A Study on the Epitaxial Germanium Thin-Film for Solar Cell Applications
A Study on the Epitaxial Germanium Thin-Film for Solar Cell Applications
- 주제(키워드) Solar cells , photovoltaics , multi-junction , tandem , power conversion efficiency (PCE) , III-V compound semiconductors , germanium , isobutylgermane (IBuGe) , thin-film , epitaxial lift-off (ELO) , flexible , epitaxial growth , metalorganic chemical vapor deposition (MOCVD)
- 발행기관 아주대학교
- 지도교수 이재진, 강호관
- 발행년도 2018
- 학위수여년월 2018. 2
- 학위명 박사
- 학과 및 전공 일반대학원 전자공학과
- 실제URI http://www.dcollection.net/handler/ajou/000000026973
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
Currently, the focus of solar cell R&D is to develop high-efficiency and low-cost devices to meet grid parity. III-V compound semiconductor multi-junction solar cells promise super high efficiencies due to the efficient use of sunlight by the tandem structure consisting of monolithically stacked two or more subcells with different band gap. Despite its high efficiency, the III-V based solar cells have been used for limited applications such as space and concentrated photovoltaic (CPV) system due to the high cost. The most expensive cost driver is the substrate for the growth of the solar cell epi layers. Manufacturing cost of III-V based solar cells can be reduced by epitaxial lift-off (ELO) processes that separates epi layer from the substrate through chemical etching process. The separated substrate can be reused for growth of the high efficiency III-V based solar cells. Additionally, it is possible to fabricate thin-film solar cells with high-efficiency and low cost. However, the most widely used InGaP/InGaAs/Ge triple-junction solar cell. Since the p-n junction of the Ge bottom subcell is realized by atomic diffusion into a p-type Ge substrate, the most commonly used InGaP/InGaAs/Ge triple-junction solar cell is difficult to reduce manufacturing cost by reusing the Ge substrate. The primary goal of this dissertation is to fabricate high-quality epitaxial Ge thin-film solar cells on flexible substrates for high-efficiency and low cost thin-film InGaP/InGaAs/Ge tandem solar cells. In order to achieve the final goals, this research has been focused on three parts: high-quality epitaxial Ge grown by metalorganic chemical vapor deposition (MOCVD) on GaAs substrate and transferred to flexible substrate using ELO processes, realization of inverted epitaxial Ge p-n junction and fabrication of epitaxial Ge thin-film solar cell, and the optimization of inverted epitaxial Ge thin-film solar cell structures. First, the high crystal quality epitaxial Ge thin-films have been grown on n-type GaAs (001) substrates with AlAs sacrificial layer by MOCVD using an isobutylgermane (IBuGe) metalorganic source which is a novel Ge precursor. The full width at half maximum values of 14 arcsec is achieved by growth of the epitaxial Ge thin-films on GaAs substrates, comparable to the value of 11.6 arcsec from the reference Ge substrate and coincident with value expected in a perfect crystal. And As grown Ge epi layer is separated from the GaAs substrate and transferred to flexible substrates by the ELO processes. The high-quality epitaxial Ge thin-films have been demonstrated on flexible substrates with a surface root mean square roughness of 1.1 nm. Second, the inverted epitaxial Ge p-n junctions have been grown by MOCVD using IBuGe without any growth techniques. The n-type epitaxial Ge layer has been grown without additional injection of n-type dopants, while the p-type epitaxial Ge layer has been grown with Ga n-type dopants. N-type doping concentration of around 1.4×1019 cm-3 and p-type doping concentration of over 1×1019 cm-3 are investigated in the undoped n-type epitaxial Ge layer and Ga continuous doped p-type Ge epitaxial layer, respectively. The first demonstrated inverted epitaxial Ge thin-film solar cell structures with extremely high doped Ge p-n junction have been fabricated by ELO processes and thin-film solar cell fabrication methods. As a result, although it achieved the low power conversion efficiency (PCE) of 3.39 %, the Ge thin-film solar cells are successfully demonstrated on flexible substrate. In addition, it is identified that n-type Ge emitter layer thickness is increased due to the internal diffusion of n-type dopants such as arsenic (As) and phosphorus (P) from the GaAs window layer and InGaP etch stop layer during the growth of the thick p-type epitaxial Ge layer. Finally, the inverted epitaxial Ge thin-film solar cells with various n-type Ge emitter layers and p-type Ge base layers have been grown to improve the PCE. Lowered doping concentrations around 3×1018 cm-3 have been obtained from the epitaxially grown p-type Ge layers by increasing the Ga injection ratio on the dicontinuous doping technique without post-annealing. The best PCE of 4.82 % which is improved over 42 % is achieved from the inverted epitaxial Ge thin-film solar cell with 50 nm-thick emitter layer and 2000 nm-thick base layer. This device has been packaged by printed circuit board and wire bonding processes and achieved the PCE of 5.46 % with Voc of 0.21 V, Jsc of 42.80 mA/cm2, and FF of 0.6082. Although the PCE of 5.46 % is lower than that of 6.57 % of the upright epitaxial Ge solar cell, the high-quality epitaxial Ge thin-film on the flexible substrate is expected to be very useful for various devices. Especially, the inverted Ge epitaxial growth technique could be applied for III-V multi-junction solar cells with excellent stability.
more목차
Chapter 1. Introduction 1
1.1. Renewable energy 1
1.2. Solar cells 4
1.2.1. History of solar cells 4
1.2.2. Solar spectrum 5
1.2.3. Principle of solar cells 7
1.2.4. Types of solar cells 14
1.3. III-V multi-junction solar cells 20
1.3.1. Fundamentals of III-V multi-junction solar cells 20
1.3.2. Lattice-matched III-V multi-junction solar cells 25
1.3.3. Lattice-mismatched III-V multi-junction solar cells 32
1.3.4. Hybrid III-V multi-junction solar cells 37
1.3.5. Applications of the III-V multi-junction solar cells 41
1.4. III-V thin-film solar cells with epitaxial lift-off technology 48
1.4.1. Epitaxial lift-off technology 48
1.4.2. III-V thin-film solar cells 54
1.4.3. Impact of the III-V thin-film solar cells and applications 58
1.5. Deposition of germanium for solar cells 61
1.6. Research objective and outline 66
Chapter 2. Experimental methods 68
2.1. Epitaxial growth using MOCVD 68
2.2. Material analysis tools 70
2.3. Epitaxial lift-off process for Ge thin-films 72
2.4. Fabrication process for thin-film solar cells 75
2.5. Photovoltaic device characterization 81
Chapter 3. Fabrication and characterization of epitaxial Ge thin-film on flexible substrate 83
3.1. Introduction 83
3.2. High-quality epitaxial Ge on GaAs substrate 86
3.2.1. Epitaxial growth 86
3.2.2. Epi layer properties 94
3.3. High-quality epitaxial Ge thin-film on the flexible substrate 96
3.3.1. ELO process 96
3.3.2. Transferred epi layer properties 98
3.4. Conclusion 102
Chapter 4. Epitaxial Ge thin-film solar cell on flexible substrate 103
4.1. Introduction 103
4.2. Inverted epitaxial Ge p-n junction on GaAs substrate 110
4.2.1. Inverted epitaxial Ge p-n junction 110
4.2.2. ECV analysis 114
4.3. Epitaxial Ge thin-film solar cell on flexible substrate 117
4.3.1. Epitaxial growth 117
4.3.2. Fabrication of thin-film solar cell 121
4.3.3. Photovoltaic device characteristics 123
4.4. Perovskite/Ge double-junction solar cell on flexible substrate 137
4.4.1. Fabrication of perovskite/Ge 2J solar cell 137
4.4.2. Photovoltaic device characteristics 140
4.5. Conclusion 143
Chapter 5. Optimization of the inverted Ge p-n junctions for high-quality epitaxial Ge thin-film solar cell on flexible substrate 144
5.1. Introduction 144
5.2. Optimization of the n-type Ge emitter layer 146
5.2.1. Epitaxial growth 146
5.2.2. Photovoltaic device characteristics 150
5.3. Optimization of the p-type Ge base layer 158
5.3.1. Epitaxial growth 158
5.3.2. Photovoltaic device characteristics 160
5.4. Conclusion 175
Chapter 6. Conclusion and Future work 176
References 179
Research achievements 193
