Non-planar perylenediimide acceptors with different geometrical linker units for efficient non-fullerene organic solar cells

Article abstract of DOI:10.1039/c6ta08739f

Three perylenediimide (PDI) acceptors (P2O2, P2N2 and P4N4) were synthesized by functionalizing the bay positions of PDI with benzil, 2,3-diphenylquinoxaline and 2,3,7,8-tetraphenylpyrazino[2,3-g]quinoxaline as linkers, respectively. The photovoltaic prop

Full text of DOI:10.1039/c6ta08739f

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1
0.1039/C6TA08739F.
Volume
4
Number
1
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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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Non-planar perylenediimide acceptors with different geometrical
linker units for efficient non-fullerene organic solar cells†
a
b
a
a
a
b
Xi Liu,‡ Tao Liu,‡ Chunhui Duan, Junyi Wang, Shuting Pang, Wentao Xiong ,
b
a
a
Yanming Sun,* Fei Huang* and Yong Cao
a
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, P. R. China. *Eꢀmail: msfhuang@scut.edu.cn
b
Heeger Beijing Research and Development Center, School of Chemistry and
Environment, Beihang University, Beijing 100191, P. R. China. *Eꢀmail:
sunym@buaa.edu.cn
†
‡
Electronic supplementary information (ESI) available. See DOI:
These authors contributed equally to this work.
Abstract: Three perylenediimide (PDI) acceptors (P2O2, P2N2 and P4N4) were
synthesized by functionalizing the bay positions of PDI with benzil,
2
,3ꢀdiphenylquinoxaline and 2,3,7,8ꢀtetraphenylpyrazino[2,3ꢀg]quinoxaline as linker,
respectively. The photovoltaic properties of the three acceptor molecules have been
investigated. The different PDI linker units show different physical and chemical
properties of the PDIs. The three PDIs display different nonꢀplanar geometrical
structures because of the different linker units, which affect the corresponding
morphology of the blend films and also influence the charge mobility and fill factor
(FF) of the organic solar cells (OSCs). Furthermore, the gradient energy levels of the
three PDIs provide an efficient research model for the relationship of device
openꢀcircuit voltage (Voc) and energy levels. As the result, the P4N4 based
nonꢀfullerene devices show the best photovoltaic performance with the power
conversion efficiency (PCE) of 5.71%, whereas the P2O2 and P2N2 based
nonꢀfullerene devices show relatively lower PCEs of 2.53% and 3.86%, respectively.
Bulkꢀheterojunction (BHJ) organic solar cells (OSCs) based on organic donor
materials and acceptor materials have attracted considerable attention in recent years
because of their outstanding potential of producing lightꢀweight and flexible
1
ꢀ5
photovoltaic devices via lowꢀcost largeꢀscale fabrication. Fullerene derivatives
acceptors, combined with the efficient polymer or small molecular donor materials,
constitute highꢀefficiency OCSs due to the good electron transport property and the
6
ꢀ20
suitable morphology in the BHJ films. However, the weak absorption in the visible

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DOI: 10.1039/C6TA08739F
region, the limited band gap variability and the high cost of the fullerene derivatives
21ꢀ25
impede the development of OSCs.
On this occasion, nonꢀfullerene acceptors
(
NFAs) have gained much attention for their superiorities such as the adjustable
21ꢀ25
energy levels, suitable absorption, and easy synthesis.
Meanwhile, the OSCs based
26ꢀ48
49ꢀ62
on NFAs, including fused aromatic imides,
calamitic (AꢀDꢀA type) acceptors,
6
3ꢀ66
67ꢀ70
polymer acceptors,
etc, have made great progress recently.
The perylenediimide (PDI) unit is a promising building block for NFAs applied
into largeꢀscale OSCs because of its favorable absorption, energy levels, electron
2
1ꢀ25
mobility, and low cost.
However, the PDI block unit with the large planar structure
show a large aggregation nanostructure and excessively large domains in the active
26
layers which severely degrade the performance of OSCs. Hence, much attention has
26ꢀ46
been devoted to nonꢀplanar PDIs acceptors to overcome that drawback recently.
There are various methods to prepare nonꢀplanar PDI derivatives, including the PDI
dimers, trimers or tetramers synthesized by the different positions
26ꢀ46
functionalization.
The PDI dimers, trimers or tetramers show the obvious
nonꢀplanar geometries compare to the PDI core unit, which is conducive to form the
suitable nanostructure in blend films for the enhancement of the shortꢀcircuit current
(Jsc) and fill factor (FF). The different linker units of the nonꢀplanar PDIs can result in
different LUMO levels of the PDIs acceptors, which closely relate to the openꢀcircuit
voltage (Voc) in OSCs. Recently, Yan and coꢀworks demonstrate a nonꢀfullerene OSC
with 9.5% efficiency and with a small driving force, and obtain a Voc of 1.11 V for a
spiroꢀfluorene linked PDI dimer (SFꢀPDI
2
) and a novel polymer (P3TEA) blend film,
69
which creates a good path towards highly efficient OSCs with a low energy loss.
Hence, it is important to obtain highꢀefficiency PDI acceptors with matched
energy levels to achieve high Voc, and suitable nonꢀplanar geometrical structure to
form good nanostructure to achieve high Jsc and FF. Herein, we report a PDI system
with three relevant PDI acceptors to access high performance nonꢀfullerene OSCs.
The relationship of structure and performance of PDI acceptors has been
systematically studied. As shown in Scheme 1, the three PDIs are named as P2O2
(
PDI dimer bay positions functionalization with benzil linker), P2N2 (PDI dimer bay
positions functionalization with 2,3ꢀdiphenylquinoxaline linker), and P4N4 (PDI
tetramer bay positions functionalization with
,3,7,8ꢀtetraphenylpyrazino[2,3ꢀg]quinoxaline linker). The different linker units
2
generate the different PDI geometrical structures and energy levels, which affect the
device performance parameters. The PDIs nonꢀfullerene devices were fabricated with
7
1ꢀ73
a wellꢀestablished wideꢀbandgap polymer (PDBTꢀT1),
sc and FF data which matched well with the design strategy. Especially, the P4N4
based OSCs devices show the best power conversion efficiency (PCE) of 5.71% with
and yield the gradient Voc,
J
ꢀ2
a Voc of 0.958 V, a Jsc of 9.40 mA cm and a FF of 63.4%.

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Scheme 1 Device architecture of nonꢀfullerene OSCs used in this study, as well as the chemical
structures of the active layer PDBTꢀT1 and the three PDI acceptors (P2O2, P2N2 and P4N4)
employed in this study.
Scheme 2 Chemical structures and synthetic routes of the PDIs. Reagents and conditions: (a)
bis(pinacolato)diboron, Pd(dppf)Cl
CO aq., THF, 80 °C, 24 h, argon; (c) oꢀphenylenediamine, AcOH, reflux, 10 h, argon; (d)
,2,4,5ꢀbenzenetetraamine tetrahydrochloride, AcOH, CHCl , reflux, 30 h, argon.
2 3 4
, potassium acetate, dioxane, 80 °C, 24 h, argon; (b) Pd(PPh ) ,
K
2
3
1
3
2
2
. Results and discussion
.1. Synthesis and characterization
Scheme 2 shows the chemical structures and the synthetic routes of these three

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nonꢀfullerene PDI acceptors. The diboronic ester monomer 1 was synthesized by
Pdꢀcatalyzed Suzuki crossꢀcoupling with 4,4’ꢀDibromobenzil and
bis(pinacolato)diboron, and the product was purified by recrystallization from
methanol. The important intermediate1ꢀbromoꢀPDI (PDIꢀBr, 2) was synthesized
2
7
according to the previously reported literature procedures. The P2O2 was
synthesized by Pdꢀcatalyzed Suzuki crossꢀcoupling with monomer 1 and PDIꢀBr, then
the P2O2 was reacted with oꢀphenylenediamine and 1,2,4,5ꢀbenzenetetraamine
tetrahydrochloride to give the product P2N2 and P4N4, respectively. The P2O2, P2N2
and P4N4 were purified by column chromatography on silica gel. The acceptors were
1
13
characterized using H and C NMR spectroscopy and MALDIꢀTOF MS. The
acceptors were easily soluble in common organic solvents, such as dichloromethane,
chloroform, and oꢀdichlorobenzene (oꢀDCB), which assure the acceptors to prepare
the neat film. The thermal stability of the acceptors was investigated by thermal
gravity analysis (TGA) under a nitrogen atmosphere, and the results are presented in
Fig. S1 (ESI†). P2O2, P2N2 and P4N4 showed good thermal stability with 5%
d
weightꢀloss temperatures (T ) of 393, 408 and 394 °C, respectively.
2
.2. Theoretical calculations
The density functional theory (DFT) calculations were performed at the
B3LYP/6ꢀ31G(d) level to optimize the ground state geometries of the acceptors
7
4
molecules using the Gaussian 09 package40. As shown in Fig. 1, for the 3 acceptors,
the dihedral angle between the benzene ring and the conjoint PDI unit (A1 or A3) is
calculated to be about 57° to 60°. The dihedral angle A2 between the two benzene
rings in the middle of the P2O2 is calculated to be 139.2°, while the diketone unit was
reacted with the oꢀphenylenediamine or 1,2,4,5ꢀbenzenetetraamine, for P2N2 and
P4N4, the A2 is calculated to be 11.7° and 11.2°, respectively. The relevant dihedral
angle data are summarized in Table 1. The different of A2 make the PDI linker units
show different geometrical types, which make the P2O2 molecular shows a
linearꢀtype linker, the P2N2 molecular exhibits a Vꢀtype linker, and the P4N4
molecular displays an Xꢀtype linker. These different geometrical type linkers make
the intermolecular nonꢀplanar structures of the PDIs molecules different, which are
showed in Fig 1. The 3 different side of view for the relevant calculate structure were
also showed in Fig 1, the moderate dihedral angle A1 and A3 of the 3 acceptors make
the three PDI molecules show the nonꢀplanar geometries is expected to effectively
prevent the three PDI acceptors from forming the large aggregates state in solid film,
which is beneficial for the PDIs blend film applied in the OSCs. In particular, the
P4N4 show a more polydirectional symmetrical geometry compared with the P2O2
and P2N2, which could make the P4N4 provide a more smooth film and good
isotropic electron transport property.

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Fig. 1 The geometric structures of the acceptors calculated by the DFT method using
B3LYP/6ꢀ31G(d); a1, a2 and a3 shows the calculate structure of P2O2, P2N2 and P4N4,
respectively; b1, b2 and b3 shows the three different view of the relevant calculate structure.
Table 1 The relevant dihedral angles of the three calculate structures.
Acceptor
Dihedral angle
A1 (°)
57.9
A2 (°)
139.2
11.7
A3 (°)
57.2
P2O2
P2N2
P4N4
59.6
59.6
59.9
11.2
60.0
2
.3. Optical and electrochemical properties
Fig. 2a shows the ultravioletꢀvisible (UVꢀvis) absorption spectra of the PDIs acceptors
ꢀ5
in 10 M chloroform solution, and Fig. 2b shows the UVꢀvis absorption spectra of the
PDIs acceptors in film state. The results are summarized in Table 2. In chloroform,
P2O2, P2N2 and P4N4 show the maximum absorption peak (λmax) at 534, 538 and
5
38 nm, respectively. In the film state, P2O2, P2N2 and P4N4 show the λmax at 538,
40 and 538 nm, respectively. While the P4N4 show an absorption peak at 457 nm in
5
chloroform and an absorption peak at 461 nm in film, which due to the absorption of
the 2,3,7,8ꢀtetraphenylpyrazino[2,3ꢀg]quinoxaline unit inside of the P4N4 molecule.

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In the film state, P2O2 shows a distinct shoulder peak of the λmax at 502 nm than
P2N2 and P4N4, which indicates that the linearꢀtype linker PDI (P2O2) shows the
large aggregates state than the Vꢀtype linker PDI (P2N2) and the Xꢀtype linker PDI
(P4N4), in agreement with the DFT calculate results. The absorption onsets of P2O2,
P2N2 and P4N4 in thin films are 592, 597 and 589 nm, respectively, which
opt
g
correspond to an optical band gap (
E
) of 2.09, 2.08 and 2.11 eV, respectively.
The energy level of the PDIs acceptors were determined by cyclic voltammetry
(
CV) versus a saturated calomel electrode (SCE), in 0.1 M tetrabutylammonium
ꢀ1
4 6
hexafluorophosphate (Bu NPF ) acetonitrile solution with the scan rate of 50 mV s
under nitrogen atmosphere. The CV curves are presented in Fig. 2c. The onsets of the
oxidation and reduction potentials are measured, and the HOMO and LUMO levels
are calculated relative to the redox potentials of the ferrocene internal standard. The
HOMO levels of P2O2, P2N2 and P4N4 are ꢀ6.04, ꢀ5.96 and ꢀ5.97 eV, respectively,
the LUMO levels of P2O2, P2N2 and P4N4 are ꢀ3.80, ꢀ3.73 and ꢀ3.69 eV, respectively,
cv
g
which correspond to an electrochemical band gap (
E
) of 2.24, 2.23 and 2.28 eV,
respectively. The results are summarized in Table 2. Based on these data, the energy
level diagram of the OSCs showed in Fig. 2d. The work functions of the ITO
electrode, Al electrode and Ca, as well as the HOMO and LUMO levels of PDBTꢀT1
71
adopted the values reported in the literature. Interestingly, for these PDIs acceptors,
the electronꢀdonating bridge units cause the elevation of the energy levels. The
LUMO offsets of the donor polymer between P2O2, P2N2 and P4N4 (ꢁLUMO) are
0
.37, 0.30 and 0.26 eV, respectively. It can be speculated that P4N4 should lead to a
relatively highest oc when used in OSCs.
V

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DOI: 10.1039/C6TA08739F
(a)
(b)
P2O2
P2N2
P4N4
P2O2
P2N2
P4N4
1.0
1.0
0
.8
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
(c)
Ferrocene
P2O2
(
d) -2.5
P2N2
P4N4
-2.9
-3.0
-
3.43
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
-3.69
3.73
-
-
3.80
-4.3
-4.7
-5.1
-
5.36
-5.96 -5.97
6.04
-
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V vs. SCE)
Fig. 2 Optical and electrochemical characterization results of acceptors: UVꢀvis absorption spectra
of the three PDI acceptors in chloroform solution (a) and in film state (b);Cyclic voltammetry (CV)
curves of the acceptors (c); and schematic energy level diagram (d) of the OSCs based on the three
PDI acceptors, the work functions of the ITO electrode, Al electrode and Ca, as well as the
HOMO and LUMO energy levels of PDBTꢀT1 adopted the values reported in the literature, the
HOMO and LUMO energy levels of the three PDI acceptors were calculated from CV
measurements.
Table 2 UVꢀvis absorption and electrochemical properties of the three PDI acceptors.
(
a)
(b)
Acceptor
Solution
Film
opt(c)
(d)
(d)
cv(e)
(f)
λ
max
λ
onset
λ
max
λ
onset
E
g
HOMO
(eV)
LUMO
(eV)
E
g
ꢁLUMO
(eV)
(
nm) (nm)
(nm) (nm) (eV)
(eV)
2.24
2.23
2.28
P2O2
P2N2
P4N4
534
538
538
567
577
578
538
540
538
592
597
589
2.09
2.08
2.11
ꢀ6.04
ꢀ5.96
ꢀ5.97
ꢀ3.80
ꢀ3.73
ꢀ3.69
0.37
0.30
0.26
(a)
ꢀ5
(b)
in 10 M chloroform solution; spinꢀcoating film on a quartz plate from the relevant 10 mg/mL
chloroform solution for UVꢀvis absorption and cast film on the electrode from that solution for CV
(
c) opt
(d)
test; Eg = 1240/λonset
;
HOMO/LUMO levels calculated from the Eonset of the first oxidation
(
E )/reduction (E ) potential, calculated according to HOMO/LUMO (eV) = ꢀe|E
ox/red
– EFerrocene
ox
red
(e) cv
+
4.8|, the onset of the first oxidation of Ferrocene (EFerrocene) is 0.25 V; Eg = LUMO – HOMO;

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(f)
ꢁLUMO = LUMO (PDBTꢀT1) – LUMO (acceptor).
2
.4. Photovoltaic properties
In order to evaluate the photovoltaic properties of P2O2, P2N2 and P4N4 as
nonꢀfullerene acceptors, solutionꢀprocessed BHJ organic solar cells based on these
PDIs
ITO/PEDOT:PSS/PDBTꢀT1:PDIs/Ca/Al, as shown in Scheme 1. The PDBTꢀT1 was
used as the donor polymer. The characteristic current densityꢀvoltage ( ) curves of
were
fabricated
with
a
conventional
device
structure:
JꢀV
the studied OSCs as well as the corresponding external quantum efficiency (EQE)
spectra are shown in Fig. 3. The best device parameters are summarized in Table 3,
and the average values and standard deviation of device statistics are listed in Table
S1 (ESI†).The P2O2, P2N2 and P4N4 based devices without DIO additive showed
the PCE of 2.27%, 2.69% and 4.28%, respectively. In particular, the P2O2 based
devices showed a
V
oc of 0.911 V, while the P2N2 and P4N4 based exhibited the
Voc of
0
.958 V and 0.976 V, respectively. The difference of the
Voc values of the three PDI
acceptors devices are in agreement with the trend of the ꢁLUMO values as listed in
Table 2, which can match up with the relationship of electronꢀdonating bridge units
for PDIs, energy levels and the device
V
oc. For the
Jsc and FF parameters, the P4N4
ꢀ2
based showed a
Jsc of 8.71 mA cm and a FF of 50.3%, while the P2O2 and P2N2
based devices exhibited the lower
based devices. The difference of the
J
sc and FF parameters compare with the P4N4
sc and FF of the different acceptors devices can
J
also verify the geometric structures of the acceptors calculated by the DFT method,
which with the device of the P4N4 acceptor showed the best geometric structure and
the efficient the Jsc and FF. Moreover, the hole and electron mobility of the devices
and the morphology of the active layers also affirm that difference, which will be
discussed below. By inclusion of 0.5% DIO additive, the PCE of the devices were
considerably enhanced. In particular, the P2O2, P2N2 and P4N4 based devices with
0
.5% DIO additive showed the PCE of 2.53%, 3.86% and 5.71%, respectively. For
P2N2 and P4N4 acceptors devices, the DIO additive plays an important role for the
increase of performance, which mainly shows in the enhancement of Jsc and FF. This
is attributed to the enhanced of the hole and electron mobility of the devices and the
optimized morphology of the active layers by DIO addition, which also will be
discussed below.
Fig. 3b shows the corresponding external quantum efficiency (EQE) spectra, the
EQE curves can match up with the enhancement of Jsc for the with/without DIO
additive devices. The OSCs devices based on P2O2 and P2N2 acceptors exhibited an
EQE the maximum value about 45% to 50% at 540 nm and showed a trough around
4
3
20 nm, while the P4N4 based devices exhibited an EQE exceeded 45% in range of
60 to 650 nm with the maximum value reached 56% at around 560 nm. The broad

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EQE of P4N4 devices indicate that the efficient electron transfer from the LUMO of
PDBTꢀT1 to the LUMO of P4N4 and the efficient hole transfer from the HOMO of
P4N4 to the HOMO of PDBTꢀT1 take place together than the P2O2 and P2N2 devices,
which can match up with the UVꢀvis absorption spectra of PDIs and PDBTꢀT1 in neat
films and blend films (Fig. S2 and Fig. S3 (ESI†)).
4
(a)
(b)
PDBT-T1:P2O2
60
PDBT-T1:P2O2 (0.5% DIO)
PDBT-T1:P2N2
PDBT-T1:P2N2 (0.5% DIO)
PDBT-T1:P4N4
2
0
2
4
6
8
50
PDBT-T1:P4N4 (0.5% DIO)
40
-
-
-
-
3
0
PDBT-T1:P2O2
PDBT-T1:P2O2 (0.5% DIO)
PDBT-T1:P2N2
PDBT-T1:P2N2 (0.5% DIO)
PDBT-T1:P4N4
20
10
PDBT-T1:P4N4 (0.5% DIO)
0
-10
300
400
500
600
700
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Wavelength (nm)
Fig. 3 (a) JꢀV characteristics curves of PDBTꢀT1 and the PDIs based OSCs without or with 0.5%
ꢀ2
DIO (v/v) under AM 1.5 G at 100 mW cm . (b) Corresponding EQE spectra of PDBTꢀT1 and the
PDIs based OSCs without or with 0.5% DIO (v/v).
Table 3 Photovoltaic properties of OSCs based on PDBTꢀT1 and the PDI acceptors under AM 1.5
ꢀ2
G at 100 mW cm .
PDBTꢀT1:PDIs
P2O2
(a)
ꢀ2 (b)
DIO additive
V
oc (V)
J
sc (mA cm )
FF (%)
35.8
41.9
38.5
51.1
50.3
63.4
PCE (%)
2.26
w/o
w/
0.911
0.871
0.958
0.950
0.976
0.958
6.94 (6.64)
6.94 (7.03)
7.27 (7.12)
7.95 (7.77)
8.71 (8.80)
9.40 (9.00)
2.53
P2N2
P4N4
w/o
w/
2.69
3.86
w/o
w/
4.28
5.71
(a)
(b)
0
.5% v/v of DIO,
Jsc in parentheses were calculated by integrating the EQE spectrum with the
AM 1.5G spectrum.
2
.5. Electron/hole mobility properties and morphology
To understand the difference of the sc and FF of the different acceptors devices, the
currentꢀvoltage ( ) characteristics of PDBTꢀT1:PDIs blend films without or with 0.5%
J
IꢀV
DIO and the PDIs neat films in spaceꢀchargeꢀlimited current (SCLC) devices was
measured. The device structures of the electron only and hole only devices are
ITO/Al/PDBTꢀT1:PDIs
(or
PDIs
neat
film)/
Al
and
ITO/MoO /PDBTꢀT1:PDIs/MoO
3
3
/Al, respectively. The currentꢀvoltage
(IꢀV)

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characteristics from the corresponding SCLC devices are shown in Fig. 4 and Fig. S4
ESI†), and the corresponding electron/hole mobility data are listed in Table 4. In the
(
ꢀ
3
PDIs neat films, P2O2, P2N2 and P4N4 exhibit the electron mobility of 2.42 × 10 ,
ꢀ3
ꢀ3
2
ꢀ1 ꢀ1
2
.60 × 10 and 2.69 × 10 cm V s , respectively, which are suitable for used as
electron acceptor materials. For the blend films, the hole mobilities are significantly
lower than electron mobilities without DIO addictive. Generally, the addition of 0.5%
DIO into PDBTꢀT1:PDIs blend films increased both the electron and hole mobilities,
which can be ascribed to that the addition of 0.5% DIO can provide better
morphology than the no DIO blend films. So that, the devices with 0.5% DIO additive
show higher
Moreover, the PDBTꢀT1:P4N4 blend films with 0.5% DIO additive show the
ratio of 1.57, which possessed a most balanced ratio compared to the others
Jsc and FF than those corresponding devices without DIO additive.
e h
ꢁ /ꢁ
e h
ꢁ /ꢁ
blend films. The betterꢀbalanced electron/hole ratio in the PDBTꢀT1:P4N4 with 0.5%
DIO additive devices is responsible for their reduced bimolecular charge
recombination, which is the reason of the corresponding devices show the best
ꢀ2
performance with a Jsc of 9.40 mA cm and a FF of 63.4%.
Since the morphology is an important factor for the device performance of OSCs,
atomic force microscopy (AFM) and transmission electron microscopy (TEM) were
used to investigate the morphologies of the active layers. The corresponding AFM
images are shown in Fig. 5. The blend films of PDBTꢀT1:P2O2, PDBTꢀT1:P2N2 and
PDBTꢀT1:P4N4 without DIO exhibited smooth and uniform morphologies with the
root mean square (RMS) values of 1.45, 1.17 and 1.03 nm, respectively. While 0.5%
DIO additive was mixed into the blend films, the RMS of the corresponding AFM
images improved, from 1.45 to 2.04 nm for PDBTꢀT1:P2O2, from 1.17 to 1.61 nm for
PDBTꢀT1:P2N2 and from 1.03 to 1.23 nm for PDBTꢀT1:P4N4, specifically. The
higher RMS values for the corresponding blend films with DIO show more obvious
aggregations and phase separations, which consistent well with the significantly
increased charge mobility and FF after adding 0.5% DIO in the PDIs based devices,
especially for the P2N2 and P4N4 devices. The TEM study results also provided more
direct evidence to support the different performance of the P2O2, P2N2 and P4N4
based devices. The TEM images of the blend films without DIO are showed in Fig. 6a,
Fig. 6c and Fig. 6e, which demonstrated that the linearꢀtype linker molecular (P2O2)
based blend film showed the obvious black aggregation than the Vꢀtype linker
molecular (P2N2) and the Xꢀtype linker molecular (P4N4) based blend films. The
direct distinction showed in TEM images of the P2O2, P2N2 and P4N4 based blend
films can match well with the corresponding RMS values, which could attribute to the
different linker units and the distinctive nonꢀplanar structures among the PDI
molecules. Moreover, the different morphology of the blend films in agreement with
the trend of the corresponding Jsc and FF. While 0.5% DIO additive was mixed into

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DOI: 10.1039/C6TA08739F
the blend films, the TEM images of the P2N2 and P4N4 based blend films (Fig. 6d
and Fig. 6f) showed the slight increased aggregate domains compared with the
corresponding blend films without DIO, which increased charge mobility and FF after
adding 0.5% DIO in the P2N2 and P4N4 based devices. The morphology of the P2O2
blend film with 0.5% DIO showed the much more homogeneous with nanoꢀphase
segregated crystalline structure, which match well with the corresponding largest
RMS change from the AFM images. Therefore, the P2O2 based blend films showed
the relative larger aggregation nanoꢀphase morphology, which reduced the charge
separation and mobility compared with the P2O2 and P4N4 based blend films. These
results illustrate the higher
J
sc and FF values for the P2N2 and P4N4 based devices
(especially for the P4N4 based devices) than that achieved with the P2O2 based
devices.
(
a)
(b)
PDBT-T1:P2O2
PDBT-T1:P2O2
4
PDBT-T1:P2O2(0.5%DIO)
PDBT-T1:P2N2
PDBT-T1:P2N2(0.5%DIO)
PDBT-T1:P4N4
PDBT-T1:P2O2(0.5%DIO)
PDBT-T1:P2N2
PDBT-T1:P2N2(0.5%DIO)
PDBT-T1:P4N4
1
0
3
1
1
1
0
PDBT-T1:P4N4(0.5%DIO)
PDBT-T1:P4N4(0.5%DIO)
2
0
3
0
1
1
0
0.1
1
0.1
1
Applied Voltage (V)
Applied Voltage (V)
Fig. 4 Currentꢀvoltage (IꢀV) characteristics of PDBTꢀT1:PDIs blend films without or with 0.5%
DIO (v/v) in spaceꢀchargeꢀlimited current (SCLC) devices, (a) electron only, (b) hole only.
Table 4 Electron/hole mobility data of PDBTꢀT1:PDIs blend films and the PDIs neat films.
(a)
2
ꢀ1 ꢀ1
2
ꢀ1 ꢀ1
Samples
DIOadditive
ꢁ
e
(cm V s )
ꢁ
h
(cm V s )
e h
ꢁ /ꢁ
ꢀ
3
ꢀ4
PDBTꢀT1:P2O2
w/o
w/
w/o
w/
w/o
w/
ꢀꢀ
1.62 × 10
1.72 × 10
1.68 × 10
1.84 × 10
1.71 × 10
1.90 × 10
2.42 × 10
2.60 × 10
2.69 × 10
2.91 × 10
3.45 × 10
3.06 × 10
4.17 × 10
8.54 × 10
1.21 × 10
5.57
4.98
5.49
4.41
2.00
1.57
ꢀ3
ꢀ4
ꢀ4
ꢀ4
ꢀ4
ꢀ3
ꢀ3
PDBTꢀT1:P2N2
PDBTꢀT1:P4N4
ꢀ3
ꢀ3
ꢀ3
ꢀ3
P2O2
P2N2
P4N4
ꢀ3
ꢀꢀ
ꢀ3
ꢀꢀ
(a)
0
.5% v/v of DIO

Journal of Materials Chemistry A
Page 12 of 22
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DOI: 10.1039/C6TA08739F
Fig. 5 AFM topographic images (2 ꢂm × 2 ꢂm) of PDBTꢀT1:PDIs blend films, (a, c and e)
PDBTꢀT1:P2O2, PDBTꢀT1:P2N2 and PDBTꢀT1:P4N4 blend films without DIO, respectively; (b,
d and f) PDBTꢀT1:P2O2, PDBTꢀT1:P2N2 and PDBTꢀT1:P4N4 blend films with 0.5% DIO,
respectively.
(a)
(c)
(e)
(b)
(d)
(f)
Fig. 6 TEM images of PDBTꢀT1:PDIs blend films, (a, c and e) PDBTꢀT1:P2O2, PDBTꢀT1:P2N2
and PDBTꢀT1:P4N4 blend films without DIO, respectively; (b, d and f) PDBTꢀT1:P2O2,
PDBTꢀT1:P2N2 and PDBTꢀT1:P4N4 blend films with 0.5% DIO, respectively.

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DOI: 10.1039/C6TA08739F
3
. Conclusions
In summary, three PDI acceptors (P2O2, P2N2 and P4N4) are synthesized by PDI bay
positions functionalization with benzil, 2,3ꢀdiphenylquinoxaline and
,3,7,8ꢀtetraphenylpyrazino[2,3ꢀg]quinoxaline linker, respectively. The three PDIs
2
display different nonꢀplanar geometrical structures because of the different linker
units, which affect the devices performance parameters. The Vꢀtype linker molecular
(P2N2) and the Xꢀtype linker molecular (P4N4) show more intermolecular nonꢀplanar
than the linearꢀtype linker molecular (P2O2), which benefit the corresponding
morphology of the blend films and promote the charge mobility and FF of the OSCs
devices. Furthermore, the gradient energy levels of the three PDIs provide an efficient
research model for the relationship of device Voc and energy levels. Meanwhile, the
P4N4 show an efficient absorption from 400 nm to 450 nm, and this absorption also
provide the efficient EQE response, which indicate that the efficient hole transfer
from the HOMO of P4N4 to the HOMO of PDBTꢀT1 take place in the P4N4 devices.
Finally, the P4N4 based nonꢀfullerene OSCs devices show the best performance, the
ꢀ2
PCE of that devices reached 5.71% with a Voc of 0.958 V, a Jsc of 9.40 mA cm and a
FF of 63.4%. The results demonstrate that the geometrical type tuning of the PDI
linker units is an efficient method to design efficient nonꢀfullerene acceptors.
4
4
. Experimental section
.1 General information
All reagents were obtained from commercial sources and used without further
purification, unless otherwise specified. Solvents were dried over before use (THF
from sodium/benzopheone, toluene was washed with H
2
SO
4
and then treated with
1
13
CaCl , acetonitrile from CaH ). H and CNMR spectra were tested using a Bruker
2 2
AVꢀ500with tetramethylsilane (TMS) as an internal reference. MALDIꢀTOFMS were
performed by BrukerAgilent1290/maXis impact. UVꢀvis spectra were measured using
a HP 8453 spectrophotometer. Thermogravimetric analyses (TGA)were measured
using a NETZSCH TG 209 under a heating rate of 10 °C/min with a nitrogen flow
rate of 20 mL/min. Cyclic voltammetry data were measured using a CHI600
Delectrochemical workstation with Bu NPF (0.1 M) in acetonitrile as electrolyte,
4
6
platinum and saturated calom elelectrode as the working and reference electrode,
respectively. The geometry was optimized with Density Functional Theory (DFT)
calculations were performed at the B3LYP/6ꢀ31G(d) level to optimize the ground
state geometries of the acceptors molecules using the Gaussian 09 package40. All
N
ꢀpentylhexyl substituents were replaced with methyl groups for calculations. The
AFM images were obtained from a NanoMan VS microscope with a tapping mode.
TEM images were characterized with a JEMꢀ2100F instrument.

Journal of Materials Chemistry A
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DOI: 10.1039/C6TA08739F
4
4
1
.2. Synthesis
.2.1.
,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethane-1,2-dione
(1): 4,4'ꢀDibromobenzil (3.68 g, 10 mmol), bis(pinacolato)diboron (7.11 g,28 mmol),
Pd(dppf)Cl (0.24 g, 0.33 mmol) and potassium acetate (4.48 g) were dissolved in 30
2
mL dioxane under argon atmosphere. The reaction solution was continuously stirred
over night at 80 °C. After cooling to room temperature, the mixture was poured into
water and extracted with dichloromethane. The organic phases were collected, dried
4
over MgSO , and the solvent was removed under vacuum. The product purified first
by a short column chromatography on silica gel and then recrystallization from
1
methanol to afford 1 as light yellow solid (4.71 g, 85%). H NMR (500 MHz, CDCl
3
)
13
δ
3
7.93 (s, 8H), 1.35 (s, 24H). C NMR (126 MHz, CDCl ) δ 194.78, 135.19, 134.76,
1
28.79, 84.48, 84.39, 24.87.
2
5
4
.2.2. P2O2:A mixture of 1 (202.4 mg, 0.440 mmol), PDIꢀBr (2) (700 mg, 0.901
(25 mg, 0.029 mmol), potassium carbonate (300 mg, 2.172 mmol),
3 4
mmol), Pd(PPh )
THF (10 mL) and water (4 mL) was heated to 80 °C for 24 h under argon atmosphere.
After cooling to room temperature, the mixture was poured into water and extracted
4
with dichloromethane. The organic phases were collected, dried over MgSO , and the
solvent was removed under vacuum. The product purified first by a short column
chromatography on silica gel (eluent: dichloromethane) to afford P2O2 as red solid
1
(
1.15 g, 81%). H NMR (500 MHz, CDCl
3
)
δ
8.79 – 8.63 (m, 8H), 8.57 (d,
= 34.0, 13.3 Hz, 6H), 7.87 (d, = 8.2 Hz, 2H), 7.76 (d,
H), 5.23 – 5.11 (m, 4H), 2.31 – 2.14 (m, 8H), 1.92 – 1.74 (m, 8H), 1.49 – 0.98
J
= 15.5
Hz, 2H), 8.22 (dd,
J
J
J = 8.3 Hz,
4
1
3
(
m,48H), 0.96 – 0.69 (m, 24H). C NMR (126 MHz, CDCl
3
)
δ
192.57, 163.71, 163.63,
1
1
1
1
63.48, 163.40, 149.40, 139.72, 135.21, 134.91, 134.21, 134.03, 132.86, 132.69,
32.03, 131.28, 130.39, 129.60, 128.89, 128.03, 127.35, 123.76, 123.35, 123.17,
23.05, 122.69, 122.60, 54.51, 54.43, 32.51, 31.82, 31.77, 26.41, 26.37, 22.70, 22.58,
4.13, 14.05.MS (MALDIꢀTOF)calcd for C H N O , 1602.853; found, 1600.007.
1
06 114
4
10
4
.2.3. P2N2: A mixture of P2O2 (240.6 mg, 0.150 mmol), oꢀphenylenediamine (16.2
mg, 0.150 mmol) and acetic acid (20 mL) was heated to reflux for 10 h under argon
atmosphere. After cooling to room temperature, the resulting mixture was extracted
with dichloromethane with surplus acetic acid removed by NaHCO
3
.The organic
phases were collected, dried over MgSO , and the solvent was removed under vacuum.
4
The product purified first by a short column chromatography on silica gel (eluent:
1
dichloromethane) to afford P2N2 as red solid (218.7mg, 87%). H NMR (500 MHz,
CDCl
3
)
δ
8.78 – 8.52 (m, 10H), 8.30 (dd,
= 8.2 Hz, 2H), 7.87 (ddd,
J
= 6.4, 3.4 Hz, 2H), 8.10 (d,
J
= 7.3 Hz,
= 8.1
2
H), 7.99 (d,
J
J
= 23.3, 13.4, 5.5 Hz, 6H), 7.61 (d, J

Page 15 of 22
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DOI: 10.1039/C6TA08739F
Hz, 4H), 5.07 (d,
J = 62.2 Hz, 4H), 2.17 (s, 8H), 1.80 (s, 8H), 1.28 – 1.12 (m, 48H),
13
0
1
1
1
3
.86 – 0.74 (m, 24H). C NMR (126 MHz, CDCl ) δ 152.09, 143.41, 141.41, 140.76,
39.39, 131.88, 130.59, 130.23, 129.33, 129.15, 128.90, 128.63, 128.12, 127.52,
23.55, 122.77, 67.99, 54.73, 54.58, 32.30, 32.22, 31.70, 29.71, 26.57, 26.55, 22.52,
118 6 8
4.02. MS (MALDIꢀTOF)calcd for C112H N O , 1676.210; found, 1677.065.
4
.2.4. P4N4: A mixture of P2O2 (320.8 mg, 0.200 mmol), 1,2,4,5ꢀbenzenetetraamine
(10 mL)
tetrahydrochloride (28.4 mg, 0.100 mmol), acetic acid (20 mL) and CHCl
3
was heated to reflux for 30 h under argon atmosphere. After cooling to room
temperature, the resulting mixture was extracted with dichloromethane with surplus
3
acetic acid removed by NaHCO .The organic phases were collected, dried over
MgSO , and the solvent was removed under vacuum. The product purified first by a
4
short column chromatography on silica gel (eluent: dichloromethane) to afford P2N2
1
as red solid (212.8mg, 65%). H NMR (500 MHz, CDCl
3
)
δ
9.26 (s, 2H), 8.63 (dd,
= 20.9, 8.1 Hz, 12H), 7.69 (d, = 8.1 Hz,
= 69.9 Hz, 16H), 1.78 (d, = 36.4 Hz, 16H),
= 6.9 Hz, 48H). C NMR (126 MHz, CDCl 157.34,
J =
1
8
1
1
1
3
0.6, 8.1 Hz, 20H), 8.15 (s, 4H), 8.02 (dd,
H), 5.11 (d, = 45.0 Hz, 8H), 2.13 (d,
.47 – 0.99 (m, 96H), 0.79 (q,
J
J
J
J
J
13
J
3
) δ
46.80, 144.20, 141.76, 139.49, 135.32, 133.22, 131.98, 130.59, 130.03, 129.31,
29.05, 128.77, 128.53, 128.12, 127.88, 123.45, 122.07, 54.34, 54.08, 32.69, 32.02,
1.89, 31.70, 29.71, 27.21 26.57, 26.54, 22.51, 14.62, 14.56.MS (MALDIꢀTOF)calcd
230 12
for C218H N O16, 3274.306; found, 3275.645.
4
.3. Fabrication and characterization of solar cells
The organic solar cells (OSCs) with a conventional device structure of
ITO/PEDOT:PSS/PDBTꢀT1:PDIs/Ca/Al were fabricated. Firstly, PEDOT:PSS
(Heraeus Clevios P VP A 4083) was spinꢀcoated on top of a cleaned ITO and annealed
in air at 150 °C for 10 min. Subsequently, the active layer with an optimal thickness of
ca. 100 nm was formed by spinꢀcoating the mixed solution of PDBTꢀT1 and PDIs
(
P2O2, P2N2 or P4N4) in oꢀdichlorobenzene (oꢀDCB) (PDBTꢀT1:PDIs, 1:1, w/w; the
ꢀ1
concentration of PDBT1 is fixed at 10 mg mL ; without/with 0.5% DIO (v/v)
additive for each PDI acceptor blend) at 1600 r.p.m. for 60 s atop the PEDOT:PSS
layer. After that, the active layers were annealed at 100 °C for 5 min for the devices
with thermal annealing treatment. Finally, a 10 nm Ca and a 100 nm Al layer were
sequentially deposited by thermal evaporation onto the active layer through a shadow
ꢀ
5
2
mask at a vacuum of ~5 × 10 Pa. The active area of the devices was 4.50 mm .
2
During the test, an aperture with the area of 3.14 mm was used. The current
densityꢀvoltage (JꢀV) characteristics were tested in a Keithley 2400 Source Measure
Unit. The devices performances were measured under an irradiation intensity of 100
ꢀ2
mW cm tested by a calibrated silicon solar cell and a read out meter (Model 91150V,

Journal of Materials Chemistry A
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DOI: 10.1039/C6TA08739F
Newport) with an Air Mass 1.5 Global (AM 1.5 G) solar simulator (ClassAAA solar
simulator, Model 94063A, Oriel). The EQE spectra were determined from a QEX10
Solar Cell IPCE measurement system (PV measurement, Inc).
4
.4. SCLC Measurements
The charge carrier mobilities of PDBTꢀT1:PDIs blend films without or with 0.5%
DIO and the PDIs neat films were determined from spaceꢀchargeꢀlimited current
(
SCLC) devices. The device structures of the electron only and hole only devices are
ITO/Al/PDBTꢀT1:PDIs (or PDIs neat film)/ Al and
ITO/MoO /PDBTꢀT1:PDIs/MoO /Al, respectively. The mobilities were determined by
fitting the dark JꢀV current to the model of a single carrier SCLC with the equation:
3
3
J
2
3
=
9
ε
0
ε
r
ꢁV /
the neat films,
of the transport medium, and
app is the applied voltage and
8
d
, when
is the permittivity of free space,
is the charge carrier mobility.
bi is the offset voltage. The carrier mobility can be
J
is the current density,
d
is the thickness of the blend films or
ε
0
ε
r
is the relative dielectric constant
bi, where
ꢁ
app
V = V – V
V
V
1
/2
calculated from the slope of the
J ꢀV curves.
Acknowledgment
The work was financially supported the Ministry of Science and Technology (No.
2
014CB643501), the Natural Science Foundation of China (No. 21520102006,
1634004 and 21490573), the Guangdong Natural Science Foundation (Grant No.
2
S2012030006232).
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Text and Graphic for the table of contents
Three perylenediimide (PDI) acceptors (P2O2, P2N2 and P4N4) functionalizing the
bay positions of PDI with different geometrical linker units were synthesized and
applied in efficient non-fullerene organic solar cells.
2
0
P2O2 Based
P2N2 Based
P4N4 Based
-2
-4
-6
-8
-10
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)