A 3-Fluoro-4-hexylthiophene-Based Wide Bandgap Donor Polymer for 10.9% Efficiency Eco-Friendly Nonfullerene Organic Solar Cells

Article abstract of DOI:10.1002/smll.201805321

Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic and industrial interest due to their potential uses for flexible and lightweight products using low-cost roll-to-roll technology. In this work, two wide bandgap (WBG) polymers, namely P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, are designed and synthesized using benzodithiophene (BDT) derivatives. Good oxidation stability and high solubility are achieved by simultaneously introducing fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh-2DBDT)-C6 films present WBG optical absorption, suitable frontier orbital levels, and strong π–π stacking effects. In addition, P(fTh-2DBDT)-C6 exhibits good solubility in both halogenated and nonhalogenated solvents, suggesting its suitability as donor polymer for NFOSCs. The P(fTh-2DBDT)-C6:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthienyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC-Th) based device processed using chlorobenzene/1,8-diiodooctane (CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of 11.1%. Moreover, P(fTh-2DBDT)-C6:ITIC-Th reaches a high PCE of 10.9% when processed using eco-friendly solvents, such as o-xylene/diphenyl ether (DPE). The cell processed using CB/DIO maintains 100% efficiency after 1272 h, while that processed using o-xylene/DPE presents a 101% increase in efficiency after 768 h and excellent long-term stability. The results of this study demonstrate that simultaneous fluorination and alkylation are effective methods for designing donor polymers appropriate for high-performance NFOSCs.

Full text of DOI:10.1002/smll.201805321

FULL PAPER
Polymer Solar Cells
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A 3-Fluoro-4-hexylthiophene-Based Wide Bandgap Donor
Polymer for 10.9% Efficiency Eco-Friendly Nonfullerene
Organic Solar Cells
Jeong Eun Yu, Sung Jae Jeon, Jun Young Choi, Yong Woon Han, Eui Jin Ko,
and Doo Kyung Moon*
developed to achieve high-efficiency OSCs
Nonfullerene organic solar cells (NFOSCs) are attracting increasing academic
and industrial interest due to their potential uses for flexible and lightweight
products using low-cost roll-to-roll technology. In this work, two wide bandgap
based on fullerene acceptors such as
[6,6]-phenylC butyric acid methyl ester
71
3–6]
[
(
PC BM).
However, fullerene accep-
71
tors present significant disadvantages,
such as morphological instability and
limited synthetic control over their elec-
tronic and structural properties, which
cause difficulty in controlling their energy
levels and limit their light absorption
(
WBG) polymers, namely P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, are designed
and synthesized using benzodithiophene (BDT) derivatives. Good oxidation
stability and high solubility are achieved by simultaneously introducing
fluorine and alkyl chains to a single thiophene (Th) unit. Solid P(fTh-
2
DBDT)-C6 films present WBG optical absorption, suitable frontier orbital
[
7–9]
ranges.
To overcome these issues,
levels, and strong π–π stacking effects. In addition, P(fTh-2DBDT)-C6 exhibits
good solubility in both halogenated and nonhalogenated solvents, suggesting
its suitability as donor polymer for NFOSCs. The P(fTh-2DBDT)-C6:3,9-
bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis-
nonfullerene OSCs (NFOSCs) using non-
fullerene acceptors (NFAs) for increased
stability and efficiency have been exten-
[10–13]
sively studied.
Recently, rapid
(
(
(
5-hexylthienyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene
ITIC-Th) based device processed using chlorobenzene/1,8-diiodooctane
CB/DIO) exhibits a remarkably high power conversion efficiency (PCE) of
advances in the field of NFAs, particularly
synthesizing acceptor–donor–acceptor
A–D–A)-type molecules such as 3,9-bis-
2-methylene-(3-(1,1-dicyanomethylene)-
(
(
1
1.1%. Moreover, P(fTh-2DBDT)-C6:ITIC-Th reaches a high PCE of 10.9%
when processed using eco-friendly solvents, such as o-xylene/diphenyl ether
DPE). The cell processed using CB/DIO maintains 100% efficiency after
272 h, while that processed using o-xylene/DPE presents a 101% increase
i n d a n o n e ) - 5 , 5 , 1 1 , 1 1 - t e t r a k i s ( 4 -
hexylphenyl)-dithieno[2,3-d:2,3′-d′]-
s-indaceno[1,2-b:5,6-b′]-dithiophene
(
1
[
14]
(ITIC)
and 3,9-bis(2-methylene-(3-(1,1-
dicyanomethylene)-indanone))-5,5,11,11-
tetrakis(5-hexylthienyl)-dithieno[2,3-
in efficiency after 768 h and excellent long-term stability. The results of this
study demonstrate that simultaneous fluorination and alkylation are effective
methods for designing donor polymers appropriate for high-performance
NFOSCs.
d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]
[15]
dithiophene (ITIC-Th)
with indaceno-
dithiophene (IDT) derivatives donors,
have reached power conversion effi-
[
16,17]
ciencies (PCEs) of over 14%,
their practical applications are now in demand.
and
[
18–20]
Organic solar cells (OSCs), which can be produced using solu-
tion processing, have been promising candidates in the field
of photovoltaic conversion because they offer advantages such
as light weight and affordability, they require simple solution
process, and present device flexibility.[1,2] In the past two dec-
ades, high-performance low bandgap donor polymers were
Unlike
fullerene derivatives, which exhibit isotropic charge transfer,
the conjugated backbones of NFAs are planar. This promotes
intermolecular interactions and π–π stacking, thereby leading
[15]
to good charge transport, and a high absorption coefficient
was achieved in the wide wavelength range of 600–800 nm
or higher. Thus, a wide bandgap (WBG) polymer with a com-
plementary absorption range and well-matched energy levels
would be advantageous for NFAs with low bandgaps.
J. E. Yu, S. J. Jeon, J. Y. Choi, Y. W. Han, E. J. Ko, Prof. D. K. Moon
Nano and Information Materials (NIMs) Laboratory
Department of Materials Chemistry and Engineering
Konkuk University
It is most effective to control the properties of polymers by
[21–24]
substituting the main backbone with fluorine atoms
and
1
20 Neungdong-ro, Gwangjin-gu, Seoul 05029, Korea
[25–27]
ring substituents
when designing donor polymers suitable
In particular, introducing fluorine lowers the
E-mail: dkmoon@konkuk.ac.kr
[
28,29]
for NFOSCs.
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201805321.
energy level of the highest occupied molecular orbital (HOMO)
without affecting the energy bandgap, thus conferring the fluor-
inated polymers higher oxidation stability compared to that of
DOI: 10.1002/smll.201805321
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nonfluorinated polymers.[30,31] In addition, the strong electron-
egativity of fluorine can enhance intramolecular and intermo-
Scheme 1 illustrates the synthesis reactions of the mono-
mers and polymers in this study. The synthesis of 2,
5-dibromo-3-fluoro-4-hexylthiophene (M1) and polymerization
of poly[[4,8-bis[5-(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithio-
phene-2,6-diyl]-(3-fluoro-4-hexylthiophene)] and P(fTh-BDT)-C6
using poly[[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]-
dithiophene-2,6-diyl]-(3-fluoro-4-hexylthiophene)] and P(fTh-
[
32]
lecular interactions and increase charge-carrier mobility. In
recent years, Beaujuge and co-workers used benzo[1,2-b:4,5-bʹ]-
dithiophene (BDT) and thiophene (Th) as polymer backbones,
and synthesized three types of polymers with differing degrees
of planarity and fluorination. Among them, PBDT(T)[2F]T with
a planar backbone and substituted 2Fs presented the highest
PCE of 9.8%.[
2DBDT)-C6 were performed using modified methods reported
33]
[41]
in previous studies.
The polymerization reactions are
However, fluorinated polymers exhibit increased back-
bone rigidity, causing strong aggregation of molecules that
limits their solubility in common organic solvents.
The solubility of donor polymers is essential for solution
processability and forming appropriate nanoscale bicon-
described in Experimental Section. As illustrated in Scheme 1,
the synthesis of M1 consisted of four steps, and a viscous pale
yellow liquid was obtained as the final product. Details of the
synthesis process and characterization data are presented in
the Supporting Information. The polymers were obtained
using Stille coupling polymerization between M1 and BDT or
2DBDT. Soxhlet purification was performed to prevent burn-
in-loss of the devices using methanol (MeOH), acetone (Ace),
hexane (Hxn), Hxn:1,2-dichloropropane (DCP), ethyl acetate
[
34–36]
tinuous interpenetrating networks to achieve high charge-
carrier mobility.[
37,38]
Introducing long alkyl chains improves
polymer solubility in common organic solvents by modulating
the intermolecular π–π interchain interactions. However, if
the solubility and intermolecular interactions of the poly-
mers are not properly balanced, the decrease in molecular
weight due to poor π–π interchain interactions may interfere
with the transport of charge carriers. Therefore, selecting
chains of appropriate length and bulkiness is very important
for designing donor polymers.[ Li and co-workers synthe-
sized three types of polymers using a backbone composed of
BDT and Th, to which fluorine, 2-hexyldecyl (HD), 2-octyl-
dodecyl (OD), and 2-decyltetradecyl (DT) chains were intro-
duced. The study reported that as the length of the chain
increased, the molecular weight increased and the molecular
packing changed. As a result, HD-PBDT2FT with appro-
priate length and bulkiness was obtained, and its PCE
was 8.7%.[
[
42]
(EA), chloroform (CF), and DCP in this particular order.
Most of the P(fTh-BDT)-C6 polymers were obtained starting
with the Hxn fraction and the P(fTh-2DBDT)-C6 polymers were
obtained using the CF fraction. The molecular structures of the
obtained polymers were confirmed using NMR spectroscopy
and elemental analysis (EA) (see Figures S1–S6 in the Sup-
porting Information). In addition, the physical properties of the
synthesized polymers were measured using thermogravimetric
analysis (TGA) and gel permeation chromatography (GPC) (see
Figures S7, S8 and Table S1 in the Supporting Information).
Introducing fluorine into P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6
resulted in good thermal stability and 5% weight loss temper-
39]
[
43]
atures (T , 5% weight loss) of 315 and 346 °C, respectively.
d
40]
The number-average molecular weight (M ) values of P(fTh-
n
Therefore, in this study, we achieve optimal performance
for BDT- and Th-based polymers by simultaneously intro-
ducing fluorine and alkyl side chain. Using this technique,
we designed and synthesized two D–A type WBG donor poly-
mers, P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6, which exhibited
good properties, such as oxidation stability and high charge-
carrier mobility. Also, the polymers presented good solubility
in common chlorinated solvents and nonhalogenated sol-
vents, enabling eco-friendly processing. In brief, a blend film
of P(fTh-2DBDT)-C6:ITIC-Th as active layer for OSCs was
prepared using chlorobenzene (CB) as halogenated solvent
and 1,8-diiodooctane (DIO) as halogenated additive. The cor-
responding OSC device presented a maximum PCE of 11.1%,
an open-circuit voltage, VOC, of 0.94 V, a short-circuit current
BDT)-C6 and P(fTh-2DBDT)-C6 were 2 and 67 kDa, and the
polydispersity indices (PDI) were 14.50 and 3.00, respectively.
Theoretical simulations of the two polymers were performed
using density functional theory (DFT) at the B3LYP/6-31G(d)
level (see Figure S9 in the Supporting Information). Two donor
polymers, P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6 featured
a dihedral angle of 0° between the BDT core and fluorinated
thiophene. The inter/intramolecular interactions exerted by
these fluorine substituents (e.g., F···H, F···S, F···π) can
have a considerable effect on the polymer backbone planarity
[
44]
and stacking of these conjugated materials in the solid state.
Furthermore, P(fTh-BDT)-C6 with an alkoxy chain featured
a dihedral angle of 91.32° between the BDT core and alkoxy
chain, while P(fTh-2DBDT)-C6 featured a lower dihedral angle
of 57.73° between the BDT core and alkylthienyl side chain,
and thus, its backbone was more planar compared to that of the
P(fTh-BDT)-C6 with an alkoxy chain. Therefore, we predicted
that P(fTh-2DBDT)-C6 can form stronger π–π stacking than
−2
density, JSC, of 17.1 mA cm , and a fill factor, FF, of 62.8%.
In addition, when o-xylene and diphenyl ether (DPE) were used
as nonchlorinated solvent and nonhalogenated additive, respec-
tively, we obtained an OSC device with a high PCE of 10.9%,
−2
[45]
VOC of 0.92 V, JSC of 17.3 mA cm , and FF of 68.4%. Both
devices exhibited excellent long-term stability and their effi-
ciencies increased by 100% and 101% after 1272 and 768 h of
stability measurements, respectively. The results of this study
indicated that donor polymers modified using the simulta-
neous fluorination and alkylation technique were effective for
producing high-performance NFOSCs. Moreover, this report
offers interesting information on the molecular design of D–A
polymers for NFOSCs.
P(fTh-BDT)-C6 in solid thin films.
Due to the relatively small size of fluorine, we expected that
disruptive steric interactions would be minimized, while the
strong electron withdrawing nature of fluorine could cause an
[46]
increase in inter- and intramolecular packing. However, these
properties of fluorine may reduce the solubilities of the poly-
[47]
mers. To prevent the decrease in solubility, we simultaneously
introduced fluorine atoms and hexyl chains to the backbone
of the polymer. Because the hexyl chains present appropriate
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Scheme 1. Synthesis of P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6.
lengths and bulkiness, we expected that the polymers would
present good solubility and high molecular interactions. Both
polymers were highly soluble in halogenated solvents such as
CF and CB, as well as common nonchlorinated organic solvents
such as toluene, o-xylene, and tetrahydrofuran (THF), and thus
they would be suitable for eco-friendly processes.[
Figure 1a illustrates the UV–vis absorption spectra of CF
solutions of the polymers and Figure 1b presents the spectra of
the spin-coated thin-film polymers and ITIC-Th. And UV–vis
absorption spectra of the polymers and ITIC-Th in solutions
and thin films are shown in Figure S10 in the Supporting Infor-
mation, respectively. As shown in Figure 1a and Figure S11
in the Supporting Information, the molar absorption
an exceptional potential for harvesting solar photons and high
JSC values.
[
48]
Both polymers exhibited absorption peaks in the 320–
650 nm range (Figure 1b), which indicated complementary
UV–vis absorption with ITIC-Th, which presented absorption
peaks in the wavelength range 500–780 nm. Moreover, the
polymers:ITIC-Th blended films displayed efficient light har-
37]
[
49]
vesting properties.
The main absorption peaks of the thin
polymers films redshifted and exhibited broader absorption
peaks compared to those of the polymers in solution. This
occurred because the chains of the thin fluorinated polymers
films were likely to have been packed more closely compared
to the polymers in solution, thus resulting in aggregation and
[
50]
coefficients (ε) of P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6 were
enhanced intermolecular charge transfer (ICT).
−1
5
4 215 and 63 333 M−1 cm at λmax of 493 and 510 nm, respec-
P(fTh-BDT)-C6 presented only 0–1 absorption peaks in the
320–650 nm range and no 0–2 or 0–0 absorption peaks. As the
alkoxy side chains influenced the tilting angle between D–A,
tively. Both polymers presented reasonable molar absorption
coefficient values, which were higher than 50 000, indicating
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Figure 1. a) Average molar absorption coefficient in different dilute CF solutions. b) UV–vis absorption spectra of thin film on quartz plate from CF
solutions. c) Energy level diagrams of the polymers and ITIC-Th.
the ICT and crystallinity values decreased.[5,51] P(fTh-2DBDT)-
C6 presented a 0–2 absorption peak (π–π* transitions) in the
Tables S2 and S3 in the Supporting Information). The current
density–voltage (J–V) curves of the polymers are illustrated in
Figure 2a and the corresponding photovoltaic parameters are
summarized in Table 2.
3
5
50–500 nm range and 0–1 and 0–0 absorption peaks in the
00–650 nm range.[ The backbone of P(fTh-2DBDT)-C6 was
52]
more planar compared to that of P(fTh-BDT)-C6; therefore, it
promoted stronger inter- and intramolecular charge-transfer
interactions.[
The electrochemical properties of the polymers were meas-
ured using cyclic voltammetry (CV), and the results are illus-
trated in Figure S12 in the Supporting Information and Table 1.
Figure 1c depicts the energy-level diagrams of the polymers
The optimized D/A ratio was 1:1.5 for both polymers when
CB and DIO were used as processing solvent and additive,
respectively. The P(fTh-BDT)-C6 based OSC device exhibited a
very low PCE of 1.1%. By contrast, the best P(fTh-2DBDT)-C6
53]
device exhibited a remarkably high PCE of 11.1% and VOC, JSC
,
−
2
and FF of 0.92 V, 17.1 mA cm , and 62.8%, respectively. The
VOC values of the two devices were 0.92 and 0.94 V, respectively,
corresponding to the difference between the HOMO energy
levels of each polymer and LUMO energy levels of ITIC-Th.
Introducing fluorine into the two polymers lowered the HOMO
and ITIC-Th. The optical bandgaps (E ) of P(fTh-BDT)-C6 and
g
P(fTh-2DBDT)-C6 were 2.08 and 2.03 eV, respectively, and the
HOMO/lowest unoccupied molecular orbital (LUMO) energy
levels were −5.47/−3.39 and −5.51/−3.48 eV, respectively. The
fluorine substituents lowered the HOMO energy levels of
[
44]
energy level and increased VOC
.
Furthermore, the parameters
of the best P(fTh-2DBDT)-C6 device were higher than those of
the P(fTh-BDT)-C6 device. This was due to the good phase sep-
aration which occurred in the active layer because of the broad-
ened absorption and higher mobility of the 2D-conjugated BDT
both polymers, which contributed to the higher V values of
OC
NFOSCs.[
54,55]
In addition, the LUMO energy levels of the two
polymers were considerably higher than that of ITIC-Th, thus
contributing to a sufficiently high driving force for the dissocia-
tion of excitons.[
[
53]
polymers.
56]
The corresponding external quantum efficiency (EQE)
curves of the devices fabricated using optimal polymers:ITIC-
Th blends are illustrated in Figure 2b, and demonstrated that
both devices absorb light over a wide wavelength range of 300–
750 nm. However, as can be in Figure 2b, EQE was >20% in
the 560–720 nm range and reached only 23% at the maximum
wavelength of 700 nm. On the other hand, EQE of the
P(fTh-2DBDT)-C6:ITIC-Th blend was >60% in the 520–710 nm
range, and presented a maximum value of 71% at 660 nm. This
We fabricated and characterized OSC devices to investigate
the photovoltaic properties of the polymers. All fabricated
devices exhibited the following inverted-cell structure: indium
tin oxide (ITO) glass/zinc oxide (ZnO)/polymer:ITIC-Th/
MoO /Ag.
3
The processing conditions for the active layer, such as the
D/A ratio and thermal annealing temperature and time, were
optimized to achieve optimal photovoltaic performances (see
Table 1. Optical and electrochemical properties of polymers.
ε [M− cm−1
1
]
E
Opt,a)
HOMOb) [eV]
LUMOb) [eV]
Polymer
Abs. [nm]
Eonset [eV]
[eV]
g
Solution
max
Film
λ
λmax
5.42 × 104
6.33 × 104
2.02 × 105
−1.16
−1.20
−1.36
−5.47
−5.51
−5.67
−3.39
−3.48
−4.06
P(fTh-BDT)-C6
P(fTh-2DBDT)-C6
ITIC-Th
493
501
2.08
350, 510
670
358, 519, 552
635, 695
2.03
1.61
a)
Calculated from the intersection of the tangent to the low energetic edge of the absorption spectrum with the baseline; b)EHOMO (or LUMO) = −[Eonset(vs Ag/AgCl) −E1/2(Fc/
Fc+ vs Ag/AgCl)] − 4.8 [eV] E1/2(Fc/Fc+ vs Ag/AgCl) = 0.49 eV (measured data).
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Figure 2. a) J–V and b) EQE curves of OSC devices processed using CB/DIO.
was consistent with the JSC values from the photovoltaic perfor-
mance measurements.
To confirm the good processability of P(fTh-2DBDT)-C6, we
fabricated OSC devices using optimized ratios of eco-friendly
solvents such as toluene and o-xylene in CB-based devices (see
Table S5 in the Supporting Information), and the results are
shown in Figure 3 and Table 2. As illustrated in Figure 3a, PCE
X-ray diffraction (XRD) measurements were performed to
observe the crystallinity and molecular orientations of the two
polymers (see Figure S13 and Table S4 in the Supporting Infor-
mation). As shown in Figure S13a in the Supporting Infor-
mation, the out-of-plane direction of pristine P(fTh-BDT)-C6
film presented a (100) peak, which was related to the lamellar
appearance at 2θ = 5.19°. For pristine P(fTh-2DBDT)-C6, the
−2
was 10.4% while VOC, JSC, and FF were 0.94 V, 16.7 mA cm ,
and 66.1%, respectively, when only o-xylene was used as pro-
cessing solvent. When 0.5 vol% DPE was added to CB/DIO,
PCE became 10.6% in o-xylene and 10.4% in toluene. More-
over, after adding 0.25 vol% DPE to o-xylene, the highest PCE
of 10.9% was achieved, while VOC, JSC, and FF were 0.92 V,
(100) peak was not observed, but the broad (010) peak associated
with π–π stacking appeared at 2θ ≈ 21.90°. According to Bragg’s
law (λ = 2dsinθ), the lamellar distance of P(fTh-BDT)-C6 is
−
2
17.4 mA cm , and 68.4%, respectively. This indicated that
P(fTh-2DBDT)-C6 fabricated using the simultaneously fluori-
nation and alkylation technique could be a promising donor
polymer for practical application because of its high PCE and
1
6.99 Å, and the π–π stacking distance of P(fTh-2DBDT)-C6
is 4.05 Å. This indicated that the edge-on tendency was domi-
nant in P(fTh-BDT)-C6, as shown in Figure S13e in the Sup-
porting Information, and the face-on tendency was dominant
in P(fTh-2DBDT)-C6, as shown in Figure S13f in the Sup-
porting Information. In addition, we measured the crystallini-
ties of the polymers and nonfullerene acceptor-blended thin
films, as shown in Figure S13c,d in the Supporting Informa-
tion. Figure S13c in the Supporting Information illustrates
the out-of-plane direction of the blended thin polymer films,
where only the P(fTh-2DBDT)-C6 blend exhibited a d(010) peak
at 3.54 Å. This revealed that P(fTh-2DBDT)-C6 maintained the
face-on tendency even when blended with ITIC-Th, and
[
61]
environment friendly fabrication process.
Additionally, the
o-xylene-processed device achieved a high performance without
using any solvent additives or post-treatments, which could
[
62]
greatly simplify the fabrication of OSC devices.
Figure 3b shows that both devices absorb light in the wide
300–750 nm range. When using o-xylene and o-xylene/DPE as
processing solvents, the maximum EQEs at 680 nm were 71%
and 68%, respectively. The calculated JSC presented the same
tendency as those of the devices listed in Table 2.
To further understand the difference in efficiency, we exam-
ined the fluorescence quenching of the polymers when mixed
with ITIC-Th through photoluminescence (PL) measurements
(see Figure S14 in the Supporting Information). The films
were excited at their maximum absorptions. Each spectrum
was corrected for the absorption of the film at the excitation
[
57,58]
the decrease in d(010) indicated improved π–π stacking.
The face-on structure is more advantageous for the transfer of
electric charge than the edge-on structure.[
59,60]
These results were
consistent with the photovoltaic properties of P(fTh-BDT)-C6
and P(fTh-2DBDT)-C6.
Table 2. Photovoltaic performances of polymers:ITIC-Th under AM 1.5G illumination, 100 mW cm−2
.
JSC [mA cm−2]
PCEavea) [%]
0.6
Polymer
Solvents/additives
CB/DIOb)
VOC [V]
0.92
FF [%]
33.5
PCEmax [%]
1.1
P(fTh-BDT)-C6
P(fTh-2DBDT)-C6
P(fTh-2DBDT)-C6
P(fTh-2DBDT)-C6
3.2
CB/DIOb)
0.94
17.1
16.7
17.3
62.8
11.1
10.6
o-xylenec)
0.94
66.1
10.4
10.3
o-xylene/DPEb)
0.92
68.4
10.9
10.6
a)The average PCE values were calculated using 10 independent cells; b)Subjected to thermal annealing at 110 °C for 10 min; c)As-cast without any post-treatments.
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Figure 3. a) J–V and b) EQE curves of P(fTh-2DBDT)-C6:ITIC-Th-based devices processed using eco-friendly solvents.
wavelength. As shown in Figure S14a,b in the Supporting Infor-
mation, the P(fTh-BDT)-C6, P(fTh-2DBDT)-C6, and ITIC-Th
films presented strong PL emissions in the 700–850, 600–800,
and 700–850 nm ranges, respectively. Both blended films exhib-
ited similar behavior which involved PL quenching in the ITIC-
Th film. However, for the donor polymers, the PL quenching
efficiencies of the P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6
blended films were 63% and 85%, respectively. This indicated
that the electron transfer from the polymers to ITIC-Th for the
excitons generated in the donor phase was more effective in
the P(fTh-2DBDT)-C6 blended film than in the P(fTh-BDT)-C6
blended film.[
Figure S14c in the Supporting Information illustrates the PL
spectra of P(fTh-2DBDT)-C6 and ITIC-Th films using o-xylene
and o-xylene/DPE as processing solvents. The PL values of the
donor polymers and acceptors were 75% and 45% quenched,
respectively, by the P(fTh-2DBDT)-C6 blended film. When DPE
was added, the PL values of the donor polymers and accep-
tors were 76% and 46% quenched. These values were slightly
higher than those obtained when using o-xylene alone. These
results were consistent with the trends exhibited by the photo-
voltaic properties of the polymers.
To improve our understanding of the performance of the
OSCs, we measured the electron and hole mobilities using a
space-charge-limited current (SCLC) method, and the results
are shown in Figure 4 and Table 3. Hole-only devices with
ITO/PEDOT:PSS/polymer:ITIC-Th/MoO /Ag structures, and
3
electron-only devices with ITO/ZnO/polymer:ITIC-Th/LiF/Al
structures were fabricated.
As shown in Table 3, the electron/hole mobility of the
−
5
2
−1 −1
P(fTh-BDT)-C6-based device was 7.50 × 10 cm
V
s /
−
7
2
−1 −1
9.82 × 10 cm V s . Thus, the difference in mobility was
more than two orders of magnitude and µ /µ was 76.37. The
e
h
63]
electron/hole mobility of P(fTh-2DBDT)-C6-based devices
was higher (1.50 × 10 cm V s /1.12 × 10 cm V s )
and their charge balance was superior compared to P(fTh-
−
4
2
−1 −1
−5
2
−1 −1
[
64]
BDT)-C6.
These differences in values originate in the pla-
narity of the backbone of the polymer and microstructural
ordering. Moreover, these differences explained why the
P(fTh-2DBDT)-C6-based devices presented superior J and
SC
[
65]
FF values.
were used as processing solvents, the electron/hole mobilities
In addition, when o-xylene and o-xylene/DPE
−
4
2
−1 −1
−5
2
−1 −1
were 1.12 × 10 cm
V
s /2.13 × 10 cm
V
s
and
−
4
2
−1 −1
−5
2
−1 −1
2.10 × 10 cm V s /4.19 × 10 cm V s , and µ /µ were
e
h
Figure 4. Jph–V curves of a) electron-only and b) hole-only devices for polymers:ITIC-Th blended films.
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Table 3. Electron and hole mobilities (µ and µ ) of polymers measured
P(fTh-2DBDT)-C6 blended film was more clear and uniform
than P(fTh-BDT)-C6. It provides not only a suitable domain
size for efficient exciton dissociation but also a continuous
interpenetrating network for transporting charge carriers in
the nanoscale. Large phase separations reduced the exciton
diffusion/separation efficiencies, and finally led to OSCs
e
h
using the SCLC method.
µe [cm V s−1
2
−1
]
µh[cm V s−1
2
−1
]
µe/µh
76.37
13.39
5.25
Conditions
P(fTh-BDT)-C6:ITIC-Tha)
7.50 × 10−5
9.82 × 10−7
P(fTh-2DBDT)-C6:ITIC-Tha)
1.50 × 10−4
1.12 × 10−5
[
9]
characterized by low PCEs. The RMS values of the P(fTh-
P(fTh-2DBDT)-C6:ITIC-Thb)
P(fTh-2DBDT)-C6:ITIC-Thc)
1.12 × 10−4
2.10 × 10−4
2.13 × 10−5
4.19 × 10−5
2
1
DBDT)-C6 blended films in Figure 5c and d were 1.12 and
.18 when o-xylene and o-xylene/DPE were used as processing
5.01
solvents, respectively. And compared to the TEM images pro-
cessed using CB/DIO, the P(fTh-2DBDT)-C6 blended film in
Figure 5g presented relatively larger crystallite domain sizes,
which could have arisen from the different solubilities of
a)Processed using CB/DIO; b)Processed using o-xylene; c)Processed using o-xylene/
DPE.
5
.25 and 5.10, respectively. The balance ratios of µ /µ when
e h
[
68]
o-xylene and o-xylene/DPE were used as processing solvents
were higher than those obtained when CB/DIO was used as
processing solvent in P(fTh-2DBDT)-C6-based devices. This
was consistent with the tendency of FF to increase.
the blended films.
Figure 5h shows finer morphology than Figure 5g due to the
The P(fTh-2DBDT)-C6 blended film in
[
69]
presence of DPE additives. The internal morphology of the
P(fTh-2DBDT)-C6 blended film processed by o-xylene/DPE
was homogeneous, consisting of nanoscale segregated crystal-
lites; this structure is known to favor effective exciton diffusion,
charge separation, and transport because it promotes the for-
mation of continuous charge-transport channels. The results
indicated higher JSC and FF values for the OSCs.
To investigate the phase separation morphologies of the
polymer:ITIC-Th blends, atomic force microscopy (AFM)
analysis and transmission electron microscopy (TEM) analysis
were performed, and the results are shown in Figure 5 and
Figure S15 in the Supporting Information. In the AFM topo-
graphic images in Figure 5, light and dark domains appeared
as the polymer and ITIC-Th were aggregated. When CB/DIO
was used as solvent, the root-mean-square roughness (RMS)
values of the P(fTh-BDT)-C6 and P(fTh-2DBDT)-C6 blended
films in Figure 5a and b were 4.23 and 1.59, respectively. These
RMS values might have been related to the strong π–π inter-
chain interaction and self agglomeration.[ The large RMS
and large domain size of the P(fTh-BDT)-C6 blended film
impaired the charge-carrier transport.[ This further explained
the weaker EQE response of the P(fTh-BDT)-C6 blended film.
Furthermore, the TEM images show that the morphology of the
We measured the long-term stability of the optimized
P(fTh-2DBDT):ITIC-Th-based device, and the results are illus-
trated in Figure 6 and Table S6 in the Supporting Information.
Figure 6 depicts the time-dependent changes in the PCE values
of devices that used CB/DIO, o-xylene, and o-xylene/DPE as
processing solvents. Figure 6a and Table S6 in the Supporting
Information demonstrate that the VOC and JSC of the device
slightly decreased in time, while FF increased. The device pro-
cessed using CB/DIO presented 100% efficiency after 1272 h
compared to the initial efficiency, and those processed using
o-xylene and o-xylene/DPE exhibited 94% and 101% efficiencies
66]
67]
Figure 5. AFM topographic(5 µm × 5 µm) and TEM images of blend films of a,e) P(fTh-BDT)-C6:ITIC-Th and b,f) P(fTh-2DBDT)-C6:ITIC-Th processed
by CB/DIO, c,g) o-xylene and d,h) o-xylene/DPE.
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weight due to the planar backbone presented the highest PCE
of 11.1% in the P(fTh-2DBDT)-C6:ITIC-Th blend when CB/
DIO was used as processing solvent. P(fTh-2DBDT)-C6 also
presented a PCE of 10.9% when o-xylene/DPE was used as
processing solvent. To the best of our knowledge, the PCE of
the P(fTh-2DBDT)-C6:ITIC-Th device was one of the highest
for NFOSCs developed using eco-friendly solvents to date. In
addition, P(fTh-2DBDT)-C6-based devices processed using CB/
DIO maintained 100% efficiency after 1272 h, and the device
processed using o-xylene/DPE presented a 101% increase in
efficiency after 768 h, indicating excellent long-term stability.
This demonstrated that the simultaneous fluorination and
alkylation technique could be an effective method for fabri-
cating donor polymers for NFOSCs with high performance
sustainability.
Figure 6. PCE-time graphs of P(fTh-2DBDT)-C6:ITIC-Th-based device
using different solvents.
Experimental Section
Materials: All reagents and chemicals were purchased from TCI Co.,
Sigma-Aldrich Co., and Acros Organics Co. and were used without
further purification unless otherwise stated. The 2,6-bis(trimethyltin)-
after 816 and 768 h, respectively. Although the tendency of the
device processed using o-xylene was to present a slight decrease
in long-term stability compared to those of the devices pro-
cessed using other solvents, this device did not use any solvent
additives and post-treatment. Therefore, the fabrication process
of this device was simplified and did not require additional
vacuum processing to remove additives. Thus, P(fTh-2DBDT)-
C6 incorporating fluorine and hexyl chains presented excellent
oxidation stability, and simultaneously exhibited good stability
4
,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-bʹ]dithiophene
(BDT)
and
(4,8-bis(4,5-didecylthiophen-2-yl)benzo[1,2-b:4,5-bʹ]dithiophene-2,6-
diyl)bis(trimethylstannane) (2DBDT) were purchased from Suna
Tech Inc.
Synthesis of P(fTh-BDT)-C6:
A
mixture of monomer M1
(34.4 mg, 0.1 mmol), BDT (76.85 mg, 0.0995 mmol), and
tetrakis(triphenylphosphine)palladium(0) (5.77 mg, 0.005 mmol)
was added to a 2–5 mL microwave vial in air. The vial was capped
and vacuumed for 20 min, and then it was refilled with nitrogen gas,
and subsequently, anhydrous toluene (3.0 mL) and anhydrous N,N-
dimethylformamide (0.3 mL) were added to the mixture. Afterward,
the reactor was degassed and refilled with nitrogen again. The
polymerization mixture was stirred and was heated to reflux for 90 h.
The polymer was end-capped by adding 2-bromothiophene (0.03 g,
[70,71]
in both halogenated and nonhalogenated solvents.
Furthermore, we also kept the devices under continuous
−2
illumination (AM 1.5G illumination at 100 mW cm ) without
encapsulation to investigate the device operational stability. The
device performance was measured periodically to observe the
illumination-dependent degradation effect in the device per-
formance for 2 h (See Figure S16 in the Supporting Informa-
tion). And Figure S17 in the Supporting Information exhibits
the change in the normalized photovoltaic parameters with the
illumination time. It can be seen that the performance of all
the devices decreased with increasing operation time; however,
the P(fTh-2DBDT):ITIC-Th-based devices demonstrated good
operational stability.[ For the device processed by o-Xylene and
o-Xylene/DPE, its PCE retains 64% of its initial efficiency. How-
ever, the device processed by CB/DIO, its PCE dropped by 45%
after 2 h of testing. It can be estimated that nonhalogenated
solvents cause less damage to donor and acceptor materials in
air with the light illumination because of the stable molecular
structures of these solvents consisting of stable carbon and
hydrogen atoms.[
0
.177 mmol) and the mixture was further heated for 4 h at 110 °C.
Subsequently, 2-tributylstannyl thiophene (0.017 g, 0.047 mmol) was
added and the mixture was heated for 4 h at 110 °C. The reaction
mixture was cooled to room temperature, poured into 300 mL MeOH,
and further purified by Soxhlet extraction using MeOH, Ace, Hxn, EA,
and CF. The Hxn fraction of the polymers was reprecipitated in MeOH,
filtered, and dried under vacuum to yield P(fTh-BDT)-C6 (43 mg, yield:
1
6
4
9%) as a red solid. H NMR (400 MHz, CDCl , δ): 7.69–6.85 (br, 2H),
3
.42–3.85 (br, 4H), 3.00–2.33 (br, 2H), 1.93–1.22 (br, 22H), 1.22–0.70
68]
(
br, 14H), 0.95–0.84 (br, 18H). Anal. calcd. for C35H49FO S : C, 68.14;
2 3
H, 8.01; F, 3.08; O, 5.19; S, 15.59; EA for C35H49FO S : C, 67.23; H, 7.68;
2
3
O, 4.27; S, 14.35.
Synthesis of P(fTh-2DBDT)-C6: P(fTh-2DBDT)-C6) was synthesized
following the same procedure used for synthesizing P(fTh-BDT)-C6.
However, the final product was further purified by Soxhlet extraction
using MeOH, Ace, Hxn, Hxn:DCP = 1:1, EA, and CF. The CF fraction of
polymers was reprecipitated in MeOH, filtered, and dried under vacuum
to yield P(fTh-2DBDT)-C6 (56 mg, yield: 73%) as a dark red solid.
72–74]
1
In this study, we designed and synthesized WBG donor
polymers employing a simultaneous fluorination and alkyla-
tion technique using BDT–Th (D–A), P(fTh-BDT)-C6 and
P(fTh-2DBDT)-C6 to produce highly efficient NFOSCs. By
simultaneously introducing fluorine and alkyl chains into
the polymers, both polymers achieved good balance between
solubility and intermolecular interactions. Accordingly, they
were determined to be suitable for fabricating highly efficient
NFOSCs using nonchlorinated organic solvents. In particular,
the inverted P(fTh-2DBDT)-C6 structure with high molecular
H NMR (400 MHz, CDCl3, δ): 7.81–7.48 (br, 2H), 7.38–7.28 (br, 2H),
7
2
.16–6.83 (br, 2H), 3.07–2.84 (br, 4H), 2.84–2.46 (br, 2H), 1.85–1.65 (br,
H), 1.65–1.14 (br, 24H), 1.14–0.74 (br, 15H). Anal. calcd. for C H59FS5:
46
C, 69.82; H, 7.52; F, 2.40; S, 20.26; EA for C46H FS : C, 69.56; H, 6.84;
59
5
S, 19.29.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
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[
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This research was supported by the New and Renewable Energy Core
Technology Program of the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) grant funded by the Ministry of Trade,
Industry, and Energy, Republic of Korea (No. 20153010140030) the
Korea Institute of Energy Technology Evaluation and Planning(KETEP)
and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of
Korea (2018201010636A) and the Human Resource Program in Energy
Technology of the Korea Institute of Energy Technology Evaluation and
Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE)
of the Republic of Korea (Grant no. 20174010201540).
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