05 Fakultät Informatik, Elektrotechnik und Informationstechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6
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Item Open Access Fluorescent materials for silicon solar cells(2012) Prönneke, Liv; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)Photovoltaic systems with fluorescent collectors use the conversion and concentration of solar photons to increase solar cell efficiencies. Fluorescent dye in a dielectric plate absorbs incoming rays and emits spatially randomized photons with a lower energy range. The acrylic plate then guides part of the emitted spectrum to the collector side surfaces due to total internal reflection. Conventional research therefore applies solar cells to the side surfaces. This work analyzes the efficiency enhancement due to fluorescent collectors on top of solar cells which promises an easier technological handling. The first part of this work uses a Monte-Carlo simulation to model photovoltaic systems with fluorescent collectors and photonic structures. The results allow the comparison between side- and bottom-mounted solar cells. Examining the systems in the radiative limit achieves maximum theoretical limits. In each system, the photon collection probability depends strongly on the scaling of cell size and distance. The side-mounted solar cells perform better for larger scales, but for small scales bottom-mounted solar cells achieve equally high efficiencies. Consideration of non-radiative loss mechanisms and the application of a photonic structure also leads to the result that the application of solar cells to the collector back side needs careful scaling but performs as good as side-mounted solar cells. The second part presents the results of five experiments which analyze basic mechanisms in the fluorescent collector. Additionally, the experiments explore the benefits of fluorescent material in photovoltaic modules. i) The reabsorption experiment directs photons from an LED with wavelength 406 nm onto the collector top surface. A camera under the collector photographs photons which leave the back side. These photons are reabsorbed at least once. An analytical description extracts the reabsorption coefficient a = 0.021 1/mm from the camera picture. ii) Light beam induced current (LBIC) measurements on an amorphous silicon solar cell show that a fluorescent collector on top increases the collected current by 7%. The additional application of a photonic structure enhances the current by 95%. An analytical description of the absorption and emission processes in the collector using the reabsorption coefficient determined in the first experiment predicts the line-scans gained in the LBIC measurements. Therefore, the reabsorption measurement is sufficient enough to predict the collection performance of photovoltaic systems with fluorescent collectors without performing long LBIC-measurements. iii) Outdoor experiments compare mono crystalline silicon (c-Si) solar cells in acrylic troughs with and without fluorescent collectors on top. Fluorescent distribution added to the geometrical concentration decreases the current gain if limited to the trough aperture. A five times larger fluorescent collecting plate leads to a current gain enhancement by at least 50% compared to the limited aperture. This shows the advantage of fluorescent concentration. Achieving an increased current gain with geometrical concentration requires a new trough and more solar cell material. The experiments also show another advantage: Fluorescent collectors concentrate photons independent of their angle. Thus, photovoltaic systems using fluorescent concentration perform best even without tracking. iv) Two parallel connected c-Si solar cells under a fluorescent plate achieve an electrical output power P = 189 mW. The same set-up with an undoped acrylic plate on top gains P = 125 mW. By varying the cell distance this experiment additionally points out that the activation of surrounding photovoltaic inactive area is crucial to compensate losses directly above the solar cell. v) The last experiment avoids unfavorable losses by applying fluorescent dye to only the optical inactive cell connectors of an industrial c-Si solar cell encapsulated under glass. The fluorescent dye covering the white painted connector distributes incoming photons at all angles. The glass-air surface guides distributed photons onto the solar cell via total internal reflection. Derived with LBIC and Quantum Efficiency measurements, the efficiency of the solar cell increases from 16.0% to 16.2%. In conclusion, this work not only finds a new characterization method for the fluorescent concentration. Additionally, it presents that applying fluorescent dye on top of photovoltaic solar modules increase efficiencies under careful consideration of the scaling.