Stepwise co-sensitization as a useful tool for enhancement of power conversion efficiency of dye-sensitized solar cells: The case of an unsymmetrical porphyrin dyad and a metal-free organic dye

Article abstract of DOI:10.1016/j.orgel.2014.03.033

A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H₂[Porph] (ZnP-H₂P), where Zn[Porph] and H₂[Porph] are metallated and free-base porphyrin units, respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs). The photophysical and electronic properties of the two compounds revealed spectral absorption features and frontier orbital energy levels that are appropriate for use in DSSCs. Following a stepwise co-sensitization procedure, by immersing the TiO₂ electrode in separate solutions of the dyes in different sequence, two co-sensitized solar cells were obtained: devices C (ZnP-H₂P/DC) and D (DC/ZnP-H ₂P).The two solar cells were found to exhibit power conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device C, which is also higher than that of the individually sensitized devices based on the ZnP-H₂P and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current (J_sc = 11.72 mA/cm²), open circuit voltage (V_oc = 0.72 V), fill factor (FF = 0.73), as it is revealed by photovoltaic measurements (J-V curves) and by incident photon to current conversion efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall performance of device C was further improved up to 7.68% (with J_sc = 13.45 mA/cm², V_oc = 0.76 V, and FF = 0.75), when a formic acid treated TiO ₂ ZnP-H₂P co-sensitized photoanode was employed (device E). The increased PCE value of device E has been attributed to an enhanced J_sc value (=13.45 mA/cm²), which resulted from an increased dye loading, and an enhanced V_oc value (=0.76 V), attributed to an upward shift and increased of electron density in the TiO ₂ CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device E revealed an enhanced electron transport rate in the formic acid treated TiO₂ photoanode, suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as shorter electron transport time (τ_d), and longer electron lifetime (τ_e).

Full text of DOI:10.1016/j.orgel.2014.03.033

Organic Electronics 15 (2014) 1324–1337
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Stepwise co-sensitization as a useful tool for enhancement
of power conversion efficiency of dye-sensitized solar cells:
The case of an unsymmetrical porphyrin dyad and a metal-free
organic dye
G.D. Sharma a, , G.E. Zervaki b, P.A. Angaridis b, A. Vatikioti b, K.S.V. Gupta c, T. Gayathri c,
⇑
c
c
P. Nagarjuna c, Surya Prakash Singh c, , M. Chandrasekharam , Ajita Banthiya ,
⇑
K. Bhanuprakash c, A. Petrou b, A.G. Coutsolelos b,
⇑
a R & D Center for Engineering and Science, JEC Group of Colleges, JEC Campus, Kukas, Jaipur 303101, Rajasthan, India
b Laboratory of Bioinorganic Chemistry, Department of Chemistry, University of Crete, Voutes Campus, P.O. Box 2208, 71003 Heraklion, Crete, Greece
c Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Tarnaka, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A tertiary arylamine compound (DC), which contains a terminal cyano-acetic group in one of
its aryl groups, and an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H2[Porph]
(ZnP-H2P), where Zn[Porph] and H2[Porph] are metallated and free-base porphyrin units,
respectively, and L is a bridging triazine group functionalized with a glycine moiety, and were
synthesized and used for the fabrication of co-sensitized dye-sensitized solar cells (DSSCs).
The photophysical and electronic properties of the two compounds revealed spectral absorp-
tion features and frontier orbital energy levels that are appropriate for use in DSSCs. Follow-
ing a stepwise co-sensitization procedure, by immersing the TiO2 electrode in separate
solutions of the dyes in different sequence, two co-sensitized solar cells were obtained:
devices C (ZnP-H2P/DC) and D (DC/ZnP-H2P).The two solar cells were found to exhibit power
conversion efficiencies (PCEs) of 6.16% and 4.80%, respectively. The higher PCE value of device
C, which is also higher than that of the individually sensitized devices based on the ZnP-H2P
and DC dyes, is attributed to enhanced photovoltaic parameters, i.e. short circuit current
(Jsc = 11.72 mA/cm2), open circuit voltage (Voc = 0.72 V), fill factor (FF = 0.73), as it is revealed
by photovoltaic measurements (J–V curves) and by incident photon to current conversion
efficiency (IPCE) spectra of the devices, and to a higher total dye loading. The overall perfor-
mance of device C was further improved up to 7.68% (with Jsc = 13.45 mA/cm2, Voc = 0.76 V,
and FF = 0.75), when a formic acid treated TiO2 ZnP-H2P co-sensitized photoanode was
employed (device E). The increased PCE value of device E has been attributed to an enhanced
Jsc value (=13.45 mA/cm2), which resulted from an increased dye loading, and an enhanced
Voc value (=0.76 V), attributed to an upward shift and increased of electron density in the
TiO2 CB. Furthermore, dark current and electrochemical impedance spectra (EIS) of device
E revealed an enhanced electron transport rate in the formic acid treated TiO2 photoanode,
suppressed electron recombination at the photoanode/dye/electrolyte interface, as well as
Received 10 December 2013
Received in revised form 21 March 2014
Accepted 21 March 2014
Available online 5 April 2014
Keywords:
Co-sensitization
Dye sensitized solar cells
Formic acid treated TiO2 photoanode
Power conversion efficiency
shorter electron transport time (
s
d), and longer electron lifetime (
se).
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding authors. Tel.: +30 2810545045 (A.G. Coutsolelos).
E-mail addresses: gdsharma273@gmail.com (G.D. Sharma), spsin-
gh@iict.res.in (S.P. Singh), coutsole@chemistry.uoc.gr (A.G. Coutsolelos).
http://dx.doi.org/10.1016/j.orgel.2014.03.033
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1325
sensitizers that exhibit strong and complementary absorp-
tion profiles [10,19]. Various combinations of dyes have
been used for DSSC co-sensitization, resulting in enhanced
photovoltaic performance relative to the systems sensi-
tized by single dyes. These include ruthenium (II) com-
plexes with organic dyes [11,12] and metal free organic
dyes [13,14], and phthalocyanines with organic dyes [15].
Porphyrins, due to their strong and moderate absorptions
in the regions of 400–500 nm (Soret band) and 550–
700 nm (Q band), respectively, are considered to be ideal
candidates for improvement of photovoltaic performance
via co-sensitization [16,17]. When they are co-adsorbed
on TiO2 films with other dyes with a complementary
absorption profile, panchromatic sensitization can be
achieved, leading to significant improvement of the solar
cell efficiencies [18–20]. In a particular case, co-sensitiza-
tion of an appropriately functionalized ‘‘push–pull’’ zinc-
1. Introduction
Dye-sensitized solar cells (DSSCs) is an emerging solar
technology that is considered to be an alternative to the
conventional first and second generation photovoltaic
devices, due to their simple structure, low cost, ease of fab-
rication, and high power conversion efficiencies (PCEs).
Since the ground-breaking report of O’Regan and Grätzel
in 1991 [1], on the fabrication of an efficient solar cell
device based on a TiO2 electrode (anode) sensitized to vis-
ible light by an adsorbed poly-pyridyl ruthenium dye in
the presence of an Iꢀ/I3ꢀ redox electrolyte, extensive
research, both in theoretical and experimental level, has
been conducted in the area of DSSCs in order to understand
the processes in a DSSC device, and to improve their effi-
ciency, durability and processability [2–6].
Among the various factors that influence the efficiency
of DSSCs, the type of sensitizer is of great importance, since
it is responsible for fundamental processes in DSSCs, such
as light absorption, injection of electrons into the TiO2 con-
duction band (CB), regeneration by the redox electrolyte,
and electron recombination. Specifically, the light harvest-
ing efficiency of a DSSC, which is related to the absorption
profile of the sensitizer, plays a significant role in the gen-
erated photocurrent. An ideal sensitizer should strongly
absorb light in a wide spectral region. In an effort to
increase the light harvesting efficiency in DSSCs, a variety
of different types of sensitizers has been utilized, which
include metal–organic, organic, and porphyrin dyes. The
most efficient dyes that have been reported to date are
based on ruthenium polypyridyl complexes, which convert
solar energy to electricity with an efficiency of 11% [7].
However, despite the fact that they exhibit a wide absorp-
tion range from visible to near infrared regions [8], their
success has been mostly attributed to the appropriately
positioned energy levels of their frontier molecular orbitals
with respect to the TiO2 CB, which result in electron injec-
tion that is faster than recombination. In addition, their
high cost and environmental concerns limit their wide
commercial application.
An effective approach to extend the light harvesting
efficiency of DSSCs is to design and synthesize molecular
sensitizers that efficiently absorb sunlight from the visible
to the near IR region so as to exploit photons of longer
wavelengths. For example, it has been estimated that by
utilizing a molecular sensitizer that absorbs ꢁ80% of inci-
dent solar light in the 350–900 nm region (in a DSSC with
Iꢀ/Iꢀ3 redox electrolyte), an overall efficiency of 15% should
be expected [9]. However, the synthesis of compounds that
absorb light over such a wide spectral range (panchromatic
sensitizers) is very difficult and expensive, as it may
involve complex, reaction and purification procedures. Fur-
thermore, molecular sensitizers with very wide absorption
profiles may not be able to provide efficient electron injec-
tion into the TiO2 CB. Due to the fact that they exhibit small
band gaps, their LUMO energy levels might be so close to
the TiO2 CB, that the there would not be sufficient thermo-
dynamic driving force for electron injection.
metallated porphyrin (with a donor-p-acceptor, D-p-A,
architecture) in combination with a metal-free organic
dye raised the overall efficiency of DSSCs to the record
value of 12.3% [21]. A similar, but not so effective method
for improvement of the photovoltaic characteristics of
DSSCs is the co-adsorption of molecules, such as chenode-
oxycholic acid (CDCA), that reduce the aggregation of the
sensitizer on the TiO2 surface. The formation of dye aggre-
gates significantly decreases the efficiency of electron
injection, and causes an adverse effect on the overall per-
formance of DSSCs. However, since co-adsorbents are gen-
erally used in large amounts, their crucial drawback is that
they occupy large section of the TiO2 surface without con-
tributing to light absorption [22].
Continuing our studies on sensitization of solar cells por-
phyrin dyes, we recently reported the synthesis of an
unsymmetrical porphyrin dyad of the type Zn[Porph]-L-H2-
[Porph], ZnP-H2P, where Zn[Porph] and H2[Porph] are met-
allated and free-base porphyrin units, respectively, and L is a
bridging triazine group functionalized with a glycine moiety
(Scheme 1) [23]. The ZnP-H2P based solar cell device
resulted in a relatively low PCE value of 4.46%, apparently
due to the lack of absorption in the 450–550 nm region. In
order to improve the light harvesting efficiency of that DSSC,
we synthesized a metal-free organic dye, i.e. a tertiary aryl-
amine compound, which contains a cyano-acetic group in
one of its aryl groups and a D-p-A architecture (DC)
(Scheme 1b), and used it as a co-sensitizer along with the
ZnP-H2P. By immersing the TiO2 photoanode successively
in solutions of the ZnP-H2P and DC dyes, the PCE value of
the resulting ZnP-H2P/DC co-sensitized device was
improved (from 4.56% for the ZnP-H2P-sensitized solar cell
and4.06%fortheDC-sensitizedsolarcell)to6.16%. Theover-
all efficiency of the co-sensitized solar cell was further
improved up to 7.68%, by utilizing a formic acid treated
TiO2 photoanode.
2. Experimental methods
2.1. General methods and materials
A second approach for extending the light harvesting
efficiency of DSSCs involves the co-sensitization of the
TiO2 film of the photoanode with two or more molecular
All manipulations were carried out using standard
Schlenk techniques under nitrogen atmosphere. All chem-
icals and solvents were purchased from usual commercial

1326
G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
Scheme 1. Structures of porphyrin dyad ZnP-H2P and metal-free organic dye DC.
sources and used as received, unless otherwise stated. Sol-
vents were distilled from appropriate reagents.
olution mass spectrometry was performed using LCQ ion
trap mass spectrometer (Thermo Fisher, Sanjose, CA,
USA) equipped with an ESI source. FTIR spectra were
recorded on a Perkin Elmer 16PC FTIR spectrometer.
UV–vis absorption spectra were measured on a Shimadzu
UV-1700 spectrophotometer using 10 mm path-length
cuvettes. Cyclic voltammetry experiments were carried
out at room temperature using an AutoLab PGSTAT20
potentiostat and appropriate routines available in the
operating software (GPES, version 4.9). Measurements
were carried out in freshly distilled and deoxygenated
CH2Cl2, at a rate of 100 mV/s with a solute concentration
of 1.0 mM in the presence of tetrabutylammonium hexa-
Porphyrin dyad ZnP-H2P, namely 5-{4-[3-(5-(4-amino-
phenyl)-10,15,20-triphenyl-porphyrinato)-5-glycine-triaz-
inyl]-aminophenyl}-10,15,20-triphenyl-porphyrin zinc, was
prepared from cyanuric chloride by adding successively
5-(4-aminophenyl)-10,15,20-triphenyl-porphyrinato zinc
Zn[TPP-NH2],5-(4-aminophenyl)-10,15,20-triphenyl-por-
phyrin H2[TPP-NH2] (prepared according to literature pro-
cedures [24]), and glycine methyl ester hydrochloride in
dry THF, in the presence of DIPEA, followed by basic hydro-
lysis with KOH, as described in our earlier communication
[21]. Satisfactory analytical and spectroscopic characteris-
tics were obtained.
fluorophosphate (0.1 M) as supporting electrolyte.
A
three-electrode cell setup was used with a platinum work-
ing electrode, a saturated calomel (SCE) reference elec-
trode, and a platinum wire as counter electrode.
2.2. Synthesis
2.2.1. Metal-free organic dye DC
To a solution of 4-(bis (9,9-dimethyl-9H-fluoren-3-yl)a-
mino) benzaldehyde (0.100 g, 0.197 mmol) and cyanoacetic
acid (0.025 g, 0.295 mmol) in acetic acid (5 mL), ammonium
acetate (10% mol) was added, under nitrogen atmosphere.
The reaction mixture was refluxed for 12 h and then cooled
to room temperature. Addition of ice-cold water resulted in
the precipitation of a solid, which was filtered-off, washed
with hexane (30 ml) and dried under vacuum. The product
was isolated as red solid in 76% yield.
2.4. Computational methods
Quantum chemical calculation were performed within
the frame work of density functional theory (DFT) [25],
using the GAUSSIAN 09 program suite [26]. Gas phase
geometry optimizations were carried out by employing
Becke three parameter exchange in conjunction with
Lee–Yang–Parr correlation functional (B3LYP) [27,28],
and the 6-311G (d,p) basis set. The optimized minimum-
energy structures were verified as stationary points on
the potential energy surface by vibrational frequency anal-
ysis calculation. Computed structures and molecular orbi-
tals were visualized and analyzed by Gauss View
software [29].
1H NMR (300 MHz, CDCl3):d(ppm) 8.15(s, 1H),
7.90–7.93(m, 2H), 7.68–7.70(m, 4H), 7.30–7.43(m, 8H),
7.11–7.22(m, 4H), 1.43(s, 12H).
13C NMR (75 MHz, CDCl3): d(ppm): 155.49,
154.98, 153.66, 153.05, 144.87, 138.36, 136.73, 133.64,
127.19, 127.10, 127.04, 126.86, 126.37, 125.09,
124.16, 123.17, 122.55, 120.99, 120.52, 119.85,
119.68,119.59,119.51,46.95, 26.96. IR (KBr, cmꢀ1): 3442,
2956, 2924, 2853, 2215, 1738, 1577, 1506, 1448, 1348,
1313, 1277, 1217, 1181, 829, 736. MS(ESI) m/z: 595
(M + Na)+.
2.5. Preparation of TiO2 electrodes and DSSCs
The DSSCs were fabricated using electrodes based on
fluorine doped tin oxide (FTO) glass substrates, which were
pre-cleaned by sonication in decon 90, distilled water, iso-
propanol (i-PrOH), ethanol (EtOH), and dried in air.
2.3. Characterization methods
The working electrodes were prepared by firstly form-
ing a blocking layer of 0.2 M titanium di-isopropoxide
bis(acetylacetonate) in i-PrOH by spray pyrolysis on
pre-cleaned FTO coated glass substrates, followed by the
1H and 13C NMR spectra were recorded on Bruker DPX-
300 MHz spectrometer as solutions in deuterated solvents
by using the solvent peak as the internal standard. Low res-

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1327
deposition of nano-crystalline layer of TiO2 using doctor
blade technique, by employing a dye sol TiO2 paste (DSL
18NR-T). The TiO2 coated electrodes were heated at
500 °C for 30 min, and then cooled at room temperature.
For the preparation of the formic acid treated TiO2 pho-
toanode, the TiO2 electrode was treated with a 0.05 M for-
mic acid solution in EtOH for 1 h, washed with EtOH, and
finally dried in ambient conditions. The thicknesses of
the TiO2 electrodes were measured using a thin film thick-
ness measurement system and they were found to be in
azine group. ZnP-H2P is also functionalized at the third
position of the triazine ring by a glycine moiety with a ter-
minal carboxylic acid group, which can be used as binding
group for anchoring onto the TiO2 surface of DSSC elec-
trodes. The synthesis of ZnP-H2P (described in our earlier
report [17]) was achieved by stepwise amination reactions
of cyanuric chloride with successive additions of Zn[TPP-
NH2],H2[TPP-NH2], and glycine methyl ester hydrochloride
in THF, in the presence of DIPEA, followed by basic hydro-
lysis with KOH.
the range of 10–12
l
m. The dye solutions were prepared
Metal-free organic dye DC (Scheme 1) is a tertiary
amine consisting of two dimethylfluorenyl groups and a
phenyl cyanoacetic group. Taking into account that the for-
mer are two electron donating groups (A), while the latter
is an electron accepting group (A), DC can be considered as
having the donor–acceptor, D–A, architecture. DC was syn-
thesized following the stepwise procedure illustrated in
Scheme 2. The intermediate 4-(bis(9,9-dimethyl-9H-fluo-
ren-3-yl)amino) benzaldehyde was synthesized according
to previously published procedures in 90% yield [30]. In
the final step, the aldehyde was converted into the DC
dye through a Knoevenagel reaction by treating an acetic
acid solution of the aldehyde with cyanoacetic acid in
110 °C for 12 h, using a catalytic amount of ammonium
acetate. 1H and 13C NMR, and ESI measurements confirmed
the identity of the resulting compound.
as follows: 3 ꢂ 10ꢀ4 M ZnP-H2P in CHCl3/EtOH (1/1 vol-
ume ratio) and 5 ꢂ 10ꢀ4 M DC in THF. Photoanodes sensi-
tized with single dyes were prepared by immersing the
TiO2 electrode into the respective solutions of dyes ZnP-
H2P and DC for 4 h, rinsed with EtOH and dried in air.
For the co-sensitization, the pure TiO2 and formic acid trea-
ted TiO2 photoanodes were dipped into the solution of the
first dye for 4 h, rinsed with EtOH, and then dipped into the
solution of the second dye for 4 h, rinsed with EtOH, and
dried in air. The counter electrode was prepared by spin
coating a H2PtCl4 solution (2 mg in 1 mL of i-PrOH) onto
a pre-cleaned FTO coated glass substrate and then heating
at 450 °C for 15 min in air. The sensitized working elec-
trode was assembled with the Pt coated FTO electrode into
a sandwich type cell and sealed with a hot-melt polymer
surlyn.
To complete the DSSC fabrication, the electrolyte solu-
tion containing LiI (0.05 M), I2 (0.03 M), 1 methyl-3-n-pro-
pylimidazolium iodide (0.6 M) and 0.5 M tert-
butylpyridine in a mixture of acetonitrile and valeronitrile
(85:15 volume ratio) was introduced into the space
between the two electrodes through the drilled hole on
Pt coated FTO by vacuum backfilling.
3.2. Photophysical properties
The UV–vis absorption spectra of the porphyrin dyad
ZnP-H2P and the metal-free organic dye DC in solutions
(0.3 mM in CHCl3/EtOH = 1/1) are shown in Fig. 1 (black
and grey color1 lines, respectively). ZnP-H2P exhibits the
typical porphyrin absorption bands, with an intense Soret
band in the 400–440 nm range and two moderate absorp-
tion bands in the 520–600 nm range. The broadening and
splitting of the Soret band can be attributed to a strong exci-
ton coupling between porphyrin units of different dyads in
their exited states or to a lowering of molecular symmetry
[31]. In case of DC, the UV–vis absorption spectrum displays
a broad and intense absorption band centered at 456 nm,
2.6. Photovoltaic measurements
The current–voltage (J–V) characteristics of the DSSCs,
in dark and under illumination, were measured using a
computer controlled Keithley source meter (model 2601
A). A solar simulator TS space system class AAA was used
to give an irradiance of 100 mW/cm2 at the surface of the
device under standard air mass (AM1.5). The incident pho-
ton to current efficiency (IPCE) spectra of the DSSCs was
measured using a Bentham IPCE system (TMc 300 mono-
chromator computer controlled). The Electrochemical
impedance spectra (EIS), in dark and under illumination,
were recorded using a CHN electrochemical workstation,
by applying a dc bias equivalent to the open circuit voltage
of the DSSC, in the frequency range of 0.1–105 Hz.
with a molar extinction coefficient of 34,500 Mꢀ1 cmꢀ1
,
which could be ascribed to an intramolecular charge transfer
(ICT) between the bis-(fluoren-3-yl)amino group (donor)
and the cyanoacetic group (acceptor) of the molecule.
In Fig. 2, the UV–vis absorption spectra of ZnP-H2P and
DC adsorbed onto the surface of TiO2 films of 10–12 thick-
ness are depicted (black and red color lines, respectively).
They display similar absorption features with the corre-
sponding spectra of the dyes in solution, but these are
broader and redshifted. The UV–vis spectrum of the co-
sensitized TiO2 film with both dyesZnP-H2P and DC (blue
color line) exhibits enhanced, broad light absorption in
the 400–650 nm region, compared with the spectra of the
individually sensitized TiO2 films. In addition, in the co-
sensitized TiO2 film, the absorption band that corresponds
to the Soret band of ZnP-H2P is broader. This suggests that
3. Results and discussion
3.1. Molecular structures and syntheses of dyes
As shown in 1, porphyrin dyad ZnP-H2P consists of two
meso-phenyl-substituted porphyrin units ZnP and H2P
(where ZnP and H2P are the zinc-metallated and free-base
forms of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin,
respectively), which are covalently bridged by a 1,3,5-tri-
1
For interpretation of color in Fig. 1, the reader is referred to the web
version of this article.

1328
G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
Scheme 2. Synthesis of metal free dye DC.
Fig. 1. Normalized UV–vis absorption spectra of ZnP-H2P and DC in CHCl3/EtOH (=1/1) solution.
the co-adsorption of DC causes a change of the orientation
of the molecules of ZnP-H2P adsorbed on the TiO2 surface,
that results in decrease of their aggregation, as a conse-
quence of steric repulsions with the co-adsorbed DC mole-
cules [16,32].
spond to their lowest unoccupied molecular orbital, LUMO,
energy levels), should be higher than the TiO2 CB edge
(ꢀ0.5 V vs NHE). Furthermore, for efficient dye regenera-
tion (after dye photo-excitation and electron injection),
the E1ox values of the dyes (which correspond to their high-
est occupied molecular orbital, HOMO, energy levels),
should be lower than the redox potential of Iꢀ/I3ꢀ couple
(+0.4 V vs NHE). As shown in Fig. 3, in which the E1ox and
E1red values of the dyes ZnP-H2P, DC, together with the
potentials of the TiO2 CB edge and the redox potential of
Iꢀ/Iꢀ3 couple, are depicted, both criteria are met, and,
therefore, there is sufficient thermodynamic force for elec-
tron injection and regeneration of the photo-oxidized dyes
[33].
3.3. Electrochemical properties
The redox properties of the porphyrin dyad ZnP-H2P
and the metal-free organic dye DC were investigated by
cyclic voltammetry measurements, in order to evaluate if
there are favorable energy offsets of the dyes with respect
to the TiO2 film and the electrolyte.
Porphyrin dyad ZnP-H2P exhibits a quasi-reversible
oxidation at E1ox = +1.16 V vs NHE and a quasi-reversible
reduction at E1red = ꢀ0.89 V vs NHE [21]. Metal-free organic
dye DC exhibits analogous oxidation and reduction pro-
cesses at E1ox = +0.80 V vs NHE and Er1ed = ꢀ1.25 V vs NHE.
For efficient electron injection from the photoexcited dyes
into the TiO2 CB, the E1red values of both dyes (which corre-
3.4. Theoretical calculations
To get a better insight into the electronic structures of
the dyesZnP-H2P and DC, DFT calculations at the B3LYP/
6-311G(d,p) level of theory were employed. The results

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1329
Fig. 2. Normalized UV–vis absorption spectra of ZnP-H2P, DC and ZnP-HP/DC adsorbed onto TiO2 films.
Fig. 3. Electrochemical potential diagram of ZnP-H2P, DC, TiO2 CB edge, and Iꢀ/Iꢀ3 couple, showing the feasibility of electron injection and dye regeneration
processes.
of the calculations for ZnP-H2P [21], revealed a ‘‘butterfly-
like’’ molecular geometry, with the porphyrin units being
in an almost perpendicular orientation with respect to
the triazine ring. The HOMO and LUMO electron densities
were found to be localized on the porphyrin units and
not on the carboxylic acid anchoring group of the glycine
moiety.
The HOMO–LUMO band gaps of both dyes ZnP-H2P and
DC, as estimated by DFT calculations, are in good agree-
ment with those obtained from the electrochemically
observed oxidation Eox and reduction potentials Ered of
the respective compounds.
3.5. Photovoltaic properties
The ground state geometry optimized structure of DC
revealed a distorted trigonal pyramidal molecular geome-
try. As shown from the electronic density distributions of
the frontier orbitals (Fig. 4), the HOMO is delocalized over
the entire molecule, but in the LUMO most of the electronic
density is localized over the cyanoacetic anchoring group.
According to the Franck Condon principle, the overlap of
3.5.1. Photovoltaic properties of co-sensitized DSSCs
For the fabrication of the co-sensitized DSSCs, a step-
wise co-sensitization procedure was followed, by sequen-
tially immersing the TiO2 electrode (with thickness of
10–12 lm) in separate solutions of the ZnP-H2P and DC
dyes [34]. In one case, the TiO2 photoanode was firstly
immersed into the ZnP-H2P solution and secondly in the
DC solution resulting in device C (or device ZnP-H2P/DC),
while in another case, the reverse order was followed, i.e.
first in the DC and then in the ZnP-H2P solution, resulting
in device D (or device DC/ZnP-H2P). The optimum soaking
time for the TiO2 photoanode film in each one of the dye
the HOMO and LUMO levels by the
p-bridging nitrogen
atom enhances the electronic transition dipole moments
between the vibrational energy levels. This suggests that
the strong intramolecular charge transfer (ICT) nature of
DC upon illumination would result in efficient electron
injection from the excited dye to TiO2 CB.

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G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
Voc = 0.72 V, and FF = 0.73), and 4.80% (with Jsc = 10.08 -
mA/cm2, Voc = 0.68 V, and FF = 0.70), respectively.
The higher PCE value of the co-sensitized device C (or
device ZnP-H2P/DC), compared with to the individually
sensitized devices A and B, is attributed to the enhanced
photovoltaic parameters Jsc, Voc and FF. Particularly, the
enhanced Jsc value is ascribed to the enhanced IPCE
response of the cell, since they are related by the equation
Z
JSC
¼
e/ph;AM1:5GðkÞIPCEðkÞdk
ð1Þ
where e is the elementary charge and
u
ph,AM1.5 is the pho-
ton flux at AM1.5 [2,35]. The IPCE response of the solar cell
can be expressed by the equation
IPCEðkÞ ¼ LHEðkÞginjðkÞgccðkÞ ¼ ð1 ꢀ 10ꢀA
ÞginjðkÞgcc
ð2Þ
where LHE is the light harvesting efficiency,
ginj is the elec-
tron injection efficiency, gcc is the charge collection effi-
ciency, and is the absorbance of the photoanode.
A
Fig. 4. The HOMO and LUMO energy levels of DC and their corresponding
electronic density distributions.
Among the three factors, LHE plays a very important role,
and it depends on the absorption profiles of the co-sensi-
tizing dyes ZnP-H2P and DC (their molar absorption coef-
ficients), the relative amounts of dyes adsorbed onto the
TiO2 surface (dye loadings) of the photoanode, and the
optical path length within the electrode. In addition, as
discussed above, since the LUMO energy levels of both
ZnP-H2P and DC lie above the TiO2 CB edge, the electron
injection from the photo-excited states of dyes to the
TiO2 CB is thermodynamically favorable.
As shown in the UV–vis absorption spectra of the dyes
ZnP-H2P, DC, and ZnP-H2P/DC adsorbed onto TiO2 films
(Fig. 2), the DC sensitized film exhibits very strong absorp-
tion in the 460–530 nm range, where the absorption of the
ZnP-H2P sensitized film is weak. Hence, the absorption
spectrum of the ZnP-H2P/DC film exhibits an extended
absorption profile with panchromatic features, which can
lead to an increased LHE, and enhanced IPCE response for
the co-sensitized solar cell device C in the wide range of
370–670 nm, with the highest IPCE value of 70% at
ꢁ440 nm (Fig. 7, light blue color line).Comparison of the
solutions was found to be 6 h. For comparison purposes,
solar cells sensitized by the individual dyes ZnP-H2P
(device A) and DC (device B), were also fabricated, under
the same experimental conditions.
The current–voltage (J–V) characteristics, under illumi-
nation (AM1.5, 100 mW/cm2), of the solar cell devices A
and Bare shown in Fig. 5 (black and red color lines, respec-
tively), while those of the solar cell devices C and D are
shown in Fig. 6 (black and red color lines, respectively).
The photovoltaic cell parameters of all devices, i.e. short
circuit current Jsc, open circuit voltage Voc, fill factor FF
and overall power conversion efficiency PCE, are summa-
rized in Table 1. The individually sensitized devices A
and B were found to exhibit PCE values of 4.56% (with
Jsc = 10.18 mA/cm2, Voc = 0.66 V, and FF = 0.68), and 4.45%
(with Jsc = 9.76 mA/cm2, Voc = 0.68, and FF = 0.67), respec-
tively, while the co-sensitized solar cell devices C and D
showed PCE values of 6.16% (with Jsc = 11.72 mA/cm2,
Fig. 5. Current–voltage (J–V) characteristics, under illumination, of devices A and B.

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1331
Fig. 6. Current–voltage (J–V) characteristics, under illumination, of co-sensitized devices C, D and E.
Table 1
as in device A. The observed IPCE enhancement may be
attributed to the improved charge collection efficiency,
which is due to dye DC impeding the electron leakage by
an increased molecular surface coverage upon co-
sensitization.
Photovoltaic parameters, i.e. short circuit current (Jsc), open circuit voltage
(Voc), fill factor (FF), and photo conversion efficiency (PCE), of devices A, B,
C, D and E.
Device
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
A
B
C
D
E
10.18
9.05
11.72
10.08
13.45
0.68
0.66
0.72
0.68
0.76
0.68
0.66
0.73
0.70
0.75
4.56
4.06
6.16
4.80
7.68
The dye loading values of the co-sensitized solar cell
device C, as well as those of the individually sensitized
solar cell devices A and B, was determined. For device C,
the amounts of ZnP-H2P and DC adsorbed onto the TiO2
photoanode were found to be 1.9 ꢂ 10ꢀ7 and 1.8 ꢂ 10ꢀ7
mol/cm2 respectively, while for devices A and B, they were
found to be 2.1 ꢂ 10ꢀ7 and 3.4 ꢂ 10ꢀ7 mol/cm2, respec-
tively. The total dye loading in the co-sensitized device C
is higher than the dye loadings in the individually sensi-
tized solar cell devices A and B. Consequently, device C
exhibits an increased LHE value, and enhanced IPCE
response, and hence to a higher PCE value. This may be
understood in the context of the difference of the molecu-
lar sizes of ZnP-H2P and DC. Upon adsorption of the larger
size ZnP-H2P molecules onto the TiO2 surface in the first
IPCE spectra of devices A, B, and C in the wavelength region
below 500 nm shows that the IPCE response of the co-sen-
sitized device C is slightly enhanced. Considering that in
this wavelength region appears the Soret band absorption
of the porphyrin dyad ZnP-H2P, while the metal-free
organic dye DC does not display any absorption, co-sensi-
tization by DC does not contribute to the observed
enhanced photocurrent generation, and the LHE and
charge collection efficiency of device C should be the same
Fig. 7. Incident photon to current conversion efficiency (IPCE) spectra of devicesA, B, C, and E.

1332
G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
step of co-sensitization, the smaller size molecules of DC
are adsorbed onto the TiO2 film in such a way that they fill
the gaps between the porphyrin dyad molecules. As a
result, an increased total dye loading is obtained. The smal-
ler molecular size of DC is also the reason for the higher DC
loading in the individually sensitized device B than the
ZnP-H2P loading in device A, since a larger number of mol-
ecules can be accommodated in the same TiO2 surface.
However, in device C, the ZnP-H2P loading was found to
be higher than the DC loading. This is a result of the step-
wise co-sensitization procedure. In the ZnP-H2P saturated
TiO2 film, there is only a small surface area available for
binding of DC. In addition, the dye loading of ZnP-H2P in
the co-sensitized device C was found to be lower than that
in the individually sensitized device A (1.9 ꢂ 10ꢀ7 vs
2.1 ꢂ 10ꢀ7 mol/cm2, respectively). This may be attributed
to a partial detachment of ZnP-H2P molecules from the
TiO2 surface, after immersing the ZnP-H2P sensitized
TiO2 film into the DC solution, in the second step of the
co-sensitization procedure. The DC molecules, due to the
presence of two anchoring groups, the carboxylic acid
and the cyano group in their structures [12,36], exhibit a
more effective binding capacity to the TiO2 surface, relative
to the ZnP-H2P molecules, which contain one carboxylic
acid anchoring group, and therefore they replace some of
the already bound ZnP-H2P molecules.
In case of co-sensitized device D (or device DC/ZnP-
H2P), the PCE value was found to be lower than that of
co-sensitized device C, and marginally higher than those
of the individually sensitized solar cell devices A and B.
This can be attributed to the lower photovoltaic parameter
values Jsc, Voc and FF of the device (Table 1 and Fig. 6).
The IPCE response of device D in the 570–630 nm
region is higher than that for device C, resulting in an
enhanced photocurrent generation. Since in this wave-
length region appears the Q band absorption of the por-
phyrin dyad ZnP-H2P and DC does not absorb light, the
improved IPCE response is a result of the enhanced LHE
value of the co-sensitized solar cell due to ZnP-H2P co-sen-
sitization. Upon immersing the DC saturated TiO2 into the
solution of ZnP-H2P, a small number of DC molecules
adsorbed onto the TiO2 surface are replaced by ZnP-H2P
molecules.
and leads the electrons to the outer circuit. The important
photovoltaic parameters Jsc, Voc, and FF are related to the
physical and electronic properties of the TiO2 film. Partic-
ularly, Jsc depends on the amount of dye loading on the
TiO2 surface, the efficiency of the electron injection from
the dye into the TiO2 CB, as well as the charge collection
at the counter electrode. Voc is governed by the energy
difference between the quasi Fermi level of TiO2 and
the redox potential of electrolyte. Moreover, both Jsc and
Voc are affected by the recombination kinetics at the
TiO2/dye/electrolyte interface. Therefore, any energy shift
of the TiO2 CB edge affects the molecular and electronic
interaction between the sensitizing dye molecules and
TiO2 surface, electrolyte component, and the surface mod-
ification of the TiO2 [37,38], and consequently influences
both Voc and Jsc of
a DSSC, affecting its overall
performance.
In an effort to enhance the photocurrent generation and
reduce the back electron transfer at TiO2/dye/electrolyte
interface a variety of methods have been employed. These
include addition of additives to electrolyte [39], use of a
blocking/tunneling layer at TiO2 surface [40], and surface
treatment or modification of the TiO2 film [41]. A success-
ful method that has been recently introduced is the modi-
fication of the interface between the FTO glass substrate
and the TiO2 film, utilizing self-assembled monolayer
(SAM) with multiple carboxylic acid functional groups
[42,43]. Along these lines, we investigated the effect of
treating the TiO2 photoanode of the co-sensitized solar cell
ZnP-H2P/DC device (device C) with formic acid on the
overall photovoltaic performance.
The current–voltage (J–V) characteristics, under illumi-
nation (AM1.5, 100 mW/cm2), of the device with the for-
mic acid treated TiO2 ZnP-H2P/DC co-sensitized
photoanode (device E) are depicted in Fig. 6 (blue color
line) and the corresponding photovoltaic parameters are
listed in Table 1. Its PCE value was found to be 7.68%,
which is higher than the PCE value of device C. This is
attributed to the enhancement of the photovoltaic param-
eters (Jsc = 13.45 mA/cm2, Voc = 0.76 V, and FF = 0.75) after
the formic acid treatment of the TiO2 film.
As shown in Fig. 7, the IPCE response of device E (blue
color line) is enhanced throughout the studied UV–vis
wavelength range, compared to the IPCE response of device
C with the untreated TiO2 ZnP-H2P/DC co-sensitized pho-
toanode (light blue color line). This can be related to the
different dye loading values of the two devices. The overall
dye loading of device E was found to be higher than that of
device C, resulting in an improved IPCE response and
enhanced Jsc value.
Furthermore, the Voc value of device E was found to be
higher relative to device C. As mentioned above, the Voc
value of a DSSC depends on the energy difference between
the quasi Fermi level of TiO2 and the redox potential of the
electrolyte. Since in devices C and E the same electrolyte
was used, the difference between the Voc values is due to
the different quasi Fermi levels of TiO2 in the two photoa-
nodes. The variation of the quasi Fermi levels of TiO2 (EF) in
the two devices could arises either from a shift of the TiO2
CB edge (ECB) or from a change in the electron density (n) in
the TiO2 CB, as described in the following expression
The overall amount of dye adsorbed onto the TiO2 pho-
toanode of device D is lower than that for device C. This can
be related to the larger molecular size of ZnP-H2P with
respect to DC. After adsorption of the small size DC mole-
cules onto the TiO2 surface in the first step of co-sensitiza-
tion, only a small space of the TiO2 is available for
adsorption of the larger size molecules of ZnP-H2P. There-
fore, a small dye loading of ZnP-H2P is obtained. The
reduced total dye loading leads to a suppressed LHE value,
and a low IPCE response, and hence to a lower PCE value.
3.5.2. Photophysical properties of co-sensitized DSSCs with
formic acid treated TiO2 photoanode
Generally, in order to improve the performance of
DSSCs, a number of aspects should be considered. Among
them, the mesoporous semiconductor oxide film of the
photoanode, e.g. TiO2, is of great importance, since it sup-
ports the sensitizing dye, collects the injected electrons,

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1333
ꢀ
ꢁ
n
Nc
range (right side of spectrum) is related to the charge
transfer impedance of diffusion processes in the electrolyte
[47]. The charge transfer rates at each interface can be
compared on the basis of the radii of the semicircles: the
larger and smaller semicircle radii represent higher and
lower impedances or resistances (R) and, hence, they cor-
respond to slow and fast charge transfer rates, respectively.
The charge transfer resistance Rt values and the charge
recombination resistance Rrec values for devices C and E
are complied in Table 2. Device E with the formic acid trea-
ted TiO2 photoanode, compared to device C with pristine
TiO2, shows a lower charge transfer resistance Rt value at
the FTO/TiO2 interface (as it is shown from the smaller
radius of the high frequency range semicircle), suggesting
that the injected electrons from the excited state of dye
into TiO2 CB are easily transported into the FTO electrode.
Moreover, the radius of the semicircle that corresponds to
the TiO2/dye/electrolyte for device E has a larger radius
than that for device C, which signifies a higher charge
recombination resistance Rrec value for the former device.
In other words, in device E the recombination of the
injected electrons into the TiO2 CB with the electrolyte is
suppressed. In total, the reduced charge transfer resistance
Rt and the increased charge recombination resistance Rrec
values for device E lead to an improved Jsc value (as
observed from the J–V characteristics of the devices under
illumination).
EF ¼ ECB þ kBT ln
ð3Þ
where kBT is the thermal energy, and Nc is the density of
states in the TiO2 CB [2,44]. In other words, the improved
Voc value of the device E results from either an upward
shift of the TiO2 CB edge and/or an increase in the electron
density in the TiO2 CB of the formic acid treated
photoanode.
In order to get information about the shift of the TiO2 CB
edge in the device E, the optical band gaps (Eogpt) of both
untreated and formic acid treated TiO2 ZnP-H2P/DC co-
sensitized films were estimated. From the onset absorption
edges of the UV–vis spectra of the respective thin films, the
optical band gaps of untreated TiO2 and formic acid treated
TiO2 were calculated to be 3.22 and 3.27 eV, respectively.
Assuming that the TiO2 valence band is ꢀ7.4 eV (as
reported in literature), the CB edge of the untreated TiO2
and formic acid treated TiO2were found to be ꢀ4.18 and
ꢀ4.13 eV, respectively. Based on these values, it can be
concluded that the CB edge of the formic acid treated
TiO2 is shifted higher in energy by 0.05 eV, with respect
to the untreated TiO2. This shift may be due to the influ-
ence of the lower pH value and surface adsorbent in device
E [45].
Considering that the upward shift of 0.05 eV of the CB
edge of the formic acid treated TiO2 is small with respect
to the relatively large enhancement of 0.08 eV of the Voc
value of device E, it is conclude that the observed Voc value
should also be influenced by the electron density in the
TiO2 CB. The quasi Fermi level of TiO2 is affected by the
change of electron density of the dye sensitized electrode
during the illumination. Upon light absorption, electrons
from the photo-excited sensitizer are injected into the
TiO2 CB, leading to a significant change of the electron den-
sity in the TiO2 CB that depends on the light harvesting
efficiency of the sensitizer and electron injection efficiency
at the TiO2/dye interface. Given that the dye loading is
higher for the formic acid treated TiO2 photoanode, it is
expected that the electron density in the CB of the formic
acid treated TiO2 is higher than that with the untreated
TiO2. Hence, the combined effect of the upward shift of
the TiO2 CB edge and the increase of the electron density
in the TiO2 CB leads to an enhanced Voc value for device E.
In an effort to gain a better understanding of the
enhancement of the Voc value in the solar cell with the for-
mic acid treated TiO2 ZnP-H2P co-sensitized photoanode,
and to investigate the kinetics of the electron transport
and recombination processes [46], electrochemical imped-
ance spectra (EIS) of devices C and E in dark conditions, at a
dc bias equivalent to the Voc value, were obtained. The
Nyquist plots of the spectra of both solar cells are shown
in Fig. 8a (black and red color lines, respectively). In each
case, three semicircles are observed, which represent
charge transfer resistances at different interfaces of the
solar cells. The semicircle in the high frequency range (left
side of spectrum) corresponds to the charge transfer
impedance at the FTO/TiO2 interface, the semicircle in
the middle frequency range is associated with the charge
transfer and recombination impedance at TiO2/dye/elec-
trolyte interface, while the semicircle in the low frequency
The electron lifetimes (se) in devices C and E were cal-
culated from the Bode phase plots of the EIS spectra of
the two solar cells (Fig. 8b, black and red color lines,
respectively),according to the relationship
1
se
¼
ð4Þ
2pfmax
where fmax is the frequency at the maximum of the curve in
the intermediate frequency region in Bode phase plot, and
the results are listed in Table 2. The electron lifetime (se
)
for the device with the formic acid treated TiO2 ZnP-H2P
co-sensitized photoanode (device E) was found to be than
for that for the solar cell with the pristine TiO2 photoanode
(device C) (38.45 vs 32.64 ms). This difference might be
expected, since the adsorbed formic acid molecules onto
the TiO2 surface of device E acts as a barrier between the
semiconductor and electrolyte and therefore reduce the
back electron transfer.
The aforementioned results are also supported by dark
current–voltage (J–V) measurements of devices C and E,
which are presented in Fig. 9 (black and red color lines,
respectively). In device E, compared to device C, dark cur-
rent J is significantly reduced while the onset voltage shifts
towards higher voltage values. These results also suggest
that in device E the back recombination between the elec-
trons in the TiO2 CB and the electrolyte is suppressed in the
upon the formic acid treatment, giving rise to Voc and Jsc,
resulting in higher overall PCE value.
To get the information about the charge transport
mechanism in devices C and E, their EI spectra under illu-
mination, at a dc bias equivalent to the Voc value, were
obtained. In the Nyquist plots of these spectra three semi-
circles are also observed. The semicircle in the intermedi-

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G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
Fig. 8. (a) Nyquist plots (b) Bode phase plots of electrochemical impedance spectra (EIS) for devices C and E, in dark conditions.
Table 2
Electron transport resistance (Rt), recombination resistance (Rrec), electron
lifetime ( e), electron transfer time ( d), and charge collection efficiency
cc) for devices C and E.
The electron transport time (
trons in the TiO2 film are related with the electron lifetime
e), the charge transport resistance (Rt) and the charge
sd) of the injected elec-
s
s
(g
(s
Device Rt (Ohm-cm2) Rrec (Ohm-cm2) se (ms) sd (ms) gcc
recombination resistance (Rrec) according to the following
C
E
13.4
8.22
35.06
48.45
32.64
38.45
12.47
6.52
0.82
0.88
equation [50]
sd
Rt
¼
ð5Þ
se Rrec
The calculated values of the electron transport time (sd
)
ate frequency region corresponds to the charge transport
resistance at the TiO2/dye/electrolyte interface and gives
for devices C and E are listed in Table 2. The electron trans-
port time ( d) is a measure of the average time taken by the
s
information about the chemical capacitance (C ) that
l
injected electron to reach the collecting FTO electrode; a
faster electron transport time is associated with a higher
photocurrent [51]. The reduced charge transport resistance
Rt and increased charge recombination resistance Rrec val-
ues for device E, relative to device C, lead to a decreased
determines the accumulation of electrons in the TiO2 CB
upon illumination and, therefore, the position of quasi
Fermi level of TiO2 [48]. The chemical capacitance C value
l
for the device with the formic acid treated TiO2 ZnP-H2P
co-sensitized photoanode (device E) was found to be
higher than that for device C. This suggests that the elec-
tron density in the formic acid treated TiO2 is enhanced
compared to the pristine TiO2 under illumination. More-
charge transport (sd) value and indicate that electrons
are collected at the FTO electrode at a faster rate for the
formic acid TiO2 treated photoanode, compared to that
for the pristine TiO2 photoanode. Furthermore, the higher
PCE value of device E can also be evidently reflected in
over, the high value of chemical capacitance C for the for-
l
mic acid TiO2 electrode results in a negative shift of the
quasi Fermi level of TiO2, as it was recently reported in lit-
erature [49].
the charge collection efficiency (
calculated by the equation
gcc) of the device, which

G.D. Sharma et al. / Organic Electronics 15 (2014) 1324–1337
1335
Fig. 9. Dark current–voltage (J–V) characteristics of devices C and E.
ꢂ
ꢀ
ꢁꢃꢀ1
The PCE of the ZnP-H2P/DC co-sensitized device was fur-
ther improved up to 7.68% when a formic acid treated
TiO2 photoanode was used (device E), which was attrib-
uted to the enhanced Jsc and Voc values, due to modification
of the electronic properties of the TiO2 CB. Dark current
and EIS measurements of device E revealed an increased
electron transport rate and suppressed electron recombi-
nation in the formic acid treated TiO2 photoanode.
sd
se
gcc
¼
1 þ
ð6Þ
The charge collection efficiency (
and E are listed in Table 2. The higher
compared to device C, leads to an enhanced PCE value for
the device with the formic acid treated TiO2 photoanode.
g
cc) values of devices C
cc value for device E,
g
4. Conclusions
Acknowledgements
In summary, a tertiary arylamine compound (DC), with
a terminal cyano-acetic group in one of its aryl groups and
an unsymmetrical porphyrin dyad of the type Zn[Porph]-L-
H2[Porph] (ZnP-H2P), with a bridging triazine group (L)
that is functionalized with a carboxylic acid group of a gly-
cine moiety, were synthesized and used as sensitizers for
the fabrication of DSSCs. Photophysical measurements of
the two dyes showed broad, strong, and complementary
absorptions in their UV–vis spectra, while electrochemis-
try experiments, and DFT calculations revealed appropriate
frontier orbital energy levels for use in DSSCs. The ZnP-
H2P/DC and DC/ZnP-H2P co-sensitized solar cells (devices
C and D, respectively) were fabricated, following a step-
wise co-sensitization procedure, by immersing the TiO2
electrode in separate solutions of the dyes in different
sequence. The ZnP-H2P/DC co-sensitized solar cell device
was found to exhibit a significant improved PCE value
(=6.16%), compared to the individually sensitized solar
cells based on the ZnP-H2P and DC dyes (4.56% and
4.06%, respectively), which is attributed to enhanced pho-
tovoltaic parameters (as shown by the corresponding J–V
curves). The enhancement of the Jsc value of the device is
in good agreement with the its enhanced IPCE response,
which is ascribed to the improved light harvesting effi-
ciency of the co-sensitizing dyes ZnP-H2P/DC with respect
to the individual dyes. Bessho et al. have already reported a
PCE of 6.9% for the DSSCs based on the porphyrin dye
cosensitized with metal free organic dye [20] and same
research group reported a record PCE of 12.3% for similar
system [21]. Diau et al. have reported a PCE of 9% for the
DSSC based on a porphyrin dye and metal free dye [10].
Financial support from the European Commission (FP7-
REGPOT-2008-1, Project BIOSOLENUTI No. 229927) is
greatly acknowledged. This research has been also co-
financed by the European Union (European Social Fund–
ESF) and Greek national funds through the Operational
Program ‘‘Education and Lifelong Learning’’ of the National
Strategic Reference Framework (NSRF)-Research Funding
Program: Heraklitos II and Operational Program ‘‘Educa-
tion and Lifelong Learning’’ of the National Strategic Refer-
ence Framework (NSRF)-Research Funding Program:
THALIS-UOA-MIS 377252. Finally Special Research account
of the University of Crete is also acknowledged. KSVG
thanks UGC, New Delhi for Senior Research Fellowship.
SPS thanks to XII FY CSIR-INTELCOAT (CSC0114) for finan-
cial support.
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