목차

I. INTRODUCTION………………………………………………………………...1
1.1. Solar cells outlook…………………...…………………………………..1
1.2. The operating principle of a solar cell…………………………………...2
1.3. Solar cell parameters……………………………………………………..4
1.4. The loss mechanisms in solar cells………………………………………5
1.5. Light trapping in solar cells……………………………...………………6
1.6. Single junction germanium (Ge) based solar cells………………………7
1.7. The contribution of this dissertation……………………...…………….10
1.8. Structure of the dissertation…………………………………………….11
2. NANOSTRUCTURE FOR SOLAR CELL APPLICATIONS………………….13
2.1. Improvement in optical absorption by nanostructures………………….14
2.2. Enhancement in photo-carrier collection by nano-rod solar cells….…...21
2.3. Microstructure and nanostructure solar cell devices……………...…….22
2.3.1. Microwire, nano-pillar, and nanocone solar cells……….…….22
2.3.2. Nanodome and nanohole solar cells…………………………...26
2.4. Summary and status……………………………………………….……28
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3. EXPERIMENTAL TECHNIQUES FOR FABRICATION AND
CHARACTERIZATION OF THE NANO-PILLAR Ge SOLAR CELLS…...........29
3.1. Fabrication tools……………………………….………………………..29
3.1.1. Metal-organic chemical vapor deposition (MOCVD)…………29
3.1.2. Inductively coupled plasma etching……………………...……34
3.1.3. Lithography techniques………………………………………..35
3.2. Characterization tools………………………………………………..…38
3.2.1. Measurement of solar cell current density – voltage curve…....38
3.2.2. External quantum efficiency………………………….……..…39
3.2.3. Spectroscopy…………………………………………..……….41
3.2.4. Scanning electron microscopy (SEM)………………………….41
3.2.5. Photoluminescence (PL)……………………………………….42
3.2.6. Electrochemical Capacitance Voltage (ECV)…………………42
3.2.7. Scanning photocurrent microscopy……………………………43
4. NANO-SPHERE LITHOGRAPHY AND REACTIVE ION ETCHING
TECHNIQUES………………………………………………….……………..46
4.1. Nano-sphere lithography technique………………………….…………46
4.2. Nano-sphere lithography processes…………………………….……....47
4.3. Reactive ion etching (RIE) process…………………………….………48
4.3.1. RIE process to tailor PS diameter…………………………..…..48
4.3.2. Deep RIE process to etch out of InGaP and Ge materials……..48
4.4. Experiment results……………………………………………………...48
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4.4.1. Spin-coating a monolayer of nano polystyrene (PS) beads on
substrates…………………………………..……………..……48
4.4.2. Tailor the size of bead “resist” by oxygen plasma etching….…49
4.4.3. Etching the exposed semiconductor areas by deep reactive ion
etching……………………………………….………………...50
A. InGaP material…………………………….…………...…50
B. Ge material……………………………….……………..…51
4.5. Conclusions……………………………………..………………..….…..53
5. LIGHT TRAPPING BY SURFACE TEXTURING…………………………….55
5.1. Light trapping by hexagonal micro-hole surface texturing……………57
5.2. Light trapping by an InGaP nano-pattern structure realized by thermally
dewetted Au nanoparticles and anisotropic dry etching……………….67
5.3. Light trapping by fabricated InGaP conical frustum nanostructures…..75
6. EPITAXIAL GERMANIUM NANO-PILLAR SOLAR CELLS……………….84
6.1. Epitaxial Ge nano-pillar solar cell fabrication process…………………84
6.1.1. Ge nano-pillar substrates preparation………………………….84
6.1.2. Single junction Ge solar cell structures grown by MOCVD…..87
6.1.3. Devices fabrication……………………………………….….…89
6.2. Influences of the front contact protection on the characteristics of nanopillar
Ge solar cell……………………………………………….…...…90
6.3. Influences of Ge nano-pillar structures on the characteristics of nanopillar
Ge solar cells…………………………………………….……….92
6.3.1. The variation in the height of Ge nano-pillars………………….92
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6.3.2. The variation in the diameter of Ge nano-pillars…………...…100
6.4. Influences of the emitter layer (n-Ge) thickness on the characteristics of
nano-pillar Ge solar cell…………………………………………....…102
6.5. Influences of the thermal cleaning temperatures on the characteristics of
nano-pillar Ge solar cell……………………………………...….……104
6.6. Influences of the window layer (n-InGaP) thickness on the characteristics
of nano-pillar Ge solar cell………………………………………...…106
6.7. Conclusions……………………………………………………………109
7. CONCLUSIONS AND PROSPECTS……….……………………………….. 110
7.1. Conclusions……………………………………………………………110
7.2. Prospects……………………………………………………………….111
REFERENCES…………………………………………………………................112
APPENDIX (Author’s academy presentations and publications)…………..……119