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Dive into the research topics where Ashley E. Morishige is active.

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Featured researches published by Ashley E. Morishige.


IEEE Journal of Photovoltaics | 2016

Engineering Solutions and Root-Cause Analysis for Light-Induced Degradation in p -Type Multicrystalline Silicon PERC Modules

Kenta Nakayashiki; Jasmin Hofstetter; Ashley E. Morishige; Tsu-Tsung Andrew Li; David Berney Needleman; Mallory A. Jensen; Tonio Buonassisi

We identify two engineering solutions to mitigate light-induced degradation (LID) in p-type multicrystalline silicon passivated emitter and rear cells, including modification of metallization firing temperature and wafer quality. Lifetime measurements on etched-back samples confirm that LID has a strong bulk component. Spatially resolved lifetime maps indicate that the defects responsible for LID are dispersed ubiquitously across the wafer. Reversibility of LID upon low-temperature annealing suggests a low-activation-energy barrier inconsistent with precipitated impurity dissolution. Lifetime spectroscopy of the LID-affected state reveals an asymmetry of electron and hole capture cross sections of ~28.5, consistent with a deep-level donor point defect (e.g., interstitial Ti, interstitial Mo, substitutional W), charged nanoprecipitate, or charged structural defect, such as a dislocation. Finally, we explain two possible root causes of this LID, including 1) a point-defect complex involving a hydrogen atom and a deep-level donor and 2) configurational change of a point-defect complex involving fast-diffusing impurities.


Applied Physics Letters | 2015

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

Mallory A. Jensen; Jasmin Hofstetter; Ashley E. Morishige; Gianluca Coletti; Barry Lai; David P. Fenning; Tonio Buonassisi

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 ...


IEEE Journal of Photovoltaics | 2016

Lifetime Spectroscopy Investigation of Light-Induced Degradation in p-type Multicrystalline Silicon PERC

Ashley E. Morishige; Mallory A. Jensen; David Berney Needleman; Kenta Nakayashiki; Jasmin Hofstetter; Tsu-Tsung Andrew Li; Tonio Buonassisi

When untreated, light-induced degradation (LID) of p-type multicrystalline silicon (mc-Si)-based passivated emitter and rear cell (PERC) modules can reduce power output by up to 10% relative during sun-soaking under open-circuit conditions. Identifying the root cause of this form of LID has been the subject of several recent investigations. Lifetime spectroscopy analysis, including both injection and temperature dependencies (IDLS and TIDLS), may offer insight into the root-cause defect(s). In this paper, to illustrate the root-case defect identification method, we apply room-temperature IDLS to intentionally Cr-contaminated mc-Si. Then, we apply this technique to the p-type mc-Si that exhibits LID in PERC devices, and we provide further insights by analyzing qualitatively the injection-dependent lifetime as a function of temperature. We quantify the sensitivity of the capture cross-section ratio to variations in the measured lifetime curve and in the surface recombination. We find that the responsible defect most likely has an energy level between 0.3 and 0.7 eV above the valence band and a capture cross-section ratio between 26 and 36. Additionally, we calculate the concentrations of several candidate impurities that may cause the degradation.


IEEE Journal of Photovoltaics | 2016

High-Performance and Traditional Multicrystalline Silicon: Comparing Gettering Responses and Lifetime-Limiting Defects

Sergio Castellanos; Kai Erik Ekstrøm; Antoine Autruffe; Mallory A. Jensen; Ashley E. Morishige; Jasmin Hofstetter; Patricia X. T. Yen; Barry Lai; Gaute Stokkan; Carlos del Cañizo; Tonio Buonassisi

In recent years, high-performance multicrystalline silicon (HPMC-Si) has emerged as an attractive alternative to traditional ingot-based multicrystalline silicon (mc-Si), with a similar cost structure but improved cell performance. Herein, we evaluate the gettering response of traditional mc-Si and HPMC-Si. Microanalytical techniques demonstrate that HPMC-Si and mc-Si share similar lifetime-limiting defect types but have different relative concentrations and distributions. HPMC-Si shows a substantial lifetime improvement after P-gettering compared with mc-Si, chiefly because of lower area fraction of dislocation-rich clusters. In both materials, the dislocation clusters and grain boundaries were associated with relatively higher interstitial iron point-defect concentrations after diffusion, which is suggestive of dissolving metal-impurity precipitates. The relatively fewer dislocation clusters in HPMC-Si are shown to exhibit similar characteristics to those found in mc-Si. Given similar governing principles, a proxy to determine relative recombination activity of dislocation clusters developed for mc-Si is successfully transferred to HPMC-Si. The lifetime in the remainder of HPMC-Si material is found to be limited by grain-boundary recombination. To reduce the recombination activity of grain boundaries in HPMC-Si, coordinated impurity control during growth, gettering, and passivation must be developed.


IEEE Journal of Photovoltaics | 2014

Sorting Metrics for Customized Phosphorus Diffusion Gettering

Jasmin Hofstetter; David P. Fenning; Douglas M. Powell; Ashley E. Morishige; Hannes Wagner; Tonio Buonassisi

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.


Journal of Applied Physics | 2016

Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

Hannes Wagner; Amir Dastgheib-Shirazi; Byungsul Min; Ashley E. Morishige; Michael Steyer; Giso Hahn; Carlos del Cañizo; Tonio Buonassisi; Pietro P. Altermatt

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivityρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.


Applied Physics Letters | 2016

Synchrotron-based investigation of transition-metal getterability in n -type multicrystalline silicon

Ashley E. Morishige; Mallory A. Jensen; Jasmin Hofstetter; Patricia X. T. Yen; Chenlei Wang; Barry Lai; David P. Fenning; Tonio Buonassisi

Solar cells based on n-type multicrystalline silicon (mc-Si) wafers are a promising path to reduce the cost per kWh of photovoltaics; however, the full potential of the material and how to optimally process it are still unknown. Process optimization requires knowledge of the response of the metal-silicide precipitate distribution to processing, which has yet to be directly measured and quantified. To supply this missing piece, we use synchrotron-based micro-X-ray fluorescence (μ-XRF) to quantitatively map >250 metal-rich particles in n-type mc-Si wafers before and after phosphorus diffusion gettering (PDG). We find that 820 °C PDG is sufficient to remove precipitates of fast-diffusing impurities and that 920 °C PDG can eliminate precipitated Fe to below the detection limit of μ-XRF. Thus, the evolution of precipitated metal impurities during PDG is observed to be similar for n- and p-type mc-Si, an observation consistent with calculations of the driving forces for precipitate dissolution and segregation g...


IEEE Journal of Photovoltaics | 2017

Evolution of LeTID Defects in p-Type Multicrystalline Silicon During Degradation and Regeneration

Mallory A. Jensen; Ashley E. Morishige; Jasmin Hofstetter; David Berney Needleman; Tonio Buonassisi

While progress has been made in developing engineering solutions and understanding light- and elevated temperature-induced degradation (LeTID) in p-type multicrystalline silicon (mc-Si), open questions remain regarding the root cause of LeTID. Previously, lifetime spectroscopy of multicrystalline silicon (mc-Si) passivated emitter and rear cell semifabricates in the unaffected and the degraded states enabled identification of the effective recombination parameters of the responsible defect. To gain further insight into the root cause of LeTID, in this paper, we measure the injection-dependent lifetime throughout degradation and regeneration and perform lifetime spectroscopy at several time points. Our analysis indicates that the change in lifetime during most of the process can be described by a corresponding change in the concentration of a single responsible defect. We also explore further exposure to light and temperature after nearly complete regeneration and a subsequent dark anneal to demonstrate that the behavior is no longer consistent with LeTID and the same defect is not detected by lifetime spectroscopy at maximum degradation. We consider our results in the context of the proposed hypotheses for LeTID and conclude that both hydrogenation and precipitate dissolution during firing are consistent with our results.


Journal of Applied Physics | 2016

Exceptional gettering response of epitaxially grown kerfless silicon

Douglas M. Powell; V. P. Markevich; Jasmin Hofstetter; Mallory A. Jensen; Ashley E. Morishige; Sergio Castellanos; Barry Lai; A. R. Peaker; Tonio Buonassisi

The bulk minority-carrier lifetime in p- and n-type kerfless epitaxial (epi) crystalline silicon wafers is shown to increase >500× during phosphorus gettering. We employ kinetic defect simulations and microstructural characterization techniques to elucidate the root cause of this exceptional gettering response. Simulations and deep-level transient spectroscopy (DLTS) indicate that a high concentration of point defects (likely Pt) is “locked in” during fast (60 °C/min) cooling during epi wafer growth. The fine dispersion of moderately fast-diffusing recombination-active point defects limits as-grown lifetime but can also be removed during gettering, confirmed by DLTS measurements. Synchrotron-based X-ray fluorescence microscopy indicates metal agglomerates at structural defects, yet the structural defect density is sufficiently low to enable high lifetimes. Consequently, after phosphorus diffusion gettering, epi silicon exhibits a higher lifetime than materials with similar bulk impurity contents but highe...


IEEE Journal of Photovoltaics | 2016

Impact of Iron Precipitation on Phosphorus-Implanted Silicon Solar Cells

Hannu S. Laine; Ville Vähänissi; Ashley E. Morishige; Jasmin Hofstetter; Antti Haarahiltunen; Barry Lai; Hele Savin; David P. Fenning

Ion implantation is a promising method to implement a high-performance emitter for crystalline silicon solar cells. However, an implanted emitter redistributes and mitigates harmful metal impurities to a different degree than a diffused one. This paper quantitatively assesses the effect of iron contamination level on the bulk diffusion length and open-circuit voltage of phosphorus-implanted solar cells manufactured with varying gettering parameters. By synchrotron-based micro-X-ray fluorescence measurements, we directly observe a process-dependent iron precipitate size distribution in the implanted emitters. We show that controlling the iron precipitate size distribution is important when optimizing final cell performance and discover a tradeoff between large shunting precipitates in the emitter and a high density of recombination active small precipitates in the wafer bulk. We present a heterogeneous iron precipitation model capable of reproducing the experimentally measured size distributions. We use the model to show that the dominant gettering mechanism in our samples is precipitation and that implanted emitters with surface phosphorus concentrations around 2×1019 cm-3 induce little-to-no segregation-based gettering. Based on this finding, we discuss optimal gettering strategies for industrial silicon solar cells with implanted emitters.

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Tonio Buonassisi

Massachusetts Institute of Technology

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Jasmin Hofstetter

Massachusetts Institute of Technology

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Mallory A. Jensen

Massachusetts Institute of Technology

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Barry Lai

Argonne National Laboratory

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Douglas M. Powell

Massachusetts Institute of Technology

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Sergio Castellanos

Massachusetts Institute of Technology

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