Physics of Thin-Film Photovoltaics. Victor G. Karpov

([email protected]) Phillip Carmical ([email protected])

      Physics of Thin-Film Photovoltaics

       Victor Karpov

      and

       Diana Shvydka

      This edition first published 2022 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA

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       Library of Congress Cataloging-in-Publication Data

      ISBN 9781119651000

      Cover image: Pixabay.com 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

      Dedication

       To our parents

Preface

      There is no longer a need to argue about the importance of solar energy and the necessity of furthering the photovoltaic (PV) industry. These issues have been addressed by many sources in the media and publications. Excellent books have been published covering the basics of photovoltaics including both the underlying classical physics and material implementations.

      Taking advantage of the above issues sufficiently presented, this book will concentrate on several subjects left beyond the scope of the exiting photovoltaic texts. These subjects are all related to thin film photovoltaics (such as CdTe, CIGS, or a-Si:H based) whose properties and operations turn out to be quite different from that of the classical PV presented mostly by the crystalline Si structures. The obvious differences lie in the device thinness (microns instead of millimeters) and its morphology (polycrystalline or amorphous instead of crystalline).

      The thinness effect may be so significant that the photogenerated charge carriers reach the electrodes without much recombination even in the imperfect non-crystalline material, which deemphasizes the classical concept of recombination limited PV performance. On the other hand, the transversal resistance not self-averaging across thin disordered structures leaves a possibility of significant lateral nonuniformities, some of which can be quite detrimental. In addition, the non-crystalline morphology leads to continuous energy spectra of localized states instead of discrete levels in crystals, which results in new transport mechanisms (hopping) and recombination features. Finally, an important part of thin film PV possess piezo-electric properties, which leads to the concept of piezo-PV unknown in the classical PV science.

      Taken together, the latter subjects form the physics of thin film PV as a distinctive field of its own. This book will for the first time provide a consistent presentation of that field. In other words, this book will not describe the material of classical PV science, which has been masterfully described in other PV books, but will instead introduce material that has never been presented in PV books before.

To better explain our book title, we would like to emphasize the term of physics as defining the filed that is not necessarily related to the object’s materials, chemistry or engineering. The physics was always related to phenomena per se, regardless of the material specificity. Think, for example, of the Newton’s second law where the material structure does not matter, think about the fact that the Planck’s constant and Boltzmann’s constant do not depend on which material they describe, and how, in the physics of disordered systems, it does not matter what is the chemical composition of a system, etc. Similarly, in the photovoltaic science, the basics (built-in field, junction, and so on) remain material unspecific. Along these lines, our book concentrates on the aspects that are not sensitive to the material structure or composition. That is not to denigrate the known great contributions to photovoltaics from the materials sciences and chemistry, which appear dominant from any query of recent publications, but rather to preface our book limitations.

      In our experience, the concept of a distinctive field of thin film PV physics may not appear obvious to everybody: quite a few in the community believe that thin film PV must be understood in the framework of classical PV science developed earlier and proven for thick crystalline systems. Such a resistance appears rather paradoxical given that thin film PV is made of materials that seem unacceptable from the classical PV perspective, and yet they often outperform classical PV. Our book solves that paradox by switching to a new physics paradigm.

      The book is naturally broken into six parts, each containing several interrelated sections.

      Part I, consisting of just section I, gives a general introduction to PV including the concepts of junctions, material requirements, and distinctive features of thin film PV.

      Part II, encompassing sections II to