Nanotextured metal copper substrates as powerful and long-lasting fuel cell anodes

Fuel cells (FCs) are promising electrochemical devices that convert chemical energy of fuels directly into electrical energy. We present a new anode material based on nanotextured metal copper for fuel cell applications. We have demonstrated that low-cost copper catalyst anodes act as highly efficient and ultra-long-lasting materials for the direct electro-oxidation of ammonia-borane and additional amine derivatives. High power densities of ca. 1W·cm(-2) (ca. -1 V vs Ag/AgCl at 1 A) are readily achieved at room temperature. We fabricate fuel cell devices based on our nanotextured Cu anodes in combination with commercial air cathodes.     

Novel tandem cell structure of dye-sensitized solar cell for improvement in photocurrent

A novel tandem cell structure is proposed to improve photocurrent of dye-sensitized solar cells (DSCs). Front and back parallel photoelectrodes are placed face-to-face; a common Pt-mesh counter electrode with transmittance is inserted between the electrodes. The short-circuit current density (J_sc) for the tandem cell is equivalent to the sum of the J_sc for the front and back photoelectrodes. A model using light energy absorbed by the photoelectrode is used to evaluate appropriate TiO₂ film thickness of the front photoelectrode. The J_sc for the tandem cell was improved to 13.3 mA/cm² for a cell with a 7.8-μm-thick front photoelectrode. The novel tandem cell has a great potential to improve DSC photocurrent and performance.     

Polycrystalline ZnO: B grown by LPCVD as TCO for thin film silicon solar cells

Conductive zinc oxide (ZnO) grown by low pressure chemical vapor deposition (LPCVD) technique possesses a rough surface that induces an efficient light scattering in thin film silicon (TF Si) solar cells, which makes this TCO an ideal candidate for contacting such devices. IMT-EPFL has developed an in-house LPCVD process for the deposition of nanotextured boron doped ZnO films used as rough TCO for TF Si solar cells. This paper is a general review and synthesis of the study of the electrical, optical and structural properties of the ZnO:B that has been performed at IMT-EPFL.The influence of the free carrier absorption and the grain size on the electrical and optical properties of LPCVD ZnO:B is discussed. Transport mechanisms at grain boundaries are studied. It is seen that high doping of the ZnO grains facilitates the tunnelling of the electrons through potential barriers that are located at the grain boundaries. Therefore, even if these potential barriers increase after an exposition of the film to a humid atmosphere, the heavily doped LPCVD ZnO:B layers show a remarkable stable conductivity. However, the introduction of diborane in the CVD reaction induces also a degradation of the intra-grain mobility and increases over-proportionally the optical absorption of the ZnO:B films. Hence, the necessity to finely tune the doping level of LPCVD ZnO:B films is highlighted. Finally, the next challenges to push further the optimization of LPCVD ZnO:B films for thin film silicon solar cells are discussed, as well as some remarkable record cell results achieved with LPCVD ZnO:B as front electrode.     

Supersonic Cold Spraying for Energy and Environmental Applications: One-Step Scalable Coating Technology for Advanced Micro- and Nanotextured Materials

Supersonic cold spraying is an emerging technique for rapid deposition of films of materials including micrometer-size and sub-micrometer metal particles, nanoscale ceramic particles, clays, polymers, hybrid materials composed of polymers and particulates, reduced graphene oxide (rGO), and metal–organic frameworks. In this method, particles are accelerated to a high velocity and then impact a substrate at near ambient temperature, where dissipation of their kinetic energy produces strong adhesion. Here, recent progress in fundamentals and applications of cold spraying is reviewed. High-velocity impact with the substrate results in significant deformation, which not only produces adhesion, but can change the particles' internal structure. Cold-sprayed coatings can also exhibit micro- and nanotextured morphologies not achievable by other means. Suspending micro- or nanoparticles in a liquid and cold-spraying the suspension produces fine atomization and even deposition of materials that could not otherwise be processed. The scalability and low cost of this method and its compatibility with roll-to-roll processing make it promising for many applications, including ultrathin flexible materials, solar cells, touch-screen panels, nanotextured surfaces for enhanced heat transfer, thermal and electrical insulation films, transparent conductive films, materials for energy storage (e.g., Li-ion battery electrodes), heaters, sensors, photoelectrodes for water splitting, water purification membranes, and self-cleaning films.     

Thin-film silicon solar cell development on imprint-textured glass substrates

In this work, we report on the fabrication of microcrystalline thin-film silicon solar cells on textured glass substrates. The development of transparent and conductive front contacts for these solar cells is presented. State-of-the-art random textures for light-trapping were replicated into a glass-like resist on glass substrates with an imprint process. We applied an industrial relevant soft polymer mold that gives excellent replication accuracy. The necessity of applying thin front contacts for enhanced incoupling of the incident light is shown. An increased series resistance of these thin front contacts caused a decrease of the fill factor of the solar cells. One way to surpass this decrease in fill factor by reducing the solar cell width is demonstrated. In addition, the light-trapping and the light-incoupling for solar cells deposited on three different types of random textures were compared.     

Electrochemical photovoltaic cells for solar energy conversion

Photoelectrochemical cells have attracted much more attention recently due to their feasibility as low-cost solar energy conversion devices and hence a number and variety of papers have appeared. Although some review papers have been published, no comprehensive review of electrochemical photovoltaic cells has been made. The present review is devoted to a survey of most of the dimensions of ECPV cells.Starting with photoelectrochemical processes, relevant theoretical background is presented including redox potentials, interface of the semiconductor-liquid junction and Butler-Volmer relation, etc. Much emphasis is given to the requirements of semiconductor electrodes, electrolytes, counter electrods and various cell configurations in ECPV cells. Results on photocorrosion, stability, etc. of the electrodes and the effect of temperature on the performance of the ECPV cell are discussed. Up-to-date data on various ECPV cells are presented and the current situation is discussed. In the light of this, various new materials are attractive. Finally, results on ECPV storage cells and their potential applications are discussed.     

Controlled oxidative nanopatterning of microrough titanium surfaces for improving osteogenic activity

To further enhance the biological properties of acid-etched microrough titanium surfaces, titania nanotextured thin films were produced by simple chemical oxidation, without significantly altering the existing topographical and roughness features. The nanotextured layers on titanium surfaces can be controllably varied by tuning the oxidation duration time. The oxidation treatment significantly reduced water contact angles and increased the surface energy compared to the surfaces prior to oxidation. The murine bone marrow stromal cells (BMSCs) were used to evaluate the bioactivity. In comparison, oxidative nanopatterning of microrough titanium surfaces led to improved attachment and proliferation of BMSCs. The rate of osteoblastic differentiation was also represented by the increased levels of alkaline phosphatase activity and mineral deposition. These data indicated that oxidative nanopatterning enhanced the biological properties of the microrough titanium surfaces by modulating their surface chemistry and nanotopography. Based on the proven mechanical interlocking ability of microtopographies, enhancement of multiple osteoblast functions attained by this oxidative nanopatterning is expected to lead to better implant osseointegration in vivo.     

Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts between PEDOT:PSS and Periodic Silicon Nanopyramid Arrays

Surface nanotexturing with excellent light‐trapping property is expected to significantly increase the conversion efficiency of solar cells. However, limited by the serious surface recombination arising from the greatly enlarged surface area, the silicon (Si) nanotexturing‐based solar cells cannot yet achieve satisfactory high efficiency, which is more prominent in organic/Si hybrid solar cells (HSCs) where a uniform polymer layer can rarely be conformably coated on nanotextured substrate. Here, the HSCs featuring advanced surface texture of periodic upright nanopyramid (UNP) arrays and hole‐conductive conjugated polymers, poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are investigated. The tetramethylammonium hydroxide etching is used to smooth the surface morphologies of the Si‐UNPs, leading to reduced surface defect states. The uniform Si‐UNPs together with silane chemical‐incorporated PEDOT:PSS solution enable the simultaneous realization of excellent broadband light absorption as well as enhanced electrical contact between the textured Si and the conducting polymer. The resulting PEDOT:PSS/Si HSCs textured with UNP arrays show a promising power conversion efficiency of 13.8%, significantly higher than 12.1% of the cells based on the‐state‐of‐the‐art surface texture with random pyramids. These results provide a viable route toward shape‐controlled nanotexturing‐based high‐performance organic/Si HSCs.     

Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings

Enhancing the light absorption in ultrathin-film silicon solar cells is important for improving efficiency and reducing cost. We introduce a double-sided grating design, where the front and back surfaces of the cell are separately optimized for antireflection and light trapping, respectively. The optimized structure yields a photocurrent of 34.6 mA/cm(2) at an equivalent thickness of 2 μm, close to the Yablonovitch limit. This approach is applicable to various thicknesses and is robust against metallic loss in the back reflector.     

Potentiometric strip cell based on carbon nanotubes as transducer layer: toward low-cost decentralized measurements

In this study, we developed a potentiometric planar strip cell based on single-walled carbon nanotubes that aims to exploit the attributes of solid-contact ion-selective electrodes for decentralized measurements. That is, the ion-selective and reference electrodes have been simultaneously miniaturized onto a plastic planar substrate by screen-printing and drop-casting techniques, obtaining disposable strip cells with satisfactory performance characteristics (i.e., the sensitivity is 57.4 ± 1.3 mV/dec, the response time is ≤30 s within the linear range from log a(K+) = -5 to -2, and the limit of detection is -6.5), no need of maintenance during long dry storage, quick signal stabilization, and light insensitivity in short-term measurements. We also show how the new potentiometric strip cell makes it possible to perform decentralized and rapid determinations of ions in real samples, such as saliva or beverages.     

Tunable electro-optic Microlens array. II. Cylindrical geometry

Although the concept of an artificial compound eye has been discussed in the literature, its optical arrangement has never been widely adopted for optical design. A design is presented for a tunable gradient-index microlens array, believed to be new, induced electro-optically inside a cylindrical shell. The transparent electrodes on the both sides of the shell are positioned such that the electrodes on the opposite side compensate the phase delay from the electrodes on the front side for a normally incident plane wave, thus suppressing the intrinsic electrode diffraction for the device without applied voltage. The original technique of the electric field calculation was developed to analyze the induced refractive index inside the shell for two types of electro-optic (EO) ceramics: with linear and with quadratic EO effects. For the linear effect it was shown that for given EO coefficients, electric field strength and intrinsic refractive index, the electrode number should exceed a certain amount to make the focal distance less than the cylinder radius. The quadratic effect provides higher sensitivity to the type of the diffracted wave polarization. It was shown how the quadratic coefficient ratio R(12)/R(11) affects the focal-length difference between TE and TM light polarization.     

Scanning electrochemical microscopy of model neurons: imaging and real-time detection of morphological changes

Living PC12 cells, a model cell type for studying neuronal function, were imaged using the negative feedback mode of a scanning electrochemical microscope (SECM). Six biocompatible redox mediators were successfully identified from a large pool of candidates and were then used for imaging PC12 cells before and after exposure to nerve growth factor (NGF). When exposed to NGF, cells differentiate into a neuron phenotype by growing narrow neurites (1-2 microm wide) that can extend > 100 microm from the cell proper. We demonstrate that carbon fiber electrodes with reduced tip diameters can be used for imaging both the cell proper and these neurites. Regions of decreased current, possibly resulting from raised features not identifiable by light microscopy, are clearly evident in the SECM images. Changes in the morphology of undifferentiated PC12 cells could be detected in real time with the SECM. After exposure to hypotonic and hypertonic solutions, reversible changes in cell height of <2 microm were measured.     

Microfluidic device for electric field-driven single-cell capture and activation

A microchip that performs directed capture and chemical activation of surface-modified single cells has been developed. The cell capture system is comprised of interdigitated gold electrodes microfabricated on a glass substrate within PDMS channels. The cell surface is labeled with thiol functional groups using endogenous RGD receptors, and adhesion to exposed gold pads on the electrodes is directed by applying a driving electric potential. Multiple cell types can thus be sequentially and selectively captured on desired electrodes. Single-cell capture efficiency is optimized by varying the duration of field application. Maximum single-cell capture is attained for the 10-min trial, with 63 +/- 9% (n = 30) of the electrode pad rows having a single cell. In activation studies, single M1WT3 CHO cells loaded with the calcium-sensitive dye fluo-4 AM were captured; exposure to the muscarinic agonist carbachol increased the fluorescence to 220 +/- 74% (n = 79) of the original intensity. These results demonstrate the ability to direct the adhesion of selected living single cells on electrodes in a microfluidic device and to analyze their response to chemical stimuli.     

Instabilities in reactive sputtering of ZnO:Al and reliable texture-etching solution for light trapping in silicon thin film solar cells

Texture etched zinc oxide is often used as transparent front contact for silicon thin film solar cells. Reactive sputtering is a potentially low-cost process. However, process stability, film uniformity, and reproducibility are challenges to be solved. Oscillations of the control signal and subsequent reaction of the plasma emission control with moving substrates from rotatable metallic targets cause fluctuations of aluminum doped zinc oxide (ZnO:Al) properties. Solutions to overcome such variations during the reactive sputtering process are discussed. However, effects on film properties, especially on etching behavior, cannot be totally removed. To achieve good light scattering properties for solar cell application ZnO:Al films are usually etched in dilute hydrochloric acid. An etch process based on hydrofluoric acid has been developed to tune the surface texture for a given ZnO:Al material. One feature of this process is the relaxed requirement on ZnO:Al film properties as the reactively sputtered ZnO:Al films do not necessarily possess optimized film structure for the HCl etch. Solar cells with optimized ZnO:Al front contacts achieved conversion efficiency well above 11%.     

Model calculation of the effectiveness of Tb<Superscript>3+</Superscript> containing glass as a wavelength converter in thin film solar cells

Wavelength converters have been proposed as one of the ways to achieve higher efficiency in third generation solar cells. The idea is to shift the wavelength of the light absorbed by the solar cell to the spectral region where the device is most efficient. Higher energy photons are often absorbed unproductively near the front contact of the solar cells. By the application of photoluminescent materials these photons are transformed into longer wavelength ones, which contribute more effectively to the generated photocurrent. In this study the improvement that a wavelength converter containing Tb³⁺ ions can produce on the efficiency of a thin film silicon single junction solar cell under AM 1.5 solar radiation is assessed by model calculations. The absorption and emission of a specified number of Tb³⁺ ions in a fluoride glass layer or plate is calculated on the basis of literature data. It is presumed that such a plate is placed in front of the solar cell and modifies the solar spectrum falling on the cell. This modified solar spectrum is used to calculate the efficiency of two model solar cells, an amorphous silicon and a microcrystalline silicon one, using the program Afors-Het 2.2. The amount of Tb³⁺ ions per unit area in the wavelength converter layer is varied. In the best case the efficiency of the a-Si:H solar cell improves by 1% and that of the microcrystalline silicon cell by 2.3%, in comparison to that calculated with the unmodified AM 1.5 spectrum.     

Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.     

Polyelectrolyte Multilayer-Treated Electrodes for Real-Time Electronic Sensing of Cell Proliferation

We report on the use of polyelectrolyte multilayer (PEM) coatings as a non-biological surface preparation to facilitate uniform cell attachment and growth on patterned thin-film gold (Au) electrodes on glass for impedance-based measurements. Extracellular matrix (ECM) proteins are commonly utilized as cell adhesion promoters for electrodes; however, they exhibit degradation over time, thereby imposing limitations on the duration of conductance-based biosensor experiments. The motivation for the use of PEM coatings arises from their long-term surface stability as promoters for cell attachment, patterning, and culture. In this work, a cell proliferation monitoring device was fabricated. It consisted of thin-film Au electrodes deposited with a titanium-tungsten (TiW) adhesion layer that were patterned on a glass substrate and passivated to create active electrode areas. The electrode surfaces were then treated with a poly(ethyleneimine) (PEI) anchoring layer and subsequent bilayers of sodium poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH). NIH-3T3 mouse embryonic fibroblast cells were cultured on the device, observed by optical microscopy, and showed uniform growth characteristics similar to those observed on a traditional polystyrene cell culture dish. The optical observations were correlated to electrical measurements on the PEM-treated electrodes, which exhibited a rise in impedance with cell proliferation and stabilized to an approximate 15 % increase as the culture approached confluency. In conclusion, cells proliferate uniformly over gold and glass PEM-treated surfaces, making them useful for continuous impedance-based, real-time monitoring of cell proliferation and for the determination of cell growth rate in cellular assays.     

陷光结构在GaAs薄膜太阳电池中的应用

陷光结构由于其独特的光学特性,在光伏器件中发挥的作用越来越重要。目前硅基太阳电池中陷光结构的应用很常见,然而在GaAs薄膜太阳电池中陷光结构的报道并不多。详细介绍了陷光结构的原理及其在GaAs薄膜电池中的研究现状和应用情况。综述了GaAs薄膜太阳能电池中常用的三类陷光结构:正面陷光结构(包括纳米颗粒、纳米线、纳米锥等)、背面陷光结构(如镜面背反射层)以及混合陷光结构。大量研究表明,陷光结构的使用可以进一步提高GaAs薄膜电池的光电转换效率,一定程度上达到降低电池生产成本的目的。     

Intertwined aligned carbon nanotube fiber based dye-sensitized solar cells

Metal wires suffer from corrosion in fiber-shaped dye-sensitized solar cells (DSSCs). We report herein that stable, ultrastrong, and highly flexible aligned carbon nanotube fibers can be used not only as catalytic counter electrodes but also as conductive materials to support dye-loaded TiO(2) nanoparticles in DSSCs. The power conversion efficiency of this fiber solar cell can achieve 2.94%. These solar power fibers, exhibiting power conversion efficiency independent of incident light angle and cell length, can be woven into textiles via a convenient weaving technology.     

影响共混结构聚合物光电池性能的因素

共混结构聚合物光电池是一种极有前途的太阳能电池, 其性能主要由器件的开路电压Voc,短路电流Isc,填充因子FF,能量转换效率η等因素决定.笔者结合自己的实验经验,从溶剂、给体与受体材料的比例、器件后处理、电极等方面综述了这些因素对器件的性能影响,为提高聚合物光电池工艺提供了有意义的信息.