Novel GaAs0.71P0.29/Si tandem step-cell design

A novel GaAs0.71P0.29/Si tandem cell is proposed and simulated. In order to grow GaAs0.71P0.29 layers on Si, Si1-yGey (SiGe) buffer layers can be used but optical losses are expected. To reduce large optical losses a wafer bonded/layer transferred structure can be used that eliminates the SiGe buffer layer. In this work we propose a novel tandem step-cell design that partially exposes the underlying Si cell for both wafer bonded and SiGe based cells. We demonstrate by experiment and simulation mitigation of the optical losses associated with SiGe buffer layers. For an optimized GaAs0.71P0.29/Si tandem cell without the step cell design, simulations estimate ~20% efficiency for the bonded structure and ~3% for the as grown structure with a SiGe buffer. With the proposed novel step-cell design, optimum efficiency of bonded structure increases to ~32% while for structures with SiGe the simulated efficiency reaches ~23%. Optimum exposure of bottom cell area increases with increasing thickness and lifetime of layers above the bottom Si cell.