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研究生：	羅喬陽 Lo, Chiao-Yang
論文名稱：	矽基太陽能電池與正面電極歐姆接觸之形成機構研究 Studies on Mechanism of Constructing Ohmic Contact between Front Electrode and Silicon based Solar Cell
指導教授：	李文熙 Lee, Wen-Hsi
學位類別：	碩士 Master
系所名稱：	電機資訊學院 - 電機工程學系 Department of Electrical Engineering
論文出版年：	2014
畢業學年度：	102
語文別：	英文
論文頁數：	69
中文關鍵詞：	正面電極、太陽能電池、歐姆接觸、網版印刷、漿料
外文關鍵詞：	front electrode, solar cell, ohmic contact, screen printing, paste
相關次數：	點閱：155 下載：2
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　　隨著綠色能源的使用逐漸成為全球趨勢，太陽能電池的需求量快速增加，因此，更多關於太陽能電池生產的細節必須被開發。本研究將探討正面電極與矽基太陽能電池間歐姆接觸的形成機構，並使用創新的漿料及製程技術來完成正面電極之製作。
　　本研究使用已完成雙層抗反射膜沉積及背鋁印製之多晶矽太陽能電池，將自製金屬漿料透過細線網版印刷印於其上，再藉由兩階段熱處理，完成正面電極之燒結和正面電極與太陽能電池間之歐姆接觸。實驗中的兩階段熱處理分別在空氣及還原氣氛下進行，第一階段在空氣中，由鉛玻璃蝕刻雙層抗反射膜並再結晶出銀在矽表面達成初步接觸；第二階段在還原氣氛中除了將氧化銅進行還原，並完成燒結外，藉由氫氣、銀與鉛玻璃的交互作用以及鉛的揮發，再結晶出奈米銀於交界處玻璃層中，形成正面電極與太陽能電池間的歐姆接觸。各階段製程結束後分別透過場放射型掃描式電子顯微鏡、透射電子顯微鏡、X光射線繞射分析儀及四點探針分析結果。
　　透過實驗與分析，推得各階段時的反應機構，同時也驗證透過創新的漿料及製程，可同時達成正電極所需之歐姆接觸與良好的片電阻值。量測結果中之最佳正電極片電阻值為0.090 Ω/□。

　　Since the use of green-energy has been one of the global trends, the demand for solar cell increases rapidly. Thus, more details about manufacturing solar cells should be developed. In this study, mechanism of constructing ohmic contact between front electrode and Si-based solar cell will be investigated. Newly-invented paste and process are used in the preparation of front electrode.
　　In this study, paste was printed with narrow line screen printing process on polycrystalline Si solar cell which has already finished the back Al printing and deposition of double anti-reflection coatings (DARCs). Then, two-step firing process was applied to sinter the front electrode and obtain the ohmic contact between front electrode and solar cell. The two-step firing process was accomplished in air atmosphere and reducing atmosphere. The first step was in air atmosphere. In this process, PbO-based glass frit etched the DARCs and Ag recrystallized at the surface of Si, constructing the preliminary contact. The second step was in reducing atmosphere. In this process, CuO reduced to Cu and sintered. Besides, Ag nanoparticles recrystallized in the glass layer at interface due to the interactions between H2, Ag and PbO-based glass frit and the volatility of Pb, constructing the ohmic contact between electrode and solar cell. Scanning electron microscope (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and four-point probe were employed to analyze the results after each process.
　　By experiment and analysis, reaction mechanism in each stage was surmised, and it was also proven that ohmic contact and good sheet resistance for front electrode could both be obtained by applying newly-invented paste and process. The lowest sheet resistance of front electrode measured was 0.090 Ω/□.

Chapter 1 Introduction	1
1-1 Si-based Solar Cells Device Overview	1
1-1.1 Structure of Typical Si-based Solar Cells	1
1-1.2 Shunt and Series Resistance	4
1-1.3 Design of Front Electrode	5
1-1.4 Current Front Ag Manufacturing Technology and Challenges	8
1-2 Motivation	10

Chapter 2 Literature Review	12
2-1 Screen Printed Metallization in Solar Cell	12
2-2 Thick Film Technology Used on Front Electrode	13
2-3 Reactions of Glass, Ag, SiNx and Si During Firing	15
2-4 Microstructural of Front-side (FS) Ag Electrode and Its Current Conduction Mechanisms	16

Chapter 3 Experiment Scheme	18
3-1 Process Equipment	18
3-1.1 Three-Roll Mill	18
3-1.2 Screen Printer and Steel Mesh Screen	20
3-1.3 Fast Firing Furnace	22
3-2 Analysis Equipment	24
3-2.1 Transmission Electro Microscopy (TEM)	24
3-2.2 Scanning Electron Microscope (SEM)	25
3-2.3 Energy-dispersive X-ray Spectroscopy (EDS)	26
3-3.4 Four-point Probe	27
3-3 Experiment Methods and Procedures	30
3-3.1 Paste Making	31
3-3.2 Screen Printing	31
3-3.3 First Step Firing Process	32
3-3.4 Second Step Firing Process	33

Chapter 4 Results and Discussion	34
4-1 Paste Characteristics and Screen Printing	34
4-1.1 Paste Characteristics	34
4-1.2 Screen Printing	38
4-2 First Step firing process	41
4-3 Second Step firing process	50
4-4 Mechanism	59

Chapter 5 Conclusion and Future Work	64
Reference	66

Figure Captions
Figure 1-1 Basic schematic of a Si-based solar cell	1
Figure 1-2 Resistive components and current flows in a solar cell	4
Figure 1-3 Schematic of top contact on a solar cell	5
Figure 1-4 Resistive loss in a finger	5
Figure 1-5 Key features of a top surface contacting	8
Figure 1-6 Scheme of current front Ag electrode manufacturing process	9
Figure 1-7 Scheme of process	11
Figure 2-1 Growth model	15
Figure 2-2 Current transport from the Si emitter to the Ag electrode via (a) direct connection between Ag crystallites and Ag bulk, tunneling through ultra-thin glass regions, and (b) conduction within the glass layer via tunneling between metal precipitates.	17
Figure 3-1 Operation of a three-roll mill	18
Figure 3-2 Three-roll mill	20
Figure 3-3 Schematic of screen printing process	21
Figure 3-4 Screen printer and mesh screen	22
Figure 3-5 Fast fire furnace	23
Figure 3-6 TEM	25
Figure 3-7 Schematic of SEM [43]	26
Figure 3-8 Four-point probe measurement of semiconductor sheet resistance	28
Figure 3-9 Four-point probe	29
Figure 3-10 Experimental procedures	30
Figure 4-1 SEM image of 1.5μm (sphere) Cu particles (a) 5000x (b) 10000x	34
Figure 4-2 SEM image of 2.5μm (sphere) Cu particles (a) 5000x (b) 10000x	35
Figure 4-3 SEM image of 1μm (sphere) Ag particles (a) 5000x (b) 30000x	35
Figure 4-4 SEM image Dupont’s front Ag paste (a)5000x (b)10000x	36
Figure 4-5 SEM image of (a) PbO-based glass (b) ZnO-based glass	36
Figure 4-6 (a) Top view [5000x] (b) Side view [5000x] of paste	37
Figure 4-7 OM image of 200μm narrow line (a) 500x (b) 1000x	39
Figure 4-8 OM image of 50μm narrow line (a) 500x (b) 1000x	39
Figure 4-9 SEM cross-section image of 50μm narrow line (a) [1250x] (b) [5000x]	40
Figure 4-10 SEM image of samples heated to different temperatures without retaining temperature (a) 400℃ (b) 500℃ (c) 600℃ (d) 700℃	41
Figure 4-11 EDS analysis of the pointed particle in figure 4-10 (b)	42
Figure 4-12 TEM image DARCs layers	43
Figure 4-13 EDS analysis of layers 1 to 3 in figure 4-12	43
Figure 4-14 Phase diagram of PbO-SiO2 system	44
Figure 4-15 SEM cross-section image of a sample which has been heated to 800℃	45
Figure 4-16 TEM cross-section image at the interface of a sample which has been heated to 800℃ and retained temperature for 5 minutes	45
Figure 4-17 EDS analysis of point 1 in figure4-15	46
Figure 4-18 EDS analysis of point 2 in figure4-15	46
Figure 4-19 EDS analysis of point 3 in figure4-15	46
Figure 4-20 EDS analysis of point 4 in figure4-15	46
Figure 4-21 EDS analysis of point 5 in figure4-15	47
Figure 4-22 EDS analysis of point 6 in figure4-15	47
Figure 4-23 TEM cross-section image around the interface of a sample fired at 800℃ for 5 minutes	47
Figure 4-24 STEM image of zone A	48
Figure 4-25 EDS analysis of point 1 in figure 4-24	48
Figure 4-26 TEM cross-section image at the interface of a sample which has been heated to 900℃ without retaining temperature	50
Figure 4-27 EDS analysis of point 1 in figure 4-26	51
Figure 4-28 EDS analysis of point 2 in figure 4-26	51
Figure 4-29 EDS analysis of point 3 in figure 4-26	51
Figure 4-30 EDS analysis of point 4 in figure 4-26	51
Figure 4-31 EDS analysis of point 5 in figure 4-26	52
Figure 4-32 EDS analysis of point 6 in figure 4-26	52
Figure 4-33 TEM TEM cross-section image at the interface of a sample which has been heated to 900℃ without retaining temperature	52
Figure 4-34 STEM image of zone B	53
Figure 4-35 EDS analysis of point 1 in figure 4-34	53
Figure 4-36 TEM image of sample which has been heated to 900℃ retaining temperature for 10 minutes	55
Figure 4-37 EDS analysis of point 1 in figure 4-36	55
Figure 4-38 EDS analysis of point 2 in figure 4-36	55
Figure 4-39 EDS analysis of point 3 in figure 4-36	56
Figure 4-40 EDS analysis of point 4 in figure 4-36	56
Figure 4-41 EDS analysis of point 4 in figure 4-36	56
Figure 4-42 TEM image of sample which has been heated to 900℃ retaining temperature for 10 minutes	57
Figure 4-43 SEM image of samples which have been heated to 900℃ retaining temperature for (a) 0 minute (b) 10 minutes	57
Figure 4-44 image of paste on the substrate (a) 100x (b) 1250x (c)5000x	59
Figure 4-45 SEM cross-section image of 400℃ ~ 500℃ (a) 250x (b) 5000x (c) 10000x	59
Figure 4-46 SEM cross-section image of 500℃ ~ 600℃ (a) 250x (b) 5000x (c) 10000x	60
Figure 4-47 SEM cross-section image of 600℃ ~ 700℃ (a) 2500x (b) 5000x (c) 10000x	60
Figure 4-48 SEM top view image of sample after first step firing (a) 2500x (b) 5000x (c) 10000x	62
Figure 4-49 Process flow and mechanism of constructing ohmic between front electrode and Si-based solar cell	63

Table Captions
Table 1-1 Concept of experiment	11
Table 3-1 Contents of paste	31
Table 3-2 Conditions of first step firing process	32
Table 3-3 Conditions of second step firing process	33
Table 4-1 Ratios of metal particles	35
Table 4-2 Final contents of paste	37
Table 4-3 Cu to O ratios of samples	58
Table 4-4 Sheet resistances of samples compared with typical Cu paste sintered only by second step firing process in reducing atmosphere	58
                                    

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