COMPARISON OF NOVEL AND STATE OF THE ART

SOLAR CELLS

By

Adegbenro Ayodeji

A Thesis submitted to

The Faculty of Engineering at

Cairo University and University of Kassel

in Partial Fulfilment of the

Requirements for the Degree of

Master of Science

in

RENEWABLE ENERGY AND ENERGY EFFICIENCY

Faculty of Engineering

University of Kassel, Kassel, Germany

Cairo University, Giza, Egypt

June 2016

COMPARISON OF NOVEL AND STATE OF THE ART

SOLAR CELLS.

By

Adegbenro Ayodeji

A Thesis submitted to

The Faculty of Engineering at

Cairo University and University of Kassel

in Partial Fulfilment of the

Requirements for the Degree of

Master of Science

in

RENEWABLE ENERGY AND ENERGY EFFICIENCY

Under supervision of

Prof. Dr. Nadia H. Rafat

Department of Engineering

Mathematics and Physics, Faculty of

Engineering, Cairo University

Prof. Dr. Hartmut Hillmer

Director, Institute of Nanostructure

Technologies and Analytics

University of Kassel, Germany

Prof. Dr. sc. techn. Dirk Dahlhaus

Head, the Department of Telecommunications Kassel University, Germany

June 2016

COMPARISON OF NOVEL AND STATE OF THE ART

SOLAR CELLS.

By

Adegbenro Ayodeji

A Thesis submitted to

Faculty of Engineering at Cairo University and Kassel University

in Partial Fulfilment of the Requirements for the Degree of

Master of Science

in

RENEWABLE ENERGY AND ENERGY EFFICIENCY

Approved by the Examining Committee

Prof. Dr. Hartmut Hillmer Thesis Advisor and Member

University of Kassel, Germany

Prof. Dr. Nadia H Rafat Thesis Advisor and Member

Faculty of Engineering, Cairo University

Prof. Dr. Dirk Dahlhaus Member

Chair of communication laboratory, Kassel University

Prof. Dr. Sayed Kaseb Member

Faculty of Engineering, Cairo University

Prof. Dr. Adel Khalil Member

Faculty of Engineering, Cairo University

JUNE 2016

Declaration

I, Ayodeji Adegbenro, hereby assure that this master thesis is my own written work and that I

have used no other sources and aids others than those indicated. To the best of my

knowledge I do hereby declare that this thesis is my own work. It has not been submitted in

any form of another degree or diploma to any other university or other institution of

education. Only the sources cited have been used. Those parts which are direct quotes or

paraphrases are identified as such.

Kassel, Germany June, 2016 _____________________

Place Date Signature

i

Acknowledgement

First and foremost, I would like to thank God almighty who has given me the grace and

strength till this very day of my master program. He has been faithful all along.

I would also like to thank my supervisors Prof. Dr. Hartmut Hillmer, Universität Kassel and

Prof. Dr. Nadia H. Rafat, Cairo University who helped to bring this research work to

realization. I am very grateful for their help, valuable guidance, consistent advice and

professionalism throughout my research work, I can not thank them enough.

Many thanks to my course mates, REMENA program advisers and my colleagues at ZAE-

Bayern for their concern and encouragement, I do greatly appreciate them.

Finally, I do express very earnest gratitude to my parents and families for providing me with

unfailing support and continuous encouragement throughout the duration of my master

program. This huge accomplishment would not have been possible without them. I sincerely

Thank you.

ii

Table of Content

1 Chapter One: Introduction .............................................................................................. 1

1.1 World Energy Crisis................................................................................................... 1

1.2 Global Warming......................................................................................................... 1

1.3 Solar Energy............................................................................................................... 2

1.3.1 Nuclear fusion in the sun (Proton-Proton reaction) ............................................... 3

1.3.2 Solar cells ............................................................................................................... 3

1.3.3 Main types of solar cells ........................................................................................ 4

1.3.4 Photovoltaic Effect: - Power generation ................................................................ 5

1.4 Electronic characteristics of solar cells ...................................................................... 6

1.4.1 Photo current and Quantum efficiency .................................................................. 6

1.4.2 Short circuit current and open circuit voltage ........................................................ 6

1.4.3 Efficiency ............................................................................................................... 7

1.4.4 Standard model of solar cells ................................................................................. 8

1.4.5 Photovoltaic market ............................................................................................... 9

2 Chapter Two: Theory ..................................................................................................... 11

2.1 Basic structure of a solar cell ................................................................................... 11

2.2 State-of- the- art production method of compared technologies ............................. 12

2.2.1 Silicon based solar cells (Crystalline silicon solar cells) ..................................... 12

2.2.2 Chemical compound based solar cells (In-organic and Organic) ........................ 15

2.2.3 Hetero-junction with intrinsic thin layer solar cell (Novel technology) .............. 19

2.3 Geometrical, compositional and operational data of the compared technologies ... 20

2.3.1 Mono-crystalline solar cells ................................................................................. 20

2.3.2 Poly-crystalline solar cells ................................................................................... 22

2.3.3 Amorphous solar cells.......................................................................................... 23

2.3.4 Inorganic solar cells (Based on the use of CdTe/CdS, CIS/CIGS and GaAs) ..... 23

2.3.5 Organic solar cells and Dye sensitized solar cell ................................................. 25

2.3.6 Heterojunction intrinsic thin layer solar cells (Novel technology) ...................... 27

3 Chapter Three: Heterojunction intrinsic thin layer solar cell .................................... 31

3.1 Design concept of hetero-junction intrinsic thin layer solar cell (HIT) ................... 31

3.2 Key features of HIT solar cell.................................................................................. 32

iii

3.3 Simulation, result and discussion of HIT solar cell ................................................. 33

3.4 Future prospects HIT solar cells .............................................................................. 39

4 Chapter Four: Weighing-factors ................................................................................... 40

4.1 Module cost of production and raw materials.......................................................... 40

4.2 Analysis of efficiency for evaluated technologies over the year till date ................ 43

4.3 Temperature effect on performance of evaluated cells ............................................ 45

4.4 Life cycle analysis and Energy analysis of evaluated solar cells ............................. 46

4.5 Environmental Issue (Toxicity and Recycling) ....................................................... 48

4.6 Sustainability............................................................................................................ 50

5 Chapter Five: Discussion and Conclusion .................................................................... 52

6 References........................................................................................................................ 54

iv

LIST OF TABLES

Table 2.1: Compositional data of compared technologies ...................................................... 27

Table 2.2: Current operational data of compared technologies based on modules ................. 28

Table 2.3 Features of evaluated cell ........................................................................................ 30

Table 3.1: Device parameters used as an input for computer simulation [33] [34] ................ 34

Table 3.2 Effect of back surface field thickness ..................................................................... 37

Table 4.1 Data used in determining the cost of production of modules.................................. 42

Table 4.2: Energy analysis of compared PV modules ............................................................ 48

Table 4.3: Major Hazards in evaluated PV technologies ....................................................... 50

v

LIST OF FIGURES

Figure 1.1 : Total anthropogenic GHG (GtCO2eq / yr) by different economic sectors. See

Ref [1] ........................................................................................................................................ 2

Figure 1.2 : Efficiency Comparison of Technologies: Best Lab Cells vs. Best Lab Modules.

See Ref [3] ................................................................................................................................. 4

Figure 1.3 : Light incident on the cell creates electron-hole pairs, which are separated by the

potential barrier, creating a voltage that drives electrons through an external circuit. See Ref

[4] ............................................................................................................................................... 5

Figure 1.4 : Graph showing short circuit current–open circuit voltage and power–voltage

solar cell characteristics, including the maximum point of both current and voltage (IMPP-

VMPP) which gives the maximum power point MPP. See Ref [5]........................................... 8

Figure 1.5 : Equivalent circuit for an ideal solar cell is represented by the full line. Dotted

line shows the non-ideal components. See Ref [6] .................................................................... 8

Figure 1.6 : Average end customer price (net system price) for installed rooftop systems with

rated nominal power from 10 - 100 kWp. See Ref [9] ............................................................ 10

Figure 2.1: Growth method of mono-crystalline silicon cell. It's called 'Czochralski growth

method'. See Ref [11]............................................................................................................... 13

Figure 2.2: Float zone growing method for manufacturing polycrystalline cell. See Ref [11]

.................................................................................................................................................. 13

Figure 2.3: Edge-defined film edge growth (EFG) is the growth method for amorphous

silicon cell. See Ref [11] .......................................................................................................... 14

Figure 2.4: Crystalline Silicon solar cell generic process diagram. See Ref [12] ................... 14

Figure 2.5: Cadmium Telluride (CdTe) solar cell generic process diagram. See Ref [12] .... 16

Figure 2.6: Copper Indium Gallium Selenide/Cadmium Sulphide solar cell generic process

diagram. See Ref [12] .............................................................................................................. 16

Figure 2.7: Gallium Arsenide (GaAs) solar cell generic process diagram. See Ref [12] ....... 17

Figure 2.8: Schematic construction of a glass/glass nanocrystalline dye sensitized cells. See

Ref [14] .................................................................................................................................... 18

Figure 2.9: Some organic molecules commonly used organic solar cells: ZnPc (zinc-

phthalocyanine), Me-Ptcdi (N,N’-dimethylperylene- 3,4,9,10-dicarboximide), and the

Buckminster fullerene C60.See Ref [29] ................................................................................. 19

Figure 2.10: Schematic representation of the process steps in fabricating HIT solar cell. See

Ref [18] .................................................................................................................................... 20

vi

Figure 2.11: Cross-sectional of a monocrystalline silicon solar cell with screen printed

contacts and another with buried contacts. See Ref [22] ......................................................... 22

Figure 2.12: Working principle of the multi-junction solar cells. See Ref [27] ..................... 25

Figure 2.13: Operating mechanism of organic solar cell. See Ref [27] .................................. 26

Figure 2.14: Schematic representation of a dye-sensitized solar cell. See Ref [28] ............... 26

Figure 2.15: Evaluated technologies layers outline ............................................................... 29

Figure 3.1: Comparisons between the HIT (a) and c-Si energy band gap (b). See Ref [30]. . 32

Figure 3.2: Key feature of HIT solar cell. See Ref [31].......................................................... 33

Figure 3.3: HIT Structure proposed by SANYO electric. See Ref [32] ................................. 33

Figure 3.4: Schematic diagram of simulated HIT solar cell using PC1D ............................... 35

Figure 3.5: Simulated current, voltage and power curve of HIT solar cell ............................. 35

Figure 3.6: Influence of the intrinsic layer thickness on efficiency....................................... 36

Figure 3.7: Impact of doping concentration of the N-type amorphous layer on efficiency

and short circuit current ........................................................................................................... 37

Figure 3.8: Impact of doping concentration of the N-type amorphous layer on open circuit

voltage ...................................................................................................................................... 38

Figure 3.9: Impact of wafer thickness on efficiency............................................................... 38

Figure 3.10: Influence of the surface area on the I-V characteristic ....................................... 39

Figure 4.1: Projections for cell and module efficiencies for 2004 to 2030 by The Japan PV

roadmap toward 2030. See Ref [37]. ....................................................................................... 44

Figure 4.2: Global potential map of PV energy generation (Ypy) of c-Si PV module [41] ... 45

Figure 4.3: Solar cells efficiency chart. NREL 2016 .............................................................. 45

Figure 4.4: Life-cycle stages of a PV module (Emtl = material production energy, Emfg =

manufacturing energy, Esol = insolation; Euse = use energy requirements, Eelm = end of life

energy, Edstr = distribution energy, Egen = energy generated).See Ref [39] .............................. 46

vii

NOMENCLATURE

SYMBOL UNIT DESCRITPION

α 1/cm Absorptivity coefficient

% Efficiency

°C - Degree Celsius

°F - Degree Fahrenheit

</> - Less than/Greater than

Al2O3 - Aluminum Oxide

bs (E) W/m2 Incident flux photon density

C - Carbon

CdTe - Cadmium Telluride

CO2 - Carbon dioxide

Cm - Centimeter

ct € Cent

CuInSe 2 (CIS) - Copper indium selenide

CuInGaSe 2 (CIGS) - Copper indium gallium selenide

e - Electron

eV - Electron Volt

FF % Fill Factor

GaAs - Gallium Arsenate

GHG - Greenhouse gas

GtCO2eq / yr - Giga-tonne Carbon-dioxide equivalent per year

GW - Giga-Watt

InP - Indium Phosphide

Isc A Short circuit current

Jsc A/cm2 Short-circuit current density

Jm A Maximum current

K - Degree Kelvin

KB eV⋅K−1 Boltzmann constant

kg - Kilogram

k/TWh - Kilo/Tera-Watt hour

kWh/m2 - Kilo-Watt hour per meter square

mm - Millimeter

Mev - Mega electron-volt

Mio.T - Million tonnes

m - Micrometer

m/kWp - Mega/Kilo-Watt peak

nm - Nanometer

p-i-n - Positive-Intrinsic-Negative

Pin W/m2 Incident radiation power

P/N - Positive/Negative

viii

PV - Photovoltaic

q C Electronic charge

QE % Quantum efficiency

RPM - Rounds per minute

Si - Silicon

T °C Temperature

TiO2 - Titanium Oxide

SiO2 - Silica

Si3N4 - Silicon Nitride

Voc V Open circuit voltage

Vm V Maximum voltage

Wm2 - Watt-meter square

ZnS - Zinc Sulphide

ix

ABSTRACT

This research is based on the comparison of novel and existing state of the art solar cells,

focusing on the materials, methods used for fabricating the evaluated photovoltaic solar cells,

life cycle (energy analysis), sustainability and cost. The technologies discussed in this

research are silicon based solar cells (monocrystalline, polycrystalline and amorphous), and

non-silicon based solar cells (organic and in-organic). This thesis also describes the physics

and theory behind the new type of silicon based solar cell called heterojunction intrinsic thin-

layer solar cell (HIT) which has been recorded to be cheaper in production and more efficient

than the crystalline solar cells. It is a combination of two different semiconducting materials,

which are hydrogenated amorphous silicon and crystalline silicon (a-Si:H and c-Si) to create

a p/n-junction. From literature, aluminum doped zinc-oxide (ZnO:Al) is usually used as the

transparent conducting oxide, then followed by layers of p-type amorphous silicon (emitter),

intrinsic amorphous silicon (passivator), n-type crystalline silicon (absorber), intrinsic

amorphous silicons (passivator) and n-type hydrogenated amorphous silicon (back surface

field). Aluminum or silver is used as the front and back contacts. Also from literature it is

stated that the methods of deposition are plasma enhanced or hot-wire chemical vapour

deposition (PE/HW-CVD) and for the contacts, the deposition method is sputtering. This

thesis also includes the environmental factors of solar cells from production to

decommissioning, toxicity and shortage of the materials, CO2 emission and other

environmental issues.

Keywords HIT solar cells, crystalline silicon solar cells, thin films solar cells, cost,

efficiency, life-cycle and energy analysis, materials, design and compositional data.

1

1 Chapter One: Introduction

1.1 World Energy Crisis

Energy demand is rapidly increasing in the world today, particularly in the demand of

electricity for households, commercial and industrial purposes. This rise is proportional to an

increase in the pace of developments in science and engineering, growing economies and in

the world’s population, which is predicted to increase by 25% in the next 20 years. The

affinity for oil, natural gas, coal and other non-renewable energy sources will increase

globally by 40% by 2035 with a decrease in energy resource. Quality of life and global

economy will be affected, with majority of the demand will be from countries like China and

India. The possibility of a huge humanitarian crisis is evident. The reserves of oil and gas

have decreased by 40% and 70% respectively, which proves that someday the world would

run out of conventional energy sources. In order to avoid this, new oil and gas fields and

other conventional sources should be discovered and better cost-effective technology should

be developed to utilize the un-ending source of renewable energy. Due to this, energy policy

makers in European countries like Germany are investing huge amount of money on

renewable energy technologies. The United States of America and the United Kingdom have

also promoted bio-fuel as an alternative to fossil fuel. The current world oil and gas reserves

can provide energy for a certain period of time at least 30 years. Within this period, in order

to preserve the climate from drastic changes and utilizing fossil fuel efficiently, specific

policies must be put into effect; energy efficiency and energy conservation, research and

development of sustainable energy technologies. There should be an agreement platform for

developed and developing countries to strive in CO2 emission and global warming.

1.2 Global Warming

Climate change is today’s biggest threat to the human existence on earth. Planetary changes

like anomalous rainfall pattern, vigorous droughts in some areas and increased rain in others,

melting of ice cape and rise in sea level are signs of global warming. The world temperature

is said to increase by 4°C by 2100, which will increase sea level by 30-100cm which is

enough to drown islands like the United Kingdom. Major steps have to be executed in other

to make the planet habitable. Global warming is simply a product of energy use. About 90%

of the world's energy is supplied through the combustion of fossil fuels (coal, oil, gas). When

this fuels are burnt to make energy, electricity for example, a unit (kWh) of electricity used

produces 0.5 kg of CO2. For transportation, a liter of petrol burnt contributes 3kg of CO2.

Carbon dioxide which is the dominant gas in greenhouse gases including methane, nitrous

oxide and fluorinated gases are responsible for global warming. In Figure 1.1, green house

emission by different sector is analyzed. The Circle shows direct percentage share of GHG

emission of five economic sectors. The extended part of electricity and heat production is

related to sectors of final energy consumption. ‘Other Energy’ means all other minor GHG

emission sources. AFOLU means emissions from Agriculture, Forestry and Other Land

include land-based CO2 emissions from forest fires and peat fires. Energy use and climate

2

change are two sides of the same coin. [Tony Blair, 2006]. As said earlier, global, economic

and population growth continue to be the most important drivers of increases in CO2

emissions from fossil fuel combustion because it leads to electricity demand. Electricity and

heat production emit more CO2 than any other sector in an economy. Greenhouse effect,

which occurs when CO2 and other atmospheric gases (nitrogen, oxygen, water vapour,

methane, nitrous oxide, and ozone) combine in large volume and form a cloud blanket over

the earth surface trapping more sun light radiation which should have been freely escaped

beyond the troposphere at a longer wavelength. The trapped long wavelength warms the earth

beyond required temperature. These greenhouse gases act like a mirror that reflects the heat

energy of the Sun’s radiation back to earth instead of the radiation escaping. Currently, most

of the CO2 in the atmosphere are due to human activities and certain measures must be taken

to cub such activities in order to have a habitable planet. Few countries like Denmark and

Germany are stabilizing and working to reduce their CO2 emission. Curbing and reducing

CO2 emission can not be addressed independently, but by interconnected global platform,

where policies will be led in order to reduce the emission of CO2 and one of the fastest

growing means is adopting renewable energy.

Figure 1.1 : Total anthropogenic GHG (GtCO2eq / yr) by different economic sectors. See

Ref [1]

1.3 Solar Energy

For billions of years, the sun has been the source of energy flow for survival in the eco-

system. The utilization of the sun’s energy started out by concentrating the sun’s heat with

glass and mirrors to light fires and today, cars, houses and electronic gadgets are solar-

powered. The energy from the sun is free but device use in converting this energy isn’t free.

There has been so many research and developments on different technologies to harness the

3

energy from the sun but the available technology produces about one tenth of one percent of

global energy demand. The sun radiates energy in the form of electromagnetic radiation in

space and the amount that reaches the earth is about 1kW/m2 depending on the latitude and

location weather pattern. The inner and surface temperature of the sun is about 2.0 × 107K

and 6000K respectively. The huge amount of temperature and unlimited energy from the sun

is produce through some fusion reaction or proton-proton reaction which occurs in core of the

sun. Three quarters of the Sun consists of hydrogen, with helium being mostly the rest and

smaller quantities of heavier elements like oxygen, carbon, neon and iron.

1.3.1 Nuclear fusion in the sun (Proton-Proton reaction)

The nuclear fusion which occurs in the sun also known as the proton-proton reaction

generates tremendous amount of energy. The energy radiated by the sun is about 4.0×1020 J/s.

The energy is a result of change in total mass of the atoms which follow Einstein’s

relationship between mass and energy; E = mc2 where E is the energy of a system, m is the

mass of the system, and c is the speed of light in a vacuum (about 3×108 m/s) [2]. The

reaction consumes millions of tonnes of hydrogen. The energy radiated by sun into space is

interlope by the earth, ranging from ultra violet radiation to infra-red radiation. Equations (1),

(2) and (3) explains the proton-proton reaction.

H11 + H1

1 → H12 + e+

10 +𝞾+ 0.42 Mev (1)

H12 + H1

1 → He23 + 𝞬+ 5.5 Mev (2)

He23 + He2

3 +→ He24 ++ 2 H1

1 + 12.8 Mev (3)

The proton-proton chain reaction in the sun releases a net energy of 26.732 MeV [2]. The

first equation involves fusing of two protons ( H11 ) resulting into a formation of deuterium

( H12 ), positron (e) and a neutrino (𝞾). The second step, which involves the deuterium ( H1

2 )

fusing into a proton, ( H11 ) to produce an isotope of helium-3, ( He2

3 ) and a gamma ray (𝞬).

Finally fusing two isotopes helium-3, ( He23 ) to generate helium-4, ( He2

4 ) and two protons,

( H11 ) takes place.

1.3.2 Solar cells

Solar cells are electronic devices which directly convert sunlight into electricity. When solar

radiation shines on a solar cell, it produces both current and voltage to generate electricity.

For this process to occur, materials that can absorb light and generate electron-hole pairs are

required. A good example is Silicon (Si). It’s the most common type of semi-conductor used

in fabricating solar cells and other electronic devices. Currently in the market there exists

three generations of solar cells used to harness the sun’s energy. The details of the three

generation will be discussed later in this thesis. The first generation solar cells have a higher

market share in comparison to the other generations. This is due to its high conversion

efficiency and due to its well established industry. Figure 1.2 shows the reason why the first

generation solar cells are still dominating the PV market. It explains the efficiency at module

and cell level. The majority of the first generation are made up of crystalline silicon and this

4

involves huge amount of energy in fabrication in comparison to other generations. The

amount of energy involved in fabrication cells affects the energy payback period and the

environment. That is, a cell that requires low energy in production has a faster energy

payback period and benefits the effect of PV on the environment. This is where the second

and third generation solar cells come in. Scientist attempt to solve the problem of high energy

consumption during fabrication by developing low temperature means of fabrication which

will therefore quicken the energy payback period and reduce the cost of purchasing solar

cells. One thing to consider about the second generation and third generation is their

efficiency and life span which is lower than the first generation solar cells.

Figure 1.2 : Efficiency Comparison of Technologies: Best Lab Cells vs. Best Lab Modules.

See Ref [3]

1.3.3 Main types of solar cells

In recent years, there have been many different solar cell designs and different materials are

being studied so as to increase the efficiency of converting solar energy into electricity.

Researches have gone as far as developing materials to produce power for spacecraft by

utilizing the solar radiation. The details of fabricating different types of solar cells are based

on solid-state physics, chemistry, and material science. Nowadays, in the market there are

majorly crystalline silicon technologies, thin-film technologies and other technologies

(organic solar cells i.e. dye-synthesize solar cells). In early 90’s, Sanyo, a Japanese company

now owned by Panasonic fabricated a solar cell called “heterojunction intrinsic thin layer

solar cell” (HIT), which is a novel idea and will be explained in details in this thesis. It’s a

combination of extrinsic amorphous and extrinsic crystalline silicon semiconductor with thin

(20nm) intrinsic silicon in between them. In 2014, the HIT solar cell reached an efficiency of

25.6% [Panasonic-HIT February 2013]. More researches are being carried out to improve the

cost-effectiveness of HIT solar cells for future use. The advantage of HIT solar cells over

crystalline solar cells is that it can be fabricated under low temperature (~ 200-300°C). This

allows the cells to be made on the silicon wafer of about 70nm in thickness without

5

destroying it. The low temperature process reduces the energy need for production, thereby

reducing the energy payback time and increasing the beneficial effect of photovoltaics on the

environment.

1.3.4 Photovoltaic Effect: - Power generation

In 1839, Edmond Becquerel was the first person to realize that when radiation from the sun

shines on a solid material, there could be a flow of electrons in that solid material, e.g. Silicon

to produce electricity. Silicon in its original state, silica (SiO2) isn’t pure enough to produce

electricity. It has to go through purification process to have intrinsic (pure) semiconductor

silicon. The unique features of semiconductors are their electrical conductivity which could

be controlled by doping, optical properties and photo-conductivity. The resulting pure silicon

is then doped with trivalent atoms or pentavalent atoms to produce a deficiency of electrons

(p-type)or an excess of electrons (n-type ) respectively to make a semiconductor capable of

conducting electricity more. The silicon ends up being glossy so an anti- reflecting coating is

used to reduce reflection of solar radiation on the surface. Photovoltaic occurs in its simplest

form, when light Photons having energy greater or equal to the band gap energy of the

semiconductor are absorbed. The photon energy strikes the electron in the valence band,

accelerating it to the conduction band. This leaves behind a positively charged hole particle,

which is a way to theorize the interaction of electrons in the vicinity of an unfilled electron

state. Then, the built- in field (due to p-n junction) pulls the excited electrons away before

they can relax, and feeds them into an external circuit-load. In crystalline silicon solar cells,

the generation of the electron-hole pair occurs mainly in the p-type layer. In the second

generation solar cells, the electric field spreads through the whole device and the electron-

hole pair is immediately separated after generation and if the photo-generated charge carriers

are have not recombined in the process, they arrive at the terminals of the device, generating

an electric charge across them. Figure 1.3 shows the creation of the electron-hole pairs, which

creates a voltage that drives the electrons to an external load.

Figure 1.3 : Light incident on the cell creates electron-hole pairs, which are separated by the

potential barrier, creating a voltage that drives electrons through an external circuit. See Ref

[4]

6

1.4 Electronic characteristics of solar cells

1.4.1 Photo current and Quantum efficiency

Photocurrent Jsc, generated by a solar cell at short circuit condition is dependent on the

photon flux. To relate the current and photon flux of a solar cell, a good understanding of the

cell quantum efficiency QE is required. Solar cell quantum efficiency is the probability that a

photon of certain energy E, can deliver an electron to an external load from the solar cell.

𝐽𝑠𝑐 = 𝑞 ∫ 𝑏𝑠 (𝐸)𝑄𝐸(𝐸)𝑑𝐸 (4)

Where q is the electronic charge in coulombs, E is the energy of incident photon, b s (E) is the

incident photons flux density incident on a unit area per time having energy ranging from E

to E+ dE. QE is determined by the solar cell material absorptivity, it’s independent of the

incident spectrum. QE and the incident spectrum can be given as a function of either photon

energy E, as in equation (4) or spectrum wavelength λ.

1.4.2 Short circuit current and open circuit voltage

The open circuit voltage (Voc) is the voltage between the terminals when zero amount of

current is drawn or load resistance is infinite as shown in Figure 1.4. The short circuit current

(Isc), is the current when the terminals are connected to each other with no load resistance as

shown in Figure 1.4. They are the most important characteristic of a solar cell. Short circuit

current increases with concentration of photons. Increase in photons concentration means

more photons on a unit square area of the cell, which in turn means more electrons and holes

generated. Since the short circuit current Isc is proportional to the area of the solar cell,

therefore the short circuit current density can be seen as;

𝐽𝑠𝑐 =𝐼𝑠𝑐

𝐴 (5)

When current flows through the load a voltage difference between the two terminals is

created then a dark (diode) current flows in opposite direction to the short circuit current.

𝐽𝑑𝑎𝑟𝑘 (𝑉) = 𝐽𝑜 (𝑒𝑞𝑉

𝐾𝐵𝑇 − 1) (6)

Whereby J0 is the reverse saturation current of the p-n junction and it depends on the doping

and on the properties of the semiconductor, KB is Boltzmann constant, T is temperature, q is

the electron charge and V is the voltage between the poles. The net current, 𝐽 can be given as

a superposition of the short circuit current and the dark current:

𝐽 = 𝐽𝑠𝑐 − 𝐽𝑜 (𝑒𝑞𝑉

𝐾𝐵𝑇 − 1) (7)

This then leads us to the open-circuit voltage, Voc, which is the maximum voltage available

from a solar cell with no current available

𝑉𝑜𝑐 = 𝑞𝑉

𝐾𝐵 𝑇ln (

𝐽𝑠𝑐

𝐽𝑜+ 1) (8)

7

Figure 1.4 shows the J-V characteristics of an ideal solar cell following equation (7) after

multiplying by the area to present the currents in Amps.

1.4.3 Efficiency

The solar cell conversion efficiency is used to compare the performance of solar cells. It’s the

ratio of output power from the solar cell to input solar radiation power. The performance of a

solar cell depends on the spectrum & intensity of the incident sunlight, air mass and the

temperature of the testing location.

𝜂 =𝑃𝑑𝑚

𝑃𝑖𝑛=

𝑉𝑜𝑐 𝐽𝑠𝑐 𝐹𝐹

𝑃𝑖𝑛 𝑥 100 (9)

Pin is the incident radiation power density which is 1 kW/m2 for AM 1.5 radiation and 𝑃𝑑𝑚 is

the maximum power density of the solar cell. The power density is the product of voltage V

and current density J, we get power density to be:

𝑃 = 𝐼𝑉; 𝑃𝐷 = 𝐽 𝑉 (10)

The maximum power density is;

𝑃𝑑𝑚 = 𝐽𝑚𝑉𝑚 (11)

Fill factor, FF is defined to be the ratio of the maximum power generated by the cell divided

by open circuit voltage and short circuit current. We have;

𝐹𝐹 =𝐽𝑚 𝑉𝑚

𝑉𝑜𝑐 𝐽𝑠𝑐 (12)

Jm and Vm are the current density and voltage respectively, at the maximum power point

(MPP) as shown in Figure 1.4 but the power in the figure is in W rather than power density in

W/m2.

8

Figure 1.4 : Graph showing short circuit current–open circuit voltage and power–voltage

solar cell characteristics, including the maximum point of both current and voltage (IMPP-

VMPP) which gives the maximum power point MPP. See Ref [5]

1.4.4 Standard model of solar cells

In an ideal solar cell, the series resistance, Rs should equal to zero and the shunt resistances,

Rsh should be large enough or equal to infinity. Unfortunately, that isn’t the case. There are

manufacturing defects in silicon during production which then creates another current path

for the light-generated current in the solar cell resulting into low power output. Rs and Rsh

influence the I-V curve slope of a solar cell. The I-V equation of a non-ideal solar is given as

𝐼 = 𝐼𝑝ℎ − 𝐼01 (𝑒𝑉+𝐼𝑅𝑆

𝐾𝐵𝑇 ) − 𝐼02 (𝑒𝑉+𝐼𝑅𝑆

2𝐾𝐵𝑇 ) −𝑉+𝐼𝑅𝑆

𝑅𝑠ℎ (13)

The equivalent circuit of such real cell is shown in Figure 1.5.

Figure 1.5 : Equivalent circuit for an ideal solar cell is represented by the full line. Dotted

line shows the non-ideal components. See Ref [6]

9

1.4.5 Photovoltaic market

The photovoltaic industry is extremely growing at a fast rate. The investment cost has fallen

to about 14% per year and almost 75 % since 2006 [7]. The total global market for solar

PV is expected to triple by 2020 to almost 700 gigawatts, with annua l demand ris ing up

to 100 gigawatts in 2019[8]. The growth of the PV has been motivated by governments

through incentives and subsidies to help attain their renewable energy targets. However, these

subsidies have been reduced as PV installations have grown rapidly worldwide, because a

decrease in system costs actually served the original plan that was very attractive and costly

to support for 20 years or more. In Germany for example the electricity generated from PV

power is well favored by the Renewable Energy Sources law (Erneuerbare Energien-Gesetz,

EEG). EEG enables PV plant owners to run their installations at profitable assured rate of

purchase. They also aim to effect a further reduction in the levelized cost of electricity

(LCOE) generation from PV by creating a market for it. The solar photovoltaic is already

today one of the cheapest renewable energy technology. The levelized cost of electricity in

Germany for example ranges between 0.078 and 0.142 Euro/kWh depending on the capacity

required. PV power plant costs ranged from 1000 to 1800 Euro/kWp, even lower prices have

been reported in sunnier regions of the world, since a major share of cost co mponents is

traded on global markets [2015, Agora Energiewing]. Crystalline silicon photovoltaic is the

drought horse of the PV market, a share of 80-90% has always been recorded. The market

dominance of crystalline silicon PV is mainly due to the abundance of silica (impure form of

silicon) which is about 25% of the earth’s crust and its non-toxicity. The longevity stability of

crystalline silicon (~ 20 years) in outdoor conditions makes it more prominent which help in

terms of cost competitiveness for PV because the payback starts in the 10th years after the

first installation. The high efficiency reduces system cost, utilizes limited space and facilitates

higher power system installation. The learning curve of crystalline silicon cost has a learning

rate of 20% (20% reduction in cost and doubling total efficiency) which will decrease further

in the nearest future. Figure 1.6 shows the average price reduction of rooftop PV systems

from 2006 to the first quarter of 2015. The reduction of the average price was from 31% to

52%. Countries like China, Japan and the U.S.A still top three largest markets in 2015 just

like they were in 2014.

10

Figure 1.6 : Average end customer price (net system price) for installed rooftop systems with

rated nominal power from 10 - 100 kWp. See Ref [9]

11

2 Chapter Two: Theory

2.1 Basic structure of a solar cell

Silicon (Si) is the most popular choice for solar cells because of its band gap of 1.12 eV at

300K [10], availability of silica (SiO2), cell efficiency of about 25.6% and module efficiency

of about 22% [3]. The evaluated technologies are monocrystalline, polycrystalline,

amorphous, inorganic (Group III-V and Group I-II-III-VI) and organics cells (Dye-sensitized

solar cell and organic polymer solar cells) which are being compared to the novel technology

(HIT solar cell). The fundamental principles of operation of solar cells are; Electron-hole pair

is generated within the cell by the effect of photons. The electron-hole pair is collected

through the contacts to generate a current for the load. Then, voltage is generated across the

terminals in solar cell, and finally, the load attached to the system is powered.

Most solar cell technologies have:-

Anti-reflecting coating (ARC), which is a very important part in solar cell fabrication. It’s

usually sprayed over bare silicon cell because silicon has a high surface reflection. ARC

increases the power output of the solar cell. Famous examples are Si3N4, TiO2, Al2O3, SiO2–

TiO2 and ZnS.

Front contacts are necessary to collect the current generated by a solar ce ll. They are

usually made of metals. The Busbars are connected directly to the external cable

which can be connected to the load or another solar panel to form an array, while the

fingers are smooth metal areas usually silver, which collects the current for delivery to

the busbars.

An emitter absorbs the incoming photons and transports their energies to the excited

state of charge carries. Pentavalent doped silicon (n-type) has a higher surface quality

than trivalent doped silicon (p-type) so it is placed at the front of the cell where

majority of the light is absorbed.

In p-n junction, in its simplest form, the base (p-type) region is joined at a junction

with emitter (n-type) region leading to majority electrons in the n-type side close to

the junction to diffuse of to the p-type side and majority hole from the p-type to

diffuse n-type side. This movement exposes positive and negative ions from the p-

type and n-type respectively, resulting into a built- in potential and a built- in electric

field at the junction and forming the depletion region. This built- in field, for the

illuminated solar cell will help the photo generated electrons and holes to cross the

junction and a photo current is created.

Rear contact is a less important than the front contact because it is much way from the

junction and does not need to be transparent. But recently, most solar cell

12

manufactures are designing a transparent back contact which would take advantage of

the reflected light from the hard surface the PV is mounted on, which then increases

the efficiency.

2.2 State-of- the- art production method of compared technologies

The silicon solar cells have dominated the PV market for so many years. They have been

produced to be used for both research and commercial purposes. They have dominated the

market because of the abundance of silicon, non-toxicity, module efficiency and excellent

cell stability. There are various levels of skills for production of the evaluated technologies

(monocrystalline, polycrystalline, amorphous, inorganic & organics cells and the novel

technology (HIT solar cell) but the current silicon based solar cells production methods will

only be discussed in the following section.

2.2.1 Silicon based solar cells (Crystalline silicon solar cells)

The rapid decrease in cost of silicon solar module in the last five years has positively affected

other system components of PV. Not long ago there have been so many improvements in

mass production of high quality silicon wafers, wafer cutting and cleaning, charge carrier life

span, surface passivation, maximization of solar radiation and device characterization.

Thousands of research institutes and companies invest in the area of silicon based solar cells

feeding their capabilities and knowledge into the manufacture of cheap and efficient silicon

based cells and modules in a commercial viable way. The production of silicon based solar

cells, starts with a fundamental material which is silica or sand rocks (contains large amount

of impurity concentrations) which is usually dung out from the earth. The silicon is subjected

to react with carbon (under heat) to get metallurgical grade silicon; SiO2 + C → Si + CO2

(removal of Oxygen). In order to get almost totally purified silicon for semiconductor,

reacting with anhydrous hydrogen chloride (HCL) is usually done. Si + 3HCl → SiHCl3 + H2,

and then with hydrogen at high temperature for numbers of hours. SiHCl3 + H2 → Si + 3HCl.

The Czochralski growth method is shown in

Figure 2.1 and Figure 2.4 it is the process that is used in growing crystalline silicon. The pure

silicon is heated in a crucible and a seed crystal is placed above it to pull and rotates in the

opposite direction of the moving crucible to produce an ingot, which matches seed’s

crystalline structure and then doped to make a crystalline semiconductor with a P/N junction.

Float zone growing method and the edge-defined film edge growth as show in Figure 2.2 and

Figure 2.3 are the growth process of amorphous and polycrystalline respectively.

13

Figure 2.1: Growth method of mono-crystalline silicon cell. It's called 'Czochralski growth

method'. See Ref [11].

Figure 2.2: Float zone growing method for manufacturing polycrystalline cell. See Ref [11]

The giant crystal growth in which all the atoms are almost aligned in a desired structure and

position is monocrystalline silicon. For polycrystalline and amorphous, the structure are not

well defined. The ingots are placed in close proximity to each and then sawed top and bottom

to have a uniform width. A square shaped wire-slicing saw with a liquid abrasive is usually

used on each ingot to cut off the rounded edges and ends up having an even four sided ingots.

A wire-saw moving sideways hundreds of times forms a web of parallel, tightly spaced

14

segments. As the wire-saw moves through the ingots placed in its way, it slices the ingots into

the required thickness of wafers. Each millimeter of sliced crystal wafer is loaded into

carriers to be transported to next phase of production. The cleaning and etching phase are

done next then followed by deposition of an anti-reflecting coating.

Figure 2.3: Edge-defined film edge growth (EFG) is the growth method for amorphous

silicon cell. See Ref [11]

Figure 2.4: Crystalline Silicon solar cell generic process diagram. See Ref [12]

The fabrication of the three silicon based solar cells follows the fundamental principles as

discussed earlier, accept for the amorphous silicon cell which as a slight difference. Instead of

growing an ingot and then slicing into wafers, the melted silicon is grown in a continuous

thing sheet at large area which is less expensive and intricate. The amorphous silicon is

actually classified as a second generation solar cell, but because it silicon based therefore its

15

discussed under first generation. Also for amorphous silicon, hydrochloric acid is used in

cleaning the thin wafers.

2.2.2 Chemical compound based solar cells (In-organic and Organic)

Inorganic solar cells :-The main inorganic solar cells are cadmium telluride/cadmium

Sulphide (CdTe/CdS), copper indium selenide (CIS), copper indium gallium selenide (CIGS)

and Gallium indium phosphide (GaInP) solar cells. They are non-silicon based solar cells and

they are heterojunction cells and are also known as the chemical compound solar cells. They

are composed of in-organics compounds from group I-II-III-V-VI elements of the periodic

table. The non-silicon based solar cell materials are earth-abundant to a fair extent and

require low-energy and low temperature in production as compared to their predecessors.

The inorganic solar cells are produced using intrinsic semiconductor thin films that utilize the

differences in band offsets to form an effective p-n heterojunction as a profitable low cost

alternative to doping the cells. They are deposited on glass, plastic or metal substrate by a

standard radio frequency sputtering equipment. The multi- junction solar cells, which

basically Group III-V are classified as the third generation solar cell, the rest are second

generation solar cells. They are built on Gallium Arsenide and Indium Phosphate substrate.

They have direct energy band gaps (conduction and valence band have same crystal

momentum), excellent light absorption coefficients. The idea of fabricating multi-junction

cells is to match different ranges of the solar spectrum. A typical 3-junction solar cell

structure for example comprises of three n–p junctions stacked on top of each other, with the

band gap energy increasing from bottom to top and assembled with low resistive tunnel

junctions. To fabricate a multi-junction device the entire materials must be built on the same

substrate, they must all have similar atomic structures in atomic spacing and provide the

multiple band gaps necessary to produce the junctions. The state-of the art techniques for

highly efficient photovoltaic energy conversion for a multi-junction cell is done by structure

combination of GaInP/InGaAs/GaAs (Gallium Indium Phosphide/ Indium Gallium Arsenide/

Gallium Arsenide) top to bottom. The fabricating process of CdTe, CIGS and multijunction

are explained graphically in Figure 2.5, Figure 2.6 and Figure 2.7 respectively.

CdTe/CdS

The first steps involve cutting and cleaning of the substrate (glass), the active layers

Cadmium Telluride and Cadmium sulphide are then deposited on the substrate. After

deposition of each layers, scribing process is done followed by screen printing and

encapsulation.

16

Figure 2.5: Cadmium Telluride (CdTe) solar cell generic process diagram. See Ref [12]

CIGS/CIS

The first steps involve cutting and cleaning of the substrate (glass), the active layers Copper

Indium Gallium Selenide and Cadmium Indium Sulphide are deposited on the substrate.

After that, it is then etched and cleaned, mounted in a frame, encapsulated. Molybdenum is

used as the back contact and a transparent conducting oxide is deposited over the window

layer.

Figure 2.6: Copper Indium Gallium Selenide/Cadmium Sulphide solar cell generic process

diagram. See Ref [12]

17

Multi-junction

The first step in processing begins with mixing gallium and Arsenic. GaAs with its bandgap

is the obvious choice for substrate layer in a triple-junction structure. GaAs is used as the

bottom cell, InGaAs is used as middle cell and GaInP as the top cells. The next step is a

lithography/etch process which outline the bypass diode and uses a selective etch to define

the area of the diode which has been grown on top of the triple junction epitaxial structure.

After that, it’s followed by a Mesa lithography/etch process and etches through the entire

epitaxial structure, down to the GaAs substrate and defines the active area of the solar cell

[13]. The front side metal grid is formed using industry standard photo patterning, metal

deposition and lift-off techniques [13]. With the grids defined on top of a highly doped n+

compound. The window is then place the top junction. The process continues with the

reactive deposition of an anti-reflection coating of a TiOx/Al2O3 dielectric stack with spectral

characteristics optimized to reduce reflections, maximizing the end-of- life performance of the

solar cells and generating more power [13]. In the final step, deposition of the backside metal

to form the p-contact and is followed by heating to ensure strong finished process.

Figure 2.7: Gallium Arsenide (GaAs) solar cell generic process diagram. See Ref [12]

Dye sensitized solar cells

The production of dye sensitized solar cells (DSC) also known as ‘Grätzel cells’ are less

expensive and less intricate in comparison to the silicon based ones. It majorly involves

preparation of glass, TiO2 colloids preparation, screen printing, drying, coloration, wiring and

framing. A schematic diagram is shown in Figure 2.8.

The first step is preparation of glass coated with a transparent conducting oxide (TCO), for

both top and bottom of the sola r cell. This has a high optical transmission and low resistance.

The standard sizes of the glasses are 7.5x10 cm2 and 10x10 cm2 [14]. After glass processing,

the TCO substrates are stacked and cleaned. TiO2 colloids which are the main substance in

DSC, has been prepared by hydrolysis of titanium isopropoxide in water [14]. The TiO2

colloid is transferred from the aqueous solution into a terpineol/ethylcellulose mixture to

obtain a screen-printable paste, by means of a pearl mill [14]. The screen printing on the

18

active layers is done in an unadulterated environment to avoid contamination on the film.

TiO2 is the photo-conductive semiconductor used on the front electrode and platinum, a

catalyst on the counter-electrode. The final step is lamination, back-side filing, wiring and

framing.

Figure 2.8: Schematic construction of a glass/glass nanocrystalline dye sensitized cells. See

Ref [14]

Organic polymer solar cells

The preparatory techniques involve in production of organic photovoltaic cells (OPV) are

done in a vacuum because the cell materials are instable at ambient temperature. The

materials are easy and cheap to process unfortunately they have short life span. Some of the

organic molecules commonly used organic solar cells are shown in Figure 2.9. Encapsulation

this material in glass could increase the life span to a very good extent but also increase the

total cost of producing the cell. The increase in the life span would help spread out cost of

manufacturing and installation over time, reducing the price per kW. The materials described

by Krebs, et. al. in their article “A simple nanostructured polymer/ZnO hybrid solar cell-

preparation and operation in air “would be perfect for this type of cost calculation [15]. They

are ZnO nanoparticles (acceptor) and the polymer poly-(3- (2-methylhex-2-yl)-oxy-

carbonyldithiophene) (P3MHOCT) (donor) which becomes poly (3-carboxy-dithiophene)

(P3CT) when heat is applied, the ZnO nanoparticles are also the semiconducting charge route

to the front electrode. Figure 2.9 shows some used in fabrication of organic polymer solar

cells. In the first step, ZnO is mixed with Poly-(3- (2-methylhex-2-yl)-oxy-

carbonyldithiophene) (P3MHOCT). Afterwards it is stirred hours then filtered [16]. ZnO and

the polymers are spun after each other on a substrate [16]. Finally, the back electrode is

added.

19

Figure 2.9: Some organic molecules commonly used organic solar cells: ZnPc (zinc-

phthalocyanine), Me-Ptcdi (N,N’-dimethylperylene- 3,4,9,10-dicarboximide), and the

Buckminster fullerene C60.See Ref [29]

2.2.3 Hetero-junction with intrinsic thin layer solar cell (Novel technology)

The combination of amorphous silicon and crystalline silicon technology in a thin layer result

into heterojunction with intrinsic thin layer (HIT) solar cell. This is the new novel solar cell

technology. The unique features of HIT solar cell are the high conversion efficiency,

Staebler-Wronski effect isn’t found yet in it and its low processing temperature, which made

it attractive for its large scale commercialization and alternative to conventional source of

power [30]. The fabrication of HIT solar cells is less intricate, the crystalline silicon, c-Si

cell is fabricated first, then followed by the deposition of an intrinsic, a-Si:H and extrinsic

amorphous silicon, n-type and p-type a-Si:H. A summarize fabricating steps is shown in

Figure 2.10. The deposition is done through plasma enhanced chemical vapour deposition

(PECVD) which is similar to fabrication process of thin-film. Currently only n-type c-Si

wafers are used in commercial HIT solar cells but some researchers focusing on HIT solar

cells on p-type wafers [30].

First step is manufacturing electronic grade crystalline silicon (SiHCL3 + H2 → Si +

3HCL) which is an almost pure form of silicon. After the silicon has been grown to an

ingot form, it’s then sliced into thin wafers doped with a pentavalent dopant, e.g.

phosphorus making it a n-type c-Si.

The c-Si requires cleaning and surface smoothing. The smoothing is done by an

isotropic etching with de- ionized water contain 8% sodium hydroxide (NaOH) at

80°C temperature for a short while [30]. The wafers are then rinsed in 10%

hydrochloric acid, HCL for 1 min, rinsed in de- ionized water for 1 min, followed by

another rinsing in 10% hydrofluoric acid, HF solution for 1 min and finally in de-

ionized water for 1 min [17].

The third step is also a cleaning step to prepare the wafer for passivation with

amorphous silicon. This cleaning step follows the radio corporation America (RCA)

cleaning standard [17]. In this step there are two standard cleaning processes. The first

is the removal of organic impurity while the second is for removal of metallic

20

impurity. The fabrication process of c-Si is called wet chemical processing of c-Si

wafers [18]

The next step is deposition of the intrinsic amorphous silicon on both sides of the c-Si

via radio frequency plasma enhanced chemical vapour deposition (RF-PECVD),

followed deposition of Phosphine (PH3) doped amorphous silicon on the top (emitter)

and diborane (B2H6) doped amorphous silicon at the bottom (BSF) via RF-PECVD.

The transparent conducting oxide, TCO is deposited next usually indium titanium

oxide is used via sputtering then followed by screen printing of the electrodes which

are 2-mm [19] in spacing.

Hotwire chemical vapour deposition method isn’t used because of the damage it could cause

on the c-Si. Research shown that surface passivation with the amorphous silicon layers

(intrinsic and extrinsic) would not deteriorate the c-Si substrates [20]

Figure 2.10: Schematic representation of the process steps in fabricating HIT solar cell. See

Ref [17]

From the description of the fabricating process of the evaluated technologies, we can then say

the HIT solar cell is less cheap and intricate in fabrication in comparison with crystalline

silicon only. HIT solar cell has gained recognition in the photovoltaic industry due to its low

process temperature (~ 200), as compared to crystalline silicon (c-Si) solar cell (1000°C) and

relatively high efficiency, η (capability of reaching efficiency up to 25%) [19]. Another

amazing feature of HIT solar cell is its good stability under light (temperature coefficient),

more power is generated in limited space due to its relatively high efficiency per watt. The

materials used are less expensive (low quality c-Si is used) and excessive in nature, excellent

Passivating property of the due of the a-Si:H(i) increases the Voc, which then result to an

improvement of the short circuit current and the fill- factor. The HIT bifacial solar module

could be mounted either slanting or horizontal because of its ability to absorb radiation from

both sides. There are also few challenges faced with HIT solar cells, i.e. optical losses,

resistance losses and recombination losses which would be discussed further in next subtopic.

2.3 Geometrical, compositional and operational data of the compared technologies

2.3.1 Mono-crystalline solar cells

The monocrystalline solar cells also known as the single-crystal silicon is the oldest, most

efficient and has been dominating the PV market since its inception. The cells are sliced

wafer from a single grown silicon crystal that has gone through a controlled growth method

called Czochralski growth method. The cylindrical shaped wafer is cut to square shape with

21

round edges in other to optimize the performance and reduce the cost of single-crystal solar

cell. The monocrystalline solar cells are recognizable by its colour and consistency in

structure which is due to the pure crystalline silicon used in production. The Silicon must be

spotless (almost no impurities); the silicon purity level should be almost 100%. The level of

the unintentional dopant content is less than one atom per Si 109 atoms [21]. This level of

purity equates to 25 apple trees planted within millions of maple trees across the United

States of America [21]. Silicon has an indirect bad gap, which leads to its high absorption

depth and its low optical absorption coefficient α ~100 cm1 [22]. In order to increase the

absorption coefficient and decrease the absorption depth, the silicon wafer must be increased

to few micrometers. The high absorption depth of silicon reduces the photon intensity by

36% of its initial intensity or drop by a factor of (1/e) [23] as it goes down the structure. In

other for photo-generation to occur in a monocrystalline cell, the pure silicon wafer must

increase in thickness to absorb the short wavelength (high energy) than the long wavelength

(low energy) and the electron-hole pair generated during this process should be able to

diffuse down to the built- in field in the depletion zone. The competency of the electron-hole

pair to diffuse down into the built- in field in the depletion zone is gotten by the minority

diffusion length L before it could recombine. The diffusion length L depends on

lifetime and mobility of the carriers and they are both perceptive of the purification of the

cell. Solar cells are designed to work best with good radiation intensity, but increasing the

solar cell thickness to a few micrometers in other to utilize the high energy radiation is at the

detriment of increasing the cells series resistance Rs and reducing the performance or

efficiency and the fill factor of the cell at increasing radiation intensity. The series resistance,

Rs of a monocrystalline solar cell should be as low as possible and the shunt or parallel

resistance Rsh should be as high as possible for effective power production of the

monocrystalline solar cell. Another option is to increase the doping of the cell which would

reduce the series resistance but excessive doping also damages the crystal structure of the

cell. When a monocrystalline cell is produce, an etching and printing of silver are done on it

for the front contact. An antireflection coating, e.g. TiO2 is sprayed over the surface which is

the N-type silicon because of its good surface quality to absorb more light and the anti-

reflection coating reduce reflection. The top layer is the negative terminal l and the rest layer

is positive terminal. Aluminum is used for the back contact and to reflect back the minority

carries back up to the junction to increase power output. In monocrystalline cell the printing

of the surface and back contacts where usually printed on top the silicon cell both front and

back but research has shown that the printing creates a bit of shading and series resistance to

the cell reducing the efficiency. Figure 2.11 shows a monocrystalline silicon solar cell with

screen printed contacts and another with buried contacts Passivating the rear surface of a

monocrystalline cell is also of great importance because it reduces back surface

recombination which then results in high power output (efficiency). Shockley-Read-Hall,

SRH recombination is the dominant recombination mechanism in c-Si. Chemical compounds

like Al2O3 are used to passivate the rear surface of the monocrystalline solar cell. The surface

of the anti-reflection coating and the back surface passivation are designed to have ridges in

order to refract the light radiation back into the cell.

22

Figure 2.11: Cross-sectional of a monocrystalline silicon solar cell with screen printed

contacts and another with buried contacts. See Ref [22]

When solar cells are soldered in series to make a solar module, it can then be set on surface at

a certain angle, which equals the latitude of that region to give best performance. The same

photovoltaic effect occurs in both monocrystalline and polycrystalline solar cell.

2.3.2 Poly-crystalline solar cells

The fabrication, geometry and characteristics of the polycrystalline are very similar to

monocrystalline solar cell. That can be seen in Table 2.1: Compositional data of compared

technologies, which shows the compositional data of evaluated cell. These cells also have a

big PV market share and its increasing each year. The fabrication of polycrystalline is less

expensive in comparison to monocrystalline solar cells and one of the reasons is the fact that

the silicon isn’t required to have a unique arrangement of atoms and a high level of purity.

The difference could be seen from the aesthetics of both monocrystalline and polycrystalline

cells. The direction of the crystal structure is patternless and each is likely to be different

from the neighboring crystals and this result to the dark blue coloration of polycrystalline

cells because each grain has different structure, therefore they reflect light differently unlike

monocrystalline the uniform structure makes it reflect light uniformly therefore showing a

uniform colour. The structure also hinders the flow electrons leading low efficiency. Most

consumers encounter difficulties from choosing either quality or cost when purchasing the

most appropriate solar PV panel for a system. One of the major disadvantages of

polycrystalline to monocrystalline is the space efficiency, for instance a meter square of both

monocrystalline and polycrystalline solar cell could generate 190W and 180W respectively

on a good sunny day. The gap isn’t so much so the type of solar system project, location and

other necessary factors would help in choosing the right type of solar panels. For example,

the efficiency of polycrystalline solar cells decreases with temperature over 25°C (Staebler-

Wronski effect) which means that location with high temperature won’t favour these type of

cells unlike monocrystalline solar cells which are more efficient under high temperature. The

recorded efficiency in the laboratory ranges between 20.8 ± 0.6 % [24].

23

2.3.3 Amorphous solar cells

In the 70s the two scientist from Dundee University, Walter Spear and Peter LeComber

where first to exhibit the semiconducting property of amorphous silicon, showing that

amorphous silicon could be doped with a pentavalent and trivalent atom to make a p- i-n

junction just as a crystalline silicon. The amorphous silicon was reacted with hydrogen in

other to meet up with electronic standard especially if it would be doped. The interesting

things about hydrogenated-amorphous silicon cells is its process of fabrication, it can be

produced over a large area and requires low temperature, making it easier to deposit on a

low-cost substrate, for example glass surface, metal plastic. It has an absorption coefficient of

about α > 105cm-1[10] because of its little micrometer thickness (ca. 100 µm). The first set

of hydrogenated-amorphous silicon cell had the conversion efficiency of 2.4% [22], since

then it has improved to an efficiency of 15% [23]. The problem with hydrogenated

amorphous silicon is that it degrades faster than crystalline cells when exposed to sunlight

within months of operation to 4-5% (Staebler-Wronski effect) [22]. Due to this issue, the

concept of multi-junction was initiated. It is a combination of two to three p- i-n layers with

different band gaps, usually the lowers band gaps is used as the bottom layer and the band

gaps increase as it goes to the top layers. The hydrogenated-amorphous silicon cell has a

market share of 9% [25] as at 2014 and about 4.4GWp (10%) global annual production. In

amorphous silicon solar cell, the p- and n-type doped layers are thin and the absorber layer is

the intrinsic layer. The low ambipolar diffusion length of the carrier, the electron and holes

are separated by the built- in-field in the intrinsic layer (absorber), which limits its thickness.

Increasing the thickness will increase the absorption of more light but at the detriment of

reducing efficiency because its poor photo carrier collection. In order to increase the

efficiency and light absorbed by the cell a transparent conducting oxide is used which makes

light to be trapped in the structure leading to multiple internal reflections in the absorber

layer, resulting in higher efficiency.

2.3.4 Inorganic solar cells (Based on the use of CdTe/CdS, CIS/CIGS and GaAs)

CdTe/CdS solar cells

Joseph J. Loferski proved that the best energy bandgap for photovoltaic solar energy

Conversion by a single junction solar cell is 1.5 eV [26]. CdTe having a direct band gap of

1.44eV at 300K and also having high optical absorption coefficient for photons is the perfect

solution for solar energy conversion. Thin layers of few micrometers were all that was

needed to fabricate CdTe which would reduce cost. Minority carrier diffusion length isn’t a

priority because all the light will be absorbed in the depletion zone. CdTe has demonstrated

the lowest cost technology among all solar cells but low efficiency. First solar is the leading

company in CdTe technology. CdTe could be doped by either group VIIA/VA (for Te) or

group IIIA/IB (for Cd). Currently CdTe/Cds solar cells are heterojunction. The CdS is the n-

type layer and CdTe is the p-type layer. They were deposited using close spaced sublimation

method [22]. Wu and co. at NREL made the highest efficiency of CdTe [22]. The structure

formation is a glass, conducting oxides, CdS, CdTe and a back contact (copper), there was an

increase in the efficiency in comparison with other CdTe produced initially, which was due to

the type of oxides layer used, which is more transmissive and more conductive than the

24

previous types of oxides used [22]. The new oxides increased the amount of light reaching

the CdS/CdTe interface and lowering surface resistance losses and also resulted in increase of

the Voc and FF. The CdTe junction is a heterojunction solar cell, the carriers are separated at

the junction, electrons are collected at the TCO layers and the holes are collected at the back

contact. CdTe could come in two forms, the “superstrate” and the “substrate”. For the

“superstrate”, the substrate in which the layers are deposited on acts as the window layer for

light passage into the solar cell (p-i-n) while the “substrate”, the back contacts act as the

substrate or back contact is deposited on the substrate as a result no light will pass through

the substrates, only through the TCO layer (n-i-p).

CIGS/CdS solar cells

Copper indium diselenide (CIS) and copper indium gallium diselenide (CIGS) are similar as

with amorphous silicon and CdTe/CdS base on their direct energy bandgap, high optical

absorption coefficients for photons with energies greater than its energy bandgap and the

recombination mechanism that dominates is the radiative recombination. This makes it

possible for a few microns of absorber layer material to absorb most of the incident light and

reducing the need for a long minority carrier diffusion length. The usual structure of CIGS is

glass/ Molybdenum (back contact, doesn’t diffuse into the semiconductor)/CIGS (P-

type)/CdS (n-type, buffer layer)/i-ZnO (TCO, front contact)/n-ZnO:Al. The light photon

enters the cell through ZnO:Al. The minority electrons in the CIGS diffuse to the CdS

interface to be separated and the hole diffuses to the molybdenum for collection. The holes

then recombine with electron from the back contact.

Group III-V (Multi-junction)

The idea of the multi-junction solar cell is for the cell to be able utilize short, medium and

long wave length of the light spectrum. The different layers have different band gaps

increasing from the bottom to the top of the cell. An example of a three- layer multi-junction

solar cell is (Gallium Indium Phosphate/ Gallium arsenide/ Germanium). The layer with the

highest bandgap e.g. Gallium Indium Phosphate (GaInP, 1.8eV), would absorb the blue

portion of the light spectrum, the layer with the medium value band e.g. gallium arsenide

(GaAs, 1.4eV), would absorb he green portion of the light spectrum and the third layer

Germanium, Ge with 0.7eV would absorb the red potion of spectrum as shown in Figure

2.12. The buildup is layers of different band gap elements. The lower layer is made up of

combination of elements with low band gap and it increases as other layers are deposited. The

top layer can absorb the short wavelength and the following layers could absorb long

wavelength. The idea of multi-junction has been shown to increase the open circuit voltage

and power of the cell in comparison to single junction. The cells are connected in series.

Disadvantage of the multi-junction is that the current density is that of the cell with lowest

current but the voltages add up. A tunnel junction is used in between each P-N junction and

the beneath one to provide low electrical resistance. The current operating data of the

evaluated cells in their module state are compared in Table 2.2.

25

Figure 2.12: Working principle of the multi-junction solar cells. See Ref [27]

2.3.5 Organic solar cells and Dye sensitized solar cells

Organic solar cells

In organic solar cell the major materials used are conductive organic molecules. The vacuum

level is the energy level of a free electron. It’s the level of alignment of two different

materials. In organic solar cells there are two orbital called highest occupied molecular

(HOMO) and lowest unoccupied molecular orbital (LUMO), they represent the valence and

conduction band respectively in both organic materials. Two types of energy are considered

here; the ionization energy, energy required to excite the electron from the HOMO (valance)

to the vacuum level and electron affinity is energy required to move an electron to the

vacuum level from the vacuum. This simply means a material with low ionization energy can

give out an electron easily from the valence band and material with high electron affinity can

accept electron in conduction band. Light excitation leads to excitons, they excited electron-

hole pair. They easily diffuse and have low life time which results into recombination due

low diffusion length. When the cell is subject to light, the excitons at the interface of the two

organic compounds are separated, the electron diffuses to electron acceptor and while the

holes diffuse to the electron donor material as shown in Figure 2.13. At each end of the

organic material are electrodes. The holes are collected at the electrode made of either metal

or indium titanium oxide and the electrons are collected the electrodes made of aluminum,

magnesium or calcium. This technology is cheap to fabricate and has very low market share

and possibly no fabrication is done on large scale.

26

Figure 2.13: Operating mechanism of organic solar cell. See Ref [27]

Dye sensitized solar cells

For dye sensitized cell, its contains a transparent conductor at both ends of the cell, titanium-

dioxide particles, dye particles and electrolyte and catalyst (platinum). Figure 2.15 show the

schematic and working mechanism of DSSC. Dye is the electron donor while the TiO2 is

electron acceptor. The dye absorbs light photons and excited electrons with its energy and

diffuses into the TiO2 they then conveyed towards the electrolyte to make complete circuit

and the back to the TiO2.The movement of the electron creates energy that is the being

utilized to produce electricity. The organic solar cell layout is quite different from other solar

cells layout. Comparisons of some of the evaluated solar cells are shown in Figure 2.15.

Organic solar cells and DSSCs are not include.

Figure 2.14: Schematic representation of a dye-sensitized solar cell. See Ref [28]

27

2.3.6 Heterojunction intrinsic thin layer solar cells (Novel technology)

The novel device developed by Sanyo is the HIT solar cell, which consists of a crystalline

silicon wafer (absorber) with ultra-thin intrinsic silicon layers (passivator), and n-type and p-

type doped amorphous silicon layers (emitter and back surface field). The deposition process

of the PN junction and the Back Surface Field is done at temperature of about 200ºC, unlike

in normal crystalline silicon (c-Si) cells the wafer has to be raised to about 800°C or more for

PN junction and BSF formation. HIT solar cells have numerous of advantage i.e. possibility

of higher efficiency than conventional c-Si, very good surface passivation which allows high

open-circuit voltages. High open circuit voltage results in higher conversion efficiency which

then require less area for mounting and number of modules to be installed are reduced. It has

a good temperature coefficient, less intricate in production, reduced energy payback time and

cheaper (€\W) in comparison to conventional silicon solar devices, because the thermal

energy in production is low. The difference in compositional data of HIT to other evaluated

solar cells is shown in Table 2.1. Thin intrinsic layers on and below the c-Si substrate

effectively passivate c-Si surface defects, and help improve the junction characteristics. The

major function of the intrinsic layer is to help reduce the recombination velocity of the holes

and electrons in the doped a-Si: H material. The diffusion length is quite small so that internal

quantum efficiency for short wavelength photons is relatively low. Therefore, a cell structure

with front hetero-emitter is always a compromise between high voltage, good transport

properties and blue response.

Table 2.1: Compositional data of compared technologies

Cell Emitter Absorber

Buffer

layer

Sub-

strate

ARC BSF TCO Front

contact

Back

contact

mc-Si N-doped silicon

P-doped silicon

- - Silicon Nitride or

titanium oxide

P+-

doped silicon

- Silver (Ag), Alumini

um (Al)

Aluminium (Al)

pc-Si N-doped

silicon

P-doped

silicon

- - Silicon

Nitride or titanium

oxide

P+-

doped silicon

- Silver

(Ag)

Alumin

ium (Al)

a-Si:H P- a-Si:H N- a-Si:H a-Si:H (i)

Glass, plastic,

metal foil

Silicon Nitride

or titanium oxide

- Indium tin

oxide

Indium tin oxide

Aluminium-

doped zinc oxide

/Aluminium

(Al)

28

CIGS/C

dS

Al-ZnO

CIGS/CIS

CdS

Glass, plastic, metal

foil

Al-ZnO

-

Indium tin oxide/i-

ZnO

i-ZnO Molybdenum

CdTe

/CIS

SnO2Cd2SnO4

CdTe /CIS

CdS

Glass, plastic,

metal foil

Zn2SnO4

-

Cd2SnO4

Cd2SnO4 Copper or

metals

HIT

a-Si:H (P)

c-Si (n)

a-Si:H

(i)

-

-

a-Si:H

(n+)

Indium

tin oxide

Silver

(Ag)

Silver

(Ag)

DSSC Dye Electrolyte/ Platinum

- Glass - - -

- -

Organic Organic compound

Organic compound

- Glass - - - Indium tin oxide Or metal

Aluminium, Magnes

ium, Calciu

m

Group

III-V

- - - - - - - Alumini

um

Alumin

ium

Table 2.2: Current operational data of compared technologies based on modules

Cell Area

cm2

Voc

[V]

Jsc

[mA/cm2]

FF [%] 𝝶 [%] Test Center

(Date)

c-Si 143.7 0.740 41.8 82.7 25.6 ± 0.5 AIST (2/14) [24]

pc-Si 243.9 0.6626 39.03 80.3 20.8 ± 0.6

FhG-ISE (11/14)

[24]

a-Si:H 1.001 0.896 16.36 69.8 10.2 ± 0.3 AIST (7/14) [24]

CIGS/ CdS 0.9882 0.752 35.3 77.2 20.5 ± 0.6

NREL (3/14)

[24]

CdTe /CIS

1.0623 0.8759 30.25 79.4 21.0 ± 0.4

Newport (8/14)

[24]

29

HIT 100.3 0.712 38.37 78.7 21.5

Sanyo Electric

[29]

DSSC 1.005 0.744 22.47 71.2 11.9 ± 0.4 AIST (9/12) [24]

Organic

solar cell 0.993 0.793 19.40 71.4 11.0 ± 0.3

A IST (9/14)

[24]

Group III-

V 1.047 3.065 14.27 86.7 37.9 ± 1.2

AIST (2/13)

[24]

These dates were measured under the global AM1.5 spectrum (1000 W/m2 ) at 25 °C. ISC -

short circuit current of solar cell, IMPP - current in maximum power point of solar cell, VMPP -

voltage in maximum power point of solar cell, VOC - open circuit voltage of solar cell, FF -

fill factor of solar cell and 𝝶- efficiency of solar cell [24].

Figure 2.15: Evaluated technologies layers outline

30

Table 2.3 Features of evaluated cell

Absorber

Emitter

Passivator or buffer layer

TCO/ front contact

Back contact

Window layer

Back surface field

31

3 Chapter Three: Heterojunction intrinsic thin layer solar cell

3.1 Design concept of hetero-junction intrinsic thin layer solar cell (HIT)

The design concept of HIT solar cell is very interesting and this chapter discusses the major

design concept of this solar cell and compares it with other technologies. HIT solar cell is

patented by Sanyo Electric Company in Japan, (now owned by Panasonic) and its currently

popular among solar cell researchers, which has a conversion efficiency of 21.5.0% (Voc:

0.712 V, Isc: 3.837 A, FF: 78.7%) with a practical size of 100.3cm2 [29]. The HIT solar cell

offers an extent of freedom in designing because both the doping and conduction-valence

band off-sets at the junction could be controlled. The HIT solar cell has the advantages of

crystalline silicon solar cells and thin amorphous silicon. Initially, the HIT solar cell was

designed with a p-type amorphous silicon deposited over a n-type crystalline silicon,

however it was noticed that the open circuit voltage Voc, fill factor and efficiency was lower

than a conventional crystalline silicon, c-Si. This was due to recombination losses at the

interface and optical losses at the front and back surface. The recombination losses were due

to mid-energy state in the p-type amorphous silicon. This created a leakage of current.

Inserting a thin intrinsic amorphous silicon in between the interface of the n-type c-Si/ helped

reduces the recombination losses which resulted in an increase of the Voc (>0.712V) and

eventually increasing the fill factor (>78.7%). The thin intrinsic layer served as a good

passivator (buffer layer). Many researchers have worked on design optimization of HIT solar

cell, but none has surpassed Panasonic efficiency. It’s currently trending in the PV industry

because of the use of thin solar cell wafers and the low-temperature in production processes,

which is about 200°C. Optical losses occurred at the a-Si:H and the TCO surface and was

reduced by using a low optical absorption material. Another loss the HIT solar cell also

encounter is the resistive losses at the electrode. It was overcome by using low resistive

silver for the electrodes. The doping concentration in the a-Si:H was seen to affect the Voc

during some simulations done by researchers. Thus increasing the doping increases the Voc

and also the defect density, but a certain amount of doping was seen to reduce to the Voc, so

a certain value of doping concentration couldn’t be exceeded in order not to decrease the

Voc.

The band off set in HIT solar cell at both the front and backsides has a huge effect on the

carrier transport and the performance of the cell. S imulating softwares like PC1D are used to

study that. With amorphous silicon having a band gap of 1.7 eV and crystalline silicon having

1.1 eV band gap, the band off set is about 0.6 eV, the valence band offset is three quarter of

the band off set (𝞓Ev= 0.45eV) while one quarter belongs to the conduction band offset

(𝞓Ec= 0.15eV). As the band offset increases the fermi level shift towards the conduction

band of the c-Si which helps in reducing the recombination at the interface, reason why a n-

type c-Si is used instead of p-type c-Si. However, when Ev increases more photons can

reach the absorber layer and more photon-generated carriers results in an increase of the Isc

and Voc. The 𝞓Ev has more effect on the cell than 𝞓Ec, the performance of the solar cell is

opposite direction to the 𝞓Ev. But when the 𝞓Ev exceeds 0.45eV, there will be a deformation

in J-V curve of the cell, especially when it’s well above 0.55 eV. This makes the energy

32

barrier so high that only a small number of the holes can get over the barrier. Eventually the

Jsc and FF reduces and then the efficiency. Figure 3.1 explains the effect of the band offset as

a hole blocking layer and electron blocking layer of a HIT solar cell and c-Si. Ef is the fermi

level of the cell at thermal equilibrium. As shown in Figure 3.1 (a), the p-type a-Si and

intrinsic a-Si have majority and minority carriers as holes and electrons respectively. The

difference in band gap results in a band offset which create an electron-blocking layer and

allows passage for holes. The electron has to transport through the blocking layer by

thermionic emission. Same principle goes for intrinsic a-Si and n-type a-Si and for Figure 3.1

(b), the reflection barriers are increased and blocking layers are reduced [30].

Figure 3.1: Comparisons between the HIT (a) and c-Si energy band gap (b). See Ref [30].

3.2 Key features of HIT solar cell

The three key features of HIT solar cells that give them an advantage based on performance

over c-Si are reduction of optical and recombination losses and minimizing electric losses. A

pictorial representation of the key features in HIT solar cell is shown in Figure 3.2

1. The recombination losses in the c-Si and at interface which could lead to losses of

charge carries (recombination) and eventually reducing the performance of the cell

has been corrected using an intrinsic amorphous silicon, which passivate the surface

of the c-Si, disallowing charge carriers from escaping and also the improved structure

of the a-Si:H reduces the recombination losses.

2. The optical losses at front and back contact, also at the TCO are being corrected by

using low optical absorption material. This is done so as to increase the short circuit

current of the cell. By using low absorption materials, the sun radiation goes straight

into the absorber for power generation.

3. The electrode at the front grid of the HIT solar cell was reduced in thickness so as to

minimize the shading effect and resistive loses. The resistive losses at the grid were

corrected using low resistive silver. Further research is being carried out in other to

improve the performance of HIT cells and also the cost reduction is being considered.

33

Figure 3.2: Key feature of HIT solar cell. See Ref [31]

3.3 Simulation, result and discussion of HIT solar cell

The importance of simulating HIT solar cell is to understand and derive crucial parameters

that explain the best working principles for HIT cells on n-type c-Si wafer. The effect of

band offset, mode of carrier transport, thickness of each layers are also explained using

simulating softwares. Here, the simulated structure will be a-Si:H(p)/ a-Si:H(i)/c-Si(n)/a-

Si:H(i)/a-Si:H(n+). Parameters in Table 3.1 was used for the simulation.

Figure 3.3: HIT Structure proposed by SANYO

The simulation will be done by using PC1D 5.9 which provides a convenient way to obtain

deeper understanding of the device physics, factors determining the performance of the

device and to evaluate the role of various parameters in the fabrication process. For example,

some writer explains that the intrinsic layer, which acts as passivator between the a-Si:H and

c-Si doesn’t have much or no benefit on the cells performance [33].

34

PC1D, Personal Computer One Dimensional, is a numerical simulating software for solar

cells developed in Australia at the university South Wales of Sydney. It allows to simulate the

behavior of photovoltaic structures based on semiconductor with respect to one-dimensional

(axial symmetry).

Assumption made for simulation

Device area –100 cm2

Light source - One sun (AM 1.5, 1000W/m2)

Front and back surface surface – Textured

Exterior front reflectance 3%

Exterior rear reflectance 95%

Ambient temperature at 300K

Emitter Contact Enabled

Base Contact 0.0015 Ω

Internal conductor 0.3S

The defect in each of the layers are not considered here. The electron and hole diffusivity,

front-surface and real-surface recombination and all other simulating parameters have been

taken from ref [34].

Table 3.1: Device parameters used as an input for computer simulation [33] [34]

Parameter a-Si(p) a-Si(i) c-Si(n) a-Si(n)

Thickness (μm) 0.01 0.01 300 0.01

Dielectric constant 11.9 11.9 11.9 11.9

Electron affinity (eV) 3.9 3.9 4.05 3.9

Bandgap (eV) 1.72 1.72 1.12 1.72

Electron mobility (cm2V-1s-1) 7 7 1140 7

Hole mobility (cm2V-1s-1) 1 1 420 1

Doping Concentration (cm-3) 5x1019 0 1.5x1016 8.2x1019

Bulk Recombination (μs) 10* 0 10* 10*

Front-Surface Recombination (cm/s) 107 107 107 107

Rear-Surface Recombination (cm/s)

107 107 107 107

*Bulk Recombination values are assumed for typical solar cells.

35

Simulation Results

PC1D simulating software have helped in determining the possible best parameters for HIT

solar cell to perform optimally. The simulate diagram is shown in Figure 3.4, similar to

Figure 3.3. Some of the parameters were varied initially in other to have an insight of their

effect on the solar cell. Effect of no buffer layer on the open circuit voltage has been studied.

Figure 3.4: Schematic diagram of simulated HIT solar cell using PC1D

Figure 3.5: Simulated current, voltage and power curve of HIT solar cell

0

0,5

1

1,5

2

2,5

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8

Power[W

]

Current[A]

Voltage[V]

lVGRAPHOFSIMULATEDHITSOLARCELLUSINGPC1D

IV

Power

36

The current, voltage and power curve of HIT solar cell can be seen in Figure 3.5. The values

of the open circuit voltage, short circuit current and maximum base power are Voc = 0.7086

V, Isc = 3.912 A, P = 2.333 W respectively. The parameters which are varied are thickness of

the intrinsic, c-Si and BSF layer, effect of the doping concentration of amorphous layers,

The impact of different intrinsic layer s thickness on cell performance

The intrinsic layer between the a-Si/c-Si serves as a passivator. The layer was varied at

different nanometers (2nm-20nm) to prove the effect of the buffer layer thickness on HIT

solar cell performance. From Figure 3.6, a maximum efficiency of 23.46% at layer thickness

of 2nm was reached. As the thickness increases a slight drop in efficiency is seen. Continuous

increase in the intrinsic thickness layer results in a decrease of efficiency. For manufacturing

processes, a thickness of 5nm or lesser might be favorable and cost effective.

Figure 3.6: Influence of the intrinsic layer thickness on efficiency

The effect of back surface field thickness

The Back surface field of a solar cell reduces effect of rear surface recombination of charge

carries on voltage and current. It usually consists of more dopant than in the emitter which

then create a barrier for the minority carriers. The BSF helps in maintaining the minority

carries concentration in the bulk of the cell resulting to good cell performance. Increasing the

BSF thickness in a HIT solar cell doesn’t have much effect on the cell’s performance as

shown in The thickness was varied from 2nm to 14nm. Slight changes were noticed in the

open circuit voltage, current and the efficiency of the cell. This means the performance of the

solar cell does not require very thick BSF as shown in Table 3.2 and cost could be saved

using very thin BSF.

37

Table 3.2 Effect of back surface field thickness

Thickness [nm] Voc [V] Current [A/cm²] Efficiency [%]

2 0.7086 3.911 23.26

4 0.7086 3.912 23.26

8 0.7086 3.9120 23.29

10 0.7086 3.9120 23.33

12 0.7086 3.912 23.33

Performance of the simulated solar cell, open circuit voltage and short circuit current as

a function of the doping concentration of the N-type amorphous layer

From Figure 3.7 and Figure 3.8 it could be seen that the effect of the doping concentration of

the BSF on the IV characteristic of the cell, which also results in efficiency of the cell. At

doping concentration of 8.2x1019 cm3 and 8.2x1020 cm3 the efficiency and short circuit current

started to smooth out, showing that a limit has been reached. A highly doped BSF creates a

blocking layer for the minority carries which explains the increase in the efficiency of the

simulated cell at high doping concentration. The open circuit voltage is also a function of

doping concentration of the BSF as seen in Figure 3.8.

Figure 3.7: Impact of doping concentration of the N-type amorphous layer on efficiency

and short circuit current

38

Figure 3.8: Impact of doping concentration of the N-type amorphous layer on open circuit

voltage

Impact of wafer c-Si thickness

The thickness of c-Si has a huge impact on the efficiency of the solar cell. A thickness of 200

micro meter should be perfect and cost effective. As shown in Figure 3.9, wafer thickness of

50-micrometer results in a lower efficiency and that is due to silicon having a low absorption

coefficient. The photons only penetrate through the wafer without being absorbed by the c-Si

layer, leading to low short current and open circuit voltage. Further increase in the c-Si could

slightly increase the efficiency but the considering the cost of fabricating thicker c-Si is

lucrative.

Figure 3.9: Impact of wafer thickness on efficiency

39

Influence of the surface area on the I-V characteristic

The surface area of the solar cell was varied at 100cm2, 200cm2 and 300cm2 as seen from

Figure 3.9. The IV characteristics is seen to be higher at 300 micrometers. As the surface area

increase, the power output also increases. At 100, 200 and 300 centimeter square, the

maximum power outputs are 2.333W, 2.328W,2.295W respectively.

Figure 3.10: Influence of the surface area on the I-V characteristic

From the simulation, it’s possible for HIT solar cell to reach higher efficiencies if the defects

in the amorphous layers are reduced during manufacturing and the c-Si wafer having a high

purity level.

3.4 Future prospects HIT solar cells

The future prospect of HIT solar cell is to produce a cell that is cost effective. A cell with low

cost in production and a high percentage of efficiency will definitely have a huge impact on

the PV market and the best choice for this is the HIT solar cell. High conversion efficiencies

have been recorded in laboratory and on the market level. Panasonic are aiming to improve

the efficiency by fabricating the heterostructure with low interface defect which is caused by

poor material, excess doping or damage during deposition which eventually reduces the

power output of the cell. In order for mass production of HIT solar module, improvement in

heterojunction interface of a-Si:H/c-Si, reduction in optical losses, reduction in resistive

losses and enlargement of effective area are areas that have to be considered in other to

improve the performance of HIT solar cell and increase the market share.

In 2003, Sanyo established a HIT panel production center in Monterrey, Mexico and in 2005

a facility for production of modules was also established in Hungary at Sanyo Hungary Kft.

in order to increase the market both in Europe and North America. Currently the market share

of HIT solar cell hasn’t been accounted for but it’s being assumed to gain a market share of

up to 10% by 2024.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Cu

rre

nt

[A

Voltage [V]

100cmˆ2200cmˆ2300cmˆ2

40

4 Chapter Four: Weighing-factors

The major focus of this thesis is to evaluate HIT solar cell with other state-of –art- solar cells

using different weighing factors. The evaluation is categorized into six parts; energy analysis,

temperature cost of production & raw material, efficiency, life cycle, sustainability and

environmental issues. The evaluations are based on module level. The balance of system is

not included due to the fact that it is dependent on the type of system in which the module is

used (e.g. grid-tied or stand-alone).

4.1 Module cost of production and raw materials

In this thesis, the cost of production does not include overhead expenses (accounting fees,

advertising, insurance, legal fees, labor burden, rent, repairs etc.). The average cost is

evaluated using the module production cost in euro per watt peak (€/Wattpeak), efficiency,

thickness and life cycle. Table 4.1, shows a comparison in the data used in evaluating the cost

production. Efficiency is important because, the higher the efficiency of a module becomes,

the cheap it gets. The cost of production is based on modules only and doesn’t include cost of

energy used in production. This helps to eliminate the ambiguity in having to make

assumptions. Cost of organic and related material won’t be considered here due to their low

share in the PV market. Over the past 30 to 40 years the cost of PV modules has decreased

but there is still a potential of each PV technologies prices reducing.

c-Si- Crystalline silicon (either mono-crystalline or poly-crystalline) PV module is

one of many silicon-based semiconductor devices. It has a huge share of the PV

market. The crystalline PV modules consist of mono and poly crystalline and most of

them range between 60-72 cells with a nominal power of 120-300 Wp [35] and cost

ranging between 0.88€ /Wp and 1.8 €/Wp [36]. The cost of production of

polycrystalline is less costly and intricate due to certain production process that was

excluded but found in production process of monocrystalline. Silicon being the basic

material and abundant in nature makes the module highly available and only few

grams per watt peak are used during production. Other materials involved in

production, e.g. silver and aluminum are ignored because few quantities of them are

used in fabrication. The factors that determine the yearly cost of production of solar

module are usually efficiency, plant size, and cost of raw material, size of wafer and

rate of consumption of raw material. Most market c-Si module has efficiency between

13-19%, thickness of between 150-500 µm are used for both practical and commercial

purposes, with 25 years lifetime and a manufacturing scale of 500-1000 MWp per

year [35]. To decrease the high production cost, increasing the efficiency, advances in

cell designs, decrease in the cost of supply of silicon feedstock, design module for

long lifetime, reduce the production steps and reduce the energy consumption of the

processes will eventually lead to low cost of c-Si PV modules.

Thin-Films (a-Si, CdTe, CIS/CIGS, Group III-V)

The four différent major types of thin-films are based on depositing thin layer of active

41

materials on large-area substrates, which are usually glass, metal foil or plastic. Most thin

films are cheaper in production, availability of material but low efficiency and low stability

under working conditions (Staebler-Wronski effect), which is really sabotaging further

improvement of thin-films.

a-Si:H- Hydrogenated amorphous silicon films are classified under thin films

technologies and has the highest share. They are only few micrometers thick with low

cost of manufacturing compared to c-Si. The production concept in which the cells are

built on a substrate (plastic or glass), this makes its cheaper in terms cost per watt.

Their efficiencies are also lower (8-10%) but they have a fair stability under high

temperature, short payback period and round about the same lifetime as c-Si [35].

Researchers are working on increasing the efficiency in the nearest future so as to

utilize the low cost of production. The production capacities of thin films have

increased in hundreds of MW in the last five years and currently the cost isn’t so low

to that of c-Si, it ranges between 0 .50€/Wp -1.2 €/Wp and the target is $1.00€/Wp per

watt by 2017 [8]. Increase in the volume of production of a-Si:H would also reduce

cost.

CdTe- This type of thin-films is also easy to fabricate using different deposition

techniques. The efficiency is said to depend significantly on deposition temperatures,

growth techniques and the substrate material used [35]. One of the factors sabotaging

the increase in production volume of CdTe is the shortage of tellurium and toxicity of

cadmium. The cost reduction and efficiency strongly depends on that. If a substitute

readily available material can replace Tellurium, then it could compete on same scale

as c-Si. Current efficiency and production volume is 6-10% and 10MWp respectively

[37].

CIGS- The cost of indium is one limiting factor for cost reduction of CIGS. The

efficiency is close to the rest of the thin-films and if indium could be replaced by an

abundant element is could compete with c-Si base on efficiency. CIGS is less intricate

and cheaper to produce, just like any other thin-films. The production volume is said

to be less than 10MWp [37]. Due to their direct band gap, between 1 and 2

micrometers is enough to absorb sunlight radiation. Module efficiency of CIGS is

similar to that of CdTe.

HIT- The most possibly logical way to reduce cost of PV module is to increase the

module efficiencies and improvise a new cell designs that cost less. HIT solar cell is

one that fits into this idea. The HIT solar is a unique hybrid solar cell that optimizes

the advantages of both amorphous and crystalline solar cells. These advantages could

help the PV industry compete with energy generation from fossil fuels. A

combination of cheap but less efficient amorphous silicon and a highly efficient but

high cost c-Si is the basic structure of this heterojunction solar cell. There are

numbers of reason why HIT would be cheaper and more efficient than c-Si in the

nearest future. Its ability to perform and convert efficiently under high temperature,

more power generated with limited space, low thermal energy, which helps reducing

42

payback period, a bifacial surface, which utilizes the front and back incident light, are

reasons why HIT will be better off in terms of cost and efficiency than c-Si. Currently

the production volume is a bout 900MWp [37] and hoping to increase in the nearest

future. A HIT module for commercial purpose under standard condition would

produce an efficiency of 19.7% (Panasonic, 2016).

Table 4.1 Data used in determining the cost of production of modules

Technology c-Si a-Si:H HIT CIGS CdTe

Cost

(€/Wattpeak)

0.88 -1.8 0.50-1.2 N/a ca. 1.0 ca 1.0

Efficiency

(%)

13-19 8-11% 19.7 10-12 6-10

Basic material

shortage

No No No Indium Tellurium

Production

scale (MWp)

>1000 >500 900 <10 10

Life cycle (yr) 25 20 20-25 20 20

Thickness

(µm) 150-500

100 200 1-2 10

Payback

period (yr)

2-4*

<1

<1

n/a

n/a

*The payback period is also dependent on the type of crystalline silicon module.

n/a-Not available

The manufacturing cost or market price of PV modules in euros per watt peak is expected to

reduce in the nearest future. A learning curve is used to measure the progress in cost of PV

both on domestic and commercial level. This is done by considering the cost and the

production scale. This will help to determine a possible futuristic cost of PV modules. An

expected increase in the volume will lead to decrease in price. Also the progress of the life

cycle, efficiency, thickness and payback period helps to determine the cost of PV modules.

For c-Si with a production scale of over 1000MW and efficiency of 19% on module level,

one would think it should be cheaper but due to its production process, the cost seems to be

the highest in the PV industry unlike the a-Si:H having a lower production scale and lower

efficiency, the cost is less. With further research it will compete better with c-Si.

43

4.2 Analysis of efficiency for evaluated technologies over the year till date

As mentioned earlier, the increase in PV modules efficiency can hugely affect the PV market

positively to extent that it can compete with conventional sources of energy. The efficiencies

of the evaluated technologies have improved since they have been invented over 30 years

ago. Based on cell efficiencies in laboratory, the multi- junction cells (Group III-V) have been

the leading of the chart, followed by the crystalline and other thin films (NREL, 2016). The

laboratory efficiencies don’t equal the efficiencies when the cells are subjected to the real life

thermal and mechanical stress or test conditions, but it has help improved the production cost.

A comparison of the module to cell efficiency could be seen in Figure 1.1, which was

compiled by Green et al. PSE AG 2015. The efficiencies of current PV modules range

between 9-19%. The high part of the range is occupied with c-Si and heterojunction modules,

while the lower part is mostly occupied by thin-films. The efficiencies of PV modules still

has the potential of going beyond 19-20% in the future with the rate at which the research and

development of solar energy is progressing. Cheaper and more efficient materials are being

studied in other to develop cost effective solar cells with higher efficiencies.

Certain common factors are seen to affect the conversion efficiency of the evaluated

technologies. Some of these factors are due to the structure of the solar module while some

are uncontrollable factors. They mostly include recombination, temperature energy of the

light, reflection, and resistance. For instance, c-Si solar cells have band gap of 1.12 eV,

photon energy lower than this band gap doesn’t add to photogeneration and it’s considered as

waste because of its incompetence to interact with the cell. But photon energy that equals the

band gap leads to photogeneration. Photon energy that’s far greater than the band gap of the

cell is re-emitted has heat or light within the cell, which leads to loss of about 20%. Solar

cells like group III-V multi-junction are designed to utilize the full light spectrum; light with

low and high energy can be harnessed by the cell. The average temperature for most solar

cells is 25°C, temperature exceeding this number damages the cell (Staebler-Wronski effect).

For every increase in the temperatures a decrease in efficiency is realized. The novel cell,

HIT which has a high temperature coefficient is designed by Sanyo, works perfectly under

increase in temperature >25°C unlike the c-Si and thin films. Efficiency reduction due to

reflection of sun light at the surface of the cell is usually minimized using anti reflecting

coating. Layers of the anti-reflecting coating reduce reflection to wide range of the light

spectrum. Texturing the surface of the cell in a pyramid like manner also help in reducing

reflection, because as the light strikes the surface and then reflects, the reflected light bounces

back and forth on the textured surface before it’s finally absorbed. Recombination is an

important factor when it comes to efficiency reduction as high rate of recombination can

render a solar cell useless in performance. The charge carries recombine before they

contribute to electricity generation due to impurities or defect in the material. Loss of charge

carries due to recombination reduces the efficiency because electrons are lost, which could

have been used in electricity generation. Electrical resistance in a solar cell could occur in

different section of the cell, it could occur at the contacts, interface and the layers. Reducing

the defects in the layers is one step to reduce electrical resistance. Passivating the interface to

seal up defects and back surface contact are often used to reduce shading effect and minimize

electrical resistance. Correcting these factors will definitely improve solar cells efficiencies in

44

the future and definitely will have an impact on the module level too. The present progress in

module efficiency is a simple sign to see the future of PV competing with conventional

energy sources. Figure 4.1 shows a chart by the Japan PV roadmap toward 2030 showing the

possible efficiencies of PV c-Si and a-Si:H cells and modules.

Figure 4.1: Projections for cell and module efficiencies for 2004 to 2030 by The Japan PV

roadmap toward 2030. See Ref [37].

Figure 4.2: Solar cells efficiency chart. NREL 2016

45

4.3 Temperature effect on performance of evaluated cells

Temperature affects the flow of electrons through any electrical circuit, this occurs when a

decrease (extremely below 25°C) or increase (extremely above 25°C) in temperature occurs.

This causes an increase in the electrical resistivity. The latest National Renewable Energy

Laboratory (NREL) for solar cells at temperature 25°C is shown in Error! Reference source

not found.. For solar modules, this effect reduces efficiency of the module. Thus, researcher

aim to design PV modules to be able to withstand the decrease and increase of the

temperature at the installed location. Most solar modules are designed to work effectively at

25°C but as there exists locations like North Africa where the temperature could exceed the

designed temperature, PV modules manufacturer are researching PV modules that could work

effectively beyond non-optimal conditions. The type of material used in fabricating PV

modules primarily affects the operation of the module at certain temperature, so materials

with good temperature coefficient are rather used. The efficiency of PV modules ranges from

8-20%, meaning that only 8-20% of the solar radiation (kW) striking an area (m2) on the

earth surface is converted to electricity. The rest is being dissipated as heat in the PV module.

Now subjecting such a module in a location with very temperature, it definitely decreases the

efficiency because of the combination of the dissipated heat and the high surrounding

temperature. Regions with high irradiation of about 2000 kWh/kW and low temperature of

about 20-25°C e.g. the south Andes and fast east Asian countries are best for PV modules

installation. Figure 4.1 shows global potential PV energy generation. Areas with light yellow

are best for PV installation because they are known for moderate temperature.

Figure 4.3: Global potential map of PV energy generation (Ypy) of c-Si PV module [41]

The PV modules are rated at standard test condition (temperature 25°C, irradiance 1000

W/m2 and air mass 1.5). For any PV module to work effectively above the standard

temperature must have a good temperature coefficient. For polycrystalline PV modules, a

factor called Staebler-Wronski effect is the definition of reduction of efficiency with increase

in temperature. For every increase of 1°C in temperature there is a decrease in efficiency and

46

voltage. It also goes the same for other conventional solar modules, unlike HIT modules,

which produce s more electricity (kWh) at an increase in temperature.

4.4 Life cycle analysis and Energy analysis of evaluated solar cells

The impact of conventional energy use in our current society is well known to everyone;

global warming and loss of terrestrial and aquatic life. In other to minimize these negative

impacts, harnessing alternative energies on a large scale is a priority. Harnessing these

alternative energies requires system that would not have a negative by-product on the

environment and the system should be able to produce more energy than that used in

production. A life cycle analysis is used to determine an overview of how sustainable an

energy generating system is. It evaluates the system from its beginning stage to its end of life

stage; the energy required in production (raw material, manufacturing process, packaging,

transportation), the energy the system its self generates and energy needed to dispose or

recycle it. The energy analysis and the payback period of PV modules explain in full details

the energy flow in production of the module and how and when the system generates more

energy than the invested energy. The payback period is dependent on the location of

installation, type of installation and the type of PV system installed. For roof-mounted c-Si

module installed in North Africa, it would have a lower payback period than that installed in

northern Europe. The energy payback period is calculated by adding all energy involved from

production to end of life and dividing by the amount of energy generated, efficiency of the

grid and energy used in maintenance [39].

The life cycle inventory will also be discussed. It explains the input and output material,

emission and energy in details. In this thesis, energy analysis, life cycle inventory, and life

cycle analysis is only based on the PV modules. The balance of systems, energy used in

transportation for both produced and used modules will be excluded and also energy used in

maintenance. In this section the energy analysis and life cycle analysis will focus mainly on

crystalline, amorphous and HIT PV modules. The life-cycle of all PV modules are mostly the

same, from manufacturing to usage and then decommissioning. Figure 4.3 helps analyses the

life cycle process for a PV module, showing the energy analysis of each life cycle phase.

Figure 4.4: Life-cycle stages of a PV module (Emtl = material production energy, Emfg =

manufacturing energy, Esol = insolation; Euse = use energy requirements, Eelm = end of life

energy, Edstr = distribution energy, Egen = energy generated).See Ref [39]

47

The material production energy is the needed energy to extract and refine the raw material for

production. This energy could be used in drilling and excavating the raw material. The

distribution energy is what is involved in transportation of the raw material from excavation

site to the company or industry for fabrication. The manufacturing energy is the energy

needed to combine all raw materials to form or fabricate a module. The energy insolation and

energy use are what the module consumes to generate energy, Egen. The Esol is the irradiance

over time. After the life time of the PV module the energy needed to move them to point

recycling and energy needed to recycle of dispose id the end of life energy. The modules

information; operational data, compositional data, size, thickness will be defined and the

energy analysis will be broken down for each PV modules based on input energy and output

energy, energy payback time, life time of module and energy generated.

c-Si- The crystalline silicon solar module discussed here will represent both monocrystalline

and polycrystalline commercial modules, having an average efficiency of 18% and active

layers of front contact/ARC/n-type c-Si/p-type-c-Si/back contact. The anti-reflecting coating,

ARC is usually TiO2, the contacts are usually silver, the modules are encapsulated in a glass

and the frames are made of aluminum.

a-Si:H- The amorphous silicon modules with an efficiency of 11% and active layers of

glass/ARC/p-type-a-Si:H/a-Si(i)/n-type a-Si:H/metal back contact. Glass is usually used as

the substrate, aluminum as metal contact and Indium tin oxide is usually used as the

transparent conducting oxide.

HIT-The HIT module is combination of both c-Si and a-Si:H. The active layers are front

contact/TCO/p-type-a-Si:H/a-Si(i)/n-type-c-Si/a-Si(i)/n-type-a-Si:H/TCO/back contact. The

TCO used in Indium tin oxide, the cells are enclosed in a glass, the contacts are made of

silver and aluminum is used for the frames.

The material production energy used in fabrication of the compared modules are usually rated

in MJ kg-1. This is the energy used in production of the basic material and it’s the primary

energy consumption of each module, i.e. Emtl→ Edstr →Emfg. The primary energy consumption

of each of the elemental material varies because some are produced using coal, natural gas or

nuclear [39].

Analyzing the energy input, Ein or the energy invested for production of the module starts by

analyzing the energy used in production of raw material, this is done by multiplying the

specific source of energy used in producing the specific raw material by the quantity used in

producing the module (including wasted material quantity) [39]. The energy for

manufacturing is next analyzed, it includes energy for distribution, energy for labor, energy

for administration, research & development and energy for fabrication. The energies used by

the modules after production is energy for installation, energy for operation and maintenance

for the life span of the module. The energy input, Ein could be simply written;

Ein = Emtl +Emfg + Edstr + Eelm (1)

48

For a c-Si PV module, the process energy and other energies involved in production of a 1m2

varies between 5300 -16,500 MJ/m2 depending on the technology of the c-Si PV module and

greenhouse gas emission, GHG of about 63g-CO2/kWh is emitted [40]. Germany with an

average solar radiation of about 1500kWh /m2 / year and c-Si module efficiency of 18%, an

energy payback period of about 2 years could be attained. For amorphous silicon PV

modules, the energy consumption used in production of a single meter square PV module is

lesser to that of crystalline as shown in Table 4.2. This leads to its low energy payback period

and GHG emission, because the thermal budget is low. According to the research case study

of Lewis and Keoleian, a-Si:H only consumes 491MJ/m2 of primary energy for production,

the other energy involve in the production is ranges between 864 and 1990 MJ/m2 [40], so a

total of about 2500 MJ/m2 is consumed depending on the technology and GHG of 34.3

gCO2eq./kWh is emitted [40]. Also for Germany with an average solar radiation of about

1500kWh /m2/ year, an energy payback period of about a year or lesser could be attained, this

values are harmonized with a-Si efficiency of 11%. The HIT solar cell is a combination of

both amorphous and monocrystalline active layers (silicon). The monocrystalline silicon used

isn’t as expensive as it is in a monocrystalline solar cell because only few micros are used in

fabrication of HIT. The second active layer is the amorphous silicon which is cheaper in

production. Harmonizing the cost and energy used in the solar cells, one could say that HIT

could be cheaper in terms of thermal budget in comparison to c-Si and a-Si:H and also less

GHG could be emitted. With a higher module conversion than crystalline and amorphous,

harmonizing all these, HIT solar cell could be assist the PV industry and reduce energy crisis.

The intrinsic layers of amorphous silicon in between the c-Si and a-Si:H is also said to be

cheaper in fabrication because it doesn’t go through doping processes and it also a few micro

meters in thickness.

Table 4.2: Energy analysis of compared PV modules

PV Technology Energy

Required (MJ/m2)

*Energy pay back

period (years)

GHG emission

(gCO2eq./kWh)

Module conversion

efficiency (%)

Crystalline PV 5300 -16,500 2-4 63 18

Thin films PV 2500- <1 34.3 11

HIT PV N/A 2 N/A 19.7

*EPBP-Energy payback period is dependent on location of installation.

*2-4 - The payback period is also dependent on the type of crystalline silicon cell.

The output energy, Eout is the total energy the PV module generate when mounted at a

particular location under a certain solar insolation. The energy generated by the PV, Egen is a

multiplication of the efficiency of the module, the aperture area, the time and the solar flux of

the location.

4.5 Environmental Issue (Toxicity and Recycling)

The aim of solar energy is to reduce pollutant, GHG emission and curb energy crisis in the

world. Solar energy is very beneficial to the environment because no pollutant or GHG is

emitted throughout its lifetime, unfortunately the primary energy used in fabricating solar PV

modules use to generate electricity from solar energy emits GHG and pollutants. So many

49

questions have been raised on how ‘green’ PV modules are. Renewable energy critics always

ask about the GHG emitted at the first phase of PV modules and the toxicity they release at

their end-of- life (disposal). During fabrication PV modules manufactures are exposed

hazardous materials or chemicals, e.g. silicon dust, hydrochloric acid, sulphuric acid, nitric

acid, hydrogen fluoride, 1,1,1-trichloroethane, and acetone. But during operation or their life

time these hazard materials are sealed up in the PV module and at their end-of- life they are

exposed during disposal which could harm the environment. The amount of hazardous

material used is dependent on the technology and size of the PV modules. Some PV modules

aren’t as hazardous as some. Table 4.3 shows the major hazards in the evaluated PV

technologies. The thin films solar modules like CIGS and CdTe have toxic substance.

cadmium selenide and cadmium telluride which pose a threat to human health. The

fabrication of these modules is done under certain regulations in other to protect the workers

from toxic materials. Discussion on the environmental issues of the evaluated technology is

done below.

For crystalline PV modules, the manufacturers are exposed to toxic material like

silicon dust, silicon dust, hydrochloric acid, sulphuric acid, nitric acid, hydrogen

fluoride, 1,1,1-trichloroethane, and acetone which is a threat to their health. When

silicon ingot is formed, it’s sliced into wafers, during this process silicon dust is

released and forms with air molecules that affects the inhalation for the workers and

possible threat to their respiratory organ. Acids used in purification of the wafers also

poses a threat to worker’s health, it reacts violently with water, causes skin burns, and

is a respiratory, skin, and eye irritant [12].

From the fabrication to end-of –life of amorphous silicon, some hazardous materials

that threaten the human health are also used. During production of a-Si cells, the

chemical compound silane or chlorosilane gas is heated and mixed with hydrogen

[58], and it is then deposited on a substrate as thin films. The chemical compound

silane or chlorosilane gas is very explosive [12], which have caused many explosions

in PV manufacturing companies. Also acidic compounds which are used in cleaning

poses threat to the workers.

Apart from Cadmium telluride (CdTe) thin-film solar modules health hazard, there is

a major concern which is the scarcity of cadmium, this could limit then supply of

CdTe solar modules. Fabrication CdTe, cadmium, cadmium sulphide, cadmium

chloride, and some organosulfur compounds are used. There are considered very toxic

by the U.S. Environmental Protection Agency (EPA) [12] and could lead to live,

kidney and other major internal organ failure.

The health risks in production of CIGS PV modules are mild. During production,

hydrogen selenide, copper dust and gallium are potential breathing risks to workers.

Accumulation of these chemical compounds in the body leads to major organ failure.

GaAs PV modules are fabricated using toxic compounds like arsenic, Phosphine and

trichloroethylene [12]. Excess exposure to these compounds affects the lungs, liver,

immune, and blood systems.

As the HIT PV module is a combination of crystalline and amorphous silicon, the

health risk is a combination of the health hazard of both silicon types. But fortunately

50

its only little amount of these silicon is needed in fabricating HIT solar modules, so

less health risk but continuous exposure will also affect major internal organs.

Table 4.3: Major Hazards in evaluated PV technologies

PV technology Types of Potential Hazards

c-Si SiH4 fires/explosions, silicon dust. Acid

burns.

a-Si:H SiH4 fires/explosions, ClH3Si, Flammable

H2

CdTe Cadmium and telluride toxicity,

carcinogenicity chemical compounds.

CIGS Cadmium and selenium toxicity,

carcinogenicity chemical compounds.

GaAs AsH3 toxicity, , carcinogenicity chemical

compounds, Phosphine.

HIT SiH4 fires/explosions, silicon dust, Acid

burns, Flammable H2

AsH3- Arsine, ClH3Si- Chlorosilane, SiH4-Silane, H2-Hydrogen

4.6 Sustainability

Energy crisis and global warming is not a new phenomenon to everyone these days.

Alternatives Sources of energy need to be harnessed. The most harnessed renewable energy

source (excluding hydro-power) is possibly solar and wind energy, having a total capacity of

177GW and 370GW globally. Solar energy seems to be the most viable out of all renewable

energy sources. There are various technologies that are being used to covert the the sun’s

energy to electricity, e.g. photovoltaic cells, concentrated solar panels and building integrated

photovoltaics (BIPV). Here, the focus is on photovoltaic cells. This technology is self-

sustaining; it can pay back the energy used in production in multiple folds but another

bothering factor is their environmental impact, ‘are the solar panels cradle to cradle or cradle

to dust’. If solar panels are cradle to cradle, environmental impact are reducing and if they are

cradle to dust, toxic material in solar panels could have negative effect on the environment

when they are being disposed after their life time. The environmental impact of PV modules

is a major concern. The PV industry is developing new materials to enhance solar energy

conversion to meet world demand. Currently most PV modules are made up of silicon,

cadmium telluride, gallium arsenide, copper indium gallium selenide. From table 5, one could

see the embodied energy used in production and GHG emission of each of the photovoltaic

module technologies. How sustainable these modules are, depends if they can actually pay

back the embodies impact they made during production. Deriving the right figures for

sustainability of solar modules isn’t an easy one. The rate of power produced using a

photovoltaic module to the amount of greenhouse gas saved (kgCO2e/kWh) is used to

51

explain how sustainable photovoltaic modules are. The rate at which GHGs are saved could

be increased if manufacturing and installation of PV modules are done in the right location.

For example if PV modules are manufactured and installed in mid-east, North Africa or other

countries with high solar irradiance, there could be a huge potential in solar power and carbon

footprint could be reduce on a larger scale and therefore proving how “green” PV cells are.

52

5 Chapter Five: Discussion and Conclusion

Majority of the solar modules in the market are crystalline silicon based. It isn’t surprising

because these solar modules have high efficiency (10-18%), in comparison to to others solar

modules. Higher efficiencies have been record in indoor or laboratory measurement. The

crystalline modules have shown good stability with variation of weather, from intense hot

weather to below zero-degree weather and exceeding more than 20 years lifespan. The

limiting factor of this technology is its cost. Fabricating crystalline modules consumes huge

amount of energy which doesn’t make cost competitive with conventional power generation.

This lead to the idea of thin films. Thin films are thin materials having thickness ranging

from few nanometers to few micrometers. Thin films solar cells offer greater design

flexibility for a variety of applications, they are produced for small applications e.g.

calculator, solar fan, solar touch light e.t.c. to larger applications like roof top PV systems and

solar farm. Thin films are cheap to fabricate, which results in a short payback period unlike

crystalline solar modules. Thin films have a low stabilised efficiency at module level. Group

III-V multi junction solar cells, which currently have the highest cell efficiency but are also

expensive to fabricate but using them as solar concentrators and space application, could

overcome the expensiveness of Group III-V multijuction solar cells. The negative side of

multijunction is their composition of toxic materials and they are rare in nature. Huge

progress has been on in improving the efficiencies, production processes and cost of CdTe

and CIGS solar cells. Also the sabotaging factor is their toxicity and non-abundance in

nature. Organic solar cells seem to have a brighter future in the PV market. Research and

development of organic solar cell that use organic materials such as dyes, semiconductor

polymers and fullerenes. Further development is that of tandem/multijunction devices as

these fully utilize the solar spectrum to generate electricity. A very interesting development is

the HIT cell. It is a combination of amorphous and crystalline solar layers. It has a low

production costs and yet achieve high efficiency. It’s very comparable to crystalline solar

cells based on efficiency, life span, durability under temperature variations and so more.

Simulating softwares has helped in understanding the physics of HIT solar cell. A

combination amorphous layers and n-type crystalline silicon has been simulated using PC1D.

The impacts of the doping concentration, the thickness of different layers and the surface area

of the cell on the performance of n-type c-Si base heterojunction with intrinsic thin layer

(HIT) solar cell have been studied using PC1D simulation software. An efficiency of 23.3%,

Voc (0.7086v) and Isc (3.912A) have been achieved at a surface area of 100 cm2 and layer

thickness of 300 µm (c-Si layer) and 10µm (a-Si:H layers). The simulated structure is a-

Si:H(p)/ a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+). The optimization of thicknesses resulted in

high efficiency, except for the intrinsic layer thickness, which reduced the efficiency of the

cell. A thickness of 10µm is suitable for the intrinsic layer. An introduction of a BSF

enhanced the open circuit voltage. Varying the BSF thickness was seen to have no effect on

the open circuit voltage likewise other characteristics of the cell.

53

A slight increase in doping concentration of the layers could possibly increase the cell’s

efficiency. The c-Si wafer thickness at 250µm was seen to result in same efficiency as 300µm

but the thickness of the c-Si wafer is very vital. A thickness of 300µm for the c-Si wafer

could guarantee good absorption coefficient and not easily prone to mechanical or thermal

stress but it could also increase the cost of the cell. HIT solar cell could be fabricated at a

temperature of about 200C or lesser and at a rapid process unlike c-Si solar cell which

requires temperature of about 1000C. The c-Si wafer required for HIT is of low quality, it

doesn’t need to be almost pure. For c-Si solar cells the silicon wafer needs to be very pure to

be used to fabricate solar cells. The deposition of the p /n junction and other parts is done by

radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) and it’s less

intricate unlike c-Si solar cells. A thickness of 200-300 µm would be perfect for a HIT solar

cell. Most c-Si solar are 500µm or more which makes them expensive and increase their

thermal budget.

The simulation results could be far from truth but it gives a very good understanding of the

device physics of the cell and possible effect of different parameters which could be used

fabricating HIT solar cell with high efficiency and could compete with crystalline solar cells

in the nearest future. Research and development of new solar photovoltaic technologies is

very important but also fabricating technologies that produce low levels of greenhouse gases

and have a low environmental impact is very vital. Sustainability is a vital factor to ensure

when developing solar cells. Lowering the cost of PV modules and increasing the efficiency

will definitely make solar energy power generation able to compete with conventional form

of power generation. Effect of temperature and environmental hazards associated with the

technologies are important a section to always consider in PV research and development.

54

6 References

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