Rahul Dewan - Jacobs University

1. INTRODUCTION
thinner than the absorption length of the material. Over the last decade, several concepts
have been proposed to enhance the absorption of light in thin-film silicon solar
cells. Most of the concepts are based on random nanotexturing of the contact layers of
the solar cells. Conversion efficiencies higher than 10% have been demonstrated for
amorphous and thin-film microcrystalline silicon solar cells by introducing randomly
textured interfaces in the solar cell [6, 7]. For micromorph (amorphous and microcrystalline
silicon) tandem solar cells stable efficiencies of up to 11.9% have been demonstrated
using textured interfaces [8]. The introduction of textured interfaces reduces
reflection losses and enhances light scattering and diffraction within the device. Due
to multiple reflections within the silicon layer the optical path length of the incident
light is greatly enhanced. This leads to a significantly enhanced short circuit current
and quantum efficiency in the red and infrared part of the optical spectrum.
1.2 Outline of the Thesis
In order to understand the optical propagation within such thin-film devices, it is imperative
to use numerical methods and solve the Maxwell’s equations rigorously. The
finite difference time domain method, finite element method, or rigorous coupled wave
analysis are commonly used to simulate the near-field and far-field wave propagation
in such devices [9–13]. By considering the near field optics, the nanotexturing process
for efficient solar cells can be understood and optimized. Within the scope of
this thesis, the optical enhancement and losses of microcrystalline thin-film silicon solar
cells with periodic surface textures were investigated. Beginning with a simplified
grating structure, the investigation in this work sequentially moved onto investigating
the influence of 3-dimensional pyramid textures on the optical wave propagation in
the thin-film devices. The different design parameters and methods are discussed in
the different chapters of this thesis. This thesis has been organized in such a way that
each of the chapters can be read as stand alone chapters as well. The structure of the
thesis is briefly described in the following:
In chapter 2, the key parameters that entail the study of thin-film silicon solar
cells are described. The optical properties of the different materials in the stratified
thin-film solar cell stack are discussed. Understanding these properties and how these
optical properties interact together in a solar cell is essential for optimization towards
efficient solar cells. Theoretical limits on the absorption enhancement and efficiency
of solar cells are derived, which serve as a benchmark for comparing the results in
the later chapters. The commonly used light confinement techniques in research and
production are also described.
In chapter 3, the algorithms for the numerical methods used in this study are described.
A brief recap on the fundamentals of electromagnetics theory was presented
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