Synthesis and investigation of triphenylamine-based double branched organic dyes for p-type dye-sensitized solar cells

Article abstract of DOI:10.1080/15421406.2017.1350040

Two double branched organic dyes (T1I and T2I) were synthesized, characterized, and employed as photosensitizers in p-type dye-sensitized solar cells (p-DSSC); we have compared with corresponding single branched dye (TC1) and reference dye P1. The absorbance of the dyes in solution was red shifted and the oxidation potential gradually decreased with increasing the donating ability. It was found that modification from single branched (TC1) to double branched (T1I and T2I) system resulted in increased photocurrent density leading to the higher photovoltaic performances. The charge transporting ability is increased with the introduction of double branching, as evidenced from electrochemical impedance analysis.

Full text of DOI:10.1080/15421406.2017.1350040

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and investigation of triphenylamine-
based double branched organic dyes for p-type
dye-sensitized solar cells
Le Quoc Bao, Ho Phuong, Chau Thi Thanh Thuy, Ji Won Lee, Jae Hong Kim &
Suresh Thogiti
To cite this article: Le Quoc Bao, Ho Phuong, Chau Thi Thanh Thuy, Ji Won Lee, Jae Hong Kim
& Suresh Thogiti (2017) Synthesis and investigation of triphenylamine-based double branched
organic dyes for p-type dye-sensitized solar cells, Molecular Crystals and Liquid Crystals, 653:1,
109-117, DOI: 10.1080/15421406.2017.1350040
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Date: 09 November 2017, At: 16:57

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis and investigation of triphenylamine-based double
branched organic dyes for p-type dye-sensitized solar cells
Le Quoc Bao, Ho Phuong, Chau Thi Thanh Thuy, Ji Won Lee, Jae Hong Kim,
and Suresh Thogiti
Department of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea
ABSTRACT
KEYWORDS
p-type dye-sensitized solar
cells; double branched;
hemicyanine; charge transfer
Two double branched organic dyes (T1I and T2I) were synthesized, char-
acterized, and employed as photosensitizers in p-type dye-sensitized
solar cells (p-DSSC); we have compared with corresponding single
branched dye (TC1) and reference dye P1. The absorbance of the dyes
in solution was red shifted and the oxidation potential gradually
decreased with increasing the donating ability. It was found that
modification from single branched (TC1) to double branched (T1I and
T2I) system resulted in increased photocurrent density leading to the
higher photovoltaic performances. The charge transporting ability is
increased with the introduction of double branching, as evidenced
from electrochemical impedance analysis.
1. Introduction
Dye-sensitized solar cells (DSSCs), developed by Gratzel and co-workers in 1990s, are a
fascinating renewable energy source and considered as a developmental low-cost alternative
to the conventional silicon-based solar cells [1–4]. Two kinds of photo-electrode DSSCs
devices exist; Devices using n-type semiconductors (such as TiO2 or ZnO as a photo-anode
i.e. n-DSSC) [5], and their inverse model, p-DSSC using a p-type semiconductor (usually
NiO as photocathode) [6]. Numerous n-DSSCs have been studied and achieved higher con-
version efficiencies [4,7–11], whereas p-DSSCs have rarely been studied and their conversion
efficiencies are much lower than that of n-DSSCs [12–16]. Despite this, the conversion effi-
ciencies of n-DSSCs still lag behind that of conventional silicon-based solar cells, which will
inhibit their large-scale application. Therefore, investigating the p-DSSCs for the fabrication
of inexpensive and efficient tandem-DSSCs is a feasible strategy for further increasing the
power conversion efficiencies [6,13].
In p-DSSCs, dye loaded on the photocathode absorbs photons of a specific wavelength and
creates electrons and holes, followed by hole-transport from the acceptor to the donor, then
injecting by an anchoring (COOH) group into the valence band of the working semiconduc-
tor. Because the dye molecule is a very crucial component in DSSCs working mechanism,
it is important to develop new dye sensitizers and elaborate on the molecular engineering
rules to design more efficient dye sensitizers. It was firstly reported by Qin et al. the sensitizer
CONTACT Suresh Thogiti
sureshyu@ynu.ac.kr
Dae-hakro , Gyeongsan, Gyeongbuk, Republic of Korea.
Department of Chemical Engineering, Yeungnam University, -,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

110
L. Q. BAO ET AL.
Figure . Chemical structures of synthesized dyes TI, TI and reference dye TC.
P1 synthesized for p-type DSSCs using triphenylamine as donor and thiophene as π-linker
which can impressively lift the quantum efficiency as high as 18% and 0.052% of efficiency
[17]. Warnan et al. reported the use of squaraine (SQ) on NiO based DSSC, which showed
considerable results of 0.034% of efficiency [18]. With the aim of developing new dyes for
p-DSSCs, this study examined the potential of well-known hemicyanine. Hemicyanine
exhibits a strong absorption band in the visible to the near IR region and present particularly
high photostabilty [19–22]. Recently, we have reported triphenylamine dyes with different
number of hemicyanine anchoring groups on NiO-based p-DSSCs, which showed satisfac-
tory results of 0.01 to 0.02 of conversion efficiencies [22]. Also, we have studied the effect
of donor in hemicyanine-based sensitizers for p-DSSC and achieved a considerable 0.02%
conversion efficiency with carabazole-based donor [19].
In this study, we designed and synthesized new dibranched hemicyanince dyes (T1I and
T2I; Figure 1) with a substituted triphenylamine as donor and carboxylic acid as anchoring
group and compared with single branched congener (Figure 1) and their optical and electro-
chemical were studied. In addition, these dyes were applied to NiO-based DSSCs and com-
pared with the reference coumarine dye C343 under similar fabrication and measurement
conditions. The optical, electrochemical, current-voltage characteristics, and impedance anal-
ysis were studied to discuss their structure property relationships.
2. Experimental
2.1. Instruments and measurements
1
All H NMR spectra were recorded on a Varian Mercury NMR 300 MHz spectrometer
using CDCl3 and DMSO-d6, which were both purchased from Alfa Aesar Company. The
chemical shifts were referenced to TMS. The absorption spectra were recorded on an
Agilent 8453 UV-Vis spectrophotometer. The redox properties of three dyes were exam-
ined by using cyclic voltammetry (Model: CV-BAS-Epsilon). The electrolyte solution
used was 0.10 M tetrabutylammonium hexafluorophosphate (TBABF6) in freshly dried
acetonitrile. The Ag/AgCl and Pt wire (0.5 mm in diameter) electrodes were used as ref-
erence and counter electrodes, respectively. The scan rate was 30 mV/s. Electrochemical
impedance spectroscopy (EIS) was performed using an electronic-chemical analyzer (Ivium-
stat Tec.). The photovoltaic I-V characteristics of the prepared DSSCs were measured under
1 sunlight intensity (100 mW cm−2, AM1.5), which was verified with an AIST-calibrated
Si-solar cell (PEC-L11, Peccell Technologies, Inc., City, State, Country). The incident
monochromatic photon-to-current efficiencies (IPCEs) were plotted as a function of the
light wavelength using an IPCE measurement instrument (PEC-S20, Peccell Technologies,
Inc.).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
111
2.2. Synthesis of sensitizers
The starting compounds, i.e., 1-bromohexane, 1-iodooctane, pyridine, methanol are prod-
ucts of Sigma Aldrich, the others ones such as carbazole, N,N- dimethylformamide
(DMF), 4-hydrazinobenzoic acid, 3-methyl-2-butanone, 2-(tri-n-butylstannyl)thiophene,
trans-dichlorobis(triphenylphosphine)palladium(II) (PdCl2(PPh3)2) are purchased from Alfa
Aesar. Phosphorus oxychloride (POCl3) is a product of Dea Jung and all the others solvents
and components are bought from Duksan the local company.
N4,N4,N4ꢀ,N4ꢀ-tetraphenyl-[1,1ꢀ-biphenyl]-4,4ꢀ-diamine (1): N,N’-Diphenylbenzidine
(2 g, 5.9 mmol), bromobenzene (3.74 g, 23.8 mmol), potassium tert-butoxide (2 g, 17.8 mmol)
and Palladium (II) acetate (0.133 g, 0.59 mmol) were added to 50 ml of o-xylene and
stirred under nitrogen atmosphere. After 0.5 ml of tri-tert-butylphosphine solution (1.0 M
in toluene) was added slowly, the reaction was kept at 110°C in 24 hours. After regulated
time, the mixture was cooled to room temperature followed by filtering. The filtrated solution
was evaporated and purified via silica gel column using hexane:MC (1:1) as elute solvent. The
obtained product is 2.4 (4.9 mmol) g of white solid (82.62%). 1H NMR (Acetone-d6, 300 MHz)
δ 7.596 (d, J = 8.4 Hz, 4 H), 7.343 (t, J = 8.4 Hz, 8 H), 7.109–7.033 (m, 16 H).
N4,N4ꢀ-bis(4-(tert-butyl)phenyl)-N4,N4ꢀ-diphenyl-[1,1ꢀ-biphenyl]-4,4ꢀ-diamine (2):
N,N’-Diphenylbenzidine (2 g, 5.9 mmol), 1-bromo-4-tert-butylbenzene (5.07 g, 23.8 mmol),
potassium tert-butoxide (2 g, 17.8 mmol) and Palladium (II) acetate (0.133 g, 0.59 mmol)
were added to 50 ml of o-xylene and stirred under nitrogen atmosphere. After 0.5 ml of
tri-tert-butylphosphine solution (1.0 M in toluene) was added slowly, the reaction was
conducted by the same process of reaction of (1). The final product was 2.54 (4.22 mmol) g
of white solid (71.11%). 1H NMR (Acetone-d6, 300 MHz) δ 7.576 (d, J = 8.7 Hz, 4 H), 7.393
(d, J = 8.4 Hz, 4 H), 7.327 (t, J = 8.4 Hz, 4 H), 7.086–7.032 (m, 14 H), 1.325 (s, 18 H).
4,4ꢀ-([1,1ꢀ-biphenyl]-4,4ꢀ-diylbis(phenylazanediyl))dibenzaldehyde (3): 1 g (2mmol) of
(1) was dissolved in DMF and stirred with 0°C of ambient temperature under nitrogen condi-
tion. POCl3 (3.067, 20 mmol) was add slowly to the solution. After 1h stirred, the mixture was
heated up to 80°C and kept for 8 h before it was quenched by being cooled and slow addition
of 2M NaOH. The mixture was extracted with DCM, then the received organic layer was dried
with anhydrous CaCl2 and filtered. The residue received after all solvent was evaporated was
purified with silica gel column with 90% Hexane:EA used as elute solvent. The obtain product
was 0.46 g (0.845 mmol) of yellow solid (41.26%). 1H NMR (Acetone-d6, 300 MHz) δ 9.858
(s, 2 H), 7.785 (t, J = 8.7 Hz, 8 H), 7.471 (t, J = 7.5 Hz, 4 H), 7.301 (t, J = 7.5 Hz, 10 H), 7.078
(d, J = 8.4 Hz, 4 H).
4,4ꢀ-([1,1ꢀ-biphenyl]-4,4ꢀ-diylbis((4-(tert-butyl)phenyl)azanediyl))dibenzaldehyde
(4): 1.2 g (2 mmol) of (3) was dissolved in DMF and stirred with 0°C of ambient temperature
under nitrogen condition. POCl3 (3.067, 20 mmol) was add slowly to the solution. The
reaction was conducted by the same process of reaction of (4). The obtain product was 0.42 g
1
(0.698 mmol) of yellow solid (31.96%). H NMR (Acetone-d6, 300 MHz) δ 9.840 (s, 2 H),
7.765 (t, J = 8.7 Hz, 8 H), 7.507 (d, J = 8.4 Hz, 4 H), 7.292 (d, J = 8.4 Hz, 4 H), 7.209 (d, J =
8.7 Hz, 4 H), 7.042 (d, J = 8.4 Hz, 4 H), 1.349 (s, 18 H).
2,2ꢀ-((1E,1ꢀE)-(([1,1ꢀ-biphenyl]-4,4ꢀ-diylbis(phenylazanediyl))bis(4,1-
phenylene))bis(ethene-2,1-diyl))bis(5-carboxy-3,3-dimethyl-1-octyl-3H-indol-1-ium)
iodide (T1I): 0.1 g (0.18 mmol) of (4), and 5-Carboxy-2,3,3-trimethyl-1-octyl-3H-indolium
iodide (0.19 g, 0.43 mmol, 2.4 equiv) were dissolved in CHCl3:MeOH (1:1) and refluxed for
6 hours. After regulated time, the mixture was cooled down to room temperature then the
solvent was removed under vacuum. The residue was dissolved in CH2Cl2 and recrystallized

112
L. Q. BAO ET AL.
by adding Et2O. The precipitate was washed several times with Et2OH, hexane, ethyl acetate
and finally with the mixture of 50% acetone/ether to afford the final product (0.21 g) as
dark purple solid (83.61%). 1H NMR (DMSO, 300 MHz) δ 8.49 (d, J = 17.1Hz, 2 H), 8.343
(s, 2 H), 8.129 (d, J = 7.8 Hz, 4 H), 7.894 (s, 2 H), 7.779 (d, J = 7.8 Hz, 4 H), 7.481 (m, 4 H),
7.320 (m, 10 H), 7.076 (s, 2H), 6.963 (d, J = 7.5 Hz, 4 H). 4.577(t, J = 6.8 Hz, 4 H), 1.796
(s, 14 H), 1.380–1.190 (m, 24 H), 0.781 (s, 6 H).
2,2ꢀ-((1E,1ꢀE)-(([1,1ꢀ-biphenyl]-4,4ꢀ-diylbis((4-(tert-butyl)phenyl)azanediyl))bis(4,1-
phenylene))bis(ethene-2,1-diyl))bis(5-carboxy-3,3-dimethyl-1-octyl-3H-indol-1-ium)
iodide (T2I Reaction was conducted according to condition of T1I while compound (5)
(0.1 g, 0.152 mmol) was used instead of (4). The obtain product was 0.177 g (0.128 mmol)
1
of dark purple solid (77.09%). H NMR (DMSO, 300 MHz) δ 8.491 (d, J = 13.2 Hz, 2 H),
8.359 (s, 2 H), 8.110 (m, 4 H), 7.901 (d, J = 7.8 Hz, 2 H), 7.777–7.705 (m, 4 H), 7.513 (d, J =
7.8 Hz, 4 H), 7.423–7.297 (m, 6 H), 7.223 (d, J = 6.9 Hz, 4 H), 7.036–6.908 (m, 6 H), 4.561
(t, J = 6.9 Hz, 4 H), 1.796 (s, 12 H) 1.305–1.217 (m, 38 H), 0.784 (s, 6 H)
2.3. Assembly and characterization of DSSCs
The conducting glass substrates (FTO) were cleaned sequentially with soapy water, ethanol
and acetone using an ultrasonic bath for 20 minutes. The compact NiO blocking layer was
deposited on cleaned FTO glass by spin-coating of 0.5 M Nickel acetate tetrahydrate in ethanol
at 3000 rpm for 30 seconds. This process was conducted 3 times. These layers were sin-
tered at 450°C for 30 minutes. The NiO paste was prepared by blending a slurry of 6 g of
NiO nanopowder in ethanol with 20 mL of 10 wt. % ethanolic ethyl cellulose solution and
40 mL α-terpineol. Then ethanol was removed via evaporation. The mesoporous NiO layer
was deposited using the doctor-blade technique, on the top of bare FTO and compact NiO
blocking layer already coated on FTO. Sequentially, the films were dried at 70°C for 45 min-
utes in air followed by sintering at 450°C for 30 minutes. Then resulting films were immersed
in a 0.3 mM solution of dyes in DMF/EtOH (1:1) for 48 hours at 25°C for dye absorption. The
active cell area was 0.24 cm2 (0.4 cm × 0.6 cm).
H2PtCl6 in an isopropanol solution was deposited on clean FTO glass and followed by
thermal decomposition by annealing at 450°C for 30 minutes to make the counter electrodes.
The complete device was assembled by placing face-to-face a platinum-coated FTO substrate
Figure . Absorption spectra of TI, and TI in a mixture of dichloromethane and methanol solvent and com-
parison with reference TC.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
113
Table . Optical and electrochemical parameter of the organic new sensitizers.
Dye
ε × −(M−cm−
)
λmax a(nm)
E– (-eV)b(abs)
EOXc(V vs NHE)
EOX-E–d(V vs NHE)
TC
TI
TI
.
.
.



—
.
.
—
.
.
—
− .
− .
and a dyed NiO working electrode using a 60 μm thick Surlyn (Dupont 1702). The device was
heated at 120°C for 1 minute to seal them. The I−/I3− based electrolyte M, contaning I2 and
0.5 M LiI in propylene carbonate, was introduced through a pre-drilled hole on the counter
electrode.
3. Results and discussion
Figure 2 show the UV-vis absorption spectra of synthesized dyes (T1I and T2I) in a mixture
of chloroform: methanol solution and Table 1 lists the corresponding results. As shown in
Figure 2, two major absorption bands in the range of 300–700 nm were observed for the
synthesized dyes. The first band (∼350 nm) comes from the spacer and the donor group,
whereas the second band (∼575 nm) is attributed to the π–π∗ electron transition of acceptor
[19]. Whereas, the single branched dye TC1 has a maximum absorption wavelength of
572 nm. When double branching introduced in T1I, the absorption maximum red-shifted
to 578 nm, which shifted further to 585 nm when an additional tert-butyl donor group was
added in T2I. The absorption differences can be attributed to increased conjugation through
double branching and the donating ability of the donor moiety in the organic dye. This is
due to the large oscillator strength of the charge transfer transition, according to Franck-
Condon principle [23]. On the other hand, the molar extinction coefficients (ε) of T1I and
T2I dyes were higher than those of single branched TC1 dye, indicating that more photons
harvested.
The ground-state oxidation and reduction potentials of the dyes were measured by cyclic
voltammetry (CV) in dry DMF solvent containing 0.1 M TBA(BF4) with 30 mV/s scan rate
(Figure 3). The ground-state oxidation potentials EOX of T1I and T2I dyes, corresponding
to the highest occupied molecular orbital (HOMO) level of dyes, are 0.67 and 0.69 V vs.
Figure . Cyclic voltammetry spectra of TI and TI in DMF solvent.

114
L. Q. BAO ET AL.
Figure . J –V characteristic spectra of p-type DSSCs used TI and TI sensitizers and compared with refer-
ence TC sensitizer.
NHE; the dyes are easier to oxidize with increasing the donating ability through tert-butyl
introduction. These values are all much more positive than the valence band of NiO (∼0.5 V
vs. NHE),[24] so there should be a substantial driving force for hole injection from the
dye to the semiconductor. The E0–0 gap for these dyes estimated from the onset absorption
spectra. The data was then used to obtain the LUMO (LUMO = HOMO− E0–0) energy levels
(Table 1). The LUMO levels (Table 1) of the dyes are higher than that of the iodide/triiodide
redox system, suggesting that the reduced dyes can efficiently reduce the oxidized electrolyte.
The TC1, T1I, and T2I sensitized p-type DSSCs were fabricated with iodide/triiodide as
a redox couple. The typical current density-voltage (J –V) curves of the devices are shown
in Figure 4 and the photovoltaic parameters are summarized in Table 2. The fill factor (FF)
and open circuit voltages (VOC) values of T1I and T2I-based cells were quite similar to each
other; whereas the short-circuit current densities (JSC) were different. Changing from single
branching to double branching in these sensitizers is reflected in the increased short circuit
current density (JSC) by approximately 28% compared to the single branched dye TC1. The
absorption data (Fig. 2), also, indicated the increased light-harvesting properties of these dyes.
This improvement in current density is due likely to the increased charge transfer, which will
be discussed in the following section. The devices based double branching dyes exhibited the
higher power conversion efficiency of 0.05% with a high JSC of 2.17 mA cm−2 and VOC of
75.5 mV, which is 25% higher than the single branched dye TC1 (0.04%).
Electrochemical impedance analysis (EIS) was performed under light conditions to study
the effects of charge transport processes in these devices. Figure 5 shows the Nyquist plots of
DSSCs and Table 3 lists the results. The EIS is a useful technique for the analysis of charge
transport processes in an electrochemical device including p-type DSSCs [25]. Upon illumi-
nation of 1000 W m−2 light under open circuit conditions, the radius of the intermediate
Table . Photovoltaic performance of DSSCs composed of organic p-type sensitizers.
Dye
Jsc (mA/cm)
Voc (mV)
FF (%)
η (%)
Ref P
TC
TI
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
TI

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
115
Figure . Electrochemical impedance analysis of DSSCs used TI and TI sensitizers and reference TC
sensitizer.
Table . Impedance analysis of p-type DSSCs of organic sensitizers.
Dye
R (ꢀ)
R(ꢀ)
R(ꢀ)
TC
TI
TI
.
.
.
.
.
.
.
.
.
frequency semicircle in the Nyquist plot shows the electron transport resistance. R1 is FTO
Interface resistance. R2 is due to the resistance at the interface between the counter electrode
and the electrolyte. R3 possibly originated from the backward charge transfer from NiO to
the electrolyte and electron conduction in porous NiO film. It was found the charge trans-
fer (R3) values for the DSSCs using T1I and T2I (55.35 and 55.21 ꢀ) were indeed smaller
than that using TC1 (65.95 ꢀ) due to the enhanced charge transfer ability. The photocurrent
improvement is more likely resulted from the enhanced charge transfer ability.
Scheme . Synthesis route for new dye-sensitizers TI and TI.

116
L. Q. BAO ET AL.
4. Conclusions
We have synthesized and characterized two new double branched organic sensitizers for
p-type dye-sensitized solar cells and obtained some insight into their structure prop-
erty relationships. The absorption spectrum red shifts and the first oxidation potential
decreases with introduction of additional donor group. These double branched dyes can
increase the charge transfer and increase JSC of DSSCs, giving the higher power conversion
efficiency of 0.05%. The DSSC based on single branched dye TC1 shows the lower pho-
tovoltaic performance compare to the double branched dye. Our work may give a useful
guidance for the molecular design of the organic dyes used in p-type dye-sensitized solar
cells.
Acknowledgements
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning
(KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No.
20163010012310).
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