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Transparent
Conductive Indium Zinc Tin Oxide Thin Films for Solar Cell Applications
Damisih,
Hee Young Lee
School
of Materials Science and Engineering, Yeungnam University, Gyeongsan, 712-749, South Korea
Abstract
Indium
zinc tin oxide (IZTO) thin films were studied as a possible alternative
to indium tin oxide (ITO) films for providing low-cost transparent
conducting oxide (TCO) for thin film photovoltaic devices. IZTO films were
deposited onto glass or flexible polymer substrates at room temperature. A dc/rf
magnetron co-sputtering system equipped with a ceramic target of the same
composition was used to deposit TCO films. Earlier studies showed that the
resistivity value of In0.6Zn0.2Sn0.2O1.5
(IZTO20) films could be lowered to approximately 6×10-4 ohm·cm
without sacrificing optical transparency and still maintaining amorphous
structure through the optimization of process variables. The
growth rate was kept at about 8 nm/min while the oxygen-to-argon pressure ratio
varied from 0% to 7.5%. As-deposited films were always amorphous and showed
strong oxygen pressure dependence of electrical resistivity and electron
concentration values. Influence of forming gas anneal (FGA) at medium
temperatures was also studied and proven effective in improving electrical
properties. In this study, the chemical composition of the targets and the
films varied around the In0.6Zn0.2Sn0.2O1.5
(IZTO20). It was the main objective of this paper to investigate how
off-stoichiometry affected TCO characteristics including electrical resistivity
and optical transmission. In addition to the composition effect, we have
also studied how film properties changed with processing variables. It was
concluded that IZTO thin films have shown their potential toward indium-saving
TCOs, so that they could be adopted in most applications where currently ITO
and IZO thin films are being used. The results are also compared to those
obtained for commercial ITO thin films from solar cell application
viewpoint.
Ⅰ. INTRODUCTION
Transparent
conducting oxides (TCOs) have been widely employed for many applications such
as solar cells, flat panel displays, low emissivity windows, thin films
transistors, light emitting diode and semiconductor lasers [1-3]. They have
been used in wide range applications because of their well known properties,
which are high conductivity and high transparency in the visible wavelength
region [4]. Until now, indium tin oxide (ITO) still become a popular choice for
TCO materials, due to its low resistivity, high stability and compatibility
with fine patterning process [5-6]. Eventhough possibly ITO is the most successful
TCO in application, its production cost is very high regarding the limited
supply and increasing price of indium. Therefore, transparent conducting oxide
that contain a reduced amount or no indium have recently received more
attention as substitute material for ITO application [7]. As a result, many
kinds of new transparent conducting oxide films are reported; polycrystalline or
amorphous thin films that show resistivity in order of 10-4 Ω.cm without
being highly toxic are possible candidates for use in transparent electrodes. Here,
we suggest indium zinc tin oxide (IZTO) as promising alternative TCO material
to substitute high cost of ITO for solar cell application due to its low
resistivity, high transparency, chemical stability, smooth surface, high work
function and low deposition temperature [3,8,9]. In addition, IZTO solubility
increase through co-substitutional of In2O3 by ZnO and
SnO2 because of isovalent substitution of a Sn4+ and Zn2+
cation for a pair of In3+ [10].
IZTO
thin films can be prepared by various kinds of deposition techniques, such as magnetron
sputtering, electron gun evaporation, chemical vapor deposition, and spray
hydrolysis [11]. In particular, magnetron sputtering is an enormously reliable
technique for thin films deposition and has been used in industry for large
scale production. The major advantage of magnetron sputtering technique are
that can continuously produce high quality film at lower processing
temperature. The thin film properties are closely dependent on the deposition
method as well as on the deposition parameter. The experimental condition and
substrate characteristics, such as RF power, working pressure, substrate
temperature and post deposition annealing significantly affect the
microstructure, electrical and optical properties.
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