# The Function ofTiO2 with Respect to Sensitizer Stability  in Nanocrystalline Dye Solar Cells

Authors: A. Barkschat; T. Moehl; B. Macht; H. Tributsch
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Dyes of characteristically different composition have been tested with respect to long-term stability in operating standardized dye sensitized cells during a time period of up to 3600 hours. Selective solar illumination, the use of graded filters, and imaging of photocurrents revealed that degradation is linked to the density of photocurrent passed. Photoelectrochemical degradation was observed with all sensitizers investigated. Sensitization was less efficient and sensitizers were less photostable with nanostructured ZnO compared to nanostructuredTiO2. The best performance was confirmed for cis-RuII(dcbpyH2)2(NCS)2 on TiO2. However, it was 7–10 times less stable under other identical conditions on ZnO. Stability is favored by carboxylate anchoring and metal-centred electron transfer. In presence of TiO2, it is enhanced by formation of a stabilizing charge-transfer complex between oxidized Ru dye and back-bonding interfacial Ti3+ states. This is considered to be the main reason for the ongoing use of expensive Ru complexes in combination with TiO2. The local surface chemistry of the nanocrystalline TiO2 turned out to be a crucial factor for sensitizer stability  and  requires further investigation.


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## Body

## 1. Introduction

The sensitization of emulsions of
semiconductor particles has played a major role in the development of
photography [1, 2]. Photoelectrochemical studies on the mechanisms of
spectral sensitization of semiconductors were performed in the late sixties [3–5]. Shortly thereafter, it was proposed to
use dye sensitization to drive a new kind of solar cell [6]. The sensitizing
Chlorophyll molecules were extracted from spinach to demonstrate solar energy
conversion in analogy to primary photosynthesis [5, 6] (this, and not, as later
claimed [7] the so called Grätzel cell, proposed 20 years later, was the first
demonstration of bioanalogue solar energy conversion using dye molecules). Although,
in the subsequent years, much scientific information on the sensitization
process became available and reasonably efficient dye sensitized solar cells
were demonstrated [8–14] on the basis of sintered oxide
ceramics, it became more and more evident that the stability of organic
sensitizers in such cells was a major problem. Nevertheless, in 1980, Matsumura
et al. showed that on the basis of zinc oxide ceramics made intentionally
porous with addition of aluminium, an efficiency for conversion of light into
electrical energy of 2.5% could be achieved (illumination occurred only in the
absorption region of the dye) [14]. Up to that time, dyes from very different
groups of chemicals and many different large gap semiconductors had been tested
for the sensitization phenomenon. In 1980, Dare-Edwards et al.  tested Ru-based
dyes by attaching them with two ester linkages to TiO2 [15]. At that
time, with respect to efficiency, no peculiar advantage of attached Ru(bipy)2(bpca)
on a TiO2 large gap semiconductor was observed. This was probably
due to the specific experimental conditions involved. Some deterioration of
current efficiency after many hours of illumination of the sensitized electrode
was specifically mentioned. Interestingly, it was a very similar system which
became the focus of attention 10 years later when high-efficiency, long-term
stable nanocrystalline dye sensitized solar cells were proposed [16]. They used
nanostructured TiO2 material sensitized via chemically attached
Ru-based dyes in combination with an I−/I3- redox
electrolyte. With such systems, solar energy conversion efficiencies of 7–10% were
demonstrated with at least several cm2 large cells (tiny cells
reached 11-12%). A
reasonable short-term stability was obvious when the cells were tightly sealed
but a long-term stability of 20 years was claimed on the basis of rather
complicated experimental and theoretical arguments [17]. Involvement of the
excited Ru-sensitizer in photochemistry was excluded due to a fast (femtosecond)
injection rate observed for the electron into TiO2
[18].A
fast regeneration of the oxidized Ru-sensitizer, on the other hand, was deduced
from the presence of a high (0.5 M) concentration of iodide in the
electrolyte. Regeneration rates sufficient for stabilization of solar cells for
over 108 cycles corresponding to 20 years lifetime were
calculated [17]. In the course of intensive research which followed in the
nineties, complications with respect to electrolyte chemistry, photochemistry,
purity of sensitizers, as well as with respect to sealing and iodine chemistry
were noted [19].  The best performing cells saw their sensitizer change from a three nuclear Ru-complex [16]
to the mononuclear cis-RuII(dcbpyH2)2(NCS)2 complex (here called N3) and the electrolyte solutions were replaced by less
volatile and more stable ones. But after 15 years of research on Ru-based dye
solar cells, and after efforts by many groups, reasonably stable dye solar
cells have still not found their way into production. In fact, several
companies have failed to produce reasonably stable commercial cells, with
larger panels still degrading too fast (e.g., on the main building of
CSIRO-Energy in Newcastle, Australia).Since
long-term stability of dye sensitized cells is a crucial issue for economic
feasibility, long-term studies of dye sensitized cells have been initiated in
increasing numbers [20–27] to take advantage of the high energy-conversion
efficiency observed [28, 29]. However, a major problem turned out to be the
sensitivity of the cells towards electrolyte loss via evaporation and the
instability of the electrolyte. The chance of a dye sensitized solar cell to
break down because of secondary phenomena not related to photochemical
stability was very high. To overcome this problem, photocurrent imaging techniques
were applied to visualize the effect of selective illumination of dye
sensitized solar cells [30]. Only part of a dye sensitized solar cell was
illuminated through a mask with simulated solar light (sulphur lamp) and the
development of photocurrents were followed over a longer period of time by
measuring spatially resolved images of the photocurrent. In these experiments,
a He/Ne-laser spot was used for scanned illumination. In this way, clear
differences between illuminated and nonilluminated areas could be observed [19].
This observation confirmed previous experiments with total reflection
FTIR-studies performed on working dye sensitized solar cells. They indicated
photo-induced generation of oxidation products of the Ru-sensitizer [31]. Significant
quantitative differences between the performances of dye sensitized solar cells
from different groups were observed. However, they behaved in a similar way
qualitatively with respect to photo-induced degradation relative to stability
in nonilluminated areas. A theoretical analysis of these preliminary studies
indicated that the branching ratio, the ratio of regeneration rate of the
oxidized sensitizer to the rate of product formation, is a crucial factor for
stability of dye sensitized solar cells. It was estimated that it may be one
order of magnitude lower than required for 20 years stability [32].Some
publications related the problem of dye cell degradation to the presence of
water in the electrolyte [33] and other influences of the
electrolyte [17, 31, 34] to higher temperatures [35] and to UV-light
incidence [35]. In a more recent paper, the degradation mechanisms in a
dye-sensitized solar cell were further investigated using UV/VIS- and
IR-spectroscopy [36]. These results indicate a loss of the isothiocyanato-ligand
of the TiO2-adsorbed dye in the presence of air, at higher
temperature or in the presence of water in the electrolyte. This effect is
accelerated by illumination. It was found that this leads to a decreased,
blue-shifted N3-dye absorption [36].It
is therefore necessary to investigate the problem of stability further,
especially with respect to the origin of dye degradation. The question should
specifically be answered is
why, after so many years of intensive research on nanostructured dye solar
cells, TiO2 and expensive Ru-based sensitizers remained the by far
preferred components of dye solar cells.The
aim of the present contribution is first to investigate different dyes, besides
of the mainly used Ru-complex (N3), to study chemical preconditions for
improved stability and, further, to find out whether the excited or the
oxidized sensitizer is a key problem in inducing photochemical side-reactions.
A key question will be the role which TiO2 and its surface chemistry
play for sensitizer photodegradation. For this reason, ZnO will be studied as a
substrate in parallel. It should be investigated to what extent the chemical
quality and surface morphology of nano-TiO2 do have an influence on
the survival of oxidized sensitizer molecules.Photo-induced
degradation, which is addressed here, is just one of several problems which can
lead to degradation of dye sensitized cells. They have been discussed in a
parallel publication [19] and include irreversible changes of
the redox electrolyte, Li insertion into the oxide, loss of electrolyte through
evaporation, and contamination from the atmosphere through the sealing
material.


## 2. Experimental

The experimental strategy of this project
consisted in fabrication of standard nanocrystalline dye sensitized solar cells
like those now produced in many laboratories [19, 37, 38].
The cells were operated with 6 different sensitizers, which were selected as
the most efficient from a larger number of dyes previously used. Both, cells
made of nanocrystalline ZnO and TiO2 were compared, and the time-dependent
performance of dye sensitized cells investigated by observing their photo-electrochemical
performance and studying space resolved photocurrent images in order to
distinguish the performance of illuminated and nonilluminated areas.While
screening a larger number of dyes (e.g., Ni-phtalocyanine, fluoresceine,
rhodamine B, methylene blue, bromothymol blue, zinkone) for
their function in standardized nanocrystalline dye sensitized cells, the work
concentrated on the 6 best performing complexes. They were:(i)
bis(tetrabutylammonium) cis-bis(isothiocyanato)bis(2,2′-bi-pyridine-4-carboxylic acid, 4′-carboxylate)ruthenium(II)
or cis-RuII(dcbpyH2)2(NCS)2,
called N3 or Ru535 (obtained from Solaronics Inc., Aubonne, Switzerland),(ii)
porphyrine,(iii)
pyrogallol red,(iv)
sodium salt of copper-chlorophylline,(v)
di-(2,2′-dipyridylmethylene)[(2,2′-dipyridylmethylene)  malonato]ruthenium(II)dihexafluorophosphate
⋅2HCL,(vi)
di-(2,2′-dipyridyl)[(2,2′-dipyridylmethylene)nomooctadecyl
malonato]ruthenium(II).With the exception of porphyrine when
chloroform was used, the dyes were dissolved in ethanol to give 0.3 mM
stock solutions which were then applied to the nanocrystalline oxides.


### 2.1. Dye Sensitized Cells

Standard dye sensitized solar cells were
prepared as described in the literature [19] and TiO2-layers were
prepared by hydrolysis of Ti(IV)isopropylate [39, 40].For
the preparation of TiO2 layers, 1 mL 65%-HNO3 was
added to 120 mL of distilled water, and, for the next 45 minutes, a
solution of 10 mL Ti(IV)isopropylate in 10 mL isopropanol was added
under constant stirring in an argon atmosphere. A white colloidal precipitation
was formed, and the isopropanol was removed by distillation. When the solvent
left was only water, the suspension was kept under reflux for 8 hours. The
suspension was then heated to 230°C while being stirred in an autoclave for 12 hours. After removing the water by vacuum distillation, 40% polyethylenglycol
20000 (40% relative to the content of TiO2 (11–20%)) was
added. This mixture was then applied onto the glass by pulling a glass rod over
the surface. The thickness of the layer was kept constant by the use of a mask,
which was fixed on the glass. The thickness of the TiO2 nanolayer
was adjusted to 5 μm, which is smaller than the optimum for high efficiency (10–15 μm) but adequate for 2–4% efficient
experimental cells (excess dye should not be acting as a buffer). After drying
for 15 minutes, the layers were tempered at 450°C for 30 minutes
(cells of Figures 3(b), 3(c), 4(b), 6, 7).For ZnO-layers, commercial powders (Alfa,
Aldrich) were used to prepare a suspension with the addition of acetyl acetone
and water (1:10) and Triton X100 as a detergent.The
suspensions were again applied to the FTO-glass by pulling a glass rod over a
mask, which was fixed on the glass. Afterwards, the samples were tempered at
450°C for 30 minutes (cells of Figures 2(a), 2(b), 3(a)).Additionally, commercially available nanocrystalline
TiO2-layers from the
Institute for Applied Photovoltaics, Gelsenkirchen (INAP) were used. (cells of Figures 4(a), 4(c), 4(d), 8).After
tempering, the layers were dipped while still hot (approx. 80°C) into a 0.2 mM or
0.3 mM solution of the dye. The second glass with the platinum back
contact was fixed above the layer and the cells were sealed with Surlyn 1702
(DuPont). The whole cell was then put into electrolyte solution in an
exsiccator. When vacuum was applied, the cell is filled with electrolyte. The electrolyte
contained 0.5 M LiI, 50 mM I2 and 0.2 M tert.-butylpyridine
in acetonitrile.


### 2.2. Measurements

Dye sensitized solar cells were
characterized by photocurrent voltage measurements before exposing them to
solar simulated light (sulphur lamp) under short-circuit conditions. A scanning
laser spot technique was used to produce images of photocurrents as described
in Chaparro et al. [41]. The scanning microscope for semiconductor characterization
(SMSC) (see Figure 1) is basically an inverted microscope provided with an x-y
scan-stage. The laser spot was focussed by the microscope lens onto the dye
sensitized solar cell, which was connected to a lock-in amplifier. The maximum
spatial resolution of the used SMSC build up at HMI Berlin is 1 μm with a
laser spot of 2 μm in diameter. In these experiments, a He/Ne-laser was
used for illumination, the light intensity was adjusted to 100 mWcm−2.
In order to study photodegradation, only well-defined surface areas (using
masks) of the dye sensitized cell were illuminated with simulated solar light
(solar simulator with sulphur lamp). The photocurrent images were recorded at
intervals to investigate the development of photodegradation.Figure 1
Scheme of the photoelectrochemical set up employing an
optical scanning microscope (SMSC).Photocurrent images of dye sensitized solar cells (nanocrystalline zinc oxide) subdivided into four sections for combinatorial testing (clock-wise from top left):
(a) neutral red, N3-dye, rhodamine B, methylene blue;
(b) N3-dye, neutral red, methylene blue, bromothymol blue.
It is seen that only the N3 dye shows significant sensitization.
(a)(b)Power output characteristic
(photocurrent voltage dependencies) of dye sensitized cells:
(a) nanocrystalline ZnO, N3 complex before and after 20 days of simulated solar
illumination; (b) nanocrystalline TiO2, pyrogallol red before and after 11 days of
simulated solar illumination; (c) nanocrystalline TiO2, copper
chlorophylline before and after 30 days of simulated solar illumination.
(a)(b)(c)Photodegradation in photocurrent
images of four regularly operating nanocrystalline TiO2 solar cells.
Sensitization with N3-dye and illumination with simulated solar light in a
round central areas only. Illumination periods: (a) 1008 hours; (b)
336 hours; (c) 1248 hours; (d) 1248 hours.
(a)(b)(c)(d)


## 2.1. Dye Sensitized Cells

Standard dye sensitized solar cells were
prepared as described in the literature [19] and TiO2-layers were
prepared by hydrolysis of Ti(IV)isopropylate [39, 40].For
the preparation of TiO2 layers, 1 mL 65%-HNO3 was
added to 120 mL of distilled water, and, for the next 45 minutes, a
solution of 10 mL Ti(IV)isopropylate in 10 mL isopropanol was added
under constant stirring in an argon atmosphere. A white colloidal precipitation
was formed, and the isopropanol was removed by distillation. When the solvent
left was only water, the suspension was kept under reflux for 8 hours. The
suspension was then heated to 230°C while being stirred in an autoclave for 12 hours. After removing the water by vacuum distillation, 40% polyethylenglycol
20000 (40% relative to the content of TiO2 (11–20%)) was
added. This mixture was then applied onto the glass by pulling a glass rod over
the surface. The thickness of the layer was kept constant by the use of a mask,
which was fixed on the glass. The thickness of the TiO2 nanolayer
was adjusted to 5 μm, which is smaller than the optimum for high efficiency (10–15 μm) but adequate for 2–4% efficient
experimental cells (excess dye should not be acting as a buffer). After drying
for 15 minutes, the layers were tempered at 450°C for 30 minutes
(cells of Figures 3(b), 3(c), 4(b), 6, 7).For ZnO-layers, commercial powders (Alfa,
Aldrich) were used to prepare a suspension with the addition of acetyl acetone
and water (1:10) and Triton X100 as a detergent.The
suspensions were again applied to the FTO-glass by pulling a glass rod over a
mask, which was fixed on the glass. Afterwards, the samples were tempered at
450°C for 30 minutes (cells of Figures 2(a), 2(b), 3(a)).Additionally, commercially available nanocrystalline
TiO2-layers from the
Institute for Applied Photovoltaics, Gelsenkirchen (INAP) were used. (cells of Figures 4(a), 4(c), 4(d), 8).After
tempering, the layers were dipped while still hot (approx. 80°C) into a 0.2 mM or
0.3 mM solution of the dye. The second glass with the platinum back
contact was fixed above the layer and the cells were sealed with Surlyn 1702
(DuPont). The whole cell was then put into electrolyte solution in an
exsiccator. When vacuum was applied, the cell is filled with electrolyte. The electrolyte
contained 0.5 M LiI, 50 mM I2 and 0.2 M tert.-butylpyridine
in acetonitrile.


## 2.2. Measurements

Dye sensitized solar cells were
characterized by photocurrent voltage measurements before exposing them to
solar simulated light (sulphur lamp) under short-circuit conditions. A scanning
laser spot technique was used to produce images of photocurrents as described
in Chaparro et al. [41]. The scanning microscope for semiconductor characterization
(SMSC) (see Figure 1) is basically an inverted microscope provided with an x-y
scan-stage. The laser spot was focussed by the microscope lens onto the dye
sensitized solar cell, which was connected to a lock-in amplifier. The maximum
spatial resolution of the used SMSC build up at HMI Berlin is 1 μm with a
laser spot of 2 μm in diameter. In these experiments, a He/Ne-laser was
used for illumination, the light intensity was adjusted to 100 mWcm−2.
In order to study photodegradation, only well-defined surface areas (using
masks) of the dye sensitized cell were illuminated with simulated solar light
(solar simulator with sulphur lamp). The photocurrent images were recorded at
intervals to investigate the development of photodegradation.Figure 1
Scheme of the photoelectrochemical set up employing an
optical scanning microscope (SMSC).Photocurrent images of dye sensitized solar cells (nanocrystalline zinc oxide) subdivided into four sections for combinatorial testing (clock-wise from top left):
(a) neutral red, N3-dye, rhodamine B, methylene blue;
(b) N3-dye, neutral red, methylene blue, bromothymol blue.
It is seen that only the N3 dye shows significant sensitization.
(a)(b)Power output characteristic
(photocurrent voltage dependencies) of dye sensitized cells:
(a) nanocrystalline ZnO, N3 complex before and after 20 days of simulated solar
illumination; (b) nanocrystalline TiO2, pyrogallol red before and after 11 days of
simulated solar illumination; (c) nanocrystalline TiO2, copper
chlorophylline before and after 30 days of simulated solar illumination.
(a)(b)(c)Photodegradation in photocurrent
images of four regularly operating nanocrystalline TiO2 solar cells.
Sensitization with N3-dye and illumination with simulated solar light in a
round central areas only. Illumination periods: (a) 1008 hours; (b)
336 hours; (c) 1248 hours; (d) 1248 hours.
(a)(b)(c)(d)


## 3. Results

Because quantitative reproducibility of dye
sensitized cells turned out to be difficult (efficiency, electrolyte stability,
durability of sealing, and long-term behavior varied from cell to cell)
combinatorial approaches to dye sensitized properties of different compounds
seemed to be appropriate. Figure2 shows the photocurrent images of two
dye sensitized cells, each of which has been separated into four sections dyed
with different sensitizers. Three of the sensitizing substances (the Ru-complex
N3, neutral red and methylene blue) were identical in order to test
the reproducibility. The additional sensitizers were rhodamine B in Figure 2(a)
and bromothymol blue in Figure 2(b). As seen in Figures 2(a) and
2(b), only the Ru-complex N3 shows significant sensitized properties in
the presence of the same iodide/triiodide-acetonitrile electrolyte. A larger
number of potential candidates for sensitized were tested (e.g.,
Ni-phtalocyanine, fluoresceine, rhodamine B, methylene blue,
bromothymol blue, zinkone) but only five additional compounds listed in
the experimental section proved to be reasonably efficient for long-term
testing. Among these were two additional Ru-complexes and the compounds
porphyrine, pyrogallol red, and copper chlorophylline. Other compounds,
such as rhodamine B or methylene blue, which had been demonstrated to
sensitize ZnO reasonably well under different conditions, turned out not to be
sufficiently effective in a nanocrystalline environment in the presence of an
iodide/triiodide electrolyte in acetonitrile.The six sensitizers mentioned were tested
under comparable conditions in solar cells which had a nanocrystalline oxide
layer of 4μm only. This is significantly smaller than in high-efficiency
dye sensitized cells (8–16 μm) so
that the solar cell efficiency reached was correspondingly smaller. In the case
of the N3 Ru-complex, the solar cell efficiency reached was between 2 and 3%.
The same sensitized cell with ZnO yielded an efficiency between 0.3% and
0.45%. Solar cells fabricated with the above mentioned six different
sensitizers were exposed to simulated solar light for periods of up to 30 days,
and in the case of the Ru-N3 dye and nanocrystalline TiO2 substrate
for periods of up to 280 days. Photochemical degradation was observed in all
cases. It was clearly more pronounced in the case of ZnO, even though
simultaneously lower solar cell efficiencies were reached.Figure3(a) shows the integral photocurrent density-voltage characteristics of a nanocrystalline
zinc oxide cell sensitized with the N3 ruthenium complex at the beginning of
the experiment and after 20 days of continuous exposure to simulated solar
light.During this period
the integral photocurrent density decreased by 35%. It is realized that
basically only the photocurrent density decreased and not the photovoltage. Figures3(b) and 3(c) show similar degradation curves of the power output
characteristics of a nanocrystalline TiO2 pyrogallol red cell and of
a nanocrystalline TiO2 copper chlorophylline solar cell. In the
first case, Figure 3(b), the period of simulated solar light exposure was 11
days, in the second case the cell was exposed to solar light for 30 days. All
investigated dyes showed a gradual photochemical degradation when exposed to
simulated solar light for a prolonged period.Among the many dyes which have been
screened, only a few approach the properties of Ru-N3, as shown in Figure2.
Ru-N3 turned out to be the most favorable sensitizer, all other dyes exhibited
lower efficiency and stronger degradation. Lower efficiencies and stronger
degradation were also observed with nanocrystalline zinc oxide solar cells.
Comparing different dyes, it was found that certain anchor groups like carboxyl
or sulphone groups improve the sensitization properties of compounds. The
interaction is apparently less efficient or of different nature with zinc oxide
nanoparticles.Special
attention was dedicated to the degradation properties of the N3 ruthenium
complex on nanocrystalline TiO2. Here, a long-term experiment, in
which sensitized cells were exposed to simulated solar light during a period of
155 days, turned out to be especially informative. The cells were illuminated
through a mask with a round opening in order to distinguish between illuminated
and nonilluminated areas. During such long-term experiments, the instability of
the iodide/triiodide electrolyte turned out to be a major problem. In some
cases, depending on small variations in the preparation, the electrolyte was
bleached so that the iodine largely disappeared from the electrolyte. This
problem was investigated and may be understood in relation to the potential-pH
diagram of iodine. Depending on traces of water which leak into the organic
electrolyte through the seal from the outside, and due to the presence of
tert.-butylpyridine, a slightly alcaline pH can be developed,  which allows oxidation of iodide to iodate (IO3−). Since this iodate could not be detected
spectroscopically in the electrolyte, it may have been adsorbed on the nanocrystalline TiO2-layer.
Whatever the detailed mechanism of this process, it causes the disappearance of
iodine and therefore the electron transport through the electrolyte became
extremely limited.In the above mentioned long-term experiment,
four dye sensitized cells showed bleaching of the electrolyte, but were
continuously operated under short-circuit conditions with a very small current
flow. This means, since the photocurrent largely collapsed, that the bleached
cells had a power output characteristic with low fill factor. At the end of the
long-term experiment, all dye sensitized cells, both those that maintained good
power output characteristics and those which were bleached and showed very
unfavorable power output characteristics were investigated using the scanning
laser technique to obtain photocurrent images. During these measurements, only
a very small photocurrent generated by the laser spot (laser light intensity
not exceeding solar light intensity) had to flow through the dye sensitized
cell. Such a small current could also be maintained by those cells which had a
bleached, iodine deficient electrolyte.Figure4 shows four dye sensitized
cells (N3 ruthenium complex on nanocrystalline TiO2) which operated
with unbleached electrolyte (Figures 4(a)–4(d)). For
comparison, four cells in which the electrolyte was bleached during long-term
(3600 hours) illumination are shown in Figure 6. In the first case (Figure 4), photodegradation of the ruthenium complex within the round areas
illuminated with simulated solar light can clearly be recognized while in the
second case there is no evidence of a difference between illuminated and nonilluminated
areas of the dye sensitized solar cell. The difference between the cells in Figures 4 and 6 is simply that in the first case photocurrent could be passed in
an uninhibited way, while in the second case photocurrent passage was severely
hindered due to lack of iodine. This shows that photons and thermal energy
alone are not the primary cause of the degradation if electrons are allowed to
react back. Because the mask for forming the light patterns was black and
placed directly onto the cell, thermal energy was also generated outside the circular
opening. The graded filter also absorbed light to generate thermal energy, so
that temperature gradients across nonhomogenously illuminated solar cell
surfaces were expected to be low.The significant 15–30%
photocurrent decrease observed during the long-term experiments (Figure4)
raises the question as to the optical visibility of the illuminated and degraded
areas. Optical transmission was indeed changed.As Figure5 shows, after contrast
amplification, the transmission pattern of the solar cell matches essentially
the photocurrent pattern (Figure 4(a)). This video image (Figure 5) was
obtained under light transmission, after cleaving of the cell, from only the
front-half which consisted of ITO-glass and a TiO2-layer with
adsorbed dye. An influence of the electrolyte or back-contact platinum layer is
therefore excluded. However, with the naked eye the transmission change has
only occasionally been observed clearly. This means it stayed within a 1-2% limit,
which leads to the conclusion that the deactivated sensitizer still absorbs
light in the visible spectral region. The degraded complex is still present and
has only moderately changed its absorption spectrum in the visible spectral
region with respect to the sensitizer.Figure 5
Video image of the cell in Figure4(a), obtained under light
transmission and after contrast amplification.Photodegradation is not detected in photocurrent images of
nanocrystalline TiO2 solar cells with bleached electrolyte, which
causes significantly decreased photocurrent densities during illumination. All
cells are sensitized with N3 dye and were illuminated with simulated solar light
in round central areas only. The illumination period was 150 days (3600 hours).
The cells were at the end not completely wetted with electrolyte.
(a)(b)(c)(d)Photocurrent profiles at the same position across a
dye sensitized solar cell with inhomogeneous photocurrent distribution.
Measurements were done after different periods of illumination with simulated
solar light. The photocurrent profiles smooth out during long-term photodegradation,
because photodegradation is faster where large photocurrents are flowing.
(Experiment performed with copper-chlorophylline on TiO2):
(a) before and (b) after 21 days, and (c) after another 93 days of
illumination with simulated solar light.
(a)(b)(c)(d)Figure 8
Nanocrystalline TiO2-N3 sensitized solar cell with
graded filter placed on its surface after 1245 hours of exposure to
simulated solar light. It is seen that photodegradation, measured as spatially
resolved photocurrent output, is proportional to solar light intensity and
thus the photocurrent passed.These
results show that photodegradation of the sensitizer must be related to the
generation of sensitization photocurrents through the cell. In the case that
these photocurrents cannot be generated, which causes the effective rapid
recycling of the electrons, photodegradation is suppressed. Therefore, it can
be concluded that only the oxidized state of the sensitizer is critical for
photodegradation.In
order to test this interesting observation, which may also be relevant for
other sensitizers, two additional experiments were performed. In the first, dye
sensitized cells with very inhomogeneous photocurrent distributions were
investigated. If a higher photocurrent density also means a higher rate of
photodegradation, a profile through such an inhomogeneous cell should smooth
out during a prolonged illumination. This means the photocurrent in high-photocurrent-density
regions should decrease faster than in lower-photocurrent-density regions. Such
an experiment is demonstrated in Figure7 showing the normalized photocurrent-profile
through an inhomogeneous dye sensitized cell versus cell operation time. It is
indeed recognized that the profiles smooth out with illumination time.To
confirm the conclusion that higher photocurrent flux means more degradation, a
graded filter was placed onto an N3 dye sensitized cell for a long-term study.
The cell with originally homogeneous photocurrent density distribution was
exposed to simulated solar light attenuated by the graded optical absorption
filter. Figure8 shows that the spatially resolved photocurrent density
measured after exposure of the cell to 1248 hours of simulated sunlight
actually has a linear degradation profile. The laser-induced photocurrent
decreases indeed proportional to the light intensity and thus to the magnitude
of the photocurrent generated at the respective location. The higher the
photocurrent density is, the higher the rate of degradation will be.Total
reflection infrared spectroscopy on functioning dye sensitized solar cells
performed in our laboratory have demonstrated the occurrence of products
parallel to photocurrent flow [31, 42]. The integrated infrared signal
disappears when the current density goes to 0 and increases with increasing
photocurrent density. These results confirm that the generation of oxidation
products is related to the extraction of electrons from the sensitizer's
environment. Depending, however, on the specific adsorption sites, sensitizer
molecules may be involved in quantitatively different reaction behavior.  The
loss peaks in the in situ spectrum at 1988 cm−1 and
2104 cm−1, with widths of 29 cm−1 and
26 cm−1, respectively, showed a somewhat smaller width than in
the ex-situ spectrum where the width was 33 cm−1. This is shown
in Figure 9 for the 2104 cm−1 line in the CN-vibration region.
It shows the line shape from an exsitu spectrum of cis-RuII(dcbpyH2)2(NCS)2 adsorbed on nanocrystals within a 1μm thin TiO2 layer. It also
shows the decrease of this absorption (plotted, however, in the opposite
direction to permit a better comparison of line shapes) from an in situ
experiment after passing the photocharge of 1.4, 5.4, 9.5, and 14 mCcm−2 through the dye sensitized cell. A clearly narrower IR line due to the
consumption of the sensitizer is realized, indicating that a limited fraction
of adsorbed molecules is selectively turned over. The molecules preferentially
consumed during photocurrent flow are those, which are positioned around the center
of the line and not those which, due to stronger interaction with the TiO2 substrate, are energetically positioned away from the center. This is clear
evidence for the role of the TiO2 adsorption site in determining the
stability of sensitizing molecules during the photoreaction.Figure 9
Shape of exsitu IR-CN stretch-line
(2104 cm−1) of N3 sensitizer (a) compared to lineshape
characteristic for photodegradation loss after passage of 1.4, 5.4, 9.5, and
14 mC cm−2 of photoelectrical charge through the dye sensitized
cell, (b) measured in total reflection FTIR-experiments.


## 4. Discussion

The main aim of this study was to identify
physical-chemical factors relevant for long-term stability or degradation of
sensitizers of nanocrystalline ZnO and TiO2 solar cells to determine
the influence of the adsorption site. The focus was on light-induced
degradation. An analytical formula describing the nature of various cell
parameters, and rate constants which may influence the photocurrent voltage
characteristic, had been presented in an earlier publication [32]. The photodegradation
of sensitizer molecules is just one possible mechanism of degradation. Other
processes involve the kinetics of the rectifying front TiO2/fluorinated
tin oxide contact (it is very critical for the performance and long-term
stability of the dye sensitized solar cell), insertion of alkali atoms into TiO2,
irreversible changes of the redox electrolyte, loss of electrolyte, and
contamination of the electrolyte from the atmosphere [19].All sensitizers investigated in the present
study, including the Ru-complex (N3), which presently is considered to be
sufficiently stable for long-term (20 years) operation in dye sensitized cells,
were found to degrade to varying extents. The N3 Ruthenium sensitizer turned
out to be the most stable one, but it nevertheless photodegrades (Figure4) and
it degrades proportional to the photocurrent passed through the dye solar cell
(Figure 8). This means that photodegradation will be the more pronounced the
higher the cell efficiency.Since this is a somewhat perturbing
conclusion and since there have been persistent claims that the Ru-dye does not
photodegrade and supports 108 turnovers, let us first discuss our
results in relation to other recent studies on dye solar cell performance and stability [20–29]. They, for example, claim significant progress by using alternative
electrolytes and improved sealing techniques (e.g., [20]). In this latter work,
0.6 M hexylmethyl imidazolium iodide was added to the acetonitrile electrolyte
and a glass
frit sealing was used.  In stead of exposing the solar cell as it is to the
solar simulator, the cell was first exposed to 80°C in the dark (!) for 1400 hours,
and subsequently exposed to light for 1700 hours at only 25°C. The claim then
was that the dye cell showed a photodegradation of less than 5% during 1700 hours.
In fact, the entire procedure (thermal treatment plus subsequent illumination
at 25°C) has decreased the solar cell efficiency by 20%, as seen
from [20, Figure 10].In reality, however, strong sunshine can
heat a dye solar cell to 80°C, and the most damaging degradation process is the
periodically daily heating and cooling of the dye solar cell under solar
illumination.For comparison, our solar cells, the
selective degradation of which is depicted in Figures4(c) and 4(d) decreased
their photocurrents during 1248 hours of illumination and solar simulator
determined higher temperature by approximately 10%. It has also to be
considered that photodegradation was found to be proportional to its efficiency
and thus to the photocurrent density passed (compare Figure 8). In addition, we
are confronting these results with nearly Redox inactive solar cells, which
show practically no degradation after 3600 hours (Figure 6). Our stability
results therefore appear to be entirely realistic and contradict possible
claims, that more recent cell configurations have solved the stability problem.
Since the most stable dye cell in [20] only had an efficiency of 2%, our results
also claim that a corresponding 8% solar cell will, under illumination, degrade
4 times faster, if a linear extrapolation is applicable. It will also degrade
faster when illumination is not performed at 25°C, but at an illumination up to 60
degree higher temperature, which can be reached in intensive sunlight.In
our work, it could be confirmed that anchoring groups of the dye support the
sensitization effect but that the nanocrystalline ZnO interface is a less favorable
substrate for electron injection and long-term stability of Ru-dyes under the
investigated conditions. It has, for example, been suggested [43]
that the N3 ruthenium complex tends to aggregate by reacting with zinc ions
dissolved from the ZnO interface.  The oxide surface is electronically and
chemically significantly different for TiO2 and ZnO, the latter of
which additionally exposes polar surfaces terminating with Zn and O,
respectively. In our studies, degradation of this ruthenium complex is
significantly faster on ZnO than on TiO2. This degradation is faster by a factor of 7 to
10 on ZnO when the same iodide/triiodide electrolyte in acetonitrile is
used.One reason why metal centered complexes
(ruthenium complexes, copper chlorophylline complex) are more stable than
sensitizers without transition metal centers may be due to coordination
chemistry. The electron transfer occurs metal centred and involves states which
are not essentially weakened by electron transfer. In addition, some
coordinative interaction between iodide and oxidized metal centered sensitizers
can occur. This allows a quite selective electron transfer which does not
excessively weaken other bonding states. Nevertheless, as shown in the
literature [44], a ruthenium complex can undergo approximately
143 transitions involving different ligand configurations. Depending on the
type of transition involved in the sensitization reaction, the oxidized product
may have variable survival lifetimes because they involve different portions of
the ligand environment. Figure 9 shows that not all sensitizer molecules
at different adsorption sites of the TiO2 environment are equally
affected. This means that the reaction kinetics of molecules is dependent on
the type of adsorption sites. Molecules which do not inject electrons or
molecules which are immediately regenerated will not participate in the
generation of the in situ infrared signal. Others, due to their peculiar
adsorption position, may show preferred tendency towards product formation, that
is, they may show a large k2 rate constant and thus contribute to a
decrease of life time (1) of dye sensitized cells, formulated below. The
interface of ZnO apparently provides more unfavorable reaction possibilities.
Alternatively, it may also be that the N3 dye has an especially favorable
affinity to TiO2. From Figure 9 it may be tentatively deduced
that the weakly interacting N3 molecules and not the strongly interacting
(large deviation from mean position of IR band) are preferentially lost to
photodegradation. It may be that these molecules, after losing the injected
electron, tend to irreversibly react, involving the chemistry of the interface.
Recent HPLC-chromatic studies combined with UV/VIS-spectroscopy of N3-dye
samples extracted from long-term illuminated solar cells have detected a
variety of products which may have formed this way [45]. In this
case, the interface of the substrate should be critical, which is indeed
supported by the much higher degradation rate of N3 on nanocrystalline ZnO.
Such an effect of the oxide interface on the rate k2 of product
formation could also explain why at the beginning of a long-term experiment the
rate of photodegradation, as reflected in the branching coefficient kbr from relation (1), appears to be larger in some experiments before levelling
off [30]. The reason may be that photo-oxidized molecules at selected surface
sites, which provide an adequate surface environment, will more easily find
reaction possibilities towards irreversible products.Interestingly, however, as seen from Figure6,
degradation ceases (at least for the time window observed) when photocurrent
flow through the cell is suppressed while all other parameters are maintained.
This is a very important observation which has to be discussed in some detail.
It explains why accelerated dye sensitized solar cell tests under high-illumination
intensity, but under open-circuit conditions (no photocurrent flow), yielded no
evident degradation and turnover numbers for electrons of the order of 108.
This would be necessary for a solar cell operation of 20
years [17]. Complementarily,
experiments in Figure 4 clearly show that when photocurrent flows, the
stability of the sensitizer is reduced. This is in agreement with the formula
for the half-lifetime of a dye sensitized solar cell which has earlier been
proposed [32]:
(1)T1/20⁢∼S0jph0kbr⁢∼S0jph0.k3I−k2with the branching coefficient kbr=k3[I−]/k2 for the assumed simplest sensitizer reaction, S0 = initial
sensitizer concentration, jph0 = initial photocurrent density, P =
product for the reaction sequence
(2)hν+S⟶g⁢S*S*⟶k1⁢S++e−S+⟶k2⁢PS++I−⟶k3⁢S+12I2.
Relation (1) shows that the half lifetime of
the dye sensitized cell is proportional to the initial concentration S0 of the sensitizer in the nanostructure and inversely proportional to the
initial photocurrent density. The perturbing fact is that the lifetime of the
cell will be the shorter the higher its efficiency (and thus the photocurrent
passing through it). This is clearly demonstrated by the graded filter
experiment of Figure 8. The most successful long-term experiment of an
optimized dye solar cell in [20] (1700 hours of solar illumination at only 25°C
yielding less than 5% of degradation) was achieved with a cell of only 2%
efficiency. A higher efficiency (photocurrent drawn) will correspondingly accelerate
photodegradation. Depending on the temperature dependence of k2 and k3 in (2), the life time of the solar cell will be correspondingly
temperature-dependent.Additionally, it has to be considered that the
rate of product formation,k2, will be influenced by the chemistry
of the oxide/electrolyte interface (compare Figures 3(a) and 4, and 9), so that k2 = k2 (oxide
interface). Considering relation (1), it has also to be pointed out that the
amount
S0
of sensitizer in the nano-oxide layer may codetermine the
initial degradation behavior of a cell.  If the absorber layer is sufficiently
thick and well be
covered with dye molecules, excess sensitizer molecules may gradually and
temporarily replace oxidized ones as sensitizers.As discussed elsewhere [32], this
branching coefficient may actually still be one order of magnitude too small to
guarantee a 20 year lifetime of an efficient dye sensitized solar cell based on
the N3 complex. But it turned out to be already surprisingly large for Ru-dyes,
when compared with other sensitizers. However, the important finding that the
sensitizer stability depends on the photocurrent density, and thus on the solar
cell efficiency, explains, why further research on sensitizer stability is
urgently needed. The instability problems are expected be the larger the higher
dye solar cell efficiency, which is a critical factor for commercialization.More
recent systematic stability studies on dye sensitized cells performed [46]
also show a clear dependence of degradation on the nature of the electrolyte
(within 2 months approximately 40% in acetonitrile, 20% in
propionitrile, 25% in methoxypropionitrile, and 75% in
methoxyacetonitrile). But another study claims stability over 1000 hours at
high efficiency [47] using a nonvolatile unspecified electrolyte. The
experiments in this case were performed at 60°C during illumination, or at 80°C
in the dark only. Real problems are to be expected and are actually found when
the cell is allowed to heat to 80°C and beyond during periodic daylight illumination,
where complex instability problems arise [48]. Only one of the problems
involved concerns, in this case, the dye.If the simplified relation (1)
is applied, the expected
rate of dye degradation is inversely proportional to the half life time and
thus
(3)kdegr0⁢∼k2jph0S0k3[I−].
This means it is determined, besides on the
iodide concentration, by the reaction
rates k2 and k3, both of which should depend on the
nature of the electrolyte as well as on the interfacial chemistry of the sensitizer
adsorbed to oxide nanoparticles, and, of course, also on the temperature. It
also follows that a negligible photocurrent will not induce significant
degradation, as observed by comparing Figures 4 with 6. This, of
course, explains why low-efficient dye solar cells yielded an apparent good
long-term stability [17, 20]. But in order to understand reported good long-term
stability at high current density [46],  we have to assume a high rate of
regeneration k2 and a low rate of product formation k3.
Both rates may depend on interfacial properties of the nano-oxides. In
addition, a surplus of sensitizer S0 in solar cells (S0 in (3)) with a thick (15–20 μm) TiO2 layer should theoretically be of advantage. However, it
was found that, simultaneously, regeneration of S+ by I− is
faced with transport problems, which lead to enhanced degradation with
increasing layer TiO2 thickness [49].A puzzling result from our studies, apart
from the fact that initial sensitizer degradation is faster than the later
ongoing one, is that the same N3 dye is from 7 to 10 times less stable on ZnO
compared with TiO2 and also appears to depend on TiO2 preparation. This means, for example, that the
oxidation product of N3 generated on TiO2 (Figure 10(b)) is
different in its behavior from that formed on ZnO. This must indeed be
concluded since, in both cases, electron transfer to the TiO2 occurs
in the pico to femtosecond range via the same carboxyl bridges, and since the
possibly slightly different bond strength of which should only have a small
influence on the life time of the large oxidized N3 molecule. Second, the
initially faster degradation rate of N3 on TiO2, and the difference
in N3 degradation on differently prepared and inhomogeneous TiO2 layers indicate that the specific surface chemistry and surface morphology of
nano-TiO2 should be critical. Therefore, it had to be investigated
how the oxidized N3 species (Figure 10(b)) could differently interact with the
TiO2 and ZnO, respectively, to which an electron has previously been
donated.Compares the N3 ruthenium
complex, both, in the state of rapid electron recovery (Figure10(a)) and
in the state of electron extraction via a reasonably high photocurrent flow (Figure 10(b)). While present in the TiO2 interphase, the injected electron
may convert Ti4+ into Ti3+ enabling a back bonding
reaction facilitating a temporary Ti3+(ligand)Ru3+ charge
transfer complex.
(a)(b)What this interaction could involve has been
studied in a parallel paper [50]. Photocapacitive techniques (oxide layers
exposed to a gas phase studied in capacitive setup) were applied to study
sensitized TiO2 and ZnO layers in contact with a controlled gas
phase. When oxygen was removed from the gas phase, formation of Ti3+ states could be spectroscopically observed within the energy gap. When oxygen
was removed in presence of the N3 sensitized TiO2, it was found that
the N3 dye forms a charge transfer complex with Ti3+ (generated by
oxygen extraction from TiO2 into the gas phase) in the TiO2 nanomaterial surface. This charge transfer complex formation is accompanied by a
significant (0.4 eV) broadening of the N3 absorption band.This phenomenon of a charge transfer complex
(Figure10(b)) turned out to be entirely reversible and disappeared when oxygen
was added again to the gas phase bordering TiO2, so that Ti3+ states were reconverted into Ti4+ states. This remarkable effect was
only observed with the N3 dye and not observed with ZnO nor was it observed
with metal cation-free dyes (Pyrogallol, Carmine) studied on the same TiO2.Our interpretation for the exceptional
stability of N3 (and related Ru-based dyes) on TiO2 is therefore
tentatively the following: when excited
N3 is transferring an electron to TiO2 (Figure 10(b)), Ti4+ may be locally reduced to Ti3+, which now, via its d-state, can get
involved in back bonding with the oxidized N3, so that a Ti3+-ligand-Ru3+ charge transfer complex is formed. This is a kind of microscopic feedback
reaction. Macroscopically, an autocatalytic (feedback) process leads to selforganization
and to a local export of entropy. This creates local order. This may also
happen on a microscopic, macromolecular level. The feedback coupling (backbonding
of the Ti3+ d-state with the oxidized N3 dye) may stabilize the
oxidized N3 species, which temporarily forms a complex with (reduced) TiO2.
This could explain the longer survival of N3 sensitizer molecules on TiO2 compared
to ZnO in an entirely consistent way. As this phenomenon of a backbonding of N3
and of N3 stabilization had been predicted [51] in 2004 long before the
relevant experiments were made [50], this adds credibility to the here
discussed charge transfer hypothesis, which is clearly different from the
traditional approach to understand electron injection from Ru(dcbpy)2X2 with (X2 = 2 SCN, 2 CN or  dcbpy) into TiO2 [52]. It occurs
within less than 100 femtoseconds, and sensitization efficiency was found to be
dependent on the redox potential of the sensitizer. The latter observation [52]
could also be consistent with the here discussed model of a charge transfer
complex.We may talk of a light-induced charge
transfer complex formation of N3 on TiO2 nanoparticles. Since TiO2 nanomaterials require a temperature treatment for favorable sensitizer
adsorption it may be that the TiO2 surface has to be especially
activated or optimized with respect to the charge-transfer interaction
proposed. There may consequently be more favorable and less favorable nano-TiO2 substrates.On the basis of these new viewpoints on the
stability of the dye solar cell, it is interesting to reconsider the
development history of dye solar cells. The discovery of the improved
efficiency and stability of dye solar cells using N3 and related Ru-based
sensitizers [16] was essentially a chance discovery, based on a favorable
combination of Ru sensitizer and with TiO2. Before, dye solar cell experiments did not encourage major efforts, because the
sensitizers were not sufficiently stable on sintered ZnO substrates [14] and
earlier studies of Ru-dyes on TiO2 [15] did not use the most favorable
electrolytes. It also explains the comparatively flat learning curve [51] for
the dye solar cell development,
resulting from not recognizing the essential research targets. It equally explains why,
after 15 years of intensive research on the Grätzel cell [16], with much effort
in direction of cheaper dyes, expensive ruthenium-based complexes (approx. 1000 $ per gram) are still the preferred sensitizers.Now the crucial role of TiO2 chemistry should be highlighted: the most essential difference between
sensitization of ZnO and of TiO2 will be a higher life time of the
oxidized N3 complex on TiO2 due to the formation of the proposed
light-induced Ti3+(ligand)Ru3+ charge transfer complex [50]. While optimizing the expensive Ru-dye further for
still higher stability is a possible strategy, a preferable initiative would be
to learn to get cheaper metal containing dyes reasonably charge transfer
stabilized while reacting on TiO2 surfaces. Can suitable alternative
charge transfer complexes be designed to work with cheaper dyes containing more
abundant transition metals?For
answering this question the here proposed mechanism will have to be studied theoretically
and experimentally in greater detail. Since the nano-TiO2 surface is
critical for the formation of the charge transfer complex and for the rate of
reaction (k2) for formation of the oxidized sensitizer, it needs
increased attention. Specifically, it should also be investigated, whether
doping with transition metal ions, which can engage in d-state backbonding with
the sensitizer, can increase sensitizer stability.Interestingly, the here demonstrated
influence of the nature of the absorption sites on the photochemical
degradation of sensitizers has already been recognized in an early study, 35
years ago, on ZnO single crystals sensitized with chlorophyll [5]. From the
degradation curve of chlorophyll sensitized electrodes, it was deduced that
sensitizer molecules with widely different stabilities were active on the oxide
surface.In the future, the electronic properties of
TiO2 nanoparticle surfaces will have to be optimized for increased
sensitizer stability and specific new sensitizer complexes should be tailored if
advantage should be taken of the potentially low costs projected for dye solar
cells.



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*Source: 814951-2008-05-07.xml*