Properties Of Amorphous Silicon Research Articles

A Parametric Study of Coupled Photo-Electro-Thermal Responses of Thin Film a-Si:H Solar Cell with Embedded Nanoparticles

A parametric investigation has been performed on a thin-film hydrogenated amorphous silicon (a-Si:H) solar cell that is enhanced with various light trapping schemes through a modelling approach. The proposed model contains a novel coupling approach and various feedback routines for a more holistic modelling treatment. The proposed optical model adopts a semi-coherent method, the electrical model extends the classical drift-diffusion model to incorporate the effects of thermal gradients, and the thermal model adopts energy conservation equations from the hydrodynamic model. Based on the simulation results, it is observed that the rise in cell temperature adversely affects the electrical performance but promotes more optical absorptions due to the unique optical properties of amorphous silicon. To obtain an optimum enhancement from the inclusion of nanoparticles, their dimensions and separation distances are essential factors. The thickness of the intrinsic active absorbing layer affects the optical performance directly which then leads to various variations in electrical and thermal responses.

Irradiation Effect on Al-Dispersed Amorphous Silicon Thin Films by Femtosecond Laser

The structural and optical properties of amorphous silicon (a-Si) and Al-dispersed amorphous silicon (a-Si:Al) thin films irradiated by femtosecond (fs) laser at various energy densities are investigated comparatively in this article. It is found that there is an uneven crystallization in both amorphous thin films by means of optical microscopy and laser Raman spectroscopy respectively. The crystallization in each pulse spot area is gradually weakened from the center to the edge along with the energy dispersion of laser irradiation. The laser induced crystallization in a-Si thin films begins early and develops more extensively compared to that in a-Si:Al thin films, and Al nanoparticles inhibit somehow the crystallization of a-Si in a-Si:Al thin films.

Effect of Collector Roughness on Properties of Amorphous Silicon Thin‐Film Anodes

AbstractUniform silicon films about 500 nm thick for lithium‐ion batteries on copper foil current collectors were prepared using a magnetron sputtering method. We discuss the surface morphology and crystal structure of the silicon films, emphasizing how the current collector affects the performance of the silicon film anode. XRD and Raman analysis demonstrate that both films are amorphous. However, the silicon film deposited onto the matte copper foil shows better cycle behavior, it can be discharged at 0.5 C with 84.1 % capacity retention about 1470 mAhg−1 after 100 cycles.

Analysis Method for the Uniformity of a-Si-Based Multijunction Thin-Film Solar Module Using Ideality Factors

A methodology was developed to analyze the uniformity properties of amorphous silicon (a-Si) based triple-junction thin-film solar module for power stations or for building-integrated photovoltaic using circuit model parameters. First, a-Si-based triple-junction thin-film solar module with a size of 5G was fabricated via the mass production method. The module was divided into small modules with ten solar cells. A total of nine samples were selected and the dark I–V characteristics for each sample were measured. A circuit model for a triple-junction solar module was proposed. One sample that could represent the triple-junction properties was selected and the circuit parameters were extracted. The circuit model with the extracted parameters was simulated using H-spice and showed good agreement between experimental and simulation results. Ideality factors and reverse saturation currents from the selected samples were extracted from the dark IV curve and resistance parameters by using the numerical method with the circuit model. The distribution of each parameter for the samples and the uniformity of the large-size module were analyzed.

Quantifying Electrochemical Reactions and Properties of Amorphous Silicon in a Conventional Lithium-Ion Battery Configuration

Despite many existing studies on silicon (Si) anodes for lithium ion batteries (LIBs), many essential questions still exist on compound formation, composition, and properties. Here we show that some previously accepted findings may have limitations in reflecting the lithiation mechanisms in the conventional charging rate. Furthermore, the correlation between structure and mechanical properties in these materials has not been properly established. Here we report a rigorous and thorough study to comprehensively understand the electrochemical reaction mechanisms of amorphous-Si (a-Si) in a conventional charging rate. In-depth microstructural characterization was performed, and correlations were established between Li–Si composition, volumetric expansion, and modulus/hardness. We have found that the lithiation process of a-Si at a conventional charging rate is a single-phase reaction while it is a two-phase reaction at high rate in in situ TEM experiments. The findings in this paper establish a reference to quantitatively explain many key metrics for lithiated a-Si as anodes in real LIBs and can be used to rationally design a-Si based high-performance LIBs guided by high-fidelity modeling and simulations.

Gap tuning and effective electron correlation energy in amorphous silicon: A first principles density functional theory-based molecular dynamics study

Abstract First principles density functional theory (DFT)-based molecular dynamics (MD) is used to study some physical and electronic properties of amorphous silicon (a-Si) samples, as-quenched and annealed containing dangling and floating bonds (pertinent to the threefold- and fivefold-coordinated defects, respectively) as well as distorted tetrahedral bonds. Surprisingly, except for the work of Pantelides (1986) who gave a rough estimate for the effective electron correlation energy, U eff of a floating bond on the fivefold-coordinated Si, to date, there are no theoretical studies in the literature for the calculation of U eff pertinent to this type of defect. In this work, U eff for each type of defect, namely, threefold- and fivefold-coordinated atoms which are present in our generated annealed a-Si sample at 300 K is calculated by the current ab initio framework. We found that, U eff for the fivefold-coordinated Si varies from + 0.32 to + 0.41 eV , whereas for the threefold-coordinated Si it ranges between - 0.33 to + 0.04 eV . The electronic, optoelectronic, and transport properties of a-Si semiconductors are directly influenced by gap tuning which in turn is controlled by the applied strains. The effects of temperature and strain on the mobility gap and the electronic density of states (DOS) for the a-Si samples are of particular interest. For the unstrained as-quenched and annealed samples at T = 0 K , the mobility gap is calculated to be equal to 1.42 and 1.47 eV , respectively; whereas, at T = 300 K these values change to 1.17 and 1.24 eV , respectively. At T = 0 K , for both samples under the uniaxial tensile strains below 0.070 , the calculated mobility gap is about 1.4 eV which sharply decreases by applying strain beyond 0.070 . As it will be seen, the gap regions for both the unstrained sample and the strained sample with ∊ 33 = 0.070 contain midgap states, but for the strained samples with the higher strains of ∊ 33 = 0.140 and 0.210 the midgap states disappear.

Study on the electromagnetic properties of thin-film solar cell grown with graphene using FDFD method

In recent years amorphous silicon solar cells have been receiving a great deal of interest due to their high energy conversion efficiency and low cost. The positions of absorption peak reflect the good absorption performance at specific frequency point or nearby spectra. However, the absorption peaks of amorphous silicon solar cell which are mainly determined by the properties of amorphous silicon and metal electrode, cannot be tuned. And the absorption efficiency can not be further enhanced also. Therefore, monolayer graphene film will be employed in the solar cells with periodic structure due to its remarkable electro-optic properties. With a suitable chemical potential applied, the dielectric constant of graphene can be tuned. This design mainly aims to tune the position of the absorption peak based on the graphene by using finite-difference frequency-domain method. Also, an approximate fitted function is developed in order to overcome the singularity in the exact expression. Numerical results show that the approximate closed form expression generates results within a maximum absolute error of 0.8%. Theoretical results provide the realistic organic thin-film solar cells with theoretical basis and technical support.

Optical Properties of Amorphous Silicon–Carbon Alloys (a-Si x C1-x ) Deposited by RF Co-Sputtering

Amorphous silicon–carbon alloy (a-SixC1-x) thin films have been deposited by radio frequency (RF) sputter deposition. These films were obtained, from a composite target consisting of silicon fragments regularly distributed on the surface of a pure graphite disc, for different values of silicon surface fraction RSi/C, at an RF power of 250 W. X-ray diffraction diagrams show that all the as-deposited SixC1-x thin films are amorphous. The optical properties of the a-SixC1-x films were investigated by optical transmission measurements in the ultraviolet–visible–near infrared wavelengths range. The optical band gap Eg varies, with RSi/C, from 1.4 to 1.9 eV and presents a wide maximum at an RSi/C value of about 35 %. This maximum value is attributed to amorphous silicon carbide a-SiC as confirmed by theoretical correlation between the molar fraction x and RSi/C. The refractive index n follows well the Cauchy law and the extrapolated value, at infinite wavelengths, increases from 2.1 to 2.65 as the RSi/C fraction increases.

Influence of electrodes' distance upon properties of intrinsic and doped amorphous silicon films for heterojunction solar cells

In this work, the influence of electrodes' distance upon the properties of amorphous silicon (a‐Si:H) deposited by plasma‐enhanced chemical vapor deposition method on both intrinsic and doped a‐Si:H films is investigated in terms of their electrical, optical, and structural characteristics. For this purpose, Fourier‐transform infra‐red and secondary‐ion mass‐spectroscopy as well as photoconductance decay, spectral ellipsometry, and conductivity measurements are employed. Electrodes' distance is varied from 20 to 120 mm. Regarding the passivation quality of the a‐Si:H film an optimum electrodes' distance of 60 mm is found. In addition, electrodes' distance is detected to have a great influence on the accelerated initial growth rate, which strongly diminishes with increasing distance. Thus, electrodes' distance also determines the overall thickness of thin intrinsic a‐Si:H films being particularly interesting for utilization in heterojunction solar cells. With doped a‐Si:H films, however, electrodes' distance influences mainly the dopant concentration in the films and therewith their conductivity.

氢稀释对FTS沉积a-Si∶H薄膜结构特性的影响

Hydrogenated amorphous silicon films were deposited by facing target sputtering with different hydrogen dilution ratio with H2 as reaction gas.The growth rate and the optical properties of a-Si∶H films with different hydrogen dilution ratio were characterized by surface profiler,Fourier-transform infrared spectroscopy,Raman scattering and ultraviolet-visible transmittance spectrum.It was found that,the low temperature and high deposition of a-Si∶H films can be deposited by facing target sputtering method.With increasing hydrogen dilution,a trend that the deposition rate of a-Si∶H films first decreases and then increases is showed.Fourier transform infrared transmission spectra show that hydrogen content of a-Si∶H films increases first and then becomes smaller.While,through analysising the Raman spectra and UV-visible light transmission spectrum,the degree of order and the optical band gap also first increase and then decrease.Obviously,the film structure can be effectively controlled by the change of the hydrogen dilution with this technology.

Substrate-plasma interaction during amorphous silicon thin films growth by sputtering technique

The present paper deals with the investigation of Argon ions – substrate interactions during film growth and their influence on sputtered hydrogenated amorphous silicon (a-Si:H) thin films structural properties. These interactions are characterized by mean of the calculation of the energy distribution of Ar ions striking the substrate and their striking force measurement in the case of Rf diode sputtering. The influence of the Ar ions bombardment on structural and physical properties of amorphous silicon thin properties is discussed. The ion bombardment affects the film growth processes and consequently, it causes films densification and evolution of film microstructure from amorphous state at low power towards a microcrystalline material with increasing the Rf power.

Improving stability of amorphous silicon using chemical annealing with helium

We report on the growth and properties of amorphous silicon (a-Si:H) materials and devices prepared using a chemical annealing technique. The chemical annealing technique consists of a layer-by-layer growth, where a thin a-Si:H layer is followed by annealing in a plasma beam of inert ions (helium). We show that high quality materials and devices can be fabricated using chemical annealing with helium. Infrared absorption data show that chemical annealing significantly improves the hydrogen microstructure and this improved hydrogen microstructure leads to improved stability of both materials and devices.

Tight-binding study of structure and vibrations of amorphous silicon

We present a tight-binding calculation that, for the first time, accurately describes the structural, vibrational and elastic properties of amorphous silicon. We compute the interatomic force constants and find an unphysical feature of the Stillinger-Weber empirical potential that correlates with a much noted error in the radial distribution function associated with that potential. We also find that the intrinsic first peak of the radial distribution function is asymmetric, contrary to usual assumptions made in the analysis of diffraction data. We use our results for the normal mode frequencies and polarization vectors to obtain the zero-point broadening effect on the radial distribution function, enabling us to directly compare theory and a high resolution x-ray diffraction experiment.

The formation and properties of amorphous silicon as negative electrode reactant in lithium systems

Essentially all of the present commercial rechargeable lithium batteries use lithium-carbons as the negative electrode reactant. However, the announcement of Fujifilm in 1997 of the potential use of convertible oxides called attention to the possibility of alternatives, and a variety of different materials and approaches has been investigated in an number of laboratories. Recent work on the potential of the use of metal–metalloid alloys has led to the recognition of several potentially attractive possibilities. Among the most interesting are a group of materials containing silicon, a boro-silicide, several silicides, and SiO. In all of these cases an irreversible reaction takes place during the first lithium loading. The result is the formation of fine particles of amorphous elemental silicon in a matrix related to the precursor. Upon further cycling, lithium reversibly reacts with this amorphous silicon. The resulting specific capacities can reach attractive values, depending upon the weight of the precursor. Their relatively flat potential profiles at low potentials make them of interest for application as negative electrodes.

Numerical Studies Of The Dynamics Of Silicon: Relaxation, Nucleation And Energy Landscape

AbstractUsing various simulation techniques, such as molecular dynamics and the activation-relaxation technique, we are slowly developing a consistent picture of the dynamical properties of amorphous silicon. For example, results of an extensive search for the activated events surrounding a single minimum, in a well-relaxed model represented by a modified Stillinger-Weber potential, confirm that barrier height at the transition point, for activated mechanisms, is determined essentially by the binding energy of a single bond and not the details of the mechanism. We will discuss these results in some detail as well as recent simulations of nucleation in liquid and amorphous silicon.

Quantum Confinement of Localized Excitons in Amorphous Silicon Nanostructures

The luminescence properties of amorphous silicon (a-Si)-based nanostructures have been studied. The photoluminescence (PL) intensity is enhanced in both a-Si nanoparticles and a-Si quantum wells compared to the case of bulk a-Si. In a-Si nanostructures, the PL intensity depends weakly on the measurement temperature and visible PL is observed at room temperature, while in bulk a-Si the near-infrared PL disappears at temperatures above 150 K. The luminescence mechanism and quantum confinement effects of localized carriers are discussed.

The Effect of Moderate Hydrogen Dilution on Stability and Structure of Amorphous Silicon Deposited by Hot-Wire CVD

ABSTRACTThe effect of moderate hydrogen dilution of the process gas, F(H2)/F(SiH4) = 0 to 3, on the properties of amorphous silicon is discussed for material and solar cells deposited by Hot-Wire CVD. Dielectric properties were obtained from spectroscopic ellipsometry and are related to stability and hydrogen bonding configuration of films deposited with varying hydrogen dilution at different substrate temperatures. The stability was determined by comparing defect densities obtained from photoconductivity spectroscopy in the constant photocurrent mode (CPM) before and after pulsed-light soaking. At low substrate temperatures, which are relevant for the prepara- tion of pin-type solar cells (160-200°C), moderate hydrogen dilution (∼0.3) improves material quality regarding density and network disorder (oscillator bandwidth) as obtained from spectro-scopic ellipsometry, resulting in a higher stability. At higher substrate temperatures (300°C), stability and hydrogen bonding configurations are generally better, but moderate hydrogen dilu-tion already deteriorates these properties compared to material prepared without dilution. The incorporation of Hot-Wire-a-Si:H into pin-type solar cells is also discussed and a good correlation of ellipsometric results with bulk-related properties of solar cell performance is observed. The optimum hydrogen dilution is found to be 0 to 0.3 for i-layer deposition yielding initial efficiencies of up to 8.9% for solar cells entirely fabricated by Hot-Wire CVD.

The influence of erbium on electrical and photoelectric properties of amorphous silicon produced by radio-frequency silane decomposition

Thermal evaporation of tris(2,2,6,6-tetramethyl-2,5-heptadionato) Er(III) inside the plasma gap was used to introduce erbium into amorphous hydrogenated silicon (a-Si:H) obtained by radio-frequency silane decomposition. The samples obtained had a pronounced layered structure due to exhaustion of the erbium source. The layer nearest to the substrate was enriched with erbium, oxygen, and carbon; gave rise to luminescence with a wavelength of 1.535 µm characteristic of 4I13/2 → 4I15/2 intra-atomic transitions of erbium; and contained a large number of defects. The top layer contained much fewer defects, was close to undoped a-Si:H in the photoelectric characteristics, and was responsible for photoconductivity in the samples obtained. The experimental data are analyzed in the context of the models for doping of a-Si:H with Er with the resulting emergence of n-type conduction and formation of heterojunction as the film grows.

Thermodynamic properties of amorphous silicon via tight binding simulations

An atomic-scale structure of amorphous silicon, generated by reverse Monte Carlo, has been used as a starting configuration for finite temperature molecular dynamics simulations performed by an orthogonal tight binding Hamiltonian. Structural, dynamic, elastic and electronic structure properties have been investigated in the range of temperatures up to and above the melting transition. The amorphous silicon structure undergoes a melting transition at a temperature sensibly smaller than that of the crystalline structure. Above this temperature, the structure has the same properties of an under-cooled liquid and it has a metallic behavior.

T.Searle: Properties of Amorphous Silicon and its Alloys

T.Searle: Properties of Amorphous Silicon and its Alloys