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Organic solar cells (OSCs) are one of the most promising cost‐effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long‐term stability remains challenging, particularly if the material choice is restricted by roll‐to‐roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT‐M) that broke the 10% benchmark when blade‐coated in air, a second donor material (PBDB‐T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small‐molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs. A highly efficient, stable, and ductile nonfullerene ternary organic solar cell by integrating two polymer donors and one acceptor is achieved. The enhanced performance and stability are mainly attributed to the suppressed crystallization of the nonfullerene acceptor by introducing a stiff donor that shows low miscibility with the acceptor and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer.
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Highly Efficient, Stable, and Ductile Ternary Nonfullerene
Organic Solar Cells from a Two-Donor Polymer Blend
Huawei Hu, Long Ye, Masoud Ghasemi, Nrup Balar, Jeromy James Rech,
Samuel J. Stuard, Wei You, Brendan T. O’Connor, and Harald Ade*
Dr. H. Hu, Dr. L. Ye, Dr. M. Ghasemi, S. J. Stuard, Prof. H. Ade
Department of Physics and ORganic and Carbon
Electronics Labs (ORaCEL)
North Carolina State University
Raleigh, NC 27695, USA
E-mail: hwade@ncsu.edu
N. Balar, Prof. B. T. O’Connor
Department of Mechanical and Aerospace Engineering and ORaCEL
North Carolina State University
Raleigh, NC 27695, USA
J. J. Rech, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201808279.
DOI: 10.1002/adma.201808279
one donor and one acceptor as the photo-
active layer, have achieved power conver-
sion efficiencies (PCEs) over 13% in many
systems due to the rapid development
of high-performance organic materials,
practically through the evolution of novel
nonfullerene small molecular acceptors
(SMAs) and matched polymer donors.[4–6]
However, the use of binary OSCs often
limits the range of light absorption and
involves a complex phase separation
dynamics that impacts device processing
windows[4,7,8] and device stability.[9–11]
To maximize the short-circuit current
densities (JSC) and overall device efficien-
cies, ternary OSCs have been developed.
Ternary devices incorporate multiple mate-
rials similar to tandem solar cells, but with
a single active layer similar to single junc-
tion OSCs, simplifying fabrication and
relaxing the current matching constraint of
tandem cells.[12–15] Recently, nonfullerene
ternary OSCs based on two SMAs as the
electron acceptors have received consider-
able interest owing to the strong absorp-
tivity for wavelengths over 800 nm,[16] as
well as the excellent compatibility of the two SMAs due to their
very similar chemical structures.[17,18] By contrast, ternary OSCs
consisting of two donor polymers usually result in relatively
low performance because even small repulsive intermolecular
interaction between the two polymer donors can lead to strong
phase separation due to the limited entropy of polymers.[19–21]
The morphology of nonfullerene SMAs can also be very sen-
sitive to the choice of donor polymer.[22,23] Furthermore, most
OSC studies focus primarily on the device performance and
ignore the mechanical durability, which is an important consid-
eration for OSC commercialization.
After considerable progress has been made on the develop-
ment of high-performance nonfullerene OSCs, improvement
in stability is of vital importance to guarantee a long operational
lifetime.[10,24,25] The lifetime of an OSC is governed by the
choice of photoactive layer, and can be limited by several pos-
sible origins, including exposure to humidity,[26] photooxidation
of the BHJ layer and morphological instability due to spinodal
demixing[9] or materials aggregation/crystallization under
thermal stress,[27] and mechanical failure.[28] Most stability
investigations to date have been focusing on fullerene-based
OSCs,[10] and given the distinct difference between nonfullerene
Organic solar cells (OSCs) are one of the most promising cost-effective
options for utilizing solar energy, and, while the field of OSCs has progressed
rapidly in device performance in the past few years, the stability of
nonfullerene OSCs has received less attention. Developing devices with both
high performance and long-term stability remains challenging, particularly if
the material choice is restricted by roll-to-roll and benign solvent processing
requirements and desirable mechanical durability. Building upon the ink
(toluene:FTAZ:IT-M) that broke the 10% benchmark when blade-coated in
air, a second donor material (PBDB-T) is introduced to stabilize and enhance
performance with power conversion efficiency over 13% while keeping
toluene as the solvent. More importantly, the ternary OSCs exhibit excellent
thermal stability and storage stability while retaining high ductility. The
excellent performance and stability are mainly attributed to the inhibition of the
crystallization of nonfullerene small-molecular acceptors (SMAs) by introducing
a stiff donor that also shows low miscibility with the nonfullerene SMA and
a slightly higher highest occupied molecular orbital (HOMO) than the host
polymer. The study indicates that improved stability and performance can be
achieved in a synergistic way without significant embrittlement, which will
accelerate the future development and application of nonfullerene OSCs.
Organic Solar Cells
Organic solar cells (OSCs) have attracted considerable attention
as a future green technology to utilize solar energy due to their
potential for large area fabrication on flexible substrates with
low cost and environmentally friendly solution manufactura-
bility.[1–3] Typical bulk heterojunction (BHJ) OSCs, consisting of
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and fullerene acceptors, the morphological stability of the non-
fullerene SMA OCSs cannot be directly generalized from data
on fullerene devices. In addition, simultaneous achievement of
both high device performance and robust morphology stability
remains a big challenge due to the difficulties in maintaining
an optimal blend morphology under continuous operation
even in harsh conditions.[10,29] Another important concern for
future technology translations of OSCs is to utilize environ-
mentally friendly solvent processing;[30,31] however, most high
performance ternary OSCs are still processed from halogen-
ated solvents[13,32] and achieving high-performance devices
using nonhalogenated solvent systems remains a big chal-
lenge. Therefore, improving performance and stability at the
same time is particularly constrained if the material choice
is restricted by fabrication parameters such as roll-to-roll and
benign solvent requirements. Such improvements are further
constraint by needs to achieve and retain favorable mechanical
properties such as high ductility, which is emerging as an
important parameter.[33,34]
To address interconnected challenges relating to performance,
morphological stablity, and mechanical stability, we report
a highly efficient ternary OSC based on a combination of a
nonfullerene acceptor (IT-M, the chemical structure is shown
in Figure 1a) and two synergistic polymer donors (FTAZ and
PBDB-T, also shown in Figure 1a) with complementary absorp-
tion, slightly different highest occupied molecular orbital
(HOMO) energy levels, and differences in ductility. The
choice was motivated by FTAZ:IT-M:toluene being the ink
that broke the 10% benchmark when blade-coating in air from
a single nonhalogenated solvent.[31] PBDB-T was chosen as a
possibly stabilizing and performance enhancing donor due to
its ability to achieve high performance and excellent thermal
stability in binary devices with a number of nonfullerene
SMAs[35,36] and its smaller bandgap and slightly higher HOMO
than FTAZ. The latter characteristics might enhance fill factor
(FF) due to enhanced hole transport in high purity “high-
ways.”[37] The best efficiency of >13% was achieved in ter-
nary OSCs with 20 wt% PBDB-T, processed from toluene (a
halogen-free solvent) without additives and yielding an open
circuit voltage (VOC) of 0.95 V, a JSC of 18.1 mA cm2, and a
FF of 73.6%. This PCE exceeds that of the corresponding tol-
uene-cast binary OSCs and is among the highest values for
any ternary OSCs, including those cast from halogenated sol-
vents.[15,20,37] The enhancement in PCE is mainly contributed
by the increased device FF, which is attributed to reduced
charge recombination and improved charge mobilities due to
a favorable morphology and electronic landscape. Furthermore,
we demonstrate that this nonfullerene ternary OSC system
exhibits improved thermal stability and storage stability as well
as favorable mechanical properties.
We first investigate the absorption properties of FTAZ,
PBDB-T, and IT-M. Normalized ultraviolet–visible (UV–vis)
absorption spectra and their corresponding absorbance and
refractive index of FTAZ, PBDB-T, and IT-M neat films are
shown in Figure 1b and Figure S1 (Supporting Information),
respectively. Both donor polymers have complementary absorp-
tion with IT-M, and while there is some absorption spectral
overlap between FTAZ and PBDB-T, the absorption peak of
PBDB-T locates between that of FTAZ and IT-M. The absorp-
tion spectra of ternary and binary blend films composed of these
materials are shown in Figure S1 (Supporting Information), sug-
gesting a broad light absorption in the range of 400–800 nm.
Adv. Mater. 2019, 1808279
Figure 1. a) Schematic diagram of device structure and chemical structures of FTAZ, PBDB-T, and IT-M. b) Normalized UV–Vis absorption spectra
and c) energy diagrams of the three materials. d) JV characteristics of OSCs with different PBDB-T contents.
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It is also noted that light harvesting from 600 to 660 nm can
be enhanced with the increase of PBDB-T content, which can
be well understood by the complementary absorption of neat
PBDB-T film.[38,39] Besides the optimal light absorption of the
ternary films, the VOC of the ternary device should be slightly
lower than that of the binary devices based on FTAZ:IT-M
because of the slightly higher HOMO level of PBDB-T com-
pared to FTAZ (Figure 1c).[31,40] The optimal amount of
PBDB-T to add to the system should be based on balanced light
harvesting, blend morphology, charge transport, and charge
collection.
To obtain the optimized amount of PBDB-T in the ternary
blend, nonfullerene OSCs were fabricated in an inverted device
architecture (Figure 1a). According to our prior benchmark,
the optimized processing condition for the FTAZ:IT-M based
binary blade-coated solar cells cast without a solvent additive is
the use of toluene with a donor/acceptor weight ratio of 1:1.[31]
This condition was kept the same for all the binary and ternary
combinations for a fair comparison and because of our goal was
to improve the benchmark FTAZ:IT-M:toluene ink. That said,
PBDB-T based binary devices can be improved with the use
of additives when cast from nonhalogenated solvents.[41] The
average device parameters from at least 10 devices are listed in
Table 1 and the current density–voltage (JV) characteristics of
the devices are shown in Figure 1d. The corresponding external
quantum efficiency (EQE) spectra are shown in Figure S1d
(Supporting Information). The binary nonfullerene OSC based
on FTAZ:IT-M exhibits a PCE of 11.5% (11.8% max) with
a VOC of 0.96 V, a JSC of 17.2 mA cm2, and an FF of 67.9%.
On the other hand, the binary OSC based on PBDB-T:IT-M
shows a similar VOC of 0.94 V, but a comparatively lower PCE
of 7.2% due to its relatively lower JSC and FF of 13.5 mA cm2
and 52.8%, respectively. The performance of the toluene-cast
PBDB-T:IT-M binary films is comparable with the reported
7.9% efficiency when cast without additives from a halogen free
solvent.[41] As summarized in Table 1, all the ternary solar cells
show comparable VOC values (between 0.94 and 0.96 V), but
the FF is substantially increased with increasing the PBDB-T
content and reaches 73.6% in the device with 20 wt% PBDB-T.
However, further increasing the PBDB-T weight ratio leads to a
decrease in the JSC values and consequently low PCEs. In these
ternary OSCs, FTAZ:PBDB-T:IT-M (0.8:0.2:1, weight ratio)
exhibits the champion PCE of 13.2%, with a VOC of 0.95 V, a
JSC of 18.1 mA cm2, and an FF of 73.6%, which is significantly
higher than the corresponding two binary OSCs. The calculated
EQE values of the devices from Figure S1d (Supporting Infor-
mation) are in good agreement with the JSC values obtained
from the JV measurement.
To understand the impact of incorporating PBDB-T on the
BHJ morphology, grazing incidence wide angle X-ray scattering
(GIWAXS)[42] was performed to reveal the molecular packing
and texture of the active layers. As observed from the GIWAXS
profiles in Figure 2 and Figure S2 in the Supporting Information,
these blend films exhibit (010) peaks at qz = 1.75 and 1.62 Å1
in the out-of-plane direction for IT-M and FTAZ (Figure S3,
Supporting Information), respectively. The corresponding out-
of-plane (010)
π
π
coherence lengths (CL) of FTAZ and IT-M of
these blend films were extracted via peak fitting by using the full
width at half-maximum of the (010) stacking peaks. Figure 2e
shows that both CLs increase for these blends with less than
20 wt% of PBDB-T and then decrease by adding 30 wt% of
PBDB-T. Moreover, a peak at qz = 0.5 Å1 (Figure 2b), corre-
sponding to the (100) lamellar stacking of IT-M, was observed in
FTAZ:PBDB-T:IT-M (0.8:0.2:1) ternary blend film, which indi-
cates strong molecular packing of IT-M in FTAZ:PBDB-T:IT-M
(0.8:0.2:1) blend film. The strong molecular stacking is benefi-
cial for charge transport in FTAZ:PBDB-T:IT-M (0.8:0.2:1) based
solar cells,[43,44] which partially explains the highest FF achieved
in the device. It is also noted that a peak at qxy = 0.62 Å1
(ascribed to the (001) peak of PBDB-T) is observed when PBDB-T
is added into the active layer. It is thus inferred that PBDB-T
possesses repulsive interactions and low miscibility with FTAZ
and that these interactions influence the observed CL of FTAZ
and IT-M in the ternary blends as a function of PBDB-T con-
tent. The charge mobilities, measured by space-charge-limited
current (Table S1 and Figure S4, Supporting Information), are
consistent with the GIWAXS results as the highest electron
and hole mobilities of 4.62 × 104 and 3.95 × 104 cm2 V1 s1,
respectively, are observed in FTAZ:PBDT-T:IT-M (0.8:0.2:1),
which also corresponds to the highest FF for the corresponding
ternary solar cells.
Resonant soft X-ray scattering (R-SoXS)[45] is employed
to probe the composition correlation characteristics related
to domain spacing and purity of these nonfullerene devices.
Figure 2f illustrates the Lorentz corrected and thickness
normalized circular R-SoXS profiles of these BHJ blends.
Under the assumption of a globally isotropic 3D morphology in
which the small molecule domains are essentially pure due to
Adv. Mater. 2019, 1808279
Table 1. Photovoltaic parameters, standard deviation of the IT-M concentration, and long period of the nonfullerene OSCs with different PBDB-T
content.
PBDB-T [donor wt%] JSC [mA cm2]VOC [V] FF [%] PCEa) [%] Relative
σ
b) [±0.01]
σ
2c) (high-q peak) Long period [nm]
Low-qHigh-q
017.2 ± 0.6 0.96 ± 0.01 67.9 ± 0.7 11.5 ± 0.3 (11.8) 0.88 0.16 25.1 19.0
10 18.2 ± 0.6 0.95 ± 0.01 70.0 ± 0.6 12.1 ± 0.3 (12.5) 0.95 0.24 28.9 19.6
20 18.1 ± 0.7 0.95 ± 0.01 73.6 ± 0.5 12.7 ± 0.4 (13.2) 0.97 0.40 31.4 19.6
30 17.5 ± 0.5 0.94 ± 0.01 72.5 ± 0.8 11.9 ± 0.3 (12.3) 1 0.28 34.9 20.9
100 13.5 ± 0.5 0.94 ± 0.01 52.8 ± 0.7 6.7 ± 0.4 (7.2) 0.65 0.20 41.9 20.3
a)The best device efficiencies are provided in the parentheses; b)The area of the R-SoXS profile over the q range probed is the integrated scattering intensity (ISI), with
ISI1/2 being proportional to the standard deviation,
σ
, of the IT-M concentration, which has been normalized relative to the highest value; c)Relative
σ
for the high q peak.
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the low molecular weight,[4,8,46] the root-mean-square standard
deviation,
σ
, of the IT-M concentration is proportional to the
square root of the normalized integrated scattering intensity
(ISI) and relates monotonically to the average IT-M concentra-
tion in the polymer rich domains.[4] The relative IT-M standard
deviations quantified from R-SoXS measurement are listed in
Table 1, where ternary blends based on FTAZ:PBDB-T:IT-M
(0.7:0.3:1) and FTAZ:PBDB-T:IT-M (0.8:0.2:1) exhibit relatively
higher
σ
of 1 and 0.97, respectively. The higher
σ
is critical in
helping to suppress bimolecular recombination as well as pro-
moting charge collection, leading to improved device FF as long
as the IT-M concentration is above the percolation limit.[4,47]
As the FF does not completely correlate to
σ
derived from all
length scales probed, we fit the R-SoXS scattering profiles with
two log-normal peaks (Figure S4, Supporting Information). The
low-q peaks for the ternary blends slightly shift toward lower q
(from 0.25 to 0.18 nm1) with the increased content of PBDB-T,
indicating that a relatively larger long period is obtained with
higher content of PBDB-T. By contrast, the high-q peaks of
FTAZ-based films are located at fixed q at 0.32 nm1 for binary
and ternary blends, corresponding to a long period of the
domains of 20 nm, which matches well with the typical exciton
diffusion length. When considering the domain purities by
analyzing the scattering intensity and standard deviation of the
composition, the FTAZ:PBDB-T:IT-M (0.8:0.2:1) blend exhibits
the largest IT-M standard deviation at the smallest length scales
(see Table 1), which agrees well with previous results that estab-
lished the importance of achieving a high
σ
(i.e., high purity) at
length scales corresponding to the exciton diffusion length.[48,49]
Interpreting the R-SoXS further is complex, as is the case with
all ternary systems. Assumptions about kinetic and thermo-
dynamic factors have to be made frequently. We will return to
this topic below after we acquire thermodynamic inferences
about miscibility. Furthermore, the influence of adding PBDB-T
on charge recombination was investigated with light inten-
sity dependent JSC measurement as function of bias voltage
(Figure S5, Supporting Information). The smaller scaling expo-
nent (
α
) for all bias conditions and the dramatic drop of it near
VOC suggests stronger bimolecular recombination in FTAZ:IT-
M binary devices compared to the optimized ternary.
Overall, the molecular packing derived from GIWAXS and
the domain purity data from R-SoXS explain the increase in FF
due to increased extraction from higher mobility and the reduc-
tion of bimolecular recombination by reducing the number of
dispersed D/A sites. The improvement in FF, combined with
the information about HUMO–LUMO levels and the VOC,
implies that the holes get trapped in PBDB-T domains that
result in reduced bimolecular recombination due to a lower
IT-M concentration relative to the FTAZ domains. This scenario
is also consistent with an interpretation of the R-SoXS resulting
Adv. Mater. 2019, 1808279
Figure 2. a–c) 2D GIWAXS patterns and d) 1D profiles of films based on FTAZ:IT-M (1:1), FTAZ:PBDB-T:IT-M (0.8:0.2:1), and PBDB-T:IT-M (1:1),
respectively. e) Out-of-plane
π
π
coherence lengths of FTAZ and IT-M. f) Thickness and Lorentz-corrected R-SoXS profiles of blend films.
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that the PBDB-T domains are purer than the FTAZ domains in
concentrations up to 20% PDBD-T. To confirm that higher IT-M
purity in PBDB-T is a thermodynamically favored morphology,
we employ differential scanning calorimetry (DSC) measure-
ment to investigate the qualitative miscibility between IT-M
and the two polymer donors. As shown in the DSC curves in
Figure S6 (Supporting Information), IT-M does not show any
melting peak (due to sublimation) but exhibits pronounced exo-
thermal peaks in the first heating cycle corresponding to cold-
crystallization of the vitrified, amorphous volume fraction of
the material.[50] Compared with the pristine IT-M, it is clear that
the cold crystallization peak of FTAZ:IT-M blends is broadened
and substantially more suppressed than that of PBDB-T:IT-
M blends. This indicates lower miscibility of IT-M in PBDB-T
than IT-M in FTAZ.[4] The increase in the standard deviation of
the IT-M distribution and domain spacing upon the addition of
PBDB-T into FTAZ:IT-M is consistent with the lower miscibility
between PBDB-T and IT-M.[8] The stronger repulsive interac-
tion with IT-M for PBDB-T leads to stronger liquid–liquid phase
separation and larger domains at the larger length-scale.[7,49]
The stronger repulsive interaction for PBDB-T also likely
leads to purer domains relative to the FTAZ, irrespective of
whether the devices overall are likely not processed to equilib-
rium but quenched.[7,9,51] We note that the devices can reach
local equilibrium most readily at the small length scale, which
is where we observe the largest IT-M standard deviation and
thus average domain purity. As a result, the mechanism for
improved device performance is likely due to preferential trans-
port of the hole polarons in the more pure PBDB-T network
that acts like a highway as previously inferred in a fullerene
based model system.[37] Here, we explicitly delineate the ther-
modynamic drivers that cause such a favorable morphology.
Having been able to improve the photovoltaic performance
with the addition of PBDB-T motivated us to investigate the rela-
tive device stability of our binary and ternary OSCs. We compared
the thermal stabilities of FTAZ:IT-M (1:1) and FTAZ:PBDB-
T:IT-M (0.8:0.2:1) and PBDB-T:IT-M (1:1) based devices. After
thermal stress at 180 °C for 10 min, the FTAZ:IT-M binary device
only gives a PCE of 6.2% (Figure 3a; Table S2, Supporting Infor-
mation), which shows a dramatic efficiency loss of 46% under
this thermal stress. The PCE loss is a result of decreases in all
three photovoltaic parameters, indicating a distinct phase organi-
zation at such a high annealing temperature.[28] This is evidenced
by the morphology changes, GIWAXS patterns clearly exhibit
multiple peaks in FTAZ:IT-M blend film after thermal annealing
(Figure 3c), indicating the crystallization of IT-M in the blend
films upon thermally annealed at 180 °C. To support the conclu-
sion that IT-M can crystallize, optical microscopy images exhibit
Adv. Mater. 2019, 1808279
Figure 3. Device stability of nonfullerene OSCs. a) JV characteristics of devices based on FTAZ:IT-M (1:1) and FTAZ:PBDB-T:IT-M (0.8:0.2:1) thermally
annealed (TA) at elevated temperatures for 10 min. b) Normalized PCE of devices based on FTAZ:IT-M (1:1) and FTAZ:PBDB-T:IT-M (0.8:0.2:1)
(annealed at 150 °C for 10 min) as a function of storage time in the nitrogen under dark. c,d) 2D GIWAXS patterns of FTAZ:IT-M (1:1) and FTAZ:PBDB-T:IT-M
(0.8:0.2:1) based blend films annealed at 180 °C.
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micrometer-sized crystals after thermal annealing at 180 °C for
FTAZ:IT-M based blend films (see Figure S7, Supporting Informa-
tion). This is very similar to many fullerene-based OSCs, where
thermally induced fullerene aggregation or crystallization has been
identified as the key mechanism for the PCE loss.[11,52] By con-
trast, the material crystallization behavior has been significantly
suppressed for the corresponding ternary blend film with 20 wt%
PBDB-T even after thermal annealing at 180 °C (Figure 3d). In
addition, the R-SoXS profiles of the FTAZ:IT-M films (Figure S8,
Supporting Information) indicate that the IT-M standard deviation
decreased from 0.88 of 150 °C annealed films to 0.78 for 180 °C
annealed ones, and the low-q peak shift to 0.02 nm1 (Figure S9,
Supporting Information), which should be attributed to reduced
IT-M concentration in mixed domains due to the extra chemical
potential of the IT-M crystals and larger domains size due to coars-
ening, respectively. The incorporation of PBDB-T prevented such
a significant morphological change under higher thermal stress
(the IT-M standard deviation decreases only from 0.97 to 0.92 and
the long period of low-q and high-q increases marginally from 31.4
and 19.6 nm to 34.9 and 20.9 nm, respectively). As a result, we can
still get 86% of the efficiency achieved from the reference devices
annealed at 150 °C (see Figure 3a). The stability is comparable to
that of binary PBDB-T:IT-M, where the devices annealed at 180 °C
get 90% of the efficiency achieved from 150 °C annealed OSCs
(Table S2, Supporting Information). We note that this direct com-
parison of the binary shows that the binary PBDB-T:IT-M devices
are more stable than binary FTAZ:IT-M device. We then compared
the shelf-life stability of these FTAZ-based OSCs. Normalized PCE
of the devices as a function of storage time is shown in Figure 3b.
FTAZ:IT-M binary solar cells attained 80% of the initial PCE after
a storage time of 1000 h in a glovebox (Table S3 and Figure S10,
Supporting Information). Ternary devices with 20 wt% PBDB-T
still get over 90% of the initial efficiency (PCE = 12.7%) and are
10% higher than the beginning binary efficiency of 11.5%. Fur-
thermore, we note that visual extrapolation of the stability data
indicates that the ternary device has reached stable performance
at 400 h, whereas the binary likely continues to decline beyond
1000 h. Overall, these results clearly indicate that solar cells based
on the FTAZ:PBDB-T:IT-M (0.8:0.2:1) ternary not only exhibit supe-
rior shelf-life stability but also show substantially improved resist-
ance to thermal stress compared to FTAZ:IT-M binary devices.
In order to elucidate the reason for the improved stability
of the ternary compared to the FTAZ:IT-M binary and because
mechanical properties also play a crucial role in roll-to-roll
fabrication and are important for eventual OSC technological
deployment, we further measured the crack onset strain
(COS) of the two binaries and the optimized ternary blends
(Figure S11, Supporting Information). Here, COS captures film
ductility, and we use it as a simple screening tool for mechanical
behavior. COS has been shown to correlate well with cohesion
and toughness in organic semiconductor neat and blend
films,[53–55] all of which are important parameters for mechan-
ical reliability.[34] FTAZ:IT-M (1:1) and FTAZ:PBDB-T:IT-M
(0.8:0.2:1) based blend films exhibit comparable good ductility
with the COS of 33 ± 5% and 29 ± 5%, respectively. By contrast,
the PBDB-T:IT-M (1:1) binary blend film exhibits a low crack
onset strain of only 10 ± 3%. It should be noted that the ductility
is dependent on the molecular weight of the polymers as well
as its molecular structure. The molecular weight of PBDB-T
(Mn 30 kDa) is lower than that of FTAZ (Mn 60 kDa), and this
may contribute to PBDB-T being more brittle. In our compara-
tive study, that dependence on molecular weight is not impor-
tant, only the existence of an absolute difference and its exploi-
tation matter. Compared with the COS of binary blend based
on host FTAZ polymer, the ductility of ternary blend was not
affected much with 20 wt% PBDB-T. We also note that these
values are for films on UV/ozone-treated poly(dimethylsiloxane)
(PDMS) substrates, which are used to increase and magnify the
differences in measured COS. Nevertheless, the film COS of
the ternary systems are comparable to reports of all-polymer
systems, which are often lauded for their ductility.[34,53,54] The
PBDB-T likely leads to lower the diffusion of IT-M due to the
more rigid nature of the polymer chains that is implied by its
lower ductility.[34,52,56] This then suppresses the ability of the
IT-M to crystallize as observed in GIWAXS (Figure 3). The
revealed mechanical property and stability relation are rather
illuminating. This relation is significant because it indicates
that there is a possible general engineering constraint in the
development and use of OSCs: For binary OSCs, the more
ductile and stretchable active layers are likely more unstable
than the more brittle active layers, a conclusion that warrants
additional detailed studies. Reaching high performance in both
parameters in binary devices will be very challenging, but the
ternary strategy presented here indicates that improved stability
and performance can be achieved in a synergistic way without
much impact on ductility.
Finally, we explored the general scope of the ternary stabili-
zation strategy using other nonfullerene SMAs. Nonfullerene
ternary solar devices based on a structurally similar acceptor
ITIC (structure shown in Figure S12a, Supporting Information)
and FTAZ with 20 wt% PBDB-T were fabricated. A higher
PCE of 11.0% (Figure S12b, Supporting Information) of the
ternary device than the PCE (10.4%) of binary device based on
FTAZ:ITIC was achieved. Furthermore, ternary OSCs achieved
a PCE of 8.8% after being thermally annealed at 180 °C for
10 min, while the corresponding FTAZ:ITIC devices only
get a PCE of 5.8% (Figure S12b and Table S4, Supporting
Information). In addition, another well-known nonfullerene
SMA (EH-IDTBR, structure shown in Figure S12a, Supporting
Information) was also used to test the applicable scope of the
ternary system. Ternary solar cell (annealed at 80 °C for 10 min)
based on this film (10.0%, Figure S12c and Table S4, Sup-
porting Information) showed slightly improved performances
to the control devices based on FTAZ:EH-IDTBR (9.8%) blend
films. Importantly, the PCE of the binary FTAZ:EH-IDTBR
drops to 8.0% and 4.0% under higher annealing tempera-
tures at 120 and 140 °C, respectively, while the corresponding
ternary solar cells can still obtain promising PCE of 10.0% and
8.8%, respectively. The encouraging results clearly support the
wide-ranging applicability to achieve a stable OSC by incorpo-
rating an incompatible brittle polymer that also exhibits low
miscibility with the nonfullerene SMA.
In summary, we report high efficiency, moderately ductile,
and relatively stable nonfullerene ternary OSCs by integrating
two polymer donors FTAZ and PBDB-T and one SMA IT-M
with an additive-free and halogen-free processing method.
A PCE over 13% can be achieved for ternary OSCs having a
weight ratio of 0.8:0.2:1 for FTAZ:PBDB-T:IT-M, which is
Adv. Mater. 2019, 1808279
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1808279 (7 of 8)
www.advmat.dewww.advancedsciencenews.com
among the highest values for nonfullerene ternary solar cells
based on dual donors. The enhancement in device performance
of the ternary OSCs is mainly attributed to the enhancement of
device FF, which is due to the reduced charge recombination
in the highly pure minority donor phase that traps the holes
into its network and improved charge mobilities. More impor-
tantly, the ternary system demonstrates good mechanical duc-
tility, superior shelf-life stability, and excellent thermal stress
tolerance. The excellent performance under high thermal stress
is mainly attributed to the inhibition of nonfullerene SMA
crystallization by introducing a stiff polymer (PBDB-T). The
results indicate that synergistic enhancements can be achieved
in more than one parameter. Given that the toluene:FTAZ:IT-
M ink achieved excellent results previously by blade-coating in
air, we expect the ternary improvements achieved here to also
translate to blade-coating and other R2R compatible methods.
The desirable design characteristics of the minority donor can
be summarized as follows: i) it should be more immiscible with
the nonfullerene than the host majority donor to form purer
mixed domains than the host, ii) it should have a slightly higher
HOMO than the host to trap and preferentially transport the
holes, and iii) it should be less ductile and more brittle than the
host. This work provides a simple yet effective approach toward
highly efficient ternary OSCs with excellent thermal stability
and mechanical properties, which would potentially be used to
accelerate the future application of nonfullerene OSCs.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The research at NCSU by the Ade and O’Connor groups was carried
out with support from NSF grant CBET-1639429 and ONR grant
N000141712204. B.T.O. and N.B. also acknowledge support from
NSF grant (CMMI-1554322). W.Y. and J.J.R. were supported by NSF
grant (CBET-1639429). S.J.S. was supported by NSF Grant (DGE-
1633587). X-ray data were acquired at beamlines 11.0.1.2 and 7.3.3 at
the Advanced Light Source, which is supported by the Director, Office
of Science, Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02-05CH11231. C. Zhu, A. Hexemer,
and C. Wang of the ALS (LBNL) provided instrument maintenance. The
authors appreciate Dr. Abay Dinku for maintaining and operating the
shared ORaCEL device fabrication facilities at NCSU.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
mechanical durability, nonhalogenated solvents, stability, ternary
solar cells
Received: December 23, 2018
Revised: February 25, 2019
Published online:
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