< Terug naar vorige pagina


Optical Characterization and Modeling of Bulk Scattering and Luminescence

Boek - Dissertatie

Bulk scattering and/or luminescent components are used in many applications, such as luminaires, displays, and photovoltaic devices. The luminescent and scattering properties of these components often have a significant influence on the application characteristics, such as efficiency, color, radiation pattern, uniformity An optical model which allows to predict the optical behavior of the bulk scattering and luminescent components is indispensable for the design of these applications. Duringthe doctoral research, two optical modeling tools were investigated: a hybrid ray tracing tool, which combines a commercial ray tracer and a programming tool, and the analytical adding-doubling method, which was extended for luminescent materials. The hybrid ray tracer allows adequate modeling of luminescent components in complex 3D designs, while the adding-doubling method is limited to layered geometries, but has the advantage that less computation time is required. To obtain useful simulation results, both optical modeling techniques require adequate parameters describing the optical behavior of the considered materials. The inverse adding-doubling method was adopted to determine scattering parametersfor optical simulations. The method was modified to allow for optimization of the scattering parameters to provide a better fit for the angularscattering profile of a bulk scattering sample. For the optical modeling of luminescent materials, the efficiency of the wavelength conversion is one of the most important parameters. An integrating sphere setup for the absolute determination of the quantum yield of luminescent materials was developed and validated. As a specific application, aplanar luminescent down-shifting (LDS) layer for solar applications wasconsidered. Due to absorption of high energy photons close to the surface and subsequent surface recombination, most solar cells exhibit low efficiency in the short wavelength range. Matching the incident spectrum on the solar cell to its spectral response by applying an LDS layer on top of the cell, can enhance the efficiency of the cell. Optical simulations are very useful in this application to screen potential luminescent materials and optimize the LDS layers. The feasibility of the adding-doubling method to model a solar module containing an LDS layer was investigated by comparing the predicted reflectance and absorption in the cell with the simulations results of the hybrid ray tracing technique. An excellent agreement between both methods was found. Due to its computational speed, the adding-doubling method is better suited for optimization purposes. Finally, a complete characterization of a solar module with an LDS layer was performed. To this end, bare solar cells were characterized by determining the external quantum efficiency and reflectance. The scattering and luminescent properties of four LDS sheets were determined. Next, single cell modules were prepared by combining the characterized solar cells and LDS sheets. The spectral response of the modules was determined. The optical models developed during this doctoral research are able to qualitatively predict the effect of the LDS layer on the external quantum efficiency of the modules.