UNSW School of Photovoltaic & Renewable Energy Engineering
Understanding boron-oxygen related light-induced degradation: Properties, kinetics and deactivation mechanisms
Nitin Nampalli - UNSW SPREE

Nitin Nampalli, at UNSW SPREE, 9 November 2017

Nitin Nampalli (52Min)


Nitin Nampalli speaks at UNSW SPREE


The boron-oxygen (BO) defect is ubiquitously present in p-type Czochralski silicon and is known to cause significant performance degradation in commercial silicon solar cells. Despite being well-known for decades, the nature and behaviour of the defect are not yet completely understood. In this presentation, three aspects of the BO defect are investigated. First, the recombination properties of the defect are studied in detail. Based on carrier lifetime modelling, it is shown that the capture cross section ratio of the donor energy level of the BO defect is higher than previously assumed. This is confirmed by temperature-and injection-dependent lifetime spectroscopy (TIDLS), which is applied for the first time to study the BO defect. TIDLS is also used to obtain a parameterisation that allows carrier lifetime at elevated temperatures to be easily determined using measurements at room temperature.

Next, the transition kinetics between two of the three known states of the BO defect are investigated. By accurate measurement and analysis of the rates of degradation and temporary deactivation (i.e. annealing) of the BO defect, the carrier dependence of the these two transitions is confirmed and quantified. Based on this, accurate estimates of the Arrhenius activation parameters relating to these transitions are obtained and are shown to explain known variations in published data. Finally, new insights into the mechanisms that lead to permanent deactivation of the BO defect are presented. It is shown that there are two independent pathways for permanent deactivation of the BO defect - a thermal pathway that is induced by firing or other high-temperature processes, and a hydrogen related pathway that explains the well-known phenomenon of regeneration that occurs under illuminated annealing at lower temperatures.

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Brief Bio

Nitin recently completed his Ph.D. degree in photovoltaics at the University of New South Wales (UNSW, Sydney, Australia) under the supervision of Prof. Stuart Wenham, Dr. Malcolm Abbott and Dr. Matthew Edwards. His Ph.D. thesis focuses on boron-oxygen defect characterisation and passivation in Czhochralski silicon, and his other research interests include defect characterisation, carrier lifetime modelling, and fabrication of high-efficiency silicon solar cells and modules. Nitin is currently a post-doctoral researcher at Aalto University (Helsinki, Finland) and works part-time at UNSW, where he continues to investigate methods to mitigate various carrier-induced defects in silicon. He has previously served as an R&D advisor for Brij Encapsulants (New Delhi, India) and has performed photovoltaics research at the Interuniversity Microelectronics Centre (Leuven, Belgium). Outside of research, Nitin is also the co-founder of BlueVolt Solar, an early-stage start-up making self-installable solar power systems, and has worked as a technology news analyst and writer for Solar Choice News. Nitin holds a Masters degree in photovoltaics from UNSW and an undergraduate degree in microelectronics from the Rochester Institute of Technology (Rochester, NY, USA).