Research on efficiency limiting defects and defect engineering in silicon solar cells ‐ results of the German research cluster SolarFocus

Riepe, Stephan; Reis, I.; Kwapil, Wolfram; Falkenberg, M. A.; Schön, Jonas; Behnken, Herfried; Bauer, Jan; Kreßner-Kiel, D.; Seifert, W.; Koch, Wolfgang

doi:10.1002/pssc.201000338

Cited by 52 publications

(40 citation statements)

References 10 publications

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“…In all Models, regions of higher structural defect density and thus nucleation site density contain more precipitated iron after crystallization, as observed experimentally [15,68,69]. During subsequent processing steps, precipitated iron can reduce device performance because precipitates can limit the lifetime [20] and dissolving precipitates release Fe, into the bulk [69][70][71].…”

Section: Crystallizationmentioning

confidence: 56%

Building intuition of iron evolution during solar cell processing through analysis of different process models

et al. 2015

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An important aspect of Process Simulators for photovoltaics is prediction of defect evolution during device fabrication. Over the last twenty years, these tools have accelerated process optimization, and several Process Simulators for iron, a ubiquitous and deleterious impurity in silicon, have been developed. The diversity of these tools can make it difficult to build intuition about the physics governing iron behavior during processing. Thus, in one unified software environment and using self-consistent terminology, we combine and describe three of these Simulators. We vary structural defect distribution and iron precipitation equations to create eight distinct Models, which we then use to simulate different stages of processing. We find that the structural defect distribution influences the final interstitial iron concentration ([Fe,]) more strongly than the iron precipitation equations. We identify two regimes of iron behavior: (1) diffusivity-limited, in which iron evolution is kinetic ally limited and bulk [Fe,] predictions can vary by an order of magnitude or more, and (2) solubility-limited, in which iron evolution is near thermodynamic equilibrium and the Models yield similar results. This rigorous analysis provides new intuition that can inform Process Simulation, material, and process development, and it enables scientists and engineers to choose an appropriate level of Model complexity based on wafer type and quality, processing conditions, and available computation time.

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Section: Crystallizationmentioning

confidence: 56%

Building intuition of iron evolution during solar cell processing through analysis of different process models

et al. 2015

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“…Details on the metal concentration in this ingot can be found in [17]. The total Fe concentration in these wafers is about 10 15 atoms/cm 3 .…”

Section: Methodsmentioning

confidence: 99%

Investigation of Lifetime-Limiting Defects After High-Temperature Phosphorus Diffusion in High-Iron-Content Multicrystalline Silicon

Fenning

Zuschlag

Hofstetter

et al. 2014

IEEE J. Photovoltaics

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Abstract-Phosphorus diffusion gettering of multicrystalline silicon solar cell materials generally fails to produce material with minority-carrier lifetimes that approach that of gettered monocrystalline wafers, due largely to higher levels of contamination with metal impurities and a higher density of structural defects. Higher gettering temperatures should speed the dissolution of precipitated metals by increasing their diffusivity and solubility in the bulk, potentially allowing for improved gettering. In this paper, we investigate the impact of gettering at higher temperatures on low-purity multicrystalline samples. To analyze the gettering response, we measure the spatially resolved lifetime and interstitial iron concentration by microwave photoconductance decay and photoluminescence imaging, and the structural defect density by Sopori etching and large-area automated quantification. Higher temperature phosphorus diffusion gettering is seen to improve metal-limited multicrystalline materials dramatically, especially in areas of low etch pit density. In areas of high as-grown dislocation density in the multicrystalline materials, it appears that higher temperature phosphorus diffusion gettering reduces the etch pit density, but leaves higher local concentrations of interstitial iron, which degrade lifetime.

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“…3,4 Iron, in particular, limits the bulk minority carrier lifetime of most as-grown p-type silicon wafers, 5,6 in part because of its large electron capture cross section, 7,8 but also because of its inevitable presence in feedstocks, 9 crystal growth crucibles and their linings, 10 and throughout the industrial growth environment. 11 Recently, several authors have investigated the macroscopic device-level effects of iron contamination, [12][13][14] updating foundational studies of metal contamination in silicon solar cells (e.g., the Westinghouse study of Davis, Jr. et al…”

Section: Introductionmentioning

confidence: 99%

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

Fenning

Hofstetter

Bertoni

et al. 2013

Journal of Applied Physics

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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.

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Research on efficiency limiting defects and defect engineering in silicon solar cells ‐ results of the German research cluster SolarFocus

Cited by 52 publications

References 10 publications

Building intuition of iron evolution during solar cell processing through analysis of different process models

Building intuition of iron evolution during solar cell processing through analysis of different process models

Investigation of Lifetime-Limiting Defects After High-Temperature Phosphorus Diffusion in High-Iron-Content Multicrystalline Silicon

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

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