Ternary Polymer Solar Cells Facilitating Improved Efficiency and Stability

Article abstract of DOI:10.1002/adma.201904601

The use of a ternary active layer offers a promising approach to enhance the power conversion efficiency (PCE) of polymer solar cells (PSCs) via simply incorporating a third component. Here, a ternary PSC with improved efficiency and stability facilitated by a new small molecule IBC-F is demonstrated. Even though the PBDB-T:IBC-F-based device gives an extremely low PCE of only 0.21%, a remarkable PCE of 15.06% can be realized in the ternary device based on PBDB-T:IE4F-S:IBC-F with 20% IBC-F, which is ≈10% greater than that (PCE = 13.70%) of the control binary device based on PBDB-T:IE4F-S. The improvement in the device performance of the ternary PSC is mainly attributed to the enhancement of fill factor, which is due to the improved charge dissociation and extraction, suppressed bimolecular and trap-assisted recombination, longer charge-carrier lifetime, and enhanced intermolecular interactions for preferential face-on orientation. Additionally, the ternary device with 20% IBC-F shows better thermal and photoinduced stability over the control binary device. This work provides a new angle to develop the third components for building ternary PSCs with enhanced photovoltaic performance and stability for practical applications.

Full text of DOI:10.1002/adma.201904601

CommuniCation
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Ternary Polymer Solar Cells Facilitating Improved Efficiency
and Stability
Yingying Dong, Yan Zou, Jianyu Yuan, Hang Yang, Yue Wu, Chaohua Cui,*
and Yongfang Li
acceptors combined with the development
The use of a ternary active layer offers a promising approach to enhance
the power conversion efficiency (PCE) of polymer solar cells (PSCs) via
simply incorporating a third component. Here, a ternary PSC with improved
efficiency and stability facilitated by a new small molecule IBC-F is demon-
strated. Even though the PBDB-T:IBC-F-based device gives an extremely low
PCE of only 0.21%, a remarkable PCE of 15.06% can be realized in the ternary
device based on PBDB-T:IE4F-S:IBC-F with 20% IBC-F, which is ≈10% greater
than that (PCE = 13.70%) of the control binary device based on PBDB-T:IE4F-
S. The improvement in the device performance of the ternary PSC is mainly
attributed to the enhancement of fill factor, which is due to the improved
charge dissociation and extraction, suppressed bimolecular and trap-assisted
recombination, longer charge-carrier lifetime, and enhanced intermolecular
interactions for preferential face-on orientation. Additionally, the ternary
device with 20% IBC-F shows better thermal and photoinduced stability over
the control binary device. This work provides a new angle to develop the third
components for building ternary PSCs with enhanced photovoltaic perfor-
mance and stability for practical applications.
of polymer donors significantly boosts
the power conversion efficiencies (PCEs)
of PSCs.[7–15] The open-circuit voltage
(Voc), short-circuit current density (Jsc),
and fill factor (FF) are three key param-
eters determining the PCE of the PSC
device. Despite the impressive progress
achieved, challenges still remain in the
field in terms of enhancing device perfor-
mance and stability. The use of a ternary
active layer offers a simple and effective
approach to enhance the PCE of the BHJ
PSCs via manipulating the energy levels,
expanding the spectral range, and facili-
tating the morphology of the active layer
by incorporating a third component.[16–20]
The introduction of
a supplementary
third component in the active layer favors
increasing their efficiency of sunlight har-
vesting that can potentially translate to
a higher Jsc in the ternary devices.[21–25]
The Voc can be easily tuned by varying
Solution-processable bulk-heterojunction (BHJ) polymer solar
cells (PSCs) are considered a promising solar energy technology,
owing to their unique features of easy fabrication, light weight,
flexibility, and semitransparency.[1–6] In recent years, the emer-
gence of narrow bandgap n-type organic semiconductor (n-OS)
the content of the third component in the ternary PSCs with
parallel and alloy working mechanisms.[26–28] As opposed to
achieving complementary absorption and matching energy
levels, it is a greater challenge to control the morphology in the
ternary blend to improve the FF in the ternary device owing to
the fact that the third component may act as a recombination
center or a morphological trap within the host binary systems.
Therefore, it is critical to select a miscible and compatible third
component that can function as a morphological modulator
to facilitate better phase separation and molecular packing to
achieve a higher Jsc and FF in ternary PSCs.[29–31] Very recently,
PCEs over 16% were achieved in the ternary PSCs using Y6 as
the acceptor.[32]
Y. Dong, Y. Zou, H. Yang, Y. Wu, Prof. C. Cui, Prof. Y. Li
Laboratory of Advanced Optoelectronic Materials
College of Chemistry
Chemical Engineering and Materials Science
Soochow University
Suzhou 215123, China
E-mail: cuichaohua@suda.edu.cn
Prof. J. Yuan
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices
Institute of Functional Nano & Soft Materials (FUNSOM)
Soochow University
In addition to a high efficiency, achieving high device
stability is critical but is still a significant challenge for the
practical application of PSCs. Generally, the device stability
suffers from the oxidation of the electrodes, degradation of
the interface layers, and intrinsic instability of the active layer
morphology driven by light and thermodynamics.[33–39] Apart
from elevating the photovoltaic performance, the use of ternary
PSCs has a positive effect in strengthening the morphology
stability of the BHJ layer for improving the device stability,
which mainly arises from the interactions of multiple com-
positions for the formation of a locked morphology.[40–42] For
instance, the ternary PSC with 20 wt% PBDB-T demonstrated
Suzhou 215123, China
Prof. Y. Li
CAS Research/Education Center for Excellence in Molecular Sciences
CAS Key Laboratory of Organic Solids
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201904601.
DOI: 10.1002/adma.201904601
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better device stability than the control FTAZ:IT-M-based
binary device after a storage time of 1000 h in a nitrogen-filled
glovebox, which was ascribed to the suppressed crystalliza-
tion of IT-M by the addition of PBDB-T in the blend.[38] In the
case of the P3HT:IDTBR-based binary device, the crystalline
IDTBR phase was effectively suppressed by the incorporation
of IDFBR for preserving a three-phase microstructure, and thus
enhanced device stability was realized in the ternary device,
so that the P3HT:IDTBR:IDFBR-based ternary device retained
80% of its initial PCE, whereas the ternary device based on
P3HT:IDTBR showed poorer stability with retaining 70% after
1200 h in air and under dark conditions.[39] Kim et al. found
that the introduction of a small amount of PCBM into the
PBDTTTPD:P(NDI2HD-T) all-polymer host blend favored the
formation of an intermixed phase, which was beneficial for
maintaining stability under thermal stress for PSCs.[40]
To realize the high-performance of the ternary device, the
most common strategy is the rational incorporation of a com-
patible third component into the host blend. It is widely recog-
nized that the third component should have good photovoltaic
properties in the corresponding binary device for improving
the efficiency of the ternary PSCs. Besides, the addition of
some third component with low efficiency in binary blend
also showed positive effect on increasing the efficiency, as
shown in Table S1 in the Supporting Information.[16–20] For
instance, Ge et al. reported a PBDB-T:ITM:ITCN-based ternary
PSC with a PCE of 12.16%, which is higher than the PBDB-
T:ITM (PCE = 10.89%) and PBDB-T:ITCN-based binary device
(PCE = 2.21%). Zhan et al. demonstrated a high performance
ternary PSC with incorporating 25% third component TR into
the binary PTB7-Th:FOIC host blend, the PCE was improved
from 12.10% for the binary device to 13.10% for the ternary
device, despite the low PCE of 0.66% for the TR:FOIC-based
device.[43] In this contribution, we report a ternary PSC with
improved efficiency and stability facilitated by a third com-
ponent IBC-F (as shown in Figure 1a) with extremely poor
photovoltaic performance in the binary devices. IBC-F has the
typical A–π–D–π–A (A = acceptor unit and D = donor unit)
main-chain architecture with a central indacenodithiophene
(IDT) donor unit, alkyl cyanoacetate end-capped groups, and
fluorine substituted phenyl units as the π-bridge. The fluorine
substituents in a π-bridge can potentially promote the macro-
molecular planarity via weak F···S intramolecular noncovalent
Coulombic interactions and enhance the inter/intramolecular
interactions.[44–46] Density functional theory (DFT) calculation
results reveal that IBC-F with fluorine substituted phenyl units
has relatively high planarity of the backbone (Figure 1a). PBDB-
T:IE4F-S (Figure 1a)[47] was chosen as the host blend, where the
same central IDT unit in the n-OS acceptor IE4F-S and IBC-F
could potentially ensure their miscibility. The IBC-F can act as
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Figure 1. a) Chemical structures of IBC-F, PBDB-T, and IE4F-S, and the optimized molecular conformation (side view) of IBC-F depicted by DFT
calculations at the B3LYP/6-31G(d, p) level. b) Synthetic routes for IBC-F.
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a sensitizer to deliver energies to the PBDB-T:IE4F-S host blend
via the Dexter or Förster resonance energy transfer (FRET).[48]
In comparison with the control PSC based on PBDB-T:IE4F-S
(1:1, w/w) with a PCE of 13.70%, the ternary PSC with incorpo-
rating 20% IBC-F into the PBDB-T:IE4F-S host blend (PBDB-
T:IE4F-S:IBC-F = 1:1:0.2, w/w/w) demonstrated a significantly
increased PCE of 15.06%, despite the extremely low PCE of
0.21% obtained for the PBDB-T:IBC-F-based binary device.
The enhancement of photovoltaic performance of the ternary
PSC mainly results from the significantly enhanced FF, which
is due to the improved charge dissociation and extraction, sup-
pressed bimolecular, and trap-assisted recombination, longer
charge carriers lifetime, and favorable fibrillar morphology
along with the enhanced intermolecular interactions for pref-
erential face-on orientation in the ternary blend. Furthermore,
the ternary device with 20% IBC-F exhibits better thermal and
photoinduced stability over the binary device, resulting in the
ternary device retaining 81.0% and 85.5% of its initial PCE
after continuous thermal aging and illumination in an inert
atmosphere for 1000 h, respectively, while the PBDB-T:IE4F-S-
based binary devices retain a lower initial PCE of 71.7% and
73.6%, respectively. These results suggest that the addition of
IBC-F into the PBDB-T:IE4F-S blend not only improves the
photovoltaic performance but also has a synergistic benefit for
enhancing both the thermal aging and photoinduced stability.
The synthetic routes toward IBC-F are shown in Figure 1b
and the detailed synthesis procedures are provided in the Sup-
porting Information. The n-OS acceptor IE4F-S was synthesized
according to our previous work.[47] IBC-F exhibits good solu-
bility in common organic solvents, such as chloroform, toluene,
and chlorobenzene. As shown in Figure 2a, the UV–vis absorp-
tion spectrum of IBC-F in the thin film state mainly appears in
the wavelength range of 450–600 nm, with a maximum peak
at 542 nm and an optical bandgap of 2.07 eV. IBC-F exhibits
a good absorption profile that is complementary to the PBDB-
T:IE4F-S blend (Figure 2a). Figure 2b shows the absorption
spectra of the PBDB-T:IE4F-S host blend with various contents
of IBC-F. Although incorporating 20% IBC-F into the PBDB-
T:IE4F-S host blend slightly decreases the absorption coefficient
in the wavelength region of 570–760 nm, the enhanced absorp-
tion coefficient can be observed in the short wavelength range
of 440–570 nm, leading to the similar absorption capability of
the two blends. This trend becomes more pronounced as the
increase of IBC-F contents. The absorption capability of the ter-
nary blends significantly decreased in the wavelength region
of 550–800 nm, while enhanced in the short wavelength of
450–500 nm, accompanying with gradually blueshifted low-
energy absorption band, which may be due to the deteriorated
packing affected by the increased content of IBC-F. The incor-
poration of 10% IBC-F in the host blend leads to a lower absorp-
tion coefficient in the region of 550–800 nm, whereas barely
influences the short wavelength absorption, which is different
from the ternary blends with 20% or more IBC-F contents.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of IBC-F
measured by cyclic voltammetry to be −5.62 and −3.53 eV, respec-
tively (as shown in Figure S1 in the Supporting Information). As
depicted in Figure 2c, IBC-F shows deeper HOMO and LUMO
levels than PBDB-T but higher LUMO and lower HOMO than
the n-OS acceptor IE4F-S, indicating incompatible energy levels
between IBC-F and IE4F-S in terms of efficient charge separa-
tion. We then performed photoluminescence (PL) measurement
to examine the working mechanism in the PBDB-T:IE4F-S:IBC-
F ternary blend. As shown in Figure 2d, the PL emission spec-
trum of IBC-F mostly overlaps with the absorption spectrum
of PBDB-T, which fulfills the requirement of sufficient overlap
between the emission spectrum of the energy donor and absorp-
tion spectrum of the energy acceptor for FRET theory, indicating
that FRET could occur in the PBDB-T:IBC-F blend.[44] To further
confirm the existence of FRET in the blend film, the PL spectra
of the thin film of PBDB-T, IBC-F, and PBDB-T:IBC-F (1:0.2,
w/w) were measured (as shown in Figure S2 in the Supporting
Information). When excited at 580 nm, the emission of PBDB-T
in the PBDB-T:IBC-F (1:0.2, w/w) blend is enhanced com-
pared to the pristine PBDB-T film, suggesting energy transfer
from IBC-F to PBDB-T rather than charge transfer. The FRET
is further confirmed using time-resolved transient absorption
spectroscopy. As shown in Figure 2e, the pristine IBC-F and
PBDB-T films show clear ground state bleaching (GSB) signals
at 550 nm (completely decays ≈1.40 ps) and 640 nm (completely
decays ≈1.10 ps), respectively. In comparison with pristine
IBC-F and PBDB-T, introducing 20% of the acceptor PBDB-T
into the donor IBC-F slightly reduces the decay time for the
GSB of IBC-F and prolongs it for the GSB of PBDB-T in the
blend (as shown in Figure S3 in the Supporting Information).
This trend becomes more obvious with the increase in acceptor
PBDB-T content in the blend, where the GSB signal of IBC-F in
the IBC-F:PBDB-T (1:0.8, w/w) blend decays at ≈0.27 ps, along
with a longer decay time at ≈4.24 ps for the GSB of PBDB-T
(Figure 2e), confirming the existence of an energy transfer pro-
cess from IBC-F to PBDB-T. Figure 2c presents an energy dia-
gram outlining the possible pathways to current generation in
the PBDB-T:IBC-F:IE4F-S ternary blend.
PSCs were fabricated with the conventional device structure of
glass/indium-tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly-
(4-styrenesulfonate) (PEDOT:PSS)/active layer/PDINO/Al to inves-
tigate the photovoltaic. The optimal treatment condition for the
PBDB-T:IE4F-S (1:1, w/w) based control device is thermal annealing
at 160 °C for 10 min.[47] We then fabricated ternary PSCs by
incorporating various contents of IBC-F into the PBDB-T:IE4F-S
(1:1, w/w) host blend. The key photovoltaic parameters of the
optimal ternary and binary devices are summarized in Table 1
and the corresponding current density–voltage (J–V) curves are
shown in Figure 3a.
The binary device based on PBDB-T:IE4F-S exhibits a PCE
of 13.70% and an FF of 0.69, which is in line with our pre-
vious work.[47] While the binary PSCs based on PBDB-T:IBC-F
(with IBC-F as acceptor) and IBC-F:IE4F-S (with IBC-F as
donor) show very low PCE of 0.21% and 0.045%, respectively
(as shown in Table S2 and Figure S4 in the Supporting Infor-
mation), indicating the poor photovoltaic performance of IBC-F
used as either a donor or acceptor in the binary devices. Notably,
with the incorporation of 20% IBC-F into the host blend, the
Voc increased from 0.869 to 0.887 V and more importantly a
higher FF of 0.744 was achieved, leading to a higher PCE of
15.06%, which is ≈10% greater than the corresponding binary
device based on PBDB-T:IE4F-S. PCEs greater than 14% were
also obtained for the ternary devices with the IBC-F contents of
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Figure 2. a) UV–vis absorption spectra of IBC-F, PBDB-T, and IE4F-S in the thin film. b) UV–vis absorption spectra of blend films with different IBC-F
weight ratios. c) Energy levels of IBC-F, PBDB-T, and IE4F-S and schematic of the charge and energy transfer mechanisms involved in the PBDB-T:IBC-
F:IE4F-S ternary blend. d) Absorption spectrum of PBDB-T and PL emission spectrum of IBC-F. e) Transient absorption spectra of pristine IBC-F,
pristine PBDB-T, and IBC-F:PBDB-T (1:0.8, w/w) blend pumped at 500 nm.
10% and 30%, mainly resulted from the improved FF of 0.734
and 0.724, respectively. With further increasing the IBC-F con-
tent over 30% (30–100%) into the ternary systems, the FF can
still reach higher than 0.70, indicating the significantly positive
effect of IBC-F third component in improving the FFs of the
ternary devices (Figure 3b). The ternary device with 100% IBC-F
(weight ratio of PBDB-T:IE4F-S:IBC-F = 1:1:1) demonstrates a
PCE of 11.58%, with an FF of 0.70 and a high Voc of 0.898 V.
The Voc of a PSC also depends on the balance of charge gen-
eration and recombination except the energy gap between the
HOMO level of donor and LUMO energy level of the acceptor.
Although the energy gap (∆E = 1.80 eV) between the HOMO
of PBDB-T and LUMO of IBC-F is large enough to obtain a
high Voc in a PSC, an unexpected low Voc of 0.887 V with large
voltage loss of 0.71 V was observed in the PBDB-T:IBC-F-based
device. We speculate that this is mainly due to the excessive
charge recombination in the PBDB-T:IBC-F blend. Note that
the Voc values of ternary devices concomitantly increase with
increasing IBC-F content in the host blend and the ternary
device with 100% IBC-F exhibit the highest Voc of 0.898 V,
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Table 1. Summary of the photovoltaic performance of PSCs based on PBDB-T:IE4F-S:IBC-F
with various IBC-F contents.
PBDB-T for charge separation in this case,
IBC-F acts as a sensitizer that delivers ener-
gies to PBDB-T rather than charge transfer.
Thus, an extremely low PCE of 0.21% is
obtained in the PBDB-T:IBC-F-based device.
In spite of the poor performance of the third
component, a notable improvement in effi-
ciency was realized in the ternary device
upon introduction of 20% IBC-F in the host
blend, which is rarely observed in highly effi-
cient ternary PSCs.
The external quantum efficiency (EQE)
curves of the PSCs are shown in Figure 3d.
The Jsc values obtained from J–V measure-
ments are rather consistent with the integral
Jsc values calculated from the EQE curves
Voca) [V]
FFa) [%]
PCEa) [%]
Jsca) [mA cm−2
]
Ratios
1:1:0
0.869 [0.866 0.002]
0.881 [0.883 0.003]
0.887 [0.887 0.003]
0.893 [0.893 0.002]
0.893 [0.896 0.002]
0.894 [0.898 0.003]
0.898 [0.900 0.002]
0.887 [0.836 0.045]
22.85 [22.49 0.32]
21.71 [21.47 0.33]
22.83 [22.42 0.38]
21.68 [20.95 0.41]
20.76 [20.63 0.17]
20.17 [19.74 0.33]
18.40 [18.27 0.28]
0.82 [0.76 0.07]
69.0 [69.4 0.79]
73.4 [73.0 0.94]
74.4 [73.8 0.53]
72.4 [73.4 0.77]
72.4 [72.3 0.19]
71.9 [71.6 0.73]
70.1 [69.2 0.69]
28.6 [28.2 2.44]
13.70 [13.51 0.18]
14.09 [13.84 0.12]
15.06 [14.68 0.21]
14.05 [13.75 0.13]
13.43 [13.35 0.06]
12.96 [12.70 0.14]
11.58 [11.38 0.12]
0.21 [0.18 0.02]
1:1:0.1
1:1:0.2
1:1:0.3
1:1:0.5
1:1:0.7
1:1:1
1:0:1
a)The statistical values in the square brackets are obtained from 10 devices.
which is 0.029 V greater than that of the binary device based
on PBDB-T:IE4F-S. The improved Voc in the ternary devices
should be ascribed to the higher LUMO level of IBC-F com-
pared to IE4F-S and the good miscibility of IBC-F and IE4F-S,
which is confirmed by the simulation (as shown in Figure S5 in
the Supporting Information).[49] In comparison with the PBDB-
T:IE4F-S (1:1, w/w) based binary device (Jsc = 22.85 mA cm−2),
a comparable high Jsc of 22.83 mA cm−2 was yielded in the
PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) based ternary device,
which should result from the similar light-harvesting capa-
bility of the active layer (Figure 2b). Due to the significantly
decreased absorption capability of other ternary blends in the
wavelength of 550–800 nm (Figure 2b), the corresponding ter-
nary devices exhibited lower Jsc values than the control binary
device. Although IBC-F showed appropriate energy levels with
with a margin of less than 7%. The ternary device based on
PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) showed a nearly identical
EQE spectrum with the binary device based on PBDB-T:IE4F-S
(1:1, w/w). Similar to the trend observed in the UV–vis absorp-
tion spectra in Figure 2b, loading higher contents of 30–100%
of IBC-F into the ternary system gradually decreased the EQE
values of the devices, which is consistent with the decreased Jsc
values from J–V measurements. High EQE values greater than
80% in the wavelength region of 550–800 nm were obtained
for both devices based on the binary PBDB-T:IE4F-S blend and
the ternary PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) blend, while
the EQE values of the ternary PSCs with less or more than
20% IBC-F were lower than 80%. This result can be explained
by the lower absorption coefficient of the ternary PSCs with
IBC-F content other than 20% in the wavelength region of
5
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3
.
5
7
.
0
2
1
.
.
.
:
:
0
0
0
0
1
1
:
:
:
:
:
1
:
:
:
1
1
1
1
1
1
:
:
:
:
1
1
1
1
1
300
400
500
600
700
800
900
Wavelength (nm)
PBDB-T:IE4F-S:IBC-F
Figure 3. a) J–V curves of the ternary and binary PSCs under AM 1.5 G illumination at 100 mW cm−2. b) FF and PCE, and c) Voc and Jsc parameters for
the devices. d) EQE spectra of the PSCs based on PBDB-T:IE4F-S:IBC-F with different IBC-F weight ratios.
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550–800 nm (Figure 2b) in comparison with that of the binary
PBDB-T:IE4F-S (1:1, w/w) and ternary PBDB-T:IE4F-S:IBC-F
(1:1:0.2, w/w) blends.
in the Supporting Information) so that the corresponding ter-
nary PSC showed higher FF and higher PCE values.
The PL measurements were carried out to investigate the
energy transfer and exciton dissociation of the host binary
blend and ternary blends (as shown in Figure S7 in the
Supporting Information). Relative to the PBDB-T:IE4F-S
(1:1, w/w) host binary blend with a quenching efficiency of
89.6% (excited at 580 nm), a higher quenching efficiency of
90.9% was observed in the ternary blend with 20% IBC-F, sug-
gesting more efficient exciton dissociation in the PBDB-T:IE4F-
S:IBC-F (1:1:0.2, w/w) ternary blend. While further increment
of IBC-F contents led to lower quenching efficiency than the
host blend (Figure S7 in the Supporting Information). In com-
parison with the ternary devices with IBC-F contents other
than 20%, a better charge transport properties combining
with a higher absorption coefficient and higher PL quenching
efficiency lead to the a higher Jsc of the ternary device based
on PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w). We then plotted the
curves of Jph (Jph = JL − JD, where JL and JD represent the light
and dark current densities, respectively) versus the effective
voltage (Veff) of the devices based on PBDB-T:IE4F-S (1:1, w/w)
and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) to further understand
the influence of IBC-F on their exciton dissociation probability
(Pdiss) (Figure 4b). It was assumed that all generated electron–
hole pairs were dissociated into free charges and collected at the
electrodes when Jph reached saturation (Jsat) at a large reverse
bias (Veff = 2 V in this case), and Pdiss is determined by normal-
izing Jph with respect to Jsat (Jph/Jsat).[50] In comparison with the
control binary device with Pdiss of 95.8%, a higher Pdiss value of
Generally, the competition between charge transport/extrac-
tion and recombination determines the FF of a PSC device.
To understand the effect of incorporating IBC-F on improving
the FF of the ternary PSCs, the charge transport and extraction
properties of the blend films were investigated (as shown in
Figure S6 in the Supporting Information). Interestingly, both
high hole (µh) and electron (µe) mobilities of 1.34 × 10−4 and
2.43 × 10−4 cm2 V−1 s−1 were determined for the IBC-F pris-
tine film, respectively, which were measured by a space charge
limited current method (Figure S6 in the Supporting Informa-
tion). The host PBDB-T:IE4F-S blend exhibits high µh and µe
of 5.56 × 10−4 and of 5.98 × 10−4 cm2 V−1 s−1, respectively, with
a µh:µe ratio of 0.93 (Table S3 in the Supporting Information).
The introduction of 10% of IBC-F into the PBDB-T:IE4F-S host
blend decreased both the hole and electron mobilities of the
blend to 3.09 × 10−4 and 3.99 × 10−4 cm2 V−1 s−1, respectively.
Interestingly, a higher µh of 5.34 × 10−4 cm2 V−1 s−1 and µe of
5.09 × 10−4 cm2 V−1 s−1 was obtained when the IBC-F content
increased to 20%, and then the hole and electron mobilities
gradually decreased with further increase the IBC-F contents
from 30% to 100% (as shown in Figure 4a, Figure S6, and
Table S3 in the Supporting Information). Although the intro-
duction of IBC-F in the PBDB-T:IE4F-S blend has a negative
effect on improving the transport properties, the PBDB-T:IE4F-
S:IBC-F (1:1:0.2, w/w) ternary blend shows a more balanced
µh:µe ratio of 1.05 than the host blend (Figure 4a and Table S3
6
(a)
(b)
6
5
5
4
3
2
1
0
4
3
2
1
10
hole mobility
electron mobility
binary blend
ternary blend
P
diss = 95.8%
Pdiss = 98.7%
1
7
3
5
.
0
1
:
2
.
1
:
1
1
.
.
0
.
0
0
:
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0
:
0
:
:
:
1
:
1
:
:
1
1
1
:
1
1
1
:
:
0.01
0.1
1
:
:
1
:
1
1
1
1
1
V
eff (V)
PBDB-T:IE4F-S:IBC-F
0.90
0.88
0.86
0.84
(d)
(c)
10
0.82
0.80
0.78
binary blend α = 0.954
ternary blend α = 0.958
binary blend slope = 1.171 kT/q
ternary blend slope = 1.140 kT/q
1
-2
10
100
10
-2
100
Light Intensity (mW cm )
Light Intensity (mW cm )
Figure 4. a) Hole and electron mobilities of the PBDB-T:IE4F-S:IBC-F blend films with various IBC-F contents. b–d) Plots of Jph versus Veff (b), light-
intensity dependence of Jsc (c), and dependence of Voc on the light intensity (d) for the PSCs based on PBDB-T:IE4F-S (1:1, w/w) and PBDB-T:IE4F-
S:IBC-F (1:1:0.2, w/w).
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98.7% was obtained in the ternary PSC with 20% IBC-F. The
results suggest that the incorporation of 20% IBC-F in the host
blend facilitates efficient charge dissociation and extraction,
contributing to the enhanced FF value in the resulting device.
In addition to efficient charge dissociation and extraction, a
high FF in the PSCs should also originate from the fine-tuned
recombination dynamics in BHJ composites. Thus, we meas-
ured the dependence of Jsc and Voc as a function of the light
intensity (Plight) to obtain deeper insight into the charge recom-
bination kinetics in the binary and ternary devices. In PSCs,
the relationship between the Jsc and Plight (Jsc ∝ Pαlight) can be
applied to evaluate the degree of bimolecular recombination in
PSCs, where α is an exponential factor that should be equal to 1
if all charge carriers are swept out before recombination.[51] As
shown in Figure 4c, the α values were fitted to 0.954 and 0.958
for the devices based on PBDB-T:IE4F-S (1:1, w/w) and PBDB-
T:IE4F-S:IBC-F (1:1:0.2, w/w), respectively. The results indicate
very weak bimolecular recombination in the two devices and the
slight impact of 20% IBC-F on restricting bimolecular recom-
bination in the ternary system. The slope of the dependence
of Voc versus logarithmic of Plight (Voc ∝ lnPlight) can be fitted
to determine the degree of trap-assisted recombination in the
two devices, a slope of 1 × kT/q (k is Boltzmann’s constant, T is
temperature, and q is the elementary charge) implies the free-
carrier bimolecular recombination in the device, while a slope
of 2 × kT/q suggests that trap-assisted or Shockley–Read–Hall
recombination is the dominating mechanism.[52] Compared to
the slope of 1.171 × kT/q for the device based on PBDB-T:IE4F-S
(1:1, w/w), a smaller slope of 1.140 × kT/q was attained in the
ternary device based on PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w)
(Figure 4d). Moreover, the trap-assisted or Shockley–Read–Hall
recombination in the ternary blend is further suppressed with
increasing IBC-F content in the host blend, and the smallest
slope of 1.003 kT/q was obtained for the ternary device based
on PBDB-T:IE4F-S:IBC-F (1:1:1, w/w) (as shown in Table S4
and Figure S8 in the Supporting Information), indicating that
the trap-assisted recombination evolves to a free-carrier bimole-
cular. Indeed, bimolecular recombination is weak in all of the
ternary devices with an α greater than 0.92 (as shown in Table S4
and Figure S8 in the Supporting Information). We subse-
quently applied the transient photovoltage technique to deter-
mine the charge carrier lifetimes (τ) of the devices. As shown
in Figure S9 in the Supporting Information, the ternary device
with 20% IBC-F exhibited a longer τ of 0.878 µs than the binary
device based on PBDB-T:IE4F-S (1:1, w/w) (τ = 0.868 µs).
Higher τ of 1.052 and 1.514 µs was observed in the ternary
blend with 50% and 100% IBC-F, respectively, indicating the
incorporation of IBC-F in the PBDB-T:IE4F-S blend has posi-
tive effect in enhancing its charge carrier lifetime, which is ben-
eficial for obtaining higher FF values of the resulting devices.
Thus, the synergetic effect of IBC-F on weakening the bimo-
lecular and trap-assisted recombination and enhancing the car-
rier lifetimes of PBDB-T:IE4F-S blend results in the higher FF
of the ternary devices.
The ordered face-on molecular packing orientation and
favorable well-defined fibrillar D/A network are critical to
obtain a high FF in PSCs. We performed 2D grazing incidence
wide-angle X-ray scattering (2D-GIWAXS) measurements to
investigate the influence of IBC-F on the molecular packing
properties of the ternary blend. As shown in Figure S10 in
Supporting Information, both neat PBDB-T and IBC-F films
exhibit a wide (010) diffraction halo, suggesting mixed orien-
tations in the solid film for both materials. In contrast, IE4F-S
exhibits dominant face-on molecular orientations, which is
beneficial for efficient charge transport in PSCs.[53,54] The
pristine PBDB-T film showed a π–π stacking (010) peak and
laminar packing diffraction (100) peak in the out-of-plane
(OOP) direction at 1.61 and 0.34 Å−1, with crystal coherence
length (CCL) estimated to be 0.84 and 5.50 nm, respectively. A
stronger (010) signal at 1.78 Å−1 (CCL = 1.13 nm) and lamellar
signal at 0.47 Å−1 (CCL = 6.90 nm) were observed in the OOP
direction of the pristine IE4F-S film. For the pristine IBC-F
film, the face-on π–π stacking is indicated by the scattering of
the (010) signal at 1.50 Å−1 in the OOP direction, with CCL of
0.86 nm. The PBDB-T:IE4F-S (1:1, w/w) binary blend showed
a lamellar stacking (100) peak both in the in-plane direc-
tion at 0.29 Å−1 (IP, CCL = 6.53 nm) and OOP direction at
0.33 Å−1 (CCL = 6.02 nm), along with preferable (010) peaks
at 1.77 Å−1 in OOP (CCL = 0.90 nm) (as shown in Figure 5).
Interestingly, the ternary blends show multiple diffraction
peaks, indicating improved ordering of these materials. As
shown in Figure 5, the CCL values of the lamellar stacking
(100) peaks increased from 6.02 to 10.53 nm for the ternary
blend when incorporating 20% IBC-F in the binary host
blend. Meanwhile, a higher CCL of 0.97 nm was also observed
in the face-on π–π stacking (010) peak in OOP direction for
Figure 5. a,b) 2D GIWAX patterns of PBDB-T:IE4F-S (1:1, w/w) (a) and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) (b). c) 1D profiles of blend films based on
PBDB-T:IE4F-S (1:1, w/w) and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w).
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the PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) compared to the
control binary blend (CCL = 0.90). Therefore, the incorpora-
tion of 20% IBC-F could effectively enhance the molecular
interactions to facilitate preferential face-on orientation in
the blend film, resulting in improved charge transport in the
ternary PSC.
higher IBC-F contents (Figure S11 in the Supporting Informa-
tion), which should correspond to the high FF values in the ter-
nary PSCs with high IBC-F contents.
The morphology stability of the BHJ layer is a critical factor
that significantly impacts the thermal and photoinduced sta-
bility of PSCs. Encouraged by the enhanced π–π stacking
coherence length and intermolecular interaction in the ter-
nary blend with 20% IBC-F, we further evaluated the influ-
ence of IBC-F in the device stability. As shown in Figure 7a,
a steep “burn-in” degradation, which only retains 70% of its
initial PCE, was observed in the unencapsulated binary device
based on PBDB-T:IE4F-S (1:1, w/w) under continuous thermal
aging at 60 °C for 8.5 h in a nitrogen-filled glovebox. Then, the
device performance recovers to 82% of its initial value after
49 h thermal aging. The “burn-in” degradation is significantly
restricted in the ternary device with 20% IBC-F, in which PCE
can remain at 91.9% under 8.5 h thermal aging, and then
recover to 95.4% after 25 h thermal aging. Impressively, the ter-
nary device retains a higher initial PCE of 81% compared to the
host binary device (71.7% of its initial PCE) after 1000 h contin-
uous thermal aging at 60 °C, which is ascribed to the lower FF
loss by incorporating 20% IBC-F (Figure S12 in the Supporting
Information). In addition to improving the thermal stability, the
incorporation of 20% IBC-F in the host blend is beneficial to
promoting the photoinduced stability. As depicted in Figure 7b,
the PCE of the binary device based on PBDB-T:IE4F-S
(1:1, w/w) decays to 73.6% of its initial, whereas the ternary
device with 20% IBC-F remained at 85.5% of its initial PCE
after 1000 h continuous illumination, resulting from the lower
Atomic force microscopy (AFM) and transmission electron
microscopy (TEM) measurements were carried out to study the
impact of IBC-F on the morphology of the ternary active layers.
As shown in Figure 6a,b, the PBDB-T:IE4F-S (1:1, w/w) and
PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) blends exhibited similar
and rather smooth surfaces with a root-mean-square roughness
(Rq) of 2.50 and 2.41 nm, respectively, implying good misci-
bility of 20% IBC-F with the PBDB-T:IE4F-S blend. However,
further increasing the IBC-F ratios in the blend gradually leads
to larger Rq values (as shown in Figure S11 in the Supporting
Information). Figure 6c,d shows TEM images of the blend
films of PBDB-T:IE4F-S (1:1, w/w) and PBDB-T:IE4F-S:IBC-F
(1:1:0.2, w/w). Due to the different electron densities between
donor and acceptor materials, the bright or dark region in the
TEM image represents the donor-rich or acceptor-rich domains,
respectively. Note that the PBDB-T:IE4F-S (1:1, w/w) blend
shows a well-developed fibrillar structure (Figure 6c). The ter-
nary blend with 20% IBC-F shows more uniform bright and
dark domains along with more evident fibrillar textures than
the binary blend, implying the positive impact of 20% IBC-F on
facilitating better morphology for obtaining a higher FF value.
Clear phase separation with more obvious fibrillar textures but
larger domain sizes were observed in the ternary films with
Figure 6. a,b) AFM topography images of the PBDB-T:IE4F-S (1:1, w/w) (a) and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) (b) blend films. c,d) TEM images
of the PBDB-T:IE4F-S (1:1, w/w) (c) and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) (d) blend films.
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1.0
1.0
0.9
0.8
0.7
0.6
0.5
Ternary device
Ternary device
0.9
0.8
0.7
0.6
0.5
85.5%
73.6%
Binary device
81.0%
71.7%
Binary device
Thermal aging at 60 °C
Illumination aging
(b)
(a)
0
100 200 300 400 500 600 700 800 900 1000 1100
Time (h)
0
100 200 300 400 500 600 700 800 900 1000 1100
Time (h)
Figure 7. a,b) Normalized PCEs of the PBDB-T:IE4F-S (1:1, w/w) based binary device and PBDB-T:IE4F-S:IBC-F (1:1:0.2, w/w) based ternary device over
the 60 °C thermal aging time (a) and illumination aging time (b) in a nitrogen-filled glovebox.
FF loss (Figure S12, Supporting Information). These results
indicate the positive effect of IBC-F on improving the thermal
and photoinduced stability of the ternary device.
Conflict of Interest
The authors declare no conflict of interest.
In conclusion, we designed and synthesized a new A–π–
D–π–A structured organic semiconductor molecule IBC-F
Keywords
containing the same IDT central fused ring D-unit with the
acceptor material IE4F-S and bifluorobenzene π-bridge which
enhances its ability to tune the morphology of the ternary
active layer of the ternary PSCs. Even though the PBDB-T:IBC-
F-based device exhibited extremely poor performance, IBC-F
plays important role in improving the charge dissociation and
extraction, suppressing bimolecular and trap-assisted recom-
bination, increasing charge carriers lifetime, and enhancing
intermolecular interactions with preferential face-on orienta-
tion, for the improved PCE of the ternary PSCs. Thus, a PCE
as high as 15.06% was achieved in the ternary device with
20% IBC-F, which is 10% greater than that of the control device
based on PBDB-T:IE4F-S (PCE = 13.70%). More importantly,
the ternary device with 20% IBC-F shows superior thermal and
photoinduced stability over the control binary device. These
results suggest that despite the extremely poor photovoltaic per-
formance of IBC-F, the addition of IBC-F third component into
the PBDB-T:IE4F-S blend not only improves the photovoltaic
performance but also has a synergistic benefit on both the
thermal aging and photoinduced stability, which demonstrates
a significant advantage for developing highly efficient and
stable PSCs for practical applications.
fill factor, power conversion efficiency, stability, ternary polymer solar
cells
Received: July 17, 2019
Revised: October 6, 2019
Published online:
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