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SURE: Shizuoka University REpository
http://ir.lib.shizuoka.ac.jp/
Title
Improved performance of dye-sensitized solar cells using a
surface modified TiO2 electrode
Author(s)
Dissanayake Mudiyanselage Buddhi Prabodha Ariyasinghe
Citation
Issue Date
URL
Version
2013-06
http://hdl.handle.net/10297/7931
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This document is downloaded at: 2015-01-26T13:04:55Z
DOCTOR THESIS
IMPROVED PERFORMANCE OF DYE-SENSITIZED SOLAR CELLS
USING A SURFACE-MODIFIED TIO2 ELECTRODE
D.M.B.P. Ariyasinghe
Graduate School of
Science and Technology, Educational Division
Department of Optoelectronics and Nanostructure Science
Shizuoka university
Japan
June 2013
1
TABLE OF CONTENTS
THE TITLE
TABLE OF CONTENTS
ABSTRACT
ACKNOWLEDGMENTS
OUTLINE OF THE THESIS
1
2
5
8
9
CHAPTER 1
1. INTRODUCTION
11
1.1 History
16
1.2 Photosensitizer
19
1.3 Redox electrolyte
21
1.4 Semiconductor film electrode
24
1.5 Fluorine doped Tin Oxide
27
1.6 Counter Electrode
27
1.7 Operational Principle of the DSSCs
27
1.8 Charge injection, transport and recombination
29
1.9 Basic parameters to evaluate the performance of DSSCs
31
1.10 Characterization techniques of DSSCs
33
References
34
CHAPTER 2
IMPROVED PERFORMANCE OF DYE-SENSITIZED SOLAR CELLS USING A
DIETHYLDITHIOCARBAMATE-MODIFIED TIO2 SURFACE
2.1 Introduction
40
2
2.2 Experimental
42
2.3 Results and Discussion
43
2.4 Conclusions
51
References
52
CHAPTER 3
IMPROVED EFFICIENCY OF DYE-SENSITIZED SOLAR CELLS BASED
ON A DOUBLE LAYERED TIO2 PHOTOANODE
3.1 Introduction
54
3.2 Experimental
55
3.3 Results and discussion
56
3.4 Conclusions
62
References
62
CHAPTER 4
THE IMPROVED PERFORMANCE OF DYE SENSITIZED SOLAR CELL
BY PYRROLIDINEDITHIOCARBAMATE MODIFIED TiO2 SURFACE.
4.1 Introduction
64
4.2 Experimental
65
3.2.1 Preparation of DSSC
65
3.2.2 Plane-wave pseudopotential DFT calculations
66
4.3 Results and Discussion
67
4.4 Conclusions
73
References
73
3
CHAPTER 5
ENHANCEMENT
OF
THE
PHOTOELECTRIC
PERFORMANCE
OF
DYE-SENSITIZED SOLAR CELLS BASED ON A SURFACE MODIFIED TiO2
PHOTOANODE WITH OXIDIZED SULFUR
5.1 Introduction
76
5.2 Experimental
77
5.2.1 Double layer
77
5.2.2 Bilayer
78
5.3 Results and Discussion
78
5.4 Conclusions
87
References
87
CHAPTER 6
SUMMARY, CONCLUSIONS
90
LIST OF PUBLICATIONS
92
4
ABSTRACT
Dye-sensitized solar cells (DSSCs) have attracted increasing interests by the pioneering
work of O‘Regan and Grätzel in 1991. Recent development of solar cells in dye-sensitized
type devices is one great step forward in the field. The DSSCs take advantages in simple
fabrication technique and low production costs in contrast to those conventional
silicon-based solar cells. However, it remains a challenge to develop better performing
DSSCs since the efficiency of DSSCs is still much lower than that of high performance
solar cells. To meet this challenge, the different types of techniques have been studied to
achieve better TiO2 photo-electrodes in DSSCs.
This thesis presents the modification of the TiO2 photo-electrode using sulfur containing
compounds. Also different types of techniques were used with sulfur containing
compounds in the dye-sensitized solar cells to develop highly efficiency solar cell for
future applications. Abstracts of the activities have orderly pointed out below.
TiO2 paste (Ti nanoxide T, Solaronix) was used to prepare TiO2 electrode using doctor
blade method. The surface modification of a TiO2 electrode with diethyldithiocarbamate
(DEDTC) in dye-sensitized solar cells was studied. Results from X-ray photoelectron
spectroscopy (XPS) indicate that over half of the sulfur atoms become positively charged
after the DEDTC treatment of the TiO2 surface. DSSCs were fabricated with TiO2
electrodes modified by adsorbing DEDTC using a simple dip-coating process. The
conversion efficiency of the DSSCs has been optimized to 6.6% through the enhancement
of the short-circuit current density (JSC = 12.74 mA/cm2). This is substantially higher
compared to the efficiency of 5.9 % (JSC = 11.26 mA/cm2) for the DSSCs made with
untreated TiO2 electrodes.
TiO2 nanocrystalline films were prepared by using the spray pyrolysis deposition technique.
Two-step spraying and sintering double layer process assists to decrease the number of
5
cracks in the TiO2 thin film. Four different TiO2 electrodes were constructed by varying the
bottom layer thickness from 6μm to 15 μm. The conversion efficiency of the DSSCs was
improved to 8.5 % with optimized charge transfer resistance. Short-circuit current was
enhanced to 15.37 mA/cm2, compared with 10.51 mA/cm2 (conversion efficiency of
5.22 %) in the single layer TiO2 electrode.
In addition spray pyrolysis deposition technique was used to construct the TiO 2 layer.
Effect of PDTC adsorption on the surface of TiO2/FTO electrodes via dip-coating process
was studied. XPS results indicate that PDTC successfully deposited on TiO2 surface with
positively charged sulfur. We applied the resulting electrodes (PDTC- TiO2/FTO) to the
photoanode of a DSSC and observed their effect on the cell performance. When
PDTC-TiO2/FTO was used as the photoanode of a DSSC, The PDTC treatment was found
to reduction in VOC due to the positive shift of TiO2 conduction band edge, which could
cause efficient electron injection from the excited sensitizer into the conduction band of
modified TiO2 and increasing of short circuit photocurrent of DSSC was observed. Also the
presence of PDTC can diminish back electron transfer but overall conversion efficiency is
improved due to short circuit current enhancement. Finally we obtained the improved
conversion efficiency when having employed PDTC-TiO2/FTO as the photoanode rather
than that without PDTC treatment.
Furthermore visible light enhanced pyrrolidinedithiocarbamate added titanium dioxide
(TiO2) thin films were prepared by the sol–gel method. TiO2-PDTC/TiO2/FTO double layer
thin film was enhanced visible light harvesting in TiO2 electrode. The performance of the
solar cell was greatly influenced by the amount of PDTC added to the sol–gel solution. The
greatest visible light absorption was observed with 0.1M of PDTC in the precursor solution.
These double layered TiO2 thin film was used as visible light harvesters. When deposited
directly on FTO electrodes, photo-conversion efficiencies were reduced. However, the
6
opposite configuration, with PDTC added thin films on top of nanoporous TiO2, yielded an
increased open circuit voltage of 0.83 V, a short-circuit current density of 17.80 mA/cm2,
and an overall conversion efficiency of 9.45% greater than that of a bare cell. These
outcomes show the PDTC treatment can also help to construct a better surface to enhance
the dye adsorption in TiO2 electrode.
7
ACKNOWLEDGEMENTS
First of all, I really would like to convey my most sincere thanks and gratitude to my
supervisor, Prof. Masaru Shimomura, Graduate School of Engineering in Shizuoka
University, Hamamatsu for his sincere support, supervision and encouragement throughout
this project. His guidance and valuable advices lead to the completion of this thesis.
I also wish to express my gratitude and thank to Prof. K. Murakami, Graduate School of
Engineering in Shizuoka University, Hamamatsu for giving me an opportunity to use his
laboratory materials and instruments during the analysis. His useful advices and
discussions assist to overcome the difficulties during the research study.
I would like to thank Prof. Akinori Konno, Department of Materials Science & Technology,
Faculty of Engineering, Shizuoka University, Hamamatsu for giving his laboratory
materials for research works.
I wish to thank Prof. R.M.G. Rajapakse, Prof. H.M.N. Bandara, Prof. O.A.Ileperuma, Dr.
Kumara Department of Chemistry, University of Peradeniya, Sri Lanka for their supportive
advices on my research work.
I would like to thank all my wonderful friends and my laboratory colleagues we have
created quite an interesting family, one that will be difficult to achieve ever again. I have a
learned a great deal from all of you, not only about science, but also about living and
working together.
Finally, I want to extend my heartfelt gratitude to my parents for their financial support and
encouragement during the PhD works.
8
OUTLINE OF THE THESIS
This thesis mainly contains five chapters under the topic of improved performance of
dye-sensitized solar cells (DSSC) using a surface-modified TiO2 electrode.
Chapter 1 explains the broad description about dye-sensitized solar cells with their
historical background. Additionally properties of the constituents are discussed in details as
well. Operational principles, characterization techniques and kinetics of charge transfer in
the DSSCs are also discussed.
Chapter 2 presents the improved performance of dye-sensitized solar cells using a
diethyldithiocarbamate (DEDTC)-modified TiO2 surface. TiO2 paste was used to prepare
TiO2 electrode using doctor blade method. The adsorption configurations of DEDTC on
the TiO2 particles were studied by XPS analysis. Furthermore the effect of the surface
treatment on the performance of DSSCs was also investigated using I–V characteristics,
UV-Visible absorption spectra and electrochemical impedance characterization.
Chapter 3 explains the improved efficiency of dye-sensitized solar cells based on a double
layered TiO2 photoanode. Conversion efficiency enhancements of DSSCs were discussed
in details using I-V parameters. Structure modifications are discussed in details using XRD,
FIB and SEM analysis methods. In addition double layered TiO2 photo anodes were further
analyzed by using dark current and electrochemical impedance characterization.
chapter 4 elaborates the improved performance of dye sensitized solar cells by
pyrrolidinedithiocarbamate (PDTC) modified TiO2 surface. Spray pyrolysis deposition
technique was used to construct the TiO2 layer. Performance of DSSCs was also examined
9
using I–V characteristics. XPS analysis was used to study the adsorption configurations of
PDTC on the TiO2 particles. Furthermore adsorption stability of PDTC on the (001)
surfaces of TiO2 anatase are studied by density functional theory calculations.
Chapter 5 presents enhancement of the photoelectric performance of dye sensitized solar
cells based on a PDTC modified TiO2 bilayered photoanode. Also the effect of the surface
treatment on the performance of DSSCs was investigated using I–V characteristics and
photocurrent action spectra.
The modified TiO2 photo anodes were further analyzed by
using XPS analysis, dark current measurements, UV-Visible absorption spectra and XRD
characterization.
10
CHAPTER 1
1. Introduction
It has been universally observed that the global energy use is presently at the level
of 13 TW, and ever mounting with increasing population and devices that use energy.
Unfortunately, a greater part of this energy comes from non-renewable sources. Fuel cells
are exceptional in producing energy. Many researchers focus on its production, delivery,
storage, and use of hydrogen. Although there is not an urgent need for a change to
renewable resources, the scarcity of such supplies could arrive as early as 2050 [1]. For
decades now, the perception of diminishing fossil fuels has motivated governments,
economists, and scientists to find an alternative. It has to be not only perpetual in supply,
but also inexpensive and comparable in terms of storage density and capacity to fossil
fuels.
The current alternative energy sources allude paradoxically to the four ancient
elements- earth, wind, water, and fire. On one hand geothermal heating currently produces
over 8,000 MW of power and also it provides the opportunity to harvest more heat from
deep beneath the Earth‘s crust. Power that originates from steam plants results in water
vapor as the primary byproduct. It also generates small amounts of NO2, CO2, and sulfur.
There are also a variety of challenges associated with maintaining an efficient geothermal
power plant including improved heat exchange and stability in corrosive environments. On
the other hand wind power results in zero emissions. But it can only be effectively utilized
in a limited number of locations. Its initial costs are large and require large areas of land to
establish an effective ―wind farm‖. Moreover hydroelectric power is also limited in sources
and its capacity has been essentially maximized.
By the year 2050, it is estimated that the global need for carbon-free energy will
be between 10 – 30 TW, depending on CO2 levels at the time. The current global
11
population consumes 13 TW of energy in all forms. In other words there will be a need to
have 1-3 times of increase in energy. Unfortunately only a little of it will be available from
fossil fuel sources. Therefore, people starts searching for new resources for energy
production. At the same time, people also realized the damages that the energy production
has done to the environment. Therefore, researches on ‗clean energy‘ have increased
rapidly within the past 30 years. One of the most prominent clean energy is solar energy.
However solar energy is a constant renewable resource (within the geological
timescale) which is available in many areas of the world and which offers a continuous
increase of realizable efficiencies. ―In this era of depleting fossil fuel resources, it would be
highly desirable to have an efficient and economical way of directly converting and storing
solar energy as a chemical fuel‖ [2]. This statement, made by Bolton shortly after the gas
crisis in the 1970‘s seems truer than ever with the nation‘s current dependence on foreign
sources of fuel and increasing costs of that fuel. The solar power reaching the Earth from
the sun is 1.76 x 105 TW and of this, 600 TW hits the Earth in areas where solar energy
collection is a feasible option [3].
The near blackbody radiation of the sun drives all solar devices and supports
terrestrial life as well. Air mass zero (AM0) radiation is defined as the intensity of light of
the sun at a distance of the Earth to the sun (92 million miles), outside the Earth‘s
atmosphere. The accepted value of AM0 sunlight is 1.353 kW/m2. But it is satisfied by
nearly 30 % inside the Earth‘s atmosphere from Rayleigh and Mie scattering back into
space and absorption by molecular gases in the atmosphere. AM1 radiation is the incident
radiation intensity on the Earth‘s surface when the sun is directly overhead whereas AM1.5
has been taken as the U.S. government terrestrial standard for incident light hitting the
Earth (37º tilt, 48.19ºs latitude), normalized to a total power density of 1 kW/m2. Along
with direct incidence, diffused radiation must be taken into account when examining the
12
makeup of sunlight. Although highest on cloudy days, 10-20 % of solar irradiance on clear
days is distributed and 50 % on days that are between clear and cloudy. This diffused
sunlight is often wasted in solar tracking systems since the concentrator modules must
always be normal to the direct radiation. Integration of novel optical designs, such as
birefringent concentrators, could lead to the capture of otherwise wasted diffused light,
thereby increasing the efficiency of these expensive systems.
Modern technologies enable solar energy to be used in two different ways. They
are thermal conversion and photo electricity conversion. Solar thermal energy can be used
on heating systems such as water cylinders or household heaters. Photo-electricity
conversion, on the other hand, can be used on household or industrial electricity power
generation. It might one day replace the conventional thermal power generation and can
decrease the amount of fossil oil required. Relaxing the existing tension of fossil fuel it
would result in an extension of the date when the fossil fuel is used up.
Photo-electricity conversion converts solar light into electricity by utilizing the
energy from photons to break off the bond between the atom and the electrons so that the
electrons become free electrons. A solar cell, silicon crystalline solar cell for example,
contains n-type silicon and p type silicon combined together, so that the free electrons from
‗n‘ type silicon can be attracted to the holes in ‗p‘ type silicon and creates electron pairs.
As the photon enters the material, it separates the electron pairs into free electrons and
holes. With a little help from the electric field at the p-n junction, the separated electron
and hole flows in opposite directions away from each other, into the electric load without
recombining with each other. Out of all the different types of solar cells, crystalline silicon
solar cell is the leading type of cells used in power generation due to its high power
efficiency. The common solar power conversion efficiencies are between 15 and 20% [3].
However, the comparatively high manufacturing cost of these silicon cells has prevented
13
their widespread use. Another disadvantage of silicon cells is the use of toxic chemicals in
their manufacture. These aspects encouraged the search for environmentally friendly and
low cost solar cell alternatives.
Driven by the motivation to search for alternatives, there is an increasing
awareness of the possible advantages of devices based on mesoscopic inorganic or organic
semiconductors commonly referred to as ‗bulk‘ junctions due to their interconnected
three-dimensional structure. These are formed, for example, from nanocrystalline inorganic
oxides, ionic liquids and organic hole-conductor or conducting polymer devices. They
offer the prospect of very low cost fabrication without expensive and energy intensive high
temperature and high vacuum processes. They are compatible with flexible substrates and
a variety of embodiments and appearances to facilitate market entry, both for domestic
devices and in architectural or decorative applications. It is now possible to depart
completely from the classical solid state cells, which are replaced by devices based on
interpenetrating network junctions. The mesoscopic morphology produces an interface
with a huge area, endowing these systems with intriguing optoelectronic properties.
Contrary to expectation, devices based on interpenetrating networks of semiconductors
have shown strikingly high conversion efficiencies, which compete with those of
conventional devices.
The model of this family of devices is the dye sensitized solar cell (DSSC),
invented in the author‘s laboratory at the Ecole Polytechnique F´ed´erale de Lausanne[4].
This accomplishes the separation of the optical absorption and the charge separation
processes by the association of a sensitizer as light absorbing material with a wide band
gap semiconductor of mesoporous or nanocrystalline morphology [4, 5]. The DSSC
currently reaches >11% energy conversion efficiencies under standard reporting conditions
(AM1.5 global sunlight at 1000 W/m2 intensity, 298 K temperature) in liquid junction
14
devices rendering it a credible alternative to conventional p-n junction photovoltaic devices
[6]. Solid state equivalents using organic hole-conductors have exceeded 4% efficiency [7]
whereas nanocomposite films composed of inorganic materials, such a TiO2 and CuInS2
have achieved efficiencies between 5 and 6% [8, 9]. New dyes showing increased optical
cross-sections and capable of absorbing longer wavelengths are currently under
development. Similarly, the performance of mesoscopic TiO2 films employed as electron
collectors is benefiting greatly from recent advances in nanomaterial research. Taking
advantage of the highly transparent nature of the sensitized nanocrystalline oxide film a
tandem structure employing a DSSC and CIGS to and bottom cell having a conversion
efficiency >15% has been realized [10].
Obtaining long term stability for DSSCs at temperatures of 80–85 0C has long
been a major challenge for over 10 years and has only recently been achieved [11]. Solvent
free electrolytes such as ionic liquids or solid polymers as hole-conductors have been
introduced to provide solutions for practical applications. Stabilization of the interface by
using self assembly of hydrophobic sensitizers in conjunction with amphiphilic
coadsorbents has been particularly rewarding. Stable operating performance under both
prolonged thermal stress (at 85 0C) and AM1.5 light soaking conditions (at 60 0C) has been
achieved [12]. These devices retained 98% of their initial power conversion efficiency
after 1000 h of high temperature aging. Long term hastened testing shows that DSSCs can
function in a stable manner for over 20 years. That is if the interfacial engineering issues
are properly addressed. The present review gives an overview of the state-of-the-art of the
academic and industrial development of this new solar cell, emphasis being placed on the
work performed in the laboratories.
15
1.1. History
The photoelectric effect was first invented in 1839 by Edmund Becquerel, a nineteen
year old French physicist. His discovery was that certain materials would produce small
amounts of electric current when they are exposed to light.
In the 1860s, while testing underwater telegraph lines for faults using a material called
selenium, an electrician called Willoughby Smith, by chance, discovered that electricity
traveled through selenium very well when it was in light. But it didn't if the selenium was
in darkness. In the late 1870s, two American scientists, William Adams and Richard Day,
became interested in this. They soon discovered that the sun's energy creates a flow of
electricity in selenium.
With the renewed interest over the next ten years, scientists worked hard to understand
more about selenium. Then in the early 1880s, Charles Fritts invented the first PV cell by
putting a layer of selenium on a metal plate and coating it with gold leaf. Placed in the
sunlight, this cell made even more electricity but not enough to be useful. One or two
scientists became very interested about this invention, but most scientists paid no attention
to it. Some thought it was just a worthless trick. Based on what they knew about black
materials capturing the sun's heat energy, they couldn't make out how a cell that was not
black could use the sun's light to make electricity. The idea that PV technology was
competing with other better developed technologies that were generating electricity was
not able to provoke the interest. Steam-driven electricity generators (or 'dynamos' as they
were called at the time) had been around since Michael Faraday invented the first
electromagnetic generator in 1831. By the end of the 19th Century, this technology had
improved a lot. In 1882, Thomas Edison opened his first electric power station in New
York. It used coal to create steam.
16
However Albert Einstein was one scientist who set his mind to understand how light
could create electricity when it hits a metal. In 1905, Albert Einstein surprised everyone.
Einstein explained how light was made of tiny packets of energy that wiggled like waves
as they sped along. He called these energy packets "lichtquant" which meant ―the light
quantum". This concept was later called ‗photons‘ (1927). Einstein further argued that
these particles of energy are much more powerful in invisible light (such as ultraviolet
light) than they are in light we can see. In fact they have enough energy to remove
electrons from some materials like selenium and silicon. It is these free electrons that move
through wires as electricity. Following the example of Einstein, other scientists tested these
ideas and found that they seemed to be right. It was these ideas that later won Einstein a
Nobel Prize. Einstein's ideas also became very important in scientists‘ efforts to make PV
cells more effective in using sunlight to generate electricity.
Modern application of photovoltaic device was initiated in 1954. The researchers at
Bell Labs in the USA discovered that a voltage was produced by the pn junction diodes
under room light. In the same year, they produced a Si pn junction solar cell with 6%
efficiency [13], which is a landmark of photovoltaic technology. Not more than within a
year, a thin-film heterojunction solar cell based on Cu2S/CdS also achieved 6% efficiency
[14]. A year later, a 6% GaAs pn junction solar cell was reported by RCA Lab in the US
[15]. With in a year, Hoffman Electronics (USA) offered commercial Si photovoltaic cells
with 2% efficient at $1500/W. The efficiency record was revived quickly by this company
– 8% in 1957, 9% in 1958 and 10% in 1959. By 1960, fundamental theories of pn junction
solar cell were developed to explain the relation between band gap, incident spectrum,
temperature, thermodynamics, and efficiency [16-19]. In 1962, the first commercial
telecommunication satellite Telstar powered by a photovoltaic system was launched. In
1963, Sharp Corporation (Japan) produced the first commercial Si modules.
17
The year 1973 marked a turning point for photovoltaics. International oil crisis
prompted many countries to seek for renewable energy including photovoltaics. Moreover,
a great improvement was made in GaAs photovoltaic device, which attained an efficiency
of 13.7% [20]. During 1970–1979, many big photovoltaic companies such as Solar Power
Corporation (1970), Solarex Corporation (1973), Solec International (1975) and Solar
Technology International (1975) were established. The first book dedicated to PV science
and technology by Hovel (USA) was also published in 1975. The photovoltaic technology
developed very fast in the 1980s. The first thin-film solar cell with over 10% efficiency
was produced in 1980 based on Cu2S/CdS. ARCO Solar was the first company to provide
photovoltaic modules with over 1 MW per year (1982). In 1985, researchers of the
University of New South Wales (Australia) formulated a Si solar cell with more than 20%
efficiency under standard sunlight [21]. In 1986, ARCO Solar produced the first
commercial thin film photovoltaic module. British Petroleum (UK) obtained a patent for
the production of thin-film solar cell in 1989. In 1990s, the market of photovoltaic
continued to grow steadily. The photovoltaic production of the entire world reached 100
MW per year in 1997 and this value increased up to 1000 MW per year by 1999. One
important event that took place during this decade was the emergence of GaInP/GaAs
multijunction solar cell with efficiency over 30% (NREL, USA, 1994) [22].
Since the mid 1980s, Gratzel‘s group has been the main driving force for development
of dye sensitized solar cells in the Laboratory of Photonics and Interfaces in the EPFL
Switzerland. They have developed successful combination of nanostructured electrodes
with efficient charge injection dyes. However, the use of dye-sensitization in photovoltaics
remained rather unsuccessful until a solar cell with energy conversion efficiency exceeding
7% in 1991 [4] and 10% in 1993 [23] was developed by Grätzel and his co-workers. This
solar cell was named ‗the dye-sensitized nanostructured solar cell‘ or ‗the Grätzel cell‘
18
after its invention. In contrast to all solid conventional semiconductor solar cells, the
dye-sensitized solar cell is a photoelectrochemical solar cell. In other words it uses a liquid
electrolyte or other ion-conducting phase as a charge transport medium. Thanks to the high
efficiencies and good long-term stability reported for the dye-sensitized solar cells, the
research interest in this technology grew rapidly during the 1990's. While the patent
holders and the licensees developed the original patented concepts towards practical
products, numerous research groups were engaged in exploring the replacement of the
original materials with new ones.
1.2 Photosensitizer
Dye molecules of proper molecular structure are used for sensitized wide band gap
nanostructured photoelectrode. Upon absorption of photon, a dye molecule adsorbed in to
the surface of the nanostructured TiO2 gets oxidized and the excited electron is injected
into the nanostructured TiO2. Polypyridyl compounds of Ru(II) which has been
investigated widely were among the first promising sensitizers. Many researches have paid
attention on molecular engineering of ruthenium compounds. Nazeeruddin et al. have
reported the ―black dye‖ as a promising charge transfer sensitizer in DSSC [23].
Figure 1.1 Ruthenium based "N3"adsorbed onto a titanium dioxide surface [24].
19
Gratzel group developed many Ru complex photosensitizers. One famous example is the
Di-tetrabutylammoniumcis-bis(isothiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylato)ruthe
ium(II), coded as N719 it has been an outstanding solar light absorber and charge-transfer
sensitizer. The red dye or N719 dye (of which the structure is shown in the figure above) is
capable of absorbing photons of wavelength ranging from 400 nm to 900 nm because of
metal to ligand charge transfer transition. Commercialized dye sensitized solar cells and
modules use ruthenium bipyridyl–based dyes (N3 dyes or N719) which have achieved
conversion efficiencies above 10% [23]. Kelly, et.al studied other ruthenium complexes
Ru(dcb)(bpy)2
[25],
Farzad
et
al.
explored
the
Ru(dcbH2)(bpy)2(PF6)2
and
Os(dcbH2)(bpy)2-(PF6)2 [26], Qu et al. studied cis-Ru(bpy)2(ina)2(PF6)2 [27] , Shoute et
al. investigated the cis-Ru(dcbH2)2(NCS) [28], and Kleverlaan et al. worked with
OsIII-bpa-Ru [29]. Calogero et al. suggested that ―Finding appropriate additives for
improving open circuit voltage VOC without causing dye degradation might result in a
further enhancement of cell performance, making the practical application of such systems
more suitable to economically viable solar energy devices for our society.‖ [30].
Theoretical Study of new ruthenium-based dyes for dye sensitized solar cells by Monari et
al., states ―The UV/vis absorption spectra have been computed within the time-dependent
density functional theory formalism. The obtained excitation energies are compared with
the experimental results.‖ [31]In fact, for dye molecule to be excellent sensitizer, it must
possess several carbonyl (C=O) or hydroxyl (-OH) groups capable of chelating to the
Ti(IV) sites on the TiO2 surface as shown in the figure above [32].
An efficient photosensitizer must fulfill certain requirements such as [33]:
•
An intense absorption in the visible region.
•
Strong adsorption onto the semiconductor surface.
20
•
Efficient electron injection into the conduction band of the semiconductor.
•
Possession of several O or –OH groups which are capable of chelating to the Ti(IV)
sites on the TiO2 surface.
Likewise, it must be rapidly regenerated by the mediator layer to avoid electron
recombination processes and be fairly stable, both in the ground and excited states. The
ideal sensitizer for a photovoltaic cell which converts standard air mass (AM) 1.5 sunlight
to electricity, must absorb all light below a threshold wavelength of about 900 nm, which
is equivalent to a semiconductor with a band gap of 1.4 eV [34]. On the whole the cell
performance is subjected to a number of factors but fundamental considerations relating to
the dye are how efficiently:
•
The molecules absorb incident photons.
•
Photons are converted to electron–hole pairs.
•
Separation and collection occurs.
Figure 1.2. Most frequently applied ruthenium polypyridyl complexes
21
1.3 Redox electrolyte
Electrolyte containing I/I3 redox ions is used in DSSC to regenerate the oxidized
dye molecules and hence to complete the electric circuit by mediating electrons between
the
nanostructured
electrode
and
counter
electrode.
NaI,
LiI
and
R4NI
(tetraalkylammonium iodide) are well known examples of mixture of iodide usually
dissolved in nonprotonic solvents such as acetonitrile, propylene carbonate and
propionitrile to make electrolyte. Cell performance is greatly affected by ion conductivity
in the electrolyte which is directly affected by the viscosity of the solvent. Thus, solvent
with lower viscosity is highly recommended. Moreover, counter cations of iodides such as
Na+, Li+, and R4N+ do affect the cell performance mainly due to their adsorption on
nanostructured electrode (TiO2) or ion conductivity. It has been found that addition of
tert-butylpyridine to the redoxing electrolyte improves cell performance [25]
In general, following are the criteria for materials to serve as electrolytes in DSSCs [35,
36]:
a) The redox potential of electrolyte should be negative as compared to the oxidation
potential or HOMO of the dye
b) The electrolyte should efficiently regenerate the dye after the process of dye
excitation and electron injection to conduction band of oxide semiconductor
c) It should have high conductivity (~10–3 S.cm–1)
d) It should infiltrate the pores of the photoanode and establish contact with both the
electrodes
e) It should not cause desorption of the dye from the photoanode
f) It should not react with the sealant and degrade it leading to poor stability of the
cell
22
g) The absorption of light by the electrolyte in the visible range, in which dye
molecules absorb, should be minimum
h) It should not undergo any chemical change leading to loss of its functionality
i) It should be stable up to ~800C.
Short circuit current density (Jsc) and open circuit voltage (Voc) considerably depend
on the electrolyte. The electrolyte used in DSSC mostly contains I−/I3− redox ions, which
mediate electrons between the TiO2 photoelectrode and the counter electrode. The first
DSSC used organic liquid electrolyte containing lithium iodide/iodine. Organic solvent is a
basic component of the liquid electrolytes as it provides an environment for I−/I3−
dissolution and diffusion. Physical parameters such as donor number, dielectric constant
and viscosity affect the efficiency of the cell [36]. The electrolyte is a neutral sink of I−/I3−
feeding the reactions at;
Cathode:
Cell:
I3− + 2e− (Pt) → 3I−
e− (Pt) + h → 3I−
It also maintains the redox potential in the bulk of the electrolyte via the fast redox reaction
of the I−/I3− pair. This redox reaction in the electrolyte is a two-electron reaction [37]:
3 I− →I3− + 2e−
Which is composed of a series of successive reactions:
(I− → I + e−) × 2 charge transfer reaction
2I → I2 fast chemical reaction
I2 + I− → I3− fast chemical reaction
23
The energy conversion efficiencies reported up to 11% [38] are typically achieved
with liquid electrolytes based on acetonitrile solvent, a low-viscosity volatile solvent, and
by using comparatively low iodine concentration. These high efficiency electrolytes are not
at the same time optimized for achieving the best long-term stability characteristics. For
those other electrolyte formulations with less volatile solvents or ionic liquids, higher
iodine concentrations are more appropriate. These latter electrolytes are often referred to as
robust electrolytes in the literature and their application leads to a lower-efficiency output
in the range of 7–9%. Next to the redox-active species and solvents, the electrolytes
contain typically some additives. The function of these additives is in most cases to reduce
the dark current by coordination to the TiO2 surface and thus to improve the VOC of the
solar cells. Table 1.1 compares the compositions of a widely used high-efficiency
electrolyte and a popular robust electrolyte.
Table 1.1: Typical current solvent-based electrolyte compositions (ACN = acetonitrile, VN
= valeronitrile, MPN = 3-methoxypropionitrile, PMII = 1-propyl-3-methylimidazolium
iodide, GuSCN = Guanidinium thicocyanate, TBP = tert-butylpyridin, NBB =
1-butyl-1H-benzimidazol)
High 
Robust
Solvent
ACN/VN (3/1)
MPN
Iodide compound
1M MPII
1M MPII
Iodine
0.03M I2
0.15M I2
Additive 1
0.1M GuSCN
0.1M GuSCN
Additive 2
0.5M TBP
0.5M NBB
Efficiency
10.5% ( < 1cm2)
9% ( < 1cm2)
1.4 Semiconductor film electrode
The semiconductor structure which is typically 15 ϻm thick with a porosity of
50% has a surface area available for dye chemisorptions bigger over a thousand times than
24
that of a flat, unstructured electrode of the same size. If the dye is chemisorbed as a
monomolecular layer, an adequate amount of dye can be retained on a given area of
electrode to provide absorption of essentially all the incident light. The need for DSSC to
absorb far more of the incident light was the driving force of the development of
mesoscopic semiconductor material (minutely structured materials with an enormous
internal surface area).
Mesoporous oxide films are made up of arrays of tiny crystals measuring a few
nanometers across. Oxides such as TiO2, zinc oxide, tin oxide, niobium oxide or
chalcogenides such as cadmium selenide are the preferred photoelectrodes. These are
connected to allow electronic conduction to take place. Between the particles, mesoscopic
pores are filled with a semiconducting or a conducting medium such as a p-type
semiconductor, a polymer, a hole transmitter and an electrolyte. The net result is a junction
of extremely large contact area between two interpenetrating, individually continuous
networks [5].
Photoelectrodes which are made of materials such as silicon and cadmium sulfide
decompose under irradiance in solution owing to photocorrosion. In contrast, oxide
semiconductor materials, especially TiO2, have good chemical stability under visible
irradiation in solution [39]. It has been found that TiO2 is a stable photoelectrode in
photoelectrochemical systems even under extreme operating conditions. It is cheap, readily
available and non-toxic and is normally used as dye in white paint and toothpastes. Its
conduction band edge coincides well with the energized electronic level of anthocyanin
containing dyes which is an important condition to be satisfied for the injection of
electrons from the dye molecule to the semiconductor [40]. The high dielectric constant of
TiO2 (C-- = 80 for anatase) provides good electrostatic shielding of the injected electron
from the oxidized dye molecule attached to the TiO2, thus preventing their recombination
25
before reduction of the dye by the redox electrolyte. High refractive index of TiO2 (n = 2.5
for anatase) results in efficient diffuse scattering of the light inside the porous
photoelectrode, which significantly enhances the light absorption [41].
The dye adsorption and microstructure of the TiO2 film are important properties
when it is used as photoelectrode for DSSCs [42]. TiO2 occurs in three crystalline forms –
rutile, anatase and brookite. Anatase appears as pyramid-like crystals and is stable at low
temperatures whereas needle-like rutile crystals are dominantly formed in high temperature
processes. Rutile absorbs 4% of the incident light in the near-UV region, and band gap
excitation generated holes that act as strong oxidants reducing the long-term stability of the
DSSCs. Brookite is difficult to produce and is therefore not considered in DSSC
application. The band-gap of anatase is 3.2 eV at an absorption edge of 388 nm and that of
rutile is 3.0 eV at an absorption edge of 413 nm [43].
There are several ways in preparing TiO2 for DSSC fabrication. TiO2 is generally
prepared in sol-gel form or colloidal form. Commercialized TiO2 sol-gel products that can
provide high performance efficiency (over 6%) are available from the industry for screen
printing deposition method. However, because the material is already prepared by the
manufacturer, changes of the controllable conditions of the material that are to be made are
very limited. Colloidal form, on the other hand, is difficult for achieving high efficiency
but there are more variables that can be controlled in fabrication. Currently, the most
common way of preparing TiO2 is by dissolving commercially manufactured TiO2 P25
powder in deionised water. This powder contains 80% of Anatase and 20% Rutile
crystalline structures. A mix of Anatase and Rutile can provide much better efficiency
because Anatase crystalline has smaller resistivity and Rutile can provide photon scattering,
while increasing the percentage of electron excitation. The prepared colloidal TiO2 can be
deposited onto the substrate by spray pyrolysis deposition technique, doctor blade
26
technique, screen printing, electrophoretic deposition and tape casting method.
1.5 Fluorine doped Tin Oxide (FTO)
FTO is another type of TCO that have been widely used, especially in solar cells.
This is due to its good stability at high temperature and its competitive cost in comparison
to ITO. SnO2 itself is a semiconductor with very low conductivity and wide band gap
(around 4 eV). An extrinsic dopant, such as Sb or F, is added into the material. Fluorine
doped SnO2 is more commonly used than the material doped with Sb. This is due to the
variation in resistivity with the amount of doped Sb. Another advantage of FTO is that it
has high transmittance (> 80 % or 85 % depending on the thickness), especially in visible
wave region [44]. Its resistivity can be as low as 2 10-4Ω-cm, depending on the thickness
of the film [45].
1.6 Counter Electrode
Counter electrode in DSSC needs to provide high conductivity as it needs to
provide the liquid electrolyte electrons to complete the redox reaction in very short time for
lifetime stability and to prevent the electron recapture. Currently, the most commonly used
material is Pt. This is because Pt has high electron mobility that can regenerate the
electrolyte rapidly. Moreover, literatures show that, for example, using gold as the counter
electrode and it has been found that the electrolyte corrodes gold [46]. Pt, on the other hand,
has high stability against electrolyte‘s corrosive characteristic.
1.7 Operational Principle of the DSSCs
Mechanism of DSSCs is quite different from conventional solar cell mechanism.
In conventional solar cells, silicon acts as both the source of photoelectrons and potential
27
barrier to separate electric charges ("electrons‖ and ―holes"). However, in DSSCs,
photoelectrons are provided by photosensitive dyes, and the semiconductor thin films
(TiO2, SnO2, and ZnO) work with the liquid electrolyte to separate the charge.
Grätzel‘s DSSC includes three main components:
(1) A transparent conductive FTO (SnO2:F) substrate as the anode and a film (~15
µm) of wide band gap semiconductor nanoparticles, such as TiO2,
(2) A monolayer of dye molecules adsorbed onto the semiconductor,
(3) A separate backing with thin layer of liquid electrolyte containing the redox couple
I- /I3Finally, the front and the back parts are sealed together to prevent the leaking of
electrolyte. During the DSSC operation, firstly the sunlight that enters the cell from the
top, travels through FTO substrate, the TiO2 film and then hits the sensitizer dye.
Photon-excited dye
to
molecules get
be "injected" directly into the TiO2
across the
band gap, and
cause electrons
films (hυ = proton, D = dye).
hυ + 2D → 2D+ + 2e-,
This step is called the electron injection process [47]. Secondly, the depleted dye is
regenerated by electron donation from the electrolyte, the iodide/ triiodide couple. The
depleted dye strips one electron from the iodide electrolyte and oxidizes it into triiodide.
2D+ + 3I- → 2D + I3This step occurs quite quickly compared to the time that takes for a neighboring
molecule to be photo excited. It means that the holes do not last very long before being
neutralized by the electrolyte. This step is called the regeneration process [48]. The iodide
is regenerated, in turn, by the reduction of triiodide at the counter electrode, which
28
reintroduces the electrons after flowing through the external circuit with Pt as catalyst.
I3- + 2e- → 3I-,
For a low-resistant electron transfer, the counter electrode is covered with some
Pt which acts as a catalyst for the redox reaction [49]. By and large, the device generates
electrical energy from sunlight without being subject to any permanent chemical
transformation of cell components.
Figure 1.3 Cell configurations of DSSC and its mechanism under illumination
1.8 Charge injection, transport and recombination
Kinetics of electron injection into the semiconductor photoelectrode after being
excited from the photosensitizer has been investigated by many researchers using
time-resolved laser spectroscopy [50]. It has been found that both the configuration of the
photosensitizer material and the energy separation between the conduction band level of
the wideband gap semiconductor and the LUMO level of the photosensitizer are greatly
affecting the electron transfer rate to the wideband gap semiconductor. The figure below
29
shows a schematic illustration of kinetics in the DSSC. The shown arrows indicate
excitation of the dye from the HOMO to the LUMO level, relaxation of the exited state (60
ns), electron injection from the dye LUMO level to the TiO2 conduction band (50 fs -1.7
ps), recombination of the injected electron with the hole in the dye HOMO level (ns -ms),
recombination of the electron in the TiO2 conduction band with a hole (I3-) in the
electrolyte (10 ms), and the regeneration of the oxidized dye by I(10 ns) [51].
Figure 1.4 Schematic illustration of kinetics in the DSSC, depicted from [51].
It has been confirmed that electron injection from the excited dye such as the
N719 dye complex into the TiO2 conduction band (CB) is a very fast process in
femtosecond scale. The reduction of the oxidized dye by the redox electrolyte‘s I- ions
occur in about 10-8 seconds. Recombination of photoinjected CB electrons with oxidized
dye molecules or with the oxidized form of the electrolyte redox couple (I3-ions) occurs in
microseconds [51]. To achieve good quantum yield, the rate constant for charge injection
should be in the picosecond range. In conclusion, Fast recovery of the sensitizer is
important for attaining long term stability. Also, long-lasting charge separation is a very
important key factor to the performance of solar cells.
30
1.9 Basic parameters to evaluate the performance of DSSCs
The performance of DSSCs is usually evaluated by the following four parameters:
(1) Open circuit photovoltage (Voc )
The open circuit photovoltage (Voc ) is the cell voltage measured when the current
within the cell is equal to zero.
(2) Short circuit photocurrent (Isc )
The short circuit photocurrent (I sc) is the cell photocurrent measured at zero voltage.
In general, it is presented in the form of the short circuit current density (Jsc ) defined as the
ratio of the short circuit photocurrent to the active cell area.
(3) Fill factor (FF)
Another important characteristic of the solar cell performance is the fill factor (FF)
and the point at which the product of current and voltage is maximized in the IV-curve is
called the maximum power point (MPP). These points are shown in the above figure in the
I-V curve. And the fill factor (FF) is defined as
FF 
VMPP  I MPP
VOC  I SC
Where VMPP and IMPP are the voltage and current at the maximum Power Point in the I-V
curve of the cell respectively.
31
(4) Energy conversion efficiency (η)
The energy conversion efficiency of the solar cell is defined as the maximum power
produced by the cell (Pmax) divided by the power of the incident light on the representative
area of the cell (Plight):

Pmax
Plight

or
VOC  I SC  FF
Pl i g h t
The efficiency of the solar cell depends on the temperature of the cell and even more on the
quality of the illumination, i.e. the total light intensity and the spectral distribution of the
intensity. For this reason, a standard measurement condition has been developed to
generalize the testing of solar cells at any laboratory. The standard condition that is used to
test a terrestrial solar cell is light intensity of 1000 W/m2 at Air Mass 1.5 when the
temperature of the cell is at 25 C. The power output of the solar cell at these conditions is
the nominal power of the cell, or module, and is reported in peak watts (Wp).
Using the sun for solar cell testing is possible, but not always practical because of
the intensity and spectral variations that occur over the Earth‘s surface daily and seasonal
weather changes in a given location. For this reason, solar simulators are often used to
mimic the solar spectrum. These simulators typically consist of Xe arc lamp housing with a
variety of filters to mimic the spectrum at a variety of air mass densities. The units also
contain a beam homogenizer that assures equivalent incident power densities across the
irradiation spot. Xe arc lamps are used exclusively for solar simulators as they most closely
match the solar spectrum, compared to metal halide lamps, and in most cases they can be
used without additional filters other than a low density heat absorbing filter (KG, Schott).
32
1.10 Characterization techniques of DSSCs
The basic characterization techniques of DSSCs are described as follows.
(1) Photocurrent-photovoltage (I-V) measurement
The photocurrent-photovoltage measurement of a DSSC is recorded using a
calibrated solar cell evaluation system (JASCO, CEP-25BX) under simulated sunlight. A
typical I-V curve is shown in the figure below.
During the I-V measurement, four parameters mentioned above (Voc, Jsc, FF and
η) will be determined.
Isc
MPP
IMPP
Current
Power curve
Voltage
VMPP
Voc
Figure 1.5. Current-Voltage (Blue) and power-Voltage (Green) characteristic of a solar cell
33
(2) Incident photon-to-electron conversion efficiency (IPCE) measurement
The sensitivity of a DSSC varies with the wavelength of the incident light. IPCE
measures the ratio of the number of electrons generated by the solar cell to the number of
incident photons on the active surface under monochromatic light irradiation:
-2
Where I (λ) is the photocurrent (μA cm ) given by the cell under monochromatic
-2
illumination at wavelength λ (nm), Pin (λ) is the input optical power (W m ) at wavelength
λ. ‗e‘ is the elementary charge, ‗h‘ is the plank instant, ‗ν‘ is frequency of light and ‗c‘ is
the speed of light is in vacuum. Even if it is not specified differently, the IPCE is measured
under short circuit conditions and displayed graphically versus the corresponding
wavelength in a photovoltaic action spectrum. IPCE measurement is also useful for
indirect determination of the short circuit photocurrent of a DSSC.
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39
CHAPTER 2
IMPROVED PERFORMANCE OF DYE-SENSITIZED SOLAR CELLS USING A
DIETHYLDITHIOCARBAMATE-MODIFIED TiO2 SURFACE
2.1 Introduction
With the impending energy crisis and the growing concern about global warming, there has
been considerable interest in the industrial sector and in the academia in dye-sensitized
solar cells (DSSCs) since their discovery in 1991 [1]. DSSCs are presently the most
cost-effective third-generation solar cell technology available with efficiencies reaching
12% [2]. Solar cells based on other thin-film technologies have efficiencies that are usually
between 5% and 13%. The commercial silicon solar cells have efficiencies of 14%-18%.
This makes DSSCs attractive as a substitute for present solar energy conversion
technologies. While silicon solar cells are highly efficient, they also utilize advanced
technology and hence have a higher cost. DSSC technology, however, is relatively simple
and may become significant if certain technological problems can be solved.
The development of DSSCs is an important research area in alternative energy
because of lower production cost and reasonable power conversion efficiency [3]. After
Grätzel and O‘Regan reported on low-cost DSSC solar cells with an efficiency of 7% [1],
numerous investigators have attempted to enhance the performance of DSSCs by
optimizing the properties of their constituents to improve their efficiency [3]. Although
other semiconductors such as ZnO, SnO2, and CdS have been used in DSSCs, none of
them have been as successful as TiO2 [4-6]. Hence, TiO2 is the most commonly used
anodic material in DSSCs because of its high efficiency in DSSCs and its chemical
stability [1].
In a DSSC, a dye molecule absorbs visible light and an electron is excited to a
40
higher energy level. The excited electron is injected into the conduction band (CB) of TiO2.
These electrons in the CB of TiO2 are transported towards the FTO surface along the
interconnected particle matrix, through trap-mediated random diffusion process. During
their journey, they could recombine with the oxidized dye molecules or the oxidized
species of the redox couple, thus reducing the cell efficiency. Attempts have been made to
change the surface properties of the TiO2 layer to reduce such recombinations. It has been
demonstrated that the charge recombination can be considerably reduced by coating the
TiO2 layer either with an ultrathin layer of an insulator [7-8] or another wide band-gap
semiconductor [9-10].
A self-assembled monolayer (SAM) of a species such as dithiocarbonate adsorbed on
a semiconductor surface plays a major role in determining the energy barriers between a
semiconductor and a sensitizer adsorbed on its surface [11]. One of the most productive
interactions involves the formation of a SAM on the TiO2 surface. Despite the decrease in
the electron conductivity due to the SAM, the overall conversion efficiency of DSSCs has
been observed to increase due to the enhancement of the short-circuit current density (JSC)
[7], [10-11].
According to the literature, nitrogen or sulfur-containing molecules have been used
as additives in the iodide/triiodide redox electrolyte in dye-sensitized solar cells [13].
These additives have contributed to a positive shift of conduction band edge and a decrease
in the charge recombination rate [12-13]. Further, a large increase in the photocurrent
density along with a small decrease in photovoltage was also demonstrated in a study using
thiourea [12-13]. Additionally, thiourea has been used as an adsorbent in the synthesis of
visible-light-responsive titanium dioxide thin films [14].
In this study, diethyldithiocarbamate (DEDTC) was coated on TiO2 via a dip-coating
process using ammonium diethyldithiocarbamate (ADEDTC) in order to modify the
41
surface electronic properties of TiO2. We hypothesized that DEDTC could possibly
generate a surface layer on the TiO2 surface resulting in an increase in JSC by reducing
recombination. The adsorption configurations of DEDTC on the TiO2 particles were
studied by X-ray photoelectron spectroscopy (XPS). The effect of the surface treatment on
the performance of DSSCs was also investigated.
2.2 Experimental
Fluorine-doped tin oxide (FTO) glass substrates were cleaned in a detergent solution
in an ultrasonic bath for 15 min and thoroughly rinsed with ethanol. A TiO2 layer with a
12-μm thickness was then deposited on the FTO glass by the doctor blade method using a
TiO2 paste (Ti nanoxide T, Solaronix). The TiO2-coated substrates were subsequently
sintered at 450 C in air for 30 min. Electrodes were soaked in the aqueous solution (0.01
M) of ADEDTC for periods varying from 20-60 min to deposit the solution onto the TiO2
particles. Afterwards, the resulting electrodes were rinsed with acetonitrile and dried at 50
C for 2 min. The TiO2/FTO and DEDTC-TiO2/FTO samples were separately immersed in
a 0.5 mM N719 dye solution (acetonitrile/tertbutyl alcohol, ν/ν = 1) for 12 h. Anhydrous
electrolyte containing I-/I3- was sandwiched between the dye-adsorbed TiO2 electrode and a
platinum-coated FTO counter electrode to construct the solar cell with an active area of
0.25 cm2. The I–V characteristics were recorded using a calibrated solar cell evaluation
system (JASCO, CEP-25BX) at AM 1.5, with a 100-mW/cm2 illumination. XPS analysis
of the S 2p peak was performed for the prepared DEDTC-TiO2/FTO samples and
ADEDTC powder. XPS experiments were performed by using a hemispherical electron
energy analyzer (ESCALAB-MkII, VG) and an Al K X-ray tube (1486.6 eV). Each
component of the S 2p core line consists of 2 p1/2 and 2 p3/2 peaks split by the spin-orbit
coupling. The peaks show a relative intensity ratio of 1:2 and are separated by 1.18 eV [15].
42
A chemical shift was evaluated by the 2 p3/2 peak position.
2.3 Results and Discussion
Chemical binding analysis surrounding sulfur atoms was performed by XPS. Figure
2.1(a) shows the XPS spectrum of the S 2p core levels obtained from an ADEDTC powder
sample. The spectrum was fitted by two components corresponding to the C-S-1 bond
(164.0 eV) and C=S bond (165.8 eV). Figures 2.1(b) to (d) show the S 2p spectra of the
TiO2 electrodes treated with ADEDTC solution for 20, 40 and 60 min respectively. After
the DEDTC treatment, two components were observed in these spectra at binding energies
of 163.0 and 169.0 eV. Dithiocarbamate (DTC) is a well-known sulfur-chelating agent that
coordinates with a wide variety of metal ions. In these cases, large chemical shifts of the S
2p core level toward lower binding energies have been reported [16]. This chelating
configuration given in Figure 2.2(a) does not agree with any of the two components
observed in our XPS results. However, the S 2p binding energies of the bidentate
configurations of DTCs on a Au surface have been reported at ~162 eV [17]. The
component observed at 163.0 eV may be related to such bidentate sulfur (Figure 2.2(b)) on
the Ti atoms of a nanocrystal. Here, it should be noted that the latter component has
drastically shifted towards a higher binding energy. This core level shift indicates that an
electronic charge transfer has taken place from the sulfur atom to surrounding atoms after
the adsorption scheme proposed in Figure 2.2(c). The binding energy of 169.0 eV is close
to that of sulfates [18].
43
Table 2.1. Peak area percentages of the S 2p components observed in the XPS
spectra of the DEDTC-treated samples.
Binding energy of
the S 2p3/2 peak
Dipping time in ADEDTC solution
20 min
40 min
60 min
163.0 ±0.2 eV
66 %
53 %
35 %
169.0 ±0.2 eV
34 %
47 %
65 %
Therefore, oxidized sulfur such as a sulfate ion should be related to the adsorption
configuration in Figure 2.2(c). Taking into account the high binding energy, 2–4 oxygen
atoms must be bonded to the sulfur atom. Therefore, we suggest the possibility of partial or
full decomposition of DEDTC on the TiO2 surface with accompanying oxidation under the
atmospheric conditions. TiO2 nanocrystals are also well-known for their photocatalytic
activity. In reality, such decomposition of monoalkyl DTC has been reported to generate
SO2, CS2, and alkyl-N=C=S on a TiO2 surface under the irradiation of UV light [19]. Thus,
sulfur atoms bonded to a number of oxygen atoms may be expected after photocatalytic
decomposition of DEDTC, as shown in Figure 2.2(d). X-ray radiation employed in the
XPS experiment may be responsible for inducing photocatalytic activity in the TiO2. The
absence of such decomposition with free DEDTC confirms the ability of the TiO2 for the
observed decomposition.
The relative amount of sulfur corresponding to a binding energy of 169 eV
increased with dipping time. This difference may have an effect on the DSSC performance.
The relative amount of sulfur related to the lower binding energy peak decreased. With
increased dipping time, sulfur atoms in the dithiocarbamate which are in the -2 oxidation
state may be getting oxidized to positive oxidation states through bonding to oxygen
atoms.
44
Figure 2.1. XPS spectra of the 2p core-level of sulfur in (a) ammonium
diethyldithiocarbamate (ADEDTC) powder, (b) ADEDTC-treated TiO2 surface (20 min),
(c) ADEDTC-treated TiO2 surface (40 min) and (d) ADEDTC-treated TiO2 surface (60
min).
45
Figure 2.2. Adsorption configurations for DEDTC on TiO2 surface, (a) bidentate chelating,
(b) bidentate bridging, (c) DEDTC filling Ti vacancy without decomposition, and
(d) sulfur atom filling Ti vacancy after decomposition.
The photocurrent–voltage curves of DSSCs using the bare and DEDTC-treated TiO2
layers are compared in Figure 2.3. Attempts were made to optimize the deposition process
of DEDTC by varying the coating time because it is one of the most important parameters
determining the coating amount in the dip-coating process [13]. We prepared three
different DEDTC-TiO2/FTO electrodes by varying the coating time from 20 to 60 min and
used them as the photoelectrodes of the DSSCs. Photo-conversion efficiencies (EFF) of the
DSSCs are presented in Table 2 together with fill factor (FF), short-circuit photocurrent
(JSC), and open-circuit voltage (VOC). The conversion efficiency with the bare TiO2/FTO
photoelectrode is 5.91%, and with the DEDTC-TiO2/FTO photoelectrode, the conversion
efficiency increased to 6.56%. In particular, it is noteworthy that JSC for the
46
DEDTC-TiO2/FTO photoelectrode increased to 12.74 mA/cm2, whereas that of the bare
photoelectrode is 11.26 mA/cm2.
Table 2.2. Performance comparison of the DSSCs with varying coating time of
DEDTC on the TiO2/FTO photoelectrode.
Time (min)
JSC (mA/cm2)
VOC (V)
FF (%)
EFF (%)
0
11.26
0 .80
65.38
5.91
20
12.31
0.78
67.51
6.51
30
12.74
0.77
66.49
6.56
60
12.33
0.75
58.43
5.42
Figure 2.3. Variation of Current-Voltage characteristics of DSSCs with the dipping time of
the TiO2/FTO photoelectrode in ADEDTC solution.
Increase in JSC can be explained as follows. It is possible that DEDTC is preferentially
adsorbed at defect sites of the TiO2 nanoporous structure, resulting in a decrease of the
47
surface states in the band-gap region. Therefore, back donation of photoelectrons from
TiO2 to the electrolyte and N719 dye would be decreased. Consequent increase in JSC by
the DEDTC-treatment resulted in the improvement in the energy conversion efficiency.
This effect is similar to the influence of 4-tertiary butyl pyridine in I-/I3- electrolyte, which
results in a lowering in VOC and an increase in JSC. In this study, the efficiencies varied
with the dipping time. A maximum efficiency of 6.56% is observed with a 30-minute
dipping time, and the efficiency and the fill factor were decreased with further increase in
the dipping time. Similar trend was reported recently where the fill factor values decreased
Figure 2.4. Absorption spectra of the TiO2 electrodes: (a) bare electrode,
(b) DEDTC-treated electrode (30 min.), (c) dye-adsorbed electrode, and (d) dye-adsorbed
electrode after the DEDTC treatment (30min).
48
with adding thiourea into the electrolyte [13]. This decrease was attributed to the decrease
in dye adsorption on the TiO2 surface. It was observed that the longer dipping time can
present multi layer structure of DEDTC that can enhance decrease in dye–TiO2 interaction.
Figure 2.4 shows the absorbance spectra of four nanoporous TiO2 electrodes: a bare
electrode, DEDTC-treated (for 30 min.) electrode, a dye-adsorbed electrode, and a
dye-adsorbed electrode after DEDTC treatment for 30 min. Compared to the bare electrode,
the apparent increase in light absorption by the DEDTC-treated TiO2 electrode (curves A
and B) is possibly due to scattering effects as DEDTC does not absorb light in the visible
region. It is also evident from the curves C and D in Figure 2.4, that DEDTC treatment has
not adversely affected the dye adsorption by the TiO2 layer. In fact, the data in Table 2
clearly show that the DEDTC treatment has positively contributed to enhance the cell
efficiency at an optimum dipping time of 30 min.
Electrochemical impedance spectroscopy (EIS) was employed to investigate the
effect of DEDTC treatment on the internal resistance of the DSSCs. The results of EIS
are shown in Figure 2.5. The charge transfer resistance at the TiO2/dye/electrolyte interface
is 195 Ω for the cell with the DEDTC-treated TiO2 electrode and 307.5 Ω for the cell with
the bare TiO2 electrode. The lower internal resistance of the cell with the DEDTC treated
electrode has clearly contributed to the improvement in fill factor.
We suggest here that DEDTC is a useful additive because of it exhibits a dual
functionality, viz., improving the visible light absorption and decreasing the TiO2
photoelectrode resistance. The probability of back donation of photoelectrons from the
TiO2 to the electrolyte or dye through the surface layer would be reduced. Therefore, after
the DEDTC treatment the energy conversion efficiency was improved by increasing the
short circuit current. Our results show that there is a slight decrease in the VOC which is
more than compensated by an increase in JSC with DEDTC treatment for the cells with
49
. Figure 2.5. Electrochemical impedance characterization of dye adsorbed with 30-min
DEDTC-treated TiO2 surface and without DEDTC-dye-sensitized solar cells.
optimum performance. A recent study [23] has shown that a super-thin AlN layer has
markedly reduced the dark reaction and greatly improved the forward electrical transport in
the intrinsic InGaN/p-InGaN solar cell where it has been suggested that the leakage current
mechanism changes from a defect related one to an interface tunneling. DEDTC may be
playing such a role by blocking the defect sites on TiO2 electrode in the DSSCs described
in this work. Yang Yu et.al [24] have discussed the role of oxygen vacancy-Ti3+ defect
sites as recombination centers in reducing both the open-circuit voltage and the fill factor.
Park et al [13] have studied the effect of incorporating thiourea into the
electrolyte of TiO2 based DSSCs and observed a small decrease in VOC, a substantial
increase in JSC, a reduction in fill factor and an overall increase in efficiency. These results
are therefore qualitatively identical to what we have observed by the incorporation of
DEDTC. They have ascribed the improvement in cell performance to (i) minimizing
recombination by adsorption of thiourea, and (ii) reaction of thiourea with I2 (present as
50
triiodide ions, I3ˉ) in the electrolyte forming H+ ions and I- ions. This reaction reduces the
concentration of triiodide ions which absorb part of the light. Therefore, a decrease in
triiodide concentration increases the photocurrent. The release of H+ ions lowers VOC due
to a positive shift of the conduction band of TiO2.
A similar reaction is possible between ADEDTC and triiodide ions as shown
below.
R
2
S
N C
R
R
+ I3
S
N
S
R
S
C
R
C
S
S
N
+3I
R
This reaction converts part of the triiodide ions into iodide ions and contributes to a higher
photocurrent as described above. Another consequence of the above reaction is a negative
shift of the redox potential of I3ˉ / Iˉ couple, due to the decrease in I3- ion concentration and
a corresponding increase in I- ion concentration. The observed decrease in VOC can be
explained as due to this negative shift of the I3ˉ / Iˉredox potential.
2.4 Conclusions
The effects of DEDTC adsorption on the surface of TiO2/FTO electrodes via a
dip-coating process were studied. XPS results indicate that DEDTC deposited on the TiO2
surface results in the creation of positively charged sulfur. Use of these electrodes
(DEDTC-TiO2/FTO) as photo-anodes in DSSCs improved the cell performance due to
enhanced visible light absorption and decreased internal resistance by reducing surface
states. Furthermore, the presence of DEDTC can reduce back electron transfer and improve
overall conversion efficiency because of short-circuit current enhancement. Finally, we
obtained improved conversion efficiency by employing DEDTC-TiO2/FTO as the photo
51
anode compared to a photo anode without DEDTC treatment.
References
1. O'Regan, B.; Graetzel, M. A. Nature 1991, 353, 737-740.
2. Lewis, N. S. Solar Energy Utilization, U.S. Department of Energy, Maryland, United
States, April 18–21, 2005.
3. Hagfeldt, A.; Grätzel, M. Acc.Chem. Res. 2000, 33, 269–277.
4. Liao, J.Y.; Ho, K.C. Sol. Energ. Mat. Sol. Cell 2005, 86, 229.
5. Chen, Z.; Tang, Y.; Zhang, L.; Lu, L. Electrochimica Acta 2006, 51, 5870–5875.
6. Sheng, X.; Zhao, Y.; Zhai, J.; Jiang, I.; Zhu, D. Appl. Phys. 2007, A-87, 715–719.
7. Frank, A.J.; Neale, N.R.; Kopidakis N.; Lagemaat, J. Solar Energy Technologies
Program, Colorado, United States, November 7-10, 2005, NREL/CP-590-38978.
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1325–1331.
9. Subasri, R.; Deshpande, S.; Seal, S.; Shinohara, T. The Electrochemical Society,
Electrochemical and Solid-State Letters 2006, 9 (1), B1-B4.
10. Subasri, R.; Shinohara, T.; Mori, K. Science and Technology of Adv. Mat. 2005, 6,
501–507.
11. Wrochem, F.; Wessels, J.; Gao, D.; Ford, W.; Rosselli, S.; Wirtz, R. Patent app. pub.,
US 2011/0031481 A1, 2011.
12. Kim, C.; Kim, J. T.; Kim, H.; Park, S. H.; Son, K.; Han, Y. Current Applied Physics
2010, 10 176-180.
13. Kim, M.; Lee, C.; Jeong, W.; Im, J.; Ryu, T.; Park, N. J. Phys. Chem. C 2010, 114,
19849–19852.
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15. Moulder, J.F.; Stickle, W.F.; Sobol, P.E.; Bomben, K.D. Handbook of X-ray
Photoelectron Spectroscopy, published by Perkin-Elmer Corp., Eden Prairie, MN,
USA, 1992,.
16. Payne, R.; Magee, R. J.; Liesegang, J. Electron Spectrosc. Relat. Phenom.1985, 35,
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T.A. Langmuir 2006, 22, 658-663.
18. Perkin-Elmer handbook.
19. Vidal, A.; Luengo, M. A. M. Applied Catalysis B: Environmental 2001, 32, 1-9.
20. Keis, K.; Magnusson, E.; Lindstrom, H.;. Lindquist, S.E ; Hagfeldt, A. Sol. Energ. Mat.
& Sol. Cell 2002, 73, 51.
21. O‘Regan, B.C.; Scully, S.; Mayer, A.C.; Palomares, E.; Durrant, J. Phys. Chem. B.
2005, 109, 4616.
22. Nazeeruddin, M.K.; Humphry-Baker, R.; Liska, P.; Gra¨tzel, M.J. Phys. Chem. B 2003,
107, 8981.
23. Sang , L.; Liao, M.; Ikeda, N.; Koide, Y.; Sumiya, M. Appl. Phys. Lett. 2011, 99,
161109.
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53
CHAPTER 3
IMPROVED EFFICIENCY OF DYE-SENSITIZED SOLAR CELLS BASED ON A
DOUBLE LAYERED TiO2 PHOTOANODE
3.1 Introduction
Dye-sensitized solar cells have attracted considerable interest as a potential low-cost, clean,
and renewable energy source since their discovery in 1991[1]. And numerous investigators
have attempted to enhance the performance of DSSCs by optimizing the properties of their
constituents to improve on their efficiencies. DSSCs consist of a sensitizing dye, a
nanoporous metal oxide film, an electrolyte and a counter electrode[2,3]. Nanocrystalline
TiO2 has been the main nanoporous metal oxide used in photoanode to obtain high
performance DSSCs [4-6].
While silicon solar cells are highly efficient, they also utilize advanced technology and
hence have a higher cost. DSSC technology, however, is relatively simple and may become
significant if certain technological problems can be solved. As opposed to silicon-based
devices where the semiconductor absorbs the light and transports the released charge
carriers, the two tasks are separated in a DSSC system[1-6]. On illumination, the
surface-adsorbed dye absorbs photons giving rise to a jump of electrons from HOMO to
LUMO which lies above the conduction band minimum of the semiconductor. The dye in
the excited state injects electrons into the conduction band of the semiconductor. Even
though it is energetically possible for the electron in the conduction band of TiO2 to
recombine with the oxidized dye, the rate at which this occurs is quite slow in general
compared to the rate at which the dye captures an electron from the neighboring electrolyte.
However, if there are surface states in the gap of TiO2, excited electrons from TiO2 may
directly recombine with the redox species in the electrolyte. This carrier recombination
54
depends on carrier path of the TiO2 nanocrystalline.
In this work, anatase titanium dioxide (TiO2) nanocrystalline films were grown on
conducting fluorine doped tin oxide (FTO) electrodes using spray pyrolysis deposition
(SPD) technique. We observed SEM images of the TiO2 film fabricated by SPD and found
a number of cracks that may be related with the carrier recombination on the surface. First,
we investigated the cross-section of the film using focused ion beam (FIB) technique.
Repetitive spraying and sintering processes were examined to decrease the number of
cracks in the TiO2 film. Then, we studied the DSSC performance and properties of the
films.
3.2 Experimental
FTO glass substrates were cleaned in a detergent solution in an ultrasonic bath for 15 min,
and thoroughly rinsed with ethanol. The sol-gel method was used to prepare TiO2 thin
films. For single layer process, TiO2 nanocrystalline film with thickness of 15 m was
formed on the heated FTO glass substrate using the SPD technique. Then, it was
subsequently sintered at 450 °C in air for 30 min. For double layer structure, the TiO 2
nanocrystalline films were formed using SPD technique with thicknesses of approximately
6, 9 and 12 m. Then, they were sintered at 450 C in air for 30 min. To keep the total
thickness of 15 m, another TiO2 layer was deposited on top of the above substrates with
thicknesses of 9, 6 and 3 m, respectively. Again the TiO2 substrates were sintered at 450
C in air for 30 min. After that the samples were immersed into a 0.5 mM of N719 dye
solution (acetonitrile/tertbutyl alcohol, v/v = 1) for 24 hours. Anhydrous electrolyte
containing I-/I3- was sandwiched between the dye-adsorbed TiO2 electrode and a platinum
coated counter electrode to construct a dye-sensitized solar cell. I–V characteristics and
photocurrent action spectra were recorded using a calibrated solar cell evaluation system
55
(JASCO, CEP-25BX) at AM 1.5, 100 mW/cm2 illumination.
3.3 Results and Discussion
A SEM image of the TiO2 nanocrystalline film that is fabricated with the single layer
process is shown in Fig. 3.1. A number of cracks are observed in the SEM image. The size
of cracks was found to be on the micrometer scale. Many reasons have been proposed for
the mechanism of crack formation in the inorganic film such as a rapid evaporation of the
solvents from the film surface during the drying process, a decrease of TiO2 particle
bonding strength as the film thickens, and a mismatch of the thermal expansion between
the FTO substrate and the TiO2 film [7-9]. The microcracks formed on the TiO2 film were
also reported by other groups [10-12].
Figure 3.1. Top view of SEM images of the TiO2 film prepared by the single layer
technique.
To investigate depth profile of the cracks, FIB is used for vertical cutting. Fig. 3.2(a) shows
the cross-section of a cracking part of the film with single layer process. The image shows
56
that the crack continues from top to bottom in the TiO2 film. Thus, the cracks may cause to
direct contact the electrolyte and FTO layer that assists to increase the carrier
recombination. In addition, it avoids interconnectivity of nanoparticles in the film. To
decrease the number of cracks in the TiO2 layer, we adopted to fabricate the double layer
structure by filling the cracks formed in the bottom layer with newly sprayed nanoparticles.
Fig. 2(b) shows the cross-section SEM image of the double layered film (top layer: 6 m,
bottom layer: 9 m). As shown in the figure, the crack does not continue into the bottom
layer. It will decrease the recombination between the FTO substrate and electrolyte. TiO 2
film thickness 10 m is facilitated around 50-70% for sufficient electrolyte film
penetration. Cracks in the top layer in double layer structure assist to penetrate the
electrolyte deeper by keeping the TiO2 particles interconnectivity at the bottom layer.
The photocurrent–voltage (I-V) curves of DSSCs using the single layer and double layers
are compared in Figure 3.3. Attempts were made to optimize by varying the bottom layer
thickness in the double-layered process. We prepared four different TiO2 electrodes by
varying the bottom layer thickness from 6 m to 15 m, and applied them to the
photoelectrode of the DSSCs. Photo-conversion efficiencies (EFF) of the DSSCs
Fig. 3.2. Cross-sectional SEM images of the TiO2 film of (a) single-layered and
(b) double-layered
ds.fdf;dsl;glsdfgm;lsdmg;lsfdmgsdf‘;lfdgkfdkg;lskg;lsdkg;lsdkg;lsdfgkls;
57
Figure 3.3. Current-voltage characteristics of DSSCs varying the thickness of TiO2 bottom
layer.
Table 3.1. Performance comparison of the DSSCs with the thickness of TiO2 bottom layer
Thickness
(m)
6
9
12
15
JSC
(mA/cm2)
VOC
(V)
FF
(%)
EFF
(%)
13.47
15.37
0.81
0.82
65.87
67.41
7.22
8.51
15.37
10.51
0.83
0.81
63.98
61.10
8.14
5.22
are presented in Table 3.1 together with a fill factor (FF), short-circuit photocurrent (JSC),
and open-circuit voltage (VOC). The number of TiO2 particles was improved after filling the
cracks with two step spraying method. Short circuit photocurrent density was significantly
improved due to more dye adsorption. Conversion efficiency of the single layer TiO2
photoelectrode was 5.22%, and with the TiO2 double layer, the conversion efficiency
increased to 8.51% with 9 m bottom layer thickness.
58
To investigate the charge transfer resistance of the DSSC, the electrochemical impedance
characterization (EIS) was measured under illumination. The obtained fitting results of EIS
are illustrated in Figure 3.4. The charge transfer in the TiO2/dye/electrolyte interface
resistances were estimated by the spectra to 16.76, 21.62, 16.49, and 28.1  corresponding
to the DSSC with the TiO2 bottom layer thickness of 6, 9, 12, and 15 m, respectively, as
shown in the Table 3.2. Although the resistance varies considerably, decreasing in the
charge transfer resistance of the TiO2 photoelectrode was found in the double-layered
structures compared with the single-layered film. Thus, it is confirmed that interconnection
of nanocrystallines are improved by the double-layered structure and JSC was increased.
Both the improvement of the charge transfer resistance and effect of filling the microcracks
to avoid the direct access of electrolyte to the FTO substrate would assist the total cell
performance.
Figure 3.4. Electrochemical impedance characterization of dye adsorbed TiO2 photoanode
by varying the thikness of TiO2 bottom layer.
59
Table 3.2. Variation of charge transfer resistance as a function of TiO2 bottom layer
thickness.
Thickness (μm)
Resistance (Ω)
6
16.76
9
21.62
12
16.49
15
28.11
Voltage (V)
0.00
0.20
0.40
0.60
0.80
0.0E+00
2
Current density (mA/cm )
-1.0E-01
-2.0E-01
-3.0E-01
Single layer
Double layer
-4.0E-01
-5.0E-01
-6.0E-01
-7.0E-01
Figure 3.5 Dark current for double layered and single layered DSSCs.
Figure 3.5 shows the current–voltage curve of the DSSCs under dark conditions.
Recombination of charge carriers by reduction of I3- at dye free TiO2 surface or bare FTO
surface exposed to electrolyte through porous electrode leads dark current. In order to keep
60
the dark current as low as possible direct contact between FTO surface and electrolyte
should be minimized. A cell made up with double layer structure have low dark current by
reducing the cracks and large pores in the surface. This result shows the full coverage of
FTO layer after the two step spaying and sintering process.
Table 3.3. Comparison of the amount of dye adsorption for single and double layered
DSSCs
Applied electrodes
Dye adsorption (mol/cm2)
1.25 x10-7
Double layered
1.43 x10-7
10
20
30
211
Double layer
Single layer
40
50
D eg ree
105
200
In ten sity/A.U.
004
101
Single layered
60
70
80
Figure 3.6 XRD patterns of the (a) double layered and (b) single layered TiO2 thin film.
Structural characterization of the double layered and single layered TiO2 thin films were
performed by XRD, as shown in Figure 3.6. Significant difference of crystal structures in
the double layered and single layered TiO2 thin films were not found. Table 3.3 compares
the amount of N719 dye adsorbed on TiO2 surface. Amount of TiO2 particles was increased
61
after filling the cracks. Therefore the amount of adsorbed dye was improved from 1.25
x10-7 mol/cm2 to 1.43 x10-7 mol/cm2 after two step spaying and sintering process.
Finally double layered structure was improved dye loading capacity of TiO2 nanocrystals.
This may course to improve the conversion efficiency with double layered structure.
3.4 Conclusions
In summary, TiO2 nanocrystalline films were prepared on FTO substrates by using the SPD
technique. The microcracks were observed in SEM images of the films. The
double-layered structure was examined to fill the cracks with the two-step spraying and
sintering process. It was confirmed that microcracks of the bottom layer were filled and the
charge transfer resistance of the TiO2 electrode was improved by the double-layered
structure. Finally, conversion efficiency of 8.5% was obtained using the double layer
structure, compared with an efficiency of 5.2% with the single-layered TiO2 electrode.
References
1. B. O'Regan, M. Graetzel, A low-cost, high-efficiency solar cell based on dye-sensitized
colloidal TiO2 films, Nature 353, 737 (1991).
2. A. Hagfeldt, M. Grätzel, Chem. Res., 33, 269 (2000).
3. Basic Research Needs for Solar Energy Utilization, U.S. Department of Energy Office
of Basic Energy Sciences, (2005).
4. J.Y. Liao, K.C. Ho, Sol. Energ. Mat. Sol. Cell 86, 229 (2005).
5. Z. Chen, Y. Tang, L. Zhang, L. Lu, Electrochimica Acta, 51, 5870 (2006).
6. X. Sheng, Y. Zhao, J. Zhai, I. Jiang, D. Zhu, Appl. Phys. A-87, 715 (2007).
7. G.W. Scherer, J. Am. Ceram. Soc. 73, 3 (1990).
8. R.C. Chiu, T.J. Garino, M.J. Cima, J. Am. Ceram. Soc. 76, 2257 (1993).
9. R.C. Chiu, M.J. Cima, J. Am. Ceram. Soc. 76, 2769 (1993).
10. C. Kaya, F. Kaya, B. Su, B. Thomas, A.R. Boccaccini, Surf. Coat. Technol. 191, 303
(2005).
62
11. T. Moskalewicz, A. Czyrska-Filemonowicz, A.R. Boccaccini, Surf. Coat. Technol. 201,
7467 (2007).
12. C.K. Lin, T.J. Yang, Y.C. Feng, T.T. Tsung, C.Y. Su, Surf. Coat. Technol. 200, 3184
(2006).
13. C. He, L. Zhao, Z. Zheng, F. Lu, J. Phys. Chem. C, 112, 18730 (2008).
14. S. Ito, P. Chen, P. Comte, M.K. Nazeeruddin, P. Liska, P. Pechy and M. Grätzel, Prog.
Photovolt: Res. Appl., 15, 603 (2007).
15. R. Kern, R. Sastrawan, J. Ferber, R. Stangl, J. Luther, Electrochimica Acta 47, 4213
(2002).
16. G. Schlichthörl, N.-G. Park, A. Frank, J. Phys. Chem. B 103, 782 (1999).
17. P.E. de Jongh, D. Vanmaekelbergh, Phys. Rev. Lett. 77, 3427 (1996).
63
CHAPTER 4
THE IMPROVED PERFORMANCE OF DYE SENSITIZED SOLAR CELL BY
PYRROLIDINEDITHIOCARBAMATE MODIFIED TiO2 SURFACE.
4.1. Introduction
Dye-sensitized solar cells (DSSCs) have been extensively studied for decades since they
were reported by Grätzel and coworkers [1]. This has been recognized as a hopeful research
field for alternative energy sources due to low production cost and high efficiency [2].
Encouragingly, many improvements have been achieved by introducing different
morphologies to the semiconductor materials [3,4], new dyes [5,6] and electrolytes [7,8].
However, further improvements in the efficiency are the key to accelerating the
industrialization of DSSCs. Specifically, nanocrystalline semiconductor oxide (typically
TiO2) films, which adsorb dye molecules and transport photo-generated electrons to the
outer circuit, serve as electron conductors and dictate the efficiency of electron transport and
collection. Therefore, some publications have started to focus more on efficient
semiconductor materials, such as new types of semiconductors ZnO, SnO2, CdS have been
used in DSSCs, none of them showed better performance than TiO2 [3,9]. Thus, TiO2 has
been most commonly used as an anodic material in a DSSC due to its predominantly
moderate charge transport capability and chemical stability [1]. Furthermore, back reaction
of photo-generated electrons into electrolyte solution and carrier recombination of excited
electrons and holes are serious problem that takes place at the TiO2 surface. Investigations
have been carried out to modify the TiO2 layer to reduce the recombination. It was found that
charge recombination reaction can be remarkably reduced by coating the TiO 2 either with an
insulator [10,11] or with another semiconductor[12,13]. These modification materials build
up to control the back electron transfer into the electrolyte or sensitizer [13].
Dipoles formed at a semiconductor surface play key roles in determining the energy
64
barriers between a semiconductor and a sensitizer, depending on the modification process
[14]. The electric dipole layer that exists at the TiO2/sensitizer interface results the
permanent dipole moment of the molecule itself. One of most successful methods is forming
a self-assemble monolayer (SAM) on the TiO2 surface. Despite of diminishing the electron
conductivity by SAM, overall conversion efficiency is improved due to the enhancement of
the short circuit current density (JSC) [10, 13, 14]. While many researchers have intensively
studied on generating energy barriers for the repression of electron recombination, little
attention
has
been
paid
to
this
approach.
Therefore,
we
tried
to
coat
1-pyrrolidinedithiocarbamate (PDTC) on TiO2 layers via dip coating process in ammonium
1-pyrrolidinedithiocarbamate (APDTC) solution. We expected that PDTC to possibly induce
the surface dipoles on the TiO2 surface resulting in the increase of JSC. In this study,
adsorption structures of PDTC on the anatase TiO2 particles are studied by X-ray
photoelectron spectroscopy (XPS). The effect of the surface treatment by PDTC on the
performance improvement of DSSCs was also investigated by comparing the cell
performances.
4.2 Experimental
4.2.1 Preparation of DSSC
FTO glass substrates were cleaned in a detergent solution in an ultrasonic bath for 15
min, and thoroughly rinsed with ethanol. The sol-gel method was used to prepare TiO2 thin
films. TiO2 nanocrystalline films were formed using SPD technique with thicknesses of
approximately 9 m. Then, they were sintered at 450 C in air for 30 min. Another TiO2
layer was deposited on top of the above substrates with thicknesses of 6 m. Again the
TiO2 substrates were sintered at 450 C in air for 30 min. One of the electrodes was soaked
in the aqueous solutions (0.01M) of APDTC for 30 minutes to deposit PDTC onto the TiO2
surface. Then, resulting electrodes were rinsed with ethanol, and dried at 50 C for 2 min.
65
The TiO2/FTO and PDTC-TiO2/FTO samples were separately immersed into a 0.5 mM of
N719 dye solution (acetonitrile/tertbutyl alcohol, ν/ν= 1) for 24 hours. Anhydrous
electrolyte containing I-/I3- was sandwiched between the dye-adsorbed TiO2 electrode and a
platinum coated counter electrode to construct a dye-sensitized solar cell. I–V
characteristics were recorded using a calibrated solar cell evaluation system (JASCO,
CEP-25BX) at AM 1.5, 100 mW/cm2 illumination. XPS analysis of S 2p peak was
performed
for
the
prepared
PDTC-TiO2/FTO
sample,
N719
dye
adsorbed
PDTC-TiO2/FTO sample, and APDTC powder. XPS experiment was performed by using a
hemispherical electron energy analyzer (ESCALAB-MkII, VG) and an Al K X-ray tube
(1486.6eV).
4.2.2 Plane-wave pseudopotential DFT calculations
All DFT calculations with plane-wave pseudopotential method were carried out
using PHASE computational code [15]. Perdew-Burke-Ernzerhof parameterization of the
generalized gradient approximation (GGA) was considered in the calculation for a
exchange-correlation scheme [16]. The electron-ion interactions were treated by using the
ultrasoft pseudopotentials [17]. Test calculations have shown that a kinetic energy cutoff
for the wave functions equal to 40 Ryd is sufficient to obtain well converged results. We
have used same slab models for the (001) TiO2 surface as in Ref. 18 with a vacuum region
in thickness of ~ 20 Å. Throughout the calculations we use the unit size of the slab based
on lattice parameters taken from the X-ray diffraction (a=3.793, c=9.510, u=0.202) [19]. A
2×2×1 k-point Monkhorst-Pack set [20] was employed within the surface Brillouin zone.
The equilibrium atomic positions were determined by relaxing all atoms in the unit cell for
the thinner (001) surface.
66
4.3. Results and discussion.
Figure 4.1a shows the XPS spectrum of the Sulfur 2p core levels obtained from
APDTC powder sample. The spectrum were fitted by two components corresponding to
the C-S-1 bond (164.0 eV) and C=S bond (166.0 eV). Figure 4.1b shows S 2p spectrum of
the PDTC-TiO2/FTO electrode. Only one component was observed in Figure 4.1b.
Therefore, two sulfur atoms of PDTC are in an identical chemical environment after
adsorption on the TiO2 surface. In addition, binding energy of S 2p at 169.2 eV, which is
clearly higher than those of APDTC powder, indicates that electronic charge transfer from
sulfur to the TiO2 surface is induced after the adsorption. It should be also noted that
PDTC related S 2p peak for the N719 dye adsorbed PDTC-TiO2/FTO sample was located
Figure 4.1. a. XPS spectrum of the Sulfur 2p peak in pyrrolidinedithiocarbamate (PDTC)
powder. b. XPS spectrum of the sulfur 2p peak which are detected in PDTC - TiO2/FTO
electrode.
67
at the same binding energy as that without dye. It means that sulfur bonding configuration
of PDTC was not changed during the dye adsorption process. Thus, we confirmed that
PDTC were successfully deposited onto TiO2 surface with positively charged sulfur linkers.
It was reported that formation of electric dipole layer at the TiO2 surface results the
permanent dipole moment of the PDTC itself [14]. This surface dipole may have an effect
of the DSSC performance. It will be discussed later. Then we fabricated DSSCs with the
PDTC-TiO2/FTO electrodes and measured their performance.
Photocurrent–voltage curves of DSSC‘s using the bare and PDTC-treated TiO2 layers are
compared in Figure 4.2. Photo-conversion efficiencies of the DSSCs are presented in Table
4.1 together with fill factor (FF), short circuit photocurrent (JSC), and open circuit voltage
(VOC). Conversion efficiency with the bare TiO2/FTO was 8.51%, and with
PDTC-TiO2/FTO photo-electrode it was increases to 9.32%. Especially, it is remarkable
that JSC for the PDTC-TiO2/FTO photo-electrode increased to 17.82 mA/cm2 while the bare
photoelectrode was 15.37 mA/cm2. Two reasons of increase of JSC are expected as follows.
One possible reason is that PDTC preferentially adsorbs at defect sites of TiO2 nano porous
structure, therefore back donation of photoelectrons from TiO2 to electrolyte and N719 dye
would be decreased. This indicates that the increase in JSC by the PDTC-treatment resulted
in the improvement in the energy conversion efficiency. Another possible reason is that
surface dipole induced by the PDTC attachment would modify the redox potential.
Table 4.1 Performance comparison of the DSSCs employing TiO2/FTO and PDTCTiO2/FTO as photoanodes.
Applied electrodes
JSC (mA/cm2)
VOC (V)
FF
EFF (%)
TiO2 /FTO
15.37
0.82
0.67
8.51
PDTC- TiO2/ /FTO
17.82
0.76
0.69
9.32
68
20.00
Current density (mA/cm2 )
18.00
00 min
30 min
16.00
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
0.00
0.20
0.40
0.60
0.80
Voltage (V)
Figure 4.2. Current-voltage characteristics of DSSCs with TiO2 layer and PDTC-coated
TiO2 layer.
Figure 4.3 illustrates how the dipole of PDTC modify the conduction band energy
(ECB) of TiO2 with respect to the redox potential (Eredox) of the electrolyte in DSSC. Our
XPS result revealed that sulfur atoms of PDTC are more positive like sulfate ions after
adsorption on the TiO2 surface. Thus, the surface dipoles at the TiO2 /PDTC interface
decrease the back donation of photoelectrons into the electrolyte. On the other hand, the
surface dipole decline of VOC would be expected as illustrated in Figure 4.3. In reality, VOC
of PDTC-TiO2/FTO photo-electrode decreased from 0.82 V to 0.76 V as listed in table 4.1.
As described above, the PDTC-TiO2/FTO photo-electrode has shown slight diminishing in
VOC due to the positive shift of TiO2 conduction band edge, which could cause, at the same
69
time, efficient electron injection from the excited sensitizer into the conduction band of
modified TiO2.
Figure 4.3. Modified energy of the TiO2 conduction band (ECB) with respect to redox
potential (Eredox) of the electrolyte [14].
Figure 4.4 shows the optimized (001) slab used for the plane-wave pseudopotentials
calculation. The (101) surface is known as the most stable surface for anatase crystal [21].
Unstable and reactive (001) surface is expected to be present as a minority surface in most
anatase crystals [22]. For the (001) surface, there are 5c-Ti and 2c-O sites for a possible
adsorption site of PDTC. Here, ―5c-Ti‖ means five-fold coordinated titanium. The
adsorption energy Eads of PDTC is estimated by the following equation.
Eads  Eslab/cluster-adsorbateEslab/cluster Eadsorbate
(1)
Here, Ecluster-adsorbate,Ecluster, and Eadsorbate are the total energies of the PDTC adsorbed TiO2
slab or cluster, clean TiO2 slab or cluster, and free PDTC radical or anion, respectively.
70
Figure 4.4 Optimized models for the clean (100) surface by the plane-wave
pseudopotential calculations
Table 4.2 Adsorption energy of PDTC on anatase surfaces
Calculation method
Plane-wave pseudopotentials
calculations
5c-Ti
-1.511 eV
2c-O
-0.814 eV
The adsorption energies for each adsorption sites are listed in Table 4.2. As shown in the
table, most stable configuration is the bidentate adsorption on the 5c-Ti sites of the (001)
surface. Figure 4.5 shows the optimized structures of PDTC adsorption on the TiO2(001)
surface by the plane-wave pseudopotential DFT calculation. Bond length of S-Ti was
2.615 Å. Since the value is slightly shorter than 2.72 Å for the physically-bonded Ti-S
distance of H2S adsorption on the 5c-Ti site [23]. The estimated adsorption energy for the
5c-Ti site is 1.511 eV. On the 2c-O sites, the S-O bond length is longer than that on the
5c-Ti site and has the adsorption energy of 0.814 eV. These results indicate that PDTC
adsorption on the 5c-Ti site is more stable than that on the 2c-O site.
71
Figure 4.5 Optimized structures of PDTC adsorption on the TiO2(001) surface by the
plane-wave pseudopotential DFT calculation (a) on the 5c-Ti and (b) 2c-O sites. Bond
lengths in Å are also noted in the figure.
We have examined two different initial structures of the bidentate models on 5c-Ti and
2c-O sites. However, the final optimized structure falls into the same structure that has
bonds between S and Ti atoms.
72
4.4. Conclusion.
In this work, surface modification of TiO2 electrode for dye-sensitized solar cell
(DSSC) by pyrrolidinedithiocarbamate (PDTC) is studied. Results by X-ray photoelectron
spectroscopy (XPS) indicates that two sulfur atoms of PDTC are in the same chemical
environment and positively charged after adsorption on the TiO2 surface. DSSC conversion
efficiency with the PDTC-treated electrode was clearly improved because of increasing of
the short circuit current.
Adsorption stability of pyrrolidinedithiocarbamate (PDTC) on the (001) surfaces of TiO2
anatase are studied by density functional theory (DFT) calculations. On well-ordered (001)
surface, PDTC adsorbs with two S atoms on five-fold coordinated Ti atoms or on two-fold
coordinated O atoms. By using pseudo potential method with periodic slab model, the
optimized bond lengths of S-Ti and S-O are 2.615 and 2.730 Å, respectively. These bond
lengths indicate that PDTC species weakly adsorbs on the (001) surface. Results suggest
that PDTC species is not strongly bonded on the ordered (001) surface with bidentate
configuration.
References
[1] O'Regan B., Graetzel M., Nature 353(1991), 737.
[2]. Hagfeldt A., Grätzel M., Chem. Res., 2000, 33 (5), 269–277.
[3] Liao J.Y., Ho K.C., Sol. Energ. Mat. Sol. Cell 86 (2005), 229.
[4] Chen Z., Tang Y., Zhang L., Lu L., Electrochimica Acta, 51 (2006), 5870–5875.
[5] A. Zaban, S. Ferrere, Gregg B.A., J. Phys. Chem. B. 102 (1998), 452.
[6] Chou1 C., Wu K., Chi1 Y., Yu S.J., Lee G.H., Lin C., Chou P., Angewandte Chemie Int.
Ed., Vol. 50(9), 2011, 2054–2058.
[7] Gregg B.A., Coordination Chemistry Reviews 248 (2004), 1215–1224.
73
[8] Kawano R., Matsui H., Matsuyama C., Sato A., Susan M.A.B.H., Tanabe N.,
Watanabe M., Photochem J., J. Photochem. Photobiol. A-164 (2004), 87.
[9] Sheng X., Zhao Y., Zhai J., Jiang I., Zhu D., Appl. Phys. A-87(2007), 715–719.
[10] Frank A.J., Neale N.R., Kopidakis N. and Lagemaat J., Conference Paper,
NREL/CP-590-38978,
November 2005.
[11] Balraju P., Kumar M., Roy M.S., Sharma G.D., Synthetic Metals 159 (2009),
1325–1331.
[12] Subasri R., Deshpande S., Seal S.,Shinohara T., The Electrochemical Society,
Electrochemical and Solid-State Letters, 9 (1), B1-B4 (2006).
[13] Subasri R.,Shinohara T.,Mori K., Science and Technology of Adv. Mat. 6 (2005),
501–507.
[14] Wrochem F., Wessels J., Gao D., Ford W., Rosselli S., Wirtz R., Patent app. pub.,
US 2011/0031481 A1, 2011.
[15] http://www.ciss.iis.u-tokyo.ac.jp/english/index.php
[16] J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.
[17] D. Vanderbilt, Phys. Rev. B 41 7892 (1990).
[18] R. Wanbayor, V. Ruangpornvisuti, Mater. Chem. Phys. 124 (2010) 720.
[19] D. T. Cromer, K. Herrington, J. Am. Chem. Soc. 77 (1955) 4708.
[20] H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13 (1976) 5188.
[21] U. Diebold, Surf. Sci. Rep. 48 (2003) 53.
[22] C. Arrouvel, M. Digne, M. Breysse, H. Toulhoat, and P. Raybaud, J. Catalysis 222
(2004) 152.
[23] R. Wanbayor, V. Ruangpornvisuti, J. Molecular Structure: Theochem 952 (2010) 103.
[24] Keis K.,Magnusson E.,Lindstrom H., Lindquist S.E., Hagfeldt A., Sol. Energ. Mat. &
Sol. Cell-73 (2002), 51.
74
[25] O‘Regan B.C., Scully S., Mayer A.C., Palomares E., Durrant J., J. Phys. Chem. B.
109 (2005), 4616.
75
CHAPTER 5
ENHANCEMENT OF THE PHOTOELECTRIC PERFORMANCE OF
DYE-SENSITIZED SOLAR CELLS BASED ON A SURFACE MODIFIED TIO2
PHOTOANODE WITH OXIDIZED SULFUR
5.1 Introduction
Dye-sensitized solar cells (DSSC) are currently attracting academic and commercial
interests with their low cost, easy fabrication, environment friendly and relatively high
energy conversion efficiency [1, 2]. Generally, DSSC comprises a dye-sensitized
nanocrystalline TiO2 film, liquid electrolyte containing an I−/I3− redox couple, and a
platinized TCO (transparent conducting oxide) glass substrate as the counter electrode. In
order to obtain high photovoltaic performance, the TiO2 solar cells generally should have
relatively large film thickness (8–17 μm), high surface area to adsorb large amount of dye
for good solar light harvesting, lower ionic resistance within the TiO2 electrode and good
interconnected crystalline network for effective electron transfer from dye molecules[3–5].
Therefore, the design and development of TiO2 films with such structure are an important
approach to improve the performance of dye-sensitized solar cells.
Generally, DSSCs have maximum absorption up to 800 nm of the total incident solar
irradiation [6, 7]. This represents a major issue for this technology, since 50% of solar
irradiation is in the ultraviolet and infrared regions, and thus is not utilized. This limits the
solar energy conversion efficiency for DSSCs. Therefore, attempting to extend the spectral
response range of a DSSC to the IR region represents an extremely important approach to
increasing the DSSC efficiency. Visible light activated TiO2 films have previously been
made using bilayer structures with low band gap materials [8–10] or, by modifying TiO2 to
render it active in the visible light region by depositing noble metals [11] or coupling it
76
with metallic oxides [12–14]. Recently, it was found that nonmetal dopants, such as sulfur
and nitrogen (N–S) can cause TiO2 to become active under visible as well as ultraviolet
irradiation [15–18]. This has been pursued for enhancing the visible light response of TiO2,
using a bilayer structure that can facilitate efficient electron transfer from the nanoporous
TiO2 to the electrode, and limit back-transfer to the electrolyte [19]. It has been reported
that growth of insulating and metal oxide layers on the surfaces of TiO2 nanoporous films
may be a valuable approach for achieving high conversion efficiencies [20–24].
Preliminary encouraging results in our laboratory encouraged us to carry out further
investigations to explore the possibility of modifying the spectral response of DSSCs by
the use of S-doped TiO2 photoelectrodes by using 1-pyrrolidinedithiocarbamate (PDTC). In
this work, we present a comprehensive understanding of a visible light enhanced TiO2
interfacial layer with PDTC, which can harvest visible light as well as create a potential
barrier between the TiO2 nanoporous layer and the electrolyte. Such films are able to
suppress back electron transfer from the FTO and TiO2 nanoporous layers to I2/ I3− redox
electrolytes without lowering electron injection from the TiO2 to the FTO susbtrate.
5.2 Experimental
5.2.1 Double layer
The precursor sol–gel solution was prepared by slowly adding 0.3 g of degussa P25
powders (provided by Nippon Aerosil Co., Ltd) to 20 ml of nano-particle sol (TKC-302;
TAYCA) with 20 ml of ethanol. For double layer structure, the TiO2 nanocrystalline films
were formed using SPD technique with thicknesses of approximately 9 m. Then, they
were sintered at 450 C in air for 30 min. To keep the total thickness of 15 m, another
TiO2 layer was deposited on top of the above substrates with thicknesses of 6 m. Again
the TiO2 substrates were sintered at 450 C in air for 30 min [25]. As a control, this sample
77
was nominated as A.
5.2.2 Bilayer
Furthermore different amounts of PDTC were added to the above sol-gel solution. 0.05,
0.1, and 0.2 mol/dm3 of PDTC added TiO2 sol-gel solutions were prepared to form three
more groups, designated B, C, and D respectively. The solutions were constantly stirred for
a period of 2 h afterwards. At the end of the preparation, the solutions were yellowish
transparent fluids ready to be used for sol–gel deposition after another 48 h of aging.
Then TiO2 nanocrystalline films were formed using SPD technique with thicknesses of
approximately 9 m with undoped TiO2 sol-gel solution. Then, they were sintered at 450
C in air for 30 min. And PDTC added thin films were prepared on top of the above
substrates with thicknesses of 6 m by using solution B, C and D. Again the TiO2
substrates were sintered at 450 C in air for 30 min [25].
The TiO2/FTO and PDTC-TiO2/ TiO2/FTO samples were separately immersed into a 0.5
mM of N719 dye solution (acetonitrile/tertbutyl alcohol, ν/ν= 1) for 24 hours. Anhydrous
electrolyte containing I-/I3- was sandwiched between the dye-adsorbed TiO2 electrode and a
platinum coated counter electrode to construct a dye-sensitized solar cell. I–V
characteristics and photocurrent action spectra were recorded using a calibrated solar cell
evaluation system (JASCO, CEP-25BX) at AM 1.5, 100 mW/cm2 illumination.
5.3 Results and Discussion
The I-V curves of dye-sensitized solar cells based on TiO2 films with different content of
PDTC are shown in Fig. 5.1. The corresponding photovoltaic characteristics of DSSCs are
summarized in Table 5.1. The content of PDTC has important effects on the short circuit
photocurrent density (ISC), fill factor (FF) and the overall conversion efficiency (EFF). The
78
short circuit photocurrent density (ISC) increases obviously as a function of PDTC content,
and a maximum value 17.91 mA·cm−2 is achieved with 0.2 mol/dm3 PDTC concentration.
The overall conversion efficiency increases as a function of PDTC content at first and
decreases then. The best performance of the solar cell is obtained with TiO2 film modified
with 0.1 mol/dm3 concentration of PDTC. The overall conversion efficiency is 9.45%, an
increase of 11% compared with the DSSCs without modification. Further increasing the
amount of PDTC leads to the decrease in the overall conversion efficiency (EFF) with
reduction of the fill factor.
The incident photon conversion efficiency (IPCE) spectrum of cells A and C were
measured in the wavelength range of 300 to 800 nm illustrated in Fig. 5.2. The IPCE
results are in good agreement with the conversion efficiency improvement shown in table
5.1. It can be seen that a maximum IPCE approximately 85% was obtained at 530 nm for
cell C with 0.1 mol/dm 3 PDTC concentration. These results reveal that the
PDTC added TiO2 layer is responsible for this red shift in the action spectra. The light
harvesting by the surface adsorbed sensitizer can be further improved by exploiting
Table 5.1 Performance comparison of the DSSCs with varying concentration of
PDTC on the FTO/ TiO2 photoelectrode.
PDTC concentration
(mol/dm3)
ISC
(mA/cm2)
VOC
(V)
FF
(%)
EFF
(%)
0
15.37
0.82
67.40
8.51
0.05
16.84
0.82
65.60
9.03
0.10
17.81
0.83
63.63
9.45
0.20
17.91
0.83
61.02
9.12
79
Figure 5.1. I–V characterization of (a) bare cell and (b) 0.05, (c) 0.1 and (d) 0.2 mol/dm3 of
PDTC added sol–gel TiO2 thin films deposited on FTO/TiO2 layer in dye sensitized solar
cells.
light localization and optical improvement effects. For example, incorporating larger sized
anatase particles enhances significantly the absorption of red or near infrared photons by
the film. These light management strategies employ scattering and photonic bandgap
effects [26-28] to localize light in the mesoporous structure, augmenting the optical
pathway significantly beyond the film thickness and enhancing the harvesting of photons
in a spectral region where the optical cross-section of the sensitizer is small. Larger
particle layer is shown to enhance the photocurrent response of the DSSC in the near
80
0.90
a
b
0.80
0.70
IPCE
0.60
0.50
0.40
0.30
0.20
0.10
0.00
300.00
400.00
500.00
600.00
700.00
800.00
Wavelength (nm)
Figure 5.2.IPCE spectra of (a) bare cell and (b) 0.1 mol/dm3 of PDTC added TiO2 thin film
deposited on FTO/TiO2 layer in the dye sensitized solar cells.
3
Figure 5.3. Absorption spectra of (a) bare electrode and (b) 0.1 mol/dm of PDTC added
TiO2 thin film deposited electrode.
81
infrared and visible regions of the solar spectrum. From these investigations, it is
understandable that the employment of PDTC is also necessary besides the tuning the
visible light absorption of TiO2 layer for enhancing the performance of DSSC. The
UV-Visible absorbance spectrum of PDTC (0.1 mol/dm3) added TiO2 with double layered
bare TiO2 electrode shown in Fig. 5.3 in the wavelength range of 300–800 nm. Compared
to a bare cell, the visible light absorption significantly increased for PDTC added TiO2 thin
films deposited on TiO2 layer, which is due to more dye adsorption by the TiO2 layer after
PDTC added TiO2 thin films were deposited.
Figure5.4 Dark current–voltages curves for double layered and
PDTC added TiO2 thin film.
82
0.1 mol/dm 3 of
The dark current–voltage curves in Figure5.4 illustrated that the dark current onset shifted
to a larger potential for DSSCs with modified TiO2 electrode. This observation indicated
that the PDTC added TiO2 thin film decreased the charge recombination. Therefore, the
photovoltage was enhanced and the dark current was reduced.
The modification of PDTC not only generates a barrier that reduces the dark current but
also shifts the conduction band potential in negative direction, revealing a lower electron
density at any applied potential Consequently, at each applied potential, the back transfer
of electrons in modified TiO2 is lower than that in unmodified TiO2. The XRD patterns of
the double layered and 0.1 mol/dm3 of PDTC added TiO2 thin film samples are shown in
Figures 5.5. There were no significant differences in XRD patterns of both double layered
and 0.1 mol/dm3 of PDTC added TiO2 thin film. Therefore the crystal structure was not
Double layer
After PDTC treatment
105
200
In ten sity/a.u .
004
211
101
modified after PDTC treatment.
10
20
30
40
50
60
70
80
D eg ree
Figure5.5 XRD patterns of the (a) double layered and (b) 0.1 mol/dm3 of PDTC added
TiO2 thin film.
83
(a)
(b)
Figure 5.6. SEM images of (a) TiO2 double layered and (b) 0.1 mol/dm3 of PDTC added
TiO2 thin film.
The nanostructures of the TiO2 double layered and 0.1 mol/dm3 of PDTC added TiO2 thin
films were obtained from the SEM image shown in Figure 5.6. Visible light absorption
improvement and red shift in the IPCE spectra may be attributed to particle size dependent
surface properties. According to the SEM images, the PDTC added TiO2 thin film has
aggregation with TiO2 particles which assists to increase the particle size. If the particle
sizes in the PDTC added TiO2 thin films were equal to the interparticle the nanoporous
TiO2 layer; hence, more dye adsorption.distances in the nanoporous TiO2 layer, then the
dye may not have adsorbed completely on the inner surface of the nanoporous TiO2 layer.
On the other hand, increased particle size means less hindrance to the penetration of dye
molecules into the inner surface of PDTC added TiO2 layer exhibitsmore dye adsorption
than the bare sample. Figure 5.7 shows 2p core-level of sulfur (a) and 1s core-level of
nitrogen (b). There was no peak for nitrogen after the sintering process. Therefore PDTC
molecule was decomposed during the drying process. XPS was exploitedin order to
identify the presence of sulfur species into the TiO2 films after the sintering process, as
shown in Figure 5.7 (a). After the PDTC treatment, one component was observed in these
2p core-level of sulfur spectra at binding energies 170 eV. Taking into account the high
binding energy, 2–4 oxygen atoms must be bonded to the sulfur atom. Therefore, we
84
suggest the possibility of full decomposition of PDTC on the TiO2 surface with
accompanying oxidation during the drying process.
Intensity
(a)
175
170
165
160
155
Binding Energy (eV)
Intensity
(b)
410
405
400
395
390
Binding Energy (eV)
Figure 5.7. XPS spectra of PDTC added TiO2 film (a) 2p core-level of sulfur
(b) 1s core-level of nitrogen
85
Figure 5.8.Adsorption configurations for the decomposed PDTC on TiO2 surface.
XPS peaks in the S 2p core-level region with binding energies of 167–170 eV are due to
the presence of S6+ and S4+ species. The binding energy of 170 eV is close to that of
sulfates. Therefore, oxidized sulfur such as a sulfate ion should be related to the adsorption
configuration in Figure 5.8.
The higher binding energy peak in sulfur doped TiO2 can be
related to either the substitution of Ti4+ ions by S6+/S4+ cations or the presence of
sulfate/sulfite groups (SO42−/SO32−) coordinated on TiO2 through bidentate bonds with
surface Ti4+ ions, similar to sulfated titania. XPS peak originate primarily on the surface of
the S doped TiO2 and thus can be related to the presence of sulfate groups anchored on
TiO2. TiO2 nanocrystals are also well-known for their photocatalytic activity. Xinchen
Wang et al group have reported that large number of Lewis acidic sites (exposed Ti 4+
cations) were created on the TiO2 surface due to the above sulfur complex. These Lewis
acidic sites were improved the number of chemically adsorbed dye on the TiO2 surface as
shown in Figure 5.8. Also this modified TiO2 surface was enhance efficient electron
injection from the excited sensitizer into the conduction band of modified TiO2.
86
UV-Visible absorbance spectrum was improved due to the Lewis acidic sites which help to
increase the number of chemically adsorbed dye on the TiO2 surface. Therefore we have
obtained 9.45% conversion efficiency with PDTC treatment by improving the visible light
absorption.
5.4 Conclusion
Dye sensitized solar cells made with visible light enhanced PDTC added TiO2 thin films.
The solar cells with PDTC show an improvement of short circuit photocurrent, opencircuit
voltage and overall conversion efficiency. The highest overall conversion efficiency is
obtained with the 0.1 mol/dm3 PDTC concentration increasing by 11% compared with the
solar cell without modification. Consequently, further detailed study of the PDTC added
TiO2 surfaces could give better pathway for improving the performance of the DSSCs.
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CHAPTER 6
SUMMARY AND CONCLUSIONS
This thesis is focused on the development of different fabrication techniques with sulfur
containing compounds to enhance energy conversion efficiency of dye sensitized solar
cells for future applications, the significant observations and improvements were
summarized as follows.
The effects of DEDTC adsorption on the surface of TiO2/FTO electrodes via a
dip-coating process were studied. XPS results indicate that DEDTC deposited on the TiO2
surface results in the creation of positively charged sulfur. Use of these electrodes
(DEDTC-TiO2/FTO) as photo-anodes in DSSCs improved the cell performance due to
enhanced visible light absorption and decreased internal resistance by reducing surface
states. Furthermore, the presence of DEDTC can reduce back electron transfer and improve
overall conversion efficiency because of short-circuit current enhancement. Finally, we
obtained improved conversion efficiency by employing DEDTC-TiO2/FTO as the photo
anode compared to a photo anode without DEDTC treatment.
Then my attempt was focused to improve the energy conversion efficiency by using
another technique. TiO2 nanocrystalline films were prepared on FTO substrates by using
the SPD technique. The microcracks were observed in SEM images of the films. The
double-layered structure was examined to fill the cracks with the two-step spraying and
sintering process. It was confirmed that microcracks of the bottom layer were filled and the
charge transfer resistance of the TiO2 electrode was improved by the double-layered
structure. Finally, conversion efficiency of 8.5% was obtained using the double layer
structure, compared with an efficiency of 5.2% with the single-layered TiO2 electrode.
Then surface modification of TiO2 electrode for dye-sensitized solar cell (DSSC) by
90
pyrrolidinedithiocarbamate (PDTC) was studied via a dip-coating process. Results by X-ray
photoelectron spectroscopy (XPS) indicates that two sulfur atoms of PDTC are in the same
chemical environment and positively charged after adsorption on the TiO2 surface. DSSC
conversion efficiency with the PDTC-treated electrode was clearly improved because of
increasing of the short circuit current.
Additional study demonstrates visible light enhanced PDTC added TiO2 thin films with
bilayer structure. These S doped TiO2 thin films were used as visible light harvesters as
well as blocking layers in dye sensitized solar cells. S–TiO2 was shown an improvement of
short circuit photocurrent, opencircuit voltage and overall conversion efficiency with bare
cell. The highest overall conversion efficiency is obtained with the 0.1 mol/dm3 PDTC
concentration increasing by 11% compared with the solar cell without modification. A
TiO2/ PDTC–TiO2/dye bilayered cell has a better performance than a TiO2/dye cell, which
is attributed to enhancement of UV-Visible absorption inthe DSSCs.
91
List of Publications
Journal Papers
1) D.M.B.P. Ariyasinghe, H.M.N. Bandara, R.M.G. Rajapakse, K. Murakami, and M. Shimomura,
Improved performance of dye-sensitized solar cells using a diethyldithiocarbamate-modified
TiO2 surface, Journal of Nanomaterials 2013, 258581, 1-6 (2013).
2) D.M.B.P. Ariyasinghe and M. Shimomura, Improved efficiency of dye-sensitized solar cells
based on a double layered TiO2 photoanode, Asian Journal of Chemistry 25, S384-S386 (2013).
Conference Proceedings
1) D.M.B.P. Ariyasinghe, K. Murakami, and M. Shimomura, Surface modification of TiO2
electrode by diethyldithiocarbamate for dye-sensitized solar cells, Proceedings of 11th
International Conference on Global Research and Education in Engineers for Better Life,
pp.111-117 (2012).
2) H. Ota, D.M.B.P. Ariyasinghe, M. Shimomura, Surface treatment of TiO2 nanoporous film by
pyrrolidinedithiocarbamate, Proceedings of Korean-Japanese Students Workshop 2012,
pp.31-34 (2012).
3) D.M.B.P. Ariyasinghe, M. Shimomura, H.M.N. Bandara, R.M.G. Rajapakse, The improved
performance of dye sensitized solar cell by pyrrolidinedithiocarbamate modified TiO2 surface,
Proceedings of the International Conference on Solar Energy Materials, Solar Cells and Solar
Energy Applications, pp. 152-157 (2011).
Conference Presentations
1) D.M.B.P. Ariyasinghe, M. Shimomura, Study on double-layered TiO2 photoanode for dye
sensitized solar cells, Conference on Sri Lanka Japan Collaborative Research - 2013
92
(SLJCR-2013), Peradeniya, Sri-Lanka, 29-31 Mar. 2013.
2) 太田紘志，D.M.B.P. Ariyasinghe，下村勝, ナノ多孔質TiO2薄膜表面におけるPDTCの吸
着，第60回応用物理学会春季学術講演会，神奈川工科大学，2012/3/27-30.
3) D.M.B.P. Ariyasinghe, M. Shimomura, Improved efficiency of dye-sensitized solar cells based
on a double layered TiO2 photoanode, International Conference on Nanoscience and
Nanotechnology (ICONN-2013) Chennai, India, 18-20 Mar. 2013.
4) D.M.B.P. Ariyasinghe, M. Shimomura, Enhanced efficiency of dye-sensitized solar cells with
TiO2 double layer thin film, 日本表面科学会中部支部学術講演会, 名城大学名駅サテラ
イト, 2012/12/22.
5) H. Ota, D.M.B.P. Ariyasinghe, M. Shimomura, Surface treatment of TiO2 nanoporous film by
pyrrolidinedithiocarbamate, Proceedings of Korean-Japanese Students Workshop 2012, Pusan,
Korea, 13-14 Nov. 2012.
6) D.M.B.P. Ariyasinghe, K. Murakami, and M. Shimomura, Surface modification of TiO2
electrode by diethyldithiocarbamate for dye-sensitized solar cells, 11th International
Conference on Global Research and Education in Engineers for Better Life
(Inter-Academia2012), Budapest, Hungary, 27-30 Aug. 2012.
7) M. Shimomura, D.M.B.P. Ariyasinghe, H.M.N. Bandara, R.M.G. Rajapakse, Adsorption of
pyrrolidine dithiocarbamate on anatase TiO2 nano-particles, 10th International Conference on
Global Research and Education (Inter-Academia 2011) Sucevita, Romania, 26-29 Sept. 2011.
8) D.M.B.P. Ariyasinghe, M. Shimomura, H.M.N. Bandara, R.M.G. Rajapakse, The improved
performance of dye sensitized solar cell by pyrrolidinedithiocarbamate modified TiO2 surface,
International Conference on Solar Energy Materials, Solar Cells and Solar Energy Applications,
Institute of Fundamental Studies, Kandy, Sri Lanka, 28-30 July, 2011.
93