Chapter 20
Transparent Conductive Oxide Semiconductors
Many of the most important semi-conductors are oxides.
A.H Wilson, 1939 [72]
Abstract Typical materials in the class of transparent conductive oxides are mentioned. Their physical
properties and the limits of conductivity versus transparency are discussed.
20.1 Introduction
Transparent conductive oxides (TCO) are semiconductors that are simultaneous transparent and highly
conductive. Therefore they can serve as transparent contacts, e.g. as a solar cell front contact or in
display applications. The materials are typically fabricated in the form of thin films on glass, polymers
or similar substrates and devices. The crystallographic structure is polycrystalline or amorphous. The
first TCO investigated was CdO in pressed powder [1662] and thin film form [38]. The recent historic
development of the resistivity of the most important TCO materials in the last 30 years is shown in
Fig. 20.1. Further information on TCO films can be found in [1663–1665].
20.2 Materials
Any wide-gap (E g > 3 eV) semiconductor that is conductive, e.g. due to intrinsic defects or by chemical
impurities (doping), can be considered to be a TCO. Pratically only a few, non-toxic materials that can
be easily deposited are of importance. The first TCO application was heating of air plane windows. As
always in semiconductor technology, price drives the suitability for applications. This in particular true
for TCO applications since they include large area devices such as solar cells, displays and also large
glass panes for electromagnetic shielding and architectural heat and IR transparency management.
Therefore large quantities of TCO are needed. The popular ITO (indium-tin-oxide) suffers from large
indium price and potential indium scarcity, opening the field for aluminum-doped ZnO (ZAO) which is
abundant. Possible other compounds including Cd are of no practical interest due to toxicity. Conductive
GaN has not been considered due to its large processing temperatures. All practical TCO materials
contain either Zn, Sn or In (Fig. 20.2). A number of TCO materials is reviewed in [1666]. TCO are
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_20
511
Transparent Conductive Oxide Semiconductors
Many of the most important semi-conductors are oxides.
A.H Wilson, 1939 [72]
Abstract Typical materials in the class of transparent conductive oxides are mentioned. Their physical
properties and the limits of conductivity versus transparency are discussed.
20.1 Introduction
Transparent conductive oxides (TCO) are semiconductors that are simultaneous transparent and highly
conductive. Therefore they can serve as transparent contacts, e.g. as a solar cell front contact or in
display applications. The materials are typically fabricated in the form of thin films on glass, polymers
or similar substrates and devices. The crystallographic structure is polycrystalline or amorphous. The
first TCO investigated was CdO in pressed powder [1662] and thin film form [38]. The recent historic
development of the resistivity of the most important TCO materials in the last 30 years is shown in
Fig. 20.1. Further information on TCO films can be found in [1663–1665].
20.2 Materials
Any wide-gap (E g > 3 eV) semiconductor that is conductive, e.g. due to intrinsic defects or by chemical
impurities (doping), can be considered to be a TCO. Pratically only a few, non-toxic materials that can
be easily deposited are of importance. The first TCO application was heating of air plane windows. As
always in semiconductor technology, price drives the suitability for applications. This in particular true
for TCO applications since they include large area devices such as solar cells, displays and also large
glass panes for electromagnetic shielding and architectural heat and IR transparency management.
Therefore large quantities of TCO are needed. The popular ITO (indium-tin-oxide) suffers from large
indium price and potential indium scarcity, opening the field for aluminum-doped ZnO (ZAO) which is
abundant. Possible other compounds including Cd are of no practical interest due to toxicity. Conductive
GaN has not been considered due to its large processing temperatures. All practical TCO materials
contain either Zn, Sn or In (Fig. 20.2). A number of TCO materials is reviewed in [1666]. TCO are
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_20
511