Guest lecture: Minority carrier lifetime in silicon photovoltaic materials

By Prof. John Murphy from University of Warwick UK

Abstract: Minority carrier lifetime is the key property of silicon substrates for all types for photovoltaic cells. As well as being a useful figure of merit, the dependence of lifetime on excess carrier density can, in well controlled sample sets, reveal a great deal about the physics of the recombination processes. I will start my talk by giving an introduction to the approach we have been using to analyse the injection-dependence of lifetime in terms of defect properties [1]. We have successfully used this approach to quantify and to understand recombination at oxygen-related extended defects in silicon which can form during processing of mono-Si cells. Our approach enables lifetime to be quantified in both p-type [1] and n-type silicon [2].

In the second part of the talk I will discuss our recent work on low temperature internal gettering of impurities in multi-crystalline silicon [3]. In this work our methodology is carefully controlled to separate thermal and bulk hydrogenation effects. Our results show that long term annealing at < 400 °C can give rise to substantial lifetime improvements in as-received mc-Si materials, in the absence of any hydrogenation which may result from a dielectric film. Additional experiments are performed with silicon nitride surface passivation and distinct differences in the evolution of both lifetime and interstitial iron with processing are found. These are discussed in terms of bulk hydrogenation and gettering of iron to the silicon nitride film.

[1]   J. D. Murphy et al. Journal of Applied Physics 111, 113709 (2012), doi: 10.1063/1.4725475.

[2]   J. D. Murphy et al. Journal of Applied Physics 118, 215706 (2015), doi: 10.1063/1.4936852.

[3]   M. Al-Amin, J. D. Murphy, Journal of Applied Physics 119, 235704 (2016), doi: 10.1063/1.4954010.

Biography: Dr John Murphy studied Physics at the University of Oxford (UK), followed by a doctorate in Materials Science at Oxford which he completed under the supervision of Prof. Peter Wilshaw in 2006. His doctoral work, part-funded by MEMC (now SunEdison), focussed on the fundamental properties of oxygen and nitrogen in silicon for integrated circuit applications. After a postdoc working on the mechanical properties of BCC metals for nuclear fusion applications, he was awarded a five Royal Academy of Engineering/ EPSRC Research Fellowship at Oxford to work on silicon materials for photovoltaics. He moved to a faculty position at the University of Warwick in early 2013, and is now an Associate Professor in the School of Engineering. He currently leads the £1.6m EPSRC SuperSilicon PV project, which unites the UK’s silicon PV materials work at Warwick, Manchester and Oxford.