Isomeric effect of fluorene-based fused-ring electron acceptors to achieve high-efficiency organic solar cells

Article abstract of DOI:10.1039/c9ta14040a

Acceptor-donor-acceptor (A-D-A) non-fullerene electron acceptors (NFEAs) using ladder-type donor structures have become the dominant n-type materials for achieving high-efficiency OSCs. In this work, two isomeric fluorene-based ladder-type structures FCTT (TT-C-F-C-TT) and FTCT (T-C-TFT-C-T) have been designed and synthesized. These two isomeric donors with the different fused-ring arrangement, molecular geometry, and side-chain placement were end-capped with the FIC acceptors to form two NFEAs FCTT-FIC and FTCT-FIC isomeric materials. Compared to FTCT-FIC using the thiophene (T)-terminal donor, FCTT-FIC with the thienothiophene (TT)-terminal donor has more evenly distributed side chains on both sides of the backbone and less steric hindrance near the FIC acceptors, which enables stronger antiparallel π-π packing among the end-groups to create a channel for efficient electron transport, as evidenced by the thin-film GIWAXS measurements. FCTT-FIC displayed a larger optical bandgap and deeper-lying energy levels than its FTCT-FIC isomer. Compared to the PBDB-T:FTCT-FIC device, the PBDB-T:FCTT-FIC device showed a higher PCE of 10.32% with an enhanced J_sc of 19.63 mA cm^-2 and an FF of 69.14%. A PM6:FCTT-FIC device using PM6 as a p-type polymer achieved the highest PCE of 12.23%. By introducing PC₇₁BM as the second acceptor to enhance the absorption at shorter wavelengths, optimize the morphology and facilitate electron transport, the ternary-blend PM6:FCTT-FIC:PC₇₁BM (1 : 1 : 0.5 in wt%) device yielded the highest PCE of 13.37% with a V_oc of 0.92 V, a higher J_sc of 19.86 mA cm^-2, and an FF of 73.2%. This result demonstrated that the TT-terminal ladder-type donor is generally a better molecular design than the corresponding T-terminal ladder-type isomer for the development of new A-D-A NFEAs.

Full text of DOI:10.1039/c9ta14040a

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DOI: 10.1039/C9TA14040A
ARTICLE
Isomeric Effect of Fluorene-based Fused-ring Electron Acceptors to
Achieve High-Efficiency Organic Solar Cells
Yung-Jing Xuea, Fong-Yi Caoa, Po-Kai Huanga, Yen-Chen Sua and Yen-Ju Cheng*ab
Received 00th January 20xx,
Accepted 00th January 20xx
Acceptor-donor-acceptor (A-D-A) non-fullerene electron acceptors (NFEAs) using ladder-type donor structures have become
the dominant n-type materials for achieving high-efficiency OSCs. In this work, two isomeric fluorene-based ladder-type
structures FCTT (TT-C-F-C-TT) and FTCT (T-C-TFT-C-T) have been designed and synthesized. These two isomeric donors with
the different fused-ring arrangement, molecular geometry, and side-chain placement were end-capped with the FIC
acceptors to form two NFEAs FCTT-FIC and FTCT-FIC isomeric materials. Compared to FTCT-FIC using the thiophene (T)-
terminal donor, FCTT-FIC with the thienothiophene (TT)-terminal donor has more evenly-distributed side chains among the
backbone and less steric hindrance near the FIC acceptors, which enables stronger antiparallel π−π packing among the end-
groups to create a channel for efficient electron transport, as evidenced by the thin-film GIWAXS measurements. FCTT-FIC
displayed a larger optical bandgap and deeper-lying energy levels than its FTCT-FIC isomer. Compared to the PBDB-T:FTCT-
FIC device, the PBDB-T:FCTT-FIC device showed a higher PCE of 10.32% with enhanced Jsc of 19.63 mA cm−2 and FF of 69.14%.
PM6:FCTT-FIC device using PM6 as a p-type polymer achieved a highest PCE of 12.23%. By introducing PC71BM as the second
acceptor to enhance the absorption at shorter wavelengths, optimize the morphology and facilitate electron transport, the
ternary-blend PM6:FCTT-FIC:PC71BM (1:1:0.5 in wt%) device yielded the highest PCE of 13.37% with a Voc of 0.92 V, a higher
Jsc of 19.86 mA cm−2, and an FF of 73.2%. This result demonstrated that the TT-terminal ladder-type donor is generally a
better molecular design than the corresponding T-terminal ladder-type isomer for the development of new A-D-A NFEAs.
DOI: 10.1039/x0xx00000x
1. Introduction
50-55
Organic solar cells (OSCs) have been considered as the deficient acceptors (A)32,
and solubilizing aliphatic side-
promising candidates for the next generation solar energy chains27, 56-58. The central donor core particularly plays a more
converter due to their intrinsic properties of lightweight, important role in determining the intrinsic characteristics of
flexibility, semi-transparency at visible region and solution- NFEAs. The mainstream design of the central symmetrical
process availability.1-13 In the past few years, n-type non- donor unit is based on the concept of molecular rigidification
fullerene acceptors (NFEAs) have been rapidly developed for and planarization.59-69 Incorporation of sp3-carbon bridges to
achieving high efficiency OSCs.14-41 The organic-based NFEAs can covalently ring-lock adjacent electron-rich aryl groups leads to
possess strong absorption covering UV-visible and near infrared a fully planarized ladder-type structure which can elongate
regions and tunable HOMO/LUMO energy levels to effective conjugation, improve charge mobility and promote
simultaneously achieve higher short-circuit current density (Jsc) intermolecular interactions.68-70 Moreover, alkyl side-chains
and open-circuit voltage (Voc).33-35 The current state-of-the-art attached on the central ladder-type donor are also crucial for
FREAs are based on an A-D-A (acceptor-donor-acceptor) tailoring intermolecular interactions between the N-type NFEA
architecture consisting of a central electron-rich donor (D) end- molecules. It is generally believed that A-D-A type NFEAs tend
capped by two electron-deficient acceptors (A). The push-pull to self-assemble through the antiparallel stacking of the
D-A system can induce intramolecular charge transfer for strong acceptor end-groups that generate a channel for electron
light-harvesting ability. The electronic and steric properties of transport.61 The function of side-chains should prevent severe
A-D-A NFEAs can be optimized by independently modifying the aggregation for solution processibility without interrupting the
central electron-rich donors (D)18, 23, 29, 42-49, end-group electron- antiparallel packing among end-groups.71 As a result, the
location of the side-chains should keep distance with the
acceptor end-groups. Various fascinating ladder-type structures
have been developed and utilized for making high-performance
A-D-A NFEAs. These lengthy conjugated ladder-type donors,
mostly comprised of thiophene (T), benzene (B) and embedded
di(4-hexylphenyl) cyclopentadienyl (C) moieties, can have
several structural isomers. Investigation of isomeric ladder-type
a. Department of Applied Chemistry, National Chiao Tung University, 1001
University Road, Hsinchu, Taiwan 30010.
b. Center for Emergent Functional Matter Science, National Chiao Tung University,
1001 University Road, Hsinchu, Taiwan 30010.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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DOI: 10.1039/C9TA14040A
Fig 1. Chemical structures of (a) BDCPDT-IC, ITIC, FTCT-FIC and FCTT-FIC; (b) the diagram of the antiparallel stacking of the terminal FIC moieties. (c) chemical structures of PBDB-T
and PM6. Polymer structures PBDBT and PM6.
structures with different main-chain and side-chain corresponding FCTT-FIC (Fig 1b) FREA. By shifting the inner
arrangements is very appealing and promising for developing thiophenes in FTCT to the terminal part, the aliphatic side-
new NFEAs. In 2015, Zhan et al first reported a high- chains are more evenly distributed at the both sides of FCTT
performance A-D-A type ITIC utilizing
a
IC (1,1- backbone. Meanwhile, the distance between the side-chains on
dicyanomethylene-3-indanone ) acceptor and a ladder-type CP rings and the FIC end-groups also becomes longer (ca. 12.21
IDTT (Fig 1a) donor which contains a central benzene ring (B), Å). FCTT-FIC displayed larger optical bandgap and deeper
two outer thienothiophenes (TT) and two embedded Cs HOMO/LUMO energy levels compared with its FTCT-FIC isomer.
(symbolized as TT-C-B-C-TT).23 By shifting the two outer Compared to the PBDB-T:FTCT-FIC-based device, the device
thiophenes in TT-C-B-C-TT to the inner part, an isomeric using PBDB-T:FCTT-FIC showed a higher PCE of 10.32%.
structure T-C-TBT-C-T (also named as BDCPDT) was designed Furthermore, the PM6:FCTT-FIC-based device exhibited
and synthesized (Fig 1a). The corresponding non-fullerene dramatically improved efficiency of 12.23%. The chemical
acceptor BDCPDT-IC was thus prepared.31 The ITIC-based structures of PBDB-T and PM6 are shown in Fig 1c. By
devices generally exhibited better device performance in introducing PC71BM as the third component to enhance the
comparison with the BDCPDT-IC-based devices. This result absorption at shorter wavelengths, optimize the morphology
implied that using TT-terminal ladder-type structure seems to and facilitate electron transport,73-77 the ternary-blend device
be a more superior design compared to its corresponding T- using PM6:FCTT-FIC:PC71BM (1:1:0.5 in wt%) yielded a highest
terminal isomer. Furthermore, Zhan et al. designed two PCE of 13.37% with a Voc of 0.92 V, a higher Jsc of 19.86 mA cm−2,
isomeric ladder-type NFEAs, FNIC1 and FNIC2.72 It is interesting and an FF of 73.2%.
to find that TT-terminal FNIC2 also showed better device
performance than T-terminal FNIC1.
2. Results and discussion
It is of great interest to systematically compare TT-terminal
and T-terminal isomeric NFEAs. We recently developed another
NFEA denoted as FTCT-FIC using a flourene-based nonacyclic
ladder-type core FTCT (Fig 1a).69 FTCT (can also be symbolized
The synthetic route of FCTT-FIC is depicted in Scheme 1. 2,7-
dibromo-9H-fluorene was reacted with 1-bromooctane under
basic conditions to form compound 1 in 77% yield. Compound
as T-C-TFT-C-T) contains a central dithienofluorene (TFT), two 1 was lithiated and then reacted with 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane to yield compound 2 in 66%
outer thiophenes and two embedded Cs. FTCT-FIC has strong
absorption in the visible and near-infrared regions but weak
crystallinity. The device with PBDB-T:FTCT-FIC (1:1.5 in wt%)
showed a decent PCE of 9.58% with a Voc of 0.86 V, a Jsc of 19.01
mA cm−2, and an FF of 58.61%. The low FF was attributed to the
improper morphology and lower electron mobility. It needs to
be pointed out that all the side chains in FTCT are situated at
the same side of the conjugated backbone, which might not be
ideal for molecular packing. Meanwhile, the geminal di(4-
hexylphenyl) groups on the sp3-carbon of the cyclopentadienyl
(CP) ring are also close to the FIC end-groups (ca. 10.52 Å),
which exerts certain steric hindrance to attenuate end-group
packing (Fig 1b).
yield. Suzuki coupling of compound
bromothieno[3,2-b]thiophene-3-carboxylate
compound in 76% yield. Reaction of
2
with ethyl 2-
afforded
3 with 4-
3
hexylphenylmagnesium bromide followed by acid-catalyzed
cyclization resulted in the formation of FCTT in an overall 45%
yield. The formylation of FCTT generated FCTT-CHO in 89% yield.
Condensation of FCTT-CHO with 1,1-dicyanamethylene-5,6-
difluoro-3-indanone (FIC) afforded the product of FCTT-FIC.
FCTT-FIC showed good solubility in common organic solvents
such as chloroform and chlorobenzene. The detailed synthetic
1
procedure, mass spectrometry, H NMR and 13C NMR of new
compounds are shown in supporting information. FCTT-FIC
exhibited a decomposition temperature (Td) at 339 °C in the
thermogravimetric analysis (TGA) measurement, showing good
thermal stability. In the differential scanning calorimetry (DSC)
measurement, FCTT-FIC did not show melting point and
In this research, we further synthesized another isomeric
TT-terminal fluorenedicyclopentathieno[3,2-b]thiophene FCTT
(also structurally symbolized as TT-C-F-C-TT) and its
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S
S
O
View Article Online
OEt
DOI: 10.1039/C9TA14040A
CO2Et
EtO2C
BpinOiPr
n BuLi
C8H17Br
NaOt-Bu
Br
S
S
Br
Br
Bpin
Bpin
Br
Br
THF
66
%
THF
77 %
Pd(PPh3)4 ,K2CO3
Toluene/H2O
76 %
S
S
C8H17 C8H17
1
C8H17
C8H17
C8H17
C8H17
2
3
C6H13
Ar =
Ar Ar
Ar Ar
Ar Ar
Ar Ar
S
S
S
S
OHC
CHO
BrMg Ar
THF
POCl3, DMF
AcOH, H2SO4
Octane
S
S
F
S
S
DCM
89 %
45 %
C8H17
C8H17
C8H17
C8H17
FCTT
FCTT-CHO
F
F
F
CN
NC
F
O
O
NC
CN
CN
FIC
Ar Ar
Ar Ar
S
S
F
CN
O
S
S
Pyridine
Chloroform
C8H17 C8H17
FCTT-FIC
82 %
Scheme 1. Synthesis of FCTT-FIC.
crystallization transition. The normalized absorption spectra of 31G(d,p) level. FTCT-FIC and FCTT-FIC with the simplified side-
PM6, PBDB-T, FTCT-FIC and FCTT-FIC in chlorobenzene solution chain (the octyl and 4-hexylphenyl groups are simplified to ethyl
and thin film are shown in Fig 2a and their intrinsic properties and 4-methylphenyl groups, respectively) are used for
are listed in Table 1. In solution state, FCTT-FIC displayed strong simulation. The optimal geometries of the simplified FTCT-FIC
absorption in 500-750 nm region with a high extinction and FCTT-FIC structures are displayed in Fig 3. Both FTCT-FIC
coefficient of 1.38 × 105 M−1 cm−1 at the λmax of 675 nm. The λmax and FCTT-FIC possessed highly planar conjugated backbone.
of FCTT-FIC was significantly red-shifted by 34 nm in thin film, Furthermore, the distance between the center of the
indicating the strong intermolecular interactions in the solid alkylphenyl side-chains and the edge of the FIC was calculated.
state. The optical bandgap of FCTT-FIC was estimated to be 1.53 FCTT-FIC showed a longer distance of 12.21 Å than FTCT-FIC
eV. The cyclic voltammogram (CV) and the energy levels of (10.52 Å), which can reduce the steric repulsion and allow more
FCTT-FIC are shown in Fig S2 and Fig 2b, respectively. The space for intermolecular antiparallel stacking of the terminal FIC
highest occupied molecular orbital (HOMO) and lowest moieties. The dipole moment of the half FTCT-FIC and FCTT-FIC
unoccupied molecular orbital (LUMO) of FCTT-FIC were molecules were also estimated to be 9.39 D and 9.66 D,
estimated to be −5.56/−4.03 eV. Compared to FTCT-FIC, FCTT- respectively. The larger dipole moments strengthen donor-
FIC showed hypsochromically shifted absorption and deeper- acceptor communication leading to the strong intramolecular
lying energy levels, indicating that rearrangement of the charge transfer and red-shifted absorption spectra.
conjugation leads to the alteration of electronic properties.
Fig 2. (a) UV-vis absorption spectra of PBDB-T, PM6, FTCT-FIC and FCTT-FIC in
chlorobenzene solution and thin film, (b) energy levels of PBDB-T, PM6, PC71BM, FTCT-
FIC and FCTT-FIC estimated by cyclic voltammetry.
To gain the understanding of the differences on the
backbone conformation and the dipole moment of the
Fig 3. Top/side view of the DFT optimized conformation and the dipole moment of half
molecule of (a) FTCT-FIC and (b) FCTT-FIC.
molecule, we applied the density functional theory (DFT)
calculations with the Gaussian09 suite15 at the B3LYP/6-
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Table1. Summary of the intrinsic properties of FTCT-FIC and FCTT-FIC.
View Article Online
DOI: 10.1039/C9TA14040A
opt
λmax (nm)
Extinction coefficient
λonset
Eg
HOMO
LUMO
Egele (eV)d
(eV)d
FREA
(×105 cm-1M-1)a
(nm)b (eV)c
(eV)d
CB
688
675
Film
766
709
FTCT-FIC
FCTT-FIC
acalculated at λ
1.80
1.38
849
799
1.46
1.55
-5.49
-5.56
-4.01
-4.03
1.48
1.53
.
bcalculated in the solid state, cEgopt = 1240/λonset, ddetermined by cyclic voltammetry.
max
Table 2. Characteristics of the devices with ITO/ZnO/active layer/MoO3/Ag.
Blend
ratio in
wt%
Voc
(V)
Jsc
FF
(%)
μhole
μele
Blend system
PCE (%)
9.58
(mA cm-2)
(cm2V-1s-1) (cm2V-1s-1)
PBDB-T:
FTCT-FIC
0.86
(0.86±0)
19.01
58.61
1:1.5
1:1
1.03×10-5
2.81×10-5
4.71×10-6
2.32×10-5
6.96×10-6
6.25×10-6
4.13×10-6
1.10×10-4
2.50×10-5
3.03×10-5
1.02×10-5
1.17×10-5
(19.19±0.29) (57.83±0.88) (9.54±0.02)
PBDB-T:
FCTT-FIC
0.76
(0.76±0)
19.63
69.14
10.32
(19.88±0.60) (66.43±1.81) (10.03±0.18)
PM6:FCTT-
FIC
0.90
(0.9±0.01)
19.49
69.73
12.23
1:1
(19.36±0.77) (69.24±1.75) (12.01±0.24)
0.92
(0.92±0)
19.86
73.20
13.37
1:1:0.5
1:1:1
1:1:1.5
(20.12±1.03) (70.97±1.85) (13.12±0.43)
PM6:FCTT-
FIC: PC71BM
0.92
(0.92±0.01)
18.48
72.71
12.36
(18.39±0.32) (71.01±0.99) (12.03±0.14)
0.92
(0.92±0)
17.83
71.48
11.72
(17.75±0.31) (70.62±1.78) (11.53±0.16)
The average values with standard deviation over 10 cells are shown in parenthesis.
To investigate the photovoltaic characteristics, the that of the PBDB-T:FTCT-FIC, indicating that the PBDB-T:FCTT-
inverted OSC devices with the configuration of ITO/ZnO/Active FIC system possessed better charge transport and negligible
layer/MoO3/Ag were fabricated. Characteristics of the devices, charge recombination. To deeply explore the charge transport,
the J-V curves and the external quantum efficiency (EQE) the hole-only devices (ITO/PEDOT:PSS/active layer/Au) and
spectra are shown in Table 2 and Fig 4. The medium-bandgap electron-only devices (Al/active layer/Al) were fabricated to
PBDBT having the suitable energy level alignment and evaluate the hole and electron mobility by space-charge limit
complementary absorption spectrum was used to blend with current (SCLC) model (Fig S2). The hole and electron mobilities
FTCT-FIC and FCTT-FIC. The optimized device using PBDB- were estimated to be 1.03×10-5/4.13×10-6 for PBDB-T:FTCT-FIC
T:FTCT-FIC (1:1.5 in wt%) exhibited a PCE of 9.58% with a Voc of blend and 2.81×10-5/1.1×10-4 cm2V-1s-1 for PBDB-T:FCTT-FIC
0.86 V, a Jsc of 19.01 mA cm−2, and an FF of 58.61%, while the blend. It is envisaged that FCTT-FIC with the side-chains at the
optimized device with PBDB-T:FCTT-FIC (1:1 in wt%) exhibited a more inner position is beneficial for the end-group stacking.
higher PCE of 10.32% with a Voc of 0.76 V, a Jsc of 19.63 mA cm−2, Therefore, PBDB-T:FCTT-FIC blend displayed higher hole and
and an FF of 69.14%. The lower Voc of the PBDB-T:FCTT-FIC electron mobilities leading to the improved Jsc and FF.
device is associated with the deeper-lying LUMO level of FCTT- It is known that precise energy level alignment between a p-
FIC. The discrepancy of Voc value can be ascribed to the type material and an n-type FREA can ensure the efficient
difference of blending ratios and morphology. It is notable that charge separation and minimize photon energy loss (Eloss) to
the device with PBDB-T:FCTT-FIC showed much higher FF than enhance the Voc. Eloss is correlated to the potential energy
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difference between the absorbed photon and the released
electron.78,79 PM6 with the similar absorption spectrum but
lower-lying HOMO/LUMO levels was used to blend with FCTT-
FIC as the p-type material. Compared to the device with PBDB-
T:FCTT-FIC, the PM6:FCTT-FIC (1:1 in wt%) device showed an
increased PCE of 12.23% and the Voc significantly enhanced
from 0.76 V to 0.90 V. Furthermore, we calculated the Eloss of
the devices based on the equation of Eloss = Egopt − eVoc where
View Article Online
DOI: 10.1039/C9TA14040A
opt
the Eg is the lowest optical bandgap among the donor and
acceptor components. The Eloss of the PBDB-T:FCTT-FIC and
PM6:FCTT-FIC devices are estimated to be 0.77 eV and 0.63 eV,
respectively. The lower Eloss of the PM6:FCTT-FIC device is
associated with the smaller HOMO level difference between the
PM6 and FCTT-FIC.
Ternary-blend system incorporating a fullerene derivative and
an FREA has been widely used to achieve high-performance
OSCs. We introduced the PC71BM as the second N-type material
to form a ternary-blend PM6:FCTT-FIC:PC71BM with different
blending ratios. The PM6:FCTT-FIC:PC71BM (1:1:0.5 in wt%)
device showed the best PCE of 13.37% with the highest Jsc of
19.86 mA cm−2 and FF of 73.2%. The increased Jsc was attributed
to the enhanced EQE from 400 nm to 500 nm. Furthermore, all
the ternary-blend devices showed the increased Voc of 0.92 V
due to the higher-lying LUMO level of PC71BM. When the
PC71BM ratio further increased to 1:1:1 and 1:1:1.5 in wt%, the
efficiency slightly decreased to 12.36% and 11.72%.
Fig 5. 2-Dimensional GIWAXS images of the films with (a) pure FTCT-FIC, (b) pure FCTT-
FIC, (c) PBDB-T:FTCT-FIC, (d) PBDB-T:FCTT-FIC, (e) PM6:FCTT-FIC and PM6:FCTT-
FIC:PC71BM with blending ratio (f) 1:1:0.5, (g) 1:1:1 and (h) 1:1:1.5 in wt% films.
in the Fig 5, the pure FTCT-FIC displayed a weak (010) π-π
stacking diffraction in the out-of-plane direction with a qz = 1.83
Å-1 corresponding to a π-π distance (dπ) of ca. 3.43 Å. On the
other hand, the pure FCTT-FIC showed a stronger π-π stacking
diffraction at qz = 1.82 Å-1 with a dπ of ca. 3.45 Å, indicating that
FCTT-FIC possesses stronger face-on stacking interactions.
In the blend films, PBDB-T:FTCT-FIC showed the face-on
orientation with the π-π diffraction at qz = 1.82 Å-1 and a dπ of
ca. 3.46 Å. In addition, PBDB-T:FCTT-FIC revealed two merged
π-π diffractions at qz = 1.74 Å-1 and qz = 1.84 Å-1 with the
corresponding dπ of ca. 3.61 Å and 3.42 Å which can be ascribed
to the crystalline PBDB-T and FCTT-FIC domains, respectively.
The shorter dπ of FCTT-FIC in the PBDB-T:FCTT-FIC blend
indicated that PBDB-T induced more compact packing of FCTT-
FIC which is beneficial to the electron transport. The face-on
orientations of the both PBDB-T and FCTT-FIC crystallines can
facilitate the hole and electron transport which is consistent
Fig 4. (a) J-V curves and (b) EQE spectra of the devices with PBDB-T:FTCT-FIC (1:1.5 in
wt%) and PBDB-T:FCTT-FIC (1:1 in wt%). (c) J-V curves and (d) EQE spectra of the devices
with PM6:FCTT-FIC:PC71BM in the ratio of 1:1:0, 1:1:0.5, 1:1:1 and 1:1:1.5 in wt%.
Based on the SCLC mobility results, all the ternary-blend devices
exhibited more balance charge transport with the hole/electron
mobility ratio of 0.77 (PM6:FCTT-FIC:PC71BM =1:1:0.5), 0.68
(PM6:FCTT-FIC:PC71BM =1:1:1) and 0.53 (PM6:FCTT-FIC:PC71BM
=1:1:1.5), respectively, than the binary device (PBDB-T:FCTT-FIC
=1:1) with the ratio of 0.19. The more balance charge transport
also reflected on the increased FF of the ternary-blend devices.
Two-dimensional grazing-incidence wide-angle X-ray diffraction
(GIWAXS) was used to study the molecular packing of the pure
FREAs and blend films. As shown
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with the mobility results. The PM6:FCTT-FIC blend presented materials. Development of new isomeric structures of the state-
View Article Online
DOI: 10.1039/C9TA14040A
similar diffraction patterns with PBDB-T:FCTT-FIC, showing face- of-the-art central ladder-type donors provides
a
on π-π stacking signals at qz = 1.76 Å-1 and qz = 1.84 Å-1 with a straightforward and promising way to effectively optimize the
dπ of ca. 3.57 Å and 3.42 Å from PM6 and FCTT-FIC, respectively. solar cell characteristics. ladder-type fluorine-based
A
When adding 0.5 wt% of PC71BM into the PM6:FCTT-FIC blend, thienothiophene-terminal FCTT (TT-C-F-C-TT), which is an
the 2D pattern showed two more obvious separated π-π isomeric structure of thiophene-terminal FTCT (T-C-TFT-C-T),
diffractions at qz = 1.73 Å-1 and qz = 1.87 Å-1 with a dπ of ca. 3.64 was designed and synthesized. The two donors were end-
Å and 3.35 Å, suggesting that PC71BM slightly enlarged the dπ of capped with FIC acceptors to form FCTT-FIC and FTCT-FIC FREAs.
PM6 and tightened the dπ of FCTT-FIC molecules.
Considering the electronic effect, FCTT-FIC exhibited a larger
The fine-tuning of the molecular packing by PC71BM effectively optical bandgap and deeper-lying energy levels than FTCT-FIC.
adjusted the hole and electron mobilities to yield more The steric effect plays a more important role in determining
balanced charge transport, thus improving Jsc and FF in the molecular interactions and packing. The distance between
PM6:FCTT-FIC:PC71BM (1:1:0.5 in wt%) device. However, when geminal di(4-hexylphenyl) side-chains and the FIC acceptor
the amount of PC71BM increased to 1 and 1.5 weight ratio to edge is estimated to be 12.21 Å for FCTT-FIC using the TT-
PM6, the 2D patterns showed the decreased diffraction terminal donor and 10.52 Å for FTCT-FIC with the T-terminal
intensity of the π-π stacking, leading to the lower charge donor. The longer distance provides more space for the
mobility, Jsc and FF. The GIWAXS results are fully consistent with antiparallel packing among the end-groups. As a result, FCTT-
the device performance. Furthermore, the 1-dimensional in- FIC exhibited stronger face-on π-π stacking in the both neat film
plane and out-of-plane GIWAXS patterns of the films are shown and blend films. The PBDB-T:FCTT-FIC device displayed a higher
in Fig S4.
PCE of 10.32%, an enhanced Jsc of 19.63 mA cm−2 and an FF of
To investigate the surface morphology of the blend films, 69.14%, compared to the PBDB-T:FTCT-FIC device.
atomic force microscopy (AFM) was used to measure the Furthermore, the PM6:FCTT-FIC device can further improve the
topography images of the blend system. As shown in the Fig 6, efficiency to 12.23%. By introducing PC71BM as the third
the surface roughness of PBDB-T:FTCT-FIC film is 5.91 nm, while component, the ternary-blend PM6:FCTT-FIC:PC71BM device
the surface roughness of PBDB-T:FCTT-FIC film greatly (1:1:0.5 in wt%) yielded the highest PCE of 13.37% with a Voc of
decreased to 0.97 nm. The much smooth surface of PBDB- 0.92 V, a higher Jsc of 19.86 mA cm−2, and an FF of 73.2%. This
T:FCTT-FIC indicates the improved miscibility between the research demonstrated that TT-terminal ladder-type structure
donor and acceptor, thus enhancing the interfacial contact and is a superior donor design for high-performance A-D-A FREAs.
facilitating the carrier transport in the device. For the PM6-
based blend systems, the surface roughness of PM6:FCTT-FIC
film is 3.39 nm, and the ternary-blend PM6:FCTT-FIC:PC71BM
Conflicts of interest
(1:1:0.5 in wt%) revealed a much smoother surface with
roughness of 1.79 nm, indicating that the introduction of
PC71BM formed the better morphology and surface contact.
There are no conflicts to declare.
Author information
Author Contributions
Y-C Xue and F-Y Cao are equally contributed to this work.
Acknowledgements
This work is supported by Ministry of Science and Technology,
Taiwan (grant No. MOST107-3017-F009-003) and Ministry of
Education, Taiwan (SPROUT Project-Center for Emergent
Functional Matter Science of National Chiao Tung University).
We thank the National Synchrotron Radiation Research Center
(NSRRC), and Dr. U-Ser Jeng and Dr. Chun-Jen Su at BL23A1
station.
Fig 6. Atomic force microscopy images and the roughness of the PBDB-T:FCTT-
FIC:PC71BM blend films with the ratio of (a) 1:1:0, (b) 1:1:0.5, (c) 1:1:1 and (d) 1:1:1.5 in
wt%.
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Table of contents
TT-terminal ladder-type donor is generally a better molecular design than the
corresponding T-terminal ladder-type isomer for development of new A-D-A FREAs.