Crystalline and active additive for optimization morphology and absorption of narrow bandgap polymer solar cells

Article abstract of DOI:10.1002/pola.28418

The morphology of active layer with an interpenetrating network structure and appropriate phase separation is of great significance to improve the photovoltaic performance for polymer solar cells. A highly crystalline small molecule named DPP-TP6 was synthesized and incorporated into the narrow bandgap polymer solar cells to optimize the morphology of PTB7:PC₇₁BM active layer. The DPP-TP6 small molecule was demonstrated to enhance the light absorbance of active layer and play the role of energy cascade to increase the exciton separation and charge transfer. What's more, DPP-TP6 facilitated forming interpenetrating network structure and increasing the phase separation size of ternary blends. These phenomena lead to a higher hole mobility and a more balanced carrier mobility, so as to increase the power conversion efficiency to 7.85% at DPP-TP6 weight ratio of 8 wt %, comparing to the pristine PTB7:PC₇₁BM system of 6.50%.

Full text of DOI:10.1002/pola.28418

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Crystalline and Active Additive for Optimization Morphology and
Absorption of Narrow Bandgap Polymer Solar Cells
Weihua Zhou,1,2 Qingyun Ai,1,2 Lin Zhang,1,2 Xiangfu Liu,3 Xiaotian Hu,1 Yiwang Chen2,3
1School of Material Science and Engineering, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
2Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
3College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
Correspondence to: Y. Chen (E-mail: ywchen@ncu.edu.cn)
Received 30 August 2016; accepted 21 October 2016; published online 00 Month 2016
DOI: 10.1002/pola.28418
ABSTRACT: The morphology of active layer with an interpene-
trating network structure and appropriate phase separation is
of great significance to improve the photovoltaic performance
for polymer solar cells. A highly crystalline small molecule
named DPP-TP6 was synthesized and incorporated into the
narrow bandgap polymer solar cells to optimize the morpholo-
gy of PTB7:PC71BM active layer. The DPP-TP6 small molecule
was demonstrated to enhance the light absorbance of active lay-
er and play the role of energy cascade to increase the exciton
separation and charge transfer. What’s more, DPP-TP6 facilitated
forming interpenetrating network structure and increasing the
phase separation size of ternary blends. These phenomena lead
to a higher hole mobility and a more balanced carrier mobility,
so as to increase the power conversion efficiency to 7.85% at
DPP-TP6 weight ratio of 8 wt %, comparing to the pristine
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PTB7:PC71BM system of 6.50%. 2016 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: additives; conjugated polymers; morphology;
polymer solar cells
the film also requires to form bicontinuous interpenetrating
network structure.8 Meanwhile, crystallization is one of the
most important driving force for the formation of interpene-
trating network structure and phase separation, especially
for crystallization of donor material.9 The active material
with high crystallinity is easy to form crystalline phase dur-
ing process of solvent evaporation. It can be considered that
polymer donor is extracted from the blending system and
form polymer crystalline region. Then, the rest of fullerene
acceptor form fullerene aggregates. Hence, most active layer
with highly crystalline donor could form a large phase sepa-
ration. In addition, molecular interaction is another driving
force, such as miscibility of donor and acceptor.10,11 If the
miscibility of donor and acceptor is very good, that is the
interaction between them is large, the blend will be easy to
form homogeneous phase, resulting in a small phase separa-
tion size. On the contrary, if the miscibility is very weak, that
is the interaction is small, the blend will form independent
phases with a big phase separation size. In a consequence,
optimizing morphology of active layer to form bicontinuous
INTRODUCTION Polymer solar cells have drawn significant
attention in past decades due to their light weight, low-
temperature solution-processing, good flexibility and large
area manufacturing.1,2 The power conversion efficiency
(PCE) based on bulk heterojunction (BHJ) has reached above
10%, through designing new materials,3 optimizing device
structure,4 applying new preparation technology, as well as
optimizing morphology of active layer.5
The morphology of active layer is of great significance for
device performance, especially for the formation of bicontin-
uous interpenetrating network structure with appropriate
phase separation.6,7 As a result of the exciton diffusion dis-
tance is only 10 nm, the phase separation size between the
donor domain and the acceptor domain should be 10–20 nm
to ensure more exciton be separated into electron-hole pair
in the interface between donor domain and acceptor domain.
In addition, the formed electron should transmit into the
cathode along the acceptor domain, and the formed hole
should transmit into the anode along the donor domain.
Therefore, except for the phase separation size at 10–20 nm,
Dedication to the celebration of Prof. Ben-Zhong Tang’s 60th birthday
Competing financial interests. The authors declare no competing financial interest.
Additional Supporting Information may be found in the online version of this article.
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interpenetrating network structure is of great significance to
improve device performance of polymer solar cell.12,13
3,6-bis{5-(4-alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-
pyrrolo[3,4-c]-pyrrole-1,4-diones (DPP-TP6) possessing
a
high crystallinity was synthesized and incorporated into
PTB7:PC71BM system to improve photovoltaic performance.
The influence of DPP-TP6 weight ratio on light absorption,
interpenetrating network structure of active layer, and pho-
tovoltaic performance was investigated. It is observed that
adding of 8 wt % DPP-TP6 small molecule into PTB7:PC71BM
solar cell could increase PCE value from 6.50 to 7.85%.
In recent years, many scholars have made a lot of research
on optimizing morphology of active layer for organic solar
cell.14–18 Generally speaking, the pristine and untreated
active layer deposited by spin coating does not reach the
thermodynamically equilibrium state. Thus, during process
of fabricating active layer, the films need some special meth-
ods to achieve the optimized morphology. It has been
reported that many physical methods, such as thermal
annealing, solvent annealing, solvent additive, adding a third
component, and so on could be applied to modulate mor-
phology of active layer to realize higher photovoltaic perfor-
mance.19–22 Among them, adding a third component not only
is an effective method to optimize morphology of active lay-
er, but also is an easier and more simple method to achieve
high performance in contrast of the tandem solar cell. The
organic solar cell with a third component is also called ter-
nary organic solar cell. Most ternary solar cells possess three
crucial advantages, which are enhancing light absorption,
playing the role of energy cascade and optimizing morphology
of active layer.23–29
EXPERIMENTAL
Materials
PTB7 was purchased from Rieke Metals, and PC71BM (99.5%
purity) was purchased from Nano-C. MoO3 (99.99% purity)
and
Pd(PPh3)4
was obtained
from
Alfa
Aesar.
Zn(CH3COO)2ꢀ2H2O, NH2CH2CH2OH, CH3OCH2CH2OH, K2CO3,
CB, and 4-hexylphenylboronic acid were purchased from
Sigma Aldrich, whereas indium tin oxide (ITO) glass was
purchased from Delta Technologies Limited. All the reagents
were used directly as received without further purification.
Synthesis of DPP-TP6
The 3,6-bis(5-bromothiophen-2-yl)22,5-di(2-ethylhexyl)-pyr-
rolo[3,4-c]pyrrole-1,4-diones(0.75 g, 1.10 mmol) and 4-
hexylphenylboronic acid (0.62 g, 3.00 mmol) were added
into 30 mL dry toluene under stirring, followed by the addi-
tion of Pd(PPh3)4 (0.075 g, 0.064 mmol) and aqueous K2CO3
(2.0 M, 5 mL; N2 bubbled before use). The mixture was vig-
orously stirred at 80 8C for 24 h. After cooling to room tem-
perature, the reaction mixture was poured into water, and
then extracted with CHCl3. The combined organic layer was
washed with water, and dried over anhydrous MgSO4. After
filtration and evaporation, the crude product was purified by
column chromatography (SiO2, CH2Cl2 as the eluent), recrys-
tallized from CHCl3/methanol, and dried under vacuum to
afford a dark violet solid. (yield 5 0.71 g, 56%). 1H NMR
(400 MHz, CDCl3): d 8.97 (d, 2H), 7.56 (d, 4H), 7.41 (d, 2H),
7.21 (d, 4H), 4.06 (m, 4H), 2.63 (t, 4H), 1.94 (m, 2H), 1.63
(m, 4H), 1.41-1.26 (m, 28H), 0.92-0.85 (m, 18H).
The polymer solar cell based on a narrow gap polymer with
thieno[3,4-b]thiophene and benzodithiophene (PTB7) and
PC71BM exhibits an excellent photovoltaic performance.30–33
It possesses a high charge carrier mobility about 102421023
cm2V21 21
, and high light absorption. But the phase separa-
s
tion size of PTB7:PC71BM film deposited from chlorobenzene
(CB) is very large.13,15 So, 1,8-diiodoctane (DIO) is a common
solvent additive for the PTB7:PC71BM system to optimize
morphology and reduce phase separation size.34 Even so,
DIO additive is of no effect on crystallization of polymer and
interpenetrating network structure. In PTB7:PC71BM system,
it is difficult to achieve an interpenetrating network struc-
ture and relative research is very infrequent, especially for
the method of adding a third component.
Based on our previous study,35 the liquid crystalline mole-
cules such as 5CT could induce the crystallization of P3HT,
in addition to the induction of aggregation of PCBM mole-
cules, leading to eventual improvement of morphological sta-
bility of active layer. As reported previously, the donor of
3,6-bis{5-(4-alkylphenyl)thiophen-2-yl}-2,5-di(2-ethylhexyl)-
pyrrolo[3,4-c]-pyrrole-1,4-diones (DPP-TP6) exhibited the
liquid crystalline behavior, showing of excellent PCE of
4.3% after blending with PC71BM.36 By analyzing the
energy levels of components, the DPP-TP6 liquid crystalline
molecule might be useful in PTB7:PC71BM system by form-
ing the cascade energy level alignment. Anyway, adding
liquid crystalline molecules into the PTB7:PC71BM system
has not been reported previously.
Fabrication of Inverted Devices
The desired geometric configuration of ITO coated glass sub-
strates were patterned by etching. The ITO glass substrates
were cleaned by detergent, deionized water and isopropyl
alcohol sequentially with ultrasound treating and dried by
nitrogen flow, followed by UV treatment for 20 min. The
cleaned ITO substrates were covered by spin-coating with a
layer of ZnO through preparing ZnO precursor (in the Sup-
porting Information) to form an electron transport layer,
thermal annealing at 200 8C for 1 h, and transferring into
nitrogen-filled glove box. The PTB7:PC71BM blends with dif-
ferent weight percentage of DPP-TP6 were spin-coated onto
the top of ZnO layer producing about a 120 nm thick active
layer. The ternary blend solution contains DIO additive with
the volume ratio of DIO and CB of 3:97. After drying and
driving off the residual solvent, 7 nm MoO3 and 90 nm Ag
were deposited on top of active layer by thermal evaporation
In this article, we are aimed to optimize morphology and
facilitate formation of interpenetrate network structure for
PTB7:PC71BM active layer by simple solution blending meth-
od. The diketopyrrolopyrrole (DPP) based small molecule
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with a mask at a pressure of ꢁ1026 Torr as an anode. The
detailed information of preparing PTB7:DPP-TP6:PC71BM
ternary blends was shown in the Supporting Information.
Characterization
The ultraviolet-visible (UV) spectra of the specimens were
recorded by a PerkinElmer Lambda 750 spectrophotometer.
Fluorescence measurement was carried out on a Hitachi F-
7000 PC spectrofluorophotometer. To study the phase transi-
tion temperature and the crystallization behavior, differential
scanning calorimetry (DSC) curves were obtained by TA DSC
Q2000 differential scanning calorimeter with an invariable
heating/cooling rate of 10 8C/min. X-ray diffraction (XRD)
study were recorded on a Bruker D8 Focus X-ray diffractom-
eter with a copper target (k 5 1.54 Å). Photocurrent/voltage
(J/V) curves were recorded by using a Keithley 2400 Source
Meter under 100 mW/cm2 simulated AM 1.5 G irradiation
(Abet Solar Simulator Sun 2000) and in the dark. The
current-voltage characterization was recorded by using a
Keithley 2400 Source Meter. The atomic force microscopy
(AFM) images were gauged by nanoscope III A scanning
probe microscope. The morphology of active layerwas con-
ducted by employing transmission electron microscopy
(TEM; JEM-2010 HR). The polarizing optical microscope
(POM) images were obtained with a NIKON E600 POL
equipped with a camera to investigate the morphology of the
films.
RESULTS AND DISCUSSION
The DPP-TP6 small molecule was synthesized as described
previously.36 The molecular structure of DPP-TP6 is shown
in Figure 1 and the corresponding 1HNMR spectra is shown
in Supporting Information Figure S1. The crystallization
behavior of DPP-TP6 was investigated by differential scan-
ning calorimetry (DSC) with the heating and cooling rate of
10 8C/min. The DPP-TP6 exhibits a conspicuous melting peak
at 168.5 8C on the heating curve. In addition, a crystallization
peak with the crystallization temperature of 134.6 8C could
be noticed on the cooling curve. It is revealed that DPP-TP6
small molecule is crystalline, possessing of a high crystallinity.
FIGURE 1 (a) DSC cooling and heating curves of DPP-TP6 at a
rate of 10 8C/min, and the corresponding chemical structure of
DPP-TP6. (b) UV–vis spectra of DPP-TP6 films without and with
thermal annealing at 140 8C.
characteristic of H-type aggregation. Therefore, upon thermal
annealing, J-aggregation of the DPP-TP6 molecules transform
into the thermodynamically favored H-aggregates with the
specific stacking changes in film. The thermoresponsive reor-
ganization behavior for DPP-TP6 is in accordance with the
appearance of liquid crystalline state upon heating, in which
the DPP-TP6 molecules retain at least some mobility. The
DPP-TP6 small molecule is sensitive to thermal annealing
within the transformation of aggregation state.
The UV–vis absorption spectra of DPP-TP6 films from CB
solution with or without thermal annealing were shown in
Figure 1(b). The as-cast film exhibits two distinct peaks and
an unconspicuous peak at 638, 584, and 543, respectively.
The maximum absorption peak at 638 nm is corresponding
to the p-p* [highest occupied molecular orbital (HOMO)-
lowest unoccupied molecular orbital (LUMO)] transition. The
structured absorption is likely intrinsic to a single molecule
with restricted intramolecular motion. The lowest vibronic
band with the maximum absorption in the film originates
from J-type aggregation, owing to excitonic coupling between
the transition dipoles of adjacent molecules. It is worth not-
ing that the absorption peak of J-aggregation band at
638 nm observed for as cast DPP-TP6 film steadily decreases
and at the same time a new absorption band centered
around 550 nm emerges upon thermal annealing at 140 8C
for 10 min. This variety of absorption is defining
The polarizing microscope images of DPP-TP6 films with or
without thermal annealing are shown in Figure 2. The as-
cast film exhibits some fascicular fibers that distributed in
the whole film. However, upon the thermal annealing at
140 8C, the DPP-TP6 film presents spherulites as shown in
Figure 2(b). The different crystalline morphology should cor-
respond to the previous UV–vis absorption with two different
aggregation states as shown in Figure 1(b). Corresponding
XRD spectra in Supporting Information Figure S2 illustrates
that the crystal form of DPP-TP6 is different before and after
thermal annealing at 140 8C, with the 2h value changing from
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blend reaches to the highest. It is believed that addition of
DPP-TP6 could enhance the light absorption of ternary
blends. The photoluminescence (PL) spectra (Supporting
Information Figure S3) of PTB7:PC71BM specimen shows a
strong PL emission peak at 480 nm assigned to radiative
decay of excitons to ground state. Incorporation of DPP-TP6
into the active layer could decrease PL emission intensity. In
addition, the specimen of PTB7:DPP-TP6:PC71BM with 8 wt
% DPP-TP6 exhibits the weakest PL emission peak, indicat-
ing that the radiative decay to ground state decreases and
the exciton dissociation efficiency increases.
The BHJ solar cells based on PTB7:DPP-TP6:PC71BM with dif-
ferent weight percentage of DPP-TP6 were fabricated. Device
FIGURE 2 POM images of DPP-TP6 film (a) without and (b)
with thermal annealing at 140 8C.
5.968 to 6.718. The interplanar distance is calculated to be
0.148 and 0.131 nm, respectively.
The molecular structure of PTB7 and PC71BM are shown in
Figure 3(a). Figure 3(b) exhibits the UV–vis absorption of
DPP-TP6, PTB7, and PC71BM in CB solution. The PTB7 speci-
men shows a strong absorption between 550 and 750 nm,
with a shoulder peak at 622 nm and a maximum peak at
680 nm. The maximum absorption of PC71BM stands on the
short wavelength (300–500 nm). However, the DPP-TP6
specimen possesses a strong absorption between 500 and
650 nm. It reveals that the absorption of these three speci-
mens is complementary on account of the different absorp-
tion in different wavelength. It could enhance the light
absorption when these three components are blended
together, to increase the current intensity. The ternary blends
based on PTB7:DPP-TP6:PC71BM with different weight
percentage of DPP-TP6 (0, 4, 8, 12, 16, and 100 wt %, the
weight ratio of donor and acceptor was maintained to be
1:0.8) were prepared and the UV–vis absorption are shown
in Figure 3(c). The pristine PTB7:PC71BM specimen exhibits
an obvious absorption between 300 and 750 nm. As the con-
tent of DPP-TP6 increases, the light absorption of ternary
blends increases and the enhanced absorption between
500 nm and 650 nm is attributed to DPP-TP6. As DPP-TP6
content increasing to 8 wt %, the absorption of ternary
FIGURE 3 (a) Molecular structure of PTB7 and PC71BM. (b) UV–
vis absorption profiles of PTB7, PC71BM, and DPP-TP6 in CB
solution. (c) UV–vis spectra of PTB7:PC71BM blend films
containing different weight ratios of DPP-TP6.
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FIGURE 4 (a) The device structure of the inverted polymer solar cell, (b) energy-level diagram of the inverted structure used in ter-
nary blend, (c) current-voltage (J-V) characteristics, and (d) EQE spectra of solar cells based on PTB7:PC71BM blends containing
different weight ratios of DPP-TP6.
structure of ITO/ZnO/PTB7:DPP-TP6:PC71BM/MoO3/Ag and
the energy-level diagram of inverted structure is shown in
Figure 4(a,b), respectively. The LUMO and HOMO of DPP-TP6
are 23.7 and 25.5 eV, which lie between the values of PTB7
and PC71BM. It indicates that DPP-TP6 plays the role of a hole
relay between PC71BM and PTB7 via its HOMO energy orbital.
It could reduce the charge transfer barrier within active layer
by decreasing recombination of photogenerated electrons and
holes, according to the theory of cascade energy level align-
ment. This energy cascade provided a suitable prerequisite for
good charge collection. Given that the energy levels of three
components were well-matched, the open-circuit voltage did
not significantly decrease in the present ternary system.
content to 8 wt %, the PCE reaches to the highest value of
7.85% for the corresponding solar cell device. As is
expected, both of Jsc and FF reach to the maximal value of
15.54 mA cm22 and 69.2%, respectively. The series resis-
tance (Rs) decreases from 4.933 to 1.023 Xꢀcm2 and the shut
resistance (Rsh) possesses a little change, in contrast to pris-
tine PTB7:PC71BM device. Consequently, the reduced Rs con-
tribute to the enhanced device performance by increasing Jsc
and FF that has to do with the morphology of active layer.
As the DPP-TP6 content increases continuously, Jsc and FF
begin to decrease. The Voc of ternary system exhibits a little
change possessing to the increasing trend with increase of
DPP-TP6 content. It can be concluded that adding a small
amount of DPP-TP6 into PTB7:PC71BM system could improve
Jsc, FF, and Voc of device, resulting in a higher photovoltaic
performance. In addition, the increased Jsc and PCE value
have been determined by external quantum efficiency (EQE)
shown in Figure 4(d). The enhanced EQE value could further
interpret improvement of Jsc and PCE.
Figure 4(c) shows the current-voltage (J-V) characteristics of
BHJ solar cell based on PTB7:DPP-TP6:PC71BM system at dif-
ferent weight percentage of DPP-TP6 that were measured
under AM 1.5G (100 mW/cm2) light intensity illumination. A
summary of photovoltaic performance parameters is given in
Table 1. The device based on PTB7:PC71BM blends exhibits a
PCE value of 6.5%, with a short-circuit current (Jsc) of 13.45
mA cm22, an open circuit voltage (Voc) of 0.726 V, and a fill
factor (FF) of 66.3%, respectively. With increase of DPP-TP6
All values represented averages from 0.04 cm2 devices on a
single chip. And all data of devices had been tested from more
than five substrates (15 chips) to ensure reproducibility.
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TABLE 1 Summary of the Photovoltaic Performance of PTB7:PC71BM Solar Cells Containing Different Weight Ratios of DPP-TP6
under AM 1.5G Solar Illumination
DPP-TP6
content (wt %)
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rs (X cm2)
Rsh (X cm2)
0
13.45 6 0.3
14.37 6 0.2
15.54 6 0.3
14.01 6 0.2
13.82 6 0.2
0.726 6 0.02
0.729 6 0.02
0.730 6 0.03
0.729 6 0.03
0.733 6 0.03
66.3 6 1
68.4 6 1
69.2 6 1
69.0 6 2
63.1 6 1
6.50 6 0.1
7.17 6 0.1
7.85 6 0.2
7.04 6 0.2
6.40 6 0.1
4.933
3.006
1.023
3.012
6.363
385.9
799.9
330.2
352.5
1833
4
8
12
16
To evaluate the apparent charge carrier mobility in the active
layer, J0.5-V characteristics of single charge carrier devices
were measured using the space charge-limited-current
(SCLC) model according to the Mott-Gurney equation:
to 3), l is carrier mobility, V is the applied voltage, and L is
the film thickness.
The thickness of BHJ blend for SCLC measurement was about
120 nm. The J0.5-Vcharacteristics of hole-only and electron-
only devices are shown in Figure 5. The summary of hole
mobility and electron mobility that were fitted with SCLC
model is shown in Table 2. The hole mobility and electron
J5ð9=8Þle0erðV2=L3Þ
(1)
where J is the current density, l is the charge carrier mobili-
ty, e0 is the permittivity of free space (8.85 3 10212 Fꢀm21),
er is the dielectric constant of PTB7 or PC71BM (assumed
mobility of PTB7:PC71BM specimen are 1.66
3
1024
cm2ꢀV21ꢀs21 and 2.84 3 1024 cm2ꢀV21ꢀs21, respectively. As
the DPP-TP6 content increases to 8 wt%, the specimen pos-
sesses a more balanced carrier mobility with hole mobility
of 2.16 3 1024 cm2ꢀV21ꢀs21 and electron mobility of 2.30 3
1024 cm2ꢀV21ꢀs21. When the DPP-TP6 content increases to
16 wt %, the carrier mobility shows of a decrease. The
enhancement of hole mobility is mainly contributed to
increased light absorbance and effect of energy cascade
which have been mentioned above. In addition, the morphol-
ogy of BHJ could be another factor resulting in the improve-
ment of device performance.
To explore the internal morphology of thin films and to
understand effect of morphology on device performance, the
corresponding TEM images are shown in Figure 6. The speci-
men of PTB7:PC71BM blend shows of almost no discernable
structure, attributing to a largely homogenous morphology
without serious phase separation. The PTB7 and PC71BM
could mix well after casting from CB containing 3% DIO sol-
vent. However, the DPP-TP6:PC71BM specimen shows a
TABLE 2 Hole Mobility and Electron Mobility of PTB7:PC71BM
Solar Cells Containing Different Weight Ratios of DPP-TP6
under AM 1.5G Solar Illumination
DPP-TP6
Hole mobility
(cm2/Vꢀs)a
Electron mobility
(cm2/Vꢀs)b
content (wt %)
0
1.66 3 1024
1.83 3 1024
2.16 3 1024
1.96 3 1024
1.53 3 1024
2.84 3 1024
2.74 3 1024
2.30 3 1024
2.62 3 1024
2.12 3 1024
4
8
12
16
a
Hole-only device configuration: ITO/PEDOT:PSS/PTB7:DPP-TP6:
PC71BM/MoO3/Ag.
Electron-only device configuration: ITO/ZnO/PTB7:DPP-TP6:PC71BM/Al.
FIGURE 5 J0.5-V characteristics of (a) hole-only and (b) electron-
b
only devices containing different weight ratios of DPP-TP6.
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FIGURE 6 TEM graphs of the PTB7:PC71BM films containing (a) 0, (b) 4, (c) 8, 12, 16, and (d) 100 wt % DPP-TP6, respectively.
remarkably different morphology, presenting a lot of black
interpenetrating network structures throughout whole film.
The black structures could be considered to be DPP-TP6
crystals and phase separation is more obvious. When 4 wt
% DPP-TP6 was added into PTB7:PC71BM blend, the speci-
men presents a lot of unconspicuous interpenetrating net-
work structures with a bigger phase separation, in contrast
to the pristine PTB7:PC71BM specimen. As DPP-TP6 content
increases to 8 wt %, the interpenetrating network structures
could be observed obviously, and the size of separated phase
becomes larger than that of PTB7:4 wt % DPP-TP6:PC71BM
specimen. As DPP-TP6 content increases to 16 wt %, the
interpenetrating network structures becomes more discern-
ible, the phase separation is serious, resulting in decrease of
charge separation efficiency. In other words, addition of DPP-
TP6 into PTB7:PC71BM system could facilitate formation of
interpenetrating network structures with a proper phase
separation, leading to enhanced charge separation efficiency
and charge transport efficiency.
PTB7:PC71BM blends. Moreover, the DPP-TP6 shows of no
beneficial contribution to the thermal stability of
PTB7:PC71BM. In Supporting Information Figure S6, the pho-
tovoltaic performance of PTB7:PC71BM devices deteriorate
after thermal annealing at 150 8C for 30 min, especially for
the specimens at high DPP-TP6 content. The corresponding
parameters are shown in Supporting Information Table S2,
the PCE value of PTB7:PC71BM system decreases to 2.0%
after thermal annealing, which further reduces to 1.3% at
DPP-TP6 content of 16 wt %. The reduction is mainly con-
tributing to the depressed Jsc value, showing that DPP-TP6 is
not beneficial to the improvement of thermal stability of
PTB7:PC71BM due to the lack of supramolecular interactions
between DPP-TP6 and PTB7 and PC71BM.37
CONCLUSIONS
Highly crystalline small molecule DPP-TP6 was synthesized
and the DPP-TP6 small molecule was easy to form nano-
fibroid structure due to its high crystallinity. The incorporation
of DPP-TP6 into PTB7:PC71BM system was demonstrated to
increase the PCE value to 7.85% at DPP-TP6 weight ratio of
8 wt %, in contrast to pristine PTB7:PC71BM system of 6.50%.
The DPP-TP6 could make up the range of light absorbance, as
to enhance light absorbance, as well as play the role of energy
cascade to promote exciton separation and charge transfer. In
addition, the addition of highly crystalline small molecule into
PTB7:PC71BM blends could facilitate the formation of inter-
penetrating network structure for the active layer and increase
phase separation size and surface roughness. The improved
device performance was contributed to the enhanced light
absorption of active layer, as well as higher and more balanced
carrier mobility. The addition of third additive such as liquid
crystalline molecules into the narrow bandgap polymer solar
cells should be a versatile strategy to improve photovoltaic
performance of corresponding devices. Anyway, the thermal
stability may deteriorate upon incorporation of DPP-TP6, due
To further investigate the morphology evolution of blend
films after blending with DPP-TP6, AFM images are shown in
Supporting Information Figure S4. The pristine PTB7:PC71BM
film shows a small surface roughness with the root-mean-
square value of 2.31 nm. As DPP-TP6 content increasing, the
RMS value of corresponding specimen increases gradually.
The variation of surface roughness should be corresponding
to change of separated phase size in TEM images. Conse-
quently, the addition of highly crystalline DPP-TP6 into
PTB7:PC71BM blends could increase the surface roughness,
resulting in a better contact between buffer layer and active
layer.
In addition, in PTB7:DPP-TP6:PC71BM ternary blend films in
Supporting Information Figure S5, no discernible diffraction
peak ascribing to DPP-TP6 could be observed, indicating that
the DPP-TP6 could not crystallize after incorporation into
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to the lack of supramolecular interactions between DPP-TP6
and other components.
€
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This work was financially supported by the National Science
Fund for Distinguished Young Scholars (51425304), and
National Natural Science Foundation of China (51303077 and
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