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.

In this study a simple route to preparing photochromic silver nanoparticles in a TiO 2 matrix is presented, which is based upon sputtering and subsequent annealing. The formation of silver nanoparticles with sizes of some tens of nanometers is confirmed by x-ray diffraction and transmission electron microscopy. The inhomogeneously broadened particle-plasmon resonance of the nanoparticle ensemble leads to a broad optical-absorption band, whose spectral profile can be tuned by varying the silver load and the annealing temperature. Multicolor photochromic behavior of this Ag-TiO 2 system upon irradiation with laser light is demonstrated and discussed in terms of a particle-plasmon-assisted electron transfer from the silver nanoparticles to TiO 2 and subsequent trapping by adsorbed molecular oxygen. The electron depletion in the nanoparticles reduces the light absorption at the wavelength of irradiation. A gradual recovery of the absorption band is observed after irradiation, which is explained with a slow thermal release of electrons from the oxygen trapping centers and subsequent capture into the nanoparticles. The recovery can be accelerated by ultraviolet irradiation; the explanation for this observation is that electrons photoexcited in the TiO 2 are captured into the nanoparticles and restore the absorption band.

lithography, provide great flexibility but are slow and expensive, and are therefore not feasible. Conventional scalable techniques, for instance random etching processes such as chemical wet etching [3,4] or plasma dry etching, [5,6] operate in a small window of parameters, thus offering only limited freedom of design and the statistics of fabricated disordered interfaces is more or less fixed. Bottom up, self-organized colloid deposition is a promising candidate for scalable interface texturing. [7][8][9] There are a number of both theoretical and experimental studies on how structures fabricated by colloid deposition can be used for light management in photo nic devices such as solar cells. [10][11][12][13][14][15] Colloid-defined samples are mostly used to produce strictly periodic structures, such as hexagonal photonic crystals and nanoparticle arrays. [7][8][9] However, partly ordered and disordered structures have been shown to possibly perform significantly better than perfectly ordered structures in recent studies. [15][16][17][18][19][20] Nevertheless, a colloid-based deposition technique to prepare disordered structures with the ability to tailor its topographical statistics, and thus a tailored optical response, is still missing.In this work, we investigate the scalable deposition of disordered arrangements of colloidal nanoparticles that selforganize on a substrate to create disordered topographies of defined statistics. The fabricated substrates may serve as templates in a subsequent fabrication process, e.g., etching, nanosphere lithography, or overcoating with optical materials such as absorber layers for solar cells or light generation layers for solid-state lighting. Irregular deposition of colloids is often governed by unwanted effects, such as ordering into regular periodic patterns, autostratification, or separation of particle sizes due to surface tension or depletion forces. [9,[21][22][23][24] Here, we introduce a self-stabilized particle deposition process to overcome these effects. The process allows us to control lateral and vertical structure dimensions by setting size distribution and interparticle spacing of a sub-monolayer of particles through experimentally easy-to-access parameters. By understanding the deposition process and the resulting statistics, we can predict the topography and thereby enable optimization of these structures for a specific application without the need for laborious trial-and-error experiments.The pattern structure of the substrates fabricated by our procedure is of correlated disorder and reveals features that resemble hyperuniformity. [25,26] Like glasses, disorderd Disordered optical substrates play a key role in photonic applications. Furthermore, structures of correlated, in particular hyperuniform, disorder are an emerging new class of photonic material enabling new ways of k-space engineering. Yet, there are little to no feasible technologies that allow fabrication of tailored disordered structures to facilitate a tailored optical response. This work ...

Silver nanoparticles embedded in titanium oxide change their color upon irradiation with visible light. Here we investigate the origin of this photochromic effect. The color change is found to result chiefly from a reduction of the optical extinction peak of the photoexcited particle plasmons. From a comparison with x-ray diffraction experiments, we conclude that this reduction is caused by a photoinduced decrease of the mean size of the silver nanocrystals.