Dopant-Free Hole-Transporting Polymers for Efficient and Stable Perovskite Solar Cells

Article abstract of DOI:10.1021/acs.macromol.9b00165

A series of novel polymers (P1-P6) derived from the combination of different units (including thiophene, triarylamine, and spirobifluorene) were successfully synthesized, completely characterized, and used as hole-transporting materials (HTMs) for perovskite solar cells (PSCs). Solar cells with some of these materials as HTMs showed very good performances of almost 13% (12.75% for P4 and 12.38% for P6) even without additives, and devices based on these new HTMs show relatively improved stability against temperature compared to those based on PTAA. The presence of dopant additives has been linked to long-term degradation, which is the main barrier to the large-scale commercialization of this innovative type of solar cell. Obtaining efficient PSCs without using dopants could represent a further step toward improvement of long-term stability and thus their introduction into the market.

Full text of DOI:10.1021/acs.macromol.9b00165

Article
pubs.acs.org/Macromolecules
Cite This: Macromolecules XXXX, XXX, XXX−XXX
Dopant-Free Hole-Transporting Polymers for Efficient and Stable
Perovskite Solar Cells
†
†
†
‡,§
Silvia Valero, Silvia Collavini, Sebastian F. Volker, Michael Saliba, Wolfgang R. Tress,‡
̈
‡
,‡
Shaik M. Zakeeruddin, Michael Gratzel,* and Juan Luis Delgado*,†,∥
̈
†
́
POLYMAT University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 Donostia - San Sebastian, Spain
‡
́
́
́
Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Federale de
Lausanne, CH-1015 Lausanne, Switzerland
§Soft Matter Physics Group, Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland
∥Ikerbasque, Basque Foundation for Science, 48013 Bilbao, Spain
S
* Supporting Information
ABSTRACT: A series of novel polymers (P1−P6) derived
from the combination of different units (including thiophene,
triarylamine, and spirobifluorene) were successfully synthe-
sized, completely characterized, and used as hole-transporting
materials (HTMs) for perovskite solar cells (PSCs). Solar
cells with some of these materials as HTMs showed very good
performances of almost 13% (12.75% for P4 and 12.38% for
P6) even without additives, and devices based on these new
HTMs show relatively improved stability against temperature
compared to those based on PTAA. The presence of dopant
additives has been linked to long-term degradation, which is the main barrier to the large-scale commercialization of this
innovative type of solar cell. Obtaining efficient PSCs without using dopants could represent a further step toward improvement
of long-term stability and thus their introduction into the market.
leading to losses in performance, although the mechanism is
not yet clear.9,10 Also, higher temperatures are detrimental for
this molecule, which crystallizes losing its efficiency as HTM.11
Several other HTMs have been used to bypass the
drawbacks of spiro-OMeTAD, both polymers and small
molecules.12,13 Polymers have emerged as good candidates
where the polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)-
amine] (PTAA) is one of the best known. The performances
obtained with this HTM are highly comparable to spiro-
OMeTAD.14 Even though PTAA showed improved stability at
higher temperatures, it still requires dopants to achieve best
performances, which is a problem for long-term stability. In
addition to the aforementioned degradation mechanisms, the
acetonitrile required to dissolve the dopant LiTFSI added to
the HTM solution could partially corrode the underlying
perovskite layer.
INTRODUCTION
■
One of the most remarkable developments in the history of
photovoltaics has been the emergence of perovskite solar cells.
After their discovery in 2009, when Miyasaka et al. obtained
3.8% of efficiency using methylammonium lead iodide
perovskite (MAPbI3),1 the performance of PSCs rose quickly
to a certified record of 23.7%.2 However, the fast growth in
performance is not enough to guarantee a spot in the market of
solar cells, which is still led by silicon solar cells.
As a matter of fact, despite their incredible performance
advancement, PSCs still suffer from lack of long-term
stability.3,4 Several adjustments have been attempted to obtain
the desired long-term stable performance.5 In this effort,
among others, hole-transporting materials (HTMs) have been
intensively studied.6 Today, the best choice as HTM is still
2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobi-
fluorene (spiro-OMeTAD), which is known and used in PSCs
For all these reasons, the use of dopant-free HTMs could be
a suitable strategy to get the required long-term stability.
Working without dopants was already shown to be a feasible
strategy, and there are few studies of dopant-free HTMs.6
Some small molecules have been tested recently as HTMs in
PSCs in the absence of dopants; not only have they reached
7
8
̈
since 2012, when Snaith et al. and in parallel Gratzel et al.
fabricated an all-solid device using this molecule as HTM.
Nevertheless, spiro-OMeTAD is far from being the perfect
HTM for different reasons: it has low conductivity and hole
mobility, and to reach the best performances the use of
dopants, such as lithium bis(trifuloromethanesulfonyl)imide
(LiTFSI) and tert-butylpyridine (tBP), is required to improve
its conductivity. These dopants are hygroscopic, and they
might migrate into the perovskite layer during operation,
Received: January 23, 2019
Revised: February 22, 2019
© XXXX American Chemical Society
A
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Figure 1. Chemical structures of monomers (M1−M5 and 2,7-dibromo-9,9′-spirobifluorene) and polymers (P1−P6).
efficiencies comparable to that of devices using doped spiro-
OMeTAD, they also showed an improved stability.15−24
With regard to polymers, they have been much less explored
than small molecules even though they offer excellent
properties, albeit most of them derive from previous OPVs
and OLEDs. Recently, Park et al.25 reported a novel polymeric
HTM based on benzo[1,2-b:4,5:b′]dithiophene (BDT) and
2,1,3-benzothiadiazole (BT) units for using it in PSCs without
using any dopant. A PCE of 17.3% was achieved, maintaining
its initial efficiency for >1400 h. The same authors have
reported a 19.8% on efficiency with a device fabricated with a
donor−acceptor type dopant-free polymeric HTM. In their
work the polymeric HTM, consisting of a BDT unit as a donor
and a BT unit as acceptor connected with a π-bridge, was
incorporated in PSCs.26
Also, in a work by Getautis, Saliba et al.27 three new
triarylamine-based polymers were tested as HTMs in the
absence of dopants leading to a PCE of 12.3% with the
polymer poly[3,5-dimethyl-N,N-diphenyl-4-(2-phenylprop-1-
en-1-yl)aniline-4′,4″-diyl], which was higher than that of
PTAA (10.8%) measured under the same conditions. More-
over, devices prepared with this dopant-free polymer were
stable for over 140 h without significant changes in the PCE,
while devices with the doped polymer showed a significant
decrease of efficiency already in the first few hours. Qiao et
al.28 reported the well-known polymer PDPP3T [poly[{2,5-
B
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Scheme 1. Synthetic Route to Monomers M1−M5
bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]-
pyrrole-1,4-diyl}-alt-{[2,2′:5′,2″-terthiophene]-5,5″-diyl}] as
dopant-free HTM in PSCs reaching a PCE of 12.3%, which
was comparable with devices prepared with doped spiro-
OMeTAD (12.34%) measured under the same conditions. In
addition, PDPP3T-based PSC devices showed better stability
in ambient air than doped spiro-OMeTAD. Neher, Stolterfoht,
and co-workers have reported the highest efficiency with a
device fabricated with a dopant-free polymeric HTM. They
employed PTAA as dopant-free HTM in inverted PSCs
reaching a 20.4% on efficiency and approaching the fill factor
Shockley−Queisser limit.29
All these results indicate the possibility of using polymers as
dopant-free HTMs for the development and commercialization
of stable PSCs avoiding the use of additives.
Herein, a series of new conjugated polymers (P1−P6)
containing different well-known hole-accepting units in the
backbone such as thiophene, triarylamine, and spirobifluor-
ene30 were successfully synthesized via Suzuki or Yamamoto
polymerization. The optical and electrochemical properties, as
well as the molecular weights and thermal properties of all
polymers, were investigated. The new polymers P1−P6 were
incorporated into PSCs to evaluate their performance as
HTMs. Moreover, the heating resistance of the champion
devices based on polymers P4 and P6 was tested.
RESULTS AND DISCUSSION
■
Synthesis. The chemical structures of the synthesized
polymers P1−P6 and the necessary monomers for preparation
are displayed in Figure 1.
The design of the six new polymers was inspired by spiro-
OMeTAD and PTAA, the latter being the only polymeric
HTM that could work as efficiently as spiro-type small
molecules.31 Polymers P1 and P2 are composed of a
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Scheme 2. Synthesis of the Target Polymers P1−P6
spirobifluorene unit in combination with two different donor
moieties, a thiophene and a triarylamine derivative, which are
known to possess high hole-transporting and electron-blocking
ability. Moreover, to ensure sufficiently high solubility for
device fabrication and good film formation, branched alkyl
chains (2-ethylhexyl) were introduced in key positions of those
donor units.
In polymers P3−P6, the possibility to introduce more donor
groups in the backbone of the polymers was explored to
modulate the HOMO energy level of the polymers for a better
hole transport from the perovskite to the HTM. Therefore, we
designed polymers P3−P6, each one containing triphenyl-
amine extended units in combination with different donor
systems as thiophene (P3), spirobifluorene (P4), or triaryl-
amine (P5) units. As in polymers P1 and P2, polymers P3−P6
will benefit from the enhanced solubility given by the
introduction of alkyl groups.
The procedure for the synthesis of monomers M1−M5 is
shown in Scheme 1, while the complete experimental details
can be found in the Supporting Information. Monomer M2
was synthesized and purified according to published
procedures,32−35 while M1 was synthesized in two steps.
First, the Kumada coupling of 3,4-dibromothiophene with 2-
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1
Figure 2. (A) H NMR spectrum and (B) MALDI-TOF of polymer P2.
ethylhexyl bromide provided thiophene 1, which after Ir-
catalyzed borylation afforded M1.
extraction with different solvents through Soxhlet extraction.
This processes yielded polymers in good (P1, P2, P4−P6) or
moderate (P3) yields. All polymers show good solubility in
common organic solvents, such as dichloromethane, chloro-
form, THF, or toluene.
The syntheses of monomers M4 and M5 were performed
following literature procedures.36−38 The synthesis of M3
started with the reaction between 4-bromophenylamine with
an excess of 1-[(2-ethylhexyloxy]-4-iodobenzene (2) to give 4-
bromo-N,N-bis[4-(2-ethylhexyloxy)phenyl)aniline (5). After
that, Buchwald−Hartwig reaction of compound 5 with
diphenylamine gave super-triarylamine 6, which finally reacted
with an excess of NBS yielding the monomer M3 (Scheme 1).
The six new polymers P1−P6 were then synthesized
recurring to either Suzuki (P1−P5) or Yamamoto (P6)
coupling polymerization. Pd-catalyzed Suzuki polymerization
of the diboronic ester monomers M1 and M2 with 2,7-
dibromo-9,9′-spirobifluorene yielded P1 and P2, respectively,
using sodium bicarbonate as base and toluene as solvent.
Following the same procedure, polymers P3, P4, and P5 were
synthesized by reaction of the corresponding monomers M1,
M4, and M5 with the super-triarylamine M3, respectively.
Homopolymer P6 was synthesized through Ni-mediated
Yamamoto polymerization of M3 in the presence of 1,5-
cyclooctadiene (COD) and 2,2′-bipyridine in a solvent mixture
of DMF and toluene (1:1) (Scheme 2).
The composition and chemical structure of all polymers
1
were confirmed by H NMR and MALDI-TOF spectrometry
(see Figure 2 and Figures S14−S24). In the latter, peaks
corresponding to the building blocks were clearly visible.
The size of the polymers was estimated by size exclusion
chromatography (SEC) in THF with polystyrene standards for
calibration, and the data are found in Table 1. The longest
molecular chain was registered for polymer P1, with a number-
average molecular weight (Mn) of 9651 g mol−1, leading to a
Table 1. Molecular Weights and Thermal Properties of
Polymers P1−P6
Mn
Mw
polymer (g mol−1
)
(g mol−1
)
Xn
PDI Td (°C) Tg (°C)
P1
P2
P3
P4
P5
P6
9651
5551
3209
4403
4681
5812
21635
11390
4746
10528
8663
15.5
8.08
3.30
4.48
4.98
8.71
2.24
2.05
1.48
2.39
1.85
2.14
302.57
371.74
327.14
392.03
390.62
390.66
147.8
241.8
76.50
198.1
141.5
103.9
The resulting polymers were purified from the residual
catalyst and low molecular weight impurities by precipitation
in MeOH (P1−P5) or MeOH:HCl (P6) and subsequent
12443
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degree of polymerization (Xn) of 16. The corresponding
weight-average molecular weight (Mw) of polymer P1 was
21635 g mol−1, showing a relatively small polydispersity (PDI
= Mw/Mn) of 2.24. Molecular weights of polymers P2−P6
showed a comparable Mn in the 5812−3209 g mol−1 range and
a Mw ranging from 4746 to 12443 g mol−1 with a PDI in the
range 1.48−2.39.
Thermal Properties. Thermogravimetric analysis per-
formed at a heating rate of 10 °C/min under a nitrogen
atmosphere indicated the high thermal stability of copolymers
P1−P5 and homopolymer P6 (Figure 3A), showing
Figure 4. (A) Normalized UV−vis spectra of P1−P6 in DCM at
concentrations of 3.03 × 10−5, 2.71 × 10−5, 2.51 × 10−5, 2.49 × 10−5,
2.21 × 10−5, and 3.27 × 10−5 mol L−1, respectively. (B) Normalized
fluorescence spectra of polymers P1−P6 in DCM.
presence of three additional phenyl rings in P2. This is also the
reason why copolymer P2 presents a higher molar absorption
coefficient (ε) than polymer P1.
As for polymers bearing the triphenylamine extended unit
P3−P6, they showed maximum absorption wavelengths in
between 350 and 380 nm. As the amount of delocalization in
the polymer increases, there is less energy gap between π and
π* orbitals, and a higher wavelength is necessary for
absorption. Therefore, polymers P3 and P6 present the lowest
maximum absorption wavelength (348 and 349 nm,
respectively) while polymers P4 and P5, with more phenyl
rings in their backbone, which implies more delocalization,
have maximum absorption at 383 and 365 nm, respectively.
From the onset of the absorption edge in the longer
wavelength region, the optical band gap (Eg) was estimated,
and the values are reported in Table 2. In addition, the
photoluminescence spectra (Figure 4B) showed emission
peaks at 451, 424, 510, 549, 494, and 491 nm for polymers
P1−P6, respectively.
Electrochemical Properties. The electrochemical proper-
ties of the polymers were investigated by cyclic voltammetry
(CV) measured in a 0.1 M solution of nBu4NPF6 in anhydrous
DCM. Their HOMO energy levels (EHOMO) were obtained
from the onset of the first oxidation (EOonxs1et) during CV
measurements using the ferrocene/ferrocinium (Fc/Fc+) redox
couple as the internal standard. The highest occupied
molecular orbital (HOMO) level is calculated according to
the equation EHOMO (eV) = −4.8 − EOonxs1et, and the data are
listed in Table 2.
Figure 3. (A) Thermogravimetric heating curves of polymers P1−P6
(heating rate 10 °C/min). (B) Differential scanning calorimetry
second heating curves for polymers P1−P6 (heating rate 20 °C/min).
decomposition temperatures (Td) (2% weight loss) ranging
from 303 to 392 °C (Table 1), which is well above operational
conditions. All polymers exhibit weight loss before decom-
position, which may be attributed to the loss of solvent traces.
Differential scanning calorimetry (DSC) measurements of
the investigated polymers P1−P6 reveal that all of them are
fully amorphous (Figure 3B and Table 1). DSC measurements
show that the glass transition temperature (Tg) of polymers
P1, P2, P4, and P5 are at 147.8, 241.8, 198.08, and 141.53 °C,
respectively, which are higher than that of spiro-OMeTAD
(125 °C),39 indicating a more stable amorphous state. The Tg
of polymer P6 was determined to be 103.85 °C, whereas the
additional thiophene unit in polymer P3 significantly
influences the Tg, lowering it to 76.5 °C.
Optical Properties. The normalized UV−vis absorption
spectra of copolymers P1−P6 in dichloromethane are depicted
in Figure 4A. Absorption peaks of all polymers are located in
the UV region, and there is no significant absorption in the
visible region, which is necessary for not competing with the
perovskite in the absorption of light.12 Upon comparison of the
absorption of the spirobifluorene-based copolymers P1 and P2,
the latter presents a maximum absorption peak at 376 nm,
which is shifted toward the red region compared to P1 (358
nm), due to a larger π-conjugated system, mostly owing to the
Polymer P2 exhibits one reversible oxidation process at a
1/2
value of 0.31 V vs Fc/Fc+, indicating its electrochemical
Ox1
E
stability, while polymer P1 shows a poorly reversible oxidation
Ox1
onset
wave at E
value of 0.61 V vs Fc/Fc+ (Figure 5A).
Comparing the electrochemical data, we could find that P2 was
more easily oxidized than P1. The HOMO energy levels
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Table 2. Optical and Electrochemical Properties of Polymers P1−P6
ε (M−1 cm−1
)
E
(V)
E
(V)
ELUMO (eV)
EHOMO (eV)
Eg (eV)
Ox1
1/2
Ox1
onset
polymer
λabs (nm)
λfl (nm)
P1
P2
P3
P4
P5
P6
358
376
348
383
369
349
451
424
510
549
494
491
27055
32284
33982
40399
45329
26586
0.61
−2.33
−2.04
−1.61
−1.81
−1.65
−1.63
−5.41
−5.01
−4.73
−4.71
−4.70
−4.68
3.08
2.97
3.12
2.90
3.05
3.05
0.31
0.21
−0.07
−0.09
−0.10
−0.12
0.01
−0.01
−0.03
−0.02
Figure 5. Cyclic voltammograms of (A) polymers P1 and P2 and (B) polymers P3−P6 in DCM/nBu4NPF6 (0.1 M) at a scan rate 250 mV s−1.
(C) Energetic levels of polymers P1−P6 and perovskite light absorber layer used in this study.
Figure 6. Statistical deviation of the photovoltaic parameters for PSCs using polymers P1−P6 and PTAA as HTMs.
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(EHOMO) of P1 and P2 calculated from CV are −5.41 and
−5.01 eV, respectively. The reported HOMO energy level for
the triple cation perovskite, used in this work, is −5.45 eV,40
revealing the favorable hole transfer from the perovskite to the
HTMs (Figure 5C). Moreover, the lowest unoccupied
molecular orbital energy levels (ELUMO) were estimated from
the difference between EHOMO and Eg derived from UV−vis
spectroscopy of the polymers as −2.33 and −2.04 eV for P1
and P2, respectively. Because of the Eg and the high ELUMO, the
compounds are suitable not only for hole transfer but also to
block electrons.
Table 3. Photovoltaic Parameters Determined from J−V
Measurements Using Polymers P1−P6 and PTAA as HTMs
HTM
VOC (V)
JSC (mA cm−2
)
FF (%)
PCE (%)
Rs (Ω)
PTAA
P1
P2
P3
P4
1.093
0.690
1.037
0.818
1.094
0.972
1.031
21.1
12.0
15.9
15.6
18.5
15.3
20.7
68
26
60
34
63
50
72
15.8
2.2
9.8
4.3
12.7
7.5
47.2
481.1
76.6
141.1
41.0
P5
P6
103.5
28.7
12.4
As in polymers P1 and P2, the voltammograms of polymers
P3−P6 show only oxidation processes, while no reduction
processes were observed in the operation window of the
solvent−electrolyte (Figure 5B). Polymers P3, P4, and P6
showed two reversible oxidations waves, while polymer P5
showed three reversible oxidations. The strong electron donor
properties of the additional diaryalmine substituents in
polymers P3−P6 compared to polymers P1 and P2 are
Ox1
1/2
reflected at the remarkably low first oxidation potentials at E
values of 0.01, −0.01, −0.03, and −0.02 V vs Fc/Fc+ for P3,
P4, P5, and P6, respectively. According to the similar redox
potentials, also the HOMO energy levels of polymers P3−P6
are nearly identical, with values around 4.7 eV. With this
values, polymers P3−P6 could be suitable for efficient hole
transfer from the perovskite to the HTM, similarly to polymers
P1 and P2. The LUMO energy levels of polymers P3−P6 lie
between −1.61 and −1.81 eV, as shown in Table 2.
Photovoltaic Performance of Devices. To evaluate their
behavior as hole-transport materials, the new polymers were
incorporated without any doping additives in regular
mesoporous PSCs. Polymer PTAA was used as HTM for
reference devices without additives as well.
The best performances obtained so far with the use of the
most common polymeric HTM (PTAA) are achieved by
doping the HTM solution with both tert-butylpyridine (tBP)
and bis(trifluoromethane)sulfonimide lithium salt (LiTFSI).14
These dopants, in fact, help enhance the performances of the
HTM with mechanisms that are not yet completely clear.
The perovskite used for this study contains three different
organic cations (cesium, methylammonium, and formamidi-
nium).40 The device structure is the following: fluorine-doped
tin oxide (FTO)/compact TiO2/mesoporous TiO2/
Figure 7. Series resistance of several devices containing polymers P1−
P6 compared with PTAA.
Here, an upper limit of Rs is approximated by finding the
minimum of the inverse differential slope of the I−V curve in
forward direction (between 1.1 and 1.2 V). Thus, the series
resistance value is linked to the FF value. The incorporation of
side alkyl chains into the polymer structure is an efficient way
to improve the solubility of the polymers; however, these alkyl
chains constitute an insulating part within the polymer. In the
case of polymers P1−P6, all of them possess alkyl chains in
their structure. It is clearly visible that polymers P1 and P3,
which give the lowest efficiency, have the highest series
resistance (Figure 7 and Table 3). Interestingly, both of them
have a thiophene unit in their backbone, in which two alkyl
chains are attached to the thiophene ring. Additionally,
polymer P3 has two more additional alkyl chains attached to
the triaryalmine extended unit, but the presence of more side
alkyl chains did not lead to higher Rs compared to the P1-
based device. In comparison to P1 and P3, polymers P2 and
P5 lead to perovskite devices with much lower Rs. In this case,
polymer P5 has two alkyl chains attached to the triarylamine
extended unit while polymer P2 only has one alkyl chain in the
p-position of the triarylamine unit. Finally, polymers P4 and
P6, which contain two alkyl chains per unit, lead to devices
with the lowest Rs, even lower than PTAA-based devices.
Taking into account that PTAA does not contain any alkyl
chain, it could be concluded that in this case the conductivity
of the layers is not affected by the presence of insulating alkyl
chains. In fact, apart from helping the material to get better
solution processability, side chains also strongly affect film
microstructures, which could also affect the conductivity of the
layer.41
Csx(MA0.17FA0.83)(100−x)Pb(I0.83Br0.17)3/HTM/Au. The de-
tailed fabrication procedure can be found in the Supporting
Information.
Figure 6 shows the photovoltaic parameters of the solar cell
devices containing the polymers P1−P6, while Table 3 shows
the highest value obtained with the several polymers. I−V
curves of the best cells can be found in the Supporting
Information.
Unexpectedly, thiophene-containing polymers P1 and P3
gave very bad efficiencies. The most affected parameters are
VOC and especially FF, the latter being an indicator of a low
conductivity of the material. I−V curves of these polymers
show an S-shape (Figure S25), which is a symptom of bad
charge extraction at the hole-transporting layer and also leads
to low VOC. This is usually due to a high energy barrier
between the HOMO of perovskite and of the HTM, but since
this is not the case, a low conductivity could explain this
behavior. This is confirmed by studies of series resistance of
the devices (Figure 7). The series resistance is extracted from
the I−V curves measured under 1 sun in a backward scan.
Apart from P1- and P3-based devices, the rest of the
polymers show very good efficiencies, which in the case of P4
and P6 are highly comparable to those of PTAA, used as
reference. The PCEs of 12.75% and 12.38% for polymer P4
and P6, respectively, are remarkable, in particular considering
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reaction mixture was degassed by bubbling nitrogen for 30 min before
and after Pd(PPh3)4 (11.4 mg, 9.86 μmol) was added, and the mixture
was stirred vigorously and refluxed for 135 h. Afterward, 30 mL of
CHCl3 and 30 mL of water were added, and upon separation of the
phases, the organic phase was washed with 50 mL of brine. Next, the
solvent was removed under reduced pressure, and the resulting
residue was dissolved in 4 mL of CHCl3 and dripped into 150 mL of
cold MeOH. The green precipitate was filtered off and washed
consecutively with MeOH (3 days), hexane (7 days), and acetone (1
day) using a Soxhlet extractor. Each washing lasted until no residue
could be found in the chosen solvent (319 mg, 77%). SEC: Mn =
9651, Mw = 21635, PDI = 2.2. MS (MALDI-TOF, DCTB): calculated
that a further optimization might lead to even higher values.
With regard to P4, the VOC of the devices containing this
polymer exceeded that of PTAA for several devices. The
slightly lower FF, which is correlated to a lower conductivity, is
the major cause of the lower PCE. P6, on the contrary,
presents astonishing FF, but both JSC and VOC are slightly
lower than those of PTAA, maybe due to higher interface
recombination because the conductivity of P6 seems to be
higher than that of PTAA, according to the series resistance
values (Figure 7). Last, P2 and P5 give efficiencies a bit lower
than the previous structures due to a low JSC.
1
for C45H48S [M+]: 620,347, found: building blocks. H NMR (300
A cross section of the devices containing P6 can be seen in
Figure 8. The HTM layer can barely be seen, since it is very
thin (∼20 nm). The concentration of the polymer solution is 7
mg/mL, while PTAA is deposited from a 10 mg/mL solution.
MHz, CDCl3), δ (ppm): 7.87−7.70 (4H), 7.40−7.24 (4H), 7.12−
6.99 (2H), 6.77−6.65 (4H), 2.42−2.14 (4H), 1.22−1.10 (2H), 1.10−
0.78 (16H), 0.76−0.65 (6H), 0.51−0.36 (6H).
Synthesis of P2. 4-(2-Ethylhexyloxy)-N,N-bis(4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (M2, 358 mg, 571
μmol) was put in a Schlenk flask under nitrogen. Next, anhydrous
toluene (14 mL) and aqueous 0.2 M NaHCO3 (5.20 mL, 1.04 mmol)
were injected together with 2,7-dibromo-9,9″-spirobifluorene (271
mg, 571 μmol) and Aliquat 336 (60.9 mg) under a nitrogen
atmosphere. The reaction mixture was purged with nitrogen for 30
min before and after Pd(PPh3)4 (9.90 mg, 8.57 μmol) was added. The
mixture was stirred vigorously and refluxed for 135 h, and 30 mL of
CHCl3 and 30 mL of water were added afterward. The phases were
separated; the aqueous phase was extracted with CHCl3 (1 × 30 mL),
and the combined organic phases were washed with 50 mL of brine,
dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The residue was dissolved in 4 mL of CHCl3 and dripped
into 150 mL of cold MeOH. The green precipitate was filtered off and
washed consecutively with MeOH (3 days), hexane (4 days), and
acetone (2 days) using a Soxhlet extractor. Each washing lasted until
no residue could be found in the chosen solvent (322 mg, 82%). SEC:
Mn = 5551, Mw = 11390, PDI = 2.1. MS (MALDI-TOF, DCTB):
Figure 8. Cross-sectional SEM images of a device with P6.
The best devices, using polymers P4 and P6, underwent a
simple high-temperature stability test to evaluate their
resistance to this kind of stress in comparison with PTAA.
As mentioned before, sensitivity to high temperatures is one of
the drawbacks of the most used HTM, spiro-OMeTAD. The
cells’ performances were measured before and after being
heated for 12 h at 65 °C in nitrogen atmosphere. The
performances given by the novel polymers are better than
those of PTAA after the stress test (Figure 9).
+
calculated for C51H43NO [M ]: 685.322, found: building blocks.1H
NMR (300 MHz, CD2Cl2), δ (ppm): 7.99−7.76 (4H), 7.66−7.48
(2H), 7.48−7.02 (16H), 6.95−6.67 (4H), 3.86−3.71 (2H), 1.77−
1.62 (1 H), 1.56−1.22 (8H), 1.01−0.80 (6H).
Synthesis of P3. The dibrominated compound M3 (446 mg, 539
μmol) was put in a Schlenk flask under nitrogen. Next, anhydrous
toluene (13 mL) and aqueous 0.2 M NaHCO3 (4.90 mL, 981 μmol)
were injected together with 2,2′-(3,4-bis(2-ethylhexyl)thiophene-2,5-
diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M1, 301 mg, 539
μmol) and Aliquat 336 (45.0 mg) under a nitrogen atmosphere. The
reaction mixture was purged with nitrogen for 30 min before and after
Pd (PPh3)4 (9.34 mg, 8.09 μmol) was added. The mixture was stirred
vigorously and refluxed for 135 h, and 30 mL of DCM and 30 mL of
water were added afterward. The aqueous phase was extracted with
DCM (1 × 30 mL), and the combined organic phases were washed
with 50 mL of brine, dried over anhydrous Na2SO4, and concentrated
under reduced pressure. The residue was dissolved in 4 mL of DCM
and dripped into 150 mL of cold MeOH. The green precipitate was
filtered off and washed consecutively with MeOH (4 days), CH3CN
(2 days), and acetone (7 days) using a Soxhlet extractor. Each
washing lasted until no residue could be found in the chosen solvent.
SEC: Mn = 3209, Mw = 4746, PDI = 1.5. MS (MALDI-TOF,
DITHRANOL): calculated for C66H88N2O2S [M+]: 972.656, found
building blocks.1H NMR (300 MHz, CD2Cl2), δ (ppm): 7.52−6.78
(20H), 3.86−3.77 (4H), 2.69−2.56 (4H), 179−1.64 (2H), 1.57−1.03
(34H), 0.99−0.64 (24H).
Figure 9. Normalized PCE for champion devices prepared with
PTAA, P4, and P6 as HTMs.
Synthesis of P4. The dibrominated compound M3 (345 g, 417
μmol) was put in a Schlenk flask under nitrogen. Next, anhydrous
toluene (10 mL) and aqueous 0.2 M NaHCO3 (3.79 mL, 758 μmol)
were injected together with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,9′-spirobi[fluorene] (M4, 237 mg, 417 μmol) and
Aliquat 336 (35.0 mg) under a nitrogen atmosphere. The reaction
mixture was purged with nitrogen for 30 min before and after Pd
(PPh3)4 (7.72 mg, 6.25 μmol) was added. The mixture was stirred
vigorously and refluxed for 135 h. Afterward, 30 mL of DCM and 30
EXPERIMENTAL SECTION
■
Synthesis of P1. 2,2′-(3,4-Bis(2-ethylhexyl)thiophene-2,5-diyl)-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M1, 368 mg, 657 μmol)
was put in a Schlenk flask under nitrogen. Next, anhydrous toluene
(16 mL) and aqueous 0.2 M NaHCO3 (5.98 mL, 1.20 mmol) were
injected together with 2,7-dibromo-9,9″-spirobifluorene (312 mg, 657
μmol) and Aliquat 336 (56.8 mg) under a nitrogen atmosphere. The
I
DOI: 10.1021/acs.macromol.9b00165
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
mL of water were added, and the phases were separated. The aqueous
phase was extracted with DCM (1 × 30 mL), and the combined
organic phases were washed with 50 mL of brine, dried over
anhydrous Na2SO4, and concentrated under reduced pressure. The
residue was dissolved in 4 mL of DCM and dripped into 150 mL of
cold MeOH. The green precipitate was filtered off and washed
consecutively with MeOH (5 days), hexane (2 days), and acetone (4
days) using a Soxhlet extractor. Each washing lasted until no residue
could be found in the chosen solvent (276 mg, 67%). SEC: Mn =
4403, Mw = 10528, PDI = 2.4. MS (MALDI-TOF, DCTB): calculated
for C71H68N2O2[M +]: 980.527, found: building blocks. 1H NMR
(300 MHz, CD2Cl2), δ (ppm): 8.04−6.68 (34H), 3.87−3.74 (4H),
178−1.65 (2H), 1.55−1.25 (16H), 1.01−0.81 (12H).
However, further investigation is required to dig deeper into
the possible advantages that these polymers could give, since
devices were not optimized and more in-depth investigations
regarding stability are required. The investigation of new
possible structures as charge transport materials is fundamental
to reach the best cell possible to open up a space for PSCs in
the market, and the possibility of working without additives is
undoubtedly an important step.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.9b00165.
Synthesis of P5. The dibrominated compound M3 (306 mg, 370
μmol) was put in a Schlenk flask under nitrogen. Next, anhydrous
toluene (9 mL) and aqueou 0.2 M NaHCO3 (3.37 mL, 673 μmol)
were injected together with 4-bromo-N-(4-bromophenyl)-N-(4-
methoxyphenyl)aniline (M5, 231 mg, 370 μmol) and Aliquat 336
(47.8 mg) under a nitrogen atmosphere. The reaction mixture was
purged with argon for 30 min before and after Pd (PPh3)4 (6.42 mg,
5.55 μmol) was added. The mixture was stirred vigorously and
refluxed for 4 days, and 30 mL of DCM and 30 mL of water were
added afterward. The aqueous phase was extracted with 30 mL of
DCM, and the combined organic phases were washed with 50 mL of
brine, dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The residue was dissolved in 4 mL of DCM and dripped
into 150 mL of cold MeOH. The brown precipitate was collected by
filtration and washed consecutively with MeOH (3 days), acetone (2
days), hexane (2 days), and MeOH (3 days) using a Soxhlet extractor.
Each washing lasted until no residue could be found in the chosen
solvent (176 mg, 46%). SEC: Mn = 4681, Mw = 8663, PDI = 1.9. MS
General methods and materials, construction and testing
of the perovskite solar cells, synthesis and character-
1
ization of monomers M1−M5, intermediates 1−8, H
NMR spectra of polymers P1−P6, monomers M1−M5,
intermediates 1−8, MALDI-TOF of polymers P1−P6,
I−V curves of devices (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: juanluis.delgado@polymat.eu.
*E-mail: michael.graetzel@epfl.ch.
■
ORCID
+
Wolfgang R. Tress: 0000-0002-4010-239X
(MALDI-TOF, HCCA): calculated for C65H69N3O3 [M ]: 939.533,
found: building blocks. 1H NMR (300 MHz, CD2Cl2), δ (ppm):
7.66−6.64 (32H), 3.89−3.66 (7H), 179−1.61 (2H), 1.58−1.21
(16H), 1.01−0.77 (12H).
̈
Michael Gratzel: 0000-0002-0068-0195
Juan Luis Delgado: 0000-0002-6948-8062
Notes
Synthesis of P6. A mixture of Ni (COD)2 (375 mg, 1.36 mmol),
2,2′-bipyridine (213 mg, 1.36 mmol), 1,5-cyclooctadiene (167 μL,
1.36 mmol), degassed anhydrous toluene (3 mL), and degassed
anhydrous dimethylformamide (3 mL) was stirred at room temper-
ature under a nitrogen atmosphere for 30 min. A solution of
compound M3 (470 mg, 568 μmol) in degassed anhydrous toluene
(9.5 mL) and degassed anhydrous dimethylformamide (9.5 mL) was
added, and the mixture was stirred at 65 °C for 7 days. The reaction
mixture was poured into MeOH/HCl (20%) (4:1, 500 mL) and
stirred for 30 min. The green precipitate was filtered off and washed
consecutively with MeOH (6 days), acetone (2 days), and CH3CN (3
days) using a Soxhlet extractor. Each washing lasted until no residue
could be found in the chosen solvent (194 mg, 51%). SEC: Mn =
5812, Mw = 12443, PDI = 2.1. MS (MALDI-TOF, DCTB): calculated
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.V. acknowledges Polymat Foundation for a PhD research
position. S.C. acknowledges Basque Government for a
PREDOC grant. J.L.D. acknowledges Ikerbasque, the Basque
Foundation for Science, for an “Ikerbasque Research Fellow”
contract, Polymat Foundation and MINECO of Spain for
CTQ 2016-81911-REDT grant, and Iberdrola Foundation for
financial support.
ABBREVIATIONS
AcOEt, ethyl acetate; DCM, dichloromethane; PE, petroleum
ether; THF, tetrahydrofuran.
■
+
1
for C46H54N2O2 [M ]: 666,417, found: building blocks. H NMR
(300 MHz, (CD2Cl2), δ (ppm): 7.54−7.30 (4H), 7.18−6.70 (16H),
3.93−3.73 (4H), 178−1.62 (2H), 1.57−1.21 (16H), 1.01−0.81
(12H).
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CONCLUSIONS
■
Several polymeric materials based on thiophene, triarylamine,
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