surface engineering of zno nanostructures for semiconductor sensitized solar cells

Then we review the approaches for growing ZnO nanostructures with controlled morphologies on trans-parent conductive oxide TCO substrates, their post-treat-ments for crystallinity and co

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 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

sur-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 ] 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,

How-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

highest reported electron mobility and the highest conduction

band edge among various candidates to electron transporters

( Table 1 ) that would respectively benefi t electron transport with

less recombination and enable the possibility for a larger

open-circuit voltage ( V OC ) As-synthesized ZnO shows n-type

con-ductivity due to oxygen vacancies and interstitial Zn which can

be further tuned by substituting Zn with Al, Ga, and In [ 84–86 ]

Moreover, the ease of crystallization and anisotropic growth of

ZnO allows the preparation of ZnO nanostructures in various

morphologies In addition to its electronic, optoelectronic, and

photocatalytic applications, recent studies on

ZnO-nanostruc-ture-based SSCs have demonstrated some new concepts and

led to a better understanding of photo-electrochemical energy

conversion

In this paper, we start with a brief discussion of the working

principle of SSCs and the factors determining the

perfor-mance of SSCs Then we review the approaches for growing

ZnO nanostructures with controlled morphologies on

trans-parent conductive oxide (TCO) substrates, their

post-treat-ments for crystallinity and conductivity enhancepost-treat-ments, and

the recently developed techniques for tuning the band

struc-ture of ZnO nanostrucstruc-tures and surface sensitization with

QDs and noble metal nanoparticles by different chemical and

physical methods Next, we highlight a number of strategies

for improving the device performance including optical

engi-neering of ZnO morphologies for enhanced light absorption,

bandgap engineering and co-sensitization of QDs for improved

photoelectron injection and transport, and suppression of

charge recombination by surface passivation We also address

the impacts of counter electrodes (CEs) and electrolyte on the

photovoltaic performance of SSCs Finally, the challenges and

perspectives of SSCs based on ZnO nanostructures for future

practical applications are discussed

2 Working Principle of SSCs

A typical SSC consists of three major components: a photoanode,

a counter electrode (CE), and electrolyte with redox couples A

Jun Xu obtained his Ph.D

degree from the City University of Hong Kong in

2012, and then continued as

a senior research associate at the Center of Super-Diamond and Advanced Films

(COSDAF), City University of Hong Kong He joined Hefei University of Technology in

2013, and currently works

as a Professor at the School

of Electronic Science and Applied Physics His research interest focuses on multinary chalcogenide photovoltaic materials, semiconductor sensitized solar cells, and opto-electronic devices

Chun-Sing Lee obtained his

Doctor of Philosophy degree

in 1991 from the University

of Hong Kong He then moved to the University of Birmingham to carry out postdoctoral research with the support of a Croucher Foundation Fellowship He joined the faculty of the City University of Hong Kong in

1994 and is currently a Chair Professor in materials science He co-founded COSDAF

in 1998 and is currently the center’s Director Prof Lee’s main research interest is on surface and interface physics, organic electronics and nanomaterials

Wenjun Zhang obtained

his Doctor of Philosophy degree in 1994 from Lanzhou University He was a postdoc

at the Fraunhofer Institute for Surface Engineering and Thin Films (1995 to 1997) and at the City University of Hong Kong (1997 to 1998) From

1998 to 2000, he worked as

a Science and Technology Agency Fellow at National Institute for Research in Inorganic Materials He joined CityU in 2000 again as a Senior Research Fellow He is cur-rently a Professor in Department of Physics and Materials Science; and he is also a core member of COSDAF His research focuses on thin fi lms, semiconducting nanomate-rials, surface science and modifi cation, and ions/materials interactions

Table 1 Structural and electronic characteristics of ZnO, SnO 2 and

TiO 2

fi lm)

4.3 × 10 −4 (nanoporous

fi lm)

7.3 × 10 −5 (nanoporous

fi lm)

[83]

schematic diagram showing the working principle of an ideal

SSC is shown in Figure 1 A The photoanode in an SSC is

typi-cally constructed with a wide bandgap MO semiconductor (such

as TiO 2 , SnO 2 , and ZnO) scaffold coated with a layer of

semi-conductor sensitizer typically in thin fi lm or QDs formats The

MO scaffolds also act as electron acceptor and transporter Upon

photoirradiation, electron-hole pairs (excitons) are generated in

QDs The excited electrons are then injected into the conduction

band (CB) of MO, leaving the QDs in their oxidized states The

injected electrons in MO are collected by the TCO substrate

(col-lector) and transported through the external load to the CE The

oxidized QDs are restored to their ground state through hole

scavenging by reduced species (e.g., S 2− in the redox couples of

S 2− /S n 2− ) in the electrolyte The oxidized species (S n 2− ) diffuse to

the CE where they are reduced by electrons from the external

circuit, resulting in electrolyte regeneration [ 7,87,88 ]

Photovoltaic performance of a solar cell is typically gauged

with its PCE , short-circuit current density ( J SC ), open-circuit

voltage ( V OC ), and fi ll factor ( FF ) The PCE of a solar cell is

defi ned as the ratio of the actual maximum electrical power

generated to the incident optical power ( P in ):

at electrodes to the number of incident photons, and IPCE ( λ )

can be given by:

LHE is the light harvesting effi ciency by the photoanode in an

SSC, which depends on the extinction coeffi cient of QDs, the amount of QDs loaded on MO surface, the optical absorption range of QDs, and the optical path length of the incident light within the photoanode ϕ inj is the electron injection effi ciency from the photoexcited QDs to the MO, and η col is the charge collection effi ciency at the electrodes A high ϕ inj requires appropriate energy band alignment at the MO/QD interfaces

It has been reported that in DSSCs an over-potential (−Δ G ) of

approximately 0.2 V between the conduction band minimum (CBM) of the MO and the lowest unoccupied molecular orbital (LUMO) level of the dye, and a −Δ G of 0.3 V between the

highest occupied molecular orbital (HOMO) level of the dye and the redox potential of the electrolyte are needed for effi cient electron injection from dye to MO and regeneration of the oxi-dized dye by hole scavenging [ 89 ] Due to the similarity in struc-ture and working mechanism of DSSCs and SSCs, these data can be considered as a reference for SSCs

Φ ET is the electron transfer yield, which is the product of ϕ inj and η col In a SSC system, Φ ET is seriously infl uenced by var-ious recombination processes, including direct recombination

of photogenerated electron-hole pairs within the QD, interfacial recombination of electrons in the CB of MO with holes in the valence band (VB) of QD and oxidized species in the electrolyte (electron capture), and interfacial recombination of electrons in the CB of QD by electron capture in electrolyte [ 90,91 ] Traps in QDs and MO also play an important role in carrier recombina-tion In an SSC, the charge-transfer and -transport processes must be much faster than recombination to obtain effi cient photovoltaic performance

V OC is determined by the potential different between the

quasi-Fermi level ( E F *) of electrons in the MO under

illumina-tion and the Fermi level ( E F ) of the photoanode in dark (being equal to the redox potential (E redox ) of the electrolyte), [ 92,93 ] as indicated in Figure 1 A It can be expressed as:

ln

OC B

n N

where k B is the Boltzmann constant, T is temperature, E C is the

CBM of the MO, n c is the free electron density in the CB of

the MO under illumination, and N C is the density of accessible states in the CB of the MO According to Equation 4 , either an

Figure 1 A) Schematic diagram showing the working principle of an SSC

B) A simplifi ed equivalent circuit model of an SSC R TCO include the sheet

resistance of the TCO and the TCO/MO contact resistance; R CT(P) and

C µ are the back electron transfer resistance and the capacitance at the

photoanode/electrolyte interface, respectively; R CT(CE) and C CE are the

charge transfer resistance and the capacitance at the CE/electrolyte

inter-face, respectively Z d is the Warburg diffusion impedance of ions transport

in electrolyte The sum of R TCO , R CT(CE) and Z d corresponds to the series

resistance ( R s ) of the SSC

upward shift of E C or an increase in n c would give rise to an

enlarged V OC It should be noted that n c is not only determined

by the photoelectron generation yield in the QDs, but also by

the electron injection rate from the QDs to the MO, which,

however, is strongly infl uenced by photoelectron recombination

in the QDs, and in particular at the MO/QDs, the

QDs/electro-lyte, and the MO/electrolyte interfaces

To further analyze the dynamic information on charge

trans-port and recombination, Figure 1 B shows a simplifi ed

elec-trochemical impedance equivalent circuit of an SSC [ 94–97 ] In

such an equivalent circuit, R CT(P) is the back electron transfer

resistance at the MO/QDs/electrolyte interfaces which display

a diode-like behavior Combination of the sheet resistance of

TCO ( R TCO ), the charge transfer resistance at the CE/electrolyte

interface ( R CT(CE) ), and diffusion resistance of the redox species

in the electrolyte (Z d ) gives the series resistance ( R s ), i.e., R s =

R TCO + R CT(CE) + Z d [ 98,99 ] The shunt resistance ( R sh ) represents

the effects that divert photogenerated carriers from fl owing in

the external circuit R CT(P) , which is associated with the

trap-ping and recombination of photogenerated carriers at

inter-faces of MO/QDs, MO/electrolyte, and QDs/electrolyte, could

be considered as a part of R sh An ideal solar cell should have

a large R sh and a small R s to attain high values of J SC , V OC and

FF Therefore, a larger R CT(P) and smaller R CT(CE) and R TCO are

desirable R CT(CE) can be regarded as an indicator to reveal the

electrocatalytic activities of CE materials, which has signifi cant

infl uences on the J SC and the FF A smaller R CT(CE) facilitates

electron transfer from the CE to the electrolyte for catalyzing

electrolyte regeneration, consequently results in less interfacial

recombination

Overall, the performance ( J SC , V OC and FF ) of an SSC

asso-ciated with Φ CT , n c , R sh , and R s is strongly infl uenced by the

surface trap states and the recombination of photoelectrons in

QDs and MO, and at MO/QDs, QDs/electrolyte, MO/electrolyte

and CE/electrolyte interfaces The least charge recombination

in the processes of photoelectron generation, and charge ration and transport is desired for pursuing high photovoltaic performance Therefore, suppression of carrier recombina-tion by surface/interface engineering is an important key for improving the PCE of SSCs

3 Controllable Synthesis and Post-Treatments of ZnO Nanostructures on TCO Substrates

ZnO has been shown both experimentally and theoretically to

be a promising electron transfer semiconductor for low-cost and high-performance SSCs [ 16 ] ZnO nanostructures offer a scaffold for effective loading of QDs, and they play important roles in light scatting, charge separation and transportation in SSCs These particular behaviors require growing ZnO nanostruc-tures with controllable and tunable morphologies, sizes and crystallinity which have direct effects on carrier transport and photon trapping and scattering [ 100,101 ] Various ZnO nanostruc-tures have thus far been developed, including particles, [ 102,103 ] rods, [ 104,105 ] wires, [ 106,107 ] belts, [ 108,109 ] tubes, [ 110–112 ] rings, [ 113,114 ] sheets, [ 115,116 ] combs, [ 117,118 ] nails, [ 119,120 ] tetrapods, [ 121,122 ] branched structures [ 123–126 ] and hierarchical structures [ 127–129 ] ZnO nanostructures have been synthesized by a variety of approaches, e.g., the sol-gel method, [ 130–132 ] hydrothermal/solvothermal growth, [ 104–106 ] physical or chemical vapor depo-sition, [ 107,108,122 ] and electrochemical deposition [ 110,111 ] For applications in solar cells, two approaches, i.e., deposition of pre-synthesized ZnO nanostructures and direct growth of ZnO nanostructures on TCO substrates, have been mostly used

Figure 2 A shows different types of ZnO nanostructures on TCO substrates, including disordered nanostructures (i and ii), 1D nanoarrays (iii and iv), and hierarchical nanostructures

Figure 2 A) Scheme of ZnO nanostructures deposited on TCO substrates including disordered nanostructures (i and ii), 1D nanoarrays (iii and iv)

and hierarchical structures based on 1D nanoarrays (v and vi) B) Typical SEM images of the corresponding ZnO nanostructures on TCO substrates: (i) Nanoparticles Reproduced with permission [ 140 ] Copyright 2013, The Royal Society of Chemistry (ii) Disordered nanorods Reproduced with permis-sion [ 105 ] Copyright 2013, The Royal Society of Chemistry (iii) Array of nanorods Reproduced with permission [ 110 ] Copyright 2007, Elsevier (iv) Array

of nanotubes Reproduced with permission [ 110 ] Copyright 2007, Elsevier (v) Array of nanoforests Reproduced with permission [ 125 ] Copyright 2008, American Chemical Society (vi) Bilayer structures Reproduced with permission [ 156 ] Copyright 2009, Elsevier

based on 1D nanoarrays (v and vi) The typical SEM images of

these ZnO nanostructures are presented in Figure 2 B

3.1 Coating of Pre-synthesized ZnO Nanostructures on TCO

Substrates

Various ZnO nanostructures (nanoparticles, nanorods,

nano-tetrapods and nanospheres, etc.) have been prepared ex situ by

different chemical and physical methods The pre-synthesized

ZnO nanostructures were deposited on TCO substrates for

SSC applications (Panel i and ii of Figure 2 A) by the methods

including spin coating, [ 131,132 ] screen printing, [ 133,134 ] spray

coating, [ 135,136 ] and doctor-blade methods [ 137–141 ] As a typical

example, ZnO nanoparticles can be synthesized by preparing

ZnO sols in a homogeneous alcoholic solution (such as

meth-anol, ethmeth-anol, propanol and butanol) containing zinc acetate

precursor and additives (such as alkali metal hydroxides,

car-boxylic acids, alkanolamines, alkylamines, acetylacetone and

polyalcohols) [ 132 ] Such sol containing ZnO nanoparticles, with

average diameter in the ten to several tens nanometers, have

been coated onto the TCO substrates by spin or dip coating

resulting in a change from liquid sol into solid wet gel Drying

and heat treatments were then used to generate a

porous-struc-ture fi lm on the TCO glass substrate [ 132 ]

Besides the spin/dip coating, screen printing and doctor-blade

methods are also used to deposit pre-synthesized ZnO

nano-structures onto the TCO substrates In general, a suffi ciently

viscous paste of ZnO nanostructures can be prepared by mixing

the nanostructures with organic binders, such as polyethylene

glycol, [ 137 ] acetyl acetone, [ 138 ] butanol [ 139 ] or mixture of ethyl

cel-lulose and terpineol [ 140,141 ] The ZnO paste can then be spread

onto the TCO substrates by the screen printing process or the

doctor-blade method Uniform fi lms of ZnO nanostructures

with controlled thickness and pore size on TCO substrates can

be obtained by suitable heat treatment to remove the residual

organic binders and solvents These deposition approaches have

advantages on manipulating the morphology and size of the

pre-synthesized ZnO nanostructures However, an additional

deposition process is required, which might also cause organic

contamination and affect the quality of the ZnO/TCO

con-tacts Moreover, the electron transport pathway in photoanodes

of such ex situ prepared ZnO nanostructures is random and

winding, which increase the probability of carrier

recombina-tion due to the increased grain boundaries and diffusion length

3.2 Direct Growth of 1D ZnO Nanoarrays on TCO Substrates

Compared with the coating of pre-synthesized ZnO

nanostruc-tures, direct growth of 1D ZnO arrays (nanorods, nanowires,

and nanotubes) on TCO substrates has obvious advantages for

photovoltaic applications Firstly, the 1D ZnO

nanorods/nanow-ires (Panel iii of Figure 2 A) can provide a direct conduction path

in the interior of a crystal bulk for electron transport, reducing

their scattering at grain-boundaries It has been shown that the

electron diffusivity in ZnO nanowires ( D n = 0.05–0.5 cm 2 s −1 )

which is several hundred times larger than that ( D n ≤ 10 −4 cm 2 s −1 )

in semiconductor nanoparticle fi lms [ 9,142 ] On the other hand,

the array structure can also enhance optical absorption due to light scattering and trapping [ 143 ]

The direct growth of ZnO nanowire arrays on TCO strates is commonly performed by seed-assisted hydrothermal process [ 9,144 ] Compared with vapor based methods, the hydro-thermal process can be conducted at low temperatures, which decrease the possibility of fi lm cracking and nanowires sepa-ration from the substrates, and enables ZnO nanowire growth even on fl exible plastic substrates [ 145–147 ] Also, it is possible to control the density, length and diameter of the ZnO nanowires via manipulating the reaction duration, precursor concentra-tion, and number of repeated growth cycles [ 9 ] In addition, ZnO nanowires prepared by hydrothermal growth are generally free

sub-of metal catalyst and other possible contaminants, which is efi cial for applications in electronic and optoelectronic devices The use of 1D ZnO nanowire arrays provide several advan-tages related to charge separation and transfer as follows The formation of ZnO/QDs core/shell nanocables with type II staggered energy band structure gives a stepwise energy band alignment [ 148 ] Electrons and holes would be preferably trans-ferred across the interface in opposite directions to achieve the formation of an excitonic charge separation state The shells

ben-in the nanocables can also provide effective passivation that inhibits non-radiative recombination of percolated electrons in 1D ZnO with electrolyte and suppresses corrosion of the ZnO cores by electrolyte More signifi cantly, the core/shell nano-cables have large-area interfacial heterojunction As a result, effi cient carrier separation occurs in the radial, instead of the long axial direction, leading to a smaller carrier collection dis-tance comparable to the minority carrier diffusion length [ 1,17,149 ] While 1D ZnO nanowire arrays provide a base for effi cient loading of QDs, the loading of QDs and the junction area can be further increased by the use of arrays of ZnO nanotubes (panel

iv of Figure 2 A) In this case not only the outer surface but also the inner surface of the tubes could be coated with sensitizers for promoting light absorption [ 150–152 ] She et al reported the synthesis of ZnO nanotube arrays using a two-step process, i.e., electrodeposition of ZnO nanorod arrays on TCO substrates, fol-lowed by selective etching of ZnO nanorods to form ZnO nano-tubes [ 110,111 ] The formation of ZnO nanotubes was proposed

to be due to the defect-selective etching of the core of the ZnO

nanorods along the c axis by high concentration OH − or H + in solutions Enhanced photo-electrochemical properties were demonstrated after ZnO nanorods were converted to ZnO nano-tubes While the CdS sensitized ZnO nanorods array showed a photocurrent density of 7.00 mA cm −2 at 0 V vs saturated cal-omel electrode (SCE), the CdS sensitized ZnO nanotube arrays increased the photocurrent density to 10.64 mA cm −2 [ 150 ] Yang et

al also observed an obvious increase of J SC from 1.86 mA cm −2 for the CdS sensitized ZnO nanorod SSC to 4.07 mA cm −2 for the CdS sensitized ZnO nanotube SSC under 1 Sun illumina-tion, correspondingly PCE increased from 0.33% to 0.87% [ 151 ]

3.3 Growth of 3D Hierarchical ZnO Nanostructures on TCO Substrates

To maintain the merit of 1D nanostructure for providing direct electron conduction pathway and meanwhile to further

increase the surface area of ZnO nanostructure for light

har-vesting and QD loading, a vertically-aligned branched-nanowire

“forest” (Panel v of Figure 2 A) has been synthesized The ZnO

“nanoforests” were typically obtained by following three steps:

i) growth of ZnO nanowires arrays on TCO substrates, ii)

depo-sition of a ZnO seeding layer on the ZnO nanowire surface,

and iii) growth of branched nanorods on the surface of ZnO

nanowires [ 123–126,153,154 ]

Bilayer architectures have also been used for SSC

aplica-tions; these consist of ZnO nanorod array as the bottom layer

and ZnO nanostructures (such as nanofl owers, nanospheres,

and nano-tetrapods) as the upper layer [ 155–157 ] In the bilayer

fl ower-rod structure illustrated in Panel vi of Figure 2 A, an

array of ZnO nanorods with a uniform density is fi rst prepared

on a TCO substrate followed by the growth of ZnO nanofl owers

on the top surfaces of the nanorods The top layer increases the

roughness factor (RF) of the ZnO photoanode and consequently

improves the effective loading density of QDs and overall PCE

of the fabricated SSCs

3.4 Post-Treatment of ZnO Nanostructures

Among the mentioned synthesis methods, the solution

approach is of considerable interest since it is

environmen-tally friendly, and has low production cost and low synthesis

temperature However, the ZnO nanostructures prepared at

low temperatures especially by the solution methods are

typi-cally featured with high defect densities, low conductivities,

and probable residual organic contamination on their surfaces

Various post-treatments, such as plasma modifi cation, [ 36,158,159 ]

UV irradiation, [ 160 ] and in particular annealing under

dif-ferent conditions, [ 161–165 ] have been demonstrated to be feasible

approaches enabling improved crystallinity, increased

conduc-tivity, and/or enhanced stability of ZnO nanostructures For

example, exposure of ZnO nanowire arrays to oxygen plasma

was shown to be effective for removing the surface

con-tamination and thus enhancing the QDs adsorption (as

dis-cussed in more details in Section 4.1.1) [ 36 ] The donor density

(5.19 × 10 19 cm −3 ) of the as-grown ZnO nanorods could be

increased to 1.79 × 10 20 cm −3 by hydrogen plasma treatment and

decreased to 1.65 × 10 19 cm −3 by oxygen plasma treatment [ 158 ]

Annealing has been demonstrated to be a powerful tool

for improving the crystallinity and thermal stability of

as-grown ZnO nanostructures; and annealing parameters, e.g.,

atmosphere, temperature, and duration, have been shown to

have signifi cant infl uences on the properties of ZnO

nano-structures [ 161–165 ] Zhang and Li et al reported that annealing

in air could signifi cantly improve the crystal structure and

reduce defects but had little effect on hole-trapping In

con-trast, annealing in hydrogen atmosphere leads to a reduction

in hole-trapping due to the passivation of Zn vacancy trap

states As a consequence, samples fi rst annealed in air

fol-lowed by hydrogen treatment showed decreased hole-trapping

and increased conductivity [ 163 ] The shape and intensity of

defect photoluminescence emission from ZnO were founded

to depend strongly on the annealing atmosphere and

temper-ature [ 161,164 ] Cabot and co-workers reported recently that the

ZnO nanowires annealed in Ar exhibited a four-fold decrease

in electrical resistivity (15.6 Ω cm down to 3.6 Ω cm) The improved conductivity was attributed to the reduced negatively charged oxygen-containing species (CO 2 , O 2 − , O 2− , O − , OH − , or

H 2 O) adsorbed on the ZnO surface and the higher tion of oxygen vacancies induced during argon Ar annealing

concentra-As a result, the DSSCs composed of Ar-annealed ZnO

nanow-ires exhibited 50% increase in J SC , and yielded 30% ment in PCE as compared with the cells based on air-annealed ZnO nanowires [ 165 ]

enhance-Furthermore, doping of ZnO nanostructures could be achieved by annealing in atmospheres containing gases such as

NH 3 [ 166,167 ] Controllable N concentrations (atomic ratio of N to Zn) up to ca 4% was achieved by varying the annealing time IPCE studies revealed that the ZnO:N nanowire arrays yielded

an obvious increase of photoresponse in the visible region pared to the undoped ZnO nanowires An increase of photocur-rent density by one order of magnitude and a photoconversion effi ciency of 0.15% at an applied potential of +0.5 V (vs Ag/AgCl) were obtained for the ZnO:N nanowires in the applica-tion for water splitting [ 166 ]

It should be noted that the post-treatments of ZnO and their impact on the applications of ZnO in electronic and optoelec-tronic devices, [ 159 ] DSSCs, [ 165 ] and water splitting [ 166,167 ] have been extensively studied However, there have been only lim-ited reports for the ZnO nanostructures employed in SSCs Further studies are still needed to explore the benefi cial effects

of post-treatments on the performance improvement of ZnO nanostructure based SSCs

4 Surface Sensitization of ZnO Nanostructures

While ZnO is an excellent electron transporting material, it cannot effectively harvest visible light due to its wide bandgap Therefore, surface sensitization of ZnO nanostructures is essential to enhance the light absorption capability, and carrier generation and separation of ZnO-based photovoltaic devices Thus far, various chemical and physical technologies have been developed to modify the surface of ZnO nanostructures with QDs and noble metal nanoparticles

4.1 Sensitization with QDs Using Solution Methods

Loading of suitable narrow bandgap QDs on the surfaces of ZnO nanostructures is an effective way to enable harvesting

of visible light Two main strategies are mostly employed to decorate nanostructured ZnO with QDs: i) ex situ growth

of colloidal QDs and subsequent attachment of the synthesized QDs to the surface of ZnO nanostructures via bifunctional linker molecules; [ 36,143,168–172 ] ii) in situ growth

pre-of QDs on the ZnO surface by chemical reaction pre-of ionic cies using the methods including chemical bath deposition (CBD), [ 32,152,173–175 ] successive ionic layer adsorption and reac-tion (SILAR), [ 115,151,176–181 ] ion-exchange, [ 48,182–187 ] and electro-chemical deposition [ 146,150,188–194 ] In comparison with ex situ process, the in situ approach involve direct nucleation and growth of QDs on ZnO surface, typically leading to improve-ments in effective loading and uniform coverage of QDs;

however, it increases the diffi culties in controlling the size

dis-tribution of the deposited QDs

4.1.1 Attachment of Pre-synthesized QDs by Molecular Linkers

The pre-synthesized QDs are typically attached to the surface

of ZnO nanostructures using bifunctional molecular linkers

Commonly used linkers include thioglycolic acid (TGA),

mer-captopropionic acid (MPA), mercaptoalkanoic acid (MAA),

methoxybenzoic acid (MBA), and cysteine (CYS), as shown in

the right column of Figure 3 A typical feature of these linkers

is that they bear simultaneously carboxylate and thiol functional

groups [ 6 ] The carboxylic acid group (–COOH) and the thiol

group (–SH) can respectively bind to ZnO and metal

chalco-genide QDs, respectively Other linker molecules such as oxalic

acid (OA), malonic acid (MA), hexandithiol (HDT), thioacetic

acid (TAA), and thiolactic acid (TLA) have also been reported

for decorating QDs on ZnO nanostructure surface [ 6,168,169 ]

The effect of molecular linkers has been studied taking the

assembly of pre-synthesized PbS QDs on ZnO porous fi lms as

an example [ 168 ] The ZnO fi lms, with a thickness between 300

and 400 nm, were prepared by spin coating ZnO nanoparticles

onto ITO (ITO-ZnO) substrates After annealing, the ITO-ZnO

substrates were put into a solution of molecule linker (e.g., OA,

MA, TAA, TGA, MPA, and HDT) in tetrahydrofuran (THF)

for surface treatment Then the linker modifi ed ITO-ZnO

substrates were immersed in a THF solution containing

pre-synthesized PbS QDs A clear color change from almost

trans-parent to a distinct brown coloration was observed, while there

was no change discernible by eye on the ITO-ZnO substrates

without linker modifi cation The degree of the coloration could

provide a visual aid to evaluate the amount of PbS adsorbed on

the surface The gained absorption spectrum of the

ITO-ZnO-linker-PbS substrate showed a weak absorption shoulder in the

NIR, which matched well with the solution phase absorption of

PbS nanoparticles

The attachment of colloidal QDs through molecular linkers

enables the use of QDs with precise control of their shape and

size of the QDs This technique has achieved great success in high performance SSCs using TiO 2 mesoporous fi lms as photo-anodes [ 37,43,71 ] However, it still faces diffi culties in achieving uniform coverage and suffi cient loading of QDs onto the ZnO nanostructured photoanode, probably due to the large dimen-sion and different surface chemical states of ZnO nanostruc-tures, which limits their light harvesting and corresponding photovoltaic performance [ 36,169,170 ] On the other hand, surface states of ZnO, such as surface charging, dangling bonds, and surface contamination, seriously affect the attachment of col-loidal QDs Therefore, treatment of ZnO surface is generally required to improve QD loading Aydil et al reported that enhanced coverage of colloidal MPA-capped CdSe QDs on ZnO nanowire surface can be achieved by exposing the ZnO nanow-ires to oxygen plasma [ 36 ] The treatment removed the surface-bound contaminants (surface hydroxyl and hydrocarbon groups) which prevented the colloidal QDs from attaching to the ZnO nanowire surface through the carboxyl group It was demonstrated that oxygen plasma treatment of ZnO nanowires

increased J SC to 2.1 mA cm −2 and PCE to 0.4%, which were more than one order of magnitude higher as compared with those of the SSC assembled using untreated ZnO nanowires The molecular linkers serve as a binding bridge between ZnO and QDs; however, they also act as in-series component in the charge transfer processes (Figure 3 ) The linker molecules impose a barrier potential between ZnO and QDs, which has

to be overcome for electron transfer [ 195 ] Therefore, the nature

of the molecular bridges is an important issue to be concerned for electron transfer processes Much effort has been devoted

to optimizing the photoelectron injection rates and electrochemical responses of the cells by changing the linker molecules, particularly by varying the alkyl chain length and

photo-by selecting molecules ending with different acid and/or thiol groups as the attachment moieties [ 169,195 ]

4.1.2 CBD of QDs on ZnO Surface

CBD is one of the most commonly used methods for direct growth of QDs onto ZnO nanostructures In this one-pot syn-thesis method, the ZnO nanostructures are immersed in an intended QD precursor solution for certain duration For SSC applications, effective loading and homogeneous coverage

of QDs on ZnO surface are desired, but aggregation of QDs should be minimized to enhance light absorption and reduce

charge recombination ( Figure 4 A,B) Aggregation of QDs on

ZnO surface (Figure 4 A) increases the diffusion length and the probability of recombination of photogenerated electrons, and thus results in a reduced injection rate of photoelectrons into ZnO [ 172,173 ]

CdS QDs have been deposited on ZnO nanowires in a chemical bath solution of CdSO 4 , thiourea, and ammonia It was shown that the quality of QDs depends strongly on the pH value of the solution, the precursor concentration, its reaction temperature, and the reaction duration in CBD process Reac-tion in dilute solutions improved the coverage of CdS QDs on ZnO surface, but led to reduced QDs loading On the other hand, prolonging the reaction duration was revealed to induce aggregation of the CdS QDs [ 175 ]

Figure 3 Schematic illustration of QDs sensitized ZnO by molecular

linker

Sulfurization of ZnO nanostructure surface has been

shown to enable a signifi cant improvement of CdS QDs

cov-erage [ 175,185 ] The SEM images in Figure 4 C and D depict CdS

QDs synthesized in ammonia/thiourea bath on the surfaces

of ZnO nanorods without and with sulfi de treatment,

respec-tively By converting the surfaces of ZnO to ZnS with an

alka-line sulfi de solution treatment, a CdS QD layer with

thick-nesses of ca 10 nm was uniformly covered on ZnO nanorod

surface Similar results were also observed in the deposition

of CdSe QDs on ZnO nanorods by CBD By incorporating

sur-face sulfurization, the coverage of CdSe QD layer was obviously

improved, as shown in Figure 4 E and F

4.1.3 SILAR Method for Depositing QDs on ZnO Surface

The SILAR method is another approach used for in situ

depo-sition of QDs on nanostructured ZnO surfaces by alternative

adsorption of cations and anions in respective solutions [ 176,177 ]

The growth of QDs is controlled by tuning the number of

cycles, solvents and precursor concentrations For CdS QD deposition, the SILAR method involves successive immersion

of ZnO nanostructure in solutions of Cd 2+ and S 2− ions and

rinsing between dips ( Figure 5 A) while the desired CdS

thick-ness is obtained by repeating the processes as needed The correlations between the number of SILAR cycles and the thick-ness of CdS QD layer can be demonstrated through high-reso-lution TEM analyses, as shown in Figure 5 B In an ideal SILAR process, the thickness should increase with cycling number, regardless of the sample surface area and dipping time [ 176 ] The absorption spectra of the ZnO/CdS core/shell nanowire arrays with different CdS shell thicknesses are presented in Figure 5 C

An absorption edge shorter than 420 nm is observed for the bare ZnO nanowire array, and it continuously red shifted to the visible light region with increasing number of SILAR cycles Such cycle-dependent layer-by-layer growth in the solution-phase SILAR process has been shown to be a powerful thin-

fi lm growth technique in current semiconductor processing During the SILAR process, the ZnO surface is fi rst converted

to ZnS by ion exchange in sulfi de solution Therefore, similar

to the CBD approach on a sulfi de-treated ZnO surface, [ 175,185 ] the SILAR method typically is characteristic with an even cov-erage of QDs on ZnO nanostructures Comparing with CBD, the SILAR process typically gives a better control on the thick-ness uniformity of the QD layer, but many repeating cycles are required to achieve suffi cient QD layer thickness

4.1.4 Ion Exchange for ZnO Surface Sensitization

Surface sensitization of ZnO nanostructures by ion exchange technique is based on the large difference in solubility product

constant ( K sp ) between the precursor and the target

semicon-ductors The K sp is the equilibrium constant for a chemical reaction in which an ionic compound dissolves to produce its

ions in a solution An ionic compound with a smaller K sp is more diffi cult to be dissolved in a solution than that with a

lager K sp For example, in an ion exchange reaction, AB + C −

= AC + B − , when the K sp value of target semiconductor (AC)

is suffi ciently smaller than that of precursor semiconductor (AB), the C − ions in solution are driven to replace the B − ions

in precursor semiconductor, leading to the formation of target semiconductor (AC) on the surface of precursor semiconductor (AB) [ 196 ]

In ion exchange reactions, ZnO can be easily converted to ZnS by surface sulfurization or to ZnSe by surface seleniz-

ation due to the much larger K sp value of Zn(OH) 2 (10 −16.5 ) with respect to those of ZnS (10 −23.8 ) and ZnSe (10 −25.4 ) [ 197–200 ] For example, the following reaction takes place in the surface sele-nization process:

ZnO Se2 H O ZnSe 2OH

A large equilibrium constant of the reaction:

[OH ][Se ]

[Zn ][OH ][Zn ][Se ]

(Zn(OH) )(ZnSe) 10

2 2

indicates that the reaction is spontaneous

Figure 4 Schematic transport path of photogenerated electron in ZnO

nanorod-based photoanodes with (A) aggregated, and (B) uniformly

cov-ered QD sensitization layer SEM images showing the effect of sulfi de

treatment on ZnO surface coverage by (C,D) CdS from

ammonia/thio-urea bath, and (E,F) CBD CdSe Left column images (C,E) are untreated

ZnO rods, right column images (D,F) show sulfi de-treated ZnO rods The

insets are higher magnifi cation backscattered images A–F) Reproduced

with permission [ 175 ] Copyright 2010, American Chemical Society

ZnSe (or ZnS) has a relatively larger K sp value as

com-pared with some other metal chalcogenides (selenides and

sulfi des), such as CdS (10 −26.1 ), CdSe (10 −35.2 ), Ag 2 S (10 −49.2 ),

Ag 2 Se (10 −63.7), CuS (10 −35.2), CuSe (10 −48.1), PbS (10 −27.1 ),

PbSe (10 −42.1 ), HgS (10 −52.4 ), HgSe (10 −59 ), CoS (10 −24.7 ), CoSe

(10 −31.2 ), NiS (10 −24 ), NiSe (10 −32.7 ), In 2 S 3 (10 −73.24 ) and Sb 2 S 3

(10 −92.8 ) Therefore, ZnSe (or ZnS) can further act as precursors

to prepare more stable metal chalcogenides, obtaining a series

of chalcogenide semiconductor sensitized ZnO photo anodes

By successive anion and cation exchange reactions, single or

double shelled semiconductor sensitizers could be coated on

ZnO surface, e.g., arrays of ZnO/ZnSe/CdSe trilayer

nano-cables [ 183 ] and bilayer ZnO/Zn x Cd 1– x Se nanocables [ 184 ]

The ion exchange method could erate a continuous and uniform layer of QD

gen-shell on ZnO surface Figure 6 A illustrates

the synthesis process of copper indium selenide (CIS) shells on ZnO nanorod sur-faces by successive ion exchange [ 182 ] In the fi rst stage, the ZnO nanorods arrays were grown on TCO substrates by the seed-assisted growth method as discussed above The Se 2− solution is prepared by reducing

Se powder with NaBH 4 in distilled water

As the K sp of Zn(OH) 2 is much larger than that of ZnSe, the ZnO nanorod array can

be used as a sacrifi cial template to size more stable ZnSe by anion exchange (Equation 5 ) Upon immersing a ZnO nanorod array into a Se 2− ion solution, ion exchange reaction between Se 2− and ZnO takes place, which produces a continuous ZnSe layer on the surface of ZnO nanorods resulting in ZnO/ZnSe core/shell nanoca-bles The ZnO/ZnSe core/shell nanocable arrays are then immersed in a Cu 2+ ion solu-

synthe-tion Due to the smaller K sp value of CuSe compared to that of ZnSe, Cu 2+ ions replace

Zn 2+ ions in ZnSe shells to form CuSe shells, leading to the formation of ZnO/CuSe core/shell nanocables Finally, CIS is synthesized by reacting CuSe shells with

In 3+ via a polyol reduction process In this step, the ZnO/CuSe core/shell nanocable array is immersed in In 3+ ion contained tri-ethylene glycol (TEG) solvent The growth of CIS shell is accompanied with the gradual dissolution of ZnO cores, and prolonging the reaction time may lead to complete etching of ZnO cores and the formation of CIS nanotube array The TEM image and corresponding electron energy loss spectro-scopic (EELS) elemental mappings in Figure

6 B further confi rm the uniform thickness of the nanotube and homogeneous distribution

of Cu, In and Se throughout the tube wall

It is interesting to note that subjecting ZnO/ZnSe and ZnO/CuSe nanocable arrays to acidic etching could be used to prepare ZnSe and CuSe nanotube arrays, respectively

4.1.5 Electrochemical Deposition of Semiconductor Sensitizers

Various semiconductors such as CdS, [ 188–190 ] CdSe, [ 191,192 ] CdTe, [ 193,194 ] and PbSe, [ 49 ] have been deposited on ZnO nanowire/nanorod arrays using electrochemical deposition The electrochemical deposition is usually carried out in a three-electrode electrochemical workstation Standard saturated calomel electrode (SCE) and Pt foil are used normally as the reference and counter electrodes, respectively, while the ZnO nanostructures grown on TCO substrate used as the working

Figure 5 A) Schematic diagram showing the SILAR deposition processes of CdS QDs on ZnO

nanowires A) Reproduced with permission [ 176 ] Copyright 2013, Elsevier B) HRTEM

observa-tions of CdS QD layer thickness upon cycle numbers in SILAR process C) UV–vis absorption

spectra of the as-prepared ZnO nanowire and the ZnO/CdS core/shell nanowire arrays, where

the shell thickness increases with the number of SILAR cycles (5, 10, 15, 30, 60, 90, 120 cycles)

The photoanodes were grown on Ti foil substrates B,C) Reproduced with permission [ 177 ]

Copy-right 2009, The Royal Society of Chemistry

electrode The electrolyte selection is a key factor for the

elec-trochemical deposition of QDs and a critical prerequisite is that

the electrolyte does not etch the ZnO nanostructures The

elec-trochemical deposition is usually performed in galvanostatic

or potentiostatic mode In contrast to the deposition methods

mentioned above, electrical current is the driving force for

pro-cessing the deposition; the deposition rate and quality of QDs,

however, are also controlled by the operation mode, precursor

concentration in electrolyte, and deposition duration

Li et al reported the electrochemical synthesis of ZnO/CdTe

core-shell nanocables [ 193 ] The deposition of CdTe was performed

at a fi xed potential of −1.0 V vs SCE Figure 7 shows a single

ZnO/CdTe nanocable, revealing a clear ZnO/CdTe core-shell

structure It should be pointed out that complete coverage of the

ZnO core by a CdTe shell was achieved without any interfacial

void formation CdTe is a promising photovoltaic material with

advantages of high optical absorption coeffi cient (ca 10 4 cm −1 )

and a band gap of ca 1.5 eV The ideal absorption properties

of the CdTe shell and the type II staggered band alignment

(Figure 7 E) would make the ZnO/CdTe core/shell nanocables a

promising photoelectrode for solar energy conversion

4.2 Sensitization of ZnO Using Vapor Phase Methods

In addition to the above chemical solution approaches, various

vapor phase methods, including chemical vapor deposition

(CVD), pulsed laser deposition (PLD), thermal evaporation,

and sputtering, have been employed to deposit narrow bandgap semiconductors onto ZnO nanostructures [ 201–207 ] As compared with the chemical solution methods, vapor phase methods typi-cally need elevated temperatures to grow the semiconductor sensitization layers resulting in high crystalline quality and even epitaxial growth of the sensitizers on the ZnO nanostructure surfaces Due to the reduced defect density, epitaxial growth of high quality semiconductors on ZnO surface could decrease the extent of non-radiative recombination and carrier scattering loss particularly at the ZnO/sensitizer interface, and benefi t the charge separation and transport [ 205,207 ] Nevertheless, much less work has been reported on gas phase synthesis of sensitizer on

Figure 6 A) Ion exchange processes for the formation of ZnO-based

nanocables and corresponding nanotubes B) TEM image of a CIS

nano-tube and the corresponding Cu, In, and Se elemental EELS mappings of

the same region A,B) Reproduced with permission [ 182 ] Copyright 2010,

American Chemical Society

Figure 7 A) TEM image of a single ZnO/CdTe nanocable B) Elemental

profi le obtained from STEM-EDX showing the distribution of the positional elements (Zn, O, Te, and Cd) along the radial direction of the nanocable (indicated by the red arrow in panel (A)) C,D) HRTEM image and SAED pattern taken from the same ZnO/CdTe nanocable E) Sche-matic of the operation of ZnO/CdTe nanocable grown on ITO substrate for SSC application A–E) Reproduced with permission [ 193 ] Copyright

com-2010, American Chemical Society

ZnO nanostructures, mainly due to the more expensive and

complicated high-vacuum deposition facilities usually required

by vapor phase methods

CVD technology has been widely used for synthesizing

nano-materials and surface coating for electronic and optoelectronic

applications This method provides a great controllability on

the composition, morphology, and crystallinity of the materials

deposited by tuning the reactive gas composition, pressure, and

substrate temperature Recently, much effort has been devoted

to synthesizing nanostructured ternary chalcogenide alloys

with controllable composition using the CVD approach ZnO/

CdS x Se 1− x , [ 201 ] ZnO/Zn x Cd 1− x Se, [ 44,202 ] and ZnO/ZnS x Se 1− x [ 204 ]

core/shell nanocables with tunable shell composition have been

successfully synthesized Park et al reported CVD synthesis of

ZnO/CdS x Se 1– x core/shell nanocables with tunable shell

com-position in a full range (0 ≤ x ≤ 1) where the ZnO nanorod

substrates were placed downstream apart from the CdS/CdSe

mixed powder precursors in a CVD reactor [ 201 ] Thickness of

the deposited CdS x Se 1− x shell was then controlled by adjusting

the growth temperature or duration Figure 8 A shows a TEM

image of a ZnO/CdS 0.5 Se 0.5 core/shell nanocable with a shell

thickness of 50 (±10) nm The lattice-resolved image of the

interface region revealed a single-crystalline wurtzite shell with

the (001) planes parallel to the ZnO (001) planes, as shown in Figure 8 B; and the Fast Fourier Transform (FFT) and electron diffrac-tion (ED) pattern also verifi ed the alignment

of [0001] CdS 0.5 Se 0.5 with [0001] ZnO (insets) Energy dispersive spectroscopic (EDS) map-ping in Figure 8 C and the line-scanning in Figure 8 D demonstrated the formation of ZnO/CdS 0.5 Se 0.5 core/shell structure with Zn confi ned in the core only The stoichiometry

of the ternary CdS x Se 1– x shell could be well controlled by changing the ratio of CdS and CdSe source powder Measurements on the composition-dependent optical absorption reveal a decrease in bandgap with increasing

Se content [ 201 ] The ZnO/CdS nanocables exhibited a bandgap of 2.35 eV with CdS shell thickness of ca 50 nm, and the ZnO/CdS 0.5 Se 0.5 nanocables showed a red shift

to 1.95 eV, matching well to that of the bulk CdS 0.5 Se 0.5 The bandgap of ZnO/CdSe nano-cables was estimated to be 1.66 eV

Pulsed laser deposition (PLD) is a physical deposition technique that uses a high power, short-pulse laser beam focused on the surface

of the source material (target) This material

is thus vaporized from the target in the form

of a plasma plume which can then deposit as

a thin fi lm onto a substrate in an ultra-high vacuum or in the presence of a back-fi lled, inert or reactive gas The PLD processing parameters include the target-to-substrate distance, deposition duration, pulse rep-etition frequency, and laser energy density Wang et al reported the use of PLD tech-nique for coating ZnSe on ZnO nanowire arrays [ 205 ] The TEM image of the resulting

ZnO/ZnSe core/shell nanocable, Figure 9 A, indicates the ZnSe

shell can grow on the ZnO nanowire surface with a thickness

of about 5–8 nm in the radial direction A sharp interface of the ZnO/ZnSe core/shell nanocable is confi rmed by the HRTEM image in Figure 9 B, which reveals that ZnO and ZnSe present Wurtzite (WZ) and zinc blende (ZB) crystalline structures, respectively Figure 9 C and D show the FFT patterns of the (WZ) ZnO core and the (ZB) ZnSe shell, with zone axes [2–1–10] and [011], respectively, which further confi rms the epitaxial growth The spatial distributions of the atomic composition across the ZnO/ZnSe core/shell nanocable are shown in the EDS line-scan analysis (marked by a line in Figure 9 A), showing the homogeneous coating of the ZnO nanowire (Figure 9 E)

4.3 Sensitization with Noble Metal Nanoparticles

Noble metal nanoparticles (NPs), such as Au and Ag, have also been decorated on ZnO NWs to enhance light absorption based

on localized surface plasmon resonance (LSPR) effects [ 208–210 ] LSPR is originated from the interaction of incident light with electrons in the metal NPs, and it has been extensively studied

Figure 8 A) TEM image of a ZnO/CdS x Se 1− x ( x = 0.5) core/shell nanorod B) HRTEM image

of the ZnO/CdS x Se 1− x interface region, showing the epitaxial growth single-crystalline wurtzite

CdS x Se 1− x ( x = 0.5) on ZnO The corresponding FFT and ED patterns confi rm the parallel

align-ment of the [0001] CdS x Se 1− x ( x = 0.5) with [0001] ZnO (insets) C) EDS mapping of the ZnO/

CdS x Se 1− x ( x = 0.5) core/shell nanocable D) Line-scan of the ZnO/CdS x Se 1− x ( x = 0.5) core/shell

nanocable, showing Zn and O elements in the core, and Cd, S, and Se elements in the shell

A–D) Reproduced with permission [ 201 ] Copyright 2010, American Chemical Society

in enhancement of Raman scattering, biomedicine, and solar

cells [ 211–213 ] The plasmon resonance wavelength depends

strongly on the size and composition of the material as well

as on its local dielectric environment, [ 211 ] which give us

oppor-tunity to design and tailor the optical properties of the noble

metal NPs sensitized photoanodes

There have been two major approaches for chemical coating

of noble metal NPs on ZnO nanostructure: ex situ growth by assembling the pre-prepared metal nanoparticles and in situ growth of the metal nanoparticles by chemical deposition Au NPs typically served as the most commonly used plasmonic material to reveal plasmon induced effects on ZnO, because its resonant wavelength is in the visible region, [ 211,212 ] which extended the wavelengths region absorbed by ZnO Some research groups reported Au NPs decorated ZnO nanostruc-tures by directly reducing HAuCl 4 solution [ 208,214,215 ] On the other hand, pre-prepared Au NPs with controlled size and shape could be attached to ZnO nanostructures by using a bifunctional molecular linker [ 209,216 ]

Recent studies showed that Au NPs coated ZnO nanorod arrays present distinct chemical and physical properties, as com-pared with uncoated ZnO nanorods arrays, due to enhanced separation of excited electron-hole pairs For example, a photovoltaic device with a single ZnO nanorod decorated with

Au NPs has been reported to show a high photocurrent of 22.6 µA at a bias of 1.0 V under UV illumination, showing the photocurrent increased nearly 1.5 times in comparison with a device using a pristine ZnO nanorod [ 209 ] ZnO nanorod arrays decorated with Au NPs have been reported to show approxi-mately 8× increase in photocatalytic activity under UV irradia-tion compared to bare ZnO [ 209 ]

Plasmonic enhancement is a useful and important approach for development of high performance photovoltaic devices Introducing Au plasmonic material onto ZnO photoanodes has been reported to markedly enhance their photovoltaic per-formance, which was proposed to involve the coupling of hot electrons formed by plasmons and the electromagnetic fi eld [ 216 ]

Figure 10 A shows the UV–vis absorption spectra of ZnO nanorod arrays coated with different amounts of Au NPs Other than the strong ultraviolet absorption, the bare ZnO nanorods showed no absorption between 400 and 800 nm In contrast, the Au-ZnO composite arrays show obvious absorption band

in the visible region due to the LSPR of Au NPs The related absorbance increases with the increasing loading of Au NPs, which was controlled by varying the deposition duration and conditions Figure 10 B is a schematic diagram showing the mechanism of photocurrent enhancement by LSPR in the Au-ZnO nanostructure Upon irradiation, electrons in the VB

LSPR-of ZnO rod will be excited to the CB Simultaneously, upon diation, plasmon will be induced on the surface of Au NPs that

irra-in turn generate hot electrons and a secondary electromagnetic

fi eld The plasmon-induced hot electrons would also be injected into the CB of ZnO leading to an increase in photocurrent On the other hand, the LSPR can generate a strong electromag-netic fi eld close to the surfaces of the Au NPs The electromag-netic fi eld can modify the band structure at the Au-ZnO inter-face and create more vacancies at the bottom of the ZnO CB

It would further facilitate the generation of photoelectrons by photoexcitation

Chen et al reported enhanced photovoltaic performance of solar cell based on Au NPs sensitized ZnO nanorod arrays [ 217 ]

Figure 10 C showed the J – V curves of the cells before and after

Au NP sensitization using iodide-based electrolyte While the device with bare ZnO nanorods did not show measurable photocurrent, the cell with Au NPs coated ZnO nanorod array

Figure 9 A) Low-magnifi cation TEM image of a ZnO/ZnSe core/shell

nanowire A thin layer of ZnSe was coated on the ZnO nanowire B)

High-resolution TEM image of the interface of the core/shell heterostructure,

enlarged from the rectangular area outlined in (A), showing the epitaxial

growth relationship of the ZnO WZ core and ZnSe ZB shell C,D) FFT

pat-terns of the rectangular areas outlined in (B) E) EDS nanoprobe line-scan

of the elements Zn, Se, and O, across the ZnO/ZnSe core/shell nanowire

as indicated by the line in (A) A–E) Reproduced with permission [ 205 ]

Copyright 2008, Wiley-VCH

presented a J SC of 1.72 mA cm −2 , a V OC of 0.37 V, and a FF

of 0.46, and yielded a PCE of 0.30% The photovoltaic

perfor-mance enhancement was due to the increased optical

absorp-tion in the visible light caused by the LSPR effects from the Au

NPs The hot electrons excited at the Au NP surfaces could be

separated and transported to the CB of the ZnO nanorods with

subsequent drift to the conductive TCO electrode (Figure 10 D)

The Schottky Au-ZnO contact enabled the injection of electrons

from Au NPs to ZnO nanorods while blocking the reverse fl ow

The excited Au ions would capture electrons donated from the

redox species in the electrolyte to compensate for their lost

electrons The oxidized redox species were then regenerated by

taking electrons from the outside circuit via the counter

elec-trode Reaction involved in the photocurrent generation process

in the Au-ZnO Schottky barrier solar cell can be summarized

as follows:

Photoanode : Au+hv→Au⊕+e Au−( )

(7) 2Au⊕+3I−→2Au I+ 3 −

(8) Counter Electrode : I3 −+2e−→3I−

(9)

5 SSCs Based on ZnO Nanostructures

As discussed in Section 2, the power conversion effi ciency

(PCE) of an SSC is determined by its current density–

voltage ( J – V ) characteristics, which includes three important operational parameters, the short-circuit current density ( J SC ),

the open-circuit voltage ( V OC ), and fi ll factor ( FF ) In this part,

we review recent efforts to enhance the performance of nanostructure-based SSCs by various approaches including improvements on optical absorption, charge separation, trans-portation, and recombination processes, as well as optimizing energy levels and gaps of the QDs Recent advances of pho-tovoltaic performance of high effi ciency ZnO nanostructures

ZnO-based SSCs are summarized in Table 2 However, it should

be noted that effi ciencies in these reports have not been

veri-fi ed by national laboratories nor other recognized third parties While the table has to be read with caution, the rapid progress achieved in recent years are unarguable

5.1 Improvement of Short-Circuit Current ( J SC ) by Enhancing Light Absorption and Charge-Injection Effi ciency

The J SC in a SSC is determined by its IPCE which depends

on the light harvesting effi ciency (LHE) and the electron tion yield ( ϕ inj ) from the photoexcited QDs into ZnO fi lm To

injec-achieve a high J SC in SSCs, some basic features are generally required These include wide optical absorption over the vis-ible and the near-infrared regions, effi cient injection of photo-generated electrons into the CB of the ZnO electrode, and effi ciently regeneration of oxidized QDs Herein, we review recent progress in SSCs with specifi c emphasis on the strat-egies for tailoring optical absorption, charge injection, and transfer

Figure 10 A) UV–vis absorption spectra of ZnO nanorod arrays decorated with Au nanoparticles prepared with various durations B) Schematic

illustra-tion of the plasmon-induced effects on Au-ZnO photoelectrode A,B) Reproduced with permission [ 216 ] Copyright 2012, American Chemical Society C)

J – V characteristics of solar cell devices with bare ZnO nanorod array and Au-coated ZnO nanorod array under illumination D) A schematic of a band

diagram corresponding to the ZnO/Au/electrolyte cell structure C,D) Reproduced with permission [ 217 ] Copyright 2009, American Chemical Society

5.1.1 Optical Engineering by Tailoring ZnO Morphologies

Good optical absorption is an obvious basic requirement for any

solar cell design The size and morphology of the ZnO

nanostruc-tured photoanode have important infl uence on its QD loading as

well as light scattering and trapping Although a larger surface area might augment surface recombination losses, it could also enhance light harvesting by enabling more effective QD loading Vertically aligned nanowire arrays have been demonstrated to have good light scattering and trapping properties resulting in

Table 2 Recent photovoltaic performance of ZnO nanostructures based SSCs

electrode

J SC [mA/cm 2 ]

V OC [V]

17.3 17.8

0.761 0.741

0.471 0.398

6.2 5.25

[218]

nanorod-nano-tetrapods

passiv-ated with TiO 2

passivated with TiO 2

passivated with TiO 2

absorption of most incident light with a relatively thin absorber

layer [ 9,143 ] Tena-Zaera et al have investigated the infl uence of

the dimensions of ZnO nanowires on light scattering [ 230 ] These

authors reported that for ZnO nanowires with a constant

diam-eter, the maximum of the total refl ectance increases from 8.6%

to 20.4% for lengths of 0.5 and 2.0 µm, respectively, with no

sig-nifi cant spectral shift observed For ZnO nanowires with a

con-stant length, there was an obvious red shift in the refl ectance

peak with increasing diameter (from 105 to 330 nm) because

the total refl ectance in the long wavelength region increases

with the nanorods diameter Therefore, optical engineering of

nanowire arrays for enhanced scattering for wavelengths where

QDs exhibit a relatively low absorption coeffi cient can result in

increased light absorption

The rational design of the ZnO nanostructures for suffi cient

QD loading and effi cient electron transport is another

impor-tant approach An obvious approach to increase QD loading

is to increase the length and decrease the diameter of the ZnO

nanowire However, it is still a challenge to grow ZnO nanowires

longer than 10 µm with diameters smaller than 100 nm While

ZnO nanowires with longer lengths have been synthesized, their

diameters became correspondingly larger, often also resulting

diminished roughness factors and poor photovoltaic performance

Yang’s group has recently reported a branched layer architecture of ZnO nanorods (NRs) and nano-tetrapods (TPs) as an effi cient photoanode, achieving a PCE as high as 5.24% [ 157 ] In the NR-TP fi lm, the ZnO TPs prepared via vapor transport growth were coated by a doctor blade technique onto

double-a 2-µm-thick ZnO NR double-arrdouble-ay to form double-a double-ldouble-ayer double-ture Additional branched structures were further put onto the double-layer to improve the roughness factor and network con-

architec-nectivity ( Figure 11 A) Such 3D structures (Figure 11 B and C)

effectively increase the surface area for effi cient QD loading CdS- and CdSe-cosensitized ZnO photoanodes of single-layer

TP fi lm, double layer NR−TP fi lm, and branched double layer

NR−TP fi lm were prepared by SILAR method, respectively

The UV–vis spectra in Figure 11 D show that the ZnO/CdS/CdSe TP fi lm, the ZnO/CdS/CdSe NR−TP fi lm and the branched ZnO/CdS/CdSe NR−TP fi lm have a similar absorp-tion onset at around 720 nm However, in the visible region from 400 to 700 nm, the absorbance of the ZnO/CdS/CdSe branched NR−TP fi lm is higher than those for the other two

fi lms The enhanced absorbance is attributed to the secondary branching which allows larger QD loading The IPCE spectra of the three photoanodes in Figure 11 E show signifi cant increase

in photocurrent density from the ZnO/CdS/CdSe single-layer

Figure 11 A) Layout of the double-layer-assembly branching processes for fabrication of the branched ZnO double-layer NR−TP fi lm photoanode Top view (B) and cross sectional view (C) SEM images of the branched ZnO double-layer NR−TP fi lm photoanode D–F) UV−vis absorbance spectra

(D), IPCE curves (E), and illuminated J − V curves (F) of the ZnO/CdS/CdSe TP fi lm (black short-dashed line), the ZnO/CdS/CdSe NR−TP fi lm (orange

dashed line), and the branched ZnO/CdS/CdSe branched NR−TP fi lm (violet solid line) A–F) Reproduced with permission [ 157 ] Copyright 2013, American Chemical Society