GaAsP/SiGe tandem solar cells on porous Si substrates

III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cells on Silicon substrates with optimum bandgap combinations. Current implementations of such devices have reached efficiencies over 20%, using thick -and thus costly- buffer layers which induce the appearance of cracks in large area samples. As a strategy to mitigate these two issues (thick buffers and cracking), a GaAsP/SiGe tandem solar cell has been developed employing group IV reverse graded buffer layers grown on Ge/Si virtual substrates with a subsurface Silicon porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses but can also induce cracks. To minimise this, a porous silicon layer has been incorporated close to the Ge/Si interface so that the ductility of this layer suppresses crack propagation. In terms of solar cell performance, this porous layer reduces the problem of cracks, not totally supressing them though. Accordingly, the low shunt resistance observed in previous designs has been increased thus improving solar cell efficiency, which is still notably behind designs using thicker forward graded buffer layers. The first results of this new architecture are presented here.