J. Am. Ceram. Soc., 92 [2] 289–301 (2009)
DOI: 10.1111/j.1551-2916.2008.02870.x
r 2009 The American Ceramic Society
Journal
Metal Oxides for Dye-Sensitized Solar Cells
Rajan Jose,w Velmurugan Thavasi, and Seeram Ramakrishna
NUS Nanoscience and Nanotechnology Initiative, Faculty of Engineering, National University of Singapore, 117576
Singapore
The incessant demand for energy forces us to seek it from sustainable resources; and concerns on environment demands that
resources should be clean as well. Metal oxide semiconductors,
which are stable and environment friendly materials, are used in
photovoltaics either as photoelectrode in dye solar cells (DSCs)
or to build metal oxide p–n junctions. Progress made in utilization of metal oxides for photoelectrode in DSC is reviewed in
this article. Basic operational principle and factors that control
the photoconversion efficiency of DSC are briefly outlined. The
d-block binary metal oxides viz. TiO2, ZnO, and Nb2O5 are the
best candidates as photoelectrode due to the dissimilarity in
orbitals constituting their conduction band and valence band.
This dissimilarity decreases the probability of charge recombination and enhances the carrier lifetime in these materials. Ternary metal oxide such as Zn2SnO4 could also be a promising
material for photovoltaic application. Various morphologies such
as nanoparticles, nanowires, nanotubes, and nanofibers have been
explored to enhance the energy conversion efficiency of DSCs.
The TiO2 served as a model system to study the properties and
factors that control the photoconversion efficiency of DSCs;
therefore, such discussion is limited to TiO2 in this article. The
electron transport occurs through nanocrystalline TiO2 through
trapping and detrapping events; however, exact nature of these
trap states are not thoroughly quantified. Research efforts are
required not only to quantify the trap states in mesoporous metal
oxides but new mesoporous architectures also to increase the
conversion efficiency of metal oxide-based photovoltaics.
I. Introduction
I
N light of rising energy demand and depleting oil resources,
alternate energy sources are actively sought. Overdependence
on fossil fuels leaves us vulnerable to air pollutants, viz. CO,
NOx, and SOx, and related health risks and global warming
due to the increased green-house gas concentration. These issues
raise severe concerns on the sustenance of life on the earth. For
example a rise of B11C in global temperature would melt a
significant portion of the polar ice thereby removing large area
D. J. Green—contributing editor
Manuscript No. 25165. Received August 28, 2008; approved October 30, 2008.
This work was partially funded from the Clean Energy Program Office, Economic
Development Board of Singapore.
w
Author to whom correspondence should be addressed. nnijr@nus.edu.sg
from lower lying lands by the seas and oceans; migration of the
living organism poleward thereby causing population imbalance; appearance of new microorganisms and diseases; etc.1
Therefore, a step forward in the pursuit of alternative energy
sources is the harness of energy from carbon-free sources such
as wind, geothermal, hydroelectricity, tidal, and solar energy.
Our primary source of clean abundant energy is the sun. More
solar energy strikes the Earth in 1 h (4.3 1020 J) than all the
energy consumed on the planet in a year (4.1 1020 J).2 Conversion of this tremendous energy into electrical power, which is
the science and technology behind photovoltaics, is an issue before scientists and engineers for long time.
Existing types of solar cells could be divided into two distinct
classes: semiconductor p–n junction solar cells and excitonic solar cells (ESCs). Excellent monographs are published by Michel
Grätzel3 and Brian Gregg4 that detail the fundamental differences between the p–n junction and ESCs. In the p–n junction
cells, light absorption results in the creation of free electrons,
which are subsequently accelerated by the in-built electric field in
the junction. The photovoltage in the p–n junction is the difference in the quasi-Fermi levels of n- and p-type regions. In the
ESCs, light absorption results in the generation of a transiently
localized excited state, known as exciton—usually a Frenkel
type that cannot thermally dissociate into free carriers in the
material it was formed due to high exciton binding energy
( kT). Excitons are the characteristics of semiconductor analogues to Fermi fluids in metals and are often characterized as a
mobile excited state with an exciton diffusion coefficient. An
exciton can be viewed as a quasi particle with an electron in the
conduction band (or lowest unoccupied molecular orbital
(LUMO) in the case of molecules and nanoclusters) and a
hole in the valence band (or highest occupied molecular orbital
(HOMO) in the case of molecules and nanoclusters). When a
semiconductor (molecule, crystals, or clusters) is anchored to
another material whose conduction band (LUMO) lies at lower
energy, then the exciton dissociates into mobile carriers at the
interface of the materials system (Fig. 1). This process is the
basis of ESCs. Examples of this type include organic solar cells,5
dye solar cells—most popularly known as dye-sensitized solar
cells (DSCs),3 and quantum dot solar cells.6 Conjugated polymers and/or organic materials such as C60, carbon nanotubes,
etc. are the materials of choice in organic solar cells; in DSCs, a
wide bandgap metal oxide semiconductor is anchored to a dye;
and in quantum dot solar cells, semiconductor nanocrystals with
size less than their exciton Bohr radius is used to harvest light.
Progress in the photovoltaic conversion efficiency is reviewed
annually; a most recent update is provided by Green et al.7
Feature
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Vol. 92, No. 2
Journal of the American Ceramic Society—Jose et al.
Fig. 1. A dye molecule (left) anchored onto a metal oxide semiconductor nanocrystal (right). Upon absorption of a photon of sufficient energy,
an electron leaves from the highest occupied molecular orbital (HOMO)
(whose position is shown in black ellipse for the dye) to the lowest
unoccupied molecular orbital (LUMO) (shown in blue circles) leaving
behind a hole (h1). This electron is injected into the conduction band of
the metal oxide, which then diffuses and makes an electric current.
For convenience the conversion efficiency data have been
summarized and shown in Table I.
Section II of this article briefly explains the basic operational
principle of DSCs, advantages of DSC over p–n junctions,
factors that control the photoconversion efficiency, techniques
to characterize DSC, and basic equations; section III explain the
metal oxides used in DSC, charge transport in DSC taking TiO2
as a specific example, morphologies explored for DSC, and the
situation when metal oxides are not sensitized with dyes.
II. DSCs
The sensitization of semiconductors using dyes dates back to
19th century by the work on the silver halide emulsion using
erythrosine dyes.8 Systematic studies to understand the mechanistic details of the dye sensitization started after a century using
RuL2(NCS)2; L 5 2.20 -bipyridyl-4,40 -dicarboxylic acid dye (N3)
and wide bandgap semiconductors such as ZnO and SnO.9
These early studies were fundamental in nature aimed to understand the charge transfer process between the LUMO of the dye
and the conduction band of the semiconductor when this material system is immersed in an electrolyte. Commercial interest
in DSC has been triggered by the discovery that mesoporous
TiO2 anchored to N3 dye gave significant energy conversion
efficiency (ZB7%).10 This breakthrough was due to large sur-
Fig. 2. Schematic of dye solar cell. Mesoporous metal oxide (nanorods
and spherical particles) act as photo electrode. Dye molecules are anchored on the surface of metal oxide and upon photo-excitation of dye,
the electron is injected into the conduction band of the electrode. The
photoelectrode is percolated with an electrolyte whose redox potential
supports the regeneration of dye after it gets reduced.
face area of the mesoporous TiO2 that allowed anchoring significantly high amount of dye molecules (B0.13 mmol/cm2)
onto it; thereby increasing the absorption cross-section.
The DSCs are fabricated by sandwiching a dye-anchored
mesoporous metal oxide, known as photoelectrode, between
two conducting glass plates (such as fluorine-doped indium tin
oxide (FTO)) in the presence of an electrolyte. One of the FTO
plates is coated with a thin layer of metal oxide to prevent
wetting of the FTO plate by liquid electrolytes and the other is
coated with a thin layer of platinum that acts as catalyst (Fig. 2).
The FTO is a cost-limiting factor of DSC; commercial DSCs use
Ti thin films in the place of FTO. The DSC has potential for
becoming a cost-effective means for producing electricity, capable of competing with available solar electric technologies and,
eventually, with today’s conventional power technologies.11
Assuming DSCs to be single junction, these cells can have
a theoretical conversion efficiency limit of B31%. In 2001,
Grätzel’s group reported ZB10.4% using the black dye,
20 nm TiO2 particles, and the iodide/triiodide electrolyte.12
The same group had broken this record and reported
ZB11.04% using N3 dye, 20 nm TiO2 particles, and guanidine
thiocyanate electrolyte.13 In 2006, Han and colleagues from
Sharp Co. (Japan) reported ZB11.1% using the black dye
Table I. State of the Art Performance Solar Cells
Type of cell
p–n (single junction)
Description
Silicon
GaAs
p–n (multijunction)
Thin film
Excitonic
w
InP
GaInP/GaAs/Ge
GaInP/GaAs
CuInGaSe2
CuInGaSe2
CdTe
Si
Si (submodule)
Dye solar cells
Dye solar cells
Organic
Quantum dot solar cells6
da, designated illumination area; ap, aperture area; t, total area.
Crystalline
Polycrystalline
Amorphous
Nanocrystalline
Crystalline
Thin film
Polycrystalline
Crystalline
Area (cm2)w
Efficiency (%)
4.00 (da)
1.00 (ap)
1.07 (ap)
1.20 (ap)
3.91 (t)
1.00 (t)
4.01 (t)
4.02 (t)
3.99 (t)
4.0 (t)
1.00 (ap)
16.0 (ap)
1.03 (ap)
4.02 (ap)
96.3 (ap)
1.00 (ap)
26.5 (ap)
1.00 (ap)
0.2
24.770.5
20.370.5
9.570.3
10.170.2
25.170.8
24.570.5
18.270.5
21.970.7
32.071.5
30.3
18.870.5
16.670.4
16.570.5
16.670.4
9.870.3
10.470.3
6.270.2
3.070.1
1.7
February 2009
Metal Oxides for Solar Cells
with increase in the haze of TiO2 electrode and the iodide/
triiodide electrolyte,14 which is currently the record holder. The
solar cell efficiencies presented in Table I clearly demonstrates
that the DSCs excel other ESCs and p–n junctions made using
amorphous and nanocrystalline silicon.
(1) Advantages/Disadvantages of DSC Over p–n Junctions
The advantages of DSCs over p–n junction cells are as follows:
1. Less sensitive to impurities; fabrication in ordinary
environments possible.
2. Easy fabrication process: inexpensive and scalable to
nonvacuum- and low-temperature-based high volume manufacturing via continuous processes (e.g., screen printing, spraying,
pressing, or roll-to-roll production) possible.
3. Operates optimally over a wide range of temperatures.
4. The efficiency is relatively insensitive to the angle of
incident light.
5. Materials choice is not fixed; tunability of photovoltaic
properties possible.
6. Cells can be made on lightweight and flexible or rigid
substrates (e.g., plastic, fabric, metal, glass, and ceramic).
7. DSCs could be potentially less expensive (o$1.0/Wp)
than p–n junctions.15
The major disadvantages of DSCs include the following:
1. The conversion efficiency of DSCs depends on a lot
number of rate limiting factors, which are explained in the
next section.
2. Less impressive cell lifetime due to the presence of volatile
chemicals compared with p–n junctions.
3. One of the major disadvantages of DSCs for practical
application is the transparent conducting glass plate onto
which they are assembled. Research in this direction are in
progress to build the DSCs on cost effective platforms such as
metal layers.16
(2) Processes and Rate Limiters in DSC
The functional diagram explaining the components of DSC and
the processes in it is shown in Fig. 3. The photovoltaic effect in
DSC occurs at the interface between a dye-anchored wide bandgap oxide semiconductor and an electrolyte. The HOMO of the
dye molecules are usually formed by relatively weak lateral
overlapping of atomic orbitals (p bonds) with loosely bond electrons (p electrons). Upon irradiation, the dye undergoes a p–p
transition thereby exciting an electron to the LUMO of the dye
(p electrons). The electron thus excited to the LUMO of the
dye undergoes a nonradiative transition into the conduction
Fig. 3. Functional diagram of dye solar cell (DSC). Each component in
DSC is represented by isolated blocks, which slightly deviates from reality. For example, the electrolyte wet the metal oxide also. The dashed
arrows show the possible losses in DSC. The vertical line on the right
shows the energy scale in eV. The energies of each functional material
are represented by horizontal lines.
291
band of the oxide material within few picoseconds thereby
getting oxidized. Progress made in dyes for DSC has been
recently reviewed by Neil.17 The oxidized dye is brought to the
normal state by reducing it using a redox electrolyte together
with a catalyst. A large number of electrolytes, both solids and
liquids, are proposed for DSC applications, an account of which
is available in a recent review.18 The electrons injected to
the conduction band of the oxide material is transported through
the mesoporous network to the counter electrode by doing a
work equal to the energy difference between the conduction
band of the electrode and redox potential of the electrolyte.
There are at least eight fundamental processes:
i. Photon absorption, which is determined by the absorption
wavelength window, intensity of solar radiation at that window,
and absorption cross-section of the dye (a).
ii. Radiative recombination, i.e., the relaxation of the excited
dye directly into its ground state, and its rate constant (k1). This
process typically occurs in a time scale of several nanoseconds.
iii. Exciton diffusion length (DEXT).
iv. Interfacial electron transfer, i.e., injection of electron
from the dye to the photoelectrode, and its rate constant (k2).
The interfacial electron transfer occurs typically in a time scale
of several picoseconds.
v. Electron back transfer, i.e., the capturing of conduction
band electrons by the oxidized species in the electrolyte, and its
rate constant (k3).
vi. Interfacial charge recombination, i.e., the capturing of
conduction band electrons by the oxidized dye molecules, and its
rate constant (k4).
vii. Electron transport through the photoelectrode material
controlled by (a) diffusion coefficient of electron (De), (b) phonon
relaxation through which an electron loses its energy through
electron–phonon interaction.
viii. Redox potential of the electrolyte and rate constant of
(k5) of electron transfer to the oxidized dye.
Among these processes, interfacial charge transfer is three
orders of magnitude faster than radiative recombination, i.e.,
k2/k1B1000; and electron back transfer is two orders of
magnitude faster than interfacial charge recombination, i.e.,
k3/k4B100.19 Therefore, it is reasonable to assume that these
two factors i.e., radiative recombination and interfacial electron
transfer, make negligible contribution to the final conversion
efficiency. The exciton is highly localized in dyes and nanoclusters such as quantum dots belong to strong confined regime,
i.e., particle radius exciton Bohr radius. In such cases the
exciton diffusion could be neglected. However, if extended
structures such quantum dots in the weak confinement regime,
i.e., particle radiusBexciton Bohr radius, are used in place of
dyes, the exciton dynamics should also be taken into account.
Therefore, the problem of eight rate limiting process reduces
into five for DSCs, viz., (i), (iv), (v), (vii), and (viii). Among the
reduced five problems (i), (vii), and (viii) are controlled by the
light harvester (dyes), photoelectrode, and electrolyte, respectively; and (iv) and (v) are processes at the light harvester–
photoelectrode and photoelectrode–electrolyte interfaces, respectively. It follows from the above description that the
metal oxide photoelectrode participate in more number of processes than electrolytes and the dyes do; and therefore, they play
a significant role on the final energy conversion efficiency of
DSC.
(3) Characterization of DSC
Three techniques are mainly used to characterize DSCs, viz.
photoelectric current (I–V ) measurements, and electrochemical
impedance spectroscopy (EIS), incident photon to conversion
efficiency (IPCE).
I–V: The electrical output power of solar cells is calculated
from photoelectric current measurements. To compare solar cell
characterized in different laboratories all over the world, the Z is
measured under a set of standard conditions.20 Essentially these
conditions specify that the temperature of the cell should be
292
Journal of the American Ceramic Society—Jose et al.
251C and that the solar radiation incident on the cell should
have a total power density of 1000 W/m2, with a spectral power
distribution characterized as AM1.5. The photovoltaic parameters for practical applications are open circuit voltage (VOC),
short-circuit current density (JSC), fill factor (FF), Z.
The VOC, as defined before, is proportional to the difference
between the Fermi level of the TiO2 electrode and the electrochemical potential of the redox couple. This definition requires
that the VOC for a given photoelectrode–electrolyte system to
be a constant. However, the VOC was found to depend on the
recombination rate in addition to the sensitizer and its adsorption mode. The sensitizers adsorption geometry induces substantial downshift of the conduction band energy of the metal
oxide thereby reducing the VOC.21
The JSC is the current per unit active area of the device when
the applied potential across the device is zero, i.e. ISC/A. Under
standard conditions, the JSC is determined by (i) the amount of
dye anchored onto the metal oxide and (ii) the electrochemical
properties of the mesoporous network in the presence of an
electrolyte. Recently, we have shown that the JSC depends on the
molecular structure of the dye also.22 Gregg4 and Bisquert
et al.23,24 contributed significantly to the understanding of
charge transport through the mesoporous network and current
density. The kinetic equation for the one-dimensional current
density Jn(x) of electrons through DSC as4
Jn ðxÞ ¼ nðxÞmn fHUðxÞ þ HmðxÞg
(2)
where n(x) is the concentration of electrons, which depends on
the amount of the electron injected by the dye; mn is the electron
mobility; U and m are the electrical and chemical potential,
respectively, which are constant for a given materials system.
The carrier motion is through either diffusion (driven by concentration gradient) or drift (driven by an electric field) whose
magnitudes are related by the Einstein’s equation
D kT
¼
mn
q
(3)
where D is the diffusion coefficient and mn is the mobility. The
electron transport in DSC occurs in the presence of an electrolyte, which screens the macroscopic electric fields; and therefore,
charge transport in DSC is primarily due to diffusion. Therefore,
primary interest for each DSC constituted by a given materials
system is to determine the electron diffusion coefficient.
The FF measures the ideality of the device and is defined as
the ratio of maximum power output to the product of VOC and
JSC, i.e.,
FF ¼
IMAX VMAX
JSC VOC
(4)
The FF is a measure of decrease in photocurrent with increase
in photovoltage and could be qualitatively determined from the
squareness of the I–V curve. Ideally, the power generated within
the cell should dissipate at the external circuit. Parallel current
paths such as electron back transfer (k3) and charge recombination (k4) within the device are possible cause of poor fill factor.
An ideal cell should offer high internal resistance (41000 O) to
these parallel currents, often characterized by a shunt resistance.
Magnitude of the shunt resistance can be determined from
the slope of the I–V curve near the short-circuit current point.
Another possible cause of poor fill factor is the magnitude of
series resistance at the cell–external circuit junction. High sheet
resistance of the FTO plate and/or high resistance at this junction
developed during processing of DSC are possible cause of high
series resistance. The series resistance, the magnitude of which
should be as low as possible, can be determined from the slop of
the I–V curve near the open circuit voltage point.
Vol. 92, No. 2
Fig. 4. Typical impedance spectrum (Nyquist plot or Cole–Cole plot)
of DSC (bottom panel) in which the imaginary part of the complex
impedance function is plotted against the real part. The electrical equivalent of a typical DSC is shown in the top panel; each circuit element is
explained in the text.
The Z is defined as
Z¼
PMAX JSC VOC FF
¼
PIN
PIN
(5)
i.e., higher JSC, VOC, and FF for lower solar irradiance are the
key for increased Z. As stated before, the mesoporous metal
oxide network directly influences all these parameters. Thus reducing the losses in the mesoporous network, a better DSC could
be fabricated.
EIS: The EIS is a useful tool to understand the kinetics of
electrical transport in the DSC. Figure 4 shows a typical impedance spectrum of a DSC together with its electrical equivalent
model.25 The DSC can be viewed electrically as a combination
of resistance and capacitors. In the transmission model shown
in Fig. 4, the rct is the charge-transfer resistance of the charge
recombination process between electrons in the mesoporous
metal oxide film and ions in the electrolyte; Cm is the chemical
capacitance of the mesoporous metal oxide film; rt is the transport resistance of the electrons in the mesoporous metal oxide
film; Zd is the Warburg element showing the Nernst diffusion of
ions in the electrolyte; RPt and CPt are the charge-transfer resistance and double-layer capacitance at the counter electrode
(platinized transparent conducting oxide (TCO) plate), respectively; RCTO and CCTO are the charge-transfer resistance and the
corresponding double-layer capacitance at the exposed CTO–
electrolyte interface, respectively; RCO and CCO are the resistance and the capacitance at the CTO–TiO2 contact, respectively; Rs is the series resistance, including the sheet resistance of
the TCO glass and the contact resistance of the cell. Steady-state
transport resistance through the mesoporous network, transient
diffusion coefficient, chemical capacitance at the metal oxide–
electrolyte interface, and recombination resistance could be
evaluated using this single technique.26
IPCE: The IPCE, or action spectra, is defined as the number
of electrons flowing through the external circuit under short
circuit conditions per incident photon at a given wavelength.
IPCEðlÞ ¼
nelectron ðlÞ
IðlÞhc
¼
nphotons ðlÞ Pin ðlÞel
(1)
where I(l) is the measured current, Pin(l) is the input power,
and l is the wavelength of irradiation in nanometer. Obviously a
February 2009
Metal Oxides for Solar Cells
293
Fig. 6. (A) Ideal atomic arrangement in bulk anatase TiO2 where the
atoms are symmetrically packed at regular lattices. When the size of the
particles is reduced to nanometer level, the bond lengths and bond angle
deviates from their equilibrium position (B). This deviation produces
additional energy states in the electronic band of nanomaterials, which
are the source of traps in nanocrystalline materials. Such defect has high
density in nanocrystalline materials.
Fig. 5. Plot showing the incident photon to current conversion efficiency as a function of the excitation wavelength for (a) (101) plane single crystal TiO2 and (b) mesoporous anatase when sensitized by the
surface-anchored ruthenium complex cis-RuL2(SCN)2. The electrolyte
consisted of a solution of 0.3M LiI and 0.03M I2 in acetonitrile. Data
reproduced from Grätzel, Nature 414, 339 (2003).
high fraction of the incident photons should be converted into
mobile electrons for enhanced performance. A mesoporous network of metal oxide with high specific surface area is a prerequisite for high IPCE. Figure 5 compares the difference in IPCE
when single crystal and mesoporous TiO2 was sensitized by the
N3 dye.
III. Photoelectrodes for DSC
The exciton generated at the dye is separated into free carriers at
the dye–photoelectrode interface. There are two driving forces
for this charge separation: (i) the charge separation is energetically favored because the conduction band of the metal oxide
is at lower energies than the LUMO of the dye; (ii) the charge
separation is entropically favored because there is larger density
of electronic energy states in the conduction band of a crystal
than molecular orbital of a dye.27
Binary metal oxides such as TiO2, ZnO, Fe2O3, ZrO2, Nb2O5,
Al2O3, and CeO2 and ternary compounds such as SrTiO3 and
Zn2SnO4 have been tested for their use as photoelectrodes in
DSC. Among them TiO2, ZnO, and Fe2O3 are 3d transition
metal oxides, ZrO2 and Nb2O5 are 4d transition metal oxides,
Al2O3 is a p-block metal oxide, and CeO2 is a f-block
metal oxide. In TiO2, the Ti ions are in a distorted octahedral
environment and formally have a Ti41(3d0) electronic confi-
guration (Fig. 6(A)). The valence band of TiO2 is composed
primarily of oxygen 2p orbitals hybridized with Ti 3d states,
while the conduction band is made up of pure 3d orbital of
titanium, i.e., electrons in valence band and conduction bands
are at different parity; and therefore, the transition probability
of electrons to the valence band is decreased which ultimately
decrease the (e–h1) recombination probability. For Fe2O3,
3d-states are present both in the valence band and conduction
band leading to states of similar parity and increase the (e–h1)
recombination probability. In the case of ZnO, which has completely filled 3d orbitals (3d10), the valence band consists of only
d orbitals and conduction band consists of hybridized s–p
orbitals. The electronic configuration of ZnO again leads to
the situation of dissimilar parity and reduced (e–h1) recombination probability. Thus in view of the electronic configuration
and recombination probability TiO2 and ZnO are the best
choice as photoelectrodes among 3d transition metal oxides.
Stoichiometric Al2O3, ZrO2, and Nb2O5 are insulators
at room temperature with bandgaps B6.6, B5, and B3.5 eV,
respectively. Their room temperature conductivities are typically
few mS/cm. However, the Nb2O5 become an n-type semiconductor at lower oxygen content and Nb2O4.978 has conductivity B3 103 S/cm.28 Similar to TiO2, conduction band
of Nb2O5 consists of mainly 4d orbitals of niobium and valence
band consists of 2p orbitals of oxygen and would lead to a situation of decreased recombination probability due to dissimilar
parities. This discussion follows that TiO2, ZnO, and Nb2O5 are
the best candidates in view of their electronic band structure.
However, it should be noted that the band structure of nanomaterials is greatly deviated from their bulk counterpart. In
agreement with this discussion, TiO2, ZnO, and Nb2O5 have
been the most popular choices as photoelectrodes in DSCs.
Among them, TiO2 was the material of choice since 1991 to
address the fundamental issues of DSCs; therefore, it is considered as a model system.
(1) TiO2 Electrodes
TiO2 is a relatively cheap, abundant, nontoxic, biocompatible,
oxide semiconducting material that is widely used in healthcare
products as well as in paint. TiO2 exists naturally in three
crystalline polymorphs, namely rutile (Eg 5 3.05 eV), anatase
(Eg 5 3.23 eV), and brookite (Eg 5 3.26 eV). In addition, TiO2
exist in many other high pressure and metastable forms, viz., the
intermediate orthorhombic,29 orthorhombic columbite30 and
cotunnite types,29 monoclinic baddeleyite type,31 and cubic
fluorite type.32 Rutile is the most common and stable TiO2
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Journal of the American Ceramic Society—Jose et al.
Fig. 7. Core/shell model and corresponding energy diagram. The shell
induces potential barrier for electrons after injected into the core. This
potential barrier prevent from electron to recombine with the electrolyte.
polymorph. Both rutile and anatase have tetragonal structure
with a 5 0.46 nm and c 5 0.29 nm (rutile); a 5 0.3782 nm and
c 5 0.9502 nm (anatase). Brookite has orthorhombic structure
with a 5 0.5456 nm, b 5 0.9182 nm, and c 5 0.5143 nm. Brookite is extremely difficult to synthesize in the laboratory but both
anatase and rutile can be readily prepared. TiO2 is known as an
n-type semiconductor, which contains donor-type defects such
as oxygen vacancies and titanium interstitials. Presence of titanium vacancies in TiO2 cause p-type properties.33
The Fermi level of anatase is 0.1 eV higher than that of the
rutile; therefore, anatase is more preferred for its applications in
DSC. However, recent studies show that rutile scatters light more
effectively and more chemically stable.34,35 Similar VOC has been
reported for anatase and rutile despite the difference in their conduction band. The observed JSC using rutile was about 30% lesser
than that of the anatase which was ascribed to the lower surface
area; and therefore, decreased the amount of dye loading.34
The DSC designs using TiO2 were continuously renovated
since 1991 to enhance the efficiency by increasing the lightscattering properties of the metal oxide film, suppressing charge
recombination, improving the interfacial energetic, and altering the particle morphology.36 Most successful TiO2 architectures so far developed features25:
(a) A TiO2 layer (thickness B100–200 nm) coating by immersing the FTO plate in precursors such as TiCl4 and subsequent sintering to prevent the electrolyte to reach the FTO plate.
(b) A thick layer (thickness B12–14 mm) of mesoporous
TiO2 of particle size 20–25 nm over the first monolayer.
(c) A thin layer (thickness B2–5 mm) of TiO2 of particle size
B400 nm to improve light scattering on the top of the mesoporous particle film.
(A) Electron Transport in TiO2: Progress made in understanding the electron transport through nanocrystalline TiO2
both in its pure form and in the presence of an electrolyte is discussed elaborately by Bisquert24 as well as Watson and Meyer.37
The electron transport through any materials is controlled by
its band structure; and therefore, inevitably linked to the bonding
characteristics. The TiO2 is an ionic compound and exhibits
strong electron–phonon coupling, known as polarons. For a
sufficiently strong electron–phonon interaction, small polarons
correspond to localized and self-trapped electrons are formed.
In such cases, the electron transport typically occurs through
thermally activated hopping from one electronic state to the next.
Large polarons, with spatially extended wave functions, are
formed for weaker coupling strength and exhibit band-type
behavior. This is typically the mechanism of electron transport
in bulk semiconductors.
On the other hand, the electron transport in sintered nanocrystalline films depends on the properties of the individual
nanoparticles, the extent of particle connectivity or electronic
coupling between the particles, and the geometrical configuration
of the assembly.24 The exciton Bohr diameter of TiO2 is 1.5 nm,
below which TiO2 exhibit quantum size effects, i.e., splitting of its
electronic band into discrete electronic levels. The TiO2 particles
considered for DSC is typically in the 10–25 nm range i.e., much
higher than its exciton Bohr radius of 1.5 nm. Therefore, the
nanocrystalline film composed of 10 nm particles can be considered as a network of bulk crystals with a band of extended electronic states arising from the high surface area of nanocrystals.
The surface states in nanomaterials, usually lie within the bandgap, arise from unsaturated bonds, deviations in the bond
lengths and bond angle from that of the bulk materials,38 and
presence of impurities (Fig. 7). However, in the case of TiO2
nature and origin of the surface states for the particles in the
10–25 nm range is not clear yet and intensive research is now
required to determine them.
To show the drastic changes in the electrical transport properties of TiO2 when particle size is decreased, four typical experiments39–42 have been summarized and shown in Table II.
Electron mobility and effective diffusion coefficients for four
TiO2 morphologies, i.e., single crystal, polycrystalline thin films,
25-nm-sized particles containing a mixture of anatase (70%) and
rutile (30%), and colloidal 9-nm-sized particles, are considered
here. The constraint here is that the experiments are not identical
for a strict comparison and conclusion. As the particle size is
much higher than its exciton Bohr diameter of 1.5 nm, they can
be considered as bulk particles with high density of defects as
shown in Fig. 7. The diffusion coefficient of single crystal anatase
was B0.5 cm2/s,42 which was reduced to 0.1 cm2/s for crystalline
thin films grown on glass substrate. The diffusion coefficient of
nanoparticles of size r25 nm is about four orders of magnitude
lower than that of the single crystals. Though all of the particles
considered here could be characterized as bulk crystals, the reported diffusion coefficients in single crystals and nanocrystals
differ in four orders of magnitude from which it could be inferred
that the mechanism of electron transport is different.
The poor diffusion coefficient of nanocrystalline TiO2 has
been explained under the framework of multiple trapping mechanism. Under this mechanism, the electron transport through
energy states arises from the surface atoms are slowed down by
trapping–detrapping events. The direct hopping, which is faster
than multiple trapping, between localized states is neglected in
this case. An electron normally pass through B107 trapping
events in nanocrystalline materials which ultimately reduces the
diffusion coefficient.43 The density of trap states influences the
kinetics of electron transport.44 The photocarrier transport measurements through TiO2 nanoparticles showed a dispersive decay
that usually occurs in materials of high trap density.45 The carrier
drift lengths (B100 nm) and charge screening lengths in the transport measurements were much larger than the particle size thereby
indicating that the electrons move through the mesoporous
network by trap filling, i.e., trapping and detrapping events.45
Table II. Comparison of Electron Mobilities of Anatase TiO2 as a Function of Particle Size
Morphology
3
Single crystal (0.6 mm )
1 mm films on glass substrate (crystalline films)
Degussa 25 (25 nm particles with 70% anatase and 30% rutile)
Colloidal TiO2 nanoparticles of size 9 nm
D (cm2/s)
Mobility (cm2/Vs)
Technique
0.5
0.1
3 104
9 104
20
4
0.01
0.034
Four probe dc resistivity42
Hall measurements41
Terahertz spectroscopy39
Microwave conductivity40
February 2009
295
Metal Oxides for Solar Cells
Table III. Photovoltaic Performances of the DSCs Those Used Composite, Doped, and Core/Shell Electrodes
Geometry
JSC (mA/cm2)
Composition
TiO2-metal oxide composite
Doped TiO2
TiO2 core/metal oxide
shell structures
95TiO2–05ZrO250
90TiO2–10GeO257
80TiO2–20Nb2O549
99TiO2–01ZrO256
TiO2:0.1%Nb2O5w
TiO2/ZrO251
TiO2/Nb2O552
TiO2/ZnO54
TiO2/SiO251
TiO2/Al2O351
TiO2/MgO53
TiO2/SrTiO355
11.2
11.4
6.83
16.5
3.1
9.1
11.4
11.7
10.6
12.1
11.7
10.2
(10.1)
(9.3)
(7.89)
(15.6)
(2.2)
(9.1)
(10.2)
(13.2)
(9.1)
(9.1)
(10.2)
(10.5)
VOC (mV)
670
721
645
715
841
675
732
620
710
760
720
708
(710)
(700)
(600)
(700)
(777)
(735)
(659)
(490)
(735)
(735)
(640)
(650)
FF
0.72
0.69
0.42
0.69
0.69
0.60
0.56
0.52
0.58
0.61
0.54
0.58
(0.68)
(0.84)
(0.29)
(0.64)
(0.54)
(0.55)
(0.51)
(0.40)
(0.55)
(0.55)
(0.47)
(0.54)
Z (%)
5.8
5.7
1.9
8.1
1.8
3.6
4.7
4.5
4.4
5.6
4.5
4.4
(4.6)
(5.5)
(1.4)
(7.0)
(1.0)
(3.7)
(3.5)
(3.3)
(3.7)
(3.7)
(3.1)
(3.8)
w
This work. Values in parentheses show performance of the control experiment that used TiO2. DSCs, dye solar cells.
The electron transport in DSC through the mesoporous electrode is coupled with the electrolyte as there is a junction capacitor in the metal oxide–electrolyte interface (Fig. 4). The capacitive
charging at this junction further retard the electron mobility, i.e.,
the electron diffusion is coupled with the ion movement in the
electrolyte. This is known as ambipolar diffusion, which typically
occurs when an electrical carrier is injected into a sea of oppositely
charged mobile carriers. Under such conditions the injected carrier is referred to as a minority carrier. The ambipolar diffusion
coefficient (Damp) is defined as
Damp ¼
ðn þ pÞ
ðn=Dp Þ þ ðp=Dn Þ
(6)
where n and Dn are the electron density and diffusion coefficient,
respectively; and p and Dp are the hole density and diffusion
coefficient. The DampBDn for p n. A packet of electrons generated by light in DSC will diffuse naturally, carrying along it a
cloud of holes such that the entire packet remains electrically
neutral. Though this fact was considered by a number of authors
before the year 2000,46,47 the ambipolar diffusion model was first
applied to DSC by Kopidakis et al.48 By using TiO2 particles of
15–20 nm, Kopidakis et al.48 measured an ambipolar diffusion
coefficient of 2 104 cm2/s which was 15% less than the corresponding electron diffusion coefficient.
(B) Electrically Modified TiO2 Photoelectrodes: Several
studies have initiated to modify the electrical transport properties of TiO2 using composite electrodes,49,50 core/shell structures,51–55and doping.56 Photovoltaic properties of the resulting
DSC are presented in Table III. Eguchi et al.49 prepared various
compositions in the TiO2–Nb2O5 system starting from pure
TiO2 to pure Nb2O5 and observed that the FF increase steadily
Fig. 8. The J–V curves of DSCs fabricated using Nb51-doped TiO2 as
photoelectrodes.
with Nb2O5 content. Increase in the FF indicates that the loss
due to carrier recombination decreased, i.e., increase in the
shunt resistance as noted in Section II(3), with increase in
the Nb2O5 content. The VOC of the composite electrode cell
was increased by increasing the Nb2O5 content; and the JSC increased by increasing the TiO2 content. Increase in the VOC is
due to the higher conduction band energy of Nb2O5 compared
with TiO2. The BET surface area of the composite materials was
smaller than TiO2, which adversely affected the amount of dye
anchored onto the composite electrodes and lowered the JSC.
More recently Kitiyanan and Yoshikawa50 prepared TiO2–ZrO2
and TiO2–GeO257 composite electrodes. The TiO2–ZrO2 composite electrodes showed increase in all the photovoltaic parameters, whereas the FF decreased in the TiO2–GeO2 composite
compared with cells fabricated using pure TiO2.
Another approach to improve the electrical transport properties was to form a shell of wider bandgap material to TiO2,
which was an idea brought from the bandgap engineering of
compound semiconductors. If the bandgap positions are in a
typical way as shown in Fig. 8, the resulting configuration confine electrons in the core material. The electron confinement
would decrease the electron transfer probability to the electrolyte and thereby increasing the FF and JSC. Palomares et al.51
coated layers of ZrO2, Al2O3, and SiO2 onto 15 nm TiO2 particles and observed significant increase in the FF and JSC for
core/shell electrode compared with the bare TiO2 electrode. The
best result was reported with optimum shell thickness for Al2O3
shell whose conduction band energy (B3.23 eV) lies between
that of the ZrO2 (B3.13 eV) and SiO2 (B2.97 eV). Although
the FF was increased with increase in the shell thickness, the JSC
was decreased. Zaban et al.52 reported enhancement in FF and
JSC for TiO2/Nb2O5 core/shell nanostructures. Ahn et al.58
recently reported diffusion coefficient and carrier lifetime of
mesoporous TiO2 and TiO2–Nb2O5 core/shell structure using
transient photocurrent measurements. The diffusion length calculated for B10 mm TiO2 film was B17.3 mm, which increased
to 23.8 mm in the TiO2/Nb2O5 core/shell structure. Jung et al.53
coated layers of mesoporous MgO onto 25 nm-TiO2 particles
and observed increase in FF and JSC. The surface area of the
TiO2/MgO core/shell structures was higher than that of the bare
TiO2, therefore, the enhanced JSC could result from the increased dye loading also. TiO2 particles were also coated using
materials of similar bandgap such as ZnO and SrTiO3 and
observed higher FF for the core/shell structure.54,55 The VOC
increased in this case also (Table III). In the case of higher
bandgap shell materials, the increased VOC results from the
higher conduction band energy of the shells. For materials of
similar band energy such as ZnO, it should be anticipated that
the dye anchoring up-shifted the conduction band energy.21
Durr et al.56 substituted Zr41 ions for Ti41 ions in TiO2 and
observed that no secondary phase was segregated up to 2% of
Zr in TiO2. The DSCs fabricated using Zr41:TiO2 showed 15%
enhancement in efficiency compared with the pure TiO2.
296
Journal of the American Ceramic Society—Jose et al.
Table IV. Photovoltaic Properties of Nb51-Doped TiO2
Electrode
TiO2
TiO21500 ppm Nb51
TiO211000 ppm Nb51
JSC (mA/cm2)
VOC (V)
FF
Z (%)
2.15
4.89
3.11
0.777
0.837
0.841
54.3
68.8
70.2
0.95
2.74
1.78
(a) Nb51-Doped TiO2: Following Durr et al.,56 we
have doped Nb51 in TiO2 and used the resultant material as
photoelectrode.
Pure TiO2 and Nb51:TiO2 were synthesized from hydrolysis
of titanium isopropoxide and a mixture of titanium isopropoxide and niobium ethoxide using acetic acid, respectively. The
amount of Nb atoms was fixed at 500 and 1000 ppm. All
the solutions yielded transparent gel upon evaporation. The
gels were dried at room temperature and heated at 5001C for 30
min. A complete characterization of the powders thus obtained
was pending at the time of writing this report.
The TiO2 particles thus obtained were sprayed on FTO plates
(area B0.28 cm2) and sintered at 5501C for 1 h. The TiO2coated FTO electrode was soaked in a solution containing
ruthenium dye (RuL2(NCS)2 2H2O; L 5 2.20 -bipyridyl-4,
40 -dicarboxylic acid, 0.5 mM, N3 Solaronix, Aubonne, VD,
Switzerland) in 1:1 volume mixture of acetonitrile and tertbutanol for 12 h at room temperature for dye anchoring. The
soaked electrodes were washed with ethanol to remove nonanchored dye molecules and subsequently dried in air. The
Pt-sputtered FTO glasses were used as counter electrodes. The
Pt counter electrode and dye-anchored TiO2 electrodes were assembled into a sealed sandwich type cell using a sealing material
(SX1170-25, film thickness 5 25 mm, Solaronix). The electrolyte
solution, acetonitrile containing 0.1M lithium iodide, 0.03M iodine, 0.5M 4-tert-butylpyridine, and 0.6M 1-propyl-2,3-dimeth-
Vol. 92, No. 2
yl imidazolium iodide, was injected into the cell through a hole
drilled on the counter electrode via vacuum backfilling. The cell
was first placed in a small vacuum chamber to remove inside air.
By exposing the cell again to ambient pressure causes the electrolyte to be driven into the cell. The hole was then sealed with a
piece of glass using epoxy adhesive. Performance of the solar cell
was evaluated using the photocurrent measurements with a 100
mW/cm2 xenon lamp (Thermo Oriel Xenon Lamp 150 W:
Model 66902, Stratford, CT) under global AM1.5 condition.
Current density–voltage curves of the cells were obtained by using a potentiostat (PGSTAT30, Autolab PGSTAT30, Eco
Chemie B.V. Utrecht, the Netherlands).
Figure 8 shows typical I–V characteristic of the DSCs fabricated in the present work. Table IV shows the photovoltaic
properties of the DSCs determined from the I–V curves. The
efficiency of the cells was improved for 500 and 1000 ppm
doping of Nb51 ions in TiO2. As anticipated, the VOC increased
for both the sample containing Nb51 for 500 and 1000 ppm,
indicating incorporation of Nb51 in TiO2 lattice. Both the fill
factor and current density increased for 500 ppm Nb51 doping
and decreased for 1000 ppm Nb51 ion doping. Impressive
increments in photovoltaic parameters were observed for
500 ppm Nb51 doping compared with pure TiO2 cells under
similar experimental conditions. Detailed investigation along
this line is currently underway.
(C) One-Dimensional TiO2 Nanostructures: The enhanced grain-boundary density and random network of nanoparticles increase the loss in DSC due to carrier recombination.
For the usual nanoparticle network used in DSC, with porosity
B60%, the average diffusion length has been estimated to be
15–20 mm.43 This inferior electron transport limits the electrode
thickness for DSC and adversely affects the final conversion
efficiency. One possible way to improve the diffusion coefficient,
and therefore, transport properties is to grow the electrode
materials as one-dimensional nanostructures such as nanotubes
Fig. 9. Different particle morphologies (A) particles showing a bimodal distribution of size, (B) nanotubes adapted from Palomares et al.,51 (C) random
nanofiber network, (D) nanofibers deposited onto FTO plate.
February 2009
Metal Oxides for Solar Cells
297
nanofibers on the FTO plate.70 This technique allowed us
fabrication of large area ( 20 cm2) TiO2 films on FTO plates.
In addition, TiO2 nanofibers were directly developed on
conducting glass plates by layer deposition method.71 The DSCs
fabricated using the electrospun nanofibers gave efficiencies B6%
using conventional N3 dyes. We have also tested metal-free indoline dyes as sensitizers in electrospun TiO2 nanofibers and
found that Z of some of the indoline DSCs are comparable to
that of the N3 dye.72 The nanorod-based DSC was further used to
understand deeply the controlling factors of the JSC; and therefore, the Z.22 Through a combined theoretical and experimental
investigation using TiO2 nanofibers we have shown that the electron injection yields from the dye to the photoelectrode is a factor
that limits JSC and Z.22 The efficiency of charge injection in DSCs;
and therefore JSC and Z, are determined by the extent to which
the LUMO of the dye lie close to the photoelectrode.
Fig. 10. Schematic of electron diffusion between (A) sintered spherical
nanoparticles and (B) one-dimensional (1-D) nanostructures. Electron
diffusion through spherical nanoparticles is according to random walk
model. Though diffusion is in accordance with the random walk in 1-D
structures also, the electrons are constrained to move directionally. The
panel ‘‘C’’ shown in the box explains how reduction of diameter of the
1-D structure make the electron flow more channeled.
and nanorods,59 nanowires,60 and nanofibers61 (Fig. 9). Figure
10 explains this concept of channeled electron transport through
one-dimensional nanostructures. Recently Frank and colleagues62,63 compared the transport and recombination properties of nanotubes and spherical nanoparticles films used in
DSCs by frequency-resolved photovoltage spectroscopy. Both
morphologies displayed comparable transport times; however,
recombination was much slower in the nanotube films. This
observation indicates that the nanotube-based DSCs have significantly higher charge collection efficiencies than their nanoparticle-based counterparts.63 It was also showed that capillary
stress in the dense TiO2 nanotube arrays could induce bundling
and microcrack formation, which would affect the dye loading
and electron transport.62 Transport became significantly faster
when the bundling of nanotubes was reduced. The photoconversion efficiency above B9.3% has been reported for DSCs
using single crystal like anatase nanowires.64 The DSCs fabricated using TiO2 nanotubes made by anodic oxidation60,65 and
using block copolymer templates66 gave Z up to 5.4% and 10%,
respectively.
Most frequently used method for fabrication of nanotubes is
the anodic oxidization of titanium.65 TiO2 nanowires are fabricated from different methods such as depositing TiO2 into
anodic alumina membranes, electrophoresis, hydrothermal
method, and oxidation of metallic titanium.60 Lengths of the
nanotubes and nanowires thus obtained are limited to few
micrometers. This drawback is eliminated in nanofibers which
are normally produced in long lengths using a relative simple
technique, known as electrospinning.67 The diameter controlled
anatase TiO2 nanofibers are fabricated by controlled electrospinning of a polymeric solution and subsequent sintering of the
as-spun fibers.68 The electrospun TiO2 nanofibers are polycrystalline and composed of densely packed TiO2 grains of several
nanometers diameters. We have shown that the nanofibers are
characterized by increased surface strain from capillary force,
the magnitude of which increases with decrease in the fiber diameter.69 The surface strain in anatase TiO2 caused the rutile
phase to nucleate at the grain boundaries at lower temperatures
than that has been observed in nanoparticles of similar size.69
Unlike other one-dimensional nanostructures such as nanowires and nanotubes, which are grown directly onto the FTO
plate, the random web structures of electrospun nanofiber impose adhesion difficulties with the FTO plate. We developed a
nanorod spraying technique to solve the adhesion issues of the
(2) ZnO Electrodes
The ZnO is II–VI semiconductor (EgB3.37 eV) that crystallizes
either in cubic zinc-blende, cubic rocksalt (NaCl), or hexagonal
wurtzite structure. The zinc-blende phase can be stabilized only
by growth on cubic substrates and the rocksalt structure forms
at relatively high pressures.73 The hexagonal wurtzite is the
lowest energy structure of ZnO. The conduction band energy
of ZnO is 4.45 eV with respect to vacuum, similar to that of
TiO2. Hexagonal ZnO has been actively sought as a replacement
for the TiO2 electrodes.74–77
Hagfeldt and colleagues78 studied the electron transport in
nanostructured and discussed their finding under the framework
of the multiple trapping diffusion model similar to that applied
for TiO2. The charge transport in ZnO also depended on the
electrolyte used. For nanoporous ZnO in 0.5M LiClO4 in ethanol, the diffusion coefficient varied from 104 to 106 cm2/s when
the potential was varied between 100 and 300 mV vs Ag/AgCl in
ethanol. On the other hand diffusion coefficient for TiO2 in 0.7M
LiClO4 in ethanol at 300 mV vs Ag/AgCl in ethanol was
B2 105 cm2/s.46 The density of traps is reported to be less
for ZnO compared with that of TiO2. This lower trap density
makes the electrons to move faster through the ZnO nanoporous
network. The carrier life time in ZnO is longer than that in
TiO2.78 Despite the superior transport properties of ZnO, the
DSCs constituted using ZnO gave inferior performance. The best
efficiency so far reported for ZnO electrode is 4.1% by using with
N719 sensitizer and iodide/triiodide electrolyte.79
The poor photovoltaic performance utilizing ZnO electrode
comes from the stability of ZnO at acidic conditions. The
protons derived from Ru complexes such as N3 and N719
make the dye loading solution relatively acidic and dissolve
the surface of ZnO, generating Zn21/dye aggregates.80 Such
aggregates lower the electron injection efficiency. This could
be overcome by immersing the ZnO film in ethanolic solution
under reflux.
Besides nanoparticles, various one-dimensional nanostructure
of ZnO including nanowires,81–83 nanotubes,84,85 nanorods,86 as
well as electrospun ZnO nanofibers87 has been explored. From
resistance measurements and by assuming electron concentration
of 1–5 1018 cm3 and mobility 1–5 1018 cm2/V/s, Law et al.81
calculated diffusion coefficient of single crystalline ZnO nanowires as 0.05–0.5 cm2/s. This diffusion coefficient of onedimensional ZnO nanowires is several hundred times larger
than that reported for TiO2 and ZnO nanoparticles. Pillarshaped and branched-shaped ZnO nanowire grown directly
on FTO substrate by thermal evaporation at temperatures of
8001–10001C were also suggested for channeled electron transport similar to one-dimensional nanostructures.88 Compared
with the pillar-shaped nanowire efficiency of 0.34%, a branched
ZnO nanowire structure has better conversion efficiency of
0.46%. This is suggested by the increase in the dye absorption
of the high-density branched structure. Owing to the fact that
many branches can be formed from a single pillar nanowire,
branched structures provide a much larger surface area for dye
298
Journal of the American Ceramic Society—Jose et al.
loading. ZnO nanosheets were also grown from precursor
templates in aqueous solutions.89 Nanosheets fabricated were
accumulated on conducting glass substrates with submicrometer
spacing, giving the film a macroporous nature. A high shortcircuit photocurrent JSC of 13.8 mA/cm2 is thus obtained. This
was attributed to the effective dye loading of 1.1 107 mol/cm2,
which is comparable to 1.3 107 mol/cm2 of a 10-mm-thick
nanoporous TiO2 photoelectrode sensitized with the N3 dyes.88
(3) Nb2O5 Electrodes
Several groups used Nb2O5 as photoelectrodes in DSSC as nanoparticles,49,90–93 nanobelts,94 and as TiO2–Nb2O5 bilayers,52,58
and as blocking layers that prevent the electron back transfer.95,96 The Nb2O5 is a wide bandgap semiconductor with bandgap energy 3.49 eV,97which is about 0.29 eV larger than that
of anatase. Lenzmann et al.91 determined the conduction band
energy of Nb2O5 from electrochemical measurements and
assuming one-electron configuration to be at 0.2–0.3 eV above
to that of anatase. Because of its larger bandgap and higher
conduction band edge compared with anatase, DSCs fabricated
using Nb2O5 as photoelectrode and N3 dye showed higher VOC
than that achieved using anatase. However, Nb2O5 sensitized by
mercurochrome dye showed VOC lower than that achieved using
anatase.98 Hoshikawa et al.90 compared the impedance characteristics of DSCs using Nb2O5 photoelectrode with that of TiO2.
They report that when the Nb2O5 particles with large BET surface area were used as the electrode, the internal resistance of the
solar cell was lower. Sayama et al.99 used porous semiconductor
films of TiO2, Nb2O5, ZnO, SnO2, In2O3, WO3, Ta2O5, and ZrO2
in DSC sensitized by the N3 dye. They observed that the Nb2O5
semiconductor cell had the next highest IPCEB18% compared
with the TiO2 cell, for which IPCEB45% was reported under
similar cell fabrication conditions, and showed the highest VOC
among them. This observation was explained on the difference in
the electronic structure of these semiconductors. The electrons
are assumed to be transferred mainly through the conjugated
orbitals of the ester linkage and semiconductor conduction band.
The Nb2O5 has a monoclinic structure (space group P2) with
a 5 2.038 nm, b 5 0.3824 nm, c 5 1.937 nm, and b 5 115.691.100
Orthorhombic Pbam symmetry with a 5 0.6168 nm, b 5 2.931
nm, and c 5 0.3936 nm has also been assigned to Nb2O5.101 Its
large unit cell dimension could reduce the surface area thereby
lowering JSC as has been observed experimentally. However, by
developing proper chemical technique nanocrystalline Nb2O5
with BET surface area 25 g/cm2 could be prepared,28 even if it
is much lower than that of mesoporous TiO2. More attention
is required to develop mesoporous Nb2O5 as well as their onedimensional nanostructures. Similar to other materials, actual
mechanism of charge transport through mesoporous Nb2O5 is
to be explored. In view of its higher conduction band energy and
high chemical stability, Nb2O5 could be a suitable alternative to
TiO2 on the way of development of high-performance DSC.
(4) Ternary Photoelectrode Materials
Besides the simple binary metal oxide systems ternary metal
oxide systems such as SrTiO391,102 and Zn2SnO4103,104 have been
considered as photoelectrode materials in DSC.
The SrTiO3 is a semiconductor with bandgap similar to TiO2
(3.2 eV). However, its conduction band is relatively higher than
that of TiO2 which could result in a higher VOC compared
with TiO2.91 Its high dielectric constant makes SrTiO3 to
behave as electrically mesoporous even with a large particle
size of B80 nm.105 The Z reported for DSCs using SrTiO3 is
relatively poor, which is explained as due to the specific surface
structure of SrTiO3. The energetically favored surface of SrTiO3
is (001) face, which has two possible terminations, SrO or TiO2.
The SrO surface is more basic; and therefore, poorly adsorb the
negatively charged carboxylate group of the sensitizer. The poor
photovoltaic performance might result from the decreased dye
loading onto the semiconductor.
Vol. 92, No. 2
Two groups reported Zn2SnO4 as a photoelectrode in DSC
almost same time.103,104 The Zn2SnO4 is a TCO with bandgap
B3.6 eV and has inverse spinel structure with octahedrally
coordinated Sn41 atoms; half of the Zn21 atoms are in tetrahedral coordination and the other half in octahedral coordination.
Research on Zn2SnO4 initiated following an observation that
Z of a DSC that used mixture of ZnO and SnO2 as working
electrode enhanced dramatically.106 The Zn2SnO4 cells reported
efficiency up to 3.8% which is close to the highest efficiency
reported for ZnO fabricated under similar conditions (4.1%)
and much higher than that reported for SnO2 (1.2%). In addition Zn2SnO4 cells also overcame the stability problem associated with ZnO against acidic dyes. In this sense, the ternary
oxide (Zn2SnO4) is more attractive than its simple binary components (ZnO and SnO2) as the electrode material for DSSC.
(5) Metal Oxide Solar Cells
Development of photocurrents upon irradiation of a semiconductor electrode has long been observed in TiO2, SnO2, SrTiO3,
Fe2O3, etc.107,108 If such electrodes are coupled with a metal
counter electrode (Schottky barrier) or with another semiconductor electrode, the system produce photoelectric current.
However, such metal oxide solar cells showed poor efficiency
typically with JSC few hundred microamperes at 1 sun irradiation because these materials are characterized by poor absorption coefficient and low carrier mobility (B0.01 cm2/Vs). A
major source that limits the JSC is the charge carrier recombination in metal oxides.109 Recently there is a revived interest in
these materials for fabrication of metal oxide solar cells.110,111
Corma et al.111 observed photovoltaic effect in CeO2,
Ce1xZrxO2, and Ce1xLaxO2 (x 5 0–0.3) nanoparticles of size
B5 nm when they are sandwiched between two conducting glass
plates and in the presence of iodide/triiodide electrolyte. The
cells were not sensitized with a dye. The CeO2 has a bandgap
of 3.2 eV arising from O2p-Ce4f transition; and therefore, bulk
CeO2 is not regard as a photoactive semiconductor. However,
nanocrystals of pure CeO2 and that doped with Zr41 and La31
exhibit a photovoltaic response.111 In the case of nanocrystalline
CeO2, the presence of significant fraction of Ce atoms lie at the
surface thereby leading to oxygen vacancies and defects. The
observed photoconductivity could be explained in terms of
hopping of electron and holes between these surface states.
The cells prepared with La31-doped CeO2 gave VOC 5 0.9 V,
Isc 5 1.5 mA, fill factor 0.41, and energy conversion efficiency of
1.4% under AM1.5 condition.
IV. Conclusions
The nanostructured metal oxide semiconductors, with particle
size greater than their exciton Bohr radius, influences the photoconversion efficiency of DSCs. Among the beneficial include
(i) its mesoporosity and large surface area allow anchoring a
large amount of dye molecules which enhances the absorption
cross-section, (ii) larger density of states in metal oxides than the
molecular orbital of dye allows faster injection of electrons
from the dye to the metal oxide. The detrimental factors are (i)
inefficient electrical transport through nanocrystalline network
and (ii) charge recombination within the materials and also with
the electrolyte. Inefficient electrical transport in nanostructured
metal oxide semiconductors arise from the trapping and detrapping of electrons at the surface atomic states in the electronic
band. The surface atoms are a large fraction in nanostructured
materials, so as the trap density. Although considerable progress
has been made in understanding the charge transport through
nanoparticles network, further efforts are required to exactly
determine the trap states in TiO2 nanoparticles of size 10–25 nm.
Trapping and detrapping reduces the kinetic energy of the
flowing electrons, which ultimately results in inferior cell performance. Quantification of the traps and their subsequent removal
could enhance the photoelectric conversion efficiency of DSCs
beyond the present record of B11%. Geometry of DSCs is well
February 2009
Metal Oxides for Solar Cells
optimized such that over 85% electrons converted from the incident photons are collected at the external electrode, which
makes the collection of high-energy electrons a more stringent
requirement.
Binary metal oxides such as TiO2, ZnO, Nb2O5, Fe2O3, ZrO2,
Al2O3, and CeO2 and ternary compounds such as SrTiO3 and
Zn2SnO4 have been tested for their use as photoelectrodes in
DSC. Among them the d-block binary metal oxides viz., TiO2,
ZnO, and Nb2O5 are the best candidates as photoelectrode due
to the dissimilarity in orbitals constituting their conduction
band and valence band. Various morphologies and approaches,
such as doping, composites, core/shell structures, have been
explored to enhance the electrical transport properties. Onedimensional morphologies show great promise in enhancing the
electrical transport properties. Well-ordered one-dimensional
nanowires synthesized through template-assisted methods
increased the recombination time and allowed enhanced charge
collection efficiency in DSCs. Doping of Nb2O5 and ZrO2 in
TiO2 is found to be beneficial in increasing the open circuit
voltage; however, the doped materials showed lesser surface area
than their parent materials. The reduced surface area adversely
affected the dye loading; and therefore, the short-circuit current
density. Owing to this drawback, the doped materials or composites made from the constituent metal oxides did not yield
high-efficiency DSCs. Efforts are to be undertaken to synthesize
mesoporous Nb2O5 and Nb-doped TiO2 with high crystallinity
to realize high-performance DSCs. In addition, cost-effective
TCOs that are stable over a wide range of temperature is
required for the commercial exploitation of DSCs.
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Metal Oxides for Solar Cells
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Prof. Seeram Ramakrishna is currently the Research Strategy VicePresident of the National University
of Singapore (NUS). After receiving
Ph.D. degree in Materials Science
and Engineering from the University of Cambridge in 1992, Seeram
joined at the Kyoto Institute of
Technology as a visiting lecturer.
He joined the NUS in 1996 as Lee
Kuan Yew Fellow where he was
promoted to Professor at the departments of Mechanical engineering as well as Bioengineering
in 2003. He served/ serving in various capacities as Directors,
Dean, chairman in advisory boards, and in the editorial board
of over 15 scholarly journals. His research interests include
biomaterials, energy, and environment. The ISI web of knowledge currently ranks him at 99 out of 3415 most cited materials
scientists in the world.
301
Dr. Rajan Jose pursued doctoral
research at the Council of Scientific
and Industrial Research (CSIR),
Trivandrum, India and received
PhD degree in Physics from the
Mahatma Gandhi University, India
in 2002. He was a postdoctoral
fellow at AIST, Japan (2003–2005)
and Toyota Technological Institute,
Japan (2005–2007). He joined the
NUS in 2007 where he is working
on nanostructured materials for excitonic solar cells and one-dimensional metal oxide nanostructures. His research interests include functional inorganic solids
for electronic, photonic, and energy applications.
Dr Velmurugan Thavasi is a
Research Fellow at the NUS Nanoscience and Nanotechnology
Initiative. He obtained MEng in
Chemical Engineering and PhD in
Chemistry from National University of Singapore. His research interests include synthesis of nanomaterials, interfaces engineering,
and the study of excitonic solar
cells and bio-nano devices. Email:
velnanotech@gmail.com
Metal Oxides for Dye-Sensitized Solar Cells
Journal of The American Ceramic Society, 2009
The incessant demand for energy forces us to seek it from sustainable resources; and concerns on environment demands that resources should be clean as well. Metal oxide semiconductors, which are stable and environment friendly materials, are used in photovoltaics either as photoelectrode in dye solar cells (DSCs) or to build metal oxide p–n junctions. Progress made in utilization of metal oxides for photoelectrode in DSC is reviewed in this article. Basic operational principle and factors that control the photoconversion efficiency of DSC are briefly outlined. The d-block binary metal oxides viz. TiO2, ZnO, and Nb2O5 are the best candidates as photoelectrode due to the dissimilarity in orbitals constituting their conduction band and valence band. This dissimilarity decreases the probability of charge recombination and enhances the carrier lifetime in these materials. Ternary metal oxide such as Zn2SnO4 could also be a promising material for photovoltaic application. Various morphologies such as nanoparticles, nanowires, nanotubes, and nanofibers have been explored to enhance the energy conversion efficiency of DSCs. The TiO2 served as a model system to study the properties and factors that control the photoconversion efficiency of DSCs; therefore, such discussion is limited to TiO2 in this article. The electron transport occurs through nanocrystalline TiO2 through trapping and detrapping events; however, exact nature of these trap states are not thoroughly quantified. Research efforts are required not only to quantify the trap states in mesoporous metal oxides but new mesoporous architectures also to increase the conversion efficiency of metal oxide-based photovoltaics....Read more
