Diketopyrrolopyrrole based highly crystalline conjugated molecules for application in small molecule donor-polymer acceptor nonfullerene organic solar cells

Article abstract of DOI:10.1016/j.orgel.2016.10.021

In order to specifically investigate the low efficiency of small molecule donor-polymer acceptor (M-P) nonfullerene organic solar cells, we have successfully modify the synthesis of a series of D-π-A-π-D conjugated molecules containing diketopyrrolopyrrol

Full text of DOI:10.1016/j.orgel.2016.10.021

Organic Electronics 39 (2016) 279e287
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Diketopyrrolopyrrole based highly crystalline conjugated molecules
for application in small molecule donor-polymer acceptor
nonfullerene organic solar cells
*
**
Jianyu Yuan , Wanli Ma
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 199
Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu, 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 September 2016
Received in revised form
In order to specifically investigate the low efficiency of small molecule donor-polymer acceptor (M-P)
nonfullerene organic solar cells, we have successfully modify the synthesis of a series of D-
conjugated molecules containing diketopyrrolopyrrole (DPP) and different end groups. By incorporation
of end group with different size of -conjugation (benzene, naphthalene and pyrene), we further
p-A-p-D
13 October 2016
p
Accepted 13 October 2016
improved the fill factor (FF) and short current density (Jsc) of the donors molecule. Our experimental
results and theoretical calculations have proven that the size of the end groups can influence the
molecule crystallinity, mobility and intermolecular packing by altering the molecular coplanarity. As the
result of improved crystallinity, morphology and fine-tuned mobilities, we demonstrated an increased FF,
a high Jsc of ~4.5 mA/cm2 and a power conversion efficiency of 2.05%, which is among the highest effi-
ciency reported for M-P nonfullerene solar cells. Our results provide opportunities and possibilities of
achieving higher performance M-P nonfullerene solar cells in the future.
Keywords:
Nonfullerene solar cells
Donor crystallinity
Morphology
Mobilities
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
scalability of the materials, stability and cost need to be further
optimized before reaching the threshold for large-scale commer-
cialization [5].
Organic solar cells made of solution-processed bulk hetero-
junction (BHJ) organic materials are potential low-cost alterna-
tives for renewable energy generation [1]. During the past decade,
synergistic efforts in donor materials (small molecules, oligomers,
polymers) design and device processing optimization have rapidly
increased the power conversion efficiencies (PCEs) of organic
photovoltaic (OPV) that approach 12% [2]. However, fullerenes are
not ideal acceptor materials due to many intrinsic issues, such as
weak light absorption, almost fixed chemical structure and energy
levels [3], further limiting the open-circuit voltage (Voc) and short-
circuit current density (Jsc) of these solar cell devices. Furthermore,
the inherent tendency of fullerene to aggregate under elevated
temperatures has been considered a key factor for deteriorated
morphology and consequently reduced lifetime of PSCs [4].
Therefore, despite the tremendous achievements to date, the
research development of OPVs has reached the stage where the
Functional photovoltaic materials applied into organic solar
cells are usually synthesized via the Suzuki, Stille or Negishi cross-
coupling reactions, which involve the preparation of organo-
boron, organo-tin or organo-zinc reagents, respectively [6]. In
recent years, attributing to the great progress of C-H activation
reaction, thiophene has been regarded as an ideal moiety in syn-
thetic organic chemistry for direct arylation with aryl halides due to
the ease of palladation through a concerted metalation deproto-
nation pathway [7]. These reactions possess numerous advantages
[8] over traditional cross-coupling reactions such as: (a) avoidance
of the use of organometallic reagents in the starting materials
leading to simpler by products and higher atom economy, (b) fewer
synthetic steps, (c) higher yields, (d) better compatibility with
chemically sensitive functional groups, and (e) the scalability
preparation of the target conjugated materials.
Meanwhile, solution-processed organic solar cells (OSCs) using
a nonfullerene electron acceptor has been rapidly improved rela-
tive to the fullerene counterparts, with PCE past 11% up to date [9].
Typically, the nonfullerene OSCs can be divided into four different
*
Corresponding author.
** Corresponding author.
E-mail addresses: jyyuan@suda.edu.cn (J. Yuan), wlma@suda.edu.cn (W. Ma).
http://dx.doi.org/10.1016/j.orgel.2016.10.021
566-1199/© 2016 Elsevier B.V. All rights reserved.
1

280
J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
types: polymer donor-small molecule acceptor (P-M) [9], polymer
donor-polymer acceptor (P-P) [3c,10], small molecule donor-small
molecule acceptor (M-M) [11] and small molecule donor-polymer
acceptor (M-P) [12]. However, among these nonfullerene OSCs,
the development of small molecule donor-polymer acceptor sys-
tem has lagged significantly behind in device performance. So far,
only a few reported M-P OSCs exhibited power conversion effi-
ciencies (PCEs) above 2% [12], with the very recent two ones
reaching 4.5% [12b,13]. The lower PCEs of M-P OSCs are generally
hypothesized to the large polymer phase separation, relatively low
electron mobility of polymer acceptors and the inefficient charge
dissociation at the donor/acceptor (D/A) interface [12a,14]. How-
ever, it is worth noting that few previous works have been focused
on the effect of initial donor molecules properties on the relevant
M-P OSCs performance. We should realize that the subtle changes
in donor molecular structure may also have significant impact on
the material optoelectronic properties, processing parameters,
blend morphology, and the resulting solar cell characteristics se-
lection. Especially, the morphology change is critical to the device
performance.
significantly different performance. The optimized M-P non-
fullerene solar cells based on DPP-P/N2200 shows a relatively high
Jsc of ~4.5 mA/cm2 (0.69 mA/cm2 for DPP-B/N2200 and 1.14 mA/
cm2 for DPP-N/N2200), and a high PCE of 2.05% (0.20% for DPP-B/
N2200 and 0.50% for DPP-N/N2200). More importantly, our re-
sults suggest that small molecule donor with higher crystallinity
can be designed for creating better charge transport and blend
morphology in M-P nonfullerene OSCs.
2. Results and discussion
2.1. Synthetic procedures
The structure and synthetic route of the donor molecules are
illustrated in Scheme 2. The detailed procedure is described in the
experimental part. All molecules were prepared via direct arylation
reaction [16], using a phosphine-free catalytic system, and the re-
action could be accelerated greatly with high yields (>90%) in the
presence of pivalic acid (PivOH). Former reports demonstrated the
preparation procedure of these DPP-based materials using
Palladium(Pd)-assisted Suzuki cross-coupling reaction with multi-
ple steps [17], however, the highly efficient one-pot synthesis of
these functional materials in this contribution gives us a promising
prospect for easier scale up production. These advantages make the
direct arylation protocol an ideal and versatile strategy for the
synthesis of structural complicated DPPs that may possess chemi-
cally sensitive functionalities. Herein, all the DPP molecules mate-
rials exhibit good solubility in common solvents at room
temperature, like chloroform (CF), chlorobenzene (CB) and o-
dichlorobenzene (ODCB).
In this contribution, a series of low band-gap donor molecules
(
DPP-B, DPP-N and DPP-P) sharing similar D-p-A-p-D backbone
structure were synthesized via one-pot direct acylation reaction.
We incorporated different end groups (benzene, naphthalene and
pyrene) into the molecular backbone, in order to investigate how
the subtle changes in molecular structure impact relevant opto-
electronic properties and the resulting M-P nonfullerene OSCs
characteristics. Finally, all these molecules were used together with
a polymer acceptor P(NDI2OD-T2) (N2200) [15] in M-P non-
fullerene solar cells. As shown in Scheme 1, to achieve higher
intermolecular contacts, DPP-N and DPP-P were synthesized by
substituting the benzene end-group with naphthalene and pyrene,
aiming to enhance charge carrier mobility and light absorption of
donor molecules. The properties of these molecules were system-
atically studied by UVevis absorption, Density Functional Theory
2.2. Optical properties
UVevis absorption spectra of DPP-based molecules and N2200
are shown in Fig. 1. The molecular solutions in chloroform display
board absorption from 450 to 680 nm. For DPP-N and DPP-P, both
solutions show distinct shoulder peaks next to the absorption
maximum in their solutions, which is typical for DPP based mole-
cules. We noticed that the absorption in region of 400e600 nm of
DPP-N and DPP-P are almost the same, indicating the similar
intramolecular charge transfer (ICT) between the donor and
(
DFT) simulation and 2D grazing incidence wide-angle X-ray scat-
tering (GIWAXS). The corresponding M-P nonfullerene solar cells
were also fabricated to investigate the effect of molecular structure
on the device photovoltaic performance. We revealed that the
structure change of small molecule donor acceptor can adjust the
molecular ordering and crystallinity in solid film, resulting in
Scheme 1. Molecular structures of donor and acceptor.

J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
281
Scheme 2. Synthetic access to DPP-B, DPP-N and DPP-P via one-pot direct arylation reaction.
Compared to the fullerene acceptors, the absorption of N2200 is
greatly improved, exhibiting a narrow optical band-gap of 1.46 eV.
Herein, the small molecule donor-polymer acceptor nonfullerene
blend extends the absorption into the near-infrared regions of the
solar spectrum, which could potentially contribute to photocurrent.
And theoretically obtain larger Jsc values with only thin films.
2.3. Theoretical calculations and structural order in solid films
DFT calculations for molecules using Gaussian 09 at the level of
B3LYP/6-31g(d) were performed [18], aiming to gain insight into
the possible molecular geometries arising from the chemical
structures. To simplify the calculations, the alkyl chains attached on
DPP moiety were replaced by CH
3
groups. In Fig. 2, our calculations
ꢁ
ꢁ
ꢁ
ꢁ
indicate that the dihedral angles are ꢀ27.8 , 31.0 , 33.6 and ꢀ23.4
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
for DPP-B, -22.9 , 11.7 , 15.8 and ꢀ21.3 for DPP-N and ꢀ2.5 , 9.2 ,
ꢁ
ꢁ
1
0.8 and ꢀ4.8 for DPP-P. The obviously smaller dihedral angles of
DPP-P than DPP-N and DPP-B imply a better planarity, which will
help to improve the intermolecular packing, resulting in reduced
optical band-gap and improved charge transport. These results
theoretically proved the proper selection of end groups can effi-
ciently modify the molecular structures of the DPP-based linear
molecules. We also calculated the materials' energy levels to
investigate how the subtle changes in molecular structure impact
relevant electronic properties. Due to the incorporation of strong
electron-deficient moiety DPP as the core, the LUMO energy levels
will be mainly decided by the acceptor block in D-A materials [19].
On the other hand, all benzene, naphthalene and pyrene units
exhibit relatively weak electron-donating ability, as shown in Fig. 2,
no obvious difference is observed in the calculated LUMO, HOMO
energy levels.
Grazing incidence wide-angle X-ray scattering (GIWAXS) mea-
surements were performed to investigate the relevant structure
information in solid state [20]. We first investigated the effect of
end group on the polymer order in film. As shown in Fig. 3, the 2D
GIWAXS patterns of a DPP-N and DPP-P thin film shows sharply
defined rings and peaks, suggesting that the packing of DPP-P is
both more crystalline and more aligned than that of DPP-B, whose
scattering pattern indicate relatively amorphous films. According to
previous report [17b], DPP-P shows a closely-packed, interdigitated
crystal structure with extensive overlap of C2-pyrene moieties. The
interplanar distance between two pyrene units is as short as 3.50 Å,
confirming strong interaction between molecules. All these results
exhibit similar trend with the DFT simulation, the pyrene end group
Fig. 1. Absorption spectra of solutions in chloroform at room temperature (a) and thin
films cast from chloroform solutions (b).
acceptor units [6a]. Blue-shift absorption is observed in DPP-B be-
tween compared to DPP-N and DPP-P in both solution and solid
state, indicating the weaker ICT when adopting benzene as the end-
groups. Besides, the absorption below 400 nm was notably
enhanced in both solution and solid state when incorporating
larger
spectra of DPP-B, DPP-N and DPP-P exhibit similar trend compared
to that in solutions. The optical band gap (E ) of DPP-B, DPP-N and
p-conjugated pyrene as the end-groups. Thin film absorption
with extended
p-conjugation can direct favourable packing
configuration for charge transport. In addition, the polymer
acceptor N2200 also exhibit more ordered solid structure
compared to other conjugated polymers, a broad and relatively
strong diffraction peak appeared in the out-of-plane direction,
which corresponds to a preferable face-on packing orientation for
g
DPP-P is 1.82 eV, 1.75 eV and 1.66 eV, respectively, as determined by
the onset of film absorption. It is speculated that different degree of
molecular packing was formed in solid state, likely due to different
molecular geometry and intermolecular contacts. UVevis absorp-
tion spectra of polymer acceptor N2200 are also shown in Fig. 1.
the polymer in the films, and the corresponding “
pep stacking”
distance is ~4.0 Å. It has been presumed that the generation and

282
J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
Fig. 2. Optimized molecular geometries and LUMO, HOMO energy levels obtained by DFT calculation.
Fig. 3. 2D-GIWAXS patterns of DPP-B, DPP-N, DPP-P and N2200 thin films.
dissociation of free charge carriers is dictated by the microscopic
structure of the donor and acceptor at the heterojunction in very
anisotropic nonfullerene system [21]. A proper crystal orientation
and degree of crystallinity in polymer domains is a prerequisite for
efficiently dissociating electron-hole pairs into free charge carriers
donor and N2200 as the electron acceptor. The structure is ITO/
PEDOT:PSS (40 nm)/blend/Al (80 nm), and the device area is
2
7.25 mm . All devices were operated under an N
2
atmosphere.
Device optimization included adjustments of solvents, donor-
acceptor blend ratio, post-treatment and active layer thickness.
CF was eventually chosen as the best processing solvent, the
optimal molecule/polymer weight ratio was determined to be 2:1,
[22].
ꢀ
1
with a total solid concentration of 16 mg mL for all small mole-
cule donor. Thermal annealing and solvent additive 1,8-
diiodooctane (DIO) has also been adopted to further enhance
2.4. Solar cell performance
Nonfullerene BHJ OSCs were fabricated using DPP molecules as

J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
283
device performance, since it has been proven to be an efficient and
universal post-treatment to improve the morphology of polymer-
fullerene films [23].
and hole mobility are crucial to achieve efficient and balanced
carrier transport. To compare the charge transport properties of
molecule/N2200 blends, hole only and electron-only diodes were
fabricated and measured [25], with a device structure of ITO/
PEDOT:PSS (40 nm)/blend/MoOx (6 nm)/Ag(80 nm) and ITO/ZnO
(40 nm)/blend/LiF (0.6 nm)/Al (80 nm). As shown in Fig. 5b, the
hole mobilities of the Optimized DPP-B/N2200, DPP-N/N2200 and
M-P BHJ blends cast from neat solvent without any treatment
exhibited a PCE of 0.04%, 0.05% and 1.18% for DPP-B, DPP-N and
DPP-P, respectively, with corresponding Voc values of 0.85 V, 0.74V
and 0.88 V. The relatively low PCEs were due to the low Jsc and FF. As
shown in Table 1, thermal annealing hardly enhances the photo-
voltaic performance in all the devices, probably due to the unfav-
ourable self-contained morphology from the initial blend films.
However, as shown in Table 1 and Fig. 4c, significant improvements
were observed upon introduction DIO as a solvent additive to the
processing solvent, giving highest PCEs of 0.20%, 0.50% and 1.75%
for DPP-B, DPP-N and DPP-P, respectively. The enhancement of PCEs
was mainly due to the improved Jsc and FF. Finally, we realized a
highest PCE of 2.05% for DPP-P/N2200 solar cell device through the
synergistic effect between thermal annealing and solvent additive,
which is also among the highest report for the less focused M-P
nonfullerene solar cells [12,13]. External quantum efficiency (EQE)
curves are shown in Fig. 4d. All M-P devices show a broad EQE
response from 300 to 800 nm, however, DPP-B/N2200 and DPP-N/
N2200 exhibit average value less than 5% across the 500e750 nm
range. In contrast, DPP-P/N2200 display quite higher quantum ef-
ficiency values, with maximum EQE value over 30% and the Jsc
values calculated from integration of the EQE with the AM1.5G
reference spectrum are in agreement (±5%) with the Jsc obtained
from the J-V measurements.
ꢀ
6
2
ꢀ1
ꢀ1
s ,
DPP-P/N2200 blends are 1.7*10
cm
V
1.6*10
ꢀ5
2
ꢀ1 ꢀ1
ꢀ4
2
ꢀ1 ꢀ1
cm
V
s
and 3.2*10 cm
V
s , respectively, which are
about one order enhancement when incorporating benzene,
naphthalene and pyrene into the molecular backbone as the end
groups. In contrast, the J-V curves of electron-only devices based on
the optimized DPP-B/N2200, DPP-N/N2200 and DPP-P/N2200
blends are quite similar, with SCLC fitting slope value of 5.0, 4.9
and 6.5, respectively. Further we can calculated the electron mo-
ꢀ5
2
ꢀ1 ꢀ1
bilities of these blend are roughly around 3.5*10
cm
V s ,
which exhibit similar value compared to the previous report for the
N2200 based blend film [11,12]. Herein, all the blends exhibit quite
similar electron mobilities, the major difference in hole mobilities
will govern the Jsc and the FF of the M-P solar cell devices. DPP-P
with enhanced crystallinity and intermolecular contacts show
higher hole mobility in the BHJ blend under the same experimental
condition, which proves the selection of highly crystalline small
molecule donor could be an efficient approach to improve the M-P
2
nonfullerene OSCs. However, the Jsc in our work (~4.5 mA/cm ) and
2
even in the best M-P solar cell device (~8.0 mA/cm ) [12] is still
relatively lower compared to other type nonfullerene OSCs (over
2
In order to understand the major effect on small molecule donor
properties on device performance, we carried out several in-
vestigations on charge extraction, generation and transport. The
dependence of the photocurrent density (Jph) on the effective
voltage (Veff) was firstly recorded under illumination at 100
15 mA/cm ) [9], which may probably due to the lower electron
affinity, limited exciton diffusion length and higher electron traps
in these polymer acceptors [12a].
2.5. Blend morphology
ꢀ
2
mWcm . Jph is equal to J
current under illumination and in dark, respectively. Veff is equal to
-V where V is the voltage when Jph is zero and V is the applied
voltage [24]. As shown in Fig. 5a, at low effective voltage below
.5 V, the Jph of DPP-P/N2200 increases drastically and reaches a
L D L D
-J , where J and J are the measured
Lots of previous reports [1,2] have demonstrated that the
exciton dissociation and charge transport process is strongly
affected by the polymer/PCBM blend morphology. Therefore, a
thorough morphological study may be helpful to understand the
less efficient carrier generation and transport process in non-
fullerene solar cells. The morphology of DPP-B/N2200, DPP-N/
N2200 and DPP-P/N2200 blend films was first examined by atomic
force microscopy (AFM) (Fig. 6). By adopting different end groups to
DPP-based donor molecules, the increased backbone rigidity and
intermolecular contacts can promote the aggregation and crystal-
lization process of the donor molecules. The enhanced packing and
crystallinity further promote the molecular/N2200 demixing,
leading to a rougher surface and large “needle” like domains
(Fig. 6c). DIO is believed to allow a slower crystallization process
during spin-coating, thus improving morphology through
enhanced intermolecular ordering and well-developed phase sep-
aration [26]. Therefore, we observed that DPP-B/N2200 and DPP-N/
N2200 blend films processed with DIO exhibited rougher surface
and larger phase separation domains, which may lead to a more
efficient charge transport. However, for DPP-P/N2200, the addition
of DIO further enhanced the mixing the donor and acceptor do-
mains, leading to a more uniformly distributed height surface. In
order to confirm the speculation, we also investigated the blend
film morphology by using GIWAXS, as shown in Fig. 7. For all the
molecule/N2200 blend films, after the addition of DIO, the
morphology showed a dramatic change from the original amor-
phous film to a semi-crystalline blend films with defined rings and
peaks, the crystalline domains is the desired morphology for effi-
cient carrier transport. However, for DPP-P/N2200, distinctive
donor molecular structure has already established the strong
intermolecular contacts and higher degree of structural order.
Compared to DPP-B and DPP-P based blend GIWAXS patterns, the
V
0
a
0
a
0
plateau at higher voltage, suggesting that free charges are swept
out efficiently. In comparison, the Jph of DPP-B/N2200 and DPP-N/
N2200 increase with a slower rate, indicating less efficient carrier
extraction. More importantly, the Jph of the latter is significantly
smaller than that of the DPP-P/N2200 at normal device operational
voltage, which is in accordance with its lower Jsc value. Thus the low
J
ph density suggests that the carrier generation process for DPP-B
and DPP-N must be less efficient than that of DPP-P/N2200 sys-
tem, which may attributed to the different blend morphology.
The charge carrier mobility is another important factor that af-
fects solar cell device performance. In addition, both the electron
Table 1
Photovoltaic properties of PSCs, under an illumination of AM1.5G, 100 mW/cm2
sc (mA/cm2)
Treatment
None
V
oc (V)
J
FF (%)
PCE (%)
DPP-B
DPP-B
DPP-B
DPP-N
DPP-N
DPP-N
DPP-P
DPP-P
DPP-P
DPP-P
0.85
0.69
0.63
0.74
0.69
0.83
0.88
0.79
0.78
0.78
0.14
0.06
0.69
0.26
0.30
1.14
2.89
3.64
4.47
4.45
33.3
46.9
43.4
27.1
43.4
53.2
46.2
45.9
50.3
58.5
0.04
0.02
0.20
0.05
0.09
0.50
1.18
1.32
1.75
2.05
ꢁ
a
TA 120
C
0.5% DIO
None
TA 120 Ca
0.5% DIO
ꢁ
None
TA 120 ꢁCa
0.5% DIO
0.5% DIOb
a
Annealed prior to cathode deposition, 20 min.
Additionally annealed at 120 C for 3 min post to cathode deposition.
b
ꢁ

284
J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
Fig. 4. Energy diagram (a) and device structures (b) of examined solar cells; (c) J-V curves of optimized molecule/N2200 based OSCs; (d) EQE plots of DPP-B, DPP-N and DPP-P based
solar cells devices under optimal processing conditions.
sharply defined rings and peaks (Fig. 7) compared to the neat DPP-P
Fig. 4) pattern indicating more crystalline and ordered domains in
the blend, which is important for hole transport. All these findings
again suggest well selection of highly crystalline molecules is an
important and essential method to improve the performance of M-
P nonfullerene OSCs.
ice water and extracted with chloroform and the combined extracts
were dried with anhydrous Na SO and evaporated. The crude
product was purified by flash column chromatography on silica gel
eluting using the mixture of CH Cl and hexane to give DPP-B as
dark purple solid. H NMR (400 MHz, CDCl ): (ppm) 9.00 (d, 2H),
8.20 (dd, 2H), 7.80e7.59 (m, 4 H), 7.62 (d, 2H), 7.60e7.57 (m, 4 H),
(
2
4
2
2
1
3
d
4
.20e4.10 (m, 4H), 2.00 (s, 2H), 1 0.50e1.27 (m, 16H), 0.99e0.87 (m,
þ
12 H); MS (m/z): [M] calcd for C42
H
48
N
2
O
2
S
2
, 676.32; found,
3
. Experimental
676.46.
3.1. General information
3.3. Synthesis of DPP-N
Nuclear magnetic resonance (NMR) spectra were obtained on
Varian 400 MHz spectrometer. Mass spectra (MS) were obtained on
Thermal Fisher Trace-ISQ spectrometer. UVeviseNIR spectra were
recorded on a Perkin Elmer model Lambda 750. Tapping-mode AFM
images were obtained with a Veeco Multimode V instrument, 2D
GIWAXS experiments were conducted at Shanghai Synchrotron
Radiation Facility (SSRF) on a diffraction beam line (BL14B1).
P(NDI2OD-T2) was prepared according to our previous report
In a 50 mL reaction tube, 3,6-Bis(5-bromothien-2-yl)-2,5-bis(2-
ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (210 mg,
0.4 mmol), 2-bromonaphthalene (207 mg, 1.0 mmol), pivalic acid
(4.6 mg, 0.02 mmol), were dissolved in 4 mL dry toluene under
ꢁ
argon. After stirred at 110 C for 4 h, the mixture was subsequently
poured into ice water and extracted with chloroform and the
combined extracts were dried with anhydrous Na SO and evapo-
2 4
(
M
n
¼ 36.0 kDa, PDI ¼ 2.5) [27]. All the chemicals were purchased
rated. The crude product was purified by flash column chroma-
from Sunatech Inc, Sigma-Aldrich and Strem Chemicals Inc. Toluene
and DMF were purchased from Adamas Reagent. Ltd and distilled
before use.
tography on silica gel eluting using the mixture of CHCl3 and
1
hexane to give DPP-N as dark purple solid. H NMR (400 MHz,
3
CDCl ): d (ppm) 9.00 (d, 4H), 7.75e7.35 (m, 14 H), 4.20e4.05 (m,
4
H), 1.99 (s, 2H), 1.50e1.27 (m, 16H), 0.99e0.87 (m, 12 H); MS (m/
þ
z): [M] calcd for C50
52 2 2 2
H N O S , 776.35; found, 776.10.
3.2. Synthesis of DPP-B
In a 50 mL reaction tube, 3,6-Bis(5-bromothien-2-yl)-2,5-bis(2-
3.4. Synthesis of DPP-P
ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(210
mg,
0
0
.4 mmol), bromobenzene (156 mg, 1.0 mmol), pivalic acid (4.6 mg,
In a 50 mL reaction tube, 3,6-Bis(5-bromothien-2-yl)-2,5-bis(2-
hexyldecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (362 mg,
0.4 mmol), 2 bromopyrene (281 mg, 1.0 mmol), pivalic acid (4.6 mg,
.02 mmol), were dissolved in 4 mL dry toluene under argon. After
ꢁ
stirred at 110 C for 4 h, the mixture was subsequently poured into

J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
285
1
dark purple solid. H NMR (400 MHz, CDCl
3
):
d
(ppm) 9.04 (d, 2H),
8
2
.40 (s, 4H), 8.20 (d, 4 H), 8.10e7.95 (m, 8H), 4.20 (d, 4 H), 2.00 (m,
H), 1.50e1.20 (m, 48 H), 0.90e0.75 (m, 12 H); MS (m/z): [M] calcd
þ
for C78
.5. Fabrication and characterization of polymer solar cells
Polymer solar cells were fabricated with a general structure of
88 2 2 2
H N O S , 1148.63; found, 1147.92.
3
ITO/PEDOT:PSS (40 nm)/Active layer/Al (80 nm). Patterned ITO
glass substrates were cleaned by sequential ultrasonic treatment in
detergent, acetone, deionized water and isopropyl alcohol. The
organic residue was further removed by treating with UV-ozone for
1
0 min. A thin film of PEDOT:PSS (Al 4083) was spin-coated on ITO
ꢁ
substrates and dried at 150 C for 10 min in ambient atmosphere. A
blend of molecule/N2200 (ratio ¼ 2/1, 16 mg/mL) was dissolved in
chloroform w/wo 0.5% (v/v) diiodooctane, filtered through a
0.45
mm poly(tetrafluoroethylene) (PTFE) filter, spin-coated at
2
000 rpm. Finally, 80 nm Al (2A/s) layers were thermally evapo-
ꢀ
6
rated on the active layer at a pressure of 1.0 ꢂ 10 mbar through a
2
shadow mask (active area 7.25 mm ). The current densityevoltage
characteristics of the photovoltaic cells were measured using a
Keithely 2400 (IeV) digital source meter under a simulated AM
1.5G solar irradiation at 100 mW/cm2 (Newport, Class AAA solar
simulator, 94023A-U). The light intensity is calibrated by a certified
Oriel Reference Cell (91150V) and verified with a NREL calibrated
Hamamatsu S1787-04 diode.
3.6. Mobility measurements by space charge limited current (SCLC)
method
Hole-only and electron-only devices were fabricated to measure
the hole and electron mobility using the space charge limited
current (SCLC) method. The hole-only device structure is ITO/
PEDOT:PSS/polymer or blend/MoOx (6 nm)/Ag (80 nm) and the
electron-only device structure is ITO/ZnO/polymer or blend/LiF
(0.6 nm) /Al (80 nm). The thickness was measured by profilometer.
The mobility was determined by fitting the dark current to the
model of a single carrier SCLC, which is described by the equation:
9
V2
d3
J ¼ 8ε ε
r
mh
;
0
Where J is the current, ε
relative permittivity of the material,
the thickness of the polymer layer, V is the applied voltage. Then
0
is the permittivity of free space, ε
r
is the
m
is the zero-field mobility, d is
1
/2
hole mobilities were calculated from the fitting slope of the J
-
Vcurves.
4. Conclusions
In order to specifically investigate the small molecule donor-
polymer acceptor (M-P) nonfullerene solar cells, we have success-
fully modified the synthesis of a series of molecules containing a D-
Fig. 5. Characteristics of photocurrent density (Jph) versus effective voltage (Veff) in
optimal molecule/N2200 solar cell devices (a); J-V curves and SCLC fit of hole-only (b)
and electron-on (c) diodes devices.
p-DPP-p-D backbone by the incorporation of different end groups
(D) (benzene, naphthalene and pyrene), aiming to further improve
the Jsc and FF of the M-P solar cells. Both experimental results and
theoretical calculations have shown that the size of the end groups
can fine-tune the polymer crystallinity and intermolecular packing
by altering the molecular coplanarity. The molecule crystallinity,
coplanarity and the use of additives play critical roles in improving
the morphology of the M-P blend. By using commercially available
n-type polymer N2200, the optimized solar cells demonstrated an
0
.02 mmol), were dissolved in 4 mL dry toluene under argon. After
ꢁ
stirred at 110 C for 4 h, the mixture was subsequently poured into
ice water and extracted with chloroform and the combined extracts
were dried with anhydrous Na
2
SO
4
and evaporated. The crude
2
increased FF of 58.5%, a Voc of 0.78 V, a Jsc of ~4.5 mA/cm and a PCE
product was purified by flash column chromatography on silica gel
eluting using the mixture of CHCl3 and hexane to give DPP-P as
above 2.0%, which is among the highest efficiency reported for M-P
nonfullerene OSCs. The synergetic effect of molecular crystallinity

286
J. Yuan, W. Ma / Organic Electronics 39 (2016) 279e287
Fig. 6. AFM height images of PPT-PCBM DPP-B (a, d), DPP-N (b, e) and DPP-P (c, f) based blend films processed from chloroform w/wo DIO.
Fig. 7. 2D-GIWAXS patterns of PPT-PCBM DPP-B (a, d), DPP-N (b, e) and DPP-P (c, f) based blend films processed from chloroform w/wo DIO.
and processing additives on blend morphology makes it an efficient
strategy to optimize the currently lower performance of M-P
nonfullerene OSCs.
Y. Chen, J. Am. Chem. Soc. 136 (2014) 15529;
c) Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin, H. Ade, H. Yan, Nat.
Commun. 5 (2014) 5293;
d) Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T.P. Russell, Y. Cao,
Nat. Photonics 9 (2015) 174;
e) J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma, Nat. Energy 1 (2016)
Acknowledgements
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3] a) X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu,
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The author thanks the Collaborative Innovation Center of Suz-
hou Nano Science and Technology, Soochow University, Shanghai
Synchrotron Radiation Facility (SSRF). This work was supported by
the National Natural Science Foundation of China (Grant No.
c) S. Shi, J. Yuan, G. Ding, M. Ford, G. Shi, Yong Li, J. Sun, X. Ling, W. Ma, Adv.
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c) T. Agostinelli, S. Lilliu, J.G. Labram, M.C. Quiles, M. Hampton, E. Pires,
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