This report discusses the project results and lessons learnt to date for the UNSW Project, Development of Novel Hydrogen Trapping Techniques for Breakthrough Si Casting and Wafering Technologies.
With silicon still a dominant cost of making solar cells, the focus of this project is to develop new techniques to heal defects that form in cheaper silicon wafers. New cheaper wafer sources such as cast quasi-mono (QM) and kerfless small-grain wafers offer the potential for significantly reduced solar cell cost, but generally have lower performance due to these defects that form during the silicon growth. Previously, testing typical advanced hydrogenation processes on such wafers was shown to be ineffective at healing these grown-in defects, likely due to the rapid dispersion of hydrogen throughout all the silicon. This project focuses on newly patented methods of controlling the hydrogen that utilise the defects to absorb more hydrogen into the silicon and trap it in the defected regions where it is needed the most.The project aims to improve the understanding of defects in these cheaper materials in order to optimise, develop and commercialise the new patented hydrogen trapping processes.
We have developed hydrogen trapping techniques that can enhance the quality of quasi-mono Si by over 70% compared to conventional hydrogenation processes. Somewhat surprisingly, we found that a hydrogen trapping processes alone provide better passivation (healing) of grown-in defects than combinations of hydrogen trapping and advanced hydrogenation processes. We found that some localised types of grown in defects (certain dislocations in QMSi) do not respond well to the trapping processes applied, or any other process applied to date.
Further investigation into these particular defects is required and they will ultimately need to be treated differently, or prevented from forming through optimised silicon growth techniques. These defects aside, the vast majority of grown-in defects respond very well to the hydrogen trapping techniques developed in this work.