Near perfect solar absorption in ultra-thin-film GaAs photonic crystals

We present designs that enable a significant increase of solar absorption in ultra-thin (100-300 nm) layers of gallium arsenide. In the wavelength range from 400-860 nm, 90-99.5% solar absorption is demonstrated depending on the photonic crystal architecture used and the nature of the packaging. It is shown that using only two hundred nanometer equivalent bulk thickness of gallium arsenide, forming a slanted conical-pore photonic crystal (lattice constant 550 nm, pore diameter 600 nm, and pore depth 290 nm) packaged with SiO2 and deposited on a silver back-reflector, one can obtain a maximum achievable photocurrent density (MAPD) of 26.3 mA cm(-2) from impinging sunlight. This corresponds to 90% absorption of all available sunlight in the wavelength range 400-860 nm. Our optimized photonic crystal design suggests that increasing the equivalent bulk thickness of GaAs beyond 200 nm leads to almost no improvement in solar absorption, while reducing it to 100 nm causes less than 10% reduction in MAPD. Light-trapping in the 200 nm conical pore photonic crystal provides solar absorption exceeding the Lambertian limit over the range of 740-840 nm. The angular dependence of the MAPD for both S- and P-polarizations is also investigated and shows no substantial degradation in the range 0-30 degrees. More dramatic light-trapping and solar absorption is demonstrated in photonic crystals consisting of conical nanowires. Using 200 nm equivalent bulk thickness of GaAs (lattice constant 500 nm, cone base diameter 200 nm, and cone height 4.77 mu m) packaged in SiO2 and deposited on a silver back-reflector, an MAPD of nearly 27 mA cm(-2) is found. This corresponds to absorption of 96% of all available sunlight in the wavelength range 400-860 nm. A clear separation of the solar spectrum along the length of each nanowire is also evident. In the absence of SiO2 packaging, this MAPD increases to 28.8 mA cm(-2), in excess of the corresponding Lambertian limit of 28.2 mA cm(-2). Most remarkably we find that, if the equivalent bulk thickness of GaAs is increased to 300 nm, nearly 100% of relevant sunlight is absorbed by the conical nanowire photonic crystal.