18.78% hierarchical black silicon solar cells achieved with the balance of light-trapping and interfacial contact

18.78% efficiency of crystalline silicon (Si) solar cells was achieved through introducing the combined nanopore/pyramid textures. These hierarchical structures possessed the ultra-low reflectivity with values <6% under the various illumination angles from 0 to 60 degrees, evidencing their remarkable omnidirectional light-trapping capability. Although the greatly improved light-trapping effect of the hierarchical structures was demonstrated, we found that the increased formation durations of Si nanopores led the substantial increase of photoluminescent characteristics that could limit the efficient separation of photogenerated carriers through charge recombination. Moreover, the positive correlation in surface roughness of nanopore arrays with respect to the elongated etching durations was evidenced, and these issues could increase the series resistance of solar cells owing to poor interfacial contact between nanopores and front electrodes. Thus, there existed the optimal combination between two-scale textures critically for both photonic and electrical management of cell design. By optimizing the etching durations for the controlled nanopore formation, the improved light-trapping characteristics and fairly unchanged contact resistance with front electrode was achieved, possessing the improved conversion efficiency with >0.58% of increased value beyond the typical microtexture-based solar cells.