Silicon E-Book

Eimutis Juzeliunas

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 565

Veröffentlichungsjahr: 2022

Beschreibung

Silicon

The expert reference on sustainable and energy-efficient production of photovoltaic-grade silicon materials

Electrochemical methods, in particular molten-salt approaches, are a cost-effective, energy-efficient, and highly sustainable approach for producing solar-grade silicon. Surface micro- and nanostructuring methods for effective light harvesting, silicon electrorefining in molten salts, electrodeposition of photoresponsive films, and other related processes are likely to replace conventional carbothermic production methods.

Silicon: Electrochemistry, Production, Purification and Applications presents an up-to-date summary of recent experimental and technological developments in the field, highlighting sustainable and energy-efficient processes for high-grade silicon production for a variety of photovoltaic and energy applications. Presented in a logical and concise format, this authoritative volume details the fundamental properties and technical processes of metal-grade silicon production and describes the various electrochemical methods for high-grade silicon production. Topics include silicon surface modification, chemical-physical structuring, porous and black silicon, electrochemical Si surface structuring and anodizing in molten salts, and more.

• Reviews the sustainable and energy-efficient production and purification of photovoltaic-grade silicon materials
• Summarizes recent progress in sustainable processes for high-grade silicon production
• Describes electrochemical methods for silicon production such as electrolysis, electrodeposition, and electrorefining
• Concludes with a discussion of future challenges and opportunities

Written by a leading researcher in the field, Silicon: Electrochemistry, Production, Purification and Applications is a valuable resource for chemists and material scientists in academia and industry, particularly those working in sustainable energy development, photovoltaics, light harvesting efficiency, solar-to-chemical conversion, and production of solar-grade silicon, batteries, photoelectrodes, or silicon-based semiconductors.

Leseprobe

Silicon

Electrochemistry, Production, Purification and Applications

Eimutis Juzeliūnas

Author

Prof. Eimutis JuzeliūnasCenter for Physical Sciences and TechnologyDepartment of Electrochemical MaterialsSauletekio Str 310257 VilniusLithuania

Cover Image: © GeorgyShafeev/Shutterstock

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Print ISBN: 978‐3‐527‐34897‐8

ePDF ISBN: 978‐3‐527‐83190‐6

ePub ISBN: 978‐3‐527‐83192‐0

oBook ISBN: 978‐3‐527‐83191‐3

To the memory of my parents – mother Zofija and father Enrikas

and

To my beloved family – wife Katerina and sons Laimis and Povilas, who inspired and supported.

Theodor von Grotthuss medal. Author: Petras Repšys. Produced by Lietuvos monetų kalykla (Lithuanian Mint)

“Though light illuminates darkness, nothing is darker than the light.”

Theodor von Grotthuss (1785–1822)

Preface

Silicon lies at the heart of modern technology. Silicon can be used in various fields, such as optoelectronics, sensors, batteries, optical fibers, photoelectrochemical water splitting, terahertz emitters, and numerous other applications. As an abundant, non‐toxic, efficient, and robust material, silicon will dominate the solar energy market at least for the next few decades.

Electrochemistry deals with the chemical transformations, which are induced by an electric current, or vice versa – with the transformations, which generate an electric current. These processes provide an opportunity to store or produce electricity with a minimum carbon footprint. Electrochemistry can, therefore, significantly contribute to low‐carbon economy; it offers an advancement in sustainable energy solutions and environment‐friendly technologies.

In the early 2000s, V. Lehman (2002) and X. G. Zhang (2004) published several books on silicon electrochemistry. Since then, various breakthrough directions in silicon electrochemistry have emerged. For instance, luminescent porous silicon nanoparticles were electrochemically produced and applied as the carriers of the drug payload in vivo. Electrochemical silicon surface modifications increased the efficiency of photovoltaic devices used for solar energy harvest or for the production of solar fuel. Silicon photoelectrodes have been successfully developed for hydrogen and oxygen production by water splitting as well as CO2 reduction. Electrochemically produced Si surface nano‐architectures showed an intrinsic quantum confinement effect. Environment‐friendly and secure solutions offered silicon electrochemistry in high‐temperature molten salts. Electrochemical silicon reduction from silica in high‐temperature molten salts has been discovered. Electrochemical deposition of doped silicon as well as formation of p‐–n junction have also been demonstrated.

This book aims to summarize the experimental and technological work done in recent decades on silicon electrochemistry, production, and purification, highlighting subjects of technological significance and future perspectives. The book aims to be highly beneficial to the communities of chemists and material scientists working in academia and industrial sectors, especially in the field of sustainable energy development: photovoltaics, light harvesting efficiency, solar‐to‐chemical conversion, production of solar‐grade silicon as well as production of batteries, photoelectrodes, or silicon‐based semiconductors. The secondary market of this book includes the education and socio‐economic sectors with focal points on such topicalities as the reduction in global climate change, replacement of fossil fuels by renewable energy, and strategies of low‐carbon economy.

Vilnius, Lithuania Eimutis Juzeliūnas

June, 2022

References

Lehman, V. (2002).

Electrochemistry of Silicon. Instrumentation, Science, Materials and Applications

. Wiley‐VCH.

Zhang, X.G. (2004).

Electrochemistry of Silicon and Its Oxide

. Kluwer Academic Publishers.

List of Abbreviations

3PI

three‐phase interface (interlines)

AFM

atomic force microscopy

AL

acetonitrile

Al‐BSF

aluminum backside field technology

ALD

atomic layer deposition

BCE

before the common (or current) era

BMIm

1‐butyl‐3‐methylimidazolium

BMPy

1‐butyl‐3‐methylpyridinium

BMPyrr

N

‐butyl‐

N

‐methylpyrrolidinium

b‐Si

black silicon

CE

contacting electrode

CNT

carbon nanotubes

COP 21

Paris Climate Conference

CV

cyclic voltammetry

CVD

chemical vapor deposition

CZ

Czochralski process

DMAE

2‐dimethylaminoethanethiol

DMS

dimethyl sulfide

DRC

Democratic Republic of the Congo

DRE

damage removal etching

EC

The European Commission

EDX (EDS)

energy‐dispersive X‐ray spectroscopy

EIS

electrochemical impedance spectroscopy

EMIm

1‐ethyl‐3‐methylimidazolium

EMPyrr

N‐ethyl‐

N

‐methylpyrrolidinium

EQCM

electrochemical quartz crystal microbalance

EU

the European Union

FAP

tris(pentafluoroethyl)‐trifluorophosphate

FBR

fluidized bed reactor

fs

femtosecond

FTIR

Fourier transform infrared spectroscopy

FTO

fluorine‐doped tin oxide

GDMS

glow discharge mass spectrometry

GDP

Gross domestic product

GI‐XRD

grazing incidence X‐ray diffractometry

ICE

initial Coulombic efficiency

ICP

inductive coupled plasma

IL

ionic liquid

IPA

isopropyl alcohol

IRENA

International Renewable Energy Agency

ISFET

ion‐sensitive field effect transistor

ITO

indium tin oxide

LCD

liquid‐crystal display

LIB

lithium‐ion battery

M, Me

metal, metallic

MACE

metal‐assisted chemical etching

MG‐Si

metallurgical‐grade silicon

MOE

molten oxide electrolysis

MS

magnetron sputtering

MT

metric ton

MWT

metal wrap through

NHE

normal hydrogen electrode

NMR

nuclear magnetic resonance

NPs

nanoparticles

ns

nanosecond

NTD

neutron transmutation doping

NWs

nanowires

P‐Si

porous silicon

PEC

photoelectrochemical cells, photoelectrochemistry

PEDOT:PSS

poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate)

PERC

passivated emitter and rear contact

PIII

plasma immersion and ion implantation

PV

photovoltaic(s)

PVD

physical vapor deposition

QCN

quartz crystal nanobalance

R&D

research and development

RE

reference electrode

RFMS

radio frequency magnetron sputtering

RIE

reactive ion etching

Si‐H

hydrogen‐terminated silicon surface

Si‐F

fluoride‐terminated silicon surface

Si‐OH

hydroxide terminated silicon surface

SCR

space‐charge region

SEM

scanning electron microscopy

SERS

surface‐enhanced Raman spectroscopy

SoG‐Si

solar‐grade silicon

SP

solubility product

STC

silicon tetrachloride

SWIR

short‐wavelength infrared

TBAB

tetrabutylammonium bromide

TBAC

tetrabutylammonium chloride

TBMA

tributyl(methyl)ammonium

TCS

trichlorsilane

TEAC

tetraethylammonium chloride

TEOS

tetraethylorthosilicate

TFO

trifluoromethylsulfonate

TFSA

bis(trifluoromethylsulfonyl)amide

TFSI

bis(trifluoromethylsulfonyl)imide

TG‐DTA

thermogravimetry and differential thermal analysis

THF

tetrahydrofuran

TMAH

tetramethylammonium hydroxide

TMHA

N

‐trimethyl‐

N

‐hexylammonium

TPAC

tetrapropylammonium chloride

TRL

technology readiness level

UPD

underpotential deposition

UTE

ultra‐high temperature electrochemistry

VLS

vapor–liquid–solid deposition

XRD

X‐ray diffraction

XPS

X‐ray photoelectron spectroscopy

About the Author

Eimutis Juzeliūnas, photo by J. Stacevičius/Lithuanian National Radio and Television, LRT.

Professor Dr. Eimutis Juzeliūnas is a principal research associate and a head of the Department of Electrochemical Materials Science at the Centre for Physical Sciences and Technology in Vilnius, Lithuania. He worked previously as a rector of Klaipėda university (2014–2018) and a director of the Institute of Chemistry in Vilnius (2001–2009). He was a Marie Curie International Fellow of the European Commission at the University of Cambridge, UK (2009–2011, 2013–2014) where he carried out research on silicon electrochemistry in high‐temperature molten salts. He also was a Fulbright fellow at the Vanderbilt University (USA), a fellow of the American Chemical Society at the Pennsylvania State University (USA), and an Alexander von Humboldt fellow at the company DECHEMA e.V. (Germany). His main research area is electrochemical materials science; current research interests are silicon electrochemistry for energy applications, environmental and microbiological degradation of metals (corrosion), physical vapor deposition of resistant alloys, and nanogravimetry of electrochemical processes.

1Introduction

World production of silicon (Si) reached (2010–2020) about eight millions of metric tonnes in the last decade. This quantity was produced mainly by the carbothermic silica (SiO2) reduction. The process requires a large supply of energy and emits carbon oxides (COx). A fundamental challenge is the electrochemical silicon extraction from silica or other solids using electricity instead of harmful chemistries. Zero carbon footprint could be attained when using electrons as absolutely clean reduction agents generated by renewable sources. Electrochemical methods can be used on a wide scale of applications: extraction, purification, surface engineering, or thin‐film technologies. Thus, silicon electrochemistry has the potential to significantly contribute to low‐carbon economy; this field offers an advancement in environmentally friendly and secure technologies of energy generation and storage.

Breakthrough research topics have emerged in silicon electrochemistry in recent decades. The electrochemical formation of porous silicon (P‐Si) was discovered in 1956 (Uhlir 1956). Canham reported in 1990 that a visible room‐temperature photoluminescence from P‐Si layer formed electrochemically on Si wafer (Canham 1990). The discovery inspired wide studies of P‐Si for applications in optoelectronics, lasers, and sensors. Luminescent porous silicon nanoparticles were applied as the carriers of the drug payload, whose infrared luminescence enabled monitoring of the particles in vivo (Park et al. 2009). Electrochemical nano‐micro‐structuring of silicon has been widely investigated. The surface modifications increased the efficiency of photovoltaic (PV) devices used for solar energy harvest or for production of solar fuel. Silicon photoelectrodes have been successfully used for hydrogen and oxygen production by water splitting as well as CO2 reduction (Sun et al. 2014). Electrochemically produced Si surface nano‐architectures showed intrinsic quantum confinement effect. Electrochemical reduction of silicon dioxide to silicon in a molten salt electrolyte has been reported, which formed the basis for new processes in silicon semiconductor technology and high‐purity silicon production (Nohira et al. 2003). Environmentally friendly and secure solutions offered silicon electrochemistry in high temperature molten salts (Juzeliūnas and Fray 2020). Electrochemical deposition of doped silicon as well as formation of p–n junction has been demonstrated (Zou et al. 2017, 2019; Peng et al. 2018). The approach has the potential of reducing capital cost and energy consumption for fabrication of solar cells when compared with the conventional manufacturing process.

This book features recent achievements in silicon electrochemistry, particularly, in electrochemical silicon extraction, purification, and processing in high‐temperature molten salts. The introductory part of the book (Chapters 2–4) is devoted to general aspects of silicon application. A historical overview of silicon production is provided, and its importance in a low‐carbon economy is considered. Chapter 4 addresses the physical and chemical properties of silicon, which are most relevant for electrochemical materials science. The subsequent material is more specific. Chapter 5 describes the major technologies used for silicon purification such as Siemens, Union Carbide, or Ethyl Corporation processes. This chapter also provides the principles of electrorefining in high‐temperature molten salts, highlighting the advantages and disadvantages when compared with conventional industrial processes.

Chapter 6 addresses electrodeposition of thin layers and discusses the possibility of replacing multiple processes of Si wafer fabrication with one‐step electrochemical deposition. Traditional manufacturing entails an energy‐intensive and environmentally unfriendly production of metallurgical grade silicon (MG‐Si), as well as its upgrade to solar grade silicon (SoG‐Si), ingot casting, and slicing. Electrodeposition from molten fluoride, chloride, and oxide electrolytes on various substrates is discussed. A recently proposed strategy for electrodeposition of photoactive silicon and p–n junction is highlighted in detail. Silicon deposition from ionic liquids – the room‐temperature molten salts – is also discussed in this chapter. Significant attention is given to the purity level of silicon electrodeposits, which are essential for photo‐electrochemical applications.

Chapter 7 discloses photoelectrochemical (PEC) properties of silicon‐oxide electrodes coated with ultrathin films of silica (SiO2), hafnia (HfO2), and alumina (Al2O3). The pivotal concept of PEC methodology is to obtain information, which correlates with that of the solid‐state cells so that there is no prior need to design a solar cell that characterizes Si surface photo‐responsiveness. Significant attention is given to studies of Si‐oxide interfacial stability by the quartz crystal nanobalance (QCN) – a sensitive mass detector, which provides information about the electrode mass change with nanogram resolution in situ and in real time.

Deoxidation of metal oxides in a molten salt electrolyte was discovered in the year 2000 (Chen et al. 2000). The process was named the FFC Cambridge process. Simplicity and rapidity of the process have attracted global interest. Over 30 metals or semimetals were extracted from solid compounds by this energy‐efficient and environment‐friendly route. Chapter 8 addresses the FFC principle and its application in silicon reduction from silica. The electrochemical extraction provides a green alternative to conventional carbo‐thermic silicon production. Chapters 9–12 provide further details on Si–SiO2 conversion in molten salts. Voltammetry, basic reactions, and in situ studies by synchrotron X‐ray diffraction are discussed, and experimental conditions used by many authors are summarized.

Technological opportunities carry out the operation at ultra‐high temperatures and at liquid state of silica feedstock. Such processes are referred to as molten oxide electrolysis (MOE). Chapter 13 discusses the MOE principles of silicon extraction in a liquid state.

This study focused majorly on electrochemical surface engineering. Chapter 14 discusses the chemical–physical methods of silicon surface structuring, such as laser engineering and various etchings: chemical, photoelectrochemical, reactive ion, plasma immersion ion implantation, and metal‐assisted chemical. The vapor–liquid–solid method is also discussed.

Chapter 15 features a comprehensive material obtained on electrochemical Si structuring at high‐temperature molten salts including formation of black silicon (B‐Si). B‐Si is a nano‐micro‐porous material, which effectively absorbs the light on a wide range of wavelengths. Electrochemical Si structuring in molten salts is attractive due to its environmental friendliness, technical simplicity, and cost‐effectiveness.

The book also outlines the perspectives of electrochemical synthesis of semiconductors (Chapter 16), the basic principles and materials for photo‐electrodes, and the preservation of solar‐fuel generators (Chapter 17).

In conclusion, while silicon electrochemistry offers a range of technological opportunities, most of the developments are still on the conceptual or bench‐scale level. As a result, viable technological developments are still pending.

References

Canham, L.T. (1990). Quantum wire array fabrication by electrochemical and chemical dissolution.

Appl. Phys. Lett.

57: 1046–1048.

https://doi.org/10.1063/1.103561

.

Chen, G.Z., Fray, D.J., and Farthing, T.W. (2000). Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride.

Nature

407: 361–364.

https://doi.org/10.1038/35030069

.

Juzeliūnas, E. and Fray, D. (2020). Silicon electrochemistry in molten salts.

Chem. Rev.

120: 1690–1709.

https://doi.org/10.1021/acs.chemrev.9b00428

.

Nohira, T., Yasuda, K., and Ito, Y. (2003). Pinpoint and bulk electrochemical reduction of insulating silicon dioxide to silicon.

Nat. Mater.

2: 397–401.

https://doi.org/10.1038/nmat900

.

Park, J.‐H., Gu, L., Maltzahn, G. et al. (2009). Biodegradable luminescent porous silicon nanoparticles for

in vivo

applications.

Nat. Mater.

8: 331–336.

https://doi.org/10.1038/NMAT2398

.

Peng, J.J., Yin, H.Y., Zhao, J. et al. (2018). Liquid‐tin‐assisted molten salt electrodeposition of photoresponsive n‐type silicon films.

Adv. Funct. Mater.

28: 1703551.

https://doi.org/10.1002/adfm.201870194

.

Sun, K., Shen, S., Liang, Y. et al. (2014). Enabling silicon for solar‐fuel production.

Chem. Rev.

114: 8662–8719.

https://doi.org/10.1021/cr300459g

.

Uhlir, A. (1956). Electrolytic shaping of germanium and silicon.

Bell System Tech. J.

35: 333.

https://doi.org/10.1002/j.1538-7305.1956.tb02385.x

.

Zou, X., Ji, L., Yang, X. et al. (2017). Electrochemical formation of a p–n junction of thin film silicon deposited in molten salt.

J. Am. Chem. Soc.

139: 16060–16063.

https://doi.org/10.1021/jacs.7b09090

.

Zou, X., Ji, L., Ge, J. et al. (2019). Electrodeposition of crystalline silicon films from silicon dioxide for low‐cost photovoltaic applications.

Nat. Commun.

10: 5772.

https://doi.org/10.1038/s41467-019-13065-w

.

2Silicon Electrochemistry – Toward Low‐Carbon Economy

Climate change is one the greatest challenges the world faces today. The renewable energy (solar, wind, water, biomass, geothermal) is considered as a climate change imperative today. Excessive exploitation of fossil energy sources had a negative impact on climate, economy, and everyday lives. The growing threat from climate change is apparent, such as natural disasters, halved mass of inland glaciers, rise of sea level, and extinction of numerous terrestrial species. Volatile oil, gas, and coal prices in recent decades and concerns about their supply from politically unstable countries are amongst the drivers of the need for alternatives, such as renewables.

The European Green Deal plan was unveiled in 2019 – a three‐decade effort to make the climate neutral by 2050 (https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal; Tamma et al. 2019; Simon 2019). The Commission's President Ursula von der Leyen characterized the initiative as “Europe's man on the moon moment” adding that “the growth model based on fossil fuels and pollution is out of date and out of touch with our planet” (Simon 2019). The overarching objective in the plan is “Climate neutral Europe,” which aims to reach net‐zero greenhouse gas emissions by 2050. The ambition for 2030 is cutting‐off the emission by 50–55%. The plan addresses major actions such as circular economy, building renovation, pollution‐free environment, strategy of ecosystems and biodiversity, green and healthier agriculture, electric vehicles, and sustainable fuels, such as biofuels and hydrogen.

Another global initiative is the Paris Agreement – an international treaty on mitigation of climate change (https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement). In 2015, 196 parties in the Paris climate conference (COP 21) adopted an agreement to limit global warming below 2 °C compared to pre‐industrial levels.

Such highly ambitious plans led to fundamental changes in the way we generate and use the energy. Electrochemical technologies have shown great potential in advancing the economy's transition toward climate neutrality. Electrochemistry provides sustainable solutions in such fields as green‐energy storage and solar‐to‐chemical or solar‐to‐electricity conversion.

Materials used in solar devices play a major role in the cost breakdown of the overall utilization process of solar energy. Creation and synthesis of new effective materials for solar‐energy applications, therefore, is very high on the agenda of materials scientists and engineers. Vast majority of the solar cells are produced from silicon where the total wafer cost dominates in the overall cell cost balance. It is assumed that silicon, being a nontoxic, efficient, and robust material, will play a key role in the solar energy market for the next few decades (Schmalensee et al. 2015; Green 2016; Polman et al. 2016). Mitigation of climate change, as a global task, could be achieved by using technologies based on the Earth‐abundant materials. The abundance of silicon in the Earth's crust (27%) makes it possible to expand the application of this material to the terawatt scale by 2050 (Schmalensee et al. 2015).

2.1 Silicon for Energy Storage – Electrochemical Batteries

Electrochemistry deals with relationship between electricity and chemical change. Typical example of an electrochemical device used in everyday life is a battery – the device, which generates electricity by chemical reactions. Batteries are widely used in portable electronics, medical devices, or e‐mobility including electric cars. A vice versa process is when electricity generates chemical reactions, for instance, electroplating of metals. The process is widely applied for the production of coatings in order to protect metals and alloys from corrosion or to improve their aesthetic appearance, as well as to decorate them.

Batteries represent a key technology for low‐carbon economy to reduce CO2 emissions from transport, power, and industrial sectors. Batteries are essential devices used to store stationary energy from sustainable sources such as solar or wind. To reach the sustainability targets, batteries must exhibit ultra‐high energy and power performance close to theoretical limits. Other requirements include outstanding lifetime, reliability, safety, and recyclability. Important requirement is also scalability to electricity grid level, as well as cost‐effectiveness and sustainable battery production. The cost target set by the European Commission (EC) for the next‐generation batteries of stationary energy storage is below 0.05 €/kWh/cycle by 2030. The growth in global battery demand is anticipated to multiplied by a factor 14× from 2018 to 2030. The greatest part of this demand goes to the electric mobility sector (Edström et al. 2020).

Lithium‐ion (Li‐ion) based technology dominates the current battery market. Lithium‐ion batteries (LIBs) are state‐of‐the‐art technology for portable electronics and electric vehicles. The Nobel Prize in chemistry has been awarded to J. B. Goodenough, M. S. Whittingham, and A. Yoshito in 2019 for the development of LIBs. These batteries, however, have several shortcomings, especially for stationary energy storage applications.

Apart from Li, cobalt (Co) is a key electrode material in Li‐ion batteries. For cathode production, various co‐containing materials have recently been investigated: LiCoO2, LiNixCoyAl1−x−yO2, LiNixMnyCo1−x−yO2, etc. (Huang et al. 2021; Chu et al. 2020; Liu et al. 2019a,b). Presently, about 60% of mined cobalt is used to produce the LIBs electrodes. Congo (The Democratic Republic of the Congo, the DRC) along with Zambia, Madagascar, Zimbabwe and the Republic of South Africa are mining about 70% of world cobalt. The DRC is a main global producer, which mines about 60% of the world's cobalt feedstock. The Amnesty International organization reported in 2016 on the violation of human rights in this country – child labor in health‐endangering mines (https://www.amnesty.org/en/documents/afr62/3183/2016/en).

The EC issues periodically the communications with a list of critical raw materials, which specifies the materials that are most important for the EC economically, alongside having high supply risk. As of 2020, the list also includes the materials, from which key components of LIBs are produced: lithium, cobalt, and graphite (Communication 2020). The list indicates 100% reliance of the European Union (EU) import on lithium, 86% on cobalt, and 98% on natural graphite. The communication states that the EU demand for lithium for electric vehicle batteries and energy storage will increase by 18 times in 2030 and almost up to 60 times in 2050. The corresponding figures for cobalt are 5 and 15 times, respectively. The limited availability of lithium makes it doubtful whether LIBs manufacturing can scale up to significantly larger production volumes.

The World Bank projected that the scenario of global warming below 2° (COP 21) will increase the demand in metals for battery applications at the level of 1000% by 2050 (World Bank 2017). The list of relevant materials for battery manufacturing includes metals such as Al, Co, Fe, Pb, Li, Mn, and Ni.

An important constraint of the stationary energy storage using LIBs is the continuous consumption of Li‐ion electrolyte, which limits both cycle and calendar life. Another restriction lies in the limitations of operating temperature window. The operation requires complex thermal management, which is impractical for stationary storage applications, particularly in “hot” countries. These batteries also contain hardly recyclable materials, such as lithium. LIBs are still facing safety issues. Also, LIBs are relatively expensive.

Thus, there is great demand and a great challenge to create effective post‐Li electrochemical energy storage systems. An alternative suggests sodium with an electrochemical charge transfer reaction Na ↔ Na+ + e−, which could be performed in a solid compound, for instance, in a chloride. Oceans provide unlimited sodium source, which is for free.

Metal‐air batteries explore air oxygen as a major reactant:

(2.1)

This approach enables battery weight reduction, at the same time, increasing the capacity for energy storage. Air–metal technologies are attractive because they are green, safe, and cost‐efficient in terms of feedstock. The technologies propose usage of an unlimited source of oxygen from the atmosphere, which is for free. The metal–air batteries can utilize various metal electrodes, e.g. Li, Zn, Al, Fe, Mg (Tong et al. 2021). Of these metals, Li‐air battery has the highest theoretical energy density and practical operation voltage (13 000 W h kg−1 and 2.4–2.7 V vs. standard hydrogen electrode (SHE), respectively). Lithium is followed by aluminum (8073 W h kg−1 and 1.2–1.6 V) and magnesium (6815 W h kg−1 and 1.2–1.4 V). Iron has the least performance (764 W h kg−1 and 0.8 V), however, the cost of iron metal anode is substantially less when compared to other metals. Tong et al. summarized that the cost of iron is about 200 times less when compared with the cost of lithium and about 15 times less when compared with magnesium or aluminum (Tong et al. 2021). The authors also estimated that Mg, Al, and Zn electrodes are nearly 10 times cheaper than the Li metal electrodes. Li–air batteries are limited due to dendrite formation, poor‐cycling efficiency, and difficulties in finding a suitable highly stable electrolyte. Fe–air battery is very attractive in terms of excellent resource‐efficiency. Note also that the electrodes can be prepared combining them in the form of alloys, for instance, Mg–Al, Mg–Al–Zn, Mg–Li, Mg–Zn, etc.

Silicon is an attractive material for LIBs anode production. The Li–Si binary system is characterized by exceptionally high Li insertion capacity. One Si atom can accommodate 4.4 Li atoms forming the alloy Li22Si5 with theoretical specific insertion capacity of 4200 mA h g−1. The analogous value for Li15Si4 is 3576 mA h g−1 (Ashuri et al. 2016; Huang et al. 2021). Such capacity values are the highest among all anode materials used in LIBs. Silicon outperforms in terms of capacity the conventionally used graphite (372 mA h g−1 for LiC6) (Liang et al. 2014). It is assumed conceptually that electrode capacity can be substantially increased when moving from classical intercalation reaction to alloying reaction (Ma et al. 2014). Lithium can react with Si‐forming Li22Si5 alloy, while graphite accommodates much less lithium forming LiC6. However, a challenge is disintegration of the electrode due to nearby 300–400% volume change during the lithiation–delithiation process, that is, the expansion–contraction cycles (Ashuri et al. 2016; Li et al. 2017; Zuo et al. 2017). This detrimental process reduces the intrinsic electrical conductivity and causes the interfacial instability, which leads to capacity fading.

The restoration of electric properties after the damage, so called self‐healing, is of paramount importance in energy‐storage device. This was stated in the BATTERY 2030+ Roadmap of the EC: “Establishing a new research community that includes a wide range of R&D disciplines to develop self‐healing functionalities for batteries” (Edström et al. 2020). Scientists are in search of effective means of surface engineering that buffers the volume changes, for instance, by graphene films, silicon‐carbon nanocomposites, Si nanowires, nanotubes, and other nanoparticles, solid core–shell structures, porous Si designs, controllable pores, or patterned silicon films on foreign metal substrates, etc. These efforts have been summarized in review articles (Liang et al. 2014; Ma et al. 2014; Ashuri et al. 2016; Li et al. 2017; Zuo et al. 2017; Huang et al. 2021).

Oxidized silicon (SiOx) has also been proposed as an alternative to pure Si. Such electrodes show a lower volume change when compared to pure silicon (Liu et al. 2019a). However, silicon monoxide possesses some problematic features, such as low intrinsic electrical conductivity, non‐negligible volume change, and low initial Coulombic efficiency (ICE) (Liu et al. 2019a). Thus, there is need to overcome these drawbacks in order to use the material for LIBs practically. The improvement of ICE could be achieved by pre‐litiation methods, for instance, using stabilized lithium metal powder (Huang et al. 2021). Various strategies have been proposed to reduce the SiO volume change: (i) SiO disproportionation into a fraction of Si nanocrystals by high‐energy milling (Hwa et al. 2013) or heating at 900 °C (Huang et al. 2021); (ii) synthesis of porous SiOx via chemical etching (Yu et al. 2014); (iii) composites of disproportionated Si–SiOx with carbon (Si–SiOx–C) (Yamada et al. 2011; Choi et al. 2012; Kim et al. 2013); (iv) core–shell porous silicon with nitrogen‐doped carbon layer (p‐Si/NC) (Xing et al. 2018); (v) silicon monoxide coated with N‐doped carbon (SiO‐NC) produced using N‐containing ionic liquid (Lee et al. 2013) and analogous system using pitch and melanine as carbon and nitrogen sources (Huang et al. 2021).

The materials based on porous silica (SiO2) have also been studied as an alternative to graphite anodes in LIBs. Porous silica has shown improved cycling stability; its discharge potential was similar to that of pure Si. Low Coulombic efficiency of SiO2 was modified by preparing a carbon‐coated C/SiO2 composition (Buga et al. 2021). The thin carbon layer of the composition diminished interfacial impedance of silica, whereby the capacity increases to 714 mA h g−1.

Despite intensive research, most of the proposed methods of the volume change buffering remain on a bench scale. Vital industrial applications are still pending.

2.2 Silicon for Energy Conversion – Photovoltaic Devices

2.2.1 Solar to Electricity

Solar energy could effectively act as a substitute to fossil fuels. The solar constant, which measures the quantity of solar energy to a square meter of the Earth surface, is estimated to be as high as 1367 W m−2, according to the World Energy Council. The constant translates into total amount of 3 400 000 EJ in a year (EJ – Exajoule, 1 EJ = 1018 J) (Breeze 2019). The solar radiation, which reaches the earth, covers more than 7000 times the world's energy needs, that is, the annual global primary energy consumption. Breeze estimates that if 0.1% of the solar energy was converted into electricity with 10% efficiency, it would provide around 10 000 GW energy, which exceeds the world energy needs (Breeze 2019). Solar energy provides the opportunity for decentralized supply of energy actually at any place around the globe. It can be accumulated (as a heat) and/or converted to electricity with no polluting emissions to the environment. Other advantages include no‐noise operation and unproblematic decommissioning.

Photovoltaic (PV) electricity generation is a rapidly expanding industry, which aims to accelerate the development of clean, sustainable, and efficient energy technologies with the scenarios to reach solar energy domination in the electricity market. In fact, creation of efficient and low‐cost devices of solar energy harvesting plays a determining role in political‐social landscape of twenty‐first century. There is a public awareness in the PV technology as a provider of clean, sustainable, and secure energy from the most abundant source, which is for free. It is not surprising, therefore, that world PV production has recently experienced close to exponential growth.

It is now close to 70 years since the invention of the first reasonably efficient silicon cells in 1954 (Green 2005). The first silicon‐based module for outdoor applications was produced at Bell Laboratories in 1955. The module was an assembly of 48 sub‐modules and attained 2% efficiency. The reader interested in historical development of silicon‐based PV devices is referred to M. Green's paper, which reviews improvement in energy conversion as well as prices of commercial modules over a 50‐year period (Green 2005).

The International Renewable Energy Agency (IRENA) comprising of about 170 member countries declares its mission “supporting countries in their transition to a sustainable energy future” (www.irena.org). IRENA is an intergovernmental organization – a driver of actions, which advance the transformation of the global energy system in the pursuit of low‐carbon economy. This organization promotes adoption of all forms of renewable energy. IRENA issued a global energy transformation paper, which aims to highlight the future of solar PV, including the key aspects, such as the deployment, investment, technology, grid integration, and socio‐economic aspects (IRENA 2019). IRENA's analysts foresee that solar PV by 2050 would represent the second‐largest power source behind wind power covering a quarter of the total electricity needs. The total growth of solar PV capacity is expected to rise from 480 GW produced in 2018 to 8519 GW projected by 2050. Asia (mostly China) has a dominating position in terms of installed capacity and will continue to dominate in the PV market reaching over 50% share in the world's production by 2050. The paper shows evolution of PV industry starting from mass production of solar cells in 1963 to 480 GW global capacity by the end of 2018. Important milestones in the provided timeline of PV development include the beginning of mass production of solar cells (1963), the first solar building (1973), the first solar plane flight around the world (2016), and the attaining of the global installed solar capacity of 480 GW (2018). Efforts have been made to beat the cost of photo‐electricity down, below that of the power generated from fossil or nuclear fuels. Cost reduction of photo‐electricity is permanently high on agenda of PV engineers and materials scientists. The IRENA's paper shows a three‐times reduction in the auction price during an eight‐year period, that is, from 241 USD/MWh in 2010 to 85 USD/MWh in 2018 (IRENA 2019).

Analysts estimated the PV capacity growth until 2050 (Figure 2.1) It is expected that installations will reach 2840 GW globally by 2030. The figure will rise up to 8519 GW by 2050 assuming utility‐scale (60–80%) and distributed rooftop (40–20%). This means 18 times higher production in 2050 when compared to the amount produced in 2018.

Vast majority of the solar cells are produced from silicon. Various structural forms of silicon are used in solar cell production: mono‐Si, poly‐Si, and amorphous. Crystalline silicon has about 95% share of the PV solar production (IRENA 2019; Fraunhofer 2019). Thanks to technological maturity and a fall in the price of raw material, silicon will keep its dominating position in the PV market. Increased outputs of the Si‐based panels also strengthen positions of silicon over other material. For instance, an average efficiency of multi‐crystalline panels increased from ∼13% in 2006 to ∼17% recently, and this positive tendency is expected to continue (IRENA 2019; Fraunhofer 2019). A study of the Massachusetts Institute of Technology (MIT) Energy Initiative has shown that silicon, as an abundant, non‐toxic, efficient and robust material, will maintain top position in the solar energy market at least for the next few decades (Schmalensee et al. 2015). Lowering of the cost of Si‐based PV devices remains very crucial.

Figure 2.1 PV capacity growth expected by 2050, as reported by IRENA analysts.

Source: IRENA 2019/IRENA/Public domain.

Production of PV module is a multistep process comprising such procedures as production of metallurgical‐grade silicon (MG‐Si), upgrading to solar‐grade silicon (SoG‐Si), ingot casting, and slicing. Traditional manufacturing includes high energy‐intensive production and purification of silicon, such as carbothermic silicon reduction from silica and purification to solar grade by the Siemens process. The cost also increases due to significant material loss when sawing the silicon ingot into wafers. Subsequent production of the PV cell from Si wafer also includes numerous steps.

First‐generation technologies (conventional solar cell architecture) explore crystalline silicon PV panels, whose evolution has covered a whole scope of technical maturity (Technology Readiness Levels, TRLs): basic research and technology development (R&D), demonstration, pilot lines, system launch and operations, enter into market and penetration, and market maturity. There are many other kinds of solar panel technologies. One modification becoming more common is the passivated emitter and rear contact (PERC) cells. Efficiency of the PERC cells is 6–12% higher when compared with conventional cells. The modification has an extra layer within the backside, which allows the reflection of some rays back into the cell. The advantages of the PERC cell includes the reduction in recombination, enhancement of light absorption, and higher internal reflectivity. From an economical point of view, it is important that the PERC cells, being a modification of the conventional cells, do not require great investment in acquiring additional equipment. These cells have recently become the new industry standard.

Figure 2.2 Major steps of solar cell production from silicon wafer according to Al back surface field (Al‐BSF) technology.

Source: Adopted from the technology route scheme applied in the “Soli Tek R&D” (Vilnius, Lithuania). Courtesy of Dr. J. Denafas.

Figure 2.2 demonstrates the major stages of the aluminum backside field (Al‐BSF) technology. The process starts from thorough wafer quality check: inspection of as‐cut wafer material, geometry, surface defects, and micro‐cracks. Bare silicon reflects more than 30% of the incident light (Singh et al. 2010; Szlufcik et al. 2005), thus, antireflection textures are formed in acidic (typically in hydrofluoric acid, HF) solutions. Further steps include p–n junction formation (e.g. by phosphorous diffusion), chemical edge isolation, removal of phosphorous silicate layer, passivation of silicon (e.g. by SiNx layer), and preparation of metallic contacts. The BSF formation provides a good contact between Si and Al with limited penetration of the AlSi alloy that is formed during the firing process, and it enables a significant reduction in the thickness of Al deposited in the rear contact. The technology of passivated emitter and rear cell (PERC) includes some additional steps: (i) edge isolation and polishing of the rear side, (ii) back side passivation by the AlOx layer, and (iii) its selective removal (e.g. by laser contact opening).

An innovative approach is metal wrap through (MWT) back‐contact solar cell technology. In this technology, the positive and negative electrodes are arranged on the rear side of the cell. The back contacts enable avoiding of application of busbars on the front side. Due to this, the shaded area is reduced, and the conversion efficiency can be increased by a few percent. The MWT processing is close to conventional fabrication sequences; thus, it does not require substantial additional investment. Photograph in Figure 2.3 demonstrates the examples of the solar cells produced in “Soli Tek R&D” (the limited liability company (LLC) in Vilnius, Lithuania) using MWT and Al‐BSF technologies.

It is obvious that each of the technological steps discussed above contributes to the cell cost and, in turn, to the cost of photo‐electricity. Thin‐film technologies provide the potential for reduction of the photo‐electricity production cost. Depositing thin silicon layers instead of thick individual cells, which have to be constructed, framed, and wired together, could reduce the consumption of material and energy.

Figure 2.3 Solar cells produced according to Al‐BSF technology (MWT cell on the left side and the other two are Al‐BSF samples). The photograph also shows the production unit in the “Soli Tek R&D” (Vilnius, Lithuania).

Source: Courtesy of Dr. J. Denafas.

Thin films provide opportunity to produce large complete modules with improved appearance for many visual applications (e.g. architectural glass, liquid crystal displays []). Mercaldo et al. presented analysis of architectural issues of thin‐film PVs, in particular, those related to applications of transparent and conductive oxides and films of silicon solar cells (Mercaldo et al. 2009). Modern architectural theories look at buildings as living organisms that should be able – during their life – to generate the energy needed to be in operation. Thin‐film technology is suitable to satisfy such advanced architectural theory. It is a challenging goal to form thin silicon films with effective light absorbance on technically important metallic substrates such as steel, aluminum alloys, and copper. This makes it possible to combine structural materials with silicon for facing panels on buildings, which could be used to harness solar energy. Putting the silicon on thin metallic foils electrochemically would expand the application fields to a wide range of subjects with flexible geometries as well as on the metal‐coated glass. Electrodeposition of thin Si layers, including the electrochemical formation of a p–n junction, which could replace multiple processes of Si wafer treatment, will be discussed in Chapter 6.

The main disadvantage of silicon is its poor intrinsic ability to absorb the light – more than 30% of incident light is reflected if silicon surface is not specifically textured or coated by antireflection coatings. Also, high‐processing complexity of silicon significantly contributes to the cost of PV devices. Light‐harvesting efficiency of the Si‐based photoelectrochemical (PEC) cells can be achieved by creating silicon nano‐micro architectures, which enhance light absorbance. To this end, various surface engineering techniques have been applied, such as femtosecond laser engineering, annealing in vacuum, coating by atomic layer deposition (ALD) as well as various etchings: chemical, reactive ion, inductively coupled plasma, and cryogenic. These methods, however, are costly and technically sophisticated. As a result, the cost‐competitive solutions for industrial‐scale solar energy applications are limited. Furthermore, etchings usually involve toxic chemicals, such as hydrofluoric acid, and rather expensive catalysts, such as gold.

Electrochemical methods of silicon surface engineering are very attractive due to environmental friendliness, technical simplicity, and cost‐effectiveness. Various methods of electrochemical Si surface texturing in aqueous and molten salt electrolytes will be surveyed in Chapters 14 and 15.

Non–silicon based thin‐film technologies have been intensively investigated as alternatives to silicon PVs. Particularly, there have been great expectations from perovskite‐based cells. This type refers to a broad class of ABX3 structures, which originate from the mineral calcium titanium oxide (CaTiO3). The component A can be either organic ions, such as methylamonium (MA, CH3NH3+) or formamidinium (FA, CH(NH2)2+), or inorganic ions, such as cesium (Cs+) or rubidium (Rb+). The component B refers to a smaller divalent cation, in most cases lead (Pb2+) or sometimes tin (Sn2+). The component X refers to halogen anions, typically iodide (I−), bromide (Br−), or chloride (Cl−). Such ABX3 structures show a high light absorption under visible light as well as an extremely long charge diffusion length. These features presuppose the high PV performance of the perovskite‐based cells. More than a dozen of firms from the USA, South Korea, China, UK, Poland, Japan Switzerland, and Netherlands strived to commercialize the perovskite solar cells (Extance 2019). The challenge to overcome, perovskite‐cells have limited lifetime due to sensitivity to air and moisture. Another issue lies in the structural transformations and performance fading when the crystals warm up and cools down. To be competitive, the operational durability of perovskite cell should be comparable with the durability of the silicon‐based cells with a 25‐year warranty. This is “now looking increasingly unlikely” according to Martyn Green from the University of New South Wales in Sydney, Australia (Extance 2019). Another limitation lies in scaling up matters: efficiency of the cells does not replicate when their size increased. Increased efficiency up to theoretical limit of 45% can be achieved in tandem cells, that is, a system of perovskite and silicon layers. Such strategy is under active investigation (Extance 2019).

The rapid expansion of PV resulted in an increasing amount of end‐of‐life solar panels, which poses a hazard for future PV development. It is a challenging goal to find solutions for PV modules recycling. An innovative opportunity has recently proposed a group consisting of 25 researchers from research establishments in Singapore, China, and Japan (Cao et al. 2022). Their idea lies in the conversion of PV waste into feedstock for thermoelectrics (Figure 2.2). Such conversion is in line with the principles of circular economy: elimination of waste and pollution as well as circulation of products and materials.

Figure 2.4 outlines the major conversion steps of Si‐based solar cells into Si suitable for thermoelectric application. Polycrystalline Si solar cells were used as a feedstock. Aluminum and silver were removed from the cell by leaching in HCl, HNO3, and H3PO4. The dried cells were pulverized into fine powder by a ball milling using the balls made of tungsten carbide. This was done in an argon atmosphere to avoid silicon oxidation by air. Furthermore, dopants (P and Ge) were added and the powders were homogenized by ball milling. Finally, the powders were consolidated by spark plasma sintering at 1150 °C, and the specimens were cut into desired shapes. The strategy provided low‐cost and environmentally friendly recycling of silicon for circular economy applications. The economic issues of the proposed process relate to minimizing the cost associated with the application of Ge and P dopants as well as the ball milling and spark plasma sintering. Cost minimization is expected when the technology is used on a large‐scale application (economy of scale).

2.2.2 Solar‐to‐Chemical Conversion

Collection of the solar energy in chemical form helps to conserve energy and makes it sustainable. Electrochemical solar energy capturing in chemical bonds (e.g. hydrogen production from water) mimics the process of photosynthesis in plants. Production of hydrogen in combination with solar energy occurs with near‐zero greenhouse gas emissions. When captured in chemical form, the energy can be converted to power in fuel cells with specifically high efficiency (up to 85%) or in combustion engines. It is also possible to combine hydrogen generation with a fuel cell to generate electricity and replace the water that has been consumed. Electrochemical hydrogen generation means the usage of electrons as absolutely clean agents, albeit at slightly lower efficiency but without pollutant emissions. The electrochemical process offers such advantages as high purity, flexibility, supply on‐site and on‐demand as well as fast start‐up and shutdown opportunities. Of particular interest is solar‐to‐fuel process, which occurs directly (wireless) on the photo‐electrodes.

Solar‐to‐chemical conversion offers important opportunity for solar energy accumulation and transmission. In this respect, the most attractive process is hydrogen generation by PEC splitting of water. This process has recently attracted the attention of researchers due to hydrogen production with a very low carbon footprint. Almost zero greenhouse emission is achieved when hydrogen is generated by splitting water using renewable energy sources such as solar, wind, hydro, or ocean. However, renewable energy is irregular and sporadic in nature; thus, hydrogen generation is applicable for energy storage and load management. Technologies demonstrate the connection of the electrolyzers to electricity and gas grids (Bertucioli et al. 2014). With the power‐to‐gas approach, excess electricity generated from renewable energy is converted into hydrogen, stored, and when needed, it is reconverted into electricity. Certainly, the produced hydrogen can also be used as a feedstock in chemical industry applications. The hydrogen can also be converted to hydrocarbon fuels, for instance, methane. An international review was published about the power‐to‐gas pilot plants for stationary applications (Gahleitner 2012). The review evaluated 41 already realized and 7 planned power‐to‐gas large‐scale systems.

Figure 2.4 The process of conversion of the waste of solar cells into thermoelectrics: (a) – solar cells; (b) – recovering of Al and Ag; (c) – pulverizing of Si in a ball milling machine in Ar atmosphere; (d) – doping of Si powder with phosphorous and germanium; (e) – final pellets for thermoelectrics.

Source: Reproduced from (Cao et al. 2022) with permission from the publisher, ©Wiley, 2021.

Silicon, as a photosensitive, chemically inert, low‐cost, and robust material, is attractive in PEC applications. Effectiveness of solar energy conversion to chemical bonds depends largely on the ability of a PEC device to absorb the light. Numerous efforts have been undertaken to increase silicon absorbance by increasing its actual surface area – the roughness at micro and nano scale. These techniques include femtosecond laser engineering with subsequent sample etching, deposition of transparent conducting oxides using ALD, reactive ion etching, chemical etching using procures metal catalysts, inductively coupled plasma and cryogenic etching, vapor–liquid–solid reactions, etc. These techniques are surveyed in detail in Section 14.2. Electrochemical methods of silicon surface engineering in aqueous and molten salt electrolytes are discussed in Chapters 14 and 15.

Of great interest is nanoporous silicon, known as black silicon, which effectively absorbs light in a wide range of wavelengths due to internal trapping by porous structures. Black silicon photocathode for hydrogen production has been reported (Oh et al. 2011) and patented (Oh and Branz 2014). The authors showed that nanostructured black silicon significantly accelerates hydrogen production with reduced need for surfactants. Techniques to fabricate black silicon, such as laser texturization, plasma treatment, metal‐assisted etching, or cryogenic deep reactive ion etching are discussed in Section 14.3. These techniques, however, are costly. There is great demand for cost‐effective and green technologies of black silicon mass production. More cost‐effective methods based on electrochemical Si surface texturing in aqueous and molten salt electrolytes are surveyed in Chapters 14 and 15.