CMOS-compatible metal-stabilized nanostructured Si as anodes for lithium-ion microbatteries

The properties of fully complementary metal-oxide semiconductor (CMOS)-compatible metal-coated nanostructured silicon anodes for Li-ion microbatteries have been studied. The one-dimensional nanowires on black silicon (nb-Si) were prepared by inductively coupled plasma (ICP) etching and the metal (Au and Cu) coatings by successive magnetron sputtering technique. The Cu-coated nb-Si show the most promising electrochemical performance enhancements for the initial specific capacity as well as their cyclability compared to pristine nb-Si. The electrochemical and microstructural properties before and after cycling of the metal-coated nb-Si compared to their pristine counterparts are discussed in detail

Light-trapping structures for planar solar cells inspired by transformation optics

Optimal light absorption is decisive in obtaining high-efficiency solar cells. An established, if not to say the established, approach is to texture the interface of the light-absorbing layer with a suitable microstructure. However, structuring the light-absorbing layer is detrimental concerning its electrical properties due to an increased surface recombination rate (owing to enlarged surface area and surface defects) caused by the direct patterning process itself. This effect lowers the efficiency of the final solar cells. To circumvent this drawback, this work theoretically explores a transformation optics (TrO) inspired approach to map the nanopatterned texture onto a planar equivalent. This offers a pattern with the same optical functionality but with much improved electrical properties. Schwarz-Christoffel mappings are used for ensuring conformality of the maps. It leads to planar, inhomogeneous, dielectric-only materials for the light trapping structure to be placed on top of the planar light-absorbing layer. Such a design strategy paves a way towards a novel approach for implementing light-trapping structures into planar solar cells

Tree-like alumina nanopores generated in a non-steady-state anodization{

Novel tree-like alumina nanopores were reproducibly obtained in non-steady-state anodization conditions by exponential decrease of anodization potential. The mechanism of pore formation was thought to be due to a combination of electrical treeing and mechanic stress in the growth process. Furthermore, some interesting properties from gold nanotrees were observed showing that the tree-like nanopores will be new templates towards fabrication of nanotrees from a variety of materials possibly exhibiting new shape-dependent properties. Nanoporous alumina formed by electrochemical anodization of aluminium in acid electrolytes has been extensively studied for more than 50 years. 1 Wood's model 2 and further modified models 3,4 can give satisfactory interpretations of many experimental phenomena, such as monodispersity of pore size distribution, linear dependence of pore diameter and inter-pore distance on the applied anodization potential. However, these models cannot easily explain well some recent findings, such as selfordered pore growth in two-step anodization, 5,6 self-ordering under high-field anodization 7,8 and at burning potential, 9 guided pore growth by imprint lithography, 10,11 etc. In order to understand the self-ordering pore growth mechanism, repulsive interactions between the pores 12 and high electric field theory 7-9 have been proposed. In contrast to the extensive research efforts on steady-state anodization, non-steady-state anodization has been given little attention. Here, it was found that unexpected tree-like alumina nanopores were generated in non-steady-state anodization when the anodization potential was decreased exponentially in a stepwise way. The development of pores is more like that of tree or root in nature, which cannot be simply explained by Wood's models. 17,18 Scheme 1 illustrates the fabrication process of tree-like alumina nanopores. Firstly, ordered nanopore arrays were fabricated via well-established two-step anodization in oxalic acid at 40 V (see details in ESI{). The nanopores have a depth of y2 mm, and a diameter of y35 nm. Then, the anodization potential was decreased exponentially from 40 V to 5 V. The change of the applied anodization potential as a function of time is shown i

Tailored Light Scattering through Hyperuniform Disorder in Self-Organized Arrays of High-Index Nanodisks

Arrays of nanoparticles exploited in light scattering applications commonly only feature either a periodic or a rather random arrangement of its constituents. For the periodic case, light scattering is mostly governed by the strong spatial correlations of the arrangement, expressed by the structure factor. For the random case, structural correlations cancel each other out and light scattering is mostly governed by the scattering properties of the individual scatterer, expressed by the form factor. In contrast to these extreme cases, it is shown here that hyperuniform disorder in self-organized large-area arrays of high refractive index nanodisks enables both structure and form factor to impact the resulting scattering pattern, offering novel means to tailor light scattering. The scattering response from the authors’ nearly hyperuniform interfaces can be exploited in a large variety of applications and constitutes a novel class of advanced optical materials

Correlated Disorder Substrate‐Integrated Nanodisk Scatterers for Light Extraction in Organic Light Emitting Diodes

A major loss mechanism in organic light emitting diodes (OLEDs) is the coupling of the emitter molecule light field to waveguide modes in the OLED thin film stack. In this work, a disordered 2D array of TiO$_{2}$ nanodisk scatterers is integrated into the OLED substrate to enable efficient light extraction from these waveguide modes. Fabrication of the nanodisks is based on a bottom-up, colloidal lithography technique and subsequent pattern transfer into high refractive index TiO$_{2}$ via reactive ion etching. The substrates are completed by spin-coating a polymer planarization layer before applying the OLED thin film stack. This ensures reproducible optoelectronic properties of the OLED through leaving the electrically active layers planar. Simultaneously, the nanodisks in close vicinity to the thin film stack ensure efficient out-of-plane scattering of waveguide modes. In a monochromatic OLED (center wavelength λ$_{0}$ = 520 nm), a 44.2%$_{rel}$ increase in external quantum efficiency is achieved in comparison to a device without scattering structure. An in-depth numerical analysis reveals that this significant enhancement is only partly due to the out-coupling of waveguide modes. Additional enhancement is suspected to result from out-coupling of substrate modes through scattering by the nanodisks. Further improvements to the scattering structure are numerically evaluated

Formation of gold nanoparticles in polymeric nanowires by low-temperature thermolysis of gold mesitylene

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The formation of polymer nanowires containing metal nanoparticle chains by low-temperature thermolyses of metal precursors has remained challenging. We report the block copolymer-assisted generation of locally regular chains of quasi-spherical gold nanoparticles with narrow particle diameter distribution by mild thermolysis of the non-polar gold precursor gold mesitylene inside the cylindrical nanopores of self-ordered anodic aluminium oxide (AAO). The block copolymer separates the gold mesitylene as well as the developing gold nanoparticles from the AAO pore walls so that surface nucleation and pinning of gold clusters are prevented. Growing quasi-spherical gold nanoparticles locally deform the polymer chains irreversibly adsorbed on the AAO pore walls, and the polymer chains are pushed into the space between the gold nanoparticles. The gold nanoparticles have, therefore, larger diameters and smaller specific surface than hypothetical pluglike gold entities with the same volume, the formation of which is suppressed.DFG, SPP 1165, Nanodrähte und Nanoröhren: von kontrollierter Synthese zur Funktio

Antireflective Huygens’ Metasurface with Correlated Disorder Made from High-Index Disks Implemented into Silicon Heterojunction Solar Cells

A large variety of different strategies has been proposed as alternatives to random textures to improve light coupling into solar cells. While the understanding of dedicated nanophotonic systems deepens continuously, only a few of the proposed designs are industrially accepted due to a lack of scalability. In this Article, a tailored disordered arrangement of high-index dielectric submicron-sized titanium dioxide (TiO$_{2}$) disks is experimentally exploited as an antireflective Huygens’ metasurface for standard heterojunction silicon solar cells. The disordered array is fabricated using a scalable bottom-up technique based on colloidal self-assembly that is applicable virtually irrespective of material or surface morphology of the device. We observe a broadband reduction of reflectance resulting in a relative improvement of a short-circuit current by 5.1% compared to a reference cell with an optimized flat antireflective indium tin oxide (ITO) layer. A theoretical model based on Born’s first approximation is proposed that links the current increase in the arrangement of disks expressed in terms of the structure factor S(q) of the disk array. Additionally, we discuss the optical performance of the metasurface within the framework of helicity preservation, which can be achieved at specific wavelengths for an isolated disk for illumination along the symmetry axis by tuning its dimensions. By comparison to a simulated periodic metasurface, we show that this framework is applicable in the case of the structure factor approaching zero and the disks’ arrangement becoming stealthy hyperuniform

Strategy for tailoring the size distribution of nanospheres to optimize rough backreflectors of solar cells

We study the light-trapping properties of surface textures generated by a bottom-up approach, which utilizes monolayers of densely deposited nanospheres as a template. We demonstrate that just allowing placement disorder in monolayers from identical nanospheres can already lead to a significant boost in light-trapping capabilities. Further absorption enhancement can be obtained by involving an additional nanosphere size species. We show that the Power Spectral Density provides limited correspondence to the diffraction pattern and in turn to the short-circuit current density enhancement for large texture modulations. However, in predicting the optimal nanosphere size distribution, we demonstrate that full-wave simulations of just a c-Si semi-infinite halfspace at a single wavelength in the range where light trapping is of main importance is sufficient to provide an excellent estimate. The envisioned bottom-up approach can thus reliably provide good light-trapping surface textures even with simple nanosphere monolayer templates defined by a limited number of control parameters: two nanosphere radii and their occurrence probability