Radio frequency magnetron sputter deposited ZnO films doped with Al, Ga and Ti

Abstract

Zinc oxide (ZnO) is a key material in the field of transparent large-area electronics. Some of the ZnO applications in large-area electronics include sensors, transistors and solar cells. In this work, ZnO films doped with aluminium (Al), gallium (Ga) and titanium (Ti) with different doping concentrations were deposited on glass substrates via the radio frequency (R.F.) plasma magnetron sputtering technique. The correlations of different doping concentration with the structural, optical and electrical properties were investigated by Atomic Force Microscopy, Ultraviolet–Visible Spectrometer and Hall Electronic Transport Measurement System, respectively.

Full text

REVIEW PAPER
Recent advances in photo-anode for dye-sensitized solar
cells: a review
Mian-En Yeoh and Kah-Yoong Chan*
,†
Centre for Advanced Devices and Systems, Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia
SUMMARY
Dye-sensitized solar cell (DSSC) attracts immense interest in the last few decades due to its various attractive features such
as low production cost, ease of fabrication and relatively high conversion efficiency, which make it a strong competitor to
the conventional silicon-based solar cell. In DSSC, photo-anode performs two important functions, viz. governs the
collection and transportation of photo-excited electrons from dye to external circuit as well as acts as a scaffold layer for
dye adsorption. The photo-anode usually consists of wide band gap semiconducting metal oxides such as titanium dioxide
(TiO
2
) and zinc oxide (ZnO) deposited on the transparent conducting oxide substrates. The morphology and composition of
the semiconductor oxides have significant impact on the DSSC photovoltaic performance. Therefore, enormous research
efforts have been undertaken to investigate the influences of photo-anode modifications on DSSC performance. The
modifications can be classified into three categories, namely interfacial modification through the introduction of blocking
and scattering layer, doping with non-metallic anions and metallic cations and replacing the conventional mesoporous
semiconducting metal oxide films with one-dimensional or two-dimensional nanostructures. In the present review, the
previously mentioned modifications on photo-anode are summarized based on the recent findings, with particular emphasis
given to published works for the past 5 years. Copyright © 2017 John Wiley & Sons, Ltd.
KEY WORDS
dye-sensitized solar cell (DSSC); photo-anode; titanium dioxide (TiO
2
); zinc oxide (ZnO)
Correspondence
*Kah-Yoong Chan, Centre for Advanced Devices and Systems, Faculty of Engineering, Multimedia University, PERSIARAN
MULTIMEDIA, 63100 Cyberjaya, Selangor, Malaysia.
†
E-mail: kychan@mmu.edu.my
Received 14 December 2016; Revised 3 April 2017; Accepted 3 April 2017
1. INTRODUCTION
Nobel laureate Richard Smalley listed energy and
environment as among the top 10 problems of humanity
for the next 50 years during his speech at Rice University
in 2003 [1]. In fact, the increment in global energy
consumption has accelerated the depletion of fossil fuels.
At the same time, the combustion of fossil fuels also leads
to undesirable environmental effects such as greenhouse
effect, acid rain and pollution. Both the energy and
environment problems spur concerns about the urgent need
of finding alternative renewable and green energy. To date,
several forms of renewable energies have been harnessed
by mankind such as hydropower, solar energy, wind
power, biofuel, biomass and geothermal. Among these
renewable energies, solar energy attracts much attention
due to its abundance and cleanliness. According to
statistics, the global energy demand can be met by just
covering 0.1% of Earth’s crust with solar cell having
efficiency of 10% [2]. This clearly shows the potential of
solar energy as the ultimate solution to growing energy
needs. The demand of solar energy led to the rapid growth
of solar industry. From Figure 1, the annual market share
for photovoltaic (PV) module accelerated rapidly from
year 2000 to 2015 and the increasing trend is expected to
continue for the next few years [3].
Solar cell or PV cell was invented to convert the
sunlight into electricity through the process of PV effect.
Nevertheless, high cost and sophisticated fabrication
process of conventional silicon-based solar cell limit its
commercialization in domestic applications. The urge of
reducing the cost of solar cell while maintaining its
efficiency leads to the development of third-generation
solar cells such as dye-sensitized solar cells (DSSCs).
The history of DSSC can be traced back to the year 1972
by using zinc-oxide photo-anode sensitized with
chlorophyll [4]. However, the cell efficiency was low due
to insufficient surface area for light harvesting. The first
significant breakthrough of DSSC came in 1991 when
Michael Grätzel and Brian O’Regan demonstrated that
INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. (2017)
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3764
Copyright © 2017 John Wiley & Sons, Ltd.

nanostructured titanium dioxide (TiO
2
)film coated with a
monolayer of ruthenium dye as photo-anode can achieve
cell efficiency of 7.1–7.9% [5]. The nanostructured TiO
2
film significantly enhanced the available surface area by a
factor of more than a thousand, which was essential for
efficient dye loading. This approach improved the light
absorption and efficiency of DSSC dramatically that
allowed DSSC to be viewed as a strong competitor to other
existing solar cell technologies [6]. Since then, the
efficiency and stability of DSSC have been steadily
improving due to extensive effort in experimental
investigations. DSSC efficiency of 10.4% was reported
by Nazeeruddin et al. [7] by using panchromatic black
dye as sensitizer. Chiba et al. investigated the use of
high-haze TiO
2
film as photo-anode, which led to high
DSSC efficiency of 11.1% [8]. Yella et al. demonstrated
that DSSC with zinc porphyrin dye (YD2-o-C8) and
cobalt-based redox electrolyte can achieve conversion
efficiency of 12.3% [9]. Efficiency as high as 13% was
achieved for TiO
2
-based DSSC sensitized with
molecularly engineered porphyrin dye, as reported by
Mathew et al. [10]. To date, the highest DSSC efficiency
achieved is 15% based on perovskite sensitizer [11].
In order to develop high-efficiency DSSC, it is essential
to understand the kinetics of photo-excited electrons in the
photo-electrochemical cell during its operation. Extensive
research are undertaken to acquire knowledge about the
behaviour of photo-excited electrons in DSSC, and the
important findings have been summarized in several
articles [12,13]. The pivotal role of photo-anode in
governing the collection and transportation of photo-
excited electrons was highlighted in these literature. As
envisaged from the literature, the modifications of photo-
anode in terms of morphology, doping and film thickness
have significant influences on the PV performance of
DSSC. Nonetheless, the existing research activities on
photo-anode are scattered, which complicate the process
of designing new experiment in the related field. Summary
of the executed work related to the development of photo-
anode is therefore necessary to provide a clear direction for
future work. In the present review, modifications of photo-
anode materials, particularly titanium oxide and zinc oxide
(ZnO), and their influences on DSSC performance are
summarized based on relevant literature reports. Prior to
the review, the working principle of DSSC is discussed
briefly for general understanding of the readers.
2. DYE-SENSITIZED SOLAR CELLS
(DSSCS)
2.1. Brief discussions of constituents and
their roles
The schematic band diagram of DSSC is represented in
Figure 2. As depicted from the figure, a DSSC normally
comprises of four components:
(a) photo-anode (semiconducting metal oxide deposited
on the surface of transparent conducting oxide
(TCO) substrate, typically fluorine-doped tin oxide
(FTO) glass);
(b) dye/sensitizer;
(c) electrolyte; and
(d) counter electrode (typically Pt-coated FTO glass)
2.1.1. Photo-anode
The photo-anode is also known as working electrode in
DSSC. The photo-anode usually consists of
Figure 1. Annual market share of photovoltaic modules [3] [Colour figure can be viewed at wileyonlinelibrary.com]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

semiconducting wide band gap metal oxide deposited on
the surface of TCO substrate, typically FTO glass. The
functions of TCO substrate in DSSC are to support the
semiconductor layer and to collect the current. An ideal
TCO substrate should possess high optical transparency
and low electrical resistivity. High transparency is essential
to allow the good transmittance of sunlight through the
substrate without unwanted adsorption, while low
resistivity is crucial in facilitating the charge transfer
process and to reduce the energy loss [14]. FTO is the most
widely used oxide in DSSC application owing to its
excellent electrical conductivity and optical transparency.
Several studies reported that FTO substrate is superior to
indium tin oxide (ITO) substrate as photo-electrode in
DSSC due its stable resistivity at elevated temperature
[15,16]. In DSSC fabrication, the preparation of photo-
anode usually involves the deposition and sintering of
TiO2paste on TCO substrate at high temperature (~450
°C) in order to improve the electrical contact [5]. However,
the sheet resistance of ITO glass will increase
tremendously when subjected to thermal treatment at
around 300°C , which will lead to poorer efficiency [15].
Therefore, FTO substrate is preferred over ITO substrate
in DSSC application. The details of semiconducting metal
oxide used in DSSC are discussed in forthcoming sections.
2.1.2. Dye/sensitizer
In DSSC, the dye/sensitizer is usually anchored on the
surface of metal oxide. Dye is one of the key components
in DSSC as it is responsible for light harvesting and
generation of photoexcited electrons. An ideal dye should
possess several requirements: (1) high molar extinction
coefficient in visible and near-infrared region; (2)
appropriate lowest unoccupied molecular orbital (LUMO)
and highest occupied molecular orbital levels for both
efficient charge injection into conduction band (CB) of
semiconducting metal oxide and dye regeneration from
electrolyte respectivelyand (3) good solubility and photo-
stability [17]. The dye can influence the cell efficiency in
three different ways, namely the efficiency in absorbing
the incident photons, efficiency in converting the incident
photon to electron–hole pairs and lastly, the efficiency in
charge transfer process [18,19]. Ruthenium-based
organometallic dyes (e.g. N3, N719, and black dyes) are
currently the most efficient dyes due to their superior light
absorption, durability and most importantly, the metal–
ligand charge transfer transition that allows the photo-
generated charges to be injected into TiO2efficiently
[20]. Both the charge transfer process and photon-to-
electron conversion are very efficient in ruthenium dyes,
leaving little room for improvement [21]. Extensive
research focused on enhancing the light-harvesting
property of ruthenium dyes. Besides the organometallic
dyes, organic dyes such as porphyrin [9], TP6CADTS
[22], RK1 [23], and ADEKA-1 [24] have been utilized in
DSSC applications. Natural sensitizers or chlorophyll dye
extracted from leaves such as Pandannus amaryllifolius
and papaya have been investigated as well [25,26].
2.1.3. Electrolyte
In DSSC, the electrolyte functions as an electrically
conducting medium that transports electronic charge
between working electrode and counter electrode, as well
as allowing the regeneration of oxidized dye. The most
popularly used electrolyte in DSSC is iodide/triiodide
(I=I3Þredox couple in an organic solvent, normally
acetonitrile [27]. The excellent performance of I=I3-based
liquid electrolyte is attributed to its several interesting
properties, namely low recombination loss, extremely fast
dye regeneration and slow penetration into semiconducting
metal oxide film [28]. However, some undesirable intrinsic
properties also exist in liquid electrolyte, which affect the
long-term stability of DSSC. The main concern is the
evaporation of volatile iodide ions that will decrease the
charge carrier concentration, resulting in cell degradation.
Besides, the leakage of toxic organic solvent will also lead
to environmental pollution [14]. In order to overcome the
disadvantages of liquid electrolyte, other types of
electrolytes such as quasi-solid state, solid state and room
temperature ionic liquid electrolytes have been
investigated for DSSC applications [29,30].
2.1.4. Counter electrode
Electrocatalytic property of the counter electrode is
important in governing the PV performance of DSSC.
Without the catalytic layer, the TCO substrate has a very
high charge transfer resistance (>106Ω=cm2Þin
iodide/triiodide electrolyte, which makes it a very poor
counter electrode [31]. Platinum is the standard catalyst
deposited on the counter electrode due to its high catalytic
activity, ability to reduce the overpotential for redox
reaction and high resistance to corrosion against electrolyte
[32,33]. Deposition of platinum on the TCO substrate can
be implemented by using a wide range of methods, namely
Figure 2. Schematic energy band diagram of dye-sensitized
solar cell. [Colour figure can be viewed at wileyonlinelibrary.com]
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

spray pyrolysis, sputtering and doctor blading technique
[34]. Although platinum is the most efficient catalyst for
counter electrode to date, its expensiveness makes it
unsuitable for low-cost approach of DSSC. Other
alternatives such as carbon black [35,36] and conducting
polymer such as poly(3,4-ethylenedioxythiophene) doped
with toluenesulfonate anions [37] were employed as
catalyst for the realization of platinum-free DSSC.
2.2. Working principle
Under illumination, the photoexcited dye molecules inject
the electrons into the CB of the semiconducting metal
oxide. Generally, the metal oxide layer is mesoporous in
nature in order to facilitate dye loading and light
absorption by providing large surface area. The electrons
flow though the semiconducting metal oxide to the TCO
substrate. The oxidized dye is restored to its ground
state through the electron transfer from electrolyte, which
contains iodide/triiodide redox couple. The rapid
regeneration of the dye by iodide is essential to intercept
the recombination of the photoexcited electrons with the
oxidized dye. The oxidized redox mediators are
regenerated at the counter electrode as the electrons are
received at counter electrode after passing through the
external circuit, and hence completing the whole cycle
[28,38].
2.3. Characterization techniques
In order to fully comprehend the working functions of
DSSC and complicated interactions of its components, it
is essential to understand the characterizations of the DSSC
as a whole, in addition to investigations of individual
components. There are three main techniques used to
characterize DSSC, viz. photocurrent-voltage (I-V)
measurement, incident photon to current conversion
efficiency (IPCE) spectroscopy and electrochemical
impedance spectroscopy (EIS). More information on these
three techniques are discussed briefly in following
sections.
2.3.1. Photocurrent-voltage (I-V) measurement
The main function of I-Vmeasurement is to determine
the electrical output power of DSSC under standard
illumination condition. The standard irradiance spectrum
used for DSSC measurement is known as air mass 1.5,
which specifies cell temperature of 25 °C and the total
power density of solar irradiation to be 1000 W=m2[39].
Four important parameters of DSSC can be obtained
conveniently from I-Vcurve as shown in Figure 3,
namely open circuit voltage (V
oc
), short-circuit current
(I
sc
), fill factor (FF) and power conversion efficiency
(PCE, η).
Open-circuit voltage (V
oc
)isdefined as the maximum
voltage that a solar cell can supply to external circuit,
which is obtained from the separation of the hole and
electron quasi-Fermi levels [40]. V
oc
is proportional to the
difference between the Fermi level of photo-anode and
electrochemical potential of redox couple [39]. V
oc
is
measured under open-circuit condition, for which no
current can flow. As shown on the I-Vcurve in Figure 3,
V
oc
is derived from I= 0 A intercept. V
oc
is independent
of the cell area and is always constant under the identical
illumination condition regardless of cell area.
Short-circuit current (I
sc
) is the maximum current output
of a solar cell. I
sc
is measured under short-circuit condition,
which the applied potential across the solar cell is zero. As
shown on the I-Vcurve, I
sc
is derived from V=0V
intercept. Under standard conditions, I
sc
is dependent on
several factors, including the amount of dye adsorption
on photo-anode, electrochemical properties of photo-anode
in the presence of electrolyte and molecular structure of the
dye [39,41]. I
sc
is proportional to the diffusion length as
shown in the succeeding equation [40]:
Isc ¼qG LnþLp
 (1)
where Gis the generation rate, while L
n
and L
p
are
diffusion lengths of electron and hole respectively.
Fill factor (FF) measures the ideality of the solar cell
and is defined as the ratio of maximum power output
(PmaxÞto the product of V
oc
and I
sc
. The value of FF varies
between 0 and 1, whereas a high value implies more
preferable rectangular shape. Ideally, the power generated
within a solar cell is dissipated at the external circuit
without internal losses. Nevertheless, in practical
application, the power in a solar cell is dissipated through
the contact resistance, charge transport, leakage current
and etc. [34]. These combined effects are electrically
equivalent to series and parallel (shunt) resistances that
reduce the FF. FF can be calculated by
FF ¼Pmax
Jsc XV
oc
¼JmaxVmax
Jsc XV
oc
(2)
Power conversion efficiency (η) of a solar cell is the
ratio of maximum generated power (P
max
) to the incident
power (P
in
). To be more specific, it is defined as the power
Figure 3. Typical I-Vcurve of dye-sensitized solar cell [40]
[Colour figure can be viewed at wileyonlinelibrary.com]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

generated by the solar cell divided by the incident power
on representative cell area under standard air mass 1.5
illumination condition. From Eqn (3) in the succeeding
texts, it can be seen that higher values of V
oc
,I
sc
and FF
will lead to increased η. The mesoporous network of
photo-anode directly influences those parameters, and
hence, reducing the loss in the mesoporous network is
essential to fabricate DSSC with higher η[39].
η¼Pmax
Pin
¼JscVoc FF
Pin
(3)
2.3.2. Incident photon to current conversion
efficiency (IPCE)
The IPCE is defined as the ratio of the number of photo-
generated electrons (N
electrons
)flowing through the external
circuit to the number of incident photons (N
photons
)ata
given wavelength:
IPCE λðÞ¼
Nelectrons λðÞ
Nphotons λðÞ (4)
IPCE is also known as external quantum efficiency.
Measuring the spectral response of DSSSC by using IPCE
monochromator is important to gain information about
how efficiently the cell converts photons into electrons at
a given wavelength. IPCE can also be expressed as a
function of three efficiency parameters as follows:
IPCE λðÞ¼LHE λðÞηinj λðÞηcol λðÞ (5)
where LHE is the light-harvesting efficiency, η
inj
is the
quantum yield of the electron injection and η
col
is the
charge collection efficiency. A highly efficient DSSC can
achieve very high IPCE over a wide range of solar
spectrum, which implies that the electron injection and
charge collection efficiencies are near to 100%, after taking
into account the reflection loss at the interface of glass
substrate [42].
The mesoporous network of photo-anode is a
prerequisite for achieving high IPCE by providing a large
surface area. The difference in IPCE of DSSCs based on
single-crystal TiO
2
and mesoporous TiO
2
was reported
by Grätzel et al. [43]. The IPCE value of single-crystal
TiO
2
photo-anode is only 0.13% near 530 nm, whereas
mesoporous TiO
2
has the IPCE value of 88%, which is
more than 600 times higher. The significant improvement
is attributed to the far better LHE of mesoporous film
compared with the single-crystal electrode, and the texture
of mesoscopic film is favourable for photo-generation and
charge collection [44].
2.3.3. Electrochemical impedance spectroscopy
(EIS)
Steady state I-Vmeasurement only provides limited
information of electron transport and recombination rates
in DSSC. In order to understand the kinetics of charge
transfer in DSSC, EIS, which is a dynamic technique, is
needed. In EIS, the potential applied to the system is
perturbed by small sine wave to generate sinusoidal
alternating potential, and the sinusoidal current output
(as represented in amplitude and phase shift) is measured
against variation in modulation frequency [34]. The
impedance is defined as the ratio of voltage to current in
frequency domain and consists of both real and imaginary
parts. The impedance of a resistor has a real value and is
independent of the frequency. On the other hand, the
impedances of capacitor and inductor have imaginary
values and vary with frequency. A DSSC can be
electrically viewed as a combination of resistances and
capacitances. Therefore, EIS is used to identify the
resistances and capacitances of each component in DSSC
during the charge transfer processes, which include (1)
series resistance (R
s
) due to the sheet resistance of TCO
substrate and contact resistance, (2) recombination
resistance (R
rec
) at the semiconductor/dye/electrolyte
interface, (3) redox couple diffusion in electrolyte, (4)
charge transfer resistance (R
ct
) of counter electrode and
(5) capacitance (C
μ
) of mesoporous metal oxide film and
more [39,40].
3. PHOTO-ANODE
3.1. General introduction to photo-anode
material
Photo-anode is an important component in DSSC as it
functions as a matrix for dye adsorption and also acts as
charge transport medium for collection and transportation
of electrons from dye molecules to TCO substrate. In order
to achieve optimum dye adsorption and smooth
transportation of electrons without undergoing
recombination, a photo-anode should possess several
characteristics as identified from literature: (1) high surface
area for maximum dye loading and hence effective light
absorption, (2) high transparency to reduce the loss of
incident photon, (3) high electron mobility to facilitate
electron transport, (4) does not react with redox electrolyte
to minimize recombination rate and (5) contains hydroxyl
group or defects for the attachment of dye molecules onto
its surface [28,45]. Many conventional semiconductor
materials have been utilized as photo-anode, including
mono-Si, poly-Si, GaAs, CdS and InP. However, those
materials suffer from photo-degradation in DSSC due to
reaction with electrolyte, which reduces the cell lifetime
considerably [3,45]. TiO
2
and ZnO were found to be the
most promising semiconductor materials as photo-anodes
in DSSC. However, due to the wide band gap of ZnO
(e:g:e3:3eVÞ[46,47] and TiO
2
(e:g:e3:2eVÞ[48], both
materials only absorb ultraviolet part of solar spectrum
and do not absorb visible light [28]. Therefore,
modifications of ZnO and TiO
2
through doping with
cations and anions are required to modulate the band
structure and shift the absorption spectra [49]. Besides
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

ZnO and TiO
2
, other metal oxides such as tin dioxide
SnO2
ðÞ, niobium pentoxide (Nb2O5Þand zinc stannate
(Zn2SnO4Þhave been studied as photo-anode materials
for DSSC application as well.
3.2. Titanium dioxide
Titanium dioxide (TiO
2
) is the most popular photo-anode
material for DSSC due to its numerous superior
characteristics such as high chemical stability, mesoporous
nature and low toxicity [50,51]. Besides, TiO
2
has higher
CB edge, electron affinity, dye loading and surface area
compared with other transition metal oxides, which makes
it the most suitable choice as photo-anode for DSSC
application [45]. TiO
2
exists in three primary crystalline
forms, namely anatase (tetragonal), brookite
(orthorhombic) and rutile (tetragonal) [49]. Rutile is the
most stable form thermodynamically. However, anatase is
more preferred in DSSC application due to its better
efficiency for solar energy conversion and photo-catalysis
[52,53]. As reported by Zhang et al., anatase belongs to
indirect band gap semiconductor category, while rutile
and brookite are direct bang gap semiconductors [54].
Due to indirect band gap of anatase, it is impossible for
photoexcited electrons to undergo direct transition from
CB to valence band (VB) of anatase. As a consequence,
the lifetime of photoexcited electrons is longer in anatase
compared with rutile and brookite. Besides, the average
effective mass of photoexcited electrons of anatase is also
the lightest among the three polymorphs. This allows faster
migration of photoexcited electrons and hence lower
recombination rate in anatase compared with rutile and
brookite [54]. Park et al. reported that anatase-based cell
has higher short-circuit photocurrent (J
sc
) than rutile-based
cell, while the open-circuit voltage (V
oc
) is the same for
both cases [55]. Lower photocurrent in rutile film is
attributed to its smaller surface area compared with anatase
film, which results in lesser amount of adsorbed dye.
Anatase TiO
2
is used in following discussions unless stated
otherwise.
In recent years, a wide variety of TiO
2
nanostructures
have been synthesized as photo-anode for DSSC
application such as nanoparticles, nanorods, nanowires,
nanotubes and nanosheets [28]. Several fabrication
methods such as sol gel, hydrothermal, spray pyrolysis,
electrochemical deposition, spin coating and chemical
vapour deposition have been reported for the synthesis of
various TiO
2
architectures [45]. Besides, the feasibility of
assembling flexible DSSC with the employment of flexible
substrate such as ITO/poly(ethylene naphthalate) substrate
in place of FTO glass has been explored as well [56,57].
3.2.1. Effect of interfacial modification
In conventional DSSC, the nanoporous nature of TiO
2
film is essential to provide a large surface area for dye
loading and light absorption [58]. However, it also offers
large TiO
2
surface site (direct route) and FTO surface site
(indirect route), which serves as recombination sites at
TiO
2
/electrolyte and FTO/electrolyte interfaces [59].
Recombination of photo-injected electrons with I
3

species
in electrolyte will cause the loss of photocurrent,
decreasing the PV performance of DSSC as a consequence
[58]. Introduction of a blocking/compact layer at FTO/
TiO
2
interface has been proven experimentally to be
effective at suppressing the electron recombination.
Besides blocking layer (BL), the DSSC performance
can be improved by coating a light-scattering layer over
the TiO
2
film in order to increase the optical absorption
of photo-anode [60]. Generally, the bilayer structure
consists of TiO
2
nanoparticles as underlying layer and
TiO
2
nanostructures with high surface area such as
nanotubes, nanowires and nanofibers as over-layer or
light-scattering layer. Besides improving the light
absorption, the light-scattering layer can reduce the grain
boundaries and enhance the dye adsorption due to its high
surface area [61].
In the forthcoming section, the influences of the
addition of BL and light-scattering layer on the DSSC
performance are discussed in detail based on the findings
from literature.
3.2.1.1. Inclusion of blocking layer. Several
materials have been utilized as BL for the fabrication of
DSSC, including TiO
2
,Nb
2
O
5
, SnO
2
and ZnO. TiO
2
has
been investigated most frequently among all the materials.
Góes et al. reported efficiency improvement up to ~20%
with the introduction of 50 nm thick TiO
2
BL at
FTO/TiO
2
interface by using radio frequency sputtering
[62]. From impedance spectroscopy analysis, the
efficiency enhancement was attributed to the reduced
recombination at FTO/electrolyte interface and
improvement in electronic contact at FTO/TiO
2
interface.
Choi et al. adopted a different approach in deposition of
TiO
2
BL, which is by hydrolysis of TiCl
4
aqueous solution
[63]. Efficiency improvement from 4.15 to 5.16% was
observed for optimized thickness at 25 nm, which was
ascribed to the suppressed electron recombination at
FTO/electrolyte interface due to increase of interfacial
charge-transfer resistance. Deposition of ultrathin TiO
2
BL (<10 nm) was investigated by Kim et al. by using
atomic layer deposition (ALD) [64]. Increase of electron
lifetime and retardation of V
oc
were observed, confirming
the retardation of electron recombination due to the
presence of BL. Prevention of back electron transfer by
F-doped TiO
2
BL in DSSC was also reported by Noh
et al. [59].
Cho et al. investigated the effect of Nb
2
O
5
BL prepared
by sol–gel method in flexible DSSC [65]. Significant
efficiency improvement from 4.43 to 6.19% was observed
after deposition of Nb
2
O
5
BL under optimum gel
processing conditions. It was found that the heat treatment
temperature and gel processing conditions have significant
impact on DSSC performance. Same fabrication technique
was also reported by Chun et al., and efficiency
enhancement from 2.87 to 3.28% was discovered [66].
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Suresh et al. compared the difference between crystalline
and amorphous Nb
2
O
5
BL on the performance of DSSC
[67]. It was found that the crystalline structure of Nb
2
O
5
BL has significant effect on DSSC performance by
suppressing electron recombination from FTO to
electrolyte through the formation of energy barrier.
Maximum efficiency of 6.94% was achieved at optimum
thickness of ~40 nm. When the thickness exceeded
40 nm, the DSSC performance was hindered due to the
decreased tunnelling probability.
Besides TiO
2
and Nb
2
O
5
, other materials such as SnO
2
[68] and ZnO [69,70] have been utilized as BL as reported
in literature. Table I summarizes the published BL
deposition techniques for different types of materials
reported to date.
3.2.1.2. Inclusion of light-scattering layer. Yu
et al. investigated DSSC performance based on four
configurations of bilayer composite film, namely
nanoparticles/nanoparticles, nanoparticles/hollow spheres,
hollow spheres/nanoparticles and hollow spheres/hollow
spheres [71]. It was discovered that the
nanoparticles/hollow spheres configuration exhibited the
highest efficiency among four configurations. The
efficiency improvement was attributed to the light-
scattering effect of the TiO
2
hollow spheres, which
enhanced the light harvesting of DSSC. Besides, the high
surface area of TiO
2
hollow spheres also facilitated dye
loading and transfer of electrolyte solution. Dadgostar
et al. reported the fabrication of DSSC with TiO
2
hollow
spheres and TiO
2
filled spheres as light scatterers [72].
TiO
2
hollow spheres were synthesized by using liquid
phase deposition of TiO
2
on carboxylate-rich carbon
microspheres with carbon microspheres as sacrificial
templates. Higher efficiency was attained by DSSC with
TiO
2
hollow spheres as light scatterers due to higher
surface area, which allowed for higher dye adsorption
and efficient light scattering.
Bakhshayesh et al. fabricated DSSC based on double
layer composite film with anatase-TiO
2
nanoparticles and
anatase-TiO
2
nanowires as under-layer and over-layer
respectively [73]. The double layer solar cell shown
improved performance compared with monolayer solar cell
due to efficient light scattering, better dye adsorption and
higher electron lifetime. Similar explanation for efficiency
improvement was also reported by Liu et al. by using TiO
2
nanorod aggregates as light-scattering over-layer [74].
Apart from the stated nanostructures, other TiO
2
nanomaterials such as nanofibers [75] and nanotubes [61]
were also reported in literature as summarized in
Table II. Recently, Song et al. investigated the influence
of CeO
2
microspheres as scattering layer on the PV
performance of DSSC [76]. Enhanced dye loading and
increased diffuse reflectance were observed after the
introduction of porous CeO
2
microspheres. Besides, the
DSSC performance was further improved through
the deposition of compact TiO
2
film on the TiO
2
–CeO
2
bilayer by using ALD. In overall, the DSSC with ALD-
modified TiO
2
–CeO
2
photo-anode (η= 9.86%) showed
31% improvement in PCE compared with reference TiO
2
photo-anode (η= 7.52%).
3.2.2. Effect of doping
3.2.2.1. Doping with metallic cations. TiO
2
doped with different metallic cations have been reported
in various literature. Durr et al. had studied DSSC
performance based on Zr-doped TiO
2
photo-anode [77].
It was discovered that increasing the amount of Zr doping
led to increased V
oc
, which was correlated to the shift of
CB edge towards higher energy. However, higher CB also
reduced the electron injection due to decreased driving
force, resulting in lower PCE. The optimum doping
concentration was found to be 1% Zr content, which led
to PCE of 8.1% compared with 7.0% of pure TiO
2.
According to Zhang et al., W doping attenuated charge
recombination and increased the driving force for electron
injection through the positive shift of CB, leading to
improved J
sc
and highest PCE of 9.1% with 0.2% doping
concentration [78]. On the other hand, Tong et al. gave a
different explanation about the role of W doping in TiO
2
photo-anode [79]. Instead of shifting the CB edge, W
doping introduced an intermediate band between VB and
CB of TiO
2
. The intermediate band acted as a medium that
allowed low-energy photon such as infrared light to excite
electrons from VB to CB. Therefore, photo-conversion is
applicable to infrared light as well and not only limited to
high-energy photon. Besides, the formation of intermediate
band also improved the electron lifetime and electron
transport, resulting in enhancement of J
sc
and PCE. Ko
et al. also observed enhancement in PCE through W
doping in TiO
2,
which was correlated to the modifications
of electrical surface-state induced by W doping [80].
Chandiran et al. reported that aliovalent doping of TiO
2
photo-anode with Nb increased the bandgap (BG) of the
material and inhibited back electron transfer to the
electrolyte [81]. As a consequence, the charge collection
efficiency was improved, which led to increased PCE from
Table I. Summary of blocking layer for DSSCs.
Material
Deposition technique
of blocking layer
η
(%) Reference
TiO
2
Radio frequency sputtering 5.81 [62]
TiO
2
Hydrolysis of TiCl
4
aqueous
solution
5.16 [63]
TiO
2
Atomic layer deposition 8.50 [64]
TiO
2
Hydrolysis of TiCl
4
aqueous
solution with NH
4
F
5.24 [59]
Nb
2
O
5
Sol–gel method 6.19 [65]
Nb
2
O
5
Sol–gel method 3.28 [66]
Nb
2
O
5
Radio frequency sputtering 6.94 [67]
SnO
2
Nanocluster deposition 8.38 [68]
ZnO Layer-by-layer chemical method 4.51 [69]
ZnO Spin-coating 6.70 [70]
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

7.4 to 8.1%. Enhanced electron injection and transport
were also observed by Lu et al. by using highly doped
(2.5–7.5 mol% Nb) TiO
2
photo-anode [82]. This was
attributed to the increased conductivity of TiO
2
powder
due to positive shift of its flat band potential (V
fb
) after
Nb doping. However, the findings of Nikolay et al.
contradicted with the results of Lu et al. According to the
studies of Nikolay et al., high Nb doping concentration
(≥2.5 mol%) degraded the V
oc
parameter due to the
narrowing of space charge region, resulting in lower PCE
[83]. They predicted that the optimum Nb doping
concentration is within the range of 1.5–2.5 mol% in order
to obtain highest PCE.
Kim et al. fabricated DSSC with Cr-doped TiO
2
as
second layer on top of TiO
2
photo-anode, creating a
Cr–TiO
2
/TiO
2
/FTO film [84]. DSSC with double layer
showed 18.3% improvement in PCE compared with
TiO
2
/FTO film, which was attributed to the attenuation of
electron recombination at double layer. Latini et al.
reported that anatase-based DSSC with 0.2 at.% scandium
(Sc) doping exhibited high efficiency of 9.6% [85].
TiO
2
doped with other metallic dopants such as Ce4þ
[86], Sn4þ[87], Sb3þ[88], In3þ[89], Ga3þ[90] and Y3þ
[90] were also reported in various literature. The PV
performance of DSSC based on TiO
2
photo-anode doped
with different types of metallic cations is summarized in
Table III.
3.2.2.2. Doping with non-metallic anions.
Besides metallic cation doping, several research groups
have studied TiO
2
photo-anode doped with non-metallic
elements such as nitrogen (N), boron (B), fluorine (F),
carbon (C) and sulphur (S). Ma et al. reported that the
efficiency improvement of DSSC through N doping was
attributed to the increased light absorption in visible light
region, enhanced light scattering and higher dye uptake
[91]. Increased dye adsorption was also observed by Guo
et al. in N-doped TiO
2
photo-anode [92,93], leading to
higher J
sc
and thus higher cell efficiency.
B doping in TiO
2
nanotube photo-anode was studied by
Subramanian et al. [94]. Enhanced performance was
observed in B-doped TiO
2
nanotube (η= 3.44%) compared
with undoped TiO
2
nanotube (η= 3.02%). The efficiency
improvement was ascribed to longer electron lifetime and
faster electron injection due to better matching of CB of
TiO
2
nanotube and LUMO of the dye molecules. Tian
et al. also reported improved electron injection kinetics in
B-doped TiO
2
photo-anode, which was attributed to the
better crystallinity of TiO
2
after doping [95].
Song et al. investigated F doping in TiO
2
hollow
spheres as scattering layer for DSSC [96]. The efficiency
was improved from 5.62 to 6.31% compared with undoped
TiO
2
hollow spheres. The improvements were claimed to
be due to higher electrolyte diffusion ability, faster electron
transport and reduced recombination at TiO
2
/electrolyte
interface. Saadi et al. also fabricated C-doped TiO
2
hollow
spheres by using hydrothermal method as scattering layer
in DSSC [97]. Highest efficiency of 8.55% was achieved
for DSSC with C-doped TiO
2
hollow spheres due to the
reduced recombination, better dye adsorption and efficient
light scattering.
Sun et al. reported 24% improvement in cell efficiency
by doping TiO
2
photo-anode with S using mechanical ball
mining method [98]. Positive shift of CB edge and
reduction of optical BG energy were observed in S-doped
TiO
2
photo-anode. The authors claimed that S-doped
TiO
2
photo-anode gave better photoelectric response in
visible light region, and hence resulting in higher PCE.
Expanded visible light harvesting was also observed by
Hou et al. in I-doped TiO
2
nanocrystals from IPCE and
UV–vis spectra, improving the efficiency by 42.9%
compared with undoped TiO
2
nanocrystals [99].
The PV performance of DSSC based on TiO
2
photo-
anode doped with different types of non-metallic anions
is summarized in Table IV.
Table II. Summary of light-scattering layer for DSSCs.
TiO
2
nanostructure Synthesis method of light-scattering layer η(%) Reference
Hollow spheres Chemically induced self-transformation 5.28 [71]
Hollow spheres Liquid phase deposition of TiO
2
on carbon microspheres 8.30 [72]
Nanowires Hydrothermal method 6.34 [73]
Nanorod aggregates Hydrothermal method 6.10 [74]
Nanofibers Electrospinning 8.40 [75]
Nanotubes Anodic oxidation of Ti sheet in perchloric acid solution 7.53 [61]
Table III. Photovoltaic performance of DSSC based on TiO
2
photo-anode doped with different types of metallic cations.
Dopant (Cation)
J
sc
(mA/cm
2
)
V
oc
(mV) FF η(%) Reference
Zirconium 16.50 715 0.69 8.10 [77]
Tungsten 19.31 610 0.77 9.10 [78]
Tungsten 15.10 730 0.67 7.42 [79]
Niobium 16.30 735 0.72 8.70 [81]
Niobium 17.67 700 0.63 7.80 [82]
Niobium 16.32 730 0.68 8.00 [83]
Chromium 15.20 780 0.71 8.40 [84]
Scandium 19.10 752 0.68 9.60 [85]
Cerium 13.50 781 0.73 7.66 [86]
Tin 16.01 722 0.71 8.31 [87]
Antimony 18.72 635 0.68 8.13 [88]
Indium 16.97 716 0.61 7.48 [89]
Gallium 13.40 755 0.79 8.10 [90]
Yttrium 15.90 739 0.77 9.00 [90]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Besides doping with metallic and non-metallic ions,
incorporation of TiO
2
with light-transmitting materials
such as SiO
2
[100–102], GeO
2
[103] and CaF
2
[104] in
order to enhance the light harvest and hence improving
the cell efficiency, has been reported in literature.
3.2.3. Effect of morphology
Typically, the photo-anode of DSSC is composed of
mesoporous TiO
2
nanoparticle films that possess large
surface area for efficient dye adsorption. However, further
enhancement in DSSC efficiency has been impeded by the
short electron diffusion length (10–35 μm) and random
electrical pathway induced by the substantial trapping
and detrapping phenomena that take place within excessive
surface states, defects and grain boundaries of
nanoparticles [105]. Besides, the lattice mismatches at the
grain boundaries and disorganized stacking of TiO
2
films
also impose limitation to electron transport and shorten
electron lifetime [106]. Therefore, one-dimensional (1D)
nanostructures such as nanowires, nanotubes and nanorods
have been investigated as photo-anode material owing to
their excellent light-scattering ability and electron
transport. However, the main limitation of these 1D
nanostructures is their poor dye loading due to low surface
area. The following section summarizes the recently
reported findings on 1D TiO
2
nanostructure photo-anode.
3.2.3.1. Nanotubes. Wang et al. reported the
fabrication of DSSC with electrospun TiO
2
nanotubes as
photo-anode [107]. The efficiency of DSSC based on
TiO
2
nanotubes (η= 3.33%) was lowered than TiO
2
nanoparticles (η= 5.98%) due to poor dye loading, which
decreased the J
sc
significantly. The influence of the
incorporation of carbon nanotube-graphene-TiO
2
nanoparticles (CNT-G-TiO
2
NPs) into TiO
2
nanotube
arrays was studied by Zhao et al. [108]. The improvement
in efficiency due to increased dye adsorption and electron
transport rate was observed through the incorporation of
0.1 wt% CNT-G composite material. However, the
efficiency decreased when the optimum concentration of
CNT-G composite material was exceeded due to electron
recombination and light shielding.
Yip et al. investigated the effect of coupling a photonic
crystal (PC) layer to the TiO
2
nanotube absorbing layer
[109]. The PC layer was fabricated by periodic current
pulse anodization, while the TiO
2
nanotube layer was
obtained by normal electrochemical anodization.
Efficiency enhancement of over 50% was achieved
(5.61% vs 3.66%) by comparing DSSCs with and without
the PC layer. The improvement was ascribed to the
enhanced light harvesting of DSSC and longer wavelength
range due to the presence of PC layer. Same fabrication
method was also reported by Guo et al. for TiO
2
nanotube
aperiodic photonic crystal [110]. The reason for using
aperiodic phonic crystal instead of periodic PC is to extend
the range of light reflection in order to maximize the light
harvesting. Nearly full-visible-spectrum light harvesting
from red to purple was achieved, as verified from
simulations and experimental results.
3.2.3.2. Nanowires. Wu et al. reported the
synthesis of hierarchically anatase TiO
2
nanowire arrays
consisting of long TiO
2
nanowire trunk and short TiO
2
nanorod branches as photo-anode for DSSC [106]. High
efficiency of 7.34% was achieved due to large surface area
of hierarchical TiO
2
nano-architecture arrays for dye
adsorption and light scattering, which led to significant
improvement in photocurrent. Increase in surface area
through the growth of secondary nanobranches on the
backbone TiO
2
nanowire was also observed by Lee et al.
[111]. The branched TiO
2
nanowire DSSC achieved
efficiency of almost four times of the non-branched TiO
2
nanowire DSSC.
Double layer structure with rutile nanowires
incorporated with anatase nanoparticles as under-layer
and disordered spherical voids as over-layer was fabricated
by Sun et al. as photo-anode in DSSC [112]. The bilayer
structure exhibited excellent light-scattering capability
and efficient dye loading, which resulted in significant
enhancement of efficiency (η= 4.07%) compared with
1D TiO
2
nanowire DSSC (η= 1.14%). Remarkably high
DSSC efficiency of 9.40% was reported by Wu et al. by
using multi-layered anatase TiO
2
nanowire arrays as
photo-anode [105]. The multi-layered configuration
consisted of three layers –densely packed TiO
2
nanowires
as bottom layer, hierarchical TiO
2
nanowires with short
nanorod branches as intermediate layer and finally loosely
packed TiO
2
nanowires as upper layer. Such configuration
allowed the combination of desirable features that were
previously incompatible such as high surface area,
excellent light scattering and fast electron transport.
3.2.3.3. Nanospindles. Qiu et al. developed
double-layered anatase TiO
2
nanospindles photo-anode
for DSSC [113]. In this bilayer configuration, one layer
was made of larger nanospindles for enhanced light
scattering, while the other layer consisted of smaller
nanospindles to increase roughness factor and hence better
dye loading. High efficiency of 8.3% was achieved based
Table IV. Photovoltaic performance of DSSC based on TiO
2
photo-anode doped with different types of non-metallic anions.
Dopant
(Anion)
J
sc
(mA/cm
2
)
V
oc
(mV) FF η(%) Reference
Nitrogen 17.90 690 0.62 8.00 [91]
Nitrogen 19.05 778 0.68 10.10 [92]
Nitrogen 12.65 796 0.72 7.27 [93]
Boron 7.85 660 0.66 3.44 [94]
Boron 14.10 691 0.62 6.10 [95]
Fluorine 11.00 754 0.76 6.31 [96]
Carbon 20.38 730 0.57 8.55 [97]
Sulphur 14.60 659 0.68 6.91 [98]
Iodine 14.10 715 0.67 7.00 [99]
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

on double-layered photo-anode, and this value was even
higher than standard TiO
2
nanoparticle photo-anode based
on Degussa P25 (η= 5.8%). Wu et al. investigated the use
of anatase nanospindles as bridges to connect the
hierarchical TiO
2
microspheres in DSSC [114]. It was
discovered that the connectivity of the film was enhanced
through the introduction of the nanospindles, leading to
improved electron diffusion coefficient. Deposition of
spherical anatase TiO
2
covered with nanospindles as
light-scattering layer on P25 photo-anode was studied by
Xue et al. [115]. Efficiency improvement from 5.14 to
6.40% was observed compared with P25 photo-anode,
which was attributed to the scattering effect and better
dye loading.
3.2.3.4. Nanorod. Yang et al. introduced the
multi-walled carbon nanotubes (MWCNTs) inside TiO
2
nanorods as photo-anode for DSSC [116]. The charge
transport rate was enhanced through the incorporation of
MWCNTs, leading to very high efficiency of 10.24 at
0.1% MWCNT concentration. However, further increase
in MWCNT concentration reduced the efficiency due to
decreased dye adsorption of the photo-anode. Chen et al.
fabricated hierarchical porous TiO
2
nanorods by using
microemulsion electrospinning method as DSSC photo-
anode material [117]. Longer electron lifetime, strong light
scattering and fast electron diffusion efficiency of TiO
2
nanorods compared with P25 nanoparticles were verified
by measurements. However, the efficiency of TiO
2
nanorods (η= 6.07%) was lower than P25 nanoparticles
(η= 7.11%) due to poor dye loading. Nevertheless, it
was discovered that the efficiency can be further improved
to 8.53% when TiO
2
nanorods were grown on P25
nanoparticles as light-scattering layer. Yang et al. studied
the synthesis of self-branching anatase TiO
2
nanorods 3D
hierarchical nanostructures for DSSC application [118].
Interestingly, high dye uptake density was observed in this
configuration due to the large percentage of exposed {010}
facets, and high efficiency of 7.17% was reported.
Besides the mentioned TiO
2
nanostructured photo-
anode, other TiO
2
nanostructures such as hollow spheres
[119,120] and nanoplates [121] were also reported in
literature, which are summarized in Table V.
3.3. Zinc oxide
Zinc oxide (ZnO) is a wide band gaps semiconductor that
is almost identical to TiO
2
in terms of energy band
structure and physical properties [122]. The most
distinctive advantage of ZnO is that ZnO has a very high
electron mobility, which is more than 1 order of magnitude
higher than anatase TiO
2
[50]. High electron mobility is
essential in facilitating electron transport by reducing the
recombination loss [122]. However, the efficiency of
ZnO-based DSSC is generally lower than that of TiO
2
,
which may be attributed to the slower electron injection
kinetics and instability of ZnO in acidic dye [45].
Nevertheless, ZnO is still regarded as a distinguished
alternative to TiO
2
due to its anisotropic growth and ease
of crystallization [122]. These characteristics allow ZnO
to be synthesized in a wide variety of nanostructures,
which can provide unique properties for photo-catalysis.
3.3.1. Effect of interfacial modification
Similar with TiO
2
-based DSSC, the efficiency of
DSSCs based on ZnO nanostructures is limited by the
charge recombination at photo-anode/electrolyte interface
[123]. Besides, ZnO is chemically unstable in acidic
solution and prone to being dissolved, resulting in the
formation of Zn
2+
/dye complexes [124]. The
agglomeration of Zn
2+
/dye complexes will form a barrier
layer on ZnO surface, preventing electron injection from
dye to ZnO CB [125]. To resolve such issue, the proposed
Table V. Various TiO
2
nanostructures and their respective efficiencies as photo-anodes in DSSC applications.
TiO
2
nanostructure Photo-anode η(%) Reference
Nanotubes TiO
2
nanotubes 3.33 [107]
Nanotubes CNT-G-TiO
2
nanoparticles/TiO
2
nanotube double-layer structure photo-anode 6.17 [108]
Nanotubes TiO
2
nanotube photonic crystal 5.61 [109]
Nanotubes TiO
2
nanotube aperiodic photonic crystal 7.87 [110]
Nanowires Hierarchically anatase TiO
2
nanowire arrays with TiO
2
nanorod branches 7.34 [106]
Nanowires Hierarchically anatase TiO
2
nanowires with densely packed branches 6.20 [111]
Nanowires Double layer TiO
2
nanowire-nanoparticle/spherical void structure 4.07 [112]
Nanowires Multi-layered anatase TiO
2
nanowire arrays 9.40 [105]
Nanospindles Double-layered anatase TiO
2
nanospindles 8.30 [113]
Microsphere/ nanospindles Hierarchical TiO
2
microspheres embedded with anatase nanospindles 8.50 [114]
Nanospindles Spherical anatase TiO
2
embedded with nanospindles on P25 photo-anode 6.40 [115]
Nanorods TiO
2
nanorods embedded with multi-walled carbon nanotubes 10.24 [116]
Nanorods Hierarchical porous TiO
2
nanorods 6.07 [117]
Nanorods Self-branching anatase TiO
2
nanorods 3D nanostructures 7.17 [118]
Hollow spheres Mesoporous TiO
2
hollow spheres 1.20 [119]
Hollow spheres/nanotube Bilayer TiO
2
hollow spheres/TiO
2
nanotube arrays 6.90 [120]
Nanoplates Hierarchical anatase TiO
2
nanoplates 6.53 [121]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

solution is to cover the surface of ZnO nanostructure with a
chemically stable shell in the so-called core-shell
configuration. The coating of several shell materials such
as TiO
2
, SnO
2
,Al
2
O
3
and SiO
2
have been reported in
literature. Among all the alternatives, TiO
2
is the most
investigated material and is considered as the most
promising candidate for shell material. The influences of
the coating of shell materials on ZnO nanostructure are
discussed in detail in forthcoming section.
3.3.1.1. Core-shell structure of ZnO DSSC.
Prabakar et al. reported the fabrication of ZnO
nanorods/TiO
2
and ZnO nanoflowers/TiO
2
core-shell
structures by sol–gel deposition [126]. It was found that
V
oc
and J
sc
were improved significantly due to the
suppressed recombination after the introduction of TiO
2
shell as an insulating layer. Enhancement in V
oc
and J
sc
due to reduced recombination was also reported by Goh
et al. in ZnO nanorods/TiO
2
core-shell structure [124].
However, the decrease in J
sc
and increase in V
oc
were
observed by Pazoki et al. in ZnO nanowires/TiO
2
core-
shell structure by using density functional rheory
calculation [127]. These phenomena can be explained by
the shift of CB and VB towards higher energies after
TiO
2
coating. When the semiconductor CB is shifted
upward, injection efficiency will decrease due to lower
driving force for electron transfer between the
semiconductor and excited dye, resulting in decreased J
sc
.
On the other hand, V
oc
will increase due to the positive
shift of semiconductor Fermi level, which corresponds
with the shift of CB and VB. Raj et al. investigated the
electrochemical properties of DSSC based on TiO
2
encapsulated ZnO nanorod aggregates [128]. High electron
diffusion length and chemical capacitance were observed
in TiO
2
/ZnO DSSC compared with bare ZnO DSSC,
which were attributed to the large electron transfer and
collection. Besides, the introduction of TiO
2
layer
improved the surface contact between ZnO and FTO glass,
leading to enhanced dye adsorption. The protection of ZnO
nanowires by TiO
2
shell against dissolution in N3 dye was
confirmed by Yeh et al. from SEM images [125]. Without
the presence of TiO
2
shell, the pristine ZnO nanowires film
was destroyed after immersion in N3 dye for 15 min. On
the other hand, the ZnO nanowires/TiO
2
film remained
intact after immersion period of 2 h.
Efficiency improvement from 2.87 to 4.71% was
reported by Zhou et al. by incorporating SnO
2
as shell
material on ZnO nanoneedle arrays [129]. Increased dye
adsorption due to larger surface area and reduced
recombination were observed after introduction of SnO
2
layer. Qin et al. investigated the coating of Al
2
O
3
and
SiO
2
on ZnO nanowire arrays as shell materials in DSSC.
It was discovered that surface modification with Al
2
O
3
and SiO
2
can enhance the acid stability of ZnO nanowire
arrays by suppressing the formation of Zn
2+
/dye
complexes [130]. Guillén et al. reported the fabrication of
ZnO/ZnO core shell structure photo-anode with ZnO
nanowires as core and ZnO nanocrystalline layer as shell.
Characterization studies using intensity-modulated
photovoltage spectroscopy and impedance spectroscopy
revealed remarkable suppressed recombination in core-
shell structure. Table VI summarizes the recent
development of ZnO-based DSSC with core-shell
structure.
3.3.2. Effect of doping
3.3.2.1. Doping with metallic cations. Similar
with TiO
2
, ZnO doped with different cations had been
reported in various literature. Raj et al. fabricated Mg-
doped ZnO photo-anode with a banyan-root structure by
using single-step drop casting method [131]. Improved
efficiency (η= 4.11%) was obtained for 5 mol% Mg-doped
ZnO compared with bared ZnO (η= 1.97%). The
continuous and porous root-like structure in Mg-doped
ZnO promoted large electron transport, improved LHE
and increased optical path length and high electrolyte
diffusion, which led to improved performance compared
with bared ZnO. Guo et al. investigated Mg doping in
nanosheet-based spherical structure ZnO photo-anode
using direct precipitation technique [132]. Improved
performance from 1.72 to 4.19% was observed, which
was ascribed to the increase of photovoltage due to BG
increment after Mg doping.
Tao et al. reported enhanced performance of DSSC
based on Al-doped ZnO nanorod arrays [133]. The
improved performance was attributed to the increase in
electron recombination resistance at
photoelectrode/electrolyte interface, resulting in the
decrease of dark current. Reduction in electron transport
resistance was observed with the increase of the diameter
of ZnO nanorods arrays by Al doping. Almost similar
efficiency (η= 0.3%) was also reported by Zhu et al. by
doping ZnO nanorod arrays with Al [134]. Reduced
electron recombination and enhanced electron lifetime are
two main factors that led to enhancement of DSSC
performance, which were confirmed by open-circuit
voltage decay and EIS characterizations.
According to Wang et al., Sn doping in ZnO spherical-
particle photo-anode enhanced dye adsorption due to the
decrease in particle size of Sn-ZnO as observed from field
Table VI. Summary of ZnO-based DSSC with core-shell
structure.
Shell
material
ZnO nanostructure
(core material) η(%) Reference
TiO
2
ZnO nanorods 3.10 [126]
TiO
2
ZnO nanorods 3.03 [124]
TiO
2
ZnO nanowires - [127]
TiO
2
ZnO nanorod aggregates 2.48 [128]
TiO
2
ZnO nanowires 1.17 [125]
SnO
2
ZnO nanoneedle arrays 4.71 [129]
Al
2
O
3
ZnO nanowire arrays 0.36 [130]
SiO
2
ZnO nanowire arrays 0.35 [130]
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

emission-SEM micrograph [135]. Table VII summarizes
the PV performance of DSSC based on ZnO photo-anode
doped with different types of metallic cations.
3.3.2.2. Doping with non-metallic anions.
Doping of ZnO photo-anode with anions has been studied
by several researchers as well. Mahmood et al. investigated
the DSSC performance based on N-doped ZnO nanorod
arrays [136]. It was observed that increasing the precursor
concentration and growth temperature enhanced the
morphological, structural and PL properties of nanorods,
which led to enhanced DSSC performance. The
performance improvement was also attributed to the
combined effects of increase of charge carrier
concentration and increase in Fermi energy level of
nanorods after N doping. Zhang et al. reported the
fabrication of N-doped ZnO using two methods, namely
annealing and solution methods [137]. N-doped ZnO
synthesized by using solution method (η= 2.64%) showed
better performance compared with annealing method
(η= 1.10%). Compared with undoped ZnO (η= 0.67%),
the efficiency improvement was observed by N doping,
which was ascribed to the reduced recombination rate.
According to Zheng et al., DSSC with I-doped ZnO
nanocrystalline aggregates as photo-anode exhibited
improved cell performance due to the longer electron
lifetime and reduced recombination in hierarchically
structured ZnO cell [138,139]. Efficiency of 3.43% was
reported by Luo et al. based on DSSC with F-doped ZnO
prism array [140]. Significant improved performance of
F-doped ZnO compared with bared ZnO photo-anode
(η= 1.04%) was attributed to the higher surface area,
prolonged electron lifetime, higher IPCE and stronger light
scattering. More importantly, the DSSC parameters
reported by Luo et al. were far from being optimized,
which mean that there is much room for improvement for
DSSC based on F-doped ZnO prism array.
Mahmood et al. fabricated DSSC based on double light-
scattering layer B-doped ZnO (DL-BZO) film, achieving
high efficiency of 7.20% [141]. The upper layer consisted
of submicron-sized BZO sphere arrays to facilitate electron
transport and to function as light-scattering centres. The
underlying layer consisted of nanoporous BZO
nanoparticulate film to increase dye adsorption and to
reduce electron recombination. DL-BZO film exhibited
improved performance compared with undoped ZnO
(DL-ZnO) film, which was ascribed to larger surface area
and improved high harvesting efficiency. Table VIII
summarizes the PV performance of DSSC based on ZnO
photo-anode doped with different types of non-metallic
anions.
3.3.3. Effect of morphology
To date, many different types of ZnO nanostructures
have been fabricated, including nanotubes, nanoparticles,
nanobelts, nanoclusters, nanowires, nanoflowers and
nanocolloids [3,142]. One-dimensional to two-dimensional
nanostructures of ZnO photo-anode provide a distinct
advantage in electron transport. They allow unidirectional
movement for photoexcited electrons in photo-anode
instead of random/zigzag movement, which facilitate rapid
injection of electrons from dye into semiconductor oxide
layer [143]. As reported by Law et al., the electron
diffusivity in ZnO nanowire DSSC was several hundred
times faster than ZnO or TiO
2
nanoparticle film [144].
ZnO nanowire also demonstrates several interesting
properties such as antireflective and light-trapping
properties, enhanced surface area and solution
processabilty as reported in literature [145]. Besides, 1D
nanostructures with large sizes can enhance the LHE due
to their superior light-scattering ability [146]. However,
1D ZnO nanostructures have limited surface area, which
inevitably lead to inferior J
sc
due to lower dye adsorption
[147].
3.3.3.1. Nanorod. Tan et al. investigated the
fabrication of DSSC based on ZnO nanorod arrays using
hydrothermal reaction [148]. It was found that the length
of ZnO nanorods increased by extending the hydrothermal
exposure time, which led to higher cell efficiency due to
large surface area for dye adsorption. Efficiency
enhancement from 3.08 to 4.13% was reported by Peng
et al. by replacing ZnO nanoparticles with ZnO
hierarchical nanorods as DSSC photo-anode. This was
attributed to significantly enhanced light scattering of
ZnO nanorods compared with ZnO nanoparticle, which
led to efficient light harvesting [146]. Longer electron
lifetime and reduced electron recombination of ZnO
nanorods were verified from EIS measurement as well.
Fang et al. studied the efficiency improvement of ZnO
Table VII. Photovoltaic performance of DSSC based on ZnO
photo-anode doped with different types of metallic cations.
Dopant
(cation)
J
sc
(mA/cm
2
)
V
oc
(mV) FF η(%) Reference
Magnesium 9.98 710 0.58 4.11 [131]
Magnesium 11.18 600 0.63 4.19 [132]
Aluminium 2.67 703 0.32 0.30 [133]
Aluminium 1.34 606 0.34 0.28 [134]
Tin 2.09 570 0.67 0.80 [135]
Table VIII. Photovoltaic performance of DSSC based on ZnO
photo-anode doped with different types of non-metallic anions.
Dopant
(anion)
J
sc
(mA/cm
2
)
V
oc
(mV) FF η(%) Reference
Nitrogen 15.00 620 0.53 5.00 [136]
Nitrogen 9.35 543 0.52 2.64 [137]
Iodine 14.10 ––4.50 [138]
Iodine 15.00 646 0.47 4.60 [139]
Fluorine 10.75 530 0.54 3.43 [140]
Boron 20.50 710 0.50 7.20 [141]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

nanorod DSSC through the optimization of dye adsorption
[149]. It was discovered that the density of nanorod arrays
was directly proportional to the thickness of seed layer. By
increasing the thickness of seed layer, the surface area of
nanorod arrays was increased and hence leading to more
dye loading. The concentration of adsorbed dye molecules
can also be improved by increasing the number of
adsorption/desorption cycle.
3.3.3.2. Nanowire. Ko et al. reported the
fabrication of high-density hierarchical ZnO nanowires
photo-anode with long-branched treelike ‘nanoforest’
configuration [150]. This study revealed that the branched
ZnO nanowires DSSC can achieve much higher efficiency
(η= 2.63) than conventional upstanding ZnO nanowires
(η= 0.45%). Efficiency improvement was ascribed to the
modified surface area, which allowed higher dye
adsorption and reduced recombination due to direct
conduction pathways. McCune et al. designed the DSSC
based on 3D multi-layered ZnO nanowire arrays with
unique ‘caterpillar-like’structure [151]. The ZnO nanowire
arrays were grown on ZnO nanofiber seed layer by using
spin coating method. It was found that increasing the
number of ZnO nanowire layers would enhance the LHEs
due to increased surface area and density, leading to
highest cell efficiency of 5.20%. Fan et al. investigated
the effect of annealing atmosphere on the performance of
ZnO nanowires DSSC [152]. The results revealed that the
argon-annealed ZnO nanowire DSSC exhibited better
properties than air-annealed ZnO nanowires in terms of
electrical conductivity, trap density and recombination
resistance, resulting in 30% increase in cell efficiency.
3.3.3.3. Nanosheet. Efficiency of 4.8% was
attained by Xu et al. by using hierarchical ZnO nanowire-
nanosheet structure in DSSC [153]. The hierarchical ZnO
structure consisted of ZnO nanosheet arrays with dense
nanowires grown on the arrays. This was due to the
consideration that ZnO nanosheet arrays alone may not
absorb the photons efficiently due to the presence of
inherent gaps in the morphology. In hierarchical ZnO
nanowire-nanosheet structure, the gaps were filled by
nanowire branches, which can offer larger internal surface
area and direct electron transport from branched nanowires
to nanosheet backbone. Lin et al. reported the fabrication
of ZnO nanosheets as photo-anode by using chemical bath
deposition (CBD) method for the formation of layered
hydroxide zinc carbonate on FTO substrate, followed by
pyrolysis process to transform layered hydroxide zinc
carbonate into ZnO nanosheets [154]. A very high
efficiency of 6.06% was achieved for ZnO nanosheet
DSSC, which was much higher than ZnO nanoparticles
DSSC (η= 2.92%). This was attributed to higher dye
loading and electron diffusion coefficient of ZnO
nanosheets compared with ZnO nanoparticles due to the
specific morphology of ZnO nanosheets. Meng et al.
adopted a different approach in synthesizing ZnO
nanosheets as DSSC photo-anode by using citric acid
assisted hydrothermal method [155]. However, much
lower cell efficiency of 1.82% was obtained by using this
method.
3.3.3.4. Other nanostructures. According to Xi
et al., ZnO nanotube arrays (η= 0.93%) exhibited better
cell efficiency than ZnO nanowire arrays (η= 0.23%) for
utilization as photo-anode material on ITO substrate in
DSSC [156]. However, it should be noted that the authors
had attained the optimum reaction conditions for the
growth of ZnO nanotube arrays prior to its implementation
into DSSC, while no optimization process was carried out
for ZnO nanowire arrays. Umar et al. synthesized ZnO
nanoflowers as photo-anode material for DSSC by using
hydrothermal method, and achieving cell efficiency of
1.38% [157]. Kilic et al. discovered the variation in the
morphology of ZnO nanostructures as a function of pH
value during the hydrothermal growth, from nanoflower
(pH 10.0) to nanowire (pH 11.0) [158]. Much higher
efficiency was attained for DSSC with 3D ZnO
nanoflowers (η= 5.12%) as photo-anode compared with
1D ZnO nanowires (η= 2.22%), which was attributed to
the superior light-absorbing ability of nanoflower structure
with much large larger surface area. Table IX summarizes
the DSSCs with different ZnO nanostructures as photo-
anode and their respective efficiencies.
3.4. Other photo-anode materials
Tin dioxide (SnO
2
) is another promising option, which
possesses two main advantages over TiO
2
, namely high
mobility and large band gap ( e:g:e3:9eVÞ[14,159].
Rashad et al. compared the PV performance difference
between two SnO
2
nanoparticle-based DSSCs synthesized
by using co-precipitation and solvothermal methods
respectively [160]. It was shown that solvothermal SnO
2
DSSC (η= 3.20%) exhibited better efficiency than co-
precipitation SnO
2
DSSC (η= 2.01%). This was attributed
to the faster electron transport in solvothermal SnO
2
film
than co-precipitation SnO
2
film, as verified by EIS
characterization. Asdim et al. reported the synthesis of
size-tunable SnO
2
nanocrystals by using microwave
hydrothermal method and its integration into DSSC as
photo-anode [161]. High IPCE of around 70% was
achieved for DSSC based on 26 nm SnO
2
nanocrystals,
resulting in PCE of 1.35%. Synthesis of macro-porous
SnO
2
using matrix-assisted method with polystyrene
spherical nanobeads as template was reported by Lee
et al. [162]. DSSC with macro-porous SnO
2
as photo-
anode achieved highest PCE of 1.53% with SnCl
4
(precursor) concentration at 0.1 M. It was discovered that
the crystallinity and surface area of macro-porous SnO
2
increased with increasing SnCl
4
concentration.
DSSCs with niobium pentoxide (Nb
2
O
5
) as photo-
anodes have been reported in literature. Jin et al. studied
the synthesis of Nb
2
O
5
microspheres by using the facile
solvothermal method [163]. Different diameters of Nb
2
O
5
Recent advances in photo-anode for DSSCs: A review Yeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

microspheres (200–900 nm) were obtained by adjusting
the NbCl
5
concentration. It was found that DSSC with
Nb
2
O
5
microspheres of 480 nm achieved highest PCE of
2.97% after the trade-off between LHE and electron
transport property. Rani et al. investigated the effect of
different thickness of Nb
2
O
5
nanochannelled photo-anode
on the PV performance of DSSCs [164]. Different
thicknesses of Nb
2
O
5
films from 5 to 25 μm were
synthesized by using anodization method. Highest PCE
of 4.48% was achieved for 10 μm thick Nb
2
O
5
film.
Further increase in film thickness resulted in faster electron
recombination and shorter electron lifetime as verified
from photovoltage decay and EIS characterization.
Fabrication of nanoforest Nb
2
O
5
photo-anode by using
pulsed layer deposition was reported by Ghosh et al.
[165]. It was discovered that the gas composition and
pressure during the deposition have significant influences
over growth of Nb
2
O
5
nanoforest. DSSC based on
optimized Nb
2
O
5
nanostructures photo-anode achieved
PCE of 2.41%.
Zinc stannate (Zn
2
SnO
4
) possesses several advantages
for its integration into DSSC as photo-anode material, i.e.
wide band gap, high electron mobility and fast electron
transport [166]. Wang et al. reported the fabrication of
hierarchically macroporous Zn
2
SnO
4
nanoparticles by
using polystyrene sphere template assisted hydrothermal
method [167]. The size of Zn
2
SnO
4
nanoparticles was
tuned from 180 to 650 nm by manipulating the sizes of
polystyrene spheres. Highest PCE of 5.01% was attained
for smallest Zn
2
SnO
4
nanoparticle size of 180 nm, which
was ascribed to faster electron transport, higher dye
loading and slower recombination rate. Surface treatment
of Zn
2
SnO
4
film using CBD method was studied by Chen
et al. [168]. Efficiency improvement from 3.04 to 3.45%
was observed after two CBD cycles, which was attributed
to the suppressed recombination rate and prolonged
electron lifetime. Hwang et al. investigated the fabrication
of hierarchically structured Zn
2
SnO
4
nanobeads by using
unique electrostatic spraying method and its integration
into DSSC [169]. The optimized DSSC achieved high
efficiency of 6.3% due to the unique morphology of
Zn
2
SnO
4
nanobeads that allowed improvements in light
scattering, dye adsorption, charge recombination lifetime
and electrolyte penetration.
4. FUTURE PROSPECT AND
SUMMARY
In the past two decades, extensive efforts have been
devoted to the development of DSSCs in order to achieve
efficiencies that are comparable to conventional silicon-
based solar cells. Photo-anode is considered as the most
crucial component of DSSC as it participates in most of
the fundamental processes of DSSC during its operation,
involving photon absorption to recombination and electron
transport. The processes involved by photo-anode are even
more than that of dye and electrolyte, and therefore, the
modifications of photo-anode have significant impact on
DSSC efficiency. Among various wide band gap
semiconductors, both TiO
2
and ZnO are considered as
promising candidates for the fabrication of DSSC photo-
anode. To date, the photo-anode for the DSSC with the
best performance is based on TiO
2
nanoparticles due to
superior surface area and dye adsorption. However, the
DSSC efficiency is still a distance away from the efficiency
of silicon solar cell. The main obstacle to the efficiency
improvement of DSSC is the energetic loss due to the
electron recombination at FTO/electrolyte interface. The
proposed solution is the introduction of blocking/compact
layer at FTO/semiconductor oxide interface or the
inclusion of core-shell configuration. Besides, it has been
experimentally verified that the introduction of light-
scattering layer on the surface of semiconductor oxide
layer can enhance the LHE of DSSC, and hence leading
to better cell efficiency. Modifications of TiO
2
and ZnO
by utilizing various metallic and non-metallic dopants
can further enhance DSSC performance by tuning the band
structure. At optimum doping concentration, the CB of
semiconductor oxide is shifted to the position such that
the photoexcited electrons can be injected efficiently from
LUMO of the dye to CB of semiconductor oxide.
However, the most suitable dopants for both TiO
2
and
ZnO photo-anodes have not been identified and the DSSC
Table IX. Various ZnO nanostructures and their respective efficiencies as photo-anodes in DSSC applications.
ZnO nanostructure Photo-anode η(%) Reference
Nanorods ZnO nanorod arrays 0.22 [148]
Nanorods Hierarchical ZnO nanorods with interconnected nanoparticles 4.13 [146]
Nanorods ZnO nanorod arrays 0.84 [149]
Nanowires Nanoforest of hierarchical ZnO nanowires 2.63 [150]
Nanowires 3D multi-layered ZnO nanowire arrays 5.20 [151]
Nanowires ZnO nanowires 1.62 [152]
Nanosheets Hierarchical ZnO nanowire-nanosheet structure 4.80 [153]
Nanosheets ZnO nanosheets 6.06 [154]
Nanosheets ZnO nanosheets 1.82 [155]
Nanotubes ZnO nanotube arrays 0.93 [156]
Nanoflowers ZnO nanoflowers 1.38 [157]
Nanoflowers 3D ZnO nanoflowers 5.12 [158]
Recent advances in photo-anode for DSSCs: A reviewYeoh M.-E. and Chan K.-Y.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

efficiencies obtained for different dopants are scattered.
Therefore, a further investigation is required to elucidate
the influences of different dopants on the band structure
and surface modification of photo-anode. Nanostructured
TiO
2
and ZnO that offer distinct advantage in electron
transport open up a new direction in efficiency
improvement for DSSC.
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