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Laser Welding of Silicon Foils for Thin-Film Solar Cell Manufacturing

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Laser Welding of Silicon Foils for Thin-Film Solar Cell Manufacturing Laser Welding of Silicon Foils for Thin-Film Solar Cell Manufacturing Laserschweißen von Siliziumfolien zur Herstellung von Dünnschicht-Solarzellen Der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg zur Erlangung des Doktorgrades Dr.-Ing. vorgelegt von Thomas Maik Heßmann aus Zschopau Als Dissertation genehmigt von der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg Tag der mündlichen Prüfung: 30.09.2014 Vorsitzende des Promotionsorgangs: Prof. Dr.-Ing. habil. Marion Merklein Gutachter: Prof. Dr. techn. Christoph J. Brabec Prof. Dr.-Ing. Rolf Brendel Abstract Thin-film solar module manufacturing is one of the most promising recent developments in photovoltaic research and has the potential to reduce production costs. As the necessity for competitive prices on the world market increases and manufacturers endeavor to bring down the cost of solar modules, thin-film technology is becoming more and more attractive. In this work a special technique was investigated which makes solar cell manufacturing more compatible with an industrial roll-to-roll process. This technique allows the creation of the first monocrystalline band substrate by welding several monocrystalline silicon wafers together, so that the size restriction of float-zone grown wafers can be overcome. Currently the size is 8 inches in diameter. Float-zone grown material is well suited as feedstock for high efficiency solar cells and it has also been very intensively studied in the past. This makes it the perfect feedstock material for thin-film solar modules. Unfortunately this material is quite expensive and therefore it should only serve as feedstock to generate the band substrate. After this step the necessary silicon layers to produce solar cells are grown epitaxially on top of the band substrate using chemical vapor deposition. To produce solar cells a silicon layer is separated from the band substrate using a layer transfer process. Subsequently the band substrate can be repeatedly reused to produce an infinite amount of silicon layers without requiring any silicon ingot feedstock. The linchpin for this technique is the welding step from single wafers to a band substrate. Thus, this work focuses on the investigation of the welding process. Welded samples were analyzed using micro-Raman and electron backscatter diffraction (EBSD). Moreover, the achievement of solar cells on top of 50 µm thick silicon foils and welded silicon foils are reported. I Kurzzusammenfassung Die Produktion von Dünnschicht-Solarmodulen ist eine der vielversprechendsten Entwicklungen in der Photovoltaik in der näheren Vergangenheit, weil diese Technik geringe Produktionskosten verspricht. Wegen der Notwendigkeit von wettbewerbsfähigen Preisen an den Weltmärkten und dem Bemühen der Hersteller die Produktionskosten zu senken gerät die Dünnschicht-Technik immer mehr in den Fokus. In dieser Arbeit wird eine spezielle Technik untersucht, die die Herstellung von Solarzellen weiter an ein industrielles Rolle-zu–Rolle- Verfahren annähern soll. Diese Technik erlaubt es, monokristalline Siliziumwafer miteinander zu dem ersten monokristallinen Bandsubstrat zu verschweißen. Dadurch kann die Größenrestriktion der Produktion von im Zonenschmelzverfahren hergestellten einkristallinen Silizium-Ingots überwunden werden, die momentan einen Durchmesser von 8 Zoll haben. Da im Zonenschmelzverfahren gewonnenes Silizium als Ausgangsmaterial für Hochleistungssolarzellen ideal ist und auch schon intensiv untersucht wurde, ist es der perfekte Ausgangspunkt für Dünnschicht-Solarmodule. Allerdings ist der hohe Preis für dieses Material ein Problem. Darum soll das hochwertige und teure Silizium nur für die Herstellung des Ausgangsbandsubstrates verwendet werden. Danach soll mittels chemischer Gasphasenabscheidung eine Epitaxie-Schicht auf dem Band gewachsen werden und diese gewachsene Schicht mittels Transferprozess vom Ausgangsband getrennt werden, um damit Solarzellen herzustellen. Das Bandsubstrat wird wiederverwendet um eine endlose Anzahl von Siliziumschichten zu produzieren ohne die Notwendigkeit von Silizium-Ingots als Ausgangmaterial. Für dieses Verfahren ist das Schweißverfahren der Dreh- und Angelpunkt, daher wurde in dieser Arbeit der Fokus auf das Charakterisieren der Verschweißung gelegt. Diese wurden mit Hilfe von Mikro-Raman und Electron backscatter diffraction (EBSD) untersucht. Außerdem wurden erfolgreich Solarzellen auf 50 µm dünnen Siliziumfolien sowie Solarzellen auf verschweißten Siliziumfolien hergestellt. II III to my family IV Contents Abstract ....................................................................................................................................... i Kurzzusammenfassung ............................................................................................................... ii 1. Introduction ........................................................................................................................ 1 2. Current Status of Crystalline Thin-Film Solar Cell Technology ....................................... 4 3. Solar Cell Basics .............................................................................................................. 13 3.1 Absorption of Light in Silicon ................................................................................... 13 3.2 Recombination of Electron-Hole Pairs ...................................................................... 13 3.2.1 Shockley-Read-Hall Recombination .................................................................. 14 3.2.2 Auger Recombination ........................................................................................ 14 3.2.3 Recombination at the Surface............................................................................. 15 3.3 Basic Equations for Solar Cells ................................................................................. 16 3.3.1 Poisson Equation ................................................................................................ 16 3.3.2 Current-Density Equations ................................................................................. 16 3.3.3 Continuity Equations .......................................................................................... 17 3.3.4 Diffusion Length ................................................................................................ 17 3.4 Characteristics of Solar Cells .................................................................................... 18 3.5 Quantum Efficiency ................................................................................................... 19 4. Solar Cell Manufacturing Concept ................................................................................... 21 5. Welding of Silicon ........................................................................................................... 24 5.1 State of the Art ........................................................................................................... 24 5.1.1 Bonding and Laser Beam Bonding .................................................................... 25 5.1.2 Laser Beam Brazing of Silicon .......................................................................... 25 5.1.3 Laser Beam Welding of Silicon ......................................................................... 26 5.2 Fundamentals and Challenges ................................................................................... 27 5.3 Sample Preparation and Validation of thin Silicon Wafers ....................................... 29 V 5.4 Process of Silicon Welding ........................................................................................ 32 5.4.1 Laser Spot Welding with a low Constant Feed Speed ....................................... 32 5.4.2 Laser Line Welding ............................................................................................ 34 5.4.3 Keyhole Welding ................................................................................................ 35 5.5 Results of Blind Welding Experiments ..................................................................... 37 6. Material Characterization of Welded Silicon Foils .......................................................... 40 6.1 Cross Section Preparation .......................................................................................... 40 6.2 Characterization Setups ............................................................................................. 41 6.2.1 Micro-Raman Setup ........................................................................................... 41 6.2.2 Electron Backscatter Diffraction Setup .............................................................. 44 6.3 Blind Welding ............................................................................................................ 46 6.4 Laser Spot Welding with a low Constant Feed Speed ............................................... 48 6.5 Laser Line Welding ................................................................................................... 55 6.6 Keyhole Welding ....................................................................................................... 60 6.6.1 Keyhole Welding of Samples Polished on One Side ......................................... 60 6.6.2 Keyhole Welding of Samples Polished on Both Sides ...................................... 65 6.7 Discussion .................................................................................................................
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