Cu2ZnSnSe4(CZTSe)鋅黃錫礦太陽能電池的晶體結構與Cu2InSe3相似，然而CZTSe中有著與Cu2InSe3截然不同的電性表現(例如在低溫時過低的短路電流以及填充因子)。此特殊電性表現的背後原因被認為是打開進一步改善其太陽能電池性能大門的鑰匙。因此，本論文致力於鋅黃錫礦太陽能電池缺陷系統的分析與控制。我們指出這些特殊的電性表現與其缺陷系統密切相關，並且我們也在CZTSSe太陽能電池中發現了深層的n型缺陷。由此可知，控制CZTSSe太陽能電池的缺陷系統對於進一步提升其電池效率是非常重要的。其中大量存在於CZTSSe太陽能銅鋅錯位缺陷被認為是低開路電壓的可能原因。為了控制CZTSSe太陽能電池的缺陷的系統，我們研發一個新的化合物 – (Ag,Cu)2ZnSnSe4以抑制銅鋅錯位缺陷的生成。我們證明了銅鋅錯為缺陷確實可藉由銀加入而抑制，我們也發現表面鈍化對於銀化合鋅黃錫礦太陽能電池的重要性。以有效的抑制銅鋅錯位缺陷輔以適當的表面鈍化，鋅黃錫礦太陽能電池性能可進一步改善。此外，銀取代銅也可以控制價帶位置，這使我們於未來研究能帶工程時能夠設計出更好的吸收層帶結構。這些研究結果為鋅黃錫礦太陽能電池的發展提供了新的途徑，並有望突破長年來鋅黃錫礦太陽能電池的發展瓶頸。

This thesis aims to gain more understanding of the defect system and the special electrical features of CZTSSe solar cells, which are believed to be the breadcrumbs to make the trustworthy strategy to improve the cell performance of kesterite solar cells. We point out that the special features (collapse of JSC and FF at low temperature) of CZTSSe solar cells are closely related to the defect system and we also find the possible existence of deep n-type defect in CZTSSe solar cells. To control the defect system of kesterite CZTSSe solar cells, we investigate a new compound – (Ag,Cu)2ZnSnSe4 to suppress the formation of CuZn antisite defects, which exist in kesterite CZTSSe in a large amount and are believed to be the reason for low VOC. We prove that the CuZn antisite defects are indeed suppressed by Ag substitution and find the importance of surface passivation in Ag-alloyed kesterite solar cells. With the effective suppression of CuZn and adequate surface passivation, the cell performance can be further improved. Moreover, the Ag substitution can also control the valence band maximum, which enables us to design a better band structure for the absorber layer. This study paves a new avenue for the development of kesterite solar cells and is promising for breaking the bottleneck of kesterite solar cells in the near future.

Abstract i
中文摘要 i
Contents iii
List of Figures vii
List of Tables xiii
Chapter 1 General Introduction 1
Chapter 2 Background 7
2.1 Physics of Photovoltaics 7
2.1.1 Physics of p-n junction 7
2.1.2 Current-voltage characteristics of a diode 9
2.1.3 Light I-V characteristics 9
2.1.3.1 Short-circuit current density 11
2.1.3.2 Open-circuit voltage 11
2.1.3.3 Fill Factor 12
2.1.3.4 Power Conversion Efficiency 12
2.1.4 Losses in solar cells 13
2.1.6 Recombination mechanism 14
2.1.6.1 Band-to-band recombination 15
2.1.6.2 Auger recombination 15
2.1.6.3 Recombination through a defect state 15
2.1.6.4 Recombination in heterojunction solar cells 16
2.2 Introduction of CZTSSe thin film solar cells 17
2.2.1 Crystal structure 17
2.2.2 Phase diagram and formation mechanism of CZTSSe 20
2.2.2.1 The Cu2Se–ZnSe and SnSe2-ZnSe system 21
2.2.2.2 The Cu2Se–SnSe2 system 21
2.2.2.3 Formation mechanism 22
2.2.3 Overview of the fabrication of CZTSSe thin film solar cell. 24
2.2.3.1 Vacuum processes 24
2.2.3.2 Non-vacuum processes 27
2.2.4 Defect system of kesterite CZTSSe solar cell 29
Chapter 3 Experimental detail 35
3.1 Device fabrication 35
3.1.1 Device structure 35
3.1.2 Molybdenum back contact 35
3.1.3 CZTSSe absorber layer 36
3.1.4 CdS buffer layer 36
3.1.5 Window layer 37
3.1.6 Al grid 37
3.2 Characterization 37
3.2.1 Material analysis 37
3.2.1.1 X-ray diffraction 37
3.2.1.2 Raman spectroscopy 38
3.2.1.3 Scanning Electron Microscopy 39
3.2.1.4 Energy Dispersive X-ray Spectroscopy (EDS/EDX) 39
3.2.1.5 Auger electron spectroscopy (AES) 39
3.2.1.6 X-ray photoelectron spectroscopy (XPS) 40
3.2.1.7 Ultraviolet photoelectron spectroscopy (UPS) 42
3.2.1.8 Ultraviolet–visible spectroscopy (UV-Vis) 43
3.2.2 Electrical characteristic analysis 43
3.2.2.1 Light current-voltage measurement 43
3.2.2.2 Capacitance-voltage measurement 44
3.2.2.3 Admittance spectroscopy 44
3.2.2.4 Temperature dependent current-voltage measurement 46
Chapter 4 Defect Characteristics of kesterite CZTSSe solar cells 49
4.1 Introduction 50
4.2 Experimental details 52
4.3 Material properties and device characteristics 56
4.4 Electrical characteristics and modeling by SCAPS 65
4.5 Summary 75
Chapter 5 Fabrication of (Ag,Cu)2ZnSn(S,Se)4 solar cells by ion beam sputtering: evaluating the formation path, thermal stability and band structure 77
5.1 Introduction 78
5.2 Experimental details 80
5.3 Phase stability and formation path of AZTSe 82
5.4 Effects of stacking sequence on the morphology of precursors. 84
5.5 Fabrication of ACZTSSe thin films 86
5.6 The effect of Ag on band structure 89
5.7 Device performance 92
5.8 Challenges 94
5.9 Summary 96
Chapter 6 Effect of Ag concentration on cell performance and defect system of Ag-alloyed kesterite solar cells 99
6.1 Introduction 100
6.2 Experimental details 103
6.3 Material properties of ACZTSSe 105
6.4 The effect of Ag concentration on ACZTSSe solar cell 110
6.5 The defect analysis of ACZTSSe solar cell 112
6.6 Summary 120
Chapter 7 Conclusion and outlooks 123
References 127

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