Enhancing Efficiency and Stability of Organic Solar Cells
by UV Absorbent
Meng Qin, Pei Cheng, Jiangquan Mai, Tsz-Ki Lau, Qianqian Zhang, Jiayu Wang,
Cenqi Yan, Kuan Liu, Chun-Jen Su, Wei You, Xinhui Lu,* and Xiaowei Zhan*
A new type of high boiling-point additive, UV absorbent benzophenone (BP),
is reported which can simultaneously improve the efficiency and stability of
fullerene and nonfullerene organic solar cells (OSCs). After the addition of
BP, the power conversion efficiencies (PCEs) of nonfullerene OSCs based on
FTAZ: ITIC-Th is increased from 8.5 to 9.4%, and is further increased to
10.3% by employing inverted geometry. Meanwhile, the photo-stability of
nonfullerene OSC is improved. After illumination-aging, the OSCs with BP
preserve 79% of the original PCEs, while the OSCs without additives or with
1,8-diiodooctane only preserve 65 and 58% of their original PCEs, respec-
tively. In addition, BP can also work in fullerene-based OSCs. After the
addition of BP, the efficiency and photo-stability of the OSCs based on PTB7-
BM are simultaneously enhanced.
As a promising technology for clean and renewable energy
conversion, organic solar cells (OSCs) have attracted considerable
attention in recent years because they present some advantages,
such as low cost, light weight, ﬂexibility, semitransparency and
Much effort has been dedicated to
enhancing thepower conversion efﬁciencies(PCEs) of OSCs, such
as new donor or acceptor materialssynthesis, morphology control,
and device engineering. Especially, morphology optimization of
active layer is an effective and indispensable step for OSCs
In recent years, researchers have developed a
number of methods to control the morphology of the polymer/
fullerene activelayers, among which, the use
of high boiling-point additives, such as 1,8-
and diphenyl ether (DPE),
most widely used approach, which can
control the solvent evaporation dynamics
and thus signiﬁcantly improve the PCEs.
However, the residue of these high
boiling-point solvent additives in OSCs is
detrimental to the device stability,
which limits the future industrial produc-
tion of OSCs.
In recent 3 years, nonfullerene acceptor
based OSCs have attracted increasing atten-
Compared with fullerene accept-
ors, nonfullerene acceptors show some
advantages, such as broad and strong
absorption and adjustable highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels. The nonfullerene OSCs show higher PCEs than
their fullerene based counterparts. Similar to the fullerene based
OSCs, high boiling-point additives (DIO, CN, DPE, etc.) also play
an important role in fabrication of high efﬁciency nonfullerene
Thus, ﬁnding new additives, which can simulta-
neously enhance the efﬁciency and stability of nonfullerene OSCs,
is timely and highly desired.
In this work, we used a new type of high boiling-point additive,
benzophenone (BP), in fullerene and nonfullerene OSCs to
control the morphology, leading to improved device efﬁciency and
stability. Due to the high boiling point (305 C), BP can control the
solvent evaporation dynamics duringthe ﬁlm formation, resulting
in optimized morphology for higher PCEs. On the other hand, BP
is a kind of low-cost and widely used ultraviolet absorbent (UV
absorbent) in industrial manufacture, which can strongly and
selectively absorb UV light (Figure S1, Supporting Information)
and dissipate the high energy of UV light through innocuous low
energy radiation. Since UV light contributes little to the
photocurrent but damages OSCs due to the high energy, the
residue of UV absorbent BP in the blended ﬁlm is potentially
beneﬁcial to the photo-stability of OSCs. After optimization of the
BP content, the PCE of nonfullerene OSC is increased from 8.5 to
9.4% (is further increased to 10.3% by employing inverted
geometry), which ishigher than that with DIO (9.0%). The photo-
stability of nonfullerene OSC is improved: after 150 min
illumination, the PCEs of the nonfullerene OSCs with BP
preserve 79% of their original values, while those without additive
or with DIO preserve 65 and 58% of their original values,
respectively. In addition, BP can also work in fullerene-based
Dr. M. Qin, Dr. P. Cheng, J. Wang, C. Yan, K. Liu, Prof. X. Zhan
Department of Materials Science and Engineering, College of
Engineering, Key Laboratory of Polymer Chemistry and Physics of
Ministry of Education, Peking University, Beijing 100871, China
J. Mai, T.-K. Lau, Prof. X. Lu
Department of Physics
The Chinese University of Hong Kong, New Territories, Hong Kong,
Q. Zhang, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill, Chapel Hill, North
Carolina 27599, United States
Dr. C. Su
National Synchrotron Radiation Research Center, Hsinchu, Taiwan,
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/solr.201700148.
Organic Solar Cells www.solar-rrl.com
Sol. RRL 2017, 1700148 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700148 (1 of 7)
OSCs. After the additionof BP, the efﬁciency and photo-stabilityof
fullerene-based OSCs are simultaneously enhanced.
The structure of nonfullerene OSCs is indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)
(PEDOT: PSS)/FTAZ: ITIC-Th/calcium (Ca)/aluminum (Al),
and the molecular structures of the wide-bandgap donor
and the narrow-bandgap acceptor ITIC-Th
shown in Figure 1. Because DIO is the most widely used and
successful high boiling-point (332 C) additive, we compared BP
with DIO in terms of both efﬁciency and stability. Figure 2 shows
the J–Vcurves and external quantum efﬁciency (EQE) spectra of
devices without or with additives under the illumination of an
AM 1.5G solar simulator, 100 mWcm
, as well as the curves of
PCEs versus illumination-aging time under nitrogen atmo-
sphere. The photo-stability tests were carried out under
continuous 150 min illumination by an AM 1.5G solar simulator,
with cooling fans to eliminate the interference of heat. The
optimized content of BP is 7% of the total weight of donor FTAZ
and acceptor ITIC-Th, while the optimized content of DIO is
0.25% of the processing solvent volume. The average and best
device characteristics are summerized in Table 1 (the average
data are calculated from 10 individual devices, and the PCE/
is deﬁned as the ratio of PCEs after and before the photo-
stability test). By employing BP into the devices, the short circuit
current density (J
) and ﬁll factor (FF) are increased, while the
open circuit voltage (V
) is not affected. The average PCE is
increased from 8.2 to 9.1%. In comparison, devices with DIO
show average PCE of 8.7%, lower than that of devices with BP. In
addition, BP was also employed in fullerene-based OSCs (PTB7-
BM) to investigate its generality. With the addition of
7% BP into the devices, the average PCE is increased from 7.4 to
8.9%. In comparison, devices with 3% DIO show average PCE of
8.3%, lower than that of devices with BP.
As shown in Figure 2b, the trend of EQE is similar to J
According to the absorption spectra of donor and acceptor
materials, the EQE under 600 nm is mainly attributed to FTAZ,
while the EQE in 600–800 nm is attributed to ITIC-Th. After the
addition of 7% BP, the EQE related to both donor and acceptor
parts are obviously enhanced. To evaluate the accuracy of the
photovoltaic results, the J
values are calculated from
integration of the EQE spectra with the AM 1.5G reference
spectrum. The calculated J
is similar to J–Vmeasurement (the
average error is 3.1%, Table 1).
Since both BP and DIO possess high boiling points, some of
them would remain in the active layers. The residue of DIO was
veriﬁed before by fourier transform infrared (FTIR).
by comparison of FTIR spectra of ﬁlms without and with BP
Figure 1. Molecular structures of FTAZ, PTB7-Th, ITIC-Th, PC
Figure 2. Photovoltaic characteristics and photo-stability tests of OSCs
based on ITO/PEDOT: PSS/FTAZ: ITIC-Th/Ca/Al without additives (w/o),
with DIO (w/ DIO), and with BP (w/ BP). a) J–Vcurves and b) EQE spectra
of devices. c) Photo-stability curves of devices under continuous
illumination-aging for 150 min (AM 1.5G solar simulator, 100mW cm
Sol. RRL 2017, 1700148 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700148 (2 of 7)
(Figure 3), it is obvious that BP remains in the ﬁlm, which is also
supported by UV absorption test (Figure S1, Supporting
Information) and gas chromatography-mass spectrometer
(GC-MS) test (Figures S2 and S3, Table S1, Supporting
Information). After illumination-aging for 150 min, FTAZ:
ITIC-Th devices without any additives, with DIO and with BP
exhibit average PCEs of 5.3, 5.1, and 7.2%, respectively,
preserving 65, 58, and 79% of original PCE values
(Figure 2c). After illumination-aging at elevated temperature
of 60 C for 150 min, the devices without additive, with DIO, and
with BP exhibit PCEs of 4.8%, 4.3%, 7.0%, preserving 59%, 49%,
77% of their original PCE values, respectively (Figure S4,
Supporting Information). Similar to nonfullerene devices, after
illumination-aging for 150 min, PTB7-Th: PC
without any additives, with DIO and with BP preserve 73, 63,
and 85% of original PCE values, respectively. The addition of BP
can signiﬁcantly enhance the photo-stability of OSCs.
High boiling-point additives can control the morphology of
active layer during ﬁlm formation. Figure 4 displays transmis-
sion electron microscopy (TEM) images of FTAZ: ITIC-Th
blended ﬁlms without or with additives to investigate the effects
of additives on morphology before and after illumination-aging.
After illumination-aging for 150 min, the morphology of ﬁlms
without additives and with DIO are changed, while the ﬁlm with
BP remains unchanged. It has been reported that the residual
DIO in the blended ﬁlm will be decomposed under illumination,
and the decrease in DIO amount will change the morphology of
Different from DIO, as a common industrial
UV absorbent, the residual BP in blended ﬁlm is stable under
illumination, and thus the active layer can maintain the original
morphology. On the other hand, the donor and acceptor
materials would suffer from photochemical reaction under
illumination of the high-energy UV light, leading to changes of
their molecular structures.
As a result, the miscibility of donor
and acceptor materials would be changed, which accordingly
affects the phase separation of active layer. The addition of BP
into the blended ﬁlm may prevent the molecular structures of
donor and acceptor materials from destroying by UV light,
resulting in stabilized morphology of the active layer.
Figure 5a–f shows the 2D grazing-incidence small-angle X-ray
scattering (GISAXS) patterns of FTAZ: ITIC-Th blended ﬁlms
without or with additives before and after illumination-aging.
The in-plane scattering intensity proﬁles are shown in Figure 5g.
By ﬁtting them with the models adopted in previous
we could roughly estimate the average domain
sizes of intermixing amorphous phases, FTAZ domains and
ITIC-Th domains, which are summarized in Table S2 (Support-
ing Information). The FTAZ domains remain almost unchanged
(4 nm) for all the ﬁlms, as manifested by the shoulder appeared
. However, the acceptor and intermixing domain
sizes suffer from large changes after illumination-aging for the
ﬁlms without additive or with DIO, consistent with the observed
signiﬁcant performance deterioration of devices made with
these two types of ﬁlms. Impressively, the ﬁlms with BP exhibit
unprecedented morphology stability under illumination with
almost unchanged donor, acceptor and intermixing domain
sizes, in support of the observed stable device performance of
FTAZ: ITIC-Th blended ﬁlms with BP under illumination-aging.
Since BP is capable of controlling and stabilizing the
morphology of active layer, which may affect charge transport,
the hole mobility and electron mobility of FTAZ: ITIC-Th
blended ﬁlms without or with additives before and after 150 min
illumination-aging were measured by space charge limited
current (SCLC) method.
Hole-only and electron-only diodes
were fabricated using the architectures: ITO/PEDOT: PSS/active
layer/Au for holes and Al/active layer/Al for electrons. As shown
in Figure S5 and Table S3 (Supporting Information), addition of
DIO or BP can increase the hole mobility from 2.1 10
, while slightly affect the electron mobil-
ity. After illumination-aging for 150 min, the blended ﬁlms
exhibit little changes of electron mobilities (from 1.5 10
Table 1. Average and best device data based on FTAZ: ITIC-Th or PTB7-Th: PC
BM without or with additives and their photo-stability after
150 min illumination-aging
Acceptor Additive V
) Calc J
) FF (%) Average Best PCE/PCE
ITIC-Th –0.91 0.01 14.0 0.2 13.6 64.5 0.8 8.2 0.2 8.5 65
ITIC-Th DIO 0.91 0.01 14.4 0.3 13.9 65.8 0.9 8.7 0.2 9.0 58
ITIC-Th BP 0.91 0.01 15.2 0.2 14.6 66.0 1.0 9.1 0.2 9.4 79
BM –0.81 0.01 14.0 0.2 13.6 65.3 0.8 7.4 0.2 7.6 73
BM DIO 0.81 0.01 15.3 0.3 14.8 67.2 0.9 8.3 0.4 8.9 63
BM BP 0.81 0.01 16.2 0.2 15.9 68.0 0.6 8.9 0.3 9.3 85
Figure 3. FTIR spectra of FTAZ films without additives and with BP. The
characteristic peak of ─C55O─at 1652 cm
demonstrates BP would
remain in the film. These films have the same thickness. ITIC-Th is not
used to eliminate its interference of ─C55O─functional group.
Sol. RRL 2017, 1700148 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700148 (3 of 7)
Figure 4. TEM images of FTAZ: ITIC-Th blended films without or with additives before (a–c) and after (d–f ) 150min illumination-aging. a and d) Without
additives. b and e) With DIO. c and f) With BP. The scale bar is 100 nm.
Figure 5. 2D GISAXS images of FTAZ: ITIC-Th blended films without or with additives before (a–c) and after (d–f) 150min illumination-aging. a and d)
Without additives. b and e) With DIO. c and f) With BP. g) GISAXS in-plane scattering intensity profiles with fitting results.
Sol. RRL 2017, 1700148 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700148 (4 of 7)
for ﬁlm without additives, unchange of
for ﬁlms with DIO or BP). However, the
hole mobilities of ﬁlms without additives and with DIO
considerably decrease from 2.1 10
to 9.2 10
, and from 7.5 10
to 9.2 10
In contrast, attributed to the stabilized morphology, the blended
ﬁlm with BP experiences smaller change in hole mobility (from
to 5.2 10
To investigate the inﬂuences of additives on the vertical phase
seperation of FTAZ: ITIC-Th blended ﬁlms, X-ray photoelectron
spectroscopy (XPS) was used to measure the ratio of atoms at the
top surface of active layer (Figure S6, Supporting Information).
Since among the donor FTAZ, acceptor ITIC-Th and additives
DIO or BP, nitrogen (N) and sulfur (S) are presented only in
FTAZ and ITIC-Th, the spectral lines of N 1s (400 eV) and S 2p
(160 eV) are attributed to both FTAZ and ITIC-Th. One FTAZ
repeated unit contains three N atoms and four S atoms, while
one ITIC-Th molecule contains four N atoms and eight S atoms.
From the N/S ratio, the polymer weight content at the top surface
can be calculated: 66, 88, and 79% for blended ﬁlms without
additives, with DIO and with BP, respectively (details in the
Supporting Information). In the blended ﬁlms, the donor FTAZ
tends to aggregate on the top surface, due to its relatively low
surface energy, which is proven by the results of contact angle
(CA) measurement (101.6 1.0for FTAZ, 44.7 0.6for ITIC-
Th, Figure S7, Supporting Information). High boiling-point
additive BP can control the solvent evaporation dynamics,
similar to DIO, leading to more aggregation of FTAZ on the top
surface. As higher content of polymer donor on the top surface is
beneﬁcial to hole extraction by the top electrode, we employed
inverted geometry (ITO/zinc oxide (ZnO)/FTAZ: ITIC-Th/
molybdenum trioxide (MoO
)/silver (Ag)) in the nonfullerene
OSCs, to further explore the potential of BP for increasing PCEs.
With addition of 7% BP, the device exhibits PCE of 10.3%
(Table S4 and Figure S8, Supporting Information). The J
17.8 mA cm
, which is higher than that of the conventional
geometry (15.2 mA cm
In summary, a new type of additive, UV absorbent BP, is
employed in nonfullerene OSCs based on FTAZ: ITIC-Th to
simultaneously enhance the PCE and stability. BP possesses two
crucial characteristics: high boiling point and absorption of UV
light. Due to the high boiling point, BP is capable of controlling the
solvent evaporation dynamics during ﬁlm formation, leading to
optimized morphology of active layer. Due to increase in J
FF, the PCE of FTAZ: ITIC-Th based devices with BP is enhanced
by a factor of 11% relative to that without additives. An even higher
PCE of 10.3% is achieved by using invertedgeometry. On the other
hand, thanks to the strong absorption in UV spectral region,
residual BP in active layer can effectively alleviate photo-damages
and stabilizethe morphology, resultingin improved photo-stability
of OSCs. After 150 min illumination-aging, devices with BP
preserve 79% of the original PCE values, while those without
additives and with DIO can only preserve 65 and 58% of their
original PCEs. In addition, BP can also work in fullerene-based
OSCs. After the addition of BP in PTB7-Th/PC
BM based OSCs,
the efﬁciency and photo-stability are simultaneously enhanced.
Our results demonstrate that additive BP presents an effective and
economic approach to fullerene and nonfullerene OSCs with high
efﬁciency and good stability.
Unless stated otherwise, solvents and chemicals were obtained
commercially and used without further puriﬁcation. FTAZ
were synthesized according to our previously
reported procedures. PTB7-Th was purchased from 1-Materials
BM was purchased from Solarmer Inc. BP, DIO, and o-
dichlorobenzene (DCB) were obtained from J&K Chemical Inc.
Organic solar cells were fabricated with the structure: ITO/
PEDOT: PSS/active layer/Ca/Al for regular geometry, and ITO/
/Ag for inverted geometry. The ITO glass
(sheet resistance ¼10 Ω&
) was pre-cleaned in an ultrasonic
bath of acetone and isopropanol, and treated in ultraviolet-ozone
chamber (Jelight Company, USA) for 20min. For regular
geometry, a thin layer (35nm) of PEDOT: PSS (Baytron PVP AI
4083, Germany) was spin-coated onto the ITO glass and baked at
150 C for 20 min. A mixture of FTAZ and ITIC-Th was dissolved
in DCB solvent (1: 1, 18 mg mL
in total) with stirring
overnight. A mixture of PTB7-Th and PC
BM was dissolved
in DCB solvent (1: 1.5, 25 mg mL
in total) with stirring
overnight. BP was also dissolved in DCB solvent (100 mg mL
To fabricate OSCs with additives, appropriate volume of BP
solution or DIO was added to the solution of active materials.
Afterwards, the solutions were spin-coated on the PEDOT: PSS
layer to form a photoactive layer. The thicknesses of the active
layers were ranging from 80 to 100 nm, as measured by
DektakXT (Bruker). A Ca (ca. 20 nm) and Al layer (ca. 80 nm) was
then evaporated onto the surface of the photoactive layer under
vacuum (ca. 10
Pa) to form the negative electrode. For inverted
geometry, a thin layer (30 nm) of ZnO precursor solution was
spin-coated onto the ITO glass and baked at 200 C for 60 min.
The fabrication method of ative layer in inverted geometry is the
same as that in regular geometry. A MoO
(ca. 10 nm) and Ag
layer (ca. 100 nm) were then evaporated onto the surface of the
photoactive layer under vacuum (ca. 10
Pa) to form the back
electrode. The active area of the device was 4 mm
. The J–Vcurve
was measured using a computer-controlled B2912A Precision
Source/Measure Unit (Agilent Technologies). An XES-70S1
(SAN-EI Electric Co., Ltd.) solar simulator (AAA grade, 70 70
photo beam size) coupled with AM 1.5G solar spectrum
ﬁlters was used as the light source, and the optical power at the
sample was 100 mW cm
reference cell (SRC-1000-TC-QZ) was purchased from VLSI
Standards Inc. The EQE spectrum was measured using a Solar
Cell Spectral Response Measurement System QE-R3011 (Enli-
tech Co., Ltd.). The light intensity at each wavelength was
calibrated using a standard single crystal Si photovoltaic cell.
Photo-stability tests of solar cells were carried out under AM 1.5
G illumination for 150 min, in an open-circuit status without a
shadow mask, with cooling fans to avoid the interference of heat
or at elevated temperature of 60 C. These tests were carried out
in glove box.
UV-vis absorption spectrum was recorded on a JΛSCO V-570
spectrophotometer. The testing samples (solution) for the
absorption spectra were prepared as below: 1) FTAZ: ITIC-Th
blend ﬁlms without or with BP were fabricated under the same
condition as the devices described above; 2) These samples were
treated under thevacuum condition, which was same asthat of the
devices; 3) The blendﬁlms without or with BP and pure BP powder
Sol. RRL 2017, 1700148 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700148 (5 of 7)
were respectively dissolved and diluted in chloroform to achieve
appropriate concentrations for the absorption tests. To prepare
samples for FTIR tests, the polymer solutions without or with BP
were ﬁrstly spin coatedon potassium bromide (KBr) with the same
thickness. Then these samples were treated under the vacuum
condition before FTIR tests. The vacuum treatment process of
FTIR samples was same as that of the devices. FTIR studies were
performed using a Bruker Tensor 27 FTIR spectrometer. The
sample for GC-MS test was preparedas below: 1) a DCB solution of
FTAZ:ITIC-Th:BP (1:1:0.14, w/w/w) was spin-coated on PEDOT:
PSS layer to form the active layer; 2) This sample was treated under
the vacuum condition, which was same as that of the devices; 3)
The active layer was soakedwith 50 mL methanol on the surface for
60 s. The methanol solution was analyzed with GC-MS. The
transmission electron microscopy (TEM) characterization was
carried out on a JEM-2100 transmission electron microscope
operated at 200kV. The samples for the TEM measurements were
prepared as follows:the active layer ﬁlms were spin-casted on ITO/
PEDOT: PSS substrates, and then substrates with the active layers
were submerged indeionized water to make the active layers ﬂoat
onto the air-water interface. Then, the ﬂoated ﬁlms were picked up
on unsupported 200 mesh copper grids for the TEM measure-
ments. The grazing incidence small-angle X-ray scattering
measurements (GISAXS) were carried out at BL23A1 of National
Synchrotron Radiation Research Center, Hsinchu. The energy of
the X-ray source was set to 10 keV (wavelength of 1.24 A
) and the
incident angle was 0.15. Hole-only or electron-only diodes were
fabricated using the architectures: ITO/PEDOT:PSS/active layer/
Au for holes and Al/active layer/Al for electrons. Mobilities were
extracted by ﬁtting the current density–voltage curves using the
Mott–Gurney relationship(space charge limited current).XPS was
performed on the Thermo Scientiﬁc ESCALab 250Xi using 200 W
monochromated Al Kαradiation. The 500 mm X-ray spot was used
for XPS analysis. The base pressure in the analysis chamber was
about 3 10
mbar. Typically, the hydrocarbon C1s line at
284.8 eV from adventitious carbon was used for energy reference.
Static contact angles were measured on a dataphysics OCA20
contact-angle system at ambient temperature (the test liquid is
Supporting Information is available from the Wiley Online Library or from
X.Z. thanks NSFC (91433114 and 21734001) for financial support. X.L.
thanks the Research Grant Council of Hong Kong (General Research Fund
No. 14303314 and Theme-based Research Scheme No. T23-407/13-N) for
financial support and the beam time and technical supports provided by
23A SWAXS beamline at NSRRC, Hsinchu. W.Y. thanks the support from
NSF (DMR-1507249 and CBET-1639429).
organic solar cells, stability, nonfullerene, UV absorbent, additives
Received: September 5, 2017
Revised: October 25, 2017
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