Green-Solvent-Processed All-Polymer Solar Cells Containing a Perylene Diimide-Based Acceptor with an Efficiency over 6.5%

Abstract

To realize high power conversion efficiencies (PCEs) in green-solvent-processed all-polymer solar cells (All-PSCs), a long alkyl chain modified perylene diimide (PDI)-based polymer acceptor PPDIODT with superior solubility in nonhalogenated solvents is synthesized. A properly matched PBDT-TS1 is selected as the polymer donor due to the red-shifted light absorption and low-lying energy level in order to achieve the complementary absorption spectrum and matched energy level between polymer donor and polymer acceptor. By utilizing anisole as the processing solvent, an optimal efficiency of 5.43% is realized in PBDT-TS1/PPDIODT-based All-PSC with conventional configuration, which is comparable with that of All-PSCs processed by the widely used binary solvent. Due to the utilization of an inverted device configuration, the PCE is further increased to over 6.5% efficiency. Notably, the best-performing PCE of 6.58% is the highest value for All-PSCs employing PDI-based polymer acceptors and green-solvent-processed All-PSCs. The excellent photovoltaic performance is mainly attributed to a favorable vertical phase distribution, a higher exciton dissociation efficiency (Pdiss) in the blend film, and a higher electrode carrier collection efficiency. Overall, the combination of rational molecular designing, material selection, and device engineering will motivate the efficiency breakthrough in green-solvent-processed All-PSCs.

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Green-Solvent-Processed All-Polymer Solar Cells Containing
a Perylene Diimide-Based Acceptor with an Effi ciency over 6.5%
Sunsun Li , Hao Zhang , Wenchao Zhao , Long Ye ,* Huifeng Yao , Bei Yang ,
Shaoqing Zhang , and Jianhui Hou*
S. Li, H. Zhang, W. Zhao, Dr. L. Ye, H. Yao,
B. Yang, S. Zhang, Prof. J. Hou
State Key Laboratory of Polymer Physics and Chemistry
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 , P. R. China
E-mail: yelong@iccas.ac.cn; hjhzlz@iccas.ac.cn
S. Li, H. Zhang, H. Yao
University of Chinese Academy of Sciences
Beijing 100049 , P. R. China
DOI: 10.1002/aenm.201501991
structures, the intrinsic material prop-
erties, including the molecular energy
levels, the optical absorption features
and the crystallinity, are relatively easy to
fi ne tune compared to fullerene deriva-
tives. Moreover, nonfullerene acceptors
can be modifi ed to show good solubility
in commonly used halogen-free solvents,
such as o -xylene, toluene, and tetra-
hydrofuran,
[ 10,11 ] thereby demonstrating
the use of green processing solvents for
realizing environmentally friendly PSC
manufacturing. As commonly used dye
intermediates, perylene and its derivatives
show good stability and good optoelec-
tronic properties; hence, they are some
of the most widely studied nonfullerene
acceptors.
[ 12,13 ]
Perylene diimide (PDI) derivatives are
the most commonly studied perylene
derivatives due to their excellent solu-
bility, strong absorption in the vis-
ible region and high electron mobility
(1.7 cm
2 V −1 s −1 ). [ 12–14 ] Among the various
types of PDI-based acceptors,
[ 3,14–19 ] bay-
modifi ed PDI dimers exhibit superior
photovoltaic properties.
[ 5,8,9,18–22 ] In 2013, Yao and collaborators
reported the fi rst PCE of over 4% in this fi eld by using a thio-
phene substituted, bay-linked PDI dimer as the acceptor and
poly{4,8-bis[5-(2-ethylhexyl)-thiophene-2-yl]benzo[1,2-b:4,5-b′]-
dithiophene-2,6-diyl- alt -2-(2-ethylhexanoyl)thieno[3,4-b]-
thiophene-4,6-diyl} (PBDTTT-C-T) as the donor.
[ 11 ] Recently,
the effi ciency of this blended system has been increased
to 6.1% via morphology optimization.
[ 9 ] Wang’s group also
designed a series of novel PDI dimers to improve the effi ciency
of PSCs step-wise from 3.6% to over 6% in only two years by
combining the efforts of the molecular modifi cation of PDI units,
the optimization of the blend fi lm morphology, and the selection
of the appropriate donor materials.
[ 20–22 ] Overall, the effi ciency of
PSCs based on PDI-based small molecular acceptors has been
boosted to more than 6%, which is comparable to the PCE values
realized by other types of high performance fullerene-free
acceptors,
[ 7,23–25 ] such as poly{[ N , N ′-bis(2-octyldodecyl)-1,4,5,8-
naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)}
(N2200) and 3,9-bis[2-(3-oxo-2-vinyl-2,3-dihydroinden-1-ylidene)-
malononitrile]-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-
d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). However,
the performance of all-polymer solar cells (All-PSCs) based on
PDI polymers is poor and the development continues to slowly
To realize high power conversion effi ciencies (PCEs) in green-solvent-pro-
cessed all-polymer solar cells (All-PSCs), a long alkyl chain modifi ed perylene
diimide (PDI)-based polymer acceptor PPDIODT with superior solubility in
nonhalogenated solvents is synthesized. A properly matched PBDT-TS1 is
selected as the polymer donor due to the red-shifted light absorption and low-
lying energy level in order to achieve the complementary absorption spectrum
and matched energy level between polymer donor and polymer acceptor. By
utilizing anisole as the processing solvent, an optimal effi ciency of 5.43% is
realized in PBDT-TS1/PPDIODT-based All-PSC with conventional confi gura-
tion, which is comparable with that of All-PSCs processed by the widely used
binary solvent. Due to the utilization of an inverted device confi guration, the
PCE is further increased to over 6.5% effi ciency. Notably, the best-performing
PCE of 6.58% is the highest value for All-PSCs employing PDI-based polymer
acceptors and green-solvent-processed All-PSCs. The excellent photovoltaic
performance is mainly attributed to a favorable vertical phase distribution,
a higher exciton dissociation effi ciency ( P diss ) in the blend fi lm, and a higher
electrode carrier collection effi ciency. Overall, the combination of rational
molecular designing, material selection, and device engineering will motivate
the effi ciency breakthrough in green-solvent-processed All-PSCs.
1. Introduction
Nonfullerene polymer solar cells (PSCs), comprising p -type
polymer as the donor and n -type small molecule or polymer as
the acceptor, have attracted extensive attention for their rapid
performance progress in recent years.
[ 1–5 ] Through the rational
molecular design of nonfullerene acceptors, the power con-
version effi ciencies (PCEs) of nonfullerene PSCs have been
increased from 6% to 7%,
[ 4–9 ] in some cases, reaching the same
performance as fullerene derivative-based PSCs.
[ 7 ] Because
nonfullerene acceptors possess versatile and easily modifi ed
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progress. To date, the highest PCE values of PDI-polymers are
limited at ≈4%.
[ 26–29 ] Therefore, it is of great signifi cance to
realize the effi ciency breakthrough in All-PSCs featuring PDI-
based polymer acceptors.
To pursue high effi ciency in All-PSCs based on PDI-based
polymer acceptors, the blend fi lm morphology should be
fi nely manipulated.
[ 30 ] Several reports demonstrated that
the elaborate selection of processing solvents is an effi cient
method in realizing favorable morphologies in All-PSCs.
[ 31–35 ]
In 2011, Tajima and co-workers introduced solvent mixtures
based on toluene and o -xylene as processing solvents to fabri-
cate PDI polymer-based All-PSCs that achieved higher PCEs
relative to those of devices utilizing conventional solvents,
such as chlorobenzene (CB), produced in parallel.
[ 35 ] Although
an optimistic trend emerged in this work, the achieved PCEs
(≈2%) still remained low at that time. Recently, Jen and his
co-workers applied o -xylene in nonfullerene PSCs employing
PDI-based small molecular acceptors to achieve a high effi -
ciency of 5.2%.
[ 22 ] Hence, as environment friendly processing
solvents, nonhalogenated solvents may show great potential
for achieving high-effi ciency in PDI-based All-PSCs, although
such work has rarely been reported. Moreover, selecting hal-
ogen-free processing solvents with lower toxicity is of great
importance in the industrialization of All-PSCs and other
types of organic photovoltaic devices. As a green solvent,
anisole possesses a lower toxicity but a similar boiling point
when compared to commonly-used, nonhalogenated solvents
such as toluene and o -xylene. [ 36 ] In the case of a polymer/
fullerene system, the performance obtained from anisole/
diphenyl ether binary solvent was comparable to that of the
devices processed by traditional o -dichlorobenzene/1,8-diiodo-
octane ( o -DCB/DIO) binary solvent.
[ 37 ] It can be anticipated
that anisole has the potential to be successfully applied in
All-PSCs.
Here, to realize the high effi ciency and green-solvent pro-
cessing of PDI-based All-PSCs, a novel nonfullerene acceptor,
poly {[ N , N ′-bis(2-octyldodecyl)-3,4,9,10-perylene diimide-1,7-diyl]-
alt -(thiophene-2,5-diyl)} (PPDIODT, see Figure 1 a), with excel-
lent solubility was synthesized by introducing bulky alkyl chain
2-octyldodecyl on PDI. Noticeably, the large dihedral angles
between thiophene and PDIs are in favor of breaking the
strong aggregation among PDI units for enhanced solubility,
[ 38 ]
and the solubility of the target polymer is further improved
by screening the long solubilizing alkyl chains on the PDI
unit. In this regard, PPDIODT exhibits superior solubility not
only in commonly used processing solvents (such as o -DCB,
CB, and chloroform (CF)) but also in green solvents (such as
toluene and anisole). A high-performance photovoltaic mate-
rial, poly{[4,8-bis[5-(octylthio)-thiophene-2-yl]benzo[1,2-b:4,5-b′]-
dithiophene-2,6-diyl]- alt -[2-ethylhexyl-3-fl uorothieno[3,4-b]-thio-
phene-2-carboxylate]} (PBDT-TS1) (Figure 1 a), was chosen to
be the polymer donor, which possesses a red-shifted absorp-
tion with an absorption peak at 720 nm and an absorption edge
over 800 nm, as well as a low-lying highest occupied molecular
orbital (HOMO) level of −5.35 eV (Figure 1 b),
[ 39,40 ] which affords
a complementary spectra and properly matched molecular
energy levels with PPDIODT. When processed by anisole, the
fullerene-free PSCs based on PBDT-TS1:PPDIODT achieved
an optimal effi ciency of 5.43% without adding any additive or
post-treatment; this effi ciency is comparable to the optimum
PCE of 5.63% acquired from the conventional binary solvent
of CB and 1-chloronaphthalene (CN) processed devices in par-
allel. The optimized morphology of the blend fi lm processed
by anisole reveals a smoother surface with a root-mean-square
(RMS) roughness of 1.23 nm but a similar internal domain size
compared to the CB/3% CN processed blend fi lm. In addition,
inverted devices were fabricated due to the unfavorable vertical
phase distribution of the blend fi lm in conventional structured
All-PSCs. An optimal PCE of 6.58% utilizing the inverted device
confi guration was recorded, with substantial enhancements
of both the short-circuit current ( J sc ) and the fi ll factor (FF).
Our study clearly indicated that the effi ciency breakthrough of
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Figure 1. a) Molecular structure of the polymer acceptor PPDIODT and the polymer donor PBDT-TS1, b) UV–vis absorption spectra and energy level
diagram of PPDIODT and PBDT-TS1, c) synthesis routes of polymer PPDIODT.

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green-solvent-processed, PDI polymer-based All-PSCs could be
realized via the rational molecular designing/selection of mate-
rials and the optimization of the device architecture.
2. Results and Discussions
2.1. Basic Properties of Polymer PPDIODT
As illustrated in Figure 1 c, the new polymer acceptor,
PPDIODT, was copolymerized by two easily accessible mono-
mers, i.e., 2-octyldodecyl substituted PDIs (PDI2OD-Br
2 ) and
2,5-bis(trimethylstannyl)thiophene via a Stille polymerization
reaction using Pd
2 (dba) 3 /P( o -tolyl) 3 as the catalyst. More details
can be found in the Section 4 and Figures S1–S3 in the Sup-
porting Information. Monomer PDI2OD-Br
2 was synthesized
according to a previous literature report.
[ 41 ] As previously men-
tioned in the report by Facchetti and co-workers, PDI2OD-Br
2
consists of 1,6- and 1,7-isomers (molar ratio ≈1:5), and it was used
without further purifi cation in this work. The number average
molecular weight ( M n ) and the polydispersity index of the target
polymer PPDIODT is 13.6 and 1.38 kDa , respectively, as deter-
mined by gel permeation chromatography (GPC) using CF as the
eluent. Due to the bulky and solubilizing alkyl chain, the target
polymer PPDIODT shows superior solubility in commonly used
solutions, such as o -DCB, CB, CF, and toluene. Interestingly, this
polymer acceptor also has a good solubility in the green solvent
anisole; thus, PPDIODT is a promising model material in the
green-solvent processing of All-PSCs. Depicted in Figure 2 a is
the thermogravimetric analysis (TGA) curve of PPDIODT. The
target polymer exhibits an excellent thermal stability with the
decomposition temperature ( T d ) at 446 °C, which is higher than
the temperature requirement for practical photovoltaic applica-
tion. The X-ray diffraction (XRD) pattern of neat PPDIODT fi lm
(Figure 2 b) shows almost no lamellar stacking peak but a weak
and broad refl ection peak in high angle region, which represents
a certain feature of
π
–
π
stacking in molecular arrangement.
Such a weak molecular aggregation of PPDIODT in solid state
was also observed in several PDI-based polymers reported previ-
ously.
[ 35,42 ] To gain further understanding of the molecular con-
fi guration, molecular modeling using density functional theory
(DFT) was performed.
[ 43 ] The two dihedral angles between thio-
phene and PDI plane are 54.2° and 57.4° (Figure S4, Supporting
Information), and the PDI unit shows a slight angular distortion
as well. The twisted structure in PPDIODT potentially leads to a
slightly larger d -spacing in the
π
–
π
direction that promotes the
reduction of strong aggregation effects among the PDI units,
thus enhancing the exciton diffusion and dissociation.
Both absorption spectra (Figure 2 c) of PPDIODT in the
diluted solution and in the fi lm state reveal strong absorp-
tion from 400 to 700 nm, and the maximum absorption wave-
length is 562 nm, with an extinction coeffi cient (solid state)
of 2.1 × 10
4 cm −1 . The polymer acceptor in the solid fi lm state
exhibits a similar absorption profi le as in the diluted solution,
which confi rms a weak interchain interaction in the fi lm.
In addition, the spectrum of the target polymer displays an
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Figure 2. Basic properties of PPDIODT: a) TGA curve, b) XRD profi le, c) normalized UV–vis spectra in the diluted solution (CF) and in the fi lm state,
d) cyclic voltammetry curves with a scan rate of 20 mV s
−1 (calibrated by Fc/Fc
+ redox couple).

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absorption edge at 694 nm, corresponding
to an optical band gap of 1.74 eV. The
absorption edge of PPDIODT is slightly
blue-shifted relative to other PDI polymers
with a similar backbone.
[ 28,35 ] In terms of
spectral coverage, a stronger absorption in
the visible region of 500–700 nm results
in PPDIODT undoubtedly outperforming
PCBM. For the molecular energy levels,
the electrochemical cyclic voltammetry
(CV) characterization of PPDIODT was
performed, with the Fc/Fc
+ redox couple
used as the external standard. As depicted
in the CV curve (Figure 2 d), the oxidation
onset potential and the reduction onset
potential are 1.10 and −0.84 V, respectively. Accordingly, the
HOMO and the lowest unoccupied molecular orbital (LUMO)
levels are estimated to be −5.90 and −3.96 eV, respectively.
Therefore, polymer PPDIODT possesses an almost identical
LUMO level to those of PCBM and N2200;
[ 44 ] the LUMO level
is an important requirement for novel electron acceptors to
properly matching with the state of-the-art donor polymers.
The above properties, including the absorption spectrum,
LUMO level, molecular conformation, and solubility, suggest
PPDIODT is a promising polymer acceptor in the fi eld of
nonfullerene PSCs.
2.2. Photovoltaic Properties
To verify the potential applicability of PPDIODT as a polymer
acceptor in All-PSCs, we select a newly designed high-perfor-
mance material, PBDT-TS1, as the polymer donor. Due to a
low-lying HOMO level of −5.35 eV and a red-shifted absorption
spectrum relative to that of PPDIODT, PBDT-TS1 is properly
matched with PPDIODT and presents a considerable HOMO
or LUMO level offset of over 0.4 eV (Figure 1 b) and a comple-
mentary spectral coverage from 400 to 800 nm in the blended
fi lm (see Figure 3 a). To understand the compatibility of the
polymer donor:polymer acceptor blend in charge transfer/
generation aspect, photoluminescence (PL) spectra of the neat
polymer fi lms and blend fi lm were obtained. As shown in
Figure 3 b, the strong fl uorescence of both the donor PBDT-
TS1 and the acceptor PPDIODT can be effectively quenched
by mixing PBDT-TS1 and PPDIODT together, indicating a very
effi cient photoinduced charge transfer between the donor and
the acceptor in the fi lm.
Herein, we select anisole as the processing solvent. All-PSCs
based on the conventional device architecture of indium tin
oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sul-
fonate (PEDOT:PSS)/PBDT-TS1:PPDIODT/Mg/Al were fab-
ricated ( Figure 4 a), and All-PSCs processed by the commonly
used binary solvent (CB/CN) were fabricated as controls. The
optimization details are shown in Figures S5,S6, Table S1 in
the Supporting Information, and Table 1 . Upon the optimum
D / A ratio (1:1) and fi lm thickness (130 nm), the best effi ciency
of 5.43% with an open-circuit voltage ( V oc ) of 0.76 V, a J sc of
14.67 mA cm
−2 , and a FF of 48.73% (Figure 4 b) was recorded
under the illumination of AM 1.5G, 100 mW cm
−2 . The
integral current density value deduced from external quantum
effi ciency (EQE) spectrum (Figure 4 c) is consistent with the J sc
of J–V test, with an error no higher than 4%. The widely used
binary solvent (CB/3% CN) was also applied in fabricating
devices in parallel (Figure 4 d), and an average effi ciency of
5.45±0.19% was obtained. Hence, All-PSC devices processed
by the single green solvent anisole and without any additives
and post-treatments are able to achieve a comparable perfor-
mance with the devices processed by conventional binary halo-
genated solvent.
2.3. Morphology Study
XRD diffraction patterns (Figure 4 e) verifi ed that the ani-
sole processed blend fi lms present the same microstructural
order features as the blend fi lms processed by the CB/CN
binary solvent. To reveal the morphological features of PBDT-
TS1:PPDIODT blend fi lms processed by the green solvent,
the surface and bulk morphology of the blend fi lms were
investigated by atomic force microscopy (AFM) and transmis-
sion electron microscopy (TEM). As revealed in Figure 5 a–c,
the anisole-processed blended fi lm shows a much smoother
surface with a RMS roughness of 1.23 nm and a quite sim-
ilar length scale of phase separation to that of the CB/CN-
processed blended fi lm (Figure S7, Supporting Information
and Figure 5 d). In addition, the ideal morphology with a
donor:acceptor interpenetrating network and an approximately
20–30 nm domain size promotes exciton dissociation and
charge transport, both of which are benefi cial for achieving a
good J sc in All-PSCs. The charge mobility of the BHJ blend
fi lm was also measured by using the space-charge-limited-cur-
rent (SCLC) method (Figure S8, Supporting Information).
[ 45 ]
The hole and electron mobilities of PBDT-TS1:PPDIODT-
based fi lm are 5.75 × 10
−4 and 1.71 × 10
−3 cm 2 V −1 s −1 , respec-
tively, corresponding to an electron/hole mobility ratio ( µ e / µ h )
of 2.97. The relatively unbalanced charge transport might be
the main origin of the poor FF. X-ray photoelectron spectros-
copy (XPS) was further utilized to analyze the top/bottom
surface (0–10 nm) compositions of the blended fi lm casted by
anisole. According to some previous works, the relative con-
tents of two components can be evaluated qualitatively from
the peak intensities of individual elements.
[ 46 ] For PBDT-TS1/
PPDIODT blend system, fl uorine (F) and nitrogen (N) can
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Figure 3. a) Absorption spectra of the PBDT-TS1:PPDIODT blend fi lm, b) photoluminescence
spectra of the neat PBDT-TS1 fi lm and the blend fi lm.

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be referred as the characteristic element of PBDT-TS1 and
PPDIODT, respectively. As depicted in Table S2 in the Sup-
porting Information, the characteristic element content of
each pure polymer was measured as the control, which is
close to the theoretical value of pure polymer. The contents of
F 1s and N 1s peaks of the top surface are 0.55% and 1.33%,
respectively, while the content of N 1s peak of the bottom
surface rises to 1.52% and almost no F atom can be detected,
which demonstrates that more PBDT-TS1 is enriched on the
top surface and almost no donor material is distributed at the
bottom surface. The accumulation of donor polymers near the
cathode interface will lead to an unfavorable vertical phase
distribution, which is detrimental to the carrier transport
and collection in the conventional device confi guration.
[ 46a ]
The XPS results clearly show that rationally applying another
novel device confi guration with an appropriate vertical phase
separation will potentially result in a high FF and thus a
high PCE.
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Figure 4. a) The architecture illustrations of conventional and inverted All-PSCs employing PBDT-TS1:PPDIODT as the active layer, b) J – V curves.
c) EQE curves of the best-performing All-PSCs based on PBDT-TS1:PPDIODT with different device architectures under the illumination of AM 1.5G,
100 mW cm
−2 , d) J–V and EQE curve (insert image) of the best-performing conventional All-PSCs based on PBDT-TS1:PPDIODT processed by the
CB/3%CN solvent, e) XRD patterns of PBDT-TS1:PPDIODT based blend fi lms processed by anisole and the CB/3%CN binary solvent.
Table 1. The photovoltaic parameters of PBDT-TS1:PPDIODT-based
conventional All-PSCs processed by different solvents.
Processing
solvent
D / A ratio V oc
[V]
J sc
[mA cm
−2 ]
FF
[%]
PCE
max
[%]
PCE
avg a)
[%]
Anisole 1.5:1 0.74 12.87 49.56 4.72 4.60 ± 0.20
Anisole 1:1 0.76 14.67 48.73 5.43 5.32 ± 0.11
Anisole 1:1.5 0.77 12.26 46.83 4.39 4.31 ± 0.16
CB/3%CN 1:1 0.77 13.74 53.22 5.63 5.45 ± 0.19
a) Average PCE values are obtained from over 10 devices fabricated in parallel.

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2.4. Device Confi guration Optimization
To overcome the issue of unfavorable phase distribution in con-
ventional devices, All-PSCs based on an inverted architecture
(see Figure 4 a) of ITO/ZnO/PFN-Br/PBDT-TS1:PPDIODT/
MoO
3 /Al were fabricated, and the blend fi lms were processed
under the same conditions as those of the conventional All-
PSC. Figure 4 b shows that the J sc and FF of the device based on
the PBDT-TS1:PPDIODT all-polymer blend can be improved
from 14.67 to 15.72 mA cm
−2 and from 48.73% to 55.11%,
respectively. The average effi ciency of 6.50 ± 0.13% is obtained
in an inverted PSC ( Table 2 ); this effi ciency is over 20% higher
than the effi ciency of the All-PSC based on the conventional
confi guration. Remarkably, the maximum PCE of 6.58% is the
highest value in PDI-based All-PSCs and also one of highest
values among single green-solvent-processed PSCs. The max-
imum EQE value located at 720 nm is increased to 75% (see
Figure 4 c), which is 21% higher than the EQE value at the same
wavelength based on All-PSCs of the conventional architecture.
The enhancement in 700–800 nm region is mainly attributed to
the enhanced charge transfer and collection due to the enrich-
ment of PBDT-TS1 near the anode interface verifi ed by XPS
characterizations. In addition, the PL spectra of two blend fi lms
spin coated on PEDOT:PSS and PFN-Br were also measured
in parallel. As shown Figure S9 in the Supporting Informa-
tion, a more effi cient PL quenching is observed in blend fi lm
spin coated on PFN-Br in comparison with that spin coated on
PEDOT:PSS, indicating more effi cient exciton dissociation at
the donor–acceptor interface occurs in inverted devices. When
different interface layers are utilized for depositing polymeric
blend layers, the difference of EQE can be potentially attributed
to the varied molecular orientations in blend fi lms.
[ 47 ]
Further studies on the basic operational mechanism were
performed to obtain the photo-generated current density
( J ph = J L – J D , J L : current density under illumination; J D : current
density in the dark) versus effective voltage ( V eff = V 0 – V a , V 0 : the
voltage when J ph is zero; V a : applied voltage) curves of devices
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Figure 5. a) AFM height and b) phase images of PBDT-TS1:PPDIODT blended fi lm processed by anisole. TEM images of PBDT-TS1:PPDIODT blended
fi lms processed by c) anisole and d) CB/CN binary solvent.
Table 2. The optimal photovoltaic parameters of All-PSCs based on
PBDT-TS1:PPDIODT under the illumination of AM 1.5G, 100 mW cm
−2 .
Device V oc
[V]
J sc
[mA cm
−2 ]
FF
[%]
PCE
max
[%]
P diss
[%]
S
Conventional All-PSC 0.76 14.67 48.73 5.43 80 0.96
Inverted All-PSC 0.76 15.72 55.11 6.58 87 0.99

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based on different architectures, as plotted in Figure 6 a. [ 21 ] The
exciton dissociation probabilities ( P diss ) of conventional and
inverted optimized devices under the short circuit condition
are 80% and 87%, respectively. Less geminate recombination
occurs in All-PSCs based on the inverted structure; this reduced
recombination could be the main reason for the high EQE. The
J sc of optimal All-PSCs under different light intensities ( P )
was also measured: the relationship between J sc and P can be
described as J sc ∝ P
S . [ 48 ] In the light intensity dependence of J sc
curves (see Figure 6 b), the scaling factor ( S ) value of optimal
devices based on conventional and inverted architectures are
0.96 and 0.99, respectively. Accordingly, the inverted All-PSCs
based on PBDT-TS1:PPDIODT exhibited reduced bimolecular
recombination and higher carrier collection effi ciency, which
are very benefi cial to achieve high J sc and high FF.
3. Conclusion
In summary, a long alkyl chain modifi ed nonfullerene acceptor
PPDIODT copolymerized by 2-octyldodecyl substituted PDIs
and thiophene was synthesized and characterized. The excel-
lent solubility of polymer PPDIODT in both commonly used
solvent (CB) and green solvent (anisole) was attributed to the
twisted conformation and long solubilizing alkyl chains. To
realize the complementary spectrum and well-matched molec-
ular energy level between the donor and the acceptor in PSCs, a
high-performance photovoltaic polymer PBDT-TS1 was chosen
to be the electron donor. By using the halogen-free solvent ani-
sole as the processing solvent, a desirable effi ciency of 5.43%
was obtained by the All-PSC based on PBDT-TS1:PPDIODT
without introducing any additives or post-treatments; this
effi ciency is comparable to the PCE acquired from a conven-
tional binary solvent system (CB/3% CN) device processed in
parallel. To avoid the unfavorable vertical phase distribution in
the conventional device, inverted All-PSCs were further fabri-
cated; the PCE was enhanced to 6.58%; the improved perfor-
mance is primarily due to the simultaneous enhancements of
J sc and FF. Photocurrent and light intensity dependence ana-
lyzes revealed that the improved performance of the inverted
devices originated from the suppression of geminate/bimo-
lecular recombination and the higher carrier collection effi -
ciency. Overall, through the rational molecular design of the
acceptor material, the elaborate selection of donor materials,
and the optimization of the device structure,
the effi ciency breakthrough of green-solvent-
processed All-PSCs based on PDI polymers
can be realized. We foresee a bright future
of high-effi ciency all-polymer solar cells with
simple environmentally-benign processing.
4. Experimental Section
Synthesis of Polymer PPDIODT : The monomer
N , N ′-bis(2-octyldodecyl)-(1,7&1,6)-dibromo-
3,4,9,10-perylene diimide (PDI2OD-Br
2 ) (240.5 mg,
0.217 mmol) and 2,5-bis (trimethylstannyl)thiophene
(88.9 mg, 0.217 mmol) were dissolved in toluene
(8 mL). After being fl ushed by argon for 5 min,
Pd
2 (dba) 3 (5 mg, 0.005 mmol) and P( o -tol) 3 (12 mg,
0.04 mmol) were added into the solution as catalyst, and then, the mixture
was fl ushed by argon for another 10 min. The reaction was stirred at 110 °C
for 48 h to precipitate crude polymer into methanol. The dried polymer was
further purifi ed by Soxhlet extraction (acetone and hexane) and column
chromatography. Next, the polymer was precipitated into methanol to
afford PPDIODT (152 mg, 46% yield) as dark purple solid. Finally, the target
material was dried in a vacuum oven for 24 h.
1 H NMR (400 MHz, CDCl
3 ,
δ
): 8.73–8.43 (m, 6H), 7.42–7.33 (m, 2H), 4.12 (m, 4H), 2.00 (m, 2H),
1.22–1.19 (m, 64H), 0.81–0.75 (m, 12H). Anal. Calcd (%) for C
68 H 90 N 2 O 4 S:
C 79.18, H 8.79, N 2.72; found: C 78.65, H 8.80, N 2.80.
Fabrication and Characterization of All-PSCs : The indium-tin oxide
(ITO)-coated glass substrate was precleaned by deionized water, acetone,
and isopropanol twice successively. Next, the substrate was transferred
to the oven and dried for 15 min at 150 °C. For the conventional
structure devices, after 20 min of ultraviolet-ozone treatment, a
35 nm layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) (Heraeus Materials, 4083) was spin coated onto the
ITO. After annealing for 15 min at 150 °C, all PEDOT:PSS coated glass
substrates were transferred to a nitrogen-fi lled glovebox. Equal weight
amounts of PBDT-TS1 and PPDIODT were dissolved in anisole with a
total concentration of 14 mg mL
−1 and then stirred at 100 °C for 8 h. The
concentration of the PBDT-TS1:PPDIODT (1:1, wt/wt) blend solution in
CB was 10 mg mL
−1 (polymer/CB); the solution was stirred at 75 °C for
6 h. The high boiling-point additive, chloronaphthalene (CN), was added
into the blend solution 30 min before the spin-coating process with a
ratio of 3% vol to the host solvent CB. After spin coating the active layer
onto the PEDOT:PSS layer, the conventional devices were completed by
vacuum evaporating Mg/Al metal electrodes, resulting in a cell area of
4.15 mm
2 . The integrated conventional device structure is Glass (ITO)/
PEDOT:PSS (35 nm)/blend fi lm (≈130 nm)/Mg (20 nm)/Al (100 nm).
For the inverted devices, a thin layer of sol-gel ZnO (≈30 nm) was spin
coated onto precleaned ITO-coated glass substrates and then annealed
at 200 °C for 1 h in air. Next, a 5 nm thick PFN-Br was cast on the top of
ZnO and then annealed at 150 °C for another 10 min in a nitrogen-fi lled
glovebox. The ZnO precursor solution and PFN-Br methanol solution
were prepared according to reports in the literature.
[ 49,50 ] Subsequently,
the PBDT-TS1:PPDIODT anisole solution was spin coated at 1800 rpm
to the optimal fi lm thickness (≈130 nm). Finally, 10 nm thick MoO
3 fi lm
and 100 nm thick Al layers were deposited sequentially to complete the
inverted device. The integrated inverted confi guration is Glass (ITO)/
ZnO (30 nm)/PFN-Br (5 nm)/active layer (≈130 nm)/MoO
3 (10 nm)/
Al (100 nm). The current–voltage curves of the devices were measured
under 100 mW cm
−2 of the standard AM 1.5G spectrum. The spectral
mismatch factor was calibrated to be unity via a National Institute of
Metrology (NIM) certifi cated silicon reference cell with a KG3 fi lter.
[ 51 ]
All EQE curves were measured through the solar cell spectral response
measurement system QE-R3011 (Enli Technology Ltd., Taiwan), which
was calibrated by monocrystalline silicon solar cell in advance. The
SCLC method was applied for hole/electron mobility measurement
of blend fi lms.
[ 45 ] The hole-only and electron-only device structure is
Adv. Energy Mater. 2015, 1501991
www.MaterialsViews.com
www.advenergymat.de
Figure 6. a) Photocurrent versus effective voltage plots and b) light intensity dependence of
short circuit intensity curves (solid lines are fi ts) of All-PSCs under optimal conditions for dif-
ferent architectures.

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1501991 (8 of 9) wileyonlinelibrary.com Adv. Energy Mater. 2015, 1501991
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ITO/PEDOT:PSS/BHJ/Au and ITO/ZnO/BHJ/Al, respectively. X-ray
photoelectron spectroscopy data was obtained on the Thermo Scientifi c
ESCALab 250Xi using 200 W monochromated Al K
α
radiation. The
500 µm X-ray spot was used for XPS analysis. The XPS takeoff angle is
90° and the probing length is 10 nm. The base pressure in the analysis
chamber was about 3 × 10
−10 mbar. Typically the hydrocarbon C 1s line
at 284.8 eV from adventitious carbon is used for energy referencing.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors gratefully acknowledge the fi nancial support from NSFC
(21325419, 91333204), the CAS-Croucher Fund for Joint Lab (CAS14601),
and the Chinese Academy of Science (XDB12030200, KJZD-EW-J01).
Received: October 6, 2015
Revised: November 18, 2015
Published online:
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