Title: Photonic Nanostructures for Advanced Light Trapping in Thin Silicon Solar Cells
Other Titles: Fotonische nanostructuren voor lichtvangst in dunne silicium zonnecellen
Authors: Trompoukis, Christos
Issue Date: 20-May-2015
Abstract: In this work we investigated the fabrication of 2D photonic nanostructures for light absorption enhancement in solar cells. The goal and simultaneously the challenge is to maintain good electrical performance minimising any degradation originating from the fabrication and integration of the photonic nanostructures, while achieving the desired absorption and current enhancement thanks to the implementation of advanced light trapping schemes in thin c-Si substrates.
Two lithography techniques, nanoimprint lithography (NIL) and hole mask colloidal lithography (HCL), were used for the definition of nanopatterns with either periodic or short-range order, respectively. Moreover, two silicon etching techniques, dry plasma and wet chemical etching, were used for the formation of nanostructures with a parabolic profile or inverted nanopyramids, respectively. By developing, fine tuning and combining each of the lithography and etching techniques we achieved and demonstrated the fabrication of nanopatterns with topographies which range from periodic to random nanostructures and from inverted nanopyramids to parabolic hole profiles. Those nanostructures were fabricated on substrates with a range of thicknesses from 1 to 700 nbsp;with various degrees of surface roughness. A similar surface area enhancement (around 1.7) and an order of magnitude lower material waste compared to the state of the art random pyramid texturing was also achieved. We showed a good control for the fabrication of the nanostructures, highlighting certain challenges and limitations.
By studying the optical performance of the photonic nanostructures on various c-Si substrates we show a light trapping mechanism consisting of the combination of two elements: i) a broadband antireflective (light in-coupling) behavior, explained from a gradual change of the refractive index at the air/c-Si interface, and ii) diffraction of light at high angles (i.e. light trapping). For all studied substrates, we achieved better absorption compared to the relevant state of the art texturing technique by nanopatterning the front surface. Apart from the absorption enhancement, a robust angular performance was shown based on which we calculated a potential improved annual energy yield compared to conventional state of the art random pyramid texturing.
Regarding the material properties after nanopatterning, dry plasma etching, normally used by the photonics community as the standard etching technique for the fabrication of photonic nanostructures caused a decrease in material quality. We identified the source of this decrease leading to low minority carrier lifetimes in: i) a high density of dangling bonds due to the surface roughness (from electron spin resonance measurements) as well as ii) the presence of sub-surface defects (from deep level transient spectroscopy measurements). In contrast to the dry plasma etched nanostructures, the inverted nanopyramids we developed in this thesis resulted in minimal material degradation enabling a conformal deposition of the subsequent films. In fact, we achieved surface recombination velocity as low as 8 cm/s for inverted nanopyramids fabricated on 40 µm thin epifoils and passivated with hydrogenated amorphous silicon (a-Si:H). Therefore, we showed that the inverted nanopyramids, apart from better light trapping performance compared to the state of the art random pyramid texturing, have a potential to be successfully integrated in high efficiency solar cells since they can enable high open circuit voltage () and fill factor () values by offering: i) low surface recombination velocities and ii) good contacting properties. Regarding the photonic-assisted solar cells, a better absorption and spectral response for short and long wavelengths was achieved for the ultra-thin 1 µm c-Si slabs thanks to the nanopattern, highlighting a better light in-coupling and light trapping behavior with respect to the reference cells. The energy conversion efficiency improved from the reference 4.4% to 4.8% because of the better light trapping. For benchmarking the nanostructures with respect to the state of the art random pyramid texturing, their integration in a 40 µm solar cell was used. A better trapping of long wavelength photons was achieved resulting in a higher calculated light path enhancement and higher equivalent thickness for the nanopatterned cells (500 µm) compared to the random pyramid textured cells (327 µm).
Table of Contents: 1 Introduction 1
1.1 Thesis motivation 1
1.2 Approach and challenges 4
1.3 Thesis outline 6
2 Fundamentals 9
2.1 Crystalline silicon solar cells 9
2.1.1 From the material to a junction 10
2.1.2 Ideal solar cells 11
2.1.3 Real solar cells 12
2.1.4 Loss mechanisms 13
2.1.5 Heterojunction solar cells 16
2.2 Light-matter interaction 18
2.2.1 At an interface 19
2.2.2 In thin films 20
2.2.3 In multilayered thin films 21
2.2.4 In waveguides 22
2.2.5 At surface corrugations 24
2.3 Light management for silicon solar cells 26
2.3.1 Conventional light management 26
2.3.2 Advanced light management 27
2.3.3 Light trapping assessment 29
2.4 State of the art light trapping in silicon solar cells 30
2.5 State of the art thin film crystalline silicon materials on foreign substrates 33
3 Fabrication of photonic nanostructures 37
3.1 Introduction 37
3.2 Overview and fundamentals of fabrication methods 38
3.2.1 Lithography: definition of the nanopattern 39
3.2.2 Silicon etching: definition of the nanostructure’s shape 42
3.3 Two dimensional periodic photonic nanostructures by nanoimprint lithography 46
3.3.1 Soft stamp fabrication 47
3.3.2 Imprint 49
3.3.3 Pattern transfer 50
3.4 Two dimensional disordered photonic nanostructures by hole mask colloidal lithography 56
3.4.1 Adsorption of PS beads 57
3.4.2 Etch mask deposition and bead removal 58
3.4.3 Pattern transfer 59
3.5 Nanopatterning fabrication challenges 62
3.6 Surface area enhancement and material waste 66
3.7 Chapter summary and conclusions 68
4 Optical performance of photonic nanostructures 71
4.1 Introduction 71
4.2 Light in-coupling 75
4.2.1 The effect of photonic nanostructure’s geometrical parameters on the light in-coupling properties 75
4.2.2 The effect of surface roughness 78
4.2.3 The effect of the antireflection coating 79
4.2.4 Introducing disorder 81
4.2.5 Benchmarking 81
4.3 Light trapping 82
4.3.1 The effect of photonic nanostructure’s geometrical parameters on the light trapping properties 83
4.3.2 The light trapping mechanism 85
4.3.3 Introducing disorder 89
4.3.4 Benchmarking 92
4.4 Parasitic absorption 93
4.5 Angular robustness and annual energy yield calculation 96
4.6 Chapter summary and conclusions 98
5 Electrical performance of photonic nanostructures 101
5.1 Introduction 101
5.2 Material properties after the fabrication of photonic nanostructures 104
5.2.1 Passivation of photonic nanostructures 106
5.2.2 The effect of surface roughness on the passivation quality 109
5.2.3 Plasma induced material degradation 111
5.3 Contacting properties of photonic nanostructures 114
5.4 Chapter summary and conclusions 118
6 Solar cell integration of photonic nanostructures 121
6.1 Introduction 121
6.2 1 µm thin film crystalline silicon solar cells 123
6.2.1 Device fabrication 123
6.2.2 Cell results and discussion 124
6.3 3 µm thin film polycrystalline silicon solar cells 128
6.3.1 Device fabrication 128
6.3.2 Cell results and discussion 130
6.4 40 µm thin epitaxially grown monocrystalline silicon solar cells 132
6.4.1 Device fabrication 132
6.4.2 Cell results and discussion 136
6.5 Light trapping assessment 142
6.6 Chapter summary and conclusions 143
7 Summary and perspectives 145
Appendix I: Soft thermal nanoimprint lithography 151
Appendix II: Annual energy yield calculation code 153
Appendix III: Deep level transient spectroscopy 155
List of publications 159
References 165
Publication status: published
KU Leuven publication type: TH
Appears in Collections:ESAT - ELECTA, Electrical Energy Computer Architectures

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