銅鋅錫硫硒是一個下世代非常有潛力的太陽能電池吸收層材料，因為此材料的組成元素皆為地表充足且便宜的元素，將有機會達到低成本高效率的薄膜太陽能電池。然而，目前銅鋅錫硫硒元件最高轉換效率達12.6%仍遠低於銅銦鎵硒元件(超過20%)，歸因於許多材料本質上的挑戰。根據熱力學的研究，能形成銅鋅錫硫硒單一相的成分區間非常小，再加上其組成元素容易在高溫時損失的問題，此皆造成了成長銅鋅錫硫硒薄膜的難度提高。硒化製程為將前驅物在高溫之下與硒氣反應形成吸收層的一種製程方法，可應用於大面積化生產，具有商業價值。雖然硒化製程早在三十年前就已經被開發，但採用此方法製備銅銦鎵硒或是銅鋅錫硫硒元件皆表現出低開路電壓，此問題大幅限制了元件轉換效率與停滯了元件發展許久。為了使元件達到商業化之水準與目的，此博士論文將致力於改善硒化製程製備銅鋅錫硫硒/銅銦鎵硒的元件轉換效率。在此博士論文，我們提出許多要達到高效率銅鋅錫硫硒元件的必要條件。首先，我們研究關於硒化製程中添加硫化錫粉末對銅鋅錫硫硒元件的影響，添加硫化錫將有助於改善硒化製程時的元素損失，並且同時調整銅鋅錫硫硒的吸收層能隙與改善背電極問題。其次，我們發展一種方法能在銅鋅錫硫硒吸收層表面引入硫含量增加的表面層，用來解決因為界面復合問題所導致的低開路電壓，我們同時深入研究表面鈍化對於元件的許多好處。最後，我們致力於解決硒化製程製備銅銦鎵硒元件所面臨的低開路電壓問題。我們提出一新穎方法使銅銦鎵硒薄膜擁有高鎵含量的表面，此能大幅度提升元件的開路電壓。我們從理論與實驗端驗證鉀摻雜是如何有效影響鎵元素擴散行為，並進一步影響吸收層的能隙梯度。基於此概念，我們達到超過15%的銅銦鎵硒太陽能電池，此效率為目前藉由硒粉進行硒化製程製備銅銦鎵硒的最高效率。這些發現皆顯示了表面工程將是採用硒化製程達到高效率銅鋅錫硫硒與銅銦鎵硒太陽能電池的關鍵因子。

Earth abundant Cu2ZnSn(S,Se)4 (CZTSSe) absorber is the promising candidate for achieving efficient solar cells with low cost. However, the highest CZTSSe efficiency of 12.6% is much lower than Cu(In,Ga)Se2 (CIGSe) solar cells (over 20%) due to several material challenges. The narrow region for pure CZTSSe phase combines with the significant element loss during high temperature, resulting in growing CZTSSe difficult. The selenization process, which selenized precursors at high temperature to form absorbers, is suitable for large-area fabrication and industrial purposes. Although selenization had been developed over 30 years ago, both CZTSSe and CIGSe devices based on this technique encounter the problem of low open-circuit voltage (Voc), limiting the efficiency and progression for a long time. This dissertation aims to improve the CZTSSe/CIGSe efficiency fabricated by selenization in order to meet the goal of commercialization. In the dissertation, the several criteria for reaching efficient CZTSSe solar cells are proposed. First, we show that adding SnS powder during selenization can improve the CZTSSe absorber by restraining the element loss from CZTSSe, tuning the bandgap of CZTSSe, and engineering the back contact simultaneously. Secondly, we demonstrate the route for forming the S-increased surface on CZTSSe film against the serious interface recombination regarded as one of limiting factors on low Voc. The beneficial effects of surface passivation are also investigated in detail. Finally, we devote to solve the problem of low Voc in selenized CIGSe. We propose the novel approach for achieving the CIGSe surface with high Ga content, which significantly boosts the Voc of devices. The mechanism, how K affects the Ga distribution, is verified theoretically and experimentally. The over 15% CIGSe solar cell is obtained, which is the highest efficiency by selenization via Se pellet. These findings strongly indicate that the surface engineering is the key to achieving high-efficiency CZTSSe and CIGSe solar cells based on selenization.

Contents

Abstract i
Contents iii
List of Figures vii
List of Tables xi
General introduction 1
1.1 Context and purposes 1
1.2 Organization of this thesis 4
Background 6
2.1 The introduction of solar cells 6
2.1.1 The operation of solar cells 6
2.1.2 Current density-voltage curve 7
2.1.3 Short circuit current density 8
2.1.4 Open circuit voltage 9
2.1.5 Fill factor 10
2.1.6 Efficiency 10
2.1.7 Shunt resistance and series resistance 11
2.1.8 Recombination in solar cells 12
2.2 The structure of thin-film solar cells 14
2.2.1 Substrate 14
2.2.2 Mo back contact 15
2.2.3 P-type absorber 16
2.2.4 N-type buffer 17
2.2.5 Window layer 18
2.2.6 Al grid 19
2.3 The reviews of chalcopyrite solar cells 19
2.3.1 History of CIGSe solar cells and development 19
2.3.2 The fabrication of CIGSe absorbers 21
2.3.3 The effects of alkali metal on CIGSe 25
2.4 The reviews of kesterite solar cells 27
2.4.1 The history of kesterite solar cells and development 27
2.4.2 The fabrication of kesterite absorbers 29
2.4.3 The current challenges of kesterite 37
Experimental Techniques 41
3.1 Sample Fabrication 41
3.1.1 Magnetron sputtering system 41
3.1.2 Selenization/Sulfurization process 41
3.1.3 Evaporation 42
3.2 Material characterization 43
3.2.1 X-ray diffraction 43
3.2.2 Scanning electron microscopy 44
3.2.3 Raman spectroscopy 45
3.2.4 X-ray photoelectron spectroscopy 46
3.2.5 Secondary ion mass spectrometry 47
3.2.6 Auger electron spectroscopy 48
3.2.7 Kelvin probe force microscope 49
3.2.8 X-ray fluorescence 51
3.2.9 Extended X-ray absorption fine structure 52
3.3 Device characterization 53
3.3.1 Current-voltage measurement 53
3.3.2 External quantum efficiency 54
3.3.3 Capacitance-voltage measurement 54
The effects of adding SnS powder during selenization on kesterite solar cells 55
4.1 Introduction 56
4.2 Experimental procedue 59
4.3 Results and discussions 61
4.4 Summary 75
The effects of sulfur-increased surface on kesterite solar cells 76
5.1 Introduction 77
5.2 Experimental procedue 80
5.3 Results and discussions 82
5.4 Summary 97
Interplay between potassium doping and bandgap profiling in selenized Cu(In,Ga)Se2 thin-film solar cells 98
6.1 Introduction 99
6.2 Experimental procedue 102
6.2.1 Fabrication of CuGa:KF target 102
6.2.2 Preparation of CIGSe absorbers and devices 102
6.2.3 CIGSe absorber and device characterization 103
6.2.4 Extended X-ray absorption fine structure technique 104
6.2.5 First-principle calculation 105
6.3 Results and discussions 107
6.3.1 Best CIGSe device with K-incorporation and tri-layer precursor 107
6.3.2 Effects of K-incorporation on growth of CIGSe absorbers 108
6.3.3 How K-incorporation affects Ga distribution 110
6.3.4 Effects of K-incorporation on CIGSe devices 116
6.3.5 Effects of CuGa:KF/In/CuGa:KF tri-layer precursors on CIGSe devices 119
6.4 Summary 122
Conclusions and outlooks 123
7.1 Conclusions 123
7.2 Suggestions for future works 124
Reference 126

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