Metals were used as compound materials or dopants (with just a few percent content).: D (D = dopant), with metals, metalloids or nonmetals.
A variety of preparation and characterization methods was applied to investigate their different chemical structures and physical characteristics. It is transparent and colorless as a thin film and yellowish to grey as bulk material.
Indium tin oxide is the most widely used transparent conducting oxide (TCO ) because of its two key properties, its electrical conductivity and optical transparency.
As n-type TCOs are of special importance for thin film solar cell production, ITO and the reasonably priced aluminum-doped zinc oxide (Zn O: Al) are discussed with view on preparation, characterization and special occurrences.
As transparent conducting oxides are usually compound semiconductors—where the nonmetal part is oxygen—they are discussed along their metal elements.
The deterioration of Zn O: Al thin films is discussed in .
Copper may be replaced by silver [136,137,138,139,140,141], palladium  or platinum .
Well known is, for example, indium tin oxide (ITO), and the doping of zinc oxide with less than 5% aluminum (Zn O: Al).
Moreover, doped delafossite and mayenite compounds are of upcoming interest (see Table 1).
: Transparent conducting oxides (TCOs) are electrical conductive materials with comparably low absorption of electromagnetic waves within the visible region of the spectrum.
They are usually prepared with thin film technologies and used in opto-electrical apparatus such as solar cells, displays, opto-electrical interfaces and circuitries.
ITO thin films are still deposited with ion assisted plasma evaporation , (low temperature) electron beam evaporation [17,18,19], direct current (DC), pulsed DC (PDC), high power pulsed magnetron sputtering (HPPMS), radio frequency (RF) magnetron sputtering [20,21,22,23,24,25], thermal evaporation  or pulsed laser deposition (PLD) [26,27,28,29].