efficient electron-
conjugated polymers, PzIIG-BDT2TC8 and PzIIG-BTT2TC10
,
withdrawing units to construct D-A conjugated polymers, due
to its large optical transition dipole moment, strong electron-
deficient characteristic and broad UV-vis absorption. IIG-based
D-A conjugated polymers usually possess strong
intramolecular charge transfer (ICT)8 and low optical band gap,
which are beneficial for charge transport and light absorption.
However, the structure of IIG is not perfect for organic
semiconductors. Steric-hindrance between the carbonyl
oxygen of the oxindole and the proton on the phenyl ring
makes the IIG unit slightly twisted from the desired planar
conformation.9 Ashraf’s group replaced the phenyl rings in IIG
with thiophene and synthesized thienoisoindigo (TIIG) to
were synthesized and applied in PSCs successfully. The PzIIG-
BDT2TC8 based PSCs showed a PCE of 5.26 % with a high Voc
up to 1.01 V.
The synthesis of the key building block brominated
pyrazine-fused isoindigo (named as BrPzIIG) is illustrated in
Scheme 1 and the details of the synthetic process are depicted
C8
H
(e)
(f)
Br
Br
+
N
N
O
O
Br
O
O
7
6
H
N
O
H
H
Br
N
(h)
O +Br
N
Br
O
Br
O
O
N
H
9
10
6
Scheme 1 Synthetic routes of BrPzIIG and BrIIG : (a) m-CPBA, EA, R.T., 24h,
(b) Ac2O, reflux, 24h, (c) MeONa, MeOH, R.T., 3h, (d) MnO2, Dioxane, reflux,
1h, (e) DMF, k2CO3, 80 oC, 2h, (f) TiCl4 / Zn, THF, (g) piperdine, EtOH, 80oC,
24 h, (h) acetic acid, HCl, reflux, 24h, (i) DMF, k2CO3, 80 oC, 2h.
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in Electronic Supplementary Information (ESI†). Pyrazine-2,5-
dialdehyde was obtained through four steps,12 which was the
most important intermediate molecule. Meanwhile, the octyl
unit was introduced in the 6-bromoisatin and one carbonyl
group was selectively reduced to obtain the alkylated 6-
bromoindolin-2-one. Then the pyrazine-2,5-dialdehyde and
alkylated 6-bromoindolin-2-one were used as substrates for
Knoevenagel condensations under alkaline condition to give
the BrPzIIG. For comparison purpose, brominated isoindigo
(named as BrIIG
) was also synthesized, the synthetic
Fig. 2 (a) HOMO and LUMO energy levels based on DFT calculations at the
B3LYP/6-311G (d, p) level of BrIIG and BrPzIIG; (b) Top view and side view
procedure is illustrated in Scheme 1 and the details are
depicted in ESI. The BrPzIIG exhibits good solubility in
common organic solvents such as dichloromethane,
chloroform and ethyl acetate, making it an ideal reactant for
further coupling reactions.
single-crystal structure of BrPzIIG
.
the calculated HOMO and LUMO energy levels of BrIIG and BrPzIIG.
The HOMO and LUMO of BrIIG show relatively homogeneous
distribution over the entire molecule. While the LUMO of BrPzIIG is
largely distributed on the pyrazine ring and the HOMO of BrPzIIG is
mainly localized on the indolone moieties. Meanwhile, the
calculated HOMO and LUMO are -6.06 eV and-3.22 eV for BrIIG,
and are -6.02 eV and -3.44 eV for BrPzIIG, respectively. Compared
with BrIIG, the HOMO of BrPzIIG was slightly raised for 0.04 eV and
the LUMO of BrPzIIG declined for 0.22 eV. The reduced band gap of
BrPzIIG may be attributed to the electronegative nitrogen atom in
the pyrazine ring. The cyclic voltammetry (CV) of BrIIG and BrPzIIG
were measured (Fig. S2, ESI†), from which the HOMO and LUMO
were estimated to be -5.77 eV and -3.66 eV for BrIIG and -5.69 eV
and -3.68 eV for BrPzIIG, respectively.
Introducing the pyrazine ring in the middle of the
isoindigo greatly influenced its absorption properties. The UV-
vis absorption spectra of BrIIG and BrPzIIG are shown in Fig. 1
(a). BrIIG and BrPzIIG exhibit similar absorption features, with
two major absorption bands located around 400 nm and 500
nm. The onset absorption edges of BrIIG and BrPzIIG are at
610 nm and 590 nm, corresponding to optical bandgaps of
2.03 eV and 2.10 eV, respectively. However, the absorbance of
the BrPzIIG are nearly twice as that of the BrIIG, in the range
between 300 nm and 600 nm. The maximum molar extinction
coefficient for BrIIG is 17218 M-1∙cm-1 at 400 nm and the
maximum molar extinction coefficient for BrPzIIG is 29151 M-
1∙cm-1 at 405 nm. The much higher absorbance of BrPzIIG can
be attributed to the extended conjugation13 due to the higher
planarity. Fig. 1 (a) indicates that BrPzIIG is a much better light
absorbing unit in the visible region compared with BrIIG. The
absorption spectrum of BrPzIIG was simulated by time-
dependent density functional theory (TD-DFT) 14 (Fig. S1, ESI†)
and the simulated spectrum is generally consistent with the
experimental data. The simulation results indicate that the
absorption band around 500 nm of BrPzIIG corresponds to the
S0–S1 transition.
The single crystal of BrPzIIG was obtained. Fig. 2 (b)
displays the single crystal structure and the molecular packing
of BrPzIIG. From the top view, an intramolecular hydrogen
bonding between the nitrogen atom on the pyrazine ring and
the hydrogen atom on the indolone can be observed. The
length of the hydrogen bonding is 2.34 Å, which is accordance
with the N-H hydrogen bonding distance. The side view
confirms that the intramolecular hydrogen bonding makes the
PzIIG very planar, which is in contrast to the twisted
conformation of IIG. In the crystal, the BrPzIIG forms slipped
face-to-face π-π stacked column with the plane distance of
3.29 Å. Both the close π-stacking and N-H hydrogen bonding
may facilitate charge transfer in PzIIG based copolymers.
To assess the electron-withdrawing capability of the PzIIG
To further understand the relationship between the
molecular structure and the frontier molecular orbital energy
level, the highest occupied molecular orbital (HOMO) level and
the lowest unoccupied molecular orbital (LUMO) level of BrIIG
and BrPzIIG were calculated by density functional theory (DFT)
at the B3LYP/6-311G (d, p) level14 and the alkyl chains were
substituted by methyl group for simplicity. Fig. 2 (a) displays
unit, two benzodithiophene (BDT) derivatives, BDT2TC8 and
BTT2TC10, were selected as the electron-donating units. Using
the BrPzIIG and the two BDT derivatives as starting materials,
(a)
(b)
4
1.4
PzIIG-BDT2T-C8-solution
PzIIG-BDT2T-C8-film
BrIIG
1.2
1.0
0.8
0.6
0.4
0.2
0.0
BrPzIIG
PzIIG-BTT2T-C10-solution
PzIIG-BTT2T-C10-film
3
2
1
0
300 400 500 600 700 800 900
300
400
500
600
700
Wavelength (nm)
Wanvelength (nm)
Fig.
1 (a) UV-vis absorption spectra of BrIIG and BrPzIIG; (b) Normalized
Scheme 2 Synthetic routes of PzIIG-BDT2TC8 and PzIIG-BTT2TC10.
absorption spectra of PzIIG-BDT2TC8 and PzIIG-BTT2TC10.
2 | J. Name., 2012, 00, 1-3
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(a)
(b)
Pd2(dba)3 and P(o-tol)3 as catalysts, Stille-coupling
polymerization in toluene gives two novel D-A conjugated
2
0
45
40
35
30
25
20
15
10
5
P
zIIG-BDT2TC8 : PC61BM=1:1
PzIIG-BDT2TC8 : PC61BM=1:2
PzIIG-BDT2TC8 : PC61BM=1:3
PzIIG-BDT2TC8 : PC61BM=1:2 (3% DIO)
PzIIG-BDT2TC8 : PC71BM=1:2 (3% DIO)
copolymers: PzIIG-BDT2TC8 and PzIIG-BTT2TC10
.
The
-2
-4
-6
-8
synthetic routes of PzIIG-BDT2TC8 and PzIIG-BTT2TC10 are
listed in Scheme 2 and the details of the synthetic process are
described in ESI. The synthesized polymers were purified by a
silica gel column (60 to 80 mesh) to afford metallic luster dark
brown solid. The polymers exhibit good solubility in common
organic solvents such as chloroform, toluene, chlorobenzene
and o-dichlorobenzene. As determined by gel permeation
chromatography (GPC) using THF as the eluent and
monodisperse polystyrene as the standard, the PzIIG-BDT2TC8
has a number-average molecular weight (Mn) of 28.2 kDa and
a polydispersity index (PDI) of 2.46, while the PzIIG-BTT2TC10
has a Mn of 23.5 kDa and a PDI of 2.18. Thermogravimetric
analysis (TGA) shows excellent thermal stability of the PzIIG-
BDT2TC8, which shows a decomposition temperature (5%
weight loss) over 352 oC; and the PzIIG-BTT2TC10 shows a
PzIIG-BDT2TC8
PzIIG-BDT2TC8
PzIIG-BDT2TC8
PzIIG-BDT2TC8
PzIIG-BDT2TC8
:
:
:
:
:
PC61BM=1:1
PC61BM=1:2
PC61BM=1:3
PC61BM=1:2 (3% DIO)
PC71BM=1:2 (3% DIO)
0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Voltage (V)
300 400 500 600 700 800
Wavelength (nm)
(c)
(d)
2
35
30
25
20
15
10
5
PzIIG-BTT2TC10 : PC61BM=1:1
PzIIG-BTT2TC10 : PC61BM=1:2
PzIIG-BTT2TC10 : PC61BM=1:3
PzIIG-BTT2TC10 : PC61BM=1:2 (3% DIO)
0
-2
-4
-6
PzIIG-BTT2TC10 : PC71BM=1:2 (3% DIO)
PzIIG-BTT2TC10 : PC61BM=1:1
PzIIG-BTT2TC10 : PC61BM=1:2
PzIIG-BTT2TC10 : PC61BM=1:3
PzIIG-BTT2TC10 : PC61BM=1:2 (3% DIO)
PzIIG-BTT2TC10 : PC71BM=1:2 (3% DIO)
0
-0.2 0.0 0.2
0.4
0.6 0.8 1.0
300 400 500 600 700 800
Wavelength (nm)
Voltage (V)
Fig. 3 (a) J–V curves and (b) IPCE curves of PzIIG-BDT2TC8-PC61BM and PzIIG-
BDT2TC8-PC71BM based polymer solar cells under AM 1.5 G illumination, 100
mW/cm2; (c) J–V curves and (d) IPCE curves of PzIIG-BTT2TC10-PC61BM and PzIIG-
BTT2TC10-PC71BM based polymer solar cells under AM 1.5 G illumination, 100
mW/cm2.
o
decomposition temperature (5% weight loss) over 325 C (Fig.
S3, ESI† ).
The UV-vis absorption spectra of PzIIG-BDT2TC8 and PzIIG-
BTT2TC10 in CHCl3 solutions and in thin films are displayed in Fig. 1
(b). The two polymers show broad absorption bands from 300 nm
to 800 nm. The absorption bands in the high energy region from
300 nm to 500 nm are attributable to the delocalized excitonic π−π*
transition. The broad bands in the low energy region from 500 nm
to 800 nm are assigned to intramolecular charge transfer (ICT)
interactions between the D and A units in the building blocks of
polymers. The PzIIG-BDT2TC8 shows the maximum absorption
wavelength (λmax) at 620 nm and has a shoulder peak at 663 nm in
solution. The PzIIG-BTT2TC10 displays the maximum absorption
wavelength (λmax) at 614 nm in solution. In solid films, the shoulder
peak of PzIIG-BDT2TC8 turn into the maximum absorption peak and
red-shifted 14 nm, while a new shoulder peak is appeared at 675
nm for PzIIG-BTT2TC10. The red-shifted shoulder peak and new
shoulder peak can be attributed to orderly π-π stacking and shorter
π-π stacking distance in the solid state. The onset absorption edges
as the thin films for PzIIG-BDT2TC8 and PzIIG-BTT2TC10 were at
762 nm and 756 nm, corresponding to optical bandgaps of 1.63 eV
and 1.64 eV, respectively.
BDT2TC8 and PzIIG-BTT2TC10, molecular simulations by DFT model
were performed on their dimers with methyl substituted alkyl
chains for simplicity (Fig. S4, ESI†).14 The two dimers possess the
same values of the HOMO and LUMO energy levels (-5.06 eV for
HOMO and -3.06 eV for LUMO). Moreover, both dimers have
similar HOMO and LUMO distribution. The HOMO of two dimers
mainly locates at the BDT2TC8 and BTT2TC10 moieties and LUMO
mostly locates around the PzIIG moieties, indicating strong ICT. The
HOMO and LUMO energy levels of the two polymers were also
estimated by CV (Fig. S2, ESI†). The HOMO and LUMO levels of
PzIIG-BDT2TC8 are estimated to be -5.48 eV and -3.64 eV,
respectively. While the HOMO and LUMO levels of PzIIG-BTT2TC10
are -5.40 eV and -3.58 eV, respectively. According to previous
literatures, a conductive polymer is air stable if the HOMO energy
level is below the air oxidation threshold (ca. -5.27 eV).15,16
Therefore, the measured HOMO energy levels of PzIIG-BDT2TC8
and PzIIG-BTT2TC10 indicate that these two polymers are air stable.
Besides, the low-lying HOMO energy levels are also beneficial for
high Voc in the PSCs device.15 The LUMO energy levels of PzIIG-
BDT2TC8 (-3.64 eV) and PzIIG-BTT2TC10 (-3.58 eV) are 0.3 eV
higher than fullerene derivatives (Fig. S2, ESI†), which afford
In order to investigate the electronic properties of PzIIG-
Table 1 The photovoltaic parameters of the solar cells from PzIIG-BDT2TC8 and PzIIG-BTT2TC10
Active layer
Additive
without
without
without
3% DIO
3% DIO
without
without
without
3% DIO
3% DIO
Voc (V)
1.04
1.03
1.02
1.02
1.01
0.96
0.95
0.95
0.95
0.94
JSC (mA/cm2)
FF (%)
50.28
53.39
44.26
62.18
65.63
44.35
50.86
43.80
61.61
60.78
PCE (%)
2.74
3.12
2.33
4.24
5.26
1.49
1.89
1.31
2.66
3.36
PzIIG-BDT2TC8:PC61BM=1:1
PzIIG-BDT2TC8:PC61BM=1:2
PzIIG-BDT2TC8:PC61BM=1:3
PzIIG-BDT2TC8:PC61BM=1:2
PzIIG-BDT2TC8:PC71BM=1:2
PzIIG-BTT2TC10:PC61BM=1:1
PzIIG-BTT2TC10:PC61BM=1:2
PzIIG-BTT2TC10:PC61BM=1:3
PzIIG-BTT2TC10:PC61BM=1:2
PzIIG-BTT2TC10:PC71BM=1:2
5.23
5.68
5.16
6.68
7.93
3.50
3.92
3.16
4.55
5.89
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sufficient driving force to transfer charge from polymers to PCBM.
morphology of the films, which may explain the relatively low
Bulk heterojunction (BHJ) PSCs were fabricated in an inverted photocurrent. Therefore, the PCEs of the two polymers can be
device structure of ITO / PFN / polymer : PC61BM / MoO3 / Ag. The further enhanced by improving the film morphology.
two polymers have good solubility in chlorobenzene (CB), so
In summary, we have designed and synthesized a novel
blended polymers with PC61BM at different weight ratios in CB at electron acceptor structure, PzIIG. The introduction of pyrazine ring
1:1, 1:2 and 1:3 were tested to optimize the devices performance. in the middle of isoindigo induces intramolecular hydrogen bonding
1,8- diiodooctane (DIO) was used as the additives and the films and high molar extinction coefficient. Using the PzIIG as electron
o
were annealed at 120 C to obtain the optimal phase separation. withdrawing unit we constructed two D-A conjugated polymers:
The J–V curves and the incident photon-to-current conversion PzIIG-BDT2TC8 and PzIIG-BTT2TC10. Inverted polymer solar cell
efficiency (IPCE) curves of PzIIG-BDT2TC8 and PzIIG-BTT2TC10 devices based on PzIIG-BDT2TC8 and PzIIG-BTT2TC10 exhibited
based PSCs are displayed in Fig. 3. The PzIIG-BDT2TC8-PC61BM gave PCEs of 5.26 % and 3.36 %, respectively. It is noted that the PSCs
the best device performance based on the weight ratio at 1:2 based on PzIIG-BDT2TC8 exhibit high Voc above 1.0 V. Further
(w/w), DIO 3 % (wt.) in CB and annealing at 120 oC, which exhibited improvement in PSCs performance may include optimizing the
a PCE of 4.24 % with a Voc of 1.02 V, a Jsc of 6.68 mA / cm2, and a FF device preparation conditions through enhancing the morphology
of 62.18 %. The PzIIG-BTT2TC10-PC61BM gave the best device of active layers such as solvent vapor annealing (SVA) methods and
performance based on the weight ratio at 1:2 (w/w), DIO 3 % (wt.) selection of the additives. Design and synthesis of PzIIG containing
in CB and annealing at 120 oC, which exhibited a PCE of 2.66 % with D-A conjugated polymers with other donor units and different side
a Voc of 0.95 V, a Jsc of 4.55 mA / cm2, and a FF of 61.61 %. As chains are currently underway. Our results indicate that PzIIG is a
PC71BM possess much stronger absorbance in the visible region promising building block for constructing D-A conjugated
compare with PC61BM, PC71BM was used to fabricate PSCs device at copolymers for optoelectronic applications.
the same conditions for performance optimization.17 The PzIIG-
This work is supported by National Natural Science Foundation
BDT2TC8-PC71BM based device exhibited a PCE of 5.26 % with a Voc of China (NSFC. 51525303, 21233001, 21572086), Fundamental
of 1.01 V, a Jsc of 7.93 mA / cm2, and a FF of 65.63 %. The PzIIG- Research Funds for the Central Universities and the 111 Project.
BTT2TC10-PC71BM based device exhibited a PCE of 3.36 % with a Voc
of 0.94 V, a Jsc of 5.89 mA / cm2, and a FF of 60.78 %. In comparison, Notes and reference
Zhang has reported that IIG and BDT based D-A copolymer gave a
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Einerhand, Synth. Met., 1997, 87, 53-59.
17. Y. He and Y. Li, Phys. Chem. Chem. Phys., 2011, 13, 1970-1983.
18. K. Cao, Z. Wu, S. Li, B. Sun, G. Zhang and Q. Zhang, J. Polym. Sci.
Pol. Chem., 2013, 51, 94-100.
PCE of 4.02 %, which has very similar molecular structure to PzIIG-
BDT2TC8.18 It suggests that using the PzIIG to construct D-A
copolymer may be an effective way to obtain high performance
PSCs. The details of PSCs performance for PzIIG-BDT2TC8 and PzIIG-
BTT2TC10 are listed in Table 1. The corresponding IPCEs of the
polymer-based solar cell devices (Fig. 3 (b) and Fig. 3 (d)) were
measured, which matched well with the results obtained from the
J–V characteristics. High Voc over 1.0 V is not easy to obtain in the
PSCs. The two polymers based PSCs devices exhibited high open
circuit voltage, which can be attributed to the deep HOMO energy
levels (-5.48 eV for PzIIG-BDT2TC8 and -5.40 eV for PzIIG-BTT2TC10)
of the polymers (Fig. S2, ESI†). The HOMO energy level of PzIIG-
BDT2TC8 is 0.08 eV lower than that of the PzIIG-BTT2TC10, which
may explain the higher Voc of the PzIIG-BDT2TC8 based PSCs device.
The hole mobilities were measured by the space charge limited
current (SCLC) method (Fig. S5, ESI†), which gave moderate hole
mobilities of 6.57x10-5 cm2 v-1 s-1 for PzIIG-BDT2TC8 and 5.92x10-5
cm2 v-1 s-1 for PzIIG-BTT2TC10.19
The thin film morphologies of the polymers blend were
characterized by tapping-mode Atomic Force Microscope (AFM) (Fig.
S6, ESI†). Both polymers blend with DIO show smaller domain size
compared with the polymers blend without DIO, which result in
improved microphase separation in the films. The root-mean-
square (RMS) roughness was 5.96 nm and 6.45 nm for the PzIIG-
BDT2TC8 blend with DIO and the PzIIG-BTT2TC10 blend with DIO,
respectively. This result suggests the PzIIG-BDT2TC8 was more
homogeneous with better phase separation compared with PzIIG-
BTT2TC10, and the morphology of the PzIIG-BDT2TC8 based film
facilitates better device performance. Although adding DIO can
effectively improve the morphology of the films, the inhomogeneity
of the films and big RMS in the AFM images indicating poor
19. G. Zhang, Y. Fu, Z. Xie and Q. Zhang, Macromolecules, 2011, 44,
1414-1420.
4 | J. Name., 2012, 00, 1-3
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