David P. Fenning

Electrode–Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights

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 yLi2-xMnO3·(1-y)Li1-xMO2, 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.

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.

Improved iron gettering of contaminated multicrystalline silicon by high-temperature phosphorus diffusion

The efficacy of higher-temperature gettering processes in reducing precipitated iron concentrations is assessed by synchrotron-based micro-X-ray fluorescence. By measuring the same grain boundary before and after phosphorus diffusion in a set of wafers from adjacent ingot heights, the reduction in size of individual precipitates is measured as a function of gettering temperature in samples from the top of an ingot intentionally contaminated with iron in the melt. Compared to a baseline 820 °C phosphorus diffusion, 870 °C and 920 °C diffusions result in a larger reduction in iron-silicide precipitate size. Minority carrier lifetimes measured on wafers from the same ingot heights processed with the same treatments show that the greater reduction in precipitated metals is associated with a strong increase in lifetime. In a sample contaminated with both copper and iron in the melt, significant iron gettering and complete dissolution of detectable copper precipitates is observed despite the higher total metal concentration. Finally, a homogenization pre-anneal in N2 at 920 °C followed by an 820 °C phosphorus diffusion produces precipitate size reductions and lifetimes similar to an 870 °C phosphorus diffusion without lowering the emitter sheet resistance.

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.

Analyses of the Evolution of Iron-Silicide Precipitates in Multicrystalline Silicon During Solar Cell Processing

We simulate the precipitation of iron during the multicrystalline ingot crystallization process and the redistribution of iron during subsequent phosphorus diffusion gettering with a 2-D model. We compare the simulated size distribution of the precipitates with the X-ray fluorescence microscopy measurements of iron precipitates along a grain boundary. We find that the simulated and measured densities of precipitates larger than the experimental detection limit are in good agreement after the crystallization process. Additionally, we demonstrate that the measured decrease of the line density and the increase of the mean size of the iron precipitates after phosphorus diffusion gettering can be reproduced with the simulations. The size and spatial distribution of iron precipitates affect the kinetics of iron redistribution during the solar cell process and, ultimately, the recombination activity of the precipitated iron. Variations of the cooling rate after solidification and short temperature peaks before phosphorus diffusion strongly influence the precipitate size distribution. The lowest overall density of iron precipitates after phosphorus diffusion is obtained in the simulations with a temperature peak before phosphorus diffusion, followed by moderate cooling rates.

Thickness-Dependent Photoelectrochemical Water Splitting on Ultrathin LaFeO3 Films Grown on Nb:SrTiO3

The performance of photoelectrodes can be modified by changing the material chemistry, geometry, and interface engineering. Specifically, nanoscale active layers can facilitate the collection of charge carriers. In heterostructure devices, the multiple material interfaces are particularly important, which at present are not well understood for oxides. Here, we report a detailed study of ultrathin (2-25 nm) LaFeO3 films grown epitaxially on Nb-doped SrTiO3. The films exhibit thickness-dependence with sensitivity to less than 10 nm in both the through-plane charge transfer conductivity and in the potential-dependent photoresponse. Supplementing photoelectrochemical measurements with X-ray photoelectron spectroscopy, spectroscopic ellipsometry, and electrochemical impedance spectroscopy, we construct a band model that accounts for this thickness dependence via a shifting valence-band offset at the film-substrate interface and the potential-dependent overlap of the depletion regions present at both the film-substrate and film-electrolyte interfaces. These results illustrate the utility of using active layer thickness and film-substrate interactions to tune the performance of photoelectrodes, providing insight for the design of efficient heterostructure oxide photoelectrochemical devices.

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 1010 cm−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 Cri 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 ...

Sorting Metrics for Customized Phosphorus Diffusion Gettering

Customized solar cell processing based on input material quality has the potential to increase the performance of contaminated regions of multicrystalline silicon ingots. This provides an opportunity to improve material yield and device efficiency without substantially reducing the overall throughput. Simulations and experiments show that in wafers from the top and border regions of an ingot containing as-grown iron concentrations ≳1014 cm-3, a high concentration of interstitial iron point defects, i.e., Fei, remains after standard phosphorus diffusion gettering (PDG), severely limiting electron lifetime and simulated efficiencies of PERC-type solar cells. It is shown that an extended PDG leads to a stronger reduction of Fei point defects, enabling high-efficiency devices, even on wafers from the red zone of the ingot. However, a satisfactory performance improvement after standard PDG is already achieved on wafers that contain as-grown total iron concentrations <;1014 cm-3, making the low-throughput extended PDG process unnecessary for a large fraction of the ingot. We propose using the total iron concentration and the corresponding photoluminescence contrast between grain boundaries and intragranular regions in the as-grown wafer as a simple sorting metric to determine when extended phosphorus diffusion is warranted.

Dislocation Density Reduction During Impurity Gettering in Multicrystalline Silicon

Isothermal annealing above 1250 °C has been reported to reduce the dislocation density in multicrystalline silicon (mc-Si), presumably by pairwise dislocation annihilation. However, this high-temperature process may also cause significant impurity contamination, canceling out the positive effect of dislocation density reduction on cell performance. Here, efforts are made to annihilate dislocations in mc-Si in temperatures as low as 820 °C, with the assistance of an additional driving force to stimulate dislocation motion. A reduction of more than 60% in dislocation density is observed for mc-Si containing intermediate concentrations of certain metallic species after P gettering at 820 °C. While the precise mechanism remains in discussion, available evidence suggests that the net unidirectional flux of impurities in the presence of a gettering layer may cause dislocation motion, leading to dislocation density reduction. Analysis of minority carrier lifetime as a function of dislocation density suggests that lifetime improvements after P diffusion in these samples can be attributed to the combined effects of dislocation density reduction and impurity concentration reduction. These findings suggest there may be mechanisms to reduce dislocation densities at standard solar cell processing temperatures.

Retrograde Melting and Internal Liquid Gettering in Silicon

COMMUNICATION Retrograde Melting and Internal Liquid Gettering in Silicon By Steve Hudelson , Bonna K. Newman , Sarah Bernardis , David P. Fenning , Mariana I. Bertoni , Matthew A. Marcus , Sirine C. Fakra , Barry Lai , and Tonio Buonassisi * Control of metal impurities has proven essential for developing modern semiconductor-based materials and devices. The prop- erties of high-performance integrated circuit, photovoltaic, and thermoelectric devices are tailored by the intentional introduc- tion of dopant species, as well as the removal and passivation of detrimental impurities. [ 1 , 2 ] In addition, the speed and uni- formity of several common semiconductor growth methods, including bulk crystal and vapor-liquid-solid (VLS) growth, are regulated by impurity-semiconductor interactions. [ 3 , 4 ] Precise control over impurity chemical states and spatial distributions requires a deep fundamental understanding of the thermodynamics and kinetics regulating impurity phase and transport. Impurity engineering in semiconductors typi- cally involves thermal annealing, as impurity solubility and diffusivity increase exponentially with temperature. However, because of the lack of suitable analytical tools for studying sub-micron-scale distributions of fast-diffusing impurities at elevated temperatures, the vast majority of experimental inves- tigations so far have been conducted at room temperature. As a result, much remains to be explored concerning fundamental impurity-semiconductor reactions at realistic processing temperatures. It was recently proposed [ 5 ] that certain silicon-impurity sys- tems can undergo melting upon cooling, a phenomenon known as retrograde melting . The controlled creation of liquid metal- silicon droplets within or on the surface of a silicon matrix of arbitrary shape could provide novel opportunities to engineer semiconductor-based systems via solid-liquid and vapor-liquid segregation. The phenomenon of retrograde melting, whereby a liquid phase forms from a solid phase upon cooling , has been observed and studied in several organic and inorganic systems, including Fe-Zr [ 6 ] and Mg-Fe-Si-O. [ 7 ] One common pathway [ ∗ ] S. Hudelson, [+] Dr. B. K. Newman, S. Bernardis, D. P. Fenning, Dr. M. I. Bertoni, Prof. T. Buonassisi Massachusetts Institute of Technology Cambridge, Massachusetts, 02139 (USA) E-mail: buonassisi@mit.edu Dr. M. A. Marcus, S. C. Fakra Advanced Light Source Lawrence Berkeley National Laboratory Berkeley, California, 94720 (USA) Dr. B. Lai Advanced Photon Source Argonne National Laboratory Argonne, Illinois, 60439 (USA) [ + ] Present address: 1366 Technologies, Lexington, MA 02421, USA for this process to occur is via the catatectic reaction, occur- ring at an invariant point on a binary phase diagram involving transformation from Solid → Solid + Liquid. [ 8 ] Many binary sys- tems exhibit such an invariant point, [ 9 ] including Ag-In, Cu-Sn, Fe-Mn, and Fe-S, [ 10 ] but very few are semiconducting mate- rials. [ 11 ] Retrograde melting in most common silicon-impurity systems cannot occur by this pathway, as these systems do not possess a catatectic point. [ 11 ] A second pathway for retrograde melting has been observed in the ternary Sb-Bi-Te system, wherein decreasing solubility of Te in Sb 2 Te 3 with decreasing temperature can lead to supersat- uration of Te and formation of liquid droplets at temperatures above the eutectic temperature. [ 12 ] We propose that a similar pathway could also produce retrograde melting in binary semiconductor-impurity systems that exhibit retrograde solu- bility. Due to the high enthalpy of formation of point defects in certain semiconductors, the solid solubility of an impu- rity within the crystal structure increases with temperature, reaching a maximum well above the eutectic temperature. Many dissolved elements in silicon demonstrate this property, [ 13 ] including many of the 3d transition metals such as iron, copper, and nickel. [ 14 ] It is hypothesized that retrograde solubility can lead to retrograde melting, [ 5 ] if supersaturation occurs at a tem- perature above the eutectic temperature (as demonstrated in Figure 1 a ). To study temperature-dependent silicon-impurity reactions at the micro-scale, we carried out synchrotron-based hard X-ray microprobe experiments at high temperatures (up to 1500 ° C). We adapted an in situ microscope hot stage (Linkam TS1500) at beamlines 10.3.2 at the Advanced Light Source [ 15 ] and 2-ID-D at the Advanced Photon Source. [ 16 ] X-ray fluorescence microscopy ( μ -XRF) mapping was used to investigate the spatial distribu- tion of transition metal-rich particles as small as 50 nm [ 17 , 18 ] in silicon matrices. The chemical state of precipitated impurities detected by μ -XRF was determined by X-ray absorption micro- spectroscopy ( μ -XAS). [ 18 ] To verify that μ -XAS can distinguish between liquid and solid phases in metal-Si systems, we prepared a standard sample (see Experimental , sample 1) consisting of a thin layer ( ∼ 1 μ m) of e-beam evaporated Cu, Ni, and Fe sandwiched between a mc-Si wafer and a thin piece ( < 15 μ m) of monocrys- talline Czochralski Si (CZ-Si). The sample was then heated to 1045 ° C, well above the Cu-Si and Ni-Si eutectic temperatures, to ensure a liquid metal-silicon mixture. μ -XRF mapping of the standard at 1045 ° C revealed that the previously continuous film had dewetted, suggesting the presence of a high-temperature liquid state. After cooling the sample to room temperature, a visual inspection revealed that the Si cap layer was fused to the