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


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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.
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1705485 (1 of 9) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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
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
Dr. Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599, USA
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
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
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1705485 (2 of 9)
step toward the ultimate commercialization of this photovoltaic
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 103 cm2 V1 s1 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 cm1, 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.,
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 mL1) 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 cm2) 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 cm2
irradiation; d) normalized PCEs of unencapsulated device as a function of storage time in the nitrogen under dark.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1705485 (4 of 9)
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 mL1
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 s1), 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 cm2 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 cm2.
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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 Å1 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
stacking reflection peaks, we estimated the out-of-
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
coherence length and Axy/Az are beneficial for charge transport
of TL devices, which can partly explain the highest Jsc and FF
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
Solvent Voc [mV] Jsc [mA cm2]FF [%] PCEa) [%] Axy/AzDomain spacing
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
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 (
) 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.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1705485 (6 of 9)
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
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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1705485 (7 of 9)
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 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.
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 and 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)
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 and 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.
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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... Thus, the film drying processes vary from one solvent to another, resulting in a differently tuned morphology. [21][22][23][24][25][26][27][28] Therefore, understanding the observed PCE variations must be substantiated by an in-depth morphology analysis, enabled by advanced characterization methods including ex situ and in situ technologies. [29][30][31][32][33][34][35][36][37] Furthermore, a tight connection between morphological features and device performance needs to be established. ...
The morphology features and the film formation processes in high-performance donor-acceptor binary photovoltaic blends cast from three representative solvents are carefully investigated and analyzed. The PM6:L8-BO system shows a very significant efficiency change when varying the solvent from chloroform (CF) to chlorobenzene (CB) and o-xylene (XY), whereas the PM6:eC9 system shows a limited influence of the used solvent. Ex-situ characterizations figure out that CB and XY cause a too-pronounced phase separation for PM6:L8-BO. In contrast, PM6:eC9 films display only slightly enlarged phase segregation in CB films and an even better mixing in XY-processed films. The in-situ observations further reveal that the PM6 aggregation-dominated stage during the film formation is longer for the eC9 system than that of L8-BO, effectively suppressing the separation of donor and acceptor. PM6 is found to be very miscible with the acceptors when processed from XY. The ex-situ analysis results correlate well with the device performance and are finely explained by the in-situ and miscibility study. Further, an excellent device efficiency of 19.10 % (verified 18.77%) is achieved by a ternary design for XY-enabled organic solar cells (OSCs) by PTQ10, while the corresponding blade coating devices present an excellent PCE of 18.25%. Thereby, this work produces a clear understanding of film morphology formation and realizes high-performance non-halogenated solvent-processed OSCs.
... [4][5][6][7] The PCE of OSCs has amplied rapidly because of the recent developments in the engineering and manufacturing of highly efficient optoelectronic semiconductors and improvement on the device fabrication. [8][9][10][11][12][13] In particular, n-type organic semiconductor (n-OS) based small acceptor molecules [14][15][16][17] with a low bandgap (E gap ), when combined with a p-type donor polymer having a medium E gap , have shown remarkable optoelectronic performance. [18][19][20] The PCE of polymer based solar cells having an n-OS as an accepting moiety has increased to a 14%, surpassing the limit for PSCs to be used in practical settings. ...
Full-text available
In the current DFT study, seven dimethoxyl-indaceno dithiophene based semiconducting acceptor molecules (ID1-ID7) are designed computationally by modifying the parent molecule (IDR). Here, based on a DFT exploration at a carefully selected level of theory, we have compiled a list of the optoelectronic properties of ID1-ID7 and IDR. In light of these results, all newly designed molecules, except ID5 have shown a bathochromic shift in their highest absorbance (λ max). ID1-ID4, ID6 and ID7 molecules have smaller band gap (E gap) and excitation energy (E x). IP of ID5 is the smallest and EA of ID1 is the largest among all others. Compared to the parent molecule, ID1-ID3 have increased electron mobility, with ID1 being the most improved in hole mobility. ID4 had the best light harvesting efficiency in this investigation, due to its strongest oscillator. The acceptor molecules' open-circuit voltages (V OC) were computed after being linked to the PTB7-Th donor molecule. Fill factor (FF) and normalized V OC of ID1-ID7 were calculated and compared to the parent molecule. Based on the outcomes of this study, the modified acceptors may be further scrutinised for empirical usage in the production of organic solar cells with enhanced photovoltaic capabilities.
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Organic solar cell (OSC) development continues to demonstrate impressive device efficiency improvements. However, the materials synthetic simplicity essential to industrialization remains seriously lacking attention, imparting inferior performance records in low-cost devices. Hence, low bandgap and completely non-fused electron acceptors (CNFEAs) having simple molecular structures are investigated herein. In contrast to typically explored fused-ring acceptors with smaller backbone conformational variations, minimizing the interface recombination sites through a greater extent of localized domains is identified as more critical in CNFEAs, leading to remarkable fill factors (FFs) approaching 75%, among the highest currently realized for low-cost systems. However, this comes with diminishing charge generation efficiency. The general ternary blend optimization strategy modifying the morphology of host components is limited in preserving such remarkably high FFs. To suppress the trade-off while keeping notable FFs, a new perspective of constructing functionally bridged components based on optical, electronic, and thermodynamic properties is introduced here. Specifically, charge generation is unrestrained from the host acceptor localized domains through the introduction of a “bridge” component while also taking advantage of the configuration to channel polarons toward the efficient transport moieties of the host components. Accordingly, this work incubates understanding-guided optimizations toward the advancement of more practical devices.
Simultaneously understanding the thermodynamic and kinetic mechanisms during film formation is very critical to enable high power conversion efficiencies (PCEs) of the blade-coated organic solar cells (OSCs) as a high-throughput...
Solution-processed organic photovoltaics (OPVs) is one of the most promising photovoltaic technologies in the energy field, due to their clean and renewable low-cost manufacturing potential. OPV has rapidly developed with the design and synthesis of highly efficient photovoltaic materials and the development of smart device engineering. To date, the majority of advanced OPV devices have been prepared using halogenated solvents, achieving power conversion efficiencies (PCE) exceeding 19% on a laboratory scale. However, for industrial-scale production, less toxic manufacturing processes and environmental sustainability are the key considerations. Therefore, this review summarizes recent advances in green solvent-based approaches for the preparation of OPVs, highlighting material design (including polymer donors and small molecule acceptors) and device engineering (co-solvent methods, additive strategies, post-treatment methods, and regulation of coating method), emphasizing crucial factors for achieving high performance in green solvent-processed OPV devices. This review presents potential future directions for green solvent-based OPVs, which may pave the way for future industrial development.
Solution-processed organic photodetectors with broadband activity have been demonstrated with an environmentally benign solvent, ortho-xylene (o-xylene), as the processing solvent. The organic photodetectors employ a wide band gap polymer donor PBDB-T and a narrow band gap small-molecule non-fullerene acceptor CO1-4F, both dissolvable in o-xylene at a controlled temperature. The o-xylene-processed devices have shown external quantum efficiency of up to 70%, surpassing the counterpart processed with chlorobenzene. With a well-suppressed dark current, the device can also present a high specific detectivity of over 1012 Jones at -2 V within practical operation frequencies and is applicable for photoplethysmography with its fast response. These results further highlight the potential of green-solvent-processed organic photodetectors as a high-performing alternative to their counterparts processed in toxic chlorinated solvents without compromising the excellent photosensing performance.
High thermal stability is crucial for the commercialization of organic solar cells (OSCs). The thermal stability of OSCs has been improved using the tailoring blend morphology of bulk heterojunctions (BHJs). Herein, we demonstrated thermally stable OSCs in a ternary blended system containing low-crystalline semiconducting polymers (asy-PNDI1FTVT and PTB7-Th) and a non-fullerene acceptor (Y6). The asymmetric n-type semiconducting polymer (asy-PNDI1FTVT) differed from general symmetric semiconducting polymers as it randomly substituted fluorine atoms at the donor moiety (TVT), resulting in significantly lower crystallinity. asy-PNDI1FTVT in PTB7-Th:Y6 exhibited a well-mixed morphology at the BHJ and efficiently facilitated the charge dissociation process with an enhanced fill factor and power conversion efficiency. Furthermore, the ternary system of PTB7-Th:Y6:asy-PNDI1FTVT suppressed phase separation with negligible burn-in loss and performance degradation under thermal stress. The experiments showed that our devices without encapsulation retained over 90% of their initial efficiencies after 100 h at 65 °C. These results show significant potential for the development of thermally stable OSCs with reasonable efficiency.
Full-text available
Green‐processable printed large‐area light‐emitting diode (LED) is the crucial component of the high quality solid lighting and flat‐panel display. Accelerating the replacement from fossil‐based solvents to sustainable ecofriendly solvent is desirable for solution‐based printed organic electronics. Herein, by simple introducing methyl to the benzene groups of polydiarylfluorene‐based polymer, this work realizes the solution processing of blue polymer light‐emitting diodes (PLEDs) with a bio‐based solvent of limonene toward large‐area printed display. Film processed from this solvent exhibits satisfactory morphology and remarkable photophysical performances, even though the polymer chains show more serious aggregation in solution. What's more, devices fabricated with spin‐coated films present higher external quantum efficiency (EQE) and better spectra repeatability. Encouraged by the good film‐forming ability of this polymer in limonene, this work realizes the fabrication of printed large‐area films (5 × 5 cm ² ) with uniform emission and surface morphology by blade coating. Furthermore, this work develops passive matrix (PM) PLEDs arrays with 20 × 20 pixels that can dynamically display monochrome letters and numbers, demonstrating the potential application of cost‐effective printing techniques in large‐area display.
Due to the characteristics of lower material waste, higher crystallinity, roll-to-roll compatibility, and high-throughput continuous processing, blade-coating has been widely applied in the preparation of large-area organic solar cells. In this paper, the technique of blade-coating is introduced, including the effects of blading speed, substrate temperature, and other technological innovations during the process of blade-coating. Besides, the recent progress of blade-coating in organic solar cells is summarized and the active layer prepared by a blade-coating method is introduced in detail, including materials, processing methods, solvents, and additives. The interface layer and electrodes prepared by the blade-coating method are also discussed. Finally, some perspectives on the blade-coating method are proposed. In the foreseeable future, blade-coating will become the core of batch production of large-area organic solar cells, so as to make organic solar cells more competitive.
Most cutting‐edge performance binary bulk heterojunction organic solar cells (OSCs) with high power conversion efficiency (PCE) over 18% generally use harmful halogenated solvent systems to guarantee the optimal morphology of active layers, urging more investigations on the non‐halogenated solvent processed OSCs and their further large‐scale devices fabrication. Herein, two BTP‐eC9 analogs, BTP‐eC9‐EH and BTP‐eC9‐HD, were synthesized to investigate the effect of alkyl chain length on the solubility and photovoltaic properties systematically. Despite the subtle side‐chain engineering, the PM6: BTP‐eC9‐HD based device exhibited an outstanding PCE of 17.5% and a remarkable fill factor of 79% in the toluene processing binary blend system, which precedes the toluene processing PM6: BTP‐eC9 based binary blend system (17.1%). The combined investigation provides a valuable insight into the modulation of branched alkyl chains, which can serve as an effective approach for adjusting the aggregation and molecular packing of A‐DA′DA‐type Y‐series based non‐fullerene acceptors to balance the materials solubility and solvents selection, thereby achieving high‐performance large‐scale OSCs processed from non‐halogenated solvents.
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Suppression of carrier recombination is critically important in realizing high-efficiency polymer solar cells. Herein, it is demonstrated difluoro-substitution of thiophene conjugated side chain on donor polymer can suppress triplet formation for reducing carrier recombination. A new medium bandgap 2D-conjugated D–A copolymer J91 is designed and synthesized with bi(alkyl-difluorothienyl)-benzodithiophene as donor unit and fluorobenzotriazole as acceptor unit, for taking the advantages of the synergistic fluorination on the backbone and thiophene side chain. J91 demonstrates enhanced absorption, low-lying highest occupied molecular orbital energy level, and higher hole mobility, in comparison with its control polymer J52 without fluorination on the thiophene side chains. The transient absorption spectra indicate that J91 can suppress the triplet formation in its blend film with n-type organic semiconductor acceptor m-ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(3-hexylphenyl)-dithieno[2,3-d:2,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene). With these favorable properties, a higher power conversion efficiency of 11.63% with high VOC of 0.984 V and high JSC of 18.03 mA cm−2 is obtained for the polymer solar cells based on J91/m-ITIC with thermal annealing. The improved photovoltaic performance by thermal annealing is explained from the morphology change upon thermal annealing as revealed by photoinduced force microscopy. The results indicate that side chain engineering can provide a new solution to suppress carrier recombination toward high efficiency, thus deserves further attention.
Full-text available
In this work, high-efficiency nonfullerene polymer solar cells (PSCs) are developed based on a thiazolothiazole-containing wide bandgap polymer PTZ1 as donor and a planar IDT-based narrow bandgap small molecule with four side chains (IDIC) as acceptor. Through thermal annealing treatment, a power conversion efficiency (PCE) of up to 11.5% with an open circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 16.4 mA cm−2, and a fill factor of 76.2% is achieved. Furthermore, the PSCs based on PTZ1:IDIC still exhibit a relatively high PCE of 9.6% with the active layer thickness of 210 nm and a superior PCE of 10.5% with the device area of up to 0.81 cm2. These results indicate that PTZ1 is a promising polymer donor material for highly efficient fullerene-free PSCs and large-scale devices fabrication.
Full-text available
A side-chain conjugation strategy in the design of nonfullerene electron acceptors is proposed, with the design and synthesis of a side-chain-conjugated acceptor (ITIC2) based on a 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']di(cyclopenta-dithiophene) electron-donating core and 1,1-dicyanomethylene-3-indanone electron-withdrawing end groups. ITIC2 with the conjugated side chains exhibits an absorption peak at 714 nm, which redshifts 12 nm relative to ITIC1. The absorption extinction coefficient of ITIC2 is 2.7 × 10(5) m(-1) cm(-1) , higher than that of ITIC1 (1.5 × 10(5) m(-1) cm(-1) ). ITIC2 exhibits slightly higher highest occupied molecular orbital (HOMO) (-5.43 eV) and lowest unoccupied molecular orbital (LUMO) (-3.80 eV) energy levels relative to ITIC1 (HOMO: -5.48 eV; LUMO: -3.84 eV), and higher electron mobility (1.3 × 10(-3) cm(2) V(-1) s(-1) ) than that of ITIC1 (9.6 × 10(-4) cm(2) V(-1) s(-1) ). The power conversion efficiency of ITIC2-based organic solar cells is 11.0%, much higher than that of ITIC1-based control devices (8.54%). Our results demonstrate that side-chain conjugation can tune energy levels, enhance absorption, and electron mobility, and finally enhance photovoltaic performance of nonfullerene acceptors.
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Significant efforts have lead to demonstrations of nonfullerene solar cells (NFSCs) with record power conversion efficiency up to ≈13% for polymer:small molecule blends and ≈9% for all-polymer blends. However, the control of morphology in NFSCs based on polymer blends is very challenging and a key obstacle to pushing this technology to eventual commercialization. The relations between phases at various length scales and photovoltaic parameters of all-polymer bulk-heterojunctions remain poorly understood and seldom explored. Here, precise control over a multilength scale morphology and photovoltaic performance are demonstrated by simply altering the concentration of a green solvent additive used in blade-coated films. Resonant soft X-ray scattering is used to elucidate the multiphasic morphology of these printed all-polymeric films and complements with the use of grazing incidence wide-angle X-ray scattering and in situ spectroscopic ellipsometry characterizations to correlate the morphology parameters at different length scales to the device performance metrics. Benefiting from the highest relative volume fraction of small domains, additive-free solar cells show the best device performance, strengthening the advantage of single benign solvent approach. This study also highlights the importance of high volume fraction of smallest domains in printed NFSCs and organic solar cells in general.
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A new polymer donor (PBDB-T-SF) and a new small molecule acceptor (IT-4F) for fullerene-free organic solar cells (OSCs) were designed and synthesized. The influences of fluorination on the absorption spectra, molecular energy levels and charge mobilities of the donor and acceptor were systematically studied. The PBDB-T-SF:IT-4F-based OSC device showed a record high efficiency of 13.1%, and an efficiency of over 12% can be obtained with a thickness of 100–200 nm, suggesting the promise of fullerene-free OSCs in practical applications.
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Organic/polymer semiconductors provide unique possibilities and flexibility in tailoring their optoelectronic properties to match specific application demands. One of the key factors contributing to the rapid and continuous progress of organic photovoltaics (OPVs) is the control and optimization of photoactive-layer morphology. The impact of morphology on photovoltaic parameters has been widely observed. However, the highly complex and multilength-scale morphology often formed in efficient OPV devices consisting of compositionally similar components impose obstacles to conventional morphological characterizations. In contrast, due to the high compositional and orientational sensitivity, resonant soft X-ray scattering (R-SoXS), and related techniques lead to tremendous progress of characterization and comprehension regarding the complex mesoscale morphology in OPVs. R-SoXS is capable of quantifying the domain characteristics, and polarized soft X-ray scattering (P-SoXS) provides quantitative information on orientational ordering. These morphological parameters strongly correlate the fill factor (FF), open-circuit voltage (Voc), as well as short-circuit current (Jsc) in a wider range of OPV devices, including recent record-efficiency polymer:fullerene solar cells and 12%-efficiency fullerene-free OPVs. This progress report will delineate the soft X-ray scattering methodology and its future challenges to characterize and understand functional organic materials and provide a non-exhaustive overview of R-SoXS characterization and its implication to date.
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To achieve efficient non-fullerene organic solar cells, it is important to reduce the voltage loss from the optical bandgap to the open-circuit voltage of the cell. Here we report a highly efficient non-fullerene organic solar cell with a high open-circuit voltage of 1.08 V and a small voltage loss of 0.55 V. The high performance was enabled by a novel wide-bandgap (2.05 eV) donor polymer paired with a narrow-bandgap (1.63 eV) small-molecular acceptor (SMA). Our morphology characterizations show that both the polymer and the SMA can maintain high crystallinity in the blend film, resulting in crystalline and small domains. As a result, our non-fullerene organic solar cells realize an efficiency of 11.6%, which is the best performance for a non-fullerene organic solar cell with such a small voltage loss.
With chlorinated solvents unlikely to be permitted for use in solution-processed organic solar cells in industry, there must be a focus on developing non-chlorinated solvent systems. Here we report high efficiency devices utilising a low-bandgap donor polymer (PffBT4T-2DT) and a non-fullerene acceptor (EH-IDTBR), from hydrocarbon solvents and without using additives. When mesitylene was used as the solvent, rather than chlorobenzene, an improved power conversion efficiency (11.1%) was achieved without the need for pre- or post- treatments. Despite altering the processing conditions to environmentally friendly solvents and room temperature coating, grazing incident X-ray measurements confirmed that active layers processed from hydrocarbon solvents retained the robust nano-morphology obtained with hot-processed chlorinated solvents. The main advantages of hydrocarbon solvent processed devices, besides the improved efficiencies, were the reproducibility and storage lifetime of devices. Mesitylene devices showed better reproducibility and shelf-life up to 4000h with PCE dropping by only 8% of its initial value.
Morphology can play a critical role in determining function in organic photovoltaic (OPV) systems. Recently molecular acceptors have showed promise to replace fullerene derivatives as acceptor materials in bulk heterojunction solar cells and have achieved >10% efficiencies in single junction devices. The nearly identical mass/electron densities between the donor (polymer) and acceptor (molecule) materials results in poor material contrast compared to fullerene-based OPVs and therefore morphology characterization using techniques that rely on mass/electron density variations poses a challenge. This inhibits a fundamental understanding of the structure–property relationships for non-fullerene acceptor materials. We demonstrate that low angle annular dark field scanning transmission electron microscopy and resonant soft X-ray scattering form a set of complementary tools that can provide quantitative characterization of fullerene as well as non-fullerene based organic photovoltaic systems.
A new acceptor–donor–acceptor-structured nonfullerene acceptor ITCC (3,9-bis(4-(1,1-dicyanomethylene)-3-methylene-2-oxo-cyclopenta[b]thiophen)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d′:2,3-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene) is designed and synthesized via simple end-group modification. ITCC shows improved electron-transport properties and a high-lying lowest unoccupied molecular orbital level. A power conversion efficiency of 11.4% with an impressive V OC of over 1 V is recorded in photovoltaic devices, suggesting that ITCC has great potential for applications in tandem organic solar cells.