(26b) Symmetry-Breaking in Light-Trapping Nanostructures on Silicon for Solar Photovoltaics

In thin-film photovoltaics, highly absorptive materials are conventionally used.  However, these materials have achieved efficiencies that are not comparable to those of thick crystalline silicon (c-Si) photovoltaics and, in some cases, suffer from their toxicity and low supply.  A viable solution to these problems would be to use c-Si for thin-film photovoltaics.  However, thin c-Si films absorb sunlight weakly because of its indirect band gap and strong light-trapping should be provided to achieve high efficiency.  For thin-film photovoltaics, nanoscale structures are typically involved for light trapping because the film thickness becomes comparable to the wavelength of sun light.  While diverse nanostructures have been studied to break the light-trapping limit of geometric optics, known as the Lambertian limit, highly efficient nanostructures that can be easily manufactured have not been demonstrated.  We have previously predicted that symmetry-breaking in light-trapping periodic nanostructures on thin films can approach the Lambertian limit very closely.  Herein, we will present how the systematic symmetry-lowering increases light-trapping in c-Si thin-film photovoltaics.  We will demonstrate the experimental realization of such low-symmetry structures using simple wet etching methods on c-Si(100) wafers without any off-cut, tilt angle.  Further, we will discuss the optical characterization of our fabricated structures on thin c-Si films.