Development of novel n-Type materials based on benzothiadiazole derivatives for organic photovoltaics: Effects of acceptor terminal substituents

Article abstract of DOI:10.1246/cl.141155

Novel n-type materials of BTD-CN (1) and BTD-CF₃ (2) were synthesized for organic photovoltaics. Both materials show approximately the same HOMO-LUMO level and position of absorption peaks. However, the organic photovoltaic performance of a device with 1 is clearly higher than that with 2. In this paper, we discuss the effects of terminal substituents on film morphology, crystallinity, and photovoltaic performance. The results suggest that cyano-modified benzothiadiazole is useful because of its crystallinity, carrier recombination, and series resistance.

Full text of DOI:10.1246/cl.141155

CL-141155
Received: December 15, 2014 | Accepted: January 29, 2015 | Web Released: February 6, 2015
Development of Novel n-Type Materials Based on Benzothiadiazole Derivatives
for Organic Photovoltaics: Effects of Acceptor Terminal Substituents
Yosei Shibata,1 Takahiro Kono,*1,2 Hiroyo Usui,1 and Yuji Yoshida*1
1Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technologies (AIST),
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
2Center for Energy and Environmental Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567
(E-mail: t-kouno@shinshu-u.ac.jp, yuji.yoshida@aist.go.jp)
Novel n-type materials of BTD-CN (1) and BTD-CF3 (2)
were synthesized for organic photovoltaics. Both materials show
approximately the same HOMO-LUMO level and position of
absorption peaks. However, the organic photovoltaic perform-
ance of a device with 1 is clearly higher than that with 2. In this
paper, we discuss the effects of terminal substituents on film
morphology, crystallinity, and photovoltaic performance. The
results suggest that cyano-modified benzothiadiazole is useful
because of its crystallinity, carrier recombination, and series
resistance.
OPV devices have similarly been reported.18,27 An understand-
ing of the effects of these electron-acceptor substituents on OPV
devices is important for the elucidation of optimal non-fullerene
molecular design. In this context, we synthesized two n-type
materials based on BTD derivatives having differing terminal
substituents: BTD-CN (1) cyano substituent and BTD-CF3 (2)
trifluoromethyl, as shown in Scheme 1. In this study, we discuss
the effects of terminal electron-accepting substituents of these
materials on OPV device performance.
Details of the synthesis methods of these compounds are
described in the Supporting Information. To investigate the
photophysical and electrochemical properties, UV-vis spectra
and cyclic voltammetry (CV) measurements were carried out.
From the results of UV-vis spectra (Figure S5), absorption
maxima of 1 were a little higher than those of 2. The peak
maxima were at 478 nm (¾, 2.3 © 104 M¹1 cm¹1) and 350 nm
(¾, 2.7 © 104 M¹1 cm¹1) for 1 and at 479 nm (¾, 1.7 © 104
Solution-processed organic photovoltaics (OPVs) have
many advantages such as potential for low cost, available
printing processes, and flexible devices. For a high-performance
OPV architecture, bulk heterojunction (BHJ) structures based
on a mixture of a π-conjugated polymer as electron donor and
fullerene derivatives as electron-acceptor molecules have been
intensively investigated during the past two decades.1-5 In recent
years, power conversion efficiencies (PCEs) in OPV devices have
drastically improved, approaching over 10%, mainly due to the
development of novel organic semiconductors.6-8 Many studies
on fullerene derivatives as electron-acceptor materials have
been reported for the control of film morphology, crystallinity,
solubility, and energy levels. Among them, [6,6]-phenyl-C61
butyric acid methyl ester ([60]PCBM)9,10 and indene-C60 bis-
adduct (ICBA)11,12 are well known as typical acceptor materials.
On the other hand, these fullerene derivatives have problems in
terms of realization of OPV device applications owing to poor
morphological stability13 and high cost.14 The pace of develop-
ment of non-fullerene acceptor materials has accelerated over
the past few years, making possible the realization of high-
performance OPV devices without fullerene derivatives.15-21
In a previous study, we have shown that the structural
control of BHJ films with terminally modified materials led to a
drastic improvement in OPV performance.22 The control of film
morphology and crystallinity is also required in the design of n-
type materials having a suitable LUMO level for efficient carrier
transport. However, for n-type materials without fullerene,
molecular design rules for the morphological control of BHJ
films have not been fully established. As candidates for an
electron-accepting backbone, we focused on acceptor materials
based on benzothiadiazole (BTD) because of their good
electron-accepting ability, which is attributable to nitrogen
atoms. Their derivatives showed high electron mobility
(® = 0.19 cm2 V¹1 s¹1) in an organic field effect transistor.23
In many cases of n-type material design, fluorinated compounds
have been reported to increase the acceptor properties.24-26
Alternatively, cyano-modified compounds and applications for
M
¹1 cm¹1) and 335 nm (¾, 2.4 © 104 M¹1 cm¹1) for 2. The
HOMO and LUMO levels of these compounds were calculated
from the CV redox peaks (Figure S6 and Table S1). The HOMO
levels of these materials are: 1 ¹5.53 eV, 2 ¹5.58 eV. The
LUMO levels are: 1 ¹3.33 eV, 2 ¹3.30 eV. On the other hand,
the HOMO level of P3HT and the LUMO level of [60]PCBM
were around ¹5.2 and ¹3.75 eV, respectively.28 It is expected
that high open-circuit voltage (Voc) devices can be obtained from
devices combining P3HT with these BTD materials, because Voc
strongly depends on the difference between the donor HOMO
level and the acceptor LUMO level.
We investigated the dependence of blend ratio (P3HT:1 or
2 = 2:1, 1:1, 2:3, and 1:2 wt %) on OPV performance. The
highest performance in both 1 and 2 was obtained at
P3HT:BTD = 1:1 wt %. Figure 1b and Table 1 show the J-V
curves of OPV devices with P3HT:1 or 2 = 1:1 (shown in
Figure 1a) and their device parameters, respectively. The OPV
fabrication procedures and J-V curves for P3HT:[60]PCBM =
1:1 and the other blend ratios in P3HT:BTD devices have been
described in the Supporting Information.
The Voc values of P3HT:1 were higher than that of
P3HT:PCBM. The device with P3HT:1 generated a Voc of ca.
1 V. This is attributed to a wide energy gap between the LUMO
level of these BTD derivatives and the HOMO level of P3HT.
However, the photovoltaic parameters (Jsc, Voc, and fill factor
1 (BTD-CN)
2 (BTD-CF3)
Scheme 1. Molecular structure of 1 (BTD-CN) and 2 (BTD-CF3).
680 | Chem. Lett. 2015, 44, 680–682 | doi:10.1246/cl.141155
© 2015 The Chemical Society of Japan

Figure 2. AFM images of spin-coated P3HT:BTD = 1:1 BHJ films
surface on ITO/PEDOT:PSS substrate after pre-annealing treatment at
130 °C for 10 min. Topography images are (a) and (b), and phase
images are (c) and (d). The scan area is 2 ¯m © 2 ¯m.
Figure 1. (a) BHJ-type OPV device configuration of ITO/PEDOT:
PSS/P3HT:BTD derivatives = 1:1 wt % (100 nm)/LiF (0.5 nm)/Al
(100 nm); (b) J-V curves under light irradiation with AM1.5G,
0.5 ³cm2 and 3.1 k³cm2, respectively. The FF of P3HT:1 was
higher than that of P3HT:2. This is attributed to low Rs and high
Rp. In addition to bimolecular recombination, these parameters
support the difference in OPV performance between P3HT:1 and
P3HT:2.
100 mW cm¹2; (c) external quantum efficiency (EQE); (d) dependence
¹2
of light intensity on Jsc ranging from 5 to 100 mW cm
.
Table 1. Estimated photovoltaic performance from J-V curves
In the following discussion, we focused on the film structure
of the P3HT:BTD = 1:1 films. Figure 2 shows the AFM images
of P3HT:BTD BHJ film surfaces. The P3HT:BTD = 1:1 surface
root-mean-square (RMS) roughness was 8.46 nm with 1 and
5.80 nm with 2. Surface morphologies of P3HT:1 and P3HT:2
were similar. The P3HT:1 film had a higher RMS value than the
P3HT:2 film. These results indicate that the BHJ film with
P3HT:1 has a higher level of crystallinity than P3HT:2. From
phase images, the formation of a homogeneous BHJ film was
confirmed (microscale aggregation structure not shown).22 The
BHJ films with P3HT:BTD = 1:1 have a homogeneous blend
structure.
For further discussion of film structure, we measured the
out-of-plane XRD patterns (shown in the Supporting Informa-
tion) of these BHJ films and of single-component films. The
diffraction angles and calculated d-spacing values from the
Bragg equation are summarized in Table 2. The diffraction
pattern of a single-component film with 2 cannot be obtained
because of its poor wettability onto the ITO-coated glass/
PEDOT:PSS substrate. In a single-component film with 1,
highly ordered peaks were observed at 2ª = 3.91, 7.80, 15.6,
and 23.3°. The d-spacing value at 2ª = 3.91° from the Bragg
equation was 22.6 ¡. This is consistent with the molecular
length of 1 (the molecular length of 1 from the DFT calculation
was obtained as around 22.14 ¡).33-35 Both first-ordered 1 and
lamellar P3HT (100) peaks were observed in the P3HT:1 film.
In contrast, P3HT:2 blend films have only a P3HT (100) peak.
Compound 2 has an amorphous configuration in P3HT:2 films.
Despite blending BTD derivatives with P3HT, the lamellar
diffraction peak of P3HT appeared in both blend films. This is
attributed to the strong molecular interactions between P3HT
molecules. On comparing P3HT:1 with P3HT:2, the morpho-
logical and crystallinity differences in BHJ films contributed to
electric properties such as internal resistance, current leakage,
and recombination ratio. The relationship between the acceptor-
aggregation state in BHJ film and OPV performance has already
Jsc
/mA cm
Fill
factor
Devicea
Voc/V
PCE/%
¹2
P3HT:1b
P3HT:2b
P3HT:[60]PCBM
= 1:1 wt %
2.53 « 0.06 0.99 « 0.01 0.35 « 0.01 0.86 « 0.04
0.58 « 0.02 0.54 « 0.03 0.30 « 0.01 0.09 « 0.01
6.05 « 0.37 0.56 « 0.01 0.65 « 0.02 2.21 « 0.13
aDevice area: 0.04 cm2. Blend ratio is 1:1 wt %.
b
(FF)) of the device with P3HT:1 were clearly higher than those
with P3HT:2. To investigate this, we measured the external
quantum efficiency (EQE) of these OPV devices, shown in
Figure 1c. The overall EQE spectra of P3HT:1 was higher than
that of P3HT:2. This means that cyano substituents are better
than trifluoromethyl substituents for n-type OPV materials.
The difference in Jsc between P3HT:1 and P3HT:2 would be
associated with the difference of the molar coefficient of BTD
derivatives (shown in Figure S5). The Voc of P3HT:2 is a
significant small value despite having approximately the same
LUMO level in both 1 and 2. This feature would suggest that
there is a possibility of carrier recombination loss. Therefore,
we measured the dependence of light intensity of Jsc, shown in
Figure 1d. The bimolecular recombination rate has previously
been discussed with the light intensity dependence of Jsc (IS,
where I and S are light intensity and slope, respectively).5,29,30
The estimated S values from power function fitting are as
follows: S = 0.89 in P3HT:1 devices, S = 0.84 in P3HT:2
devices (The dependence of J-V curves on light intensity is
shown in Figure S9, Table S2, and Table S3.). These results
indicate that the bimolecular recombination ratio in P3HT:1
devices is lower than that in P3HT:2. The series resistance (Rs
means the internal resistance in devices) and parallel resistance
(Rp means the resistance to current leakage in devices) of these
devices were extracted from the dark-current using the Shockley
diode model.31,32 The Rs and Rp of P3HT:2 are 2.5 ³cm2 and
1.8 k³cm2, respectively. In case of P3HT:1, the Rs and Rp are
Chem. Lett. 2015, 44, 680–682 | doi:10.1246/cl.141155
© 2015 The Chemical Society of Japan | 681

Table 2. Diffraction profiles of thin films on ITO-coated glass/
PEDOT:PSS
9
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P3HT:1 = 1:1 wt %
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have crystalline and amorphous configuration, respectively.
Consequently, P3HT:1 shows higher OPV performance than
P3HT:2. It is concluded that the crystallinity of BTD derivatives
in BHJ films with P3HT was drastically influenced by the
terminal substituent of BTD.
In summary, we synthesized two novel n-type materials, 1
and 2, based on benzothiadiazole in order to understand the
effects of acceptor substituents on organic photovoltaics.
Devices with P3HT:1 have higher open-circuit voltages (Voc)
than those with P3HT:[60]PCBM. This can be attributed to the
LUMO levels of these BTD derivatives. The terminally modified
substituents have a significant effect on the crystallinity of BHJ
structure. The suitable film structure led to the improvement of
Rs, Rp, and bimolecular recombination. Consequently, the PCE
of P3HT:1 is higher than that of P3HT:2. For the molecular
design of n-type materials, the cyano group is a better substituent
than the trifluoromethyl group in terms of homogeneous blend
and ordered structure in BHJ films.
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This work was supported by the Japan Science and
Technology Agency (JST-CREST) and investment from Re-
search Center for Photovoltaic Technologies at AIST as frame-
work for the priority issue. We appreciate Dr. Junichi Murakami
(Tsukuba Innovation Arena Headquarters, AIST) for HRMS
measurements.
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30 D. Mori, H. Benten, I. Okada, H. Ohkita, S. Ito, Energy Environ.
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Supporting Information is available electronically on J-STAGE.
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682 | Chem. Lett. 2015, 44, 680–682 | doi:10.1246/cl.141155
© 2015 The Chemical Society of Japan