Formation,rapid thermal oxidation and passivation of solar grade silicon nanowires for advanced photovoltaic applications

Uniform and regular silicon nanowires (SiNWs) arrays are fabricated on both sides of solar grade silicons (SiGS) by silver assist-electrochemical etching. SiNWs arrays exhibit an excellent antireflection character with an overall reflectance of 2% in the range from 300 to 1000 nm. More importantly, the effective lifetimes of the symmetric SiNWs/Si structures decreased due to the high densities of dangling bonds and surface defects. Surface passivation to overcome lifetime degradation is realized by means of rapid thermal oxidation (RTO). Following rapid oxidation, Fourier Transform Infrared spectroscopy reveals that oxygen diffusion is enhanced inside silicon nanowires where the morphological structure is preserved during RTO. Moreover, it is shown that even the rapid thermal oxidation process is not effective to recover initial τ_eff due to the high density of imperfections involved during nanowires formation and the contamination level induced by silver. The interdiffusion between residual silver and metal contaminants in the core of the nanowire can probably limit the passivation effect due to the segregation of metal atoms at SiO₂ and to the redistribution of both impurities across the wire.     

Structure and optical properties of ZnO nanowires coated with silica

Abstract

Abstract

It is essential to passivate one-dimensional nanostructures with insulating materials to protect them from contamination and oxidation as well as to avoid cross-talking between the building blocks of complex nanoscale circuits. The ZnO nanowires synthesised by the thermal evaporation of ZnO powders were coated with SiO₂ by the sputtering technique. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the ZnO core–SiO₂ shell nanowires were single crystal wurtzite type ZnO and amorphous SiO₂ respectively. Photoluminescence measurements at room temperature showed that the passivation of the ZnO nanowires was successfully achieved with SiO₂ without nearly degrading the near band edge emission from the wires. However, subsequent thermal annealing treatment was found to be undesirable owing to the degradation of the near band edge emission in intensity.     

Structural and electrochemical study of the reaction of lithium with silicon nanowires

The structural transformations of silicon nanowires when cycled against lithium were evaluated using electrochemical potential spectroscopy and galvanostatic cycling. During the charge, the nanowires alloy with lithium to form an amorphous Li_xSi compound. At potentials <50 mV, a structural transformation occurs. In studies on micron-sized particles previously reported in the literature, this transformation is a crystallization to a metastable Li₁₅Si₄ phase. X-ray diffraction measurements on the Si nanowires, however, show that they are amorphous, suggesting that a different amorphous phase (Li_ySi) is formed. Lithium is removed from this phase in the discharge to form amorphous silicon. We have found that limiting the voltage in the charge to 70 mV results in improved efficiency and cyclability compared to charging to 10 mV. This improvement is due to the suppression of the transformation at low potentials, which alloys for reversible cycling of amorphous silicon nanowires.     

Nano-sized SiOx/C composite anode for lithium ion batteries

Nano-sized SiO_x/C composite with core-shell structure is prepared by a modified Stöber method. After heat-treatment, the O/Si ratio in SiO_x/C composite is near 1 and the core of SiO_x presents a structure composing of amorphous Si clusters and ordered SiO₂ domains. SiO_x/C composite anode shows high specific capacity (ca. 800 mAh g⁻¹), excellent cycling stability, good rate-capability but low initial coulombic efficiency. Li₂O and Li₄SiO₄ may generate in the initial lithiation process, which, combining with the carbon shell, can buffer the volume change caused by the alloying of Si with Li, and thereby improving the cycling stability of electrode. The nano feature of SiO_x/C particle and the electronic conductive nature of carbon coating layer ensure the good rate-capability of SiO_x/C electrode.     

Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires

In this work, Mg doped zinc oxide (Mg_xZn_1−xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV–Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg_xZn_1−xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H₂ at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed.     

Fabrication and electrochemical characterization of a vertical array of MnO2 nanowires grown on silicon substrates as a cathode material for lithium rechargeable batteries

A complementary metal-oxide-semiconductor (CMOS) compatible process for fabricating on-chip microbatteries based on nanostructures has been developed by growing manganese dioxide nanowires on silicon dioxide (SiO₂)/silicon (Si) substrate as a cathode material for lithium rechargeable batteries. High aspect-ratio anodized aluminum oxide (AAO) template integrated on SiO₂/Si substrates can be exploited for fabrication of a vertical array of nanowires having high surface area. The electrolytic manganese dioxide (EMD) nanowires are galvanostatically synthesized by direct current (dc) electrodeposition. The microstructure of these nanowire arrays is investigated by scanning electron microscopy and X-ray diffraction. Their electrochemical tests show that the discharge capacity of about 150 mAh g⁻¹ is maintained during a few cycles at the high discharge/charge rate of 300 mA g⁻¹.     

Field effect surface passivation of SiO2/Si interfaces by heat treatment with high-pressure H2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H₂O vapor heating. Heat treatment with 2.1×10⁶ Pa H₂O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO_x film/Si. Additional deposition of 140 nm-SiO_x films (x<2) with a high density of fixed positive charges on the SiO₂/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO₂. Additional deposition of 210 nm SiO_x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO_x film and the heat treatment with high-pressure H₂O vapor is effective for low-temperature passivation of the silicon surface.     

WO3−x nanowires based electrochromic devices

A simple method was developed to fabricate tungsten oxide (WO_3−x) nanowires based electrochromic devices. The WO_3−x nanowires are grown directly from tungsten oxide powders in a tube furnace. The WO_3−x nanowires have diameters ranging from 30 to 70 nm and lengths up to several micrometers. The WO_3−x nanowires based device has short bleach-coloration transition time and can be grown on a large scale directly onto an ITO-coated glass that makes it potential in many electrochromic applications. The structure, morphology, and composition of the WO_3−x nanowires were characterized using the scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive spectrometer. The optical and electrochromic performance of the nanowires layer under lithium intercalation was studied in detail by UV–VIS–NIR spectroscope and cyclic voltameter.     

Manganese oxide with different morphology as efficient electrocatalyst for oxygen evolution reaction

Three-dimensional (3D) manganese oxides consisted of tetragonal phase Mn₃O₄ and α-MnO₂ with different morphology have been directly grown vertically on Ti foil by a simple electrochemical method without any template and used as the catalysts for oxygen evolution reaction (OER). The results show that manganese oxides with different morphology show high activity and good stability for OER and the manganese oxide (MnO_x) nanowire arrays obtained at 70 °C show higher activity and better stability than MnO_x with cotton wool structure and MnO_x nanosheet arrays.     

Full process for integrating silicon nanowire arrays into solar cells

A novel process was developed for integrating silicon nanowire arrays into solar cells. n-Type silicon nanowires were grown by chemical-vapour deposition via the gold-catalysed vapour-liquid-solid method, on a p-type silicon substrate. After the growth, the nanowire array was planarized, by embedding the nanowires in a spin-on glass matrix and subsequent chemical-mechanical polishing of the front surface. This planarization step allows to deposit a continuous and uniform conductive film on top of the nanowire array, and thus to form a high-quality front electrical contact. For an illumination intensity of 100 mW/cm², our devices exhibit an energy conversion efficiency of 1.9%. The main performance limiting factor is a high pn junction reverse current, due to contamination by the growth catalyst or to a lack of passivation of surface electronic defects.     

Performance of superstrate multijunction amorphous silicon-based solar cells using optical layers for current management

The performance of multijunction amorphous silicon-based thin film solar cells has been reported using thin layers of TiO₂ and SiO_x acting as refractive index matching optical layers for different interfaces of the superstrate device structure. Improvement of short-circuit current from the sub-cells of a-Si/μc-Si cells is demonstrated with TiO₂ as anti-reflection layer at TCO/Si interface and SiO_x as intermediate-reflector layer between two sub-cells. An initial efficiency of 11.8% is achieved by applying both the TiO₂ and SiO_x optical layers in a-Si/μc-Si solar cell.     

Enhanced electrocatalytic performance of NiOx@MnOx@graphene for oxygen reduction and evolution reactions

Oxygen electrodes for oxygen evolution reaction and oxygen reduction reaction were intensively investigated due to high overpotential required to drive the four-electron process. A NiOx@MnOx@G nanostructure supported homogenously on graphene nanosheets through an easy and scalable self-assembly method was studied. The NiOx@MnOx@G exhibited a nanostructured NiO_x nanocrystalline with size around 2.3 nm and an amorphous MnO_x with controllable thickness. The 25% NiOx@MnOx@G showed remarkable activity for oxygen reduction reaction with a 4-electron process, the half-wave potential was 50 mV, but the stability of 25% NiOx@MnOx@G was better than Pt/C-JM. NiOx@MnOx@G nanostructure exhibited significantly better activity for oxygen evolution reaction compared with MnO_x, which can demonstrate that NiO_x could tune the activity of surface amorphous MnO_x and dramatically increased oxygen evolution reaction activity. NiOx@MnOx@G is demonstrated with superior oxygen catalysts performance for reversible oxygen evolution and oxygen reduction reaction, due to the synergistic effect of NiO_x and amorphous MnO_x.     

Fast and highly-sensitive hydrogen sensing of Nb2O5 nanowires at room temperature

The single-crystalline Nb₂O₅ nanowires with tetragonal phase structures were synthesized through thermal oxidation process. The Nb₂O₅ nanowires were grown along [001] orientation and formed a layer of free-standing nanowire membrane. A pair of platinum electrodes was deposited on the surface of the nanowire layer to fabricate a Pt/Nb₂O₅ nanowire hydrogen sensor. The Pt/Nb₂O₅ nanowire hydrogen sensor exhibited fast, highly-sensitive and selective hydrogen response at room temperature, which may be attributed to the hydrogen induced interface and surface effects together with the high specific surface area of the Nb₂O₅ nanowires.     

Highly stable and effective Pt/carbon nitride (CNx) modified SiO2 electrocatalyst for oxygen reduction reaction

We report a durable and active electrocatalyst, Pt/carbon nitride (CN_x) modified silicon dioxide (SiO₂) composite (donated as CN_x/SiO₂), for oxygen reduction reaction (ORR). CN_x/SiO₂ composite is synthesized by calcination of polypyrrole coated SiO₂ (Ppy/SiO₂) at 800 °C. The structure and composition are assessed using Fourier transform infra-red spectroscopy, X-ray diffraction, transmission electron microscope and energy dispersive spectroscopy. Voltammetry is used to study the activities of Pt immobilized on Vulcan XC-72R and CN_x/SiO₂, respectively. The electrochemical data indicate that Pt supported on CN_x/SiO₂ possesses higher electrocatalytic activity and durability for ORR compared with those of Vulcan XC-72R. All these demonstrate that CN_x/SiO₂ is a promising ORR electrocatalyst support for low temperature fuel cells.     

Performance of p-type silicon-oxide windows in amorphous silicon solar cell

a-SiO_x films have been prepared using silane and pure oxygen as reactive gases in plasma CVD system. Diborane was introduced as a doping gas to obtain p-type conduction silicon oxide. Infrared absorption spectra show the incorporation of Si–O stretch mode around 1000 cm⁻¹. The optical bandgap increases with the oxygen to silane gas ratio, while the electrical conductivity decreases. Hydrogenated amorphous silicon solar cells have been fabricated using p-type a-SiO_x with around 1.85 eV optical bandgap and conductivity greater than 10⁻⁷ S/cm. The measured current–voltage characteristics of the solar cells under 100 mW/cm² artificial light are V_oc=0.84 V, J_sc=14.7 mA/cm², FF=0.635 with a conversion efficiency of 7.84%.     

Construction of three-dimensional hierarchical Pt/TiO2@C nanowires with enhanced methanol oxidation properties

Three-dimensional (3D) hierarchical Pt/TiO₂@C core-shell nanowire networks with high surface area have been constructed via wet chemical approaches. The 3D TiO₂ nanowire framework was in situ synthesized within a porous titanium foam by hydrothermal method followed by carbon coating and self-assembled growth of ultrathin Pt nanowires. Structural characterization indicates that single crystalline ultrathin Pt nanowires of 3–5 nm in diameter were vertically distributed on the anatase TiO₂ nanowires covered with a 2–4 nm thin carbon layer. The 3D hierarchical Pt/TiO₂@C nanostructure demonstrates evidently higher catalytic activities towards methanol oxidation than the commercial Pt/C catalyst. The catalytic current density of the hierarchical catalyst is 1.6 times as high as that of the commercial Pt/C, and the oxidation onset potential (0.35 V vs. Ag/AgCl) is more negative than the commercial one (0.46 V vs. Ag/AgCl). Synergistic effect between the ultrathin Pt nanowires and the TiO₂@C core-shell nanostructure accounts for the enhanced catalytic properties, which can be determined by X-ray photoelectron spectroscopy (XPS) investigation. The obtained hierarchical Pt/TiO₂@C nanowire networks promise great potential in producing anode catalysts for direct methanol fuel cells applications.     

Fine cathode particles prepared by solid-state reaction method using nano-sized precursor particles

Fine-sized Li–Co–Mn–O cathode particles with various ratios of cobalt and manganese components were prepared by conventional solid-state reaction method using the nano-sized precursor particles. The nano-sized precursor particles of cobalt and manganese components were prepared by spray pyrolysis. The LiCo_1−xMn_xO₂ (0.1 ≤ x ≤ 0.3) particles had finer size than that of the pure LiCoO₂ particles. Manganese component disturbed the growth of the LiCo_1−xMn_xO₂ cathode particles prepared by solid-state reaction method. The initial discharge capacities of the layered LiCo_1−xMn_xO₂ (0 ≤ x ≤ 0.3) cathode particles decreased from 144 to 136 mAh g⁻¹ when the ratios of Co/Mn components were changed from 1/0 to 0.7/0.3. The mean sizes of the spinel LiMn_2−yCo_yO₄ (0 ≤ y ≤ 0.2) cathode particles decreased from 650 to 460 nm when the ratios of Mn/Co components were changed from 2/0 to 1.8/0.2. The initial discharge capacities of the LiMn_2−yCo_yO₄ (0 ≤ y ≤ 0.2) cathode particles decreased from 119 to 86 mAh g⁻¹ when the ratios of Mn/Co components were changed from 2/0 to 1.8/0.2.     

Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–Y alloys

Ti₄₅Zr₃₀Ni₂₅Y_x (x = 1, 3, 5 and 7) alloys were prepared by melt-spinning at wheel velocity of 20 m s⁻¹. The effect of additive Y on phase structure and electrochemical performance of melt-spun alloys was investigated. Ti₄₅Zr₃₀Ni₂₅Y_x melt-spun alloys were composed of I-phase and amorphous phase. The amorphous phase increased with increasing x value, indicating amorphous forming ability improved with increasing Y content. The maximum discharge capacity and high-rate dischargeability decreased with increasing x value, which may be ascribed to the decrease of nickel content. Cycling stability first increased with increasing x from 1 to 3, and then decreased when x increased to 7, which was resulted from the combined effect of the decrease of nickel content and the increase of amorphous phase.     

SiOx(C)/SiNx dual-layer anti-reflectance film coating for improved cell efficiency

Light-induced plating (LIP), in which the current driving the metal reduction process is derived from illuminated solar cells, is an attractive technique for solar cell metallization because of its potential simplicity. However, applying the LIP techniques on standard acidic-textured multicrystalline silicon wafers with a silicon nitride-coated surface presents a challenge. The use of a spray-on carbon-doped non-stoichiometric silicon oxide [SiO_x(C)] dielectric film before nickel and silver plating can greatly reduce background plating while helping decrease the reflectance on the front of silicon solar cells. The sprayed dielectric films have low refractive indices of 1.3–1.4, depending on the annealing temperature. Simulation studies show that the SiO_x(C)/SiN_x dual-layer anti-reflective coating has a lower weighted reflectance against an AM 1.5 G spectrum compared with the SiN_x single coating. Finally, the performance of the laser-doped solar cells with a standard SiN_x as an anti-reflectance coating were compared with those with the SiO_x(C)/SiN_x double-layer stack. An efficiency of 16.74% on a large, commercial-grade, p-type, multicrystalline silicon substrate was achieved.     

Oxygen evolution reaction on Ni-substituted Co3O4 nanowire array electrodes

Nanowire arrays of mixed oxides of Co and Ni freely standing on Ni foam are prepared by a template-free growth method. The effects of Ni content on the morphology, structure and catalyst performance for oxygen evolution reaction are investigated by scanning electron microscopy, X-ray diffraction spectroscopy and electrochemical techniques including cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. A transformation from nanowire arrays to nanoplate arrays is found with the increase of the atomic ratio of Ni to Co in the preparation solution. The Ni_xCo_3−xO₄ electrode obtained at 1:1 of Ni:Co in the preparation solution exhibits nanowire array structure and has better catalytic performance for oxygen evolution reaction than other Ni_xCo_3−xO₄ and Co₃O₄ electrodes. The catalytic activities of the Ni_xCo_3−xO₄ and Co₃O₄ electrodes are correlated with their surface roughness. Superior stability of the Ni_xCo_3−xO₄ nanowire array electrode is demonstrated by a chronopotentiometric test. The reaction orders with respect to OH⁻ on the Ni_xCo_3−xO₄ electrode are close to 2 and 1 at low and high overpotentials, respectively.