Quadrupolar D–A–D diketopyrrolopyrrole-based small molecule for ternary blend polymer solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.05.048

A quadrupole diketopyrrolopyrrole (DPP)-based small molecule (DPP4T-Cz) was designed and synthesized to enhance absorption coefficient, and then employed as the third component to improve the light harvesting of polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Because of an enhanced absorption coefficient of more than 10⁵ cm^?1, the photon harvesting efficiency was improved effectively in the near infrared (near-IR) range by using only a small amount of DPP4T-Cz (3.4 wt%) into the P3HT:PCBM binary blend polymer solar cells. Interestingly, the photocurrent generation was also enhanced in the visible range by the long-range energy transfer from P3HT to DPP4T-Cz molecules. As a result, the short-circuit current density (J_SC) and power conversion efficiency (PCE) of P3HT:PCBM:DPP4T-Cz ternary blend devices were enhanced by more than 30% compared to those of P3HT:PCBM binary control devices. These findings suggest that quadrupole DPP-based molecules are one of the effective light-harvesting materials for ternary blend polymer solar cells.

Full text of DOI:10.1016/j.dyepig.2018.05.048

DyesandPigments158(2018)213–218
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Quadrupolar D–A–D diketopyrrolopyrrole-based small molecule for ternary
blend polymer solar cells
Yanbin Wanga,∗, Teng Wanga, Jinxing Chena, Hyung Do Kimb, Penghan Gaoa, Biaobing Wanga,∗∗
Ryo Iriguchib, Hideo Ohkitab,∗∗∗
,
a
Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of
Photovoltaic Science and Engineering, Changzhou University, Changzhou, Jiangsu, 213164, People's Republic of China
b
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
A quadrupole diketopyrrolopyrrole (DPP)-based small molecule (DPP4T-Cz) was designed and synthesized to
enhance absorption coefficient, and then employed as the third component to improve the light harvesting of
polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM). Because of an enhanced absorption coefficient of more than 105 cm−1, the photon harvesting
efficiency was improved effectively in the near infrared (near-IR) range by using only a small amount of DPP4T-
Cz (3.4 wt%) into the P3HT:PCBM binary blend polymer solar cells. Interestingly, the photocurrent generation
was also enhanced in the visible range by the long-range energy transfer from P3HT to DPP4T-Cz molecules. As a
result, the short-circuit current density (JSC) and power conversion efficiency (PCE) of P3HT:PCBM:DPP4T-Cz
ternary blend devices were enhanced by more than 30% compared to those of P3HT:PCBM binary control
devices. These findings suggest that quadrupole DPP-based molecules are one of the effective light-harvesting
materials for ternary blend polymer solar cells.
Exciton harvesting
Exciton separation
Dye location
Energy transfer
Ternary solar cells
1. Introduction
of > 14% has been reported very recently [16].
Ternary blend polymer solar cells can be classified into sensitizer
Polymer solar cells have attracted more and more attention as one
of the most efficient technologies for utilizing solar energy, because of
their outstanding advantages such as ease of processing, potentially low
cost and large scale production [1–6]. The power conversion efficiency
(PCE) has been improved steadily, and increased up to more than 13%
even for single-junction polymer solar cells [7,8], but it still lags behind
those of commercialized inorganic solar cells. For further improvement
in the PCE, many more photons should be absorbed over a broad range
from the visible to near infrared (near-IR) region. However, it is im-
possible to cover such a wide wavelength region of the terrestrial solar
radiation by using only one or two organic materials, because the ab-
sorption bandwidth of organic materials is typically as narrow as a few
hundreds of nanometers at most. As such, ternary solar cells have at-
tracted huge attention because the third component material exhibits
complementary absorption bands, which can broaden the absorption
spectrum of the host binary blend [9–15]. With this strategy, the pho-
tovoltaic performances have been improved effectively, and a PCE
type and parallel type. In the case of the sensitization, small molecules
have been employed as a third sensitizing material. For efficient ternary
blend solar cells, of particular importance is a large absorption coeffi-
cient of the third small molecules. The small molecules reported so far
include dye molecules and conjugated small molecules. Dye molecules
typically exhibit relatively sharp and narrow absorption bands with a
large absorption coefficient. For example, phthalocyanines and squar-
aines have an intense absorption band in a longer wavelength region
compared to most of wide-bandgap conjugated polymers [17–20]. Such
dye molecules are likely to form aggregates in polymer:fullerene blend
films [21–23]. We therefore have employed silicon phthalocyanine
derivatives with bulky ligands for ternary blend polymer solar cells
[24–27]. As a result, we found that bulky ligands can effectively sup-
press dye aggregation and hence improve the photocurrent generation
owing to the additional absorption. Furthermore, we have shown that
dye location in blends can be controlled by careful selection of ligands.
More specifically, we found that hexyl ligands are compatible with poly
∗
Corresponding author.
Corresponding author.
Corresponding author.
∗∗
∗∗∗
E-mail addresses: wangyanbin@cczu.edu.cn (Y. Wang), biaobing@cczu.edu.cn (B. Wang), ohkita@photo.polym.kyoto-u.ac.jp (H. Ohkita).
https://doi.org/10.1016/j.dyepig.2018.05.048
Received 13 April 2018; Received in revised form 22 May 2018; Accepted 23 May 2018
Availableonline24May2018
0143-7208/©2018ElsevierLtd.Allrightsreserved.

Y. Wang et al.
DyesandPigments158(2018)213–218
(3-hexylthiophene) (P3HT) but benzyl ligands are compatible with a
fullerene derivative (PCBM). On the basis of these findings, we de-
monstrated that a heterostructured dye with hexyl and benzyl ligands
can most effectively improve short-circuit current density (JSC) in
ternary blend polymer solar cells [28]. On the other hand, conjugated
small molecules exhibit relatively broad absorption bands with a
modest absorption coefficient. Typically, such conjugated small mole-
cules are compatible with conjugated donor polymers. Thus, it is re-
quired to develop conjugated small molecules with a large absorption
coefficient for highly efficient ternary blend solar cells.
In this study, we have designed a conjugated small molecule with a
quadrupole donor–acceptor–donor (D–A–D) structure for a sensitizer in
ternary blend polymer solar cells. Such D–A–D molecular structures can
enhance transition dipole moments and hence give a large absorption
coefficient. Indeed, most of squaraine molecules have a large absorp-
tion coefficient because of D–A–D quadrupole structures [17,20]. Here,
we selected a diketopyrrolopyrrole (DPP) moiety as an acceptor unit
and a carbazole (Cz) moiety as a donor unit and synthesized a quad-
rupole D–A–D molecule 3,6-bis{5'-(9-(2-ethylhexyl)-9H-carbazol-2-yl)-
[2,2′-bithiophen]-5-yl}-2,5-bis(2-ethylhexyl)-pyrrolo[3,4-c]pyrole-1,4-
dione (DPP4T-Cz). Because of the D–A–D quadrupole structure, DPP4T-
Cz exhibited a wide absorption band in the near-IR region with an
absorption coefficient of more than 105 cm−1. This DPP4T-Cz molecule
was incorporated as the third sensitizer in P3HT:PCBM solar cells. Fig. 1
shows chemical structures of these three photovoltaic materials. Con-
sequently, the JSC was increased from 10.5 to 12.9 mA cm−2 and hence
the PCE was improved from 3 to 4% by using a small amount of DPP4T-
Cz. Furthermore, the sensitization mechanism and the location of the
sensitizer molecule in the active layer are also studied.
(DPP4T) was purchased from 1-Material Inc. 2-Bromocarbazole (Cz-Br),
1-bromo-2-ethylhexane, tri(o-tolyl)phosphine (P(o-tol)3), and tris(di-
benzylideneacetone)dipalladium(0) (Pd2(dba)3) were purchased from
Tokyo Chemical Industry. All the materials were used without further
purification.
2.2. Synthesis
Scheme 1 shows a general synthetic route for the conjugated mo-
lecule DPP4T-Cz. 2-Bromo-9-(2-ethylhexyl)-9H-carbazole (M1) was
synthesized from 2-bromocarbazole (Cz-Br) and 1-bromo-2-ethyl-
hexane through the Hofmann alkylation reaction. 3,6-Bis{5'-(9-(2-
ethylhexyl)-9H-carbazol-2-yl)-[2,2′-bithiophen]-5-yl}-2,5-bis(2-ethyl-
hexyl)pyrrolo[3,4-c]pyrole-1,4-dione (DPP4T-Cz) was synthesized from
M1 and DPP4T through the Stille coupling reaction. The chemical
structures of intermediate M1 and the final dye molecule DPP4T-Cz
have been confirmed by 1H-NMR measurements.
2.2.1. Synthesis of M1
Potassium hydroxide powder (1.35 g, 24 mmol), 2-ethylhexyl bro-
mide (4.26 mL, 24 mmol), and tetrabutylammonium hydrogen sulfate
(0.2 g, 0.59 mmol) were added to Cz-Br in acetone (30 mL,
0.4 mol L−1). The reaction mixture was heated slowly to reflux for
overnight, and then poured into water and extracted with di-
chloromethane. The organic layer was washed with water and dried
over anhydrous magnesium sulfate. Further purification was performed
by column chromatography on silica gel with hexane: ethyl acetate (10:
1, v/v) as eluent, yielding 3.86 g (85%) of 2-bromo-9-(2-ethylhexyl)
carbazole (M1) as colorless oil. 1H-NMR (400 MHz, CDCl3, δ): 8.02 (d,
1H), 7.87 (d, 1H), 7.45 (m, 2H), 7.32 (d, 1H), 7.29 (d, 1H), 7.20 (t, 1H),
4.00 (m, 2H), 2.00 (m, 1H), 1.40–1.15 (m, 8H), 0.95–0.78 (m, 6H).
2. Experimental section
2.2.2. Synthesis of DPP4T-Cz
2.1. Materials
DPP4T (200 mg, 0.197 mmol), M1 (141 mg, 0.394 mmol), P(o-tol)3
(7.2 mg, 0.0236 mmol) were dissolved in 15 mL anhydrous toluene. The
solution was purged with argon for 30 min, and then Pd2(dba)3
(10.8 mg, 0.0118 mmol) was added. The reaction mixture was heated
slowly to reflux for 72 h, and then poured into water and extracted with
dichloromethane. The organic layer was washed with water and dried
over anhydrous magnesium sulfate before the solvent was evaporated.
Further purification was performed by column chromatography on si-
lica gel with hexane: dichloromethane (1: 1, v/v) as eluent, yielding
195.5 mg (80%) of DPP4T-Cz as purple solid. 1H-NMR (400 MHz,
CDCl3, δ): 8.92 (d, 2H), 7.77–7.72 (m, 4H), 7.57–7.54 (m, 4H), 7.40 (d,
2H), 7.32–7.28 (m, 4H), 7.20–7.14 (m, 4H), 7.03 (d, 2H), 4.32 (t, 4H),
4.23 (m, 2H), 4.08 (m, 2H), 2.05 (m, 4H), 1.80–1.55 (m, 8H), 1.50–1.18
(m, 24H), 1.06–0.82 (m, 24H).
Zinc acetate dehydrate ethanolamine, 2-methoxyethanol, and P3HT
were purchased from Sigma–Aldrich. A fullerene PCBM was purchased
from Frontier Carbon. 3,6-Bis(5'-(trimethylstannyl)-[2,2′-bithiophen]-
5-yl)-2,5-bis(2-ethylhexyl)-dihydropyrrolo[3,4-c]pyrrole-1,
4-dione
2.3. Materials characterization
1H-NMR spectra were acquired at room temperature for M1 and
DPP4T-Cz on an Avance III 400 M NMR (Bruker, Rheinstetten,
Germany) in deuterochloroform (CDCl3) containing tetramethylsilane
as an internal reference. Absorption and PL spectra of blend films were
investigated by spectrophotometer (Hitachi, U-3500) and spectro-
fluorometer (Horiba Jobin Yvon, NanoLog), respectively. Surface
morphology was performed by atomic force microscopy (Shimadzu,
SPM-9600) in a tapping mode using a high resolution cantilever
(MikroMasch, Hi’Res-C14/Cr-Au) with a force constant of ≈5 N m−1
and a resonance frequency of 160 kHz.
2.4. Device fabrication and characterization
Patterned indium tin oxide (ITO)-coated glass substrates with a
sheet resistance of 10 ohm per square were cleaned consecutively in an
ultrasonic bath containing toluene, acetone, ethanol, and deionized
Fig. 1. Chemical structures of photovoltaic materials: a) P3HT, b) PCBM, and c)
DPP4T-Cz.
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Y. Wang et al.
DyesandPigments158(2018)213–218
Scheme 1. Synthetic scheme of a quadrupole D−A−D molecule DPP4T-Cz.
water for 15 min each, and then blow-dried by high purity nitrogen,
finally cleaned with UV–ozone cleaner for 30 min. A precursor solution
of ZnO was prepared by dissolving zinc acetate dihydrate (Zn
(CH3COO)2·2H2O, 1 g) and ethanolamine (NH2CH2CH2OH, 0.28 g) in 2-
methoxyethanol (CH3OCH2CH2OH, 15 mL) under vigorous stirring for
one night for the hydrolysis reaction in air. The ZnO precursor solution
was spin-casted on top of the ITO glass substrate at 800 rpm for 10 s and
3000 rpm for 60 s in sequence, and then were anneal at 180 °C for 1 h in
air. During this process, the precursor was converted to solid-state ZnO
to give transparent nanoparticle ZnO thin film, which was used as an
electron-transporting layer. Subsequently, P3HT:PCBM (with and
without DPP4T-Cz) solutions was spin-coated at a spin rate of 600 rpm
for 60 s in the glove box, films were naturally dried under N2 atmo-
sphere for at least 12 h. Then, the device fabrication was completed by
thermally evaporating 10-nm-thick MoO3 and 100-nm-thick Au under
vacuum at a pressure of 3 × 10−4 Pa. The effective device area was
0.07 cm2.
The PCE was determined from J–V curve measurements (using a
Keithley 2611B source meter) under a 1 sun, AM1.5G spectrum from a
solar simulator (Oriel model 91192; 1000 W m−2). The solar simulator
illumination intensity was determined using a monocrystal silicon re-
ference cell (Bunkoukeiki, BS-520). The EQE data were obtained using a
solar cell spectral response measurement system (Bunkoukeiki, ECT-
250D).
Fig. 2. Absorption spectra of DPP4T-Cz in chloroform solution (solid line) and
in solid film on a quartz substrate (broken line).
Cz. With the two light-harvesting materials of P3HT and DPP4T-Cz, the
solar light ranging from 400 to 850 nm can be collected effectively.
Furthermore, the photoluminescence (PL) spectrum of P3HT is well
overlapped with the absorption spectrum of DPP4T-Cz, suggesting ef-
ficient energy transfer from P3HT to DPP4T-Cz. On the other hand, the
highest occupied molecular orbital (HOMO) level of the photovoltaic
materials used in this study was evaluated by photoelectron yield
spectrometer (PYS) (see the Supporting Information S1), and the lowest
unoccupied molecular orbital (LUMO) level was estimated from the
optical bandgap and the HOMO level. As shown in Fig. 3b, the HOMO
and LUMO levels of DPP4T-Cz are evaluated to be −5.2 and −3.5 eV,
respectively, which are located in between those of P3HT and PCBM. It
has been reported that such a cascade energy structure would be ben-
eficial for the charge transfer at the interfaces of P3HT/DPP4T-Cz and
DPP4T-Cz/PCBM [31]. In other words, there is a competition of energy
transfer and charge transfer between P3HT and DPP4T-Cz at the in-
terfaces of P3HT/DPP4T-Cz as described below.
3. Results
3.1. Optoelectronic properties
Fig. 2 shows the absorption spectra of DPP4T-Cz in chloroform so-
lution and in solid film on a quartz substrate. As shown in the figure,
DPP4T-Cz molecules exhibit two major absorption regions at 400 nm
and at 623 and 658 nm in solution. The longer absorption peaks are
ascribed to the intramolecular charge transfer transition [29,30]. In the
solid film state, these two peaks are observed at 660 and 725 nm, which
are red-shifted by about 60 nm compared to that in solution. This red-
shifted absorption suggests that the DPP4T-Cz molecules have strong
intermolecular packing interaction in the solid state. More importantly,
DPP4T-Cz molecules exhibit a high absorption coefficient in the near-IR
region both in solution and in film state, indicating that it can be used
as an effective sensitizer for improving the exciton harvesting in com-
bination with wide-bandgap materials.
3.2. Energy transfer
To discuss the energy transfer from P3HT to DPP4T-Cz, we eval-
uated the Förster radius by equation (1) assuming point-dipoles.
9000κ2 (ln 10)ηD
128π5n4NA
͠
dν
R06
=
͠ ͠
f (ν)ε (ν)
A
D
∫
4
(1)
͠
ν
where κ is the orientation factor between P3HT and DPP4T-Cz (κ2 = 2/
3), ηD is the fluorescence quantum yield of P3HT in the absence of
DPP4T-Cz (ηD = 0.01) [32], n is the refractive index of the medium
As shown in Fig. 3a, P3HT exhibits a strong absorption band around
550 nm, which is complementary to the absorption spectrum of DPP4T-
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Y. Wang et al.
DyesandPigments158(2018)213–218
Fig. 5. The current density–voltage (J–V) curves of P3HT:PCBM:DPP4T-Cz
ternary (solid line) and P3HT:PCBM binary (broken line) blend devices under
AM1.5G simulated solar illumination.
3.3. J–V characteristics
To examine the sensitization effect of DPP4T-Cz, we fabricated
binary and ternary blend polymer solar cells with an inverted layered
structure of ITO/ZnO/active layers/MoO3/Au under the same condi-
tions. As shown in Fig. 5, the P3HT:PCBM binary control device gave a
PCE of 3% with JSC of 10.5 mA cm−2, VOC of 0.53 V, and FF of 0.54.
With addition of only 3.4 wt% of DPP4T-Cz molecules into the binary
blends, the photovoltaic performance of P3HT:PCBM:DPP4T-Cz ternary
blend devices was improved significantly as summarized in Table 1. It is
noteworthy that the JSC was improved obviously up to 12.9 mA cm−2
.
In addition, the fill factor (FF) was slightly improved from 0.54 for the
binary control device to 0.56 for the P3HT:PCBM:DPP4T-Cz ternary
blend device, suggesting that charge transport in ternary blends is
better than in binary blends. On the other hand, the open-circuit vol-
tage (VOC) of P3HT:PCBM:DPP4T-Cz ternary blend devices remained
the same as those of the binary devices. Hence the PCE was improved
from 3.0% for the P3HT:PCBM binary control devices to 4.0% for the
P3HT:PCBM:DPP4T-Cz ternary blend devices. Further addition of
DPP4T-Cz rather decreased the photocurrent generation and hence
degraded the overall photovoltaic performance. A similar sensitization
effect was found for another ternary blend solar cell based on poly[5,5′-
bis(2-butyloctyl)-(2,2′-bithiophene)-4,4′-dicarboxylate-alt-5,5′-2,2′-bi-
thiophene] (PDCBT), PCBM, and DPP4T-Cz. In this case, the JSC was
increased from 10.2 to 12.4 mA cm−2, thus PCE was improved by 17%
from 5.2% for PDCBT/PCBM binary blend solar cells to 6.1% for
PDCBT/PCBM/DPP4T-Cz without further optimization (see the
Supporting Information S2). This finding suggests that our DPP4T-Cz is
a versatile sensitizer for improving the efficiency of polymer solar cells.
Fig. 3. a) Normalized UV–visible absorption spectra of DPP4T-Cz (solid line)
and P3HT (broken line), and photoluminescence (PL) spectrum of P3HT (da-
shed-dotted line) in solid films. b) Energy level diagram of photovoltaic ma-
terials P3HT, DPP4T-Cz, and PCBM.
3.4. External quantum efficiency (EQE) spectra
To address the origin of the improvement in JSC, we measured the
external quantum efficiency (EQE) spectra of P3HT:PCBM binary con-
trol and P3HT:PCBM:DPP4T-Cz ternary blend devices (see the
Supporting Information S3). With the addition of 3.4 wt% DPP4T-Cz
into the binary blend, the EQE signal was observed in the near-IR region
for the ternary blend polymer solar cells, which is consistent with the
improved absorption as shown in Fig. 6a. Interestingly, the EQE signal
ascribed to P3HT was also enhanced at around 550 nm from 61 to 72%
as shown in Fig. 6b, although there is no difference in the absorption
Fig. 4. Photoluminescence (PL) spectra of P3HT (solid line) and P3HT:DPP4T-
Cz (broken line) with a weight ratio of 3: 4 thin films excited at 500 nm.
(n = 1.6) [33], NA is the Avogadro's number, fD is the normalized donor
fluorescence spectrum, εA is the molar absorption coefficient of DPP4T-
Cz, and v(∼) is the wavenumber. On the basis of equation (1), a Förster
radius of 3.5 nm is obtained for P3HT and DPP4T-Cz. The long Förster
radius indicates that there is efficient energy transfer from P3HT to
DPP4T-Cz. Indeed, as shown in Fig. 4, the PL intensity of P3HT in the
P3HT:DPP4T-Cz binary blend film decreased significantly while the PL
intensity ascribed to DPP4T-Cz was observed clearly in the near-IR
region, although the absorption of DPP4T-Cz is negligibly small at the
excited wavelength. These findings suggest that P3HT excitons can be
efficiently harvested through the energy transfer from P3HT to DPP4T-
Cz.
Table 1
Device parameters of ternary blend polymer solar cells.
P3HT: PCBM: DPP4T-Cz
JSC/mA cm−2
VOC/V
FF
PCE/%
20: 20: 0 (0 wt%)
10.5
11.3
12.9
11.7
0.53
0.54
0.56
0.54
0.54
0.56
0.56
0.51
3.00
3.42
4.04
3.22
20: 20: 0.7 (1.7 wt%)
20: 20: 1.4 (3.4 wt%)
20: 20: 2.1 (5.0 wt%)
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Y. Wang et al.
DyesandPigments158(2018)213–218
Table 2
Surface energy of P3HT, DPP4T-Cz, and PCBM.
P3HT
18.9
DPP4T-Cz
22.0
PCBM
30.6
γ/mJ m−2
Fig. 8. Photoluminescence (PL) spectra of P3HT (black solid line), P3HT:PCBM
with a weight ratio of 1: 1 (black broken line), and P3HT:PCBM:DPP4T-Cz (grey
solid line) ternary blend films excited at 500 nm.
Fig. 6. a) Thin film UV–visible absorption spectra and b) external quantum
efficiency (EQE) characteristics of P3HT:PCBM binary (broken line) and
P3HT:PCBM:DPP4T-Cz ternary (solid line) blend devices.
0.6 mA cm−2. Furthermore, the internal quantum efficiency (IQE) of
the P3HT:PCBM:DPP4T-Cz ternary blend device was estimated to be
93% from the EQE and absorption spectra at 710 nm, suggesting that
more than 93% of DPP4T-Cz molecules are located at the interface of
P3HT/PCBM, this is because that only DPP4T-Cz molecules at the in-
terface can contribute to the photocurrent as shown in Fig. 7. In order
to figure out such spontaneous location of DPP4T-Cz molecules in
ternary blend films, we measured surface energy (γ) of P3HT, PCBM,
and DPP4T-Cz by the contact angle method. As a result, we found that
the surface energy is estimated to be 18.9 mJ m−2 for P3HT,
22.0 mJ m−2 for DPP4T-Cz, and 30.6 mJ m−2 for PCBM as summarized
in Table 2. Consequently, a wetting coefficient (ω) derived from the
surface energy was estimated to be 0.46 for DPP4T-Cz in the
P3HT:PCBM:DPP4T-Cz ternary blend film from Neumann's and Young's
equations [35]. Previously, it has been reported that if −1 < ω < 1,
the third component material would be located at the interface of the
host binary blend [28,36]. These results support that the location of
DPP4T-Cz plays a key role in the sensitization of ternary blends.
Now, we focus on the energy transfer from P3HT to DPP4T-Cz. As
shown in Fig. 8, although the PL intensity of P3HT decreased sig-
nificantly for the P3HT:PCBM binary blend film with a weight ratio of
1: 1, 20% of P3HT excitons radiatively were deactivated to the ground
state before arriving at the interface because of the large P3HT do-
mains. Typically, an exciton diffusion length of conjugated polymers
has been reported to be only about 10 nm [37,38], which limits the
exciton diffusion efficiency, especially for highly crystalline polymers.
On the other hand, it has been confirmed that the exciton diffusion
between P3HT:PCBM binary and P3HT:PCBM:DPP4T-Cz ternary blend
films, as reported previously [24,34]. This is probably because there is
efficient energy transfer from P3HT to DPP4T-Cz, which can collect the
P3HT exciton lost in the P3HT:PCBM binary blend. On the other hand,
as shown in Fig. 6a, the vibronic band at around 600 nm was observed
in the both blend films, indicating that P3HT crystalline domains are
not disturbed in the P3HT:PCBM:DPP4T-Cz ternary blend films. This is
consistent with almost the same hole mobility and surface morphology
observed for both binary and DPP4T-Cz blended ternary films (see the
Supporting Information S4 and S5).
4. Discussion
As mentioned above, the photovoltaic performance of
P3HT:PCBM:DPP4T-Cz ternary blend devices has been improved com-
pared with P3HT:PCBM binary control devices. There are two reasons
for this: one is the direct light-harvesting ascribed to DPP4T-Cz, and the
other is the improved exciton-harvesting of P3HT due to the energy
transfer.
Firstly, we focus on the high absorption coefficient molecule of
DPP4T-Cz. As shown in Fig. 6a and b, with the addition of only 3.4 wt%
DPP4T-Cz into the binary blend, the photocurrent generation was im-
proved obviously in the near-IR region. By integrating the product of
the AM1.5G solar spectrum and the absorption spectrum of DPP4T-Cz,
the direct photocurrent generated by DPP4T-Cz is estimated to be
Fig. 7. Charge transfer of DPP4T-Cz molecules a) at the interfaces of P3HT and PCBM, b) in the domains of P3HT, and c) in the domains of PCBM, respectively.
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Y. Wang et al.
DyesandPigments158(2018)213–218
efficiency can be improved through the long-range energy transfer [39].
Indeed, with the addition of 3.4 wt% of DPP4T-Cz molecules in the
P3HT:PCBM binary blend, the P3HT excitons was quenched com-
pletely. The improved exciton diffusion efficiency is useful for
achieving a higher photocurrent in the visible region. By integrating the
product of the AM1.5G solar spectrum and the absorption spectrum of
P3HT, the photocurrent ascribed to the energy transfer is estimated to
be 1.8 mA cm−2. In other words, the photocurrent ascribed to the en-
ergy transfer is three times as high as that ascribed to the direct light
harvesting from DPP4T-Cz.
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Acknowledgments
This work was partly supported by the Natural Science Foundation
of Jiangsu Province (BK20160280), Double Plan of Jiangsu Province,
and the Japan Science and Technology Agency ALCA program (Solar
Cell and Solar Energy Systems).
Appendix A. Supplementary data
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