Optical Modeling of Gold nanoparticles (Au NP) for efficiency improvement of a-Si:H photovoltaic cells
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Optical Modeling of Gold nanoparticles (Au NP) for efficiency improvement of a-Si:H photovoltaic cells Peiqing Yu1, Jean-Philippe Blondeau1, Caroline Andreazza2, Esidor Ntsoenzok1, Julien Roussel3, Perrine Dutheil4, Anne Lise Thomann4, Elyaakoubi Mustapha5, Jacque Meot5 1 CEMTHI - CNRS, Site Cyclotron, 3A rue de la Férolerie, 45071 Orléans, France 2 CRMD - CNRS, 1 Rue de la Ferollerie, 45100 Orléans, France 3 PRISME - Université d’Orléans, 8 Rue Léonard de Vinci, 45072 Orléans, France 4 GREMI - Université d’Orléans, 14 Rue d'Issoudun, 45067 Orléans, France 5 SOLEMS, 3 Rue Léon Blum 91120 Palaiseau, France ABSTRACT The efficiency of thin-film solar cells using a-Si:H is limited by the decrease in a-Si:H layer optical path length and its poor light absorption at red and NIR wavelengths. Metal NP such as Au have been shown to increase the absorption in the active material and then cell performances, by exhibiting localized surface plasmon (LSP) resonances. Our work’s goal is to understand NP influence in such cells, to perform an optimal structure by increasing the amount of light absorbed within the cell using NP scattering and luminescence. Modeling based on Mie theory is first carried out using bulk Palik data for Au spheres with various diameters and refractive medium indexes. Using modeling parameters, Au layers were deposited on glass and SnO2 substrates respectively by thermal evaporation in vacuum and sputtering, followed by thermal annealing (200 ~ 500°C) in order to promote the NP growth. MEB pictures show quasispherical Au NP shape with a mean size of 150nm. This diameter range switches extinction of NP in scattering regime. Annealing temperature (T) strongly affects the NP morphology. Surface coverage decreases and sphericity appears to increase with T. UV-Visible spectroscopy displays distinct LSP resonances around 600nm after annealing with a red shift while T increases. INTRODUCTION Thin-film solar cells using a-Si:H offer the benefit of reducing material consumption and fabrication costs. Additionally, benefits include advantages of light-weight and possible flexible devices by roll-to-roll deposition processing. However, such thin absorbing layers reduces the photovoltaic efficiency, due to the decrease in a-Si:H layer optical path length and its poor light absorption at red and near-infrared (NIR) wavelengths. Metal NP such as Au can exhibit strong localized surface plasmon (LSP) resonances at UV, visible and NIR wavelengths[1]. Once excited, surface plasmons decay, resulting in scattering and in light absorption as well. The optical properties of NP can be turned by changing their size, shape, or by altering the local dielectric environment [2]. Metal NP have been shown to increase the absorption in the active material and then solar cells performances [3-5]. The process involved is based on two approaches: i) the increase of the electromagnetic field in the vicinity of the metal NP of small size (
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