Physics of Thin-Film Photovoltaics. Victor G. Karpov

V, concentrates on one-dimensional concepts relevant to thin film PV. It presents a densely populated zoo of various diodes, some quite different from p-n junctions, including the ones that play a significant role in thin film PV operations. It then extends that diode consideration on one-dimensional analysis of solar cells, including some important case studies.

      Part III, made up of sections VI to XIII, takes the consideration beyond one-dimensional physics considering lateral effects, such as shunting, random micro-diodes, weak diodes, etc., addressing both the related theory and observations.

Part IV, including sections XIV to XVIII, concentrates on electronic processes in disordered materials of thin film PV which addresses morphology, fluctuations, transport properties, recombination in the quasi-continuous spectrum of localized states, operations of nonocrystalline junctions, and, finally, piezo photovoltaics.

      Part V, formed by sections XIX to XXIV, presents the subject of electro-thermal instabilities in thin film PV starting from the exactly solved two-diode model and current hogging to spontaneously formed hot spots conducive of performance degradation.

      Part VI includes sections XXV to XXVIII and discusses a sensitive issue of device degradation for thin film PV, including both solar cells and PV modules. It then discusses the problematic of accelerated life testing and risk analyses.

      Finally, our book has an Appendix that comprise three parts of methodical value” the validity of in-series representation of complex band diagrams, the nature of saturation current in diode models, and accurate estimates for the electron potential fluctuations.

      We are grateful to many people who have helped us to learn the subjects presented in this book. Our exposure to PV science and industry started with First Solar LLC (Perrysburg, OH) before the year 2000, at which time its research and production groups combined many enthusiastic contributors including H. McMaster, G. Dorer, G. Nelson, R. C. Powell, D. Rose, U. Jayamaha, E. Bykov, T. Maxon, R. Harju, T. Colman, L. McFaul, N. Reiter, T. Kahle, G. Khouri, G. Rich, M. Steel, and many others, to all of whom we are grateful for their intelligent friendly support. We would like as well to acknowledge our interactions with the PV group of the University of Toledo including A.D. Compaan, X. Deng, A. Vijh, D. Giolando, A. Vasko, Y. Roussillon, V. Parikh, L. Attigalle (Cooray), M. Mitra, M. Nardone, X. Li, V. Plotnikov, K. Wieland, and J. Drayton. Finally, we would like to express our gratitude to the members of CdTe PV National Team at NREL, starting with their remarkable managers K. Zweibel, B. von Roedern, H. Ulllal, and brilliant contributors B. McCandless, D. Albin, V. Kaydanov, T. Ohno, J. Sites, T. Sampath, A. Fahrenbruch, and so many others.

      This book is intended for three readership categories. One is that of graduate and advanced undergraduate students with some understanding of the general physics and familiarity with the basic concepts of condensed matter. We hope that our book will bring to them a flavor of live physics in an informal manner as used by practicing researchers. It may help to open their eyes to a more general fact that the science of physics often evolves in its own ways quite different from those implied by the standard curriculum textbooks, and, for some, to become an evidence that physics can be relevant for PV.

Our next category of readers belongs to those in academic teaching and research revolving around semiconductors, device physics, and thin film structures. For that category, we aim to provide a fresh look at a number of subjects so established that they appear “carved in stone” for ages as dictated by the classical photovoltaic science developed during the times when thin film technology PV did not exist. Our book will invite that category of readers to pay more attention to the issues of variability and statistics apparent to those working for industry rather than for academia. Finally, we hope that some of our book topics and approaches can become a part of university curricula for PV/device physics or related disciplines.

      We identify our third category of readers as the industrial R&D professionals of all levels. Their business responsibilities and hectic environment do not often leave much time to follow the published research and appreciate new concepts. We believe that our book will help the curious readers of that category to broaden their horizons and open new approaches towards technology goals. On top of that, adopting some concepts of our book could create beneficial synergy between physics and solar technology.

      Victor Karpov Diana Shvydka Sylvania, Ohio, USA February 2021

Part I

      General and Thin Film PV

      I. Introduction to Thin Film PV

      A number of brilliant texts are readily available describing the general photovoltaic (PV) principles and their classical implementations [1–6], as well as various specifics of thin film PV [7–11], and we are not going to duplicate them in any form. This section will briefly overview main book concepts especially addressing the underlying physics of PV functionality different between the crystalline and thin film devices. Such a synthetic view will help to inter-relate various sides of the physics of thin film PV as a subject of its own.

      A. The Origin of PV. Junctions

We recall that PV effect in general is presented by the light generated voltage. The PV voltage when the system absorbed light creates electric asymmetry in the form of spatially separated opposite electric charges, electrons and holes, which can be extracted and utilized in the form of electric current. For the charge separation to occur, there must be a built-in electric field as illustrated in Fig. 1 presenting a basic design of PV devices made of two semiconductor layers. The built-in field originates from the junction formed by those layers. Historically, such junctions were between p- and n-types of semiconductor layers; hence, the name of p-n junctions traditionally associated with PV devices. However, p-n junctions are neither necessary nor sufficient elements of systems with built-in fields as discussed below. For example, semiconductor/metal junctions in Fig. 1 will generate their own built-in fields that can be either beneficial or detrimental to PV device functionality.

In general, the built-in electric fields always emerge with junctions of any two chemically different materials. The underlying physics is that one of the materials will be energetically more favorable for the electrons than the other. To minimize the system energy, the electrons will therefore move there leaving the one with unbalanced positive charge; hence, the built-in field between the spatially separated opposite charges of electrons and holes, qualitatively similar to that of electric capacitor. The number of electrons moving across the junction is determined by the balance between the above mentioned energy gain and the energy loss due to the necessity of overcoming the Coulomb attraction to the positive charges left behind. The built-in electric fields of that nature are omnipresent and are not limited to photovoltaics, or p-n junctions, or other artificial structures.

Fig. 1 Conceptual design of a solar cell.