Panchromatic dyes having diketopyrrolopyrrole and ethylenedioxythiophene applied to dye-sensitized solar cells

Article abstract of DOI:10.1016/j.orgel.2016.07.022

Novel panchromatic donor-acceptor-π-acceptor dyes (DPP1–4) containing diketopyrrolopyrrole (DPP) and 3,4-ethylenedioxythiophene (EDOT) have been designed and synthesized. Introduction of both DPP and EDOT units into a dye framework induced a remarkable red-shift of absorption bands and thus photoactive spectral regions of the dye-sensitized solar cells (DSSCs) based on the panchromatic DPP1–4 dyes were extended far above 800?nm. Influences of alkyl substituents in DPP1–4 on the performances of the DSSCs were discussed, and also those of co-adsorbate on TiO₂ and additive in the electrolyte used in the DSSC preparation were studied to optimize photovoltaic performances of the DSSCs.

Full text of DOI:10.1016/j.orgel.2016.07.022

Organic Electronics 37 (2016) 465e473
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Panchromatic dyes having diketopyrrolopyrrole and
ethylenedioxythiophene applied to dye-sensitized solar cells
Ichiro Imae*, Yohei Ito, Shun Matsuura, Yutaka Harima
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527,
Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel panchromatic donor-acceptor-p-acceptor dyes (DPP1e4) containing diketopyrrolopyrrole (DPP)
Received 10 June 2016
Received in revised form
12 July 2016
and 3,4-ethylenedioxythiophene (EDOT) have been designed and synthesized. Introduction of both DPP
and EDOT units into a dye framework induced a remarkable red-shift of absorption bands and thus
photoactive spectral regions of the dye-sensitized solar cells (DSSCs) based on the panchromatic DPP1e4
dyes were extended far above 800 nm. Influences of alkyl substituents in DPP1e4 on the performances
of the DSSCs were discussed, and also those of co-adsorbate on TiO2 and additive in the electrolyte used
in the DSSC preparation were studied to optimize photovoltaic performances of the DSSCs.
© 2016 Elsevier B.V. All rights reserved.
Accepted 18 July 2016
Keywords:
Dye-sensitized solar cells
Diketopyrrolopyrrole
Ethylenedioxythiophene
Panchromatic absorption
1. Introduction
available industrial pigment for the application to car-paint pig-
ments and polymeric organic photovoltaics [15e17], and is ex-
pected to have a strong light-harvesting ability. From this
viewpoint, several DPP-containing D-A-p-A dyes have been syn-
thesized [12,15,18e30].
€
Since the first report of Gratzel et al., in 1991 [1,2], dye-
sensitized solar cells (DSSCs) have attracted a considerable atten-
tion due to their high conversion efficiency of incident light to
electricity and low production cost [3]. In order to improve the
power conversion efficiency, many researchers have focused on the
We have recently developed novel D-
ethylenedioxythiophene (EDOT) in the oligothiophenes as
linkers [31,32]. These D- -A dyes demonstrated a successful
p-A dyes containing 3,4-
p
-
development of novel organic dyes [4e6]. Since donor-
(D- -A) dyes consisting of an electron donor (D) and an electron
acceptor (A) linked through a -conjugated bridge ( -linker)
p-acceptor
p
p
expansion of spectral response due to a remarkable red-shift of
absorption bands caused by introduction of EDOT [33e37]. In our
p
p
possess high molar absorption coefficients and their electronic
properties can be easily tuned by molecular modifications, many
present study, a novel class of D-A-
EDOT units (DPP1e4) have been designed and synthesized (Fig. 1),
in order to further improve the light-harvesting ability of the D- -A
dyes by utilizing both characteristics of DPP and EDOT. The per-
formances of DSSCs based on the D-A- -A dyes are discussed in
comparison with those for PE4T, which has an EDOT-containing
p-A dyes having both DPP and
types of D-
In 2010, Tian et al. reported a new concept of a donor-acceptor-
acceptor (D-A- -A) structure for designing a new generation of
efficient dye-sensitizers [12]. In this D-A- -A structure, an addi-
p-A dyes have been enthusiastically synthesized [7e11].
p
p-
p
p
p
tional electron-acceptor unit such as diketopyrrolopyrrole (DPP),
benzothiadiazole, benzotriazole, and quinoxaline has been intro-
quaterthiophene as a
2 known well as a D-
p
-linker with no DPP unit [31], and for MK-
p-A dye having oligothiophene for the high-
duced into the
p-linker of the D-
p-A framework, so that the ab-
performance DSSC [38].
sorption spectral response has been extended to the near infrared
region [13,14]. DPP was originally developed as a commercially
2. Experimental
2.1. Materials
n-Hexane, toluene, tetrahydrofuran (THF), N,N-dime-
* Corresponding author.
E-mail address: imae@hiroshima-u.ac.jp (I. Imae).
thylformamide (DMF), acetic acid (AcOH), chloroform (CHCl3),
http://dx.doi.org/10.1016/j.orgel.2016.07.022
1566-1199/© 2016 Elsevier B.V. All rights reserved.

466
I. Imae et al. / Organic Electronics 37 (2016) 465e473
Fig. 1. Chemical structures of DPP1e4, PE4T, and MK-2.
dichloromethane (CH2Cl2), methanol (MeOH), and acetonitrile
were purified by standard methods and used immediately after
purification. Tetrabutylammonium perchlorate (TBAP) and N-
bromosuccinimide (NBS) were purified by recrystallization from
ethanol and benzene, respectively, and dried under vacuum.
2.2. Synthesis
The synthetic routes of D-A-p-A dyes having both EDOT and DPP
units are shown in Scheme 1. The starting compounds, N-(4-
bromophenyl)-N,N-diphenylamine (1a) [39], N,N-bis(9,9-dihexyl-
EDOT,
5-formyl-2-thiopheneboronic
acid
(7a),
4-
9H-fluoren-2-yl)-N-(4-bromophenyl)amine
(1b)
[40],
2-
formylphenylboronic acid (7b), and cyanoacetic acid (9) were
purchased from Tokyo Chemical Industry and used without
further purification. Tetrakis(triphenylphosphine)palladium (0)
(Pd(PPh3)4) purchased from Kanto Chemical was used as the
catalyst of the Stille and Suzuki reactions. 5-(2-Carboxy-2-
cyanovinyl)-[5000-(9-ethyl-9H-carbazol-3-yl)-30,300,3000,4-tetra-n-
hexyl-2,20:50,200:500,2000-quaterthiophene (MK-2) was purchased
from Aldrich.
tributylstannyl-3,4-ethylenedioxythiophene (2) [35], and 3,6-
bis(5-bromo-2-thienyl)-2,5-dialkyl-2,5-dihydropyrrolo[3,4-c]pyr-
role-1,4-dione (5) [41] were synthesized according to previously
published procedures. The Stille coupling reaction of bromo-
triarylamine (1) and stannyl-EDOT (2) gave the compounds 3.
Compounds 4 were obtained by a reaction between tributyltin-
chloride with an organolithium compound obtained from 3 and n-
BuLi, and were used for the Stille reaction with bis(bromothienyl)-
DPP (5) to give the compounds 6. Suzuki coupling reaction between

I. Imae et al. / Organic Electronics 37 (2016) 465e473
467
Scheme 1. Synthetic routes of DPP1e4.
6 and formylaryl boronic acid (7) gave the compounds 8, which
were converted to the corresponding sensitizing dyes by Knoeve-
nagel condensation with cyanoacetic acid (9). Detailed synthetic
processes for some intermediate chemicals and target dyes and
their characterizations are described below.
After 1 h, tributyltin chloride (0.40 g, 1.2 mmol) was added in one
portion. The reaction mixture was stirred at ꢁ78 ꢀC for 30 min and
then at room temperature for 1 h. After the solvent was removed
using a rotary evaporator, n-hexane was added and the solution
was stirred for 1 h. After filtration of the solution, the solvent was
removed using a rotary evaporator. A yellow liquid was obtained
upon solvent removal. Product was used without further
purification.
2.2.1. 2-[4-(N,N-Diphenylamino)phenyl]-3,4-
ethylenedioxythiophene (3a, Ar ¼ phenyl)
Under a nitrogen atmosphere, a mixture of 1a (1.1 g, 3.4 mmol),
2 as-prepared from EDOT (0.73 g, 5.6 mmol), and Pd(PPh3)4 (0.47 g,
0.44 mmol) were dissolved in DMF (10 mL). The resulting solution
was stirred at 80 ꢀC for 12 h. The mixture was poured into water
and then extracted with CH2Cl2. The organic extract was dried over
Na2SO4. After removal of the solvent, the residue was purified by a
column chromatography (silica gel, n-hexane/CH2Cl2 (1/2)). Com-
pound 3a (0.47 g, 1.2 mmol) was obtained as a pale yellow liquid.
2.2.3. 3-(5-Bromo-2-thienyl)-6-{50-[4-(N,N-diphenylamino)
phenyl]-30,40-ethylenedioxy-2,20-bithien-5-yl}-2,5-di(n-octyl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (6a, Ar ¼ phenyl, R ¼ n-
octyl)
Under a nitrogen atmosphere, a mixture of 4a obtained from 3a
(0.47 g, 1.2 mmol), 5a (0.83 g, 1.2 mmol), and Pd(PPh3)4 (0.14 g,
0.13 mmol) were dissolved in toluene (20 mL). The resulting solu-
tion was stirred at 80 ꢀC for 12 h. The mixture was poured into
water and then extracted with CHCl3. The organic extract was dried
over Na2SO4. After removal of the solvent, the residue was purified
by a column chromatography (silica gel, toluene). Compound 6a
Yield: 36%. 1H NMR (500 MHz, CD2Cl2,
d, ppm): 4.21e4.31 (m, 4H,
OCH2CH2O), 6.25 (s, 1H, thienyl-H), 7.00e7.05 (m, 2H, phenyl-H),
7.03 (d, J ¼ 8.83 Hz, 2H, phenylene-H), 7.06e7.11 (m, 4H, phenyl-H),
7.23e7.29 (m, 4H, phenyl-H), 7.57 (d, J ¼ 8.83 Hz, 2H, phenylene-H).
(60 mg, 61
(500 MHz, CD2Cl2,
J ¼ 7.14 Hz, 3H, CH3), 1.22e1.48 (m, 20H, (CH2)5CH3), 1.62e1.81 (m,
4H, N-CH2CH2), 3.98 (t, J ¼ 7.80 Hz, 2H, N-CH2), 4.07 (t, J ¼ 7.80 Hz,
2H, N-CH2), 4.34e4.50 (m, 4H, OCH2CH2O), 7.04 (d, J ¼ 8.80 Hz, 2H,
m
mol) was obtained as a purple solid. Yield: 6%. 1H NMR
2.2.2. 2-[4-(N,N-Diphenylamino)phenyl]-5-tributylstannyl-3,4-
ethylenedioxythiophene (4a, Ar ¼ phenyl)
d
, ppm): 0.86 (t, J ¼ 7.14 Hz, 3H, CH3), 0.88 (t,
To a THF (15 mL) solution of 3a (0.47 g, 1.2 mmol), 0.74 mL of n-
butyl lithium (1.65 M in n-hexane, 1.2 mmol) was added at ꢁ78 ꢀC.

468
I. Imae et al. / Organic Electronics 37 (2016) 465e473
phenylene-H), 7.03e7.08 (m, 2H, phenyl-H), 7.08e7.13 (m, 4H,
phenyl-H), 7.24 (d, J ¼ 4.26 Hz, 1H, thienylene-H), 7.25e7.31 (m, 4H,
phenyl-H), 7.36 (br, 1H, thienylene-H), 7.60 (d, J ¼ 8.80 Hz, 2H,
phenylene-H), 8.61 (br, 1H, thienylene-H), 9.05 (d, J ¼ 3.91 Hz, 1H,
thienylene-H).
2.2.7. 3-[50-(2-Carboxy-2-cyanovinyl)-2,20-bithien-5-yl]-6-{50-[4-
(N,N-diphenylamino)phenyl]-30,40-ethylenedioxy-2,20-bithien-5-
yl}-2,5-di(2-octyldodecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
dione (DPP3, Ar ¼ phenyl, R ¼ 2-octyldodecyl, Ar0 ¼ 2,5-thienylene)
DPP3 was prepared according to the same procedure as
described for DPP1 by using 3,6-bis(5-bromo-2-thienyl)-2,5-di (2-
octyldodecyl)-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione as
a
2.2.4. 3-(50-Formyl-2,20-bithien-5-yl)-6-{50-[4-(N,N-
diphenylamino)phenyl]-30,40-ethylenedioxy-2,20-bithien-5-yl}-2,5-
di(n-octyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (8a,
Ar ¼ phenyl, R ¼ n-octyl, Ar0 ¼ 2,5-thienylene)
starting material. 1H NMR (500 MHz, acetone-d6,
d,
ppm):
0.78e0.88 (m, 12H, CH3), 1.10e1.50 (m, 64H, methylene-H),
1.98e2.03 (m, 2H, N-CH2CH), 3.98e4.08 (br, 4H, N-CH2), 4.40e4.60
(m, 4H, OCH2CH2O), 5.57 (s, 1H, vinylene-H), 7.04e7.74 (m, 18H, Ar-
H), 9.01 (d, J ¼ 4.12 Hz, 1H, thienylene-H), 9.19 (d, J ¼ 4.12 Hz, 1H,
thienylene-H). HRMS (ESI) m/z calcd for C86H108O6N4S4 1420.71462
(Mþ), found 1420.71375 (Mþ).
Under a nitrogen atmosphere, a mixture of 6a (56 mg, 57
mmol),
7a (15 mg, 0.60 mmol), and Pd(PPh3)4 (10 mg, 9.5
mmol) was dis-
solved in THF (9 mL). To this solution, Na2CO3 (16 mg, 0.15 mmol) in
water (3 mL) was added. The resulting solution was stirred at 80 ꢀC
for 16 h. The mixture was poured into water and then extracted
with CHCl3. The organic extract was dried over Na2SO4. After
removal of the solvent, the residue was washed by acetone. Com-
2.2.8. 3-{5-[4-(2-Carboxy-2-cyanovinyl)phenyl]-2-thienyl}-6-(50-
{4-[N,N-bis(9,9-dihexyl-9H-fluoren-2-yl)amino]phenyl}-30,40-
ethylenedioxy-2,20-bithien-5-yl)-2,5-di(n-octyl)-2,5-dihydropyrrolo
[3,4-c]pyrrole-1,4-dione (DPP4, Ar ¼ 9,9-dihexyl-9H-fluoren-2-yl,
R ¼ n-octyl, Ar0 ¼ 1,4-phenylene)
pound 8a (30 mg, 29
m
mol) was obtained as a reddish purple solid.
Yield: 52%. 1H NMR (500 MHz, CD2Cl2,
d
, ppm): 0.86 (t, J ¼ 6.88 Hz,
3H, CH3), 0.87 (t, J ¼ 6.88 Hz, 3H, CH3), 1.24e1.41 (m, 20H,
(CH2)5CH3),1.71e1.81 (m, 4H, N-CH2CH2), 4.07 (t, J ¼ 8.72 Hz, 2H, N-
CH2), 4.09 (t, J ¼ 8.72 Hz, 2H, N-CH2), 4.35e4.48 (m, 4H, OCH2CH2O),
7.03 (d, J ¼ 8.80 Hz, 2H, phenylene-H), 7.04e7.08 (m, 2H, phenyl-H),
7.08e7.12 (m, 4H, phenyl-H), 7.25e7.31 (m, 4H, phenyl-H), 7.36 (d,
J ¼ 4.34 Hz, 1H, thienylene-H), 7.39 (d, J ¼ 3.97 Hz, 1H, thienylene-
H), 7.49 (d, J ¼ 4.16 Hz, 1H, thienylene-H), 7.60 (d, J ¼ 8.80 Hz, 2H,
phenylene-H), 7.71 (d, J ¼ 3.97 Hz, 1H, thienylene-H), 8.88 (d,
J ¼ 4.16 Hz, 1H, thienylene-H), 9.09 (d, J ¼ 4.34 Hz, 1H, thienylene-
H), 9.86 (s, 1H, CHO).
DPP4 was prepared according to the same procedure as
described for DPP2 by using 4-formylphenylboronic acid for the
synthesis of compound 8.1H NMR (500 MHz, DMSO-d6,
d, ppm):
0.70e0.92 (m, 18H, CH3), 0.94e1.16 (m, 52H, methylene-H),
1.58e1.72 (m, 4H, N-CH2CH2), 1.80e1.94 (m, 8H, fluorene-CH2),
3.96e4.07 (br, 4H, N-CH2), 4.32e4.54 (m, 4H, OCH2CH2O),
5.75e5.78 (br, 1H, vinylene-H), 6.92e8.02 (m, 24H, Ar-H),
8.76e8.81 (br, 1H, thienylene-H), 8.98e9.01 (br, 1H, thienylene-H).
HRMS (ESI) m/z calcd for C102H118O6N4S3 1590.82080 (Mþ), found
1590.81983 (Mþ).
2.2.5. 3-[50-(2-Carboxy-2-cyanovinyl)-2,20-bithien-5-yl]-6-{50-[4-
(N,N-diphenylamino)phenyl]-30,40-ethylenedioxy-2,20-bithien-5-
yl}-2,5-di(n-octyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione
(DPP1, Ar ¼ phenyl, R ¼ n-octyl, Ar0 ¼ 2,5-thienylene)
2.3. Characterizations
1H NMR spectra were recorded by a 500 MHz spectrometer
(Varian Inc., NMR System 500). Measurements of mass spectros-
copies were made using a GC-TOFMS (gas chromatographdtime-
of-flight mass spectrometry, JEOL, JMS-T100GCV (AccuTOF GCv
4G)) and an ESI-FTMS (electron spray Fourier transformed mass
spectrometry, Thermo Fisher Scientific, LTQ Orbitrap XL). UVevis
absorption and fluorescence spectra were taken on a Shimadzu UV-
3150 spectrophotometer and a Hitachi F-4500 fluorescence spec-
trophotometer, respectively. Cyclic voltammetry (CV) was carried
out with a potentiostat/galvanostat (Hokuto Denko, HZ-3000) us-
ing a three-electrode system with a Pt working electrode, a Pt wire
counter electrode, and a Ag/Agþ reference electrode. The support-
ing electrolyte was 0.1 M TBAP in dichloromethane. The potential of
the reference electrode was calibrated by ferrocene after each set of
measurements. Potentials referred to ferrocene were then con-
verted to the Normal Hydrogen Electrode (NHE) standard by
addition of 0.63 V [42].
A mixture of 8a (30 mg, 30 mmol), 9 (25 mg, 0.30 mmol) and
ammonium acetate (2.3 mg, 0.029 mmol) in acetic acid (2 mL) was
refluxed for 16 h under a nitrogen atmosphere. The mixture was
cooled down to room temperature and the solvent was evaporated
under reduced pressure. The crude product was washed with acetic
acid (20 mL) and diethylether/n-hexane (1/1) (40 mL) in this order.
DPP1 (17 mg, 16
m
mol) was obtained as a dark-green solid. Yield:
52%. 1H NMR (500 MHz, acetone-d6,
d
, ppm): 0.82 (t, J ¼ 7.25 Hz, 6H,
CH3), 1.05e1.22 (m, 20H, (CH2)5CH3), 1.78e1.82 (m, 4H, N-CH2CH2),
4.12e4.18 (br, 4H, N-CH2), 4.46e4.63 (m, 4H, OCH2CH2O), 5.62 (s,
1H, vinylene-H), 7.06e7.78 (m, 18H, Ar-H), 8.98 (d, J ¼ 3.94 Hz, 1H,
thienylene-H), 9.19 (d, J ¼ 3.94 Hz, 1H, thienylene-H). HRMS (ESI)
m/z calcd for C62H60O6N4S4 1084.33902 (Mþ), found 1084.33809
(Mþ).
2.2.6. 3-[50-(2-Carboxy-2-cyanovinyl)-2,20-bithien-5-yl]-6-(50-{4-
[N,N-bis(9,9-dihexyl-9H-fluoren-2-yl)amino]phenyl}-30,40-
ethylenedioxy-2,20-bithien-5-yl)-2,5-di(n-octyl)-2,5-dihydropyrrolo
[3,4-c]pyrrole-1,4-dione (DPP2, Ar ¼ 9,9-dihexyl-9H-fluoren-2-yl,
R ¼ n-octyl, Ar0 ¼ 2,5-thienylene)
2.4. Fabrication of DSSCs and photovoltaic measurements
The TiO2 paste (JGC Catalysts and Chemicals Ltd., PST-18NR) was
deposited on a fluorine-doped-tin-oxide (FTO) substrate by doctor-
DPP2 was prepared according to the same procedure as
described for DPP1 by using N,N-bis(9,9-dihexyl-9H-fluoren-2-yl)-
4-bromoaniline as a starting material. 1H NMR (500 MHz, acetone-
blading and sintered for 50 min at 450 ꢀC. The 9
mm thick TiO2
electrode (0.5 ꢂ 0.5 cm2 in a photoactive area) was immersed into a
0.1 mM dye solution of CH2Cl2/MeOH (10/1, v/v) for adsorption of
the photosensitizer. The DSSCs were fabricated using the dye-
adsorbed TiO2 electrode, Pt-coated glass as a counter electrode,
and a solution of 0.05 M iodine, 0.1 M lithium iodide, and 0.6 M 1,2-
dimethyl-3-propylimidazolium iodide (DMPrII) in acetonitrile as
the electrolyte. The photocurrent (J)-voltage (V) characteristics
were measured using a potentiostat under a simulated solar light
(AM 1.5,100 mW cmꢁ2) supplied by a solar simulator (Asahi Spectra
d6, d, ppm): 0.76e0.94 (m, 18H, CH3), 1.04e1.24 (m, 52H, methy-
lene-H), 1.78e1.86 (m, 4H, N-CH2CH2), 1.94e2.00 (m, 8H, fluorene-
CH2), 4.10e4.20 (br, 4H, N-CH2), 4.45e4.62 (m, 4H, OCH2CH2O),
5.62 (s, 1H, vinylene-H), 7.06e7.80 (m, 22H, Ar-H), 8.98 (d,
J ¼ 3.86 Hz, 1H, thienylene-H), 9.19 (d, J ¼ 3.86 Hz, 1H, thienylene-
H). HRMS (ESI) m/z calcd for C100H117O6N4S4 1597.78505 ([MþH]þ),
found 1597.78198 ([MþH]þ).

I. Imae et al. / Organic Electronics 37 (2016) 465e473
469
HAL-302). The incident photon-to-current conversion efficiency
(IPCE) spectra were measured under monochromatic irradiation
using a tungsten-halogen lamp (Shimadzu AT-100HG) and a
monochromator (Shimadzu SPG-120 S).
2.5. Estimation of amount of dye molecules adsorbed on TiO2
A specific surface area of the sintered TiO2 powder was deter-
mined by nitrogen adsorption experiments at 77 K. When the
temperature of the TiO2 electrode was cooled to 40 ꢀC, the sintered
electrode was immersed into CH2Cl2/MeOH (10/1, v/v) solutions
containing various concentrations of the DPP dyes for adsorption of
the dyes. In all cases, the dye adsorption was performed for 18 h at
room temperature. The dye-adsorbed TiO2 electrode was cautiously
soaked in pure CH2Cl2/MeOH (10/1, v/v) for several seconds to
remove a trace of dying solution from the void of nanoporous TiO2
electrode. Subsequently, dye molecules adsorbed on TiO2 were
desorbed in a mixed solution (CH2Cl2/MeOH/Bu4NOH (37% in
MeOH) 10/0.9/0.1, v/v/v) and the absorption spectra of the solutions
were measured to determine the amount of adsorption of the DPP
dyes on the TiO2 surface.
Fig. 3. Optical absorption (solid line) and fluorescence (dashed line) spectra.
light-absorption wavelength regions, but enhanced the molar ab-
sorption coefficients (εs). On the other hand, the replacement of a
thiophene ring placed between DPP and CA units with a benzene
ring (DPP4) caused the blue-shift of absorption bands. We note also
that the ICT bands of DPP1e4 (650 nm) are located at wavelengths
longer than that of PE4T (500 nm). This demonstrates that the
introduction of an additional electron acceptor of DPP into the D-p-
A framework induced a remarkable red-shift of the ICT band by
200 nm due to a strong accepting ability of the DPP unit. Absorption
band edges of the D-A-p-A dyes (DPP1e4) developed in this study
were extended beyond 800 nm compared with those of 700 nm or
below for most of DPP-dyes reported earlier [12,18e30]. The band-
edge extinction due to the introduction of EDOT unit in p-linkers is
3. Results and discussion
Density functional theory (DFT) calculations were performed
using a Gaussian 09 software at the Becke three-parameter hybrid
functional combined with LeeeYangeParr correlation functional
(B3LYP) with a polarized 6-31G(d) basis set [43]. The theoretically
calculated energy levels and electronic-density distribution of
DPP1 are shown in Fig. 2. The calculation results demonstrated that
the electronic density of the HOMO of DPP1 was delocalized from
the triarylamine (TArA) to DPP unit, whereas that of the LUMO was
delocalized from DPP unit to cyanoacrylic acid (CA), indicating that
an intramolecular charge transfer (ICT) can occur from the TArA
group to the CA unit upon illumination. Electron densities of HO-
MOs and LUMOs of other DPP-dyes were similar to those of DPP1,
indicating that the introduction of fluorene and bulky alkyl chains
did not affect their electronic structures considerably. It was found
that four alkyl groups introduced in fluorene and DPP units of DPP2
responsible, in part, for the panchromatic nature of DPP1e4.of
DPP1e4 and PE4T in dichloromethane.
Cyclic voltammetry (CV) was conducted to evaluate the feasi-
bility of electron injection and the possibility of dye regeneration
for DPP1e4 developed in this study. Fig. 4 displays the CV curves of
DPP1e4 in dichloromethane, measured with an initial anodic scan
of potential from ꢁ0.5 V vs. Fc/Fcþ. The HOMO energy levels of
DPP1e4 evaluated from the half-wave potential (EO1/X2) for the first
redox step were 0.84, 0.81, 0.86, and 0.77 V, slightly less positive
than 0.89 V vs. NHE for PE4T. The LUMO energy levels were
calculated to be ꢁ0.93, e0.96, e0.91, and ꢁ1.00 V from the HOMO
energy and the bandgap energy (E0-0) estimated from the inter-
section of the normalized absorption and fluorescence spectra.
These energy levels of DPP1e4 and PE4T are summarized in Table 1
and they are illustrated also in Fig. S2, along with the positions of
the conduction band edge (ECB) of TiO2 (ꢁ0.5 V vs. NHE) and the
and DPP3, respectively, were perpendicularly located to the
p-
plane of fluorene and DPP units (Fig. S1, see supporting
information).
Optical absorption and fluorescence spectra of DPP1e4 and
PE4T in dichloromethane are shown in Fig. 3 and their spectral data
are summarized in Table 1. DPP1e4 exhibited two major absorption
bands at ca. 400 nm and ca. 650 nm, where the former band is
attributed to the p-p* transition and the latter is assigned to the ICT
from TArA to CA unit. Compared with DPP1, the introduction of
bulky groups on TArA (DPP2) and DPP (DPP3) did not change the
Fig. 2. Frontier molecular orbitals optimized by Gaussian 09 at B3LYP/6-31G(d) level for (a) HOMO and (b) LUMO of DPP1.

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I. Imae et al. / Organic Electronics 37 (2016) 465e473
Table 1
Optical and electrochemical properties of DPP1e4 and PE4T.
Dye
Absorptiona
Fluorescencea
Energy levels/V vs. NHE
10ꢁ4 εmax/Mꢁ1 cmꢁ1
lmax/nm
EHOMO
E0-0
ELUMO
b
c
d
lmax/nm
DPP1
DPP2
DPP3
DPP4
PE4Te
658/408
651/405
656/381
638/383
510/425
3.56/1.88
5.27/2.77
6.04/4.18
7.42/6.23
6.33/3.78
724
727
724
727
590
0.84
0.81
0.86
0.77
0.89
1.75
1.77
1.77
1.77
2.22
ꢁ0.93
ꢁ0.96
ꢁ0.91
ꢁ1.00
ꢁ1.33
a
Absorption and fluorescence spectra were recorded in dichloromethane.
Determined by cyclic voltammetry.
Determined from the intersection of absorption and emission spectra.
b
c
d
e
Calculated from EHOMO ꢁ E0-0
.
Data from Ref. [31].
Fig. 5. Adsorption isotherms of DPP1e3 on nanoporous TiO2 electrodes.
for PST-18NR [44]. The abscissa denotes concentrations of dyes in
equilibrium with dye-adsorbed TiO2 electrodes in CH2Cl2/MeOH
(10/1, v/v). These adsorption isotherms were analyzed on the basis
of the following equation:
Fig. 4. Cyclic voltammograms of DPP1e4 (1 mM) in dichloromethane containing TBAP
(0.1 M) at a scan rate of 50 mV sꢁ1
.
ꢁ1
C
¼ ðK C0Þ ½Dyeꢃꢁ1 þ C0ꢁ1
(1)
ꢁ1
ad
Fermi level energy of the Iꢁ/I3ꢁ redox couple (ca. 0.4 V vs. NHE). It is
seen from Fig. S2 that the HOMO and LUMO levels are similar for
DPP1e3, whereas both energy levels of DPP4 in which a thiophene
ring placed between DPP and CA units in DPP2 is replaced with a
benzene ring are shifted slightly upward relative to those of
DPP1e3. On the other hand, the LUMO levels of DPP1e4 (ꢁ0.91
to ꢁ1.00 V) lie below that of PE4T (ꢁ1.33 V) due to the introduction
of the strong electron-accepting DPP unit into PE4T. The estimated
HOMO levels of all the dyes are located well above the ECB of TiO2
and the LUMO levels are below the Fermi level of the redox couple.
These results indicate that an electron injection from the photo-
excited sensitizer into the conduction band of TiO2 and a regener-
ation of the oxidized dye by iodide in the electrolyte are feasible in
the DSSCs based on DPP1e4 and PE4T.
In order to investigate the steric effects of the alkyl substituents
on TArA and DPP units in the DPP dyes, the amounts of dyes
adsorbed on the nanocrystalline TiO2 surface were measured as a
function of the dye concentration for DPP1e3. Fig. 5 depicts
adsorption isotherms for the three dyes in a double-logarithmic
representation. The amount of adsorbed dyes (Cad) on the ordi-
nate is expressed by the number of dye molecules per unit area of
the TiO2 surface calculated using a specific surface area of 83 m2 gꢁ1
derived from a Langmuir isotherm [45e51].
Cad=C0 ¼ K½Dyeꢃ=ð1 þ K½DyeꢃÞ
(2)
where Cad, C0, K, and [Dye] denote the amount of adsorbed dye,
adsorbed amount at saturation, adsorption equilibrium constant,
and equilibrium conceꢁn1 tration of dye in solution, respectively. Fig. 6
represents plots of Cad against [Dye]ꢁ1 obtained from the data of
Fig. 5. The plots fit straight lines well, demonstrating that adsorp-
tions of these dyes on TiO2 surfaces follow a Langmuir isotherm.
The amounts of dyes adsorbed at saturation, C0s, which can be
estimated from the intercepts of these lines, were found to be
13.2 ꢂ 1013, 8.2 ꢂ 1013 and 6.8 ꢂ 1013 cmꢁ2 for DPP1e3, respectively
(Table 2). We have already found that the density of Brønsted-acid
sites on which of TiO2 surface organic dyes with carboxyl anchor
are adsorbed is around 1.5 ꢂ 1014 cmꢁ2 for PST-18NR [44]. This
value is comparable to the Co value of DPP1, suggesting that DPP1
molecules are almost fully adsorbed on available Brønsted-acid
sites of TiO2 surface. On the other hands, the decreased C0 values for
DPP2 and DPP3 are indicative of the increased bulkiness of alkyl
groups introduced into the two dyes. Although C0 value of DPP4
was not estimated, it will be similar to that of DPP2. These results

I. Imae et al. / Organic Electronics 37 (2016) 465e473
471
Fig. 6. Double-reciprocal plots for the data shown in Fig. 6. Fitting lines were obtained
from the linear-least-squares method.
Table 2
Adsorption parameters of DPP1e3 obtained from Langmuir isotherms.
Dye
C0/cmꢁ2
K/mMꢁ1
DPP1
DPP2
DPP3
13.2 ꢂ 1013
8.2 ꢂ 1013
6.8 ꢂ 1013
16.2
46.2
74.4
suggest that the introduction of bulky substituents in TArA or DPP
units decreases the C0s and will suppress the interaction between
dye molecules on TiO2.
Fig. 7. (a) IPCE spectra and (b) J e V characteristics of DSSCs based on DPP1e4 and
PE4T.
Fig. 7 depicts the IPCE spectra and J e V curves of DSSCs based on
DPP1e4 and PE4T, where any additives such as chenodeoxycholic
acid (CDCA) and tert-butyl pyridine (TBP) used conventionally for
optimizing DSSC performances [52e54] were not employed in or-
der to get an insight into essential natures of the respective dyes
used in the DSSCs as photosensitizers. Detailed photovoltaic pa-
rameters are listed in Table 3, together with those for DSSCs pre-
pared using co-adsorbate on TiO2 (CDCA) and/or additive to the
electrolyte (TBP). Photovoltaic performances for a typical organic
dye, MK-2, are also included in Table 3 for a reference. It is seen
clearly from the IPCE spectra of Fig. 7(a) that the photoactive
spectral regions of the DSSCs based on DPP1e4 are extended far
above 800 nm with extremely broad peaks centered at 600 nm in
comparison with that for PE4T which has no DPP unit in the dye
framework, demonstrating that we are successful in the prepara-
tion of panchromatic dyes for DSSCs. Fig. 7(b) shows that the Jsc
values of the DSSCs prepared with DPP1e4 are greater than that for
PE4T. This can be ascribed to the enhanced spectral response of the
DSSCs based on the panchromatic DPP1e4 dyes. We note also in
Fig. 7(b) that the Jsc values are increased systematically on the order
of DPP1e4. According to the adsorption study of DPP1e3 on TiO2
described above, the maximum adsorbed amounts of these dyes (C0
values) are decreased from DPP1 to DPP3, corresponding to the
order of the sizes of alkyl substituents introduced. In view of this,
the increased Jsc values from DPP1 to DPP3 are likely to be attrib-
uted, at least in part, to the suppression of dye aggregation due to
the increased steric hindrance of the bulky substituents. On the
other hand, DPP4 in which one of thiophene rings of DPP2 is
replaced with a benzene ring gives the largest Jsc value among
DSSCs based on DPP1e4. Since the sizes of the two dye molecules
are similar with each other, the dye aggregation cannot be
responsible for a proper reason for the difference in Jsc between
DPP2 and DPP4. Based on the energy schemes of DPP2 and DPP4
illustrated in Fig. S2, we presume that the upward shift of the LUMO
level of DPP4 relative to that of DPP2, induced by the replacement
of thiophene with benzene, leads to the enhanced probability of an
electron injection from LUMO to the conduction band of TiO2 and
thus results in the Jsc value of DPP4 larger than that of DPP2.
In order to optimize photovoltaic performances of the DSSCs
based on DPP1e4 as well as to get a deeper insight into these
panchromatic dyes, possible influences of the co-adsorption of
CDCA on TiO2 and/or the addition of TBP into the redox electrolyte
were examined. At first, we will discuss influences of CDCA on the
photovoltaic performances. The experiments were made with
different concentrations of CDCA added in the dye solution and
only the results which gave the best performances are listed in
Table 3. We note that the addition of CDCA enhanced the
h values
for all the DSSCs: 1.1e2.1% for DPP1,1.4e2.1% for DPP2,1.8e2.0% for
DPP3, and 2.2e3.3% for DPP4. As is seen from Table 3, the principal
reason for the enhanced
h values is ascribable to the increase of Jsc:
5.4e8.8 mA cmꢁ2 for DPP1, 6.2e9.0 mA cmꢁ2 for DPP2,
7.8e8.9 mA cmꢁ2 for DPP3, and 9.2e13.7 mA cmꢁ2 for DPP4. It is
interesting to note in Table 3 that the Cad values for DPP1e3 are
decreased considerably by the addition of CDCA irrespective of the
obvious increase of Jsc. The observed increase of Jsc accompanied by
the decrease of the adsorbed amount of dye demonstrates that
CDCA acts effectively as co-adsorbate so as to suppress aggregation
of dye molecules on TiO2 responsible for the deactivation of
photoexcited dyes. On the contrary, this implies that relatively
small IPCE values below 50% observed with DPP1e4-based DSSCs
without CDCA shown in Fig. 7(a)) are due, mostly, to the

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I. Imae et al. / Organic Electronics 37 (2016) 465e473
Table 3
Photovoltaic parameters of DSSCs based on DPP1e4, PE4T, and MK-2.
Dye
[CDCA]/[dye]a
[TBP]/Mb
Cad/cmꢁ2
Jsc/mA cmꢁ2
Voc/V
FF
h (%)
DPP1
0
0
0
0.5
0.5
0
0
0
0
0
6.7 ꢂ 1013
5.4
8.8
0.7
0.9
6.2
9.0
7.8
8.9
9.2
13.7
0.8
4.4
3.3
13.7
0.36
0.41
0.48
0.52
0.41
0.43
0.42
0.44
0.45
0.48
0.52
0.55
0.37
0.60
0.54
0.57
0.61
0.66
0.55
0.54
0.54
0.52
0.52
0.50
0.62
0.64
0.52
0.42
1.1
2.1
0.2
0.3
1.4
2.1
1.8
2.0
2.2
3.3
0.8
1.6
0.6
3.4
1000
0
1000
0
200
0
20
0
200
0
200
0
2.9 ꢂ 1013
6.7 ꢂ 1013
2.9 ꢂ 1013
DPP2
DPP3
DPP4
6.1 ꢂ 1013
2.9 ꢂ 1013
5.0 ꢂ 1013
3.3 ꢂ 1013
e
e
e
e
e
e
0
0.5
0.5
0
PE4T
MK-2c
0
0
a
Molar ratio of CDCA/dye in the solution for (co)adsorption on TiO2.
Concentration of TBP in the electrolyte.
Data were obtained from the DSSC prepared by authors themselves.
b
c
aggregation of dye molecules adsorbed on TiO2 and the bulky alkyl
chains in DPP1e4 do not serve necessarily well as units capable of
hindering the dye aggregation. The Voc values for DPP1e4 were
increased slightly when CDCA was added into the electrolyte:
0.36e0.41 V for DPP1, 0.41e0.43 V for DPP2, 0.42e0.44 V for DPP3,
and 0.45e0.48 V for DPP4. However, they are still as small as or
below 500 mV compared with those of DSSCs sensitized by other
organic dyes [7,9,10]. The small Voc values are likely to be accounted
for in terms of the large dark currents, i.e., recombination of elec-
trons injected from excited dyes to the conduction band of TiO2
with the reducible species, Iꢁ3 ions, in the electrolyte. To suppress
such undesirable recombination process, TBP is widely used as an
additive in the electrolyte solution [53]. When the TBP-containing
electrolyte was used for DPP1-DSSC, Voc and FF were improved
from 0.36 to 0.48 V and from 0.54 to 0.61, respectively, but Jsc was
drastically reduced from 5.4 to 0.7 mA cmꢁ2. This is also the case for
the electrolyte including both TBP and CDCA: the Jsc value, for
example, decreased from 8.8 to 0.9 mA cmꢁ2 for DPP1 and from
13.7 to 4.4 mA cmꢁ2 for DPP4, respectively, upon addition of TBP in
the electrolyte containing CDCA. It is known that the addition of
TBP into the electrolyte induces the negative shift of the ECB of TiO2
by adsorption of pyridyl group of TBP onto the TiO2 surface [55]. As
Fig. 5 shows, the LUMO levels of DPP1e4 lie only 0.4e0.5 eV above
the ECB of TiO2, which is close to the threshold energy difference for
the efficient electron injection from the photoexcited dyes to TiO2
in the DSSCs without CDCA and TBP [2,12,56,57]. Therefore, when
the ECB of TiO2 is shifted upwards by the addition of TBP, the
electron injection probability may be greatly reduced because of
the smaller energy difference and thus the Jsc may be decreased as
shown in Table 3.
the EDOT-containing quaterthiophene p-linker. It was found that
the introduction of the DPP unit induced a red-shift of absorption
bands of the photosensitizing dyes. Accordingly, the photoactive
regions of the DSSCs based on DPP1e4 were extended far beyond
800 nm with extremely broad peaks centered at around 600 nm,
although the very strong electron-withdrawing property of the DPP
unit lowered the LUMO levels of these dyes. Substitution of a
thiophene ring with a benzene ring in
p-linker between DPP and
CA units in DPP4 caused an upward shift of the LUMO level, which
enhanced injection probabilities of electrons from dye to TiO2,
resulting in the improvement of Jsc and
h for DPP4-DSSC.
Acknowledgements
The authors are grateful to Drs. Tomoko Amimoto and Daisuke
Kajiya, the Natural Science Center for Basic Research and Devel-
opment (N-BARD), Hiroshima University for the measurement of
mass spectroscopy.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.orgel.2016.07.022.
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