Enhanced solar absorption in thin film photovoltaic cells via embedded silica-coated silver nanoparticles
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Thin-film photovoltaic cells are a promising technology that can harvest solar energy at a low cost. The main drawback of this technology is its low efficiency in comparison to conventional photovoltaics. This deficiency is due to poor absorption of long wavelengths in the solar spectrum. Plasmonic nanostructures can be tuned to resonantly interact with these wavelengths in order to enhance a solar cell’s absorption of these wavelengths and improve its efficiency. Historically, the two key factors limiting the success of plasmonically-enhanced photovoltaics have been parasitic absorption of light by the nanoparticle lost to heating, and recombination of charge carriers at the interface of the nanoparticle and the photovoltaic medium. Here we propose that these deficiencies can be overcome by employing nanospheres with a silver core and silica shell. Through experimentation supported by simulations, this thesis outlines how these plasmonic nanostructures can be applied to significantly improve the performance thin-film solar cells through experimentation supported by simulations. The plasmonic enhancement of photovoltaic devices can be studied and optimized computationally; however, highly uniform nanoparticles are necessary to validate these simulations.. The colloidal synthesis of plasmonic nanoparticles can achieve this at a low cost. We present several methods for the synthesis of silver nanoparticles with diameter of 5 to 50 nm and compare the monodispersity and yield of the colloids that they produce. These colloids are then adapted to synthesis processes enabling the formation of silica shells of 2 to 20 nm onto the silver cores. To facilitate the integration of silver-core, silica-shell nanoparticles into semiconductor thin films, we also develop procedures to deposit these nanoparticles onto silicon substrates with precisely-controlled inter-particle spacing. Finally, we experimentally integrate silver-core, silica shell nanoparticles into sub-micron layers of silicon. Absorption measurements reveal that integration of these nanoparticles can nearly double the amount of light absorbed by the silicon. The absorption spectra indicate the strong presence of interference effects within the thin films, which we account for in our simulations. We use the simulations to show how parasitic absorption by the nanoparticle only accounts for a small percentage of the absorption gains that we measure. Therefore, most of the optical absorption happens within the silicon, and would potentially improve the efficiency of a silicon solar cell.