NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 43 of 144
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0.4 0.9 1.4 1.9 2.4 2.9
Figure 2: Modeled transmittance of a TCO film: the
parameters being the same as in Figure 1.
o 1o 20 30 40 50 60 70 80 90 100
Carrier concentration (x1019 crm)
Figure 3: Modeled resistivity and mobility of 'TCO
A previous report by Nozik  indicated that high
mobility could be achieved in this material, possibly
because of a low free-carrier effective mass. There are
only two ways to obtain a higher mobility: improve the
carrier relaxation time, or use a material with a lower
effective mass. The lower limit of resistivity is very
similar to values obtained in practice.
Many investigators have published data on TCOs
and the most striking feature is the similarity of the
optimized electrical and optical properties. This
appears to apply to films made from many materials,
different deposition techniques, different deposition
Thus, we originally concluded (a conclusion that
was later shown by Mulligan  to be incorrect) that
the option of improving the relaxation time was
probably not realistic. This was the primary
motivation for searching for materials with lower
effective masses. Careful analysis , later showed that
the improvement in the properties in general, and the
mobility in particular, was in fact due to an
improvement in the relaxation time of CIO films,
beyond that typically obtained for more traditional
5x10 9 cm-3
- - 1020 c-3
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Figure 4: Measured free-carrier absorbance of films of
tin oxide and CTO.
Notice that the increasing carrier concentrations in
Figure 1 lead to the absorption band gradually
extending further into the part of the spectrum to which
the solar cells respond. This leads to a reduction of the
short-circuit-current density and also causes the TCO
film to appear brown. Excessive free-carrier
concentration is responsible for the well-known brown
appearance of non-optimal TCO films.
3. TCO DEPOSITION AND PERFORMANCE
Films of CIO were deposited by radio-frequency
sputtering, in pure oxygen, on room-temperature
substrates of soda-lime glass (although higher-quality
substrates have also been used). They were then
annealed in pure argon, or argon/CdS, at a temperature
of up to 680 C. This sequence has previously been
shown to give the highest performance films. It has
been shown that the structure of films prepared in this
way is single-phase spinel but if other processing
sequences were used, then multiple phases resulted,
which never performed as well as single phase
materials. These comments apply to both the
conductivity and optical transmittance of the films.
When optimized, single-phase films had a resistivity
as low as 1.1x104 2 cm. A comparison of the optical
absorbance of a typical research-quality CIO film and a
sample of commercially-available SnO2, deposited onto
a soda-lime substrate, using chemical vapor deposition
from a SnCl4 precursor, is shown in Figure 4. The
carrier concentrations for the SnO2 and the CIO were
5x1020 cm' and 3.2x1020 cm3, respectively, and the
film thicknesses were about 500 nm for both materials.
The mobilities of the films were 15 and 54 cm2 V-' s.'
for the tin oxide and CI'O, respectively.
Clearly, the performance of the CTO is considerably
superior to that of the tin oxide. The reason for this, in
accordance with the modeling data reported above, is
the extraordinarily high electron mobility, which was
more than 60 cm2 V-' s' in some cases.
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/43/: accessed January 23, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.