Conditions Extrêmes et Matériaux : Haute Température et Irradiation

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P.Yu, J.P.Blondeau, C.Andreazza, E.Ntsoenzok, J.Roussel, P.Dutheil, A.L.Thomann, E.Mustapha, J.Meot, 'Optical Modeling of Gold nanoparticles (Au NP) for efficiency improvement of a-Si:H photovoltaic cells', Mat. Res. Soc. Symp. Proc. (2015)

Thin-film solar cells using a-Si:H offer the benefit of reducing material consumption and fabrication costs. Additional, benefit includes advantages of light-weight and possible flexible devices by roll-to-roll deposition processing. However, such thin absorbing layer 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 resonances at UV, visible and NIR wavelengths. Once excited, surface plasmons decay, result 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. Metal NP have been shown to increase the absorption in the active material and then cell performances. 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 (<50nm) when irradiated with sunlight having a wavelength close to the resonance excitation wavelength; or ii) the diffusion of incident light from metal particles of bigger size (~100nm). However, the optimal parameters of NP in such cell are not actually well determined. Therefore, our work deals to understanding 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(optical trapping). Modeling based in Mie theory is first carried out with bhmie program using bulk Palik data. The extinction, scattering and backscattering efficiencies of Au sphere are calculated for various diameters and refractive medium indexes. The resonance wavelength is observed around 600nm for NP having diameter of 100nm. A red shift of the surface plasmon resonance is detected while NP size increases and/or refractive medium index. In addition, normalized angular scattering distribution at SPR shows the influence of NP size on the backscattering cross section of light. This distribution allows defining the optimal position of NP in photovoltaic cells. Using these parameters, 10nm thickness gold layer has been deposited on glass and Transparent Oxide Conductor substrates respectively by thermal evaporation in vacuum and sputtering, followed by thermal annealing from 200°C to 500°C in order to promote the NP growth. MEB pictures show quasi-spherical Au NP shape with a mean size of 100nm. This diameter range switches extinction of NP in scattering regime. Annealing temperature (T) strongly affects the morphology of NP. Surface coverage decreases and sphericity appears to increase with T. UV-Visible spectroscopy displays distinct localized surface plasmon resonances around 600nm after annealing with a red shift while T increases. Acknowledgments: The research project leading to these results has received funding from the region Centre, France under the name ARPPCM (Efficiency Improvement of Thin Layers Photovoltaic Panels).