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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-24189-8
Cover image: Top figures: RENA Technologies GmbH – BatchTex machine for solar wafer texturing Bottom row left: Technological Institute of Microelectronics (TiM), University of the Basque Country (UPV/EHU), Bilbao, Spain
Bottom row right: Fraunhofer Institute for Microstructure of Materials and Systems IMWS, Research Unit Center for Silicon Photovoltaics CSP, Halle (Saale), Germany
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
The last two decades were groundbreaking for photovoltaic (PV) technology. Countless researchers, engineers, technicians, politicians, and individuals all over the world contributed with their work and enthusiasm to the progress of this field. In this time, silicon PV cells increased their efficiency to 26.1% [1], being close to their theoretical limit for real cells of 29.8% [2]. PV technologies such as multijunction solar cells achieved a maximum of 39.2% efficiency in nonconcentrated applications [1], and new emerging technologies such as perovskites evolved. Figures 1 and 2 visualize the impressive progress in photovoltaics, depicting the best research cell efficiencies (Figure 1) and the champion module efficiencies (Figure 2). Both figures start with a few technologies, remarkable achievements, and, especially in the case of modules, a somewhat steady progress. The first cell type ever recorded in these data, an a:Si:H cell, evolved from 2.4% efficiency in 1976 to 14.1%. However, shortly around the year 2000, 10 new photovoltaic technologies evolved, increasing the record PV efficiency from 31.9% in 2000 by more than 50% to 47.1%. Not only were 40% of the technologies of today developed during the last two decades but also their efficiency trends are improving much steeper than ever before in the PV history.
The same is true for the record PV module efficiencies depicted in Figure 2. The efficiencies demonstrated with small-scale modules in the year 2000 increases with some module technologies doubling or even tripling their module record efficiencies within a decade and less. This is especially remarkable as some the technologies were scaled up to large module productions. Again, more than 40% of today’s module technologies depicted in Figure 2 were not evolved at module or even research scale just 20 years ago.
The global solar module production reached the Gigawatt range soon after 2000 and ramped up to 165 GW in 2020 [3].
The use of PV technology has changed from test and research sites and fist operational installations to being an essential part of national energy strategies worldwide. In many regions, PV is part of the landscape in both rural and urban areas. The levelized cost of energy (LCOE) is a common measure to compare the average net cost of different electric energy production technologies over their lifetime. The LCOE of PV per MWh has taken a breathtaking journey from about 400 USD/W in 2010 to 50 USD/MWh and lower globally with some projects reaching values as low as 23 USD/MWh [4, 5]. This trend is expected to continue with a predicted LCOE of 20 USD/MWh for 2030.
Figure 1 Conversion efficiencies of best research solar cells worldwide from 1976 through 2020 for various photovoltaic technologies. Efficiencies are determined by certified agencies/laboratories. Image by Nikos Kopidakis, National Renewable Energy Laboratory (NREL), Golden, CO, 2021, under public domain.
Figure 2 NREL chart of the highest confirmed conversion efficiencies for champion modules for a range of photovoltaic technologies, plotted from 1988 to the present. Image by Nikos Kopidakis, National Renewable Energy Laboratory (NREL), Golden, CO, 2021, under public domain.
Despite this impressive progress, the processing of PV still is far from having reached its limits, and new challenges have to be addressed. Emerging developments, such as black silicon, provide a huge potential to make PV even more competitive in the field of energy conversion. Production efficiency requires a minimization of material and process losses, reproducible results, and economic scaling of the technology. The long-term nature of most PV applications and their large-scale implementation increases the importance of recycling. Ideally, this is included as part of the processing and manufacturing strategy.
The processing of PV today follows well-established standards, but as anyone involved knows, the detailed result will be highly dependent on the local machines and processing steps. Any difference in the settings might make a critical difference in the PV product performance and might distinguish the market leader from its competitors. This book introduces the readers to the theory and practical aspects of solar processing.
Metal-assisted chemical etching (MacEtch) for black silicon (b-Si) is expected to be the leading solar manufacturing technology in the future. Chapter 1 introduces this micro-/nanofabrication approach as one of the most promising prospects to further reduce the costs of photovoltaic devices while increasing their efficiency. The origin of MacEtch and the underlying mechanism are explained with a special focus on b-Si. The history, the state of the art, and an outlook toward the large-scale deployment in silicon photovoltaic industry are given.
Chapter 2 introduces the reader to alkaline texturing for the reduction of optical losses in monocrystalline silicon solar cells. The underlying process and the most important factors, parameters, and issues are explained. In addition, the texturing process is located in the whole manufacturing process of the solar cell, highlighting the importance of the previous steps for a high-quality result.
Chapter 3 provides a detailed introduction to advanced texturing with metal-assisted chemical etching in silicon solar wafers in general. The underlying electrochemical mechanisms are explained. Common methods, typical process steps, and structure characteristics obtained by metal-assisted chemical etching methods are introduced. Examples of the characteristics of topography and anti-reflection of the structures obtained using different metal catalysts and different etchant ratio are discussed.
Wet chemical cleaning of wafer surfaces and the most common cleaning technologies are outlined and discussed regarding their potential in the solar manufacturing process in Chapter 4. The reader is introduced to types and impact of contamination and to the concept of “contamination management.” Examples for this innovative approach are given. The chapter closes with an economic perspective on the topic.
Reliable quality control, reproducibility, and the development of