《非晶硅_晶体硅异质结太阳能电池》Wolfgang

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SPRINGER BRIEFS IN APPLIED SCIENCES AND TECHNOLOGY Wolfgang Rainer Fahrner Editor Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells SpringerBriefs in Applied Sciences and Technology For further volumes http//www.springer.com/series/8884 Wolfgang Rainer Fahrner Editor Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells 123 Editor Wolfgang Rainer Fahrner University of Hagen Hagen Germany and School of Photovoltaic Engineering Nanchang University Nanchang China ISSN 2191-530X ISSN 2191-5318 electronic ISBN 978-3-642-37038-0 ISBN 978-3-642-37039-7 eBook DOI 10.1007/978-3-642-37039-7 Springer Heidelberg New York Dordrecht London Jointly published with Chemical Industry Press, Beijing ISBN 978-7-122-15935-9 Chemical Industry Press, Beijing Library of Congress Control Number 2013934387 C211 Chemical Industry Press, Beijing and Springer-Verlag Berlin Heidelberg 2013 This work is subject to copyright. All rights are reserved by the Publishers, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publishers’ locations, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publishers can accept any legal responsibility for any errors or omissions that may be made. The publishers make no warranty, express or implied, with respect to the material contained herein. Printed on acid-free paper Springer is part of Springer ScienceBusiness Media www.springer.com Preface The need to replace conventional energiescoal, oil and nuclear powerby alternative ones has been emphasised many times and underlined only recently in the Durban Climate Change Conference. Among these alternatives, photovoltaic devices play a leading role. This book here deals with one important representa- tive, the heterojunction solar cell. As its name points out, it consists of two different materials, crystalline and amorphous silicon. While the former one was brought to a high standard already shortly after World War II, amorphous silicon was investigated in detail only in 1968, in Romania. In contrast, heterojunction solar cell production of today is a flourishing business as seen by the example of Sanyo or Meyer Burger. This book deals with some typical properties of the heterojunction cell. Its history, schematic cross-sections, and production tools will be shown. A special chapter is devoted to the challenges of the cell such as texturization, interface defects, passivation, lifetime and surface velocity, epitaxial layer formation, emitter, and back surface field conductivity. Some important measurement tools are presented. Today no electronic device will be produced any more before it is not simu- lated. Thus, we present a few of the simulation programmes available on the market. The book is completed with a brief survey of the state of the art as represented by the efficiencies. Because China is the strongest emerging market in the solar cell field a col- lection of related publications and their discussion appeared to be mandatory. Nanchang, December 2011 Wolfgang Rainer Fahrner v Acknowledgments The authors thank Mrs. K. Meusinger and Dipl.-Ing. B. Wdowiak, University of Hagen, for technical assistance, A. Denker, affiliated to the Helmholtz Zentrum Berlin HZB formerly Hahn-Meitner Institut Berlin HMI and B. Limata, M. Romano and L. Gialanella from the Physics Department of Naples University for the proton irradiation, R. Scheer HZB for the EBIC measurements and M. Ferrara affiliated to the ENEA research center in Portici for the electrolumi- nescence measurements. The authors gratefully acknowledge the fruitful discus- sions with M. Kunst HZB. vii Contents Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells. 1 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Basic Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 History of a-SiH/c-Si Device Development. . . . . . . . . . . . . . 3 1.3 Economic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 Useful Material Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Useful Data of Monocrystalline Silicon. . . . . . . . . . . . . . . . . 7 2.2 Useful Data of Multicrystalline Silicon . . . . . . . . . . . . . . . . . 9 2.3 Useful Data of Microcrystalline Silicon. . . . . . . . . . . . . . . . . 9 2.4 Useful Data of Amorphous Silicon with Respect to Heterojunction Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . 11 3 Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.1 Lapping and Polishing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2 Texturing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.3 Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.4 PECVD of i-, n-, and p-Layers. . . . . . . . . . . . . . . . . . . . . . . 17 3.5 TCO. 19 3.6 Metallization and Screen Printing . . . . . . . . . . . . . . . . . . . . . 19 4 Concepts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1 Conventional n a-SiH/p c-Si Cell. . . . . . . . . . . . . . . . . . . . . 21 4.2 Bifacial a-SiH/c-Si Heterojunction Solar Cell with Intrinsic Thin Layer, HIT Structure . . . . . . . . . . . . . . . . 22 4.3 a-SiH/c-Si Heterocontact Cell Without i-Layer . . . . . . . . . . . 22 4.4 Other Concepts for Improved Entrance Windows . . . . . . . . . . 23 4.5 a-SiH/c-Si Heterocontact Cell with Inverted Geometry. . . . . . 23 4.6 Interdigitated HIT Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5 Problems and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.1 Choice of the Base Material, Impact of the Doping, n/p Versus p/n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.2 Surface States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5.3 Surface Passivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 5.4 PECV-Deposited Emitter and Back Surface Field. . . . . . . . . . 34 ix 6 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 6.1 Absorption, Reflection, and Transmission . . . . . . . . . . . . . . . 37 6.2 Excess Charge Carrier Lifetime . . . . . . . . . . . . . . . . . . . . . . 38 6.3 Electroluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 6.4 a-Si Characterization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 6.5 Electronic Device Characterization . . . . . . . . . . . . . . . . . . . . 55 7 Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 7.1 AFORS-HET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 7.2 Comparison with Experiments . . . . . . . . . . . . . . . . . . . . . . . 60 8 Long-Term Stability and Degradation. . . . . . . . . . . . . . . . . . . . . . . 64 8.1 Long-Term Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 8.2 Radiation Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 9 State of the Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 10 Silicon-Based Heterojunction Solar Cells in China. . . . . . . . . . . . . . 78 10.1 Introduction of the Active Groups in this Area in China . . . . . 79 10.2 Experimental Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 10.3 Theoretical Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 References 99 x Contents Contributors Wolfgang Rainer Fahrner Chair of Electronic Devices, University of Hagen, Haldener Str. 182, 58084 Hagen, Germany; School of Photovoltaic Engineering, Nanchang University, Xuefu Ave. 999, 330031 Nanchang, China, e-mail wolfgang.fahrnerfernuni-hagen.de Haibin Huang School of Photovoltaic Engineering, Nanchang University, Xuefu Ave. 999, 330031 Nanchang, China, e-mail Haibinhuangncu.edu.cn Thomas Mueller Chair of Electronic Devices, University of Hagen, Haldener Str. 182, 58084 Hagen, Germany, e-mail thomas.muellernus.edu.sg Stefan Schwertheim Chair of Electronic Devices, University of Hagen, Haldener Str. 182, 58084 Hagen, Germany, e-mail s.schwertheimweb.de Frank Wuensch Chair of Electronic Devices, University of Hagen, Haldener Str. 182, 58084 Hagen, Germany, e-mail Frank.Wuenschalumni.TU-Berlin.de Heinz-Christoph Neitzert University of Salerno, Via Ponte Don Melillo 1, 84084 Fisciano, SA, Italy, e-mail neitzertunisa.it xi Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells Wolfgang Rainer Fahrner 1 Introduction 1.1 Basic Structure Like any other semiconductor solar cell, the amorphous silicon / crystalline silicon heterojunction solar cell consists of a combination of p-type and n-type material, that is, a diode structure. However, while in the usual case the n-type and the p-type semiconductors are identical and just differ in the doping, a hetero- junction is built on two different materials, crystalline and amorphous silicon in our case. Its basic structure is given in Fig. 1. A crystalline wafer acts as a substrate for the amorphous layer on its top. In the following, the abbreviations c-Si and a-Si are used for crystalline and amorphous silicon, respectively. To obtain the required diode structure, it is evident that the two materials must be of opposite doping type. The amorphous layer acts as emitter and the wafer as the base of the solar cell. During the development of the heterojunction cell, the range of emitter and base materials has been expanded. For instance, the initially monocrystalline silicon had been replaced by multicrystalline material of various origins edge-defined ribbon growth, block-cast silicon, etc.. Similarly, microcrystalline silicon had been tried instead of amorphous silicon. Of course, a cell according to Fig. 1 would not operate very efficiently. For instance, incoming light would be reflected to a good deal. As a first counter- measure, an antireflection coating ARC, cf. Sect. 3.2 is deposited on the a-Si. The ARC is highly transparent and highly conductive. Together with a texture W. R. Fahrner ammonium hydroxide removes particles; metallic impurities are removed by hydrochloric hydroxide; diluted HF removes the native oxide films; and for rinsing, de-ionized DI water is used. The mechanisms of hydrogenation of Si surfaces in fluoride-containing solutions diluted HF lead also to an H-termination of the c-Si surface. After rinsing the wafer with DI water, the sample is immediately transferred into the plasma deposition chamber for deposition of a-SiH or its counterparts by means of RF-PECVD. The time period between the last HF dip and the transfer into the PECVD chamber averages less than 5 min. It is assumed that during this procedure, the H-termination is still present and re-growth of native oxides is reduced to a minimum, cf. [72]. 3.4 PECVD of i-, n-, and p-Layers Depositions of amorphous silicon layers, whether they are intended to be used as passivation, emitter or BSF layer, or for material characterization, are usually performed in a parallel plate, mostly capacitively coupled plasma-enhanced chemical vapor deposition PECVD system. In this technique, a plasma occurring during the decomposition of the gaseous precursors, which can be i.e., SiH 4 ,H 2 , CH 4 , and doping gases such as PH 3 and TMB. During the decomposition inelastic Amorphous Silicon / Crystalline Silicon Heterojunction Solar Cells 17 collisions between high-energetic electrons and the gaseous precursor atoms result in a dissociation into atomic and ionic species. The pathways for the chemical reactions of SiH 4 and its plasma products occurring during the operation of PECVD systems can be found in [75]. For sake of simplicity, a plasma deposition as used at the LGBE, Hagen, is described in the following. In general, the samples are placed into a 10 9 10 cm 2 squared sample holder, which is suitable for up to 4-inch wafer substrates, and transferred via a load lock into one of the three chambers each chamber for either intrinsic, p-doped, or n-doped layers to prevent contamination. The sample holder is attached to the upper electrically grounded electrode, while the RF-power is capacitively cou- pled to the lower electrode. A plasma power as high as 100 W can be adjusted. The RF-PECVD system operates either at a fixed frequency of 13.56 MHz or is driven by a very-high-frequency VHF generator, at frequencies ranging up to very high frequencies at 110 MHz. Changing the excitation frequency does not necessarily lead to higher deposition rates, but possible changes in the micromorph structure are likely. The distance between the parallel electrodes affects the emerging network of the deposited amorphous layers and has to be adjusted precisely. Prior to deposition, the samples are heated up before being purged with Argon gas. The Ar pressure is typically 5 mTorr for 5 min. After igniting the plasma in the chamber, a transient effect may uncontrollably influence the sample morphology in the beginning of the deposition process. Therefore, the sample holder is transferred out of the plasma field before igniting the plasma. The plasma ignition is supported by a piezo-element, after flushing the chamber with the precursors for 2 min. The deposition pressure and the gaseous precursors can be varied via the mass flow controllers MFCs depending on the experimental needs. The samples are radi- atively heated from above with actual substrate te