Polymeric liquids in mesoporous photonic structures: From precursor film spreading to imbibition dynamics at the nanoscale.

Polymers are known to wet nanopores with high surface energy through an atomically thin precursor film followed by slower capillary filling. We present here light interference spectroscopy using a mesoporous membrane-based chip that allows us to observe the dynamics of these phenomena in situ down to the sub-nanometer scale at milli- to microsecond temporal resolution. The device consists of a mesoporous silicon film (average pore size 6 nm) with an integrated photonic crystal, which permits to simultaneously measure the phase shift of thin film interference and the resonance of the photonic crystal upon imbibition. For a styrene dimer, we find a flat fluid front without a precursor film, while the pentamer forms an expanding molecular thin film moving in front of the menisci of the capillary filling. These different behaviors are attributed to a significantly faster pore-surface diffusion compared to the imbibition dynamics for the pentamer and vice versa for the dimer. In addition, both oligomers exhibit anomalously slow imbibition dynamics, which could be explained by apparent viscosities of six and eleven times the bulk value, respectively. However, a more consistent description of the dynamics is achieved by a constriction model that emphasizes the increasing importance of local undulations in the pore radius with the molecular size and includes a sub-nanometer hydrodynamic dead, immobile zone at the pore wall but otherwise uses bulk fluid parameters. Overall, our study illustrates that interferometric, opto-fluidic experiments with mesoporous media allow for a remarkably detailed exploration of the nano-rheology of polymeric liquids.

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.

Atomic Layer Deposition of Cobalt Phosphide for Efficient Water Splitting.

Transition-metal phosphides (TMP) prepared by atomic layer deposition (ALD) are reported for the first time. Ultrathin Co-P films were deposited by using PH3 plasma as the phosphorus source and an extra H2 plasma step to remove excess P in the growing films. The optimized ALD process proceeded by self-limited layer-by-layer growth, and the deposited Co-P films were highly pure and smooth. The Co-P films deposited via ALD exhibited better electrochemical and photoelectrochemical hydrogen evolution reaction (HER) activities than similar Co-P films prepared by the traditional post-phosphorization method. Moreover, the deposition of ultrathin Co-P films on periodic trenches was demonstrated, which highlights the broad and promising potential application of this ALD process for a conformal coating of TMP films on complex three-dimensional (3D) architectures.

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.

Toward a planar black silicon technology for 50 µm-thin crystalline silicon solar cells.

Auger and surface recombinations are major drawbacks that deteriorate a photon-to-electron conversion efficiencies in nanostructured (NS) Si solar cells. As an alternative to conventional frontside nanostructuring, we report how backside nanostructuring is beneficial for carrier collection during photovoltaic operation that utilizes a 50-µm-thin wafer. Ultrathin (4.3-nm-thin) zinc oxide was also effective for providing passivated tunneling contacts at the nanostructured backsides, which led to the enhancement of 24% in power conversion efficiency.

Photoelectrochemical hydrogen evolution of tapered silicon nanowires.

The origin of the photocurrent enhancement and the overpotential reduction in solar water splitting employing nanostructured silicon is still a matter of debate. A set of tapered Si nanowires (SiNWs) has been designed for clarifying the impact of nanostructured Si on the hydrogen evolution reaction (HER) while precisely tailoring several interference factors such as surface area, light absorption and surface defect density. We find that defect passivation by KOH achieved by tapering is much more beneficial than the optical gain. Surfactant-mediated modification of SiNWs is capable of engineering the band structure. As a result, we suggest a guideline for nanostructured Si photoelectrodes optimized for the HER.

Influence of black silicon surfaces on the performance of back-contacted back silicon heterojunction solar cells.

The influence of different black silicon (b-Si) front side textures prepared by inductively coupled reactive ion etching (ICP-RIE) on the performance of back-contacted back silicon heterojunction (BCB-SHJ) solar cells is investigated in detail regarding their optical performance, black silicon surface passivation and internal quantum efficiency. Under optimized conditions the effective minority carrier lifetime measured on black silicon surfaces passivated with Al(2)O(3) can be higher than lifetimes measured for the SiO(2)/SiN(x) passivation stack used in the reference cells with standard KOH textures. However, to outperform the electrical current of silicon back-contact cells, the black silicon back-contact cell process needs to be optimized with aspect to chemical and thermal stability of the used dielectric layer combination on the cell.

Automated spray coating process for the fabrication of large-area artificial opals on textured substrates.

3D photonic crystals, such as opals, have been shown to have a high potential to increase the efficiency of solar cells by enabling advanced light management concepts. However, methods which comply with the demands of the photovoltaic industry for integration of these structures, i. e. the fabrication in a low-cost, fast, and large-scale manner, are missing up to now. In this work, we present the spray coating of a colloidal suspension on textured substrates and subsequent drying. We fabricated opaline films of much larger lateral dimensions and in much shorter times than what is possible using conventional opal fabrication methods.

The Role of Silicate Enrichment on the Discharge Duration of Silicon-Air Batteries.

Silicon-air batteries are candidates for next generation batteries from non-critical raw materials. However, current silicon-air batteries with alkaline electrolytes suffer from premature termination of the discharge process. To understand this process, we investigated the correlation of dissolved silicon in the electrolyte and the discharge duration until passivation. The air- and Si-electrode could be excluded as the source of the voltage drop, while the concentration of silicates in the electrolyte was identified as the decisive factor. A low silicate concentration in the electrolyte was found to be crucial for a sustained discharge of silicon-air batteries with alkaline electrolytes and full consumption of the silicon electrode used in these conditions was shown.

Experimental data showing the influence of different boron nitride particles on the silica network, the butyl stearate and the porogens in shape-stabilized phase change materials.

Shape-stabilized phase change materials (ss-PCMs) based on silica and butyl stearate were thermally enhanced via the addition of different hexagonal boron nitride particles (BN) to the in situ sol-gel synthesis. The dataset is used in conjunction with the experimental data of the influence of the particle size and surface area of BN on the thermal and mechanical properties of ss-PCMs discussed in Marske et al. (2021). To study the effect of the different BN particles on the hydrolysis degree of the silica network and on the chemical nature of the porogens sodium dodecyl sulfate and poly(vinyl alcohol) used for the ss-PCM synthesis, the ss-PCM samples are measured via High Power Decoupling (HPDEC) Magic Angle Spinning (MAS) 29Si NMR and attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, respectively. Additionally, data of the influence of BN on the thermal properties is presented as thermogravimetric analysis (TG). The 29Si MAS NMR spectra are referenced to tetramethylsilane and show the different silica species in ppm. The different value of wavenumber and intensity of each reference and ss-PCM sample is listed in the IR spectra. The decomposition points of the ss-PCMs are calculated from the TG data via OriginLab. The spectra and data can be used as a reference for other researchers and engineers to use in synthesizing ss-PCMs based on silica and other polymeric materials or as reference for pure BN, SDS, stabilized silica sol and PVA.

Losses and group index dispersion in insulator-on-silicon-on-insulator ridge waveguides.

We present polarization-dependent optical transmission properties of a completely symmetric silicon-on-insulator (SOI) microphotonic material system. In contrast to typical SOI based photonic materials, here an insulator-on-silicon-on-insulator (IOSOI) material system has been fabricated. This symmetric structure exhibits average losses between 1510 and 1630 nm of around 0.5 dB/mm for TE and 0.3 dB/mm for TM-polarization. The good transmission for TM-polarization can be explained by the thick insulting cladding layer of 3 microm thickness. Moreover, group index dispersion diagrams are presented and discussed for both polarizations.

Strain dependence of second-harmonic generation in silicon.

Strained silicon is a versatile new type of material, which has found application in microelectronics and integrated optics in the last years. Unlike ordinary silicon, it does not possess a centrosymmetric lattice structure. This allows for stimulation of nonlinear optical processes that involve second-order nonlinear susceptibility. Here, the dependence of the nonlinear susceptibility on the applied strain by means of reflected second-harmonic generation is investigated. This surface-sensitive technique is suitable for the investigation of bulk silicon strained by a layer of thermal oxide. The obtained relation between applied stress and susceptibility enhancement is compared to theoretical prediction based on an analytical model for the deformed silicon orbital. The knowledge of the stress-susceptibility dependence can be used to develop suitable photonic devices that benefit from second-order nonlinear processes in silicon.

Synthesis of monolithic shape-stabilized phase change materials with high mechanical stability via a porogen-assisted in situ sol-gel process.

The confinement of phase change materials (PCMs) in construction materials has recently solved leakage, supercooling and low thermal conductivity problems in the industrial use of PCMs as monolithic thermal energy storage materials. To produce shape-stabilized PCMs (ss-PCMs) as crack-free monoliths, less than 15-30% v/v pure or encapsulated PCMs can be used in construction materials. Therefore, the heat storage capacity of these monolithic ss-PCM boards is comparatively low. In this study, we synthesized a novel class of monolithic ss-PCM boards with high compressive strength of 0.7 MPa at 30 °C (1.2 MPa at 10 °C), high PCM loadings of 86 wt%, and latent heats in the range of 100 J g-1 via a porogen-assisted in situ sol-gel process. We confined butyl stearate (BS) as PCM in a core-shell-like silica matrix via stabilized silica sol as silica source, sodium dodecyl sulfate as surfactant and poly(vinyl alcohol) as co-polymer. The ss-PCMs obtained are hydrophobic, thermally stable up to 320 °C and perform 6000 state transitions from solid to liquid and vice versa, without losing melting or freezing enthalpies. We analyzed the silica structure in the ss-PCMs to understand in detail the reasons for the high mechanical stability. The silica structure in ss-PCMs consists of spherical meso- and macropores up to 10 000 nm filled with PCM, formed mostly by BS droplets in water as templates during gelation. With an increasing BS amount in the synthesis of ss-PCMs, the total nanopore volume filled with PCM in ss-PCMs increases, resulting in higher compressive strengths up to 500% and thermal conductivities up to 60%.

Intermediate reflectors for enhanced top cell performance in photovoltaic thin-film tandem cells.

We have investigated the impact of three types of intermediate reflectors on the absorption enhancement in the top cell of micromorph tandem solar cells using rigorous diffraction theory. As intermediate reflectors we consider homogenous dielectric thin-films and 1D and 3D photonic crystals. Besides the expected absorption enhancements in cases where photonic band gaps are matched to the absorption edge of the semiconductor, our results distinguish between the impact of zero order Bragg-resonances and diffraction-based enhancement at larger lattice constants of the 3D photonic crystal. Our full-spectrum analysis permits for a quantitative prediction of the photovoltaic conversion efficiency increase of the a-Si:H top cell.

Seebeck-voltage-triggered self-biased photoelectrochemical water splitting using HfOx/SiOx bi-layer protected Si photocathodes.

The use of a photoelectrochemical device is an efficient method of converting solar energy into hydrogen fuel via water splitting reactions. One of the best photoelectrode materials is Si, which absorbs a broad wavelength range of incident light and produces a high photocurrent level (~44 mA·cm-2). However, the maximum photovoltage that can be generated in single-junction Si devices (~0.75 V) is much lower than the voltage required for a water splitting reaction (>1.6 V). In addition, the Si surface is electrochemically oxidized or reduced when it comes into direct contact with the aqueous electrolyte. Here, we propose the hybridization of the photoelectrochemical device with a thermoelectric device, where the Seebeck voltage generated by the thermal energy triggers the self-biased water splitting reaction without compromising the photocurrent level at 42 mA cm-2. In this hybrid device p-Si, where the surface is protected by HfOx/SiOx bilayers, is used as a photocathode. The HfOx exhibits high corrosion resistance and protection ability, thereby ensuring stability. On applying the Seebeck voltage, the tunneling barrier of HfOx is placed at a negligible energy level in the electron transfer from Si to the electrolyte, showing charge transfer kinetics independent of the HfOx thickness. These findings serve as a proof-of-concept of the stable and high-efficiency production of hydrogen fuel by the photoelectrochemical-thermoelectric hybrid devices.

High strength in combination with high toughness in robust and sustainable polymeric materials.

In materials science, there is an intrinsic conflict between high strength and high toughness, which can be resolved for different materials only through the use of innovative design principles. Advanced materials must be highly resistant to both deformation and fracture. We overcome this conflict in man-made polymer fibers and show multifibrillar polyacrylonitrile yarn with a toughness of 137 ± 21 joules per gram in combination with a tensile strength of 1236 ± 40 megapascals. The nearly perfect uniaxial orientation of the fibrils, annealing under tension in the presence of linking molecules, is essential for the yarn's notable mechanical properties. This underlying principle can be used to create similar strong and tough fibers from other commodity polymers in the future and can be used in a variety of applications in areas such as biomedicine, satellite technology, textiles, aircrafts, and automobiles.

Nondestructive mechanical release of ordered polymer microfiber arrays from porous templates.

The fabrication of one-dimensional (1D) nanostructures and microstructures inside the pores of porous templates is intensively investigated. The release of these structures is commonly accomplished by etching and destroying the templates. The 1D nanostructures and microstructures tend to condense because of the occurrence of capillary forces during drying of the specimens. It is shown that highly ordered arrays of polymer microfibers can be easily detached from silanized porous templates by mechanical lift-off. This procedure leaves the templates intact, thus allowing their recycling, and does not involve the use of solutions or solvents, thus circumventing condensation. Therefore, mechanical lift-off may enable the up-scaling of template-based approaches to the fabrication of highly ordered assemblies of 1D nanostructures and microstructures.

Nanotubes by template wetting: a modular assembly system.

The wetting of porous templates with polymer melts and solutions or polymer-containing mixtures is a simple and versatile method for the preparation of tubular structures with diameters ranging from a few tens of nanometers to micrometers. The tube walls can be made of a multitude of materials, some of which have thus far been altogether impossible to use or very limited in their ability to be incorporated into nanostuctures. Template wetting also makes it possible to modify the nanotubes in a variety of ways, for example through the controlled generation of pores or the embedding of nanoparticles into the walls. This method offers a promising approach to functionalized nanotube-template hybrid systems and free-standing nanotubes.

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.