Micro Solid Oxide Fuel Cells on Glass Ceramic Substrates

Miniaturized solid oxide fuel cells are fabricated on a photostructurable glass ceramic substrate (Foturan) by thin film and micromachining techniques. The anode is a sputtered platinum film and the cathode is made of a spray pyrolysis (SP)‐deposited lanthanum strontium cobalt iron oxide (LSCF), a sputtered platinum film and platinum paste. A single‐layer of yttria‐stabilized zirconia (YSZ) made by pulsed laser deposition (PLD) and a bilayer of PLD–YSZ and SP–YSZ are used as electrolytes. The total thickness of all layers is less than 1 µm and the cell is a free‐standing membrane with a diameter up to 200 µm. The electrolyte resistance and the sum of polarization resistances of the anode and cathode are measured between 400 and 600 °C by impedance spectroscopy and direct current (DC) techniques. The contribution of the electrolyte resistance to the total cell resistance is negligible for all cells. The area‐specific polarization resistance of the electrodes decreases for different cathode materials in the order of Pt paste > sputtered Pt > LSCF. The open circuit voltages (OCVs) of the single‐layer electrolyte cells ranges from 0.91 to 0.56 V at 550 °C. No electronic leakage in the PLD–YSZ electrolyte is found by in‐plane and cross‐plane electrical conductivity measurements and the low OCV is attributed to gas leakage through pinholes in the columnar microstructure of the electrolyte. By using a bilayer electrolyte of PLD–YSZ and SP–YSZ, an OCV of 1.06 V is obtained and the maximum power density reaches 152 mW cm⁻² at 550 °C.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Enhancing the Performance of Solid‐State Dye‐Sensitized Solar Cells Using a Mesoporous Interfacial Titania Layer with a Bragg Stack

High efficiency dye‐sensitized solar cells (DSSCs) are fabricated with a heterostructured photoanode that consists of a 500‐nm‐thick organized mesoporous TiO₂ (om‐TiO₂) interfacial layer (IF layer), a 7 or 10‐μm thick nanocrystalline TiO₂ layer (NC layer), and a 2‐μm‐thick mesoporous Bragg stack (meso‐BS layer) as the bottom, middle and top layers, respectively. An om‐TiO₂ layer with a high porosity, transmittance, and interconnectivity is prepared via a sol‐gel process, in which a poly(vinyl chloride)‐g‐poly(oxyethylene methacrylate) (PVC‐g‐POEM) graft copolymer is used as a structure‐directing agent. The meso‐BS layer with large pores is prepared via alternating deposition of om‐TiO₂ and colloidal SiO₂ (col‐SiO₂) layers. Structure and optical properties (refractive index) of the om‐TiO₂ and meso‐BS layers are studied and the morphology of the heterostructured photoanode is characterized. DSSCs fabricated with the heterostructured IF/NC/BS photoanode and combined with a polymerized ionic liquid (PIL) exhibit an energy conversion efficiencies of 6.6% at 100 mW/cm², one of the highest reported for solid‐state DSSCs and much larger than cells prepared with only a IF/NC layer (6.0%) or a NC layer (4.5%). Improvements in energy conversion efficiency are attributed to the combination of improved light harvesting, decreased resistance at the electrode/electrolyte interface, and excellent electrolyte infiltration.     

Natural Hematite for Next‐Generation Solid Oxide Fuel Cells

Natural hematite ore is used as a novel electrolyte material for advanced solid oxide fuel cells (SOFCs). This hematite‐based system exhibits a maximum power density of 225 mW cm^?2 at 600 °C and reaches 467 mW cm^?2 when the hematite is mixed with perovskite‐structured La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ. These results demonstrate that natural materials for next‐generation SOFCs can influence the multiutilization of natural resources, thereby affecting the environment and energy sustainability.     

An Easily Sintered,Chemically Stable,Barium Zirconate‐Based Proton Conductor for High‐Performance Proton‐Conducting Solid Oxide Fuel Cells

Yttrium and indium co‐doped barium zirconate is investigated to develop a chemically stable and sintering active proton conductor for solid oxide fuel cells (SOFCs). BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} possesses a pure cubic perovskite structure. The sintering activity of BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} increases significantly with In concentration. BaZr_0.8Y_0.15In_0.05O_{3‐ δ} (BZYI5) exhibits the highest total electrical conductivity among the sintered oxides. BZYI5 also retains high chemical stability against CO₂, vapor, and reduction of H₂. The good sintering activity, high conductivity, and chemical stability of BZYI5 facilitate the fabrication of durable SOFCs based on a highly conductive BZYI5 electrolyte film by cost‐effective ceramic processes. Fully dense BZYI5 electrolyte film is successfully prepared on the anode substrate by a facile drop‐coating technique followed by co‐firing at 1400 °C for 5 h in air. The BZYI5 film exhibits one of the highest conductivity among the BaZrO₃‐based electrolyte films with various sintering aids. BZYI5‐based single cells output very encouraging and by far the highest peak power density for BaZrO₃‐based proton‐conducting SOFCs, reaching as high as 379 mW cm^?2 at 700 °C. The results demonstrate that Y and In co‐doping is an effective strategy for exploring sintering active and chemically stable BaZrO₃‐based proton conductors for high performance proton‐conducting SOFCs.     

Properties of Flame Sprayed Ce0.8Gd0.2O1.9‐δ Electrolyte Thin Films

Thin films of Ce_0.8Gd_0.2O_1.9‐δ (CGO) are deposited by flame spray deposition with a deposition rate of about 30 nm min^?1. The films (deposited at 200 °C) are dense, smooth, and particle‐free and show a biphasic amorphous/nanocrystalline microstructure. Isothermal grain growth and microstrain are determined as a function of dwell time and temperature and correlated to the electrical conductivity. CGO films annealed for 10 h at 600 °C present the best electrical conductivity of 0.46 S m^?1 measured at 550 °C. Reasons for the superior performance of films annealed at low temperature over higher‐temperature‐treated samples are discussed and include grain‐size evolution, microstrain relaxation, and chemical decomposition. Nanoindentation measurements are conducted on the CGO thin films as a function of annealing temperature to determine the hardness and elastic modulus of the films for potential application as free‐standing electrolyte membranes in low‐temperature micro‐SOFCs (solid oxide fuel cells).     

High‐Performance,Air‐Stable,Top‐Gate,p‐Channel WSe2 Field‐Effect Transistor with Fluoropolymer Buffer Layer

High‐performance, air‐stable, p‐channel WSe₂ top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al₂O₃ as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe₂, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe₂ FET is realized. The top‐gate p‐channel WSe₂ FET demonstrates a high hole mobility of 100 cm²_­ V^?1 s^?1 and a I_ON/I_OFF ratio > 10⁷ at low gate voltages (V_GS ca. ?4 V) and a drain voltage (V_DS) of ?1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe₂ FETs, which will lead the way toward future electronic nanodevices and their integration.     

Template‐Directed Liquid ALD Growth of TiO2 Nanotube Arrays: Properties and Potential in Photovoltaic Devices

Dense and well‐aligned arrays of TiO₂ nanotubes extending from various substrates are successfully fabricated via a new liquid‐phase atomic layer deposition (LALD) in nanoporous anodic alumina (AAO) templates followed by alumina dissolution. The facile and versatile process circumvents the need for vacuum conditions critical in traditional gas‐phase ALD and yet confers ALD‐like deposition rates of 1.6–2.2 Å cycle^?1, rendering smooth conformal nanotube walls that surpass those achievable by sol–gel and Ti‐anodizing techniques. The nanotube dimensions can be tuned, with most robust structures being 150–400 nm tall, 60–70 nm in diameter with 5–20 nm thick walls. The viability of TiO₂ nanotube arrays deposited on indium tin oxide (ITO)–glass electrodes for application in model hybrid poly(3‐hexylthiophene) (P3HT):TiO₂ solar cells is studied. The results achieved provide platforms and research directions for further advancements.     

One‐Step All‐Solution‐Based Au–GO Core–Shell Nanosphere Active Layers in Nonvolatile ReRAM Devices

Nonvolatile resistive random‐access memory devices based on graphene‐oxide‐wrapped gold nanospheres (AuNS@GO) are fabricated following a one‐step room‐temperature solution‐process approach reported herein for the first time. The effect of the thickness of the GO layer (2, 5, and 7 nm) and the size of the synthesized AuNS (15 and 55 nm) are inspected. Reliable bistable switching is observed in the devices made from a flexible substrate and incorporating 5 and 7 nm thick GO‐wrapped AuNS, sandwiched between two metal electrodes. Current–voltage measurements show bipolar switching behavior with an ON/OFF ratio of 10³ and relatively low operating voltage (?2.5 V). The aforementioned devices unveil remarkable robustness over 100 endurance cycles and a retention of 10³ s. Conversely, a 2 nm thick GO layer is shown to be insufficient to allow current passage from the bottom to the top electrodes. The resistive switching mechanism is demonstrated by space charge trapped limited current due to the AuNS in AuNS@GO matrix. The proposed device and methodology herein applied are expected to be attractive candidates for future generation flexible memory devices.     

CuOx/a‐Si:H heterojunction thin‐film solar cell with an n‐type µc‐Si:H depletion‐assisting layer

We showed that thin n‐type CuO_x films can be deposited by radio‐frequency magnetron reactive sputtering and demonstrated the fabrication of n‐CuO_x/intrinsic hydrogenated amorphous silicon (i‐a‐Si:H) heterojunction solar cells (HSCs) for the first time. A highly n‐doped hydrogenated microcrystalline Si (n‐µc‐Si:H) layer was introduced as a depletion‐assisting layer to further improve the performance of n‐CuO_x/i‐a‐Si:H HSCs. An analysis of the external quantum efficiency and energy‐band diagram showed that the thin depletion‐assisting layer helped establish sufficient depletion and increased the built‐in potential in the n‐CuO_x layer. The fabricated HSC exhibited a high open‐circuit voltage of 0.715 V and an efficiency of 4.79%. Copyright © 2015 John Wiley & Sons, Ltd.     

Single‐Crystalline Films of the Homologous Series InGaO3(ZnO)m Grown by Reactive Solid‐Phase Epitaxy

Single‐crystalline thin films of the homologous series InGaO₃(ZnO)_m (where m is an integer) are fabricated by the reactive solid‐phase epitaxy (R‐SPE) method. Specifically, the role of ZnO as epitaxial initiator layer for the growth mechanism is clarified. High‐temperature annealing of bilayer films consisting of an amorphous InGaO₃(ZnO)₅ layer deposited at room temperature and an epitaxial ZnO layer on yttria‐stabilized zirconia (YSZ) substrate allows for the growth of single‐crystalline film with controlled chemical composition. The epitaxial ZnO thin layer plays an essential role in determining the crystallographic orientation, while the ratio of the thickness of both layers controls the film composition.     

Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr_0.85Y_0.15O_1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.     

Ligand‐Free Synthesis of Aluminum‐Doped Zinc Oxide Nanocrystals and their Use as Optical Spacers in Color‐Tuned Highly Efficient Organic Solar Cells

The color of polymer solar cells using an opaque electrode is given by the reflected light, which depends on the composition and thickness of each layer of the device. Metal‐oxide‐based optical spacers are intensively studied in polymer solar cells aiming to optimize the light absorption. However, the low conductivity of materials such as ZnO and TiO₂ limits the thickness of such optical spacers to tenths of nanometers. A novel synthesis route of cluster‐free Al‐doped ZnO (AZO) nanocrystals (NCs) is presented for solution processing of highly conductive layers without the need of temperature annealing, including thick optical spacers on top of polymer blends. The processing of 80 nm thick optical spacers based on AZO nanocrystal solutions on top of 200 nm thick polymer blend layer is demonstrated leading to improved photocurrent density of 17% compared to solar cells using standard active layers of 90 nm in combination with thin ZnO‐based optical spacers. These AZO NCs also open new opportunities for the processing of high‐efficiency color tuned solar cells. For the first time, it is shown that applying solution‐processed thick optical spacer with polymer blends of different thicknesses can process solar cells of similar efficiency over 7% but of different colors.     

Highly Sensitive Low‐Bandgap Perovskite Photodetectors with Response from Ultraviolet to the Near‐Infrared Region

It is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C₆₀ layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C₆₀ layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under ?0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R ) of over 0.4 A W^?1, high specific detectivity (D* ) of over 10¹² Jones in the near‐infrared region under ?0.2 V. Such highly sensitive broadband response PPDs, which can work well as self‐powered conditions, offer great potential applications in multicolor light detection.     

Dual‐Function Scattering Layer of Submicrometer‐Sized Mesoporous TiO2 Beads for High‐Efficiency Dye‐Sensitized Solar Cells

Submicrometer‐sized (830 ± 40 nm) mesoporous TiO₂ beads are used to form a scattering layer on top of a transparent, 6‐µm‐thick, nanocrystalline TiO₂ film. According to the Mie theory, the large beads scatter light in the region of 600–800 nm. In addition, the mesoporous structure offers a high surface area, 89.1 m² g^?1, which allows high dye loading. The dual functions of light scattering and electrode participation make the mesoporous TiO₂ beads superior candidates for the scattering layer in dye‐sensitized solar cells. A high efficiency of 8.84% was achieved with the mesoporous beads as a scattering layer, compared with an efficiency of 7.87% for the electrode with the scattering layer of 400‐nm TiO₂ of similar thickness.     

A High Power Density Intermediate‐Temperature Solid Oxide Fuel Cell with Thin (La0.9Sr0.1)0.98(Ga0.8Mg0.2)O3‐δ Electrolyte and Nano‐Scale Anode

Solid oxide fuel cells (SOFCs) with thin (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3‐δ (LSGM) electrolytes are primary candidates for achieving high (> 1 W cm^‐2) power density at intermediate (< 650 °C) temperatures. Although high power density LSGM‐electrolyte SOFCs have been reported, it is still necessary to develop a fabrication process suitable for large‐scale manufacturing and to minimize the amount of LSGM used. Here we show that SOFCs made with a novel processing method and a Sr_0.8La_0.2TiO_{3‐ α} (SLT) oxide support can achieve high power density at intermediate temperature. The SLT support is advantageous, especially compared to LSGM supports, because of its low materials cost, electronic conductivity, and good mechanical strength. The novel process is to first co‐fire the ceramic layers – porous SLT support, porous LSGM layer, and dense LSGM layer – followed by infiltration of nano‐scale Ni into the porous layers. Low polarization resistance of 0.188 Ωcm² was achieved at 650 °C for a cell with an optimized anode functional layer (AFL) and an (La,Sr)(Fe,Co)O₃ cathode. Maximum power density reached 1.12 W cm^?2 at 650 °C, limited primarily by cathode polarization and ohmic resistances, so there is considerable potential to further improve the power density.     

Vertically Aligned Nanocomposite Thin Films as a Cathode/Electrolyte Interface Layer for Thin‐Film Solid Oxide Fuel Cells

A thin layer of a vertically aligned nanocomposite (VAN) structure is deposited between the electrolyte, Ce_0.9Gd_0.1O_1.95 (CGO), and the thin‐film cathode layer, La_0.5Sr_0.5CoO₃ (LSCO), of a thin‐film solid‐oxide fuel cell (TFSOFC). The self‐assembled VAN nanostructure contains highly ordered alternating vertical columns of CGO and LSCO formed through a one‐step thin‐film deposition process that uses pulsed laser deposition. The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode. Low cathode polarization resistances of 9 × 10^?4 and 2.39 Ω were measured for the cells with the VAN interlayer at 600 and 400 °C, respectively. Furthermore, anode‐supported single cells with LSCO/CGO VAN interlayer demonstrate maximum power densities of 329, 546, 718, and 812 mW cm^?2 at 550, 600, 650, and 700 °C, respectively, with an open‐circuit voltage (OCV) of 1.13 V at 550 °C. The cells with the interlayer triple the overall power output at 650 °C compared to that achieved with the cells without an interlayer. The binary VAN interlayer could also act as a transition layer that improves adhesion and relieves both thermal stress and lattice strain between the cathode and the electrolyte.     

Surface‐Modified Low‐Temperature Solid Oxide Fuel Cell

This paper reports both experimental and theoretical results of the role of surface modification on the oxygen reduction reaction in low‐temperature solid oxide fuel cells (LT‐SOFC). Epitaxial ultrathin films of yttria‐doped ceria (YDC) cathode interlayers (<10–130 nm) are grown by pulsed laser deposition (PLD) on single‐crystalline YSZ(100). Fuel cell current–voltage measurements and electrochemical impedance spectroscopy are performed in the temperature range of 350 °C ≈ 450 °C. Quantum mechanical simulations of oxygen incorporation energetics support the experimental results and indicate a low activation energy of only 0.07 eV for YDC, while the incorporation reaction on YSZ is activated by a significantly higher energy barrier of 0.38 eV. Due to enhanced oxygen incorporation at the modified Pt/YDC interface, the cathodic interface resistance is reduced by two‐fold, while fuel cell performance shows more than a two‐fold enhancement with the addition of an ultrathin YDC interlayer at the cathode side of an SOFC element. The results of this study open up opportunities for improving cell performance, particularly of LT‐SOFCs by adopting surface modification of YSZ surface with catalytically superior, ultrathin cathodic interlayers.     

The electrical and physical analysis of Pt gate/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/p-Si (100) with dual high-k gate oxide thickness for deep submicron complementary metal-oxide-semiconductor device with low power and high reliability

In order to examine the electrical and physical properties of Al₂O₃ layers with dual thickness on a chip, Pt gate/Al₂O₃ with dual thickness/p-type Si (100) samples were fabricated using atomic-layer deposition, separation photolithography, and 100:1 HF wet etching to remove the first Al₂O₃ layer. Dual metal-oxide-semiconductor (MOS) capacitors with thin (physical thickness, ∼4.5 nm, equivalent oxide thicknesses (EOT): 2.8 nm) and thick (physical thickness, ∼8.2 nm, EOT: 4.3 nm) Al₂O₃ layers showed a good leakage current density of −5.4×10⁻⁶ A/cm² and −2.5×10⁻⁹ A/cm² at −1 V, respectively; good reliability characteristics as a result of the good surface roughness; low capacitance versus voltage measurements (C-V) hysteresis; and a good interface state density (∼7×10¹⁰ cm⁻²eV⁻¹ near the midgap) as a result of pre-rapid thermal annealing (pre-RTA) after depositing the Al₂O₃ layer compared with the single MOS capacitors without the pre-RTA. These results suggest that dual Al₂O₃ layers using the dual gate oxide (DGOX) process can be used for the simultaneous integration of the low power transistors with a thin Al₂O₃ layer and high reliability regions with a thick Al₂O₃ layer.     

Gas Sensing of NiO‐SCCNT Core–Shell Heterostructures: Optimization by Radial Modulation of the Hole‐Accumulation Layer

Hierarchical core–shell (C–S) heterostructures composed of a NiO shell deposited onto stacked‐cup carbon nanotubes (SCCNTs) are synthesized by atomic layer deposition (ALD). A film of NiO particles (0.80–21.8 nm in thickness) is uniformly deposited onto the inner and outer walls of the SCCNTs. The electrical resistance of the samples is found to increase of many orders of magnitude with the increasing of the NiO thickness. The response of NiO–SCCNT sensors toward low concentrations of acetone and ethanol at 200 °C is studied. The sensing mechanism is based on the modulation of the hole‐accumulation region in the NiO shell layer upon chemisorption of the reducing gas molecules. The electrical conduction mechanism is further studied by the incorporation of an Al₂O₃ dielectric layer at NiO and SCCNT interfaces. The investigations on NiO–Al₂O₃–SCCNT, Al₂O₃–SCCNT, and NiO–SCCNT coaxial heterostructures reveal that the sensing mechanism is strictly related to the NiO shell layer. The remarkable performance of the NiO–SCCNT sensors toward acetone and ethanol benefits from the conformal coating by ALD, large surface area of the SCCNTs, and the optimized p‐NiO shell layer thickness followed by the radial modulation of the space‐charge region.