1

Solid-state Physics, Interfaces and Nanostructures

Department of Physics

University of Liège

Prof. Ngoc Duy NGUYEN Head of Laboratory

Lab’Insight ‘Surface Treatments and Solar Energy’

Brussels – 21st November 2013

SOLID-STATE PHYSICS, INTERFACES AND NANOSTRUCTURES

SPIN

Watching the Lab’s video

Fields of expertise • Material characterizations

Electrical and optical measurements • Electron device simulations

Physical interpretation of device characteristics • Atomistic simulations

Prediction and optimization of material properties

Industrial applications fields • Quality control, in-line monitoring • Defect assessment • Device characterization

2 Brussels – 21st November 2013

Fields of expertise and industrial targets

3

Applied research projects

Material characterization Device simulation Electrical and opto-electrical characterization of thin film semiconducting heterostructures

Aim Defect assessment

Identify defect signatures in electrochemical

impedance spectroscopy Correlate with material quality Implement measurement as an in-line

monitoring tool Provide feedback to process steps

Brussels – 21st November 2013

Cu(In,Ga)Se2-based layer stack

as thin-film photovoltaic cell

N. D. Nguyen, ULg 2011-2012 PHYS3013-1 | PHYS3016-1 | #2

Other active topics

17

Solar water-splitting for hydrogen production

1. Introduction

Energy from the Sun can easily provide enough power for all

of our energy needs if it can be efficiently harvested. While

there already exist a number of devices that can capture and

convert electromagnetic energy, the most common—a photo-

voltaic cell—produces electricity, which must be used immedi-

ately or stored in asecondary device such asabattery or a fly-

wheel. Amore elegant, practical, and potentially more efficient

route to storing solar power is to convert the electromagnetic

energy directly into chemical energy in the form of molecular

bonds, analogous to the photosynthesis process exploited by

nature.[1] Biologic photosynthesis effectively rearranges elec-

trons in H2Oand CO2 to store solar energy in the form of car-

bohydrates. However, the extremely low overall efficiency that

natural photosynthesisexhibits implies the requirement of vast

amounts land and farming resources to meet our energy de-

mands.[2–3] Because of this, artificial photosynthetic routes in-

cluding photoelectrochemical (PEC) and photocatalytic (PC)

solar energy conversion have been intensely investigated over

the last four decades. Given the abundance of H2O on Earth,

the water splitting reaction, H2O! 1=2O2+ H2 (E0= 1.23V), is the

most appealing pathway for artificial photosynthesis. Indeed

solar water splitting would form the basis for asustainable hy-

drogen-based energy economy.

Adistinction can bemade between the different approaches

to artificial photosynthesis: PC water splitting systems use a

dispersed material in pure water and accordingly produce hy-

drogen and oxygen homogeneously throughout the solution.

This approach is under examination with both inorganic col-

loid materials[4–5] and molecular complexes.[6] In contrast, PEC

systemsemploy photoactive materialsaselectrodes. As in con-

ventional water electrolysis, oxidation (O2 evolution) occurs at

the anode, reduction (H2 evolution) occurs at the cathode, and

an aqueous electrolyte completes the current loop between

the electrodes and an external circuit. One or both of the elec-

trodes can be a photoactive semiconductor, in which a space-

charge (depletion) layer is formed at the semiconductor/liquid

junction (SCLJ). Upon irradiation, photogenerated carriers are

separated by the space-charge field and the minority carriers

(holes for an n-type photoanode and electrons for a p-type

photocathode) travel to the SCLJ to perform one half of the

water splitting reaction. The schematic for an n-type photo-

anode is shown in Figure1a. The advantage of the PECroute

isthat it allowsthe spatially separate production, and therefore

collection, of H2 and O2.

Photoelectrochemical (PEC) cells offer the ability to convert

electromagnetic energy from our largest renewable source, the

Sun, to stored chemical energy through the splitting of water

into molecular oxygen and hydrogen. Hematite (a-Fe2O3) has

emerged as a promising photo-electrode material due to its

significant light absorption, chemical stability in aqueous envi-

ronments, and ample abundance. However, its performance as

a water-oxidizing photoanode has been crucially limited by

poor optoelectronic properties that lead to both low light har-

vesting efficiencies and a large requisite overpotential for pho-

toassisted water oxidation. Recently, the application of nano-

structuring techniques and advanced interfacial engineering

has afforded landmark improvements in the performance of

hematite photoanodes. In this review, new insights into the

basic material properties, the attractive aspects, and the chal-

lenges in using hematite for photoelectrochemical (PEC) water

splitting are first examined. Next, recent progress enhancing

the photocurrent by precise morphology control and reducing

the overpotential with surface treatments are critically detailed

and compared. The latest efforts using advanced characteriza-

tion techniques, particularly electrochemical impedance spec-

troscopy, are finally presented. These methods help to define

the obstacles that remain to be surmounted in order to fully

exploit the potential of thispromising material for solar energy

conversion.

Figure 1. a)Energy diagram for photoelectrochemical water splitting with an

n-type photoanode (hematite) performing the oxygen evolution (oxidation)

reaction and acathode performing the hydrogen evolution (reduction) reac-

tion. The photoanode bandgap, Eg, flat band potential, Vfb, and the applied

bias, Vb, are indicated. b) Photoanode/photovoltaic water splitting tandem

cell concept, shown with adye-sensitized solar cell (DSC).

[a] Dr. K. Sivula, F. LeFormal, Prof. Dr. M. Grätzel

Instituteof Chemical Sciencesand Engineering

ÉcolePolytechniqueFØdØraledeLausanne

Station 6, 1015 Lausanne (Switzerland)

Fax: (+ 41)21 693 4111

E-mail:kevin.sivula@epfl.ch

ChemSusChem 2011, 4, 432–449 2011 Wiley-VCHVerlag GmbH&Co. KGaA, Weinheim www.chemsuschem.org 433

Solar Water Splitting: ProgressUsing a-Fe2O3 Photoelectrodes

CIS/CIGS materials for thin film photovoltaics

TCO/CdS

CIGS

Mo

Substrate

Coating Thickness Material TestingMicrohardnessMaterial Analysis

CIGS-solar panels on a roof

photo: Würth Solar, Germany

Inline FISCHERSCOPE®

CONTI 4000-V

Thin Film Solar Cells.

How to measure thickness and

composition of CIGS and CdTe coatings.

Offline FISCHERSCOPE®

CONTI 4000-DPP

Transparent conductive oxide nanostructures

Please cite this article in press as: N. Karst, et al., Mater. Sci. Eng. B(2011), doi:10.1016/j.mseb.2011.02.009

ARTICLE IN PRESSGModel

MSB12734 1–7

2 N. Karst et al. / Materials Science and Engineering Bxxx (2011) xxx–xxx

Fig.1. Schematic representation of aDSSCbased on nanostructured ZnOcomposite.

nanoparticles. By changing the nanoparticle concentration in the63

solution or by addingsurfactantsand binder, Baxter et al. [13] man-64

aged to fill the interstices between nanowires or to deposit the65

nanoparticles at their top. The approach described in this paper is66

rather closeto theonegiven by Ku et al. [14].Acomparison and dis-67

cussion between these two research works isreported at theend of68

Section 3.4.69

So far, the highest efficiency of ZnO-based-DSSCs has been70

obtained by using ZnO nanostructures such as nanosheets (3.9%71

[15]) or hierarchically structured spheres composed of ZnO72

nanocrystallites (5.4% [16]), but not with ZnO nanowires or73

nanorods. However, the nanowires can help in finding an opti-74

mized structure since the transport is expected to be much more75

efficient as compared to nanoparticles. This can be explained by76

the fact that so far no nanocomposite hasbeen found for which all77

the key parameters have simultaneously been optimized: among78

them, one can for instance mention the complete filling of the79

interstices between NWs, the surface area, the interface quality,80

the NW density and diameters. Therefore, some thorough exper-81

imental investigations are required to achieve such an optimized82

structure. It should be also noted that the use of ZnO NWs in a83

DSSC may increase the crack resistance since the ZnO nanowire-84

based-film can be bended to a very low radius of curvature85

[17].86

In thispaper, ZnOnanocomposite based on both ZnOnanowires87

and nanoparticles were grown by wet chemical route in order88

to fabricate a DSSC as schematically depicted in Fig. 1. In order89

to better take into account the key roles of nanowires and90

nanoparticles in a ZnO nanocomposite-based-DSSC, the struc-91

tural properties of ZnO nanowires and nanostructured ZnO92

sheet have separately been investigated by scanning (SEM) and93

transmission electron microscopy (TEM) and X-ray diffraction94

measurements. Vertically aligned ZnO nanowires were grown by95

chemical bath deposition (CBD) fluorine-doped tin oxide (FTO)96

thin films seeded with ZnO nanoparticles, as reported by Bax-97

ter et al. [18]. Subsequently, the empty space between each98

nanowire was filled with layered hydroxide zinc acetate nanopar-99

ticles from a methanolic solution of zinc acetate [19]. In order100

to transform the LHZA nanoparticles into ZnO nanoparticles, an101

annealing under air atmosphere was performed. One of the main102

advantages is that ZnO nanoparticles are directly sintered on103

ZnO nanowires, favouring a direct pathway for the electrons104

from ZnO nanoparticles into the FTO thin film. Another impor-105

tant aspect of the present paper is the direct comparison of106

the electrical properties between ZnO nanowire-based-DSSC and107

ZnO nanocomposite-based-DSSCs by dark and illuminated inten-108

sity/voltage measurements as well as electrochemical impedance109

spectroscopy (EIS).

2. Experimental 110

2.1. Fluorine-doped tin oxide thin films 111

All ZnO depositions were performed onto FTO thin film with a 112

thicknessof 250nm.FTOthin filmswere grown on glasssubstrates 113

at 410 ◦Cby atmospheric spray pyrolysis from achemical precursor 114

consisting of 0.16M tin chloride pentahydrate 98%(Sigma-Aldrich) 115

and0.04M ammonium fluoride98%(Sigma-Aldrich) in amethano- 116

licsolution. TheSheet resistance of FTOthin films isabout 10 per 117

square while their overall transmittance with glass equals nearly 118

85%in thevisible range. 119

2.2. ZnOnanowiregrowth by chemical bath deposition 120

Before the ZnO nanowire growth, FTO thin films were seeded 121

by dip coating in an ethanolic solution of 0.375M of zinc acetate 122

99.99%(Sigma-Aldrich) and 0.375M of monoethanolamine (MEA) 123

(Sigma-Aldrich). An annealing wascarried out for 20min at 400 ◦C 124

under controlled humidity nitrogen flux,which leadstothedecom- 125

position of thezinc acetate into oriented ZnOnanoparticles [18]. It 126

is worthy of noticing that this seed layer is a key step to get ver- 127

tically aligned ZnO nanowires [20]. The growth of ZnO nanowires 128

was then performed in a sealed batch by dipping the seeded FTO 129

substrate in an aqueous solution of 0.025M of zinc nitrate hexahy- 130

drate99%(Sigma-Aldrich) and0.025Mof hexamethylenetetramine 131

(HMTA) (Sigma-Aldrich) kept at 90 ◦ C. The solution was refreshed 132

every 3h in order to retain a constant growth rate since the 133

reagent concentration decreases as growth proceeds while zinc 134

oxide homogeneously precipitates. Several baths are then neces- 135

sary to get nanowires with significant lengths of about several 136

microns. 137

2.3. Nanostructured ZnOsheet growth by CBD 138

Nanoporous ZnO that is used later on to fill in the interstices 139

between ZnO nanowires was also grown by CBD as reported by 140

Hosono et al. [19]. Two consecutive steps were involved to fabri- 141

cate such a layer. The first step consists in the growth of a layered 142

hydroxide zinc acetate by CBD: FTO thin films were immerged in 143

amethanolic solution of zinc acetate dihydrate 99.5%(Merck) at a 144

concentration of 0.15M kept at 60 ◦ Cfor several hours. Thesecond 145

step results in the conversion of the LHZA film into awurtzite ZnO 146

film through an annealing under air atmosphere first at 200 ◦Cfor 147

5min and then at 450 ◦ Cfor 10min. 148

2.4. ZnOcomposite growth 149

In order to fill in thenetwork of ZnOnanowires with nanoparti- 150

cles, a similar deposition as described in Section 2.3 was achieved 151

on ZnO nanowires. The immersion time was adjusted depending 152

on the nanowire length. 153

2.5. Fabrication of DSSC 154

After the deposition of either single ZnO nanowires or ZnO 155

nanocomposites, the sample was immersed for 10min in an 156

ethanolic solution consisting of 0.25mM of N719 dye. Theduration 157

of 10min wasfound tobetheoptimal dipping timeand yielded the 158

highest DSSCefficiency. A 100nm thick platinum layer deposited 159

by sputtering on a glass substrate was used as the counter elec- 160

trode. The front and rear faces are spaced by means of a 25 m 161

thick plastic film and assembled with binder clips. A commercial 162

electrolyte from Dyesol (EL-HSE) with high boiling point solvents 163

favouring the time stability of the DSSCproperties was used to fill 164

Please cite this article in pressas: N. Karst, et al., Mater. Sci. Eng. B(2011), doi:10.1016/j.mseb.2011.02.009

ARTICLE IN PRESSGModel

MSB12734 1–7

N. Karst et al. / Materials Scienceand Engineering Bxxx (2011) xxx–xxx 3

in the empty space between the two faces. The active DSSCarea165

was25mm2.166

2.6. Characterization of ZnOdeposit and DSSCs167

The morphology of the film structure was investigated by168

SEM and TEM imaging recorded with a Hitachi S-4500 and JEOL169

JEM-2010 equipments (operating at 200kV with a 0.19nm point-170

to-point resolution), respectively. The structural properties of the171

film structure were determined by XRD measurements according172

to the Bragg–Brentano configuration with a Bruker D8 Advance173

diffractometer using CuK radiation. The AC impedance spectra174

were collected by using a multichannel potentiostat equipped175

with an impedance module (VMP2 from Bio-Logic SAS) in poten-176

tiostatic mode: a 10mV amplitude sinusoidal signal was used177

while the frequencies were ranged from 200kHz to 0.4Hz.178

Photocurrent density–photovoltage curves and impedance mea-179

surements were achieved under an AM 1.5 simulated sunlight180

at 100mW/cm2 (150W Xe lamp, AM 1.5 filter, UV attenuator)181

with a Newport 150W solar simulator. It is emphasized that182

the DSSCfabrication was highly reproducible, since each step for183

the fabrication of all the parts composing the DSSCs have been184

controlled several times. The growth of the FTO thin films, ZnO185

NWs and the filling the interstices between NWs are therefore186

very reproducible. A Jasco V530 UV–vis spectrophotometer was187

employed to determine the amount of adsorbed dye onto ZnO188

semiconductor.189

The dye desorption was performed by dipping the sensitized190

sample in a 10−2 M of NaOH in ethanol–water (1:1) solu-191

tion [21]. The effective dye loading was determined from the192

absorption value at 540nm for each NaOH/dye solution accord-193

ing to the Beer’s law. The extinction coefficient at 540nm of194

the dye solution used to sensitize the DSSCs was found to be195

ε=6560M−1 cm−1.

3. Results and discussion 196

3.1. Structural propertiesof ZnOnanowires 197

The morphology of ZnOnanowires grown in an aqueous solu- 198

tion are shown in Fig. 2a and b from cross section and top view 199

SEM images. ZnO nanowires exhibit a diameter between 70 and 200

300nm and a length of about 4 m. They are vertically aligned 201

and well faceted with a hexagonal base. As revealed by the high- 202

resolution TEM image in Fig. 2c, the ZnOnanowires have a single 203

crystal structure, which is defect-free. From the HRTEM image, 204

the lattice constant was found to be about 0.52nm for the (002) 205

planesasexpected for bulk ZnO. TheXRDpattern in Fig. 2d clearly 206

shows that the layer composed of ZnOnanowires is strongly tex- 207

turedalongthepreferential <002>crystallographicorientation.The 208

presence of other crystallographic directions on the XRD pattern 209

originates from ZnOnanoparticles below ZnOnanowires. Another 210

explanation is due to the growth of ZnO nanowires: some aggre- 211

gates stems from homogeneous nucleation in solution, settle on 212

ZnOnanowirefilmandtheseaggregatescanbeoriented indifferent 213

crystallographic directions. From the peak intensity, an experi- 214

mental average texture can be determined [22,23]. The texture 215

coefficients, noted as Chkl , are determined by the following equa- 216

tion:217

Chkl =Ihkl / I0,hkl

1N

N

Ihkl / I0,hkl

218

where Ihkl and I0,hkl arethe integrated intensitiesof the<hkl>crys- 219

tallographic direction, respectively, for the involved ZnOnanowire 220

sample and for arandomly oriented wurtzite ZnOsamplegiven by 221

theJoint Committeefor Powder Diffraction Standard (JCPDS) values 222

[24]; and Ncorresponds to thenumber of considered peaks. In our 223

case,fivepeaksshould beconsidered so that N=5.Arandomly ori- 224

Fig. 2. Characterization of a 8 bath CBD nanowires sample, (a) cross section SEM image, (b) top view SEM image, (c) TEM image of a single nanowire and (d) XRD diagram,

note that thisfigure is in log-scale for the sake of clarity.

Please cite thisarticle in pressas: N. Karst, et al., Mater. Sci. Eng. B(2011), doi:10.1016/j.mseb.2011.02.009

ARTICLE IN PRESSGModel

MSB127341–7

N. Karst et al. / MaterialsScienceand Engineering Bxxx (2011) xxx–xxx 3

in the empty space between the two faces. The active DSSC area165

was 25mm2.166

2.6. Characterization of ZnOdeposit and DSSCs167

The morphology of the film structure was investigated by168

SEM and TEM imaging recorded with a Hitachi S-4500 and JEOL169

JEM-2010 equipments (operating at 200kV with a 0.19nm point-170

to-point resolution), respectively. The structural properties of the171

film structure were determined by XRD measurements according172

to the Bragg–Brentano configuration with a Bruker D8 Advance173

diffractometer using CuK radiation. The AC impedance spectra174

were collected by using a multichannel potentiostat equipped175

with an impedance module (VMP2 from Bio-Logic SAS) in poten-176

tiostatic mode: a 10mV amplitude sinusoidal signal was used177

while the frequencies were ranged from 200kHz to 0.4Hz.178

Photocurrent density–photovoltage curves and impedance mea-179

surements were achieved under an AM 1.5 simulated sunlight180

at 100mW/cm2 (150W Xe lamp, AM 1.5 filter, UV attenuator)181

with a Newport 150W solar simulator. It is emphasized that182

the DSSCfabrication was highly reproducible, since each step for183

the fabrication of all the parts composing the DSSCs have been184

controlled several times. The growth of the FTO thin films, ZnO185

NWs and the filling the interstices between NWs are therefore186

very reproducible. A Jasco V530 UV–vis spectrophotometer was187

employed to determine the amount of adsorbed dye onto ZnO188

semiconductor.189

The dye desorption was performed by dipping the sensitized190

sample in a 10−2 M of NaOH in ethanol–water (1:1) solu-191

tion [21]. The effective dye loading was determined from the192

absorption value at 540nm for each NaOH/dye solution accord-193

ing to the Beer’s law. The extinction coefficient at 540nm of194

the dye solution used to sensitize the DSSCs was found to be195

ε=6560M−1 cm−1.

3. Results and discussion 196

3.1. Structural properties of ZnOnanowires 197

The morphology of ZnO nanowires grown in an aqueous solu- 198

tion are shown in Fig. 2a and b from cross section and top view 199

SEM images. ZnO nanowires exhibit a diameter between 70 and 200

300nm and a length of about 4 m. They are vertically aligned 201

and well faceted with a hexagonal base. As revealed by the high- 202

resolution TEM image in Fig. 2c, the ZnO nanowires have a single 203

crystal structure, which is defect-free. From the HRTEM image, 204

the lattice constant was found to be about 0.52nm for the (002) 205

planes asexpected for bulk ZnO. The XRD pattern in Fig. 2d clearly 206

shows that the layer composed of ZnO nanowires is strongly tex- 207

turedalongthepreferential <002>crystallographic orientation. The 208

presence of other crystallographic directions on the XRD pattern 209

originates from ZnOnanoparticles below ZnOnanowires. Another 210

explanation is due to the growth of ZnO nanowires: some aggre- 211

gates stems from homogeneous nucleation in solution, settle on 212

ZnOnanowirefilm and theseaggregatescan beoriented in different 213

crystallographic directions. From the peak intensity, an experi- 214

mental average texture can be determined [22,23]. The texture 215

coefficients, noted as Chkl , are determined by the following equa- 216

tion:217

Chkl =Ihkl / I0,hkl

1N

N

Ihkl / I0,hkl

218

where Ihkl and I0,hkl are the integrated intensities of the<hkl>crys- 219

tallographic direction, respectively, for the involved ZnOnanowire 220

sample and for a randomly oriented wurtzite ZnOsamplegiven by 221

theJoint Committeefor Powder Diffraction Standard (JCPDS) values 222

[24]; and Ncorresponds to the number of considered peaks. In our 223

case, fivepeaksshould beconsidered so that N=5.A randomly ori- 224

Fig. 2. Characterization of a8 bath CBD nanowires sample, (a) cross section SEM image, (b) top view SEM image, (c) TEM image of a single nanowire and (d) XRD diagram,

note that this figure is in log-scale for the sake of clarity.

N. D. Nguyen, ULg 2011-2012 PHYS3013-1 | PHYS3016-1 | #2

18

!" #$%&' ( )*&+*&' , - . #/ 0- $12 34

4

Applied research projects

Material characterization Emerging transparent conductive materials as electrodes for solar cells

Aim Investigation of the percolation mechanism in metallic nanowire networks

Understand electrical conductivity and

optical transmittance Compute collection efficiency Solve thermal stability issue Integrate in dye-sensitized solar cells

Brussels – 21st November 2013

Encapsulated Ag nanowire network

as transparent conductive material

5

Applied research projects

Modeling Atomistic simulations of nanostructures (ab initio)

Aim Optimisation of photovoltaic properties

Provide model structures (compositions,

geometries) Provide spectral fingerprints Estimate the effect of the environment (i.e.

effect of ligands and solvant on chemical stability and spectral properties)

Brussels – 21st November 2013

Light-induced charge separation in

a Si-Ge pyramidal wire (2 nm

diameter)

6

Special equipments

Brussels – 21st November 2013

7

Examples → Consultancy service in electrical and optical characterization of thin films → Expertise on impedance spectroscopy → Device simulation works → Experimental development in semiconductor metrology for industry-oriented

long-term applied projects → On-demand calculations (collaboration-based) of atomistic structures,

electronic and optical properties → Training of scientists

Services provided to companies

Brussels – 21st November 2013

Prof. Ngoc Duy NGUYEN Chargé de cours

ngocduy.nguyen@ulg.ac.be, phone : +32 4 366 3604

8

Contact information

University of Liège|Department of Physics Solid-state Physics, interfaces and nanostructures Institut de Physique (B5a), Allée du Six Août 17, 4000 Liège (Sart-Tilman) http://www.spin.ulg.ac.be

Brussels – 21st November 2013

Dr. Jean-Yves RATY Senior Research Associate FRS-FNRS

jyraty@ulg.ac.be, phone : +32 4 366 3747

SOLID-STATE PHYSICS, INTERFACES AND NANOSTRUCTURES

SPIN