Modeling the Performance of Biaxially-Textured Silicon Solar Cells
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Modeling the Performance of Biaxially-Textured Silicon Solar Cells Joel B. Li,1 Bruce M. Clemens2 1Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA 2Department of Materials Science & Engineering, Stanford University, Stanford, CA 94305, USA
ABSTRACT Grain boundaries (GBs) in polycrystalline silicon (poly-Si) thin film solar cells are frequently found to be detrimental for device performance. Biaxiallytextured silicon with grains that are well-aligned in-plane and out-of-plane can possess fewer GB defects. In this work, we use TCAD Sentaurus device simulator and known experimental work to investigate and quantify the potential performance gains of biaxially-textured silicon. Simulation shows there can be performance gain from well-aligned grains when GB defects dominate carrier recombination or when grains are small. On the other hand, when intra-grain defects dominate recombination and grains are large, well-aligned grains do not lead to much performance gain. Another important result from our simulation is when intra-grain and GB defects are few, Jsc is almost independent of grain size while Voc drops with decreasing grain size. INTRODUCTION In recent years, biaxially-textured thin-film solar cells have been actively pursued by various groups[1-8] with the aim of reducing the detrimental effects that GBs can have on solar cell performance. It has been shown that biaxiallytextured films have enhanced hall carrier mobility[6,8-10] compared to films with randomly-oriented grains. This enhancement in hall carrier mobility is attributed to the smaller GB barrier height that results from fewer GB traps in biaxiallytextured films. It is postulated that biaxially-textured films with its less deleterious GB[3,11], can result in higher efficiency solar cells. This work uses two-dimensional TCAD Sentaurus simulation to numerically solve poisson and continuity equations for a finite-element mesh model, providing quantitative analysis of the benefits that biaxially-textured silicon can bring to solar cell performance, an area which has not been explored by previous solar cell device modeling work[12-16]. PHYSICAL MODEL The solar cell model used in the simulation has a p-type silicon absorber layer with doping level of 1e16 cm-3, p+ doped front emitter layer with doping level of 5e19 cm-3 and n+ doped back surface field with doping level of 5e19 cm-3. The heavily doped layers have a Gaussian doping profile with depth of 100nm and the total thickness of the silicon layer is 2µm. The chosen thickness values lie within the range reported for polycrystalline thin-film silicon solar cells[17-19]. This silicon layer lies on top of calcium fluoride, CaF2, which serves as the biaxiallytextured template layer. CaF2 is well lattice-matched to silicon and exhibits
biaxial texture when grown by ion-beam assisted deposition (IBAD)[1,2] or oblique angle deposition[20-23] techniques. These suggest that CaF2 can be a good template layer to deposit biaxially-textured silicon and has been done in previous work
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