March 12, 2015 16:7 PSP Book - 9in x 6in 02-Valenta-c02
Chapter 2
The Dielectric Function and
Spectrophotometry: From Bulk to
Nanostructures
Caterina Summonte
Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e i Microsistemi,
via Gobetti 101, 40129 Bologna, Italy
caterina.summonte@cnr.it
2.1 Introduction
The world of group IV nanomaterials for applications in photo-
voltaics is vast and heterogeneous. It includes silicon and germa-
nium quantum dots and nanowires (NWs) and their combinations;
quantum wells; graphene and carbon nanotubes.
Silicon and germanium nanoparticles are candidates in tunable
band-gap absorbers in third-generation multijunction solar cells
[1]; NWs exhibit remarkable scattering and are used to enhance
the absorption well beyond the value of the corresponding bulk
materials [2]; if fabricated in quantum dimensions, they combine a
tunable band gap with electrical transport properties [3]; graphene
Nanotechnology and Photovoltaic Devices: Light Energy Harvesting with Group IV Nanostructures
Edited by Jan Valenta and Salvo Mirabella
Copyright
c
2015 Pan Stanford Publishing Pte. Ltd.
ISBN 978-981-4463-63-8 (Hardcover), 978-981-4463-64-5 (eBook)
www.panstanford.com
March 12, 2015 16:7 PSP Book - 9in x 6in 02-Valenta-c02
28 The Dielectric Function and Spectrophotometry
sheets have been proposed as transparent conducting material in
organic photovoltaic devices [4]; and Ge nanoparticles are used to
enhance photocurrent in dye-sensitized solar cells [5].
The different materials cover different roles, have different
experimental characteristics, and are treated in different ways. For
materials of optical quality, spectroscopic ellipsometry (SE) and
reflectance and transmittance (R&T) spectroscopy can be used to
retrieve the dielectric function (DF), detect features such as the
crystallized fraction, or investigate the surface quality. SE can be
applied to nondepolarizing materials, whereas R&T spectroscopy is
the only option for depolarizing, highly scattering materials, such as
NWs or structured surfaces.
In photovoltaics, the relevant spectral range is determined by
the range of highest intensity of the solar spectrum, that is, photon
energies from 0.8 to less than 4 eV, as can be seen in Fig. 2.1 (yellow
pattern), which is described in Section 2.3.1. For bulk silicon, most
of this region corresponds to the range of medium-low absorption
(Fig. 2.1). This region is best analyzed by R&T rather than SE, which
is only moderately sensitive to low absorption. In contrast, in the
opaque range where T = 0 and R&T bears limited information, SE
performs at best. SE detects the spectral shape of the DF around
the critical points, and gives a fundamental insight into the material.
R&T spectroscopy is the best choice to determine the absorption
edge of materials, the optical gap, and its direct or indirect character.
R, T are parameters of direct interest for those devices whose
performance is related to absorption or transparency.
SE and R&T do not detect the sub-band-gap absorption related
to defect states. Knowledge of such parameter allows us to gain
an insight into material quality rather than being of interest in
photovoltaic conversion, and will not be discussed in this text. This
chapter is mainly focussed on R&T spectroscopy. Reviews on the
state of the art of SE and polarimetry applied to the nanoscale can
be found in Refs. [6, 7].
When speaking of nanoparticles applied to photovoltaics, an
emerging topic is the application of metal nanoparticles for plasmon
induced light trapping. This exciting topic is however out of the
scope of this review.
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