The document discusses silicon nanowires and their application in solar cells. It begins by introducing semiconductor nanowires and methods of fabricating them, including bottom-up and top-down approaches. It then discusses different crystal structures of silicon used in solar cells, including crystalline, amorphous, and polycrystalline silicon. The objective is to review the application of silicon nanowires in solar cells using different silicon crystal structures. It provides literature on these topics and discusses advantages and challenges of different solar cell technologies using various silicon materials.
2. 1.Introduction
Semiconductor nanowires are 1-D structures where the magnitude of the
semiconducting material is confined to a length of less than 100nm.
Semiconductor nanowires can be fabricated by a range of methods which
can be categorised into one of two patterns, bottom-up or top-down.
Bottom-up processes can be defined as those where structures are
assembled from their sub-components in an additive fashion.
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3. Cont’d
Top-down fabrication strategies use sculpting or etching to carve structures from
a larger piece of material in a subtractive fashion
A silicon nanowire is an extended single crystal of silicon with a diameter of tens
to a few hundred nanometres and with a length of several micrometres.
Both solar cell and nanowire research have become hot topics within science
and engineering.
The need for higher solar cell efficiencies at lower cost has become apparent, and
at the same time synthetic control in nanoscience has improved such that high
performance electronic devices are becoming possible.
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4. Objective
The objective of this seminar work was to review the application of
silicon nanowire for solar cells/photovoltaic cells for different crystal
structure of silicon.
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5. 2. LITERATURE REVIEW
Silicon is one of the most abundant elements in the Earth’s crust.
It is usually found in the form of oxides and silicates, such as sand
and quartz.
Possibly one of the more important and multipurpose elements,
silicon can be used to produce everything from elements to computer
chips.
Silicon is the basis for current electronics and photovoltaic, so it is the
most widely studied and described semiconductor.
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6. 2.1.2. Crystalline silicon
Crystalline silicon(c-Si) forms the basis of many of today’s
integrated circuits as it is a readily available semiconductor which is
relatively easy to process and dope to form semiconductor devices.
Devices made from crystalline silicon include many integrated
circuits, diodes or solar cells.
High purity crystalline silicon is typically produced using the
Czochralski process (Lide, 2005).
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7. 2.1.2 Amorphous silicon
Amorphous silicon (a-Si) is a glass like materials that
structurally does not have long rang structural order and is
randomly oriented.
Due to the random orientation of the material, the absorption
coefficient of light in amorphous silicon tends to be higher than
that of crystalline silicon.
This allows thin film devices to be fabricated from amorphous
silicon that can absorb similar quantities of light to much thicker
crystalline silicon slices.
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8. Cont’d
One of the main uses of amorphous silicon is in the solar cell
industry for the fabrication of cheap thin film solar cells.
In addition it is a relatively straightforward process to create doped
layers of amorphous silicon.
The dopant layers can be produce by introducing dopant gases
during the deposition of amorphous silicon.
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9. 2.1.3. Polycrystalline silicon
Polycrystalline silicon is variant of silicon that consists of a series of
small crystallites that have grown together with an extremely high
crystallinity or proportion of crystalline material.
Typically, polycrystalline silicon is defined as having a grain size
(cryatallite diameter) of between 10 and 30 micrometres and a
crystalline fraction of close to 100% (Cabrrocas, 2004).
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10. 2.2. Silicon nanowire
A nanowire is a nanostructure, with the diameter of the order of a
nanometer (10−9 meters).
Alternatively, nanowires can be defined as structures that have a
thickness or diameter constrained to tens of nanometers or less and an
unconstrained length.
Many different types of nanowires exist, including metallic (e.g., Ni,
Pt, Au), semiconducting (e.g., Si, InP, GaN, etc.), and insulating (e.g.,
SiO2, TiO2).
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11. Cont’d
Several growth mechanisms are used to describe the growth of silicon
nanowires.
The most widely used of these is vapour liquid solid (VLS) mechanism.
This was first proposed by Wagner and Ellis in 1964 and uses a liquid
metal catalyst to aid the growth.
Gold is the catalyst most frequently used although some other catalysts,
such as titanium, have also used.
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13. 2.3. Application of silicon nanowires
2.3.1 Photovoltaic cells/ solar cells
The photovoltaic effect was first reported by Edmund Becquerel in
1839 when he observed that the action of light on a silver coated
platinum electrode immersed in electrolyte produced an electric
current.
A solar cell (also called a photovoltaic cell) is an electrical device
that converts the energy of light directly into electricity by the
photovoltaic effect.
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14. Cont’d
There are a number of different ways to produce solar cells and a range
of materials from which they can be produced.
Silicon is a commonly used semiconductor material for producing
solid state solar cells.
Solar cells producing using silicon have different properties when they
are made using different types of silicon.
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16. Cont’d
Solar cells are often electrically connected and encapsulated as a
module.
Photovoltaic modules often have a sheet of glass on the front (sun
up) side, allowing light to pass while protecting the semiconductor
wafers from abrasion and impact due to wind-driven debris, rain,
hail, etc.
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17. Cont’d
Solar cells convert three-quarters of the energy contained in the Sun‘s
spectrum into electricity yet the infrared spectrum is entirely lost in
standard solar cells.
In contrast, black silicon solar cells are specifically designed to
absorb this part of the Sun‘s spectrum – and researchers have recently
succeeded in doubling their overall efficiency.
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18. Cont’d
Much of the early use of silicon photovoltaic was in power supply
systems for space vehicle.
However, in recent times, there is an increasing market for
photovoltaic remote area power supplies and in distributed
generators on the electricity grid.
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19. Cont’d
In the marketplace there are several types of solar cell technologies
available including crystalline, microcrystalline, nanocrytalline and
amorphous silicon.
The world market is currently dominated by crystalline silicon solar
cells which held some 90.9% of the market in 2004 and 93.5% in 2005
(Singh and Jennings, 2007).
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20. 2.3.1.1. Crystalline solar cells
Traditionally most solar cells have been made from doped crystalline
semiconductors such as crystalline silicon (c-Si).
Crystalline semiconductors have a very well defined structure with
both a high long range and high short range order.
The characteristics of crystalline solar cells include a well-defined
band gap and high quantum efficiency.
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21. Cont’d
However, style of cell is very expensive.
It is also time consuming to make involving as it does wafers of
single crystal.
These drawbacks and the rising costs of solar grade crystalline
silicon have encouraged the development of other materials
particularly thin film for solar cells.
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22. 2.3.1.2. Nanocrystalline solar cells
o Thin film solar cells can be produced using nanocrystalline silicon as
core component.
o Nanocrystalline silicon solar cells have been produced using a
combination of amorphous silicon and nanocrystalline silicon in triple
junction device.
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23. Cont’d
o Nanocrystalline silicon films have recently attracted attention for use
in photovoltaics solar cells since they show promise for providing an
approach that results in lower cost and higher efficiency than the
conventional solar cells.
o Its wider band gap is much more suitable for solar cell applications
than the narrow indirect band gap of single crystalline silicon.
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24. Cont’d
o In addition crystalline silicon has low absorption coefficient and narrower
band gap that desired for solar cell applications.
o Solar light with energy much larger than the band gap, when absorbed by
the materials is converted into heat rather than electricity thus reducing the
efficiency.
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25. 2.3.1.3. Amorphous silicon solar cell
• Amorphous silicon (a-Si) has been used as a photovoltaic solar cell
material for devices which require very little power, such as pocket
calculators, because their lower performance compared to traditional
c-Si solar cells is more than offset by their simplified and lower cost
of deposition onto a substrate.
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26. 2.4. Band gap
The band gap also known as the optical gap, energy gap or mobility
gap, is an important properties of semiconductors that determines the
optoelectronics properties of devices created from such
semiconductors.
The band gap is the minimum amount energy needed for an electron
to jump from the valence band to the conduction band is as shown in
figure below.
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27. Cont’
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Schematic of the semiconductor band gap showing the valance band,
conduction band and mid gap states (Parlevliet, 2008)
28. Cont’d
Photovoltaic devices, the band gap energy needs to be close to the peak of
the energy range of visible light (1eV to 3eV) or the spectrum of light
emitted by the sun.
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29. 2.4.1. Types of band gap
In semiconductor physics, the band gap of a semiconductor is always one of
two types, a direct band gap or an indirect band gap.
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Indirect band gapDirect band gap
30. 3.Conclusion
The Monocrystalline silicon cell is produced from pure silicon.
Since the Monocrystalline silicon is pure and defect free, the
efficiency of cell will be higher.
In polycrystalline solar cell, liquid silicon is used as raw material
and polycrystalline silicon was obtained followed by solidification
process.
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31. Cont’d
Amorphous silicon was obtained by depositing silicon film on the
substrate like glass plate.
The efficiency of amorphous cells is much lower than that of the
other two cell types.
As a result, they are used mainly in low power equipment, such as
watches and pocket calculators, or as facade elements.
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