Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Nanoscience and
Nanotechnology Letters
Vol. 4, 471–482, 2012
Conformal Growth of Anodic Nanotubes
for Dye-Sensitized Solar Cells:
Part I. Planar Electrode
Lidong Sun1 2 , Sam Zhang1 ∗ , Qing Wang2 ∗ , and Dongliang Zhao3
1
School of Mechanical and Aerospace Engineering, Nanyang Technological University,
50 Nanyang Avenue, Singapore 639798, Singapore
2
Department of Materials Science and Engineering, Faculty of Engineering, NUSNNI-NanoCore,
National University of Singapore, Singapore 117576, Singapore
3
Central Iron and Steel Research Institute, No. 76 Xueyuan Nanlu, Beijing 100081, P. R. China
In this review, we dissect nanotube growth under a systematic changing of electrode configurations
and analyze relevant solar cell constructions as well as performances, in an attempt to explore
efficient approaches to harvest solar energy. It is divided into two parts for discussion: planar and
nonplanar electrodes, as a conformal coating of anodic nanotubes can be formed on an electrode
regardless of its geometric shape. The first part is presented in this paper. To date, the most efficient
dye-sensitized solar cells (DSCs) based on anodic nanotubes exhibit a power conversion efficiency
of 7∼8%, whereas those based on nanoparticles show a higher efficiency of 11∼12%. This is due
to a lower surface area per photoanode volume for nanotubes with respect to nanoparticles. It is
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calculated that, for a given
photoanodeOn:
volume,
requires
the nanotube
IP: 155.69.4.4
Wed,it 06
Feb 2013
10:26:23diameter to go down to
∼30 nm to generate a comparable
surface
area
with
nanoparticles
of ∼20 nm. For single-sided
Copyright American Scientific Publishers
tube growth, three dominant fabrication routes render two major cell configurations: backside and
frontside illuminations. The relevant cell structures and performances are discussed and compared.
For double-sided tube growth, a parallel DSC is constructed for doubled surface area and power
output.
Keywords: Dye-Sensitized Solar Cells, Nanotubes, Nanoparticles, Double-Sided.
CONTENTS
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Comparison of Surface Area between Nanoparticle and
Nanotube Photoanodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Single-Sided Tube Growth for General Solar Cells . . . . . . . . . .
4. Double-Sided Tube Growth for Parallel Solar Cells . . . . . . . . .
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
471
474
475
478
481
481
481
1. INTRODUCTION
One-dimensional nanostructures have generated worldwide
interest since Iijima discovered carbon nanotubes.1 Among
the diverse nanostructures and various materials, titanium
dioxide (titania) nanotube array has been studied intensively due to its semiconducting and functional features.
∗
Authors to whom correspondence should be addressed.
Nanosci. Nanotechnol. Lett. 2012, Vol. 4, No. 5
In 1996, Hoyer first reported the formation of titania nanotube arrays using an electrodeposition approach with the
assistance of nanoporous templates.2 In 1999, a columnar porous film was directly formed on titanium via electrochemical anodization in a fluorinated electrolyte by
Zwilling and co-workers.3 In 2001, fabrication of anodic
titania nanotube arrays was first published by Gong et al.
as a rapid communication.4 Thereafter, the anodic nanotubes have been vigorously investigated for versatile applications. A complete literature survey can be found in a
number of review papers from Grimes’,5–8 Schmuki’s,9–12
and also other groups.13–15 In this review, we focus on
conformal growth of the anodic nanotubes with reference
to application in dye-sensitized solar cells (DSCs), in an
attempt to explore new approaches to harvest solar energy.
The current version of dye-sensitized solar cells was
established by O’Regan and Grätzel in 1991.16 The key
component of a typical DSC is a mesoporous film of titania nanoparticles on transparent conducting oxide (TCO)
1941-4900/2012/4/471/012
doi:10.1166/nnl.2012.1355
471
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
glass, which provides high surface area for dye adsorption. An electrolyte solution containing redox shuttles
(e.g., I − /I3− penetrates through the nanopores of the
film and, at the other end, contacts with a platinized
counter electrode. Under illumination, photon energy is
absorbed by dye molecules that attached on the surface
of the nanoparticles. This produces electron-hole pairs in
the dye molecules. The electrons subsequently inject into
Sun et al.
the conduction band of the TiO2 film from the excited
states of the molecules, and percolate through the nanoparticle network until being collected by the TCO glass. In
the meanwhile, the holes are scavenged through electron
donation from the redox shuttles in the surrounding electrolyte. The oxidized species of the redox shuttles thereafter diffuse to the counter electrode and collect electrons
to regenerate. The whole circuit completes via electrons
Lidong Sun received his Ph.D. degree in 2012 from Nanyang Technological University, Singapore. He is currently a postdoctoral research fellow in National University of
Singapore, Singapore. His research interests include synthesis of nanostructured materials
by electrochemical methods and relevant application in energy conversion devices.
Sam Zhang received his Ph.D. degree in Ceramics in 1991 from The University of
Wisconsin-Madison, USA and is a tenured full professor at the School of Mechanical
and Aerospace Engineering, Nanyang Technological University. Professor Zhang serves as
Editor-in-Chief for Nanoscience and Nanotechnology Letters and Principal Editor for Journal of Materials Research (USA). Professor Zhang has been in processing and characteriDelivered by Publishing Technology to: Nanyang Technological University
zationIP:
of 155.69.4.4
nanocomposite
films
for 20 years and has authored/co-authored
On:thin
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06 and
Febcoatings
2013 10:26:23
around 250
peer
reviewed
international
journal
papers with an average of about 12 citaCopyright American Scientific Publishers
tions per paper, 7 books, 20 book chapters and guest-edited more than 20 journal volumes.
His book on “Materials Characterization Techniques” has been adopted as textbook by
6 American and 1 European universities: Purdue University, New York University, University of New Orleans, Louisana State University, California Polytechnic State University,
University of Missouri, and in Europe: Univ of Southern Denmark. This book is translated into Chinese and published
by China Science Press in October 2010. His other books are: NANOCOMPOSITE THIN FILMS AND COATINGS:
Processing, Properties and Performance, Nanostructured Thin Films and Coatings: Mechanical Properties; Nanostructured
Thin Films and Coatings: Functional Properties; Organic Nanostructured Thin Film Devices and Coatings for Clean
Energy; Biological and Biomedical Coatings Handbook: Processing and Characterization; Biological and Biomedical
Coatings Handbook: Applications. In addition, two more books are in the pipeline: Hydroxyapitatite Coatings (ed. Sam
Zhang, CRC Press, 2013) and Surface Technologies (ed. Guojun Qi & Sam Zhang, Springer, 2014). Professor Zhang
has founded the Thin Films conference series in 2002 and has been the Chair of this very successful conference series
ever since. Professor Zhang has founded and has been the President of Thin Films Society established in Singapore in
2009. Professor Zhang’s current researches centre on the four aspects: Hard yet tough nanocomposite coatings; Biological
Coatings and Drug Delivery Application; Electronic Thin Films and Energy Films and Coatings. Details are accessible
at http://www.ntu.edu.sg/home/msyzhang.
Qing Wang is an assistant professor at the Department of Materials Science & Engineering,
National University of Singapore (NUS). He obtained his Ph.D. of condensed matter physics
at Institute of Physics, Chinese Academy of Sciences in 2002. Before joined NUS, he has
been working with Prof. Michael Grätzel at Swiss Federal Institute of Technology (EPFL),
and Dr. Arthur Frank at National Renewable Energy Laboratory (NREL) in the fields of
nanostructured solar cells. Currently he is leading a group to conduct fundamental studies
on “Charge Transport in Mesoscopic Energy Conversion and Storage Systems.”
472
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
Sun et al.
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
Dongliang Zhao received his Ph.D. degree in 1993 from the Central Iron and Steel Research
Institute (CISRI), Beijing, China. He is the chief engineer at CISRI since 2009 and the
director of CISRI’s Institute of Functional Materials. Professor Zhao’s research interests
include computational material science, magnetic materials, superalloys as well as energy
materials. Prof. Zhao has been the leading principle investigator of or participated in more
than twenty Chinese national research projects, funded by National Natural Science Foundation of China (NSFC) program, National Key Basic Research program, National High Tech
Research and Development Program of China or General Armament Department. Aside
from classified researches, he published some 40 journal papers and was granted 6 patents.
In 2003, Professor Zhao was conferred the title of “Beijing Outstanding Young Engineer”
by Beijing City Government. In 2006 he was recognized by the State Department as one
of the National Star Researchers and in 2008, he was conferred the title of “National Defense Science and Technology
Innovation leader.”
raise the overall efficiency to about 15%.20 In conventraveling through the external circuit and thus providing
tional nanoparticle photoanodes, large anatase particles
electric energy.
(200∼400 nm radius) are usually used to increase light
There are two primary motivations to replace the conscattering by either mixing in the film or preparing on the
ventional nanoparticles with the anodic nanotubes: reductop.21 Accordingly, a power conversion efficiency () of
ing dark current and enhancing energy absorption. In a
11.1% was achieved by controlling the haze with submiconventional nanoparticle photoanode, the tiny particle size
cron particles (400 nm diameter).22 The nanotube photoan(∼20 nm on average)17 produces numerous connections
ode was reported to exhibit an intrinsic property of internal
between particles. This in turn incorporates a large numlight scattering, and therefore increasing light harvesting
ber of trapping sites in the band gap of the semiconductor.
efficiency by at least 20% as compared to the nanoparticle
It is reported that the electrons that percolate through the
counterpart.19 23
nanoparticles undergo an average of about 106 trapping/
18
In view of these, it is expected that the use of anodic
detrapping events before being collected. In contrast, the
Delivered by Publishing Technology to: Nanyang Technological University
nanotubes
should
be better than the conventional nanoparphotoanode of anodic nanotubes exhibits
highly
oriented
IP: 155.69.4.4 On: Wed, 06 Feb 2013
10:26:23
alignment that perpendicular to the substrate.
This
gives
ticles.
However,
the
Copyright American Scientific Publishers former is less efficient. Table I sumrise to a vectorial electron transport and hence minimizes
marizes the photovoltaic performances of the most efficient
the trapping sites. As such, recombination in the nanoDSCs based on the nanotubes. A stable power convertubes is an order of magnitude slower than that in the
sion efficiency of 7∼8% has been reported by different
nanoparticles.19 It is of paramount importance to harvest
groups. In comparison, much higher efficiency has been
solar energy in the red and near infrared region, as this
achieved with the nanoparticles, for example, from the
could promote the photocurrent significantly and hence
initial 7.12% in 1991,16 to 10.0% in 1993,24 10.4% in
Table I. Efficient dye-sensitized solar cells based on anodic titania nanotube arrays (tested under AM 1.5 illumination with a power density of 100
mW/cm2 .
No.
1
2
3
4
5
6
7
8
Photoanode
(Illumination)
L
(m)
Ti foil (Backside)
Ti foil (Backside)
Ti foil (Backside)
Ti foil (Backside)
FTO (Front side)a
FTO (Front side)b
FTO (Front side)d
FTO (Front side)e
20
19
14
30
17.6
35
20.8
63 + 3f
(70 + 3)g
D
W
Jsc
(nm) (nm) (mA/cm2 132
—
90
120
95
100
99
140
24
—
—
—
10
15
27
50
12.72
14.25
15.44
14.63
15.8
16.8c
15.46
18.5h
(11.7)
Voc
(V)
FF
(%)
Dye
0.817 0.663 6.89 N719
0.771 0.64 7.0
N3
0.77
0.62 7.37 N719
0.741 0.70 7.6 N719
0.73
0.59 6.86 N719
0.733 0.62 7.6 N719
0.814 0.641 8.07 N719
0.770 0.64 9.1h N719
(0.714) (0.63) (5.3)
electrolyte
I − /I3−
I − /I3−
I − /I3−
I − /I3−
I − /I3−
I − /I3−
I − /I3−
I − /I3−
Counter
Electrode
Active
Area (cm2 Refs.
Pt (0.6 nm) on FTO (sp)
—
Pt (2.3 nm) on FTO (sp)
0.28
Pt on ITO (td)
0.125
Pt on ITO (td)
0.25
Pt (100 nm) on FTO (sp)
0.2
Pt on FTO (td)
0.03∼0.15c
Pt on FTO (td)
—
Pt on FTO (sp)
0.25
29
30
31
23
32
33
34
35
Note: L: tube length, D: pore diameter, W : wall thickness, Jsc : short-circuit photocurrent density, Voc : open-circuit photovoltage, FF: fill factor, : power conversion
efficiency, sp: sputtering, td: thermal decomposition.
a
The nanotubes were directly produced on FTO glasses by anodizing Ti films. b The nanotubes were detached from Ti substrate and then stuck onto FTO glasses with 100
mM Ti-isopropoxide. c The small active area may overestimate the cell performance. In addition, it is difficult to determine the active area accurately based on the photos
shown in the supporting information. d The nanotubes were detached from Ti substrate and then adhered onto FTO glasses with a drop of TiO2 sol containing Ti(OBu)4
and polyethylene glycol. e The nanotubes were detached from Ti substrate, etched with oxalic acid to penetrate the tube bottoms, and then adhered onto FTO glasses using
sol-gel processed TiO2 nanoparticle paste. f The photoanode consists of 63-m-long open-ended nanotube arrays and 3-m-thick nanoparticles. g The photoanode consists
of 70-m-long closed-ended nanotube arrays and 3-m-thick nanoparticles. The relevant cell performance is given in the parentheses. h The nanoparticle film of 3 m alone
contributes a Jsc of 3.4 mA/cm2 and exhibits a of 1.7%.
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
473
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
2001,25 11.18% in 2005,26 and very recent 12.3% with
different sensitizers by Grätzel’s group.27 One of the key
factors influencing is the short-circuit photocurrent density (Jsc , which is about 15 mA/cm2 for the nanotubes
in Table I whereas approximately 17∼20 mA/cm2 for the
nanoparticles.22 24–27 The photocurrent is, in turn, determined by the following parameters at a given incident photon flux density:22 28 Energy loss in light absorption and
reflection by TCO glass, optical absorption cross section
of sensitizer (), dye loading amount per projected surface
area of photoanode ( , charge injection efficiency (inj ,
and charge collection efficiency (cc . Two of these parameters are intimately dependent on the photoanode, i.e., and cc . The charge collection efficiency is quite efficient
in a typical DSC with nanoparticles and reported to be
improved by 25% with the nanotubes as compared to the
nanoparticles.19 As such, it is that limits the photocurrent in a DSC based on the nanotubes, which is directly
related to the surface area of a photoanode.
Sun et al.
A face-centered cubic (FCC) structure is the most
closely packed form of spheres with ABCABC arrangement in space, as illustrated in Figure 1(a). In this
architecture, the packing factor of nanoparticles (NP can
be calculated as
n=
1
1
×8+ ×6 = 4
8
2
√
4r = 2a
(1)
(2)
VNP
4/3r · n
= 7405%
(3)
=
V
a3
Here n is the number of nanoparticles in a unit cell, r is
the radius of a nanoparticle, a is the lattice constant of
the unit cell. As for nanotubes, the space arrangement can
only be ABABAB sequence, as shown in Figure 1(b). To
facilitate further calculation, the packing factor of nanotubes (NT is calculated with the inner tube being solid
instead of hollow (i.e., similar to a nanorod structure),
3
NP =
n = 1+
2. COMPARISON OF SURFACE AREA
BETWEEN NANOPARTICLE AND
NANOTUBE PHOTOANODES
1
×6 = 3
3
2R = a
NT =
(4)
(5)
VNT
R h · n
= √ = 9069%
V
3/4 a2 · 6 · h
2
(6)
To demonstrate this point, surface areas of nanoparticle
and nanotube photoanodes
are calculated
basedTechnology
on the fol- to:where
n is Technological
the number of solid
nanotubes (i.e., nanorods)
Delivered
by Publishing
Nanyang
University
lowing assumptions:
in aFeb
hexagonal
column (see dashed line in Figure 1(b)), R
IP: 155.69.4.4 On: Wed, 06
2013 10:26:23
(1) nanoparticles and nanotubes are regular
spheres
and Scientific
is the radius
of a nanotube, a is the lattice constant of the
Copyright
American
Publishers
hexagonal unit cell, h is the height of the column.
cylindrical tubes, respectively;
The key point concerned herein is how to obtain a com(2) wall thickness and bottom area of the nanotubes are
parable surface area between nanoparticle and nanotube
negligible;
photoanodes under the same volume. Assuming there are
(3) contact areas between nanoparticles or nanotubes are
x nanoparticles and y nanotubes in the respective photoanignored.
ode under the same volume V , therefore
4 3
r · x = NP V = 7405%V
3
R2 h · y = NT V = 9069%V
4r · x = 2R · h · 2 · y
2
Fig. 1. (a) Face-centered cubic structure of close packed nanoparticles, (b) hexagonal close packed structure of an ideal nanotube array.
Reprinted with permission from [36], L. Sun et al., Handbook of Nanostructured Thin Films and Coatings, edited by S. Zhang, Taylor & Francis,
New York (2010), Vol. 3, pp. 57–108. © 2010.
474
(for nanoparticles)
(7)
(for nanotubes)
(8)
(for the same surface area) (9)
Equation (9) denotes that x nanoparticles and y nanotubes have the same surface area. Note that the second
number 2 on the right of the equation originates from the
two surfaces (i.e., the inner and outer surfaces) of each
tube. On the basis of Eqs. (7)∼(9), the following relation
can be attained
r
3
≈
(10)
R 5
Equation (10) indicates that, to obtain a comparable surface area under the same photoanode volume, the radius
ratio has to be 3:5 for a particle with respect to a tube.
In conventional titania nanoparticle based dye-sensitized
solar cells, the particle size is normally in the range of
15∼30 nm. Taking the particle size of 18 nm for example, it requires the tube diameter to go down to 30 nm to
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
Sun et al.
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
generate a comparable surface area. Zhu and co-workers
and bottom area (i.e., assumption #2). If these factors
prepared nanotubes with an average inner diameter of
are considered, in the case of funnel shaped tube inner
30 ± 4 nm. Relevant dye loading measurements indicate
walls,54–57 Eq. (10) becomes
that, for a given film thickness, the nanotubes have comr
3
parable surface areas with nanoparticles of an average size
>
(11)
19
R
5
of 24 nm. These results are in good agreement with the
above calculations when considering a tube wall thickness
This is because the second number 2 on the right of the
of 8 ± 1 nm.
Eq. (9) should be substituted with a relatively smaller numPore diameter of the nanotubes has been reported to
ber due to reduced inner surface area. In this scenario, it
be affected by a number of parameters, such as applied
requires even smaller tube diameter to generate comparapotential,4 9 37–39 electrolyte composition,40 water content
ble surface area with regard to the conventional nanoparin an organic solution,41 42 and temperature.43 To fabriticle photoanode, which is even more difficult.
cate anodic nanotubes of small diameters, two essential
Similarly, given the same device area, the minimum tube
factors underlying these parameters should be considered:
length required can also be estimated from Eqs. (7)∼(9).
electric field and chemical dissolution. The electric field,
Ito and co-workers reported that the optimum film thickoriginating from the potential drop over an oxide layer
ness (TiO2 particle size: 20 nm, sensitizer: N719 dye) to
at the tube bottom, is the driving force for tube growth.
produce highly efficient DSCs is 12∼14 m.58 AccordThe lateral component of the field contributes to evolution
ingly, using particle size of 20 nm, film thickness of
of pore diameter.44 45 It is reported that the pore diameter
12 m, and the smallest pore diameter in Table I (i.e.,
decreases linearly with the field strength.46 Accordingly,
90 nm for Cell #3), the tube length is calculated to be
small diameter (e.g., 30 nm as calculated above) can only
∼33 m to create a comparable surface area under the
be achieved under low field (e.g., 5 V vs. Ag/AgCl in 1 M
same device area. As discussed above, the actual length
H3 PO4 + 03 wt% HF,38 15 V vs. Pt in ethylene glycol,47
should be longer than this value. Obviously, all the tubes in
and 20 V vs. Pt in glycerol).19 However, the growth rate,
Table I cannot satisfy this requirement except for Cell #8.
on the other hand, exhibits an exponential decrease with
As such, the key challenge in DSCs based on anodic
the field strength46 (e.g., a maximum length of 150 nm
nanotubes
is to promote dye loading amount of the phoat 15 V,47byand
5.7 m Technology
at 20 V).19 to: Nanyang Technological University
at 5 V,38 4.89 m Delivered
Publishing
toanode and hence increase the photocurrent as well as
In other words, it is difficult to IP:
produce
anodicOn:
nano155.69.4.4
Wed, 06
Feb 2013
10:26:23
efficiency.
This
can be achieved by controlling tube growth
American
tubes with small diameter (e.g., 30 nm)Copyright
and enough
length Scientific Publishers
and
treatment
judiciously
to enhance the surface area. In
(e.g., 30 m as estimated).13 48 This is further demonthe following sections, we dissect nanotube growth under
strated in Table I, where a minimum size of 90 nm is
a systematic changing of electrode configurations and anaused for efficient DSCs. The other factor of chemical
lyze relevant DSC constructions, aiming to open up new
dissolution, or field-assisted dissolution at the tube botways for efficiency enhancement.
toms to be exact, is dependent on the electrolyte composition. It is well-known that fluorine ion is crucial in
formation of the anodic nanotubes, without which, in gen3. SINGLE-SIDED TUBE GROWTH FOR
eral, only compact layer rather than highly ordered nanoGENERAL SOLAR CELLS
tubes can be obtained.9 41 Nonetheless, high-aspect-ratio
A two- or three-electrode electrochemical cell is normally
anodic nanotubes of small pores (15∼30 nm in diameter,
used to grow anodic nanotubes. The major difference
up to 60 m in length) can be produced with fluoridebetween these two systems is the value of applied potential
free electrolytes in a short time (only a few minutes), for
which is reported with respect to Pt counter electrode for
example, in perchlorate,49 50 chloride,50–52 or nitrate conthe former while to reference electrode for the latter. In this
taining electrolyte.53 Even though the required 30 nm can
section, a typical two-electrode configuration is adopted
be achieved with this method, utilization of both inner
for discussion, where a commercial titanium foil or sputand outer nanotube surfaces becomes another challenging
tered titanium thin film on TCO glass is employed as the
issue, as the resulting structures are bundles of closely
working electrode and a platinum mesh (or foil, gauze,
packed nanotubes. Moreover, it is difficult to prepare unietc.) as the counter electrode. In the case of a titanium
form and rigid nanotubes over a large scale; therefore
foil, the face opposite the counter electrode is usually profew applications in dye-sensitized solar cells have been
tected with a polymer coating,5 as illustrated in Figure 2;
reported. In view of these, generally the effective surface
otherwise the kinetics of tube growth is different, as will
area per unit photoanode volume is relatively smaller for
be discussed in Section 4.
nanotubes with respect to the nanoparticles, thus resulting
The two choices of working electrode provide three difin less dye loading amount which, in turn, gives rise to
ferent routes to construct dye-sensitized solar cells, and
low photocurrent and conversion efficiency.
eventually render two major cell configurations: backside
It is noteworthy that the above discussion is based on
the calculation without taking into account wall thickness
and frontside illuminations. In backside illumination,59
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
475
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
Sun et al.
Fig. 2. General procedures to fabricate dye-sensitized solar cells based on anodic nanotubes (NT) with configurations of backside (Cell I) and frontside
(Cells II and III) illuminations.
nanotube arrays are grown directly on commercial Ti foils
In general, either thick or thin TiO2 nanoparticle film is
and then assembled into devices, as shown in Route I of
used as the adhesive layer, such as TiO2 nanoparticle paste
Figure 2. Due to the opaque nature of Ti foil (e.g., usually
(thick layer, about 2∼3 m),35 65 73–75 Ti-isopropoxide
with a thickness of 100 or 250 m), light has to illu(thin layer, 2 drops),33 and TiO2 sol containing Ti(OBu)4
minate from the side of counter electrode (see Cell I in
and polyethylene glycol (thin layer, 1 drop).34 The thick
Fig. 2). As electrons are collected from the foil which is
layer makes substantial contributions to the cell perforopposite the counter electrode, this cell structure is named
mance, while the thin layer shows negligible influence. For
as backside illumination. Efficient cells for this category
example, Cell #8 (Table I) adopts a 3-m-thick nanoparare exemplified in Table
I
(see
Cells
#1∼4).
In
the
case
film as
the adhesiveUniversity
layer and exhibits a Jsc of
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Nanyang
Technological
2
of frontside illumination, nanotube IP:
arrays
are produced
on
18.5
mA/cm
and of 9.1%, whereas the nanoparticle
155.69.4.4
On: Wed,
06
Feb
2013 10:26:23
TCO glasses and then fabricated into devices,
as displayed
film alone
shows a Jsc of 3.4 mA/cm2 and of 1.7%,
Copyright
American Scientific
Publishers
in Figure 2 (Cell II or III). Since irradiation can be perequivalent to 18.4% and 18.7% contributions to Jsc and ,
formed from the TCO glass where the electrons are colrespectively. Note that the real contributions may be even
lected simultaneously, this architecture is called frontside
larger considering the light scattering effect of the upper
illumination.
nanotube layer. Cell #7 (Table I) employs a thin adhesive
The frontside illuminated DSCs can be further subdilayer and presents a Jsc of 15.46 mA/cm2 and of 8.07%,
vided into two categories in terms of preparation methods:
and only negligible portion originates from the interlayer,
in-situ and ex-situ. The in-situ method sputters titanium
i.e., 0.18 mA/cm2 (1.2% of the total Jsc for Jsc and 0.06%
thin film on TCO glass first, anodizes the film into nano(0.7% of the overall ) for . In addition, the use of
tube arrays,60 and then assembles the tubes into frontside
thick nanoparticle interlayer attenuates the virtue of vectorial electron transport in the upper nanotubes, because of
illuminated DSCs,61 as illustrated in Route III of Figure 2.
increased electron scattering in the underlying nanopartiAn efficient cell is presented as Cell #5 in Table I. The
cles. This is evidenced from the shorter electron lifetime
ex-situ method starts with Ti foil to grow nanotube arrays,
in the presence of the nanoparticles, i.e., 31 ms vs. 69 ms
detaches the nanotube layer from the Ti substrate to form
for nanotubes (25 m) adhered with nanoparticles (3 m)
a free-standing nanotube membrane, subsequently transon TCO glass and nanotubes (25 m) on titanium foil,75
fers the membrane onto TCO glass, and then applies to
dye-sensitized solar cells. The complicated processes are
respectively. As such, thin adhesive layer is desirable. On
shown in Route II of Figure 2. Formation of the nanotube
the other hand, a better interface between nanoparticles
membrane is crucial for this method and has become a hot
and nanotubes can be obtained using thick one especially
topic since demonstrated in 2007 with three different feafor rough tube bottom or surface,74 which should also be
62
tures: closed-ended membrane by ultrasonication, opentaken into account.
Besides, the effect of thickness of the nanoparticle
ended membrane by dissolving Ti substrate and exposing
interlayer becomes more prominent when closed-ended
to HF vapors,63 large-scale, flat, and robust membrane of
tubes are used as the photoanodes. Open-ended tube struceither type by critical point drying with carbon dioxide.64
ture is expected to transcend the closed-ended counterThereafter, a number of follow-up works have been perpart, because trapped air at the tube bottoms can be
formed using different approaches, as listed in Table II.
expelled easily. Accordingly, higher dye loading amount is
Another important step for the ex-situ method is to stick
expected in open-ended tubes. Nevertheless, a comparable
the membranes onto TCO glasses to form photoanodes.
476
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
Sun et al.
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
Table II. Methods to prepare free-standing membrane of anodic titania nanotube arrays with different configurations.
Membrane Configuration
Methods
Refs.
closed-ended
Ultrasonication
Soaking and rising with methanol
Immerging in 0.1 M aqueous HCl for 1 h
Dissolving Ti substrate with a mixture of Br2 and dry methanol, and exposing to HF vapors
Detaching by ultrasonication, and etching with a dilute hydrofluoric acid/sulfuric acid solution
Detaching by immersing in 33 wt% H2 O2 solution, and etching in a 0.5 wt% oxalic acid solution
Potential reduction at the end of anodization, and sonication in methanol
Potential shock at the end of anodization
Two-step anodization: growing and annealing nanotubes of the first layer, and then anodizing at
the same conditions to peel off the fist layer (Note: annealing temperature is a critical factor)
Detaching with the method presented in Ref. [75], and opening the tube bottoms with dry etching
in a highly dense plasma reactor under BCl3 /Cl2 for periods 0∼300 s
Drying in a critical point dryer with carbon dioxide
Two-step anodization: growing and annealing nanotubes of the first layer, and then anodizing at a
lower (or higher) potential to grow the second layer, thereafter immersing in 10% H2 O2 solution
(or other low surface tension solvent) to detach the first layer
62, 34
66
33
open-ended
non-curving and robust
63
64
67, 35
68, 69
70, 71
72, 73
74
64
75, 76
performance with the closed-ended nanotubes (cf. the two
dye loading was obtained in both structures,35 74 suggestcells of Cell #8 in Table I). Accordingly, a thin adhesive
ing that air trapping may not be as serious as that estilayer should be adopted for closed-ended nanotubes, as
mated initially.5 59 Only under the circumstance that a
presented in Figure 3(c). In this scenario, a small portion
thick layer of TiO2 nanoparticles is employed to adhere
of nanoparticles barely influences the cell performance,
the membranes onto TCO glasses, the open-ended nanothereby achieving high efficiency (Cell #7 in Table I).
tubes exhibit superior performance to the closed-ended
The illumination configuration affects the photovoltaic
ones, since mass transport of the redox shuttles in the
characteristics of nanotube-based DSCs significantly,
underlying nanoparticles is insufficient for the latter.35 73
comparable
powerUniversity
conversion efficiencies have
In frontside-illuminated
DSCs, the
photon-electron
conver- to:though
Delivered
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Technology
Nanyang
Technological
been
up to date, as exemplified in Table I. The
IP: 155.69.4.4
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sion takes place predominantly close
to the TCOOn:
glass.
Copyright
backsidePublishers
illuminated DSCs can be fabricated with a facile
The oxidized species of the redox shuttles
has toAmerican
diffuse Scientific
and simple method, as commercial titanium foils are used
from this region to counter electrode to regenerate. When
directly as the substrates. This on the other hand renopen-ended nanotubes are used, both inner tube pore and
ders a number of novel DSC structures, as will be disinter tube spacing are available for mass transport, as illuscussed in Sections 4 and Part II of this review, and
trated in Figure 3(a). In contrast, only the inter tube spacmakes it possible to apply in flexible DSCs.77 Additioning, which is much smaller than the inner tube pore, can be
ally, a better contact exists at TiO2 nanotubes and titanium
exploited for the transport when using closed-ended nanometal interface.78 However, the backside illumination suftubes, as shown in Figure 3(b). As such, the inadequate
fers undesirable energy loss. When light irradiates from
dye regeneration in the nanoparticle layer hampers the cell
the platinized counter electrode, partial photon energy is
first reflected by the platinum thin film acting as catalyst
for redox mediator regeneration. A thinner film provides
higher transmittance but degrades the activity of Pt catalyst, and hence gives rise to a low fill factor. Ito and
co-workers reduces the loss to 2∼3% with an electrodeposited Pt film; however, the charge transfer resistance at
this counter electrode is over 4 times larger as compared to
the one prepared by thermal decomposition.77 Chen et al.
diminishes the loss to 5∼7% by optimizing the sputtering
condition (see Cell #2 in Table I).30 Other materials of high
transmittance and catalytic property are possibly promising alternatives.79 Another energy loss emanates from the
absorption of triiodide ions (I3− in the electrolyte, which
cuts the incident light below 500 nm significantly.77 80 This
Fig. 3. Schematic representation of frontside illuminated dye-sensitized
energy loss is predominant and vital to the backside illusolar cells with open-ended (a) and closed-ended (b, c) anodic nanotube
minated DSCs, therefore optimized electrolyte with low
(NT) membranes adhered to TCO glasses using thick (a, b) and thin (c)
iodine concentration is necessary.30 80 The use of other
nanoparticle (NP) films. The arrow pairs denote redox shuttles in the
electrolyte with high transmittance in visible region may
electrolyte. Sensitizers are not shown in all the illustrations.
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
477
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
Sun et al.
configurations, the major difference comes from the phosolve the problem.81 After passing through the electrolyte,
photon energy is absorbed by the sensitizers on the nanotocurrent and it gives rise to ∼9.7% lower efficiency for
tubes. A majority of electrons are produced at the top of
backside illuminated DSCs relative to the frontside one
the nanotubes (tube mouths), as illustrated in Figure 4(a).
with anodic nanotubes.34 This difference in efficiency is
As such, most of the electrons have to percolate through
consistent with that of nanoparticle based DSCs using visthe photoanode from tube tops all the way to the botcous ionic liquids (ca. 8.3%∼10%).83 Even so, frontside
65
toms via a trap-limited diffusion. In contrast, a major
illuminated DSCs normally need multiple procedures to
part of electrons are generated at tube bottoms in frontside
fabricate the devices, as shown in Figure 2 (Routes II
illumination without the above mentioned energy losses,
and III). Although the in-situ method takes relatively fewer
as shown in Figure 4(b), therefore exhibiting a combined
steps to produce the solar cells, it on the other hand
transport mode of trap-free and trap-limited diffusion.65 It
requires quite a long time (∼61 h for 20 m film)32 to
is calculated that, for higher photon energy (i.e., 530 nm
sputter the titanium thin film of high quality that suitable
light pulse), the average distance to travel is about 62.5%
for anodization. It is also difficult to control the termiand 37.5% of the total tube length for electrons created in
nation time of the anodization.60 A premature terminabackside and frontside illumination, respectively.82 As for
tion leaves a thick titanium film which converts into rutile
lower photon energy (i.e., 660 nm light pulse), the average
titania upon annealing around 430 C.84 This, in turn,
distance is roughly the same in both cases, about 50% of
facilitates rutile formation in tube walls and thus affects
the total tube length, as a result of low absorption coefcell performance. A delay termination minimizes the barficient. However, the electron diffusion coefficient is the
rier layer thickness and even peels off the nanotubes,69 82
same at a constant photoelectron density, regardless of illuwhich may be one of the reasons that a smaller elecmination configuration and light wavelength. On the other
tron lifetime is present in transparent nanotubes on FTO
hand, mass transport in the electrolyte is more efficient for
glass in comparison to that on Ti foil.85 A better solubackside illumination, owing to shorter diffusion distance
tion is proposed by Kim et al. through depositing an addias compared to that in frontside illumination,83 as distional Nb-doped TiO2 layer between the Ti metal and TCO
played in Figure 4. The distance is spanned from tube tops
substrate.82 The ex-situ method is even more complicated
to counter electrode for backside illumination in contrast
and needs special cares for certain steps, as discussed
to that from the tubeDelivered
bottoms for
illumination.
by frontside
Publishing
Technology to:above.
Nanyang Technological University
155.69.4.4
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As a consequence of the aboveIP:factors,
for theOn:
same
In short,
backside illuminated DSCs can be fabricated
American Scientific
Publishers
piece of an efficient solar cell under Copyright
different irradiation
with a facile and simple method but with relatively lower
power conversion efficiency, as a result of energy losses
which can be minimized by choosing proper counter electrode and redox electrolyte. Frontside illuminated DSCs
provide a little higher efficiency due to advanced architecture for energy absorption and electron transport, but it
requires multiple steps and special cares to manufacture
the devices.
4. DOUBLE-SIDED TUBE GROWTH FOR
PARALLEL SOLAR CELLS
Fig. 4. Schematic diagram of excitation processes in nanotube-based
dye-sensitized solar cells under backside (a) and frontside (b) illuminations. Sensitizers are not shown in both illustrations. The red regions
on the nanotubes indicate the location where a majority of electrons are
generated in the respective cell. Black dots: electrons, arrow pairs: redox
shuttles in the electrolyte, dashed line: trap-limited diffusion, solid line:
trap-free diffusion.
478
For general solar cell structures, only single-sided anodic
nanotubes are needed, as discussed in Section 3. Consequently, in the case of a titanium foil as the substrate, the
face opposite the platinum electrode is usually protected
during anodization.60 66 31 Otherwise, the current transient
is distinctly different if both sides of the foil are exposed
to the electrolyte, due to unequal anodization process proceeding at each side. As for water splitting, double-sided
nanotubes are exploited for higher hydrogen generation,86
which are produced with two platinum counter electrodes,
as presented in Figure 5(a). The tube lengths at both sides
are equal when keeping the titanium foil in the centre of
two platinum electrodes. This is understandable because
the anodizing environment is the same for both sides of the
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
Sun et al.
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
counter electrode only (referred to as DG-1C, Fig. 5(b)).
It shows that the DG-1C exhibits the largest growth rate,
with the SG being the smallest and the DG-2C in between.
This facile method with one counter electrode to grow
double-sided nanotubes is thus employed to fabricate dyesensitized solar cells of parallel configuration.
In view of application in DSCs, it is essential to obtain
identical nanotubes on both sides of a foil. Figure 6 discloses that no appreciable differences are present in inner
and outer tube diameters. Similar tube length is attained
in an organic electrolyte after anodizing at 25 V and
40 C for different durations with indispensable ultrasonic cleaning.88 Crystallographic structures are examined
without detecting any obvious differences between FCFig. 5. Double-sided growth of anodic nanotubes with two (a) or one
and OP-side. Thereafter, the double-sided nanotubes on
(b) platinum counter electrode.
a titanium foil are assembled into a dye-sensitized solar
cell of parallel configuration, as shown in Figure 6. This
foil. In contrast, using one counter electrode only, doublecell structure is similar to bifacial solar cells that have
sided nanotubes can also be produced. Grimes’ group pre62
been investigated since 1966,89 especially for a doublepared double-sided nanotube membranes of 720 m and
sided n+ pn+ transistor-like solar cell.90 A bifacial solar
even over 2000 m.64 They stated that the membranes
cell can absorb photon energy incident on both sides of
consisted of two back-to-back nanotube arrays separated
the cell, hence producing about 50% more electric power
by a thin barrier layer, and thought that the tube lengths
than the monofacial cell by collecting reflected light from
were fairly the same on both sides. However, the pertinent
the surroundings.91 Using these bifacial cells, photovoltaic
images show that the tube length is quite different.
modules can be installed with fewer limitations (generated
In our previous work,87 double-sided nanotube growth
power is insensitive to facing direction) and less required
was investigated with a typical two-electrode configuraby Publishing
Nanyang
Technological
University
tion, as displayed inDelivered
Figure 5(b).
The resultsTechnology
reveal that to:area
(installed
module is normal
to the ground).92 As a
On:elecWed, 06
Feb the
2013
10:26:23
the tube length at the side facingIP:
to 155.69.4.4
the platinum
result,
bifacial
architecture provides a cost-effective
Copyright American Scientific Publishers
trode (referred to as FC-side) is longer than that opposite
way to generate solar electricity.
(referred to as OP-side) regardless of applied potential and
Besides these common similarities, the parallel strucanodizing duration, as illustrated in Figure 5(b). This is
ture proposed in Figure 6 also exhibits some differences
attributed to a higher ionic flux at the FC-side induced by
as compared to conventional bifacial photovoltaic cells.
additional potential drop in an organic electrolyte solution.
Firstly, the parallel cell consists of two individual cells and
The flux at the FC-side (JFC consists of two parts: ion
thereby two carrier collecting junctions. In contrast, most
diffusion under concentration gradient and ion migration
of the bifacial cells have one junction only in each cell,
under electric field, whereas only ion diffusion exists at
e.g., inorganic p+ nn+ 93 and n+ pp+ structures,94 95 and
the OP-side (JOP , as follows:87
dye-sensitized schemes.83 79 Secondly, the performance of
each individual cell is independent on each other, as charge
c
(12)
JFC = −pD + ucE
carriers are transported separately before being collected.
x
For inorganic bifacial cells, the rear surface recombination
c
velocity affects the front cell significantly due to a shared
(13)
JOP = −pD
n- or p-type base;94 95 therefore a good passivation at rear
x
surface is necessary to create a back surface field. For dyewhere p is the porosity of the nanotube surface layer, D is
sensitized bifacial cells, the performance is also influenced
the diffusion coefficient, c is the concentration, c/x is
owing to a shared electron and hole channel in a single
the concentration gradient, u is the mobility, and E is the
cell. Thirdly, the structure lowers the series resistance as
electric field strength resulting from the potential drop in
a result of the exclusive parallel arrangement. Finally, it is
the electrolyte. It is evident that the JFC will equal to the
easy to fabricate a symmetrical cell (i.e., the same power
JOP as the field strength E approaching zero. Accordingly,
conversion efficiency at each side) by controlling nanotube
the tube length is tailored to be comparable on both sides
growth with the experimental setups in Figure 5.
through reducing the potential drop and hence the field
In Section 2, it is calculated that the lower surface
strength, echoing the above conclusion. The tube growth
area of nanotube photoanode hinders further efficiency
rate at the FC-side is compared with three different conenhancement of the corresponding DSCs. The parallel
figurations: single-sided growth (referred to as SG, Fig. 2),
architecture is anticipated to double the surface area of a
double-sided growth with two counter electrodes (referred
to as DG-2C, Fig. 5(a)), and double-sided growth with one
photoanode and thus the dye loading amount as well as
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
479
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
Sun et al.
Delivered by Publishing Technology to: Nanyang Technological University
Fig. 6. (a) An optical microscope image IP:
showing
double-sided
nanotube
on a 2013
titanium
substrate, which are assembled into a dye-sensitized
155.69.4.4
On:
Wed,arrays
06 Feb
10:26:23
solar cell of parallel configuration, as illustrated
below the American
image accordingly.
The performances
of single and parallel cells are exhibited on the
Copyright
Scientific
Publishers
down-left and -right corner, respectively. (b, c) Top and (d, e) cross-sectional views of the nanotubes at respective FC- (b, d) and OP-side (c, e).
Reproduced with permission from Ref. [48], L. Sun, et al., Energy Environ. Sci. 4, 2240 (2011). © 2011, The Royal Society of Chemistry.
photocurrent. To test the photovoltaic behaviors of the parallel cell, a dielectric mirror with known transmittance and
identified reflected intensity is used to reflect the light onto
the rear cell. The results show that the parallel cells give
rise to an average 70% increase in photocurrent instead
of an expected 100% due to lower light intensity at the
rear surface (i.e., 38 mW/cm2 vs. 100 mW/cm2 . The
photocurrent of a parallel solar cell benefits from three
contributions:48 the cell at the front, the cell at the rear,
and the interaction between the two individual cells, which
reduced the series resistance through the parallel connection. The maximum power output increases by a factor of
two for parallel DSCs with respect to the single ones, as
evidenced in Figure 7. Eventually, on average 30% increment in efficiency is obtained, emanating from increased
photocurrent and reduced series resistance.
The bifacial DSCs with parallel configuration render
a general and promising route to reduce the cost of
solar electricity. The double-sided nanotubes are prepared
directly on commercial titanium foils without any additional prerequisites, just exposing both surfaces to an
electrolyte solution during anodization. In other words,
the nanotubes can be produced using exactly the same
experimental setups as the normal single-sided nanotubes.
480
The concentrating system96 and the packing method97 for
the double-sided solar cells have been studied since the
1970s, which are not critical problems. Moreover, as discussed in Section 3, the cell performance can be improved
Fig. 7. Maximum power outputs of single and parallel dye-sensitized
solar cells as a function of nanotube length. Data are extracted from the
J–V curves in Figure 6.
Nanosci. Nanotechnol. Lett. 4, 471–482, 2012
Sun et al.
Conformal Growth of Anodic Nanotubes for Dye-Sensitized Solar Cells: Part I. Planar Electrode
further by optimizing the counter electrode and the redox
electrolyte.
10. A. Ghicov and P. Schmuki, Chem. Commun., 2791 (2009).
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5. SUMMARY
13. L. Sun, S. Zhang, X. Sun, and X. He, J. Nanosci. Nanotechnol.
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15. J. Yan and F. Zhou, J. Mater. Chem. 21, 9406 (2011).
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(2001).
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(2007).
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21. M. Grätzel, Chem. Lett. 34, 8 (2005).
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cated with a facile and simple method but with relatively
24. M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
lower power conversion efficiency, as a result of energy
E. Müller, P. Liska, N. Vlachopoulos, and M. Grätzel, J. Am.
Chem. Soc. 115, 6382 (1993).
losses which can be minimized by choosing proper counter
25. M. K. Nazeeruddin, P. Péchy, T. Renouard, S. M. Zakeerudelectrode and redox electrolyte. The frontside illuminated
din, R. Humphry-Baker, P. Comte, P. Liska, Le Cevey, E. Costa,
DSCs can be realized with either in-situ or ex-situ method.
V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi, and
Both methods need multiple steps and special cares, while
M. Grätzel, J. Am. Chem. Soc. 123, 1613 (2001).
the cells exhibit superior performances due to the advanced
26. M. K. Nazeeruddin, F. De Angelis, S. Frantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, and M. Grätzel, J. Am. Chem. Soc.
architecture for energy absorption and electron transport.
127, 16835
(2005).
Delivered
by Publishing
Technology
Technological
University
Double-sided nanotubes
are obtainable
when both
sides of to: Nanyang
27. Feb
A. Yella,
Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K.
IP: 155.69.4.4
On:durWed, 06
2013H.-W.
10:26:23
a titanium foil are exposed to the electrolyte
solution
Nazeeruddin,
E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, and
Copyright American Scientific
Publishers
ing anodization. The double-sided nanotubes on the foil
M. Grätzel, Science 334, 629 (2011).
can be directly assembled into a dye-sensitized solar cell
28. M. Grätzel, Inorg. Chem. 44, 6841 (2005).
which renders an intrinsic parallel configuration. The par29. K. Shankar, G. K. Mor, H. E. Prakasam, S. Yoriya, M. Paulose, O. K.
Varghese, and C. A. Grimes, Nanotechnology 18, 065707 (2007).
allel scheme gives rise to an average 70% increase in pho30. C.-C. Chen, H.-W. Chung, C.-H. Chen, H.-P. Lu, C.-M. Lan,
tocurrent and 30% enhancement in efficiency. The relevant
S.-F. Chen, L. Luo, C.-S. Hung, and E. W.-G. Diau, J. Phys. Chem. C
power output is doubled, thus providing a general and
112, 19151 (2008).
promising way to produce solar electricity.
31. J. Wang and Z. Lin, Chem. Mater. 22, 579 (2010).
32. O. K. Varghese, M. Paulose, and C. A. Grimes, Nat. Nanotechnol.
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Acknowledgments: Support of this work by the NUS
33. J. H. Park, T.-W. Lee, and M. G. Kang, Chem. Commun., 2867
URC grant R284000075112 is gratefully acknowledged.
(2008).
34. B.-X. Lei, J.-Y. Liao, R. Zhang, J. Wang, C.-Y. Su, and D.-B. Kuang,
J. Phys. Chem. C 114, 15228 (2010).
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