ICMCTF 2012! - CD-Lab Application Oriented Coating Development

carrier concentration, Hall mobility and their stability in long-term
moisture-resistance tests as well as the surface texture structure formed by
wet-chemically etching and its resulting haze properties. It should be noted
that differences in the obtainable Hall mobility among AZO, GZO and BZO
thin films prepared with a low resistivity on the order of 10 -4 Ωcm by the
different deposition methods were attributed to the content rather than the
kind of impurity doped into the thin films. In addition, the influence of rapid
thermal annealing (RTA) on surface texture formation as well as on light
management was investigated in the impurity-doped ZnO thin films. The
RTA treatment significantly improved the obtainable light management in
thin-films prepared on substrates at 200 o C over that found in thin-films
prepared at RT. Comparing AZO, GZO and BZO, the AZO thin films
prepared with an appropriate Al content on substrates at 200 o C were found
to be the most suitable for transparent electrode applications in Si-based
thin-film solar cells.
2:10pm C2-2/F4-2-3 Comparative physical properties of Ga-, In-, Zr-
and Sn-doped ZnO semiconductor thin films fabricated via sol-gel
method, C.Y. Tsay (cytsay@fcu.edu.tw), W.C. Lee, S.S. Lo, C.J. Chang,
C.K. Lin, Feng Chia University, Taiwan
Transparent ZnO semiconductor thin films doped with In, Ga, Sn, and Zr
were deposited on glass substrate via sol-gel method. The doping
concentration defined as M/(Zn+M) atomic ratio where M is the dopant, is 2
at.% for all doped ZnO thin film samples. In this study, authors reported the
influence of dopants on the structures, optical, and electrical properties of
ZnO thin films. Moreover, composition, chemical bonds, and
photoluminescence spectra of undoped and doped ZnO thin film samples
were examined. These ZnO-based sol-gel films were preheated at 300 o C
for 10 min, and then annealed in ambient air at 500 o C for 2 hr. XRD results
show that the doped ZnO thin film exhibited broaden diffraction peaks, and
the broadening features were significant in the cases of In, Zr, and Sn
doping. All doped ZnO thin films exhibited higher optical transmittances in
the visible range than the undoped ZnO thin film. E lectrical properties of
each ZnO-based thin film were characterized by Hall measurements using
the van der Pauw configuration.
2:30pm C2-2/F4-2-4 Temperature Dependence of Electrical Properties
of Ga-Doped ZnO Films Deposited by Ion-Plating with DC Arc
Discharge, T. Terasako (terasako.tomoaki.mz@ehime-u.ac.jp), Graduate
School of Science and Engineering, Ehime University, Japan, H.-P. Song,
H. Makino, Kochi University of Technology, Japan, S. Shirakata, Graduate
School of Science and Engineering, Ehime University, Japan, T. Yamamoto,
Kochi University of Technology, Japan
Ga-doped ZnO (GZO) is one of the promising materials for transparent
electrodes in flat panel displays and solar cells. Highly transparent GZO
polycrystalline films with low resistivity can be deposited at low
temperatures below 473 K. Very little work is currently available in the
published literature on temperature dependence of electrical properties;
resistivity, r, carrier concentration, n, and Hall mobility, m, of GZO films.
In this paper, we have investigated the subject to discuss carrier transport
through grain boundaries in the films.
GZO films with a thickness of 200 nm were deposited by ion-plating with
DC-arc discharge. Sintered ZnO tablets with different Ga2O3 contents
ranging from 0.003 to 4 wt.% were used as resources. During the growth
process, an oxygen gas was introduced into the deposition chamber to
compensate oxygen deficiencies. Hall measurements were carried out using
the van der Pauw method at 83-343 K under the magnetic field of 0.47 T.
Analysis of data obtained by the Hall measurements shows that the
characteristics of μ-T curve gradients, ⊿μ/⊿T, plotted as a function of n can
be divided into three regions: (1) Region I; n values below n=1×10 20 cm -3 ,
the ⊿μ/⊿T > 0 (grain-barrier-limited transport) and the values decreased
with increasing n, (2) Region II; n values ranging from 1×10 20 to 5×10 20 cm -
3
, ⊿μ/⊿T=0: μ values were independent of T and (3) Region III;
n>1×10 21 cm -3 , the ⊿μ/⊿T < 0 (metal-like behavior). This tendency is
probably due to the change in dominant scattering mechanism from grain
boundary scattering to phonon scattering with increasing n.
In the case of Region I described above, based on the model that carrier
mobility is mainly determined by the potential barrier, VB, at the grain
boundary, we applied a charge trapping model to understand the nature of
grain boundary scattering on carrier transport for polycrystalline GZO
films. As a result, we find the values of the VB of less than 0.02 eV for all
samples in Region I. Considering that even GZO films in the Region I have
the VB values smaller than kT at room temperature, tunneling current can
makes a major contribution to the carrier transport trough grain boundaries
for all GZO films under investigation.
Monday Afternoon, April 23, 2012 12
2:50pm C4-2/F4-2-6 Doped Cadmium Oxide as a High Performance
Transparent Conductive Oxide, R. Mendelsberg, K.M. Yu, Y. Zhu, D.
Speaks, S. Lim, Lawrence Berkeley National Laboratory, US, S. Zhao,
University of California, Berkeley, US, J. Reichertz, S. Mao, W.
Walukiewicz, A. Anders (aanders@lbl.gov), Lawrence Berkeley National
Laboratory, US
Cadmium oxide has a high intrinsic electron mobility compared to all other
conventional transparent conductive oxides (TCOs) such as ZnO:Al and
indium-tin-oxide. Unintentionally doped CdO films can have Hall
mobilities greater than 150 cm 2 /Vs and carrier concentrations in the high
10 19 cm -3 resulting in resistivities in the 10 -4 Ω cm range. However, the
small band gap (2.2 eV) severely restricts the visible transmission of
undoped CdO, thus limiting its application as a TCO. However, the optical
absorption edge can be considerably widened by increasing the electron
concentration due to the Burstein-Moss shift which occurs when the Fermi
level shifts to higher energy and more conduction band states are filled with
electrons from the donor dopants. In this work we report on high quality
films of CdO, undoped and doped with indium, deposited by filtered
cathodic arc deposition (FCAD) and by pulsed laser deposition (PLD). The
properties of films synthesized by both methods will be compared. Using
these highly non-equilibrium synthesis methods, we have achieved
extremely low resistivities (less than 5 x 10 -5 Ω-cm) for In-doped CdO films
with an electron concentration greater than 1x10 21 cm -3 and mobilities as
high as 200 cm 2 /Vs. Moreover, due to the high electron concentration, these
films have a band gap >3.2 eV. The high optical transmittance (>85%) of
this material in the spectral range extending from the visible to 1300 nm in
the infrared makes it an ideal TCO for thin film photovoltaics, especially in
multijunction solar cells where high infrared transmittance is essential.
3:10pm C2-2/F4-2-7 Current Status and Future Prospects of the CIGS
PV Technology, S. Niki (shigeru-niki@aist.go.jp), S. Ishizuka, H. Komaki,
S. Furue, K. Matsubara, H. Shibata, A. Yamada, Research Center for
Photovoltaic Technologies, AIST, Japan, N. Terada, Kagoshima University,
Japan, T. Sakurai, K. Akimoto, Tsukuba University, Japan INVITED
Overview of the research activities in the CIGS PV technology will be
presented. In addition, the current status and future prospects of the CIGS
technology will be discussed based upon our research results.
High-efficiency CIGS solar cells and submodules have been developed in
our research group. In order to fill the efficiency gap between small-area
cells and commercial modules, the multi-stage evaporation technique has
been applied to fabricate the monolithically integrated CIGS submodules.
The conversion efficiencies of integrated submodules on 10x10cm 2
sodalime glass and flexible ceramics substrates have been improved up to
h=16.6% and h=15.9%, respectively. These results indicate that the CIGS
technologies are competitive with the current Si and CdTe technologies in
terms of both cost as well as performance.
3:50pm C2-2/F4-2-9 Reactive magnetron sputtering of precursors for
CZTS solar cells, T. Kubart (Tomas.Kubart@angstrom.uu.se), T. Ericson,
J.J. Scragg, C. Platzer-Björkman, The Angstrom Laboratory, Uppsala
University, Sweden
At the moment, CIGS (CuInGaSe2) solar cells show the highest efficiency
among industrial scale produced thin film solar cells. Given the present
strong increase in production, however, the availability and price of indium
will become an issue because of its low abundance in Earth's crust.
Therefore, there is strong interest in alternative indium free absorber
materials. Kesterites CZTS (Cu2ZnSn(SxSe1-x)4) attracted most attention
owing to the fact that relatively high efficiencies have already been
demonstrated and also due to the similarity to CIGS.
In this contribution we report on reactive sputtering for deposition of CZTS
precursors. In order to avoid Sn loss at elevated temperatures a two stage
process, synthesis of precursor films at relatively low substrate temperature
followed by annealing, is used. Depositions are performed by pulsed DC
magnetron sputtering from two targets, a CuSn alloy and Zn, in a mixture of
Ar and H2S. The film structure is evaluated by X-ray diffraction and Raman
spectroscopy while the composition is analysed by RBS, XRF, EDS and
EPMA. Internal stress is measured by deflection of thin substrates.
Characteristics of the deposition process are discussed with respect to the
discharge power, total pressure, substrate temperature and H2S flow on the
film structure and composition.
Sulphur incorporation can be readily controlled by H2S flow with the
structure changing from amorphous to columnar with increasing S content.
The main issue encountered in the depositions is related to the film
composition as the ratio between Cu and Sn does not correspond to the
target composition. This effect is discussed in detail with respect to the
sputtering and transport through the gas phase. Finally, material properties
after annealing are briefly summarized.