Iron Pyrite Absorbers for Solar Photovoltaic Energy Conversion

208 Pages·2015·58.27 MB·English

by  Moritz Limpinsel

UC Irvine
UC Irvine Electronic Theses and Dissertations
Title
Iron Pyrite Absorbers for Solar Photovoltaic Energy Conversion
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Author
Limpinsel, Moritz
Publication Date
2015
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University of California
UNIVERSITY OF CALIFORNIA,
IRVINE
Iron Pyrite Absorbers for Solar Photovoltaic Energy Conversion
DISSERTATION
submitted in partial satisfaction of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
in Chemical and Materials Physics
by
Moritz Limpinsel
Dissertation Committee:
Professor Matt Law, Chair
Professor John Hemminger
Professor Robert Corn
2015
Part of section 4.1 adapted with permission from [22]. (cid:13)c 2012 American Physical Society
Part of section 3.1, section 2.1.3 and section 5.3.2 adapted with permission from [6]. (cid:13)c
2012 WILEY-VCH Verlag
Part of section 3.1 adapted with permission from [97]. (cid:13)c 2013 American Chemical Society
Part of section 4.4 and section 2.1.2 adapted with permission from [70]. (cid:13)c 2014 The Royal
Society of Chemistry
All other materials (cid:13)c 2015 Moritz Limpinsel
DEDICATION
This work is dedicated to my parents, Hans-Dieter Limpinsel and Christel Limpinsel.
None of this would have been possible without their emotional and financial support,
adamant belief in me, and their boundless and unconditional love for me.
Danke!
ii
TABLE OF CONTENTS
Page
LIST OF FIGURES vi
LIST OF TABLES viii
ACKNOWLEDGMENTS ix
CURRICULUM VITAE x
ABSTRACT OF THE DISSERTATION xiii
1 Introduction 1
1.1 Solar Photovoltaics as a Renewable Energy Source . . . . . . . . . . . . . . . 1
1.2 Earth-Abundant Semiconductors for Large Scale Photovoltaics . . . . . . . . 4
1.3 Iron Pyrite FeS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
1.3.1 Review of Material Properties . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 Open Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.3.3 Scope of This Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2 Methods 15
2.1 Synthesis of Iron Pyrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.1.1 Chemical Vapor Transport (CVT) . . . . . . . . . . . . . . . . . . . . 15
2.1.2 Flux Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.1.3 Chemical Vapor Deposition (CVD) . . . . . . . . . . . . . . . . . . . 19
2.1.4 Pyrite Ink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 Deposition and Patterning of Thin Films . . . . . . . . . . . . . . . . . . . . 22
2.2.1 Sputter Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 Thermal Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2.3 Atomic Layer Deposition (ALD) . . . . . . . . . . . . . . . . . . . . . 27
2.2.4 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3 Preparation of Single Crystal Samples . . . . . . . . . . . . . . . . . . . . . . 29
2.3.1 Sectioning and Polishing . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 Wet Passivation Treatments . . . . . . . . . . . . . . . . . . . . . . . 30
2.3.3 Fabrication of Pellets . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.3.4 Electron Beam Lithography (EBL) . . . . . . . . . . . . . . . . . . . 31
2.3.5 Ohmic Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
iii
2.4 Structural & Elemental Characterization . . . . . . . . . . . . . . . . . . . . 34
2.4.1 X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4.2 Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.4.3 Mass Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.4.4 Energy-Dispersive X-ray Spectroscopy (EDS) . . . . . . . . . . . . . 36
2.4.5 Photoelectron Spectroscopy (UPS/XPS) . . . . . . . . . . . . . . . . 37
2.5 Optical & Morphological Characterization . . . . . . . . . . . . . . . . . . . 37
2.5.1 Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . 37
2.5.2 Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . . 37
2.5.3 Optical Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . 38
2.5.4 Spectral Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.6 Electrical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.6.1 Temperature-Dependent Resistivity and Hall Effect . . . . . . . . . . 38
2.6.2 Seebeck Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.6.3 Carrier Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.6.4 Solar Cell Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.7 Data Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.7.1 Electrical Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.7.2 Hall Effect Data and Multilayer Model . . . . . . . . . . . . . . . . . 44
2.7.3 Band Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.8 Solar Cell Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.8.1 Single Crystal Junctions . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.8.2 Thin Film p-n Heterojunctions . . . . . . . . . . . . . . . . . . . . . 48
3 Pyrite and Other Relevant Thin Films 51
3.1 Pyrite Thin Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.1.1 Comparison of Thin Films Made by Different Routes . . . . . . . . . 51
3.1.2 Effect of Marcasite Impurity . . . . . . . . . . . . . . . . . . . . . . . 52
3.1.3 Electrical Transport in Polycrystalline Thin Films . . . . . . . . . . . 57
3.2 MoS Films Obtained by Sulfurization from Mo . . . . . . . . . . . . . . . 61
2−x
3.3 NiO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4 Pyrite Single Crystals 68
4.1 Crystals Found in Nature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.2 Growth by Chemical Vapor Transport (CVT) . . . . . . . . . . . . . . . . . 72
4.3 Mesoscopic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.3.1 Microcrystalline Pellets . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3.2 Single Nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4.3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.4 Flux Grown Single Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.4.1 Structural and Elemental Characterization . . . . . . . . . . . . . . . 82
4.4.2 Hall Effect Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
4.4.3 Assessment of the Bulk Band Gap of Pyrite . . . . . . . . . . . . . . 108
4.5 Carrier Lifetimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
4.6 Surface Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
iv
4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5 Pyrite Solar Cells 132
5.1 Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.2 Wet Junction Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.3 Solid-State Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.3.1 Single Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.3.2 Thin Film Heterojunction Solar Cells . . . . . . . . . . . . . . . . . . 137
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
6 Summary and Conclusions 159
Bibliography 163
A Elemental Purity of Materials Used 173
B Additional Data and Calculations 177
C Igor Pro Fitting Procedure 183
v
LIST OF FIGURES
Page
1.1 Three Approaches to Harvest the Sun’s Energy . . . . . . . . . . . . . . . . 2
1.2 Basic Principles of a Photovoltaic Cell . . . . . . . . . . . . . . . . . . . . . 3
1.3 Cumulative PV Installations and Economies of Scale . . . . . . . . . . . . . 3
1.4 Potential for Electricity Production and Raw Material Cost . . . . . . . . . . 5
1.5 Iron Pyrite Unit Cell and Fe-S Phase Diagram . . . . . . . . . . . . . . . . . 7
1.6 Electronic Structure of Bulk Pyrite . . . . . . . . . . . . . . . . . . . . . . . 7
1.7 XPS Study of Sulfur Species at the Pyrite 100 Surface . . . . . . . . . . . . 9
1.8 XPS Study of Iron Species at the Pyrite 100 Surface . . . . . . . . . . . . . . 10
1.9 Oxidation of the Pyrite Surface . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 Chemical Vapor Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2 Na-S Binary Phase Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Technique of Magnetron Sputtering . . . . . . . . . . . . . . . . . . . . . . . 23
2.4 Shadow Mask for Sputter Deposition and Resulting Patterns . . . . . . . . . 25
2.5 Target Surface Control and Partial Pressure Hysteresis in Reactive Sputtering 26
2.6 Pyrite Films Patterned by Photolithography . . . . . . . . . . . . . . . . . . 28
2.7 Polishing of Pyrite Slices to Atomic Flatness . . . . . . . . . . . . . . . . . . 30
2.8 Electron Beam Lithography (EBL) . . . . . . . . . . . . . . . . . . . . . . . 33
2.9 Hall Effect and Van der Pauw Method . . . . . . . . . . . . . . . . . . . . . 39
2.10 Conductivity and Hall Effect in a Multi-Layer System . . . . . . . . . . . . . 44
3.1 Comparison of Pyrite Thin Films Grown by CVD and Ink Methods . . . . . 53
3.2 Elemental Impurity Content of Pyrite Thin Films From Different Methods . 53
3.3 Marcasite Impurity in Pyrite Thin Films: Morphology and Distribution . . . 54
3.4 Marcasite Impurity in Pyrite Thin Films: Optical and Electrical Effects . . . 56
3.5 Electrical Transport in Polycrystalline Pyrite Thin Films . . . . . . . . . . . 60
3.6 Sputter Deposition of Mo Films on Glass . . . . . . . . . . . . . . . . . . . . 61
3.7 Mo Films Before and After Sulfurization: Topography and Morphology . . . 62
3.8 Phase Transition from Mo to MoS . . . . . . . . . . . . . . . . . . . . . . . 64
2
3.9 Properties of NiO Films Made by Reactive Sputtering . . . . . . . . . . . . . 66
4.1 Characterization of Natural Crystal Used for SE Study . . . . . . . . . . . . 70
4.2 Dielectric Spectrum and Critical Point Energies from SE and DFT . . . . . . 71
4.3 Pyrite Single Crystals Grown by Chemical Vapor Transport . . . . . . . . . 73
4.4 Transport in Single Crystals vs. Thin Films . . . . . . . . . . . . . . . . . . 74
vi
4.5 Microcrystalline Pyrite Pellets . . . . . . . . . . . . . . . . . . . . . . . . . . 76
4.6 Single Nanocrystals of Pyrite Contacted by Electron Beam Lithography . . . 78
4.7 Comparison of Pyrite Single Crystals, Pellets and Thin Films . . . . . . . . . 80
4.8 Impact of Precursor Purity on Electrical Transport in Pyrite Crystals . . . . 81
4.9 Pyrite Single Crystals Grown in a Na S-Sulfur Flux . . . . . . . . . . . . . . 83
2
4.10 SIMS Depth Profile of Flux Grown Pyrite Crystals . . . . . . . . . . . . . . 87
4.11 Hall Effect and Conductivity Data for Flux Grown Pyrite Slab . . . . . . . . 89
4.12 Mobility and Concentration of Free Carriers in the Unipolar Approximation 91
4.13 Calculations of R (T) for a Homogeneous Semiconductor, All Scenarios . . . 93
H
4.14 Low-Temperature Resistivity Data and Fits . . . . . . . . . . . . . . . . . . 94
4.15 Transverse Magnetoresistance (MR) of a Pyrite Single Crystal . . . . . . . . 97
4.16 Calculated DOS(E): Parabolic Band Approximation vs. DFT . . . . . . . . 100
4.17 Fermi Energy and Carrier Concentrations for Bulk and Surface Layers . . . . 101
4.18 Dependence of Hall data on Sample Thickness. . . . . . . . . . . . . . . . . . 101
4.19 Dependence of the Hall Data on Surface Modifications. . . . . . . . . . . . . 103
4.20 Upper Valence Band Spectra of a Polished Pyrite (111) Slab . . . . . . . . . 104
4.21 Calculated Equilibrium Band Diagram of the Pyrite Surface . . . . . . . . . 107
4.22 Conductivity of Pyrite Crystals in the Intrinsic Region . . . . . . . . . . . . 110
4.23 XRD Patterns of Pyrite Single Crystal Before and After Heating . . . . . . . 112
4.24 Integrated DOS(E) of Pyrite Ruling out Burstein-Moss Shift . . . . . . . . . 113
4.25 Optical Extinction Spectra of a Pyrite Crystal as a Function of Temperature 115
4.26 Compilation of Literature Values for the Pyrite Band Gap . . . . . . . . . . 115
4.27 Carrier Lifetime in Pyrite Single Crystals Determined by TRMC . . . . . . . 118
4.28 Magnitude of Surface Effects for Various Pyrite Materials . . . . . . . . . . . 120
4.29 Surface Passivation of Single Crystals, Evaluated by Hall Effect . . . . . . . 121
4.30 Piranha Etch for Surface Passivation . . . . . . . . . . . . . . . . . . . . . . 123
4.31 Hydrogenic Treatments for Surface Passivation . . . . . . . . . . . . . . . . . 124
4.32 Annealing and Plasma Treatments for Surface Passivation . . . . . . . . . . 125
4.33 Coverage with ZnS for Surface Passivation . . . . . . . . . . . . . . . . . . . 127
5.1 Optical Absorptivity of Pyrite, Theoretical and Experimental . . . . . . . . 133
5.2 Liquid Junction Solar Cell with Pyrite Absorbers . . . . . . . . . . . . . . . 134
5.3 Solid-State Junctions with Pyrite Single Crystals . . . . . . . . . . . . . . . 136
5.4 Initially Proposed Design for Pyrite Heterojunction Solar Cells . . . . . . . . 138
5.5 FeS Thin Films on Mo-Coated Glass Substrates . . . . . . . . . . . . . . . . 139
2
5.6 SEM Images of pn-Heterojunction Device Stack . . . . . . . . . . . . . . . . 140
5.7 Comparison of FeS /ZnS to Other pn-Heterojunctions . . . . . . . . . . . . . 141
2
5.8 H S Content in Sputter Plasma and Effect on Target and Film Composition 143
2
5.9 Attempts to Improve Thin Film Diode Rectification . . . . . . . . . . . . . . 146
5.10 Rectification, Stability and Temperature-Dependence of Thin Film Solar Cells.148
5.11 Thin Film Solar Cell Champion Devices . . . . . . . . . . . . . . . . . . . . 150
5.12 Thin Film Solar Cells: Reproducibility and Punch-Through Issues . . . . . . 152
5.13 SEM Cross-Sectional View of Fe Back Contacts . . . . . . . . . . . . . . . . 153
5.14 Illustration of Solar Cell Architecture and Fabrication Steps . . . . . . . . . 154
5.15 Delamination Issues with Mo/MoS Back Contacts . . . . . . . . . . . . . . 156
2
vii
LIST OF TABLES
Page
2.1 Target and Deposition Parameters Used for Magnetron Sputtering . . . . . . 24
4.1 Elemental Analysis of Pyrite Flux Crystals . . . . . . . . . . . . . . . . . . . 86
4.2 Fit Parameters Used in Figure 4.11 and Figure B.3 . . . . . . . . . . . . . . 99
viii