D-π-A organic dyes with various bulky amine-typed donor moieties for dye-sensitized solar cells employing the cobalt electrolyte

Article abstract of DOI:10.1016/j.orgel.2015.06.011

SGT dyes containing various amine-typed donors as triphenylamine, bis-fluorenylamine and bis-phenothiazinylamine as the electron donor and a cyanoacrylic acid moiety as electron acceptor in D-π-A system, were developed to use in dye-sensitized solar cells

Full text of DOI:10.1016/j.orgel.2015.06.011

Organic Electronics 25 (2015) 1–5
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
D–
p
–A organic dyes with various bulky amine-typed donor moieties
for dye-sensitized solar cells employing the cobalt electrolyte
⇑
Kang Deuk Seo, In Tack Choi, Hwan Kyu Kim
Global GET-Future Laboratory, Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Jochiwon, Sejong 339 700, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2015
Received in revised form 1 June 2015
Accepted 4 June 2015
SGT dyes containing various amine-typed donors as triphenylamine, bis-fluorenylamine and
bis-phenothiazinylamine as the electron donor and a cyanoacrylic acid moiety as electron acceptor in
D–p–A system, were developed to use in dye-sensitized solar cells (DSSCs). The SGT-102 dye containing
bis-fluorenylamine had a better prevented charge recombination than other SGT dyes; leading to
improvement in Voc. As a result, the conversion efficiency of 7.22% was achieved with a Jsc of
12.1 mA cmꢀ2, Voc of 865 mV and a FF of 69.1 for the DSSC employing a dye containing the bulky
Keywords:
Triphenylamine
Bis-dimethylfluorenylamine
Bis-phenothiazinylamine
Organic dye
bis-fluorenylamine donor unit, while the DSSC based on
a
dye containing the bulky
bis-phenothiazinylamine donor unit showed a lower Jsc and Voc, leading to a lower efficiency of 5.16%,
due to slow charge recombination associated with differently geometric structure orientations.
Ó 2015 Elsevier B.V. All rights reserved.
Dye-sensitized solar cell
1. Introduction
and bis-phenothiazinylamine, as shown in Fig. 1, and we investi-
gated the structure–property relationship between the donor
To date, the cell efficiencies of DSSCs with a cobalt(II/III) redox
couple are reported 13.0% for zinc porphyrin dyes [1], 12.8% for
metal-free organic dyes [2,3], while ruthenium dyes-based DSSCs
group and conversion efficiency in the DSSCs. The organic sensitiz-
ers based on the D– –A structural motif compose of various
amine-typed units as donor, the bithiophene group as -bridge
and cyanoacrylic acid as acceptor. The oligothiophene derivatives
are intensively exploited as -bridges [11–13]. Further extension
of the -conjugation increases the absorption but unexpected
additional processes, like self-quenching or recombination pro-
cesses, occurred to reduce the photocurrent and photovoltages,
thus leading to the decrease of the overall efficiency of the DSSCs
[14]. Also, the bulky dihexyloxyphenyl group was introduced into
the donor moiety for suppression of dye aggregation and charge
recombination [15].
p
p
with
a cobalt(II/III) redox couple have showed conversion
efficiency of about 12% [4]. Recently, Murakami et al. has found
that metal-free organic sensitizers containing triphenylamine
donors performed very well with cobalt-based redox shuttles
because the two phenyl groups at the tip of triphenylamine
donor group play a significant role in blocking the recombination
p
p
reaction [5]. Also, the sensitizers containing
a
bulky
bis-dimethylfluorenylamine donor are able to achieve a relatively
high molar extinction coefficient as well as efficient charge injec-
tion and electron lifetime [6]. Recently, phenothiazine derivatives
have also been used as a promising donor in the organic dyes used
as sensitizers in DSSCs [7–9]. Phenothiazine is a well-known
heterocyclic compound with electron-rich sulfur and nitrogen het-
eroatoms, and the phenothiazine ring is nonplanar with a butterfly
conformation, which can impede the molecular aggregation and
the formation of intermolecular excimers [10]. However, the
relationship between the bis-dihexylphenothiazine amino groups
and DSSC performance has not been studied.
2. Experimental
2.1. Materials and synthesis
All reactions were carried out under a nitrogen atmosphere.
Solvents were distilled from appropriate reagents. All reagents
were purchased from Aldrich. All reactions were carried out
under
a nitrogen atmosphere. Solvents were distilled from
Here, we prepared SGT organic dyes containing various
amine-typed donors, such as triphenylamine, bis-fluorenylamine
appropriate reagents. All reagents were purchased from Aldrich.
7-(2,4-bis(hexyloxy)phenyl)-N-(7-(2,4-bis(hexyloxy)phenyl)-9,9-
dimethyl-9H-fluoren-2-yl)-N-(4-bromophenyl)-9,9-dimethyl-9H-
fluoren-2-amine [6], (50-(1,3-dioxolan-2-yl)-2,20-bithiophen-5yl)-
tributylstannane [16] were synthesized following procedures as
⇑
Corresponding author.
E-mail address: hkk777@korea.ac.kr (H.K. Kim).
http://dx.doi.org/10.1016/j.orgel.2015.06.011
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

2
K.D. Seo et al. / Organic Electronics 25 (2015) 1–5
O
O
O
O
S
S
N
N
COOH
COOH
S
S
NC
NC
O
O
O
O
SGT-101
SGT-102
O
O
N
N
S
S
S
N
COOH
S
NC
O
O
SGT-103
Fig. 1. Chemical structures of SGT dyes.
described previously. The detailed synthetic procedure and charac-
terization are shown in the ESI.
the 6-31G(d,p) basis set in GAUSSIAN03 program [19]. We focused
on transitions occurring in the range of 350–800 nm, specifically
those whose oscillation strengths were greater than 0.1. In order
to treat the low wavelength excitations correctly around 350 nm,
we made extensive calculations up to 100 singlet ? singlet
transitions.
2.2. Measurements
1H NMR and 13C NMR spectra were recorded at room tempera-
ture with Varian Oxford 300 spectrometers and chemical shifts
were reported in ppm units with tetramethylsilane as the internal
2.4. Fabrication and testing of DSSC
standard. FTIR spectra were taken on
a JASCO, 4200+ ATR
Pro-450-S spectrophotometer. UV–visible absorption spectra were
obtained in THF on a Shimadzu UV-2401PC spectrophotometer.
FTO glass plates (Pilkington) were cleaned in a detergent solu-
tion using an ultrasonic bath for 1 h, then rinsed with water and
ethanol. The FTO glass plates were immersed in an aqueous solu-
tion of 40 mM TiCl4 at 70 °C for 30 min and then washed with
Cyclic voltammetry was carried out with
a Versa STAT3
(AMETEK). A three-electrode system was used and consisted of a
reference electrode (Ag/AgCl), a working electrode, and a platinum
wire electrode. The redox potential of dyes on TiO2 was measured
water and ethanol. The first TiO2 layer with a thickness of 5.3 lm
was prepared by screen-printing TiO2 paste (Solaronix, 13 nm
anatase), and the second scattering layer containing 400 nm sized
anatase particles was deposited by screen printing. The TiO2
electrodes were immersed into the dye solution (0.3 mM) in
THF/EtOH (2:1) with and kept at room temperature overnight.
Counter-electrodes were prepared by coating with a drop of
H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) on an FTO plate.
The dye-adsorbed TiO2 electrode and Pt counter electrode
in CH3CN with 0.1 M TBAPF6 with a scan rate between 50 mV sꢀ1
.
2.3. Density functional theory (DFT)/time-dependent DFT (TDDFT)
calculations
Structural optimization of SGT dyes was done with a PBE
exchange–correlation function using the Vienna ab initio simula-
tion package (VASP) [17,18]. We used 400 eV as the cut-off energy,
and the conjugate gradient method was employed to optimize the
geometry until the force exerted on an atom was less than
0.03 eV/Å. In order to calculate the absorption spectra, we per-
formed the TD-DFT calculations for more stable one between the
two configurations. Calculations were done in the gas phase using
were assembled in
a sealed sandwich-type cell. A drop of
electrolyte solution (0.22 M [Co(II)-(bpy)3](B(CN)4)2, 0.05 M
[Co(III)(bpy)3](B(CN)4)3, 0.1 M LiClO4, and 0.8 M TBP in ACN) was
placed on the drilled hole in the counter electrode of the assembled
cell and was driven into the cell via vacuum backfilling. Finally, the
hole was sealed using additional Surlyn and a cover glass.

K.D. Seo et al. / Organic Electronics 25 (2015) 1–5
3
7
2.5. Photoelectrochemical measurements of DSSC
SGT-101 UV
SGT-102 UV
SGT-103 UV
SGT-101 PL
SGT-102 PL
SGT-103 PL
6
Photoelectrochemical data were measured using a 1000 W
xenon light source (Oriel, 91193) that was focused to give
1000 W/m2, the equivalent of one sun at AM 1.5G, at the surface
of the test cell. The light intensity was adjusted with a Si solar cell
that was double-checked with an NREL-calibrated Si solar cell (PV
Measurement Inc.). The applied potential and measured cell cur-
rent were measured using a Keithley model 2400 digital source
meter. The current–voltage characteristics of the cell under these
conditions were determined by biasing the cell externally and
measuring the generated photocurrent. This process was fully
automated using Wavemetrics software.
5
4
3
2
1
0
3. Results and discussion
300
400
500
600
700
800
Wavelength/ nm
The absorption and emission spectra of SGT dyes are summa-
rized in Table 1. In the UV–vis spectra (Fig. 2), the three SGT dyes
exhibited two main prominent bands, appearing at 300–400 and
400–550 nm, respectively. The intense peak positioned at ca.
Fig. 2. The UV–visible absorption (solid lines) and emission spectra (dashed lines)
of SGT-101 (red), SGT-102 (blue) and SGT-103 (green) in 2 ꢁ 10ꢀ5 M in THF. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
300–400 nm is attributed to the localized
p–
p⁄ transition, and
low energy absorption occurring at above 450 nm is attributed to
the intramolecular charge transfer (ICT) transition between the
donor and the cyanoacrylic acid acceptor. The SGT-101 and
SGT-102 dyes showed a maximum absorption at 479 nm. Also,
the SGT dyes and the geometrically structural orientations with
various donor groups in the SGT dyes are shown in Fig. 3 (see
GA). The electrons in the ground state of all the dye was homoge-
neously distributed in the donor, and upon light illumination, elec-
trons were excited to an anchoring group through the change from
the HOMO to the LUMO by ICT. It indicates that photoinduced elec-
trons are able to transfer from the dye to the TiO2 electrode
efficiently.
Fig. 4a shows the IPCE spectrum of SGT dyes-based cells. The
onset wavelengths of the IPCE spectra for the DSSC based on
SGT-102 appeared at 690 nm. IPCE values of higher than 60% were
observed in the range of 400–550 nm with a maximum value of
70% at 500 nm for the DSSC based on SGT-102. The maximum
IPCE value of the DSSC based on SGT-101 was slightly lower than
the value of the DSSC based on SGT-102 due to lower molar extinc-
tion coefficient. On the other hand, the SGT-103-sensitized DSSC
remarkably reduced the IPCE value compared to two other dyes.
The lower IPCE value of the DSSC based on SGT-103 is probably
due to its less negative HOMO level than that of two other
dyes, which might lead to the relative decrease of the
electron-regeneration yield [23]. The photovoltaic performance of
the SGT dye-based DSSCs are summarized in Table 1 (Fig. 4b).
Under the standard global AM 1.5 solar condition,
SGT-101-sensitized cell gave a short circuit photocurrent density
(Jsc) of 11.9 mA cmꢀ2, an open circuit voltage (Voc) of 862 mV,
and a fill factor (FF) of 69.4, corresponding to an overall conversion
the molar absorption coefficient (e) of the SGT-102 dye was higher
than SGT-101 dye due to -extended conjugation. Unlike expecta-
p
tion, the SGT-103 dye containing the bulky phenothiazine donor
group showed a maximum absorption of 484 nm, which was
red-shifted by 5 nm, compared to the other dyes, since it has been
recently reported that the blue-shifts in solution with the number
of the donor groups in the starburst 2D–p–A organic dye would be
expected due to the higher ICT energy levels resulting from the
increase of donor moieties [20,21]. However, increasing the num-
ber of the donor moiety improves the molar extinction coefficients
of the ICT absorption peaks of the dyes. The molar absorption coef-
ficient of the SGT-103 dye was higher (e )
max = 36,800 Mꢀ1 cmꢀ1
than the other dyes, indicating that the SGT-103 dye has a good
light-harvesting ability.
The electrochemical properties were investigated by cyclic
voltammetry (CV) to obtain the HOMO and LUMO levels of the pre-
sent dyes (Table 1). HOMO values (1.16, 1.10, 1.22 V vs. NHE) were
more positive than the cobalt redox couple (0.5 V vs. NHE).
Electron injection from the excited sensitizers to the conduction
band of TiO2 should be energetically favorable because of the more
negative LUMO values (ꢀ1.15, ꢀ1.19, ꢀ1.06 vs. NHE) compared to
the conduction band edge energy level of the TiO2 electrode [22].
Structural optimization of SGT dyes was done with
a PBE
efficiency (g) of 7.11%. For the SGT-102-sensitized DSSC, compared
exchange–correlation function using the Vienna ab initio simula-
to the SGT-101-sensitized DSSC, Jsc increased from 11.9 to
tion package (VASP). The shapes of HOMO and LUMO levels of
Table 1
Photophysical, electrochemical and photovoltaic performance data of SGT dyes.
Dye
Absorptiona
kmax (nm),
Emission
(kmax/nm)
Potentials and energy level
C
/10ꢀ7
Photovoltaic performancee
e
(Mꢀ1 cmꢀ1
)
(mol cmꢀ2
)
Eoxb/V
E0–0c/V
ELUMOd/V
(vs. NHE)
Jsc
Voc (V)
FF
g (%)
(vs. NHE)
(mA cmꢀ2
)
SGT-101
SGT-102
SGT-103
337 (34,100), 479 (29,900)
380 (63,100), 479 (30,700)
351 (50,400), 484 (36,800)
588
597
593
1.16
1.10
1.22
2.31
2.29
2.28
ꢀ1.15
ꢀ1.19
ꢀ1.06
0.51
0.38
0.34
11.9
12.1
8.95
862
865
793
69.4
69.1
72.7
7.11
7.22
5.16
a
b
c
Absorption and emission spectra were measured in THF.
Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M TBAPF6 with a scan rate of 50 mV sꢀ1 (vs. NHE).
E0–0 was determined from the intersection of absorption and emission spectra in THF.
d
e
LUMO was calculated by Eox ꢀ E0–0
.
TiO2 thickness 8.3
l
m, (5.4 + 3), TiCl4 cell active area 0.16 cm2; electrolyte conditions 0.22 M [Co(bpy)3]2+, 0.05 M [Co(bpy)3]3+, 0.8 M tert-butylpyridine and 0.1 M LiClO4
in an acetonitrile solution; dye was dissolved in THF 1/EtOH 2.

4
K.D. Seo et al. / Organic Electronics 25 (2015) 1–5
Fig. 3. Isodensity plots of the HOMO and LUMO levels and the geometrically structural orientations of SGT dyes.
the differently geometric structure orientations, we measured the
80
70
60
50
40
30
20
10
0
(a)
electrochemical impedance spectroscopy (EIS) under dark condi-
tions at a forward bias of ꢂ0.81 V (see Fig. S1). The fitted Rct values
at the TiO2/dye/electrolyte interface corresponding to the second
semicircle of the Nyquist plot were in the order of SGT-102
SGT-101
SGT-102
SGT-103
(150.6
X) > SGT-101 (140.2 X) > SGT-103 (65.92 X), indicating
that the recombination rate is decreased in the order of
SGT-103 < SGT-101 < SGT-102 in the dark. SGT-101 and SGT-102
had almost the same recombination resistance. From the EIS
results, it is clear that the charge recombination rate is the
rate-determining step of the DSSC in terms of device performance,
since SGT-102 DSSCs decreased the charge recombination rate and
resulted in an improvement of the device performance. This insu-
lating molecular layer, associated with differently geometric struc-
ture orientations, effectively inhibits back electron transfer from
the TiO2 to cobalt electrolyte and gives a higher Voc
.
300
400
500
600
700
Wavelength (nm)
14
12
10
(b)
4. Conclusions
SGT-101
SGT-102
SGT-103
We have successfully synthesized organic sensitizers containing
various amine-typed donors with interesting optical and photo-
voltaic performance. The SGT-102 had a better prevented charge
recombination than other SGT dyes; leading to improvement in
Voc. As a result, the solar cell based on the SGT-102 dye showed
better photovoltaic performance with a Jsc of 12.1 mA cmꢀ2, a Voc
of 865 mV, and an FF of 69.1, corresponding to an overall conver-
8
6
4
2
0
sion efficiency
g of 7.22% under standard AM 1.5 irradiation.
Acknowledgments
This work was supported by the National Research Foundation
of
Korea
(NRF)
Grant
(2014R1A2A1A10051630
and
0.0
0.2
0.4
0.6
0.8
1.0
2014R1A6A1030732) and the Human Resources Development
Program (No. 20124010203190) of the Korean Institute of Energy
Technology Evaluation and Planning (KETEP) Grant funded by
funded by the Korea Governments.
Voltage (V)
Fig. 4. Photon-to-current conversion efficiencies (IPCE) (a) and photocurrent–
voltage characteristics (b) of representative TiO2 electrodes sensitized with SGT
dyes.
12.2 mA cmꢀ2 and 7.11–7.22% of
g
due to high light-harvesting
Appendix A. Supplementary data
ability, while the DSSC based on SGT-103 dye showed a lower Jsc
and Voc, leading to a lower efficiency of 5.16%.
To further characterize the electron transport and recombina-
tion effect on photovoltaic performance for the SGT dyes with
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.orgel.2015.06.
011.

K.D. Seo et al. / Organic Electronics 25 (2015) 1–5
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