2014
ACLN
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P.Yu, J.P.Blondeau, C.Andreazza, E.Ntsoenzok, J.Roussel, P.Dutheil, A.L.Thomann, A.Caillard, E.Mustapha, J.Meot, 'Optical Modeling of Gold nanoparticles (Au NP) for efficiency improvement of a-Si:H photovoltaic cells', European PV Solar Energy Conference and Exhibition (2014)
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 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 Ag can exhibit strong localized surface plasmon resonances at UV, visible and NIR wavelengths. Once excited, surface plasmons decay, resulting in scattering and in light absorption as well. The optical properties of NP can be tuned 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 the 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 cells are not actually well determined. Therefore, our work’s goal is to understand NP influence in such cells and 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 Ag spheres are calculated for various diameters and refractive medium indexes. With index 1.9, corresponding to the transparent electrode SnO2 in such cells, the principal resonance wavelength is observed between 500nm and 700nm for NP of diameters from 50nm to 100nm. A red shift of the surface plasmon resonance is detected while NP size and/or refractive medium index increase. In addition, normalized angular scattering distribution at SPR shows the influence of NP size on the backscattering cross section of light. This distribution allows to define the optimal position of NP in photovoltaic cells.
Using these parameters, 6nm and 12nm thick Ag layers have been deposited on glass and different SnO2 substrates (smooth, textured and less textured), by Plasma sputtering magnetron at 300°C respectively during 1 minute and 2 minutes, to get the different sizes of Ag NP.
UV-Visible spectroscopy displays distinct localized surface plasmon resonances around 550nm and 600nm respectively for the 6nm and 12nm thick Ag layers. The presence of these resonances in these areas, but not around 400 nm, allows us to assume that the Ag NP inserted into the SnO2 layer during the deposition. According to the modeling with the index of SnO2 (1.9), these resonances correspond respectively to 60nm and 80nm-diameters Ag NP. The red shift of the resonance corresponds to an increase of NP size, according to the modeling. The X-ray diffraction studies also show us the evidence of Ag crystallization.
In the future work, these layers will be studied by SEM in order to determine the NP sizes and verify our quasi-spherical modeling, and AFM study will also be carried out in order to understand the NP position (in SnO2 or air). The results will be presented in the full paper.
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).
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