  We systematically investigate the optical behaviors of an amorphous silicon
solar cell based on a core-shell nanograting structure. The horizontally
propagating Bloch waves and Surface Plasmon Polariton (SPP) waves lead to
significant absorption enhancements and consequently short-circuit current
enhancements of this structure, compared with the conventional planar one. The
perpendicular carrier collection makes this structure optically thick and
electronically thin. An optimal design is achieved through full-field numerical
simulation, and physical explanation is given. Our numerical results show that
this configuration has ultrabroadband, omnidirectional and
polarization-insensitive responses, and has a great potential in photovoltaics.
