Thiazolothiazole-based conjugated polymers for blade-coated organic solar cells processed from an environment-friendly solvent

Article abstract of DOI:10.1016/j.tetlet.2020.152037

High efficiencies of organic solar cells (OSCs) are generally achieved for small-area devices processed from halogenated solvents, which are hazardous to human health and pollute the environment. Commercialization of OSCs requires the development of novel photovoltaic materials, which are suitable for large-scale fabrication of devices through printable techniques using environmentally-friendly solvents. In this work, we report the synthesis of two novel polymers comprising thiazolothiazole, benzothiadiazole (P1), and benzoxadiazole (P2) moieties and the fabrication of polymer-fullerene OSCs and photovoltaic modules by blade coating under ambient conditions. The P1-based solar cells and larger area modules showed tolerance to processing solvents and delivered encouraging efficiencies illustrating polymer P1 as a promising material for environment friendly large-area photovoltaics.

Full text of DOI:10.1016/j.tetlet.2020.152037

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Thiazolothiazole-based conjugated polymers for blade-coated organic
solar cells processed from an environment-friendly solvent
Petr M. Kuznetsov a, Sergey L. Nikitenko a, Ilya E. Kuznetsov a, Pavel I. Proshin a,b, Daria V. Revina c,
Pavel A. Troshin d,a, Alexander V. Akkuratov a,
⇑
a Institute for Problems of Chemical Physics of the Russian Academy of Sciences (IPCP RAS), Academician Semenov Avenue 1, Chernogolovka 142432, Russian Federation
b Dmitry Mendeleev University of Chemical Technology of Russia, Higher Chemical College of RAS, Miusskaya sq. 9, 125047 Moscow, Russian Federation
c Moscow State University, Faculty of Fundamental Physical and Chemical Engineering, GSP-1, 1 Leninskiye Gory, 119991 Moscow, Russian Federation
d Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld.1 Moscow, 121205, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
High efficiencies of organic solar cells (OSCs) are generally achieved for small-area devices processed
from halogenated solvents, which are hazardous to human health and pollute the environment.
Commercialization of OSCs requires the development of novel photovoltaic materials, which are suitable
for large-scale fabrication of devices through printable techniques using environmentally-friendly sol-
vents. In this work, we report the synthesis of two novel polymers comprising thiazolothiazole, benzoth-
iadiazole (P1), and benzoxadiazole (P2) moieties and the fabrication of polymer-fullerene OSCs and
photovoltaic modules by blade coating under ambient conditions. The P1-based solar cells and larger
area modules showed tolerance to processing solvents and delivered encouraging efficiencies illustrating
polymer P1 as a promising material for environment friendly large-area photovoltaics.
Received 22 April 2020
Revised 6 May 2020
Accepted 11 May 2020
Available online xxxx
Keywords:
Organic solar cells
Photovoltaic modules
Blade coating
Thiazolothiazole
Polycondensation
Ó 2020 Elsevier Ltd. All rights reserved.
Introduction
[6]. Thus, successful commercialization of OSCs requires the design
and development of novel materials showing minimal efficiency
In the last decade, rapid progress has been made in improving
the performance of organic solar cells (OSCs). Recently, Chen and
co-workers reported the champion tandem OSCs with power con-
version efficiency (PCE) of 17.3% [1]. The achieved PCEs are already
higher than that of multiple-junction inorganic solar cells based on
amorphous silicon [2,3] and reached the threshold for the commer-
cialization of OSCs. Nevertheless, some drawbacks of state-of-
the-art OSCs have impeded their large-scale production and appli-
cation. In particular, record-high photovoltaic performances were
achieved for laboratory OSCs of only 2–5 mm2 in size, while
increasing the device active area leads to a severe roll-off in their
efficiency. Moreover, the active layers of high-efficiency solar cells
are deposited predominantly using spin-coating techniques, which
is not compatible with roll-to-roll production techniques [1,4,5].
Another challenge is related to the fact that the most efficient
devices are fabricated using chlorinated organic solvents
(chlorobenzene, dichlorobenzene, or chloroform), which are haz-
ardous for the environment and toxic for humans, which limits
their application for manufacturing of OSCs on industrial-scale
roll-off upon upscaling and processable from non-toxic halogen-
free solvents using roll-to-roll coating techniques suitable for mass
production.
Thiazolo[5,4-d]thiazole-based (Tz) conjugated polymers have
attracted significant attention in recent years as promising
semiconductor materials for efficient and large-area OSCs [7].
Conjugated polymers with Tz units exhibit the tendency for strong
p–p stacking in the solid state, which results in intense and broad
absorption in the visible region as well as high charge carriers
mobility [8–11]. Encouragingly PCEs > 10% were obtained recently
for large-area devices demonstrating the high potential of Tz
blocks for constructing efficient donor polymers [12].
In this work, we designed two novel Tz-based polymers with
A-D-A moieties (A-acceptor and D-donor units) and explored them
as absorber materials for OSCs and large-area photovoltaic mod-
ules. Both small-area solar cells and modules were fabricated using
doctor blade coating from standard dichlorobenzene (DCB) and
environment-friendly m-xylene solvents. The devices showed sig-
nificant tolerance to processing solvents and upscaling while deliv-
ering encouraging efficiencies thus addressing the challenge of
moving OSCs from research labs to industry.
⇑
Corresponding author.
E-mail address: akkuratow@yandex.ru (A.V. Akkuratov).
https://doi.org/10.1016/j.tetlet.2020.152037
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.
Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037

2
P.M. Kuznetsov et al. / Tetrahedron Letters xxx (xxxx) xxx
The thermal properties of the polymers were investigated by
Results and discussion
thermal gravimetry analysis (TGA) and differential scanning
calorimetry (DSC) under a nitrogen atmosphere with a heating rate
of 15 °C/min (Fig. S10a, ESI). The polymers showed high decompo-
sition temperatures (Td, corresponding to 5% weight loss) of 335 °C
for P1 and 320 °C for P2 indicating good thermal stability for their
application in OSCs. The differential scanning calorimetry
(Fig. S10b, ESI) profile exhibits an endothermic peak of 278 °C
and an exothermic peak of 244 °C corresponding to the melting
(Tm) and solidification (Tc) temperatures of polymer P1, respec-
tively. At the same time, no signals were recorded for polymer
P2, thus indicating the less ordered structure of polymer P2 in
the solid state compared to polymer P1.
Optical properties of the polymers P1 and P2 were investigated
in solution and in thin films. The normalized optical spectra are
shown in Fig. 1 and the corresponding data are listed in Table 1.
Spectra of P1 and P2 in dichlorobenzene solutions show absorp-
tion maxima at 525 nm and 529 nm, respectively. A shoulder peak
around 572 nm for polymer P2 can be seen, which suggests the
aggregation of polymer molecules in solution at room temperature.
While going from solution to thin films, a strong bathochromic
Conjugated polymers P1 and P2 were designed according to the
‘‘push–pull” concept with alternating donor (thiophene units) and
acceptor blocks (benzothiadiazole or benzoxadiazole and Tz units).
The synthesis of key monomers and polymers P1 and P2 is shown
in Scheme 1. Condensation of aldehyde 1 [13] with dithiooxamide
in N,N-dimethylformamide afforded 2,5-bis(3-(2-ethylhexyl)thio-
phen-2yl)-thiazolo[5,4-d]thiazole (2). Lithiation of compound 2
with n-butyllithium and further nucleophilic reaction of the
dilithium intermediates with trimethylchlorostannane afforded
the key monomer M1. Monomers M2 and M3 were synthesized
in three steps. First, 5,6-bis(octyloxy)-2,1,3-benzothiadiazole [14]
(3a) or 5,6-bis(octyloxy)-2,1,3-benzoxadiazole [15] (3b) were
brominated with 2 equiv. of N-bromosuccinimide (NBS) in acetic
acid. Further palladium-catalyzed Stille coupling of compounds
4a-b and 2,5-bis(trimethylstannyl)thiophene gave monomers M2
and M3. Conjugated polymers P1 and P2 were synthesized via
Stille polycondensation reaction.
Polymers P1 and P2 were purified by Soxhlet extraction with
hexane, acetone, dichloromethane, and chlorobenzene followed
by concentration of the chlorobenzene fractions and precipitation
of the polymers with ethanol. Both polymers P1-P2 exhibit good
solubility in common organic solvents such as chloroform,
dichlorobenzene (DCB), tetralin, and xylenes. The molecular
weight characteristics of P1 and P2 were estimated by gel perme-
ation chromatography at 50 °C in chlorobenzene as eluent (Fig. S1)
[16].
shift (Dkmax = 98–135 nm) of the absorption maxima can be seen
in the spectra of both polymers, thus indicating good supramolec-
ular ordering of the polymer chains in the solid state. The optical
bandgaps (Eogpt) of 1.7 eV and 1.66 eV were estimated from the
low energy band onset (kedge) in thin film spectra for polymers
P1 and P2, respectively (Table 1).
The electrochemical properties of polymer thin films were
investigated using cyclic voltammetry as reported previously [16]
(Fig. 1b). The oxidation onset potentials (Eoonxset) of P1 and P2 were
estimated as 0.52 V and 0.69 V, respectively. The HOMO energy
The weight average molecular weights (Mw) of 51 kDa and
78 kDa, with polydispersity indexes of 1.3 and 1.8 were obtained
for P1 and P2, respectively (Table 1).
Scheme 1. Synthesis of conjugated polymers P1 and P2. Reagents and conditions: (i) dithiooxamide, DMF, reflux; (ii) BuLi, À60 °C, Me3SnCl, RT; (iii) NBS, AcOH, 60 °C; (iv)
2,5-bis(trimethylstannyl)thiophene, Pd(PPh3)4 (0.002 equiv.), toluene, reflux.
Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037

P.M. Kuznetsov et al. / Tetrahedron Letters xxx (xxxx) xxx
3
Table 1
Physicochemical, optical, and electrochemical properties of polymers P1 and P2.
Eogpt, eV
Eooxnset,V vs. Fc+/Fc
HOMO, eV
LUMO, eVb
Td, °C
Tm/Tc, °C
a
sol
max
k
/kfmilamx/kefidlmge, nm
Mw, kDa
Mw/Mn
P1
P2
51
78
1.3
1.8
525/623/729
529/664/745
1.70
1.66
0.52
0.69
À5.62
À5.79
À3.92
À4.13
335
320
278/244
n/a
a
HOMO energy was estimated from onset of the oxidation potential using Fermi energy of À5.1 eV for the Fc+/Fc redox couple.
b
LUMO level was estimated as Eogpt + HOMO.
(60 nm)/Al (50 nm). The active fullerene derivative ([70]PCBM) –
conjugated polymer blend films were blade-coated in air from
1,2-dichlorobenzene as a standard solvent and from eco-friendly
m-xylene. It is well known that the morphology of active layer
films and consequently the photovoltaic performance of the OSCs
can be improved by using additives with high-boiling points [22].
Therefore, screening of different compounds revealed diphenyl
ether (DPE) as a promising processing additive for P1(P2)/[70]
PCBM blends.
On the other hand, enhancing the performance of OSCs as well
as their stability is closely related to interface engineering [23,24].
Therefore, we inserted polyhydroquinone [25] (PHQ) or fullerene
derivative BLF-P [26] as interfacial buffer layers between the active
layer and top electrode (Fig. 2b). These compounds were selected
because their LUMO energy levels match the LUMO of the acceptor
component [70]PCBM and the work function of the metal
electrodes.
The parameters of devices fabricated under different conditions
tested during optimization are shown in Table S1 (ESI). The photo-
voltaic characteristics of optimized solar cells are presented in
Table 2. Current density–voltage (J-V) curves and external quan-
tum efficiency spectra (EQE) measured under simulated AM1.5G
illumination (100 mW cmÀ2) are shown in Fig. 3. Small-area OSCs
based on polymer P1 showed efficiencies of ~7%. When using
dichlorobenzene as a solvent, the best performance was achieved
by adding 1% of DPE and introducing a PHQ buffer layer, while
xylene-processed OSCs showed the best characteristics without
using any interfacial layers (Table S1, ESI). Both devices demon-
strated high and comparable short-circuit current densities (JSC),
which match well with photocurrents obtained by integration of
the corresponding EQE spectra.
As illustrated in Fig. 3 and Table 2, benzoxadiazole-containing
polymer P2 showed lower efficiencies in solar cells: 6.4% and
5.7% when processed from dichlorobenzene and xylene, respec-
tively. Solar cells exhibited lower JSC values and fill factors (FF)
compared to that of P1-based devices that might be originating
from poor charge transport or inappropriate morphology of the
composite active layer [27]. Notably, OSCs with polymer P2 dis-
played significantly enhanced VOC that is in accordance with the
low-lying HOMO energy of this donor polymer. It is worth noting
that PHQ or BLF-P buffer layers did not improve the performance
of P2-based solar cells (Table S1, ESI).
Fig. 1. Absorption spectra (a) and cyclic voltammograms of conjugated polymers
P1 and P2 (b).
ox
levels of the polymers were calculated from E
potentials and
onset
using the value of À5.1 eV as the potential of the Fc+/Fc redox couple
in the Fermi energy scale [17]. Conjugated polymer P2 comprising
benzoxadiazole acceptor blocks showed 0.17 eV lower HOMO energy
compared to that of benzothiadiazole-containing P1. The observed
effect can be related to the higher electronegativity of the oxygen
atom [18,19]. Considering this fact, higher open-circuit voltages
can be obtained for solar cells based on P2 [20]. The LUMO energies
were calculated based on HOMO energies and the optical bandgap of
P1 and P2.
We investigated charge career mobilities for optimized poly-
mer-fullerene blends using the space-charge limited current
(SCLC) method [28]. The measurements were performed for the
hole-only
(ITO/PEDOT:PSS(60
nm)/blend/MoO3(22
nm)/Ag
(120 nm) and the electron-only (ITO/Yb(15 nm)/blend/Ca) devices.
It was found that the optimized P1/[70]PCBM blends have higher
and more balanced hole (lh) and electron (le) mobilities than
P2/[70]PCBM blends (Table 2). Thus, a more efficient charge trans-
port in P1-based films can explain the higher JSC and FF values
achieved for devices.
In order to understand the morphology of P1/[70]PCBM and P2/
[70]PCBM blend films processed from DCB and m-xylene, atomic
force microscopy (AFM) measurements were performed. AFM
topography height images are shown in Fig. 4 and Figs. S11 and
S12 (ESI).
Fig. 2 shows the energy levels diagram of the photovoltaic
device components. The difference between HOMO and LUMO
levels of the polymers and [70]PCBM ([6,6]-phenyl-C71-butyric
acid methyl ester) are greater than 0.3 eV, which is beneficial for
exciton dissociation into free charge carriers [21].
We used standard bulk heterojunction organic solar cell config-
uration ITO/PEDOT:PSS (60 nm)/active layer (100–200 nm)/Mg
Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037

4
P.M. Kuznetsov et al. / Tetrahedron Letters xxx (xxxx) xxx
Fig. 2. Energy levels diagram for components of organic solar cells (a) and chemical structures of the used interfacial buffer layer materials PHQ and BLF-P (b).
Table 2
Processing conditions and photovoltaic properties of optimized solar cells and modules based on P1/[70]PCBM and P2/[70]PCBM blends.
Area, cm2
0.3
Conditions
VOC, mV
JSC mA/cm2
FF, %
PCE, %
lh cm2VÀ1sÀ1
le cm2VÀ1sÀ1
lh/le
EQE
J
, mA/cm2
SC
a
P1
P2
DCB, DPE 1 vol%, PHQ
m-xylene, DPE 1 vol%
DCB, DPE 1 vol%
783
745
2230
2150
15.3
15.5
4.1
14.9
15.1
n/a
59
62
38
38
7.0
7.2
3.4
3.3
3.8 Â 10À4
4.2 Â 10À4
n/a
3.6 Â 10À4
4.3 Â 10À4
n/a
1.05
0.97
n/a
16.4
m-xylene, DPE 1 vol%
4.1
n/a
n/a
n/a
n/a
b
0.3
DCB
912
907
3269
3624
13.3
11.9
2.0
11.6
10.7
n/a
53
52
37
46
6.4
5.7
2.4
3.3
2.2 Â 10À4
1.3 Â 10À4
n/a
1.9 Â 10À4
2.4 Â 10À4
n/a
1.16
0.54
n/a
m-xylene, DPE 0.5 vol%
DCB
m-xylene
16.4
2.0
n/a
n/a
n/a
n/a
a
P1: [70]PCBM ratio À 1:2 (by weight), film thickness ca 170 nm, films annealing temperature 95 °C.
P2: [70]PCBM ratio À 1:1.5(by weight), film thickness ca 120 nm, films annealing temperature 95 °C.
b
Generally, the blend films based on benzothiadiazole-contain-
processed from both DCB and xylene. The modules based on poly-
mer P2 showed higher efficiency when processed from xylene and
delivered the PCE of 3.3%. Interestingly, these results were
obtained without any buffer layers, which suggests a better inter-
facial contact between the metal electrode and photoactive layer.
These results demonstrate polymers P1 and P2 as promising mate-
rials for fabrication of large-area OSCs processed from halogen-free
solvents under ambient conditions and using R2R-compatible film
coating techniques.
Unfortunately, the efficiency of the fabricated modules is still
low for practical applications. This issue can be addressed by a
rational design of new polymeric materials, e.g. random copoly-
mers comprising benzothiadiazole and benzoxadiazole blocks to
reach a good balance between VOC and JSC of the devices. Advan-
tages of using statistical copolymers have been demonstrated in
a number of previous reports [29,30,31]. Moreover, the module
design can also be improved to reduce losses e.g. due to ITO series
resistance. Furthermore, the combination of the developed poly-
mers P1 and P2 with low-bandgap non-fullerene acceptors might
also improve the performance of solar cells.
ing polymer P1 are more uniform and exhibit smoother surface
than the P2/[70]PCBM films. The root mean square (RMS) sur-
face roughness values were 1.65 nm and 2.22 nm for P1/[70]
PCBM films deposited from DCB and m-xylene, respectively. On
the contrary, P2-based films processed from DCB as well as m-
xylene had
a rougher surface with RMS of 2.96 nm and
2.85 nm, respectively. Such higher surface roughness is unfavor-
able for effective exciton dissociation and charge transport to the
electrodes. Thus, non-optimal composite morphology seems to
be the main reason for the lower JSC and FF values observed
for P2-based solar cells.
Moving toward the development of the large-area devices, we
fabricated modules with the total area of 25 cm2 consisting of four
series-connected cells. The active area of the modules was
16.4 cm2. Active layers were blade-coated in air under the opti-
mized conditions found for small-area OSCs. The photovoltaic
parameters of large-area devices are summarized in Table 2, J-V
curves are shown in Fig. S13 (ESI). The modules based on conju-
gated polymer P1 exhibited modest PCE of ~3.3–3.4% for devices
Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037

P.M. Kuznetsov et al. / Tetrahedron Letters xxx (xxxx) xxx
5
Conclusion
In conclusion, we have synthesized and characterized two novel
conjugated polymers comprising thiazolothiazole building block
combined with thiophene donor and benzothiadiazole (P1) or ben-
zoxadiazole (P2) acceptor units. Organic solar cells and larger-area
photovoltaic modules based on polymers P1 and P2 were success-
fully fabricated from DCB and halogen-free m-xylene solvents
using scalable blade coating technique. These devices delivered
encouraging efficiency of ~7%, thus demonstrating the designed
polymers as promising materials for large-area organic photo-
voltaics processed from environment-friendly solvents.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the Russian Science Foundation
(project No.18-73-00095). We thank Dr. A Kozlov and Dr. E. Khak-
ina for providing interfacial buffer layer materials.
Fig. 3. J À V curves (a) and EQE spectra (b) of solar cells based on P1 and P2 under
Appendix A. Supplementary data
optimal conditions.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2020.152037.
Fig. 4. AFM height images for (a) P1/[70]PCBM blends processed from DCB with 1% DPE and coated with PHQ buffer and (b) the same films processed from m-xylene with 1%
DPE; (c) P2/[70]PCBM blends processed from DCB and (d) the same films processed from m-xylene with 0.5% DPE.
Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037

6
P.M. Kuznetsov et al. / Tetrahedron Letters xxx (xxxx) xxx
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Please cite this article as: P. M. Kuznetsov, S. L. Nikitenko, I. E. Kuznetsov et al., Thiazolothiazole-based conjugated polymers for blade-coated organic solar
cells processed from an environment-friendly solvent, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2020.152037