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).