Utilizing Difluorinated Thiophene Units to Improve the Performance of Polymer Solar Cells

Article abstract of DOI:10.1021/acs.macromol.9b01168

While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and DFT-HTAZ, which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while DFT-HTAZ utilizes difluorinated thiophene (DFT) linkers. These polymers serve as the model system to explore the impact of DFT units in OSCs; additionally, this is the first publication to investigate polymers containing DFT units paired with non-fullerene acceptors. Compared to HTAZ, the incorporation of the DFT units maintained the optical properties while lowering the energy levels by a0.4 eV, which allowed for a much improved V_oc value of a1 V. Importantly, when compared with the appropriate non-fullerene acceptor, DFT-HTAZ:ITIC-Th1 blends reached an efficiency of a10%, which is nearly 3× that of the nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using DFT units could serve as a proficient method to increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the acceptor moiety.

Full text of DOI:10.1021/acs.macromol.9b01168

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Cite This: Macromolecules XXXX, XXX, XXX−XXX
Utilizing Difluorinated Thiophene Units To Improve the
Performance of Polymer Solar Cells
Jeromy J. Rech,† Liang Yan,† Zhengxing Peng,‡ Shuixing Dai,§ Xiaowei Zhan,§ Harald Ade,‡
and Wei You*,†
†Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
‡Department of Physics and Organic and Carbon Electronics Lab (ORaCEL), North Carolina State University, Raleigh, North
Carolina 27695, United States
§Department of Materials Science and Engineering, College of Engineering, Key Laboratory of Polymer Chemistry and Physics of
Ministry of Education, Peking University, Beijing 100871, China
S
* Supporting Information
ABSTRACT: While there are numerous approaches to functionalize conjugated polymers for organic solar cells (OSCs), one
widely adopted approach is fluorination. Of the many different locations for fluorination, one of the least studied is the
conjugated linker which connects the donor and acceptor moieties; further, all existing reports primarily explore
monofluorinated thiophene units. Herein, we synthesize and compare two conjugated polymers, HTAZ and dFT-HTAZ,
which have different thiophene linkers. In HTAZ, a bare thiophene unit connects the donor and acceptor moieties, while dFT-
HTAZ utilizes difluorinated thiophene (dFT) linkers. These polymers serve as the model system to explore the impact of dFT
units in OSCs; additionally, this is the first publication to investigate polymers containing dFT units paired with non-fullerene
acceptors. Compared to HTAZ, the incorporation of the dFT units maintained the optical properties while lowering the energy
levels by ∼0.4 eV, which allowed for a much improved Voc value of ∼1 V. Importantly, when compared with the appropriate
non-fullerene acceptor, dFT-HTAZ:ITIC-Th1 blends reached an efficiency of ∼10%, which is nearly 3× that of the
nonfluorinated HTAZ. As most OSC polymers have thiophene linkers, using dFT units could serve as a proficient method to
increase OSC performance in many polymer systems, especially those that do not have locations for functionalization on the
acceptor moiety.
offers negligible steric hindrance and deepens energy levels,
while maintaining a similar band gap.18−20 Another advantage
to fluorination is an increase in inter- and intramolecular
interactions (e.g., F···H, F···S, and F···π), which have
demonstrated to favorably change the polymer backbone
planarity as well as the packing in OSC devices.21−25 Most high
performance conjugated polymers used in OSCs are
copolymers with a characteristic “donor−linker−acceptor−
linker” alternating architecture; therefore, the location of
fluorination needs to be considered.
1. INTRODUCTION
Organic solar cells (OSCs) based on conjugated polymers have
been extensively researched for reasons that include low cost,
flexibility, lightweight, semitransparency, and ease of process-
ing on an industrial scale, making them candidates to supply
the world’s renewable energy needs.1−4 The continued
efficiency improvements for OSCs is due in part to the
development of new conjugated organic materials and blending
strategies (e.g., ternary blend),5−9 and a both popular and
reliable method to improve device efficiency is fluorination of
conjugated polymers.10−15 Fluorine is both the smallest
electron-withdrawing group (van der Waals radius of 1.35 Å)
and the most electronegative element (χr = 3.98).16−18
Therefore, when added along the polymer backbone, fluorine
Received: June 6, 2019
Revised: August 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.9b01168
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While the donor moiety can be fluorinated to improve the
photovoltaic performance of the devices, in many cases, adding
fluorine substituents to the donor moiety shows a detrimental
impact in device performance.12,26 However, one fluorinated
donor moiety that has consistently performed well is 3,3′-
difluoro-2,2′-bithiophene (2TF).27−36 Compared to a non-
fluorinated bithiophene donor moiety, the 2TF has been
shown to have a higher torsional barrier, which promoted a
planar structure with enhanced π−π interactions, and results in
improved morphology of the bulk heterojunction blend and
photovoltaic properties of the corresponding devices.31,33
There have also been a few studies that investigated the
fluorination of the solubilizing side chains of conjugated
polymers. Therein fluorination seems to generally increase
mobilities, suppress triplet formation and charge recombina-
tion, and slightly improve thermal stability, which can be
attributed in all cases to fluorine interactions increasing
ordering of the side chains into a more optimal orienta-
tion.37−42 However, because of the limited scope of materials
made with fluorinated side chains, a complete understanding
on the impact of this approach is still elusive. Given the
electron-withdrawing nature of fluorine, it is not surprising that
the most common fluorination location is the electron-
deficient acceptor moiety. We have previously demonstrated
with our FTAZ polymer that the fluorine substituent on the
benzotriazole acceptor moiety increased all three major device
characteristics: open circuit voltage (Voc), short circuit current
density (Jsc), and fill factor (FF).10,11 The increase in Voc was
due to the deepening of the highest occupied molecular orbital
(HOMO) level from the electron-withdrawing strength of the
fluorine substituent, and a higher hole mobility allowed for
improvements in both Jsc and FF. This same trend has been
demonstrated by many other polymer systems as well.43−47
While fluorination of the acceptor moiety has been
established as a valuable technique to increase the OSC device
performance, a limitation of this approach is that not all
acceptor moieties have locations for fluorination. A notable
example would be the benzodithiophenedione (BDD) accept-
or moiety in the commonly used PBDB-T polymer.48
Moreover, to synthesize a new fluorinated acceptor moiety,
typically a new synthetic route needs to be developed
beginning with a fluorinated starting material; this de novo
synthesis increases the number of steps/time required to make
the final polymer. Therefore, to circumvent these issues,
fluorination on one final location should be considered: the
conjugated linker (often thiophene) that connects the donor
and acceptor moieties together. The idea of fluorinating
thiophene units has shown success in the donor moiety of 2TF
(vide supra) and in various monofluorinated thiophene units
(3′FT and 4′FT) which we used to link benzodithiophene
donor moieties and benzotriazole acceptor moieties.21 Therein,
we demonstrated that relocating the fluorine substituent from
acceptor moiety to the thiophene linker maintained the
advantageous properties from fluorination. There have been
multiple other reports that have explored using monofluori-
nated thiophene units,17,49 but there are very few reports for
OSC polymers that simultaneously utilize fluorination at both
the 3′- and 4′-position of the thiophene linker, i.e., a
difluorinated thiophene (dFT) unit.
example, Hu’s lab reported an OFET polymer containing
dFT units with mobilities over 6 cm2 V−1 s−1 and on/off ratio
over 105, which makes it among the highest values in OFETs
fabricated on a flexible substrate.59 However, of the reports
that use dFT units in OSCs,60−65 the impact of fluorination
remains ambiguous. For example, Takimiya’s lab used a
difluorinated thiophene unit in their PNTz polymers, and
compared to the monofluorinated version, there was a sharp
decrease in performance (10.5% vs 6.5%), which is attributed
to increased recombinationthe polymers containing the dFT
unit had a much lower solubility and thus formed crystallites.60
Conversely, when Heeney’s lab incorporated the dFT unit into
their germanium-containing polymers, they observed a 50%
improvement in efficiency, attributed to a substantially higher
hole mobility and deeper energy levels.61,62 Furthermore, there
have not been any reports of a dFT-containing conjugated
polymer that was paired with non-fullerene acceptors (NFAs),
even as NFAs have rapidly boosted the performance of OSCs
within the past few years.66−71
To further understand the impact of fluorination, in
particular, the dFT unit, we designed and synthesized a new
conjugated polymer, dFT-HTAZ, which differs from the
original HTAZ polymer10 by having two difluorinated
thiophene linkers (see Scheme 1 for structures of HTAZ and
dFT-HTAZ). The incorporation of the dFT units maintained
the optical properties while lowering the energy levels of the
dFT-HTAZ polymer by ∼0.4 eV, which allowed for a much
improved Voc value of ∼1 V in the dFT-HTAZ based devices.
While multiple electron acceptors were considered and tested,
the champion device of dFT-HTAZ:ITIC-Th1 reached an
efficiency of ∼10%, which is nearly 3× that of the
nonfluorinated HTAZ:ITIC-Th1 blend. As most OSC
polymers have thiophene linkers, we envision that strategically
using the dFT units can serve as an effective method to
increase OSC performance in many conjugated polymer
systems.
2. RESULTS AND DISCUSSION
2.1. Synthesis of Monomers and Polymers. HTAZ was
synthesized according to literature procedures,10,21 and the
synthetic pathway for dFT-HTAZ is depicted in Scheme 1.
The starting material, tetrabromothiophene (1), can be
prepared straightforwardly by reacting thiophene with excess
liquid bromine; however, recrystallization is necessary before
the next reaction can proceed. As the desired locations for
fluorination is on the less reactive 3′- and 4′-positions of
thiophene, 1 must first be protected at the 2′- and 5′-positions
with trimethylsilyl (TMS) groups to render 2. Once purified, 2
was then treated with n-butyllithium (n-BuLi) and underwent
electrophilic fluorination with N-fluorobenzenesulfonimide
(NFSI) to offer 3; however, the order of addition is very
important. Upon addition of 2 mol equiv of n-BuLi, then 2
equiv of NFSI resulted in unacceptably low yields (<10%).
Fortunately, we discovered that the yield for this reaction could
be improved by adding a small amount of n-BuLi and NFSI in
several portions rather than all at once. Multiple different
orders of addition were explored and are summarized in the
Supporting Information. Next, selective deprotection and
bromination of the 5′-position of 3 can be achieved in one
pot with N-bromosuccinimide (NBS). The desired precursor 4
can then be made through lithium−halogen exchange followed
by stannylation. Product 4 is a key immediate, since it can be
paired with any acceptor moiety of choosing via Stille coupling.
Throughout the literature, dFT units have a prominent place
in the realm of organic field-effect transistors (OFETs). There
are numerous reports that utilize dFT units in conjugated
polymers to achieve high OFET mobilities.24,50−58 For
B
DOI: 10.1021/acs.macromol.9b01168
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Scheme 1. Synthesis of the Fluorinated Thiophene Units, Monomers, and Stille Coupling Based Polymerization To Make
HTAZ and dFT-HTAZ Polymers
In our case, the fluorinated thiophene units were attached
through a Stille cross-coupling reaction between 4 and a
brominated benzotriazole 5 to yield the monomer precursor 6.
The final step to produce the dFT-HTAZ monomer 7 is to
remove the TMS protecting group and brominate at that
location, which can be done similarly to step 1 in converting 3
to 4. The resulting monomer was recrystallized multiple times
to yield a high purity yellow crystalline powder appropriate for
polymerization. Finally, the monomers (TAZ monomer and
BnDT monomer) were subjected to a microwave assisted Stille
polycondensation to yield the target polymers HTAZ and
dFT-HTAZ. Full details for each reaction, nuclear magnetic
resonance (NMR) characterization, and polymerization
conditions are described in the Supporting Information. The
number-average molar mass (Mn) and dispersities (Đ),
reported in Table 1, were acquired through high-temperature
gel permeation chromatography (HT-GPC) at 150 °C with
1,2,4-trichlorobenzene (TCB) (stabilized with 125 ppm BHT)
as the eluent. Both polymers have Mn values appropriate for
high current density while remaining soluble; the dispersity
values are also appropriate for such step growth polymerization
methods.
2.2. Optical and Electrochemical Properties. To
understand the impact of the dFT units, we first looked at
variations in the optical and electrochemical properties of the
resulting polymers. First, the optical properties of the polymers
HTAZ and dFT-HTAZ were compared with solution UV−vis
absorption of the polymer dissolved in chloroform in Figure
1a. Both polymers absorb in the same range and have an
absorption onset at 618 nm, which equates to an optical band
gap of 2.01 eV. While the HTAZ polymer shows a slightly
stronger aggregation shoulder around 580 nm, both polymers
have a similar λmax around 535 nm. When thin films of
polymers are deposited via spin-coating, similar results are seen
in the absorption when comparing HTAZ and dFT-HTAZ
(Figure 1b). In short, with the exception of a slight difference
in aggregation strength, both polymers have very similar optical
properties.
Next we estimated the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels by cyclic voltammetry (CV) of a thin polymer
film for both polymers (Figure 1c showing the voltammo-
grams). Full details for the CV setup can be found in the
Supporting Information. Key optical and electrochemical data
are summarized in Table 1. Unlike the optical properties, there
are large differences when comparing the electrochemical
properties of the two polymers. First, the conversion of
oxidation onset (EOX) to HOMO energy level is done through
the ferrocene standard (EFC) by using the equation HOMO =
+
−(4.8 eV + e(EOX − EFc/Fc )). The HTAZ polymer has an
oxidation onset of 0.72 V, which corresponds to a HOMO
energy level of −5.44 eV, while the dFT-HTAZ polymer is
shifted to an oxidation onset of 1.11 V, corresponding to a
HOMO energy level of −5.78 eV. The LUMO was estimated
through the optical band gap, and using the information
learned from the optical and electrochemical properties, we
can construct an approximate energy level diagram for the
devices, as shown in Figure 1d. In comparison, it is clear that
the addition of the electron-withdrawing fluorine substituents
to the thiophene linkers lowers the HOMO energy levels by
∼0.4 eV. The deeper HOMO energy level is advantageous as
the Voc is linked to the energy level difference between the
HOMO and LUMO levels of the donor and acceptor
materials. Lowering the HOMO level of HTAZ by an
additional 0.4 eV would indicate that the dFT-HTAZ based
photovoltaic devices should have a much larger Voc than that of
the HTAZ based ones.
2.3. Photovoltaic Properties. To determine the efficacy
of the fluorinated thiophene units as a means to improve
device efficiency, the photovoltaic properties were next
investigated in a bulk heterojunction (BHJ) solar cell with a
normal device configuration: ITO/CuSCN/dFT-
HTAZ:PCBM/Ca/Al and ITO/PEDOT:PSS/HTAZ:PCBM/
C
DOI: 10.1021/acs.macromol.9b01168
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Table 1. Molar Mass, Optical Properties, Band Gaps, and Measured Energy Levels for Both HTAZ and dFT-HTAZ Polymers
a
b
c
d
e
f
polymer
HTAZ
dFT-HTAZ
Mn (kg/mol)
Đ (Mw/Mn)
λmax,sol (nm)
λmax,film (nm)
Eg,opt (eV)
HOMO (eV)
LUMO (ev)
44.1
59.2
3.0
2.1
535/570
534
537/580
530
2.01
2.01
− 5.44
− 5.78
− 3.43
− 3.77
a
b
c
Measured by high-temperature gel permeation chromatography. Absorption maximum in chloroform solution. Absorption maximum in thin
film. Optical band gap is found by dividing 1240 by the absorption onset (618 nm for both polymers). Measured by cyclic voltammetry.
Calculated by electrochemical HOMO and optical band gap: LUMO = HOMO + Eg,opt
d
e
f
.
Figure 1. Normalized UV−vis spectra of the HTAZ and dFT-HTAZ polymers (a) in chloroform solution and (b) as thin films cast from
chlorobenzene. (c) Cyclic voltammogram of each polymer as a thin film, with a scan rate of 0.1 V s−1. (d) Energy levels of both polymers calculated
from cyclic voltammetry.
Ca/Al. The polymer:PCBM ratio was 1:2, and the active layer
thickness was ∼250 nm for both types of devices. The solar cell
characteristics are shown in Table 2, and a representative J−V
curve is found in Figure 2a, with different optimization
conditions summarized in Table S1. Note that we used two
different hole transport layers (HTLs): PEDOT:PSS and
CuSCN. Given the low HOMO energy level of the dFT-
HTAZ polymer, CuSCN, which has the more appropriate
work function (5.5 eV),72 was chosen as the HTL for dFT-
HTAZ based devices. The HTAZ polymer blend used
PEDOT:PSS at the HTL, as it had a more appropriate work
function (5.1 eV). To illustrate the impact of the HTL, the
dFT-HTAZ polymer was also tested with PEDOT:PSS, and
the results are shown in Figure S4. While similar Voc and Jsc are
obtained, there is a drastic difference in the FF (41.7% vs
62.9%), which is attributed to the energy level misalignment
with the PEDOT:PSS interlayer.
Consistent with the prediction above, the Voc value for the
dFT-HTAZ blend is significantly improved when compared to
the HTAZ devices as the HOMO energy level of dFT-HTAZ
is deepened upon addition of the fluorine substituents.
Specifically, the Voc of HTAZ is 0.741 0.001 V while dFT-
HTAZ has a Voc of 0.990 0.002 V. A strong increase in Voc is
often seen upon fluorination, and this increase of 0.25 V in Voc
shows the same trends continue to hold when the fluorination
occurs on the thiophene linker. It is important to note that the
energy levels measured by CV would suggest a 0.4 V change in
the Voc value, meaning there is ∼0.15 V additional energy loss
exists in the dFT-HTAZ:PCBM blend. This energy loss is
likely from either a change in the CT state energy, which can
be attributed to a change at the donor/acceptor interfacial
morphology, or a change in charge recombination loss.
Regardless, while the Voc loss appear to be ∼0.1 V higher for
the dFT-HTAZ blends, the larger increase in Voc results in a
net improvement in the performance of the dFT-HTAZ
blends. Next, the Jsc is often related to the absorption spectra of
the polymers, which, as highlighted previously, are nearly
identical for HTAZ and dFT-HTAZ. Yet, the HTAZ polymer
has a slightly higher Jsc (11.10
HTAZ (9.74
0.33 mA/cm2). Finally, the FF is increased,
0.25 mA/cm2) than dFT-
from 53.3 1.3% for HTAZ to 61.9 1.4% for dFT-HTAZ.
Our previous work has shown that the increasing of hole
mobility through fluorination strongly influences the FF,10,20,73
which can be attributed to the more planar backbone. To
confirm the difference in backbone planarity, we performed
density functional theory (DFT) calculations at the DFT
B3LYP/6-311+G(d) level of theory on one repeat unit for
both HTAZ and dFT-HTAZ, the results of which are shown in
Figure S3. The fluorination of the thiophene linker has a large
impact on the most stable conformation and the dihedral angle
between units. The dFT-HTAZ polymer has a conformation
that is very planar, with the largest dihedral angle of 0.2°, while
the HTAZ polymer has a significantly larger dihedral angle of
11.6° between the BnDT donor moiety and the thiophene
linker. This change in the backbone planarity can be facilitated
through noncovalent inter- and intramolecular interactions,
such as F···H, F···S, and F···π; the fluorinated thiophene units
can impart the same interactions, thus explaining the increase
in FF from 53.3 to 61.9%. The impact on mobility is also
directly measured through space charge limited current
(SCLC) measurements (Figure S5 and Table S2). The hole
mobility value for the dFT-HTAZ:PCBM blend (15.5 × 10−4
cm2 V−1 s−1) is nearly a magnitude higher than the
HTAZ:PCBM blend (1.70 × 10−4cm2 V−1 s−1), which is
attributed to the more planar backbone from the dFT units as
demonstrated above. In summary, when paired with PCBM,
the HTAZ blends can reach a PCE of 4.40% while dFT-HTAZ
can achieve a higher value over 6.41%, a very large 46%
increase in efficiency, mainly due to much improved Voc and
FF.
While these device results show an optimistic outlook for
fluorinated thiophene units, fullerene electron acceptors, such
as PCBM, have been rapidly replaced with the new non-
fullerene acceptors (NFAs). Compared to PCBM, these new
NFAs, such as ITIC and ITIC-Th1 shown in Chart 1, have
superior light absorption in the visible and near-infrared range
D
DOI: 10.1021/acs.macromol.9b01168
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