Excitonic and Band-to-Band Transitions in
Temperature-Dependent Optical Absorption
Spectra of Cu
Naoya Aihara, Hideaki Araki, and Kunihiko Tanaka*
Temperature-dependent optical absorption spectra from transmittance and
reflectance measurements of Cu
(CTS) thin films which are promising
material for solar cells were investigated. Thin film CTS samples with Cu-poor,
near-stoichiometric, and Cu-rich compositions were prepared on glass sub-
strates by thermal co-evaporation and sulfurization. At low temperature, three
band-to-band (BB) transitions that are allowed between triple upper valence
bands and a single lower conduction band were observed for all samples. The
square of the product of the absorption coefficient and the photon energy was
plotted to estimate the band gap energy for three bands of the Cu-poor sample.
On the other hand, excitonic (EX) transitions that corresponded to three bands
were observed for the Cu-rich and near-stoichiometric samples. Temperature-
dependent optical absorption spectra with the triple BB and EX transitions were
resolved using a simplified fitting equation with Lorentzian functions as the EX
transitions. The band gap, EX transition, and exciton binding energies for the
lowest energy band of the Cu-rich sample were determined to be 0.945 eV,
0.936 eV, and 8.9 meV at 6 K, respectively. In the low-temperature region,
anomalous blue-shifts of the estimated band gap energy with increasing
temperature were obtained for three bands of all samples.
Compound semiconductors such as Cu(In,Ga)Se
have recently been considered to be good
candidates for the absorber layer in thin-ﬁlm solar cells. Cu(In,
-based thin-ﬁlm solar cells have
been demonstrated with respective power conversion efﬁcien-
cies of 22.6
However, these photovoltaic materials
contain scarce (In, Ga) and toxic (Se) elements.
(CTS), which is composed of
earth-abundant and non-toxic elements,
has been reported to have a direct band gap
and a high optical absorption coefﬁcient,
which makes it a promising material for
solar cells. CTS-based thin-ﬁlm solar cells
with power conversion efﬁciencies of over
4% have been reported by several research
and Chantana et al.
reported an improved efﬁciency of 4.8%.
Although CTS-based device fabrication
and CTS thin ﬁlm synthesis have been
extensively reported, there has been insuf-
ﬁcient investigation of the fundamental
properties of CTS. An understanding of the
optical and electronic properties of CTS is
important to further improve the power
conversion efﬁciency of CTS-based thin-
ﬁlm solar cells. In particular, the band gap
energy is one of the most signiﬁcant
parameters for photovoltaic absorber mate-
rials because it determines the theoretical
limit of the solar cell efﬁciency.
crystal structure of CTS used for solar cell
absorbers is monoclinic with Cc symmetry.
The band gap energy for monoclinic CTS
has been experimentally determined to be
0.91–0.94 eV and 0.98–1.04 eV based on double absorption bands
from optical absorption spectra of thin-ﬁlm samples
external quantum efﬁciency measurements of photovoltaic
In theoretical investigations, ﬁrst-principles
calculations of monoclinic CTS have been reported by several
Using a combination of theoretical calculations
and spectroscopic ellipsometry measurements, Crovetto et al.
determined that the double absorption onset originates from
optical transitions at the Brillouin zone center G-point from three
energetically close-lying valence bands to a single conduction
band in monoclinic CTS. In addition, Wild et al.
valence band splitting (VBS) effect for the three valence bands in
monoclinic CTS by room-temperature optical absorption
measurements. We have previously reported free-exciton
luminescence at 0.9317 eV at a temperature of 4.2 K from
chemical vapor transport grown CTS bulk crystals, and the
thermal activation energy of the free-excitons was determined to
be 6.6 meV.
These observation results imply that the
transition in the low energy region of experimentally observed
double absorption spectra originates not from absorption caused
N. Aihara, Dr. K. Tanaka
Nagaoka University of Technology, 1603-1,
Kamitomioka, Nagaoka, Niigata 940-2188, Japan
Prof. H. Araki
National Institute of Technology, Nagaoka
College, 888, Nishikatakai, Nagaoka, Niigata 940-
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/pssb.201700304.
Optical Absorption www.pss-b.com
Phys. Status Solidi B 2018, 255, 1700304 © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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