Abstract
A central quantity to assess the high quality of monocrystalline silicon (on scales beyond mere purity) is the minority charge carrier lifetime. We demonstrate that the lifetime in high purity float zone material can be improved beyond existing observations, thanks to a deeper understanding of grown-in defects and how they can be permanently annihilated. In a first step we investigate the influence of several process sequences on the lifetime by applying a low temperature superacid passivation treatment. We find that a pre-treatment consisting of an oxidation at 1050 °C followed by a POCl3 diffusion at 900 °C can improve the lifetime by deactivating or eliminating grown-in defects. Then, pre-treated wafers of different float zone materials are passivated with three state-of-the-art layer stacks. Very high effective lifetime values are measured, thereby demonstrating the high quality of the surface passivation schemes and the pre-treated silicon wafers. The measured effective lifetimes exceed previous records, and we report an effective lifetime of 225 ms measured on a 200 µm thick 100 Ω cm n-type silicon wafer symmetrically passivated with a layer stack of a thin thermally grown oxide and a polycrystalline layer (the TOPCon layer stack).
Highlights
In the recent years the photovoltaic community has seen significant improvements in the efficiency of both industrially produced solar cells and record efficiencies of elaborate research scale cells, e.g. [1,2,3]
The measured effective lifetimes exceed previous records, and we report an effective lifetime of 225 ms measured on a 200 μm thick 100 Ω cm ntype silicon wafer symmetrically passivated with a layer stack of a thin thermally grown oxide and a polycrystalline layer
These measurements confirmed that bulk defects in the as-grown state do limit the bulk lifetime, as indicated by moderate τeff,SA on samples investigated without any thermal processing after wafer purchase
Summary
In the recent years the photovoltaic community has seen significant improvements in the efficiency of both industrially produced solar cells and record efficiencies of elaborate research scale cells, e.g. [1,2,3]. FZ silicon is typically assumed to be virtually free of recombination-active bulk defects, and it is the gold standard material to measure the maximum attainable effective charge carrier lifetimes τeff and assess the influence of surface recombination This assumption is weakened by the finding that FZ wafers are often affected by bulk defects introduced during crystal growth or typical sample fabrication, e.g. The underlying defects are known to be affected by thermal treatments and distort the comparability of different process schemes due to different thermal budget [8,10] Such unexpected changes of the bulk lifetime during processing, including dielectric deposition and subsequent annealing, can hinder the optimization of surface passivation layers, the quantification of intrinsic recombination in silicon and loss analyses in high performance solar cells
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
