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Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 10¹⁰cm⁻³. In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cr[subscript i] point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi₂ growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cr[subscript i]] by three orders of magnitude to 10¹⁰cm⁻³. Some Cr-rich precipitates persist after both processes, and locally high [Cr[subscript i]] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible., United States. Department of Energy (Contract DE-EE0005314), National Science Foundation (U.S.) (Contract EEC-1041895), United States. Department of Energy (Contract EEC-1041895), National Science Foundation (U.S.) (Grant 1122374)

Control of detrimental metal impurities is crucial to silicon solar cell performance. Traditional silicon solar cell emitters are diffused in an infinite-source regime and are known to cause strong point defect segregation towards the emitter and thus enhance bulk minority carrier diffusion length. With the advent of ion-implantation and chemical vapor deposition (CVD) glasses, finite-source diffused emitters are attracting interest. This contribution aims to increase their adoption by elucidating the dominant gettering mechanisms present in finite-source diffused emitters. Our findings indicate that infinite-source diffusion is critical for effective segregation gettering, but that high enough surface phosphorus concentration can activate segregation gettering via finite-source diffusion as well. In the case of ion-implanted emitters, the traditional segregation gettering may be considerably enhanced by impurity precipitation in the implanted layer., United States. Department of Energy. Office of Basic Energy Sciences (Contract No. DE-AC02-06CH11357), National Science Foundation (U.S.) (CA No. EEC-1041895), Finnish Cultural Foundation, Fulbright-Technology Industries of Finland Grant, University of California, San Diego. Start Up Funds

Understanding reactions at the electrode/electrolyte interface (EEI) is essential to developing strategies to enhance cycle life and safety of lithium batteries. Despite research in the past four decades, there is still limited understanding by what means different components are formed at the EEI and how they influence EEI layer properties. We review findings used to establish the well-known mosaic structure model for the EEI (often referred to as solid electrolyte interphase or SEI) on negative electrodes including lithium, graphite, tin, and silicon. Much less understanding exists for EEI layers for positive electrodes. High-capacity Li-rich layered oxides yLi[subscript 2–x]MnO[subscript 3]·(1–y)Li[subscript 1–x]MO[subscript 2], which can generate highly reactive species toward the electrolyte via oxygen anion redox, highlight the critical need to understand reactions with the electrolyte and EEI layers for advanced positive electrodes. Recent advances in in situ characterization of well-defined electrode surfaces can provide mechanistic insights and strategies to tailor EEI layer composition and properties., BMW Group, MIT/Battelle postdoctoral associate program, Taiwan. Ministry of Science and Technology (02-2917-I-564-006-A1), National Defense Science and Engineering Graduate (NDSEG) Fellowship, United States. Department of Defense (32 CFR 168a DoD), United States. Air Force. Office of Scientific Research, United States. Department of Energy. Office of Science (Contract No. DE-AC02-05CH11231)

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2013., Cataloged from PDF version of thesis., Includes bibliographical references (pages 175-203)., Efficiency is a major lever for cost reduction in crystalline silicon solar cells, which dominate the photovoltaics market but cannot yet compete subsidy-free in most areas. Iron impurities are a key performance-limiting defect present in commercial and precommercial silicon solar cell materials, affecting devices at concentrations below even one part per billion. The lack of process simulation tools that account for the behavior of such impurities hinders efforts at increasing efficiency in commercial materials and slows the time-to-market for novel materials. To address the need for predictive process modeling focused on the impact of impurities, the Impurity-to-Efficiency kinetics simulation tool is developed to predict solar cell efficiency from initial iron contamination levels. The modeling effort focuses on iron because it is known to limit most industrial solar cells. The simulation models phosphorus diffusion, the coupled diffusion and segregation of iron to the high phosphorus concentration emitter, and the dissolution and growth of iron-silicide precipitates. The ID process simulation can be solved in about 1 minute assuming standard processing conditions, allowing for rapid iteration. By wrapping the kinetics simulation tool with a genetic algorithm, global optima in the high-dimensional processing parameter space can be pursued for a given starting metal concentration and distribution. To inform and test the model, synchrotron-based X-ray fluorescence is employed with beam spot sizes less than 200 nm to identify iron-rich precipitates down to 10 nm in radius in industrial and research materials. Experimental X-ray fluorescence data confirm model predictions that iron remains in heavily-contaminated multicrystalline materials after a typical industrial phosphorus diffusion. Similar measurements of the iron-silicide precipitate distribution in multicrystalline silicon samples before and after higher-temperature gettering steps confirm that the higher the, by David P. Fenning., Ph. D.

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.

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 FeSi[subscript 2] 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., United States. Dept. of Energy (Contract DE-FG36-09GO1900), Spanish Ministry of Science and Innovation (Thincells Project TEC2008-06798-C03-02)

Synchrotron-based X-ray fluorescence microscopy is applied to study the evolution of iron silicide precipitates during phosphorus diffusion gettering and low-temperature annealing. Heavily Fe-contaminated ingot border material contains FeSi2 precipitates after rapid in-line P-diffusion firing, suggesting kinetically limited gettering in these regions. An impurity-to-efficiency (I2E) gettering model is developed to explain the results. The model demonstrates the efficacy of high- and medium-temperature processing on reducing the interstitial iron population over a range of process parameters available to industry., United States. Dept. of Energy (contract number DE-FG36-09GO1900), National Science Foundation (U.S.) (NSF Graduate Research Fellowship), Barcelona Chamber of Commerce (MIT-Spain/La Cambra de Barcelona Seed Fund)

The extraction of metal impurities during phosphorus diffusion gettering (PDG) is one of the crucial process steps when fabricating high-efficiency solar cells using low-cost, lower-purity silicon wafers. In this work, we show that for a given metal concentration, the size and density of metal silicide precipitates strongly influences the gettering efficacy. Different precipitate size distributions can be already found in silicon wafers grown by different techniques. In our experiment, however, the as-grown distribution of precipitated metals in multicrystalline Si sister wafers is engineered through different annealing treatments in order to control for the concentration and distribution of other defects. A high density of small precipitates is formed during a homogenization step, and a lower density of larger precipitates is formed during extended annealing at 740º C. After PDG, homogenized samples show a decreased interstitial iron concentration compared to as-grown and ripened samples, in agreement with simulations.