Σπύρος Γλένης Επίκ. Καθηγητής ΕΚΠΑ (Επιβλέπων) , Σπύρος Γαρδέλης Κύριος Ερευνητής ΕΚΕΦΕ ΄΄Δημόκριτος΄΄, Ανδρούλα Νασιοπούλου Δ/ντρια Ερευνών ΕΚΕΦΕ ΄΄Δημόκριτος΄΄
The present thesis was devoted to the investigation of the different properties
of Si nanocrystals that are of interest for their use in solar cells. The
properties that were studied involve structural, optical and optoelectronic
properties, as well as electrical transport phenomena in single- and multilayer
arrays of Si nanocrystals. Finally, a novel Si-based photovoltaic cell was
studied and fabricated, with the objective of its future use in combination
with Si nanocrystals.
The properties of Si nanocrystals are very different from those of bulk Si and
they are highly dependent on crystal size. For sizes below few nm, Si
nanocrystals show quantum confinement, charging and Coulomb blockade effects.
Macroscopically, they show enhanced light emission in the visible range at room
temperature and enhanced absorption, both depending on nanocrystal size.
The first chapter of the present thesis is devoted to an introduction to
photovoltaic systems and to a bibliographic presentation of the use of Si
nanocrystals in efficient photovoltaic devices.
The second chapter is devoted to sample preparation. We first give an overview
of the different techniques used to form Si nanocrystals and oxidise them. We
then present the characterisation techniques used in the present thesis for the
structural and morphological characterisation of the nanocrystal arrays,
together with the obtained results for the sample structure and morphology.
Chapter three is devoted to the absorption in semiconductors in general and
more specifically in Si nanocrystals. The main results from this study are as
-Observation of the quantum confinement phenomenon.
-It was found that the nanocrystalline silicon shows increased absorption
coefficient compared with the crystalline silicon.
-The refractive indexes of the samples were calculated.
-The phenomenon of the photoluminescence was observed in some samples.
Chapter four is devoted to the extended study of electrical transport phenomena
in arrays of Si nanocrystals arranged in single and multilayered structures.
This study is very relevant for the use of Si nanocrystals in solar cell
devices. We present the different mechanisms involved and conclusions are drawn
for the specific nanocrystal layers studied. The main results are as follows:
-Interpretation of the conductivity in nanocrystalline silicon.
-Observation of the space charge limited current.
-Observation of Coulomb blockade in the smaller Si nanocrystals for
temperatures lower than 200 K.
In chapter five the optoelectronic properties of Si nanocrystals are presented.
More specifically, the theory of photo-transport is reviewed, together with the
main results obtained within the thesis on the mechanism of phototransport and
the different parameters that are relevant for photovoltaics. The main
conclusions from this study are as follows:
-Interpretation of the photoconductivity mechanisms.
-Observation of the dependence between the photoconductivity and the dimensions
of the nanocrystals.
-The life time of the photo-excited carriers was found to be few µsec and the
recombination was of linear type.
The last part of the thesis is devoted to the fabrication of a novel Si-based
solar cell and the optimization of its performance. The specific solar cell can
be used in combination with Si nanocrystals for optimization of its performance
through "down shifting".