Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

Abstract

The commercialization of nonfullerene organic solar cells (OSCs) critically
relies on the response under typical operating conditions (for instance, temperature
and humidity) and the ability of scale-up. Despite the rapid increase
in power conversion efficiency (PCE) of spin-coated devices fabricated in a
protective atmosphere, the efficiencies of printed nonfullerene OSC devices
by blade coating are still lower than 6%. This slow progress significantly
limits the practical printing of high-performance nonfullerene OSCs. Here, a
new and relatively stable nonfullerene combination is introduced by pairing
the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over
12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a
single halogen-free solvent. More importantly, chlorine-free, blade coating of
FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of
≈50%. X-ray scattering results reveal that large π–π coherence length, high
degree of face-on orientation with respect to the substrate, and small domain
spacing of ≈20 nm are closely correlated with such high device performance.
The material system and approach yield the highest reported performance
for nonfullerene OSC devices by a coating technique approximating scalable
fabrication methods and hold great promise for the development of low-cost,
low-toxicity, and high-efficiency OSCs by high-throughput production.

Full text

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Surpassing 10% Efficiency Benchmark for Nonfullerene
Organic Solar Cells by Scalable Coating in Air from Single
Nonhalogenated Solvent
Long Ye, Yuan Xiong, Qianqian Zhang, Sunsun Li, Cheng Wang, Zhang Jiang,
Jianhui Hou, Wei You, and Harald Ade*
DOI: 10.1002/adma.201705485
renewable and low-cost energy technology
that possesses the advantages of light-
weight, flexibility, variable or controlled
absorption/color, lead-free, and low energy
loss.[3–12] Despite considerable advances
in the last few years, up to now most pro-
gress in nonfullerene organic solar cells
with power conversion efficiency (PCE)
over 10% has been made by spin coating
in a protective nitrogen atmosphere, which
is not scalable for the mass production of
solar panels. Recent studies[13–15] indicated
that transitioning the high efficiency of
existing top-performing OSCs from con-
ventional spin-coated devices to devices by
scalable printing methods (as illustrated in
Figure 1a), such as blade coating (solution
shearing), slot-die coating, and roll-to-roll
coating, is not that straightforward due
to the different processing temperatures
and film formation mechanisms. It is
shown that the morphologies and drying
dynamics of films made by blade coating,
slot-die coating, and roll-to-roll processing
are almost identical,[13] while the morpho-
logical features and device optimization
strategies in blade-coated devices are largely different from
those in conventionally spin-coated devices. Through these
studies, it became clear that precisely optimizing the mor-
phology and device performance of nonfullerene OSCs made
by blade coating in conditions similar to those during mass
production (e.g., humidity and elevated temperatures) is a key
The commercialization of nonfullerene organic solar cells (OSCs) critically
relies on the response under typical operating conditions (for instance, tem-
perature and humidity) and the ability of scale-up. Despite the rapid increase
in power conversion efficiency (PCE) of spin-coated devices fabricated in a
protective atmosphere, the efficiencies of printed nonfullerene OSC devices
by blade coating are still lower than 6%. This slow progress significantly
limits the practical printing of high-performance nonfullerene OSCs. Here, a
new and relatively stable nonfullerene combination is introduced by pairing
the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over
12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a
single halogen-free solvent. More importantly, chlorine-free, blade coating of
FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of
≈50%. X-ray scattering results reveal that large
π
–
π
coherence length, high
degree of face-on orientation with respect to the substrate, and small domain
spacing of ≈20 nm are closely correlated with such high device performance.
The material system and approach yield the highest reported performance
for nonfullerene OSC devices by a coating technique approximating scalable
fabrication methods and hold great promise for the development of low-cost,
low-toxicity, and high-efficiency OSCs by high-throughput production.
Solar Cells
Dr. L. Ye, Y. Xiong, Prof. H. Ade
Department of Physics
Organic and Carbon Electronics Lab (ORaCEL)
North Carolina State University
Raleigh, NC 27695, USA
E-mail: hwade@ncsu.edu
Dr. Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599, USA
Q2
S. Li, Prof. J. Hou
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, P. R. China
Dr. C. Wang
Advanced Light Source
Lawrence Berkeley National Laboratory
Berkeley, CA 94720, USA
Dr. Z. Jiang
Advanced Photon Source
Argonne National Laboratory
Argonne, IL 60439, USA
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201705485.
Nonfullerene organic semiconductors have achieved signifi-
cant breakthroughs in the field of photovoltaics and transis-
tors.[1–3] Due to intensive research efforts in chemical tailoring
of nonfullerene small molecular acceptors (NFAs), NFA-based
organic solar cells (OSCs) are now outperforming conven-
tional fullerene-based OSCs and have remarkable potential as a
Adv. Mater. 2018, 1705485

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step toward the ultimate commercialization of this photovoltaic
technology.
Among the variety of donor polymers used in nonfullerene
PSCs, the wide band-gap (≈2.0 eV) polymer donor FTAZ[16,17]
(see Figure 1b) stands out as one of the best options largely due
to its high hole mobility on the order of 10−3 cm2 V−1 s−1 and pos-
sible morphological insensitivity.[18] In addition, FTAZ exhibits
strong absorption at the wavelength range of 400–600 nm with
a high absorption coefficient of 1 × 105 cm−1, which is highly
complementary with the absorption of many newly developed
NFAs.[19,20] For example, Bauer et al.[21] paired FTAZ as the
polymer donor with a perylene-based NFA, but the PCE of such
blend-based devices (below 4%) was noticeably lower compared
to that of FTAZ:PC61BM blend due to the large recombination
loss. Very recently, Zhan and co-workers[20] reported an impres-
sively high efficiency of 12.1% in spin-coated single junction
OSCs from chloroform/1,8-diiodooctane by blending a new
fluorinated NFA with FTAZ. Their results suggest that care-
fully synthesized NFAs with fluorine substituents can provide
significantly higher efficiency compared with the nonfluori-
nated counterparts. However, these high-efficiency fluorinated
NFAs usually require multiple step syntheses and thus the
limited quantities/yields produced severely restrict the material
accessibility and device upscaling by researchers. Another
important aspect for commercialization is to realize environ-
mentally benign fabrication of OSC devices. This requires
the use of low-toxicity, halogen-free solvents. In all previous
studies of FTAZ:fullerene and FTAZ:NFA blends, only chlo-
rinated solvents such as trichlorobenzene, chloroform, and
chlorobenzene (CB) were used as the processing solvent. It is
generally acknowledged that these halogenated solvents are det-
rimental to the environment and should be avoided for printing
devices,[22] in particular, in large-scale production. As such, real-
izing high efficiency in printed nonfullerene OSCs via scalable
materials and less toxic solvents remains a grand challenge.
To address this key challenge, we report here a chlorine-
free upscaling of nonfullerene OSCs (see Figure 1) based
on a new photoactive polymer:NFA combination by using
a single chlorine-free solvent, in the absence of solvent addi-
tives. The nonfullerene combination comprises FTAZ and a
nonfluorinated NFA named IT-M (the chemical structure is
shown Figure 1d),[23,24] which is now commercially available.
An average efficiency up to ≈12% can be consistently achieved
in spin-coated FTAZ:IT-M devices in a nitrogen atmosphere
despite using a single halogen-free solvent. We are able to
breach the 10% efficiency benchmark by blade coating in air
with an average humidity of 50%, and the highest efficiency
achieved (≈11%) is the best PCE for blade-coated OSCs to
date. Furthermore, we investigate the relationship between
processing solvent, drying dynamics, molecular packing, thin-
film morphology, and device performance in these blade-coated
films. Comparative studies of three hydrocarbon solvents
(Figure 1c), namely, toluene (TL), o-xylene (XY), and 1,2,4-tri-
methylbenzene (TMB), indicate quantified morphological
parameters (e.g.,
π
–
π
coherence length, face-on to edge-on
ratio, domain spacing and purity) correlate well to the key per-
formance metrics of these printed devices. Our results sug-
gest that FTAZ:IT-M is not only a model system for identifying
key solvent–morphology–performance relations, but also a
promising candidate for upscaling in industrial printing. Sig-
nificantly, the upscaling practice shown here is expected to be
widely applicable for a large class of polymer:NFA combinations,
considering FTAZ and its derivatives are becoming increasingly
studied in the field of nonfullerene OSCs.[4,19,20,25,26]
We started out our investigation by evaluating the basic
properties of FTAZ and IT-M. Figure 2a shows the ultraviolet–
visible (UV–vis) absorption spectra of the films of FTAZ and
IT-M, suggesting a broad and complementary light-harvesting
range (300-800 nm) of the FTAZ:IT-M blend (see Figure S1,
Supporting Information). Also, their energy levels (Figure 2b)
Adv. Mater. 2018, 1705485
Figure 1. a) Schematic illustration of widely used spin coating and scalable coating of organic films; b) chemical structure of polymer donor FTAZ;
c) chemical structures of a chlorinated solvent and several nonhalogenated solvents used in the nonfullerene OSC processing; d) chemical structure
of a nonfluorinated NFA (IT-M).

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are quite matched with a sufficient energy offset for charge
creation. Through simple solubility tests at room temperature,
we find both FTAZ and IT-M can be well dissolved in several
hydrocarbon solvents, such as toluene, o-xylene, and 1,2,4-tri-
methylbenzene. Good solubilities (>8 mg mL−1) afforded by
those hydrocarbon solvents allow us to make uniform FTAZ:IT-
M films. To assess the potential of this new combination for
OSC applications, nonfullerene OSCs with TL and CB respec-
tively as process solvents are fabricated in a regular device
architecture (Figure 2b), where PEDOT:PSS was used as a
hole-transporting layer and an alcohol-soluble material PFN–
Br[27] as the electron-transporting layer buffer layer to the metal
cathode. After optimizing the processing parameters such as
D/A weight ratio (Table S1, Supporting Information) and film
thickness, 1:1 weight ratio was used to make a 105 ± 5 nm thick
film. Notably, no solvent additive was needed. The spin-coated
films were dried/annealed at 150 °C for 10 min. The detailed
descriptions of the device fabrications are available in the Sup-
porting Information. The current density–voltage (J–V) charac-
teristics of the nonfullerene OSCs measured under simulated
AM 1.5 G irradiation (with the intensity of 100 mW cm−2) are
shown in Figure 2c and the device parameters are summarized
in Table S2 (Supporting Information). Using the halogen-
free solvent TL, we were able to achieve an average PCE of
12.0 ± 0.1% in spin-coated devices. As a control, a lower PCE
of 10.5 ± 0.3% is obtained when CB is used due to the relative
lower fill-factor (FF) and short-circuit current density (Jsc). The
external quantum efficiency (EQE) values shown in Figure 2c
are in a good agreement with the observed Jsc from J–V tests,
indicating the advantage of nonhalogenated solvents in this
system. We note that the low absorption range (350–450 nm)
of nonfullerene OPVs can still deliver a high EQE of ≈60%,
which has been observed in other literature reports[20,25] and are
mainly due to the efficient exciton dissociation and charge col-
lection as reflected by the morphology. Unlike previous reports
on FTAZ:fluorinated NFA[19,20] where fluorination was believed
to play an important role, it is intriguing to note that matching
FTAZ with a nonfluorinated IT-M in our case is able to yield
over 12% efficiency by spin coating in the nitrogen atmosphere.
This implies fluorinated NFA is not a prerequisite for achieving
high-efficiency by pairing with FTAZ. We also note that the
TL devices without encapsulation can maintain over 80% of
the original performance after storing in the nitrogen atmos-
phere under the dark condition for 1200 h (Figure 2d), while
the PCE of CB devices drops much faster, which might be due
to the relatively poor morphological stability (e.g., larger size
phase separation occurs after aging) of CB devices, as shown
in a recent study.[28] These results clearly indicate that one can
use greener solvents to replace the dominating chlorinated sol-
vents currently used in fabricating nonfullerene OSCs. Overall,
our new combination of FTAZ:IT-M is a promising candidate
system for scalable coating due to halogen-free processing and
good stability.
Inspired by these results from spin coating, we directed our
attention to chlorine-free blade coating of this new system in
air (Figure 3a). The processing was done in lab air with ≈50%
relative humidity (see Figure S2 in the Supporting Informa-
tion) highlighting that ability to translate to cost-effective
Adv. Mater. 2018, 1705485
Figure 2. a) Absorption spectra of neat FTAZ and IT-M films; b) energy diagrams of FTAZ and IT-M, and device configuration used in this work; c) repre-
sentative J–V curves and EQE curves of FTAZ:IT-M devices processed by CB and TL in the nitrogen atmosphere recorded under AM 1.5G 100 mW cm−2
irradiation; d) normalized PCEs of unencapsulated device as a function of storage time in the nitrogen under dark.

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fabrication. Optimization of the conditions for blade coating
of the photoactive layers is summarized in the Supporting
Information. For successive optimization, a chlorine-free TL
solution of FTAZ:IT-M blend was deposited using a custom-
built blade coater (the schematic setup is shown in the inset of
Figure 3a) and the coating temperature (i.e., the temperature of
the hot stage) was screened first (Figure S3 and Table S3, Sup-
porting Information) and the PCE of the respective devices was
recorded. As shown in Figure 3b, we were able to achieve an
average PCE greater than 10% when the coating temperature
was 70—80 °C and the optimum performance of nearly 11%
can be achieved at 70 °C, while the device FF was significantly
lower when the coating temperature was below 70 °C or above
80 °C. Our data clearly suggest that the coating temperature is
a critical parameter for scalable coating of the blend films, and
this finding is in line with the previous reports of blade-coating
polymer:fullerene OSCs.[13,14,29] Additionally, the film thick-
ness can be readily tuned from 80 to 200 nm by altering the
blade speed for a fixed solution concentration of 16 mg mL−1
in total. As shown in Figure 3c and Table S4 (Supporting Infor-
mation), the thickness and blade speed follow a power-law rela-
tion for the speed range investigated (80–250 mm s−1), which
implied our deposition process is in close agreement with the
Landau–Levich regime[30] due to the positive power exponent.
A plot of the PCE against the thickness of TL-processed devices
is displayed in Figure 3d and the corresponding J–V curves are
shown in Figure S4 (Supporting Information). The device PCE
is insensitive to the thickness of the active layer in the range
of 90–150 nm, and ≈9% can be obtained for these thicknesses
with the highest efficiency of nearly 11% appearing at a thick-
ness of ≈110 nm.
To reveal the process–morphology–function relation in
FTAZ:IT-M by this chlorine-free blade-coating, we performed a
comparative investigation of the drying dynamics, photovoltaic
properties, molecular packing, and phase separation of blade-
coated films by using three halogen-free solvents TMB, XY, and
TL. The difference of these halogen-free solvents is the number
of methyls attached to the benzene ring, which varies with the
boiling points and drying dynamics. Our in situ variable angle
spectroscopic ellipsometry[15] experiments (see Figure S5 in
the Supporting Information and additional details) suggest the
drying times after the blade casting at the same temperature are
10, 6, and 2 s for TMB, XY, and TL, respectively. Correspond-
ingly, such a distinct drying time may lead to different device
performances. Using the same device architecture shown in
Figure 2b, the J–V characteristics of blade-coated devices from
different solvents and the associated EQE curves are shown in
Figure 3e,f. As listed in Table 1, the open-circuit voltage (Voc) is
0.94–0.95 V, irrespective of the solvent used, while the average
Jsc is gradually increased from 14.4 to 16.8 mA cm−2 and the
average FF from 61.9% to 66.1%, when a faster-drying solvent
(TL > XY > TMB) is applied. As a result, the average and cham-
pion PCE of our blade-coated FTAZ:IT-M devices using TL are
10.6% and 11%, respectively.
To understand the performance differences of blade-coated
devices resulting from three different solvents, quantification
Adv. Mater. 2018, 1705485
Figure 3. a) Schematic representation of the adopted scalable processing, i.e., chlorine-free blade-coating in air (having an average humidity of 50%)
with a single solvent; b) device PCE of the blade-coated FTAZ:IT-M film with TL as a function of the temperature of hot stage; c) thickness of the
TL-processed FTAZ:IT-M film as a function of blade speed, and the error bars of thickness are calculated from 4 spots; d) device PCE of the blade-
coated FTAZ:IT-M film with TL as a function of thickness; e) representative J–V curves and f) EQE curves recorded under the illumination of AM
1.5G 100 mW cm−2.

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of both nanoscale and mesoscale features is critical to esta-
blish a complete solvent–morphology–function relation. First,
high-resolution grazing incidence wide-angle X-ray scattering
(GIWAXS)[31] at Argonne National Laboratory was used to probe
the molecular packing and texture of their active layers, where
the volume fraction of the disordered portion is much larger than
that of the ordered part.[32] As observed from the 2D GIWAXS
patterns in Figure 4a and 1D profiles shown in Figure S6
(Supporting Information), the molecular order of blend films
is gradually improved (TL > XY > TMB) when the boiling
point of solvent was decreased. Particularly, we observed
more pronounced (010) diffraction peak at qz = 1.7 Å−1 and
high orders of lamellar peaks in the in-plane (qxy) direction,
which reveals a preferential face-on orientation with respect
to the substrate. By using the full width at half-maximum of
the
π
–
π
stacking reflection peaks, we estimated the out-of-
plane
π
–
π
coherence lengths of FTAZ and IT-M of these blend
films via peak fitting, and both coherence lengths (Figure 4b)
show the same trend: TL > XY > TMB. Here, pole figures of
(100) peaks are extracted and corrected for geometry,[32,33] and
a quantitative parameter Axy/Az as schematically described in
Figure 4c is used for comparing the face-on to edge-on ratios
across samples. We noted that the face-on to edge-on ratio is
the highest (3.1) for the TL-processed devices, indicative of
a more face-on tendency, while XY and TMB devices show a
lower ratio of 2.2 and 1.9, respectively. It has been shown that
face-on orientation enhances intermolecular charge transport
and is beneficial for efficient OSCs.[33,34] Thus, the high
π
–
π
coherence length and Axy/Az are beneficial for charge transport
of TL devices, which can partly explain the highest Jsc and FF
obtained.
Resonant soft X-ray scattering (R-SoXS) was utilized to quan-
tify the domain characteristics of these blade-coated films fol-
lowing detailed protocols previously established.[15,35] Figure 4d
displays the Lorentz-corrected scattering profiles at a resonant
X-ray energy where the scattering contrast is relatively large.
Adv. Mater. 2018, 1705485
Table 1. Photovoltaic device parameters of FTAZ:IT-M-based nonfullerene OSCs by chlorine-free blade-coating in air having an average humidity of
≈50%.
Solvent Voc [mV] Jsc [mA cm−2]FF [%] PCEa) [%] Axy/AzDomain spacing
[nm]
ISIb)
TMB 940 ± 5 14.4 ± 0.5 61.9 ± 1.0 8.4 ± 0.3 (9.1) 1.9 32.9 0.90
XY 943 ± 5 15.7 ± 0.7 62.3 ± 1.2 9.3 ± 0.2 (9.6) 2.2 34.0 0.92
TL 950 ± 2 16.8 ± 0.3 66.1 ± 1.1 10.6 ± 0.2 (11.0) 3.1 19.9 1.00
a)The standard deviations of solar cell performances are averaged from ten devices and the maximum values are shown in the parentheses; b)Here the area of the whole
lognormal peak is used as integrated scattering intensity (ISI) and this ISI is normalized to the highest value, which is set as 1. The film thickness is 105 ± 5 nm for these
blade-coated samples.
Figure 4. a) GIWAXS 2D pattern and b) out-of-plane
π
–
π
coherence lengths of FTAZ and IT-M in the blade-coated films from different solvents; c) pole
figures extracted from the (100) diffraction of blade-coated FTAZ:IT-M films with TL, XY, and TMB. The inset shows the close-up of the lamellar (100)
region and the definitions of the polar angle (
χ
) range corresponding to the edge-on (Az) and face-on (Axy) crystallites; d) Lorentz-corrected scattering
profiles from azimuthally averaged R-SoXS images of blade-coated films at a photon energy of ≈284 eV.

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We note that all R-SoXS profiles are dominated by a single
lognormal function, similar to prior polymer:fullerene studies
using FTAZ.[18] Under an assumption of globally 3D isotropic
morphology, 2
π
over q of the dominant peak gives the center-to-
center domain spacing of a given system, and the integration of
the R-SoXS profile (the whole lognormal peak), i.e., integrated
scattering intensity (ISI), represents the mean-square variance
of the composition, which can be used to quantify the average
domain purity. The details of this procedure and its justification
have been documented in some recent reports.[36] The quanti-
fied parameters from R-SoXS tests are listed in Table 1, and
the ISI of TMB and XY devices is respectively 90% and 92%
of that of the TL device. It is shown that high domain purity is
often desirable for suppressing bimolecular recombination and
increase device FF.[36] Therefore, the relative domain purity as
reflected by ISI here also positively correlates with the device
FF. In addition, domain spacing is strongly affected by the type
of solvent. The XY film shows the largest domain spacing of
34 nm and the spacing of the TMB film is very similar. In stark
contrast to the XY and TMB cases, the domains of the TL film
has the smallest spacing (19.9 nm), which is very close to the
typical exciton diffusion length.[37] As small domains close to
this length are considered necessary to provide large enough
D/A interfacial area, exciton dissociation and charge creation
will be facilitated.[36] On the basis of the above results, a halogen-
free solvent with lower boiling point enables increased face-on
orientation, larger coherence lengths, and smaller domains.
We conclude that halogen-free blade coating in air with TL is
able to achieve the most face-on ordering and smallest domain
spacing, thereby affording nearly 11% efficiency in devices.
The above findings motivated us to further understand the
stability of the printed FTAZ:IT-M devices using TL. After a
storage time of ≈1000 h in a glovebox, blade-coated devices
using TL can attain ≈85% of the initial PCE (Figure 5a; Table S5,
Supporting Information). Utilizing an aged FTAZ:IT-M solu-
tion in air after storing in dark for 20 days, we can still get
≈91% of the efficiency achieved from a freshly prepared solu-
tion (see Figure 5b and Table S6 in the Supporting Informa-
tion, and experimental details are shown in the Supporting
Information). We completed blade-coated devices after heating
the active layers at 150 °C for different time periods from 10 to
240 min (Figure 5c), and the PCE can still maintain over 10%
after being heated at 150 °C for 240 min. Under such a high
annealing temperature, the devices Jsc and Voc are independent
of the annealing time (see Table S7 in the Supporting Infor-
mation), resulting in a largely unchanged PCE (10–11%). We
further fabricated a larger-area (0.56 cm2) FTAZ:IT-M device
(Figure 5d) using the same halogen-free blade-coating in
air approach. The device efficiency is 9.8%, and the slightly
decreased PCE and Voc (Table S8 in the Supporting Informa-
tion) are mainly due to the resistance loss of the top electrodes
utilized (see the inset of Figure 5d). To make a comparison with
the results shown in the literature, we carried out a survey (see
Table 2) of state-of-the-art nonfullerene OSCs to date[14,15,38,39]
Adv. Mater. 2018, 1705485
Figure 5. a) Storage stability of FTAZ:IT-M devices by halogen-free blade coating in air (average humidity: ≈50%). TL was used as the solvent. Devices
were unencapsulated and stored in a glovebox; b) J–V characteristics of FTAZ:IT-M devices by halogen-free blade coating in air using freshly prepared
solution and solution aged at room temperature for 20 d; c) J–V characteristics of blade-coated FTAZ:IT-M devices with active layers annealed at
150 °C for different time periods; d) J–V characteristics of FTAZ:IT-M devices with a larger area (0.56 cm2). The inset is the photograph of the device
and effective area.

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Adv. Mater. 2018, 1705485
by blade-coating techniques. The highest value shown in the
literature is below 6% after taking into account all critical fac-
tors such as effective device area, solvent, and atmosphere. As
a consequence, the PCE of our blade-coated nonfullerene OSC
is substantially higher, by ≈90%, than that of the best blade-
coated nonfullerene device reported in the literature.[15] Lastly,
it is worth pointing out that more than 10 derivatives of FTAZ
(the chemical structures are shown in Figure S7 in the Sup-
porting Information) have been recently applied to spin-coated
nonfullerene OSC devices. As most of these derivatives have
an identical backbone (all of them are consisting of benzodith-
iophene and fluorinated benzotriazole) and presumably com-
parable solubility in nonchlorinated solvents to that of FTAZ,
our method should be applicable to the morphology optimiza-
tion of blade-coated devices based on these structurally similar
FTAZ derivatives. Similarly, our chlorine-free blade-coating
approach can be applied to other emerging NFA systems of
interest, such as ITCC[40] and EH-IDTBR.[9,28,41] We note that
the FF values of our new FTAZ:IT-M system are relatively low
(<70%) compared with some reported high-efficiency non-
fullerene OSC systems,[12,20] and the FF of FTAZ:IT-M devices
might be limited by the intrinsic polymer:acceptor misci-
bility[24] that will depend on the donor polymer and the mole-
cular acceptor. Further molecular engineering the chemical
structures is anticipated to reduce the FF and efficiency loss.
In summary, we report spin-coated and printed OSCs
of high efficiency using a new nonfullerene polymer:small
molecule blend (FTAZ:IT-M) and an additive-free and hal-
ogen-free solvent processing. Our results suggest fluorinated
NFAs are not necessarily required for high-performance
nonfullerene OSCs, and a nonfluorinated NFA (IT-M) that is
now commercially available is capable of yielding a compa-
rable PCE with those of the state-of-the-art fluorinated NFAs
when pairing with FTAZ. By blade coating with chlorine-free
solvents in air, FTAZ:IT-M OSCs yields a PCE of up to 11%,
which outperforms any reported nonfullerene and fullerene
devices to date by blade coating. Comparative studies suggest
the choice of nonchlorinated solvents not only alters the face-
on orientation, but also significantly affects the domain puri-
ties and length scales of phase separation of the blade-coated
nonfullerene OSCs. Among the halogen-free solvents studied,
toluene with the lowest boiling point is the best processing
solvent for this new FTAZ:IT-M combination due to the more
preferable molecular ordering and optimized domain char-
acteristics. Furthermore, we discovered that the FTAZ:IT-M
film blade-coated from TL is stable against high-temperature
(150 °C) heating and its aged solution up to 20 days in air
shows only a very small performance loss. Both aspects should
bode well for translation into an industrial setting and for mor-
phological stability. Together, our results represent important
progress for printed nonfullerene OSCs and underscore that
FTAZ:IT-M is a viable candidate for use in highly efficient and
stable photovoltaic cells. Further engineering the morphology
and interface characteristics of this blend system is underway.
Experimental Section
For GIWAXS characterizations, samples are placed in vacuum at
beamline 8-ID-E[31] of the Advanced Photon Source, Argonne National
Laboratory. The energy of the hard X-ray beam is ≈11 keV. GIWAXS
data were collected with a 2D area detector (Pilatus 1M). The sample
to detector distance was ≈228 mm, and the bulk was probed using an
incident angle of 0.16° (i.e., above the polymer critical angle). R-SoXS[35]
and reference spectrum[42] measurements were performed at beamline
11.0.1.2 and 5.3.2 at Advanced Light Source, Lawrence Berkeley National
Laboratory. R-SoXS images were collected in vacuum on a 2D charge-
coupled device cooled to −45 °C (Princeton Instrument PI-MTE). For
the chlorine-free blade coating, a new blade-coater is used and the
design is similar to our previously used blade coater[14] except that this
new blade coater is more compact (see Figure S8 and details in the
Supporting Information). Other experimental details can be found in
the Supporting Information.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
L.Y. and Y.X. contributed equally to this work. This research was carried
out at NCSU with supports from UNC-GA Research Opportunity
Initiative grant and the NSF INFEWS grant CBET 1639429. Q.Z. and
W.Y. were supported by the NSF (CBET-1639429, DMR-1507249).
J. Hou gratefully acknowledges the financial support from National
Nature Science Foundation of China (Grant Nos. 91633301, 91333204
and 21325419), and the Chinese Academy of Science (Grant No.
XDB12030200). Beamlines 5.3.2.2 and 11.0.1.2 at the Advanced Light
Source are supported by the Director of the Office of Science, Office of
Basic Energy Sciences, of the U.S. Department of Energy under Contract
No. DE-AC02-05CH11231. Use of the Advanced Photon Source was
Table 2 . A survey of photovoltaic parameters and processing conditions of the state-of-the-art nonfullerene OSCs by blade-coating techniques.
Nonfullerene system Atmosphere Solventa) Halogen free Device areab) [mm2] PCEmax [%] Ref.
PBDT-TS1:PPDIODT Air o-MA Yes 6.9 5.6 [15]
PBDT-TS1:PPDIODT Air o-MA Yes 13.0 5.1 [15]
PiI-tT-PS5:P(TP) Air CB No 4.0 3.2 [38]
PTB7-Th:tPDI-Hex Air 2Me-THF Ye s 4.0 4.8 [39]
FTAZ:IT-M Air TL Yes 6.9 11.0 This work
FTAZ:IT-M Air TL Yes 56.0 9.8 This work
a)o-MA is o-metylanisole, and 2Me-THF represents 2-methyltetrahydrofuran; b)Effective device area.

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1705485 (8 of 9)
www.advmat.dewww.advancedsciencenews.com
Adv. Mater. 2018, 1705485
supported by the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Contract DE-AC02-06CH11357. We
gratefully acknowledge the beamline support at beamlines 5.3.2.2 and
11.0.1.2 provided by C. Wang, Y. Yu, and A.L.D. Kilcoyne. Prof. Brendan
O’Connor and Nrup Balar are acknowledged for the discussion and help
with the in situ ellipsometry data analysis. We appreciate Dr. Abay Dinku
for maintaining and operating the shared device fabrication facilities.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
blade coating, film morphology, nonfullerene acceptors, nonhalogenated
solvents, organic solar cells
Received: September 21, 2017
Revised: November 2, 2017
Published online:
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