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STUDY OF SURFACE PLASMONS FOR ENHANCED THIN FILM
SOLAR CELL
A DISSERTATION SUBMITTED TO
DEPARTMENT OF APPLIED PHYSICS, ELECTRONICS AND COMMUNICATION
ENGINEERING,
UNIVERSITY OF DHAKA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
BACHELOR OF SCIENCE
SUBMITTED BY
EXAM ROLL: 2228
REGISTRATION: HA 1447
SESSION: 2007-2008
CERTIFICATE OF APPROVAL
NAME OF THE PROJECT: STUDY OF SURFACE PLASMONS FOR ENHANCED
THIN FILM SOLAR CELL
It is done under my supervision, meets acceptable presentation standard and can be submitted
for evaluation to the Department of Applied Physics, Electronics & Communication Engineering
in partial fulfillment of the requirements for the degree of Bachelor of science in Applied
Physics, Electronics & Communication Engineering.
Supervisor
Date…………………..
Subrata Das
Lecturer
Dept. of Applied Physics, Electronics and Communication Engineering,
University of Dhaka.
DECLARATION
I declare that this project entitled is the result of my own research except as cited inthe references. The project has not been accepted for any degree and is notconcurrently submitted in candidature of any other degree.
Signature…………. ………..
Date……………………
Signature…………. ………..
Date……………………
ACKNOWLEDGMENTS
I would like to express my gratitude to my supervisor Mr. Subrata Das for his guidancein this project work and for showing how to extract the essence out of vague ideas. Iwould like to thank him for his continuous support and guidance throughout my project.Not only his support has been crucial to this work, but also I am grateful that Mr. Dasalways provides extra motivation to get things done. I greatly benefited from hisphilosophy and method of performing the pioneering research work. This work wouldnot have been possible without the mentoring of him. His expertise in photonics andnanoelectronics made the completion of the work possible.
Thanks to all my friends for their pleasant cheers for me along the way. Finally I can’tjust say thanks to my father, mother and sister. You people took care of me at everysingle moments of my existence - truly you are my creator.
Lists of Figures
Figure Page
Figure 2.1 Block diagram of a solar cell 6
Figure 2.2 Band diagram of a silicon solar cell 8
Figure 2.3 Current, voltage and power curves of a solar cell 9
Figure 2.4 A typical dye sensitized solar cell 14
Figure 2.5 AM 1.5 solar spectrum and the portion of the spectrum whichcan be utilized by (a) Si solar cells (b)Ga0.35In0.65P/Ga0.83In0.17As/Ge solar cells
16
Figure 3.1 (a)Schematic of a surface plasmon at the interface of a metal anddielectric showing the exponential dependence of the field E in the zdirection along with charges and (b) electromagnetic field of surfaceplasmons propagating on the surface in the x direction.
21
Figure 3.2 Definition of a planar waveguide geometry 26
Figure 4.1 AM1.5 solar spectrum, together with a graph that indicates thesolar energy absorbed in a 2-μm-thick crystalline Si film(assuming single-pass absorption and no reflection). Clearly, alarge fraction of the incident light in the spectral range 600–1,100 nm is not absorbed in a thin crystalline Si solar cell.
33
Figure 4.2 Energy-crystal momentum diagram of a (a) direct bandgap
material showing excitation of an electron from the valence to
the conduction band by absorption of a photon (b) indirect
bandgap material showing absorption of a photon by a two step
process involving a phonon emission or absorption.
35
Figure 4.3 Light trapping schemes 36
Figure 4.4 The high efficiency PERL cell with the inverted pyramid
structures on the top surface for light trapping.
38
Figure 4.5 Plasmonic light-trapping geometries for thin-film solar cells. a,
Light trapping by scattering from metal nanoparticles at the
surface of the solar cell. Light is preferentially scattered and
trapped into the semiconductor thin film by multiple and high-
angle scattering, causing an increase in the effective optical path
length in the cell. b, Light trapping by the excitation of localized
surface plasmons in metal nanoparticles embedded in the
semiconductor. The excited particles’ near-field causes the
creation of electron–hole pairs in the semiconductor. c, Light
trapping by the excitation of surface plasmon polaritons at the
metal/semiconductor interface. A corrugated metal back surface
couples light to surface plasmon polariton or photonic modes
that propagate in the plane of the semiconductor layer
41
Figure 4.6 SPPs are bound waves at the interface between a semiconductor and
a dielectric. This dispersion diagram, plotting the relationship between
frequency and wavevector (2π/λ) for SPPs on a Ag/Si interface. The
‘bound’ SPP mode occurs at energies below the surface plasmon
resonance energy of 2.07 eV (600 nm)
43
Figure 4.7 Fraction of light scattered into the substrate, divided by total scattered
power, for different sizes and shapes of Ag particles on Si. Also plotted
is the scattered fraction for a parallel electric dipole that is 10 nm from
a Si substrate
46
Figure 4.8 Photocurrent enhancement from a 1.25 m thick SOI test solar cell for
particle size corresponding to 12 and 16nm mass thickness of Ag
relative to the cell without silver particles
47
Figure 4.9 Extinction efficiency of Au nanoparticles of different diameters
suspendedin aqueous solution48
Figure4.10
Photocurrent of Si pn junction diode with and without Au nanoparticles 49
Figure4.11
Photocurrent enhancement plots for the case with metal islands and
for thecase of metal islands overcoated with ZnS for 95nm Si on SOI
with 35nm top oxide.Inset shows the corresponding bare island
resonances for the two cases clearly showing the red shifting with the
ZnS overcoating.
51
Figure 5.1 Cost efficiency Trade off for photovoltaics 57
Figure 5.2 Plasmonic tandem solar-cell geometry. Semiconductors with different
bandgaps are stacked on top of each other, separated by a metal
contact layer with a plasmonic nanostructure that couples different
spectral bands of the solar spectrum into the corresponding
semiconductor layer.
60
Figure 5.3 Plasmonic quantum-dot solar cell designed for enhanced
photoabsorption in ultrathin quantum-dot layers mediated by coupling
to SPP modes propagating in the plane of the interface between Ag
and the quantum-dot layer. Semiconductor quantum dots are
embedded in a metal/insulator/metal SPP waveguide
61
List of Tables
Table Page
Table 2.1 Efficiency of different thin film solar cell 15
Table 2.2 Bandgaps of different multijunctions. 16
Table 2.1 Efficiency of different multijunctions. 17
Table 5.1 Photovoltaic resource requirements: materials by production andreserve
59
i
TABLE OF CONTENTS
ABSTRACT……………………………………………………………………………...iii
CHAPTER 1 Introduction
Motivation of the project……………………………………………………….….…..2 Objective of the project …………………………...………………………………......2 Organization of the project…….……………………………………………………....2 References……………………………………………...………………………….......3
CHAPTER 2 An overview to solar cell technology
Introduction..........................................................................................................................5 Solar cell history..................................................................................................................5 Structure ….........................................................................................................................6 Solar cell operation........................................................................................................7 Solar cell efficienc .y.....................................................................................................8 Solar cell generation .....................................................................................................9 First generation solar cell..……………………………...………….………..............10
Monocrystalline silicon ……………………....…………..................10 Polycrystalline silicon/multicristalline silicon ..………………..…..10
Second generation solar cells ……………..……………………………...................10 Thin film silicon ....……………………………….……....................11
o Advantages ...…………………….……………...…..11o Disadvantages……………………….……………….11
Cadmium telluride(CdTe) solar cell……………………….................11o Advantages…………………….……………………..12o Disadvantages………………….……...……………...12
Copper indium gallium selenide(CIGS) solar cell ..….………….......12o Advantages ……..…………………..…..............…...13o Disadvantages ..…………………….…......................13
Dye sensitized solar cell(DSSC).……………….…….………..…....13o Advantages …...……………………....…………......14o Disadvantages ..……………………….………….......14
Other products of second generation …………….…………….….....14 Third generation solar cells ……………...……………………………………..........15
Advantages ………………..………………………..…………....….17 Disadvantages .………………………………………...………….....17
References ………………………………………………………..…............…….…17
CHAPTER 3Intrroduction to surface plasmons
Introduction.................................................................................................................19 What are plasmons……………………………………………….……….………....19
ii
Surface plasmon………………..…………………………...……….20 Volume plasmons……………………………………….….………..20 Surface Plasmon polariton……………………...………….….……..20 Localized surface plasmon…………………………………….….....23
Mathematical background……....................................................................................25
References……………...............................................................................................31
CHAPTER4Plasmonic solar cell
Introduction……………….........................................................................................32 Plasmonics for photovoltaic…....................................................................................32
Light trapping technique in conventional solar cell………................33 Light trapping technique in thin film solar cell...................................38 Light trapping for photovoltaics……………………….…………….40
Choice of metals..........................................................................................................44
Effect of various parameters in plasmonic thin film solar cell....................................45 Effect of size and shape of nanoparticle.………………………….....45 Effect of dielectric coating…………………………………………..52 Effect of different materials………………………………………....53
Conclusion………………………………………………………………….……......54 References…………………………………………………………………………...54
Chapter 5Conclusion and future perspective
Discussion……………...……………………………………………………………57 Limitations and future considerations………………………….………………........60 Future perspective…………………………………………………………...............61
References…………………………………………………………………………...63
Abstract
iii
Abstract
Photovoltaics (PV) are the fast emerging as an attractive renewable energy
technology due to concerns of global warming, pollution and scarcity of fossil fuel
supplies. However to compete in the global energy market, solar cells need to be
cheaper and more energy efficient. Silicon is the favorite semiconductor used in solar
photovoltaic cells because of its ubiquity and established technology, but due to its
indirect bandgap silicon is a poor absorber and light emitter. Thin film cells play an
important role in low cost photovoltaics, but at the cost of reduced efficiencies when
compared to wafer based cells. Plasmonic nanostructures have been recently
investigated as a possible way to improve absorption of light in solar cells. The strong
interaction of small metal nanomaterial with light allows control over the propagation of
light at the nanoscale and thus the design of ultrathin solar cells in which light is
trapped in the active layer and efficiently absorbed.
Chapter 1
1
INTRODUCTION
The solar photovoltaics industry is a rapidly growing business, with value of $10 billion
per year and annual growth of over 30% [1], which involves a great deal of research
both in the industrial and academic level. Academic research is a key factor in
developing applications for domestic enterprises, so that they would be able to
compete in international markets. As photovoltaic technologies are just breaking
through commercially, there is a good opportunity for domestic enterprises to take
position on the edge of the solar photovoltaic industry. Silicon is and has for long been
the most widely used material for photovoltaic cells. In 2004, at least 94% of
commercial photovoltaic devices shipped were manufactured of silicon [2]. The status
of silicon as the dominant commercial photovoltaic material is due to it’s abundancy in
nature, stability, non-toxicity and well established refining and processing technologies
[3]. Although commercial single crystalline silicon solar cells have high efficiencies [2],
their high manufacturing costs prevent them from breaking through in the energy
markets.
The thin-film silicon solar cells [4, 5] are cheaper to manufacture, but their efficiencies
are low compared to those achieved with single crystalline silicon wafer based cells
[6]. Therefore, the trend in the photovoltaic research is to reach for even higher
efficiencies and less material consuming cell designs. One promising approach to
achieve these goals, and to lower the price of solar produced electricity to a
commercially competitive level, is the exploitation of optical properties of metal
nanoparticles in photovoltaic cells. The purpose of this thesis is to provide an overview
of the research conducted, until present day, on metal nanoparticles as means to
improve silicon solar cell efficiency. The research was focused on silicon as
photovoltaic material because of the already existing wide industry built around it; the
knowledge, processes and materials are already commercially available; and it can
provide a part of the solution for the acute need of renewable energy.
Chapter 1
2
1.1 Motivation of the project
Plasmonics forms a major part of the fascinating field of nanophotonics, which explores
how electromagnetic fields can be confined over dimensions on the order of or smaller
than the wavelength. The foremost motivation to choose this field comes from the fact that
plasmonics is firmly grounded in classical physics, so that solid background knowledge in
electromagnetism and material science at undergraduate level are sufficient to
understand main aspects of this topic. Plasmonics is the field that deals with a
numerous applications like chemical and bio-sensing, efficiency of thin film solar cell,
fluorescence effect for sensing and a lot others. Our motivation to work on this field also
lies in these numerous applications.
After deciding the field of interest, we have selected one of the most promising
applications of this sector as our topic of interest- utilization of nanoparticles to enhance
the solar cell efficiency. Allthough the research in this sector has already been started,
but a complete sustainable tradeoff between solar cell efficiency and the price yet not
done.
1.2 Objectives of the project
The main objectives of this project are-
To study about different types of solar cell generations.
To study the physics behind the plasmonics structures
To study about the utilization of different nanoparticles in thin film solar cells.
1.3 Organization of the project
A general introduction of this project is presented in Chapter 1 along with the
motivation behind this project.
Chapter 1
3
Chapter 2 summarizes different types of solar cell generations including the crystalline
solar cell to the thin film multijunction solar cells with their advantages and
disadvantages.
Chapter 3 summarizes the most important facts and phenomena that form the basis
for a study of surface plasmons. Mathematical derivation of the study behind
plasmonics is shown in this chapter.
Chapter 4 gives the idea of implementing plasmonic nanoparticles in solar cells. The
variation of different parameters using different types of nanoparticles are delineated
in this chapter.
Chapter 5 discusses about future prospects of plasmonic solar cells.
1.4 References
[1] N. S. Lewis. Toward cost-effective solar energy use. Science, 315:pp. 798–
801, 2007.
[2] L. L. Kazmerski. Solar photovoltaics R&D at the tipping point: A 2005
technology overview. Journal of Electron Spectroscopy and Related Phenomena,
150:pp. 105–135, 2006.
[3] S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green. Surface plasmon
enhanced silicon solar cells. Journal of Applied Physics, 101:p. 093105,
2007.
[4] M. A. Green. Recent developments in photovoltaics. Solar Energy, 76:pp.
3–8, 2004.
[5] A. V. Shah, H. Schade, M. Vanecek, J. Meier, E. Vallat-Sauvain, N.Wyrsch,
Chapter 1
4
U. Kroll, C. Droz, and J. Bailat. Thin-film silicon solar cell technology.
Progress in Photovoltaics: Research and Applications, (12):pp. 113–142,
2004.
[6] M. A. Green, K. Emery, Y. Hishikawa, and W. Warta. Solar cell efficiency
tables (version 31). Progress in Photovoltaics: Research and Applications,
16:pp. 61–67, 2008.
Chapter 2
5
AN OVERVIEW TO SOLAR CELL TECHNOLOGY
2.1 INTRODUCTION
Solar cells convert light energy into electrical energy through a direct process known
as photovoltaic effect. Photovoltaic technology is one of the most effective
technologies for producing energy. Sunlight is abundant and is renewable and free.
So utilizing sunlight can reduce the energy demand. All we need is to develop better
technology to efficiently generate high efficient solar cells.
2.2 Solar cell history
The development of the solar cell stems from the work of the French physicist
Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect while
experimenting with a solid electrode in an electrolyte solution; he observed that
voltage developed when light fell upon the electrode. About 50 years later, Charles
Fritts constructed the first true solar cells using junctions formed by coating the
semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts's
devices were very inefficient, transforming less than 1 percent of the absorbed light
into electrical energy.
By 1927 another metal semiconductor-junction solar cell, in this case made of copper
and the semiconductor copper oxide, had been demonstrated. By the 1930s both the
selenium cell and the copper oxide cell were being employed in light-sensitive
devices, such as photometers, for use in photography. These early solar cells,
however, still had energy-conversion efficiencies of less than 1 percent. This impasse
was finally overcome with the development of the silicon solar cell by Russell Ohl in
1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and
Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-
conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as
well as those made of gallium arsenide, with efficiencies of more than 20 percent had
Chapter 2
6
been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is
concentrated onto the cell surface by means of lenses, achieved an efficiency of 37
percent due to the increased intensity of the collected energy.
2.3 Structure
Modern solar cells are based on semiconductor physics -- they are basically just P-N
junction photodiodes with a very large light-sensitive area. The photovoltaic effect,
which causes the cell to convert light directly into electrical energy, occurs in the three
energy-conversion layers. The first of these three layers necessary for energy
conversion in a solar cell is the top junction layer (made of N-type semiconductor).
The next layer in the structure is the core of the device; this is the absorber layer (the
P-N junction). The last of the energy-conversion layers is the back junction layer
(made of P-type semiconductor).
Figure 2.1 Block diagram of a solar cell
Chapter 2
7
As may be seen in the above diagram, there are two additional layers that must be
present in a solar cell. These are the electrical contact layers. There must obviously
be two such layers to allow electric current to flow out of and into the cell. The
electrical contact layer on the face of the cell where light enters is generally present in
some grid pattern and is composed of a good conductor such as a metal. The grid
pattern does not cover the entire face of the cell since grid materials, though good
electrical conductors, are generally not transparent to light. Hence, the grid pattern
must be widely spaced to allow light to enter the solar cell but not to the extent that the
electrical contact layer will have difficulty collecting the current produced by the cell.
The back electrical contact layer has no such diametrically opposed restrictions. It
need simply function as an electrical contact and thus covers the entire back surface
of the cell structure. Because the back layer must be a very good electrical conductor,
it is always made of metal.
2.4 Solar cell operation
When photon in sunlight strikes the solar cells, one of the three things can happen:
1. The photon can pass straight through silicon.
2. The photon can replace the surface.
3. The photon can be absorbed by silicon, if the photon energy is higher than the
silicon bandgap value.
In a solar cell, the third phenomenon is taken account. When the photon is absorbed,
its energy is given to an electron in crystal lattice. Usually this electron is in valence
band. Whenever it gets energy from the photon, it excites and jumps to the conduction
band. This creates negatively charged electron (e-) in conduction band and positively
charged hole (h+) in valence band. The asymmetry of the structure leads to the
Chapter 2
8
gradients of concentration of electrons and holes and develops an electric field in
junction. The band diagram of a silicon solar cell is portrayed in figure 2.2.
Figure 2.2 Band diagram of a silicon solar cell[8].
2.5 Solar cell efficiency
Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident
energy in the form of sunlight. The most efficient operating point of a solar cell is at its
maximum power point, Pmpp which is the product of the voltage and current at the
point. The efficiency is related to open circuit voltage Voc and short circuit current Isc
via the fill factor FF, given by
η = = = (2.1)
Where Pin is the incident power on the solar cell.
Chapter 2
9
Figure 2.3 Current, voltage and power curves of a solar cell
Not all the light incident on the solar cell is absorbed and there exist detrimentalmechanisms that prevent a solar cell from attaining its ideal efficiency. The longestwavelength that can be absorbed is limited by the bandgap of the semiconductor,which is 1.1 eV for Si. This effect alone limits the maximum efficiency for a Si cell to44%. Overall the conversion efficiency value has a maximum theoretical limit of 31%[Shockley ‘61] for a single junction semiconductor device with a bandgap of 1.3eV. thetheoretical energy conversion efficiency limits for Si solar cell remains at 29%. Overthe past decade much of the cell efficiency improvements have resulted from themove towards multi-junction devices. Using this approach , the limiting efficiency forunconcentrated sunlight is increased to nearly 70%.
2.6 Solar cell generation
Despite the solar cells are effective to power generation, the application of this
technology is limited. The main obstacle behind this is high price of photovoltaic
Chapter 2
10
modules. Also, there exists the scarcity of manufacturing materials in the earth. So,
researches have been done to utilize the limited materials and produce high efficiency
solar cells which can be categorized by three different generations,
1. First generation solar cells- bulk silicons
2. Second generation solar cells- thin film technology
3. Third generation solar cells- multijunction technology
2.7 First generation solar cells
The most extensive materials for solar cells, crystalline silicons were used in thegeneration. Bulk silicon is separated into various categories according to crystallinityand crystal size.
2.7.1 Monocrystalline silicon (c-Si)
This type of Si is often prepared by using Czochralski method. This type of silicons areopposite of amorphous silicon in which short range atomic order is observed. Thistype of solar cells can achieve upto 17% efficiency[1].
2.7.2 Polycrystalline silicon/ multicrystalline silicon( poly Si or mc- Si)
This type of silicons are as much as 99.9999% pure[2]. This type of solar cells areless expensive than single crystal silicon cell. However, the efficiency is less thanmonocrystalline Si cells.
2.8 Second generation solar cells
Photovoltaic products of first generation were efficient enough but limitation of thematerials and high costs led to the second generation solar cell technology in whichthin film technology was the salient feature. The cost of the silicon can be reduced byreducing thickness of the cells. Different type of thin film solar cells are seen now-a-days which usually are categorized according to the material used.
Chapter 2
11
2.8.1 Thin film silicon ( TF-Si )
In contrast to bulk silicons, this type of silicon includes amorphous silicon(a- Si),protocrystalline silicon(p- Si), nanocrystalline(nc- Si). The silicon is mainly depositedby plasma enhanced chemical vapour deposition PECVD.
2.8.1.1 Advantages
Strong absorption in visible part of the solar spectrum. Easier to manufacture. Less material is used in this type of cell.
2.8.1.2 Disadvantages
Lower efficiencies. Shorter lifetime.
2.8.2 Cadmium telluride(CdTe) solar cell
This type of solar cells use thin film of cadmium telluride semiconductor. This is the
only thin film photovoltaic technology which exceeds crystalline Si solar cells in
cheapness[3].Sun illumination takes place from the backside of the original glass
deposition substrate. Scribing steps are used to allow for a monolithically integrated
module, and the module is laminated on the backside with a low-cost glass sheet. The
CdTe layer can be deposited by low-cost deposition methods such as closed-space
sublimation and sputtering. The key technological challenges for CdTe PV technology
are associated with improving device performance and environmental and health
concerns Associated with Cd in manufacturing and at the end of module life. The
record efficiency for CdTe was obtained by the U.S. National Renewable Energy
Laboratory (NREL) at ~16.5. Module-level efficiencies are lower for several reasons,
with the main contribution being from short-wavelength (<500 nm) absorption in the
CdS window layer.It has been shown that reducing the CdS thickness improves the
short circuit current density, though typically at a loss of Voc and FF . Losses
Chapter 2
12
associated with grain boundary space charge and related effects also strongly
contribute to reduction in efficiency. Interestingly, the efficiency of polycrystalline CdTe
solar cells is equal to or higher than that of single-crystal devices, which have been
argued to be related to doping type inversion in the region near the heterojunction.
The purity of grains compared to bulk materials may also play arole in this
observation. Concerns about the hazardous nature of Cd are addressed at the end of
module life by robust buy-back and recycling programs, and it is well known that Cd
within the CdTe crystal phase (and embedded in glass sheets) does not pose the
health hazards associated with pure Cd. Appropriate environmental health and safety
(EHS) measures must be in place, however, during manufacture of CdTe PV
modules. The CdTe layer can be deposited by low-cost deposition methods such as
sublimation-based deposition and sputtering
2.8.2.1 Advantages
Easier to manufacture. Cadmium telluride absorbs solar spectrum at almost at almost ideal
wavelength. Cadmium is abundant in earth.
2.8.2.2 Disadvantages
Low efficiency levels. Limited supply of tellurium in earth. Cadmium in the cell could be toxic if released.
2.8.3 Copper indium gallium selenide(CIGS) solar cell
The Cu (In,Ga)Se2 (CIGS) are other materials systems of great interest in thin-film
form for photovoltaic. These materials, which can also substitute sulfur for selenium
(so-called CIGSSe), have been studied for the past couple of decades, and record
efficiencies of nearly 20% have been shown by NREL. Unlike CdTe, CIGS solar
Chapter 2
13
cellsare fashioned in a standard substrate configuration, and it is also possible to
deposit CIGS at relatively low temperatures (approximately 500°C)on metal or
polymer substrates to enable flexible solar products. CIGS thin films are primarily
deposited using co-evaporation/evaporation or sputtering, and to a lesser extent
electrochemical deposition, or ion-beam-assisted deposition. Absorption coefficient of
CIGS is higher than other semiconductors used in photovoltaic technology[8].
2.8.3.1 Advantages
Does not contain toxic materials.
Scalable and cost-effective.
Can be deposited on flexible materials.
2.8.3.2 Disadvantages
Low efficiencies.
Limitation of indium in earth.
2.8.4 Dye sensitized solar cells(DSSC)
This type of cells are based on photoelectrochemical system. Dye sensitized solar
cells, also known as Grätzel cell has a number of attractive features. Earlier dyes were
sensitive only in high frequency of solar spectrum but newer dyes have wider
frequency response. A typical DSSC is shown in figure
Chapter 2
14
Figure 2.4 A typical dye sensitized solar cell.
2.8.4.1 Advantages
Easy production.
Components are less harmful to the environment.
2.8.4.2 Disadvantages
Low efficiencies.
DSSC use liquid electrolytes which is temperature sensitive.
2.8.5 Other products of second generation
The thin film solar cell technology experienced various techniques. Beside the four
technologies described above, there are also some technologies which contributed
much in improving solar cell efficiency. Quantum dot solar cells(QDSC) are based on
Chapter 2
15
DSSC which employ low bandgap semiconductor nanoparticles, fabricated in such a
way that they form quantum dots( such as CdS, CdSe, PbS etc). The efficiency of
QDSC has increased rapidly recently[4]. Another technology which is relatively novel
is organic solar cells which are built from organic semiconductors including
polyphenylene vinylene and small molecule compound like copper phthalocyanine[8].
Copper zinc tin sulphide(CZTS) semiconductors are also used in thin film technology,
although the efficiency is trivial. In table-2.1 efficiencies of different solar cells in thin
film technology is demonstrated[7].
Classification Efficiency (%)
Amorphous silicon (a- Si) 10.1 ± 0.3
CdTe solar cell 16.7 ± 0.5
CIGS solar cell 17.4 ± 0.5
DSSC 11.0 ± 0.3
CZTS solar cell 10.1 ± 0.2
Organic solar cell 10.0 ± 0.2
Table 2.1 Efficiency of different thin film solar cell
2.9 Third generation solar cells
This type of solar cells also known as tandem cells contain several p-n junctions. Twomechanically separate thin film solar cells are used and wired together separatelyoutside the cell. The technique is widely used in amorphous silicon solar cells. Thematerials are ordered with decreasing bandgaps, Eg. Table 2.2 shows the bandgaps ofdifferent materials used for multijunction solar cells[8].
Chapter 2
16
Material Eg (eV)
c- Si 1.12
InGaP 1.86
GaAs 1.4
Ge 0.65
InGaAs 1.2
Table 2.2 Bandgaps of different multijunctions.
Usually the materials with high bandgaps are used in top sub-cells and the materialswith low bandgaps are used in bottom sub-cell. Several multijunction solar cells aretuned to different wavelength of light. Figure shows how multijunction solar cells canbe more efficient than Si solar cell.
Figure 2.5 AM 1.5 solar spectrum and the portion of the spectrum which can beutilized by (a) Si solar cells (b) Ga0.35In0.65P/Ga0.83In0.17As/Ge solar cells[9].
Chapter 2
17
2.9.1 Advantages
The efficiencies of multijunction solar cells high as they are tuned to differentwavelength of light. The theoretical efficiency of solar cells is 86.8% for an infinitenumber of p-n junctions[8]. Laboratory testing has shown that, multijunction solar cellscould be twice as efficient as their single junction. Efficiencies of differentmultijunctions are depicted in table 2.3[7].
Material Efficiency (%)
GaInP/GaInAs/Ge 34.1 ± 1.2
GaAs 28.3 ± 1.2
InP 22.1 ± 0.7
GaInP/GaAs/GaInNAs 43.5 ± 2.6
GaInP/GaInAs/Ge 41.6 ± 2.5
GaInP/GaAs/Ge 27.0 ± 1.5
Table 2.3 Efficiency of different multijunctions.
2.9.2 Disadvantages
Multijunction solar cells are more complex comparing to the single junction solar cells.This complexity highly increases the manufacturing cost of the cells. As a result theprice of multijuction solar cell is as high as, it is not possible to use it in normalprojects. It is preferred in space and in the projects in which high power generation issalient factor rather than the manufacturing cost.
2.10 References
[1] Upadhaya, A.D. ; Yelundur, Vijay; Rohatgi, Ajeeti ,“High efficiency monocrystallinesolar cells with simple manufacturing technology” Georgia Tech-SmartTech.
Chapter 2
18
[2] Kolic,Y(1995); “Electron powder ribbon polysrystalline silicon plates used forporous layer fabrication”, Thin solid films 255:159.
[3] “Solar power lightens up with thin film technology”, Scientific American, April 2008.
[4] Kaimat, Prashantv,(2012),” Boosting the efficiency of quantum dot sensitized solarcells through modulation of interfacial charge transfer”, Accounts of chemicalresearch: 120411095315008.
[5] “Dye sensitized solar cells(DSSC) based on nano crystalline oxide semiconductorfilms”. Laboratory for photonics and interfaces, Ḗcole Polytechnique Fédérale deLousanne-2 February 1999.
[6] Martin A, Green; Emery, Keith; Hishikawa, Yoshiro; Ewan D, Dunlop; ”Solar cellefficiency tables(version 39) Progress in photovoltaics: research and applications.
[7] N. V. Yastrebova; “ High efficiency multijunction solar cells: current status andfuture potential”.
[8] Wikipedia- www.wikipedia.org
[9] Dimroth, Frank; Kurtz, Sarah; “ High efficiency multijunction solar cells”, mrsbulletin, volume 32 , march 2007.
Chapter 3
19
INTRODUCTION TO SURFACE PLASMONS
3.1 Introduction
Metals provide low resistance electrical contact for solar cells, connecting them to the
external circuits. Metals make good conductors because of their large free electron
density. For this characteristics they can act good recombination center as well, which
causes absorption in the metal, one of the loss mechanism in solar cell. That’s why
great attention has been paid to metal contacts in the fabrication of solar cells in order
to optimize the metal area to minimize recombination, shading and contact resistance
losses.
Now question arises, how can metal nanoparticles then improve light trapping in solar
cells? The optical properties of metal nanoparticles are totally different from the bulk
metals. In this chapter the concept of surface Plasmon is introduced to the reader.
3.2 What are plasmons
Plasmon is a quantum of plasma oscillation. Or simply we can call the Plasmon, is a
quasiparticle resulting from the quantization of plasma oscillation just as photons.
Let us consider a bulk metal in equilibrium condition, where the density of mobile
negative charges (moving under the influence of internal electric field) is equal to the
density of fixed positive ion.now if the equilibrium condition is distorted slightly by an
internal field, the nonuniform charge distribution sets up an electric field to restore
neutrality. The negative charge gain momentum from the field and will overshoot the
equilibrium condition resulting in a collective oscillation of the conduction band
electrons called a Plasmon.
Chapter 3
20
Plasmon can exist in the bulk, can be in the form of propagating waves on thin metal
surface or can be localized to the surface. Accordingly, the Plasmons are termed
volume or bulk Plasmoms, Surface Plasmon Polaritons (SPP) and Localized Surface
Plasmons (LSP) respectively.
3.2.1 Surface Plasmon
Surface Plasmons are those Plasmons confined to surface and interact strongly with
light and create Polaritons. They occure at the interface of a vaccum or material with a
positive dielectric constant.
3.2.2 Volume Plasmons
volume plasmons are known as bulk plasmons which caused by longitudinal
oscillation of free electron in the bulk of a metal and the frequency in which plasma
oscillates are termed as plasma frequency which is indicated by p
ωp =²
(3.1)
where N is density of electron, m is the mass of electron, e is the electron charge and
is the permittivity of free space.
Visible light may used for the excitation of volume Plasmons but it might not be useful
because the momentum which is transferred to the crystal electron by incident light is
negligible and hence the probability of plasma excitation is small. So the conduction
electrons in the bulk behave like relaxator system.
3.2.3 Surface Plasmon Polariton
Surface Plasmons polaritron (SPP) are combined excitations of the conduction
electrons and photons, and form a propagating mode bound to the interface between
Chapter 3
21
a thin metal and dielectric travelling perpendicular to the thin film.[atwater’03].these
plasmons are analogous to bulk plasmons except they are restricted to surface
electrons.
Surface Plasmons occur at interface between metals and dielectric where the
Re( ) [where is the dielectric constant] have opposite sign, and decay
exponentially with distance from interface. Re( ) <0 occurs where a material is
strongly absorbing; which is responsible for high reflectivity of metals. This oscillation
are accompanied by transverse and longitudinal electromagnetic field which has its
maximum at the surface Z=0 , and decays exponentially and disappears at Z=∞ .
This field that is perpendicular to the surface and decays exponentially with distance
from the surface is said to be evanescent or near field in nature.
Figure 3.1: (a) Schematic of a surface plasmon at the interface of a metal and dielectric
showing the exponential dependence of the field E in the z direction along with charges and
(b) electromagnetic field of surface plasmons propagating on the surface in the x direction.
Chapter 3
22
Thus SPP is a surface electromagnetic wave, whose electromagnetic field is confined
to the near vicinity of the dielectric metal interface. This confinement leads to an
enhancement of the EM field at the interface ,resulting in an extraordinary sensitivity
of SPPs to the surface condition. This sensitivity is extensively used for studying
adsorbates on a surface , surface roughness and related phenomena. SPP based
devices exploiting this sensitivity are widely used in chemo- and bio- sensors. The
enhancement of the electromagnetic field at the interface is reasonable for surface
enhanced optical phenomena such as Raman scattering, second harmonic generation
(ShG), fluorescence etc.
The intrinsically two-dimensional nature of SPPs provides significant flexibility in
engineering SPP based all optical integrated circuits needed for optical
communications and optical computing. The relative ease of manipulating SPPs on a
surface opens an opportunity for their application to photonics and optoelectronics for
scaling down optical and electronic devices to nanometric dimensions. Most
importantly , active plasmonic element based on nonlinear surface Plasmon polariton
optics , which allows controlling optical properties with light much easier to realize with
suitably patterned metal surface , due to the SPP related electromagnetic field
enhancement near a metal surface.
The resonant interaction between oscillating electrons and electromagnetic field of
light gives rise to its unique optical properties.[1] This is because SPP’s have a higher
momentum for a given frequency than light which prevents power from propagating
away from the surface, the principle behind Surface Plasmon waveguide[Maier’04].
The surface Plasmon frequency of a thin flat metal surface can be easily determined
from the bulk Plasmon as it corresponds to Re m( sp ) = − d where d > 0 is the
dielectric of the adjacent medium. Therefore the free electron Plasmon frequency for a
metal film in contact with vacuum is modified to
Chapter 3
23
spp = √2 (3.2)
These propagating waves can travel up to 10-100 µm in the visible region for silver
owing to its low absorption losses and can increase up to 1mm in the near infrared
(NSR) [Barenas]. Generally the surface Plasmon resonant frequency sp is in the ultra
violet (UV) for metals and infra-red (IR) for heavily doped semiconductor. .
3.2.4 Localized surface plasmons
In contrast, localized surface plasmons (LSPs) are oscillations of the electrons in
confined geometries, such as small metal particles or voids in metallic structures.
Movement of the conduction electrons upon excitation with incident light leads to a
build up of polarization charges on the particle surface. This acts as a restoring force,
allowing a resonance to occur at a particular frequency, which is termed the dipole
surface plasmon resonance (SPR) frequency sp to distinguish it from bulk
resonances and resonance on metal surfaces. Surface plasmons are known solutions
of Maxwell’s equations. We can understand many of the properties of small metal
particles by looking at the quasi-static response. Under these circumstances, the
electric field of the incident light can be assumed to be spatially constant, and the
interaction is governed by electrostatics rather than electrodynamics. This is strictly
applicable for particles with dimension much smaller than the wavelength of light, but
can also provide physical insight for larger particles, with a diameter of up to 100nm
for incident light of optical wavelengths. For a small metal particle, the positive
charges are assumed to be fixed and the negative charges are assumed to be moving
under the influence of an external field as mentioned earlier. Therefore a displacement
of the negative and positive charges occurs under the influence of an external electric
field [2]. The internal field is
= E ∈∈ ∈ (3.3)
where the relative permittivity of the medium is and ∈ is the complex relative
Chapter 3
24
permittivity of the metal particle given by = ′ + ′′ . The real part describes the
polarizability whereas the imaginary part gives the energy dissipation in the metal. The
polarization of a sphere due to the presence of an external field can be calculated
using boundary conditions and is given by
= 4πϵ R ∈ ∈∈ ∈ (3.4)
The dielectric function of a free electron metal is given by the Drude formula
= 1 − ω²ω² ωγ
(3.5)
where is the angular frequency of the incident radiation and is the damping
coefficient.
We can see from equation (2.2) and (2.3) that very strong interaction of the spheres
with
the incident field occurs at the frequency where = -2 and is | +2 |minimum.
This occurs when ′= -2 and ′′= 0 . This condition corresponds to the surface
plasmon resonance frequency in vacuum, given by
= ω√( ϵ )(3.6)
Materials with Re( )<0 have high reflectance and propagate light internally mainly by
decaying evanescent waves [Smith '03] and dissipation is small (i.e. ′′/ ′<<1) [3]. A
metal exhibits this property below its bulk plasma frequency (i.e. in the optical regime)
and this is responsible for the high reflectivity of metals.
Chapter 3
25
3.3 Mathematical background
In order to investigate the physical properties of surface plasmon polaritons (SPPs),
we have to apply Maxwell’s equations to the flat interface between a conductor and a
dielectric. To present this discussion most clearly, it is advantageous to cast the
equations first in a general form applicable to the guiding of electromagnetic waves,
the wave equation. As we have seen in chapter 1, in the absence of external charge
and current densities, the curl equations can be combined to yield
∇ ×∇ ×E = − ( ∂2D/∂t2 ).) (3.7)
Using the identities ∇ × ∇ × E ≡ ∇(∇ · E) − ∇2E as well as ∇ · (εE) ≡ E · ∇ε + ε∇ · E, and
remembering that due to the absence of external stimuli ∇. = 0 the central equation
changes to
∇(- E · ∇ε)−∇²E = − (∂²E/∂t²) (3.8)
For negligible variation of the dielectric profile ε = ε(r) over distances on the order of
one optical wavelength, (3.2) simplifies to the central equation of electromagnetic
wave theory,
∇²E−²∂²E/∂t² = 0. (3.9)
Practically, this equation has to be solved separately in regions of constant ε, and the
obtained solutions have to been matched using appropriate boundary conditions. To
cast (3.3) in a form suitable for the description of confined propagating waves, we
proceed in two steps. First, we assume in all generality a harmonic time dependence
E(r, t) = E(r) of the electric field. Inserted into (3.3), this yields
Chapter 3
26
∇²E + ² εE = 0 (3.10)
where k0 = ω/c is the wave vector of the propagating wave in vacuum. Equation (3.4)
is known as the Helmholtz equation.
Next, we have to define the propagation geometry. We assume for simplicity a one-
dimensional problem, i.e. ε depends only on one spatial coordinate. Specifically, the
waves propagate along the x-direction of a Cartesian coordinate system, and show no
spatial variation in the perpendicular, in-plane y-direction (see Fig. 3.1); therefore ε
=ε(z). Applied to electromagnetic surface problems, the plane z = 0 coincides with the
interface sustaining the
Figure 3.2. Definition of a planar waveguide geometry.
The waves propagate along th propagating waves,which can now be described as
E(x, y, z) = E(z) The
Chapter 3
27
complex parameter β = kx is called the propagation constant of the traveling waves
and corresponds to the component of the wave vector in the direction of propagation.
Inserting this expression into (3.4) yields the desired form of the
wave equation
²²+ (k²0ε − β²)Ey = 0 (3.11)
Naturally, a similar equation exists for the magnetic field H.
Equation (3.5) is the starting point for the general analysis of guided electromagnetic
modes in waveguides, and an extended discussion of its properties and applications
can be found in and similar treatments of photonics and optoelectronics. In order to
use the wave equation for determining the spatial field profile and dispersion of
propagating waves, we now need to find explicit expressions for the different field
components of E and H.
For harmonic time dependence ( = −iω), we arrive at the following set of coupled
equations
− = i Hx (3.12)
− = i Hy (3.13)
− = i Hz (3.14)
− = -i Ex (3.15)
Chapter 3
28
− = -i Ez (3.16)
For propagation along the x-direction ( =i ) and homogeneity in the y-direction
( = 0) this system of equation simplifies to
−i = i Hy (3.17)
i = i Hz (3.18)
= i Ex (3.19)
− =-i Ey (3.20)
= -i Ez (3.21)
It can easily be shown that this system allows two sets of self-consistent solutions with
different polarization properties of the propagating waves. The first set are the
transverse magnetic (TM or p) modes, where only the field components Ex , Ez and
Hy are nonzero, and the second set the transverse electric (TE or s) modes, with only
Hx , Hz and Ey being nonzero.
For TM modes, the system of governing equations (2.7) reduces to
Ex =− (3.22)
Ez =− (3.23)
Chapter 3
29
and the wave equation for TM modes is
With the TE Equation
² + ( ² − ²) = 0 (3.24)
with these equations at our disposal, we are now in a position to embark on the
description of surface plasmon polaritons. The most simple geometry sustaining SPPs
is that of a single, flat interface between a dielectric, non-absorbing half space (z > 0)
with positive real dielectric constant ε2 and an adjacent conducting half space (z < 0)
described via a dielectric function ε1(ω). The requirement of metallic character implies
that Re [ε1] < 0. As shown for metals this conditiofulfilled at frequencies below the bulk
plasmon frequency . We want to look for propagating wave solutions confined to the
interface, i.e. with evanescent decay in the perpendicular z-direction.
(z) = (3.25)
(z) = (3.26)
(z) = − (3.27)
For z<0. ki≡kz,i (i=1,2) is the component of the wave vector perpendicular to the
interface in the two media. Its reciprocal value, ^z= 1/ |kz|, defines the evanescent
decay of fields perpendicular to the interface. Which quantifies the confinement of the
wave. Community of and i Ez at the interface requires that A1 = A2 and
Chapter 3
30
= − (3.28)
Note that with our convention of the signs in the exponents in (2.10,2.11),
confinement to the surface demands Re [ε1] < 0 if ε2 > 0 - the surface waves exist
only at interfaces between materials with opposite signs of the real part of their
dielectric permittivities, i.e. between a conductor and an insulator. The expression for
Hy further has to fulfill the wave equation, yielding
² = ²− ² (3.29)
² = ²− ² (3.30)
Combining this and (2.12) we arrive at the central result of this section, the dispersion
relation of SPPs propagating at the interface between the two half
Spaces
= (3.31)
Before discussing the properties of the dispersion relation (2.14) in more detail, we
now briefly analyze the possibility of TE surface modes. Using (2.9), the respective
expressions for the field components are
(z) = (3.32)
(z) = − (3.33)
(z) = (3.34)
For z>0 and
(z) = (3.35)
Chapter 3
31
(z) = (3.36)
(z) = (3.37)
For z < 0. Continuity of Ey and Hx at the interface leads to the condition
A1 (k1 + k2) = 0.
Since confinement to the surface requires Re [k1] > 0 and Re[k2] > 0, this condition is
only fulfilled if A1 = 0, so that also A2 = A1 = 0. Thus, no surface modes exist for TE
polarization. Surface plasmon polaritons only exist for TM polarization.
3.4 Referances
[1] W. L. Barnes, A. Dereux and T. W. Ebbesen, "Surface Plasmon subwavelength
optics," Nature, 424, pp. 824-830 (2003).
[2] U. Kreibig and M. Vollmer, "Optical properties of metal clusters," Wiley, NY,
(1995).
[3] A. K. Sarychev and V. M. Shalaev, "Optical Properties of Metal- Dielectric Films,"
in Introduction to Complex Mediums for Optics
and Electromagnetics. SPIE, Pg. 397 (2003).
Chapter 4
32
PLASMONIC SOLAR CELLS
4.1 Introduction
Plasmonics is an emerging field that makes use of the nanoscale properties of metals.
Though plasmonics is a wide area of study, its application for solar cells has a recent
surge of interest. Metals support surface Plasmons that are the collective oscillation of
excited free electrons and characterized by a resonant frequency. They can be either
localised as for metal nanoparticles or propagating as in the case of planar metal
surfaces. By manipulating the geometry of the metallic structures, the surface plasmon
resonance or plasmon propagating properties can be tuned depending on the applica-
tions. The resonances of noble metals are mostly in the visible or infrared region of the
electromagnetic spectrum, which is the range of interest for photovoltaic applications.
The surface plasmon resonance is affected by the size, shape and the dielectric
properties of the surrounding medium. Silver and gold have dominated experimental
research in this area although other metals also support surface plasmons.
4.2 Plasmonics for photovoltaic
Conventionally, photovoltaic absorbers must be ‘optically thick’ to allow near-complete
light absorption and photocarrier current collection. Figure 4a shows the standard AM1.5
solar spectrum together with a graph that illustrates what fraction of the solar spectrum is
absorbed on a single pass through a 2-μm-thick crystalline Si film. Clearly, a large
fraction of the solar spectrum, in particular in the intense 600–1,100 nm spectral range, is
poorly absorbed. This is the reason that, for example, conventional wafer-based
crystalline Si solar cells have a much larger thickness of typically 180–300 μm. But high-
efficiency solar cells must have minority carrier diffusion lengths several times the
Chapter 4
33
material thickness for all photocarriers to be collected a requirement that is most easily
met for thin cells. Solar-cell design and materials-synthesis considerations are strongly
dictated by these opposing requirements for optical absorption thickness and carrier
collection length.
Figure 4.1 AM1.5 solar spectrum, together with a graph that indicates the solar energy
absorbed in a 2-μm-thick crystalline Si film (assuming single-pass absorption and no
reflection). Clearly, a large fraction of the incident light in the spectral range 600–
1,100 nm is not absorbed in a thin crystalline Si solar cell.
Chapter 4
34
4.2.1 Light trapping technique in conventional solar cell
Silicon is an indirect bandgap semiconductor i.e. it requires both a photon and a phonon
to be involved in the near-bandgap absorption process. This makes silicon a relatively
weak absorber of light. A schematic of the absorption process for a direct and indirect
bandgap semiconductor material is shown in Fig. 1.4. Reflection and parasitic absorption
tend to lower the absorption further. Suitable processing techniques and choice of
semiconductor materials can help reduce the losses and enhance absorption .Optical
losses in solar cells are reduced by using an antireflection coating to reduce the top
reflection and minimizing shading by the contacts. However other mechanisms need to
be considered to improve the absorption and hence the current density of silicon solar
cells, the most important being light trapping. Light trapping becomes crucial at longer
wavelengths closer to the band gap of Si (1.1eV). The reason is the absorption coefficient
_ of silicon, which is high for short wavelength light but decreases close to the bandgap.
There are two possible methods of increasing the light generated current density in solar
cells :
1. Increase the photon density by using lenses and mirrors to concentrate the light
incident on the modules. This forms the basis of solar concentrators [1].
2. Increase photon absorption within the solar cell by reducing optical losses and giving
light multiple opprotunites to get absorbed.
Though the former case is an important area of research for thin-film solar cells, it is not
discussed in this section. The latter case is however more relevant to this work in terms
of light trapping and hence will be discussed in more detail.
Chapter 4
35
Figure 4.2: Energy-crystal momentum diagram of a (a) direct bandgap material
showing excitation of an electron from the valence to the conduction band by absorption
of a photon (b) indirect bandgap material showing absorption of a photon by a two step
process involving a phonon emission or absorption.
The minimum photon energy required to excite an electron hf =Eg - Ep where Eg is the
bandgap energy and Ep the energy of the absorbed phonon. A very popular method of
light trapping with conventional solar cell fabrication is surface texturing [2]. Texturing of
semiconductor surfaces has two advantages. It can be used to enhance optical
absorption by (1) reducing surface reflection by increasing the chances of the light
bouncing back onto the surface rather than out into the air and (2) increasing the average
Chapter 4
36
optical path length of weakly absorbed long wavelength light or in other words increasing
the ‘effective’ thickness of the device. This process by which light is trapped by a path
length enhancement (usually by total internal reflection) is termed light trapping. Light
trapping is typically quantified by the path length enhancement factor Z, where Z is
defined as the ratio of the optical or ‘effective’ thickness Weff to that of the actual cell
thickness W.
= (4.1)
is proposed as the figure of merit of the optical design. The incorporation of light
trapping features can experimentally be seen using spectral response (A/W), reflectance
or the overall light generated current [3].
(a) (b)
Figure 4.3 Light trapping schemes
Chapter 4
37
Figure 4.3 show two light trapping schemes. One highly efficient way of confining light in
a thin film silicon cell is the use of diffuse back reflector with cosine characteristic which
should be a simple choice for confinement by randomising the direction of light as shown
in Fig. 4.4 (a) Here the light that is within the escape cone of the semiconductor will be
coupled out of the top surface (1/n2).However the light that is not coupled out will be
reflected off the top surface to the rear,where the direction would be re-randomised,
enhancing the path length by a factor of 4n2 n[1]. However an even more efficient
scheme is the geometrical light trapping shown in Fig. 4.4(b) where the textured surface
enhances the pathlength to a greater extent [1]. However textures have to be designed
with care so that maximum passes across the cell are obtained. Random pyramid
texturing along with pressing these features in glass is expected to increase energy
conversion efficiency even further. A 3.3μm Si cell using this technology could potentially
increase the short circuit current by 56% [Campbell '02]. Other light trapping methods
have been discussed elsewhere [2]. The highest performance silicon solar cell yet
fabricated – the PERL (passivated emitter rear locally-diffused) solar cell [4], used
inverted pyramids along the top surface to reduce top reflection and enhance light
trapping as shown in Fig 1.6. Photolithography technique using UV beam was used to
etch inverted pyramid structures on mono crystalline PERL cells and holes on oxide to
form a honeycomb structure on a multi crystalline solar cell which resulted in the
fabrication of the 24.7% record efficiency mono crystalline solar cell and 19.8% efficient
multi crystalline cells respectively [4]. However due to the expense involved, the
application of these techniques are limited to laboratory based cells. The textures have
also been attempted by reactive ion etching.
Chapter 4
38
Figure. 4.4: The high efficiency PERL cell with the inverted pyramid structures on the
top surface for light trapping.
4.2.2 Light trapping technique in thin film solar cell
Apart from increasing absorption, light trapping is attracting interest in photovoltaic
energy conversion because it allows reduction in the active cell material [1]The light
trapped in the active layer by total internal reflection increases the effective pathlength,
making it possible to reduce the semiconductor layer thickness by incorporating light
trapping schemes. Light trapping becomes particularly important when the cells get
thinner. Because thin film solar cells are only a few microns thick standard methods of
increasing the light absorption which use surface textures that are typically around ten
microns in size cannot be used. Plasma etch techniques, which can be used to etch
Chapter 4
39
submicron sized features, can damage the silicon thereby reducing the cell efficiency.
Another alternative to direct texturing of Si is the texturing of the substrate upon which the
solar cell is deposited. A recent development has been the use of nano-textured
scattering interfaces for thin-film solar cells. A 15% increase in the current density was
reported from an a-Si-H solar cell that incorporated reflecting and scattering substrates
into the cell. These were from regularly textured substrates with average feature heights
close to 180nm on glass substrates [5]
Textured transparent conducting oxide (TCO) surfaces are also used to introduce and
control the roughness of the internal interfaces and investigate the scattering effects in
terms of haze (ratio of the diffuse scattering to total scattering i.e. diffuse + specular)
[6]. Transparent conducting oxides in general are n-type degenerate semiconductors with
good electrical conductivity and high transparency in the visible spectrum. The textured
TCO in this case provides index grading for lower front reflection and scattering beyond
the escape angle providing light trapping. Another similar approach is using silica beads
to texture glass using a patented technique for 2μm crystalline silicon on glass to provide
light trapping[7].
However all the above methods result in increased recombination losses through
increased surface area of the semiconductor material. Hence electronic surface
passivation becomes very important to the achievement of high efficiencies to avoid
surface recombination losses dominating over any light trapping gains. The impact of
surface recombination on cell performance is greatly affected by the thickness of the
cell. It has been shown that for larger thickness the impact on cell efficiency is small but
as cell thickness reduces the limiting efficiency is severely affected[8].
Though in practice it has been experimentally proven to be very difficult to reduce
recombination losses beyond a certain limit, theoretically energy conversion efficiency
Chapter 4
40
of above 24% even for 1μm cells can be achieved [8]. This highlights the need to
incorporate better light trapping mechanisms that do not increase recombination losses in
thin-film solar cells to extract the full potential of the cells. Also, textured surfaces face
handling problems and problems with metallisation [8].
The use of pigmented dielectric reflectors (PDR) in the rear of the cell to reflect the light
that is otherwise transmitted is another viable method of achieving light trapping [Cotter
'99, Shaw '03]. Surface plasmons offer a novel way of light trapping by using metal
nanoparticles instead of conventional texturing to explore possibilities of enhanced
absorption or light extraction in thin film silicon solar cell structures. By manipulating their
size, the particles can be used as an efficient scattering layer as discussed in the
following section. Metal particles can act as high recombination centres but in this work
the metal particles are separated from the active silicon layer with a passivating oxide.
One of the benefits of this light trapping approach is that the surface area of silicon and
surface passivation layer remain the same as for a planar cell, so surface recombination
losses are not expected to increase.
4.2.3 Plasmonics light trapping for photovoltaics
A new method for achieving light trapping in thin-film solar cells is the use of metallic
nanostructures that support surface plasmons: excitations of the conduction electrons at
the interface between a metal and a dielectric. By proper engineering of these metallodi-
electric structures, light can be concentrated and ‘folded’ into a thin semiconductor layer,
thereby increasing the absorption. Both localized surface plasmons excited in metal
nanoparticles and surface plasmon polaritons (SPPs) propagating at the
metal/semiconductor interface are of interest.
Chapter 4
41
Figure 4.5 Plasmonic light-trapping geometries for thin-film solar cells. a, Light trapping
by scattering from metal nanoparticles at the surface of the solar cell. Light is
preferentially scattered and trapped into the semiconductor thin film by multiple and high-
angle scattering, causing an increase in the effective optical path length in the cell. b,
Light trapping by the excitation of localized surface plasmons in metal nanoparticles
embedded in the semiconductor. The excited particles’ near-field causes the creation of
electron–hole pairs in the semiconductor. c, Light trapping by the excitation of surface
plasmon polaritons at the metal/semiconductor interface. A corrugated metal back
surface couples light to surface plasmon polariton or photonic modes that propagate in
the plane of the semiconductor layer. [9]
Plasmonic structures can offer at least three ways of reducing the physical thickness of
the photovoltaic absorber layers while keeping their optical thickness constant. First,
metallic nanoparticles can be used as subwavelength scattering elements to couple and
trap freely propagating plane waves from the Sun into an absorbing semiconductor thin
film, by folding the light into a thin absorber layer (Fig. 4.6a). Second, metallic
nanoparticles can be used as subwavelength antennas in which the plasmonic near-field
is coupled to the semiconductor, increasing its effective absorption cross-section
Chapter 4
42
(Fig. 4.6b). Third, a corrugated metallic film on the back surface of a thin photovoltaic
absorber layer can couple sunlight into SPP modes supported at the
metal/semiconductor interface as well as guided modes in the semiconductor slab,
whereupon the light is converted to photocarriers in the semiconductor (Fig. 4.6c).
As will be discussed in detail in the next section, these three light-trapping techniques
may allow considerable shrinkage (possibly 10- to 100-fold) of the photovoltaic layer
thickness, while keeping the optical absorption (and thus efficiency) constant. Various
additional ways of using plasmonic nanostructures to increase photovoltaic energy
conversion are described in the section on other plasmonic solar-cell designs. [9] A small
metal nanoparticle embedded in a homogeneous medium is nearly symmetric in the
forward and reverse directions. This situation changes when the particle is placed close
to the interface between two dielectrics, in which case light will scatter preferentially into
the dielectric with the larger permittivity. The scattered light will then acquire an angular
spread in the dielectric that effectively increases the optical path length ( Fig. 4.6a).
Moreover, light scattered at an angle beyond the critical angle for reflection (16° for the
Si/air interface) will remain trapped in the cell. In addition, if the cell has a reflecting metal
back contact, light reflected towards the surface will couple to the nanoparticles and be
partly reradiated into the wafer by the same scattering mechanism. As a result, the
incident light will pass several times through the semiconductor film, increasing the
effective path length. The enhanced incoupling of light into semiconductor thin films by
scattering from plasmonic nanoparticles was first recognized by Stuart and Hall, who
used dense nanoparticle arrays as resonant scatterers to couple light into Si-on-insulator
photodetector structures. They observed a roughly 20-fold increase in the infrared
photocurrent in such a structure. This research field then remained relatively dormant for
many years, until applications in thin-film solar cells emerged, with papers published on
enhanced light coupling into single-crystalline Si, amorphous Si, Si-on-insulator, quantum
well and GaAs solar cells covered with metal nanoparticle
Another light trapping geometry, where light is converted into SPPs, which are
electromagnetic waves that travel along the interface between a metal back contact and
Chapter 4
43
the semiconductor absorber layer (Fig. 4.7). Near the plasmon resonance frequency, the
evanescent electromagnetic SPP fields are confined near the interface at dimensions
much smaller than the wavelength. SPPs excited at the metal/semiconductor interface
can efficiently trap and guide light in the semiconductor layer. In this geometry the
incident solar flux is effectively turned by 90°, and light is absorbed along the lateral
direction of the solar cell, which has dimensions that are orders of magnitude larger than
the optical absorption length. As metal contacts are a standard element in the solar-cell
design, this plasmonic coupling concept can be integrated in a natural way.
At frequencies near the plasmon resonance frequency (typically in the 350–700 nm
spectral range, depending on metal and dielectric) SPPs suffer from relatively high
losses. Further into the infrared, however, propagation lengths are substantial. For
example, for a semi-infinite Ag/SiO2 geometry, SPP propagation lengths range from 10
to 100 μm in the 800–1,500 nm spectral range. By using a thin-film metal geometry the
plasmon dispersion can be further engineered61–64. Increased propagation length
comes at the expense of reduced optical confinement and optimum metal-film design
thus depends on the desired solar-cell geometry. Detailed accounts of plasmon
dispersion and loss in metal–dielectric geometries are found in refs 61–64.
Chapter 4
44
Figure 4.6 SPPs are bound waves at the interface between a semiconductor and a dielectric.
This dispersion diagram, plotting the relationship between frequency and wavevector (2π/λ) for
SPPs on a Ag/Si interface. The ‘bound’ SPP mode occurs at energies below the surface plasmon
resonance energy of 2.07 eV (600 nm). [9]
4.3 Choice of metalsNoble metal like gold, copper and silver are considered suitable for surface Plasmon
research because of their low resistivity and strong interactions with visible light through
resonant excitations. While gold is the best in terms of stability and inertness to oxidation,
copper is the worst and needs to be overcoated to prevent oxidation. However the
plasma frequency for gold and copper lie in the visible (~2.5eV for gold and ~2.1eV for
copper) whereas for silver the corresponding transition is outside the visible region
(~4eV) [10]. This leads to a highly dispersive dielectric function at optical frequencies for
silver on excitation with light with strong scattering throughout the visible regime without
much absorption in the metal (at resonance frequency the metal absorption is high) which
is crucial for solar cell applications. Also silver is the metal of choice because of its low
Chapter 4
45
absorption losses and high radiative efficiency when compared to Au and Cu. Silver
tends to form a silver sulphide on contact with air and has led to significant differences in
the results when measured a few days apart as mentioned in the references .Formation
of an inert layer on silver has not been ruled out in our case and hence all measurements
were performed within a few days of the deposition of silver. However measurements
repeated a few months later were consistent with the results taken immediately after the
deposition indicating that for our case any layer formation of silver was not affecting the
performance of the device.
4.4 Effect of various parameters in plasmonics thin film solar cellThere are various parameters that plays an important role in solar efficiency of a thin film
solar cell. Among them size and shape of nano particles, dielectric medium, diameter of
the silicon layer etc are noticeable. In this section we discuss effect of some parameters
in thin film solar cell in brief.
4.4.1 Effect of size and shape of metal nanoparticle
Both size and shape of metal nanoparticles are key factors determining the incoupling
efficiency. Maxwell’s equations predict strong absorption and scattering for metal
nanoparticles, the intensity of which is size dependant. The strong dependence of the
scattering crosssection on the size of the particles makes it necessary that the size of the
particles be optimized for the desired application. Metallic particles that are much smaller
than ~20nm in radius, tend to absorb more and hence extinction is dominated by
absorption in the metal particles. Absorption dissipates heat and this property is utilized in
applications like solar glazing, nanoscale lithography and therapeutic applications [11].
However as the size of the particles increases, extinction is dominated by scattering and
Chapter 4
46
we take advantage of this property for our application oflight trapping. Beyond certain
limits, however, increasing the particle size results in increased retardation effects (due to
inhomogeneous polarization) and higher order multi-pole excitation modes, which
decreases the efficiency of the scattering process. In addition to the particle size, other
factors affecting the absorption enhancement are the particle shape and variation in the
dielectric function of the substrate and the surrounding medium, which determine the
position and width of the surface Plasmon resonance.
Stuart et. al. [12] saw an increase in the photocurrent enhancement from silicon-on-
insulator devices with metal islands of various sizes. Their experiments indicate that if all
other conditions remain the same, the size of the particles decides the scattering intensity
of the particles. This is because the scattering cross-section of the particles increases
thereby increasing the interaction with the incident light. They also showed that particles
that oscillate as dipoles can be expected to have high scattering efficiency, where
scattering efficiency is the fraction of scattered light as a proportion of the total extinction.
However there is a limit to the increase in enhancement because larger particles cease to
act as dipoles and may excite multipole oscillations which candecrease the scattering
efficiency.
Chapter 4
47
Figure 4.7: Fraction of light scattered into the substrate, divided by total scattered power, for
different sizes and shapes of Ag particles on Si. Also plotted is the scattered fraction for a parallel
electric dipole that is 10 nm from a Si substrate [9]
Figure 4.8, which shows that smaller particles, with their effective dipole moment located
closer to the semiconductor layer, couple a larger fraction of the incident light into the
underlying semiconductor because of enhanced near-field coupling. Indeed, in the limit of
a point dipole very near to a silicon substrate, 96% of the incident light is scattered into
the substrate, demonstrating the power of the particle scattering technique.[9]
Chapter 4
48
Figure 4.8: Photocurrent enhancement from a 1.25 m thick SOI test solar cell for particle size
corresponding to 12 and 16nm mass thickness of Ag relative to the cell without silver
particles.[13]
Fig. 4.9 shows the photocurrent enhancement from the 1.25 micron SOI test cells before
and after deposition of the silver islands. The interesting feature here is an overall
increase in current throughout the visible and near IR and a close to 16 fold
enhancement at around 1050 nm with particle sizes corresponding to 16nm mass
thickness of silver. These results correspond to a 33% increase of the total current of the
device, when averaged over the AM1.5 global spectrum for particle sizes corresponding
to 12nm mass thickness of silver and 16% increase for particle sizes corresponding to
16nm silver thickness.
Chapter 4
49
Another example can be given where Au nanoparticle is used. Also in this case
difference is viewed in efficiency with various diameter of Au particle.
Figure 4.9: Extinction efficiency of Au nanoparticles of different diameters suspendedin aqueous
solution [14].
Chapter 4
50
Figure 4.10: Photocurrent of Si pn junction diode with and without Au nanoparticles
The photocurrent response of the Si pn junction diode was measured for devices with
and without Au nanoparticles. Results are shown in fig.4.10, with fig. 4.11 showing the
absolute photocurrent in respect to the photocurrent without Au particles. All the spectra
have been normalized to account for the source illumination spectrum, and scaled so that
the photocurrent response is similar for wavelengths of 950 - 1100 nm, in order to
remove variations due to external factors of the measurements. Schaadt et al. presume,
that in this wavelength area the added Au nanoparticles do not contribute significantly
into the photocurrent, based on the extinction spectra in fig.4.10. Comparison of, for
example, figs. 4.10 and 4.11implies that the metal particles can also affect the
photocurrent response of the Si semiconductor at the wavelengths far away from the
bare metal particle resonance. From fig 4.11 considerable photocurrent enhancement
over a wide wavelength range can be observed for all Au particle sizes. The maximum
peak enhancement of factor~1.8 is observed for 80 nm particle diameter, but particle
diameter of 100 nm produces the widest enhancement peak, extending from ~900 nm
Chapter 4
51
downwards, with maximum enhancement factor of roughly 1.5. For particle diameter of
50 nm, the enhancement factor drops below unity – meaning a decrease in photocurrent
– at wavelengths of approx. 650 to 800 nm. All particle sizes show a decrease in
photocurrent at wavelengths above ~950 nm. The enhancement peak shifts towards
longer wavelengths as the particle size increases. Schaadt et al. credit the observed
photocurrent enhancement to increased optical absorption in Si, due to the surface
plasmon excitations in the Au nanoparticles. Two primary reasons for this are considered:
First, the amplitude of electric field near a metal nanoparticle, due to the LSP excitation,
is significantly larger than the amplitude of the incident field, giving rise to enhanced
optical absorption in semiconductor region in close proximity to the particles. Second, the
duration of the interaction between the incident electromagnetic field and the
semiconductor is increased near the metal particles, as the life-time of an LSP excitation
is some ~ 5 - 10 times greater than the photon transit time through the distance of the
LSP near field ( ~100 nm). Apart from the LSP excitations, an alternate explanation for
the observed photocurrent enhancement could, be generation of charge carriers inside
the metal nanoparticles, which are then injected into the semiconductor. This carrier
generation could occur either directly via photoexcitation in the metal particle, or via
plasmon decay, which generates an electron-hole pair. However, the direct carrier
generation in the metal nanoparticles is ruled out by the observation that the photocurrent
enhancements occur only at wavelengths corresponding to the LSP resonances. The
plasmon decay is eliminated as a possibility, due to the fact that no charging of the
nanoparticles was observed; if electron-hole pairs were generated in the particles and
electrons then injected to semiconductor, the metal particles would accumulate positive
charge.
Chapter 4
52
4.4.2 Effect of dielectric overcoating
Dielectric overcoating of the metal nanoparticles is used as a way to tune the Plasmon
resonance.
Figure 4.11: Photocurrent enhancement plots for the case with metal islands and for thecase of
metal islands overcoated with ZnS for 95nm Si on SOI with 35nm top oxide.Inset shows the
corresponding bare island resonances for the two cases clearly showing the red shifting with the
ZnS overcoating.[13]
The effect of using the dielectric coating is to red-shift the Plasmon resonance ,thereby
increasing the possibility of absorption at the longer wavelengths. This is because
scattering is enhanced at resonance frequency, and hence closer to the resonance of
the particle is to the bandgap of Si, the greater the absorption will be at those
Chapter 4
53
wavelengths. The thickness of the dielectric layer is chosen so that it is thick enough to
form a dielectric overcoating layer but thin enough not to introduce any waveguiding
effect. Since the particle were found to be 40-60nm in height, a thickness of 30nm ZnS
was arbitrarily chosen. The photocurrent enhancement result with ZnS overcoating on the
silver nanoparticle (Figure 4.12) clearly shows the red shift and an increase in
photocurrent enhancement.
4.4.3 Effect of different materials
The particle material also affects the resonance frequency. For metals like gold and
copper the plasma frequency lies in the visible whereas for silver the corresponding
transition is outside the visible region leading to a highly dispersive dielectric function at
optical frequencies on excitation with light [15] i.e. for a particle size corresponding to the
same mass thickness of gold and silver deposited, gold nanoparticles have a lower
frequency resonance than silver nanoparticles. Alloying the two metals in the different
ratios can yield resonances in between the resonances of the pure gold and silver
particles. The resonance can also be tuned using nanoshells that have a dielectric as the
core and a metal as the shell. Here the localised surface plasmon resonance tunability
was achieved in the visible from 550nm to 2000nm in the NIR regions by varying the core
to shell ratio which coincides with the important telecommunication wavelength.
Chapter 4
54
4.5 Conclusion
The ability to construct optically thick but physically very thin photovoltaic absorbers could
revolutionize high-efficiency photovoltaic device designs. This becomes possible by using
light trapping through the resonant scattering and concentration of light in arrays of metal
nanoparticles, or by coupling light into surface plasmon polaritons and photonic modes
that propagate in the plane of the semiconductor layer. In this way extremely thin
photovoltaic absorber layers (tens to hundreds of nanometres thick) may absorb the full
solar spectrum As we see from figure 4.9, applying nanoparticle on thin film Si solar cell
can improve its absorption efficiency by a huge margin. Different particle has their
resonance peak at different wavelength range. Ag nanoparticle has its peak over 900-
1200nm, on the other hand Au nanoparticle has it over 600-900nm.But it is pretty clear
that nanoparticle on top do increase solar cells efficiency.
4.6 References[1] P. Campbell and M. A. Green, "Light Trapping properties of pyramidally textured
surfaces," J. Appl. Phys., 62 (1), pp. 243-249 (1987).
[2] P. Campbell, "Enhancement of light absorption from randomizing and geometric
textures," J. Opt. Soc. Am. B, 10 (12), pp. 2410-2415 (1993).
[3] J. A. Rand and P. A. Basore, "Light-trapping silicon solar cellsexperimental results
and analysis," in Proc. of the 22nd IEEE Photovoltaic Specialists Conference, Las Vegas,
USA, 1991, p. 192- 197.
Chapter 4
55
[4] J. Zhao, A. Wang, P. Campbell and M. A. Green, "A 19.8% Efficient. Honeycomb
Multicrystalline Silicon Solar Cell with Improved Light Trapping," IEEE Trans. on Electron
devices, 46 (10), pp. 1978-1983 (1999a).
[5] V. T. Daudrix, J. Guillet, F. Freitas, A. Shah, C. Ballif, P. Winkler, M. Ferreloc, S.
Benagli, X. Niquille, D. Fischer and R. Morf, "Characterisation of Rough Reflecting
Substrates incorporated Into Thin-film Silicon Solar Cells," Prog. in photovoltaics,
[6] C. Beneking, B. Recha, S. Wieder, O. Kluth, H. Wagner, W. Frammelsberger, R.
Geyer, P. Lechner, H. Rubel and H. Schade, "Recent developments of silicon thin film
solar cells on glass substrates," Thin Solid Films, 351, pp. 241-246 (1999).
[7] P. A. Basore, "CSG-1: Manufacturing a new polycrystalline silicon PV technology,"
Proc. of 4th World Conference on Photovoltaic Energy Conversion, Waikoloa, Hawaii,
(2006a).
[8] M. A. Green, "Solar Cells - Operating principles, Technology and System
Applications," UNSW, Sydney, (1992).
[9] Harry A. Atwater, Albert Polman “Plasmonics for improved photovoltaic devices”
Review article, Nature material.
[10] D. J. Nash and J. R. Sambles, "Surface plasmon-polariton study of the optical
dielectric function of silver," Journal of Modern Optics, 43 (1), pp. 81-91 (1996).
[11] D. Pissuwan, S. Valenzuela and M. B. Cortie, "Therapeutic possibilities of
plasmonically heated gold nanoparticles.," Trends in Biotechnology, 24 (2), pp. 62-67
(2006).
Chapter 4
56
[12] H. R. Stuart and D. G. Hall, "Island size effects in nanoparticle enhanced
photodetectors," Appl. Phys. Lett., 73 (26), pp. 3815-3817
[13] S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green. Surface Plasmon enhanced
silicon solar cells. Journal of Applied Physics, 101:p. 093105, 2007
[14] D. M. Schaadt, B. Feng, and E. T. Yu. Enhanced semiconductor optical absorption
via surface plasmon excitation in metal nanoparticles. Applied Physics Letters, 86:p.
063106, 2005.
[15] U. Kreibig and M. Vollmer, "Optical properties of metal clusters," Wiley, NY,(1995).
Chapter 5
55
CONCLUSION AND FUTURE PERSPECTIVE
5.1 DiscussionPhotovoltaics have the potential to make a significant contribution to solving the
energy problem that our society faces in the next generation. To make power from
photovoltaics competitive with fossil fuel technologies, the cost needs to be reduced
by a factor of 2-5. Currently a large fraction of the solar cell market is based on
crystalline silicon wafers with a thickness of 180-300 μm, and a major fraction of the
cell price is due to Si materials and processing costs. Because of this, there is great
interest in thin-film solar cells, with film thicknesses in the range 1-2 μm, that can be
deposited on cheap module-sized substrates such as glass, plastic or stainless steel.
Thin-film solar cells are made from a variety of semiconductors including amorphous
and polycrystalline Si, GaAs, CdTe, and CuInSe2, as well as organic semiconductors.
A major limitation in all thin-film solar cell technologies is that the absorbance of near-
bandgap light is small, in particular for the indirect bandgap semiconductor silicon.
Therefore, structuring the thin-film solar cell so that light is trapped inside in order to
increase the absorbance (“light trapping”) is very important. A significant reduction in
thin-film solar cell thickness would also enable the large-scale use of scarce
semiconductor materials that are only available in the earth’s crust in limited
quantities, such as In and Te.
In our early discussion we have demonstrated a mechanism for localized
enhancement of semiconductor optical absorption via excitation of surface plasmon
resonances in proximate metal nanoparticles.The absorption coefficient of Si is low. It
absorb light at short wavelength.But metal nanoparticles absorb light at long
wavelength and it can be shifted to near IR region by proper tuning. The relative
contributions from radiative damping through resonant scattering and absorption
strongly depend on the particle size. Small particles are highly absorbing, and
generally have smaller, sharper peaks. As particle size is increased the peak
becomes red-shifted and broadened. This is due to dynamic depolarisation effects,
which occur when the electric field is not constant across the particle surface. Each
Chapter 5
56
particle size has various advantages for solar cell applications: small particles have
long plasmon lifetimes, while large particles scatter the incident light more and have
higher extinction efficiencies. For silver, whereas particles smaller than 30 nm exhibit
only absorption, light extinction of particles larger than about 50 nm is dominated by
resonant scattering. At 50 nm, both the absorption and scattering become equal but
their spectral maxima are shifted relative to each other. At frequencies near the
plasmon resonance frequency (typically in the 350–700 nm spectral range, depending
on metal and dielectric) SPPs suffer from relatively high losses. Further into the
infrared, however, propagation lengths are substantial. For example, for a semi-infinite
Ag/SiO2 geometry, SPP propagation lengths range from 10 to 100 μm in the 800–
1,500 nm spectral range.Increased propagation length comes at the expense of
reduced optical confinement and optimum metal-film design thus depends on the
desired solar-cell geometry. For a Si/Ag interface, with smaller optical absorption in Si
owing to the indirect bandgap, plasmon losses dominate over the entire spectral
range, although absorption in the 700–1,150 nm spectral range is still higher than
single-pass absorption through a 1-μm-thick Si film.
Nanotechnology is a more cost-effective solution and uses a cheap support onto
which the active component is applied as a thin coating. As a result much less
material is required (as low as 1% compared with wafers) and costs are decreased.
Conventional crystalline silicon solar cell manufactured by high of using a low
temperature process similar to printing. Nanotechnology reduced installation costs
achieved by producing flexible rolls temperature vacuum deposition process but
nanotechnology . Reduced manufacturing costs as a result instead of rigid crystalline
panels. Cells made from semiconductor thin films will also have this characteristic
Nanosolar company have successfully created a solar coating that is the most cost-
efficient solar energy
Chapter 5
57
Figure 5.1: Cost efficiency Trade off for photovoltaics.[1]
source ever. Their Power Sheet cells contrast the current solar technology systems by
reducing the cost of production from $3 a watt to a mere 30 cents per watt. This
makes, for the first time in history, solar power cheaper than burning coal.
Photovoltaic devices are limited in their practical efficiencies governed by the
thermodynamic limits and production costs that involve tradeoffs in materials,
production processes, and PV device packaging. The Lewis Group as a result of
higher efficiency or lower production provides a thorough illustration of the efficiency
trends for various PV devices materials such as crystalline silicon used in
semiconductors as well as the new approaches to thin film PV including amorphous
silicon, cadmium telluride (CdTe), copper indium deselenide (CIS) and copper indium
gallium deselenide materials (CIGS). These thin film material could offer substantial
PV devices price reductions costs. Most such cells utilize amorphous silicon, which,
as its name suggests, does not have a crystalline structure and consequently has a
much lower efficiency (8%), however it is much cheaper to manufacture.
Chapter 5
57
Figure 5.1: Cost efficiency Trade off for photovoltaics.[1]
source ever. Their Power Sheet cells contrast the current solar technology systems by
reducing the cost of production from $3 a watt to a mere 30 cents per watt. This
makes, for the first time in history, solar power cheaper than burning coal.
Photovoltaic devices are limited in their practical efficiencies governed by the
thermodynamic limits and production costs that involve tradeoffs in materials,
production processes, and PV device packaging. The Lewis Group as a result of
higher efficiency or lower production provides a thorough illustration of the efficiency
trends for various PV devices materials such as crystalline silicon used in
semiconductors as well as the new approaches to thin film PV including amorphous
silicon, cadmium telluride (CdTe), copper indium deselenide (CIS) and copper indium
gallium deselenide materials (CIGS). These thin film material could offer substantial
PV devices price reductions costs. Most such cells utilize amorphous silicon, which,
as its name suggests, does not have a crystalline structure and consequently has a
much lower efficiency (8%), however it is much cheaper to manufacture.
Chapter 5
57
Figure 5.1: Cost efficiency Trade off for photovoltaics.[1]
source ever. Their Power Sheet cells contrast the current solar technology systems by
reducing the cost of production from $3 a watt to a mere 30 cents per watt. This
makes, for the first time in history, solar power cheaper than burning coal.
Photovoltaic devices are limited in their practical efficiencies governed by the
thermodynamic limits and production costs that involve tradeoffs in materials,
production processes, and PV device packaging. The Lewis Group as a result of
higher efficiency or lower production provides a thorough illustration of the efficiency
trends for various PV devices materials such as crystalline silicon used in
semiconductors as well as the new approaches to thin film PV including amorphous
silicon, cadmium telluride (CdTe), copper indium deselenide (CIS) and copper indium
gallium deselenide materials (CIGS). These thin film material could offer substantial
PV devices price reductions costs. Most such cells utilize amorphous silicon, which,
as its name suggests, does not have a crystalline structure and consequently has a
much lower efficiency (8%), however it is much cheaper to manufacture.
Chapter 5
58
5.2 Limitations and future considerationMaterials resources are a significant limitation for large-scale production of two of the
most common thin-film solar-cell materials: CdTe and CuInSe2. Manufacturing costs
for these cells have fallen, and solar-cell production using these semiconductors is
expanding rapidly. Table 1 lists the (projected) annual solar-cell production per year,
as well as the materials feedstock required for the production of the corresponding
solar-cell volume using Si, CdTe or CuInSe2. As can be seen, the materials feedstock
required in 2020 exceeds the present annual world production of Te and In, and in the
case of In is even close to the total reserve base. If it were possible to reduce the cell
thickness for such compound semiconductor cells by 10–100 times as a result of
plasmon-enhanced light absorption, this could considerably extend the reach of these
compound semiconductor thin-film solar cells towards the terawatt scale. Earth-
abundance considerations will also influence plasmonic cell designs at large-scale
production: although Ag and Au have been the metals of choice in most plasmonic
designs and experiments, they are relatively scarce materials, so scalable designs will
need to focus on abundan metals such as Al and Cu.
Reducing the active-layer thickness by plasmonic light trapping not only reduces costs
but also improves the electrical characteristics of the solar cell78. First of all, reducing
the cell thickness reduces the dark current (Idark), causing the open-circuit voltage
Voc to increase, as Voc = (K T/q) ln(Iphoto/Idark + 1), where K is the Boltzmann
constant, T is temperature, q is the charge and Iphoto is the photocurrent.
Consequently, the cell efficiency rises in logarithmic proportion to the decrease in
thickness, and is ultimately limited by surface recombination. Second, in a thin-film
geometry, carrier recombination is reduced as carriers need to travel only a small
distance before being collected at the junction. This leads to a higher photocurrent.
Greatly reducing the semiconductor layer thickness allows the use of semiconductor
materials with low minority carrier diffusion lengths, such as polycrystalline
semiconductors, quantum-dot layers or organic semiconductors. Also, this could
render useful abundant and potentially inexpensive semiconductors with significant
impurity and defect densities, such as Cu2O, Zn3P2 or SiC, for which the state of
electronic materials development is not as advanced as it is for Si.
Chapter 5
59
5.3 Future perspective
The previous section has focused on the use of plasmonic scattering and coupling
concepts to improve the efficiency of single-junction planar thin-film solar cells, but
many other cell designs can benefit from the increased light confinement and
scattering from metal nanostructures. First of all, plasmonic ‘tandem’ geometries may
be made, in which semiconductors with different bandgaps are stacked on top of each
other, separated by a metal contact layer with a plasmonic nanostructure that couples
different spectral bands in the solar spectrum into the corresponding semiconductor
layer (figure 5.2).
Chapter 5
60
Figure 5.2 Plasmonic tandem solar-cell geometry. Semiconductors with different bandgaps
are stacked on top of each other, separated by a metal contact layer with a plasmonic
nanostructure that couples different spectral bands of the solar spectrum into the
corresponding semiconductor layer.[2]
Coupling sunlight into SPPs could also solve the problem of light absorption in
quantum-dot solar cells (Fig.5.3), which is another example of new plasmonic solar
cell. Although such cells offer potentially large benefits because of the flexibility in
engineering the semiconductor bandgap by particle size, effective light absorption
requires thick quantum-dot layers, through which carrier transport is problematic. As
we have recently demonstrated80, a 20-nm-thick layer of CdSe semiconductor
quantum dots deposited on a Ag film can absorb light confined into SPPs within a
decay length of 1.2 μm at an incident photon energy above the CdSe quantum-dot
bandgap at 2.3 eV. The reverse geometry, in which quantum dots are electrically
excited to generate plasmons, has also recently been demonstrated.
Chapter 5
61
Figure 5.3 Plasmonic quantum-dot solar cell designed for enhanced photoabsorption in
ultrathin quantum-dot layers mediated by coupling to SPP modes propagating in the plane of
the interface between Ag and the quantum-dot layer. Semiconductor quantum dots are
embedded in a metal/insulator/metal SPP waveguide.[3]
5.4 References
[1] Harry A. Atwater, and Albert Polman2 , Plasmonics for improved photovoltaic
devices
[2] Fahr, S., Rockstuhl, C. & Lederer, F. Metallic nanoparticles as intermediate
reflectors in tandem solar cells. Appl. Phys. Lett. 95, 121105 (2009
[3] Walters, R. J., van Loon, R. V. A., Brunets, I., Schmitz, J. & Polman, A. A silicon-
based electrical source of surface plasmon polaritons. Nature Mater. 9, 21–25 (2010).
Chapter 5
61
Figure 5.3 Plasmonic quantum-dot solar cell designed for enhanced photoabsorption in
ultrathin quantum-dot layers mediated by coupling to SPP modes propagating in the plane of
the interface between Ag and the quantum-dot layer. Semiconductor quantum dots are
embedded in a metal/insulator/metal SPP waveguide.[3]
5.4 References
[1] Harry A. Atwater, and Albert Polman2 , Plasmonics for improved photovoltaic
devices
[2] Fahr, S., Rockstuhl, C. & Lederer, F. Metallic nanoparticles as intermediate
reflectors in tandem solar cells. Appl. Phys. Lett. 95, 121105 (2009
[3] Walters, R. J., van Loon, R. V. A., Brunets, I., Schmitz, J. & Polman, A. A silicon-
based electrical source of surface plasmon polaritons. Nature Mater. 9, 21–25 (2010).
Chapter 5
61
Figure 5.3 Plasmonic quantum-dot solar cell designed for enhanced photoabsorption in
ultrathin quantum-dot layers mediated by coupling to SPP modes propagating in the plane of
the interface between Ag and the quantum-dot layer. Semiconductor quantum dots are
embedded in a metal/insulator/metal SPP waveguide.[3]
5.4 References
[1] Harry A. Atwater, and Albert Polman2 , Plasmonics for improved photovoltaic
devices
[2] Fahr, S., Rockstuhl, C. & Lederer, F. Metallic nanoparticles as intermediate
reflectors in tandem solar cells. Appl. Phys. Lett. 95, 121105 (2009
[3] Walters, R. J., van Loon, R. V. A., Brunets, I., Schmitz, J. & Polman, A. A silicon-
based electrical source of surface plasmon polaritons. Nature Mater. 9, 21–25 (2010).