Black thin film silicon

Abstract

“Black etching” has been proposed previously as a method for the nanoscale texturing of silicon surfaces, which results in an almost complete suppression of reflectivity in the spectral range of absorption relevant for photovoltaics. The method modifies the topmost 150 to 300 nm of the material and thus also is applicable for thin films of silicon. The present work is focused on the optical effects induced by the black-etching treatment on hydrogenated amorphous and microcrystalline silicon thin films, in particular with respect to their application in solar cells. In addition to a strong reduction of the reflectivity, efficient light trapping within the modified thin films is found. The enhancement of the optical absorption due to the light trapping is investigated via photometric measurements and photothermal deflection spectroscopy. The correlation of the texture morphology (characterized via atomic force microscopy) with the optical effects is discussed in terms of an effective medium with gradually varying optical density and in the framework of the theory of statistical light trapping. Photoconductivity spectra directly show that the light trapping causes a significant prolongation of the light path within the black silicon films by up to 15 μm for ∼1 μm thick films, leading to a significant increase of the absorption in the red.

Black silicon: fabrication methods, properties and solar energy applications

“…The morphologies of the resulting silicon surfaces can be controlled by varying the process parameters, such as the size, shape and surface coverage of noble metal nanoparticles, etchant concentration and the etching time. 103 This etching technique can be applied to c-Si, mc-Si, and amorphous Si (a-Si), 104,105 as well as other materials, such as GaAs, GaN, and SiC. 92 It has grown increasingly popular over the last decade, especially in a research context, and remains as a mainstream etching method.…”

Low surface damage dry etched black silicon

“…These layers can act as an effective optical medium because the Si nanostructures are not distinguished from the surrounding medium by light with effective wavelength higher than k Si $ 100 nm, which is equivalent to k air > 400 nm, 22 since k Si ¼ k air =n Si . This is also known as the so-called "zero order effective medium requirement."…”

“…Hence, the diffraction mechanism based on Lambertian randomized light trapping is the dominant mechanism according to Yablonovitch. 22,24,25 With diffraction, solar radiation is smoothly in-coupled from air to Si leading to nearly zero reflectance. According to Fig.…”

“…5 and Table II, the pitch between nanostructures is in the range of 180-280 nm, which is smaller than or similar to the wavelength in the range (200-1100 nm), and then light interferes and forms a transmissivity diffraction pattern behind the nanostructures acting as a diffraction slit. Following Koynov, 22 the transmittivity diffraction grid of nanostructures is characterized by an average distance D between scatters. According to Bragg's equation, the direction of constructively interfering rays can be defined as k air =n Si ¼ D sin ðbÞ after passing the grid, where k air is the wavelength of the incident light in air k air =n Si and b is the effective light wavelength and the scattering angle within the Si layers, respectively.…”

Improved Black Silicon for Photovoltaic Applications

“…Nano‐textured silicon, also referred to as black silicon (b‐Si), is a material with an optically graded surface, which shows a reflectivity as low as 1–5% in the whole range of Si absorption 1–3. Nano‐texturing of thin silicon wafers and films results also in an additional enhancement of the optical absorption due to efficient light trapping 4. The unique optical properties of b‐Si, as well as the opportunity to produce nano‐textures on different silicon substrates (single‐ or multicrystalline wafers, amorphous or microcrystalline Si thin films)2–5 are of significant interest for photovoltaic applications.…”