urea and rgo additives to iodine/triiodide electrolyte for
TRANSCRIPT
South Dakota State University South Dakota State University
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Electronic Theses and Dissertations
2018
Urea and rGO Additives to Iodine/Triiodide Electrolyte for Higher Urea and rGO Additives to Iodine/Triiodide Electrolyte for Higher
Efficiency Dye-sensitized Solar Cells Efficiency Dye-sensitized Solar Cells
Salem Abdulkarim South Dakota State University
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Recommended Citation Recommended Citation Abdulkarim, Salem, "Urea and rGO Additives to Iodine/Triiodide Electrolyte for Higher Efficiency Dye-sensitized Solar Cells" (2018). Electronic Theses and Dissertations. 2945. https://openprairie.sdstate.edu/etd/2945
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UREA AND rGO ADDITIVES TO IODIDE/TRIIODIDE ELECTROLYTE FOR
HIGHER EFFICIENCY DYE-SENSITIZED SOLAR CELLS
BY
SALEM ABDULKARIM
A dissertation submitted in partial fulfillment of the requirements for the
Doctor of Philosophy
Major in Electrical Engineering
South Dakota State University
2018
iii
ACKNOWLEDGEMENTS
My deep gratitude goes first to Dr. Qiquan Qiao as dissertation advisor, who expertly
guided and supported me through my PhD research at the Center of Advanced
Photovoltaics, South Dakota State University. His valuable suggestions and continuous
encouragements have helped me to conduct my research successfully and attain skills to
be an independent researcher.
My appreciations extend to Dr. Parashu Kharel, Dr. Robert McTaggart, Dr. Yue Zhou,
and Dr. Michael Hildreth for their important role and support as committee members.
I am also thankful to all my colleagues, especially Dr. Hytham Elbohy in helping me
to complete my experiments.
Finally, I am grateful to my family for their support, patience during my PhD studies.
iv
TABLE OF CONTENTS
LIST OF FIGURES ...............................................................................................................viii
LIST OF TABLES ...................................................................................................................xi
ABSTRACT ...........................................................................................................................xii
Chapter 1: Introduction ............................................................................................................. 1
Overview ......................................................................................................................... 1
Solar Photovoltaic Cells ........................................................................................... 2
DSSC Overview .............................................................................................................. 4
DSSC Substrate Characteristics ............................................................................... 5
Metal Oxide Electrodes ............................................................................................ 6
Photosensitizers (Dyes) for DSSCs ......................................................................... 7
Counter Electrodes (Cathode) ................................................................................ 10
DSSC Electrolytes ................................................................................................. 11
Advantages and Disadvantages of Dye-Sensitized Solar Cells .................................... 16
Advantages ............................................................................................................. 17
Disadvantages ........................................................................................................ 17
Previous Work .............................................................................................................. 18
Dye-Sensitized Solar Cell Liquid Electrolytes ...................................................... 18
Iodide/Triiodide Electrolyte Additives .................................................................. 22
Motivations ................................................................................................................... 23
v
Objective ....................................................................................................................... 24
Chapter 2: Theory ................................................................................................................... 25
Solar Radiation and Air Mass ....................................................................................... 25
Solar Cells and Photovoltaic Effect .............................................................................. 27
i. Charge Carrier Generation Due to Photon Absorption in Junction Materials .............. 28
ii. Separation of Photo-Generated Charge Carriers in the Junction .................................. 30
iii. Accumulation of Photo-Generated Charge Carriers at the Junction Terminals ........... 30
Classical P-N Junction Solar Cell ................................................................................. 31
DSSC Operating Principles........................................................................................... 32
Key Efficiency Parameters Representing DSSC Performance ..................................... 34
Short-Circuit Current Density ................................................................................ 35
Open-Circuit Voltage ............................................................................................. 36
Fill Factor ............................................................................................................... 36
Conversion Efficiency ........................................................................................... 37
Equivalent Circuit Model for Conventional PN-Junction Solar Cells .......................... 38
Characterization Techniques ......................................................................................... 39
Current Density-Voltage Characteristics (J-V) ...................................................... 39
External Quantum Efficiency (EQE) ..................................................................... 40
Electrochemical Impedance Spectroscopy (EIS) ................................................... 41
Cyclic Voltammogram Measurements (CVs) ........................................................ 43
UV-Visible Absorption Spectra ............................................................................. 46
vi
Mechanism of Urea Reaction with I-/I-3 Redox Electrolyte .......................................... 48
Mechanism of rGO Reaction with I-/I-3 Redox Electrolyte .......................................... 49
Chapter 3: Experimental Procedures ...................................................................................... 51
Material and Device Fabrication Procedures ................................................................ 51
Materials ................................................................................................................ 51
Device Fabrication – Glass Substrate Preparation ................................................. 51
Device Fabrication – Photoanode Fabrication ....................................................... 52
Device Fabrication – Counter Electrode (CE) Fabrication .................................... 52
Device Fabrication – Final Assembly .................................................................... 53
Device Fabrication – Electrolyte Preparation ........................................................ 54
Materials and Device Characterizations ....................................................................... 54
Current Density-Voltage Characteristics (J-V) ...................................................... 54
External Quantum Efficiency (EQE) Measurements ............................................. 56
Electrochemical Impedance Spectroscopy (EIS) Measurements .......................... 57
Cyclic voltammetry (CV) Measurements .............................................................. 57
UV-Visible Absorption Spectra Measurements..................................................... 58
Chapter 4: Results and Analysis ............................................................................................. 61
DSSC Performance with Urea-Treated Electrolyte ...................................................... 61
J-V Characteristics ................................................................................................. 61
UV-Vis Absorbance Spectra .................................................................................. 63
External Quantum Efficiency (EQE) ..................................................................... 64
Electrochemical Impedance Spectroscopy (EIS) ................................................... 66
vii
Cyclic Voltammogram (CV) Measurement ........................................................... 68
UV-Vis Absorption of Photoanodes Immersed in Urea-Treated Electrolyte (dye
loading) ..................................................................................................................... 69
J-V Hysteresis Effects ............................................................................................ 70
DSSC Performance Using rGO-Treated Electrolyte .................................................... 72
J-V Characteristics ................................................................................................. 72
UV-Vis Absorbance Spectra .................................................................................. 74
External Quantum Efficiency (EQE) ..................................................................... 74
Electrochemical Impedance Spectroscopy ............................................................ 76
Cyclic Voltammogram Measurement .................................................................... 78
UV-Vis Absorption of Photoanodes Immersed in rGO-Treated Electrolyte (dye
loading) ..................................................................................................................... 80
J-V Hysteresis Effects ............................................................................................ 81
Chapter 5: Summary and Conclusions .................................................................................... 83
Summary ....................................................................................................................... 83
Conclusions ................................................................................................................... 84
Future work ................................................................................................................... 85
References ………………………………………………………………………….. … ....... 86
viii
LIST OF FIGURES
Figure 1.1 Solar Cell Technology Development _______________________________ 3
Figure 1.2 Solar Cell Research Efficiency Reported Over Last Four Decades ________ 4
Figure 1.3 Classification of DSSC Electrolytes _______________________________ 12
Figure 2.1 Solar Radiation Spectrum _______________________________________ 25
Figure 2.2 Solar Zenith Angle ____________________________________________ 26
Figure 2.3 Solar Reference Spectra ________________________________________ 27
Figure 2.4 Photon Absorption in a Semiconductor with Bandgap Energy Eg ________ 29
Figure 2.5 Direct and Indirect Bandgap Semiconductor Characteristics ____________ 30
Figure 2.6 Energy Band Diagram of a p-n Junction in Thermal Equilibrium, with
Depletion Region ______________________________________________________ 32
Figure 2.7 Schematic of DSSC with Energy Level Diagram _____________________ 34
Figure 2.8 J-V Characteristic Curves Under Dark and Illuminated Conditions _______ 37
Figure 2.9 Conventional P-N Junction Solar Cell Equivalent circuit model _________ 38
Figure 2.10 Changes in I-V Response with Changes in Rsh and Rs ________________ 39
Figure 2.11 Example DSSC Nyquist Plot with Equivalent Circuit Model ___________ 43
Figure 2.12 Cyclic Potential Sweep and (b) Shape of a Cyclic Voltammogram Curve _ 45
Figure 2.13 Energetics Between Electrolyte Solution and Electrode _______________ 46
Figure 2.14 Possible Transitions of π, σ, and n Electrons _______________________ 47
Figure 2.15 UV-Vis Spectroscopy _________________________________________ 48
Figure 2.16 Mechanism of Urea Reaction with I-/I-3 Redox Electrolyte ____________ 49
Figure 2.17 Mechanism of rGO Reaction with I-/I-3 Redox Electrolyte _____________ 50
Figure 3.1 Schematic of DSSC device assembly ______________________________ 54
ix
Figure 3.2 Schematic of J-V Measurement Setup _____________________________ 55
Figure 3.3 Schematic of EQE Measurement Setup ____________________________ 56
Figure 3.4 Functional Diagram of EIS Setup _________________________________ 57
Figure 3.5 Functional Diagram of Cyclic Voltammetry (CV) ____________________ 58
Figure 3.6 Schematic of UV-Visible Absorption Spectroscopy __________________ 60
Figure 4.1 DSSC Current Density-Voltage (J-V) Curves Due to Urea-Treated Electrolyte
_____________________________________________________________________ 62
Figure 4.2 UV-Vis Absorbance Spectra of Urea-Treated Electrolyte ______________ 64
Figure 4.3 EQE Spectral Response Due to Urea-Treated Electrolyte ______________ 65
Figure 4.4(a) Nyquist Plots of DSSCs Due to Urea-Treated Electrolyte ___________ 67
Figure 4.5 Cyclic Voltammograms of Counter Electrodes Immersed in Urea-Treated
Electrolyte ____________________________________________________________ 69
Figure 4.6 UV-Vis Absorbance Spectra of Dye Solutions Desorbed from Electrolytes
Treated with Urea ______________________________________________________ 70
Figure 4.7 Hysteresis Effects for DSSC Devices Fabricated with Urea-Treated
Electrolyte ____________________________________________________________ 71
Figure 4.8 DSSC Current Density-Voltage (J-V) Curves Due to rGO-Treated Electrolyte
_____________________________________________________________________ 73
Figure 4.9 UV-Vis Absorbance Spectra of rGO-Treated Electrolyte ______________ 74
Figure 4.10 EQE Spectral Response Due to rGO-Treated Electrolyte ______________ 75
Figure 4.11(a) Nyquist Plots of DSSCs Due to rGO-Treated Electrolyte ___________ 77
Figure 4.12 Cyclic Voltammograms of Counter Electrodes Immersed in rGO-Treated
Electrolyte ____________________________________________________________ 79
x
Figure 4.13 UV-Vis Absorbance Spectra of Dye Solutions Desorbed from Electrolytes
Treated with rGO ______________________________________________________ 81
Figure 4.14 Hysteresis Effects for DSSC Devices Fabricated with rGO-Treated
Electrolyte ____________________________________________________________ 82
xi
LIST OF TABLES
Table 4.1 DSSC Performance Parameters Due to Urea-Treated Electrolyte _________ 62
Table 4.2 EQE Peaks of DSSCs Due to Urea-Treated Electrolyte _________________ 65
Table 4.3 Predicted Rs and Rr from Nyquist Plots of Urea-Treated Electrolyte ______ 68
Table 4.4 Anodic and Cathodic Peak Intensities of Urea-Treated Electrolyte ________ 69
Table 4.5 DSSC Performance Due to rGO-Treated Electrolyte ___________________ 73
Table 4.6 EQE Peaks of DSSCs Due to rGO-Treated Electrolyte _________________ 76
Table 4.7 Predicted Rs and Rr from Nyquist Plots of rGO-Treated Electrolyte _______ 78
Table 4.8 Anodic and Cathodic Peak Intensities of rGO-Treated Electrolyte ________ 79
xii
ABSTRACT
UREA AND rGO ADDITIVES TO IODIDE/TRIIODIDE ELECTROLYTE FOR
HIGHER EFFICIENCY DYE-SENSITIZED SOLAR CELLS
SALEM ABDULKARIM
2018
This research investigated urea and reduced graphene oxide (rGO) as potential
additives to a dye-sensitized solar cell (DSSC) iodide/triode electrolyte, in order to enhance
overall electrical performance. Additive concentrations of 0, 2, 5, 10, and 15 wt% were
investigated. In general, both additives were found to enhance DSSC electrical
performance with respect to open circuit voltage Voc and DSSC fill factor FF. The greatest
enhancement in these parameters occurred at an additive concentration of 10 wt%. Urea
produced a greater degree of enhancement than rGO; the overall efficiency was increased
by 10.17±0.01% for urea vs 9.57±0.01% for rGO. The specific mechanisms by which
performance was increased differed between the additives. This degree of performance
enhancement may be near the limits of what is achievable given current DSSC technology
and development.
1
Chapter 1: Introduction
Overview
Since the start of the Industrial Revolution, fossil fuel usage (e.g. crude oil, coal, natural
gas, or heavy oils) has contributed more carbon dioxide (CO2) to the air than the Earth's
plants can use, resulting in increasing temperatures worldwide. For example, from 1890 to
2015, atmospheric carbon dioxide emissions increased from 270 to 400 ppm,
corresponding to a temperature increase that contributes to the greenhouse effect [1]. This
temperature increase is prompting more research into use of renewable energy resources
(e.g., wind, biomass, and solar energy [2]) that do not emit air pollution or greenhouse
gasses. Wind power generation depends on wind speed, which is affected by altitude,
terrain and/or man-made features; it cannot reliably supply steady electricity to power
systems [3, 4]. Biomass includes all organic wastes and all land and water-based
vegetation. Biomass has been a major energy source, contributing on the order 14% of the
world’s energy supply [5]. However, biomass is a fuel that needs to be manufactured for
use and still produces carbon dioxide during the processing.
Solar energy takes approximately 8 minutes to reach the Earth. It is produced through
nuclear fusion of hydrogen into helium. All forms of fossil fuel energy were produced
through photosynthetic processes, making solar energy the primary energy source. Even
wind energy has a solar origin, since it comes from temperature differences in different
regions on the Earth. Considering solar energy by itself, the power generated from sunlight
striking the Earth's surface is approximately 1.05 10 TW. If only 1% of the irradiance
at the Earth’s surface could be directly converted into electrical energy with just 10%
efficiency, it would provide 105 TW, supplying more than the expected worldwide need
2
of 25-30 TW [6].
Solar Photovoltaic Cells
Solar photovoltaic cells (PVs), which convert sunlight directly into electrical energy,
are a promising alternative to existing fossil fuel energy sources [7] and will likely
contribute to future energy production by significantly reducing atmospheric CO2 levels.
Figure 1.1 shows the various solar cell technologies and their historical and recent
development trends [8]. The technologies considered in Figure 1.1 include multi-junction
cells, single-junction GaAs, crystalline Si cells, thin film technologies, and emerging PV
technologies.
In 1839, Becquerel first observed the photovoltaic (PV) effect in selenium. In 1883,
Fritte introduced the first solid state solar cell with a power conversion efficiency (PCE) of
less than 1% [9]. In 1954, Bell Laboratories in the U.S. constructed a solar PV device with
a PCE of approximately 6% [6]. In 1958, solar cells were used in small-scale and
commercial applications. After much research, the conversion efficiency was enhanced to
approximately 19.8% for a multicrystalline Si cell and approximately 24.4% for a
monocrystalline Si cell [10]. This first commercially viable generation of solar cells was
fabricated from Si wafers and used primarily as passive rooftop solar collectors. The
advantage of these solar cells is good photoconversion efficiency. However, they are
mechanically rigid, and they require a lot of energy to fabricate.
The second generation of solar cells was fabricated using amorphous or
microcrystalline Si or other materials such as copper indium gallium selenide (CIGS) and
cadmium telluride (CdTe). These cells avoid the use of Si wafers and consume less raw
material during fabrication, significantly reducing their production costs compared to the
3
earlier crystalline Si cells. However, their fabrication also requires large amounts of
energy.
The current generation of solar cells is fabricated with advanced polymers, oligomers,
synthetic dyes, and organic/inorganic hybrid perovskites. Due to high fabrication costs at
the current time [11], these cells are mostly used in large-scale commercial applications
only.
Figure 1.1 Solar Cell Technology Development
Figure 1.2 shows the highest achieved photoconversion efficiencies for different
photovoltaic technologies, covering a time period from 1976 to 2017. Efficiencies vary
from approximately 10.6% for organic tandem cells to 46.0% for a four-junction
(concentrator) cell. For dye-sensitized solar cells, the highest achieved efficiency is
4
approximately 11.9%. For perovskite and CIGS cells, the highest achieved efficiencies are
approximately 22.1% and 22.4%, respectively.
Figure 1.2 Solar Cell Research Efficiency Reported Over Last Four Decades
(Source: National Renewable Energy Laboratory (NREL), 2017)
Because of their higher costs and energy usage required for fabrication, Si-based solar
cells are no longer used in commercial applications. Organic solar cells are promising as
potential replacements for silicon-based cells [12, 13]. With respect to efficiency and ease
of fabrication, the dye-sensitized solar cell (DSSC) is one of the most promising
alternatives. The next section considers these cells in additional detail.
DSSC Overview
DSSCs have attracted significant attention as a solar cell due to lower material costs
and ease of fabrication. The first DSSC was constructed by O'Regan and Gratzel in 1991.
5
Its photoconversion efficiency was 7.1%, and its incident photon to current conversion
efficiency (IPCE) was approximately 80% [14]. Their original design consists of two glass
plates coated with a transparent conducting oxide, typically a fluoride-doped tin oxide
(FTO), forming transparent conducting oxide-glass (TCO) substrates. A mesoporous TiO2
semiconductor thin-film “paste” is deposited onto one of the plates, which is immersed in
a photosensitive ruthenium-polypyridine dye solution. After soaking in the dye solution,
dye molecules are covalently bonded within the mesoporous TiO2 layer. A thin layer of
platinum is spin coated onto the second glass plate. The plates are then sealed together,
and an I-/I-3 redox electrolyte solution is injected between the sealed plates.
Recently developed DSSC cells are generally fabricated following this initial design.
They differ primarily in the electrolyte/dye solution and electrode/photoanode material that
is used [15].
DSSC Substrate Characteristics
As described in the previous section, the DSSC can be considered to have a “sandwich”
structure, with the electrolyte/dye solution between the two TCO substrates. The substrates
should possess low sheet resistance and high transparency in the visible and IR regions of
the electromagnetic spectrum. The typical sheet resistance for a well fabricated TCO is
approximately 5 to 15 Ω / square [16].
There are two types of TCO substrates typically used for DSSCs: indium-doped tin
oxide (ITO) and FTO. FTO substrates tend to be more efficient with respect to overall
photoconversion efficiency than ITO substrates; this is due to lower resistivity (8.5 Ω/sq)
in the FTO substrate than in the ITO substrate (18.5 Ω/sq), resulting in lower ohmic losses.
In addition, FTO substrates exhibit better thermal stability than ITO substrates; after
6
annealing at 450 °C, the ITO resistivity increased to approximately 52 Ω/sq, while the FTO
resistivity remained unchanged [17, 18].
Metal Oxide Electrodes
Many metal oxide semiconductor materials, such as TiO2, SnO2, ZnO, and ZrO2, have
been used for the DSSC photoanode [19]. TiO2 is the most commonly used material due to
its superior performance, lower material cost, relative abundance, relative nontoxicity,
biocompatibility, and chemically stable nature [20, 21]. The mesoporous TiO2 film has a
high surface area, allowing it to absorb a large amount of dye in the monolayer. This i)
helps to ensure consistent photoelectron generation; and ii) facilitates dispersion of
electrolyte in the photoanode, which ensures the regeneration of oxidized dye by the redox
couple in the electrolyte [22]. In a typical DSSC device, the film thickness is between
approximately 2 µm and 15 µm.
TiO2 exists in three different phases dependent on temperature: rutile, anatase and
brookite. Rutile is the most stable form; it has a slightly lower bandgap compared to anatase
and can absorb at near-ultraviolet wavelengths. Anatase is more chemically active than
rutile. Anatase and brookite are stable at normal temperatures but slowly convert to rutile
upon heating to temperatures above 550° C and 750° C, respectively [15]. For a given film
thickness at an intensity level corresponding to normal sunlight, the measured photocurrent
voltages of rutile-based and anatase-based DSSCs are quite similar. Voc in both cells is
essentially the same; the short-circuit photocurrent of the rutile-based cell is approximately
30% less than that of the anatase-based cell, due to a smaller film surface area per unit
volume resulting in less adsorbed dye. The overall conversion efficiency of the rutile-based
cell is less than the efficiency of the anatase-based cell [21].
7
Photosensitizers (Dyes) for DSSCs
A critical component of the DSSC is the photosensitizer dye. Ideally, the
photosensitizer should meet the following requirements [15, 23]:
• It should be strongly absorbing in the visible and near-infrared (NIR) regions.
• Its molecular structure should include anchoring “groups” (e.g. -COOH, -
H2PO3, -SO3H, etc.) to promote effective entry of dye molecules into the
semiconductor mesoporous film.
• For n-type DSSCs (based on n-type semiconductors), the excited state energy
of the photosensitizer should be higher than the semiconductor conduction
band edge, so that efficient electron transfer between the excited dye and
semiconductor can occur. For p-type DSSCs (based on p-type
semiconductors), the highest occupied molecular orbital (HOMO) level of the
photosensitizer should be at a more positive potential than the semiconductor
valence band level, in order for efficient hole transfer to occur.
• For dye regeneration, the oxidized state level of the photosensitizer must be
more positive than the redox potential of the electrolyte.
• The photosensitizer should be chemically and thermally stable.
• The photosensitizer should have good solubility in organic solvents for facile
deposition from stock solutions within a few hours.
Many types of photosensitizer dyes have been synthesized since DSSC technology was
first introduced. These dyes can be classified into four groups [24-27]. Each group will be
considered in greater detail in this section.
• metal complex dyes
8
• metal-free organic dyes
• perovskite-based dyes
• “natural” dyes derived from plant material
Metal Complex Dyes
Photosensitizer dyes containing metal complexes are the most commonly used dyes in
DSSCs. Dyes with metal complexes based on Ru, Os [28], Re[29], Fe[30], Pt[31], and
Cu[32]. have been synthesized. DSSCs using Ru-based dyes have been shown to provide
a maximum overall photoconversion efficiency of 13% [33]. However, their use is limited
by high production costs, scarcity and toxicity of Ru metal, and inefficient absorption in
the NIR region. Research into modifying the basic molecular design in order to increase
absorption in the visible region has been conducted; these variant dyes include N3 (“red
dye”), N719, N749 (“black dye”), K19, and Z907 [34]. At the present time, only N3 and
N719 have demonstrated increased visible and NIR region absorption.
Metal-Free Organic Dyes
Metal-free organic dyes have been synthesized as potential replacements for Ru based
dyes. They are generally non-toxic, relatively inexpensive and easy to synthesize, offer
very high molar extinction coefficients, and can be readily “tuned” to enhance absorption
in different regions of the spectrum. In addition, they are generally stable at high
temperatures and under prolonged illumination [35, 36]. These dyes have reached
maximum photoconversion efficiencies of 12.75% [37].
Metal-free organic dyes are generally divided into three major sub-types with respect
to molecular structure: “donors”, “linkers”, and “acceptors”. Linkers are usually a π-
conjugated system that link electron-donating (D) and electron- accepting (A) groups by
9
bridges, the so-called “D-π-A” sensitizers [38, 39]. In metal-free organic dye sensitizers,
the donor groups should consist of electron-rich compounds such as phenylamine,
aminocoumarin, and indoline. The most successful π-conjugated groups are selected from
compounds containing thiophene units such as oligothiophenes, thienylenevinylenes, or
dithienothiophene. Cyanoacrylic acid, rhodamines, and pyridines are also suitable acceptor
compounds.
Perovskite Dyes
Perovskite dyes have specific crystalline structures following the general formula
ABX3, where X can be oxygen, carbon, nitrogen, or a halide element. Current halide
perovskites with the general structure CH3NH3PbX3 (X=Cl, Br, or I) have attracted
considerable attention due to low bandgap energies on the order of 1.5 eV. This class of
perovskite was first used as an inorganic sensitizer in DSSCs in 2009; they demonstrated
photoconversion efficiencies of approximately 3.1% for X=Br and 3.8% for X=I [40]. In
general, perovskite solar cells offer greater photoconversion efficiency at lower cost and
ease of manufacturing.
Natural Dyes
Natural dyes can be easily extracted from plant and bacterial sources, and employed in
DSSCs [41]. They provide increased absorption in the visible region. In addition, they are
a relatively abundant dye source and are environmentally friendly in that they are
completely biodegradable [42]. Most importantly, synthesis of natural dyes for DSSC use
is cost effective in that they do not rely on noble metals such as Ru. However, they are not
particularly efficient; a maximum photoconversion efficiency of 2% has been reported for
a pomegranate dye [43].
10
Counter Electrodes (Cathode)
The counter electrode (CE) or cathode is the site in the DSSC where reduction
processes occur. Typically, the counter electrode is a glass substrate coated with platinum
(Pt) or other conductor; it effectively functions as an electron collector. The ideal CE
material should meet the following requirements [44, 45]:
• High catalytic activity
• High conductivity
• High reflectivity
• Large surface area
• Optimum thickness
• Chemical, electrochemical and mechanical stability
• Resistance to chemical corrosion resistance
• Energy levels that match the potential of the redox couple electrolyte
• Good adhesivity with TCO
• Low cost
However, one material cannot easily meet all of these requirements simultaneously.
Modern DSSCs use composite CEs produced by various material deposition methods
including thermal decomposition, electrochemical deposition, chemical reduction,
chemical vapor deposition, hydrothermal reaction, and sputter deposition. The deposition
method significantly influences CE particle size, surface area and smoothness, as well as
its catalytic and electrochemical properties. Smaller particle sizes and larger surface areas
produce more efficient catalytic activity. Pt satisfies nearly all the requirements for CEs;
consequently, it is the most frequently used CE material. However, its increasing cost and
11
chemical instability limit its use. Among Pt-free CEs, carbon-based materials have been
the most researched material due to their low cost, ease of manufacturing, and good
stability. However, they possess relatively low conductivity and catalytic activity
compared to Pt CEs, and do not adhere well to various substrates.
DSSC Electrolytes
The selection of an electrolyte has a significant impact on overall DSSC
photoconversion efficiency, to the extent that it facilitates the charge transport between the
photoanode and the counter electrode. The ideal DSSC electrolyte material should possess
the following characteristics [46]:
• Very low viscosity
• Negligible vapor pressure
• High boiling point
• High conductivity
• Minimal absorption in the visible region of the spectrum
• Long-term mechanical, chemical, electrochemical, thermal and optical
stabilities
• Chemically non-reactive with the sensitized dye material
The electrolytes used in DSSCs can be classified into three categories, as shown in
Figure 1.3: (i) liquid electrolytes; (ii) quasi-solid electrolytes; and (iii) solid-state
conductors. The following sections consider each category in greater detail.
12
Figure 1.3 Classification of DSSC Electrolytes
Liquid Electrolytes
Liquid electrolytes are the most widely utilized transport medium for DSSCs, yielding
photoconversion efficiencies as high as 13% for traditional DSSC designs [33]. They are
easy to synthesize, are highly conductive, and exhibit low viscosity. Their high efficiency
is provided through efficient interfacial wetting to the electrodes [47].
In general, a liquid electrolyte consists of three main components: the solvent, redox
couples, and various additives. Each component is considered in greater detail below.
Solvent:
The solvent is the major component in a liquid electrolyte, as it provides the
environment for the dissolution and diffusion of the redox couples. Solvents typically used
in DSSCs possess the following characteristics [48, 49]:
13
• Melting point below −20 °C and boiling point above 100 °C
• Sufficient chemical stability whether illuminated or not
• High dielectric constant
• Good solubility
• Low viscosity, so that redox mediators possess high diffusion coefficients and the
liquid electrolytes will have high conductivity
• Minimal light absorption
• Chemically inert with the surface-attached dye, including the dye−metal-oxide (or
another semiconductor) bond
• Poor solubility to the sealant materials
• Low toxicity
• Low cost
Two types of solvents meeting the above requirements have been used in DSSC designs:
polar solvents, and highly conductive ionic liquids (IL) or molten salts.
Redox Couples:
Redox-couples (RCs), or redox mediators, play an important role in DSSCs. The RCs
are responsible for charge transport between the photoanode and the counter electrode. The
electrochemical potential of the RC determines the DSSC’s open-circuit voltage (Voc) with
respect to the Fermi energy of the conducting substrate with the attached photoanode. With
regard to reaction kinetics, the RC should exhibit “asymmetric behavior” [50], i.e. electron
donation to the photo-oxidized dye should be sufficiently fast to ensure efficient dye
regeneration, and electron acceptance from the mesoporous TiO2 film should be slow
enough to decrease electron recombination losses [47].
14
To date, the most efficient RC for DSSC use is the I-/I-3 redox couple traditionally
dissolved in volatile organic solvents. This RC has good solubility, high conductivity, long-
term stability, and ability to efficiently penetrate into the mesoporous semiconductor film.
However, several drawbacks limit the industrial application of the I-/I-3 RC. First, iodine is
extremely corrosive toward metals used as current collectors in some DSSCs, such as
copper or silver, which are. With a relatively high vapor pressure, proper encapsulation of
the iodine-based cells is challenging. In addition, the I-3 ion absorbs a significant part of
visible light, stealing photons from the sensitizing dye. Finally, the redox potential of the
I-/I-3 limits the photovoltage [51].
Many alternative redox couples have been investigated as replacements for the current
I–/I–3 system. The goal is to minimize the driving force needed for dye regeneration while
increasing the device photovoltage, which affects the photoconversion efficiency. These
redox couples include halide and sulfur/selenium-organic redox couples (e.g. Br-/Br3-,
SCN-/(SCN)3-, SeCN-/(SeCN)3
- [47]), as well as transition-metal complexes (e.g.
ferrocene/ferricenium/ (Fc/Fc+), copper (I/II), cobalt (II/III), and nickel (III/IV) complexes
[52]).
Additives
The additive optimizes the DSSC’s photovoltaic performance. Specific cations or
compounds in the electrolyte are expected to adsorb onto the mesoporous semiconductor
surface. This adsorption has been found to affect fundamental photovoltaic parameters
such as the semiconductor surface charge, the shift of the conduction band edge (CB),
overall recombination kinetics etc., which in turn affects the overall photoconversion
efficiency [53].
15
Quasi-Solid-State Electrolytes
For any substance, the quasi-solid or semisolid state is a special state between the
normal solid and liquid states. Quasi-solid-state electrolytes possess, simultaneously, the
cohesiveness of solids and the diffusivity of liquids [54-56]. Consequently, quasi-solid-
state electrolytes exhibit greater long-term stability than liquid electrolytes, with higher
ionic conductivity and excellent interfacial contact. Due to these desirable characteristics,
quasi-solid electrolytes are typically used in DSSCs [57].
Quasi-solid-state electrolytes are synthesized through solidification of liquid
electrolytes with various gelating compounds; the resulting electrolytes are generally
grouped into three major kinds [46]. Organic gelators, such as polyacrylonitrile (PAN),
polystyrene (PS), polyethylene oxide (PEO or PEG), polyvinyl pyrrolidinone (PVP),
polyvinyl chloride (PVC), etc) [58] form thermoplastic or thermosetting polymer
electrolytes. Inorganic gelators, such as SiO2 or nanoclay, form composite polymer
electrolytes. Any gelating compound can be used to synthesize a quasi-solid ionic liquid
electrolyte [59] from an existing ionic liquid electrolyte.
In general, the photoconversion efficiency of a quasi-solid-state DSSC is less than that
of a DSSC employing a liquid electrolyte. Research into optimized designs and selection
of a suitable quasi-solid electrolyte should yield significant improvements in photovoltaic
performance.
Solid-State Conductors
DSSCs employing liquid electrolytes cannot avoid solvent exudation under conditions
of long-term storage or exposure to air. To address this issue, several solid-state transport
materials have been developed for replacing liquid or quasi-solid electrolytes, including
16
ionic conductors and organic/inorganic hole-transport materials. Such solid-state transport
materials exhibit good long-term mechanical and operational stability compared to dye-
sensitized photoelectrochemical cells. However, the photoconversion efficiency of
traditional DSSCs based on solid-state transport materials (including solid-state
electrolytes and hole-transport materials) is lower than for DSSCs based on liquid
electrolyte due to poor interfacial contact with the counter electrode.
The semiconductor-dye pair should possess the following characteristics [60]:
• The semiconductor has a wide band gap, which corresponds to transparency in the
portion of the visible spectrum where the dye most strongly absorbs light.
• The semiconductor deposition does not dissolve or degrade the dye monolayer on the
TiO2 nanocrystallites.
• The dye’s lowest unoccupied molecular orbital (LUMO) level is greater than TiO2’s
lowest conduction band level, and its highest occupied molecular orbital (HOMO)
level is less than the p-type semiconductor’s highest valence band level.
The most common approach to fabricate solid-state DSSCs involves the use of p-type
semiconductor materials. Fujishima, et al [61] found that copper compounds such as CuI
could effectively serve as a p-type semiconductor. In particular, CuI can be cast from
solution or vacuum deposition to form a complete hole-transporting layer, and it exhibits
conductivity in excess of 10-2 Scm-1.
Advantages and Disadvantages of Dye-Sensitized Solar Cells
DSSC technologies are among the third-generation technologies in solar cell
development. Their development addresses deficiencies in previous cell designs and
production. This section summarizes the advantages and disadvantages inherent to this
17
particular design.
Advantages
DSSC cell designs offer the following advantages [62, 63]:
• The photoanode is fabricated from a nanoparticle material. As a result, the cell can
absorb almost all photons in the incident solar beam. The dye absorber within the
nanoparticles efficiently converts photons to electrons.
• DSSCs can more easily and efficiently absorb diffuse sunlight and fluorescent light,
in addition to direct sunlight. This allows operation in overcast conditions or even
indoors to drive small devices.
• Heat is more easily radiated from the thin layers of the photoanode and counter
electrode, allowing greater operating performance and efficient reduction in internal
temperatures during operation.
• Individual components in a DSSC generally do not need frequent replacement.
• They are more mechanically robust, allowing easier handling.
• They are more economical to fabricate than solar cells based on bulk semiconductor
materials.
Disadvantages
The following issues currently limit use of DSSC technologies [64]. For DSSC’s using
liquid electrolyte solutions, the issues are related to the temperature sensitivity of the
electrolyte solution:
• At low temperatures, the electrolyte can freeze, thus rendering the DSSC completely
inoperable.
• At higher temperatures, the liquid electrolyte expands, resulting in issues with sealing
18
the photoanode and counter electrode. There may also be issues with respect to stability
of the operating electrochemical potential, and a risk of partial or complete evaporation
of the electrolyte solution, which will also result in the DSSC becoming inoperable. To
avoid these issues, the maximum safe operating temperature must be limited.
DSSCs using aqueous solid and/or solid electrolytes are more resistant to the effects of
electrolyte temperature. Their primary disadvantage is the decrease in operating efficiency
with respect to liquid electrolyte DSSCs.
Previous Work
This section summarizes previous work with respect to DSSC design and fabrication,
emphasizing research into liquid electrolyte-based dyes and additives used for the I-/I-3
redox couple. The maximum reported photoconversion efficiency resulting from the
reported research will also be given.
Dye-Sensitized Solar Cell Liquid Electrolytes
O’Regan’s and Gratzel’s original DSSC design [14] used a rudimentary liquid
electrolyte consisting of I-/I-3 redox couple dissolved in an organic solvent, without extra
additives. The reported photoconversion efficiency for this design was between
approximately 7.1% and 7.9%.
In 2000, Oskam et al. [65] reported on their efforts to prepare and characterize two
pseudohalogen redox couples for dye-sensitized TiO2 photoelectrochemical cells, and
investigated the effect of the redox potential on Voc. The equilibrium potentials of the
(SeCN)2/SeCN- and (SCN)2/SCN- couples were 0.19 and 0.43 V, respectively, more
positive than for the I-/I-3 couple. With the N3 sensitizer (cis-Ru(dcb)2(NCS)2), the incident
photon-to-current conversion efficiency decreased by a factor of 4 for (SeCN)2/SeCN- and
19
a factor of 20 for (SCN)2/SCN- compared with I-/I-3.
In 2003, Nusbaumer, et al [66] synthesized a group of cobalt (Co) complexes and
evaluated their suitability as redox mediators. The one‐electron‐transfer redox mediator
[Co(dbbip)2] (ClO4)2 (dbbip = 2,6‐bis(1′‐butylbenzimidazol‐2′‐yl) pyridine) was found to
achieve the best performance among the compounds investigated. Photovoltaic cells
incorporating this redox mediator reported a maximum photoconversion efficiency of
approximately 8%, and incident photon-to-current conversion efficiency of up to 80%.
However, the cells demonstrated poor dye regeneration kinetics.
In 2004, Gratzel, et al [67], synthesized and studied a solvent-free ionic liquid
electrolyte based on a SeCN−/(SeCN)-3 redox couple as a mediator. DSSCs using this redox
couple mediator demonstrated maximum photoconversion efficiencies between
approximately 7.5% and 8.3% at illumination levels of 1.5 times full sunlight, as good as
or better than the standard I-/I-3 redox couple.
In 2005, Wang, et al [68] characterized the performance of halide-based redox couples.
Under illumination levels corresponding to 1.5 times normal sunlight, DSSCs using 0.4 M
LiBr + 0.04 M Br2 electrolyte in acetonitrile yielded a 2.61% photoconversion efficiency.
DSSCs using 0.4 M LiI + 0.04 M I2 in acetonitrile as the electrolyte solution and tested
under the same conditions produced a photoconversion efficiency of 1.67%. Replacement
of I-/I3- with Br-/Br-
3 in eosin Y yielded a significant increase in Voc that was offset by slight
decreases in Jsc and FF, leading to an increase in photoconversion efficiency by 56%.
In 2005, Yanagida, et al [69], synthesized and investigated a new series of Cu
complexes for their effectiveness as redox mediators, and compared their photochemical
responses to the standard I-/I-3 couple. Under standard solar illumination levels
20
(approximately 100mW/cm2), DSSCs using the mediators [Cu(phen)2]2+/+,
[Cu(dmp)2]2+/+, and [Cu(SP)(mmt)]0/- exhibited photoconversion efficiencies of
approximately 0.1%, 1.4%, and 1.3%, respectively. A maximum photoconversion
efficiency of approximately 2.2% was obtained for a DSSC using the [Cu(dmp)2]2+/+
redox couple at a reduced illumination level of 20 mW/cm2 intensity, with a higher Voc
than achieved with the conventional I-/I-3 couple.
In 2007, Gorlov, et al [70] synthesized and investigated new halide/halide redox
couples such as I/IBr2 and I/I2Br for use as ionic liquid electrolytes, in combination with
Ru-based photosensitizing dyes. DSSCs using the new halide/halide couples achieved
maximum photoconversion efficiencies of approximately 6.4%. However, their
characterization was more difficult due to the complex equilibrium conditions between
various halide ions.
In 2008, Zhang, et al [71] synthesized an organic redox couple based on the 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) radical. The photovoltaic performance of this new
redox couple was evaluated by employing nanocrystalline TiO2 films of varying thickness.
They estimated an overall photoconversion efficiency for such devices of approximately
5.4% under illumination levels 1.5 times normal sunlight.
In 2010, Whang, et al [72] synthesized a disulfide/thiolate redox couple with little
absorption in the visible spectrum, a very attractive feature for flexible DSSCs using
transparent conductors as current collectors. Using this new iodide-free redox electrolyte
with a sensitized heterojunction, they achieved a maximum photoconversion efficiency of
approximately 6.4% under standard illumination conditions. This redox couple may
provide a viable pathway for developing efficient DSSCs that can be scaled for practical
21
applications.
In 2010, Tina, et al [73] synthesized a nickel Ni(III)/(IV) bis(dicarbollide) redox couple
and investigated its potential usability with varying electrolyte concentrations and
additives. The Ni (III)/(IV) redox couple was identified as a promising, non-corrosive
redox couple with good solubility and redox properties. DSSCs using a mixture of reduced
nickel III (0.030 M) and oxidized nickel IV (1.8 × 10-3 M) in acetonitrile, exhibited
maximum photoconversion efficiencies of approximately 0.9% with tert-butylpyridine and
TMABF4 as solution additives.
In 2011, Daeneke et al [74] reported maximum photoconversion efficiencies of
approximately 7.5% at illumination levels 1.5 times normal sunlight in DSSCs using the
ferrocene/ferrocenium (Fc/Fc+) single-electron redox couple combined with the metal-free
organic donor-acceptor sensitizer Carbz-PAHTDTT. Under comparable conditions, these
Fc/Fc+-based devices exceeded the efficiency of devices using the standard I-/I-3
electrolytes, suggesting their potential as a replacement electrolyte in future DSSC
applications. This improvement was thought to result from a more favorable matching of
the redox potential of the ferrocene couple to that of the new donor–acceptor sensitizer.
In 2012, Kato, et al [75] synthesized an organic redox mediator based on the 2-
azaadamantan-N-oxyl (AZA) radical. This mediator exhibited appropriate redox potential
and high diffusivity, electron transfer and electron self-exchange reaction rates, which
resulted in decreased cell resistance. Consequently, DSSCs using this mediator achieved
maximum photoconversion efficiencies of approximately 8.6%, due to a significantly
higher FF (> 0.75) with the AZA mediator, which also exhibited more effective charge
propagation and regeneration.
22
Iodide/Triiodide Electrolyte Additives
In 1993, Gratzel, et al [76] studied 4-tert-butylpyridine (4TBP) as a potential additive
to the standard I-/I-3 electrolyte. This additive causes a negative or upward shift in the TiO2
conduction band [77], resulting in a higher Voc that enhances the DSSC’s overall
performance.
In 2000, Park, et al [78] investigated the photovoltaic performance of dye-sensitized
nanocrystalline rutile TiO2 DSSCs using additives containing Li+ and 1,2-dimethyl-3-
hexyl imidazolium ions in the standard I-/I-3 electrolyte. The electrolyte containing Li+ ions
produced a lower Voc and higher short-circuit photocurrent Isc than the electrolyte
containing 1,2-dimethyl-3-hexyl imidazolium ions. The adsorption of Li+ ions to the
nanocrystalline film was shown to have a significant effect on the band-edge position,
which, in turn affects Voc, Isc, and IPCE. This study indicated that for future development
of high efficiency DSSCs, the nature of the cation must be a major consideration when
optimizing the electrolyte composition.
In 2009, Zhang, et al [79] investigated the influence of guanidinium thiocyanate
(GuSCN) on the photoelectron injection efficiency and interface dynamics of electron
transfer. Analysis of the short circuit current density Jsc vs light intensity suggested that
higher electron injection efficiency could be achieved with GuSCN as the electrolyte
additive. Increasing the GuSCN concentration from 0.1 to 0.4 M had no significant effect
on electron injection efficiency. Based on analysis of impedance spectra of the solar cells,
they concluded that addition of GuSCN to the electrolyte could suppress surface
recombination effects on the TiO2 electrode, resulting in a shift of the conduction band
towards positive potentials.
23
In 2010, Yu, et al [80] investigated the effects of GuSCN and N-methylbenzimidazole
(NMPI) additives on DSSC photovoltaic performance using ionic-liquid and organic-
solvent based electrolytes. They observed only a slight improvement in Voc and a
significant enhancement in Isc with GuSCN as the only electrolyte. They also observed a
significant improvement in Voc with NMPI as the only electrolyte. Synergistic
enhancements of Voc and Isc were observed when both GuSCN and NMPI were added to
the ionic-liquid electrolyte; these effects resulted in optimal photovoltaic performance.
In 2010, Kim, et al [81] investigated the potential of thiourea as an additive to the
standard I-/I-3 redox electrolyte. They observed simultaneous positive conduction band
edge shifts and decreases in the charge recombination rate. For a thiourea concentration of
0.05M, adding 0.7M of 1-methyl-3-propylimidazolium iodide (MPII) and 0.05 M I2 in
acetonitrile significantly enhanced Jsc (from approximately 7.7 mA/cm2 to 10.8 mA/cm2)
while reducing Voc (from approximately 0.78 V to 0.71 V). The resulting overall
photoconversion efficiency increased approximately 23%, from 4.7% to 5.8%.
In 2012, Kim, et al [82] studied replacement of GuSCN with ordinary urea as a long-
term stabilizer additive. They found the urea-based additive to be less reactive than
GuSCN, resulting in greater long-term thermal and electrical stability.
Motivations
This work investigates the use of novel electrolyte additives to DSSCs using the
standard I-/I-3 redox electrolyte. DSSC performance using varying additive concentrations
is characterized, with the goal of further optimizing photovoltaic characteristics and overall
photoconversion efficiency.
24
Objective
Improve the DSSC efficiency using commercially available liquid I-/I-3 redox
electrolyte by more than 1%, from approximately 8.5% to more than 9.5%, using urea
(CH4N2O) as the sole electrolyte additive. In addition, achieve similar improvement in
DSSC efficiency using reduced graphene oxide (rGO) as the sole electrolyte additive.
The implementation plan for this work consists of the following tasks:
1. Fabricate three DSSC devices with urea doped electrolyte for each concentration:
0, 2, 5, 10, and 15 wt%.
2. Fabricate three DSSC devices with rGO doped electrolyte for each concentration:
0, 2, 5, 10, and 15 wt%.
3. Measure Current density-voltage (J-V), external quantum efficiency (EQE),
electrochemical impedance spectroscopy (EIS), cyclic voltammogram (CVs), and
UV-Vis absorption spectra for all fabricated devices.
25
Chapter 2: Theory
Solar Radiation and Air Mass
The solar spectrum typically covers wavelengths corresponding to the ultraviolet and
visible / near infrared electromagnetic spectrum (0.2 µm to 3.0 µm). The intensity is non-
uniform, sharply increasing through the ultraviolet region to a maximum in the visible
region, then decreasing more slowly into the infrared region (Figure 2.1). The solar
spectrum at the top of the atmosphere is approximated with a Planck (blackbody) radiation
curve corresponding to a temperature of approximately 5250° C, as shown in Figure 2.1.
Figure 2.1 Solar Radiation Spectrum [83]
Solar spectra measured at the top-of-atmosphere (TOA) are not the same as spectra
measured at the surface, due to atmospheric scattering and absorption effects. The path
length of solar energy in the atmosphere depends on the solar zenith angle (Figure 2.2),
26
which varies with the time of day. This path length is given by the air mass number (AM),
which is the secant of the solar zenith angle θ:
sec 1cos
(2.1)
Figure 2.2 Solar Zenith Angle. Modified from ref [84]
With respect to solar cell technology, solar spectra are considered at three different air
mass values: AM0, AM1.5global, and AM1.5direct. The different spectra are plotted in Figure
2.3. With the sun directly overhead 0°, radiation striking the ground passes through
all of the atmosphere. This direct radiation is known as "Air Mass 1 Direct" (AM1direct)
radiation. Similarly, the global radiation at 0° is known as "Air Mass 1 Global"
(AM1global) radiation. Because it passes through no air mass at the top of the atmosphere,
the extraterrestrial spectrum is called the "Air Mass 0" spectrum or AM0 [85]. AM1.5direct
is the spectrum related to the air mass corresponding to θ=48.2ο due to rays directly
transmitted to the surface that cast a hard shadow. Similarly, AM1.5global is the spectrum
related to the air mass corresponding to θ=48.2ο due to directly transmitted rays and rays
27
scattered by the atmosphere to the surface. Air mass estimates corresponding to a specific
solar zenith angle value can still vary by season, location, local weather conditions, etc.
Figure 2.3 Solar Reference Spectra [86]
Solar Cells and Photovoltaic Effect
The working principle of solar cells is based on the photovoltaic effect, i.e. the
generation of an electrical potential difference at the junction of two different materials in
response to electromagnetic radiation. The photovoltaic effect is closely related to the
photoelectric effect, in which electrons are emitted from a material absorbing light at a
frequency higher than a threshold frequency dependent on the material. In 1905, Albert
Einstein hypothesized this effect could be understood by assuming that light consists of
well-defined energy quanta called photons. The energy of such a photon is given by
(2.2)
28
where h is Planck’s constant and ν is the frequency of the light. For his explanation of the
photoelectric effect, Einstein received the Nobel Prize in Physics in 1921 [87].
The photovoltaic effect can be divided into three basic processes:
i. Charge Carrier Generation Due to Photon Absorption in Junction Materials
When a photon with energy is absorbed by bulk semiconductor material, its energy
excites an electron from an initial energy level Ei to a higher energy level Ef, and creates a
void in the valence band behaving as a positively charged particle known as a “hole”, as
shown in Fig. 2.4. Photons can only be absorbed if the difference between Ei and Ef equals
the incident photon energy . In an ideal semiconductor, electrons can populate energy
levels less than the valence band edge, EV, and greater than the conduction band edge, EC.
Between these two energy levels is the bandgap, where no energy levels exist in which
electrons can populate:
Eg = EC − EV (2.3)
Incident photons with energy hν < Eg traverse the material unabsorbed. Photons with
incident energy hν > Eg emit an electron with an excited energy state above the minimum
conduction band energy; the electron then emits the energy difference (hν - Eg) as thermal
energy (heat).
29
Figure 2.4 Photon Absorption in a Semiconductor with Bandgap Energy Eg
In a real semiconductor, the valence and conduction bands are not flat as shown in
Figure 2.4, but vary depending on the k-vector describing the crystal momentum of the
semiconductor, as shown in Figure 2.5. If the valence band maximum and conduction band
minimum energies share the same k-vector, an electron can be excited from the valence to
the conduction band without a change in crystal momentum. Such a semiconductor is
called a direct bandgap material. If the electron cannot be excited without changing the
crystal momentum, the semiconductor is called an indirect bandgap material. The
absorption coefficient in a direct bandgap material is much higher than in an indirect
bandgap material, thus the bulk material can be much thinner [88].
For any semiconducting material, if an electron is excited from Ei to Ef, an electron-
hole pair is created. The radiative energy of the photon is converted to the electrical energy
of the electron-hole pair. The maximum conversion efficiency from radiative energy to
chemical energy is limited by thermodynamics. For sunlight, the thermodynamic limit
ranges from approximately 67% for non-concentrated sunlight to approximately 86% for
fully concentrated sunlight [89].
30
Figure 2.5 Direct and Indirect Bandgap Semiconductor Characteristics
ii. Separation of Photo-Generated Charge Carriers in the Junction
Usually, the electron-hole pair recombines shortly after its generation, i.e. the electron
falls back to the initial energy level Ei. The energy is released either as a photon, transferred
to other nearby electrons or holes, or released as thermal energy through lattice vibrations
in the semiconductor material. To use the energy stored in the electron-hole pair for
performing work in an external circuit, a semipermeable membrane must be present on
both sides of the junction such that electrons leave through one membrane and holes leave
through the other [89]. In most solar cells, these membranes are formed by doped n- and
p-type materials. A solar cell must be designed such that the electrons and holes can reach
the membranes before they recombine, i.e. the time required for charge carriers to reach
the membranes must be shorter than the recombination lifetime. This requirement limits
the thickness of the junction region.
iii. Accumulation of Photo-Generated Charge Carriers at the Junction Terminals
Finally, the charge carriers are extracted from solar cells with electrical contacts so that
they can perform work in an external circuit. The chemical energy of the electron-hole
31
pairs is finally converted to electrical energy. After the electrons pass through the circuit,
they will recombine with holes at a metal absorber interface.
Classical P-N Junction Solar Cell
P-N junctions are formed by joining n-type and p-type semiconductor materials, as
shown in Figure 2.6. Since the n-type region has a high electron concentration and the p-
type region a high hole concentration, electrons diffuse from the n-type side to the p-type
side. Similarly, holes diffuse from the p-type side to the n-type side. If the electrons and
holes were not charged, this diffusion process would continue until the concentrations of
electrons and holes on both sides were equal. However, in a P-N junction, electron/hole
diffusion creates a diffusion current, leaving behind exposed charges on dopant atom sites,
which are fixed in the crystal lattice and unable to move. In the n-type material, positive
ion cores are exposed, and in the p-type material, negative ion cores are exposed. An
electric field Ê forms between the positive and negative ion cores, resulting in a carrier
drift current in the opposite direction of the diffusion current. Carrier diffusion continues
until the drift current balances the diffusion current, thereby reaching thermal equilibrium
as indicated by a constant Fermi energy (Figure 2.6). The region where the electric field
occurs is called the "depletion region" since the field quickly sweeps free carriers out.
32
Figure 2.6 Energy Band Diagram of a p-n Junction in Thermal Equilibrium, with Depletion Region. Modified from ref [90]
DSSC Operating Principles
Nanocrystalline TiO2 is deposited on the conducting electrode (photoelectrode) of a
DSSC to provide a sufficiently large surface area to adsorb sensitizers (dye molecules).
Upon photon absorption, dye molecules are excited from the highest occupied molecular
orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) states as shown
schematically in Figure 2.7. This excitation process is represented as:
→ ∗ (2.4)
Once an electron is injected into the conduction band of a wide bandgap semiconductor
nanostructured TiO2 film, oxidation of the dye molecule (photosensitizer) occurs:
∗ !" → #$%&'( ) (2.5)
33
The injected electron is transported between the TiO2 nanoparticles and then extracted to a
load in the counter electrode (cathode) where the work done is delivered as electrical
energy:
#$%&'( *. . → !" #+.,.( #-#./ .0-##/12 (2.6)
Electrolytes containing, I-/I-3 redox ions are used as an electron mediator between the TiO2
photoelectrode and the carbon coated counter electrode. Therefore, the oxidized dye
molecules (photosensitizer) are regenerated by receiving electrons from the I- ion redox
mediator as it oxidizes to I-3:
) 325( → 12 56(
(2.7)
The I-3 ion substitutes the internally donated electron with that from the external load and
reduces back to an I- ion:
12 56( #+.,.( → 3
25( *. . (2.8)
Electron movement in the conduction band is accompanied by the diffusion of charge-
compensating cations in the electrolyte layer close to the nanoparticle surface. Therefore,
a DSSC can generate electric power without causing any permanent chemical change or
transformation (Gratzel, 2005).
As illustrated in Figure 2.7, the maximum potential produced by the cell is determined
by the energy separation between the electrolyte redox potential (Eredox) and the Fermi level
EFermi of the TiO2 layer. The small energy separation between the HOMO and LUMO
ensures absorption of low energy photons in the solar spectrum. Therefore, the
photocurrent level is dependent on the HOMO-LUMO energy difference of the dye,
analogous to the bandgap energy Eg in inorganic semiconductor materials. Electron
34
injection into the TiO2 conduction band of is enhanced with the increase of energy
separation of the dye LUMO and the TiO2 conduction band minimum. Furthermore, for
the HOMO level to effectively accept donated electrons from the redox mediator, the
energy difference between the HOMO and redox chemical potential must be more positive
(Hara & Arakawa, 2003) [91].
Figure 2.7 Schematic of DSSC with Energy Level Diagram. Modified from ref [14, 92]
Key Efficiency Parameters Representing DSSC Performance
The following sections consider the four main parameters used to characterize the
performance of DSSCs. These parameters–the peak power Pmax, the short-circuit current
density Jsc, the open-circuit voltage Voc, and the fill factor FF–are determined from the
illuminated J-V characteristic curve, as shown in Figure 2.8. The conversion efficiency η
can be determined from these parameters as shown in Section 2.5.4.
35
Short-Circuit Current Density
The short-circuit current Isc is the current flowing through an external circuit when the
solar cell electrodes are short-circuited. It depends on the cell’s area and the incident photon
flux density determined by the incident light intensity, standardized to the AM 1.5
spectrum.
To remove the dependence of the solar cell area on Isc, the short-circuit current density
Jsc is used to describe the maximum current. The maximum current in general depends on
the cell’s optical properties, such as its reflectivity and the absorber layer’s absorption
characteristics. In a DSSC, Jsc also depends on the intensity of incident light and absorption
spectrum of the dye:
789 # : ;<= >?@AB
(2.9)
where Nph is the photon flux of incident light per unit wavelength (counts/[m2-s-nm]), Eg
is the material bandgap in electron volts, e is electronic charge in Coulombs, E is the photon
energy in electron volts, and EQE is the external quantum efficiency.
The net current density, J(V), within the cell is given as:
JV JEFGHV I J<= (2.10)
where Jph is the photocurrent density, and the dark current density Jdark, is
JEFGHV JJ Ke LMNOPQ I 1R (2.11)
where KB is Boltzmann`s constant, J0 is the saturation current density, e is the electronic
charge, and T is the temperature in degrees Kelvin. Substituting Equation 2.11 into
equation 2.10 allows J(V) to be expressed as follows:
36
JV JJ Ke LMSOPQ I 1R I JTU (2.12)
When the load resistance is zero, corresponding to V=0, J(V) is just Jsc:
J J89 (2.13)
Open-Circuit Voltage
The open-circuit voltage Voc is the voltage at which no current flows through an
external circuit (7 = 0). It is the maximum voltage that a DSSC can deliver. Voc corresponds
to the forward bias voltage of the cell’s internal P-N junction such that the dark current
density balances the photocurrent density. Voc can be calculated by solving equation 2.12
for V and assuming a zero net current density:
VW9 XY# ln JTU7J 1 \ XY
# ln JTU7J (2.14)
where the approximation is justified because JTU >> 7J.
Equation 2.14 shows that Voc depends on the ratio of the photocurrent and saturation
current densities in the DSSC. While Jph typically exhibits minimal variation, J0 can vary
by several orders of magnitude, as it depends primarily on recombination effects.
Therefore, Voc is a measure of the amount of recombination in the device. Laboratory
crystalline silicon solar cells have a Voc of up to 720 mV under standard AM1.5
illumination, while commercial DSSCs under similar illumination typically have Voc
exceeding 600 mV [88].
Fill Factor
The fill factor (FF) is a measure of the maximum power output from a DSSC and is
defined as the ratio of the maximum power to the product of Voc and Jsc. FF is calculated
as follows:
37
F^ _789VW9 7_V_789VW9 (2.15)
where Jm is the maximum current density (A/cm2) and Vm is the maximum voltage (V). It
can be approximated by rectangles in a J-V curve (Figure 2.8). In the ideal case, FF is equal
to 1.
The fill factor is a function of the DSSC’s series and shunt resistances, and reflects the
extent of electrical and electrochemical losses during cell operation. Higher FF values can
be obtained through i) increasing the shunt resistance; ii) decreasing the series resistance;
and/or iii) reducing the “overvoltage” [88] related to diffusion and charge transfer.
Figure 2.8 J-V Characteristic Curves Under Dark and Illuminated Conditions
Conversion Efficiency
The light-to-electricity photoconversion efficiency (η) is the total amount of electrical
38
power produced for a given amount of solar energy incident on the cell. It is calculated as
the ratio of the maximum generated power and a reference incident power Pin of 100
mW/cm2 assuming an AM1.5 spectrum. Additional information relating to the conversion
efficiency is presented in Section 2.7.1.
Equivalent Circuit Model for Conventional PN-Junction Solar Cells
The electrical properties of conventional P-N junction solar cells can be characterized
using a simple equivalent DC circuit similar to Figure 2.9. In this equivalent circuit, a
resistance (Rs) is in series with a diode representing the electrical properties of the junction;
a shunt resistance (Rsh) and a constant-current source (Iph) are in parallel with the junction.
Series resistance occurs along the path the current takes through the cell; as it increases,
Voc decreases, which in turn reduces FF (Figure 2.10). Losses occur at the contacts and the
interfaces. No other parameters are affected until this resistance is large enough to reduce
Jsc. The shunt resistance is due to defects introduced during cell fabrication that reduce the
available photocurrent by allowing it to flow along multiple paths. As the shunt resistance
decreases, Jsc is reduced, which in turn reduces FF. The source current Iph results from the
excitation of excess carriers by incident light.
Figure 2.9 Conventional P-N Junction Solar Cell Equivalent circuit model [93]
39
Figure 2.10 Changes in I-V Response with Changes in Rsh and Rs
Characterization Techniques
This section describes the measurements used in this work to characterize DSSC cell
performance. These measurements include the current density-voltage (J-V) characteristic,
the incident photon-to-current-efficiency (IPCE) spectrum, the electro-chemical
impedance spectrum (EIS), cyclical voltammetry (CV), and the UV-visible absorption
spectrum.
Current Density-Voltage Characteristics (J-V)
Current density-voltage (J-V) measurement is the most important and conventional
technique for assessing solar cell photovoltaic performance. A standard illumination of AM
1.5 with an irradiance of 100 mW/cm2 is normally used for characterization. The J-V
characteristics are determined under constant illumination, by increasing an external load
from zero (short-circuit conditions) to infinite (open-circuit conditions). There is a point
on the knee of the J-V curve where maximum power is produced, the Maximum Power
40
Point (MPP) [94]. The voltage and current density at the Maximum Power Point are
designated as Vm and Jm (Figure 2.10).
J-V characteristic curve measurements provide the key parameters needed to estimate
a DSSC’s light-to-electricity photoconversion efficiency (η), defined as the ratio of the
product of Jsc, Voc, and FF to the reference intensity Pin under AM1.5 illumination:
η JabVcbFFPeN 100
(2.16)
Current density-voltage measurements should be carried out with a sufficiently slow
voltage scanning rate such that the cell response is free of hysteresis effects.
External Quantum Efficiency (EQE)
The external quantum efficiency (EQE) is another key parameter used in quantitative
characterization of DSSC performance. The EQE describes the efficiency of converting
incident energy to electron-hole pairs as a function of wavelength. More specifically, the
EQE is the ratio of the number of electrons generated by the cell and collected at an external
circuit to the number of incident photons with a given wavelength striking the cell’s
photoanode:
> /0#fgh#/i.--#.#?#-#./j/0#fgh#/i . ?#kj
(2.17)
Alternatively, the EQE can be calculated as a scaled ratio of the short-circuit current density
to an input irradiance of 100 mW/cm2 assuming AM1.5 illumination. The scaling factor
varies inversely with wavelength:
> . 789# l %S 1240 789l %S (2.18)
where h is Planck’s constant (6.626 ×10-34 J.s), c is the speed of light in vacuum (2.998
×108 m s-1), and e is the electronic charge.
41
Electrochemical Impedance Spectroscopy (EIS)
According to Ohm’s law, a DC current I flowing through a resistor R produces a voltage
drop V across the resistor terminals:
V 5n (2.19)
For a sinusoidal varying current at an angular frequency ω flowing through its terminals,
the resistor has a characteristic impedance consisting of its DC resistance R and a reactance
X that is a function of ω; X represents the resistor’s opposition to changes in applied current
or voltage due to its internal inductance and/or capacitance. The impedance of the resistor
is represented by a complex number, with the resistance as the “real” component and the
reactance as the “imaginary” component [95]:
o n pq (2.20)
By applying a harmonically modulated small amplitude AC potential V(ω, t) at a given
frequency i r"s to the resistor and measuring the resulting current I(ω, t), the resistor’s
impedance can be expressed according to a generalized version of Ohm’s law:
ot Vt, 5t, (2.21)
The same current-voltage relationship exists in DSSCs, with R defined as the total DC
resistance of the cell. However, this value of R represents a “black box” measure of
resistance across the cell terminals; there is no knowledge about resistances associated with
individual cell components, e.g resistance associated with charge transfer at the counter
electrode, resistance associated with electron transport in the TiO2 mesoporous layer, or
resistances due to recombination and diffusion in the electrolyte. Measuring the cell’s
electrochemical impedance spectrum (EIS) provides information on these internal
42
resistances.
If the applied voltage is time-independent (ω=0), DC current flows through the cell,
and the resulting impedance is just its DC resistance (RDC):
o0 V0, 50, nvw (2.22)
Using complex number notation, a small-amplitude sinusoidal varying AC voltage and
current can be described as: Vt, VJexppt and 5t, 5Jexppt I
respectively, where j=√ (−1) and V0 and I0 are the amplitudes of the voltage and current
signals, respectively. Thus, equation (2.21) can be written as:
opt Vt, 5t, VJ5J #| (2.23)
Using the magnitude Z0, equation (2.23) can be written as:
opt oJ#| (2.24)
By applying Euler’s relationship and replacing Z0 with |o|, equation (2.24) can be written
as:
opt |o|cos p sin (2.25)
The general expression for impedance is:
ot o po%_ (2.26)
where Zre = |Z| cos(θ) and Zim = |Z| sin(θ).
EIS data are usually displayed in the form of a Nyquist plot. In the complex plane, Zim
is plotted against Zre for different values of ω. The result is three semi-circular arcs
representing the low-frequency, mid-frequency, and high-frequency resistance
characteristics of the cell.
43
Figure 2.11 shows a standard Nyquist plot of a DSSC with equivalent circuit model. nj
is the series resistance of the DSSC, n.t is the charge transfer resistance at the counter
electrode, n/ is the recombination resistance of electrons in the TiO2 conduction band to
the redox couple in the electrolyte, and n is the diffusion resistance for the redox couple
in the electrolyte.
DSSCs can be modeled as an equivalent circuit of passive elements as shown in Figure
2.11. CPE is a constant phase element replacing the double layer capacitance Cdl on the
interface between an electrode and the electrolyte [96]. This double layer is formed as ions
from the solution adsorb onto the electrode surface. The charged electrode is separated
from the charged ions by an insulating space on the order of a few angstroms in thickness.
Figure 2.11 Example DSSC Nyquist Plot with Equivalent Circuit Model. Modified from ref [97, 98].
Cyclic Voltammogram Measurements (CVs)
Cyclic voltammetry (CV) is a powerful electrochemical measurement technique
44
commonly employed to characterize reduction and oxidation processes in various
molecular compounds. CV is widely used in DSSC research for characterization of dyes,
redox mediators and charge transfer processes at interfaces. In this technique, a cyclic
potential (Figure 2.12(a)) is applied to the CE immersed in a stable solution of the redox
couple electrolyte under study. The redox analyte’s oxidation/reduction reaction is
represented as:
! #( n#? (2.27)
where Ox and Red are the oxidized and reduced species (redox couples) in solution.
Figure 2.12(b) shows the corresponding response to the cyclic potential. When the
potential increases from an initial value (Ei), a small, transient double-layered capacitive
current appears. As the potential increases linearly with time beyond Eo (a threshold
voltage), the Fermi-level of the electrode is raised above the energy level of vacant orbitals
in the Ox species. The electrons in the electrode cross the electrode-solution interface and
reduce Ox to Red in the solution via reaction (2.27); as a result, a Faradaic current flows
in the electrode. A further increase in the negative potential accelerates the reduction and
depletion of Ox at the electrode/electrolyte interface, creating a significant concentration
gradient between the electrode/electrolyte interface and bulk solution. This concentration
gradient enhances fast diffusion of Ox from the bulk solution towards the interface, causing
the Faradaic current to reach a maximum called the cathodic peak current (Ipc) at the applied
voltage Epa. Even though the applied voltage increases beyond Epa, the cathodic current
decreases continuously from this point because the concentration gradient decreases with
time. As the cyclic potential reaches a peak value of EV, the potential begins decreasing
with time. When the potential approaches E0, the Fermi-level in the electrode is lowered;
45
electron transfer from Red into the electrode becomes more favorable, and Red is oxidized
back to Ox.
Figure 2.12 Cyclic Potential Sweep and (b) Shape of a Cyclic Voltammogram Curve. Modified from ref [14, 92]
Figure 2.13 shows that an electrolyte analyte solution will exchange an electron with
an electrode when it reaches certain threshold potentials. The electrolyte undergoes
reduction when the electron potential energy in the electrode is higher than the potential
energy of the electrolyte’s empty molecular orbitals. Conversely, the electrolyte undergoes
oxidation when its highest energy electron is higher than the electron potential energy in
the electrode. In both cases, electrons are transferred in order to minimize the potential
energy of the total system. A potentiostat can be used to change the electrode potential and
observe the direction of current flow in order to learn about the electrolyte’s electron
transfer energetics.
46
Figure 2.13 Energetics Between Electrolyte Solution and Electrode. Modified from
ref [99]
UV-Visible Absorption Spectra
UV-visible absorption spectroscopy is a technique used to measure electronic
transitions within a material when it is illuminated. The absorption of UV-visible light is
related to outer electron excitations. The material absorbs light with a wavelength matching
an energy level difference corresponding to the allowed transition of an σ-electron, n-
electron, or π-electron from its ground state to a higher anti-bonding molecular orbital level
(Figure 2.14).
47
Figure 2.14 Possible Transitions of π, σ, and n Electrons. Modified from ref [100]
UV-Vis spectroscopy analyzes the absorption characteristics of a sample of dye and/or
electrolyte solution across the ultraviolet, visible and near infrared regions of the
electromagnetic spectrum. Figure 2.15 shows a representative UV-Vis spectrometry
measurement setup. The system uses a deuterium arc lamp and tungsten lamp as light
sources generating wavelengths in the range of 190 – 1100 nm. The monochromator
consists of an entrance slit, a dispersion device and an exit slit, which produces a
monochromatic beam at a given wavelength. When illuminated by the monochromatic
beam, electrons in the sample are excited to an energy equal to the difference between
allowed transitions within the sample molecules (e.g., between the bonding and
antibonding orbital levels). The principle behind UV-Vis absorption spectroscopy is the
Beers-Lambert law, which is represented as:
l 55ₒ #( (2.28)
where I0 is the incident light intensity, d is the sample thickness, I is the transmitted light
intensity and α(λ) is the absorption coefficient. The total absorbance of the material, A(λ),
48
is given by:
Aλ I-1JTλ (2.29)
Substituting eq. 2.28 for the transmittance into eq. 2.29, the absorption coefficient of the
sample can be determined:
α 1d lnT (2.30)
Figure 2.15 UV-Vis Spectroscopy [101]
Mechanism of Urea Reaction with I-/I-3 Redox Electrolyte
Adding urea to a I-/I-3 redox electrolyte solution initiates a reaction with neutral iodine
to produce oxalyl dihydrazide, H+ ions, and iodide ions [81, 102, 103]:
2*;"! 5" →*";!" 2) 25( (2.31)
Generation of the H+ ions results in a positive shift in the electrolyte’s redox potential
(Figure 2.16), which in turn increases Voc as it is inversely proportional to the tri-iodide
concentration [104, 105]:
49
V&w # ln ∅%SJX56(
(2.32)
where k is Boltzmann’s constant, e is the electron charge in Coulombs, T is the absolute
temperature in degrees Kelvin, Φinj is the charge flux resulting from electron injection by
the dye, n0 is the electron concentration at the TiO2 surface when the cell is not illuminated,
and Ket is the I-3 reduction rate by conduction band electrons.
Figure 2.16 Mechanism of Urea Reaction with I-/I-3 Redox Electrolyte
Mechanism of rGO Reaction with I-/I-3 Redox Electrolyte
Reduced graphene oxide (rGO) is a conductive material that is insoluble in an I-/I-3
redox electrolyte solution. I-3 ions are large and heavy ions, with low mobility and a
tendency to concentrate near the photoanode (Figure 2.17). I-3 ions are electron acceptors,
promoting recombination reactions between the TiO2 conduction band and the electrolyte,
which reduces FF and Voc.
50
Adding rGO to the electrolyte cause intermolecular interactions between I-3 ions and
rGO due to the electrostatic interactions. In the crystal structure, I-3 was arranged as rigid
rods and arrays in parallel to rGO molecules [106, 107]. This positive interaction allows
the removal of I-3 ions far from the photoelectrode interfaces, recovering the Voc and FF
values. Therefore, adding rGO to the electrolyte increases Voc and accelerates the charge
transfer rate by reducing the electrolyte’s resistance and decreased charge back-
recombination rate.
Figure 2.17 Mechanism of rGO Reaction with I-/I-3 Redox Electrolyte
51
Chapter 3: Experimental Procedures
Material and Device Fabrication Procedures
This section considers the materials and particular procedures used to fabricate the
devices characterized in this thesis work.
Materials
Fluorine-doped tin oxide (FTO) substrates (sheet resistance of 7 Ω / sq and a thickness
of 2.2 mm) were ordered from Hartford Glass (http://www.hartfordglass.com, Hartford
City, IN, USA). Nanocrystalline TiO2 (Ti-Nanoxide HT/SP) (pore size ~15-20 nm),
scattering TiO2 (Ti-Nanoxide R/SP) (particle size ~100 nm), N719 dye (Ruthenizer 535-
bisTBA), iodide/triiodide electrolyte (Iodolyte AN 50), and Meltonix thermoplastic sealant
were purchased from Solaronix (http://www.solaronix.com, Aubonne, Switzerland). Urea
was obtained from Sigma-Aldrich (http://www.sigmaaldrich.com, St. Louis, MO, USA).
Reduced graphene oxide (rGO) (particle size ~ 6-10 µm) was obtained from Graphenea
(http://www.graphenea.com, Cambridge, MA, USA). All materials were used as received
except that TiCl4 was diluted in deionized water (DI) to prepare a 40 mM aqueous solution.
All other materials (i.e. isopropyl alcohol, acetone, acetonitrile, and valeronitrile) were
purchased from Fisher scientific (http://www.fisherscientific.com, Pittsburgh PA, USA).
Device Fabrication – Glass Substrate Preparation
FTO coated glass substrates were cleaned ultrasonically while bathed in solutions of
detergent, de-ionized (DI) H2O, acetone, and 2-propanol. Each bath lasted approximately
25 minutes. Once the baths were completed, the glass substrates were exposed to UV O3
for approximately 20 minutes.
52
Device Fabrication – Photoanode Fabrication
The working electrodes were prepared as follows. A compact TiO2 layer was deposited
onto the conductive side of one of the treated FTO glass substrates by spin coating. The
TiO2 compact layer solution consisted of a 75% ethanol solution containing a 0.3 M
solution of titanium diisopropoxide bis(acetylacetonate).
Once the TiO2 compact layers were applied by spin coating, a two-step annealing
process was performed, first at 125° C for approximately 15 minutes, followed by another
annealing at 470° C for approximately 20 minutes; both annealings were performed under
normal atmospheric conditions. After annealing the compact layer, a 0.16 cm2 x 12 µm
thick layer of nano-crystalline TiO2 was deposited onto the top of the compact layers using
the “doctor blading” technique [108]. The substrates were subjected to the same annealing
process as before, again under normal atmospheric conditions. The TiO2 light scattering
layer was prepared by doctor blading Ti-Nanoxide R/SP (particle size > 100 nm) and
sintering at 470° C for 30 min under normal atmospheric conditions.
After application of the final scattering layer, the photoanodes were soaked for
approximately 30 minutes in a 40 mM aqueous solution of TiCl4 heated to a temperature
of 70°C, rinsed with DI H2O and ethanol, and then dried for approximately 30 seconds in
a compressed nitrogen atmosphere. Finally, the photoanodes were soaked in a 0.3 mM N-
719 dye solution in acetonitrile/valeronitrile (volume ratio 1:1) for approximately 24 hours,
in order to insert dye molecules into the mesoporous TiO2 layer.
Device Fabrication – Counter Electrode (CE) Fabrication
The Pt CE was fabricated onto the conductive side of the other glass substrate by spin
coating an ethanol solution containing a 20 mM solution of hydrogen hexachloroplatinate
53
(IV) hexahydrate (99.9% trace metal basis) at 2000 rpm for approximately 10 seconds. The
substrates were then annealed at 400° C for approximately 15 minutes. A second layer of
Pt was added to the substrate using the same process.
Device Fabrication – Final Assembly
Figure 3.1 shows the final assembly of a DSSC. The dye-sensitized photoanodes were
washed with acetonitrile to remove unanchored dye from the TiO2 film and then dried with
compressed nitrogen gas for approximately 30 seconds. Both the photoanode and CE were
then joined together using Meltonix thermoplastic sealant, cut into a 2.5 cm x 1.5 cm
rectangular shape. A circular hole approximately 9 mm in diameter was punched into the
sealant. Outside the hole, two narrow channels approximately 1 mm in width were made
along the line of the diameter of the hole for injection of the electrolyte. The sealant was
positioned on the FTO surface so that the dye-sensitized TiO2 film was located at the center
of the hole. The glass side of the photoanode was kept on a hot plate at 80° C and gently
pressed with a cotton swab, in order to allow the sealant to bond to the photoanode.
The Pt CE was placed on the sealant, on top of the photoanode. The assembled layers
of the photo-anode, the sealant, and the CE were heated on the hot plate from the glass side
of the CE for less than 2 minutes at approximately 110° C, while pressed together from the
glass side of the photoanode. The assembled cell was then immediately removed from the
hot plate and allowed to cool to room temperature.
54
Figure 3.1 Schematic of DSSC device assembly
Device Fabrication – Electrolyte Preparation
The final step in the fabrication process involved addition of the urea and rGO agents
to the base I-/I3- electrolyte solution. Five batches of urea-doped electrolyte at
concentrations of 0%, 2%, 5%, 10%, and 15% by wt. were prepared. Similarly, five batches
of rGO-doped electrolyte were prepared with the identical concentrations. Three random
samples were then extracted for each concentration. Each sample was injected into a cell
through one of the channels cut into the sealant block. Once the photoanode-counter
electrode gap was filled with the electrolyte solution, the channels were sealed closed with
Adhesive Tech hot glue.
Materials and Device Characterizations
Current Density-Voltage Characteristics (J-V)
Figure 3.2 shows the schematic of the experimental arrangement used to measure the
J-V for DSSCs. A Xenon lamp (Newport Model 67005) with an AM 1.5 filter was used as
the illumination source. The power supply for the lamp (Newport Model 69911) was set to
55
300 W. The lamp was allowed to warm up for approximately 20 minutes before a
calibration measurement of its intensity was performed using an NREL calibrated
Hamamatsu photodetector (S1133). Calibration was achieved when the distance between
the lamp and the photodetector resulted in an equivalent Isc of 753 μA and Voc of 0.65 V.
The DSSCs were positioned at the same distance and orientation as the photodetector.
AM 1.5 illumination was incident on the solar cells through the transparent FTO photo-
anode side. An Agilent 4155C semiconductor parameter analyzer equipped with current
and voltage source meters was used to sweep the voltage levels between 0 V and 1 V at
increments of 10 mV and simultaneously obtain the current measurements. A Microsoft
Windows-based PC running Agilent Desktop software was used to control the
semiconductor parameter analyzer. From the resulting set of (V, J) measurements, Jsc, Voc,
FF, and efficiency were then determined according to the equations given in Section 2.5.
Figure 3.2 Schematic of J-V Measurement Setup
56
External Quantum Efficiency (EQE) Measurements
External quantum efficiency (EQE) was measured using the Newport EQE
measurement kit (Figure 3.3). The same Xenon lamp used in the J-V measurements was
used as the illumination source. The photodetector was again used to determine the correct
focusing distance before measuring the EQE of a DSSC. A PC-controlled monochromator
(Oriel Monochromator 74001) and focusing lenses were used to produce monochromatic
beams with wavelengths between 350 nm and 800 nm, in increments of 5 nm. A reference
voltage Vref was produced by passing the photodetector output through a trans-impedance
amplifier. As with the J-V measurements, the semiconductor parameter analyzer recorded
the measurements of the photodetector / amplifier and DSSC outputs. The photodetector
was replaced by the DSSC under test and its output voltage Vsample at each wavelength was
recorded. The EQE of the tested DSSC was then determined as follows:
EQE8_< V8_< >V (3.1)
Figure 3.3 Schematic of EQE Measurement Setup
57
Electrochemical Impedance Spectroscopy (EIS) Measurements
The catalytic properties of urea and rGO treated electrolyte were compared to those
with electrolyte-only reference DSSC substrates with EIS measurements.
Figure 3.4 shows the functional diagram of the EIS experiment. A PC-controlled
Ametek VERSASTAT3-200 Potentiostat with a frequency analysis module (FDA) was
used for the EIS measurement. The EIS was carried out for each DSSC as follows. A
reference probe and the CE probe from the Potentiostat were connected together at one of
the cell electrodes, and the working electrode and sensor probes of the Potentiostat were
connected together at the other cell electrode. The Nyquist plots were obtained at a -0.7 V
open-circuit bias using a 10 mV AC signal automatically swept between 0.1 Hz and 1000
KHz, under illumination from a Fiber-Lite MI-150 High Intensity Illuminator (Dolan –
Jenner, Boxborough, MA, USA).
Figure 3.4 Functional Diagram of EIS Setup [109]
Cyclic voltammetry (CV) Measurements
Figure 3.5 shows a functional diagram of a cyclic voltammetry (CV) experimental
58
setup used to evaluate the catalytic properties of the different concentrations of urea and
rGO treated electrolyte. The cyclic voltammetry was carried out on test solutions of
acetonitrile containing 10 mM lithium iodide (LiI) and 1 mM of iodine (I2), combined with
0.1 M lithium perchloride (LiClO4) and the same urea or rGO concentrations used in
fabricating the DSSCs. Using the three-electrode system, the substrate Pt CEs, solid
Ag/AgCl, and a Pt wire placed in each solution were used as the working electrode, the
reference electrode, and the CE, respectively. The voltage applied to the working electrode
was swept from -0.5 V to a maximum of 1.5 V at a rate of 0.05 V/s.
Figure 3.5 Functional Diagram of Cyclic Voltammetry (CV)
UV-Visible Absorption Spectra Measurements
UV-Visible absorption measurements were conducted using an Agilent 8453
spectrophotometer running ChemStation software, with a deuterium lamp as the ultraviolet
(UV) source and a tungsten lamp as the visible/near infrared (VNIR) source (Figure 3.6).
Two sets of measurements were performed – one on the dye absorbed in the
photoanode containing the urea or rGO additive, and the other on the electrolyte solution
containing the urea or rGO additive.
59
The electrolyte measurements were obtained as follows. First, a 3 ml Isopropyl Alcohol
(IPA) solution was loaded in a sample holder, and its properties measured. In this case, the
data processing software was set to operate in “Blank” measurement mode to establish a
“reference” absorption spectrum. Then, the samples of additive-treated electrolyte solution
were combined with the IPA solution and the data processing software was set to operate
in “Sample” measurement mode. The net absorption spectrum was determined in the data
processing software by subtracting the IPA absorption spectrum from the electrolyte/IPA
solution spectrum.
The dye absorption measurements were performed as follows. After a DSSC was
fabricated, an electrolyte redox solution with the given urea or rGO concentration was
injected into it, which was allowed to “rest” for approximately 24 hours. The DSSC was
then disassembled; the photoanode was rinsed with acetonitrile and dried using compressed
N2 gas for approximately 30 seconds, then soaked in a 3 mL, 1 M NaOH solution for 3
hours [110]. The spectra of desorbed dye solution from the photoanode was obtained as
follows:
1. A 3 mL NaOH solution was loaded into a sample holder to obtain a reference
measurement.
2. The solution containing the NaOH /dye/additive was loaded into a sample
holder to obtain a sample measurement.
3. The net absorption spectrum was determined by subtracting the reference
(NaOH) spectrum from the total NaOH /dye/additive solution spectrum.
60
Figure 3.6 Schematic of UV-Visible Absorption Spectroscopy [111]
61
Chapter 4: Results and Analysis
This chapter presents the results obtained from analyses of DSSC performance given
the use of urea and rGO as iodide/triiodide electrolyte additives. As mentioned earlier, five
weight percentage concentrations (wt%) of urea / rGO were prepared: 0%, 2%, 5%, 10%,
and 15%. Each additive will be considered in separate sections.
DSSC Performance with Urea-Treated Electrolyte
J-V Characteristics
Figure 4.1 shows the current density-voltage (J-V) curves for one of the DSSCs
fabricated with the electrolyte treated with each urea concentration. This particular DSSC
exhibited acceptable performance between the highest and lowest performances of the
other two. Table 4.1 summarizes the average resulting photovoltaic performance of all
three devices.
The open circuit voltage Voc increased with increasing urea concentration, from 0.72 V
at 0 wt% concentration to 0.83 V at 15 wt% concentration. The increase in Voc could be
attributed to the H+ ions produced by the reaction of urea with iodine in the electrolyte that
also yields oxalyl dihydrazide and I- ions, as shown in eq. (2.31). The H+ ions can lead to
a positive shift in the redox potential of the electrolyte [81]. Another possible reason for
the observed increase in Voc is a decrease in the I3- ion content in the electrolyte when the
increased urea concentration increases the iodide ion concentration that can transfer faster
which increased the ionic conductivity, as shown in eq. (2.31) and (2.32) [104].
The FF also increased with increasing urea concentration, from 0.61 at 0 wt%
concentration to 0.68 at 15 wt% concentration. This increase may be due to a lower
62
recombination rate at the TiO2/electrolyte interface caused by the same mechanism
accounting for the increase in Voc – the increase in I- ions and corresponding decrease in
I3- ions due to the increased urea concentration. This reduces the back recombination of the
electrolyte with I3- ions, and thus increases the FF.
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Cur
rent
den
sity
(m
A/c
m2 )
Voltage (v)
Figure 4.1 DSSC Current Density-Voltage (J-V) Curves Due to Urea-Treated Electrolyte
Table 4.1 DSSC Performance Parameters Due to Urea-Treated Electrolyte
Urea Conc. (wt %)
Jsc (mA/cm2)
Voc (volt)
FF
ηηηη (%)
0 -19.14±0.03 0.72 0.61 8.51±0.01
2 -18.70±0.02 0.76 0.63 8.95±0.02
5 -18.99±0.02 0.79 0.65 9.78±0.01
10 -18.76±0.03 0.81 0.66 10.17±0.01
15 -16.79±0.01 0.83 0.68 9.56±0.03
63
While both Voc and FF increased with increasing urea concentration in the electrolyte,
the short circuit current density Jsc density remained virtually unchanged at approximately
-19 mA/cm2, until the urea concentration increased to 15 wt%. At this concentration, Jsc
significantly decreased to approximately 16.79 mA/cm2 due to dye molecule desorption by
excess H+ ions in the electrolyte. Urea concentrations above 15 wt% resulted in
solidification of the electrolyte, which will also reduce Jsc.
As a result of the increased Voc and FF, the overall PCE also increased with increasing
urea concentration, from ~8.51 % at 0 wt% concentration to ~10.17% at 10 wt%
concentration. Interestingly, the PCE decreased slightly to ~9.56% at 15 wt%
concentration, most likely due to the observed decrease in Jsc. With respect to this
parameter, the “optimum” urea concentration was found to be 10 wt%.
UV-Vis Absorbance Spectra
Figure 4.2 shows the UV-Vis absorption spectra of I-/I-3 electrolyte at each urea
concentration. The absorption peak wavelength was found to be ~360 nm, and is related to
the presence of I3- ions. [81, 112]. Increasing the urea concentration in the electrolyte did
not shift the peak wavelength location, but it did decrease the peak intensity. This
observation is consistent with previously reported analysis [81]. Adding urea to I-/I-3
electrolyte decrease the numbers of I-3 ions in the electrolyte.
64
300 400 500 600 700 8000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Abs
orba
nce
(a.u
.)
Wavelength (nm)
Figure 4.2 UV-Vis Absorbance Spectra of Urea-Treated Electrolyte
External Quantum Efficiency (EQE)
Figure 4.3 shows the external quantum efficiency (EQE) as a function of wavelength
at each urea concentration. To validate the J-V measurement results presented in Section
4.1.1, Jsc was estimated from the EQE as follows [113]:
J : eη λNUλdλ¢£
¢¤
(4.1)
where ηIPCE (λ) and Nph (λ) are the IPCE and photon flux at wavelength λ, respectively, and
e is the magnitude of electronic charge in Coulombs. Using equation 4.1, the values of Jsc
calculated from the EQE measurements are shown in Table 4.2. The calculated Jsc values
are consistent with those measured from the J-V curves.
65
400 500 600 700 800
0
20
40
60
80
100
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
EQ
E (
%)
Wavelength (nm)
Figure 4.3 EQE Spectral Response Due to Urea-Treated Electrolyte
Table 4.2 EQE Peaks of DSSCs Due to Urea-Treated Electrolyte
Urea Conc. (wt%)
Jsc (mA/cm2)
From EQE
Jsc (mA/cm2) From J-V
Peak EQE (%)
0 19.15 19.14 89.04
2 18.98 18.70 88.22
5 18.96 18.99 87.67
10 18.89 18.76 86.77
15 16.35 16.79 77.15
Figure 4.3 shows a slightly decreased EQE near 360 nm at 0% urea concentration. This
could be attributed to the absorption of I-3
ions in the electrolyte. Increasing the urea
concentration up to 10% in the electrolyte resulted in an increasing EQE. These results are
consistent with the UV-Vis absorption spectra results shown in Figure 4.2, indicating that
66
the I3- concentration decreased with increasing urea concentration. However, the overall
EQE spectra of DSSC devices decreased when the urea concentration in the electrolyte
increased from 10% to 15%.
Electrochemical Impedance Spectroscopy (EIS)
Figure 4.4(a) shows the resulting Nyquist plots of both extracted and fitted
electrochemical impedance data for the prepared DSSCs with active area 0.2 cm2, obtained
under typical illumination and open-circuit conditions using the equivalent circuit model
shown in Figure 4.4(b). Two frequency-dependent arcs were obtained. The high-frequency
arc represents the Pt/electrolyte interface impedance, while the mid-frequency arc
represents the TiO2/N719 dye/redox electrolyte interface impedance, which is a measure
of the electron back recombination resistance or back charge transfer resistance Rr.
The overall device series resistance Rs values at each concentration are shown in Table
4.3. The decrease in Rs with increasing urea content is consistent with the increase in fill
factor values from the J-V curves.
From the Nyquist plots, no significant change is observed in the diameter of the high-
frequency arcs. However, a noticeable change in mid-frequency arc diameters occurs with
changing urea concentration, indicating an increase in Rr, which results in less
recombination of the photo-generated electrons with triiodide (I3-) ions in the electrolyte at
the TiO2 / electrolyte interface. This reduction in recombination occurs whether the cell is
operating under illumination or open-circuit conditions.
The Rr value was fitted and calculated using the equivalent circuit model shown in
Figure 4.4(b). The predicted values for each urea concentration are given in Table 4.3. The
increase in Rr with increasing urea concentration in the electrolyte was attributed to the
67
decrease in the concentration of heavy triiodide (I3-) ions at the photo-anode /electrolyte
interface. The decrease in I3- ion concentration was due to the increase in iodide ion
concentration as mentioned in equation 2.31, which acted to inhibit recombination at the
photo-anode/electrolyte interface.
15 20 25 30 35 40 45 500
5
10
15
20
25
-Zim
(Ω
)
Zre (Ω)
0 wt% fit 2 wt% fit 5 wt% fit 10 wt% fit 15 wt% fit
4.4(a) Nyquist Plots of DSSCs Due to Urea-Treated Electrolyte
4.4(b) Equivalent Circuit Model Used to Fit Nyquist Plot
68
Table 4.3 Predicted Rs and Rr from Nyquist Plots of Urea-Treated Electrolyte
Urea Conc. (wt%)
Rs (Ω)
Rr (Ω)
0 18.9 10.3
2 18.4 22.2
5 17.9 26.1
10 17.7 25.3
15 17.6 19.6
Cyclic Voltammogram (CV) Measurement
Figure 4.5 shows the cyclic voltammetry (CV) curves for the Pt CEs immersed in the
I-/I3- electrolyte at each urea concentration. Table 4.4 shows the corresponding anodic and
cathodic peak current density intensities measured at each concentration.
The CV plots show a pair of cathodic and anodic peaks on the left side, where Jpa and
Jpc are the anodic and cathodic peak current densities, respectively. These correspond to
the following electrochemical reactions:
56( 2#( → 35(cathodicreaction (4.4) 35( → 56( 2#(anodicreaction (4.5)
The catalytic performance of the device can be determined by these two reactions [114].
Figure 4.5 shows that both anodic and cathodic peak currentdensities increase with
increasing urea concentration in the electrolyte. An overall positive shift in the oxidation
potential of all anodic peaks can also be observed, and is believed to be due to the positive
shift of the redox potential due to the introduction of H+ ions according to equation 2.31,
which results in an increase in Voc. This behavior is consistent with the observed increase
in Voc in the J-V characterization.
69
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-3
-2
-1
0
1
2
3
4
5
I3
-+2e
- 3I
-
3I - I
3
-+2e
-
Cur
rent
den
sity
(mA
/cm
2 )
Potential (V vs. Ag/AgCl)
0 wt % 2wt % 5 wt % 10 wt % 15 wt %
4.5 Cyclic Voltammograms of Counter Electrodes Immersed in Urea-Treated Electrolyte
Table 4.4 Anodic and Cathodic Peak Intensities of Urea-Treated Electrolyte
Urea Conc. (wt %)
Jpa (mA/cm2)
-Jpc (mA/cm2)
0 2.553 1.378
2 3.034 1.708
5 2.853 1.555
10 3.200 1.829
15 3.502 2.055
UV-Vis Absorption of Photoanodes Immersed in Urea-Treated Electrolyte
(dye loading)
To better understand the decrease in Jsc as a function of increasing urea content in the
electrolyte, dye loading UV-Vis absorbance measurements were performed for dye
solutions desorbed from photo-anodes at each urea concentration, prepared according to
the procedure described in section 3.2.5. Figure 4.6 shows the resulting UV-Vis spectra of
70
the desorbed dye solutions from the disassembled photoanodes for each electrolyte
solution. The two absorption peaks at ~372 nm and 512 nm are attributed to the N719 dye.
The absorption peak intensities decrease linearly with increasing urea concentration. The
obtained results are consistent with the Jsc values obtained from the J-V measurements and
indicate that dye molecules are desorbed by the urea-containing electrolytes, with more
desorption typically occurring at higher urea concentrations.
300 400 500 600 700 800
0 wt % 2wt % 5 wt % 10 wt % 15 wt %
Abs
orba
nce
(a.u
.)
Wavelength(nm)
4.6 UV-Vis Absorbance Spectra of Dye Solutions Desorbed from Electrolytes Treated with Urea
J-V Hysteresis Effects
Hysteresis effects have been observed in the J-V characteristics of DSSC and
perovskite solar cells [115, 116]. One possible reason for this behavior is that the capacitive
nature of the mesoporous TiO2 layer allows storage of heavy ions such as I-3 in the DSSC
electrolyte [117] and halide ions in the perovskite cell [118]. The hysteresis effect is
71
significant at very fast J-V scan rates (~1500 mv/s) due to a decrease in the delay time
[119].
Figure 4.7 shows the resulting J-V curves with hysteresis effects for DSSC devices
fabricated with electrolyte solutions at each urea concentration. The scans were performed
from 0 V to 1 V then from 1 V to 0 V in steps of 50 mV. The hysteresis effect in Fig. 4.7
is substantial for the DSSC devices fabricated without added urea. Increasing the urea
content results in a decrease in the observed hysteresis, consistent with the previous
observed behavior. At higher urea concentrations, more I-3 ions are converted to I- ions;
fewer I-3 ions will penetrate and be stored in the mesoporous TiO2 layer.
-50
-40
-30
-20
-10
0
10
20
30
40
500.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Voltage (V)
Cur
rent
den
sity
(m
A/c
m2 )
4.7 Hysteresis Effects for DSSC Devices Fabricated with Urea-Treated Electrolyte
72
DSSC Performance Using rGO-Treated Electrolyte
J-V Characteristics
Figure 4.8 shows the current density-voltage (J-V) curves for one of the DSSCs
fabricated with the electrolyte treated with each rGO concentration. This particular DSSC
exhibited acceptable performance between the highest and lowest performances of the
other two. Table 4.5 summarizes the average resulting photovoltaic performance of all
three devices.
Voc increased from 0.71 V to 0.75 V as the rGO concentration increased from 0% to
10%. While similar in overall trend to that observed with urea, a decrease was observed at
the 15% concentration, and the range of Voc increase was smaller.
The FF changed over a narrow range, between 0.59 and 0.62 between 0 wt% and 10
wt% concentrations, and decreasing back to 0.59 at 15 wt% concentration. As with Voc, the
range of increase was smaller than the FF increase measured with the urea-treated
electrolyte. The potential mechanism for this interaction differs from that of the urea-
treated electrolyte: a lower recombination rate at the TiO2/electrolyte interface. As
mentioned in section 2.9, I-3 ions are large and heavy ions, with low mobility and a
tendency to concentrate near the photoanode (Figure 2.17), which promotes recombination
reactions between the TiO2 conduction band and the electrolyte. This reduces both FF and
Voc. Adding rGO to the electrolyte slightly increases Voc and accelerates the charge transfer
rate by reducing the electrolyte’s resistance and charge back-recombination rate. This
behavior is related to the strong interaction of I-3 ions with the rGO particles. This positive
interaction allows the removal of I-3 ions far from the photoanode interfaces, permitting
some recovery of FF and Voc.
73
The short circuit current (Jsc) showed a very slight increase. This indicates that rGO
works as a bridge, accelerating the electron transfer rate by reducing the electrolyte
resistance.
The overall PCE (%) was improved from ~ 8.54% at 0% concentration to ~9.57% at
10 wt% concentration. The PCE decreased back to ~8.79% at 15 wt% concentration.
-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
00.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Voltage (v)
Cur
rent
den
sity
(m
A/c
m2 )
4.8 DSSC Current Density-Voltage (J-V) Curves Due to rGO-Treated Electrolyte
Table 4.5 DSSC Performance Due to rGO-Treated Electrolyte
rGO Conc. (wt%)
Jsc (mA/cm2)
Voc (volt)
FF
ηηηη (%)
0 -20.34±0.04 0.71 0.59 8.54±0.01
2 -20.72±0.05 0.72 0.59 8.8±0.02
5 -21.71±0.04 0.73 0.58 9.29±0.03
10 -20.33±0.03 0.75 0.62 9.57±0.01
15 -20.37±0.02 0.72 0.59 8.79±0.01
74
UV-Vis Absorbance Spectra
Figure 4.9 shows the UV-Vis absorption spectra of I-/I-3 electrolyte at each rGO
concentration. As with the urea additive, the absorption peak at ~360 nm did not shift
location. However, increasing the rGO concentration increased, rather than decreased, the
peak intensity, resulting in an increase in the electrolyte’s ionic conductivity [120].
300 400 500 600 700 8000.0
0.1
0.2
0.3
0.4
Abs
orba
nce
(a.u
.)
Wavelength (nm)
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
4.9 UV-Vis Absorbance Spectra of rGO-Treated Electrolyte
External Quantum Efficiency (EQE)
Figure 4.10 shows the external quantum efficiency (EQE) as a function of wavelength
for DSSC electrolyte treated with each rGO concentration. Again, the Jsc of the tested
DSSCs was estimated by integrating the EQE using equation 4.1. The estimated values and
corresponding Jsc values measured from the J-V characterization are shown in Table 4.5.
The calculated Jsc values are consistent with those measured from the J-V curves. EQE
75
increased with increasing rGO concentration; the maximum EQE value was 92.47% when
the rGO concentration was 10 wt%. The increase in EQE could be due to increased ionic
conductivity of the electrolyte and/or decreased TiO2/electrolyte recombination at the
interface. Table 4.6 summarizes the values of EQE at each rGO concentration.
350 400 450 500 550 600 650 700 750 8000
20
40
60
80
100
EQ
E (
%)
Wavelength (nm)
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Figure 4.10 EQE Spectral Response Due to rGO-Treated Electrolyte
76
Table 4.6 EQE Peaks of DSSCs Due to rGO-Treated Electrolyte
rGO Conc. (wt%)
Jsc (mA/cm2)
From EQE
Jsc (mA/cm2) From J-V
Peak EQE (%)
0 19.89 20.34 84.19
2 19.77 20.72 89.53
5 19.82 21.71 90.12
10 20.91 20.33 92.47
15 19.92 20.37 87.88
Electrochemical Impedance Spectroscopy
Figure 4.11(a) shows the resulting Nyquist plots of both extracted and fitted
electrochemical impedance of DSSCs with active area 0.2 cm2. The data obtained under
typical illumination and open-circuit conditions, using the equivalent circuit model shown
in Figure 4.11(b). Similar to the results obtained for the urea-treated electrolyte, two sets
of frequency-dependent arcs were obtained. These arcs tended to be larger than the
corresponding arcs observed with the urea-treated electrolyte.
Table 4.7 gives the overall device series and recombination resistances Rs and Rr.
Again, Rs decreased and Rr increased as the rGO concentration increased, consistent with
the increase in FF demonstrated in the J-V characterization.
From the Nyquist plots in Figure 4.11(a), there is no significant change in the diameter
of arcs. However, a noticeable change in midfrequency arc diameters occurs after varying
rGO content, indicating an increase in Rr as the rGO concentration increased, which results
in a decrease in TiO2 / electrolyte interface recombination.
77
15 20 25 30 35 400
2
4
6
8
10
12
14 0 wt% fit 2 wt% fit 5 wt% fit 10 wt% fit 15 wt% fit
-Zim
(Ω)
Zre (Ω)
Figure 4.11(a) Nyquist Plots of DSSCs Due to rGO-Treated Electrolyte
Figure 4.11(b) The Equivalent Circuit Model Used to Fit Nyquist Plot
78
Table 4.7 Predicted Rs and Rr from Nyquist Plots of rGO-Treated Electrolyte
rGO Conc. (wt%)
Rs (Ω)
Rr (Ω)
0 18.8 11.6
2 18.4 12.6
5 18.2 13.0
10 18.2 16.1
15 16.2 12.45
Cyclic Voltammogram Measurement
Figure 4.12 shows the CV curves for the Pt CEs immersed in the I−/I3− electrolyte as a
function of electrolyte rGO concentration. Table 4.8 shows the corresponding anodic and
cathodic peak intensities measured at each concentration. Similar to the CV plots for urea-
treated electrolyte shown in Section 4.1.5, the plots show Jpa and Jpc peak intensities
corresponding to the electrochemical reactions described by equations 4.4 and 4.5. Figure
4.12 shows that both anodic and cathodic reactions are enhanced by adding rGO to the
electrolyte, with higher peak anodic and cathodic current densities measured at rGO
concentrations less than 15 wt%. Unlike the urea-treated electrolyte, no shift in the peak
positions was observed.
79
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
I3
- +2e-------3I-
3I- ------I-
3+2e-
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Cur
rent
den
sity
(m
A/c
m2 )
Potential (V vs. Ag/AgCl)
Figure 4.12 Cyclic Voltammograms of Counter Electrodes Immersed in rGO-Treated Electrolyte
Table 4.8 Anodic and Cathodic Peak Intensities of rGO-Treated Electrolyte
rGO Conc. (wt %)
Jpa (mA/cm2)
-Jpc (mA/cm2)
0 1.859 1.015
2 1.905 1.196
5 1.982 1.269
10 2.037 1.377
15 1.739 1.140
80
UV-Vis Absorption of Photoanodes Immersed in rGO-Treated Electrolyte (dye
loading)
Dye loading UV-Vis absorbance measurements were performed for dye solutions
desorbed from photoanodes at each rGO concentration, prepared according to the
procedure described in section 3.2.5. Figure 4.13 shows the resulting UV-Vis spectra of
the desorbed dye solutions from the disassembled photoanodes for each electrolyte
solution. As with the urea concentration, the two absorption peaks at ~372 nm and 512 nm
correspond to the N719 dye. However, the absorption peak intensities increased linearly
with increasing rGO concentration, with a maximum peak intensity observed at 10 wt%
concentration. This is likely due to the higher reflectivity of rGO, which allows the dye to
absorb more light. The greater absorption results in an increased Jsc. The higher reflectivity
effect appears to be offset at a 15 wt% rGO concentration, as the measured peak intensity
was reduced.
81
300 400 500 600 700 800
Abs
orba
nce
(a.u
.)
Wavelength(nm)
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Figure 4.13 UV-Vis Absorbance Spectra of Dye Solutions Desorbed from Electrolytes Treated with rGO
J-V Hysteresis Effects
Figure 4.14 shows the J-V curves with hysteresis effects obtained for the DSSC devices
fabricated with electrolyte solutions containing varying rGO concentrations, with the scans
performed from 0 VTO 1 V then from 1 V to 0 V in steps of 50 mV. Adding rGO to the
electrolyte resulted in a decrease in the observed hysteresis. This is due to greater I3-
reaction with the rGO, which prevents the I3- ions from penetrating the TiO2 mesoporous
layer.
82
-50
-40
-30
-20
-10
0
10
20
30
40
500.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 wt% 2 wt% 5 wt% 10 wt% 15 wt%
Voltage (V)
Cur
rent
den
sity
(mA
/cm
2 )
Figure 4.14 Hysteresis Effects for DSSC Devices Fabricated with rGO-Treated Electrolyte
83
Chapter 5: Summary and Conclusions
Summary
The use and increasing cost of energy are among the most important problems the
world is currently facing. Solar energy, fossil fuels and nuclear energy are currently
available sources. Fossil fuels and nuclear energy have issues relating to ongoing
availability and safety questions that make their use problematic. Solar PVs show great
promise as an alternative energy source. However, Si-based PV cells require creation of
high purity Si wafers, increasing their cost and limiting their practical applicability. Other
solar PV types, such as single junction GaAs or thin films (CIGS), are also more costly to
fabricate. DSSCs, Perovskite, and polymer cells are less costly to fabricate, and their
properties have been extensively researched. With respect to DSSCs, a key research
question has been how to improve the overall energy conversion efficiency; much of the
research has focused on enhancing the electrochemical properties of the electrolyte
solution.
This work investigated the use of novel electrolyte additive materials to DSSCs using
the standard I-/I-3 redox electrolyte. DSSC performance using varying additive
concentrations is characterized, with the goal of further optimizing photovoltaic
characteristics and overall photoconversion efficiency. The objective of this work was to
improve the DSSC efficiency by more than 1% using commercially available liquid I-/I-3
redox electrolyte, using different concentrations of urea (CH4N2O) or rGO as the sole
electrolyte additive. J-V, EQE, EIS, CV and UV-Vis absorption spectra measurements of
all fabricated devices demonstrated improvement in DSSC electrical performance to the
84
required degree with either additive. For the DSSCs characterized in this work, urea added
to the electrolyte solutions resulted in a greater than 1% increase in electrical performance.
Conclusions
Adding urea to the DSSC iodide/triiodide electrolyte led to measurable increases in Voc
and FF. The increase in Voc could be caused by the introduction of H+ ions into the
electrolyte, as urea reacts with iodine and causes a positive shift in the redox potential of
the electrolyte. The increase in FF with increasing urea concentration resulted from a lower
recombination rate at the TiO2 / electrolyte interface. The I- ion concentration increases
while the I-3 ion concentration at the TiO2/ electrolyte interface decreases, which inhibits
back charge recombination at the interface. The overall PCE (%) was significantly
increased, from ∼8.51% for 0 wt% urea to ∼10.17% for 10 wt% urea. However, the Jsc
value decreased at 15 wt% urea. This decrease could be due to dye molecule desorption
due to excess H+ ions in the electrolyte.
Adding rGO to the iodide/triiodide electrolyte of DSSCs lead to increases in both Voc
and FF, but these increases were less than those measured with urea. Both Voc and FF were
most improved with a 10 wt% rGO concentration; this could be due to reduction of the
electrolyte’s resistance and charge back-recombination rate, which increases the charge
transfer rate. This behavior is related to the strong interaction of I-3 ions with the rGO
particles, which allows the removal of I-3 ions far from the photoanode interface. The
overall PCE (%) was significantly increased from ~ 8.54 % at 0 wt% rGO concentration to
~9.57 % at 10 wt% rGO concentration. However, the Jsc value shows relatively little change
with increasing rGO concentration.
85
Higher rGO concentrations do not appear to have a similar level of effect to the
corresponding urea concentrations. However, for both tested additives, the performance
enhancement objectives were successfully achieved.
Future work
Given the demonstrated improvement in DSSC performance with either of these
electrolyte additives, additional characterization of DSSC performance over longer
intervals of time could be performed, especially with respect to the overall
photoconversion efficiency (e.g. the degree of degradation of the additive in the
electrolyte and/or dye, etc). Another potential research direction would consider the
efficacy of adding either additive to the electrolyte of Li ion batteries in order to
enhance their capacity and stability above the levels currently achieved.
86
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