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Characteristics of broadband light absorption in nanostructures

초록/요약

Photovoltaic solar cells have been considered as promising sources of electricity. Both the power conversion efficiency and correlated efficiency-to-cost metric must be further increased. In the past, most researches were focused on anti-reflection and hundreds of micrometers thick wafers were used for sufficient light absorption. Due to their thickness, carrier recombination and high cost were inevitable problems. Nanostructures have several unique advantage in photovoltaics. First, the junction distance is less than diffusion length. As a result, the carrier recombination rate is significantly low. Second, because the amount of material to be used decrease, cost also decreases. Third, small changes of structure can lead to dramatic changes in their optical properties as compared to conventional structures and thus can enable efficient photonic devices. Recently, how to increase light absorption in solar cells by using this properties has been attracting a great interest. In this thesis, solutions for increasing light absorption efficiency by using nanostructures are proposed and their broadband light absorption characteristics is demonstrated numerically and experimentally. First, the design and synthesis of core/shell p-type/intrinsic/n-type (p/i/n) Si nanowires (NWs) with various sizes and cross-sectional morphologies are proposed. Measured photocurrent spectra exhibit well-defined diameter-dependent peaks. The corresponding external quantum efficiency (EQE) spectra calculated from these data show good quantitative agreement with finite-difference time-domain (FDTD) simulations. A rectangular NW with a diameter of 260 nm yields a dominant mode centered at 570 nm with near-unity EQE in the transverse-electric polarized spectrum. Second, various substrate structures are proposed for broadband light absorption in single Si nanowires. FDTD simulation showed that when a silver bottom mirror was adjacent to a Si nanowire, the optical resonances were significantly amplified at every resonant wavelength beyond the classical limit given by ray optics, thereby significantly enhancing the absorption efficiency. Third, a nanowire photovoltaic (NW PV) device with amorphous Si shell structure is proposed. The FDTD simulations revealed that a crystalline Si nanowire with an embedded amorphous Si shell yields 40% enhancement of absorption as compared to a homogeneous crystalline Si nanowire, under air-mass 1.5 global (AM1.5G). Fourth, laterally oriented Si nanowire array structures are proposed. Comparison of a nanowire array with a single nanowire shows that the current density (JSC) is preserved for a range of nanowire morphologies. The JSC of a nanowire array depends on the spacing of its constituent nanowires, which indicates that both diffraction and optical antenna effects contribute to light absorption. Fifth, NW PV devices coated with dielectric shells are proposed. Scattering and absorption measurements on Si NWs coated with shells of SiNX or SiOX exhibit a broadband enhancement of light absorption by ∼50−200% and light scattering by ∼200−1000%. The increased light-matter interaction leads to a ∼80% increase in short-circuit current density in Si photovoltaic devices under 1-sun illumination. Sixth, efficient input couplers for various Si thin film solar absorbers are proposed. In the simulation, a dielectric coating on Si thin film led to enhanced light absorption at near-ultraviolet to blue wavelengths, while the absorption peaks at longer wavelengths were nearly preserved. For broadband absorption, we introduced two-dimensional square-lattice periodic patterns. The periodic pattern exhibited tunable and pronounced absorption peaks that are identified as horizontally-propagating waveguide modes. In summary, solutions for increasing light absorption efficiency are proposed and demonstrated numerically and experimentally. The results obtained from our demonstration will be useful for designing nano light absorbers efficiently operating at specific or broadband wavelengths toward the development of next-generation ultra-small photodiodes and solar cells.

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목차

Abstract i
Contents iii
List of Figure vii
1. Introduction 1
2. Finite-difference time-domain algorithm for quantifying light absorption in silicon nanowires 5
2.1. Abstract 5
2.2. Introduction 6
2.3. Verification of the method 8
2.3.1. Dispersion Model for the Dielectric Constants of Silicon 8
2.3.2. Light absorbance in a silicon thin film 10
2.4. Light absorption in silicon nanowires 12
2.4.1. Calculation of Absorption Cross-Section in Nanoscale Optical Cavities 12
2.4.2. Light absorption spectra in a single silicon nanowire 12
2.4.3. Calculation of Light Absorption in a Silicon Nanowire Using a Broadband Light Source 15
2.5. Summary 17
2.6. Reference 18
3. Tuning light absorption in core/shell silicon nanowire photovoltaic devices through morphological design 23
3.1. Abstract 23
3.2. Introduction 25
3.3. Strategy 26
3.4. Fabrication 27
3.5. Size-dependence of hexagonal cross-sectional NWs 29
3.5.1. Experimental result 30
3.5.2. Numerical result 30
3.5.3. Analysis 32
3.6. Morphological design 34
3.6.1. Synthesis 34
3.6.2. Experimental & Numerical result 34
3.6.3. Analysis 36
3.7. Summary 38
3.8. Reference 39
4. Substrate-dependent broadband light absorption in transferrable single nanowires 45
4.1. Abstract 45
4.2. Introduction 46
4.3. FDTD simulation 47
4.3.1. Simulation condition 47
4.3.2. Substrate dependence of light absorption spectrum 47
4.3.3. Introducing a silver mirror to increase the light absorption 49
4.3.4. Substrate-dependence of broadband light absoption 52
4.4. Summary 54
4.5. Reference 55
5. Design of Nanowire Optical Cavities as Efficient Photon Absorbers 59
5.1. Abstract 59
5.2. Introduction 60
5.3. Result and discussion 63
5.3.1. Light absorption in core/shell Si NWs 63
5.3.2. Multishell Si NW with an a-Si inner shell 65
5.3.3. NWs with hexagonal and rectangular cross sections 67
5.3.4. Vertical NW stacks 69
5.3.5. Close-Packed NW Array 72
5.4. Summary 74
5.5. Reference 75
6. Laterally assembled nanowires for ultrathin broadband solar absorbers 79
6.1. Abstract 79
6.2. Introduction 80
6.3. Simulation result and analysis 82
6.3.1. Close-packed NW array with various cross-sectional morphologies 82
6.3.2. Current density of NW arrays with various pitch sizes 85
6.3.3. Resonance features of multi-layered NW arrays 87
6.4. Summary 92
6.5. Reference 93
7. Doubling Absorption in Nanowire Solar Cells with Dielectric Shell Optical Antennas 97
7.1. Abstract 97
7.2. Introduction 98
7.3. Nanowire solar cell with dielectric shell optical antennas 100
7.3.1. Fabrication 100
7.3.2. Scattering property of NW with dielectric shell 100
7.3.3. Relation between absorption and scattering in NW cavity 103
7.3.4. Broadband absorption property of NW with dielectric shell 105
7.4. Summary 109
7.5. Reference 110
8. Design of input couplers for efficient silicon thin film solar absorbers 117
8.1. Abstract 117
8.2. Introduction 118
8.3. Anti-reflection coating on Si thin film 120
8.4. Two-dimensional SiO2 or Si3N4 periodic patterns on Si absorber 123
8.5. Two-dimensional periodic pattern in Si absorber 127
8.6. Summary 130
8.7. Reference 131
9. Conclusion 135
Acknowledgements 139
국문 초록 141

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