Modelling textured GaAs nanocrystals on silicon for III-V/Si tandem solar cells
Résumé
This work reports optical and electronic numerical modelling of a novel emerging structure which is the GaAs nanocrystal on Si tandem solar cell. This is based on a technological breakthrough1 which is the growth of GaAs crystals on silicon. The growth proceeds via nanometric scale seeds, which are small enough not to break the Matthews-Blakeslee strain relaxation constraint, and thereby allows GaAs growth in micron scale hexagonal crystals without strain relaxation through generation of material defects. The focus of this presentation is the application of these nanocrystals in the very active field of silicon based multijunction solar cells2, and design of a GaAs/Si two terminal tandem. To these ends, we model the complete structure in two dimensions, consisting of a GaAs high bandgap junction connected with a tunnel junction to the low-bandgap Si junction. The elaboration of models of these non planar structures is described, with an emphasis on the CaAs hexagonal crystal featuring a non-planar pn-junction, and a focus on the optical properties of this lattice of micrometric GaAs crystals and in particular their light trapping properties. The question of GaAs surface coverage is addressed, given that neighbouring GaAs crystals have differently crystal orientations on a (111) Si surface, such that any coalescence of neighbour GaAs crystals leads to crippling defects at their interface. The result is that some high energy incident light above the GaAs bandgap is nevertheless transmitted directly to the Si cell, such that the resulting photogenerated carriers thermalise to the Silicon bandgap, and result in a loss of efficiency. This work therefore presents a theoretical framework for evaluating the potential of GaAs nanocrystal growth on Si for light trapping, for GaAs silicon two terminal tandem cell performance including tunnel junctions, and quantifies the penalties expected from incomplete surface coverage.
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