Broadband Absorbers And Their Applications On Solar Energy Devices

Solar energy has been widely applied to generate electrical power, mainly in two ways, namely, solar photovoltaics and solar thermal power generation. In order to achieve a high power conversion efficiency in either way, broadband absorbers are indispensable and of critical importance to absorb all the photons in the solar spectrum. It is easy to realize broadband absorbers based on meta-materials and surface plasmons, which have strong ability of light trapping and localization. In the thesis, we demonstrate different kinds of meta-material and plamonic broadband absorbers as well as their possible applications in solar power conversion systems.Firstly, two kinds of novel broadband absorbers are demonstrated, including gold absorbers based on plasmonic tapered coaxial holes (PTCHs) and multi-layered broadband absorber based on hyperbolic meta-material both theoretically and experimentally. For the PTCH-based absorber, an average absorption of over 0.93 is obtained theoretically in a broad wavelength range from 300 nm to 900 nm without polarization sensitivity. Strong scattering of the incident light by the tapered coaxial holes and gap surface plasmon polaritons propagating along the taper dominate the absorption machenism in different wavelength ranges. A sample is fabricated by focused ion beam milling and the optical characterization verifies well the simulation result. For the multi-layered broadband absorber, polarization-insensitive broadband light absorption is realized by using a hyperbolic meta-material composed of alternating aluminum-alumina thin films. Broadband slow-light effect dominates the absorption machenism. A simulated absorption of over 90% is obtained over the wavelength range from 500 nm to 2500 nm. The experimental result matches the simulation well. For these two kinds of absorbers, their absorption bandwidth can be further extended by integration of different-size unit-cells in one period.Then, we investigate a new amorphous silicon solar cell based on a core-shell nanograting structure to improve its power conversion efficiency. We get an optimized design through numerical simulation and perform systematic analysis of its optical property. Our results show that sunlight will be transformed into horizontal bloch modes and plasmonic waves in our structure, which results in ultrabroadband, omnidirectional and polarization-insensitive responses. Our structure has about 45% enhancement in the short-circuit current compared with conventional planar one, and has a great potential in photovoltaics.For application in solar thermophotovoltaic systems, spectral selectivity must be considered. Here, we demonstrate a selective absorber and a selective emitter for planar solar thermophotovoltaic system. A selective absorber with excellent spectral selectivity is demonstrated both theoretically and experimentally based on a germanium (Ge) checkerboard on top of a tantalum (Ta) substrate. The strong cavity resonances in the Ge nanosquares and their interactions with adjacent nanocavities and the bottom Ta substrate dominate the spectrum selectivity machenism. Such excellent selectivity of our structrue is verified very well by both the experimental results and the thermal analysis. A new high-efficiency selective emitter based on a tungsten (W) spherical core-shell nanostructure is proposed. This structure consists of silicon dioxide (SiO2)-coated W nanospheres periodically distributed on a W substrate and a thin W layer deposited on top. Its excellent emission selectivity is attributed to the strong photonic interaction within the gaps between the adjacent core-shell nanospheres, the tightly-confined optical field in both the Ω-shaped W-SiO2-W nanocavity and the bottom nanocavities formed by the W nanospheres and the W substrate. Its selectivity property is much better than other similar structures.