Integrating diffractive rear side structures for light trapping into crystalline silicon solar cells

Diffractive rear side light trapping structures significantly enhance the path length for near infrared light. We present a way of integrating such diffractive rear side structures into crystalline silicon solar cells. Two types of diffractive structures, hexagonal sphere gratings and binary gratings have been simulated and fabricated. For both structures absorption enhancements corresponding to a photo current density gain of 1-2 mA/cm2 have been verified in optical measurements. To integrate these diffractive structures into fully processed silicon solar cells, we developed a passivation concept using a thin Al2O3 layer that provides a high-quality passivation for the rear side of the active silicon bulk and has a negligible influence on the optical properties. We also present a planarization process that prevents parasitic absorption in the rear side metal layer.