A nanostructure for crystalline silicon is imitating the surface morphology of moth eyes and is decreasing optical losses in solar cells.
Scientists at Fraunhofer Center for Silicon Photovoltaic CSP and the University of Applied Science Anhalt have developed a nanostructuring for silicon solar cells which is imitating the optical characteristics of moth eyes. In nature, this structure protects the moth from predators. Goal of the technological development is an implementation in current industrial processes. Due to this technology low reflectivity values can be achieved leading to an improved efficiency of the solar cells.
The physical fundamental of moth-eye structures is a surface morphology with widths of 400 nm and smaller. The width of the texture is in the order of the visible light, making the texture works as a so-called effective medium. This effective medium causes a gradual transition of the refractive index between air and silicon. The incident light is almost entirely absorbed.
Since the moth-eye structures are formed by an isotropic plasma etching by SF6 and oxygen, this technology is also suitable for both mono-crystalline and multi-crystalline silicon. Moreover, the plasma etching process is independent of the surface characteristics of the wafer. As a result, the technology is particularly suitable for texturing of diamond wire sawn wafers and wafers from kerf-less technologies.
The next step is to investigate the influence of the moth-eye structures on subsequent process steps in the industrial manufacturing of solar cells. As an example, surface passivation and metallization must be adapted to the moth-eye structure. Therefore, the SiN surface passivation and the cleaning of the nanostructures are being optimized. The goal of this optimization is the complete abandonment of wet chemical process steps, as these represent a cost factor and safety issue in industrial production.
This development is the result of the cooperation of the University for Applied Science Anhalt and the Fraunhofer Center for Silicon Photovoltaics CSP within the framework of the research project StrukturSolar (BMBF 03SF0417A).