Furthermore, these internal multiple scattering processes are responsible for a strong photoluminescence (PL) and an enhanced Raman scattering from the Si NW forest, paving the way toward a new class of light-emitting devices. Despite their ultra-small dimensions (only a few microns long with a diameter of few nanometers), this type of fractal NW array allows for a very high light-trapping efficiency across the entire visible and near-infrared range, reaching high values of apparent absorbance due to light absorption driven by multiple scattering inside the structure. Here, a forest of ultrathin and vertically aligned silicon NWs, arranged in a 2D fractal array, is reported this array is obtained via a silicon-compatible technology without the use of any mask or lithographic process. Our approach involves the fabrication of a 2D random fractal structure of Si NWs as a unique material that meets all of the previously described demands. Moreover, strong light-trapping and absorption properties in terms of an enhanced short-circuit current have also been demonstrated in Si NW solar cells, leading to a path-length enhancement exceeding the randomized scattering Lambertian limit 22, 23, 24, 25.
These needles act as an almost perfect graded index layer on the Si surface, thus strongly suppressing reflectivity of the incoming light regardless of the propagation direction 5. This type of material exhibits conical-shaped needles with typical cross sections varying from a few to hundreds of nanometers. In particular, ‘black-silicon’, which is obtained through the exposure of a crystalline silicon surface to a reactive-ion etching (RIE) process with various gases 5, 21, has been reported to exhibit exceptional antireflection properties, with extremely low reflectance values (below 1%) and high absorbance over a wide spectral range in the visible and near-infrared regions. Indeed, the production of smart Si NW materials to enhance light scattering and absorption is currently the most convenient approach. Recently, plasmonic fractal-like structures have been proposed to improve photovoltaic device performances indeed, through an efficient coupling of the incident light at different frequency bands into both the cavity modes and the surface plasmon modes 14, a broadband absorption enhancement can be reached 15.Īlternatively, high refractive index-textured materials, in particular semiconductor nanostructures 16 and nanowires (NWs) 17, 18, 19, are good candidates to scatter, trap and localize light, minimizing the parasitic optical losses typical of metallic structures 20.Ĭurrently, silicon is certainly the most important and well-known semiconductor because Si-based devices have dominated microelectronics for many decades. In this scenario, the production of a fractal pattern presents the possibility of achieving a complex disorder with strong structural heterogeneities correlated on all length scales 11, 12, 13. These new structures allow for strong and broad optical resonances, leading to in-plane multiple scattering phenomena, efficient light trapping and absorption enhancement beyond the theoretical limit dictated by ray optics 8, 9, 10.
Recently, a new strategy of designing two-dimensional (2D) random patterns of submicron size holes in thin films has been demonstrated 6, 7. Novel concepts of thin films textured at the micro- and nanoscale, and assemblies of nanostructures with peculiar spatial arrangements, both ordered and disordered, have a key role on the light transport inside the materials and, consequently, on their optical properties 1, 2, 3, 4, 5. The development of new materials for light trapping, emission and amplification of light is an ever-growing research field.
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