銅銦鎵硒薄膜太陽能電池具有可撓、高吸收、可調控能隙以及直接能隙的特性，被認為是次世代太陽能電池候選人之一，其特性可應用於雙面吸光太陽能電池、彎曲基板以及堆疊型太陽能電池，即使銅銦鎵硒薄膜太陽能電池具有高於其它薄膜太陽能電池之轉換效率，然而其製造成本遠高於傳統矽基太陽能電池，使其較少被使用於商業模組，合金後硒化法符合大量生產以及工業目的，其製程包含了濺鍍前驅層以及高溫硒化，雖然合金後硒化法已經開發超過三十年，受限於較低的開路電壓，使得其製程開發進度仍然較緩慢，在此論文中，我們致力於引入新的技術與概念，以改善鎵成分梯度、二次相於銅銦鎵硒以及鉬背電極接觸面以及不足的晶粒成長，首先，我們在藉由在硒化過程當中利用高硒分壓創造出有序缺陷化合物，使其促進鎵成分擴散，其次，為了改善有序化合物於背電極處形成電洞阻礙層，我們藉由銅鎵銫靶額外添加銫元素於銅銦鎵硒中抑制其成長，最後，我們致力於藉由設計的前驅層結構創造銀成分梯度，使其表面能隙增加並且促進晶粒成長，其中我們發現銀成分會促進表面形成有序化合物並形成反轉層以及藉由形成缺陷化合物抑制反位缺陷，我們並且討論有序化合物的形成機制以及鉀元素增加載子濃度於銀合金之銅銦鎵硒太陽的機制，最終我們達到超過19%的元件最高轉換效率，此為透過硒粉硒化的最高轉換效率，我們認為我們提出的方法能容易的被現今製成技術所使用。

Cu(In,Ga)Se2 (CIGS) solar cells has been regarded as the candidate for next generation solar cells due to their flexibility, high absorption coefficient, tunable band structure and direct bandgap, which can be developed into several wide ranging application such as bifacial solar cells, tandem solar cells and flexible solar cells. However, the manufacturing cost of CIGS solar cells are significantly higher than existing Si-based solar cells, making CIGS rarely use in commercial module even though the cell efficiency is higher than other thin film solar cells. The sequential process, which involves sputtering the precursor first and then selenized under high temperature annealing, is a widely approach to meet the mass production purposes and industrial aspects. Although the sequential process was proposed over 30 years ago, the progress of sequential process is slow and limited by the open circuit voltage. In this dissertation, we aim to introduce a key concept and technology to address the uncontrollable Ga grading, undesirable secondary phase at CIGS/Mo, and insufficient grain growth. First, an ordered vacancy compound (OVC) can be formed during the early stage of the selenization by high selenium pressure, which promotes Ga diffusion and modifies Ga grading. Second, the undesirable OVC phase at the CIGS/Mo interface can be reduced by Cs incorporation by CuGa:CsF, which further eliminates the hole barrier. Finally, we aim to form the notch band structure by Ag grading to enhance the surface bandgap and promote the grain growth by the designed precursor stacking. The incorporation of Ag also induces the OVC phase on the top surface, which is the type inversion layer and reduces the antisite defect by forming the defect complex. The mechanism, how Ag affects OVC phase, and how K affects the carrier density in Ag alloyed CIGSe were discussed by the first-principle calculation. Over 19% cell efficiency were achieved, which is the highest efficiency achieved by Se pellet. We believe that our concept can easily be adopted in the existing manufacturing process.

Abstract i
Chapter 1 1
General introduction 1
1.1 Context and purpose 1
1.2 Organization of this thesis 3
Chapter 2 5
Literature review 5
2.1Background 5
2.1.1 The operation of Cu(In,Ga)Se2 solar cells 5
2.1.2 Diode equation 6
2.1.3 Open circuit voltage 7
2.1.4 Short circuit current 8
2.1.5 Fill factor 9
2.1.6 Efficiency 9
2.1.7 Shunt resistance and series resistance 10
2.1.8 Recombination 11
2.2 The structure of Cu(In,Ga)Se2 solar cells 13
2.2.1 Substrate 14
2.2.2 Back contact 15
2.2.3 P type absorber 16
2.2.4 Buffer layer 17
2.2.5 Transparent conductive oxide (TCO) 17
2.2.6 Front contact 18
2.3 The history and key developments of Cu(In,Ga)Se2 18
2.3.1 Co-evaporation process 18
2.3.2 Sequential process 22
2.4 Design rule of the Cu(In,Ga)Se2 solar cells 26
2.4.1 Rule 1: Back surface grading should be greater than 0.5 eV to avoid the backside recombination 26
2.4.2 Rule 2: The VBM should be downshift over 0.3 eV to avoid the front surface recombination. 27
2.4.3 Rule 3: The carrier concentration should not exceed 1017cm-3 and not lower than 1015 cm-3 30
2.5 The history of alkali metal effect 30
2.6 The phase diagram of CIGSe: Chalcopyrite CIGSe phase (α-phase) and ordered vacancy compound (β-phase) 32
2.7 Ga profile in selenized CIGSe 34
2.8 Ag alloyed CIGSe solar cells 35
Chapter 3 Experiment Techniques 38
3.1 Sample preparation 38
3.1.1 Magnetron sputtering system 38
3.1.2 Selenization process 39
3.1.3 Evaporation 41
3.2 Material Characterization 42
3.2.1 Scanning electron microscopy (SEM) 42
3.2.2 Transmission electron microscopy (TEM) 43
3.2.3 X-ray diffraction (XRD) 44
3.2.4 Atomic force microscopy (AFM) 46
3.2.5 X-ray photoelectron spectroscopy (XPS) 47
3.2.6 Secondary ion mass spectroscopy (SIMS) 48
3.2.7 Raman spectroscopy 48
3.2.8 Photoluminescence (PL) and time-resolved PL spectroscopy 49
3.2.9 X-ray fluorescence (XRF) 50
3.3 Device Characterization 51
3.3.1 Current -voltage measurement 51
3.3.2 Admittance spectroscopy 51
3.3.3 External quantum efficiency 52
3.3.4 Capacitance-voltage measurement 52
Chapter 4 53
Adjusting the Ga grading by the formation of ordered vacancy compound in sequential process 53
4.1 Introduction 54
4.2 Experimental section 56
4.2.1 Device Preparation: 56
4.2.2 Characterization: 57
4.3. Results and Discussion 58
4.3.1. Benefits of K-doped precursor layer 58
4.3.2. Effects of selenium partial pressure on structure and Ga grading of CIGSe films 58
Chapter 5 80
Incorporation of Cs in selenized Cu(In,Ga)Se2 solar cells by CuGa:CsF precursor layer 80
5.1 Introduction 80
5.2 Experimental section 84
5.2.1 Fabrication of the CuGa:CsF target 84
5.2.2 Device fabrication 84
5.2.3 Characterization 85
5.3 Results and discussion 85
5.3.1 Sputtering Cs-containing precursor layer 85
5.3.2 Effects of Cs-containing precursor on CIGSe Solar Cell Performance 92
5.4 Conclusion 95
Chapter 6 96
Band engineering and defect elimination by Ag-grading in high Ag alloyed Cu(In,Ga)Se2 thin film solar cells 96
6.1 Introduction 97
6.2Experimental section 101
6.2.1 Device fabrication 101
6.2.3 Computational methods 103
6.3 Results and discussion 103
Chapter 7 128
Conclusion and outlooks 128
7.1 Conclusions 128
7.2 Suggestions for future development 129
Reference 131

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