Mimicking moth's eyes for photovoltaic applications with tapered GaP nanorods

We demonstrate experimentally that ensembles of conically shaped GaP nanorods form layers of graded refractive index due to the increased filling fraction of GaP from the top to the bottom of the layer. Graded refractive index layers reduce the reflection and increase the coupling of light into the substrate, leading to broadband and omnidirectional antireflection surfaces. This reduced reflection is the result of matching the refractive index at the interface between the substrate and air by the graded index layer. The layers can be modeled using a transfer-matrix method for isotropic layered media. We show theoretically that the light coupling efficiency into silicon can be higher than 95% over a broad wavelength range and for angles up to 60° by employing a layer with a refractive index that increases parabolically. Broadband and omnidirectional antireflection layers are specially interesting for enhancing harvesting of light in photovoltaics.