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.     

Reaction temperature of solid particles undergoing an endothermal volatilization. Application to the fast pyrolysis of biomass

The present paper describes the thermal conditions under which a solid particle (biomass) undergoes an endothermic decomposition caused by an external heat flux. The results derived from a mathematical model concern two extreme cases of conditions following the size of the particle (chemical and ablation regimes). The sensitivity of the results is studied as a function of several experimental parameters (particle size; heat transfer coefficient; heat source temperature) and chemical characteristics (kinetics and enthalpy). The sensible parameter governing the reaction temperature is the activation energy. The enthalpy has a minor effect except on heating rates. The reaction temperature is always much lower than the heat source temperature. In the chemical regime it is shown that after a simple heating phase, the temperature at which the reaction starts, varies between relatively close limits (less than 80 K). A temperature stabilization is then observed as the decomposition proceeds in such a way that the reaction may be considered as quasi-isothermal mainly for high enthalpies. In the ablation regime, the particle shrinks rapidly at a constant velocity, the reaction occurring inside an external thin layer. The reaction temperatures and heating rates are always lower than in the chemical regime.

Data derived from previous experiments on fast pyrolysis of wood (ablation conditions) are in good agreement with the predictions of the model (reaction temperature, ablation rate and ablation layer thickness). The results bring confirmation of the effect of fusion observed during the thermal decomposition of biomass.     

Two-Dimensional Effects in Laser Ablation of Carbon

This article studies the importance of two-dimensional effects in laser ablation of carbon. It describes the process by using the kinetic theory model of laser ablation based on the moment solution of the Boltzmann equation for arbitrary strong evaporation, and compares the predictions of the full two-dimensional model and of the two other models that use quasi-one-dimensional approximation in the solid or in the gas. All models estimate the total ablated mass reasonably well. However, comparison of their predictions shows that, surprisingly, two-dimensional effects are more important for the heat transfer in the solid than for the gas dynamics.     

Local aluminum-silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm² on a phosphorous diffused emitter with a peak doping density of 2×10²⁰ cm⁻³. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.     

连续CO2激光熔凝太阳能硅材料

采用大功率连续ＣＯ２激光熔凝硅粉末，生长得到纯度较高的太阳能多晶硅材料，探讨了影响太阳电池效率的少子复合中心和微观组织结构以及激光消除复合中心和改善微观组织结构的机理。在实验中获得了杂质浓度低且呈较整齐分布的柱状晶太阳能多晶硅材料。     

Effect of the dielectric transition on laser-induced phase explosion in metals

Phase explosion is an explosive liquid to vapor phase change that occurs during laser ablation as the surface approaches 90% of the thermodynamic critical temperature (0.9T_c), which is the upper limit of superheating. Large variations in properties are expected to occur near 0.8T_c, transforming the electrically conductive metal into a nearly transparent dielectric, an effect that has been neglected in previous models of laser ablation. The work presented in this paper numerically investigates the possible effect of the dielectric transition using a one dimensional heat transfer model. The results show that accurate knowledge of the absorption coefficient above 0.8T_c is critical for predicting the laser fluence at which phase explosion occurs.     

Effect of temperature on dark current characteristics of silicon solar cells and diodes

The influence of temperature on the dark forward current–voltage characteristics of a single crystalline silicon solar cell and a small silicon diode within the range from 295–373 K has been analysed. It was shown that the forward voltage of the solar cell degrades 2 mV and in the case of diode 1 mV per 1 K temperature increases at constant forward current of 100 mA. Thermal resistance and heat transfer from the solar cell by using a thick copper plate as a heat sink have also been discussed. For the series resistance determination the current–voltage I–U characteristics of single crystalline silicon solar cells in different temperatures were measured in the dark. It was proved that series resistance of the silicon solar cells and diodes is temperature dependent and increases with temperature increase 0.65% K^?1. Therefore, protection of silicon solar cells as well as silicon diodes against overheating is essential during their exploitation. Copyright © 2005 John Wiley & Sons, Ltd.     

Electromagnetic field-induced thermal management of biological materials

The study of temperature profiles and heat transport within the human body when subjected to electromagnetic waves is crucial for development and improvement of radiofrequency cardiac ablation treatments (radio frequency ablation). The present study provides an analytical solution for computing the temperature profiles for blood and tissue for various biological media along with heat transfer behavior during various ablation processes. The local thermal nonequilibrium model is used to characterize the bioheat transport through the biological medium. The two energy equation model for tissue and blood phase is considered. To understand the effects induced by imposed electromagnetic field, the specific absorption rate of body tissues is also studied. The results obtained have been validated against the pertinent numerical results in the literature. This study provides benchmark analytical solutions for heat transport through biological media, thereby helping in understanding the thermophysiologic response of human body toward imposed electromagnetic radiation.     

The numerical simulation of coupled unsteady ablation and heat transfer in the porthole region during reentry

The complicated phenomena in the porthole region of a vehicle, which consists of coupled thermolysis/ablation with heat transfer in different thermal protection materials, are numerically simulated in this paper. The different characteristics of temperature increment at some important positions in the porthole region are, respectively, given under both constant heating and variable heating on the upper surface of the porthole. Also, the mechanism of the heat transfer in the porthole region is given, which is significant for the design of local thermal protection structure. © 1999 Scripta Technica, Heat Trans Asian Res, 28(7): 597–605, 1999     

Heat transfer and phase change during picosecond laser ablation of nickel

This work investigates heat transfer and phase change during picosecond laser ablation of nickel. In this study, ablation of nickel is studied using a mode-locked 25 ps (FWHM) Nd:YAG laser. The threshold fluence for mass removal (ablation) is experimentally determined. Numerical calculations of the transient temperature distribution and kinetics of the solid-liquid and liquid-vapor phase change interfaces are performed. The results show that evaporation is negligible at the free surface, resulting in superheating of liquid to near 0.9T_cr, at which temperature homogeneous nucleation will result in an explosive phase transformation, removing part of the molten layer.     

TEMPERATURE RESPONSE OF SILICON MEMS CANTILEVERS DURING AND AFTER Nd:YAG LASER IRRADIATION

This article reports a two-dimensional, finite-difference heat transfer model for calculating the transient temperature distribution in a polycrystalline silicon cantilever during and after irradiation by a Nd:YAG laser. Results include the peak surface temperature after irradiation and the uniform temperature increase in the microcantilever following subsequent heat conduction through the thickness. The calculations reveal that the time scale after which the temperature is uniform through the thickness is on the order of hundreds of nanoseconds and that the microcantilever cools in the order of tens of milliseconds. The effects of energy transfer to the environment by convection and radiation on the cooling time are also investigated. The accuracy of the model predictions are shown through high-speed temperature measurements using a novel MEMS temperature sensor.     

Molecular dynamics studies of ultrafast laser-induced phase and structural change in crystalline silicon

In this work, thermodynamic phenomena in crystalline silicon irradiated by an ultrafast laser pulse were studied using the method of molecular dynamics simulations. The Stillinger–Weber potential was used to model the crystalline silicon. The temperature development in silicon when heated by an ultrafast laser pulse was tracked. Melting and resolidification processes and the resulting structural change were investigated. Radial Distribution Functions were used to track the liquid-amorphous interface during resolidification. It was found that the temperature at the solid–liquid interface could deviate from the equilibrium melting temperature by several hundred degrees. After the melted layer was solidified, some melted material became crystalline and the rest of the material remained in an amorphous state. The difference in the final state was associated with the rate of resolidification and both of the qualitative and quantitative analyses of the relationship between the final atom structure and resolidification rate were made.     

Precise dental ablation using ultra-short-pulsed 1552 nm laser

A desktop diode pulsed laser having pulse width of 1.3 ps and wavelength of 1552 nm is utilized for precise targeted ablation of dentin, enamel, and composite material while minimizing thermal damage to the surrounding healthy tissue and nerve endings. A thermal imaging camera is used to measure the dental surface temperature rise during ablation. Following ablation, scanning electron microscopy (SEM) and optical microscopy are used to determine the quality of ablation and the volumetric ablation rate as a function of laser parameters. Surface temperature measurements are compared with the numerical modeling results obtained using the transient heat conduction equation. A good agreement between experimental and modeling results for the surface temperature is obtained which ensures accurate prediction of the temperature distribution throughout the tooth using numerical models. The SEM generates images of precise ablation of each dental material when the optimal laser parameters are used and the sample is scanned at a velocity to limit the number of overlapping pulses. During the ablation process there is minimal collateral damage to the surrounding healthy tissue and minimal heat spread throughout the tooth thus preserving the integrity of the pulp.     

Characterization of pulsed laser crystallization of silicon thin film

Increases in the melt duration of silicon films were achieved by electrical current heating during and after pulsed excimer laser heating. When 50 nm thick amorphous silicon films formed on glass substrate were irradiated by 28-ns-pulsed excimer by applying 1.8 μs long pulsed-voltage at 100 V to the films, the silicon films were melted for the duration of the voltage pulse. The power threshold for heat energy for this long melting by the self-heating effect of the silicon films was 3.0×10⁵ W/cm². The high electrical conductivity of the silicon film (2.9×10⁻² S/cm) was found after regrowth of the silicon using a laser energy density of 360 mJ/cm² and a pulsed voltage of 150 V. The advantages of the long melt duration for large crystalline growth are discussed.     

FINITE ELEMENT ANALYSIS OF COUPLED HEAT. MASS,AND PRESSURE TRANSFER IN POROUS BIOMATERIALS

Abstract

The finite element method was used to solve Luikov's system of partial differential equations for neat, mass, and pressure transfer in capillary porous bodies. The finite element predictions were validated by comparing with exact solutions and the analytical results given by Mikhailov and Shishedjiev [1]. An application of the finite element method to the drying of wood (spruce) and a comparison based on an eigenvalue solution for simultaneous heat and mass transfer [2] are also provided. This technique was applied to study the coupled transport process in a silicon gel. The simulation indicated that the results obtained from the heat, mass, and pressure transfer model showed a marked difference from the results obtained by the heat and mass transfer model.     

Modeling of coupled spectral radiation,thermal and carrier transport in a silicon photovoltaic cell

Photovoltaic conversion efficiency of a crystalline silicon cell is investigated as a function of its temperature and taking into account complete thermal and irradiation operating conditions. The spectral radiative transfer problem is solved through a gray per band approach and a separated treatment of the collimated and diffuse components of radiation fluxes. The heat transfer modeling includes local heat sources due to radiation absorption and thermal emission, non-radiative recombinations and excess power release of photogenerated carriers. Continuity equations for minority carriers are solved to provide the current–voltage characteristic. A detailed analysis of the electrical and thermal behaviors demonstrates that proper adjustment and control of both thermal and surroundings radiative operating conditions are likely to provide guidelines for the improvement of photovoltaic cell performances.     

RADIATION MODELING OF STATIONARY FUSED SILICA RODS AND FIBERS HEATED BY CO2 LASER IRRADIATION

This study presents a heat transfer model for a stationary fused silica rod heated by a CO₂ laser. During laser heating, the effect of fused silica being modeled to be opaque or semitransparent to laser irradiation is studied. The radiative heat transfer caused by the emission of fused silica is modeled using the zonal method, and compared to the Rosseland diffusion approximation. The spectral dependence of the fused silica absorption coefficient in semitransparent wavelengths is approximated by a two-band model. The weighted-sum-of-gray-gas (WSGG) method is used to calculate the radiative source term. The governing equation with conduction and radiation heat transfer is solved by the finite-volume method. The importance of modeling the effects of laser energy penetration below the fused silica surface during heating, especially for small diameter fibers, is discussed. The importance of radiative heat transfer in fused silica is also discussed. Around 25 K in temperature difference is observed when the diffusion approximation is used in place of the zonal method to model the radiative transfer in fused silica.     

From monosilane to crystalline silicon, Part I: Decomposition of monosilane at 690–830 K and initial pressures 0.1–6.6 MPa in a free-space reactor

Thermal decomposition of undiluted monosilane at high pressures produces amorphous spherical silicon particles, with a mean diameter of 3 μm, clustered together. The size of the particles remains unchanged regardless of monosilane initial pressures between 0.13 and 6.61 MPa. This may reflect that the particle nucleation process is completed at an early stage of the decomposition reaction and/or demonstrate growth inhibition caused by the high hydrogen pressure resulting from the decomposition. The hydrogen content of the particles goes through a maximum of 5 at% at 2 MPa initial pressure and seems to flatten at some 3.5 at% hydrogen at the highest monosilane initial pressures studied. After dehydrogenation by heat treatment, the present raw-silicon powder is easily melted and crystalline silicon obtained on cooling.     

NUMERICAL SIMULATION OF SHORT-PULSED LASER PROCESSING OF MATERIALS

A numerical study of the short-pulsed laser-induced evaporation process is presented. For short-pulsed laser operation, the radiation penetration depths of most nonmetallic engineering materials are of the same order of magnitude as or of higher order of magnitude than the heat diffusion depth. Thus, the materials must be treated as semitransparent media during short-pulsed laser material processing. A quasi–one-dimensional model is developed to predict the two-dimensional heat conduction inside the solid. It is assumed that the conduction losses are normal to the surface and the ablation velocity is governed by an Arhennius equation. The model is solved using an integral method. The numerical simulations of the laser processing of ceramics are carried out. The results indicate that the radiation penetration depths of the materials make significant differences in the groove shape, heat losses, and the temperature field during short-pulsed laser operations.     

临近空间太阳电池模型修正与发电量预测研究

由于在飞行过程中,温度、辐照度和倾角变化都会对临近空间飞行器上太阳电池的输出功率及效率产生影响,该文利用太阳光模拟器及薄型晶体硅太阳电池,进行多组测量实验,得到在不同温度、辐照度和倾角条件下,太阳电池的短路电流、开路电压等参数,并通过与模型仿真结果进行对比,对已有太阳电池电模型进行修正,得到更接近真实飞行工况的临近空间飞行器用薄型晶体硅太阳电池的模型。最后,基于修正后的模型通过仿真对太阳电池阵列在临近空间的全天发电功率变化趋势进行预测,可为临近空间飞行器用太阳电池阵列设计与功率预测提供重要参考。