New photosensitizer with phenylenebisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.electacta.2011.04.011

A new low band gap photosensitizer, D, which contains 2,2′-(1,4- phenylene) bisthiophene central unit and cyanovinylene 4-nitrophenyl terminal units at both sides was synthesized. The two carboxyls attached to the 2,5-positions of the phenylene ring act as anchoring groups. Dye D was soluble in common organic solvents, showed long-wavelength absorption maximum at 620-636 nm and optical band gap of 1.72 eV. The electrochemical parameters, i.e. the highest occupied molecular orbital (HOMO) (-5.1 eV) and the lowest unoccupied molecular orbital (LUMO) (-3.3 eV) energy levels of D show that this dye is suitable as molecular sensitizer. The quasi solid state dye-sensitized solar cell (DSSC) based on D shows a short circuit current (J_sc) of 9.95 mA/cm², an open circuit voltage (V_oc) of 0.70 V, and a fill factor (FF) of 0.64 corresponding to an overall power conversion efficiency (PCE) of 4.40% under 100 mW/cm² irradiation. The overall PCE has been further improved to 5.52% when diphenylphosphinic acid (DPPA) coadsorbent is incorporated into the D solution. This increased PCE has been attributed to the enhancement in the electron lifetime and reduced recombination of injected electrons with the iodide ions present in the electrolyte with the use of DPPA as coadsorbant. The electrochemical impedance spectroscopy (EIS) results indicated that the augment ascribes to inhibited interfacial charge recombination between the conduction band electrons and the tri-iodide ions in the electrolyte.

Full text of DOI:10.1016/j.electacta.2011.04.011

Electrochimica Acta 56 (2011) 5616–5623
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
New photosensitizer with phenylenebisthiophene central unit and
cyanovinylene 4-nitrophenyl terminal units for dye-sensitized solar cells
J.A. Mikroyannidisa,∗, P. Sureshb, M.S. Royd, G.D. Sharmab,c,∗∗
a Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece
b Physics Department, Molecular Electronic and Optoelectronic Device Laboratory, JNV University, Jodhpur (Raj.) 342005, India
c R & D Centre for Engineering and Science, Jaipur Engineering College, Kukas, Jaipur (Raj.), India
d Defence Laboratory, Jodhpur (Raj.) 342011, India
a r t i c l e i n f o
a b s t r a c t
A new low band gap photosensitizer, D, which contains 2,2ꢀ-(1,4-phenylene) bisthiophene central unit
and cyanovinylene 4-nitrophenyl terminal units at both sides was synthesized. The two carboxyls
attached to the 2,5-positions of the phenylene ring act as anchoring groups. Dye D was soluble in common
organic solvents, showed long-wavelength absorption maximum at 620–636 nm and optical band gap of
1.72 eV. The electrochemical parameters, i.e. the highest occupied molecular orbital (HOMO) (−5.1 eV)
and the lowest unoccupied molecular orbital (LUMO) (−3.3 eV) energy levels of D show that this dye is
suitable as molecular sensitizer. The quasi solid state dye-sensitized solar cell (DSSC) based on D shows a
short circuit current (Jsc) of 9.95 mA/cm2, an open circuit voltage (Voc) of 0.70 V, and a fill factor (FF) of 0.64
corresponding to an overall power conversion efficiency (PCE) of 4.40% under 100 mW/cm2 irradiation.
The overall PCE has been further improved to 5.52% when diphenylphosphinic acid (DPPA) coadsor-
bent is incorporated into the D solution. This increased PCE has been attributed to the enhancement in
the electron lifetime and reduced recombination of injected electrons with the iodide ions present in
the electrolyte with the use of DPPA as coadsorbant. The electrochemical impedance spectroscopy (EIS)
results indicated that the augment ascribes to inhibited interfacial charge recombination between the
conduction band electrons and the tri-iodide ions in the electrolyte.
Article history:
Received 29 January 2011
Received in revised form 2 April 2011
Accepted 2 April 2011
Available online 12 April 2011
Keywords:
Dye-sensitized solar cells
Organic sensitizer
Low band gap
Cyanovinylene
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
organic DSSC is the formation of dye aggregate on the semiconduc-
tor surface [4]. High-performance dyes for DSSC are requested to
Dye-sensitized solar cells (DSSCs) with mesoporous TiO2 elec-
trodes have been regarded as one of the most promising candidates
for solar energy conversion because of the potential of low-cost pro-
duction and high power conversion efficiency (PCE) [1]. Although
ruthenium polypyridyl complexes have proven to be excellent TiO2
sensitizers that have achieved the highest power conversion effi-
ciency (PCE) value up to 11.5%, their extensive application would be
hampered considerably owing to the limited availability and high
cost of Ru metal [2]. In this context, studies on metal-free organic
chromophores or inexpensive metal complexes for DSSC have pro-
gressed rapidly [3]. One of the factors for the low efficiency of an
display broad light absorption capability that can cover the whole
solar light spectrum, along with fast injection of electrons from
the excited dyes to a conduction band (CB) of the TiO2 electrode,
and slow charge recombination between the injected electrons and
−
resulting dye cations and/or I3 in the electrolyte. Organic dyes
composed of ␲-conjugative molecules such as coumarin [5], indo-
line [6], polyene [7], thiophene [8], cyanine [9], hemicyanine [10],
squaraine [11], phthalocyanine [12], triphenylamine [13] and pery-
lene [14] derivatives have been developed as potential sensitizers
for DSSC.
The sensitizer is a crucial element in DSSCs, exerting signif-
icant influence on the PCE as well as the stability of the cells.
Generally, organic dyes used for efficient solar cells are required
to possess high charge carrier mobility and broad and intense
spectral absorption in the visible light region [8]. Recently, we
have synthesized two simple sensitizers of low band gap based on
4-nitro-␣-cyanostilbene prepared from a one step reaction only,
which gave quasi solid state DSSCs with PCE up to 4.8% [15].
Moreover, we have synthesized three organic sensitizers with car-
∗
Corresponding author. Tel.: +30 2610 997115; fax: +30 2610 997118.
Corresponding author at: Physics Department, Molecular Electronic and Opto-
∗∗
electronic Device Laboratory, JNV University, Jodhpur (Raj.) 342005, India.
Tel.: +91 0291 2720857; fax: +91 0291 2720856.
E-mail addresses: mikroyan@chemistry.upatras.gr, mikroyan@googlemail.com
(J.A. Mikroyannidis), sharmagd in@yahoo.com (G.D. Sharma).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.04.011

J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
5617
boxy, acid anhydride and hydroxyl anchoring groups and terminal
2.2.2. Compound 5
cyanovinylene 4-nitrophenyl segments [16]. The sensitizer with
carboxy anchoring group afforded DSSC with the highest PCE 4.58%,
which was improved to 5.80% when chenodeoxyacholic acid was
added as coadsobent. Finally, this PCE was further improved to 6.9%
with the incorporation of poly(vinylidene fluoride) in the polymer
gel electrolyte [16].
The present investigation describes the synthesis and character-
ization of a new metal free ␲-conjugated organic photosensitizer,
D, of low band gap for DSSCs. The molecule of D is symmetrical
and contains phenylenebisthiophene central unit and cyanoviny-
lene 4-nitrophenyl terminal units at both sides. The two carboxyls
attached to the phenylene act as anchoring groups. This dye was
successfully synthesized by a six-step reaction sequence starting
from p-xylene which is a low cost starting material.
A flask was charged with a solution of 4 (0.36, 0.93 mmol) in
1,2-dichloroethane (20 mL), DMF (0.22 g, 3.01 mmol) and POCl3
(0.47 g, 3.06 mmol) were added dropwise and the mixture was
refluxed for 17 h. After cooling to room temperature and addition
of dichloromethane (15 mL) and a saturated aqueous solution of
CH3COONa (30 mL), the mixture was stirred for 2 h at room tem-
perature. The organic phase was then washed with water and dried
over Na2SO4. Solvent removal and column chromatography (silica
gel/dichloromethane) gave 5 (0.31 g, 75%).
FT-IR (KBr, cm−1): 1730 (C O stretching of formyl and ester);
2954, 2922 (C–H stretching of ethyl); 1578, 1463 (aromatic).
1H NMR (CDCl3) ppm: 9.94 (s, 2H, formyl); 7.81–7.78 (m, 2H,
phenylene and 4H, H3, H4 of thiophene); 4.22 (m, 4H, COOCH2CH3);
1.16 (t, 6H, COOCH2CH3).
The dye D of the present investigation has certain common
structural characteristics with the sensitizers, which have been
previously synthesized in our laboratory [15,16]. In particular, all
sensitizers contain cyanovinylene 4-nitrophenyl segments, which
broadened their absorption spectrum. However, dye D contains two
carboxyls (anchoring groups) in the middle of the molecule, while
the previously synthesized sensitizers contained only one anchor-
ing group (carboxyl or acid anhydride). Moreover, the central unit
in the molecule of each sensitizer is entirely different. We have
used D as a sensitizer for quasi solid state dye sensitized solar cells
(DSSCs) and achieved PCE of 4.4%, which has been further improved
to 5.52%, when diphenylphosphinic acid (DPPA) is added to D solu-
tion as coadsorbent. The aim of the present study is to compare
the influence of the presence of DPPA coadsorbent during the dye
sensitization in DSSCs, and to study its effect on the parameters,
such as electron lifetime, dye loads, semiconductor energy level
and overall photovoltaic (PV) performance.
Anal. Calcd. for C22H18O6S2: C, 59.71; H, 4.10. Found: C, 59.23;
H, 3.97.
2.2.3. Compound D
A flask was charged with a solution of 4 (0.41 g, 0.93 mmol)
and 4-nitrobenzyl cyanide (0.30 g, 1.86 mmol). Sodium hydrox-
ide (0.20 g, 5.00 mmol) dissolved in ethanol (10 mL) was added
portionwise to the stirred solution. The mixture was stirred and
heated at 80 ◦C for 3 h under N2. Then, dilute hydrochloric acid
was added to the solution and D precipitated as a dark green solid.
The crude product was purified by column chromatography (silica
gel/dichloromethane) (0.49 g, 78%).
FT-IR (KBr, cm−1): 1674 (C O stretching of carboxyl); 3374 (O–H
stretching of carboxyl); 2207 (cyano); 1524, 1344 (nitro); 1606,
1466 (aromatic).
1H NMR (CDCl3) ppm: 13.3 (br, 2H, carboxyl); 8.18–8.12 (m, 4H,
phenylene ortho to nitro and 2H, phenylene ortho to carboxyl); 7.72
(s, 2H, cyanovinylene); 7.50 (m, 4H, phenylene ortho to nitro); 7.10
(m, 4H, H3, H4 of thiophene).
2. Experimental
Anal. Calcd. for C34H18N4O8S2: C, 60.53; H, 2.69; N, 8.30. Found:
C, 59.91; H, 2.83; N, 8.12.
2.1. Reagents and solvents
2.3. Characterization methods
2-Tributylstannylthiophene was prepared from the reaction
of thiophene with n-BuLi in hexane and subsequently with trib-
utylchlorostannane according to the literature [17]. 4-Nitrobenzyl
cyanide was synthesized from the nitration of benzyl cyanide with
concentrated nitric and sulfuric acid [18]. It was recrystallized from
ethanol. N,N-dimethylformamide (DMF) and tetrahydrofuran (THF)
were dried by distillation over CaH2. All other reagents and solvents
were commercially purchased and were used as supplied.
IR spectra were recorded on a Perkin-Elmer 16PC FT-IR spec-
trometer with KBr pellets. 1H NMR (400 MHz) spectra were
obtained using a Brucker spectrometer. Chemical shifts (ı values)
are given in parts per million with tetramethylsilane as an inter-
nal standard. UV–vis spectra were recorded on a Beckman DU-640
spectrometer. Elemental analysis was carried out with a Carlo Erba
model EA1108 analyzer.
Cyclic voltammetry (CV) measurement of D was performed
with an Autolab analyzer using a three electrodes electrochemical
cell in a solution of tetra-butyl ammonium hexafluorophosphate
(Bu4NPF6) (0.1 M) in anhydrous acetonitrile at a scan rate of
100 mV/s at room temperature. Dye D coated on carbon glassy was
used as the working electrode. A Pt wire and an Ag/Ag+ were used
as counter and reference electrodes, respectively.
2.2. Preparation of compounds
2.2.1. Compound 4
A flask was charged with a mixture of 3 (0.35 g, 0.93 mmol),
2-tributylstannylthiophene (0.83 g, 2.22 mmol), PdCl2(PPh3)2
(0.0195 g, 0.03 mmol) and freshly dried THF (25 mL). The flask
with the reagents was evacuated several times and filled with
N2. The mixture was stirred and refluxed for 24 h. It was subse-
quently filtered and the filtrate was concentrated under reduced
pressure to afford 4. The crude product was purified by silica gel
chromatography (ethyl acetate/hexane 1:9, v/v) (0.29 g, 80%).
FT-IR (KBr, cm−1): 1732 (C O stretching of ester); 2954, 2922
(C–H stretching of ethyl); 1582, 1462 (aromatic).
2.4. DSSCs fabrication and characterization
TiO2 paste was prepared by mixing 1 g of TiO2 powder (P25,
Degussa), 0.2 mL of acetic acid and 1 mL of water. Then, 60 mL of
ethanol was slowly added while sonicating the mixture for 3 h.
Finally, Triton X-100 was added to obtain a dispersed colloidal
(TiO2) paste. The mixture was stirred vigorously for 2–4 h at room
temperature and then stirred for 4 h at 80 ◦C to form a transparent
colloidal (TiO2) paste. The TiO2 paste was deposited on the pre-
cleaned fluorine (F) doped tin oxide (FTO) coated glass substrate
by the doctor blade technique. The TiO2 coated FTO films were
sintered at 450 ◦C for 30 min and cooled down up to room tem-
1H NMR (CDCl3) ppm: 7.81 (s, 2H, phenylene); 7.39 (m, 2H,
H5 of thiophene); 7.08 (m, 4H, H3, H4 of thiophene); 4.22 (m, 4H,
COOCH2CH3); 1.16 (t, 6H, COOCH2CH3).
Anal. Calcd. for C20H18O4S2: C, 62.15; H, 4.69. Found: C, 61.85;
H, 4.52.

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J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
Scheme 1. Synthesis of dye D.
perature. Before immersing into the dye solution, these films were
soaked in a 0.2 M aqueous TiCl4 solution overnight in a closed cham-
ber. After being washed with deionized water and fully rinsed with
ethanol, the films were again sintered at 450 ◦C for 30 min, followed
by cooling at 60 ◦C.
2,5-dibromo-p-xylene (1) [19]. The reaction of the latter with
KMnO4 in pyridine and subsequent acidification with hydrochlo-
ric acid gave 2,5-dibromoterephthalic acid (2) [20]. This compound
reacted with ethanol in the presence of sulfuric acid to yield the
corresponding diethyl ester 3 [21]. The coupling of 3 with 2-
tributylstannylthiophene gave 4. The formylation of 4 by means of
DMF and POCl3 afforded bisaldehyde 5. Finally, the condensation
of 5 with 4-nitrobenzyl cyanide in the presence of sodium hydrox-
ide was followed by hydrolysis of the ester and acidification with
hydrochloric acid to give dye D.
Even though D lacked solubilizing aliphatic chains, it was soluble
in common organic solvents such as chloroform, dichloromethane
and THF. It was characterized by FT-IR and 1H NMR spectroscopy.
The IR spectrum showed characteristic absorption bands at 1674
(C O stretching of carboxyl); 3374 (O–H stretching of carboxyl);
2207 (cyano) and 1524, 1344 cm−1 (nitro). On the other hand,
the 1H NMR spectrum displayed upfield signals at 13.3 (car-
boxyl); 8.18–8.12 (phenylene ortho to nitro and carboxyl); 7.72
(cyanovinylene) and 7.50 ppm (phenylene ortho to nitro). Finally,
the thiophene resonated at 7.10 ppm.
The dye (D) was dissolved in THF solution (0.5 mM) and the
TiO2 electrode was immersed into this solution overnight and
after sensitization the sensitized electrode was washed and dried.
20 mM of DPPA was included in the dye bath solution as coad-
sorbent. A quasi solid state polymer gel electrolyte containing
0.0383 g of P25 TiO2 powder, 0.1 g of LiI, 0.019 g of I2, 0.264 g of
poly(ethylene oxide) (PEO) and 44 ␮L of 4-tert-butyl-pyridine (TBP)
in 1:1 accetonitrile/propylene carbonate was prepared. The mix-
ture was continuously stirred at 80 ◦C in a water bath for about
4–5 h. The polymer gel electrolyte was spread on sensitized TiO2
film by spin coating to form the hole transport layer. The counter
electrode was platinized by spin coating H2PtCl6 solution (2 mg in
1 mL of isopropanol onto the FTO coated glass substrate and then
heated at 450 ◦C for 30 min in air. The dye sensitized photoelectrode
containing the quasi solid state polymer electrolyte and the counter
electrode were clamped together and separated in a distance of
15 ␮m.
3.2. Photophysical and electrochemical properties
The current–voltage (J–V) characteristics of the devices in dark
as well as under illumination were measured by Keithley electrom-
eter equipped with built in power supply. A xenon lamp light source
was used to give an irradiance of 100 mW/cm2 (the equivalent of
one sun at 1.5 AM) at the surface of the device, measured by luxme-
ter equipped with silicon detector. The IPCE spectra of the devices
were measured using a monochromator (Spex 500 M, USA) and the
resulted photocurrent was measured with Keithley electrometer
(model 6514), which is interfaced to the computer by LABVIEW
software.
Fig. 1 shows the UV–vis absorption spectra of D in dilute
(10−5 M) THF solution and thin film. The long-wavelength absorp-
The electrochemical impedance spectra (EIS) measurements
were carried out by applying bias of the open circuit voltage (Voc
)
and recorded over a frequency range of 10−1 Hz to 105 Hz with
amplitude of 10 mV. The above measurements were recorded with
an electrochemical analyzer equipped with FRA.
3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 outlines the six-step reaction for the synthesis of
the target dye D. Particularly, p-xylene was brominated to afford
Fig. 1. UV–vis absorption spectra of dye D in THF solution and thin film.

J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
5619
Fig. 2. Absorption spectra of dye D and D with DPPA coadsorbent on 10 ␮m porous
TiO2 nanoparticle film.
tion maximum (ꢀa,max) appeared at 636 nm in solution and 620 nm
in thin film. This could be attributed to an intramolecular charge
transfer (ICT) between the electron-donating phenylenebisthio-
phene central unit and the electron-withdrawing cyanovinylene
4-nitrophenyl terminal units. Both spectra displayed almost the
same absorption onset, which indicates that there are not sig-
nificant intermolecular interactions in solid state. The thin film
absorption onset was located at 720 nm corresponding to an optical
band gap (Egopt) of 1.72 eV.
The position of the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of the organic dye
used as photosensitizer play an important role in the PV perfor-
mance of the DSSCs. The oxidation potential (Eooxnset) and reduction
potential (Eroendset) corresponding to the HOMO and LUMO levels,
respectively, of dye D were measured by cyclic voltammetry (CV)
according to the following expressions [22]:
Fig. 3. Current–voltage (J–V) characteristics under (a) illumination intensity of
100 mW/cm2 and (b) in dark for DSSCs based on with and without DPPA coadsorbent.
ELUMO = −(Eonset + 4.70) eV
red
(1)
broadened as compared to that for D in solution. Such phenom-
ena were commonly observed in spectral response of other organic
dyes, which may be ascribed to the formation of dye H-aggregates
on the TiO2 surface and/or the interaction between the dye and TiO2
[26]. The blue shift is attributed to the deprotonation of carboxylic
acid upon the adsorption onto the TiO2 surface since the result-
ing carboxylate–TiO2 unit is a weaker electron acceptor compared
to the carboxylic acid [27]. Comparing the absorption onset of the
dye in solution and the adsorbed dye onto TiO2 film, a red shift of
the absorption onset was observed in the latter. Such red shift in
the absorption onset can maintain a high absorption in the visible
region (450–700 nm) and also an extension into the near infrared
(NIR) region (800 nm), which is beneficial to high light utilization.
EHOMO = −(Eooxnset + 4.70) eV
The HOMO and LUMO energy levels of D are −5.1 eV and −3.3 eV,
respectively and the electrochemical band gap (Egel) is 1.80 eV. The
value of HOMO for D is sufficiently more positive than the redox
potential of I−/I3− redox mediator (approximately −4.83 eV vs. vac-
uum)intheelectrolyte[23], indicatingthat the oxidizeddye formed
after the electrons injection into the conduction band of TiO2 could
accept electrons from I− ions in the electrolyte thermodynamically.
Moreover, it is evident that the LUMO level of D is more negative
than that of the conduction band edge of TiO2 (ECB = −4.1 eV vs.
vacuum) [24]. Consequently, the dye can complete the process of
electron injection into the conduction band of TiO2 to form the
excited state of oxidized D, upon the absorption of the light. Thus,
this dye can be regenerated from the reduced species in the elec-
trolyte to give an efficient charge separation.
3.3. Application in DSSCs
We have measured the absorption spectra of thin TiO2 film
loaded with a monolayer of D (Fig. 2). Generally, when sensitizers
anchor onto the nanocrystalline TiO2 surface, the deprotonation
and aggregation of dye molecules affect the optical absorption
profile. Typically, the molecular aggregation contains H- and J-
aggregates. Deprotonation [25] and H-aggregates always result in
blue shift in the absorption peak, while J-aggregates mainly lead to
the red shift in the absorption peak. The absorption spectra of dye
adsorbed onto TiO2 film, was slightly (∼15 nm) blue shifted and
PV performance was investigated to evaluate D as sensitizer in
DSSC. The J–V characteristics of the DSSCs based on D, under illu-
mination intensity of 100 mW/cm2 are shown in Fig. 3(a). The PV
parameters, i.e. short circuit current (Jsc), open circuit voltage (Voc),
fill factor (FF) and power conversion efficiency (PCE) were esti-
mated from the J–V characteristics under illumination. The DSSC
showed Jsc of 9.95 mA/cm2, Voc of 0.70 V and FF of 0.64 leading to
an overall PCE of 4.4%. This value of the PCE is lower than that
for DSSC based on dyes containing cynovinylene 4-nitrophenyl

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J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
icantly. The value of Jsc is increased from 9.95 to 10.85 mA/cm2,
when the DPPA is added into the dye solution. The enhancement
in the IPCE value is attributed to the break-up of the dye aggre-
gates upon coadsorption. The increased Jsc is due to the higher IPCE
value caused by the higher electron injection efficiency resulting
from the relatively independent dye molecules arraying on TiO2
surface. Here, the increased Voc upon the coadsorption is due to the
retarded charge recombination as evidenced by the dark current
onset potential shift from 0.41 V to 0.48 V (as shown in Fig. 3b).
One of the possible reasons for the increased Jsc in the presence
of DPPA is the reduction of charge recombination rate. This is evi-
denced from the observation that the dark current, which is the
resultant of recombination of electrons with iodide ions in the elec-
trolyte, has been reduced. Similar results have been also observed in
a triphenylamine dye containing C C bond in ␲-space [36]. Finally
all parameters (Jsc, Voc, and FF) are optimized to the best overall PCE
of about 4.4% and 5.52% for the DSSCs without and with coadsorbent
respectively.
Generally, the influence of the dye on Voc has mostly been
attributed to the electron lifetime, which is related to certain fac-
tors such as molecular size and dye adsorption behavior. The value
of Voc is determined by the potential difference between the redox
species of TiO2 (EFn) and the chemical potential of the redox species
(Ered) in the electrolyte, which could be described by the following
expression [37].
Fig. 4. IPCE spectra for DSSCs based on with and without DPPA coadsorbent.
segments with carboxyl anchoring group (4.58%) [16] and 4-nitro-
␣-cyanostilbene (4.8%) [15].
ꢀ
ꢁ
ꢀ
ꢁ
ECB
q
kT
q
Nc
n
Ered
q
Voc
=
+ ꢁ
ln
−
(2)
Many organic dyes are aggregating on the semiconductor sur-
face via molecular stacking, leading to self quenching and reducing
electron injection into TiO2. Several methods have been attempted
to restrain this processes, including oxide coating of TiO2 film [28],
and utilization of small organic molecules defined as coadsorbents.
Coadsorbents commonly utilize anchoring groups such as carboxyl
and phosphonyl to anchor onto TiO2 surface. Several kinds of small
organic molecules (coadsorbents), including phosphonic acids [29],
carboxylic acids [30] have proven to be effective in improving the
PV performance of the DSSC. Generally, cheno-deoxy cholic acid
(CDCA) and deoxycholic acid (DCA) have been used as coadsorbents
to dissociate the ␲-stacked dye aggregates and improve the elec-
tron injection yield, thus affording higher Jsc value [31]. Recently,
phosphinic acid has attracted research interest as a coadsorbent
for DSSCs [32]. This type of coadsorbent has proven beneficial
for DSSCs that incorporate sensitizers with small injection driv-
ing forces, such as porphyrins [33], by lowering the conduction
band edge, thereby increasing Jsc. It has been reported theoretically
that this kind of coadsorbent improves not only the efficiency but
also the long-term stability of DSSCs, since the bonding between
phosphinic acid and TiO2 is salient and difficult to break [34]. Here,
diphenylphosphinic acid (DPPA) was employed as a coadsorbent
to improve the PCE of the DSSC based on D. A two benzene based
adsorbent was chosen in the D dye system not only because the two
phenyls posses more pronounced hydrophobicity, meaning a larger
current is expected, but also the presence of ␲-bonds strengths the
link between DPPA and TiO2.
where ꢁ is a characteristic constant of TiO2 tailing states, k is
the Boltzmann constant, T is the temperature, q is the elementary
charge, and Nc is the effective density of states at the TiO2 con-
duction band edge. In Eq. (2), the sum of the first two terms is the
quasi Fermi level of TiO2. Eq. (2) thus shows that the Voc is the
difference between the quasi Fermi level in TiO2 and Ered/q. Gener-
ally, as the number of the injected electrons in TiO2, is smaller than
Nc, the maximum quasi Fermi level is ECB/q and the maximum Voc
becomes (ECB − Ered)/q, i.e. the difference between the ECB in TiO2
and Ered [38]. According to Eq. (2), the Voc can be enhanced either by
increasing the conduction band of TiO2 or down-shifting the Ered
[39]. Since we have used the same electrolyte in both DSSCs, we
assume that the Ered remains the same for both DSSCs. In an open
circuit, the number (n) of electrons in TiO2, is determined by the
balance between electron injection and charge recombination. An
improvement in the light absorption capability and electron injec-
tion efficiency will increase the number of injected electrons in TiO2
and then will increase the Voc, while the charge recombination pro-
cesses will reduce the electron population in TiO2 thus decreasing
Voc. Therefore, the Voc is affected by the potential of conduction
band edge (ECB) of TiO2 and the electron density (n) in the TiO2.
These two parameters are closely related to the surface charge
and charge recombination, respectively. Possible factors for the
enhanced value of Voc for DSSCs with D and DPPA coadsorbent as
compared to D only, involve the TiO2 surface blocking, conduction
band movement, and electrolyte–dye interaction.
The J–V characteristics of the DSSC under illumination inten-
sity of 100 mW/cm2 using DPPA as coadsorbent in D bath is shown
in Fig. 3a. The DSSCs based on DPPA coadsorbed D showed Jsc of
10.85 mA/cm2, Voc of 0. 76 V and FF of 0.67, giving an overall PCE of
5.52%. It is well known that alkyl phosphinic acids can bind strongly
onto the oxide surface via P–O–metal bonds, occupying hydrophilic
sites settled by redox species [35]. This leads to an increase in the Jsc
of the DSSC, since a large number of this kind of sites are blocked
by DPPA. The IPCE spectra of the DSSCs based on with and with-
out DPPA coadsorbent D are shown in Fig. 4. It can be seen from
this figure that upon coadsorption with DPPA, the IPCE values in
the wavelength region (500–700 nm) have been increased signif-
Electrochemical impedance spectroscopy (EIS) is a powerful tool
for identifying electronic and ionic transport processes in DSSCs,
which provides valuable information for the understanding of PV
parameters [40]. Typical EIS Nyquist plots for the DSSCs based on
with and without DPPA coadsorbent, measured in dark under for-
ward bias of −0.7 V are shown in Fig. 5. For the frequency range
investigated (0.1–105 Hz), in both DSSCs, a larger semicircle occurs
in the lower frequency range (0.1–100 Hz) and a smaller semi-
circle occurs in the higher frequency range. The larger semicircle
observed in lower frequencies corresponds to the charge transfer
processes at the TiO2/dye/electrolyte interface, while the smaller
semicircle at higher frequencies corresponds to the charge trans-

J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
5621
Fig. 6. EIS spectra, i.e. Nyquist plots DSSCs based on with and without DPPA coad-
sorbent measured at bias voltage −0.7 V, under illumination.
for the DSSCs with and without DPPA as coadsorbent, respectively.
These values are comparable to those estimated from Bode plots.
Fig. 5(b) shows the EIS Bode plots (i.e. the phase angle of the
impedance vs. the frequency) of the DSSCs based on D only and D
with DPPA coadsorbent. For the frequency range investigated, both
EIS Bode plots also exhibit two peak features; one at higher fre-
quencies corresponding to the charge transfer at the Pt/electrolyte
interface and another one at lower frequencies corresponding to
the charge transfer at the TiO2/dye/electrolyte interface. It can be
seen from Fig. 5(b) that the differentiation in Bode plots of both
DSSCs mainly occurs in the lower frequencies range. The charac-
teristic frequency of the lower frequency peak in the Bode plot is
related to the charge recombination rate, and its reciprocal is asso-
ciated with the electron lifetime [40b,41]. As it can be seen from
Fig. 5(b), the lower frequency peak of the DSSC based on D with
DPPA coadsorbent shifts to a lower frequency as compared to DSSC
based on D only. The electron lifetime (ꢂn) for DSSC based on D and
DPPA-D is estimated from the frequency peak using ꢂn = 1/2ꢃfp, fp is
the peak frequency in lower frequency region. The value of electron
lifetime for the DSSC based on D and DPPA-D is 16 ms and 25 ms,
respectively. The increase in the electron lifetime for DSSC based on
DPPA-D sensitizer indicates an improvement in both electron injec-
tion efficiency and collection efficiency, leading to an enhancement
in Jsc. The reduction is the dark current (Fig. 3b) for the DSSC based
on DPPA-D as compared to D also indicates that the coadsorption of
DPPA leads to a suppression of the charge recombination between
Fig. 5. EIS spectra (a) Nyquist plots and (b) Bode plots for DSSCs based on with and
without DPPA coadsorbent, measured in dark under −0.7 V bias. Top corner of (a) is
the equivalent circuit used to fit the impedance spectra.
fer processes at the Pt/electrolyte interface. The two devices hardly
show any difference in smaller semicircles at higher frequencies
but the difference in larger semicircles at lower frequencies is sig-
nificant. The larger width of the lower frequency semicircle of the
DSSC based on D indicates a larger charge transfer resistance or
recombination resistance at the TiO2/D/electrolyte interface. The
equivalent circuit presented in Fig. 5(a) was used to fit the exper-
imental data of DSSCs. Rs is the series resistance accounting for
the transport resistance of the FTO and the electrolyte, C and R
−
the injected electrons and the I3 ions in the electrolyte [42]. We
assume that due to the addition of coadsorbent in the dye sensi-
tization process, a thin compact layer can be formed on the TiO2
layer [43]. The suppression of charge recombination and enhanced
electron lifetime may be attributed to the higher Voc of the DSSC
based on D with DPPA coadsorbent than that for D. Various coad-
sorbents have also been used for metal free organic dyes, such as
bisindolymalemide [44] and indoline [45] dyes to increase both Jsc
␮
CT
are the chemical capacitance and the charge recombination resis-
tance at the TiO2/electrolyte interface, respectively. CPt and RPt are
the interfacial capacitance and charge transport resistance at the
Pt/electrolyte interface, respectively. The fitted value of the RCT
is higher for the DSSCs based on D with DPPA coadsorbent with
respect to that for D only, which is consistent with the sequence of
Voc values in the devices. The smaller value of the RCT means that the
electron recombination from the conduction band to the electrolyte
occurs more easily and lowers Voc. Clearly, electron recombina-
tion in the DSSC based on D is faster than that for D with DPPA
coadsorbent. The results can be cross-checked by measuring the
J–V characteristics of the DSSCs in dark (Fig. 3b). The onset poten-
tials for dark current (potential value at the dark current value
−0.076 mA/cm2) for DSSCs based on D only and D with DPPA coad-
sorbent are 0.41 V and 0.48 V, respectively, the sequence of which
is in accordance with that of RCT. By fitting the EIS curves, another
parameter for DSSCs, electron lifetime (ꢂe), could be extracted from
the C and R using ꢂe = C ·RCT and the values are 14 ms and 28 ms
and Voc
.
In order to gain more information about the electron transport
in DSSCs, the EIS, i.e. Nyquist plot of the DSSCs was also measured
under illumination (100 mW/cm2). As shown in Fig. 6, the radius of
the semicircle in the middle frequency region is larger for the DSSC
based on D than that for DPPA-D. These results indicate that the
charge transport and injection into the conduction band of TiO2 in
the DSSC based on DPPA-D is faster than that for D, which coincides
with the overall PCE of the DSSCs [46].
To investigate the influence of the shift on the conduction band
of TiO2 and charge recombination on Voc, impedance spectra were
also measured by varying the applied potential at equal intervals
in the vicinity of Voc. The fitted C and RCT values were estimated
␮
␮
␮
CT

5622
J.A. Mikroyannidis et al. / Electrochimica Acta 56 (2011) 5616–5623
bias of −0.7 V, the DSSCs based on D with DPPA coadsorbent shows
a decrease of C (from 207 to 168 ␮F) and an increase of R (from
␮
CT
68 ꢄ to 148 ꢄ) compared to that of D only, yielding a higher ꢂn of
25 nm. The electron lifetime for D is shorter than that for D with
DPPA coadsorbent.
The Voc and PCE values were effectively enhanced by the
coadsorption procedure, when the original dye suffered from the
aggregation. The dark current measurement showed that charge
−
carrier recombination between the injected electrons and I3 ions
was reduced as a result of the coadsorption [49].
The adsorbed amount of dye on the TiO2 film (determined by dye
desorption) with and without DPPA is about 5.6 × 10−8 mol/cm2
and 4.3 × 10−8 mol/cm2, respectively. It was found that the dye
amount of D sensitizer adsorbed onto the TiO2 surface was reduced
by 23% in the presence of DPPA, as compared to that without DPPA
in the dye bath. Despite of this, higher Jsc was observed. The main
possible explanation is that the coadsorption of DPPA prevents dye
aggregation, which can cause intermolecular energy transfer and
subsequently results in the excited state quenching of the dye [50].
Hence, the reduction of dye load on the TiO2 surface in the pres-
ence of DPPA in the dye bath results in more efficient electron
injection from the excited dye to the TiO2 conduction band [51].
A more efficient electron injection compensates the least amount
of the adsorbed dye. Similar results have been also reported for
CDCA coadsorbent with triphenylamine based organic sensitizers
[52]. On the basis of the above results, it was suggested that the
presence of coadsorbent DPPA in the dye solution during the sensi-
tization process resulted in shielding of the surface against electron
recombination as well as provided a higher concentration of func-
tional dye on the TiO2 surface contributing to an efficient electron
injection into the TiO2 conduction band.
4. Conclusions
A new photosensitizer, D, with two carboxy anchoring groups
attached to the central phenylene ring was synthesized. The
cyanovinylene 4-nitrophenyl terminal units broadened the absorp-
tion spectrum of D, which showed long-wavelength absorption
maximum at 620–636 nm and optical band gap of 1.72 eV. The
HOMO (−5.1 eV) and LUMO (−3.3 eV) levels estimated from cyclic
voltammetry reveal that D is a suitable sensitizer for DSSC. The
overall PCE of the quasi solid state DSSC using D as sensitizer is
about 4.4%. The PCE of the DSSC has been further improved to 5.52%
by the introduction of DPPA as coadsorbent in the D bath. In par-
ticular, although DPPA reduced the dye loads into the TiO2 film,
the increase in Jsc and IPCE comes partially from the effects of the
reduction in the electron recombination and the improved electron
injection efficiency. On the other hand, shift in the conduction band
edge of the TiO2 leads to an increase in the Voc of the DSSC with the
inclusion of DPPA in the dye sensitization process.
Fig. 7. (a) Chemical capacitance (C ) and (b) interfacial charge transfer (RCT) fitted
from impedance spectra under different applied bias voltages for DSSCs with and
without DPPA as coadsorbent.
␮
at different biasing voltages. We observed an exponential rise of
C
␮
and a fall of R with the increase in forward bias (as shown in
CT
Fig. 7a and b). The exponential behavior C with bias potential is
␮
described by the following expression [47].
ꢂ
ꢃ
ꢄ
ꢅ
q2
kT
˛
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