鹼金屬離子添加對於銅銦鎵硒太陽能電池的性能提升具有巨大潛力。本研究著眼於銅銦鎵硒（CIGS）太陽能電池的前接觸面，探討添加鹼金屬離子對其性能的影響。在第一部分，我們著重於重鹼金屬離子添加至含硫之CIGS吸收層的效果。透過在CIGS吸收層上後沉積不同量的銫（Cs-PDT），我們觀察到光電轉換效率從15.88％提升至16.68％。時間解析光激發螢光（TRPL）測量得到的載子壽命τ1和τ2分別從8.3 ns和27.8 ns增加至9.6 ns和34.4 ns。在照光下的變溫電壓電流測量顯示，在Cs後處理後，載子複合途徑由界面復合主導轉變為中性區域的復合主導。開爾文探針力顯微鏡（KPFM）的結果顯示，在Cs-PDT處理後，CIGS表面整體功函數增加了432.1 meV，這表示吸收層內的載子濃度增加，這與電容-電壓（CV）測量的結果一致，其載子濃度在經過Cs-PDT處理後，從2.05×1016 cm-3增加至9.2×1016 cm-3。而Cs-PDT後的晶界處其功函數則提升，表示Cs-PDT處理後晶界處的價帶下移。無光照的低溫測量顯示，PN接面之間的載子注入的活化能從81.6 meV增加至104.8 meV，顯示界面處的導帶偏移（CBO）有所增加。基於KPFM和CBO測量，我們提出了Cs-PDT處理後可能的能帶模型，並利用紫外光電子能譜（UPS）結合低能量反光電子能譜（LEIPS）驗證了此能帶變化。證實Cs-PDT處理後，吸收層前接觸面的價帶明顯向下移動，使能隙從1.44 eV增加至Cs-PDT處理後的2.55 eV。而此向下移動價帶可以有效鈍化CIGS太陽能電池的PN接面。在第二部分之中專注於研究經過Cs-PDT處理的吸收層對CdS緩衝層的影響。我們首先分析了經不同程度Cs-PDT的CIGS吸收層樣品上的CdS緩衝層。我們發現CIGS吸收層一旦經歷了Cs-PDT，從CIGS向外擴散進入CdS層的Cu即會減少。自CdS擴散進CIGS的Cs也展現了與其與PDT相似的離子交換現象，即較高的Cs將排斥吸收層中的Na含量。此外，CdS緩衝層中雜質的分佈也會影響功函數，Na傾向於降低其功函數，而Cs傾向于增加其功函數。為了驗證這些效應，我們特意將鹼金屬離子加入CdS緩衝溶液中進行檢測。結果顯示添加Na後CdS的功函數增加，而添加Cs後CdS的功函數減少。因此，我們將Na-doped CdS 緩衝層做在Cs-PDT CIGS上，與Cs-PDT CIGS搭配未摻雜CdS 樣品相比，Na-doped CdS有較高的的 CdS 載子濃度，元件效率上也有微幅的提升。最後利用SCAPS軟件檢測CdS緩衝層中載子濃度對於元件表現的影響。此論文研究了Cs-PDT影響CIGS光電轉換效率的各種面向，透過電性量測以及材料分析進行驗證。後續並深入Cs-PDT對於CdS緩衝層的影響，最後提出了一種以Na摻雜的CdS作為未來調整緩衝層以增加元件表現的可能方案。

The incorporation of alkali metal ions holds tremendous potential for enhancing the performance of sulfur-incorporated copper indium gallium selenide (CIGS-based, CIGSSe) solar cells. This study centers on the front interface of CIGS solar cells and examines the consequences of alkali metal ion integration. In Part 1, the attention is directed towards the influence of introducing heavy alkali metal ions into the CIGSSe absorber layer. Through the post-deposition of varying quantities of Cs (Cs-PDT) onto the CIGS absorber layer, an increase in efficiency was observed, elevating it from 15.88% to 16.68%. Time-resolved photoluminescence (TRPL) measurements revealed that the carrier lifetimes τ1 and τ2 increased from 8.3 ns and 27.8 ns to 9.6 ns and 34.4 ns, respectively. Temperature-dependent JV measurement under illumination demonstrated that a transition from interface-dominated recombination to bulk-dominated recombination through an appropriate Cs-PDT. The results obtained through Kelvin probe force microscopy (KPFM) revealed a notable increase of 432.1 meV in the surface work function of CIGS after Cs-PDT. This increase points to an enhanced carrier concentration within the absorber layer. This observation aligned with capacitance-voltage (CV) measurements, where the carrier concentration increased from 2.05×1016 cm-3 to 9.2×1016 cm-3 after Cs-PDT. On the other hand, the work function at grain boundaries increased after Cs-PDT treatment, suggesting a downward band bending in the valence band. Low-temperature measurements under dark conditions demonstrated that the activation energy for carrier injection across the PN junction increased from 81.6 meV to 104.8 meV, implying an elevated conduction band offset (CBO) at the interface. Based on the data obtained from KPFM and CBO measurements, potential energy band models resulting from Cs-PDT were formulated. The validation of these energy band models was carried out using ultraviolet photoelectron spectroscopy (UPS) in combination with low-energy inverse photoemission spectroscopy (LEIPS). This verification process affirmed that Cs-PDT induced a significant downward shift in the valence band at the CIGSSe front interface, resulting in an expansion of the bandgap from 1.44 eV to 2.55 eV following Cs-PDT. This downward shift in the valence band effectively passivates the PN junction in CIGSSe solar cells. The focus of part 2 lies in studying the impact of Cs-PDT absorber layers on the CdS buffer layer. The analysis commences with an examination of the CdS buffer layer on CIGS absorber samples exposed to different levels of Cs-PDT. Upon Cs-PDT treatment of the CIGS absorber layer, a decrease in the diffusion of Cu from CIGS into the CdS layer is observed. Similarly, the Cs diffusing from CdS into CIGS demonstrates ion exchange behavior akin to its behavior within CIGS, where higher Cs levels lead to lower Na content. Furthermore, the distribution of impurities in the CdS buffer layer also affects its work function, with Na tending to lower it and Cs tending to increase it. To validate these effects, alkali metal ions were deliberately introduced into the CdS buffer solution for examination. The findings suggest that the addition of Na elevates the work function of CdS, whereas the addition of Cs diminishes it. As a result, Na-doped CdS buffer layers on Cs-PDT CIGSSe were fabricated and compared them to samples of Cs-PDT CIGSSe paired with non-doped CdS. The Na-doped CdS exhibited a marginal enhancement in device efficiency when contrasted with the samples featuring non-doped CdS. Finally, employing SCAPS software, an examination of the impact of carrier concentration in the CdS buffer layer on device performance was conducted. According to simulation results, the increase of carrier concentration in CdS can enhance the FF and subsequently improve overall efficiency. To recap, part 1 emphasized the improvement in efficiency resulting from Cs incorporation into CIGSSe, coupled with an investigation into the driving factors. On the other hand, part 2 was delved into the potential side effects on the upcoming CdS buffer layer stemming from Cs-PDT and introduced a strategy involving Na-doped CdS to adjust buffer condition further to enhance PV performance.

中文摘要 i
ABSTRACT iii
ACKNOWLEDGMENT v
Contents vi
List of Figures ix
List of Tables xvii
Chapter 1 Introduction 1
1.1 Research Motivation 1
Chapter 2 Literature Review 2
2.1 Basic Principle of Photovoltaic Devices 2
2.1.1. Photovoltaic Device Material Properties 2
2.1.2. Charge Carrier Generation and PN Junction 3
2.1.3. Current-Voltage Characteristic (J-V) Curves 4
2.1.3.1. Short Circuit Current (Isc) 4
2.1.3.2. Open Circuit Voltage (Voc) 5
2.1.3.3. Fill factor (FF) 5
2.1.3.4. Power Conversion Efficiency (Eff) 5
2.1.3.5. Parasitic Resistance 6
2.1.3.6. Quantum Efficiency (Q.E.) 7
2.2 Introduction to Cu(In,Ga)(S,Se)2 Solar Cells 8
2.2.1. Device Structure of CIGS-based Solar Cells 9
2.2.2. Device Properties of CIGS Solar Cells 11
2.2.3. Carrier recombination mechanism in CIGS solar cells 16
2.2.4. Production of CIGS-based Absorber 17
2.3 Alkali Metal Treatment in CIGS Solar Cells 20
2.3.1. Effect of Sodium Incorporation in CIGS Solar Cells 22
2.3.2. Effect of Cesium Incorporation in CIGS Solar Cells 23
2.3.3. Concluding the effect of alkali metal ions in CIGS 25
2.3.4. Alkali Element Incorporation in CdS Buffer Layer 27
Chapter 3 Experimental Methods 30
3.1 Device Fabrication and Characterization 30
3.1.1. Device Fabrication 30
3.1.1.1. Molecular Beam Epitaxy (MBE) system 30
3.1.1.2. Sputter System 30
3.1.2. Device Characterization 31
3.1.2.1. Semiconductor Analysis System 31
3.1.2.2. External Quantum Efficiency (EQE) 31
3.1.2.3. Photoluminescence (PL) & Time-resolved PL (TRPL) 31
3.1.2.4. Temperature-dependent J-V Measurement 32
3.1.2.4.1 Light Temperature-dependent J-V Measurement 32
3.1.2.4.2 Dark Temperature-dependent J-V Measurements 33
3.1.2.4.3 Temperature-dependent Admittance Measurement 33
3.1.2.5. Time-of-flight Secondary Ion Mass Spectroscopy 35
3.1.2.6. Kelvin Probe Force Microscopy 36
Chapter 4 Results and Discussions 38
4.1 Alkali Metal Ion Incorporation in CIGS-based Absorber 38
4.1.1. Material Analysis on Cs-PDT CIGSSe 38
4.1.2. Electrical and Optical Properties after Cs-PDT on CIGSSe 41
4.1.3. Surface potential analysis after Cs-PDT on CIGSSe 51
4.1.4. Grain Boundary Model of CIGSSe after Cs-PDT 56
4.2 Alkali Metal Ion Incorporation in CdS Buffer Layer 61
4.2.1. Phenomenon of CdS Buffer Layer after Cs-PDT on CIGS-based Solar Cells 61
4.2.2. Alkali Metal Ion Doped CdS Buffer Layer on the CIGSSe Solar Cells 67
Chapter 5 Conclusions 82
Reference 84
Appendix 97

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