LT) can be achieved for weakly absorbed photons with energies close to the absorption edge of silicon. [ 15 ] These properties of b-Si are particularly useful for photovoltaic applications.The limiting effi ciency of a solar cell is given by the detailed balance of absorption and radiative recombination [ 16 ] and by nonradiative processes like Auger-and impurity recombination. [17][18][19] b-Si can help to approach those limits in two ways. On the one hand b-Si improves the coupling of light into the solar cell and the absorption of near band edge photons. This in turn increases the short circuit current and on a logarithmic scale also the open circuit voltage. On the other hand, due to excellent light-trapping properties b-Si might also allow reducing the solar cell thickness substantially below 100 µm while sustaining a high light absorption. This reduces nonradiative bulk recombination losses that scale linearly with the solar cell thickness [ 17,18 ] and hence, increases the open-circuit voltage. Of course, reducing the solar cell thickness also increases the cost effi ciency. Decreasing the amount of required silicon feedstock is a major industry concern as can be seen by the growing interest in kerf-free crystalline silicon solar cell technologies. [20][21][22] Unfortunately, besides bulk effects, surface recombination imposes a very critical limit to the solar This article presents an overview of the fabrication methods of black silicon, their resulting morphologies, and a quantitative comparison of their optoelectronic properties. To perform this quantitative comparison, different groups working on black silicon solar cells have cooperated for this study. The optical absorption and the minority carrier lifetime are used as benchmark parameters. The differences in the fabrication processes plasma etching, chemical etching, or laser processing are discussed and compared with numerical models. Guidelines to optimize the relevant physical parameters, such as the correlation length, optimal height of the nanostructures, and the surface defect densities for optoelectronic applications are given.

We present experimental results and rigorous numerical simulations on the optical properties of Black Silicon surfaces and their implications for solar cell applications. The Black Silicon is fabricated by reactive ion etching of crystalline silicon with SF6 and O2. This produces a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range and a pronounced light trapping effect at the silicon absorption edge such Black Silicon surface textures are beneficial for photon management in photovoltaic applications. We demonstrate that those light trapping properties prevail upon functionalization of the Black Silicon with dielectric coatings, necessary to construct a photovoltaic system. The experimental investigations are accompanied by rigorous numerical simulations based on three dimensional models of the Black Silicon structures. Those simulations allow insights into the light trapping mechanism and the influence of the substrate thickness onto the optical performance of the Black Silicon. Finally we use an analytical solar cell model to relate the optical properties of Black Silicon to the maximum photo current and solar cell efficiency in dependence of the solar cell thickness. The results are compared to standard light trapping schemes and implications especially for thin solar cells are discussed

35 words): An overview and comparison of different fabrication methods of black silicon is presented. Guidelines to optimize relevant parameters such as spatial frequencies and surface defect densities for optoelectronic applications such as photovoltaics will be given. Summary:The potential of silicon surfaces structured in the micro-and nanometer regime are widely known, e.g. in photovoltaic (PV) crystalline silicon (c-Si) solar cells [1][2], watersplitting by photo-electrochemical-catalysis (PEC) [3], phododiods [4], terahertz emitters[5], highly sensitive optical[6] and chemical detection devices [7], amongst many others. Black silicon offers extraordinary and broadband antireflection properties [8]. Additionally, strong randomizing light-trapping is achieved for weakly absorbed photons close to the band edge of c-Si, i.e. light trapping [9]. The enlarged area for 3-D structured p/n-junctions might be advantageous for charge carrier separation. But due to the strongly enlarged surface and a usually high surface defect density black silicon nanostructures exhibit rather low electronic surface quality leading to strong surface recombination. This drawback limits the performance of potential devices [10], [11]. Therefore, effective surface passivation is essential for nanostructured silicon and investigations to find a well suited passivation scheme are going on ever since b-Si has been used for electro-optical devices. In this work, different black silicon fabrication methods are compared. Investigated are the resulting morphologies, as well as optical and electronic properties. Different groups working on black silicon have cooperated for this study. As benchmark parameters, we used the optical absorption and the minority carrier lifetime after the passivation of the samples with thermal-atomic layer deposited Al2O3 [12]. The differences of the obtained black silicon samples fabricated by different proceesses, such as plasma etching, chemical etching or laser processing, are discussed. Based on our findings, we will give guidelines to optimize the relevant physical parameters, e.g. the spatial frequencies and the surface defect densities for optoelectronic applications. PTu2C.2.pdf