Solar light trapping in slanted conical-pore photonic crystals

摘要

We show that with only one micron, equivalent bulk thickness, of crystalline silicon, sculpted into the form of a slanted conical-pore photonic crystal and placed on a silver back-reflector, it is possible to attain a maximum achievable photocurrent density (MAPD) of 35.5mA/cm~2 from impinging sunlight. This corresponds to absorbing roughly 85% of sunlight in the wavelength range 300-1100nm and exceeds the Lambertian limit suggested by previous "statistical ray trapping" arguments. When the silicon volume is reduced to an equivalent thickness of only 380nm, the MAPD remains as high as 32mA/cm~2. This suggests the possibility of very high efficiency, ultra-thin-film silicon solar cells. Our one-micron structure consists of a photonic crystal square lattice constant of 850nm and slightly overlapping inverted cones with upper (base) radius of 500nm and 1600nm cone depth. When the solar cell is packaged with silica (each pore filled with SiO_2 and modulation on the top is added), the MAPD in the wavelength range of 400-1100nm becomes 32.6mA/cm~2 still higher than the Lambertian 4n2 benchmark of 31.2mA/cm~2 . Thinner structures are considered by keeping the lattice constant and cone radius fixed but by decreasing the cone depth. The MAPD dependence on the overall depth of nanopores indicates that using roughly half the amount of silicon leads to only about 5% drop in the MAPD. In the near infrared regime light is absorbed within slow group velocity modes, that propagate nearly parallel to the interface and exhibit localized high intensity vortex-like flow in the Poynting vector-field.