Microsoft Word - Thesis_Jongsung Park_2013-revisedWritten by
JongSung Park
A thesis submitted to the University of New South Wales in fulfilment of the requirements for the degree of
Master of Photovoltaic Engineering
School of Photovoltaic and Renewable Energy Engineering The University of New South Wales
Sydney NSW 2052 Australia
March 2013
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D E D I C A T I O N
This thesis is dedicated to my wife AhRum Lee
and my parents ,
“ ”
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Abstract
Thin film photovoltaics (PV) can potentially have a lower manufacturing cost by
minimising the amount of a semiconductor material used to fabricate devices. Thin-film solar
cells are typically only a few micrometres thick, while crystalline Silicon (c-Si) wafer solar
cells are 180 - 300 micrometers thick. Incident light is not fully absorbed in such thin-film
layers, resulting in lower energy conversion efficiency compared to c-Si wafer solar cells.
Therefore, effective light trapping is required to realise commercially-viable thin film cells,
particularly for indirect-band-gap semiconductors such as crystalline silicon. An emerging
method for light trapping in thin film solar cells is the use of metallic nanostructures that
support surface plasmons. Plasmon-enhanced light absorption is shown to increase cell
photocurrent in many types of solar cells.
This thesis presents the author’s results on plasmonic polycrystalline silicon (poly-Si) thin
film solar cells. It can be categorised into three parts, which are the optimum cell’s surface
condition for nanoparticle (NP) fabrication, optimisation of Ag NP fabrication process to
enhance energy conversion efficiency and a wet-etching method for re-using metallised
polycrystalline silicon thin film solar cells after NP deposition.
The first part (Chapter 3.2) introduces the optimum surface condition for silver NPs. NPs
are formed on Si film, a native SiO2 and a thermal SiO2 layer, and absorption, scattering cross
section and potential short-circuit current density are compared for varying surface conditions.
The sample with NPs on the thermal SiO2 layer shows better absorption at 500 – 700 nm
wavelength range, whilst the sample with NPs on the native SiO2 and with NPs directly on Si
show higher absorption at greater than 700 nm. The sample with NPs on the native SiO2 layer
indicates 62.5% potential short circuit current density enhancement, which is 0.7% and 12%
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higher enhancement than that of the sample with NPs directly on Si and NPs on the thermal
SiO2 layer, respectively.
The second part (Chapter 3.3) is a systematic study of optimisation of Ag NP fabrication
process for enhancing efficiency of poly-Si thin film solar cells. Three factors are studied: the
Ag precursor film thickness, annealing temperature and time. The thickness of the precursor
film was 10, 14 and 20 nm; annealing temperature was 190, 200, 230 and 260°C; and
annealing time was varied between 20 to 95 min. NPs formed from 14 nm thick Ag precursor
film annealed at 230°C for 53 min result in the highest photocurrent enhancement, 33.5%,
efficiency enhancement 32% and the plasmonic cell efficiency of 5.32% without a back
reflector and 5.95% with the back reflector which is the highest reported efficiency for
plasmonic poly-Si thin film solar cells.
The last part (Chapter 3.4) introduces a wet-etching-based method for re-using metallised
poly-Si thin film solar cells after NP deposition. Nitric acid is used to etch Ag NPs on the
metallised cells. The optical and electrical properties of the metallised cell are compared
before and after etching. The optical and electrical properties of the cell after etching are well
matched with the initial value, and the Si film and the aluminium contacts are not damaged
by the etching solution even after five times etching.
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Acknowledgements
It has been a great adventure for me to research at SPREE, UNSW. Two years ago, I was
working at a steel mill company as an engineer on a completely different career path from
one I am on now. My time at UNSW changes me from an engineer to a researcher that I have
always dreamed. This postgraduate work would not have been successful without the kind
assistance and advices from people.
First and foremost, I would like to express my gratitude to my supervisor Dr. Sergey
Varlamov. You suggested me a great topic for my research, so that I can successfully finish
masters by research with good achievements. Thank you for being my guide to do solar cell
research and for expanding my professional knowledge. Not only my academic knowledge
and skills but also I, myself was enriched and nourished further by a big step during the study.
I would also like to extend my thanks to all officials of Chung-Nam provincial
government. They selected me as a provincial government study abroad scholarship student,
and have provided financial supports to pursue master’s degree at UNSW. Without their help,
I could not finish my study at UNSW, also cannot expect to do PhD.
A huge “Thank you” goes to TaeKyun Kim. You suggested me to do masters by research
rather than masters by coursework. Without your advice, I was not able to do research on
solar cells. You also gave me guide all the processes for the metallisation and characterisation.
Thanks so much for your mentorship and more importantly, friendship. Lots of thanks go to
KyungHun Kim who helped me prepare samples for my experiments and tough me how to
use several equipment for characterising the solar cells that I fabricated. I would also like to
express my thanks to the “Howard worriers”, , , , , ,
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and Miga for activities and discussions, and thanks to all the others who contributed to this
work.
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1.3 Plasmonic solar cells……………………………………………………………. 4
1.4 Thesis outline………………………………………………...………..………… 5
Chapter 2. Background…………………………………………. 6
2.3.3 Progress in plasmonic solar cells…………………………………………….. 16
2.4 Nanoparticle fabrication……………………………………………………….. 18
2.5 Characterisation methods……………………………………………………… 18
2.5.1 Optical measurement……………………………………………………….... 18
2.5.2 Electrical measurements……………………………………………………... 19
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Thin Film Solar Cells…………………………………………….
3.1 Introduction……………………………………………………………………… 25
3.2 Solar cell surface condition for plasmonic enhanced light trapping…………. 26
3.2.1 Motivation……………………………………………………………………… 26
3.2.4 Summary……………………………………………………………………….. 37
3.3 Enhanced efficiency of plasmonic polycrystalline silicon thin film solar cells
by optimisation of nanoparticle fabrication process………………………….
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3.3.4 Summary………………..……………………………………………………… 61
3.3.1 Motivation……………………………………………………………………… 62
3.3.4 Summary……………………………………………………………………….. 71
4.1 Thesis summary…………………………………………………………………. 74
1.1 Motivation
For last few decades, renewable energy technologies as an alternative to fossil fuel energy
have been developing actively and effectively to prepare a countermeasure of fossil fuel
shortage in near future that is imminent and to minimise carbon emission which causes global
warming. Photovoltaic, solar thermal, biomass, wind energy as forms of renewable energy
sources have a high energy production cost [1] and therefore it is highly unlikely that
renewable energy sources satisfy the global energy demand if there is no technology
breakthroughs in significant cost reduction. There are several other candidates that can meet
the global energy demand which are not renewable energy sources but have low carbon
emission, nuclear power and hydrogen nuclear fusion reaction. Nuclear energy has several
advantages such as low carbon dioxide, already developed technology - ready for market and
especially low costs of generating electricity [2]. Due to its low costs of generating electricity
and operation [3], shown table 1.1.1, over 30 countries operate nuclear power stations in 2010
[4].
Technology Region or country Cost (US cents / kWh) Nuclear OECD Europe 8.3 - 13.7 Black coal with CCS OECD Europe 11 CCGT with CCS OECD Europe 11.8 Large hydro-electric OECD Europe 14.0 - 45.9
Onshore wind OECD Europe 12.2 - 23.0
China 7.2 - 12.6 Offshore wind OECD Europe 18.7 - 26.1
Solar photovoltaic OECD Europe 38.8 - 61.6
China 18.7 - 28.3
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However, nuclear power has significant disadvantages which are high- accident risks and
radioactivity contamination [3]. Japan’s nuclear power plant accident caused by an
earthquake in 2011 highlighted the risk associated with nuclear power. A hydrogen nuclear
fusion reaction is also a possible candidate because it could generate power in the terawatt
range compared to fission power plants which generate power in the giga-watt range and it
emits low carbon dioxide as well as radioactivity. Nevertheless, this technology is not
possible yet. Hydrogen nuclear fusion reaction still remains theoretical [5].
Photovoltaic (PV) is the most promising energy source for the future. It has a number of
advantages over other renewable energy sources and other possible energy sources. First of
all, resource of generating electricity, sun-light, is unlimited and abundant. Sun light is
directly converted into electricity in PV modules. Therefore, PV has a huge possibility of
clean and low cost electricity generation. The other advantages that PV has been low
maintenance cost, long lifetime, totally silent compared to wind power, easy to set up and
flexibility to small and large scale installations. In spite of these advantages, there are several
problems to be overcome. Currently, the production cost of PV modules is far exceeding
other form of energy sources. Compared to coal and nuclear power, the cost of generating
electricity by PV modules is more than 3 to 6 times higher as shown in Table 1.1.1.
Furthermore, PV market is still small and the market growth is slow. Production cost of PV
modules highly depends on manufacturing cost of PV devices. In order for the market to be
grown and become competitive, the manufacturing cost of PV devices should be decreased.
In addition, energy conversion efficiency of PV devices is relatively low compared to other
energy sources. The highest efficiency of the Si wafer solar cell, PERL cell made by UNSW,
is 25%, and commercial Si wafer solar cells show around 20% energy conversion efficiency.
Therefore, both reducing manufacturing cost and improving energy conversion efficiency are
the motivation of this thesis.
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1.2 Poly-Si Thin Film Solar Cells at UNSW
In order to reduce the manufacturing cost of PV devices, many technologies have been
introduced. One way to reduce the manufacturing cost is to minimise the amount of materials
used to fabricate PV devices. In this technology, only few micrometre range of thickness is
required to make the devices compared to the thickness of Si wafer solar cells, few hundred
micrometres. This kind of solar cells refers to thin film solar cells, and sometime refer to as
“second-generation” solar cells. There are many types of thin film solar cells such as
hydrogenated amorphous Si (a-Si:H), poly-Si, Cadmium telluride (CdTe), Copper indium
gallium selenide (CIGS) , III-V compound-based solar cells, organic solar cells and etc.
These cells have competitive manufacturing cost due to low usage of materials for fabrication
as well as the potential of producing large area on various substrates including glass or plastic
[6].
At UNSW, poly-Si thin film solar cells on glass have been developed and researched.
Poly-Si thin film solar cells on glass combine the advantages of crystalline Si technology
with low material usage [7]. The advantages of poly-Si thin film solar cells are good
durability, non-toxic materials, low material costs and the possibility to produce large area
monolithic modules [8]. In order to form the cells, the precursor amorphous Si (a-Si) is
deposited onto a glass by plasma enhanced chemical vapour deposition (PECVD) or e-beam
evaporation and crystallisation process is followed. Solid phase crystalline (SPC) is
commonly used to form poly-Si from a-Si. This method showed open-circuit voltage (Voc) of
533 mV and efficiency of 9.7% [9]. Recently, our group has established diode laser
crystallisation process using a continuous wave (CW) diode layer, simplifying the process by
avoiding the long SPC step [10]. Voc of 557 mV and 8.4% of efficiency were achieved by this
method, and 10% of efficiency cells are expected in the near future [11].
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Due to the fact that the film is only 2 µm thick and because Si is an indirect band gap
semiconductor, incident light is not fully absorbed in the cells. Therefore, effective light
trapping technology is required. Aluminium induced glass texturing (AIT) [12] and white
paint back reflector are used for light trapping in our cells [13]. The state of the art light
trapping scheme for our cells is light scattering by metal NPs formed on the cell surface. This
is referred to as plasmonic NP light trapping. Our group demonstrated 38% short-circuit
current density (Jsc) enhancement for a fully functional plasmonic poly-Si thin film solar cell
[14].
1.3 Plasmonic solar cells
Plasmons are collective oscillations of the free electron gas density in conductors like
metals. Plasmonic solar cells are defined as solar cells using plasmonic NPs for light trapping.
Plasmon enhanced light absorption in thin film solar cells with metal NPs has been recently
developed because it is a new approach which avoids surface texturing and may, in principle,
achieve better results [15]. In conventional Si wafer based solar cells, surface texturing is
used for light trapping such as inverted pyramid light trapping scheme in PERL cell [16]. For
wafer-based cells, inverted or upright pyramids can be used for light trapping, which have a
typical feature size of around 10 μm [17]. However, for thin film solar cells, a thickness of
only 2 - 3 μm, such textures are not suitable.
A new method for achieving light trapping in thin film solar cells is the use of metallic
nanostructures that support surface plasmons. Recent reviews document a surge of interest in
the use of plasmons [18, 19, 20]. By proper engineering of these metallodielectric structures,
light can be concentrated and ‘folded’ into a thin semiconductor layer, thereby increasing the
absorption. Both localized surface plasmons excited in metal NPs and surface plasmon
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polaritons (SPPs) propagating at the metal/semiconductor interface are of interest. In the past
few years, the field of plasmonics has emerged as a rapidly expanding new area in solar cell
research [21]. There are many solar cells which can make use of plasmons for their
performance enhancement, such as Si wafer based cells [15, 22, 23], a-Si cells [24, 25], III-V
compound solar cells [26], dye-sensitised solar cells [27] and organic solar cells [28], resulted
from better light trapping by NPs.
1.4 Thesis outline
Chapter 1 discusses motivation of the research and reviews thin film solar cells, poly-Si
thin film solar cells on glass at UNSW and plasmonic solar cells.
Chapter 2 introduces background information on the fabrication processes of poly-Si thin
film solar cells and metallisation scheme used in the research. Moreover, NP fabrication
method is reviewed. Characterisation methods for the cells such as optical measurement,
external quantum efficiency (EQE) and illuminated I-V are described. NP characterisation
methods are also discussed.
In chapter 3, optimisation of Ag NP fabrication process is presented. Enhanced light
absorption and Jsc of the cells with Ag NPs are highlighted. Additionally, surface condition of
poly-Si thin film solar cell which affects plasmonic enhanced absorption, parasitic absorption
in Ag NPs and a re-use method for the metallised cells with NPs are discussed.
Chapter 4 summarises the thesis and presents a conclusion obtained from this research. In
this chapter, a prospective possible approach to improve absorption and Jsc as well as energy
conversion efficiency for poly-Si thin film solar cells at UNSW are suggested.
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This chapter introduces necessary background information for better understanding of the
thesis. It consists of four parts. In the first part (section 2.2), cell fabrication processes
including poly-Si film fabrication and metallisation process are introduced. The second part
(section 2.3) is an introduction to plasmonics. In this chapter basic principles of plasmonic
metal NP and design principles of plasmonic thin film solar cells are mentioned. The third
part (section 2.4) is an introduction to a NP fabrication method which is a Ag precursor film.
In the last part (section 2.5), characterisation methods used in this thesis are introduced.
Optical measurement which determines how much incident light is absorbed in the cells is
introduced; electrical measurements which are EQE and illuminated I-V are also mentioned;
NP characterisation including average particle size, shape and coverage is introduced.
2.2 Cell fabrication
2.2.1 General process
At UNSW, poly-Si cell on glass is in superstrate configuration, deposited onto a glass by
either PECVD or e-beam evaporation. Poly-Si thin films used for this thesis are all deposited
by PECVD.
The general processes for the structure are as follows: 3.3 mm thick Schott Borofloat
glass is used as a substrate; silicon nitride barrier layer (SiNx) acts as antireflection coating
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(ARC), 80 nm; n+ layer (emitter layer, ~100 nm,~1x1020cm-3), p- absorber layer (1800 nm,
~4x1016cm-3) and p+ back-surface layer (BSF, ~100 - 200 nm, ~1x1019cm-3). The incident
light can enter the cell through the glass (superstrate configuration) or from the air side
(substrate configuration). Then the film underwent solid phase crystallisation (SPC) at 600°C
for 40 hours, rapid thermal annealing (RTA) at 950°C for 5 min and defect passivation by
remote hydrogen plasma. The fabrication steps and the cell structure are illustrated below;
Fig. 2.2.1.2. Schematic of Poly-Si diode structure.
Fig. 2.2.1.1. General fabrication steps of Poly-Si thin film solar cells.
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2.2.2 Metallisation scheme
In order to characterise effects of plasmonic light-trapping on cell electrical performance,
metallised poly-Si thin film solar cells are required. The metallisation scheme used in this
thesis is the interdigitated aligned metallisation developed at UNSW which allows bifacial
cell configuration [29]. The bifacial metallisation scheme makes it possible for NPs to be
formed on both front- and rear-surfaces of the cells. In this thesis, NPs are only formed on the
rear Si surface (illumination from the front surface which is glass side) of the cells. The
reasons for this are discussed in Chapter 3. Figure 2.2.2.1 shows the schematic cross section
of the metallised cell.
Fig. 2.2.2.1. Schematic cross-section of the metallised Poly-Si thin film solar cell.
The following steps show the sequence of aligned metallisation.
(1) Poly-Si film is etched by hydrofluoric (HF) acid to remove a thermal SiO2 layer
formed during RTA process. After HF dipping, photoresist (S1818) is spin-coated on the film
at 3000 rpm for 30 sec, and pre-baking at 110 °C for 90 sec is followed. The photoresist is
patterned using photolithography, followed by development in MF26A for 1 min. Then, it is
post-baked at 130 °C for 5 min before plasma etching. (Fig. 2.2.2.2. (a), (b) and (c))
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(2) Reactive ion etching using SF6 plasma is used to etch down to the emitter layer. The
main focus of this step is to obtain a uniformly etched emitter layer. (Fig. 2.2.2.2. (d))
(3) Photoresist deposited on the sample in step (1) is removed by acetone. To make better
contact between Al contacts and Si film, a native SiO2 layer is removed by HF and then the
2μm Al film is deposited immediately afterwards. (Fig. 2.2.2.2. (e) and (f))
(4) Photoresist is again spun-on to develop a metallisation pattern (fingers and busbars).
Similarly, photoresist is pre-baked and patterned with photolithography using a photo-mask,
and this is followed by post-bake at the same temperature as in step (1). (Fig. 2.2.2.2. (g))
(5) The cell with patterned photoresist obtained in step (4) is etched in diluted phosphoric
acid (H3PO4) solution at 65 °C for 5 to 10 mins. Finally, the remaining photoresist on Al film
is removed with acetone. (Fig. 2.2.2.2. (h))
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Fig. 2.2.2.3. Top view of the aligned metallisation finished poly-Si thin film solar cell.
2.3 Plasmonics
2.3.1 Basic principle of plasmonics
When incident light irradiates a small metallic NP, the oscillating electric field causes the
conduction electrons to oscillate [31]. This is shown in the figure below:
Fig. 2.3.1.1. Schematic of plasmon oscillation for a sphere, showing the displacement
of the conduction electron cloud relative to the nuclei [31].
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When the electron cloud is displaced relative to the nuclei, a restoring force arises from
Coulomb attraction between electrons and nuclei, resulting in oscillation of the electron cloud.
The collective oscillation of the electrons is referred to the dipole plasmon resonance of the
particle distinguished from plasmon excitation that can occur in bulk metal or metal surfaces.
Plasmonic structure can provide at least three ways to reduce the thickness of the PV
layers, whereas their optical thickness stays constant. First of all, metallic NPs can be used as
sub-wavelength scattering elements to couple freely propagating plane waves from the Sun
into the solar cells, by folding the light into a thin absorber layer (Fig. 2.3.1.2. a). Second,
metallic NPs can be used as sub-wavelength antennas in which the plasmonic near-field is
coupled to the semiconductor, increasing its effective absorption cross-section (Fig. 2.3.1.2.
b). Finally, a corrugated metallic film on the back surface of a thin photovoltaic absorber
layer can couple sun-light into surface plasma polaritrons (SPPs) 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. 2.31.2. c) [18].
Fig. 2.3.1.2. Plasmon light trapping geometries for thin film solar cells (a) light trapping by
scattering from metal NPs at the surface of the solar cell. (b) light trapping by the excitation
of localised surface plasmons in metal NPs embedded in the semiconductor. (c) light trapping
by the excitation of surface plasmon polaritons at the metal/semiconductor interface [18].
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Figure 2.3.1.2. (a) shows the metal NPs on the front surface of the solar cells. This type of
plasmonic structure can lead to scattering of the incident light into the absorber layer of the
solar cells over an increased angular range, enhancing the optical path length. However,
Plasmons on the front surface of solar cells tend to lead to reduction of current at short
wavelengths, due to destructive interference. Localised surface plasmons (LSPs) shown in
Figure 2.3.1.2. (b) are a suitable structure for hydrogenated amorphous silicon (a-Si:H) solar
cells due to the excitation of NPs by electro-magnetic radiation which enhance light
absorption by the cells. LSPs can cause strong local electro-magnetic field and light
enhancement within certain sections of frequency spectrum, since the absorption is
proportional to the intensity of the local field [32]. SPPs also referred to as the propagating
waveguide mode or photonic waveguide mode shown in Figure 2.3.1.2. (c) are based on the
scattering within the absorber layers. The power of SPPs mode is partially absorbed by the
semiconductor layer, hence resulting in the generation of additional electron-hole pairs and
an enhancement of the photocurrent of the cells [33]. Plasmonic structures are also used as
scattering back reflectors. Unlike the NP structure as shown in Figure 2.3.1.2. (a), NPs are
formed on the rear surface of the cells for the purpose of enhancing the absorption of long
wavelengths greater than 600 nm. This structure is particularly suitable for the thin film solar
cells which are made of indirect band gap semiconductors, like c-Si. Therefore, this structure
can lead to enhanced performance of our cell, poly-Si thin film solar cells. In poly-Si thin
film solar cells, incident light is not fully absorbed due to its physically thin absorber layer, 2
µm. Short wavelengths smaller than 600 nm are absorbed on first path through the cells while
long wavelengths are not. This structure thus can increase optical path of the wavelengths
greater than 600 nm, resulting in enhanced photocurrent of the cells.
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2.3.2 Design principle of plasmonics
Plasmonic structures are the main factor of plasmonic enhanced light trapping. Moreover,
it is also affected by several characteristics of plasmonic NPs such as its shape, size,
dielectric environment and NP materials. Therefore, it is obvious that designing plasmonic
structures by varying the above characteristics is critical to achieving enhanced performance
of the solar cells. The fraction of light that is scattered into the substrate, fsubs, defined as the
power scattered into the substrate divided by the total scattered power is the most important
factor for plasmonic enhanced light trapping [34]. When the NPs are formed on the solar cells,
it is required that the light is scattered in the cell direction for better light trapping. In other
words, the value of fsubs should be as high as possible. The value of fsubs varies between 0 and
1 by definition. For fsubs < 0.5, light is mostly scattered outside the cell that is undesired for
plasmonic enhanced light trapping [35]. This feature basically depends on the particle size,
shape and the surrounding medium. This is well indicated in the figure below;
Fig. 2.3.2.1. Fraction of light scattered into the substrate, fsubs, for Ag particles [34].
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Figure 2.3.2.1 shows fsubs calculated for Ag particles. As can be seen, fsubs is varied
depending on the particle shape as well as size. The cylinder and hemisphere show much
higher fraction scattered into the substrate for the entire wavelength range at 500 – 800 nm
than the spheres. The hemisphere shows better fsubs than cylinder at 500 – 570 nm; on the
other hand, cylinder shows better fsubs at great than 570 nm. Different shape of the particles
causes different pattern of fsubs at 500 – 800 nm wavelength ranges. For spheres, the value of
fsubs increases with decreasing particle size for the entire wavelength range. Therefore, the
particle size is also crucial factor for designing a plasmonic solar cell.
In this thesis, the particle size is the most important factor for enhancing photocurrent of
the cells, even though the particles are formed on the rear surface (BSF layer, see figure
2.2.2.1) of the cells. This is because the particle size influences the scattering cross-section
(Cscat) and the absorption cross-section (Cabs). Cscat is defined as the area over which the
particle scatters light, normalised to the geometrical cross-sectional area of the particle and
Cabs is defined as the area over which the particle absorbs the incident light, normalised to the
geometrical cross-sectional area of the particle. With increasing particle size, Cscat becomes
dominant; on the other hand, Cabs becomes dominant in smaller particles. For Cabs >> Cscat,
the dominant process induced by the particles is absorption of photons in metal NPs; on the
contrary, strong scattering of incident light is dominant process induced by the particles for
Cscat >> Cabs. To effectively enhance light absorption in the cells using NPs, Cscat must be
maximised and Cabs has to be minimised. Since NPs are located on the cells, it is desired that
the scattered incident light to be highly peaked to the cell direction. In principle, this feature
depends on the NP size. Therefore, there is an optimum particle size which leads to better
performance of the cells. This is further discussed in Chapter 3.
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2.3.3 Progress in plasmonic solar cells
Plasmonic NPs have been employed for the purpose of enhancing light absorption in
various solar cells such as Si wafer based solar cells, poly-Si thin film solar cells, a-Si:H solar
cells and etc and actively been research by many of research groups. Recently, Dr. Miro
Zeman’s group have reported energy conversion efficiency of 7.9% in plasmonic a-Si:H solar
cells [36]. In their research, Ag NPs were formed on the Ag/AZO layer from an Ag precursor
film deposited by thermal evaporation, and the particles are acted as back surface reflector
(BR). Three types of BR including plasmonic BR, flat BR and textured BR were compared to
confirm enhanced Jsc by each BR. In order to evaluate the potential of plasmonic light
trapping in this solar cells, it was compared to state-of-the-art random textures. They have
demonstrate that Jsc of plasmonic cells was enhanced from 13.1 mA/cm2 to 15.1 mA/cm2 and
Jsc of textured cells was enhanced from 13.1 mA/cm2 to 14.8 mA/cm2. Consequently, it has
been shown by Dr. Miro Zeman’s group that self-assembled Ag NPs based plasmonic BR can
provide light trapping performance comparable to the cutting-edge of the random textures in
a-Si:H solar cells. The conclusion is based on the comparison to high performance n-i-p solar
cells and high efficiency p-i-n solar cells. The plasmonic BR showed good reflection above
80% in the wavelength range 520−1100 nm and can provide efficient light trapping over
broad spectral range. The excellent light trapping is a result of strong light scattering of Ag
NPs. Their promising results open the route to use plasmonic metal nanoparticles to obtain
high efficiency and low-cost thin film Si solar cells.
In addition, Dr Reinhard Carius’s group has also showed outstanding experimental result
in plasmonic thin film solar cells [37]. In this experiment, microcrystalline (μc) Si:H solar
cells in n-i-p substrate configuration was prepared, and Ag NPs were formed from Ag
precursor film deposited on Si wafer and performed as a back surface reflector. They
controlled the size of Ag NPs by depositing various thickness of Ag precursor films. As a
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result of this, they achieved the radius of 50, 100, 150, 200 and 300 nm of NPs. Among them,
200 nm Ag NPs showed highest Jsc enhancement, Jsc was enhanced form 18.4 mA/cm2 to
20.5 mA/cm2. They have concluded that the μc-Si:H n-i-p solar cell with plasmonic Ag BR
which is non-ordered half-ellipsoidal Ag nanostructure showed significant enhancement in
EQE in wavelengths longer than 500 nm compared with flat back contacts. The associated
light-trapping effect is attributed to localized surface plasmon induced scattering at
nanostructures on the back contact.
At UNSW, Ag NPs have actively been researched and employed to our poly-Si thin film
solar cells. The fabrication method for Ag NPs employed in our cell is discussed in later
chapter. Performance enhancement of our poly-Si thin film solar cell using plasmonic NP
have been reported by many of our colleagues. The initial study of Plasmonic Ag NPs for
poly-Si thin film solar cells has successfully been done by Dr. Zi Ouyang [14], and a good
review of the plasmonic light trapping was done by Prof. Martin Green [20]. In addition,
dielectric environment for plasmonic Ag NPs has been studied by Dr. Jing Rao [38]. Finally,
the highest efficiency of plasmonic poly-Si thin film solar cells is achieved by the author. The
result of this is discussed in chapter 3.
18
2.4 Nanoparticle fabrication
In this thesis, Ag is chosen for forming NP because; it is proven to work for plasmonic
poly-Si thin-film cells , and the plasmonic resonance frequency, if well-tuned, can be in the
spectral range, which is important for poly-Si photovoltaic devices (600 – 1200 nm) [39]. In
addition, Ag is promising for plasmonic solar cells because of its low parasitic loss-it behaves
more like an ideal Drude metal than other metals. Recently, many plasmonic nanostructures
incorporating metal NPs on the front- and rear-surface of the cells have been proposed. To
form NPs either on front or rear surface, several methods have been developed. Spin coating
is used to apply NPs on the cell surface. NPs are dispersed in a suspension solution and the
solution is coated by a spin coater [40]. Dipping is also used. Cells are dipped into a solution
containing NPs, and then the particles are situated on the cells [41]. Dropping is one way to
be positioning NPs. NP containing solution is directly dropped onto the cells; this produces
NP deposited solar cells [34]. Metal precursor films are used for forming metallic NPs [42].
The particle formation method used in this thesis is annealing of silver precursor films.
Ag NPs are formed from the Ag precursor film deposited by thermal evaporation at the rate
of approximately 0.5 /s at 2x10-5 Torr onto the cell surface, annealing in an oven in nitrogen
atmosphere. As a result of annealing, Ag precursor film breaks down into NPs due to the
surface tension. Since particle shape, size and coverage are crucial elements for designing
plasmonic solar cells [43], these are controlled by varying the thickness of the precursor Ag
film, annealing temperature and time.
2.5 Characterisation methods
2.5.1 Optical measurement
19
In order to measure how much of incident light is absorbed in the solar cells, optical
measurement is required. Spectrophotometer is commonly used to determine the light
absorption A, reflectance R and transmittance T. Cell reflectance R and transmittance T are
measured separately by a spectrophotometer and used to calculate absorption A from the
equation below;
where, λ is the wavelength of light (in nm).
In this thesis, Perkin Elmer Lambda 1050 spectrophotometer equipped with 150 mm
InGaAs integrating sphere is used. The schematic diagrams of transmittance and reflectance
measurements using the intergrating sphere are shown below.
Fig. 2.5.1.1. Schematics of a spectrophotometer transmittance (left) and reflectance (right) measurements using integrating sphere.
2.5.2 Electrical measurements
Electrical measurements are required to characterise electrical properties of metallised
solar cells. These include short-circuit current (Isc) or Jsc, Voc, fill-factor (FF), energy
20
conversion efficiency (Eff) and EQE. In this chapter, EQE measurement and illuminated I-V
measurement are introduced.
2.5.2.1 External quantum efficiency (EQE) measurement
The quantum efficiency (QE) is classified into Internal Quantum Efficiency (IQE) and
External Quantum Efficiency (EQE). The IQE is the ratio of the number of charge carriers
collected by the cell to the number of photons of a given energy absorbed by the cell. The
EQE is the ratio of the number of charge carriers collected by the cell to the number of
photons of a given energy incident on the cell. Therefore, the IQE is always larger than the
EQE. The EQE is calculated by the equation below.
where, (h : Plank’s constant, c : the vacuum velcity
of light) and .
The IQE is calculated divided by the EQE to absorption (1-R-T);
The EQE is the most useful characterisation method for light trapping investigation and
derivation of Jsc because it is relatively independent from cell metallisation which is often
affected by unintentional variations in processing steps. The 1-Sun Jsc can be calculated from
the EQE data;
21
where, q = the electron charge and S(λ) is the standard spectral photon density of
sunlight at the earth’s surface (AM 1.5).
At UNSW, the EQE system is a custom-made system with a xenon arc lamp that provides
a steady-state light source. The monochromatic light generates photocurrent and voltage
across the sample and the current response of the sample to the monochromatic light beam of
known spectral power is measured. In this thesis, the EQE of the cells were measured
spectrum between 300 - 1200 nm and 2.5 nm interval. The basic structure of a simple EQE
tester is shown below.
2.5.2.2 Illuminated I - V measurement
The illuminated I-V measurement is one of the most important methods for solar cell
characterisation. The I-V curve plotted by I-V measurement is the set of all possible operating
points of the device. An ideal diode’s I-V curve in the dark is described by the diode equation
below.
where, I0 = dark saturation current. Under illumination condition, the diode equation becomes
22
where, IL = the light generated current. From the equation above, Isc and Voc are
determined.
Therefore,
Through the measurement, several other parameters of the solar cell can be calculated
such as fill-factor (FF), energy conversion efficiency (Eff), series resistance (Rs) and shunt
resistance (Rsh).
An illuminated I-V curve is measured by varying the solar cell’s load and measuring
current and voltage on the load under standard test condition (AM 1.5, 1000W/m2, 25°C cell
temperature). The I-V tester used in this thesis is a custom made system and PV module
measuring device from Spire. Xenon lamps and housing unit containing a transformer are
equipped in this system. The basic structure of the I-V tester used for this thesis is shown
below;
23
2.5.3 Nanoparticle Characterisation (size, shape and coverage)
The size, coverage and shape of the NPs are characterized from Scanning Electron
Microscopy (SEM) images, FEI Nova NanoSEM 230. The shape of NPs is characterised
directly from SEM images. The size and coverage of NPs were calculated by statistical
analysis of SEM images using “Image J” software assuming a circular particle. A sample
SEM image of NPs is shown below;
24
Fig. 2.5.3.1. A sample SEM image of NPs and their average size and coverage calculated by Image J.
Average particle size : 62nm Coverage : 40% Calculated by Image J
25
Film Solar Cells
3.1 Introduction
This chapter describes the application of Ag NPs to poly-Si thin film solar cells. First of
all, the surface conditions for poly-Si thin film solar cells that influence plasmon-enhanced
light trapping are introduced. NPs are formed on Si films, a thermal SiO2 layer and a native
SiO2 layer and absorption is compared for varying surface conditions. In addition, scattering
cross section (Qscat) which depends on surface conditions is simulated using Finite Definition
Time Domain (FDTD) program, so-called potential Jsc of each condition is then calculated to
determine the optimum surface condition for plasmonic enhanced light trapping.
Secondly, the optimisation of NP fabrication parameters for plasmon-enhanced light
trapping is presented. Ag NPs are formed from Ag precursor films with varying film
thickness, annealing temperature and time. Different fabrication parameters can lead to
different NP shape, size and coverage, resulting in varying plasmonic effects. Enhanced
absorption by NPs formed from different parameters is compared in order to select
prospective candidates for maximising the photocurrent of the cell. The selected parameters
are then applied to metallised cells to confirm actual photocurrent enhancement as well as
efficiency enhancement.
Finally, a wet-etching-based method for re-using poly-Si films and metallised poly-Si
solar cells with NPs is introduced. Film absorption and solar cell electrical properties before
particle formation and after wet etching are compared.
26
trapping 3.2.1 Motivation
The optical environment is an important key factor to consider when designing plasmonic
solar cells because it affects the surface plasmon resonance wavelength λSPR, scattering angles
θscat, scattering cross-section Qscat and coupling efficiency fsubs. The environment is normally
a dielectric layer. Layers such as silicon oxide, silicon nitride, titanium oxide, magnesium
fluoride and tantalum pentoxide can be used in principle for either isolation between the
semiconductor and NPs or coating NPs to tune the spectral shift.
In the case of our cell, a SiO2 layer is formed on the rear surface during rapid thermal
annealing (RTA) and/or metallisation processes. The rear surface of the cell, which contains
the BSF (p+), is in contact with atmospheric oxygen during these processes. When SiO2 is
present on the rear surface of the cell and NPs are formed on this layer, the plasmon-
enhanced light absorption is expected to be different from the absorption as it is enhanced by
NPs formed directly on the Si surface. This is because Si and SiO2 have different refractive
indices (3.7 and 1.7, respectively) as well as wetting properties of Ag are also different on Si
or SiO2 which can affect NP formation; this value determines plasmonic factors such as Qscat,
fsubs and λSPR.
In this section, we explore the best surface conditions for poly-Si thin film solar cells. In
order to find the optimum surface condition for plasmonic enhanced light absorption,
absorption is compared between films or cells with NPs formed on Si, a native SiO2 layer and
a thermal SiO2 layer. In addition, the Qscat of the NPs is calculated, and the potential Jsc of
each surface condition with NPs is compared to determine the best surface condition. We also
investigate how and why these surface conditions affect plasmonic enhanced light trapping.
27
3.2.2 Experimental design
In this experiment, 5 cm x 5 cm poly-Si thin films were used. The films were deposited
by PECVD. The films underwent the standard material preparation process as described in
Chapter 2. The poly-Si films were used to demonstrate absorption variation given different
NP surrounding environments.
Spectrophotometry was performed to characterise the light absorption of the samples
before and after NP formation. Scanning electron microscopy (SEM) was used to confirm
whether the Ag films were fully broken down into NPs. Ellipsometry was used to measure
the thickness of the SiO2 layer formed on the surface of the cells. The potential Jsc calculation
was used to estimate enhanced Jsc by NPs on Si, a thermal SiO2 layer and a native SiO2 layer.
The definition and calculation method are introduced in the results and discussion chapter.
The experiment was conducted in the following steps.
(1) RTA process finished three samples were prepared.
(2) Two samples were dipped in Hydrofluoric (HF) acid for 15 min until the surface
became hydrophobic.
(3) One sample dipped in HF was stored in atmosphere for 100 hr to form a native SiO2.
(4) Reflectance (R) and transmittance (T) of the samples were measured by a
spectrophotometer, and absorption (A) was calculated from R and T. In addition, the
thickness of SiO2 layers was measured by ellipsometry.
(5) Three samples were placed in a thermal evaporator and then Ag precursor films were
deposited onto the samples.
(6) The samples were annealed in an oven at 200°C for 60 min in a nitrogen atmosphere.
(7) R and T of the samples were measured again, and then A was calculated.
28
3.2.3 Results and discussion
First of all, ellipsometry was used to measure the thickness of the SiO2 layers of each
sample. The reason for measuring the thickness of the SiO2 layers is that dielectric
environments of NPs are influnced by the thickness of dielectric layer. Table 3.2.3.1 shows
the thickness of SiO2 layers formed on the rear surface of the films.
Sample Lot Thermal SiO2
Thickness of SiO2
Table. 3.2.3.1. Thicknesses of SiO2 layers of three different surface conditions.
As can be seen, the thicknesses of the thermal SiO2, native SiO2 and SiO2 free are 45 nm,
3 nm and 0 nm, respectively. The thermal SiO2 layer on the Si film was formed during RTA
process, so that it showed the highest thickness. Since the Si film contacted with oxygen in
the air, the native SiO2 layer was formed on Si film which underwent HF dipping process
before storing in the atmosphere.
In order to confirm initial absorption of each sample before NP formation and absorption
after NP formation, spectrophotometry was used to measure absorption in the samples with
and without NPs. Figure 3.2.3.1 shows the absorption graphs of the samples before and after
NP formation. As can be seen in Figure 3.2.3.1. (a) and (b), the light absorption at 300 - 500
nm is different from each other even the samples were fabricated in the same batch. The
absorption at short wavelengths, 300 - 500 nm, might be dominantly affected by
unintentional differences in anti-reflection coating (ARC), highly doped emitter (1020
atoms/cm2) or variations in total film thickness rather than the existence of a SiO2 layer on
29
the rear Si surface. In this case, the absorption difference at this wavelength range is due to
non-uniform thickness of ARC. Therefore, it is better to consider absorption at wavelengths
greater than 500 nm which is less affected by non-uniform thickness of ARC.
30
Fig. 3.2.3.1. Absorption of the three different surface conditions (a) before (b) after NP
formation.
(b)
(a)
31
Figure 3.2.3.1. (a) shows the absorption of each sample without NPs. In this figure, the
absorption pattern of each sample is nearly the same in 500 - 1100 nm wavelength range.
Since their initial absorptions are similar to each other, we can simply compare the absorption
of the samples after NP formation.
Figure 3.2.3.1 (b) shows the absorption graphs of the samples with NPs. As can be seen in
the figure, the sample with NPs on the native SiO2 layer and the sample with NPs directly on
Si show similar absorption over entire wavelengths. On the other hand, the sample with NPs
on the thermal SiO2 shows different absorption pattern compared to that of other two samples.
The sample with NPs on the thermal SiO2 layer shows better absorption in 500 - 600 nm
wavelength range, whilst less absorption in 600 - 1100 nm than other two samples. This is
because such a thick thermal SiO2 layer acted as a spacer layer, and the light absorption in the
sample with NPs was affected by presence of this layer. The sample with NPs directly on Si
has a spacer layer of the zero thickness or of the high refractive index (nSi = 3.7), and
therefore absorption pattern after NP formation is different from that of the sample with NPs
on the thermal SiO2 layer. In case of the sample with NPs on the native SiO2 layer, this has a
3 nm thick spacer layer, however, this shows same absorption pattern to the sample with NPs
directly on Si.
To confirm absorption differences of the samples after NP formation, it would be
necessary to analyse reflectance and transmittance of each sample.
32
Fig. 3.2.3.2 Reflectance of three different surface conditions (a) without NPs, (b) with NPs.
Fig. 3.2.3.3. Transmittance of three different surface conditions (a) without and (b) with NPs.
Figure 3.2.3.2 (a) and (b) show reflectance of the samples without and with NPs. It can be
seen in the figures, reflectances of the samples without NPs are similar in the 500 - 1100 nm
wavelength range. After NP formation, the sample with NPs on the thermal SiO2 layer shows
a slightly different reflection pattern compared to that of sample with NPs on the native SiO2
layer and with NPs directly on Si. Even reflection patterns of the samples are similar, the
sample with NPs on the thermal SiO2 layer show lower reflection in the 550 - 900 nm region.
The highest reflection of the sample with NPs on the native SiO2 and with NPs directly on Si
is approximately 30% at around 800 nm, but that of the sample with NPs on the thermal SiO2
(a) (b)
(a) (b)
33
layer is small by factor of 2. If the reflectance is smaller than that of the sample with NPs on
the native SiO2 layer and with NPs directly on Si, absorption of the sample should be higher
than that in the other sample. However, the sample with NPs on the thermal SiO2 layer
showed lower absorption in 600 - 1100 nm than that of other two samples. Therefore, this is
not the reason why the sample with NPs on the thermal SiO2 layer showed lower absorption
than the sample with NPs on the native SiO2 and with NPs directly on Si.
Figure 3.2.3.3 (a) and (b) show transmittances of the samples without and with NPs.
Figure 3.2.3.2 (a) shows transmittance of each sample before NP formation. Initial
trasmittances are similar in the samples. After NP formation, the sample with NPs on the
native SiO2 layer and with NPs directly on Si show nearly same tramsmission, on the other
hand, the sample with NPs on the thermal SiO2 layer shows lower tramission at 450 - 650 nm
and much higher transmission at 650 - 1100 nm than that of other two samples. Therefore, it
is clear that absorption differences between the sample with NPs on the thernal SiO2 sample
and the smple with NPs on the native SiO2/with NPs directly on Si are due to the different
transmittion pattern after NP formation. It can be also said that the existence of the thick SiO2
layer causes this different absortion, resuting in different transmission pattern in the samples.
To confirm experimental results above, FDTD numerical simulations were performed
using the FDTD solutions package. The Qscat was calculated for single hemispherical
particles of diameter 150 nm (detailed in Figure 3.2.3.3) located either directly on a semi-
infinite Si substrate or separated from Si by a 3 nm and 45 nm SiO2 layer. The particle
diameters correspond to the median of the particle sizes calculated from SEM images of the
samples. The SEM images of the samples are shown in Figure 3.2.3.3. As can be seen in the
images, NPs formed on the thermal SiO2 layer and directly on Si film are nearly the same in
shape as well as average particle size which is approximately 150 nm in diameter.
34
Fig. 3.2.3.3. SEM image of NP formed on (a) the native SiO2 layer (b) the thermal SiO2 layer
from 14 nm thick Ag precursor film annealed at 200°C for 60 min.
Fig. 3.2.3.4. Calculated Qscat spectra for single Ag NPs 150 nm diameter on Si substrates, located
directly on the Si, on a 3 nm and 45 nm SiO2 layer.
(a) (b)
35
Figure 3.2.3.4 shows calculation result of scattering cross section of NPs on each sample.
NPs on the native SiO2 and directly on Si show similar Qscat over entire wavelength range.
This represents the result from absorption measurement that the sample with NPs on the
native SiO2 showed similar absorption with the sample with NPs directly on Si. However, NP
on the native SiO2 layer shows larger Qscat in 450 - 1000 nm wavelength range than that of
NP directly on Si film. Clear peak from NPs on the thermal SiO2 in the Qscat data can be seen,
corresponding to the localised surface plasmon resonance. It is observed that the height of the
Qscat peak is doubled for the 150 nm diameter particle on the native SiO2 and directly on the
Si compared to the case where the oxide layer is present. Without the SiO2 layer the
resonance is clearly red-shifted from a wavelength of 675 nm to 1200 nm. This is a
consequence of the better coupling of electromagnetic near-field of the particle with the high
reflective index Si [14]. This trend agrees with absorption patterns of the samples (see Figure
3.2.3.1 b). The sample with NPs on the thermal SiO2 layer showed higher absorption at 500 –
700 nm due to their higher Qscat than that of the sample with NPs on the native SiO2 and
directly on Si. Similarly, the sample with NPs on the native SiO2 layer and directly on Si
demonstrated higher absorption at wavelengths longer than 700 nm in comparison to the
sample with NPs on the thermal SiO2 layer because of its high Qscat at this wavelength.
In order to confirm how electrical characteristics such as Jsc and energy conversion
efficiency can be affected by the cell surface conditions, potential Jsc is calculated. A concept
of potential Jsc is used to compare possible performance of each type of the samples. Potential
Jsc is the method to estimate Jsc of a solar cell by spectrally integrating the product of
absorption with the AM 1.5G solar spectrum (photons/cm2) and multiplying by the electronic
charge (1.6 x 10-19 coulombs) [45].
36
Potential Jsc is used as a criterion to assess light trapping capability of the corresponding
glass substrate [48]. In this experiment, the product of absorption and the AM 1.5 spectra
were integrated in the wavelength range 400 - 1100 nm to calculate the potential Jsc,
assuming unity IQE at each wavelength. The potential Jsc of the samples with NPs on the
thermal SiO2, the native SiO2 and with NPs directly on Si were calculated respectively and
potential Jsc enhancements were also calculated in order to compare potential Jsc of each type
of the samples.
Potential Jsc with NPs 11.68 mA/cm2 12.45 mA/cm2 12.38 mA/cm2
Potential Jsc without NPs 7.76 mA/cm2 7.66 mA/cm2 7.65 mA/cm2
Potential Jsc enhancement 50.5% 62.53% 61.8%
Table. 3.2.3.2. Potential Jsc of the samples and Jsc enhancements.
Table 3.2.3.2 shows potential Jsc of the samples and potential Jsc enhancements depending
on the each surface condition. Before NP formation, potential Jsc of the samples with the
native SiO2 layer and without SiO2 layer is about 7.65 mA/cm2 and this is similar to that of
the thermal SiO2 sample. After NP formation, the sample with NPs on the native SiO2 layer
shows the highest potential Jsc, 12.45 mA/cm2 and also highest potential Jsc enhancement,
62.53%. The sample with NPs directly on Si shows 12.38 mA/cm2 of potential Jsc and 61.8%
of potential Jsc enhancement. Even though potential Jsc is not conventional method to
estimate Jsc using absorption, and is not commonly accepted, it has been published in peer
reviewed journal papers by other researchers. Potential Jsc estimation can only be applied if
the real cells used for calculation of potential Jsc are made from exactly the same materials
37
and exactly the same processes. This is because the assumption which IQE is the same for all
the cells can only be achieved when the cells are made from the same materials and processes.
In this thesis, the cells used for potential Jsc estimation made from the exactly the same
materials and processes, and potential Jsc showed comparable results to that of real cells.
Therefore, we have convinced that potential Jsc estimation is valid in our work.
For NPs on the thermal SiO2 layer, the peak of Qscat is located at the visible light
wavelengths < 700 nm, which are less important for light trapping than for wavelengths >
700 nm in our cells. Therefore, the sample with NPs on the thermal SiO2 layer showed lower
potential Jsc enhancement. On the other hand, peak of Qscat of NPs on the native SiO2 layer is
situated at the longer wavelengths > 700 nm, which are weakly absorbed in 2 µm thick poly-
Si thin film solar cells. For these reasons, the sample with NPs on the native SiO2 layer
showed higher potential Jsc enhancement than that of the sample with NPs on the thermal
SiO2 layer.
3.2.4 Summary
We investigated the optimum surface condition for maximising plasmonic enhanced light
absorption in the cells, hence enhanced photocurrent of the cells. The experiment was
conducted over three different surface conditions for NPs, which are the thermal SiO2, native
SiO2 and an oxide-free Si film. We found that the existence of the thick SiO2 layer between
Si and NPs has a major effect on Qscat. We also found that the thin SiO2 layer has a feeble
effect on Qscat. NPs on the thermal SiO2 layer showed that the peak of Qscat is located at the
visible light wavelengths < 700 nm, NPs on the native SiO2 layer and directly on Si sample
showed that their peaks of Qscat are positioned at the longer wavelengths > 700 nm. The
sample with NPs on the native SiO2 layer showed higher potential Jsc enhancement which is
38
62.5%. On the other hand, the sample with NPs on the thermal SiO2 layer showed 50.5%
enhancement of potential Jsc. Consequently, NPs on the native SiO2 layer are the best for
plasmonic enhanced light absorption for our cells.
39
3.3.1 Motivation
Poly-Si thin film solar cells are physically too thin to fully absorb incident light as
discussed in Chapter 1. Therefore, the cells need to have effective light-trapping for optical
path-length enhancement in order to achieve more complete light absorption for better
performance. The light-trapping is particularly important for poly-Si thin film solar cells.
This is because the cells are made of indirect bandgap semiconductor, Si, so that the near
infrared light still abundant in the solar spectrum is not fully absorbed in the cells. The use of
plasmonic NPs is one of the efficient approaches to achieve effective light trapping for our
cells.
As mentioned in Chapter 2, many methods have been introduced to form NPs on the
surface of the cells. Spin coating, dip coating, drop coating are used to apply NPs to the cells,
more commonly precursor metal films is commonly used. This is because it has been proven
to work the best and a precursor metal film has several benefits over other fabrication
methods; 1) simple processes; deposition and annealing, 2) easy to get high surface coverage.
This standard NP formation process has been shown to increase photocurrent up to about
45% [47, 48] but there are no reports on if and how this process has been optimised,
particularly for enhancing the performance of poly-Si thin-film cells on glass. We conducted
comprehensive NP fabrication process optimisation that included the precursor Ag film
thickness, annealing temperature and time. We found a correlation between absorption
enhancements in a specific wavelength range and Jsc enhancement.
40
3.3.2 Experimental design
As mentioned in chapter 2, Ag is chosen for this experiment. Ag precursor films were
deposited by thermal evaporation at the rate of approximately 0.5 /s at 2×10-5 Torr onto the
Si cell surface. The thicknesses of 10, 14 and 20 nm were produced. To form NPs, the films
were annealed at temperatures ranging from 190 to 260 °C and with annealing times of 20 to
95 min in a nitrogen atmosphere. Varying these parameters is known to affect particle shape,
size and coverage, which are crucial elements for designing plasmon-enhanced solar cells
[34]. The sample batches are shown in the table below.
Film thickness (nm) Annealing temperature (°C) Annealing time (min)
10
14
20
Table. 3.3.2.1. Sample batches for optimisation process.
41
In the previous work from our group, NPs formed from 14 nm thick Ag precursor film
annealed at 200 °C for 50 min showed competent Jsc enhancement even the parameters were
not optimized. Based on this, we assumed that the optimised parameters should not be quite
different from the parameters previously used. In addition, we also assumed that thickness of
Ag precursor film less than 10 nm may produce very small size of Ag NPs (may be less than
30 nm in diameter) which is not suitable for back reflector. On the other hand, Ag NPs
formed from the Ag precursor film more than 20 nm thickness are too large, so that surface
plasmon polariton resonance is reduced. In case of annealing temperature, we designed its
range from 190 – 260 °C. The reason for this is that we assumed that at the annealing
temperature lower than 190 °C, Ag precursor films may not be fully broken down into Ag
NPs. Also, the annealing temperature more than 260 °C may produce large Ag NPs which
reduce surface plasmon polariton resonance. In case of annealing temperature, we designed
190 °C with longer annealing time and 260 °C with shorter annealing time. This is because
we assumed that 190 °C with shorter annealing time such as 20 – 35 min may not sufficient
for Ag films to effectively break down into Ag NPs. On the other hand, annealing
temperature of 260 °C with longer annealing time such as 55 – 100 min may produce much
larger particles which is not suitable for our cells.
In order to compare the absorption in different cells, it was integrated over the spectral
region of interest, namely 500 - 1100 nm, given that for λ < 500 nm all light is absorbed in a
single pass and it does not take part in light-trapping. This integrated value (in arbitrary units)
was used as an absolute measure of absorption as presented in [41]. Absorption values are
calculated by direct integration of the absorption curves and they are not weighed by the solar
spectrum. The initial absorption in the cells prior to NP formation may differ from sample to
sample due to unintentional differences in cell fabrication steps (such as ARC, film thickness,
etc.), which makes comparison between different cells inaccurate. Because of this, the
42
absorption enhancement was then calculated as a difference between the average values of
integrated absorption before and after NP fabrication for the same cell normalised to the
initial absorption. This sort of measure facilitates more accurate comparison of the large
number of samples (48) and ranking their potential for photocurrent enhancement because is
it impractical to process all samples into metallised cells.
Spectrophotometer was used to measure the light absorption in the samples. SEM was
used to characterise NP shape and average particle size and coverage are systematically
calculated by “Image J” software. EQE and illuminated I-V measurements were conducted to
characterise the electrical properties of the metallised solar cells with and without NPs.
3.3.3 Results and discussion
Prior to discussing results of the experiments, sample labelling should be explained. The
first two digits of the sample name refer to the thickness of the Ag precursor film (nm); the
following three digits indicate the annealing temperature (°C); the last two or three digits give
the annealing time (min).
1. Nanoparticle size and coverage differences depending on the thickness of Ag films. Figure 3.3.3.1 shows SEM images of NPs formed from 10, 14 and 20 nm Ag precursor
films. These three samples were annealed at 260°C and for approximately 25 min. Ag
precursor films of different thicknesses (tAg) yield NPs of different size and coverage, as
shown in Figure 3.3.3.1. With increasing tAg, the average particle size increases: the average
particle size of (a), (b) and (c) are 27, 167 and 233 nm, respectively. However, particle
coverage decreases from 45.7% to 41.5% and then to 37.1% respectively.
43
Fig. 3.3.3.1. SEM images of NPs formed from (a) 10 nm, (b) 14 nm and (c) 20 nm thick Ag
precursor films and annealed at 260°C for 25 min (14 nm for 26 min).
Different annealing temperatures and times have shown to yield different NP size and
coverage. Unlike to the case of different Ag precursor film thicknesses, a definite trend could
not be found between the particle size, coverage and annealing temperature and time. In some
samples, the average particle size increased and the particle coverage decreased with
increasing annealing temperature and time. However, not all samples showed this trend. In
addition, these parameters, temperature and time, have less influence over particle size and
coverage compared to that of the Ag precursor film thickness. We assume that this is because
both the annealing temperature and time interact with each other, and both simultaneously
affect to the particle size and coverage. In the other words, for a particular tAg more or less the
(a) (b)
(c)
44
same particle size, shape and coverage can be obtained for different combinations of the
annealing temperature and the time.
2. Absorption enhancements by different nanoparticle fabrication parameters.
Variation in NP size and coverage produces variation in plasmonic light trapping
characteristics. Absorption enhancements from 48 different parameter combinations—
calculated as described above—are compared in Figure 3.3.3.2. Each absorption
enhancement in the graph is an average of 3 measurements. The highest absorption
enhancement is 111.16% achieved for 10_190_70 parameter combination, and 20_190_70
shows the lowest absorption enhancement, 83.11%. As it can be seen, the NPs formed from
tAg = 14 nm have higher absorption enhancements on average than the particles formed from
tAg = 10 and 20 nm. In the cases of tAg = 10 and 20 nm, absorption enhancements fluctuate
more with annealing temperature and time than for tAg= 14 nm. The average sizes and
coverage of the particles formed from tAg= 14 nm are relatively insensitive to annealing
temperature and time; tAg = 10 and 20 nm, however, showed large variation in particle sizes
and coverage with different parameter combinations, explaining the large fluctuation in
enhancement.
45
Fig. 3.3.3.2. Absorption enhancements by NPs fabricated with different Ag thicknesses: (a)
10 nm, (b) 14 nm and (c) 20 nm.
(a)
(b)
(c)
46
Five samples were chosen to be processed into metallised cells to measure Jsc
enhancements. Note that none of these combinations includes samples for tAg = 20 nm, due to
the low absorption enhancements observed for such thick precursor films. Large particles that
result from the 20 nm thick Ag films (an average radius of 233 nm) are known to suffer from
the presence of multiple degenerate modes that reduce the strength of the fundamental
surface plasmon polariton resonance [50]. Therefore, only samples with particles formed
from tAg= 10 and 14 nm were among those with highest absorption enhancement.
Figure 3.3.3.3 shows SEM images of the NPs of the five selected samples, along with the
average particle radius and coverage. Particles formed from Ag layers of the same thickness
show similar size, shape and coverage. However the mean size varies with tAg, annealing
temperature and time.
47
Fig. 3.3.3.3. SEM images of the NPs for the highest absorption enhancements.
(a) 10_190_70 Ave. radius : 40nm Coverage : 41.3%
(b) 14_260_26 Ave. radius : 167nm Coverage : 41.5%
(c) 14_200_70 Ave. radius : N/A Coverage : 44.8%
(d) 10_190_85 Ave. radius : 50nm Coverage : 40.5%
(e) 10_230_40 Ave. radius : 51nm Coverage : 39.1%
48
3. Short-circuit current density (Jsc) enhancement
Figure 3.3.3.4 and Table 3.3.3.1 show the absolute Jsc and the Jsc enhancements of the
selected samples. Jsc was measured before and after NP formation in order to calculate its
enhancement. Each Jsc shown in the graph is an average of six different cells with the same
NPs.
Fig. 3.3.3.4. Jsc (before and after NP formation) and Jsc enhancement of five selected samples
with the highest absorption enhancement.
Jsc without NPs (mA/cm2)
Jsc with NPs (mA/cm2)
Jsc enhancement (%)
10_190_70 13.66 16.69 22.22 10_190_85 13.92 16.60 19.25 10_230_40 14.02 17.48 24.73 14_260_26 13.69 17.89 30.70 14_200_70 13.60 17.61 29.46
Table 3.3.3.1. Jsc (before and after NP formation) and Jsc enhancements of best selected
samples with the highest absorption enhancement.
49
As can be seen in Figure 3.3.3.4, sample 14_260_26 shows the highest Jsc enhancement,
30.7%. On the other hand, sample 10_190_70 shows approximately 10% lower Jsc
enhancement—22.22%—than sample 14_200_70 even though it showed higher absorption
enhancement. Also, two other samples with tAg = 10 nm demonstrate smaller Jsc
enhancements than that of the sample with tAg = 14 nm.
Figure 3.3.3.5 shows the absorption graphs of two samples with NPs formed from 10 and
14 nm thick Ag precursor films. It can be seen, sample 10_190_70 shows higher average
absorption in the 550 - 750 nm range than sample 14_260_26, though beyond 750 nm the
situation reverses. For this reason, two samples may show similar total absorption
enhancements even though their absorption can differ over certain wavelength ranges. This is
likely to be a result of a different particle size, as is described by the Mie theory.
F
50
4. Parasitic absorption in nanoparticles.
For NPs which size is small compared to the incident wavelength and with a dielectric
permittivity ε, dipole modes alone are sufficient to determine the extinction, i.e., the
absorption and scattering [50]. The corresponding cross-sections, Cscat and Cabs, are given by
(1)
(2)
where, a = the radius of NP, nm = the reflective index of the surrounding medium, λ = the
wavelength of incident light and ε = the dielectric permittivity of the particle. According to
the equations above, the scattering and absorption cross-sections are influenced by the
particle size. Figure 3.3.3.6 shows computed values for scattering and absorption cross-
section for Ag NPs as a function of the particle radius. With increasing particle size, Cscat
becomes dominant; on the other hand, Cabs dominates for smaller particles. When the particle
size is 40 and 167 nm and λ = 800 nm, Cscat is greater than Cabs by the factor of 2 and 200
respectively.
51
Fig. 3.3.3.6. Computed values for scattering and absorption cross-section for Ag NPs in air as
a functions of particle radius a.
The average size of the NPs formed by 10_190_70 is 40 nm. For this case, Cabs greater
than Cabs by the factor of only about 2 to, so that a fraction of scattered light is not much
greater than a fraction of absorbed light by NPs. However, the particles formed by sample
14_260_26 are 167 nm, for which Cscat >> Cabs. Therefore, it is possible that the enhanced
light absorption between 550 and 750 nm shown by the NPs formed on sample 10_190_70 is
parasitic absorption in the particles and it does not contribute to enhanced photocurrent.
Another possible reason for this is that absorption at wavelengths longer than 750 nm for the
NPs with tAg = 14 nm is more important for Jsc enhancement than higher absorption at
wavelengths shorter than 750 nm for the NPs with tAg = 10 nm.
52
The EQEs and EQE enhancements of both sample 10_190_70 and sample 14_260_26 are
compared in Figure 3.3.3.7. Without NPs, both cells show similar EQE for λ > 500 nm.
However, sample 14_260_26 shows higher EQE than sample 10_190_70 for λ > 700 nm after
NP formation. This owes to the stronger absorption demonstrated by sample 14_260_26 in
this wavelength range due to better scattering by larger NPs. On the other hand, sample
10_190_70 shows a similar EQE to sample 14_260_26 over the 500 - 700 nm range even
though it absorbs more strongly than sample 14_260_26 in this region. This trend is more
clearly shown in Figure 3.3.3.7 (b) with EQE enhancements for both samples. In this graph,
EQE enhancements in the 300 - 700 nm range are similar in both samples even though
sample 10_190_70 shows greater absorption. Sample 14_260_26, however, shows better
EQE enhancement than sample 10_190_70 for λ > 700 nm due to increased absorption in this
wavelength range. Therefore, it is most likely that the enhanced absorption in the 550 - 750
nm range for sample 10_190_70 is parasitic absorption in the particles themselves or this
wavelength range is less important than longer wavelengths above 750 nm for sample
14_260_26. Therefore, the NPs formed from precursor film with tAg = 14 nm are more
suitable for improving cell performance than those formed with tAg = 10 nm.
53
Fig. 3.3.3.7. (a) EQEs and (b) EQE enhancements of samples 10_190_70 and 14_260_26.
(a)
(b)
54
The optimum particle size should lead to resonance at wavelengths of a sufficiently high
photon flux in the AM 1.5G solar spectrum and a moderate absorption coefficient in Si, for
which the optical path length enhancement is most beneficial. This corresponds to the range
of 550 - 1100 nm where the light attenuation depth is two to a few tens of microns, i.e., one to
a few times the cell thickness. The NPs formed from the 14 nm thick Ag films meet these
criteria, resulting in the best photocurrent enhancement [49].
5. Matching orders of absorption enhancement and Jsc enhancement
In the selection process above, the highest absorption enhancement did not give the
highest total Jsc enhancement. Therefore, it is useful to find a selection, or ranking criterion
based on absorption enhancement for estimating the parameter combination which gives the
highest Jsc enhancement. This is because it is impractical to fabricate a very large number of
metallised cells including all parameter combinations. When the absorption enhancement
from any particular wavelength range is matched to the total Jsc enhancement, the
selection/ranking criterion can be set.
NPs formed from tAg = 14 nm showed better Jsc enhancement due to stronger absorption
at 700 - 1100 nm. In addition, the EQE enhancement in the 300 - 700 nm range is similar for
samples with tAg = 10 nm and 14 nm. In order to find if there is any particular absorption
wavelength range, where the absorption enhancement and Jsc enhancement are in the same
order, the absorption enhancement for all 48 samples was re-calculated for the wavelength
ranges of 700 - 1100 nm.
In Table 3.3.3.2, total Jsc enhancements of the top five samples from table 3.3.3.1 are
compared with the absorption enhancement in 500 - 1100 nm and 700 - 1100 nm wavelength
ranges separately. As it can be seen, the ranking order of total Jsc enhancement does not
match that of the absorption enhancement for the 500 - 1100 nm range. On the other hand,
55
the ranking order of total Jsc enhancement matches well that of the absorption enhancement
for the 700 - 1100 nm range. Therefore, we can assume that the sample which shows the
highest absorption enhancement in 700 - 1100 nm wavelength range has the best potential to
achieve the highest Jsc enhancement.
Sample Lot. Jsc enhancement (%), (ranking)
Abs_enh at 500 - 1100 nm (%), (ranking)
Abs_enh at 700 – 1100 nm (%), (ranking)
10_190_70 22.22 (4) 111.2 (1) 239 (4) 10_190_85 19.25 (5) 110.1 (2) 207 (5) 10_230_40 24.73 (3) 109.1 (3) 251 (3) 14_260_26 30.70 (1) 107.9 (4) 316 (1) 14_200_70 29.46 (2) 106.5 (5) 291 (2)
Table. 3.3.3.2. Comparison of Jsc enhancement ranking order and the ranking order of
absorption enhancement at 500 - 1100 nm and 700 - 1100 nm.
After re-calculation, the top four samples which show highest absorption enhancement in
700 - 1100 nm range were selected. Table 3.3.3.3 shows the integrated absorption (before and
after NP formation) and absorption enhancements of the selected samples. The highest
absorption enhancement at 700 - 1100 nm is achieved by 14_230_53, 330%. Interestingly,
absorption enhancement decreases with increasing average particle radius.
Sample Lot Absorption
Table. 3.3.3.3. The top four parameter combinations selected from re-calculation of
absorption enhancement in 700–1100 nm range.
56
In order to confirm Jsc enhancement by the re-ranked samples, the top four samples which
show highest absorption enhancement were processed into metallised cells. Figure 3.3.3.8
and Table 3.3.3.4 show absolute Jsc (before and after NP formation) and the Jsc enhancements
for the selected samples. Each Jsc shown in the graph is an average of six different cells with
the same NPs.
Fig. 3.3.3.8. Jsc (before and after NP formation) of the selected parameter combinations based
on the re-calculated absorption enhancement in 700–1100 nm range and their Jsc
enhancements.
Jsc enhancement (%)
Absorption enhancement
ranking 14_230_53 13.42 17.91 33.5 1 14_200_60 13.99 18.46 31.9 2
14_190_100 13.98 18.35 31.2 3 14_260_26 13.69 17.90 30.8 4
Table. 3.3.3.4. Jsc (before and after NP formation) of the selected parameter combinations
based on the re-calculated absorption enhancement in 700–1100 nm range and their Jsc
enhancements.
57
It can be seen, sample 14_230_53—which showed the highest absorption enhancement in
700-1100 nm range upon re-calculation—demonstrates the greatest Jsc enhancement, 33.5%
on average. In addition, the ranking of Jsc the enhancements of other samples are consistent
with the absorption enhancements. This indicates the validity of estimating Jsc enhancement
by calculating absorption enhancement in the 700 - 1100 nm range. Therefore, it can be
concluded that an average particle size of about 157 nm is the optimum size in this
experiment for maximising photocurrent enhancement. Also, particle shape, size and
coverage of the four best samples are similar, as shown in Fig 3.3.3.
Fig. 3.3.3.9. SEM images of selected samples based on the re-calculation.
14_230_53 Ave. radius : 157 nm Coverage : 41%
14_200_60 Ave. Size : 162 nm Coverage : 48.3%
14_190_100 Ave. Size : 166 nm Coverage : 41.3%
14_260_26 Ave. Size : 167 nm Coverage : 41.5%
58
Due to the similar characteristics of these NPs, there is no significant difference between
their Jsc enhancements. Consequently, it can be said that sample 14_230_53 is the best
parameter combination for enhancing Jsc of the cells.
6. Illuminated I-V measurement
We also measured the illuminated I-V to confirm efficiency enhancements by applying
the best NP parameter combinations. The results of the measurements are shown in Table
3.3.3.5. Note that the cells were measured without a back reflector (except best cell
14_230_53) and some light is lost due to transmission through the cells. With a back reflector,
such as white paint typically used for poly-Si cells, both Jsc and the efficiencies are expected
to be about 10-15% higher as it can be seen for cell 14_230_53. The cell efficiency is very
much dependent on the quality of cell metallisation which can unintentionally vary from cell
to cell. Therefore, the initial efficiencies of the cells in Table 3.3.3.5 are not the same. For
this reason, it is preferable to compare efficiency enhancements. As can be seen in Table
3.3.3.5, sample 14_230_53 shows the highest efficiency enhancement, 32%, as well as the
highest efficiency, 5.32%. It also shows the highest Jsc enhancement, the same seen in EQE
measurements. In addition, the ranking of Jsc enhancements from the illuminated I-V
measurement reproduces the results of the previous EQE measurements.
There is a slight but consistent increase in both Voc and Rs for the samples in Table 3.3.3.5
after NP formation. The reasons for such increase are not apparent and they are being
currently investigated.
14_230_53_With NPs 5.32 446 18.88 63.1 Infinite 2.18 32.85 32.00
14_230_53_w/o NPs 4.03 441 14.21 64.2 Infinite 1.44
14_230_53_With NPs
+ back reflector 5.95 446 20.91 63.8 Infinite 2.10 45.81 47.64
14_200_60_With NPs 5.07 449 18.85 59.9 Infinite 3.5 32.36 25.50
14_200_60_w/o NPs 4.04 442 14.24 64.1 Infinite 3.14
14_190_100_With
32.15 29.19
14_260_26_With NPs 5.27 448 18.75 62.7 Infinite 2.04 31.74 28.85
14_260_26_w/o NPs 4.09 446 14.23 64.4 Infinite 1.29
Table. 3.3.3.5. Efficiency (before and after NP formation) of the selected parameter
combinations measured by illuminated I-V, and their efficiency enhancements.
Figure 3.3.3.10 shows the illuminated I-V curves for best performing sample 14_230_53,
which showed the highest Jsc and efficiency enhancement. Jsc increases from 14.21 mA/cm2
to 18.88 mA/cm2, or 32.7%. This enhanced Jsc and efficiency is achieved without a back
reflector and the efficiency of 5.95% is achieved with a detachable back reflector (GORE
Diffuse reflector: www.gore.com/MungoBlobs/659/489/DRP-brochure.pdf].
Fig. 3.3.3.10. I-V curves of 14_230_53, which showed the highest Jsc and efficiency
enhancement.
Cell efficiency enhancements are similar to their Jsc enhancements. However, sample
14_200_60 shows the lowest efficiency enhancement even though its Jsc enhancement is
higher than that of both 14_190_100 and