High efficiency silicon solar cells require substrates with carrier lifetimes well into the millisecond range. High bulk lifetimes can be extremely difficult to measure accurately because of recombination at the sample’s surfaces. Although it is possible, in principle, to passivate the surface well by oxidation or by deposition of a dielectric, such surface passivation approaches often change the bulk lifetime which is being measured. This can be because: (i) the passivation occurs at elevated temperature hence creating or destroying bulk defects; (ii) hydrogen enters the bulk and passivates recombination centres; or (iii) impurities are gettered to the passivating film at the sample’s surface.

It has recently been discovered that the thin film which forms on the silicon surface when samples are immersed in ionic solutions containing superacids can provide exceptional surface passivation [1-3]. As this passivating layer is formed at room temperature, the bulk lifetime of the silicon is not modified and hence the artefacts identified above are avoided. In this talk it will be shown how surface recombination velocities of 0.7 cm/s can be achieved using room temperature ionic passivation, and it will be demonstrated how this can be used to measure extremely long bulk lifetimes (>43 ms) [3]. Results of experiments which aim to establish the mechanism by which passivation occurs will be reported, and factors such as the solvent polarity, humidity and solution storage conditions will be reviewed.

The talk will then highlight three applications of the ionic passivation for improving the performance of silicon materials. The first will be in the development of high efficiency interdigitated back contact (IBC) solar cells, in which ionic surface passivation is used to understand process-induced bulk lifetime degradation [4]. The second will be in the understanding of light-induced degradation which has recently been found to occur in high performance p-type float-zone silicon [5]. The final example will involve using the superacid-based passivation to measure bulk lifetimes up to and beyond the current intrinsic lifetime limit [6]. It will be shown that the true limit of the bulk lifetime of silicon must be higher than previously assumed, and results from a study which aims to re-evaluate the fundamental limit of silicon’s performance will be presented.

References:
1. J. Bullock, D. Kiriya, N. Grant, A. Azcatl, M. Hettick, T. Kho, P. Phang, H. C. Sio, D. Yan, D. Macdonald, M. A. Quevedo-Lopez, R. M. Wallace, A. Cuevas, A. Javey, ACS Applied Materials & Interfaces 8, 24205 (2016), doi: 10.1021/acsami.6b07822.
2. N. E. Grant, T. Niewelt, N. R. Wilson, E. C. Wheeler-Jones, J. Bullock, M. Al-Amin, M. C. Schubert, A. C. van Veen, A. Javey, J. D. Murphy, IEEE Journal of Photovoltaics 7, 1574 (2017), doi: 10.1109/JPHOTOV.2017.2751511.
3. A. I. Pointon, N. E. Grant, E. C. Wheeler-Jones, P. P. Altermatt, J. D. Murphy, Solar Energy Materials & Solar Cells, in press (2018), doi: 10.1016/j.solmat.2018.03.028.
4. T. Rahman, A. To, M. E. Pollard, N. E. Grant, J. Colwell, D. N. R. Payne, J. D. Murphy, D. M. Bagnall, B. Hoex, S. A. Boden, Progress in Photovoltaics: Research and Applications 26, 38 (2018), doi: 10.1002/pip.2928.
5. T. Niewelt, M. Selinger, N. E. Grant, W. M. Kwapil, J. D. Murphy, M. C. Schubert, Journal of Applied Physics 121, 185702 (2017), doi: 10.1063/1.4983024.
6. A. Richter, S. W. Glunz, F. Werner, J. Schmidt, A. Cuevas, Physical Review B 86, 165202 (2012), doi: 10.1103/PhysRevB.86.165202.