Nanotechnology and Photovoltaic Devices. Nanotechnology and Photovoltaic Devices. Light Energy Harvesting with Group IV Nanostructures

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Silicon is an abundant element and is produced in large quantities for the electronic industry. The falling price of this commodity also feeds the growth of solar photovoltaics (PV).

Therefore, new principles and materials are being investigated in order to build the third generation of SCs with improved conversion efficiency achieved by the optimized harvesting of the solar

The unique properties of semiconductor nanostructures (tuning of optoelectronic properties by the quantum confinement effect, stronger interaction with light, etc.

In view of the increasing research effort devoted to nanostructures applications in PV, this book aims to provide a background to students and newcomer researchers as well as to point out some open

1 Silicon is an abundant element and is produced in large quantities for the electronic industry. The falling price of this commodity also feeds the growth of solar photovoltaics (PV). However, solar cells (SCs) based on bulk semiconductors have quite limited maximum attainable performance. Therefore, new principles and materials are being investigated in order to build the third generation of SCs with improved conversion efficiency achieved by the optimized harvesting of the solar spectrum, improved carrier generation, better light management, etc. The unique properties of semiconductor nanostructures (tuning of optoelectronic properties by the quantum confinement effect, stronger interaction with light, etc.) can be exploited to fabricate novel types of high-efficiency solar cells. Here, again, silicon along with carbon and germanium (group IV elements) is about to play a major role. In view of the increasing research effort devoted to nanostructures applications in PV, this book aims to provide a background to students and newcomer researchers as well as to point out some open questions and promising directions for future development. It presents a useful overview of group IV nanostructures for PV, which includes the theoretical background, presentation of main solar cell principles, technological aspects, and nanostructure characterization techniques, and finishes with the design and testing of prototype devices. It is not intended to be just a review of the most up-to-date literature, but the authors aim to provide an educative background of the field. All authors are renowned researchers and experienced teachers in the field of semiconductor nanostructures and photovoltaics. Salvo Mirabella received his laurea (1999) and PhD (2003) in physics from the University of Catania, Italy, and is now researcher at the Institute for Microelectronics and Microsystems, National Council of Research (CNR IMM), Italy. His research activity is mainly experimental, focusing on group IV advanced materials for applications in photovoltaics (light absorption mechanisms in Si- or Ge-based nanostructures, sunlight-energy conversion, and transparent conductive electrodes) and microelectronics (point-defect engineering and dopant diffusion in crystalline or amorphous semiconductors and ion beam modification of materials). V411 V411 ISBN Valenta Mirabella Jan Valenta is professor of quantum optics and optoelectronics at the Department of Chemical Physics and Optics, Charles University, Prague. His research is oriented toward optical properties of semiconductor nanostructures, especially silicon. He is developing special spectroscopy set-ups and methods to measure photo- and electroluminescence spectra (down to single nano-objects), optical gain, and absolute quantum yields. His other interests include the history of science, scientific photography, and science-for-art applications. He is co-author (with I. Pelant) of the textbook Luminescence Spectroscopy of Semiconductors (Oxford, 2012). Nanotechnology and Photovoltaic Devices It is commonly accepted that nanostructures, whose properties can be conveniently tuned by size adjustments, will provide the materials basis for the next generation of highly efficient solar energy solution. That is in particular true for photovoltaics, possibly the most elegant solar energy harvesting strategy. There are many reasons why the first-generation PV is dominated by silicon; most of them will apply also to the next-generation solutions and that defined importance of nano-si for the future photovoltaics. This book provides an excellent introduction to the field and a comprehensive overview of the state of the art in this vividly developing discipline, with experimental as well as theoretical advancements being presented in parallel. Prof. Tom Gregorkiewicz University of Amsterdam, the Netherlands edited by Jan Valenta and Salvo Mirabella Nanotechnology and Photovoltaic Devices Light Energy Harvesting with Group IV Nanostructures

2 Nanotechnology and Photovoltaic Devices

4 Pan Stanford Series on Renewable Energy Volume 2 Nanotechnology and Photovoltaic Devices Light Energy Harvesting with Group IV Nanostructures editors Preben Maegaard Anna Krenz Wolfgang Palz edited by Jan Valenta and Salvo Mirabella The Rise of Modern Wind Energy Wind Power for the World

5 Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore editorial@panstanford.com Web: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Nanotechnology and Photovoltaic Devices: Light Energy Harvesting with Group IV Nanostructures Copyright c 2015 Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN (Hardcover) ISBN (ebook) PrintedintheUSA

6 Contents Preface xiii 1 Introduction to Photovoltaics and Potential Applications of Group IV Nanostructures 1 Jan Valenta and Salvo Mirabella 1.1 Energy from the Sun The Basic Principles of Photovoltaic Solar Cells Energy Balance Energy Conversion: Efficiency and Limits Advanced Concepts for Photovoltaics The Multijunction Approach Up- and Down-Conversion Wavelength conversion Intermediate-band SCs Carrier multiplication Hot-Carrier Extraction Group IV Nanostructures Prospects of Nanomaterials in Photovoltaics Light Management in Solar Cells Conclusions 21 2 The Dielectric Function and Spectrophotometry: From Bulk to Nanostructures 27 Caterina Summonte 2.1 Introduction The Dielectric Function: Why do we Need an Approximation? Electromagnetic Mixing Formulas The Dielectric Function at the Nanoscale 31

7 vi Contents Silicon Nanoparticles Germanium Nanoparticles Nanowires Graphene Measurements and Elaboration Volume Fractions of Composite Materials R&T Spectroscopy Experimental Setup Elaboration of R&T Spectra Determination of absorption Determination of the optical gap Qualitative evaluation of R&T spectra Single layer on a transparent substrate Spectral forms for the DF The Generalized Transfer Matrix Approach R&T Spectroscopy Applied to Nanoparticles Single-Layer Approach Management of the unknown parameters Determination of the dielectric function of nc-si Volume fractions and Si crystallized fractions Detection of a low-density surface layer Phase separation in silicon-rich oxides Single Layers and Multilayers Conclusions 53 3 Ab initio Calculations of the Electronic and Optical Properties of Silicon Quantum Dots Embedded in Different Matrices 65 Roberto Guerra and Stefano Ossicini 3.1 Introduction Structures Embedded Silicon Quantum Dots Freestanding Quantum Dots Results 72

8 Contents vii Amorphization Effects Size and Passivation Embedding Insulating Materials Optical Absorption Applicability of Effective Medium Approximation Strain Local-Field Effects Ensembles of Quantum Dots Beyond DFT Conclusions 90 4 Silicon Nanoclusters Embedded in Dielectric Matrices: Nucleation, Growth, Crystallization, and Defects 99 Daniel Hiller 4.1 Introduction Silicon Quantum Dot Formation Preparation Methods Phase Separation for Matrix-Embedded Si QDs Silicon Quantum Dot Crystallization Silicon Nanocrystal Size Control and Shape The Superlattice Approach Silicon Nanocrystals: The Role of Point Defects Identification and Quantification of Defects Classification of Point Defects Defects in the Si/SiO 2 system Defects in the Si/Si 3 N 4 system Defects in the Si/SiC system Influence of Interface Defects on PL Interaction of defects with PL in SiO 2 -embedded Si NCs Interaction of defects with PL in Si 3 N 4 -embedded Si NCs Influence of Interface Defects on Electrical Transport Conclusions 130

9 viii Contents 5 Excited-State Relaxation in Group IV Nanocrystals Investigated Using Optical Methods 145 FrantišekTrojánek,Petr Malý, and Ivan Pelant 5.1 Introduction Experimental Methods Pump and Probe Technique Up-Conversion Technique Transient Grating Technique Time-Resolved Terahertz Spectroscopy Femtosecond Phenomena Picosecond and Nanosecond Phenomena Carrier Multiplication in Isolated and Interacting Silicon Nanocrystals 177 Ivan Marri, Marco Govoni, and S. Ossicini 6.1 Introduction Carrier Multiplication and Auger Recombination in Low-Dimensional Nanosystems Theory One-Site CM: Absolute and Relative Energy Scale Two-Site CM: Wavefunction-Sharing Regime Conclusions The Introduction of Majority Carriers into Group IV Nanocrystals 203 Dirk König 7.1 Introduction Theory of Conventional Nanocrystal Doping Thermodynamics: Stable vs. Active Dopant Configurations Electronic Properties: Quantum Structure vs. Point Defect Phosphorous as an Example: Hybrid Density Functional Theory Calculations Survey on Experimental Results of Conventional Si Nanovolume Doping Si Nanovolumes in Next-Generation Ultra-Large-Scale Integration 226

10 Contents ix Free-Standing Nanocrystals Embedded Nanocrystals Formed by Segregation Anneal Alternatives to Conventional Doping Modulation Doping Exploiting Interface Energetics: Nanoscopic Field Effect Conclusion and Outlook Electrical Transport in Si-Based Nanostructured Superlattices 255 Blas Garrido, Sergi Hernández, Yonder Berencén, Julian López-Vidrier, Joan Manel Ramírez, Oriol Blázquez, and Bernat Mundet 8.1 Introduction and Scope Superlattices and Minibands Amorphous and Nanocrystal Superlattices Transport in Nanocrystal Superlattices Semiclassical Miniband and Band Transport Transport with Field-Assisted Carrier Exchange between Localized and Extended States Conduction through Localized States (Hopping by Tunneling) Injection and Space Charge Limited Currents Horizontal Transport Vertical Transport in SRO/SiO 2 Superlattices Transport in SRON/SiO 2 and SRC/SiC Superlattices Horizontal Transport in SRC/SiC Superlattices Vertical Transport in SRON/SiO 2 Superlattices Conclusions 299 Appendix A Band Structure of Nanocrystal Superlattices 300 Appendix B Semiclassical Conduction in the Extended States of a Superlattice 306 Appendix C Generalized Trap-Assisted Tunneling Model 310

11 x Contents 9 Ge Nanostructures for Harvesting and Detection of Light 317 Antonio Terrasi, Salvatore Cosentino, Isodiana Crupi, and Salvo Mirabella 9.1 Introduction Light Absorption, Confinement Effects, and Experimental Methods Synthesis of Ge Nanostructures Light Absorption in Germanium QWs Confining Effects in Germanium QDs Matrix Effects: SiO 2 vs. Si 3 N QD QD Interaction Effects Light Detection with Germanium Nanostructures Conclusions Application of Surface-Engineered Silicon Nanocrystals with Quantum Confinement and Nanocarbon Materials in Solar Cells 355 Vladimir Svrcek and Davide Mariotti 10.1 Introduction Si NC Surface Engineering in Liquids Surface Engineering of Doped Si NCs Tuning Optoelectronic Properties of Si NCs by Carbon Terminations Functionalization of Surface-Engineered Si NCs with Carbon Nanotubes Solar Cells Based on Si NCs and Nanocarbon Materials Conclusions and Outlooks Prototype PV Cells with Si Nanoclusters 381 Stefan Janz, Philipp Löper, and Manuel Schnabel 11.1 Introduction Motivation Material Selection Current Collection Doping Device Concepts for Si NC Test Structures Device Results 398

12 Contents xi 11.8 Tandem Solar Cell Development Current Matching Future Trends Thermal Budget Compatible Processing Increased Conductivity of the Si NC Material Reduction of Electronic Defects Conclusion 415 Index 425

14 Preface The increasing energy demand of humankind on the Earth cannot be reasonably sustained by prolonged exploitation of fossil fuels. Therefore we have to turn toward efficient usage of the most abundant renewable supply of energy it means the Sun. When considering Photovoltaics aim, the direct transformation of solar photon flux into electrical energy, the most practical materials for this transformation are semiconductors whose absorption matches quite well solar photons energy and whose conductivity can be adjusted so that photogenerated charge carriers are separated and directed to make useful work in an external circuit. Fortunately, some of these materials are very abundant, especially silicon, but other elements from group IV of the periodic table of elements are also extremely interesting. However, the maximum efficiency in energy conversion of the solar spectrum by a single semiconductor material is limited, as described by the famous Shockley Queisser limit. To overcome this constraint, most of the proposed ideas, commonly labeled as third-generation Photovoltaics, are based on Nanotechnology employing materials whose energy scheme is more complex and variable. There are such materials, namely, semiconductor nanostructures, that enable us to tune their energy levels, density of electronic states, transition probabilities, etc., with large potential benefits for light energy conversion. The purpose of this book is to summarize the knowledge and current advances of group IV semiconductor nanostructures potentially applicable in the next generations of solar cells. Considering the increasing research efforts devoted to nanostructure applications in Photovoltaics, our intention was to provide a clear background to students and newcomer researchers as well as to point out some open questions and promising directions of future development.

15 xiv Preface The book presents a broad overview on group IV nanostructures in Photovoltaics, beginning with a theoretical background, presentation of main solar cell principles, technological aspects, and nanostructure characterization techniques and finishing with the design and testing of prototype devices. The limited space of one book did not allow us to include some special nanostructure-related subjects, such as nanocrystal-sensitized solar cells (Grätzel cells or polymer cells), microcrystalline and amorphous silicon materials, rare-earth-doped nanostructures, plasmonic structures, etc. It is not intended to be just a review of the most up-to-date literature, but the contributing authors ambition was to provide an educative background of the field. In view of the harsh economic competition in the solar cell business it might be that nanostructures will never be a commonly used material in Photovoltaics massive production; still the solid background knowledge gained by researchers and summarized in this book will help in applying nanostructures to this and other fields. The idea to compile this book was born in 2012 within the framework of a successful European research project (NASCEnT, Silicon nanodots for solar cell tandem, , 7FP project contract ), and in fact, many authors of the book participated in that project. Therefore we shall thank the European Commission for the support and Pan Stanford Publishing for its effort and helpful cooperation. The main acknowledgment goes to all chapter authors, who invested a lot of time and effort into the success of this book. Jan Valenta and Salvo Mirabella Prague and Catania January 2015

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