Amorphous and Microcrystalline Silicon Solar Cells: Modeling, Materials and Device Technology

Transcription

1 Amorphous and Microcrystalline Silicon Solar Cells: Modeling, Materials and Device Technology

2 Amorphous and Microcrystalline Silicon Solar Cells: Modeling, Materials and Device Technology By Ruud E.I. Schropp Utrecht University, Debye Institute MiroZeman Delft University oftechnology ~. " SPRINGER SCIENCE+BUSINESS MEDIA, LLC

3 Library of Congress Cataloging-in-Publication Data Schropp, Ruud E. I., 59- Amorphous and microcrystalline silicon solar cells: modeling materials and device technology I by Ruud E.l. Schropp, Miro Zeman. p. em. Includes bibliographical references and index. ISBN ISBN (ebook) DOl / Solar cells -- Materials. 2. Silicon crystals. 3. Thin film devices--design and construction. 4. Amorphous semiconductors. I. Zeman, Miro, 1957 II. Title. TK2960.S '244--dc CIP Copyright 1998 Springer Science+ Business Media New York Originally published by Kluwer Academic Publishers in 1998 Softcover reprint of the hardcover 1st edition 1998 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, mechanical, photo-copying, recording, or otherwise, without the prior written permission of the publisher, Springer Science+ Business Media, LLC Printed on acid-free paper.

4 Contents List of Figures List of Tables Preface Introduction IX xv XVII XIX Part I Technology of Amorphous and Microcrystalline Silicon Solar Cells 1. INTRODUCTION 1.1 History of amorphous and microcrystalline silicon 1.2 Applications References 2. DEPOSITION OF AMORPHOUS AND MICROCRYSTALLINE SILICON 2.1 Plasma enhanced chemical vapor deposition Large area deposition Production systems Compatible PECVD techniques 2.2 Alternative CVD approaches Hot-wire deposition ECR, microwave deposition, and plasma-beam deposition References 3. OPTICAL, ELECTRONIC AND STRUCTURAL PROPERTIES 3.1 Undoped materials Hydrogenated amorphous silicon Micro- and polycrystalline silicon Hydrogenated amorphous silicon germanium alloys v

5 vi AMORPHOUS AND MICROCRYSTALLINE SILICON SOLAR CELLS 3.2 Doped materials Doped amorphous silicon and silicon alloys Doped microcrystalline silicon References 4. TECHNOLOGY OF SOLAR CELLS 4.1 Principles of solar cell operation 4.2 Superstrate solar cells Front electrode technology Semiconductor multilayer structure Back electrode technology 4.3 Substrate solar cells Back electrode technology Semiconductor multilayer structure Front electrode technology 4.4 Multijunction technology 4.5 Fabrication of large area modules Series-connected stacked multijunctions Parallel-connected stacked multijunctions Encapsulation 4.6 Production and markets for amorphous silicon photovoltaics References 5. METASTABILITY 5.1 Light-induced changes in films 5.2 Light-induced changes in solar cells Single junction cells Multijunction cells References Part II Modeling of Amorphous Silicon Solar Cells 6. ELECTRICAL DEVICE MODELING 6.1 Introduction 6.2 Specific issues in amorphous silicon device modeling 6.3 Set of device model equations Basic set of semiconductor equations Band diagram Carrier statistics Set of model equations

6 Contents vii Boundary conditions Density of states model for amorphous silicon Standard model of density of states distribution in a-si:h Extended and localized states in a-si:h Defect pool model for dangling bond states distribution Recombination-generation statistics in amorphous silicon R-G statistics of the continuously distributed states in the band gap R-G statistics of VB and CB tail states R-G statistics of DB states Total R-G rate and space charge in the localized states of the band gap Numerical solution of the semiconductor equations 142 References OPTICAL DEVICE MODELING Introduction Optical properties of the layers and interfaces in a-si:h solar cell Lambert-Beer's formula Optical system with smooth interfaces Multilayer optical system Optical system with rough interfaces Light trapping Scattering at a rough interface Multi-rough-interface optical model 166 References INTEGRATED OPTICAL AND ELECTRICAL MODELING Calibration of model parameters Calibration procedure of model parameters Extraction of model parameters by inverse modeling Sensitivity study of a-si:h solar cell model parameters Set of model input parameters Understanding a-si:h solar cell performance 8.3 Optimization of a-si:h solar cells Optical design of a-si:h solar cells Multijunction a-si:h alloy solar cells Band gap profiling of narrow band gap materials References 199

7 List of Figures Schematic representation of the glow-discharge deposition process (By courtesy of H. Meiling, Utrecht University). Examples of plasma box configurations as proposed by Schmitt et al., Co, Ci, and Cp are conductances; Po and Pp are the base pressure and the process pressure, respectively. The configuration on the left side uses dual pumping and that on the right side uses internal throttling. Estimated peak Si-ion energies for SiH4 plasmas as a function of excitation frequency (after Dutta et al., 1992). The initial efficiency of single-junction a-si:h solar cells plotted as a function of intrinsic layer deposition rate. The open symbols refer to cell areas of less then 0.1 cm 2, the filled symbols refer to cell areas of at least 0.1 cm 2. (Reprinted from Schropp et al., 1989, with permission from Elsevier Science. For sources of data, see the same literature reference). Density of deep states Nd versus hydrogen concentration CH in the fully light-soaked state (By courtesy ofh. Mahan, National Renewable Energy Laboratory). Schematic cross section of an HWCVD reactor. The same chamber can be used for PECVD. Schematic diagram of an ECR microwave CVD reactor (Shing, 1989). Schematic diagram of a cascaded arc plasma beam CVD reactor. (By courtesy of M.C.M. van de Sanden, Eindhoven University of Technology). Typical Raman scattering spectra for (a) a mixed phase, or microcrystalline film, (b) a purely polycrystalline film. 10 l ix

8 x AMORPHOUS AND MICROCRYSTALLINE SILICON SOLAR CELLS 3.2 Cross-sectional TEM of a polycrystalline thin film that exhibits only (220) orientation in XRD spectra. To provide immediate nucleation on glass, the first 20 nm of the film was made under high hydrogen dilution conditions Comparison of the optical absorption coefficients of single crystal silicon, JLc-S:H, and a-si:h. Also shown is a curve calculated from the effective media approximation (Ward, 1994), using 10 % a-si:h and 90 % crystalline silicon. (reprinted from Vanecek et al, 1997, with permission from Elsevier Science) Infrared absorption spectra of polycrystalline silicon thin films. Curve (a) is for a film made using a H2/SiH4 ratio of 100 (high dilution), and (b) is for a film, made using a H2/SiH4 ratio of 10 (low dilution). For both cases the wire temperature, substrate temperature, and process pressure were 1800 C, 480 C, and 0.1 mbar, respectively Potential distribution in a pti-n+ solar cell calculated using the defect pool model, under short circuit conditions and at the maximum power point Cross-sectional diagram of a superstrate single junction and tandem solar cell Scanning Electron Micrograph of Asahi type-u TCO (By courtesy of K. Adachi, Asahi Glass Company) Cross-sectional Transmission Electron Micrograph of Asahi type-u TCO (By courtesy of K. Adachi, Asahi Glass Company) Scanning Electron Micrograph of a 500 nm thick complete a- Si:H pin multilayer structure deposited on a textured TCO layer Cross-sectional diagram of a substrate triple junction solar cell and corresponding schematic band diagram Multijunction cell under forward bias voltage, left: with a good tunnel-recombination junction right: showing the potential loss of built-in voltage due to an inadequate tunnelrecombination junction Typical relative changes in Vae, Jse, FF, and efficiency during continuous light soaking of a single junction pti-n+ solar cell with a AM1.5 Global, 100 mw/cm2 spectrum (Von der Linden, 1994). 104

9 LIST OF FIGURES xi 5.2 Schematic band diagram of a pti-n+ solar cell after light soaking. The electric field distribution is distorted due to positively charged defects near the p/i interface and negatively charged defects near the i/n interface Normalized solar cell output for single junction and multijunction devices illustrating the enhanced stability for multijunction devices and the positive effect of other design improvements over the years (reprinted from Kazmerski, 1997, with permission from Elsevier Science) The energy band diagram of a typical single junction a-si:h solar cell under equilibrium conditions The standard model of the DOS distribution in a-si:h on a linear scale The standard model of the DOS distribution in a-si:h on a logarithmic scale The DOS distribution in the mobility gap of intrinsic a-si:h at different positions in the solar cell according to the defect pool model for the single electron DB states distribution. Three cases with different position of Fermi level are illustrated: near the p-type layer (p-type and EF(p)), in the center of the intrinsic layer (i-type and EF(i)), and near the n-type layer (n-type and EF(n)) Different types of the localized states in the band gap of a-si:h and the models that are used to calculate the recombination rate and charge occupation (after Willemen, 1998) Electronic transitions in the recombination process between a single energy level in the band gap of semiconductor and the energy bands Possible electronic transitions in the recombination process between the energy bands and an amphoteric R-G center represented by two energy levels in the band gap The refractive index and the extinction coefficient of individual layers that comprise a typical superstrate single junction a- Si:H solar cell The reflection and transmission of light at a flat interface The reflection and transmission of light in a single layer Schematic representation of light trapping in an a-si:h solar cell The reflection and transmission processes at a rough interface Solid angle distribution of incoming and outgoing light from a rough interface. 167

10 xii AMORPHOUS AND MICROCRYSTALLINE SILICON SOLAR CELLS 7.7 The reflection and transmission processes at a rough interface when light incidents on a rough interface from both sides Incoming and outgoing light from a single layer with two rough interfaces Generation rate profiles in the reference solar cell (as in Fig. 8.1) for the case with only flat interfaces and the case with rough interfaces Schematic structure of the a-si:h p-i-n reference cell The measured (dots) and simulated (continuous curve) illuminated J-V characteristics of the a-si:h p-i-n reference cell The defect density profiles in the intrinsic layer of a-si:h p-i-n solar cell The dark J-V characteristics for different defect density profiles in the intrinsic layer of a-si:h p-i-n solar cell The J-V characteristics of a-si:h p-i-n solar cell under AM1.5 illumination for different defect density profiles in the intrinsic layer The electric field profile in the intrinsic layer of an a-si:h p-i-n solar cell under short circuit current conditions The recombination rate profile in the intrinsic layer of an a- Si:H p-i-n solar cell under short circuit current conditions The total absorption in the separate layers and total reflection of the reference a-si:h single junction cell expressed as quantum efficiency. Abbreviations: i is intrinsic a-si:h layer, pis p-type a-sic:h layer, n is n-type a-si:h layer, Tis Sn02:F TCO layer, M is metal contact, and R is the total reflection of the cell. a) all interfaces in the cell are flat b) n/metal interface is rough and considered as a perfect diffuser c) TCO/p interface is rough and only the light reflected back to the TCO is scattered d) TCO/p interface is rough and only the light transmitted to the p-layer is scattered Scattering coefficients of reflected light for different textured interfaces in the reference solar cell The measured (symbols) and calculated QE of the reference a-si:h solar cell. The filling patterns are the same as in Fig The efficiency of a-si:h/a-si:h (a) and a-si:h/a-sige:h (b) tandem cells as a function of the thickness of the top (il) and bottom (i2) intrinsic layers. 196

11 LIST OF FIGURES xiii 8.12 Band gap profiling configurations. (a) no profiling, (b) normal profiling, (c) reverse profiling, (d) double profiling or V shape, and (e) U shape. 197

12 List of Tables 1.1 Definitions for various morphologies of thin film silicon materials Typical deposition conditions used for hot-wire deposition of amorphous and polycrystalline intrinsic silicon films Characteristic quantities of various deposition techniques Criteria for 'device quality' amorphous silicon films Criteria for 'device quality' polycrystalline silicon films Criteria for 'device quality' a-sige:h films Criteria for doped a-si:h layers for application in solar cells Properties of microcrystalline doped films Typical properties for various TCO front electrode coatings Record stabilized laboratory cell aperture-area efficiencies Record stabilized total-area prototype module efficiencies Current module testing procedure Description of a semi-continuous production line for a-si:h modules Description of a roll-to-roll production line for a-si:h modules Production facilities for a-si:h and related solar cells and modules The time rates of change in the carrier concentrations for transitions involving VB and CB tail states at arbitrary energy level ET in the band gap The time rates of change in the carrier concentrations for recombination processes involving DB states The set of input parameters for modeling the reference single junction a-si:h solar cell The simulated external parameters of p-i-n a-si:h solar cell for different defect density profiles in the intrinsic layer 186 xv

13 Preface Amorphous silicon solar cell technology has evolved considerably since the first amorphous silicon solar cells were made at RCA Laboratories in Scientists working in a number of laboratories worldwide have developed improved alloys based on hydrogenated amorphous silicon and microcrystalline silicon. Other scientists have developed new methods for growing these thin films while yet others have developed new photovoltaic (PV) device structures with improved conversion efficiencies. In the last two years, several companies have constructed multi-megawatt manufacturing plants that can produce large-area, multijunction amorphous silicon PV modules. A growing number of people believe that thin-film photovoltaics will be integrated into buildings on a large scale in the next few decades and will be able to make a major contribution to the world's energy needs. In this book, Ruud E. I. Schropp and Miro Zeman provide an authoritative overview of the current status of thin film solar cells based on amorphous and microcrystalline silicon. They review the significant developments that have occurred during the evolution of the technology and also discuss the most important recent innovations in the deposition of the materials, the understanding of the physics, and the fabrication and modeling of the devices. DAVID E. CARLSON xvii

14 This book is dedicated to all who devote their time and effort to promoting a clean and sustainable energy source for mankind

15 INTRODUCTION Ruud Schropp Electricity is an energy source that greatly contributes to the quality of life. It makes our household appliances work, it enhances our communication facilities, it powers electric public transportation systems, it accelerates our information retrieval and processing capabilities. Roughly two thirds of the world's electricity needs are generated by burning fossil fuels. Apart from the fact that this source will become exhausted, the detrimental environmental effects demand that cleaner, sustainable electricity sources are developed and expanded. Within the variety of sustainable energy sources, photovoltaic energy will play a considerable role. The beauty of it is that it can be applied in systems of any scale, from the m W to MW ranges, so that in principle nobody on this planet would have to be fully dependent on centrally organized power plants. Photovoltaic panels should be made of abundant, cheap, and non-toxic materials, while these materials should allow handling and processing such that manufacturing at low cost is feasible. Silicon thin film photovoltaic technology offers these prerequisites. Since the first amorphous silicon solar cells were prepared in 1974 by David E. Carlson, the simple low temperature thin film formation process and the feasibility of coating extremely large areas have stimulated entrepreneurial spirits to start production plants of various sizes. Although photovoltaic sales have been increasing at a % annual growth rate, the low dollar rate at which fossil fuels are available makes it hard to compete economically. Political appreciation of the avoided social costs of fossil fuel consumption would help to reach the break-even point much sooner. Meanwhile researchers around the world continue to work hard to further improve the efficiency and reliability of solar cells, with the help of the physical insight obtained by computer modeling, and to develop yet cheaper manufacturing methods. In this book, we chose to study the most promising cross section of the wide variety of deposition technologies, the many kinds of silicon-hydrogen thin films, the fundamentally different solar cell structures, and the multidisciplinary physics and technology attributes that are presently available. The book is by no means complete; neither is the development of the ultimate thin film solar cell. Therefore its primary aim is to encourage scientists and industrialists to promote the introduction of photovoltaic energy by pushing the technology further. In our personal quest for a breakthrough technology in the field of photovoltaic xix

16 xx AMORPHOUS AND MICROCRYSTALLINE SILICON SOLAR CELLS energy conversion, which has recently gained momentum, Miro Zeman and I have interacted with many researchers, industrialists, and project monitoring authorities. Therefore, first of all, we want to express our gratitude to all who showed their confidence in what we have been trying to achieve, by actively cooperating or by directly supporting our research. This research requires the long term commitment that society can provide through government institutions such as our Universities, NOVEM and NWO in the Netherlands, and the European Union RTD programmes. We have also benefited from the exchange of insights and results through scientific collaboration world wide, as well as close to home, with our nearest colleagues. It is impossible to adequately thank every individual, who contributed to our enterprises. Of the many interactions that have taken place, we wish to acknowledge Utrecht University, Delft University of Technology, Eindhoven University of Technology, TNO Institute of Applied Physics, National Renewable Energy Laboratory, Asahi Glass Co., Neste Corporation, Microchemistry Ltd., Naps France, Glasstech Solar, MVSystems, Elettrorava, FZ Jiilich, Ecole Polytechnique Palaiseau, IMT Neuchatel, and Philips Research Laboratories Redhill and Eindhoven for their scientific contributions. We also wish to acknowledge the recent cooperation with our partners at Akzo Nobel. My co-author Miro Zeman wishes to express his thankfulness to his parents for providing him continuous love and care that gave him peace and confidence for this challenging work. The greatest support for me came from my wife Mechtild, my son Floris and my daughter Gwendolijn: "Thank you for understanding what I want to accomplish in life and for helping me in every aspect".