Diffractive optics for thin-film silicon solar cells : doctoral thesis accepted by the University of York, UK

This thesis introduces a figure of merit for light trapping with photonic nanostructures and shows how different light trapping methods compare, irrespective of material, absorber thickness or type of nanostructure. It provides an overview of the essential aspects of light trapping, offering a solid basis for future designs. Light trapping with photonic nanostructures is a powerful method of increasing the absorption in thin film solar cells. Many light trapping methods have been studied, but to date there has been no comprehensive figure of merit to compare these different methods quantitatively. This comparison allows us to establish important design rules for highly performing structures; one such rule is the structuring of the absorber layer from both sides, for which the authors introduce a novel and simple layer-transfer technique. A closely related issue is the question of plasmonic vs. dielectric nanostructures; the authors present an experimental demonstration, aided by a detailed theoretical assessment, highlighting the importance of considering the multipass nature of light trapping in a thin film, which is an essential effect that has been neglected in previous work and which allows us to quantify the parasitic losses. Supervisor’s Foreword......Page 8 Abstract......Page 11 Acknowledgements......Page 12 Contents......Page 15 1.2 The Potential of Photovoltaics......Page 17 1.3.1 The NIR Problem......Page 19 1.3.2 The Light-Trapping Potential......Page 22 1.4 Scope and Outline of This Thesis......Page 25 References......Page 26 2.2.1 Non-diffractive Techniques......Page 27 2.2.2 Refractive Approaches......Page 29 2.2.3 Diffractive Approaches......Page 30 2.3 The Lambertian Scatterer......Page 34 2.3.1 Absorption Enhancement by a Lambertian Backscatterer......Page 36 2.3.2 The Maximum Absorption Enhancement......Page 41 2.4.1 How to Assess Light-Trapping Structures for Solar Cells ?......Page 44 2.4.2 The Light-Trapping-Efficiency (LTE)......Page 47 2.5.1 Important Milestones......Page 50 2.5.2 The State of the Art......Page 54 2.6 Concluding Remarks......Page 58 2.6.1 Phase Engineering......Page 59 2.6.2 Dual Structuring......Page 60 References......Page 61 3.2.1 Electron Beam Lithography......Page 68 3.2.2 Nanoimprint Lithography......Page 70 3.3 Pattern Transfer by Reactive Ion Etching (RIE)......Page 73 3.3.1 Dry-Etching in the RIE-System......Page 75 3.4.1 Reference Measurement -- Thin Films......Page 76 References......Page 78 4.2 Dual Gratings by a Simple Layer Transfer Technique......Page 80 4.2.1 Why Dual Gratings?......Page 81 4.2.2 A Novel Layer-Transfer Technique......Page 82 4.2.3 Proof of Principle Demonstration......Page 83 4.3 Plasmonic and Diffractive Nanostructures ƒ......Page 87 4.3.1 Methodology......Page 88 4.3.2 Results......Page 90 4.3.3 Discussion......Page 92 4.3.4 Comparison of the Two Different Approaches......Page 95 4.3.5 Conclusions......Page 97 4.4 Summary......Page 98 References......Page 99 5.1 General Conclusions and Remarks......Page 102 5.1.1 Limitations and Future Work Left......Page 104 5.1.2 Outlook......Page 106 5.2 Can Solar Power Solve the World's Energy Crisis?......Page 108 References......Page 109 Appendix A A Home-Made UV Nanoimprint Tool......Page 110 Appendix B Absorption Measurement Setup......Page 118