Liv Prönneke. Fluorescent Materials for Silicon Solar Cells - PDF

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Universität Stuttgart Liv Prönneke Fluorescent Materials for Silicon Solar Cells Institut für Physikalische Elektronik Prof. J. H. Werner Pfaffenwaldring 47 D Stuttgart Fluorescent Materials for

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Universität Stuttgart Liv Prönneke Fluorescent Materials for Silicon Solar Cells Institut für Physikalische Elektronik Prof. J. H. Werner Pfaffenwaldring 47 D Stuttgart Fluorescent Materials for Silicon Solar Cells Von der Fakultät Informatik, Elektrotechnik und Informationstechnik der Universität Stuttgart zur Erlangung der Würde eines Doktor-Ingenieurs (Dr.-Ing.) genehmigte Abhandlung Vorgelegt von Liv Prönneke geboren am in Engelskirchen Hauptberichter: Prof. Dr. rer. nat. habil. Jürgen H. Werner Mitberichter: Prof. Dr. rer. nat. habil. Uwe Rau Tag der Einreichung: 6. Juli 2011 Tag der mündlichen Prüfung: 23. Februar 2012 Institut für Physikalische Elektronik der Universität Stuttgart 2012 ...two roads diverged in a wood, and I - I took the one less traveled by, and that has made all the difference. Robert Frost Inhaltsverzeichnis Zusammenfassung 1 Abstract 4 1 Introduction Motivation Objective Outline System components Solar Cell Fluorescent Collector Photonic Band Stop Filter Component Matching Monte-Carlo ray-tracing simulation Simulation method Systems in their radiative limits System geometries and idealized components Classical set-up: side-mounted solar cells Novel concept: bottom-mounted solar cells Influence of loss mechanisms Conclusion and Outlook Experimental results Measuring and modeling reabsorption Measuring reabsorption in fluorescent collectors Comparison to Monte-Carlo simulation i ii INHALTSVERZEICHNIS Analytical fit Conclusion and Outlook Current increase of a-si cell under fluorescent collector Measuring concentration in fluorescent collectors Comparison to Monte-Carlo simulation Link to Reabsorption Conclusion and Outlook Geometrical and dispersive concentration Geometrically concentrating troughs under fluorescent collectors Outdoor measurement results for July, Conclusion and Outlook Output power increase of c-si module under fluorescent collector % power increase due to fluorescent collector Threshold area Conclusion and Outlook Efficiency increase in photovoltaic modules by colored cell connectors Experimental set-up Results for calculated efficiency Conclusion and Outlook A Exemplary component matching 97 B Evenly filled spherical surfaces 99 C Solar Cell Parameter 100 Nomenclature 102 List of Tables 107 List of Figures 108 Bibliography 111 Danksagung 116 Erklaerung 118 Zusammenfassung Photovoltaische Systeme mit Fluoreszenzkollektoren benutzen die Konversion und Konzentration solarer Photonen, um Solarzellenwirkungsgrade zu erhöhen. Fluoreszenzmoleküle in einer Acrylglasplatte absorbieren einfallende Strahlung und emittieren räumlich isotrop verteilte Photonen in einen niedrigeren Energiebereich. Die Acrylglasplatte totalreflektiert die gestreuten Photonen und leitet sie zu den Kollektorseitenflächen. Daher beschäftigt sich die Forschung üblicherweise mit Solarzellen, die an den Kollektorseitenflächen angebracht sind. Diese Arbeit analysiert Wirkungsgradsteigerungen in photovoltaischen Systemen mit Fluoreszenzkollektoren, die oben auf der Solarzelle liegen. Der erste Teil dieser Arbeit nutzt eine Monte-Carlo Simulation, um Vergleiche zwischen Fluoreszenzkollektorsystemen mit seitlich angebrachten und mit darunter liegenden Solarzellen zu ziehen. Zusätzlich untersucht die Simulation den positiven Einfluss einer photonischen Struktur über dem Kollektor. Die Ergebnisse zeigen, dass die Sammelwahrscheinlichkeit für Photonen in beiden Systemen abhängig ist von der Skalierung der Zellflächen und Zellabstände. Systeme mit seitlich angebrachten Solarzellen liefern höhere Erträge bei größeren Skalierungen. Für kleine Skalierungen ist der Fluoreszenzkollektor mit darunter liegenden Solarzellen jedoch genauso gut. Die Berücksichtigung von nicht strahlenden Verlusten und die Verwendung einer photonischen Struktur zeigen ebenfalls, dass unten liegende Solarzellen genauso viele Photonen einsammeln wie seitlich angebrachte, jedoch empfindlicher in der Skalierung sind. Der zweite Teil der Arbeit präsentiert fünf experimentelle Ergebnisse, die einerseits grundsätzliche Mechanismen in Fluoreszenzkollektoren analysieren. Andererseits zeigen sie, wie Fluoreszenzfarbstoffe gewinnbringend in photovoltaischen Solarmodulen eingesetzt werden können. i) Zur Messung der Reabsorption fallen LED-Photonen der Wellenlänge λ = 406 nm auf die Kollektoroberfläche. Eine Kamera hinter dem Kollektor fotografiert 1 2 ZUSAMMENFASSUNG mindestens einmal reabsorbierte Photonen, die die Rückseite verlassen. Eine analytische Beschreibung der Absorption und Emission im Kollektor liefert aus den Kamerabildern den Reabsorptionskoeffizienten α reabs = mm 1. ii) Light beam induced current (LBIC-) Messungen an einer amorphen Siliziumsolarzelle zeigen, dass ein aufliegender Fluoreszenzkollektor den gesammelten Strom um 7% erhöht. Eine zusätzliche photonische Struktur erhöht den Strom um 95%. Eine analytische Beschreibung für dieses Experiment sagt unter Verwendung des im ersten Experiment ermittelten Reabsorptionskoeffizienten den Verlauf der Intensität vorher. Damit ist die Messung des Reabsorptionskoeffizienten ausreichend, um die Photoneneinsammlung in photovoltaischen Systemen mit Fluoreszenzkollektoren zu beschreiben, ohne, dass langwierige LBIC-Messungen nötig sind. iii) Feldexperimente unter realer Sonneneinstrahlung vergleichen Solarzellen aus monokristallinem Silizium (c-si) in Acrylglaströgen mit und ohne aufliegenden Fluoreszenzkollektor. Ist der Fluoreszenzstoff auf die Trogapertur begrenzt, so verringert er den Stromertrag. Eine fünffach größere Kollektorfläche erhöht den Stromertrag um 50% im Vergleich zu der begrenzten Fläche. Dieses Ergebnis verdeutlicht den Vorteil von streuender Konzentration gegenüber geometrischer Konzentration. Um den Stromertrag mit geometrischer Konzentration zu erhöhen, müssen neue Konzentratorsysteme mit zusätzlicher Solarzellenfläche gebaut werden. Das Experiment zeigt noch einen anderen Vorteil auf: Da die Fluoreszenzplatten die Photonen unabhängig von ihrem Winkel einsammeln, ist ihr Ertrag in Systemen mit und ohne Nachführung gleich. iv) Zwei parallel geschaltete 2 2 cm 2 c-si Solarzellen erreichen unter einem Fluoreszenzkollektor eine elektrische Leistung P el = 189 mw. Derselbe Aufbau mit einer undotierten Acrylglasplatte erreicht nur P el = 125 mw. Durch die Variation der Solarzellenabstände zeigt dieses Experiment außerdem, dass die Aktivierung der umliegenden photovoltaisch inaktiven Fläche unbedingt notwendig ist, um die Verluste durch Streuung direkt über der Solarzelle zu kompensieren. v) Alle vorherigen Experimente zeigen, dass Fluoreszenzstoffe direkt auf Solarzellen in der Praxis immer zu Verlusten führt. Das letzte Experiment vermeidet diese unerwünschten Verluste, indem der Fluoreszenzstoff nur die optisch inaktiven Zellverbinder einer cm 2 großen industriellen c-si Solarzelle bedeckt, die mit Glas verkapselt ist. Der Fluoreszenzstoff auf den geweißten Zellverbindern streut einfallende Photonen in alle Richtungen. Die Oberfläche des Glases zur Luft totalreflektiert gestreute Photonen und lenkt sie auf die Solarzelle. Der mithilfe von LBIC- und Quanteneffizienzmessungen errechnete Wirkungsgrad der Zelle steigt von ZUSAMMENFASSUNG 3 η = 16.0% auf η = 16.2%. Zusammenfassend findet diese Arbeit nicht nur eine neue Charakterisierungsmethode für Konzentration durch Fluoreszenz. Sie zeigt außerdem, dass bei einer sorgfältigen Skalierung die Anwendung von Fluoreszenzkollektoren auf photovoltaischen Solarmodulen höhere Wirkungsgrade liefert. Abstract 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 performs better for larger scales, but for small scales bottommounted 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 λ = 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 α reabs = mm 1 from the camera picture. ii) Light beam induced current (LBIC) measurements on an amorphous silicon 4 ABSTRACT 5 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 2 2 cm 2 c-si solar cells under a fluorescent plate achieve an electrical output power P el = 189 mw. The same set-up with an undoped acrylic plate on top gains P el = 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 a cm 2 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 η = 16.0% to η = 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. Chapter 1 Introduction 1.1 Motivation Electricity generation with photovoltaic systems is a growing market everywhere in the world. Figure 1.1 presents the data for Germany over the last 20 years. The total consumption as well as the fraction of photovoltaic generated electricity increases. An enhanced production of solar modules decreases the costs and enables the consumer to install photovoltaic systems with low financial investment. Not only in Germany with a well established electricity grid, photovoltaic modules gain interest but also in developing countries with dispersed population and often no electricity supply at all. Here, solar power generation shows its main advantages of local installation and easy scaling of power plant size. Research and development therefore reaches for lower costs via reducing solar cell material or enhancing the solar cell efficiency. Increasing the efficiency requires either better electrical or optically improved properties of the solar cell. Unfortunately, optical improvement of the solar cell top surface often vanishes in the module, because the module glass is the actual surface towards the sun. Furthermore, in a module, areas like cell interspaces [2] and cell connectors [3, 4] are photovoltaic inactive, because they reflect impinging photons. Therefore, special interest lies in improving optical properties of solar modules for lower costs and higher efficiencies. 1.2 Objective Fluorescent collectors are acrylic plates containing fluorescent dye. The collector concentrates incoming photons since the dye emits absorbed photons spatially ran- 6 1.3 OUTLINE 7 total electricity consumption [GWh] year fraction of PV generated electricity [%] Fig. 1.1: Fraction of photovoltaic generated electricity on annual electricity consumption [1]. domized and the acrylic plate guides photons due to total internal reflection to the collector edges. Because of this side-guiding behavior, research follows mainly the objective to mount solar cells at the collector sides [5 7]. The main object of this work is the examination of solar cells under the fluorescent collector, a geometry which is easier to construct technologically. First, this work uses a Monte-Carlo simulation to model photovoltaic systems with fluorescent collector in the radiative limit. The results allow the comparison of the maximum efficiencies of side-mounted with bottom-mounted solar cells. Second, five experiments aim to understand and to use the beneficial concentration of fluorescent collectors for underlying solar cells. 1.3 Outline Chapter 2 presents a short introduction to the examined system components solar cell, fluorescent collector and photonic structure. Chapter 3 explains the Monte-Carlo simulation and presents the comparison between fluorescent collectors with side- and bottom-mounted solar cells and an optional photonic structure. The systems are in the radiative limit. The simulation 8 CHAPTER 1. INTRODUCTION therefore provides maximum theoretical limits. The efficiency of the systems depends strongly on the scaling of the size and the distance of the solar cells. Sidemounted solar cells reach larger efficiencies for large scales. However, for small scales bottom-mounted solar cells achieve equally high efficiencies. Therefore, using solar cells under a fluorescent collector needs careful scaling but reaches efficiencies of the same value as side-mounted solar cells. Chapter 4 presents five experiments sketched in Figs. 1.2a-e. The reabsorption measurement in Fig. 1.2a takes place by directing LED photons with the wavelength λ = 406 nm onto the collector surface. A black circle absorbs directly transmitted photons. A camera photographs the photons which are reabsorbed at least once and leave the back surface outside the circle. An analytical description extracts the reabsorption coefficient α reabs from the camera picture. The light beam induced current (LBIC) measurement in Fig. 1.2b on an amorphous silicon (a-si) solar cell under a fluorescent collector impressively shows the concentration effect. The application of a photonic structure increases the concentration even more. The reabsorption coefficient determined in the first experiment predicts the contribution of the formerly inactive area around the solar cell. The outdoor experiments in Fig. 1.2c explore the advantages of fluorescent concentration over geometrical concentration. In the experiment in Fig. 1.2d the fluorescent collector enhances the efficiency of a solar module with two parallel connected mono crystalline silicon (c-si) solar cells. In the experiments in Figs. 1.2b-d, fluorescent collectors cover the whole solar cell area. Above the cell the distribution of photons is always of disadvantage. Figure 1.2e shows an experiment where the fluorescent concentration limited to the optical inactive cell connector area increases the efficiency by distributing photons, such as they are totally internal reflected at the module glass. 1.3 OUTLINE 9 Fig. 1.2: a) Reabsorption measurement: Photons leaving the bottom surface of the collector outside the circle are reabsorbed once. An analytical fit determines the reabsorption coefficient. b) light beam induced current (LBIC) measurement of fluorescent collector with and without photonic structure on top above amorphous silicon (a-si) solar cell. c) Geometrical concentrator with and without fluorescent collector on top of mono crystalline silicon (c-si) solar cell measured outdoor. d) Efficiency measurement of two parallel connected c-si solar cells. e) Quantum efficiency and LBIC measurements on encapsulated c-si solar cell with fluorescent cell connectors. Chapter 2 System components This section describes in short the theoretical background for the components used in the analyzed photovoltaic systems: the solar cell, the fluorescent collector and the photonic band stop (PBS) filter. A fluorescent collector -as understood in this thesis- is an acrylic glass plate doped with fluorescent dye molecules. Figures 2.1a-e sketch the functionality of a photovoltaic system with fluorescent collector, side-mounted solar cells and an optional PBS filter. Figure 2.1b indicates that the fluorescent dye absorbs incoming photons with energy range E 1. Subsequently, the dye emits fluorescence photons which areshifted spectrally dueto thestokes shift ( [8], p. 695f). The shift produces photons with a lower energy rangee 2 which partly overlaps with the incident energy range E 1 ( [9], p.38). Aside from being spectrally shifted, the emitted photons are distributed with spatially randomized spherical angles (θ, φ) defined in Fig. 2.1a. Figure 2.1balso shows that the acrylic plate with refractive index n ag r 1.5 ( [10], p. 1156) guides emitted photons with angles θ larger than the angle of total internal reflection θ c = 42.2 ( [8], p.487f). In contrast, photons with θ θ c leave the collector as implied in Fig. 2.1c. Guided photons travel through the collector until they either are reabsorbed by the dye or reach an optically coupled solar cell. The reabsorption occurs for photons which are emitted into an energy range where the dye absorbs, because then the fluorescence spectrum overlaps with the absorption spectrum of the dye. If an emitted photon reaches a solar cell attached to the fluorescent collector (see Fig. 2.1b), the solar cell needs to be optically coupled to the collector. The optical coupling disables the total internal reflection. For the experiments in Sects. 4.2, 4.3, 4.4 and 4.5 I choose Glycerin, because its refractive index n oc r 1.5 [11] lies between the refractive index of the acrylic glass n ag r and 10 2.1 SOLAR CELL 11 the refractive index of the solar cell (n c-si r amorphous silicon [12]). 3.9 for crystalline
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