Isomerizing thieno[3,4-: B] thiophene-based near-infrared non-fullerene acceptors towards efficient organic solar cells

Article abstract of DOI:10.1039/d0tc00269k

With the rapid growth in the requirement for emerging photovoltaic technology like semitransparent solar cells applied for integrated smart windows, there is an urgent need to develop near-infrared (NIR) non-fullerene acceptors (NFAs). To address this issue, thieno[3,4-b]thiophene (TT), which has a stable quinoid structure to minimize the energy difference between two resonance structures corresponding to the band gap, is introduced into the push-pull molecular architecture as a bridge unit to narrow the band gap of the derived acceptors. Due to the different linkage positions (4-or 6-position) of asymmetric TT, these acceptors are classified into two types of isomers, namely 4TIC, 4T4F, 6TIC and 6T4F, of which all have strong absorption in the NIR range. By incorporation with polymer donor PTB7-Th, the devices based on 6-position isomers exhibit superior photovoltaic performance, wherein a champion device based on 6T4F is obtained with a power conversion efficiency of 10.74%. With detailed investigations on inherent optoelectronic properties as well as structural and morphological variation, this performance diversity induced by isomerism is determined by the evident difference in the packing order, which will impact the charge mobility and fill factor. This work presents a class of high-performance NIR acceptors in which the regioisomeric backbone will significantly impact the optoelectronic properties.

Full text of DOI:10.1039/d0tc00269k

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ARTICLE
Isomerizing Thieno[3,4‑b]thiophene-Based Near-Infrared Non-
fullerene Acceptors towards Efficient Organic Solar Cells
Zeqi Zhang, Tong Shan, Yi Zhang, Lei Zhu, Lingwei Kong, Feng Liu and Hongliang Zhong*
Received 00th January 20xx,
Accepted 00th January 20xx
With the rapid growth of the requirement for emerging photovoltaic technology like semitransparent solar cells applied for
integrated smart windows, there is an urgent need to develop near-infrared (NIR) non-fullerene acceptors (NFAs). To address
this issue, thieno[3,4-b]thiophene (TT), which has a stable quinoid structure to minify the energy difference between two
resonance structures corresponding the band gap, is introduced into the push-pull molecular architecture as the bridge unit
to narrow the band gap of derived acceptors. Due to the different linkage positions (4- or 6-position) of asymmetric TT, these
acceptors are classified to two types of isomers, namely 4TIC, 4T4F, 6TIC and 6T4F, of which all have strong absorption in
NIR range. By incorporation with polymer donor PTB7-Th, the devices based on 6-position isomers exhibit superior
photovoltaic performance, wherein a champion device based on 6T4F is obtained with a power conversion efficiency of
10.74%. With detailed investigations on inherent optoelectronic properties as well as structural and morphological variation,
this performance diversity induced by isomerism is determined by the evident difference in packing order which will impact
the charge mobility and fill factor. This work presents a class of high-performance NIR acceptors in which the regioisomeric
backbone will significantly impact the optoelectronic properties.
DOI: 10.1039/x0xx00000x
electron-rich fused indacenodithienothiophene (IDTT) or
indacenodithiophene (IDT) core and two strong electron-
Introduction
withdrawing
2-(3-oxo-2,
3-dihydro-1H-inden-1-
Recently, remarkable progresses have been achieved in the
community of organic solar cells (OSCs) attributed to
continuous advancements from both material and device
approaches, in particular the innovation of non-fullerene
acceptors (NFAs) which possess outstanding advantages, such
as facile accessibility and structure flexibility, compared to
fullerene derivatives.1-6 The power conversion efficiencies
(PCEs) over 16% have been achieved for single junction solar
cells.7-10 Thus, some potential applications for OSCs in consumer
market is emerging and might become mature in near future,
e.g. building-integrated photovoltaics (BIPVs) in which
traditional windows are replaced by transparent OSCs to
provide an opportunity to supply additional energy with less
effect on the origin functions of buildings. To ensure the visible
transparency, it is essential to develop organic semiconductors
mainly absorbing the near-infrared light with high extinction
coefficient.11-30
ylidene)malononitrile (IC) as the terminals, wherein fluoro-
substituted IC terminal groups with a stronger electron-
withdrawing ability are used to further enhance the ICT
effect.36, 37 Another strategy to lower the band gap of an organic
semiconductor is to construct a conjugated system wherein
both the origin state and quinoid resonance structure possess
aromatic rings.38-40 Consequently, the energy difference of
varied resonance structures with varied aromatic states, as an
indication of its band gap, is reduced. Therefore, we reasons
that it might further minify synergistically the band gap by
combining these two methods, to introduce a moiety with
unique quinoid resonance structure into the push-pull
conjugated molecules.
Thieno[3,4-b]thiophene (TT) is a building block with exactly
required properties since the fused thiophene ring can stabilize
42
its quinoid resonance structure (conductive state).41,
In
addition, thieno[3,4-b]thiophene derivatives substituted by
ester or carbonyl groups are commonly used as electron-
deficient acceptor in organic semiconductors.12, 43-46 However,
it is notable the asymmetric configuration of TT raises an issue
of isomerism to complicate structural definition. Our previous
study has revealed there are two different repeat units in PTB7-
Th, as one of many successful donor polymers, due to the
asymmetry of fluoro-substituted TT (FTT), while the ratio of
It is an effective way to narrow the band gap to manipulate
the absorption in near-infrared region by intramolecular charge
transfer (ICT) between donor (D) and acceptor (A) moieties in
push-pull conjugated molecules.21, 31-33 For example, a class of
the most successful NFAs is based on an A-D-A configuration,
such as middle band gap acceptors ITIC34 and IDIC15, 35 with an
these two units plays a key role in determining the material
School of Chemistry and Chemical Engineering, Shanghai Key Lab of Electrical
Insulation and Thermal Aging, Shanghai Jiao Tong University, Shanghai 200240, P.
R. China. Email: hlzhong@sjtu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
48
properties and device performance.47,
Zhu et al. have
reported a series of small molecules and oligomers containing
TT units wherein the regioregularity significantly affects the
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X
DOI: 10.1039/D0TC00269K
COOEH
S
X
S
R
R
R
HEOOC
CHO
O
R
R
S
CN
CN
S
S
S
NC
S
NC
S
S
O
S
NC
CN
R
R
S
S
X
X
OHC
COOEH
R
X
HEOOC
X
6T4F, X=F
6TIC , X=H
O
1a
C8H17
R=
EH=2-ethylhexyl
X
COOEH
X
R
S
HEOOC
OHC
CHO
S
R
R
R
S
S
O
S
CN
CN
S
S
S
S
S
NC
NC
S
O
R
COOEH
R
R
R
S
HEOOC
X
X
4T4F, X=F
4TIC, X=H
1b
Scheme. 1 Synthetic route of 6T4F, 6TIC, 4T4F and 4TIC.
44, 49, 50
charge mobility and photovoltaic property.42,
In the
Results and discussion
Synthesis and characterization
meanwhile of this work is carrying out, Li et al. have synthesize
a family of isomers with FTT as bridge unit and revealed the
impact of the fluorine orientation in derived OSCs.51 Hence, it is
still challenging to develop new near-infrared NFAs involving of
TT which causes the isomerism of relevant materials.
The synthetic routes of the four NFAs are shown in Scheme. 1
and Scheme S1. The precursors 1a and 1b are prepared by Stille
cross-coupling involving of two regioisomers, 2-ethylhexyl 4-
bromo-6-formylthieno[3,4-b]thiophene-2-carboxylate and 2-
Herein, we introduce TT into A-D-A molecules as conjugated
bridge to form A-TT-D-TT-A configuration to obtain four
acceptors 4TIC, 4T4F, 6TIC and 6T4F. The four compounds
comprise a fused IDT core and two strong electron-withdrawing
IC(2F) bridged by quinoidal ester-substituted TT (Scheme. 1),
wherein the asymmetry of TT creates two types of derivatives,
4-position isomers 4TIC and 4T4F, and 6-position isomers 6TIC
and 6T4F. With the help of a strong push-pull effect and the
unique resonance structure of TT, these acceptors show
extended absorption in near-infrared region wherein the
absorption edge of 4TIF and 6TIF with fluorinated terminal
group is over 1000 nm. More importantly, the choice of linkage
position in TT unit play a key role in affecting the optoelectronic
properties and crystallinity, and consequently determining the
photovoltaic performance. The champion device based on 6T4F
shows the best PCE of 10.74% with a high short circuit current
(JSC) of 25.79 mA cm−2, an open-circuit voltage (VOC) of 0.60 V,
and a fill factor (FF) of 69.4%, but 4T4F-based device shows a
much lower PCE of 6.58%. The detailed study reveals the
superior performance of 6-position is ascribed to their higher
packing order when blended with PTB7-Th, which enhances the
charge mobility, and thus improving the fill factors. This work is
one of few examples wherein NIR acceptors with absorption
edge exceeding 1000 nm can achieve a PCE over 10%.
ethylhexyl
6-bromo-4-formylthieno[3,4-b]thiophene-2-
carboxylate, respectively. Then the final products are obtained
by Knoevenagel reaction of 1a (1b) and IC (IC-2F). According to
the linking position between IDT core and TT, these acceptors
are classified to two types: 4-position isomers 4TIC and 4T4F,
and 6-position isomers 6TIC and 6T4F. The four NFAs are able to
be dissolved in common organic solvents such as chloroform
(CF), chlorobenzene (CB) and 1, 2-dichlorobenzene (DCB), which
favors the solution processability. Their chemical structures
1
have been confirmed by H NMR, 13C NMR, and mass spectra
which are shown in Electronic Supplementary Information (ESI).
Optical, electrochemical properties and theory calculation
The optical property of these NFAs has been characterized by
UV–vis–NIR absorption. The spectra of the NFAs in chloroform
solution and film are shown in Fig. 1 and detailed parameters
are summarized in Table 1. All of these new acceptors show
broad absorption in near-infrared region. Compared to 4-
position isomers 4TIC (818 nm) and 4T4F (833 nm), the
maximum absorption of 6-position isomers 6TIC (798 nm) and
6T4F (813 nm) in solution shows hypsochromic shifts
approximate 20 nm, respectively, indicating the linkage position
has significantly impacted on the push-pull structure. Compared
to the solution, the absorption of films exhibits evident
bathochromical shifts with the edge extending to over 1000 nm,
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(b)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
4TIC
4T4F
6TIC
6T4F
4TIC
4T4F
6TIC
6T4F
0.8
0.6
0.4
0.2
0.0
400
600
800
1000
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
(c)
(d)
4TIC
4T4F
6TIC
6T4F
-1.5
-1.0
-0.5
0.0
0.5
1.0
Potential (V)
Fig. 1 The normalized UV–vis absorption of the four acceptors in solution (a) and film (b). Cyclic voltammetry curves (c) and energy levels (d) of 4TIC, 4T4F, 6TIC and
6T4F.
potentials (Fig. 1c). The corresponding energy level diagram and
implying the strong aggregation in solid state, in particular for data are listed in Fig. 1d and Table 1. It is obvious that different
6T4F which shows a red-shift of 106 nm. In addition, the fluoro- linkage positions affect energy levels of the four isomers.
substituent in the IC terminal group is helpful to prompt the Compared to 6TIC and 6T4F, 4TIC and 4T4F have downshifted
electron-withdrawing ability to result in the red-shift in relevant LUMOs, respectively. With introducing fluorine into IC
absorption spectra. The optical band-gap (Eg) of 6T4F and 4T4F terminals, the LUMOs show apparent downshifts attributed to
estimated from the absorption edge of corresponding films are the strong electron-withdrawing ability of fluorine. The
1.23 and 1.22 eV, which are slightly lower than Eg of 6TIC (1.30 electrochemical band gaps are consistent with the optical band
eV) and 4TIC (1.26 eV), respectively.
gap except 6T4F (1.45 vs 1.25 eV), since the films are prepared
To study the electrochemical property, the cyclic by different methods for cyclic voltammetry (drop casting) and
voltammetry measurement has been conducted. The highest absorption (spin casting), while 6T4F shows a strong tendency
occupied molecular orbital (HOMO) and lowest unoccupied to aggregate wherein the packing mode is significantly
molecular orbital (LUMO) energy levels of 4TIC 4T4F, 6TIC and impacted by the processing condition.
6T4F are estimated to be -5.36/-4.11 eV, -5.38/-4.13 eV, -5.31/-
3.96 eV and -5.45/-4.00 eV, respectively from the onset
Table 1 Optical and electrochemical properties of 6T4F, 6TIC, 4T4F, 4TIC.
sol.
sol.
film
opt a)
b)
b)
cv b)
εmax
[M-1cm-1]
λmax
λmax
Eg
EHOMO
[eV]
ELUMO
Eg
Mater.
[nm]
[nm]
[eV]
[eV]
[eV]
5
×
×
×
×
4TIC
4T4F
6TIC
6T4F
1.91 10
818
833
798
813
881
920
853
919
1.26
1.22
1.30
1.23
-5.36
-5.38
-5.31
-5.45
-4.11
-4.13
-3.96
-4.00
1.25
1.25
1.35
1.45
5
5
5
2.20 10
1.85 10
2.09 10
a) Estimated from the absorption. b) Estimated from the electrochemical measurement.
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Fig. 2 (a) J-V curves and (b) corresponding EQE spectra of the optimal PTB7-Th:acceptors devices.
value over 70% in the region from 600 to 900 nm with the
maximum value reaching 73.7%, indicating both polymer donor
Photovoltaic properties.
and acceptor make
a considerable and complementary
In order to investigate the photovoltaic properties of the four contribution to the whole Jsc values. The Jsc values calculated
acceptors, bulk heterojunction (BHJ) OSCs were fabricated with from the EQE curves are 14.93, 16.97, 19.14, and 24.28 mA/cm-2
an inverted device architecture of ITO(indium tin oxide)/ZnO for the 4TIC, 4T4F, 6TIC, and 6T4F based devices, respectively.
/PTB7-Th:acceptor/MoO3/Al. The active layers were prepared All the integrated Jsc values are within about 5% errors
by using chloroform as solvent. The optimized weight ratio of compared to those values obtained from J-V curves, indicating
PTB7-Th:acceptors is 1:1.3 and the temperature of thermal that the measurements are reliable for the PCEs. The EQE
o
annealing (TA) is 110 C. Fig. 2a shows the current density- response of devices based on 6-position isomers is much higher
voltage (J-V) curves of PTB7-Th:acceptors solar cells under the than that of 4-isomers. To rationalize this, the absorption of
optimized conditions and the detailed device parameters are various blend films are measured (Fig.S1). However, the
listed in Table 2. Interestingly, 6-position isomers outperform 4- absorption spectra show approximate intensity for the
position isomers in photovoltaic performance. 4TIC based analogues. Furthermore, photoluminescence (PL) experiments
device shows the lowest PCE of 5.26% with a Voc of 0.70 V, a Jsc have been carried out to probe the origin of the varied
of 14.58 mA/cm2 and a FF of 48.7%, while changing the linkage performance as shown in Fig. S2. For the four blend films, the
position dramatically prompts all parameters leading to a higher PL emissions of PTB7-Th (excited at 685 nm) are completely
PCE of 8.13%. Meanwhile, fluorine substitution on IC group quenched by acceptors respectively, indicating efficient charge
decreases the Voc around 0.1 V which is consistent with the transfer from polymer donor PTB7-Th to electron acceptors.
tendency of lowering LUMO levels in relevant acceptors, but The PL emissions of 4TIC, 4T4F, 6TIC and 6T4F were efficiently
significantly enhances the Jsc and FF, and then synergistically quenched in blend films with quenching efficiencies of 72.6%,
realizing the best PCE of 10.74% with a Jsc of 25.79 mA/cm-2, a 79.8%, 87.4%, and 93.9%, suggesting better charge transfer
Voc of 0.60 V and a FF of 69.4% in the champion device PTB7- between 6-position isomers and PTB7-Th, thus leading to higher
Th:6T4F. The external quantum efficiency (EQE) curves are photocurrent.
shown in Fig. 2b. The 6T4F-based device exhibits a higher EQE
Table 2 Photovoltaic performance of the OSCs based on PTB7-Th:acceptors.
b)
Voc
(V)
Jsc
Jcal
FF
(%)
PCEc)
(%)
μhole
(cm2 V-1 s-1)
μelectron
(cm2 V-1 s-1)
Acceptorsa)
(mA/cm2)
(mA/cm2)
4TIC
4T4F
6TIC
6T4F
0.70±0.00
0.60±0.00
0.74±0.00
0.60±0.00
14.58±0.53
17.32±0.60
19.22±0.67
24.87±0.70
14.93
16.97
19.14
24.28
48.7±0.5
60.0±0.6
54.1±0.5
68.6±0.6
5.26 (5.00±0.25)
6.58 (6.26±0.30)
8.13 (7.74±0.38)
10.74 (10.22±0.50)
7.97×10-4
1.13×10-3
1.35×10-3
1.41×10-3
4.36×10-4
4.76×10-4
9.76×10-4
1.27×10-3
a) The donor:acceptor weight ratio is 1:1.3 with 1% CN as additive and thermal annealed at 110 oC; b) The integral Jsc calculated from the EQE curves; c) The
average values and standard deviations in parentheses are statistical data from ten independent cells
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Fig. 3 (a) JPh-Veff characteristics and (b) light intensity dependence of Jsc for the optimal PTB7-Th:acceptors devices.
6TIC and 6T4F based devices, respectively. These results imply
that 6-position linkage and fluoro-substituent are able to
improve the Pdiss/Pcoll in relevant devices. Thus, the 6T4F based
Charge transport, exciton dissociation and charge recombination
The charge transport properties of the PTB7-Th:acceptors films device exhibits the most efficient exciton dissociation and
are evaluated for hole-only devices ITO/PEDOT:PSS/PTB7- charge collection process and this contributes to its enhanced
Th:acceptors/MoO3/Al and electron-only devices ITO/ZnO/ values of Jsc and FF.
PTB7-Th:acceptors /ZnO/Al by the space-charge limited current
The charge carrier recombination mechanism was further
(SCLC) method (Fig. S4 and Table 2). The blend films based on examined by measuring the correlation between Voc or Jsc and
6-postion acceptors shows higher mobilities, in particular for light intensity (Plight) as shown in Fig. S3 and Fig. 3b. The slope
the electron which typically transports along the pathway of Voc versus light intensity can help to verify the main
formed by acceptor. Furthermore, 6-position isomers exhibit recombination mechanism in the OSCs. When the slope is kT/q
more balanced charge transport than 4-position isomers. In where k is the Boltzmann constant, T is the temperature and q
addition, the fluorination in terminal groups is able to enhance is the elementary charge, bimolecular recombination is the
both hole and electron mobilities. Consequently, the hole and primary mechanism. A slope of 2 kT/q implies that the trap-
electron mobilities for the PTB7-Th:6T4F film are calculated to assisted or Shockley-Read Hall (SRH) recombination is
be 1.41×10-3 and 1.27×10-3 cm2 V-1 s-1, respectively. This dominant, The values of slopes for 4TIC, 4T4F, 6TIC and 6T4F-
enhanced and more balanced mobility leads to the champion based devices are about 1.09 kT/q, 1.24 kT/q, 1.11 kT/q and
device performance with the best FF and Jsc. The diversity of 1.37 kT/q (Fig. S3), respectively, indicating that the trap-assisted
charge mobility is interrelated with morphology and recombination is suppressed and bimolecular recombination is
microstructure which will be discussed in following section.
domain in all fabricated devices. The dependence of Jsc with Plight
In order to understand the mechanisms of exciton can be described as Jsc∝(Plight)α. In general, when the value of α
dissociation and charge extraction in the devices based on is close to unity, bimolecular recombination of charge carriers is
varied acceptors, the variation of photocurrent density (Jph) negligible. The values of α for 4TIC, 4T4F, 6TIC and 6T4F based
with effective voltage (Veff) is employed and shown in Fig. 3a. Jph devices are 0.934, 0.936, 0.911 and 0.975, respectively, which
is defined as Jph = JL-JD, where JL and JD are the current densities manifests that bimolecular recombination is more suppressed
under illumination and in dark, respectively. Veff is described as in the 6T4F based device compared to another three devices.
Veff = Vo-Vbias, where V0 is the voltage at which Jph is zero and
Morphology characterization
Vbias is the applied external voltage bias. In general, it is assumed
that at high Veff, photogenerated excitons can be dissociated To investigate the impact of isomerism on the morphology of
into free charge carriers and collected by the individual devices, 6T4F, 6TIC, 4T4F and 4TIC neat films as well as blend
electrodes efficiently. Herein, for these four devices, the Jph films were characterized by atomic force microscopy (AFM). The
reaches the saturation current density (Jsat) at high Veff (> 2V). neat films of 6TIC and 6T4F show greater root mean square
The exciton dissociation probability (Pdiss) and collection (RMS) roughness with values of 2.66 nm and 10.8 nm
probability (Pcoll) were estimated by the ratio of Jph/Jsat in which respectively, compared with 4TIC (0.49 nm) and 4T4F (0.90 nm)
Jph is obtained under short circuit and maximal power output (Fig. S5), indicating stronger aggregation. From the AFM height
conditions, respectively. The Pdiss/Pcoll are 78.2%/67.7%, images (Fig. 4), we observed uniform surfaces for all four blend
92.6%/73.4%, 89.9%/67.6% and 94.5%/82.1% for 4TIC, 4T4F, films. The PTB7-Th:4TIC and PTB7-Th:4T4F blend films show
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Fig. 4. AFM height and phase images of the optimized blend films based on PTB7-Th:4TIC (a, e), PTB7-Th:4T4F (b, f), PTB7-Th:6TIC (c, g) and PTB7-Th:6T4F (d, h).
Fig.5. 2D GIWAX patterns of (a) PTB7-Th:4TIC, (b) PTB7-Th:4T4F, (c) PTB7-Th:6TIC, (d) PTB7-Th:6T4F. (e) Corresponding 1D line-cut profiles.
especially Jsc and FF. As for PTB7-Th:6TIC and PTB7-Th:6T4F
relatively small root mean square (RMS) roughness with values blend films, the aggregate becomes evident and the RMS is
of 1.68 and 1.25 nm, respectively. No large phase separation increased up to 2.30 and 3.15 nm, respectively, indicating the
appears for both films, indicating the excessive miscibility existence of phase separation induced by 6TIC or 6T4F with
between polymer donor and 4-position acceptors. It can be improved crystallinity.
deduced that the extremely low crystallization in the blend films
To get a deep insight into the molecular packing structures,
is unfavorable for efficient charge separation and the four blend films are studied by grazing incidence wide-angle
transportation, and consequently limiting device performance X-ray scattering (GIWAXS), as shown in Fig. 5. The key
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parameters calculated from the GIWAXS line-cut profiles for the
packing behavior of these films are displayed in Table S1. It is
well known PTB7-Th tends to pack with face-on orientation,
leading to a lamellar packing (100) peak at around 0.27 Å−1 in
the in-plane (IP) direction and a π-π stacking (010) diffraction
peaks at 1.57 Å−1 in the out-of-plane (OOP) direction,47 both of
which are observed in Fig. 5 with relatively weak intensity.
Compared to the peaks for PTB7-Th, much stronger diffraction
peaks resulted from acceptors are evident at approximate 0.34
Å−1 in IP and 1.86 Å−1 in OOP direction in all four blend films,
corresponding to lamellar packing and π-π stacking. The π-π
distances are calculated to 3.39 Å (4TIC), 3.36 Å (4T4F), 3.38 Å
(6TIC) and 3.36 Å (6T4F), respectively. Such exceptionally tight
π-π stacking implies a very strong intermolecular interaction in
these NFAs which may be resulted from their rigid planar
skeletons. Notably, the blend film based on 6-position isomers
show stronger and sharp scattering peaks, compared to those
of 4-position isomers. As depicted in Table S1, the coherence
lengths evaluated form full width at half maximum (FWHM) of
6-position isomers are bigger, indicating 6-position isomers
possess higher packing order even in blend film. This feature is
believed to be beneficial for constructing a well-organized
active layer, and thus helping 6T4F to achieve high efficiency.
Acknowledgements
The authors thank the fund support by the National Natural
Science Foundation of China (Grant No. 21604053).
View Article Online
DOI: 10.1039/D0TC00269K
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