2D Periodic Gratings by Laser Processing

As the thickness of crystalline silicon wafers for use in solar cells is reduced, transmission-related losses become increasingly important. Very thin silicon solar cells will benefit from surface structures exhibiting efficient light trapping, such as 2D periodic structures with diffractive properties. Furthermore, multicrystalline silicon solar cells will benefit from a front surface texture which exhibits lower reflectance than the standard iso-etch. In this paper, we investigate three new methods for creating such structures based on the formation of a 2D periodic template from a colloidal suspension of microspheres and subsequent laser processing and etching.Polystyrene spheres with 1 μm diameter are spin-coated onto a silicon wafer, and forms a hexagonal pattern through self-ordering. The wafer is irradiated with an ultrashort-pulse laser with sufficient power to form pits in the wafer surface below the individual polystyrene spheres, thus transferring the hexagonal pattern to the silicon wafer. The maximum obtained diameter and depth of the pits are 600 nm and 100 nm respectively.To increase the depth of the pits we deposit the self-ordering microspheres on a silicon nitride etch barrier, followed by laser irradiation for local opening of the barrier and an isotropic etch through these holes. With this approach we are able to create structures with a base diameter of 800 nm and a depth of 350 nm. These structures are found in simulations to have good light-trapping properties and the method may therefore be suitable for the fabrication of 2D photonic crystals for light-trapping in silicon solar cells.