Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells
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Plasma Texturing and Porous Silicon Mirrors for Epitaxial Thin Film Crystalline Silicon Solar Cells Izabela Kuzma-Filipek, Filip Duerinckx, Kris Van Nieuwenhuysen, Guy Beaucarne, and Jef Poortmans IMEC, Kapeldreef 75, Leuven, Belgium ABSTRACT Thin film silicon solar cells, consisting of an epitaxially grown active layer on a low quality highly doped silicon substrate, incorporate many attractive features usually associated with their sister cells based on bulk silicon. However, the efficiency of the current epitaxial semiindustrial screen printed cells is limited to 11-12% mainly due to optical shortcomings. This paper will give an overview of our work aimed at tackling the 2 most important problems: (i) Finding and implementing an adequate front surface texture and (ii) the simulation, fabrication and incorporation of an intermediate reflector. The former issue has been addressed by the development of plasma texturing based on halogen species. This method allows us to fulfil the sometimes contradictory requirements for the textured surface, i.e. a uniform and reduced reflection, a strong lambertian character to scatter the light and a limited removal of silicon. It will be shown that the scattering efficiency is dependent on both the wavelength of the impinging light and on the silicon removal during the texturing process. The second and main issue of this work is the limited absorption volume of the epitaxial layer. To resolve this drawback, an intermediate reflector is placed at the epi/substrate interface to enhance the path length of the low energy photons through the epi-layer. In practice, a multilayer porous silicon stack is created by electrochemical anodization of the substrate. The reflection at the epi/reflector/substrate interface is a combination of several different effects including a Bragg mirror and Total Internal Reflection (TIR). Measurements of the external reflectance as well as extraction of the internal reflection parameters are used to clarify the issue. Advanced structures, including chirped porous silicon stacks, are introduced. Finally, the benefits of the reflector on the level of the epitaxial silicon solar cell are analysed. Efficiencies close to 14% are obtained for epitaxial cells incorporating an advanced porous Si reflector. INTRODUCTION Silicon solar cells based on mono- or multicrystalline silicon wafers continue to dominate the photovoltaic market. Nevertheless, when analyzing the route to produce a silicon wafer, it becomes apparent that it is very energy consuming and inefficient. Indeed, to produce a high quality wafer based on electronic grade silicon (EG-Si), the silicon has to pass several stages: (i) Creation of Metallurgical Grade (MG) silicon from sand (quartz) (ii) Purification by distillation of trichlorosilane (TCS) (iii) Chemical vapour deposition (CVD) in the form of a polycrystalline rod (Siemens process) (iv) Crystallization in the form of a Cz-Si cylinder or multicrystalline ingot (v) Sawing/wafering
Since the Si material is at one stage already
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