With only one micron (equivalent bulk thickness) of silicon it is possible to absorb nearly 85% of all available sunlight in the wavelength range of 300-1100 nm. This enables a one-micron thick silicon photonic crystal to achieve a power conversion efficiency of 17.5%, surpassing commercial silicon solar cells that use up to 300 microns of silicon. With 10-micron thickness of silicon it is possible to reach a photo-current density of 42.5 mA/cm2 (from sunlight in the 300-1100 nanometer wavelength range), not far from the 100% solar absorption limit of 43.5 mA/cm2. By including solar absorption over the full 300-1200 nm range and using an optimized doping profile that minimizes Auger and surface recombination, it possible to reach as high as 30% power conversion efficiency using a thin and flexible 10-micron-thick sheet of silicon. In a 15-micron-thick silicon solar cell with interdigitated back contacts, it is possible to achieve 31% power conversion efficiency, well above the world record of 26.7% and close to the Shockley-Queisser thermodynamic limit of 32.33%.
We have also shown near perfect solar absorption in ultra-thin-film Gallium Arsenide photonic crystals. With only 300 nm of GaAs, it is possible to absorb 99.5% of all available sunlight in the wavelength range of 300 nm- 860 nm. Using only 200 nm equivalent bulk thickness of GaAs, it is possible to realize a world record power conversion efficiency of 30% for a single-junction solar cell.
We are considering ways to improve the efficiency of Perovskite Solar Cells using light-trapping effects.