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UNIVERSITY OF SOUTHAMPTON
Faculty of Engineering,Science and Mathematics
School of Electronics and Computer Science
A project report submitted for the award of
MEng Electronic Engineering
Supervisor:
Dr Darren Bagnall
Examiner:
Dr Paul Lewin
Antireflection and Light Trapping Scheme
Development with Biomemetic
Metamaterials
by
Sean E.Nuzum
May 22,2007
UNIVERSITY OF SOUTHAMPTON
ABSTRACT
FACULTY OF ENGINEERING,SCIENCE AND MATHEMATICS
SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE
A project report submitted for the award of MEng Electronic Engineering
by
Sean E.Nuzum
Biologically inspired structures and designs are investigated,with emphasis on their
structural properties and potential uses in synthetic development of photovoltaic devices
as metamaterials.Physical biological samples exhibiting properties including black,iri-
descence,fluorescence,white and transparent are investigated using optical experimen-
tal procedures involving reflectance,polarisation,angle of incidence and transmittance.
Nano-structural properties of the samples are investigated in detail through Scanning
Electron Microscopy and the significant importance of structures in the generation of
striking optical effects is highlighted.Fourier transforms are used to characterise and
compare structural layouts.Full experimental preparation and procedural information
is provided,with results and conclusions formed in relation to current techniques and
concepts.
Keywords:Solar cells,metamaterials,nano-structures,structural colour,photonic struc-
ture,butterfly
Contents
List of Figures
iv
List of Tables
vi
List of Symbols
vii
Acknowledgements
ix
1 Introduction
1
1.1 Project Goals
..................................
1
1.2 Organisation of Report
.............................
2
2 Background Knowledge and Literature Search
3
2.1 Introduction to Solar Cells
...........................
3
2.2 Solar Cell Efficiency Limitations
.......................
5
2.3 Optical Interfaces in Photovoltaics
......................
7
2.3.1 General Principles
...........................
7
2.3.2 Reflective Affects in Solar Cell Design
................
9
2.4 Antireflection and Light Trapping in Biology
................
10
2.4.1 Antireflection
..............................
10
2.4.2 Texturing Schemes
...........................
11
2.4.3 Structural Colours
...........................
12
2.4.3.1 Black Colouration
......................
13
2.4.3.2 White - Spectrally Diffuse Reflectance
...........
13
2.4.3.3 Transparent Characteristics
.................
15
3 Experimental Procedures - Optical Measurements
16
3.1 Biological Sample Preparation
.........................
16
3.2 Specular Reflectance Measurements - White Light Source
.........
17
3.2.1 Experimental Apparatus
........................
17
3.2.2 Experimental Procedure
........................
18
3.2.3 Results and Discussion
.........................
20
3.2.3.1 Black Samples
........................
20
3.2.3.2 Other Samples
........................
22
3.2.4 Conclusions
...............................
26
3.2.5 Limitations of Experiment
.......................
26
3.3 Polarisation and Incident Angle Reflectance Measurements - 633nm Laser
28
3.3.1 Experimental Apparatus
........................
28
ii
CONTENTS iii
3.3.2 Experimental Procedure
........................
29
3.3.3 Results and Discussion
.........................
30
3.3.4 Conclusions
...............................
32
3.4 Angle of Incidence Transmittance Experiments - White Light Source
...
34
3.4.1 Experimental Apparatus
........................
34
3.4.2 Experimental Procedure
........................
35
3.4.3 Results and Discussion
.........................
35
3.4.4 Conclusions
...............................
36
3.5 Conclusion of Optical Experiments
......................
38
4 Experimental Procedures - Biological Structural Investigation
40
4.1 Scanning Electron Microscopy
.........................
40
4.1.1 Sample Preparation
..........................
41
4.1.2 Black Sample Investigation
......................
41
4.1.3 Spectral Colour Investigation
.....................
44
4.1.3.1 Blue Samples
.........................
44
4.1.3.2 Other Samples
........................
47
4.1.4 Transparency Investigation
......................
49
4.1.5 Summary and Conclusions
.......................
51
4.2 Fourier Transform Investigation
........................
53
4.2.1 Basic Principles
.............................
53
4.2.2 Structural Black Sample Transforms
.................
54
4.2.3 Structural Colour Sample Transforms
................
55
4.2.4 Non Structural Colour Sample Transformations
...........
57
4.2.5 ‘Moth-wing’ Structure Transform
...................
57
4.2.6 Summary and Conclusions
.......................
58
4.3 Conclusion of Biological Structural Investigations
..............
59
5 Conclusions and Further Work
61
5.1 Evaluation and Conclusions
..........................
61
5.2 Further Developments
.............................
62
A Biological Samples
64
B Biological Sample Summary
68
C Experimental CD Table of Contents
72
C.1 Reflectance Data
................................
72
C.2 Polarisation Data
................................
72
C.3 Transmittance Data
..............................
73
Bibliography
74
List of Figures
2.1 Diagram showing a typical first generation screen-printed single-crystal
silicon solar cell (from [
1
]).
...........................
4
2.2 Schematic diagrams of thin filmCdTe,CIGS and a-Si thin filmPVdevices
(from [
1
]).
....................................
5
2.3 Boundary between two media of refractive indicies n1,n2.a) reflected
path of light,b) reflection at interface and c) transmitted path of light.
.
7
2.4 Diagram to show the layers present in a typical silicon bulk wafer solar cell.
8
2.5 Diagramto show light interaction with different interfaced material (using
Figure
2.4
):A) light reflecting from metal contact/light entering silicon
B) Light reflection frommetal back contact,increasing photon path length
C) Example of transition between thin filmsolar cell layer (using Figure
2.2
).
8
2.6 Diagram showing the utilisation of reflectance:a) the dispersion of light
entering a medium and b) reflectance at boundary n
2
leading to total
internal reflection.
...............................
9
2.7 Optical effects of light entering a material at normal incidence:a) Light
randomly diffused b) Incident light transmits through immediate boundary.
10
2.8 Representation of light reflection optimisation for light trapping within a
solar cell (CIGS from [
1
]).
...........................
10
2.9 A diagram showing single layer ARC destructive interference (from [
2
]).
.
11
2.10 a) Scanning Electron Microscope (SEM) image of nipple array on the
cornea of a nightflying moth (from [
3
]) b) SEMimage of replicated moth-
eye structure on silicon substrate (from [
4
]).
.................
12
2.11 a) A collection of black butterfly samples and b) Diagrammatic repre-
sentation of part of a fractured scale showing the relationship between
the lower lamina,trabeculae (T),ridges (R),lamellae (**),ribs (r),and
microribs (mr).This view also shows pigment granules (*),not present
in all scales.(from [
5
])
.............................
14
2.12 a) A large white,Pieris Brassicae b) SEM image of Pieris Brassicae but-
terfly wing (images extracted from [
6
]).
...................
14
2.13 Natural transparent butterfly wing structure cross section,an example of
light travelling through the material is shown.
................
15
2.14 A collection of transparent winged biological specimines.(from [
7
]
....
15
3.1 Top view of RTC-060-SF integrating sphere (from [
8
]).
...........
17
3.2 Outline of the experimental apparatus for reflectance experiments.
....
18
3.3 Diagram to show path of sample beam (red) and reference beam (blue)
when taking measurements(extension of Figure
3.1
from [
8
]).
.......
19
3.4 A collection of black samples and their corresponding reflective charac-
teristics.
.....................................
20
iv
LIST OF FIGURES v
3.5 Graphical reflection properties of one red sample and two blue samples
under test.
....................................
22
3.6 Visual representation of the reflection characteristics of the PERL cell
surface and standard untextured silicon surface,black P.ulysses sample
included for reference.
.............................
23
3.7 Spectral reflectance characteristics for a collection of samples,namely
yellow,green and white.
............................
25
3.8 Spectrum of visible light spectrum.
......................
27
3.9 Diagram of the clip style center-mounted sample holder (from [
8
]).
....
28
3.10 Image showing P-plane and S-plane incident light on a surface (from
http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/tayu/ACT05E/img/05IMG/05
11.jpg).
29
3.11 Schematic of laser experiment of polarisation and angles of incidence.
...
29
3.12 a) overall image of apparatus b) the integrating sphere c) collection of
lenses/polarisers.
................................
30
3.13 Reflectance of Papilio Ulysses sample for S and P plane polarisation and
changing angles of incidence.
.........................
31
3.14 Matlab generated cubic curve to model polarisation result data.
......
32
3.15 Graph to show increase of gradient for the polarisation experimental results
33
3.16 Transmission experiment schematic.
.....................
34
3.17 Graph showing transmission at varied angles of incidence on moth wing
sample number 7.
................................
35
3.18 Graph showing average transmission across different angles of incidence.
.
37
3.19 Standard single,double and triple glazed window transmittance charac-
teristics over a range of angles of incidences (from [
9
]).
...........
37
4.1 JSM 6500F thermal field emission scanning electron microscope layout,
Universtiy of Southampton (from http://www.micro.soton.ac.uk/).
....
41
4.2 Images to show a) samples mounted on carbon stubs and b) samples
mounted into SEM holder after sputtering.
.................
42
4.3 Papilio Ulysses black scales:a) overall view of serrated black scales b)
closer view of individual scale c) image of broken scale d) closer image of
broken scale revealing the substrate honeycomb structures lie on.
.....
42
4.4 SEM Images to show Papilio Phorcas black scale ridges and honeycomb
structure (Sample 3b in Table
A.3
).
.....................
43
4.5 Using scale from Figure
4.3
b):revealing honeycomb 3D photonic struc-
ture in greater detail a) shows the Papilio Ulysses,sample 1b) X22,000
zoom,with ridge period approximately 2.5µm.b) shows the Papilio
Nireus,sample 2b) at X7,000 zoom,revealing a similar black honeycomb
structure.
....................................
43
4.6 The Troides Rhadamantus black scale X5,500 revelling lack of honeycomb
structure.
....................................
44
4.7 Black and iridescent colourations of the Papilio Ulysses and appropriate
scales in relation to Section
4.1.3
,as extracted from Appendix
A
.
.....
45
4.8 Cross section of ulysses blue scale,showing absorbing and reflecting layers.
(from [
10
])
....................................
45
4.9 Papilio Ulysses blue sample (Table
A.3
sample 1a ) scales.a) iridescent
blue scale X370 b) underlying black scale X370 c) enlarged image of ‘a)’
- blue scale.X8,500.
..............................
46
LIST OF FIGURES vi
4.10 Papilio Nireus blue sample (Table
A.3
sample 2a) a) underlying black
scales b) overlapping iridescent blue scales.
.................
47
4.11 Papilio Nireus blue scale SEM image showing high intensity honeycomb
structure.
....................................
48
4.12 P.Nireus SEM image of Photonic crystal slap (PCS) (from [
11
])
......
48
4.13 SEM Images showing a) Troides Rhadamantus black scale zoom X5,500
and b) Cymothoe Sangaris red butterfly scale,zoom X5500
........
48
4.14 Cymothoe sangaris,white nano-scales.
....................
49
4.15 Moth wing ‘nipple’ array structures zoom from X4,000 to X50,000.
....
50
4.16 Cross-sectional view of moth wing nipple array structures and interme-
diate substrate.a) cross section approx.depth 1.6µm b) nipple height
approx.200nm.
.................................
51
4.17 Structure outlines:P = perodicy,H = Height,W = Width:a) motheye
structure cross section outline (from [
12
]) b) moth wing structure cross
section outline.
.................................
51
4.18 Honeycomb structure - 3D Photonic Crystal (from [
13
]
...........
53
4.19 Fourier Transform of Figure
4.9
b),(P.Ulysses black scale SEM image).
.
54
4.20 Fourier transform of Figure
4.10
a),P.Nireus black scale.
..........
55
4.21 The white Pryoneris Philonome (Figure
4.14
) butterfly fourier transform.
56
4.22 a) Red Cymothoe Sangaris SEM and fourier transform b) Black Troides
Rhadamantus SEM and fourier transform.
..................
57
4.23 ‘Moth-wing’ structure fourier transform
...................
58
A.1 Collection of biological samples and the sections extracted for experimen-
tal purpose
...................................
65
A.2 Collection of biological samples and the sections extracted for experimen-
tal purpose (continued)
............................
66
B.1 Samples and their corresponding SEM images and Fourier Transforms 1/3
69
B.2 Samples and their corresponding SEM images and Fourier Transforms 2/3
70
B.3 Samples and their corresponding SEM images and Fourier Transforms 3/3
71
List of Tables
3.1 Black samples and corresponding reflectance average across wavelengths
of 450nm to 700nm - spectral reflectance experiment.
............
21
3.2 Black samples and corresponding reflectance average across wavelengths
of 450nm to 700nm - spectral reflectance experiment.
............
24
3.3 Colour samples and average reflection characteristics.
............
25
3.4 Samples and corresponding colours - spectral reflectance experiment.
...
27
3.5 Polarisation and Angle of incidence reflectance results for Papilio Ulysses
(black) butterfly wing (using integrating sphere with center mounted sam-
ple holder [
8
]).
.................................
30
3.6 Average transmittance over different angles of incidence.
..........
36
4.1 Period of ridges for a variety of butterfly wings.
...............
53
4.2 Period of ridges for a variety of butterfly wings,calculated using accurate
fourier transform plots.
.............................
59
A.1 Butterfly samples imported via Worldwide Butterflies and description of
prominent characteristics
...........................
64
A.2 Moth wing samples imported via The Insect Company and description of
prominent characteristics
...........................
64
A.3 Samples taken from biological samples
....................
67
vii
List of Symbols
R Normal incidence reflectance factor
n Refractive index
t Thickness
λ Wavelength of light
R(λ) Reflectance factor
R
Std
(λ) Reflectance data for calibrated reflectance standard
S
Sample
(λ) The detector signal recorded with the sample beam lit
S
Ref
(λ) The detector signal recorded with the reference beam lit
S
Dark
The detector signal recorded for the dark current measurements
Subscript “S
￿￿
indicates measurements taken with the sample loaded at the sample port
Subscript “Std
￿￿
indicates measurements taken with the calibrated standard loaded at
the sample port
Transmittance:
T(λ) Reflectance factor
subscript “Empty
￿￿
indicates measurements taken with an empty transmittance port
subscript “S
￿￿
denotes measurements taken with the sample loaded at the
transmittance port
viii
Acknowledgements
There are a number of people that have aided the progress and development of my
project.Firstly I would like to thank my project supervisor Dr.Darren Bagnall for his
help and guidance from the start of the project and maintaining my course of direction
throughout.Stuart Boden a PhD student has invested much time and effort into the
experimental aspects of my project,mainly setting up and using devices in both the
electronics and chemistry laboratory.I would also like to thank Paras Bhagtani for aiding
my sections on the Fourier Transform and introducing me to the necessary software
applications.
ix
Chapter 1
Introduction
Technological development worldwide is leading to an increasing demand for electri-
cal power,with the need to generate renewable energy of increasing importance as we
continue to deplete our limited sources of fossil fuels.Just 18% of the world’s power
generation is renewable [
14
] and there is particular emphasis on developing this field
of energy production for the sake of the environment and a sustainable future.Solar
cells are an important and well established part of this renewable energy industry and
there has been large investment in areas of research and development.This project will
look at developing and enhancing the scope of antireflective and light trapping scheme
development,through the use of biological samples as case studies.
1.1 Project Goals
The fundamental goal of this project is to provide useful data and results that can be
used to aid our humanitarian endeavour to produce higher efficiency photovoltaic devices
and sources of sustainable energy.The method of approach is to provide a comprehen-
sive report through documentation of experimental procedure and discussion;providing
foundations for implementation and further development of the concepts explored.
Providing foundations will be achieved through particular concentration on the investiga-
tion into metamaterials
1
,establishing and investigating structures present on specimens
such as butterflies and moths,with specialisation into subwavelength structures and
their potential implementation in synthetic system development.
With the concepts of metamaterials in solar cell development being highly structural,
there is aim to place particular emphasis on structural properties where possible;since
this constitutes to the phototonic properties of interest.Attributable tasks include,
1
Composite artificial material that gains its properties from its structure rather than directly from
its composition.
1
Chapter 1 Introduction 2
gaining experimental results and analysing critically from a structural point of view;
enabling formation of useful conclusions.
1.2 Organisation of Report
The report is organised into five chapters,namely,Introduction,Background Research,
Optical Experimental,Structural Experimental and Conclusions,with accompanying
appendices and a CD containing all experimental data obtained.
Chapter 2:Provides an introduction to the background knowledge behind solar cell
technologies and the problems faced in the development of such technologies.Graphical
representations are used to show how layers are interfaced,with detail into the opti-
cal effects experienced by photons.Literature behind antireflection and light trapping
technologies are discussed and description of current research into metamaterials is iden-
tified.To conclude the section,structurally induced optical effects are discussed from a
biological perspective,using examples.
Chapter 3:The purpose of this chapter is to obtain experimental data through optical
investigations.Details of the biological samples obtained for experimental purpose are
given,explaining the preparation process and experimental procedures in detail.Re-
sults are discussed individually in detail and where appropriate compared with current
techniques used in industry.In order to provide comprehensive result data,experiments
are conducted in areas of reflectance,polarisation,angle of incidence and transmission.
Full results obtained are documented in the accompanying CD (See Appendix
C
).
Chapter 4:Begins to explore the structural properties exhibited by the biological sam-
ples via Scanning Electron Microscopy techniques to highlight the underlying nano-
structural contents.The sample preparation process and the experimental procedure is
documented in detail,collections of images are presented and discussed,finally they are
processed using the fourier transform in order to relate the natural structural properties
to the spectral reflectance attributes;highlighting the potential future benefits of these
concepts in our synthetic long-term progression of solar cell development.
Chapter 5:Summarises and concludes the report as a whole,detailing places for further
work and areas of additional interest on the basis of the work explored in this project.
Appendices:A & B,Detail the biological samples in further detail,documenting the
samples extracted for experimental purposes.Appendix C provides the table of contents
for the accompanying CD.
Chapter 2
Background Knowledge and
Literature Search
2.1 Introduction to Solar Cells
The principle purpose of a solar cell is to convert incident light energy into electrical
power.Light can be analysed as quantifiable ‘packets’ of energy called photons,with the
energy of each photon proportional to its wavelength.This energy can be absorbed by a
material to stimulate generation of carriers (electron-hole pairs).This forms the basis of
the Photovoltaic effect,first discovered by Becquerel in 1839 [
15
].The first photovoltaic
cells (solar cells) were successfully fabricated by the 1950’s [
16
].They consisted of P
doped and N doped material layered to form a P-N junction (essentially a P-N diode);
with surrounding metal contacts and an upper window to allow entry of light photons.
Initially a fraction of incident light is lost due to reflection at the surface,the remaining
photons pass into the cell with anticipation to be of absorbed.Photons with sufficient
energy equal or greater than that of the material band gap (difference in energy between
the conduction band and the valence band) can be absorbed.This allows electrons to
be excited into the conduction band,giving rise to mobile electron-hole pairs.When
the device is connected to a load the free carriers can flow between the layers,providing
current.The combination of this current flow and the electric field generated across the
P-N junction gives rise to electrical power.
Despite the many years of solar cell development the price per watt (£/W) is still
approximately four times too expensive [
1
] and there is certainly room for development.
Solar cell fabrication techniques have evolved over time and essentially they can be
classified into three generations.
First generation solar cells are based on the use of bulk crystalline N and P doped silicon
semiconductor wafers.Figure
2.1
shows the typical layout of a first generation cell,with
3
Chapter 2 Background Knowledge and Literature Search 4
a large P-type base layer and thin N-type layer,complete with antireflective coating
1
and
metal contacts.The simplicity of applying the metal contact,reliability and durability
in this solar cell generation has led to market dominance with over 90% of the market
share [
17
].
Figure 2.1:Diagram showing a typical first generation screen-printed single-crystal
silicon solar cell (from [
1
]).
Single-crystalline silicon is grown in the form of ingots,sheets or ribbons [
18
].These
uniformly arranged continuous crystalline structures are then sliced into wafers that can
be fabricated into solar cells.The costs related to the production of bulk silicon wafers
has led research into new more cost effective second generation approaches;such as thin
film solar cells.This is due to over 40% of the final fabrication costs being attributable
to the bulk starting silicon wafer.[
19
]
A second generation cell is formed by depositing an arranged series of thin film semi-
conductor layers on low-cost substrates.This provides a low mass solution and greatly
reduced production costs.The first thin film solar cells were formed using multiple P-N
junctions that introduce multiple band gaps and ability to absorb a wider spectrum of
light.Single junction devices will perform optimally when the incident light wavelength
provides sufficient energy to equal that of the bandgap;inherently losing efficiency at all
other wavelengths.Multi-junction devices are therefore utilising a greater bandwidth of
the solar spectrum and theoretical studies have revealed that these devices could reach
maximum efficiencies of up to 60% [
20
].
Thin filmapproaches provide a significant reduction in material usage whilst still achiev-
ing similar efficiencies to that of the first generation cells at a fraction of the cost.The
fabrication of thin film solar cells can be carried out using amorphous silicon or other
alternative semiconductors such as Cadium,Telluride,Indium and Selenide as shown
in Figure
2.2
.Poly-crystalline silicon can be used to fabricate thin film solar cells,this
1
Here antireflection is achieved through the use of pyramid texturing as shown in the diagram,these
concepts will be discussed further in Section
2.4
Chapter 2 Background Knowledge and Literature Search 5
formation of silicon is made up of “many grains of single crystal silicon whose size and
quality determine solar cell performance.” [
18
] Light trapping schemes can be used to
increase the effective thickness of the silicon layer and improve the overall efficiency;
this will be discussed further in Chapter
2
.Aside from being less abundant,the use of
alternative semiconductors can produce more efficient solar cells as they exhibit better
bandgap properties and absorption capabilities than that of silicon.
As second generation solar cells reach costing limits and the issue of improved efficiency
continues;the introduction of future third generation solar cells is becoming more fea-
sible.These are generally defined as semiconductor devices which do not rely on a
traditional p-n junction to separate photogenerated charge carriers.These new devices
include photoelectrochemical cells,Polymer solar cells,and nanocrystal solar cells.[
21
].
Figure 2.2:Schematic diagrams of thin filmCdTe,CIGS and a-Si thin filmPVdevices
(from [
1
]).
2.2 Solar Cell Efficiency Limitations
An ideal solar cell would convert 100% of the light energy incident on its surface into
electrical power.In reality this is an impossible goal to achieve with the efficiency of a
solar cell being affected by several factors,both internally and externally.Namely,

Reflection of light

Short photon paths

Limitations of the material(s)
Light is reflected by the front metal contacts present on the surface of the cell and
during its transitions through the cell layers (see Figure
2.1
).The optical route taken
by a packet of light through the layers of a solar cell is known as a photon path.A
shorter photon path will result in reduced probability of absorption,therefore reducing
Chapter 2 Background Knowledge and Literature Search 6
the overall cell efficiency capability.The bandwidth of light absorption is limited by the
material(s) that constitute the cell layers.A degree of energy loss will occur as a result of
electrical resistance exhibited by all materials and the maximum obtained energy from
incident photons is governed by the photovoltaic effect (described in Section
2.1
).
Chapter 2 Background Knowledge and Literature Search 7
2.3 Optical Interfaces in Photovoltaics
Photovoltaic devices and their operational characteristics are governed by the elements
they are comprised of and how they are interfaced during the fabrication process.The
purpose of this section is to explore a variety of optical effects experienced by light in
the content of solar cell development.
2.3.1 General Principles
It can be shown that the reflectance R at normal incidence to the boundary between
two materials with refractive indices n
1
and n
2
is given by:
R =
￿
n
1
−n
2
n
1
+n
2
￿
2
(2.1)
Absorption capability depends on reflection of light and photon path lengths (as dis-
cussed in Section
2.2
).Equation
2.1
shows that as the refractive indices present at an
interface between two materials diverge,the amount of reflection at normal incidence
is increased.When light reaches the surface of a solar cell or passes between layers,a
portion of its photons will be reflected due to this effect.A typical example of reflection
and transmission is shown in Figure
2.3
.
Figure 2.3:Boundary between two media of refractive indicies n1,n2.a) reflected
path of light,b) reflection at interface and c) transmitted path of light.
Light incident upon a surface will have a percentage of light loss due to reflectance,as
shown by a) in Figure
2.3
.An increased angle of incidence into the material (angle Y

)
will result in greater reflectance and reduced efficiency.Reflectance at b) however is not
Chapter 2 Background Knowledge and Literature Search 8
such a problem,since the light is effectively trapped into the cell (this will be explained
in further detail later).The basic representation of a typical silicon solar cell and its
layers is shown in Figure
2.4
,Figure
2.5
shows some example illustrations of optical
effects that can be experienced by incident light.
Figure 2.4:Diagram to show the layers present in a typical silicon bulk wafer solar
cell.
Figure 2.5:Diagram to show light interaction with different interfaced material (us-
ing Figure
2.4
):A) light reflecting from metal contact/light entering silicon B) Light
reflection from metal back contact,increasing photon path length C) Example of tran-
sition between thin film solar cell layer (using Figure
2.2
).
Chapter 2 Background Knowledge and Literature Search 9
2.3.2 Reflective Affects in Solar Cell Design
It has been shown that reflection can reduce efficiency at the surface,similarly efficiency
can be improved through the use of reflection at cell boundaries to prevent photons
escaping.Prevention and utilisation of reflective effects forms the basis of antireflective
and light trapping techniques (as discussed in further sections).
Within a solar cell there are particular places where transmittance,reflectance and
diffusion properties are best preferred.Figure
2.6
shows an example situation of optical
dispersion of light upon entry to a solar cell.If (n
1
) is a photon absorbing material
then a reflective property at the interface between material n
2
is preferred,since this
allows the concept of total internal reflection and increased photon path lengths;since
the photons spend increased time in the material for absorption to occur.
Figure 2.6:Diagram showing the utilisation of reflectance:a) the dispersion of light
entering a mediumand b) reflectance at boundary n
2
leading to total internal reflection.
An alternative option to standard reflectance at b) in Figure
2.6
is for diffuse reflectance,
this will scatter incident light uniformly.Scattered light will travel at more oblique
angles,therefore increasing the optical path length.Also more light will undergo total
internal reflection being trapped for multiple passes through layers,again increasing
optical path length.This optical effect is shown in Figure
2.7
and is achieved using
a white,spectrally diffuse surface.The basis of uniform light scattering is linked to
the concepts of a lambertian diffuse reflector and the concepts discussed are shown
practically in Figure
2.8
(an adaptation of Figure
2.2
CIGS cell) to show the need for
transmission and reflectance.
Chapter 2 Background Knowledge and Literature Search 10
Figure 2.7:Optical effects of light entering a material at normal incidence:a) Light
randomly diffused b) Incident light transmits through immediate boundary.
Figure 2.8:Representation of light reflection optimisation for light trapping within a
solar cell (CIGS from [
1
]).
2.4 Antireflection and Light Trapping in Biology
This section is a continuation of that discussed in the previous,now describing the prin-
ciples of antireflection and light trapping in more formal detail for solar cell development
and relating this to where biology has naturally developed these optical systems.
2.4.1 Antireflection
The concept of antireflection is to reduce reflection at an optical interface to a minimum,
therefore increasing transmission capability.Untreated silicon reflects more than 30%
of incident light [
16
] and the simplest method to incorporate antireflection into the
development of solar cells is by depositing single,double or multiple thin film coating(s)
on the surface.Introducing an anti-reflective layer reduces the amount of reflection by
Chapter 2 Background Knowledge and Literature Search 11
taking advantage of thin filmdestructive interference.Using Figure
2.9
,the intermediate
thin film layer (represented as n
1
,with thickness t) has a refractive index between that
of the two materials n
0
& n
2
,as light passes through the boundaries it creates two
identical waves A & B which destructively interfere;in effect cancelling each other out,
providing antireflection.
Figure 2.9:A diagram showing single layer ARC destructive interference (from [
2
]).
The problems faced by conventional ARC’s are mainly due to its dependence on wave-
length,ARC coatings are tuned to a narrow band of frequencies at which it can ef-
fectively provide antireflection (Section
2.4.1
).Since solar cells are designed to absorb
the broad range of light from the solar spectrum,limitations of this technology arise.
The use of multilayer ARC’s reduces the extent of this problem by introducing multiple
wavelengths of antireflection,although multiple layers can cause mixing and degrading
through cracking and delamination of the cell’s connection between the substrate and
layers.
2.4.2 Texturing Schemes
An alternative to thin film coating as a method of antireflection is the use of sub-
wavelength structures,the principle of subwavelength-scale (SWS) texturing is to create
features that encompass dimensions and periods smaller than that of the wavelength
concerned.The nature of an SWS texture is to provide antireflection rather than a
method of light trapping.Subwavelength structures introduce a gradual change in re-
fractive index from air into the material,this gradual change in refractive index reduces
the amount of reflection and allows better coupling of light into the cell.Nature has
been developing these natural subwavelength structures that exhibit striking optical ef-
fects for over 500 million years [
22
].Initial studies by C.G.Bernhard in 1967 has lead
research into understanding these natural formations of sub-wavelength structures on
Chapter 2 Background Knowledge and Literature Search 12
many of nature’s surfaces,including beetles,butterflies,birds,moths,worms and fish
[
23
][
3
].
Moths have been studied in particular detail for the structures present on their cornea
2
surface;a grainy structure that results in antireflection properties for the purpose of night
camouflage [
4
].With the increasing ability to operate on a nano-scale,it is becoming
more feasible to integrate these structural concepts on silicon and other elements in
the process of solar cell development.Figure
2.10
a) shows these natural ‘moth-eye’
biological structures and b) synthetically produced replicas on the surface of silicon.
Figure 2.10:a) Scanning Electron Microscope (SEM) image of nipple array on the
cornea of a nightflying moth (from [
3
]) b) SEM image of replicated motheye structure
on silicon substrate (from [
4
]).
2.4.3 Structural Colours
Some moths and butterflies have known optical effects through arrays of precisely fab-
ricated structures present on their wings [
23
].For the purpose of this report,I will
investigate the properties/characteristics that give rise to a diversity of optical coloura-
tions present on a variety of biological wings from a collection of butterfly samples and
a moth sample.
Our interest in biological optical systems as engineers is purely structural;since coloura-
tions stimulated by pigment have limited benefit in developing solar cells with our mod-
ern linear approach to implementation.The optical dependence on structural property
for colouration present on butterfly wing samples has been proven through the work of P.
Vukusic
3
.This was achieved through suppressing the structurally induced optical effect
by immersing a butterfly sample in bromoform fluid which had a refractive index that
matched that of the scale architecture (effectively this eliminates the optical functioning
of the structures present on the scale(s)).Revealing that optical absorption was highly
dependent on the structural characteristics [
24
].
2
The cornea is an outer layer of the eye,this texture is often referred to as a ‘moth-eye’ structure
3
Thin Film Photonics,School of Physics,University of Exeter,UK.
Chapter 2 Background Knowledge and Literature Search 13
The useful optical characteristics exploited by biological wing samples to explore in this
report and for the purpose of solar cell development are mainly:

Black - antireflective and efficient absorber of broadband light

White - spectrally diffuse reflectance

Transparent - antireflective properties at both incident surfaces of the material
and poor absorber in cross section,allowing effective transmission of light
2.4.3.1 Black Colouration
Black,antireflective,efficient absorbers have been discussed in Section
2.4.1
with the
basis of the motheye.Black colourations are also exhibited by a variety of different
butterfly species.Some black samples are shown in Figure
2.11
a),with the structural
properties shown below in b).The necessity for such high light absorption has multiple
functions,namely:thermal regulation
4
,attraction for reproduction
5
and for camouflage
purpose [
4
] (as described in Section
2.4.2
).
2.4.3.2 White - Spectrally Diffuse Reflectance
A white surface is provides spectrally diffuse reflectance,as shown in the schematic
representations in Section
2.3.2
.White colouration in butterfly wings is significantly
dependent upon structural properties,the P.rapae and it’s structural beads are shown
in Figure
2.12
.This butterfly was found to be have reflective properties significantly
higher than that of the white wing areas of H.melpomene,a butterfly lacking these scale
beads) [
6
].
Randomly textured subwavelength surfaces provide white optical effects.The methods of
subwavelength-scale texturing described in Section
2.4.2
exhibit properties of uniformity
or regularly in the shapes and orientation of the structures created.Therefore the path
of incident light can be predicted to an element of accuracy.When light is incident on a
randomly textured surface it will have no directly predictable path(s) since it’s scattered
into a random directions.This concept was explored in Section
2.3.2
and the lambertian
diffuse reflector optical effect illustrated in Figure
2.7
.
4
“As butterflies do not possess internal thermal regulation like mammals...they have to spend con-
siderable time to heat their bodies using the energy of solar radiation.” [
25
]
5
“the females recognize potential mates (i.e.,of their own species) by their ultraviolet patterning.”
[
5
]
Chapter 2 Background Knowledge and Literature Search 14
Figure 2.11:a) A collection of black butterfly samples and b) Diagrammatic repre-
sentation of part of a fractured scale showing the relationship between the lower lamina,
trabeculae (T),ridges (R),lamellae (**),ribs (r),and microribs (mr).This view also
shows pigment granules (*),not present in all scales.(from [
5
])
Figure 2.12:a) A large white,Pieris Brassicae b) SEM image of Pieris Brassicae
butterfly wing (images extracted from [
6
]).
Chapter 2 Background Knowledge and Literature Search 15
2.4.3.3 Transparent Characteristics
Many layers within solar cells have the requirement to be good transmitters with low
absorption,in order for the photons to reach the necessary layer where absorption takes
place.The basis of transmission was covered in Section
2.3.2
and its requirement shown
in a practical illustration of a thin film solar cell in Figure
2.8
.
In nature,transparent surfaces are formed through the concept of forming antireflective
structures on the surfaces directly interfaced with incident light.If a poor absorbing
material is placed in the cross section,effective light transfer will occur.
Figure
2.14
shows a variety of flying species with transparent wing characteristics.Part
of this project will be to look at the optical properties of a transparent moth sample.
Figure 2.13:Natural transparent butterfly wing structure cross section,an example
of light travelling through the material is shown.
Figure 2.14:A collection of transparent winged biological specimines.(from [
7
]
Chapter 3
Experimental Procedures -
Optical Measurements
In order to investigate the properties and uses of biological nanostructures present on the
wings of moths and butterflies further a variety of world-wide samples were imported via
the internet - Worldwide Butterflies (http://www.wwb.co.uk) and The Insect Company
(http://www.insectcompany.com).See Appendix
A
for details and images of the biolog-
ical samples used in this chapter.
3.1 Biological Sample Preparation
The butterfly samples were received dried and set in their open state,a range of samples
were selected and sliced from particular sections and placed into Specac
1
9mm sample
holders ready for experimental testing.In order to gain a comprehensive set of results
a collection of black samples were taken along with iridescent blues,red,green,yellow
and white:full details of samples are documented in Appendix
A
.
In order to measure and investigate the spectral reflectance and transmittance of the dif-
ferent samples there was need to use an integrating sphere.As one of the most versatile
and useful devices in photonics;particularly for reflectance and transmittance measure-
ments [
26
] an integrating sphere provided a suitable testing device for the samples.The
following experiments were carried out using the Labsphere RTC-060-SF integrating
sphere as shown in Figure
3.1
.
1
Specac Limited,River house,Kent
16
Chapter 3 Experimental Procedures - Optical Measurements 17
Figure 3.1:Top view of RTC-060-SF integrating sphere (from [
8
]).
3.2 Specular Reflectance Measurements - White Light Source
3.2.1 Experimental Apparatus
The experiment was carried out by following the instructions for ‘Hemispherical Re-
flectance Measurements in Double Beam Mode
2
’ (as detailed in the labsphere guide [
8
]).
The experiment consists of three main stages namely,the light source (input),integrat-
ing sphere (process) and spectrometer (output).The light source (as shown by ‘a)’ in
Figure
3.2
) is provided by a white filament
3
directly coupled through a collimator to a
fiber optic cable.The fiber optic is fed into a holder ‘b)’ where light is then transmitted
into the sphere through an achromatic doublet lens and a focusing lens
4
of focal length
15mm at ‘c)’.The light can then be appropriately focused into an area of approximately
5mm in diameter at the rear of the sphere ‘d)’ (Sample Reflectance Port 2 in Figure
3.1
)
where the sample under test is loaded.The light reflected from the sample due to the
incident beam reaches the edge of the sphere where it is diffusely scattered in uniform
directions
5
,ensuring light is evenly distributed entirely around of the sphere’s inner
surface.Therefore the amplitude of light received by the fiber optic located at the base
2
The double beam mode provides a greater accuracy in reflection/transmission measurements over
single beam mode.
3
Mikropack halogen HL-2000-FHSA
4
equipment Thorlabs
5
An example of this is shown in Figure
3.2
by ‘e)’),this is due to the ultra diffuse surface - Spectraflect
with a reflectance of %98
Chapter 3 Experimental Procedures - Optical Measurements 18
of the sphere ‘f)’ is of a set proportion of the light reflected from the sample;giving
an accurate reading proportional to the sample’s reflectance.The data received at f)
can then be processed by a spectrometer at ‘g)’ controlled by a computer system where
results can be processed (normalised and plotted).
Figure 3.2:Outline of the experimental apparatus for reflectance experiments.
3.2.2 Experimental Procedure
The reflectance factor R(λ) in double beam mode is shown by equation
3.1
:
R(λ) = R
std
(λ)
￿
S
Sample
(λ) −S
Dark
S
Ref
(λ) −S
Dark
￿
S
￿
S
Ref
(λ) −S
Dark
S
Sample
(λ) −S
Dark
￿
Std
(3.1)
The reflectance standard,R
std
is a collection of data values from a known reflectance
standard over a range of wavelengths λ relative to the experiment.This is used to
calibrate the integrating sphere and ensure accurate results can be obtained.The Spec-
traflect
6
surface was used as a reflectance standard for the purpose of this experiment,
since data regarding it’s reflectance over a range of wavelengths was available on-line
(approx.96% to 98%).The values documented on their website had an appropriate
range to cover that of the spectrometer used in these experiments,although not specific
reflectance data for the wavelengths sampled by the spectrometer,therefore Matlab was
used to interpolate the values in order to provide a full range of reflectance standard
data (further details and data listings can be found on the CD indexed in Appendix
C
).
The first step in this experiment was to mount the reflectance standard (spectraflect
painted surface) at the sample port 2 and illuminate light onto the outer sphere through
the reference port 1,scattering it uniformly.(thus obtaining S
Ref
(λ)
Std
) (the path
of the reference beam and sample beam are shown diagrammatically in Figure
3.3
).
6
Labspheres Spectraflect is a specially formulated white reflectance coating for use over the UV-VIS-
NIR wavelength region.http://www.labsphere.com/productdetail.aspx?id=232
Chapter 3 Experimental Procedures - Optical Measurements 19
Following this,the light source is repositioned to illuminate directly onto the sample
(which is holstered at the sample port) through the transmittance port 5,allowing data
collection.(thus obtaining S
Sample
(λ)
Std
).
When collecting data readings for the samples under test,there was need to carry out
all measurements with the light source directed at the reference port and then after
reposition collection of all data with the light source present incident on the sample
port
7
.
Measurements for the Standard (as represented by the brackets enclosed by
Std
in eqn
3.1
) were taken with the reflectance standard
8
at the sample port for both light and
dark
9
measurements.The light measurements are taken through the reference port 1
and the transmittance port 5,once for the reflectance Standard (
Std
) and each time for
the Sample (
S
) under test.
All result data is stored on an MS Excel spreadsheet and can be found on the accom-
panying CD detailed in Appendix
C
.
Figure 3.3:Diagram to show path of sample beam (red) and reference beam (blue)
when taking measurements(extension of Figure
3.1
from [
8
]).
7
Ideally this process could be done with two separate light sources each directed into the two ports
(sample and reference) and illuminating individually when required.
8
As explained previously,Spectraflect is used as a standard with known reflectance characteristics
for use in the normalisation process when calculating the reflectance factor of the samples under test.
9
S
Sample
(λ) and S
Dark
respectively
Chapter 3 Experimental Procedures - Optical Measurements 20
3.2.3 Results and Discussion
3.2.3.1 Black Samples
Figure 3.4:A collection of black samples and their corresponding reflective charac-
teristics.
Initial experiments are carried out on the black samples to investigate and compare
the antireflective properties further.Figure
3.4
shows the black reflectance plot of four
different biological samples with a y-axis from 0 to 4% reflectance.Data displayed has
been cropped to the range of 400nm to 700nm due to the limitations of the spectrom-
eter,noise is present at the initial and final values.Noise reduces to an acceptable
level at approximately 450nm,where the results stabilise allowing more accurate data
processing.For samples that exhibit higher reflection properties discussed further in
this Section,the data is acceptable from the 400nm point onwards.Studying the data
in Figure
3.4
reveals four different reflective characteristics,the three Papilio
10
samples
follow a similar path across the range of wavelengths,with differing amplitude of re-
flectance.The Papilio ulysses sample shows highest reflection at approximately 2% and
Nireus/Phorcas following a similar characterstic at 1.5%,increasing more rapidly from
600nm past 2% reflectance by 700nm.The Troides (shown in red) however maintains
10
Refering to the Ulysses,Nireus & Phorcas investigated in this section
Chapter 3 Experimental Procedures - Optical Measurements 21
an almost flat path at just above 1% reflectance,with spikes existant at 470,550 and
620nm.The papilio nireus also presents three data peaks at similar values to that of
the Troides,this is mainly due to experimental error persistent for these samples under
test.These minor data errors can be neglected,since they don’t have an overall affect
on the average reflection across the broadband spectrum of wavelengths.The average
reflectance for each of the black samples is summerised in Table
3.1
.
Sample Name
Reflectance average (%)
Papilio Ulysses
1.90
Papilio Nireus
1.38
Papilio Phorcas
1.42
Troides Rhadamantus
1.21
Table 3.1:Black samples and corresponding reflectance average across wavelengths
of 450nm to 700nm - spectral reflectance experiment.
In summary the Troides Rhadamantus shows the greatest overall antireflective prop-
erty with the lowest reflection characteristic at normal angle of incidence for a white
light source.Further sections in this project will investigate whether these samples ex-
hibit colour through structure or pigments.For the purpose of this report structural
colouration is appropriate in the ability to create synthetic photovoltaic systems for use-
ful purpose.The initial acquiring of the majority of the Papilio butterfly samples for
investigation was carried out with the known fact that they exhibit a variety of proven
structural colourations (black,blue,green etc.) through the previous work of other
scientists such as P.Vukusic [
27
][
28
][
24
].
Chapter 3 Experimental Procedures - Optical Measurements 22
3.2.3.2 Other Samples
Figure 3.5:Graphical reflection properties of one red sample and two blue samples
under test.
Here red and iridescent blue samples are investigated for reflectance properties.The
reflectance results obtained are directly correlated to what is visible via.the human eye,
since wavelengths collected are across the visible light spectrum (approx.400-700nm).
As expected,the result in Figure
3.5
shows a peak at the upper end of wavelengths for
the red sample and a peak at the lower end of wavelengths for the blue samples.It is also
clear that the red sample has low reflectance at approximately 1-2% until 600nm where
the reflectance begins to increase rapidly,giving rise to a strong red visible colouration.
Likewise the P.ulysses blue sample maintains a low reflectance at approximately 5%after
it has peaked in the blue region of the visible light spectrum,giving rise to a strong blue
colouration with iridescence as a result of structural manipulation of incident light.The
visibly more dull blue sample P.nireus has a significantly reduced peak approximately
half of the P.ulysses with a maximum reflection of 30%.Investigation into structural
properties will aid the explanation of this in further sections the report.
Chapter 3 Experimental Procedures - Optical Measurements 23
Figure 3.6:Visual representation of the reflection characteristics of the PERL cell
surface and standard untextured silicon surface,black P.ulysses sample included for
reference.
Figure
3.6
shows a graphical comparison of the reflective characteristics of the PERL
cell
11
surface,a standard silicon linear polished surface and the P.ulysses black sample.
The reflectance of polished silicon is shown to be high in comparison to the other samples;
reflectance of approximately 60%and dropping to a consistant 40%from500nmonwards.
This proves the requirement for surface texturing schemes in silicon solar cell fabrication
as discussed in the literature review,to prevent this effect of loosing half the light incident
due to an untreated silicon surface.The PERL cell provides a much improved reflective
characteristic across the range of wavelengths than that of silicon.This is mainly due
to the micron-scale pyramidal texturing present on the cell surface,providing effecive
light trapping and antireflection (surface texturing was discussed in the literature review
Section
2.4.2
).
The P.ulysses black sample was added to this graphical representation for reference to
what reflectance characteristics an ideal black absorbing biological surface provides.The
PERL cell begins to match the performance of the black sample from wavelengths above
11
Currently the world’s most efficient solar cell
Chapter 3 Experimental Procedures - Optical Measurements 24
approximately 600nm.The average reflectance for each of the samples is summarised
in Table
3.1
.
Sample Name
Reflectance average (%)
Untreated silicon surface
36.88
PERL cell surface
3.56
Papilio Ulysses black sample 1b)
1.90
Table 3.2:Black samples and corresponding reflectance average across wavelengths
of 450nm to 700nm - spectral reflectance experiment.
Chapter 3 Experimental Procedures - Optical Measurements 25
Figure 3.7:Spectral reflectance characteristics for a collection of samples,namely
yellow,green and white.
Figure
3.7
shows the comparison between a variety of different colours exhibited by
different biological samples.The graph clearly shows that the white Pryoneris Philonome
sample exhibits a broadband average reflectance factor of 40%.The green P.phorcas
sample reveals a rise and drop of reflectance across the spectrum,leaving a peak at
approx 520nm providing green reflection on the visible light spectrum.The yellow
Troides shows low reflectance until approximately 475nm where a sharp rise to 20%
reflectance for the remainding wavelengths in the range.
Sample Name
Reflectance average (%)
Yellow Sample 4a)
17.00
Green Sample 3a)
24.26
White Sample 5
39.27
Table 3.3:Colour samples and average reflection characteristics.
Chapter 3 Experimental Procedures - Optical Measurements 26
3.2.4 Conclusions
The ability for a material or surface to give rise to particular colouration is dependent on
it’s ability to absorb and reflect particular wavelengths of light as efficiently as possible.
These samples investigated begin to showhowthis colouration can be achieved effectively
through structural impact.The results show that all samples generally stabilise in
reflectance characteristics after 450nm;this is mainly due to experimental apparatus
error.
The results show a direct correlation to the visible light spectrum (i.e.what is visible
by the human eye) as expected,with peaks in reflectance at the appropriate wavelength
regions.For example the blue samples showed a peak in reflection percentage towards
the lower wavelengths,the red sample showed a peak in the upper end of wavelengths and
the black samples have a low reflectance factor across the broadband of the spectrum as
desired.The estimated peak values of wavelength and the corresponding visible colour
present on each of the samples are shown in Table
3.4
,the visible light spectrumis shown
in Figure
3.8
for reference purpose.Part of the reason for such optical phenomena
discussed in this section is linked directly to the visible spectrum for the purpose of
attraction and reproduction as described in the literature review (Section
2.4.3
).
Current technologies in production have been investigated with the test of a PERL cell
surface and a basic silicon layer,comparing with that of the biological samples and their
optical effects.Although the PERL cell is made from silicon,which is more reflective
than the butterfly chitin substrate,results have proven that overall the antireflective
properties of the dark biological samples are more effective than that of the PERL cell.
Therefore revealing more about the structural properties these black samples encompass
and analysing ways to adapt these concepts on silicon could prove beneficial.
Solar cell absorption needs to be efficient over a large range of wavelengths,angles of
incidence and polarisational differences.The graph data collected for the black samples
shows a consistent absorption of light with reflection between 1% and 2% over a large
range of wavelengths,therefore exhibiting useful properties for the purpose of solar cell
development.The results gained for the other samples and their taylored reflectance
characteristics matching the colourations they exhibit can be useful in the discovery of
what is structurally happening and how factors vary between biological samples.Further
sections explore other reflectional factors involved with biological samples and moves into
structural imaging and analysis of the structural layout in further detail.
3.2.5 Limitations of Experiment
The integrating sphere is a useful piece of apparatus for reflectance and transmittance
experiments.Although there are limitations:
Chapter 3 Experimental Procedures - Optical Measurements 27
Sample Name
Colour
Wavelength Peak (nm)
Papilio Ulysses
Blue
450
Papilio Ulysses
Black
no peak
Papilio Nireus
Blue
470
Papilio Nireus
Black
no peak
Papilio Phorcas
Green
520
Papilio Phorcas
Black
no peak
Troides Rhadamantus
Yellow
580
Troides Rhadamantus
Black
no peak
Pryoneris Philonome
White
Spectral
Cymothoe Sangaris
Red
650
Table 3.4:Samples and corresponding colours - spectral reflectance experiment.
Figure 3.8:Spectrum of visible light spectrum.

Small section/area of the sample is placed under test.

A small portion of the light will be lost through reflection before reaching the
sample.

The angle of incidence is constant,therefore only appropriate for a single angle.
In order to get a wider range of experimental data and more comprehensive conclusions
regarding the collection of biological samples under test,there is need to carry out similar
experiments that involve other factors such as changing angles of incidence,polarisations
and transmittance.The following experiments detail these investigations,results and
conclusions.
Chapter 3 Experimental Procedures - Optical Measurements 28
3.3 Polarisation and Incident Angle Reflectance Measure-
ments - 633nm Laser
The following experiment explores the effects of incident laser light (at 633nm wave-
length) on the Papilio ulysses (sample 1b),changing polarisation over a range of incident
angles (full sample details in Appendix
A
).This black sample is chosen individually for
experimentation due to its useful absorbing property in solar cell fabrication and much
of the current research has involved this particular sample.This experiment will lead to
a more comprehensive understanding of the structural optical absorption characteristics
that this sample encompasses.
3.3.1 Experimental Apparatus
For the purpose of this experiment,the integrating sphere detailed in the previous
reflectance measurement experiment is used.Laser light is used at a specific wavelength
rather than a broad wavelength white light source.Variable angles of incidence are
provided through the use of a center-mounted sample holder as shown in Figure
3.9
.A
photodetector is used for measurement of reflected laser light at a specific wavelength
of 633nm - in the previous experiment a spectrometer was used to collect data over a
range of spectral wavelengths.
Figure 3.9:Diagram of the clip style center-mounted sample holder (from [
8
]).
A set of linear polarisers are used to manipulate the laser light into S-plane and P-plane.
The concept of S and P-plane polarisation is shown diagramatically in Figure
3.10
,here
this is polarisation is achieved through the use of linear & circular polarisers.The laser
light is transmitted through two linear polarisers b) and d) that sandwich one quarter
wave plate c),as shown in Figure
3.11
.The initial linear polariser at b) ensures the laser
light from a) is completely polarised,the quarter wave plate then circularly polarises
the light (using principles of phase difference) where it travels through the final linear
polariser c) and the polarisation plane (S or P) can be chosen without affecting the
intensity of laser light.The light then travels into the sphere reflecting from the sample
Chapter 3 Experimental Procedures - Optical Measurements 29
present at e),illuminating the sphere and a proportional reflectance measurement is
detected at the photodetector reading g).A photograph of the apparatus is shown in
Figure
3.12
.
Figure 3.10:Image showing P-plane and S-plane incident light on a surface (from
http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/tayu/ACT05E/img/05IMG/05
11.jpg).
Figure 3.11:Schematic of laser experiment of polarisation and angles of incidence.
3.3.2 Experimental Procedure
Measurements are carried out using the principles of the double beam experiment (as
detailed in the initial reflectance experiment Section
3.2.2
) with Reflectance factor R
described by eqn
3.1
.Measurements are taken in both the sample position and reference
position by repositioning the laser source in relation to the integrating sphere,as detailed
previously.Angles of incidence are varied by rotation of the sample holder (position e)
in Figure
3.11
),as explained in the experimental apparatus above.Angles of incidence
are used in the range from 8

to 80

.For each angle of incidence investigated,individual
measurements are taken with the laser polarisation in S-plane and then repeated for
polarisation in P-plane.
Chapter 3 Experimental Procedures - Optical Measurements 30
Figure 3.12:a) overall image of apparatus b) the integrating sphere c) collection of
lenses/polarisers.
3.3.3 Results and Discussion
The results of the experiment are summarised below in Table
3.5
.It is clear from
the results that the reflectance across the range of angles and polarisations is low at
approximately 1%.Towards the higher angles of incidence the reflectance begins to
increase,this effect is to be expected since the probability of light entering the material
is reduced (as described in the literature review Section
2.3
of this report).
Angle
S-plane Reflectance %
P-plane Reflectance %
Average Reflectance %
8
1.2311
1.1936
1.2124
10
1.2766
1.2701
1.2734
20
1.2328
1.1761
1.2045
30
1.2945
1.1735
1.2340
40
1.4698
1.2757
1.3728
50
1.4712
1.3035
1.3874
60
1.9478
1.6614
1.8046
70
3.9650
2.8451
3.4050
80
5.7121
3.6905
4.7013
Table 3.5:Polarisation and Angle of incidence reflectance results for Papilio Ulysses
(black) butterfly wing (using integrating sphere with center mounted sample holder
[
8
]).
Chapter 3 Experimental Procedures - Optical Measurements 31
Figure 3.13:Reflectance of Papilio Ulysses sample for S and P plane polarisation and
changing angles of incidence.
The results are shown graphically in Figure
3.13
,the y-axis is scaled from 0-6% re-
flectance and angles of incidence no greater than 90

.Both the S & P-plane results
show the following similar trend;just above 1% reflectance initially between 0

and 50

,
then a rise in reflectance from 45

onwards.The reflectance begins to increase approx-
imately linearly until the measurements become inaccurate past 80

due to limitations
of apparatus.It is shown that the S-plane reflectance is greater across all angles of inci-
dence and the divergence between S-plane reflectance and P-plane reflectance increases
with larger angle of incidence.This effect is related to the structural property of the
sample,since light incident on the sample in one plane will make contact with different
aspects of the structure compared with that of the other plane.In this case investigated,
the S-plane (as shown by Figure
3.10
) experiences less optical affect due to the struc-
tural property of the sample,resulting in greater reflectance capabilities in this plane of
propagation.The average S and P-plane reflectance lies inbetween the two traces,this
reflectance trace can be modelled by a cubic function,using plot funcitions in Matlab
the data can be represented by Equation
3.2
.(The data and modelled curve are shown
in Figure
3.14
.)
Chapter 3 Experimental Procedures - Optical Measurements 32
Reflectance = 2.2943e
(−5)
x
3
−0.0017035x
2
+0.036543x +1.0292 (3.2)
Where x = Angle of Incidence
Figure 3.14:Matlab generated cubic curve to model polarisation result data.
Differentiation of Equation
3.2
allows calculation of the changing gradient,as shown by
Equation
3.3
.
Gradient = 6.8829e
(−5)
x
2
−0.003407x +0.036543 (3.3)
Plotting the gradient equation with angle of incidence for x reveals an almost linear
increase in gradient (Figure
3.15
).Therefore increasing the angle of incidence has a
linear effect on the gradient of the black surface absorption capability.
3.3.4 Conclusions
This experiment has proven that the structural characteristics exhibited by the sample
investigated does exhibit varied optical effects through it’s dependence on the polarsation
of incident light.The results show that the black Papilio Ulysses biological sample has a
reflectance between 1%and 6%over a broad range of angles and polarisations,this result
is relevant to solar cell development
12
,since these are the very properties necessary for
absorbing layers in modern photovoltaics.
The increase in reflectance has been modelled as a cubic function and the gradient
increase shown to be linear.Although the angle of incidence does produce varied optical
effects,the reflectance did not rise above 6% indicatin that it is an efficient antireflective
surface.This investigation has now aided the goal of obtaining a more comprehensive
12
For a fixed solar cell the incident light changes angles of incidence and absorbs a variety of polari-
sations as the sun’s position changes throughout the day.
Chapter 3 Experimental Procedures - Optical Measurements 33
Figure 3.15:Graph to show increase of gradient for the polarisation experimental
results
understanding of the reflective characteristics
13
of a black biological sample.From this
result along with the reflectance experiment in Section
3.2
that proved a reflectance
maximum of 3% over a broad range of angles,we can conclude that black biological
samples do exhibit good overall comprehensive absorbing characteristics.The following
investigations will involve analysis of the structural property contributing to this efficient
black absorption capability.
13
Namely data of:reflectance,angle of incidence and polarisational.
Chapter 3 Experimental Procedures - Optical Measurements 34
3.4 Angle of Incidence Transmittance Experiments - White
Light Source
Here the Cryptotympana Aquila (as detailed in Appendix
A
/Table
A.2
) is investigated
for transmission properties using the integrating sphere in Transmittance Measurements
Double Beam mode (as detailed in [
8
]).
3.4.1 Experimental Apparatus
The procedure for transmission experiments is similar to that of reflection as detailed
in Section
3.2
,although the sample under test is loaded at the transmittance port 5 (as
shown by ‘d
￿
in Figure
3.16
) rather than the reflectance port 2 (see Figure
3.3
for layout
of integrating sphere ports).The sample is loaded at this port to allow transmission
through the sample and for the light to be collected by the integrating sphere with
the detector receiving data proportional to that transmitted through the sample.The
schematic for the experiment is shown in Figure
3.16
,notice the achromatic doublet lens
and 15mm focal lens are positioned at the midpoint c) between the white light source
holder b) and the sample at d) coupled with the integrating sphere.The light transmitted
through the sample gets uniformly distributed at e) and light is detected via the same
process detailed for reflectance measurements.The angle of incidence in relation to the
sample loaded at d) is varied by re-positioning the white light source (b)) and lenses (c))
around the angle shown by ‘Z
￿
whilst ensuring the light remains incident on the sample.
The angle was accurately measured using a protractor positioned directly below d) and
the instruments aligned in the appropriate direction for the each experimental result.
Figure 3.16:Transmission experiment schematic.
Chapter 3 Experimental Procedures - Optical Measurements 35
3.4.2 Experimental Procedure
The total transmittance T(λ) is shown by equation
3.4
.Notice there is no require-
ment for a reflection standard as for reflectance measurements;since the Empty sphere
measurement calibrates the transmittance T(λ) measurements sufficiently.
T(λ) =
￿
S
Sample
(λ) −S
Dark
S
Ref
(λ) −S
Dark
￿
S
￿
S
Ref
(λ) −S
Dark
S
Sample
(λ) −S
Dark
￿
Empty
(3.4)
Similarly in this experiment,measurements are taken from the Sample position and the
Reference position in order to gain measurements for S
Sample
(λ) and S
Ref
(λ) by repo-
sitioning the white light source between measurements.(the sample and reference beam
alignments are shown in Figure
3.3
).Results are obtained between 0

and 70

to main-
tain accuracy in the experiment,with the transmittance measured by the spectrometer
and data stored on a PC spreadsheet.
3.4.3 Results and Discussion
Figure 3.17:Graph showing transmission at varied angles of incidence on moth wing
sample number 7.
The results are shown graphically in Figure
3.17
,8 plots for each of the angles of in-
cidence investigated are shown.Initially at lower wavelengths where incident light has
higher energy,more transmittance occurs at each of the angles of incidence.At increased
Chapter 3 Experimental Procedures - Optical Measurements 36
incident wavelengths there is a non-linear increase in transmittance,approximately fit-
ting that of an inverse exponential curve shape.
At normal incidence (0

) transmittance is greatest as expected,then as the angle of inci-
dence increases the traces show a gradual increase in the rate of reduction in percentage
transmittance due to light lost through increased reflection.The transmittance remains
at a relatively consistent approximate value of 90% for angles of incidence between 0

and 40

,then rapidly falls to around 60% transmittance between 40

and 70

.After
70

limitations of the apparatus introduce inaccuracies in the results,hence no results
exist from 70

to 90

angles of incidence.In order to represent the results in a collective
manner,average transmittance has been calculated for each of the angles of incidence
investigated,the values are documented in Table
3.6
and plotted in Figure
3.18
.This
data shows that transmittance varies by approximately 30% (from 94% to 67%) over
the range of angles investigated,a clear drop in transmittance is shown from 50

before
results become inaccurate to measure.
Angle of Incidence
Transmittance average (%)
0

93.21
10

91.86
20

91.23
30

89.40
40

88.99
50

88.36
60

74.67
70

67.15
Table 3.6:Average transmittance over different angles of incidence.
3.4.4 Conclusions
Here the final optical experiment was conducted,resulting in transmission data for a
biological sample.The results have shown useful data,with transmission properties of
approximately 90% upto 50

angle of incidence.The transmittance at normal incidence
of standard glass over a range of manufacturing companies was found to lie between 90-
91% in the visible light spectrum (380-780nm) [
29
].This biological sample matches this
quality over a range of incident angles,providing improved antireflective property.A
variety of glass surfaces and their reflective characterisitics are shown Figure
3.19
.The
results gathed in this experiment (as shown in Figure
3.18
) show better transmission
properties than that of conventional single pane glass.Therefore patterning glass with
‘moth-wing’ (or moth-eye) antireflective structures could lead to better transmissional
properties compared with planar glass.
Chapter 3 Experimental Procedures - Optical Measurements 37
Figure 3.18:Graph showing average transmission across different angles of incidence.
Solar cells are generally encased by a glass case,or grown on a glass substrate (as
shown in Figure
2.8
).These ‘moth-wing’ structures could provide optical transmission
properties better than that of conventional materials,improving efficiency capabilities.
Figure 3.19:Standard single,double and triple glazed window transmittance charac-
teristics over a range of angles of incidences (from [
9
]).
Chapter 3 Experimental Procedures - Optical Measurements 38
3.5 Conclusion of Optical Experiments
The optical experiments carried out in this Chapter were to gather conclusions regarding
the optical characteristics of a variety of biological samples,collecting data and mak-
ing comparisons with current techniques in solar cell development where appropriate.
Investigations into effects of changing wavelength,angle of incidence & polarisation in
reflectance and transmittance experiments have been explored.
Samples were obtained from a variety of countries world-wide through internet services,
then mounted and prepared for measurement procedures.The main optical properties to
investigate of particular interest were black,white and transparent sample colouration,
although a collection of other colours were investigated for comparison on the wavelength
spectrum and further analysis in Chapter
4
will reveal where structural properties con-
tribute to the results obtained.
Initial experiments in spectral reflectance revealed promising results that confirmed pre-
dictions with quantifiable data.Coloured samples provided a variety of characteristics,
with reflectance peaks in the predicted regions for particular visible colourations.The
white sample investigated provided a spectrally broad reflectance characteristic and the
black samples presented very low reflectance across the whole spectrum of wavelengths
investigated.This low reflectance result provided a useful outcome,proving that the
samples exhibit low reflectance qualities across the complete spectrum,which has a
greater benefit in the development of solar cells that must absorb light throughout the
spectrum.The effectiveness of the samples absorption capabilities were compared to
that of polished silicon and the PERL solar cell,providing intriguing results.The black
samples all matched the performance of the PERL cell and even provided improved ab-
sorption capability at the lower wavelengths,where higher energy photons are received.
The next step in optical data collection was to investigate what effects light polarisation
and angles of incidence have on selected samples.Firstly it was proven that up to angles
of approximately 50

there is limited effect on the absorption capability of the P.Ulysses
black sample 1b).Also S and P-plane polarisation have similar effects on reflective
characteristics,with S-plane proving slightly more destructive to absorbing potential.It
was then shown using modelling techniques in matlab that the gradient of reflectance
increases in a virtually linear fashion with changing angle of incidence.
Finally the transmission capabilities of the transparent Cryptotympana Aquila moth
wing were investigated by applying a different technique to the integrating sphere.Re-
sults again proved intriguing,with transmittance remaining high throughout the angles
of incidence investigated.A comparison to that of conventional methods in the produc-
tion of transparent glass window layers was carried out showing improved performance
characteristics.
Chapter 3 Experimental Procedures - Optical Measurements 39
These optical measurements are the foundations of the report,with further work in
Chapter
4
involving the physical structures involved with the generation of these striking
optical effects.Conclusions can then be formed relating the feasibility of these unique
colourations to their structural properties.
Chapter 4
Experimental Procedures -
Biological Structural
Investigation
The purpose of this chapter is to investigate the nanostructure properties present in
a variety of black,white,transparent and coloured samples further
1
.Comparisons are
made between the different colours present and the analysis of their relation to structural
property is identified and quantified into further detail,forming useful conclusions.Peter
Vukusic
2
,Andrew Parker
3
and Helen Ghiradella
4
are some of the leading researchers in
analysing nanostructures present on a variety of different biological specimens;with the
basis of their work stemming from that of C.G.Bernhard in 1967.
4.1 Scanning Electron Microscopy
The use of conventional white light visible by the human eye through lenses to view a
surface in greater detail has limitations and poor resolution for the purpose of micro/-
nano structures.However with a Scanning Electron Microscope (SEM) one can magnify
images up to X200,000/a resolution of 5 nanometres through the use of electrons to
view the image.The SEM used in the following experiments is the JSM 6500F thermal
field emission scanning electron microscope
5
as shown in Figure
4.1
.
1
The Troides Rhadamantus butterfly is investigated via SEMand no current literature exists for this
biological sample
2
Thin Film Photonics,School of Physics,University of Exeter,UK
3
Department of Zoology,University of Oxford,UK
4
Department of Biological Sciences,University of Albany,US
5
The Science and Engineering Electron Microscopy Centre,Universtiy of Southampton UK.
40
Chapter 4 Experimental Procedures - Biological Structural Investigation 41
Figure 4.1:JSM 6500F thermal field emission scanning electron microscope layout,
Universtiy of Southampton (from http://www.micro.soton.ac.uk/).
4.1.1 Sample Preparation
In order for scanning electron imagery to function effectively the surface must first be
sputtered with an element such as carbon,silver or gold to increase conductivity and so
prevent charging.This layer allows an energy exchange between the electron beam and
the sample,resulting in an emission of electrons and electromagnetic radiation which
can be detected to produce an image [
30
].For the purpose of this experiment a carbon
coating is used;this is carried out by placing sections of each sample for investigation
onto carbon stickers mounted on metal stubs.A small section of each sample is scraped
using a scalpel,to remove microscales present on the surface of the wings,provide
greater surface area and reveal underlying/embedded scales.Each metal stub contains
a complete section of the wing and a collection of scraped scales to experiment.Figure
4.2
shows a collection of samples before sputtering and the stubs mounted into the holder
ready for SEM (full sample details in Appendix
A
).
4.1.2 Black Sample Investigation
The literature review in Chapter
2
explores the useful properties of high absorption
(black) surfaces in photovoltaics,the black samples investigated in Section
3.2.3
for
spectral reflectance properties were the Papilio Ulysses,Phorcas,Nireus and the Troides
Rhadamantus.This SEM investigation explores these black samples in further detail,
making quantitative comparisons between structures.
Chapter 4 Experimental Procedures - Biological Structural Investigation 42
Figure 4.2:Images to show a) samples mounted on carbon stubs and b) samples
mounted into SEM holder after sputtering.
Initial viewing of sample 1b) (Papilio Ulysses black scale) revealed a collection of serrated
scales as shown by a) in Figure
4.3
.Closer viewing revealed the honeycomb structure
embedded in a series of periodical linear ridges as shown in b).Images c) and d) detail
the scrapped scales revealing the smooth substrate on which the structure is mounted.
Figure 4.3:Papilio Ulysses black scales:a) overall view of serrated black scales b)
closer view of individual scale c) image of broken scale d) closer image of broken scale
revealing the substrate honeycomb structures lie on.
Chapter 4 Experimental Procedures - Biological Structural Investigation 43
Further viewing of the other black samples within the Papilio family reveal the same
fundamental honeycomb structure,with varying periodicity between ridges and average
honeycomb spacing.The P.Phorcas is shown in Figure
4.4
,revealing the honeycomb
structure in greater detail and it’s cross sectional view,revealing the ‘trabeculae’ [
5
]
(structural supports).A more detailed view of the Papilio Ulysses & Nireus black scale
crystalline structures are shown in Figure
4.5
.
Figure 4.4:SEM Images to show Papilio Phorcas black scale ridges and honeycomb
structure (Sample 3b in Table
A.3
).
Figure 4.5:Using scale fromFigure
4.3
b):revealing honeycomb 3Dphotonic structure
in greater detail a) shows the Papilio Ulysses,sample 1b) X22,000 zoom,with ridge
period approximately 2.5µm.b) shows the Papilio Nireus,sample 2b) at X7,000 zoom,
revealing a similar black honeycomb structure.
These structures presented are primarily designed to scatter incident radiation towards
the ridging and about the scale interior,this has the effect of increasing the path length
of incident light through the absorbing pigmentation [
24
] and therefore increasing ab-
sorption capability.Research into the optical functioning of the scales’ nanostructure
has been conducted by P.Vukusic,revealing that when immersing the structure into
bromoform (which effectively eliminates the optical functioning of the scales nanostruc-
ture,due to a matching refractive index) the optical absorption of the scale is reduced
by 40%.This is a significant amount and without the nanostructure present,even with
Chapter 4 Experimental Procedures - Biological Structural Investigation 44
the same quantity of absorbing pigment,each scale would be a less efficient absorber of
incident radiation.[
24
]
Therefore this proves that scattering and a gradual change in refractive index are pro-
vided by the structure,leading to lower reflectance.This evidence shows that if we were
able to replicate this structure in the synthetic development of solar cell surfaces we
could see a large reduction in reflectance.
Investigation into the Troides Rhadamantus (Figure
4.6
) revealed ridges and ribs,like
that of the Papilio family,although an absence of honeycomb structures.This sample
did prove low reflectance during the experimental procedures in Section
3.2
although
due to the lack of these honeycomb scattering structures,this would suggest that this
particular sample inherits much of its optical blackness through high absorbing pigmen-
tation,with less reliance on structural qualities than that of the papilio family.This
leads to the conclusion that for the purpose of this report we can neglect this sample,
as its structural properties that we are interested in provide little benefit in developing
methods of antireflection and light trapping in solar cell development.
Figure 4.6:The Troides Rhadamantus black scale X5,500 revelling lack of honeycomb
structure.
4.1.3 Spectral Colour Investigation
In order to investigate the properties of butterfly wings that exhibit diverse colourations,