Low cost photovoltaic modules based on dye sensitized nanocrystalline titanium dioxide and carbon powder

A new type of photovoltaic module based on monolithically series connected dye sensitized photoelectrochemical cells is described. Each solar cell element consists of three porous layers on a transparent conducting substrate, namely a photoelectrode of dye sensitized nanocrystalline TiO₂ (anatase), a spacer of electrically insulating, light reflecting particles of TiO₂ (rutile), and a counterelectrode of graphite powder and carbon black. The pores of these layers are filled with a redox electrolyte containing iodide for hole transport between photo- and counterelectrode. The monolithic series connection on the transparent conducting substrate, e.g., SnO₂ coated glass, is achieved by simple overlap of each carbon counterelectrode with the back contact of the adjacent photoelectrode. Such modules may be produced in a continuous non-vacuum process by simple printing techniques. In this paper we present the first results on energy conversion efficiency and long term stability obtained with this new type of solar cell.     

Formation of a low reflective surface on crystalline silicon solar cells by chemical treatment using Ag electrodes as the catalyst

A new method was developed for making a porous silicon layer as an anti-reflective coating on the top of crystalline silicon solar cells. The porous silicon layer was formed in a mixed solution of H₂O₂ and HF by using screen-printed Ag front electrodes as the catalyst. With the help of the catalytic effect, porous silicon layers were formed by treatment in a solution chemically milder than conventional solutions. The multi-crystalline silicon solar cell covered with the porous silicon layer showed a surface reflectance below 15% in a wavelength region of 400–800 nm.     

Doubled layered ITO/SnO2 conducting glass for substrate of dye-sensitized solar cells

The effects of indium tin oxide (ITO) and ITO/SnO₂ conducting substrates on photovoltaic properties of dye-sensitized solar cells (DSCs) using nanocrystalline TiO₂ were studied. The decrease in fill factor of the DSCs was correlated to the increase in resistance of conducting substrate. The heat stability of ITO conducting glass was improved by depositing SnO₂ on ITO layer. The efficiency of the cells using double layered ITO/SnO₂ substrate remarkably increased comparing with that of the cells using ITO substrates. It is worth mentioning that increasing in sintering time, which enhanced the electronic contact between substrate and TiO₂, also modified the cell performance of MP-TiO₂ cells. Our experimental finding suggests that 3000 Å ITO substrate, which was covered by 1000 Å SnO₂ layer, exhibited the best properties for the DSCs.     

Simulation of a Turbulent Impinging Jet into a Layer of Porous Material Using a Two–Energy Equation Model

This article presents numerical results for a turbulent jet impinging against a flat plane covered with a layer of permeable and thermally conducting material. Distinct energy equations are considered for the solid porous material attached to the wall and for the fluid that impinges on it. Parameters such as Reynolds number, porosity, permeability, thickness, and thermal conductivity of the porous layer are varied in order to analyze their effects on the local distribution of Nu. The macroscopic equations for mass, momentum, and energy are obtained based on volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer eliminates the peak in Nu at the stagnation region. For highly porous and highly permeable material, simulations indicate that the integral heat flux from the wall is enhanced when a thermally conducting porous material is attached to the surface.     

A three-dimensional tin-coated nanoporous copper for lithium-ion battery anodes

Nanoporous copper (NPC), as a new kind of porous metal prepared by dealloying, is introduced into the lithium-ion battery as both the current collector and substrate of active material. The nanoporous copper has three-dimensional structure composed of large channels (hundreds of nanometers) and small pores (tens of nanometers) on the channel walls. Anodes were prepared by electroless depositing of a thin layer of tin on NPC and copper foil. By comparing the electrochemical performance of both electrodes, the nanostructured electrode exhibits much higher areal capacity and better Coulombic efficiency than planar electrode.     

Extremely thin absorber solar cells based on nanostructured semiconductors

Abstract

Extremely thin absorber (eta) solar cells aim to combine the advantages of using very thin, easily and cheaply produced absorber layers on nanostructured substrates with the stability of all-solid-state solar cells using inorganic absorber layers. The concept of using nanostructured substrates originated from the dye-sensitised solar cell, where having a very high surface area allows the use of very thin layers of dye while still absorbing sufficient sunlight. However, both the dye and liquid electrolyte used in these devices demonstrated poor stability, and efforts were made to replace them with very thin inorganic absorber layers and solid state hole collectors respectively. The combination of these concepts – a nanostructured substrate coated with a very thin inorganic absorber and completed with a solid state hole collector – is known as an eta solar cell. This review summarises the development of both the inorganic absorbers and solid state hole collectors in porous TiO₂ and ZnO nanorod based cells, focusing on the material properties and growth/deposition methods. Future possibilities for eta solar cells are discussed, including utilisation of a wider range of materials, synthesis methods and novel materials such as quantum dots to produce tuned band gap and multijunction solar cells.     

Characterization and single cell testing of Pt/C electrodes prepared by electrodeposition

Electrodes for proton exchange membrane fuel cells (PEMFC) have been prepared by the electrodeposition method. For this task, the electrodeposition of platinum is carried out on a carbon black substrate impregnated with an ionomer, proton conducting, medium. Before electrodeposition, the substrate is submitted to an activation process to increase the hydrophilic character of the surface to a few microns depth.Electrodeposition of platinum takes place inside the generated surface hydrophilic layer, resulting in a continuous phase covering totally or partially carbon substrate grains. Cross sectional images show a decay profile of platinum towards the interior of the substrate, reflecting a deposition process limited by diffusion of PtCl₆²⁻ through the porous substrate. Electrodes with different platinum loads have been prepared, and membrane electrode assemblies (MEA) have been mounted with the electrodeposited electrodes as cathode and other standard components (commercial anode and Nafion^R 117 membrane). The electrochemically active surface area determined from hydrogen underpotential deposition charge, is lower on the electrodeposited electrodes than on standard electrodes. However, single cell testing shows higher mass specific activity on electrodeposited cathodes with low and intermediate Pt load (below 0.05 mg Pt cm⁻²).     

First demonstration of photoelectrochemical water splitting by commercial W–Cu powder metallurgy parts converted to highly porous 3D WO3/W skeletons

Hydrogen evolution through photoelectrochemical (PEC) water splitting by tungsten oxide-based photoanodes, as a stable and environmental-friendly material with moderate band gap, has attracted significant interest in recent years. The performance of WO₃ photoanode could be hindered by its poor oxygen evolution reaction kinetics and high charge carrier recombination rate. Additionally, scalable and cost-effective commercial procedure to prepare nanostructured electrodes is still challenging. We present, for the first time, a novel and scalable method to fabricate highly efficient self-supported WO₃/W nanostructured photoanodes from commercial W–Cu powder metallurgy (P/M) parts for water splitting. The electrodes were prepared by electrochemical etching of Cu networks followed by hydrothermal growth of WO₃ nanoflakes. Interconnected channels of W skeleton provided high active surface area for the growth of WO₃ nanoflakes with a thickness of ~40 nm and lateral dimension of ~250 nm. The optimized photoelectrode having 35% interconnected porosity exhibited an impressive current density of 4.36 mA cm⁻² comprising a remarkable photocurrent of 1.71 mA cm⁻² at 1.23 V vs. RHE under 100 mW cm⁻² simulated sunlight. This achievement is amongst the highest reported photocurrents for WO₃ photoelectrodes with tungsten substrate reported so far. Impedance and Mott-Schottky analyses evidenced fast charge transfer, low recombination rate, and accelerated O₂ detachment provided by optimum 3D porous WO₃/W electrode. Due to the nature of the commercial P/M parts and low-temperature hydrothermal processing, the procedure is cost-effective and scalable which can pave a new route for the fabrication of highly porous and efficient water splitting electrodes.     

Patternable solution process for fabrication of flexible polymer solar cells using PDMS

PH 500, a highly conducting poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), is a typical conducting polymer anode material used in organic electric devices. However, it has the disadvantages of low conductivity and poor surface roughness and requires a patterning method for the electrode through including the laser and plasma. In this paper, therefore, the conducting polymer ink for a transparent anode was formulated by adding dimethyl sulfoxide (DMSO) and BYK-333 as the surfactant to enhance the conductivity and surface roughness. The conducting polymer anode was patterned through the application of a new patterning method that used polydimethylsiloxane (PDMS) on a flexible substrate. In addition, a photoactive layer was formed by applying the new patterning method to the conventional brush painting method in which patterning had previously been impossible. The resulting material was compared with the device fabricated by the spin coating method. The fabricated flexible polymer solar cells (PSCs) exhibited short-circuit current density (J_sc), open-circuit voltage (V_oc), fill factor (FF) and power conversion efficiency (PCE) values of 4.2 mA/cm², 0.878 V, 26.5% and 0.98%, respectively, which represented an efficiency improvement of 38% over those fabricated by the spin coating method. Meanwhile, the J_sc value was increased when the series resistance (R_s) decreased to 150 Ω cm².     

SiO2 ceramic nanoporous substrate-reinforced sulfonated poly(arylene ether sulfone) composite membranes for proton exchange membrane fuel cells

Porous substrate-reinforced composite membranes have been extensively investigated due to their promising application to proton exchange membrane fuel cells (PEMFC). In this study, we develop a new ceramic-based reinforcing porous substrate, which consists of hygroscopic silica (SiO₂) nanoparticles interconnected by 3-glycidoxypropyltrimethoxysilane (GPTMS)-based silicate binders and a poly(paraphenylene terephthalamide) (PPTA) nonwoven support. This unusual ceramic substrate is featured with the strong mechanical strength, well-developed nanoporous structure (i.e., nanosized interstitial voids formed between the close-packed SiO₂ nanoparticles), high hydrophilicity, and more notably, good water retention capability. The nanostructured pores of the ceramic substrate are subsequently impregnated with sulfonated poly(arylene ether sulfone) (SPAES, degree of sulfonation = 49.3%). In comparison to a pristine SPAES membrane, the ceramic substrate-reinforced SPAES composite membrane offers the significantly improved dimensional change and also effectively mitigates the steep decline of proton conductivity at low humidity conditions, which is further discussed by considering the state of water in the reinforced composite membrane.     

Fabrication of CuO film with network-like architectures through solution-immersion and their application in lithium ion batteries

A novel CuO negative electrode with network-like architectures was fabricated on copper substrate by a simple solution-immersion step and subsequent heat treatment, which avoids the use of binder and conducting agent that necessary to the conventional electrode-preparation process. The as-prepared CuO electrodes exhibit not only high reversible capacity but also long cycling life, high rate capability in Li ion batteries. The result of this approach creates a new and attractive negative electrode with good electrochemical performance, which is simple, mild conditions, low cost, and easy control. It also opens a pathway for the application of other nanostructured materials of transition metal oxides in lithium ion batteries.     

Electrodeposited nanostructured lead dioxide as a thin film electrode for a lightweight lead-acid battery

Thin films of nanostructured lead dioxide are investigated as a positive electrode material for a lightweight lead-acid battery. The films are obtained by constant current deposition from electrolytes of lead methanesulfonate in methanesulfonic acid. The films are tested in two conditions namely (a) cyclic voltammetry and (b) constant current battery cycling in sulfuric acid. The charge and discharge current density, charge density and charge efficiency are measured as a function of cycle number. The effect of deposition conditions, such as solution temperature (295 and 333 K), type of substrate and electrolyte additive (hexadecyltrimethylammonium hydroxide), on the electrochemical performance of the PbO₂ in sulfuric acid is investigated. It is found that the as-deposited lead dioxide film is compact and nanostructured β-phase structure. Following successive cycling in sulfuric acid, the compact thin film gradually transforms into a porous microstructure consisting of positive active material (PbO₂ and PbSO₄), several tens of nanometres size. The charge density, discharge density and peak discharge current density of the PbO₂ improve with cycling of the thin film electrode.     

Silver coated cathode for molten carbonate fuel cells

Within this study, a layered cathode for use in a Molten Carbonate Fuel Cell (MCFC) has been developed. The substrate layer and reference MCFC cathode made of porous nickel was covered by a porous silver film with defined porosity and pore size. Both layers were fabricated using the tape casting method and further fired in a reductive atmosphere. The new cathode was assembled with other reference cell components to form a single MCFC, which was subjected to performance and durability tests. Scanning electron microscopy was used to analyze the microstructure of the materials before and after tests. The reference cathode was also studied for the comparison.The results show that the porous silver layer was able to enhance the electron transport between the cathode and current collector. It was also found that oxygen reduction is enhanced due to the presence of silver in the gas supply. As a result, the power density of the cell was increased by 50%. On the other hand, due to the separation from electrolyte by the NiO layer, no significant degradation of the silver layer, identified by SEM or electrochemical tests, was found after 1000 h.     

Oxygen evolution at ultrathin nanostructured Ni(OH)2 layers deposited on conducting glass

Ultrathin and transparent nanostructured Ni(OH)₂ films were deposited on conducting glass (F:SnO₂) by a urea-based chemical bath deposition method. By controlling the deposition time, the amount of deposited Ni(OH)₂ was varied over 7 orders of magnitude. The turnover number for O₂ generation, defined as the number of O₂ molecules generated per catalytic site (Ni atom) and per second, increases drastically as the electrocatalyst amount decreases. The electrocatalytic activity of the studied samples (measured as the current density at a certain potential) increases with the amount of deposited Ni(OH)₂ until a saturation value is already obtained for a thin film of around 1 nm in thickness, composed of Ni(OH)₂ nanoplatelets lying flat on the conductive support. The deposition of additional amounts of catalyst generates a porous honeycomb structure that does not improve (only maintains) the electrocatalytic activity. The optimized ultrathin electrodes show a remarkable stability, which indicates that the preparation of highly transparent electrodes, efficient for oxygen evolution, with a minimum amount of nickel is possible.     

Porous silicon in solar cells: A review and a description of its application as an AR coating

In this paper, we first review the potential applications of porous Si in solar cell structures. Then we describe the fabrication of this material by both electrochemical and chemical etching methods, providing some guidelines for a better comprehension of the influence of each process parameter. After that, the properties of porous Si in terms of morphology, structure, photoluminescence, and electroluminescence emissions are summarized together with their actual photovoltaic applications.The results of our study specifically address the creation of an antireflection (AR) coating for polycrystalline Si based solar cells. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, chemical etching method. The formation of a porous Si layer about 0.5 μm thick on the polycrystalline wafer results in an effective reflectance coefficient R_eff lower than 5% in the wavelength region from 350 to 1150 nm. The drastic reduction of the optical losses is controllable by the process parameters and is almost independent of the starting substrate.     

Thin-film crystalline silicon mini-modules using porous Si for layer transfer

Layer transfer processes yield Si films of high electronic quality on low-cost non-Si carriers such as glass: a crystalline Si film grows on a Si–substrate wafer, is detached from that substrate and transferred to a low-cost non-Si device carrier. The substrate wafer is re-used for further growth cycles. Electrochemical etching of a porous Si (PSI) layer system into the substrate wafer enables homoepitaxial growth of monocrystalline Si films and facilitates the detachment of the film. We discuss the potential of crystalline thin-film cells from layer transfer and review the layer transfer work conducted at ZAE Bayern. The sevenfold use of a substrate wafer and the transfer of a 10 × 10 cm² epitaxial film from a 6″-wafer is demonstrated. A new module process that permits an integrated series connection by a single metallization step is demonstrated to yield a module efficiency of 10%.     

TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Fabrication of highly porous Pt coated nanostructured Cu-foam modified copper electrode and its enhanced catalytic ability for hydrogen evolution reaction

The nanoporous copper foam was prepared by electrochemical reduction of copper ion at the copper substrate. The as-prepared substrate was used as three-dimensional templates for preparation of Pt coated nanostructured Cu-foam by galvanic replacement of Cu with platinum by simply immersing the prepared nanoporous copper foam in a K₂PtCl₆ aqueous solution. The structure and nature of the fabricated Pt coated nanostructured Cu-foam was characterized by scanning electron microscopy and energy dispersive X-ray spectrometry. Pt coated nanostructured Cu-foam modified copper electrode exhibited remarkable electrocatalytic activity for the hydrogen evolution reaction. The effect of electrodeposition time during Cu-foam formation on the kinetic constants for hydrogen evolution reaction was comparatively investigated.     

Simulation of laminar impinging jet on a porous medium with a thermal non-equilibrium model

This work shows numerical simulations of an impinging jet on a flat plate covered with a layer of a porous material. Macroscopic equations for mass and momentum are obtained based on the volume-average concept. Two macroscopic models are employed for analyzing energy transport, namely the one-energy equation model, based on the Local Thermal Equilibrium assumption (LTE), and the two-energy equation closure, where distinct transport equations for the fluid and the porous matrix follow the Local Non-Thermal Equilibrium hypothesis (LNTE). The numerical technique employed for discretizing the governing equations was the finite volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure–velocity coupling. Parameters such as porosity, porous layer thickness, material permeability and thermal conductivity ratio were varied in order to analyze their effects on flow and heat transport. Results indicate that for low porosities, low permeabilities, thin porous layers and for high thermal conductivity ratios, a different distribution of local Nusselt number at the wall is calculated depending on the energy model applied. The use of the LNTE model indicates that it is advantageous to use a layer of highly conducting and highly porous material attached to the hot wall.     

Appropriate position of porous insert in a heat exchanger by thermohydraulic analysis

This paper presents numerically an appropriate position of a porous insert to get a better thermohydraulic performance from a porous heat exchanger. The simulation is based on the Darcy‐Brinkman‐Forchheimer model in the porous field. Two‐dimensional continuity, momentum, and energy equations with incompressible, laminar, steady assumptions have been solved using a finite volume approach. The analysis is performed for different values of porous layer thickness, length, and porosity at a fixed value of Reynolds number (Re = 100) and thermal conductivity ratio (Rc = 5). The results showed that there is about a 48% and 13% reduction in pressure drop and Nusselt number, respectively, by decreasing horizontal porous substrate thickness from 1 to 1/2 for δ_v = 1/3 at ε = 0.7. As a result, the pressure drop reduces considerably with a reasonable reduction in heat transfer rate by decreasing horizontal porous substrate thickness from 1 to 1/2.