# Influence of Different Cations of

Authors: Luísa Andrade; Shaik M. Zakeeruddin; Mohammad K. Nazeeruddin; Helena Aguilar Ribeiro; Adélio Mendes; Michael Graetzel
Journal: International Journal of Chemical Engineering
 (2009)

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## Abstract

The N3 dye was modified by substituting two of its protons by potassium or sodium cations. The performance and stability of dye-sensitized solar cells incorporating the new dyes were evaluated under light soaking(1000W⋅m-2) at
50∘C. Photocurrent measurements demonstrated that proton substitution by potassium cations rends the system more stable. Further characterization of the potassium-based devices was performed by electrochemical impedance spectroscopy to investigate the charge-transfer phenomena occurring at the different interfaces of the cells.


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## Body

## 1. Introduction

Nowadays, a particular interest in the development of
alternative energy sources arises, especially motivated not only by the need of
reducing the dependency on fossil fuel resources, but also for providing the
reduction of the CO2 emissions. An attractive strategy to
overcome the present energy problem is using renewable energy sources, such as
the direct solar radiation,  for producing clean energy. In this sense, the
direct conversion of sunlight into electricity by means of photovoltaic systems
makes an important contribution to this energy
contend in an
environmentally friendly way [1, 2]. In conventional solar cells, the charge
separation occurs at the interface of two materials of different conduction
mechanisms, exploiting the photovoltaic effect [3]. More recently, a new generation
of cells emerged, the dye-sensitized solar cells (DSCs). DSCs are considered
very promising since they use low cost, abundant and environmentally safe raw materials, showing relatively high-energy efficiency [4].A DSC is made of a nanoparticulated titania film coated
with an adsorbed dye monolayer. This thin film is applied on a glass substrate
coated with a transparent conducting oxide (TCO) that collects the photoinjected
electrons. A back electrode consists of the same conducting glass coated with
platinum. This serves as a catalyst for the redox reaction occurring in the
electrolyte present in between the two electrodes (Figure1).Figure 1
Schematic representation of a
dye-sensitized solar cell.In DSCs, the dye is a crucial component to achieve high
overall photoelectric conversion efficiency. Up to now, ruthenium complexes
have been widely investigated due to their advantageous spectral properties, device photostability and high conversion efficiency [5–7]. An example of these
ruthenium polypyridine complexes with better performance as sensitizer is the
cis-di(thiocyanato)bis(4,4′-dicarboxilic acid-2,2′-bipyridine)ruthenium(II),
commonly known as N3 [8]. Since the development of the N3 dye in 1993 [8], its
tetrabutylammonium (TBA) salt N719 has been used as the standard red dye
because of its unmatched performances. Similarly to the N719 dye, new dyes were
developed bearing in mind that the number of protons in the dye influences the
open-circuit potential and the short circuit current of the DSC [9]. In this
work we modified the N3 dye by partially substituting its protons with
different cations, namely, sodium and potassium. The modified dyes were
subsequently incorporated in final DSC devices, which were then submitted to
accelerated thermal/light soaking aging tests for performance and stability
evaluation. The cells were subjected to full sunlight irradiation at 50C°
for about 1000 hours, during which time their photovoltaic parameters were
periodically monitored.Electrochemical  impedance spectroscopy (EIS) was used to
investigate the charge transfer phenomena occurring at the different interfaces
of the DSCs. This
technique has been widely used for the characterization of several
electrochemical systems and, in particular, to analyze internal resistances in the DSCs
[10–19]. In 2000 Bisquert et al. applied their models in the
analysis of the mechanisms of electron recombination in nanoporous TiO2 dye-sensitized solar cells [18]. After this first approach, EIS became more and more useful, playing a crucial role in what concerns modeling
and understanding the complex charge phenomena occurring in DSCs [10–19]. The
charge-transfer resistance at the TCO layer, the charge-transfer resistance at
the counter-electrode/electrolyte interface, and the charge-transfer resistance at
TiO2/dye/electrolyte interface can therefore be obtained by fitting
the EIS results to appropriate equivalent electrical circuits [20].


## 2. Experimental
Section

### 2.1. Dye Preparation

N719 dye was
synthesized as reported earlier [21]. The dipotassium [2K+(N3, 2H+)]
and disodium [2Na+(N3, 2H+)] dyes were prepared as
follows. First, N719 dye was dissolved in acetonitrile solvent and to this an
excess amount of potassium triflate or sodium triflate in acetonitrile was
added. Immediately, the triflate counter ion was precipitated, filtered and
washed with acetonitrile and dried under vaccum.


### 2.2. Composition of Electrolyte E1

0.1 M iodine, 0.5 M N-methylbenzimidazole in a mixture of BMII; PMI TFSI;γ-BL (2; 3; 1) vol/vol.


### 2.3. TiO2 Electrode Preparation

A screen-printed
double layer film of interconnected TiO2 particles was used as mesoporous
negative electrode. A 7-μm thick film of
20-nm-sized TiO2  particles was first printed on the fluorine-doped
SnO2 conducting glass electrode and further coated by a 5-μm thick second layer of 400-nm-sized light
scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing and double-layer nanostructured TiO2 film have been reported elsewhere [22].


### 2.4. Dye-Sensitized Solar Cell Fabrication

The working electrode, described above, was sealed to the counter
electrode (FTO glass—15 Ω/square—coated with a platinum solution chemically
deposited at 450C° for 15 minutes) by means of a 25 μm-thick transparent Surlyn
ring (from DuPont) at 130C° for 15 seconds. The cells were
filled with an electrolyte solution through a predrilled hole in the counter
electrode. The hole was then sealed with a Bynel disc and a thin glass to avoid
leakage of the  electrolyte.


### 2.5. Photovoltaic Measurements

All photovoltaic measurements were
performed under a 450 W xenon light source able to provide1000W⋅m−2 sunlight equivalent
irradiation (AM 1.5). The spectral output of the lamp was matched in the region
of 350–750 nm with the aid of a Schott K113 Tempax
sunlight filter (Präzisions Glas & Optik GmbH, Iserlohn, Germany) so as to reduce the
mismatch between the simulated and true solar spectra. Various incident light
intensities were regulated with wavelength neutral wire mesh atenuators. I-V curves
were obtained by applying an external bias to the cells and measuring the
respective photocurrent response with a digital source meter (Keithley
Instruments Inc., Ohio, USA Model 2400).
The incident photon-to-current conversion efficiency (IPCE) was recorded by a
data-collecting system as a function of excitation wavelength. The incident
light from a 300 W xenon lamp (ILC Technology, Calif, USA) was focused through a
Gemini-180 double monochromator (Jobin Yvon., UK) onto the cell under test. The monochromator output was incremented through the visible
spectrum to generate the IPCE (λ) as defined by IPCE(λ)=12400(Jsc/λϕ),
where λ is the wavelength (nm), Jsc  is short-circuit photocurrent density (mA·cm−2), and ϕ is the incident radiative flux (mW·cm−2). Photovoltaic performance was determined
for an active area of 0.158 cm2 defined by a metal mask.


### 2.6. Electrochemical Impedance Measurements

Impedance experiments were
carried out with a computer-controlled potentiostat (EG&G M273) equipped with a frequency response analyzer (EG&G
M1025). The frequency range was 0.005 Hz–100 kHz and the magnitude of the
modulation signal was 10 mV. All the measurements were performed at room
temperature in the dark at −0.75 V bias. The EIS spectra
were fitted to an appropriate electrical analogue by means of the Z View
software (v2.5b, Scribner Associates Inc, NC, USA).


### 2.7. Stability Tests

Hermetically sealed
cells were used to check the long-term stability under visible light soaking at
50C°. The light soaking
experiments employed a polymer film of 50-μm thickness (Preservation Equipment Ltd,
UK), as a 400 nm UV cut-off filter. Two cells
of each type with matched photovoltaic performances were exposed at open
circuit to a Suntest CPS lamp (ATLAS Material Testing Solituions, GMBH, 1000W⋅m−2, 50C°) over a period of 1000 hours.
The cells were taken out at regular intervals to record the
photocurrent-voltage curve.


## 2.1. Dye Preparation

N719 dye was
synthesized as reported earlier [21]. The dipotassium [2K+(N3, 2H+)]
and disodium [2Na+(N3, 2H+)] dyes were prepared as
follows. First, N719 dye was dissolved in acetonitrile solvent and to this an
excess amount of potassium triflate or sodium triflate in acetonitrile was
added. Immediately, the triflate counter ion was precipitated, filtered and
washed with acetonitrile and dried under vaccum.


## 2.2. Composition of Electrolyte E1

0.1 M iodine, 0.5 M N-methylbenzimidazole in a mixture of BMII; PMI TFSI;γ-BL (2; 3; 1) vol/vol.


## 2.3. TiO2 Electrode Preparation

A screen-printed
double layer film of interconnected TiO2 particles was used as mesoporous
negative electrode. A 7-μm thick film of
20-nm-sized TiO2  particles was first printed on the fluorine-doped
SnO2 conducting glass electrode and further coated by a 5-μm thick second layer of 400-nm-sized light
scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screen-printing and double-layer nanostructured TiO2 film have been reported elsewhere [22].


## 2.4. Dye-Sensitized Solar Cell Fabrication

The working electrode, described above, was sealed to the counter
electrode (FTO glass—15 Ω/square—coated with a platinum solution chemically
deposited at 450C° for 15 minutes) by means of a 25 μm-thick transparent Surlyn
ring (from DuPont) at 130C° for 15 seconds. The cells were
filled with an electrolyte solution through a predrilled hole in the counter
electrode. The hole was then sealed with a Bynel disc and a thin glass to avoid
leakage of the  electrolyte.


## 2.5. Photovoltaic Measurements

All photovoltaic measurements were
performed under a 450 W xenon light source able to provide1000W⋅m−2 sunlight equivalent
irradiation (AM 1.5). The spectral output of the lamp was matched in the region
of 350–750 nm with the aid of a Schott K113 Tempax
sunlight filter (Präzisions Glas & Optik GmbH, Iserlohn, Germany) so as to reduce the
mismatch between the simulated and true solar spectra. Various incident light
intensities were regulated with wavelength neutral wire mesh atenuators. I-V curves
were obtained by applying an external bias to the cells and measuring the
respective photocurrent response with a digital source meter (Keithley
Instruments Inc., Ohio, USA Model 2400).
The incident photon-to-current conversion efficiency (IPCE) was recorded by a
data-collecting system as a function of excitation wavelength. The incident
light from a 300 W xenon lamp (ILC Technology, Calif, USA) was focused through a
Gemini-180 double monochromator (Jobin Yvon., UK) onto the cell under test. The monochromator output was incremented through the visible
spectrum to generate the IPCE (λ) as defined by IPCE(λ)=12400(Jsc/λϕ),
where λ is the wavelength (nm), Jsc  is short-circuit photocurrent density (mA·cm−2), and ϕ is the incident radiative flux (mW·cm−2). Photovoltaic performance was determined
for an active area of 0.158 cm2 defined by a metal mask.


## 2.6. Electrochemical Impedance Measurements

Impedance experiments were
carried out with a computer-controlled potentiostat (EG&G M273) equipped with a frequency response analyzer (EG&G
M1025). The frequency range was 0.005 Hz–100 kHz and the magnitude of the
modulation signal was 10 mV. All the measurements were performed at room
temperature in the dark at −0.75 V bias. The EIS spectra
were fitted to an appropriate electrical analogue by means of the Z View
software (v2.5b, Scribner Associates Inc, NC, USA).


## 2.7. Stability Tests

Hermetically sealed
cells were used to check the long-term stability under visible light soaking at
50C°. The light soaking
experiments employed a polymer film of 50-μm thickness (Preservation Equipment Ltd,
UK), as a 400 nm UV cut-off filter. Two cells
of each type with matched photovoltaic performances were exposed at open
circuit to a Suntest CPS lamp (ATLAS Material Testing Solituions, GMBH, 1000W⋅m−2, 50C°) over a period of 1000 hours.
The cells were taken out at regular intervals to record the
photocurrent-voltage curve.


## 3. Results and
Discussion

Figure2(a)  shows the photocurrent density-voltage curves obtained under
AM 1.5 simulated sunlight (1000W⋅m−2) for DSCs prepared with [2K+(N3,
2H+)], [2Na+(N3, 2H+)], and N719 dyes in association
with electrolyte E1. The cells were labelled as device A, B and C,
respectively. For simplicity, the results for one single cell of each type are
presented hereafter.(a)
Photocurrent intensity-voltage characteristics for devices A, B, and C, measured
at 1 sun(1000W⋅m−2),
AM 1.5 global sunlight illumination.  (b) Photocurrent action
spectra of the same devices.
(a)(b)Comparing the three systems under study, device C shows the
best initial photovoltaic performance. In fact, it is already known by its unmatched
performance. On the basis of cations’ substitution in the N3 dye, the anchoring
groups of the adsorbed sensitizer transfer most of its protons to the
semiconductor surface, charging it positively. This change in the TiO2 surface enhances the adsorption of the anionic ruthenium complexes and favors
electron injection from the excited state of the dye into the conduction band
of the semiconductor, resulting in higher photocurrents. Nevertheless, this
positive shift of the Fermi level induced by surface protonation leads to a low
open-circuit potential. In fact, if the sensitizer has less protons, it is
expected to obtain high open-circuit potential and low photocurrents, and the
other way round if the sensitizer is fully protonated. So, an optimal degree of
protonation of the sensitizer should be considered to reach maximum overall
conversion efficiency [9]. In this study we observed that the sodium-based
system (device B) presents higher values of short-circuit current, Jsc, and open-circuit voltage, Voc, than cells with the potassium-based system (device A)—Figure 2(a). However,
the latter system has higher overall power conversion efficiency, η,  due to a higher value of the fill
factor, FF. This fact is explained by
a reduction in the series resistance of the cell with potassium salt dye.In Figure2(b) the incident photon-to-current conversion efficiency (IPCE) for devices A, B, and C can be compared.
The IPCE is plotted as a function of the
excitation wavelength. In line with the better photovoltaic performance, device
C shows the highest IPCE: maximum of
about 67% at 530 nm. At the same wavelength, device A reaches a maximum of 54%, while device B reaches approximately 63%.Apart from efficiency, the long-term stability is also a key
issue regarding the industrial development and commercialization of DSCs. Thus,
an intimate relation between photovoltaic performances and stability should be
achieved. The evolution of the photovoltaic performances throughout the aging
process of the three systems under study is presented in Figure3. Device A
reveals a very good stability when compared with the two other systems.
Actually, this device kept close to 90% of its initial performance after 1000 hours of light soaking at 50C°. In contrast, devices B and C showed a drop in
efficiency of about 23% and 25%, respectively. The higher stability of device
A is mainly due to an increase in the photocurrent (6%) during the first week
of aging, which was then  maintained at the same level for the rest of the
aging period. Additionally, this system shows a quite stable evolution of the FF values, indicating constancy in
series resistance during the aging process. Despite its high photovoltaic
performance as a fresh cell, the N719 system was unstable, exhibiting a marked
decrease in Jsc and Voc soon after the first week.
This may be due to desorption of dye from the TiO2 surface. Moreover,
the sodium salt containing device B does not rend the system very stable, as
suggested by the strong oscillation in all its photovoltaic parameters.Evolution of photovoltaic parameters for
device A(▴), device B (×),  and device C (□). The
cells were kept under one sun visible-light soaking at 5C° for approximately 1000 hours.
(a)(b)(c)(d)The stability tests allow us to
conclude that the number and type of substituting cations will strongly
influence the photovoltaic performances of the devices. Considering the
ionic potentials of the cations we can realize how strongly they will be
electrostatically attracted to ions of opposite charge and to what extent the
cations will repel other ions of like-charge. As the sodium cation has higher
ionic potential than the potassium cation, we may be induced to say that the
stability is improved when substituting the N3 protons by sodium cations.
However, this conclusion is not straightforward since we have to considerer the
DSC system with all its components. The present work proves this fact since
potassium substitution renders a better stability. The positive shift of the
Fermi level upon adsorption of the dye decreases the gap between the redox
couple and the Fermi level. This fact
will strongly influence the TiO2/dye/electrolyte interaction,
which is not totally understood.Bearing in mind the promising results in terms
of performance and long-term stability of the potassium-based DSC, a deeper
characterization of the system was performed. In this sense, the
effect of the aging process in the overall performance of device A was analyzed
by means of electrochemical impedance spectroscopy. This technique allows us to
determine the charge-transfer
resistances at the platinum counter-electrode and at the TiO2/dye/electrolyte
interface as well as to determine the Nernstian diffusion of I−/I3− ions within the electrolyte [10, 20]. The Bode
and Nyquist (Z′′-imaginary part of impedance versus Z′-real part of
impedance) plots for the potassium-based system, before and after the aging
process, are shown in Figure 4.Bode (a) and
Nyquist (b) diagrams obtained for device A before
and after 1000 hours  under thermal/light soaking stress. In the Nyquist diagram,
symbols correspond to the impedance data obtained experimentally in the dark
under −0.75 V bias, while solid lines represent the fittings according to the
equivalent circuit present in Figure5.
(a)(b)Several physical models have been developed in an effort to understand all the complex
charge-transfer processes that take place in DSCs [15, 18, 23]. These works employ the
diffusion-recombination model to study the electronic processes taking place at
the semiconductor, whereas the electron transfer phenomena at the electrolyte
and contact interfaces are described by simple RC electrical arrangements. Actually,
the electron transport and charge recombination in nanocrystalline TiO2 films have been widely studied by Bisquert et al. [18], who suggested an
infinite transmission line model to describe the charge-transfer phenomena
occurring at the mesoscopic TiO2 film, as presented in Figure 5.Figure 5
Transmission line model used to fit the EIS experimental data.The transmission line
model assumes the TiO2 photoanode as an
interconnected network where electrons, after excitation, can diffuse toward
the external circuit with a resistance Rw or recombine at
the TiO2/electrolyte interface. Assuming that the recombination phenomenon is only related to
electron transfer by back reaction with the electrolyte (dark current), it can
be described by a charge transfer resistance, Rk, and a chemical capacitance, Cμ. If L is the thickness of the
mesoscopic TiO2 film, the electron transport resistance through the
semiconductor is defined as Rw=rwL,
while the interfacial charge recombination resistance and the chemical
capacitance at the interface are, respectively, Rk=rk/L and Cμ=cμL.The other charge-transfer processes occurring in
DSCs are also considered in the electrical analogue, as shown in Figure5.
Regeneration of I3− at the counter electrode is characterized by RCE and CCE, which represent the
charge-transfer resistance and the double-layer capacitance at the platinized
FTO glass, respectively.Finally, the ZView software was used to fit the
experimental data to the equivalent circuit presented in Figure5. For better fitting, all capacitor elements
were replaced by constant phase elements (CPEs), non-ideal
capacitances associated with a nonuniform distribution of current in the
heterogeneous TiO2 film [24].Table1 presents some parameters useful to understand the underlying mechanisms related
to the aging process. Besides Rk and Rw, the electron lifetime τn (τn=RkCμ), the effective electron
diffusion length Ln (Ln=LRk/Rw), and the effective diffusion coefficient of electrons in the TiO2 semiconductor Dn(Dn=Ln2/τn) can also be obtained [23]. According to the diffusion-recombination
model, the ratio Rk/Rw (≫1) clearly shows that the
charge-transfer resistance associated to recombination of electrons at the TiO2/electrolyte
interface (Rk) is much higher than the electron transport
resistance through the semiconductor (Rw). This means that
the cell exhibits carrier collection efficiency near unity [23].Table 1
Parameters
determined by fitting the EIS experimental data of device A to the equivalent
circuit as shown in Figure5.
Exposure
time/hoursRk/ΩRw/ΩRk/RwLn/μmτn/msDn/cm2⋅s−1067.25.113.325.511.15.90× 10−4100032.31.818.430.04.52.00× 10−3Moreover, Table1
shows that Ln is much
higher than the thickness of the mesoscopic TiO2 film (L≈12μm)
in both fresh and aged cells, confirming the excellent transport
properties.
The increased Jsc for the
aged sample can be attributed to the longer electron diffusion length compared
to the fresh sample (Jsc0hours=12.1 mA⋅cm−2 and Jsc1000hours=12.9 mA⋅cm−2). As shown in Figure 4(a), the middle frequency
peak position of the aged sample was slightly shifted to higher frequencies,
revealing a decrease in the electron lifetime. The decreased electron lifetime
is mainly ascribed to a major decrease in Rk (in Table 1) for the aged sample compared to the fresh one. This
means that electrons recombine more easily with the electrolyte upon aging. A faster electron recapture taking place in the aged device prevents the electron accumulation in the TiO2 nanoparticles from reaching the same density as for the fresh devices. As a result, the drop of Voc was observed in aged devices (Figure 3). The same
conclusions can be drawn from the significant decrease of the intermediate
semicircle in Figure 4(b), which corresponds to a lower value of the
charge-transfer resistance (Rk)
at this interface. The increased electron diffusion coefficient was probably due to the positive
shift of the conduction band energy level edge.A decrease in the redox charge-transfer resistance
at the platinum counter electrode was observed upon aging (RCE0hours=9.1Ω and RCE1000hours=5.5Ω). This explains the decrease of the left-hand
side semicircle in the Nyquist diagram. This semicircle corresponds to the
high-frequency peak in the Bode plot, which has moved toward larger values
(meaning lower electron lifetimes and shorter redox reaction times).
Additionally, an overlap between the photoelectrode impedance (middle
semicircle) and that related to the Nernstian diffusion within the electrolyte (right-hand semicircle) can
be noticed. This was probably due to a larger overpotential for the I−/I3− redox reaction on the platinum electrode of
the aged sample, accompanied with a fill factor problem (FF decreases from 0.67 to 0.59 upon
aging).


## 4. Conclusion

Two N3-based dyes were synthesized bearing in mind that the
number of protons in the sensitizer influences the open-circuit potential and
the short circuit current of the DSC. Potassium and sodium cations were used to
substitute two protons of the N3 dye and the resulting dyes were compared with the
well-known N719 dye that contains two TBA (tetrabutylammonium)
cations. Despite presenting the highest initial photovoltaic performance, the N719
system was rather unstable, exhibiting a strong decrease inJsc  and Voc upon aging, which might be due to dye desorption. In
addition, sodium substitution in place of TBA does not help to enhance the
stability of devices, whereas substitution with potassium salt revealed better
stability compared to the other two systems. In fact, this system kept close to
90% of its initial performance after 1000 hours of light soaking at 50C°.



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