Coupling Optical and Electrical Modelling for the Study of a-Si:H-Based Nanowire Array Solar Cells

Coupled optical/electrical simulations have been performed on solar cells consisting in arrays of p-i-n radial nanowires based on crystalline p-type silicon (c-Si) core/hydrogenated amorphous silicon (a-Si:H) shell heterojunctions. Three-dimensional (3D) optical calculations based on rigorous coupled wave analysis (RCWA) are firstly performed and then coupled to a semiconductor device simulator that exploits the radial symmetry of the nanowires. By varying either the doping concentration of the c-Si core, or the work function of the Al-doped ZnO (AZO) back contact we can separate and originally highlight the contribution to the cells performance of the nanowires themselves (the radial cell) from the planar part in between the nanowires (the planar cell). We show that the short-circuit current density (J_sc) only depends on the doping of the c-Si core indicating that it is mainly influenced by the radial cell. On the contrary the open-circuit voltage (V_oc) is strongly affected by the back contact conditions (AZO work function), revealing an important impact of the interspacing between the nanowires on the characteristics of the entire nanowire array. We explain this strong influence of the back contact conditions by the fact that it determines the band-bending in the a-Si:H absorber shell touching the AZO, i.e. in the planar part. Therefore, it directly impacts the potential drop (V_bi) in the same area. For low AZO work functions, the dark current density (J_dark) is increased in the planar region, where V_bi is lower, which degrades the V_oc of the entire cell.