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2014Surface Engineering of ZnO Nanostructures for SemiconductorSensitized Solar Cells

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REVIEW

Surface Engineering of ZnO Nanostructures for
Semiconductor-Sensitized Solar Cells
Jun Xu, Zhenhua Chen, Juan Antonio Zapien, Chun-Sing Lee,* and Wenjun Zhang*

via quantum confinement effects and surface-area effects.[18–20] These advantages
offer new possibilities for a variety of new
solar cell structures with reduced cost and
improved efficiency. It is expected that the
nanostructured cells, through band structure engineering of the nanomaterials and
new device design concepts, could achieve
a power conversion efficiency (PCE) even
greater than the thermodynamic limit
of bulk single junction solar cells (33%
under 1 Sun illumination).[21]
Among the nanostructured solar cells,
the DSSC has shown to be an important solar cell design
with considerable superiority given its simple device structure
as well as facile, scalable, and low cost fabrication.[7–10] However, the advances of DSSC techniques have been seriously
obstructed by stability and lifetime issues often caused by
degradation of organic dyes.[22,23] Semiconductor sensitizers,
in particular semiconductor quantum dots (QDs), have been
regarded as a superb alternative to replace dye sensitizers.
Compared to organic dyes, QDs in SSCs have: i) better stabilities; ii) higher optical absorption coefficients; iii) lower costs
and iv) more tunable properties as they can be easily prepared
with controllable size, shape and composition at low costs.[23–26]
Furthermore, QDs may also enable utilization of hot electrons
or generating multiple charge carriers with a single photon,
which could boost the theoretical PCE of SSCs by up to 44%
higher than the Shockley and Queisser limit (33%).[27–30] So
far, various QDs with their bandgaps covering a wide spectrum
range such as CdS,[31–34] CdSe,[35–38] CdTe,[39,40] CdSxSe1−x,[41,42]
CdSexTe1−x,[43] ZnxCd1−xSe,[44] PbS,[45–47] PbSe,[48,49] Bi2S3,[50,51]
In2S3,[52–55] InP,[56,57] InAs,[58] and CuInS2[59–61] have been
employed in SSCs.
A number of metal oxide (MO) semiconductors such as
TiO2,[35,43,62–64] ZnO,[32,36] SnO2,[65,66] Zn2SnO4,[67,68] Nb2O5,[69]
W2O3,[70] and In2O3,[54,55] have also been used as scaffolding for
hosting semiconductor sensitizers and to provide efficient electron transport in SSCs. TiO2-nanostructure electron transporter
has been studied most comprehensively following its successful
application in DSSCs, and a record PCE beyond 6% has been
achieved in TiO2-based SSCs very recently.[43,71] However...
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2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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REVIEW
Surface Engineering of ZnO Nanostructures for
Semiconductor-Sensitized Solar Cells
Jun Xu , Zhenhua Chen , Juan Antonio Zapien , Chun-Sing Lee ,* and Wenjun Zhang*
DOI: 10.1002/adma.201400403
1. Introduction
With the advance of nanotechnology, a variety of novel photo-
voltaic (PV) devices based on nanostructures have been devel-
oped in recent years.
[ 1–7 ]
These include dye-sensitized solar cells
(DSSCs),
[ 8–11 ]
colloidal nanocrystal thin-fi lm solar cells,
[ 12,13 ]
and three-dimensional (3D) nanostructured semiconductor
junction solar cells,
[ 14–17 ]
among others. These new types of
devices can be classifi ed as the third-generation solar cells, fol-
lowing the fi rst generation of crystalline silicon bulk solar cells
that followed by a second generation of thin fi lms cells based
on a variety of materials including amorphous Si, polycrystal-
line cadmium telluride, or copper indium gallium selenide
(CIGS). As compared with their bulk and thin-fi lm counter-
parts, nanomaterials often have unique and, importantly, tun-
able electronic and optical properties resulting from their sizes
Semiconductor-sensitized solar cells (SSCs) are emerging as promising
devices for achieving effi cient and low-cost solar-energy conversion. The
recent progress in the development of ZnO-nanostructure-based SSCs is
reviewed here, and the key issues for their effi ciency improvement, such as
enhancing light harvesting and increasing carrier generation, separation, and
collection, are highlighted from aspects of surface-engineering techniques.
The impact of other factors such as electrolyte and counter electrodes on
the photovoltaic performance is also addressed. The current challenges and
perspectives for the further advance of ZnO-based SSCs are discussed.
Dr. J. Xu, Dr. Z. Chen, Dr. J. A. Zapien, Prof. C.-S. Lee,
Prof. W. J. Zhang
Center of Super-Diamond and
Advanced Films (COSDAF)
Department of Physics and Materials Science
City University of Hong Kong
Hong Kong SAR, P. R. China;
Shenzhen Research Institute
City University of Hong Kong
Shenzhen P. R. China
E-mail: apcslee@cityu.edu.hk; apwjzh@cityu.edu.hk
Dr. J. Xu
School of Electronic Science and Applied Physics
Hefei University of Technology
Hefei 230009 , P. R. China
via quantum confi nement effects and sur-
face-area effects.
[ 18–20 ]
These advantages
offer new possibilities for a variety of new
solar cell structures with reduced cost and
improved effi ciency. It is expected that the
nanostructured cells, through band struc-
ture engineering of the nanomaterials and
new device design concepts, could achieve
a power conversion effi ciency (PCE) even
greater than the thermodynamic limit
of bulk single junction solar cells (33%
under 1 Sun illumination).
[ 21 ]
Among the nanostructured solar cells,
the DSSC has shown to be an important solar cell design
with considerable superiority given its simple device structure
as well as facile, scalable, and low cost fabrication.
[ 7–10 ]
How-
ever, the advances of DSSC techniques have been seriously
obstructed by stability and lifetime issues often caused by
degradation of organic dyes.
[ 22,23 ]
Semiconductor sensitizers,
in particular semiconductor quantum dots (QDs), have been
regarded as a superb alternative to replace dye sensitizers.
Compared to organic dyes, QDs in SSCs have: i) better stabili-
ties; ii) higher optical absorption coeffi cients; iii) lower costs
and iv) more tunable properties as they can be easily prepared
with controllable size, shape and composition at low costs.
[ 23–26 ]
Furthermore, QDs may also enable utilization of hot electrons
or generating multiple charge carriers with a single photon,
which could boost the theoretical PCE of SSCs by up to 44%
higher than the Shockley and Queisser limit (33%).
[ 27–30 ]
So
far, various QDs with their bandgaps covering a wide spectrum
range such as CdS,
[ 31–34 ]
CdSe,
[ 35–38 ]
CdTe,
[ 39,40 ]
CdS
x
Se
1 x
,
[ 41,42 ]
CdSe
x
Te
1 x
,
[ 43 ]
Zn
x
Cd
1 x
Se,
[ 44 ]
PbS,
[ 45–47 ]
PbSe,
[ 48,49 ]
Bi
2
S
3
,
[ 50,51 ]
In
2
S
3
,
[ 52–55 ]
InP,
[ 56,57 ]
InAs,
[ 58 ]
and CuInS
2
[ 59–61 ]
have been
employed in SSCs.
A number of metal oxide (MO) semiconductors such as
TiO
2
,
[ 35,43,62–64 ]
ZnO,
[ 32,36 ]
SnO
2
,
[ 65,66 ]
Zn
2
SnO
4
,
[ 67,68 ]
Nb
2
O
5
,
[ 69 ]
W
2
O
3
,
[ 70 ]
and In
2
O
3
,
[ 54,55 ]
have also been used as scaffolding for
hosting semiconductor sensitizers and to provide effi cient elec-
tron transport in SSCs. TiO
2
-nanostructure electron transporter
has been studied most comprehensively following its successful
application in DSSCs, and a record PCE beyond 6% has been
achieved in TiO
2
-based SSCs very recently.
[ 43,71 ]
However, as an
electron transporter, ZnO presents attractive properties which
in some aspects are superior to those of TiO
2
and has received
increasing research interest in the past few years. ZnO is a
direct bandgap semiconductor with similar band structure and
physical properties as those of TiO
2
. Signifi cantly, ZnO has the
Adv. Mater. 2014,
DOI: 10.1002/adma.201400403
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