Highly crystalline new benzodithiophene-benzothiadiazole copolymer for efficient ternary polymer solar cells with an energy conversion efficiency of over 10%

Article abstract of DOI:10.1039/c8tc00817e

A new alternating polymer P(BDTSi-DFBT), poly(4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene-alt-5,6-difluoro-4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole), was prepared via Stille polymerization. The determined absorption maximum and optical band-gap (E_g) of P(BDTSi-DFBT) were 590 nm and 1.74 eV, respectively. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTSi-DFBT) were -5.45 eV and -3.71 eV, respectively. The X-ray diffraction (XRD) analysis confirmed that P(BDTSi-DFBT) is a crystalline polymer. The binary-polymer solar cells, ITO/PEDOT:PSS/P(BDTSi-DFBT):PC₇₀BM (1:1.5 wt%) + 3 vol% DIO/Al, made from P(BDTSi-DFBT) gave a power conversion efficiency (PCE) of 5.02% with an open-circuit voltage (V_oc) of 0.81 V, a short-circuit current (J_sc) of 11.68 mA cm^-2, and a fill factor (FF) of 53%. Conversely, the ternary-polymer solar cells, ITO/PEDOT:PSS/PTB7-Th:P(BDTSi-DFBT):PC₇₀BM (0.8:0.2:1.5 wt%) + 3 vol% DIO/Al, made with a synthesized medium band-gap P(BDTSi-DFBT) and low band-gap PTB7-Th, offered a maximum PCE of 10.05%, with a V_oc of 0.79 V, a J_sc of 17.92 mA cm^-2, and a FF of 71%.

Full text of DOI:10.1039/c8tc00817e

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC00817E
Journal Name
ARTICLE
Highly crystalline new benzodithiophene–benzothiadiazole
copolymer for efficient ternary polymer solar cells with energy
conversion efficiency over 10%
Received 00th January 20xx,
Accepted 00th January 20xx
a,†
a,†
a
b
c
DOI: 10.1039/x0xx00000x
Vellaiappillai Tamilavan, Yanliang Liu, Jihoon Lee, Yun Kyung Jung, Semo Son , Junghyun
a
a
Jeong, and Sung Heum Park*
www.rsc.org/
A new alternating polymer P(BDTSi-DFBT), poly(4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene-alt-5,6-
difluoro-4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole), was prepared via Stille polymerization. The determined
absorption maximum and optical band-gap (E ) of P(BDTSi-DFBT) were 590 nm and 1.74 eV, respectively. The calculated
g
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P(BDTSi-
DFBT) were ‒5.45 eV and ‒3.71 eV, respectively. The X-ray diffraction (XRD) analysis confirmed that P(BDTSi-DFBT) is a
crystalline polymer. The binary-polymer solar cells, ITO/PEDOT:PSS/P(BDTSi-DFBT):PC70BM (1:1.5 wt%)+3 vol% DIO/Al,
made from P(BDTSi-DFBT) gave a power conversion efficiency (PCE) of 5.02% with a open-circuit voltage (Voc) of 0.81 V, a
2
short-circuit current (Jsc) of 11.68 mA/cm , and a fill factor (FF) of 53%. Conversely, the ternary-polymer solar cells,
ITO/PEDOT:PSS/PTB7-Th:P(BDTSi-DFBT):PC70BM (0.8:0.2:1.5 wt%)+3 vol% DIO/Al, made with synthesized medium band-
gap P(BDTSi-DFBT) and low band-gap PTB7-Th, offered a maximum PCE of 10.05%, with a Voc of 0.79 V, a Jsc of 17.92
2
mA/cm , and a FF of 71%.
electron-donating polymers and electron-accepting organic
small molecules, is required to further enhance the
performance of PSCs and OSCs devices. We have been
interested in preparing a new polymeric donor material,
because an efficient polymeric material is required for all three
types of solar cells mentioned above.
1
. Introduction
Organic solar cells (OSCs), fabricated via solution-processed
printing techniques using a blend of electron-donating
polymers and electron-accepting fullerene derivatives or π-
conjugated organic small molecules, have attracted great
interest as promising low-cost, lightweight, and large-area
devices on flexible substrates for solar-to-electrical energy
The literature suggests that the electron-deficient
benzo[c][1,2,5]thiadiazole (BT)2
4,25
derivatives are promising
building blocks to prepare efficient polymeric donor materials
for solar cell applications. The broad absorption in the entire
visible part of the solar spectrum and the relatively high PCE
achieved for BT-based polymers have pushed researchers to
tune the photo-physical properties of BT-based polymers via
side or main chain engineering. Consequently, various electron
donor and acceptor units were inserted into the BT
1-23
conversion.
PSCs) made from polymer and [6,6]-phenyl C71 butyric acid
methyl ester (PC70BM) offered a maximum power conversion
The conventional binary-polymer solar cells
(
1
-9
efficiency (PCE) of ~11%. On the other hand, ternary-PSCs,
made from wide- or medium band-gap polymer , a low band-
gap polymer , and PC70BM, gave an improved PCE of ~12%.
Recently, OSCs made by using a blend of polymer and π-
conjugated organic small molecules have also displayed an
1
1
0-
2
18
2
6-41
th
backbone.
Interestingly, the substituents on the 5 and 6
th
positions of BT and the 4 position of the thiophene attached
with BT greatly alter the properties and photovoltaic
performance of resulting BT-based polymers.
1
9-23
impressive PCE over ~13%.
properties of the π-conjugated material used on the
photoactive layer of the solar cell devices (PSCs and OSCs) are
crucial determining factors of the device performance.
It is well documented that the
2
4-41
Among them,
the fluoro substituent is found to be the best choice for the 5
1
-23
th
and 6 positions of BT and alkyl groups are more suitable for
Therefore, a new efficient π-conjugated material, such as
th
the 4 position of the thiophene attached with BT to obtain
42-51
high energy converting BT-based polymers.
Consequently,
we chose 5,6-difluoro-4,7-bis(4-octylthiophen-2-
a.Department of Physics, Pukyong National University, Busan 608-737, Republic of
Korea. Fax: 82-51-629-5549; Tel: 82-51-629-5774; *E-mail: spark@pknu.ac.kr
School of Biomedical Engineering, Inje University, Gimhae 50834, Republic of
yl)benzo[c][1,2,5]thiadiazole (DFBT) as an electron accepting
unit to prepare new donor-acceptor (D–A) polymers for PSCs.
Several studies have shown that DFBT-based polymers offer a
b.
Korea.
c.
Department of Graphic Arts Information Engineering, Pukyong National
42-51
University, Busan 608-737, South Korea
These two authors contributed equally to this work.
maximum PCE over 10% in binary-PSCs.
Fig. 1a shows the
†
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different positions of the BT derivatives for an easy and clear properties of the new polymer, P(BDTSi-DFBT), containing
View Article Online
DOI: 10.1039/C8TC00817E
understanding.
BDTSi and DFBT.
On the other hand, the rigid and planar structured
benzo[1,2-b:4,5-b′]dithiophene (BDT) is an efficient electron
rich unit used to prepare high energy converting D–A
2
. Experimental section
5
2-54
polymers.
units have been reported for PSCs.
studies suggest that the substituents on the 4 -and 8
position of the BDT unit significantly affect the properties of Aldrich. A Varian Mercury Plus spectrometer (300 MHz) was
used for nuclear magnetic resonance (NMR) analysis. Gel
Many D–A polymers incorporating BDT and DFBT
2
.1 Materials and measurements.
16,17,44-46
These careful
th
th We obtained the required chemicals and reagents from Sigma-
24,25,52-54
the resulting polymers.
The 4- and 8-positions of the
BDT unit are also shown in Fig. 1a. For example, the 4,8- permeation chromatography (GPC) was performed on the
alkoxy–substituted BDT unit offered a relatively low band-gap Agilent 1200 Infinity Series separation module. The JASCO V-
and highly crystalline polymer when it was copolymerized with 570 spectrophotometer and a CH Instruments Electrochemical
the DFBT derivative compared to that of the 4,8-thienyl– Analyzer were used to obtain the absorption spectrum and
substituted BDT unit. As a result, a large difference was cyclic voltammogram (CV) of the polymer. The X-ray diffraction
51
(XRD) and small-angle X-ray scattering (SAXS) analysis were
observed in their photovoltaic performances. The chemical
structure of the respective DFBT-based polymers P(BDT-DFBT) performed on the XPert-MPD (Philips, Netherland) with sealed
and P(BDTT-DFBT) are shown in Fig. 1b.
proportional counter and X'celerator PSD for high speed data
collection detector. The XRD and SAXS instruments were
calibrated using standard silicon crystal. The incident photon
to current efficiency (IPCE) was measured as a function of the
wavelength in the wavelength range 300 to 1000 nm using a
xenon lamp as the light source, and the calibration was
performed by using a silicon reference cell. Atomic force
microscopy (AFM) and transmission electron microscopy (TEM)
were performed using Seiko instruments SPI 3800N-SPA 400
and Hitachi H-7500, respectively.
2.2. PSC device fabrication and characterization.
The PSCs were fabricated with a device structure of ITO-coated
glass substrate/PEDOT:PSS/P(BDTSi-DFBT):PC70BM/Al. The pre-
cleaned ITO-coated glass substrate was dried overnight in an
oven. A 40-nm thick layer of PEDOT:PSS (Baytron PH) was spin-
casted from an aqueous solution on to an ITO-coated glass
substrate. The substrate was dried for 10 min at 140°C in air
and then transferred to a glove box to spin-cast the
photoactive layer. A solution containing a mixture of P(BDTSi-
DFBT):PC70BM (1:1.5 wt%) in chlorobenzene (CB):1,8-
diiodooctane (DIO) at a volumetric ratio of 97:3 (CB :DIO) was
then spin-casted on top of the PEDOT/PSS layer. The film was
then dried for 30 min at RT in a glove box. Subsequently, an
aluminum (Al, 100 nm) electrode was deposited by thermal
Fig. 1. (a) The different positions of BT and BDT units for substitution. (b) The chemical
structure of the DFBT-based polymers P(BDT-DFBT) and P(BDTT-DFBT).
‒6
It is widely known that the bulky triisopropylsilylethynyl evaporation in a vacuum of approximately 3 x 10 Torr. The
(
TIPS) on acene derivatives improves the solubility, oxidative current density-voltage (J‒V) characteristics of the PSC devices
stability, and crystallinity of π–conjugated materials, as well as were measured using a Keithley 2400 Source Measure Unit.
the π-orbital overlap between π-conjugated molecules.55,56 The solar cell performance was determined using an Air Mass
Consequently, the 4,8-triisopropylsilylethynyl–substituted BDT 1.5 Global (AM 1.5 G) using a Peccell (PEC-L01) solar simulator
−
2
(
BDTSi) derivative was copolymerized with a variety of with an irradiation intensity of 1000 Wm . The spectral
electron-deficient units, such as benzo[c][1,2,5]oxadiazole mismatch factor was calculated by comparing the solar
5
7
58
59
(
BO), diketopyrrolo[3,4-c]pyrrole (DPP), quinoxaline (QX),
simulator spectrum with the AM 1.5 spectrum at RT.
6
0
61
benzo[d][1,2,3]triazole (BTz), thieno[3,4-b]thiophene (TT),
2
.3. Synthesis of P(BDTSi-DFBT) [polymer poly(4,8-
62
and thiazolo[5,4-d]thiazole (TTz) derivatives and explored in
PSCs. However, the combination of BDTSi and DFBT has not
yet been studied, though DFBT is a more promising acceptor
bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene-alt-
,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)]
5
unit than the above-mentioned acceptor units. Here, we A solution of BDTSi 59 (0.26 g, 0.30 mmol) and DFBT63 (0.22 g,
report the synthesis, optical, electrochemical, and photovoltaic 0.30 mmol) in chlorobenzene (15 mL) was purged well with
2 3 3
argon for 20 min. Pd (dba) (7 mg) and P(o-tol) (15 mg) were
2
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then added in one portion. The entire mixture was stirred and Fig. 2a presents the absorption spectra of PV(BiewDTArStiic-leDOFnBliTne)
refluxed for 18 h under argon atmosphere. Subsequently, the solution in CB together with its thin filmD. OAIs: 1s0h.1o0w39n/Ci8nTCFi0g0.8217aE,
reaction mixture was cooled to RT and then added dropwise the polymer P(BDTSi-DFBT) in solution and film state exhibited
into vigorously stirred methanol (200 mL). The precipitates a broad absorption band between 300–700 nm, with
were recovered by filtration, and the crude polymer was maximum absorption at 586 nm and 595 nm, respectively,. We
extracted with methanol and acetone for 24 h in a Soxhlet could not see any big difference in the absorption spectra of
apparatus. The pure polymer P(BDTSi-DFBT) was obtained as a P(BDTSi-DFBT) except the shoulder peak at 650 nm. The
deep blue solid. Yield (0.30 g, 91%). Anal. Calc. for relatively increased shoulder peak in film suggests that the
C
62
H
82
F
2
N
2
S
5
Si
2
: C, 67.10; H, 7.45; N, 2.52; S, 14.45. Found: C, inter-chain interactions between the P(BDTSi-DFBT) chains are
comparatively better in film than solution. The calculated
optical band gap (E ) is 1.74 eV. It is worth noting that P(BDTSi-
6
6.21; H, 6.87; N, 2.12; S, 13.93.
g
DFBT) exhibited a slightly higher band-gap and blue-shifted
absorption maximum compared to that of the structurally
3
. Results and discussions
similar DFBT-based polymers, P(BDT-DFBT) and (P(BDTT-DFBT),
which contain 4,8-alkoxy or thienyl–substituted BDT units.51
3
.1 Synthesis and characterization
The monomers,
2,6-bis(trimethyltin)-4,8- Cyclic voltammetry analysis was performed with P(BDTSi-
bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene
DFBT) and the corresponding cyclic voltammogram (CV) is
59
(BDTSi)
and
4,7-bis(5-bromo-4-octylthiophen-2-yl)-5,6- shown in Fig. 2b. The estimated onset oxidation potential (Eox,
63
difluorobenzo[c][1,2,5]thiadiazole (DFBT), were prepared as onset) values for P(BDTSi-DFBT) and standard ferrocene were
shown in Scheme 1. The Stille copolymerization between 1.17 V and 0.52 V, respectively. The highest occupied and
BDTSi and DFBT using the procedure described in the lowest unoccupied molecular orbital (HOMO and LUMO)
experimental section resulted in the new polymer P(BDTSi- energy levels of P(BDTSi-DFBT) were calculated using the
DFBT). The synthetic route for the synthesis of P(BDTSi-DFBT) following standard equations: EHOMO = [‒(Eox,onset vs. Ag/AgCl
‒
is outlined in Scheme 1. Polymer P(BDTSi-DFBT) exhibited Eferrocene vs. Ag/AgCl) ‒ 4.8] eV and ELUMO = EHOMO + E . The
g
excellent solubility in most common chlorinated organic calculated HOMO and LUMO levels of P(BDTSi-DFBT) were
solvents like chloroform (CF), chlorobenzene (CB), and ‒5.45 eV and ‒3.71 eV, respectively. Interestingly, the HOMO
dichlorobenzene (DCB). The determined weight average level of P(BDTSi-DFBT) was found to be deeper (0.12~0.14 eV)
molecular weight (M
w
), number average molecular weight than that of P(BDT-DFBT) and P(BDTT-DFBT) (HOMO
 ‒5.31
4
51
(
M
n
), and polydispersity for P(BDTSi-DFBT) were 4.30 x 10
eV and ‒5.33 eV). A deeper HOMO level is favorable to get
higher Voc when the respective material is used as an electron
material in PSCs.
4
g/mol, 2.22 x 10 g/mol and 1.93, respectively.
3
.2 Opto-electrical properties
Scheme 1. Synthetic route for P(BDTSi-DFBT).
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DOI: 10.1039/C8TC00817E
XRD spectrum for P(BDTSi-DFBT) exhibited several peaks at
 14.18°, 18.02°, 24.17° and 39.14°, as well as in the SAXS
2
spectrum at 2  3.75° and 5.45°. The presence of these peaks
confirm that P(BDTSi-DFBT) is a crystalline polymer. It is worth
noting that the DFBT-based polymers, P(BDT-DFBT) and
P(BDTT-DFBT), incorporating alkoxy- or thienyl–substituted
BDT units, showed only two peaks at 2  4° and 24° in their
XRD spectrum; on the other hand, P(BDTSi-DFBT) displayed
several
peaks.
These
results
proved
that
triisopropylsilylethynyl substituents significantly alter the
crystallinity of the resulting polymer compared to that of the
frequently used alkoxy and thienyl substituents.51 The
calculated in-plane spacing between polymer backbones (d
and d -spacings) of P(BDTSi-DFBT) are 23.54 Å and 16.20 Å. In
contrast, the estimated p-stacking distance, d –d -spacings, for
P(BDTSi-DFBT) are 6.24 Å, 4.92 Å, 3.68 Å and 2.30 Å.
1
2
3
6
3
.4 Binary-polymer solar cell properties
Fig. 4. shows the PSC device structure (a), the energy level
diagram (b), the current-voltage (J–V) curves (c) and the
incident photon to the current efficiency (IPCE) spectra (d) of
the PSCs prepared by using a P(BDTSi-DFBT):PC70BM (1:1.5
wt%) blend. The device made with 3 vol% DIO displayed an
improved power conversion efficiency (PCE) of 5.02% with an
open-circuit voltage (Voc) of 0.81 V, a short-circuit current (Jsc
of 11.68 mA/cm , and a fill factor (FF) of 53% compared to that
)
2
of the device made without additives (PCE
 1.01%, Voc  0.68
2
V, Jsc

4.58 mA/cm , and FF

32%). As seen in Fig. 4d, both
devices made with and without 3 vol% DIO showed quite
broad IPCE response from 300 nm to 720 nm. However, the
maximum IPCE values significantly increased when the device
was processed with DIO. The IPCE responses for the device
made without any additives were over 20% in the range of
Fig. 2. (a) Absorption spectra of P(BDTSi-DFBT) in CB and as thin film on glass. (b) The
cyclic voltammogram of P(BDTSi-DFBT).
3
50–620 nm, with a maximum IPCE of 27% at 410 nm. While
the device made with 3 vol% DIO showed an increased IPCE
maximum of 67% at 410 nm, with responses found to be over
5
0% in the range of 360–650 nm.
The dissimilar performances of the PSC devices are
attributed to the difference in the surface morphology of their
photoactive layers. The atomic force microscopy (AFM) and
transmission electron microscopy (TEM) images of the films
processed using the P(BDTSi-DFBT):PC70BM (1:1.5 wt%) blend
with and without 3 vol% DIO are shown in Fig. 5. The films
processed without additives showed a uniform size of cluster
formation between polymer and PC70BM. However, the cluster
formation was significantly decreased with the use of 3 vol%
DIO. The root mean square (RMS) values of the films made
with and without 3 vol% DIO were found to be quite similar.
The 3D topology images reveal that the grain size notably
altered when the film was processed under different
conditions, such as with and without 3 vol% DIO. Overall, the
film processed with DIO shows better morphology for efficient
carrier transport than the film processed without DIO. We
think that the high crystalline nature of P(BDTSi-DFBT) could
be a possible reason for the high roughness obtained. The TEM
Fig. 3. Powder XRD and SAXS spectra of P(BDTSi-DFBT).
3
.3 X-ray diffraction analysis
The powder X-ray diffraction (XRD) and small-angle X-ray
scattering (SAXS) analyses were performed with P(BDTSi-DFBT)
to study the nature of P(BDTSi-DFBT). The XRD and SAXS
spectra of P(BDTSi-DFBT) are shown in Fig. 3. Interestingly, the
4
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images presented in Fig. 5 firmly support our argument. The reasonable Voc values compared to P(BDT-DFBT)-VaienwdArtPic(leBODnTliTne-
dark color observed on the TEM image arises from PC70BM and DFBT)-based devices. However, the lowDerOIJ: s1c 0.a1n03d9/FCF8TlCim00it81o7nE
white color is related to P(BDTSi-DFBT). In this instance, the the P(BDTSi-DFBT)-based device resulted in a performance
PC70BM aggregation was clearly observed for the film made within 5.1%. The device made with a lower concentration of
without DIO and a much-improved nanoscale network P(BDTSi-DFBT) gave relatively higher FF (
between P(BDTSi-DFBT) and PC70BM was observed for the film value was significantly lower (~ 8.50 mA/cm ) and

58%) but the Jsc
2
made with 3 vol% DIO.
consequently, there was no improvement on the PCE. We
The overall PCE for P(BDTSi-DFBT) is comparable with that think that the blue-shifted absorption and the relatively high
51
of P(BDTT-DFBT) but lower than that of P(BDT-DFBT). The RMS value might be the reasons for the moderate
maximum PCE reported for P(BDTT-DFBT) and P(BDT-DFBT) performance of P(BDTSi-DFBT). Nevertheless, the photovoltaic
2
was 4.46% (Voc
.30% (Voc
respectively.

0.68 V, Jsc

11.87 mA/cm , and FF

55%) and parameters obtained for P(BDTSi-DFBT) are quite similar to
2
8

0.78 V, Jsc

15.38 mA/cm , and FF

69%), those of the DFBT-based polymers incorporating 2D π–
5
1
24,25
The P(BDTSi-DFBT)-based device showed conjugated BDT units.
Fig. 4. The PSC device structure used in this study (a), the energy level diagram for the materials used for PSC fabrication (b), the J-V curves (c) and IPCE spectra (d) of the binary-
PSCs prepared using the P(BDTSi-DFBT):PC70BM (1:1.5 wt%) blend without and with 3 vol% DIO.
Fig. 5. Topology AFM and TEM images of the films made from P(BDTSi-DFBT):PC70BM (1:1.5 wt%) blend without and with 3 vol% DIO.
6
4
3
.5 Ternary-polymer solar cell properties
2 2
from p-DTS(FBTTH ) :PC70BM and PTB7-Th:PC70BM blends. In
this instance, the use of crystalline P(BDTSi-DFBT) along with
the PTB7-Th:PC70BM blends is also expected to improve the
PCE of binary-PSCs prepared from the PTB7-Th:PC70BM blend.
The chemical structure (a), the energy level diagram for the
materials used for ternary PSC fabrication (b), and normalized
absorption spectra of P(BDTSi-DFBT) and PTB7-Th (c), the J-V
curves (d) and IPCE spectra (e) of the ternary PSCs prepared
using PTB7-Th:P(BDTSi-DFBT):PC70BM (at various ratio) + 3
vol% DIO blends are shown in Fig. 6.
The high crystallinity of P(BDTSi-DFBT) allows its application as
co-absorber along with low band-gap PTB7-Th in ternary-PSCs.
A recent study suggests that the addition of the highly
crystalline π–conjugated small molecule p-DTS(FBTTH
2 2
) on the
PTB7-Th:PC70BM blend resulted in improvement of
a
absorption, crystallinity and the face-on orientation of the
ternary blend. Consequently, the Jsc and FF of ternary-PSCs are
considerably enhanced, and the overall performance of
ternary-PSCs greatly increased compared to binary-PSCs made
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Journal Name
View Article Online
DOI: 10.1039/C8TC00817E
Fig. 6. The chemical structure of polymers used for ternary PSCs (a), the energy level diagram for the materials used for ternary-PSC fabrication (b), and the normalized absorption
spectra of the polymers P(BDTSi-DFBT) and PTB7-Th (c), the J-V curves (d) and IPCE spectra (e) of the ternary-PSCs prepared using PTB7-Th:P(BDTSi-DFBT):PC70BM (at various ratio)
+
3 vol% DIO blends.
Table 1. Photovoltaic properties of PSCs made with the device structure of ITO/PEDOT:PSS/PTB7-Th:P(BDTSi–DFBT):PC70BM (at different ratio) + 3 vol% DIO/Al.
2
a
J
sc (mA/cm ) / (Ave.
Ternary blend ratio
.0 : 0.0 : 1.5
[cal] Jsc (mA/cm2)b
V
oc (V) / (Ave. Value)
c
f
FF (%) / (Ave. Value)
d
f
PCE (%) / (Ave. Value)
e
f
f
Value)
1
16.43 (16.30±0.21)
17.16 (16.95±0.25)
17.92 (17.71±0.32)
16.54 (16.10±0.46)
15.69 (15.20±0.53)
16.09
16.89
17.40
15.96
15.10
11.40
0.76 (0.75±0.01)
0.77 (0.76±0.01)
0.79 (0.78±0.01)
0.79 (0.78±0.01)
0.80 (0.79±0.01)
68 (67±1)
69 (68±1)
71 (70±2)
67 (65±2)
63 (60±3)
53 (50±3)
8.49 (8.22±0.27)
9.12 (8.83±0.29)
10.05 (9.70±0.35)
8.75 (8.30±0.45)
7.91 (7.32±0.59)
0
.9 : 0.1 : 1.5
0
.8 : 0.2 : 1.5
0
0
.7 : 0.3 : 1.5
.6 : 0.4 : 1.5
0
.0 : 1.0 : 1.5
11.68 (10.85±0.91)
b
0.81 (0.80±0.01)
5.02 (4.60±0.42)
a
Short-circuit current density. Calculated Short-circuit current density from IPCE. c Open-circuit voltage. Fill factor. e Power conversion efficiency. Average values
d
f
obtained from 15 devices.
As indicated in Fig. 6c, the polymers P(BDTSi-DFBT) and XRD and SAXS analysis were performed for the polymer films
PTB7-Th showed better complementary absorption between prepared using a chloroform (5mg/0.5 mL) solution containing
3
00 and 800 nm. In addition, the HOMO levels of P(BDTSi- PTB7-Th, PTB7-Th:P(BDTSi-DFBT) (0.8:0.2 wt%) and P(BDTSi-
DFBT) and PTB7-Th (see Fig. 6b) are found to be similar and DFBT) with the aim of studying the variation on the crystallinity
therefore expected to have better charge transport on the of ternary blend film compared to their corresponding binary
ternary systems. A recent study indicated that the matched blend films. The corresponding XRD and SAXS (insert) spectra
HOMO levels of the two donor polymers are favorable to get are shown in Fig. 7. The comparison suggests that P(BDTSi-
high FF, via the improved carrier mobility, and PCE for ternary- DFBT) is highly crystalline compared to PTB7-Th. It is very clear
PSCs.6 Consequently, combined effects such as improved that the intensities of the peaks (at 2  24°, 18° and 14°) are
absorption via complementary absorption, and high carrier considerably increased for the film made using the PTB7-
mobility via the improved crystallinity, are expected for the Th:P(BDTSi-DFBT) blend compared to the film made only from
ternary-PSCs made from the PTB7-Th:P(BDTSi-DFBT):PC70BM PTB7-Th. Also, a sharp characteristic peak at 2  18°, which
blend. The photovoltaic parameters obtained for the ternary- comes from P(BDTSi-DFBT), is observed for the film made from
PSCs are summarized in Table 1. The ternary-PSC made with the PTB7-Th:P(BDTSi-DFBT) blend. The SAXS spectra for the
5
0
.8:0.2:1.5 wt% blend offered a maximum PCE of 10.05% with polymer films also exhibit a similar trend. High-intensity peaks
2
a Voc of 0.79 V, a Jsc of 17.92 mA/cm , and a FF of 71%. The for the film made from the PTB7-Th:P(BDTSi-DFBT) blend
overall PCE of ternary-PSCs is 20% higher than that of the compared to the film made using only PTB7-Th were observed
2
binary-PSCs (PCE
FF
DIO blend. Interestingly, all three parameters such as Voc, Jsc

8.49%, Voc

0.76 V, Jsc

16.43 mA/cm , and with XRD. Overall, the crystallinity of PTB7-Th is found to be

68%) made from PTB7-Th:PC70BM (1:1.5 wt%) + 3 vol% notably improved with the addition of P(BDTSi-DFBT), which is
expected to be favorable to acquire better photovoltaic
,
and FF notably improved when P(BDTSi-DFBT) is added to the performances.
PTB7-Th:PC70BM blend.
6
| J. Name., 2012, 00, 1-3
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(
08:0.2:1.5 wt%) + 3 vol% DIO blends using AFM and TEM. The
View Article Online
DOI: 10.1039/C8TC00817E
corresponding AFM and TEM images of these films are shown
in Fig. 8. The AFM images suggest that the morphology of the
films processed with binary and ternary blends are very
similar. No notable changes are observed on the AFM images.
However, the surface roughness of the film processed with the
ternary blend is slightly higher than that of the film prepared
using a binary blend. Sometimes, higher roughness is favorable
to get a better charge collection via an improved contact
66,67
between the active layer and electrode.
According to the
TEM images shown in Fig. 8, the nanoscale network between
the ternary blend (PTB7-Th:P(BDTSi-DFBT):PC70BM) is found to
be better than the network between their corresponding
binary blend (PTB7-Th:PC70BM). As seen in the figure, the dark
areas, which refer to PC70BM are somewhat visible for the film
made using the PTB7-Th:PC70BM (1:1.5 wt%) + 3 vol% DIO
blend, but the visibility of the dark areas are much lower for
the film made from PTB7-Th:P(BDTSi-DFBT):PC70BM (08:0.2:1.5
wt%) + 3 vol% DIO blend. Overall, we think that the inclusion
of crystalline P(BDTSi-DFBT) on PTB7-Th:PC70BM blend
improves the morphology which is favorable for better charge
transport. Consequently, the improved crystallinity and
morphology of ternary blends offer enhanced photovoltaic
performance compared to binary blends.
Fig. 7. Thin film XRD and SAXS spectra of PTB7-Th, PTB7-Th:P(BDTSi-DFBT) (0.8:0.2 wt%)
and P(BDTSi-DFBT).
To get insight into the morphological modifications for
films processed from ternary blends compared to that of the
films made form binary blends, we examined the surface
morphology of the films made from PTB7-Th:PC70BM (1:1.5
wt%) + 3 vol% DIO and PTB7-Th:P(BDTSi-DFBT):PC70BM
Fig. 8. Topology AFM and TEM images of the films made from PTB7-Th:PC70BM (1:1.5 wt%) + 3 vol% DIO and PTB7-Th:P(BDTSi-DFBT):PC70BM (0.8:0.2:1.5 wt%) + 3 vol% DIO blends.
wt%)+3 vol% blend offered a PCE of 5.02% with a Voc of 0.81 V,
2
a Jsc of 11.68 mA/cm and a FF of 53%. While, the ternary-PSCs
4
. Conclusions
made with
a
blend of PTB7-Th:P(BDTSi-DFBT):PC70BM
0.8:0.2:1.5 wt%) gave a maximum PCE of 10.05% with a Voc of
.79 V, a Jsc of 17.92 mA/cm , and a FF of 71%. This study
confirm that the crystallinity of benzodithiophene–
benzothiadiazole copolymer is greatly improved by inserting
the triisopropylsilylethynyl group on the 4 and 8-position of
benzo[1,2-b:4,5-b′]dithiophene unit. The high crystallinity of
P(BDTSi-DFBT) was found to increase the surface roughness of
the film made with P(BDTSi-DFBT):PC70BM blend via solution
processing. Consequently, P(BDTSi-DFBT) exhibited moderate
The Stille polymerization of the di-stannyl derivative of 4,8-
bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b′]dithiophene
(
0
2
and
octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole resulted in a new
polymer P(BDTSi-DFBT). The estimated E , HOMO and LUMO
levels of P(BDTSi-DFBT) are 1.74 eV, ‒5.45 eV and ‒3.71 eV,
respectively. Several peaks appear in the XRD and SAXS spectra
of P(BDTSi-DFBT) indicates that it is a crystalline polymer. The
binary-PSCs made with the P(BDTSi-DFBT):PC70BM (1:1.5
di-bromo
derivative
of
5,6-difluoro-4,7-bis(4-
g
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Journal Name
2
2
2
2
0 H. Bin, Y. Yang, Z.-G. Zhang, L. Ye, M. GhasemVii,ewS.ArCtichleeOnn,linYe.
photovoltaic performance on binary-PSCs. However, the
inclusion of crystalline P(BDTSi-DFBT) on the PTB7-Th:PC70BM
blend was found to significantly improve the photovoltaic
performance of the resulting ternary-PSCs.
Zhang, C. Zhang, C. Sun, L. Xue, C. Yang, H. Ade and Y. Li, J.
DOI: 10.1039/C8TC00817E
Am. Chem. Soc., 2017, 139, 5085.
1 Q. Fan, Y. Wang, M. Zhang, B. Wu, X. Guo, Y. Jiang, W. Li, B.
Guo, C. Ye, Wenyan Su, J. Fang, X. Ou, F. Liu, Z. Wei, T. C.
Sum, T. P. Russell and Y. Li, Adv. Mater., 2018, 30, 1704546.
2 B. Kan, H. Feng, X. Wan, F. Liu, X. Ke, Y. Wang, Y. Wang,. H.
Conflicts of interest
There are no conflicts to declare
Zhang, C. Li, J. Hou and Y. Chen, J. Am. Chem. Soc., 2017, 139,
4
929.
3 H. Bin, L Gao, Z.-G. Zhang, Y. Yang, Y. Zhang, C. Zhang, S.
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Acknowledgements
This research was supported by the NRF Grant funded by the
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6 A. Casey, S. D. Dimitrov, P. Shakya-Tuladhar, Z. Fei, M.
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