Photophysical properties of near-IR cyanine dyes and their application as photosensitizers in dye sensitized solar cells

Article abstract of DOI:10.1016/j.ica.2017.08.001

The photophysical properties of eight structurally diverse near-infrared cyanine dyes were investigated with respect to their structural features and potential use as light-harvesting sensitizers of dye sensitized solar cells. The absorption, emission, and lifetime characteristics of the dyes were assessed under a variety of conditions. The photophysical properties and sensitizing efficiency of three of the eight dyes, with an additional six-membered cyclic ring, were markedly different from the other five. The dyes absorbed light of wavelength within the range of 779–823 nm and emitted within the range of 832–876 nm.The dyes had fluorescence quantum yield in the range of 14–22%. The surface morphology and elemental analysis of the cyanine dye sensitized photoanodes were characterized via Field-Emission Scanning Microscopy Imaging (FESEM) and Transmission Electron Microscopy (TEM). The performance of the fabricated cyanine dye sensitized solar cells was assessed via current-voltage and electrochemical impedance spectroscopic measurements. The solar-to-electric power conversion efficiency ranged from 0.04% to 0.24%.

Full text of DOI:10.1016/j.ica.2017.08.001

Accepted Manuscript
Research paper
Photophysical Properties of Near-IR Cyanine Dyes and their Application as
Photosensitizers in Dye Sensitized Solar Cells
William Ghann, Hyeonggon Kang, Edward Emerson, Jiyoung Oh, Tulio
Chavez-Gil, Fred Nesbitt, Richard Williams, Jamal Uddin
PII:
S0020-1693(17)30443-7
http://dx.doi.org/10.1016/j.ica.2017.08.001
ICA 17795
DOI:
Reference:
Inorganica Chimica Acta
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31 July 2017
1 August 2017
Please cite this article as: W. Ghann, H. Kang, E. Emerson, J. Oh, T. Chavez-Gil, F. Nesbitt, R. Williams, J. Uddin,
Photophysical Properties of Near-IR Cyanine Dyes and their Application as Photosensitizers in Dye Sensitized Solar
Cells, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.08.001
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Photophysical Properties of Near-IR Cyanine Dyes and their Application as
Photosensitizers in Dye Sensitized Solar Cells
William Ghann1, Hyeonggon Kang1, Edward Emerson1, Jiyoung Oh1, Tulio Chavez-Gil1, Fred
Nesbitt1, Richard Williams2 & Jamal Uddin*1
1 Center for Nanotechnology, Department of Natural Sciences, Coppin State University, 2500 W.
North Ave., Baltimore, MD21216
2Department of Chemistry, Morgan State University, 1700 E. Cold Spring Ln. Baltimore, MD
21251
*Correspondence and requests for materials should be addressed to J.U. (email:
juddin@coppin.edu)
ABSTRACT
The photophysical properties of eight structurally diverse near-infrared cyanine dyes were
investigated with respect to their structural features and potential use as light-harvesting
sensitizers of dye sensitized solar cells. The absorption, emission, and lifetime characteristics of
the dyes were assessed under a variety of condition. The photophysical properties and
sensitizing efficiency of three of the eight dyes, with an additional six-membered cyclic ring,
were markedly different from the other five. The dyes absorbed light of wavelength within the
range of 779 - 823 nm and emitted within the range of 832 – 876 nm. The surface morphology
and elemental analysis of the cyanine dye sensitized photoanodes were characterized via Field-
Emission Scanning Microscopy Imaging (FESEM) and Transmission electron microscopy (TEM).
The current-voltage characteristics and the electrochemical impedance spectroscopic studies
were also carried out to calculate solar cell efficiency. The dyes had fluorescence quantum yield
1

in the range of 14 – 22%. The solar-to- electric power conversion efficiency ranged from 0.04 %
to 0.24 %
Key words: NIR, Cyanine dyes, UV-VIS, Emission, Lifetime Decay, IR, FESEM, TEM, I-V and
Impedance, DFT calculation
1. Introduction
Dye sensitized solar cells (DSSCs) are a class of photovoltaic devices that have been widely
studied due to their advantages, such as ease of fabrication, low cost, and appreciable solar-to-
electric energy conversion efficiency [1-7]. DSSCs convert sunlight to electricity using dye
molecules which absorb photons and generate electrons [6, 8-10]. Many dye molecules,
especially ruthenium polypyridyl complexes, have been used in the fabrication of DSSC, but
their wavelength region is only in the UV-Visible range [11-13]. Others have used the co-
sensitization of dyes to reduce energy gap sand cover wider regions of the electromagnetic
radiation, but the method is quite burdensome [14-18]. Cyanine dyes are a group of dyes that
are not frequently explored for use in DSSC due to their relatively long absorption/emission
wavelengths [19-20]. They are fluorescent organic dyes made up of two nitrogen-containing
heterocycles joined together by a polymethine chain [21-28]. They have been used in various
biomedical applications as sensors and imaging agents and have specifically been used for real
time detection of cellular function or malfunction [23-25].
Cyanine dyes have shown promise as excellent photosensitizers for DSSC [4, 31-33]. It has been
shown that upon adsorption of cyanine dyes to TiO2 film, the absorption band of the cyanine
2

dye broadens to the red and blue regions in comparison with their absorption spectra of the
dyes in plain solvent. A larger absorption intensity and a broader absorption spectra insure a
greater light to electricity conversion efficiency, which clearly establish the proof of principle of
fluorescence redox-switching for cyanine derivatives, with more intense fluorescence in the
“on” state that is certainly desirable for any practical use compared to other systems developed
so far [10].
Heptamethine cyanine dyes, in particular, are promising additions to DSSC technology due to
their high molar absorption coefficient in the near infrared (near-IR) region. Heptamethine
cyanine dyes are dinitrogen heterocycles linked together by seven methine bridges. In this
study, eight heptamethine cyanine dyes, displayed in Figure 1, have been synthesized,
characterized and tested for potential DSSC applications. The heptacyanine chromophores,
structure were derived from reactions between 2, 3, 3-trimethylindolenine and eight different
alkyl halides. Three of the eight heptamethine products are formed by six-membered cyclic
ring, attached to a particular polymethine chain. The alkyl groups for the first five heptamethine
cyanine derivatives are without the additional six-membered ring are: methyl (Cy1), Ethyl (Cy2),
Propyl (Cy3), 2-hydroxy-ethyl (Cy4), and 4-carboxy-butyl (Cy5) Iodide. The remaining three bi-
phenyl alkyl three cyanine derivatives are P-methyl (Cy6), P-ethyl (Cy7), and P-butyl (Cy8)
iodide, with P=phenyl, respectively.
This study focuses on the photophysical properties of absorption, emission and fluorescence
lifetime measurements for the eight structurally diverse near-IR heptamethinecyanine dyes and
their potential as photosensitizers in the fabrication of DSSCs. The photophysical properties of
3

the cyanine dyes were compared by structure and their influence on the efficiency of DSSCs.
The photophysical properties of these near-IR cyanine dyes are summarized in Table 1.
2. Experimental Section
2.1 Materials
Titanium dioxide powder (Degussa P25) was purchased from the institute of chemical
education. Fluorine tin oxide (FTO) conducting glass slides were purchased from Harford glass
company, Hartford City, Indiana. Sodium Hydroxide (NaOH), acetone (C3H6O), ethanol
(C2H5OH), and acetic acid (CH3COOH) were purchased from Sigma-Aldrich and were used
without further purification. Graphite used in making cathode slides was purchased from TED
PELLA, INC.
The photoanode was prepared using a fluorine-doped SnO2 (FTO) conducting glass substrate.
The FTO glass slides were cleaned by first washing them a detergent solution and rinsing them
with water and ethanol. The FTO glass substrates were subsequently spin coated with TiO2
paste prepared from TiO2 powder, acetic acid, and soap water. The TiO2 coated FTO slides were
then annealed at 450 °C for an hour. Using Scanning Electron Microscopy cross-section imaging,
the thickness of the TiO2 layer was found to be approximately 9 μm. To prepare the cathode,
graphite paint was spread uniformly on the cleaned FTO glass and allowed to dry in room
temperature.
4

2.2 Synthesis of Cyanine Dyes
The near infrared cyanine dyes were prepared via a microwave assisted organic synthesis. The
protocol for the synthesis has been discussed elsewhere [19, 20, 22]. The protocol was
optimized using the Biotage single-mode microwave system. For each of the cyanine dyes, one
equivalence of N-((E)-(2-chloro-3-((E)-(phenylimino)methyl)cyclohex-2-enylidene)methyl)aniline
was made to react with 2 equivalence of the corresponding2,3,3-trimethyl-1-R-3H-indolium
iodide in the presence of sodium acetate and ethanol for 20 min at 120oC. Upon completion of
each reaction, the reaction vial was cooled to 0°C, filtered, and washed with diethyl ether to
obtain greenish-gold crystals with an average yield of 79%. The purified products were
characterized using 1H and 13C NMR [19, 20].
2.3 Fluorescence Lifetime Measurements
Cyanine dyes were dissolved in 3 mL of ethanol for fluorescence lifetime solution. To prevent
inner filter effect, absorption measurements were carried out to ensure the absorbance of the
dyes was less or equal to 0.15 absorbance unit. Fluorescence decays were measured using
Horiba Delta-flex fluorescence lifetime system using the time-correlated single-photon counting
(TCSPC) technique with the PPD-850 picosecond photon detection module. The excitation
source was 785 nm light-emitting diodes (Delta LED) with 785 nm.
2.4 Fabrication of Solar Cell
The photoanode was prepared by depositing a thin film of TiO2 on the conductive side of
fluorine doped tin oxide (FTO) glass using a spin coater and sintered at 380oC for 2 hours [34-
5

36]. The TiO2 covered FTO glass was then immersed in a freshly prepared 5mM cyanine dye
solution for a period of two hours. The counter electrode (cathode) was prepared by painting
FTO glass with colloidal graphite. The cyanine dye-sensitized glass (anode) and the carbon
electrode (cathode) were assembled to form a solar cell sandwiched with a redox (I-/I3-)
electrolyte solution.
2.5 Characterization Techniques
Steady-state absorption spectra of cyanine dyes in solution were acquired using UV-3600 Plus
from Shimadzu. Steady-state fluorescence spectra were recorded on the fluorescence Nanolog
Spectrofluorometer System from Horiba Scientific (FL3-22 iHR, Nanolog). ATR spectra were
obtained with a Thermo Nicolet iS50 FTIR. The morphology of each film was analyzed using field
emission scanning electron microscopy (FESEM; JSM-7100FA JEOL USA, Inc.). Transmission
Electron Microscopy (TEM) images were acquired on JEM-1400 Plus (JEOL USA, Peabody,
Massachusetts). The images were viewed using Digital Micrograph software from Gatan (Gatan,
Inc, Pleasanton, CA). HOMO and LUMO calculations were carried out using Spartan’14 software
from Wavefunction, Inc. Irvine, CA, USA. TiO2 paste was printed on FTO glass using WS-650
Series Spin Processor from Laurell Technologies Corporation.
2.6 Photovoltaic Properties Measurement
The energy efficiencies of the fabricated cyanine DSSCs were measured using 150 W fully
reflective solar simulator with a standard illumination with air-mass 1.5 global filter (AM 1.5 G)
having an irradiance corresponding to 1 sun (100 mW/cm2) purchased from Sciencetech Inc.,
6

London, Ontario, Canada and Reference 600 Potentiostat/Galvanostat/ZRA from Gamry
Instruments (734 Louis Drive, Warminster, PA 18974). The tested solar cells were masked to an
area of 5 cm2. Each cell performance value was taken as the average of three independent
samples. The solar energy to electricity conversion efficiency (η) was calculated based on the
equation, η = FF × Isc × Voc, where FF is the fill factor, Isc is the short-circuit photocurrent density
(mA cm–2), and Voc is the open-circuit voltage (V) as listed in Table 3.
3. Results and Discussion
The photophysical properties of the eight near-IR cyanine dyes were studied to assess their
usefulness as sensitizers for dye sensitized solar cells. Absorption and emission spectra
measurements, stoke shift calculation, quantum yield, molar absorptivity, and detection
sensitivity were determined and are summarized in Table 1. The properties of Cy1-Cy5 were
markedly different from the Cy6 to Cy8 which had the additional phenyl group.
3.1 UV-Vis Absorption Studies
The optical properties of the cyanine dyes were investigated using UV-vis spectrometry and the
results exhibited in Figure 2. The overall performance of a DSSC depends on the light absorption
capability of the dye sensitizer. The absorption characteristics of Cy1 to Cy5 were different from
the absorption characteristics of Cy6 to Cy8 which occurred at longer wavelength. The light
absorption bands shifted to longer wavelength by 40 nm with the increase in the length of
conjugated chain. The absorption peaks of cyanine dyes are known to shift to the red with
increasing lengths of the conjugated methine chain [10]. The attachment of the additional six-
membered ring increases the photostability of the near-IR cyanine dyes. The rich pi electron
7

conjugated region of the heptamethine cyanines increases stability in a way that translates into
stronger absorption at longer wavelengths. Near-IR absorption and large molar absorptivities
are two desired features of sensitizer dyes in DSSC systems. The wavelength of maximum
absorption for Cy1 to Cy5 was around 785 nm whereas the wavelength of maximum absorption
for Cy6 toCy8 was in the range of 818 - 823 nm. Both of these absorption wavelength regions
are desirable as good wavelengths for DSSC sensitizers as well as the accompanying large molar
absorption coefficients.
3.2 Steady State Fluorescence Studies
Fluorescence spectra of the cyanine dyes utilized in the fabrication of the DSSC were obtained
in order to examine the emission characteristics of the eight dyes. The measurements were
carried out with an exciting light (λexc) of 700 nm. The emission pattern of the first five cyanine
dyes (Cy1- Cy5) was slightly different from that of Cy6 to Cy8 as shown in Figure 3. While the
wavelengths of maximum emission for Cy1 - Cy5 were within the region of 840 nm -843 nm, the
wavelength of maximum emission for Cy6 -Cy8 was in the range of 869 – 876 nm. The Stokes
shifts at the longer wavelengths are interesting to note. This and the modestly low quantum
yields of these dyes, as presented in Table 1, suggest a substantial degree of self-quenching,
which competes with the excitation energy needed for electron capture at the anode. Self-
quenching occurs when these dyes aggregate. These dyes are known to aggregate, an
undesired property for sensitizers, even at low concentrations. However, the aggregation can
be minimized by the addition of the addition of long alkyl chains and /or polar constituents such
as carboxylic, phosphate, sulfonate, and or hydroxyl groups to the substituent regions or the
indolenic nitrogen atoms of the cyanine moles [33, 37]. These substituent groups, in particular,
8

the carboxylic and hydroxyl groups, also efficiently serve as anchors to the semiconductor
electrode (i.e. TiO2) surface [37].
Self-quenching and the broad absorption spectra of these dyes along suggest favorable Forster
Resonance Energy Transfer (FRET) interactions (provided stacking from dye aggregation can be
minimized), which can occur with highly energetic luminescent compounds and be used to
transfer additional energy to these dyes when they are used as sensitizers [38, 39].
3.3 Fluorescence Lifetime Studies
Fluorescence lifetime (FLT) measurements were carried out to evaluate the length of time the
molecules stay in the excited state and how lifetime impacts various parameters associated
with dye sensitized solar cells constructed with them. The structural relationship of cyanine
dyes to their fluorescence lifetimes has been studied by some number researchers [14]. A
scattering solution, ludox, was used to measure the instrument response time (IRF) before the
lifetime measurement of the samples. The FLT properties of the dyes are summarized in Table
2. With concentrated solutions, the photoluminescence decay show fast and a slower
component of lifetime of the near-IR cyanine dyes in ethanol solution. The two lifetimes
suggest the presence of two species in the solution. The shorter lifetime is associated with self-
quenching aggregation or coplanar “stacking”, while the longer lifetime is associated with
“free” dye [14]. However, with very dilute solution, a single exponential decay is observed as
shown in Figure in Figure 4 and Table 1. Four spectra, Cy4 representing the first five different
structures (Cy1- Cy5) and Cy6 - Cy8 are displayed in Figure4. In general, the indolenic structure
of Cy1 - Cy5 had a longer lifetime (0.18 ns - 0.68 ns) than the benzoindolenic structure of Cy6 –
9

Cy8 (0.32 ns – 0.43 ns). The short lifetimes of these near-IR dyes pose a potential problem
because of the implied competition with electron injection half times. Longer fluorescence
lifetimes and shorter electron injection half times are requirements for efficient DSSC’s,
especially in terms of long term stability a requirement for commercially successful solar cells
[40]. In addition to minimizing aggregation, the relationship between dye synthesizers lifetime
and electron injection halftimes must be optimized for the effective incorporation of near-IR
dyes in DSSC’s.
3.4 Field Emission Scanning Electron Microscopy Imaging
The morphological characteristics of the TiO2 electrode were further investigated using Field-
Emission Scanning Microscopy Imaging (FESEM), Energy dispersive X-ray spectroscopic (EDS),
and line profile map analysis. Cy5 was used for the SEM and EDS imaging studies because of its
carboxylate anchoring groups which interacts well with TiO2 film. The carboxylate groups
present in Cy5 enhance the adsorption of the dye to the titanium dioxide semiconductor
surface providing an efficient coupling for the ultrafast injection of electrons [35-38]. Figure 5
shows the FESEM microscopic morphology of the FTO slide with nanocrystalline mesoporous
TiO2 film and a layer of cyanine attached to the surface. No significant change in the surface
morphology of the samples was observed before and after dye application. Energy dispersive X-
ray spectroscopic (EDS) was also used for the chemical characterization of the cyanine dye
sensitized TiO2/FTO glass (Figure 5b) in comparison with the bare TiO2 coated FTO glass (Figure
5a). An intense peak for carbon in the dye-sensitized sample as compared to that of the blank
was observed. Cyanine dye contains a high percentage of carbon by virtue of the fact that it is
10

an organic compound. EDS mapping analysis of the cyanine dye sensitized TiO2/FTO slide
(Figure 5 c & d) shows presence of carbon indicative of the presence of organic compound and
also Titanium which is suggestive of the presence of TiO2. It is seen from the line profile image
(Figure 5 c &d) that the portion of TiO2 substrate covered with dye show huge peaks of carbon
(red) whereas the portion without dye mostly had only Titanium (green).
3.5 Transmission Emission Microscopy Imaging
To confirm the adsorption of the cyanine dyes on the TiO2 nanoparticles, the high-resolution
images were taken by transmission electron microscopy (TEM, JEM-1400, JEOL Co.) as shown in
Figure 6. To get the TEM images, after cyanine dye was dissolved in the ethyl alcohol, the TiO2
powder was added to the solution. The mixture was suspended for 20 min for the adsorption of
the TiO2 powder and the dye. After then, one drop of the dye mixture was dried on TEM grid in
a hood overnight. In addition, as a control, a mixture of TiO2 and ethyl alcohol was also dried on
another TEM grid. Anatase TiO2 nanoparticles are observed with a particle size range of 20 -25
nm as shown in Figure 6. Rutile TiO2 nanoparticles are also rarely observed (Figure 6(c)). Small
spherical dyes of 1 - 3 nm decorated on the anatase TiO2 nanoparticles are observed (Figure
6(a) and (b), the arrows in the images guide to the dyes), while any dye shapes are absent in the
control as shown in the Figure 6 (c) and (d). It is speculated that the adsorption between the
TiO2 nanoparticles and the dyes results from the electrostatic binding because the dyes do not
have any functional groups which are able to induce any binding between them.
11

3.6 IR Spectroscopy Analysis
The IR spectra of the films annealed at room temperature shows three vibrational modes which
are ascribed to TiO2 [39-41]. The IR measurements, as displayed in Figure 7, were carried out on
Cy4 and Cy6 basing of the fact that the behavior of the first five cyanine dyes (Cy1 – Cy5) are
slightly different from that of the last three dyes (Cy6 – Cy8). However, the differences between
the IR spectra of the two dye-sensitized TiO2 photoanodes were not significant. An interesting
observation in these studies was that the stretching intensities were increased with the
deposition of cyanine dyes, with a significant change on the TiO2 IR modes, which indicates that
there occurred an interaction that shifts to low energies the initially TiO2 stretching. This
observation shows that the deposition of cyanine dye undergoes strong phase transition due to
an electronic environment change around the TiO2 energies [32, 36, 37].
3.7 Current-Voltage Characteristics
The I-V characteristics curves of the all eight fabricated cyanine dye sensitized solar cell were
measured and analyzed as exhibited in Figure 8. The parameters related to the cell
performance such as the short circuit current, open circuit voltage, and fill factor are displayed
in Table 3. The I-V measurements were conducted under illumination of 100 mW/cm2. The
highest short circuit current density was obtained for the cell sensitized with Cy1. The higher
efficiencies observed in Cy1 to Cy5 may be due to the higher percentage of electron donating
groups compared to Cy6 to Cy8 as has been previously established for other cyanine molecules
[38].
12

3.8 Electrochemical Impedance Spectroscopy
The electrochemical impedance spectroscopy (EIS) of the fabricated cells was investigated. EIS
is a valuable technique for studying the kinetics and energetics of charge transport and
recombination in dye sensitized solar cells [44]. Impedance measurements were carried out at
frequencies between 0 and 100 KHz. Figure 9 shows the Nyquist plots and Figure 10, the Bode
plot of all the eight cyanine dye employed in the study. Cy2 (30Ω) and Cy3 (55Ω) have the least
resistance to current according to the Nyquist plot. Cy6 (130Ω) and Cy8 (110 Ω) had higher
resistance. The cyanine dyes with similar structures therefore have similar resistance. The two
phenyl groups in Cy6 to Cy8 increase the resistance to flow of current. Cy5 is however an
outliner shows the greatest resistance among all the dyes. This is also consistent to the IV data
that show Cy5 as having lower efficiency compared to all of the other cyanine dyes with similar
structures. Figure 10 shows the Bode phase plot for the cyanine dye sensitized solar cells. Cy2
and Cy3 show a relatively small capacitance compared to that of the other cyanine dyes. The
transfer resistance of Cy6 and Cy8 are far higher than that of the other cyanine dyes with the
exception of Cy5.
3.9 Density Functional Calculations for Cy5
Density functional calculations were used to optimize the geometry of Cy5 molecule using the
software Spartan 16 from Wavefunction. All geometries were computed using the ΩB97X-D
density functional theory method. The geometry optimization was performed using 6-31G*
basis set. This model was used to calculate the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) energies of Cy5. The calculations gave a result
for the HOMO of -8.86 eV and the result for the LUMO of -3.95 eV . The difference in the HOMO
13

and LUMO, the band gap, is 4.91 eV. Figure 11 (a & b) shows the Lewis and 3D structures of
Cy5. The HOMO and LUMO surfaces and orbital energy diagrams are shown in Figure 12 (a) and
Figure 12 (c), respectively. In Figure 12(a) and Figure 12(c) the blue and red regions represent
positive and negative values of the orbitals, respectively.
When exposed to radiant energy, dye molecules (S) adsorbed on the TiO2 film absorb photons
and make a transition from the ground state or the highest occupied molecular orbital (HOMO)
to an excited state or the lowest unoccupied molecular orbital (LUMO) state as displayed in
Figure 12. The photoexcited dye species (S*) injects an electron into the conduction band of
TiO2 electrode and becomes oxidized (S+). The oxidized dye species subsequently accept an
electron from the electrolyte (I-), and then the ground state of the dye (S) is restored. The
injected electron is transported through the mesoporous TiO2 film to the FTO layer and is
conducted through an external circuit to a load where the work done is delivered as electrical
energy. The electron from the external load diffuses to the cathode where it gets transferred to
-
the electrolyte (I3 ), so the electrolyte system is regenerated.
4. Conclusion
In this paper the photophysical properties of synthesized cyanine dyes were evaluated for
application as sensitizers for dye sensitized solar cells. Preparation and characterization of
cyanine dye sensitized solar cells are presented in this work. The devices were characterized
14

using absorption spectroscopy, Fluorescence spectroscopy, Field Emission Scanning Electron
Microscopy, and Transmission Electron Microscopy. The absorption spectra, electrochemical,
and photovoltaic properties were accordingly investigated. The performance characteristics of
the devices in terms of current and voltage measurements and Impedance measurement show
that near-IR heptamethine cyanines are promising sensitizer dye candidates for DSSC. Overall,
Cy1 - Cy5 were found to possess photophysical properties that were different from that of Cy6
to Cy8. In general, the indolenic cyanine dyes (Cy1 – Cy5) showed better photovoltaic
performance than the benzo indolenic dyes (Cy6 – Cy8). In all, a short-circuit photocurrent
density (Isc) in the range of 0.55 – 2.69 mAcm-2, an open-circuit photovoltage (Voc) of 170 - 390
mV, fill factor of 0.24–0.39, and corresponding conversion efficiency in the range of 0.04- 0.24
% was obtained under the simulated AM 1.5G solar light condition with irradiation of 100
mW/cm2. The relatively low efficiencies can be linked to the short lifetimes exhibited by the
dyes. When the excitation lifetimes of the sensitizers approach the half-life times of electron
injection, competition between the two processes becomes more relevant, and lower
efficiencies result. The lower efficiencies can also be linked to the absence of anchoring groups
on the dyes. The anchoring groups aid the adsorption of the dye to semiconductor TiO2 which
facilitates the injection of electrons.
15

Acknowledgement
The work was financially supported by the University of Maryland System Wilson E. Elkins
Professorship (2485897), Constellation, an Exelon Company, E2- Energy to Educate grant
program (163893), and Dept. of Education, SAFRA Title III Grant (2486005). The authors are also
grateful to the Institution of Advancement, Coppin State University, for administrative help. The
content is exclusively the responsibility of the authors and does not necessarily represent the
official views of the funding agencies.
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20

Cy1
Cy2
Cy5
I-
Cy6
I-
Cy3
Cy7
-
I
Cy8
Cy4
Figure 1: Structures of the eight cyanine dyes investigated: Cyanine 1 (Cy1); Cyanine 2 (Cy2);Cyanine
(Cy3); Cyanine 4 (Cy4); Cyanine 5 (Cy5); Cyanine 6 (Cy6); Cyanine 7 (Cy7); Cyanine 8 (Cy8)
21

1.2
1.0
0.8
0.6
0.4
0.2
0.0
Cy1
Cy3
Cy5
Cy7
Cy2
Cy4
Cy6
Cy8
600
650
700
750
800
850
900
Wavelength (nm)
Figure 2: Absorption spectra of eight cyanine dyes measured in ethanol (10 µM) showing differences in
absorption between Cy1 – Cy5 and Cy6 – Cy8
22

1.2
1
Cy1
Cy3
Cy5
Cy7
Cy2
Cy4
Cy6
Cy8
0.8
0.6
0.4
0.2
0
700
800
900
1000
1100
1200
Wavelength (nm)
Figure 3: Emission spectra of eight cyanine dyes measured in ethanol showing differences in absorption
between Cy1 – Cy5 and Cy6 – Cy8. Wavelength of excitation is 700 nm.
23

Figure 4: Fluorescence Lifetime Measurement of selected cyanine dyes: Cy4, Cy6, Cy7 &Cy8 and the
instrument response function. Blue curve: Instrument response function; black curve: fit for various
fluorescence decays.
24

a.
c.
b.
d.
Figure 5: SEM and EDS Analysis: (a) EDS of bare TiO2 coated FTO glass; (inset) TiO2 film partially covered
with cyanine dye (b) EDS of cyanine dye sensitized TiO2 /FTO glass; (inset) Cyanine dye sensitized TiO2
/FTO glass (c) Partially dyed TiO2 film/FTO glass for line profile analysis ;(d) Line profile analysis of
partially dyed TiO2 /FTO; insets for (a) and (b) corresponding to SEM images used for EDS analysis
25

(b)
(a)
(d)
(c)
Figure 6: High-resolution TEM images of the cyanine dye/TiO2 and the TiO2 nanoparticles: (a) (b) high
resolution TEM images of Cyanine dye/TiO2, and (c) (d) of TiO2 nanoparticles without dye
26

110
100
90
2360 cm-1
C
Cy
y
a
4
nine 4
Cy
Cy
a
6
nine 6
80
70
60
3310 cm-1
3420 cm-1
50
40
30
20
3900
3400
2900
2400
1900
1400
900
400
Wavenumber (cm-1)
Figure 7: FT-IR spectrum of Cy4 (red) and Cy6 (blue) showing variation in wavelength of peak
transmittance between the two different cyanine dye-sensitized TiO2 representing the two categories of
dyes investigated.
27

3.0
2.5
2.0
1.5
1.0
0.5
0.0
Cy1
Cy3
Cy5
Cy7
Cy2
Cy4
Cy6
Cy8
0.00
0.10
0.20
0.30
0.40
0.50
Voltage (V)
Figure 8: Photocurrent-voltage characteristics for cyanine dye sensitized solar cell measured
under illumination of 100mW/cm2 (1.5 AM)
28

70
60
50
40
30
20
10
0
Cyanine 2
Cyanine 3
Cyanine 4
Cyanine 5
Cyanine 6
Cyanine 8
0
20
40
60
80
Z'(Ω)
100 120 140 160
Figure 9: Nyquist plots for the fabricated Cyanine dye Sensitized Solar Cells showing differences in the
resistances to charge transfer.
29