CITY UNIVERSITY OF HONG KONG

DEPARTMENT OF

PHYSICS AND MATERIALS SCIENCE

BACHELOR OF ENGINEERING (HONS) IN MATERIALS ENGINEERING

2008-2009

DISSERTATION

Arrays of ZnO nanowires for photovoltaic devices

by

WANG Yunqi

March 2009

Arrays of ZnO nanowires for photovoltaic devices

By

WANG Yunqi

Submitted in partial fulfilment of the

requirements for the degree of

BACHELOR OF ENGINEERING (HONS)

IN

MATERIALS ENGINEERING

from

City University of Hong Kong

March 2009

Project Supervisor : Prof. Igor Bello

i

Acknowledgement

Despite a short period of seven months, the project has been the most invaluable and

constructive experience in my starting scientific career. The final year project provides me an

opportunity to carry out research work independently and systematically. From designing

experiments to analyzing results and drawing conclusions, I have gained precious research

experience and realized the importance of applying scientific knowledge into practice. Here, I

write to extend my heartfelt thanks to those who have assisted me to complete this

dissertation.

Firstly, I would like to express my sincere gratitude to my project supervisor, Prof. Igor Bello,

for his continuous support and encouragement throughout the entire work. Prof. Bello is not

only my mentor but also my friend. He was always there to meet and give invaluable advices,

to raise thoughtful questions, and to proofread and mark up my dissertation. Secondly, I

would like to thank my assessor, Dr. C H Shek, who talked and discussed with me on the

progress of the project and encountered problems. Also, I wish to thank the research student

Mr. Chen Zhenhua who assisted me in experiment designs and trained me in various

characterization techniques. He always shared his ideas and gave me constructive

suggestions.

ii

Table of Contents

Acknowledgement i

Table of Contents ii

List of Figures v

List of Tables viii

Abstract ix

Page

1 Introduction and Objectives 1

2 Literature Review 3

2.1 Nanotechnology 3

2.2 Nanowires 4

2.3 Synthesis of One-dimensional (1D) Semiconductor Nanomaterials 4

2.3.1 Vapor-Liquid-Solid (VLS) Mechanism 4

2.3.2 Solution-Liquid-Solid (SLS) Mechanism 5

2.3.3 Vapor-Solid (VS) Mechanism 5

2.3.4 Oxide-Assisted Growth (OAG) Mechanism 6

2.3.5 Other Growth Mechanisms 7

2.4 Synthesis of Heterostructures 7

2.4.1 Axial Nano-heterostructures 7

2.4.2 Radial Nano-heterostructures 8

2.4.3 Branched Nano-heterostructures 9

2.5 Zinc Oxide nanowires 9

2.5.1 General Properties of Zinc Oxide 9

2.5.2 Growth of Aligned ZnO Nanowires 10

2.5.3 Application of Zinc Oxide in Solar Cells 11

3 Experimental Procedures 13

3.1 Fabrication of ZnO Nanowire Arrays 13

3.1.1 Synthesis of ZnO Nanowire Arrays by a Solution Method 13

3.1.1.1 Cleaning the Substrates 13

iii

3.1.1.2 ZnO Seeding and Nanowire Growth 13

3.1.2 Synthesis of ZnO Nanowire Arrays by a Thermal Evaporation Method 14

3.1.2.1 Preparation of Aluminum Doped Zinc Oxide Buffer Layer 14

3.1.2.2 Growth of ZnO Nanowire Arrays on AZO/Si Substrates 14

3.2 Design and Fabrication of Photovoltaic Devices 15

3.2.1 Fabrication of Dye Sensitized Solar Cells (DSSCs) 15

3.2.1.1 Adsorption of Dye 15

3.2.1.2 Fabrication of Solar Cells 16

3.2.2 Fabrication of Hybrid Dye Sensitized Solar Cells 16

3.2.2.1 Coating Platinum Films on the Counter Electrodes 16

3.2.2.2 Deposition of Silver Pad on the Bottom Electrode 17

3.2.2.3 Attachment of Gold Nanoparticles to ZnO Nanowire Arrays 18

3.2.2.4 Adsorption of Dye and Construction of Hybrid Dye Sensitized

Solar Cells 18

4 Results and Discussions 19

4.1 Characterization of Synthesized ZnO Nanowire Arrays 19

4.1.1 Morphological and Topographic Features of the Prepared ZnO Nanowire

Arrays 19

4.1.2 The Structure of Synthesized Nanowires on Nanometer and Atomic Scales 20

4.1.3 Structural Analysis of ZnO Nanowires by X-ray Diffraction 21

4.1.4 Photoluminescence (PL) Induced in Prepared ZnO Nanowires 23

4.2 Characterization of ZnO Nanowire-based Solar Cells 24

4.2.1 Characterization of Conventional Dye Sensitized Solar Cells 24

4.2.1.1 The Effect of Dye Coating on the Light Absorption 24

4.2.1.2 Current density–Voltage (J-V) Characteristics of Dye Sensitized

Solar Cells 25

4.2.1.3 Effect of the Light Intensity on the Solar Cell Performance 28

4.2.2 Dye Free Schottky Barrier Solar Cells 29

4.2.2.1 Analysis of Distribution of Gold Nanoparticles 29

4.2.2.2 Analysis of Current Density–Voltage (J-V) Characteristics of Dye

Free Schottky Barrier Solar Cells 30

4.2.2.3 Performance of Dye Free Solar Cells with Schottky interfaces

under Different Light Intensities 32

4.2.3 Characterization of Hybrid Dye Sensitized Solar Cells 33

iv

4.2.3.1 Performance of Designed Hybrid Dye Sensitized Solar Cells 33

4.2.3.2 The Effect of the Characteristics of Gold Coatings on the

Hybrid Solar Cell Performance 35

5 Conclusions 36

6 References 37

v

List of Figures

Page

Fig.2-1 Growth of a silicon nanowire by VLS. (a) Initial condition with a

liquid droplet on a substrate. (b) Growing nanowire with a liquid

droplet at the top tip of a nanowire.

5

Fig. 2-2 A schematic diagram contrasting the OAG method and VLS growth

method of nanowires.

6

Fig. 2-3 Synthesis of nanowire heterostructures. (a) First reactant material leads

to 1D axial growth. (b) A change in the reactant leads to either (c) axial

heterostructure growth or (d) radial heterostructure growth. Alternating

reactants will produce (e) axial superlattices or (f) core-multi-shell

structures.

8

Fig. 2-4 Branched nano-heterostructures. The colors indicate regions with

distinct chemical composition and/or doping.

9

Fig. 2-5 Crystal structure of ZnO.

10

Fig. 2-6 Schematic diagram of a dye sensitized solar cell, based on ZnO

nanowire arrays.

11

Fig. 3-1 Schematic diagram of the furnace with a horizontal double-tube

configuration.

15

Fig. 3-2 Schematic diagram of the constructed solar cell. Light enters the

cell through the bottom electrode.

16

Fig. 3-3 Schematic diagram of the electrochemical deposition setup.

17

Fig. 3-4 Thermal evaporation system used in deposition of silver pads on the

base electrodes.

17

Fig. 3-5 Schematic diagram of the constructed hybrid solar cell. Light enters

the cell via the bottom transparent electrode.

18

Fig. 4-1 SEM micrographs of synthesized ZnO nanowires prepared on ITO

buffer layers at 90 o

C by a solution method: (a) Cross-sectional

19

vi

image; (b) Magnified image shows faceted nanowire structures

indicating the single crystalline nature of individual nanowire.

Fig. 4-2 SEM micrograph depicts synthesized ZnO nanowires prepared on

an AZO buffer layer at 700 oC by thermal evaporation.

19

Fig. 4-3 TEM micrograph of ZnO nanowires: (a) Conventional TEM bright

field image of a single ZnO nanowire with a hemispherical tip; (b)

high-resolution TEM (HRTEM) image of a typical ZnO nanowire

showing its c-axial growth direction. The inset shows the

corresponding selected area electron diffraction (SAED).

21

Fig. 4-4(a) XRD pattern of ZnO nanowire arrays synthesized by solution

method.

21

Fig. 4-4(b) XRD pattern of ZnO nanowire arrays synthesized on AZO film by a

thermal evaporation method.

22

Fig. 4-5(a) Room temperature PL spectrum collected from the ZnO nanowire

arrays prepared by a solution method.

23

Fig. 4-5(b) Room temperature PL spectrum acquired from the ZnO nanowire

arrays prepared by a thermal evaporation method.

23

Fig. 4-6 Band structure: the levels of defects in ZnO.

24

Fig. 4-7 UV-Vis absorption spectra of as-grown and dye-coated ZnO

nanowire arrays.

24

Fig. 4-8(a) TEM image of dye-coated ZnO nanowires. The inset shows a single

ZnO nanowire without coating dye.

25

Fig. 4-8(b) Schematic illustration of exciton dissociation at the ZnO/dye

interface and electron injection into the conduction band of ZnO.

25

Fig. 4-9 Schematic band diagram describing the charge transfer processes

involved in DSSC.

26

Fig. 4-10 (a) J-V curves for the solar cell in the dark and under simulated

AM1.5G illumination with intensity of 100 mW/cm². (b)

Corresponding schematic photovoltaic device structure.

27

vii

Fig. 4-11 (a) J-V characteristics of the solar cell with the contact treatment in

dark and under simulated AM1.5G illumination with intensity of

100 mW/cm². (b) Corresponding schematic photovoltaic device

structure.

28

Fig. 4-12 J-V characteristics of the contact treated solar cell under

illumination with intensities of 80, 100 and 120 mW/cm².

28

Fig. 4-13 (a) SEM image of ZnO nanowires loaded with Au nanoparticles; (b)

bright field TEM image of abstracted ZnO nanowires; (c) EDX

spectrum acquired from ZnO nanowires coated with Au

nanoparticles.

29

Fig. 4-14 J-V characteristics of the photovoltaic device with bare ZnO

nanowires and the device with Au-coated ZnO nanowires. The

characteristics were obtained under simulated AM1.5G illumination

with intensity of 100 mW/cm².

31

Fig. 4-15 UV-Vis absorption spectra collected from bare and Au coated ZnO

nanowires.

31

Fig. 4-16 Schematic band diagram describing the charge transfer processes

involved in the dye free Schottky barrier solar cell.

32

Fig. 4-17 Performance of dye free Schottky barrier solar cells under light

illumination with intensities of 40, 60 and 80 mW/cm².

32

Fig. 4-18 (a) Typical J-V characteristics of the hybrid solar cell referenced to

that based on bare ZnO nanowires measured under simulated

AM1.5G illumination with intensity of 100 mW/cm². (b) The

corresponding schematic of hybrid solar cell structure.

33

Fig. 4-19 Energy level diagram and mechanism of photocurrent generation in

the hybrid DSSC. C.B. and V.B. are the conduction and valence

bands of ZnO, respectively.

34

Fig. 4-20 J-V characteristics of the solar cells constructed with bare ZnO

nanowires, and ZnO nanowires coated thick Au layer and thin layer

35

viii

of Au nanoparticles under simulated AM1.5G illumination with

intensity of 100 mW/cm².

List of Tables

Page

Table 4-1 Performance of the contact treated solar cell under different

illumination intensities.

29

Table 4-2 Parameters of dye free solar cells illuminated with different light

intensities.

32

ix

Abstract

The photovoltaic cells, also called solar cells, are electronic devices operated on principle of

conversion of solar energy to electricity. These novel sources of energy becomes more

increasingly important because firstly traditional fossil recourses will be exhausted for

several decades, and secondly the conversion of solar energy to electricity do not produce

environmentally unfriendly emission and represent unlimited source of energy. Therefore this

work focuses on design, fabrication and study solar cells. The solar cells are based on zinc

oxide nanowires with different devices architectures.

Zinc oxide (ZnO) nanowire arrays were synthesized directly on electrically conducting and

optically transparent indium tin oxide (ITO) films using solution and thermal evaporation

methods. The ITO films were deposited on glass substrates. ZnO nanowires have been grown

on the ITO in hydrothermal environment at 90 °C. Alternatively ZnO nanowires have also

been grown on aluminum doped zinc oxide (AZO) buffer layers prepared on silicon (Si)

substrates by thermal evaporation at 700 °C using a double-tube furnace system.

Different characterization techniques were employed to investigate the properties of

fabricated ZnO nanowires, which demonstrated that the ZnO nanowires synthesized by

thermal evaporation method show higher degree of the vertical alignment on the substrates,

and they comprise lower crystal defect densities. However, the low temperature environment

of solution method which can induce less damage to the electrical property of the bottom

electrode is more favorable to synthesize ZnO nanowire arrays used for constructing dye

sensitized solar cells (DSSCs).

Conventional DSSCs were designed and fabricated with using N719 dyed ZnO nanowire

arrays and iodine based redox electrolyte. After coating platinum thin films on the counter

electrodes, silver contact pads were deposited. The pads were used to bond wire leading for

external electronic circuit to determine the device parameters and overall performance. The

fill factor (FF) and energy conversion efficiency (η) of the DSSCs were improved from 0.28

to 0.36 and from 0.48 to 0.69 %, respectively, due to reducing photoelectron recombination at

the contact interfaces. The performance investigation of the fabricated DSSCs under different

illumination intensities indicated raising the power conversion efficiency η from 0.34 to

0.85 % as the intensity increased from 80 to 120 mW/cm².

x

Gold (Au) nanoparticles coated ZnO nanowire arrays have been employed to fabricate dye

free Schottky barrier solar cells. The Au nanoparticles enhanced the optical absorption of

ZnO nanowires in the visible light region due to their surface plasmon resonance. Upon

illumination, the photoelectrons generated within the plasmon excited Au nanoparticles were

injected into the conduction band of ZnO, and the electron donors (such as I -) from the

electrolyte would compensate the loss of electrons in Au nanoparticle surfaces,

simultaneously. The fill factor FF of the fabricated dye free Schottky barrier solar cell

reached about 0.5.

Hybrid DSSCs have also been designed and fabricated using ZnO nanowires coated Au

nanoparticles and applied ruthenium dye (N719) and redox electrolyte. Compared with

DSSCs with bare ZnO nanowires, the open circuit voltage (VOC) was improved from 0.51 to

0.64 V due to blocking effect of the Schottky barrier formed at ZnO/Au interface. As a result,

electron density increased at ZnO conduction band. The reduction of the short circuit current

density (JSC) can probably be ascribed to the increased surface defects of ZnO nanowires after

coating Au nanoparticles, and poor electron injection efficiency implying unsuitable bindings

of ZnO and dye molecules. Controlling the characteristics of Au nanoparticles coatings is

proposed to be vital for fabrication of hybrid DSSCs with enhanced performance.

1

1 Introduction and Objectives

Dye sensitized solar cells (DSSCs) are considered as one of the most promising photovoltaic

device architectures. Their potentially high solar-to-electrical power conversion efficiency

and a low fabrication cost make them very attractive for producing electricity in large scales

[1-3]. In some aspects, they are superior to traditional silicon solid-state solar cells based on

the p-n junction diodes. Instead of acting as the source of photoelectrons and provide electric

field to separate electrical charges, the semiconductor in DSSCs provides the path for

electron transport and the photoelectrons are generated by a photosensitive dye. The

interfaces between the electrolyte, semiconductor and dye play an important role in charge

separation.

It was reported that the solar energy conversion efficiency of DSSCs can achieve 11% by

using a titanium dioxide (TiO2) nanoparticle film as the photoanode [4]. Ruthenium

complexes and porous TiO2 structures have been mostly used as sensitizers and

semiconductor materials in DSSCs, respectively [5]. Although TiO2 nanoparticle films have

sufficiently large surface area for dye adsorption, they limit the device efficiency by

enhancing electron recombination probability and slowing electron transport rate which

essentially depends on the process of electrons diffusion to the anode through particles.

Consequently, vertical single crystalline nanowires which can provide a direct path to anode

are anticipated to replace nanoparticle films for further increasing the electron diffusion rate.

Over recent years, it was shown that Zinc oxide (ZnO), because of its suitable electronic and

optical properties, is one of the materials that could be incorporated into construction of in

DSSCs to improve their performance. ZnO has comparable band gap (3.37 eV) to TiO2,

much higher room temperature mobility (ZnO: 115-155 cm2V

-1s

-1, TiO2: < 10

-5 cm

2V

-1s

-1), a

high binding energy (60 meV), high breakdown strength, cohesion, and exaction stability. It

is also one of the hardest materials in the family of II–VI semiconductors. Various ZnO

nanostructures, comprising arrays of nanorods and nanowires, can be prepared on optically

transparent and electrically conducting oxide substrates using various methods. The surfaces

of these ZnO nanowires can be functionalized. It was reported that the arrays of highly

ordered ZnO nanowires in DSSCs design could provide a more open structures for filling

2

with hole-transporting materials [6-8]. The conversion efficiencies of the ZnO nanowire

based DSSCs are about 1.5% up to now.

Obviously in further development of ZnO DSSCs, the principal goals are to increase their

energy conversion efficiency. Primarily, the higher conversion efficiency can be provided by

spectral absorbance over a broader light range covering the visible and near-IR regions.

Particularly, modifying dyes can not only assist to higher absorbance but also to increase of

the open-circuit voltage which is influenced by the metal oxide electronegativity and the dye

ionization potential. The development of conversion efficiency also focuses on increasing of

the electron diffusion length and improvement of electron transport.

In this project, the main objectives are to synthesize ZnO nanowire arrays directly on

electrically conducting electrodes and design of photovoltaic structures that will lead to

higher conversion efficiencies. In the fundamental technological step, different

characterization techniques were employed to investigate the morphology, crystal structures,

optical and electrical properties of fabricated ZnO nanowires. DSSCs were built based on

dense arrays of ZnO nanowires. The efficiency of ZnO nanowires based DSSCs was further

improved via charge transport in the DSSCs. The counter electrode was coated with platinum

to enhance the hole transport as well as improving the catalyst behaviors of redox reaction in

electrolyte. A silver pad used for leading out wires was also deposited on the bottom

electrode and thermally treated to reduce the contact barrier. A hybrid structure of DSSCs,

ZnO nanowires coated with gold (Au) nanoparticles, was designed and constructed to

improve the light absorbance in visible range. Without involving dye, a Schottky barrier solar

cell based on Au nanoparticles coated ZnO nanowirs was also characterized and its working

principle was investigated in this project.

3

2 Literature review

2.1 Nanotechnology

Nanotechnology refers to a field of applied science which deals with matters on an atomic

and molecule scale and involves developing devices within that size [9]. Nanostructures

which correspond to the structures having at least one physical dimension between 1 to 100

nm have been constantly gaining interests for their peculiar and fascinating properties [10].

Recently, there are various nanostructures being under research, like carbon nanotubes,

nanowires [11], nanobelts, nanopropellers, nanosprings, nanorings, nanobows, nanorods [12].

It has been reported that various techniques can be used to synthesize one dimensional

nanostructures, for example, chemical vapor deposition, physical vapor deposition (PVD),

laser ablation, hydrothermal [14].

The synthesized nanomaterials are extremely useful in both present and future semiconductor

industry. Recently, electronic devices are being made in smaller and smaller sizes and of

better performance. However, traditional top-down approaches, refer to assembling small

scale devices from large ones directly [15], like etching and lithographic, are facing the

problem of a near-exponential increase in cost associated with each new level improvement

for manufacture. On the contrary, bottom-up approaches, which correspond to arranging

nano-objects with well defined chemical or physical properties into more complex

components, have been applied to alternate the conventional top-down methods. It is much

like the way nature constructs complex biological systems using proteins and other

macromolecules [16].

Lieber suggested three key areas that enable bottom-up approaches for nanotechnology. First,

by rational design and predictable synthesis, chemical properties and the corresponding

physical properties of nanoscale building blocks need to be preciously controlled. Second, the

limits of functional devices based on these building blocks should be developed and explored

critically. Third, hierarchical assembly methods that can organize building blocks into the

architectures which enable high density of integration with predictable function should be

developed [16].

4

2.2 Nanowires

Since late 1990s, the field of semiconductor nanowires has been gaining interest among

science and engineering communities. High carrier mobility and large surface area are two

main priorities of nanowires. As a result, nanowires have various applications including high

performance field effect transistors (FETs), dye sensitized solar cells [17], chemical sensors

[18, 19] and ultrasensitive bio-sensors [20, 21]. Besides, nanowire-based on-chip photonic

devices, including light-emitting diodes [22], lasers [23–26], active waveguides [27, 28] and

integrated electro-optic modulators, have been explored as well for the optical cavity and

waveguide properties of semiconductor nanowires.

2.3 Synthesis of One-dimensional (1D) Semiconductor Nanomaterials

The rational design and synthesis of nanoscale materials are critical to work directed towards

understanding fundamental properties, creating nanostructured materials, and developing

nanotechnology [46]. Wagner and Ellis first investigated the catalyst-assisted chemical vapor

deposition (CVD) for growing single silicon crystal in 1964 [46]. Among the numerous

methods that have been developed to design and rationally synthesize nanowires with

predictable control over the key structural, chemical, and physical properties, the strategy that

involves exploiting a “catalyst” to confine growth in 1D has received increasing focus in the

past several years [16].

Classified by the different phases involved in the reaction, this strategy is cataloged into

vapor–liquid–solid (VLS) [13], solution–liquid–solid (SLS) [30], and vapor–solid (VS)

growth [36].

2.3.1 Vapor-Liquid-Solid (VLS) Mechanism

The VLS growth mechanism was first proposed by Wagner and Ellis to explain the

unidirectional silicon single crystal growth [29]. In VLS growth, a nanometer-scale catalyst

particle which can form a liquid solution with the nanowire material is involved. The size of

the initial nucleation event and the nanowire diameter are determined by the catalyst particle

size. The crystalline nanowire can be nucleated when the liquid solution, a nanoscale droplet,

becomes supersaturated. Then, by adding reactant selectively to this nanoscale droplet, the

nanowire can be grown [13]. The orientation of the nanowire is determined by the surface

5

lattice of the substrate on which an epitaxial relationship can be built [16]. Fig. 2-1 illustrates

the growth of a silicon crystal by VLS.

An important feature of this approach to nanowire growth is that phase diagrams can be used

to select catalyst, composition of catalyst and nanowire material, and temperature for

successful growth for different nanowire materials. Having this general idea, it is possible to

grow nanowire with all of the periodic table’s main group elemental, binary, and related alloy

semiconductors in a controlled fashion essentially. This general approach also enables control

of nanowire diameter and length [13].

2.3.2 Solution-Liquid-Solid (SLS) Mechanism

The Solution-Liquid-Solid mechanism is quite similar to VLS mechanism. The difference is

that the solution phase in the SLS reaction takes place of the vapor phase in the reaction of

VLS mechanism. This strategy has been reported to synthesize InP, InAs and GaAs

nanowires [30], and AlxGa1- x As nanowhiskers [31]. Buhro’s research group reported that

they found the diameter of a GaAs nanowire was smaller than the diameter of the catalyst

nanoparticle from which it grew [30]. This contrasts with the results of VLS growth which

shows the diameter of the nanowires being similar or greater than the initial diameters of the

catalyst nanoparticles [32-35]. They concluded the difference was due to the high reaction

temperature of VLS (≥500 oC), which increases the size of the catalyst nanoparticle prior to

nanowire growth. On the contrary, smaller nanowire diameters can be achieved by the

low-temperature (203 oC) SLS method [30].

2.3.3 Vapor-Solid (VS) Mechanism

Unlike VLS and SLS mechanism, there is no additional metal employed as the catalyst in

vapor-solid (VS) process. Vapors of a material for the nanowire growth are produced by

(b) (a)

Fig.2-1 Growth of a silicon nanowire by

VLS. (a) Initial condition with a liquid

droplet on a substrate. (b) Growing

nanowire with liquid droplet at the top tip of

a nanowire [46].

6

thermal evaporation at high temperature. As carried by the inert gas (e.g. N2, Ar) to the

downstream regions of lower temperature, the vapors begin to condense on the substrate

surface and crystallize. A competition in growth rate among different crystallographic

directions starts at the initial stage of crystallization. Once the condition of a favored

anisotropic one-dimensional growth occurs, the nanowire material atoms will strike on the

specific favorable advancing plane to invoke and continue the one-dimensional growth and

hence forming nanowires [36]. For the vapor-solid (VS) mechanism, the supersaturation ratio

of the condensing species in the gas is critical to the formation of one-dimensional

nanostructures. The supersaturation ratio of the condensing species must be below some

critical value for anisotropic growth, while, a medium supersaturation ratio results in bulk

crystals. For high supersaturation ratios, homogeneous nucleation in vapor phases leads to

powder formation [36].

2.3.4 Oxide-Assisted Growth (OAG) Mechanism

Oxide-assisted growth (OAG) method was developed by Lee’s group in 1998 [37]. This

method can produce large quantities of silicon nanowires without the need of a metal catalyst,

which hence yields high-purity nanowires that are free of metal impurities [37, 38-41].

The comparison between OAG method and VLS growth method are demonstrated in Fig.

2-2. The mechanism of the Si nanowires growth was explained by Lee’s group. First, the

deposited silicon sub-oxide cluster can be fixed on the substrate because of the strong bonds

between some of the reacted silicon atoms and substrate atoms. Non-bonded reactive silicon

atoms in the same cluster provide their dangling bonds directed outward from the surface as

nuclei to absorb additional reactive silicon oxide cluster and facilitate the formation of Si

nanowires. The growth of the silicon domain after nucleation may be crystallographically

Fig. 2-2 A schematic diagram

contrasting the OAG and VLS

growth methods of nanowires

[42].

7

dependent subsequently [42]. The mechanism of the one-dimensional growth is explained

that the oxygen atoms in the silicon sub-oxide clusters may be expelled by the silicon atoms

during the growth of Si nanowires and diffuse to the outer surface forming a chemically inert

silicon oxide layer which stops the nanowires from the diameter growth [43]. The defects of

the Si nanowires play a key role in controlling the growth rate in certain crystallographic

directions [42]. The quantity, diameter, and morphology of the nanowires are observed to be

affected by growth parameters [44].

2.3.5 Other Growth Mechanisms

Besides the strategies of synthesis of the one-dimensional (1D) semiconductor nanomaterials

discussed above, there are also other mechanisms to produce nanostructures with 1D

morphologies. Template-directed synthesis (TDS) mechanism makes use of carbon nanotubes

as template for nanowires to be reacting aligned. The orientation, diameter, and length of the

oriented nanowires are similar to the original aligned nanotubes [45]. Laser ablation is used

to produce pure single-walled carbon nanotubes and boron nitride nanotubes [47]. Nanowires

can also be synthesized by photolithographic and etching processes, which are fundamentally

top-down approaches [48].

2.4 Synthesis of Nano-heterostructures

CVD methods have been employed in synthesis of silicon and germanium core shell and

multishell nanowire homostructures and hereostructures. The ratio of surface-to-volume is

important factor of these nanostructures to control the material properties with novel

functions [49]. The ability to control the growth heterostructures will open up many exciting

opportunities in both nanoscience and nanotechnology for many years to come [13].

2.4.1 Axial Nano-heterostructures

Axial nano-heterostructures refer to the structures that have different materials with the same

diameter growing along the wire axis. Axial growth is achieved when reactant activation and

addition occurs at the catalyst site and not on the nanowire surface through epitaxial growth

[16]. The axial growth process was demonstrated by Lieber’s group. First, a length of

material (red) is prepared by reactant A (Fig. 2-3(a)). Then, a reactant B is used to produce a

8

length of the second material (blue) for a fixed interval (Fig. 2-3(c)). If reactants are changed

in a regime favoring axial growth repeatedly, a nanowire superlatttice can be formed (Fig.

2-3(e)) [13]. Axial heterostructures demonstrate that vapor decomposition/adsorption

continues exclusively at the surface of the catalyst nanocluster site, which leads to the

crystalline growth of new semiconductor along the axial direction [50].

2.4.2 Radial Nano-heterostructures

Radial nano-heterostructures are materials with morphologies which have core/shell and

core/multi-shell forms along the radial direction [50]. When the reactant vapor changed once

nanowire growth has been established, the radial heterostructures growth takes place. Unlike

axial nano-heterostructures, the deposition of the new vapor/reactant on the surface of the

semiconductor nanowire cannot be neglected, which leads to the growth of new material on

the original nanowire surface (Fig. 2-3(d)) [50]. Changing reactants in a radial-growth regime

will result in core-multi-shell radial structures (Fig. 2-3(f)) [13]. The growth and

characterization of different Si-Ge nanowire structures, like single-crystal germanium

nanowire cores with epitaxial shells of doped silicon, have been demonstrated by Lieber’s

group [13].

Fig. 2-3 Synthesis of nanowire heterostructures. (a) First reactant material leads to1D

axial growth. (b) A change in the reactant leads to either (c) axial heterostructure growth

or (d) radial heterostructure growth. Alternating reactants will produce (e) axial

superlattices or (f) core-multi-shell structures [49].

(a) (b)

(c)

(e)

(d)

(f)

9

2.4.3 Branched Nano-heterostructures

Branched nano-heterostructures can be formed by depositing metal catalyst nanoclusters on

the surfaces of initial nanowires. Then, reactants are introduced to facilitate nucleation and

directional growth of nanowire branches. If the trunk and branches have similar chemical

composition and amorphous overcoat (e.g. oxides) developed before branch growth, epitaxial

growth of the branches can occur [50]. The materials for the trunk and the side branches can

be different which results in the formation of 3D junction arrays (Fig. 2-4) [16]. Branched

nano-heterostructures were independently studied by Wang et al [51] in the case of Si and

GaN; and Dick et al [51] in the case of GaP using MOCVD and a sol–gel method to deposit

Au catalyst nanoclusters on the nanowire trunks.

2.5 Zinc Oxide Nanowires

2.5.1 General Properties of Zinc Oxide

For most common III-V and II-VI compound semiconductors, their band gap energy varies as

a function of the lattice constant. GaN, AIN, InN, GaAs, InAs and AlAs are III-V compound

semiconductors. Zinc Oxide (ZnO) is one of the group II-V compound semiconductors which

also include ZnS, ZnSe, CdSe, ZnTe and CdTe. Because ZnO has a direct wide band-gap

(3.37eV) and a large excitation binding energy (60 meV) at room temperature, it becomes

one of the most important semiconductor materials for applications in optoelectronics,

sensors and actuators [52, 53]. Among the known one-dimensional (1D) nanomaterials, ZnO

nanowires and other nanostructures are extensively studied due to their excellent intrinsic

properties [54, 55].

Fig. 2-4 branched nano-heterostructures.

The colors indicate regions with distinct

chemical composition and/or doping [16].

10

ZnO has three fundamentaly advantages for applications in the nanotechnology [56]. (i) It is

semiconductor with a direct wide band gap and a large excitation binding energy. It is an

important functional oxide which exhibit near UV emission and transparent conductivity at

room and higher temperatures. (ii) It is a piezoelectric material due to its non-central

symmetric crystal structure (Fig. 2-5). It shows the capacity to built electro-mechanical

couple sensors and transducers on nanoscales. The piezoelectric coefficient of a polar

nanobelt is about three-times that of the bulk. It can be applied in construction of nanoscale

electromechanical coupling devices [57] (iii) ZnO is biologically safe and biocompatible

material, which can be used in biomedical applications without any coating. With these

attributes, ZnO could be one of the most important nanomaterials in future research and

applications [56].

However, the difficulties in p-type doping of ZnO hamper the application of ZnO

nanomaterials in the field of photonic devices [58]. Several mechanisms leading to doping

difficulty are known: low solubility, compensation by low-energy native defects, deep

impurity level, and structural instability.

2.5.2 Growth of Aligned ZnO Nanowires

ZnO nanowires can be grown by various techniques, such as thermal evaporation, solution

method, molecular beam epitaxy (MBE), MOCVD and chemical beam epitaxy (CBE)

[49-54].

Both growths with and without assistance of catalysts are commonly used in synthesis of

ZnO nanowires. In the catalyst assisted process, a metal catalyst is employed. The saturation

and diffusion of the dissolved precursor in the liquid metal cluster can lead to nucleation and

growth of nanowires. VS is non-catalytic growth which can form small diameter nanowires

easier and provide higher purity without metal catalyst.

Fig. 2-5 Crystal structure of ZnO.

11

Different substrates for the growth of vertically aligned ZnO nanowire arrays have been

studied for various applications, such as Al2O3 and GaN. The technologically important

substrates SiC and GaN were reported to grow ZnO nanowires used for light emitting diodes,

laser devices and light detectors [66].

2.5.3 Application of Zinc Oxide in Solar Cells

Solar power can be converted directly into electrical power in photovoltaic (PV) cell,

commonly called solar cell [67]. Among excitonic solar cells, the dye sensitized cell (DSSC)

which is a relatively new class of low-cost solar cell is considered currently the most efficient

[68] and stable [69] photocell. DSSC is a photoelectrochemical system that a semiconductor

is constructed between a photo-sensitized anode and an electrolyte. The dense array of

oriented and crystalline ZnO nanowires which maintain very rapid carrier collection is used

as a dye scaffold. The working mechanism of DSSCs is explained. Under illustration, the dye

monolayer creates excitations that are rapidly split. The electrons injected into the

nanocrystalline film and holes leaving the opposite side of the device by means of redox

species in a liquid or solid-state electrolyte [65]. Better electron transport within the

nanowires photoanode is determined by its higher crystallinity and an internal electric field

that can assist carrier collection by separating injected electrons from the surrounding

electrolyte and sweeping them towards the collecting electrode [65].

In Yang’s paper [65], the ZnO nanowire arrays are grown on conducting F/SnO2 layer (FTO)

deposited on glass by chemical solution method. The substrate with dye coated ZnO

nanowires are then sandwiched and bonded with a platinum coated glass electrode. A

standard 30 μm spacer is used to separate the two electrodes. The internal space of the cells is

filled with a liquid electrolyte by a capillary action (see Fig. 2-6). I–V measurements under

white light illumination can be recorded by varying an external load resistance [65].

Fig. 2-6 Schematic diagram of a

dye-sensitized solar cell, based on

ZnO nanowire arrays [65].

12

The limitation of the DSSC is the poor absorption of low energy photons using available

dyes. JSC is the current density at short circuit. In principle, the maximum JSC of a DSSC is

determined by how well the absorption window of its dye overlaps the solar spectrum [65].

Depending on the exact dye used, the cells can achieve their current densities that are

between 55% and 75% of their theoretical maxima at full sunlight [70]. Although

considerable efforts have been used to develop dyes and dye mixtures which can absorb more

light at long wavelengths, little success has been showed [71, 72]. To raise the efficiency of

the nanowires based DSSCs further, higher dye loadings through an increase in surface area

should be achieved [65].

13

3 Experimental Procedures

3.1 Fabrication of ZnO Nanowire Arrays

3.1.1 Synthesis of ZnO Nanowire Arrays by a Solution Method

Zinc oxide nanowire arrays were fabricated by using a solution method. The route to form

ZnO colloids and nanocrystallites by hydrolyzing zinc acetate at ~ 90 °C in aqueous solution

was adapted to synthesize ZnO nanowires directly on a substrate [73].

3.1.1.1 Cleaning the Substrates

The indium tin oxide (ITO) coated glass substrates with a size of 2 cm × 2 cm were used for

growing ZnO nanostructures. The transparent ITO coated substrates were chosen because of

their high electrical conductivity and optical transparency. First of all, the substrates were

cleaned thoroughly to eliminate contaminants of mineral and organic origin. The substrates

were cleaned in acetone (CH3COCH3), ethanol (CH3CH2OH) and distilled water baths

assisted with ultrasonic agitations for 5 minutes in each bath. Then the substrates were dried

in a dry nitrogen flow.

3.1.1.2 ZnO Seeding and Nanowire Growth

Before seeding, ITO sides of the substrates were tested for electric conductivity to distinguish

the functional area coated by ITO from the glass surface. The sheet resistance of the ITO

films is 10 Ω/cm². The ITO side of the substrate surface was wetted with 3-5 droplets of

0.0088 M zinc acetate dehydrate dissolved in ethanol. After 10 s, the substrates were rinsed

using ethanol and dried in a dry nitrogen flow. The whole procedure was repeated five times

to coat a thin film of zinc acetate crystallites on each substrate. The coated ZnO nanocrystal

seed particles were 3–4 nm in diameter. After that, the substrates were annealed at 350 °C in

air for 20 minutes for ZnO layers to yield their favored crystallographic orientation.

The ZnO nanowire arrays were grown by dipping ZnO seeded substrates in aqueous solutions

containing 25 mM zinc nitrate hydrate, 25 mM hexamethylenetetramine and 5–7 mM

polyethylenimine (branched, low molecular weight, Aldrich) at 90 oC for 3 hours.

14

3.1.2 Synthesis of ZnO Nanowire Arrays by a Thermal Evaporation Method

The synthesis of ZnO nanowires was carried out in a horizontal tube furnace with two

thermal zones. The zinc oxide and graphite powder mixture is a source materials loaded

being placed in the high temperature zone of the furnace. At temperature ~ 950 oC, zinc oxide

is reduced to zinc via following carbon redox reactions

)()()()(

)()()()(

2 gCOvZngCOsZnO

gCOvZnsCsZnO

The reduced zinc is vaporized and is transported to the low-temperature zone. The zinc

vapors condense and oxidize on the aluminum-doped zinc oxide (AZO) substrate and form

nuclei for further growth of zinc oxide nanowires.

3.1.2.1 Preparation of Aluminum Doped Zinc Oxide Buffer Layer

Aluminum doped zinc oxide (AZO) buffer layers were prepared on Si substrates by a reactive

radio-frequency (RF) magnetron sputter deposition. The sputter disc target with a diameter of

3 inches was made by sintering ZnO and Al2O3 (~ 2 wt %) powders. During deposition, the

substrates were heated to 500 oC. Argon and oxygen with purity of 99.995％ and 99.99％,

were fed into the reactor at mass flow rates of 50 and 1.0 sccm (sccm stands for cubic

centimeter per minute at STP), respectively. The total pressure in the reactor was maintained

at 6 mTorr. The magnetron plasma was induced at a radio-frequency power of 120 W.

3.1.2.2 Growth of ZnO Nanowire Arrays on AZO/Si Substrates

The ZnO nanowire arrays were grown in a double-tube system which enables a higher growth

rate due to better vapor confinement [74]. The dimension of the large quartz tube is 40 mm in

diameter and 70 cm in length and the values of the small quartz tube are 12 mm × 30 cm,

respectively. The mixture of ZnO (99.9%) and graphite in a molar ratio of 1:1 was placed at

the closed end of the small tube. Silicon substrates coated with AZO (1cm × 2cm) were

located 4 cm apart from the open end of the small tube. Fig. 3-1 illustrates the experimental

setup.

15

The furnace was evacuated to ~ 5 × 102 mbar by a rotary pump, and subsequently argon

with purity of 99.995% and oxygen with purity of 99.99% were leaked into it at flow rates of

30 and 1sccm, respectively. Feeding the gas mixture and continuous pumping maintained a

dynamic pressure of 200 mbar. Thereafter the furnace was heated to the preset temperature.

In the central zone the temperature increased from room temperature to 950 oC with a

ramping rate of ~ 46.3 oC/min (20 mins) while in the substrate zone, the temperature raised to

700 oC with a ramping rate of ~33.8

oC/min ( 20 mins). In both the temperature zones, these

temperatures were maintained during the growth process for one hour. Finally the furnace

was switched off and was naturally cooled down to room temperature.

3.2 Design and Fabrication of Photovoltaic Devices

3.2.1 Fabrication of Dye Sensitized Solar Cells (DSSCs)

3.2.1.1 Adsorption of Dye

First indium tin oxide (ITO) substrates (1cm × 3cm) with grown arrays of ZnO

nanostructures were cleaned thoroughly to eliminate surface organic contaminants. The

substrates were cleaned in acetone (CH3COCH3) and ethanol (CH3CH2OH) baths with

assistance of ultrasonic agitations for 3~4 minutes in each bath. Subsequently they were dried

in oven at 80 oC to remove residual solvent from their surfaces. The arrays of ZnO nanowires

were then dipped in 0.5 mM ethanolic dye solution for 12 hours and dried in oven at 80 oC for

Fig. 3-1 Schematic diagram of the furnace with a horizontal

double-tube configuration.

16

30 minutes. The dye used here was ruthenium based (Bu4N)2Ru(dcbpyH)2 (NCS)2 dye

commercially known as N719 dye.

3.2.1.2 Fabrication of Solar Cells

The substrate with dyed ZnO nanowire arrays was sandwiched with a transparent ITO

counter electrode. The two electrodes were separated by a 30 μm polypropylene spacer and

bonded with binder clips. The internal space of the cells was filled with liquid electrolyte by

capillary forces. The electrolyte was an ethanolic solution composed of 0.5 M LiI, 50 mM I2,

and 0.5 M 4-tertbutylpyridine. Curent-voltage (J–V) measurements were taken under white

light illumination (light intensity: 100mW/cm²; wavelength: 380-790 nm). The J-V curves

were recorded by varying an external load resistance. The designed DSSC mode is

demonstrated in Fig. 3-2.

3.2.2 Fabrication of Hybrid Dye Sensitized Solar Cells

3.2.2.1 Coating Platinum Films on the Counter Electrodes

A thin platinum film was deposited on the ITO substrate by electrochemical process. The 100

ml electrolyte was prepared by dissolving 0.5g H2PtCl6·6H2O, 5g (NH4)2HPO4 and 15g

Na2HPO4 in distilled water. In the electrolyte, H2PtCl6·6H2O is the effective solute to provide

platinum ions which obtain electrons and reduce to platinum at the cathode. (NH4)2HPO4 and

Na2HPO4 were used to adjust the pH value of the electrolyte to be 7-8. The graphite plate

(1cm × 4 cm) and ITO substrate (1 cm × 3 cm) serving as anode and cathode, respectively,

Fig. 3-2 Schematic diagram of the constructed solar cell. Light enters

the cell through the bottom electrode.

Glass

ITO (n+)

Aligned ZnO nanowires (n-type)

Pt

Dye (N719)

I

Glass

ITO (n+)

Aligned ZnO nanowires (n-type)

Pt

Dye (N719)

Glass

ITO (n+)

Aligned ZnO nanowires (n-type)

Pt

Dye (N719)

I

ITO Counter Electrode

Electrolyte

Aligned dye-coated ZnO

nanowires ITO (n+)

17

were submerged in the 100 ml electrolyte maintained at 80 oC as illustrated in Fig. 3-3. The

graphite plate and ITO substrate were connected to positive and negative terminals of the

power supply, respectively. The current density was preset to 0.030 A/cm² and for 2 minutes.

The thickness of the obtained platinum thin film was about 0.4 μm.

3.2.2.2 Deposition of Silver Pad on the Bottom Electrode

A potential barrier can be formed at the junction of metal and semiconductor, which can

inhibit photoelectrons transport. To reduce the resistance at the electrode/ITO substrate

interface, pads of silver (Ф = 4.52-4.74 eV) with a comparable work function to that of ITO

(Ф = 4.6-4.7 eV) was deposited at the edge of ITO substrates and used for wire bonding of

electrical leads. During silver deposition the ITO substrate was masked except for a narrow

slit (2-3 mm) determining the size of the pad. The silver pad was deposited by thermal

evaporation using a deposition system illustrated in Fig. 3-4. The silver film was deposited at

pressure of ~610 mbar. The deposited thickness of the silver film was about 100 nm. Finally,

the ITO substrate with deposited silver pad was annealed at 450 oC in furnace under N2

atmosphere for 30 minutes to provide an ohmic contact.

Fig. 3-3 Schematic diagram of the

electrochemical deposition setup.

Fig. 3-4 Thermal evaporation system

used in deposition of silver pads on

the base electrodes.

18

3.2.2.3 Attachment of Gold Nanoparticles to ZnO nanowire arrays

The sol-gel solution was prepared by dripping 2 ml of a 1% solution of trisodium citrate

dehydrate (Na3C6H5O7.2H2O) into 10 ml of 0.03 M chloroauric acid (HAuCl4). The pH value

of this sol-gel solution was maintained at around 8-9. The array of ZnO nanowires grown on

the ITO substrate were dipped into the solution. Then the solution with a substrate was heated

to 100 oC in an oven for 30 minutes to assist the reduction process of gold.

3.2.2.4 Adsorption of Dye and Construction of Hybrid Dye Sensitized Solar Cells

The methodology of dye adsorption and fabrication of the dye sensitized solar cells (DSSCs)

exploiting ZnO nanowires and gold nanoparticles hybrid structures is similar to that of the

DSSCs fabrication which has been described above. The difference is only introduction of Au

nanoparticles on ZnO surfaces of nanowires just before dye coating. Then the bottom

electrode was also provided with an Ag pad while the counter electrode was coated with a

thin platinum film. The structural diagram of the designed and fabricated hybrid DSSC is

presented in Fig. 3-5. Photocurrent density and photovoltage of the solar cell was measured

under white light illumination.

Fig. 3-5 Schematic diagram of

the constructed hybrid solar

cell. Light enters the cell via the

bottom transparent electrode.

19

4 Results and discussions

4.1 Characterization of Synthesized ZnO Nanowire Arrays

Morphology and structure of the prepared ZnO samples were analyzed with a Philips XL30

FEG scanning electron microscope (SEM) and a Philips CM20 transmission electron

microscope (TEM). High-resolution transmission electron microscopic (HRTEM) images

were obtained with a Philips CM200 FEG TEM operated at 200 kV. X-ray diffraction (XRD)

spectra were recorded with a Siemens D500 diffractometer. Photoluminescence (PL) spectra

were measured at room temperature with a Perkin Elmer luminescence spectrometer LS50B.

4.1.1 Morphological and Topographic Features of the Prepared ZnO Nanowire

Arrays

The surface structure of ZnO nanowire arrays were examined with Philips XL30 SEM. The

working pressure was below 6108.9 mbar and the electron beam voltages were 5 kV and

20 kV in our study.

Fig. 4-1 SEM micrographs of synthesized ZnO nanowires prepared on ITO buffer layers at

90 o

C by a solution method: (a) Cross-sectional image; (b) Magnified image shows faceted

nanowire structures indicating the single crystalline nature of individual nanowire.

(a) (b)

Fig. 4-2 SEM micrograph depicts

synthesized ZnO nanowires prepared

on an AZO buffer layer at 700 o

C by

thermal evaporation.

20

Figs. 4-1 (a) and (b) show the SEM images of ZnO nanowires fabricated by a solution

method. They were grown from the nanocrystal seeds (seeding by 0.0088 M zinc acetate) on

ITO substrates in hydrothermal environment at 90 °C for 3 hours. The cross-sectional image,

in Fig. 4-1(a), shows a dense array of ZnO nanowires. The nanowires are well aligned and

preferentially parallel to the surface normal of the substrate. However the degree of the

alignment is lower than that of nanowires being grown by a thermal evaporation technique as

illustrated in Fig. 4-2. The SEM image, in Fig. 4-1(b), taken at higher magnification reveals

faceted surfaces, termination of nanowires by flat hexagonal heads and faceted walls

implying single crystalline nature of each nanowire. The height and diameter of width of

nanowires prepared by the solution method are fairly uniformed, and are about 300 nm and 3

μm, respectively.

The morphology of the ZnO nanowire arrays fabricated by a thermal evaporation method in a

double-tube system at 700 oC for 1 hour is shown in Fig. 4-2, ZnO nanowires are uniform in

their size and grown vertically to the AZO substrate surface. The vertical alignment of

nanowires is presumed to be associated with the epitaxial growth emerging due to

crystallographic matching between the grown ZnO nanowires and AZO buffer layer beneath.

The average diameter and length of ZnO nanowires are about 400 nm and 9 μm, respectively.

The ZnO nanowires synthesized by the described thermal evaporation method have a larger

aspect ratio and higher degree of alignment than that of nanowire arrays prepared by the

solution method.

4.1.2 The Structure of Synthesized Nanowires on Nanometer and Atomic Scales

ZnO nanowires on nanometer and atomic scales were characterized with a Philips CM20 and

CM200 TEMs operating at 200 kV.

The TEM bright field image shown in Fig. 4-3(a) reveals that the synthesized high-purity

ZnO nanowires are straight with a smooth surface, uniform diameters (~ 40 nm) and

hemispherical tips. The high resolution TEM image, in Fig. 4-3(b), taken nearby the edge of

the ZnO nanowire illustrates single crystalline nature of an examined nanowire. In TEM view

field the ZnO nanowire comprises neither observable dislocations nor stacking faults. While,

other types of defects like vacancies and interstices are hard to be observed in HRTEM

images.

21

The lattice space between two adjust plans is 0.26 nm which corresponds to the d-space of

(0002) planes of wurtzite crystal structure, confirming the preferred growth direction of the

ZnO nanowire is along [0001]. The sharp diffraction spots in the selected area electron

diffraction (SAED) pattern inset in Fig. 4-3(b) also substantiates that the ZnO nanowire is

single crystalline with c-axis growth direction and (10 -1 0) side faces.

4.1.3 Structural Analysis of ZnO Nanowires by X-ray Diffraction

The chemical and crystallographic structures of ZnO nanowire arrays were analyzed with a

Philips X’Pert XRD (at anode parameters of 40kV/30mA). The step was set to be 0.05° (See

Fig. 4-4(a, b)).

Fig. 4-3. TEM micrograph of ZnO nanowires: (a) Conventional TEM bright field

image of a single ZnO nanowire with a hemispherical tip; (b) high-resolution TEM

(HRTEM) image of a typical ZnO nanowire showing its c-axial growth direction.

The inset shows the corresponding selected area electron diffraction (SAED).

Fig. 4-4(a) XRD pattern of ZnO

nanowire arrays synthesized by

solution method.

(a)

b

)

[0001]

0002 0-110

(b)

22

The XRD spectrum of the ZnO nanowires synthesized by a solution method is shown in Fig.

4-4(a). The peaks were diffracted from wurtizite crystal structure of ZnO nanowires with

lattice parameters of a = 0.325 nm and c = 0.521 nm. The (0002) diffraction peak with

full-width half-maximum (FWHM) of ~ 1.1o dominates in the spectrum. The sharp peak

demonstrates that the growth direction c-axis of the nanowires is aligned vertically to the

surface of the ITO substrate, which supports the SEM and TEM analyses. The low intensity

peaks at low 2 , namely (1010) and (10-11), correspond to the plane diffraction of nanowires

orientated at other angles. If the ZnO nanowires in the array are uniform and “perfectly”

aligned in the [0001] crystallographic direction, other prominent diffraction peaks cannot be

observed in the XRD spectrum.

Fig. 4-4(b) shows the XRD spectrum of ZnO nanowire arrays synthesized by a thermal

evaporation method on the AZO film which was deposited on a p-Si substrate. The intense

ZnO (0002) diffraction peak indicates the prepared ZnO nanowires are grown along c-axis

and aligned perpendicular to the substrate. Compared with Fig. 4-4(a), the diffraction

intensity of other peaks of ZnO crystallographic planes are almost negligible in Fig. 4-4(b).

Thus, the spectral analysis indicates that the vertical alignment of ZnO nanowires synthesized

by thermal evaporation method is considerably better than that of prepared by the solution

method, which is consistent with the above SEM observations. The red peak arising from

AZO buffer layer coincides with the black peak corresponding to ZnO nanowire arrays being

indexed to be the (0002) plane diffraction. This implies that AZO film is crystalline and

matches the lattice parameters of nanowires. The Si (400) peak originates from the bottom

single crystal Si substrate.

Fig. 2 XRD of ZnO nanowires

Fig. 4-4(b) XRD pattern of ZnO

nanowire arrays synthesized on

AZO film by a thermal evaporation

method.

23

4.1.4 Photoluminescence (PL) Induced in Prepared ZnO Nanowires

Photoluminescence (PL) spectra were measured at room temperature with a Perkin Elmer

luminescence spectrometer LS50B. An argon laser with a wavelength of 244 nm was used as

an excitation source.

In addition to the analysis of faceted structures of ZnO nanowires by SEM and study of their

crystalline natures by TEM and XRD, the properties of these materials can further be

examined by photoluminescence. The photoluminescence spectrum in Fig. 4-5(a) is collected

from the ZnO nanowire arrays prepared by a solution method. The spectrum is characteristic

with a relatively sharp peak at a wavelength of about 380 nm and an intense broad peak

centred at about 495 nm. On the other hand the spectrum in Fig. 4-5(b) acquired from the

ZnO nanowire arrays prepared by an evaporation method is dominated with a single sharp

peak at a wavelength of about 375 nm. The narrow peaks in both the spectra correspond to

the near band edge emission of the wide bandgap ZnO. The later broad peak in Figure 4-5(a)

is located in the visible spectral range and is identified as green band emission. It has been

suggested that the green band emissions are ascribed to the singly ionized oxygen vacancies

in ZnO structures arising from the recombination of photogenerated holes with the single

ionized charge states of the defects [75]. The levels of the defects in the band structure of

ZnO can be referred to Fig. 4-6. The intensity of the green luminescence indicates the defect

density in ZnO structure in a certain degree. Hence, the almost flat green band emission peak

in Fig. 4-5(b) indicates that the oxygen vacancies in the thermal evaporation synthesized ZnO

Fig. 4-5(a) Room temperature PL

spectrum collected from the ZnO

nanowire arrays prepared by a solution

method.

Fig. 4-5(b) Room temperature PL

spectrum acquired from the ZnO

nanowire arrays prepared by a thermal

evaporation method.

24

nanowires are fairly low. It can be seen that the optical properties of ZnO nanowires are

sensitive to the synthesis methods and conditions.

4.2 Characterization of ZnO Nanowire-based Solar Cells

4.2.1 Characterization of Conventional Dye Sensitized Solar Cells

This section characterizes and investigates the performance of constructed conventional dye

sensitized solar cells that are based on dyed ZnO nanowires and electrolyte.

4.2.1.1 The Effect of Dye Coating on the Light Absorption

Investigation of light absorption by ZnO nanostructures with and without the N719 dye

coating shows that the dye coating enhances the light absorption in the visible range of light.

The light absorption increases in the range of 400 to 700 nm by up to 100% when ZnO

Fig. 4-6 Band structure: the

levels of defects in ZnO [76].

Fig. 4-7 UV-Vis absorption spectra

of as-grown and dye-coated ZnO

nanowire arrays

25

nanowires are coated with N719 dye as illustrated in Fig. 4-7. Thus solar energy particularly

in this light region is more effectively absorbed and therefore higher power conversion

efficiency can be expected.

Dipping the ZnO nanowires in dye solution leads to anchoring dye molecules. The dye

molecules are distributed over the nanowire surface rather uniformly and form continuous

film as it is evident from TEM image in Fig. 4-8(a). It has been ascertained that the N719 dye

molecules are firmly grafted onto the metal oxide via two carboxylic groups as demonstrated

in Fig. 4-8(b). Upon photon absorption, excitons are generated within the excited dye

molecules and then diffused into dye/ZnO interface, where they are dissociated due to built-in

energy gradient. The oxidation potential of the excited dye (LUMO) is in-line with the

conduction band of ZnO, which facilitates the freed electrons into ZnO [77]. The carboxylate

mode of the ZnO/dye interface in DSSCs is important for effective electron injection. The

larger amount of dye loadings and more packed arrangement of dye molecules are desired for

the higher absorption of solar light, which can then lead to the higher power conversion

efficiency of DSSCs.

4.2.1.2 Current density–Voltage (J-V) Characteristics of Dye Sensitized Solar Cells

The working principle of DSSCs can be explained by the energy band diagram of the

ZnO/Dye/electrolyte interfaces as illustrated in Fig. 4-9. In the DSSCs, the dye molecules

Fig. 4-8(a) TEM image of dye-coated

ZnO nanowires. The inset shows a

single ZnO nanowire without coating

dye.

Fig. 4-8(b) Schematic illustration of

exciton dissociation at the ZnO/dye

interface and electron injection into the

conduction band of ZnO.

26

become excited and inject electrons to the ZnO nanowires after absorbing photon energy. The

electron transfer occurs due to favourable energy difference between the lowest unoccupied

molecular orbital (LUMO) of the dye and the conduction band of ZnO. The photo-generated

electrons are drifted through the ZnO nanowire and collected by the conducting ITO buffer

layer, which is the bottom electrode of the device. The highest occupied molecular orbital

(HOMO) of the dye is energetically lower than the redox potential. The energy difference

provides the driving force for holes to inject into the electrolyte. The involved redox process

can be demonstrated as follows

Anode: Dye + hv Dye*

Dye* Dye+ + e

- (ZnO)

2Dye+

+ 3I- 2Dye + I3

-

Cathode: I3- + 2e

- (counter electrode) 3I

-

Recombination of charge carriers is also minimized in these devices because only a single

type of carriers, electrons, can energetically be transported from the dye to the

semiconductor.

Current density-voltage (J-V) characteristics were measured in dark and under air mass 1.5

global (AM1.5G) illumination with a simulated intensity of a sun (100 mW/cm²). Fig. 4-10(a)

shows the current density-voltage (J-V) characteristics of the DSSC and the corresponding

photovoltaic device structure is shown in Fig. 4-10(b). The DSSC behaves like a diode in

dark. Upon illumination of the simulated sunlight through the bottom electrode, photocurrent

is induced in the device. The important fundamental parameters to evaluate the performance

of the DSSC are short circuit current density JSC, open circuit voltage VOC, fill factor FF, and

solar energy-electrical energy conversion efficiency η. The FF is a measure of quality of the

solar cell which is attributed to function of the ZnO/electrolyte interface. The lower

Fig. 4-9 Schematic band

diagram describing the charge

transfer processes involved in

DSSC.

27

recombination between the conduction band electrons of ZnO and the electrolyte implies the

larger FF, while the J-V curve takes more square-like shape. The fundamental current-voltage

and FF and η parameters are interrelated as follows

OCSCVJ

VJFF maxmax and

in

OCSC

P

FFVJ

where JSC is the short circuit current density obtained at the beginning of the sweep when the

voltage is zero; VOC is the open voltage which is the maximum voltage at zero current; Jmax

and Vmax are the current density and voltage at the point of current-voltage characteristic

where the power output is maximal; and Pin is the solar radiation intensity. From the J-V

curve, the short circuit current JSC, related to the charge injection and transport is found to be

3.45 mA/cm². The open voltage VOC, depending on the energy difference between the Fermi

level of ZnO and the Nernst potential of the redox couple in the electrolyte, is 0.48 V [77].

Accordingly, the fill factor FF and energy conversion efficiency η are calculated to be 0.28

and 0.48%, respectively.

Further improvement of the device performance was obtained by contact treatment. The

counter electrode was coated with a Pt thin film to accelerate holes injection and transport

them at the electrolyte/counter electrode interface. An Ag pad which is used for contacting

electric leads was also prepared to reduce the potential barrier between the ITO

semiconductor and metal contact interface as shown in Fig. 4-11(b). The corresponding J-V

Fig. 4-10 (a) J-V curves for the solar cell in the dark and under simulated

AM1.5G illumination with intensity of 100 mW/cm². (b) Corresponding

schematic photovoltaic device structure.

(b)

(a)

28

curve, in Fig. 4-11(a), indicates that short circuit current JSC is 3.77 mA/cm². This JSC value is

greater than that of the device without the contact treatment. This demonstrates that the

performance of the charge injection and transport is improved owing to lower electron-hole

recombination at the contact interface. However, the open voltage VOC being 0.51 V is almost

unaffected by the contact treatment. This is consistent with the observation that the open

voltage VOC is mainly influenced by the electronegativity of ZnO and the ionization potential

of dye [78-79]. As a result, both the determined fill factor FF and energy conversion

efficiency η increase to the values of 0.36 and 0.69%, respectively, by the described

modification.

4.2.1.3 Effect of the Light Intensity on the Solar Cell Performance

Fig. 4-11 (a) J-V characteristics of the solar cell with the contact treatment

in dark and under simulated AM1.5G illumination with intensity of 100

mW/cm². (b) Corresponding schematic photovoltaic device structure.

Fig. 4-12 J-V characteristics of the

contact treated solar cell under

illumination with intensities of 80,

100 and 120 mW/cm².

(b)

(a)

29

Table 4-1 Performance of the contact treated solar cell under different illumination

intensities.

Illumination intensities

(mW/cm²)

VOC (V) JSC

(mA/cm²)

FF η (%)

80 0.50 2.00 0.34 0.34

100 0.51 3.79 0.36 0.69

120 0.48 5.04 0.35 0.85

The performances of the DSSC with contact treatment under various light intensities were

investigated by adjusting the relative distance between the device and the illumination source.

Fig. 4-12 shows the J-V characteristics of the DSSC under different illumination intensities.

The parameters designated for the performance of the solar cell are listed in Table 4-1. The

short circuit current JSC exhibits an approximately linear relationship with illumination

intensity. On the other hand, the Voc and FF appear to be constant within the measured

intensity range of 80 - 120 mW/cm². The energy conversion efficiency of the DSSC increases

from 0.34 to 0.85% as the intensity increased from 80 to 120 mW/cm².

4.2.2 Dye Free Schottky Barrier Solar Cells

This section describes and evaluates the performance of Schottky barrier solar cells that are

based on ZnO nanowires coated with gold nanoparticles and an electrolyte.

4.2.2.1 Analysis of Distribution of Gold Nanoparticles

Fig. 4-13 (a) SEM image of ZnO

nanowires loaded with Au nanoparticles;

(b) bright field TEM image of abstracted

ZnO nanowires; (c) EDX spectrum

acquired from ZnO nanowires coated

with Au nanoparticles.

(b)

(c)

(a)

30

The ZnO nanowire arrays were immersed in a mixed solution of Na3C6H5O7.2H2O (2ml, 1%)

and HAuCl4 (10ml, 0.03M). The gold ions (Au3

) reduced to neutral atoms by the citrate,

and the Au nanoparticles started to precipitate as the solution becoming supersaturated with

gold atoms. The precipitated Au nanoparticles were attached and distributed over the surfaces

of the immersed ZnO nanowires. The SEM image, in Fig. 4-13(a), shows precipitated and

anchored Au nanoparticles on the surfaces of ZnO nanowires. Obviously the surfaces of the

nanowires have become rough. However, the distribution of the Au nanoparticles over

individual nanowires is fairly uniform and the amount of Au nanoparticles occupying the

surface is conformed to the size of nanowire surface. The bright field TEM image in Fig.

4-13(b) illustrates this fact. The diameters of the precipitated Au nanoparticles on ZnO

nanowire surfaces are about 20-30 nm. The EDX compositional analysis of the nanowires in

different spots indicates chemical nature of the precipitated nanoparticles. A representative

EDX spectrum accumulated in TEM instrument in Fig. 4-13(c), comprises three peaks which

correspond to Au, Zn and O. The highly localized EDX analysis thus proves that

nanoparticles are gold, while the zinc-to-oxygen peak intensities with consideration of

corresponding atomic correction numbers give value of about one

1

/

/

0 O

znZn

O

Zn

ZI

ZI

n

n

which indicates nearly stoichiometric zinc monoxide (ZnO).

4.2.2.2 Analysis of Current Density–Voltage (J-V) Characteristics of Dye Free

Schottky Barrier Solar Cells

Current density-Voltage characteristics were measured for both Schottky barrier solar cells

based on bare ZnO nanowires and ZnO nanowires coated by gold. The device characteristics

were measured using a calibrated solar simulator under a light intensity of 100 mW/cm² (see

Fig. 4-14). The representative current density-voltage characteristic J-V measured on the light

illuminated device with Au nanoparticles shows typical values of short circuit current JSC and

open circuit voltage VOC to be 1.72 mA/cm² and 0.37 V, respectively. The fill factor FF and

energy conversion efficiency η were calculated to be 0.47 and 0.3 %, respectively. On the

other hand the J-V characteristic acquired from the device with pure ZnO nanowires almost

goes through the origin, demonstrating that practically no photocurrent is generated. This

dramatic difference is certainly associated with absence of Schottky Au/ZnO interface.

31

The mechanism of this phenomenon was investigated and explained. As confirmed in Fig.

4-15, the effect of the surface plasma resonance of Au nanoparticles can improve the optical

absorption of the ZnO nanowires in the visible light region [80]. Upon illumination, the

induced excitons in the plasma excited Au nanoparticles are dissociated and the free electrons

with equivalent energy level as that of the conduction state of the ZnO are injected into the

conduction band of the ZnO. Meanwhile, the Schottky barrier formed between the ZnO and

Au can inhibit back electron transfer from ZnO into the Au and the electrolyte. The forward

transferred electrons are transported through the ZnO nanowires and collected at the bottom

of ITO electrode. Then they are exported to an external load to complete the loop of electric

circuit. At the same time, the Au ions capture the electrons donated by the redox species

(

3/ II ) of the electrolyte to compensate their lost electrons. The triiodide (

3I ) then obtain

electrons at the counter electrode and regenerate oxidized iodide (I ). The electrons transfer

process of the redox couple should be fast and their redox potential should be sufficiently

Fig. 4-14 J-V characteristics of the

photovoltaic device with bare ZnO

nanowires and the device with Au-coated

ZnO nanowires. The characteristics

were obtained under simulated AM1.5G

illumination with intensity of 100

mW/cm².

Fig. 4-15 UV-Vis absorption

spectra collected from bare and

Au coated ZnO nanowires.

32

negative to provide electrons to Au continuously. The whole process of generating

photocurrent within the dye free Schottky barrier solar cell is demonstrated in Fig. 4-16.

4.2.2.3 Performance of Dye Free Solar Cells with Schottky Interfaces under Different

Light Intensities

Table 4-2 Parameters of dye free solar cells illuminated with different light intensities. Illumination intensities

(mW/cm²) VOC (V) JSC

(mA/cm²)

FF η (%)

40 0.367 0.548 0.498 0.100

60 0.367 0.918 0.478 0.161

80 0.368 1.326 0.456 0.168

100 0.370 1.720 0.468 0.298

The J-V characteristics of dye free Schottky barrier solar cells under light illumination with

various intensities have also been investigated. The representative current density–voltage

J-V characteristics are plotted in Fig. 4-17. Obviously, the energy conversion efficiency η

raises with increasing the light intensity. The fundamental parameter of the fabricated free

dye solar cells are listed in Table 4-2. The short circuit current JSC is approximately in a linear

relationship with the light intensity, whereas the open circuit voltage VOC arising from the

Fig. 4-16 Schematic band

diagram describing the charge

transfer processes involved in

the dye free Schottky barrier

solar cell.

Fig. 4-17 Performance of dye

free Schottky barrier solar cells

under light illumination with

intensities of 40, 60 and 80

mW/cm².

33

differences of the energy levels between ZnO/Au/electrolyte interfaces is maintained almost

constant. Similarly the fill factor FF is also nearly constant, since it is within the error of the

measurement accuracy of couple precent. If the value of the fill factor systematically reduces

with the increase in light intensity with small increments, then this behaviour is probably

caused by the enhanced recombination process of the electrons in the conduction band of

ZnO and triiodide in the electrolyte.

4.2.3 Characterization of Hybrid Dye Sensitized Solar Cells

This section is focused on measurement and analysis of fabricated hybrid dye sensitized solar

cells. These solar cells are based on ZnO nanowire arrays coated with Au nanoparticles and

subsequently by dye interfacing an electrolyte. The fabrication process is described above.

4.2.3.1 Performance of Designed Hybrid Dye Sensitized Solar Cells

In contrast to the discussed Schottky barrier solar cells, N719 dye was used in construction of

hybrid DSSCs. Gold nanoparticles were precipitated on the bare ZnO nanowires employing a

sol-gel method before applying the dye. The solar cell devices with the structure

schematically illustrated in Fig. 4-18(b) were analyzed to determine the device parameters,

current-voltage characteristic, and overall performance. A representative current

density–voltage (J-V) characteristic is shown in Fig. 4-18(a).

Fig. 4-18 (a) Typical J-V characteristics of the hybrid solar cell referenced to that based on

bare ZnO nanowires measured under simulated AM1.5G illumination with intensity of 100

mW/cm². (b) The corresponding schematic of hybrid solar cell structure.

(a)

(b)

34

The analysis of the J-V characteristics unambiguously indicates that the modification of the

ZnO surfaces does not improve the induced photocurrent. Compared with the J-V

characteristic of the conventionally designed DSSC, apparently, the fill factorr FF, which can

be roughly estimated form the area of the rectangle, becomes smaller for the devices

modified by Au nanoparticles. This is probably caused by the increased dissipation of power

across internal resistances. The short circuit current JSC of gold modified device is 3.08

mA/cm² which is the lower value than that of 3.8 mA/cm² measured for the DSSC based on

bare ZnO. The reduction of short circuit current density JSC may be associated with the

increased defect densities on the surfaces of ZnO nanowires produced during the Au

nanoparticles synthesis process. Although the open circuit voltage VOC is to 0.64 V, the

calculated fill factor FF and power conversion efficiency η are 0.29 and 0.58 %, respectively.

Drop of these parameters for gold modified structures is certainly caused by the lower current

densities which are evident from the J-V characteristics.

The mechanism of the photocurrent induced in the hybrid DSSCs is explained as follows.

Upon photon absorption, two simultaneous processes are involved in the process of

transferring the generated electrons from the excited dye into the conduction band of ZnO.

Firstly, the LUMO level of the dye is sufficiently negative implying that electrons can be

directly injected into the ZnO across the thin layer of Au nanoparticles by tunneling effect.

Secondly, the electrons transfer into the Au nanoparticles first, and they begin to accumulate

within Au nanoparticles. When the Fermi level of Au is raised in-line with the Fermi level of

ZnO, a quick shuttling of electrons from Au to ZnO takes place. The Schottky barrier formed

at the Au/ZnO junction can inhibit the back electron transfer event. The ZnO nanowires

provide direct paths for electrons to transport to the bottom of the ITO substrate and generate

an anodic photocurrent. Meanwhile, the generated holes in the oxidized dye are transported to

the counter electrode in the electrolyte. The whole processes are schematically illustrated by a

diagram in Fig. 4-19.

Fig. 4-19. Energy level diagram

and mechanism of photocurrent

generation in the hybrid DSSC.

C.B. and V.B. are the conduction

and valence bands of ZnO,

respectively.

35

The poor performance of the hybrid DSSCs is ascribed to the increased defects on the surface

of ZnO nanowires. The surface defects probably arising from the in-situ synthesis of Au

nanoparticles, which might act as electron recombination sites. This recombination process

then reduces the energy conversion efficiency. Annealing treatment the structure did not lead

to significant improvement in the device performance. Another possibility is the fact that Au

nanoparticles can affect the adsorption and arrangement of dye molecules over the surface of

ZnO nanowires. Aggregation and unfavourable binding of dye molecules on the ZnO surface

can result in an undesirable energy level of LOMO, which in return can induce low current

density as a consequence of the poor electron injection efficiency [77].

4.2.3.2 The Effect of the Characteristics of Gold Coatings on the Hybrid Solar Cell

Performance

The performance of the solar cell also depends on the characteristics of the thin layer of Au

nanoparticles. It was found that dense and thick layer can reduce the current density and the

fill factor FF as shown in Fig. 4-20. The reasons can originate in several effects. (i)

Aggregations of Au nanoparticles can partially block the incident light. (ii) Dense Au

nanoparticles cause high defect density on the ZnO surfaces and affect the crystallinity of

ZnO. (iii) Large density of Au nanoparticles can result in poor bindings between dye

molecules and ZnO surface. (iv) Thick Au coatings can reduce the tunnelling effect when

electrons transfer from excited dye to ZnO nanowires. Thus important implication from this

analysis is the requirement for well controlled processes associated with evolution and

characteristics of Au nanoparticles on ZnO surfaces to obtain optimal performance of hybrid

solar cells.

Fig. 4-20 J-V characteristics of the

solar cells constructed with bare ZnO

nanowires, and ZnO nanowires coated

thick Au layer and thin layer of Au

nanoparticles under simulated AM1.5G

illumination with intensity of 100

mW/cm²

36

5 Conclusions

This work presents design, fabrication, characterization and study of the produced solar cells

based on ZnO nanowires with different structural architectures. The synthesized ZnO

nanowires were aligned with the surface normal of the electrical conducting substrates. Each

nanowire is single crystalline. Although the ZnO nanowires fabricated by a thermal

evaporation method were observed to have better vertical alignment and less oxygen

vacancies, the low temperature synthesis environment and the transparent property of ITO

substrate used in the solution method are more favorable for growing ZnO nanowire arrays

used in fabrication of photovoltaic devices.

The performance of constructed conventional DSSCs has been improved after contact

interface treatment. The counter electrode was coated with platinum thin film by

electrochemical deposition to assist regeneration process of the oxidized iodide in the

electrolyte. The metal of silver with comparable work function to that of ITO was selected to

deposit on the bottom electrode to relieve contact barrier between the ITO substrate and the

electric wire leads. This way, the energy conversion efficiency η of the DSSCs improved and

was about 0.69%. It was also shown that the power conversion efficiency of the fabricated

DSSCs raised with increasing the illumination intensity.

It was found that the devices exploiting ZnO nanowires coated with Au nanoparticles can

generate photocurrent even without dye. The presence of Au nanoparticles enhanced optical

absorption in the visible range of light due to the surface plasmon resonance. The fill factor

of the designed dye free Schottky barrier solar cells achieved 0.5. After adsorption of

ruthenium dye (N719), the hybrid DSSC structures with Au nanoparticles and ZnO

nanowires showed different parameters. The open circuit voltage VOC was increased from 0.5

to 0.63 V when compared with bare ZnO nanowire based DSSCs. The reduction of short

circuit current density JSC was however reduced owning to the higher density of surface

defects on the ZnO nanowires. These defects acted as electrons recombination sites.

Therefore the overall device performance was not marginally improved. The thickness of the

Au nanoparticles coating also influenced the performance of the designed hybrid DSSCs,

which implied that further engineering and study of devices structure would be required.

37

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