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中 華 大 學 碩 士 論 文

太陽能電池製作與效能改進之研究 Process-related Solar Cells Performance

系 所 別: 電機工程學系碩士班 學號姓名: M09701008 葉沐詩 指導教授: 謝 焸 家 博 士 鳳 德 博 士

中 華 民 國 九 十 九 年 七 月

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中 華 大 學 (博碩士論文授權書)

本授權書所授權之論文為本人在中華大學

電機工程學系(所)電子電路組 98 學年度第二學期取得碩士學位之論文。

論文題目: 太陽能電池製作與效能改進之研究

指導教授: 謝焸家教授,荊鳳德教授

研究生姓名: 葉沐詩

指導教授:謝焸家教授,荊鳳德教授 授 權 人:葉沐詩

簽 名:______________________ (請親筆正楷簽名)

中 華 民 國 九 十 九 年 七 月 二 十 五 日

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中 華 大 學 碩 士 班 研 究 生 論 文 指 導 教 授 推 薦 書

電機工程學系碩士班葉沐詩君所提之論文太陽 能電池製作與效能改進之研究,係由本人指導撰 述,同意提付審查。

指導教授 (簽章) 指導教授 (簽章)

中 華 民 國 九 十 九 年 五 月

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中 華 大 學 碩 士 班 研 究 生 論 文 口 試 委 員 會 審 定 書

電機工程學系碩士班葉沐詩君所提之論文太陽 能電池製作與效能改進之研究,經本委員會審 議,符合碩士資格標準。

論文口試委員會 召集人

(簽章)

委 員

(簽章)

(簽章)

系主任

(簽章)

中 華 民 國 九 十 九 年 五 月 三 十 一 日

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太陽能電池製作與效能改進之研究

研究生:葉沐詩 指導教授:謝焸家教授 荊鳳德教授 中 華 大 學

電機工程學系 電子電路組 碩士班 摘 要

太陽能是一種極易取得的天然能源,它是不分地域的一種最便捷之能源,而且 沒有污染的問題產生,目前未能被普遍應用,源於成本高、效率低,以及方便性的 問題。

本人以磷擴散法製作太陽能電池的製程與其特性分析。以 P 型矽晶圓作為基 板,利用磷擴散在高溫爐作高溫固態擴散製程,做出單晶矽太陽能電池之 p-n 接 面。之後使用熱阻絲蒸鍍金屬鋁,形成太陽能電池正極及負極。在表面未做任何 改善處理的太陽電池,由於折射率與入射介質之折射率不同,而導致入射光在電 池表面的光學反射損失相當的大,所以必須針對表面作改善才能有效地降低光在 表面的反射損失,進而提升入射光至電池內部的機會,能夠將光有效地完全利 用,進而提升太陽能電池的轉換效率。

本研究有兩大方向:

第一,針對表面作改善處理(表面粗糙化),期望可在所選定的頻譜範圍內獲

得反射率的降低。在單晶矽太陽能電池中的表面粗化,為使受光面積增大,入射 光能夠多重反射、多重利用,使光線被吸收機會增加,太陽電池表面粗糙結構化

(texture)設計是必然步驟,而逆金字塔凹槽結構為最佳之光封存表面結構,並 且在高效率單晶矽太陽電池的製作中被廣為利用,本論文使用 P 型晶向的矽晶片

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作為基板,利用 KOH 溶液異向性蝕刻後可得到晶向之逆金字塔凹槽結構。對於 KOH 溶液在不同的溫度、時間下會有不同的蝕刻速率等製程參數進行研究。

第二,針對正電極和負電極,使用無電電鍍鎳和無電電鍍銅,具有低溫且製 程簡易,在金屬鋁上增加鎳鍍層和銅加鎳鍍層,而電極金屬遮蔽所造成影響,是 將遮蔽效應降低,必須減少金屬電極在主動區面積,但是太少的遮蔽面積則會造 成在高電流時充填因子(Fill Factor)下降。所以試著去妥協這兩個效應以達到最佳 的轉換效率,降低串聯電阻和並聯電阻,來提升轉換效率。

最後將表面粗糙化、無電電鍍法應用於太陽能電池上,藉著降低入射光在表面 的反射,增加入射至電池內部的機會,還有降低串聯電阻和並聯電阻,進而提升太 陽電池之效率。

關鍵字:太陽能電池、表面粗糙化、無電電鍍鎳、無電電鍍銅

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Process-related Solar Cells Performance

Graduate Student:M. S. Yeh Thesis supervisor:Dr. I. J. Hsieh Dr. Albert Chin

Department of Electrical Engineering Chung Hua University

ABSTRACT

The solar energy is a natural energy easily obtained, it is one of the most convenient, regardless of region of energy, and there is no pollution produced, it has not been widely used, due to high costs, low efficiency, and convenience issues.

In this study, phosphorus diffusion solar cells with characteristics of the manufacturing process. In this study, the p-type silicon wafer as a substrate, phosphorus diffusion at high temperature furnace diffusion process to make single-crystal silicon solar cell p-n junction. After using the thermal evaporation of aluminum wire to form a solar battery and the negative.

Without making any improvement in the surface treatment of the solar cell, due to the refractive index of incident medium refractive index and different, which led to the incident light in the cell surface of the optical return loss is quite large, it is necessary for improvement for the surface to effectively reduce the light on the surface reflection loss, enhance opportunities for the incident light to the internal battery can be fully effective use of light, thereby enhancing the conversion efficiency of solar cells.

Re-use of I-V measurement system voltage and current measurement curves, and measured open-circuit voltage and short-circuit current.

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Traditional solar cells surface, did not make any improvement, incident light on the surface of the optical return loss is quite large. Therefore, we have to improve the surface of solar cells, can effectively reduce the surface reflection loss of light to enhance the opportunities within the incident light to the battery will be fully effective use of light to enhance the conversion efficiency of solar cells.

This study has two main directions:

First of all, for improvement for the surface treatment (surface roughness structured), hoping in the chosen spectral range, were lower reflectivity. In the single crystal silicon solar cells in the surface roughness, for the larger area affected by light, incident light to multiple reflection, the opportunity to increase light absorption. Solar cell structure of the surface roughness is an important step in the formation of inverted pyramids in the surface groove structure, and high efficiency single crystal silicon solar cell production is often used. In this study, using the P-type crystal to the silicon as a substrate. Reverse of etching using KOH solution, the crystalline structure of the inverse pyramid groove.

KOH solution at different temperature and time, there is a different etching rate and process parameters were studied.

The second for the positive electrode and negative electrode, the use of nickel electroless plating and electroless plating of copper. This experiment has a low temperature and simple process, increasing the thickness of the metal electrode. The impact of metal shielding electrodes, metal electrodes must be reduced in the active area, but too little of the shadow area, resulting in high current, fill factor decreased. So try to compromise these two effects, to achieve the best conversion efficiency, to reduce series resistance and shunt resistance, to improve conversion efficiency.

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Finally, the surface roughness, non-electrical plating method used in solar cells, by reducing the surface reflection of incident light and increase the chance the incident to the internal battery, as well as reduce the series resistance and shunt resistance, to improve the efficiency of solar cells.

Keywords: solar cell, surface roughness structured, nickel electroless plating, copper electroless plating.

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Acknowledgement

The completion of this paper, first of all thank professor I. J. Hsieh, and professor Albert Chin, and the careful guidance and encouragement. Two years, the choice of direction from the study, conceptual framework of the building and design, and until the writing of this paper, with love for constant guidance and inspiration to make this thesis has been completed smoothly. Thanks to the instructor at this also many valuable suggestions and corrections, I would like to convey my deepest thanks.

Grateful to the National Nano Device Laboratories, and National Chiao Tung University Nano Facility Center of the enthusiastic assistance of technical personnel, there is a good environment in which this research can be carried out smoothly.

Also, thank laboratory seniors and students, the conceptual and methodological guidance to help resolve the difficulties encountered have helped solar energy experiments completed, hereby record our appreciation.

Finally, I would like to express my deep gratitude to my parents and family whose continuous encouragement and support enable me to complete this work.

Yeh, Mu-Shih June 14, 2010 Department of Electrical Engineering

Chung Hua University

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Contents

Chinese Abstract --- i

English Abstract --- iii

Acknowledgements --- vi

Contents --- vii

List of Table --- ix

List of Figure --- x

Chapter 1 Introduction 1

1.1 Overview --- 1

1.2 Basic Principles of Solar Cell --- 5

1.3 Fundamental Parameters of Solar Cell --- 9

1.4 Quantum Efficiency of Solar Cells Measurement --- 12

Chapter 2 Experimental Procedure 13

2.1 Experimental Procedure of Basic Solar Cells --- 13

2.2 Experimental Procedure of Surface Texture for Solar Cells --- 25

2.3 Experimental Procedure of Electroless Nickel Plating and Electroless Copper Plating --- 31

2.3.1 Principles of electroless nickel reaction --- 32

2.3.2 The Composition of Electroless Nickel Plating Solution --- 34

2.3.3 Aluminum Electroless Nickel Plating Substrate Activation Methods --- 36

2.3.4 Principles of Electroless Copper Reaction --- 38

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2.4 Measurement Tool for Solar Cells --- 40

Chapter 3 Results and Discussion 48

3.1 Basic Solar Cells Characteristics Measured and Analysis --- 49

3.2 Surface Texture for Solar Cells Characteristics Measured and Analysis --- 51

3.3 Electroless Nickel Plating and Electroless Copper Plating for Solar Cells Characteristics Measured and Analysis --- 53

3.4 Incident Photon Conversion Efficiency Measured and Analysis --- 54

3.5 Scanning Electron Microscope Measured and Analysis --- 55

3.6 Summary --- 59

Chapter 4 Conclusions 60

4.1 Conclusions --- 60

4.2 Future Work --- 62

References

--- 63

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List of Table

Chapter 1 Introduction 1

Table 1-1 Confirmed terrestrial cell and sub module efficiencies measured under the global AM1.5 spectrum (1000W/m2) at 25℃ --- 4

Chapter 2 Experimental Procedure 13

Table 2-1 Replacement of steps to process the second zincate --- 37

Table 2-2 Details and specs for full spectrum solar simulator --- 41

Table 2-3 System performance spec for incident photon conversion efficiency --- 43

Table 2-4 System performance spec for scanning electron microscope --- 45

Chapter 3 Results and Discussion 48

Table 3-1 Comparison of the same mask size in different solar cell performance in the p-type substrate --- 59

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List of Figure

Chapter 1 Introduction 1

Fig. 1-1 Solar cell development status --- 2

Fig. 1-2 Solar radiation spectrum --- 3

Fig. 1-3 Schematic representation of a silicon p-n junction solar cell --- 5

Fig. 1-4 Schematic diagram of solar light energy conversion --- 6

Fig. 1-5 Diagram of photovoltaic effect --- 6

Fig. 1-6 Energy band diagram of a p-n junction solar cell under solar irradiation - 7 Fig. 1-7 The three generations of solar cells --- 8

Fig. 1-8 Equivalent circuit of an ideal solar cell --- 10

Fig. 1-9 Current-voltage characteristics of a p-n junction diode in the dark and when illuminated --- 10

Chapter 2 Experimental Procedure 13

Fig. 2-1 The process flow for basic solar cell --- 13

Fig. 2-2 RCA clean above the silicon substrate --- 14

Fig. 2-3 Nitride 1000 Å deposition by PECVD --- 14

Fig. 2-4 Phosphorous doping at 850 ℃ and 950 ℃ --- 15

Fig. 2-5 Etch nitrideon silicon wafer by BOE --- 15

Fig. 2-6 Nitride 850 Å deposition by PECVD --- 16

Fig. 2-7 Vacuum oven by HMDS --- 16

Fig. 2-8 Photoresist deposition by FH-6400 --- 17

Fig. 2-9 Soft bake at 90 ℃, 90 seconds --- 17

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Fig. 2-11 Mask size --- 18

Fig. 2-12 Develop by FHD-5 --- 19

Fig. 2-13 Fixing by water --- 19

Fig. 2-14 Hard bake at 120 ℃, 3 minutes --- 20

Fig. 2-15 Etch nitride on silicon wafer by BOE--- 20

Fig. 2-16 Al 5000 Å deposition by thermal evaporation coater --- 21

Fig. 2-17 Removing PR by ACE --- 21

Fig. 2-18 Al 5000 Å deposition by thermal evaporation coater --- 22

Fig. 2-19 Al sintering at 380 ℃ for 30 minutes --- 23

Fig. 2-20 Basic solar cell experimental complete graph --- 24

Fig. 2-21 Basic solar cell process results --- 24

Fig. 2-22 The process flow for surface texture for solar cell --- 26

Fig. 2-23 KOH solution and stirrer --- 27

Fig. 2-24 Heating of KOH solution --- 28

Fig. 2-25 Add IPA 20 cc to the KOH solution --- 28

Fig. 2-26 KOH anisotropic etching --- 29

Fig. 2-27 Surface texture for solar cell experimental complete graph --- 30

Fig. 2-28 Surface texture for solar cell process results --- 30

Fig. 2-29 Electroless nickel plating deposition --- 37

Fig. 2-30 Electroless copper plating deposition --- 39

Fig. 2-31 Full spectrum solar simulator --- 42

Fig. 2-32 Incident photon conversion efficiency --- 44

Fig. 2-33 Scanning electron microscope (Hitachi FE-SEM model S-4160) ---

--

47

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Chapter 3 Results and discussion 48

Fig. 3-1 Four different mask sizes, area is divided into 4 cm ² and 1 cm ² --- 48

Fig. 3-2 The illuminated I-V characteristics of basic solar cells at different size (drive-in time:30 minutes) --- 49

Fig. 3-3 The illuminated I-V characteristics of basic solar cells at different size (drive-in time:40 minutes) --- 50

Fig. 3-4 The illuminated I-V characteristics of surface texture for solar cells at different size (surface textured time: 20 minutes) --- 51

Fig. 3-5 The illuminated I-V characteristics of surface texture for solar cells at different size (surface textured time: 30 minutes) --- 52

Fig. 3-6 The illuminated I-V characteristics of electroless plating and surface texture for solar cells --- 53

Fig. 3-7 Certified AM1.5 illumination quantum efficiency measurements of the basic / surface texture / electroless nickel plating and electroless copper plating for solar cells --- 54

Fig. 3-8 SEM images of Al layer / nickel layer --- 55

Fig. 3-9 SEM images of Al layer / copper layer --- 56

Fig. 3-10 SEM images of Al layer / Nickel-copper layer --- 57

Fig. 3-11 SEM images of the proposed surface texture structure (3,000×) --- 58

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Chapter 1 Introduction

1.1 Overview

In recent years, the problem of energy consumption becomes more important because the fossil fuel will be exhausted. Combustion of fossil fuels produces carbon dioxide that causes the greenhouse effect. With the improvement of human's standard of living and prosperous development in social economics, the demand for energy is ever increasing. In order to retard the greenhouse effect, the major task is to find the technology of renewable energy. The green energy is an important issue to research in the academic and industrial field. With the recent soaring in petroleum price, people start to pay attention to the importance of alternative energies.

Solar cells are devices that convert sunlight directly into direct current electricity, and they have been an important part of the space program for over a decade. Solar cells are also capable of making a significant impact on terrestrial energy need. The ultimate objective in the development of solar cells is to replace the traditional energy. It knows that solar is an unlimited energy source, and the energy emitted from the sun is approximately equivalent to 3.8×1023 kW of electric power. The energy of sunlight that reaches the earth is about 1.8×1014 kW. This energy is about one hundred thousand times higher than the average power generation in the world. If we can utilize this energy effectively, it not only solves the issue of exhausted petrochemical energy, but also the environmental protection issue. For this reason, solar energy is a plentiful energy for people living on earth.

Historically, silicon was the first commercially used solar cell material and is today the most extensively studied semiconductor. With high-purity silicon and optimized solar

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cell designs, efficiencies of 23% under normal sunlight can be achieved today in laboratory experiments. With polycrystalline silicon used in commercial cells and modules, efficiencies of about 17% are obtained now. [1] [2]

There are many different kinds of solar cells and their features have many plusses and minuses. Efficiency that can be achieved is usually balanced by the complexity of the cell and therefore cost. The Fig.1-1 reveals several different classes of solar cell showing how our experience in making the cells has led to improvements in efficiency over time.

Fig.1-1 Solar cell development status [3]

Fig.1-1 taken from an NREL publication [3] shows a historical review of solar cell materials and structures along with their efficiencies at converting solar radiation into electricity. Photovoltaic (PV) technology makes use of the abundant energy in the sun for energy production, with only little impact on our environment. Importance for the world environment of developing high-optical efficiency and inexpensive PV cells and thereby producing clean energy cannot be over-valued. [5]

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Fig.1-2 Solar radiation spectrum [4]

In the sunshine, the amount of solar radiation over a given surface area and a specified time, at the surface of the earth may be reduced up to 45% by our atmosphere, primarily due to reflection and absorption. About half of the sunshine finally reaching the earth's surface is in the visible portion of the electromagnetic spectrum. Even considering this, the global potential for solar energy is huge. The amount of energy that reaches the earth's surface every year exceeds the total energy consumption by roughly a factor of 10,000.

A significant percentage of the total losses in a PV cell (more than 50%) are associated with spectral mismatch (the inability of the semiconductor material bandgap to absorb energy across the full solar spectrum). This illustrates the importance for solar simulation systems to match, as close as possible, the spectral distribution of solar radiation incident on the surface of the earth.

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Table 1-1 Confirmed terrestrial cell and sub module efficiencies measured under the global AM1.5 spectrum (1000W/m2) at 25℃ [4]

The device operation principles of a solar cell are described in Chapter 1. The device fabrication processes of solar cells and measurement techniques are given in Chapter 2.

The experimental results and discussion are presented in Chapter 3.

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1.2 Basic Principles of Solar Cell [6]

For solar cell semiconductors, the most common device structures are the p-n junctions. Fig.1-3 is a schematic representation of a silicon solar cell. It consists of shallow junction formed near the front surface, a front ohm contact in the form of stripes and fingers, covered with an antireflection coating to reduce optical losses, and a back ohm contact that covers the entire back surface.

Fig.1-3 Schematic representation of a silicon p-n junction solar cell

Solar cells require a particular p-n junction design, which is depicted in the schematic representation (see Fig.1-4). Photovoltaic energy conversion requires the separation of electrons and holes by an internal electrical field. We carry out a load RL to a solar cell, and get the photocurrent after photon illumine.

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Fig.1-4 Schematic diagram of solar light energy conversion

Fig.1-5 Diagram of photovoltaic effect [6]

Fig.1-5 shows the diagram of the photovoltaic effect. The first step of the conversion is photons impinging on and absorbed in a semiconductor transfer their energy to electrons. If the energy of a photon (hv) is greater than or equal to the energy-gap (Eg) of the semiconductor (hv E≧ g), electrons could be promoted from valence-band to conduction-band to generate electron-hole pairs. Then, if the electron-hole pairs can be separated by a suitable built-in electric-field or voltage, this would result in a

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photocurrent (Iph). The p-n junction is one of the most common structures of solar cells.

When a solar cell is exposed to the solar spectrum, electrons will be excited to conduction band from valence band and result the electron-hole pair whose energy is the same as the bandgap, while the photon energy (hv) is greater than the Eg of the solar cell.

Fig.1-6 depicts the energy-band diagram of a p-n junction in thermal equilibrium.

Electrons in the conduction-band of the n-type semiconductor try to move into the conduction-band of p-type semiconductor, they would see a potential barrier. This barrier is the built-in voltage (Vbi). The direction of the corresponding build-in electric-field of a p-n junction is also shown in the figure. A p-n junction solar cell usually would be

illuminated on the top of the p-type region. The electron-hole pairs are photo-generated in the p, depletion, and n regions. In the p and n regions, there is no electric-field to separate the photo-generated electron-hole pairs. Therefore, the photocurrent of a solar cell mainly comes from photo-generated carriers in the depletion region. [7]

Fig.1-6 Energy band diagram of a p-n junction solar cell under solar irradiation [6]

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First-generation cells are based on expensive silicon wafers and make up 85% of the current commercial market. Second-generation cells are based on thin films of materials such as amorphous silicon, nanocrystalline silicon, cadmium telluride, or copper indium selenium.

The materials are less expensive, but research is needed to raise the cells’ efficiency to the levels shown if the cost of delivered power is to be reduced.

Third-generation cells are the research goal: a dramatic increase in efficiency that maintains the cost advantage of second-generation materials. Their design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials. The horizontal axis represents the cost of the solar module only; it must be approximately doubled to include the costs of packaging and mounting. Dotted lines indicate the cost per watt of peak power (Wp). [8]

Fig.1-7 The three generations of solar cells [8]

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1.3 Fundamental Parameters of Solar Cell

A solar cell is formed in general by a p-n diode. When sunlight falls incident on the surface of a semiconductor, electron-hole pairs are generated which are then separated by the potential barrier across the p-n junction. Since the solar cell is a diode, the ideal I-V characteristics of such a device are given by

( )

=

0 V /VT

- 1

I I e

(1.4.1)

Where I is the diode current, I0 is the saturation current, V is the applied voltage and VT=kT/q. Based on illumination, the diode equation is modified as:

( )

⎡ ⎤

⎢ ⎥

⎣ ⎦

=

0 V /VT

- 1 -

L

I I e I (1.4.2)

We obtain for the open voltage (VOC), when I=0:

⎛ ⎞

⎜ ⎟

⎝ ⎠

=

T L

+ 1

OC

0

V V ln I

I (1.4.3)

Fig.1-8 is the idealized equivalent circuit of a solar cell, and the ideal I-V curve is shown in is Fig.1-8, it contains the light current I0 (eV/VT-1). By illumination, the I-V curve shifts down to the fourth quadrant, hence, the power can be extracted from the corresponding area. The most important parameter of a solar cell is the efficiency. ISC is the short-circuit current equal to IL and VOC is the open-circuit voltage of the cell; the shaded area in the figure is the maximum-power rectangle.

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Fig.1-8 Equivalent circuit of an ideal solar cell

Also defined in Fig.1-9 are the quantities Im and Vm that correspond to the current and voltage, respectively, for the maximum power output Pm (= Im × Vm).

Fig.1-9 Current-voltage characteristics of a p-n junction diode in the dark and when illuminated

Hence, for a given IL, VOC increases logarithmically with decreasing saturation current (I0). The output power is given by

( )

⎡ ⎤

⎢ ⎥

V /VT

L

P = IV = I V e0 - 1 - I V (1.4.4)

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The output power of solar cell can be written as Pm = Im × Vm where Im and Vm are the respective current and voltage corresponding to maximum power point. Pm can also be defined as Pm = ISC × VOC× FF, where ISC is the short circuit current (V=0), and VOC is the open circuit current (I=0), and FF is the fill factor defined as the measure of rectangle.

Then, the efficiency can be expressed as efficiency can be expressed as:

× ×

m m

×

m SC OC

in in in

I V FF

P I V

η = = =

P P P (1.4.5)

We can use fill factor to determine a solar cell quality. Fill factor is defined as the rate of maximum power and the product with open circuit voltage, VOC and short circuit current, ISC.

×

×

m m

×

m

SC OC SC OC

P I V

FF = =

I V I V

(1.4.6)

The E is the energy that incident light illuminates to the surface of solar cell,

PH

E = N (hc / λ)dλ

(1.4.7) Therefore, with the sun irradiating perpendicular to a solar cell’s surface. Its average value is approximately 100 mW/cm2.

The value of AC is referred to as the effective areas of solar cell. [7] The solar cell conversion efficiency can then be rewritten as:

× ×

×

SC C OC

FF I V

η = E A

(1.4.8)

(29)

1.4 Quantum Efficiency of Solar Cells Measurement

A photovoltaic spectral responsively describes its ability to convert light of various wavelengths into electricity. It is often reported as the ratio of device current divided by incident-beam power or device current divided by incident photon flux.

The function of the spectral response is :

( ) [ ( ) ( ) φ ( ) ]

L

q 0 G χ,λ - R χ,λ dx SR λ =

λ hc / λ (1.5.1)

ψ(χ,λ) is the photon flux when the intensity of incident light is air mass 1.5 at wavelength λ. G(χ,λ)is the speed caused by the light with different wavelength λ. R(χ,λ) is the recombination velocity of light with different wavelength λ. Its unit is A/W.

However, the outer quantum efficiency of solar cell is defined by the next equation:

( )

q

QE = SR λ hc / λ (1.5.2)

The physical meaning is the ability of generating electron-hole pair which is caused by the incident photon.

Thus, the unit is electron/photon. Plight is incident power. IPH is photocurrent of solar cell. Even so, another expression is:

× ×

PH

× ×

light

I hc 100%

QE = P λ e

(1.5.3)

(30)

Chapter 2 Experimental Procedure

2.1 Experimental Procedure of Basic Solar Cells

In this study, phosphorus diffusion solar cells with characteristics of the manufacturing process. In this study, the p-type silicon wafer as a substrate, phosphorus diffusion at high temperature furnace for high temperature solid state diffusion process to make single-crystal silicon solar cell p-n junction. After using the thermal evaporation of aluminum wire to form a solar battery and the negative.

A. Experimental flow chart:

Fig.2-1 The process flow for basic solar cell

(31)

B. Experimental procedure steps:

Step A-1: RCA clean. (The RCA clean is a standard set of wafer cleaning steps.

RCA cleaning includes RCA-1 and RCA-2 cleaning procedures. RCA-1 involves removal of organic contaminants, while RCA-2 involves removal of metallic contaminants.)

Fig.2-2 RCA clean above the silicon substrate

Step A-2: PECVD. (Nitride 1000 Å)

Fig.2-3 Nitride 1000 Å deposition by PECVD

(32)

Step A-3: Phosphorous doping at 850 ℃ and 950 ℃, drive-in time:30/40/50 minutes

Fig.2-4 Phosphorous doping at 850 ℃ and 950 ℃

Step A-4: Etch nitrideon silicon wafer by BOE. (Both sides non-stick water)

Fig.2-5 Etch nitrideon silicon wafer by BOE

(33)

Step A-5: PECVD. (Nitride 850 Å)

Fig.2-6 Nitride 850 Å deposition by PECVD

Step A-6: Vacuum Oven.

Fig.2-7 Vacuum oven by HMDS

(34)

Step A-7: Photo resist spinner. (FH-6400)

Fig.2-8 Photoresist deposition by FH-6400

Step A-8: Soft bake at 90 , ℃ 90 seconds.

Fig.2-9 Soft bake at 90 , 90 seconds

(35)

Step A-9: Mask aligner.

Fig.2-10 Exposure by mask

Fig.2-11 Mask size

(36)

Step A-10: Develop. (FHD-5)

Fig.2-12 Develop by FHD-5

Step A-11: Fixing.

Fig.2-13 Fixing by water

(37)

Step A-12: Hard bake at 120 , 3 minutes℃ .

Fig.2-14 Hard bake at 120 , 3 minutes

Step A-13: Etch nitrideon silicon wafer by BOE. (Both sides non-stick water)

Fig.2-15 Etch nitride on silicon wafer by BOE

(38)

Step A-14: Al 5000 Å deposition by thermal evaporation coater.

Fig.2-16 Al 5000 Å deposition by thermal evaporation coater

Step A-15: Removing PR by ACE. (lift-off)

Fig.2-17 Removing PR by ACE

(39)

Step A-16: Silicon on the back coat by BOE.

Step A-17: Al 5000 Å deposition by thermal evaporation coater.

Fig.2-18 Al 5000 Å deposition by thermal evaporation coater

(40)

Step A-18: Al sintering at 380 for 30 minutes℃ .

Fig.2-19 Al sintering at 380 for 30 minutes

(41)

Fig.2-20 Basic solar cell experimental complete graph

Fig.2-21 Basic solar cell process results

(42)

2.2 Experimental Procedure of Surface Texture for Solar Cells

Without making any improvement in the surface treatment of the solar cell, due to the refractive index of incident medium refractive index and different, which led to the incident light in the cell surface of the optical return loss is quite large, it is necessary for improvement for the surface to effectively reduce the light on the surface reflection loss, enhance opportunities for the incident light to the internal battery can be fully effective use of light, thereby enhancing the conversion efficiency of solar cells.

For improvement for the surface treatment (surface roughness structured), hoping in the chosen spectral range, were lower reflectivity. In the single crystal silicon solar cells in the surface roughness, for the larger area affected by light, incident light to multiple reflection, the opportunity to increase light absorption. Solar cell structure of the surface roughness is an important step in the formation of inverted pyramids in the surface groove structure, and high efficiency single crystal silicon solar cell production is often used. In this study, using the P-type crystal to the silicon as a substrate. Reverse of etching using KOH solution, the crystalline structure of the inverse pyramid groove.

KOH solution at different temperature and time, there is a different etching rate and process parameters were studied. [9, 10, 11]

The chemical reaction is as follows:

Si + 2NaOH + H

2

O → Na

2

SiO

3

+ H

2 (2.1.1)

(43)

Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching. The chemical reaction is as follows: [12, 13]

3Si + 4HNO3 → 3SiO

2

+ 2H

2

O + 4NO

(2.1.2)

SiO

2

+ 6HF → H

2

(SiF

6

) + 2H

2

O

(2.1.3)

A. Experimental flow chart:

Fig.2-22 The process flow for surface texture for solar cell

(44)

B. A simple and high efficient wet etching technique for fabricating pyramid textures on Si wafer is proposed. Experimental procedure steps:

Step B-1: KOH: DI water = 75g: 1400cc.

1400 cc

stirrer

Fig.2-23 KOH solution and stirrer

(45)

Step B-2: Heating temperature 75 ~ 80 ℃.

stirrer

Fig.2-24 Heating temperature of KOH solution

Step B-3: Isopropyl Alcohol (IPA) is often added to the solution to abate the bubbling effect caused by hydrogen released from the Si surfaces during reaction.

[14]

Fig.2-25 Add IPA 20 cc to the KOH solution

(46)

Step B-4: Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching.

75 ~ 80 ℃

Fig.2-26 KOH anisotropic etching

Step B-5 ~ Step B-22: Repeat the experimental procedures before the step A-1 ~ step A-18.

(47)

Fig.2-27 Surface texture for solar cell experimental complete graph

Fig.2-28 Surface texture for solar cell process results

(48)

2.3 Experimental Procedure of Electroless Nickel Plating and Electroless Copper Plating

The second for the positive electrode and negative electrode, the use of nickel electroless plating and electroless plating of copper. This experiment has a low temperature and simple process, increasing the thickness of the metal electrode. The impact of metal shielding electrodes, metal electrodes must be reduced in the active area, but too little of the shadow area, resulting in high current, fill factor decreased. So try to compromise these two effects, to achieve the best conversion efficiency, reduce series resistance and shunt resistance, to improve conversion efficiency.

The axiom of so-called electroless nickel-plating stems from the oxidization of the deoxidant in the plating liquor. As oxidizing, the deoxidant give forth charges which force metal anion deoxidize into metal and then precipitate on the surface with catalysis and activation. And the precipitated metal with autocatalytic reaction can be regarded as catalyst of reaction, and the there will be chain reaction proceeding. Therefore, the process of plating without the need of imposed current is so-called electroless plating or chemical plating. [15]

As Brenner and Riddell-the generally acknowledged inventors of electroless nickel plating-conducted the plating of nickel-tungsten alloys in the internal surface of pipe by using the non-soluble internal anode, they accidentally found out the extraordinary deoxidance of hypophosphite and then applied the patent of it in 1950.

(49)

2.3.1 Principles of electroless nickel reaction

Electroless nickel reaction depicted is as follows: [16]

(1) Dehydrogenation:

H

2

PO

2- catalyst

HPO

2-

+ H

ads (2.1.4) (2) Oxidation:

HPO

2-

+ H

2

O → HPO

32-

+ H

+

+ H

ads (2.1.5) (3) Liberation:

H

ads

→ H

+

+ e

- (2.1.6)

(4) Recombination:

H

ads

+ H

ads

→ H

2

(2.1.7)

(5) Reduction:

Ni

2+

+ 2e

-

→ Ni

(2.1.8)

(6) Formation of Phosphorous:

3H

2

PO

2-

+ H

+

→ HPO

32-

+ 2P + 3H

2

O

(2.1.9)

(50)

The total reaction type as follows:

6H

2

PO

2(aq)

+ 2Ni

2+

(aq)

→ 2P

(s)

+ 2Ni

(s)

+ H

2

+ 6H

+

+ 4HPO

32-

(aq) (2.1.10) Judging from the reactive, electroless nickel plating of alloy composition and its use

of the reducing agent, sodium hypophosphite as a reducing agent for electroless nickel plating is a nickel-phosphorus alloy, with hydrazine as the reducing agent is available to high-purity nickel.

(51)

2.3.2 The Composition of Electroless Nickel Plating Solution

Electroless nickel coating of the material properties under conditions of solutions composition and plating control. According to the actual needs of plating solutions, including Nickel Source, reductive agent, complexing agents, stabilizer, PH adjuster and addition agent. Its function is as follows:

(1) Nickel source:

Nickel plating reaction of salt required to provide nickel ions, increasing concentration of nickel can increase the plating rate, but will reduce the stability of the plating solutions. Nickel source are commonly used: Nickel Sulfate, Nickel Chloride.

(2) Reducing agent:

Reducing agent in nickel plating solutions can be reduced to Ni atomic ions, increase the concentration of reducing agent can increase the plating rate. Likely to cause an excessive amount of reducing agent instead of solutions decomposition. Commonly used reducing agent, including: Sodium Hypophosphite, Sodium Borohydride, Dimethylamine Borane, Hydrazine.

(3) Complexing agent:

Chelators with solutions complexes of nickel ions clamp. For instance: citric acid, sodium succinate.

(4) Stabilizer:

Stabilizer can coating solutions may be the emergence of some nuclear activity.

(52)

(5) PH adjuster:

Different solutions ph value will change the reaction of electroless nickel plating results. Plating process will continue to release hydrogen ions change the ph value of the solutions, must use the buffer and the ph adjusting agent to adjust the bath ph.

(6) Additive;

Commonly used additives are generally high molecular weight organic compounds brightener, add in the plating solutions may lead to decreased coating roughness and grain refinement, it can also be seen as brightener.

(53)

2.3.3 Aluminum Electroless Nickel Plating Substrate Activation Methods

Electroless nickel reaction, the substrate and the analysis of metal-coated by the catalyst, the reaction will make continuous plating.

Aluminum substrate in this experiment, though not with the inner electron orbital’s of the atomic structure of space. The positively charged aluminum stronger than that of nickel, it can and replacement reaction of nickel, but because of the high affinity of aluminum and oxygen, thus forming a layer of dense oxide film, which impedes the electroless nickel reaction.

In this study, electroless nickel plating reaction on the aluminum substrate, it must use zincate replacement dip method. [17] A layer of zincate metal film deposited on aluminum substrate, further acid oxidation, provide the required catalytic, can increase the plating coating and aluminum substrate bonding strength between. Replacement reaction of zincate dissolution institutions, including aluminum and zincate anode cathodic reaction reaction, its wholly-reactive as: [18]

2Al + 3Zn

2+

→ 2Al

3+

+ 3Zn

(2.1.11)

Study found that in the underlying aluminum metal gasket for the fine pitch when, with the increase in the number of zincate replacement, zincate plating nucleation of particles is smaller and more smooth zincate coating, thus making the completion of the long point significant enhancement of shear strength.

(54)

Table 2-1 Replacement of steps to process the second zincate 1.

Mechanical machining of substrate

2.

Alkaline cleaning

3.

Acid cleaning

4.

First zincating

5.

Nitric acid strip

6.

Second zincating

Therefore, the product of hypophosphite is nickel- phosphorous alloy which hen gives forth a part of its hydrogen (hydrogen).

Electroless nickel deposition: [19]

Fig.2-29 Electroless nickel plating deposition

(55)

2.3.4 Principles of Electroless Copper Reaction

Traditional Pd colloids, Pd-based catalysts for electroless copper deposition. [20, 21]

A Pd system was first synthesized with surfactant, sodium alkyl sulfates, which acted as both protecting agent and reducing agent. The gradual decomposition of surfactant into alcohol provided the reducing power for metal ions, as shown below:

R

n

SO

4

Na + H

2

O

Re flux

R

n

OH + SO

42-

(2.1.12)

R

n

OH + Pd

2+

→ R

n-1

COOH + Pd

0

+ 2H

+ (2.1.13)

Both electroplating and electroless plates use oxidation-reduction reaction to deposit copper. If soluble anode is used, the chemical reaction is as follows: [22, 23]

Cathode Reaction:

Cu

2+

+ 2e

-

→ Cu

(2.1.14)

Anode Reaction:

Cu → Cu

2+

+ 2e

- (2.1.15) If insoluble anode is used, such as platinum, the anodic reaction would be:

2H

2

O → O

2

↑ + 2H

+

+ e

-

(2.1.16)

(56)

Electroless copper deposition: [24]

Fig.2-30 Electroless copper plating deposition

(57)

2.4 Measurement Tool for Solar Cells

This research focused on the improvement of solar cells conversion efficiency by modifying the measuring configurations. The measuring configurations are composed of hardware including the methods of testing. The goals are to assure the result of testing data and are in coordinate with the solar cell manufacturer data. Because the testing equipment of solar cell manufacturer was calibrated regularly, it should be much more accurate.

At first, we need to understand the series resistances and shunt resistances how to influence the performance parameters of solar cells, which include conversion efficiency, fill factor, open-circuit voltage and short-circuit current.

By reducing series resistances through changing those key factors, improving the conversion efficiency and fill factor. In analytical part, solar cells study electrical characteristics, efficiency, shunt resistivity, series resistivity, and quantum efficiency.

(58)

1. Simulator Type: Full Spectrum Solar Simulator

The 91160 Full Spectrum Solar Simulator produces power equivalent to about 2 suns. With optional air mass filters, you can simulate various solar conditions. This 300 W Solar Simulator has a 2 x 2 inch (50.8 × 50.8 mm) collimated output.

Table 2-2 Details and specs for full spectrum solar simulator

Model 91160

Simulator Type Full Spectrum Solar Simulator Beam Size 2 × 2 in. (51 × 51 mm)

Typical Power Output 2 suns

Lamp Wattage 300 W

Collimation Angle <±10 °

Type Solar Simulators

Power Requirements 95 - 264 VAC, 8 A, 47 - 63 Hz Light Ripple <1 % rms

Line Regulation 0.01 %

Lamp Type Xenon, Short Arc Beam Uniformity ±5 %

Collimation <±10 °

(59)

Fig.2-31 Full spectrum solar simulator

(60)

2. Simulator Type: Incident photon conversion efficiency

The QE-PV-SI QE/IPCE Measurement Kit allows researchers to measure Quantum efficiency (QE) and Incident Photon to Charge Carrier Efficiency (IPCE) measurement for solar cells, detectors, or any other photon-to-charge converting device.

Table 2-3 System performance spec for incident photon conversion efficiency Light Source 250W Quartz Tungsten Halogen Lamp Spot Size 1mm × 2.5mm rectangular at focus

Working Distance 50mm

Wavelength Range 350-1100nm (Extended range available) Monochromator Path Lengths 1/8M

Resolution 5nm (adjustable)

Repeatability <±0.5

Accuracy with Si detector 350-900 <±2%, 900-1100 <±5 Order Sorting Filters

(Automated Filter wheel)

5 filters max

(standard configuration uses 2)

Signal Acquisition Chopper with Lock-in Amplifier.

Modulation frequency 8-1100Hz

Measurement Type Simultaneous EQE and IQE

measurement Optical Output Power 10.6υW @ 600 QE Calibration test cell Included Computer Included Dell Latitude

(61)

Fig.2-32 Incident photon conversion efficiency

(62)

3. Simulator Type: Scanning Electron Microscope (SEM)

Model: Hitachi FE-SEM model S-4160

Table 2-4 System performance spec for scanning electron microscope specifications Performance:

Secondary electron image resolution

1.5 nm at 15kv

Magnification 20X - 500KX Electron Optics:

Electron gun Cold field emmission source Lens type electromagnetic

Objective aperture 4 position externally selectable Stigmator Octopole electromagnetic Scanning coil 2-stage electromagnetic Sample Chamber:

Size Type I

Airlock prepumped, max sample size:50 mm

diameter

X/Y

25 mm, Z: 3-28 mm, T: -5Deg - +45 Deg., R: 360 deg continuous

Stage motion 5 axis manual

Draw-out door Max sample size:150 mm dia.

Size Type II

(63)

Airlock

prepumped, max sample size:100 mm or 150 mm diameter

X/Y

100mm/50 mm, Z: 3-33 mm, T: -5 Deg - +60 Deg., R: 360 deg continuous

Stage motion 5 axis manual Display system:

Image display Dual 12" monitors

Scanning mode Normal, reduced area , line scan, photo scan, spot position, split screen

Scanning speed TV, 0.3, 2, 9, 25, 35, 100, 160 320 s/frame

Signal processing

Real-time processing, auto-brightness and contrast control, dynamic stigmator, sutofocus, a

Frame averaging, frame integration, contrast conversion, Vacuum system:

Full automatic operation with pneumatic valve control

Ultimate vacuum

10-7 Pa in electron gun chamber, 10-4 Pa in specimen chamber

Ion pump 60 L/s X1, 20 L/s X2

Speciment chamber DP (570 L/s); Turbo Optional

Foreline Rotary pump X2

(64)

Accessories (Optional) :

EDX at 30 Deg. take off angle

Digital Image capturing Orion-6 software, Optional

Chilled water circulator 10 - 20 deg C, 1.0 -1.5 L/m for DP only Installation:

AC Single phase AC 220 or 240 volt, 50/60 Hz Grounding Independent grounding 100 Ohms or less

Fig.2-33 Scanning electron microscope (Hitachi FE-SEM model S-4160)

(65)

Chapter 3 Results and Discussion

Finally, we using two different sizes of mask, measure the optical characteristics with a photoluminescence system, and the electrical performance solar cell with I-V measurement system.

Fig.3-1 Four different mask sizes, area is divided into 4 cm ² and 1 cm ²

(66)

3.1 Basic Solar Cells Characteristics Measured and Analysis

Phosphorous doping at 850 ℃, drive-in time:30 minutes. Under AM1.5, one sun at 25 ℃, basic solar cells.

-1.0 -0.5 0.0 0.5 1.0

-200 -100 0 100 200 300 400 500

M

Fill-Factor =42.274%

Efficiency =6.986%

Voc =0.500(V) Isc = 129.288(mA)

L

Fill-Factor =41.499%

Efficiency =7.282%

Voc =0.500(V) Isc =133.928(mA)

Current (A/cm2 )

Voltage (V)

Fig.3-2 The illuminated I-V characteristics of basic solar cells at different size (drive-in time:30 minutes)

(67)

Phosphorous doping at 850 ℃, drive-in time:40 minutes. Under AM1.5, one sun at 25 ℃, basic solar cells.

-1.0 -0.5 0.0 0.5 1.0

-200 -100 0 100 200 300 400 500

M

Fill-Factor =38.472%

Efficiency =7.815%

Voc =0.500(V) Isc =155.035(mA)

L

Fill-Factor =39.411 % Efficiency = 8.148%

Voc = 0.550(V) Isc =143.449(mA)

Current (A/cm2 )

Voltage (V)

Fig.3-3 The illuminated I-V characteristics of basic solar cells at different size (drive-in time:40 minutes)

(68)

3.2 Surface Texture for Solar Cells Characteristics Measured and Analysis

Phosphorous doping at 850 ℃, drive-in time:50 minutes.

Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching. Surface textured time: 20 minutes, at heating temperature 75 ~ 80 ℃.

Under AM1.5, one sun at 25 ℃, surface texture for solar cells.

0.0 0.5

-100 0 100 200

Current (A/cm2 )

Voltage (V)

L

Fill-Factor =41.287%

Efficiency =8.151%

Voc = 0.45(V) Isc =43.873(mA)

Fig.3-4 The illuminated I-V characteristics of surface texture for solar cells at different size (surface textured time: 20 minutes)

(69)

Phosphorous doping at 850 ℃, drive-in time:50 minutes.

Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching. Surface textured time: 30 minutes, at heating temperature 75 ~ 80 ℃.

Under AM1.5, one sun at 25 ℃, surface texture for solar cells.

0.0 0.5 1.0

-50 0 50 100

Current (A/cm2 )

Voltage (V)

L

Fill-Factor =47.915%

Efficiency =9.847%

Voc = 0.520(V) Isc =39.519(mA)

Fig.3-5 The illuminated I-V characteristics of surface texture for solar cells at different size (surface textured time: 30 minutes)

(70)

3.3 Electroless Nickel Plating and Electroless Copper Plating for Solar Cells Characteristics Measured and Analysis

Phosphorous doping at 850 ℃, drive-in time:50 minutes.

Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching. Surface textured time: 30 minutes, at heating temperature 75 ~ 80 ℃.

Under AM1.5, one sun at 25 ℃, electroless plating for solar cells.

0.0 0.5 1.0

-100 0 100 200 300 400 500 600

M

Fill-Factor =55.148%

Efficiency =11.895%

Voc =0.540(V) Isc =39.944(mA)

L

Fill-Factor =56.248%

Efficiency =13.808%

Voc =0.540(V) Isc =45.459(mA)

Current (A/cm2 )

Voltage (V)

Fig.3-6 The illuminated I-V characteristics of electroless plating and surface texture for solar cells

(71)

3.4 Incident Photon Conversion Efficiency Measured and Analysis

Quantum efficiency (QE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell. QE therefore relates to the response of a solar cell to the various wavelengths in the spectrum of light shining on the cell. The QE is given as a function of either wavelength or energy.

400 500 600 700 800 900 1000 1100

0 10 20 30 40 50 60 70 80 90 100

Quantum efficiency (%)

Wavelength (nm)

basic solar cells

surface texture for solar cells

electroless nickel plating and electroless copper plating for solar cells

Fig.3-7 Certified AM1.5 illumination quantum efficiency measurements of the basic / surface texture / electroless nickel plating and electroless copper plating for solar cells

solar cell without and with single layer AR coating aimed at 600 nm.

(72)

3.5 Scanning Electron Microscope Measured and Analysis

Use of thermal coater, plated aluminum, the thickness of 400 nm. Using SEM, the actual thickness of 333.3 nm.

Nickel electroless plating solution time: 60 minutes, heating temperature: 70~80℃.

Use of nickel electroless plating, the thickness of 10 um. Using SEM, the actual thickness of 9.442 um.

Fig.3-8 SEM images of Al layer / nickel layer

(73)

Use of thermal coater, plated aluminum, the thickness of 400 nm. Using SEM, the actual thickness of 333.3 nm.

Copper electroless plating solution time: 1 minutes, heating temperature: 30~40℃.

Use of copper electroless plating, the thickness of 1 um. Using SEM, the actual thickness of 935.5 nm.

Fig.3-9 SEM images of Al layer / copper layer

(74)

Use of thermal coater, plated aluminum, the thickness of 400 nm. Using SEM, the actual thickness of 444.4 nm.

Nickel electroless plating solution time: 60 minutes, heating temperature : 70~80℃.

Copper electroless plating solution time: 1 minutes, heating temperature: 30~40℃. Use of copper electroless plating and nickel electroless plating, the thickness of 11 um. Using SEM, the actual thickness of 10.80 um.

Fig.3-10 SEM images of Al layer / Nickel-copper layer

(75)

Pyramid textures were formed on Si wafers to reduce reflections using KOH anisotropic etching. Surface textured time: 20 minutes, at heating temperature 75 ~ 80 ℃.

Fig.3-11 SEM images of the proposed surface texture structure (3,000×)

(76)

3.6 Summary

In this chapter, when the sunlight, in the basic solar cells, surface texture for solar cells and electroless nickel and electroless copper for solar cells, three different parameters, from the theoretical and experimental results show that the fill factor and efficiency of parameter changes.

Table 3-1 Comparison of the same mask size in different solar cell performance in the p-type substrate

Model

Device Area (cm2)

Optimum operating

voltage , Vm (V)

Optimum operating current, Im (mA)

Open-circuit voltage, Voc (V)

Short-circuit current, Isc (mA)

Fill-Factor (%)

Efficiency (%)

Basic Solar Cells

(Phosphorous doping drive-in time:30 minutes)

4 0.299 92.683 0.500 133.928 41.499 7.282

Basic Solar Cells

(Phosphorous doping drive-in time:40 minutes)

4 0.299 103.705 0.550 143.449 39.411 8.148

Surface Texture for Solar Cells

(surface textured time:

20 minutes)

1 0.300 27.170 0.45 43.873 41.287 8.151

Surface Texture for Solar Cells

(surface textured time:

30 minutes)

1 0.330 29.839 0.520 39.519 47.915 9.847

Electroless Plating

for Solar Cells 1 0.389 35.411 0.540 45.459 56.248 13.808

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