Micro-Crystal GaAs Array Sub-Cells for Si Tandem Solar Cells

This work reports optical and electronic numerical modelling of a novel emerging structure which is the GaAs nanocrystal on Si tandem solar cell by epitaxial lateral overgrowth, a technique which allows defect free material growth. The techniqueconsists of creating nucleation sites in a silicon surface SiO2 layer and initiating growth of nanoscalescale seeds, whereby strain energy remains below the Matthews-Blakeslee strain relaxation limit. This leads to AlₓGaAs growth in micro-crystals without generation of material defects. The focus of this presentation is optical and electrical modelling of nanocrystals for applications in the very active field of silicon based multijunction solar cells, and design of a AlₓGaAs/Si two terminal tandem, for compositions ranging from x=0 to x=30% in absorber layers. We present a model of the complete structure in two dimensions, consisting of a AlₓGaAs high bandgap subcell connected with a tunnel junction to the low bandgap Si junction. The elaboration of models is described, with an emphasis on the AlₓGaAs crystal featuring a non-planar pn-junction, and a focus on the optical properties of this lattice of micrometric AlGaAs crystals and in particular their light trapping properties from the resulting surface texture. The question of AlₓGaAs surface coverage is addressed, given that neighbouring AlₓGaAs crystals have different crystal orientations on a (111) Si surface, such that any coalescence of neighbour AlₓGaAs crystals leads to crippling defects at their interface. The result is that some high energy incident light above the AlₓGaAs 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. The interface between AlₓGaAs and Si subcells is addressed, with an emphasis on current transport efficiency through the nanoseeds and tunnelling currents through appropriately designed SiO2 buffer layers. This work therefore presents a theoretical framework for evaluating the potential of AlₓGaAs nanocrystal growth on Si for light trapping, for GaAs silicon two terminal tandem cell performance including tunnel junctions, and provides models and design rules for efficient AlₓGaAs microcrystal arrays as high bandgap subcells for tandem solar cells on silicon.