Isoindigo-3,4-Difluorothiophene Polymer Acceptors Yield “All-Polymer” Bulk-Heterojunction Solar Cells with over 7 % Efficiency

Article abstract of DOI:10.1002/anie.201709509

Poly(isoindigo-alt-3,4-difluorothiophene) (PIID[2F]T) analogues used as “polymer acceptors” in bulk-heterojunction (BHJ) solar cells achieve >7 % efficiency when used in conjunction with the polymer donor PBFTAZ (model system; copolymer of benzo[1,2-b:4,5

Full text of DOI:10.1002/anie.201709509

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Accepted Article
Title: Isoindigo-3,4-Difluorothiophene Polymer Acceptors Yield "All-
Polymer" BHJ Solar Cells with >7% Efficiency
Authors: Shengjian Liu, Yuliar Firdaus, Simil Thomas, Zhipeng Kan,
Federico Cruciani, Sergei Lopatin, Jean-Luc M. Bredas, and
Pierre M. Beaujuge
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709509
Angew. Chem. 10.1002/ange.201709509
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10.1002/anie.201709509
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COMMUNICATION
Isoindigo–3,4-Difluorothiophene Polymer Acceptors Yield
“All-Polymer” BHJ Solar Cells with >7% Efficiency
Shengjian Liu,†,& Yuliar Firdaus,†,& Simil Thomas,‡ Zhipeng Kan,† Federico Cruciani,† Sergei Lopatin,§
Jean-Luc Bredas,‡ and Pierre M. Beaujuge*,†
Abstract:
Poly(isoindigo–alt–3,4-difluorothiophene)
(PIID[2F]T)
all-PSCs are some important parameters that have received
significant attention in recent years.2 While a large part of that
effort has been directed to extending the range of efficient
polymer acceptors, analogues based on perylenediimide (PDI)
and naphthalenediimide (NDI) motifs remain the most efficient
thus far, achieving PCEs of 7%–9% in all-PSCs with selected
polymer donors.2,4,5 Polymer acceptors based on B←N bridged
bipyridine (BNBP) represent another emerging class of
promising fullerene alternatives that has recently been shown to
reach PCEs of ca. 6%.6 Several other acceptor motifs are being
analogues used as “polymer acceptors” in bulk-heterojunction (BHJ)
solar cells achieve >7% efficiency when used in conjunction with the
polymer donor PBFTAZ (model system; copolymer of benzo[1,2-
b:4,5-b’]dithiophene and 5,6-difluorobenzotriazole). Considering that
most efficient polymer acceptor alternatives to fullerenes (e.g.
PC61BM or its C71 derivative) are based on perylenediimide or
naphthalenediimide motifs thus far, branched alkyl-substituted
PIID[2F]T polymers are particularly promising nonfullerene
candidates for “all-polymer” BHJ solar cells.
examined,
such
as
diketopyrrolopyrrole,7a
2,1,3-
benzothiadiazole,7b and various nitrile (CN)-derived motifs;7c with
reported PCEs in range of 1-4%. However, to date, polymer
acceptor developments remain synthetically challenging, and the
manifold of electron-deficient motifs and polymer acceptor
candidates that can rival fullerenes for efficient BHJ solar cells
with polymer donors remains modest. Thus, broadening the
class of polymer acceptors for further examinations of the “all-
polymer” BHJ concept is a critically important step to take in the
improvement of device performance beyond currently reported
PCEs.
Isoindigo (IID) motifs may serve as alternatives to their PDI
and NDI counterparts.8,9 These synthons have primarily been
used in the design of low-bandgap polymer donors for BHJ solar
cell8 and thin-film transistor8a applications. Only few studies have
discussed their possible use in the design of polymer acceptors,
mainly because of the high-lying HOMO (“highest occupied
molecular orbital”; i.e., low IP) that IID motifs tend to induce
when incorporated in the polymer main-chain.9 The two lactam
moieties in IIDs impart their substantial electron-withdrawing
character, and the various alkyl substituents that can be
appended to the nitrogen sites provide a handle on solubility and
polymer self-assembly.8 Turning to polymer acceptor designs,
systems incorporating IIDs should however include adequate
electron-deficient co-monomers that can result in alternating
sequences with sufficiently high IP and EA values
(corresponding to low-lying HOMO and LUMO (“lowest
unoccupied molecular orbital”) energy levels) to be used as
nonfullerene acceptors in BHJ solar cells.2,9,10
In the early years of research on “all-polymer” bulk
heterojunction (BHJ) solar cells (all-PSCs), devices made from
blends of π-conjugated polymer donors and acceptors met with
limited power conversion efficiencies (PCEs).1 While in all-PSCs,
polymer acceptors are used as alternatives to fullerenes (e.g.
PC61BM or its C71 analogue), the ability to concurrently achieve
donor-acceptor networks with adequate domain sizes (within the
lengthscale of exciton diffusion) and efficient charge transfer
between the donor and acceptor counterparts is paramount.
Therefore, only a few polymer acceptors have been shown to
yield BHJ device PCEs >7% with polymer donors to date.2 In
comparison, PSCs composed of either
a fullerene or a
nonfullerene molecular acceptor can achieve PCEs >11%,3
although their lack of morphological stability and mechanical
conformability remains a matter of examination at this time.4
Meanwhile, polymer acceptors are more synthetically accessible
than PCBM analogues and come with the perspective of lower
synthetic costs in light of the more standard purification
protocols followed.2 In parallel, the tunability in their synthetic
design can give access to specific optical and electronic
properties (i.e., electron affinities (EAs), ionization potentials
(IPs), spectral absorption, etc.) that are not readily accessible
with fullerene acceptors.2
The active-layer material selection, choice of electron/hole-
transporting interlayers, and the device processing conditions in
[&]
[†]
These authors contributed equally to this work.
Dr. S. Liu, Dr. Y. Firdaus, Dr. Z. Kan, F. Cruciani, Prof. P. M.
Beaujuge*
In this contribution, we report on a set of branched alkyl-
substituted polymer acceptors composed of alternating IID8,9 and
3,4-difluorothiophene [2F]T motifs,10 and show that the
Physical Science and Engineering Division, Solar & Photovoltaics
Engineering Research Center (SPERC)
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900, Saudi Arabia.
appropriately
functionalized
poly(isoindigo–alt–3,4-
difluorothiophene) analogue, namely PIID[2F]T (Figure 1a), can
achieve PCEs as high as 7.3% in BHJ solar cells with PBFTAZ11
as the polymer donor (model system; Figure 1b). PBFTAZ
(Eopt~1.9 eV) and PIID[2F]T (Eopt~1.7 eV) provide spectral
complementary across the UV-vis spectrum (300-750 nm),
yielding high short-circuit current densities (JSC) of ca. 13.2
mA/cm2 and some of the best open-circuit voltage (VOC) values
(ca. 1.0 V) achieved thus far in single-cell BHJ devices. Given
the extent of synthetic modularity known for IID motifs,8
adequately substituted PIID[2F]T analogues effectively broaden
E-mail: pierre.beaujuge@kaust.edu.sa.
Dr. S. Thomas, Prof. J.–L. Bredas
[‡]
School of Chemistry and Biochemistry, Center for Organic
Photonics and Electronics (COPE)
Georgia Institute of Technology
Atlanta, Georgia 30332-0400, United States.
Dr. S. Lopatin
[§]
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900, Saudi Arabia.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.2017xxxxx
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10.1002/anie.201709509
Angewandte Chemie International Edition
COMMUNICATION
the class of systems with tunable electronic and optical spectra
for all-PSCs with improved PCEs.
offsets between the DFT-calculated IPs and EAs of the polymer
donor PBFTAZ and the nonfullerene acceptor PIID[2F]T are
found to be 0.37 eV and 0.57 eV (Figure S7), respectively; these
results indicate that the polymers have appropriate energy band
offsets to be used as donor and acceptor counterparts in BHJ
solar cells.
R2
N
R1
N
F
F
PIID[2F]T(R1/R2)
O
O
S
m
2BO n:m = 1:0
2HD n:m = 0:1
S
The PIID[2F]Ts with various branched alkyl side-chains shown
in Figure 1a (2-butyloctyl: 2BO, 2-hexyldecyl: 2HD, and
2BO/2HD [2:1] in random sequences) were synthesized via
Stille cross-coupling polymerization (see experimental details in
SI).10 Here, we note that the relatively limited solubility of the
2BO derivative in common organic solvents made us first
consider turning to the longer branched 2HD analogue. However,
the higher branched 2HD analogue showing complete solubility
in dichloromethane, we envisioned the random polymerization of
the two motifs 2BO and 2HD in various stoichiometric ratios
(details in SI). The 2BO/2HD [2:1] analogue proved to provide
sufficient solubility in the common organic solvents used in BHJ
active-layer processing steps. The thermal analyses in Figure
S8 indicate that PIID[2F]T and PBFTAZ are thermally stable until
ca. 400 °C and that no visible characteristic peak for glass
transition, crystallization, or melting are observed in the range
25–250 °C. Hence, the polymers can be considered sufficiently
stable to be subjected to post-processing thermal annealing in
BHJ device optimization steps (detailed in later sections).
The normalized thin-film UV–vis absorption spectra of the
PIID[2F]T analogues and that of the polymer donor PBFTAZ
(model system; later used in the device study) are superimposed
in Figure 1d. The PIID[2F]T derivatives have equivalent
absorption spectra across the range 400-750 nm (peaking at ca.
624 nm) and optical gaps (Eopt) of ~1.7 eV. The spectral
absorption of PBFTAZ falls in the range 400-650 nm, where
PIID[2F]T does not absorb as effectively –thus adding in
complementarity, a substantial benefit in the optimization of BHJ
solar cell efficiency. The IP of PIID[2F]T estimated by
photoelectron spectroscopy in air (PESA) is ~5.6 eV, vs. ~4.8 eV
for PBFTAZ (see Figure S12 and Table S3). These notably large
IP values for PIID[2F]T are comparable to benchmark polymer
acceptors such as N2200 (~5.7 eV estimated by PESA). From
Figure S13, the electrochemically estimated IPs and EAs can be
inferred as follows: ~6.2 eV for PIID[2F]T (vs. ~5.5 eV for
PBFTAZ) and ~4.0 eV (vs. ~3.9 eV for PBFTAZ), respectively
(results reported in Table S4). Here, it is worth noting that the
EA estimate is 0.1 eV lower than that of the fullerene acceptor
PCBM (4.1-4.2 eV) and slightly lower than those of PDI/NDI-
based polymer acceptors.4-5 The lower EA values inferred for
PIID[2F]T should be conducive to high VOC values in BHJ solar
cells (see below).
O
O
N
N
Fn
2BO/2HD n:m = 2:1
F
R2
R1
R1 =
2BO
2HD
R2
(a)ꢀ R2 =
N
N
N
R1
R1 =
Oct
S
S
S
S
F
F
n
R2 =
2HD
PBFTAZ
R1
S
S
(b)ꢀ
(c)ꢀ
(d)ꢀ
Figure 1. Chemical structures of (a) PIID[2F]T with various branched alkyl
substituents (2BO, 2HD, and 2BO/2HD [2:1] in random sequences) and (b)
PBFTAZ (model system). (c) Representations of the TD-DFT tuned-ωB97X-D
natural transition orbitals (NTOs) with the largest contribution to the vertical
S0-S1 transition for a IID[2F]T hexamer (bottom: hole NTO; top: electron NTO).
(d) Superimposed thin-film UV−vis optical absorption spectra of PIID[2F]Ts
and the polymer donor PBFTAZ (normalized).
Prior to synthesizing the PIID[2F]T analogues, density
functional theory (DFT), at the ωB97XD/6-31G(d,p) level of
theory, was employed to probe the main-chain conformation and
the extent of π-electron delocalization along the alternating IID
and [2F]T motif sequences.12 Figure S2 gives the torsion
potential energy surface as a function of rotation of the IID motif
with respect to the [2F]T unit; two minima occur, corresponding
to the anti/155° and syn/25° conformations. The syn/25°
Thin-film BHJ solar cells with the inverted device configu-
ration ITO/ZnO/PEOz/PBFTAZ:PIID[2F]T/MoO3/Ag (device area:
0.1 cm2) were fabricated and tested under AM1.5G solar
illumination (100 mW/cm2). The devices with optimized
PBFTAZ:PIID[2F]T blend ratios of 1:1 (wt/wt) were cast from hot
conformation
is
slightly
more
stable
than
the
o
chlorobenzene (CB; ca. 100 C). Systematic device fabrication,
anti/155°conformations by some 0.5 kcal/mol (which is close to
thermal energy at room temperature). Figure 1c depicts the
time-dependent (TD) DFT tuned-ωB97XD natural transition
orbitals with the largest contribution to the vertical S0-S1
transition for a IID[2F]T hexamer, with the excitation delocalized
over ca. three repeat units (cf. additional details in SI). The
optimization steps, and statistics are provided in the SI. As
shown in Figure 2a and Table 1, optimized all-PSC devices
made with the PIID[2F]T analogues appended with various
substituents (2BO, 2HD, and 2BO/2HD) and the polymer donor
PBFTAZ achieve very distinct performance characteristics.
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10.1002/anie.201709509
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While optimized BHJ devices made with PIID[2F]T(2BO) (limited
and acceptor counterparts in the range 350-750 nm, the average
solubility) reach promising JSC and fill-factor (FF) values of ca.
IQE of ca. 73% (up to 77% at 570 nm) obtained with the
Figure 2. (a) Characteristic J-V curves and (b) EQE and IQE spectra of
optimized all-PSCs fabricated with the polymer donor PBFTAZ and the
PIID[2F]T polymer acceptors (here named 2BO, 2HD, and 2BO/2HD).
Integrated EQEs are in agreement (±0.6 mA/cm2; ±5%) with the JSC values
reported in Table 1.
Figure 3. (a-c) Dark field STEM images of the optimized BHJ active layers
and (d-f) corresponding EELS maps depicting the phase-separation patterns
between donor-rich domains (green; PBFTAZ) and acceptor-rich domains
(red; PIID[2F]T). (a,d: 2BO, b,e: 2BO/2HD and c,f: 2HD); see experimental
details in SI.
Table 1. PV Performance of the PIID[2F]Ts Derivatives in Inverted BHJ
Devices with the polymer donor PBFTAZ.a,b
Avg.
PCE
[%]
Max.
PCE
[%]
JSC
[mA/cm2]
VOC
FF
Acceptor
2BO
[V]
[%]
9.3
13.2
8.2
0.99
0.97
1.00
0.70
50
55
42
60
4.4
7.1
3.4
4.5
4.6
7.3
3.7
4.8
2BO/2HDc
2HD
PC71BM
11.4
[a] Average values across >10 devices. [b] Device statistics in SI, Table S5.
[c] Randomly substituted alkyl chains (2BO/2HD, 2:1).
9.3 mA/cm2 and 50%, respectively, the same figures of merit
obtained for devices made with PIID[2F]T(2HD) (the more
soluble counterpart) remain relatively modest: ca. 8.2 mA/cm2
and 42%. Overall, PCEs remain in the 3-5% range, in spite of
the high VOC of 1.0 V achieved. Turning to optimized BHJ solar
cells made with PIID[2F]T(2BO/2HD) (adequate solubility in CB),
Table 1 shows markedly improved figures of merit, with a JSC as
high as 13.2 mA/cm2, a FF nearing 55%, and PCEs of up to 7.3%
(avg. 7.1%) –representing a stark ca. twofold PCE improvement
over devices made with the 2BO and 2HD derivatives. The VOC
of 1.0 V – consistent with the reduced electrochemically-inferred
EA estimates for PIID[2F]T compared to PC71BM (Table S4) – is
0.3 V greater than that of the control PC71BM-based devices
with PBFTAZ (ca. 0.7 V; Table 1).11 The higher JSC values for the
2BO/2HD devices (13.2 mA/cm2) compared to those of the
control PC71BM-based devices (11.4 mA/cm2) result in part from
the more efficient spectral absorption of the all-PSC solar cells
(PC71BM’s absorption being limited to the short-wavelength
region where the photon flux is only modest). Thus, importantly,
all-PSCs based on 2BO/2HD achieve PCEs higher by ca. 50%
relative to those of the PC71BM-based BHJ solar cells (max.
4.8%).
Figure 4. Dark current density-voltage characteristics for (a) hole-only and (b)
electron-only diodes made with optimized BHJ active layers composed of
PBFTAZ and the PIID[2F]T polymer acceptors (2BO, 2HD, and 2BO/2HD).
The experimental data are fitted using the SCLC model (solid lines; cf. details
in the SI).
2BO/2HD derivative emphasizes the effectiveness of the
photon-to-current conversion process. In comparison, the 2BO-
and 2HD-based devices averaging 50% IQE across the same
wavelength range point to the existence of charge recombination
and/or collection losses (analyses beyond the scope of this
concise report).
Given the solubility differences observed between the
PIID[2F]T analogues (stated in earlier sections), we turned to a
systematic analysis of their phase-separation patterns with the
polymer donor PBFTAZ across optimized BHJ active layers –
using
a
combination of scanning transmission electron
microscopy (STEM) and spatially resolved electron energy-loss
spectroscopy (EELS) methods (see experimental details in SI).13
The external quantum efficiency (EQE) and internal quantum
efficiency (IQE) spectra provided in Figure 2b are consistent with
the trend in JSC values described in Table 1. PIID[2F]T-based all-
PSCs show prominent spectral contributions from both donor
Figure
3 shows three distinct donor-acceptor segregation
patterns, with the most evidently “adequate” corresponding to
that of optimized 2BO/2HD-based active layers: smaller domain
sizes indicative of a higher degree of donor-acceptor mixing.
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Optimized BHJ Thin Films with PBFTAZ and PIID[2F]Ts.
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µe
Acceptor
2BO
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[a] Randomly substituted alkyl chains (2BO/2HD, 2:1).
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2 provide the hole (µh) and electron (µe) mobility
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cm2V−1s−1 and 2.9×10−5 cm2V−1s−1, respectively.
[6]
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In summary, we have shown that the nonfullerene PIID[2F]T
polymer analogues (Eopt~1.9 eV) used with the polymer donor
PBFTAZ (model system; Eopt~1.6 eV) can achieve PCEs >7%
and some of the highest reported JSC and VOC values for all-
PSCs (>13 mA/cm2, 1.0 V). Given that, to date, most efficient
polymer acceptors are based on PDI or NDI motifs, branched
alkyl-substituted PIID[2F]T alternatives to PC61BM (or its C71
derivative) expand the class of efficient material systems for
further developments of the “all-polymer” BHJ solar cell
approach. The examination of other polymer donors will be of
importance in future work with the nonfullerene PIID[2F]T
polymer analogues.
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Acknowledgements
The authors acknowledge financial support under Baseline
Research Funding from King Abdullah University of Science and
Technology (KAUST), from ONR (Award N00014-17-1-2208 to
JLB), and from the Guangdong Natural Science Foundation (No.
2016A030310428 to S. Liu). The authors thank Xin Song for
some contributions in the early project developments. The
authors thank KAUST Analytical Core Labs for technical support.
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Keywords: all-polymer solar cells • polymer acceptor • Isoindigo
• 3,4-Difluorothiophene
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10.1002/anie.201709509
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
S. Liu,†,& Y. Firdaus,†,& S. Thomas,‡ Z.
Kan,† F. Cruciani,† S. Lopatin,§ J.-L.
Bredas,‡ and P. M. Beaujuge*,†
Page No. – Page No.
Isoindigo–3,4-Difluorothiophene
Polymer
Acceptors
Yield
“All-
Polymer” BHJ Solar Cells with >7%
Efficiency
Poly(isoindigo–alt–3,4-difluorothiophene) (PIID[2F]T) analogues can serve as
fullerene alternatives in “all-polymer” bulk heterojunction (BHJ) solar cells with
efficiencies as high as 7.3%. Employing a wide-bandgap polymer donor (PBFTAZ;
commonly used with fullerenes) yields some of the best VOC figures (ca. 1.0 V)
reported to date for BHJ solar cells.
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