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Enhancing Efficiency and Stability of Organic Solar Cells by UV Absorbent

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Enhancing Efficiency and Stability of Organic Solar Cells by UV Absorbent

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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, respectively. 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-Th: PC71BM are simultaneously enhanced.
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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-
Th: PC
71
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, exibility, semitransparency and
large-area fabrication.
[17]
Much effort has been dedicated to
enhancing thepower conversion efciencies(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
fabrication.
[8,9]
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-
diiodooctane (DIO),
[10]
chloronaphthalene
(CN)
[11]
and diphenyl ether (DPE),
[12]
is the
most widely used approach, which can
control the solvent evaporation dynamics
and thus signicantly improve the PCEs.
[1320]
However, the residue of these high
boiling-point solvent additives in OSCs is
detrimental to the device stability,
[2126]
which limits the future industrial produc-
tion of OSCs.
[2729]
In recent 3 years, nonfullerene acceptor
based OSCs have attracted increasing atten-
tion.
[3044]
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 efciency nonfullerene
OSCs.
[4551]
Thus, nding new additives, which can simulta-
neously enhance the efciency 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 efciency and
stability. Due to the high boiling point (305 C), BP can control the
solvent evaporation dynamics duringthe lm 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 lm is potentially
benecial 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
xwzhan@pku.edu.cn
J. Mai, T.-K. Lau, Prof. X. Lu
Department of Physics
The Chinese University of Hong Kong, New Territories, Hong Kong,
China
E-mail: xhlu@phy.cuhk.edu.hk
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,
China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/solr.201700148.
DOI: 10.1002/solr.201700148
Organic Solar Cells www.solar-rrl.com
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OSCs. After the additionof BP, the efciency 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
FTAZ
[52]
and the narrow-bandgap acceptor ITIC-Th
[41]
are
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 efciency and stability. Figure 2 shows
the JVcurves and external quantum efciency (EQE) spectra of
devices without or with additives under the illumination of an
AM 1.5G solar simulator, 100 mWcm
2
, 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/
PCE
0
is dened as the ratio of PCEs after and before the photo-
stability test). By employing BP into the devices, the short circuit
current density (J
SC
) and ll factor (FF) are increased, while the
open circuit voltage (V
OC
) 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-
Th: PC
71
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
SC
.
According to the absorption spectra of donor and acceptor
materials, the EQE under 600 nm is mainly attributed to FTAZ,
while the EQE in 600800 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
SC
values are calculated from
integration of the EQE spectra with the AM 1.5G reference
spectrum. The calculated J
SC
is similar to JVmeasurement (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
veried before by fourier transform infrared (FTIR).
[21]
Similarly,
by comparison of FTIR spectra of lms without and with BP
Figure 1. Molecular structures of FTAZ, PTB7-Th, ITIC-Th, PC
71
BM,
and BP.
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) JVcurves 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
2
).
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(Figure 3), it is obvious that BP remains in the lm, 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
71
BM devices
without any additives, with DIO and with BP preserve 73, 63,
and 85% of original PCE values, respectively. The addition of BP
can signicantly enhance the photo-stability of OSCs.
High boiling-point additives can control the morphology of
active layer during lm formation. Figure 4 displays transmis-
sion electron microscopy (TEM) images of FTAZ: ITIC-Th
blended lms 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 lms
without additives and with DIO are changed, while the lm with
BP remains unchanged. It has been reported that the residual
DIO in the blended lm will be decomposed under illumination,
and the decrease in DIO amount will change the morphology of
active layer.
[21,25,53]
Different from DIO, as a common industrial
UV absorbent, the residual BP in blended lm 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.
[21]
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 lm may prevent the molecular structures of
donor and acceptor materials from destroying by UV light,
resulting in stabilized morphology of the active layer.
Figure 5af shows the 2D grazing-incidence small-angle X-ray
scattering (GISAXS) patterns of FTAZ: ITIC-Th blended lms
without or with additives before and after illumination-aging.
The in-plane scattering intensity proles are shown in Figure 5g.
By tting them with the models adopted in previous
studies,
[54,55]
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 lms, as manifested by the shoulder appeared
at q
r
0.08 A
1
. However, the acceptor and intermixing domain
sizes suffer from large changes after illumination-aging for the
lms without additive or with DIO, consistent with the observed
signicant performance deterioration of devices made with
these two types of lms. Impressively, the lms 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 lms 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 lms without or with additives before and after 150 min
illumination-aging were measured by space charge limited
current (SCLC) method.
[56]
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
5
to
7.5 10
5
cm
2
V
1
s
1
, while slightly affect the electron mobil-
ity. After illumination-aging for 150 min, the blended lms
exhibit little changes of electron mobilities (from 1.5 10
5
to
Table 1. Average and best device data based on FTAZ: ITIC-Th or PTB7-Th: PC
71
BM without or with additives and their photo-stability after
150 min illumination-aging
PCE (%)
Acceptor Additive V
OC
(V) J
SC
(mA cm
2
) Calc J
SC
(mA cm
2
) FF (%) Average Best PCE/PCE
0
(%)
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
PC
71
BM 0.81 0.01 14.0 0.2 13.6 65.3 0.8 7.4 0.2 7.6 73
PC
71
BM DIO 0.81 0.01 15.3 0.3 14.8 67.2 0.9 8.3 0.4 8.9 63
PC
71
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 C55Oat 1652 cm
1
demonstrates BP would
remain in the film. These films have the same thickness. ITIC-Th is not
used to eliminate its interference of C55Ofunctional group.
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Figure 4. TEM images of FTAZ: ITIC-Th blended films without or with additives before (ac) and after (df ) 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 (ac) and after (df) 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.
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1.0 10
5
cm
2
V
1
s
1
for lm without additives, unchange of
1.2 10
5
cm
2
V
1
s
1
for lms with DIO or BP). However, the
hole mobilities of lms without additives and with DIO
considerably decrease from 2.1 10
5
to 9.2 10
6
cm
2
V
1
s
1
, and from 7.5 10
5
to 9.2 10
6
cm
2
V
1
s
1
, respectively.
In contrast, attributed to the stabilized morphology, the blended
lm with BP experiences smaller change in hole mobility (from
7.5 10
5
to 5.2 10
5
cm
2
V
1
s
1
).
To investigate the inuences of additives on the vertical phase
seperation of FTAZ: ITIC-Th blended lms, 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 lms without
additives, with DIO and with BP, respectively (details in the
Supporting Information). In the blended lms, 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
benecial to hole extraction by the top electrode, we employed
inverted geometry (ITO/zinc oxide (ZnO)/FTAZ: ITIC-Th/
molybdenum trioxide (MoO
3
)/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
SC
is
17.8 mA cm
2
, which is higher than that of the conventional
geometry (15.2 mA cm
2
).
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 lm formation, leading to
optimized morphology of active layer. Due to increase in J
SC
and
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
71
BM based OSCs,
the efciency 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
efciency and good stability.
Experimental Section
Unless stated otherwise, solvents and chemicals were obtained
commercially and used without further purication. FTAZ
[52]
and ITIC-Th
[40]
were synthesized according to our previously
reported procedures. PTB7-Th was purchased from 1-Materials
Inc. PC
71
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/
ZnO/active layer/MoO
3
/Ag for inverted geometry. The ITO glass
(sheet resistance ¼10 Ω&
1
) 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
1
in total) with stirring
overnight. A mixture of PTB7-Th and PC
71
BM was dissolved
in DCB solvent (1: 1.5, 25 mg mL
1
in total) with stirring
overnight. BP was also dissolved in DCB solvent (100 mg mL
1
).
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
5
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
3
(ca. 10 nm) and Ag
layer (ca. 100 nm) were then evaporated onto the surface of the
photoactive layer under vacuum (ca. 10
5
Pa) to form the back
electrode. The active area of the device was 4 mm
2
. The JVcurve
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
mm
2
photo beam size) coupled with AM 1.5G solar spectrum
lters was used as the light source, and the optical power at the
sample was 100 mW cm
2
.A22cm
2
monocrystalline silicon
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 lms 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 blendlms without or with BP and pure BP powder
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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 rstly 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 lms were spin-casted on ITO/
PEDOT: PSS substrates, and then substrates with the active layers
were submerged indeionized water to make the active layers oat
onto the air-water interface. Then, the oated lms 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 tting the current densityvoltage curves using the
MottGurney relationship(space charge limited current).XPS was
performed on the Thermo Scientic 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
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
water).
Supporting Information
Supporting Information is available from the Wiley Online Library or from
the author.
Acknowledgements
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).
Keywords
organic solar cells, stability, nonfullerene, UV absorbent, additives
Received: September 5, 2017
Revised: October 25, 2017
Published online:
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... The identification of material and device structure strategies that enable the generation of OSCs with improved UV-light stability is an interesting approach to promote future commercial applications of OSCs. In the past, several studies have investigated the stability of OSC devices using fullerene and NFAs, respectively, under unfiltered-UV sunlight conditions [10,[21][22][23][24][25][26][27][28][29]. For instants, UV instabilities of solar cells using [6,6]-Phenyl-C71-butyric acid methyl ester (PC 71 BM) and NFAs based blends toward the processing additive 1, 8-diiodooctane (DIO) have been demonstrated [21,[25][26][27][28]. ...
... In the past, several studies have investigated the stability of OSC devices using fullerene and NFAs, respectively, under unfiltered-UV sunlight conditions [10,[21][22][23][24][25][26][27][28][29]. For instants, UV instabilities of solar cells using [6,6]-Phenyl-C71-butyric acid methyl ester (PC 71 BM) and NFAs based blends toward the processing additive 1, 8-diiodooctane (DIO) have been demonstrated [21,[25][26][27][28]. A photochemical reaction product of PC 71 BM with UV-induced radicals of DIO was identified as the cause of the device degradation [25]. ...
... A photochemical reaction product of PC 71 BM with UV-induced radicals of DIO was identified as the cause of the device degradation [25]. To improve the stability of the device under unfiltered UV light, replacing DIO with a UV-absorbing additive, benzophenone, proved to be a promising approach [26]. It was also found that thermal annealing of the fullerene-based solar cells at 85 • C clearly improved the stability of the device under UV-light by improving the crystal order in the acceptor domains [21]. ...
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Organic solar cells (OSCs) are a promising technology with the potential for low-cost manufacturing. However, to translate to economically viable applications, long-term stability is a fundamental requirement. Amongst intrinsic degradation pathways the sensitivity of OSC to ultraviolet (UV) light severely limits their photostability. Here, we focus on the impact of UV on the stability of solar cells based on well-known fullerene-based blends processed with 1,8-diidooctane (DIO) as additive. The post-annealed devices resulting in DIO-free blends are directly compared to as-cast devices containing residual DIO. After a pronounced initial burn-in, as-cast devices demonstrate a self-healing effect leading to stable solar cells under prolonged exposure to UV light. This initial burn-in can be considerably reduced in annealed devices with a suitable heating process, resulting in very stable solar cells under UV-containing light over a long time period. Under UV-free LED light, solar cells are stable, which implies a direct impact of UV on the performance evolution of devices. Advanced characterization techniques were used for in-depth morphological analyses under light exposure to distinguish the observed UV-related processes in the polymer blends. Our results point thus towards the presence of two processes occurring under UV-light within as-cast devices involving fullerenes, one causing a performance degradation and the other allowing a repair tending towards a performance stability. Due to an improved initial crystal order within annealed devices, the process related to the degradation is in the minority. The UV stability of devices can be attributed to the UV light-induced diffusion of fullerenes, leading jointly to the enlargement of the initial existing fullerene domains and to their crystallization under UV light. These results path the way for a better understanding of the stability of efficient normal OSCs under simulated sunlight.
... [16] Because the key components of OSCs, such as the photoactive layer and interfacial layer, are mainly constructed from organic conjugated compounds, highenergy ultraviolet (UV) rays from sunlight can cause serious decomposition of these compounds, making OSCs intrinsically unstable under sunlight illumination. [17][18][19] Although the use of a UV filter may help reduce the UV damage, the installation of an additional optical filter not only complicates the structure of the device but also has a negative effect on the light-harvesting of OSCs. [20] Currently, there is no effective approach to protect Page 2 of 23 CCS Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 This article presented here has been accepted for publication in CCS Chemistry and is posted at the OSCs from UV-induced deterioration, making it very challenging to fabricate devices with high operational stability. ...
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Due to the poor stability of organic solar cells (OSCs), especially ultraviolet light (UV) stability, they are not yet up to the standard for commercial applications. Inverted devices based on the ZnO electron transport layer (ETL) are relatively thermal‐ and air‐stable, while their UV light stability is poor. Moreover, the active layer materials undergo severe degradation under UV light, which also causes the degradation of the device. Here, aggregation‐induced emission (AIE) molecules as an optical barrier layer are coated between ZnO‐ETL and active layer to fabricate efficient OSCs with excellent UV photostability. AIE molecules can avoid direct photodegradation of the active layer materials by absorbing UV light. At the same time, they can passivate the oxygen defects on the ZnO surface and prevent photocatalytic degradation of the active layer materials. Therefore, the ZnO/TPIZ‐based devices maintain 84% of the original power conversion efficiency (PCE) with continuous irradiation by 5 mW cm–2 UV light (365 nm) for 1500 h, and similarly T80 for more than 700 h under 20 mW cm–2 UV light exposure. Moreover, the ZnO/TPIZ‐based devices also preserve excellent thermal stability accompanying the upgrading of PCE. Aggregation‐induced emission molecular layer is coated on ZnO‐electron transport layer as an optical barrier layer to fabricate efficient organic solar cells. Through this strategy, the ZnO/TPIZ‐based devices exhibit over 1500 h of T80 under continuous exposure to 365 nm, 5 mW cm–2 UV light, which is far more stable than the ZnO‐based devices, which have only less than 24 h of T80.
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Currently, the two exocyclic vinyl bridges in the acceptor–donor–acceptor (A–D–A)-type nonfullerene acceptors (NFAs) have been widely recognized as one of the most vulnerable sites under external stresses. Embedding the exocyclic vinyl bridges into an aromatic ring could be a feasible solution to stabilize them. Herein, we successfully develop a phenalene-locked vinyl bridge via a titanium tetrachloride—pyridine catalytic Knoevenagel condensation, to synthesize two new A–D–A-type unfused NFAs, EH-FPCN and O-CPCN, wherein malononitrile is used as the electron-deficient terminal group while fluorene and carbazole rings are used as the electron-rich cores, respectively. These two NFAs possess wide bandgaps associated with deep energy levels, and significantly enhanced chemical and photochemical stabilities compared to the analogue molecule O-CzCN with normal exocyclic vinyl bridges. When pairing with a narrow bandgap polymer donor PTB7-Th, the fabricated EH-FPCN- and O-CPCN-based organic solar cells achieved power conversion efficiencies of 0.91 and 1.62%, respectively. The higher efficiencies for O-CPCN is attributed to its better film morphology and higher electron mobility in the blend film. Overall, this work provides a new design strategy to stabilize the vulnerable vinyl bridges of A–D–A-type NFAs.
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A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (JSC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing JSC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC71BM (PCE = 5.22%).
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A new, easy, and efficient approach is reported to enhance the driving force for charge transfer, break tradeoff between open-circuit voltage and short-circuit current, and simultaneously achieve very small energy loss (0.55 eV), very high open-circuit voltage (>1 V), and very high efficiency (>10%) in fullerene-free organic solar cells via an energy driver.
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We design and synthesize four fused-ring electron acceptors based on 6,6,12,12-tetrakis(4-hexylphenyl)-indacenobis(dithieno[3,2-b;2',3'-d]thiophene) as the electron-rich unit and 1,1-dicyanomethylene-3-indanones with 0 to 2 fluorine substituents as the electron-deficient units. These four molecules exhibit broad (550-850 nm) and strong absorption with high extinction coefficients of (2.1-2.5) ×10(5) M(-1) cm(-1). Fluorine substitution down shifts LUMO energy level, red shift absorption spectrum, and enhance electron mobility. The polymer solar cells based on the fluorinated electron acceptors exhibit power conversion efficiencies as high as 11.5%, much higher than that of their nonfluorinated counterpart (7.7%). We investigate the effects of the fluorine atom number and position on electronic properties, charge transport, film morphology, and photovoltaic properties.
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Simutaneously high open circuit voltage and high short circuit current density is a big challenge for achieving high efficiency polymer solar cells due to the excitonic nature of organic semdonductors. Herein, we developed a trialkylsilyl substituted 2D-conjugated polymer with the highest occupied molecular orbital level down-shifted by Si–C bond interaction. The polymer solar cells obtained by pairing this polymer with a non-fullerene acceptor demonstrated a high power conversion efficiency of 11.41% with both high open circuit voltage of 0.94 V and high short circuit current density of 17.32 mA cm−2 benefitted from the complementary absorption of the donor and acceptor, and the high hole transfer efficiency from acceptor to donor although the highest occupied molecular orbital level difference between the donor and acceptor is only 0.11 eV. The results indicate that the alkylsilyl substitution is an effective way in designing high performance conjugated polymer photovoltaic materials.
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Triarylamine (TAA) and related materials have dramatically promoted the development of organic and hybrid photovoltaics during the past decade. The power conversion efficiencies of TAA-based organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), and perovskite solar cells (PSCs) have exceeded 11%, 14%, and 20%, which are among the best results for these three kinds of devices, respectively. In this review, we summarize the recent advances of the high-performance TAA-based materials in OSCs, DSSCs, and PSCs. We focus our discussion on the structure–property relationship of the TAA-based materials in order to shed light on the solutions to the challenges in the field of organic and hybrid photovoltaics. Some design strategies for improving the materials and device performance and possible research directions in the near future are also proposed.
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The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open-circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG-based OSCs faces great challenges. A novel acceptor-donor-acceptor structured non-fullerene acceptor is reported with an ultra-narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short-circuit current density of 25.3 mA cm(-2) is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG-based OSC demonstrates that the design and application of ultra-narrow materials have the potential to further improve the PCE of OSC devices.
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Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.