Eivind Øvrelid

Effect of iron in silicon feedstock on p- and n-type multicrystalline silicon solar cells

The effect of iron contamination in multicrystalline silicon ingots for solar cells has been investigated. Intentionally contaminated p- and n-type multicrystalline silicon ingots were grown by adding 53 ppm by weight of iron in the silicon feedstock. They are compared to reference ingots produced from nonintentionally contaminated silicon feedstock. p-type and n-type solar cell processes were applied to wafers sliced from these ingots. The as-grown minority carrier lifetime in the iron doped ingots is about 1–2 and 6–20 μs for p and n types, respectively. After phosphorus diffusion and hydrogenation this lifetime is improved up to 50 times in the p-type ingot, and about five times in the n-type ingot. After boron/phosphorus codiffusion and hydrogenation the improvement is about ten times for the p-type ingot and about four times for the n-type ingot. The as-grown interstitial iron concentration in the p-type iron doped ingot is on the order of 1013 cm−3, representing about 10% of the total iron concentra...

Characterisation of the surface films formed on molten magnesium in different protective atmospheres

Abstract Molten magnesium oxidises rapidly during casting and handling unless it is protected by an atmosphere that stabilises the surface. In this article, results from the analysis of magnesium melt surfaces exposed to SO 2 and different fluorine-containing atmospheres are reported. The microstructure of the surface films, formed during controlled exposure in laboratory scale experiments, have been characterised using X-ray diffraction (XRD), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). Both SO 2 and the fluorine-containing gases were found to protect the melt from burning and vaporisation in oxidising atmospheres. The protected surfaces generally had a shiny metallic appearance, but turned dull grey after extended exposure to high concentrations of fluorine containing gases. All the surface films initially consisted of small crystallites of MgO forming a thin continuous film. This film was found to contain some sulphur when the melt was protected by SO 2 , while fluorine was the only element detected in the oxide when SF 6 or other fluorine-containing gases were used for protection. With increasing exposure time, the films gradually grew thicker and the fluorine/oxygen-ratio of the films formed in fluorine-containing atmospheres increased. Finally, after long term exposure to fluorine containing atmospheres, the thermodynamically stable MgF 2 -phase was formed. In a N 2 atmosphere, SO 2 and SF 6 -additions did not protect the magnesium, indicating that a rapid initial formation of MgO is necessary to obtain protective films.

Distribution of iron in multicrystalline silicon ingots

The distribution of iron in multicrystalline silicon ingots for solar cells has been studied. A p- and a n-type multicrystalline ingot were intentionally contaminated by adding 53ppmwt (μg∕g) of iron to the silicon feedstock and compared to a reference p-type ingot produced from ultrapure silicon feedstock. The vertical total iron distribution was determined by neutron activation analysis and glow discharge mass spectrometry. For the intentionally Fe-contaminated ingots, the distribution can be described by Scheil’s equation with an effective distribution coefficient of 2×10−5. The interstitial iron concentration was measured in the p-type ingots. In the Fe-contaminated ingot, it is almost constant throughout the ingot and constitutes about 50% of the total concentration, which is in conflict with the previous studies. Gettering had a large impact on the interstitial iron levels by reducing the concentration by two orders of magnitude. Considerable trapping was observed at crystal defects on as-cut wafers...

Thermochemical and Kinetic Databases for the Solar Cell Silicon Materials

The fabrication of solar cell grade silicon (SOG-Si) feedstock involves processes that require direct contact between solid and a fluid phase at near equilibrium conditions. Knowledge of the phase diagram and thermochemical properties of the Si-based system is, hence, important for providing boundary conditions in the analysis of processes. A self-consistent thermodynamic description of the Si-Ag-Al-As-Au-B-Bi-C-Ca-Co-Cr-Cu-Fe-Ga-Ge-In-Li-Mg-Mn-Mo-N-Na-Ni-O-P-Pb-S-Sb-Sn-Te-Ti-V-W-Zn-Zr system has recently been developed by SINTEF Materials and Chemistry. The assessed database has been designed for use within the composition space associated with the SoG-Si materials. An assessed kinetic database covers the same system as in the thermochemical database. The impurity diffusivities of Ag, Al, As, Au, B, Bi, C, Co, Cu, Fe, Ga, Ge, In, Li, Mg, Mn, N, Na, Ni, O, P, Sb, Te, Ti, Zn and the self diffusivity of Si in both solid and liquid silicon have extensively been investigated. The databases can be regarded as the state-of-art equilibrium relations in the Si-based multicomponent system. The thermochemical database has further been extended to simulate the surface tensions of liquid Si-based melts. Many surface-related properties, e.g., temperature and composition gradients, surface excess quantity, and even the driving force due to the surface segregation are possible to obtain directly from the database. By coupling the Langmuir-McLean segregation model, the grain boundary segregations of the nondoping elements in polycrystalline silicon are also possible to estimate from the assessed thermochemical properties.

Thermo-Mechanical Analysis of Directional Crystallisation of Multi-Crystalline Silicon Ingots

Multi-crystalline silicon ingot casting using directional crystallisation is the most costeffective technique for the production of Si for the photovoltaic industry. Non-uniform cooling conditions and a non-planarity of the solidification front result, however, in the build-up of stresses and viscoplastic deformation. Known defects, such as dislocations and residual stresses, can then occur and reduce the quality of the produced material. Numerical simulation, combined with experimental investigation, is therefore a key tool for understanding the crystallisation process, and optimizing it. The purpose of the present work is to present an experimental furnace for directional crystallisation of silicon, and its analysis by means of numerical simulation. The complete casting procedure, i.e., including both the crystallisation phase and the subsequent ingot cooling, is simulated. The thermal field has been computed by a CFD tool, taking into account important phenomena such as radiation and convection in the melt. The transient thermal field is used as input for a thermo-elasto-viscoplastic model for the analysis of stress build-up and viscoplastic deformation during the process. Numerical analysis is employed to identify process phases where further optimisation is needed in order to reduce generated defects.

Investigating minority carrier trapping in n-type Cz silicon by transient photoconductance measurements

Minority carrier trapping was investigated in n-type Cz silicon by means of transient-photoconductance (PCD). A simplified Hornbeck and Haynes model was developed for fitting results from transient-PCD to calculate trap density, and it was found to be identical to the model developed for quasi-steady-state photoconductance technique. This indicates that the model can be applied to all photoconductance techniques for lifetime measurement. The results revealed that the trap density is dependent on the concentration of interstitial oxygen and thermal donors, indicating a good agreement with reported results and the results from annealing experiments in this work. Meanwhile, a deep trap energy level was revealed, probably implying that traps also act as recombination centers in n-type silicon. By studying detrapping processes, the concentration of the trapped holes was found to decrease exponentially with time, resulting in a detrapping constant of 167 s.

Investigating thermal donors in n-type Cz silicon with carrier density imaging

A new method to map the thermal donor concentration in silicon wafers using carrier density imaging is presented. A map of the thermal donor concentration is extracted with high resolution from free carrier density images of a silicon wafer before and after growth of thermal donors. For comparison, free carrier density mapping is also performed using the resistivity method together with linear interpolation. Both methods reveal the same distribution of thermal donors indicating that the carrier density imaging technique can be used to map thermal donor concentration. The interstitial oxygen concentration can also be extracted using the new method in combination with Wijaranakulas model. As part of this work, the lifetime at medium injection level is correlated to the concentration of thermal donors in the as-grown silicon wafer. The recombination rate is found to depend strongly on the thermal donor concentration except in the P-band region.

EBIC, EBSD and TEM study of grain boundaries in multicrystalline silicon cast from metallurgical feedstock

Grain boundaries in multicrystalline silicon material grown from metallurgical feedstock, were investigated in detail using Electron Beam Induced Current (EBIC), Electron Back-Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) techniques. The EBSD analysis showed that small angle grain boundaries, with misorientation angles lower than 2°, gave high EBIC contrast, i.e., high recombination activity. EBIC combined with TEM showed that at low temperatures, silicon oxide was found to be recombination centers both at grain boundaries and on decorated dislocations in the bulk. The grain boundaries containing multi-metallic silicides were found to have random misorientations and showed strong contrast in the EBIC image. Clean twins showed less or no contrast in the EBIC image. The metallic precipitates observed in the sample contain mainly nickel silicide with an iron rich core.

Solidification of Silicon for Solar Cells

Silicon is the dominating material in solar cells. Monocrystalline and multicrystalline cells have approximately equal market shares and are produced from wafers, cut from single crystals produced by Czochralski (CZ) pulling or from polycrystalline ingots made by directional solidification, respectively. The present paper reviews how demands for lower cost, better yield, higher efficiency and use of less pure silicon in solar cells are addressed by advanced solidification processing. In monocrystalline solar silicon, careful growth control results in less point defects, and better efficiency. Continuous- or semi-continuous CZ growth processes are being developed for better productivity and lower cost. In multicrystalline solar silicon, extended defects such as dislocations and grain boundaries decrease efficiency, particularly in combination with new, less expensive, but more contaminated silicon feedstock. This problem is addressed by control of nucleation and growth of ingots with larger grains, preferred grain orientation and lower dislocation density.

Modeling of Lifetime Distribution in a Multicrystalline Silicon Ingot

Lifetime distribution of a multicrystalline silicon ingot of 250 mm diameter and 100 mm height, grown by unidirectional solidification has been modeled. The model computes the combined effect of interstitial iron and dislocation distribution on minority carrier lifetime of the ingot based on Shockley Read Hall (SRH) recombination model for iron point defects and Donolato’s model for recombination on dislocations. The iron distribution model was based on the solid state diffusion of iron from the crucible and coating to the ingot during its solidification and cooling, taking into account segregation of iron to the melt and back diffusion after the end of solidification. Dislocation density distribution is determined from experimental data obtained by PVScan analysis from a vertical cross section slice. Calculated lifetime is fitted to the measured one by fitting parameters relating the recombination strength and the local concentration of iron