Optical phenomena of plasmonic nanostructures and their applications in energy conversion
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Metallic nanostructures such as nanoparticles, nanowires and nanoapertures exhibit extraordinary optical properties in absorption, scattering and transmission of electromagnetic radiation due to the excitation of surface plasmons. This allows them to provide applications in converting photon energy to other forms of energy such as heat, mechanical work and electricity in a more efficient or controlled manner. When incorporated into an amorphous silicon thin film solar cell, nanoparticles were found to substantially increase the light absorption in the photoactive layer within certain wavelength range. The mechanism of this optical absorption was studied using three-dimensional finite element method. It was found that intensified Fabry-Perot resonance in the active layer due to the addition of the nanostructures and enhanced light scattering by the plasmonic nanostructures were both responsible for this phenomenon. Interestingly, higher absorption only occurs at wavelength range outside the surface plasmons resonance of the nanostructures. A further study on the absorption of the nanoparticles themselves revealed that enhanced near field associated with the SP resonance of particles causes extraordinary energy dissipation in the particles, resulting in decreased light scattering. Strong power dissipation accompanied with the surface plasmons resonance becomes desirable when nanostructures are used as heat generator. Using the new technique of three-dimensional localization of the metallic nanoparticles on polymer microstructures, wavelength dependent controlling of a light-driven microactuator was achieved by selectively coating it with nanoparticles of different materials. Another important plasmonic nanostructure is the subwavelength hole arrays perforated on a metal film. The optical transmission through these nanometer scaled apertures whose dimensions are smaller than the wavelength of the incident light can be several orders of magnitude larger than expected. Based on this property, a novel tandem solar cell structure was proposed. A metal film perforated with periodic subwavelength hole arrays was inserted in a tandem solar cell as a light transmittable intermediate common electrode for the top and the bottom cell. The perforated electrode removes the current matching restriction in conventional tandem cells and allows active materials with different energy conversion and charge transport mechanisms to be combined in the same device. If used in a multi-junction solar cell, the new design can also save the power loss across the tunnel junction. The perforated intermediate electrode was modeled and its optical performance in the tandem solar cell was investigated.