Rules and tools for understanding, modelling and designing textured silicon solar cells

Abstract

This thesis is an expansive and in-depth discourse on the textured front surface of silicon solar cells. Systematic and comprehensive analyses of both optical and recombination behaviours are undertaken to arrive at a thorough understanding of the impacts of surface texture. These analyses provide the basis for the development of several novel methods for the precise simulation of photovoltaic devices and modules. A rigorous approach to the optics of surface texture, incorporating polarisation effects, is shown to advance the accuracy of standard reflectance-absorptance-transmittance analyses. Using this approach, assessment of any texture morphology is possible; of particular utility are applications to isotexture and random upright pyramids, as well as extensions to non-ideal morphologies typical on practical devices. The methodology proposed is computationally inexpensive, as it decouples geometric ray tracing from the Fresnel equations, and is hence well suited to routine application. Ray tracing is applied to determine a one-dimensional profile of photogeneration with respect to the distance to an isotextured front surface. An analytical approximation to this profile is developed. The approximation is suitable for routine analysis of multicrystalline devices, and facilitates the simulation of short circuit current in typical wafered silicon solar cells with less than 3.5% error. By comparison with ray traced photogeneration profiles beneath pyramidal texture, an established approximation is validated; application of this widely-used approximation results in the prediction of short circuit current in typical devices to within 6% accuracy. In further exploration of optical behaviour, measurements of the angular distribution of reflection from textured surfaces are shown to provide critical new information for accurate modelling of photovoltaic modules. An example application demonstrates that, in the case that cells are encapsulated, current generation beneath isotexture approaches 99% of that calculated beneath random pyramids. This implies that cast-mono silicon cells should be isotextured, rather than pyramidally textured, when they possess less than 85% monocrystalline surface area. The paucity of experiments dedicated to the recombination mechanisms at textured surfaces is recognised in this work and is rectified by a thorough study. This is particularly pertinent given that the current trend towards cells featuring lighter front diffusions elevates the relative importance of front surface recombination. Compared to a planar surface, it is shown that isotexture incurs little or no recombination penalty. At pyramidal texture, however, increased surface area drives increased recombination according to conventional understanding. It is demonstrated that an additional increase due to the orientation of texture facets occurs when an Si-SiO2 interface is present. Further, results of this study repudiate the common conjecture that vertices and edges necessarily induce supplementary recombination when passivated with common dielectrics; an experimental methodology developed in this thesis is applied to show that any increase in recombination can be avoided by judicious choice of passivation. Finally, it is shown that recombination is commonly greater at a regular array of inverted pyramids than at an equivalently prepared random array of upright pyramids. -- provided by Candidate. Show more