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Spectroscopic Ellipsometry of InAs/(Al)GaAs Quantum Dots for Intermediate Band Solar Cells

Drøyli, Maja Bjerke
Master thesis
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17851_FULLTEXT.pdf (10.10Mb)
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http://hdl.handle.net/11250/2463526
Utgivelsesdato
2017
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  • Institutt for fysikk [1817]
Sammendrag
As a step towards reducing the CO2 emissions to the atmosphere, generating electricity

from renewable energy resources instead of fossil fuels is a good contributor. Solar cells

convert energy from sunlight directly into electricity, and they have long lifetimes and

require little maintenance as they contain no mechanically moving parts. Even though

efficiency has increased and prices have dropped significantly during the last decade, there

are still many steps that can be made towards making solar cells even more attractive.

Intermediate band solar cells (IBSCs) have a theoretical potential of being 50 % more

efficient than the theoretically most efficient single band-gap solar cells, which is what

is used in today s commercial cells, without any major increase in cost. Many steps are

yet to be taken to develop highly efficient IBSCs, and one way to increase the speed and

reduce the costs of this development is to perform simulations of the cells, which can lead

to a better understanding of how the cells work and which compositions provides the most

efficient cells.

To simulate a solar cell, a good optical model is needed for that cell. Quantum dot (QD)

IBSCs currently have no physically explainable, good optical models, and one way to create

a model is by using spectroscopic ellipsometry (SE). In this thesis, InAs/(Al)GaAs

QD-IBSCs are investigated using this technique. As a QD-IBSC has a relatively complex

structure, different parts of this structure are studied separately to reduce the amount of

unknowns in each measurement. This work looks into GaAs with varying doping, samples

with two and ten layers of InAs QDs with GaAs and AlGaAs spacers, as well as

full solar cell structures. Most emphasis is put into understanding the two-layer QD samples,

as these are the simplest QD structures and therefore contain the least number of

unknowns. Also, these samples have previously been measured using photoluminescence

spectroscopy, atomic force microscopy, scanning electron microscopy and transmission

electron microscopy, providing useful information for SE analysis.

Different models from literature are applied in the modeling of QDs and wetting layer.

The QD samples are first modeled only using bulk GaAs, oxide and surface roughness, before

a macroscopic model with bulk InAs values and an effective medium approximation

(EMA) is applied, and at last an oscillator model of Lorentz oscillators is tried. This thesis

finds that the simple model outperforms the more detailed models, indicating that the

assumptions made in the detailed models are not correct. Applying the oscillator model

gives some surprising results indicating a negative contribution to the pseudo-dielectric

function (PDF) of the sample although the oscillators have positive amplitude. No explanation

to this has been found, and it is not known if this is caused by a bug in the

software used for analysis or if it has a physical explanation. However, when combining

the two detailed models by making an EMA of InAs and an oscillator model, the oscillator

model contributes as expected to the PDF, outperforming the simple model for some of

the measurements.

Although the oscillator model showed some unexpected behaviour, it is still believed

to be the model with most potential to make a good and physically explainable model for

QDs.
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