Peter Wilshaw

Professor Peter Wilshaw
Professor of Materials
+44 1865 273736
peter.wilshaw@materials.ox.ac.uk

My interests are those of the Semiconductor and Silicon Photovoltaics group. They are described in more detail at http://semiconductor.materials.ox.ac.uk/ 

In brief the group researches the properties and processing of semiconductors particularly silicon and particularly with reference to the production of improved solar cells. The research comprises four main areas.

 

  • The use of permanent charges introduced into dielectrics for surface passivation and application to novel cell geometries, passivated contacts etc.
  • The development of shielded hydrogen passivation for improved surface and bulk passivation.
  • Understanding and improving gettering processes which includes saw damage gettering, developing novel high temperature gettering processes and atom by atom analysis of electrically active defects (as characterised by electron beam induced current, EBIC) using atom probe tomography.
  • The development of a new vapour phase texturing technique for all silicon types including diamond wire sawn HP-multi.

Selected Publications

  • Imaging and quantifying carrier collection in silicon solar cells: A submicron study using electron beam induced current

    November 2020
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    Journal article
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    Solar Energy
    © 2020 International Solar Energy Society In this work electron-beam-induced current (EBIC) is used to study the collection efficiency of emitters in industrial silicon solar cells. Laser-doped local emitters have been deployed industrially, yet in mas production they are designed wider than the screen-printed silver fingers to allow alignment tolerances. EBIC has allowed to image and quantify the laser-induced damage that occurs in these local emitter regions. A model is developed to account for such damage, so that losses in EQE could be quantified from the observed EBIC collection characteristics. The damaged regions present ~12% lower collection efficiency at short wavelength (300–500 nm) than the homogenous emitter. Sentaurus TCAD simulations reveal that eliminating such damage would improve cell efficiency by ~0.12%. Additional degradation is found in a region 1–2 µm wide adjacent to the silver fingers, which has not been detected before. It is also found that pulsed laser doping leads to ~15 µm long un-doped gaps, along the direction of laser movement. As laser doping becomes a key part of industrial cell fabrication, it is crucial to develop a better understanding of the potential pitfalls, and future improvements to the process. The versatility of EBIC imaging is also demonstrated using FIB milling to improve lateral resolution and study the depth profile of boron emitters in newly developed industrial i-TOPCon cells. EBIC imaging, in combination with advanced device simulations, have proven powerful tools to elucidate carrier collection characteristics and drawbacks, thus helping to understand and improve fabrication processes at industrial level.

  • Optoelectronic properties of ultrathin ALD silicon nitride and its potential as a hole-selective nanolayer for high efficiency solar cells

    November 2020
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    Journal article
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    APL Materials

  • Special issue: Surface and interface passivation in crystalline silicon solar cells

    August 2020
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    Journal article
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    Solar Energy Materials and Solar Cells

  • Atom Probe Tomography Study of Gettering in High-Performance Multicrystalline Silicon

    May 2020
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    Journal article
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    IEEE Journal of Photovoltaics
    © 2011-2012 IEEE. During the production of high-performance multicrystalline silicon (HPMC-Si) solar cells, gettering occurs inherently during the formation of an emitter. The material benefits with an increase in minority carrier lifetime from the external gettering of impurities into the diffused layer. However, depending on the thermal budget and parameters of the emitter diffusion, as well as the specific material properties, the process can also be detrimental in terms of increased recombination activity of specific crystallographic defects. Thus, it is important to understand the root causes behind the change in recombination activity of defects following gettering. Here, we present a correlative atom probe tomography study of grain boundaries in both p- and n-type HPMC-Si before and after gettering. The presence of nitrogen was found to directly correlate with the increase in recombination activity at grain boundaries. Additionally, an estimation of the atom probe tomography detection limit for transition metals in silicon is made and found to be greater than known impurity levels in commercial HPMC-Si.

  • Understanding and optimizing EBIC pn-junction characterization from modeling insights

    January 2020
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    Journal article
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    Journal of Applied Physics
    © 2020 Author(s). In this paper, the physical mechanisms involved in electron-beam-induced current (EBIC) imaging of semiconductor pn-junctions are reviewed to propose a model and optimize the acquisition of experimental data. Insights are drawn on the dependence of the EBIC signal with electron accelerating voltage and surface conditions. It is concluded that improvements in the resolution of EBIC are possible when the surface conditions of the specimens are carefully considered and optimized. A lower accelerating voltage and an increase of the surface recombination velocities are quantitatively shown to maximize the EBIC lateral resolution in locating the pn-junction. The effect of surface band bending is included in the model, and it is seen to primarily affect the surface recombination. Introducing controlled surface damage is shown as a potential method for resolution enhancement via focused ion beam milling with Ga+ ions. These findings contribute to the understanding of this technique and can produce further improvements to its application in semiconductor device technology.

  • Direct Observation of Hydrogen at Defects in Multicrystalline Silicon

    August 2019
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    Journal article
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    Progress in Photovoltaics

  • Identification of colloidal silica polishing induced contamination in silicon

    June 2019
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    Journal article
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    Materials Characterization
    © 2019 This paper presents a multiscale characterisation approach, analysing the effect of colloidal silica polishing on crystallographic defects in multicrystalline silicon. Colloidal silica polishing for as little time as 30 min was found to significantly increase the recombination activity of all defects, as measured by electron beam induced current mapping. The impurities responsible for the room temperature contamination of defects due to colloidal silica polishing were Cu and Ni, as measured by atom probe tomography.

  • Microstructural Evolution of Mechanically Deformed Polycrystalline Silicon for Kerfless Photovoltaics

    May 2019
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    Journal article
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    Physica Status Solidi (A) Applications and Materials Science
    © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Silicon wafers for photovoltaics could be produced without kerf loss by rolling, provided sufficient control of defects such as dislocations can be achieved. A study using mainly high resolution electron backscatter diffraction (HR-EBSD) of the microstructural evolution of Siemens polycrystalline silicon feedstock during a series of processes designed to mimic high temperature rolling is reported here. The starting material is heavily textured and annealing at 1400 °C results in 90% recrystallization and a reduction in average geometrically necessary dislocation (GND) density from >1014 to 1013 m−2. Subsequent compression at 1150 °C – analogous to rolling – produce sub-grain boundaries seen as continuous curved high GND content linear features spanning grain interiors. Post-deformation annealing at 1400 °C facilitates a secondary recrystallization process, resulting in large grains typically of 100 μm diameter. HR-EBSD gives the final average GND density in as 3.2 × 1012 m−2. This value is considerably higher than the dislocation density of 5 × 1010 m−2 from etch pit counting, so the discrepancy is investigated by direct comparison of GND maps and etch pit patterns. The GND map from HR-EBSD gives erroneously high values at the method's noise floor (≈1012 m−2) in regions with low dislocation densities.

  • Scalable Techniques for Producing Field-Effect Passivation in High-Efficiency Silicon Solar Cells

    January 2019
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    Journal article
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    IEEE JOURNAL OF PHOTOVOLTAICS
    Field-effect passivation, ion charged dielectrics, Silicon photovoltaics, surface passivation

  • The Behavior and Transport of Hydrogen in Silicon Solar Cells Observed through Changes in Contact Resistance

    November 2018
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    Conference paper
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    2018 IEEE 7th World Conference on Photovoltaic Energy Conversion, WCPEC 2018 - A Joint Conference of 45th IEEE PVSC, 28th PVSEC and 34th EU PVSEC
    © 2018 IEEE. This paper presents a simple method of studying the transport of hydrogen in crystalline silicon, as well as its release from bound forms during annealing at temperatures between 350-450°C. This technique is based upon observations of an increase in the contact resistance of industrial solar cells, which displays the reversibility and dependence upon electric field commonly associated with hydrogen. It is shown that the increase in contact resistance depends strongly upon whether the device is fabricated on multi-or mono-crystalline silicon and upon the device structure.
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