www.advenergymat.de
www.MaterialsViews.com
Xin-Hong Lu, Yan-Zhen Zheng, Shi-Qing Bi, Yuan Wang, Xia Tao,* Liming Dai,
and Jian-Feng Chen
Dye-sensitized solar cells (DSCs) are important low-cost photovoltaic devices that show great promise to solve many environmental
and energy problems.[1] DSC is typically a sandwich-structure
solar cell consisting of a porous photoanode film coated with dye
molecules, an iodide/triiodide electrolyte and a platinum counter
electrode.[2] Apart from the search for more efficient and stable
dyes, the design and modification of the photoanode architecture
has also attracted great attention.[3] In general, high specific surface area, fast electron transport and outstanding light-scattering
capability are prerequisites for an excellent photoanode in highefficiency DSCs.[4] In the past decade, ZnO has attracted extensive
interest owning to its much higher electronic mobility in comparison with the traditional TiO2, which may facilitate more effective
electron transport in DSCs.[5] More importantly, its unique merit,
namely ease of crystallization and anisotropic growth, opens wide
possibilities to accommodate all of the above favorable characteristics through control of morphology and structure.[5–9] To
date, various ZnO nanostructures, including zero-dimensional
(0D) nanoparticles, 1D nanowires, nanotubes and nanobelts,
2D nanosheets, 3D hollow spheres, nanocrystalline aggregates
and nanoforests, have been fabricated.[5–14] Among them, several exceptional structures, such as 2D nanosheets (NSs) and
3D nanocrystalline aggregates (NAs), are of particular interest in
performance enhancement for DSCs. For DSCs based on ZnO
NSs, high photo-to-current conversion efficiencies (PCEs) above
6% have been reported mainly due to their high surface area,
effective electron transport, and high porosity.[10–12] Aside from
NSs, the 3D ZnO NAs assembled with 0D ZnO nanocrystals
have also been one of the hotspot nanostructures to yield high
PCEs of 5.3−6.3% because of their superior light-scattering effect
Dr. X.-H. Lu, Dr. Y.-Z. Zheng, Dr. S.-Q. Bi,
Dr. Y. Wang, Prof. X. Tao
State Key Laboratory of
Organic-Inorganic Composites
Beijing University of Chemical Technology
15 Beisanhuan East Road, Beijing 100029, P. R. China
E-mail: taoxia@mail.buct.edu.cn
Prof. L. Dai
Department of Macromolecular Science and Engineering
Case School of Engineering
Case Western Reserve University
10900 Euclid Avenue, Cleveland, OH 44106, USA
Prof. J.-F. Chen
Research Center of the Ministry of Education for
High Gravity Engineering & Technology
Beijing University of Chemical Technology
15 Beisanhuan East Road, Beijing 100029, P. R. China
DOI: 10.1002/aenm.201301802
Adv. Energy Mater. 2014, 4, 1301802
arisen from the sub-micrometer size and large surface area provided by the nanocrystals.[8,13] In photoanodes constructed even
with high-performance 2D ZnO NSs and 3D ZnO NAs, however, it is difficult to simultaneously achieve the aforementioned
three prerequisites for a high efficiency since the 2D NS structure
always possesses nanoscale size with poor light-scattering capability while the zigzag electron transport pathway along the 3D
NAs often retards the electron transport and thus inflicts electron
recombination within the electrode film to some extent.[8,10] Generally speaking, ZnO architectures cannot provide all the abovementioned properties required for high-performance DSCs, and
hence few DSCs based on ZnO could have PCEs exceed 7%.[13]
Therefore, the fabrication of photoanodes based on multidimensional (MD) ZnO architectures composed of different monodimensional nanostructures of specific features with synergistic
effects offer a better approach to high PCEs.[4,15,16] Indeed, we
have previously demonstrated that DSC based on a MD ZnO
architecture consisting of a 3D-nanorod-aggregate overlayer and
a 0D-nanocrystallite underlayer yielded a much higher PCE than
those based on single-layered nanorod-aggregates or nanocrystallites.[4] Nevertheless, the overall PCE of the MD ZnO cells is still
low and it is still a great challenge to finely design and tune the
ZnO photoanode architecture to simultaneously maximize the
key features for outstanding PCEs.
Here, we report the preparation of an innovative MD ZnO
architecture with combination of 2D ZnO NSs and 3D ZnO
NAs, in which the ZnO NSs act not only as bridges connecting
the ZnO NA-framework to form favorable electron transport
channels within the film, but also as building blocks to enlarge
the overall surface area for dye-adsorption and to provide a randomly-opened structure for a better concurrent electrolyte penetration. In this context, a key trick in this work is to synchronously tune the surface area and electron transport property of
the MD ZnO architecture by controlling the density of the ZnO
NSs with retention of relatively strong light-scattering provided
by the ZnO NAs. Under optimal condition, an overall PCE of
6.66% was achieved for a photoanode based on the ZnO NA/
NS composite architecture, a noticeable improvement by 64.0%
in comparison with that of the corresponding cell fabricated
with ZnO NA photoanode. Various analyses confirmed that the
photovoltaic performance enhancement in such a MD ZnO NA/
NS composite photoanode was mainly due to the excellent combination of large surface area, effective electron transport, and
strong light-scattering capability. To further improve the PCE,
a compact layer of 0D ZnO nanocrystal was then paved on the
substrate before fabricating the optimized ZnO NA/NS film.
This led to an impressive PCE of 7.35%, which is very close to
the record PCE (7.5%) of ZnO-based DSCs ever reported.[17]
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
(1 of 6) 1301802
COMMUNICATION
Multidimensional ZnO Architecture for Dye-Sensitized
Solar Cells with High-Efficiency up to 7.35%
www.advenergymat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. Top-view SEM images a) ZnO NA and b) ZnO NA/NS−020 films (the scale bar of the inset is 100 nm). c) Cross-sectional SEM image of the
ZnO NA/NS−020 film. d) TEM and e) high-resolution TEM images together with the corresponding selected area electron diffraction pattern (inset) of
a typical NS in the ZnO NA/NS−020 film. f) Schematic diagram of typical NS assembly and NS bridges between NA particles. g,h) Magnified crosssection SEM images taken from the regions in (c) labeled with the green and orange rectangles, respectively.
The MD ZnO architecture was fabricated via suspending the
ZnO NA film in an aqueous solution of zinc nitrate and urea at
90 °C for 2 h (see Experimental Section for details). The scanning electron microscopy (SEM) images in Figure 1a–c clearly
show the variation of the ZnO NA film after hydrothermal
treatment in a 0.20 M Zn-precursor solution (designated as
ZnO NA/NS−020 for convenience, similarly hereinafter) to produce a 9 μm-thick vertically-grown ZnO NS layer on top of the
ZnO NA layer. Note that the NSs atop the NA layer are accumulated forming many large gaps, which could benefit the penetration of electrolyte into the photoanode. A magnified SEM
image (inset of Figure 1b) reveals 2D morphological characteristic, featuring individual NSs with a thickness of 20−30 nm.
Figure 1d shows a transmission electron microscopy (TEM)
image of a single NS, showing numerous mesoscopic pores.
Such morphological characteristics are favorable to possess
large surface area for dye adsorption.[18] Further, a high-resolution TEM image (Figure 1e) and the corresponding selected
area electron diffraction pattern (inset) clearly reveals that the
ZnO NS is a single-crystalline wurtzite structure, which may
serve as a highway for electron transport.[6,19]
Instead of just growing on top of the ZnO NA film, ZnO
NSs tended to also grow within the ZnO NA film framework
(Figure 1g,h), in which a small amount of NS bridges between
ZnO NAs were formed as illustrated in Figure 1f. Since NSs
within the NA film suffered much greater spatial hindrance to
grow, the NSs would preferentially grow on top of the NA film.
Nevertheless, the NS bridges thus formed throughout the whole
1301802 (2 of 6)
wileyonlinelibrary.com
NA film could provide direct pathways, rather than a zigzag way
in the ZnO NA photoanode, for electron transport.[10] Although
these NS bridges could inevitably cause disruption of the closed
light loops locally to weaken the light scattering of the NA
photoanode, the large-scale spherical morphology of ZnO NA
was preserved after the hydrothermal process, leading to the
retention of strong light scattering capability (>70%) (Figure S1,
Supporting Information) and good crystallinity of the ZnO NA
film (see Figure S2, Supporting Information). As such, the wellcrystallized ZnO NA/NS−020 film with a dense NA underlayer
and porous NS overlayer, interparticle connecting NS bridges,
and sub-micron size of NAs possesssed all the aforementioned
morphological properties required for high-efficiency DSCs, as
discussed in more details below.
To investigate morphological changes of the as-prepared
multidimensional films, ZnO NA/NS films were also fabricated
in other two Zn-precursor growth solutions (i.e., ZnO NA/
NS−010: 0.10 M; ZnO NA/NS−015: 0.15 M). For comparison,
ZnO NA film without further treatment was also prepared as
a reference. The top-view SEM images of the as-prepared films
(Figure 1, also see Figure S3 in the Supporting Information)
clearly show that the packing density of the ZnO NSs on top
of the ZnO NA film increased with increasing Zn-precursor
concentration. When the Zn-precursor concentration is up to
0.20 M, the NA film is uniformly covered with ZnO NS assemblies, as shown in Figure 1b. Meanwhile, the top ZnO NS layer
also increased in thickness from about 5 to 9 μm (see Figure 1c
and Figure S3b,d, Supporting Information). In addition, the
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Energy Mater. 2014, 4, 1301802
www.advenergymat.de
www.MaterialsViews.com
COMMUNICATION
Figure 2. Photovoltaic performances: a) J−V curves, b) Nyquist plots, c) Bode phase plots, and d) IPCE spectra of the ZnO NA/NS and ZnO NA cells.
(VOC) of 0.603 V, respectively. In comparison with that of the
ZnO NA cell, the JSC and VOC of the ZnO NA/NS cells both
increased with increasing Zn-precursor concentration, leading
to a continuous enhancement of PCE. Impressively, a high JSC
of 17.90 mA cm−2 and VOC of 0.631 V were obtained for the
ZnO NA/NS−020 cell, leading to an encouraging PCE of 6.66%,
a 64.0% increment from that of the ZnO NA cell. The observed
photovoltaic performance enhancement is attributable to the
unique morphology change (i.e., tailored density and thickness
of the NS layer as well as ameliorative interparticle connections) with the Zn-precursor concentration shown in Figures 1
and Figure S3 (Supporting Information).
To reveal the influence of the NSs in the ZnO NA/NS composite films on the DSC performance, the dye loading amounts
into the photoanode films were first characterized (Table 1).
Compared with the ZnO NA film (1.58 × 10−7 mol cm−2), the
ZnO NA/NS films exhibited much better capabilities in dye
enlarged cross-sectional SEM images (Figure S3c,f, Supporting
Information) of the NA/NS films also revealed that the number
of the interparticle NS bridges increased with increasing Znprecursor concentration too. It was noted that further increase
in the Zn-precursor concentration to 0.25 M caused the top NS
layer to become so dense and thick that the whole ZnO NA/
NS−025 film easily fell off from the substrate to be inapplicable
for the DSC application, and hence no further discussion on
ZnO NA/NS−025.
Figure 2a shows the current density−voltage (J−V) curves
for the DSCs based on the abovementioned ZnO NA/NS
films (i.e., ZnO NA/NS−010, ZnO NA/NS−015, ZnO NA/
NS−020) and ZnO NA film under AM 1.5 sunlight illumination (100 mW cm−2), and the photoelectric characteristics
are summarized in Table 1. As can be seen, the ZnO NA cell
exhibited only a low PCE of 4.06% with a short-circuit current
density (JSC) of 12.92 mA cm−2 and an open-circuit voltage
Table 1. Characteristics of the ZnO NA/NS and ZnO NA photoanodes together with the photovoltaic parameters of the corresponding DSCs.
Cells
Dye uptake
[×10−7 mol
cm−2]
SBET
[m2 g−1]
Rct2
[ohm]
τe
[ms]
IPCE
[%]
13
1.58
36.9
22.7
2.38
56.5
22
2.65
−
40.4
0.60
−
18
2.63
47.0
17.9
3.80
65.6
5.30
21
3.60
55.0
13.8
4.87
72.9
FF
[%]
PCE
[%]
0.603
52.1
4.06
0.607
50.9
4.04
15.13
0.611
49.7
4.61
16.64
0.617
51.6
JSC
[mA cm−2]
VOC
[V]
NA
12.92
NA*
13.06
NA/NS−010
NA/NS−015
Film thickness
[µm]
NA/NS−020
17.90
0.631
59.0
6.66
22
3.87
56.3
12.8
6.32
87.0
NA/NS−020/
CL
20.72
0.622
57.0
7.35
−
−
−
−
−
−
Adv. Energy Mater. 2014, 4, 1301802
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
(3 of 6) 1301802
www.advenergymat.de
COMMUNICATION
www.MaterialsViews.com
adsorption with the dye loading amount increased from 2.63 ×
10−7 mol cm−2 to as high as 3.87 × 10−7 mol cm−2 by increasing
the content of NSs in the ZnO NA/NS films from the NA/
NS−010 to NA/NS−020. This significant enhancement in the
dye loading can be attributed to the concurrent increment of
the density and thickness of the NS layer as well as the augment of overall surface area of the ZnO NA/NS films associated
with the newly-formed high-surface-area NSs.[10] The increase
in surface area has also been well verified by the Brunauer–
Emmett–Teller (BET) specific surface area (SBET) measurements as summarized in Table 1, in which a distinct increase
in SBET from 36.9 to 56.3 m2g−1 is observed with increasing the
content of NSs in the ZnO NA/NS films. A larger dye loading
amount could ensure that the incident photons are sufficiently
absorbed, resulting in a larger photocurrent and thus the high
overall PCE.[10]
We further performed electrochemical impedance spectroscopy (EIS) to study the impact of the NSs in the ZnO NA/NS
composite films on the electric behaviors of the DSCs.[18] As
shown in Figure 2b, the Nyquist plots spectra obtained from
the ZnO NA/NS and ZnO NA cells exhibit three distinguishable semicircles, which are related to the electrochemical reaction at the Pt counter electrode in the 103 Hz range (Rct1),
at the ZnO film/dye/electrolyte interface in the 100−103 Hz
(Rct2), and the Warburg diffusion process of I−/I3− at low frequency range <1 Hz (Rdiff ) from the left to the right, respectively.[20] The extent of electron transport in the photoanode
can be judged by Rct2, which is defined by the diameter of
the middle semicircle.[15,16] According to the fitted values of
Rct2 listed in Table 1, it can be seen that the electron transport resistance decreases dramatically from 22.7 ohm for the
ZnO NA cell to 12.8 ohm for the ZnO NA/NS−020 cell with
the increasing content of NS, implying an easier and faster
electron transport in the ZnO NA/NS films with a higher NS
content. This observation can be ascribed to the interparticle
NS bridges serving as transport highway for electrons in the
photoanodes to reduce the charge recombination, and hence a
higher electron diffusion and collection efficiency in ZnO NA/
NS cells with a higher NS content.[6] The corresponding Bode
phase plots (Figure 2c) can be drawn from the Nyquist plots
under illumination in Figure 2b. According to the reported
EIS model, the lifetime (τe) of the injected electrons in photoanodes can be acquired from the following equation: τe =
1/ωmax = 1/(2πfmax), where fmax is the maximum frequency of
the intermediate frequency regime.[21] As listed in Table 1, the
τe is remarkably increased from 2.38 ms for the ZnO NA cell
to 6.32 ms for the ZnO NA/NS−020 cell. A prolonged electron
lifetime and lower electron transport resistance are directly
related to the improved JSC and VOC of the ZnO NA/NS cells.
To better understand the significance of NSs to electron transport, we also fabricated ZnO NA photoanode with an identical
film thickness as that of the ZnO NA/NS−020 film (denoted
as ZnO NA*). As expected, the photovoltaic performance
of the ZnO NA* cell is not improved with increasing of the
film thickness and adsorbed dye because the thicker NA film
causes a great increase of Rct2 up to 40.4 ohm and a much
lower τe of 0.60 ms (Table 1, also see Figure S4, Supporting
Information), which fully proves the advantage of NSs on electron transport over NAs.
1301802 (4 of 6)
wileyonlinelibrary.com
In addition, the incident photon-to-current conversion efficiency (IPCE) spectra provide further evidence for the improved
electron transport property of the ZnO NA/NS cells than the
ZnO NA cell. Figure 2d shows IPCE spectra of the ZnO NA/NS
cells and ZnO NA cell as a function of the illuminated wavelength, in which strong photoactivity for all the cells is observed
in the visible region from 470 to 620 nm that coincides with
the absorption maximum wavelength of the D205 dye used in
this work. In contrast to the ZnO NA cell (56.5%), the ZnO
NA/NS cells show higher photoresponse (>65%) in the entire
visible wavelength region with the highest IPCE value up to
87.0% at around 500 nm ± 20 nm observed for the ZnO NA/
NS−020 cell. IPCE is known to predominantly determined by
the harvesting efficiency of the incident light, the injection efficiency and the electron collection efficiency; the first of which
is proportional to dye loading of the photoanodes.[8] Since it is
rational to assume the same injection efficiency for the given
ZnO/dye system, the improvement of IPCE can be deduced to
originate from enhancement in light harvesting efficiency and/
or charge collection efficiency, which are mainly determined
by the dye-loading amount and electron transport property.[7,8]
In this work, the variation of dye uptakes for the ZnO NA cell,
ZnO NA/NS−010 cell and ZnO NA/NS−015 cell is remarkable. Therefore, the augment of IPCE for the ZnO NA/NS−010
cell and ZnO NA/NS−015 cell can be mainly attributed to the
increased adsorbed dye molecules on the photoanodes despite
of their slightly lowered light-scattering capability with respect
to the ZnO NA cell (Figure S1, Supporting Information). In
view of the slight discrepancy of dye loading amounts between
the ZnO NA/NS−015 cell and ZnO NA/NS−020 cell and even
a little weaker light scattering for the ZnO NA/NS−020 cell
(Figure S1, Supporting Information), however, the higher IPCE
for the ZnO NA/NS−020 cell could be mainly resulted from the
enhanced electron transport (i.e. electron collection efficiency)
attributed to the incorporation of more ZnO NSs into the electrode film, as discussed above.
To further improve the overall PCE, we introduced an
ultrathin compact layer between the photoanode film and the
conductive FTO substrate, which has been proved to be effective in suppressing the charge recombination and thus boosting
PCE of DSCs.[22] Specifically, a ZnO nanocrystallite compact
layer with thickness of around 100 nm was subsequently
deposited on the substrate via a spin-coating method before
doctor-blading the ZnO NA film. The photovoltaic performance
of ZnO NA/NS−020 cell with compact layer (denoted as NA/
NS−020/CL cell) is given in Figure 3, which shows a noticeable
improvement of JSC from 17.90 to 20.72 mA cm−2, leading to an
outstanding PCE up to 7.35%. This is very close to the record
PCE of 7.5% ever reported for DSCs based on ZnO. According
to the EIS analysis and J−V measurement performed in dark
condition (Figure S5, Supporting Information), such enhancement of JSC and thus PCE can be mainly ascribed to the suppressed charge recombination, and hence improved electron
collecting efficiency.
In conclusion, a class of novel ZnO photoanodes with
multidimensional architecture composed of 2D ZnO NSs
and 3D ZnO NAs has been developed for DSCs to show a
much improved PCE of 6.66% under optimal condition. Various materials and device characterization demonstrated that
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Energy Mater. 2014, 4, 1301802
www.advenergymat.de
www.MaterialsViews.com
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Figure 3. J−V curve of the ZnO NA/NS−020/CL cell.
the ZnO NSs played a key role in remarkably enhancing the
PCE by connecting ZnO NA-framework for favorable electron transport within the film and simultaneously enlarging
the overall surface area for efficient dye loading with retention of the superior light-scattering provided by ZnO NAs. A
further enhanced PCE up to 7.35%, which is among the top
records of the ZnO-based DSCs, was achieved by utilizing a
ZnO compact layer to suppress possible charge recombination in the newly developed photoanode. This work provides
a facile and feasible way to attain outstanding DSC cell efficiencies by rational design and tune the ZnO photoanode
architecture.
Experimental Section
Preparation of ZnO NA Films: All chemicals were received from
Sigma-Aldrich and used as received. The synthesis of spherical ZnO
NAs were achieved by hydrolysis and condensation of zinc acetate
dehydrate (0.1 M) in diethylene glycol at 160 °C for 8 h under reflux,
followed by repeating centrifugation/sonication/dispersion in ethanol.[8]
After drying at 60 °C in air overnight, the NAs were then mixed with ethyl
cellulose and terpineol with a weight ratio of 1:0.2:1.5 to form a viscous
ZnO paste.[23] The ZnO NA film was sequentially fabricated by doctorblading the as-prepared ZnO paste on the FTO/glass substrate (2.3 mm
thickness, 8 ohm sq−1, GEAO, China), following a programmed heating
of 350 °C for 1 h and then stepwise rising to 450 °C at 10 °C min−1.[24,25]
Preparation of ZnO NA/NS Photoanode Films: The ZnO NA/NS
film was prepared via suspending the above-synthesized NA film
upside-down in a sealed bottle containing an aqueous solution of
zinc nitrate with certain concentration (0.10 M, 0.15 M and 0.20 M)
and 16.7 wt% of urea at 90 °C for 2 h to form layered hydroxide zinc
carbonate (Zn4CO3(OH)6·H2O), which further transforms into ZnO NS
in the following annealing step (300 °C) with loss of H2O and CO2.[18] For
comparison, the ZnO NA photoanode without hydrothermal treatment
was also prepared as reference.
DSC Fabrication: The ZnO photoanode with an active area of
0.16 cm2 was immersed into a 0.5 mM D205 solution in a mixture of
acetonitrile and tert-butyl alcohol (V : V = 1 : 1) containing 1.0 mM
chenodeoxycholic acid for 50 min at 60 °C. Then, the photoanode was
assembled with a Pt-coated photocathode into a sandwich cell using a
60 µm-thick surlyn sheet, followed by filling of electrolyte composed of
0.03 M I2, 0.5 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII) and
0.5 M tertbutylpyridine in acetonitrile.
Adv. Energy Mater. 2014, 4, 1301802
Acknowledgements
The authors are grateful for the financial support from National Natural
Science Foundation of China (Nos. 21121064, 21176019, 21377011), and
863 project (2013AA031901).
Received: November 25, 2013
Revised: February 3, 2014
Published online: March 5, 2014
[1] A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, M. Nazeeruddin,
E. Diau, C. Yeh, S. Zakeeruddin, M. Grätzel, Science 2011, 334,
629.
[2] B. O’Regan, M. Grätzel, Nature 1991, 353, 737.
[3] a) D. W. Chang, H. J. Lee, J. H. Kim, S. Y. Park, S.-M. Park, L. Dai,
J.-B. Baek, Org. Lett. 2011, 13, 3880; b) K. Keis, E. Magnusson,
H. Lindstrom, S. E. Lindquist, A. Hagfeldt, Sol. Energy Mater. Sol.
Cells 2002, 73, 51.
[4] Y.-Z. Zheng, J. Zhao, H. Zhang, J.-F. Chen, W. Zhou, X. Tao, Chem.
Commun. 2011, 47, 11519.
[5] a) Q. Zhang, C. S. Dandeneau, X. Zhou, G. Cao, Adv. Mater. 2009,
21, 4087; b) F. Xu, L. Sun, Energy Environ. Sci. 2011, 4, 818.
[6] Y. Z. Zheng, X. Tao, L. X. Wang, H. Xu, Q. Hou, W. L. Zhou,
J. F. Chen, Chem. Mater. 2010, 22, 928.
[7] W.-Q. Wu, Y.-F. Xu, H.-S. Rao, C.-Y. Su, D.-B. Kuang, Nanoscale
2013, 5, 4362.
[8] Q. F. Zhang, T. R. Chou, B. Russo, S. A. Jenekhe, G. Z. Cao, Angew.
Chem. Int. Ed. 2008, 47, 2402.
[9] C.-X. He, B.-X. Lei, Y.-F. Wang, C.-Y. Su, Y.-P. Fang, D.-B. Kuang,
Chem. Eur. J. 2010, 16, 8757.
[10] C.-Y. Lin, Y.-H. Lai, H.-W. Chen, J.-G. Chen, C.-W. Kung, R. Vittal,
K.-C. Ho, Energy Environ. Sci. 2011, 4, 3448.
[11] Z. Li, Y. Zhou, G. Xue, T. Yu, J. Liu, Z. Zou, J. Mater. Chem. 2012, 22,
14341.
[12] Y. Shi, K. Wang, Y. Du, H. Zhang, J. Gu, C. Zhu, L. Wang, W. Guo,
A. Hagfeldt, N. Wang, T. Ma, Adv. Mater. 2013, 25, 4413.
[13] J. A. Anta, E. Guillén, R. Tena-Zaera, J. Phys. Chem. C 2012, 116,
11413.
[14] S. H. Ko, D. Lee, H. W. Kang, K. H. Nam, J. Y. Yeo, S. J. Hong,
C. P. Grigoropoulos, H. J. Sung, Nano Lett. 2011, 11, 666.
[15] S. Yodyingyong, Q. Zhang, K. Park, C. S. Dandeneau, X. Zhou,
D. Triampo, G. Cao, Appl. Phys. Lett. 2010, 96, 073115.
[16] J. Zhang, W. Que, Q. Jia, P. Zhong, Y. Liao, X. Ye, Y. Ding, J. Alloys
Compd. 2011, 509, 7421.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
(5 of 6) 1301802
COMMUNICATION
Characterization: The morphology and structure of ZnO photoanode
films were observed by SEM (JEOL, JSM−6701F) and TEM (JEOL,
JEM−3010). The photoelectrochemical characterization of DSCs was
performed using electrochemical station (CHI660C, ShangHai) under
simulated AM 1.5 sunlight illumination with 100 mW cm−2 light output. A
1000 W xenon lamp (Thermo Oriel, America) served as the light source.
The electrochemical impedance spectroscopy (EIS) was performed on
an electrochemical workstation (Zennium Zahner, Germany). The EIS
measurement was recorded at VOC with ac amplitude of 10 mV and
a frequency ranging from 10−1 to 105 Hz. Monochromatic light in the
range of 400−800 nm was obtained by using a series of filters and the
IPCE measurements were performed on a Keithley Model 2000 Source
Meter.
www.advenergymat.de
COMMUNICATION
www.MaterialsViews.com
[17] N. Memarian, I. Concina, A. Braga, S. M. Rozati,
A. Vomiero, G. Sberveglieri, Angew. Chem. Int. Ed. 2011, 50,
12321.
[18] Y.-H. Lai, C.-Y. Lin, H.-W. Chen, J.-G. Chen, C.-W. Kung, R. Vittal,
K.-C. Ho, J. Mater. Chem. 2010, 20, 9379.
[19] Y. Shi, C. Zhu, L. Wang, W. Li, C. Cheng, K. M. Ho, K. K. Fung,
N. Wang, J. Mater. Chem. 2012, 22, 13097.
[20] S. J. Lim, Y. S. Kang, D.-W. Kim, Electrochem. Commun. 2010, 12,
1037.
1301802 (6 of 6)
wileyonlinelibrary.com
[21] Q. Hou, Y.-Z. Zheng, J.-F. Chen, W. Zhou, J. Deng, X. Tao, J. Mater.
Chem. 2011, 21, 3877.
[22] S. Ahmed, A. D. Pasquier, T. Asefa, D. P. Birnie III, Adv. Energy
Mater. 2011, 1, 879.
[23] H.-M. Cheng, W.-F. Hsieh, Energy Environ. Sci. 2010, 3, 442.
[24] Y. Ren, Y.-Z. Zheng, J. Zhao, J.-F. Chen, W. Zhou, X. Tao, Electrochem. Commun. 2012, 16, 57.
[25] X.-H. Lu, Y.-Z. Zheng, S.-Q. Bi, J.-X. Zhao, X. Tao, J. Power Sources
2013, 243, 588.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Energy Mater. 2014, 4, 1301802