C. del Cañizo

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Iron distribution in silicon after solar cell processing: Synchrotron analysis and predictive modeling

The evolution during silicon solar cell processing of performance-limiting iron impurities is investigated with synchrotron-based x-ray fluorescence microscopy. We find that during industrial phosphorus diffusion, bulk precipitate dissolution is incomplete in wafers with high metal content, specifically ingot border material. Postdiffusion low-temperature annealing is not found to alter appreciably the size or spatial distribution of FeSi2 precipitates, although cell efficiency improves due to a decrease in iron interstitial concentration. Gettering simulations successfully model experiment results and suggest the efficacy of high- and low-temperature processing to reduce both precipitated and interstitial iron concentrations, respectively.

Chemical Vapor Deposition Model of Polysilicon in a Trichlorosilane and Hydrogen System

The traditional polysilicon processes should be refined when addressing the low energy consumption requirement for the production of solar grade silicon. This paper addresses the fluid dynamic conditions required to deposit polysilicon in the traditional Siemens reactor. Analytical solutions for the deposition process are presented, providing information on maximizing the rate between the amount of polysilicon obtained and the energy consumed during the deposition process. The growth rate, deposition efficiency, and power-loss dependence on the gas velocity, the mixture of gas composition, the reactor pressure, and the surface temperature have been analyzed. The analytical solutions have been compared to experimental data and computational solutions presented in the literature. At atmospheric pressure, the molar fraction of hydrogen at the inlet should be adjusted to the range of 0.85-0.90, the gas inlet temperature should be raised within the interval of 673 and 773 K, and the gas velocity should reach the Reynolds number 800. The resultant growth rate will be between 6 and 6.5 μm min -1 . Operation above atmospheric pressure is strongly recommended to achieve growth rates of 20 μm min -1 at 6 atm.

Analysis of a technology for CZ bifacial solar cells

A bifacial cell technology for Cz Si and evaporated contacts is presented. A p/sup +/nn/sup +/ structure on high resistivity material gives 17.7% for n/sup +/ side illumination and 15.2% for p/sup +/ side illumination. Cell performance is analyzed by fitting experimental measurements with PC1D. Analysis shows that p/sup +/ layer puts a limit to cell performance, mainly due to a high surface recombination velocity. The boron depleted zone near the surface also enhances recombination, but its effect can be reduced by performing a boron etch-back step in the process. Cells with boron etch-back give higher short-circuit current and a reduction of open-circuit voltage of around 10 mV. These results are consistent with the PC1D model.

Enhancing Silicon Solar Cell Efficiency by Modifying the Solar Spectrum

Two approaches can be followed to reduce thermalisation and transmission losses in solar cells and thereby better exploit the solar spectrum. Firstly, modification may be done to lower energies, which can be via down-conversion, where one high energy photon is split into two or more low energy photons, or photoluminescence, where photons are shifted into different wavelength regions. Secondly, modification may be done to higher energies using up-conversion, where two or more low energy photons are combined to form one high energy photon. In this paper, the state of the art of these methods and the suitability of materials available today for application to silicon solar cells are presented

Effect of thickness on bifacial silicon solar cells

The influence of the thickness reduction of bifacial solar cells has been investigated using Czochralski (Cz) substrates with thicknesses of 140 and 240 mum. Thickness reduction has increased cell performance for both illumination modes, and unlike the conventional structure, the thin one presents higher efficiency when illuminated by the n+n junction. The good light trapping properties and low rear recombination have revealed efficient once the influence of bulk recombination is reduced thanks to thinning.

Optimisation of SiNx:H anti-reflection coatings for silicon solar cells

The deposition of SiNx:H as anti-reflection coating has become a standard step in industrial production of silicon solar cells. In the present work the improvement of the anti-reflection properties and thus the improvement of the short circuit current density by deposition of a SiNx:H double layer coating are investigated. It is shown that an optimised double layer coating with indices n1 ap 1.8 and n2 ap 2.1 only leads to an 1.3% improvement of short circuit current in comparison to an optimised single layer coating with n ap 1.9. But the deposition of a SiNx:H layer with higher refractive index on the silicon surface is supposed to lead to a good surface and bulk passivation. The passivating properties of the optically optimised double layer coating will be investigated in following experiments.

Increase on Siemens Reactor Throughput by Tailoring Temperature Profile of Polysilicon Rods

Siemens process productivity can be limited by non homogeneous temperature profile in polysilicon rods. To overcome this limitation high frequency current sources have been proposed. An analysis is presented which, based on electromagnetic and heat transfer theory, studies temperature and current density profiles within the rods. Two linked differential equations have been numerically solved by use of non-linear methods. The solution of these equations shows that by means of an increase in current frequency, skin effect takes place, heat generation in the inner part of the rod is decreased and therefore temperature homogeneity increases. The effect of high frequency current sources in the rod stability is also analyzed, and it can be derived that resonance problems could appear.

Influence of Depth-Inhomogeneity of Lifetime in Silicon Solar Cells

It is usually assumed that the bulk lifetime is constant throughout a silicon wafer. In this paper, we show that as a result of contamination and gettering processes during solar cell fabrication, lifetime-killing impurity profiles can he nonhomogeneous throughout the wafer, producing a nonuniform lifetime profile that can have a relevant effect on the behavior of solar cells. The existence of steep nonhomogeneous impurity profiles is predicted by simulations of gettering processes in silicon. Solar cell equations are solved for nonuniform lifetime profiles, showing that their electrical performance cannot he accurately described using a constant lifetime model.

Photon converters for Si solar cells

To assess the potential of photon converters to increase the efficiency of state-of-the-art silicon solar cells, a number of cases is analyzed through modeling. The AM1.5G spectrum is modified, according to the change in energy and number of photons produced in the converter, and applied to a PC1D model of a cell structure. For a good bifacial device, it is shown that efficiency gains in the range of 18% (relative) can be achieved with photon down-converters, while the efficiency increases more than 17% with an optimized up-converter. By combining optima down-converter al the front and up-converter at the rear, the gain in efficiency for the bifacial cell exceeds 30%.