Pros and cons of iron precipitates in crystalline silicon solar cells

Hannu Laine

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Meeting the increasing global energy demand in a sustainable manner requires vast amount of low carbon energy production. This Thesis begins by discussing several energy-intensive industries that are expected to grow significantly in the future, and that are intrinsically well suited to be served via a particular low carbon energy source: photovoltaics (PV). These industries include: electric vehicles, desalination and space cooling. All of these industries could sustain several TW of PV capacity during the course of the 21st century, mitigating the problems incurred by the inherent seasonal and diurnal variability of PV electricity production.  This Thesis seeks to help the PV industry realize its TW-scale potential via enhancing the cost-efficiency of silicon PV through better control of unwanted iron impurities, and in particular, iron silicide precipitates. First, this Thesis increases the productivity of a state-of-the-art iron precipitate characterization technique, synchrotron-based X-ray-fluorescence, by benchmarking a new scanning method, called on-the-fly-scanning, which enables an order-of-magnitude throughput for characterized samples. In additional to experimental techniques, a similar productivity increase is realized for state-of-the-art iron precipitation simulation tools. More specifically, it is shown that up to an order-of-magnitude faster models produce adequately accurate results compared to more complex and slower models.  Next, these advanced techniques are leveraged to improve both established and incipient silicon solar cell processes via enhanced control of iron silicide precipitates. First, a potentially industrially-feasible scheme to decrease the yield loss of the industry-standard silicon substrate material, multi-crystalline silicon, by up to 30 % is presented. The scheme, called dissolution-gettering, adds a high-temperature anneal in the beginning of typical silicon solar cell process. This high temperature anneal leaves iron precipitates in a more mobile state for the rest of the solar cell process, making them easier to mitigate.  Lastly, a comprehensive elucidation of iron precipitation in ion-implanted emitters during silicon solar cell processing is presented. This Thesis shows that in the case of implanted emitters, iron precipitates can in certain cases be beneficial rather than deleterious. The lattice damage created by the implantation process can be used as heterogeneous precipitation sites for iron point defects and hence reduce iron point defects from the silicon wafer bulk and increase solar cell efficiency potential. This mechanism is verified experimentally and a quantitative physics-based model for the phenomenon is described. The validated model can be applied in future phosphorus- and boron-implanted silicon solar cell processing.
Translated title of the contributionRautaerkaumien hyödyt ja haitat kiteisissä piiaurinkokennoissa
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Savin, Hele, Supervising Professor
Publisher
Print ISBNs978-952-60-7909-7
Electronic ISBNs978-952-60-7910-3
Publication statusPublished - 2018
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • silicon
  • solar cell
  • metal impurities
  • gettering

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