Suzuki polycondensation for the synthesis of polytriarylamines: A method to improve hole-transport material performance in perovskite solar cells

Article abstract of DOI:10.1016/j.tetlet.2020.152317

Polytriarylamines have attracted significant interest as hole transport materials for organic electronics. Herein we report the synthesis of a polytriarylamine (PTAA) using the Suzuki polycondensation reaction. The Pd-based catalytic system was optimized to obtain a polymer with a high molecular weight and low polydispersity. The synthesized PTAA polymer was evaluated as a hole transport material in perovskite solar cells with n-i-p configuration. The devices showed an improved power conversion efficiency of 17.6% and an open-circuit voltage of 1.06 V, whereas the referenced cells based on commercial PTAA delivered a lower efficiency of 16.7% and an open-circuit voltage of 1.02 V. Thus, the proposed synthetic approach yields high-quality PTAA with improved electrical properties.

Full text of DOI:10.1016/j.tetlet.2020.152317

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Suzuki polycondensation for the synthesis of polytriarylamines: a method to
improve hole-transport material performance in perovskite solar cells
Marina M. Tepliakova, Alexander V. Akkuratov, Sergey A. Tsarev, Pavel A.
Troshin
PII:
S0040-4039(20)30790-5
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152317
TETL 152317
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Tetrahedron Letters
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8 June 2020
26 July 2020
30 July 2020
Please cite this article as: Tepliakova, M.M., Akkuratov, A.V., Tsarev, S.A., Troshin, P.A., Suzuki
polycondensation for the synthesis of polytriarylamines: a method to improve hole-transport material
performance in perovskite solar cells, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.
2020.152317
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polytriarylamines: a method to improve hole-
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Marina M. Tepliakova, Alexander V. Akkuratov, Sergey A. Tsarev, and Pavel A. Troshin

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Tetrahedron Letters
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Suzuki polycondensation for the synthesis of polytriarylamines: a method to improve
hole-transport material performance in perovskite solar cells
Marina M. Tepliakovaa,b, Alexander V. Akkuratovb, Sergey A. Tsareva, and Pavel A. Troshina,b
a Skolkovo Institute of Science and Technology, Nobel street, Building 3, Moscow, 143026, Russia
b Institute for Problems of Chemical Physics of Russian Academy of Science, Academician Semenov avenue, Building 1, Chernogolovka, 142432, Russia.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Polytriarylamines have attracted significant interest as hole transport materials for organic
electronics. Herein we report the synthesis of a polytriarylamine (PTAA) using the Suzuki
polycondensation reaction. The Pd-based catalytic system was optimized to obtain a polymer
with a high molecular weight and low polydispersity. The synthesized PTAA polymer was
evaluated as a hole transport material in perovskite solar cells with n-i-p configuration. The
devices showed an improved power conversion efficiency of 17.6% and an open-circuit voltage
of 1.06 V, whereas the referenced cells based on commercial PTAA delivered a lower efficiency
of 16.7% and an open-circuit voltage of 1.02 V. Thus, the proposed synthetic approach yields
high-quality PTAA with improved electrical properties.
Available online
Keywords:
Hole transport material
Organic Electronics
Perovskite solar cells
Polytriarylamine
2009 Elsevier Ltd. All rights reserved.
Suzuki polycondensation
Yamomoto polycondensation18 were also used for the synthesis
of polytriarylamines such as PTAA.
Introduction
Perovskite solar cells have demonstrated remarkable progress
with efficiency rising from 3.8% in the first report1 to the current
record of 25.2%2 within less than a decade. One of the important
components of perovskite solar cells is a hole transport layer
(HTL), which selectively extracts and transports holes from the
photoactive layer to a hole-collecting electrode. The most popular
family of polymeric HTLs is represented by polytriarylamines
with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA)
providing the highest efficiencies of up to 22.1% using
appropriate doping.3, 4
There are various approaches to the synthesis of
polytriarylamines. One of them is based on the oxidative
polymerization of monomeric triarylamines with reagents such as
FeCl3.5-7 On one hand, this reaction is rapid and the oxidizing
reagents are inexpensive. On the other hand, due to the
abundance of various side reactions, oxidative polycondensation
produces low-quality polymers suffering from poor
regioregularity, low molecular weights and multiple types of
defects (e.g. crosslinking, carbon-halogen bond formation,
Scheme 1a).8-12 Therefore, such materials are not optimal for
electronic applications, in particular in photovoltaic devices.
On the contrary, metal-catalyzed polycondensation reactions
afford high-quality polymers at the cost of additional synthetic
steps required to obtain auxiliary monomers and the need for
expensive metal-based catalysts. Among the different metal-
catalyzed cross-coupling reactions, Buchwald-Hartwig C-N
coupling represents the most prevalent pathway for
polytriarylamine synthesis.13-16 Ullman polycondensation,8, 17 and
Another type of metal-catalyzed reaction is the Suzuki-
Miyaura polycondensation, which requires halogen-substituted
and boron-substituted derivatives. The initial monomers might
include both functional groups and in this case, they are referred
to as AB-type monomers, where A and B represent halogen and
boron
functional
groups,
respectively.
The
Suzuki
polycondensation using AB-type monomers was previously
applied for the synthesis of PTAA as shown in Scheme 1b.19
Typically, the AB-type monomer is prepared from an AA-type
monomer by lithiation with a stoichiometric amount of n-
butyllithium and quenching of the intermediate with the
20
appropriate boronic ester.19,
The AB-type monomer, in this
case, should be extensively purified from the unreacted AA-type
reagent, BB-type byproduct, and also dehalogenated products
with only one functional boronic ester group, which can play the
role of end cappers in polycondensation reactions. The high
purity of AB-type monomer can be achieved with column
chromatography, which in turn can damage the boronic ester
functional groups.
Herein, we report the synthesis of PTAA via the Suzuki
polycondensation reaction starting from AA-type and BB-type
monomers. We demonstrate that purification of the BB-type
monomer can be performed with simple recrystallization and
precipitation, which is faster, easier, and more reliable for
boronic esters as compared to column chromatography.
Moreover, the yield of the BB-type monomer (65%) is superior
compared to the yield of the AB-type monomer after column
chromatography purification (49%).19

2
Scheme 1. Synthetic routes to polytriarylamines: oxidative polymerization (a); Suzuki polycondensation using AB-type monomers (b); Suzuki polycondensation
using AA-type and BB-type monomers presented in this work (c).
and commercial PTAA (PTAA ref), most probably synthesized
by Suzuki polycondensation from the AB-type monomer,15 were
obtained using gel permeation chromatography (GPC). These
characteristics are summarized in Figure 1 and the GPC traces
are presented in Figure S4 (ESI). According to the GPC analysis,
polymer 4a had a lower average molecular weight (Mn) and
narrower molecular weight distribution (Mw/Mn) as compared to
PTAA ref, whereas polymer 4b had twice lower Mn compared to
4a.
All three batches of PTAA were evaluated as hole transport
layer (HTL) materials in perovskite solar cells with n-i-p
configuration (Fig. 1a). The optimized perovskite solar cell
architecture was comprised of ITO (indium tin oxide) covered
Results and Discussion
The synthetic route used for the preparation of PTAA applied
in our work is depicted in Scheme 1c. Compound 1 was
synthesized via Buchwald-Hartwig coupling between
commercially
available
2,4,6-trimethylaniline
and
bromobenzene. The subsequent bromination of 1 produced the
AA-type monomer 2 with a quantitative yield.19 Monomer 2 was
additionally sublimated under reduced pressure leading to 99%
purity according to high-performance liquid chromatography
(HPLC). The BB-type monomer 3 was obtained by the lithiation
of 2 with 4 equivalents of n-butyllithium followed by the addition
of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.21 The
high purity of the monomer 3 (97% by HPLC) was achieved
simply by precipitation from isopropanol and further
recrystallization from chloroform upon the addition of methanol.
Finally, the Suzuki polycondensation reaction was performed
with various catalytic systems. The first catalyst
tetrakis(triphenylphosphine)palladium was used previously for
the synthesis of low molecular weight triarylamine-based
compounds.22 However, this catalytic system failed to initiate the
polycondensation of PTAA in our experiments. An alternative
catalyst based on the combination of palladium acetate and
triphenylphosphine led to low molecular weight polymer 4b. The
with an electron transport layer of SnO2 passivated with a
24
fullerene derivative phenyl-C61-butyric acid (PCBA).23,
The
film of conventional lead halide perovskite material CH3NH3PbI3
was deposited atop SnO2/PCBA. A polymeric HTL based on one
of 4a, 4b, or PTAA Ref polymers was spin-coated from the
third
catalytic
system
comprised
of
tris(dibenzylideneacetone)dipalladium and tri(o-tolyl)phosphine
led to polymer 4a. Polymers 4a and 4b were extensively purified
with sequential Soxhlet extractions using methanol, acetone, and
chloroform.
The relative molecular weight characteristics and
polydispersity indexes of the chloroform fractions of 4a and 4b
Figure 1. Configuration of the perovskite solar cell (a); J-V characteristics of
perovskite solar cells with PTAA 4a, 4b and PTAA ref applied as the hole
transport material (b).

3
5. Lin, H.-Y.; Liou, G.-S.; Lee, W.-Y.; Chen, W.-C. J. Polym. Sci.,
Part A: Polym. Chem. 2007, 45, 1727.
chlorobenzene solution on the top of the photoactive layer.
Optimal film deposition conditions for each PTAA batch were
evaluated in the preliminary experiments.
Finally, the hole-collecting electrode consisting of MoOx(10
nm) and Ag (100 nm) was deposited by thermal evaporation
under vacuum thus completing the configuration of the
6. Qin, P.; Tetreault, N.; Dar, M. I.; Gao, P.; McCall, K. L.; Rutter, S.
R.; Ogier, S. D.; Forrest, N. D.; Bissett, J. S.; Simms, M. J.; Page, A.
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K.; Kim, S. Y. Polym. Bull. 2020.
8. Kisselev, R.; Thelakkat, M. Macromolecules 2004, 37, 8951.
9. Lewis, R. Photoinduced Charge Transfer - Proceedings Of The 10th
Annual Symposium Of The Nsf Center; World Scientific Publishing
Company, 2000.
perovskite
solar
cell
ITO/SnO2/PCBA/CH3NH3PbI3/HTL/MoOx/Ag as illustrated in
Figure 1a.
The J-V characteristics of the best devices fabricated with all
three polymer batches are presented in Figure 1b. The cells
incorporating polymer 4b demonstrated low power conversion
efficiencies (PCE) of 8.2% with a modest fill factor (FF) of 43%.
These results confirmed the poor electronic quality and,
consequently, the suboptimal charge transport properties of low
molecular weight polymer 4b. On the contrary, the devices with
HTL based on PTAA Ref and 4a delivered much higher
efficiencies of 16.7% and 17.6%, respectively. To the best of our
knowledge, these are among the highest efficiency values
demonstrated for perovskite solar cells with an undoped PTAA
layer. The fill factors (FF) and current densities (JSC) were
comparable for devices with both polymer batches. The current
density values were additionally reconfirmed with external
quantum efficiency measurements (Fig. S6). Importantly, the
open-circuit voltage (VOC) of the devices with 4a approached
1.06 V, which was notably higher compared to the cells with
PTAA Ref yielding a VOC of 1.02 V. The higher VOC reached for
devices with 4a indicates a decrease in the density of the defects
at the perovskite/HTL interface suppressing trap-assisted
recombination of charge carriers.25
10. Mark, H. F. Encyclopedia of Polymer Science and Technology,
Concise; Wiley, 2013.
11. Bai, W.; Yao, R.; Guan, M.; Shang, X.; Lai, N.; Xu, Y.; Lin, J. Eur.
Polym. J. 2017, 95, 105.
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Tetrahedron Lett. 2014, 55, 2932.
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Macromol. Rapid Commun. 2007, 28, 449.
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Vol.02No.04, 10.
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Polym. Sci., Part A: Polym. Chem. 2012, 50, 4155.
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1999, 102, 1125.
18. Kim, Y.; Jung, E. H.; Kim, G.; Kim, D.; Kim, B. J.; Seo, J. Adv.
Energy Mater. 2018, 8, 1801668.
19. Ialn McCulloch, M. H. Material Matters 2009, 4.3, 4.
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Song, K.; Watkins, S. E.; Anthopoulos, T.; McCulloch, I. J. Am.
Chem. Soc. 2009, 131, 10814.
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Xiong, Z.-J.; Xie, N.-H.; Tang, B. Z.; Zhu, M.-Q. Macromolecules
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Conclusion
23. Tsarev, S.; Dubinina, T. S.; Luchkin, S. Y.; Zhidkov, I. S.; Kurmaev,
E. Z.; Stevenson, K. J.; Troshin, P. A. J. Phys. Chem. C 2020, 124,
1872.
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Wilkins, C. L. J. Org. Chem. 1995, 60, 532.
To conclude, the proposed approach for PTAA synthesis from
AA- and BB-type monomers using the Suzuki polycondensation
reaction led to a high-quality polymer with narrow molecular
weight distribution. The obtained polymer was applied as a hole-
transport material in dopant-free perovskite solar cells, which
demonstrated improved performance with the efficiency reaching
17.6% due to the reduced defect density at the perovskite/PTAA
interface. Our results emphasize the importance of the synthetic
route choice for designing high electronic quality materials for
photovoltaic applications.
25. Zhao, P.; Kim, B. J.; Jung, H. S. Mater. Today Energy 2018, 7, 267.
Acknowledgements
This work was supported by Russian Science Foundation
(project No. 19-73-30020) at IPCP RAS and Skolkovo
Foundation grant at Skoltech.
Supplementary Material
Experimental procedures, material characterization, device
fabrication data, and external quantum efficiency spectra can be
found in the Electronic Supplementary Information.
References and notes
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Timerbulatov, R. S.; Dremova, N. N.; Frolova, L. A.; Stevenson, K.
J.; Troshin, P. A. Sustain. Energy Fuels 2019, 3, 2627.

4
PTAA is synthesised from AA-type and BB-type
monomers via Suzuki polycondensation
The Pd-based catalytic system for PTAA synthesis is
optimized
Perovskite solar cells with PTAA hole-transport material
show 17.6% efficiency
Superior performance of the synthesized PTAA is
attributed to its improved quality