隨著日益加劇的能源危機和環保意識的抬頭，太陽能因其容易取得且無汙染性等等諸多的優點，已引起廣泛的關注。然而，因性價比的緣故，光伏元件相較於石化能源還無法被普遍地取代。因此，發展低成本或高效率的太陽能裝置是必要的。此研究的目的就是想透過結合矽奈米結構和二維材料所製備的異質接面來發展具有潛力的太陽能電池。
此實驗會分為兩個部分。在第一部分，我們設計了基於二硫化鉬(MoS2)/矽奈米結構的異質接面太陽能電池。首先，利用金屬輔助化學蝕刻(MaCE)形成P型的矽奈米線(SiNWs)基板，可以大幅地降低矽的反射率，同時提供非常大的表面積對體積的比例，以獲得出色的光捕獲能力。接著，我們使用相對容易的旋塗熱裂解方法，在矽奈米線上形成多層的二硫化鉬層，透過拉曼光譜儀(Raman)、X射線光電子能譜儀(XPS)、能量色散X射線光譜儀(EDS)等等的分析，了解所合成二硫化鉬的性質，並繪出異質接面的能帶圖，最後量測了太陽能電池的效能，獲得了0.52%的轉換效率。
為了探索二硫化鉬在太陽能電池中的更多應用，實驗的第二部分設計了基於聚(3,4-乙烯二氧噻吩)-聚苯乙烯磺酸(PEDOT:PSS)/二硫化鉬/矽奈米線的有機異質接面太陽能電池。我們嘗試了多種方式將二硫化鉬奈米片加入元件中，最後發現在旋鑄PEDOT:PSS之前先旋塗二硫化鉬奈米片分散液並烤乾可以提升元件的效率。最後我們獲得了超過11%的光電轉換效率，相較起沒有添加的元件提升了約18.1%。在研究量測的數據後，我們的解釋是二硫化鉬相較於PEDOT:PSS有著較高的電洞遷移率，可以較快地傳輸光生電洞，同時在繪製能帶圖後，發現二硫化鉬的導帶和價帶位置符合矽和PEDOT:PSS的能帶排列。此外，二硫化鉬還能在一定程度上的抑制矽和PEDOT:PSS之間的氧化反應。
總之，此基於矽奈米結構異質接面的太陽能電池可以使用簡易且低溫的濕式蝕刻和溶液旋塗法來製作，因而降低了製造成本。我們相信若再進一步的優化電極或是加上背電場層等等的改善，可以獲得更高的效率，相信會是具有潛力的光伏元件。

Nowadays, with a further aggravative energy crisis and the rise of environmental protection awareness, solar energy has caught much attention due to its non-polluting, accessible, and other plentiful properties. However, the efficiency-to-cost ratio of photovoltaic techniques is still too low to replace the fossil energy. As a consequence, developing a high-efficient or cost-effective solar device has attracted people’s interest and become a must in the field of energy harvesting. Here, a potential candidate that, based on the silicon substrate with nanostructure, combines a TMDCs material MoS2 as a heterojunction solar cell will be elaborated.
This work can be divided into two parts. In the first part, a MoS2/SiNWs heterojunction solar cell is proposed and examined. In the beginning, the p-type SiNWs substrates were fabricated by metal-assisted chemical etching (MaCE), and such an etching process can significantly decrease the reflectivity of Si, and provide a high surface-to-volume ratio to obtain higher light-trapping ability. We adopted the pyrolysis method to form multilayer MoS2 on the SiNWs, forming a radial p-n junction. Through the results of Raman, XPS, EDS, etc., we realized the qualities of the MoS2 and depicted the band diagram of the heterojunction solar cell, reaching an efficiency of 0.52%.
To explore more possibilities of MoS2 applying to the solar cell, we demonstrated an organic PEDOT:PSS/MoS2/SiNWs heterojunction solar cell. We have tried several ways to add the MoS2 nanoflakes into the device. After many attempts, we discovered that spin-coating the MoS2 nanoflakes dispersion on the SiNWs substrate and baking until it dries before the drop-casting of the PEDOT:PSS will show a better device performance. The device with the MoS2 addition resulted in an efficiency of 11.05 %, which increased by 18.1 % compared to the device without it. Our explanation is that the MoS2 nanoflakes with higher hole mobility compared to PEDOT:PSS can improve the hole transport ability; meanwhile, from the band diagram we depicted, we considered that the MoS2 fits the band alignment between n-type silicon and PEDOT:PSS. Besides, the MoS2 at the interface can suppress the oxidation reaction between Si and PEDOT:PSS to some content.
To summarize, such Si nanostructure heterojunction solar cells can be simply fabricated via low-temperature wet etching and spin-coating process methods that can dramatically reduce fabrication costs. We believe that with further optimization of electrodes or the addition of the back surface field, it has the potential to reach higher efficiency, applying to practical usage in the photovoltaics industry.

摘　要 I
Abstract III
Acknowledgments V
Content VI
List of Figures X
List of Tables XVI
Chapter 1 Introduction 1
1.1 Background 1
1.2 Motivation 3
Chapter 2 Literature Review 4
2.1 Silicon-based nanostructure used as the light-absorbing layer 4
2.1.1 Axial vs. Radial junction structures 4
2.1.2 Comparison of different silicon nanostructures 6
2.1.3 Fabrication of SiNWs 8
2.2 2D materials—Transition metal dichalcogenide 13
2.2.1 Properties of TMDCs 13
2.2.2 Pyrolysis method 17
2.2.3 MoS2-Based Heterojunction Solar Cells 20
2.2.4 Other Applications of MoS2 in Solar Cells 24
2.3 PEDOT:PSS hole transport layer 27
2.3.1 Chemical structures of PEDOT:PSS 27
2.3.2 Approaches to Improve the Electrical Conductivity of PEDOT:PSS 28
2.3.2.1 Solvent Additive Method 29
2.3.2.2 Polar Solvents Post-Treatment Method 30
2.3.2.3 Surfactant Treatment 33
2.4 Performance-Enhancing Approaches for PEDOT:PSS-Si Hybrid Solar Cells 35
2.4.1 Modification of PEDOT:PSS 38
2.4.1.1 Organic Solvent Treatment 38
2.4.1.2 Addition of Nanomaterials in PEDOT:PSS 41
2.4.2 Optimization of the Light-Trapping Effect 43
2.4.3 Surface Passivation of Silicon 46
2.4.4 Others 47
Chapter 3 Design of Experiment 48
3.1 Fabricate nanowires on Si substrate 48
3.2 MoS2 formation by pyrolysis method 49
3.3 PEDOT:PSS solution preparation 50
3.4 Fabrication of the SiNWs heterojunction solar cells 51
3.5 Process instruments 52
3.5.1 Furnace 52
3.5.2 Electron beam evaporator 53
3.6 Analysis instrument 54
3.6.1 Raman spectrometer 54
3.6.2 UV-Visible spectrophotometer 54
3.6.3 Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) 55
3.6.4 X-ray photoelectron spectrometer (XPS) 56
3.6.5 AES/ESCA Scanning Microprobe 56
3.6.6 Photovoltaic properties measurement 58
3.6.7 External Quantum Efficiency measurement 59
Chapter 4 Results and discussions 60
4.1 MoS2/SiNWs inorganic heterojunction solar cell 60
4.1.1 The length of the SiNWs substrate 60
4.1.2 Properties of the MoS2 fabricated by the thermal decomposition process 62
4.1.2.1 Raman spectrum 63
4.1.2.2 X-ray photoelectron spectroscopy (XPS) 64
4.1.2.3 EDS analysis 66
4.1.2.4 UV-Visible measurement 66
4.1.2.5 TEM images 68
4.1.3 Band diagram of the MoS2/p-SiNWs heterojunction 69
4.1.4 Solar cell performance measurement 71
4.2 Organic PEDOT:PSS/MoS2/SiNWs hetero-junction solar cell 75
4.2.1 Design of the heterojunction solar cell 75
4.2.2 The basic properties of the MoS2 nanoflakes 77
4.2.3 SEM images 78
4.2.4 Photovoltaic performance measurement 79
4.2.5 External Quantum Efficiency (EQE) measurement 82
Chapter 5 Conclusions 83
Reference 85

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