Laser ablation modelling of aluminium,silver and crystalline silicon for applications in photovoltaic technologies

Abstract

Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (10⁸–10⁹ W cm^?2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.     

Investigation into ultrathin CdTe solar cell Voc using SCAPS modelling

Abstract

Ultrathin CdTe photovoltaic solar cells were produced by metal organic chemical vapour deposition in a single horizontally configured growth chamber. Solar cell activation was investigated by varying the duration of the CdCl₂ layer deposition and 420°C thermal anneal to promote Cl diffusion into the CdTe. Thicker CdCl₂ layers used in activation treatment resulted in a greater degree of sulphur interdiffusion, up to 2 at.-%, into the CdTe layer. The thicker CdCl₂ activation layer was necessary to lower the reverse saturation current density for obtaining optimum experimental photovoltaic (PV) device performances. Modelling of the PV performances with equivalent solar cell structure for optimised devices using solar cell capacitance simulation software resulted in an overestimated open circuit voltage (V_oc). The simulations showed that reduced acceptor states at the CdTe interface with the intermixed region resulted in the largest decrease in V_oc when considering large back surface recombination velocities.     

Molecular dynamic simulation: Studying the effects of Brownian motion and induced micro-convection in nanofluids

ABSTRACT

Nanofluids are suspensions of nanoparticles into convectional heat transfer fluid to enhance the thermal conductivity of its base fluid. The roles of Brownian motion of nanoparticles and induced micro-convection in base fluid in enhancing the thermal conductivity of nanofluids were investigated using molecular dynamic (MD) simulation. The roles were determined by studying the effect of particle size on thermal conductivity and diffusion coefficient. Results show that the Brownian motion and induced micro-convection have insignificant effects on enhancing the thermal conductivity. The hydrodynamic effect is restricted by an amorphous-like interfacial fluid structure in the vicinity of the nanoparticle due to its higher specific area.     

Fabrication and characterisation of Si micropillar PV structures

Abstract

Arrays of vertical silicon micropillar radial junction solar cells have been fabricated by diffusion of direct application spin on dopant and from the vapour phase through proximity rapid thermal diffusion. The micropillars were fabricated by optical lithography and deep reactive ion etching. The micropillar arrays show superior antireflective properties over the measured spectrum and good correlation to finite difference time domain modelling of identical geometry arrays. Junctions formed by a conventional spin on doping process of phosphorus containing dopant solution produced Suns-V_oc values in the region of 0·3 V. This value is likely due to difficulties encountered in achieving an even distribution of dopant over the entire surface of the arrays. An alternative method utilising spin on dopant but employing an intermediate vapour phase diffusion step produced promising results with Suns-V_oc values reaching 0·5 V following a post-diffusion drive-in step.     

TRANSIENT THERMAL STRESS ANALYSIS AND BENDING BEHAVIOR OF AN ANGLE-PLY LAMINATED SLAB

Abstract

This paper is concerned with a theoretical treatment of thermal stress and bending behavior in a transient state of a multilayered, nonisotropic, laminated slab. As an analytical model, we consider an infinitely long, laminated slab, which consists of obliquely directed layers with orthotropic material properties; the model corresponds to the so-called angle-ply laminate. We solve the thermoelastic problem for the slab under the condition of uniformly distributed heat supply from its one surface. Introducing a method of Laplace transforms to the temperature field, we obtain the temperature solution using the residue theorem, and we evaluate the thermal stresses in a transient state by using the elementary plate theory. As an example, we carry out numerical calculations for the five-layered angle-ply laminate, evaluate the thermal stress distributions and the bending behavior, and examine the influence of the ply angle on the thermal stress distribution.     

THE USE OF GENERAL CURVILINEAR ORTHOGONAL MESHES IN TLM HEAT CONDUCTION MODELS

Abstract

An extension to the transmission line matrix (TLM) numerical modeling technique is presented that enables this method to be used for diffusion problems with arbitrary two-dimensional domains. The practical problems of mounting TLM on a general orthogonal grid are discussed, in particular, the elemental parameters of resistance and capacitance, and solutions are given. Errors in calculation of resistance are considered, and the proposed methods are validated by running a TLM model of thermal diffusion problem and comparing the results with those produced by a standard finite-element package.     

Modeling of Substrate Remelting,Flow, and Resolidification in Microcasting

ABSTRACT

A millimeter-sized molten droplet impacting on a substrate of the same material was modeled. Numerical simulations of microcasting experiments were conducted with the different thermal contact resistances. It was found that the thermal contact resistance affects not only the droplet spreading but also the substrate remelting volume and remelting front configurations. Under the assumption of an appropriate thermal contact resistance, the numerical results of spreading factors, bump heights, and substrate remelting volumes agree well with the experimental data. The effects of droplet impacting velocity, superheat, and substrate temperature were also investigated in detail and some nonintuitional phenomena were discovered.     

Analysis of high-pressure hydrogen, methane, and heptane laminar diffusion flames: Thermal diffusion factor modeling

Direct numerical simulations are conducted for one-dimensional laminar diffusion flames over a large range of pressures (1?P₀?200 atm) employing a detailed multicomponent transport model applicable to dense fluids. Reaction kinetics mechanisms including pressure dependencies and prior validations at both low and high pressures were selected and include a detailed 24-step, 12-species hydrogen mechanism (H₂/O₂ and H₂/air), and reduced mechanisms for methane (CH₄/air: 11 steps, 15 species) and heptane (C₇H₁₆/air: 13 steps, 17 species), all including thermal NO_x chemistry. The governing equations are the fully compressible Navier-Stokes equations, coupled with the Peng-Robinson real fluid equation of state. A generalized multicomponent diffusion model derived from nonequilibrium thermodynamics and fluctuation theory is employed and includes both heat and mass transport in the presence of concentration, temperature, and pressure gradients (i.e., Dufour and Soret diffusion). Previously tested high-pressure mixture property models are employed for the viscosity, heat capacity, thermal conductivity, and mass diffusivities. Five models for high-pressure thermal diffusion coefficients related to Soret and Dufour cross-diffusion are first compared with experimental data over a wide range of pressures. Laminar flame simulations are then conducted for each of the four flames over a large range of pressures for all thermal diffusion coefficient models and results are compared with purely Fickian and Fourier diffusion simulations. The results reveal a considerable range in the influence of cross-diffusion predicted by the various models; however, the most plausible models show significant cross-diffusion effects, including reductions in the peak flame temperatures and minor species concentrations for all flames. These effects increase with pressure for both H₂ flames and for the C₇H₁₆ flames indicating the elevated importance of proper cross-diffusion modeling at large pressures. Cross-diffusion effects, while not negligible, were observed to be less significant in the CH₄ flames and to decrease with pressure. Deficiencies in the existing thermal diffusion coefficient models are discussed and future research directions suggested.     

Aging and Thermal Conditioning of Modified Heat Exchanger Surfaces—Impact on Crystallization Fouling

ABSTRACT

In many research studies diamond-like-carbon coatings are used to change the wetting behavior by varying the solids´ surface free energy of heat exchanger surfaces to mitigate crystallization fouling. For future industrial application, the stability of their specific surface properties, exposed to fluidic, thermal, and chemical stresses, determines their efficiency. Therefore, fluidic thermal and cleaning stresses applied to the coating are investigated. Cleaning procedures with acid, base, and heat treatment over multiple cycles were conducted in order to investigate the solids´ surface free energy over time and thereby the stability of the coating. From this information an optimal conditioning to set constant surface properties was derived. Furthermore, the fouling behavior of CaSO₄ on new and conditioned coatings was investigated in order to identify repeatable and favorable surface properties for fouling reduction. For all coatings the cleaning treatments and fouling experiments provided changes in the energetic surface properties, dominated by the change of polar/γ^? content. Most probably these changes originate from varying elementary composition and structure of the coating.     

Fundamental solution of steady oscillations equations in nonlocal thermoelastic medium with voids

Abstract

A new nonlocal theory of generalized thermoelasticity with voids based on Eringen’s nonlocal elasticity is established. The propagation of plane harmonic waves in nonlocal thermoelastic medium with voids is investigated in the context of dual-phase-lag model of generalized thermoelasticity. There exist three longitudinal waves, namely elastic (E-mode), thermal (T-mode) and volume fraction (V-mode) in addition to transverse waves which get decoupled from the rest of motion and not affected by thermal and volume fraction fields. The fundamental solution of the system of differential equations in case of steady oscillations in terms of the elementary functions has been constructed. The effect of nonlocal parameter and the effect of voids on phase velocities, attenuation coefficients and penetration depths are presented graphically.     

Use of Fluoroscein-EDTA System in Photogalvanic Cell for Solar Energy Conversion

Abstract

Fluoroscein has been used as a photosensitizer in photogalvanic cell for solar energy conversion. EDTA was used as an electron donor. The photopotential and photocurrent generated by this cell were 418 mV and 42 μA, respectively. The effect of various parameters like pH, light intensity, diffusion length, reductant concentration, dye concentration, etc. on the electrical output of the cell has been studied. The current voltage (i–V) characteristics of the cell has also been observed and a tentative mechanism for the generation of photocurrent has been proposed. Performance of the cell was determined in dark at its power point.     

Residual thermal strains and stresses in organic matrix composite materials

ABSTRACT

Residual strain/stress may arise in organic matrix composite materials due to the intrinsic heterogeneity of their elementary constituents (polymer matrix and fibrous reinforcements), for instance, during material processing, thermal cycling, for harsh in-service conditions. This article illustrates some method to “measure” residual/internal strains/stresses of thermal origin—at both the microscopic and the mesoscopic levels—by inverse analysis of the matrix shrinkage profiles between fibers in unidirectional composite (at the microscopic scale) and the measure of the deflection (curvature) of 0/90 unsymmetric plates (mesoscopic scale). The analysis is carried out by using multiphysical phenomenological models.     

Au/Porous silicon‐based sodium borohydride fuel cells

The Au/Porous silicon structure (Au/PS) was developed as hydrogen fuel cell. The use of a porous silicon filled with hydrochloric acid as a proton‐conducting membrane and thin gold film as a catalyst in Au/PS/Si fuel cell is demonstrated. The devices were fabricated by first creating 10–20 µm thick porous silicon layer by anodization etching in a standard silicon wafer and then depositing the gold catalyst film onto the porous silicon. Using sodium borohydride (NaBH₄) solution as the fuel, generation of the open‐circuit voltage of 0.55 V and the fuel cell peak power density of 13 mW cm⁻² at room temperature was achieved. Moreover production of hydrogen by evolution (out‐diffusion) of hydrogen from solid sodium borohydride during thermal annealing at 30–120°C was investigated. Data on the effective diffusion coefficient of the hydrogen in NaBH₄ were determined from intensity changes of infrared vibration peaks of B–H bond (2280 and 3280 cm⁻¹), as a result of thermal annealing of NaBH₄ samples. The relatively high values of the diffusion coefficient of hydrogen, increasing from 1×10⁻⁶ cm² s⁻¹ to 2×10⁻⁴ cm² s⁻¹ suggest that a thermo‐stimulated evolution process can be used for producing hydrogen from NaBH₄. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetic incentive of hydrogen addition on nonpremixed laminar methane/air flames

In order to find out the respective influences of chemical reactivity and physical transport of hydrogen additive on nonpremixed flame, two fabricated hydrogen additions were introduced into nonpremixed methane/air flame modeling. Hydrogen addition was assumed as inert gas or partial reactivity fuel to respectively explore the kinetic reasons by the three aspects: the elementary reaction route, heat release, and physical diffusion of hydrogen addition. The analyses were implemented in terms of OH and H production. Results showed that, hydrogen addition can enhance OH and H production via elementary reactions, and causes flame reaction zone migration through the coupling interaction between the low-temperature heat enthalpy release and diffusion behavior of hydrogen addition. R84 (OH + H₂=H + H₂O) and R38 (H + O₂=O + OH) are the most important elementary reactions related to OH and H production. The physical incentive of hydrogen addition can hardly work without the chemical effects of hydrogen addition.     

Design of an optical thermal sensor for proton exchange membrane fuel cell temperature measurement using phosphor thermometry

Internal temperatures in a proton exchange membrane (PEM) fuel cell govern the ionic conductivities of the polymer electrolyte, influence the reaction rate at the electrodes, and control the water vapor pressure inside the cell. It is vital to fully understand thermal behavior in a PEM fuel cell if performance and durability are to be optimized. The objective of this research was to design, construct, and implement thermal sensors based on the principles of the lifetime-decay method of phosphor thermometry to measure temperatures inside a PEM fuel cell. Five sensors were designed and calibrated with a maximum uncertainty of ±0.6 °C. Using these sensors, surface temperatures were measured on the cathode gas diffusion layer of a 25 cm² PEM fuel cell. The test results demonstrate the utility of the optical temperature sensor design and provide insight into the thermal behavior found in a PEM fuel cell.     

A novel integration approach for combining the micro thermal sensor and stainless steel foil as gas diffusion layer in micro fuel cell

Fuel cells will be used extensively in the future as renewable energy sources and they are the subject of substantial research. However, various problems are encountered with their mass production, such as the bipolar plate, the flow channel, the catalyst, the membrane electrode assembly (MEA), and the gas diffusion layer (GDL). Given the present uneven gas reactions and the difficulty of obtaining information on the temperature in a fuel cell, this novel investigation utilized micro-electro-mechanical-systems (MEMS) to integrate a micro thermal sensor and a stainless steel foil as a gas diffusion layer. The reaction inside a micro fuel cell must be controlled and adjusted in real time. The results of the experiment demonstrated that the accuracy and sensitivity of the micro thermal sensor were 0.5 °C and 1.805 × 10^?3/°C, respectively.     

Measurement of solutal and thermal diffusion in systems subjected to rapid cooling under microgravity during parabolic flight

In the process of crystal growth from solution under normal gravity, double diffusive convection occurs due to thermal and solute gradients. This natural convection disturbs the pure thermal and solutal diffusion field around a crystal, and phase change phenomena during the diffusion process cannot be seen directly. Microgravity environments, generated by parabolic flight, were used during experiments in order to suppress the double diffusive convection. In-situ measurements of the diffusion fields were carried out using a real-time phase-shift interferometer. The saturated solution around a seed crystal of NaClO₃ was subjected to rapid cooling during the duration of microgravity by Peltier elements in the test cell. A number of temperature profiles were applied during the microgravity experiments. In particular, the diffusion phenomena measured for a system subjected to rapid cooling to −17 °C cannot be explained in terms of a conventional diffusion process nor by the kinetics of crystal growth. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 114–129, 1998     

Diffusion and influence of Cu on properties of CdTe thin films and CdTe/CdS cells

The effective diffusion coefficients of Cu for thermal and photodiffusion in the CdTe films have been estimated from resistivity versus duration of thermal or photoannealing curves. In the temperature range 60–200°C the effective coefficient of thermal diffusion (D_t) and photodiffusion (D_ph) are described as D_t=7.3×10⁻⁷exp(−0.33/kT) and D_ph=4.7×10⁻⁸exp(−0.20/kT).It is found that the diffusion doping of CdTe thin films by Cu at 400°C results in a sharp decrease of resistivity up to 7 orders of magnitude of p-type material, depending on thickness of Cu film. The comparative study of performance of CdTe(Cu)/CdS and CdTe/CdS cells has been studied. It is shown that the diffusion doping of CdTe film by Cu increases efficiency of CdTe(Cu)/CdS cells from 0.9% to 6.8%. The degradation of photovoltaic parameters of CdTe(Cu)/CdS cell, during testing under forward and reverse bias at room temperature, proceeds at a larger rate than those of CdTe/CdS cell without Cu. The degradation of performance of CdTe(Cu)/CdS cells is tentatively assigned to electrodiffusion of Cu in CdTe, resulting in redistribution of concentration of Cu-related centers in CdTe film and heterojunction region.     

Transient hygrothermoelastic response in a porous cylinder subjected to ramp-type heat-moisture loading

Abstract

The classical theory of heat conduction (Fourier theory) predicts an infinite speed for thermal disturbance propagation, which is physically unrealistic. By extending the classical Fourier heat conduction and Fick’s diffusion, this article develops hyperbolic diffusion/heat conduction laws with phase lags of heat/moisture flux to simulate coupled heat-moisture diffusion-propagation behavior with the Defour and Soret effects. A porous cylinder subjected to a ramp-type heating and humidifying at the surface is studied. The Laplace transform is used to obtain a closed-form solution of the temperature, moisture, displacements and stresses in the cylinder. Numerical results are calculated via the inversion of the Laplace transform. Obtained results show that the thermal/moisture relaxation time or phase lag plays a significant role in affecting transient hygrothermoelastic field. For a non-vanishing phase lag, non-Fourier and non-Fickian effects exist and hygrothermal waves have finite propagation speeds. The influences of the phase lag of heat/moisture flux and ramp-type time parameter on the transient response of hygrothermoelastic field are presented graphically. A comparison of the numerical results based on the classical model and the present one is made. The non-Fourier heat conduction and non-Fickian diffusion can effectively avoid the shortcomings induced by the classical Fourier and Fick laws.     

Thermal diffusion of Cu into In2Se3: A better approach to SEL technique in the deposition of CuInSe2 thin films

This paper reports the modifications made in the preparation techniques of getting CuInSe₂ thin films starting with chemical bath deposited (CBD) selenium films. In the present study, CBD Se film was converted into CuInSe₂ by stacked elemental layer (SEL) technique and also by thermal diffusion of Cu into In₂Se₃. In both the cases CBD Se films were used to avoid toxic Se vapor and H₂Se gas. Improvements were made in these techniques through a detailed study, varying the composition of the films over a wide range by changing the Cu/In ratio. Structural, optical and electrical characterizations were performed. On comparing the material properties of CuInSe₂ deposited by these two techniques, it was found that photosensitivity was better for samples prepared by thermal diffusion of Cu into In₂Se₃. So the technique of thermal diffusion of Cu into In₂Se₃ was found to be better than SEL technique in the preparation of CuInSe₂ using CBD Se. Cu-rich, In-rich and nearly stoichiometric samples could be prepared by thermal diffusion of Cu into In₂Se₃. These samples were analyzed using energy dispersive spectroscopy, Raman spectroscopy and atomic force microscopy also.