Applications of Spectrally-Resolved Photoluminescence in Silicon Photovoltaics
Date
2016
Authors
Nguyen, Hieu Trong
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Abstract
In broad terms, this thesis is devoted to measuring and
interpreting the photoluminescence spectra emitted from different
structures in crystalline silicon wafers and solar cells. Based
on the knowledge accumulated, it also establishes a variety of
applications of photoluminescence spectroscopy in silicon
photovoltaics. The thesis may be divided into 3 main categories:
band-to-band luminescence from wafers, deep-level luminescence
from defects and impurities, and composite luminescence from
different structures and layers in solar cells.
First, this thesis utilizes band-to-band photoluminescence
spectra emitted from planar silicon wafers to determine the
values of the band-to-band absorption coefficient and the
radiative recombination coefficient as a function of temperature
with high precision. Parameterizations of these two coefficients
are established to allow convenient calculations. Based on the
newly established temperature data, the impacts of surface
geometries and excess carrier profiles on luminescence spectra
emitted from various silicon wafers are investigated via both
modeling and experiments as a function of temperature. The
results suggest that, the accuracy of many
photoluminescence-based techniques, established mainly at room
temperature in the literature, can be further improved by
performing the measurements at higher temperatures due to the
increasing impacts of surface reflectivities and excess carrier
profiles on luminescence spectra with rising temperatures. These
applications highlight the significance of the established data
of the two coefficients for spectral fitting techniques.
Next, the thesis investigates the deep-level luminescence from
defects and impurities distributed around sub-grain boundaries in
multicrystalline silicon wafers. The thesis shows that, the
dislocations at sub-grain boundaries and the defects and
impurities trapped around the dislocations emit separate
luminescence peaks at low temperatures. The luminescence
intensity of the trapped defects and impurities is found to be
altered significantly after phosphorus gettering, whereas the
dislocation luminescence is not changed throughout different
solar cell processing steps. Also, the trapped defects and
impurities are found to be preferentially distributed on one side
of the sub-grain boundaries due to the asymmetric distribution of
their luminescence intensity across the sub-grain boundaries. In
addition, the thesis also demonstrates that the damage induced by
laser doping is related to dislocations, since its deep-level
luminescence spectrum has similar properties to those emitted
from dislocations in multicrystalline silicon wafers. The
interface between the laser-doped and un-doped regions is found
to contain more damage than the laser-doped regions.
Furthermore, the thesis reports a new photoluminescence-based
method to separate the luminescence signatures from different
layers and structures in a single silicon substrate, courtesy of
the well-resolved luminescence peaks at low temperatures from
different layers. In particular, the technique is applied to
characterize the doping level of both locally-diffused and
laser-doped regions on various silicon solar cells and cell
precursors, utilizing band-gap narrowing effects in heavily-doped
silicon. The results show that, the interface between the
laser-doped and un-doped regions is much more heavily-doped that
the doped regions. In addition, the technique is also applied to
evaluate and the parasitic absorption of different surface
passivation films on finished solar cells, due to the correlation
between the sub band-gap luminescence intensity from these
passivation films and the optical absorption in the films. The
technique is contactless and nondestructive, requires minimal
sample preparation, and provides micron-scale spatial
resolutions.
Finally, the thesis combines the advantages of
spectrally-resolved photoluminescence (PLS) and photoluminescence
excitation spectroscopy (PLE) to develop a PLS-PLE-combined
technique for characterizing wafers and solar cells. In
particular, the entire photoluminescence spectrum from a silicon
wafer or solar cell is captured and monitored while the
excitation energy is varied. This technique allows us to
quantitatively evaluate both the doping level and the junction
depth of various diffused silicon wafers, the defects induced by
the post-diffusion thermal treatment at different depths below
the wafer surface, and the enhanced diffusion at grain boundaries
and sub-grain boundaries in multicrystalline silicon wafers. The
results show that, the enhanced diffusion happens at both grain
boundaries and sub-grain boundaries.
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Keywords
photoluminescence, silicon, photovoltaics
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Thesis (PhD)
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