Silicon Based Thin Film Solar Cells


by

Roberto Murri

DOI: 10.2174/97816080551801130101
eISBN: 978-1-60805-518-0, 2013
ISBN: 978-1-60805-456-5



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Silicon Based Thin Film Solar Cells explains concepts related to technologies for silicon (Si) based photovoltaic applications. Topics...[view complete introduction]
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Table of Contents

Foreword

- Pp. i-ii (2)

Daniel L. Rode

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Preface

- Pp. iii-iv (2)

Roberto Murri

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List of Contributors

- Pp. v-vi (2)

Roberto Murri

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Introduction

- Pp. vii-x (4)

Sergio Pizzini

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Basics of Thin Film Solar Cells

- Pp. 3-28 (26)

Marco Ficcadenti and Roberto Murri

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Deposition of Thin Films: PECVD Process

- Pp. 29-57 (29)

Armando Menéndez, Pascal Sánchez and David Gómez

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Sputtering of Thin Films

- Pp. 58-80 (23)

Paolo Rava

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Molecular Beam Epitaxy (MBE)

- Pp. 81-107 (27)

Lorenzo Morresi

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Infrared and Raman Spectroscopies

- Pp. 108-145 (38)

Ubaldo Coscia, Deepak K. Basa and Giuseppina Ambrosone

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Morphological and Structural Properties

- Pp. 146-176 (31)

Angelica M. Chiodoni and Elena Tresso

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Optical Properties of Semiconductors

- Pp. 177-242 (66)

Marian Nowak

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Electrical Properties of Semiconductors

- Pp. 243-276 (34)

Nicola Pinto, Marco Ficcadenti and Lorenzo Morresi

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Heterojunction for Silicon Photovoltaics

- Pp. 277-358 (82)

Mario Tucci, Luca Serenelli, Massimo Izzi, Enrico Salza, Simona De Iuliis, Pietro Mangiapane, Giampiero de Cesare and Domenico Caputo

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Micromorph Cells

- Pp. 359-394 (36)

Maurizio Acciarri

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Light Trapping in Thin Silicon Solar Cells

- Pp. 395-473 (79)

Mario Tucci, Luca Serenelli, Simona de Iuliis, Domenico Caputo and Giampiero de Cesare

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Index

- Pp. 474-508 (35)

Roberto Murri

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Foreword

For eons, Earth’s water was allowed to fall naturally from mountains to seas, converting vast stores of potential energy primarily and wastefully into heat. The discovery of methods to put water power to use in mills of various sorts led eventually to its industrial exploitation on a grand scale in the eighteenth and nineteenth centuries. With the discovery of electricity, the power of falling water led to hydroelectric generation in the nineteenth and twentieth centuries, culminating in today’s huge enterprises at Itaipu in Brasil (14 GW) and Three Gorges in China (22.5 GW).

Similarly, today we allow much of the Sun’s solar insolation (800 watts per square meter, peak), mostly in the form of infrared radiation, to fall unproductively onto Earth as heat, adding to the already worrisome heat burden of the Planet. Thus, analogous to earlier experience with falling water, it behooves one to utilize at least part of this infrared radiation productively, to create the useful artifacts of civilization, before depositing a lesser amount of it on the Planet as heat.

Solar photovoltaic energy conversion addresses precisely this goal, in the form of photovoltaic cells made of various semiconductor materials, of which silicon is predominant. Thin films of silicon used for the construction of solar photovoltaic cells comprise the focus of the present volume.

One might do well to take a “top-down” view of one of the semiconductor materials challenges of solar photovoltaic energy conversion on a massive scale, to gather an overall sense of the magnitude of the endeavor. For this purpose, we may posit a few simple estimates, which leads to the scope of the issue at hand. To wit, there are 7 billion human inhabitants on Earth. Insofar as there is a general and historical movement toward improved standards of living, including electrical energy usage at a fully loaded, annual average level of about one kilowatt per person, and about 1% per year of the population experiences this relative advancement, there is an estimated need for 60 GW of additional electrical energy generating capacity worldwide each year.

It would be a pleasant environmental happenstance to have a significant fraction of this 60 GW arise from solar photovoltaic energy conversion, say 10 GW. However, due to diurnal effects, cloud cover, solar cell conversion efficiency, and so on, a useful estimate is that one square meter of solar cells (unconcentrated) provides an average of 25W of electrical power as an annual average. Therefore, 10 GW implies 400 million square meters annual production of solar cell materials. Due to the relatively small infrared absorption coefficient of silicon, the thickness of the silicon thin film must be at least about 40 microns.

Combining the above numbers, 4x108 m2 and 4x10-5 m, one finds that the annual production rate of photovoltaic silicon must be around 37,000 tonnes. While this may seem a daunting figure for a nascent enterprise, it does not greatly exceed current world-wide production of single-crystal silicon for electronics applications, generally.

Thus, one is led to conclude that impressive levels of production of thin-film photovoltaic silicon will be necessary if the enterprise is to succeed, giving cause to pay special attention to such large-area deposition methods as chemical vapor deposition (CVD) and its plasma variants, as well as the proven large-area method of choice, physical vapor deposition (PVD, or sputtering), which among others are well treated in the present volume.

Stepping back from the above grand overview, and observing a related technology, one cannot help but be impressed by the magnificent improvements wrought by the use of compositional heterostructure technology for photovoltaic cells in the case of the rather expensive compound semiconductors, which is their drawback. Alternatively, structural (as opposed to compositional) heterostructure technology is well-established in high-frequency low-noise compound semiconductor transistor technology. Unfortunately, Mother Nature’s periodic table of the elements leaves silicon with a relatively paltry offering consisting primarily of carbon and germanium for compositional heterostructure technology. However, as is discussed in this volume, silicon does allow a myriad of possibilities for structural heterostructure technology, with silicon in its microcrystalline and amorphous states.

The microcrystalline forms of silicon exhibit increased energy gaps so that band-gap engineering may be usefully possible, i.e. heterostructure technology. Solid-state theorists are rather far behind the front lines of this initiative and their assistance might be helpful. Perhaps a hint could be offered? Pauling showed long ago (“The Nature of the Chemical Bond”) that dangling bonds, such as are widespread in microcrystalline silicon, cause strengthening of back-bonds and contraction of interatomic bond lengths. This type of behavior has also been observed for the near-surface atomic layers of semiconductors. However, reduced interatomic bond length in semiconductor structures causes increased energy band gap. Therefore, there may be an essential interplay between the grain size of microcrystalline silicon (for grain sizes comparable to the de Broglie wavelength) and energy band gaps, notwithstanding Tamm states and deep-trap recombination.

There is ample opportunity here to do good for humanity and the environment, as well as to advance the solid-state and materials sciences. Further great work needs to be accomplished on a grand scale, comprising the microscopic nature of thin-film silicon and its myriad device possibilities, its commercial production on an impressive scale, and its insertion into the marketplace.

One can clearly see that there is an ongoing need for educational materials to advance the state of the art, and editor Professor Murri and his colleagues who have written this volume provide us with an important and generous contribution in this regard, for which we are grateful.

Daniel L. Rode, Prof.
Shangri-La, Paraná-Brazil &
Washington University in St. Louis
USA


Preface

Thin film silicon solar cells present all the basic physical pre-conditions for a successful development of the next generation of low cost, high efficiency photovoltaic devices, capable to fulfill the needs of a future Green Energy Society.

Their industrial development still presents, however, a number of problems associated to the diverse nature of these devices, with respect to the today generation of solar cells based on the use of crystalline silicon. In fact, here a multidisciplinary micro- and nano-technological approach is requested, applied to thin film modellization, large scale deposition, material characterization and solar cells fabrication, paying attention, as well, to quanto-mechanical issues, whose full application will only allow to go above the classical Shockley and Queissser limit of photovoltaic efficiency.

While the physics and engineering of crystalline silicon solar cells are covered by the content of a relevant number of Handbooks and Textbooks, the same does not apply to thin film solar cells, and this is the main reason why the publication of this eBook seems appropriate to supply, on time, to the absence. It is intended for non-specialists, i.e. for graduate and PhD students with a sufficient background in solid-state physics as well as for professional physicist and engineers who are not fully familiar with all the aspects of thin film technology, but who might be potential contributors to the development and industrialization of this opportunity. This eBook critically covers, in fact, in its three Sections all the main aspects of thin film solar cells physics and engineering and is co-authored by recognized experts in the field, whose view is not at all a personal view, but reflects the state of the art of the field and the opinion of the international thin film community.

Section I contains four chapters describing, respectively, the basic elements of thin film solar cells (Ch.1) and deposition of thin films by (Ch.2), plasma enhanced chemical vapor deposition (PECVD), sputtering (Ch.3) and, finally, Ch.4, molecular beam epitaxy (MBE).

Experimental techniques to measure film and device parameters are discussed in the four Chapters of Section II. In details, Infrared and Raman spectroscopies (Ch. 5), Morphological and structural properties (Ch.6), while Ch.7 and Ch. 8 present how to measure optical and electrical parameters, respectively.

The last three Chapters of Section III discuss the physics of two classes of devices, Heterojunctions for silicon photovoltaics (Ch. 9) and Micromorph cells (Ch.10), widely studied as structures able to improve the conversion efficiency of a solar cell. Finally, Ch.11 underlines the role of light trapping increasing the capture of solar radiation and then, again, the conversion efficiency.

I would like to thank all Authors, Prof. Daniel Rode for writing the Foreword, Prof. Sergio Pizzini for his introduction discussing the general aspects of the subject of the eBook, Mrs. Lisa Kramer Taruschio for language revision of Chapters 1,4,7 and 8 and Bentham Science Publishers, in particular Mrs. Salma Sarfaraz for her support, efforts and patience. The University of Camerino - School of Science and Technology, togheter with Elena Tresso -DISAT, Department of Applied Science and Technology, Politecnico di Torino, and Maurizio Acciarri - Department of Materials Science and Solar Energy Research Center (MIBSOLAR) Università degli Studi Milano Bicocca, partially supported the publication costs of the eBook for its open access.

Roberto Murri
University of Camerino
School of Science and Technology
Physics Division
Italy

List of Contributors

Editor(s):
Roberto Murri
University of Camerino
Italy




Contributor(s):
Angelica M. Chiodoni
Center for Space Human Robotics (CSHR@POLITO)
Italian Institute of Technology
Corso Trento 21
Torino
IT-10129
Italy


Armando M. Estrada
Energy Area, ITMA Materials Technology
Parque Empresarial P.E.P.A.
c/Calafates 11, Parcela L.3.4
Avilés (Asturias), 33417
Spain


David Gómez
Energy Area, ITMA Materials Technology
Parque Empresarial P.E.P.A.
c/Calafates 11, Parcela L.3.4
Avilés (Asturias), 33417
Spain


Dipak K. Basa
Department of Physics
Utkal University
Bhubaneswar, 751004
India


Domenico Caputo
DIET Department of Information
Electronics and Telecommunication Engineering
University of Rome "Sapienza
Via Eudossiana 18
00184 Rome
Italy


Elena Tresso
DISAT, Dipartimento di Scienza Applicata e Tecnologia
Politecnico di Torino
Corso Duca degli Abruzzi 24
10129 Torino, and Center for Space Human Robotics (CSHR@POLITO)
Italian Institute of Technology
Torino
Italy


Enrico Salza
ENEA, Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Giampiero de Cesare
DIET Department of Information, Electronics and Telecommunication Engineering
University of Rome "Sapienza,
Via Eudossiana 18
Rome, 00184
Italy


Giuseppina Ambrosone
Department of Physical Sciences
University of Naples "Federico II"
Complesso Universitario MSA
Via Cintia, 80126 Napoli
Italy and CNR-SPIN
Complesso Universitario MSA, Via Cintia
Napoli, 80126
Italy


Lorenzo Morresi
School of Science and Technology
Physics Division, University of Camerino
Via Madonna delle Carceri
CAMERINO (MC), 9-62032
Italy


Luca Serenelli
ENEA, Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Marco Ficcadenti
School of Science and Technology
Physics Division, University of Camerino
Via Madonna delle Carceri
9-62032 Camerino (MC) Italy
now at CESI Spa,Via Rubattino 54
Milan, 20134
Italy


Marian Nowak
Institute of Physics
Silesian University of Technology
8 Krasińskiego
Katowice
PL, 40019
Poland


Mario Tucci
ENEA, Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Massimo Izzi
ENEA, Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Maurizio Acciarri
Department of Materials Science and Solar Energy Research Center (MIB-SOLAR,)
Università degli Studi Milano Bicocca
Milan
Via Cozzi, 53-20125
Italy


Nicola Pinto
School of Science and Technology
Physics Division, University of Camerino
Camerino (MC)
Via Madonna delle Carceri, 9-62032
Italy


Pascal Sánchez
Energy Area, ITMA Materials Technology
Parque Empresarial P.E.P.A.
c/Calafates 11, Parcela L.3.4
Avilés (Asturias), 33417
Spain


Pietro Mangiapane
ENEA, Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Roberto Murri
University of Camerino
School of Science and Technology
Physics Division
Via Madonna delle Carceri 9
Camerino (MC), 62032
Italy


Paolo Rava
Elettrorava SpA
Via Don Sapino 176
Venaria
Torino, 10078
Italy


Sergio Pizzini
Department of Materials Science
Università degli Studi Milano Bicocca
Via Cozzi
Italy


Simona de Iuliis
ENEA
Research Center Casaccia
Via Anguillarese 301
Roma, 00123
Italy


Ubaldo Coscia
CNISM Naples Unit
Complesso Universitario MSA
Via Cintia,
Napoli, 80126
Italy
/
Department of Physical Sciences
Complesso Universitario MSA
Via Cintia
Napoli, 80126
Italy




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