Handbook of Transparent Conductors
.
David S. GinleyEditor
Hideo Hosono l David C. PaineAssociate Editors
Handbook of TransparentConductors
EditorDr. David S. GinleyNRELPhotovoltaics & Electronic MaterialsCenter & Basic Sciences Ctr.Cole Blvd. 161780401-3393 Golden [email protected]
Associate EditorsDr. Hideo HosonoTokyo Institute of TechnologyMaterials & Structures Lab.Nagatsuta 4259226-8503 [email protected]
Prof. David C. PaineBrown UniversityDivision Engineering610 Barus & HolleyHope Street 18202912 Providence Rhode [email protected]
ISBN 978-1-4419-1637-2 e-ISBN 978-1-4419-1638-9DOI 10.1007/978-1-4419-1638-9Springer New York Heidelberg Dordrecht London
Library of Congress Control Number: 2010935196
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Preface
Transparent Conducting Oxides (TCOs) are a unique class of materials that exhibit
both transparency and electronic conductivity simultaneously. These materials
have found wide spread use in displays, photovoltaics, low-e windows, and flexible
electronics. In many of these applications, the TCO’s, are enabling in their role as
transparent contacts. However, increasingly, the demands required extend beyond
the combination of conductivity and transparency, where indeed higher perfor-
mance is needed, but now include work function, morphology, processing and
patterning requirements, long term stability, lower cost and elemental abundance/
green materials. As these needs have begun to emerge over the last 5 years they
have stimulated a dramatic resurgence of research in the field leading to many new
materials and processes. Overall it is the purpose of this book to provide both a
snapshot of the new and enabling work in the field and to provide some indications
of what might be coming next. We note that now the field of Transparent Con-
ductors (TC’s) includes not only conventional TCOs but also metal and carbon
nano-composites, grapheme and polymer based TC materials. While the book
primarily focuses on the TCOs some comparisons are made to the newer materials.
To do this we have assembled a group of authors representing most of the leading
groups in the field.
Historically, TCOs were limited primarily to tin oxide with fluorine doping, zinc
oxide with aluminum doping and Indium tin oxide. Over the past 5–10 years the
field has exploded to include a vastly increased number of n-type materials and to
add in a class of new p-type materials. In addition, the historically held view that
crystalline materials have superior properties, has been challenged by an emergence
of new amorphous TCOs that have properties as good as or better than their
crystalline counterparts. These materials have led to the development of amorphous
oxide transistors which offer the advantage of low temperature processing and the
promise of flexible electronics on polymer substrates. In their role as a channel
material in thin film transistor structures, TCO’s with controlled carrier densities
are often termed transparent oxide semiconductors (TOS) since their key properties
v
may lie in the limited to non-conductive regime. To capture this diversity of
materials, processing and applications, we have organized the book as follows.
Chapter 1 introduces TCOs and covers the historic materials and their properties
and uses this background to put some of the newly emergent materials into a
technological context.Chapter 2 presents a detailed discussion of the basic electronicstructures of TCO materials emphasizing the key properties which give them their
unique properties. Chapter 3 then provides an overview of methods for the measure-
ment and interpretation of transport properties in TCOs based of the Drude model
with a focus on the method of four coefficients for the determination of critical
parameters such as carrier type, mobility and scattering mechanisms in multinary
oxides. Chapter 4 covers the basic physics of, and practical tools for, the characteri-zation of important TCO parameters including atomic structure, optical properties,
electrical transport, work function and other properties that must be better understood
as TCO’s become used in novel applications such as thin film transistors. Chapter 5presents a picture of the current In based TCOs covering both the traditional InSnOx
materials which have been the gold standard of TCOs and the emerging amorphous
materials. Chapter 6 presents an overview of the tin oxide based TCO materials.
While historically these materials have been produced in exceptionally large areas
newwork has begun to improve their properties.Chapter 7 reviews the state of the artfor ZnO. This material, due to its natural abundance and the ease with which it can be
deposited via both physical and chemical routes, has important applications both as a
traditional transparent contact and great potential as an active optoelectronic
material. To realize this potential, a great deal of work has been done to identify
new approaches to both n and p-type doping. Chapter 8 looks at the rapidly expand-ing class of multi-cation TCO materials. Recent work shows that much higher
performance can be achieved in some TCO materials by the addition of elements
that serve to modify defect and electronic band structure. This ability to create multi-
component TCO materials without significantly degrading key transport parameters
(e.g., carrier mobility) is a characteristic of the TCO class of materials. Chapter 9looks at the theoretical framework used to describe the band structures of both n- and
p-type oxide materials and includes a discussion of emerging non-oxide based
transparent conductors. This fundamental background provides the basis for a dis-
cussion on considerations for the discovery of new high performance transparent
conducting materials. Chapter 10 considers new materials that have emerged in the
transparent conductor field over the last few years. Historically, the set of elements
whose oxides provide useful TCO properties have been constrained to single or
mixed oxides of In, Ga, Zn, Sn, and Cd. This chapter discusses how the pallet of
useful elements for TCO applications has grown to open whole new classes
of materials.
The second half of the book begins to address the applications of TCOs and how
new materials can significantly change the paradigm for a technology or be
enabling for another. Chapter 11 discusses the application of TCO materials for
solar energy and energy efficiency applications. In fact, though a key focus is the
active devices like PV, the reality is that in terms of energy efficiency, the use of
TCO’s in energy conservation applications are greater in the near term than
vi Preface
production. In any case it needs to be looked in an integrated way which is the
theme of the chapter. Chapter 12 considers the idea that TCOs need not be planar
films but that in many cases the films can be enabling or integrated into a more
complex hybrid (organic/inorganic for example) device by having a nanostructured
morphology. Enabling this is a broad set of solution and PVD approaches to
creating controlled nanostructures in TCO materials from texture to nano-rods
etc. Chapter 13 explores the application of amorphous TCOs and their semicon-
ducting/insulating TOS counterparts to develop new flexible and transparent elec-
tronics for displays and more. The demonstration of TOS materials as a channel
materials in thin film transistor applications has dramatically altered the potential
for amorphous oxides in an increasingly diverse set of technologies. Chapter 14considers the potential for making true oxide based p/n junctions to realize active
devices that are entirely based on TCO/TOS materials. The ability to make such
junctions expands the potential for oxide based electronics including transparent
electronics, oxide based solar cells and LED/lasers. Finally, Chap. 15 discusses thescaling of TCO materials to large area industrial processing. This is a key issue as it
addresses some of the critical properties dependence on process parameters.
We note that there is increasing interest in solution processed transparent con-
ductors consisting of nanostructures of carbon (nanotubes), oxides (nanorods i.e.,
ZnO) and metals (such as Ag nanorods). However, thus far although they are very
interesting, these materials still have conductivities approximately an order of
magnitude below those for high performance TCOs. Over the next few years we
expect these materials will become increasingly important perhaps in combination
with TCO materials. Their inclusion in this volume at present is, however, beyond
the intended scope of this publication.
This book presents a picture of an important class of materials that has, in recent
years, drawn increasing interest for applications in active devices and as a critical
component in any structure that requires both electrical connectivity and optical
transparency. Despite their technological importance and relatively long history of
use, our understanding of the existing set of TCO materials are only now receiving
the kind of combined fundamental/experimental materials research attention that
will inevitably lead to new materials discoveries and novel applications. Overall, it
is clear that transparent conductive oxides and transparent conductors are a vibrant
field that is advancing rapidly across an ever broadening spectrum of applications.
We hope this book will provide a valuable reference for those interested in the
topic and stimulate additional development of new TCO materials and their
applications.
David Ginley (Editor)
Hideo Hosono and David Paine
(Associate Editors)
Preface vii
.
Contents
1 Transparent Conductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
David S. Ginley and John D. Perkins
2 Electronic Structure of Transparent Conducting Oxides . . . . . . . . . . . . 27
J. Robertson and B. Falabretti
3 Modeling, Characterization, and Properties of Transparent
Conducting Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Timothy J. Coutts, David L. Young, and Timothy A. Gessert
4 Characterization of TCO Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
David C. Paine, Burag Yaglioglu, and Joseph Berry
5 In Based TCOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Yuzo Shigesato
6 Transparent Conducting Oxides Based on Tin Oxide . . . . . . . . . . . . . . . 171
Robert Kykyneshi, Jin Zeng, and David P. Cann
7 Transparent Conductive Zinc Oxide and Its Derivatives . . . . . . . . . . . . 193
Klaus Ellmer
8 Ternary and Multinary Materials: Crystal/Defect
Structure–Property Relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
Thomas O. Mason, Steven P. Harvey, and Kenneth R. Poeppelmeier
9 Chemistry of Band Structure Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
Art Sleight
ix
10 Non-conventional Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
Hideo Hosono
11 Applications of Transparent Conductors to Solar Energy
and Energy Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353
Claes G. Granqvist
12 Nanostructured TCOs (ZnO, TiO2, and Beyond) . . . . . . . . . . . . . . . . . . . . 425
Dana C. Olson and David S. Ginley
13 Transparent Amorphous Oxide Semiconductors for Flexible
Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
Hideo Hosono
14 Junctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489
Hiromichi Ohta
15 Process Technology and Industrial Processes . . . . . . . . . . . . . . . . . . . . . . . . 507
Mamoru Mizuhashi
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527
x Contents
Contributors
Joseph Berry National Renewable Energy Laboratory, Mail-stop 3211, 1617
Cole Blvd, Golden, CO 80401, USA
David P. Cann Associate Professor of Materials Science, Department of
Mechanical Engineering, 303D Dearborn Hall, Oregon State University, Corvallis,
OR 97331, USA, [email protected]
Dr. Timoth J. Coutts Research Fellow Emertus, National Renewable Energy
Laboratory, 1617 Cole Blvd. Golden, CO 80401, USA
Dr. Klaus Ellmer Dept. solar fuels, Helmholtz-Zentrum fur Materialien und
Energie Berlin GmbH, Hahn-Meitner-Platz 1, 14109, Berlin, Germany, ellmer@
helmholtz-berlin.de
Barbara Falabretti Department of Engineering, University of Cambridge, Trum-
pington Street, Cambridge, CB2 1PZ, UK, [email protected]
Dr. Timoth A. Gessert Group Manager Thin Film Photovoltaics, NREL National
Center For Photovoltaics, 1617 Cole Blvd, Golden, CO 80401, USA, Tim.
David S. Ginley Research Fellow Group Manager Process Technology and
Advanced Concepts, NREL SERF W102, 15313 Denver West Pkwy, Golden, CO
80401, USA, [email protected]
Claes G. Granqvist Professor Solid State Physics, Department of Engineering
Sciences, The Angstrom Laboratory Uppsala University, Uppsala, SE-75121,
Sweden, [email protected]
Steven P. Harvey Institute of Physical Chemistry, RWTH Aachen University,
Landoltweg 2, Aachen D-52056, Germany
xi
Hideo Hosono Professor at Frontier Research Center & Materials and Structures
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan, [email protected]
Robert Kykyneshi Department of Materials Science, Oregon State University,
Corvallis, OR 97331, USA
Thomas O. Mason Department of Materials Science and Engineering North-
western University, Materials Research Science and Engineering Center, Evanston,
IL 60208, USA, [email protected]
Dr. Mamoru Mizuhashi School of Science and Engineering, Aoyama Gakuin
University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan,
Dr. Hiromichi Ohta Associate Professor Department of Molecular, Design &
Engineering, Graduate School of Engineering Nagoya University, Furo-cyo,
Chikusa-ku, Nagoya 464-8603, Japan
Dana C. Olson National Renewable Energy Laboratory, Mail Stop 3211 National
Center for Photovoltaics, 1617 Cole Blvd. Golden, CO 80401-3393, USA, dana.
David C. Paine Professor of Engineering Brown University, Division of
Engineering, Box D, Providence, RI 02912, USA, [email protected]
Dr. John D. Perkins National Renewable Energy Laboratory, Mail Stop 3211
National Center for Photovoltaics, 1617 Cole Blvd. Golden, CO 80401, USA
Kenneth R. Poppelmeier Professor of Chemistry Northwestern University,
Room: Tech GG35 Clark Street, Evanston, IL 60208, USA, [email protected]
John Robertson Department of Engineering University of Cambridge, Trum-
pington Street, Cambridge, CB2 1PZ, UK
Yuzo Shigesato Professor Graduate School of Science and Engineering Aoyama
Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8558, Japan,
Art Sleight Chemistry Department Oregon State University, 339 Weniger Hall,
Corvallis, OR 97331, USA, [email protected]
Burag Yaglioglu Plastic Logic Limited, 296 Cambridge Science Park, Milton
Road, Cambridge, CB4 0WD, UK, [email protected]
xii Contributors
David L. Young Senior Scientist National Renewable Energy Laboratory
National Center for Photovoltaics, Silicon Materials and Devices, 1617 Cole
Blvd. M.S. 3219, Golden, CO 80401, USA, [email protected]
Jin Zeng Materials Science Oregon State University, Corvallis, OR 97331, USA,
Contributors xiii
.
Chapter 1
Transparent Conductors
David S. Ginley and John D. Perkins
1.1 Basics
Over the last 6 years the field of transparent conducting oxides has had a dramatic
increase in interest with a huge influx in the number of active groups and the diversity
of materials and approaches. Why? There are a number of primary motivators for
this, some of the most compelling are the increase in portable electronics, displays,
flexible electronics, multi-functional windows, solar cells and, most recently, tran-
sistors. The diverse nature of the materials integrated into these devices, including
semiconductors, molecular and polymer organics, ceramics, glass, metal and plastic,
have necessitated the need for TCO materials with new performance, processibility
and even morphology. The remarkable applications dependent on these materials
have continued to make sweeping strides. These include the advent of larger flat-
screen high-definition televisions (HDTVs including LCD, Plasma and OLED based
displays), larger and higher-resolution flat screens for portable computers, the
increasing importance of energy-efficient low-emittance (“low-e”), solar control
and electrochromic windows, a dramatic increase in the manufacturing of thin film
photovoltaics (PV), the advent of oxide based transistors and transparent electronics
as well as a plethora of new hand-held, flexible and smart devices, all with smart
displays. Driven by the increased importance and potential opportunities for TCO
materials in these and other applications, there has been increasing activity in the
science of these materials. This has resulted in new n-type materials, the synthesis
of p-type materials and novel composite TCO materials as well as an increased set of
theoretical and modeling tools for understanding and predicting the behavior of
TCOs. Considering that, over the last 20 years, much of the materials work on
TCOs has been empirical with a focus on minor variants of ZnO, In2O3 and SnO2,
it is quite remarkable how dramatically this field has grown recently in both basic and
D.S. Ginley (*)
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA
e-mail: [email protected]
D.S. Ginley et al. (eds.), Handbook of Transparent Conductors,DOI 10.1007/978-1-4419-1638-9_1, # Springer ScienceþBusiness Media, LLC 2010
1
applied science. This is reflected in the thousands of papers published over the last 5
years. This may be a function of not only the need to achieve higher performance
levels for these devices, but also of the increasing importance of transition-metal-
based oxides in devices in a broader sense including ferroelectric, piezoelectric,
thermoelectric, gas sensing superconducting and other materials applications. An
important realization is that despite a very long history in the application of TCOs,
there is still not a complete theoretical understanding of the materials nor an ability to
reliably predict the properties of newmaterials. This has been emphasized recently by
the emergence of amorphous mixed metal oxide TCO materials, typified by amor-
phous In-Zn-O [1, 2], where even the basic transport physics is not understood.
Broadly, this book will summarize the current state of the art across a broad range
of TCO science including materials, theory, thin film deposition and applications.
At this point, a brief summary of the relevant opto-electronic properties of
conventional TCO materials will provide a useful baseline for comparison
and discussion. The left panel in Fig. 1.1 shows optical reflection, transmission
and absorption spectra for a typical commercial ZnO TCO on glass which, collec-
tively, show the key spectral features of a TCO material. First, the material is quite
transparent, �80%, in the visible portion of the spectrum, 400–700 nm. Across this
spectral region where the sample is transparent, oscillations due to thin film
interference effects can be seen in both the transmission and reflection spectra.
The short wavelength cut off in the transmission at �300 nm is due to the
fundamental band gap excitation from the valence band to the conduction band as
depicted in the right panel of Fig. 1.1. The gradual long wavelength decrease in the
transmission starting at �1,000 nm and the corresponding increase in the reflection
starting at �1,500 nm are due to collective oscillations of conduction band elec-
trons known as plasma oscillations or plasmons for short. There can also be
substantial absorption due to these plasma oscillations as is the case for this
particular sample with the maximum absorption occurring at the characteristic
plasma wavelength, lP, as shown in the figure. As the number of electrons in the
Fig. 1.1 Optical spectra of typical (ZnO) transparent conductor (left side) and schematic electronic
structure of conventional TCO materials (right side)
2 D.S. Ginley and J.D. Perkins
conduction band, N, is increased, such as by substitutional doping, the plasma
wavelength shifts to shorter wavelengths as lP / 1� ffiffiffiffi
Np
which creates a funda-
mental tradeoff between conductivity and the long wavelength transparency limit.
At very high electron concentrations, this can even decrease the visible wavelength
transparency. The left panel in Fig. 1.2 shows how the infrared transparency
increases for SnO2 TCOs as the sheet resistance is increased from 5 to 100 O/sq.Even though both of these SnO2 samples have similar visible wavelength transpar-
ency, the 5 O/sq. sample would be unusable as a transparent conductor for telecom
applications at 1,500 nm or for giving a high solar throughput. The right panel in
Fig. 1.2 shows how the plasma wavelength varies with the dopant level in Ti doped
In2O3 and hence how TCO properties can be tuned [3]. Collectively, the examples
shown in Figs. 1.1 and 1.2 should make it clear that there is no such thing as a single
“best” TCO and that TCOs must be tailored to the constraints of the specific
application.
The current use of TCOs in industry is dominated by just a few materials. We
will present an overview of the current state of the field in order to help the reader
develop an appreciation for the size and demands of the industry as well as the need
for new materials. At present, the dominant markets for TCOs are in architectural
window applications and flat panel displays (FPDs), followed closely by the rapidly
growing photovoltaics industry.
The architectural use of TCOs is predominately for energy efficient windows.
Fluorine-doped tin oxide (SnO2:F), deposited by a chemical vapor deposition
(CVD) process, is the TCO most often used in this application [4, 5]. Metal-oxide/
Ag/Metal-oxide stacks such as ZnO/Ag/ZnO are also common [6, 7]. Windows with
tin oxide coatings are efficient in preventing radiative heat loss, due to their low
thermal emittance, �0.15, compared to �0.84 for uncoated glass [8]. Such “low-e”
windows are ideal for use in cold or moderate climates. In addition, pyrolytic tin
oxide is also used for heated glass freezer doors in commercial use. In this applica-
tion, the doors can be defrosted by passing a small current through the slightly
Fig. 1.2 Optical spectra of TCO materials: SnO2 (left side) and Ti-doped In2O3 (right side) fromvan Hest et al. [3]
1 Transparent Conductors 3
resistive TCO coating. In 2007, the annual demand for low-e coated glass in Europe
was 60 � 106 m2 (�23 square miles) and this is projected to increase to about
100 � 106 m2 in a few years [9]. Rapid growth in China is also increasing the
demand for low-e glass with a projected demand of 97 � 106 m2 in 2010 and a
projected domestic production capacity of 50 � 106 m2 in 2010, up significantly
from 3 � 106 m2 in 2004 [10]. Added to these demands for low-e coated glass are
increasing amounts of TCO coated glass for solar control applications [5, 9] as well
as the increasing amounts used in displays and PVs [11]. Multilayer stacks as
referred to above represent an increasing market for TCOs in the conventional
low-e applications but also in an expanding automotive and specialty market.
Pyrolytic tin oxide is also used in PV modules, touch screens, and plasma
displays. However, for the majority of flat panel display (FPD) applications,
crystalline tin doped indium oxide (indium-tin-oxide, ITO) and, more recently,
amorphous In-Zn-O (IZO) are the TCOs used most often in these higher value
added products. In FPDs, the primary function of the TCO is as a transparent
electrode. However, often, the TCO will also have additional functions such as an
antistatic electromagnetic interference shields or as an electric heater.
The annual volume of FPDs produced, and hence the volume of TCO (ITO)
coatings required, continues to grow rapidly. New analysis from Frost & Sullivan
(http://www.electronics.frost.com) World Flat Panel Display Markets, reveals that
the FPD market earned revenues of $65.25 billion in 2005 and estimates this to
roughly double to $125.32 billion in 2012 as shown in Fig. 1.3. This by far exceeds
the initial market projections and arises from the rapid worldwide adoption of flat
panel displays in place of conventional vacuum tube display.
Recently, a significant fraction of the FPD industry has begun to use amorphous
indium-zinc-oxide (IZO) in place of ITO as the main TCO. Amorphous IZO has the
advantages of being amorphous along with room temperature deposition, easy
patterning and improved thermal stability relative to ITO. While it is currently
not known exactly what fraction of the display industry uses IZO, it is estimated to
be between 30 and 40%.
Fig. 1.3 TCO markets vs.
year from http://www.
electronics.frost.com
4 D.S. Ginley and J.D. Perkins
Currently, the third, and fastest growing segment, of the TCO market is for
photovoltaic (PV) cells, predominately driven by crystalline and polycrystalline
silicon solar cells which represent more than 93% of the PV market at present.
Even for PV cells based on bulk Si material TCOs are important. For example, the
two-sided Sanyo HIT cell (Heterojunction with Intrinsic Thin-layer) shown in
Fig. 1.4 actually uses TCO layers on both the front and back. In addition, thin
film photovoltaics based on amorphous-Si (a-Si), CdTe and Cu(In,Ga)Se2 (CIGS)
absorber layers all depend on one or more layers of a high performance TCO as
shown in Figs. 1.4 and 1.5. This thin film PV application represents a growing and
important market for TCO materials. Figure 1.6 shows the projected market growth
of the PV industry based on various growth rates ranging from 15 to 30% per year.
In actuality, the PV market is currently (2007) growing at over 50% per year with
no sign of slowing down.
There are potentially two other areas where there could be a rapid increase in
TCO use on a large scale. These are electrochromic windows and oxide based thin
film transistors (TFTs). In the former, this technology is either all metal oxide based
or organic/inorganic based. In either case, there is a key reliance on TCOs as the
transparent electrodes. The second area, oxide based TFTs, is embryonic at present,
but could rapidly mature [12]. There has been a longstanding search for higher
mobility transistors for displays and flexible electronics and recently amorph-
ous metal oxide based transistors have emerged as a promising alternative to
the conventional a-Si TFTs. Here, the key driver is higher electron mobility,
m � 10–40 cm2/V s whereas m < 1 cm2/V s for a-Si as well as processibility,
easy integration into flexible electronics due to low temperature (room temperature)
processing, and the potential mechanical advantages of an amorphous material. In
addition to these major applications, there is also use of TCOs in the emerging areas
of opto-electronic components, other electrochromic devices (automobile windows,
mirrors, sun roofs, displays etc.) and flexible electronics. Together, all these needs
Fig. 1.4 Typical configuration for (a) the Sanyo HIT cell and (b) for an amorphous-Si nip cell
1 Transparent Conductors 5
provide the driving impetus for the increasing importance of TCO materials both
technologically and economically across a wide variety of applications.
Cleary, the driving forces for new TCOs are quite diverse and are being driven
by a large number of concerns. Some of these are summarized in Table 1.1. All or
Fig. 1.5 CIGS (top) and CdTe (bottom) PV structures seen in cross-section using SEM (left side ofpanel) and viewed schematically (right side of panel)
Fig. 1.6 Projected growth of PV markets
6 D.S. Ginley and J.D. Perkins
any of these criteria may be important for a particular TCO application. This
diverse matrix of needs may also necessitate new individual TCO materials and/
or the development of new multilayer stacks incorporating TCOs. Table 1.2 gives a
partial list of some of the properties and some of the representative TCOs based on
the conventional materials as well as a column showing the role of some of the new
materials. This, in part, is driving the exploration of new and improved materials as
well as the increasing emphasis on improved processibility and environmental
properties.
Table 1.1 Properties relevant to TCO materials and applications
General criteria Opto-electronic criteria Processing criteria
Green materials Visible transparency Deposition temperatures and conditions
Green processing Conductivity Annealing stability
Cost Carrier concentration Compatibility with vacuum or
non-vacuum processing
Availability Mobility Chemical stability
Ease of application Infrared transparency Etchability, patterning and electrical
contacts
High mobility Interfacial chemistry and surface states
High mobility with low carrier
concentration
Ionic diffusion properties
Suitability to flexible electronics Temperature sensitivity
Work function Atmospheric sensitivity
Table 1.2 TCO materials for various applicationsa
Property application Material
Simple Binary Ternary
Highest transparency ZnO:F Cd2SnO4
Highest conductivity In2O3:Sn
Highest plasma frequency In2O3:Sn
Highest work function SnO2:F ZnSnO3 Zn0.45In0.88Sn0.66O3
Lowest work function ZnO:F
Best thermal stability SnO2:F Cd2SnO4
Best mechanical durability SnO2:F
Best chemical durability SnO2:F
Easiest to etch ZnO:F
Best resistance to H plasmas ZnO:F
Lowest deposition temperature In2O3:Sn
ZnO:B
a-InZnO
Least toxic ZnO:F, SnO2:F
Lowest cost SnO2:F
TFT channel layer ZnO a-InZnO, a-ZnSnO InGaO3(ZnO)5,
a-InGaZnO
Highest mobility CdO, In2O3:Ti
In2O3:Mo
Resistance to water SnO2:FaAdapted from Gordon [4]
1 Transparent Conductors 7
Thus, in the last few years, there has been an increasing realization that the
conventional TCO material set of substitutionally doped crystalline SnO2, ZnO and
In2O3 materials are no longer sufficient to meet the needs of all TCO applications.
As is the case in many technological areas, this is a consequence of the acknowl-
edgment of the limitations of the existing materials as well as a realization that new
materials can open the way to new and improved devices. Amplifying this is the
need for TCO materials with certain specific properties other than just high trans-
parency and conductivity as applications are emerging where work function,
surface roughness, nano-structure, thermal and chemical reactivity/diffusivity or
ease of patterning are critical TCO functionalities.
1.2 History of Transparent Conducting Oxides
The list of TCO materials in Table 1.3, though not fully inclusive, clearly shows the
wide diversity of current TCO materials. As one can see, there was a dramatic and
on-going increase in the number of TCO materials starting after 1995. This rate of
materials discovery has continued with more new materials every year. Further-
more, transparent conductors now also include thin metal films, sulphides, sele-
nides, nitrides, nanotube composites, graphenes and polymers in additional to the
traditional metal oxide based TCOs.
As we have seen, there are numerous technological drivers for the development
of new and improved TCOs and we have also seen that worldwide the field has
expanded dramatically in the last 10 years with the number of researchers and their
efforts increasing substantially every year. There are also global societal drivers for
the development of improved TCOs due to their critical role in the development of
various energy related technologies. For example, Fig. 1.7, which shows the world
energy consumption by region, makes very clear the rapidly increasing energy use
worldwide [13]. Furthermore, as the undeveloped world rapidly becomes more
technological with the associated increasing energy needs and vehicular traffic, the
total global energy consumption will continue to rise rapidly. One clear conse-
quence of this is that global atmospheric CO2 levels which are a major cause of
global warming are increasing dramatically. In Fig. 1.8, it is clear that the present
on-going rapid increase in CO2, which appears instantaneous on the 45,000 year
time span of the graph, is significantly beyond any previous short term event in the
history spanned by the figure [14–18]. These two interrelated facts are increasingly
leading to a view that society must drive towards a truly sustainable life style. To
achieve this, sustainability must be a consideration in all aspects of a technology
including design, processing, delivery, application and, finally, end of service life
and recycling. So how do TCOs relate to this global problem? First off, they are
key elements in a number of “green” technologies. In particular, they are critical to
low-e and solar control windows, photovoltaics, OLEDs for indoor lighting and
vehicle heat management. Collectively, this combination of technologies which
depend on TCO materials has the potential to significantly change the energy use
8 D.S. Ginley and J.D. Perkins
balance by both enabling new energy generation technologies and improving
energy efficiency technologies. Again, this provides further motivation to move
to new TCO materials for less environmental impact, lower cost, sustainability and
efficiency improvements in important devices.
1.3 Diversity of Transparent Conductors
As stated previously, TCOs have historically been dominated by a small set of
oxide materials including predominately SnO2, In2O3, InSnO and ZnO. However,
stimulated by the concerns enumerated above, the field has more recently expanded
not just into a broader spectrum of oxides, but also into other materials as well. The
traditional TCO oxide composition space is nominally focused on oxides and
Table 1.3 Selected historical TCO references
Material Year Process Reference
Cd-OCdO 1907 Thermally
Oxidation
K. Badeker, Ann. Phys. (Leipzig) 22, 749 (1907)
Cd-O 1952 Sputtering G. Helwig, Z. Physik, 132, 621 (1952)
Sn-OSnO2:Cl 1947 Spray pyrolysis H.A. McMaster, U.S. Patent 2,429,420
SnO2:Sb 1947 Spray pyrolysis J.M. Mochel, U.S. Patent 2,564,706
SnO2:F 1951 Spray pyrolysis W.O. Lytle and A.E. Junge
SnO2:Sb 1967 CVD H.F. Dates and J.K. Davis, USP 3,331,702
Zn-OZnO:Al 1971 T. Hada, Thin Solid Films 7, 135 (1971)
In-OIn2O3:Sn 1947 M.J. Zunick, U.S. Patent 2,516,663
In2O3:Sn 1951 Spray pyrolysis J.M. Mochel, U.S. Patent 2,564,707 (1951)
In2O3:Sn 1955 Sputtering L. Holland and G. Siddall, Vacuum III
In2O3:Sn 1966 Spray R. Groth, Phys. Stat. Sol. 14, 69 (1969)
Ti-OTiO2:Nb 2005 PLD Furubayashi et al., Appl. Phys. Lett. 86, 252101 (2005)
Zn-Sn-OZn2SnO4 1992 Sputtering Enoki et al., Phys. Stat. Solid A 129, 181 (1992)
ZnSnO3 1994 Sputtering Minami et al., Jap. J. Appl. Phys. 2, 33, L1693 (1994)
a-ZnSnO 2004 Sputtering Moriga et al., J. Vac. Sci. & Tech. A 22, 1705 (2004)
Cd-Sn-OCd2SnO4 1974 Sputtering A.J. Nozik, Phys. Rev. B, 6, 453 (1972)
a-CdSnO 1981 Sputtering F.T.J. Smith and S.L. Lyu, J. Electrochem. Soc. 128,
1083 (1981)
In-Zn-OZn2In2O5
a-InZnO
1995 Sputtering Minami et al., Jap. J. Appl. Phys. P2 34, L971 (1995)
In-Ga-Zn-OInGaZnO4 1995 Sintering Orita et al., Jap. J. Appl. Phys. P2. 34, 1550 (1995)
a-InGaZnO 2001 PLD Orita et al., Phil. Mag. B 81, 501 (2001)
CVD chemical vapor deposition; PLD pulsed laser deposition
1 Transparent Conductors 9
combinations of oxides of Zn, Sn, In, Ga, and Cd. These main TCO materials tend
to fall in to groups that can be defined by structure type as shown in Table 1.4. The
building blocks in the first two rows of Table 1.4 can then be combined as shown in
Table 1.5 to form most of the known TCO materials. This basic composition space
is depicted pictorially in Fig. 1.9.
This has created a very diverse set of crystalline and, more recently, amorphous
transparent conducting materials. There has been considerable modeling and spec-
ulation of the range of crystalline oxide TCO materials with both empirical and first
principles models having been developed recently [19–23]. This has led both to a
much better understanding of the n-type materials and to an emerging understand-
ing of the limits of p-type materials. Thus far however, the theory has not been
applied extensively to the amorphous mixed-metal-oxide n-type systems such as
the prototypical In-Zn-O and Zn-Sn-O systems. This is largely due to the difficulty
in applying electronic structure calculation approaches to non-period amorphous
materials but some initial work has been done on In-Ga-Zn-O [24].
Fig 1.8 Atmospheric CO2 vs.
year. Data compiled from
refs. [14–17]
Fig. 1.7 Energy consumption
vs. year by global region
(mtoe: millions of tons oil
equivalent) [13]
10 D.S. Ginley and J.D. Perkins
The generality of the amorphous TCOs is further illustrated by the ternary cation
InGaZnO [24, 25] and the binary cation CdSnO [26] systems. In all of these
amorphous materials, the electronic transport mechanism appears to be complex
Table 1.4 Cation coordination and carrier type of TCO materialsa
Structural feature Carrier type Examples
Tetrahedrally-coordinated cations n-Type ZnO
Octahedrally-coordinated cations n-Type CdO, In2O3, SnO2, CdIn2O4, Cd2SnO4, etc.
Linearly-coordinated cations p-Type CuAlO2, SrCu2O2, etc.
Cage framework n-Type 12CaO-7Al2O3
aFrom Inger et al. Journal of Electroceramics 13, 167 (2004)
Table 1.5 Doping of TCO
materialsaMaterial Dopant or compound
SnO2 Sb, F, As, Nb, Ta
In2O3 Sn, Ge, Mo, F, Ti, Zr, Mo, Hf, Nb,
Ta, W, Te
ZnO Al, Ga, B, In, Y, Sc, F, V, S, Ge,
Ti, Zr, Hf
CdO In, Sn
Ga2O3
ZnO-SnO2 Compounds Zn2SnO4, ZnSnO3
ZnO-In2O3 Zn2In2O5, Zn3In2O6
In2O3-SnO2 In4Sn3O12
CdO-SnO2 Cd2SnO4, CdSnO3
CdO-In2O3 CdIn2O4
MgIn2O4
GaInO3, (Ga, In)2O3 Sn, Ge
CdSb2O6 Y
Zn-In2O3-SnO2 Zn2In2O5-In4Sn3O12
CdO-In2O3-SnO2 CdIn2O4-Cd2SnO4
ZnO-CdO-In2O3-SnO2
aFrom Minami, Semiconductor Science and Technology 20, S35
(2005)
Fig. 1.9 Composition space
for conventional TCO
materials
1 Transparent Conductors 11
but nevertheless, the performance is very good, especially the electron mobilities
which can be as high as 50 cm2/V s [2], better than many commercial crystalline
TCOs. These new amorphous TCO materials are amorphous mixtures of the
composition phase space shown in Fig. 1.9 in which all the single metal oxide
basis members have filled d-shells and the conduction band states come mostly
from the empty metal atom s-states. Hosono et al. [27, 28] have proposed that the
high electron mobility in these amorphous TCO materials is due to direct overlap of
these large and non-directional metal atom s-states as depicted in Fig. 1.10. Candi-
date metallic elements which form oxides which satisfy these basic criteria are
highlighted in Fig. 1.10. However, materials synthesis work over the past decade
has made clear that the specific metal element mixtures selected from this candidate
set can make an order of magnitude or more difference in the maximum achievable
conductivity.
Recently, attention has also been directed to Nb and Ta doped anatase TiO2 as a
potential new TCO material. In this material, which falls outside the conventional
TCO materials shown in Fig. 1.9, the conduction band is formed largely from Ti 3d
states instead of metal atom s-states as discussed above. Highly conducting films
have been deposited onto single crystal SrTiO3 by both pulsed laser deposition
(PLD) (s ¼ 4,300 S/cm) [29] and sputtering (s ¼ 3,000 S/cm) [30]. For films on
glass, a conductivity of s ¼ 2,200 S/cm has been obtained for films deposited by
PLD and then subsequently annealed in H2 [31]. Another novel class of unconven-
tional oxide based transparent conductors system is the 12CaO·7Al2O3 (“C12A7”
Fig 1.10 Effect of disorder on electron orbital overlap in metal oxide semiconductors (top) andcandidate metal ions for amorphous mixed metal oxide transparent conductors (bottom) fromHosono [82]
12 D.S. Ginley and J.D. Perkins
or Ca12Al14O33) cage compounds based on electride materials [32–34] as shown in
Fig. 1.11. The conductivity in these materials must be activated. In the initial photo-
activated materials, conductivities of order 1 S/cm were obtained [33]. Recently,
using a multi-step film growth, crystallization and in-situ annealing process, a
conductivity of �800 S/cm has been obtained [35].
Generally TCOs are predominately n-type because of the ease of forming
oxygen vacancies or cation interstitials in the oxides such as those depicted in
Fig. 1.9 [36]. However one of the current major research challenges for TCO
materials is the development of p-type materials with comparable conductivities
to their n-type counterparts, i.e., of order 103 S/cm. The on-going search for p-type
TCOs was pushed to a much higher level by the work of Kowazoe and Hosono in
the 1990s on the Cu based materials such as CuAlO2 [37] and SrCu2O2 [38]. These
materials were clearly p-type, but the doping levels and mobilities were low,
typically N � 1018/cm3 and m < 1 cm2/V s. To date these problems have not
been solved in these copper based metal oxides.
Starting in 1999 [39, 40], there was a great flurry of activity trying to make ZnO
p-type which would have yielded a very versatile opto-electronic material similar to
GaAs. To date, while p-type materials have been observed, they do not seem to be
stable and reproducible [34, 41–47]. Many groups are still working on this area
because of the promise of ZnO which can have the band gap increased or decreased
by the addition of Mg or Cd respectively and can be made a spintronic material by
the addition of Co. This, along with a viable p-type material, would make a
remarkably versatile optoelectronic system. Towards this end, Tsukazaki et al.
have developed a laser MBE deposition with in-situ repeated temperature modula-
tion during growth that can consistently make p-type nitrogen doped ZnO [48] as
well as ZnO/MgZnO quantum well structures of sufficient quality to enable obser-
vation of the quantum hall effect in a ZnO based heterostructure [49].
The search for p-type materials has expanded beyond oxides to copper based
sulfides and selenides such as LaCuOS [50], BaCuSF [51] and related materials
Fig. 1.11 Structure of
12CaO·7Al2O3 (C12A7) from
Medvedeva et al. [32]
1 Transparent Conductors 13
[52–54]. Several precious metal based transparent oxides have also shown p-type
conduction including crystalline ZnM2O4 (M = Ir, Rh, Co) [55–57] as well as
amorphous Zn-Rh-O [58, 59].
Outside of the range of the basic oxide materials and primarily driven by the
OLED and nanomaterials communities is an emerging interest in organic based
transparent conductors. The focus has been on intrinsically conducting polymers
[60], charge transfer polymers like PEDOT:PSS [61–64] and, more recently, on
carbon nanotube composites [65–68] and graphenes [69–73]. All of these materials
of are of great interest for the OLED, flexible electronics and polymer/thin film
photovoltaics communities because of the potential to write, print, or spin on
coatings at, or near, room temperature from liquid based precursors at atmospheric
pressure. Typically, the conductivities are around 2–1,200 S/cm, about an order of
magnitude less than for ITO. Still, the materials may be of use as an intermediate
contact layer to a conventional TCO in order to provide a better electronic interface
to a polymer or molecular electronic device.
Overall, as transparent conductors are used in an increasingly broad array of
applications, each with their own particular needs, nearly everything in the trans-
parent conductors materials tool box will likely become important.
1.4 Emerging Applications
In addition to the conventional TCO applications discussed above, there are a
number of emerging applications that have the potential to significantly impact
the use of TCOs and, in some cases, clearly require very different TCO perfor-
mance characteristics. Several of these currently emerging applications are briefly
described here.
1.4.1 Transistors and Flexible Transparent Electronics
One high profile and rapidly emerging area is that of oxide based thin film
transistors (TFTs) including transparent thin film transistors (TTFTs). Conducting
TCOs can be combined with amorphous semi-insulating high mobility TCO mate-
rials to create a viable alternative to the conventional amorphous Si TFTs currently
used in flat panel displays. This has the potential to create a whole new class of
transparent electronics. At present, most of the schemes employ an amorphous
semi-insulating transparent oxide semiconductor (TOS) such as indium zinc oxide
(IZO) for the channel layer as shown in Fig. 1.12 [74]. This simple back-gated test
structure designed for channel layer materials studies is not transparent due to both
the Si substrate and the Ti gate. However, transparent top-gated structures utilizing
TCOs for the source, drain and gate electrodes have been demonstrated on both
glass and flexible plastic substrates as shown in Fig. 1.13 [12]. The semiconducting
14 D.S. Ginley and J.D. Perkins
IZO films used as the channel layer can be sputtered from the same target as
conducting IZO simply by adding oxygen to the sputtering gas, about 10% O2 in
Ar is typical [2]. These amorphous TOS layers can have a high electron mobility,
m � 10–50 cm2/V s, across a wide range of doping levels compared to m < 1 cm2/V s
in typical amorphous Si TFTs [75]. So, while the process technology for oxide TFT
fabrication is not yet up to the reliability and device density of a-Si, it is a rapidly
developing area with the potential for not just faster TFTs to replace a-Si TFTs, but
transparent flexible electronics are possible. This will be covered in more detail in
Chaps. 13 and 14.
Fig. 1.12 Schematic (left side) and top-down image (right side) of IZO based TFT structure on Si
substrate. Schematic image from Yaglioglu et al. [74]
Fig. 1.13 Transparent TFT on flexible PET substrate from Nomura et al. [12]
1 Transparent Conductors 15
1.4.2 Electrochromic Windows
Electrochromic devices have been emerging as a commercial reality over the last
10 years first finding application in the automotive arena as self dimming and then
smart rear view mirrors as well as in smart windows with electronically adjustable
transmission for building applications [7, 76, 77]. For these electrochromic applica-
tions either one TCO layer is needed for reflective applications and two are needed
for transmissive applications such as the window illustrated in Fig. 1.14 [78].
1.4.3 Optical Arrays
The operation of liquid crystal displays is based on the change in the liquid crystal
index of refraction when a voltage is applied and as the index of refraction changes,
so does the effective optical path length. Based on this effect, electronic optical
beam steerers can be made by using pixelated contacts to apply a spatially varying
voltage across a liquid crystal resulting in an electronically reconfigurable optical
wedge, an optical phased array in short. Such devices require a transparent contact
at the operating wavelength, typically 1,500 nm for free space laser based telecom
which requires high mobility, low-carrier concentration TCO materials to obtain
sufficient IR transparency [79] such as the Ti-doped In2O3 shown in Fig. 1.2 [3].
Fig. 1.14 Schematic structure of an electrochromic window showing front and back TCO contacts
from Granqvist [78]
16 D.S. Ginley and J.D. Perkins
Recommended
