Optical Simulation of Multijunction Solar Cells Based on III-V Nanowires on Silicon

Arrays of III-V direct-bandgap semiconductor nanowires are promising candidates for future photovoltaic devices due to their high optical absorption and their ability to be grown on low cost semiconductor substrates like silicon. The core-shell structure is particularly interesting as the electron-hole pair separation occurs in the radial direction and the photogenerated minority carriers have to travel short distances (the radius of the nanowires) thus improving the collection probability in case of well passivated nanowire surfaces.The aim of this study is to find the optimal geometry (length, height and diameter) of a GaAs nanowire array grown on a silicon substrate in order to have the best absorption of the incident photons. For this purpose, we have performed electromagnetic simulations with a homemade Rigorous Coupled Wave Analysis (RCWA) software. Our simulations take into account the core-shell structure, the passivation layer (GaAlAs) and the anti-reflection coating, but also the necessity to achieve current matching between the GaAs nanowire-based and the silicon substrate solar cells. This requirement is justified by the fact that the final goal is to process a tandem solar cell with junctions connected in series.