σχολη εφαρμοσμένων μαθηματικων και φυσικων ... - DSpace

Abstract
In this master thesis are presented the results of structural and optical
characterization of thin film poly-Si, which is used in micromorph tandem solar
cells.
The work was performed within the Program “Competitiveness and
Entrepreneurship” and was co-financed by Hellenic Funds and the European
Regional Development Fund (ERDF) under the Hellenic National Strategic
Reference Framework (NSRF) 2007-2013.
The active region of a micromorph solar cell is composed of two thin layers of
amorphous (200nm) and polycrystalline silicon (800nm) in p-i-n arrangement.
The films were deposited on a glass substrate coated with ZnO (playing the role
of the front electrode) by chemical deposition of silicon from vapor in the
presence of plasma (PECVD).
The Efficiency of a micromorph solar cell, as well as its overall behavior are
determined by the structure and the individual characteristics of the thin films that
compose it, since they determine the quantum efficiency of the absorption of the
sunlight, the electrical mobilities of the photoinduced carriers in the structure and
the chance of reconnection. Thus, the detailed characterization of these films
using techniques such as SEM, XRD and Raman spectroscopy was deemed
necessary.
From the SEM images emerged the rough structural characteristics of our
samples in micrometric scale, while from their XRD diagrams the average size of
nanocrystallites for the entire thickness of the polycrystalline silicon film was
determined. From the Raman spectra and their analysis, which is the main part of
this thesis, the rates of the different phases that constitute the polycrystalline
silicon were obtained. Moreover, the inverted conical growth of this structure was
emerged with the characterization of samples with increasing thickness of
polycrystalline silicon. By the use of a Phonon Confinement Model (PCM), we
were able to estimate the sizes and shapes of nanocrystallites from the Raman
spectra of the samples with the successive thicknesses of μcSi and to monitor
the dependence that they present with increasing thickness.
In order to improve the performance of the structure, we proceeded to the
manufacture of polycrystalline silicon by laser annealing of amorphous silicon.
For that purpose, an Nd:YAG Laser at 1064nm and 266nm was used, in an
appropriate setup where a motorized stepper was utilized for the creation of a
large enough, evenly annealed area. Preliminary annealing-processes, with
different annealing parameters, have been tested, such as fluence and number of
pulses. The annealed material studied by micro-Raman mapping, reveals areas
of different quality ranging from amorphous to nanocrystalline Si.
Keywords: Micromorph, polycrystalline silicon, amorphous silicon, laser
annealing, SEM, XRD, Raman spectroscopy, Phonon Confinement Model (PCM),
aggregates of nanocrystallites, induced heating, phase’s rates, sizes and shapes
of nanocrystallites, mapping, Kriging method, Variograms.
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