以單一靶材濺鍍製作銅銦硒薄膜在超過三十年前即被提出，和目前主流的共蒸鍍法以及合金後硒化法約略同時期開始發展。然而，共蒸鍍法與合金後硒化法在三十年來有長足的演進，兩者都能夠製作元件效率超過 20% 的銅銦鎵硒 (CIGS) 太陽能電池，單一靶材濺鍍製程的進步卻十分有限。

為了有效運用單一 CIGS 靶材濺鍍製程的優點，如大面積均勻、不需使用毒性氣體、不需後硒化等，本論文致力於提升單一四元靶材濺鍍薄膜的元件性能，縮小其與主流製程間的差距。本論文點出此製程的關鍵缺點為硒供給不足，並嘗試以三種方法緩解硒不足以及轉換效率過低的問題：

1. 以濺鍍 Sb2Se3 靶材提升硒供應量：在共蒸鍍與後硒化製程中，硒的供給量可以獨立控制。然而，過量硒供給在四元靶材濺鍍製程中卻難以達成。本論文利用 Sb2Se3 在高溫會分解產生硒蒸氣的特性，成功以濺鍍 Sb2Se3 做為額外硒的來源，並探討在 CIGS 濺鍍的前、中、後三個不同階段提供額外硒供給的效果。以此方法，元件轉換效率成功的提升至 10.4%。

2. 以鈉摻雜減少硒不足時易生成的缺陷：除了藉由提供額外的硒來消除硒不足時產生的缺陷，本研究發現這些缺陷也可以藉由額外的鈉摻雜來消除。這一部分的研究探討鈉摻雜方式對 CIGS 薄膜以及元件電性表現的影響，並以此方法達成 11% 的元件轉換效能。

3. 以鈉、鉀共摻雜更進一步提升元件性能：近期，人們發現鈉鉀共摻雜可以更近一步提升 CIGS 的元件轉換效率，然而其機制仍待探索。本研究的第三部分，探討了鈉、鉀共摻雜對四元靶材濺鍍 CIGS 薄膜之影響，並提出背後可能的機制。藉由鈉、鉀共摻雜，元件效能更進一步提升至 14% 以上，超越了只有一種額外鹼金屬摻雜的元件效能。

本論文提出的方法在不需要後硒硫化、且未做能隙工程的前提下，得到了超過 14% 的元件轉換效能，證實了四元靶材濺鍍足以成為工業生產上可行的製程。現階段，我們仍未看到此製程的限制，並認為在不久的將來可以有更進一步突破。

Depositing CuInSe2 absorbers by sputtering from a single compound target has been proposed over 30 years ago, as old as the current mainstream processes, co-evaporation and post-selenization. Single target sputtering is a simple and straightforward process. Uniform CuInSe2 or Cu(In,Ga)Se2 (CIGS) thin films can be deposited over a large area without the need of toxic gases or post-selenization. However, while both co-evaporation and post-selenization processes have evolved and reached efficiency over 20%, single target sputtering seems to be abandoned, with little progress made in the past thirty years. This dissertation aims to improve the efficiency of the CIGS absorbers deposited by sputtering from a CIGS target in order to fully utilize the advantages of the process. In the dissertation, we point out that the key drawback of the quaternary-sputtering process is the limited Se supply, and several approaches are proposed to resolve the Se deficiency. The first approach is to supply extra Se by sputtering an Sb2Se3 target, which is a stable and up-scalable method to provide extra Se during sputtering. The influences of extra Se supply before, during and after CIGS sputtering are investigated. Secondly, we show that the Se deficiency can also be compensated by extra Na doping, which significantly improves cell efficiency without the need of extra Se supply. Finally, we show that Na and K co-doping further improves the quality of quaternary-sputtered CIGS absorbers, and the possible mechanisms are proposed. With these approaches to minimizing the impact of Se deficiency and improving cell performance, an efficiency of over 14% can be obtained by sputtering from a single CIGS target without post-selenization or introducing compositional grading in the absorbers. The feasibility of the quaternary-sputtering process is proved, and, at the current stage, we do not see any limitation of the quaternary-sputtering process and expect for higher cell efficiency in the near future.

Abstract iii
Acknowledgements vi
Contents vii
List of Figures xi
List of Tables xv
Abbreviations xvii
Symbols xix

1 General introduction 1
1.1 Context and objectives . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Organization of this thesis . . . . . . . . . . . . . . . . . . . . . . . 4

2 Background 7
2.1 Status of photovoltaics . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.1 Photovoltaic industry . . . . . . . . . . . . . . . . . . . . . . 7
2.1.2 Research and development . . . . . . . . . . . . . . . . . . . 8
2.2 Device structure of CIGS solar cells . . . . . . . . . . . . . . . . . . 10
2.3 Material properties of Cu(In,Ga)Se2 . . . . . . . . . . . . . . . . . . 13
2.3.1 Composition and phases of CIGS . . . . . . . . . . . . . . . 13
2.3.2 Intrinsic defects in CI(G)S . . . . . . . . . . . . . . . . . . . 16
2.3.3 Effect of oxygen . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.4 Effect of Na doping . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.5 Effect of K doping . . . . . . . . . . . . . . . . . . . . . . . 21
2.4 Device physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.1 Solar cell operation . . . . . . . . . . . . . . . . . . . . . . . 25
2.4.2 Band diagram and current transport . . . . . . . . . . . . . 26
2.4.3 Band engineering . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.4 Grain boundaries . . . . . . . . . . . . . . . . . . . . . . . . 31
2.5 Fabrication of Cu(In,Ga)Se2 . . . . . . . . . . . . . . . . . . . . . . 33
2.5.1 Three-stage co-evaporation process . . . . . . . . . . . . . . 33
2.5.2 Sulfurization after selenization . . . . . . . . . . . . . . . . . 34
2.5.3 One-step sputtering process . . . . . . . . . . . . . . . . . . 36
2.5.4 Other one-step processes . . . . . . . . . . . . . . . . . . . . 38
2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Experiment techniques 41
3.1 Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1.1 High-vacuum magnetron sputtering system . . . . . . . . . . 41
3.1.2 Evaporators . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.1 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . 43
3.2.2 Kelvin force microscope . . . . . . . . . . . . . . . . . . . . 44
3.2.3 X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2.4 Scanning electron microscope . . . . . . . . . . . . . . . . . 47
3.2.5 Surface analysis . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2.6 Performance measurement . . . . . . . . . . . . . . . . . . . 49
3.2.7 Capacitance-voltage analysis . . . . . . . . . . . . . . . . . . 50
3.2.8 Admittance . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4 Excess Se supply for quaternary sputtering process 53
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.3.1 Importance of Se supply in quaternary sputtering process . . 59
4.3.2 Co-sputtering of CIGS and Sb2Se3 . . . . . . . . . . . . . . 61
4.3.3 Sequential sputtering of CIGS and Sb2Se3 . . . . . . . . . . 65
4.3.4 On the differences between co-sputtering and sequential sput-
tering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

5 Na-induced efficiency boost for Se-poor CIGS absorbers 75
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
5.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . 78
5.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.3.1 CIGS growth conditions . . . . . . . . . . . . . . . . . . . . 79
5.3.2 PV device characteristics . . . . . . . . . . . . . . . . . . . . 81
5.3.3 CIGS film characteristics . . . . . . . . . . . . . . . . . . . . 82
5.3.4 Transport and defect properties . . . . . . . . . . . . . . . . 85
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

6 Effects of K on quaternary-sputtered CIGS absorbers 95
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.2 Experimental details . . . . . . . . . . . . . . . . . . . . . . . . . . 98
6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.3.1 Individual effects of annealing, NaF-PDT and KF-PDT . . . 99
6.3.2 Effects of sequential PDT of NaF and KF . . . . . . . . . . 107
6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

7 Conclusions and outlooks 119
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.2 Suggestions for future works . . . . . . . . . . . . . . . . . . . . . . 120

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